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2020 Quarter 2 Vol. 6, No. 2

EB Curing in Flexible Packaging UV-Cured Coatings in Aerospace A Review of RadTech 2020 Eliminating Safety Hazards

Official Publication of RadTech International North America


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FEATURES 14

22

ON THE COVER

This issue’s cover is inspired by the RadTech student poster competition winner, Dana Pousty. Pousty is a student with the Water Research Center, School of Mechanical Engineering, Tel-Aviv University. 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.

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Comparison of UV- and EB-initiated Polymerizations Based on Equivalent Radical Concentration A protocol was developed to investigate UV- and EBpolymerized films of equivalent radical concentrations. This protocol then was applied to an acrylate/methacrylate pair to characterize the impact of the initiation mechanism. Monomer chemistry was shown to be a key variable. By Nicole L. K. Thiher, Chemical and Biochemical Engineering Department, University of Iowa; Erin Peters, Rockridge High School; Sage M. Schissel, PCT Ebeam and Integration, LLC; and Julie L. P. Jessop, School of Chemical Engineering, Mississippi State University

UV Curing Advances Printed and Novel Electronics at PARC

A diverse set of projects at the Palo Alto Research Center (PARC) are shaping the future through the use of UV technology. By Liz Stevens, contributing editor, UV+EB Technology

High-Energy Discussions at RadTech UV+EB Conference

An overview of the event, held March 9-11 in Orlando, Florida, and the award honorees who were celebrated. Reprinted with permission from Richard Romano, WhatTheyThink.com

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DEPARTMENTS

2 | UV+EB Technology • Quarter 2, 2020

In this study, the application of EB in the production and destruction of a mono-material, compostable flexible food packaging structure was investigated. Low doses of EB were used to cure matte and gloss OPVs and then tested to determine if the OPV inhibited compostability. By Sage M. Schissel, PCT Ebeam and Integration, LLC

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President’s Message............................................. 4 Association News................................................. 6 Technology Showcase........................................ 31 Regulatory News................................................ 50 Calendar.............................................................. 52 Advertising Index............................................... 52

Attainable Sustainable: Using Electron Beam Technology in Compostable Flexible Packaging

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Aerospace UV Cured Coatings: Yesterday, Today and Tomorrow

During nearly three decades of development, several hurdles have impeded UV cure technology progress and use in the aerospace market. This paper reports on these hurdles and potential solutions to further develop the UV cure aerospace coatings market. By Michael J. Dvorchak and Melanie L. Clouser, Dvorchak Enterprises LLC, and A. David Harbourne, Harbourne Consulting LLC

UV LED curing systems: Measuring accurately and eliminating safety hazards The advance in UV LED technologies make it important that measurement equipment and methods for UV LED curing systems are well understood. The risks and safety concerns of UV also are discussed. By Geri Tandiongga, Don Moncy Dominic and Dominik Stephan, Dymax Asia Pacific Pte. Ltd.

uvebtechnology.com + radtech.org


TECHNOLOGY 2020 Quarter 2 Vol. 6, No. 2

CHAMPIONS THIS ISSUE

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.

Susan Bailey

COLUMNS 8

UV Curing Technology

I’m with the Band By Jim Raymont, EIT LLC

10

Innovations

HARP Changes the Game in 3D Printing By Liz Stevens, UV+EB Technology

12

Professor’s Corner

Syed T. Hasan

Editorial Board Co-Chair Business Development Manager, Digital & Specialty Printing Michelman, Inc.

Editorial Board Co-Chair Key Account Manager, Security Inks BASF Corporation

JianCheng Liu

Sudhakar Madhusoodhanan

Understanding Glass Transition Temperature: Part 2 By Byron K. Christmas, Ph.D., Professor of Chemistry, Emeritus Senior Scientist PPG Industries

UV+EB TECHNOLOGY EDITORIAL BOARD Susan Bailey, Michelman, Inc. Co-Chair/Editor-in-Chief Syed Hasan, BASF Corporation Co-Chair/Editor-in-Chief Darryl Boyd, US Naval Research Laboratory Byron Christmas, Professor of Chemistry, Retired Amelia Davenport, Colorado Photopolymer Solutions Rachel Davis, Azul 3D, Inc. Charlie He, Glidewell Laboratories Mike Higgins, Phoseon Technology Molly Hladik, Michelman, Inc.

uvebtechnology.com + radtech.org

Mike J. Idacavage, Colorado Photopolymer Solutions JianCheng Liu, PPG Industries Sudhakar Madhusoodhanan, Applied Materials Gary Sigel, Armstrong Flooring Maria Muro-Small, Spectra Group Limited, Inc. Jacob Staples, Wausau Coated Products Chen Wang, National Renewable Energy Lab Huanyu Wei, Chase Corp. Sheng “Sunny” Ye, Facebook Reality Labs

Technical Project/Program Management Applied Materials

Huanyu Wei

Research and Development Manager Chase Corporation

UV+EB Technology • Quarter 2, 2020 | 3


PRESIDENT’S MESSAGE

W

elcome to our new abnormal.

Eileen Weber President

Normally, the UV+EB Technology issue following our biennial RadTech North America conference would bring the pleasure of a show wrap-up message speaking to the many successes of the event. Albeit this year’s show was another success – especially considering the circumstances – these aren’t normal times.

During a recent 60 Minutes interview, Mary Barra, CEO of General Motors, referred to it as a “new abnormal” when asked when automakers will ramp up production in the United States again. I think this is a much more appropriate assessment. It has been so very difficult to watch these past several weeks as many of our employers, customers, restaurants, shops, family, friends and neighbors are severely impacted by the Coronavirus. And, at this writing, the virus still seems to be setting the rules that we are all expected to follow. But a bright spot during a dark time, UV/EB – a process that can be identified with disruption – seems to be well placed to assist as our nation begins to build our new abnormal.

are rethinking their processes and supply chains to ensure reliable sourcing and responsive product development and production. Many of these trends were highlighted in our recent RadTech webinar on UV/EB Trends. (Please visit www.radtech.org if you missed it.) The webinar presented results from a late April survey of you, our members. Perhaps the most striking results from this survey show the extensive and insightful view of the potential future of UV/EB in these times. Among the many thoughts you shared were new directions and accelerated efforts for UV/EB in sustainably made products, antimicrobial coatings, 3D printing and additive manufacturing, and the possible trend of many manufacturing processes returning to the US – which UV/EB may also help enable. In the weeks to come, RadTech will be identifying the ways we might meet these new imperatives. In times like this, it is not about just one company getting a new contract, but it is about our industry coming together as a whole with innovative solutions that help us all. We hope to report back on new ideas soon.

In fact, companies in the UV/EB space have been very well placed to assist in supplying urgent needs. From the additive manufacture of swabs and masks, to meeting the increased demands of labels for medical disposables and equipment, to ensuring our food and consumer goods manufacturers are able to ramp up and meet increased and changing demands, UV/EB has had a visible role in the efforts.

But first, it is our priority to help our communities and get through these difficult times. RadTech would like to thank you for your efforts! We also would like to tout the good work of our members by posting your contributions on our website. Whether that involves making hand sanitizer – as several of our members have been doing – or pitching in at a local healthcare or nursing facility, please give us the opportunity to celebrate your good work. Contact me or email info@radtech.org. We look forward to working with you, as together we help build our new abnormal.

Moving forward, the emphasis for manufacturing is poised to focus on innovation and fast, flexible processes – a clear match to the many advantages offered by UV/EB. It is reported that, during this time of crisis, manufacturers – including the automotive industry –

Be well and stay safe. Eileen Weber President, RadTech Board of Directors Global Marketing Manager, PC&I Radcure, allnex USA, Inc.

BOARD OF DIRECTORS President Eileen Weber – allnex USA., Inc.

TECHNOLOGY An official publication of: RADTECH INTERNATIONAL NORTH AMERICA 6935 Wisconsin Ave, Suite 207 Chevy Chase, MD 20815 240-497-1242 radtech.org EXECUTIVE DIRECTOR Gary M. Cohen gary@radtech.org SENIOR DIRECTOR Mickey Fortune

Published by:

President-elect Jo Ann Arceneaux – allnex USA Inc. Secretary Susan Bailey – Michelman Treasurer Paul Elias – Miwon North America Immediate Past-President Lisa Fine – Ink Systems, Inc. Board of Directors Evan Benbow – Wikoff Color Corporation David Biro – Sun Chemical Mike Bonner – Saint Clair Systems, Inc. Todd Fayne – Pepsico Michael Gould – Rahn USA Jeffrey Klang – Sartomer Diane Marret – Red Spot Paint & Varnish Jim Raymont – EIT LLC Chris Seubert – Ford Motor Company P.K. Swain – Heraeus Noblelight America Karl Swanson – PCT Ebeam and Integration Hui Yang – Procter and Gamble Sheng “Sunny” Ye – Facebook Reality Labs

4 | UV+EB Technology • Quarter 2, 2020

2150 SW Westport Drive, Suite 101 Topeka, Kansas 66614 785-271-5801 petersonpublications.com Publisher Jeff Peterson

National Sales Director Janet Dunnichay

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Contributing Editors Nancy Cates Liz Stevens

Circulation Manager Brenda Schell

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ENews Kelly Adams

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ASSOCIATION NEWS RadTech Consultant Offers Assistance to Members

As many businesses and academics began working to assist those on the “front lines” in combatting COVID-19, Marcy Gainey of TechCheck LLC, the RadTech regulatory consultant, began offering complimentary support to RadTech members for such projects. For example, assistance was offered to member companies making hand sanitizer or building ventilators and 3D printing companies working on personal protection equipment, such as masks or shields. Gainey offered the service to RadTech members through the end of May, when she plans to reassess the situation based on the status of COVID-19 and supply shortages. For more information, contact Gary Cohen at gary@radtech.org.

IUVA Releases Fact Sheet on UV Disinfection for COVID-19

The International Ultraviolet Association (IUVA) believes that UV disinfection technologies can play a role in a multiple-barrier approach to reducing the transmission of the virus causing COVID-19, SARSCoV-2, based on current disinfection data and empirical evidence. UV is a known disinfectant for air, water and surfaces that can help to mitigate the risk of acquiring an infection via contact with the COVID-19 virus when applied correctly. “The IUVA has assembled leading experts from around the world to develop guidance on the effective use of UV technology, as a disinfection measure, to help reduce the transmission of COVID-19 virus,” said Ron Hofmann, a professor at the University of Toronto and president of the IUVA. The fact sheet is available for download at http://iuva.org/.

Ultraviolet Association (IUVA), have developed a new website to document and track UV LED technology. The site – uvledsource. org – consolidates technical papers and articles on diverse UV LED subjects in a convenient, easy-to-search online resource. It is designed to comprehensively educate those interested in UV LED technology, with new papers added regularly to reflect the ongoing work of RadTech and IUVA members. UV LED innovations are leading to more efficient, economical and environmentally responsible processes, offering scientific advancements to foster groundbreaking industrial and public health initiatives.

Rachel Davis Joins UV+EB Technology Editorial Board

Rachel Davis, senior chemist with Azul 3D, Inc., has been named as the newest member of the UV+EB Technology editorial board. Davis received a degree in writing and materials science and engineering from the Massachusetts Institute of Technology, Cambridge, Massachusetts, where she completed advanced courses in polymer science, including work in macromolecular chemistry, mechanical behavior of materials, biomaterials and industrial ecology of materials. She was previously employed as a researcher at the University of Berlin and as a materials scientist and operations training specialist at Formlabs. Her current responsibilities include overseeing the development of a research laboratory for photopolymer resins, investigating new UV-curable thermoset polymers and determining test methodology for a variety of applications. u

Website Consolidates UV LED Information

With accelerating technical and market developments – along with the fast-growing number of conference papers and trade journal articles focused on UV LEDs for industrial curing, public health and life science applications – RadTech, the nonprofit for ultraviolet and electron beam technologies, and the International

For more information on upcoming events, member news, learning opportunities and more, visit uvebtechnology.com.

6 | UV+EB Technology • Quarter 2, 2020

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UV CURING TECHNOLOGY QUESTION & ANSWER

I’m With the Band y Syms, from Syms Clothing, ran commercials from 1974 through 2011 with the slogan: “An educated consumer is our best customer.” This column uses Sy’s approach to educate consumers to turn them into better customers. Understanding the source type, photoinitiator package and desired end properties in the cured product will guide measurement strategy and instrument selection. This column is about bands: not the musical type, but “bands” as they relate to UV measurement and process control. Broad band source Mercury-based sources emit energy over a “broad” band (UV, Visible, IR) of the electromagnetic spectrum. The UV portion is normally broken down into four areas (UVA, UVB, UVC and UVV), as shown in Table 1. The numerical values for each area may vary slightly among authors, but the way each UV area influences the product properties is well established.

emission specified by their center wavelength (CWL). The CWL is narrow (1 to 2 nm), with manufacturers typically specifying it falling within a +/- 5 nm window. A 395 nm LED is expected to have its narrow peak output at 390 to 400 nm. Commercial arrays have hundreds – even thousands – of individual diodes. Based on how the LED manufacturer selects (or “bins”) the diodes, the spectral distribution at the 50% power level (Full Width at Half Max, or FWHM) may be 15 to 20 nm wide. The distribution of energy from an LED at lower power levels (10%) may be 40 nm or more. (See Figure 2.) 100 90 80 70

LED Output, %

S

60

UV Area (short to long)

Cured Properties

UVC (200 to 280 nm)

Surface cure properties, such as stain & scratch resistance

UVB (280 to 320 nm)

Coating toughness

UVA (320 to 390 nm)

Through cure

UVV (395 to 445 nm)

Used to penetrate opaque & white colors, provide depth of cure

Table 1. UV area and impact of cured properties. (nm = nanometers)

Additives to the mercury, such as iron (Fe) or gallium (Ga), can change the spectral output of the broad band source, as shown in Figure 1, to achieve or enhance desired cure properties. It is a common practice to use multiple sources, such as a mercurygallium additive lamp, for through cure followed by a mercury lamp for robust surface properties. Narrow band source UV LEDs are narrow band sources with their peak spectral

50 40 30 20 10 0

360 365 370 375 380 385 390 395 400 405 410 415 420 425 430 435 440

Figure 2. X axis is nanometers. Y axis is normalized intensity

In designing their instruments, radiometer manufacturers determine the responsivity, or the bandpass width and shape, of their devices. Two approaches they follow include: 1. Wide Band (Instrument) Response. I classify an instrument with optic response of greater than 100 nm as a wide band instrument. 2. Narrow Band (Instrument) Response. Narrow band instruments have an optic response of less than 100 nm wide, with some manufacturer bands as narrow as 10 nm. Each approach has advantages and disadvantages. Both approaches have been used for years with broad band sources and, more recently, with LEDs. Broad band mercury lamps have repeatable peak emission lines in defined spectral areas, i.e. the 365 nm line.

Figure 1. Broad Band (mercury) based lamp showing mercury (Hg), mercury-gallium (Ga) and mercury-iron (Fe) outputs

8 | UV+EB Technology • Quarter 2, 2020

The peak output of an LED will vary based on the source type (i.e. 365, 385, 395, etc.) and the CWL (+/- 5 nm) in that source. Obtaining accurate, repeatable LED measurements from different LED sources, each with its own CWL variations, requires a radiometer with a flat response. Fabricating a flat optic response over a narrow band is easier than trying to fabricate a flat response over a wide band. There is less variation in the overall flatness of a narrow band than over a wide band. uvebtechnology.com + radtech.org


Wide Band Response • One instrument: a one-size-fits-all approach • Harder to achieve flatness of optics across entire width of the band • Need to pick a point to calibrate the wide band response, which could lead to variations if the response is not flat

Narrow Band Response • Selective approach that may require more than one instrument or a multiband instrument (see below) • Easier to control the shape (flatness) of the optic response and point of calibration • Narrow band response can focus on desired cure properties in Table 1

Figure 4. Wood line with multiple LEDs with mercury lamps at the end for surface cure. Illustration: Efsen UV & EB Technology

A narrow band approach allows some instruments to incorporate all optical components in the stated instrument response and not just the optical filter. Multi-band instruments Multi-band (UVA, UVB, UVC, UVV) instruments for broad band sources have been commercially available for over 30 years. They are popular for production lines with multiple mercury and mercury-additive sources. Measuring the irradiance and energy density values with a multi-band instrument allows the educated customer to: • identify lamp types by examining lamp output across different bands, • ensure that the correct lamp is in the correct position and • monitor reflector cleanliness by looking at the UVA:UVC ratio. (UVC values will drop compared to UVA values in a system with a dirty reflector.) Figure 3 shows the output of two lamps collected with a profiling radiometer. Based on the ratio of the UV bands, the first lamp is mercury-gallium (high UVV output) and the second lamp is mercury. The example below clearly and easily shows the differences between source types. Similar information can be obtained from the display of a multiband radiometer. Individual lamps need to be measured one at a time and the irradiance values compared to one another. This approach is more work compared to using a one-pass profiling radiometer, especially with multiple lamps. Customers utilizing LEDs also have started to optimize production lines using multiple and/or a mixture of different

Figure 5. Profile of the wood line illustrated in Figure 4 collected with a multi-band LED radiometer. Individual LED lamps are labeled. The X axis is time (seconds), Y axis is UV irradiance (mW/cm2). Data: Efsen UV & EB Technology

wavelength LEDs to achieve the desired product characteristics. Figure 4 shows the design of a wood coating production line developed by Efsen UV & EB Technology with multiple types of LED sources followed by mercury sources. Multi-band radiometers optimized for different LED emissions now are commercially available. The response in each of the narrow bands is flat within the LED band. Each band is calibrated to the appropriate LED source type. A multi-band LED radiometer will allow you to identify the different types of LEDs, which cannot be done with a wide band radiometer. Summary Don’t forget the words of Sy Syms. Educate yourself and decide if a wide, narrow or multi-band radiometer best meets your needs. There are advantages and disadvantages with each. Parting thought: This quarter’s column focused on “bands.” Being a rock star is not always glamorous. In some respects, it matches the day-to-day glamour of being a UV process control star: u

Jim Raymont

Figure 3. Mercury-gallium (Hg-Ga) lamp followed by mercury (Hg) lamp, with the X axis as time (seconds) and Y axis as UV irradiance (mW/cm2) uvebtechnology.com + radtech.org

