Editor’s note: A portion of this article was recognized with the 2023 CoatingsTech Conference Best Paper Award, held in Cleveland, OH, in June. The authors have since expanded the article with the addition of new data and updated information.

1. Introduction

Binders are the essential ingredients in every coating formulation and primarily define the properties, applications, and curing technologies of the coating system. The most popular and commonly used binders in conventional coatings are usually designed to be cured by one specific curing technique, such as thermal curing, radiation curing, or ambient temperature curing. Systems with binders that can be cured with only one curing mechanism have inherent limitations, such as difficulty balancing hardness and flexibility, substrate characteristics, flow, leveling, and wettability, complete surface, through curing with no apparent defect, and poor adhesion to substrates in some cases. These shortcomings often require more application space.

UV-curable coatings are among the fastest growing systems due to their low energy input, rapid curing, and low- to zero-VOC compositions, making them sustainable alternatives. The most commonly used UV-curing processes are based on the free radical polymerization mechanism, which brings many challenges to the system, such as oxygen inhibition leading to an under-cured surface, volume shrinkage, and adhesion issues.¹ Some researchers have tried to overcome the challenges related to free radical systems mainly by replacing them with curing processes based on ionic polymerization. Cationic polymerization has been developed as a sustainable solution, as it is not sensitive to oxygen and benefits from full cure potential. However, besides limited availability and high price, cationic polymerization is also very sensitive to environmental humidity; although a small amount of water can be helpful for the reaction—as it can play a major role as a proton carrier and transfer agent—high humidity can inhibit the reaction and terminate the polymerization.² Systems using an anionic polymerization mechanism could be an option, as they have not been observed to have had this challenge, but their drawbacks include low quantum yields and relatively low basicity of the generated bases.³

Some efforts have been made to address the challenges related to the ionic system, such as using a humidity blocker approach, which involves adding a hydroxyl-functional reactive diluent and an epoxy siloxane; this combination creates a hydrophobic system that can block humidity.⁴ Yet, taking full advantage of ionic polymerization requires a novel product. The organic-inorganic hybrid (OIH) system is one of the major products developed recently, especially for UV-curable coatings. The OIH system is based on a sol-gel reaction triggered by a strong acid or strong base catalyst. However, so many developments have been made in trying to initiate a sol-gel reaction by UV irradiation.

One such development is the synthesis of organosilanes containing epoxy and alkoxysilane functional groups and the use of photopolymerization, which uses photo acids and photo bases to cure.^5-8 Nayini et al. investigated the gel time and shrinkage of a UV-cured coating based on the OIH system. In their work, the organosilane oligomers have been synthesized based on tetraethoxysilane (TEOS) and tetraethylene glycol diacrylate. They used photo acids to initiate the sol-gel reaction after exposure under a UV mercury lamp. The results showed delayed gelation and reduced final film shrinkage as the portion of inorganic moieties increased in the formulation.⁹ A review by Liu et al. discusses different preparation methods for OIH systems, including blending, sol-gel, intercalation polymerization, and in-situ reaction methods.¹⁰ The authors’ research group, Mannari et al., at the Coating Research Institute, Eastern Michigan University, has done extensive research in exploring UV-cure systems that can effectively address the aforementioned limitations inherent to free-radical and ionic polymerization mechanisms. They reported the development of OIH coatings that are cured by a UV-initiated sol-gel mechanism that is triggered by photo-latent catalysts. Specifically, they used coating compositions containing multi-functional organosilane precursors as oligomers and super photo-acid or photo-base catalysts to bring about a cure by sol-gel reaction using ambient moisture.^11-12

Continue reading in the September-October 2023 digital issue of CoatingsTech.

By Michael J. Dvorchak, Dvorchak Enterprises LLC

INTRODUCTION

Over the past decade, the automotive-refinish industry has been forced to look at innovative technologies to reduce volatile organic compound (VOC) content and hazardous air pollutants (HAPs) while providing a rapid return to service of the consumer’s vehicle.

UV-A-cured one-component (1K) auto-refinish primers were first introduced in the mid-1990s. UV-A clearcoats were subsequently introduced in the late 1990s.

Materials have continued to be developed and pushed to mimic the classic two-component (2K) solvent-based polyurethanes (PURs); however, slow acceptance by the auto-refinish market over the past two decades is indicative of a market that is difficult to change.

The automotive-refinish coatings market is forecast to surpass U.S. $6.3 billion globally in 2021.¹ This market is expected to increase by 5.4% CAGR between 2021 and 2031. The main technology types are solventborne, waterborne, and UV cure. The classic coating layers are primers, basecoats, topcoats, and clearcoats.¹ A specific parameter in the refinish area that must be addressed is the bottleneck of a 2-hour cure for the primer before it can be sanded. Current UV-cure primers can be sanded within 2 minutes. The need to lower VOCs and volatile HAPs (VHAPs) is among the current constraints for all technologies. A hurdle that was recently cleared in the UV-cure sector is the price barrier for UV light equipment. Reports for the market have UV LED units priced under $1,000.² This market continues to consolidate and will be required to decrease refinishing speeds to remain competitive.

This article will review the history of the UV-cured 1K and 2K auto-refinish market and formulations for primers and clearcoats. It will also attempt to look at current UV-cured 1K and 2K auto-refinish primers and clearcoats in the global market, new formulations, and new developments in UV equipment.

CHANGES IN THE AUTOMOTIVE OEM AND REFINISH MARKETS

The automotive OEM and refinish markets have undergone incredible changes in both polymer technologies and substrates over the past several years. The original markets used nitrocellulose lacquers when the only color you could specify was black. Today, the number of 2K reactive primers and clearcoats, as well as basecoats, has pushed the limits of polymer chemistries. With the pressures to lower VOCs and VHAPS, solvent-based systems have shifted to water-based chemistries. The OEM’s substrates have evolved from the traditional steel metals to composites and aluminum.

INTRODUCTION OF UV-A-CURABLE AUTO REFINISH

Early attempts to develop a UV refinish clearcoat

The earliest paper that reviews the use of UV-cure clearcoats for auto refinish was focused on the use of a UV Flash lamp (Xe lamp).³ The idea was that after application, the fully formulated UV clearcoat would be flashed several times (by the Xe lamp) to activate the photoinitiator (PI) for this dual-cure system. The dual-cure crosslinking of this system was done with a polyol that had acrylate and hydroxyl functionality in combination with a dual-cure crosslinker that possessed acrylate and polyisocyanate functionality. This system was a 2K system. Due to the Xe lamp wavelength occurring around 480 nm, the use of a bis-acylphosphine oxide photoinitiator was specified for this use. Cure was done by using 10 to 20 flashes at 20 °C.

