Uv coatings basics, recent developments and new applications

UV Coatings Basics,

Recent

Developments

Applications

Reinhold Schwalm

Publisher: Elsevier Science (December 21, 2006) Language: English ISBN-10: 0444529799 ISBN-13: 978-0444529794

and

New

Table of Contents

Preface, Pages v-vii Chapter 1 - Introduction to Coatings Technology, Pages 1-18 Chapter 2 - The UV Curing Process, Pages 19-61 Chapter 3 - Network Formation and Characterization, Pages 62-93 Chapter 4 - Raw Materials, Pages 94-139 Chapter 5 - Formulations, Pages 140-159 Chapter 6 - Structure-Property Relationships, Pages 160-178 Chapter 7 - Tackling the Drawbacks of UV Systems, Pages 179-194 Chapter 8 - Classical Applications, Pages 195-205 Chapter 9 - Recent Developments, Pages 206-251 Chapter 10 - New Applications, Pages 252-290 Chapter 11 - Health, Safety and Environment, Pages 291-302 Subject index, Pages 303-310

Preface Curing a car coating as fast as lightning – dream or becoming reality soon?

Few flash lights are sufficient to completely convert falling drops of liquid UV-curable materials into hard balls before they reach the table top. This experiment has been recorded for a 3sat TV documentation and is depicted in the different stages in Figure P1. UVcurable formulations respond so fast to a complete phase change from liquid to solid because the radiation employed forms an active species as initiator for a catalytic curing reaction. The predominantly used radiation out of the electromagnetic spectrum is in the UV (wavelength range: 200 to 400 nm). Such liquid formulations applied to a substrate are termed UV-curable coatings and after curing upon UV exposure solid UV-cured coating are generated. The traditional applications of UV coatings are on thermo-sensitive substrates, like wood and paper. Since UV curing is a very eco-efficient and low-energy curing method, the growth rates of UV-curable coatings are in the range of 10% per year. The typical UV coatings containing multifunctional oligomers are solvent free (100% solids), thus helping industry to reduce significantly emissions into the atmosphere and hence protect the environment. Recently, the automotive industry has discovered that UV-cured coatings are very scratch resistant, which stimulated very extensive work into the development of UV coatings for automotive applications.

This book reviews in a comprehensive fashion the basics of the technology, describing the decision process from the application requirements over the choice of the appropriate chemistry, the raw materials (resins, diluents, photoinitiators) and formulations employed, to the curing process including network formation as well as the equipment necessary. Not only the advantages but also the drawbacks of this fast emerging technology are discussed together with proposed technical solutions to tackle these disadvantages. The main part will focus extensively on recent developments, like UV powder coatings, dual cure systems, curing under inert conditions or UV plasma curing and new applications, like DVD bonding, ink-jet printing and various automotive applications. These new systems are extending the application areas considerably, for example to the curing of three-dimensional substrates. These progressions will enable applications in the automotive industry, at first on trim parts, then for coating repair and finally in OEM car body coating. The last chapter addresses health, safety and environmental (HSE) aspects, which are of considerable importance not only to the manufacturers and operators, but also to the end users. Furthermore, manufactures, suppliers and end users have to respond to current and emerging chemical regulations, like REACH, VOC- and Food-Contact legislation. The ecological and economical benefits of the UV-curing technology are demonstrated on some examples using the tool of eco-efficiency analysis. v

vi

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . P1. Flash light curing of a falling “UV-curable drop”.

The televised experiment shows impressively the potential of UV-curable coatings. Not only the very fast curing speed, but also the low curing temperature, as well as the solvent free composition of the formulations are the highlights of this technology. UV curing has the potential to replace thermal hardening as the curing technology of the future.

PREFACE

vii

The author appreciates the contributions of several colleagues working in the field of UV technology for providing text, figures, discussions and/or corrections of the manuscript and thanks especially the experts in specific fields for their help in the design and revisal of the corresponding chapters: Rainer Königer (GE Plastics) Heinz-Hilmar Bankowsky (BASF AG) Nick Gruber (BASF AG) Oscar Lafuente (BASF AG) Erich Beck (BASF AG) Rolf Müller (IST-Metz) Bernward Röttgers (Fusion Corp.)

Nazire Dogan (AKZO Nobel Coatings) Thomas Fey (DuPont Performance Coatings) Chapter 2: Christian Decker (Mulhouse) and Richard W. Stowe (Fusion Corp.)

Chapter 3: Georg Meichsner (FH Esslingen) Chapter 5: Wolfgang Schrof, Klaus Menzel (BASF AG) Chapter 10: Michael Kutschera (BASF AG), Andreas Poppe (BASF Coatings), Bradley Richards (BASF Corp.)

C HAPTER 1

Introduction to Coatings Technology Coatings are found almost anywhere in daily life, the most prominent examples are architectural wall coatings and automotive paints. They are applied in order to provide: • decorative appearance, and/or • protective barrier.

The main functions of a coating are thus on the one hand to ensure the desired appearance (colour, gloss) and on the other hand the necessary protection, against corrosion, stone chipping, scratches, abrasion or chemical attack, like red wine, coffee or mustard on furniture coatings or acid rain, tree resins or bird excrements on automotive coatings, as shown in Figure 1.1.

F IG . 1.1. General function of a coating (e.g., automotive).

1

2

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

Whereas the do-it-yourself architectural coatings are almost all water-based, the vast majority of industrially used coatings, applied in factories on various substrates, like vehicles, furniture, metal cans, paperboards, etc., still contain solvents.

The coatings and application spectrum discussed in this book are predominantly based on the industrial coatings sector, which had a share of about 40% of the whole worldwide coatings market (60% architectural).

1.1 COATING MARKETS AND MARKET PROSPECTS The market prospects of future coating technologies in the industrial paint sector are reflecting the environmental concerns about the use of solvents, and hence governed by VOC (volatile organic carbon) regulations. According to these regulations, the market share of solvent-based coatings is declining significantly and the share of alternative, environmentally friendly systems, especially water-based, powder, and radiation curable (UV/electronbeam) coatings is steadily increasing, as depicted in the chart in Figure 1.2 (Paulus, reported at RadTech Conference, Barcelona 2005).1 Further information about market developments in the specific sectors of the coatings industry is available by numerous market research institutes.2 As can be seen from the technology split in Figure 1.2, the predictions of the total amount of solvent-based systems for 2015 are only slightly lower than the amounts in 2003, however, a considerable part of the classical solvent-based systems containing from 50–70% solvents are shifted to “higher solids” systems with up to 80% solids. The reluctance to switch away from solvent-based systems is often related to the

F IG . 1.2. Market development of industrial coatings by technology and region.

INTRODUCTION TO COATINGS TECHNOLOGY

3

excellent properties of such coatings, as well as the ease of handling and the high comfort factor gained over the years of working with solvent-based coatings.

A comparison of the most suitable future coating technologies reveals that all alternatives to the classical solvent-borne coatings have specific advantages and drawbacks. “High solids� systems are closest to conventional solvent-based coatings and hence most easily adopted by manufacturers of solvent-borne coatings. However, they still consist of up to 30% solvents and have to be replaced in the long run. Water-based systems are well developed, however, they still lack performance when directly exposed to the environment, mainly due to their sensitivity to humidity, which is a consequence of the use of water compatible groups for solubilizing or dispersing the systems in water. Furthermore drying of water-based systems requires more energy and specially designed drying units.

The most environmentally friendly coatings are powder and radiation curable (UV/EB) systems, which are based on 100% solid or liquid formulations. The drawbacks of these coating systems are related to their performance. Due to the interference of melting and film formation with the cross-linking reaction, powder coatings often exhibit an orange peel structure. Radiation curable systems struggle with oxygen inhibition reactions of the radical induced polymerisation, mainly at the surface. Furthermore, UV light absorbing components that are present in the formulation like pigments, additives or UV absorbers can cause through-cure issues.

The science and technology of organic coatings, including the chemistry, processing, applications, properties and performance, is described in detail in a number of comprehensive books.3

TABLE 1.1. Comparison of advantages and drawbacks of future coating technologies Coating

Advantages

Drawbacks

High solids

Excellent properties Ease of handling Users are familiar with solvent-based coatings

Still solvent containing Long curing time

Waterborne

Low VOC Wide range of chemistries, properties, application techniques

Weak chemical resistance Difficult (cleanup) to dry Foaming

Powder

100% solids Environmentally nearly ideal

Narrow process window Orange peel structure Expensive Long curing time

UV/EB

100% liquid Low energy consumption Low emission Low capital investment Low space consumption Marginal substrate heating

Higher raw material costs Difficult surface cure (due to oxygen inhibition) Difficult cure of pigmented coatings No cure in shadow areas

4

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

1.2

UV COATING MARKETS AND MARKET PROSPECTS

Compared with the total resins market for industrial coatings and inks the share of radiation curable products is still small (<5%) and will remain rather small even at higher growth rates than the average market. Other compliant technologies like powder and waterborne will also take their share at the expense of solvent-borne coatings. The development of the total volume of industrial coatings in Europe and the share of UV systems, as well as the main topics and trends has been described by Bankowsky (Figure 1.3).4 An estimation of the regional split of the global UV curing market is given in Table 1.2.

In UV technology, as in almost all sectors, the fastest growth rates are in Asia. However, for Europe and NAFTA a growth rate in the range of 9% p.a. is expected for UV coatings, which is considerably higher compared with the average growth of the coatings industry. These data can furthermore be split into the most important industrial market sectors, wood, graphic arts, industrial and automotive applications.

As can be seen from Table 1.3, the highest growth rates will be expected in new applications, like automotive and general industry, due to a high substitution potential of solventborne coatings. The UV technology will be established in automotive coatings within a reasonable time-frame, starting with refinish applications and finally hopefully penetrate into OEM applications.

The usage of UV coatings technology is different in the three major regions, NAFTA, Europe and Far East (Figure 1.4). Whereas in Europe the industrial wood coatings dominate, the major applications in the US are graphic arts (inks and overprint varnishes) and TABLE 1.2. Regional split of global UV radcure market (Paulus, ref. 1) 1995 (tons)

2000 (tons)

2004 (tons)

2008 (tons)

2015 (tons)

Growth 1995–2004

Growth 2004–2015

Europe NAFTA Asia S. Amer.

Others

32,000 35,000 13,000

46,000 51,000 26,000

1000

4000

63,200 64,700 40,000 4400 7000

77,400 81,500 56,200 5700 10,900

138,000 147,000 132,000 11,500 34,500

8% 7% 13% – 24%

7% 8% 12% 9% 16%

Total

81,000

127,000

177,000

230,000

463,000

9%

9%

TABLE 1.3. Sectorial split of global UV radcure market (Paulus) 1995 (tons)

2000 (tons)

2004 (tons)

2008 (tons)

2015 (tons)

Growth 1995–2004

Growth 2004–2015

Wood Graphic Industry Auto

30,000 32,000 19,000

38,000 56,000 33,000

47,300 81,000 48,800

58,000 96,000 73,000 2000

102,000 153,000 182,000 26,000

5% 11% 11% –

7% 6% 12% –

Total

81,000

127,000

177,000

230,000

463,000

9%

9%

INTRODUCTION TO COATINGS TECHNOLOGY

5

F IG . 1.3. Market of industrial coatings in Europe and share of UV acrylates.

F IG . 1.4. World market of UV curable formulations in main segments (2004).

in Far East the area of electronic and display applications (others). The graphics area of inks and overprint varnishes also has a considerable share in all regions and has by far the highest market share of UV coatings on a worldwide basis.

These figures give a rough understanding of the radiation curing market, however, the absolute values of different market studies vary, due to different definitions of the market

6

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

TABLE 1.4. UV coating market consumption in 2004

Japan China

Total (to)

Coatings

Inks

Adhesives

Photoresists

46,000 32,000

16,000 23,000

9800 9000

1700

18,000

segments (for example sometimes industrial coatings contain wood coatings, but separate opto-electronics), as well as different definitions and synopsis of resins, diluents and formulations.

Looking at the trends across the regions Frost& Sullivan5 groups the applications into graphic arts (printing inks, plates and overprint varnishes), industrial coatings (wood, plastics, metal and others), opto-electronics (photoresists, resists, optical fibre and liquid crystal display (LCD)) and adhesives (structural, laminating and pressure sensitive adhesives (PSA)). In 2003 in US the biggest application is graphics (58%), followed by industrial (29%) and opto-electronics (9%), in EU the biggest is industrial (47%, including wood), followed by graphics (35%) and opto-electronics (17%). The forecasts of the development in the regions are estimated in US to 5%, in Europe to 3% and to 10% in Asia. A detailed insight into the development of the emerging Asian radiation curing markets has been given recently for Asia in general,6 Japan,7 Thailand8 and China.9 In these market studies, the total Asian UV market for resins was estimated to about 76,000 tons in 2004 with a Japanese share of 40% and a Chinese share of 25%. The market segments dominating are wood coatings including floorings, plastics (computer components, mobile phones and cosmetic/personal care packaging), and electronics, like photoresists and display applications. Therefore, these figures also include special UV curable or patternable systems, like photoresists for semiconductors and resists for LCDâ&#x20AC;&#x2122;s.

In Japan the total UV market volume in 2004 was estimated to 46,000 tons, whereof 18,000 tons are photoresists, 16,000 tons coatings (wood 7000; hard coatings 4000; ceramic 1700 and optical fiber 1100 tons), 9800 tons inks (offset inks 6800 tons) and 1700 tons adhesives (digital versatile disk (DVD) bonding and display). The raw materials (reactive diluents and oligomers) summed up to about 36,500 tons. Outstanding annual growth rates in Japan will be expected for photoresists in the semiconductor industry, photoresists for LCD, as well as barrier rib for plasma displays and film coatings (anti-reflection for displays, scratch resistant hard coatings).

In Thailand the graphic arts market contributes 80% of the use of UV curable systems and wood coatings the remaining 20%. It is dominated by coating and printing plastics for packaging, about 60% of all printing inks applied by screen printing.

In China, coatings and inks are the main applications with a total output of 23,000 tons of UV curable coatings and 9000 tons of inks. The coatings were used for wood/bamboo (14,500), overprint varnishes for paper (4300), plastics (1900), PVC (1300), automotive (800) and others. The total growth rate of raw materials was 11.8% and of UV curable products 27.4% in 2004. The growth and the perspectives stem from increasing penetration of UV technology in the various applications as well as from a buoyant overall market.

INTRODUCTION TO COATINGS TECHNOLOGY

7

F IG . 1.5. Electromagnetic energy spectrum.

1.3 UV TECHNOLOGY AND APPLICATIONS UV curing has now been established as an alternative curing mechanism to thermal hardening, contrary to the past, where it was only considered for the curing on temperature sensitive substrates, like wood, paper and plastics. This alternative curing technology uses the energy of photons of radiation sources in the short wavelength region of the electromagnetic spectrum in order to form reactive species, which trigger a fast chain growth curing reaction.

Out of the electromagnetic spectrum (shown in Figure 1.5 is the range from the nearinfrared (NIR), over visible and ultraviolet (UV) to electron beams and X-ray) the UV region, further classified into UV-A, UV-B, and UV-C radiation, is mainly used for this technology.

The energy content of a photon is defined by the equation E = hν = hc/λ, where ν is the frequency and λ is the wavelength (nm). This equation tells us, that the shorter the wavelength, the higher the energy of a photon. UV light in the wavelength region of 300–400 nm should already be able to cleave C–C bonds. The high energy photons of e-beam and X-ray are sufficient to cleave C–C or C–H bonds, thus, they do not need a special photoinitiator for forming the desired radical species as initiators for polymerization. In the case of UV exposure, however, photoinitiators are commonly used, since the direct cleavage processes are not efficient enough. The photoinitiators are excited and after a cascade of reactions form the desired reactive species. In the case of using longer wavelength exposures, more complicated energy transfer reactions are needed.

From the spectrum of usable radiation energy sources, UV technology is by far the most common one. From the higher energy radiation sources, e-beam technology has been widely explored for coatings technologies. It is still the most economical technology for industrial applications with very high volumes. However, the high safety requirements related to the use of e-beam technology and the high investment costs hamper the widespread

8

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 1.6. Energies as a function of wavelenght in comparison to bond energies.

use of this technology. At the RadTech Conference 2005 in Barcelona, considerable interest has been expressed in the session dealing with e-beam technology for “printing, varnishing and laminating for the packaging industry”.10 The reasons for this alertness are new developments of compact and less expensive EB equipment and new formulation advances in flexographic printing inks, coatings and adhesives. Especially in the packaging printing for food contact applications the use of EB technology has advantages over UV coatings since no photoinitiator is needed, which can migrate, if the coating is inadequately cured.

As can be seen from the few application examples shown in Figure 1.7, UV curable coatings are traditionally used on temperature sensitive substrates, like wood, paper and plastics, for example, clear coats for parquet, furniture, vinyl flooring, on plastic substrates (crash helmet, boards), compact discs, headlight lenses or overprint varnishes (posters, high gloss packaging). However, since coatings are used almost everywhere, the UV coatings market is expanding to new applications, where traditionally thermal curing systems have been the workhorses. Applications like UV curable coatings on metals (automotive, coil coating) and exterior uses on windows, on glass, bikes, on appliances, like refrigerators, washing machines, and most prominently on cars are good examples. A multiplicity of coating applications is often less noticed, such as adhesives and protective coatings for DVD and CD’s, protective coating on glass fiber wires, inside and outside of beverage cans, on automotive parts, like headlight mirrors and in multiple functions on electronic parts. This list can easily be extended even further.

Up to now, UV curable systems are mainly used in clear coat applications, thus posing high demands on the performance of this layer; at the surface of the coating it is exposed to attack by mechanical or chemical stresses, like scratches, household chemicals (detergents, red wine, coffee, mustard), by air polluents (acids, water, bird excrements) as well as stone chipping or many other impacts. The formulations used for radiation curable coatings depend therefore on the specific performance requirements and on the application technique.

The traditional formulations of UV curable coatings are still 100% liquids (or also commonly referred to 100% solids, despite used in liquid form, in order to point out that they

INTRODUCTION TO COATINGS TECHNOLOGY

9

F IG . 1.7. UV coatings – from traditional to new applications.

contain no solvents or other volatiles). However, in the meantime, due to the consideration of UV curing as an alternative to thermal hardening, the use of small amounts of solvents in order to reduce the viscosity, the formulation of UV curable water-based systems and the development of UV powder have been pursued.

The market penetration of UV coatings is up to now still regarded as a niche technology.

This is due to several factors, one decisive reason is based on the curing technology itself, which is still stamped as a two-dimensional curing process, in which only planar substrates are feasible. Up to now there are only few applications involving three-dimensional objects.

As shown schematically in the diagram of Figure 1.8, the application processing is mainly based on industrial applications on two-dimensional substrates. The substrate is first coated, exemplarily shown is a casting line, and then passed under lamp units, where it is exposed to intense radiation. Within a fraction of a second, the liquid low molecular mass is thereby transferred via a photo-induced radical polymerization to a solid crosslinked network. After the end of the line, the fully cured and dry substrate can be stacked and further processed immediately.

Typical compositions of UV curable coatings (Table 1.5) contain in the range of 25% to 90% oligomeric resins, which are responsible for the film formation and the basic coating properties. Reactive diluents are low molecular weight compounds, which are incorporated into the polymer network, and used instead of solvents (in conventional lacquers) in order to adjust the viscosity to the requirements of the application process. Typical application viscosities range from 3000–5000 MPa s (Pascal × second) for roller application to 100–

10

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 1.8. Scheme of the UV curing process and UV induced cross-linking.

TABLE 1.5. General composition and function of an UV lacquer Component

Share (%)

Function

Oligomeric resin

25–90

Film formation Basic properties

Reactive diluents

15–60

Viscosity adjustment X-link density

Photoinitiator

1–8

Initiation

Additives

1–50

Surfactants, pigments, fillers, stabilizers, etc.

200 mPa s for spray applications. In UV curable lacquers, about 1–8% photoinitiators, as well as several other additives (from 1% up to 50%), like leveling agents, stabilizers, UV absorbers, radical scavengers, pigments and so on, are used to tailor the formulation to the application process and coating property requirements.

This general composition of UV curable coatings applies to radically polymerizable coatings as well as to cationically curable systems and EB curable coating, which, however, do not need photoinitiators.

INTRODUCTION TO COATINGS TECHNOLOGY

11

1.4 ADVANTAGES AND DRAWBACKS OF UV COATINGS Some general advantages and disadvantages of UV curable coatings have already been mentioned in Table 1.1. From the many advantages and disadvantages mentioned in the literature, some of the most important are listed below: Economical advantages • Energy saving (commonly rapid cure at room temperature) • High production speed • Small space requirements • Immediate post cure processing possible Ecological advantages • In general solvent free formulations (VOC reduction) • Possibility of easy recycling (waste reduction) • Energy saving Performance advantages • Low substrate heating • High product durability • Application versatility • High scratch resistance and chemical resistance • Exceptional abrasion, stain and solvent resistance • Superior toughness Drawbacks • Material costs are higher than, e.g., alkyds, polyesters or epoxies • 3D curing equipment development is in its infancy • UV curing in the presence of UV stabilizers decelerated • Oxygen inhibition at the surface (in many radical curing systems) • Sensitivity to moisture (cationic curing system) • Difficult through-cure of pigmented coatings (at thicknesses >5 µm) Topics to eliminate weaknesses • Improving adhesion to metal, plastics • Minimizing skin irritation caused by some reactive diluents • Reducing odor (of the formulations) • Reducing extractables of cured coatings • Improving photoinitiators (cost, migration, volatility) • Direct food contact packaging approval While the advantages and good performance characteristics of this technology are very obvious, the reasons for the limited penetration into large volume coating applications must lie in some substantial disadvantages. Major reasons are the limited availability of three-dimensional curing equipment, the very limited use of UV cured coatings in exterior applications, due to the existing paradigm that UV curing would not be possible in the presence of UV exterior durability stabilizers, and higher material costs compared to conventional coatings.

12

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

One of the major reasons for the almost exclusive application of UV curing in twodimensional curing systems is the fact that the radiant power of the lamp decreases with the square of the distance. Thus, it is difficult to control the effective energy (radiant power arriving at the surface/curing time) necessary to cure the coating sufficiently at every point of a three dimensional substrate. Major advances in the design and radiometric control of three-dimensional curing equipment have already been achieved, but still have to be improved.

The development of UV curable coatings for exterior applications had been disregarded in the past, since such coatings have to be stabilized with UV absorbers and radical scavengers (HALS types) in order to provide enough long-term stability. This was due to the preconception, that the UV induced radical polymerization could not be possible in the presence of UV absorbers and radical scavengers. Since this prejudice has been disproved, the whole field of exterior applications opened to UV coatings, which had seemed closed to UV curing forever. Especially the high scratch resistance obtainable with UV cured coatings, as proven in parquet flooring, has attracted the attention of automotive companies, which are looking for coatings which can withstand the typical scratches resulting in car wash units.

When thinking about curing of such complex geometries as available in car bodies (doors, hoods) it is getting obvious that solutions have to be found to cure areas within three-dimensional shapes, which are in the shadow of the exposure source. Dual cure coatings for example are developed in order to cure also shadow areas of complex threedimensional objects. They may use as a second functionality, besides the UV curing chemistry, the complete range of available thermal curing chemistries, like isocyanates (in combination with hydroxyl-functional compounds, known as 2 component PU systems) or carbamate groups (curable with melamine chemistry, known as 1 C coatings). The problem of curing in shadow areas has also been tackled from the equipment side, which resulted in the development of UV curing in a plasma chamber or under inert conditions.

Further improvements have to be achieved in order to overcome the oxygen inhibition effect, which leaves a tacky surface, unless very intense radiation or other measures are employed. This effect is caused by the high reactivity of oxygen with radical species and the formation of an unreactive peroxy radical, which does not continue the curing chain reaction. Therefore, the cross-linking reaction and the formation of a solid network are retarded until all oxygen is consumed. Since the use of high energy density radiation is undesirable several alternative measures will be discussed.

The discussion of tackling the drawbacks of UV technology, the evaluation of structure property relationships, relating mainly to mechanical and scratch resistant properties, constitutes the basis for understanding the advantages of using this technology for exterior, especially for automotive and industrial applications.

The economical and ecological benefits of UV curable coatings often appear when the question arises “which coating system should be chosen in order to coat a specific surface area” and different coating alternatives may be considered. This is preferably done by comparing the whole process of coating a substrate from cradle to grave with the ecoefficiency method.

INTRODUCTION TO COATINGS TECHNOLOGY

13

1.5 ECO-EFFICIENCY The method of eco-efficiency11 has been developed by BASF and Roland Berger&Partners starting in 1996 as a strategic instrument in order to identify products combining the optimum in the desired application with good environmental performance in conjunction with the lowest costs. It compares economical and ecological advantages and drawbacks across various solutions based on the view of the end-user, for example, for the application of â&#x20AC;&#x153;painting a square meter of furnitureâ&#x20AC;?. With this method it is possible to study the complete life-cycle of a product, starting from crude oil as a resource for the raw materials all the way to recycling after use. Besides the ecological factors, like the pollution of the environment or the hazard potential caused by the products, also the economic picture, for example as a cost consideration, is evaluated and the economical and ecological advantages and disadvantages are compared. This list is very valuable in order to be used to identify the decisive influence factor in order to design alternative scenarios and eventually develop more suitable products.

The environmental impact is evaluated by five main criteria, the consumption of energy (25% weighting), the consumption of raw materials (25%), the emissions (20%), the toxicity potential (20%) and the risk potential for misuse and hazard (10%). Each of these categories covers a large number of individual criteria, for example the emissions are evaluated as air emissions (50%), water emissions (35%) and soil or wastes (15%), whereof the air emissions are classified by the greenhouse warming potential (50%), the ozone depletion potential (20%), the photochemical ozone creation potential (20%) and the acidification potential. In parallel the economic data are compiled in the form of costs for all alternatives.

From the materials and energy flow chart, weak points, driving costs and potentials for cost reductions can be identified. These data are plotted as a two-dimensional graph, known as an eco-efficiency portfolio. This portfolio classifies the different alternatives based on the assumptions put into the evaluation of the data. Several consequences can be derived from this portfolio: products with high degree of eco-efficiency can be sold accordingly in order to improve market share, products with lower eco-efficiency can be optimized depending on the necessity of improvement on the ecological or economical side, or products with low eco-efficiency may be abandoned or substituted by an alternative. For the example of the analysis of several production lines running with different coating technologies to coat wooden door fronts,12 the resulting portfolio is shown in Figure 1.9.

For the door front analysis different coating alternatives are compared, an UV formulation based on low-viscous polyether acrylates, an aqueous varnish, a two component polyurethane, an acid curing varnish and a nitro cellulose varnish. From the evaluation of the economical (cost) and ecological data, the UV roller coating shows the best results in the environmentally relevant points, due to lowest raw material and low energy consumption, as well as high productivity and lowest production costs.

Further eco-efficiency examples are discussed in Chapter 10. Although the UV technology is already known as an environmentally friendly technology, the eco-efficiency analysis can support the decision of companies to switch over to UV technology, if it is shown that the cost situation is also favorable for an UV solution.

14

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 1.9. Eco-efficiency portfolio for coating of 1000 wooden door fronts.

F IG . 1.10. Driving forces of UV technology.

1.6 DRIVING FORCES OF UV TECHNOLOGY From the discussion of the advantages and drawbacks, as well as the eco-efficiency of the UV technology the driving forces for future developments of this environmentally friendly technology become evident (Figure 1.10).

In general the main driving forces are listed below: Performance, exemplarily mentioned are: • High surface quality • Chemical and mechanical resistance • Gloss, scratch and abrasion resistance

INTRODUCTION TO COATINGS TECHNOLOGY

15

Economy, exemplarily mentioned are: • Energy and material saving process • Cold cure, no additional heating Ecology, exemplarily mentioned are: • Nearly no VOC’s • Very low emissions after curing • Very low extractables after curing These driving forces result in double digit growth rates in classical applications. Due to the high curing speed of UV polymerization other eco compatible coating systems, besides the classical 100% solids UV coatings, such as water-based, powder and dual cure systems, are also being modified in such a way to be curable by UV polymerization. And ultimately, general new exterior applications and especially automotive usage will contribute to further vital growth.

Innovative concepts are introduced which will further trigger the use of UV coatings in three-dimensional curing. UV curing under inert (nitrogen, carbon dioxide) atmosphere has been described long ago; however, the influence on the scratch resistance, especially with test methods relevant to automotive top coats, has been investigated in detail only recently.

Furthermore, since radiation curing has mainly been used for two-dimensional industrial substrates, the equipment, conditions and influencing factors of three-dimensional UV curing still has to be developed. The investigation of several influencing factors has triggered the development of 3D UV exposure equipment, which resulted in the establishment of the Larolux® curing process in a carbon dioxide filled pool with conventional tanning lamps, which makes the UV curing process easily accessible to everybody, particularly for craftsmen, like joiners.

Another very innovative concept for 3D curing even in shadow areas is realized in the UV plasma processing, which can be visualized by the picture of placing the 3D object into the “enlarged lamp”, as described in Chapter 9.

In the whole coatings industry, the introduction of nanomaterials improved the performance of coatings considerably. For instance, the introduction of a thermal curable 2C polyurethane clear coat by PPG (Ceramiclear® ) improved the scratch resistance of 2C PU coatings dramatically. Thus, also a lot of work to incorporate nanoparticles into UV curable systems has been done and will as well be described in Chapter 9.

1.7

CURRENT R&D TOPICS AND TRENDS IN UV TECHNOLOGY

The following spot list (according to Paulus1 and Bankowsky4 ) covers some of the hot topics worked on within raw materials and coating companies.

Wood coatings: • Products with less extractables • Products with high scratch resistance • Spray coating of 3 D objects

16

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

Graphic arts: • Inks and overprint varnishes (OPV’s) with approval for food packaging • Inks and adhesives for DVD’s • Ink jet Plastic coatings: • Adhesion promoters • Adhesives for DVD and flat panel displays Metal coatings: • Products with good adhesion • Products for coil coatings • Weather resistant coatings for 1. Automotive OEM 2. Automotive refinish Most of these new targets require new products, like UV curable waterborne systems or dual cure systems, relying on UV cure and a second independent curing mechanism, for example thermal or oxidative curing. A surely incomplete selection shows what future trends have been identified so far and what developments can be expected in Europe.

Trend at raw material suppliers: • Become a solution provider to customer’s problems • Work on segment and customer specific innovations • Active participation in panels to follow the changes in legislation related to health, safety, working hygiene and environment • Globalization of services and standards Trend at coatings manufacturers: • Short lead-time to meet new legislative regulations (VOC, Preparation Directive, etc.)

• Reduced numbers of suppliers and raw materials • Transfer formulation development towards the raw material supplier • Provide package of services and problem solutions • Even smaller companies must become global Trend at end-users: • Short lead-time to meet new legislative regulations (VOC, Preparation Directive, etc.)

• Reduced numbers of suppliers • Transfer of development activities towards the coating manufacturer

1.8 PERSPECTIVES In order to provide a basis for the understanding of the technology of the new developments and applications, a brief description of the basic process technology, the raw materials and formulation chemistries involved, the network structures obtained, as well as structure– property relationships are given. A more detailed insight into the chemistry, processes,

INTRODUCTION TO COATINGS TECHNOLOGY

17

F IG . 1.11. Perspectives of UV curing technology to catch a bigger share of the coatings market.

equipment, formulations and mainly classical applications of radiation curable coatings is given in a number of excellent reviews and books already published.13 The technical perspectives of the UV curing technology leading to the expected increase in market share are drafted in Figure 1.11. The application field will be widened significantly by extending the employment from the classical wood and paper substrates to metal, plastics, leather, minerals etc., from almost exclusively interior and industrial use to exterior and handcraft applications. The development of novel equipment and processes facilitates the curing of three-dimensional shapes. The progression of advanced materials and formulations enable the through-cure of pigmented coatings, the creation of flexible and elastic coatings as well as change the whole coatings process from liquid application to prefabricated shape-able coating films, an approach that may pave the way to completely substitute painting of objects by foil coating, for example wallpapering a car as a vision.

REFERENCES 1. Paulus, W., Status of UV/EB in Europe, RadTech Europe 2005, Barcelona (paper available at the RadTech web site: http://www.radtech-europe.com/download/paperpaulusrteconference2005.pdf).

2. (a) Information Research Limited (IRL), a division of Business Research Group, UK, www.informationsresearch.co.uk; (b) SKEIST Incorp., Whippany, NJ, www.skeistinc.com; (c) Lovett, P.D. & Company, Business planning and market research, www.pdlovett.com/coatingsresearch.html; (d) Kansai Research Institute (KRI, JP); Stanfort Research Institute (SRI, USA).

3. Comprehensive standard works about organic coatings: Wicks Jr., Z.W., Jones, F.N. and Pappas, S.P., “Organic Coatings, Science and Technology”, Vol. 1 and 2. John Wiley & Sons, New York, 1994. Mischke, P., Groteclaes, M. and Brok, T., “European Coatings Handbook”. Vincentz, 2000. Goldschmidt, A. and

18

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS Streiberger, H.J., “BASF Handbook on Basics Coatings Technology”. Vincentz, 2003. Streitberger, H.J. and Kreis, W. (Eds.), “Automotive Paints and Coatings”, 2nd edn. Wiley–VCH, 2006.

Bankowsky, H.H., Radiation curing in Europe, RadTech USA, May 2004, Charlotte, Conference Proceedings. 2004, on CD. Zhao, E., Radcure markets: trends across regions, RadTech Asia 2005, Proceedings. 2005, pp. 420–426.

Luo, V., RadTech Asia 2005, Proceedings. 2005, pp. 24–25.

Ukachi, T., Otaka, T. and Shinohara, N., RadTech Asia 2005, Proceedings. 2005, pp. 26–33.

Kiatkamjornwong, S., Phattanarudee, S. and Jiratumnikul, N., RadTech Asia 2005, Proceedings. 2005, pp. 34–39. (a) Shi, W., Jin, Y. and Jin, Y., RadTech Asia 2005, Proceedings. 2005, pp. 40–47. (b) Shi, W. and Jin, Y., RadTech 2006, Conference Procedings, Chicago, IL. 2006, on CD. Come-back of EBC at the RadTech ’05, Coating, 12/2005, pp. 518–520. (a) Overview about the method is available at the web page: http://corporate.basf.com/en/sustainability/ oekoef/fizienz/veroeffentlichungen.htm?id=K*4JH7ixmbcp.l2; (b) Seufert, W., Umwelt 4/5, 3 (2001); and (c) Chem. Daily (2002).

Biehler, M., Menzel, K. and Saling, P., UV spectrum, Fusion UV, Winter 2003, www.fusionuv.com/news_ events/spectrum_newsletter.

Comprehensive standard works about UV curing: Pappas, S.P. (Ed.), “UV-Curing: Science and Technology”. Technology Marketing Corp., Stanford, 1978. Holman, R. (Ed.), “UV& EB Curing Formulations for Printing Inks, Coatings & Paints”. SITA Technology, London, 1984. Allen, N.S. (Ed.), “Photopolymerisation and Photoimaging Science and Technology”, Elsevier Applied Science, New York, 1989. Fouassier, J.P. and Rabek, J.F. (Eds.), “Radiation curing in Polymer Science and Technology”. Vol. I-IV. Elsevier Applied Science, 1993. Fouassier, J.P., “Photoinitiation, Photopolymerization and Photocuring – Fundamentals and Applications”. Hanser Publishers, New York, 1995. Garett, P.G., “Strahlenhärtung”. Hrsg. U. Zorl, Vincentz, Hannover, 1996. “Chemistry and Technology of UV and EB Formulations for Coatings, Inks and Paints”, In Wiley/SITA Series in Surface Coatings Technology, Vol. I-VI, (Oldring, P.K.T. ed.), John Wiley and Sons, New York, 1991–1996. Mehnert, R., Pincus, A., Janorsky, I., Stowe, R. and Berejka, A., “Chemistry and Technology of UV and EB Formulations for Coatings, Inks, and Paints: UV and EB Curing Technology and Equipment”. John Wiley and Sons, New York, 1998. Davidson, R.S., “Exploring the Science, Technology and Applications of UV and EB-Curing”. SITA Technology Limited, 1999.

C HAPTER 2

The UV Curing Process The UV curing process is predominantly determined by the desired application of the coating. The intended end-product governs the substrate to be coated. This may be an abrasion resistant clear coat for ready-to-install parquet or an overprint varnish for paper cards, a coloured base coat and a clear coat for plastic automotive parts or metal coils, as well as a flexible protective coat for window frames. The function of the coating, for instance the colouration of the part, the protection against corrosion, scratching, chemical attack or against weathering deterioration, determines the type and property requirements of the coating as well as the thickness required.

The targeted properties, like high gloss appearance, abrasion resistance, colour effects, hardness, flexibility, resistance against chemicals or scratches, have to be provided by the chemical formulation, consisting of base resins, diluents, photoinitiators and various additives. Furthermore, an appropriate selection of the components has to be done in order to enable an effective curing process; for instance, in coatings containing pigments or UV light stabilizers, the spectral absorbance of the photoinitiator has to be adjusted to a spectral region where the pigments or UV absorbers are fairly transparent. This fine tuning is necessary to match the characteristics of the lamp system with the chemistry of the coating to provide an economic curing process.

Besides the physical properties of the cured material to be obtained, the economics of the coating process is the most important variable which decides over the type of coating used. Thus, in order to calculate the total costs of a coating process, not only materials costs but the whole process design and the equipment set-up have to be considered in order to compare different coating processes with each other. UV curable coatings are always in competition with thermally curable systems of the classical solvent-type, water-based or powder coatings.

Some economic factors of UV curing have been discussed by Stowe,1 with cost examples for ink, coating and adhesive applications in comparison with thermal hardening, if applicable. UV curing in general offers a number of advantages over competitive coatings, while some can be related to costs, others relate to performance, environmentally compliance or processes not achievable with other methods. However, no general comparison of process economics can be made; it has to be done rather in a case to case study.

Thus, the UV curing process relies crucially on an efficient cogging of the required application properties with the chemistry chosen to fulfill the performance requirements as 19

20

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 2.1. Interaction of UV process design parameters.

well as the UV curing equipment applied to provide a fast and complete cure in order to meet the economical and ecological aspects of coating technology (Figure 2.1).

UV curing in its basics is a fast, room temperature curing process indicating low energy consumption and requiring little space for the equipment.

2.1 APPLICATION (PROPERTY REQUIREMENTS) From the application point of view, UV curable coatings are mainly used in such industrial applications where thermal curing is hardly possible, like curing of coatings on temperature sensitive substrates, like wood, paper and plastics, and in imaging applications, where only selected areas should be polymerized, like in polymer printing plates and photoresists.

Specific applications of photocurable coatings are clear coats for parquet, furniture, vinyl flooring, on plastic substrates (skies, boards), compact discs, headlight lenses, overprint varnishes (posters, high gloss packaging), adhesives, protective coatings for optical fibres, electronic parts. Applications of photocurable coatings on metals (automotive, coil coating) and exterior uses are just emerging. These applications cover a large range of properties.

The function of the coating and the desired end product properties determine the chemistry to be used in order to fulfill the application requirements. For instance, for overprint varnishes (OVP) on book covers, art prints, post cards, photos, etc. used to protect the printed image and increase the appearance by a high gloss finish, the OVP layer thickness is in the range of 8–10 µm, cured at a speed of about 60–80 m/min. Such a clear coat of moderate thickness can be composed of standard acrylate resins and diluents; merely the photoinitiator system has to be selected to comply with the high curing speed. The curing

THE UV CURING PROCESS

21

F IG . 2.2. Interaction of UV process parameters.

can be performed with a standard mercury lamp set-up. UV coatings on wood, for example, fibre boards, plywood or veneer, have different functions. UV primer may have to stabilize the wooden support and ensure adhesion. This layer is also very thin and will be cured with standard equipment. If a wooden décor, however, is printed on top of the primer or printed on a foil and laminated onto the particle board, this décor layer will contain pigments and therefore the photoinitiator has to be chosen to absorb at longer wavelength in order to match with a transparent area of the pigment. In this case, a lamp system should be chosen, which also has significant emissions in the longer wavelength range. The same applies if a clear coat is used for exterior applications, for instance a clear coat for polycarbonate headlamps. Here a UV light absorber has to be used, which also absorbs at least in the UV-B range.

UV printing inks are used for example in the offset, flexographic and gravure printing process at a layer thickness in the range of up to 2 µm. Since these inks are highly pigmented, even at the relatively low thickness, the through-cure is often difficult to achieve.

These few examples have been selected to demonstrate, that the required properties of the coating determine the chemistry to be used, and that photochemistry and the exposure equipment then have to be adjusted to achieve the target properties in an efficient cure process (Figure 2.2).

2.2 CHEMISTRY (PHOTOCHEMICAL PROCESS) This section describes briefly the principles of the chemistry and kinetics involved in the UV curing process.

22 2.2.1

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

Photoinduced Curing Chemistry

Photoinduced curing can be realized as in the preparation of conventional linear polymers by a step like process, as used in polyaddition and polycondensation reactions or by a chain process occurring in polymerization reactions (Figure 2.3).

The photoinduced polyaddition technology has been for a long time the workhorse of photoresist technology,2 for example, the crosslinking of resins was achieved by photoinduced dimerization of cinnamates. This photodimerization is an example of a direct photoreaction where every step of polymer built-up is initiated by an absorbed photon, thus every single reaction step is dependent on the quantum yield of the photoreaction (generally very much smaller than 1).

On the contrary, in polymerization reactions induced by light only the initiating step is dependent on the photoreaction (ÎŚ < 1). The photopolymerization reaction then is a chain reaction, where one produced initiator radical can add up to several thousand monomer units, thus the overall quantum yield of the total reaction is much bigger than 1. Whereas the photoinduced radical polymerization is now the mainstream technology, the photoinduced ionic curing reactions are not so well explored and developed, mainly due to the lack of easily available photoinitiators. In recent years, considerable progress has been made in the development of new cationic photoinitiators,3,4 however, there are only a few anionic type photoinitiators described.5 The basic principles of curing and network formation are similar in radical and cationic induced curing. The cationic curing has its main advantages in the oxygen insensitive curing and in the good adhesion mainly to metals achieved with the cationic curable epoxy systems. The cationic curing will be described briefly in the section of the raw materials. The main focus will be placed on the photoinduced radical polymerization.

The UV curing technology is based on the photoinitiated rapid transformation of a reactive liquid formulation into a solid coating film. The initiating species may be a cation, an anion or a radical. The vast majority of UV curable coatings are based on radical producing photoinitiators. The main components of such formulations based on radical polymeriza-

F IG . 2.3. Possibilities of photoinduced curing.

THE UV CURING PROCESS

23

tions are: • Reactive resins containing a plurality of polymerizable double bonds, which govern mainly the desired properties of the final coating; • Copolymerizable, monomeric diluents, which are responsible for the reduction or adjustment of the viscosity of the formulation, a function taken by the solvent in conventional formulations; • Photoinitiators or a photoinitiating system containing photoinitiator and photosensibilizer or coinitiators; and, if necessary, other coating additives, like surface active additives, slip additives, fillers, pigments, light stabilizers, etc.

The chemistry involved in the radical initiated UV induced crosslinking can be divided into the three steps, initiation, propagation and termination. Although the UV energy applied in photocuring may cleave C–C and C–H bonds, the commonly used monomers do not produce sufficient amounts of initiating species, which is due to low absorbance and poor cleavage efficiency. Thus, a special photoinitiator is usually applied, which is excited and ultimately yields via intersystem crossing, accompanied by various deactivation reactions, the formation of a radical species, which can initiate radical polymerization. The following polymerization reaction follows almost exactly the rules of conventional radical polymerization. Thus, only the initiation step is different to thermal initiated radical polymerization. The basic principles of photoinitiation, photopolymerization and photocuring are described in detail in a book edited by Fouassier.6 The light absorption and the following processes are commonly pictured in a Jablonski diagram (Figure 2.4). The process starts with the absorption of a photon by the photoinitiator molecule, which results in excitation of an electron into higher singlet states. From

F IG . 2.4. Jablonsky-type diagram for photoinduced radical photoinitiation.

24

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 2.5. Photoinitiator types.

these excited states, various processes can follow. First, deactivation can proceed by radiationless internal conversion and evolution of heat back to the ground state or by emission of fluorescence. Second, by intersystem crossing (ISC) an electron spin inversion leads to the excited triplet state. The photochemical processes which lead to the desired active species (e.g., free radicals) often take place from the excited triplet state, where the molecule posses two unpaired electrons, rather than from the singlet state. The formation of the reactive species, namely free radicals, competes with further deactivation processes, like monomer quenching, oxygen quenching and phosphorescence. The direct oxygen quenching of the photoinitiator excited states is not very likely in the case of the extremely shortlived triplet states of α-cleavable type photoinitiators, but much more pronounced in the hydrogen abstraction type owing to the relatively long-lived triplet states.7 From the triplet state two main reactions can lead to initiating species, the intramolecular scission of an α-bond, or the intermolecular abstraction of a hydrogen atom. The intramolecular scission is the most effective process in the formation of radicals, since the hydrogen abstraction is a bimolecular type reaction, which is diffusion controlled and may be accompanied by several deactivation reactions.

The quantum yield of initiation, representing the number of growing chains per photon absorbed reflects the importance of the processes leading to initiation over all the indicated processes of deactivation.

The efficiency of the photoinitiation is a function of different quantum yields, since several side reactions can occur in every step. Thus, the overall yield of initiation is a complex function of different quantum yields, represented exemplarily in Figure 2.4. Two

THE UV CURING PROCESS

25

F IG . 2.6. Propagation and transfer.

F IG . 2.7. Termination reaction.

examples of photoinitators, an alpha type scission initiator and a hydrogen abstraction type photoinitiator are shown in Figure 2.5.

The chemical structures of photoinitiators are discussed in more detail in the raw materials section (Chapter 4, photoinitiators). Furthermore, photoinitiator chemistry, the formation of the excited states, and the reactivity of radical and cationic photoinitiators are very well described in the literature (Fouassier, ref. 8, Chapters 3 and 4).

Propagation (Figure 2.6) is the key step to very efficient curing, since it is a chain reaction where for instance one produced radical can add more than 1000 monomer units within a fraction of a second. The steps after the initiation are very similar to the normal radical polymerization of monofunctional monomers, which are widely used to synthesize thermoplastic polymers, like polyethylenes, polypropylene or polystyrenes. The main difference in coating systems is the use of multifunctional monomers or oligomers, which

26

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

leads to the formation of networks. In the propagation reaction transfer reactions also often play a significant role, where the growing radical chain does not add to another monomer unit, but abstracts hydrogen radical from a neighbouring R–H group. The remaining Rradical can then start another growing chain, thus leading to the termination of the growing polymer chain, but not to the termination of the chain reaction. The reaction of the radicals with oxygen does not play a significant role in the polymerizations of linear polymers, since they are normally conducted under inert conditions. However, the curing of coatings is normally performed under atmospheric conditions, thus, the oxygen interference plays a major role.

The termination reactions are also manifold (Figure 2.7). Besides the termination with an initiator radical, several other termination reactions play a role, especially the recombination of growing radical species or elimination reaction of the chain end.

2.2.2

Kinetics

indexKinetics Conventional UV curing lines are operating at curing speeds of 5 m/min to 100 m/min, corresponding to a residence time of roughly a fraction of a second under a typical lamp setup. Thus, the polymerization kinetics has to be very fast. Why are these reactions so fast, while typical radical polymerizations of linear homopolymers take hours?

The kinetics of the photopolymerization of multifunctional monomers is in principle similar to the classical radical polymerization of monomers. For those, the kinetics has been well investigated and for a vast number of classical monomers used in the synthesis of linear homopolymerization, kinetic constants have been determined.9 However, the kinetics and mechanism of the curing of multifunctional monomers is much more complex due to different basic conditions. For instance, the number of initiating radicals produced per second is much higher in photopolymerizing systems, having pronounced effects on the rate of polymerization and termination. The mobility of the active species changes dramatically while the medium turns from a liquid over to a viscoelastic rubber, and, finally, to a glassy solid, and the formation of microgels at an early stage has considerable influences on the structural composition, the local heat evolution, volume shrinkage and/or radical trapping reactions. Therefore it is not advisable to use published kinetic constants of monomers, despite multifunctional monomers being structurally similar, in order to estimate the photopolymerization kinetics of coatings.

Rather than taking data from literature compiled under various conditions, it is better to compare the different systems under the conditions to be used, in the case of UV curable coatings, for example in a thin film and irradiation at room temperature. A lot of kinetic studies have been done by Decker and coworkers.10 They have followed the conversion of the reactive groups, for example the acrylate double bonds, for a number of resins and reactive diluents used in UV curing with real-time IR spectroscopy (RT-IR) (see 2.2.5.1), and determined the kinetic constants under UV curing process conditions.

In the polymer growth reaction (−MA∗ is a radical species), kp =kAA

P−MA∗ + MA → P−MA −MA∗ ,

(2.2.1)

THE UV CURING PROCESS

27

the rate of polymerization (rp ) is calculated as follows: rp = kp /(2kt )0.5 [M0 ]ri0.5 ,

(2.2.2)

where kp is the rate constant of polymerization, kt is the rate constant of termination, [M0 ] is the monomer concentration, and ri is the initiation rate.

In general, the high propagation rate constants of acrylates (kp ≈ 104 l mol−1 s−1 ) together with a relatively slow termination process are the reason why the curing of acrylate based UV curable coatings proceeds so fast.11 The photopolymerization reactions proceed within some seconds, and from the rate constants Decker has calculated the average time for the addition of one monomer unit to the growing polymer chain being about 20 µs.

The propagation reaction drops slightly as the polymer network is formed, whereas the termination reaction drops rapidly due to reaction diffusion control of the termination step.

Besides C. Decker (University of Mulhouse, France), kinetic studies of UV curable systems have been a major research topic in the research group of C. Bowman (University of Colorado, US). Some of the most important publications are dealing with the reaction diffusion during the photopolymerization, the volume relaxation and mechanistic studies of Iniferter (initiation and transfer) photopolymerizations.12 Recent studies are also dealing with modelling, for instance the effects of chain length on the termination kinetics in crosslinking photopolymerization.13 In this work a model has been developed which allows the prediction of multivinyl free-radical photopolymerization kinetics, incorporating termination reactions dependent on the chain lengths (at low conversions) and diffusion controlled termination (at high conversions).

The rate of initiation (ri ) is defined by the quantum yield of initiation (Φi ) and the irradiance absorbed (Ia ): ri = Φi Ia , and is directly related to the photoinitiator concentration [PI]:

ri = Φi I0 1 − e−2.3[PI]ε A/V ,

(2.2.3)

(2.2.4)

where I0 is the incident irradiance, ε is the PI extinction coefficient, l is the sample thickness, A is the exposed area and V is the exposure volume.

Addition rate constants of some of the most important photoinitiator radicals to acrylate and methacrylate monomers have been compiled recently14 and evaluated as a function of the polymerization medium. The benzoyl radical, produced in alpha type photoinitiators adds with a relatively low rate constant to the (meth)acrylate monomers (in the order of 105 M−1 s−1 ), whereas the second radicals formed in the cleavage reaction (aliphatic ketyl radicals) add with a much higher rate constant in the order of about 107 M−1 s−1 to the monomers.

The kinetic chain length (kcl) in crosslinking reactions have to be shown as considerably high in spite of the high rate of initiation, due to the high number of radicals produced during intense illumination and to the short exposure.

The kinetic chain length can be expressed as: kcl = rp /ri .

(2.2.5)

28

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

In an example presented in the paper of Decker,10 the kinetic chain length of a formulation based on Lucirin® TPO as photoinitiator, has been determined to be a value of 7700 mol per radical. This high value surprises to some extent, since the high exposure energies used in UV illumination should increase the rate of initiation (2.2.3), which would consequently, decrease kcl (2.2.5).

The long kinetic chain length observed in UV curable acrylate based coatings, however, is considerably attributed to the high monomer concentrations used (in a typical formulation in the order of [M0 ] = 3 mol/l). In the Lucirin based formulation 1 wt% of photoinitiator was used, thus, each photoinitiator molecule is surrounded by about 100 acrylate double bonds. After a short exposure time, about 1% of the initiator is destroyed (forming two radicals), thus resulting in a situation where each radical produced is surrounded by 5000 acrylate bonds. Finally about 10,000 double bonds were found to be polymerized by every produced radical.

2.2.2.1 Influence of the irradiance The effect of irradiance (mW/cm2 ) (often misleading termed light intensity (I0 )) on the polymerization kinetics has been described in several studies.15 The increase of irradiance has been shown not only to increase the rate of polymerization, but also the conversion of double bonds, thus the extent of cure is much higher. This effect has been explained by the increase of the sample temperature at higher irradiance, due to higher heat production of the lamp itself as well as to higher heat of polymerization produced within the short polymerization time, leading to higher mobility and thus higher conversion.

2.2.2.2 Copolymerization The curing kinetics is also dependent on the type of reactive groups used.

The polymerization rate of acrylates is for instance considerably higher than for methacrylates, since the α-methyl group stabilizes the radical. Data found in the Polymer Handbook16 for the thermal initiated polymerization roughly give a difference of a factor of 5 in the polymerization rate at room temperature. However note that the termination reactions are also faster with acrylates: Methyl methacrylate

kp = 310 l/(mol s) (25 ◦ C)

kt = 17 × 10−6 l/(mol s)

Methyl acrylate

kp = 1580 l/(mol s) (25 ◦ C)

kt = 55 × 10−6 l/(mol s)

Similar results, obtained by photocalorimetric measurements,17 have been published for the photoinitiated polymerizations of acrylates and methacrylates (I = 5 mW/cm2 , 4 wt% PI):

TetrahydrofurfurylTrimethylolpropane tri-

Acrylate kp (l/min)

Methacrylate kp (l/min)

1.94 6.41

0.54 1.55

In homopolymerizations, these differences result in a higher polymerization rate of acrylates compared to methacrylates. If different monomer types are used, like acrylates and

29

THE UV CURING PROCESS

methacrylates (for example, when using methacrylates as reactive diluents), the rules of the classical copolymerization apply. In a simple copolymerization model, it is assumed, that the reactivity of an active center only depends upon the monomer unit in the chain where the active center is located. The kinetic scheme is shown in Figure 2.8.

These reactivity ratios define the relative reactivities of the growing radical center towards the comonomers. Reactivity ratios of comonomers have been determined for the classical copolymerization and several values can be extracted from the Polymer Handbook.

Examples are presented in the tabular form below: Monomer A

rA

Monomer B

rB

Acrylic acid Acrylic acid butyl ester Acrylic acid butyl ester Acrylic acid ethylhexylester Acrylic acid hydroxybutylester Acrylic acid methylester

0.42 0.15 3.07 0.26 0.73 0.35

Acrylic acid, methyl ester Styrene Vinyl acetate Styrene Styrene Methylmethacrylate

0.98 0.8 0.06 0.94 0.72 1.8

The reactivity ratios of the methyl esters of acrylates and methacrylates in a copolymerization clearly show, that the acrylate radical as well as the methylacrylate radical react faster with the methacrylate monomer than with the acrylate monomer, because of the formation of the most stabilized (methyl acrylate) radical. Thus, in a copolymerization, every formed radical reacts faster with methacrylates than with an acrylate monomer. However, at least the further addition of another methacrylate proceeds with the slower polymerization rate of methacrylates, thus often leading to an overall reduced rate. Furthermore, this behaviour leads to polymer structures, which should be more block like rather than statistical.

F IG . 2.8. Propagation possibilities in copolymerization and reactivity ratios of different monomer units.

30

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 2.9. Comparison of acrylate to methacrylate conversion in photoinitiated homopolymerization of HEA and HEMA, respectively.

In Figure 2.9 the polymerization behaviour of acrylates and methacrylates in air and inert atmosphere (CO2 ) is shown.18 The homopolymerization of Hydroxyethyl methacrylate (HEMA) did not occur at all in the presence of air in a 10 Âľm thick film at low irradiance (30 mW/cm2 ), and only about 10% of the Hydroxyethyl acrylate (HEA) is polymerized under air. This is due to the oxygen inhibition reaction, discussed in more detail in Chapter 7. The great differences in reactivity of acrylates and methacrylates can be revealed when exposed under inert conditions, for example, under carbon dioxide. These monofunctional acrylates or methacrylates are generally used as reactive diluents in oligomeric acrylate resins. The kinetics of the reaction of urethane acrylate resins in the presence of 30% of the HEA and HEMA, respectively, is shown in Figure 2.10. In the inert atmosphere, again, the differences can be shown clearly. The formulation containing the acrylate (HEA) diluent polymerizes readily within 1.5 s to a conversion plateau of about 95%. When the methacrylate (HEMA) is used as a reactive diluent, the polymerization is significantly retarded and is similar to the polymerization behaviour of the pure HEMA. After most of the HEMA reactive diluent is consumed, the polymerization rate increases significantly.

This example demonstrates that the curing behaviour is dependent on the type of the functional groups used.

In radical based UV curing, the number of different functional groups used is limited.

Still, unsaturated polyesters, based on maleic acid as the unsaturated group, rely on the use of styrene as a diluent. Due to toxicological reasons, the use of styrene is rapidly decreasing. Styrene has been substituted by acrylates, even in systems based on unsaturated polyesters. Since the vast majority of resins are acrylate functionalized, the preferred reactive diluents are also acrylates (tri- or dipropyleneglycol diacrylate, hexandiol diacrylate, trimethylolpropane triacrylate). Since these low molecular weight acrylates are eye and skin irritating, higher molecular weight acrylates as well as other functionalities have been

THE UV CURING PROCESS

31

F IG . 2.10. UV curing of UA acrylate resin-acrylate diluent (HEA) and UA acrylate resin-methacrylate (HEMA) diluent.

tested. The methacrylates are less irritating, however, the kinetics of the copolymerization with the acrylate resins has to be considered.

There are also other functional groups besides acrylates and methacrylates employed in UV curing systems. Very effective in the reduction of the formulation viscosity are vinyl ether (VE) monomers,19 which at the same content are more effective in viscosity reduction of, for example, an epoxy acrylate resin than TPGDA (tripropylene glycol diacrylate).

The kinetics of the radical-induced copolymerization of divinyl ethers with acrylate resins has been evaluated by Decker.20 In acrylate/vinylether systems the copolymerization behaviour is much different than in acrylate/acrylate or acrylate/methacrylate systems, since the vinyl ethers do not homopolymerize radically. Here, the reactivity ratio rA = kAA /kAB was found in the order of RA = 1.8, indicating, that an acrylate radical is about twice as reactive towards an acrylate double bond than towards a vinyl ether bond.21 Depending on the monomer feed ratio, unreacted vinyl ether bonds may be present in the coating, since no homopolymerization occurs. In order to get close to 100% conversion of the vinyl ether groups, the vinyl ether content should not exceed about 10% of the resin composition with acrylates as copolymerization partners and about 50% in unsaturated polyesters (ref. 19).

Vinyl ethers also undergo a radical copolymerization with monomers having electron poor double bonds, like maleimides and unsaturated esters (e.g., maleates or fumarates).22 Such monomers also do not homopolymerize either, but polymerize radically in an alternating fashion with high efficiency if the stoichiometric mixture based on the double bond content was kept.23 The polymerization of the VE–maleimide or maleate systems proceeds through a donor– acceptor mechanism, which is responsible for a less oxygen inhibited polymerization behaviour,24 but still present as shown by Dias.25 The charge transfer complex furthermore can directly be photolyzed to an exciplex (VE-MA)∗ , which initiates the polymerization

32

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

without the use of a photoinitiator. The polymerization, however, proceeds much slower than in the presence of an additional photoinitiator since the absorbance of the donor– acceptor complex is very low.

The absorbance of the donor–acceptor complex is higher with systems based on vinyl ether–maleimide monomers, resulting in a more efficient copolymerization in such photoinitiator free systems.26 This is mainly due to the higher absorbance of the maleimide monomer as well as the strong hydrogen abstractability of the excited maleimide, which is considerably higher than benzophenone. The maleimides thus can act simultaneously as a photoinitiator and as a (co)monomer. If the curing is performed under inert atmosphere (nitrogen) and readily abstractable hydrogens are present, like aliphatic maleimide structures with a methylene group next to the imide-nitrogen and ideally with even better abstractable hydrogens, such systems can be competitive to conventional hydrogen abstraction type systems.

2.2.3

Frontal Polymerization

Because of the limited penetration of UV light into a coating, UV polymerization is mainly used in thin film (up to 200 µm) applications. The term frontal polymerization27 refers to a polymerization process, where the UV light is absorbed by the photoinitiators at the surface layers of UV curable coatings and upon bleaching the photopolymerization front moves into the deeper layers of the coating. Up to 2 mm thick samples have been cured by such frontal polymerizations in a system containing 5 wt% triarylacylphosphine oxides as strongly absorbing photoinitiators and polyurethane acrylates as resins.28 Further systems described therein are based on the use of N-substituted maleimides as photobleachable components in photoinitiator-free coatings, and interpenetrating networks (IPN) based on the mixture of radically (acrylates) and cationically (epoxides) polymerizing systems.

These polymerizations can be initiated simultaneously or consecutively, whereby the polymerizing front velocity is directly controlled by the bleaching rate of the corresponding photoinitators. Photoinitiated frontal polymerisation can for example be used for the repair of holes in many materials or the curing of composites.29

2.2.4

Heat of Polymerization

For a polymerization to occur, the (Gibbs) free energy difference between the monomer and the polymer must be negative G = H − T S. For a polymerization process, the gain in enthalpy ( H ) caused by a conversion of a π to a sigma bond is a highly exothermic reaction (–34 to –160 kJ/mol) and dominates the loss of translational entropy, when monomers are bound to chains. The entropy loss is in the range of –100 J/mol, thus at normal temperatures, the enthalpy term will be the dominating and favouring the polymerization.

THE UV CURING PROCESS

33

Thus, the curing process of acrylates and methacrylates is exothermic and the change in enthalpy for acrylates and methacrylates is typically in the range of minus 50–90 kJ/mol.

In general, the most important factors for the enthalpy gains are the resonance stabilization of the monomer double bonds and the steric strains in the polymer formed. Thus, the better the resonance stabilization of the monomer, the less exothermic the reaction (for example, α-methyl styrene exhibits only a low heat of polymerization of 35 kJ/mol), on the other hand, tetrafluorethylene produces a polymer with little steric strain, thus exhibiting a heat of polymerization of 155 kJ/mol.30 In acrylate based photopolymerizing coatings this exothermic reaction can increase the sample temperature cured at room temperature up to 90 ◦ C within a fraction of a second. The rate at which the temperature rises was found to correlate well with the rate of polymerization.31 Recording the maximum temperatures reached upon curing of a sample of a urethane diacrylate under different light intensities: • 10 mW/cm2 : 40 ◦ C at final conversion of 80% • 40 mW/cm2 : 65 ◦ C at final conversion of 90% • 80 mW/cm2 : 90 ◦ C at final conversion of 95% shows, that the final conversion is dependent on the maximum temperature reached for a UV cured sample. At the same energy (energy = irradiance × exposure time, in the past often expressed as dose = light intensity × exposure time), the final conversion increases with increasing light intensity, since the heat of polymerization is released in a much shorter time, thus resulting in the higher sample temperature. The temperature profiles have been recorded by Decker with IR spectroscopy, using a temperature sensitive IR band of poly (propylene) substrates at 842 cm−1 .32

2.2.5 Evaluation of Cure Extent 2.2.5.1 Real-time infrared (RT-IR) spectroscopy As mentioned before, the cure conversion can be measured with infrared spectroscopy, either classically by measuring the IR spectrum before and after cure, or with a method developed by Decker, the real-time FTIR,33 following the course of acrylate double bond conversion during UV polymerization (Figure 2.11). With this method, the temporal reaction conversion could be followed in the millisecond range by the decreasing absorption of the acryl ester double bonds at 810, 1410 or 1636 cm−1 , while simultaneously curing with UV light. Important information on the polymerization rate, content of the unreacted double bonds, inhibition by oxygen, and so on can be gained with this method.

The decreasing absorption for example of the 1410 cm−1 band can be directly converted to conversion of the acrylate bands as a function of exposure time, as shown in Figure 2.12.

The rate of polymerization (Rp ) can easily be obtained from the slope of the conversion versus exposure time curve and the initial monomer concentration Rp = [M0 ]

dx . dt

(2.2.6)

34

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 2.11. Set-up of real-time Fourier transform-infrared spectroscopy (RT-FTIR).

F IG . 2.12. Transfer of RT-IR spectra to acrylate double bond conversion.

Furthermore, the final conversion can be extracted from the conversion–time curve and the residual remaining unreacted double bonds calculated.

The IR measurement gives a good correlation with the total double bond conversion of the coating layer, but no information about the cure extent throughout the film thickness can be deduced. Thus, another technique has been developed, which allows the measurement of acrylate double bonds throughout the film thickness, Confocal Raman Microscopy.

2.2.5.2 Confocal Raman Microscopy Confocal Raman Microscopy combines the chemical information from vibrational spectroscopy with the spatial resolution of confocal microscopy.34 The spatial resolution of this technique is in the range of 1 µm3 . This allows a mapping of the surface as well as a depth profiling of, for example, the remaining acrylate double bonds. The set-up of the Raman microscope is shown in Figure 2.13.

THE UV CURING PROCESS

35

F IG . 2.13. Confocal Raman spectroscopy set-up.

For example, Raman microscope LABRAM (Dilor company) can be equipped with a He– Ne laser (632.8 nm, 10 mW power at the surface). The Confocal Raman Spectroscopy has been used for evaluating the oxygen inhibition effect (see Figure 7.2, Chapter 7), the light attenuation effect described by Lambert–Beer’s law, the cure extent within pigmented coatings, or the effects of additives on the curing conversion (UV absorbers, HALS radical scavengers, etc.).35 Raman spectra mapping of a coating into the deeper layers with one micron steps is shown exemplarily in Figure 2.14. The indicated peaks are attributed to acrylate double bond vibrations. As a function of the depth of the cured coating the remaining double bonds are diminishing with cure depth.

Thus, real-time IR spectroscopy and confocal Raman microscopy are two complementary non-destructive techniques useful for the characterization of UV curable and cured coatings. Whereas the RT-IR gives information about the conversion development during curing and the residual double bonds over the whole film thickness, the confocal Raman spectroscopy is able to give a detailed picture about the spatial distribution of residual double bonds within the coating depth as well as a surface mapping. Thus, IR and Confocal Raman Spectroscopy are complementary techniques applied to gain information about conversion, residual monomer content, oxygen inhibition effects and so on (Figure 2.15).

2.2.5.3 In-line monitoring by near-infrared (NIR) spectroscopy The in-line monitoring in process control of various reactions including the radical polymerization with nearinfrared spectroscopy has gained widespread use.36 Since the extinction coefficients in the near-infrared are relatively low, the NIR spectroscopy measurements are usually performed with samples having layer thicknesses in the mm range. Due to calibration of the NIR spec-

36

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 2.14. Depth profile of a UV cured polyether acrylate by confocal Raman microscopy.

F IG . 2.15. Characterization of cure extent by IR and confocal Raman.

troscopy with either FTIR spectroscopy or specific properties of the coating, like hardness, it has been shown that NIR spectroscopy can be used as an effective tool for monitoring the conversion of acrylate double bonds directly in the running UV curing line.37 2.2.5.4 Photo DSC The differential scanning calorimetry (DSC) is the most widely used technique to study photocuring reactions (Figure 2.16).38 It is very well suited for the determination of kinetic parameters, like enthalpy, degree of conversion, rate constants, Arrhenius parameters, etc., however, since the DSC has relatively long response times (2 s),

THE UV CURING PROCESS

37

F IG . 2.16. Differential photocalorimetry set-up.

the monitoring of fast UV polymerizations is less accurate, unless low irradiance exposures are chosen.

2.3 EQUIPMENT (FOR UV CURING PROCESS) 2.3.1 Coating Application Processes The application of the UV coating to the substrate is usually done in automated processes.

There are several application processes in operation, which are very well described in the literature39 : 1.

2.

3.

4.

Roll coaters and curtain coaters, used for flat-panel production; Airless or conventional spray guns, used for three-dimensional or shaped objects; Vacuum coaters; Electrostatic application.

The typical UV formulations are in a viscosity range (<4000 mPa s) to be used preferably in roll and curtain coating applications. The very low viscosities needed for spray coatings (less than 500 mPa s) are hard to obtain without the use of solvents or high amounts of diluents.

The transfer efficiency of the liquid coating to the substrate is very high (about 100%) in roll, curtain and vacuum coaters. Similarly, in spray lines overspray of the 100% solid UV lacquers can also be recovered and reused.

2.3.1.1 Equipment advances New automatic, computer-controlled, carousel-type robotic spraying units have been introduced recently, which are equipped with a photo eye which â&#x20AC;&#x153;readsâ&#x20AC;? the substrate and fires the precise amount of coating to an exact configuration. Furthermore, reciprocating sprayers which use separate heads that traverse the conveyor and employ the photoeye technology within an enclosed structure are providing exceptional finishing capabilities. This new application technology make it increasingly likely that manufacturers who already make limited use of UV coatings will expand the use of these systems to more intricate or detailed furniture. Kitchen cabinet manufacturers, for example, who already use UV coatings on end panels are likely to find that equipment

38

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 2.17. Robot applications: spraying (left) and curing (right: robot equipped with a microwave powered lamp).

advances now make it practical for them to apply UV coatings to doors. A 150-meterlong finishing line, with 25 employees using conventional spray-applied coatings, could be shortened to 30 m and would require only two or three employees if redesigned for UV coatings. Production time in such a scenario would be reduced from 1.5 h to 10â&#x20AC;&#x201C;20 min or less.

2.3.1.2 UV robot For three-dimensional spray coating applications robots are often used. Whereas the robot application for spray coating is well established in the automotive production lines, the UV curing with robots is in development.40 Instead of the spray heads, lamps have also been placed on a robot in order to pass along the contour of the object (Figure 2.17). Recent developments using UV lamps mounted onto robot arms in order to cure large and complex three-dimensional parts have been reported by P. Mills.41

2.3.2

Exposure Process

2.3.2.1 Terms related to emission and deposition of light at the surface The quality of the UV curing process is characterized by the desired properties of the coating. These properties can often be realized by exposure to very different lamp systems or configurations, for instance, by exposure to a standard mercury UV lamp, by extended exposure with a lamp having only low irradiance in the photoinitiator absorption bands, or by very short exposure with intensive flash lights. The optimization and design of the most efficient curing process, however, is dependent on the match of the photoinitiator absorption characteristics and the spectral output characteristics of the lamp. The characterization of the energy applied at the surface of the coating can be measured by radiometric methods.

THE UV CURING PROCESS

39

F IG . 2.18. Terminology used for UV curing related to light emission.

Since this measurement does not give any feedback about the interaction with the chemistry going on in the coating film, a thorough understanding of the optical properties of the film (photoinitiator absorption, optical density, absorption of additives), the key exposure conditions (spectral distribution, irradiance profile, cure speed, heat evolution of the lamp) and the radiometric measurement conditions (wavelength range, responsivity, saturation, sampling rate) has to be gained.

When dealing with UV exposure, several terms are used to characterize the emission and the deposition of light at the surface to be cured, as well as the interaction of light within the coating film.

In order to provide common and technical meanings of terms appropriate for UV process design, measurement and specification, the RadTech (North America) organization has elaborated a glossary of terms.42 Some of these terms are explained within this chapter in order to get a general idea about the processes involved in UV curing (Figure 2.18).

By describing the emission of a light sources one has to consider several physical parameters, e.g., the electrical input power, which stimulates the emission, the radiance, which refers to the radiant output of the source, and the radiant power, which is the rate of the energy transfer emitted in all directions.

The irradiance describes the radiant power arriving at a surface (to be cured) from all angles, including the light coming from the reflector installed to direct the light to the location of interest, per unit area. The radiant energy is the radiant power multiplied with the exposure time, and relating the radiant energy to a unit area (irradiance Ă&#x2014; exposure time) results in the energy density (J/cm2 ), which is applied at the surface.

40

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 2.19. Energy output of a mercury bulb relative to electrical input.

This energy density is applied at the surface in the form of the complete emission spectrum of the lamp. The spectrum of the lamp is composed of the radiant output versus wavelength, which is a characteristic of the lamp used, but can be altered by the composition of the emitting atoms in the lamp. Since only the energy of a wavelength or wavelength range absorbed by the photoinitiating species contribute to the curing process, this energy within a specified wavelength range is termed the effective energy density. This effective energy density arriving at the surface can be measured by radiometric methods, as explained later, and used for the characterization of the UV curing process.