Director of Sales EIT LLC jraymont@eit.com UV+EB Technology • Quarter 2, 2020 | 9


INNOVATIONS INDUSTRY ADVANCES WITH RADLAUNCH WINNERS

HARP Changes the Game in 3D Printing By Liz Stevens, contributing editor, UV+EB Technology

A

mid the wide range of 3D printers these days – for everything from chocolate to human skin to bridges to rockets – a promising new technology is coming into view, one that overcomes the limitations of its predecessors, such as materials and applications, size and speed. High Area Rapid Printing (HARP™), pioneered by researchers David Walker and James Hedrick in Professor Chad Mirkin’s laboratory at Northwestern University, is in a promising ascent. Mirkin, director of the International Institute for Nanotechnology and Rathmann Professor of Chemistry, is an expert in nanoscience. Among his many achievements, Mirkin advanced nanolithography with his development of dip-pen technology and two-dimensional arrays of atomic force microscope tips. He has written more than 760 manuscripts, 1,100 patent applications worldwide (over 350 issued) and is the founder of multiple companies, including AuraSense, Exicure, TERA-print and Azul 3D™. As researchers in Mirkin’s Evanston, Illinois, laboratory, Walker and Hedrick, a doctoral candidate at the time, were not initially working toward the technology that would become HARP™. “The tech arose as a combination of my own background knowledge in interfacial physics and fluoro-chemistry,” explained Walker, “along with James’ background in 2D nano-scale printing techniques. We originally combined these two skill sets to develop a 3D nano-printer.” But in 2015, after learning about Continuous Liquid Interface Production (CLIP) – which builds and solidifies an object in a pool of liquid resin continuously using UV from a projector – Walker and Hedrick recalibrated their mission. “We realized that our techniques for 3D nano-printing might have some advantages if they were scaled up,” said Walker.

technology) for its Start-up Technology Accelerator Class of 2020. Azul 3D™ and the rest of the 2020 class gave presentations at RadTech 2020 and the co-located IUVA Americas 2020 on March 8-11, 2020, in Orlando, Florida. Innovation with HARP™ Mirkin, Hedrick and Walker’s innovation looks to be a gamechanger for 3D printing. The original High Area Rapid Printing (HARP™) -enabled printer is a giant at 13 feet tall with a 2.5 square-foot print bed. The scaled-up size is matched by an equally astounding increase in speed: HARP™ prints at about 1 1/2 feet per hour. The team has demonstrated continuous vertical print output of more than 430 mm per hour, processing a volume of up to 100 liters. HARP™ is bottom up stereolithography, using high-resolution continuous UV irradiation and light patterning in the form of customized, proprietary DLP optics to build objects. “The light patterning sets the shape of the liquid resin,” Walker explained. “Various forms of post-curing can be used to complete the reaction to ensure safety to the end consumer.” Walker described how the Azul 3D™ team incorporated a buffer of fluorinated oil to eliminate the usual repetitive step of separating objects that have adhered to the print plane and also to address the problem of UV-initiated heat generation. “The moving fluorinated oil creates a ‘slip-boundary’ that makes the adhesive forces very small between the oil layer and the emerging solid part,” said Walker. “Consequently, we can print the part in a continuous manner as opposed to a laminated part. At the same time, the oil also carries away all the heat,” he said, “by being routed through a cooling system.”

And scale it up they did, in an effort that eventually led the pair of researchers to create a start-up company with Professor Mirkin. At Azul 3D™ in Skokie, Illinois, Mirkin serves as chairman of the board, Hedrick is CEO and Walker is the company’s CTO. The start-up has garnered wide interest with its HARP™ technology, including recognition by RadTech International North America.

Generated heat, said Walker, “is not a large issue until you begin to print faster and over larger build areas, since the rate of heat production is proportional to volumetric throughput.” In fast printing, generated heat can damage the 3D printers themselves, and frequently can cause damage to solid membranes and interfaces. Thanks to HARP’s™ fluorinated oil, there is nothing to damage. “With the use of a liquid that is stable at incredibly high temperatures,” he said, “there’s really not much that we have to worry about in terms of damage to our printer hardware.”

Azul 3D™ is one of the five key technology developers chosen by RadLaunch (RadTech’s idea accelerator for UV and EB

“Our vision therefore became more focused on the printed part getting damaged or deformed by heat, and trying to avoid this,”

10 | UV+EB Technology • Quarter 2, 2020

uvebtechnology.com + radtech.org


faced during their development of HARP™, Walker pivoted to a topic that is daunting to many an entrepreneur. “James and I are both researchers at heart – developing the tech was a fun venture,” he said. “The largest challenge has been in developing a company around the technology. Designing promising technology is only the first step of that journey. “When you are working at a lab bench, it is not entirely clear how private industry might best make use of any technology,” he continued. “We have spent the last two years honing our business plan and approach, and proving out just how enabling this technology can be. It’s exciting because the more we learn about the field, the more we realize the power of what we’ve been able to develop.” Walker continued. Firsthand experience with the effects of heat made this clear for the researcher. “When I observed our first parts smoking with the earliest embodiments of our technology, I knew heat mitigation and dissipation would be key. “It is such an issue that new printers coming onto the market are beginning to integrate IR thermal imaging of the emerging object,” he said. With the flowing oil technique, Walker explained, “HARP™ offers the first way to mitigate and control this issue while maintaining rapid print speeds.” In the materials that can be used and the objects that can be built, HARP™ once again shows great promise. “HARP™ can operate without the need of any oxygen present because it does not rely upon an oxygen ‘dead-layer,’” Walker said. “This is what enables us to use a broader range of oxygen-insensitive chemistries.” HARP™ can handle all light-driven resins, including those with flexible and rubber-like characteristics, as well as hightemperature ceramics and machinable compounds. And, the technology can build one very large object or multiple, disparate objects. “We can print a large array of nonidentical objects at the same time,” Walker explained. “Mass customization is one of the key boons of 3D printing for manufacturing. “The key limiting factor here is software for designing customized objects in a high throughput manner,” he continued, “but we are working on this. Right now, an object takes longer to design than to print. I believe AI and machine learning will drastically change this in the coming years.” From technology development to commercial capitalization When asked to pinpoint the biggest challenge the researchers uvebtechnology.com + radtech.org

On the journey from the laboratory to industry, the founders of Azul 3D™ have capitalized on an association with RadTech, as Walker detailed. “Since exiting stealth mode, our company has had a presence at the RadTech/NIST PAM 2019 Workshop in Boulder, Colorado, and at the 2019 RadTech Fall Meeting in Dearborn, Michigan,” he said. “We found these meetings and the connections made during them to be immensely helpful. “The RadLaunch award allows us to be active members within the community while we are in a pre-revenue state,” he continued. “We are excited to introduce this technology to the field and to help push the photopolymer additive manufacturing community forward.” Azul 3D™ is making strides toward commercializing the HARP™ technology. “We are finalizing a few key beta users and expect to be launching a product at the end of next year,” said Walker. The company is poised to announce some early adopters in 2020. “Importantly, these early adopters are all targeting the long-term manufacture of goods,” stated Walker, “and not just short-run productions with 3D printing as a novelty.” The manufacturers that are set to implement HARP™ are varied. “Some of them are producing fun and flashy luxury goods, as the field has come to expect,” said Walker, “but others are less obvious and are in more commonplace products where you might not expect a 3D-printed part.” This fits in with Walker’s articulated goal for HARP™: “We don’t want to see 3D printing as a unique novelty in the manufacturing space, but rather a new tool that becomes ingrained within the manufacturing supply chain.” u

UV+EB Technology • Quarter 2, 2020 | 11


PROFESSOR’S CORNER BACK TO THE BASICS OF UV/EB

Understanding Glass Transition Temperature: Part 2 I

n the last edition of Professor’s Corner, we began a discussion of the glass transition temperature (Tg).1 In this issue, we will continue that discussion by investigating three methods for experimental measurement of this all-important polymer property: 1) Dilatometry; 2) Differential Scanning Calorimetry (DSC); 3) Dynamic Mechanical Analysis (DMA). These three methods are based on three different polymer properties that change significantly and in a measurable way when the temperature approaches the Tg. These include changes in specific volume, changes in heat capacity and changes in the ability of the polymer to store or dissipate mechanical energy.

Free Volume Increase. Since the Tg of a polymer is the temperature at which the onset of long-range segmental motion occurs,2 perhaps the most obvious physical change it must undergo as the temperature nears the Tg is that its “free volume” must increase to provide room or space for the segments to actually move. Free volume is the space within a sample that is not occupied by any molecule or atom. It is literally “empty space.” The spaces seen among the spaghetti strands in the model shown in Figure 1 of the previous edition of Professor’s Corner represent the free volume.3 So, as we think about the occurrence of increased movement of the relatively large segments of polymer chains at the Tg, additional space for these movements is required. Therefore, any method that allows one to measure the increase in volume of a given polymer sample with increasing temperature can be useful in determining the Tg. Dilatometry is a widely used method for measuring changes in specific volume, the reciprocal of the density, in polymers.4. As the density decreases with increasing temperature, the specific volume increases. It turns out that, for polymers, the specific volume increases in approximately linear fashion with temperature. At the Tg, a linear relationship continues, but at a distinctly different slope and higher rate of increase. These Figure 1. Specific volume vs. effects are depicted temperature for amorphous domains schematically in Figure in a polymer 1. The inflection point 12 | UV+EB Technology • Quarter 2, 2020

in the curve in Figure 1 is indicative of the Tg. (Recall from the previous article that only the amorphous regions of the polymer will exhibit a Tg.)5 Thermal Energy Changes. Since the structure of a polymer changes as it passes through the Tg, many of the fundamental properties, in addition to specific volume, also change. These include thermal properties that can be measured by several common thermoanalytical methods, including DSC, DMA and others. A detailed discussion of these methods of analysis is beyond the scope of this column. However, a brief discussion of DSC and DMA is provided. DSC is based on the principle that, as a material goes through various thermodynamic phase transitions, both “primary” and “secondary,” thermal energy is either absorbed (endothermic) Figure 2. Example DSC thermogram or emitted (exothermic). Since the Tg involves a structural change at the molecular level, the sample is expected to have a measurable change in enthalpy (∆H) during that transition. DSC methodology measures such changes for relatively small samples (~0.5 to 10 mg).6 Stevens gives a good summary of DSC instrumentation.7 While raising the temperature of the sample and a reference at a constant rate, the instrument records the thermal changes occurring. These changes are then recorded as a DSC “thermogram.” Figure 2 is a schematic representation of a thermogram. As the temperature is increasing, the absorption or emission of thermal energy is recorded. In Figure 2, a downward change in the curve represents an endothermic change, the first of which is an indication of the Tg for the sample. The second endothermic change indicates the crystalline melting temperature (Tm). Note that the Tm endotherm is much larger than the Tg. A third transition – exothermic in this case – is depicted between the other two. This is known as the heat of crystallization (∆Hcryst). Some polymers, once having passed through their Tg, will experience a realignment of chains, resulting in an increase in microcrystallinity. The resulting attractive forces among the chains will produce an exothermic uvebtechnology.com + radtech.org


the molecular level, the “net” formed by the crosslinks eventually reaches its maximum extension. To expand farther would break covalent chemical bonds, and decomposition would occur. So, the storage modulus reaches a minimum and then becomes independent of temperature. This region of the thermogram is known as the “rubbery plateau.” The storage modulus value in the rubbery plateau is a function of the crosslink density of the polymer.

Storage Modulus Tg

Rubbery Plateau Loss Modulus

Tan δ Evidence of Heterogeneity

Figure 3. DMA thermogram

change in enthalpy. UV/EB-polymerized materials typically will not exhibit a ∆Hcryst. DMA is a very sensitive, though more expensive, method for determining the Tg of polymers. The technique involves sinusoidally straining and relaxing a sample at a given frequency as the temperature is raised. Viscoelastic materials, such as polymers, are those that have both “solid-like” and “liquid-like” properties. During a DMA scan, the solid-like properties of the sample are recorded as the “storage modulus” – the ability of the sample to store mechanical energy during testing. Conversely, the liquid-like properties of the polymer are recorded as the “loss modulus” – the ability of the sample to dissipate mechanical energy through molecular frictional interactions. Figure 3 provides a representative DMA thermogram for a UVpolymerized material. The DMA properties were measured using three samples to test reproducibility. The green curves represent the storage modulus change with temperature, while the blue curves represent the loss modulus change. The red curves (known as “tan δ”) are calculated from the ratio of the loss and storage moduli rather than by direct measurement. The maximum in the tan δ is generally taken to represent the Tg! It is the temperature at which the liquid-like properties are at a maximum relative to the solid-like properties. (Note that the loss modulus scale is linear and relatively small while the storage modulus scale is logarithmic.) When comparing Figure 2 with Figure 3, it is obvious that DMA provides a much more sensitive measurement of the Tg. As the temperature increases during a DMA scan, the UVpolymerized and crosslinked sample represented in Figure 3 expands. With this expansion, the sample loses the ability to store energy, and the green curve descends. As the sample expands, at uvebtechnology.com + radtech.org

What about the loss modulus? As the sample begins to expand, its ability to dissipate energy through frictional interactions increases since there are more energetic collisions occurring. But then those frictional forces reach a maximum, after which they decline rapidly as the polymer chains move farther apart. In the case of a crosslinked polymer, when the network has reached the maximum allowable extension, the frictional energy loss drops to near zero. DMA is an awesome thermoanalytical method that can provide a wealth of information about the structure and nature of polymers. Figure 3 provides one more piece of information relevant to these Tg discussions. Note that the half-height peak width has been highlighted. This measurement is related to the relative heterogeneity of the polymer film at the molecular level.8 The tan δ curve can be thought of as representing a mixture of different Tgs for different domains within the polymer sample. Were the polymer perfectly homogeneous, this curve would be a “spike,” and the entire sample would have a single Tg. What produces this heterogeneity? Ah! That is a topic for a future article. Stay tuned to the Professor’s Corner! u References: 1. Christmas, B. K., “Professor’s Corner,” UV+EB Technology, 6, No. 1, 1st Quarter, 2020, pp. 16,17. 2. ibid., p. 16 3. ibid. 4. van der Beek, Maurice H.E. Specific Volume of Polymers: Influence of the Thermomechanical History, Technical University of Eindhoven, Eindhoven, The Netherlands, 2005, p. 2. https://doi. org/10.6100/IR590887 5. op. cit., Christmas, pg. 16 6. Stevens, Malcolm P., Polymer Chemistry: An Introduction, 3rd Edition, Oxford University Press, 1999, p. 150 7. ibid., pp. 150-152. 8. Morales, Timothy; Rueda, Elda; and Christmas, Byron. “The Relative Effects of AC- and DC- Powered UV Lamp Systems on the Properties of UV-Polymerized Films,” Proceedings, UV&EB Technical Conference, RadTech International, North America, Chicago, 2008.

Byron K. Christmas, Ph.D.

Professor of Chemistry, Emeritus University of Houston-Downtown b4christmas@gmail.com

UV+EB Technology • Quarter 2, 2020 | 13


BEST PAPER By Sage M. Schissel, PCT Ebeam and Integration, LLC

Recipient of the Best Paper Award at RadTech 2020 in Orlando, Florida, March 9-11.

Attainable Sustainable: Using Electron Beam Technology in Compostable Flexible Packaging Editor’s Note: In addition, the test results are ongoing for this study. An update will be posted at www. uvebtechnology.com. Due to print space limitations, the article references can be found online in the Article Archive.

I

n recent years, the packaging industry has experienced a consumer backlash regarding its use of plastics.1-3 In response, companies have made sustainability pledges that include goals such as reducing their plastic consumption, reducing production waste and increasing the amount of recycled plastic in products.4-6 These strides toward a more environment-friendly future are commendable but are not always easily achieved. While consumers call for less single-use packaging waste, their expectations of how packaging performs – that food remains fresh, products remain undamaged and marketing remains eye-catching – have not waivered. Electron beam (EB) technology is a multifaceted tool that is well suited for aiding the achievement of sustainability goals.7-12 For instance, EB crosslinking can improve mechanical properties, allowing for reduced plastic consumption by down-gauging film. It also can be used to increase compatibility between different polymers, as well as virgin and recycled polymer blends. EB-induced chain scission can be used to recycle materials through degradation. Scrap polytetrafluroethylene (PTFE), for example, is irradiated to create micropowders that become ingredients in lubricants, inks and coatings. In flexible packaging production, EB is a useful tool in accomplishing one of the industry’s more recent trends – mono-layer and/or mono-material packaging. Traditionally, flexible packaging has relied on multilayer, nonrecyclable structures to achieve the wide array of demands placed on a single pouch, from moisture and oxygen barriers to tear strength and sealability.13,14 Mono-material structures are expected to meet these demands with a single type of plastic so that they can be more easily recycled or composted, but it is an ambitious challenge. One way Sample Film OPV Ebeam Settings EB can help meet this challenge is by A NK120 – – altering a polymer without any added chemistry. Electron beam crosslinking B NK120 – 30 kGy / 115 kV and chain scission provide property tuning C NK120 EG 30 kGy / 115 kV that can be controlled through a selected D NK120 EM 30 kGy / 115 kV substrate depth. A common example of Table 1. Unprinted samples tested for compostability. this application is crosslinking an outer layer of polyethylene (PE) to increase heat resistance without Post-treatment affecting the sealing Sample Film OPV Ebeam Settings Ebeam Settings temperature of the inner A NK120 EG 30 kGy / 115 kV – PE layer. Additionally, EB can be used to B NK120 EG 30 kGy / 115 kV 150 kGy / 200 kV polymerize inks and C NK120 EG 30 kGy / 115 kV 300 kGy / 200 kV overprint varnishes (OPVs) without the need Table 2. Printed and post-treated samples tested for compostability.

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A.

B.

C.

D.

