2017 Quarter 2 Vol. 3, No. 2
Exploring UV-Curable Composite Formulations UV LED for Floor Coatings The Challenges of Measuring UV Photoinitiators for LED-Cured Coatings
Official Publication of RadTech International North America
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Photoinitiator Selection for LED-Cured Coatings Proper photoinitiator selection and concentration can be used to optimize the performance of LED-cured coatings, particularly in regard to depth of cure. By Michael Wyrostek, Director of Sales and Marketing and Matthew Salvi, Hampford Research
Emerging UV Curing Applications in Additive Manufacturing A technology from Continuous Composites allows continuous fiber to be 3D-printed with a UV-curable resin. The company was presented an Emerging Technology Award at RadTech 2016. By Dianna Brodine, Managing Editor, UV+EB Technology
ON THE COVER
Cover photo: A 3D printing process to print continuous fiber with UV-curable resin was developed by Continuous Composites (CC3D). The technology received an Emerging Technology Award at the RadTech 2016 awards dinner from RadTech International North America. Photo courtesy of Continuous Composites The cover was finished by Royle Printing Company, Sun Prairie, Wisconsin, using a multi-step UV-curing process called Rough Reticulated Strike-Through. First, the 4-color process was laid down and a UV varnish was applied as a spot application in the areas that did not receive the gloss UV treatment (photograph and copy). The UV varnish was cured with UV lights, and then an LED curing system was used to cure the 4-color process inks. A flood gloss UV was applied over the entire cover, which “reacted” to the UV varnish and created the matte varnish – staying glossy in the areas that were knocked out to receive the gloss UV. The final step was a pass under another UV curing system to cure the coating. This process was performed in one pass on press.
DEPARTMENTS President’s Message............................................. 4 Association News................................................. 6 Technology Showcase........................................ 24 Industry News..................................................... 42 Regulatory News................................................ 63 Calendar.............................................................. 64 Advertisers’ Index............................................... 64
2 | UV+EB Technology • Issue 2, 2017
Floor Coatings with UV LED Curing: A Focus on Performance and Properties Methods to mitigate oxygen inhibition by utilizing reactive chemicals and nitrogen atmosphere to improve surface cure for UV/LED-cured floor coatings are explored. By Gary Sigel, Ph.D., Senior Principle Scientist, Armstrong Flooring
uv.eb WEST 2017 Celebrates Record Attendance with Focus on Emerging Technologies and Materials With several standing-room only presentations on a variety of newer technologies, uv.eb WEST 2017 was a success. By Gary Cohen, Executive Director RadTech International North America
Total Measured Optic Response: A New Approach to UV LED Measurement Measuring the output of an UV LED source presents challenges in capturing the source’s output and then transforming the optical energy into a value that can be stored and displayed. By Jim Raymont, Joe May, Mark Lawrence and Paul Mills, EIT Instrument Markets
51 Microfabrication of Riblets for Drag Reduction: A Novel UV Approach Utilizing Photolithograhic Methods Drawing on photolithographic methods used in computer chip fabrication, riblets or other repeating microstructures are “printed” onto an external aircraft surface. By H. Bilinsky, CEO, MicroTau Pty Ltd.
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CHAMPIONS THIS ISSUE TECHNOLOGY 2017 Quarter 2 Vol. 3, No. 2
RadTech International North America’s Editorial Board facilitates the technical articles featured in UV+EB Technology. Smaller teams of Issue Champions review and approve articles and provide overall content management for each issue, as needed. If you are a member of RadTech and are interested in serving on the Editorial Board, contact Gary Cohen at firstname.lastname@example.org.
Editorial Board Co-Chair Technical Development Manager, Acrylates IGM Resins
Syed T. Hasan
Charlie (Chunlin) He
Editorial Board Co-Chair Key Account Manager, Security Inks BASF Corporation
Lead Materials Scientist Full Spectrum Laser LLC
UV Curing Technology Question & Answer
Which is more important – irradiance or exposure? By R.W. Stowe, UV Applications Engineering Consultant, Heraeus Noblelight America LLC
EB Curing Technology Question & Answer
Is there a technique to evaluate the performance of low-voltage electron beam processors? By Im Rangwalla, Market Development Manager, Energy Sciences, Inc.
Business Development Manager Sartomer
UV+EB TECHNOLOGY EDITORIAL BOARD Susan Bailey, IGM Resins Co-Chair/Editor-in-Chief Syed Hasan, BASF Corporation Co-Chair/Editor-in-Chief Brian Cavitt, Lipscomb University Byron Christmas, Professor of Chemistry, Retired Charlie He, Full Spectrum Laser LLC Mike Higgins, Phoseon Technology Molly Hladik, Michelman, Inc.
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Mike J. Idacavage, Colorado Photopolymer Solutions Jin Lu, Sartomer Sudhakar Madhusoodhanan, Valspar Gary Sigel, Armstrong Flooring Maria Muro-Small, Spectra Group Limited, Inc. R.W. Stowe, Heraeus Noblelight America LLC Huanyu Wei, ITW Sports Branding Division Jinping Wu, PolyOne Corporation Sheng “Sunny” Ye, 3M
Senior Principle Scientist Armstrong Flooring
Applications Engineering Consultant Heraeus Noblelight America LLC
UV+EB Technology • Issue 2, 2017 | 3
President’s Message TECHNOLOGY
recently attended a gathering of UV and LED printers and some of their suppliers. One of the things that impressed me about the event was the two days of nonstop Q&A, enthusiasm and discussion amongst the attendees. The energy and collaboration displayed by the group throughout were palpable. There were panel discussions relative to certain types of printing (for example, printing on plastics) and it was amazing to see presentations that may have lasted 20 minutes – but the ensuing Q&A went on for an hour. There were printers on one side of the room answering questions from other printers in attendance, providing ideas and solutions to each other. It is heartening to experience firsthand this kind of drive and commitment in the energy-curable printing sector!
An official publication of: RADTECH INTERNATIONAL NORTH AMERICA 6935 Wisconsin Ave, Suite 207 Chevy Chase, MD 20815 240-497-1242 radtech.org
On the regulatory side of things, toxic substance control act (TSCA) reform and renewal were discussed at length at uv.ebWEST in February. Though advances have been made in LED hardware for mixed wavelength outputs, attendees discussed the possibility that new chemistries might be necessary to optimize LED curing with this equipment. The challenge of introducing new molecules is the roadblock currently being faced at the Environmental Protection Agency, along with concurrent TSCA legislative matters. It is not yet clear how it will all resolve, but we are keeping our ears to the ground on the matter. So…enthusiasm meets the unknown. It seems to be a recurring theme lately. We are being asked to change – constantly – and to be positively engaged in the experience. However, change, even good change, is challenging for the human organism. One thing is sure: Resisting it is futile and only leads to excessive stress. The point is to embrace it and shape change in a way that suits our goals and experiences. Sometimes, expanding our core competencies to meet change halfway also is in order. Over just the past few months, RadTech has developed new offerings to help us keep some of those key competencies foremost in our minds and to serve as important references. For example, in recognition of the significance of sharing UV/EB research to enable new developments, the RadTech board directed that past conference proceedings should be made open and available to all, in full text format. You can find this wealth of information on our website, www.radtech.org. And, while there, please check out some of our other new resources, such as our UV/EB Industry News RSS Feed to get the latest news and also our UV/EB Resource Guide for Printers – it is packed with recent articles, presentations and information. Finally, as noted, collaboration is perhaps our best resource, and I am pleased to announce our next major RadTech gathering: UV+EB Packaging Conference and RadTech Fall Meeting, October 24 and 25 in Philadelphia, Pennsylvania. This is our first time in Philadelphia, and this new event will feature the latest regulatory updates. Additionally, we will hear from users of our technology about their UV/EB experiences – so, we look forward to seeing you there. And as always, please contact RadTech if we may ever be of service! Lisa Fine President, RadTech Board of Directors Joules Angstom UV Printing Inks
4 | UV+EB Technology • Issue 2, 2017
EXECUTIVE DIRECTOR Gary M. Cohen email@example.com SENIOR DIRECTOR Mickey Fortune
BOARD OF DIRECTORS
President Lisa Fine – Joules Angstrom UV Printing Inks President-elect Eileen Weber – Red Spot Secretary Jennifer Heathcote – Phoseon Technology Treasurer Paul Elias – Miwon North America Immediate Past-President Peter Weissman – Quaker Chemical Corporation Board of Directors Jo Ann Arceneaux – Allnex USA Inc. Susan Bailey – IGM Resins Mark Gordon – INX International Ink Company David Biro – Sun Chemical Michael Gould – Rahn USA George McGill – Coatings and Adhesives Alexander Polykarpov – AkzoNobel Beth Rundlett – Katecho, Inc. Chris Seubert – Ford Motor Company Xiasong Wu – DSM Functional Materials Hui Yang – Procter and Gamble
2150 SW Westport Drive, Suite 101 Topeka, Kansas 66614 785-271-5801 petersonpublications.com Publisher Jeff Peterson
National Sales Director Janet Dunnichay
Art Director Becky Arensdorf
Managing Editor Dianna Brodine
Contributing Editors Circulation Manager Lara Copeland Brenda Schell Nancy Cates firstname.lastname@example.org ENews & Website Developer Jen Clark
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Association News Hui Yang Joins RadTech Board of Directors Hui Yang, senior technologist at Procter & Gamble, has been appointed to the RadTech Board of Directors. During her 16-year career at P&G, she has led technical development in printing, fluid deposition, measurement capability for transformation understanding and process control. Hui Yang is honored to serve the board in RadTech, and looking forward to contributing to the advancement and adoption of energy curable technology and process in industry. IKEA and Others to Present During RadTech Session at AWFS Fair, Las Vegas RadTech will present its first ever UV/EB technical session at the AWFS Fair in Las Vegas, Nevada, from 1:30 to 5 p.m. on Thursday, July 20. The AWFS Fair, a premier conference and exposition for the woodworking industry, takes place July 19 through 22. The session, titled “Latest Advancements in UV Finishing Technology: LED & Beyond,” will feature presentations from Benjamin Keesee, IKEA; Jim Raymont, EIT Instrument Markets; Gloria Valtorta, Superfici Italy; and Larry Van Iseghem, Van Technologies, Inc. Presentations will wrap up with a panel discussion, and RadTech will have a booth in the exhibition. Find details at www.awfsfair.org. Dates Set for UV+EB Packaging Conference and Fall Member Meeting RadTech is proud to announce the UV+EB Packaging Conference, an exciting, one-day conference in Philadelphia, Pennsylvania, on Tuesday, October 24. At the conference, RadTech will host presentations on migration, regulatory issues and the latest developments in UV- and EB-curable inks, coatings and adhesives for food and consumer goods packaging. Please save the date and plan to join us. Also, the RadTech Fall Member Meeting will be held the day after the UV+EB Packaging Conference, October 25. Go online now to register and find additional information at www.RadTech.org. Call for Papers Opens for RadTech 2018 RadTech invites the submission of abstracts for RadTech UV+EB Technology and Conference 2018, scheduled for May 7 through 9, 2018, at the Hyatt Regency O’Hare, Rosemont, Illinois. The technical conference paper presentation should be no longer than 25 minutes, with five minutes for Q&A. Those interested in presenting can fill out the online Abstract Submission Form before September 8, 2017. See details at www.RadTech2018.com. SUNY-ESF Offers Radiation Curing Program State University of New York College of Environmental Science and Forestry (SUNY-ESF), Syracuse, New York, announced its Radiation Curing Program (RCP). RCP is a two-course suite of graduate-level online courses led by SUNY-ESF and RadTech International for current employees in radiation-curingrelated industries, as well as those preparing to enter the field. RCP introduces fundamentals of polymer chemistry pertinent 6 | UV+EB Technology • Issue 2, 2017
to functional inks, coatings, resins and adhesives. For more information, visit www.esf.edu. RadTech Seeks Speakers for Upcoming Events RadTech is partnering with three upcoming conferences, and speakers are needed for special RadTech UV/EB technical sessions. Those interested in speaking should send speaker name, presentation title and high-quality abstract to the contact noted below as soon as possible. Please note that abstracts must be approved by the conference organizers, and not every submission will be accepted due to limitations on the number of speakers per conference. •
Western Coatings Symposium October 15-18, 2017 – Las Vegas, Nevada We are looking for three to six speakers in total for this conference, so space is limited. The audience is traditionally focused on architectural coatings, but the conference organizers would like to expand into UV/EB coatings, so please keep that in mind. The submitted abstract should be 100 to 200 words, with no commercial product names. Send a speaker name, presentation title and abstract as soon as possible to email@example.com. We must have all details to consider your presentation.
AIMCAL R2R Conference October 15-18, 2017 – Naples, Florida We are looking for up to six presentations total of about 20 to 30 minutes each. The conference organizers have asked for presentations related to formulation, chemistry and applications for converters of metallized, laminated and coated flexible substrates and their suppliers. There also is interest in a presentation on UV LED curing systems. Send a speaker name, presentation title and abstract as soon as possible to firstname.lastname@example.org. We must have all details to consider your presentation.
17th Symposium on Radiation Curing Technology – UV LED June 1-3, 2017 – Guangzhou, China Presented by: RadTech China The symposium will focus on cutting-edge UV LEDcuring technology. Renowned UV LED curing research and application experts, both in China and outside China, will be invited to deliver a keynote speech and communicate with the delegates. The symposium will cover UV LED light source devices, UV LED photoinitiators, UV LED-curing printing inks, UV LED-curing optical fiber coatings, UV LED-curing electronic inks and UV LED-curing technology application and development in the fields of optical fiber, printing, electronics, medicine, PCB, anti-fake and environmental protection. If interested in presenting, contact Prof. Xiaoxuan Liu (email@example.com) AND Xiaosong Wu (xiaosong. firstname.lastname@example.org). u
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hybrid... ...UV systems driven by RHINO power electronics ...the only future-proof UV technology with true arc lamp and LED compatibility, supported by the security of a 5-year warranty.
TWO UV Curing Technologies ONE RHINO Power Supply
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UV CURING TECHNOLOGY QUESTION & ANSWER
Q. Which is more important –
irradiance or exposure?
They are both important, but the relative effectiveness depends on the physical chemistry of an ink, coating, adhesive or paint – as well as on the optical thickness of the UV-curable material and the properties required of the end application. A higher flux rate, or irradiance, intitates a higher rate of simultaneous radical generation, and consequently, more short polymer chains – usually characteristic of harder, less flexible and more resistant properties. Noting that exposure* is the time-integral of irradiance, a longer duration of lower irradiance may yield the same degree of exposure, but the resulting longer chains may exhibit different physical properties, such as flexibility or lower resistance. At increased depth within an optically thick material, a higher effective irradiance (greater than a minimum threshold) can affect adhesion. A high “intensity” or peak irradiance will have a beneficial effect on the depth of cure of most UV-curable materials. The effective irradiance, or photon flux rate, at any depth within the film to be cured follows a definite relationship between irradiance at the surface and the spectral absorbance of the film (at any specific wavelength), according to the Bouguer-Lambert law:
Ia λ =
Io λ (1 – 10 –Aλ) d
Io is the incident irradiance (flux rate) at wavelength λ. Ia is the flux rate at depth d, Aλ is absorbance at wavelength λ, and d is the depth from the surface or film thickness. The optical thickness of a UV-curable material is a combination of the physical thickness and the opacity of the film at specific wavelengths of interest. It also may be represented by the ratio of the photon flux rate at the bottom of the film (adhesion interface) compared to that measured at the top. A cure ladder** of a material can reveal the relative benefits of irradiance and time of exposure for a specific curable film and substrate by identifying the upper and lower limits of the two and assessing the process window of successful cure. This, of course, is essential to process and equipment design of curable systems. Figure 1 illustrates the concept of different process windows for several classes 8 | UV+EB Technology • Issue 2, 2017
FIGURE 1. Illustration of the concept of process windows related to several application types
of applications. Of course, the spectral distribution of the source is a fundamental variable of UV exposure, but cure ladders can explore the response of the UV-curable material with a specific selected type of UV lamp – by varying power, distance and speed to find the irradiance and exposure process limits. Power and energy ladder analyses can be conducted with wavelength distribution nearly constant. Radiometers Radiometers measure only the instantaneous value of irradiance (mW/cm² in a specific wavelength band), while integrating radiometers can calculate exposure (mJ/cm² in the same band). An electronic “sample-and-hold” type of electronic memory can report the maximum (or peak) irradiance observed during a dynamic pass under a lamp or lamps. Integrating radiometers will sample irradiance at an internal clock rate and will sum all the instantaneous irradiance measurements to calculate exposure. Irradiance Profile Although peak irradiance is an important component of exposure, the irradiance profile is more significant. This is because differing regions of the irradiance profile will have different effects on cure and depth of cure. Irradiance and peak irradiance may fall into any one of these useful categories:
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the work surface, and most instruments will calculate it with an accuracy and repeatability of ±10 percent. Essentially, it is represented by the area under the irradiance profile curve. It is the time-integral of irradiance.
FIGURE 2. Irradiance profiles of a 385 nm UV-LED at three distances VERY LOW: LOW: HIGH: VERY HIGH:
1 to 100 mW/cm² 100 mW/cm² to 1 W/cm² 1 W/cm² to 10 W/cm² More than 10 W/cm²
The illustration in Figure 2 shows a comparison of different peak irradiance curves of a typical 385 nm UV-LED array (without optics) at several distances. Most UV LEDs will have a “soft” irradiance profile, and the irradiance very close to the face window of the lamp has become an important primary measure of the “output” of the UV-LED products. Figure 3 is the characteristic irradiance profile of many UV-LED arrays The peak of irradiance of medium-pressure mercury UV lamps is affected by factors such as power input to the bulb, bulb diameter, lamp focus, type of reflector FIGURE 3. Typical UV-LED and distance from irradiance profile in the the lamp. The higher near-field the peak of focus, the narrower it may be; and the time of exposure at the peak can be short, so the precision of measuring it becomes less important. The general or average irradiance may be important to curing, but most radiometers cannot measure that. Exposure Exposure is a measure of the total photon flux delivered to uvebtechnology.com + radtech.org
A measurement of UV exposure (mW/cm²) is the integration of the irradiance profile over time, but information about neither irradiance nor speed can be extracted from it. A specification of exposure alone can be misleading or insufficient in terms of material cure dynamics. Most UVcurable materials do not exhibit exposure reciprocity or linearity.*** A simple “specification” of exposure may not be sufficient because it does not provide enough information for the selection or design of the most effective UV exposure or process configuration. Further, often a material supplier’s data sheets do not provide sufficient information. That fact, however, may be a practical necessity, owing to the fact that there is no way to know – at the supply level – what type of UV lamp, film thickness, substrate or production speed will be required in a user’s application. For this reason, the process development step of production design must include a series of test exposures to determine the optimum exposure as well as verification of the achievement of all specific physical and chemical properties of the final cured material. Accordingly, a supplier’s “specification” of exposure can be, at least, useful to provide some guidance and approximation of optimum exposure. u *Note: “Exposure” is the optical term for radiant energy density at a plane. “Dose” is a term defined for high-energy and ionizing radiation, for example x-ray and electron beam energy. In electron beam technology, the units are Mrads or kGy. See “Is there a technique to evaluate the performance of low-voltage electron beam processors?” in this issue. **See “What is a “Cure Ladder and How is it Used in UV Curing?” UV+EB Technology, Issue 3, 2015; pp 8-9. ***See “Non-Reciprocity of Exposure of UV-Curable Materials and the Implications for System Design,” RadTech 2016.