In UV curing equipment, the lamps are often characterized by the electrical input power, related to the unit length of a lamp, the power given as W/cm, whereas this input power of the lamp does not have a direct meaning to the output efficiency of a lamp system, to spectral conversion efficiency, to curing performance, nor to the UV irradiance delivered to the surface. Typical data for the mainly used mercury high pressure lamps are in the range of 80–120 W/cm and up to 275 W/cm for special types. In order to give a rough estimation for the output efficiency of a mercury lamp, the output in the UV range is about 35% of the input power (Figure 2.19).

Terms related to the energy transfer, the total emission from a light source: The radiant power is the rate emitted in all directions per time unit (s), measured in W (J/s). The radiant intensity is then the total radiant power emitted in a given direction defined by a steradian (W sr−1 ). The spectral radiant power is the radiant power at each wavelength. Plotting the spectral radiant power as a function of wavelength results in the spectrum of the radiation source.

The amount of radiant power impinging on a unit surface area from all forward angles, is accordingly the irradiance measured in (W/cm2 ). The irradiance arriving at the surface is dependent on the distance from the lamp. Sometimes the irradiance values are given by the manufacturers for selected distances (e.g., 150 mW/cm2 at 20 cm distance and 75 mW/cm2 at 50 cm distance), however, often without characterization of the wavelength range. Thus they are better measured with radiometry (as discussed later on) for the actual UV system.

41

THE UV CURING PROCESS

TABLE 2.1. Terms and SI units of light emission Quantity

SI unit

Symbol

Radiant energy

joule

J

Radiant flux = radiant power

watt

W

radiant energy/unit time

Radiant intensity

watts per steradian

W sr−1

power/unit solid angle

Radiance

watts per steradian per square meter

W sr−1 m−2

power/(unit solid angle × unit area)

Irradiance

watts per square meter

W m−2

power/unit area

Energy density

joules per square meter

J m−2

Spectral radiance

watts per steradian per square meter per nanometer (wavelength) watts per square meter per nanometer

Spectral irradiance

Remarks

measured in W sr−1 m−2 n m−1 measured in W m−2 nm−1

One of the most important parameters is the energy density, which is the radiant energy arriving at the surface per unit area, the time integral of irradiance, usually expressed in J/cm2 or mJ/cm2 . Terms used in other technologies for the energy density are “light dose” or “radiant exposure”.

Thus, the total UV energy to which the coating is exposed is the time integral of irradiance (I )

Eλ range = Iλ range dt.

Since these considerations are all valid for the whole electromagnetic spectrum, the radiance, irradiance or energy density are also related to a specified wavelength range, for instance in the UV-A, UV-B or UV-C, called accordingly effective irradiance or effective energy density, or related to a single wavelength, called then spectral radiance and spectral irradiance.

Since the light consists of photons, the radiant power can also be defined as radiant flux, which represents the flow of photons, expressed in Einstein/s, where 1 Einstein corresponds to one mole of photons.

2.3.2.2 Excursion to illumination In the case of illumination these output parameters of a radiation source have to be related to the optical parameters of light, based on the sensitivity of the human eye. From this correlation, the luminous parameters result, the equivalent of the radiant power is the luminous power, measured in lumen (lm), of the radiant intensity it is the luminous intensity, measured in candela (cd), the radiance has its equivalent in the luminance, measured in candela/square meter and the irradiance in the illuminance, measured in lumen per square meter or lux. Just for an example, the electrical power of a 60 W light bulb results in a luminous flux of 730 lm, corresponding to a luminous efficiency of 12 lm/W. This light is irradiated in all directions, resulting in a luminous

42

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

intensity of 730 lm/(4π sr) = 58 cd. The luminous intensity related to the electrical power (60 W) is therefore about 1 cd/W. In UV curing, the luminous characteristics do not play a significant role, however, they are often listed as a characterization of a bulb or lamp.

2.3.2.3 Interaction of light within the coating In UV curing, the just defined terms relate to the emitted energy from a light source and the energy density arriving at the surface.

When the light impinges on a coating surface, it may be transmitted without interaction, or if it interacts with the material it may be reflected or absorbed.

In order to get a photoreaction started, the applied energy has to interact with the coating materials, especially with the photoactive compound.

For an exact evaluation of the interaction of light with materials, one has to consider that light consists of photons (=quant), which contain different energies related to their wavelength (λ).

The energy of a photon can be calculated from Planck’s equation E = hc/λ, with h = 6.625 × 10−34 J s and c is the velocity of light, which results in values of 4.9 × 10−19 J (400 nm) to 7.1 × 10−19 J (280 nm) for the electromagnetic UV spectrum.

In chemistry, energy is often related to one mole, thus related to 6.023 × 1023 molecules and expressed as Einstein 1 Einstein = 6.023 × 1023 quants or photons.

Einstein is not an energy equivalent unit, but 1 Einstein of quants transports an energy according to the wavelength of the light used.

The whole emitted irradiation of a lamp is called radiant flux, the number of photons per second or in energy equivalents in watts (W). Related to the lamp area is the radiant flux density in photons per cm2 and s, expressed in energy of W/cm2 . As an indication for the order of magnitude, the incident photon flux, emitted in the 250–400 nm UV range has been measured to be 1.5 × 10−6 E s−1 cm−2 (for a medium pressure Hg lamp).43 From the applied radiant power (P0 ) of a single wavelength, a fraction is transmitted (P ) and the other part absorbed (Pa ).

The fraction of transmitted light is called transmittance (T = P /P0 ), and the fraction of absorbed light is called absorbance (A = Pa /P ), commonly expressed in % values: (P /P0 ) × 100 = % transmittance, (Pa /P ) × 100 = % absorbance.

The absorbance can furthermore be expressed as A = log10 1/T = log10 P0 /P , commonly called optical density.

For the interrelationship with the physical parameters of the coating material, the Lambert–Beer law applies44 P = P0 e−εcd ,

(2.3.1)

where ε is the molar extinction coefficient in l mol−1 cm−1 , c is the concentration of absorbing material in mol/l and d is the thickness (depth layer in cm).

THE UV CURING PROCESS

43

From Beer’s law the radiant power at a given depth can be calculated by the following equation:

(2.3.2) Pa = P0 1 − 10−A /d.

2.3.2.4 Effect of radiation attenuation throughout coating thickness In the UV curing process, however, the most important part is the physics and chemistry occurring in the film to be cured. Among many others, the spectral absorbance (a function of the wavelength) of the photoactive compounds is essential, but the efficiency of the photoinitiator absorption is also dependent on the absorption of all materials contained in the film, governing the “optical film thickness”, which determines the light available at every point of the film.

It has been described already that the first step of a photoreaction to take place is the absorption of light. However, due to this absorption, the amount of light getting to the bottom of a coating layer is reduced. The attenuation of light throughout the coating thickness is described by Beer’s law. The question arises, which will be the optimum absorbance or optical density of the photoactive compounds in the coating layer. It has been mathematically calculated and verified in a model photosensitive layer, that the optimum spectral response of a photoactive system is given at an optical density of the photoactive component absorbing actinic radiation of about 0.43.45 This consideration is dependent on the assumption, that the maximum response of the photoactive system is determined by the photoreaction in the incremental volume element nearest to the substrate (base). Two extreme cases can be considered, first, the optical density is 0, thus all light would pass, but no photoreaction occurs, whereas in the other case at extremely high optical density all light is absorbed at the surface and no light can penetrate to the base. According to Beer’s law the energy density absorbed per unit time by the base volume element (Pb = P0 e−εcd ) can be calculated. This results finally in the expression: Optical density (O.D.) = εcd/2.303.

The maximum of the function appears to be when εcd = 1, thus resulting in an optical density of 1/2.303 = 0.43. This is valid only for systems where absorption does not change during exposure. If bleaching (decreasing absorption during exposure) occurs, the optical density may be much higher and if absorbing photoproducts are formed, the optical density should be lower. These considerations can also predict the wavelength of maximal response to be the wavelength at which the density due to the photoactive component is 0.43. To keep the optical density of a coating at this value is extremely important in positive acting photoresists, where the solubility change of the incremental base volume defines the needed energy density, whereas in UV curable systems exact cure conversion of the base layer does not play such an important role, since usually an extreme overexposure is applied to compensate for the oxygen inhibition effect. However, in inert curing systems, the amount of necessary photoinitiator and energy density can be optimized by complying with this theoretically optimal optical density.

2.3.2.5 Effective energy and peak irradiance As has been shown, that the same energy can be applied by a low power lamp and high exposure times, or high power lamp and

44

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

low exposure times. Whereas this should in theory result in the same curing efficiency, this is often not the case. This ostensible unexpected behaviour has been attributed in the literature to a nonlinear relationship between needed energy density (dose) for curing and radiant power.46 However, one has to consider several other factors. In the curing reaction a transition of a liquid to a solid phase is performed. Therefore, during the curing reaction, the glass transition temperature of the coating increases. This leads to a vitrification of the coating and finally to a limitation of the conversion, due to restricted mobility. However, if the same energy is applied in a shorter time (high power, low time), more radicals are produced, which result in higher polymerization rate and therefore more polymerization heat evolved, which may lead to higher conversion, if the glass transition temperature of the final coating is above room temperature and the mobility limitating factor. If the same dose is applied in a longer time, the polymerization rate is slower, thus the heat evolved lower, which may lead to reduced mobility of propagating radicals and therefore lower conversion. Furthermore, the heat evolved from the lamp itself, which will be different at different lamp powers may also affect the mobility.

The peak irradiance, the maximum point of the irradiance profile, is described to play an important role on the curing efficiency.47 A higher ratio of the peak irradiance to energy can be obtained by a smaller diameter of the bulb (9 mm to 23 mm at 120 W/cm and 13 mm compared to 23 mm at 240 W/cm electrical input). In the experiments described in the literature 47 the conversion of a formulation based on a monomer (ethyldiglycol acrylate) was compared by exposure to lamps with different electrical input and diameter (100 W/cm Hg 23 mm compared to 240 W/cm Hg 9 mm) exhibiting similar energy (535 and 565 mJ/cm2 , respectively), obtained by controlling speed, but different peak irradiance (96 and 780 mW/cm2 ). The higher peak irradiance system exhibited a conversion of 86%, whereas the other achieved only 17% conversion.

The example demonstrating the effect of peak irradiance shows, that it is important for the final curing result not only to pay attention to the effective UV energy of the lamp system to be used, but also to the spectral distribution as well as to spectral and peak irradiance.

2.3.3

UV Line Equipment

A UV curing line for coatings consists of: • • • • • •

One or more UV lamps; Lamp housings and reflectors; Power supply and electric or electronic control; Shielding and cooling; Conveyor, printer or other transport units; And auxiliary parts.

All these components are available individually or as complete systems. The major companies involved in the supply of such equipment are listed at the end of the chapter.

THE UV CURING PROCESS

45

2.3.3.1 UV lamps As mentioned before, one important factor for UV curing is the applied energy density at the coating surface, which is characterized by the irradiance multiplied with the exposure time in seconds (common unit is mJ cm−2 ).48 Thus, important factors to be known about the UV lamp are the irradiance, the profile of radiant power (W/cm2 ) and the wavelength range, as well as the spectral distribution, which is the radiant power as a function of wavelength.

The second factor, which determines the energy density (the time integral of irradiance), is the curing time. The curing time is often characterized in UV curing lines by the curing speed, determined by the velocity of the conveyor belt. The curing speed also influences the heat applied on the surface of the coating, since the heat evolution of the lamp is a factor to be considered. Depending on the coating system and the substrate, the amount of emitted IR radiation may be useful (heating up the coating and increasing conversion) or harmful (plastic substrates may be destroyed).

For the design of an exposure unit, the information of the irradiance profile, the wavelength range and curing time has therefore to be considered in order to design the whole exposure process, since different irradiance profiles can produce different physical properties in the UV cured coating.

2.3.3.1.1 Mercury based lamps Almost all lamps used in UV units available on the market are based on mercury tubes.49 The mercury is enclosed in a quartz tube and excited by electrodes (arcs) or by microwaves. Mercury is used in UV technology, because about 35% of the emitted radiation is in the UV range. The commonly used lamps are medium pressure mercury vapour arc lamps, operated at approximately 1 bar and around 80 W/cm.

The low-pressure lamps operate at 10−6 bar (12 W/cm) and emit mainly wavelength of 185 and 254 nm. The pressure in the tube can be increased up to 100 bar in high pressure versions with 240–360 W/cm, however with the disadvantage of intensity fluctuations.

The pressure increase reduces the mean free path for electron–atom collisions and leads to more emission wavelength in the UV region, mainly at 256, 303, 313 and 356 nm. The nominal input power of these medium pressure lamps is about 80 W/cm, but the total output (integral of emitted wavelength) is in the order of 38 W/cm and since only about 35% are in the UV, only about 14 W/cm are emitted in the UV region. The remaining energy is lost in the form of heat. The output of such bulbs is reduced after about 1000 to 2000 h lifespan, due to degradation at the electrodes and fogging onto the quartz walls.

The microwave powered mercury lamps, developed by Fusion UV Systems, are also of the medium pressure mercury vapour type, but operated by microwave energy, transmitted directly into the emitting plasma, instead of using two electrodes to stimulate the plasma.

Created by microwaves, a high frequency and high intensity electrical field excites the mercury to emit the characteristic wavelength of the mercury lamp. The output in the UV (35%) and visible (25%) is similar to the mercury arc lamp, but the IR radiation is about half (15% instead of 30%). As in the case of the arc lamps, the emitted light and the energies are a function of the used molecules in the plasma. The operating lifetime is described to be in the range of 5000 to 8000 h without changes in the spectral energy output (Figure 2.20).50 The selection of photoinitators therefore must be performed in order to match the photoinitiator absorption with the peak wavelength of the lamp source, in order to obtain the

46

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 2.20. Stability of microwave powered lamp bulbs.

maximum conversion of light energy into the chemical process of excitation, and, finally, formation of radicals, which can start the polymerization.

This tuning can be done easily when the photoinitiator is the only absorbing material in the coating layer. However, in a coating system other components, like pigments or light stabilizers can absorb light and therefore compete with the photoinitiator. Thus, lamps have been developed to provide enough energy and absorption at wavelengths outside of the absorption bands of pigments and light stabilizers. In such lamps the emission spectrum of the mercury lamps is altered by addition of halides of other metals, such as iron or gallium.

The standard mercury lamp is used in almost 80% of the existing applications. The main spectral output is in the wavelength range below 300 nm and at 365 nm (Figure 2.21).

In transparent clearcoat applications, this spectrum can well be used to tune the spectral output with photoinitiator absorption.

By adding iron to the mercury lamp, the lamp spectrum contains additional bands in the range of 350 to 400 nm, thus in the UV-A range. Such types of lamps are used when additives in the coating have strong absorption in the UV-C range, like UV absorbers, and the best overlap with the photoinitiators has to be tuned to longer wavelength ranges.

A further shift to longer wavelength absorption can be done with Gallium and Indium addition. Here intense output is added in the wavelength range of 400â&#x20AC;&#x201C;450 nm. Such lamp systems are mainly used for pigmented coatings.

Such standard or metal halide modified (doped) mercury lamps are available with electric inputs of about 80 to 200 W/cm.

THE UV CURING PROCESS

47

F IG . 2.21. Spectral power of different mercury bulbs (13 mm).

The spectral output power of such lamps is given at a plurality of wavelengths in a specific range. In order to evaluate, if it is important to have a high output at a specific wavelength, the influence of peak irradiance on curing performance has been determined (ref. 47).

2.3.3.1.2 Excimer lamps An introduction in the basics and technology of standard and excimer lamps is given in a presentation by Honig and Bopp (Heraeus Noblelight, www.hi-techlamps.com/pdfs/uv/uvbasics.pdf).

Excimer lamps emit at a single wavelength: 172 nm, used for UV matting; 222 nm, used for UV curing for printing applications; 308 nm, used for UV curing for printing applications.

Their major benefits are described as not warming up the substrate, thus being used beneficially in printing applications, resulting in reduced smell from the print and no warping of the printed medium, and since they are operated under inert conditions, not producing ozone as a side product.

Excimer lamps are typically monochromatic, which also may have some disadvantages in some curing applications, for example if the absorption of UV absorbers or pigments is very high at these wavelength, or the photoinitiators do not absorb significantly at the specific excimer wavelength output.

2.3.3.1.3 LED based light sources Light emitting diodes (LED) have been developed several years ago. Since they emit in the UV region, they are attractive as light sources for UV curing, because of low heat production, portability and low power consumption. These advantages are attractive in applications where dimensional stability of the substrates is required. However, current LED devices are limited in wavelength (>380 nm) and power in the UV region. This leads to difficulties in the curing of some materials, especially in the cationic curing, since the standard cationic photoinitiators all have absorption bands below 300 nm.

The technical feasibility of LEDâ&#x20AC;&#x2122;s has been evaluated by comparing the properties of a urethane acrylate adhesives cured with an LED light vs. a multiple wavelength mercury arc

48

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 2.22. Reflector geometries.

lamps, showing that similar properties can be obtained. Curing studies with epoxies, on the other hand, illustrate that curing with LED systems is limited by the absorption of common cationic photoinitiator systems. Current LEDs are commercially available at wavelengths of 400 nm or higher. In order to match the spectral emission of the LED light, the cationic photoinitiators may have to be modified.51 UV LED’s are, however, still in the development stage and have not yet developed high power or shorter wavelength sources.

2.3.3.2 Reflectors Since a UV lamp emits in all directions, a reflector is used to direct the light emitted in the “wrong direction” onto the surface of the coating. Well designed reflectors collect and project about 75% of the radiant energy from the bulb to the surface, whereas parabolic reflectors collect no more than 55% (Figure 2.22). The material of the reflector is mainly aluminium. Aluminium, however, also reflects IR, which may lead to unwanted heat up of the surface. Therefore, reflectors have been developed, which also reflect UV radiation very well, but not visible and IR radiation. Furthermore, the geometry of the reflector is responsible for the light distribution at the surface, whether homogeneously distributed or well focused.

2.3.4

Exposure Parameter Measurement

Why measure the exposure parameters? When a UV cured coating has been developed to exhibit satisfactory performance characteristics with a specific equipment setup, the operator often does not know what UV exposure conditions are required to obtain this cure.

Furthermore, it is difficult to define the necessary conditions for repeating the process later or even at a different curing line.

For practical reasons it is important to measure and compare the total energy density applied on the coating for the setup of an exposure unit. However, since a lot of other

THE UV CURING PROCESS

49

equipment specific parameters may not be the same, this may not lead to exactly the same curing results, assessed by curing extent and coating performance. Reasons for such deviations may be that the wavelength spectrum of the lamp, which in conjunction with the absorbance characteristics of the coating determines the rate of polymerization, can be different. Second, the configuration of the unit determines the distance to the coating and thus the heat distribution at the coating surface. Furthermore it is important how the energy is measured, since the tools are registering an integral spectral distribution.

Thus, the identification of the key exposure parameters, which have the most significant effect on the performance of the final coating is most important.

These parameters are the total energy, irradiance profile, the spectral distribution and the temperature of the coating by infrared energy. Infrared (IR) energy is primarily emitted by the quartz envelope of the UV source, but also absorbance of UV and visible light and any exotherm of the curing reaction contribute to the temperature profile of the coating.

Thus, it is important for process design to measure these key parameters needed for the UV curing process. For process control a reduced measurement package may be sufficient.

How to measure process relevance? The challenge of measuring the UV energy is not the energy measurement per se, but the correlation of the measured values with the performance of the UV curing reaction, since the measured energy only represents the energy density applied at the surface. The interaction with the photoactive material and the curing process occurring is not reflected. Thus, as mentioned before, the radiometric measurement must be relevant to the process, thus, for example, if the photoinitiator absorption is in the range below 300 nm, the measurement of the radiant power in the UVA range of 315 to 400 nm would not be very relevant.

In Figure 2.23 the situation is elucidated and for the purpose of explanation divided into 3 stages, which are related to the optimization of the effective energy arriving at the surface, the interaction of this energy with the photoactive components in the coating and the control and monitoring of the optimized process. The design of the process is related to an excellent correlation of the desired cure result with the employed UV energy. The difficulty related to the control and monitoring of the process is due to the fact, that the cure extent can not be monitored easily, but instead the radiometric measurements are measuring only the energy density arriving at the surface. Thus, it is assumed that the same energy density arriving at the surface results in the same cure extent. Therefore, in the design of the process the â&#x20AC;&#x153;process windowâ&#x20AC;? 52 also has to be defined, in order to determine which variations in the energy density still give a satisfactory curing result and at which extent of deviations adjustments have to be done.

In the process design, the choice of the appropriate UV lamp type is dependant on the matching of the photoinitiator (PI) absorption and lamp spectrum. After the choice of the proper lamp type, the effective energy arriving at the surface can be optimized by several factors, like reflector design, irradiance, or peak irradiance. These factors have to be correlated with the efficiency of curing. Thus, the photoinitiator type and content, the absorption characteristics of interfering additives, like pigments or UV absorbers, the cure speed and other factors have to be correlated with the exposure parameters and optimized to result in the desired coating properties.

50

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 2.23. Phases of process design, optimization, characterization, and monitoring/control.

F IG . 2.24. Spectral absorption of an aliphatic UA resin and a α-hydroxy-ketone (HK) photoinitiator.

If the key parameters are identified, the “process window” can be determined and the control and monitoring of the curing can then be done by monitoring only few key parameters.

The complexity of energy measurement and significance to process design and control can be exemplified by the examples given in Figures 2.24 and 2.25.

THE UV CURING PROCESS

51

The example shown in Figure 2.24 corresponds to a common clear coat formulation, where only the absorption of the resins and the photoinitiators plays a major role. In this case, the photoinitiator absorption is in the range of 220–280 nm, the UV-C range, which is defined from 200 to 280 nm. So, the most meaningful measurements to the process will be the energy density applied in the UV-C range. However, the measurement of the total energy in this range may not be sufficient, since the distribution of the different wavelengths within the spectrum may be considerably different with two lamps. As shown in the spectral distribution of the different bulbs (Figure 2.21), the radiant power applied below 300 nm is much different with the standard mercury compared to the gallium added bulb. Thus, even if the total energy in the UV-C is the same the curing result will not be the same; for example, in the case of the standard bulb, 1000 mJ/cm2 is applied within 4 s (at 250 mW/cm2 radiant power) and with the gallium bulb 1000 mJ/cm2 is applied within 20 s (at 50 mW/cm2 ). It is therefore also important at what radiant power the energy was delivered. The PowerPuck® , for example, measures the total energy and the peak radiant power per transition band defined. This presents another difficulty, namely that the wavelength range is defined different by the radiometer types and manufacturers (UV-C with the PowerPuck® is measured between 250 and 260 nm).

In Figure 2.25 the absorption spectra are plotted of compounds contained in a pigmented coating formulation, representing a more complicated situation concerning the absorption behaviour of the photoinitiators. There are two photoinitators employed, one type (acylphosphine oxide, e.g., TPO) absorbing at long wavelength outside the absorption spectrum of the pigment, responsible for through cure, and one photoinitiator absorbing in the short wavelength region responsible for surface cure. In principle, the TPO type photoinitiator could also be used for surface cure, however, it is the more expensive type. Thus, in this case the wavelength ranges in the UV-A (315–400 nm) and UV-C play a role and

F IG . 2.25. Overlap of lamps spectrum (Hg–Fe) with absorbance curves of a pigmented UV formulation.

52

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

are the most meaningful for the process, however, the exact impact of the measured data to the process can not be deduced. The overlay of the output spectrum of the ferrum doped mercury lamp with the absorbance curves of the coating components illustrates the importance of matching the spectral output of the lamp with the absorption characteristics of the photoinitiators (Figure 2.25). For a process control, the measurement of the total energies in these ranges can be kept constant by increasing the exposure time (or decreasing belt speed) in order to maintain the curing result. If the radiant power of the lamp decreases for any reason, the curing result will be satisfactory only as long as the radiant power is high enough for the specific optimized curing process. Thus, for process control it is important to track energy density and peak radiant power, in order to correctly run the process and have the right measures at hand to respond, if the curing results are no longer satisfactory.

These examples represent only a simplification of designing and monitoring the UV curing process. A more detailed description of radiometric methods for UV process design and process monitoring has been given recently by Stowe 53 as well as an insight into three-dimensional curing design processes.54 A short description of main aspects is given in the section below.

How to measure the exposure parameters? With the use of an appropriate radiometric process control, the UV curing process can be quantified sufficiently. Ideally, after the coating has been cured satisfactory, the process will be repeated under the same conditions by exposing a dynamic radiometer instead of the coating. Since there is not the radiometer to be used for all setups on the market, the question arises which type of instrument will be suited for the specific process.

There are radiometers available, which are positioned at a specific place in an exposure unit and can perform static measurements, mainly used to monitor the stability of a lamp system. They measure irradiance and spectral distribution, but not energy, since they are not coupled to the exposure time. Radiometers recording the data of the whole exposure process are moving under the lamps. Such dynamic measurements record the total energy, irradiance profile and spectral distribution. Most UV radiometers do not measure IR irradiance. The usual method of determining the effect of IR is the measurement of the surface temperature. The heat produced by the lamp may be a benefit or a nuisance, but it is an inseparable factor in the curing process. Currently, a non-contacting optical thermometer55 is recommended for surface temperature measurement.

Radiometers give the total UV energy density in a given wavelength range, but since the energy is the time-integral of irradiance, information about neither irradiance nor time can be extracted from it. To illustrate this statement, for example, consider a total energy in the UV-A range of 1000 mJ/cm2 , the energy can be applied by an irradiance of 250 mW/cm2 with a mercury lamp within 4 s, or by daylight exposure in the sun at an irradiance of about 2.5 mW/cm2 within 400 s. In both cases, the total energy is the same, but the final properties of the coating exposed to the different conditions would differ very much. Consequently, data on energy alone is less important to design, but more useful in monitoring or control, where the exposure unit is kept constant and only the variation or decay of the energy during operation is monitored.

Thus, information about the peak of irradiance or of the entire exposure profile is important to the design. Figure 2.26 illustrates several irradiance profiles, all of which exhibit approximately the same energy content, but distinctly different profiles â&#x20AC;&#x201C; radiant peak power

THE UV CURING PROCESS

53

F IG . 2.26. Different exposure profiles with the same total energy.

as a function of time. The fact that these exposures produce different physical properties in most UV-curable materials is the reason that profile information is needed in the process design stage. The exposure profile is characteristic of any lamp design. It is not possible to increase the peak-to-energy ratio of a lamp â&#x20AC;&#x201C; this is determined by its physical design.56 For process design, it may be necessary to evaluate a significant number of parameters and variables in order to optimize a process and assure a wide operating â&#x20AC;&#x153;process windowâ&#x20AC;?. Process optimization means matching the lamp system and the required exposure variable to the coating and its application, and matching the UV curable chemistry to the performance required. In order to evaluate the effects on the physical properties of the final cured product, correlation with exposure variables is essential. As mentioned, these variables can be expressed in terms of irradiance profile, spectral distribution, total energy, and infrared energy (or temperature). Multi-band radiometers, mapping radiometers (to evaluate profile), and spectrophotometers can record information on a significant number of these parameters. In addition to facilitating the optimization process, these measurements are used to determine the operating limits for production process control. Once designed and optimized, monitoring the process in production may be limited to monitoring only a few key parameters; namely those which, when out of pre-determined limits, affect the result. From process design, these critical parameters were identified.

Relatively inexpensive, simple tools and methods can be used in production monitoring.

These may be on-line monitors, dosimeter strips, single band radiometers, and the like.

Ultimately, these measurements must be related to target properties to be achieved and must be correlated with measurements of the optimized process from the process design parameters.

Selecting a method of measurement or a particular radiometer should be based on the specific process and the identification of the key variables which have the greatest effect on the process. For further information see also ref. 57.

A wide variety of radiometric instruments (EIT, Inc., International light) is available for measuring the radiant characteristics of industrial and laboratory UV lamps. Relating these characteristics to the performance of a UV-cured product depends on how well the selected parameters match the critical factors of the cure process. Effective radiometry must reflect the understanding of the interactions of the lamp characteristic with the chemistry needed

54

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

for the specific application related to the development of the physical properties of the final coating.

The first step of consideration of an UV equipment setup is related to the process design, the optimization of the UV exposure conditions based on the performance requirements of coating. This is done by varying the formulation ingredients and/or UV exposure, and by evaluation of the performance of the resulting cured coating. In this respect, the optical characteristics of the film, the spectral absorption mainly determined by the photoinitiator type and concentration, the film thickness, other absorbing additives, like pigments, UV absorbers, etc., in conjunction with the spectral characteristic of the used UV lamp play a major role. These parameters determine what exposure is most effective in obtaining the desired physical properties of the cured coating. To characterize this design information together with the limits where the process still works, is important to achieve an efficient process with an adequate process window. Radiometric measurements are useful in quantifying the successful exposure parameters, so the process can be reliably repeated.

2.3.4.1 Radiometric devices There are a lot of instruments and devices available for radiometric measurements, the selection of the proper instrument is dependant on the compatibility with the process equipment (e.g., UV line, on-line monitoring or 3D exposure) and the relevant exposure parameters.

Radiometers measure irradiance (W/cm2 ) over a uniquely defined wavelength band range. Most of them are narrow band radiometers, differing in construction, filters, detectors and mode of operation. Therefore, different radiometers give different results when measuring broad-band sources, due to the different responsivity or wavelength sensitivity. Even if comparing data from instruments of the same type, the actual UV energy may be different if the type of source is different. The predominant number of commercial radiometers measures in the UV-A wavelength range (320–390 nm), mainly due to the fact, that most applied chemistries (photoinitiators) respond in this wavelength region.

The most important parts to know about the radiometer are: 1. Responsivity (wavelength band or range) (EIT UVC (240–260 nm), EIT UVA (320– 390 nm), International Light, IL 390B (250–400 nm)), 2. Dynamic range (W/cm2 ), and 3. Capacity for recording energy (J/cm2 ), and if it is a sampling type instrument: 4. Sampling rate (slow sample rates may reduce accuracy at high speeds; very fast sampling rates can cause errors under lamps that pulse with line (mains) frequency), and if peak irradiances are measured; 5. Threshold (if exposures are in the low irradiance level, for example, as it applies in 3D exposures, where the irradiance may be in the order of less than 100 mW/cm2 , a radiometer with a 50 mW/cm2 threshold would be subject to significant errors).

The responsivity is the degree of response of a radiometric detector to the different wavelength applied. Due to the design and filters used, the responsivity of every instrument

THE UV CURING PROCESS

55

F IG . 2.27. Selected examples of responsivity and mercury lamp UV emission (R.W. Stowe).

is different, and therefore leads to different collected data despite exposure to the same lamp (Figure 2.27). The responsivity data are given by the equipment manufacturers.

For high energy applications, almost all radiometers can be used, however, for energy measurements below 100 mW/cm2 , only few instruments are available (e.g., UVRAD from EIT; low power version of PowerPuck).

A widely used radiometer is for instance the EIT PowerPuck® . The UV Power Puck also has the distinct advantage of being the only radiometer able to monitor the peak irradiance in W/cm2 in each bandwidth. This allows the operator to determine not only the total energy density, but also how that energy density is delivered; i.e., what irradiance at what wavelength.

The PowerPuck® measures four separate UV transmission band ranges (UV-V: 445– 395 nm; UV-A: 390–320 nm; UV-B: 320–280 nm; UV-C: 260–250 nm) simultaneously and indicates the total energy density (J/cm2 ), as well as the peak irradiance (W/cm2 ) of each band range.

Dosimeters measure the accumulated energy (J/cm2 ) over the whole exposure time, also at a uniquely defined wavelength range. Since this instrument type also includes the time of exposure, it is commonly used (e.g., RADCHECK UV EB DOSIMETER).

Spectroradiometers measure spectral irradiance at a very narrow bandwidth, thus they are used when the effect of the irradiance in a specific wavelength on the curing behaviour has to be evaluated.

Radiachromic dosimeters are tabs or strips that measure the time integrated energy by changing colour. They exhibit normally broad band response. The values of colour change can be correlated with a radiometer and then these strips can be used as an effective tool for process monitoring. One example are UV FastChek® strips,58 composed of 5 separate color changing zones, which indicate the applied level of energy by a simple visual inspection, measuring energies from 0 to 1500 mJ/cm2 . They are very versatile and can especially

56

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 2.28. Typical custom engineered UV system (Fusion UV Systems, Inc.).

be placed on positions where the use of a radiometer is difficult (e.g., three-dimensional objects).

Correlating parameters measured with different radiometers? In the development of a coating it happens very often, that the optimization of the formulation and the curing conditions were done with laboratory equipment and afterwards the process has to be transferred to production lines with different lamps and equipment. Often, the type of the radiometer also has to be changed, for instance when the device may not be attached to 3D substrates easily. Thus, it is a very difficult task to correlate different radiometers (may have different responsivities) with different lamps (may have different spectral emissions).

Some correlations can be done with different radiometers when the results are correlated with the exact same lamps and spectral distribution.

2.3.5

UV Coating Line Examples

The setup of a typical 2D curing UV line is shown schematically in Figure 2.28, consisting of a conveyor belt for the transport of the substrate, a coating unit for roller or curtain coating, the lamp system, consisting of one or multiple lamp arrays, an exhaust integrated in the lamps system to remove ozone and other volatiles generated during the exposure and often cooling units for the lamp and eventually the substrate.

There are several case studies about operating UV lines, available by the US EPA (Environmental Protection Agency) in cooperation with the Midwest Research Institute, on UV curable wood coatings, covering different companies (e.g., Columbia Forest Products, Chatham, VA, Lane, Altavista, VA; Loewenstein, Pompano Beach, FL), different products (e.g., panels, office furniture, cabinets, seatings), and different UV cured layers (e.g., UV

THE UV CURING PROCESS

57

sealer, UV topcoat, UV filler).59 Some of the lines and their characteristics are introduced briefly.

One example of existing UV coating lines is at Columbia Forest Products, where a wide variety of hardwood plywood panels are produced, with a clearcoat on one or both sides.

The coating process includes a UV-cured sealer and a UV-cured topcoat. The panels are either multi-ply or medium density fibreboard (MDF). The UV coating line consists of two sanders, two roll coaters, and two UV exposure units, connected by conveyor belts.

The panels first pass through a multi-head sander that also cleans the panels for a smoother coating application. The panels then pass through the roll coater where the sealer is applied.

Afterwards, the coating is cured by UV lamps. The number of lamps and cure time vary depending on the product, however cure time is only a few seconds. In the second half of the line the top coat is applied by a roll coater and cured by two UV lamps. There are several advantages of using the UV-cured coating system instead of traditional solventborne nitrocellulose coatings. The short curing time reduces the amount of space required in the facility to house the UV line. The UV equipment also provides a highly automated coating process and requires a smaller labour force than hand spraying traditional coatings.

Coors Can Manufacturing Plant (Golden, CO), the largest single aluminium can producer in the world, producing approximately 4 billion cans per year, converted to UV-cured coatings in 1975 to increase can printing speeds and reduce energy consumption. Printing speeds are approximately 1600â&#x20AC;&#x201C;1800 cans per minute, and cure time is 0.7 s. The UV ovens require less floor space and less start-up time than thermal ovens meeting the same throughput requirements. Coors estimated that VOC emissions resulting from UV-curable coatings was 1.6 tons (1 t) per billion cans cured, whereas thermally cured cans resulted in 28.5 tons (26 t) of VOCâ&#x20AC;&#x2122;s per billion cans cured.

Sutherland Golf Inc. (Barberton, OH) manufactures golf balls, typically coated with a waterborne primer and a urethane clearcoat. The company chose an UV clearcoat instead because it provides a glossy, tough coating but remains more elastomeric.

Hussey Seating, the world leader in spectator seating, switched already 1994 from a polyurethane finishing line to an automated UV-cured system. VOC emissions decreased from 199 tons/year to 99 kg/year, even though production increased from 9000 units/week to greater than 14,000 units/week. Switching to the new system averted costs for constructing a new storage area ($200,000) and reduced labour costs by two-thirds.

Since UV-curable coatings are 100% solids, coating costs decreased by 17% on a per-unit basis. The UV coatings proved more durable than the polyurethane coatings when exposed to sunlight, heavy use, and water.

Paintbox Ltd., located in Banbury, UK, is a contract coater and decorator of plastic components. Paintbox coats parts for customers in the automotive, domestic appliance, and electronics industries. They used to coat their parts with solvent-based products and cured the parts in thermal curing ovens. However, due to customer concerns, they switched to UV technology, which offers superior scratch and scuff resistance while improving aesthetics.

By switching to UV-curable coatings, Paintbox improved their overall rejection rates. This improvement can be attributed to reduced contamination from dust particles on the coated parts. UV curing dries much quicker, which reduces the possibility of contamination.

IKEA has converted many of its wood coating lines to UV coatings. The resulting conversion helped IKEA reduce the cost per-square-meter finish surface for flat production.

58

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 2.29. Typical UV printing press set-up (IST-Metz, GmbH).

The UV coating system resulted in a more environmentally friendly system that improved quality and productivity. UV technology enabled IKEA to produce products with a better quality finish, as well as better scratch resistance and resistance to liquids, such as water and coffee. IKEA’s line productivity increased due to UV’s faster drying and curing times.

The UV system also provided a safer work environment (i.e., no solvent in the air) with reduced cleaning time.

What are the equipment costs/benefits? While the investment required in UV application equipment is steep, the potential benefits are tremendous. Costs to outfit a UV coating line typically range from $50,000 at the very low end to $1 million, depending on the sophistication of the system. It is unusual for a manufacturer to recoup that investment in a relatively short time from production benefits.

Typical coating lines are schematically shown in Figures 2.28 and 2.29.

2.3.6

Equipment Suppliers

An overview about UV equipment supplier companies, mainly in the US, is given at the GlobalSpec website.60

THE UV CURING PROCESS

59

Main suppliers are listed below.

UV lines and lamps: • • • •

Fusion UV Systems, Inc. (www.fusionuv.com) Hönle (www.hoenle.com) IST Metz (www.ist-uv.de) Nordson (www.nordson.com/Businesses/UV) Lamps:

• Primarc (www.primarc.com) • Heraeus amba (www.heraeusamba.com/uv_curing_lamps.html) • Heraeus Noblelight Excimer lamps (www.heraeus-noblelight.com/HNG/eng/uvfb/uvfb_ Home.nsf/$frameset/start) Instrument suppliers: • • • • • •

Solatell Instruments, 4D Controls Ltd., Cornwall, UK (www.solatell.com) EIT Instruments, Sterling, VA (www.eitinc.com) International Light, Newburyport, MA (www.intl-light.com) UV/EB Labels; UV Process Supply, Inc. Chicago, IL (www.uvprocess.com) Rad Check Dosimeter; UV Process Supply, Chicago, IL (www.uvprocess.com) Far West Technology, Inc., Goleta, CA (www.fwt.com) REFERENCES

1. Stowe, R.W., Some economic factors of UV curing, RadTech (1994), Conference Proceedings, North America, Orlando, FL. 1994, pp. 353–359.

2. “Materials for Microlithography”, In ACS Symposium Series, Vol. 266 (L.F. Thompson, C.G. Willson and J.M.J. Frechet, eds.). 1984.

3. (a) Crivello, J.V. and Ahn, J., J. Polym. Sci. Polym. Chem. Ed. 41 (16), 2556–2569 (2003); (b) Dietliker, K. and Crivello, J.V., In “Photoinitiators for Free Radical, Cationic and Anionic Photopolymerization”, 2nd edn, Vol. 3, In Chemistry and Technology of UV and EB Formulation for Coatings, Inks and Paints (G. Bradley, ed.). John Wiley and Sons/SITA Technology, London, 1998, chapter IV; (c) http://www.rpi.edu/dept/chem/faculty/crivello/crivello.html.

4. Irgacure 250 – Ciba Specialty Chemicals.

5. http://www.cibasc.com/first_commercial_photolatent_base_catalyst_for_uv__a_clearcoat_applications. pdf.

6. Fouassier, J.P., “Photoinitiation, Photopolymerization and Photocuring, Fundamentals and Applications”.

Hanser Publishers, Munich–Vienna–New York, 1995.

7. Allen, N. (Ed.), “Photopolymerization and Photoimaging Science and Technology”. Elsevier Applied Science, London, 1989, Chapter 1, pp. 40–44.

8. Fouassier, J.P., “Photoinitiation, Photopolymerization and Photocuring, Fundamentals and Applications”.

Hanser Publishers, Munich–Vienna–New York, 1995, Chapters 3 and 4.

9. Brandrup, J. and Immergut, E.H., Polymer Handbook, 2nd edn, Wiley–Interscience, 1975, II-45.

10. Decker, C., Macromol. Rapid Commun. 23 (18), 1067–1093 (2002).

11. Decker, C. and Moussa, K., Eur. Polym. J. 26, 393 (1990).

12. (a) Anseth, K.S., Wang, C.M. and Bowman, C.N., Macromolecules 27, 650 (1994); (b) Anseth, K.S., Bowman, C.N. and Peppas, N.A., J. Polym. Sci. A Polym. Chem. 32 (1), 139–147 (1994); (c) Anseth, K.S., Kline, L.M., Walker, T.A., Anderson, K.J. and Bowman, C.N., Macromolecules 28, 2491 (1995); (d) Kannurpatti, A.R., Lu, S., Bunker, G. and Bowman, C.N., Macromolecules 29, 7310 (1996).

60

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

13. Lovestead, T.M., Berchtold, K.A. and Bowman, C.N., Macromol. Theor. Simulat. 11, 729–738 (2002).

14. Lavevée, J., Allonas, X., Jradi, S. and Fouassier, J.P., Macromolecules 39, 1872–1879 (2006).

15. (a) Klosterboer, J.G., Adv. Polym. Sci. 84, 1 (1984); (b) Decker, C., Elzaouk, B. and Decker, D., Macromol.

Sci. Pure Appl. Chem. A33, 173 (1996); (c) Decker, C., Polym. Int. 45, 133–141 (1998).

16. Brandrup, J. and Immergut, E.H., “Polymer Handbook”, 2nd edn. John Wiley & Sons, New York, 1975.

17. Abadie, M.J.M. and Voytekunas, V.Y., Eurasian ChemTech J. 6, 67–77 (2004).

18. Studer, K., Decker, C., Beck, E. and Schwalm, R., Progr. Org. Coat. 48, 101–111 (2003).

19. Schwalm, R., Binder, H., Funhoff, D., Lokai, M., Schrof, W. and Weiguny, S., RadTech Europe 1999, Conference Proceedings. 1991, pp. 103–109.

20. Decker, C. and Decker, D., J. Macromol. Sci. A34, 4 (1997).

21. Decker, C. and Decker, D., J. Macromol. Sci. Pure Appl. Chem. A34 (4), 605–625 (1997).

22. Decker, C. and Decker, D., Polymer 38 (9), 2229–2237 (1997).

23. Noren, G.K., Tortello, A.J. and Vanderberg, J.T., Proceedings RadTech Conference USA, Northbrook, IL, Vol. 2. 1990, p. 201.

24. Jönsson, S., Sundell, P.E., Ericsson, J., Hultgren, J., Sheng, D., Hoyle, C.E., Clark, C.C., Miller, C. and Owens, G., RadTech 96, Conference Proceedings. 1996, pp. 377–392.

25. Dias, A.A., Jansen, J.F.G.A. and vanDijck, M., RadTech Europe 1999, Conference Proceedings. 1999, pp. 473–482.

26. (a) Hoyle, C., Proceedings RadTech Conference USA, Northbrook, IL, Vol. 1. 1990, p. 194; (b) Decker, C., Morel, F., Jönnson, S., Clark, S.C. and Hoyle, C., Polym. Mater. Sci. Eng. 75, 198 (1996).

27. Rytov, B.L., Ivanov, V.B. and Anisimov, V.M., Polymer 37, 5695 (1996).

28. Decker, C., Polym. Int. 45, 133–141 (1998).

29. Pojman, J.A., RadTech 2006, Conference Proceedings, Chicago, IL. 2006, on CD. 30. Allen, G. and Bevington, J.C. (Eds.), “Chain Polymerization”, Part I. Vol. 3. Comprehensive Polymer Science, Pergamon Press, 1989.

31. Decker, C. and Decker, D., F-Morel, In “Photopolymerization Fundamentals and Applications” (A.B. Scranton, C.N. Bowman and R.W. Pfeiffer, eds.), In ACS Symposium Series, Vol. 673. Am. Chem. Soc., Washington, 1997, p. 63.

32. Decker, C., Macromol. Rapid Commun. 23 (18), 1067–1093 (2002).

33. (a) Decker, C. and Moussa, K., Macromol. Chem. 189, 2381 (1988); (b) Decker, C. and Moussa, K., J. Coat.

Technol. 62, 55–61 (1990).

34. Williams, K.P.J., Pitt, G.D., Batchelder, D.N. and Kip, B.J., Appl. Spectrosc. 48, 232–235 (1995).

35. Schrof, W., Beck, E., Königer, R., Meisenburg, U., Menzel, K., Reich, W. and Schwalm, R., Characterization of radiation curable coatings by confocal Raman microscopy, RadTech 1998, North America, Conference Proceedings. 1998, pp. 363–374.

36. Siesler, H.W., Ozaki, Y., Kawata, S. and Heise, H.M. (Eds.), “Near-Infrared Spectroscopy: Principles, Instruments, Applications”. Wiley–VCH, Weinheim, 2002.

37. Scherzer, T., Mehnert, R., Volland, A. and Lucht, H., Process and quality control during UV curing of acrylate coatings using near-infrared reflection spectroscopy, Proceedings RadTech Asia 2005, Shanghai, China.

2005, pp. 574–582.

38. Abadie, M.J.M., Eur. Coat. J. 11, 788–795 (1991).

39. Wicks Jr., Z.W., Jones, F.N. and Pappas, P. (Eds.), “Organic Coatings – Science and Technology”, In Applications, Properties, and Performance, Vol. 2. John Wiley & Sons, 1994, Chapter XXII, pp. 65–82.

40. Raith, Th., Bischof, M., Degner, M. and Gemmler, E., 3-D UV technology for OEM coatings, RadTech Report, November/December 2001. 2001, pp. 26–29.

41. Mills, P., Robotic UV curing: A cost-effective way to cure large, 3D parts, RadTech 2006, Conference Proceedings. Chicago, IL, 2006 on CD. 42. http://www.radtech.org/RadTechGlossary.pdf.

43. Laser induced polymerization: Decker, C. In “Materials for Microlithography” (L.F. Thompson, C.G. Willson and J.M. Frechet, eds.), In ACS Symposium Series, Vol. 266. 1984, Chapter 9, pp. 207–223.

44. http://www.shu.ac.uk/schools/sci/chem/tutorials/molspec/beers1.htm.

45. Thommes, G.A. and Webers, V.J., J. Imaging Sci. 29, 112–116 (1985).

46. Seao, N., RadTech Asia 1991, Osaka, Proceedings. 1991, pp. 478–484.

THE UV CURING PROCESS

61

47. Schaeffer, W., Jönsson, S. and Amin, M.R., Greater efficiency in UV curing through the use of high peak energy sources, Proceedings, RadTech Europe, 1995.

48. Stowe, R.W., Aspects of radiometry and process verification for 3D UV processing, RadTech Europe 2003.

2003, pp. 821–828.

49. (a) see Chapter 2, pp. 41–65, in Roffey, C.G., “Photopolymerization of Surface Coatings”. Wiley Interscience, 1982; (b) Calvert, J.G. and Pitts, J.N., “Photochemistry”. Wiley, New York, 1966.

50. Stowe, R.W., Equipment for UV curing, International Coatings Expo (ICE) and Technology Conference 2000, Chicago, IL, USA, October 18–20. 2000, pp. 1–17.

51. Dake, K., Montgomery, E., Koo, Y.C. and Hubert, M., LED curing versus conventional UV curing systems: Property comparisons of acrylates and epoxies, RadTech North America, UV&EB Technology, Charlotte, NC, May 2004, Technical Conference Proceedings. 2004, on CD. 52. Stowe, R.W., “Relationship of the Significant Elements of UV Lamps to the UV Curing – Process Window. Paint Line Design and Implementation”. Society of Manufacturing Engineers, Nashville, TN, USA, December 1998.

53. Stowe, R.W., J. Coat. Technol. March (2003), www.radtech-europe.com/download/stowemarch.pdf.

54. Stowe, R.W., Verifying UV exposure in 3D curing systems, RadTech North America, UV&EB Technology, Charlotte, NC, May 2004, Technical Conference Proceedings. 2004, on CD. 55. Raytek, Inc.; Santa Cruz, CA (www.raytek.com).

56. Stowe, R.W., Practical aspects of irradiance and energy in UV curing, RadTech 1998. Proceedings, North America, Chicago, IL. 1998, pp. 640–645.

57. (a) Stowe, R.W., Practical relationships between UV lamps and the UV curing process window, Proceedings, RadTech North America, 1994; (b) Stowe, R.W., Practical aspects of irradiance and energy density in UV curing, Proceedings, RadTech North America, 2000.

58. www.uvprocess.com/products/Curecon/UVmon/FastCheck.asp.

59. http://www.epa.gov/ttn/atw/wood/low/ultravio.html.

60. http://process-equipment.globalspec.com/LearnMore/Manufacturing_Process_Equipment/Industrial_ Assembly/UV_Curing_Systems.

C HAPTER 3

Network Formation and Characterization In the year 2000 about 180 mln. tons of polymers were sold, whereof about 30 mln. tons are coatings (lacquers), glues and dispersions. According to their functions and structures these polymers can be classified into different categories, for instance, natural (proteins, polysaccharides, rubber) and synthetic polymers (thermoplastics, elastomers, duromers).

The main group of synthetic polymers belongs to the class of thermoplastics and fibres, which are linear, uncrosslinked polymers, which can be processed into different shapes by thermal treatment. Besides other classifications, one classification of the synthetic polymer types can be done by the increasing degree of crosslinking, whereof the elastomers are slightly or moderately crosslinked, occurring very often as rubber-like flexible materials, and duromers, which are highly crosslinked, available for instance as hard coating materials on kitchen worktops, on cars or on industrially produced parquet floorings (Figure 3.1).

This type of classification however reflects only the structural composition, whereas the names of the different classes (thermoplastic, elastomer, duromers) reflect the mechanical behavior of the polymer classes at the normal temperatures of use. Thermoplastics appear at room temperature in a glassy state, but become plastic upon heating at temperatures

F IG . 3.1. Classification of polymer subgroups.

62

NETWORK FORMATION AND CHARACTERIZATION

63

above the so-called glass transition temperature, the elastomers are used at temperatures above their glass transition and therefore are elastic at room temperature, and, finally, the duromers are very durable and exhibit extraordinary mechanical properties. The mechanical properties of materials are those properties that involve a reaction to an applied stress or load, and determine the range of usefulness of a material and establish the product life that can be expected. They are often used to help classify and identify materials. The most common properties considered are hardness, tensile strength, ductility, impact resistance, and fracture toughness. These properties are dependent on the physical state of the materials, polymers or coatings in particular.

The mechanical properties of a material are not constants and often change as a function of temperature, rate of loading, and other conditions. For illustration, H2 O can appear in the physical state of a solid (snow), in the liquid state (water) or in the gaseous phase (steam), depending on the surrounding temperature. As it is with water, the mechanical properties of polymers are also very temperature dependent, thus they appear as hard or soft materials. Solid polymers are either in a glassy (amorphous) or crystalline state. In a melt polymers are liquid. In between, in the transition between solid and liquid a more or less broad transition state exists, where the polymers become softer with increasing temperature, called the glass transition region.

So, the most important question is, what are the requirements put on a material (or coating) and how do they behave at the application temperature and conditions. For example, an automotive topcoat should withstand mechanical attack during the lifetime of several impacts, like scratching during car wash (and ideally also scratches resulting from vandalism) or gravel chipping, as well as chemical attack of acid rain, spilled gasoline, tree resin or bird droppings, and this within a huge temperature and climate range, from hot and humid conditions in Florida to cold and dry conditions in Siberia or Canada. So, the coatings have to be designed to fulfill a bunch of properties, some of them are diametrically opposed, like hardness and flexibility, and the test methods have to be designed to match the reality as close as possible (and enable a fast screening under time lapse conditions).

The scientific classification of the material states according to their flow properties is similar to the classification according to their uses. Very common test method for the characterization of material properties are stress-strain experiments, where a specimen is subjected to a stress (force/area) and the resulting strain (elongation, compression, bending, torque or shear strain) is measured. In the simplest approximation (which is almost always good enough) the relation between stress and strain is taken to be linear (at low deformations). In this linear region, the relationship is defined by Hooke’s law where the ratio of stress to strain is a constant. The slope of the line in this region where stress is proportional to strain is called the modulus of elasticity or Young’s modulus. The modulus of elasticity (E) defines the properties of a material as it undergoes stress, deforms, and then returns to its original shape after the stress is removed. It is a measure of the stiffness of the material.

Depending on the way the material is being stretched, bent, or otherwise distorted, further moduli can be derived, like the torsion modulus (T ) or shear modulus (G). In the temperature dependent curves of the moduli different regions can be identified, at “low” temperatures and high moduli the glass region, where the cooperative mobility of the chain segments is frozen and the material reacts on stress like an elastic solid (the material is hard). Then the glass transition region where the molecular mobility increases. This re-

64

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 3.2. Modulus (shear G) as a function of temperature for different polymer classes.

gion is defined by a glass transition temperature which, however, is not a thermodynamic defined â&#x20AC;&#x153;sharpâ&#x20AC;? temperature like the melting temperature, but rather a more or less broad region. The definition of the glass transition temperature is dependent on the determination method used (see below). After the glass transition region often a rubber-elastic plateau appears, and at higher temperatures a further drop in the modulus leads into the viscous flow region.

As shown exemplarily for the shear (G)-modulus in Figure 3.2, the modulus of the elastomers drops significantly below room temperature. The uncrosslinked elastomer precursors become very soft at temperatures well below room temperature and pass directly into viscous flow, whereas slightly crosslinked (typical) elastomers keep a relatively low, but nearly constant rubber-elastic plateau at temperatures around and above room temperature. This plateau formation is due to the slight crosslinking, which is necessary for the function as elastomers, since the elastic deformation of the polymer backbone is only reversible if crosslinks prevent the sliding and relaxation of the polymer chains, as happens in the uncrosslinked elastomer precursor. Entanglements of polymer chains are a special form of (temporary) crosslinks. The temperature region at which the drastic drop occurs is called the glass transition temperature, since the material drops from a glassy into a soft or rubber-like state.

The thermoplastics usually exhibit a high modulus up to well above room temperature, until they reach their glass transition temperature, where the modulus drops steeply and steadily.

In the highly crosslinked duromers the modulus drop is shifted further to higher temperatures and the rubber-elastic plateau is on a much higher level than obtained with the elastomers. It will be shown later that the crosslink density can be derived from the hight of the rubber-elasticity plateau. However, with higher crosslink density the drop of the modulus is getting less pronounced and disappears completely in a ideal duromers (since Tg is approaching infinity for x = 1).

NETWORK FORMATION AND CHARACTERIZATION

65

F IG . 3.3. Network structures.

Thus, the main influencing factors on the classification and on the mechanical properties of polymers are the physical states of the polymers at the temperature of use (normally room temperature), as well as on the niveau of the modulus beyond the glass transition temperature.

Thus, the polymer classes range from (at room temperature) soft uncrosslinked elastomers, which upon slight crosslinking remain soft, but posses restoring forces, over (at room temperature) hard, but above its glass transition temperature formable thermoplastics, to the highly crosslinked duromers. On the one hand these highly crosslinked duromers can be considered similar to thermoplastics with extended usable temperature range, and on the other hand they exhibit extraordinary properties with respect to solubility, swelling, chemical and abrasion resistance, etc., as will be discussed further in the book.

Most of the polymer networks (Figure 3.3) belong to the imperfect network type, thus, they are inhomogeneous, contain cycles (or loops), dangling ends and entanglements of molecular chains between crosslinks. However, as shown later, highly crosslinked systems with short chains between crosslinks are approaching the ideal network type, which should behave perfectly energy-elastic (a deformation is completely restored) and do not show plastic flow. As can be seen in parts of a cobweb, nature almost builds perfect networks.

3.1 FILM FORMATION Preliminary to network formation is film formation, which in case of UV curable coatings is not disturbed by drying processes, as in the case of solvent- or water-borne coatings.

The first step in the film formation is the wetting of the substrate and the spreading of the droplets or the liquid applied. This process can be measured by the determination of the wetting angle Îą (>90â&#x2014;Ś for a drop sitting on the surface), which should approach zero for ideal spreading. In order to obtain a good spreading the surface tension of the paint should be equal or lower than that of the surface.1 After wetting the droplets or the liquid flows to

66

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

form a smooth, continuous film. The driving force behind the flow is the surface tension, surface tension gradients and gravity, facilitated by low viscosity. Although certain terms may be disregarded depending on the situation.2 On horizontal surfaces gravity helps for ideal film formation, whereas on vertical surfaces and edges sagging and runaway from edges occurs. The sagging and edge flow can be reduced or prevented by controlling the viscosity and introducing thixotropic behaviour.

The transition of a liquid lacquer to a solid coating passes through various intermediate stages, which can be described by examining the changes in viscosity

lg η = k Mn NV,

(3.1.1)

where NV is the solids content. Thus, the viscosity is dependent on the molecular weight and the solids content. However, this relationship is linear only at low enough concentrations where individual polymer molecules and their associated solvent molecules are isolated from each other. For concentrated solutions of polymers and resins as used in the coatings field the relationship can be complex, however, viscosity generally increases with increasing molecular weight. Among the factors controlling the viscosity are the molecular weight, the molecular weight distribution, the resin structure and the concentration, which determine the Tg of the resin solution.4 As resin solution viscosity increases, for example due to molecular weight increase by crosslinking, at some concentration the Tg of the solution becomes the dominating factor, and above that concentration the viscosity increases rapidly.

Since the application viscosity depends on the application method (e.g., ∼100 mPa s for spray or 3000 mPa s for roller coating), in solvent based systems the higher molecular weights of the resins can be compensated by the solvent content. In 100% liquid UV coatings the molecular weights of the resins have to be in the lower molecular weight range.

The resin is then diluted with reactive diluents to reach the application viscosity range. Finally, upon curing the molecular weight and the viscosity increase into the area of desired properties, like mechanical strength and chemical resistance (Figure 3.4).

Thus, during the coating process (of a UV-curable system) the physical state of the coating changes dramatically from a liquid to a solid state, with two major processes involved, film formation and buildup of a network during the curing reaction.

3.2

NETWORK FORMATION

General principles of polymer network formation, structures and properties are well described.5 The network formation during the curing reaction can be described by two major transitions, the gelation, which involves the transformation of a liquid into a gel-like state and in the case of duromers, the vitrification, the transition of the rubber state into a solid glassy state.

This change in the physical properties is caused by a molecular weight increase during the curing reaction. In the build-up of molecular weight, there is a big difference between chain growth polymerization reactions and stepwise polycondensation or polyaddition reactions. Starting from low molecular weight monomers, the polymerization yields

NETWORK FORMATION AND CHARACTERIZATION

67

F IG . 3.4. Viscosity changes from monomer to UV cured coating.

F IG . 3.5. Molecular weight built-up at low conversions for polymerization (chain process) and polyaddition/-condensation (step process).

already high molecular weight chains at rather low conversions, whereas the step growth polyaddition/-condensation does not builds up high molecular weights until very high conversions (Figure 3.5).

The classical polymerization process of monofunctional monomers normally leads to linear polymers. Some side reactions, like abstraction of hydrogen atoms from a polymer chain and subsequent growth from this reactive site can lead to branching or even forma-

68

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 3.6. Network formation during radical induced UV polymerization.

tion of some crosslinks by dimerization reactions. Intended crosslinking reactions are used widely in low crosslinked systems of natural rubber, elastomers or polyacrylic acids, like in tires or superabsorbent polymers, and in highly crosslinked coatings. However, to obtain high crosslink densities (XLD) in polycondensation or polyaddition type reactions the network formation is usually not done by the reaction of multifunctional monomers, but rather by reaction of preformed multifunctional resins, like in two-component polyurethanes by the use of polyacrylate resins containing hydroxyl functions, with multifunctional isocyanates. In this case, the network formation can already proceed at much lower conversions. This benefit, however, has to be traded-off by the higher viscosity of the resins, which must be dissolved in solvents.

The network build-up of radically initiated polymerizations of multifunctional monomers has been illustrated by Klosterboer22 (Figure 3.6). It is characterized by the fact that the gel points in radically polymerization reactions occur at a very early stage and therefore microgels are formed and with increasing conversion of reactive groups the number of microgels increases until at the final stage the microgels grow together and only few unreacted monomer units or dangling double bonds exist.

Depending on the chemistry used different types of polymer networks can be generated (Figure 3.7). By using the same type of (multi)functional groups, like (meth)acrylates, epoxides, polyenes or vinyl ethers, homopolymer networks are obtained. Applying copolymerizable different functional groups, for example, radically polymerizable acrylates and polyenes, acrylates and vinyl ethers or cationically polymerizable epoxides and vinyl ethers, copolymer networks result. If the networks are formed independently, like in the case where in a coating a radically polymerized acrylate network and a cationically polymerized network are generated together, interpenetrating networks are generated.

The structure of such interpenetrating networks is symbolized in Figure 3.8. The interpenetrating networks are characterized by the interpenetration of molecular chains, where no chemical links exist between the different networks. The networks cannot be separated unless chemical bonds are broken.

NETWORK FORMATION AND CHARACTERIZATION

69

F IG . 3.7. Network types formed with different chemistries.

F IG . 3.8. Display detail of an interpenetrating network structure.

The reaction possibilities of a chain radical and the formation of networks using multifunctional monomers is depicted in Figure 3.9. There are several possibilities of the addition of an initiator or a growing chain radical to the double bonds. If one chain is added and the thus formed radical abstracts a hydrogen atom, the molecular chain is terminated (however, the kinetic chain propagation can go on with the corresponding radical produced!). The remaining double bond, however may join in another chain building process (Xi,k ). This nomenclature according to Macosko and Miller indicates the functionality of the structural unit with (i) and the maximum functionality of the net point with (k) (e.g.,

70

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 3.9. Possibility of chains connected to difunctional monomers.

F IG . 3.10. Soluble fraction and fraction of crosslinked diacrylate (HDDA) with thre- and four-connected polymer chains.

HDDA = X1,4 to X4,4 ). If the second double bond reacts in the same way (addition and hydrogen transfer), only two sites are connected together resulting in linear growth, but no crosslinking. The same is the case, if a growing chain adds to one double bond and this radical starts another chain, but the remaining second double bond is trapped in the network and does not react. A real network formation is only achieved with the reactions X3,4 and X4,4 . With increasing conversion during a polymerization reaction, the amount of the structures X3,4 and X4,4 increases (Figure 3.10). Thus, a difunctional acrylate containing two double bonds functions as a tetrafunctional branching point in the ideal case.

The amount of the network formed can be determined by extraction methods. After exceeding the gel point, which is characterized by the transition of a hyperbranched into a crosslinked molecule, the coating consists of gel, which is insoluble in all solvents, and a soluble fraction. With increasing conversion the soluble fraction decreases. This has been calculated by Meichsner et al.6

NETWORK FORMATION AND CHARACTERIZATION

71

F IG . 3.11. Insight into a network structure.

Crosslinked polymers contain per chain at least two bridges to another chain. If the crosslink density is not too high, these networks can be swollen by a solvent to a so-called gel. The transition of a soluble polymer to a gel is often so sharp, that it is entitled as the “gelpoint”. Gel formation occurs if the functionality of the monomers exceeds two (a monoacrylate has a functionality of 2, a di-acrylate of 4). A closer insight into a network (Figure 3.11) reveals the existance of junction points to form a network structure, the network density determined by the molecular weight between X-links, but also inhomogeneities caused by dangling ends (radical termination by hydrogen abstraction), residual unsaturations and long living radicals.

Since the networks consist of “infinite” molecular weight molecules (Mw approaches infinity, whereas Mn stays finite), which are virtually unsoluble, the characterization of crosslinked polymers can not be done by molecular weight determinations, but rather by crosslink density and molecular weight between crosslinks. The nomenclature is often inconsistent. The crosslink density is expressed by ν (mol/g; ν 0) or by the degree of crosslinking (Xc ; 0 Xc 1). The degree of crosslinking can be related to the molecular weight between crosslinks Xc = 1/Mc , which is also defined by the number of moles of elastically effective network chains per cubic centimeter of film (also abbreviated as νe ).

Mc can be calculated by the following equation7 : Mc =

M f −2

(3.2.1)

where f is the functionality of the molecule.

The subtraction of 2 from the functionality is due to the fact that a functionality of 2 contributes only to linear molecular weight build-up, but not to branching or crosslinking.

If several components are used, the molecular weight M is calculated by the medium molecular weight M0 M0 =

n 1 M1 + n 2 M2 + · · · + n i Mi n1 + n2 + · · · + ni

(3.2.2)

72

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

and f0 =

n1 f1 + n2 f2 + · · · + ni fi n1 + n2 + · · · + ni

(3.2.3)

M0 f0 − 2

(3.2.4)

resulting in Mc =

and in the case the conversion of the functional groups (e.g., double bonds) is considered: Mc =

M0 pf0 − 2

(3.2.5)

where p is the fraction of converted groups (p 1).

To give some examples about the values of Mc and Xc , the crosslinking of an oligomer of the molecular weight 1000 g/mol, endcapped with two acrylic groups, in combination with the equivalent molecular weight of a monofunctional acrylate (10 mol of ethyl acrylate: 100 g/mol) can be calculated to:

Mc = (1 × 1000 + 10 × 100)/11 (1 × 4 + 10 × 2)/11 − 2 = 181/0.18 = 1010, thus resulting in a crosslink density of Xc = 1 × 10−3 .

If the same oligomer is reacted with hexanediol diacrylate instead of ethyl acrylate with approximately the same molecular weight (4 molecules of 226 g/mol)

Mc = (1 × 1000 + 4 × 226)/5 (1 × 4 + 4 × 4)/5 − 2 = 380/2 = 190, then the resulting (higher) crosslink density is Xc = 5.26 × 10−3 .

Some further values, all calculated for a (theoretical) double bond (DB) conversion of 100%, starting with a long-chain diacrylate with molecular weight of 1000 g/mol, over a trifunctional oligomer and the pure hexandiol diacrylate as a highly crosslinked system to the even higher crosslinked dendrimers are given in Table 3.1. It should be noted, that it will be difficult in practice to drive the high functional systems to complete conversion.

The calculation of the network quantities can be done according to the theories developed by Macosco and Miller.8 The prerequisites of this theory are: • Equal reactivity of all functional groups; • The reactivity of the groups being independent from each other; and • Cyclizations are not considered.

The network characterizing values can be experimentally determined by dynamical mechanical analysis, based on the theory of rubber elasticity.9 The crosslink density (Xc ) is: Xc = E /3ρRT ,

(3.2.6)

73

NETWORK FORMATION AND CHARACTERIZATION

TABLE 3.1. Theoretical calculation of crosslink density of different functional molecules Xc (×10−3 )

Molecule

Molecular weight

Functionality

Mc

Difunctional oligomer Trifunctional Oligomer Hexanediol diacrylate Dendrimer∗ of Generation 1 Generation 2 Generation 3

1000 750

4 (2 DB) 6 (3 DB)

500 188

2.0 5.3

226

4 (2 DB)

113

8.8

74 80 83

13.5 12.5 12.0

1040 2416 5168

16 (8 DB) 32 (16 DB) 64 (32 DB)

∗ Pentaerythritol as a core and esterification with dimethylolpropane carboxylic acid (DMPA).

where E is the E-modulus in the rubber elastic region, R is the gas constant, ρ is the density and T is the absolute temperature.

With this equation, the network densities can be calculated from the E-modulus data in the rubber elasticity region. However, it has been shown by Meichsner et al.6 that equation (3.2.6) holds not true for highly crosslinked networks. They found considerable deviations from the linear behavior and derived an exponential correlation between E-modulus and crosslink density (Xc ). This deviating behavior may be attributed to the short molecular weights between crosslinks, where the theory of rubber elasticity, which reflects the entropic elastic behavior of the chain segments between the X-links may not be valid anymore and instead energy elastic deformations may become dominant, which can be described by deformations of valence bonds or bond angles. They found the following correlation for modulus and X-link density: ln E = mXc + b, where m is the slope and b is the axis intercept; or expressed as: E = b emXc .

(3.2.7)

3.3 MONITORING FILM FORMATION AND CURING Numerous methods exist which are able to describe film formation and detect the chemical changes occurring during curing. Among the methods are infrared and Raman spectroscopy, differential scanning calorimetry, differential thermal analysis, measurement of dielectric constants or dielectric loss (see ref. 1).

Recently, two novel methods have been developed to monitor the film formation and curing, exemplified with UV curable coatings.10 Real-time dynamic mechanical analysis at low frequencies, as well as ultrasonic frequencies have been developed to monitor the physical changes occurring, like gelation and vitrification, and have been combined with near-infrared analysis to relate them to the chemical conversion of reactive bonds caused by the polymerization reaction.

74

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

Low-frequency dynamical mechanical analysis is able to monitor gelation, while the high-frequency ultrasonic reflection technique has been used to characterize the vitrification. In the investigated UV-cured coatings, it could be shown, that the cure of the coating is frozen due to vitrification at a high, but not complete conversion. The increase of the shear modulus during cure of almost two orders of magnitude (to about 1 GPa, which is typical for glassy materials) has been related to the cure conversion, which stops at about 80% conversion in the coating studied.

The ultrasonic method has also been used to study drying and film formation of waterborne coatings.11

3.4 NETWORK CHARACTERIZATION Networks elude most of the chemical characterization methods used for linear polymers, like molecular weight determination, end group analysis, etc., because of their insolubility.

The insolubility of coatings or of parts of the coatings occurs after a certain degree of crosslinking, defined by the gel point, has been exceeded. The gel point, which is defined as the conversion degree where the weight average of the molecular weight approaches infinity, can occur at very low conversions (below 10%), as has been shown before in the case of radically polymerizing acrylate systems. Thus, the characterization of networks is mainly restricted to the analysis of physical and mechanical properties and their response to the impact of chemicals, like solvents, or physical impacts, like particles, causing scratches or abrasions, on the coating surface. Some of the characteristic properties and responses to impacts are listed in Table 3.2. The characterization of coatings can be roughly classified TABLE 3.2. Characterization of coatings Physical

Chemical

Network characterization Viscoelastic properties Modulus Elongation Transition temperatures Glass transition Brittleâ&#x20AC;&#x201C;ductile transition Hardness and flexibility Crosslink density Elastically effective chain length Curing gradient

Network characterization Extractables

Response to impacts Abrasion resistance Adhesion Scratch resistance Stone chipping

Response to impacts Solvent resistance Swelling behavior Chemical resistance Household chemicals Acid rain and other automotive chemicals Exterior durability Photooxidation Hydrolysis

75

NETWORK FORMATION AND CHARACTERIZATION

F IG . 3.12. Network structures of low- and high-crosslink density and effect of stress application on network structure.

in the physical and the chemical characterization, and subdivided into the characterization of the network structure and in the response of the network to different impacts, be it the resistance to chemicals (solvent, water, household chemicals, like mustard, coffee, etc.), or to physical attack, like stone chipping or scratching.

Most of the properties and their test methods for coatings are described very well in the literature, for example, in “BASF Handbook on Basics of Coating Technology, Chapter 3.2”,12 or in “Organic Coatings, Science and Technology, Vol. 2, chapter 24”.13 Some of the test methods often used for the characterization of the networks are described briefly below.

3.4.1 Test Methods for Analyzing Physical Properties The physical properties of the coatings will be dependent on the network structures (Figure 3.12) obtained as a function of chemical composition and crosslink density.

3.4.1.1 Viscoelastic properties The viscoelastic properties of coatings are considerably dependent on temperature, rate of deformation and time or frequency, and often measured in stress-strain analysis (tensile or shear stress) as a function of temperature (Figure 3.13).14 The behavior of the material upon the applied stress in the initial, essentially straight line, is characterized by the proportionality constant, upon tensile stress by the elasticity (E )- and upon shear stress by the torsion (G )-modulus, σ = Eε, where σ is the tensile stress and ε is the elongation in relation to initial length ( l/ l); τ = Gγ , where τ is the shear stress and γ is the distortion.

(3.4.1)

76

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 3.13. Determination of mechanical properties (typical diagram for viscoelastic polymers).

The thus obtained moduli can be converted into each other by the following equation: G=

E , 2(1 + Îź)

where Îź is the Poisson ratio, the quotient of the relative transverse and longitudinal contraction, typical values are in the region of 0 to 0.5 (glassy polymers: 0.33).