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Radiation-cured coatings continue to experience moderate growth and find expanded applications. These coatings have the ability to increase productivity while offering enhanced environmental compliance and improved physical performance. They hold the potential to offer innovative solutions and the instruments used to produce them are becoming more advanced and cost-effective.

Indeed, according to the RadTech Biennial ultraviolet (UV)+electron beam (EB) Market Survey performed in 2020, the most important advances during the past two years include improvements in UV-LED technology and better UV technology overall, as well as the ability to achieve enhanced adhesion and the introduction of more sustainable materials.

RadTech’s 2020 survey also found that the sales volume of UV+EB-formulated products in North America increased by 4.6% over the previous year, and the annual average for volume sales growth in 2021-2024 is predicted to be even higher at 5.0%. Asia dominates the market with a 38% share, followed by Europe (27%), North America (26%), and the rest of the world (9%).

In 2019, the greatest end-use by far for UV+EB formulated products was graphic arts (OPV and inks), which accounted for 47% of the market. The next two important applications were wood (18%) and plastic coatings (12%). Applications predicted to experience the most rapid growth over the next two years include 3D printing/additive manufacturing, inkjet printing, barrier/conductive films, food packaging, and field-applied solutions.

The bulk (74%) of products are cured using conventional UV technology, but UV LED curing now accounts for 15% of the market. EB and dual/hybrid curing solutions are used for 6% and 5% of products, respectively. Areas for further development identified by the RadTech 2020 survey include specifically designed photoinitiators for UV LEDs, improved weatherability and adhesion, reduced migration of resin components, material sustainability, and regulatory issues.

EFFICIENCY, PRODUCTIVITY, PERFORMANCE

UV-cured materials are most widely used in the graphics and industrial wood markets due to their high speed and efficiency, according to Michela Fusco, global marketing director for Radcure at allnex. They also offer a smaller footprint for application, a very attractive return on investment, as well as the avoidance of volatile organic compounds (VOCs), according to Michael Kelly, chief customer officer at Allied Photochemical.

Chris Puccio, business director Radcure—Americas with allnex, agrees that overall growth of radiation cure is above typical gross domestic product (GDP) rates due to the mix of markets. He notes that interest in the technology’s near-instant curing is driving the market growth, along with technology’s ability to reduce the work in process, use 100% solids or waterborne formulations, produce less emissions, and consume
less energy.

Radiation-curable coatings also offer these environmental benefits along with excellent mechanical and chemical resistance, says Masahito Furo, group manager of the DIC Corporation Global Marketing Strategy Group. Combined, these attributes keep work-in-progress goods out of inventory, reduce scrap, and facilitate quicker lead times while simultaneously producing desirable finished goods properties, according to Jennifer Heathcote, business development manager for GEW.

In addition to 3D printing/additive manufacturing, metal and composites are also markets with real potential, according to Fusco. The trend, according to Dustin Kurath, technical director at Northern Coatings and Chemical, is more toward unusual applications for novel substrates such as metal, glass, and plastics.

“Growth seems to be moderate but steady, but interest and growth potential appears stronger in areas where UV coatings have traditionally not been used because of performance limitations that are now being addressed,” he explains.

Van Technologies looks for surface-area opportunities that can benefit from higher performance, higher quality, lower cost, and minimal environmental impact, according to company President and CEO Lawrence Van Iseghem.

“It’s difficult to state any application that can’t be supported by those benefits,” he observes. “It used to be that when considering performance, quality, and cost, you could only target two of the three and acknowledge that the third would be compromised. Using radiation-curable coatings, it is possible to target all three. The biggest end-use applications? Use your imagination. The graphic arts (printing), paper, film, foils, wood, direct to metal (DTM), glass, ceramic, and the automotive industries all can benefit from radiation-curable coatings.”

For instance, Furo notes, in Asia, automotive applications and consumer electronics are expected to continue to drive the market for UV coatings and coating resins.

“Radiation curing is an environmentally friendly manufacturing tool that enables companies to produce products faster, more efficiently, and with better end use product performance and longevity,” Heathcote says.

As new consumer and industrial goods and related manufacturing processes emerge, the application of ultraviolet curing and all its benefits can be transferred. “It is simply a matter of engaging with evolving markets and the supply chain to repurpose the technology for new uses,” she says.

Heathcote points to considerable activity and innovation in UV digital inkjet, 3D printing, UV LED curing, electronic assemblies, and electric vehicle (EV) batteries, low-migration formulations for food packaging, and mattification of coatings using excimer lamps.

In addition, overall demand for many commercial print products is declining, but commercial printing, labels and packaging, and pre-coated substrates used in those applications still represent the largest markets by volume for radiation-cured coatings, Heathco notes. Home-related structural goods, furnishings, and appliances are also strong.

Allnex is also receiving more requests for sustainable solutions, such as renewable material content, and solutions for more circular products, according to Fusco.

Van Technologies has observed growing interest in waterborne UV-curable coatings because they can be applied manually in a normal spray booth and therefore provide an excellent starting point for those interested in achieving high-performance, high-quality finishes without major capital investment in automation.

“Customers want the shortest possible processing time for painted goods they can get,” Kurath adds. “And this expectation is driving growth in both 100% solids UV and water-based UV as well, particularly on plastics.”

Van Iseghem has also seen growing interest in exterior grades of both waterborne and 100% UV-curable coatings. Recent advances in UV LED technology that provide energy savings and very low heat generation, coupled with high intensity for rapid cure, is also driving interest in its applicability in the finishing markets, he notes.

MARKET SHIFTS RELATED TO COVID-19

Some unusual activity was observed in 2020 due to the COVID-19 pandemic that is carrying over into 2021. “The COVID-19 pandemic changed the way business is conducted in most every market sector,” Van Iseghem says. “Many businesses experienced significant impact and a decline in year-over-year revenue, while those in the ‘essential’ business areas were able to weather the pandemic better with some experiencing improved performance relative to the prior year.”

Lockdowns, shutdowns, travel restrictions, and import-and-export trade issues led to issues with material availability.

For instance, many sectors within graphics printing that serve tradeshows, conferences, entertainment venues, and restaurants were hit hard by pandemic lockdowns while a rise in home-improvement activity led to higher-than-expected growth in demand for hardwood flooring, kitchen and bathroom cabinets and fixtures, molding, millwork, doors, and furniture, many of which are finished using UV-cured coatings.

The pandemic also led to greater use of packaging, which led to surges in demand for UV materials in that market. The demand for consumer electronics was also up dramatically as people transitioned to working from home, leading to greater demand for the UV-cured coatings used on these products.