UV Applications Engineering Consultant Heraeus Noblelight America LLC firstname.lastname@example.org
UV+EB Technology • Issue 2, 2017 | 9
EB CURING TECHNOLOGY QUESTION & ANSWER
Q. Is there a technique to evaluate the performance of low-voltage electron beam processors?
Radiation-induced in situ polymerization reactions offer significant advantages over conventional thermal processes. The biggest advantage is the use of 100 percent reactive and compliant chemistry so that no thermal drying is required.
7.00 6.00 5.00 DOSE, MRad
4.00 3.00 2.00 1.00
Since the introduction of electron beam 0.00 curing in the early ’60s, polymer chemists 0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48 have been intrigued by the ability of LOCATION, INCHES the electrons to initiate polymerization reactions and crosslink plastic films High _ Dose − Low _ Dose Percent _ Uniformity = 100 ⋅ without adding any photoinitiators, High _ Dose + Low _ Dose photosensitizers or peroxides. Immediate applications were sought FIGURE 1. Uniformity of a typical 48-inch electron beam unit in packaging using the free-radicalinitiated chemistries, since electron beam processing offers high-speed curing. In the development cycle begins at the chemistry supplier particular, food packaging was of interest because electron laboratory. Then the application goes through standard beam processing results in the following: development in pilot phase, pre-commercial and then • a high degree of conversion (low migration); commercial. In each case, a different electron beam unit is • packaging in which no photoinitators or other additives, used. Questions arise as to how to relate the established such as peroxides, are required; and • quality control that meets National Institute of Standards dose in the lab through various phases of development and Technology (NIST) traceable dosimetry techniques. and then into ongoing production — especially after any maintenance on the EB unit. The answer is dosimetry. Applications for electron beam processing include curing of coatings, inks, pressure-sensitive and laminating adhesives, A dosimeter can be anything that undergoes an observable and consistent physical change that can be correlated and crosslinking of films. For each of these applications,
Questions arise as to how to relate the established dose in the lab through various phases of development and then into ongoing production — especially after any maintenance on the EB unit. The answer is dosimetry. 10 | UV+EB Technology • Issue 2, 2017
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125 kV 100 kV 80 kV
100.0 Dose % Front Surface
with the dose of radiation it has received. For example, some dosimeters change color when irradiated. This color change is due to the presence of a radiochromic dye that is blended in a polyamide matrix. The polyamide film then is cast out of solution, cut to a 1 x 1 cm2 area – called a radiochromic film dosimeter – and used to measure the output of electron beam accelerators. These dosimeters are either about 50 grams/m2 (used to measure dose from > 150 kV EB processors) or about 10 grams/m2 (used to measure dose of < 125 kV EB units).
80.0 60.0 40.0 20.0 0.0
FIGURE 2. Electron beam unit depth-dose profiles from 70 to 125 kV
Electron beam processors using these dosimeters are characterized to measure three aspects of the EB unit. 1. Yield measurements: The EB processor has no direct knowledge of the dose being delivered to the product. It can, however, regulate the electron current density (beam current) proportional to the line speed to maintain a constant desired dose. This proportionality constant is measured with dosimetry and then programmed into the microprocessor of the EB unit. This constant “k” is specific to that particular EB unit and is dependent on the operating high voltage. The equation governing this relationship is as follows:
D⋅S k= I
Range gram \m2
These nylon films start off as clear, transparent films that change to dark blue color after irradiation. The dose they receive can be correlated to change in optical density, which is measured by a densitometer or a spectrophotometer at a light of a specific wavelength, usually at 510 nm or 600 nm, depending on the type of dosimeter used. The measured change in optical density is correlated to dose by appropriate calibration to international standards. NIST is used in the United States, and National Physics Laboratory (NPL) is used in Europe. These international standards facilities irradiate the dosimeters at pre-determined doses, using Co60 gamma rays to make the calibration curve specific to that dosimeter and reading instrument.
110 kV 90 kV 70 kV
Beam current (I) is in mA K is Mrad.FPM/mA or kGy.MPM/mA
2. Beam uniformity: These dosimeters are spaced every one inch across the width of the EB unit to ensure uniform electron distribution across the width. Figure 1 is a typical uniformity of a 48-inch EB unit. Typical uniformity is < +/-10%. 3. Depth dose: Dosimeters stacked on top of each other are used to measure the high-voltage calibration of the EB unit, which determines the electron’s penetration capability. Figure 2 shows the depth-dose profiles from 70 to 125 kV. The measured values are compared with expected profiles to ensure the EB unit is in highvoltage calibration. u
Market Development Manager Energy Sciences, Inc. email@example.com
Dose (D) is in Mrads or kGy Speed (S) is in FPM or MPM
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UV+EB Technology • Issue 2, 2017 | 11
PHOTOINITIATOR SELECTION By Michael Wyrostek, Director of Sales and Marketing and Matthew Salvi, Hampford Research
Photoinitiator Selection for LED-Cured Coatings Background ED sources are rapidly gaining in popularity in the UV-cured coatings market as they offer reduced cost, longer life and environmentally friendly alternatives to conventional lamps. Transitioning to UV LED, however, often requires more than just a simple equipment change. Modifications to the chemistry also might be needed to effectively compensate for the lower energy levels and narrower wavelength range. This is particularly true in coatings applications, where oxygen inhibition can have a detrimental effect on cure properties1. In this paper, we will examine how proper photoinitiator selection and concentration can be used to optimize the performance of LED-cured coatings.
Oxygen inhibition Poor surface cure, due to oxygen inhibition, is one of the most challenging aspects associated with LED-cured coatings. Oxygen, in its ground state, has a “diradical” nature and is highly reactive toward radical species.2 As a result, oxygen can scavenge radicals to form less reactive peroxy compounds, which can terminate the growing chain via radical-to-radical interaction. The result of oxygen inhibition is observed as a decreased rate of polymerization and, ultimately, compromised coating performance.3 Formulators have tried to overcome curing issues in various ways1, each with different degrees of success, but also having its own drawbacks. Some of the frequently discussed remedies include the following: • isolating the coating from oxygen, • increasing the amount of energy to which the surface is exposed, • increasing the concentration of photoinitiator or • modifying the coating chemistry/photoinitiator package. Isolating the coating from the atmosphere is the most straightforward method of mitigating oxygen inhibition, but it also is the most difficult – particularly when curing large areas. Applicators have attempted to reduce the oxygen exposure by blanketing the exposed area with inert gas or covering with waxes and films. This approach works well in laboratory settings but can be impractical for large, industrial applications. Increased energy is another way of improving curing of coatings under UV LED. Upon inception, LED lamps were limited in the amount of energy they could produce, but as the technology evolved, higher energy lamps capable of producing more free radicals and faster cure speeds were developed. While this significantly improves surface cure, the higher rate of polymerization can have a detrimental effect on depth of cure, depending on the photoinitiator package the formulator has chosen. This is particularly true with thicker coatings, where the poor depth of cure can lead to poor coating performance in the field. Similarly, increasing the concentration of photoinitiator in the coating allows for more free radical formation which, in turn, provides for better through-cure. Depending on the type of photoinitiator chosen, this approach can have a detrimental effect on depth of cure as well as obvious economic implications. Care also should be taken that the concentration of free radicals produced does not exceed available sites, as this could result in a reduction of cure speed.
Excited singlet state
Excited Triplet State
FIGURE 1. Photoinitiator radical production process 12 | UV+EB Technology • Issue 2, 2017
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FIGURE 2. α-scission mechanism of acyl-phosphine oxides
Photoinitiator technology To determine how photoinitiator selection and concentration affect cure performance, it is important to understand first how photopolymerization works. The process begins with a molecule being exposed to radiation, creating a reactive species resulting in photopolymerization of monomers. The two most common types used in coatings are free-radical photoinitiators, used to polymerize acrylatebased monomers, and cationic photoinitiators, used in epoxybased formulations. In this paper, we will focus only on acrylate-based systems. Upon light absorption, a photoinitiator transitions from the neutral ground state to an electronically excited singlet state. Once in the singlet state, a rapid intersystem crossing occurs to form the excited triplet state, which is where radical production most often results4 (Figure 1). The exact mechanism of radical formation can vary, depending on the molecule’s chemical configuration. A (substituted) alkyl group at the R1 (as is the case with acyl-phosphine oxide) undergoes a Type I scission, producing two radical species with different reactivity and oxygen sensitivity characteristics5 (Figure 2).
FIGURE 3. HABI radical formation One variable that has yet to be completely explored is the effect of the type of photoinitiator used in the formulation. Formulators frequently use combinations that have succeeded in the past, but photoinitiator performance under conventional UV lamps is not necessarily indicative of how it will do under UV LEDs. In some cases, particularly thick coating applications, entirely new combinations that work using a completely different mechanism can produce significantly better results. In this paper, we will evaluate not only how the concentration of photoinitiators can affect coating performance but, more importantly, why the choice of material also is critical.
Hexaarylbiimidazole, or HABI as it is more commonly known, utilizes a different mechanism to produce free radicals and initiate polymerization. Upon exposure to UV radiation, the HABI molecule is activated to form an excited intermediate compound. Radicals are then produced by hydrogen abstraction or electron extraction from a second compound in the formulation. The secondary compound then becomes the initiating radical, resulting in polymerization (Figure 3). Numerous HABIs are possible by modifying the substituent groups “R” attached to the aryl groups of the molecule. For example, the structure is called a lophine dimer when “R” is hydrogen. However, when chlorine replaces hydrogen, the compound now is called o-Cl-HABI. The addition of the Cl page 14 u
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UV+EB Technology • Issue 2, 2017 | 13
PHOTOINITIATOR SELECTION t page 13
FIGURE 4. HABI molecule
changes the physical properties and performance of the molecule. Hundreds of HABIs can be synthesized by the addition of functional groups to various and/or multiple positions on the aryl groups. Each may exhibit different physical properties and performance. The performance of each material, however, is system- and applicationdependent (Figure 4).
Reaction mechanism is only one consideration when choosing a photoinitiator. Solubility, color, photo speed and cost all play important roles. Researchers also rely on a compound’s UV spectra to determine the suitability of material for the specific application (Figure 5). Recently, much attention has been given to optimizing photoinitiator packages for LED applications. In the next section, we will explore how photoinitiator selection affects the curing properties of both thick and thin coatings under UV LED.
FIGURE 5. Typical UV spectra Photoinitiator effect on surface cure To evaluate the effect of photoinitiator substitution on surface cure, a series of standard formulations was created. Each formula was composed of a main acrylate monomer, a diluting monomer and a photoinitiator package at the following percentages: Main acrylate monomer--------------------87% by weight Diluting monomer---------------------------5% by weight Photoinitiator package----------------------8% by weight The main acrylate monomer was chosen from the following three materials commonly used in UV-curable coating applications: • EO-TMPTA Ethoxylated TMPTA (Allnex) • CN 131B Aromatic monoacrylate oligomer (Sartomer) • IBOA Isobornyl acrylate (various) • HDODA 1,6-hexanediol diacrylate (various)
• • • • •
Unit powers up quickly, saving time Quiet operation Multiple UV distribution points make curing more even and cooler Full lamp curing up to 1000” wide Optional Inteli-GuardTM control allows system to power down while not in use A .W.T. World Trade Inc.
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14 | UV+EB Technology • Issue 2, 2017
The diluting monomer was added to improve the solubility of the photoinitiator package, as well as to facilitate coating drawdown. In all cases, the diluting monomer used was n,n-dimethyl acrylamide (DMA), a nonionic acrylic monomer. DMA also was chosen as the nitrogen atom, positioned on the backbone to facilitate curing in the presence of oxygen. The photoinitiator package used for this series experiments made up of three components: Free radical photoinitiator --------------------- 12.5 parts Substituted thioxanthone sensitizer------------25 parts Electron donor------------------------------------62.5 parts The photoinitiators chosen for this study were selected from the either the imidazole family (Hampford Research) or phosphine oxides (BASF). In all cases, the substituted thioxanthone coinitiator chosen was 2,4-Diethylthioxanthone (DETX), and the electron donor used was 2-Mercaptobenzoxazole (2 MBO). page 16 u uvebtechnology.com + radtech.org
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You’re passionate about your formulations. We are, too. Our tech support team contributes ingenious chemistry to advance performance in a wide host of applications. Think electronics, soft-touch coatings and automotive adhesives, just to name a few. Formulators come to us for our unique expertise in acrylate and methacrylate chemistry. We lead the world with the broadest product portfolio for UV/EB/dual cure technology. But, it’s our interactive chemist-to-chemist support program that distinguishes us from the rest. Our technologists understand your needs and create customized and innovative solutions to reach your specific formulation, processing and performance goals.
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PHOTOINITIATOR SELECTION t page 14 Rating
No evidence of cure
Dry but smears easily
Does not smear, but scratches with fingernail
Resistant to finger scratch
TABLE 1. Rating surface cure
FIGURE 8. Surface cure comparisons #3 oxide photoinitiator packages dissolved readily into the diluting monomer, and the resulting solution added to the TMPTA. The coating material was applied to an aluminum plate and cured under UV lamp (as detailed previously).
FIGURE 6. Surface cure comparisons #1
Both the imidazole-based, as well as the acyl-phosphine oxide system, showed little surface cure after the first pass and comparable results through four passes. After the fifth and final pass, the imidazole-initiated coating was completely cured while the PO coating could still be scratched (Figure 6). The same tests were performed, this time substituting Isobornyl acrylate for the main monomer. IBOA is commonly used for coatings due to its hardness and flexibility characteristics. It is somewhat less reactive than the TMPTA, which was demonstrated, as neither formulation achieved full cure even after five passes. The phosphine oxide-initiated system did show a slight advantage in surface cure after the second and fourth pass (Figure 7).
FIGURE 7. Surface cure comparisons #2 Testing was performed using a series of 0.8-mil wet film drawdowns on 0.010-inch aluminum plates. The coated aluminum plates were irradiated using a Phoseon “Starfire Max” LED lamp at 395 nm wavelength. The line speed was maintained at 12 feet per minute throughout the experiment. After each pass, the panels were removed and the degree of surface cure evaluated per a set scale (Table 1). The first formulation tested used EO-TMPTA (Ethoxylated TMPTA) as the main monomer. This material is commonly used in acrylic coating formulations due to its low toxicity and fast cure speed. Both the imidazole-based and phosphine 16 | UV+EB Technology • Issue 2, 2017
Sartomer’s CN-131B was the primary acrylate for the third and final series of surface cure tests. This particular aromatic monoacrylate oligomer also was chosen due to its high reactivity and fast cure speed. As expected, both photoinitiator technologies exhibited full surface cure after only one pass (Figure 8). While there were some slight differences in surface cure characteristics (the imidazole-based being slightly more cured), the two photoinitiator systems gave very similar performances. Repeating this same series of tests substituting a traditional mercury lamp, all of the formulations tested demonstrated full cure by the second pass. This clearly illustrates the challenges associated with oxygen inhibition under LED lamps normally not seen with mercury lamps. Photoinitiator effect on depth of cure The relationship between surface cure and depth of cure is complex and not always completely understood. Normally, one would expect good surface cure to be an indication of complete uvebtechnology.com + radtech.org
FIGURE 9. Cured coating
FIGURE 10. Thickness measurement
through cure as well. While this is the case with conventional lamps, the opposite can occur with LED systems. Additionally, modifications that intuitively would help (i.e. higher photoinitiator concentration) can actually reduce the depth of cure. In the next series of tests, we evaluated how photoinitiators affect the depth of cure in both LED- and conventionally cured coatings.
To evaluate how photoinitiator selection and concentration affect through cure, we started with a standard coating formulation composed of equal parts CN-964, IBOA and CN-131B. Two photoinitiators were chosen from the acyl phosphine oxide class as well as two from the imidazole family. For this series of tests, page 18 u
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PHOTOINITIATOR SELECTION t page 17
Photoinitiator Solution #1 (o-ethoxy HABI) Solution #2 (TCDM HABI) Solution #3 (TPO)
Concentration 0.5% bw 1.0% bw 2.0% bw 0.5% bw 1.0% bw 2.0% bw 0.5% bw 1.0% bw 2.0% bw 0.5% bw 1.0% bw 2.0% bw
LED (395 nm) Percent change
Depth of cure Depth of cure 145 142 150 142 150 135 145 115 150 78 150 50 145 130 150 70 150 40 148 90 150 42 150 21
D vs LED -2% -5% -10% -21% -48% -67% -10% -53% -73% -39% -72% -86%
Conclusions The transition from broad spectra mercury lamps to lower cost, more environmentally friendly LEDs represents one of the most important changes in recent UV/ EB technology. However, formulators face significant challenges in maintaining coating performance within the limited wavelengths these lamps produce.
Poor surface cure due to oxygen inhibition is one of the more Solution #4 common challenges applicators face when converting to LED. (BAPO) This can be mitigated a number of different ways, either by TABLE 2. Effect of photoinitiator on surface and through cure isolating the coating from * Thicknesses 145 to 150 mils were considered complete through cure. the environment, or through modification of the chemistry itself (both monomers as well as photoinitiators). the photoinitiator concentration ranged from 0.5 percent to 2 percent by weight. As before, 2-Mercaptobenzoxazole was added Incomplete through cure is another problem related to UV to all test solutions as an electron donor. LED. It generally occurs when insufficient energy is produced to pass through the cured surface and seems to worsen as its The four photoinitiators chosen for this study were: concentration is increased. • Mono ethoxy substituted imidazole (Test solution #1) • Poly methoxy substituted imidazole (Test solution #2) As LED lamps become more and more popular in mainstream • Monoacylphosphine oxide (Test solution #3) coating applications, formulators will continue to find ways to • Biacylphosphine oxide (Test solution #4) optimize overall coating performance. u Exactly 0.5 gram of each formulation was weighed out into a References ceramic Coors evaporating dish and irradiated under UV LED 1. J. Arceneaux. Mitigation of Oxygen Inhibition in UV-LED, UVA and (395 nm) for a single pass at 72 feet per minute. The cured Low-Intensity UV Cure, UV+EB Technology, Issue 3, 2015 coating was removed (Figure 9) and the thickness measured to the 2. Crivello, Dietliker. Photoinitiators for Free Radical, Cationic & nearest 0.001" using a micrometer (Figure 10). nd The chart above (Table 2) details the results obtained from the second series of tests. As was the case with the first series of tests, complete cure was achieved in all cases. When exposed to UV LED, there was anywhere from a two percent reduction in through cure (Solution #1) to as much as an 86 percent reduction (Solution #4).