Figure 1. Results of the compostability testing for unprinted samples. After 6 weeks, the OPV-coated samples (C and D) have the same amount of uncomposted material left in the frame as the plain film (A). Sample conditions are listed in Table 1.

for initiators or solvents and with comparatively little energy.15-18 OPVs are a sustainable alternative to lamination. Because inks and OPVs make up such a small fraction of the overall package, they’ve been shown to not hinder recyclability.19,20 Accompanying the mono-material trend to make flexible packaging recyclable is a movement for compostable flexible packaging. Instead of relying on the ability to effectively repurpose packaging plastics, compostable packaging seeks to optimize the disposal of packaging waste by using biodegradable polymers. EB is less tested as a production solution in compostable packaging; however, there are several ways this technology may benefit the industry. Similar to recyclable packaging, it is expected that EB-cured inks and OPVs can be utilized without impacting the biodegradability of compostable packaging. Furthermore, EB could possibly result in accelerated disintegration if the packaging is exposed to high doses after consumer use. Molecular weight reduction by chain scission is a known effect of EB irradiation on cellulose, a common compostable material.21 It is hypothesized that this degradation could be leveraged to uvebtechnology.com + radtech.org

decrease composting time. As compostable plastic packaging gains popularity, a reduction in the multi-week disintegration process may be imperative for the current infrastructure to keep up with the increasing supply. According to a survey conducted by BioCycle in 2018, of the 185 identified food waste compost centers in the US, only 53 reported the ability to accept compostable plastics.22 In addition, accelerating plastic disintegration should make it a more profitable enterprise; high volume turnover will help offset costs, such as sorting noncompostable plastics out of the feedstock. In this study, the application of EB in the production and destruction of a mono-material, compostable flexible food packaging structure was investigated. Low doses of EB were used to cure matte and gloss OPVs on the compostable film, which was then tested to determine if the OPV inhibited or impeded compostability. Moreover, high doses of EB were used to induce chain scissioning in the packaging structure (film/ink/EB-cured OPV) to ascertain whether such degradation at the end-of-life could efficiently reduce compost times. Puncture strength was measured as a means of analyzing the EB degradation. page 16 u UV+EB Technology • Quarter 2, 2020 | 15


BEST PAPER t page 15

0 kGy

50 kGy

100 kGy

150 kGy

200 kGy

250 kGy

300 kGy

350 kGy

400 kGy

Figure 2. Visual effects of EB post-treatment. Samples are coated with EB-cured EG OPV (Table 2).

Experimental Materials The substrate material used in this study was NatureFlex™ NK 120 gauge (NK120, Futamura).23 NK120 is a transparent cellulose film coated with polyvinylidene chloride (PVDC) for moisture and gas barrier properties. It was selected because it is suitable both as a laminate and for mono-layer flexible packaging applications. NK120 is certified as industrial and home compostable. The primer used was DigiPrime® 050 (Michaelman). The ink used was the CMYK Indigo ElectroInk digital ink set (Hewlett-Packard).20 These digital inks are indirect food-contactsafe and certified as industrial and home compostable.24,25 In addition, the digital printing process has some environmental advantages over analog methods, including less material waste, low energy consumption and no printing plates or cylinders. The overprint varnishes (OPVs) used in this study were EHG2601 (EG, DBT Coatings) and EMQ-3710 (EM, DBT Coatings) with high-gloss and matte finishes, respectively.26,27 These EBcurable OPVs were chosen because they are used in the flexible food packaging industry and are formulated to protect and highlight the digital inks. Both OPVs meet ultra-low migration standards, are indirect-food-contact safe and are free of initiators and solvents. Methods Sample Preparation NK120 film was coated with primer and printed using an HP Indigo 20000 digital press. Unprinted NK120 film was used for some samples. Both the printed and unprinted film were then corona treated at 13.8 W/in, OPV was applied with an indirect gravure coater, and the OPV was cured at 30 kGy and 115 kV

using a Broadbeam EP electron beam pilot line (PCT Ebeam and Integration). Oxygen levels were kept to < 200 ppm using a flow of 99.999% pure N2 in the beam. The EM OPV was applied using a 5 BCM, 400 line ceramic anilox with a resulting coat weight of approximately 2.0 g/m2. The EG OPV was applied using a 10 BCM, 200 line steel anilox with a resulting coat weight of approximately 3.5 g/m2. Additionally, some samples were exposed to EB for a second time for a post-treatment, which was intended to reduce composting time through degradation of the cellulose film. The post-treatment was done in air, at 200 kV, 50 ft/min, and at doses ranging from 50 to 400 kGy. To ensure an equal dose distribution through the complete thickness of the film, 200 kV was chosen as an accelerating voltage. Compostability Testing To determine the influence of the samples on the composting process, samples were composted by Organic Waste Systems (OWS) following the test method detailed in ISO 16929:2013.28 The pilot-scale aerobic composting test consisted of organic biowaste (a mixture of vegetable, garden and fruit waste) in a 200-L composting bin monitored through temperature and exhaust gas composition for 12 weeks. The mixture was turned by hand every 1 to 2 weeks. In order for the test to be deemed valid, the temperature must remain between 60 and 75°C during the first week and below 65°C thereafter, with the minimum temperature remaining above 40°C for at least four consecutive weeks. Photographs were taken on a weekly basis to visually evaluate the percentage of disintegration the sample material had undergone. Because of this qualitative evaluation metric, the results of this compostability testing serve only as an indicator of whether the sample will pass a quantitative (mass balance) test. The unprinted page 18 u

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BEST PAPER t page 16 means of accelerating the disintegration during composting. High doses were used to weaken the compostable film through chain scission. Packaging Compostability An integral aspect of using EB-cured OPVs in compostable flexible packaging is demonstrating that the OPVs do not inhibit or significantly impede the disintegration of the compostable film. In addition, because EB is well known to interact with cellulose, it is also important to establish what effect an EB curing dose (30 kGy) has on the compostability of the film.21,29 To this end, select samples (Table 1) were composted, and their disintegration progress visually documented (Figure 1). Comparing the control film (Figure 1A) to a film that has received a curing level dose (1B), the EB dose does not appear to Figure 3. Puncture resistance of the compostable packaging film decreases as the EB have a significant effect. After two weeks post-treatment dose is increased. The grey square (n) and triangle (p) are samples composting, the EB sample (1B) appears A and B of Table 1, respectively. The black circles (l) represent samples of the to have slightly more disintegration than construction listed in Table 2. the control (1A), but those impressions flip after three weeks. After four weeks, samples tested for compostability are listed in Table 1 and the both samples are almost completely disintegrated, with only a few printed and post-treated samples are listed in Table 2. small pieces of film still left at the edges of the test frame. Puncture Testing Puncture resistance was used as a measure of film strength after EB exposure. Each 6.5-inch square of sample material was held taut in a fiberglass board frame (Micarta) using a rubber O-ring with a 5-inch outer diameter and 0.210-inch width. The frame had a 4-inch diameter circular window exposing the sample. A compressive load was applied to the center of the exposed sample at a rate of approximately 3 to 4 lb/s using a rounded probe with an arc 1.094 inches wide and 0.270 inches high. A broad probe was chosen to gain better resolution of the film strength lost at varying EB doses. The applied load was measured using a Uline platform dial scale (model no. H-176). The recorded value was taken as the maximum load applied before the sample ruptured, and the probe was able to push through the sample. For each sample condition, five repetitions were completed, and the values reported are an average of those repetitions. The error reported is the standard deviation of the five repetitions. Results and Discussion The purpose of this study is to establish EB-curable OPVs can be used in the production of compostable flexible food packaging without significantly impeding the compostability of the packaging. Furthermore, EB exposure was investigated as a 18 | UV+EB Technology â&#x20AC;˘ Quarter 2, 2020

The addition of an EB-cured OPV also does not appear to significantly impact the disintegration time of the film. The majority of both the EM-coated (Figure 1C) and EG-coated (1D) samples was disintegrated after four weeks. Both samples retained slightly more film at the edges of the test frame after four weeks than the control (1A); however, by the end of six weeks (a five-week photo not being included in the test results), the disintegration levels of the OPV-coated samples and the control are visually the same. Post-treatment of Packaging With the compostability testing providing positive qualitative results and demonstrating that EB-cured OPVs can be effectively used in the production of compostable flexible food packaging, the potential of EB to affect a package after consumer use was considered. Composting, even on an industrial scale with controlled conditions, is a time-intensive, multi-week process. Accelerating the disintegration of material could allow compost facilities to efficiently convert a higher volume of packaging with little to no change in infrastructure. Visual Effects The degradation of the compostable packaging structure (film/ uvebtechnology.com + radtech.org


An integral aspect of using EBcured OPVs in compostable flexible packaging is demonstrating that the OPVs do not inhibit or significantly impede the disintegration of the compostable film. print/OPV) caused by EB irradiation was first evaluated visually (Figure 2). Samples exposed to an EB post-treatment dose of 50 to 400 kGy were compared to a control (Figure 2, 0 kGy). Remarkably, there are no discernible effects of the EB posttreatment until 150 kGy. At 150 kGy, there is some slight discoloration of the film and noticeable cracking of the OPV. As higher post-treatment doses are applied, the yellowing of the film intensifies and the film shrinks and wrinkles. Puncture Strength Reduction Puncture strength was used as quantitative measure of the scissioning effect caused by high doses of EB. As chain scission increases, the strength of the film is expected to decrease. Figure 3 shows a clear correlation between the puncture resistance of the compostable film and the dose level of the EB posttreatment. Comparing the plain NK120 film to the plain film after receiving a curing-level dose (Figure 3, grey square and triangle, respectively), there is an approximately 5 lb decrease in puncture resistance. Note, there is some overlap of the error in these measures. No significant difference is seen when print and EG OPV is added to the construction (black circle, 0 kGy). As the EB post-treatment dose is increased, the puncture force decreases. The relationship of these two variables follows the trend line of a second order polynomial with an R2 value of 0.9869. Interestingly, there is almost no loss of puncture resistance between the 0 kGy and 100 kGy post-treatment dose. While EB dose levels are generally kept quite low for curing (~30 kGy), this result, along with the visual results, demonstrates there is potentially a much larger EB operating window than previously thought. The effect of EB on other mechanical properties would need to be investigated to confirm. However, a larger operating window could be a beneficial option for improving ink and coating performance through enhanced crosslinking.

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Compost Results Based on the results of the puncture strength testing, three samples were chosen to test the effect of EB post-treatment on the compost rate of the packaging structure, a control and two different post-treatment doses (Table 2). Currently, the test is ongoing; however, significant differences among the samples already have been observed (Figure 4). After one week, the control and 150 kGy post-treatment samples turned brown, but no disintegration had yet occurred. Contrastingly, the 300 kGy post-treatment sample had already experienced a significant amount of disintegration. After two weeks, a large portion of the 150 kGy post-treatment sample had disintegrated, while the control sample was still almost completely intact. The compostability test will last 12 weeks, but as of this writing, using a high dose EB post-treatment to increase the rate of disintegration of compostable material looks promising. Conclusions In conclusion, a qualitative compostability test was conducted and showed that EB-cured OPV did not make a significant impact on the compost rate of a cellulose film. These results indicate that the reviewed structures would likely pass a quantitative, mass balance test. page 20 u

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BEST PAPER t page 19

A.

B.

Furthermore, a post-treatment EB dose was evaluated as a potential means of increasing the composting rate of the flexible packaging structure. The degradation of the film was confirmed visually as well as by demonstrating that puncture strength decreased as the post-treatment dose was increased. The qualitative compostability testing of these post-treated structures is ongoing; however, after four weeks of testing, high-dose EB post-treatment was shown to positively impact the composting rate. In addition to quantitative compostability testing, future work in this arena includes broadening the scope of OPVs, inks and

C. Figure 4. Results of the compostability testing for printed samples with EB post-treatment. After one week, the 300 kGy post-treatment sample (4C) shows significant disintegration, while the control (4A) and 150 kGy post-treatment samples (4B) are completely intact. This test will be completed after 12 weeks. Sample conditions are listed in Table 2.

compostable substrates investigated. The optimal EB dose necessary to degrade a compostable structure is expected to be dependent on the chemistry of the film and should also be evaluated for a wide variety of compostable material. The broad applicability of EB was demonstrated by establishing the technology as a tool for both the production of compostable packaging as well as the degradation of it after use. As the packaging industry endeavors toward a more sustainable future, versatile technologies, such as EB, provide companies flexibility in developing new avenues to achieve their recycling and composting targets. u

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20 | UV+EB Technology â&#x20AC;˘ Quarter 2, 2020

Acknowledgements The author would like to acknowledge DBT Coatings for initiating and funding the first round of compostability testing and sharing the results (Figure 1). These results provided the impetus for further exploration of electron beamâ&#x20AC;&#x2122;s role in compostable packaging. The author would also like to acknowledge Futamura for donating the NK120 film and GOpak for donating printing press time and materials. uvebtechnology.com + radtech.org


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BEST STUDENT PAPER By Nicole L. K. Thiher, Chemical and Biochemical Engineering Department, University of Iowa; Erin Peters, Rockridge High School; Sage M. Schissel,  PCT Ebeam and Integration, LLC; and Julie L. P. Jessop, School of Chemical Engineering, Mississippi State University 

Recipient of the Best Student Paper Award at RadTech 2020 in Orlando, Florida, March 9-11.

Comparison of UV- and EB-initiated Polymerizations Based on Equivalent Radical Concentration Editor’s Note: Due to print space limitations, the article references can be found online in the Article Archive at www.uvebtechnology.com. Abstract n this study, a protocol was developed to investigate UV- and EB-polymerized films of equivalent radical concentrations. This protocol then was applied to an acrylate/methacrylate pair to characterize the impact of the initiation mechanism. Raman spectroscopy was used to determine differences in polymer conversion. Monomer chemistry was shown to be a key variable in the comparison of the two initiation mechanisms.

I

Introduction Two common techniques for radiation-induced polymerization are ultraviolet (UV) and electron beam (EB). Both techniques have advantages over thermal initiation and can be used to create similar products,1,2 including inks, adhesives and coatings.3 Although UV and EB polymerization techniques are similar in many respects, there are differences between the two initiation mechanisms that can affect the polymerization kinetics and polymer properties.4-7 A major difference in UV and EB initiation is how primary radicals are formed. UV requires a photoinitiator, which absorbs light and decomposes to form predictable radical structures.8,9 In contrast, EB has enough energy to form radicals on any molecule in the formulation – even on the body of a polymer chain – and does not require an initiator.10,11 Radical formation during EB exposure is much less predictable than during UV exposure due to this ability to form radicals nearly anywhere. Non-selectivity can result in secondary reactions, such as crosslinking, instead of the desired secondary initiation reaction that is required for polymer formation.12 Other studies have shown that monomer chemistry is a key factor in radical formation and secondary reactions during EB polymerization.12,13 In one study, an increase in labile bonds in a monomer led to a higher concentration of primary radicals. However, this increase in primary radical concentration did not necessarily lead to an increase in the rate of polymerization or a higher final conversion.12 Furthermore, this study also showed differences when comparing the conversion of acrylates and methacrylates. These differences are consistent with major kinetic differences observed during UV-initiation of acrylates and methacrylates in the past.14-18 These previous studies show there is a difference between the kinetics of acrylate and methacrylate polymerization; however, the authors have not found any studies that compare the impact of initiation mechanism on the kinetics of radiation-induced polymerization of acrylates and methacrylates. In fact, few studies have focused on comparing the kinetics of UV and EB polymerization in general.5-7 Schissel et al. found differences in conversion and physical properties of polymers initiated with UV and EB using equal energy deposition.19 Comparing UV and EB with equal initiation energy highlighted some of the differences in the initiation methods, but other questions remained unanswered. It is known that radical formation, concentration and secondary reactions are reliant on monomer chemistry, which affects polymerization kinetics. The comparison of UV and EB polymerization at equal primary

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radical concentrations will reveal the differences in kinetics and network formation driven by monomer chemistry and initiation mechanism. To compare initiation mechanisms, an acrylate/ methacrylate pair was exposed to UV and EB, resulting in equal primary radical concentrations. Using Raman microscopy to measure conversion, a relationship among formulation chemistry, initiation mechanism and polymerization kinetics was elucidated for these acrylate/methacrylate systems. Experimental Materials An acrylate/methacrylate pair was chosen to further study the significance of monomer chemistry and initiation mechanism on radiation-induced polymerization kinetics:12,19 2- phenylethyl acrylate (PEA, TCI America) and 2-phenylethylmethacrylate (PEMA, Polysciences) (Figure 1). To induce UV polymerization, the photoinitiator 2,2-dimethoxy-2phenylacetophenone (DMPA, Sartomer) was used. The free-radical inhibitor 2,2,1-diphenyl1picrylhyrazl (DPPH, TCI America) was used to quantify radical formation. All materials were used as received and stored at room temperature.