UV-curable technology is an excellent fit for these manufacturers, according to Van Iseghem, and the pandemic provided ample opportunity to explore its potential. “They all were interested in automation to reduce dependency on labor while also improving operational efficiencies, quality, and cost effectiveness,” he says.

“They needed to remain competitive, and the COVID-19 pandemic made many operational activities unpredictable. The need to sustain operations, lessen the impact of labor, improve efficiencies and quality, and become more cost-effective were driving forces toward the consideration of radiation-
curable coatings.”

On the flip side, lockdowns, shutdowns, travel restrictions, and import-and-export trade issues led to issues with material availability. This situation only compounded problems with already existing photoinitiator shortages and raw material feedstock shortages, according to Kurath. As a result, he notes, prices for photoinitiators, especially those based on phosphine oxide, and some other key additives have increased dramatically in the past few years.

The damaging winter storms in early 2021 in Texas further compounded the shortages, adds Puccio.

“The biggest COVID-19-related impact on radiation-cured coatings is that it has negatively impacted the supply chain while simultaneously driving up demand for many products,” Heathcote says. “Many manufacturers that use UV-curable coatings are running lines at capacity and even adding new equipment. At the same time, these same users are being rationed by suppliers who are having freight and raw material supply challenges of their own. The entire chemical industry, not just radiation curing, has been impacted.”

One of the biggest challenges today, Kelly says, is the lack of confidence to invest in private companies due to changing tax laws, rules, and investment regulations.

Fusco expects recovery to be different across regions and application. “China is leading the recovery and already back to pre-COVID levels in 2021. The United States also started strong in 2021, but the recovery in Europe is slower,” she says. “These differences are in part due to changes in consumer confidence impacting markets for UV from packaging for consumer goods to furniture and consumer electronics.”

Kurath believes, in fact, that the next few years will be difficult to predict. “Most of the impact has been related to supply-chain disruptions in Europe and Asia. Monomers and certain oligomers, such as epoxy acrylates, have become harder to keep in stock due to COVID restrictions,” he says. “We expect given the current situation combined with tightening environmental regulations and rising labor costs in Asia that production of many raw materials will move back domestically, which should normalize the market.”

A FEW HEADWINDS

In addition to raw-material supply and cost issues, another headwind impacting the growth of radiation curing includes the high initial costs for new users, according to Furo.

“The perception that capital equipment costs are high is a real obstacle to implementation and is unfortunate,” says Van Iseghem, “because if the return on investment is calculated, it will often be determined that payback is well under two years.” He adds that the issue is further confused if purchasing departments look at cost per gallon as opposed to cost-per-finished surface area, unit, or component.

“When comparing the costs of applied, dried, and cured chemistry, there is no cheaper way to finish than using radiation-curable coatings,” Van Iseghem says. “The return on investment is rapid and enhanced profit margins are assured.”

For existing manufacturing lines not currently using radiation curing, the business case must be made and the ROI calculated in order to justify the capital investment, Heathcote says. Widespread unfamiliarity with radiation curing and the associated learning curve for new users have always been an impediment to growth, she says.

In addition, as with all technologies that employ chemicals to manufacture goods, navigating the ever-changing and increasingly restrictive regulatory and sustainability landscape is also a challenge, Heathcote notes.

It is essential, Fusco agrees, to analyze the overall impact of radiation-curing technologies.

WHERE DO EB, UV, AND UV LED STAND?

The three primary radiation-curing technologies include EB, UV, and UV LED. EB is mostly used on large food-packaging-printing machinery where the benefits of radiation-cured coatings are required; however, converters are not willing or able to accept any risk of photoinitiator and migration in their products, according to Robert Rae, managing director for sales at GEW.

The large capital investment required makes EB suitable only for high-speed printing operations and packaging production items that can be offset, or screen printed, and produced in large quantities, Kurath adds.

Allnex has experienced increased interest in EB curing in deco film and cabinets applications that have traditionally been coated with conventional acid-catalyzed coating systems, according to Eileen Weber, global marketing manager for Radcure at allnex.

UV technology continues to provide huge advantages over solvent- and water-based processes for total energy consumption of the finished product, Rae says. “Reduction in running costs, as well as the ability to achieve sustainability goals for brands and converters, are increasingly important drivers for moving to UV coatings, and we see our significant growth in wide-web, UV-coating applications as evidence of this trend,” he observes.

UV LED is experiencing the greatest growth, however. “As LED lamps have become more powerful, more capable, and more economical, we are seeing an acceleration in adoption of LED, particularly in graphics applications,” says Weber. She notes that in wood coatings systems, LED has found application in high-volume furniture markets, mostly for primer and sealer coats.

“UV LED curing seems to be most popular with customers that have issues working with heat-sensitive, two-dimensional substrates that would be damaged by heat from a traditional mercury lamp,” Kurath says.

LED cure can offer numerous advantages, including lower energy consumption and lower heat, but it also has some limitations especially related to surface cure. The main issue, according to Rae, is a lack of UVA photoinitiators suitable for highly functional formulations. “Further advances are needed to achieve UV LED emitters of sufficient power at UVC wavelengths before significant progress will be seen,” he says.

While much work is being done on the development of LEDs with emission in the UVB/UVC wavelengths, Kurath sees the greater industry focus on the development of more powerful UVA LEDs, which he notes slowly seem to be overcoming the surface cure problem with raw power.

One possible approach suggested by Kurath is the development of a good Norrish type 2 photoinitiator with good quantum efficiency in the UVA region between 350 and 400 nm, which could accelerate adoption of UV LED in the next couple years.

“Current UVA initiators are helped by synergists like tertiary amines and mercaptide functional materials, but good performance is tough to achieve, especially with low-molecular-weight monofunctional monomers and difunctional oligomers crucial to achieving film flexibility commonly needed for plastics,” he explains. “Type 2 synergists already exist and are available from several large resin manufacturers, so Norrish type 2 photoinitiators would help coating manufacturers better address surface cure issues inherent with LED cure coatings.”

Finally, Kelly notes that UV LED is already coming of age particularly as it sees increasing use for 3D-curing applications.

The interest in UV LED is very high, agrees Van Iseghem, but not necessarily practical for many to implement outside of the graphic arts and printing industries. “With higher LED power, however,” he says, “the application to non-flat surfaces will expand. We are on that threshold presently. The energy savings, applicability to heat-sensitive substrates, coupled with being more environmentally responsible will draw many new users.”

INSTRUMENT ADVANCES

There have been a few advances in instrument technology for radiation curing worth noting.

For traditional UV curing, monitoring technologies for mercury vapor lamps are increasing the good manufacturing practice oversight that converters have on their printing/coating processes—especially when these technologies have real-time feedback, according to Rae. “In general, UV monitoring improves yields across all radiation-cured coating market segments. It also enables converters to feel comfortable using UV technology and coatings containing photoinitiators in final-use products, which demand low migration,” he explains.