Thus, the value of the elastic modulus is generally about two to three times the shear modulus.

At the end of the stress-strain curves in Figure 3.13, the sample breaks. This point defines the elongation-at break (percents of elongation) and the tensile-at-break (or tensile strength). The area under the curve represents the work-to-break. Viscoelastic materials are often characterized by the fact that at an intermediate strain less stress has to be applied to further elongate the sample. The maximum stress at that point is called the yield point.

Whereas ideal elastic deformation is (over a wide range) almost independent of temperature, viscous deformation is time (frequency) and temperature dependent, thus viscoelastic deformation is very dependent on the time and temperature over which the stress is kept. At fast rates of stress application or at low temperatures, elastic response dominates, however resulting in significantly lower elongations-at-break, whereas at low rates of applied stress and high temperatures the viscous response (plastic flow) will be proportionally greater.

Therefore, the same viscoelastic response curves can be obtained at low temperature or high stress rates or on the other hand at high temperatures or low stress rates. Thus, it is possible to calculate the data, for example, of high rate of stress application from low temperature measurements or vice versa with the method of time-temperature superpositioning.15 This relationship is relevant to all mechanical responses of a coating upon

NETWORK FORMATION AND CHARACTERIZATION

77

F IG . 3.14. Elastic (E ) and loss (E ) modulus as a function of temperature.

application of stresses as a function of temperature and time interval of applied stress. To illustrate this further lets consider an automotive top coat and its response to scratches.

During hand wash of a car at normal conditions (25 ◦ C) small gravel particles on the surface of a top coat will be pushed into the surface with low shear rates. The coating will respond mainly elastic and probably no scratches will emerge, whereas in automatic car wash units the brushes exert high impacts on the gravel particles. This high impact causes a brittle response of the coating and fracture type scratches will remain. Similar behaviour can be expected if stones are catapulted onto a coating surface. The coating may behave elastic and/or plastic in summer (high temperatures) with no visible damage, but brittle in winter (low temperatures) with significant scratches produced.

Stress-strain analysis can also be done dynamically. Figure 3.14 shows the typical diagrams obtained in dynamic mechanical analysis (DMA) measurements as a function of temperature. The dynamic mechanical analysis has the advantage over linear stress-strain curves (as shown in Figure 3.13) that the elastic and plastic components of the modulus can be separated, however, neither tensile-at-break nor elongation-at-break can be determined, since the coating must stay intact during the measurement. The stress is usually applied in an oscillating way and the phase angle difference between applied stress and resultant strain and amplitude of deformation is measured and calculated into the storage modulus (E = σ0 cos δ/ε0 ) and the loss modulus (E = σ0 sin δ/ε0 ). In the low temperature regions the network is in a glassy state, where only local molecular motions (vibrations or rotations) can take place which may give rise to so called β-transitions. With increasing temperature, the main transition region (also called α-transition) defined by the glass transition temperature is reached. Above the glass transition temperature the E -(storage) modulus, which measures the recoverable part of the energy imparted by the applied stress,

78

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 3.15. Temperature dependence of elasticity modulus.

drops considerably (note that the scale is logarithmic) by several orders of magnitude, thus indicating the transition of a glassy into a rubber-elastic state. The glass transition temperature is not as sharp as a melting point (it is a second order “kinetic” transition in contrast to a first order thermodynamic transition of Tm ), instead it appears as a transition region.

Hence, the value of Tg is dependent on the cooling rate of the sample as well as the fundamental chemical/molecular composition, the molecular weight, and junction point density.

The most important influencing factors on the glass transition temperature are given in terms of structure–property relationships in Chapter 6. The loss modulus, which is a measure of the viscous response, reflects that a part of the imparted stress is dissipated as heat in the form of viscous flow. The maximum of the tangent δ (= E /E ) is used to define the Tg . This value, however corresponds to a temperature, where the E-modulus already has dropped by more than one order of magnitude. Therefore, often instead of the tangent δ, , is used as the T . The maximum of the E curve the maximum of the loss modulus, Emax g corresponds more to the onset of the decay of the E -modulus and therefore to the beginning of the glass transition region. Thus, in coatings where a broad glass transition region appears, the differentiation if a coating is in the glassy or already in a soft state, is better

when using the Emax value16 as the indicator for the glass transition temperature (Tg ).

According to equation (3.2.6) the crosslink density can be calculated from the E-modulus in the rubber-elastic region. This can be shown by the E-modulus curves of Figure 3.15.

We have seen from Figure 3.13 that the stress strain analysis gives information about the modulus, the elongation at yield (where viscous flow starts) and the elongation at break.

Wu17 has studied the modulus and elongation data of linear polymers (e.g., polymethyl methacrylate) and superimposed the curves (Figure 3.16) as a function of temperature.

He has also shown that the modulus drops above the glass transition temperature. The elongation is very low at low temperatures and there is no yield point, that means that the

NETWORK FORMATION AND CHARACTERIZATION

79

F IG . 3.16. Temperature dependence of mechanical properties, elongation and modulus in a linear polymer (e.g., PMMA).

polymer breaks by brittle failure. At around 50 ◦ C (still below Tg ) greater elongations show up and a yield point appeared. At somewhat higher temperatures the elongation of break of this polymer starts to increase rapidly still at a temperature below Tg . Wu defined the temperature at the intercept of the elongation at break and the elongation at yield curves as the brittle–ductile transition temperature (Tb ). Below this temperature the polymer is brittle, between Tb and Tg it is hard and ductile and above Tg it is getting soft. Theses transitions have been determined for linear polymers, in Chapter 5 it will be discussed how Tb is influenced by crosslinking.

The homogeneity of polymer networks may also be evaluated by DMA measurements.

It has been shown for the polycondensation of polyol-melamine (1 K) thermal curing systems, that the E exhibits a relatively sharp drop at a baking temperature of 110 ◦ C, and the transition curve shifts parallel to higher temperatures up to a baking temperature of 130 ◦ C. Beginning at 130 ◦ C and higher baking temperature the onset and Tg remain constant, but the transition region broadens, which can be explained in this case with the beginning selfcondensation of the melamines, resulting in inhomogeneous network structures.18 Thus, the evaluation of the broadness of the glass transition region may also be used to determine the homogeneity of network structures.

A complete phase diagram of curing (phase transformations from liquid to glass) has been provided in the case of thermal curing systems with the TTT (transition, time, temperature)-diagram analysis (Gilham,19 Figure 3.17). In these (isothermal cure schedule) diagrams the vitrification line marks the time necessary for the Tg to coincide with the cure temperature. However, the same Tg can be obtained by isochronal cure schedules (not designed to be depicted in the TTT diagram). It has been found that samples cured isothermally or isochronically, despite having the same Tg , exhibited different shear strength. Thus, the choice of the cure schedule, in the case of radiation cured systems, the irradiance, exposure time, cure temperature, may greatly influence the ultimate mechanical properties.20 A time-temperature-energy (TTE) diagram for e-beam cured coatings has

80

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 3.17. What happens during cure? Temperature–time–transition (TTT) diagram: A map of thermal curing (from liquid over rubber to glass).

been proposed.21 For UV cured coatings the influence of the irradiance on the Tg has been shown by DMA analysis of the UV induced homopolymerization of hexanediol diacrylate (HDDA). The Tg (tangent delta) increases from −23 to 62 ◦ C as a result of increasing irradiance.22 Furthermore, the conversion limitation due to vitrification of room temperature UV cured systems will be discussed in Chapter 6 (see Figure 6.3). By increasing the cure temperature an increase in conversion and Tg can also be obtained. To get a full phase diagram about UV curable systems an irradiance(time)-temperature diagram should be generated, since conversion limitations of room temperature cured UV systems due to vitrification can be overcome by increasing irradiance (leads to temperature increase due to polymerization heat) or increasing cure temperature.

3.4.1.2 Hardness and flexibility What is hardness? Hardness can be defined as the resistance of a material to localized deformations, caused, for example, from indentation, scratching, cutting or bending. In metals, ceramics and most thermoplastics and coatings, the deformation considered is plastic or fracture deformation of the surface. For elastomers and some polymers, hardness is defined as the resistance to elastic deformation of the surface. The lack of a fundamental definition indicates that hardness is not a basic property of a material, but rather a composite one with contributions from the yield strength, true tensile strength, modulus, and others factors. In practice, hardness is measured in terms of the size of an impression made on a specimen by an indenter of a specified shape when a specified force is applied for a specified time, the indent being measured after the force has been removed. There are three principal standard methods for expressing the relationship between hardness and the size of the impression, these being Brinell, Rockwell, and Vickers. For practical and calibration reasons, each of these methods is divided into a range

NETWORK FORMATION AND CHARACTERIZATION

81

of scales, defined by a combination of applied load and indenter morphology, to cover the range of hardness. Recently, with the introduction of instrumented indentation hardness, it has become possible to measure the indent under the applied force.

In the coating industry very common for measuring the hardness of coatings are pendulum, pencil, and indentation hardness.

The pendulum method evaluates hardness by measuring the damping time of an oscillating pendulum. These tests are performed according to ASTM D 4366 and DIN EN ISO 1522 or DIN 53157 (König). The pendulum rests with two steel balls on the coating surface. When it is kicked off, the viscoelastic behavior of the coating determines the oscillation time. Hard coatings exhibit long oscillation times. The pendulum hardness tester may be equipped with “König” or “Persoz” pendulum: König: 200 g, ball weight, 5 mm ball diameter; deflection start at 6 ◦ and end at 3 ◦ with a period of oscillations of 1.4 s (reference: damping on glass 250 s); Persoz: 500 g ball weight, 8 mm ball diameter; deflection start at 12◦ and end at 4 ◦ with a period of oscillations of 1 s (reference: damping on glass 430 s).

In the literature values are mostly expressed as König or Persoz hardness in seconds, however, sometimes also as number of oscillations, which may be recalculated into seconds (one oscillation with König corresponds to 1.4 s). The rapid and inexpensive Pencil hardness (ASTM D 3363) test relies on the comparison of film hardness with the hardness of calibrated drawing leads (hardness scale from soft to hard: 6B-5B-4B-3B-2B-B-HB-FH-2H-3H-4H-5H-6H). A coating is placed on a horizontal surface and the pencil is held against the film at an angle of about 45 ◦ and pushed away from the operator in a stroke (about 7 mm). The pencil hardness given is the hardness of the pencil which does not cut into the film.

The indentation hardness (ASTM D 1474) is the quotient of the applied load and the surface area of the impression that is present underneath at a given load. The indentation can be done with a point or ball or in the case of nano-indentation with tips of different shapes (Fischerscope, Berkovich, Cube corner, see Figures 3.19 and 3.21). Instrumented indentation hardness23 provides the ability to measure the indenter penetration h under the applied force F throughout the testing cycle and is therefore capable of measuring both the plastic and elastic deformation of the material under test (see, for example, Figures 3.21 and 6.16).

Flexibility is tested in a mandrel bend test (ASTM D 522), where the coated panels are bent over a mandrel and the resistance to cracking is determined or in a test where a ball type mandrel is forwarded into the backside of a coated metal plate and the penetration depth without cracking of the coating is expressed in mm (Erichsen cupping; ISO 1520; DIN 53156).

3.4.1.3 Crosslink density The crosslink density can be experimentally determined by measuring the modulus in the rubber elastic state as described before (3.2.6) by Zosel.9 Furthermore, the crosslink density can be determined by swelling experiments. The swelling can be measured by gravimetric methods (weight before and after immersed in a (good) solvent for a given period), or by measuring the change in height as the sample swells.

82

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 3.18. Laboratory scratch testing apparatuses.

According to Floryâ&#x20AC;&#x2122;s theory the crosslink density can be calculated from the volume ratio of the swollen and unswollen network.24 3.4.1.4 Test methods for abrasion resistance The most common test method for determination of abrasion resistance is the Taber Abraser method (Figure 3.18). It consists of measuring the weight loss occurring when the coated (wooden) substrate is subjected to rotating abrasive wheels with defined load. This method is chosen in order to simulate the abrasion occurring on wooden flooring during walking. The tests are standardized by many methods (e.g., DIN EN 438, ASTM D 4060-90, DIN 53754, DIN 53799). The most common used wheels are Calibrase resilient wheels composed of rubber and aluminum oxide abrasive particles, whereof the CS-10 is relatively soft and causes mild abrasion, whereas CS-17 is harder and causes harsh abrasion (an overview about the wheels available is given in ref. 25). Another method for differentiating very abrasion resistant coatings is the use of sand paper. Paper of the size no. 180 is extremely abrasive and can also be employed to assess the coating sandability.

The resistance to abrasion can be expressed in several ways, the most common are the weight loss (in mg) per number of cycles or the number of cycles (per unit of film thickness) needed to wear through. Since the abrasive paper gets clogged easily, the DIN EN 438 requires the changing of the paper after 500 cycles or after getting clogged. Based on this, the reproducibility is rather low and therefore in Scandinavia the method known as falling sand has been developed (SIS 923509). In this test a fixed amount of corund particles of

NETWORK FORMATION AND CHARACTERIZATION

83

specific size fall into the path of a leather covered wheel that rotates in contact with the coated substrate to be examined. The abrasion resistance is given in weight loss per cycles.

The falling sand test is a simple method of testing abrasion. Standardized sand is released and guided through a tube onto the test specimen. The volume of sand required to obtain total erosion of a known coating thickness indicates the abrasion resistance (ASTM D 968, DIN 1164).

Since the mechanism involved in abrasion and the rates of stress application vary so widely one single test will be comprehensive to predict the wear life of different coatings for different applications. Therefore, for every application the best simulation test has to be chosen and the mechanical properties of the coatings to be optimized to perform well in the application and the simulation test.

3.4.1.5 Test methods for scratch resistance A multitude of test methods exist to simulate the scratching conditions to which a coating may be subjected to, for example, the scratches occurring during car wash or on parquet flooring due to high heel shoes.

Among them are the multi scratch tests, like the laboratory car wash simulation unit of Amtec–Kistler26,27 or Elcometer 1730, modified Crockmeter28,29 tests, Rota–Hub30 or ScotchBrite® scratching tests (Figure 3.18). The abrasive tests, like Taber test,31,32 or falling sand tests, are rarely used in automotive clear coat testing, but in clear coats for parquet, detecting the abrasive materials loss. The single scratch tests for indentation only (Fischerscope), scratch (CSM-Nanoscratch) or combined indenter and scratch (Hysitron Triboscope) method are used to characterize the material properties. Furthermore many other more or less standardized tests exist. Most of these tests have a drawback in common: they typically damage the surface more severely than typical real life cycles in order to generate optical visible and distinguishable damage in short time scales. The extent of the damages is dependent on the forces, velocities, and geometries applied as well as on the test conditions, like humidity and temperature. In all tests, some sort of abrasive bodies (sandpaper, particles, brushes) are moved with specified normal force several times across the sample surface.

Classification of scratch resistance is then done by visual inspection, optical gloss (Figure 3.19), color change (greying) or abrasive material loss detection. For automotive coatings, nearly every automotive company uses different tests or test parameters which sense different but not completely clarified aspects of scratch resistance. Since it simulates the car wash reality very closely, the laboratory car wash unit AMTEC-Kistler is used very often as a standard test method.33 In the AMTEC test, usually two types of coatings exhibit good scratch resistance, highly crosslinked and rubber-type materials, which is due to the high restoring forces caused by the “rubber-like” elastic behavior. Since the rubber-type materials, however, are susceptible to swelling when contacted with typical “automotive” chemicals, which may interact with the coating surface, like gasoline, they exhibit poor chemical resistance. In order to distinguish between hard and the rubber-type coatings, often a second scratch test method reflecting the hardness of the surface, like the Crockmeter test or methods which reflect the crosslink density, like dynamical mechanical analysis (DMA), are applied in addition to the AMTEC test. The DMA method, which evaluates the viscoelastic properties of the material has also been used to correlate crosslink density with scratch resistance.34

84

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 3.19. Principle of gloss and haze measurement.

In order to simulate the scratching and simultaneously measure the mechanical response parameters of coatings occurring during indentation or scratching, a variety of single contact measurement systems has been developed. A review of different test methods for generating scratches and evaluating the scratch resistance of the coating has been published recently.35 A widely used instrument to evaluate hardness and mechanical parameters is the Fischerscope, which measures the surface mechanics of coatings by vertical indentation on the surface with a diamond tip under defined forces, whereas the nano-scratcher (e.g., CSM Nano-Scratch Tester) evaluates the slip properties by lateral scratching with defined tip geometries and forces. The newest generation of scratch test equipment is able to perform both, indentation vertical to the surface, lateral scratch set-ups as well as imaging of the resulting damage.36 Kutschera et al.37 for instance describe the single contact testing using a Hysitron TriboIndenter® system mounted on a Digital Instruments Nanoscope Dimension 3100 AFM, a Berkovich type indenter tip with sharp edges (pyramidal diamond 142.3◦ opening angle) to perform the indentation of different coating surfaces with load, hold and unload cycles (Figures 3.20 and 3.21). This method allows the determination of the mechanical response as a function of elastic, plastic and fracture response ratio. The correlation of the detailed mechanical responses with scratch data can therefore help to optimize the coating properties in order to develop scratch resistant systems (see chapter 10 for scratch resistant UV cured automotive top coats).

3.4.1.6 Internal stress and shrinkage38 ent reasons:

The build-up of internal stress can have differ-

• Temperature stress, caused by different thermal expansion coefficients of coating and substrate, resulting in a different volume contraction; • Polymerization shrinkage in a gel or glassy state, resulting in a stress due to insufficient relaxation; • Volume expansion or contraction due to humidity or solvent/water evaporation in a gelled or glassy state.

NETWORK FORMATION AND CHARACTERIZATION

85

F IG . 3.20. AFM tool with nanoindenter set-up.

F IG . 3.21. Sketch of the nanoindentation measurement principle (top) and analysis scheme (elastic, plastic, fracture according to Shen et al.).

86

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 3.22. Shrinkage as a function of functionality per volume.

Although UV curing proceeds normally at room temperature, the heating of the coating may be substantial, since the curing reaction proceeds very fast and therefore considerable heat is released which can heat up the coating by more than 50 ◦ C. Thus, the Tg of the final coating will rise above room temperature and if the expansion coefficients of the coating and the substrate are different, during cooling, and even more if the adhesion of the coating to the substrate is high, internal stress will develop.

The thermal stress can be described by the following equation39 :

σ =

E (αf − αs ) dT ,

from T1 to T2 ,

(3.4.2)

where αf and αs are the expansion coefficients of the film and substrate, respectively.

In radically polymerizable acrylate coatings the acrylate double bonds are converted during cure to covalent bound single bonds. Due to this coupling reaction, the total volume changes from the van der Waals distance of the acrylates to the shorter covalent bonds formed, thus, resulting in a shrinkage due to volume contraction. According to the literature, there is a correlation between the internal stress and the gap of the temperature of measurement to the glass transition temperature of the coating.40 As shown in Figure 3.22, the cure of highly crosslinked UV materials can result in shrinkages of up to 35%.

Although shrinkage occurs during the whole curing process, internal stress will only develop after gelation has occurred. However, since the gel point of radically cured UV systems occurs at a very early stage, the shrinkage of the material can result in large internal stresses during the cure after gelation. These developed internal stresses can lead to deformation or cupping, as well as to delamination or adhesion failure.

NETWORK FORMATION AND CHARACTERIZATION

87

F IG . 3.23. Determination of internal stress by the cantilever-beam method.

The internal stress can be measured by optical41 or mechanical methods, whereof the cantilever beam method42 has been used for the shrinkage and stress determination in model UV-curable formulations.43 The latter method and the equations for the determination of the internal stress σi are shown in Figure 3.23. The internal stress developed in the model UV coating based on the difunctional hexanediol diacrylate (HDDA) and the monofunctional trimethylolpropan-monoformal-acrylate (TMPFMA) as a function of the HDDA content and the temperature is shown in Figure 3.24. In this example it is shown that considerable stress is built-up due to shrinkage occurring during polymerization. The internal stress decreases with increasing temperature, since relaxations are occurring. The internal stress increased exponential with increasing glass transition temperature of the coatings (Figure 3.25), corresponding to a higher content of the difunctional resin (HDDA), similar to the curves reported by Zosel.44 The glass transition temperatures plotted are based on Tg measured with DSC (differential scanning calorimetry) and with DMA (dynamic mechanical analysis), using the maximum of the E loss modulus.

In Figure 3.26, the internal stress of the same HDDA/TMPFMA (0% HDDA to 100% HDDA) coatings is plotted as a function of the shrinkage. From this figure it is obvious that the coating with no HDDA crosslinker develops no stress despite a shrinkage of about 10% (obtained by the density method: shrinkage = 100% × (density cured film − density liquid)/density cured film). The glass transition temperature of this linear homopolymer is below 20 ◦ C, thus below the measurement temperature. A considerable internal stress develops not until shrinkage occurs at Tg ’s above room temperature. Figure 3.27 shows the internal stress as a function of humidity and temperature.

Firstly, the internal stress is higher the lower the temperature is, secondly, it looks

88

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 3.24. Internal stress as a function of temperature in coatings based on HDDA and TMPFMA.

F IG . 3.25. Internal stress as a function of Tg in coatings based on HDDA (%) and TMPFMA (Tg by Emax and DSC).

like water penetrating into the film has a softening effect and hence reduces internal stress.

A crosslinked polymer that, upon exposure to light, exhibits stress and strain relaxation without changes in materials properties has been reported recently by the Bow-

NETWORK FORMATION AND CHARACTERIZATION

89

F IG . 3.26. Internal stress as a function of shrinkage in coatings based on HDDA (%) and TMPFMA (shrinkage by cantilever beam method).

F IG . 3.27. Internal stress as a function of humidity in coatings based on HDDA (60%) and TMPFMA (40%).

man group.45 The approach is based on addition-fragmentation chain transfer reactions, similar to the RAFT mechanism in controlled radical polymerization,46 with allyl sulfides as addition-fragmentation agents. The crosslinked polymer is build-up with the thiol-ENE chemistry containing a tetrafunctional thiol (pentaerythritol tetra-

90

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

(3-mercapto-propionate), triethylenglycol divinylether, the ring opening monomer 2methyl-7-methylene-1,5-dithiacyclooctane (MDTO47 ) and 1,6-hexanedithiol. The MDTO was introduced into the backbone as the reversible cleavage allyl sulfide functionality.

The stress relaxation during exposure has been shown. This effect is not due to photodegradation, since the moduli of the samples are the same before and after irradiation.

3.4.1.7 Measurement of the curing and hardness gradient within a coating The radiation attenuation within the coating layer (according to Beer’s law) causes a gradient of energy density throughout the coating layer resulting in a decrease of conversion of functional groups from the surface to the bottom layer. Such a gradient can be measured by Confocal Raman Microscopy (see Chapter 2, Figure 3.13).48 As a result of this gradient the cure conversion and thus, the crosslink density decreases from the surface to the bottom, if negative effects of oxygen inhibition at the surface are eliminated (e.g., by inert curing). Added pigments or UV absorbers can enhance such a gradient formation. With decreasing X-link density also the hardness should decrease. Since the hardness gradient can not be followed directly by Confocal Raman microscopy and furthermore added pigments may cause complications in the Raman determination, Meichsner and coworkers evaluated the hardness gradient by micro- (Fischerscope® H100) and nano-indentation (Hysitron two-dimensional triboscope® ) methods.49 The micro-indentation hardness can only be measured at film thicknesses above 40 µm, because at lower thicknesses the hardness measurement is influenced by the hardness of the substrate. Thus the gradient was determined by evaluating the difference of the hardness of a normally exposed sample (under inert conditions) to a sample, where the exposure was done from the backside through quartz glass (transparent to UV light). This exposure setup should introduce a gradient from the quartz interface to the surface, with higher X-link density at the interface and decreasing hardness in the direction of the surface. The measurement of the hardness here was also done at the surface, but the surface layer in this setup corresponds to the bottom layer of a normal exposure. This setup showed that the hardness of the films with thicknesses higher than 40 µm exhibited a gradient in hardness. Whereas the hardness stayed constant in the case of the normally exposed sample, as expected, the hardness decreased with increasing film thickness in the quartz setup. At a thickness of 105 µm the hardness corresponding to the bottom layer was about two-third and at 160 µm about half of the hardness of the surface. With nano-indentation the hardness can be measured at very thin films and this allows the measurement of a gradient directly of a cross-section of the exposed film layer. The hardness and elasticity modulus measurements corroborated that a hardness gradient exists. In both cases the absolute values decrease to about 40% at the bottom of a 200 µm thick film.

3.4.2

Chemical characterization

The chemical characterization of networks is pretty difficult since they are not soluble.

Thus, test methods evaluate the extractables or the swelling behaviour. The choice of the

NETWORK FORMATION AND CHARACTERIZATION

91

F IG . 3.28. Chemical resistance test methods.

extraction or swelling solvent is usually made by durability needs of the coating. Extraction tests are done in order to characterize the sol fraction of the network. They are often done with good solvents for the employed raw materials (e.g., methylene chloride, tetrahydrofurane, N-methyl-pyrollidone). The extractable can then be characterized by standard analytical methods (NMR, IR, SEC, . . .). Standard tests with a specific solvent, like methylethyl-ketone (MEK) are used in double rub tests for the characterization of the network quality. The test is done with a tissue soaked with MEK, rubbing over the coating (one double rub is a stroke forth and back). An uncrosslinked thermoplastic rubs away within few double rubs, whereas with increasing crosslinking the number of rubs increases. Highly crosslinked coatings withstand 200 double rubs (where the test is usually stopped) and more.

3.4.2.1 Chemical resistance The chemical resistance of the coating surface is usually tested according to occurring or anticipated stresses. Automotive coatings, for example, are subjected to a variety of chemicals, like acid rain, bird excrements, tree resins, gasoline and so on, which may act upon the surface at various climate conditions, for example, at temperatures as high as 80 â&#x2014;Ś C. These chemicals should not leave any marks or damage at the coating surface. Therefore, the chemical resistance test for automotive applications determines the highest temperature where no visible marks of applied droplets after treatment are occurring (with the gradient oven method). Applied chemicals are sulfuric acid (simulation of acid rain (1 or 10% concentration)), pancreatine (as a model for bird excrements), tree resin, caustic soda, water, and gasoline.

For wood coatings typical household chemicals, like coffee, mustard, red wine or ballpen ink, are applied and tested at room temperature (Figure 3.28).

92

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

REFERENCES 1. Goldschmidt, A. and Streitberger, H.-J., “BASF Handbook on Basics of Coating Technology”. Vincentz Network, Hannover, 2003, Chapter 3, p. 323.

2. (a) Myers, T.G., Surface tension driven thin film flows, In “The first European Symposium on The Mechanics of Thin Film Coatings” (P.H. Gaskell, M.D. Savage and J.L. Summers, eds.). World Scientific, 1996, pp. 259– 268; (b) Orchard, S.E., Appl. Sci. Res. A 11, 451–464 (1962); (c) Smith, N.P.D., Orchard, S.E. and RhindTutt, A.J., J. Oil Col. Chem. Assoc. 44, 618 (1961); (d) Overdiep, W.S., Prog. Org. Coat. 14, 159–175 (1986); (e) Wilson, S.K., IMA J. Appl. Math. 50 (2), 149–166 (1993).

3. Mercurio, A. and Lewis, S., J. Paint Technol. 47 (607), 37 (1975).

4. Wicks, Jr., Z.W., Jones, F.N. and Pappas, S.P. (Eds.), “Organic Coatings, Science and Technology”, Vol. II. John Wiley & Sons, New York, 1994, pp. 15–21.

5. Stepto, R.F.T. (Ed.), “Polymer Networks, Principles of their Formation, Structure and Properties”. Blackie Academic&Professional, London, 1998.

6. Meichsner, G., Deuter, F., Groß, Th., Beck, E. and Menzel, K., Farbe&Lack 103 (8), 45–50 (1997).

7. Berger, J. and Huntgens, F., Angew. Makromol. Chem. 76/77, 109 (1979).

8. (a) Macosko, C.W. and Miller, D.R., Macromolecules 9, 199 (1976); (b) Miller, D.R. and Macosko, C.W., Macromolecules 9, 206 (1976).

9. Zosel, A., In “Lack und Polymerfilme” (U. Zorl, ed.). Vincentz Verlag, Hannover, 1996.

10. Steeman, P.A.M., Dias, A.A., Wienke, D., Alig, I. and Lellinger, D., Recent developments in monitoring film-formation and cure of coatings, In “Coatings Science International (CoSi 2005), Book of Abstracts”.

2005, pp. 24–29.

11. Alig, I. and Lellinger, D., Chem. Innov. 2, 12–18 (2000).

12. Goldschmidt, A. and Streitberger, H.-J., “BASF Handbook on Basics of Coating Technology”. Vincentz Network, Hannover, 2003.

13. Wicks, Jr., Z.W., Jones, F.N. and Pappas, S.P., “Organic Coatings, Science and Technology”, Vol. 2. Wiley, 1994, p. 105.

14. Schnecko, H., Makromol. Chem. 76/77, 1 (1979).

15. Hill, L.W., “Mechanical Properties of Coatings”. Federation of Societies for Coating Technology, Blue Bell, PA, 1987.

16. Frey, Th., Große-Brinkhaus, K.-H. and Röckrath, U., Progr. Org. Coat. 27, 59–66 (1996).

17. Wu, S.J., J. Appl. Polym. Sci. 20, 327 (1976).

18. (a) ref. 16; (b) Hill, L.W. and Kozlowski, K., J. Coat. Technol. 59 (571), 63 (1987).

19. (a) Enns, J.B. and Gillham, J.K., J. Appl. Polym. Sci. 28, 2831 (1983); (b) Wisanrakkit, G., Gillham, J.K. and Enns, J.B., J. Appl. Polym. Sci. 41, 1895 (1990).

20. Zumbrunn, M.A., Wilkes, G.L. and Ward, Th.C., In “Radiation Curing in Polymer Science and Technology, Vol. III, Polymerization Mechanism” (J.P. Fouassier and J.F. Rabek, eds.). Elsevier Applied Science, 1993, Chapter 4, pp. 101–151.

21. Kim, H.-C. and Wilkes, G.L., J. Radiat. Cur. 16 (1), 8 (1989).

22. Klosterboer, J.G., Adv. Polym. Sci. 84, 1 (1988).

23. http://www.npl.co.uk/force/guidance/hardness/references.html.

24. http://campoly.com/notes/005.pdf (Cambridge Polymer Group, Boston, MA).

25. http://www.labomat.com/english/Taber.html.

26. http://www.amteckistler.de.

27. Meier-Westhues, U., Klimmasch, Th. and Tillack, J., Eur. Coat. J. 9, 258 (2002).

28. (a) American Association of Textile Chemists and Colorists, AATCC 8, 116 & 165; (b) ASTM D6279–98 (Method for rub abrasion mar resistance of high gloss coatings).

29. Trumbo, D.L., Rudelich, J.C. and Mote, B.E., In “Perspectives on New Crops and New Uses” (J. Janick, ed.). ASHS Press, Alexandria, VA, 1999, pp. 267–271.

30. Used, for example, in “UV curable clearcoats” by Jung, T. and Valet, A., RadTech Report, November/December, 2001, pp. 30–35.

31. Gregorovich, B. and Hazan, I., Progr. Org. Coat. 24, 131 (1994).

32. ASTM Committee in Annual Book of ASTM Standards, American Society for Testing and Materials, West Conshohocken, 1998, p. 6.

NETWORK FORMATION AND CHARACTERIZATION

93

33. ISO/FDIS 20566:200, Paints and varnishes-determination of scratch resistance of a coating system using laboratory car wash (also German Standard DIN 55665).

34. AIF project 12567: http://www.fpl.uni-stuttgart.de/Aktuelles/AiF12567.pdf.

35. Courter, J.L. and Kamenetzky, E.A., Eur. Coat. J. 7, 100 (1999).

36. (a) Schwalm, R., Beck, E. and Pfau, A., Eur. Coat. J. 1, 107 (2003); (b) Meichsner, G., Burk, T., Zhang, H., Larbig, H., Sander, R. and Kutschera, M., Farbe&Lack 111 (7), 27–31 (2005).

37. Kutschera, M., Sander, R., Herrmann, P., Weckenmann, U. and Poppe, A., http://www.coatings-science. chem.tue.nl/2005/speakers/Poppe.htm.

38. Meichsner, G., Mezger, T. and Schröder, J., “Lackeigenschaften Messen und Steuern – Fließeigenschaften, Grenzflächen, Kolloide”. Vincentz Verlag, Hannover, 2003.

39. Perera, D. and van den Eynde, D., J. Coat. Technol. 53 (677), 39 (1981).

40. (a) Perera, D. and van der Eynde, D., Progr. Org. Coat. 28, 21 (1996); (b) Pang, Z. and Ling, X., Beijing Huiagong Daxue Xuebao, Ziran Kexueban 22, 22 (1995).

41. Askadskii, A., “Physical Properties of Polymers”. Overseas Publishers Association, Amsterdam, 1996.

42. Corcoran, E., J. Paint Technol. 41 (538), 635 (1969).

43. Maday, M., Diploma Thesis, FH Esslingen, 1998.

44. Zosel, A., In “Lackeigenschaften Messen und Steuern – Fließeigenschaften, Grenzflächen, Kolloide” (G. Meichsner, T. Mezger and J. Schröder, eds.). Vincentz Verlag, Hannover, 2003.

45. Scott, T.F., Schneider, A.D., Cook, W.D. and Bowman, C.N., “Photoinduced Plasticity in Crosslinked Polymers”, Vol. 308. Science, June 2005, pp. 1615–1617.

46. Meijs, G.F., Rizzardo, E. and Tang, S.H., Macromolecules 21, 3122 (1988).

47. Evans, R.A. and Rizzardo, E., J. Polym. Sci. A 39, 202 (2001).

48. Schrof, W., Farbe&Lack 103 (7), 22 (1997).

49. Meichsner, G., Burk, T., Zhang, H., Larbig, H., Sander, R. and Kutschera, M., Monitoring the cure gradient of UV-cured coatings by microindentation hardness and atomic force microscopy, 8th Nürnberg Congress, Creative Advances in Coating Technology, Congress Proceedings, Vol. 2. Vincentz Network, Hannover, 2005, pp. 173–180.

C HAPTER 4

Raw Materials The raw materials described in this section are classified into the categories of radical polymerization systems, cationic polymerization systems and radical polyaddition systems (Thiol-ENE).

The raw materials used for the formulation of UV coatings consist of low molecular weight resins, typically in the molecular weight range of 300–5000 g/mol. The main resins types are radically polymerizable unsaturated polyesters, acrylate terminated molecules, like polyepoxides, polyesters, polyethers and polyurethanes as well as epoxides and vinylethers. Epoxide terminated oligomers and vinyl ethers are cured cationically. An overview of the main resin types used in UV curable systems is given in Figure 4.1. By far the most applied resins are the radical polymerization type acrylates and unsaturated polyesters, the thiol-ene systems are often discussed because of their insensitivity against oxygen inhibition, but yet only used in few niche applications.1 The cationic polymerization

Radical Acrylates Epoxy acrylates Polyester acrylates Urethane acrylates Polyether acrylates Acrylated polyacrylates Acrylated oils

Cationic Epoxides Bisphenol Adiglycidyl-ether 3,4-epoxycyclohexylmethyl-3 ,4 epoxycyclohexane carboxylate (ECC)

Photoinitiator free Donor–acceptor systems Maleimide-acceptor N-ethyl-MI N-phenyl-MI Vinyl ether-donor Triethylenglycol di-vinylether Allyl ether-donor

Unsaturated Polyesters

Vinyl ethers VE oligomers (RadTech’98 US, Conf. Proc. p. 53)

Self initiating β-keto-Michael addition oligomers

Poly-ENE-Thiol systems Isoprene–styreneblock copolymers F IG . 4.1. Resin types for UV curing systems.

94

RAW MATERIALS

95

systems are also used only in a few metal coating applications, whereas the photoinitiatorfree systems are just in the developmental stage.

Since most of these resins are often too high in viscosity, they are diluted with reactive diluents to adjust the application viscosity. Mono- or multifunctional acrylates are mainly used as reactive diluents, whereas methacrylates, or non-acrylate monomers, like styrene, vinyl pyrrolidone, divinyl ethers and few others are less frequently applied. Vinyl ethers and monoepoxides are used as reactive diluents in cationically curable coatings.

The photoinitiators are responsible for the effectiveness of the curing reaction. The vast majority of used photoinitiators are forming radicals, which add to unsaturated double bonds, whereas the cationic photoinitiators upon exposure form a Broensted (H+ ) or Lewis acid, which initiates the cationic polymerization of epoxides or vinyl ethers. Photoinitiatorfree systems rely on the direct formation of radicals upon exposure, for example, with electron beams, where Ď&#x192; -bonds are cleaved and radicals formed. The donorâ&#x20AC;&#x201C;acceptor systems form radicals from the excited complex by rearrangements. Such systems therefore do not need an external photoinitiator.

Besides these essential raw materials of a UV curable formulation, additives, like surfactants, defoamers, leveling agents, flow regulators, flexibilizers, pigments (mainly in printing inks), UV stabilizers (UV absorbers and HALS type radical scavengers), or fillers (clay, calcium carbonate, silica) as well as nanoparticles, which are transparent and may provide higher scratch resistance, are used according to requirements.

4.1 RADICAL POLYMERIZATION SYSTEMS 4.1.1 Standard Resins 4.1.1.1 Unsaturated polyesters The unsaturated polyesters were the first class of resins used in UV curing, now their overwhelming use is in fibre reinforced composites and only a small fraction is used in UV curable laminating adhesives and wood fillers. Such unsaturated polyesters (UPE), derived by the condensation reaction of maleic or fumaric acid with various diols, dissolved in styrene, were the earliest used UV curable resins. Because of the toxicity of styrene, these systems are not used extensively any more. Multifunctional acrylates, like TPGDA or TMPTA, have been used instead of styrene as a reactive diluent in UPE resins for adhesives and ink applications. Recently, powder resins based on unsaturated polyesters have been introduced, obtained by mixtures of UPE with vinyl ether polyurethane crosslinkers2 or mixtures of UPE with allyl ether polyesters.3 4.1.1.2 Standard acrylate terminated oligomers The acrylate resins now dominate the market. The schematic structure of the main acrylate terminated resin classes is shown in Figure 4.2.

From the resin classes the epoxy acrylates are the biggest on the market, prepared by the reaction of epoxides, e.g., Bisphenol-A diglycidylether, with acrylic acid (Figure 4.3).

These Bisphenol-A type acrylated epoxides are the dominant products on the market and account for about 70% of all epoxy acrylates used. Phenol-formaldehyde resin based (Novolac)-glycidylethers acrylates are used as a speciality in solder film resists, because

96

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 4.2. Schematic chemical structure of main acrylate resin types for acrylate based radically curing UV coatings.

F IG . 4.3. Synthetic scheme of aromatic epoxy and urethane acrylates.

RAW MATERIALS

97

TABLE 4.1. Synthesis of an epoxy acrylate4 Using a vessel equipped with an agitator, a reflux condenser, an inert gas inlet and a charging port, there was charged 660 g Epon 828. Epon 828 is a diglycidyl ether of Bisphenol-A, having an epoxy equivalent weight of about 185–192 g and an epoxide value of 0.50–0.54. Through the gas inlet was introduced a nitrogen flow which was maintained throughout the resin preparation cycle. While under agitation, 238 g of glacial acrylic acid was added and stirred until epoxy resin dissolved. Upon dissolution of the acrylic acid, 1.3 g of triethyl amine and 0.1 g of hydroquinone were added. Under continued agitation, heat was raised to 95–100 ◦ C and held at this temperature until an acid number of 5–10 was obtained (∼10 h). Temperature was reduced to 65–70 ◦ C and 600 g hydroxypropyl acrylate was added and the mixture was stirred until uniform. The final product was a clear solution of prepolymer in monomer at a 60/40-weight ratio.

of their high temperature resistance. The epoxy precursors are to a lesser extent modified with fatty acids or amines. Similar products to the fatty acid modified epoxies can also be obtained by the reaction of epoxidized soybean oil with acrylic acid. Due to the high viscosity of especially the Bisphenol-A type epoxy acrylates, they are delivered usually diluted with reactive diluents, like hydroxypropyl acrylate, dipropylene glycol diacrylate, tripropylene glycol diacrylate or hexanediol diacrylate. Besides these aromatic epoxide precursors, more expensive aliphatic epoxy resins play only a minor role in the market.

The epoxy acrylates are distinguished by a high reactivity and the cured coatings exhibit good chemical stability. Main uses are paper coatings and inks as well as wood coatings.

An example for the synthesis of such a resin is given in Table 4.1.

Urethane acrylates are simple addition products of multifunctional isocyanates, like toluene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate or their condensation products, e.g., isocyanurates, biurets, allophanates, with polyols and hydroxyalkyl acrylates, for instance, hydroxyethyl acrylate, hydroxybutyl acrylate or pentaerythritol triacrylate. Since the addition reaction proceeds very well, the coatings or ink formulating companies produce a large portion of the urethane acrylates captively. However, also a large variety of different urethane acrylate resins are available by the raw materials suppliers. The applications are mainly on plastics, with the dominant application on PVC floor coverings, wooden parquet, screen inks and optical fibres. These applications require good optical properties and non-yellowing behaviour, thus more than 80% of the used urethane acrylates are based on aliphatic isocyanates. Urethane acrylates with low functionality exhibit a high flexibility and are often based on flexible polyester or polyether diols, which are reacted with bifunctional isocyanates and endcapped with hydroxyalkyl acrylates (Figure 4.3).

A typical synthesis procedure of a urethane acrylate is given in Table 4.2.

Since the viscosity of the urethane acrylates is relatively high, they are often diluted with reactive thinners like TPGDA or HDDA. However, if the flexibility of the coatings should be increased, rather than using flexible diols, monofunctional diluents, like ethylhexyl acrylate, 2-(2-ethoxyethoxy) ethyl acrylate or trimethylolpropane-formal-monoacrylate are also used.

98

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

TABLE 4.2. Synthesis example of an aromatic urethane acrylate5 A polycaprolactone (98 g, produced as described in US Patent 3,169,945), having an average molecular weight of 530, and 52 g of 2-hydroxypropyl acrylate were placed in a 500 ml flask that was equipped with a stirrer, thermocouple and two dropping funnels. After the addition of one drop of dibutyl tin dilaurate the mixture was heated to 70 ◦ C in an oil bath. The solution was stirred while 69.6 g of an 80/20 mixture of 2,4- and 2,6-tolylene diisocyanates was added in a drop wise manner over a period of 2 h. The mixture was stirred for an additional 1.5 h and then 0.002 g of 4-methoxyphenol was added as a stabilizer. This product was a clear liquid.

A solution containing 73 wt% of the acrylate capped polycaprolactone and 27 wt% of 2-butoxyethyl acrylate had a Brookfield viscosity of 2270 cps at 23 ◦ C.

F IG . 4.4. Toolbox to tune the properties of urethane acrylates.

The higher functional urethane acrylates are often used to obtain hard, scratch and chemical resistant coatings. Examples of such multifunctional type of urethane acrylate resins are given in Figure 4.3.

Besides the good mechanical properties, these aliphatic type urethane acrylates exhibit good weatherability and do not yellow upon exposure to exterior conditions. Thus, they are the preferred class of resins for exterior applications.

The structure of the urethane acrylates can be designed to the required properties by choosing the right balance of hard phase and soft phase, by tuning the setscrews molecular weight, glass transition temperature and crosslink density (Figure 4.4). The compilation of the desired properties of urethane acrylates, however, reveals that the individual measures are often diametrically opposed and that a compromise always has to be made in order to adjust the most desired properties.

Examples of available aliphatic urethane acrylate resins by raw materials suppliers are listed in Table 4.3.

99

RAW MATERIALS

TABLE 4.3. Available flexible and hard urethane acrylates BASF AG

Bayer AG

Cognis

Cytec Surf

Sartomer/CV

Rahn

Laromer® UA

Desmolux® UA

Photomer®

Ebecryl®

Sartomer® (SR) Craynor® (CN)

Genomer®

CN966H90 CN965 CN9001 CN936

4188/EHA 4215 4269/1122

CN922 CN925 952B75 CN 975

4302 4590/PPTTA

Flexible 19T 9028V 9030V 9033V

VP LS 2220 VP LS 2258 VP LS 2220 XP LS 2413 XP LS 2430 XP LS 2989

6008 6210 6891

401 8307 8402 8406

Hard LR 8987 9029V 9048V 9050V

VP LS 2265 VP LS 2308

6623 6613

1290 8305 264

F IG . 4.5. Conventional and non-NCO route to urethane acrylates (UA).

Recently, novel routes to urethane acrylates have been published which do not rely on the reaction of hydroxyalkyl acrylates with isocyanates, but make use of the well known reaction of amines with cyclic ethylene or propylene carbonate to form hydroxyethyl- or hydroxypropyl urethanes, which are then acrylated by classical transesterification reactions6 or by enzymatic esterification7 (Figure 4.5). The use of this route has the clear advantage

100

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 4.6. Examples of the synthesis of polyester and polyether acrylates.

TABLE 4.4. Synthesis example of a polyester acrylate8 A mixture of pentaerythritol (78.0 g), stabilized methacrylic acid (344.0 g) (molar ratio of alcohol:acid = 1:8), concentrated sulfuric acid (7.75 g) and cupric oxide (0.5 g), dry toluene (300 ml) was stirred under reflux. The water formed during the reaction was continuously removed using a Dean and Stark trap. After 24 h the required quantity of water (32.0 ml) had been removed. Some charring and polymer formation was evident. The organic layer was washed with water, dried (magnesium sulphate) and the solvent removed in vacuum to afford viscous brown oil. Upon triturating with ethanol, a white crystalline solid (163 g) was obtained, which, after recrystallisation from the same solvent, had an mp. 66–67 ◦ C (70% yield).

that the portfolio of available products is not restricted any more to the limited number of available isocyanates, but rather on the multitude of available amines.

Polyester and polyether acrylates are synthesized by esterification of polyester/ether polyols with acrylic acid (Figure 4.6). Examples of polyesterols are the condensation products of adipic acid with diethylene glycol, 1,6-hexane diol or trimethylol propane or their ethoxylated or propoxylated derivatives.

Higher functional resins can be obtained by the use of triols, like trimethylolpropane or glycerol, or tetrols, like pentaerythritol, in the reaction with for example adipic acid and acrylic acid.

The molecular weights of such resins are typically in the range of 500–2000 g/mol.

There is a large variety of polyester acrylates available on the market. These resins are mainly used in wood coatings and paper coatings, and to a lesser extend in inks. The polyester acrylates used in wood coatings are mainly applied in top and undercoats.

RAW MATERIALS

101

F IG . 4.7. Amine modification of acrylates.

Polyetherols often used are ethoxylated or propoxylated glycerol or trimethylol propane.

Such polyether acrylates represent a class of resins of low viscosity, which can be used as sole resins as well as reactive diluents. Due to their higher molecular weight the skin irritation levels are lowered significantly compared to conventional diluents. Applying the substance on rabbit eyes and evaluating the redness often determine the irritancy. The irritancy is expressed with a Draize value (Draize rating: 0–2: slightly irritant; 2–5: irritant; 6–8: severe irritant). Rabbits are used because they are inexpensive, have large eyes, and are easy to handle. However, the transferability to humans is disputed, since the rabbit is considered as an inappropriate and inaccurate model for human ocular damage.9 The Draize values of such ethoxylated and/or propoxylated reactive diluents are in the range of 0–2.

The amine modification of acrylate resins is usually done in order to incorporate the amine synergist, which is used mainly in combination with hydrogen abstraction type photoinitiators, into the polymer backbone, and therefore increases reactivity, and reduce odour and extractables. The amines are introduced into the resins by a Michael Addition type reaction of primary or secondary amines with acrylates (Figure 4.7).10 The amine content used is normally rather low (in the 2–10% range). This type of resins is mainly used in graphic and wood applications, where high reaction speeds are required and yellowing does not play a significant role.

Acrylated polyacrylates are obtained preferably by the polymer analogous modification of functional polyacrylates containing pendant hydroxyl, epoxide, acid, or anhydride groups. These groups are subsequently reacted with complementary functional group containing unsaturated monomers by esterification or addition reactions, as pointed out in Figure 4.8.

The acrylated acrylic prepolymers offer good exterior durability, low colour and good chemical resistance.

102

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 4.8. Synthesis of acrylated polyacrylates by transesterification (left) and acrylic acid addition (right).

Aromatic epoxy acrylates – Based on Bisphenol-A diglycidether – High viscosity – High reactivity – Diluted with reactive thinners – Good chemical stability – Good cost/performance ratio – Tendency to yellowing

Aliphatic urethane acrylates – Pronounced toughness and flexibility – Non-yellowing – High weathering resistance – High viscosity – High price

Polyester acrylates – Good overall performance (allrounders) – Large variety of hard or flexible resin – Low residual monomers – Low odour – Good compatibility (with other resins)

Polyether acrylates – Low viscosity – Applicable as resins or reactive diluent – High reactivity

Amine modified acrylates – Coinitiators – High reactivity – Low odour, low migratables

Acrylated polyacrylates – Large variety of polyacrylatols available – High exteriour stability

F IG . 4.9. Principle property spectrum of standard type acrylate resins.

Since the requirement spectrum of a coating is so manifold, in most of the cases one resin type cannot fulfill all the demands. This is the reason why so many different resin types exist. The principle property spectrum of the different standard resin types is given in Figure 4.9.

RAW MATERIALS

103

F IG . 4.10. Specialty resins: Example of a dendritic acrylate.

4.1.2 Specialty Resins 4.1.2.1 Dendritic and hyperbranched resins Dendritic polymers have gained attractiveness because of their unique structure as well as unique properties compared to their linear or branched homologues. They are classified into the two categories of dendrimers (a typical structure is shown in Figure 4.10), characterized by a perfect symmetrical globular shape and monodisperse molecular weight distribution, and hyperbranched polymers, which are less perfect in their structure and shape, exhibiting a polydispersity, but are easier to prepare at a lower cost level.

Hyperbranched polyacrylates exhibit a considerable advantage against conventional acrylates as they offer low viscosity at relatively high molecular weights and high functionalities. Comparisons of the properties of linear and hyperbranched polymers have been published recently in general reviews.11 Low-viscosity hyperbranched polymers have been developed for radiation curing applications.12 A new product has been described to be based on acrylated hyperbranched polyester/polyether polyol blend, which is hard and flexible, with hardness and scratch resistance as good as dipentaerythritolhexaacrylate (DPHA), but lower viscosity and better flexibility than DPHA. Hyperbranched polyacrylates described earlier are based on the polycondensation of dimethylolpropane carboxylic acid onto a polyol core (trimethylolpropane) and about 50% acrylation of the third generation polyester with about 24 hydroxyl groups.13 Dendritic polyols are commercially available under the trade name BoltornÂŽ by Perstorp Corp. The series number H20, H30, H40 indicates the generation of the dendritic build up (second, third, fourth) showing an average hydroxyl number of 16, 32, and 64, respectively.

Hyperbranched urethane acrylates have also been prepared on the basis of similar hyperbranched polyester structures, but functionalization with the monoadduct of isophoronediisocyanate with (alkoxylated) hydroxyalkyl (meth)acrylates.14

104

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

Boltorn® -type hyperbranched aliphatic polyesters have further been used for the design of waterborne polyurethane acrylate dispersions.15 The described hyperbranched acrylates are synthesized in at least two-steps by first producing the hydroxyl or carboxyl terminated hyperbranched molecule, which is then functionalized with unsaturated acrylate groups. A one-step synthetic process has been reported by the use of Diels–Alder reaction of polyfunctional sorbic and polyfunctional acrylates, where for example the condensation products of sorbic acid with trimethylolpropane and dipropylene glycol-diacrylate are reacted in a Diels–Alder reaction.16 The acrylates have to have at least one function less than the sorbates in order to yield dendritic polymers under reproducible conditions. The viscosities are low compared to linear polyester acrylates (in the range of 1–10 Pa for molecular weight in the range of 1000–3000 g/mol). They exhibit good UV reactivity and low UV energies are sufficient for curing.

Dendritic acrylate oligomers of a different structure type have been prepared by the Michael-type reaction of ethylenediamine with trimethylolpropane-triacrylate in a molar range of 1 to 5 under mild conditions.17 In order to use acrylated Boltorn type polyesters for powder applications, the end groups have been modified with semi crystalline (octadecyl) groups, in order to introduce a crystalline melting point at 50–60 ◦ C.18 4.1.2.2 Photoinitiator-free vinyl ether/maleimide systems A limited number of UVcurable systems containing a combination of electron donor and electron acceptor monomers, which are capable of being cured without an additional photoinitiator have been recently developed by Jönsson.19 The best studied and best performing systems are based on a N-substituted maleimide as the acceptor monomer and a vinyl ether donor monomer. The reactivity of such systems without any photoinitiator was found to be in the order of the acrylate resins containing a photoinitiator.20 The maleimide has an absorption in the 300 nm wavelength range, whereas the vinyl ether does not absorb above 250 nm.

The mechanism proposed is based on two types of initiation, the intermolecular excimer formation to generate diradical species and the hydrogen abstraction type from the triplet state of the maleimide (Figure 4.11).

The formed maleimide radical as well as the hydrogen donor radical can add to another maleimide or to vinyl ether. Since the vinyl ether does not homopolymerize, the structure of the resulting network is composed of alternating MI and VE units, as well as homopolymer MI sequences and excess MI with minor isolated VE units. The aliphatic maleimides exhibit the highest reactivity, especially those containing readily abstractable hydrogens, like propoxylated or ethoxylated derivatives; however, the toxicity of the specific maleimides has to be evaluated carefully. Other monomers used in the photoinitiator-free polymerization of donor and acceptor molecules are given in Table 4.5.20 4.1.2.3 Miscellaneous resin types Self-initiating UV-curable resins of the structure shown in Figure 4.12 have been introduced recently.21 They are obtained by the Michaeladdition-type reaction of beta keto esters to diacrylates. This chemistry is very versatile to the design of new UV-curable resins that do not need an additional photoinitiator. This is, because upon UV exposure these resins undergo alpha cleavage next to the ketone moiety

RAW MATERIALS

105

F IG . 4.11. Donor–acceptor-type photoinitiator-free initiation mechanism.

TABLE 4.5. Donor–acceptor combinations Vinyl ethers

Maleimides

Triethyleneglycol divinyl ether 4-Hydroxybutyl-vinyl ether

Triethyleneglycol bismaleimide (Ciba) C36-alkyl bismaleimide (Q-bond, Quantum) 2-Ethylcarbonate ethylmaleimide

to split off an acetyl radical and generate another radical on the polymer backbone. Both radicals can then polymerize the acrylate groups.

Another new raw material type of resins is based on metal containing monomers (Figure 4.12). Such polymers, like crosslinked zinc diacrylate22 promote adhesion to metal and glass substrates and may provide reversible crosslinking at the ionic site in order to be used for UV hotmelt adhesives or self-healing films.

4.1.2.4 Silicon based oligomers Silicon based oligomeric (meth)acrylates are used in so-called hybrid polymers based on sol–gel reactions, where an inorganic network is obtained via the siloxane condensation and an organic network via the UV or thermal polymerisation of reactive groups like acrylates or methacrylates. Such systems can also be classified as dual cure systems. Oligomers of this type can be synthesized by the reaction of functional (trialkoxy)-siloxanes (–OH, –NH2 , epoxy) with functional (meth)acrylates.

UV-curable hybrids, like ORMOCER® (organic modified ceramic) systems,23 as described in Chapter 9, can, for example, be based on the commercially available 3-methacryloxypropyl-trimethoxy silane24 as part of the system (Figure 4.12).

106

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 4.12. Specialty resins: Self-initiating, metal ion bridged and silicone acrylates.

4.1.2.5 Polyenes The photocrosslinking of thermoplastic elastomers combines the properties of thermoplastics and elastomers.25 Polymers used are based on block copolymers building a hard phase (e.g., polystyrene, glassy domain) and an elastomeric phase (e.g., polybutadiene), which can be crosslinked via the unsaturation in the elastic phase.

Such crosslinking reactions have been described for styrene-butadiene26 and acrylonitrilebutadiene (ABA).27 The crosslinking reaction can be greatly enhanced by the addition of mulitfunctional acrylate monomers or multifunctional thiols. The reaction leads to rapid insolubilization of the thermoplastics and to the improvement of other properties, like higher adhesion failure temperatures and moderate hardening of the elastomeric phase, however, the elastic character was retained. Such systems can advantageously be used in patterning applications, like flexography and photolithography.

4.1.2.6 Dual cure resins Dual curing (UV initiated and thermal) can be performed by using blends of the classical resins, like acrylate terminated oligomers and polyisocyanates/polyol or melamine–formaldehyde/polyol combinations. According to this concept a large variety of viable chemistries is around. Discussed here briefly are resins, which contain two functional groups in “one” molecule, where one group is accessible to UV and the other to thermal curing. Belonging to this class are the described epoxyacrylates, which contain the unsaturated acrylate group and a secondary hydroxyl group, which can for instance be reacted with isocyanates or melamine resins.

Complementary to such hydroxy acrylates are isocyanato acrylates, containing the acrylate and an isocyanate group in the same molecule. Such compounds can be prepared by the understochiometric reaction of hydroxyalkyl acrylates with polyisocyanates, like the trimers (isocyanurate, biuret) of hexamethylene diisocyanate or isophorone diisocyanate.

Since such molecules suffer from relatively high viscosity, novel molecules based on allo-

RAW MATERIALS

107

F IG . 4.13. Specialty resins: Dual-cure resins based on isocyanato acrylates.

F IG . 4.14. Composition scheme of PU (PUD) and dual cure dispersions.

phanates, synthesized by the allophanatization of hexamethylene-diisocyanate and hydroxyethyl acrylate as the alcohol component have been commercialized.28 Different synthetic routes have obtained similar structures.29 The intramolecular instead of intermolecular hydrogen bridging, as explained in Figure 4.13, is responsible for the reduced viscosity of this type of isocyanato acrylates.

4.1.2.7 Water-based systems The composition scheme of polyurethane acrylate and dual curable dispersions is shown schematically in Figure 4.14. The polyurethane acrylate

108

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

dispersions typically comprise polyisocyanates reacted with diols (and/or small amounts of polyols), hydroxyalkyl acrylates and compounds containing acid groups, like carboxylic or sulfonic acids, as well as a functional groups which reacts with the isocyanate, e.g., amino or hydroxyl groups. Such UV-curable polyurethane dispersions are very well known in the field (see ref. 30 and references therein).

Dual curable dispersions furthermore contain blocking groups for the isocyanates and functional groups that are able to react with the (finally) released isocyanates.

4.1.2.8 Polymer definition According to the new EU regulation REACH, new chemical species have to be registered (see also Chapter 11) and subjected to a bunch of testing procedures. However, there are exemptions to the registration obligation, and one exemption refers to polymers according to the OECD polymer definition, which just have to be notified and require only a reduced test package. This exemption will foster the future development of resins to fulfill the polymer criteria. In order to be considered as polymers according to the OECD definition the following four prerequisites on a substance are required. Polymer means a substance consisting of: a) Molecules characterized by the sequence of one or more types of monomer units; b) A simple weight majority of molecules containing at least three monomer units that are covalently bound to at least one other monomer unit or other reactant; c) Less than a simple weight majority of molecules of the same molecular weight; and d) Molecules distributed over a range of molecular weights; wherein differences in the molecular weights are primarily attributable to differences in the number of monomer units.

There are also several exclusions from the exemption (e.g., cationic polymers, degradable or unstable polymers, polymers containing more than 2% of a monomer not listed in EINECS). Furthermore, several other constraints have to be considered, for example, if a multifunctional starter molecule, like glycerol is used in an ethoxylation reaction, the sequence of monomers has to be 3 mol of oxirane attached to one alcohol group of the glycerol, thus for statistical reasons (if not otherwise evidenced), a polymer is only formed if more than 6 mol of oxirane is applied per mol glycerol, see, for example, ref. 31.

4.1.2.9 Stabilizer/inhibitors The stabilization of the acrylate resins during synthesis and storage against undesired polymerization has been described in several publications;32 most of the occurring reactions are also involved in the degradation and autooxidation of polymers. The stabilization issue can be separated into the stages of the synthetic process and long-term stabilization, as well as differentiated by the conditions of handling under aerobic or inert atmosphere. In all many different radicals may be involved in the reaction scheme, as depicted in Figure 4.15.33 As indicated, these different radical types need and enable various paths of possible stabilization measures. The formation of an initial radical may be triggered by heat, shear force induced by pumping, by catalyst residues in redox reactions or by exposure to light. Such primarily generated carbon-centered radicals form in the presence of oxygen (right side of Figure 4.15) in a very fast reaction a hydroperoxide radical (R–OO• ). The further addition of a monomer unit is very slow, thus a very effective way of stabilization of the monomers against unwanted polymerization in the presence of

RAW MATERIALS

109

F IG . 4.15. Types of radicals produced during processing or storage and stabilization possibilities.

oxygen is the scavenging of the hydroperoxide radical by phenolic antioxidants. The thus formed (hindered) phenolic radicals do not initiate polymerization. If the hydroperoxide radicals are not captured by the phenolic antioxidants they abstract further hydrogen and form hydroperoxides (Râ&#x20AC;&#x201C;OOH) and regenerate alkyl radicals. The hydroperoxides can also produce further radicals by decomposition to the very reactive alkoxy and hydroxy radicals under the impact of UV light, heat or catalytic residues (left side of Figure 4.15). Thus, in order to prevent the decomposition of hydroperoxides to the detrimental radical, often hydroperoxide decomposers are added, which destroy the hydroperoxide to form harmless products. Very effective hydroperoxide decomposers are organophosphorus compounds, like phosphites and phosphonites.34 Under inert conditions the primarily generated C-radical or the reaction products of these radicals with monomers must be scavenged effectively. There are only few effective C-radical scavengers mentioned in the literature.

4.1.2.9.1 Inhibition under aerobic conditions The inhibition of the undesirable polymerization of acrylates during synthesis or storage in the presence of air or reduced air content (of about 6% air in nitrogen) is mainly due to the formation of less reactive peroxide radicals, which are then completely deactivated by inhibitors reacting with oxygen centered radicals. The mechanism of the inhibition under oxygen conditions is explained in Figure 4.16. The most commonly used aerobic inhibitors belong to the chemical class of phenols, like monomethylether of hydroquinone (MEHQ), hydroxyanisol or 2,6-di-tertbutyl-kresol. There are a numerous number of phenolic antioxidants available, either partly or fully substituted with steric bulky substituents in the 2, 4, and/or 6 position. These

110

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 4.16. Mechanism of inhibition of polymerization under oxygen conditions (MEHQ inhibitor).

F IG . 4.17. Inhibition mechanism of phenolic antioxidants and side reactions.

phenolic antioxidants inhibit the polymerization effectively, however, reaction products formed may also contribute to discolouration (Figure 4.17).

Whereas the phenols are mostly applied as long-term stabilizers during storage, MEHQ is also used in the synthesis, for example, the esterification with acrylic acid, because of its high volatility, which contributes to the stabilization of the vapour phase. The deactivation

RAW MATERIALS

111

is mainly based on the scavenging of the low reactivity peroxy radicals. In the absence of oxygen MEHQ does not inhibit the polymerization at all.

A very effective natural occurring antioxidant is Vitamin E, which has also been used as stabilizer in synthesis of acrylates35 and has been evaluated as a model for improving the action of synthetic phenolic antioxidants,36 showing, that the introduction of an alkoxy group in any of the 2, 4, or 6 positions should have a superior stabilizing power.

Other molecules described to scavenge both; alkyl and alkoxy radicals under low oxygen concentrations are benzofuranone derivatives.37 Hydroperoxide decomposers are often used during synthesis, in order to decompose hydroperoxides formed during the course of the reaction.

An example is represented by the following equation: P–[O–Ar]3 + ROOH → O=P–[O–Ar]3 + ROH.

(4.1.1)

Since these compounds are susceptible to hydrolysis, in practise mainly substituted hydrolytically stable phosphites or phosphonites are used. These stabilizers are especially useful if polyetherols are used in the esterification reaction with acrylic acid, since the ethers are very susceptible to the formation of hydroperoxides.

Further effective hydroperoxide decomposers frequently used are copper salts, thioesters or metal complexes of thiocarbamates.38 4.1.2.9.2 Inhibition under anaerobic (inert) conditions The mechanism associated with inhibition of the polymerization under anaerobic or inert conditions is related to two reactions occurring, the addition of the inhibitor molecule to a primarily formed radical, and second, the stabilization of the unpaired electron. The technically most used inhibitor for the stabilization under inert conditions is phenothiazin.39 Another class of very effective C-radical scavengers are the nitroxyl radicals, for example the 2,2,6,6-tetramethyl piperidine-N-oxyl (TEMPO), which are very reactive towards carbon centered radicals, but not towards the unpaired electron at an oxygen.

Despite the fact that phenothiazin or N-oxyles are very effective inhibitors, their widespread use is hampered by two disadvantages. The first being related to their effectiveness, since in the desired photopolymerisation, the inhibitor has to be overrun by a higher concentration of photochemically produced radicals, in order to start the crosslinking reaction.

And the second is due to the discoloration associated with the use of these compounds.

4.1.3 Reactive Diluents As reactive diluents monomers and oligomeric acrylates or vinyl ethers are used in order to adjust the application viscosity (Figures 4.18 and 4.19). Despite the fact that monomers like styrene, N-vinyl pyrrolidone40 and monofunctional esters of acrylic acid, belong to the best diluents, their use is decreasing due to their high volatility, strong odour, skin irritation and flammability. Since their mono-functionality provides a higher molecular weight between crosslink’s or lower crosslink density, which results in a better flexibility, monomers with

112

Methacrylates Hydroxypropyl-MA Isobornyl-MA Dicyclopentenyl-oxy-ethyl-MA Hydroxyethyl-MA

Cationic: Mono-epoxides, Vinyl ethers, allyl ethers, oxetanes F IG . 4.18. Reactive diluent types for UV-curing systems.

Others Styrene N-Vinyl caprolactam N-Vinyl pyrrolidone N-Vinyl formamid Acrylamidomorpholine Silanes Vinyl ethers Divinylether of Tripropylenglycol Divinylether of cyclohexanedimethanol Vinylether capped urethanes

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

Acrylates Monofunctional acrylates Isobornyl acrylate (IBOA) Trimethylolpropane-formal-mono-acrylate Phenoxyethyl acrylate (POEA) Difunctional acrylates Tripropylene glycol diacrylate (TPGDA) Dipropylene glycol diacrylate (DPGDA) Hexandiol diacrylate (HDDA) Neopentylglycol diacrylate (NPGDA) Multifunctional acrylates Trimethylolpropane triacrylate (TMPTA) (Ethoxylated/propoxylated) TMPTA Propoxylated glycerole triacrylate Pentaerythritol triacrylate (PETIA) Pentaerythritol tetraacrylate (PT4 A) Dipentaerythritol penta/hexa acrylate

RAW MATERIALS

113

F IG . 4.19. Structure of main acrylate reactive diluents.

lower volatility and odour, like isobornyl acrylate or trimethylol propane formal monoacrylate, have been developed. Multifunctional monomers are available in a broad range since they are produced by esterification reactions of acrylic acid with polyols; the main representatives are tripropylene glycol diacrylate (TPGDA), trimethylolpropane triacrylate (TMPTA), propoxylated glycerol triacrylate (GPTA, like OTA480), and hexanediol diacrylate (HDDA). Most of the others have only a small market share. Higher functional acrylates such as pentaerythritol tetraacrylate (PTA) or dipentaerythritol hexaacrylate (DPHA) are also available, but only used as minor ingredients since they increase dramatically the crosslink density resulting rapidly in hard but brittle films. Such resins and diluents are commercially available from Cytec, BASF, Akcros, Cray Valley/Sartomer and Henkel.

Reactive diluents used in cationic polymerizing systems are mainly mono- or difunctional epoxides and vinyl ethers.

The idealized structure of these compounds however does not represent exactly the real composition of such reactive diluents, since some side reactions occur during the esterification with acrylic acid, mainly Michael-type additions of the acrylic acid as well as Michael addition of the various alcohols on the double bonds of the formed multifunctional acrylates (Figure 4.20).

The vinyl ethers have been evaluated as reactive diluents in radical polymerizable acrylate and unsaturated polyester41 systems, as well as in cationic polymerizable epoxy coatings.42 It has been shown, that the advantages of vinyl ethers compared to acrylates are their high diluting power, which is often better than acrylates (e.g., tripropylene glycol diacrylate

114

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 4.20. Structure of trimethylolpropane triacrylate (TMPTA) and addition by-products formed during synthesis.

(TPGDA)), and their lower toxicity (no labelling, no irritancy). The performance of the systems evaluated, triethylene glycol divinylether (DVE-3) compared to TPGDA as a reactive diluent, revealed similar mechanical properties, but higher reactivity and scratch resistance.

The disadvantages are due to the kinetics, since the vinyl ethers do not homopolymerize radically, but rather react in an alternating polymerization, the content should be no higher than 50-mol%. Furthermore, it has been shown by Decker43 that in such formulations upon radical photoinitiation the acrylate radical is twice as reactive towards the acrylate double bond than towards the vinyl ether double bond. Thus, in order to be fully converted, and therefore avoid unreacted residual vinyl ether groups in the coating, which are prone to acidic hydrolysis, resulting in the formation of acetaldehyde, the content of the vinyl ether reactive diluents should not exceed about 10% in acrylate systems, and the range of 10– 20% in epoxy systems, however, in unsaturated polyesters the vinyl ether content can be up to 50%.

4.1.4

Radical Photoinitiators

Photoinitiators are molecules that absorb photons upon irradiation with light and form reactive species from their excited state, which initiate consecutive reactions (see Figure 2.4). The initiating species may be radicals, cations or anions. Radical photoinitiators, which represent more than 90% of commercially used initiators, are available in large number from companies like Ciba Specialties (trade names Irgacure® and Darocure® ), Lamberti (Esacure® ), BASF (Lucirin® ), and many others. Available in smaller number are cationic photoinitiators, mainly sulfonium salts, from Union Carbide (Cyracure® ), Degussa (Degacure® ) as well as iodonium salts from General Electric and iron complexes from Ciba. Examples of commercially available photoinitiators are listed in reviews published by Crivello,44 Fouassier,45 and Davidson.46

RAW MATERIALS

115

F IG . 4.21. Examples of “alpha-cleavage type” and non-cleavable “electron transfer-hydrogen abstraction type” photoinitiators.

4.1.4.1 Standard radical type photoinitiators Almost all radical photoinitiators contain the benzoyl(phenyl–CO–) structure element. The two most important classes are the αcleavable (Norrish type I) and the non-cleavable (“hydrogen abstraction” type II) photoinitiators (Figure 4.21 and Table 4.6).

A photoinitiator for radical polymerization should exhibit a bunch of properties, from which the following are the most important: a) High absorption at the exposure wavelength and high molar extinction coefficient; b) High quantum yield of formation of initiating species; c) High reactivity of the radical to the monomer.

The α-cleavage-type photoinitiators are very versatile, exhibiting higher efficiency compared to hydrogen abstraction types due to the unimolecular cleavage reaction and consequently they are the most widely used.

The quantum yields of dissociation of the cleavable photoinitiators are for example 0.3 (2-benzyl-2-dimethylamino-1-[4-(4-morpholinyl) phenyl]-1-butanone), 0.7 (2,4,6trimethyl-benzoyl)-diphenyl phosphine oxide), 0.8 (1-hydroxy-cyclohexyl-phenyl-ketone), 2-hydroxy-2-methyl-1-phenyl-1-propanone) or 0.95 (α, α-dimethoxy-α-phenylacetophenone). The addition rate constants (M−1 s−1 ) of the formed radicals to monomers (acrylates and methacrylates) are shown in Figure 4.22.