Anionic Photopolymerisation, 2 Edition, p259-264, John Wiley and Sons, 1998 3. R. Dessauer. Photochemistry, History and Commercial Applications of Hexaarylbiidazoles: All about HABI. Elsevier, 2006 4. S. Finson. HABI, Enabling Photolithography for 30 years in Electronics, PCB Magazine, 2012 5. W. Arthur Green. Industrial Photoinitiators, A Technical Guide, p35,82,103,192,196, Taylor and Francis Group, 2010
Although the mechanism is not completely understood, it has been proposed that achieving a high rate of surface cure is actually detrimental to depth of cure, as it prevents radiation to reach deep into the coating. It also appears that smaller, more mobile photoinitiators – as found in solutions #1 and #3 – performed better than their larger, bulkier counterparts. Overall, solution #1 should offer the best overall performance under UV LED.
Michael Wyrostek is the director of sales and marketing for Hampford Research. He is a graduate of the University of Massachusetts with a bachelor’s degree in chemistry. Wyrostek has worked in the specialty chemical business for over 30 years, holding positions in research, sales, marketing and product management. For more information, email mwyrostek@ hampfordresearch.com or visit www.hampfordresearch.com.
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THE SOLUTION IN ENERGY CURING
Miwon Specialty Chemical Co., Ltd. is a world-class producer of specialty acrylate and methacrylate monomers and oligomers. Miwon maintains a global presence – in Asia, North America and Europe – with full-scale production facilities, regional technical support laboratories, regional customer sales/supply support offices and local warehouses. We are a key raw materials supplier to the inks, coatings, adhesives and electronics industries. As manufacturing raw materials for UV and EB curing is our core business, we offer one of the broadest product lines for formulators utilizing this advanced and environmentally friendly technology. Products are engineered to support changing market needs: • Cross contamination free • Toluene free (HAP free) • Consistency in quality • BPA-free systems • High purity • Low residuals • Low extractables • High refractive index • Tin-free oligomers • Captive EO/PO capability As a member of the American Chemistry Council, we are committed to the principles of Responsible Care. Miwon Specialty Chemical Co., Ltd. Miwon North America 696 W. Lincoln Highway Exton, PA 19341
Phone: 484-872-8711 Fax: 484-872-8717 email@example.com www.Miramer.com
APPLICATION By Dianna Brodine, managing editor, UV+EB Technology
Emerging UV Curing Applications in Additive Manufacturing A
t the RadTech 2016 awards dinner, creative users of UV/EB technologies were honored with Emerging Technology Awards. One of the 2016 honorees was Continuous Composites (CC3D), a Coeur d’Alene, Idaho-based company that has developed a 3D printing process to print continuous fiber with UV-curable resin. CC3D is reportedly the only company in the world 3D printing continuous fiber with thermoset epoxy and the only company to successfully print continuous fiber into free space. With a focus on functional composites additive manufacturing to scale, CC3D is providing an industrial solution to advanced manufacturing techniques of mid- to large scale. Adding strength and flexibility Ken Tyler is the chief technology officer for Continuous Composites and inventor of the foundational technology. Originally a craftsman in the boat industry, Tyler first used a stereolithography (SLA) machine at a rapid prototyping center at Boise State University and later witnessed a fused deposition modeling (FDM) machine in action. “The first time I watched one of the FDM machines melting plastics together, I thought it was crude – there had to be a better way,” he explained. “In the boat industry, I was working with epoxies and fiberglass every day, and one day I had an epiphany that we should introduce these materials to the additive manufacturing industry.” Tyler Alvarado is Continuous Composites’ chief executive officer. “I’ve been partners with Ken for two years now,” he said. “When we met, Continuous Composites was a patentpending technology with limited resources, and it made sense for the me and my business partners to invest in the technology and work with Ken to bring it to the next level. “There are three primary elements to our technology – materials (i.e., resins and fibers), software and hardware,” Alvarado continued. “Until recently, we had been using the same UVcurable resin for all the various fibers we’ve printed with – it was a onesize-fits-all approach. Now, we’re developing specialized UV-curable formulations for each fiber and/or application – fiberglass, carbon fiber, etc. Our technology has applications across many different industries, and those applications each have their own material requirements.”
20 | UV+EB Technology • Issue 2, 2017
RadTech editorial board member Mike Idacavage presented an Emerging Technology Award to Ken Tyler, Continuous Composites. uvebtechnology.com + radtech.org
wire – to form a part with properties unavailable with traditional 3D printing. “Our thermoset resins are cured using UV light, which causes a molecular-level change, and the parts cure without porosity” Alvarado said. “The UV curing process not only ‘sets’ the resin quickly, but also enables us to print into free space without supports.” He continued, “This allows us to orient fibers in unique ways to reinforce stress points and not deposit materials where materials are not needed. Our technology opens up the realm of design and manufacturing capabilities. We no longer need molds and autoclaves to manufacture composite structures.” High material deposition also is a major benefit of the technology. Continuous Composites has the ability to not only print with single nozzles extruding a single tow of fiber, but also multichannel nozzles with multiple tows of fiber – often different types of fibers – simultaneously. “We’ve always had a plan to put our print head on a robot arm, but it’s currently on a gantry system,” said Alvarado. “We’re designing and manufacturing a 4th and 5th axis to act as a ‘wrist’ to angle and rotate the nozzle(s) which expands our motion control capabilities.” page 22 u “Early on in the technology development, the resin had been the missing piece,” said Tyler. “None of the resins I could find would work with our process, so I attended a conference held by RadTech International North America, where I met a great group of formulators. I had a breakthrough on the resin with them.”
UV Curable Acrylate and Epoxy Silicones Provide Flexibility and Elongation Servicing the Energy Cured Coatings and 3D Printing Industries with Unique Silicone Building Blocks
The Continuous Composites process uses low-viscosity, UVcurable thermoset resins as opposed to the thermoplastics that are used in conventional 3D printing. With thermoset resins, there is no heating and cooling process like there is with thermoplastics. The thermoset resin is extruded with the continuous fiber – ranging from carbon fiber and fiberglass to fiber optics and copper
The thermoset resin is extruded with the continuous fiber – ranging from carbon fiber and fiberglass to fiber optics and copper wire – to form a part with properties unavailable with traditional 3D printing. uvebtechnology.com + radtech.org
Your Technology Our Chemistry
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UV+EB Technology • Issue 2, 2017 | 21
APPLICATION t page 21 Taking the technology to industry “I filed the first patent for the technology in 2012, and it was granted in December of 2016,” said Tyler (see box, right). “Now, we are working to license our technology to companies from various industries,” Alvarado continued. “We are having good discussions with major players from industries such as automotive, aerospace and construction.” In addition to the uniqueness of the fiber/resin combination, low energy usage and higher speeds are attracting attention. “We are focusing UV LEDs at the point of extrusion, so we’re able to apply really high energy – as high as 14 watts per cm2 – but we’re not consuming much power, because LEDs have low power usage,” Tyler explained. “And, we can cure up to 1,500 inches per minute.” Alvarado’s vision for the technology is impressive. “We’ve been able to showcase our technology’s ability to print strong composite parts unsupported in free space while embedding other functional materials, such as fiber-optics and copper wire. As our technology matures, we will be printing production-ready parts quickly, which will include sensors and other electronic components,” he said. “It’s not hard to see a future where the technology allows us to print an entire airplane.”
As for Tyler, his goal for the technology leads back to where he first had the idea that sparked Continuous Composites. “One of these days, I’ll be able to 3D print my own boat,” he laughed. u Method and apparatus for continuous composite three-dimensional printing Patent No. US 9511543 B2 Granted to Kenneth Tyler, December 6, 2016 ABSTRACT: A method and apparatus for the additive manufacturing of three-dimensional objects are disclosed. Two or more materials are extruded simultaneously as a composite, with at least one material in liquid form and at least one material in a solid continuous strand completely encased within the liquid material. A means of curing the liquid material after extrusion hardens the composite. A part is constructed using a series of extruded composite paths. The strand material within the composite contains specific chemical, mechanical or electrical characteristics that instill the object with enhanced capabilities not possible with only one material.
Contract Manufacturing Precision Custom Coating Services
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Wikoff Develops Laminating White, Solution for Folding Carton Market Wikoff Color Corporation, Fort Mill, South Carolina, developed a high-performance laminating white that exhibits very high opacity levels. Compass Ultra White provides ink transfer, combined with very low solvent retention; hiding properties over challenging metallized substrates; adhesive laminating bonds and the ability to reduce white application volumes by up to 50 percent. The company also introduced AccuLith, the latest oil-based Litho solution for the folding carton market. This low-tack system maintains stability on press and achieves color rapidly on startup and restarts, making it a great option for printers looking to reduce their makeready times and the associated waste. AccuLith conforms to ISO 12647-2, GRACoL, GMI and G7 standards. For more information, visit www.wikoff.com.
Phoseon Technology Increases Curing Solution Power Phoseon Technology, Hillsboro, Oregon, increased the peak intensity of the FireJet™ FJ100 LED curing solution by 50 percent, up to 12W/cm2. The FireJet FJ100 provides a combination of size and power for space-constrained environments requiring highintensity curing performance. With WhisperCool™ technology and TargetCure™ technology, the FJ100 provides reliable, consistent performance at a quiet operating level. The FireJet line of air-cooled UV LED curing lamps is aimed primarily at UV inkjet wide-format systems and is capable of curing at the highest speeds for small-, medium- and grand-format digital printing systems. It also is ideal for many large, single-pass UV inkjet and wood-coating applications. For more information, visit www. phoseon.com.
Fujifilm Introduces LED-UV Retrofit System Fujifilm North America Corporation, Graphic Systems Division, Hanover Park, Illinois, introduced Illumina, an LED-UV retrofit system for converting any traditional UV or waterbased flexo press to LED-UV curing. Patented LED technology delivers up to 44 percent more energy toward the substrate, resulting in faster curing. The UV-LED cure is an instant on/off process, lowering the energy usage and stress on lamp bulbs experienced in conventional “always-on” UV mercury lamp curing. UV-LED also eliminates the costs generated by cooling air blowers, ozone extraction and heat makeup systems. For more information, visit www.fujifilmusa.com/northamerica.
Dow Corning Unveils Bonding Solution Dow Corning, Midland, Michigan, a wholly owned subsidiary of The Dow Chemical Company, unveiled an innovative UV-curable optical bonding solution engineered to enhance the reliability and performance of automotive displays. Available in Asia and Europe, Dow Corning® VE-6001 UV Optical Bonding Material delivers higher thermal stability vs. organic materials, and its one-part silicone chemistry offers simpler processing than most competitive silicones. VE-6001 UV Optical Bonding Material adheres a variety of cover window materials – including glass, acrylic and polycarbonate – to automotive LCD display modules. Compatible with common industry processes – such as dam and fill, patterning and slit coating – it exhibits controlled flow during processing and improved deep section cure, depending on structures, with exposure to UV light. For more information, visit www.dowcorning.com.
Dymax Launches Maskant Dymax Corporation, Torrington, Connecticut, launched SpeedMask® 9-7001, a light-curable maskant designed for protecting connectors and board-level areas from both solventbased and light-curable conformal coatings. Cured masks withstand wave solder and reflow temperatures and are easily removed in one piece, saving time and eliminating the concern of ionic contamination or residue left behind by other masking methods. The residue-free surface after removal results in passing SIR testing per IPC-TM-650 and zero keep-out violations. This halogen and silicone-free maskant is compatible with gold and copper connector pins, cures upon exposure to light and is designed to provide protection of connectors with no discoloration or corrosion. It is thixotropic and is ideal for manual or automated dispensing on boards or components that may be difficult to mask. For more information, visit www.dymax.com.
Baldwin Introduces New Solid-State LED Curing Technology Baldwin Technology Company Inc., St. Louis, Missouri, introduced its proprietary UVed™ LED UV, representing the latest in solid-state LED curing technology. Featuring a lightweight design, an ultracompact UV lamp head, widthswitching capabilities and instant on/off pure UV output, the UVed produces no ozone and virtually no heat, and it offers more than a 50 percent reduction in power consumption. For more information, visit www.baldwintech.com.
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Hamamatsu Offers Quick-Drying UV-LED Light Sources The UV-LED light sources from Hamamatsu Corporation, Bridgewater, New Jersey, offer many advantages over traditional metal-halide lamps used for drying UV inks, such as compact size, low-power consumption, instantaneous lighting and on/off operation, air cooling and easy maintenance. Hamamatsu’s LC-L5 G series of UV-LED light sources are available in various configurations to fit many types of printers, and they are offered with 365nm, 385nm, 395 nm and 405 nm UV emission. For more information, visit www.hamamatsu.com.
needed when adding Curalite. For more information, visit www. perstorp.com. Codimag Introduces LED Press Codimag, Bondoufle, France, introduced its VIVA 340, a modular press that mixes the flexibility of digital printing with the performance of conventional printing technology. The machine is equipped with MBS LEDcure, an air-cooled, high-power LED series, which can perform all the major printing or finishing processes inline in intermittent mode: waterless offset, letterpress, screenprinting, hot foil stamping, embossing, flexo varnishing, wet laminating and diecutting. All labels, from four-color labels to the more elaborate labels for wine or cosmetics, can be printed inline in one pass on the press. For more information, visit www. codimag.fr.