GROWING INTEREST IN EXCIMER UV CURING

The introduction of excimer-cure equipment is also getting a lot of interest in the market, particularly in Europe, according to Fusco. “This technology offers a solution for achieving ultra-low gloss, high-performance deco surfaces,” she says, but does require finding the optimal resin/formulation fit to fully realize the equipment potential.

It is also used in furniture decorating, flooring, automotive, and display applications because it can add soft touch, matte finishes, and other desirable properties, such as antiglare and anti-fingerprint, without any real increase in the cost of the coating, Rae says. “Most importantly, the added durability and stain resistance of excimer mattifying is of significant benefit to flooring and furniture coatings versus impregnation of the coating with a typical matting agent,” he says.

WATERBORNE TOPS MARKETS

Among all coating types, waterborne dominates (40%), followed by solventborne (20%), powder (20%), radiation curing (10%), and all others (10%), according to Furo. “We expect waterborne, powder, and radiation-cured coating demand to increase and solventborne demand to decline, but the changes will not be drastic,” he says.

Demand for 100% solids products is also increasing. Van Iseghem notes that within five years the market for radiation-curable coatings is expected to grow faster than the markets for 100% solids and powder, but still well below waterborne coatings. “Radiation-curable coatings will be viewed as a very versatile quality option for most any substrate as time passes,” he asserts.

Within the radiation-curable coatings market, 100% solids formulations dominate, but waterborne products are experience increasing demand. Very few UV-curable coatings are formulated as solvent-based systems. Kurath expects water-based UV to experience the highest growth since it has properties that are most similar to those of traditional coatings with respect to safety and application characteristics and only comes at a slight expense with respect to processing time.

Waterborne UV-curable coatings also allow for better control of applied dry film thickness and surface gloss, according to Van Iseghem.

With regards to the long term, Kurath says he believes that UV powder may enjoy stronger demand, but currently it is a niche product that requires special equipment to use and has limited applications.

MONOMER AND RESIN DEVELOPMENTS

With respect to monomer and resin technologies, epoxyacrylates in combination with monomers are still the workhorse of the radiation-curing technology because they combine good reactivity with hardness and chemical resistance, according to Fusco.

Free-radical-initiated acrylate functionalized chemistries dominate the market and will remain in a preeminent position for some time to come, agrees Van Iseghem.

There are a wide range of acrylate monomers and functionalized oligomers including epoxy, urethane, polyester, acrylic, siloxane, melamine, and others. These chemistries have interior and exterior capabilities; they also have a broad array of mechanical and physical properties from low to high modulus of elasticity that have low odor potential and can be formulated in pigmented and clear coatings.

There has been discussion, however, about the use of bisphenol-A with these systems, and Fusco notes that requests for more sustainable alternatives, including biobased products, are increasing, not only in wood coating applications, but also for printing inks. “Customers are switching to waterborne UV solutions to reduce the need for monomers,” she adds. “Self-curing resins are also gaining popularity due to their low label and migration. The shift to UV LED curing, meanwhile, has created demand for booster systems to increase reactivity.”

Cationic-cured chemistries show continued progress as well, according to Van Iseghem, but remain somewhat limited in their applicability. “We expect this situation to change over time as further advances in this chemistry are made,” he says. Van Iseghem also notes that their ability to achieve effective cure in highly pigmented coatings may give cationic systems advantages in these applications. In addition, he believes that the ability of UV LED systems to target wavelengths beneficial to cationic reins could contribute to the mutual growth of both.

Some new monomers have been introduced to the market that offer advantages over older chemistries, but Kurath notes they are expensive, which limits their usefulness in price-sensitive applications.

The most attractive new resin technologies, according to Kurath, tend to be based on thermoplastic acrylics and thus experience limited shrinkage and provide adhesion and flexibility properties that are tough to achieve with fully reactive resins.

He would like to see a new resin technology that does not rely on acid groups for bonding to metal substrates, perhaps with a grafted silane functionality that could bind to metals with the aid of photoacid generators. “Such a UV-curable coating technology would have improved corrosion resistance and possibly have the ability to bond to difficult, non-metallic inorganic substrates,” he explains.

ADDITIVE AND PIGMENT IMPROVEMENTS

Given that most UV-curable coatings are based on acrylate chemistry, additives, and pigments play an important role in establishing differentiated properties of applied coatings. UV absorbers to prevent yellowing in exterior coating applications and pigments that do not block UV transparency for thick-film coatings or high-concealing paints are two important examples, according to Furo.

Additives commonly used in radiation-cured coatings include surfactants and dispersants, antifoamers and defoamers, mar and slip agents, ceramic media, silicas, silicones, and siloxanes, and even UV absorbers and inhibitors, Van Iseghem says. “Additives can make a dramatic impact but must not conflict with the ability of the system to cure,” he adds.

Recent developments that Van Iseghem says have presented new options include nano titanium dioxides, aluminum oxides and silicas, conductive nano and micro metals, carbon nanotubes, graphene, and other materials.

Antimicrobial additives are seeing greater use today in response to the COVID-19 pandemic, although in the United States most manufacturers target antibacterial and antifungal claims versus antiviral claims because there is significantly greater cost associated with the efficacy testing required before an application can be submitted to the Environmental Protection Agency, according to Van Iseghem.

Sourcing is another consideration. Additive suppliers have become much more reluctant to sell additives in small quantities during the past couple years, so additives that can be used in traditional coatings as well as UV are very attractive, Kurath says. He adds that sample lead times and product quality are ongoing issues with pre-dispersed pigment pastes because UV-cure color pastes tend to have poor shelf stability.

Furthermore, the monomer commonly used to dilute the pastes—trimethylolpropane triacrylate (TMPTA)—undergoes high shrinkage and can cause film cracking and reduced adhesion to difficult substrates when used at high levels.

Areas for improvement include additives to improve adhesion on new, more sustainable substrates and that can enhance the sprayability of 100% UV coatings, according to Fusco.

Kurath would like to see more effective wetting aids and defoamers as well as the development of dyes suitable for use in UV-cure coatings. “Currently available dyes have an inherent and dramatic negative impact on curing performance, even when used at low levels in translucent films,” he explains. “There is a desire for transparent, clean colors that can only be achieved with dyes if appropriate technology could be developed.”

Finally, as mentioned above, the development of optimal photoinitiators for UV LED curing (along with the improvement of diode technology) is needed to support the growth of this segment of the radiation-curing market. “Such advances will enhance the applicability of UV LED into profiled surfaces and other complex shapes that can be finished with radiation-curable coatings,” Van Iseghem says.