116

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

TABLE 4.6. Most important photoinitiator classes, product examples and main uses Chemical class

Examples

Physical state

Main use

Benzophenones

Benzophenone (Genocure® BP)

Powder

General: low cost; in combination with amine synergists

α-Hydroxy ketones (α-HK)

1-Hydroxy-cyclohexylphenyl-ketone (Irgacure® 184)

Powder

General surface cure; non-yellowing

2-Hydroxy-1-[4(2-hydroxyethoxy) phenyl]-2-methyl-1propanone (Irgacure® 2959)

Powder

Clear coats

2-Hydroxy-2-methyl1-phenyl-1-propanone (Darocure® 1173)

Liquid

Benzil-dialkylketal

α, α-Dimethoxy-αphenylacetophenone

Powder

Wood, PCB, printing plates

(BDK)

(Irgacure® 651)

α-Amino ketones

2-Benzyl-2dimethylamino-1[4-(4-morpholinyl) phenyl]-1-butanone (Irgacure® 369)

Powder

Electronics (PCB), inks

Phenyl glyoxylates (PG)

Methyl-benzoylformate (Genocure® MBF)

Liquid

Labelling free

Thioxanthones (ITX)

Isopropyl-thioxanthone (Genocure® ITX)

Powder

Inks

Acylphosphine oxides (APO)

2,4,6-Trimethylbenzoyl)-diphenyl phosphine oxide (Lucirin® TPO) Phenyl-bis-(2,4,6trimethylbenzoyl) phosphine oxide (Irgacure® 819)

Powder TPO-L (liquid)

General through cure, inks (pigmented)

Powder, mix: liquid

Exterior coatings (UV-absorber containing)

RAW MATERIALS

117

F IG . 4.22. Dissociation quantum yields of photoinitiators and addition rate constants of the corresponding radicals to monomers.

The benzoyl radical formed in almost all cleavable type photoinitiators adds at a low rate to the acrylate or methacrylate monomers. The aliphatic ketyl or phosphinoyl radicals react at a two orders of magnitude higher rate with (meth)acrylate monomers.47 To increase reactivity amine-containing coinitiators (synergists) are used which have two effects. First, the C–H group adjacent to the nitrogen is a good hydrogen atom donor and the thus formed radical can initiate the polymerization and/or second, the radical can scavenge oxygen very effectively and the initially formed hydroperoxide also abstracts a hydrogen from another amine moiety, forming again the reactive amine radical, which can initiate polymerization or add oxygen in another cycle until all oxygen is consumed (Figure 4.23).

The photoinitiators are typically differentiated by their absorption profiles (Figure 4.24), characterized by the absorption wavelength range and the molar absorption strength defined by the molar absorption coefficient. Typical loadings of photoinitiators are in the range of 1–5%, often 2–3% are reasonable. Such high concentrations are often only necessary in order to overcome oxygen inhibition effects, since under inert curing conditions loadings of 0.5–1% are enough. The loading should not be too high in order to avoid filter effects that may prevent light penetration to the bottom layers resulting in through cure problems.

The photoinitiators also have to be selected in order to match with the output spectrum of the UV light source. The medium pressure mercury lamps are the standard lamps used since they provide high power and emission lines where most of the commercially available photoinitiators absorb (see Figure 4.25).

118

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 4.23. Amine synergist action (hydrogen donor; initiation and oxygen scavenging).

F IG . 4.24. Absorption spectra of different photoinitiators in acetonitrile at a concentration of c = 0.01 g/l (LucirinÂŽ TPO, c = 0.025 g/l).

4.1.4.2 Low yellowing photoinitiators The yellowing during photoinitiation is a general problem to which several factors contribute, the photoinitiators, the resins, stabilizers, exposure conditions and so on. The effect of the various influencing factors on the yellowing behaviour will be discussed in detail in Chapter 7. The structures of the photoinitiator as well as the photoproducts, however, also contribute to yellowing. Especially the formation of semiquinoid or aromatic bi-ketonic structures contributes to yellowing, as, for example, in the case of benzildimethylketal (BDK).48 Thus, one strategy in the design of non-yellowing photoinitiators was the design of systems that generate non-absorbing

RAW MATERIALS

119

F IG . 4.25. Chemical structure of aromatic and cycloaliphatic epoxides, standard sulfonium salt and novel-type of cationic photoinitiator (arylthianthrenium salt).

(non-yellowing) photoproducts. Photoinitiators whose benzoyl moiety is linked to aliphatic residues are following this design rule, as it is the case with hydroxyalkylacetophenones (α-HK). 1-Hydroxy-cyclohexyl-phenyl-ketone is therefore one of the best non-yellowing type photoinitiator.

Another approach is based on the photobleaching, represented by the class of acyl phosphine oxides, which are yellowish per se, but turn into colourless products after photocuring.

4.1.4.3 Photoinitiators for pigmented, UV-stabilized coatings and through cure The longer wavelength (>300 nm) absorbing photoinitiators of the acyl phosphine oxide type (Lucirin® TPO or Irgacure® 819) have been developed for the use in pigmented or UV absorber containing films as well as to provide good through cure in thicker coatings. The normal loading of this type of photoinitiators is in the range of 0.5–1%. They are often used in combination with hydroxyketone type photoinitiators that provide the surface cure while phosphine oxides provide the through cure.

4.1.4.4 Low emission photoinitiators Novel photoinitiators for low odour and low migration have been developed by Ciba, especially for graphics applications,49 based on a difunctional hydroxyketone, a special substituted aminoketone and a difunctional phenyl glyoxylate.50 Polymer bound photoinitiators are described in an overview by Carlini.51 Special types, based on the coupling of hydroxy benzophenone or hydroxy-thioxanthone to a polytetrahydrofurane diole52 have been introduced recently under the trade names Omnipol BP and Omnipol TX.

120

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

4.2

CATIONICALLY-CURABLE SYSTEMS

Interest and research activity in cationic UV curing has increased due to the insensitivity to oxygen inhibition. Until now this technology has not yet found the widespread use in many industrial applications compared to radical systems. Decorative and protective coatings, printing inks and adhesives are just a few examples of applications in which photoinitiated cationic polymerizations have experienced the most commercial growth.53 As with radically polymerizing UV coatings, the cationic curing is also a very rapid polymerization consuming little energy without the need for an inert atmosphere, hence providing important economic incentives. The cationic polymerization process is shown schematically on the example of epoxide polymerization (4.2.1). A proton or a Lewis acid activates the oxygen atom in order to facilitate a nucleophilic attack of epoxide oxygen onto the (positivated) carbon next to the activated oxygen. Upon ring opening a secondary hydroxide is formed and the chain reaction continues at the next activated oxirane ring.

(4.2.1) The thermal, mechanical, chemical resistance, low shrinkage and adhesion characteristics of the network polymers that are formed are excellent. However, their limited use up to now may be due to other problems of the cationic polymerization system than oxygen inhibition; for example, moisture can terminate the chain reaction. The industrial impact of cationic UV curing is predicted to increase markedly in the future as this technology undergoes further maturation.54 4.2.1

Cationically Curable Resins and Monomers

In general, two main groups of monomers are used in cationic polymerization, the first being ethylenic monomers containing groups that stabilize the carbocation by resonance, like aromatic rings (styrene, Îą-methyl styrene), a double bond (butadienes, isoprenes) or an ether group (vinyl ethers, oxetanes), and the second being heterocyclic monomers containing at least one heteroatom (O, S or P, like epoxides or caprolactones), for which the propagation reaction proceeds via onium ions.

A large number of cationic resins and monomers have been evaluated.55 By far the main resin types used in radiation curable cationic coatings are based on epoxides, for example, the bis-glycidyl ether of Bisphenol-A or the cycloaliphatic 3,4-epoxy-cyclohexylmethyl3 ,4 -epoxy-cyclohexane-carboxylate (Cyracure UVR 6110 from Dow) and a large number of other cycloaliphatic epoxides,56 followed by vinyl ether terminated resins (Figure 4.25).

RAW MATERIALS

121

As reactive diluents epoxides (1,6-hexanediol-diglycidylether, HDDGE) or vinyl ethers (triethyleneglycol-divinyl ether, TEGDVE) are used.

The cationic curing of epoxies together with vinylether57 or oxetane groups58 containing monomers has been evaluated. Triethylene glycol divinyl ether was shown to be more effective as reactive diluent in cationic curing epoxy systems (Cyracure UVR 6110) than HDDGE. With optimum TEGDVE concentrations in the range of 10–20% the viscosity could be decreased to the range of 50–100 mPa s, cure rate and conversion increased as well as solvent resistance. The use of 3-ethyl-3-hydroxymethyl oxetane (EHMO) also reduced the viscosity down to 150–100 mPa s at concentrations of 10–25%. Here also the cure rate and the solvent resistance are increased, as shown with typical formulations for clear coats and inks.

Novel hyperbranched polymers, based on dipentaerythritol as the core molecule and 4,4 -bis-(4-hydroxyphenyl) valeric acid as AB2 monomer and functionalized with hydroxy, epoxy or oxetane groups have been evaluated in cationic UV curing59 of epoxy and oxetane systems as additives. They were incorporated into the network either by copolymerization or via chain transfer involving the hydroxy group. They showed an increase of toughness and modulus and better thermal stability, expressed as increase of weight residue above 450 ◦ C in TGA experiments.

4.2.2 Cationic Photoinitiators The most widely employed cationic photoinitiators are compounds belonging to two classes of onium salts, the diaryliodonium and triarylsulfonium types. Less important are diazonium salts and organometallic complexes, like ferrocenium salts (Irgacure® 261).

Fouassier has reviewed the chemistry, excited state processes and reactivity of a large number of such initiators.60 The iodonium and sulfonium compounds are very efficient photoinitiators, producing Broensted acids as initiating species, however, they exhibit some disadvantages. Their spectral sensitivity is predominantly in the short wavelength UV, their toxicity, especially of the iodonium salts, has to be considered, and in general their solubility, in particular of the cheaper sulfonium salts, in coating formulations is rather low.

Furthermore, these diaryl- or triaryl-onium salts release undesirable benzene during the photoreaction, which prompted regulation of these types in food packaging.61 New developments have been undertaken to tackle the problems described. A new photoinitiator recently introduced by Ciba is based on long alkyl chain substituted diaryl iodonium salts, thus avoiding the release of benzene (Ciba Irgacure® 250).

Recent work published by Crivello62 makes use of the class of Dialkylphenacylsulfonium salts (DPS, see Figure 4.26), which can be easily synthesized in high yields by simple, straightforward reactions. The photoreactivity is comparable to the reactivity of the iodonium and sulfonium salts (Figure 4.26). Due to this new, easy synthetic route, it has been feasible to optimize the structure in order to produce highly photosensitive photoinitiators that are both very soluble in a wide variety of polar and non-polar monomers, thus, are suitable for the polymerization of all kinds of available cationic monomers. The DPS salts prepared by this method are stable, colourless, light-sensitive compounds. Since the crystalline DPS salts do not undergo thermal decomposition at temperatures up to their melting

122

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 4.26. Comparison of the curing conversion of different cationic photoinitiators.

points, they can also be used as latent thermal initiators for the cationic polymerization of reactive monomers and oligomers.

Another approach in order to avoid the release of benzene was published by Lamberti S.p.A.63 with the development of cyclic sulfonium salts of the Arylthianthrenium type (Figure 4.25). They are characterized by a fast cure and absence of toxic by-products during the photoreaction. Due to the good performance of the monothianthrenium salts and their good solubility in the formulations, a series of 5-alkoxyphenyl-thianthrenium derivatives particularly suitable for food packaging applications has been developed.

4.2.3

Photosensitization of Cationic Photopolymerisation

Diaryliodonium, triarylsulfonium and dialkylphenacylsulfonium salt cationic photoinitiators typically possess major absorption bands in the short wavelength region (<300 nm) of the UV spectrum. For this reason, much of the energy emitted by the conventional light sources is not absorbed by the onium salt photoinitiator and, therefore, wasted. The use of photosensitizers64 can dramatically improve the speed of onium salt induced cationic photopolymerizations by broadening the range of the spectral absorption of these photoinitiators. These photosensitizations are either based on energy transfer, electron transfer or free-radical induced decomposition. The well-known hydrocarbons, like anthracene, perylene, or pyrene, are working through electron transfer.65 Carbazole compounds, like polymers and copolymers of N-vinylcarbazole, have been found to be excellent and highly general photosensitizers for onium salts used in cationic UV curing.

RAW MATERIALS

123

4.3 THIOL-ENE SYSTEMS The thiol-ENE curing reaction is also a radical induced process, however, proceeds via a step growth type polyaddition in contrast to the radical induced chain growth polymerization. This process has the distinct advantage that it is not oxygen inhibited, unlike the acrylate based polymerizations. The thiol-ENE UV curable coatings are based on a mixture of a polyene and multifunctional thiols that undergo a polyaddition reaction upon UV irradiation. The UV irradiation produces a photoinitiator radical (PI• ), which in a preliminary step abstracts hydrogen from a thiol under formation of a thionyl radical (R–S• ). The step growth process, which is propagated by chain transfer reaction involving as the key molecule the thionyl radical (R–S• ) is depicted in Figure 4.27. The basic principles and almost all aspects of this type of coating system have been reviewed recently.66,67 The general mechanism shown in Figure 4.27 also explains why the oxygen does not have a detrimental effect on the curing process. After the formation of a thionyl radical (R–S• ) by hydrogen abstraction of the photoinitiator radical, the thionyl radical adds to an ENE double bond and forms a new sigma bond and a carbon centered radical. This addition reaction is the first and already crucial step for the crosslinking process. The second step is the subsequent hydrogen abstraction from a further thiol molecule by the carbon centered radical produced in the first step and formation of another thionyl radical. In the presence of air, the carbon centered radical will be scavenged by oxygen to yield a peroxy radical, which, however, is also able to propagate by hydrogen abstraction from a thiol, yielding a hydroperoxide and a thionyl radical. A crosslinked coating is only produced if the combined average functionality of the thiol and ENE compound is higher than two. Besides the insensitivity to oxygen inhibition, the thiol-ENE systems described in the literature exhibit fast cure,68 and exceptional properties.69 As expected from the step growth polyaddition network formation, the gel point of such systems occurs at high conversions, unlike in acrylate systems, where already at low con-

F IG . 4.27. X-Linking of multifunctional thiol and poly-ENE via thiol-ENE addition.

124

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 4.28. Structures and relative reactivity of typical ENE’s used in thiol-ENE reactions.

versions microgelation occurs (see Chapter 3). Therefore, such systems do not suffer as much as acrylate based coatings from shrinkage problems occurring after a crosslinked solid network has formed, since theses systems stay in a viscoelastic state up to high conversions, which allows for easier relaxation.

The usable types of enes are manifold, including simple alkenes, like 1,6-hexadiene, allyl ethers, propenyl ethers, vinyl esters, unsaturated esters, vinyl ethers, conjugated dienes, acrylates and even modified nanoparticles, like vinyl group containing silsesquioxanes.61 This wide range of monomers makes this type of chemistry very versatile, since a great variety of oligomers containing the mentioned groups is available,70 which has been summarised in an excellent review by Hoyle,71 some of the typical structures mentioned are shown in Figure 4.28 (ENE’s) and Figure 4.29 (thiols). Furthermore the reaction of thiols with multifunctional norbornens, dendritic allyl ethers, with vinyl acrylate, with polymeric enes, like styrene-butadiene copolymers and other elastomers, with ene functionalized siloxanes, with multifunctional vinyl esters and acrylates are described.

Surprisingly, such mixtures of acrylates and thiols also exhibited the beneficial properties of being insensitive to oxygen inhibition.72 It was found, that both, homopolymerization of acrylates and step growth polyaddition of thiols onto acrylate double bonds occurred. The explanation for the oxygen insensitivity was based on the assumption that the formed peroxy radicals in the acrylate polymerization also abstract a hydrogen from the thiol, thereby generating a thionyl radical capable of continuing the step growth process.

Despite these considerable advantages over acrylate based systems, the polyene-thiol systems are not yet widely used in the UV-curing technology. This is mainly due to the limited number of available multifunctional thiol molecules, to the often unpleasant odour and higher costs of these thiol compounds.

However, by using higher mol weight thiols, improving the synthesis and purification, the odour can be minimized and the THEIC-tri-thiol shown in Figure 4.29 from Yodo Chemicals has essentially no thiol odour.

RAW MATERIALS

125

F IG . 4.29. Structures of typical thiols used in thiol-ENE reactions.

As the traditionally raised objections have been dispelled, this old type of chemistry may get a new chance with a promising future, due to the insensitivity towards oxygen inhibition, delayed gellation, low shrinkage, high conversion and uniform crosslink densities, resulting in coatings with excellent physical and mechanical properties.

4.4 COATING ADDITIVES The basic components of a UV-curable formulation (resins, diluents, photoinitiators) alone hardly ever produce a coating of acceptable quality. The wetting of the surface or pigments, which is often facilitated by the solvent, and the removal of entrained air, which is difficult due to short or no flash off times before cure, are only two examples of specific difficulties related to the film quality of UV-curable coatings. Therefore, as in classical coatings, various additives are used. The additives are in principle the same as used in classical solvent-based formulations, however, often tailor made for specific needs.73 De-aeration additives are based on polyacrylates as silicon-free additives and organically modified siloxanes. They replace surfactants, which stabilize the bubbles, and due to a balanced incompatibility lead to a raise and burst of the bubbles at the surface. In almost all powder coatings benzoine is used as a de-aeration additive.

Substrate wetting additives have to reduce the surface tension of the coating system and maintain these properties in highly dynamic processes like printing. For UV-curable formulations often low molecular weight silicones, hydrocarbon organics or fluorinated surfactants are used.

Surface control additives are used to improve flow and levelling as well as slip and scratch resistance. Here also silicones, however, with higher molecular weights and often

126

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

TABLE 4.7. Common characteristic values given for UV raw materials Oligomeric resins

Diluents

Photoinitiators

Functionality Viscosity Density Molar mass Colour Hydroxyl value Acid value

Viscosity Density Colour Molar mass Acid value

Absorbance spectrum Appearance Melting point Solubility

containing polymerizable groups are used. These compounds show a strong tendency to migrate to the surface and provide improved slip and scratch resistant properties.

Surface-active additives, such as flow, levelling, de-aeration, slip additives, are available from Byk, Degussa (Tego), Dow Corning, DuPont and many others.

Further additives, like inhibitors have already been discussed, and UV absorbers and HALS radical scavengers, applied in coatings designed to provide exterior stability, will be discussed together with their mode of action in Chapter 9.

4.5

CHARACTERIZATION OF RAW MATERIALS

Since the coating materials have to meet numerous requirements regarding performance, processing and storage consistency, they have to be well characterized in order to enable constant product quality. A good compilation of the various characterization methods available for coating raw materials is given in Handbook on Basics of Coating Technology.74 In this chapter some of the most relevant characterization methods for UV-curable raw materials are described briefly.

The most common characteristic values for the raw materials used in UV-curable systems are given in Table 4.7.

4.5.1

Viscosity

The viscosity of a coating formulation governs the film formation and surface characteristics, like the appearance of a film.

The viscosity of coating formulations or raw materials is measured by the kinematic viscosity, when flow is driven by gravity [measured in m2 /s or stokes (St): 1 m2 /s = 10,000 St]. A simple equipment to determine the kinematic viscosity often used in the coatings industry is a 100 cm3 beaker (DIN 53211 and ASTM D-4212) with defined nozzle geometry (e.g., 4 mm for DIN beaker 4). The flow time is determined and used as a measure for the viscosity. Today, more common is the dynamic viscosity, where viscosity is a measure of resistance of the fluid to flow, defined by the applied shear stress divided by shear rate. It is measured in a rheometer where the fluid is placed between two plates,

RAW MATERIALS

127

where one plate is moveable at different shear rates (velocity per thickness). The dynamic viscosity is measured in the SI unit Pascal seconds (Pa s = kg/(m s2 )) or Poise (1 Pa s = 10 P, 1 mPa s = 1 cP).

The dynamic viscosity can be calculated from kinematic viscosity by multiplication with density (kg/m3 ).

Ideal fluids exhibit Newtonian flow, thus the viscosity is constant for any applied shear stress. If the viscosity of fluids is increasing with shear stress, they are called dilatant; if it is decreasing, they are called thixotropic. The flow behaviour of fluids and coatings is well discussed in Chapter XIX of Organic Coatings, Science and Technology,75 and Basics of Coating Technology.76 Typical viscosities of reactive diluents at room temperature are in the range of 3 to 150 mPa s (water at 20 ◦ C 1 mPa s). The viscosity of oligomeric resins used in the 100% liquid formulations may vary in a larger range of 0.05 to 100 Pa s, where the polyether acrylates are at the lower limit (0.05–2 Pa s), whereas epoxy and urethane acrylates are in the higher viscosity ranges.

4.5.2 Chemical Characterization The total number of double bonds present in unsaturated fats and oils is often determined by the iodine value. It is generally expressed in terms of “number of grams of iodine that will react with the double bonds in 100 g of fats or oils”. Iodine solutions have a violet colour. In the reaction with a double bond, one iodine molecule will lose its colour. In iodine number determinations (titration) the quantity of iodine is determined which will just be decoloured by the fat or oil. This provides a direct measure of the number of double bonds present in the sample.

Not only unsaturated double bonds in fats and oils, but also unsaturated acrylate double bonds can be determined by the method of halogen addition. The reaction of Iodine bromide with the double bond results in the addition of bromine to the double bond and the formation of Iodine (DIN 53241-1). Excess iodine bromide, the remainder of the titration solution is then reacted with iodide to form also iodine, which is then titrated with thiosulfate. The iodine value is related to 100 g test sample and calculated by: Iodine value = (B − S)1.26/WS, where S is the consumption thiosulfate of the sample (ml), B is the consumption of the blank (ml) and WS is the weight of the sample (g).

Further methods to determine the double bond content are based on spectroscopy, for example, 1 H NMR, where the hydrogen atoms of the double bond (at 5–7 ppm chemical shift) are related to other prominent hydrogens.

To determine the acid number 1 g of the solid resin is dissolved in 50 ml acetone and titrated with 0.1 N KOH (in ethanol) with phenolphthalein as indicator, until the red colour stays for some seconds. The consumption of the KOH in acetone is determined as a blank value. The acid number is defined as the number of milligrams of KOH required to neu-

128

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

tralize 1 g of resin solids (DIN EN ISO 3682): Acid number = 5.61f (S − B)/WS, where f is the factor of the ethanolic KOH, S is the KOH consumption of the sample (ml), B is the consumption of the blank (ml) and WS is the weight of the sample (g).

The acid number of 56 corresponds therefore to 1 mmol carboxylic groups per 1 g of resin. The equivalent weight equals to 56,100/acid number.

The OH number 77 is determined by the titration of residual unreacted acetic anhydride left from the reaction of the free OH groups of the sample with an excess of acetic anhydride (DIN EN ISO 4629). For the determination about 2.5 g of the OH-containing resin are weighted into a 250 ml Erlenmeyer flask and dissolved in 10 ml pyridine. To this solution 10 ml of a fresh prepared acetylation liquid (prepared by 150 g acetic anhydride in 350 g dry pyridine) are added and reacted for 70 min at 110 ◦ C in a drying oven. After cooling down the solution is titrated with 1 N NaOH with Nilblauchloride as an indicator.

The blind value is also determined with 10 ml pyridine and 10 ml acetylation solution. The OH number is defined as the equivalent of mg KOH in 1 g of the solid resin: OH number = 56.1f (B − S)/WS, where f is the factor of the ethanolic NaOH, S is the NaOH consumption of the sample (ml), B is the consumption of NaOH in the blank (ml), and WS weighted sample (g).

4.5.3

Spectroscopic Characterization

The spectroscopic methods used to characterize the raw materials are based on the interaction of electromagnetic radiation with the molecules and recording the changes of the response parameters (e.g., extinction, transmission, absorption) at the corresponding frequency or wavelength.

UV spectra are based on the excitation of electrons from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) in the wavelength region of between 200 and 400 nm. In this region, only chromophores having π -electrons and heteroatoms having non-bonding electron pairs are absorbing. An isolated double bond, like ethene has an absorption maximum at 171 nm, thus only conjugated double bonds are absorbing in the UV region. UV spectroscopy is used to characterize the absorption spectra of the photoinitiators, the UV absorbers, the double bond containing monomers, as well as other additives, like pigments. Comparing the absorption spectra of the different ingredients in UV curable systems allows the tuning of the components in order to provide an effective UV curing process. UV spectra of radical photoinitiators are shown for example in Figure 4.24.

Infrared spectroscopy relies on the excitation of molecular vibrations in the wavelength region of 2.5 to 15 µm, more commonly given as wavenumbers (proportional to frequency) in the region of 4000 to 600 cm−1 . In this spectral region molecular dipoles are excited,

129

RAW MATERIALS

and the specific groups absorb at specific wavelength. The acrylate double bonds for example show distinct bands at 810 and 1410 cm−1 . These bands can therefore be used to characterize the unsaturated acrylate containing materials, but also to follow the curing process, the decrease of the double bonds during exposure. This latter method has been developed by Decker,78 known as real-time infrared spectroscopy (RT-IR), and used in order to characterize the curing process and kinetics.

Other stretching bands can be used for example to follow the photoaging process of coatings by monitoring the decay or increase of characteristic molecular bands, like: Decrease of stretching band N–H C–H C–N

cm−1

3345 2951 cm−1 1531 cm−1

Increase of stretching band OH C=O

3500 cm−1 1780 cm−1

Nuclear magnetic resonance (NMR) is based on the spin reversal of odd numbered species (1 H, 13 C, 15 N) in a magnetic field. The frequency of the spin reversal is bond specific and dependent on the electron shielding of the nucleus results, thus the neighbourhood of the nucleus determines the resonance frequency. This shift is determined relative to an internal standard, usually tetramethyl silane. The 1 H NMR, for example, can tell whether a proton is assigned for instance to a pure aliphatic carbon (around 1–3 ppm shift), to a carbon next to an ether oxygen (3–5 ppm shift), or to an acrylate double bond (5–7 ppm shift), which enables the determination of the chemical structure.

Another method to determine the structure of a compound or a mixture, as it is the case with most of the resins used in UV curing, is mass spectrometry (MS). Mass spectrometry79 is a powerful analytical technique that is used to identify unknown compounds, to quantify known compounds, and to elucidate the structure and chemical properties of molecules.

This technique measures the masses of individual molecules that have been converted into ions, i.e., molecules that have been electrically charged are ionized by a commonly used electron ionization (EI) source. Bombarding the gaseous sample molecules with a beam of energetic electrons generates ions. For those molecules that can be vaporized without decomposition, EI is often used to generate ions for mass analysis. However, ionization by electrons accelerated through a potential of 70 V is a highly energetic process that may lead to intensive fragmentation. To avoid this decomposition lower energy ionization techniques have been developed based on chemical and desorption ionization. Desorption ionization is a term used to describe the process by which a molecule is both evaporated from a surface and ionized although the exact mechanism may not be understood. Desorption/ionization methods based on this principle are “fast atom bombardment (FAB), secondary ion MS (SIMS), matrix assisted Laser desorption/ionization (MALDI)”. The bombardment of the sample with high velocity atoms, ions, fission fragments, or photons of relatively high energy, deposits an impact energy into the sample, either directly or via the matrix, and leads to both sample molecule transfer into the gas phase and ionization.

In order to determine the structure of molecules, a compilation of tables about the specific response of structural groups in 13 C and 1 H NMR, IR, MS, UV/VIS is available.80

130

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

Further techniques for the characterization of mixtures are based on combined chromatographic separation methods and molecular analysis, like gas chromatography and mass spectrometry (GC/MS).81 4.5.4

Chromatographic Characterization

Chromatographic methods are used to separate material mixtures by the different interaction of molecules transported in a mobile phase with a stationary phase. These specific interactions result in different retention times, which can be used to identify the substances, either by comparison with the retention times of the pure molecules or by combination with structure identification techniques, like IR or MS. In gas chromatography (GC) only low molecular weight samples can be analysed, which can be vaporized (without decomposition). Higher molecular weight compounds are better characterized with high performance liquid chromatography (HPLC), size exclusion chromatography (SEC) or superfluid chromatography (SFC), where supercritical carbon dioxide is used as the mobile phase.

In most cases, a combination of methods is very helpful in order to examine the composition of UV curable resins as well as reactive diluents. The reactive diluents available on the market, like 1,6-hexanediol diacrylate (HDDA), trimethylolpropane triacrylate (TMPTA) or pentaerythrol triacrylate (PETIA) are not as pure as the structure suggest. Methods in order to analyse reactive diluents, e.g., TMPTA, based on a joint chromatographic-spectroscopic approach in order to separate, quantify and identify every component have been published.82 The authors showed that TMPTA, for example, produced by the azeotropic esterification of trimethylolpropane with acrylic acid may contain several by-products, like TMPTA-acrylic acid (Michael) adduct, and several other Michael adducts of incompletely esterified TMP products, adding with the alcohol group to acrylated compounds, like TMPTA-TMP-diacrylate (TMP-DA) adduct, TMP-monoacrylate on TMPTA or TMP-DA (Figure 4.20). These addition reactions result in the formation of higher functional molecules, and contribute also to variations in viscosity. In radiation curable systems, these by-products also participate in the curing reaction; however, a good understanding of the composition of the mixture is very helpful in optimization of performance, for example crosslink density, or evaluation of toxicological properties of such monomers (irritation, sensitization).

4.5.5

Thermal Analysis

The thermal analysis of raw materials analyses the temperature dependent conversions of the materials, which can be used for the analysis of transition temperatures (melting, boiling, glass transition), for evaluation of safety features (specific heat evolution or storage stability (isothermal heating at the desired temperature), as well as for detecting decomposition reactions. The common methods used are differential thermal analysis (DTA) or differential scanning calorimetry (DSC). The DTA compares the temperature changes occurring during heating of a sample and a reference and records the temperature difference as a function of temperature at which the differences occur, whereas in the DSC the tem-

131

RAW MATERIALS

TABLE 4.8. Selection criteria for raw materials to fulfill wood application requirements Wood coatings General requirements: high abrasion resistance Resin class

Reactive diluents

Epoxy acrylates Amine modified PE or PO acrylates Urethane acrylates

High functionality: DPHA, PTA

Photoinitiators

Additives Fillers (e.g., corundum)

Special requirements: adhesion (primer) Resin class

Reactive diluents

Water-soluble or dispersed resins

Photoinitiators

Additives

IrgacureÂŽ 2959

perature of sample and reference is kept at the same level and the difference in heat flow in order to compensate the temperature differences is recorded.

The thermogravimetric analysis (TGA) records the weight fluctuation, normally weight loss during heating as a function of temperature. With these measurements impurities within the sample (loss of solvent, additives, etc.), the drying behaviour of formulations or decomposition of the materials can be followed.

4.5.6 Particle Size Measurement The particle size of the dispersions can be measured with standard scattering (laser scattering) or acoustic methods, instruments available from Malvern83 or Quantachrome,84 for example.

4.6 SELECTION OF RAW MATERIALS The right selection of raw materials is a crucial issue in order to meet the desired coating property requirements. An important role is attributed to the resins, since the resin is to a high degree responsible for the basic properties of the coating. But also the selection of the reactive diluent (e.g., functionality), as well as the photoinitiators (absorption, solubility) and additives (UV protection, matting) are important. Some general remarks about the selection criteria of raw materials for wood applications (Table 4.8) and graphic applications (Table 4.9) are summarized in the Tables; further selection criteria are given in the formulation Chapter 5.

4.6.1 Raw Materials for Exterior Applications In many coatings applications the use of light stabilization is already state-of-the-art.85 However, the belief that UV curing cannot be combined with light stabilization is still

132

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

TABLE 4.9. Selection criteria for raw materials to fulfill graphic application requirements Graphic arts General requirements: high reactivity Resin class

Reactive diluents

Epoxy acrylates Amine modified PE or PO acrylates

High functionality: DPHA, PTA

Photoinitiators

Additives

Additives

Special requirements: Overprint varnishes Resin class

Reactive diluents

Photoinitiators

All Epoxy acrylates PO acrylates

TPGDA

Non-yellowing type: Irgacure® 184 Lucirin® TPO

Special requirements: UV inks Resin class

Reactive diluents

Photoinitiators

Additives

Grinding resins, all

High functionality, HDDA, TMPTA, PTA, DPHA

Acylphosphine oxides: Lucirin® TPO Irgacure® BAPO

Colour pigments

prevalent, despite literature86 to the contrary. Recent results have shown, that by carefully formulating photoinitiators and complimentary light stabilizers, coatings can be obtained which show curing properties that are analogous to UV coatings without light stabilizers. In general the use of light stabilizers not only leads to a reduced yellowing behavior but also results in greater durability of the coatings. The first step is the selection of resins, which do not contain chemical structures that are destroyed by UV light nor exhibit tendency towards yellowing or cracking under typical weathering conditions.

4.6.1.1 Selection of resin types for outdoor use87 The applications of UV-curable systems for outdoor use88 put higher demands on the raw materials, especially with respect to yellowing and weatherability. The weathering and yellowing behaviour of a variety of different radiation curable resin classes has been evaluated under thermal as well as photochemical stress. The four common acrylate resin classes were tested: epoxyacrylates (EA), polyether acrylates (PO), polyester acrylates (PE) and urethane acrylates (UA). Apart from the epoxyacrylates, the other resin types were chosen such that they are predominantly aliphatic and contained only small amounts of aromatic species, if any. In the case of the epoxyacrylates aromatic systems were tested because aliphatic epoxyacrylates are of only limited commercial importance. In general, when considering outdoor applications aromatic systems are disadvantageous as they have a stronger tendency to yellow and degrade photochemically due to their stronger absorption of light in the ultraviolet region. Apart from the four common acrylate resin classes amine-modified systems were also tested to determine the influence of amine functionalization on the weathering characteristics.

RAW MATERIALS

133

F IG . 4.30. Thermal yellowing of UV-resins at different temperatures (according to CIE L∗ a ∗ b∗ colour space; b∗ = negative values indicate blue, positive values yellow).

4.6.1.2 Thermal yellowing Coatings for outdoor use may also be exposed to relatively high thermal stresses, depending on the climatic conditions and the exposure to sun light.

To simulate these stresses under accelerated conditions 100 µm thin sample films of the different resins were stored at elevated temperatures for 48 h (Figure 4.30). Up to temperatures of 60 ◦ C only relatively small changes in colour could be detected which didn’t allow a significant differentiation of the samples. At temperatures above 80 ◦ C the amine modified products show a distinctly stronger tendency towards yellowing than the other resins. The polyether-, polyester- and epoxy-acrylates discolour moderately while the urethane acrylate shows the best temperature colourfastness. The thermal yellowing of the amine modified systems seems to be the result of oxidative processes, since the heating of the UV cured samples under an inert atmosphere (Figure 4.31) show a less pronounced yellowing of all resin classes, whereas the amine modified systems show a dramatic increase in discoloration in air. Therefore, the relatively strong thermal yellowing observed for amine modified systems could be explained by an oxidation at the amine nitrogen. Of all the radiation curing resins, amine modified acrylate resins are certainly the least suitable for coatings which experience high thermal stresses.

4.6.1.3 Photochemical yellowing For outdoor applications, the yellowing under sunlight conditions plays a major role. Therefore, the different radiation curable resin classes were submitted to dry (without humidity) SUN-test conditions, reflecting the exposure to the natural sunlight spectrum. Whereas for the evaluation of the yellowing behaviour it is sufficient to follow the changes in the b∗ value, the total colour variation is normally

134

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 4.31. Thermal yellowing under inert and air atmosphere.

F IG . 4.32. Accelerated weathering tests on UV resins with 2% UV absorber and 1% HALS light stabilizers ( E = 1 is slightly visible; E < 1 required for automotive clear coats).

expressed with the E value: E =

( L∗ )2 + ( a ∗ )2 + ( b∗ )2 ,

(4.6.1)

where L is the brightness and a and b are the red–green and the yellow–blue deviations, respectively.

135

RAW MATERIALS

TABLE 4.10. Selection criteria for raw materials to fulfill exterior application requirements Exterior applications General requirements: high weathering stability, non-yellowing Resin class

Reactive diluents

Photoinitiators

Additives

Urethane acrylates

Hydrocarbon type, HDDA, DDDA

Non-yellowing type: Irgacure® 184, Lucirin® TPO

UV absorbers, HALS

Special requirements: high scratch resistance Resin class

Reactive diluents

Photoinitiators

Additives

Higher functional urethane acrylates

High functionality, HDDA, TMPTA, PTA, DPHA

Non-yellowing type: Irgacure® 184, Lucirin® TPO

UV absorbers, HALS

Special requirements: high flexibility Resin class

Reactive diluents

Photoinitiators

Additives

Difunctional urethane acrylates with high molecular weight

Monofunctional monomers

Non-yellowing type: Irgacure® 184, Lucirin® TPO

UV absorbers, HALS

As shown in Figure 4.32, where the different radiation curable resins are formulated with 2% UV-absorber and 1% of a radical scavenger, the (aromatic) epoxy acrylates remain the least colour-fast of all the tested resin classes. The polyether acrylates show good durability and low yellowing with light stabilizers, whereas the light stabilized amine modified polyether acrylates yield equally good results, comparable with the analogous, unstabilized coatings, but no better. This observation suggests that amine modification may also have a light stabilizing effect on polyethers. By far the best durability and very good colour fastness is displayed by the urethane acrylates, which outclass all other UV resins by at least a factor of 5 in their durability.

In conclusion, resins containing amines show a significant yellowing under thermal stress, while aromatic systems display strong photochemical yellowing. Conventional polyether and polyester acrylates show only limited stability in the presence of moisture so that only the aliphatic urethane acrylates can be recommended unconditionally for exterior applications.

Table 4.10 summarizes the selection criteria for raw materials to be used in exterior applications.

4.7 RAW MATERIAL SUPPLIERS Some representative links to raw materials suppliers are given in the following list, which does not raise a claim on completeness (Table 4.11).

136

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

TABLE 4.11. Raw material supply Company

Link

Resin families

Photoinitiators

BASF AG

www.basf.de/en/produkte/ farbmittel/farben/pigmente/coatings/ holz/harze

Laromer®

Lucirin®

Bayer AG

http://www.bayerls.com/ls/lswebcms.nsf/id/030100_EN

Desmolux® Roskydal®

Byk

www.byk-chemie.com

Ciba

Additives

Byk® Irgacure®

http://www.cibasc.com/index/ ind-index/ind-paints_and_coatings.htm

Darocure®

Cognis

www.cognis.com/framescout.html?/ BusinessUnits.html

Photomer®

CrayValley

http://www.crayvalley.com/upload/ hTechnical_Brochure_Photocures.pdf

Craynor® (CN)

Cytec Surface Specialties

http://www.surfacespecialties.com/ radcureproducts2.htm

Ebecryl® Omnipol®

Beijing Insight http://www.insight.net.cn/ High Techn.

asp-bin/EN/index.asp?page=8&id=146

Eternal

www.eternal.com.tw/english/html/ products/Chemical.htm

Etermer®

Rahn AG

http://www.rahn.ch/site/index__144gast-e-urlvars-level-3-topnav-136.html

Genomer®

Sartomer

http://www.sartomereurope.com/prod.asp

Sartomer® (SR)

Tego

http://www.tego.de/en/prod/prod2/prod8.html

Genocure®

Tego® Rad

REFERENCES 1. Allen, N.S. and Edge, M., UV and electron beam curable pre-polymers and diluent monomers: Classification, preparation and properties, In “Radiation Curing in Polymer Science and Technology”, Vol. I (J.P. Fouassier and J.F. Rabek, eds.). Elsevier Applied Science, London–New York, 1993, Chapter 5, pp. 225–262.

2. Udding-Lourier, S., Baijards, R.A. and Feima, N. W., RadTech Europe 1999, Conference Proceedings. 1999, pp. 607–613.

3. Burget, D., Mallein, C., Mauguiere-Guyonnet, F., Fouassier, J.P., Varelas, C.G., Apostolatos, S., Charalambopoulou, G., Manea, M. and Lundmark, S., RadTech Europe 2003, Conference Proceedings, Vol. I. 2003, pp. 57–62.

4. According to US 4,072,592, Mobile Oil Corporation.

5. According to US 3,700,643, Union Carbide Corporation.

6. Stone, V.W., Burie, P., van den Bergen H. and van Holen, J., Novel routes to urethane acrylates, RadTech North America, UV&EB Technology, Charlotte, NC, May 2004, Technical Conference Proceedings. 2004, on CD.

RAW MATERIALS 7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

17.

18.

19.

20.

21.

22.

23.

24.

25.

26.

27.

28.

29.

30.

31.

32.

33.

34.

35.

36.

37.

38.

137

WO 2004/050888 (BASF).

According to Oke, R.A. and Wragg, R.T., Chem. Ind. 597 (1970). www.curedisease.com/Perspectives/vol_1_1989/Problems%20with%20the%20Draize.html.

Allen, N.S., Lo, C.K. and Salim, M.S., RadTech Europe 1989, Florence. (a) Hult, A., Adv. Polym. Sci. 143, 1–34 (1999); (b) Frechet, J., J. Macromol. Sci., Pure Appl. Chem. 33, 1399 (1996). (a) James, D., Hyperbranched polymers for hardcoat with superior performance, Proceedings, RadTech Asia 2005, Shanghai, China. 2005, p. 139–145; (b) James, D., European Coatings Conference, Parquet Coatings II, 14–15, November 2002, Berlin.

Hult, A., Malstroem, E. and Johansson, M., Polym. Mater. Sci. Eng. 72, 528 (1995).

Dunjic, B., Tasic, S. and Bozic, B., Eur. Coat. J. 06/04 36–41 (2004).

Asif, A. and Shi, W., Photopolymerization and properties of waterborne polyurethane acrylate dispersions based on hyperbranched aliphatic polyesters, RadTech Asia 2005, Proceedings. 2005, pp. 146–154.

Frings, R.B. and Wend, M., DIC Technical Rev. 9, 43–51 (2003).

Xu, D.-M., Zhang, K.-D. and Zhu, X.-L., J. Appl. Polym. Sci. 92, 1018–1022 (2004).

Schwalm, R., Acrylate functionalized polymers in coating applications, Proceedings of the American Chemical Society, Division of Polymeric Materials: Science and Engineering, Vol. 84, April 2001, San Diego.

2001, pp. 3380–3381. (a) Jönsson, S., Schaeffer, W., Sundell, P.E., Shimose, M., Owens, J. and Hoyle, C.E., Proc. RadTech Conference Orlando, 1994, p. 194; (b) Shimose, M., Hoyle, C.E., Jönsson, S., Sundell, P.E., Owens, J. and Vaughn, K., ACS Polym. Mater. Sci. Eng. 72, 470 (1995); (c) Shimose, M., Hoyle, C.E., Jönsson, S., Sundell, P.E., Owens, J. and Vaughn, K., Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 32 (2), 485 (1995); (d) Jönsson, S., Sundell, P.E., Hultgren, J., Sheng, D. and Hoyle, C.E., Progr. Org. Coat. 27, 107 (1996); (e) Jönsson, S., Ericsson, J.E., Sundell, P.E., Shimose, M., Clark, S.C., Miller, C., Owens, J. and Hoyle, C.E., Proc.

RadTech Conf., Nashville, 1996; (f) Jönsson, S., Doucet, G., Mattson, G., Clark, S., Miller, C., Hoyle, C.E., Morel, F. and Decker, C., Proc. RadTech Europe Conf. 1997, p. 103; (g) Hoyle, C.E., Clark, S.C., Jönsson, S. and Shimose, M., Polymer 38, 5695 (1997).

Decker, C., Morel, F., Jönsson, S., Clark, S. and Hoyle, C., Macromol. Chem. Phys. 200, 1005–1013 (1999).

Gould, M.L., Narayan-Sarathy, S., Hammond, T.E. and Rechter, R.B., RadTech Europe 2005, Conference Proceedings, Vol. I, pp. 245–251.

Ceska, G.W., Leroy, C. and Schaeffer, B., RadTech Europe 2005, Conference Proceedings, Vol. I. 2005, pp. 255–260. http://www.microresist.de/poster_ormocer.pdf. http://www.fluorochemsilanes.co.uk/commercial%20scale.htm.

Decker, C., Macromol. Rapid Commun. 23, 1085 (2002). (a) Decker, C. and Nguyen Thi Viet, T., Macromol. Chem. Phys. 200, 358 (1999); (b) Decker, C. and Nguyen Thi Viet, T., Macromol. Chem. Phys. 200, 1965 (1999); (c) Decker, C. and Nguyen Thi Viet, T., Polymer 41, 3905 (2000).

Decker, C. and Nguyen Thi Viet, T., J. Appl. Polym. Sci. 82, 2204 (2001).

EP 1144476 and US 6617413.

Ludewig, M., Stöckel, N. and Weikard, J., RadTech Europe’05, Conference Proceedings, Vol. I. 2005, pp. 263–268. (a) Tielemans, M., Roose, P., DeGroote, P. and Vanovervelt, J.-C., Progr. Org. Coat. 55, 128–136 (2006); (b) Masson, F., Decker, C., Jaworek, T. and Schwalm, R., Progr. Org. Coat. 39, 115–126 (2000); (c) Decker, C., Masson, F. and Schwalm, R., Polym. Degrad. Stab. 83 (2), 309–320 (2004).

“Polymer Exemption Guidance Manual”. http://www.epa.gov/oppt/newchems/pubs/polyguid.pdf. (a) Fried, M.J., Farbe&Lack 100 (8), 604–609 (1994); (b) Zweifel, H., “Stabilisation of Polymeric Materials”.

Springer-Verlag, Berlin, 1998.

King III, R.E., “Introduction to Polymer Stabilization”. ACS Chemical Society, San Diego, CA, 2001.

Schwetlick, K., Pure Appl. Chem. 55, 1634 (1983).

Patent WO 90/07485.

Breese, K.D., Laméthe, J.-F. and DeArmitt, C., Polym. Degrad. Stab. 70, 89–96 (2000).

Hinsken, H., Mayerhöfer, H., Müller, W. and Schneider, H.J., US Patent 4,325,863.

Zweifel, H., “Stabilization of Polymeric Materials”. Springer-Verlag, Berlin, 1998, pp. 54–56.

138

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

39. Levy, L.B., J. Polym. Sci., Part A: Polym. Chem. 30, 569–576 (1992).

40. White, T.J., Liechty, W.B. and Guymon, C.A., RadTech 2006, Conference Proceedings, Chicago, IL. 2006, on CD. 41. Schwalm, R., Binder, H., Funhoff, D., Lokai, M., Schrof, W. and Weiguny, S., Vinyl ethers in UV curing: Copolymers with acrylates and unsaturated polyesters, RadTech Europe 1999, Conference Proceedings.

1999, pp. 103–109.

42. Schrof, W., Binder, H., Schwalm, R. and Weiguny, S., Cationic UV curing: Benefits of formulating epoxides and vinyl ethers together, RadTech Europe 1999, Conference Proceedings. 1999, pp. 99–102.

43. Decker, C. and Decker, D., Pure Appl. Chem. 34 (4), 605–625 (1997).

44. Crivello, J.V. et al., “Photoinitiators for Free Radical, Cationic & Anionic Polymerization”, Vol. III (Bradley, ed.), 2nd edn. J. Wiley and Sons, 1998, p. 327.

45. Fouassier J.P., “Photoinitiation, Photopolymerization, and Photocuring”. Hanser Publishers, Munich– Vienna–New York, 1995.

46. Davidson, R.S., J. Photochem. Photobiol. A: Chem. 73, 81–96 (1993).

47. Lalevée, J., Allonas, X., Jradi, S. and Fouassier, J.P., Macromolecules 39, 1872–1879 (2006).

48. Kirchmayr, R., Berner, G., Huesler, R. and Rist, G., Farbe&Lack 88, 910 (1982).

49. Fuchs, A., Bolle, T., Ilg, S. and Hüsler, R., New raw materials for UV inks and varnishes, RadTech Europe 2003, Conference Proceedings, Vol. I. 2003, pp. 507–511.

50. Macromolecules 31, 322–327 (1998). EP 956280 (Ciba).

51. Carlini, C. and Angiolini, L., Polymeric photoinitiators, In “Radiation Curing in Polymer Science and Technology”, Vol. II (J.P. Fouassier and J.F. Rabek, eds.). Elsevier Applied Science, 1993, Chapter 5, pp. 283–321.

52. Shidaqn, H., Wenchao, Z., Lixiu, Y., Bertens, F. and Qingjin, Y., Proceedings, RadTech Asia, 2005, Shanghai, China. 2005, pp. 61–70.

53. Crivello, J.V., In “Developments in Polymer Photochemistry-2” (N.S. Allen, ed.). Appl. Sciences Pub., London, 1981, pp. 1–38.

54. Casatelli, L.M., Amer. Ink Maker 79, 18 (2001).

55. Fouassier, J.P., “Photoinitiation, Photopolymerization and Photocuring”. Hanser Publisher, München, 1995, pp. 157–160.

56. Carter, W. and Jupina, M., Tougher cycloaliphatic epoxide resins, RadTech Europe 2003, Conference Proceedings, Vol. I. 2003, pp. 243–248.

57. Schrof, W., Binder, H., Schwalm, R., Weiguny, S., Chomienne, F. and Carroy, A., Cationic UV curing: Benefits of formulating epoxies and vinyl ethers together, RadTech Europe 1999, Conference Proceedings. 1999, pp. 95–102.

58. Carroy, A., Chomienne, F. and Nebout, J.F., RadTech Europe 1999, Conference Proceedings. 1999, pp. 499– 505.

59. Sangermano, M., Di Gianni, A., Bongiovanni, R., Priola, A. and Voit, B., RadTech Europe 2005, Vol. II. 2005, pp. 241–247.

60. Fouassier, J.-P. (Ed.), “Photoinitiation, Photopolymerization, and Photocuring – Fundamentals and Applications”. Hanser Publisher, Munich, 1995, Chapter 4.

61. RadTech Regulatory Alert 1, 1 (2000).

62. Crivello, J.V., Kong, S., Jiang, M. and Gomurashvili, Z., RadTech North America, UV&EB Technology, Charlotte, NC, May 2004, Technical Conference Proceedings. 2004, on CD. 63. Visconti, M., Cattaneo, M. and Norcini, G., RadTech North America, UV&EB Technology, Charlotte, NC, May 2004, Technical Conference Proceedings. 2004, on CD. 64. (a) Crivello, J.V., In “Ring-Opening Polymerization” (D.J. Brunelle, ed.). Hanser, Munich, 1993, p. 157; (b) Sahyun, M.R., DeVoe, R.J. and Olofson, P.M., In “Radiation Curing in Polymer Science and Technology” Vol. II (J.P. Fouassier and J.F. Rabek, eds.). Elsevier, New York, 1993, p. 505.

65. (a) Pappas, S.P., Progr. Org. Coat. 13, 35 (1985); (b) Crivello, J.V., Adv. Polym. Sci. 62, 1 (1984).

66. Hoyle, C.E., Lee, T.Y. and Roper, T.M., J. Polym. Sci.: Polym. Chem. Ed. 42, 5301 (2004).

67. Jacobine, A.F., In “Radiation Curing in Polymer Science and Technology” Vol. 3 (J.P. Fouassier and J.F. Rabek, eds.). Elsevier Applied Science, 1993, pp. 219–268.

68. Decker, C., Macromol. Rapid Commun. 23 (18), 1067–1094 (2002).

69. Hoyle, C., Clark, T., Lee, T.Y., Roper, T., Pan, B., Wei, H., Zhou, H. and Lichtenhan, J., RadTechEurope 2005, Conference Proceedings, Vol. 1. 2005, pp. 289–293.

RAW MATERIALS

139

70. Hoyle, C.E., Roper, T., Pan, B., Nason, C., Lee, T.Y. and Turner, T., Advances in the polymerization of Thiol-Ene formulations, RadTech Europe 2003, Conference Proceedings, Vol. I. 2003, pp. 573–578.

71. Hoyle, C.E., Lee, T.Y. and Roper, T., J. Polym. Sci. A Polym. Chem. 42, 5301–5338 (2004).

72. Gush, D.P. and Ketley, A.D., Mod. Paint Coat. 68, 61 (1978).

73. Oestreich, S. and Struck, S., Macromolecular Symposium 187, 333–342 (2002). Adler, H.-J.P. and PotjeKamloth, K. (Eds.), “Quo Vadis Coatings?” Wiley–VCH, Weinheim, Germany, 2002.

74. Goldschmidt, A. and Streitberger, H.-J., “BASF Handbook on Basics of Coating Technology”. Vincentz Network, 2003, Chapter 2.3, p. 254.

75. Wicks, Jr. Z.W., Jones, F.N. and Pappas, S.P., “Organic Coatings, Science and Technology, Vol. 2: Application, Properties, and Performance”. John Wiley & Sons, Inc., 1994.

76. Goldschmidt, A. and Streitberger, H.-J., “BASF Handbook on Basics of Coating Technology”. Vincentz Network, 2003, p. 274.

77. Houben-Weyl 14 (2), 17 (1963).

78. (a) Decker, C. and Moussa, K., Makromol. Chem. 189, 2381 (1988); (b) Decker, C. and Moussa, K., J. Coat.

Technol. 62, 55 (1990).

79. http://www.asms.org/whatisms/p11.html.

80. Pretsch, E., Seibel, J., Simon, W. and Clerc, T., “Tabellen zur Strukturaufklärung organischer Verbindungen”.

Springer-Verlag, 1986.

81. van den Berg, J.D.J., van den Berg, K.J. and Boon, J.J., Progr. Org. Coat. 41, 1443 (2001).

82. Hall, R.H., van der Maeden, F.P.B. and Willemsen, A.C.C.M., Just how pure are monomers? A chemical analysis of some common reactive diluents, RadTech 1985, pp. 57–64.

83. http://www.malvern.co.uk/home/index.htm.

84. http://www.quantachrome.de/partikelgroessen/index_dispersion.html.

85. Böhnke, H. and Hess, E., Farbe & Lack, 95 (10), 715–719 (1989).

86. (a) Gatechair, L.R., In “UV Curing: Science and Technology” (S.P. Pappas, ed.). Technology Marketing Corporation, 1985, pp. 283–323; (b) Misev, L., Schmid, O., Udding-Louwrier, S., de Jong, E.S., Bayards, R., J. Coat. Technol. 71 (891), 37–44 (1999).

87. Beck, E., Enenkel, P., Königer, R., Lokai, M., Menzel, K., Schwalm, R., Weathering of radiation curable coatings, RadTech Europe 1999, Conference Proceedings. 1999, pp. 531–536.

88. Fouassier, J.-P. (Ed.), “Photoinitiation Photopolymerisation and Photocuring”. Hanser Publishers, 1995, p. 298.

C HAPTER 5

Formulations Formulations used for radiation curable systems depend on the specific performance requirements of the coatings and have to be adjusted to the application techniques (e.g., viscosity needs). Typically, formulations for UV curable coatings contain 25–90% oligomeric resins, 15–60% reactive diluents, 0.5–8% photoinitiators, 1–5% additives, like levelling agents, defoamers, and optionally pigments, fillers and matting agents.

The application fields are very wide and therefore the requirements on the coating properties differ very much. A coarse overview of main applications is given in Figure 5.1. From this broad span of applications it is obvious that formulations also differ very much in their chemistry and composition. Classical applications in wood and paper coating often require highly crosslinked coatings for abrasion, scratch and chemical resistant surfaces. Whereas for future applications, such as exterior wood coating, like window frames, or coil coating

F IG . 5.1. Application overview of uses of UV curable formulations.

140

141

FORMULATIONS

TABLE 5.1. General composition of wood coatings and function of the components Component

(%)

Function

Oligomer resin

25–95

Film formation Basic performance properties

Reactive diluent (monomers)

0–60

Film formation Viscosity adjustment

Photoinitiator

1–5

Initiation of curing; speed

Fillers/pigments

0–50

Increased abrasion resistance Increased scratch resistance Cost reduction (e.g., talc) Coloration

Matting agents Additives

Gloss reduction 0–3

Defoaming Wetting, levelling, slip, . . .

much more flexible coating properties are needed. A general discussion about all types of coatings for metal and non-metal substrates including major performance requirements is given in reference1 (Chapters XXXIII and XXXIV).

Formulations available to the paint users are developed and tailored by the paint manufacturers (e.g., BASF Coatings, DuPont Performance Coatings, Kansai Paint, PPG for automotive; AKZO, Beckers, DuPont, Valspar, Sherwin Williams for industrial and wood coatings; Dainippon Inks, Sun Chemicals and Flint for graphic arts), who usually do not publish the exact recipes. However, for starting new developments guiding formulations are often made available by raw material producers (BASF AG, Cognis, Cytec Surface Specialities, Eternal and Sartomer) and Paint Research Institutes. Despite published already more than 20 years ago, a basic understanding of UV curable formulations for coatings, inks and paints has been compiled by Holman.2 The purpose of this chapter is to give a brief insight into the formulations for the classical applications and discuss the development of formulations for future applications, such as in food packaging, as well as in exterior uses in automotive, wood or coil coating.

5.1

FORMULATIONS FOR WOOD COATINGS

Examples given below are formulations of high gloss topcoats on wood surfaces. In the past styrene based unsaturated polyesters were used which are still common in some countries, but the more versatile and faster curable acrylics are becoming dominant. Polyester or epoxy acrylates either alone or in combination in amounts up to 60%, diluted with 35% of monomers are mainly used for wood coatings. For parquet and flooring applications the tougher and more abrasion resistant urethane acrylates along with monomers are also employed.

A general formulation for wood coatings is shown in Table 5.1, further explaining the typical functions of the ingredients.

142

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

TABLE 5.2. Layer composition of parquet flooring coating Layer

Approx. amount (g/m2 )

Curing

1. Waterbased adhesive primer 2. Undercoat (ctg., corundum) 3. Undercoat (ctg., corundum) 4. Undercoat 5. Top coat

10 20 20 20 5–10

Gellation (incomplete cure) Gellation Gellation Fully cured + grinding Fully cured

TABLE 5.3. Composition of a waterbased adhesion primer for wood Component

(%)

Type

Oligomer resin

50

Emulsion or dispersion Photoinitiator Additives

50 1–3 1–3

Watersoluble 100% UV resin; aliphatic epoxy acrylate, polar polyester acrylate or urethane acrylate Polyester acrylate emulsion (50% solids) All Defoaming, wetting

5.1.1

Industrial Parquet Coating

In order to be able to meet the various requirements of parquet flooring, the construction of prefabricated (multilayer) parquet needs a combination of different layers. The composition of these layers depends on the production line available as well as on the substrate quality. Regardless of the solution chosen, it has to guarantee good adhesion to the substrate, good interlayer adhesion, low abrasion as well as high scratch resistance and high resistance against chemicals, like cleaning agents, red wine, coffee and other typical household chemicals.

A typical coating layer construction is shown in Table 5.2.

5.1.2

Adhesion Primer (to the Wooden Substrate)

In order to enable a good adhesion to the wooden substrate, mainly waterbased UV curable primers are used. The main function of this waterbased primer is swelling of the wood, which results in the erection of wood fibres, further increasing the available surface and favouring an entanglement of the wood fibres by the coating to improve the adhesion.

A typical formulation used for waterbased primers is given in Table 5.3.

In order to improve the interlayer adhesion two strategies are pursued, the incomplete curing of the primer, which results in a gellation of the layer, but only to such an extent, that there are sufficient double bonds remaining for the reaction with the next UV curable layer, and second the grinding of the primer layer in order to increase the roughness and therefore the surface area of the layer.

143

FORMULATIONS

TABLE 5.4. Topcoat formulations Component

Oligomer resin

Monomer containing

Monomer free

(%)

Type

(%)

Type

55

Epoxy acrylate, Laromer® LR 8986

43

Epoxy acrylate, Laromer® LR 8986 Polyetheracrylate, Laromer® LR 8967

43 Diluent

26.5

Tripropylenegylcoldiacrylate (TPGDA)

Matting agents

10

7 Talcum 10MOOS 3 Syloid® 162C

Wax

3

Ceraflour® 950

Photoinitiator

3

Irgacure® 500

Synergist

2

Amine-synergist, Laromer® LR 8956

Additives

0.5

Defoamer, Tego® Airex 920

– 10

Syloid ED 80

3.5

Iragcure® 184

0.5

Levelling agent, Byk 361

In the production of top-quality parquet up to three undercoat layers are used. The first two can be formulated with very hard abrasive fillers (e.g., corundum) and are only cured to a low conversion, whereas the third one does not contain the filler. After full curing of the three layers, a consecutive grinding step to prepare the surface for topcoat application is added, where fillers in the third layer would destroy the grinding belt. Finally, the topcoat finishes the construction of the parquet coating.

Topcoats for parquet can be based on the combination of trifunctional polyether and polyester acrylate resins,3 as well as on epoxyacrylates, which provide high chemical stability. Besides the high gloss formulations also matt top coats are used.

Although matt formulations are difficult to obtain with UV curable 100% liquid formulations, formulations have been developed which yield topcoats with matt appearance (Table 5.4). Such formulations developed for roller coater applications contain matting agent (silicagel and talc), small amounts of solvent can also be used. Such formulations for example are applied in a thickness of about 8–12 µm (8–12 g/m2 ), and exposed to a 120 W/cm lamp at a conveyor belt speed of 5 m/min. The resulting abrasion resistant coating exhibit gloss values of about 30% (Gardner, 60◦ ).

Further starting formulations for obtaining high scratch or abrasion resistance have been published.4,5 Special formulations for obtaining high scratch resistance (on beech wood, for example) use large amounts of highly functionalized resins6 or are formulated with nanoparticles (Table 5.5).

144

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

TABLE 5.5. Highly scratch resistant top coat formulations Component

(%)

Type

Formulation based on high functional acrylates Dipentaerythrolpenta-acrylate Propoxylated neopentylglycol-diacrylate Urethane acrylate Photoinitiator Photoinitiator Oligomer resin

Sartomer® SR 399 Sartomer® SR 9003 Sartomer® CN 965 Benzophenone Irgacure® 184

57 28.5 9.5 3 2

Formulation based on nanoparticles 30

Diluent

22

Nanoparticles (dispersed in resin) Matting agent Photoinitiator Additives

35 9 3.5 0.4 0.4

Epoxy acrylate, Laromer® LR 8986 Polyetheracrylate, Laromer® LR 8967 Laromer® PO 9026V Syloid® ED 80 Irgacure® 184 Levelling agent, Byk 361 Defoamer, Tego® Airex 920

TABLE 5.6. Composition of a typical printing ink formulation Component

(%)

Function

Oligomer resin

40–60

Film formation Basic performance properties

Reactive diluent

10–20

Film formation Viscosity adjustment

Photoinitiator

3–8

Initiation of curing; speed

Pigments or dyes

15–25

Colour image formation

Additives

0–3

Surfactants Waxes

5.2 FORMULATIONS FOR GRAPHIC INKS UV curable systems used in graphic arts applications are divided into the categories of printing inks, containing pigments or dyes, and clear coat overprint varnishes (OPV).

Ink formulations have to deal with two main requirements, the performance requirement of the printing process, like offset, letterpress or screen printing, which for instance require different pigmentation and viscosities and the performance requirement of the final printed image.

145

FORMULATIONS

TABLE 5.7. Beige pigmented UV ink Component

(%)

Function

Oligomer resin

75

Polyesteracrylate (Laromer® LR 8800)

Reactive diluent

8.82 4.4

Hexanediol diacrylate Trimethylolpropane triacrylate

Photoinitiator

0.75

Bisacylphosphineoxide (Irgacure® 819, Ciba)

Photoinitiator

1.23

Irgacure® 184 (Ciba)

Pigment

8.82

Titanium dioxide, Rutil

Pigment

0.98

Isoindoline (PY 110; Irgazingelb RTL, Ciba)

TABLE 5.8. UV offset ink UV offset ink components

Yellow (wt%)

Magenta (wt%)

Cyan (wt%)

Black (wt%)

Pigment Permanent yellow GR 01 Litho Rubin D 4574 DD Heliogen® Blue D 7092 Special black 250

15.8 – – –

– 17 – –

– – 17 –

– – 2 15

Resin Laromer® LR 9013 Laromer® LR 9004 Laromer® LR 8986

18.5 40 15.7

20 20 33

20 43 10

20 43 10

4 4 2 100

4 4 2 100

4 4 2 100

4 4 2 100

44 180 40

39 170 60

45 175 55

40 100 50

Photoinitiator Lucirin® TPO-L Irgacure® 369 Irgacure® 907 Viscosity (Pa s, 23 ◦ C) Yield (Pa, 23 ◦ C) Reactivity (m/min; 120 W/cm)

5.2.1 Printing Inks (UV Offset, UV Flexo and UV Screen Inks) The film thicknesses of the printing inks are in the range of up to 3 µm for offset and flexo inks and up to 15 µm for screen inks. The printing processes are discussed in Chapter 8.1.2.

A general formula for printing inks contains about 40 to 60% resins, and between 15 and 25% of colorant (pigment or dye) as shown in Table 5.6.

A specific example of beige pigmented UV ink is given in Table 5.7 (ref. 3).

The pigmented UV coating relies on a mixture of photoinitiators, where the short wavelength absorbing Irgacure® 184 is responsible for surface cure and the longer wavelength

146

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

TABLE 5.9. UV flexo ink UV flexo ink components

Yellow (wt%)

Magenta (wt%)

Cyan (wt%)

Black (wt%)

Pigment Permanent yellow GR 01 Litho Rubin D 4574 DD Heliogen® Blue D 7092 Special black 250

15.5 – – –

– 15 – –

– – 15 –

– – 2 13

Resin/diluent Laromer® LR 9013 Laromer® PO 94F Dipropyleneglycol-diacrylate

15.5 45.6 13

15 51.6 8

15 51.6 8

15 51.6 8

0.2 0.2

0.2 0.2

0.2 0.2

0.2 0.2

Additives Levelling, Byk 307 Defoamer, Foamex N Photoinitiator Lucirin® TPO-L Irgacure® 369 Irgacure® 907 Viscosity (Pa s, 23 ◦ C) Yield (Pa, 23 ◦ C) Reactivity (m/min; 120 W/cm)

4 4 2 100

4 4 2 100

4 4 2 100

4 4 2 100

2.9 41 >65

2.0 4 >65

3.2 36 >65

2.4 20 >65

absorbing acylphosphine oxides (either Irgacure® 819 or Lucirin® TPO can be used) enables the throughcure. The coating is exposed to two ferrum added lamps (80 W/cm) at curing speed up to 10 m/min.

Starting formulations for further optimization according to the specific needs are given for UV offset, UV flexo and UV screen inks, recommended for paper, board and film applications (e.g., BASF brochure: Make up your print products7 ) in Tables 5.8–5.10. The formulations contain either yellow, red, blue and/or black pigments, usually a grinding resin, like Laromer® LR 9013, polyester or epoxy acrylate binder resins, a photoinitiator combination containing standard photoinitiators facilitating surface cure and an acylphosphine oxide type photoinitiator (e.g., Lucirin® TPO-L), responsible for through cure, and optionally levelling agents and defoamers.

5.2.2

Formulation for Overprint Varnishes (OPV)

Overprint varnishes are applied to impart high gloss and protection to a printed surface, like post cards, catalogue covers, cosmetic cartons, etc.

147

FORMULATIONS

TABLE 5.10. UV screen ink UV screen ink components Pigment Heliogen® Blue D 7092 Special black 250 TiO2 Kronos® 2063S Resin Laromer® LR 9013 Laromer® LR 9019 Laromer® UA19T Laromer® LR 8986 Dipropyleneglycol diacrylate

Cyan (wt%)

Black (wt%)

White (wt%)

5 – –

1 4 –

– – 34

5 – 54 7 15

5 – 54 7 15

– 37 – 12

Photoinitiator Lucirin® TPO-L Irgacure® 369 Irgacure® 907 Darocure® 1173

1.5 1 0.5 2

1.5 1 0.5 2

4 – – 3

Co-initiator Laromer® LR 8956

6

6

–

2 1 – 100

2 1 – 100

3 6 1 100

Additives Aerosil® 200 CAB 551-001 (20% in DPGDA) Byk® 164 Viscosity (Pa s, 23 ◦ C) Yield (Pa, 23 ◦ C) Reactivity (m/min; 120 W/cm)

4 – –

3.7 – –

3.5 15 60

The basic performance requirements on overprint varnishes are characterized by: • • • •

High reactivity; High gloss; Excellent wetting of the ink; Low price.

These overprint varnishes are applied with a thickness of about 6–10 µm (g/m2 ) typically in a coating unit as part of a printing press or in a separate step on a coating machine.

A typical formulation is based on a high content of multifunctional crosslinkers (for high reactivity), oligomeric resins (epoxy-, polyester- or amine-modified polyether acrylates) and high photoinitiator amounts (Table 5.11). Thus, a general formula for overprint varnishes contains about 30–70% of highly functional reactive diluent, 10–20% resins and photoinitiator amounts up to 8% and often also amine-synergist, in order to obtain high cure speeds.

148

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

TABLE 5.11. General composition of an overprint varnish formulation Component

(%)

Function

Oligomer acrylate resin

10–20

Film formation Basic performance properties

Reactive diluent

20–30

Film formation Viscosity adjustment

Trifunctional X-linkers

30–70

Coating properties

Photoinitiator

3–8

Initiation of curing; speed

Photosynergists

2–5

Cure speed increase

Additives

0–3

Surfactants Wetting, levelling agent, slip, . . .

TABLE 5.12. Comparison of standard OPV formulation and OPV for inert exposure Component

OPV (%)

Type

Standard

Inert

Oligomer resin

52

52

Epoxy acrylate (high reactivity and chemical resistance), Laromer® LR 8986

Reactive diluent

35

35

Viscosity adjustment, tripropyleneglycol diacrylate (TPGDA)

Photoinitiator

8 –

– 1

Benzophenone Lucirin® TPO

Additives

0.4

0.4

Surfactant and slip additives

In order to reduce the photoinitiator content, for example for formulations, which are designed for packages with indirect food contact (where only very small amounts of migratables are tolerated), curing can be done under inert conditions. The same cure speed can be achieved with formulations containing only 1% photoinitiator compared to 8% when curing under air (Table 5.12). Furthermore, due to the reduced photoinitiator contents, migratables and odour are reduced with such formulations cured under inert conditions.

5.3 SPECIALTY FORMULATIONS (FOR GLASS, POLYCARBONATE, METAL) In the following three examples of specialty formulations applicable as starting formulations for coating glass, plastics or metal surfaces are given.

149

FORMULATIONS

TABLE 5.13. Scratch resistant topcoat for glass Component

(%)

Type

Dipentaerythrolpenta-acrylate Propoxylated neopentylglycol-diacrylate Urethane acrylate Photoinitiator Photoinitiator

64.2 16.4 10 7 2

Sartomer® SR 399 Sartomer® SR 9003 Sartomer CN 965 Benzophenone Darocure® 1173

TABLE 5.14. Flexible starting formulation for UV coating of polycarbonate Component

(%)

Type

Aliphatic epoxyacrylate Trimethylolpropane formal-monoacrylate

60 10

Sartomer® CN 132 Sartomer® SR 531 or Laromer® LR 8887

1,6-Hexanediol diacrylate (HDDA) Photoinitiator Photoinitiator

25 2 3

Irgacure® 184 Darocure® 1173

TABLE 5.15. Formulation for cationic metal coatings Component

(%)

Function

Epoxy resin Inert flexibilizer Photoinitiator

60–70 20–30 1–3

Dow UVR-6160 Polycaprolactone triol Sulfonium salt (e.g., Dow UVI-6990) or Iodonium salt (e.g., Irgacure® 250)

Pigments or dyes Additives

1–2 0–3

Surfactants

Coatings suggested for high scratch resistance on glass (ref. 6) are based on high functional pentaerythrol pentaacrylate (Table 5.13). The reported thickness was about 100 µm and no basecoat was used.

The starting formulation proposed for coating plastic polycarbonate (ref. 6) is much more flexible than the formulations described before. It is pointed out, that the use of a flexible basecoat improves abrasion resistance, as well as adhesion and flexibility.

Since cationic coating systems based on epoxy resins exhibit little shrinkage, the adhesion of such coatings on metal surfaces is generally better than that of acrylate based formulations. Most of the cationic coatings rely on epoxy resins, flexibilizers and sulfonium or iodonium salts as photoinitiators (Table 5.15).

5.4 FORMULATION SCREENING FOR NEW APPLICATIONS Frequently asked questions are about how to start the formulation screening for a new application. For the traditional applications, like wood, plastic coatings and graphic arts

150

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

the formulation examples discussed before can be used as a starting point. Some general considerations for the design and selection of raw materials for formulations are given in Figure 5.2.