Perstorp Launches Cationic UV Boosters Perstorp, headquartered in Perstorp, Sweden, launched Curalite™, a new range of boosters for cationic UV curing formulations that can increase speed of reactivity up to 15 times, enabling more efficient production. Curalite also brings through cure and hardness, which are crucial in many applications. Formulation costs can be reduced, as lesser amounts of photoinitiator are
Laboratory Unit LABcure Celebrates World Premiere IST METZ, a group of companies with head offices in Nürtingen, Germany, has introduced a UV laboratory system, page 26 u
EIT® LEDCure™ L395
Optics designed specifically for UV LEDs Profiling & Non-Profiling Versions 40 Watt/cm2 Dynamic Range L395 shipping, other bands released soon
New L395 Band (above) with Total Measured Optics Response Provides Accurate, Repeatable & Absolute Values USB Download, Works with Updated PowerView II Software
New Address: 309 Kelly’s Ford Plaza SE Leesburg, VA 20175 Email: firstname.lastname@example.org
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UV+EB Technology • Issue 2, 2017 | 25
Technology Showcase t page 25 LABcure. The compact unit can be positioned on a table and is capable of transporting samples up to 200mpm. With a lamp length of 370mm, a continuously adjustable lamp power of up to 200W/cm, high-quality URS reflectors and the fast lamp change system FLC®, LABcure is user-friendly and service-friendly and is suitable for a wide range of research and development tasks. For more information, visit www.ist-uv.com/en. Chromaflo Unveils Colorants for UV Coatings Chromaflo Technologies, headquartered in Ashtabula, Ohio, unveiled colorant solutions for UV coatings that can be customized. The colorants contain organic and inorganic pigments finely milled in monomeric carriers, which can be customized depending on the application. The selection of carriers provides for compatibility in a wide variety of coatings formulas and substrate requirements, including industrial wood and furniture. For more information, visit www.chromaflo.com. Sartomer Launches Additive Manufacturing Resins, Adhesion Promoters Sartomer, Exton, Pennsylvania, a subsidiary of Arkema Group, launched NextDimension™, a range of solutions for UV-curable additive manufacturing that consists of two grades developed
to allow the end-user to meet and exceed performance levels of current market standards. One grade has been designed to enable high print-quality prototyping while the other provides improved mechanical performance for high-end industrial applications. Additionally, Sartomer introduced PRO22019, an adhesion promotion oligomer that achieves superior adhesion properties on rigid and flexible plastic substrates. For more information, visit www.americas.sartomer.com. Mimaki Reveals New UV-LED Tabletop Printer Model Mimaki USA, Suwanee, Georgia, announced the UJF-3042 Mark II (MkII) EX model, expanding the company’s series of UV-LED tabletop printers. This new model supports additional ink configurations to, and increases capacity on, an already popular platform. The UJF-3042 MkII EX printer includes eight ink channels, enabling users to take advantage of all available ink colors. The UJF-3042 MkII EX printer can print onto media up to 11.8x16.5x6" thick. The Kebab MkII option is available for printing on cylindrical objects such as stainless steel tumblers, bottles, cans, vases, packaging and shipping tubes. The UJF3042 MkII EX printer is expected to be available for order from Mimaki authorized representatives by summer 2017. For more information, visit www.mimakiusa.com. Duplo USA Launches Digital Spot UV Coater Duplo USA Corporation, Santa Ana, California, launched its DDC-810 Digital Spot UV Coater, which will be available in the US, Canada and Latin America. Utilizing inkjet technology, the DDC-810 applies a gloss varnish to defined areas of the substrate, giving images a raised effect with texture and depth. Its CCD camera recognition system ensures image-to-image registration and its PC Controller software offers an easy-to-use operation. Designed for short-run applications, the DDC-810 can process up to 21spm (A3), UV thickness from 20 to 80 microns and paper weights from 157 to 450gsm (coated paper). For more information, visit www. duplousa.com. u
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WOOD FLOORING By Gary Sigel, Ph.D., Senior Principle Scientist, Armstrong Flooring
Floor Coatings with UV LED Curing: A Focus on Performance and Properties F
loor coating compositions were prepared from monofunctional, difunctional and trifunctional acrylates, as well as urethane acrylates (UA), and used to determine the effect of double-bond equivalent weight (DBEW) mercapto-based resin, high-viscosity UA and LED photoinitiators on final cured film properties when cured under an atmosphere of air vs. nitrogen. The least amount of yellowing was observed using a combination of TPO/ITX as the UV LED photoinitiator package. Scuff, stain and abrasion properties sought in floor coatings were met using LED/UV 365 nm and 385 nm lamps. The use of nitrogen inerting resulted in an improvement in surface properties, such as stain resistance and scuff resistance. Thermal and mechanical properties, such as tensile elongation and storage modulus, are described. Introduction Armstrong’s first UV-cured floor coating was introduced in 1976, and coating systems that provide improvements in resisting stains and abrasions have continued to evolve for the past 40 years. In 1976, the company introduced its first UV-curable coating, purchased from W. R. Grace, based on “thiol-ene” chemistry for Solarian tile. The coating brought to market a no-wax “do it yourself” installation. This encompassed the reaction between difunctional norebornenes with R = 6 with tetrathiol. A schematic of the “thiol-ene” cure process is shown in Figure 1. These systems cured quite well due to the nature of the thiol-ene reaction that mitigated oxygen inhibition1,2. As published by Hoyle and Johnson, oxygen has little effect on the polymerization rate of thiol-ene free radical polymerization process. Upon addition of oxygen to the carboncentered radical formed from the radical propagation step, the peroxy radical readily abstracts a hydrogen from the thiol, producing a thiyl radical that adds to the carbon-carbon double bond, thereby reinitiating the propagation step (Scheme O O O O 1).3 These successes R O SH O SH with the photoinitiated O O SH O O SH polymerization of thiolenes led to the first truly O O large-scale uses of radiation curing in the United States, ensuring the expanding Photoinitiator UV future of this green technology. For a wide variety of reasons, both economic and technical, O O O O thiol-ene photocuring gave R O O S O O S way to acrylate-based S O O S photocurable systems. n Objections at the time to O O thiol-ene-based ultravioletFIGURE 1. Schematic of curing ene-thiol coatings for flooring that curable resins included were UV cured by conventional medium-pressure Hg vapor lamps page 28 u
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UV+EB Technology • Issue 2, 2017 | 27
WOOD FLOORING t page 27
FIGURE 2. Photo showing topcoats being cured in air using LED 385nm followed by LED 365nm modules Scheme I odor, as well as the incorrect perception that all thiol-ene coatings were subject to rapid yellowing and discoloration upon weathering. Coating requirements for flooring are very different from requirements of other coating systems used in the UV/EB industry for inks, varnishes, furniture and plastics or wall coverings. Several surface properties of the coating must be retained after UV-initiated polymerization, including resistance to common household stains; ease of surface cleaning after spills; extended wear and scuff resistance to rubber heels or other types of footwear; and ability to retain original aesthetics after everyday wear from foot traffic. This paper will explore methods to mitigate oxygen inhibition by utilizing reactive chemicals and nitrogen atmosphere to improve surface cure for UV/LED cured floor coatings. The types of materials studied can be broken down into highviscosity urethane acrylates, mercapto-modified polyester acrylate resin and LED photoinitiators within a base formulation for each series of formulations (Tables 1, 2). Within these comparative studies using a base formulation with the same materials, the effect of atmosphere processing conditions (i.e., air vs. nitrogen) on degree of cure; glass transition (Tg); mechanical properties; and scratch, scuff and stain performance will be described. Properties of cured UV/ LED coatings include double-bond conversion determined by FTIR, mechanical tests to determine percent elongation at break, DMA properties to determine level of cross-linking and thermal analysis to determine glass transition temperatures. Three studies are presented: 1. Effect of double-bond equivalent weight (DBEW) within the formulation on degree of cure, mechanical testing and performance data (Table 3). 28 | UV+EB Technology â€˘ Issue 2, 2017
2. Effect of photoinitiator type: phosphine oxide-based TPO, TPO-L vs. 3-ketocumarin on cure of LED formulations (Table 4). 3. Effect of high-viscosity urethane acrylate and thiol-based acrylate on LED cure (Table 5). Experimental Materials used in this study are designated in Table1. Tables 2 through 5 show the various formulations used in each of the four studies. Table 2 summarizes the UV-cured formulation used in this study for comparison to LED/UV cured formulations. Table 3 shows the formulation composed of a combination of polyester acrylate; urethane acrylate; and mono-, di- and trifunctional acrylates, along with overall double-bond equivalent weight (DBEW)/gm resin. The DBEW is calculated by dividing the C=C gm equivalent weight for each material by the amount of material (gms) to get equivalents of C=C. The summation C=C equivalents for each material divided by total weight gives double-bond equivalent weight. All additives have been left out of this calculation, including photoinitiators and hard particles. To simplify formulations, the DBEW/g has been multiplied by 1,000 to give a whole number as DBEW/1000g. Testing Methods Test Tile Preparation All performance testing for resistance to scuffs, stains and abrasion was performed on floor tile substrate to isolate the properties of the topcoats. All formulations were coated onto tile substrate using a #6 wire wound rod to give approximately 0.6 to 0.7 mils and subsequently cured by the conditions outlined in Tables 6 and 7. The coated tile substrate or wood was preheated to 85Â°F after application of coating to allow for flow of the coating prior to UV or LED/UV cure processing. Laminate films were prepared by coating 2.6 mil rigid PVC film with the coating that was mounted on a glass plate before processing. uvebtechnology.com + radtech.org
Control UV Cured
Highly functional urethane acrylate
Highly functional urethane
Ebecryl LED 02, mercapto based acrylate resin, Allnex
Ebecryl 8807, Aliphatic UA high viscosity, Allnex
Ebecryl 8811, Aliphatic UA, High Viscosity
Ebecryl 8811, Aliphatic UA, High Viscosity, Allnex
ITX, isopropylthioxanthone Amine co-initiator
Diphenyl(2,4,6-trimethylbenzoyl) phosphine oxide (TPO)
LED/UV photo initiator
Diphenyl(2,4,6-trimethylbenzoyl) phosphine oxide (TPO)
Ominirad BL723 , IGM Resins
LFC3644 IGM experimental 3ketocumarin, IGM Resins
Additives Hard Particles
TABLE 1. Monomer and oligomer descriptions for UV/LED base formulations (base formulation in purple) Gloss was measured using a BYK-Gardner 60 degree gloss meter set in statistical mode for a total of 10 measurements to give an average gloss value. Viscosity was measured using a Brookfield RVT Viscometer #6 spindle at 100rpm, 77°F. Scratch resistance was measured using a modified BYK Gardner abrasion tester. Each coating layer was abraded using a proprietary method. After abrading, each sample was visually assessed using the scale of “1” for minimal damage and “2” for severe damage. The percent gloss retained was calculated based on initial gloss and final gloss after the test. The iodine stain test was conducted by placing a dropper full of iodine (size of a dime) on the substrate and allowing it to remain for one minute. The sample was wiped with a damp rag and the color was measured with a sphere spectrometer from X-rite, Model SP64. The CIE L*a*b* color scale was used for color measurements. A uvebtechnology.com + radtech.org
Viscosity cps 77F
TABLE 2. Effect of high MW aliphatic UA on surface cure with LED lamps 385, 365 Delta b (Δb) value was obtained (a measure of the difference in b values between the control and a sample) to indicate the degree of yellowing. The degree of yellowing was measured with a calorimeter that measures tristimulas color values of “a,” “b” and “L,” where the color coordinates are designated as +a (red), -a (green), +b (yellow), -b (blue), +L (white) and -L (black). Double-bond Conversion The samples were scanned using a Cary 620 FTIR spectrometer with a diamond ATR accessory with a ZnSe engine. The samples were placed directly on the sample compartment base plate of the spectrometer without a salt crystal. Analysis was performed in absorbance mode, using 64 scans at 8 cm-1 resolution. Degree of double-bond cure for the LED-cured coatings was estimated by comparing the intensity of the IR stretch for the C=C group around 1400 cm-1 of uncured liquid (0% cure) to cured coating (100 percent degree of cure). This is based on the relative ratio of the measured intensity of each peak relative to carbonyl intensity at ~1700cm-1 (constant). The 1400cm-1 stretch is absent in a fully cured coating, indicating the polymerization of the C=CH2 in the acrylic resin. This would be 100 percent degree of cure. The results are presented as the IR degree of cure. UV Process Parameters for UV and LED/UV Cure Two types of UV cure equipment were used for studies described page 30 u UV+EB Technology • Issue 2, 2017 | 29
WOOD FLOORING t page 29 Description
Highly functional urethane acrylate
Highly functional urethane acrylate
Ebecryl 8811, Aliphatic UA
ITX, 0.3 ISOPROPYLTHIOXANTHONE
Diphenyl(2,4,6trimethylbenzoyl)phosphine oxide (TPO)
Diphenyl(2,4,6trimethylbenzoyl)phosphine oxide (TPO)
Viscosity cps 77F
TABLE 3. Effect of DBEW on LED properties cured in air and nitrogen atmosphere in this paper. 1. Miltec UV System, Inc., with conveyor system, equipped with four 320 watts/inch mercury standard medium-pressure Mercury Hg bulbs (Table 6). Samples were run with one pass for pre-cure and two passes for final cure, for a total of 1160mJ/cm2 UVA energy density. 2. Baldwin 365 nm and 385 nm LEDs side-by-side, equipped with SF50 optics (Figure 2). Data recorded using an EIT Power Mapper for each specific UV regime: UVA, UVB, UVC and UVV. All samples coated utilized four passes of LED 385 nm and LED 365 nm at 20 fpm at a distance of 1.5 inches with no additional UV cure (Figure 2). The processing parameters and radiometry values are given in Table 7.
Ominirad BL723 nonyellowing blend PI for LED 365nm
LFC3644 IGM experimental 3-ketocumarine
Viscosity cps 77F
TABLE 4. Effect of TPO vs. 3-ketocumarin photoinitiators on LED properties heating the samples from -50°C to 190°C at a rate of 20°C/min. in a nitrogen atmosphere. The samples were quench-cooled using the RCS-90 (chiller) between the initial and reheat scans. Mechanical Testing All mechanical testing was conducted using a Instru-Met instron at a rate 0.5"/min. Samples were prepared by using machined 0.5.00-inch X 6.00-inch template to prepare samples of coating/ film composites.
Samples were cured in air or under a nitrogen atmosphere by using an enclosed metal chamber equipped with a quartz plate on top with an inlet and outlet for nitrogen purge and a removable sealed plate to allow for inserting samples. All samples were purged with nitrogen for a period of 30 seconds prior to LED/UV exposure to 385 nm and 365 nm LED lamps.
Mechanical Analysis of LED Cured Formulations Using DMA An approach to estimating the degree of cross-linking utilized a TA Instruments Model Q-800 Dynamic Mechanical Analyzer (DMA) with tension film clamp. Nominally, 0.6 mil coatings were prepared on a 2.6 mil PVC carrier film and cured by UV and LED in air and nitrogen atmosphere. A strain sweep at 100C was conducted to determine the degree of cross-link density of various UV LED formulations.
Differential Scanning Calorimetry DSC experiments were conducted using a TA Instrument Model Q-2000 Differential Scanning Calorimeter (DSC). About 5.0 mg of the samples were weighed into an aluminum pan and analyzed using a TA Instrument. Initial and reheat data were obtained while
Results and Discussion Effect of Formulation and Air vs. Nitrogen LED Cure on Scuff Resistance An important goal of floor coating technology is to keep floors page 32 u
30 | UV+EB Technology • Issue 2, 2017
uvebtechnology.com + radtech.org
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WOOD FLOORING t page 30 looking spectacular longer through improved scuff resistance, scratch resistance, stain resistance and cleanability. Scuff resistance is the ability of the flooring structure to withstand marks from shoe soles or high heels by providing a coating surface that exhibits superior wear resistance and easy cleaning. The scuff test was developed to determine how effectively coating formulations performed when marked by a rubber sole. After scuffing a sample, the sample is wiped with a dry cloth and Description
Highly functional urethane acrylate
Ebecryl LED 02, Thiol based resin
Ebecryl 8807, Aliphatic UA high viscosity 2686000, f=2
ITX, ISOPROPYLTHIOXANTHONE 0.4
Diphenyl(2,4,6-trimethylbenzoyl) phosphine oxide (TPO)
Viscosity cps (77F)
Figure 3 provides a bar chart of scuff resistance for each formulation LED cured under air and nitrogen. Overall, the LED/ UV formulation tested and atmosphere of cure, air vs. nitrogen, had little effect on scuff resistance, as noted by the “1” rating. This indicates that sufficient surface cure was achieved by using UV/LED arrays that resulted in a hard-cured surface to resist scuff marks. The control UV-cured using medium-pressure Hg lamps was found to give a rating of “1,” indicating no marks present. The exception was Formulation A – cured in air – that exhibited scuff marks (Figure 4). This same coating cured in a nitrogen atmosphere did not show scuff marks, indicating a higher degree of cross-linking is achieved at the surface. Effect of Formulation and Air vs. Nitrogen LED Cure on Gardner Scratch Figure 5 summarizes the impact of formulation and atmosphere, air vs. nitrogen LED cure on Gardner Scratch as percent gloss retained in bar graph. All formulations within this study contained the same type and wt percent of hard particles. Virtually no effect from air vs. nitrogen atmosphere during LED cure was observed on Gardner scratch performance for LED formulations. Almost all formulations were rated as “light” damage, with gloss retention values after testing at greater than 89 percent, based on initial gloss before testing and final gloss measurements after testing (Figure 6). The influence of DBEW for Formulations A,
TABLE 5. Effect of oligomers on LED properties UVA
rated based on visual appearance of the remaining scuff mark. A rating of “1” is given indicating no sign of scuff; rating “2” is the presence of scuff marks.
Line speed, fpm
TABLE 6. UV process conditions used for control UV sample UVA
Line speed, fpm
Baldwin 385nm LED SP50 Optics
Baldwin 365nm LED SP50 Optics
TABLE 7. LED/UV processing conditions at lamp head to substrate of 1.5 inches 32 | UV+EB Technology • Issue 2, 2017
uvebtechnology.com + radtech.org
2 1.5 1 0.5 0
Air: Rate 2, Mark present
FIGURE 3. Effect of formulation and air vs. nitrogen LED cure on scuff resistance 60; B, 67; and C, 76, did not have a dramatic effect on surface scratch properties, indicating that the resin materials were sufficiently cured to hold the hard particles within the matrix. Similarly, no effect from type of LED photoinitiator, TPO/ITX for formulation D, phosphine oxide-based TPO-L formulation E vs. 3-ketocumarine for formulation F was observed on Gardner scratch results. The exception was curing the UV control formulation by LED/UV arrays, where severe scratch damage was observed (Figure 7). This is due to insufficient cure of the resin matrix, giving rise to failure of the surface to hold the hard particles and leading to catastrophic failure. Effect of Formulation on Initial Yellowing Color stability of the coatings after LED cure was determined by using an XRite SP64 flash spectrometer to record initial tristimulus L*a*b* values. In general, the bar graph indicates formulations D and E containing 3-ketocumarine initiators show 120 more yellowing vs. formulations containing TPO and ITX (Figure 8) 100 by the higher b* values. The control cured by UV was found to have the 80 lowest b* value of 10.2, whereas formulations D and E, containing 60 five percent of BL723 and LFC 40 3644, were found to have higher b* values of 14 and 19, respectively, for 20 the same base resin compositions. The exception was Formulation H, containing TPO/ITX and a high MW aliphatic urethane acrylate, that was found to have a high color b* value of 16.3. uvebtechnology.com + radtech.org
Nitrogen: Rate 1, No sign of marks
FIGURE 4. Shows initial scuff damage for Formulation A cured in air vs. nitrogen atmosphere
Overall, the staining observed, with the exception of Formulations F and H, would probably not impact the color of medium or dark wood stains – such as gunstock, Sumatra, cherry, saddle, mocha or midnight. Lighter colors – such as natural and harvest – would be affected by the coating’s initial b* values. One-Minute Iodine Staining: Effect of Air vs. Nitrogen Cure Effect of process conditions on 1-minute iodine stain resistance as determined by the delta b* color value shows that UV LED cure under nitrogen performed better than LED cure under air atmosphere due to improved surface cure. The lower the delta b stain value, the better resistance to iodine staining observed, as shown in the bar graph in Figure 8. It is important to note that formulations having a higher initial b* value than the UV control page 34 u
Air Gardner % ret
Nitrogen Gardner % ret
UV A Cured
FIGURE 5. Effect of formulation, air vs. nitrogen LED cure on gardner scratch UV+EB Technology • Issue 2, 2017 | 33
WOOD FLOORING t page 33 will result in masking of the staining due to iodine and result in a false b* value. Air LED/UV Cure The best 1-minute iodine stain-resistance in air is observed for the UV control cured vs. the remainder of the LED formulations, indicating better surface cure using Hg lamps. Although Formulation F had the lowest recordable Delta b value after the 1-minute stain test, it also had the highest initial b* color that would mask the delta b value of the iodine stain. In comparing Formulations A, B and C in the series of increasing DBEW from 62 to 76, no trend in improved iodine stain-resistance was observed. Formulation A, with DBEW of 60, and formulation C, with DBEW of 76, have similar initial b* color values of ca 5.6, which is three times that of the UV control. Similarly,
FIGURE 6. Showing light scratch damage for formulation C LED when cured under air (LHS) and nitrogen (RHS). Note that damage between the joint of the sample and the edge effects are not considered in this evaluation
in comparing the LED photoinitiator series D, E and F, the TPO/ITX formulation D and phosphine oxide TPO-L based photoinitiator E were similar on stain resistance. Nitrogen LED/UV Cure The improvement in stain resistance under a nitrogen atmosphere is due to mitigation of oxygen inhibition during the UV-initiated polymerization (Scheme 1). Again, no trend was observed in increasing DBEW in the series A, B and C on stain resistance. In comparing the LED photoinitator series D, E and F, an improvement was noticed for Formulation E, containing the phosphine oxide blend (TPO-L) BL723 Db=0.74 vs. Formulation D, containing TPO, Db=2.7. In comparing the effect of mercaptobased resin vs. high-viscosity UA in the series G and H, an improvement in stain was observed for Formulation G, containing the mercapto-based resin LED2; Db= 0.8 vs. 4 for UA EB8807. The other factor contributing to improved stain resistance is the increase in formulation viscosity that helps mitigate oxygen diffusion into the coating. Effect of Formulation Cured by LED in Air vs. Nitrogen on C=C Cure by Infrared Analysis The control UV formulation cured in air was found to have 100 percent carbon-carbon double-bond conversion by IR using standard medium-pressure Hg arc lamp processing conditions of 1330mj/cm2 and 900mW/cm2. The same formulation designated as Base C1 cured by LED 385 nm and LED 365 nm in air was found to have a low IR conversion of only seven percent after eight passes to get a tack-free surface, which is twice as many as other formulations in this study. In contrast, curing the UV formulation under a nitrogen atmosphere using LEDs resulted in a high double-bond conversion of 88 percent. The difference in cure in air vs. nitrogen is well documented, where oxygen inhibition is interfering with radical formation-quenching reactions, and scavenging reactions (Scheme 1, Figure 9).5 The formulation G was found to have the highest double-bond conversion of 75 percent when cured under LED/UV air. This formulation has a DBEW of 76 and contains 47 percent of a highly functional urethane acrylate and ca. 25 percent of the mercapto-based resin LED 2 that results in a high degree of reactivity.
FIGURE 7. Showing severe scratch damage to the “control” formulation when cured using LED/UV under air due to insufficient cure after 8 passes through 385nm and 365nm LED arrays 34 | UV+EB Technology • Issue 2, 2017
The formulation with the least amount of C=C double-bond conversion change was Formulation F, which went from 58 percent conversion in air to page 36 u uvebtechnology.com + radtech.org
If you are an academic or industrial participant in the field of photopolymerization, please plan to attend the premier scientific conference for the photopolymerization industry.