(A)

(B)

Figure 1. Chemical structures of the acrylate/methacrylate monomers used in this study: (A) PEA and (B) PEMA

Methods Radical concentration The method to quantify primary radical concentration during EB exposure has been described in detail previously,20 and only a brief outline of the method is presented here. In previous ionizing radiation literature, the radical concentration was reported in terms of a radiation chemical yield or G-value. Here, radical concentration is reported in mol/L as opposed to radicals per 100 eV or mol/J, as are typically used to report G-values. The unit change was made to accommodate discussion of both EB and UV initiation mechanisms since G-values are associated with EB polymerization but not with UV polymerization. To determine the primary radical concentration in mol/L, the rate of radical formation (đ?&#x2018;&#x2026;R) was multiplied by the reaction time. đ?&#x2018;&#x2026;R was determined by adding an inhibitor to the monomer formulations, which reacted with primary radicals upon their uvebtechnology.com + radtech.org

Monomer

PEA

PEMA

Effective Irradiance (mW/cm2)

600

675

Table 1. Effective irradiance of the UV lamp required to match the radical concentrations for each monomer during EB exposure

formation during EB exposure. The disappearance of inhibitor, marked by a color change, was directly proportional to the rate of radical formation (đ?&#x2018;&#x2026;R) and was monitored using UV-Vis spectroscopy (DU-62 Spectrophotometer, Beckman). Once the primary radical concentration of each monomer was determined for EB exposure, a program written by Kenning et al. was used to determine the UV requirements necessary to match the radical concentration during UV exposure.21 This program used a set of differential equations to model polychromatic illumination based on initiator concentration, absorbance and efficiency. For each monomer, the concentration of photoinitiator was set at 0.14 mol/L, which was the lowest concentration that allowed the UV radical concentration targets to be achieved with the available lamp. Low initiator concentration was desirable because high concentration of initiator is known to block UV light penetration.8 The exposure time was set at 1 s to match the exposure time of the EB reactions. Matching the exposure time meant that not only would the total radical concentration of the UV and EB reactions be the same, but the average rate of radical formation also would be equivalent. For the program, the quantum yield of DMPA was estimated to be 0.2.22 Conversion measurements EB sample preparation. Neat monomer was pipetted onto a glass slide, and a tape spacer was used to achieve a sample thickness of ~100 Âľm. Samples were polymerized using an EBLab unit (Comet Technologies, Inc.) or an EB Pilot Line (BroadBeam EP Series, PCT Ebeam and Integration, LLC). The voltage of both EB units was set at 200 kV to ensure uniform energy deposition throughout each sample. Nitrogen flow was used to reduce the oxygen concentration to less than 200 ppm to minimize the effects of oxygen inhibition. Ten exposure conditions were used for the EBLab experiments, all at a constant dose rate of 197Âą4 kGy/s. The dose rate was held constant because altering the dose rate has been shown to have an impact on conversion.13,23,24 The dose and line speed combinations match those used to determine the G-values on the EBLab unit and are shown in Table 2. A single 400 kGy exposure at 1.5 m/min was carried out on the EB Pilot line, which was used to gather conversion data rather than determine radical concentration. UV sample preparation. Monomer containing 0.14 mol/L DMPA was pipetted onto glass slides with a tape spacer used to achieve a sample thickness of ~100 Âľm. Samples were polymerized page 24 u UV+EB Technology â&#x20AC;˘ Quarter 2, 2020 | 23


BEST STUDENT PAPER t page 23 Dose (kGy)

200

100

67

50

40

33

29

25

22

20

Line speed (m/min)

3

6

9

12

15

18

21

24

27

30

Table 2. Dose and line speed combinations used to create EB samples for conversion profiles

using an OmnicureÂŽ S1000 Ultraviolet/Visible Spot Cure System (Excelitas, 250-450 nm band pass filter) with a 3-mm liquid light guide at ambient temperature. The tip of the light guide was inserted into a nitrogen-purging chamber. The air gap between the sample surface and the light guide was ~1 mm. Nitrogen flow was used to reduce the oxygen level below 200 ppm to minimize the effects of oxygen inhibition. The effective irradiance was measured by a radiometer (Versaprobe Pro, Con-Trol Cure). Raman microscopy. Raman microscopy was used to determine conversion of the samples after polymerization. In order to eliminate error from instrumental variation and EB bombardment, a reference peak was used. Previous work has established the reaction peak at 1636 cm-1 (indicative of the -C=C- bond in the (meth)acrylate moiety) and a reference peak at 1613 cm-1 (indicative of the -C=C- bonds in the phenyl ring).25 Fractional conversion, Îą, was calculated using the following equation: (1)

where Irxn(đ?&#x2018;&#x192;) and Iref(đ?&#x2018;&#x192;) are the peak intensities of the reaction and reference peak of the polymer, respectively; Irxn( (đ?&#x2018;&#x20AC;) and Iref (đ?&#x2018;&#x20AC;) are the peak intensities of the reaction and reference peak of the monomer, respectively.26 Samples were transferred to aluminum Q-panels for analysis. Raman spectra of the samples were collected using an optical microscope (DMLP Leica) connected to a modular research Raman spectrograph (HoloLab 5000R, Kaiser Optical Systems, Inc.) via a 100-Âľm collection fiber. A single-mode excitation fiber carried an incident beam of 785-nm near-infrared laser to the sample through a 10x objective with a numerical aperture of 0.25 and a working distance of 5.8 mm. Laser power at the samples was ~8 mW. Spectra were collected with an exposure time of 30 s and 3 accumulations. Ten monomer spectra were collected and averaged to provide accurate values for Irxn(đ?&#x2018;&#x20AC;) and Iref (đ?&#x2018;&#x20AC;) to use in Equation 1. The error in the conversion measurements due to instrumental variation is expected to be Âą0.05. Results and Discussion To compare UV- and EB-initiated polymerizations, an acrylate/ methacrylate pair was examined. Polymerization kinetics were studied for UV and EB initiation with equal primary radical concentration. After samples were exposed to either UV or EB radiation, Raman microscopy was used to determine monomer conversion. 24 | UV+EB Technology â&#x20AC;˘ Quarter 2, 2020

Comparison of radical concentration and initiating energy Using the free-radical inhibitor (as described in the Methods section), primary radical concentration during EB exposure was quantified (Table 3). The methacrylate PEMA has three additional labile C-H bonds compared to its otherwise identical acrylate counterpart (PEA) due to the added methyl group. Although these additional labile bonds provide additional locations for primary radical formation, PEA forms approximately the same concentration of radicals (within error) as PEMA. Radical Concentration (mol/L) PEA

0.0115Âą0.0007

PEMA

0.0122Âą0.0006

Table 3. The primary radical concentration for the monomers used for both UV and EB initiation are shown. Note that the error values are for EB initiation. There are no error values associated with UV initiation because equivalent radical concentrations were determined through theoretical modeling.

In order to generate this same number of primary radicals, the energy required for EB is much different than that required for UV. For EB, the monomers received a dose of ~200 J/g. When using UV, the energy was varied to match the radical concentration of the EB samples. The more radicals produced during EB exposure, the higher the effective irradiance required to form the same quantity of radicals. The effective irradiance (I) was converted to dose (đ??ˇ) in using the exposure time (đ?&#x2018;Ą), in sample mass (đ?&#x2018;&#x161;), and area (đ??´) as follows:19 (2) Note that Equation 2 contains the implicit unit conversions: and . The EB and UV equivalent doses are compared in Table 4. EB Energy (J/g)

UV Energy (J/g)

PEA

200

58

PEMA

200

69

Table 4. The EB and UV equivalent doses required for initiation with equivalent primary radical concentration

EB initiation requires ~3 to 4 times the amount of energy as that required by UV to form the same number of primary radicals. The smaller energy requirements for UV initiation are likely the page 26 u uvebtechnology.com + radtech.org


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BEST STUDENT PAPER t page 24 conversion was observed for UV initiation compared to EB initiation (Figure 2A). The lower conversion of PEA during EB initiation compared to UV initiation could be the result of differences in primary radical structures that cause differences in secondary radical reactions. UV initiation typically results in predictable radical structures, while EB initiation tends to form many radicals with structures that are much harder to predict.8,10 During EB polymerization, some of the primary radical structures may undergo secondary radical reactions – such as crosslinking, recombination or termination – which do not lead to chain initiation.28 If fewer primary radicals are used to initiate polymerization, conversion is suppressed. In contrast to PEA, PEMA achieved nearly zero conversion regardless of initiation mechanism (Figure 2B). Previous studies have shown that methacrylates typically react much slower than their acrylate counterparts during UV illumination,14-18 and this trend holds under EB exposure as well. The lack of conversion of PEMA compared to PEA is likely due to the stability of the initiating methacrylate radical compared to the initiating acrylate radical. Conclusions The conversion of an acrylate/methacrylate pair was studied for UV and EB polymerization initiated with Figure 2. Conversion profiles are shown for UV- and EB-initiated PEA (A) and PEMA (B). UV initiation results in faster reactions and higher equal radical concentrations. Higher conversion of the final conversion compared to initiation with EB for PEA, while PEMA is acrylate monomer was achieved with UV initiation essentially unreactive for both UV and EB initiation. Note that the x- and when compared with EB initiation. Although both y-axes do not intersect at zero and the negative conversion values are monomers start with equivalent primary radical within the error of the measurement technique and indicate essentially no concentration, EB may be less likely to form initiating reaction. radicals compared to UV because of the nonselectivity of the accelerated electrons, which results in less secondary reactions leading to monomer conversion. result of differences in bond lability of photoinitiatiors compared to monomers. For example, the photoinitiator DMPA requires 52.1 kcal/mol to generate radicals during UV exposure;27 breaking For both UV and EB initiation, monomer chemistry has a large impact on polymerization kinetics. Methacrylates had relatively weak C-H bonds on monomer molecules requires ~100 significantly lower conversion than their acrylate counterpart due kcal/mol during EB irradiation.12 Additionally, there are many to the increased stability of the tertiary methacrylate propagating other reactions that can happen besides radical formation when radical compared to the secondary acrylate propagating radical. electrons interact with matter.11 The inefficiency of accelerated Knowing the differences that initiation mechanisms and monomer electrons is not well established; however, photon loss to other chemistry can make on polymer development can help guide reactions is characterized through the quantum yield of the formulation practices and extend the use of radiation-cured photoinitiator. materials in industrial processes. u Comparison of monomer conversion Acknowledgments For this study, EB and UV polymerizations were initiated at This work was supported by the National Science Foundation conditions to produce equal primary radical concentrations [grant number 1264622]. The authors would like to acknowledge in order to compare the impact of initiation mechanisms on Stan Howell for his contribution to the development of the radiation-induced polymerization kinetics. The conversion was nitrogen inerting chamber for UV polymerization. then measured using Raman microscopy (Figure 2). UV initiation of PEA was faster than EB initiation. Additionally, higher final 26 | UV+EB Technology • Quarter 2, 2020

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APPLICATION By Liz Stevens, contributing editor, UV+EB Technology

UV Curing Advances Printed and Novel Electronics at PARC R

esearcher Janos Veres and his colleagues at the Palo Alto Research Center (PARC) are shaping the future with their research and development – and with a helping hand from UV technology. Across projects as diverse as a “smart” mouth guard for athletes, a printed and embedded circuit with its own NFC antenna, and a multi-technique printer that combines three printing technologies and two curing methods, UV technology can be found playing a part in many of the innovations in Veres’ portfolio. The Palo Alto Research Center, located in Palo Alto, California, is a subsidiary of Xerox Corporation, initially founded in 1970 and established as an independent company in 2002. The center has six main foci: • AI and human-machine collaboration • Digital workplace • Novel printing • IoT and machine intelligence • Digital design and manufacturing • Microsystems and smart devices   At PARC, 175 researchers from 22 countries provide R&D and other services for clients ranging from Fortune 500 and Global 1000 firms to government agencies and start-ups worldwide. The researchers’ work has led to more than 2,000 patents, 4,000 scientific papers and 100 books.   Veres is program manager, Printed and Novel Electronics at PARC. His research revolves around creating electronics in new form factors, including large, flexible and conformal image sensors and detector arrays; exploring flexible, hybrid printed electronics for use in customized IoT devices; employing smart inks that can add electronic functionality to automotive and wearable devices; and investigating microchip inks from which electronics can be created and instructed to configure themselves, change shape or even disintegrate.   Of PARC’s six areas of focus, Veres concentrates on two: Microsystems and Smart Devices, and Novel Printing. His passion for pushing the envelope is contagious, and so is his enthusiasm for the possibilities – probabilities – for flexible, conformable electronics, seamlessly integrated electronics and the on-demand manufacture of electronics.   Veres summarized the nature of his recent research. “For the last two years or so, we’ve been working on leveraging the printing know-how that already exists with the innovation that exists at PARC,” he said. He hopes to see how the combined expertise can be deployed in the electronics industry. Veres anticipates the development of new printing techniques that use novel materials to build electronics “with new form factors that are flexible, accessible and foldable, and that may be thermoformable.”   His goal is not to create things by squeezing electronics into a particular shape – which is typically how people view microelectronics – but, rather, to create what could be called macroelectronics, “relevant for things that interface with the world and with people, like automotive interiors, wearable devices or sensors on aircraft wings.” The macroelectronics that Veres envisions, he said, “will require new form factors, often new materials, and new deposition techniques.”   Veres described the environment at PARC in vivid terms. “What we are doing at PARC is that we are operating, let’s say, a time machine,” he said. “We are thinking ahead – what could the world look like if

28 | UV+EB Technology • Quarter 2, 2020

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A customizable lactate biosensor in a mouthguard can help athletes monitor their performance.

we had these technologies at our fingertips? That’s why we experiment and develop things that could change electronics manufacturing.” Veres’ experiments and development projects are difficult to categorize as strictly a quest for this cutting-edge device or that novel process. For Veres, R&D on new devices can lead to the creation of fresh problem-solving processes, and exploration of emerging processes can open doors for imagining new functional devices.   In some cases, Veres explores existing but relatively new technology, such as 3D printing, and uses it as a jumping off point. “Most of 3D printing is directed to creating structures,” he said, “shapes that are difficult to obtain with conventional techniques.” Veres pushes the envelope, to take 3D printing to a new incarnation. “What we envision is that there will be a need to make these things smart, to actually add functionality to them so that they carry current or so that they send signals.”   In other cases, Veres seems to be reimagining a process altogether, as with his work in developing a smart label with printed sensors, a printed antenna and bare die chips. Veres also described a futuristic UV-cured embedded circuit. “It’s an NFC antenna,” he explained, “and this is built up as a monolithic piece from a UV-curable dielectric with the silver printed in the middle of the structure.” In product creation, designers often must make a choice as to what takes precedence – form or function. For Veres, the choice apparently is an emphatic “Both!” His impetus to design and refine flexible, conformable electronics results in high functionality that is molded (literally) into the desired form.   One such device that Veres is refining is a wearable device – a three-dimensional, customizable lactate biosensor in a mouth uvebtechnology.com + radtech.org

guard. Envisioned for athletes to help monitor their performance and now in the piloting stage, this sensor is built into a mouth guard structure that initially is UV-cured. The electronics for the biosensor feature enzyme-based printed biosensors, flexible and conformable circuitry, communications to send the sensed lactate levels to a smart watch via Bluetooth Low Energy and a wirelessly rechargeable battery. After the integration of its technology, the mouth guard structure can be thermoformed to the shape of an individual’s mouth.   Another device that Veres is refining is both device and process. It is an integrated printing platform that includes a multi-technique printer along with multiple curing technologies. Arrayed in one platform are heads for inkjet, aerosol jet and extrusion printing, a heated platen for thermal curing, and blocks for UV and radiant thermal curing. Engineers and software experts at PARC collaborated to develop this platform, which also included developing the workflow for driving the printers. In addition to controlling the printing and curing via software, the platform allows for computerized control of XYZ motion, and for droplet and surface visualization.   Veres notes that prototyping printers do exist on the market, but not in anything like the configuration that PARC has produced. “There are prototyping printers out there which you can buy from various equipment manufacturers that, say, give you an inkjet platform or an extrusion platform,” he said. “Or they separately sell you flash cure equipment or high-intensity photonic curing or aerosol jet. What we are doing is combining them because then we can do sequential printing of different materials.”   The platform’s UV curing block is of great benefit. “As we print inkjet dielectric, we can UV cure them right away,” Veres explained, noting that UV curing instantly produces the required solidified interior for dielectric. The integrated platform offers additional benefits. “Another reason why you need the different types of techniques is that you want to bring together different material systems,” said Veres, “and they will have different viscosities.” Some structural materials that may be utilized include urethanes, silicones, epoxies, acrylics, phase changes and hydrogels. Particle-based and precursor conductive materials also are common in the type of printing envisioned for Veres’ platform.   The inkjet, spray and extrusion printing methods bundled into the platform are compatible with their own range of viscosities. “Aerosol jet actually is a very unique technique,” said Veres, because it spans the viscosities of ink jet and extrusion printing. This technique is still being fine-tuned. “It’s very powerful,” he said, “but it’s typically a single nozzle technique and we don’t know yet how it is going to scale.”   page 30 u UV+EB Technology • Quarter 2, 2020 | 29


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30 | UV+EB Technology • Quarter 2, 2020

t page 29 The PARC integrated printing platform is configured, as of now, for printing sheets. “But it does have the ability to move on to rolls,” said Veres. “Ultimately, when we truly understand the need for high-volume manufacturing, we can transition it to rolls, but we probably won’t be stopping there.” While UV curing technology is only a part of Veres’ arsenal of techniques for fabricating the future, he points out some unique and important benefits of using it. “We use UV curing, in particular,” explained Veres, “for dielectric and structural materials for electronics, in a very similar way that UV curing is used for 3D printing and with inkjet-based curing.”   In his work on printed electronics, Veres seeks to replace the many discrete components that typically are assembled and affixed to a circuit board. “Quite a few of them can be readily transitioned to printable versions,” he said, “and that means electronics boards that you now see with discrete resistors can instead use printed resistors which become seamless with the board itself.” Seamless printing and integration of components, however, poses problems. “There is an important challenge in printing electronics that is actually very similar to printing on nonporous surfaces in the conventional printing industry. Because when it comes to printing on plastic, the surfaces are non-absorbing.   “You want to print fast and so there must be some technique to solidify the ink as it coats the product,” he continued, noting that the packaging industry already has solved this dilemma by using UV-curable inks. “We can deploy similar techniques for UV curing when it comes to printing dielectric layers,” he explained, “and that is very useful.”   Veres went on to describe another option for photonically curing nonoparticle metals (silver or copper) as part of the process of printing electronics. “The traces that are required for these electronics, very often they are photonically cured, and people use Xenon flash which is mostly visible light,” he said. “It might have on the tail end a little bit of UV, but the benefit is that you provide the energy to melt nanoparticle metals very rapidly. The metal particles in the inks are made from 10- to 15-millimeter size metal nanoparticles. When the particles are tiny, their melting temperature is much lower, so if you present a very high-intensity flash, you almost instantly can melt the particles without melting the plastic substrate underneath.   In a comment to the consortium of UV curing scientists and professionals, Veres offered this: “If there is one message for the UV curing community, it is this: There is this interesting area of printing electronics where UV-curable material can give a benefit because it allows for immediately solidifying the dielectrics that are not in between traces in the X and Y dimensions.”   At PARC, the future looks bright thanks to the hard work and fertile imagination of researchers like Janos Veres – and with a little help from UV. u uvebtechnology.com + radtech.org