For interior applications, the selection of the raw materials is governed by the demands of the application segment, the required properties and the application method. There are no overall general restrictions. On the other hand, exterior applications and UV curing of pigmented systems require a careful selection of raw materials as well as of the exposure equipment. Since pigments absorb more or less intensely in the UV, the photoinitiators and the lamp have to be tuned to allow enough through-cure. Thus, usually a combination of photoinitiators, a standard hydroxyalkyl ketone (HAK) for surface cure and an Acylphosphine oxide (APO) type for through-cure, is selected. The exposure lamp also should exhibit considerable output at longer wavelength, as do gallium or ferrum added mercury lamps (see Figure 2.21). For exterior applications the selection of all components is critical. The resins have to be “weather-resistant”, particularly oxidative and hydrolytically stable. Hence, the only appropriate resin types are aliphatic urethane acrylates (UA) or acrylated polyacrylates (APA), to be used in combination with hydrocarbon type acrylate diluents (e.g., hexanediol diacrylate, decanediol diacrylate). In order to stabilize the coating and the substrate against degradation and discolouration, the coatings have to contain UV absorbers and radical scavengers, usually of the hindered amine light stabilizer (HALS) type. Since these compounds, similar to pigments, also absorb in the UV, the photoinitiator package is also composed of a (non-yellowing) surface cure type (e.g., Irgacure® 184) and an acylphosphine oxide type photoinitiator. The exposure lamp is preferably of the ferrum added mercury type.

The main performance requirements in wood coatings are good adhesion (flexible waterbased primers or reactive acrylates, like isocyanato acrylates) and especially for parquet coatings high abrasion resistance (corundum fillers). The graphic applications often ask for high reactivity (high functional oligomers or amine modified resins) and high gloss. Can and coil coating as well as plastics demand excellent adhesion (low shrinkage, reactive primers) and good flexibility (low functional higher molecular weight resins and/or monofunctional reactive diluents). High scratch resistance can be obtained with high crosslink densities and/or added nanoparticles. High hardness can be achieved with high Tg coatings (high crosslink density or rigid components) and high flexibility by low crosslink density (high molecular weight between crosslinks) and flexible chains (polyether, polysiloxanes).

Last but not least the desired application method determines the choice of the raw materials. Powder applications require a glass transition temperature of the powder above room temperature, preferably above 40 ◦ C, and melting temperatures below 120 ◦ C. Thus, high Tg resins and/or crystalline resins/diluents have to be used in combination with low volatile, solid photoinitiators. In liquid systems, the application viscosity decreases from roller over curtain and vacuum coating to spray coatings. The viscosity adjustment can be done preferably with reactive diluents, however, where low molecular weight monomers are undesirable or prohibited (porous substrates, spraying), solvents or waterbased emulsions or dispersions may be used.

FORMULATIONS

F IG . 5.2. Considerations for selection of starting formulations.

151

152 UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 5.3. Schematic structures of flexible and highly crosslinked urethane acrylate resins and different functional diluents for formulation screening with high-throughput methods.

153

FORMULATIONS

TABLE 5.16. HTS set-up for screening urethane acrylates Varied components

Components kept fixed in all formulations

Binder

Reactive thinner

Reactive thinner (wt%)

Photoinitiator

Additives

UV protection

UA 1 ↓ UA 26

TMPMFA HDDA TMPTA

0, 10, 30, 50 0, 10, 30, 50 0, 10, 30, 50

3.5% Irgacure® 184 0.5% Lucirin® TPO

1% defoamer 0.3% levelling

1.3% UV absorber 0.7% HALS

5.5

INCREASING EFFICIENCY IN FORMULATION SCREENING: HIGH-THROUGHPUT SCREENING (HTS)

In order to illustrate the method of high-throughput screening (HTS), the course of the development of weathering and scratch resistant aliphatic urethane acrylate resins is given in the following example.8 The screening was based on the formulation of aliphatic urethane acrylate resins together with reactive diluents of varying functionality. The aliphatic urethane acrylate resins used were synthesized by the reaction of a low viscous isocyanato-acrylate (ICA: Laromer® 9000 (BASF AG)) with commercially available diols and hydroxyalkyl acrylates. Molecular weight and acrylate functionality of the urethane acrylates was varied systematically. Twenty-six different aliphatic urethane acrylates were synthesized and formulated with the 3 reactive thinners as shown in Figure 5.3.

The combination of 26 UA resins with the 3 diluents in 4 different concentrations resulted in 260 formulations. As fixed components 4 wt% photoinitiator (mixture of Irgacure® 184, Lucirin® TPO), 1 wt% defoamer, 0.3% levelling additive and 1.3 wt% UV absorber and 0.7 wt% HALS (UV protection) were added to the formulations (Table 5.16).

The workflow of the high-throughput formulation testing is elucidated in Figure 5.4.

This tool setup can be used for formulation screening with already existing raw materials, however, a parallel synthesis robot can also be applied to synthesize tailor-made new components (resins) in a very efficient way before the formulation step. A modular coatings robot (Figure 5.6) was used to dispense the raw materials and stir the formulations, then to draw down the coating films and finally to cure them using UV light. Various semiautomated application tests yielded materials properties like hardness, elasticity, scratch and chemical resistance as well as UV conversion. Sophisticated data management supported the design of experiments, configures the robot, captures the recipes, process parameters and measurement results in a database and offers data visualization. This allows selection of promising formulations and to harvesting of structure property correlations of coating properties, process parameters and chemistries applied. In subsequent steps more focused libraries can be generated.

The resins can be synthesized in a classical manner or with a HTS reactor with automated dosing and parallel setup as shown in Figure 5.5.

The formulations were coated with the HTS equipment shown in Figure 5.6 onto glass substrates by doctor blading (slit width 200 µm) with a wet paint thickness of 150 µm.

After 15 min flash-off at room temperature the coatings were subjected to a 20 min treatment at 100 ◦ C. Subsequently, UV curing was performed at 60–80 ◦ C with 5 J/cm2 under

154

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 5.4. Scheme of high throughput formulation testing.

F IG . 5.5. Parallel reactors for synthesis.

inert conditions in a CO2 bath in order to eliminate oxygen inhibition. All the properties were measured with the HTS equipment (Figure 5.7). The mechanical properties of the cured films, like hardness and elasticity, were determined by Fischerscope® measurements

FORMULATIONS

F IG . 5.6. HTS parallel coating equipment.

F IG . 5.7. HTS combinatorial application testing.

155

156

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 5.8. Determination of hardness and elasticity (fischerscope® ).

F IG . 5.9. Fischerscope measurement (hardness versus elastic work fraction) of highly X-linked UA formulations from HTS.

FORMULATIONS

157

F IG . 5.10. Fischerscope measurement (hardness versus elastic work fraction) of UA formulations from HTS of all 260 formulations.

(Figure 5.8). Scratch resistance was evaluated by a sand scrub test. UV conversion was determined by confocal Raman microscopy at the surface (0 Âľm) and 15 Âľm layer depth.

Chemical resistance was evaluated for H2 SO4 , NaOH, Pancreatin, tree resin and H2 O at two different temperatures each.

At first glance, one might be overwhelmed by the flood of data generated from the screening. With visualization tools, however, interesting correlations can be revealed.

A plot of hardness vs. elasticity, as shown in Figure 5.9, for a subset of highly crosslinked urethane acrylate resins (see Figure 5.3, top right) as a function of the used hydroxyacrylate building block on one hand and the type (i.e., functionality) and concentration of the reactive thinner on the other hand. The picture becomes even more complex (Figure 5.10), if additionally urethane acrylate resins are incorporated which contain flexibilizing diols in the center of the resin molecule (see Figure 5.3, top left). From the data in Figure 5.10 molecular resin structures can be identified which result in coatings of high elastic work (>70% of total) at low hardness (highly flexible systems) or at high hardness (highly crosslinked coatings).

The evaluation of the scratch testing showed high scratch resistance for the coatings which exhibited high elastic work, either the flexible or the highly crosslinked. The chemical resistance data (Figure 5.11) were plotted in the same type of graph (hardness versus elastic work) and the differentiation of good and bad chemical resistance is shown by the shape (filled circle, empty circle and plus (+), respectively). The results demonstrate that the best chemical resistance is in the upper right corner of every plot, thus produced by the coatings with high crosslink density. The highly crosslinked urethane acrylate coatings identified by the HTS screening have then been subjected to intensive classical scratch resistant tests. According to a model published by Jones et al.9 , there are three different ways

158

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 5.11. Chemical resistance results of UA formulations from HTS (the formulations are plotted the same as in diagram in Figure 5.10).

TABLE 5.17. “Three response, two mechanism model” for evaluation of scratch resistance Responses to marring stress

Mechanisms of marring

Mechanisms of healing

Fracture Plastic deformation Elastic deformation

Fracture Plastic deformation

Viscoelastic creep

coating surface can respond to marring stress, addressing two different mechanisms of marring, and one mechanism of healing. In this model, fracture and plastic deformation lead to observable marring, while elastic deformation does not. Instead, elastically deformed mars recover their original dimensions almost instantaneously. Thus, the three responses in this model lead to only two marring mechanisms (Table 5.17).

According to the model described above and the results of the high-throughput screening, the urethane acrylates which combine a high ratio of elastic work (We /Wtot ) and high universal hardness (Hu ) in Fischerscope testing should exhibit a very interesting combination of hardness and elasticity.10 After further optimization of the respective formulations these products have been tested as automotive clear coats, especially on plastic parts against the conventional two pack (2C PU) formulations. The results are shown in Table 5.18.

Such highly crosslinked coatings based on aliphatic urethane acrylates, identified by the high-throughput screening, showed excellent chemical resistance and extreme scratch resistance.

159

FORMULATIONS

TABLE 5.18. Applications test for automotive clear coats High functional aliphatic urethane acrylate (UV) based on Laromer® 9000 3.5% Irgacure® 184/0, 5% Lucirin® TPO, UV curing Larolux® Scratch resistance Initial gloss at 20◦ AMTEC-Kistler 10 cycles at 20◦ (%) DIN 55668 10 cycles after reflow 2 h 80 ◦ C at 20◦ (%) 40 cycles at 20◦ (%) Chemical resistance Gradient oven Base coat brilliant silver

H2 SO4 (◦ C) NaOH (◦ C) Pancreatin (◦ C) Tree resin (◦ C) H2 O (◦ C)

UV

89 85 87 83

2C PU scratch resistant (bench mark) 85 74 87 48

75 57 69 75 75

44 69 71 69 75

Such HTS methods are now often used for the screening of formulations. Further examples describing the development of UV refinish primers and clear coats have been published recently.11

REFERENCES 1. Wicks Jr., Z.W., Jones, F.N. and Pappas, S.P. (Eds.), “Organic Coatings, Science and Technology, Vol. 2, Applications, Properties, and Performance”. John Wiley, 1994, Chapters XXXIII and XXXIV, pp. 273–311.

2. Holman, R. (Ed.), “UV&EB Curing Formulations for Printing Inks, Coatings and Paints”. SITA Technology, London, 1984.

3. Müller, B. and Poth, U., “Lackformulierung und Lackrezeptur: Das Lehrbuch für Ausbildung und Praxis”.

Vincentz Verlag, 2003, Chapter 2.

4. Bankowsky, H.H., Paint Coat. Ind. June, 44–48 (1997).

5. Bankowsky, H.H., Enenkel, P., Beck, E., Lokai, M. and Sass, K., The formulation and testing of radiation curing binders for abrasion-resistant parquet varnishes, RadTech, Baltimore, MD, End User Workbook. 2000, pp. 210–222.

6. Silberzan, I., Magny, B., Elie, K., Roche, S. and Loubert, J.L., An insight in scratch resistance of UV cured coatings, RadTech Europe, Conference Proceedings, Vol. I. 2003, pp. 171–177.

7. www.basf.com/pigment and www.basf.com/resin.

8. Heischkel, Y., Schwalm, R., Kutschera, M., Schrof, W., Koltzenburg, S., Beck, E., Larbig, H., Menzel, K. and Gruber, N., RadTech Europe 2005, Conference Proceedings, Vol. I. 2005, pp. 297–303.

9. Jones, N., Shen, W., Smith, S.M., Huang, Z. and Ryntz, R.A., Progr. Org. Coat. 34, 119 (1998).

10. Schwalm, R., Polym. Paint Colour J. 189 (4421), 18 (1999).

11. Bach, H., Gambino, C., Galeza, L., Dvorchak, M.J., Fäcke, T., Detrembleur, C., Ehlers, M., Mundstock, H., Schmitz, J. and Weikard, J., UV refinish primer and clearcoat, RadTech Europe 2003, Conference Proceedings. 2003, pp. 731–738.

C HAPTER 6

Structure–Property Relationships We have seen in Chapter 3, that the properties of networks and thus of the coatings depend strongly on the physical state of the materials, being at the application temperature in a glassy state, in a rubber-like soft region or even in the transition between glass and rubber. In this transition region, the modulus is heavily influenced by temperature and may drop by several orders of magnitude. This transition is defined by the glass transition temperature (Tg ). Since almost all crosslinked coatings are amorphous polymer networks, the glass transition temperature has the most prominent influence on the mechanical properties. The Tg describes the deflection from linearity of the increase or decrease of volume of a polymer during heating, from a glassy to a soft state or during cooling from the soft to a glassy state. This behaviour is not characterized by a sharp discontinuity of the increase of the specific volume with temperature, as it is with a melting transition, but just as a discontinuity identified by a change in the rate of increase of the specific volume with temperature. Thus, the transition of a polymer into an amorphous glass is more a kinetic than a thermodynamic transition. The glass transition temperature increases with increasing molecular weight, since the mobility of the chain segments decreases. In linear polymers the glass transition temperature reaches a plateau at a polymerization degree above about 100–600, depending on the molecular structure. Consequently it does not further increase with increasing molecular weight. Below the Tg , the polymer behaves like an immobile glassy material, whereas above the glass transition temperature the chains are able to move to a certain extent, and the material behaves like a rubber. The absolute value of the glass transition temperature depends on the structure of the polymer and can vary from Tg ’s well below room temperature, like in linear polysiloxanes to above 300 ◦ C in polyimides, polybenzimidazole and others.

Crosslinking restricts the mobility of chain segments, if the distance between the crosslinks is shorter than the segment length required for mobility. It has been described, that the mobility of about 30–50 chain-links is necessary to observe a glass transition. Elastomers (rubbers) are used above their glass transition and may exhibit elongations beyond 500%, and in addition to that, they are able to restore the original shape, due to the few crosslinks. Highly crosslinked polymers, like duromers, which are used below their Tg , often exhibit elongations less than 10%, and are therefore not really extendable or formable.

Structure–property relationships of highly crosslinked UV curable duromers have been evaluated for many different systems. A review of characterization methods and under160

STRUCTUREâ&#x20AC;&#x201C;PROPERTY RELATIONSHIPS

161

standing of the relationships between exposure conditions, chemical structure and network properties are given by Zumbrunn.1 The properties of radiation curable materials can be tailored to the application needs. Some fundamental influencing factors on the main properties, like mechanical behaviour, are discussed in the next section.

The main influences of the acrylate (resin) functionalities and resin types on the polymerization process and general properties are exemplarily shown in Figures 6.1 and 6.2. Increasing the monomer (diluent or resin) functionality leads to higher cure speed, higher Tg , higher crosslink density, higher hardness and higher chemical and scratch resistance, but lower flexibility and lower conversion, causing a higher content of uncured residual unsaturated groups. Generalized structureâ&#x20AC;&#x201C;property relationships of the typical resins used in radiation curable acrylate systems are given in Figure 6.2. The performance criteria shown

F IG . 6.1. Influence of resin/diluent functionality on the polymerization process and some properties.

F IG . 6.2. Generalized properties of typical resins of the different UV curable acrylate resin classes (spider diagram: 0, worst; 10, best).

162

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

in the diagram are rated relative to a defined scale, ranging from poor (=0) to excellent (=10). The (aromatic) epoxy acrylates provide high hardness, reactivity and chemical resistance, whereas urethane acrylates are known for their high flexibility, toughness and abrasion resistance. The polyester acrylates have a well balanced property profile, making them typical all-rounder, and the polyether acrylates are very good reactive diluents due to their low viscosity but also usable as pure resin materials. It will be discussed in this chapter how these properties are influenced by chemical composition, cure extent and crosslink density.

6.1

EFFECT OF CROSSLINKING ON MECHANICAL PROPERTIES

Mechanical properties relevant for coatings are hardness, stiffness, toughness, elasticity and many others. All these properties are not only material properties, but also depend on the molecular interactions, morphological structures (amorphous, crystalline) as well as on the processing conditions. The basic concepts of the physical behaviour of coating films have been adopted from the concepts developed by the plastics and rubber industries, for example, stress-strain analysis as an effective tool to characterize mechanical film properties, as discussed in Chapter 3.

A brief and good introduction into determination and methods of mechanical properties of coatings is given in Vol. 2 of “Organic Coatings”.2 The fact that coating films are viscoelastic materials is fundamental for the understanding of coating properties. Upon deformation they respond with elastic restoring and/or viscous flow. The ideal elastic behaviour (illustrated by the deformation of a spring) is described by Hook’s law. The elongation of the material is proportional to the stress applied, and if the stress is released, the material returns to its original shape. On the other hand, an ideal viscous material will show elongation by viscous flow upon stress applied, and remain at this elongated stage even after the stress is released. The coating films are viscoelastic materials, thus the behaviour can be regarded as intermediate between full recovery and viscous flow. Thermoplastics generally do not recover their initial state after deformation, since the viscous flow results in permanent deformation. In crosslinked films, the stress on the crosslinks produces restoring forces to partly or completely reverse viscous flow. The yield point in the stress strain diagram (Figure 3.13) corresponds to the point where the material begins to have permanent (unrecoverable) deformation. The stress-strain analysis can be done in a static way, where strain is applied to elongate the sample and trace the stress relaxation or in a dynamic oscillating strain application at a specific frequency, where the resulting stress development and phase angle difference is measured. From these experiments the storage modulus (E ), which is a measure of the restorable energy imparted by the applied strain, the loss modulus (E ), which reflects the loss of the applied strain due to viscous flow of the sample, and the ratio of loss modulus to storage modulus (E /E ), called tan δ, can be determined.

The drop in the modulus of E is caused by transition of the material from a glass into a rubber-like material. This transition is characterized by the glass transition temperature, which can be determined by the maximum of the tangent δ, which corresponds more to the mean value of the glass transition region or by the maximum of the loss modulus (E ), which corresponds more to the beginning of the glass transition region. In a sharp drop of

STRUCTURE–PROPERTY RELATIONSHIPS

163

the modulus (E ) these values do not differ significantly, but in highly crosslinked heterogeneous networks, the Tg region can be relatively broad and in these cases the values will differ significantly.

Especially in the cases where the glass transition region is within or near to the use temperature it is very important to know if the network (or coating) is or behaves like it is in the physical state of a glass or a rubber.

Since the mechanical properties of crosslinked coatings are strongly influenced by the glass transition temperature,3 the structural influences on the Tg are discussed.

If the Tg of the coating is below the application temperature, the coatings often exhibit some of the following properties: • • • • •

Soft and flexible, Difficult to sand (rubber-like), Exhibiting poor blockability (tacky), Low barrier properties (against chemicals, oxygen, water), High water uptake and swelling.

If Tg is above the application temperature, the coating can often be characterized as:

• • • • •

Hard, to some extent brittle, Exhibiting low water uptake and swelling, High barrier properties against chemicals and water, High scratch resistance, and High chemical resistance.

Physically, Tg can be explained as the lowest temperature where segments of polymer molecules start moving relative to neighbouring segments and start occupying a larger volume. The glass transition is not a sharp point, but rather a transition range depending on the thermal history of the sample. With fast heating rates the Tg appears at higher temperatures and if the material was cooled down rapidly, the Tg appears to be lower than with samples that are cooled down more slowly.

What are the main structural influencing factors on the glass transition temperature?

Most relevant factors are those that influence the thermal energy necessary for the maintenance of the rotation of chain segments. These are the chain flexibility, the molecular constitution (sterical effects), the molecular weight, as well as branches and crosslinks. The relative flexibility of the polymer chains will determine the magnitude of the glass transition temperature. It is a measure for the ability of the chain segments to rotate around the bonds. Therefore a rigid chain needs high thermal energy in order for the chain segments to rotate, thus is has a high Tg , whereas a flexible chain can maintain rotations already at low temperatures, thus exhibiting low glass transition temperatures. Since the mobility of chain segments is also dependent on the molecular weight, values for Tg are often given as the glass transition temperature at infinite molecular weight. Tg increases with increasing molecular weight, but the Tg versus molecular weight curve approaches a constant value (Tg∞ ). The Tg∞ can vary over a wide temperature range (Table 6.1), as shown for different linear polymers.

Besides the chemical constitution of the backbone, including the steric hindrance effects on rotation, which is reflected by Tg∞ , the glass transition is influenced by the molecular

164

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

TABLE 6.1. Tg values of linear polymers at infinite molecular weight Polymer

Tg∞ ( ◦ C)

Poly(dimethyl siloxane) Polypropylene oxide Polybutyl acrylate Polyethyl acrylate Polymethyl acrylate Polycyclohexyl acrylate Polymethyl methacrylate Polyvinyl carbazol

−123 −65 −54 −24 +10 +19 +105 +150

weight (Mn ), and by the crosslink density (X) (see Chapter 3 for definition), as expressed by the simple equation (6.1.1)4 : Tg = Tg∞ − [K/Mn ] + [Kx X],

(6.1.1)

where Tg∞ is the limiting glass temperature of the uncrosslinked polymer at infinite molecular weight, K is a constant; the term [K/Mn ] characterizes the decrease in Tg due to the remaining end groups of the uncrosslinked polymer chains, since the end groups require a higher volume than the chain segments and therefore contribute to an easier rotation. Kx is a constant characterizing the increase of Tg due to the presence of crosslinks and X is the crosslink density.

Equation (6.1.1) holds true only for linear polymers with low crosslink densities, since it assumes that the reference glass transition temperature of the linear polymer does not change with the degree of cure (end group conversion). A better fit of data with higher crosslinked systems was derived with an equation published by Stutz,5 taking into account that the reference glass temperature is not a constant, but dependent on the cure conversion

Tg = Tg∞ − K1 (1 − p) 1 + K2 X/(1 − X) ,

(6.1.2)

where Tg∞ is the true backbone Tg without endgroups or crosslinks, K1 characterizes the influence of endgroups, thus reflecting the degree of cure (p), and K2 is another constant accounting for the influence of crosslinks. Thus, the Tg of the linear polymer is lowered by endgroups if the degree of cure is not 100%, and increased due to the crosslinking reaction. This equation also explains how Tg changes during cure. As the cure conversion increases the term lowering Tg gets smaller and with increasing crosslink density the last term increases exponentially.

6.1.1

Cure Conversion and Glass Transition Temperature

The degree of cure and the dependence on the glass transition temperatures of different resins has been evaluated,6 and will be described in the following.

STRUCTURE–PROPERTY RELATIONSHIPS

165

F IG . 6.3. Cure conversion as a function of cure temperature for three different Laromer® resins.

In order to verify the degree of cure of the resins, the conversion of the acrylic double bonds as a function of cure time has been followed for resins of different molecular structure. According to equation (6.1.2), Tg is dependent on the conversion of the functional groups. The conversion of the acrylate groups was determined by real-time infrared spectroscopy as described by Decker.7 It is shown in Figure 6.3 (top left) that the conversions of the three coatings are relatively high (>60%), however, never close to completion. The limitation of acrylate conversion is mainly due to vitrification of the system, analogous to thermosetting systems published by Gillham.8 According to equation (6.1.2) Tg increases with increasing curing conversion. It has been known from thermoset coatings that the curing reaction stops when Tg approaches or exceeds the curing temperature by about 20 ◦ C. In order to evaluate if this observation also holds true for radiation curable systems three UV curable coatings of different resin structure were chosen.

When UV cured at room temperature the obtained Tg ’s of the coatings based on the three resin types were −3 ◦ C (LR 8907), 51 ◦ C (LR 8861) and 75 ◦ C (LR 8713), respectively.

The corresponding acrylate conversions were 90, 62 and 59%. The measurement of the acrylate conversion as a function of cure temperature showed, that already nearly complete conversion is obtained in the case of the resin where the backbone Tg is below RT (LR 8907), while almost no change occurs with increasing cure temperature (Figure 6.3, right top). The situation changes with experimental product LR 8861. The cure conversion of the room temperature curing experiment is restricted to about 70% conversion (Tg of 51 ◦ C

166

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 6.4. Increase of Tg as a function of cure temperature for a Laromer® resin.

reached), however is steadily increased with increasing cure temperature up to 90% final conversion (Figure 6.3, right middle). The LR 8713 resin (Tg of 75 ◦ C at RT cure) clearly shows that the cure reaction stops when the increasing Tg (during curing) approaches or slightly exceeds cure temperature, demonstrated by the large plateau up to ∼80 ◦ C cure temperature. Not until the cure temperature was increased above 80 ◦ C the conversion increased again (Figure 6.3, right bottom). Thus, the higher curing temperatures increased conversion, and also resulted in a Tg increase, as shown in Figure 6.4.

According to the demonstrated limitation of cure conversion and Tg one could expect that the final Tg ’s of the room temperature cured resins should be limited to upper values of about 30–40 ◦ C. Nevertheless Tg ’s of 51 and 75 ◦ C were obtained at room temperature cure. However, this can be explained by the experiments of Decker,9 who recorded the temperature profiles of photocuring at room temperature. These measurements revealed that a temperature increase ( T ) of up to 70 ◦ C might occur during curing, due to the evolved heat of polymerization. This temperature increase within the coating film can explain the obtained Tg ’s in the range up to 75 ◦ C in the case of room temperature curing.

The influencing factors on Tg have been discussed in such detail, since it is of paramount importance to know the physical state of the coating when discussing the mechanical behaviour.

6.1.2

Hardness and Flexibility as a Function of Tg

For a series of UV curable resins of different types (epoxy, polyether, polyester and urethane acrylates of Laromers) the hardness (Figure 6.5) and flexibility (Figure 6.6, as determined by the Erichsen cupping method) have been determined as a function of the glass transition temperature. The basic mechanical properties of this series are summarized in Table 6.2.

STRUCTURE–PROPERTY RELATIONSHIPS

167

F IG . 6.5. Hardness as a function of Tg of UV cured Laromer® resins.

F IG . 6.6. Flexibility as a function of Tg of UV cured Laromer® resins.

The hardness measurements of the coatings can be considered as single temperature modulus determinations and were done with an indentation (Vickers) and a pendulum test.

The Erichsen cupping represents a reverse impact test, where a ball is pressed into the coated substrate from the backside until the coating breaks. This test reflects the flexibility of the coating and is measured as the extent of elongation in mm.

The hardness and flexibility data of the various acrylate resins were plotted against the glass transition temperature. From the data in Figure 6.5 it is very obvious that there is a significant increase in hardness for coatings with Tg ’s above room temperature, which

Laromer

Type

Conv.h (%)

Tg i ( ◦ C)

4.78 2.64 5.49

59 60 93

(−10) 54 75 (−8) 24

234 176 145 615 143

4.27 5.68 6.90 1.63 6.99

70 72 58 90 61

19 14 34 −3 27

1.5 4.6 4.7 2

275 164 91 161

3.60 6.10 10.99 6.21

29 82 81 78

66 (−10) 19 (−9) 26 (−15) 25

12 172

8 4.0

1100 285

0.91 3.5

60 75

26 65

157 225

5 6.8

121 793

8.26 1.26

66 57

56 57

Func.c

PDd (s)

Vickerse (N/mm2 )

Erichsenf (mm)

HDDA (20) BuAc –

2 2 2

186 144 31

194 193 8.8

1 1.1 4.1

209 379 182

– – – DPGDA (25)

2.5 3.5 3.5 2.5 3.5

42 45 116 29 97

11.9 16.7 95 3 89

6.5 4.5 2.7 6.8 2.6

– – – –

3 3 3 3

113 52 73 55

161 19.6 47.5 22.9

TPGDA (35) HDDA (30)

2 2.5

43 164

50% 40%

3.5 2.2

154 192

1/Mc ×10−3

a All acrylates used are Laromer® types (BASF AG) and were pure or with the specified diluent containing 4 wt% of the photoinitiator Irgacure® 500. Coating thickness was 50 µm. The films were exposed with 1280 mJ/cm2 . b Diluent: HDDA, hexanediol diacrylate; PO 33F, oligoetheracrylate; D(T)PGDA, di(tri)propyleneglycol diacrylate. c Func., functionality. d PD, pendulum hardness. e Vickers, vickers hardness. f Erichsen cupping. g M , elastically effective chain length. c h Conv., conversion of acrylates. i T , glass transition temperature (a second T in brackets). g g

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

Epoxy acrylates EA 81 Aromatic LR 8713 Aromatic LR 8765 Aliphatic Polyester acrylates PE 55F Aliphatic LR 8799 Aliphatic LR 8800 Aliphatic LR 8907 Aliphatic LR 8912 Aliphatic Polyether acrylates PO 33F PO 84F LR 8894 PO 83F Urethane acrylates UA 19T Aliphatic LR 8987 Aliphatic Water based systems LR 8895 LR 8949

Mc g (g/mol)

Diluentb (%)

168

TABLE 6.2. Mechanical data of various radiation cured acrylatesa

STRUCTURE–PROPERTY RELATIONSHIPS

169

appears with both hardness measurement methods. The data for flexibility show nearly the opposite behaviour. There is a drop of flexibility when Tg rises above room temperature.

The step is not as pronounced as in the hardness diagram, probably due to the existence of a second Tg in several coatings below room temperature, which indicates phase separations. However, there are two exceptions with the water-based systems, which exhibit extraordinarily high flexibility at high Tg ’s.

6.1.3 Influence of Mc (Molecular Weight Between Crosslinks) The principle influence of the molecular weight between crosslinks (Mc ) on the flexibility has been shown schematically already in Figure 3.12. The mechanical data of the waterbased systems as listed in Table 6.2 exhibit high hardness in agreement with the high Tg ’s, however, considerably more flexibility than expected according to a pure Tg consideration (Figure 6.6). Therefore the flexibility as a function of Mc has been evaluated. In the case of the investigated resins there was no exact linear correlation between flexibility and Mc as shown in Figure 6.7, but a clear trend that the flexibility decreases with decreasing molecular weight (1/Mc ). There are some coatings, which deviate from the trend line, but this deviation behaviour could be explained by the specific structures of these resins (see ref. 6). Generally, the water-based coatings had a high Tg , which is responsible for the high hardness, and a relatively high molecular weight between crosslinks, which is responsible for the high flexibility. The explanation for the high flexibility of the chain segments can be derived from investigations of the mechanical behaviour of linear polymers.

The mechanical analysis has been done for linear polymers to a much greater extent than for coating networks. In such polymers (e.g., polymethyl-methacrylate (PMMA) investigated by Wu,10 Figure 6.8), it has been detected, that a considerable change of the mechanical properties occurs at the glass transition temperature, however, a considerable

F IG . 6.7. Flexibility versus 1/Mc (molecular weight between crosslinks).

170

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

Transition temperatures of homopolymers Polymer Tg (◦ C) Tb (◦ C) Polystyrene 100 90 PMMA 105 45 PVC 80 10 Bisphenol-A–PC 150 −200

Tg –Tb (◦ C) 10 60 70 350

F IG . 6.8. Transition temperatures of linear polymers.

increase of the elongation behaviour (elongation at yield as well as elongation at break) occurs already well below the glass transition temperature. This elongation behaviour is responsible for the observed mechanical performance of the polymer being ductile (or flexible in a coating sense) in a region below Tg . The lower boundary of this region has been defined by Wu by another transition temperature, the brittle–ductile transition (Tb ), which can be detected from stress-strain analysis as a function of temperature. Wu has defined this Tb transition temperature at the intercept of the elongation at break and the elongation at yield point. Thus, in the range between Tb and Tg , the polymer material exhibits a ductile behaviour.

This transition (Tb ) is due to starting chain segment mobility (sometimes also described as second order β-transition) and renders the deep-drawing of plastics possible well below Tg . The mechanical properties of UV cured coatings can be well explained by modulus and elongation curves as shown in Figure 6.9. According to the mechanical property values listed in Table 6.2, the polyester acrylate (LR 8907) appears soft (3 N/mm2 ) and flexible (6.8 mm Erichsen), which can be correlated with the low Tg (−3 ◦ C), which dominates the mechanical properties. In the case of the water-based LR 8949, the Tg is above room temperature (57 ◦ C), but due to a lower crosslink density (see high Mc value) a Tb transition exists below room temperatures, which is responsible for the relatively high flexibility (6.8 mm Erichsen) at high hardness (225 N/mm2 ). With increasing crosslinking, the mobility of segments is reduced and the difference of Tb and Tg decreases, until in highly crossliked materials only one transition, the glass transition exists (schematically shown in Figure 6.10). In these cases, the Tg virtually marks the transition temperature below which the coating is brittle and above which it is flexible. An example is the resin LR 8987 with a Tg of 65 ◦ C and a reduced, but moderate flexibility of 4.0 mm (probably due to the broad Tg region) and even more pronounced in the case of the resin LR 8713 with a Tg of 75 ◦ C and a elongation of only 1.0 mm (Erichsen).

STRUCTUREâ&#x20AC;&#x201C;PROPERTY RELATIONSHIPS

171

F IG . 6.9. Superimposing modulus and elongation curves of three UV cured resins with different composition.

F IG . 6.10. Effect of crosslinking on brittle-ductile transition (Tb ).

Although the mechanical behavior can be explained by the physical state of the coatings and the position of the transition temperatures relative to the application temperatures, it has to be considered that the glass transition temperature and therefore the properties is influenced by the velocity of the impact in reality as well as the velocity of the measurements performed for the determination of the values (see Chapter 3.4.1).

The obtained Tg distinguishes between hard and soft coatings. If Tg is above room temperature, the coating is hard and turns soft at temperatures above Tg , which is reflected in the modulus curves. The flexibility can be determined by elongation measurements.

Linear polymers often have brittleâ&#x20AC;&#x201C;ductile transitions below room temperature, they are tough (flexible) at room temperature. With increasing crosslink density the elastically effective chain length between crosslink decreases and the Tb transition increases until at high crosslink densities only one transition, the glass transition temperature, exists. Since

172

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

most of the radiation cured coatings are highly crosslinked only Tg determines the mechanical properties and that’s the reason why most of the coatings are either hard/brittle or soft/flexible. Thus, in order to get hard and flexible coatings, the crosslink density should be decreased to allow a Tb -transition below room temperature and the glass transition has to be increased above RT by means of chemical composition and crosslinking during photopolymerization. Low crosslink densities are difficult to realize in 100% coatings without using large amounts of monofunctional reactive diluents, since viscosity also increases with molecular weight. Therefore, water-based systems, where the viscosity is independent of molecular weight of the resin, or systems applicable by melting, like hot melt or powder coatings, are option towards hard and flexible coatings.11 6.2 EFFECT OF CROSSLINKING ON COATING PROPERTIES In the preceding section some structure–property relationships have been discussed influencing the mechanical properties of UV cured coatings. In daily life however, the application properties of the coatings, like abrasion-, scratch- or chemical resistance, are the crucial key factors. For linear polymers a lot of data exist characterizing the structural influences on mechanical properties, however much less data exist for crosslinked coatings.

And even less data are available for correlating property–property relationships, like the influences of mechanical properties on scratch resistance, pendulum hardness, elasticity or Taber abrasion (Figure 6.11).

The reason why such correlations hardly exist is based on the fact, that many of these application properties, like scratch resistance, are not a defined material property. The scratch resistance of a material is dependent on the scratch test, which means on the scratching material (hardness, shape, etc.) as well as on the impact (velocity, mass) affecting the coating.

Thus, depending on the scratch test different material properties will be required to obtain optimal scratch resistance, for instance, in a car wash test or an abrasive Taber test. Several scratch test methods used in the evaluation of coatings have been described in Chapter 3 (see Figure 3.18).

F IG . 6.11. From structure–property to property–property relationships.

STRUCTURE–PROPERTY RELATIONSHIPS

173

In the following we will discuss the effects of X-linking on some performance data, like hardness, flexibility and scratch resistance of a series of model coatings based on aliphatic urethane acrylate chemistry. The scratch resistance was evaluated in a ScotchBrite® test, where a ScotchBrite® fleece is moved under a specific load (750 g) with 50 double strokes over the surface and the gloss difference of the coating measured before and after scratching.

Urethane acrylates have been identified as the most promising candidates of UV curable raw materials for outdoor applications.12 The resins used to obtain valuable structure– property relationships are based on a bifunctional resin containing a linear diol to introduce flexibility (UA1, flex) and a three-functional aliphatic urethane resin (UA2, hard). These resins were formulated with different functional monomers (monofunctional to hexafunctional) to vary the crosslink density (Figure 6.12 and Table 6.3). The crosslink density has been calculated from the molecular composition, the functionality and the conversion with the DryAdd program of OxMat13 : the crosslink density of the gel is the cycle rank divided by the weight of the gel. Cycle rank is the number of “cuts” required to reduce the network structure to a tree.

Some basic properties as a function of crosslink density are shown in Figure 6.13. In the diagram the performance values are given as relative values. The 100% values are defined for the scratch resistance, given as the gloss retention after scratching (Gloss 100%), the flexibility (Erichsen 10 mm), the hardness (Pendulum 200 s), the elongation (Elongation at break 100%) and the chemical resistance as grades ranging from 0 to 5 (0 refers to no visible mark after chemical treatment).

For the bifunctional, flexible resin in combination with reactive diluents, the relative values show, that at the lowest crosslink density, the flexibility and elongation at break are at a high level, the scratch resistance starts at about 60% gloss retention, but approaches almost 100% at high X-link densities. The hardness starts at a very low level, but increases to about 45%, the chemical resistance does not change very much on a low level (Fig-

F IG . 6.12. Structure of urethane acrylates and reactive diluents used for the determination of basic coating properties as a function of crosslink density.

174

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

ure 6.13). Using the tri-functional, hard urethane acrylate resin as a basis, the diagram (Figure 6.13) clearly shows the pronounced influence of crosslink density on the increase of scratch resistance (gloss retention), pendulum hardness and chemical resistance. The gloss retention is increased from about 80% up to about 97% at crosslink densities around TABLE 6.3. Crosslink (X) density of UA resins (70%) in combination with 30% reactive diluents of different functionality Resin

Diluent

X-link density (×10−3 mol/cm3 )

UA 1 (flex) UA 1 (flex) UA 1 (flex) UA 1 (flex) UA 1 (flex) UA 1 (flex)

TMPMFA (1) DDDA (2) HDDA (2) TMPTA (3) PTA (4) DPHA (6)

0.31 0.88 1 1.88 2.4 2.22

UA 2 (hard) UA 2 (hard) UA 2 (hard) UA 2 (hard) UA 2 (hard) UA 2 (hard)

TMPMFA (1) DDDA (2) HDDA (2) TMPTA (3) PTA (4) DPHA (6)

0.9 1.7 2 2.7 2.4 2.6

F IG . 6.13. Correlation of coating performance with crosslink density of a bifunctional and a trifunctional resin, diluted with 30% diluents (of functionalities 1–6).

STRUCTURE–PROPERTY RELATIONSHIPS

175

F IG . 6.14. Correlation of scratch resistance with crosslink density of a bifunctional and a trifunctional resin, diluted with 30% diluents (of functionalities 1–6).

F IG . 6.15. Correlation of E-modulus with crosslink density of a bifunctional and a trifunctional resin, diluted with 30% diluents (of functionalities 1–6).

2.5×10−3 (corresponds to Mc < 330 g/mol). The trade-off by increasing crosslink density is in flexibility (Erichsen cupping), which decreases significantly with increasing crosslink density. Depending on the application a compromise between high scratch resistance and enough flexibility has to be found.

The scratch resistance, tested in the ScotchBrite® test (see Figure 3.18), is depicted in more detail in Figure 6.14 as a function of the crosslink density of the two different resin type series.14 It is shown that a high scratch resistance in the test can be obtained with both concepts if the crosslink density is high enough. This scratch resistance behaviour, however, is not due to the same “hardness” of the surface, since the E-moduli of the two series are significantly different (Figure 6.15). While the moduli of both coating series

176

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 6.16. Elastic work (W elast ) and plastic displacement (hplast ) as a function of crosslink density from nano-indentation measurements.

F IG . 6.17. Hardness vs. contact depth during nano-indentation of a UA coating exposed under air and nitrogen atmosphere.

are increasing with increasing X-link density, the absolute values of the hard UA1 series are higher by at least a factor of three. Thus, the high restoring forces induced by high crosslink density cause high elastic response in hard as well as in soft coatings, resulting in high gloss retention after scratching.

In order to more directly simulate the mechanical stress induced by scratching15 these samples were also investigated with a nano-scratcher/indenter system.16 This system is equipped with two orthogonal actuator/sensor systems that are designed to apply a welldefined load (p) perpendicular to the surface for indentation and parallel to the surface for scratching and to sense the penetration depth at the same time. During indentation load displacement curves are generated (see insert in Figure 6.17). From these curves apart from the reduced modulus of elasticity and the hardness,17 several other parameters can be

STRUCTURE–PROPERTY RELATIONSHIPS

177

derived: hmax The maximal indentation depth under load, consisting of the elastic deformation and the plastic deformation including viscous flow; helas The elastic part of the deformation or the elastic recovery; hplast The remaining plastic deformation when the load is fully removed; Welast The work which during the deformation is transferred to potential energy and which is finally recovered during release; Wplast The work that is lost in the plastic and viscous processes during deformation.

Altogether this set of parameters allows an assessment of the “hardness-flexibility balance” of the respective material.

In Figure 6.17 the elastic work and the plastic displacement as a function of crosslink density are illustrated. With increasing X-link density, the material reacts less plastic, which is also reflected in an increasing share of recovered elastic work (W elast ).

The conclusions of the indentation measurements can be summarized in the way that increased crosslink density increases the E-modulus, hardness and scratch resistance of the material – as expected. At the same time the share of plastic deformation is reduced and the “shape” recovery is enhanced, by the reactive interlocking of the chains. This interpretation is – on first glance – clearly not in line with the standard notion, that crosslinking hinders reflow. However the latter classical picture hints at reflow kinetics driven by surface tension, whereas here the crosslinking seems to induce a – so to speak – thermodynamic restoring force by interlocking strained polymer chains, which then try to release the strain by recovering their original positions.

It has to be noticed, however, that besides the structural influences, in UV curing systems the exposure atmosphere, due to the oxygen inhibition effect, has a considerable effect on the curing result, especially when scratching is addressed, since the scratching happens only at the uppermost surface layer. This inhibition effect is most pronounced at the surface, probably leading to a reduced crosslink density. At least a difference in hardness has been found in hardness measurements with nanoindentation showing a higher hardness at the surface of a coating exposed under nitrogen compared to air curing (Figure 6.17).

REFERENCES 1. Zumbrunn, M.A., Wilkes, G.L. and Ward, T.C., The characterization of the dynamical mechanical and dielectric properties of UV- and EB-cured coatings, In “Radiation Curing in Polymer Science and Technology”, Vol. III (J.P. Fouassier and J.F. Rabek, eds.). Elsevier Applied Science, 1993, pp. 101–152.

2. Wicks, Jr., Z.W., Jones, F.N. and Pappas, S.P. (Eds.), Chapter XXIV: Mechanical properties, In “Organic Coatings, Science and Technology, Vol. 2: Applications, Properties, and Performance.” John Wiley & Sons, New York, 1993, pp. 105–131, and main references: e.g., (a) Hill, L.W., J. Coat. Technol. 64 (808), 29, (1992); (b) Hill, L.W., “Mechanical Properties of Coatings”. Federation of Societies for Coating Technology, Blue Bell, PA, 1987; (c) Hill, L.W., Prog. Org. Coat. 5, 277 (1977).

3. Zosel, A., Farbe&Lack 83, 804 (1977).

4. Fox, T.G. and Loshaek, S., J. Polym. Sci. 15, 371 (1955).

5. Stutz, H., Illers, K.-H. and Mertens, J., J. Polym. Sci., Part B: Polym. Phys. 28, 1483 (1990).

6. Schwalm, R., Polym. Paint Colour J. Okt., 18–22 (1999).

7. Decker, C. and Moussa, K., J. Coat. Technol. 62 (786), 56 (1990).

178 8.

9.

10.

11.

12.

13.

14.

15.

16.

17.

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS Simon, S.L. and Gillham, J.K., J. Appl. Polym. Sci. 53, 709–727 (1994).

Decker, C., Elzaouk, B. and Decker, D., Preprints RadTech Europe 1995, p. 115.

Wu, S.J., J. Appl. Polym. Sci. 20, 327 (1992).

Schwalm, R., Häußling, L., Reich, W., Beck, E., Enenckel, P. and Menzel, K., Progr. Org. Coat. 32 (1–4), 191–196 (1997).

Königer, R., Beck, E. and Menzel, K., Farbe&Lack 4, 233 (1999).

DryAdd Simulation software (Intelligensys), http://www.intelligensys.co.uk/sim/dryadd.htm.

Gruber, N., presented at API Conference Ulm 2004. (a) Shen, W., Ji, C., Jones, F.N., Everson, M.P. and Ryntz, R.A., Surf. Coat. Intl. 79, 253 (1996); (b) Oliver, W.C. and Phar, G.M., J. Mater. Res. 7, 1564 (1992).

Schwalm, R., Beck, E. and Pfau, A., Eur. Coat. J. 1–2 (03), 39–46 (2003).

Klinke, E., Kordisch, M., Kunz, G. and Eisenbach, C.D., Farbe&Lack 4, 54–60 (2002).

C HAPTER 7

Tackling the Drawbacks of UV Systems The most important intrinsic drawbacks of the radically induced radiation curing technology together with proposed and practiced solutions to overcome these disadvantages are summarized in this chapter. Two of these drawbacks are related to the radical intermediates itself, which firstly hamper the curing reaction at the surface (oxygen inhibition) and secondly contribute to yellowing (initial yellowing), until they are completely decayed. The third main drawback is associated with the lack of radical formation in areas to which the light does not penetrate (shadow areas) and therefore no curing occurs.

7.1 OXYGEN INHIBITION The reaction of oxygen with the different species formed during the photopolymerisation and the effects on the network formation throughout the film thickness of a coating are

F IG . 7.1. Scheme of the oxygen inhibition reactions.

179

180

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

sketched in Figure 7.1. The termination of the reaction by hydroperoxides, the incomplete network formation at the surface and the remaining acrylate double bonds are commonly called the “problem of oxygen inhibition”.

In the photoinduced radical polymerization of multifunctional monomers, the efficiency of crosslinking is due to the efficiency of the propagation step. In the presence of air, the oxygen diradical reacts much faster with the photoinitiator or propagating radical to form a relatively stable peroxy-radical,1 which does not initiate the acrylate polymerization, but rather acts as an inhibitor. This inhibition results in an induction period of the polymerization until all oxygen is consumed.2 Therefore, in thin films the complete polymerization is retarded and in thicker films the acrylate conversions at the air–coatings interface are very low, resulting in tacky surfaces. This detrimental action of oxygen inhibition is shown schematically in Figure 7.1, depicting the lower acrylate consumption especially at the interface to air. The extension of the inhibited layer thickness is dependent on the oxygen diffusion into the coating. The penetration or diffusivity of oxygen in the coating layer itself is dependent on several factors, for instance the type and polarity of the materials used, as well as the viscosity, which is a major influencing factor. This oxygen penetration into the film can be derived from an approximate solution of Fick’s diffusion equation (7.1.1).

1/2 d = 6D( t) ,

(7.1.1)

with d = distance (cm), D = oxygen diffusivity (cm2 /s) and t = exposure time.

The oxygen diffusivity in water-like liquids (viscosity 1 mPa s) is on the order of 10−5 cm2 /s, and in typical UV resins with increasing viscosity from 10−6 to 10−8 cm2 /s.

Thus in typical UV polymerizations times ranging from 0.5 to 5 s, the oxygen molecules can penetrate distances of 0.1–10 µm. This theoretical estimate3 can be confirmed, as shown in Figures 2.14 and 7.2, where a polyether acrylate has been photopolymerized and the remaining concentration of double bonds has been determined by confocal Raman microscopy4 as a function of layer thickness. During curing of the unmodified resin,

F IG . 7.2. Double bond conversion as a function of depth into the films with a neat polyether acrylate resin, with amine modified resin (oxygen consumption) and with wax additive (oxygen barrier).

TACKLING THE DRAWBACKS OF UV SYSTEMS

181

a polyether acrylate, up to a layer depth of about 10 µm the double bond conversion remains very low, until in deeper layers the conversion increases significantly. By modifying the resin with amines or by applying a wax additive, the conversion is already high at the surface and almost as high as in the bulk regions. It has been shown,5,6 that the detrimental effects of oxygen inhibition are less pronounced when the diffusion of atmospheric oxygen into the liquid coating film is decreased by: • decreased oxygen concentration in the atmosphere, • increased formulation viscosity or • decreased sample temperature; and the cure speed increased by • high photoinitiator concentrations and/or efficient photoinitiators, • highly reactive formulations or • high light irradiance.

Several studies have shown that this inhibitory effect of oxygen in the photopolymerisation of acrylate based formulations depend on the type and concentration of the photoinitiators selected, the formulation reactivity and the irradiance.7,8 The Bowman group has presented a detailed study into the impact of oxygen on photopolymerisation kinetics recently.9 They found that the inhibition rate constant is ∼106 times greater than the propagation rate constant. They also investigated the effect of dissolved oxygen on the mechanical properties of the film and found, that it has a negligible effect on glass transition and modulus, since the concentration of oxygen terminated short-chain species is very low compared to the crosslinked polymer chains.

There are several methods known to reduce the effect of oxygen inhibition.

7.1.1 Physical Methods 1. High irradiance and/or high energy density 2. Inerting the exposure atmosphere 3. Physical barriers, like wax or protective films 7.1.1.1 High irradiance and high energy density The most applied method for overcoming oxygen inhibition is the use of high irradiance and high energy density in order to produce a high concentration of radicals, which quench the oxygen effectively and finally result in a high curing speed and a tack-free surface. The disadvantage associated with this method is the multiple overexposure of the coating compared with the energy density needed to cure the bulk of the coating.

The effect of high irradiance on the double bond conversion of a urethane acrylate formulation is shown in Figure 7.3. The influence of the irradiance on the conversion of a urethane acrylate formulation under air shows, that the total conversion improves significantly when the irradiance is increased from 15 to 90 mW/cm2 . A comparison of the effect of irradiance of curing under inert conditions (carbon dioxide) with air is given in Figure 9.41.

182

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 7.3. Conversion as a function of irradiance.

7.1.1.2 Inerting the exposure atmosphere The detrimental effects of atmospheric oxygen can be successfully overcome by inerting the exposure atmosphere.10 It does not play any significant role, which type of inerting medium will be applied, either nitrogen, argon, carbon dioxide or other inert gases. The comparison of nitrogen and carbon dioxide, as well as the different influence factors, like oxygen content, sample temperature, monomer viscosity, film thickness, type and concentration of photoinitiator, monomer reactivity, light irradiance have been evaluated, with special emphasis on carbon dioxide as the inert atmosphere are described in more detail in Chapter 9.2.1 (see refs. 5,6).

Carbon dioxide has the advantages over nitrogen that it is: • easily available and cheaper than nitrogen, • heavier than air and can therefore be maintained in a container without much loss.

In recent years, the use of inert gas was extended to several applications, especially in foil coating and in the printing sector.11 The motivation behind this was to achieve faster curing, to reduce the photoinitiator content, to reduce the number of lamps and to improve the quality. The cost for producing the inert atmosphere in belt systems is offset by savings in the plants and for the photoinitiators, so that this scenario may be worthwhile even independently of the improvement in quality.12 The influencing factors on the coating performance by working under carbon dioxide atmosphere and application examples are discussed in Chapter 9.

7.1.1.3 Physical barriers Simple but not widely usable possibilities for preventing the admission of oxygen include the use of floating waxes, as used in the 1960s for UV curable styrene/UP resin systems in combination with relatively low-power 30 W/cm lamps.13 The effect is demonstrated in Figure 7.2.14 Furthermore, inerting can be achieved by covering the coated substrate with a transparent protective foil,15 an evident example is realized in the UV curing of adhesive through a transparent foil.

TACKLING THE DRAWBACKS OF UV SYSTEMS

183

7.1.2 Chemical Methods 1.

2.

3.

4.

5.

6.

7.

Amine synergists High photoinitiator (PI) concentrations and PI type Acrylate monomer structure High reactivity Formulation viscosity Oxygen scavenging by a dye sensitizer Other additives

Hoyle has recently published an overview covering several of these methods to reduce the detrimental effects of oxygen inhibition on photocuring.16 He also demonstrated the effect of various additives on the polymerization exotherms by polymerizing acrylates together with additives in a photo-DSC (differential scanning calorimetry) in air with and without the additive in comparison to nitrogen atmosphere.

7.1.2.1 Amine synergists A long and well-known method for overcoming the oxygen inhibition is the addition of amines to the formulation, either as additives or chemically bound to acrylates via Michael addition.17 The proposed mechanism of the amine-cosynergist has been discussed in Chapter 4 (Figure 4.23).

The main effect is the oxygen scavenging reaction of the amine, based on the good hydrogen atom donor properties of the C–H-group adjacent to the nitrogen. The once formed C-centered radical, produced by hydrogen abstraction of a photoinitiator or propagating acrylate radical, either can scavenge an oxygen molecule to form a peroxy radical, which itself can further abstract a hydrogen from another amine, or initiate the polymerization directly. Despite the excellent oxygen scavenging properties of the amines, they exhibit some distinct disadvantages, like yellowing of the coating, poor weatherability, plasticizing effect, which limit their general usage.

7.1.2.2 High photoinitiator concentrations Since oxygen reacts readily with photoinitiator or propagating radicals, a high concentration of radicals in the system consumes oxygen and prevents diffusion into deeper layers. The effect of the photoinitiator concentration on the polymerization kinetics is given in Figure 9.41 as a comparison of the effect of curing under air and inert atmosphere. Furthermore, of course, the quantum yield of the radical formation is dependent on the photoinitiator type (see Chapter 4.1.4).

7.1.2.3 Acrylate monomer structure and viscosity Effective monomer structures in order to reduce the effect of oxygen inhibition are based on considerations to provide either labile hydrogen atoms as discussed in the case of amines, or to increase viscosity.

As discussed in Chapter 2, the type and structure of the resins and diluents have to be chosen by the application requirements, however, if the requirements allow the use of ethylene or propylene glycol or their thioether analogues to be used, the oxygen inhibition effect is significantly reduced.18 The proposed mechanism is similar to the mechanism demonstrated in Figure 4.23, however with the substitution of the –N–CH– group by the –O–CH– counterpart.

184

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

7.1.2.4 High reactivity The reactivity of the monomers or oligomers governs the cure speed and therefore the reaction time during which oxygen can penetrate into the sample. However, photopolymerization will not occur if the diffusion of oxygen is very high (e.g., low viscosity), even if the reactivity of the resin is very high, until the concentration of the oxygen dissolved in the sample has dropped by two orders of magnitude,19 and the monomer can compete successfully with the oxygen for the addition to the initiating radicals.

7.1.2.5 Formulation viscosity The rate of oxygen diffusion into the un-cured liquid film is significantly determined by the viscosity of the UV-curable formulation.20 The effect of the viscosity has been demonstrated by comparing the polymerization rates of films of an urethane resin (Laromer LR 8987) as a function of temperature, which changes dramatically the viscosity. The film thickness of 5 µm has been chosen in a range where the oxygen inhibition is most pronounced. An increase in the temperature causes a decrease in formulation viscosity. At −19 ◦ C, where the viscosity is rather high the polymerization rate (per [M]) is on the order of 1, and drops continuously to 0 by increasing the curing temperature over 6, 25, 50 ◦ C to 80 ◦ C. This behaviour is plotted in Figure 9.42 in comparison to the curing under carbon dioxide. The behaviour under air is due to decreased viscosity and therefore enhanced oxygen diffusion, leading to reduced polymerization rates. Similar results were obtained by photo-DSC evaluations.21 7.1.2.6 Oxygen scavenging by a dye The conversion of dissolved oxygen into singlet oxygen in the presence of a dye sensitizer and the scavenging of the singlet oxygen by 1,3-diphenylisobenzo-furan to generate 1,2-dibenzoyl-benzene, which can work as a photoinitiator has been described.22 However, associated with the use of this dye, the coating is slightly coloured, and therefore this approach has not been used extensively, probably due to the limited application range for slightly coloured coatings.

Recently, a novel system of singlet oxygen generator (zinc 2,9,16,23-tetra-tert-butyl29H ,31H -phthalocyanine (Zn-ttp)) and singlet oxygen scavenger (dimethylanthracene (DMA)), has been published.23 The combination of Zn-ttp/DMA and pre-illumination can effectively consume the molecular oxygen dissolved in the system. As a result, the inhibition period was significantly reduced and the rate of polymerization increased dramatically.

7.1.3

Conclusions

As discussed in this chapter, there are several possibilities to overcome the detrimental effects of oxygen inhibition, especially at the surface of an UV-cured coating. The most convenient way for working in an air atmosphere is to compensate the oxygen inhibition reaction by high irradiance and energy density, by selection of efficient photoinitiators and use of high photoinitiator concentrations, by selection of highly reactive formulations or by incorporation of additives, like amines or waxes. Each of these methods has its own advantages and disadvantages.

The very simple, yet, more expensive way is the inerting of the curing atmosphere.

There are exposure units available where nitrogen is cycled around the exposure set-up.

TACKLING THE DRAWBACKS OF UV SYSTEMS

185

F IG . 7.4. Possibilities to cure in shadow areas.

Well known, however, only recently further developed is the inerting with carbon dioxide.

The influencing factors on the curing conversion in the presence of carbon dioxide as the inerting medium are discussed in Chapter 9. Since carbon dioxide is heavier than air, a new process called Larolux® has been introduced, where the exposure can be performed in a carbon dioxide pool with very low-irradiance lamps. This process, as well as the curing under vacuum conditions (UV plasma curing), which can favourably be used for threedimensional curing, will also be described in more detail in the Chapter 9.

7.2 SHADOW AREAS “The brighter the light, the deeper the shadow” and “every cloud has a silver lining”. These worldly wisdoms also hold true for UV curing. The curing of relatively complex threedimensional objects is non-trivial. Such objects may appear for instance in car bodies or car doors, exhibiting hollow regions, the inside of which can hardly be reached by UV radiation and will therefore remain uncured. To resolve this issue, several approaches have been published: • Light tunnels with lamps illuminating the object from all sides • Placing lamps on robots This approach has been pursued in several evaluations (see Figure 2.17).

• Curing under inert conditions using reflective walls The curing under inert conditions using reflective (aluminium) walls will be described in Chapter 9 (Larolux® process).

186

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

• Dual cure systems: • UV and thermal, • UV and oxidative drying.

Dual cure systems combining UV-radiation with thermal curing have recently been developed.24–26 They contain two types of functional groups: usually UV curable acrylate double bonds, and thermally curable groups, for example, polyol and isocyanate or melamine-amino functionalities. The basic principle of these chemistries is depicted in Figure 7.4. There are several possibilities to design such systems. The simplest approach of just combining the individual molecules of multifunctional acrylates, polyols and polyisocyanates for example, results in the formation of interpenetrating networks, which are often inhomogeneous. This is due to a phase separation of the independently formed relatively unpolar polyacrylate network and the relatively polar polyurethane network. Therefore molecules have been designed which combine the different functionalities in one molecule, like acrylate double bonds and isocyanates and on the other hand hydroxyl groups and acrylate double bonds. The newly developed chemistries are discussed in more detail in Chapter 9.

The dual curing approach can also be transferred to water-based systems.27 Furthermore, dual curing can also be obtained with one type of chemistry, but different initiator types, namely a photoinduced radical initiator and a thermal activatable peroxide. With these initiators pure acrylate based coating systems can be used for dual curing.

• UV plasma curing As shown in Fig. 7.4, the UV plasma curing is as well as the Larolux® process a novel innovative concept of “inert” curing, which is based on the UV curing of (preferably threedimensional) parts in an evacuated plasma chamber. The process is described in more detail in Chapter 9.

7.3 INITIAL PHOTOYELLOWING28 Certain effects that are unique to radiation curing hamper the development of new UV coating applications. One such effect is initial photoyellowing that develops in the coating directly after exposure to ultraviolet (UV) or electron beam (EB) radiation (Figure 7.5), which has to be distinguished from the yellowing occurring (with almost all coatings) during aging and weathering. This initial photoyellowing is at least in part reversible and will, therefore bleach to some extent in the first few hours after the exposure. This variability in the colour of the coating makes exact colour matching very difficult until several hours after the radiation curing step, which, in turn, makes an inline quality control difficult.

To expand the use of radiation curing into new applications this effect needs to be better understood.

There are several influencing factors, which have been evaluated (ref. 28). The initial yellowing resulting from the curing reaction diminishes in the hours after exposure, the rate depending on the storage conditions (Figure 7.6). The bleaching rate is faster the higher the storage temperature. The yellowing development is not exclusively dependent on the presence of a photoinitiator, but rather due to the retarded decay of the formed radicals.

TACKLING THE DRAWBACKS OF UV SYSTEMS

187

F IG . 7.5. Different stages of photoyellowing (due to photocuring and photoaging, respectively).

F IG . 7.6. Initial photoyellowness decay as a function of storage conditions.

The effect of the photoinitiator on initial photoyellowing is shown in Figure 7.7, where the same resin is exposed to e-beam radiation in the absence of a photoinitiator, and to UV in the presence of an Îą-hydroxy alkyl acetophenone photoinitiator. The absolute values of yellowing (bâ&#x2C6;&#x2014; ) are a function of the e-beam dose (7 mrad, 6.8; 3 mrad, 4.0; 15 mrad, 11) or the photoinitiator concentration. Attempts at measuring the initial photoyellowing with various concentrations of photoinitiator were realized. As expected, the higher the concentration, the more coloured the film becomes up to a certain limit, after which the initial yellowing plateaus or even starts to decline again. A similar dependence of the photoinitiator concentration on the reactivity of UV-printing inks has also been observed.29 This effect is not well understood, however, it is possible that at high photoinitiator concentrations and therefore high radical concentrations termination reactions (radical combination) could remove radicals from the system or shorten the average chain length of the polymer.

This effect in turn would yield less long-lived radicals in the polymer matrix. Therefore,

188

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 7.7. Comparing initial photoyellowness and yellowness decay of e-beam exposed with UV exposed resin.

F IG . 7.8. Photoproducts of the decomposition of irgacure® 184 photoinitiator.

it is not the presence of photoinitiators per se which is responsible for the initial yellowing, but rather, the initial photoyellowing seems to be an inherent effect of the radical concentration.

The GC-MS spectrum of a cured solution of Irgacure® 184 in methanol showed the presence of a variety of scission products (Figure 7.8): benzene, cyclohexanol, cyclohexanone, benzaldehyde, 2-hydroxycyclohexanone, 1,1-dimethoxy-cyclohexane, methyl benzoate, 2-hydroxy-1-phenylethanone, Irgacure® 184, 2-diphenylethanone, benzoic acid, benzil,

TACKLING THE DRAWBACKS OF UV SYSTEMS

189

F IG . 7.9. Initial photoyellowing due to photoinitiator type in UA resin laromer® LR8987 under nitrogen.

o-benzoylbenzoin. It is conceivable that many of these by-products could lead to yellowing upon undergoing secondary reactions such as oxidation or condensation. However, none of the fragments found can directly account for the initial photoyellowing phenomenon.

Similar experiments were performed for other α-hydroxy alkyl acetophenones: Darocur® 1173 and Irgacure® 2959 were found to behave very similarly to Irgacure® 184. Consequently, all the α-hydroxy alkyl acetophenone photoinitiators, which have been investigated, also showed similar initial photoyellowing values. Acylphosphine oxides were introduced in radiation curable systems more than a decade ago by BASF and more recently by Ciba. 2,4,6-Trimethylbenzoyldiphenylphosphine oxide (Lucirin® TPO) and 2,4,6-trimethylbenzoylethoxyphenylphosphine oxide (Lucirin® TPO-L) are commonly used for pigmented systems or systems with high film thickness. Despite being yellow, these photoinitiators display photobleaching upon irradiation due to the destruction of the visible-light chromophore group of TPO. As the absorption of the photoinitiator bleaches, UV or visible light may penetrate deeper and deeper into the film. The initial photoyellowing of these photoinitiators is similar to the initial photoyellowing of α-hydroxy alkyl acetophenones (see Figure 7.9). However, the initial discoloration presented a slightly red coloration instead of the typical yellow coloration. Furthermore, combinations of Lucirin® TPO with other Photoinitiators may display synergistic effects and therefore show less initial photoyellowing than would be expected by simply adding the contributions from each photoinitiator. In order to avoid the formation of the yellow-coloured benzil, two other classes of photoinitiators have been tested. These are photoinitiators that undergo a primary process of hydrogen atom abstraction from the environment (or an intramolecular H-abstraction) and onium salt photoinitiators, which are primarily used for cationic polymerisation, but may also be used to initiate radical reactions. Benzophenone presents the disadvantage of developing significant discoloration when exposed to sunlight. Moreover, the yellowing during ageing becomes even worse if amines are used as co-initiator. Fortunately, some resins may also function as H-donors, so that the use of amines may be avoided.

Despite the poor long-term performance of benzophenone yellowing, it shows only a very slight initial photoyellowing (see Figure 7.9). No benzil was found in GC-MS

190

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

analysis of a cured solution of benzophenone in propan-2-ol, which further confirmed that no α-cleavages took place. It has been reported30 that the initial photoyellowing of isopropyl-thioxanthone (ITX) with N-methyldiethanolamine as co-initiator is due to photoproducts from ITX. Unfortunately, ITX already exhibits a yellow coloration and shows an even stronger initial photoyellowing. For these reasons thioxanthones are usually only used in pigmented systems such as screen inks.31 Similar to benzil dimethyl ketal (BDK), the initial photoyellowing for ITX decreases in the presence of amines when N,N-dimethylethanolamine is added as co-initiator. Phenylglyoxylates have, unfortunately, only been the subject of a limited number of studies probably because of its non-trivial photochemistry. It has been claimed32 that these molecules undergo an intramolecular Habstraction followed by a fragmentation similar to a Norrish type II. It may, however, be feasible that this structure could yield a Norrish type I reaction. Initial photoyellowing results were excellent with this initiator. However, the initial photoyellowing of this photoinitiator depends on the conditions under which the curing takes place. The initial photoyellowing was very slight with a urethane acrylate under air. However, the initial photoyellowing was significant under inert conditions. Furthermore, the real time-IR spectra showed that the polymerisation realized under inert conditions achieved a high conversion of 71%, whereas, the reaction under air only reached a conversion of 44% (in thin films (film thickness ca. 10 µm)). The presence of benzaldehyde found in the GC-MS spectrum of a photolyzed solution of methyl phenylglyoxylate (Nuvopol® PI 3000) suggests that (apart from H-abstraction) a reaction type Norrish I may also take place.

Segurola et al.33 drew similar conclusions by evaluating the photoyellowing and discoloration of the photoinitiator types, summarizing that 1-hydroxy-cyclohexyl-phenyl ketone (Irgacure 184) exhibited very little discoloration and gave the best results of all photoinitiators studied and from the near UV absorbing photoinitiators Lucirin TPO (acyl phosphine oxides) caused lower yellowing compared to α-aminoalkyl ketone photoinitiators due to photobleaching.

Amines are usually used to improve the reactivity of a system or as hydrogen donors.

However, as systems containing amines present a strong discoloration when exposed to high temperatures it is advantageous to avoid their use in formulations. However, amines can also have a positive influence on the initial discoloration; as mentioned above, BDK shows a lower initial photoyellowing in the presence of amines, as does Nuvopol® PI 3000.

It has been claimed34 that the reduction of the initial photoyellowing in the presence of amines with BDK could be explained by the photoreduction of the carbonyl chromophore to carbinol compounds through intermolecular hydrogen abstraction. However, in the case of most Norrish Type I photoinitiators the presence of amines leads to significantly stronger initial photoyellowing.

Triarylsulfonium salts are commonly used as cationic photoinitiators. Cyracure® UVI6990 is a mixture of sulfonium salts. In the unexposed form it already exhibits a slight red colour. The acid obtained with this photoinitiator after curing leads to a very violent reaction with some vinylethers. However, it is suitable for initiating a polymerisation with epoxides. As in the case of iodonium salts urethane acrylates could be polymerised radically too. After UV curing a film of the aliphatic epoxide Basoset® 162 and a film of Laromer® LR 8987 with Cyracure UVI-6990 as photoinitiator, the films showed no increased initial photoyellowness. However, a slight red colour was observed. This initial

TACKLING THE DRAWBACKS OF UV SYSTEMS

191

discoloration may be a result of (i) coloured by-products which are formed from radicals or (ii) side reactions of the super acid HPF6 with the radiation curable resin (e.g., condensation reactions or rearrangements). Dektar and co-workers have already studied the photoproducts resulting from the photolysis of triphenylsulfonium salts by GC-MS.35 Cyracure UVI-6990 is expected to yield additional photolysis products as a result of being a mixture and because of its more complex structure.

The initial photoyellowing may depend on the quantity of photoinitiator which is photolyzed and thus on the quantity of light received by the sample. The more the sample is irradiated, the more yellow the film is. This observation is true for low irradiation doses, however, at higher doses this simple correlation no longer holds – in fact it reverses. It is feasible that at very high irradiation doses, the coloured photoproducts, which are themselves sensitive to UV-light, are further photolyzed. Also at higher radical concentrations, radical combination reactions become more favoured, thereby removing reactive species from the system. This is also reflected in the dependence of the photoyellowing on the photoinitiator content (Figure 7.10). The photoyellowing goes through a maximum, indicating that with higher photoinitiator contents more radicals are produced, however, under these conditions more radical termination reactions also become more likely.

The binder seems to play an important role in the initial photoyellowing. Four classes of radiation curable resins have been tested: (i) urethane acrylates, UAs (aliphatic polyurethane acrylates are the most suitable for industrial outdoor applications), (ii) oligoether acrylates, POs, (and amine modified oligoether acrylates, POAs), (iii) epoxy acrylates, EAs, and (iv) polyester acrylates, PEs. Using Irgacure® 184 as photoinitiator, different behaviours were observed between the different binders, which suggest that the initial photoyellowing depends on both the photoinitiator and the matrix it is in. The best initial photoyellowing result was obtained with the aliphatic urethane acrylate Laromer® UA19T (see Figure 7.11). Tripropylene glycol diacrylate (TPGDA) and diethylene glycol diacrylate (DEGDA) gave better results, however, TPGDA and DEGDA are very rarely used as sole binders in a formulation.

F IG . 7.10. Influence of the content of the photoinitiator on the initial photoyellowing.

192

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 7.11. Initial photoyellowing of different (Laromer® ) acrylate resins types exposed under air and nitrogen atmosphere.

In most resin classes a slight trend is detectable favouring either air or inert conditions for curing, e.g., aromatic epoxy acrylates (EAs) show less initial photoyellowing under nitrogen, while UV monomers show less initial yellowing under air. These differences are generally considered to be too small to be significant for normal applications. Furthermore, the data also show that amine modified polyether acrylates (POAs) exhibit a stronger initial discoloration than the polyether acrylates (POs). UAs and POs yielded the best results.

Investigations employing Lucirin TPO as the photoinitiator gave very similar results to the results obtained with Irgacure® 184.

Further experiments investigating the reactivity, absorption and viscosity of different radiation curable resins showed that there was no direct correlation between these properties and the observed initial photoyellowing. It is worth noting that in many cases the extent of initial photoyellowing does not correlate with long-term yellowing from artificial weathering or ageing experiments. On the contrary, the systems known for their low yellowing in ageing experiments (urethane acrylates) seem to be particularly prone to initial photoyellowing.

7.3.1

Conclusions

Thus, both the photoinitiator and the binder play an important role for the initial photoyellowing of radiation cured coatings. Except for BDK, photoinitiators undergoing photoscissions exhibit very similar initial photoyellowing results – qualitatively and quantitatively – presumably because they all have the benzoyl radical in common. With the exception of ITX, which exhibits a very strong discoloration after UV exposure, photoinitiators that undergo H-abstraction present low levels of initial photoyellowing. However, it is these photoinitiators that present yellowing upon ageing, especially, benzophenone and its derivatives. Cationic onium photoinitiators show only a relatively slight initial photoyellowing;

TACKLING THE DRAWBACKS OF UV SYSTEMS

193

F IG . 7.12. Scheme for explanation of initial photoyellowness in radical initiated coatings.

their low solubility as well as the reddish colouring of the cured films make these cationic photoinitiators suitable for only a small number of applications. The complete absence of photoinitiators in the case of electron beam curing does not solve the problem of the initial photoyellowing effect either. On the contrary, electron beam curing has led to relatively strong initial photo-yellowing effects.