Photopolymerization Fundamentals 2017
Abstract Submission Opening: February 1, 2017 Early Registration Deadline: June 30, 2017 Topics will include: • • • • • • • • • • •
Highlights of the meeting include:
• Numerous scientific presentations on various photopolymerization topics • An open atmosphere where presentation of difficult, unexplained results is encouraged • A poster session and vendor exhibit • Reduced rates for students to promote interaction between industrial scientists and students • A short course consisting of tutorial or review lectures from leaders in the photopolymerization community
Oral and Poster Session presentation submissions are welcome from academic, industrial and student attendees. Student poster competition with cash prize sponsored by Polymer Chemistry (Royal Society & Chemistry)
Novel Concepts & Emerging Applications Radical and Cationic Polymerizations Thiol-Ene Polymerizations Oxygen Inhibition Polymerization Kinetics Novel Initiation Systems Hydrogels and Biomaterials Dental Materials Additive Manufacturing Composites, Smart and Responsive Networks Photo Responsive Materials
Speakers to date who plan to present:
Christopher Bowman, Univ of Colorado; Allan Guymon, Univ of Iowa; Jeffrey Stansbury, Univ of Colorado; Robert Mcleod, Univ of Colorado; Christopher Ellison, Univ of Minnesota; Roberta Bongiovanni, Politecnico di Torino; Hadley Sikes, MIT; Tim Scott, Univ of Michigan; Christopher Barner-Kowalik, Queensland Univ of Technology; Hansjörg Grutzmacher, ETH Zürich; Céline Croutxé-Barghorn, Univ de Haute Alsace; Xavier Allonas, Univ de Haute Alsace; Darryl Boyd,US Naval Research Laboratory; Marco Sangermano, Politecnico di Torino; Derek Patton, Univ of Southern Mississippi; Christopher Kloxin, Univ of Delaware; Brent Summerlin, Univ of Florida; Stuart Rowan, Univ of Chicago
Exhibitors to date:
Heraeus Noblelight, IGM Resins, Flacktek, Sartomer, Colorado Photopolymer Solutions This conference is presented by RadTech-The Association for UV & EB Technology-and Colorado Photopolymer Solutions. The conference chair is Professor Chris Bowman from the University of Colorado.
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THE ASSOCIATION FOR UV&EB TECHNOLOGY :• .: •
WOOD FLOORING t page 34
20 18 16 14 12 10 8 6 4 2 0
Initial b value Air Cure; iodine 1 min Nitrogen Cure; iodine 1 min
FIGURE 8. Effect of air vs. nitrogen cure on 1 minute iodine staining (Delta B) along with initial color *b value on beige substrate
120 100 80 60
IR conversion air
IR conversion nitrogen
Delta c=c change
FIGURE 9. Infrared degree of conversion for air vs. nitrogen LED cure
Break Elongation % of PVC Film and UV Cured Coated Film 140.0 120.0 100.0 80.0 60.0 40.0 20.0 0.0 PVC film
UV Coated PVC film
FIGURE 10. Interval plot showing break elongation percent for the UV cured composite coating/PVC at 37% and the neat UV treated PVC film at 125% 36 | UV+EB Technology â€˘ Issue 2, 2017
61 percent conversion in nitrogen. This formulation contained the 3- ketocumarin photoinitiator LFC3644 at 5 percent wt. In contrast, Formulation E contained the LED phosphine oxide TPO-L photoinitator blend identified by IGM as Omnirad BL723 that was found to give a higher conversion of 69 percent in air and 87 percent in nitrogen. These differences in cure are attributed to the combination of benzophenone derivatives for surface cure and the phosphine oxide-based photoinitiator TPO-L mixture vs. LFC3644.7 In comparing the degree of doublebond conversion for the series A, B and C, no trend of increasing double-bond conversion with increasing DBEW was observed for the range 60 to 76. The range of double-bond conversion was 58 to 66 percent for LED air cure and 78 to 86 percent for LED nitrogen cure. The expected outcome would have been a higher double-bond conversion for Formulation C, with the lowest DBEW, due to less of an effect of diffusion limitations during acrylate polymerization.
In the series containing different LED initiators D, E and F, the highest double-bond conversion was observed for formulation E, containing the phosphine oxidebased photoinitiator Omirad BL724, with 69 percent conversion in air and 87 percent conversion in nitrogen atmosphere. An improvement in double-bond conversion was noted in the series G and H, where the mercapto-based resin LED2 gave a higher double-bond conversion than the highviscosity urethane acrylate EB8807 when cured in air or nitrogen atmosphere. In this instance, the mercapto-based resin had more impact on doublebond conversion than the high-viscosity UA. It was expected that the high-viscosity UA would have the potential to slow oxygen diffusion into the coating by increasing the overall viscosity of the formulation. The delta C=C change for air vs. nitrogen shows that the worst is the UV Base C1 formulation, followed by D (TPO/ITX), C (DBEW 76) and E (phosphine oxide-based photoinitiator). uvebtechnology.com + radtech.org
Bar graphs for break elongation percent for each LED coating series studied are illustrated in Figure 11. The base coating/PVC laminate that was UV-cured was found to have a break elongation three to four times greater to that of LED-cured coatings. The elongation at break for the UV control was 37 percent versus 11 percent or less for all other LED-cured coatings tested. These differences are attributed to the overall total UVA and UVV energy density and peak irradiance by the two different cure methods.
Air Break elongation % Nitrogen Break elongation %
9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 A
Both the energy density and peak irradiance are dramatically FIGURE 11. Interval plot showing break elongation percent for LED series cured in higher for LED-cured coatings air vs. nitrogen vs. conventional UV-cured films. The total energy density for LED/ UV cured films is UVA, 13.6J/cm2, vs. 0.9J/ cm2 for the UV-cured film. UV LEDs have been reported to transfer 15 to 25 percent of the received electrical energy into light, with the remaining 75 to 85 percent transferred as heat.4 Even though the DBEW for the UV-cured coating is 60 vs. 62 to 76 for higher functionality LED coatings, the magnitude of difference is not expected.
FIGURE 12. Strain sweep for PVC film, UV cured, LED formulation B, and LED formulation G measured at 100°C and 0.1Hz Mechanical Properties of LED-Cured Coatings on RVF: Instron Studies To gain an understanding regarding the cross-link density of the acrylatecured coatings, nominally 0.6 mil films were prepared on a 2.6 mil rigid polyvinylchloride (PVC) carrier film and evaluated by mechanical tests. Percent elongation at break was determined at the point of breakage of the film composite structure. The function of the PVC film carrier is to provide a backing to allow for relative comparisons to be made between coating formulations that would otherwise be too brittle to test by using an Instron. All coatings were applied in machine direction of the film. The stress strain curve for the carrier PVC film exposed to UV process shows the yield elongation at 5.5 percent and break elongation of 125 percent. Test results for the UV-cured “control” coating on the RVF show a dramatic elongation at break to 37 percent, thus showing the influence of the coating on mechanical properties (Figure 10). uvebtechnology.com + radtech.org
In comparing the effect of DBEW on break elongation in air, formulation B with a DBEW of 68 was found to have an elongation of 7.9 percent, which is similar to formulation C, with a DBEW of 76 and elongation at break of 6.9 percent. Comparison of PVC film composites composed of LED formulations E vs. F, with two different photoinitiators at five percent wt., had no effect on percent elongation at break. Similarly, virtually no effect on break elongation was observed utilizing a mercapto-based acrylic resin LED2 formulation G vs. the high MW aliphatic urethane acrylate in formulation H. Both formulations cured in air gave six to seven percent break elongation. In comparing break elongation of LED coatings cured in nitrogen atmosphere for the DBEW series A, B and C, the expected decrease in elongation is observed. The elongation decreased from 6.6 percent to 5.2 percent as the DBEW increased from 59 for A to 66 for B and 76 for C.
page 38 u UV+EB Technology • Issue 2, 2017 | 37
Tg Deg C
t page 37
Air Cure Tg (deg F) Nitrogen Cure Tg (Deg F) DBEW/10000g
FIGURE 13. Effect of LED cure in air vs. nitrogen on glass transistion temp (Tg)
Air Cure Tg (deg F)
Nitrogen Cure Tg (deg F)
TABLE 8. Effect to DBEW on Tg (deg C) inflection for coatings cured in air vs. nitrogen
Plot of DBEW Vs. Tg For Air & Nitrogen Cure 80 75 70 65 60 55 50 45 40 35 30
Air Cure Tg (deg F) Nitrogen Cure Tg (Deg F)
FIGURE 14. Plot of double-bond equivalence vs. Tg (deg C) for air vs. nitrogen cure
38 | UV+EB Technology â€˘ Issue 2, 2017
Dynamic Mechanical Analysis of LED-Cured Formulations An approach to estimate the degree of cross-linking utilized a TA Instruments Model Q-800 Dynamic Mechanical Analyzer (DMA) with tension film clamp. Nominally 0.6mil coatings were prepared on a 2.6mil PVC carrier film and cured by UV and LED in air and nitrogen atmosphere. A strain sweep at 100Â°C to determine the degree of cross-link density of various LED formulations is illustrated in Figure 12. As expected, the carrier PVC film displayed the lowest storage moduli at 6.46MPa in comparison to the UV-cured control/PVC laminate, with a storage moduli of 18.1MPa. Increasing the DBEW from 67 to 76 for the LED-cured formulations B and G resulted in an increase in storage modulus from 23.3MPa to 27.5MPa for the laminate films as a result of higher functionality and crosslinking. Effect of DBEW on Glass Transition Temperatures for Coatings cured in Air vs. Nitrogen Figure 13 and Table 8 summarize the effect of LED cure in air vs. nitrogen atmosphere on glass transition temperature (Tg) for various DBEW formulations. The expected trend of higher Tg for coatings cured in nitrogen vs. air was not observed for all coatings. Formulation A, with a DBEW of 59, shows a slight drop in Tg on going from air to nitrogen cure, whereas formulation D, with a DBEW of 76, shows the largest increase in Tg on going from air to nitrogen. For formulation C, DBEW 68, the Tg remained about the same, regardless of air vs. nitrogen cure. An attempt was made to correlate the glass transition temperature of each coating with the DBEW. The plot of DBEW vs. Tg in Figure 14 shows, for the most part, the expected trend is observed. As the uvebtechnology.com + radtech.org
FIGURE 15. Photo of Formulation A on solid wood coating structure showing good Gardner scratch test results
DBEW increases within the formulation, the glass transition temperature also increases. LED Topcoat on Wood Based on overall properties, double-bond conversion in air and low DBEW, formulation A was applied onto wood substrate that had the coating stack layers applied up to the topcoat layer and cured by UV LED in air. Gardner scratch results show little to no damage after 30 cycles using the BYK Gardner machine, indicating a hard surface was formed (Figure 15). Abrasion testing using the ASTM method D4060 for Taber abrasion showed final wear-through at 1058 cycles vs. the UV control at 1105 cycles, indicating no significant changes are observed due to the LED/UV topcoat. Conclusions UV LED formulations with DBEW ranging from 60 through 76 containing high-viscosity urethane acrylates, mercapto-modified resin and photoinitiators specific for 365 nm and 385 nm LED spectral outputs can be used to give good surface properties for abrasion resistance and stain resistance for flooring applications – as long as initial yellowing is taken into consideration. Yellowing was found to be more prevalent using the 3-ketocumarin photoinitiator based on five percent wt used in formulations presented in this paper. Good scuff-, Gardner scratch- and iodine stain-resistance can be achieved using a combination of 365 nm and 385 nm LED arrays under an atmosphere of nitrogen. Curing in nitrogen atmosphere resulted in an increase in double-bond conversion and improvement in stain performance. The highest reactivity as determined by double-bond conversion was achieved using a combination of ITX and TPO photoinitiator followed by the phosphine oxide-based photoinitiator BL723. Mechanical properties of laminate films prepared and cured by 365 nm and 385 nm LED arrays could be related to DBEW for percent elongation at break and storage modulus. The large gap in percent elongation at break for the UV control, 37 percent, vs. LED-cured coatings, 5 to 11 percent, is attributed to the significantly higher amount of energy density and peak irradiance used for this LED/ UV study. u
Acknowledgements This work was supported by Dr. Brian Beakler as one of the innovation platforms to investigate UV/EB radiation cure technologies, as well as input from Dr. Brett Diehl. The formulation work and processing were carried out by the coatings lab group: Dan Baker, Rebecca Winey, Lauren Miller, Brenda Erisman and Jose Rodriquez. All analytical studies were carried out by the Testing and Analytical Group: Kayla Lowrie, Kerry Eckman, Susan Gramlich and Tiffany Hilliker. References 1. J. S. Ross, L. W. Leininger, G. A. Sigel, and D. Tian. “A Brief Review of Radiation Cure Systems Used in Flooring,” RadTech Conference Proceedings, 241 – 250; Baltimore, MD, April 9-12, 2000. 2. “Armstrong Wood Coatings Quality Journey,” J. S. Ross and G. A. Sigel; RadTech Report, May/June2006, pgs. 39-47. 3. Charles Nason, M. Cole, C. E. Hoyle, Sonny Johnson, Fusion Systems, Inc. “Photocuring of Hard Thiol-Enes,” Rad Tech 2002 (Scheme used to show oxygen reaction) 4. Jennings, Sara, “UV-LED Curing Systems: Not Created Equal,’ Presented at 2016 Rad Tech International). 5. Jo Ann Arceneaux, “Mitigation of Oxygen Inhibition in UV-LED, UVA and Low Intensity UV Cure”, UV + EB Technology, Vol. 1 No. 3, pp48-56. 6. E. V. Sitzmann. “Critical Photoinitiators for UV-LED Curing: Enabling 3D Printing, Inks and Coatings,” Proceedings of Radtech UV.EB West 2015, Redondo Beach, CA, March 10, 2015. 7. A. Freddi, M. Morone, G. Norcini. “Design of New 3-ketocoumarins for UV/LED Curing,” UV+EB Technology, Vol 2, No 3, pp 46-51.
Gary A. Sigel holds a B.S. in chemistry/biology from Western Washington University and a Ph.D. in inorganic chemistry from the University of California, Davis. Post doctoral research on preceramic polymers to AlN/SiC solid solutions was conducted at Rensselaer Polytechnic Institute. He is a senior principle scientist with Armstrong Flooring and has 28 years of UV/EB coating and adhesive formulation experience with Armstrong, including six years as a UV coatings specialist in the company’s wood division. He currently is responsible for the development and deployment of new UV coating systems/processes for residential and commercial segments of the flooring business. For more information, email email@example.com.
* Additional tables and figures, including more text detail on UV LED characteristics, can be found online at www.uvebtechnology. com.
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UV+EB Technology • Issue 2, 2017 | 39
uv.eb WEST 2017 Celebrates Record Attendance With Focus On Emerging Technologies and Materials W
ith several standing-room-only conference presentations on 3D printing materials, UV LED, UV technology for touch displays, food packaging, disinfection and UV inkjet, uv.eb WEST 2017 Materials + Manufacturing Summit was a definite success. Held February 27 through March 1 at the Embassy Suites San Francisco Airport Waterfront in San Francisco, California, the event marks the largest such gathering ever in Northern California, with more than 300 attendees and 50 exhibitors. Formlabs’ Maximilian Zieringer helped keynote the 3D printing session with a presentation on “Material Science: Advancing the Future of Digital Manufacturing and 3D Printing,” while Jake Hundley, HRL Laboratories, discussed “Process-Microstructure Relationships in Additively Manufactured Photopolymer-Derived Ceramics.”
Act.” A session on Next-Generation UV Inkjet Technology included talks on new materials and enabling low migration, with a special presentation by Matt Hirsch of Lumii, an MIT start-up company, on “Lumii Light Field 3D Prints: A New Dimension for UV Printers.” A special session on UV Materials for Displays: Touch + Beyond included a review of the soaring automotive touch display market, as well as new materials and processes for fabrication. The event wrapped up with a series of presentations on the application, materials, equipment and measurement advancements that are enabling the fast-emerging implementation of UV LEDs. The event was presented by RadTech, the nonprofit trade association dedicated to the advancement of UV/EB technologies. u
The role that UV (ultraviolet) and EB (electron beam) technologies are playing in food packaging and disinfection was presented in depth, with Connie Williams, Mars Chocolate NA, offering a review of “Mars and the Food Safety Modernization
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UV+EB Technology • Issue 2, 2017 | 41
Sartomer Offers Free Technical Booklet A free technical booklet from Sartomer Americas, Exton, Pennsylvania, a business unit of Arkema Inc., contains six articles by industry experts to help compounders enhance their coatings formulations to advance performance in a wide variety of applications. “Coatings and Concepts” contains new information on the economics of UV curing; radiation-curable components and their use in hard, scratchresistant coating applications; influence of various matting agents on abrasion-resistant UV-cured coatings; new UV PUDs enhancing performance of waterborne UV-curable coatings; and more. For more information, visit www.sartomer.com or www. arkema.com. ICP Announces Acquisition Innovative Chemical Products (the ICP Group), Andover, Massachusetts, announced the acquisition of MinusNine Technologies, a formulator and manufacturer of UV/EB coatings, adhesives, primers and specialty products for graphic arts and industrial applications. MinusNine, headquartered in Birdsboro, Pennsylvania, will be integrated into ICP Industrial, Inc., one of the three divisions of the ICP Group. ICP Industrial currently manufactures Nicoat® coatings and adhesives and, with the successful completion of the acquisition of MinusNine, is poised to provide even broader offerings to the packaging, labeling, graphic arts and industrial markets. For more information, visit www.icpgroup.com. ebeam Technologies Launches R&D Center ebeam Technologies, Flamatt, Switzerland, launched its first research and development center in Yokohama, Japan. Known as an atelier, the innovation center is the latest in a growing list of ebeam Technologies’ ateliers and pilot lines around the world, which are part of the company’s Access ebeam program. The atelier is fully equipped with an EB laboratory and offers a team of experts who are on hand to aid in the exploration of new possibilities in the field of print and packaging, among other industries. For more information, visit www.ebeamtechnologies. com. ISO Celebrates 70 years The International Organization for Standardization (ISO) tuned 70 this year. In 1946, delegates from 25 countries gathered in London to discuss standardization and a year later, ISO became official. In 1947, the purpose of the fledgling organization was to facilitate the coordination and unification of standards developed by its member bodies, all of which were national standardization 42 | UV+EB Technology • Issue 2, 2017
entities in their respective countries. The founders decided that the organization would be open to every country wanting to collaborate – with equal rights and equal duties. These founding principles still hold true today and the ISO family has blossomed to include 163 members (as of December 2016) from almost every country in the world. Standardization has come a long way, and ISO International Standards – which now cover almost all aspects of technology and business – will continue to ensure positive change in an evolving world. For more information, visit www.iso.org. Sun Chemical Announces Acquisition Sun Chemical Corporation, Parsippany, New Jersey, has acquired the assets and business of RJA Dispersions LLC, effective March 1, 2017. Based in Hudson, Wisconsin, RJA Dispersions specializes in ultra-fine particle and pigment dispersions for the digital inks market. Primarily used for energy cure (UV), ecosolvent and aqueous inkjet inks, RJA’s full range of dispersions will join Sun Chemical Performance Pigments’ product lineup. RJA Dispersions will become part of Sun Chemical’s Performance Pigments’ digital business unit, led by Peter O’Loughlin. As the director of digital, O’Loughlin will lead the RJA Dispersions integration and be responsible for the global growth of the combined product portfolio of the unit. For more information, visit www.sunchemical.com. New York Vehicle Composites Program Completes X-Ray Cured Carbon Fiber Chassis The New York State Vehicle Composites Program extended its work on X-ray curing of composites to cure a structural automotive component, a chassis, using X-rays derived from high-current industrial accelerators. In curing a non-structural component, performance vehicle hoods, a full-scale commercial X-ray curing facility would demand 40 percent less input power per unit than conventional thermal curing, as in using fully loaded commercial autoclaves. With X-rays, curing would take place in less than a minute using shelf-stable matrix materials. Hoods were made using an X-ray-curable pre-preg and commercial thermosetting pre-pregs and power demand determinations were made. X-rays can penetrate metals embedded within plies and bond to them without heat. Non-thermal ultraviolet was used to cure a pigmented coating onto the vehicle hoods. In its initial work, non-thermal X-ray curing produced vehicle fenders with Class A finishes and no heat distortion. For more information, contact Tony Berejka at firstname.lastname@example.org. TheIJC 2017 Announces First Program Details The world’s biggest inkjet event, TheIJC (The Inkjet Conference), will take place October 24 and 25, 2017, in Swissôtel Düsseldorf, Germany. Entering its fourth edition, TheIJC is organized by ESMA with an ongoing support of drupa and sponsorship of MS Printing. For two days, OEMs, brands, engineers, chemists, researchers and all technology users gather together with suppliers – printhead manufacturers, ink producers, software and hardware designers. TheIJC offers 25 hours of technical talks that address uvebtechnology.com + radtech.org
topics ranging from printheads and ink improvements to inkjet in packaging, textiles and pharmaceuticals. Speakers who have confirmed their attendance so far are all acknowledged experts in their fields. TheIJC 2017 will feature 50 presentations, an exhibition of 70 tabletops and an expected total of more than 400 attendees. For more information, visit www.theijc.com. SONGWON Opens Technology Innovation Center SONGWON Industrial Co., Ltd., Ulsan, South Korea, announced it has opened a new Technology Innovation Center, enabling SONGWON to enter new business areas with high value and sophisticated technology. The center encompasses 32,000 square meters and is currently three stories high. However, the facility has been especially designed with future expansion in mind and is constructed to allow the company to add two more floors later, as required. The new facility will host R&D, Global Application Community and Technical Service. It will enable the organization to leverage the synergies among the groups, consolidate SONGWON’s technologies and support the complete development of new products from the design phase to final customer applications. The center will include analysis labs, synthetic rooms, clean rooms, kilos lab scale rooms, polymer processing and application labs. For more information, visit www. songwon.com. u
NEW FACES Wikoff Color, Fort Mill, South Carolina, has elected Theodore E. Lapres III to its board of directors. Lapres joins Harvey Lowd, Pat Burns, Veda Clark, Karl Warnke, Phil Lambert and Geoff Peters as the seventh member of the board. Lapres served as the president, CEO and director of Nypro, Inc., from July 2006 until his retirement in 2013. Lapres George Vance has joined the company as director of manufacturing. Most recently, he was director of operations for a large-scale manufacturing company where he oversaw the silicone injection molding production.