TECHNOLOGY SHOWCASE Roland DGA Introduces VersaUV LEF2-300D Flatbed UV Printer Inkjet printer manufacturer Roland DGA, Irvine, California, has launched the VersaUV® LEF2-300D flatbed UV printer to give those selling their wares online a way to stand out by offering personalized and customized products. The LEF2-300D offers expanded workspace that supports a height of up to 7.87 inches (200 mm) – twice that of standard LEF2 models. It makes product customization easy and versatile, enabling users to print text and full-color graphics directly onto an even broader range of objects. For more information, visit www.rolanddga.com/products/printers/versauvlef2-300-flatbed-printer. Gigahertz-Optik UVC Radiometer Measures for Disinfection Safety Gigahertz-Optik Inc., Amesbury, Massachusetts, a manufacturer of innovative UV-VIS-NIR optical radiation measurement instrumentation, now offers the X1-1-UV-3726 radiometer, which enables the effectiveness of UV germicidal irradiation (UVGI) to be accurately determined for both low-pressure mercury (254 nm) germicidal lamps and UVC LEDs. Additionally, the device has sufficient sensitivity to detect if undesired exposure poses a photobiological safety risk to users. The X-1-1-UV-3726 radiometer measures UVC irradiance over a wide dynamic range to beyond 100 mW / cm² with a resolution of 0.0001 µW / cm². It is calibrated for its spectral responsivity from 250 nm to 300 nm. For more information, visit www.gigahertz-optik.com. Sirrus and Sartomer Advance Fast-Curing 3D Printing Resins Sartomer – a pioneer in advanced photocurable resin solutions – with US headquarters in Exton, Pennsylvania, and Sirrus – a developer of novel methylene malonate monomers and oligomers – in Loveland, Ohio, are partnering to create new fast-curing methacrylate 3D printing resins. The new 3D printing resins are based on the copolymerization of methylene malonates and methacrylates. “Research has demonstrated that methylene malonate comonomers can significantly enhance the UV cure rate of some methacrylates,” said Mark Holzer, Sirrus vice president of application development. For more information, visit www. sirruschemistry.com. Nagase Crosslinker Allows High-Performance Coatings Nagase America, LLC, a specialty chemical manufacturer and distributor with US headquarters in New York, has announced Denacol™ EX-622, a unique crosslinker that provides valueadded solutions to many coatings formulation challenges. Denacol EX-622 is a sorbitol polyglycidyl ether – its aliphatic epoxy uvebtechnology.com + radtech.org

backbone enables formulators to create coatings that are nonyellowing, weather resistant and low in color. Due to its tetrafunctionality, Denacol EX-622 increases reactivity and crosslink density, resulting in faster cure speeds and better resistance to water staining, chemicals and mechanical wear. The crosslinker can be used both in traditional epoxy systems and non-isocyanate (NISO) systems. For more information, visit www. nagaseamerica.com. Innovations in Optics Releases 6000B-100 Solar Simulator LED light source provider Innovations in Optics, Inc., Woburn, Massachusetts, has introduced the Model 6000B-100, a compact, multiwavelength LED solar simulator for PV manufacturers to test steady-state I-V measurements of photovoltaic devices. It meets Class AAA solar simulator requirements of IEC 609049 for spectral match, uniformity of irradiance and temporal stability. The 6000B-100 includes a digital driver/controller with RS-485 MODBUS RTU protocol. Embedded chip-scale spectral sensors provide feedback monitoring to stabilize source irradiance. The field of illumination is 50 x 50 mm at a working distance of 155 mm. For more information, visit www.innovationsinoptics.com. IST Metz UV and LED Technology for Printed Electronics UV equipment manufacturer IST Metz, Nürtingen, Germany, offers UV and LED units for fast, reliable curing of printed circuits. With a maximum lamp length of 550 mm, the MBS LI achieves a maximum output of 270 W/cm. Optimized air flow and a quartz glass pane between the lamp chamber and the substrate guarantee effective heat management in the UV. IST Metz, with subsidiary Integration Technology, presents the latest LED technology – systems consisting of freely configurable and addressable modular units, adaptable to the specific requirements of an application. The SPOTcure series for precise curing also is part of the product portfolio. The highpower, multispectral systems combine the radiant power and spectral characteristics of a mercury arc lamp with the TCO and process advantages of LED technology. For more information, visit www.ist-uv.com. u UV+EB Technology • Quarter 2, 2020 | 31


EVENT REVIEW

High-Energy Discussions at RadTech UV+EB Conference Reprinted with permission from Richard Romano, WhatTheyThink.com

A

t what may well have been the last in-person industry event for a while, professionals in the world of energy-curing technology gathered in Orlando in early March for RadTech’s 2020 UV+EB Technology Conference, three days of sessions devoted to ultraviolet (UV) and electron beam (EB) technologies. UV and EB straddle a number of disparate industries – a prominent one in particular at this year’s conference was the use of UV in healthcare as a disinfecting agent, using UV to kill pathogens that may be living on hospital floors and other surfaces. Indeed, one highly relevant panel discussion was “UV vs. the Coronavirus: What’s the Situation and What Are We Doing”? Dr. John Boyce of Boyce Consulting gave an overview of the virus in general and the (then) current situation which was exceedingly informative. The session included a video report from Zhiming He of Foshan Comwin Light, who was supposed to have appeared in person but, as he is based near Wuhan, remained in China and gave an update of the situation at “ground zero.” This being a UV conference, the thrust of the session was the extent to which UV might be effective in killing the virus. Obviously, it’s far too soon to draw any solid conclusions, but Dr. Boyce looked at the efficacy of UV on similar pathogens, such as SARS CoV, and cited four studies that found that varying UV exposure times and distances were effective in reducing the prevalence of the virus. “We need more definitive studies on what role UV can play,” said Dr. Boyce.

32 | UV+EB Technology • Quarter 2, 2020

An interesting further – if disquieting – application was presented by Dr. Art Kreitenberg of Dimer UVC Innovations. He began his presentation with the question, “How often do you think the plane you flew here on is disinfected?” Given the airline I primarily fly, my assumption was never, but as it turns out, no commercial airplane is ever disinfected, as there is no practical way of doing so. At least until now. Dimer UVC Innovations is developing the GermFalcon, a UV-based aircraft disinfection system that is in the process of being tested and is proving to be fast and effective at reducing the germiness of airplane seats and tray tables. It wasn’t designed for COVID-19 in particular, but rather for flu and other common viruses that can be the bane of both passengers and crews. Dimer hopes to have the GermFalcon take flight by the next flu season. UV and EB have many graphic communications applications, most commonly in energy-curing inks and coatings. Flatbed wideformat and industrial printing, for example, have been completely enabled by UV-curing inks, and packaging is a top application for UV and EB inks. During a lively panel discussion called “Trends in UV + EB Packaging,” the conversation quickly turned to that perennial bugaboo of food packaging: migration. UV and EB inks are used for food packaging, but not on what are called primary surfaces – the part of a package that has direct contact with food – and only on secondary surfaces if the material has the appropriate barrier properties and the ink has low migration tendencies,

uvebtechnology.com + radtech.org


meaning it won’t permeate through the packaging into the food. This is why corrugated is one of the top applications for UV inks, since it has high barrier characteristics, and food items generally aren’t placed loose in corrugated containers. As for digital printing of packaging, it’s “a cultural shift, especially for large brands,” said James Gill, Fujifilm/Dimatix. “Brands are not ready to change over to digital” despite the fact that, he added, “there is a compelling argument to go digital.”

RadTech Awards Given for Papers, Posters, Emerging Tech and StartUps

Several awards were presented at RadTech 2020 in Orlando, Florida, held March 9 to 11. Thank you to Molly Hladik, chair of the Radtech Technical Committee, for sharing her comments from the RadTech and IUVA 2020 Awards Dinner, which became the basis for this report.

A topic that came up in this session, as well as at a later one called “Innovative Directions in Flexible Packaging” (full disclosure: I moderated this panel), was sustainability, and the top “innovative directions” that flexible packaging is moving is, according to the panelists – who included representatives from Pepsi, EST (electron beam curing equipment), Comexi (flexo and offset printing systems), and Ashland (chemicals used for inks, coatings, substrates, etc.) – in more sustainable directions. The consensus is that reducing or eliminating plastic in favor of other materials like paper isn’t necessarily the best or even most sustainable strategy, but rather developing more compostable, recyclable and reusable flexible packaging materials will satiate the need for flexible packaging, while at the same time addressing environmental concerns. 3D printing also was a big topic at the event. One panel discussion – “Emerging Applications in Additive Manufacturing/3D Printing” – ran down the veritable “explosion of opportunities” in industries such as healthcare (3D printing prosthetics/orthotics and even implants and organs), entertainment (one of the panelists was from Walt Disney Imagineering, who spoke about how Disney uses 3D printing in sets and costuming for various theme park attractions), to military applications (uniforms, helmets, armor, even weaponry), to a variety of other “consumer industrial” applications. There also was a discussion about digital vs. “analog” manufacturing (3D printing vs. injection molding, for example), which had a familiar ring to those of us who have been witnessing the analog to digital transition in the various segments of our own industry. “3D printing is just another tool in the tool box,” said Michael Brady of 3D Systems. “If you want one million parts all the same, injection molding is the way to go. If you want shorter-run, more customized parts, 3D printing is the way to go.” This originally appeared on WhatTheyThink.com. Copyright © 2020 WhatTheyThink. All rights reserved. Reprinted with permission. WhatTheyThink is the global printing industry’s leading independent media organization with both print and digital offerings, including WhatTheyThink.com, PrintingNews.com and WhatTheyThink magazine versioned with a Printing News and Wide-Format & Signage edition. Our mission is to provide cogent news and analysis about trends, technologies, operations, and events in all the markets that comprise today’s printing and sign industries including commercial, in-plant, mailing, finishing, sign, display, textile, industrial, finishing, labels, packaging, marketing technology, software and workflow.

Sage M. Schisse was the recipient of the Best Paper Award for “Attainable Sustainable: Using Electron Beam Technology in Compostable Flexible Packaging.”

Best paper awards Well over 100 abstracts were received, and all were thoroughly reviewed and discussed by the committee for inclusion in the RadTech 2020 Technical Programs. Committee members included Molly Hladik, Michelman; Julie Jessop, Mississippi State University; Joel Schall, Henkel; Alexander Polykarpov, Sirrus, Inc.; and Susan Bailey, Michelman. Of the 100+ abstracts, more than 70 papers were submitted for review. From those 70+ papers, the technical committee reviewed and selected the best overall paper and the best student paper submitted for the conference. Unique to this year is that both papers awarded focused on the use of electron beam technology. This year’s Best Paper designation was awarded to Sage Schissel and Karl Swanson for their paper titled, “Attainable Sustainable: Using Electron Beam Technology in Compostable Flexible page 34 u

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UV+EB Technology • Quarter 2, 2020 | 33


EVENT REVIEW t page 33 Packaging.” The Best Student Paper designation was awarded to Nicole Thiher, Erin Peters, Sage M. Schissel and Julie L.P. Jessop (faculty) for a paper titled, “Comparison of UV- and EB-Initiated Polymerizations Based on Equivalent Radical Concentration.”

Student poster competition For the past few conferences, RadTech has partnered with the Technical Association for the Graphic Arts (TAGA) to request that graphic arts students compete in creating a poster to tout the benefits of energy curable technology. These posters then are voted on for the best representation of the technology. This year’s winner is an IUVA member: Dana Pousty is a student with the Water Research Center, School of Mechanical Engineering, TelAviv University. The second-place winner is Prashant Koktar, a Ph.D. student in Paper and Printing Science at Western Michigan University, doing research in Industrial Inkjet Printing. These honorees received cash awards from RadTech International North America, and the first-place winner’s design is featured on the cover of this issue of UV+EB Technology magazine. Emerging technology awards The emerging Technology Awards go to novel, creative and/or impactful uses of curing technologies. This year, the awards were given to a diverse group of end users. Winners included: Electron Beam Technology for Food Packaging, PepsiCo As innovative solutions are being sought for the development of a circular economy, PepsiCo is leading the way by working with the flexible packaging supply chain to develop advanced materials and processes for food packaging. As part of these PepsiCo initiatives, the company is exploring electron beam curing, as a fast, clean and energy-saving way to dry inks, coatings and adhesives for PepsiCo’s flexible packaging operations. Additive Manufacturing of High-Performance Material for Teeth Replacement, Myerson LLC One of the largest and fastest-growing opportunities in additive 34 | UV+EB Technology • Quarter 2, 2020

A team from the University of Colorado, including Jeff Stansbury (center), received a RadLaunch award for a solvent-free radical photopolymerization that continues its post-conversion in the dark.

manufacturing is the dental market. While the printing of teeth as temporary replacements is feasible, if not yet widely adopted, reaching the very high performance required for long-term tooth replacement has been out of reach. Myerson, LLC (Chicago, Illinois), in collaboration with Hybrid Ceramic (San Francisco, California), has developed a high-performance UV-curable dental material that can be printed and used as long-term teeth replacements. In vitro and in vivo trials are underway. Surface Treatment of Intraocular Lens Injectors, AST Products, Inc. LubriMATRIX™ technology has become a “Hidden Champion” for cataract surgery, providing a novel and innovative surface treatment technology specifically developed to enable a safer, simpler and more effective intraocular lens (IOL) delivery. An IOL is an artificial lens made of polymeric or silicone materials, and it is used to replace the patient’s natural lens that has become opaque, a.k.a. cataract. This technology is a patented surface treatment that utilizes an electron-beam-induced grafting method to append a hydrophilic layer onto the surface of an IOL. UV cured transparent films for Advanced Process Control (APC), Materials Business Unit, Applied Materials The drive to smaller, faster and more versatile electronic devices such as cellphones has accelerated innovation in state-of-the-art computer chips. To support this innovation, Applied Materials has integrated a high-energy stationary UV cure system to fabricate the UV transparent polymer films. The optical uniformity of the uvebtechnology.com + radtech.org


film and time to cure make UV curing a critical technology for these processes. RadLaunch awards Awards also were presented to students, universities, start-ups or others doing novel work in UV/EB curing. HARP (High-Area Rapid Printing), Azul 3D This three-year-old start-up spun out of Northwestern University and is creating a paradigm shift in the plastics manufacturing sector via a revolutionary 3D-printing technology called HARP (High-Area Rapid Printing). HARP enables the use of both oxygen-sensitive and oxygen-insensitive photo-chemistries at rapid speeds, broadening the scope of UV/EB technology for 3D printing. A Solvent-Free Radical Photopolymerization that Continues its Extensive Post-Conversion in the Dark, University of Colorado A new photoinitiating system with unprecedented photoefficiency and extensive post-conversion in the dark has been developed. This innovation allows reduced light exposure times helping to cure irregular surfaces, or thicker samples that might

otherwise not work. This discovery promises to increase the scope of photocuring technologies into new application areas. This award goes to a team including Kangmin Kim, Jasmine Sinha, Charles Musgrave and Jeff Stansbury. Next-Generation Energy Storage with UV Curing of Novel Polymer Electrolyte Materials, The Hosein Research Group, Syracuse University The ever-increasing demand for Li-ion battery storage solutions soon will present an imminent crisis with the availability of mineral lithium to support the clean energy economy. The challenge with next-generation batteries is developing suitable electrolytes. To this end, polymer electrolytes are an attractive solution owing to their high conductivity, as well as mechanical and thermal conductivity. UV curing may be used to produce both acrylic- and epoxy-based polymer electrolytes, to effectively operate in prototype next-generation batteries. Bio-based 1,5-Pentanediol: A New Renewable Monomer for the Radcure Industry, Pyran The vast majority of chemicals are made from nonrenewable and often expensive petroleum (oil) resources. Pyran set out to find a better way and discovered a new pathway to make a chemical called 1,5-pentanediol (1,5-PDO) from renewable resources, such as corncobs. This technology allows the renewable 1,5-PDO to be made at costs that can be 30% to 50% lower than that of similar oil-based chemicals. The 1,5-PDO product is a major component in many ultraviolet (UV) cure coatings. Real-Time Feedback Controlled Monomer Conversion: A New Paradigm for UV Curing Process Control, Eindhoven University of Technology in The Netherlands To prepare UV technology for the imminent fourth industrial revolution, a new paradigm is proposed for UV curing process control. The proposal consists of acquiring in-situ measurement data of monomer conversion from a spectrometer to compute a corrective action for the UV light source if needed. This innovation makes UV curing more robust to disturbances and may prove to be an indispensable solution for quality control.

David Walker and Rachel Davis, Azul 3D, accepted a RadLaunch award for the company’s revolutionary 3D-printing technology – HARP (High-Area Rapid Printing). uvebtechnology.com + radtech.org

A special RadLaunch University award was presented to Hamidreza Asemani and his professor adviser, Vijay Mannari, from the Coatings Research Institute at Eastern Michigan University for Novel UV-Initiated Dual-Curing Thermoset Materials Suitable for 3D Printing. While 3D printing is fast emerging, one major challenge faced by 3D-printed products is their suboptimum performance due to poor inter-layer adhesion. A team from Eastern Michigan University has designed a 3D printing material that cures by two independent cure mechanisms – one providing rapid green strength development for faster processing, while the second one allows for chemical bonding between the layers – thus significantly enhancing final product performance. The proposed system is enabled by UV curing technology, making it efficient and environmentally responsible. u UV+EB Technology • Quarter 2, 2020 | 35


AEROSPACE By Michael J. Dvorchak and Melanie L. Clouser, Dvorchak Enterprises LLC, and A. David Harbourne, Harbourne Consulting LLC

Aerospace UV Cured Coatings: Yesterday, Today and Tomorrow Editor’s Note: Due to print space limitations, the article references can be found online in the Article Archive at www.uvebtechnology.com.

O

ver the past three decades suppliers to the aerospace coatings industry, paint companies, the USAF, US Army, US Navy, US Coast Guard and civil aviation entities have – with limited success – tried to implement the use of UV cure coatings. The benefits of the UV cure technology are well understood, in that it will provide for the end user the following: 1) immediate dry to fly, 2) ultra-low volatile organic compounds (VOCs) / volatile hazardous air pollutants (VHAPs), 3) one component and 4) technology that could potentially meet military specification MIL- PRF-85285D. From the early uses of UVA cure stencils for aerospace UV coatings, the technology has evolved to the UV Cure Shark Skin coatings that have the promise of delivering drag reduction values in the range of 6% to 7%. Both the MicroTau Direct Contactless Microfabrication (DCM) technology and the Lufthansa Technik Airbus UV Cure Shark Skin coatings have the promise of accelerating these developments of aerospace UV cure coatings acceptance in the marketplace. These UV cure technologies can be based on 100% oligomer as well as water-based polyurethane dispersion technology. During the nearly three decades of development, several hurdles have impeded UV cure technology progress and use in the aerospace market. This paper will report on these hurdles and potential solutions to further develop the UV cure aerospace coatings market. (In this paper we will specifically look at intermediate and topcoats, and not primers. Primers for both aluminum and composite aerospace materials are undergoing a dramatic change due to the need to reduce or eliminate chrome primers.) Changes in the Aerospace UV Cured Markets Over the last several years, the aerospace coatings market has gone through an incredible number of changes in both polymer technologies and substrates. Today, the current number of 2K reactive primers and clear coats, as well as base coats, have increased, pushing the limits of polymer chemistries. With the pressures to lower VOCs and VHAPs, solvent-based systems have begun to evolve to water-based chemistries. Substrates used by the OEMs have evolved from the traditional aluminum metals to composites.