Binders show varying trends in their initial photoyellowing depending on their chemical composition or resin class. Stabilisers, amines and the atmosphere during curing influence the strength of the initial photoyellowing to some extent and may allow incremental modifications of the initial photoyellowing. However, these differences in initial photoyellowing are minor in comparison with the contribution from the combination of resin and photoinitiator used in a formulation. The chemical nature of the polymer network as well as it’s density seems to be strong influencing factors regarding initial photoyellowing. Both electron beam and thermal curing also show strong initial yellowing which suggests that the yellowing is inherent to these highly crosslinked polymers. It is conceivable that the coloration is due to radicals or highly reactive species that are trapped in the polymer matrix and therefore have a significant lifetime of several hours to weeks36 (Figure 7.12).

Any modification of the conditions that increases the mobility (i.e., delay vitrification) of these reactive centres (temperature, solvents) will reduce initial photoyellowing. Furthermore, reducing the number of radical species also reduces the observed photoyellowing (photoinitiator, energy density, stabilizers). Unfortunately, the phenomenon of initial photoyellowing is very complex and there is no universal solution. Therefore, curing conditions and raw materials need to be carefully examined for each application to reduce initial photoyellowing to a minimum.

REFERENCES 1. Decker, C., “Handbook of Polymer Science and Technology”, Vol. 3. 1989, p. 541.

2. Kloosterboer, J.G., Adv. Polym. Sci. 84, 1–61 (1988).

3. Krongauz, V.V. and Chawla, C.P., Photopolymerization in compact and digital versatile disks manufacturing: Peculiarities of oxygen effects, RadTech Report September/October 2001. pp. 34–46.

194 4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

17.

18.

19.

20.

21.

22.

23.

24.

25.

26.

27.

28.

29.

30.

31.

32.

33.

34.

35.

36.

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

Schrof, W., Beck, E., Königer, R., Reich, W. and Schwalm, R., Progr. Org. Coat. 35, 197–204 (1999).

Studer, K., Decker, C., Beck, E. and Schwalm, R., Progr. Org. Coat. 48, 92–100 (2003).

Studer, K., Decker, C., Beck, E. and Schwalm, R., Progr. Org. Coat. 48, 101–111 (2003).

Wight, F.R. and Nunez, I.M., J. Radiat. Curing 16 (1), 3 (1989).

Müller, U. and Vallejos, C., Angew. Makromol. Chem. 206, 171 (1993).

O’Brien, A. and Bowman, C., The impact of oxygen on photopolymerization kinetics, RadTech North America, eI5, UV&EB Technology, Charlotte, NC, May 2004, Technical Conference Proceedings. 2004, on CD. (a) Müller, R., Proceedings of RadTech Europe Conference, Basel, 2001, p. 149; (b) Henke, T., Proceedings of RadTech Europe Conference, Basel, 2001, p. 145; (c) Beck, E., Proceedings of RadTech Europe Conference, Basel, 2001, p. 643.

Menzel, K., Bankowsky, H.-H., Enenkel, P. and Lokai, M., RadTech Europe 1999, Conference Proceedings.

1999, pp. 165–170.

Jung, J., RadTech Europe 1999, Conference Proceedings. 1999, p. 41.

Bolon, D.A. and Webb, K.K., J. Appl. Polym. Sci. 22 (9), 2543 (1978). (a) Schrof, W., Beck, E., Königer, R., Meisenburg, U., Menzel, K., Reich, W. and Schwalm, R., RadTech North America, 1998. pp. 363–374; (b) Schrof, W., Häußling, L., Schwalm, R., Reich, W., Menzel, K., Königer, R. and Beck, E., RadTech Europe, 1997, Conference Proceedings. 1997, pp. 535–547.

Davidson, S., “Exploring the Science, Technology and Applications of UV and EB Curing”. SITA Technology Limited, London, 1999, p. 249.

Hoyle, C., An overview of oxygen inhibition in photocuring, RadTech North America, eI5, UV&EB Technology, Charlotte, NC, May 2004, Technical Conference Proceedings. 2004, on CD. Davidson, R.S., The role of amines in UV-curing, In “Radiation Curing in Polymer Science and Technology: Polymerization Mechanism”, Vol III (J.P. Fouassier and J.F. Rabek, eds.). Elsevier Applied Science, London, 1993, p. 153.

Pappas, S.P. (Ed.), “UV Curing: Science and Technology”, Vol. II. 1985, Technology Marketing Corporation, p. 125.

Decker, C. and Jenkins, A.D., Macromolecules 18, 1241 (1985).

Kunz, M., Strobel, R. and Gysau, D., Proceedings of RadTech North America Conference, 1996, p. 278.

Abadie, M.J.M. and Voytekunas, V.Y., Eurasian ChemTech J. 6, 67–77 (2004). (a) Decker, C., Faure, J., Fizet, M. and Rychla, L., Photogr. Sci. Eng. 23 (3), 137 (1979); (b) Decker, C., Makromol. Chem. 180, 2027 (1979).

Gou, L. and Scranton, A., A photochemical method to eliminate oxygen inhibition in photocured systems, RadTech North America, eI5, UV&EB Technology, Charlotte, NC, May 2004, Technical Conference Proceedings. (2004), on CD. Königer, R., Farbe&Lack 105, 233 (1999).

Maag, K., Lenhard, W. and Löffles, H., Progr. Org. Coat. 40, 93 (2000).

Fischer, W. and Weikard, J., Farbe&Lack 107, 120 (2001).

Decker, C., Masson, F. and Schwalm, R., Macromol. Mater. Eng. 288, 17–28 (2003).

Königer, R. and Studer, K., Eur. Coat. J. 1–2, 26–58 (2001).

Seng, H.-P., Coatings 5, 199 (2000).

Hult, A. and Ranby, B., Polym. Degr. Stab. 8, 89 (1984).

Crivello, J.V. and Dietliker, K., “Chemistry and Technology of UV & EB Formulation for Coatings, Inks & Paints, Photoinitiators for Free Radical Cationic and Anionic Photopolymerisation”, Volume III, 2nd edn.

Wiley, 1998, p. 107.

Crivello, J.V. and Dietliker, K., “Chemistry and Technology of UV & EB Formulation for Coatings, Inks & Paints, Photoinitiators for Free Radical Cationic and Anionic Photopolymerisation”, Volume III, 2nd edn.

Wiley, 1998, p. 186; Encinas, M.V., Lissi, E.A., Zanocco, A., Stewart, L. and Scaiano, J.C., Can. J. Chem.

62, 386 (1984); and Huyser, E.S. and Neckers, D.C., J. Org. Chem. 29, 276 (1964).

Segurola, J., Allen, N.S., Edge, M., McMahon, A. and Wilson, S., Polym. Degrad. Stab. 64, 39–48 (1999).

Herlihy, S., Proceedings of RadTech Europe 1999 Berlin, November 8–10. 1999, p. 489.

Dektar, J.L. and Hacker, N.P., J. Chem. Soc., Chem. Commun. 1591 (1987). (a) Atherton, N.M., Melville, H.W. and Whiffen, D.H., Trans. Faraday Soc. 54, 1300 (1958); (b) Selli, E., Trends Photochem. & Photobiol. 4, 55 (1997).

C HAPTER 8

Classical Applications The traditional applications of UV-curable systems are in the market sectors where temperature sensitive substrates are coated, like wood, paper and plastics as well as in imaging applications, like electronics and printing plates (Figure 8.1).1 The worldwide consumption of UV curable acrylate formulations in the year 2000 was approximately 140,000 tons. The dominating market sector worldwide is the graphic arts, with a share of about 50% in NAFTA, 42% in Europe and 35% in Japan, the wood sector being the second largest with 50% in Europe and about 14% each in NAFTA and Japan (Figure 8.2).

8.1 ON WOODEN SURFACES The wood finishing industry today deals with solid wood constructions, composition boards, like particle-boards or hard-boards, as well as with moulded polymers or foams,

F IG . 8.1. Classical applications of radiation curable coatings.

195

196

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 8.2. The World market for UV-curable acrylate resins in the traditional applications (wood and graphic arts).

F IG . 8.3. Wood coatings market in Europe by chemistries.

decorated with wood imitating finishes. The main segments of wood processing are the direct processing of solid wooden products, like solid wood furniture, window/door frames or construction components, articles made of wooden pieces, like (veneer) coated fibre boards or chipboards, and wooden floorings, like parquet. The main markets for furniture production are in Europe, America and China. The market for wood coatings in Europe in the year 2000 was about 460,000 tons, polyurethane coatings accounting for about 34%, followed by nitrocellulose based (16%) and acid catalyzed coatings (15%) (Figure 8.3).

CLASSICAL APPLICATIONS

197

Wood processing in Europe is mainly done in Italy (33%), Germany (19%) and Spain (18%). In Europe UV-curable coatings have a considerable market share of about 12%.2,3 The main applications of UV coatings in the wood sector are on furniture and on flooring.

The furniture wood coatings sector is dominated by large furniture manufacturers consuming large quantities of coatings for mass production, like IKEA, and to a lesser extent by artisans producing smaller production runs or pieces of furniture. Examples are residential furniture, like drawers, chairs, Tables, kitchen cabinets, bathroom vanities, hotel furniture, office furniture, doors, or parquet hardwood flooring.

The coating of wooden flat panels with UV-curable materials started at least 30 years ago, mainly with filler sealer applications using unsaturated polyester-styrene coatings, few basecoat formulations and several topcoat applications,4 based on unsaturated polyesters and, due to higher speed and better performance, acrylate based coatings.

Nowadays, a large variety of several different types of acrylate coatings are available, the major classes being:5 • 100% liquids for flat panel coating with roller application, and coating mouldings or shaped panels; • Water-based UV coatings for flat panels with curtain coating and coating mouldings or shaped panels with spray applications; • UV powder coats.

Flat panels are coated mainly with 100% liquid UV systems. Complete solutions for UV staining, UV sealers and UV topcoats are available as well as the adequate equipment, like curtain and roller coaters and exposure systems. Large scale production is realized with parquet finishes, flat panel wood with open or closed grain, either mat or high gloss, and flat doors. For coating of shaped panels with 100% solids, sprayable formulations have to be used, which either contains appropriate amounts of monomers or low viscous oligomers, which can be applied with hot spraying systems (“unisprayer”, CEFLA).

Industrially manufactured parquet is one of the domains of UV curable coatings, since they can be easily roller coated and cured at high speed. They exhibit high-abrasion and scratch resistance. Formerly used coatings based on unsaturated polyesters diluted with styrene have been replaced by acrylic type systems. As already discussed in the formulations section (Chapter 5.1), such parquet coatings are usually composed of: • a primer layer, responsible for the adhesion, • filler/sealer, and/or • several undercoats, which may contain abrasive resistant hard particles (corundum), responsible for abrasion resistance (Figure 8.4), and • a top coat layer, responsible for scratch and chemical resistance and the optical appearance, the entire layer thickness being up to 100 to 150 µm.

Water-based UV coatings are often used as primers in order to enhance adhesion of the base and/or top coat, as well as for top coats with matt finishes, since the traditional 100% liquid UV coatings are more difficult to matt6 due to marginal shrinkage compared to solvent-borne and water-based coatings (Figure 8.5).

198

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 8.4. Abrasion resistance of parquet flooring coatings containing filler and unfilled.

F IG . 8.5. Matting process.

Unsaturated polyesters/styrene or epoxy acrylate resins are often used in filler/sealers.

Furthermore, clay, talcum or calcium carbonate are commonly used as fillers in these systems. Typically, antibubbling and leveling additives are also included in the formulation to yield coatings with a layer thickness of up to 50 µm.

The parquet top coatings are predominantly 100% liquid acrylic systems based on polyester or urethane acrylates applied at thicknesses of around 50 µm.

As alternatives to the 100% liquids, UV powder coats are heavily discussed as a viable coating alternative on MDF (medium density fibre boards) products, since they melt at low temperatures. When heated with IR and exposed to UV relatively short curing times (1– 2 min) prevent excessive heating stress on the substrate. This is important to avoid flaking problems due to heating up the MDF board and helps reduce the required cooling capacity needed. The chemistry of UV powder coatings will be discussed in Chapter 9.

The coverage of wooden boards (chipboards, fiberboards) with foils, consisting of impregnated paper, which is printed with wood pattern and sealed with clear coats, is exem-

CLASSICAL APPLICATIONS

199

plified in Chapter 10. The clear coats used are often UV curable systems because of high production speed and good chemical and scratch resistance of the final coating.

The challenges of industrial wood finishing beyond environmental concerns, to also address performance issues. For instance wear resistance of floorings, including new test methods to predict wear resistance, similarly, increased durability for external joinery and once again methods to predict durability. Increase of wear resistance by application of nanoparticles is an area of extensive research, since the nanoparticles introduce higher strength into the coating without negatively influencing transparency and gloss. Nanoparticles have also been used to introduce antibacterial properties into wood coatings,7 thus enabling the use of wooden articles in hygienic applications, like hospitals. The unsatisfactory durability of exterior coatings is not related to the topcoat performance but mainly attributed to poor adhesion. Therefore, the improvement of adhesion of the coating to wooden surfaces is a major issue. Beside the use of waterborne primers and low surface tension coatings by plasma treatment, crosslinking of the coating with the wood by reaction with isocyanato-acrylates has been discussed.8

8.2

IN GRAPHIC ARTS INDUSTRY

UV-curable systems are used in graphic arts in various applications, in imaging, as inks, as well as overprint coatings/varnishes. This market is by far the largest of all UV coatings applications (see also Figure 1.4).

8.2.1 Imaging Applications Photocuring is one possibility out of a wide range of photochemical induced reactions applied in imaging technology, such as typical copying processes (electrophotography, diazotyping), in graphic arts applications (diazo systems, colloid-dichromate emulsions, photopolymer printing plates) and in photoresists (photosolubilizing diazoquinone systems, photoacid catalyzed systems, azide crosslinking, photodimerization, photopolymerization).

The process relies on applying a coating on a substrate, exposing through the image mask and subsequent removal (development) of the unexposed (negative) or exposed (positive) areas (Figure 8.6). Photocurable materials are only used in negative acting systems, where they induce a crosslinking reaction in the exposed areas and the unexposed parts can be dissolved (developed) in the solvent, which has been used in the coating formulation. In the negative process swelling during development is often observed, which may reduce the resolution capabilities. In positive imaging usually the solubility is changed by exposure from a hydrophobic to an alkaline soluble state, for example, blocked phenolic resins are deblocked to restore the alkaline soluble phenol resin. The imaging process is similar for photoresists and printing plates. The required resolution is much lower in printing plate applications (several Âľm pattern size) than in photoresists (from <1-Âľm to <100 nm). The different resolution requirements is the reason why the negative process still dominates in printing plate fabrication while positive processes are mainly used in photoresists.

200

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 8.6. Relief/pattern formation in photoresists and printing plates.

Printing plates are classified in the main categories of gravure, letterpress and flexo, screen-printing and lithography (offset). The printing methods are shown schematically in Figure 8.7.

In gravure printing plates the image is generated by a series of pitched areas (cells) which are formed with an etcherâ&#x20AC;&#x2122;s needle in a copper based top layer of a steel cylinder. The ink is delivered to the pitches and transferred to the printing stock. The precise control over process dots makes gravure the ultimate fine line process printing, for example, applied for printing catalogues and magazines. However, the high cylinder costs make the process unalluring for general purpose printing. Recent advances in curing technology by efficient inerting the surface layer enable the application of UV overprint varnishes in high speed web printing applications, like gravure inline UV printing with speeds up to 15 m/s.9 Letterpress (including flexo) printing plates are produced by image-wise exposure of a photopolymer plate, where the exposed parts result in hard relief pattern, and the unexposed areas are washed away. The printing plates work like the lead letters of a Gutenberg printing press. In letterpress negative photocurable systems are dominating due to significantly higher layer thickness compared with lithographic plates, which requires high quantum efficiency only available with photopolymerizing chain reactions.

A flexographic printing plate looks very similar to an enlarged rubber stamp. It is produced by an image-wise exposure in the same way as in letterpress, however, softer patterns are generated. The ink is transferred from the raised pattern to the (in most cases) flexible substrate.

The formulations for the production of photopolymer printing plates contain as a main component a polymeric binder, which determines the important application properties hardness, flexibility and resistance to printing inks. Binders used in letterpress plates are poly(vinyl alcohol), polyamides, polyacrylates and for flexographic plates elastomers, like

CLASSICAL APPLICATIONS

201

F IG . 8.7. Scheme of the printing techniques.

styrene-butadiene and nitrile rubbers. The photoinitiators and crosslinkable oligomers and monomers are predominantly the same as described for radiation curable coatings.

Screen-printing is an easy printing process where an image is produced on a metal screen by blocking the nonprinting pores with waxes or coatings. The ink is then pressed through the open meshes of the screen with the aid of a doctor blade or a flexible squeegee. Screen printing is the most versatile of all printing processes. A wide variety of substrates can be used, for example paper, plastics, metals glass, textiles, and many other materials. Examples of common products from the screen printing industry include posters, labels, decals, signage, and all types of textiles and printed circuit boards for electronic applications. A big advantage of screen-printing over other print methods is it can print on substrates of any size, shape, and thickness.

Lithographic printing plates are hydrophilic, oxidized aluminium plates containing a 1â&#x20AC;&#x201C;5 Âľm light sensitive layer, defined as presensitized plates. The processing results in an image-wise differentiation of hydrophilic and hydrophobic areas. For instance, negative light sensitive layers use photoinitiated polymerization of carboxylic group containing acrylate oligomers, which can be developed with alkaline solutions. After crosslinking with unsaturated polyesters or other chemistries the solubility of these carboxylic group containing acrylate oligomers is significantly reduced in the exposed areas. Additionally the light sensitive layers contain dyes in order to provide a good contrast between exposed and unexposed areas. Negative plates are used mainly for newspapers and positive plates for art prints, brochures and advertising materials.

202

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

Other imaging applications are photoresists, which are used in the production of printed circuit boards (PCB) and integrated chips (IC) to define the circuit elements in a chip or PCB. The term photoresist stems from the two functions it has to fulfill, namely to enable a photoinduced pattern generation, which is used to mask the underlying areas during subsequent image transfer steps, thus, to resist the attack of chemicals. Thompson10 and Steppan11 give a comprehensive overview about theory, materials and processing. The types of photoresists are classified by their physical constitution (liquid, dry film), radiation response (X-ray, e-beam, UV), mode of operation (positive/negative) or number of main components (1C, 2C). Contrary to PCB applications where dry film resists dominate, in IC manufacturing only liquid resists are used. Until recently negative two component resists consisting of a cyclized rubber matrix and a bis-arylazide sensitizer, forming nitrenes upon exposure which crosslinked the matrix, had more than 50% market share. Since several years the workhorse of the IC manufacturers are two component positive resists, based on Novolac and a diazonaphthoquinone, which becomes alkaline soluble upon photoinduced rearrangement to an indene carboxylic acid. These resists respond to near UV radiation and since the required pattern sizes have decreased steadily below 0.5 µm shorter exposure wavelength in the deep UV (DUV, 254 nm eximer line) are needed, which prompted higher sensitivity requirements. Thus, completely new resist materials utilizing a “chemical amplification reaction”, based on photoacid generators (PAG) evolved. The resins used in DUV resists are based on acetal and/or carbonate (t-BOC) protected polyhydroxystyrenes in conjunction with onium salt sulfonates as the major PAG class. Upon exposure the PAG releases a proton, which cleaves the protective groups in an acid catalyzed reaction. The exposed parts become alkaline soluble and can be dissolved with appropriate developers. Bases have been added in order to stabilize the image and prevent the cleavage of the complete resist layer within storage times between exposure and development. Reviews of the newest material generations and future challenges were published recently by Reichmanis12 and for the latest generation of Deep UV resists.13 Deep UV lithography, operating at 248 nm, has become the technology of choice for the production of 130 nm patterns and will coexist with 193 nm lithography in order to manufacture sub 100 nm design features.

8.2.2

Inks

Inks contain a colorant (dye or pigment), resin binders, carrying vehicles (solvent or water) and additives (catalysts, wetting agents, waxes). The printing ink market worldwide accounted for about 4% of the entire coating consumption, calculated to about 1 million tons in the year 2000. In the US the value of shipped inks was about $ 5 billion in 2001, the most expensive components being the coloured pigments (about $ 800 million), whereof carbon black accounts for about 70%. The ink demand of end-use markets was largest for packaging (36%), commercial printing (33%) and publishing (23%).14 From a technology point of view, solvent-based inks have been popular in the past, but due to environmental concerns printers have been driven to water-based inks wherever possible. However, nowadays UV curable inks are desirable because of low VOC, fast cure, higher gloss and better chemical and rub resistance.

CLASSICAL APPLICATIONS

203

The issues related to the use of UV curable inks (as well as for coatings) in packaging printing could be divided into the two sectors of food packaging and non-food packaging.

The use of UV-curable systems in non-food packaging applications is uncritical, whereas UV printing of food packaging is in general critical, since the total amount of migratables have to be below a specified value (<60 mg/kg). It is generally possible to obtain such low extractable values with radiation cured coatings, however, the photoinitiators or photoinitiator residues contribute to some extent to the extractable portions, and thus have to be carefully selected. In this context, polymer-bound or polymerizable photoinitiators as well as electron beam curable coatings are less critical, since they do not need any photoinitiator.

UV-curable coating formulations intended for food packaging with indirect food contact need to be accredited by authorized test institutes.

UV-curable inks are mainly used in commercial screen, lithographic (offset) and letterpress/flexographic printing. See Chapter 5.2 for ink formulations.

UV inks for screen-printing is the largest printing application. With screen-printing, unlike other printing methods, all types of substrates, like paper, plastics, textiles, metals, leather, wood, ceramics or printed circuit boards, can be coated. Major applications of screen-printing are labels, signs, decals, optical disks (CD, DVD) and book covers.

The advantages of screen-printing are low cost and fast fabrication of the screen image, and due to high viable layer thicknesses, high color strength and high gloss are achievable.

Typical formulations are based on 15–60% of acrylated resins (urethane, epoxy or polyester acrylates), 15–40% monomers (mono- to trifunctional), 5–30% pigments and 1–5% photoinitiators (benzophenone, isothioxanthone (ITX) or others). The viscosity is in the medium range of about 1–5 Pa s in order to provide good wetting and no foaming. The cure speed in screen-printing is about 30 m/min.

Flexographic printing is mainly used for flexible substrates, cardboards, paper, or labels. UV inks are experiencing high growth rates in flexographic printing applications.15 Due to low viscous, 100% materials, exceptional image sharpness and excellent end-use properties, like rub and chemical resistance can be achieved. UV flexo inks do not change consistency during processing in contrast to solvent-borne (evaporation) or water-based (pH changes) inks. The UV ink does not dry in the cells of the anilox rollers and therefore needs fewer cleaning cycles. The drawbacks related to UV flexo inks are the higher operating costs due to the need for a wider assortment of anilox rollers in order to adjust colour strength, skin irritation of UV ink chemistry, adhesion problems on some polymeric substrates and cost as well as heating associated with the use of UV lamps units. The formulations are based either on epoxy acrylate resins or on blends of epoxy and urethane acrylates, the viscosity being in the range of 300–500 mPa s. Weighing the advantages in image quality, handling and environmental issues against drawbacks in view of water and solvent alternatives will give rise to further growth of UV flexo inks.

Lithographic or offset printing is based on the use of hydrophilic oxidized aluminium plates containing the images brought about with the aid of a photolithographic process.

The image areas are therefore hydrophobic and the hydrophilic areas are wetted with water in order to repel the hydrophobic (often oil-based) ink. During the printing process the ink is offset (transferred) from the plate to a rubber cylinder and finally to the substrate.

Offset is the largest printing application, however, UV-curable systems still have a very small share. The formulations are similar in composition to that of screen inks, where

204

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

• Performance requirements – low cost – high reactivity – high gloss – good wetting behaviour of the ink • Coating – Epoxy, polyester and/or amine modified polyethers – high photoinitiator content (benzophenoen, ITX) – high content of levelling and wetting agents • Application – about 6–10 µm; in-line with the printing equipment F IG . 8.8. Major requirements for overprint varnishes (OVP).

epoxy acrylates are the most preferred resins and the viscosities are slightly lower, in the range of 1–2 Pa s. In order to speed up drying behaviour of classical oil-based (alkyd) inks, hybrid inks consisting of blends of oil-based and UV-curable inks are recently enjoying great popularity.

Digital printing (ink–jet) will be discussed in Chapter 10.

8.2.3

Overprint Varnishes (OVP)

Overprint varnishes are clear coats applied, for example, on printed cartons (e.g., cosmetic boxes), magazines, catalogues or book covers, greeting cards, labels or CD, DVD jackets.

The varnishes are usually applied inline after the lithographic/offset (cartons and covers) or flexographic printing (labels, flexible packaging) process in which interactions between ink and coating systems have to be carefully adjusted. Less often the coatings are applied in a separate coating step after printing. The major features of overprint varnishes are depicted in Figure 8.8. The performance requirements of high curing speed and low cost necessitate the employment of relatively cheap resins (epoxy acrylates or amine modified acrylates), and since applied in high concentrations, cheap photoinitiators (benzophenone, ITX). Formulation examples for OVP’s have been discussed in Chapter 5.

UV curing in graphic arts is described in more detail in Fouassier, Vol. IV, Chapter 7.16 REFERENCES 1. Pfeiffer, R., Photopolymerization, In “Applications of Photopolymer Technology” (A.B. Scranton, C.N. Bowman and R.W. Pfeiffer, eds.). ACS, 1997, Chapter 1.

2. Bankowsky, H.H., et al., Farbe&Lack 106 (10), 50 (2000).

3. Goldschmidt, A. and Streitberger, H.-J., “BASF Handbook on Basics of Coating Technology.” Vincentz Network, 2003, Chapter 7.10, p. 735.

4. Gruber, G.W., In “UV Curing: Science and Technology” (S.P. Pappas, ed.). Technology Marketing Corporation, Norwalk, CT, US, 1978, Chapter 7.1.

5. Baroncini, B., General overview of UV applications on wood furniture, RadTech Europe 2003, Conference Proceedings, Vol. I. 2003, pp. 469–474.

CLASSICAL APPLICATIONS

205

6. Ferner, U., How to matt 100% radiation curing systems, RadTech Europe 2003, Conference Proceedings, Vol. I. 2003, pp. 427–435.

7. Morrison, S., http://www.specialchem4coatings.com/resources/editorials/editorial.aspx?id=2770.

8. Weikard, J. and Fischer, W., http://www.bayer-ls.de/ls/lswebcms.nsf/files/brochure_de/$File/Radtech_2001_ WDJ_GB.pdf.

9. Hahne, L., Innocure – Curing technology for high speed web applications, RadTech Europe 2005, Conference Proceedings, Vol. II. 2005, pp. 165–168.

10. Thompson, L.F., Willson, C.G. and Bowden, M.J., “Introduction to Microlithography”, In ACS Symposium Series, Vol. 219. American Chemical Society, Washington DC, 1983.

11. Steppan, H., Buhr, G. and Vollmann, H., Angew. Chem. 94, 471–485 (1982).

12. Reichmanis, E., Nalamasu, O. and Houlihan, F.M., Polym. Mater. Sci. Eng. 80, 301 (1999).

13. Lee, D.K. and Pawlowski, G., J. Photopolym. Sci. Technol. 15 (3), 427–434 (2002).

14. Thayer, A.M., C&EN Cover Story May 6, 23–30 (2002).

15. Lanska, D., A new light on UV inks, Paintindia, October 2003, pp. 65–68.

16. Fouassier, J.P., “Photoinitiation, Photopolymerization, and Photocuring,” Vol. IV. Hanser Publishers, Munich–Vienna–New York, 1995.

C HAPTER 9

Recent Developments 9.1 CHEMISTRY RELATED DEVELOPMENTS 9.1.1

Solvent-Borne UV Coatings

Typical UV-curable coatings do not contain any solvents and are often therefore called “100% solids” systems, although they are liquid in the uncured state. They are composed of low molecular weight resins and “monomeric” reactive diluents, which take over the function of reducing viscosity instead of traditionally used solvents. However, the use of solvents is not strictly forbidden. In case of spray applications the viscosity of the coatings has to be very low (<500 mPa s). Therefore solvents may be used instead of monomers, since monomer spraying mist may cause pollution and toxicological problems. The thinning power of solvents is similar to that of the reactive diluents (Figure 9.1). With less than 20% solvents (thus very high solids systems) viscosities can be reduced to the range required for spray application. This low viscosity is due to the relatively low molecular weight of the resins used. Furthermore, solvents as thinners may be used for UV primer applications, since the preferred very thin layers are obtained with very low viscosities or low solids contents. Solvents may also help to improve adhesion, especially to plastics, where they are able to swell the surface and facilitate interpenetration.

9.1.2

Water-Based UV Coatings

The use of aqueous binder emulsions for radiation-curing coatings has increased within the last 20 years. A technology that began with aqueous emulsions based on unsaturated polyesters, polyester or epoxy acrylates, has advanced with new, physically drying, radiationcurable polyurethane dispersion systems. Compared to radiation curing 100% systems, water-based products enable a wide variety of application techniques to be employed. Even those requiring low viscosity, such as spraying, are possible without the use of monomers or organic solvents. But why use water-based UV coatings which need an additional drying step compared to 100% UV systems?

From a UV technology standpoint, the original idea behind radiation curing – the in situ polymerization of a solvent-free coating material by high-energy radiation – had to

206

RECENT DEVELOPMENTS

207

F IG . 9.1. Thinning power of butyl acetate compared to HDDA for a urethane acrylate resin.

struggle with technical problems in the wood coating area: • Low molecular weight components of the coatings can penetrate into the porous wood and remain uncured; • To overcome this problem higher molecular weight resins have been proposed, but they exhibit high viscosity and are difficult to apply; • Lack of volumetric shrinkage interferes with matting of the coatings.

On the other hand, classical aqueous dispersion-based coatings (e.g., emulsion paints) rely on high molecular weight resins, which are, however, usually uncrosslinked and therefore thermoplastic in nature. As a consequence, the dry films exhibit poor blocking and scratch resistance and low gloss. These disadvantages can be overcome or improved by a radiation-curable component, which introduces crosslinking into the coating structure. In dispersions the viscosity is independent of the molecular weight of the resin, only governed by particle size and particle concentration. Therefore, UV-curable dispersions can be based on high molecular weight resins without monomeric reactive diluents, and the thinning with water makes it possible to obtain the low viscosities necessary for spray applications (Figure 9.2). Because of volumetric shrinkage these coatings also exhibit better matting properties than 100%-systems.

Furthermore, adhesion problems on certain substrates (e.g., metal) can be overcome by adding high molecular modifiers, but only at the expense of a sharp increase in the viscosity of the formulation. Aqueous dispersions on the other hand have the important advantage that their viscosity is largely independent of the molecular weight of the dispersed polymer.

UV-curable water-based systems can be divided into three categories, water-soluble resins, emulsions, or dispersions (Figure 9.3). The first generation of aqueous radiationcurable systems was based on conventional binders. Unsaturated polyesters, polyester acry-

208

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 9.2. Why water-based UV coatings?

lates, epoxide acrylates or urethane acrylates were emulsified with the aid of a polymeric amphiphilic surfactant (protective colloid). These emulsions were composed of the classical resins and could be diluted without the use of monomeric reactive diluents. Since water forms the continuous phase, the emulsions can easily be thinned with more water or combined with water-soluble (or dilutable) resins.

There are not many water-soluble UV-curable resins on the market. Some aliphatic epoxy acrylates are partially water-soluble, which permit the use of water as a thinner. For example, the viscosity of butanediol-diglycidylether diacrylate, can be reduced from about 900 mPa s to a sprayable, yet high solids, solution with a viscosity of less than 100 mPa s by adding up to 25% water.1 Furthermore, highly ethoxylated or higher molecular weight polyethylene glycol acrylates can be diluted with water.

The second generation of waterborne UV-curable systems was based on dispersions, where the dispersing groups, either hydrophilic or ionic, are incorporated into the resinâ&#x20AC;&#x2122;s backbone. Polyurethane acrylates can be made self-emulsifying by incorporating sulfonate or carboxyl groups. These can then be neutralized with bases and dispersed in an aqueous medium.2,3 An important milestone was reached with the development of radiation-curable, physical-drying aqueous dispersions. These were used for 3D objects that have surfaces inaccessible to the radiation and so must dry to give a tack-free surface even though they are not crosslinked.

Blends of classical, primary or secondary dispersions with UV-curable reactive acrylates are another trivial possibility to extend the range of aqueous radiation-curing systems.

As with non-aqueous systems, the properties of aqueous formulations can be varied widely, from hard coatings based on polyester acrylic emulsions to the highly flexible ones made from urethane acrylate dispersions.

RECENT DEVELOPMENTS

209

F IG . 9.3. Design of water-based UV systems (water-soluble, emulsions and dispersions).

The advantages of aqueous, radiation-curing systems will be exemplified in more detail in the following examples: • Lower extractables content; • Better and easier mattability; • Better adhesion on specific substrates, e.g., wood.

9.1.2.1 Advantages of water-based coatings: lower extractables A wood varnish must be applied in a very thin layer if the wood pores are to remain open. This implies that the coating formulation must have a low viscosity and a low solid content, which is only achievable by thinning the resin with solvent or by employing an aqueous system. In any case, formulations for spray or vacuum coating must necessarily be of low viscosity. The problem with conventional radiation-curing systems is that the organic solvent thinners or reactive monomers used to reduce the viscosity can be absorbed by the wood and released later, which can lead to greasy surfaces and odour nuisance.

As extraction tests have proven, aqueous radiation-curing systems represent a promising alternative. Various types of wood veneer were coated to a dry-film weight of about 100 g/m2 with different formulations (see Table 9.1) and then UV cured at room temperature (120 W/cm, 5 m/min).

The most important components extracted from these different formulations coated on beech veneer are shown in Figure 9.4 (the amounts are expressed as grams per square meter of veneer). Tests on other wood types, such as oak and pine, produced similar re-

210

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

TABLE 9.1. Formulations used in the extraction tests Formulation (parts)

1

2

3

Polyester acrylate (PE)

67

100

Polyester acrylate emulsion (PE-EM) (50% in water)

4

100

Polyurethane acrylate dispersion (PUD 1) (40% in water) TPGDA

100 33

Butyl acetate Photoinitiator 1 (PI 1)

15 3

3

1.5

1.2

F IG . 9.4. Extractables from a wood coating formulated with different coating alternatives.

sults. The results also correspond with those of volatile emissions tests.4 Apart from photoinitiator, the main substances extracted were TPGDA (reactive thinner), low molecular acrylates and the solvent butyl acetate. The overall amount of extractable compounds in aqueous radiation-curing systems is much less than in non-aqueous ones because of the higher molecular weight of the binder used. This is a significant advantage over monomercontaining formulations. The high proportion of extractable photoinitiator in aqueous systems is unavoidable unless a polymer bound photoinitiator is used, or curing is performed under inert conditions (which would require less initiator).

9.1.2.2 Advantages of water-based coatings: easy matting Wood varnishes have to exhibit volumetric shrinkage in order to enhance the surface structure of the wood. To make this possible, the viscosity of the varnish has to be reduced with solvents that evaporate

RECENT DEVELOPMENTS

211

out of the film once the coating has been applied. Nowadays organic solvents are undesirable for environmental reasons. Water offers a promising alternative. It is desirable that coatings for porous substrates like wood do not contain low molecular weight reactive thinners. These monomers penetrate into the substrate, where they escape the curing process, but migrate out of the surface later during the lifetime of the object. However, volatile thinners are often invaluable: it is very difficult to achieve matting effects or enhancement of the wood grain without them (see Figure 8.5). Matte or satin matte varnishes are of great interest particularly in the wood industry. These are made by adding a matting agent, usually amorphous silica, to the binder to give roughness to the surface of the film. The silica used is sometimes wax-coated. To achieve the matting effect, some degree of film shrinkage must take place. In aqueous systems, this occurs when the water evaporates.

In 100%-systems the polymerization only accounts for 5â&#x20AC;&#x201C;10% shrinkage, not enough to produce a significant matting effect.

9.1.2.3 Advantages of water-based coatings: improved adhesion The adhesion and mar resistance of coatings on beech-veneered MDF board are compared by a typical coin test.

This test is performed on the final coating containing a water-based primer, an intermediate coat cured by UV, and an UV-cured topcoat. In the coin test a coin held at an angle of about 45 â&#x2014;Ś is drawn across the surface of the specimen followed by examination of the specimen for signs of marring. The tests, which were carried out on a whole series of formulations, with and without wet and uncured priming coat, showed that the surface adhesion of UV-curing varnishes can be greatly improved by application of an aqueous primer.

If a high-solids water-based primer is chosen, time allowed for drying by evaporation is unnecessary since the small amount of water that is present in such thin films is absorbed by the substrate.

A major drawback of water-based radiation-curing systems is related to the need of airdrying the film before it is irradiated. However, it has been shown that forced drying at higher temperatures can bring benefits if the film is irradiated immediately afterwards. The

F IG . 9.5. Influence of the curing temperature on the curing conversion of a water-based polyurethane dispersion.

212

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 9.6. Influence of the relative humidity on the curing conversion of a water-based polyurethane and a 100% UA acrylate.

apparent drawback of having to air dry the film before irradiating has a positive side in that the higher thermal activity from force drying results in higher conversion of acrylic double bonds, which is particularly important for polymers with high glass transition temperatures (see Figure 9.5). A similar picture emerged from Raman analysis,5 the higher the temperature before irradiation, the higher the conversion rate. The conversion during UV cure may also be greatly influenced by the humidity of the exposure environment, as shown in Figure 9.6. Whereas a 100% liquid urethane acrylate does not feature any sensitivity of the conversion to the humid environment, the conversion of the water-based coating increases from 25% at 0% humidity to 55% at 100% humidity, presumably due to increased mobility by the plasticizing effect of the absorbed water.

The difference in the plasticizing effect of solvent-borne and water-based coatings has also been demonstrated in completely cured coatings by the following hardness development during storage in humid and dry environment (Figure 9.7).

Water-based systems will contribute to a wider use of radiation-curing technology. For the reasons that have been mentioned, the rate of growth of water-based radiation-curing systems will exceed that of the technology in general, since it is a suggestive supplement to a technology with future.

Advantages and disadvantages of waterborne UV-curing systems have been compared to 100% liquid UV coatings and traditional aqueous dispersions,6 the main points are summarized in Table 9.2.

Water-based UV-curable coatings are often applied by spray coating, however, UVcurable polyurethane dispersions have also been developed which could be deposited very homogeneously by electrodeposition on conductive surfaces, showing good stability and good adhesion on steel panels.7

RECENT DEVELOPMENTS

213

F IG . 9.7. Influence of humidity on the hardness of UV-cured water- and solvent-based PUA coatings.

TABLE 9.2. Main advantages and disadvantages of water-based UV curing systems compared to 100% liquid UV and traditional aqueous dispersions Advantages

Disadvantages

No reactive diluents

Water as solvent/dispersion medium

• Reduced toxicity, flammability, odour and shrinkage

• Water evaporation prior to UV curing (energy, time, costs) • Wetting problems

Low viscosity • Spraying, roller and curtain coating possible Water as solvent/dispersion medium • Easy clean up

Sensitivity of cured film to swelling • Humidity, polar chemicals Lower gloss and lower productivity compared to 100% liquid UV (sometimes)

Water as swelling agent for wood surfaces • Adhesion facilitation Lower costs Higher gloss than traditional dispersions

9.1.3 UV Powder Coatings Conventional powder coatings have gained a considerable market share in industrial coatings applications because of their high environmental compliance and good recyclability.

The global powder coatings consumption in the year 2000 was about 720,000 tons, about 50% of it used in general metal finishing.8 Thermosetting powders are mainly used on

214

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 9.8. Viscosity development in thermal and UV-cured powder coatings (providing better flow properties with UV powders).

metal substrates, since they need high curing temperatures above 140 ◦ C and curing times of up to 30 min. If the curing times need to be shortened to the range of minutes the temperatures have to be increased up to 300 ◦ C. Improving the curing conditions by other measures are limited, since increasing the reactivity is hardly possible, because the stability during powder formulation, which is normally done in an extrusion process at temperatures around 100 ◦ C, would be negatively affected. The main advantages of powder coatings are associated with 100% solids, dry handling, easily recyclable overspray, almost no emissions and the application of thick layers in one pass. The main problem associated with the use of powder coatings is the smoothness of the final coating surface, which is due to the flow characteristic. Thermosetting powders start melting upon thermal heating, as explained in Figure 9.8.

Increasing the temperature further results in reduced viscosity and start of flow. But, because of the high temperature, the curing reaction begins and restricts mobility before the flow has provided a smooth surface. The alternative of reducing the resin viscosity by lowering the molecular weight in order to provide better flow characteristics is also limited, since this results in a lower Tg and therefore in storage stability problems of the powder particles due to agglomeration by sticking together (sintering/blocking). This trade-off is the reason why UV powders became of interest. In UV-curable powders the melting and flow can be decoupled from the curing reaction. Hence, at the point where flow is completed and an optimal surface is obtained the curing reaction can be triggered by switching on the UV lamps. UV powder coatings provide excellent coating properties and combine the advantages of powder coating technology with the advantages of UV curing, like fast cure speed, low energy consumption and low floor space requirements. In order to melt UV powders temperatures as low as 90 to 120 ◦ C are sufficient, and this fact has enabled the use of powder coatings on heat sensitive substrates, like wood, plastics or heat sensitive alloys. Of the wood based substrates the most interesting for coating with UV powders is medium density fibreboard (MDF), because of its uniform density, controllable humidity, dimensional stability and stability at processing temperatures up to 140 ◦ C.9

RECENT DEVELOPMENTS

215

Thus, the two main applications developed to date are the UV powder coating of preassembled articles (e.g., motor housing) and MDF coating.

Besides the described advantages of UV-curable powders: • • • • •

Solvent free system; Low temperature cure; Excellent flow characteristics; Faster curing speed (compared to thermal powders); Excellent properties.

Limitations are still remaining with few pigmented coatings (yellow shades) and realization of matt surfaces.

The potential and technology of UV powder coatings has recently been described in various publications.10–12 How do UV powders compare with conventional powders, liquid UV-curable coatings or solvent based coating technologies?

Conventional powder coatings are the first choice when metal articles, able to withstand heating for up to 30 min at temperatures up to 200 ◦ C, have to be cured and the requirements on the surface quality are not of the highest importance. If temperature sensitive materials, like wood, plastics or fully-assembled articles have to be coated, for example, an electric motor or a pump containing plastic parts, wires or seals, conventional powders can not be applied. However, if a manufacturer is used to application of powder coatings with all the powder processing equipment installed, the coating of temperature sensitive substrates and the coating of articles combining metal and temperature sensitive parts provides an opportunity for UV powders. Due to the fast curing reaction and the reduced heat application only for melting the powder, the process cycle times can be shortened.

If powder process equipment is not already installed, liquid UV-curable coatings are also an alternative. They are especially suitable as clear coats on temperature sensitive substrates and two-dimensional curing. However, due to considerable advances in the development of materials and process equipment, the extension of this technology to pigmented systems and three-dimensional curing makes liquid UV coatings a competitive alternative to UV powders for painting fully assembled articles containing heat sensitive parts.

The classical choice of solvent based two-component polyurethane coatings is also still an alternative. These coatings have excellent properties and durability, however suffer from the use of solvents (VOC problem) and costs associated with overspray, cautions to be taken for isocyanate handling and treatment of the spray booth water.

How is the handling and processing of UV powders?

The application of the UV powders to the substrate is preferably done in an electrostatic application process, which is essentially the same as in the conventional powder application process. The coated substrate is then heated in an oven or with IR lamps until the powder forms a smooth surface film, which normally only takes about 1–2 min and the hot film is passed under UV lamps to be cured completely within a few seconds. The whole process may be run continuously and takes 5 min at the most including the cooling down phase (Figure 9.9).

216

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 9.9. UV powder coating process line.

What is the chemistry behind UV powder formulations?

The UV powder coating formulations developed to date are almost all based on radically polymerizable systems essentially consisting of UV-curable powder resins, photoinitiators, additives and optionally pigments. The chemistry used in UV powder coating resins is mainly based on polyesters, either unsaturated or (meth)acrylate functionalized, or other acrylate functionalized polymers, like hyperbranched polyesters or polyacrylates. Examples are unsaturated (maleic/fumaric) polyesters in combination with semi-crystalline reactive diluents, like vinyl ether urethanes or (meth)acrylate functionalized amorphous polyesters as the sole resins,13,14 methacrylated polyesters15 (UV-coat types, formerly UCB, now Cytec Surface Specialities), unsaturated polyacrylates as the basic resins,16 combinations of amorphous urethane acrylates and crystalline urethane acrylates17 (Figure 9.10) or acrylated hyperbranched polyesters.18 The glass transition temperatures of the resins are typically above room temperature in the range of 40 to 60 ◦ C. While most of the (also conventional) powder resins are amorphous, the use of semi-crystalline structures has the advantage that above the melting point the molten crystalline region causes a strong decrease in viscosity and therefore contributes to better flow characteristics,19 a prerequisite for low temperature curing powder coatings.

Useful photoinitiators have to be compatible with the formulations, display low volatility and absorb at different wavelength than a pigment (if present). In clear coats the hydroxy acetophenone type photoinitiators, like Irgacure® 184 or 2959 can be used as sole initiators and in pigmented systems in combination with acyl phosphine oxide type photoinitiators, like Lucirin® TPO or Irgacure® 819.

The choice of pigments has to be done carefully according to their absorption characteristics. A standard pigment for white powder coatings is titanium dioxide, for example Kronos® 2160 (Kronos). Other commercial color pigments are also commonly used.

The through cure of pigmented coatings can be predicted by a numerical simulation tool, which provides insight into the physical interactions associated with the curing process of pigmented coatings.20 It enables the choice of proper process parameters (type of photoinitiator, lamp type, maximum film thickness) in order to achieve optimum cure results.

Additives used are mainly flow additives, like Acronal 4F, Modaflow and BYK Powderflow, and degassing or anti-popping agents, like benzoine or benzoine ethers.

RECENT DEVELOPMENTS

217

F IG . 9.10. Chemistry examples of UV powder coatings.

The formulation of the powders, for example the addition of photoinitiators and additives has to be done in an extrusion process, which should be carried out, especially in the case of semi-crystalline powders, at temperatures above Tg , but below Tm , usually in the range of 50â&#x20AC;&#x201C;70 â&#x2014;Ś C. As in classical powder applications metal surfaces can be easily coated by the electrostatic spray process, while the non-conducting surfaces, like wood, are more difficult to coat. Here best results are obtained with a triboelectric powder gun, where the particles are charged by friction in a Teflon tube, rather than by commonly used corona guns, which rely on an external high voltage source for charging. It also helps if the surface is charged with the opposite polarity before application of the powder. Furthermore care has to be taken to avoid too high film thicknesses which might not be cured through due to limited light penetration, especially in pigmented UV powder coatings. This may result in an extreme gradient of highly crosslinked material at the surface and almost uncured resins at the interphase, hence effecting the mechanical properties. In order to optimize the throughcure the choice of the right lamp system also plays a major role. As is the case with liquid UV-curable systems, clear coats may be well through-cured with typical mercury lamps, whereas pigmented or weathering stabilized coatings should be cured with lamps having higher outputs at longer wavelength (Ga or Fe added mercury lamps).

The weathering stability of UV-cured powder coatings21 and the performances reported on UV-cured powder coatings22 is comparable with that of conventional thermosetting outdoor durable powder coatings systems. Conventional powder coatings are already applied as automotive top coats (BMW) and developments are under way to replace the conventional powders by UV powders.

218

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 9.11. Dual curing: UV for high scratch resistance and thermal cure for shadow areas.

It has been emphasized that a well running UV powder coatings line, more than any other aspect, has to be based on a close cooperation in the design and full back-up service of end-user, powder formulator as well as equipment supplier.

The total costs of a UV powder process are best assessed by an eco-efficiency analysis, since the comparison with other coating processes should be based on the price per coated square meter. This is, because several influencing factors have to be considered, like lower energy consumption and shorter cycle time, realization of thick layers, whereas with other processes often multiple coating processes are necessary or higher material costs have to be weighted with higher material utilization (95 vs. 60% for most wet processes).

9.1.4

Dual Cure Coatings (UV and Thermal)

The dual cure coatings discussed here combine the fast curing mechanism of the UV-curing reaction with an independent thermal cure, especially necessary in areas where sufficient light is not available, or where special effects, like improved adhesion have to be obtained (Figure 9.11).

The miscellaneous options of the design of dual cure systems have been overviewed by Peeters.23 In contrast to Hybrid-Cure systems (two mechanism, like radical and cationic polymerization, occurring at the same time), the dual cure systems are defined by the curing reaction taking place in two separate stages. Some of the systems proposed are summarized in Figure 9.12.

In order to improve the scratch resistance of conventional automotive clear coats UVcurable systems have been identified as a viable alternative, since they have proven high scratch resistance in parquet floorings. However, automotive bodies are much more com-

RECENT DEVELOPMENTS

219

F IG . 9.12. Examples of possible dual cure systems.

plex in geometry than even parquet floorings, a second light independent curing mechanism has to be installed in order to cure coated shadowed areas.

9.1.4.1 Two component dual cure coatings Design concepts of dual cure systems similar to the two-pack polyurethanes (2K PU), which are the workhorse of automotive clear coat painting, are based on blends of poly-isocyanate/poly-ol combinations with UVcurable acrylate crosslinkers as schematically shown in Figure 9.13. These systems form two independent networks, which are interpenetrating each other. Such dual cure coatings have been shown to exhibit better scratch and chemical resistance than pure thermal curing 2K systems.24 In Figure 9.14 the development of the performance of different generations of such UV dual cure systems is compared to a standard 2K clearcoat as a reference. The spider diagram shows that the 2K PU has still the best process stability (very well optimized) and best behaviour in shadow regions, however, the dual cure systems are much better in chemical and scratch resistance. The scratch resistance of such dual cure systems is discussed in more detail in Chapter 10.1.5, where different coatings are compared in accelerated scratch tests and car wash field tests.

In order to link the different polyurethane networks together novel molecules have been designed which combine isocyanate and acrylate unsaturations in one molecule. The easiest way to come to isocyanato acrylates is the partial reaction of classical polyisocyanates, like the isocyanurates, biurets or allophanates of hexamethylene diisocyanate or isophorone diisocyanates, with hydroxyalkyl acrylates.25 Isocyanato acrylates based on this chemistry, however, exhibit relatively high viscosities. Therefore isocyanato acrylates based on allophanate structures derived from the reaction of hexamethylene diisocyanate with hydroxyalkyl acrylates have been developed exhibiting low viscosities,26 commercially available for example as LaromerÂŽ LR 9000. Such isocyanato acrylates can be used on their own in

220

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 9.13. Scheme of resin design for two component dual cure coating.

F IG . 9.14. UV clearcoat developments for automotive OEM coatings; UV dual cure systems based on blends of UV and 2K PU components.

moisture curing dual cure (moisture and UV) systems as well as in combination with all kinds of complementary reaction partners of the isocyanate group, like polyols, polythiols, polyamines, hydroxyalkyl acrylates or epoxyacrylates.

The independent cure of the thermal OH-NCO reaction and the polymerization of the acrylate double bonds is shown in Figure 9.15. Within 10 min at 80 ◦ C the isocyanates react

RECENT DEVELOPMENTS

221

F IG . 9.15. Conversion of a dual cure coating: 10 min 80 ◦ C thermal cure, afterwards 30 s UV cure.

almost completely with the hydroxyl resin, and subsequently upon irradiation the acrylates are driven within seconds to above 80% conversion.

Dual cure coatings can also be advantageously used in order to improve the adhesion by the reaction of the isocyanates with hydroxy groups of the substrate, for example, on parquet flooring applications.27 9.1.4.2 One component dual cure coatings Since one component systems offer easier processing, 1K dual cure coatings have also been developed. One example is based on resins obtained from the reaction of isocyanato acrylates with hydroxy-carbamates. Formulating the resulting carbamato acrylates with etherified melamine crosslinkers (HMMM) and, optionally, further acrylate resins one component dual cure systems result, where the thermal crosslinking proceeds via the condensation reaction of the hexamethoxy melamine with the carbamate groups and the UV curing by polymerization of the acrylates (Figure 9.16).28 Also dual curable water-based coatings have been described, which are based on the reaction of polyisocyanates with diols, hydroxyalkyl acrylates, hydroxy carboxylates, isocyanate blocking groups and polyols29 (Figure 9.17).

Such dual cure water-based coatings cure well in the dual cure mode, but also in the pure thermal curing reaction at 150 ◦ C, shown in Figure 9.18.

At 150 ◦ C the blocking agent is lost completely within 60 min and at that time span also about 50% of the acrylate groups are polymerized. With increasing conversion of both reactive groups the hardness also increased simultaneously.

Dual cure coatings have also been developed which are based on consecutive radical and cationic curing. In the first step the systems based on acrylate-oxetane monomers are transferred from a liquid into a solid, formable state, and after thermoforming they are completely cured by cationic cure of the oxetane groups.30 The efficiency of the second curing step is influenced by the crosslink density obtained by the first radical curing step.

Furthermore, dual cure powder coating hybrids based on radical and cationic UV curing have been described, which rely on the “dark curing” mechanism of the cationic reaction

222

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 9.16. Scheme of a one-component dual cure coating.

F IG . 9.17. Composition scheme of UV-curable polyurethane dispersions (PUD) and dual cure dispersions (DC-PUD).

in order to cure the film in areas not fully exposed to UV. The combination of solid epoxy resins and solid urethane acrylates in the presence of radical and photo-acid generators provided flexible pinhole free powder coating films with good adhesion.31

RECENT DEVELOPMENTS

223

F IG . 9.18. Thermal curing of a dual cure PUA dispersion at 150 ◦ C.

9.1.5 UV-Curable Nanocoatings/Nanocomposites Composites are designed in order to combine properties which can not be reached with single materials, for example, the good barrier properties and high hardness and scratch resistance of inorganic materials and the high flexibility, high toughness and easy processability of organics. Such composite materials are processed in large amount in typical thermal curing fibre-reinforced composites. Since the commonly used inorganics are nontransparent the UV curing of such composite materials appeared to be critical in terms of through cure. With the increased evaluation of transparent nanoparticles the possibility of curing such systems with UV has become a reality.

In this section UV-curable coatings containing inorganic moieties, which are usually below 100 nm in size are discussed briefly. The mentioned systems aggregated under the term “nanotechnology” are organic–inorganic hybrid coatings formed by sol–gel technology, nanoparticle containing coatings, which are often also operate under the term nanocomposites and nanocomposites based on layered nanostructures.

Inorganic–organic nanocoatings by the sol–gel technology have been developed at the Fraunhofer Institute of Würzburg32 and the INM at Saarbrücken.33 Organic modified ceramics (ORMOCER® ) based on such processes rely on the formation of an inorganic network by the polycondensation reaction (sol–gel) of organic modified siloxanes. The organic groups on the siloxane contribute to higher flexibility or can be designed to additionally form an organic network, for example by UV-curable modified ORMOCER® coatings34 (Figure 9.19).

ORMOCER® coatings in general can be applied to many surfaces (polymers, ceramics, glass, metal, paper, wood) in order to improve mechanical and chemical resistance of the substrates. ORMOCER® laquers are commercially available (e.g., by T_O_P Oberflächen GmbH in Würzburg) which can be processed with nearly all of the customary application procedures, like dip and spray coating, curtain coating, roller coating, inkjet or screen printing, and cured by thermal and IR, redox-initiated polymerization or UV irradiation.

224

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 9.19. Scheme of UV-curable ORMOCER® coatings.

Applications of such coatings are, scratch and abrasion resistant protective coatings, decorative coatings, antireflective coatings, hydrophilic, hydrophobic and oleophobic functional (anti-adhesive coatings) and antistatic coatings.

An example of the high abrasion resistance of ORMOCER® coatings on polycarbonate is shown in Figure 9.20.

In contrast to the described sol–gel coatings, which form inorganic networks throughout the whole film, preformed nanoparticles are also used as fillers or functional components in coatings.

A nanoparticle filled thermally cured system has recently been introduced in automotive coatings and has set a benchmark regarding the scratch resistance in car wash cycles (CeramiclearTM by PPG). The coating is based on 2 C polyurethane technology containing 20 nm SiO2 particles, predominantly located at the surface.35 A high concentration of particles at the surface is obviously necessary in order to obtain a considerable improvement of scratch resistance, as shown in Figure 9.21, where only bulk loadings of nanoparticles (20 nm SiO2 ) in a UV-curable urethane acrylate coating in excess of 30% exhibit considerable increase of scratch resistance (decreased gloss loss).

Nanoscaled silica in situ incorporated to a polyester acrylate also yields very scratch resistant surfaces36 (Figure 9.22).

It has been reported that high scratch resistance can be obtained by the addition of predispersed nanoadditives (20 nm Al or silica in amounts of 0.25–2%), and even enhanced scratch resistance is obtained by the further addition of silicone or fluorinated surfactantlike molecules, which favour the alignment of the nanoparticles at the surface.37

RECENT DEVELOPMENTS

225

F IG . 9.20. Abrasion resistance of ORMOCER® coatings on polycarbonate (left side, coated; right side, uncoated).

F IG . 9.21. Gloss loss of a urethane acrylate resin (Laromer® LR 8987) filled with increasing contents of nanoparticles (∼20 nm).

Special UV-curable systems containing up to 20% reacted fumed silica, which leads to low viscosity formulations have been reported to result in excellent scratch resistant parquet flooring coatings, which can be improved even more by the further addition of corundum.38

226

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 9.22. Residual gloss of a polyester acrylate resin (Laromer® PE 9026) as a function of nanoparticle (∼20 nm) content in a ScotchBrite® scratch test (50 double strokes).

The influence of different nanoparticle types (SiO2 , TiO2 , Al2 O3 , CaCO3 ) on appearance, hardness, adhesion and abradability has been evaluated with UV-curable white powder coatings based on Uvecoat 3000 and 9010.39 The best performance in terms of abradability was seen with the use of Al2 O3 nanoparticles.

UV-curable acrylate-metal oxide (silica, tin oxide, or antimony doped tin oxide) nanocomposite coatings have been evaluated where the particles have been surface modified with silane coupling agents.40 The silane coupling agents were grafted on the metal oxide nanoparticles to increase the compatibility of the particles with the acrylate matrix.

A polymerizable grafting group (3-methacryloxypropyltrimethoxysilane; MPS) has been compared to an inert grafting group (n-octyltrimethoxysilane; OTS). The increase of the hardness and E-modulus was largest with a large amount of grafted MPS. Moreover, when MPS was grafted on the fillers, the coatings showed more elastic recovery after the indentation than when OTS was grafted on the fillers. Tests with a Taber abrader revealed that the abrasion resistance is strongly enhanced by the nanoparticles.

Furthermore, several approaches have been described to prepare UV-curable nanocomposites, which are based on the UV induced Thiol-ENE chemistry.

The Thiol-ENE photocuring reaction has been used to prepare inorganic-organic composites by incorporation of nanoparticles derived from functionalized (ENE-group containing) polyhedral oligomeric silsesquioxane (POSS) into crosslinked networks.41 If the POSS content in such coatings containing further triallylether and trimethylolpropane tris(mercapto propionate) is well balanced (20%), high hardness and limited flame spread could be obtained.

Thiol siloxane colloids based on mercaptopropyltrimethoxysilane (MPTS) were prepared with sol–gel methods as the thiol moiety and formulated together with norbornene linseed oil derivatives as the ENE-component in order to establish a range of UV-curable organic–inorganic hybrid systems.42 Nanocomposites on the basis of ZnS nanoparticles have been incorporated into UV systems by modification of the particles with thiol groups and using hyperbranched acrylated polyesters (Boltorn® H20) as the resin. Upon UV curing the thiol modified nanoparticles are immobilized into the coating via a free radical addition reaction. In 1 µm thick films

RECENT DEVELOPMENTS

227

these coatings containing semiconductor nanoparticles are optically transparent and can be used in optical applications, like lenses, optical waveguides or reflectors.43 Particle modified polymers containing well dispersed (layered) particles in the nanometer scale exhibit superior mechanical, thermal and optical properties compared to conventionally filled polymers.44 Such materials are usually obtained by intercalation of a polymer into the layered structure of silicates, like clay. A comprehensive overview about such nanocomposites has been published recently.45 Whereas nanocomposites have been obtained by e-beam curing,46 the evaluation of the production of nanocomposits by an UVcuring process just recently came into focus. First examples are the fabrication of montmorillonite filled UV polymerized methylmethacrylates47 and bentonite mineral fillers readily dispersed in a crosslinked polyurethane-acrylate matrix cured by photoinitiated polymerization.48 Up to 2 mm thick disks of nanocomposites have been produced by a 4 s UV exposure. In these UV-cured nanocomposites the silicate layers form a skeleton like superstructure, resulting in a significant improvement in polymer properties.

9.1.6 Weathering Resistance of UV-cured Acrylics Very many industrial clear coats used in outdoor applications contain light stabilizers in order to protect the clear coat as well as the underlying (coloured) coats or substrates from photo-oxidative degradation induced by exposure to sunlight. Light stabilizers are commonly used in a package of UV absorber and radical scavenger. While the main function of the UV absorber is the protection of the coated material from detrimental solar UV radiation, in order to prevent degradation or discoloration, radical scavengers on the other hand, mainly of the hindered amine light stabilizer (HALS) type, serve primarily to prevent the oxidative degradation of the top coating layer itself. Classical UV absorbers are based on the benzotriazole (BTZ), hydroxy-phenyl-triazine (HPT) or hydroxy-benzophenone (HBP) type, whereas the radical scavengers are mainly based on the hindered amine light stabilizer (HALS) type (Figure 9.23).

The function of the UV absorbers is explained in Figure 9.24. Upon absorption of a photon the UV absorber molecule is excited. The energy dissipation from the excited state can follow different pathways, by direct radiation-less deactivation, by deactivation after intersystem crossing or by tautomerization, all processes leading to energy release as heat without the formation of any radical species. Such UV absorbers have been reported to be used already in UV-curable coatings.49 But how can UV polymerization work if the UV light is absorbed and dissipated by the UV absorber?

Until recently, it has been a common belief that UV curing in the presence of UV absorbers and radical scavengers should not be possible. This belief is based on the fact that UV absorbers compete for the UV light, which is required for exciting the photoinitiator and producing radical species, which in turn initiate the polymerization process. Furthermore, besides the interference of the UV absorber with the initiation process, the HALS radical scavengers should rapidly terminate the few radicals which are formed during photoinitiation (Figure 9.25).

Since the UV absorber itself degrades upon weathering, novel types of UV absorbers have been developed which generate an UV absorber from a precursors (Figure 9.26)

228

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 9.23. Examples of light stabilizers: UV absorbers and radical scavengers (hindered amine light stabilizers (HALS)).

F IG . 9.24. Energy dissipation of UV absorbers by excitation, tautomerization and radiationless deactivation (absorbed energy is released as heat).

upon solar exposure, and thus are able to compensate the degradation of the initial absorber.50,51 One of this novel type of precursor UV absorber (2-acetate substituted phenyl-benzotriazol), which is converted into the hydroxy-phenyl-triazine UV absorber, can be used advantageously in UV-curing systems, since the absorption spectrum of the

RECENT DEVELOPMENTS

229

F IG . 9.25. Challenges for exterior durable UV coatings.

F IG . 9.26. UV spectra of UV absorber (Tinuvin® 900), a precursor UV absorber (Acetate BTA) and acylphosphine oxide photoinitiator (Lucirin® TPO).

230

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 9.27. Conversion (by RT-IR) of a UV-curable UA-resin in the absence and presence of HALS (Tinuvin® T123) or UV absorber (Tinuvin® T400).

precursor is in the short wavelength region. This is helpful, since in the presence of UV absorbers in UV-curable coatings there is a competition of the photoinitiator and the UV absorber for scavenging the photons designated for the photoinitiator to produce polymerization initiating radicals.

Despite the absorption characteristics of UV absorbers, the appropriate selection of UV absorber, photoinitiator type and lamp output spectrum can enable the proper UV curing of weathering resistant protected UV coatings. In Figure 9.26 the absorption spectrum of the UV absorber Tinuvin 900 (BTZ) reveals that only light above 350 nm can be used for the photoinitiator excitation. If an acyl phosphine oxide type photoinitiator is used in combination with the appropriate lamp (Fe or Ga added) the UV curing should be possible.

The UV absorber precursor (Acetate BTA), which finally will be converted into the BTZ absorber, leaves a larger window for the photoinitiator to absorb radiation.

It has been shown that in the presence of such a precursors the drop in the polymerization rate is less pronounced (25%) than in the presence of the UV absorber itself (66%) when using a Benzilketal photoinitiator (Irgacure® 651).

The cure conversion diagram (Figure 9.27) of an urethane acrylate resin in the presence of an UV absorber or a radical scavenger (HALS) shows the typically observed effects.52 In the presence of the UV absorber the cure conversion drops compared with the polymerization curve of the pure resin. This behaviour is observed with all photoinitiator types, where the drop of the curve is more pronounced with the short wavelength absorbing photoinitiators (e.g., hydroxy-ketone types) than with the longer wavelength absorbing acyl phosphine oxides (mono- or bis-phosphine oxides).

Surprisingly, the HALS radical scavengers do not have any retarding effect on the cure conversion.53 This behaviour can be explained by the HALS amine action scheme (Figure 9.28). The hindered amine introduced into the formulation is not a radical scavenger

RECENT DEVELOPMENTS

231

F IG . 9.28. Scheme of HALS amine action: Formation of nitroxyl radicals, deactivation of C-radicals and regeneration of nitroxyl radical.

F IG . 9.29. Raman microscopy of a polyesteracrylate film with increasing photoinitiator (D1173) concentration and UVA and HALS.

per se, but is converted to a radical scavenger upon weathering by the action of light and oxygen. Thus it does not interfere in the polymerization process. Only when it is exposed to weathering conditions it starts to scavenge detrimental radicals produced and thus prevents photooxidation.

The through cure can be easily determined with confocal Raman microscopy. In Figure 9.29, the influence of the photoinitiator concentration on the through cure of a sta-

232

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 9.30. Example of photoinitiators used in combination with light stabilizer package (UV absorber/HALS).

bilized coating (fixed concentrations of UV-abs and HALS) is shown on the example of a short wavelength absorbing hydroxy-ketone photoinitiator (Darocure® 1173). Although hydroxy-ketone photoinitiators may be used (in higher concentrations) alone, it is recommended to use the (cheaper) hydroxy-ketones (for surface cure) in combination with longer wavelength absorbing photoinitiators (acyl phosphine oxides) for through cure in the presence of UV absorbers, in order to better circumvent the filtering effect of the UV absorbers (Figure 9.30).54 Hence, by selecting appropriate photoinitiator/UV absorber combinations according to the absorption characteristics, effective UV polymerizations are possible in the presence of UV absorbers. Especially the use of acyl phosphine oxide photoinitiators enabled a good through cure of UV-curable clear coats as well as an excellent weathering stability.55 Up to now we have only discussed how to cure UV coatings properly in the presence of stabilizers for outdoor exposure.

But how do such stabilized UV coatings behave upon weathering?

It is known in the literature, that in general highly crosslinked coatings are more resistant to photooxidation than uncrosslinked systems, since the lower mobility of the polymer chains in a network structure reduce the extent of initial “radical” species, because recombinations by cage effects are more likely, as well as limited propagation of the species.

However, the structure of the resins or the used unsaturated oligomers/monomers also plays a significant role on the photo-oxidative stability.56 It has been found that the ether functionality is the main site of photo-oxidative attack. However, the incorporation of better H-atom donors, like amines used as coinitiator synergist can stabilize the resins. The weathering stability of coatings is further enhanced by the addition of UV-Absorbers (UVAbs) and radical scavengers (e.g., hindered amine light stabilizers (HALS)).57

RECENT DEVELOPMENTS

233

F IG . 9.31. Tracing of changes of different chemical bonds during aging time in accelerated QUV-weathering tests.

234

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

The general suitability of radiation-cured coatings for outdoor durability has been evaluated by Holman.58 It has been concluded, that the photocuring reaction itself in an air atmosphere as well as the photoinitiator type and concentration have only a negligible effect on long-term durability. The only resin class which survived the QUV-A tests for 4000 h were the aliphatic urethane acrylates.

With highly crosslinked UV coatings based on aliphatic urethane acrylates containing UV absorbers and HALS, Decker59 has shown outstanding weathering performance, even compared to two-pack polyurethanes considered to have the best performance for exterior clear coat applications. Decker has evaluated the weathering behaviour with QUV accelerated weathering tests in wet and dry cycles with QUV-A or QUV-B filters, whereof the QUV-A filter corresponds at least in the shorter wavelength range to the natural solar spectrum, whereas the QUV-B provides even harsher conditions with short wavelength exposure (Figure 9.31).

Decker used the method of IR spectroscopy in order to follow the integrity of chemical bonds during the weathering experiments. This method has been proven to be more sensitive to changes going on in the coating film than just following the gloss retention, as commonly done in such experiments. With this method the coating was applied on a BaF2 crystal, which was placed in the QUV chamber and the IR spectrum recorded as a function of weathering time.

The methodology is explained in Figure 9.31. The IR spectra are recorded in the course of the aging experiment, and by differential FT-IR of the spectrum before and after aging, the loss of specific groups (C–H and C–N) and the build-up of oxidation products (C–O and O–H) can be followed.

This method can be used for all types of coatings. With UV coatings, also the disposition of unreacted double bonds can be followed. Exemplarily shown in Figure 9.31 is the weathering behaviour of a UV coating which exhibited an initial cure conversion of 65%. It is shown, that the conversion of the remaining acrylate double bonds increases above 90% within the first hours in the QUV environment and to complete conversion within about 200 h. With this experiment, however, it cannot be determined if this reaction leads to further crosslinking or to oxidation or other decomposition reactions of the acrylate groups.

A closer look into the reactions associated with the increased conversion of the remaining double bonds could be gathered by following the mechanical properties. Excellent weathering stabilities of UV coatings, if properly stabilized with UV absorber and HALS, are shown by the example of an aliphatic urethane acrylate 100% liquid coating (Laromer® LR 8987; Figure 9.32).

The photocuring and photostabilization of coatings for exterior applications, like glassreinforced composites, laminates, factory-applied wood coatings and exterior wood coatings, has also been evaluated, with special attention given to the topics of adhesion and yellowing60 as well as to weatherfastness and scratch resistance.61 Such clearcoats are at least as weatherfast as thermally cured coating systems, but show significantly improved scratch resistance as well as chemical resistance.

9.1.6.1 Weathering resistance of water-based coatings The weathering resistance of water-based coatings is a critical point, since water-compatible groups have to be used in order to disperse the coating materials in the water phase. The hydrophilicity finally also

RECENT DEVELOPMENTS

235

F IG . 9.32. Weathering stability of a well-stabilized urethane acrylate under QUV exposure.

F IG . 9.33. Surface energy measurements as a quantification of wettability of a urethane acrylate containing different diols.

determines the stability against water or water-based chemicals as well as the weathering stability. Figure 9.33 shows the hydrophilicity of coating surfaces, measured by the polar component of the surface energy, as a function of the acid number of the carboxylic groups in the dispersion. In the series of dispersions with the same composition, containing polycaprolactone as a diol, the polar component increases with increasing acid number.

A polyurethane acrylate of the same composition, but without carboxylic groups was incorporated in the study (acid number 0) as the solvent-borne comparison. It had been shown, that dispersions could be designed with the same polar component value as the solvent-borne coating of the caprolactone (CAP) series by changing the diol from the ester type (caprolactone) to a hydrocarbon type (D9). The polar component measurements cor-

236

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 9.34. Water pick-up at 100% humidity of urethane acrylate dispersions and solvent-borne PUA (CAP 0).

F IG . 9.35. Weathering stability of unstabilized and stabilized polyurethane dispersion PUA D9 in QUV-A test.

relate well with the water uptake of the dispersions (Figure 9.34). The CAP 24 takes up about 18% water within 90 min storage in 100% humid atmosphere, whereas the dispersion D9 takes up water as low as the solvent-based coating (CAP 0).

Figure 9.35 shows that well-stabilized water-based aliphatic polyurethane coatings show excellent weathering stability in QUV-A test cycles.

RECENT DEVELOPMENTS

237

F IG . 9.36. Stability of stabilized and unstabilized UV dispersion (PUA-D9) and dual cure dispersion under QUV-B exposure.

F IG . 9.37. Comparison of weathering stability of water-borne UV to conventional coatings followed by CH decrease in QUV-A.

The stabilizer effect in water-based and dual curable dispersions reveals remarkable stability even under the harsher QUV-B conditions with about 50% remaining urethane groups (Câ&#x20AC;&#x201C;NH) after 3700 h (Figure 9.36).