EDUCATION KEY TO GROWTH

While the growth of the radiation-cured coatings market is healthy, given the myriad benefits this technology offers, there is potential for an even higher rate of expansion.

Of all the limiting factors mentioned previously, perhaps lack of awareness is the greatest issue impeding that faster growth rate. “Growth would certainly increase if more people knew UV-cure coatings existed,” says Kurath. “Many of the customers who approach us have just recently heard of UV-curable coatings. There are many application opportunities in many market segments for existing radiation-curing technologies, and the industry must make a concerted effort to broadcast the various advantages and benefits for end-users.”

Kelly sums it up well: “Radiation-cured coating technologies offer efficiency and productivity combined with high quality and performance. They also enable environmental compliance combined with sustainability in the form of reduced emissions and energy consumption. On top of all that, the ROI is incredible.”

COATINGSTECH JUNE 2021 | Vol. 18, No. 6

For pipe and tubing, Allied Photochemical reports on two case studies that highlight the advantages of UV-cured over water-based systems. “In today’s competitive manufacturing environment, it is necessary to drive process efficiencies, improve sustainability, and deliver a high return on investment (ROI) for stakeholders while also manufacturing a superior product,” says Michael Kelly, chief customer officer at PhotoChemical. “UV coatings deliver on all four points,” he asserts. It is, therefore, essential for applications where UV curing is practical to consider the advantages that this technology may provide, according to Kelly. “In pipe and tube applications, for instance, UV coatings technology offers a unique opportunity for users to dramatically improve their manufacturing processes and sustainability footprints and deliver real ROI in the form of less coating cost per linear foot of pipe or tube,” he explains.

In pipe and tube applications, for instance, UV coatings technology offers a unique opportunity for users to dramatically improve their manufacturing processes and sustainability footprints and deliver real ROI in the form of less coating cost per linear foot of pipe or tube.

One of the perceived concerns regarding adoption of UV curing technology is that it has both higher upfront investment and ongoing operating costs and thus higher overall product costs. In two customer case examples in which each customer had to choose between investing in an upgrade to an existing water-based coating system or a new UV coating system, Allied Photochemical was able to show that this perception is faulty. The examples included coating of OCTG (Oil Country Tubular Goods) / line pipe and mechanical tube products, and for both, it was found based on quotations from various partner companies that the overall capital costs for waterborne and UV-curing systems were comparable, according to Kelly. “The main difference was the heating system used for each: induction heating and microwave / UV lights, respectively. While both have similar capital investment costs, they have very different operating costs and uptime costs,” he observes.

For the OCTG / line pipe example, the customer was looking to increase the line speed of the coating application process, improve corrosion resistance and reduce end-user complaints, and eliminate wasted floor space due to the need for drying tables. The existing water-based coating (18.5% solids) cost $11.89/gal and covered 296.7 ft² at a 1.0 mil DFT (Dry Film Thickness). This product was proposed to be replaced with a 100% solids UV-cured coating priced at $39.70/gal but offered coverage at 1.0 mil DFT of 1,604 ft². Using a proprietary functional pipe coating model, Allied Photochemical estimated the cost per linear foot for the existing water-based coating and the proposed UV coating solution for OCTG / line pipe with a diameter of 9.625 in. applied at thickness of 1.0 mil. According to the model, for a 2,000,000 million linear foot run, switching to the UV coating would afford cost savings of $77,197.74 ($771,977.36 at 20,000,000 million linear feet).

[The UV coating] also outperformed the waterborne coating in humidity, adhesion, and UV resistance testing.

In the case of the mechanical tube, the customer wanted to improve the corrosion resistance and aesthetic appearance of the coated tubes in order to reduce end-user complaints, increase the line speed, minimize humidity and temperature impacts on the coating process, and eliminate VOCs and HAPs. Although a water-based coating was used at the time, it contained ~11% of a flammable cosolvent. At $19.79/gal, this coating comprised 30.5% solids by volume and covered 489.2 ft² at 1 mil DFT. The proposed 100% solids UV coating alternative priced at $59.25/gal but offered coverage at 1.0 mil DFT of 1,604 ft². “Once again,” says Kelly “the performance of the UV coating exceeded that of the water-based system and met all of the higher target specifications established by the customer. The line speed was increased from 175 fpm to over 275 fpm and ~70% less coating had to be manipulated. In addition, handling/exposure of the flammable water-based system was eliminated.” The other process advantages outlined for the first example were also realized, he adds. In this case, the cost analysis using the functional pipe coating model (2.0-in. diameter tube coated at a thickness of 0.3 mil) predicted savings of $2,759.29 for a 5,000,000 million linear-foot run and $27,592.94 for a 50,000,000 million linear-foot run. Based on all of these results, the customer elected to implement a new UV coating system.

“In both examples, UV coatings allowed the customer to run much faster, with a smaller physical footprint, lower WIP, and reduced quality and energy costs, resulting in a much cleaner, green, sustainable process while delivering an improved ROI to the bottom line,” Kelly concludes.

CoatingsTech | Vol. 17, No. 8 | August 2020

Introduction

Waterborne (WB) UV-curable polyurethane dispersions (PUDs) are the binders of choice in many industrial wood applications. These resins are formulated with photoinitiators that absorb energy and initiate consecutive free radical-curing reactions. Traditional photoinitiators include both Type I (photo cleavage) and Type II (H- abstraction) classes, which are effective for surface curing.^1�� Because  these products are classified as environmentally damaging (toxic to aquatic life) and can be a health hazard (suspected carcinogen), it is essential to fully react these components during curing because of potential migration of unreacted residual amounts. Such concerns are especially important considering the use of these materials for skin- or food-contact applications. New-generation UV-curable PUDs have been developed with initiating sites based on Type II photoinitiators incorporated into the backbone of the polymer. These new self-initiating (SI) polymers have been evaluated for use in industrial wood applications and benchmarked against traditional WB UV PUDs formulated with both conventional and polymeric photoinitiators. The performance of both clear and pigmented coatings has been investigated.

A series of five SI WB UV resins has been developed. These polymers are SI versions of existing WB UV resins. These WB UV resins are all acrylic/polyester polyurethane dispersion hybrids with a minimum film formation temperature (MFFT) of approximately 0 ^oC. Table 1 summarizes the physical properties and characteristics of the resins evaluated.

Experimental—Clear Self-Sealing Topcoats

A study has been conducted to compare the properties of the SI WB UV resins with traditional WB UV resins. These coatings were tested according to Kitchen Cabinet Manufacturers Association (KCMA), Architectural Woodwork Standard, and individual office furniture manufacturer specifications.