36 | UV+EB Technology • Quarter 2, 2020

Photo 1. C-130 UV cured black stencil coating uvebtechnology.com + radtech.org


Introduction of UV-curable Aerospace Coatings – The Past A. UV-Curable Aerospace and Aircraft Coatings (SBIR/ SERDP) A contract was awarded in 2005 - 2006 via Small Business Innovative Research (SBIR) and Strategic Environmental Research and Development (SERDP). During this testing protocol it was found that the normal 3 mils wet film thickness (WFT) did not cure properly. Investigators found that thinner layers resulted in the proper cure. These particular UV cure coatings were one component (1K), which is the traditional concept for a UV cure aerospace coating. These researchers decided to go the dual-cure route, which uses the free radical cure produced by the UV light and then the socalled dark cure, where the dual cure molecule has both acrylate functionality and polyisocyanate functionality. The so-called dark cure occurs through the moisture cure of the residual polyisocyanate on the molecule. These dual cure systems result in a 2K (twocomponent) system with a limited pot life1. Test results via the military specification (MIL SPEC) passed, giving only concerns with percent elongation at 275°F. No artificial weather testing was performed.2

Application

Current Dry Time

Time Saved

4 to 6 hours

30 minutes to 1 hour

3 to 5.5 hours

Topcoat

16 to 24 hours

30 minutes to 1 hour

15 to 23.5 hours

Stencils

72 hours

30 minutes to 1 hour

71 to 71.5 hours

Potential time saved per Aircraft

89 to 100.5 hours

Table 1. Full repaint benefits (F-16)

Coating System

X Y A

Coating System

B C D E F G H

Color Camo Gray 36173 Camo Gray 36173 Camo Black 37038

Color Camo Black 37038 Camo Black 37038 Camo Black 37038 Gloss White 17925 Gloss White 17925 Camo Gay 36173 Camo Gray 36173

Weathering (500 hours)

Color Match

Gloss Match

Wet Tape

Cross Hatch

Low Temp Flex

GE Impact

Pencil Hardness

P

P

P

P

P

F

P

P

P

P

P

F

F

P

P

Color Match

Gloss Match

Wet Tape

F

F

F

Color Change

Gloss Change

Post Test Low Temp Flexibility

Post Test GE Impact

HB

P

P

P

N/A

F

HB

P

P

P

N/A

P

F

F

P

N/A

P

N/A

Cross Hatch

Low Temp Flex

GE Impact

Pencil Hardness

P

F

P

F

F

P

P

P

F

F

P

F

F

F

F

F

F

F F

Weathering (500 hours) Color Change

Gloss Change

Post Test Low Temp Flexibility

Post Test GE Impact

HB

P

N/A

P

N/A

F

HB

P

N/A

P

N/A

P

F

HB

F

N/A

P

N/A

F

P

F

B

P

N/A

F

N/A

P

F

P

F

HB

F

N/A

F

N/A

P

P

P

P

F

2B

P

F

P

N/A

P

P

P

P

F

< 6B

P

N/A

P

N/A

Table 2. USAF/AFRL UV cure pigmented topcoat test results (F=fail & P=pass)

B. Concept of UV Cure Black Stencil Earned 2007 Funding for Testing at UDRI. This black stencil then was carried forward to a C-130 unit to see if the system could meet the criteria needed to perform in the field. In 2008, a demonstration was performed on a C-130 Hercules to evaluate its performance (Photo 1). Stencils were applied and cured via a 2,000 Watt H & S Auto Shot UV lamp.3 An additional test area was applied on the wing flap directly in line with the jet engine blast. Since this C-130 was an operational aircraft, it was deployed in multiple missions around the globe and in specific austere, hot and dirty environments. After 600 flying hours and 14 months in theater, the stencil coating performed quite well. The stencil displayed promising physical properties, with room for improvement in the areas of flexibility uvebtechnology.com + radtech.org

Using UV Cure

Primer

and gloss. In the color change area, the C-130 had Delta E values comparable to conventional 2K polyurethane fluoropolymers topcoats.3 C. Early USAF Requirements to Develop UV Cure Aerospace Coatings in 2009 The USAF has worked hard to develop a UV cure system for Department of Defense aircraft. In 2009, a report was published reviewing why the time and money were being spent on trying to develop a UV cure system for DoD aircraft. The specific requirement stemmed from the current USAF-required 72 hour “dry to fly” time for the 2K polyurethane topcoat. Shown in Table 1 are references to the ability of the UV cure technology to dramatically drop the “dry to fly” time by cutting off nearly 89 page 38 u UV+EB Technology • Quarter 2, 2020 | 37


AEROSPACE t page 37

Lube Oil Resistance

Hydraulic Fluid Resistance (24-hr)

Jet Fuel Resistance (7-day)

P

P

P

P

P

P

P

P

P

Not Required

P

P

P

P

P

Camo Black 37038

Not Required

P

P

P

P

P

C

Camo Black 37038

Not Required

P

P

P

P

P

D

Camo Black 37038

Not Required

P

P

P

P

P

E

Gloss White 17925

P

P

P

F

P

P

F

Gloss White 17925

P

F

F

P

P

P

G

Camo Gray 36173

F

P

P

P

P

P

H

Camo Gray 36173

P

P

P

P

P

P

Cleanability

Heat Resistance (1-hr 250°F)

Opacity

Camo Gray 36173

F

P

Y

Camo Gray 36173

F

A

Camo Black 37038

B

Coating System

Color

X

Table 3. USAF/AFRL UV cure pigmented topcoat test results (F=fail & P=pass)

to 100.5 hours of processing time to fully repaint an F-16. The obvious benefit of UV technology would be even more dramatic for the legacy transport aircraft (C-130, C-5 and C-17) since the surface area needing painting is much greater. Under this program, UV cure Camo Black, Gloss White and Camo Gray were evaluated. As can be seen in Table 2, coating systems X and Y are 2K polyurethane coatings currently used by the DoD on military aircraft and, in this test protocol, were the controls. It is interesting to note that under this test protocol, not even the 2K polyurethane coating system X and Y passed the tough GE Impact test. Color match / gloss match values were negative for the UV systems and could be corrected with reformulating. A caveat would be in the Black systems, since getting them to thoroughly cure requires pigmentation that the color eye has problems evaluating. UV cure coating systems A, F and G results were closest to the DoD aerospace color requirements. Again, it is interesting that the controls X and Y fail the cleanability test, which is probably the result of the low gloss requirements for these systems, as can be seen in Table 3. Coating systems A, B, C, D and H results were closest to the DoD aerospace requirements4. In all of this original work, very little is mentioned on the UV cure light sources that are being used with the UV-curable systems.1 The first of these systems would be the traditional UV arc lamp (electrode), which is a mature technology. The UV energy 38 | UV+EB Technology • Quarter 2, 2020

is developed by arc across the electrode that excites the mercury vapor to emit at a certain wavelength. The units are in wide use and have issues with UV bulb life. However; researchers in the early development of UV cure automotive refinishing discovered that a full spectrum light source would have industrial hygiene (IH) issues when used in an open shop environment.4,5,6,7 Working hand-in-hand with the UV light manufacturers, the researchers developed UVA light sources that were essentially UV arc lamps that were doped with Gallium so they would emit in the UVA region and not the UVB and UVC wavelength regions.

These UVA lamps were much safer for open shop use than the full spectrum UV light sources. Of course, proper personal protective equipment (PPE) needed to be followed, according to regional safety standards. Microwave electrodeless UV lamps were also considered, but the cost and being a full-spectrum light source were concerns in IH issues for the open shop. Light emitting diodes (LED) also were considered, but at this point in time the energy density and costs made it impossible to use with a large aircraft. Cooling of the LEDs was another issue, since on a larger scale the LEDs would need to be cooled to prevent them from becoming overheated and destroying themselves. D. Early Attempts to Develop UVA Aerospace Coatings – June 2010 Early work by the Air Force Research Lab (AFRL) on the development of a UV cure coating was funded as an Environmental Security Technology Certification Program (ESTCP) project. This project looked at the ability of the UV cure systems to perform the following criteria: adhesion, flexibility, color/gloss match, color/gloss retention, fluid resistance and repairability. This Joint Test protocol can be seen in Table 4. Evaluations were performed on “commercial off-the-shelf” (COTS) coatings on a flat black and gloss white coating. The results for the flat black coating were: Adhesion equal to the performance of the controls, flexibility needing improvement, uvebtechnology.com + radtech.org


hardness in the desired range, fluid resistance passing MIL-PRF85285 and weathering passed 3,000 hours for <1 delta E color change. The results of the gloss white system showed negative results in adhesion and flexibility, and marginal on fluid resistance and weathering.5 Eventually work was continued along this avenue and resulted in a US patent being issued that had an oligomer, monomer, PI, black pigmentation along with solvent, resulting in an 85-degree gloss of less than 15.7

project was to develop UV cure clear and pigmented artwork for commercial aviation. Requirements by the CAM for this project were the following: reproduce the entire CAM-approved color gamut, spray properties close to thermally curable paints, meet “hang time” requirements, cure process requirements, overspray cure requirements, and engineering and appearance requirements. The project developed page 40 u

Since the testing of the high gloss white was a priority the approach up until now was based on oligomer, monomer, PI and solvent for adjusting spray viscosity. The new vendor suggested that a water-based UV cured polyurethane dispersion (UV-PUD) be evaluated to try and meet the MIL SPEC since it offered low intensity UV cure, flexible coating, high gloss and ultra-low volatile organic compounds (VOCs) and hazardous air pollutants (HAPs). The performance of the gloss white UV cure polyurethane dispersion met everything in the specification except for the 60-degree gloss level, which needs to be at 90 or above. Unfortunately, the gloss value for this system came in at 80. Due to funding limitations, this project was stopped. The idea that a water-based technology could come this close to the MIL- PRF 85285 specification shows great promise for the technology in the future.9 LED UV Cure Aerospace Topcoats 2011 As discussed previously, UV LED light sources have been evaluated for aerospace primers and topcoats. An AFRL contract in 2010-2011 evaluated the performance of the UVA LED source in combination with UV cure primers and topcoats. Five different UV cured primers were evaluated against a gray UV cured topcoat. Four of the most promising stacks passed the fluid resistance, 1,000-hour salt spray, jet fuel, skydrol and cryogenic bend tests. With the UVA LED light source footprint, cure speed would be one square foot per minute. At this rate the UVA LED light source would take days to cure an entire C-17 aircraft. Other UVA light sources have been developed Test that have a much larger footprint, but support GE Impact Test for the UVA FL bulb technology has waned. 10 Development of UV-Curable Pigmented and Clear Coats for Complex Commercial Artwork In 2011, a commercial aircraft manufacturer (CAM) put together a team of individuals that included the following: two UV paint formulators, raw material suppliers and UV cure equipment suppliers. The purpose of the uvebtechnology.com + radtech.org

Table 4. Joint test protocol for MIL-PRF-85285 and MILPRF-32239 MIL -PRF 85285

UV PUD White Coating

> 60 %

60%

Dry / Wet Adhesion

> 4B / 4A

4B / 4A

Gloss

60 > 90

80

Initial Pencil Hardness

>2B

HB/F

Mobil Jet Oil

-2 pencils

-1

Hydraulic Fluid

-2 pencils

-1

JP-8 Jet Fuel Humidity Resistance; 14 days

-2 pencils 30 days

-2 No blisters

Table 5. Gloss white UV cure polyurethane dispersion

UV+EB Technology • Quarter 2, 2020 | 39


AEROSPACE t page 39 a UVA source that was used to scan over the aircraft body. The project lasted for 6 years and terminated in 201511. E. UV-curable Coatings for Aerospace Photo 2. UV cure black stencil Applications stickiness (ESTCP) 2012 From the previous project USAF/AFRL funded a major project to see if UV cure paint technology could meet the rigors of DoD aerospace specifications. This project was funded via ESTCP from July 2008 until August 2012.

Current Products and Innovation in the UV Cure Aerospace Market – The Present Under the Air Force Research Laboratory’s Engineered Surfaces, Materials and Coatings (ESMC) program, a project was started in 2013. The program is targeting skin-friction drag, which is reported to be an issue for 50% of DoD legacy aircraft. The Ohio Aerospace Institute (OAI) is the prime contractor, with Lockheed Martin as the main subcontractor. A professor at Stevens Institute of Technology has reported that the largest legacy aircraft in DoD have the following fuel burn: Boeing C-17, 461 million gallons in 2014; Lockheed Martin C- 130, 86 million gallons; and the C-5, 71 million gallons. The professor stated that the USAF wants passive technologies that would not change the structure or surface of the aircraft.

Lab testing was done at a private laboratory without oversight, especially on the UV cure light source and application. In general, most samples were tested above the accepted wet film thickness, which resulted in poor through-cure. This poor through-cure resulted in erroneous results. An example of this was the UV Gray 36173 and UV Black: After being subjected to 2,000 hours of salt spray, the UV coating was peeling of the panel at the scribe. A demonstration/validation trip was made to Hill Air Force Base to evaluate the potential for these UV cure systems. Applications of both the UV cure gray and black systems were made with limited success. Again, problems were encountered, with poor cure manifested by a surface that was sticky. This stickiness was attributed to lack of enough energy hitting the coating and overriding the oxygen inhibition to which all UV cure systems are susceptible. This problem with the black stencil can be seen in Photo 2. Unfortunately, the problem that occurred was that the power needed to run the UV lights was over 200 ft away, and a large extension cord was used. It was calculated that this large extension cord reduced the energy Figure 1. Riblet shapes: output of the UVA lights sawtooth, scalloped and blade by 20%. This reduction in energy caused incomplete cure of the UV Black and Gray coatings. Since this was at the end of the funding for this project, these obvious issues could not be resolved12. 40 | UV+EB Technology • Quarter 2, 2020

Figure 2. Optical micrograph of the UV cured riblets

OAI conducted an online InnoCentive search to uncover technologies that might meet the criteria. Out of 95 submissions and through additional screening, MicroTau Ltd. was awarded the contract in 2016 and developed a Shark Skin (riblet-like structure) that was manufactured from a UV-curable aerospace paint. This UV cure coating technology has its roots in the automotive UVA cure technology. The twist is that, utilizing photolithographic methods currently used in computer chip fabrication, the MicroTau technique directly prints riblets onto the external surface of the aircraft. The uniqueness of the MicroTau technology is that is doesn’t contact the wet UV paint and allows the formation of 3D geometries, as shown in Figures 1 and 2. This technology is the new hope in the development of a surface technology that might truly mimic the surface of a shark skin and allow for drag reduction in legacy DoD aircraft. Results of wind tunnel testing have confirmed that the MicroTau UV Cure riblet page 42 u uvebtechnology.com + radtech.org


Miwon…the derivation flows from “Original Beauty” As the leading-edge supplier of the highest market quality materials, Miwon now offers unique Monomers and Oligomers as compliant alternatives (HAP/Toluene - Free materials) to meet increasingly stringent downstream enduse ink, coating & adhesive market formulation requirements. Get in the “flow” and experience the Beauty of Market Compliance with Miwon Products and Technology! Contact toll-free – 1-877-44 MIWON www.Miramer.com


AEROSPACE t page 40

Photo 3. Application of a topcoat to an F-117A Nighthawk

coating gave 6% viscous drag reduction. It is calculated that just a 2% reduction in aviation fuel usage would reduce the annual CO2 emissions by 20 million tons annually.13 This direct contactless microfabrication (DCM) technique results in a 3D-printed profile that can be varied from the wing tip to the fuselage to allow for the air flow performance to result in the lowest drag possible for legacy DoD aircraft at altitude.14,15 Others have tried to manufacture these riblets for commercial aviation, with limited success. This commercial aviation technique for the manufacturing of riblet structures utilizes a UV cure aerospace paint applied to the aircraft surface. A silicon film mold that has the inverse of the riblet structure is then laid into the wet UV paint, and a UV system cures the riblet structures in place. This technology sounds impressive but has a major issue with trying to vary the shape and size of the riblets, since the silicon film has a structure that can’t be real time varied as can the DCM techniques described previously.16 Outlook for UV Cure Aerospace Coatings Technology – The Future With all of the resources that the US government has implemented for the development of a 1K UV cure aerospace coating over the last two decades, the future definitely has a great foundation. a. 1K UV-curable Non-isocyanate Polyurethane Aerospace Coating A recent Strategic Environmental Research and Development Program (2018 to 2019) project looked at the development of a 1K UV-curable non-isocyanate polyurethane (NIPU) aerospace coating. This work resulted in coatings that had flexibility at -54°F, good chemical resistance of methyl ethyl ketone (MEK) double rubs > 90. These UV-NIPU’s also showed no significant change in appearance regarding aerospace fluids. These particular systems were all based on clear coats. When the best clear coats 42 | UV+EB Technology • Quarter 2, 2020

were selected and pigmented, problems occurred with lubricating oil and aromatic fuel B. For some reason the researchers decided to build the dry film thickness (DFT) to 10 mils, whereas most aerospace coatings are applied at a 3 mil DFT. The clear coats were done in 5 DFT increments to reach 10 mils DFT. The pigmented systems were done in 1 mil DFT to reach the 10 mil DFT. The UV cure light source was a Fusion H-bulb, which works well for clear coat applications but not for those with pigment. The researchers used the right photoinitiator package for the pigmented system but needed to expose the system to a V-bulb (Gallium doped bulb to get the proper shift above the pigmentation absorbance) and immediately expose it to the H-bulb for the complete cure.17 It would be useful to retest these UV-NIPU systems with the proper UV sources to see what significant improvements in performance would occur. However, if future work is going to be attempted on the pigmented UVNIPU systems, it should be done with either a UVA electrode lamp or a UVA LED. The UVA and UVA LED units, at this point in time, are more practical than the electrodeless UV source for aerospace coatings applications.18 b. Developing superhydrophobic coatings that will keep the Shark Skin (riblets) clean Another project was funded 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) and the Air Force Research Lab (AFRL), in conjunction with Ohio Aerospace Institute. The project’s focus was on developing superhydrophobic coatings that could be overlaid onto the UV cured Shark Skin (riblets) coatings to keep the riblets clean. If you can’t keep the riblets clean, they will lose their ability to reduce drag. Following is the process developed under this program: 1) clean the riblet surface with an atmospheric plasma pressure jet (APPJ), 2) one-pass spray coating of a silane coupling agent, 3) apply the fluoroganosiloxane film with the APPJ, using hexamethyldisiloxane (HMDSO) and hexafluropropylene oxide (HFPO). This technique results in a superhydrophobic surface that was subjected to weathering, dirt accumulation, impact, cleaning,