Water-based UV-cured coatings exhibit excellent weathering stability on the same order as two-pack polyurethane solvent-borne coatings, and much better than thermoset melamines. This observation is shown in Figure 9.37 on the example of unstabilized coating films.

238

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 9.38. Weathering stability of model clear coats in Florida test.

The excellent weathering stability of UV-cured model clear coats compared with thermally cured standard clear coats has also been demonstrated in Florida results (Figure 9.38).62

9.2 PROCESS RELATED DEVELOPMENTS 9.2.1

UV Curing under Inert (Nitrogen, Carbon Dioxide) Atmosphere

Up to now UV coatings are mostly applied onto planar substrates where the lamp distance to the substrate is constant and the energy density applied is evenly distributed. When it comes to curing of 3D substrates the process is much more complex. Because of different distances between surface and lamps the energy density varies. Since in air atmosphere it is very important to have relatively high energy densities in order to overcome the oxygen inhibition effect, the curing efficiencies at different points of a part may vary significantly.

The measures to compensate the detrimental effects of oxygen inhibition all have disadvantages (high photoinitiator content – high price, odour, migration; high UV energy density – substrate heating, price; amine synergists – yellowing, odour, etc). Most of the mentioned drawbacks can be overcome by curing under inert conditions. Therefore, different inert media have been evaluated in the UV-curing process.

9.2.1.1 Comparison of curing under nitrogen and carbon dioxide The UV curing under inert conditions is well known, but due to higher costs for the replacement of air by nitrogen

RECENT DEVELOPMENTS

239

F IG . 9.39. Influence of the cure environment on the polymerization kinetics of a UV-curable PUA resin.

or other inert gases, like carbon dioxide, not extensively practiced. The kinetics of the photopolymerization of UV-curable acrylates under different curing conditions has been evaluated. The curing under pure nitrogen atmosphere is obviously a very efficient way to overcome oxygen inhibition.63 However, on an industrial curing line, which can not be made completely airtight, nitrogen losses can be very important, thus making the process expensive, and even more so, if argon is used to achieve an oxygen free environment.

It has been shown recently that nitrogen can be advantageously replaced by carbon dioxide to achieve a fast UV curing under inert conditions,64 in a process introduced under the Tradename Larolux® , which will be described in more detail below. In the following, the various influence factors on the curing kinetics under inert (carbon dioxide) atmosphere are discussed.65 The use of carbon dioxide as the inerting medium presents the following advantages66 : • • • • • •

CO2 is widely available, and at least as cheap as nitrogen; CO2 is heavier than air and can be easily maintained in a container; 3D substrates can be readily UV-cured in containers using reflecting aluminium walls; Sufficient cure is obtained with lower UV energy; Higher lamp to substrate distances are tolerable; Simple equipment may be used.

To be attractive, the inerting process by carbon dioxide must show similar performances as that based on nitrogen. This has been demonstrated by comparing the kinetics of the polymerization under CO2 and N2 atmosphere. As shown in Figure 9.39, the curves of nitrogen and carbon dioxide are perfectly superimposed, shown for a thin polyurethane acrylate (PUA) film exposed to UV. In both cases, the induction time is on the order of 0.1 s, the time needed for the initiating radicals to consume the stabilizer and to reach their steady state concentration. Once it is initiated, the polymerization proceeds rapidly. It starts to slow down only above 40% conversion, once molecular mobility restrictions brought upon by the viscosity increase and progressive gelation of the sample become important.

Whereas in the presence of air, the polymerization starts after an induction period of 0.2 s,

240

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 9.40. Influence of the oxygen concentration on the polymerization kinetics of a UV-curable polyurethane acrylate resin.

at a speed four times as low as in an inert atmosphere. After 0.35 s, once 15% of the acrylate double bonds have polymerized, the reaction starts already to slow down because of the continuous diffusion of air into the sample. To prevent the diffusion of oxygen into the sample, a transparent polypropylene film was placed on top of the UV-curable coating (laminated sample) for comparison. The induction period is slightly shorter than in air (oxygen is still dissolved in the film) and the polymerization proceeds 3 times as fast as in air to reach 85% conversion by the end of the UV-irradiation. The curves clearly show the importance of the O2 inhibitory effect on the photopolymerization.

9.2.1.2 Influence of the oxygen content Thus, curing under inert conditions is beneficial, but “how inert” must the atmosphere be? Neither on a typical UV-line used for flat substrates nor in a CO2 container used for 3D substrates, can samples be cured in a sealed reactor, therefore contamination with air cannot be completely prevented. A certain amount of oxygen is expected to still be present in the UV exposure atmosphere. Therefore, the influence of the oxygen content on the polymerization kinetics has been evaluated. Figure 9.40 shows the curves obtained for an oxygen volume content in the atmosphere above the sample varying between 0.4 and 50%. As expected, the induction period increases and the polymerization is slowing down as the oxygen content of the atmosphere was increased. When the O2 concentration was increased to 50%, polymerization did not occur at all. Under those conditions, gaseous oxygen diffuses more rapidly into the sample so that the concentration of dissolved O2 cannot drop low enough to allow the monomer to compete successfully with oxygen for the scavenging of the initiating radicals.

Compared to curing under air, it has been shown in the literature that the scratch resistance already improves significantly at oxygen contents less than 15%67 and also other properties are positively influenced at levels less than 5% oxygen.68 9.2.1.3 Influence of curing conditions It has been pointed out already, that the film thickness plays a significant role in the extent of the oxygen inhibition. The Raman spectra discussed in Chapter 7 showed, that the oxygen inhibition is well pronounced in the top

RECENT DEVELOPMENTS

241

F IG . 9.41. Conversion under air and CO2 atmosphere for a PUA resin at different conditions.

10 µm. Thus, one can expect the gap between the UV curing in air and CO2 to increase as the film thickness is decreased. This trend appears clearly in Figure 9.41 (top left), thin films being much more sensitive to oxygen inhibition than thicker ones. While a 3 µm thick coating hardly reaches 10% conversion after a 2 s irradiation under air (I = 30 mW/cm2 ), it polymerizes readily under carbon dioxide atmosphere, no difference in conversion kinetics being observed between the 3–33 µm thick films (80% conversion after 5 s). Consequently, even if the acrylate conversion through the whole thickness of a thick film reaches a high value after UV curing under air, its surface remains tacky at that UV (low) energy density. A longer exposure under intense radiation is then necessary. Another option is to work under inert conditions, especially if such acrylic coatings are to be cured at relatively low light irradiance.

Performing the UV curing of coatings with powerful lamps not only speeds up the curing process and increase the productivity, but also yields tack-free surfaces. The influence of the UV irradiance on the scratch resistance for exposure under air has already been investigated.7 Applying high irradiance, thus generating a large amount of free radicals within a short period of time enables the curing system to overcome the O2 -inhibition.

This constraint disappears when the UV curing is performed in an inert atmosphere, thus allowing an effective polymerization to be achieved by means of low intensity UV lamps, in particular at the coating surface, which receives the most intense light. The gap between the polymerization curves recorded in air and CO2 is thus expected to widen as the light intensity is decreased. This is exactly what was observed with a 6 µm thick polyurethane

242

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

acrylate coating when it was UV-irradiated at irradiances ranging from 15 to 90 mW cm−2 , as shown in Figure 9.41 (bottom, left). The reactivity ratio CO2 /air was found to increase from a value of 1.3 at an irradiance of 90 mW cm−2 up to 20 at 15 mW cm−2 , while the final conversion of the coating cured in air dropped concomitantly from 90 to 70%. It should be noted that the conversion values are determined by transmission IR spectroscopy and correspond to the average conversion of the whole sample. In coatings UV-cured in air, the very top layer may not be fully cured and may remain tacky, even for “well-cured” thick samples, if the light intensity is not high enough.

Besides the measure of using high irradiance radiation, also the concentration of the photoinitiator can contribute to overcome the oxygen inhibition effect. In a light induced polymerization, the rate of initiation (ri ), as well as the rate of polymerization (Rp ), depends on the photoinitiator concentration [PI].69 When the UV-curable formulation contains a photoinitiator in low concentration, there will be a small number of free radicals generated upon photolysis. Under those conditions, it will be more difficult to overcome oxygen inhibition because there are not enough free radicals left to efficiently initiate the polymerization. Most of the initiator radicals are scavenged by the O2 molecules either dissolved in the sample or diffusing into the film from the air during the UV exposure (Figure 9.41, top right). Contrarily, under inert carbon dioxide atmosphere photoinitiator concentrations as low as 0.15 wt% already give reasonable conversions of about 50%.

At first glance the effect of the cure temperature gives a surprising result. As expected, the conversion is higher at the higher cure temperature in carbon dioxide atmosphere. However, curing in air exhibited the opposite effect, the conversion being higher at lower temperature. The temperature effect has to be discussed in correlation with the viscosity of the coating. Most of the UV-curable resins have a relatively high viscosity, typically between 200 and 4000 mPa s. The viscosity decreases at higher temperatures, thus accelerating the oxygen diffusion into the film and amplifying its inhibitory effect. This phenomenon will compete with a higher molecular mobility, which accelerates the polymerization and increases the final conversion. By selecting the irradiation conditions intentionally so as to amplify oxygen inhibition (low light intensity: 15 mW cm−2 ), the temperature effect can be demonstrated more dramatically. An increase in temperature will decrease the viscosity of the monomer and the oxygen diffusion into the sample will thus be affected. Figure 9.42 shows the correlation, which exists between the reactivity of the system and the viscosity of a polyurethane acrylate resin, for different temperatures between −19 and 80 ◦ C. The gap between curing experiments performed in air and in carbon dioxide was progressively reduced when the sample temperature was lowered and the viscosity of the resin increased.

At −19 ◦ C where the viscosity is very high, oxygen diffusion hardly occurs and has therefore not much effect on the polymerization process (reactivity ratio CO2 /air = 1.3). At −19 ◦ C, the resin is frozen and the polymerization proceeds significantly slower than at ambient temperature under inert conditions. In the presence of air, the diffusion of oxygen is much reduced in the solid sample, so that the photopolymerization occurs nearly as fast.

An intermediate behavior was observed when the UV curing was performed at a temperature of 6 ◦ C. The curve obtained under CO2 inerting conditions shows the actual reactivity of the resin (the increased mobility at higher temperature leading to a faster cure), whereas oxygen inhibition governs the polymerization kinetics when the irradiation is performed under air

RECENT DEVELOPMENTS

243

F IG . 9.42. Correlation between reactivity and viscosity with a PUA under air and CO2 at different temperatures.

(the lower the viscosity, the slower the polymerization because of an enhanced oxygen diffusion). It should be noted that the film thickness shown here (5 µm) is in the range where O2 inhibition is more pronounced than in the commonly used UV-cured coatings where the thickness ranges typically lie between 20 and 50 µm. However, the top layer of such coatings is likely to experience a similar O2 inhibitory effect as thin films used in the present study, thus making it impossible to obtain a tack-free surface upon exposure to low intensity light in the presence of air.

9.2.1.4 Benefits of inerting and measures for reduction of oxygen inhibition UV curing in carbon dioxide (or other inert gases) for reducing the oxygen inhibition permits: • • • • • •

Curing of UV coatings at larger distances; Using low energy, even UV-A lamps which are easy and safe to handle; Reducing the photoinitiator concentration; Curing of formulations which do not cure in air; Curing of thin films (<15 µm); Curing of spray-mist deposits.

Oxygen inhibition is less pronounced when the diffusion of oxygen into the coating is hampered by: • Decrease of oxygen concentration in the atmosphere; • Increase of resin viscosity; • Decrease of the sample temperature.

The curing under inert conditions opens up new application possibilities besides the well known industrial application processes, especially for 3D curing for instance in small (handicraft) businesses. Applications and curing line design of this new technology will be discussed in following (3D curing under inert atmosphere).

244

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 9.43. Scheme of the Larolux® three-dimensional carbon dioxide inerted UV curing process.

9.2.2

Larolux® : UV Inert Curing with Tanning Lamps

An innovative UV curing process overcoming the problems of oxygen inhibition has been established recently as the “Larolux® process”. This process was developed by Beck70,71 (Figure 9.43) and overcomes the need for high intensity lamps associated with ozone generation and short wavelength UV emission, allowing the use of inexpensive equipment and reduced materials costs (low photoinitiator contents). The process relies on the use of carbon dioxide as a readily available, non-polluting, non-toxic, transparent inert gas which is heavier than air. Since carbon dioxide is 1.5 times heavier than air, it can be used for inerting containers, either in the form of gas in cylinders or as dry ice, which produces CO2 by evaporation. The gas consumption is much less than with nitrogen, for example. 1 kg of CO2 is equivalent to a gas volume of 550 l under standard temperature and pressure conditions. The gas itself is non-toxic, however, its hazard potential lies in the displacement of atmospheric oxygen. It is produced on an industrial scale without environmental pollution.

In CO2 filled containers, residual oxygen contents of less than 1% can be achieved. This is considered to be sufficient for conventional free-radical UV-curable coatings.72 Investigations performed by us on the scratch-resistance of weather-resistant UV coatings showed that an oxygen reduction below 15% is already sufficient to lead to substantial improvements.73 To maintain such a low oxygen content is relatively easy in the case of discontinuous exposure conditions, like in the case of piece goods with small throughput rates. In continuous exposure lines, however, oxygen is introduced easily through turbulence at the surface in contact with the air, in particular by external air flows, by immersion and withdrawal of the substrates, by introduction of CO2 and by heat sources, such as UV lamps. Depending on substrate and throughput, corresponding adaptations of the plant are required for producing oxygen-poor exposure zones.

Since such lines are difficult to inert completely, the influence of the O2 concentration in the atmosphere on the curing performance has been studied.74 Furthermore, the effect of the sample temperature, the resin viscosity, the resin reactivity, the film thickness, the

RECENT DEVELOPMENTS

245

F IG . 9.44. The layout of a Rippert 3D-UV curing pilot unit working with Larolux® process in side and top view.

Objects are hanging on a belt; the mid part of the container is filled with CO2 , and contains UV lamps.

concentration of photoinitiator, the light intensity and the radiation wavelength on the photoinitiated polymerization of acrylate resins under air and inert exposure atmosphere has been examined.

Under a reduced oxygen atmosphere, lower radiation intensities are sufficient for curing.

In addition to reducing the curing time and the number of lamps – something which has scarcely been considered to date – it is also possible to use UV lamps of lower power or to increase the lamp–substrate distance.

Lamps of lower power open up a wide range of potentially suitable lamps since many lamp types contain spectral emission in the UV range. In the case of lamps for room lighting for instance, the short UV wavelength range is filtered out for hygienic reasons. Economical and portable UV lamps are used in particular in the tanning sector and in highpower flash lamps.75 A first pilot project for 3D UV curing of car body parts under CO2 has already been realized.76 Small parts can be cured in a pilot line, where the objects are hanging on a belt for transporting. They are dipped into the deeper part of the container which is filled with CO2 and contains the UV lamps (Figure 9.44).

The alignment of lamps can be specifically adjusted to the objects to be cured, shown in Figure 9.45 for the curing of chairs using cheap illumination lamps.

A further pilot UV device for working under carbon dioxide has been published by Tikkurila Coatings,77 where the geometries of the curing chamber have been optimized in order to maintain the inert conditions at a low oxygen level in the range of 2–4%, which

246

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 9.45. UV curing of a chair with the Larolux® 3D CO2 inerted UV-curing process.

is sufficiently low to minimize the effects of oxygen inhibition. Furthermore, due to the reflecting walls, the extent of “shadow areas”, if present, can be reduced.

9.2.3

UV Curing of Pigmented Systems78

To enter into additional market segments with pigmented UV coatings some obstacles have to be overcome. Unfortunately there is competition in the coating between pigments and photoinitiators when exposed to UV light. The ideal pigment should have good UV transparency in order not to interfere with the photoinitiator reaction and good absorption in the range of 400 to 700 nm in order to provide good hiding power. With a smart combination of photoinitiators absorbing UV light between 380 to 410 nm, suitable lamps (Ga- or Feadded) and curing conditions most of the pigmented coatings can be fully cured. By using a dual cure process the remaining restrictions can be eliminated, as demonstrated with a UV cured coloured chair (Figure 9.46).

9.2.4

UV Plasma Curing

UV curing in a plasma reactor is another innovative concept to overcome the obstacles of curing in shadow zones. In the new UV plasma curing process the coated substrates are

RECENT DEVELOPMENTS

247

F IG . 9.46. 3D application of UV-cured pigmented coatings.

F IG . 9.47. UV plasma curing process in comparison to UV lamp curing.

placed in a vacuum chamber containing gases like nitrogen, helium or argon, at pressures below 0.1 mbar. Thus, the curing also proceeds under inert conditions. A microwave with a rating of 800â&#x20AC;&#x201C;900 W ignites the plasma of the chamber, which emits electromagnetic radiation in the UV. After a processing time of about 90 s the UV-curable coating is fully crosslinked. The curing results of urethane acrylate formulations in respect to durability and curing data (conversion, yellowing) are comparable to the UV lamp cured samples.79 The UV emission of the plasma described above is not much different from the UV emissions of mercury arc lamps. The main difference is founded in the direction of the emission

248

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 9.48. UV Plasma Cure® : Scheme of pilot body curing plant.

F IG . 9.49. UV Plasma Cure® : Real size chamber to cure entire car bodies.

RECENT DEVELOPMENTS

249

F IG . 9.50. UV Plasma Cure® : Comparison of space requirement with conventional drying oven.

of the arc lamps, which obeys the wave propagation rules, thus the light may be reflected or diffracted, however, shadow areas can not be avoided. In the plasma chamber, however, the atoms emitting the UV are distributed within the entire volume, and therefore no shadow areas exist. The picture of “placing the to be cured object into the lamp” describes the process colourfully (Figure 9.47). It has been shown on the example of an U-shaped profile that the cure conversion of an urethane acrylate coating is almost as high inside the profile compared to outside. The hardness measurements in the “shadow areas” revealed, that the hardness of the plasma cured sample was even slightly better compared to a dual cure coating. This process therefore offers the advantage of curing three-dimensional substrates without shadow areas.

The first intentions to use such are process are reflected by the development of a pilot plant by the automotive equipment supplier Duerr GmbH (scheme of pilot body plant see Figure 9.48). This pilot plant is constructed to cure entire car bodies under plasma conditions (Figure 9.49). The big advantage of using UV curing of clear coats by such a plasma process is pointed out in the space requirement comparison to a conventional thermal curing process used in the automotive production today (Figure 9.50). The plasma curing process should give comparable curing results, since the objections regarding the curing of shadow zones are eliminated with this process.

REFERENCES 1. Beck, E., Keil, E. and Lokai, M., Farbe&Lack 98, 165–170 (1992).

2. Lange, H., Farbe&Lack 99, 597–601 (1993).

3. Tauber, A., Scherzer, T. and Mehnert, R., J. Coat. Technol. 72 (911), 51–60 (2000).

250

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

4. Beck, E., Lokai, M., Keil, E. and Nissler, H., RadTech 1995, Conference Proceedings. 1995, pp. 351–357.

5. Schrof, W., Häußling, L., Schwalm, R., Königer, R., Reich, W., Menzel, K. and Beck, E., RadTech 1997, Conference Proceedings. 1997, pp. 535–547.

6. Uminski, M., Pigm. Resin Technol. 26 (3), 149–152 (1997).

7. Reis, O. and Fieberg, A., Macromol. Symp. 187, 605–615 (2002).

8. C&EN, Cover Story 79 (45, November 5) (2001).

9. Buysens, K., Tielemans, M. and Randoux, T., Surf. Coat. Int. A 05, 179–186 (2003).

10. Vandendriessche, J., Bontinck, B. and Buysens, K., Surf. Coat. Int. A 226–229 (2004/2005).

11. www.radtech-europe.com/download/uvpapers.pdf.

12. Binder, H., Blum, R., Königer, R., Paulus, W., Schrof, W. and Schwalm, R., Farbe&Lack 105 (11), 38 (1999).

13. Udding-Louwrier, S., Baijards, R. and Sjoerd deJong, E., J. Coat. Technol. 72 (904), 71–75 (2000).

14. DSM EP 636669 (1995) and www.dsmcoatingresins.com.

15. Zune, C. and Buysens, K., Eur. Coat. J. 20–30 (2000); UCB EP 739922 (1995).

16. BASF EP 650978 (1994); BASF EP 678562 (1995); Bayer EP 350730 (1990).

17. Wenning, A., Macromol. Symp. 187, 597–603 (2002).

18. Hult, A., Johansson, M., Jansson, A. and Malstroem, E., Proceedings RadTech Europe 1999, pp. 634–639.

19. Johansson, M., Falken, H., Irestedt, A. and Hult, A., J. Coat. Technol. 70 (884), 57–62 (1998).

20. Frey, T., Biehler, M. and Graf, K., UV Meets Colour: A numerical simulation of the through-cure of pigmented UV coatings, RadTech Europe’05, Conference Proceedings, Vol. I. 2005, pp. 515–520.

21. Valet, A., Farbe&Lack 102 (1), 40–46 (1996).

22. Vandendriessche, J., Bontinck, D. and Buysens, K., Surf. Coat. Int. 87 (Part A) (2004).

23. Peeters, S., Overview of dual-cure and hybrid-cure systems in radiation curing, In “Radiation Curing in Polymer Science and Technology, Vol. III, Polymerization Mechanism”, J.P. Fouassier and J.F. Rabek, eds.

Elsevier Applied Science, 1993, Chapter 6.

24. Fey, T. and Muhle, J., UV-dual cure systems for automotive applications, RadTech Europe 2003, Vol. II. 2003, pp. 741–747.

25. Desmolux range, http://www.bayer-ls.com/ls/lswebcms.nsf/id/radiation_en.

26. Beck, E., Low viscous dual cure concepts with new type of isocyanato acrylate, RadTech Europe 2003, Vol. I. 2003, p. 215.

27. Fischer, W. and Weikard, J., Farbe&Lack 107 (3), 120–124 (2001).

28. US Patent 6,916,854.

29. Schwalm, R., Decker, C., Masson, F., Heischkel, Y., Jaworek, T., Beck, E., Enenkel, P. and Menzel, K., Exterior stability of UV-curable polyurethane dispersions, RadTech Europe 2003, Vol. II. 2003, pp. 725– 730.

30. El-Ghayoury, A., Boukaftane, C., de Ruiter, B. and van Linde, R., Macromol. Symp. 187, 553–561 (2002).

31. Boncza-Tomaszewski, Z., UV dual cure powder coatings, RadTech Europe 2005, Vol. II. 2005, pp. 425–429.

32. http://www.isc.fhg.de/.

33. http://www.inm-gmbh.de/.

34. Gigant, K., Posset, U., Schottner, G., Baia, L., Kiefer, W. and Popp, J., J. Sol–Gel Sci. Technol. 26, 369–373 (2003).

35. http://www.ppg.com/car_autocoat/pressrelease_1_8_03_Ceramiclear.htm.

36. Biehler, M., Proceedings RadTech Basel, 2001.

37. Sawitowski, Th., The use of nanoadditives to improve scratch resistance of radiation curable coatings, RadTech 2006, Conference Proceedings, Chicago, IL. 2006, on CD. 38. Borup, B., Edelmann, R. and Mehnert, R., Eur. Coat. J. 06, 21–27 (2003).

39. Wie, J., Li, H., Sun, Z. and Wang, L., RadTech Asia 2005, Proceedings. 2005, pp. 683–688.

40. Posthumus, W., PH.D thesis, TU Eindhoven, http://alexandria.tue.nlextra2/200411175.pdf.

41. (a) Hoyle, C.E., Clark, T., Lee, T.Y., Roper, T., Pan, B., Wei, H., Zhou, H. and Lichtenhan, J., RadTech Europe 2005, Vol. I. 2005, pp. 289–293; (b) Clark, T.S., Hoyle, C.E. and Nazarenko, S., RadTech 2006, Conference Proceedings. Chicago, IL, 2006, on CD. 42. He, J. and Soucek, M.D., RadTech Asia 2005, Proceedings. 2005, pp. 470–483.

43. Zhaoa, Y., Yangb, J., Shia, W. and Mingb, H., RadTech Asia 2005, Proceedings. 2005, pp. 485–491.

44. Kojima, Y., Usuki, A., Kawasumi, M., Okada, A., Fukushima, Y., Karauchi, T. and Kamagaido, O., J. Mater.

Res. 6, 1185 (1993).

RECENT DEVELOPMENTS

251

45. Alexandre, M. and Dubois, P., Mater. Sci. Eng. 28, 1 (2000).

46. Gläsel, H.J., Hartmann, E., Mehnert, R., Hirsch, D., Büttcher, R. and Hormes, J., Nucl. Instr. Math. Phys.

Res. B 151, 200 (1999).

47. Huimin, W., Minghua, M., Yongeai, J., Qingshan, L., Xiachong, Z. and Shikang, W., Polym. Intern. 51, 7 (2002).

48. Decker, C., Zahouily, K., Keller, L., Benfarhi, S., Bendaikha, T. and Baron, J., Mater. Sci. 37, 4831–4838 (2002).

49. Valet, A., Jung, T. and Köhler, M., “Creative Advances in Coatings Technology, Congress Papers”. 4th Nürnberg Congress, 1997, p. 33.

50. Olson, D.R., J. Appl. Polym. Sci. 28, 1159 (1983).

51. Decker, C., Biry, S. and Zahouily, K., Polym. Degrad. Stab. 49, 111–119 (1995).

52. Decker, C. and Biry, S., Progr. Org. Coat. 29, 81–87 (1996).

53. Hult, A. and Ränby, B., Polym. Degrad. Stab. 9, 1–13 (1984).

54. Valet, A. and Wostratzky, D., Conference Proceedings, RadTech 1998, Chicago, IL, 1998, p. 396.

55. Valet, A., Progr. Org. Coat. 35, 223–233 (1999).

56. Allen, N.S., Robinson, P.J. and White, N.J., Polym. Degrad. Stab. 23, 245–255 (1989).

57. Rabek, J.F., “Photostabilization of Polymers”. Elsevier Applied Science, London, 1990.

58. Holman, R. and Kennedy, R., RADnews Issues 19–20, 2–9 (1997).

59. Decker, C. and Zahouily, K., Conference Proceedings, RadTech 1998, Chicago, IL, 1998, p. 396.

60. Megert, S., Köhler, M., Rogez, R., Burglin, M. and Litzler, A., Surf. Coat. Int. A 06, 254–260 (2004).

61. Jung, T. and Valet, A., Macromol. Symp. 187, 531–542 (2002).

62. Jung, T. and Valet, A., UV curable clearcoats, RadTech Report, November/December 2001, pp. 30–35.

63. Henke, T., RadTech Europe 2001, Proceedings, 2001, p. 145; and Müller, R., RadTech Europe 2001, Proceedings. 2001, p. 149.

64. Beck, E., UV curing for everybody, RadTech Europe 2001, Proceedings. 2001, pp. 643–648.

65. (a) Studer, K., Decker, C., Beck, E. and Schwalm, R., Progr. Org. Coat. 48, 92–100 (2003); (b) Studer, K., Decker, C., Beck, E. and Schwalm, R., Progr. Org. Coat. 48, 101–111 (2003).

66. Beck, E., Deis, O., Enenkel, P. and Schrof, W., Lichthärtung von strahlungshärtbaren Massen unter Schutzgas, German Patent 199 57 900, 1999.

67. Schwalm, R., Farbe&Lack 106 (4), 58–69 (2000); (b) EP 1218462.

68. de Ruiter, B. and Meertens, R.M., Macrmol. Symp. 187, 407–416 (2002).

69. Decker, C., Prog. Polym. Sci. 21, 593 (1996).

70. Beck, E., Deis, O., Enenkel, P. and Schrof, W., German Patent 199,57,900, 1999.

71. Beck, E., UV curing under carbon dioxide, Proceedings RadTech Berlin 2003, pp. 857–863.

72. Bolte, G., Coating 2, 44 (2001).

73. EP 1218462; US 6777458; and WO0114483.

74. Studer, K., Decker, C., Beck, E. and Schwalm, R., Progr. Org. Coat. 48, 92–100 (2003).

75. Stöhr, A. and Renschke, J., RadTech Europe 1999, Conference Proceedings. 1999, pp. 353–356.

76. Ortmeier, J., Stand der UV Lackentwicklung für dreidimensionale Kunststoffteile, Neue Automobillacke und Lackaufbauten für PKW und Nutzfahrzeuge in Entwicklung und Anwendung, Bad Nauheim, March 2003. pp. 54–67.

77. Biehler, M., Beck, E., Menzel, K., Titusson, S., Daiss, A., Soljamo, K. and Fagerholm, K., UV becomes three-dimensional, RadTech Europe 2005, Conference Proceedings, Vol. I. 2005, pp. 59–65.

78. Menzel, K. et.al., RadTech North America, eI5, UV&EB Technology, Charlotte, NC, May 2004, Technical Conference Proceedings. 2004 on CD. 79. Jung, T., Simmendinger, P. and Tobisch, W., UV plasma curing: Capture the third dimension, RadTech Europe 2005, Conference Proceedings, Vol. I. 2005, pp. 171–175.

C HAPTER 10

New Applications Due to the many advantages of UV curing, a lot of new applications for radiation curable coatings are emerging. Among them are printing applications, like UV Inkjet, label printing, gravure and wide web flexo, adhesives, like pressure sensitive adhesives or CD bonding, clear coats for metallized plastics, exterior coil coating of steel and aluminium, and automotive applications.1 In Asia, especially electronic applications, like photoresists and flat panel displays, are the drivers, while Asian companies are also the market leaders in CD’s and DVD’s, where adhesives and protective coatings are UV cured.2 UV applications for 3D substrates, such as TV sets and vehicle parts, have been reported recently.3 Especially the major advances in the equipment development for 3D applications and curing of weather resistant coatings have opened up new application fields with automotive applications providing the entry to many other industrial coating possibilities (Figure 10.1).

Besides UV applications already in use in automotive parts, like motor sealings, sensor encapsulates, electronic parts, headlamp assemblies, lens and reflector coatings, graphic identification labels, name boards, dashboard screen printings or battery labels, new automotive applications under development are: • Primer or sealers of plastic and SMC parts; • Automotive refinish coatings; • Clear coats with high scratch resistance.

10.1 AUTOMOTIVE APPLICATIONS As UV coatings enter the market as an alternative to almost all thermally cured coatings, the issue has been raised, what are the basic demands on automotive coatings. There is no general and simple answer, since the specifications depend on the substrate used, the location of the part (exterior, interior), and the automotive company involved, as well as many others. A more general overview about basic Performance Requirements for Automotive Coatings with emphasis on physical properties, general film and coating properties, durability and chemical resistance, has been given recently.4 In this chapter the various tests, their methods and the tests demanded by major automotive companies are listed. Some of the tests are mentioned briefly below. Except for the chip resistance (ASTM D3363), where a gravel-o-meter propels gravel at the coated surface and the number and size of chips is 252

NEW APPLICATIONS

253

F IG . 10.1. Major advances to extend the application spectrum of UV-curable coatings.

evaluated, most basic physical property tests, like adhesion, abrasion, gloss, mar resistance, scratch resistance are not restricted to automotive coatings. The same is true for general film and coating properties, like film thickness, distinctness of image (ASTM D5767-95), odour and viscosity. There are a lot of durability tests required, which include environmentally exposure, grouped into accelerated laboratory tests, accelerated outdoor simulation and outdoor tests, thermal and humidity aging, cold cracking and salt spray tests. For accelerated laboratory tests, currently the Xenon Arc test is the preferred test method. The Xenon Arc light source used is modified by filters to closely match the spectral power distribution of sunlight at UV wavelengths, and is currently the closest simulation of sunlight across the entire UV spectrum. Other weathering conditions can also be changed, like water spray to the coated surface, temperatures and humidity, so the overall effect is similar to natural weathering. The acceleration factor is provided by higher radiant intensities, longer light-on duration, higher temperatures, and more prolonged moisture cycling. The xenon arc light source should be used whenever good agreement with outdoor testing is the major goal of testing. The xenon arc conforms to many specifications including ASTM G26, SAE J 1960, SAE J 1885 and AATCC Method 16E, F. For outdoor exposure several test sites exist, which are located in different climate areas in order to cover the extreme weather conditions occurring naturally. Well known test sites are in Miami (FL), providing high ambient temperatures and UV exposure, as well as high humidity. The other extreme is the desert, either Arizona or Kalahari (Africa), with low annual rainfall and humidity, with high temperatures in summer and wide day and night temperature differences. For automotive exterior parts, often also the Jacksonville (FL),

254

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

exposure is required, a test site where a large paper and textile industry generates acid rain and other pollutions. These tests are used to screen and to select coatings, able to withstand the harsh environmental conditions, and do not fail by adhesion loss, crazing, cracking, gloss or colour loss. Some of the test sites are operated by, for example, Sub-Tropical Testing Services (http://www.sub-tropical.com). Several standard test methods required by the automotive companies are also listed at this web-site.

Associated with durability tests are the various tests required for chemical resistance, where the coating is exposed to specific chemicals at different temperatures. The most common test chemicals are sulphuric acid, tree resin, pancreatine, water, caustic soda, and gasoline. The Chrysler laboratory procedure LP-463PB-31-01 is a good reference for the chemical testing required. The various test methods can be downloaded from ASTM (www.astm.org) and SAE (www.sae.org).

The opportunities for UV-curable coatings on automotive plastics have already been described in 1987.5 At this time, the use of plastics, like polyacrylates (PMMA) and polycarbonate, was discussed heavily as a substitute for glass in headlamps, reflectors or windows, attributed to new designs, weight savings, and fewer broken lenses. The use of polymers required protection of the plastics by hard clear coats, since these plastics suffer from low scratch resistance, solvent sensitivity, thermal sensitivity, UV degradation and/or yellowing due to sunlight exposure. UV coatings proved to be the best choice because of their economics. These coatings cure within seconds, whereas the low temperature curing applied at that time required 3–5 h curing time.

Another recent application was an UV-curable sealer coating for fibre reinforced plastics, like sheet moulding compounds (SMC) or bulk moulding compounds (BMC). The main problem to be solved with such sealers is the porosity popping. During the moulding of the compounds voids are formed which can entrap air or solvents. This air is afterwards released during basecoat or clearcoat baking, resulting in the formation of unwanted craters or other defects. UV-curable sealers were based on low viscosity formulations, which can flow into the voids, and seal the voids eithin seconds upon curing to prevent air release.

Furthermore the sealer smoothes the surface out and covers fibres sticking up of the plastic.

Recently, current and planned UV applications in the automotive industry have been described from the view of equipment suppliers at the RadTech Asia Conference.6 The current applications described are varnishes on headlamp lenses, varnishes on reflectors and on instrument panels. The potential application which might come pretty soon are car refinish coatings, primers or sealers on plastics and SMC components, and finally scratch resistant top clear coats.

10.1.1

Suitability of UV Coatings for Automotive Applications

Despite the use of UV-curable coatings in specific automotive applications, like sealers or headlamp coatings, the general question arises, if the chemistry used in UV coatings is well suited for high gloss surfaces with “class A” quality for flat horizontal substrates with excellent scratch and chemical resistance as well as long term durability as required for top clear coats or refinish systems.

NEW APPLICATIONS

255

F IG . 10.2. Chemistry of typical automotive top coats (1K and 2K systems).

The two major clear coat systems used in automotive coatings today are the onecomponent (1K) amino and the two-component (2K) polyurethane coatings (Figure 10.2).

Both systems are based on polyacrylatol resins, which are copolymers of hydroxyl group containing (meth)acrylates (hydroxyethyl acrylate) with other comonomers, such as other (meth)acrylates and/or styrene. They are usually tailor made by the automotive coatings formulators and suppliers in order to provide the basic properties for high performance clear coats.

In the case of 1K systems the polyacrylatols are crosslinked with hexafunctional melamines providing high crosslink densities, however, reduced chemical stability (etch resistance) due to the formation of acid labile Nâ&#x20AC;&#x201C;O acetal structures compared with the 2K polyurethanes, which are obtained by crosslinking with trifunctional isocyanates. The 2K PUâ&#x20AC;&#x2122;s are excellent in etch resistance performance, but exhibit declined scratch resistance due to reduced crosslink density.

Besides these basic chemistries a lot of specific formulations exist which combine the crosslinker chemistries in order to introduce higher crosslink densities (addition of HMMM) or increased etch resistance and flexibility (addition of blocked isocyanates).

Another major class of automotive coatings is based on epoxy-carboxy-crosslinking chemistry.7 The 2K polyurethanes are considered top class automotive coatings, and therefore the benchmark for the comparison with the chemistry of UV coatings, urethane acrylates in special. As shown in Figure 10.3, the polyacrylatol resins of the 2K systems are radically polymerized by copolymerizing hydroxyethyl acrylate with comonomers in a reaction vessel. They are formulated and applied together with polyisocyanate crosslinkers in 2K systems and thermally crosslinked on the car body to yield a durable network.

256

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 10.3. Network formation with 2K polyurethane and UV urethane acrylate chemistry.

The urethane acrylates for UV systems are built-up in an opposite manner. The hydroxyethyl acrylate reacts first with the polyisocyanate in a reaction vessel and is then formulated with reactive monomers and crosslinked on the car body by means of UV-induced radical polymerization. Thus, by either thermal or UV-induced polymerization similar networks are formed.

The suitability of UV-cured and dual cure UV systems for automotive applications has been demonstrated recently by evaluating the scratch/mar resistance and the chemical resistance in comparison with traditional 2K polyurethanes.8 DuPont showed that the UVcrosslinked systems exhibit improved resistance to scratches and improved chemical stability compared to conventional 2K PU. An optimized process for dual cure and UV monocure has been developed and a pilot line for curing real car bodies has been implemented in the DuPont TechCenter in Wuppertal9 (Figure 10.4).

Thus, since a similar network can be provided, UV coatings offer several advantages for automotive clear coats compared to thermal curing: • • • • •

Lower VOC emissions (100% liquids); Reduced energy costs and floor space (low temperature UV unit instead of 140 ◦ C oven); Reduced process time (seconds instead of up to 30 min); Improved scratch resistance (higher crosslink density); Recycling of overspray in the case of 1K UV-cure coatings.

10.1.2

UV-Curable Clear Coats (Head Lamps, Reflectors, Alu Wheels)10

Some examples of automotive applications of UV-curable clear coats are shown in Figure 10.5.

NEW APPLICATIONS

F IG . 10.4. UV-curing pilot line in the DuPont TechCenter in Wuppertal.

F IG . 10.5. Examples of UV-cured parts in automotive applications.

257

258

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

The protection of polycarbonate headlamp lenses by scratch resistant UV coatings has been used for a long time because of its economic processing compared to the commonly used silicon hard coats. The Japanese automotive companies were the first to introduce this UV process. Now it is also in widespread use in the US and Europe.11 In the plastic reflector housing a thin UV coat is applied to give a very smooth surface for the subsequent aluminium metallizing to guarantee for a mirror like finish. In order to protect the soft aluminium layer from scratches and hazing, finally a protective UV clear coat is applied. These UV processes can be completed within minutes compared with more than three hours for conventional coatings. Furthermore, the assembling of the head lamps has also been shortened due to the use of UV curable adhesives.

Developments have been pursued for the protection of aluminium wheels with UVcurable coatings, both liquid and powder coats. The 100% liquids are processed like any other UV-curable liquid materials. The flow and curing speed can be increased by short IR heating before UV cure. The UV powder coats can be molten at temperatures as low as 95 â&#x2014;Ś C to obtain good levelling before UV cure. The processing times are significantly reduced compared to thermal hardening and the energy required is about 10% that of a convection ovens.

10.1.3

UV-Curable Primer/Sealer (Eco-Efficiency)

Today most repairs of automotive coatings are done by starting with a primer application, consisting of either two-component (2K) urethane, showing fast drying times and good sanding characteristics, or epoxy-polyamides, exhibiting superior adhesion, corrosion protection and long pot life. On top of this primer the usual base coat/clear coat assembly is applied, in which the clear coat is a 2K urethane. Recently, several UV-curable products for automotive refinish applications have been introduced, for example an UV-curable primer-sealer over sheet moulded compounds (SMC) has been implemented by Ford12 and described for the priming of SMC sheets.13 Since automotive refinish coaters are expressing growing interest in UV technology and finding significant advantages over conventional systems, the economical and environmental impacts of these new UV products can be compared to conventional solutions by performing an eco-efficiency study. The following eco-efficiency analysis of UV-curable refinish primers has been performed by Richards et al.14 and will be discussed in detail in order to show the many influencing factors to be considered in finding the economically and ecologically best solution for coating a specific component part.

An eco-efficiency study is a methodology that assesses the environmental and economic impacts of products and processes over their life-cycle.15 The methodology is based upon the ISO14040 standards for life-cycle analysis and contains some additional enhancements that allow for expedient review and decision-making at all business levels.16 UV-cured coatings promise enhanced performance, higher productivity and an improved environmental footprint vs. traditional thermally cured coatings. Eco-efficiency is one tool that can be used to strategically develop and pursue the best products. Thus, it is a natural fit that eco-efficiency quantifies and demonstrates the benefits of UV technology. In the future, eco-efficiency will be used to further optimize coatings and other technologies.

259

NEW APPLICATIONS

TABLE 10.1. Definition of customer benefit and product alternatives Customer benefit (CB)

New products

Comparable products

Application of primer to 1 m2

1K UV Cure, Primer (UV-A) 1K UV Cure, Primer (UV-A) as an aerosol

Conventional 2K polyurethane primer: a) Thermal cure b) IR cure

of a car body, for lifetime use of 200,000 miles

Epoxy primer: a) Thermal cure b) IR cure

TABLE 10.2. Coating data of new refinish products and conventional alternatives Coating data Transfer efficiency Film thickness Total solids Specific gravity Coverage at 1 Mil VOC Pot life

% Mil wt% Lb/gal Mils ft2 /gal Lbs/gal h, 20 â&#x2014;Ś C

1K UV cure

1K UV aerosol

2K polyurethane

Epoxy

65 3 80 12.3 1022 1.7 No limit

75 3 43 5.9 354 4.5 No limit

65 3 56 11 248 4.7 0.5

65 3 58 10.7 616 4.4 24

The first step is to define the customer benefit (CB), so that the application and use for all of the alternatives considered can be compared. Here, application and curing of a primer on one square meter of automobile surface area is considered. The six primer technologies compared are described in Table 10.1. The study encompasses production, application and curing of the coatings, cleaning of application equipment between applications, and fuel consumption of the automobile over its lifetime, resulting from the weight of coating on the vehicle.

Coating application and curing parameters are based upon manufacturersâ&#x20AC;&#x2122; recommendations. Key data are shown in Table 10.2.

The most significant differences in the coatings are in their application. As can be seen in Table 10.3, preparation time is shorter for the UV-cured coatings because there is no need for combining two separate components prior to mixing. In addition, the time of UV curing is significantly shorter than for thermal curing. Finally, cleaning of spray equipment is either non-existent, for the UV-cured primer in aerosol cans, or drastically less than for the urethane and epoxy coatings. This is due to advantages in pot life of the UV-cured primer, which means primer can be left in the spray gun between applications without the concern of solidification.

In order to assess the economic portion of eco-efficiency, the costs of using each of the products are considered. Table 10.4 depicts the costs for a body shop that currently processes 5 m2 jobs per day. Because of the efficiencies of the UV-cured coatings in coating preparation, application, equipment cleaning and cure times, body shops are able

260

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

TABLE 10.3. Application data of new refinish products and conventional alternatives Application data

1K UV cure

2K PU thermal

Epoxy thermal

2K PU IR

Epoxy IR

1K UV aerosol

Mixing of components Coats Flash time between coats (min) Flash time before lamp cure (min) Curing time (min) Spray equipment cleaning frequency

No 1 0 5 3 1/day

Yes 2 10 n/a 25 1/job

Yes 2 10 n/a 20 1/job

Yes 2 10 n/a 13 1/job

Yes 2 10 n/a 13 1/job

No 2 1 5 3 n/a

TABLE 10.4. Cost categories for processing a 1 m2 job Cost category

Units

1K UV

2K PU thermal

Epoxy thermal

2K PU IR

Epoxy IR

1K UV aerosol

Materials Labour cost per job Energy cost per job Total cleanup per job Rework costs Total cost per CB

$/job $/job $/job $/job $/job $/job

15.29 9.66 0.024 0.346 – 25.32

7.47 13.62 3.693 2.185 6.74 33.71

6.64 13.62 2.110 2.119 6.12 30.61

7.47 12.03 0.065 2.185 2.72 24.48

6.64 12.03 0.049 2.119 2.61 23.45

52.14 11.56 0.024 – – 63.72

Total time per job Base no. of jobs Capacity increase Revenue per job Total annual revenue Total revenue increase/year

Min/job Jobs/yr Jobs/yr $/job T§/yr T$/yr

36 1250 500 200 350 100

84 1250 – 200 250 –

69 1250 – 200 250 –

57 1250 – 200 250 –

57 1250 – 200 250 –

42 1250 500 200 350 100

to process individual jobs more quickly and consequently increase their productivity. Increased productivity leads to higher profitability. A University of Michigan study17 found a 40% capacity increase based on cycle-time decreases from 14 to 9 h for a shop handling five vehicles per day, using UV-cured coatings. Assuming a base number of 1250 jobs per year, this means that 500 additional jobs could be processed. The resulting potential revenue increases by using UV-cured coatings are $100,000 per year.

Figure 10.6 shows the relative impact of the primers in six environmental categories.

The ecological fingerprint is composed of several criteria resulting in the subsumed six values, where the worst alternative is set to 1, and all others are relative to this worst alternative. The criteria energy consumption and emissions are explained in more detail in Figures 10.7 and 10.8. The most significant differences occur in emissions, energy and resource consumption, land use and risk potential. The overall health effect potential for the primers is similar.

The greatest environmental benefit of UV-cured coatings is the outstanding efficiency of the curing process. Figure 10.7 shows the individual energy consumption data, which are summarized on the ecological fingerprint.

NEW APPLICATIONS

261

F IG . 10.6. Ecological fingerprint for automotive refinish primer.

F IG . 10.7. Energy consumption (MJ) of the refinish primer alternatives.

The thermally cured coatings have longer, more energy-intensive curing processes, which consume large quantities of natural gas and result in correspondingly higher environmental impact and utility costs. This characteristic is also the reason for the high resource consumption of the thermally cured coatings. The overall emission results are primarily impacted by air emissions. Figure 10.8 shows the global warming (GWP) and photochemical oxidant (i.e., smog) creation potentials (POCP) for the alternatives. The thermally cured coatings have the highest GWP due to the natural gas produced and used for curing. The UV-cured primer in cans has the lowest smog creation potential (POCP) because it has the lowest VOC content. Figure 10.9 shows the costs per customer benefit for each of the coatings.

The labour and material costs are the most significant. The efficiencies in preparation, application and curing of the UV-cured coating in cans results in similar total costs com-

262

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 10.8. Air emissions: Global warming potential and photochemical ozone depletion potential.

F IG . 10.9. Costs of refinish coating alternatives.

NEW APPLICATIONS

263

F IG . 10.10. Total costs of ownership (costs + revenue loss).

pared to the thermal curing urethane and epoxy coatings. In addition, with the urethane and epoxy coatings there is potential for dust uptake, which results in rework and additional cost. Although the material cost of the UV-cured coating in aerosol cans is high, its advantage is its convenience. As shown in Table 10.4, an annual revenue increase of $100,000 is possible if a body shop switches to UV-cured coatings. If, however, the shop continues to use the urethane or epoxy coatings, and the lost revenue per job is combined with the costs depicted in Figure 10.9, the resulting total cost of ownership is shown in Figure 10.10. It can clearly be seen that the UV-cured coating in cans has a much lower total cost of ownership.

The eco-efficiency portfolio consolidates all of the individual environmental and economic results into one portfolio, allowing for an overall picture of which products are best.

The most eco-efficient products lie in the upper right-hand quadrant, which corresponds to the lowest environmental and cost impacts. Figure 10.11 shows the portfolio for automotive refinish primers when the total cost of using the products is considered. The UVand IR-cured primers in cans are the most eco-efficient. Due to the more efficient curing processes they have less overall environmental impact and lower costs than the thermally cured coatings. The UV-cured coating in aerosol cans has similar environmental impact to the non-aerosol product; however, the higher material cost results in lower eco-efficiency.

When the potential revenue increase for switching to the UV-cured coatings is considered, their eco-efficiency improves significantly. Figure 10.12 show that the UV-cured coating in cans truly differentiates itself from the other products, while the aerosol becomes much more comparable to the IR- and thermally-cured products.

The key to a successful product is both environmental and economic advantage over competing products. UV-cured primers provide both of these and are successful products that support industrial sustainability initiatives.

This eco-efficiency analysis showed that the UV primer cures 10 times faster than conventional thermally cured products. Since UV curing is light-activated and does not depend on heat, it eliminates the need for bake ovens. As a result the energy consumption in a body shop is reduced by approximately 80%. Therefore, the U.S. Environmental Protection

264

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 10.11. Eco-efficiency portfolio (base case).

F IG . 10.12. Eco-efficiency portfolio including potential lost revenues.

Agency has awarded its coveted Presidential Green Chemistry Challenge Award to BASF’s Automotive Refinish Coatings business for the company’s eco-efficient UV-curable primer.

10.1.4

UV-Curable Coatings for Car Refinish

UV-curable coatings for automotive refinish applications enabling a fast curing process and improvement of productivity have also been developed by DuPont Performance Coatings.18 They focus on the development of an UV curing refinish primer system which is UV-cured with equipment based on flash light technology. The UV equipment generates UV flashes in a very short interval with high intensity and energy, emitting over a wide

265

NEW APPLICATIONS

wavelength range down into the UV-B19 region, providing a homogenous light distribution to cure three-dimensional objects. This optimized process is designed for spot- and microrepair applications, enabling a UV curing time of less than 30 s, which is about half of the conventional primer process. The UV refinish primer can be applied on different surfaces like steel, aged coatings or plastics and is compatible with over-coated conventional base coats.

In contrast to the high energy, short wavelength responsible systems described by DuPont, the approach reported by BASF Corporation is based on the use of lower energy UV-A lamps. UV-A lamps are commonly used as tanning exposure equipment, and are therefore much safer to use in paint shops, however, the low energy density provided by the lamp requires highly reactive UV coatings in order to be cured in reasonable times.20 The development of UV refinish primer and clear coat for UV-A exposure systems has also been achieved by Bayer employing high throughput experimentation (Bach et al., Bayer AG).21 They published formulation guidelines based on proprietary urethane acrylates with the primer exhibiting good sandability and excellent adhesion, the clear coat exhibiting high hardness, high gloss, good weathering characteristics, solvent and impact resistance, but ultimately the suitability of a commercial refinish clear coat needs to be fully explored by end-users.

In order to provide fast curable UV-A clearcoats, Akzo Nobel Car Refinishes (Dogan et al.) has developed a novel UV-A curing, high solids clear coating, which can be used for refinishing spots, panels and even total resprays. The novel technology is based on a room temperature hardening two component curing technology activated by a photochemically produced base as a catalyst.22 The crosslinking mechanism used in this approach is essentially a regular catalysed 2K polyurethane curing chemistry, however based on a thiol-isocyanate crosslinking instead of the usual polyol-isocyanate reaction, but comparable with other car refinish systems. The key characteristic referring to an “UV coating” is the fact, that the specific catalyst used is activated by UV-A irradiation, leading to a very fast reaction. This catalyst is called a “photo latent base”, which is essentially a masked base (e.g., tertiary amine) which generates the base upon a photochemical reaction.23 The initially used photo latent base generator PLA 1, which works well as a catalyst for the carboxylic acid-epoxy addition reaction, had to be modified in order to be effective in the catalysis of the thiol-isocyanate reaction:

⇒

⇒

⇒

PLA-1

PLA-2 modification of amine

modification of steric shielding

modification of chromophore

Thus modified chromophore of the latent amine generator exhibits optimal absorption in the UV-A, high photochemical efficiency and does not contribute to yellowing in the cured coating.24 Since this system is based on a thermal (2K) polyaddition curing chemistry it has the advantage, compared to radical curing UV-systems, that there is no oxygen inhibition

266

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 10.13. Curing of a 2K UV-A curable coating (AKZO Nobel).

involved. Furthermore, since the uncatalysed thiol-isocyanate reaction O

|=

Photolatent catalyst

polymer–[–NCO]n + polymer–[–SH]m −−−−−−−−−−−→ polymer– N UV-A, room temperature H

S–polymer

thio-urethane network

n, m > 2, proceeds relatively slow, the cure of shadow spots occurs at a rate which is acceptable from a practical car refinish point of view, and is therefore a clear advantage over pure standard UV-curing systems.

For UV-A refinish applications a handheld UV-A lamp can be used (e.g., the Panacol H254, an iron-doped high intensity discharge lamp), which is equipped with a specific filter that transmits only long-wavelength UV-A light and some visible light only, thus, the short wavelength UV radiation in the UV-B and UV-C range is filtered out. Curing of spots or larger areas can also be done with a mobile unit system (e.g., the INP UV Curebooster) equipped with TL tubes emitting low energy UV-A light. These mobile units can also be placed around the geometry repair area and can be used for curing horizontal and vertical parts of the vehicle (Figure 10.13).

10.1.5

Scratch Resistant Coatings for Automotive Applications

10.1.5.1 Scratch resistance of automotive clear coats The attention directed to scratch resistance of automotive clear coats is mainly due to the slight damages caused by conventional car wash units, which leave their marks in the form of fine scratches in automotive top coats (Figure 10.14). The dimensions of theses scratches are on the order of 0.1–0.3 µm

NEW APPLICATIONS

267

F IG . 10.14. Typical scratch pattern after conventional car wash.

in depth and 1–4 µm in width. Scratches applied by vandalism, which usually destroy the whole coating structure down to the substrate, are almost impossible to inhibit by the usual chemistry used in automotive coatings. Strategies against vandalism are not discussed in this context. The improvement of the “car wash” scratch resistance, sometimes also called “mar resistance”, of automotive clear coats is of great interest for car manufacturers, since improvements can lead to a better value perception of the car. Since the testing of this scratch behaviour in field tests would absorb tremendous time and resource input, a wide variety of laboratory test methods have been developed (see Chapter 3.2). All the test methods are designed to simulate the situation during normal car wash and try to accelerate the damage and provide measures to improve the coating performance. The question arises, why do the car wash brushes leave these scratches? It has been pointed out, that the brushes themselves do not severely damage the coating surface, but they exert an impact on dust (sand) particles on the coating surface. Depending on the geometry of the particles, which can exhibit different (sharp) shapes, the velocity of the rotating brushes and the resulting forces acting on the particles, different damage patterns result (Figure 10.15). Further factors influencing the extent of scratches resulting in a car wash unit are the amount of flushing water (to remove dust particles), surfactant content or temperature. It has been shown, that the optical perception of the plastic pile-up has the strongest effect on the visibility of the scratches, followed by fracture pattern and color changes, whereas the elastic rubber-like behaviour does not result in any scratches or optical changes.25,26 The damage pattern of scratch tests can be evaluated by visual inspection, gloss measurement as well as with profiling measurements like AFM and nano-scratch/indentation methods. By classification and interpretation of the AFM data the mechanical characteristics of the coatings, like the elastic, plastic and fracture response, can be calculated analogously to the methods developed by Shen et al. (Fig. 3.21).27 The damage type’s change with variation of the nor-

268

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 10.15. Response behaviour of coatings to scratching impacts, perception of scratches and reasons for the different perception extent.

mal force applied. With higher forces the elastic part decreases and the fracture increases.

The experimental conditions can be chosen in such a way, that all response types (elastic, plastic, fracture) are tested simultaneously, which is often the case at a normal force of 5 mN.28 What can not be simulated so far in the indentation tests is the velocity observed during car wash, which is in the order of >10,000 mm/min, whereas the nano-indentation covers velocities from 0.1 Âľm/min to 100 mm/min. Since it is known from stress-strain measurements of polymers, that higher draw speeds increase the brittleness of the polymer, an exact simulation of the conditions in car wash units in terms of velocity can not be provided with the indentation set-up so far. Now, is it possible to correlate the mechanical properties of coatings with scratch test results?

10.1.5.2 Comparison of UV coatings to standard 1K and 2K clears In order to get an overview of the scratch resistance of several different clear coatings, Poppe and Kutschera29 compared the scratch resistance of thermal and UV-cured coatings described in Table 10.5. They performed field tests of the coatings in a car wash unit and correlated these results with scratch results of laboratory scratch tests and with mechanical response properties determined by nanoindentation measurements.

The field tests were performed on coated small panels which were mounted on a car roof. By frequently changing the positions of the panels on the car roof, differences in the influence of the washing brush due to the positioning were eliminated. The car wash testing

269

NEW APPLICATIONS

TABLE 10.5. Clear coat (CC) systems used Number

Clear coat

Curing condition

Chemistry Epoxy–carboxy

1

Powder CC

145 ◦ C, 30 min

2

1K conventional

135 ◦ C, 20 min

OH-functional acrylic/melamine resin/ blocked isocyanate

3

Water borne CC

155 ◦ C, 23 min

OH-functional acrylic/blocked isocyanate

4

2K conventional

140 ◦ C, 22 min

OH-functional acrylic/polyisocyanate

5

2K variant

140 ◦ C, 22 min

OH-functional acrylic/polyisocyanate melamine

6

2K UV dual cure

1500 mJ/cm2 , air, 140 ◦ C, 14 min

OH-functional resin/polyisocyanate, acrylic functional resin

7

2K evolution

140 ◦ C, 22 min

Nanocomposite clear coat based on acrylic resins

8

UV cure

1500 mJ/cm2 N2 inert atmosphere

Acrylic/functional resins

F IG . 10.16. Car wash field test procedure.

was performed once per week in a public car wash facility, as shown in Figure 10.16. After 30 weeks field tests were finished and the panels finally evaluated.

All panels were visually inspected and ranked (Table 10.6). In parallel, the scratch resistance was evaluated by microscopic investigation (Figure 10.17) and gloss measurements.

The gloss of the damaged panels was measured at 20 ◦ angle and compared to the initial gloss values (see Figure 3.20 for gloss measurement). From the gloss data it can be seen that only the 1 K conventional clear coat and the 2 K conventional clear coat show a rather poor performance. All other clear coats exhibited the same residual gloss values within the tolerance of the measurement. Thus, a clear ranking of the higher scratch resistant coatings on the basis of gloss measurements only was not possible. From visual inspection, how-

270

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

TABLE 10.6. Field test results after 30 cycles field test Clear coat

Ranking/ visual inspection

Ranking/ microscopic investigation

Gloss loss (%)

Ranking/CSEM nanoscratch test

UV monocure 2K evolution 2K variant Water-borne clear coat 2K UV dual cure Powder clear coat 2K conventional 1K conventional

1 2 3 4 5 6 7 8

1 2 3 4 5 6 7 8

4 3 3 3 3 4 7 10

1 2 3 5 4 6 7 8

F IG . 10.17. Microscopic investigation of panels after 30 cycles field test (ranking was done by evaluating the number and area covered by scratches).

NEW APPLICATIONS

271

ever, huge differences could be detected, which yielded the same ranking as was found with optical microscope inspection.

These eight clear coats where also subjected to laboratory scratch tests and nanoindentation measurements in order to determine the mechanical response data of the coatings. To find the decisive mechanical material properties for extremely scratch resistant surfaces, AMTEC and Crockmeter results can be plotted versus the mechanical response data of nanoindentation investigations. The results of the AMTEC and Crockmeter loadings (dry scratch resistance employing an Atlas AATCC Crockmeter CM 5 according to EN ISO 105-X12) are shown in Figure 10.18, plotted exemplarily against the elastic response values from the nano-indentation evaluations. It can be seen that the best ranked coatings in the field test feature a high elastic response, but looking only at the elastic response does not reflect the same ranking of the coatings observed in the visual inspection. However, correlations were found between residual gloss, elastic and fracture response, as well as plastic deformation (see Figure 3.21: determination of mechanical response). Corresponding data are plotted in Figure 10.19. The remaining gloss in the AMTEC test increases with increasing elasticity and decreasing plastic as well as fracture response of the coatings. Similar to the described elastic response, the ability of a coating to return to its original shape has also been defined by the “Recovery” (R = (penetration depth − residual depth)/penetration depth; R = 1 corresponds to no scratch) as shown in Figure 10.20.30 The analysis of the nanoscratch indentation data of a similar series of clear coats reported by Fey,31 comparing the scratch resistance of different clear coats with special focus on UV-dual cure coatings, gave the same ranking in terms of recovery as in the field tests. It has been shown that the recovery increases32 with decreasing tan δ (increasing storage modulus) and coatings with high recovery exhibited

F IG . 10.18. Correlation of AMTEC residual gloss vs. elasticity (left) and crockmeter residual gloss vs. elasticity (right).

272

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 10.19. Comparison and correlation of nano-scaled, single contact indent results with AMTEC remaining gloss level.

F IG . 10.20. Dynamic hardness and recovery against normal force.

high scratch resistance. As is well known from the literature, UV-technology33 offers one promising way to enhance the scratch resistance of clear coats. The excellent performance of the UV-cured clear coats can also be seen in the laboratory scratch test results. The UV (mono) cured coating showed by far the best performance, with residual gloss values up to 94% in the AMTEC and 96% in the Crockmeter-test.

NEW APPLICATIONS

273

F IG . 10.21. Benchmarking of UV and conventional clears: AMTEC results by gloss retention and brightness measurements in comparison to field test car wash.

However, it was found that the relative ranking of the different clear coats was not the same in AMTEC and Crockmeter compared to the car wash field tests. In car wash field tests the total damage is by far not as severe as for AMTEC testing (Figure 10.21, bottom).

Therefore, Fey31 proposed a modified AMTEC test, relying on low numbers of AMTEC double runs (2 DR), instead of the DIN conditions (10–50 double runs). He showed that the ranking of different highly mar resistant coatings, UV monocure, UV-dual cure coatings, and standard 2K polyurethanes, is the same in the modified (2 DR) AMTEC, in Crockmeter, in car wash and in single-scratch indenter tests. Furthermore he also pointed out that the impressions of the visual inspection are correlated much more with optical measurement of the brightness (dL 15 ◦ ) than with gloss measurements. Since coatings exhibiting more scratches appear gray on black panels, the dL 15 ◦ values proved to be a more sensitive tool to describe the scratch results of highly scratch resistant coatings. The dL 15 ◦ results of the (2DR) modified AMTEC corresponded very well with the ranking of the field test car wash runs (Figure 10.21).

It was obviously shown that clear coats exhibiting high levels of residual gloss after scratching in the AMTEC test are mainly characterized by an elastic response pattern.

Elasticity has to be interpreted in this context in terms of rubber-like elasticity and not flexibility. Moreover, as a result of microscopic investigations, there seem to be more abrasive fracture than plastic deformation scratches in the real car wash machines than in the AMTEC Kistler apparatus. Thus, the description of the pure elastic/plastic material properties as measured by indentation is not sufficient to characterize the scratch performance

274

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 10.22. Classification of the results of nanoscratch experiments of different clear coats (corresponds to elastic versus fracture response).

properly. Instead, single scratch tests have to be done to estimate the field performance of the surfaces more reliable. In those tests, in addition to the elastic properties, the susceptibility of the surfaces towards fracture is described more accurately. At the end, a combination of enhanced elasticity and resistance against abrasion-type scratches seems to be decisive for extremely scratch resistant surfaces. Therefore, the clearcoats evaluated by Poppe and Kutschera29 were also subjected to nano-indentation measurements in order to determine the fracture load at first crack (abrasive behaviour) and the residual depth pattern after application (elastic response or recovery behaviour) of a constant load (5 mN load).

With this method developed by Lin et al.,34 it is possible to evaluate different clear coats regardless of their curing conditions with respect to their scratch performance. This classification is shown in Figure 10.22 for the eight clear coats under investigation. The UV coatings are by far the best in this classification. As a general result, there was no clear correlation found for the individual mechanical properties, such as E-modulus, creep, elasticity, plastic deformation or fracture load. However, the ranking obtained from Figure 10.22 coincides with that of the field test results indicating that one single parameter alone is not sufficient to describe the field test scratch performance properly.

Field and AMTEC test studies have demonstrated, that both, extreme elasticity and strong resistance against fracture-like scratches are the most important requirements extremely scratch resistant clear coats have to fulfill.

For optimizing the elasticity, the corresponding networks have to be highly crosslinked, and since the rubber elasticity is an entropic effect, minimizing the interactions of the

NEW APPLICATIONS

275

chains segments between crosslinks increases the probability of conformational changes.

Such high recovery or elastic responses35 can be obtained with highly crosslinked or low crosslinked elastomeric formulations. However, with the low crosslinked elastomeric formulations the chemical resistance of the coating is worse. Due to the fact that extremely elastic, scratch resistant surfaces are realized through high crosslink densities, internal tensions may appear in those coatings. UV-cured systems have demonstrated the ability to exhibit extremely high scratch resistance, however, in view of the overall requirements, OEM clear coats have to fulfill, there are certain limitations in the development regarding scratch resistance. The most important one is a loss in the ability of stress relaxation and consequently an increased susceptibility towards cracking. Both phenomena can be explained by an excess crosslink density. Because extremely scratch resistant clear coats have to meet a number of other contrary requirements, at the end a reasonable compromise has to be achieved, for which UV-curable coatings are promising candidates. The potential has already been demonstrated with several different clear coats subjected to scratch tests before and after Jacksonville aging, where a UV-cured top coat, used as sealer in a two layer top clear coat, exhibited the best performance by far after aging36 (Figure 10.23, left). This behaviour is extremely encouraging for the further development of UV-curable coatings, since the second best coating in this test series, a highly crosslinked polyurethane coating used as sealer, can also exhibit high scratch resistance, but suffered from lower etch resistance (Figure 10.23, right). The reasons for the lower etch resistance are not discussed in the publication, but may be due to the increased hydrophilic urethane group content.

If this assumption holds true, the diametrically opposed relationship of scratch resistance

F IG . 10.23. Scratch resistance of different clear coats before and after Jacksonville test (left) and diametrically opposed relationship of scratch and acid etch resistance (right).

276

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 10.24. Scheme of standard and UV-dual cure processing for automotive top coat application.

and etch resistance may be overcome by UV-curable coatings, where more hydrophobic crosslinkers can be employed.

Furthermore, UV technology allows faster cure and requires less space. The comparison to standard thermal OEM coating processes reveals, that a combined thermal-UV dual cure process will already reduce the space requirements by 50% at much faster cycle times (Figure 10.24, ref. 8).

10.1.6

Plastic Applications in Automotive10

Several applications of UV-curable coatings on plastics for automotive parts are already realized, like headlamp lenses and head lamp reflector housings. Other plastic applications are in implementation or development, like tail lens assemblies, exterior plastic trims, body side mouldings, and interior coatings.

The major reasons for using the UV process are reduction of parts-in-process, lower energy costs, and reduced floor space.

Due to a superior abrasion resistance compared to PU and acrylic thermal clear coats and a deeper “wet” look, UV-curable clear coats for acrylic tail lenses evolved in the decorative application process. The UV clear coat had to exhibit enough flexibility to match the thermal expansion of the underlying basecoats during heating and cooling cycles.

Exterior plastic trim coatings with UV systems are in development for plastic wheel covers. Painting plastic parts with conventional basecoat/clearcoat combinations can account for up to 50% of the costs of such parts. Cheaper alternatives are moulded-in-color (MIC) plastic parts; however, they suffer from a cheap and poor quality image. High gloss

NEW APPLICATIONS

277

UV-curable clear coats are an option to improve the design and optics as well as the performance, like scratch resistance, gloss and exterior stability, of such low cost parts.

An example of a target application for interior automotive parts is vinyl applique decorated plastic parts. The major challenge for UV coatings to replace 2K polyurethanes is adhesion and the low gloss requirement. Therefore water-based UV-curable clearcoats have been developed which exhibit excellent scratch and chemical resistance at very low gloss, however flexibility of up to 300% elongation is critical.10

10.2 INDUSTRIAL APPLICATIONS 10.2.1 UV-Curable Coatings for Hard Topcoats on Plastic The main applications of UV coatings for plastics are mobile phones, disc players, electronic devices, TV’s, cameras, Pac’s, audio players, cosmetic packaging.37 In order to protect theses devices against mechanical damage, hard topcoats are applied. Besides hardness, abrasion and scratch resistance as well as certain flexibility, further key requirements are good adhesion, no yellowing and high gloss without haze. Preliminary formulations have been developed with proprietary resin types for hard topcoats for mobile phone coatings, disk players, cameras and small electronic devices and topcoats for vacuum metallization coatings and cosmetic packaging.

10.2.2 UV-Curing of Highly Flexible Coatings Recently the question was raised, if “UV-cured and elastomeric coatings” are a contradiction?38 UV-cured coatings are usually attributed with high crosslink density, high hardness and limited flexibility. Most of the applications realized today with UV coatings do not ask for flexibilities above 50% elongation. Since urethane chemistry is known to provide high flexibility in textile or leather coatings, Weikard38 and coworkers have evaluated the potential of UV-cured urethane acrylates in order to provide high flexible coatings.

The approach is based on the use of polymeric diols in the molecular weight range of about 2000 g/mol, which are subjected to polyaddition reactions with different diisocyanates and terminated with hydroxyalkyl acrylates. The resulting molecular weights of these resins were in the range of 16,000 g/mol. These resins were then diluted with monofunctional monomers and UV-cured. The viscosities were ranging from 6 to 100 Pa s at 60 ◦ C, influenced by the polymeric diol, the diisocyanate used and the urethane content (hydrogen bridging). The elongation at break could be extended to values from 150 up to 750%, the latter, however, at viscosities of about 36 Pa s at 60 ◦ C (Figure 10.25).

Such coatings could be used on leather or textiles, if they are optimized to also exhibit good resistance and adhesion properties.

278

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 10.25. Structure scheme of elastomeric UV-cured coatings.

10.2.3

Coil Coatings

Coil coating is a technology where “prefinished” sheets of galvanized steel or aluminium are produced, which are stamped, deep drawn, bent or otherwise formed into the final shape by the processing industry. The coating process (see Figure 10.26) and with it the environmental regulations are shifted to the coil coaters. Four major market segments are served by coil coated materials. Each market segment is mainly using one or two specific resins types. The market segments are construction (roofing, panels, garage doors), transportation (truck and bus body panels, automotive exterior trim components, gas tanks, engine components), consumer products (appliance “white goods”, like refrigerators or washing machines, office furniture, shelving, computer components, signs, industrial equipment) and packaging (cans, containers, crowns, barrels, drums). The biggest market in terms of coating usage is the construction sector, followed by consumer products and transportation.

The coatings used are either thermoplastic or thermosetting. With existing technology the line speeds are in the range of >200 m/min, the curing times in the range of 10 to 60 s, in which the coating must have good levelling properties and exhibit good adhesion to the metal substrate as it is bent, punched or drawn during fabrication. The coatings are almost all solvent-based. Up to now, water-based, powder and UV-curable systems have not succeeded to gather a significant market share for a variety of reasons. The water-based systems are difficult to clean when switching from one colour to another and increased use of gas to fuel the incinerators reduces economy, whereas powder coatings suffer from the

NEW APPLICATIONS

279

F IG . 10.26. Coil coating process.

difficulty to obtain the smooth, low coating thicknesses commonly demanded. UV-curable coatings show poor adhesion due to high volume shrinkage. The resin types used shifted in their market prominence from alkyd, vinyl and epoxies to polyesters. For exterior metal buildings the polyesters were not durable enough, whereby siliconized polyesters and fluorocarbons are rapidly emerging.

The basic resin systems, polyesters (60% share), PVC plastisols (30%), PVDF (5%) and others, as well as their performance profile regarding a few key requirements, such as flexibility, surface hardness, metal adhesion, corrosion protection, weathering resistance and heat resistance have been compared.39 The high curing speed of the coil coating process and the two-dimensional application should be well suited for UV-curable systems, however, the technical problems, mainly adhesion, flexibility and through cure issues in pigmented coatings, have to be solved.

Despite the fact, that a complete shift from the high temperature curing (up to 230 â&#x2014;Ś C) to the low temperature UV curing should be should be an important economic driver for change, the use of UV-curable coatings is developed in order to substitute one layer (primer) or introduce a third layer in the conventional two-layer (primer, topcoat) coating assembly.

Earlier developments are cationically curing systems based on hydroxyl terminated polyesters and epoxide crosslinkers (Cyracure UVR 6110, Dow).39 A recent study on the adhesion and flexibility in radiation cured coil coatings evaluated the differences of conventional and radiation curable systems and point out ways to better understand and improve UV-curable formulations for coil coating.40 It has been the common perception in conventional coil coatings that the solvent helps to improve adhe-

280

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 10.27. Coil coating line. Courtesy of Fusion UV.

sion due to good wetting characteristics and the high curing temperature helps to relieve stresses introduced by the curing process, both factors being absent in UV-curable systems.