Panel Preparation

Approximately 3 wet mils of coating were sprayed over an 18 x 18-in. stained birch plywood panel. The coating was allowed to air dry for 10 min and force dry for 10 min at 50 ^oC, and then cured with a mercury bulb at 800 mJ/cm². The coating was sanded with a 3M Superfine Sanding Sponge before a second coat was applied at approximately 3 wet mils. The coating was allowed to air dry for 10 min and force dry for 10 min at 50 ^oC, and then cured with a mercury bulb at 800 mJ/cm². Testing began after seven days unless otherwise indicated in the test method.

For edge soak, all sides of a 4 x 4-in. solid oak panel were coated and cured with a mercury bulb at 800 mJ/cm². The panel was then sanded, recoated, cured, and allowed to air dry for seven days.

Table 2 shows the clear self-sealing topcoat formulations used in Part 1 of the study.

Procedures and Results

KÖnig Pendulum Hardness

A 150-micron drawdown was prepared on a glass panel, allowed to air dry for 10 min and force dry at 50 ^oC for 10 min, and cured with a mercury bulb at 800 mJ/cm². The topcoat was measured for König pendulum hardness before UV cure and again one and seven days after cure. Figure 1 shows the König pendulum hardness results.

All of the coatings had excellent hardness properties, with SI WB UV 1 and SI WB UV 4 having the highest film hardness due to the high crosslink density of the polymers. As expected, UV curing resulted in rapid hardness development unlike other technologies that rely on self-crosslinking mechanisms that take several days to develop.² The SI polymers all had similar hardness values compared to their traditional counterparts.

Boiling Water Resistance

Ten mL of boiling water was applied to the test panel. A ceramic coffee cup full of boiling water was then placed on top of the 10 mL of water. After one hour, the cup was removed and wiped with a paper towel. Figure 2 shows the boiling water resistance results, with zero representing complete destruction of the film and five representing no effect on the film.

All of the SI polymers showed comparable performance to their traditional counterparts for boiling water resistance. Formulations based on UV 2 and UV 5 showed slight blushing when exposed to boiling water due to small amounts of hydrophilic materials in the composition, which are not present in the other polymers.

Chemical/Stain Resistance

A 0.875-in. diameter filter paper was placed on the test panel and saturated with the chemical/stain and covered with watch glass. After the recommended time according to the specification, the chemical/stain was removed  and the surface of the panel was washed with water. As shown in Figure 3, each chemical/stain was rated according to the following scale:

5 = No impact on film

4 = Very light discoloration or gloss change and no softening or film deterioration

3 = Moderate discoloration and/or moderate film softening and no film deterioration

2 = Heavy discoloration and/or
moderate film softening allowing easy deformation

1 = Heavy film softening with little loss of adhesion

0 = Complete deterioration of the film

All of the coatings performed very well and had excellent resistance to all of the reagents. 100-proof vodka left a slight gloss change on SI WB UV 5.

Figure 4 shows the resistance to chemicals in office furniture specifications, which are set by the individual office furniture manufacturers.

Exposure to these chemicals showed more differentiation in resistance level. In most cases, the SI WB UV coatings had comparable performance to their traditional counterparts. Acetone and isopropyl alcohol proved to be more difficult chemicals to resist. While the coatings based on UV 1 performed exceptionally well, UV 3 and UV 5 resulted in film defects upon exposure.

The durability of the coatings was tested by both scrape adhesion and ball point pen indentation using a BYK Balanced Beam Scrape Adhesion and Mar Tester. It is essential for furniture coatings to resist scratches from sharp objects or from abrasion over the surface of the finish.

Scrape Adhesion

A 4 x 4-in. piece was cut from each test panel and evaluated with a 1–5 Kg weight using the loop stylus. This test method is a modified version of ASTM D 2197. Figure 5 shows the scrape adhesion results reported in Kg passed.

SI WB UV 1–4 had equal or better scrape adhesion than their traditional counterparts. SI WB UV 5 had slightly inferior scrape adhesion as evidenced by whitening of the film.

Ball Point Pen Indentation

Similarly, for ball point pen indentation, a 4 x 4-in. test panel was evaluated with 100–500 g of weight using the small pen stylus #5785. Any indentation that could be seen from a distance of 24 in. is considered a failure. Results are reported in grams passed after a one-hour recovery period. Figure 6 shows the ball point pen indentation results.

The best resistance to indentation was achieved by coatings 1, 3, and 4. SI WB UV 2 and 5 had slightly inferior results than their traditional counterparts.������

Several other tests were conducted to evaluate the performance of the coatings for industrial wood finishes. The results are summarized in Table 3. Performance was excellent for all formulations, and no differentiation was found.

Green Print Resistance

One hour after the test panel was cured, a 2-in. square piece of #10 cotton duck cloth was applied to the finish. A force of 2 lb/in.² was applied directly to the cotton duck cloth. The cotton duck cloth was removed after 24 h and then evaluated for printing.

Hot Print Resistance

Approximately 14 days after the test panel was cured, a 2-in. square piece of #10 cotton duck cloth was applied to the finish. A force of 1 lb/in.² was applied directly to the duck cloth. The specimen was placed in an oven at 60 ^oC for 24 h.  The duck cloth was removed, and the specimen was allowed to cool for one hour and then evaluated for printing.

Edge Soak

A cellulose sponge was placed in a plastic container. The container was leveled and and filled with detergent solution (1% Dawn® dish soap by weight in water) to 0.5 in. below the top level of the sponge. The panel was placed on the sponge cut side down and allowed  to stand for 24 h. The panel was then removed and allowed to dry for one hour before it was evaluated for blushing and blistering.

Plasticizer Resistance

A 2-in. square piece of red vinyl was applied to the test panel. A force of ½ lb/in.² was then applied. The specimen was placed in an oven at 50 ^oC for 72 h. After cooling at room temperature for one hour, the vinyl square was removed and then evaluated for softening and blistering.

Hot and Cold Check Resistance

A 4 x 4-in. piece was cut from each panel. A panel was placed in the humidity cabinet at 50 ^oC and 70% humidity for one hour. The panel was removedd and allowed to reach original room temperature and humidity. The panel was then placed in the freezer at -10 ^oC for one hour. The panel was removed and allowed to reach original room temperature and humidity. The cycle was repeated five times.

Discussion

In the clear self-sealing topcoats, the SI WB UV resins had very good performance and compared well to their traditional WB UV counterparts formulated with traditional photoinitiator. SI WB UV 5 had the weakest performance overall. While its resistance to KCMA stains is quite good, it showed poor resistance to isopropyl alcohol and acetone and showed the poorest scrape adhesion. The ball point pen indentation and boiling water resistance were also not perfect. The cure response was calculated using FTIR spectroscopy.³ The SI WB UV 5 was found to have lower cure (78%) compared to its traditional counterpart (85%). In an effort to improve the cure and boost performance, SI WB UV 5 was reformulated with a small amount of Type I photoinitiator and retested. Table 4 shows the formulation used.