This technology is the new hope in the development of a surface technology that might truly mimic the surface of a shark skin and allow for drag reduction in legacy DoD aircraft. uvebtechnology.com + radtech.org


solvents, jet fuel and abrasion resistance. The results showed that this coating held up very well to the durability testing. Open questions about these coating are: 1) What impact will adding a coating to the UV cured riblet have on the performance for drag reduction since it modifies the surface of the riblet? 2) How long will these coatings maintain their superhydrophobic characteristics when subjected to the hysteresis of a military aircraft during a PDM cycle?19 c. UV Cure 2K Water-based Chemical Agent Resistant Coating (CARC) The Army Research Lab was awarded a patent that could potentially be used in aerospace applications for the chemical agent resistant coating (CARC) as well as being low observable (LO). This technology has its roots in the development of 2K waterborne polyurethanes.20 It also borrows from the UVA cure technology for UVA automotive refinish using the H & S Autoshot 1200 W light source. In addition; it uses UV-curable polyurethane dispersion (UV PUD). What the technology teaches is that a polyol that has hydroxyl functionality is combined with a UV PUD, and then a water dispersible polyisocyanate is stirred in just prior to use. Included in the formulation are transparent (transparent to UV radiation) iron oxide pigments, as well as pigments that result in low IR signature. Also important in the

formulation is a PI that operates in the 365 nm and above range so that the pigmentation does not affect through-cure. One would have to wonder that such a unique coating system could not be utilized in stealth-style aircraft, as shown in Photo 3.21 Conclusions • One of the most important limiting factors for this UV cure aerospace paint technology is the design and size of the UV light for curing the paint on the aircraft. • The future is bright for this technology, especially considering the incredible foundation that exists for UV cure aerospace coatings. • DCM 3D UV cure printing technology has the potential to obtain a 2% reduction in aviation fuel usage and reduce annual CO2 emissions by 20 million tons. • The US government and private industry have spent incredible sums in furthering development of UV cure coatings for aerospace. The actual dollar amount is not known but must be in the multi-millions of dollars. • It is recommended that RadTech/AFRL host a joint summit to facilitate cross-fertilization and eliminate silos that exist due to the types of contracts used in developing UV cure coatings for aerospace. u

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TECHNOLOGY

UV+EB Technology • Quarter 2, 2020 | 43


SAFETY By Geri Tangdiongga, Don Moncy Dominic and Dominik Stephan, Dymax Asia Pacific Pte. Ltd.

UV LED curing systems: Measuring accurately and eliminating safety hazards Editor’s Note: Due to print space restrictions, the full article and references can be found online in the Article Archive at www.uvebtechnology.com. Abstract ltraviolet (UV) LED technologies are progressing very rapidly in recent years, both in performance and cost. Therefore, the measurement equipment and methods for UV LED curing systems must be well understood. This paper focuses on the measurement and characterization of the UV LED curing systems, particularly in the UVA range. Moreover, the risks and safety concerns of UV will be discussed, with a focus on illuminating the inherent advantage of LED.

U

Introduction In recent years the demand to incorporate UV LED in curing systems – instead of conventional UV lamps – has increased significantly. Although applications using UV sources still are dominated by conventional UV lamps, it was reported by Yole Développement that the forecast of market share of the UV LED has increased from 19.1% in 2014 to 41.4% in 2019.1 Furthermore, the total market size of UV LED has increased from $20.2 million (US) in 2008 to $127.0 million in 2015 and was predicted to have further increased to $505.5 million in 2019. Consistent with Yole Développement’s report, the study done by Allied Market Research showed that the global UV LED market size is expected to reach $1.2 billion by 2026, from $271.1 million in 2018, growing at a CAGR of 17.3% from 2018 to 2026.2 UV curing – such as inks, coatings and adhesives applications – accounts for nearly 60% of UV LED’s market size, and more than 50% of the users are in Asia. UV curing is a process of utilizing UV energy to initiate the photochemical reaction that generates a crosslinked network of polymers. The strong growth of UV LEDs is mostly determined by the attractiveness of the technology over conventional UV lamps. Some of UV LED’s advantages are discussed below: a. Compactness UV LED can be utilized in system surface mounted device (SMD) package form or in die form factor, using chips on board (COB) technology. As a result of its “flexibility,” the design of the UV LED emitter for the curing system becomes compact. Different types of curing systems, such as flood, spot or modular systems, can be designed. This flexibility is difficult to achieve if the curing system utilizes traditional UV lamps. For example, consider an end-of-wand, spot-based UV curing system: A UV lamp-based UV curing system needs a long guide to divert the light into the spot, making the system appear bulky, while the UV LED-based curing system can be designed in such a way that it uses only an aspheric lens to converge the light, resulting in a simple and compact design with higher efficiency output. b. Longer product lifetime The typical lifetime of conventional UV lamps is about 2,000 hours or up to 8,000 hours for microwave UV lamps, while the UV LED’s life is usually reported to be in the range of 20,000 hours.3 Typically, UV LED manufacturers follow the test standards (LM-80 and TM-21) that are applied to the visible wavelength LEDs to determine the lifetime. The lifetime of the UV LED is defined as the number of operating hours until the degradation of output reaches the defined level, noted as Lp, where p is the percentage of the initial output power. In visible LED, L70 is commonly used to determine the lifetime of LED. A longer lifetime of the light source in the system is beneficial for the end user because of its longer replacement cycle, less downtime for the system and greater stability over time. The lifetime can be extended if the UV LED system is operated 44 | UV+EB Technology • Quarter 2, 2020

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under intermittent mode. Furthermore, the lifetime of the light source depends highly on the driving current, as well as device and environment temperature. Currently, the technology of UV LEDs in the UVA region is more advanced and mature compared to that of UV LEDs operated in the UVB and UVC region. When the UV LED is integrated into the system – for instance, emitter and curing system – the lifetime of other parts must also be considered. Therefore, the rated lifetime of the UV LED systems, such as UV light curing systems, will be shorter at the UV LED package level compared to the system level. Hence, the actual lifetime should be determined as use-condition in the application. However, the root cause of the system’s failure is often due to the environment, the auxiliary system design4 and the possibility that the end user does not follow the system’s installation requirement. c. Environmentally friendly (i.e. ozone and mercury free) The UV lamp’s emission spectrum shows multiple peaks spreading from UVC to the visible spectrum. At the short wavelength, which is typically from 160 to 240 nm, the UV light can convert the oxygen into ozone (O3). The produced ozone can be a health issue if it is inhaled for prolonged periods. Therefore, it is suggested that the ventilation system surrounding the process should be set up properly to reduce risk. Furthermore, some conventional UV curing systems use mercury-based UV lamps as the emitter. Mercury contamination can occur if the UV lamp is broken. Hence, safety instruction is necessary to handle the breakage of mercury-based UV lamps. UV LED’s emission has a single peak, with the typical bandwidth of 9 to 15 nm, which can be selected. Doing so avoids the wavelength ranges that can produce ozone, especially for curing applications utilizing UV light at UVA and UVB bands. Additionally, the UV LEDs are made of semiconductor material (InGaN type), which is much safer and does not contain harmful materials or complicated disposal procedures. On top of the above-mentioned points, there are additional benefits supporting environmental friendliness: less electrical power consumption, no warm-up time, immediate performance availability and ease of maintenance, with fewer service intervals. With the increased demand for UV LED for curing applications, end users gradually shift from UV lamp-based curing systems to UV LED-based systems. While there are many advantages to using UV LEDs over UV lamps in curing applications,5-8 there still are some technological challenges in performance and remaining safety risks that UV curing system integrators, metrology device providers and end users will encounter. For instance, the end user must study and carefully match the adhesive and the UV LED system in terms of the adhesive’s absorption and the LED’s wavelength. The end user needs to characterize the system by evaluating peak irradiance, energy and curing time needed to achieve the optimized curing process. Another key challenge involves the proper measurement method of output for curing applications from the UV LED system. With uvebtechnology.com + radtech.org

the proper method and measurement tools – such as a UV radiometer – the output of the UV curing system can be measured accurately. This measurement of output in the manufacturing line is highly recommended Figure 2. Intraocular filtering of UV for production radiation by ocular tissue.11 consistency. The measurement is aimed: • To ensure reliable application performance • To ensure repeatability and stability of the curing process • As a preventive measure to monitor the degradation of the curing system Therefore, this report aims to give an overview and recommendations to the end users of UV LED curing systems – such as experienced process engineers, manufacturing engineers and technicians – on the performance measurement of UV LED curing systems using an appropriate radiometer. The report also covers the risks and safety concerns in operating UV LED curing system. UV radiation and safety concerns UV radiation is in the form of electromagnetic waves, with wavelength range between x-ray and visible light (10 to 400 nm). Sources of UV radiation can be natural, like the sun, or artificial, such as mercury-vapor lamps, black lights, metal-halide lamps, UV LEDs and UV lasers. Based on its wavelength range, the UV radiation can be classified into: • UVA: 315 to 400 nm • UVB: 280 to 315 nm • UVC: 180 to 280 nm • Vacuum/extreme UV: 10 to 180 nm Although there are negative effects of UV radiation on human health and the environment (ozone creation), UV radiation also can be beneficial for our lives.9 It can be a source of Vitamin D. In industrial and medical applications, UV radiation is widely used in curing polymers and ink, as well as in medical phototherapy treatment, forensic analysis, water disinfection, sterilization of certain viruses, horticultural lighting for optimal photosynthesis, etc. This section discusses the safety of using UV in curing applications. Applications in adhesive curing, typically utilize radiation in the UVA region because a majority of the polymers and inks can only be cured by UVA due to the specific properties of the photoinitiator used in the formulations. The biggest hazard of the conventional UV lamp-based system is the UV radiation page 46 u UV+EB Technology • Quarter 2, 2020 | 45


SAFETY t page 45 from the emitter. Being exposed to excessive UV radiation can be dangerous for humans. UV radiation, especially in the UVA and UVB spectra, can lead to cataract and other eye diseases, skin cancer, sunburn and accelerated skin aging. There also is evidence that UV radiation reduces the effectiveness of the immune system8. The curing systems available in the market use conventional UV lamps as the source, and these lamps emit not only in UVA but also UVB and some visible spectra. However, using a UV LED system is inherently safer: The emitter’s wavelength is controlled in the UVA region, with a narrow bandwidth. Thus, from a safety perspective, the risks of UV LEDs are more manageable with simple precautions. When it comes to the maximum value of UV exposure, most authorities follow the UV exposure threshold limit values (TLVs) recommended by the American Conference of Governmental Industrial Hygienists (ACGIH). The recommended TLV value for the UVA wavelength region (315 to 400 nm) should not exceed 1.0 mW/cm2 for a period greater than 1,000 seconds (approximately 16.7 minutes), and for exposure time less than 1,000 seconds, the total energy should not exceed 1.0 J/cm2. To put it into perspective, 1 mW/cm2 is the typical intensity level in a cloudless spring day in New England, measured using a radiometer that is pointed directly at the sun.10 Effects on the eye UV exposure to eyes has been associated with cataract and retinal degeneration. The cornea and intraocular lens are the most important tissues of the eye to absorb UV radiation. As reported by Walsh,11 the cornea absorbs the most UVB (below 300 nm), and the intraocular lens absorbs UVA (below 370 nm), as illustrated in Figure 2. It is, therefore, recommended that users wear UV grade eye protection when operating a UV source or while in close proximity to the UV curing system. The safety eye protection must be qualified under the OSHA and ANZI Z87.1 standard. For UV radiation protection, the safety eyewear code is indicated in letter “U” followed by a number on the scale 2 to 6, with 6 being the lowest transmission of UV radiation through the eyewear. It is advisable to consider the following factors in choosing the appropriate safety eye protection: (1) the ability of the safety eyewear to protect against specific workplace hazards other than UV radiation, (2) provision of unrestricted vision and movement and (3) any interference with or restriction of the function of any other personal protection equipment the employee wears. Besides safety eyewear, a protective shield also can be used as protection to minimize the end user’s exposure to UV, since the UV emitter in the curing machine might emit stray radiant energy away from the application surface. The possibility that stray radiant energy can leak is high because it can arise from the reflection on various surface materials and improper installation or design of the system. The UV protective shield can be made of black anodized or black coated metal sheets, rigid plastic films – 46 | UV+EB Technology • Quarter 2, 2020

typically acrylic and polycarbonate – or flexible films, such as UV blocking flexible urethane film.12 Plastic materials melt at very low temperatures compared to metals, so the shields made from acrylic or polycarbonate materials must be used at a sufficient distance from the source to overcome the heat generated by UV sources – especially conventional UV lamps – to avoid any form of softening. Effect on the skin The effects of UV radiation on skin can be classified into two categories; acute, with effects appearing within a few hours of exposure, and chronic, with long-lasting and cumulative effects, which may not appear for years. An acute effect of UV radiation can appear as redness of the skin: erythema or sunburn. Chronic effects include accelerated skin aging and skin cancer. Furthermore, UVB radiation’s effect in skin tissue is associated with the skin burning, which increases the likelihood of developing skin cancer13. The effect of UVA radiation, which penetrates deeper into the skin tissue, is associated with premature skin aging and can also lead to skin cancer.13,14 To protect the skin from UV radiation, appropriate gloves and coats are recommended for end users. Glove materials such as nitrile, latex or tightly woven fabric are suitable to protect the skin against large amounts of UVA and UVB. Dark and shiny finishes also can prevent UV radiation from penetrating the skin by absorption and reflection, respectively. These types of gloves have low UV transmission compared to vinyl-based gloves. During the operation of UV devices, wearing a long-sleeved coat is recommended to ensure that the skin is not exposed to UV radiation. To mitigate the risks of UV radiation exposure, control measures must be designed carefully to minimize exposure to eyes and skin and to prevent cumulative exposure. The precautions needed depend on the risk assessment. If end users are unsure about the quality of the UV curing system’s safety equipment, they should use a UV meter/radiometer to measure stray radiant energy at the location of interest. The measurement of UV using a radiometer is discussed in next section. As discussed, UV LED sources are inherently safer to operate than conventional UV sources. However, an important element in eliminating the risk of UV radiation exposure is training and awareness for end users. Guideline for measuring UV LED curing system The quality and consistency of the end result in the polymerization of an adhesive depends on many factors. One factor is the stability and consistency of the UV output, including the radiant power or irradiance. It is important that the output of the curing system is checked and monitored regularly to ensure its power or irradiance value is maintained at the desired level. This section will provide guidelines for measuring the UV curing system – especially for the UV LED-based curing system. uvebtechnology.com + radtech.org


Optical parameters of UV LED curing systems Generally, basic optical parameters of UV LED curing systems consist of: a. Peak wavelength The wavelength emitted from UV LED is controlled by the selected/engineered semiconductor and doping materials.15,16 Unlike conventional UV lamps, the emission of UV LEDs has a “reasonably monochromatic” spectrum (or rather one peak) with specific peak wavelength and relatively narrow bandwidth, typically between 9 to 15 nm, compared to visible LED bandwidth. A spectrometer is required to characterize the wavelength parameters of the UV LED curing system. b. Peak irradiance Irradiance is a measurement indicating the intensity of the radiant power emitted from a UV LED onto a specific application surface. It is expressed in units of W/cm2 or mW/cm2. Irradiance decreases exponentially with increasing distance. In the specification of a UV LED curing system, peak irradiance value is used as a key parameter. Per definition, peak irradiance is the highest irradiance

(a)

(b)

Figure 3. Radiometer configuration available (a) handheld type and (b) puck type

at a point of reference measured by a light meter or a radiometer. Datasheets of UV LED curing systems often indicate the measurement location of peak irradiance at the emitting window or at a specific distance from the emitting window. However, some datasheets do not indicate the exact measurement location in terms of distance and lateral position – for example at the center, edge or corner of the referenced area. This results in confusion for the end user and might void effective datasheet performance comparisons. c. Energy density Energy density measurement is the collection of the irradiance over a specific time and is expressed in Joules per square centimeter (J/cm2). In a conveyor system, the measurement of energy density is often used as a parameter to determine the UV cure ability. The end user can adjust the speed of the conveyor to set the correct energy density while the source emits the constant intensity. d. Uniformity Another important parameter is the uniformity of the UV distributed over specified locations. Not all UV curing systems emit consistent energy across the entire curing area. Uneven distribution can lead to poor adhesive curing quality or even lack of cure. Uniformity is the variation of energy distribution on the specified distance and area. It is measured as a ratio between highest and lowest irradiance values in the area of interest and expressed in percentage. To establish process uniformity, end users can adjust the measurement location during measurement and tune the UV curing system accordingly. Further important parameters that are not highlighted in this paper are the operating and ambient temperature. As mentioned in the earlier section, UV LEDs are temperature-sensitive devices.17,18 The output power decreases as the function of the temperature, hence the lifetime of the emitter will be reduced. The UV LED page 48 u

Figure 4. Typical spectral responsivity of Dymax’s radiometers and the UV sources spectral intensity: (a) detector spectrum of ACCU CAL 50 /160 and spectrum of metal halide and mercury lamps, and (b) detector spectrum of ACCU CAL 50L /160L and spectrum of UV LEDs uvebtechnology.com + radtech.org