Miltec UV, Stevensville, Maryland, announced the promotion of Herbert Freyre to manufacturing engineering manager. He has years of experience in several industries, including aerospace and oil and gas. Freyre has experience with cost reduction, continuous improvement, training of staff, quality systems, and equipment maintenance and reliability.
IGM Resins USA, Inc., Charlotte, North Carolina, has appointed Tony Pirro to the new position of sales director uvebtechnology.com + radtech.org
Print + Digital Don’t miss this chance to stay up-to-date on new technologies and learn about the latest developments in the industry.
Subscribe or manage your subscription at www.uvebtech.com
for North America. He will be responsible for driving new business development through IGM’s direct sales team and distribution partners in the US, Canada and Mexico. Scott Burns Pirro Burns has been added to IGM Resins’ commercial team as sales manager for the Northeast US. Most recently, Burns was the commercial market development manager in the engineered surfaces business of Omnova Solutions. Sartomer Americas, a business unit of Arkema Group, has appointed René Neron and Nick Ferraco as plant managers in West Chester, Pennsylvania, and Chatham, Virginia, respectively. They will oversee all production operations and activities of Sartomer’s high-performance specialty chemicals. Neron began working with Arkema in 2015 as the plant manager after 25 years in the manufacturing industry, during which he earned certifications of Six Sigma Black Belt and Lean Leader. Ferraco got his start with Arkema in the developing engineer program in 2005 and, after leaving in 2007, returned to Arkema in 2013 as the operations manager for the Chatham plant. u
UV+EB Technology • Issue 2, 2017 | 43
MEASUREMENT By Jim Raymont, Joe May, Mark Lawrence and Paul Mills, EIT Instrument Markets
Total Measured Optic Response: A New Approach to UV LED Measurement The Fundamental Paradox of Measurement easurement is important to set process parameters and to identify if the process is running within those parameters. When things change in the process, measurement provides valuable clues that help locate the source of the problem(s). Without measurement, we cannot optimize the process for greatest profit and efficiency. Measurement allows communication with precision – within a facility or between customers and their supply chain partners.
However, the act of measurement frequently presents its own problems. For instance, suppose you wish to measure the air pressure in your car’s spare tire (Figure 1). How can you measure the pressure within the tire without changing the pressure itself? Opening the tire’s valve opens a can of worms that form the fundamental paradox of measurement. Scientists call the problem of changing something’s properties through the act of measuring it the observer effect. The engineer’s challenge is to devise an instrument that causes the least distortion of whatever we intend to measure. The Challenges of Measuring UV Measuring the output of a UV LED source presents similar challenges. To measure UV energy, we must first capture the source’s output and then transform the “optical” energy into an electronic signal with a value that can be stored and displayed. Figure 2 illustrates a simplified, generic chain of components needed to perform this task. In this example, UV energy is collected by the optical components (or optical stack) of the radiometer. Usually, a high-quality optical window is used to the condition the incoming energy and protect the sensitive components beneath. To ensure that all the energy from the source is captured and presented evenly, the UV passes through a series of diffusers and apertures. Since the detector also is prone to measure wavelengths other than UV, such as visible or infrared, this unwanted energy is removed using filters. Conditioned (uniform and filtered) UV energy is presented to the surface of a sensitive electronic photodiode detector. The photodiode converts the UV energy into an electronic potential with a magnitude that depends on the UV energy intensity. This signal is processed, stored and displayed. But, the accuracy of the UV measurement depends on the spectral response of each of the components that come between the original signal and the display. And, the radiometer designer must engineer the system to be affordable, easy to use and robust FIGURE 1. Even for a measuring something as simple as the to the harsh conditions frequently air in a car tire, the range of instrument choices depends on the encountered in industrial UV desired cost and accuracy curing processes. 44 | UV+EB Technology • Issue 2, 2017
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Band Name Identifier
Approximate Wavelength Range (nm)
TABLE 1. The common UV band designations developed for measuring traditional mercury-based UV light sources
FIGURE 2. A generic “optic stack’”for a UV radiometer Over several decades of development, UV radiometers have evolved to meet these technical and commercial objectives. Instruments are available to measure the output of a wide FIGURE 3. To improve range of conventional UV accuracy, a multiband sources. Some devices try radiometer divides a broad UV to accomplish this using spectrum into smaller, more a single set of optical and uniform bands electronic components to capture the entire (100nm to 400nm) range of UV wavelengths. However, the spectral response of the components across such a large spectrum may not always be “flat.” This could cause errors in measurement. An alternative approach uses separate optical and electronic devices that each measure smaller portions of the broad spectrum. With broad band sources, measuring in different spectral areas allows the user to better understand the efficiency of the UV source. Concerned about adhesion or penetration through opaque coatings? Measuring the longer wavelengths (UVA, UVV) may be useful. If you are concerned about surface cure properties, measuring the short wave UVC or UVB will be more important. Comparing the values in multiple UV bands allows users to identify bulb types, spectral changes in the bulb and when maintenance is required. For example, by comparing readings, a user can detect short wave UV values dropping while long wave UV values remain the same. With a little experience, multiband radiometers also allow users to confirm the correct bulb type has been installed in a lamp housing.
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As a bonus, the optics used in narrower spectral “bands” generally are more uniform. Figure 3 illustrates data gathered using this multi-band approach from a radiometer designed to measure UV output that has been segmented into the four distinct bands shown in Table 1. These designations were originally derived by CIE, the International Commission on Illumination, based on the observed health effects of various wavelengths. In practice, radiometer designers may use different definitions of these bands in their own instrument design that correspond more closely to the spectral output of commercial UV sources. This approach is popular because the engineer can select devices with a predictable, repeatable, uniform spectral response within each band. This method solves the problem of trying to design a “one size fits all” solution that requires a set of components that must perform over a wide spectrum. This solution has served the industry well for decades of UV curing by measuring UV energy coming from arc and microwave UV sources. LED Light Sources are Different In the last ten years, the landscape for UV curing has been changing as new UV LED sources have been adopted for commercial applications. The output of a UV LED source has substantially different spectral characteristics than that of broad band mercury-based lamps. Figure 4 compares the output of a traditional mercury light source with the output of a UV LED array. While the output of mercury lamps appears as a continuous spectrum with many sharp peaks, UV LEDs produce a single peak, centered at a specific wavelength. Today, although development is occurring at other wavelengths, most UV LEDs used for curing have output ranging from 365nm to around 405nm. Industrial UV LED sources are not a single tiny UV LED diode, but are arrays that contain hundreds – or thousands – of individual diodes (or dies) arranged in geometric columns and rows to provide uniform irradiance over the cure surface. To assemble these arrays, the UV LED source manufacturer typically purchases individual diodes in large quantities. These are specified to have a given desired output wavelength. For example, page 46 u UV+EB Technology • Issue 2, 2017 | 45
MEASUREMENT t page 40
FIGURE 4. Comparison of the continuous broadband spectral output of a conventional mercury UV source and the sharp narrow-spectral output of UV LED sources to build a commercial 6" x 6" 395 nm UV array the manufacturer must purchase and mount hundreds of individual 395 nm diodes to the array. In reality, however, the output of any of these individual diodes may differ from another one depending on how precisely the semiconductor chemistry and manufacturing is controlled. In practice, UV LED diodes are created from a complex sandwich of semiconductors and other materials layered together in a delicate architecture whose FIGURE 5. Almost all of the exact size and geometry energy from a UV LED falls can vary the spectral output within a narrow bandwidth when the wafer is sliced of the central wavelength into tiny diodes. Thus, a 395 nm array also will contain diodes with a central peak at 390 nm, 400 nm or any of a wide range of other possible wavelengths. Diode suppliers often price their components by how tight the tolerance of individual diodes in a batch may be (through a process known as “binning”). The tighter the binning specification, the higher the price, since the diode manufacturer must reject more non-conforming diodes to meet the guaranteed tolerance. A “395 nm” source is sold with the Center Wave Length (CWL) typically specified as +/- 5 nm. In a competitive market, low-cost, low-quality manufacturers may be tempted to “loosen up” binning standards and assemble arrays
FIGURE 6. In practice, the output of UV LEDs described as “395 nm” diodes may vary, depending on the supplier’s quality and screening (or binning) procedures made of a wide range of individual diodes. They may still refer to these diodes as though they are a single wavelength, but the measured output of a 395 nm array could vary considerably from the expected CWL of +/- 5 nm. This variation in diode wavelengths presents an additional concern for UV radiometer design, since the spectral response must be kept flat over a wider range of wavelengths. As shown in Figure 5 a diode with output at a central wavelength, say 395 nm, actually emits energy over a substantially wider range. With “good” binning, the variation in the CWL from 390 to 400 nm on a 395 nm source, the output of the energy may be as low as 370 nm and as high as 425 nm (Figure 6). The effects of poor LED binning may have the energy even further outside of the bandwidth shown. The bands proposed in Table 2 incorporate and capture almost all but a very tiny percentage of the energy from different LEDs. Keeping the bands narrow has several optics advantages, as
TABLE 2. The new “L-band” designations are based on the observed variation of a UV LED’s central wavelength page 48 u
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MEASUREMENT t page 46
FIGURE 7. Total Measured Optic Response considers the influence that each device in the radiometer imposes on the overall instrument’s response described in the next section. It also would let end users know when a claimed “395 nm” LED is closer to having energy output closer to 405 nm or more. Total Measured Optic Response: A Different Approach What if a radiometer design incorporated all the optical optic components shown in Figure 3 in the instrument response instead of just the filter? This would require incorporating all components in the optic stack in the instrument response.
FIGURE 8. The result of the Total Measured Optic Response approach: A flat instrument response across the L-band’s entire spectrum.
Working Distance (mm) 5 10 15 20 25
Primary Standard Integrating Sphere (W/cm2) 9.01 7.74 6.66 5.74 5.04
LEDCure L395 (W/cm2)
9.23 7.74 6.63 5.83 5.08
2.4% 0.0% -0.5% 1.6% 0.8%
TABLE 3. Test results show good instrument performance measuring to primary, traceable standards under various process conditions. Data courtesy Excelitas Lumen Dynamics.
48 | UV+EB Technology • Issue 2, 2017
A Total Measured Optic Response requires that the overall response of the instrument take into account the contribution of each component (e.g., the optical window, diffuser, aperture, spacers, filters and detector), each of which has its own response characteristics, as shown in Figure 7. This approach results in a flat overall instrument response. While this integrated approach seems conceptually straightforward, until now radiometer design did not consider the Total Measured Optic Response of the sum of these individual components. One reason is that the output spectra of conventional mercury lamps is very predictable, due to the natural structure of the mercury atom. Since all mercury atoms are identical to all others, there is no need for “binning” atoms used in mercury lamps. Second, as shown in Figure 4, the mercury spectra is very broad, and the consequences of cutting off a few nanometers of output are not as great as with the sharp spectral output that characterizes the output of LED light sources. As can be seen in Figure 6, a filter that cuts off at 410 nm instead of 405 nm would have substantial impact on the measured UV output, since much of the energy would be cut off. To solve this problem, a new approach considers the Total Measured Optic Response of the instrument. To assure uniformity within the range that commercial LEDs may vary due to differences from diode to diode in the array, the commercial UV LED spectrum is segmented into a series of LED or “L” bands. Each L band is constructed by considering a central peak wavelength that might vary by ±5nm. Thus, each band is 50 nm wide, as shown in Table 2. Thus, an L395 radiometer is equipped page 50 u uvebtechnology.com + radtech.org
MEASUREMENT t page 48 UV energy, indicating that L395 spectral response has a sharp response in the L-395 band. The radiometer also demonstrated consistent peak irradiance and energy density measurements at various scan speeds varying from 1.2 to 6.0 meters/min. Finally, the unit had strong correlation with a NIST traceable meter from another manufacturer. These results have been replicated by others. 2
FIGURE 9. Total Measured Optic Response provides exceptional repeatability in run-to-run or instrument-toinstrument measurement comparisons to measure diodes whose center wavelength vary between 390 nm and 400 nm using filters that capture all the energy emitted between 370 nm and 422 nm at the 50 percent power responsivity point. The widths of the L bands were chosen to balance flatness with performance at an affordable cost. Figure 8 shows the overall measured response achieved for the popular L395 band. Notice that the achieved response is exceptionally uniform over the desired region. The Results By considering the total optical response, not only are readings highly accurate within each designated L-band, but there is vast improvement in the correspondence of measurements made from instrument to instrument. This means that a process measured in the lab with one instrument may be replicated in the field with little error. Figure 9 illustrates the accuracy and reliability of two production radiometers1. The pair of radiometers made 20 successive laboratory measurements of a single 395 nm UV LED array. The variation in absolute UV output from run to run, which averages substantially less than one percent, may even be due in part to small intertemporal fluctuations in the source itself. The difference between instruments, approximately 0.2 percent overall, is extremely narrow and significantly better than results with traditional, filter-only designs. Table 3 illustrates how closely absolute measurements of a 395 nm LED array match a national, traceable, primary standard at various working distances (data courtesy Excelitas Lumen Dynamics.)
This robust evidence points to the success of re-engineering the radiometer to meet the reality of man-made semiconductor light sources. Although LED proponents are quick to point to the longevity and stability of the arrays, irradiance, until now the effects of diode variation have been somewhat ignored, though every array is a mixture of diodes with different spectral emission. By creating L-bands across which the instrument response may be very flat, and by considering the individual contributions of each optical and electronic component on the radiometer’s overall response, a new generation of LED radiometers has been designed to help characterize the UV curing processes of the future. u References 1. EIT LEDCure™ L395 radiometers were utilized. 2. Integration Technology Ltd has tested the EIT LEDCure L395 and found it produces consistent readings on a variety of sources, with the profiler function being an extremely useful feature for research and product development.
Jim Raymont is director of sales for EIT LLC. He joined EIT in 1993 and has been involved with the worldwide sales and marketing of EIT’s UV products since 1996. Raymont also is involved in product development and co-holds four patents on UV measurement. Paul Mills is an industry consultant in the paint and coatings field with years of expertise in industrial applications for UV curing. He works closely with EIT to help foster an understanding of the important role of UV measurement in commercial process development and quality control. For more information, contact Raymont at email@example.com or visit www. eit.com.
The performance benefits of this new UV LED radiometer design are already being proven in the field. For example, Phoseon Technology found good spectral response in testing done using 385 nm, 395 nm and 405 nm light sources with an L-395 band instrument. The lamps were calibrated using a third-party meter with a known (measured) spectral response. When exposed to a 365 nm UV LED lamp, the L-395 radiometer measured very little 50 | UV+EB Technology • Issue 2, 2017
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PHOTOLITHOGRAPHY By H. Bilinsky, CEO, MicroTau Pty Ltd.
Microfabrication of Riblets for Drag Reduction: A Novel UV Approach Utilizing Photolithographic Methods I. Introduction
lthough drag-reducing riblet microstructures have been researched for over two decades and reliably demonstrate up to 10 percent reduction in skin friction1 attempts to apply them to aircraft as a fuel consumption reduction measure have not yet been fully successful. An economically viable implementation has so far been prevented by cost of application, maintenance and a lack of durability.2 The current paper proposes a novel method of microfabrication to overcome these issues. Drawing on photolithographic methods currently used in computer chip fabrication, the method directly “prints” riblets or other repeating microstructures onto an external aircraft surface. As a continuous and contactless method, the process is scalable to large areas, allowing reductions in time and cost of application.