Figures 7–10 show the resistance comparisons of WB UV 5, SI WB UV 5, and SI WB UV 5 with additional photoinitiator. The addition of 0.5% photoinitiator to SI WB UV 5 improved all of the resistance properties, making them equal to the traditional WB UV 5.

Another way to minimize migration issues with residual photoinitiator is with the use of polymeric initiators. The next step in this investigation was to compare the properties of the SI WB UV resins with the traditional WB UV resins formulated with polymeric Type II photoinitiator. It is key that the polymeric photoinitiator has good compatibility with the UV resin. Thus, a small amount of cosolvent was required in the formulation to ensure good solubility. SI WB UV 1 and SI WB UV 2 were included in this evaluation. Hardness development and chemical resistance were assessed. Figure 11 shows the König pendulum hardness comparison.

Coatings made from the SI WB UV resins were significantly harder than coatings made from the traditional WB UV resins with polymeric photoinitiator. It is possible that the polymeric form impacts the mobility of the initiator, reducing initiation and, thus, impacting crosslink density. The addition of a second photoinitiator is likely needed to improve performance.

SI WB UV coatings had similar chemical resistance to coatings made from the traditional WB UV resins with polymeric photoinitiator as shown in Figure 12.

Experimental—White Self-Sealing Topcoats

White coatings were formulated with SI WB UV 1 and SI WB UV 2, and the properties were compared to coatings formulated with traditional WB UV 1 and WB UV 2. Pigmented UV coatings are commonly formulated using two different types of photoinitiators. Type I and II photoinitiators that absorb at short wavelengths (< 290 nm) are used for surface cure. Acyl phosphine oxide is used for cure at the coating/substrate interface because it absorbs at longer wavelengths (> 320 nm). In this study, the SI WB UV resins were tested with the addition of acyl phosphine oxide. The traditional WB UV resins were formulated using Type I and II photoinitiator and acyl phosphine oxide. All coatings were cured with both gallium and mercury bulbs. Table 5 shows the white self-sealing topcoat formulations used in Part 2 of the study.

Figures 13–15 show a comparison of data from the white topcoats.

The pigmented SI WB UV formulations performed very well compared to the controls.

Conclusions

Novel UV-curable SI polymers have been developed for use in industrial wood finishes. These SI polymers showed good photo-efficiency leading to high chemical resistance, hardness, and good mechanical properties compared to their traditional counterparts formulated with conventional photoinitiator. They are easily formulated and open the door for use in more varied applications without the risk of compromising health and safety. Future work will explore the use of these materials in other market areas.

Acknowledgements

Special thanks to the R&D group at Alberdingk Boley GmbH who contributed to the development of the technology: Mr. Markus Dimmers, head of technical marketing coatings and Dr. Matthias Hölderle, R&D polymer dispersions.

References

1. Cowan, D.O. and Drisco, R.L. Elements of Organic Photochemistry, Plenun Press, Ch. 3 and 4 (1976).
2. Carlini, C., Ciardelli, F., Dohati, D., and Gurzoni, F. Polymer, 24, 599–606 (1983).
3. Morris, L. “A Cure for Wood Protection,” European Coatings Journal, 9, 18–23 (2016).

CoatingsTech | Vol. 17, No. 7 | July 2020

Infrared-(IR) based weaponry and surveillance systems utilizing the short-wave, mid-wave, and long-wave IR emissions produced by ambient to high-temperature surfaces are an existing and continually evolving threat to military platforms. The survivability and sustainability of aircraft, helicopters, ships, and land vehicles depend on adequate protection against these threats. IR emissions from an object can be reduced by the use of an easily applied, low-weight, and passive low-emissivity coating for a relatively low cost. Several low-emissivity coatings with visual camouflage colors used by the Australian Defence Force (ADF) have been formulated that have lower emissivity in the critical IR transmission windows in the thermal IR when compared with conventional military coatings. A low-emissivity IR coating based on the camouflage color Aerospace Material Specification Standard 595 36375, a color that is employed by the Royal Australian Air Force on a number of ADF platforms, was formulated and tested against two topcoat specifications, MIL-PRF-85285E and DEF(AUST) 9001A.

Introduction

Camouflage is the method of using a natural or artificial material on personnel, platforms, or tactical positions with the aim of confusing, misleading, or evading an enemy.1 It is an essential attribute for any modern military platform, which, together with operational tactics, greatly improves the survivability and operational capability of the platform and personnel involved. Selected visual camouflage colors from the Aerospace Material Specification Standard 595A series (AMSS),² used for Royal Australian Air Force (RAAF) aircraft and Australian Army (Army) vehicles, or the Australian Standard (AS)³ series, some of which are used for Royal Australian Navy (RAN) vessels, are listed in Table 1.

Visual camouflage coatings, apart from providing protection from corrosion and weathering, can also be formulated to provide camouflage in other parts of the electromagnetic spectrum (EMS) outside the range detectable by the human eye (having an operating range of 0.38–0.78 mm), such as the infrared (IR). Since the 1990s, research at the Defence Science and Technology (DST) Group on the IR properties of military topcoats has focused on the near IR (NIR) for purposes of reflecting incoming solar radiation to reduce heating of military equipment⁴ and active night vision goggle camouflage.⁵ More recently, work has been completed in formulating coatings that assist with camouflage in other parts
of the IR spectrum.

This article presents results of the work undertaken to test the performance of ambient cure low-emissivity (LE) versions of the camouflage colors listed in Table 1 designed for operational performance for temperatures ≤ 250°C. LE coatings for high-temperature (> 250°C) applications rely on a different technology that will not be covered in this article. One color, AMSS 36375, was selected due to its wide operational use in the RAAF, a branch of the Australian Defense Force (ADF). The color was formulated as an LE coating and tested against both the MIL-PRF-85285E and the DEF(AUST) 9001A specifications.

Infrared Theory

Thermal Infrared

Many definitions exist for the boundaries within the IR component of the EMS.^6-8 For this article, these wavebands will be based on semiconductor material responses, as listed in Table 2.

At longer wavelengths beyond the NIR, self-emissions from objects (referred to as “thermal” radiation) become dominant. Due to the quantum nature of our universe, all objects above absolute zero emit radiation.