However, it has been shown that these factors were not valid in the case of the investigated systems, for instance cationically UV-cured coating. The authors have identified segregation of components, in both UV curing and thermal curing coatings which contributed to the level of adhesion. In the cationic system they identified the sulfonium salt photoinitiator to migrate to the metal interface, limiting the adhesion potential of this system. In the thermal curing coating the amino melamine crosslinker was identified at a much higher level at the interface than in the bulk, resulting in this case in the enhancement of the adhesion properties. Thus, the classical adhesion tests are useful to classify on pass/fail criteria, but are improper in defining either the mode of failure or its cause. A closer insight into the chemistry present and acting at the interface is necessary in order to design UV coatings which can overcome the performance problems and ensure durability without suffering adhesion failures. The design of such a UV-curable line (Figure 10.27) demonstrates that the low space requirements allows such a line to be placed between or in addition to conventional thermal curing ovens in order to introduce another coating layer or replace for instance the thermal primer by a UV coating.

Further evaluations of UV-curable systems for direct metal use41â&#x20AC;&#x201C;44 and the through cure of pigmented UV-curable coatings45 have been reported.

NEW APPLICATIONS

281

10.2.4 Adhesives Adhesives can be classified by many criteria, by function (structural, non-structural), by chemical composition (thermoplastic, thermosetting, elastomeric), by mode of application or reaction (hardening by drying, by cooling from melt, or by chemical reaction, like moisture, radiation or heat) or by physical form (liquid, paste, powder, film).

Chemical (covalent bonds), physical (hydrogen bonding, dipoles, van-der Waals forces) and mechanical (jams, surface fissure) forces are involved in adhesion. Adhesive failure can be caused by different modes, depending on the type of force interacting or by cohesive failure of the adhesive layer.

In adhesives, generally, the tack, peel strength and cohesion can not be varied independently. With increasing molecular weight the cohesive strength of the adhesive layer increases and the tack decreases, due to an increase of the glass transition temperature.

The peel strength, however, passes through a maximum. At low molecular weights the tack is high enough to get good adhesion to the substrates, but the cohesive strength is low since the chain links can slide easily from each other and facilitate easy pull apart of the substrates. At high molecular weights the cohesive strength is high; however, the high Tg or even brittleness reduces the adhesion to the substrate significantly (Figure 10.28). Therefore, the molecular weight of the specific adhesives plays a decisive role, if one likes an adhesive which sticks permanently or one which can be removed again easily.

Hence, for every application the molecular weight has to be controlled carefully. Therefore adhesives consist of long chain polymers, often containing functional groups for interaction with the surface. In order to obtain high molecular weight systems preformed polymers, like thermoplastics can be used. However, these systems need solvents to reduce the application viscosity, or thermoset polymers are applied which start from lower molecular weight compounds, and lower viscosity, and are reacted after the application. Due to environmental concerns other alternatives are emerging and UV-curable systems have been focused on.

Radiation or UV-curing adhesives have been developed to take advantage of their easy application possibilities, low viscosity, high curing speed, while simultaneously eliminating mixing, heat curing, as well as solvents.46 The use of UV-curable adhesives is limited to applications where only the surface of a substrate has to be coated with the adhesive or where transparent materials are used when the adhesive has to be cured between substrates.

Examples of applications where only one side is coated are labels. Here pressure sensitive adhesives (PSA) are often used, with which the substrates can be joined with very little pressure. PSAâ&#x20AC;&#x2122;s can be applied from solvent solutions, dispersions or hot melts. They exhibit good flexibility, permanent tackiness, and peel strength. UV-curing has already been used in these applications for ten years.47 UV-curable systems can be applied at low viscosities without solvents, however the conventional UV systems react very fast to very high molecular weight crosslinked networks and the molecular weight control is difficult.

However, by functionalizing relatively low molecular weight polymer molecules with hydrogen abstraction type photoinitiators, like benzophenone, the viscosity remains at a relatively low level and the molecular weight build-up can be easily controlled by the UV energy density (Figure 10.29).

282

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 10.28. Principle of the crosslinking reaction of AcResin® adhesives and effect on adhesive and cohesive forces.

F IG . 10.29. Chemistry of the crosslinking reaction of AcResin® adhesives.

When these commercially available acResins® (BASF) are exposed to large amounts of UV light, large numbers of the photo-reactive groups become active and a large number of crosslinking bonds are created between the polymers. This makes the glue more internally cohesive and thus less adhesive to other surfaces, which means that a label treated in this way is easier to remove without leaving any residues.

NEW APPLICATIONS

283

By contrast, less ultraviolet exposure leads to less bonding between the polymers. If the label is now removed, the polymer chains tend to separate more easily than the bonds between the label and the surface to which it is adhered. The label remains firmly stuck until it tears. Adhesive residues or even large areas of the label remain behind, enabling retailers, for example, to detect when a label has been tampered with.

The application of such UV acrylates for adhesive tapes has been adapted to suitable production equipment in order to achieve coating line speeds of at least 350 m/min on PET with coating thicknesses between 20 and 80 Âľm.48 Recently, water-dispersible UV-curable polyurethane PSAâ&#x20AC;&#x2122;s based on a similar concept containing benzophenone in the backbone have also been described.49 Structural adhesives have taken over a lot of bonding applications that had formerly been dominated by metal-joining techniques, because they distribute loads over wider surface areas and join assemblies made from dissimilar materials like plastic, aluminium or steel.

The chemistry used for these adhesives is based on epoxies, silicones, cyano-acrylates and acrylics. Silicones have the lowest bond strength, but it remains constant over a wide temperature range constant. The most common structural acrylics are acrylated urethanes and also acrylated elastomers. They can be cured with heat activators or with UV light.

The advantages of theses systems lie in the instantaneous fixture, the bond-on-demand characteristics, induced by the UV cure. They exhibit high peel strength, high toughness and compared to cyano-acrylates better moisture and heat resistance. Furthermore they allow long open times and do not suffer from the handling problems of moisture curable cyano-acrylates. Assembled parts can be handled as soon as the light has been switched off.50 One of the fastest growing applications for UV-curable adhesives is optical disks and displays. Digital Versatile Disk or Digital Video Disk (DVD) proved to be a massively popular data storage medium, since its introduction in 1997. DVDs are made of 1 or 2 sandwiched polycarbonate disks of 8 or 12 cm diameter, like a CD. Storage capacities range from 4.7 GB (DVD-5) to 17 GB (DVD-18), depending on disk type. DVD-9 is a current successful format for recording and replaying films and other video entertainment.

After metallizing, the substrates are bonded together with the metallized surfaces on the inside of the sandwich, normally using a UV-cured lacquer. The adhesive bonding layer must be optically transparent, uniform and of the correct thickness with no bubbles or other defects. The UV curing presents no problem for bonding DVD-5 and DVD-9, since up to DVD-9 one disk is unmetallized and the curing can be done through this disk. Polycarbonate is highly absorbing from 180 to 300 nm, but becoming transparent above 300 nm with transmissions up to 60%. This is sufficient for conventional UV curing. However, DVD-10 and DVD-18 contain two metallized disks (aluminium or silicon) with thicknesses in the range of 50 nm (Figure 10.30). Only about 0.1% of the UV light passes the metal layer, thus high energy densities have to be applied in order to cure the adhesive. The use of high energy exposure systems may, however, cause problems with heating and destroying the disk. Special lamps operating with only reflected and filtered light are reported to optimally match the absorption spectrum of the photoinitiators in the transparent region of the polymer and produce minimal heating.51 In DVD-R (recordable) the recording layer contains a dye. According to strong requirements for cost reduction, pure silver has been introduced as the semi-transparent layer

284

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 10.30. Scheme of DVD disks with different storage capacities based on Al or Si data layers.

instead of silver–gold alloys. Thus the adhesive must be compatible with various recording dyes, exhibit excellent heat and corrosion resistance of pure silver, and non-yellowing behaviour. The corrosion of silver is caused by oxidizing chemicals as well as water. Thus, improved UV adhesives, often urethane acrylates, achieve reduced permeability of chemicals and water. Free radical polymerizing UV-curable adhesive are available to meet these demands (e.g., DataShield 6-005, Borden Chemicals).

While DVD (±R) use a red laser to read and write data, a new format uses a blue–violet laser instead, named Blu-ray disk.52 The benefit of using a blue–violet laser (405 nm) is that it has a shorter wavelength than a red laser (650 nm), which makes it possible to focus the laser spot with even greater precision resulting in data storage capacities of about 25 GB/50 GB for the same disk space. In contrast to a 1.2 mm thick substrate for CD or two 0.6 mm thick substrates (bonded together) for DVD, the blue ray disk will be moulded as a 1.1 mm thick substrate. This substrate will then be coated with a 0.1 mm thick protective layer, which must be clean and optically pure. This protective layer will most likely be applied as a scratch resistant UV-cured lacquer in a spin-coating process (Figure 10.31), but it could also be applied as a plastic film that is laminated to the Blue-Ray substrate.

10.2.5

UV Inkjet

UV-curable ink-jet inks are the focus of recent developments in order to achieve faster drying behaviour and better durability. The main requirements of ink-jet printing are related to a low viscosity, substrate wettability and absence of solvents (VOC). Liquid UVcurable coatings with solid contents of 100% are a good choice, if the low viscosity of 10– 20 mPa s can be obtained, which is required by the ink formulation to be ejected by Piezo-

NEW APPLICATIONS

285

F IG . 10.31. Scheme of blue ray disk with UV protective coating.

driven ink-jet equipment. Low-viscosity resins, like hyperbranched or dendritic acrylates are therefore screened in combination with appropriate monomers. Furthermore, waterbased UV-curable coatings are an option, since there the viscosity is not a function of the resin molecular weight, but of the solids content. Water would be absorbed by many substrates, like paper or textiles, and would not need any drying step prior to UV curing. The cure rate of the UV ink-jet formulation also has to be adjusted in order to comply with the speed and latitude of printing. Key factors for UV curing of ink-jet printing, especially with relevance to the UV exposure process, the determination of the cure â&#x20AC;&#x153;windowâ&#x20AC;? without loosing key physical properties, like adhesion, solvent and scratch resistance has been evaluated recently.53 Furthermore, evaluating the influences of optimized additives, like photoinitiators, and pigment selection on the performance of UV ink-jet inks has resulted in the development of new raw materials54 and optimized UV-curable Inkjet Inks.55

10.2.6 UV Systems for Dental Applications In dental applications for fillings and restorative work metal alloys have been dominant in the past, however, due to the toxicity of these mercury containing systems replacements are desired. UV-curable glassy materials mainly based on poly-methacrylates have been introduced for such dental applications.56 The main issue of improvement of such polymer materials is related to the shrinkage behaviour and the long term abrasive stability. By the use of oligomeric methacrylates as model polymers for dental applications a relatively

286

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 10.32. Scheme of furniture foil production by thermal curing (AC) and UV/EB cured coatings.

low volume shrinkage of less than 5% has been obtained, however, due to vitrification the overall conversions of monomers did not exceed 93% and therefore residual monomers may be left. In order to be used in dental applications the polymerization conditions have to be optimized.57 In order to reduce shrinkage formulations for dental fillings therefore often contain already polymerized PMMA, which is dissolved in dimethacrylates. Furthermore in order to reduce the abrasion resistance inorganic particles are added.

10.2.7

Furniture Foil Coatings

Furniture foil (laminated surface) is a printed and coated paper that is glued onto chip board as decorative surface. Coating of furniture foils is conventionally done with an acid catalyzed water–borne system of melamine resin/emulsion combination. Major drawbacks are high energy consumption for the curing process and emission of formaldehyde. With a radiation curable coating the process is much simplified and more economical (Figure 10.32).

10.3

FILM COATING INSTEAD OF PAINTING: AN INNOVATIVE CONCEPT

In the Daimler Chrysler High Tech Report 2005,58 it has been published that the film coating technology is on the way to readiness for series production. The roof module of the Mercedes-R-Class has been tested for qualification purposes under Kalahari climate conditions at temperatures up to 90 ◦ C.

NEW APPLICATIONS

287

F IG . 10.33. Scheme of the foil coating technology.

What is film coating and what was special about that roof module? It was the type of the roof module applied. Normally such modules consist of shaped steel or plastic body parts, which are spray-coated and thermally cured. With the new roof module, the production process is reversed. A thin (engineering) plastic substrate film is coated with an uncured, but solid paint film and shielded from contamination with a protective foil. This compound is then moulded into the shape of the body part, subsequently the top coat cured with UV radiation and the substrate film reinforced with the same engineering plastic material used for the substrate (Figure 10.33).

The production of this “painted film” is outlined in Figure 10.34. It has been done in a sophisticated coating plant at the Daimler Chrysler Research Center in Ulm, where a 65cm-wide thermoplastic film runs from a roll into a 16-m-long machine, where it is coated subsequently with the base coat that provides color and then with a “thermoplastic” UV clear coat, which is not cured yet, but physically dried to a solid film, and finally covered with a transparent protective foil. Afterward, the “painted film” can be cut into sheets, stacked and processed further elsewhere. On the way to becoming a body part, the sheets of the painted film are shaped in a deep-drawing tool, where the film is heated and deep drawn with the help of vacuum, like the processes well known in the plastics industry, for example yogurt cups are produced in the same way. This step gives it the shape of the desired component that of a roof element, for instance, or a vehicle bumper or piece of side rail trims. In the next step, the top coat of the film is hardened using ultraviolet light (UV light), resulting in a top performance clear coat with a highly durable and scratch resistant surface. Since the 3D shape that results already exhibits the contours of the component

288

UV COATINGS â&#x20AC;&#x201C; BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 10.34. Foil coating process.

to be produced, it is not stiff enough to be used as a body part. That is why the moulded and cured paint films then go into special injection, compression or foam moulds made of steel, where they are reinforced with engineering plastic by back injection or stabilized with (polyurethane) foams. Foam backing is a process well suited to flat parts like roof modules and hatch doors, because the elements produced in this fashion are heat-resistant.

Back injection is a typical process for robust add-on parts like side rail trim or bumpers.

At this stage, the plastic parts coated with paint films can be delivered to the appropriate conveyor lines of the automotive plants for immediate assembly and without further postprocessing.

The advantages of this technology open up new manufacturing processes at the automakers and suppliers, offering enormous economic and ecological advantages. Ecological advantages of paint films are primarily due to the fact that they dispense with the usual liquid painting, since no â&#x20AC;&#x153;oversprayâ&#x20AC;? occurs, that must be recycled or disposed in environmentally sound ways. Liquid painting is furthermore costly. Painting lines are among the most expensive components at plants where cars and automotive parts are manufactured. The result is that the painting costs of a plastic part can make up 30 to 50% of its total cost. Aside from the cost savings and the variety of components that will be possible, the paint films offer also other potential advantage, for instance the integration of further functions, like embedding electrical lines, antennas and even sensors into the paint film elements during component production, which would not be possible for metal substrates. The paint films can be used for even the difficult metallic hues of the body colour palette, that there are not any discernible differences between these and components painted in the conventional manner. The colour matching to the spray coated body parts has also been achieved by means of a specially developed paint application technique during the film-coating process,

NEW APPLICATIONS

289

and many tests and improvements have meanwhile led to approval for the paint film system.

The key to the success of this film technology is the use of UV-curable formulations as top clear coat, which can be applied as a solid thermoplastic film, to be moulded into the desired shape, and cured to a highly crosslinked coating, exhibiting the desired performance requirements, like high scratch and chemical resistance.

REFERENCES 1. Harbournc, D., Overview and update of the radiation curing market and technology in North America, RadTech Asia 2005, Proceedings. 2005, pp. 3–22.

2. Luo, V., Perspectives on radiation curing industry in Asia, RadTech Asia 2005, Proceedings. 2005, pp. 24–25.

3. Burger, P., Besser Lackieren 15, 1 (2000); Schneider, M., Klein, W. and Schröder, C., RadTech Europe 1999, pp. 711–716.

4. Snowwhite, P., Performance requirements overview for automotive coatings, RadTech Report, RadTech North America, November/December 2001, pp. 43–45.

5. Near, W.R., Polym. Paint Colour J. 177 (4203), 766–770 (1987).

6. Starzmann, O., UV applications in the automotive industry, Proceedings, RadTech Asia 2005, Shanghai, China. 2005, pp. 271–282.

7. Poth, U., Topcoats for the Automotive industry, In “Automotive Paints and Coatings” (G. Fettis, ed.). VCH Verlagsgesellschaft, Weinheim, 1995, Chapter 5, pp. 119–147.

8. Fey, Th. and Muhle, J., UV-DualCure Systems for automotive applications, RadTech Europe2003, Conference Proceedings, Vol. II. 2003, pp. 739–748.

9. Siever, F.-L., 21st International Conference on Automobile Body Finishing, SURCAR Conference Book, 26–27, Cannes, 2003.

10. Joesel, K., Current and future automotive applications for UV curing technology, http://www.radtech. org/industry/Automotives/current_future.pdf.

11. Rekowski, V., J. Oberflächentechnik 6, 26–31 (1999).

12. Braun, D., Shortening the gap between automotive refinish and OEM coatings, RadTech Europe 2003, Conference Proceedings, Vol. II. 2003, pp. 709–712.

13. Meisenburg, U., J. Oberflächentechnik 12, 40–46 (2000).

14. Wall, C.A., Richards, B.M. and Bradlee, C., The ecological and economical benefits of UV curing technology, RadTech (USA) Report, March/April 2004, pp. 25–29.

15. Saling, P., Kicherer, A., Dittrich-Kraemer, B., Wittlinger, R., Zombik, W., Schmidt, I., Schrott, W. and Schmidt, S., Int. J. Life Cycle Assess. 7 (4), 203 (2002).

16. The methodology was created in partnership with an external consultant, and has been further developed by BASF. BASF’s eco-efficiency group has conducted over 220 analyses.

17. University of Michigan Business School, MAP Team 1, April 2002.

18. (a) Muhle, J., Fey, T. and Wulf, M., Farbe&Lack 10, 18 (2003); (b) Maag, K., Löffler, H. and Lenhard, W., Eur. Patent Appl. EP 1032474; (c) Maag, K., Löffler, H. and Lenhard, W., Progr. Org. Coat. 40, 93 (2000).

19. Meichsner, G. and Vogg, K., Farbe&Lack 105 (8), 48–53 (1999).

20. Richards, B., RadTech Europe, Conference Proceedings. 2003, pp. 773–778.

21. Bach, H., Gambino, C., Galeza, L., Dvorchak, M.J., Fäcke, T., Detrembleur, C., Ehlers, M., Mundstock, H., Schmitz, J. and Weikard, J., UV refinish primer and clearcoat, RadTech Europe, Conference Proceedings.

2003, pp. 731–738.

22. (a) Dogan, N., Klinkenberg, H., Reinerie, L., Ruigrok, D., Wijnands, P., Dietliker, K., Misteli, K., Jung, T., Studer, K., Contich, P., Benkhoff, J. and Sitzmann, E., Fast UV-A curable clearcoat, RadTech Europe 2005, Conference Proceedings, Vol. I. 2005, pp. 203–214; (b) Studer, K., Jung, T., Dietliker, K., Benkhoff, J., Sitzmann, E. and Dogan, N., RadTech 2006, Conference Proceedings, Chikago, IL. 2006, on CD.

290

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

23. Dietliker, K., Misteli, K., Jung, T., Studer, K., Contich, P., Benkhoff, J. and Sitzmann, E., A novel photo latent base catalyst for UV-A clearcoat applications, RadTech Europe 2005, Conference Proceedings, Vol. II. 2005, pp. 473–478.

24. Klinkenberg, H. and Oorschot, J.C., v. PCT Pat. Appl WO 0192362 (2001); US patent 6579913.

25. Hara, Y., Mori, T. and Fujitani, T., Progr. Org. Coat. 22, 27–37 (1992).

26. Hara, Y., Mori, T. and Fujitani, T., Progr. Org. Coat. 40, 39–47 (2000).

27. Shen, W., Smith, S.M., Jones, F.N., Ji, C., Ryntz, R.A. and Everson, M.P., J. Coat. Technol. 69 (873), 123 (1997).

28. Lin, L., Blackman, G.S. and Matheson, R.R., Progr. Org. Coat. 40, 85–91 (2000).

29. (a) Poppe, A. and Kutschera, M., COSI Conference, 2005; (b) Kutschera, M., Sander, R., Hermann, P., Weckenmann, U. and Poppe, A., J. Coat. Technol. Res. 3 (2) (2006).

30. Nothhelfer-Richter, R., Klinke, E. and Eisenbach, C.D., Macromol. Symp. 187 853–860 (2002).

31. Fey, Th., UV DualCure Systems for Automotive Applications, RadTech Europe 2003, Conference Proceedings, Vol. II. 2003, pp. 741–747.

32. Nothhelfer-Richter, R., Klinke, E. and Eisenbach, C.D., Farbe&Lack 111 (12), 42–46 (2005).

33. Schwalm, R., Farbe&Lack 106 (4), 58 (2000).

34. Lin, L., Blackman, G.S. and Matheson, R.R., Progr. Org. Coat. 40, 85–91 (2000).

35. Treloar, L.R.G., “The Physics of Rubber Elasticity.” Clarendon Press, Oxford, 1940.

36. Frigge, E., Farbe&Lack 106 (7), 78–80 (2000).

37. Muzeau, E. and Yan, P., New developments of UV resins and formulations for plastic hard topcoats, Proceedings, RadTech Asia 2005, Shanghai, China. 2005, pp. 331–338.

38. Weikard, J., Fischer, W., Lühmann, E. and Rappen, D., UV cured and elastomeric coatings – A contradiction?, RadTech USA, e/5 2004, Technical Proceedings. 2004, on CD. 39. Schmitthenner, M., Eur. Coat. J. 9, 618–625 (1998).

40. Simmons, G.C. and Lowe, C., Surf. Coat. Int. B: Coat. Transact. 87 (B1), 33–39 (2004).

41. Heylen, M., New developments in UV resins for metal coatings, RadTech Europe 2005, Conference Proceedings, Vol. I. 2005, pp. 181–184.

42. Weikard, J., Urethane acrylates on metal substrates, RadTech Europe’05, Conference Proceedings, Vol. I. 2005, pp. 187–192.

43. Amigo, J., Innovative developments in UV pigmented low viscosity (100% solids) for the automotive and metal coatings industry, RadTech Europe 2005, Conference Proceedings, Vol. I. 2005, pp. 195–200.

44. Pietschmann, N., UV curable metal coatings – Special possibilities and problems, RadTech Europe 2005, Conference Proceedings, Vol. I. 2005, pp. 523–530.

45. Frey, T., UV meets colour: A numerical simulation of the through-cure of pigmented UV coatings, RadTech Europe 2005, Conference Proceedings, Vol. I. 2005, pp. 515–520.

46. Bachmann, C., Adhesive Age April (1995).

47. Müller, R., Ten years of UV crosslinking of hotmelt PSA – A success story, RadTech Europe 2005, Conference Proceedings, Vol. I. 2005, pp. 597–605.

48. Merklinger, D., Coating 12, 498–500 (2005).

49. Czech, Z. and Kocmierowska, M., Coating 12, 521–523 (2005).

50. Langer, N., Dymax Corp. Maschine Design August 4 (2005).

51. Bisges, M., Adhäsion 11.04, 12–16 (2004); www.arccure.de.

52. http://www.cdteam.co.uk/pdfs/bluray.pdf; www.blue-ray.com.

53. Stowe, R.W., Key factors of UV curing of ink jet printing, IS&T’s NIP19: International Conference on Digital Printing Technologies, Final Programm and Proceedings, New Orleans, LA, US, Sept. 28–Oct. 3, 2003. 2003, pp. 186–189.

54. Fuchs, A., Richert, M., Biry, S., Villeneuve, S. and Bolle, T., IS&T’s NIP19: International Conference on Digital Printing Technologies, Final Programm and Proceedings, New Orleans, LA, US, Sept. 28–Oct. 3, 2003. 2003, pp. 268–271.

55. Edison, S., Optimization of UV curable inkjet ink properties for jet stability, RadTech 2006, Conference Proceedings, Chicago, IL. 2006, on CD. 56. Miyazaki, K. and Horibe, T., J. Biomed. Mater. Res. 22, 1011–1022 (1988).

57. Bland, M.H. and Peppas, N.A., Biomaterials 17, 1109–1114 (1996).

58. “Am Anfang ist der Lack”, DaimlerChrysler HighTech Report. 1, 2005, pp. 10–15.

C HAPTER 11

Health, Safety and Environment Health, safety and environmental (HSE) aspects are of considerable importance for the whole UV coatings value chain, from the manufacturers of raw materials over formulators, all the way to the applicators and end users. Although UV-curable coatings are regarded as the most environmentally friendly coating systems, several precautionary measures have to be taken during handling and application. On the one hand this is due to the reactive chemistry of the formulations involved, where the liquid lacquers can be harmful to health, due to skin or eye irritation as well as inhalation, and on the other hand due to the exposure with UV light, which is harmful to skin and eyes. Furthermore, the short wavelength UV-C light can form ozone during exposure, which has to be exhausted during the curing process.

Manufactures, suppliers and end users have to respond to current and emerging chemical regulations, like REACH, VOC and food-contact legislation. Additionally, the European Integrated Pollution Prevention & Control (IPPC) directive demanding the determination of best available techniques (BAT) will be a challenge but also offers opportunities for UV technology.1 In order to facilitate the safe handling and use of UV-curable systems several organisations have been dealing with the compilation of guidelines for labelling, safe handling and safe application. The RadTech Europe Organisation2 together with the European Chemical Industry Council (CEFIC), the European Association of Paint and Ink Manufacturers (CEPE), and other organisations, e.g., German Berufsgenossenschaft of Printers, have taken lots of actions (Table 11.1). The guidelines published will support the sustainable development of the environmentally friendly (VOC reduced and energy saving) UV technology.

TABLE 11.1. Handling guides for the safe use of UV technologies Labelling guide of UV/EB acrylates: UV/EB Acrylates Sector Group at CEFIC http://www.cepe.org/eas/easpdf/0104AC.pdf UV Protocol of safe handling of coatings and inks: German Berufsgenossenschaft Printers http://www.radtech-europe.com/UVProtokol.html Uvitech Project that proves safe application of inks: Supported by EU http://www.radtech-europe.com/uvitech2004.html

291

292

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

11.1 SAFE HANDLING OF ACRYLATES (LABELLING GUIDE) Following the “responsible care” commitment of the Chemical Industry, the UV/EB Acrylate Resins Sector group of CEFIC has elaborated harmonized guidelines for the classification and labelling of UV/EB curable acrylates.3 In future, the classification of acrylates will be divided into the two categories of welldefined substances and polymers, according to the OECD polymer definition.

Compounds listed in the European Inventory of Existing Chemical Substances (EINECS) or in the European List of Notified Chemical Substances (ELINCS) belong to the first category. This will be also the case for oligomeric substances, which were considered polymers in the past and therefore are listed as “no longer polymers (NLP)”, but do not any more comply with the OECD polymer definition, after the adoption of the REACH System.

Acrylates, which fulfil the OECD polymer definition, belong to the second category.

For the labelling of dangerous compounds in general, the following hazard classes are defined in Table 11.2.

The acrylates used in the UV-curing technology have been allocated to the two groups of stenomeric or eurymeric acrylates.

Classified under the category of stenomeric acrylates, the name derived from the greek word for low molecular weight part, are the commonly called “reactive diluents” belonging to the well defined substances, which are neither polymers nor oligomers or NLP’s. The most common substances belonging to this category are listed in Table 11.3.

The second category of acrylates combines the acrylate resins, which are summarized under the eurymeric acrylates, the word derived from the greek meaning of large molecular weight parts. These acrylates are oligomeric or polymeric in nature, exhibiting a wide molecular weight distribution and differ significantly in chemical composition from different suppliers.

Among the category of eurymeric acrylates which may be mono-, di-, tri-, tetra- or higher functional are: • • • •

Epoxy acrylates, Polyether acrylates, Polyester acrylates, urethane acrylates, Melamine acrylates, TABLE 11.2. Hazard classes and labelling requirements Hazard

Label

Hazard

Label

1. Carcinogenic 2. Mutagenic 3. Reprotoxic 4. Very toxic 5. Toxic 6. Sensitizing 7. Irritant 8. Harmful

T Xn T Xn T Xn T+ T Xi Xi Xn

9. Hazardous to environment 10. Explosive 11. Extremely flammable 12. Highly Flammable 13. Flammable 14. Oxidizing 15. Corrosive

N E F+ F R10 O C

TABLE 11.3. Stenomeric acrylates ATP 28th

Symbols Xi+N

R-Phrases 36/37/38+51/53

Annex I number 607-133-00-9

CAS RN

EINECS

19th 25th 19th 19th 19th 19th 19th 19th 19th 19th 25th 19th 19th 25th 21st 24th

Xi Xi+N C C Xi T Xi T Xi Xi Xi+N Xi Xn T+N T Xi+N

37/38+43 36/37/38+50/53 21+34+43 21+34+43 36/38+43 24+36/38+43 36/38+43 24+36/38+43 36/38+43 36/38+43 36/37/38+43+51/53 36/38+43, 21+38+43 24+34+43+50 23/24/25+34+43 37/38+51/53

607-107-00-7 607-244-00-2 607-118-00-7 607-119-00-2 607-109-00-8 607-120-00-8 607-126-00-0 607-112-00-4 607-110-00-3 607-122-00-9 607-249-00-X 607-111-00-9 607-121-00-3 607-072-00-8 607-108-00-2 607-116-00-6

103-11-7 29590-42-9 19485-03-1 1070-70-8 13048-33-4 4074-88-8 1680-21-3 2223-82-7 3524-68-3 4986-89-4 42978-66-5 15625-89-5 10027-06-2 818-61-1 25584-83-2 3066-71-5

203-080-7 249-707-8 243-105-9 213-979-6 235-921-9 223-791-6 216-853-9 218-741-5 222-540-8 225-644-1 256-032-2 239-701-3 â&#x20AC;&#x201C; 212-454-9 247-118-0 221-319-3

HEALTH, SAFETY AND ENVIRONMENT

Substance Monoalkyl, monoaryl or monoalkylaryl esters of acrylic acid with the exception of those specified elsewhere in this annex 2-Ethylhexyl acrylate (2-EHA) Isooctyl acrylate 1,3-Butylene glycol diacrylate 1,4-Butylene glycol diacrylate 1,6-Hexanediol diacrylate (HDDA) Diethylene glycol diacrylate (DEGDA) Triethylene glycol diacrylate (TEGDA) Neopentyl glycol diacrylate Pentaerythritol triacrylate (PETIA) Pentaerythritol tetraacrylate (PETA) Tripropylene glycol diacrylate (TPGDA) Trimethylolpropane triacrylate (TMPTA) 2-Norbornyl acrylate 2-Hydroxyethyl acrylate (2-HEA) Hydroxypropyl acrylate (HPA) Cyclohexyl acrylate

293

294 • • • • •

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

Silicone acrylates, Acrylated acrylics, Amine modified polyether or polyester acrylates, Amine synergists, Alkoxylated acrylates, etc.

These acrylates have to be labeled according to data available from toxicity tests and according to the hazard classes. Since these eurymeric acrylates are difficult to compare with respect to toxicity tests, the CEFIC UV/EB Acrylates Sector Group recommended the labelling in case of incomplete data according to the guideline listed in Table 11.4.

In the RadTech 2003 paper of Raulfs3 the rules for classification of preparations as well as the classifications according to the German Water Hazard Classes were also discussed.

11.2 PRINCIPLES FOR SAFE USE OF UV COATINGS AND INKS: UV PROTOCOL In an UV protocol,4 jointly developed by Berufsgenossenschaft (BG, D), Health and Safety Executive (HSE, UK), Caisse Nationale del’Assurance Maladie des Travailleurs Salaries (CNAMTS, F), and Istituto Superiore per la Prevenzione e la Sicurezza del Lavoro (ISPESL, I), and signed recently by the RadTech Europe,5 improved (health, safety and environmental) conditions for the use of UV technology in the printing industry have been note down.

TABLE 11.4. Labelling recommendations in case of incomplete data Classified as

Labelling

Skin irritant

Eye irritant

No No Yes Yes No test results No No test results

No Yes No Yes No No test results No test results

S24 S25 S26 S28 S37 Xi R36 R41 R38 R36/38

S24+S25+S26+S28+S37 Xi+R36 or R41 Xi+R38 Xi+R36/38 or R38+R41 Xi+R38 Xi+R36 Xi+R36/38

Avoid contact with skin Avoid contact with eyes In case of contact with eyes, rinse immediately with water and consult physician In case of contact with skin rinse with (. . . to be specified by producer) Wear appropriate gloves Irritant Eye irritant Danger of severe eye damage Skin irritant Irritant to skin and eyes

HEALTH, SAFETY AND ENVIRONMENT

295

The protocol describes the best practise for using UV-curable systems, however, it is no legal binding. It details measures for safe application of UV-curable systems with respect to: • Contact or inhalation of dangerous substances; • Inhalation of ozone; • Exposure to radiation.

Precautions regarding the contact with UV inks, varnishes and lacquers, the contact with or inhalation of ink mist are described in detail. The harmful effects are mainly due to some ingrediants which may cause skin irritation or in the case of some (low molecular weight) polyfunctional acrylates may lead to sensitisation. If small ink droplets become airborne, these ink mists are also presenting an inhalation and irritation hazard. Although most of the UV units are connected to exhausters, the ozone produced by short wavelength UV-C radiation can lead to eye, nose and throat irritation. The exposure to direct UV light may result in irritation of the eyes as well as deleterious effects on the skin.

The basic principles for the precautions to be considered are: • Elimination or avoidance of chemical substances or processes giving rise to hazards (e.g., excluding carcinogenous substances, e.g., NMP); • Substitution as soon as less hazardous substitutes are available; • Limiting exposure in terms of quantity and time; • Personal precautions: protective gloves, face masks, . . . ; • Monitoring of actual exposure.

The issue of safe use of UV/EB technology in the coating and printing industry has also been a topic in the 2005 RadTech Conference.6

11.3 UVITECH (RISK ASSESSMENT OF UV CURING IN PRINTING INDUSTRY) The aim of the UVITECH Project (an initiative of RadTech Europe and BG Druck und Papierverarbeitung) was a risk assessment of health/safety and environmental performance as well as air quality due to ink fly (particulates, acrylate vapours, etc.) generated from UV printing compared to other risks associated with the printing process.7 The results of the study of the health and safety performance at six printing factories within four European countries showed, that there is low risk in respect to total inhalable dust, particulate ink fly and ozone, medium risk due to multifunctional acrylate vapours in workplaces where poor general ventilation and high solvent levels are present (but not exceeding the recommended exposure levels) and high risk due to working practices, e.g., lack of personal protective equipment as well as smoking, eating, drinking practises in the factory workplaces. Besides health and safety, the environmental assessment of multifunctional acrylates also reveals an insignificant emission level and therefore leads to low risk classification. As a result of the project, a risk assessment matrix and an environmental impact assessment matrix have been generated (Figures 11.1 and 11.2).

296

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

Printing process

UV printing Normal running (lithographic) Cleaning operations

Risk assessment matrix Exposure Oxone Actinic to UV Solvents

Inkfly particulate

Inkfly vapour MuFa’s

Noise

Working practices

L

L

L

L

L

M

H

NA

M

H

NA

NA

NA

H

Conventional printing Normal running (lithographic) Cleaning operations

L

NA

L

NA

NA

M

H

NA

NA

H

NA

NA

NA

H

Small-scale print-proofing (UV) Normal running Cleaning operations

L NA

M M

H H

L NA

L N/A

L N/A

H H

L NA

L NA

M NA

H H

UV label printing L L Normal running L Cleaning operations NA M H NA – not applicable.

Pollutants environmental impact: L, low; M, medium; H, high.

F IG . 11.1. Risk assessment matrix of the UV printing process (UVITECH project).

Printing process

Environmental impact assessment Particulate emissions

VOC emissions

Ozone emissions

MuFa emissions

Paper recycling

UV printing Includes normal running and cleaning operations (lithographic)

L

M

M

L

H

Conventional printing Includes normal running and cleaning operations (lithographic)

L

M

NA

NA

M

NA

NA

L

NA

H

L

L

H

L

H

Small-scale print-proofing (UV) Normal running UV label printing Includes normal running and cleaning operations NA – not applicable.

F IG . 11.2. Environmental impact assessment of the UV printing process (UVITECH project).

The information contained in the risk assessment matrix will also be useful for other industries using UV curing, like wood, plastic, metal coatings. The matrices are available from RadTech containing hyperlinks to further embedded documents (link in Table 11.1).

11.4 REDUCTION OF EMISSIONS (VOC REGULATION) Another challenge for the end user industry is the reduction of VOC’s. A great number of end-users will have to switch over to compliant technologies in order to meet the deadline for requirements for VOC reduction in October 2007 (EU Council Directive 1999/13/EC).

HEALTH, SAFETY AND ENVIRONMENT

297

A pre-requisite for this change is that existing technical problems have been solved.

For raw material producers, the formulators and the equipment designers this legislation results in a lot of efforts in R&D. They have to widen the existing applications with product modifications or new products. New applications with new products and processes must be elaborated. A lot of work in this direction has already been done. The impact of the VOC directive on the coating process in the furniture industry has been discussed under consideration of several possibilities, among them the radiation curing alternative.8 The basic requirements of this directive are covered in Article 5, stating that all installations shall comply with either the emission limit values in waste gases and the fugitive emission values, or the total emission limit values laid down in Annex IIA, or the requirements of the reduction scheme specified in Annex IIB. The purpose of the reduction scheme is to allow the operator the possibility to achieve by other means emission reductions, equivalent to those achieved if the emission limit values were to be applied. Annex IIA specifies for every application limit values for existing and new installations. These VOC values are also different in Europe and NAFTA. The differences have been discussed under the view of more globally uniform VOC requirements on the example of automotive refinish applications.9 A major difference is the nature of the solvent, whereas in Europe all solvents, which have the tendency to be reactive in the atmosphere, are regarded as VOC, in NAFTA only solvents, which are regarded as reactive to the atmosphere, are considered. This leads to the exemption of solvents like acetone, chlordifluoro-methane or methylenchloride in NAFTA. The VOC limits for the various applications in refinish are also different, for example, the solvent content for primers (580 g/l NAFTA; 250 g/l EU), 1pack/2pack systems (600 g/l NAFTA; 420 g/l EU), clear coats (varies in NAFTA; 420 g/l EU), and threshold (none in NAFTA; 0.5 to/a EU).

And to add some further numbers as an indication for the dimension of the limits, existing installations, which do not exceed: • 50 mg C/Nm3 in the case of incineration, • 150 mg C/Nm3 in the case of any other abatement equipment are exempted for a period of 12 years.

The emission limit values in waste gases for vehicle coating and vehicle refinishing as well as coil coating are also at 50 mg C/Nm3 , for other coatings, like metal, plastic, textile, film and paper coatings at 100 mg C/Nm3 are laid down. For discharges of halogenated VOC’s an emission limit value of 20 mg C/Nm3 , and for carcinogens, mutagens, or toxic to reproduction compounds (CRM’s), the limit is at 2 mg C/Nm3 . The unit Nm3 refers to a “normal cubic meter”, which in this connection means a temperature of 0 ◦ C and a pressure of 1.013 bar, the conditions at which one mole of an ideal gas has a volume of 22.413837 l.

11.5 REGISTRATION, EVALUATION AND AUTHORIZATION OF CHEMICALS (REACH) Environmentally friendly coating technologies, like radiation curing technology, have been supported for further development by the European legislation regarding chemistry and safe use of chemicals and will continue to be supported over the next years. One example

298

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

of important legislative regulation that will also affect importers to Europe in the coming years is the future EU chemicals policy. It started when the EU Commission published the so called White Paper in 2001 that proposes a uniform regulatory system for existing and new substances under the principle of REACH: Registration Evaluation and Authorisation of Chemicals The motivation behind REACH is to protect human health and the environment. All substances with consumption above 1 ton/year have to be registered by manufacturers and importers and there will no longer be a differentiation between new and existing substances anymore. After a transition period for existing products of 11 years the European Inventory of Existing Chemical Substances (EINECS) will be obsolete. Polymers according to the definition worked out by the Organisation of Economic Co-operation and Development (OECD) will be initially exempted from this system.

Every company producing and processing chemicals will have to face this new situation.

Every chemical substance, which is produced or imported in a volume above 1 tonne per year, will require registration in a central database, to communicate and take adequate measures to control the identified risks. Chemical Safety Assessments (CSA’s) will need to be prepared and will include information on Exposure Scenarios (ES’s).

REACH will constitute a uniform system for the registration, evaluation and authorization of chemicals. The impact of REACH on the production chain of the coatings industry has been evaluated.10 It is estimated that more than 30,000 chemicals will have to be registered, and depending on their production volume and use a chemical safety report is required. For volumes above 10 tons/year, more than 5500 chemicals have to be evaluated by national authorities and a decision made about further testing requirements. An authorization of exceptional uses is required for CMR (carcinogenic, mutagenic, reprotoxic), persistent, bio accumulative, toxic chemicals as well as for endocrine disrupters (Figure 11.3).

The regulation shall apply to all industrial chemicals with volumes >1 ton/year produced or imported. Relevant to UV curable coatings are exemptions, that the regulation does not apply to non isolated intermediates, and partly exempt in the strict sense to onsite isolated or transported intermediates, as well as to polymers, but not monomers used for polymer synthesis.

The registration obligations and the timeline are as follows: Prior to manufacturing or importing, the manufacturer or importer has to register the substances at the central agency. Substances produced or imported >1000 tons/year and CRM’s (>1 ton/year) have to be registered between 2007 and 2010. Their evaluation period extends to 2012. Substances produced or imported >100 tons/year have to be registered between 2007 and 2013, the evaluation done by 2016. The registration period for substances >1 ton/year ends 2018, when all of the existing substances >1 ton/year have to be registered. Up to now substances subject to authorization are limited to CMR, PBT, vPvB and endocrine disrupters, for which the producing or importing companies have to apply for an authorization for the foreseen use. If the risk is adequately controlled the EU

HEALTH, SAFETY AND ENVIRONMENT

299

F IG . 11.3. A uniform system for registration, evaluation and authorization of chemicals in the EU (REACH).

commission does grant an authorization, or if the socio-economic benefits outweigh the risks the commission may grant authorization with a time-limit.

The impact of the REACH process on the production chain, as examined by Bias, considers the registration requirements of all chemicals used in the production chain (process chemicals for production of raw materials, e.g., solvents; raw materials, preparations, e.g., formulations from lacquer components, process chemicals, e.g., for surface treatment, etc.) compared with low or moderate level requirements put on the registration of imported final articles. It is therefore more attractive to import articles or finished goods instead of importing or producing within the EU individual substances or preparations.

According to internal studies by companies about 20–100% of the formulations used in plastic additives, paints, electronics and cosmetics have to be re-examined and re-qualified due to higher costs. For some substances in the 10–100 tons/year range production may be stopped entirely. A rough estimate of the testing and registration costs was given (Figure 11.4).

However, considering pros and cons of the REACH system, it may be a burden as well as an opportunity in respect to the changes in markets and competition profiles, alternative product developments, customer relationships and comprehension of uses, product developments and markets.

The direct effects of REACH on the UV coatings market, and especially on the availability of the numerous raw materials are not yet conceivable. The trends, however, are foreseeable: Since UV-curable resins are low molecular weight oligomers, the sustainability of existing resins and the development of new types will shift to polymeric resins, which are exempt from registration. The reactive diluents portfolio will be cut down to a few commodities. The use of additives, like photoinitiators, synergists or other functional additives, is in the range of 10–100 tons (for about 10%), the range of 1–10 tons (for the

300

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 11.4. Cost estimation of testing and registration (REACH).

other 90%). Thus, the number of available additives will most likely decrease significantly and/or the raw materials prices will increase. This scenario will create high cost burdens for reformulations. EuPia has assessed the impact on the European printing industry.11

11.6 UV COATED MATERIALS FOR FOOD CONTACT A new regulation of the EU (EC 1935/2004) on materials and articles intended to come into contact with food entered into force in November 2004.

The general requirements of this regulation are laid down in Article 3, stating that materials coming into contact with food shall be manufactured such that under normal and foreseeable conditions of use, they do not transfer their constituents to food in quantities, which could endanger human health or bring about an unacceptable change in the composition of food or deterioration in the organoleptic characteristics thereof. In Article 5 specific measures for groups of materials and articles are defined, but no positive list of allowed substances with specific migration limits for printing inks exist. Therefore the compliance of UV inks can only be assessed on the basis of the overall migration limit (60 mg/kg food), the level of benzophenone as photoinitiator (0.6 mg/kg food) and the sum of acrylic acid esters (6 mg/kg food).

Migration tests with UV printed materials have been performed and the results reported,12 showing that the migration levels in UV systems regarding the photoinitiators and acrylates are not as bad as often expected. The migration data of a large number of investigated photoinitiators was below the 10 ppb level (proposed for unknown substances in the “super directive draft”) and the level of acrylic acid was also below the proposed sum for acrylates of 6 mg/kg food.

HEALTH, SAFETY AND ENVIRONMENT

301

11.7 CONCEPT OF BEST AVAILABLE TECHNIQUE (BAT) The Directive on the Integrated Pollution Prevention and Control (IPPC 96/61/EG), adopted by the EU gives a list of industrial activities with a significant contribution to environmental pollution, for which an information exchange on the Best Available Technology (BAT) was organized.

One of these activities is the field of surface treatment,13 which includes coatings. A concept of integrated technique assessment14 has been developed, where some fundamental aspects for the determination of the BAT have to be considered: • • • • • • •

Use of low-waste technology, Use of less hazardous substances, Consumption and efficient use of raw materials and energy, Recovery and re-use of raw materials, Technological and scientific advances, Nature and amount of emissions, Cost benefit relationship.

In a Case Study presented in this paper the application of different refinish primers for vehicle coating has been evaluated. Among them were, 1K UV cure, 2K PU thermal cure, Epoxy thermal cure, 2K PU infrared cure, Epoxy infrared cure, and 1K UV cure using aerosol cans. Considering the total costs, energy, global warming potentials, photochemical oxidant creation, health effects, risk, and resources, the 1K UV curing coating for vehicle refinish applications has been identified as the best alternative.

REFERENCES 1. (a) Jansen, I., The “Sevilla Process”: A driver for environmental performance in industry, RadTech Europe 05, Conference Proceedings, Vol. II. 2005, pp. 149–152; (b) http://eippcb.jrs.es.

2. RadTech Europe, http://www.radtech-europe.com/orghse.html.

3. (a) Raulfs, F.W., Classification and labelling of UV/EB acrylates, RadTech Europe 2003, Conference Proceedings, Vol. I. 2003, pp. 37–42; (b) http://www.cefic.be/Templates/shwAssocDetails.asp?NID=473&HID= 26&ID=39.

4. “UV protocol” download at the website: www.bgdp.de/e/pages/health-safety/UV-technology.htm or available at the institutions noted in the text.

5. http://www.radtech-europe.com/download/rte05signinguvprotokol.pdf.

6. (a) Knoop, R., UV protocol – requirements related to occupational health and safety stipulated consistently in Europe, RadTech Europe 2005, Conference Proceedings, Vol. II. 2005, pp. 113–118; (b) Riester, M., Küter, B. and Mayer, A., Safe use of UV/EB Technology in the Coating and Printing industry, RadTech Europe 2005, Conference Proceedings, Vol. I. 2005, pp. 27–29.

7. Nuijten, L. and Mayer, A., REACH – Exposure Scenarios (Categories versus UVITECH data), RadTech Europe 05, Conference Proceedings, Vol. II. 2005, pp. 105–110.

8. Roux, M.L., The impact of the European VOC Directive on the coating process of the furniture industry, RadTech Europe 2003, Conference Proceedings, Vol. I. 2003, pp. 459–465.

9. Drexler, H.J. and Sell, J., Eur. Coat. J. 04, 2–8 (2002).

10. Bias, W.-R., REACH – Impact and workability affecting the production chain, RadTech Europe 05, Conference Proceedings, Vol. II. 2005, pp. 87–102.

11. www.eupia.org/doc/easnet.dll/GetDoc?APPL=2&DAT_IM=0201C7&TYPE=PDF.

302

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

12. Rüter, M., Benz, H. and Piringer, O., UV Printing Inks in food contact material – Migration and set-off problems, RadTech Europe 2005, Conference Proceedings, Vol. II. 2005, pp. 139–146.

13. (a) Jansen, I., The “Sevilla Process” – Best available techniques in surface treatment using organic solvents, In “Integrated Scenario Analysis and Decision Support for the Modern Factory” (J. Geldermann, H. Schollenberger, I. Hubert and O. Rentz, eds.). French–German Institute for Environmental Research (DFIU/IFARE), Karlsruhe, 2004, pp. 127–136; (b) Schollenberger, H., Treitz, M., Geldermann, J. and Rentz, O., “Integrated environmental protection: Best Available Coating Techniques, International Coater congress: The Power of Surfaces.” Vincentz Network, Hannover, 2004, pp. 329–338.

14. Treitz, M., Schollenberger, H., Schrader, B., Geldermann, J. and Rentz, O., Multi-criteria decision support for integrated technique assessment, RadTech Europe 2005, Conference Proceedings, Vol. II. 2005, pp. 155– 160.

Subject Index

Automotive applications, 252 Automotive clear coats, 159

3D-UV curing pilot unit, 245 α-cleavage-type photoinitiators, 24, 115 Abrasion resistance, 224 of ORMOCER® coatings, 225 of parquet flooring coatings, 198 Absorbance, 42 Absorption spectra of different photoinitiators, 118 Accelerated weathering tests on UV resins, 134 Acid number, 127 Acrylate reactive diluents, 113 Acrylate resin type: effect on photoyellowing, 192 Acrylate terminated oligomers, 95 Acrylated polyacrylates, 101 Acrylates, 28 Adhesion, 211 Adhesion primer, 142 Adhesives, 281 Advantages and drawbacks of UV coatings, 11 of water-based UV coatings, 213 Advantages of UV coatings, 11 ecological, 11 economical, 11 performance, 11 Advantages of UV coatings for automotive clear coats, 256 Advantages of UV-curable powders, 215 Advantages of water-based coatings, 209–211 Aluminum can, 57 Amine modification, 118 of acrylate resins, 101 synergist action, 118 Amine synergist action, 118 Amine synergists, 183 Amtec–Kistler test, 83, 271 Application determines substrate and properties, 20 Application fields of UV-curable formulations, 140 Application spectrum of UV-curable coatings, 253 Application viscosity, 66

Best available technique (BAT), 301 Blue ray disk with UV protective coating, 285 Brittle–ductile transition temperature, 79 C-radical scavengers, 111 Calculation of crosslink density, 73 Cantilever-beam method, 87 Car wash field test, 269 Carbamato acrylates, 221 Cationic curing of epoxies, 121 Cationic metal coatings, 149 Cationic photoinitiators, 119, 121 avoiding the release of benzene, 121 Cationically curable systems, 120 Ceramiclear, 224 Chain type radical polymerization, 67 Challenges for exterior durable UV coatings, 229 Characterization of coatings, 74 Characterization of raw materials, 126 Chemical characterization, 90 Chemical methods to reduce oxygen inhibition, 183 Chemical resistance for automotive applications, 91 for wood applications, 91 Chemistry being determined by required properties, 21 Chemistry of typical automotive top coats, 255 Chromatographic characterization, 130 Classification of polymers duromers, 62 elastomers, 62 thermoplastics, 62 Classical UV applications, 195 in graphic arts, 199 on wooden surfaces, 195 Classification of the results of nanoscratch experiments, 274 Coating additives, 125 303

304

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

Coating application processes equipment advances, 37 roll and curtain coating, 37 spray coating, 37 UV robots, 37 Coating line examples aluminum cans, 57 hardwood panels, 57 plastic components, 57 printing press, 58 Coating markets, 2 Coil coating process, 279 Coil coatings, 278 Combinatorial application testing, 155 Comparison of UV coatings to standard 1K and 2K clears, 268 Components of UV formulations, 22 Composition of a water-based adhesion primer, 142 Composition of wood coatings, 141 Compositions of UV curable coatings, 9 Confocal Raman Microscopy, 34 Considerations for selection of starting formulations, 151 Conversion limitation by Tg , 165 Conversion limitation due to vitrification, 80 Copolymerization, 28 Correlation of coating performance with crosslink density, 174 E-modulus with crosslink density, 175 scratch resistance with crosslink density, 175 Crockmeter, 83 Crosslink density: correlation with modulus, 73 Crosslink density, 68, 71, 72, 78, 81, 173 Crosslinking reaction of AcResin® adhesives, 282 CSM-Nanoscratch, 83 Cure conversion in the presence of HALS radical scavenger, 230 of UV absorber, 230 Cure extent, 33, 36 Curing and hardness gradient, 90 Curing under inert conditions, 185 Curing under nitrogen and carbon dioxide, 238 Current R&D topics, 15 E value, 134 Deep UV photoresists, 202 Degree of crosslinking, 71 Dendritic acrylate oligomers, 104 Dendritic resins, 103 Depth profile of acrylate conversion, 36 Differential scanning calorimetry (DSC), 130 Differential thermal analysis (DTA), 130 Discolouration, 110

Dissociation quantum yields of photoinitiators, 117 Donor–acceptor mechanism, 31 Donor–acceptor systems, 104, 105 Dosimeters, 55 Double bond conversion, 34 Drawbacks of UV coatings, 11, 179 Driving forces of UV technology, 14 Dual cure coatings, 218 adhesion improvement, 221 Dual cure dispersions, 108, 222 Dual cure powder coating hybrids, 221 Dual cure resins, 106 Dual cure systems, 186 Dual cure water-based coatings, 221 Durability tests, 253 Duromers, 62 DVD bonding, 283 Dynamic mechanical analysis, 77 Dynamic viscosity, 126 e-beam technology, 7, 8 E-modulus, 78 Easy matting, 210 Eco-efficiency, 13 analysis, 258 of automotive primer/scaler, 258 of wooden door fronts, 13 portfolio, 263 Economic factors of UV curing, 19 Effect of crosslinking on coating properties, 172 Effect of crosslinking on mechanical properties, 162 Effective energy, 40, 43 Efficiency of photoinitiation, 24 Elastic response, 268, 271–274 Elastic response pattern, 273 Elasticity modulus, 75 Elasticity modulus as function of crosslink density, 78 Elastomers, 62 Electrical input power, 39 Electromagnetic spectrum, 7 Elongation at break, 76 Elongation at yield, 76 Energies as a function of wavelenght, 8 Energy density, 39, 41 Energy of a photon, 42 Energy output of a mercury bulb, 40 ENE’s: structures and reactivities, 124 Environmental benefit of UV-cured coatings, 260 Environmental impact assessment matrix, 296 Enzymatic esterification, 99

SUBJECT INDEX Epoxy acrylates, 95–97 synthesis, 97 Equipment, 37 Equipment suppliers, 58 Evaluation of scratch resistance, 158 Excimer lamps, 47 Explanation of initial photoyellowness, 193 Exposure parameter measurement, 48 Exposure process, 38 Exposure profile, 52 Exterior applications, 135 Exterior durable UV coatings: challenges, 229 Falling sand test, 83 Fe-added mercury lamp spectrum, 47 Fe-additive, 47 Field test car wash, 273 Field test scratch performance, 270, 273, 274 Film coating instead of painting, 286 Film coating technology, 286 Film formation, 65 Fischerscope® , 83 Fischerscope® measurements, 154 Flash light curing, vi Flexibility, 81 Flexibility as a function of Tg , 167 Flexographic printing plate, 200 Foil coating process, 288 Foil coating technology, 287 Food contact, 300 Formulation robot, 155 Formulation screening for new applications, 149 Formulations, 140 Formulations for graphic inks, 144, 145 overprint varnish, 147 pigmented UV ink, 145 UV flexo ink, 146 UV offset ink, 145 UV screen ink, 148 Formulations for wood coatings, 141 Fracture response, 268, 271, 272, 274 Frontal polymerization, 32 Function of UV absorbers, 227 Functions of a coating, 1 Furniture foil coatings, 286 Furniture wood coatings, 197 Future coating technologies, 3 advantages, 3 drawbacks, 3 Ga-added mercury lamp spectrum, 47 Ga-additive, 47 Gelpoint, 71 Glass region, 63

305

Glass transition region, 63 Glass transition temperature (Tg ), 64, 160, 162 determines mechanical properties, 171 influence of crosslink density, 164 influence of cure conversion, 164 influence on flexibility, 167 influence on hardness, 167 structural influencing factors, 163 Gloss and haze measurement, 84 Graphic application requirements, 132 Graphic arts industry, 199 Gravure printing plates, 200 Guidelines for classification and labelling of UV acrylates eurymeric acrylates, 292 stenomeric acrylates, 292 Guides for the safe use of UV technologies, 291 H-abstraction type photoinitiators, 24, 115 HALS radical scavengers, 227, 230, 231 Hardness, 80 Hardness and flexibility as a function of Tg , 166 Hardness as a function of Tg , 167 Hardness at the surface as a function of UV exposure atmosphere, 176 Hardness-flexibility balance, 177 Hardwood panels, 57 Hazard classes, 292 Health, safety and environmental (HSE) aspects, 291 Heat of polymerization, 32 High performance liquid chromatography (HPLC), 130 High-throughput screening (HTS), 153, 154 Homogeneity of polymer networks, 79 Hybrid-Cure systems, 218 Hydrogen abstraction type photoinitiators type II, 115 Hydroperoxide decomposers, 109, 111 Hyperbranched resins, 103 Illumination, 41 Imaging applications, 199, 202 Indentation hardness, 81 Industrial parquet coating, 142 Inert atmosphere, 182 Influence of curing conditions, 240 Influence of Mc (molecular weight between crosslinks) on flexibility, 169 Influence of resin/diluent functionality on polymerization process and properties, 161 Influence of the oxygen content on curing reaction, 240

306

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

Infrared spectroscopy, 128 Inhibition under aerobic conditions, 109 Inhibition under anaerobic (inert) conditions, 111 Initial photoyellowing, 186 decay of yellowing, 187, 188 effect of photoinitiator content, 191 effect of photoinitiator type, 189 effect of resin type, 192 Initial yellowing, 179 Inks, 202 Interaction of UV process parameters, 20, 21 Internal stress, 84, 87 as a function of shrinkage, 89 as a function of Tg , 88 as a function of temperature, 88 Interpenetrating networks, 68 Iodine value, 127 Irradiance, 39, 40 Isocyanato acrylates, 106, 219 Jablonsky-type diagram, 23 Key exposure parameters, 49 Kinematic viscosity, 126 Kinetic chain length, 27 Kinetics of the photopolymerization, 26 Labelling guide, 291 Labelling recommendations in case of incomplete data, 294 Laboratory scratch tests, 271 Lambert–Beer law, 42 Larolux® , 239 process, 185 UV inert curing, 244 Layer composition of parquet flooring coating, 142 LED based light sources, 47 Letterpress printing plates, 200 Light stabilizers, 227 Lithographic printing plates, 201 Low emission photoinitiators, 119 Low yellowing photoinitiators, 118 Lower extractables, 209 Macosko–Miller theory, 69, 72 Maleimides, 31 Mar resistance, 267 Market development of industrial coatings, 2 Market UV acrylate resins, 196 Market of UV coatings, 4, 5 Mass spectrometry, 129

Matching the spectral output of the lamp with the absorption characteristics of the photoinitiators, 52 Matt formulations, 143 Matting process, 198 Measurement of exposure parameters, 52 Measures to decrease detrimental oxygen effect, 181 Mechanical properties of coatings, 162 Mechanical properties of materials, 63 Mechanical properties of UV cured coatings brittle–ductile transition (Tb ), 170 effect of crosslinking on Tb , 170 effect of Tg (hard or soft), 171 measures to obtain hard and flexible coatings, 172 Mechanical response, 271 Mechanism of monomer inhibition and stabilization possibilities, 109 Mercury based lamps, 45 Metal containing monomers, 105 Methacrylates, 28 Microgel formation, 68 Microwave powered mercury lamps, 45 Modulus as a function of temperature, 64 Modulus of elasticity, 63 Molecular weight between crosslinks, 71 Molecular weight build-up during curing, 66 Monitoring film formation and curing, 73 Monitoring of optimized UV process, 50 Monomers, 111 Multifunctional monomers, 113 Nano-indentation, 176 Nano-Scratch Tester, 84 Nano-scratching, 176 Nanocoatings, 223 Nanocomposites, 223 Nanoindentation measurements correlation to laboratory tests, 272 correlation with field tests, 274 principle, 85 Nanoparticles, 226 Near-infrared (NIR) spectroscopy, 35 Network characterization, 74 Network formation, 62, 66, 68 Network formation by chain connections, 70 Network structures, 65, 71 Network types, 69 New applications for radiation curable coatings, 252 Norrish type I photoinitiators, 115 Novel routes to urethane acrylates, 99 Nuclear magnetic resonance (NMR), 129

SUBJECT INDEX OH number, 128 One-component dual cure coatings, 221, 222 Onium salts, 121 Optical density, 43 ORMOCERÂŽ coatings, 223 ORMOCERÂŽ systems, 105 Overcoming the detrimental effects of oxygen inhibition, 184 Overprint varnishes (OPV), 146 application, 204 for inert exposure, 148 formulations, 148, 204 performance requirements, 204 Oxetane groups, 121 Oxygen diffusion, 184 Oxygen diffusion scheme, 179 Oxygen inhibition, 179 influence of viscosity on polymerization rate, 184, 243 Oxygen inhibition as function of layer depth, 180 Oxygen inhibition reaction, 30 Oxygen penetration, 180 Oxygen scavenging by a dye, 184 Painted film, 287 Parquet coatings, 197 Parquet flooring UV coatings: scratch resistance, 225 Particle size measurement, 131 Peak irradiance, 43, 44 Pendulum hardness, 81 Perception of scratches, 268 Performance requirements for automotive coatings, 252 Perspectives of UV curing technology, 17 Phases of process design, 50 Phenolic antioxidants, 109 Photo DSC, 36 Photo latent base, 265 Photochemical yellowing, 133 Photoinduced curing chemistry, 22 Photoinitiation, 23 Photoinitiator classes, 116 Photoinitiator content: effect on photoyellowing, 191 Photoinitiator-free polymerization, 104 Photoinitiators, 115 hydrogen abstraction type II, 115 Norrish type I, 115 Photoinitiators for, 119 cationic curing, 119, 121 pigmented coatings, 119

307

radical curing, 114 through cure, 119 UV-stabilized coating, 119 Photoinitiators used in combination with light stabilizer package (UV absorber/HALS), 232 Photopolymer printing plates, 199 Photopolymerization kinetic chain length, 27 propagation, 25 rate of initiation, 27 termination, 25 Photoresists, 200, 202 Photosensitization of cationic photopolymerisation, 122 Photoyellowing, 186 Physical methods to reduce oxygen inhibition, 181 Pigmented UV coating formulation absorption spectra, 51 Pigmented UV coatings, 246 Pigmented UV ink, 145 Pigmented UV-curable coatings, 280 Plastic applications in automotive, 276 Plastic components, 57 Plastic response, 268, 271, 272, 274 Polycarbonate headlamp lenses, 258 Polyenes, 106 Polyester acrylates, 100 property spectrum, 161 synthesis, 100 Polyester and polyether acrylates, 100 Polyether acrylates, 101 Polymer bound photoinitiators, 119 Polymer definition, 108 Polymerization behaviour of acrylates and methacrylates, 30 Polymerization rates, 28 acrylates, 28 methacrylates, 28 Polymerization shrinkage, 84 Polyurethane acrylate dispersions, 108 Printed circuit boards (PCB), 202 Printing ink formulation, 144 Printing inks, 145 Printing plates, 200 Printing press, 58 Printing techniques, 201 Process control, 49 Process design, 53 Process window, 49, 50 Processing of UV powders, 215 Propagation, 25 Properties of UV curable acrylate resins fundamental influencing factors, 161

308

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

generalized property spectrum, 161 Property spectrum of standard acrylate resins, 102, 161 Property–property relationships, 172 Radiachromic dosimeters, 55 Radiant flux, 42 Radiant power, 39, 40 Radiation attenuation throughout coating thickness, 43 Radiation cured coil coatings, 279 Radical initiated UV induced crosslinking, 23 photoinitiation, 23 propagation, 24 termination, 24 Radical photoinitiators, 114 Radical polymerization systems, 95 Radical scavengers, 227, 228 Radical types occurring during processing/storage, 109 Radiometers, 52, 54 Radiometric devices, 54 Radiometric methods, 52 Rate of initiation, 27 Rate of polymerization, 33 Raw material suppliers, 135 Raw materials for exterior applications, 131 Raw materials for UV coatings, 94 REACH (Registration, Evaluation and Authorisation of Chemicals), 297, 298 impact on production chain, 298 motivation, 298 registration obligation and timeline, 298 Reactive diluent types, 112 Reactive diluents, 111 Reactivity ratios of comonomers, 29 Real-time infrared (RT-IR) spectroscopy, 26, 33 Recovery, 271, 272 Recovery behaviour, 274 Reduction of oxygen inhibition, 243 by chemical methods, 183 by physical methods, 181 Reflectors, 48 Registration, evaluation and authorization of chemicals (REACH), 297, 298 Relationship of scratch and acid etch resistance, 275 Resin types for UV curing systems, 94 Responsivity, 54 Risk assessment matrix, 296 Risk assessment of UV curing, 295 Rota–Hub, 83 Rubber-elastic plateau, 64

Safe application of inks, 291 Safe application of UV-curable systems, 295 Safe handling of acrylates, 292 Scratch laboratory test methods, 267 Scratch resistance, 175, 224, 271, 275 coating response to impacts, 77 correlation of field tests with nanoindentation measurements, 274 requirements coatings have to fulfill, 274 Scratch resistance after Jacksonville testing, 275 Scratch resistance of automotive coatings correlation with mechanical response, 271 Scratch resistance of different clear coats, 275 Scratch resistance of weather-resistant UV coatings, 244 field tests, 270 Scratch resistant coatings for automotive applications, 266 Scratch resistant top coat formulations, 144 Scratch resistant UV coatings, 258 Scratch resistant UV coatings for DVD/Blue ray, 284 Scratch tests, 267 Selection criteria for raw materials for exterior applications, 135 Selection of raw materials, 131 exterior applications, 131, 135 graphic applications, 132 wood applications, 131 Selection of resin types for outdoor use, 132 Self-initiating UV-curable resins, 104 Shadow areas, 179, 185, 249 Shrinkage, 84, 86 Shrinkage as a function of functionality, 86 Side reactions contributing to discolouration, 110 Side reactions during esterification, 113 Silicon based oligomers, 105 Single scratch tests, 83 Sol–gel technology, 223 Solvent-borne UV coatings, 206 Spectroradiometers, 55 Spray application, 206 Spray coating, 37 Stabilizer/inhibitors, 108 Standard mercury lamp, 46 spectrum, 47 Stenomeric acrylates, 293 Step type polyaddition/condensation, 67 Stress-strain analysis, 77, 162 Stress-strain curves, 76 Stress-strain experiments, 63 Structural adhesives, 283

SUBJECT INDEX Structure–property relationships, 160, 172 Structures of typical thiols, 125 Suitability of UV coatings for automotive applications, 254 Superfluid chromatography (SFC), 130 Surface tension, 65, 66 Taber Abraser method, 82 Tangent δ, 78 Temperature dependence of mechanical properties, 79 Termination in radical polymerization reactions, 25, 26 Terminology, 39 Terminology related to emission of light, 38, 39 electrical input power, 38 energy density, 39, 41 irradiance, 39, 40 radiant power, 39, 40 Terms and SI units of light emission, 41 Test methods for abrasion resistance, 82 coatings, 75 scratch resistance, 83 AMTEC-Kistler, 83 Crockmeter, 83 Indentation, 83 Rota–Hub, 83 Thermal analysis, 130 Thermal stress, 86 Thermal yellowing, 133 Thermogravimetric analysis (TGA), 131 Thermoplastics, 62 Thiol-ENE systems, 123 general mechanism, 123 insensitivity to oxygen inhibition, 123 Thiols structures, 125 Time-temperature superpositioning, 76 Topcoat for glass, 149 Topcoat formulations, 143 Torsion modulus, 75 Traditional applications of UV-curable systems, 195 Transesterification, 99 Transmittance, 42 Trends in UV technology, 15 at coatings manufacturers, 16 at end-users, 16 at raw material suppliers, 16 TTT (time, temperature, transition)-diagram, 79 Two component dual cure coatings, 219 Unsaturated esters, 31

309

Unsaturated polyesters, 95 Urethane acrylates, 96–100, 135, 173, 256, 277 enzymatic esterification, 99 exterior applications, 98, 132–135 flexible, 99 hard, 99 novel routes to, 99 property spectrum, 161 property tuning, 98 synthesis, 98 transesterification, 99 UV absorbers, 227, 228 UV spectra, 229 UV clear coat, 287 UV clearcoat developments for automotive OEM coatings, 220 UV coated materials for food contact, 300 UV coating line examples, 56 UV coating markets, 4, 5 regional split, 4 sectorial split, 4 trends across the regions, 5, 6 UV coating of polycarbonate, 149 UV coatings application examples, 8 from traditional to new applications, 9 UV equipment setup, 54 UV flexo ink, 146 UV inkjet, 284 UV inks for flexographic printing, 203 for lithographic printing, 203 for screen-printing, 203 UV lamps, 45 UV line equipment, 44 UV offset ink, 145 UV plasma cure® , 248, 249 UV plasma curing, 186, 246, 247 UV powder coating formulations, 216 UV powder coating process, 216 UV powder coatings, 213, 258 advantages, 215 applications, 214 chemistry, 216 processing, 215 why UV powder?, 214 UV powder coatings on MDF, 198 UV protocol, 294 UV Protocol of safe handling of coatings, 291 UV refinish primer, 258, 265 UV robot, 37 UV screen ink, 147 UV spectra of photoinitiators, 128

310

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

UV systems for dental applications, 285 UV technology, 7 UV wood coatings 100% liquid coatings, 197 UV powder coatings, 197 water-based UV coatings, 197 UV-A clearcoats, 265 UV-A curable coating, 266 UV-curable adhesives for optical disks and displays, 283 UV-curable clear coats, 256 UV-curable coatings, 275 for car refinish, 264 for hard top coats on plastic, 277 key for film technology, 286–289 on automotive plastics, 254 UV-curable dispersions, 208 UV-curable emulsions, 208 UV-curable functional groups, 31 UV-curable inks, 202, 203 UV-curable ink-jet inks, 284 UV-curable nanocoatings, 223 UV-curable nanocomposites, 226 UV-curable ORMOCER® coatings, 224 UV-curable polyurethane dispersions (PUD), 222 UV-curable primer/sealer, 254, 258 UV-curable refinish primers, 258 UV-curable water-based systems, 207 UV-cured parts in automotive applications, 257 UV-curing adhesives, 281 UV-curing in the presence of UV absorbers and radical scavengers, 227, 230 UV-curing of highly flexible coatings, 277 UV-curing of pigmented systems, 246 UV-curing pilot line for automotive bodies, 257 UV-curing process, 10, 19 application, 20 chemistry, 21 equipment, 37 UV-curing under inert atmosphere, 238 advantages of carbon dioxide, 239 comparison nitrogen versus carbon dioxide, 238

influence of curing conditions, 240–243 influence of oxygen content, 240 influence of viscosity, 243 UV-dual cure processing for automotive top coat application, 276 UVITECH Project, 291, 295 Vinyl ether reactive diluents, 114 Vinyl ether (VE) monomers, 31 Vinyl ether/maleimide systems, 104 Viscoelastic materials, 162 Viscoelastic properties, 75 Viscosity, 66, 126, 243 Viscosity changes, 67 Vitrification, 165 VOC regulation, 296 Water-based systems, 107 Water-based UV coating, 206 types of (soluble, emulsion, dispersion), 208, 209 why water-based UV?, 206 Water-soluble UV-curable resins, 208 Weathering performance of UV coatings, 234 aging followed with IR spectroscopy, 234 Florida tests of UV clear coats, 238 of water-based UV coatings, 234–237 Weathering resistance of UV-cured acrylics, 227 Weathering resistance of water-based coatings, 234 Weathering stability of coatings, 232 Weathering stability of polyurethane dispersion, 236 Weathering stability of UV-cured clear coats, 238 Wood application requirements, 131 Wood coatings market in Europe, 196 Wooden door fronts: eco-efficiency, 13 Years in Florida, 238 Yellowing, 186 Yellowness decay, 188