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SAFETY t page 47 designed using conventional UV lamp or UV LED. Typically, the UV emitter consists of the energy source, Sensor A tuned at Sensor B tuned at additional optics in form of reflector, lens or UV grade 385 and 405 nm 365, 385, 405 nm optical window, and a thermal system in form of air or 365 nm 27 0.46 0.53 liquid cooling mechanism. All sources emit UV but in 385 nm 27 0.95 0.96 different spectral ranges. The conventional UV lamps 405 nm 27 0.89 0.88 have a wide spectral range across UV spectrum and small portion in the visible spectrum, while UV LEDs Table 1. Measurement result of UV LED module using different radiometers. The modules are measured at a distance of 25 mm. emit radiant energy in the narrow UV spectral range. The end-user should know that different tools might need to curing system needs to be placed and operated based on the be used to measure the irradiance or energy of the conventional manufacturers’ instruction for the cooling mechanism in the UV lamp and UV LED. Radiometers for a UV broadband lamp system to operate at maximum capacity. can only measure the conventional UV lamp-based curing system and cannot be used to measure a UV LED curing system because In the following sections, the guidelines on measuring the they measure one a particular wavelength range. For measuring a irradiance of UV LED curing system using a radiometer are UV LED system, a specific measurement tool tuned for this use explained. case is recommended. Peak Wavelength of UV LED module

PCB Temperature (°C)

Irradiance (W/cm2)

Radiometer as a measuring device As mentioned in the previous sections, a radiometer is used to measure irradiance values at a given location and distance. It is a useful device to monitor and ensure a stable UV curing process. A radiometer also can be used for: a. Maintaining a reliable UV curing process It can ensure that a UV curing system is providing good irradiance and dosage levels required for successful curing. b. Acting as a safety measurement tool during the UV curing process Radiometers are sufficiently sensitive to measure the intensity of stray or reflected energy. c. Measuring transmission rates through substrates It measures the transmission rates of various wavelength ranges through substrates that sometimes absorb various frequencies of energy. To ensure an effective curing process, it is critical to measure the intensity reaching the cure site below any intervening substrate. Based on the form factor and application, the radiometers available in the market can be classified into handheld/end-ofwand and disc/puck type. For manufacturing lines, it is necessary for the design of both types to be compact and easy to operate. Handheld types of radiometers are suitable for spot and end-ofwand curing system configuration. Often, they come with adapter kits to fit into the light guide or connector of the UV curing system. The puck-based radiometer is more suitable for measuring irradiance or energy density in conveyor and flood chamber UV curing system. Figure 3 shows the example of Dymax’s UV radiometers. In selecting a suitable radiometer for a curing application, several factors need to be considered to get accurate and repeatable measurements: The Emitter of the UV curing system The emitter is the UV source part in a curing system. It can be 48 | UV+EB Technology • Quarter 2, 2020

Responsivity of a radiometer’s detector The key parameter in selecting a radiometer is identifying the wavelength of the emitter’s source. It is extremely important that the spectrum of the emitter’s source is within the spectral responsivity of radiometer’s detector. However, a radiometer for conventional UV lamps shall not be used to measure the UV LED lamps due to the different range of quantifying the radiant energy. While it is desirable that the radiometer’s detector has flat top responsivity over UV spectrum, such a detector would be very costly and would not be handy, hence it is not usually applied in industrial applications. For applications in adhesive curing using UVA, such as metal halide and mercury lamp, the detector used has highest responsivity at 360 to 365nm because the spectrum of UV metal halide and mercury lamp has a higher peak at that range, as depicted in Figure 4. For a UV LED-based curing system, it is recommended that the end users understand at which wavelength the radiometer is calibrated. Various types of radiometers can be used to measure irradiance and dosage at single or multiple wavelengths. If the radiometer’s measurement mode is tuned at 385 nm but is being applied to measure irradiance of the UV LED curing system, which has a peak wavelength at 365 nm, the measurement result will not be reflected as the true value and can only be assumed as a relative value. This still can be used for process stability analysis. Simple measurements are performed to find the difference in irradiance measured between specifically tuned radiometers. The set-up is constructed in such a way for the measurement to be reliable and repeatable. It is seen from the result in Table 1, at peak wavelength of 365 nm, that the deviation measured using two radiometers is about 13%. Spot and flood UV curing system Another important factor in achieving accurate results is the ability of a radiometer to measure spot and flood type UV curing systems. The emitter of a UV spot curing system is coupled with a light guide or collimated lens, resulting in a focused and narrow uvebtechnology.com + radtech.org


output beam. The spot-type curing system is typically used to cure small parts with high intensity, thus it provides faster curing time. Unlike the spot curing system, the flood curing system is used to cure a larger Figure 5. Example of measurement part or small parts location of the radiometer at a fixed working distance to determine uniformity at the same time. To measure the irradiance of the UV curing system, the end user should ensure that the emitter and radiometer’s detector are in parallel position. A slanted or tilted radiometer sensor will cause an inaccurate measurement. To measure the irradiance value of the spot curing system, the radiometer is attached perpendicular onto an adapter that fits the aperture of the light guide or connector. The adapter and connector must be designed accurately so that the total radiant energy illuminates the entire detector window without loss. It also must be uniform across the measurement window. Some radiometer manufacturers provide standard and customized adapters and connectors or a measurement mode specifically designed to measure the UV in flood mode or certain spot sizes. Lateral position and vertical location during measurement At a production line, the radiometer is used to monitor the output of the UV curing system with the goal of ensuring the system operates within the specification. For that reason, a consistent irradiance value is desired. Because the irradiance measured is distance-dependent, to achieve repeatable and consistent measurements, the location and the orientation must be equal at each measurement. Furthermore, not all flood systems emit consistent energy laterally, which results in different irradiance values across the illuminated area. For this reason, the uniformity must be determined at working distance to achieve good and repeatable curing results. Few measurement points across the illumination area can be taken using the radiometer to map the irradiance uniformity, as illustrated in Figure 5. If the irradiance map is constructed, the dimension of an effective curing area can be determined and matched to the curing application. Temperature All semiconductor-based devices are temperature sensitive. In general, the operating temperature of the radiometer should not be more than 70°C. If the temperature of the UV curing system is built up with long exposure of UV radiation on the detector, the performance of the detector starts to attenuate.19,20 Current technologies enable radiometers to perform the irradiance or energy density calculation at the milliseconds interval to avoid uvebtechnology.com + radtech.org

temperature error. Additionally, when the UV emitter turns on, it is recommended that users measure its irradiance after the temperature reaches a steady state to ensure the reading is consistent and accurate, and ensure that the radiometer has cooled down before performing the next measurement. One of the advantages of using a UV LED-based curing system is its shorter warm-up time in comparison to the conventional UV curing system. Radiometer calibration and maintenance Components of the radiometer will deteriorate over time, resulting in reduced measurement accuracy – even more with frequent use. The radiometer is a delicate system, particularly its optical parts. Slight changes or debris in the optical parts can affect results significantly. Therefore, periodic maintenance and calibration are recommended, including cleaning and proper storage. Periodic calibration of the radiometer is necessary to maintain the lowest possibility of error. During calibration, the error of all important parameters is determined. The calibration process is usually performed on a standard artifact, traceable to national standards, and measured under defined environmental conditions. Most calibration laboratories follow the standard set by National Institute of Standards and Technology (NIST). The result and associated measurement uncertainties are recorded on a calibration certificate. The key factor in the calibration process is that it should display an unbroken chain of transfer comparisons originating at a national standards laboratory through the final products of the radiometer.21 Conclusion The market share of UV LED-based curing systems is expected to continue increasing every year as the technologies become more mature. The inherent advantages of UV LEDs over the conventional UV bulb are not only due to performance but also environmental friendliness and safety. Some end users may not yet employ the proper methods to measure the output using a radiometer, which can lead to false measurement results that may seem correct or close to the desired value. An appropriate measurement method for the UV LED curing system is needed to ensure consistency of the result as well as to eliminate the risks of using UV. Furthermore, it is important to evaluate data in a UV curing system datasheet that does not list radiometer type, the location where irradiance is measured and means to couple the UV energy to the detector. This report can be utilized as a guideline for end users to obtain accurate and repeatable measurements of a UV LED curing system – especially those measurements using a radiometer. u Disclaimer

Technical data provided is of a general nature and is based on laboratory test conditions. Dymax does not warrant the data contained in this bulletin. Any warranty applicable to the product, its application and use, is strictly limited to that contained in Dymax’s standard Conditions of Sale. Dymax does not assume responsibility for test or performance results obtained by users. It is the user’s responsibility to determine the suitability for the product application and purposes and the suitability for use in the user’s intended manufacturing apparatus and methods. The user should adopt such precautions and use guidelines as may be reasonably advisable or necessary for the protection of property and persons. Nothing in this bulletin shall act as a representation that the product use or application will not infringe a patent owned by someone other than Dymax or act as a grant of license under any Dymax Corporation Patent. Dymax recommends that each user adequately test its proposed use and application before actual repetitive use, using the data contained in this bulletin as a general guide.

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REGULATORY NEWS

Doreen M. Monteleone, Ph.D., director of sustainability & EHS initiatives, RadTech International North America doreen@ radtech.org

US EPA Releases Final Amendments on CDR Rule, Extends Reporting Period On March 17, 2020, the US Environmental Protection Agency (EPA) announced the availability of a final rule amending the Chemical Data Reporting (CDR) rule. The amendments are intended to reduce the burden for certain CDR reporters, improve the quality of CDR data collected and align reporting requirements with the Frank R. Lautenberg Chemical Safety for the 21st Century Act’s (Lautenberg Act) amendments to the Toxic Substances Control Act (TSCA). Additionally, EPA is extending the reporting period for CDR data submitters from Sept. 30, 2020, to Nov. 30, 2020, to provide additional time for the regulated community members to familiarize themselves with the amendments and for reporters to familiarize themselves with an updated public version of the reporting tool. For more information, see https://www.epa.gov/chemical-data-reporting. TSCA’s 20 High-Priority Substances Requirement Update In late January 2020, the Environmental Protection Agency (EPA) published a US Federal Register notice identifying the preliminary list of manufacturers, including importers, of the 20 high-priority substances for which fees will be charged. Two months later, a proposed rule was developed to allow for exemptions. The three categories of exemptions are as follows: (1) importers of articles containing one of the 20 highpriority substances, (2) domestic producers of one of the 20 high-priority substances as a byproduct and (3) domestic producers or importers of one of the 20 high-priority substances as an impurity. These manufacturers (importers) will not need to self-identify and will not be subject to the risk evaluation fees. In addition, since the rule isn’t final, and won’t be until 2021, EPA also issued a “no action assurance” memo, promising that the agency will exercise its enforcement discretion for those three categories. This essentially means that, even though the law hasn’t been formally changed, EPA will not pursue any companies that didn’t self-identify because they fall into one of those three categories. To summarize, if any of the 20 high-priority substances is present in products imported into the US, either as part of an article or as an impurity, then no action is required for self-identification, and importing companies will not be subject to the fees rule. US manufacturers also may be absolved from self-identification if they manufacture one of the 20 high-priority substances as a by-product. SGP Takes Steps to Deal with COVID-19 Impact While Community Continues to Grow The Sustainable Green Printing Partnership (SGP) is taking additional steps to deal with the impact of the COVID-19 pandemic. For printers that are required to have a certification or recertification audit before June 1, 2020, SGP is working with its teams to find an alternative, remote method for the auditing system. Each facility will be considered on a case-by-case basis to find the best solutions and will be contacted by the SGP lead auditor. In the first quarter of 2020, the SGP community increased by two new SGP Brand Leaders, a new SGP Patron and a new certified SGP Printer. SGP Brand Leader Brandkey Graphics works with its clients to improve workflow. InnerWorkings, another SGP Brand Leader, works with numerous top brands to maximize messaging. Neenah, Inc., joined the SGP Community as a silver level SGP Patron. Neenah offers premium paper products with responsible fiber sourcing. The newest SGP Certified Printer is Hub Labels of Hagerstown, Maryland. Hub, a flexographic printer and manufacturer of pressure-sensitive labels, is the second facility in Maryland to become certified. To learn more about becoming part of the SGP Community, visit www.sgppartnership.org.

News From the West Coast

More Stringent Regulations for Solvent Systems Add-on control devices traditionally have been the standard set by regulators in volatile organic compound (VOC) emission reductions from conventional solvent operations. The devices also are considered best available control technology (BACT). The devices typically use natural gas as the combustion fuel. No specific regulations were related to these combustion devices until 2006, when California enacted legislation to reduce climate change. According to the California Air Resources Board (CARB), natural gas combustion generates greenhouse gases (GHGs). The state law requires the reduction of GHG emissions to 1990 levels by 2020 (approximately a 30% reduction) and an 80 percent reduction below 1990 levels by 2050. Facilities that emit GHGs are subject to the regulation. Facilities can stay under certain thresholds to avoid applicability, but any GHG emissions – including those from add-on controls – enter into the equation. 50 | UV+EB Technology • Quarter 2, 2020

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Additionally, the South Coast Air Quality Management District (SCAQMD) has recently announced that – for the first time – it will require add-on control devices to comply with the BACT regulation. Businesses must use BACT for new sources, relocated sources and modifications to existing sources that may result in an emission increase of any nonattainment air contaminant, any ozone-depleting compound (ODC) or ammonia. While add-on controls are listed as BACT for many processes, the emissions from the controls themselves now will be subject to employing BACT. Rita Loof, director of regional environmental affairs, RadTech International North America rita@radtech.org

SCAQMD is proposing the following requirements for add-on controls that use natural gas: • Regenerative thermal oxidizers at major polluting facilities (those that emit or have potential to emit 10 tons per year or more of any pollutant) will need to meet a nitrogen oxide (NOx) limit of 30 parts per million (on a dry basis @ 3% O2). • Recuperative thermal oxidizers at major polluting facilities will be subject to an NOx limit of 30 parts per million (ppm) and a carbon monoxide (CO) limit of 250 ppm (on a dry basis @ 3% O2). • Regenerative thermal oxidizers at minor source facilities will be subject to a 30 ppm NOx limit and a CO limit of 400 ppm (on a dry basis @ 3% O2). Cost-effectiveness will be evaluated for minor sources. The proposal was presented in late February 2020, and a period of 30 days originally was allowed for public comments. Due to the COVID-19 outbreak, it is unclear when the proposal will be finalized. CARB Proposes New Category for Photovoltaic Coatings The California Air Resources Board (CARB) plans to amend its “Architectural Coatings Suggested Control Measure.” The agency proposes the addition of a new category for photovoltaic coatings. This category would sunset on January 1, 2024, and there would be no exemption for “small containers.” Photovoltaic coatings would be subject to a VOC limit of 600 grams per liter, and there would be notification and reporting requirements. CARB will leave it up to each local air district to impose any coating volume limits. CARB staff has recognized the benefits of photovoltaic coatings by indicating that “applying the coating to existing uncoated solar panels increases electricity generation. Increased electricity generation leads to avoided criteria pollutants and GHG emissions” and that the panels remain cleaner longer. PCBTF in Califorinia In early 2020, California’s Office of Environmental Health Hazard Assessment (OEHHA) presented a draft document summarizing the carcinogenicity and derivation of a proposed cancer inhalation unit risk factor for p-chloro-α,α,α-trifluorotoluene, also known as pchlorobenzotrifluoride (PCBTF). OEHHA is required to develop guidelines for conducting health risk assessments under the Air Toxics Hot Spots Program (California Health and Safety Code Section 44360 (b)(2)). The values proposed are as follows: Unit Risk Factor 8.6 × 10-6 (µg/m3)-1 Inhalation Slope Factor 3.0 × 10-2 (mg/kg-day)-1 More detailed information can be found at the following link: https://oehha.ca.gov/air/crnr/p-chloro-aaa-trifluorotoluene-p-chlorobenzotrifluoride-pcbtf-cancer-inhalationcancer-unit?utm_medium=email&utm_source=govdelivery. BPA Safe for Eyewear Products California’s Office of Environmental Health Hazard Assessment (OEHHA) recently issued a “Safe Use Determination for Exposures to Bisphenol A from Certain Polycarbonate Eyewear Products Manufactured, Distributed or Sold by The Vision Council Member Companies” related to exposures to BPA from certain polycarbonate prescription glasses and sunglasses, over-the-counter (OTC) reading glasses and nonprescription sunglasses as well as safety glasses. Such glasses have acetonitrile extractable concentrations of BPA in the temple, nose pad, frame and lens at or below 25 micrograms per gram (µg/g), 68 µg/g, 120 µg/g and 302 µg/g, respectively. Thus, exposures to BPA from use of such eyewear products, under the conditions described in OEHHA’s assessment, would not require a Proposition 65 warning. Additional documents can be found at: https://oehha.ca.gov/proposition-65/proposition-65-safe-use-determinations-suds. u

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UV+EB Technology • Quarter 2, 2020 | 51


CALENDAR SEPTEMBER 15-17: Labelexpo Americas 2020 and Brand Print Americas, Stephens Convention Center, Rosemont, Illinois. For more information, visit www.labelexpo-americas.com.

OCTOBER 11-13: TLMI Annual Meeting 2020, The Broadmoor Hotel, Colorado Springs, Colorado. For more information, visit www.tlmi.com. 18-21: AIMCAL R2R Conference USA and SPE FlexPackCon 2020, Hilton DoubleTree Hotel at Universal Studios, Orlando, Florida. For more information, visit www.aimcal.org/2020-r2r-usa-conference.html.

NOVEMBER 8-11: PackExpo International, McCormick Place, Chicago, Illinois. For more information, visit www.packexpointernational.com

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ADVERTISING INDEX American Ultraviolet.................................................................. americanultraviolet.com......................................................................................... 7 BCH North America Inc............................................................. bch-bruehl.com..................................................................................................... 27 EIT Instrument Markets............................................................. eit.com............................................................................................................. 21, 30 Excelitas Technologies.............................................................. excelitas.com..........................................................................................Back Cover Heraeus...................................................................................... heraeus-noblelight.com/technicalservice........................................................... 25 Honle UV America Inc............................................................... honleuv.com............................................................................................................ 5 IST America................................................................................ ist-uv.com.............................................................................. Inside Front Cover, 20 Miwon Specialty Chemical Co., Ltd......................................... miramer.com.......................................................................................................... 41 Nagase....................................................................................... nagaseamerica.com/UV-EB.................................................................................. 17 RAHN.......................................................................................... rahn-group.com...................................................................................................... 1 Siltech Corporation................................................................... siltech.com............................................................................................................. 19 UV+EB Technology.................................................................... uvebtechnology.com............................................................43, Inside Back Cover

52 | UV+EB Technology â&#x20AC;˘ Quarter 2, 2020

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