The contactless microfabrication method draws on several fields of knowledge including functional microstructures, photolithographic computer chip fabrication technology, UV-curable coatings and classical optics.3 Microstructures Nomenclature h = riblet height s = riblet spacing t = riblet thickness y = dimensional wall units y+ = non-dimensional wall units Engineered microstructures such as riblet and lotus leaf structures hold great promise to reduce aircraft skin friction and provide self-cleaning properties to maintain hydraulically smooth surfaces, respectively. These microstructures have yet to be successfully implemented on aircraft due to problems with cost, maintenance, durability and application. FIGURE 1. From top: sawtooth, scalloped and blade riblet geometries uvebtechnology.com + radtech.org
Riblets are small surface protrusions aligned with the direction of flow and spacing in the order of 50 to 150 μm.4 These microstructures have been studied for over two decades and have page 52 u UV+EB Technology • Issue 2, 2017 | 51
PHOTOLITHOGRAPHY t page 51
FIGURE 2. Uncured photopolymer thickness vs. exposure time
FIGURE 3. Theoretical bottom-up curing relationship between exposure/irradiance and cured height due to O2 inhibition
reliably demonstrated turbulent flow skin friction reduction of up to 10 percent.1 Blade, sawtooth and scalloped are the most studied riblet geometries (see Figure 1). Their required dimensions are given in terms of non-dimensional wall units (y+). Optimal dimensions are spacing of s = 15±2y+; height h = 0.5-1s and thickness t = 0.02-0.04s.1 Dimensional wall units y change according to the fluid mechanics of the environment. For the wind tunnel testing, parameters used in this project optimal riblet spacing is s = 117μm. Final riblets for an aircraft at speed Mach 0.8 and altitude 30kft will have spacing s = ~50μm and increase in size going aft from the leading edge.5
Photolithography utilizes a class of photocurable materials known photopolymers, or “negative photoresists” that consist of monomers, oligomers and a photoinitiator. When exposed to UV, the photoinitiator catalyzes a polymerization reaction, “curing” the monomers and oligomers into a strong network polymer.
Appliqué riblet films developed by the 3M Company demonstrated a two percent drag reduction in flight tests however failed to prove economically viable due to significant application time and cost requiring specialized workers;2 issues with durability and maintaining drag reduction; and an inability to cover more than 70 percent of the aircraft without adversely impacting flight characteristics.6 The current microfabrication method aims to provide greater coverage, a more durable material and lower cost of application as well as riblet size optimization for drag reduction. The proposed method may also be used to fabricate lotus leaf structures that impart self-cleaning super-hydrophobic properties.7 These consist of regular 2D “hierarchical” microstructures that repeat their structure on different micro- or nano-scales.8 Lotus leaf structures have been suggested for anti-fouling and anti-icing applications to maintain hydraulically smooth aircraft surfaces.9 B. Photolithography The microfabrication method draws on a mature and commercially successful photolithographic technology from computer chip manufacturing that has achieved structural features orders of magnitude smaller than required for riblets. 52 | UV+EB Technology • Issue 2, 2017
Microstructures can thus be made by applying a thin layer of photopolymer to a substrate and exposing it to UV in the desired pattern. This is typically achieved by passing UV through a photomask and then removing the unexposed photopolymer. The Fraunhofer Institute has attempted to repurpose this photolithographic technology to print riblets directly onto aircraft with a continuous contact process.4 This involves a tool consisting of a flexible UV-transparent mask with the inverse of the desired riblet structures stenciled into it. This mask contains a UV source and rolls over the photopolymer-coated aircraft surface. The riblet structures are formed out of the photopolymer with the stencil pattern and cured whilst in contact with the mask. This method places requirements on the photopolymer material used10 due to mask contact at the point of exposure and can only apply structures with the one geometry as defined by the mask. This method has not been commercially implemented to date. By using a contactless optical system, the current method avoids issues with photomask contact and eliminates associated restrictions on the photopolymer material used. C. UV-curable Coatings Without the restrictions associated with a contact exposure process, an extensive range of photocurable materials may be selected from to meet aircraft coating requirements. UV-curable coatings based on the same photopolymer combination of monomers, oligomers and photoinitiators are commercially used as a quick-cure and low-VOC alternative to traditional dry-touvebtechnology.com + radtech.org
The optical system to produce the exposure irradiance profile is designed to be robust and for exposure at a distance to transition photolithographic technology from a cleanroom environment of computer chip manufacturing to a hangar for application to aircraft surfaces. cure coatings in wood flooring11 and automotive applications. Success in the automotive industry led to development of the coatings for commercial12 and military3 aircraft. The current project investigated whether such coatings could be used for microfabrication using the same selective exposing and developing method used in photolithographic computer chip manufacturing discussed above. An ideal candidate material for this method is a UV-curable coating developed by the AFRL aimed to reduce the 72-hour minimum “dry to fly” time to increase production throughput and reduce the environmental burden of coating aircraft.13 The Air Force UV-Curable Coatings Program developed a UVcurable coating for MIL-PRF-85285 specifications that has been successfully flown for 600 hours over 14 months on a C-130H.3 By applying said UV-curable coating to an aircraft and then patterning with the proposed optical system, riblet microstructures may be fabricated from a material that already meets strict military aircraft coating specifications. D. Oxygen Inhibition (Bottom-up Curing) Oxygen inhibition has been a longstanding problem for the photocurable coatings industry. When the photoinitiator in the coating is exposed to curing, it produces free radicals that catalyze a polymerization reaction, curing the photopolymer. However, as oxygen from the atmosphere diffuses into the coating it consumes these free radicals before they can begin the photopolymerization reaction, thus inhibiting curing14, which can cause uncured, liquid or tacky coating on the surface rather than a complete cure. The closer to the surface of the coating, the higher the oxygen concentration and the harder it is to cure. As a result UV-curable coatings tend to cure from the bottom up. In order to overcome this, and to cure the photopolymer despite the presence of oxygen within it, higher exposure to curing (either by increasing irradiance or exposure time) is used. This is due to the fact that with more UV, more free radicals are produced and eventually enough free radicals are present to successfully photopolymerize the coating despite oxygen inhibition.14 As the oxygen inhibition effect is more pronounced nearer to the surface (where oxygen concentration higher), greater irradiance is required to cure to that height. The current method proposes taking advantage of this phenomenon to be able to control the heights and profiles of riblet microuvebtechnology.com + radtech.org
structures in a single exposure. By designing an irradiance profile to expose and cure the coating, different regions can be made to cure to different heights. Therefore, a two-dimensional exposure pattern can be used to fabricate microstructures in three dimensions by varying the irradiance across said pattern. Previous research suggests this bottom-up curing effect should reflect Fick’s law of diffusion15 as the inhibition is dependent on the concentration of oxygen in the coating at any given point. This is exhibited in Figure 2 which shows the increasing exposure time required to cure the last (topmost) coating. As the current method uses photocurable coatings that are designed to have complete through-cure, one does not expect an asymptotic approach to complete surface cure, but still a similar exposure/height profile with increasing exposure or irradiance required to cure upper page 54 u 2015 Quarter 3 Vol. 1, No. 3
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PHOTOLITHOGRAPHY t page 53
FIGURE 4. Continuous riblet exposure regions is expected. The theoretical relationship between exposure or irradiance and cured height is shown in Figure 3, with a similar shape as the prior data. E. Optical System The optical system to produce the exposure irradiance profile is designed to be robust and for exposure at a distance to transition photolithographic technology from a cleanroom environment of computer chip manufacturing to a hangar for application to aircraft surfaces. The proposed method aims to achieve this by use of a diffraction grating. A diffraction grating is an opaque material with small apertures at regular intervals that allow light to pass through. By illuminating the grating with a single wavelength source, an interference pattern is produced of bright and dark fringes. Grating parameters can be designed such that the irradiance peaks match the spacing s of desired riblet microstructures and used to expose the photocurable coating even at large distances. Features of the pattern scale linearly with the distance of the diffraction grating to the exposed surface, akin to moving a projector closer or farther from a screen to shrink or grow the image (see Hecht16 for a detailed description of diffraction grating optics). The profile is then drawn out across the coated surface in a continuous exposure process that is suited to scaling for exposing large areas quickly.
FIGURE 5. Patterned UV-curable automotive paint sample B. Expose Photocurable Coating An optical system is kept at a predetermined distance from, and parallel to, the coated aircraft surface during exposure. The system projects a one- or two-dimensional interference pattern that is traversed perpendicular to said pattern to draw out the desired two-dimensional riblets. The system travels across the surface of the aircraft drawing out the riblet pattern in a continuous exposure (Figure 4). C. Develop The unexposed photocurable coating is then removed using an appropriate solvent or “developer.” The developer depends on the material used, e.g. mineral alcohol for unexposed UV-curable coatings.17 This may be assisted with some physical removal processes, e.g. spraying of the developer, compressed air or rubbing/wiping.
IV. Coating Investigation
Fabrication consists of three key steps: (1) application of the photocurable (PC) coating; (2) exposure in the desired pattern; and (3) developing (i.e. removing unexposed material).
A. Initial Investigation The goal of the initial investigation was to test the patterning process of selective exposing of a photocurable coating and then removing the unexposed with a developing solvent. A commercial-off-the-shelf (COTS) UV-curable automotive primersurfacer18 was applied to aluminum samples. Exposing with a 405 nm wavelength laser diode through a patterned photomask, the sample was then developed using mineral spirits and some light rubbing. The patterning process was successfully demonstrated (Figure 5). Methyl ethyl ketone (MEK) was used for all following developing as a more powerful solvent that required no physical rubbing or force.
A. Apply Photocurable Coating The photocurable coating is applied to the external aircraft surface using existing coating methods such as spray painting. The coating must be applied to the thickness of desired riblet height or greater (50 to 150 μm). The present proof of concept testing used a drawdown method for experimental purposes.
B. Military Aircraft Topcoat and Reformulation The photocurable military aircraft top coat discussed previously was then formulated using newly developed oligomers, and tested. This was a gray pigmented topcoat designed to have the same properties as the current C-130H topcoat used by the USAF.3 Applied at the required thickness for wind tunnel riblet
III. A Novel Microfabrication Method
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FIGURE 6. Illustration of Fresnel-Fraunhofer optical effect producing interference patterns producing repeating images of the photomask pattern at each diffracted order (top), which can be aligned to be “in focus” (middle) or misaligned to be “out of focus” (bottom) dimensions (i.e. greater than their height) we failed to achieve adhesion with the 405 nm laser diode source. Adjustments to the formulation were made as necessary to achieve through-cure and adhesion at the required coating thickness. The resulting formulation achieved through-cure at the required coating thickness. C. Adhesion In order for the coating to adhere to the substrate, the bottommost layer of coating that is in contact with the substrate must cure. The UV used to cure the coating is coming from the top (the exposed surface) and therefore has to travel through the entire uncured coating thickness first. This will cause attenuation of the irradiance that is dependent on the composition and thickness of the coating. Therefore, for a given coating and coating thickness, a minimum critical exposure is required for coating adhesion. Failing to achieve this critical exposure may result in surface curing that lifts off during developing of the sample. D. Bottom-up Curing Investigation The bottom-up curing effect was investigated to determine how exposure, irradiance and/or time could be utilized to control the final cure height. Experimental results supported the relationship hypothesized in Figure 3; however, time constraints and issues with the reliability of our coating process prevented determining the exact relationship between exposure parameters and cure height. Preliminary findings suggest this should depend on oxygen concentration in the atmosphere above, temperature and the coating formulation. uvebtechnology.com + radtech.org
FIGURE 7. Four profiles produced by different grayscale masks as measure by the CMOS camera: clockwise from top left increase in peak irradiance width and gradient from light to dark, period of peaks is ~120 μm
V. Optics Investigation
The goal of the optics investigation was to determine a method to reliably produce an irradiance profile that could be used to expose a photocurable coating in a riblet microstructure pattern. This needed to be at a distance (non-contact), robust (able to handle ambient room vibrations and small variations in distance to coated surface) and create the desired pattern spacing (117 μm for wind tunnel samples). A. Equipment The optical system was mounted on a breadboard held at the vertical so as to project the irradiance profile down onto the coating. The curing source used was a 405 nm, 40 mW, Ø5.6 mm, B Pin code, Sanyo laser diode, driven by a LTC100-A Laser/Tec Driver Kit. Despite photocurable coatings typically described as “UV-curable” the near-UV visible regime was tested first for a number of reasons, including better penetration ability of longer wavelengths and versatility in the photomask material (soda-lime glass is transparent to visible light but absorbs UV). The output beam shape was adjusted using various combinations of mirrors, lenses and pinholes to tidy up the beam and adjust the focal point. The photomask design contained a matrix of different diffraction grating designs with varying slit periods and widths. The mask was made of soda-lime glass (made possible with the visible 405 nm wavelength source) and a chrome coating that had been etched with desired diffraction grating designs. Some masks included grayscale designs to slowly vary the profile (i.e. the gradient from page 56 u UV+EB Technology • Issue 2, 2017 | 55
PHOTOLITHOGRAPHY t page 55 pixel or ~5-10 μm peak width); projected from distances over a range of 100 to 600 mm and demonstrated focus tolerance of 1mm to 5mm. Note that being “in focus” was determined with the CMOS – i.e. that the image remained unchanged and therefore the pattern remained the same within the tolerance of the camera’s pixel pitch (5.2 μm).
FIGURE 8. Grayscale mask design (top left); CMOS irradiance profile (top right); profilometry of hierarchical structures produced (bottom) peak to minimum irradiance). A monochrome USB CMOS Camera with pixel size 5.2 μm was used to measure the profile produced. Neutral density filters of varying optical densities were used to reduce the intensity of the exposure pattern so that the pattern could be viewed on the CMOS sensor. B. Theory The initial plan to produce the riblet irradiance profile using an optical component known as a diffraction grating. This produces bright and dark fringes of light due to interference of diffracted light passing through the grating in the Fraunhofer regime. After encountering issues with producing the diffraction pattern predicted by theory, it was determined this was a result of a failure to meet the optical requirement of incident normal plane waves16 hitting the diffraction grating and that such a requirement was not feasible in a laboratory context. An unexpected result was, however, observed in which the image of the incoming light beam was projected through the photomask (as in the Fresnel regime) and replicated at each diffraction peak as predicted by the Fraunhofer diffraction grating equations. This combination of the Fraunhofer and Fresnel regimes, as well as the symmetry of the photomask pattern in the direction of the diffracted patterns, provided a means to create the desired profiles for exposing the photcurable coating (Figure 6). Whilst it is beyond the scope of the current paper to delve into the optics theory behind this and further research is required to conclusively determine the mechanism of action – we were however able to demonstrate reliable profiles with desired spacing (117 μm); sharpness (1-2 56 | UV+EB Technology • Issue 2, 2017
C. Results Sharp irradiance profiles were achieved as measured on the CMOS that matched the riblet spacing of 117 μm. Peak irradiance was achieved down to the limit of the CMOS pixel size of 5.2 μm. The spacing of the peaks was able to be changed through a combination of focal adjustment and distance from mask-to-CMOS sensor to any desired spacing (within the CMOS resolution limit). Profiles were exposed onto the CMOS at distances ranging from 100 to 600 mm. This distance could be increased, as we were simply limited by the size of the optical breadboard holding the components. Depending on the optical setup, a tolerance of up to 5mm in that distance whilst maintaining the same irradiance pattern as observed on the CMOS camera (pixel size 5.2 μm). It appeared as though this could be improved upon by exposing from larger distances. As can be seen in Figure 7, different photomask designs were able to produce profiles with varying gradients from light to dark. This reflects the photomask designs and is only possible through the projection characteristic of the Fraunhofer-Fresnel phenomenon discussed previously. As the resulting pattern is essentially a projection of the photomask, arbitrarily shaped profiles can be produced and high control over microstructure profiles can theoretically be achieved.
VI. Experimental Setup
The current project aimed specifically at making riblet microstructures for testing in a Lockheed Martin Corporation (LMCO) wind tunnel facility. The goal was to produce riblets with spacing s = 117 um on aluminum panels of dimensions 24"x23" and 0.012" thickness. A pair of these panels were applied to LMCO’s NACA 0012 2D Airfoil (top and bottom) for wind tunnel testing. A. Experimental Rig The experimental rig consisted of the described optical system, a programmable XY table and a black corflute enclosure with yellow lighting to protect the coating from dust and ambient light. B. Coating method Aluminum panels were pre-coated off site at the University of Dayton Research Institute (UDRI) with a Deft 02-Y40 primer currently used by the USAF on C-130 aircraft. A drawdown method using wire wound rod was used for convenience, as it enabled coating application within the uvebtechnology.com + radtech.org
profile by the optical system and measured by the CMOS camera. Samples were printed on aluminum pre-coated with a C-130 primer.
FIGURE 9. Stationary slit CMOS irradiance profile (left) and optical microscopy of fabricated riblets showing sloped ends (right).
FIGURE 10. Continuous riblet profiles (to scale) lab and for control over coating thickness. This method, however, caused multiple issues with coating coverage and evenness over the whole panel. Future investigations should use an alternative coating method that can be transferred to aircraft application, e.g. spray painting. C. XY Table and Exposure Program The continuous exposure method to cure the photocurable coating was achieved with a custom-built programmable XY translational table that allowed for automated exposures of large areas. The described profiles were traversed across the coated sample to “draw out” the extended riblet microstructures. D. Developing Methyl ethyl ketone (MEK) was used as a developing agent. After exposure the sample panel was placed in a tray or tub, covered in MEK and gently agitated for ~1 minute, then removed from MEK to allow drying. Post-curing in sunshine or under UV lamp was conducted to ensure completely cured structures.
The following results are of some of the microstructures fabricated with the above experimental setup. Each sample was produced with a different uvebtechnology.com + radtech.org
A. Stationary Grayscale Exposures The first attempt was made using a grayscale mask and a stationary exposure. The grayscale mask design (see top left Figure 8) consisted of bars of increasing thickness on 5.5 μm period. The smallest bar is 0.5 μm, increasing 0.5 μm to the largest at 5 μm. This was used to produce a profile with peak spacing of ~130 μm. The resulting riblet profiles were far too short at ~2 μm in height, however they exhibited a fine structure pattern that matched the fine structured bars of the grayscale mask (see bottom Figure 8). This was an unexpected result as it appears that feature sizes down to ~1-2 μm in size are achievable with this method. The structures are also “hierarchical” in that they consist of ~1 μm structures superimposed on ~100 μm structures. This may be useful in self-cleaning superhydrophobic surfaces such as lotus leaf microstructures or perhaps a combination of riblets and lotus leaf structures to impart both drag-reducing and self-cleaning properties. B. Stationary Slit Exposures Given the grayscale mask riblets were far too flat, the “sharpest” irradiance profile was attempted using a slits-only design of regularly spaced apertures of 4 μm (i.e. without any slow-changing grayscale). A stationary exposure was used to produce a profile at the scale required for riblets on an aircraft (i.e. spacing s = ~50 μm). Optical microscopy revealed that regular riblet structures were produced of said spacing as well as in interesting effect at the end of the riblet segments. Where the profile faded to dark, the fabricated riblets sloped down to the substrate (Figure 9). This suggests the bottom-up curing effect is replicating the profile accurately and that adjustment of irradiance can be used to manipulate riblet heights for drag reduction optimization or variable height riblets. C. Continuous Exposures The process was then adjusted and aligned for a continuous exposure at the dimensions required for the wind tunnel riblets (i.e. s = 117 μm).5 Testing was completed on small 3" size panels to allow for metrology prior to fabricating full wind tunnel panels. Exposure speeds were successfully run at 2.5 to 700 mm/min with successful adhesion. Profilometry of riblets produced indicated a very sharp (~1-2 μm) peak radius and a reliable sawtooth profile of ~55 μm height (Figure 10). page 58 u UV+EB Technology • Issue 2, 2017 | 57
PHOTOLITHOGRAPHY t page 57
Proof of concept of a continuous and contactless microfabrication process was demonstrated with better than anticipated results. Riblet microstructures were fabricated from a durable military aircraft topcoat and achieved a six percent viscous drag reduction. This is a very encouraging first step toward a successful implementation of drag-reducing riblets microstructures on aircraft and overcoming past issues with cost of application, FIGURE 11. SEM images of riblets fabricated reveal far sharper, smoother and narrower maintenance and microstructures than anticipated. Scale bars in first 5 images are 100μm. The last image is a durability. The six magnified riblet profile showing a 9μm width. percent viscous drag reduction from Scanning electron microscopy (SEM) of the sample was also wind tunnel testing has the potential to translate to a significant conducted for high resolution images of the riblet profiles reduction in USAF’s $8B+ annual expenditure on aircraft fuel. produced. These images revealed far smoother and more even structures than anticipated (Figure 11). This may be a fortunate All photocurable coatings tested were able to be microfabricated, consequence of the continuous fabrication process averaging out suggesting the process may work with a large range of coatings the exposure irradiance profile any given point in the coating. that could be selected to provide desired characteristics. High print speeds of up to 700 mm/min were achieved and should The SEM images also suggest far sharper and narrower (~9 μm) be able to be improved upon with higher irradiance exposures. riblets of a more blade-like geometry than the profilometry results The number of riblets printing at once is also easily scalable indicate. It is suspected the profilometry data are in error as they through either magnification of a scaled photomask design, or by are unreliable for horizontal measurements, use a 5 μm radius running a horizontal array of optical systems in parallel. There stylus tip and may have systematic error in scanning riblet peaks are thus multiple attractive avenues for greatly reducing time of and troughs. application. Exposures were conducted at distances of 100 to 700 mm from the coated surface with a robust tolerance of up to 5 mm D. Wind Tunnel Testing variation in that distance. After optimization of coating formulation, thickness, exposure irradiance profile and exposure speed full 24" x 23" wind tunnel Feature sizes down to the order of single microns were reliably samples were produced. Wind tunnel testing was conducted by produced, exceeding expectations by an order of magnitude. LMCO and a viscous drag reduction of six percent was measured. Bottom-up curing was demonstrated, allowing for “single exposure 3D printing” or 3D manipulation of microstructure E. Low-gloss Riblets designs in a single step. Manipulating heights and profiles of riblet It was also observed that the riblet samples produced had a dull microstructures was demonstrated in this way. This also enables finish when compared to non-patterned cured coating, which had a live-manipulation of riblet heights, which may be used to optimize high gloss finish. This may prove useful for achieving a low-gloss riblet parameters across the aircraft surface or in fabricating finish without the use of downglossing agents or pigmentation. variable height riblets for further drag reduction. Preliminary page 60 u 58 | UV+EB Technology • Issue 2, 2017
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PHOTOLITHOGRAPHY t page 58
The six percent viscous drag reduction from wind tunnel testing has the potential to translate to a significant reduction in USAF’s $8B+ annual spend on aircraft fuel. findings suggest the process can be used to fabricate hierarchical structures in single or multiple exposures, holding promise as a possible method of fabricating self-cleaning superhydrophobic microstructures to improve maintenance and durability. Future investigations toward the ESMC program goal of practical riblet application to the USAF legacy transport aircraft fleet should focus on building a robust, mountable optical unit with tolerances required for hangar environment; coating with existing AFRL spray painting methods and qualification testing of riblet structures at the AFRL Coatings, Corrosion & Erosion Laboratory (CCEL). Adjustments to microstructure design and formulation of the photocurable military aircraft topcoat should be made as necessary. u Acknowledgments This work is supported by the Operational Energy Capability Improvement Fund (OECIF) from the Office of the Assistant Secretary of Defense for Operational Energy Plans and Programs, ASD (OEPP). The MicroTau Pty Ltd authors would like to acknowledge the Ohio Aerospace Institute, through which they were funded. The author would like to thank Mr. Nathan Apps and Mr. Ralf Wilson for their assistance in project management, mechanical design and technical support. This work was done in part at the OptoFab Node of the Australian National Fabrication Facility with the assistance of Ethel Ilagan, process engineer, and David O’Connor, foundry manager. The author would also like to thank the following individuals for their contributions to this project: Dr. Joe Khachan and Dr. J. Scott Brownless for their consultation on matters of experimental design and optical theory respectively; Michael J. Dvorchak, technical director of Dvorchak Enterprises, LLC for his insight into formulating UV curable coatings for this unique application; and Scott Smith, research chemist of R & D Coatings, Inc. for his formulation skills in being able to meet the parameters required.
2. Bushnell, D. M., “Aircraft Drag Reduction – a Review, Proceedings of the Institution of Mechanical Engineers,” Part G: Journal of Aerospace Engineering, Vol. 217, 2003, pp. 1-18 3. Williams, C.T., Dvorchak, M., and Gambino, C., “Development of UV-A Curable Coatings for Military Aircraft Topcoats,” Radtech Report [online journal], Spring 2011, URL: http://www.radtech.org/images/ pdf_upload/development-of-uv-a-curable-coatings-for-military-aircrafttopcoats-spring2011.pdf [cited December 2015]. 4. Malas, J., Jines, L., Ontko, N., Richey, K., and Manter, J., Introduction and Qualification of New and Improved Engineered Surfaces, Materials, and Coatings for Aircraft Drag Reduction, United Technologies Corporation, 2014. 5. Smith, B., “Wind Tunnel Validation Testing Concepts,” ESMC TIM Presentation, 20 October 2015, slide 6. 6. Lynch, F. and Klinge, M., “Some Practical Aspects of Viscous Drag Reduction Concepts,” SAE Technical Paper 912129, 1991. 7. Ensikat, H.J., Ditsche-Kuru, P., Neinhuis, C., and Barthlott, W., “Superhydrophobicity in perfection: the outstanding properties of the lotus leaf,” Beilstein J. Nanotechnol., 2011, 2, 152–161. 8. Vorobyev, A.Y., and Guo, C., “Multifunctional surfaces produced by femtosecond laser pulses,” Journal of Applied Physics, 117, 033103, 2015. 9. Malas, J., Jines, L., Richey, K., Manter, J., Ontko, N., Allport, C., and Folck, J., Engineered Surfaces, Materials and Coatings for Drag Reduction (Phase 0), United Technologies Corporation, 2014. 10. Stenzel, V., Kaune, M., and Da Silva Branco Cheta, M.R., Fraunhofer-Gesellschaft Zur Forderung Der Angewandten Forschung E.V., U.S Patent for a “Tool for generating microstructured surfaces,” Patent Number (US 7736570), filed 30 Sep 2004. URL: https://www. google.com.au/patents/US7736570 11. Dvorchak, M.J., “UV Curing of Pigmented High-Build Wood Coatings Based on Non-Air-Inhibited Unsaturated Polyesters,” Journal of Coatings Technology, 1995, URL: http://infohouse.p2ric.org/ ref/25/24087.pdf 12. Baird, R.W., “Heat-Resistant UV-Curable Clearcoat for Aircraft Exteriors,” The Boeing Company, 2011, URL: https://goo.gl/spHP9Y 13. Naguy, T., and Straw, R., “Ultraviolet (UV)-Curable Coatings for Department of Defense (DoD) Applications AFRL,” AFRL, 2009, URL: https://goo.gl/YqZHvO 14. Arceneaux, J.A. “Mitigation of Oxygen Inhibition in UV LED, UVA, and Low Intensity UV Cure,” Allnex USA Inc., 2014, URL: https://goo. gl/hDSHNZ 15. Alvankarian, L., and Majlis, B.Y., “Exploiting the Oxygen Inhibitory Effect on UV Curing in Microfabrication: A Modified Lithography Technique,” PLoS ONE 10(3), URL: https://www.ncbi.nlm.nih.gov/ pubmed/25747514 16. Hecht, E., “Optics,” 3rd ed, Addison Wesley, 1998, Chap 10. 17. Pfanstiehl, J., “The 2-minute Cure,” Radtech Report [online journal], November/December 2003, pp.46-50 URL: http://radtechintl.org/ resources/Documents/2minutecurenovdec03.pdf 18. See product page URL: http://products.axaltacs.com/dcat/us/en/dr/ product/A-3130S.html
* Additional information, tables and figures can be found online at www.uvebtechnology.com. References 1. Dean, B., and Bhushan, B., “Shark-skin surfaces for fluid-drag reduction in turbulent flow: a review,” Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences, vol. 368, 2010, pp. 4775–4806.
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Regulatory News EHS Committee Update The RadTech Environmental Health and Safety (EHS) Committee, led by chairman Michael Gould of RAHN, met in San Francisco, California, on March 2. Rita Loof gave an update of activities in the South Coast Air Quality Management District (SCAQMD). She is helping to represent RadTech member interests in several areas with SCAQMD and has just been approved for a two-year term as representative of RadTech on the Best Available Control Technology Scientific Review Committee (BACT SRC). Doreen M. Monteleone, Ph.D., director of sustainability & EHS initiatives, RadTech International North America doreen@ radtech.org
The committee again discussed bisphenol A, which has appeared in the news numerous times recently. Of particular note is that the European Chemicals Agency (ECHA) Member State Committee has unanimously agreed on identification of bisphenol A and three other substances of very high concern (SVHCs). Substances were added to the SVHC list in January 2017. Progress on implementation of the Lautenberg Chemical Safety Act – the “new Toxic Substances Control Act (TSCA)” – and feedback from a meeting of the American Chemistry Council were reviewed. Also, a bullet point primer on the pending “TSCA Reset,” scheduled to begin June 22, 2017, was shared. Guide to Proper Handling of UV-Curable 3D Printing Resins UV-curable resins for 3D printing/additive manufacturing cure rapidly when exposed to UV sources. As with all chemicals, UV-curable resins must be handled in a safe manner. A fact sheet now available is meant as a guideline for the handling of UV-curable resin materials (photopolymers) used in 3D printing systems, such as stereolithography (SLA), digital light processing (DLP) and UV inkjet. The guide can be downloaded from RadTech’s website at http://www.radtech.org/health-safety/proper-handling-of-uv-resins. Hazardous Waste Generator Improvements Rule Update 2017 In late 2016, the US Environmental Protection Agency (US EPA) administrator signed the final Hazardous Waste Generator Improvements Rule. This rule finalizes an update to the hazardous waste generator regulations and is meant to make the rules easier to understand, facilitate better compliance, provide greater flexibility in how hazardous waste is managed and close important gaps in the regulations. Two key provisions with which US EPA is finalizing flexibility are as follows: 1. allowing a hazardous waste generator to avoid increased burden of a higher generator status when generating episodic waste, provided the episodic waste is properly managed; and 2. allowing a very small quantity generator (VSQG) to send its hazardous waste to a large quantity generator under control of the same person. In addition to finalizing key flexibilities, the rule enhances the safety of facilities, employees and the public by improving hazardous waste risk communication and ensuring that emergency management requirements meet today’s needs. Further, US EPA is finalizing a number of clarifications without increasing burden, including a reorganization of the hazardous waste generator regulations so that all such regulations are in one place. More information can be found at https://www.federalregister.gov/documents/2016/11/28/2016-27429/hazardouswaste-generator-improvements-rule. Evaluating Risk of Existing Chemicals Under TSCA Under the Toxic Substances Control Act (TSCA), the US Environmental Protection Agency (US EPA) now is required to evaluate existing chemicals to determine whether they “present an unreasonable risk of injury to health or the environment.” Under the conditions of use for each chemical, US EPA will assess the hazard(s), exposure(s) and the potentially exposed or susceptible subpopulations(s). This information will be used to make a final determination as to whether the chemical presents an unreasonable risk. Learn more at https:// www.epa.gov/assessing-and-managing-chemicals-under-tsca/evaluating-risk-existing-chemicals-under-tsca.
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Regulatory News News From The West Coast Advocacy for Exemption Continues RadTech has stepped up advocacy efforts to obtain a permit exemption for UV/EB/LED materials from the South Coast Air Quality Management District (SCAQMD). The SCAQMD board directed its staff to work with industry to minimize regulatory barriers to the implementation of UV/EB technology. Although this is a regional rule, the policy decisions of SCAQMD tend to influence national policy. Rita Loof, director of regional environmental affairs, RadTech International North America email@example.com
When the agency’s governing board met April 7, 2017, RadTech took the opportunity to discuss its proposal for a permit exemption for UV/EB/LED materials. Two days before the meeting, district staff released new language for Rule 219 – Permit Exemptions. RadTech expressed its appreciation for the change in the exemption level from 25 grams per liter to 50 grams per liter of volatile organic compounds (VOC’s), as requested, but expressed concerns with the newly added record-keeping requirements. The new recordkeeping requirements will render the exemption useless. The new language specifically targets printing and coating operations by requiring yet another annual report every March for materials that contain less than 50 grams per liter. These are the cleanest facilities, which have reduced their emissions beyond rule requirements. Staff does not believe it will be burdensome for businesses to submit an annual report to the district, but RadTech respectfully disagreed, explaining that “wood coaters and print shop operators want to be able to focus on their businesses rather than on filling out forms. The whole point of providing a permitting exemption is to make it easier for them to continue to operate in Southern California. The issue was scheduled to be heard in committee April 21, 2017, and the final adoption hearing was set for early May in Diamond Bar, California. RadTech will continue to urge the SCAQMD board to support its exemption request for UV/EB/LED materials that contain less than 50 grams per liter of VOCs. BACT Update Several weeks ago, the SCAQMD board adopted RadTech’s proposed resolution language that directs staff to continue work on updating the Best Available Control Technology Guidelines, with an emphasis on UV/ EB inks and coatings technology, and report back to the agency’s Stationary Source Committee by June 2017 on proposed updates. As part of the commitment, agency staff convened a meeting of the BACT Scientific Review Committee. RadTech was officially appointed as an adviser on the committee in March. BACT is required for new and relocated sources and for modifications that increase emissions. In response to requests by RadTech, SCAQMD is now proposing inclusion of UV/EB technology as a compliance option for the categories listed below. • Major Sources, Printing (graphic arts) flexographic • Major Sources, Fiberglass operations; cultured marble products; application hand and spray lay up (use of polyester resin with monomer content less than 34 percent by weight) • Minor Sources, Printing (graphic arts) flexographic • Minor Sources, Printing (graphic arts) screen printing and drying The proposal also included a requirement for facilities to use “super compliant” [less than 50 grams per liter volatile organic compound (VOC) content] cleaning solvents. The current limit in Rule 1171—Solvent Cleaning is 100 grams per liter. RadTech expressed concerns, which were echoed by the Printing Industry Association, that printers may not be able to meet the cleanup solvent requirement, as there have been reports of problems with the lower VOC cleaning solvents. The complete listings can be found at http://www.aqmd.gov/docs/default-source/Agendas/bact/bact-srcagenda-4-4-17/bact-src-full-draft-package.pdf. u
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6-7: Sink or Swim, the Cleveland Coatings Society Conference, Cleveland Airport Marriott, Cleveland, Ohio. For more information, visit www.clevelandcoatingssociety.org
25-28: Label Expo Europe 2017, Brussels Expo, Brussels, Belgium. For more information, visit www.labelexpo-europe. com.
19-22: AWFS Fair, Las Vegas Convention Center, Las Vegas, Nevada. For more information, visit www.awfsfair.org
10-12: SGIA, Ernest N Morial Convention Center, New Orleans, Louisiana. For more information, visit www.sgia.org.
17-19: RadTech Europe Conference and Exhibition, Clarion Congress Hotel, Prague, Czech Republic. For more information, visit www.radtech2017.com.
10-14: PRINT 17, McCormick Place South, Chicago, Illinois. For more information, visit www.ppiassociation.org. 17-20: Photopolymerization Fundamentals, St. Julien Hotel & Spa, Boulder, Colorado. For more information, visit www.radtechintl.org/Photopolymer2017.
24-25 UV+EB Packaging Conference and RadTech Fall Meeting, DoubleTree by Hilton Philadelphia Airport, Philadelphia, Pennsylvania. For more information, visit www.radtech.org
25-27: PACK EXPO Las Vegas, Las Vegas Convention Center, Las Vegas, Nevada. For more information, visit www.packexpolasvegas.com.
Advertisers Index American Ultraviolet.................................................................. americanultraviolet.com....................................................................................... 49 AWFS Fair................................................................................... awfsfair.org............................................................................................................. 40 A.W.T. World Trade Inc.............................................................. awt-gpi.com........................................................................................................... 14 BASF........................................................................................... basf.us/dpsolutions....................................................................Inside Front Cover Carestream................................................................................. tollcoating.com..................................................................................................... 22 Dymax......................................................................................... dymax-oc.com/clearmove.................................................................................... 31 EIT Instrument Markets............................................................. eit.com................................................................................................................... 25 Excelitas Technologies.............................................................. excelitas.com..........................................................................................Back Cover GEW............................................................................................ gewuv.com............................................................................................................... 7 Gurun Technology Co., Ltd....................................................... hbgrkj.com............................................................................................................. 26 Heraeus...................................................................................... heraeus-noblelight.com....................................................................................... 47 Honle UV America Inc............................................................... honleuv.com............................................................................................................ 5 IGM Resins................................................................................. igmresins.com/contact...............................................................Inside Back Cover Keyland Polymer........................................................................ keylandpolymer.com............................................................................................. 17 Miwon Specialty Chemical Co., Ltd......................................... miramer.com.......................................................................................................... 19 Phoseon Technology................................................................. phoseon.com/uv-eb.............................................................................................. 23 Photopolymerization Fundamentals 2017............................... radtechintl.org/photopolymer2017..................................................................... 35 PRINT 17..................................................................................... print2017.com........................................................................................................ 61 RAHN.......................................................................................... rahn-group.com...................................................................................................... 1 Sartomer Arkema Group........................................................... sartomer.com......................................................................................................... 15 SGIA Expo.................................................................................. sgiaexpo.org.......................................................................................................... 59 Siltech Corporation................................................................... siltech.com............................................................................................................. 21
64 | UV+EB Technology â€˘ Issue 2, 2017
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