For a blackbody (a theoretical object that absorbs all the energy of all the wavelengths of the incident radiation⁹) at thermal equilibrium, the spectral radiance of the thermal radiation emitted from the blackbody can be calculated and plotted (Figure 1) using Planck’s radiation law¹⁰ as given by equation (1):

�� �ă�ă(1)

where l is the wavelength (m), c is the speed of light (2.998 × 10⁸ m s^-1), k is the Boltzmann constant (1.38 × 10^-23 J K^-1), S is the spectral radiance (W sr^-1 m^-2 mm^-1), h is Planck’s constant (6.63 × 10^-34 J s), and T is the absolute temperature in Kelvin.

The wavelength of the peak radiation can be calculated using Wien’s displacement law,¹¹ given by equation (2):

(2)

where b is Wien’s displacement constant (2.898 × 10^-3 m K).

Planck’s radiation law demonstrates that the effect of increasing temperature on spectral radiance is nonlinear and that emittance increases rapidly with temperature. For simulated temperatures where the blackbody temperature increases from 300 to 700 K, the peak thermal emission wavelength shifts from the LWIR into the MWIR waveband and the spectral radiance increases. Thermal emissions of an object in these wavebands are referred to as thermal infrared (TIR) emissions. In these wavebands, the average solar thermal output from the Sun¹² (itself a blackbody at 5780K¹³) is negligible, providing a method to detect objects free from solar interference. As only minor amounts of TIR radiation are emitted in the VLWIR waveband, this contribution to the total TIR will be ignored in this article as it has no military value in this technology field.

Sources of TIR self-emissions from military platforms include hot exhaust plumes, hot end components near plumes, aerodynamic heating of leading edges, and heating of the skin.¹⁴ Minor levels of TIR radiation are also generated by reflection of radiation from warm terrestrial sources. The type of TIR self-emissions generated by military equipment is dependent upon its design and purpose, e.g., an aircraft might display elevated TIR emissions due to aerodynamic heating of the airframe or from operation of its afterburner; a warship might exhibit strong TIR emissions from its heated funnel or exhaust plumes.

Because an object emits TIR radiation, it produces a TIR signature allowing its detection by passive TIR sensors that acquire this radiation. The advantages of using TIR imaging technology is that it is useful for in-field detection (IR waves do not refract over the horizon), identification, and tracking. TIR radiation is also less scattered by fog, smoke, or dust particles as compared with visible wavelengths. As TIR sensors are passive systems, the power requirements and the probability of detection of a passive TIR detector are relatively low when compared with active source systems, such as LIDAR and RADAR.¹⁵ Thus, TIR targeting and tracking devices are difficult to detect and eliminate.

Molecules of water and carbon dioxide in the Earth’s atmosphere significantly attenuate IR radiation within the TIR wavebands of the EMS. Carbon dioxide absorbs radiation at wavelengths of 2.7, 4.3, and 15 mm while water vapor absorbs radiation at wavelengths of 1–2 and 5–8 mm.¹⁶ Attenuation effects can be observed when the atmospheric absorption¹⁷ is overlayed on the spectral radiance plots (Figure 2).

Many of the TIR self-emissions are attenuated, but three main transmission windows exist at 1–3, 3–5, and 8–14 mm (called Bands I, II, and III, respectively). TIR radiation within the VLWIR and FIR wavebands are completely attenuated. Transmission windows allow emitted TIR radiation to propagate through the atmosphere and be detected by TIR sensors attached to trackers and guided missiles. The transmission windows (Bands), their wavelength ranges, typical sensor materials used to detect these emissions, and the common sources of these emissions are shown in Table 3.

Band I emissions, derived from extremely hot engine parts and from plumes of an aircraft, ship, or land vehicle can only be observed from discrete aspect angles. For example, Band I emissions from an aircraft are usually emitted from the rear sector of an aircraft; therefore, Band I-guided missiles detect, then approach an aircraft from behind. Band II sensors are a higher-priority threat for aircraft¹⁸ due to strong plume emissions and aircraft skin emissions from leading edges in that band. Plume and aircraft skin emissions allow an all-aspect profile in this transmission band; therefore, the aircraft can be tracked and targeted as it moves toward, or tangentially, to a Band II detector or guided missile. For “cooler” objects, such as personnel and the skin of aircraft, land, and naval vessels, detection by Band III detectors is of concern, as Band III emissions also allow an all-aspect profile. Aircraft are the most exposed, as they are usually imaged against a cold sky, whereas a land vehicle or naval vessel may have outer skin temperatures closer to that of the surroundings.

The ease with which objects can be imaged by a TIR sensor can be demonstrated with an F-111 aircraft as viewed with a Band III thermal camera (Figure 3). Intense TIR emissions are observed from extremely hot exhaust plumes and hot metal parts on the nacelles. The exhaust plumes, being composed of gaseous combustion products, are dominated by the carbon dioxide and water vapor emission bands. While intense at the source, they are absorbed by the atmosphere and are, therefore, more rapidly attenuated with distance, unlike emissions from the hot surfaces near the aircraft nacelles or the skin of the aircraft, which are heated by aerodynamic friction.

Examples of TIR-guided missiles include the AIM-9 Sidewinder and FIM 92 Stinger from the United States and the 9K333 Verba from Russia.^20-22 The use of TIR-guided missiles has influenced battle tactics and outcomes throughout the latter half of the 20th Century.^23,24

There are several countermeasures that may be used against TIR-guided trackers and missiles. Engineering modifications, such as shrouding hot components and plumes, have been employed.²⁵ Active cooling, the process of reducing the engine and plume temperatures to shift the wavelength of maximum emittance to those attenuated by water and carbon dioxide, is achieved by using a turbofan that can pass incoming cool air over the engine components and into the plume, or by using cooler recirculated on-board fuel as a heat sink.^26,27 These methods reduce Band I emissions, but emissions from Bands II and III remain. In many cases, these modifications add weight, reduce operability, are costly, and are difficult to upgrade. Chemical-based pyrotechnic flares are used (Figure 4) to produce a spectral emittance profile similar to that of the hot parts of military equipment to confuse and seduce a thermal-seeking missile. However, smart missile imagers can differentiate between flares and aircraft.²⁹ IR-jamming utilizes lasers to blind TIR detectors and break the target lock.³⁰ Even if dazzled, advanced sensors can still detect the target.³¹

Low-Emissivity Coatings

If the temperature of an object cannot be lowered, a simple, low-cost, passive method of reducing the TIR signature of an aircraft is to apply an LE coating over the hot areas to suppress TIR emissions. If Planck’s radiation law is integrated over all wavelengths, the total radiant exitance by a blackbody per-unit time and per-unit area is given by the Stefan-Boltzmann law³² [equation (3)]:

(3)

where I is the radiant exitance (W m^-2), s is the Stefan-Boltzmann constant (5.67 ×  10^-8 J s^-1 m^-2 K^-4), and e is the emissivity of the material. In this equation, s is fixed and T⁴ is the main contributor to radiant exitance. Emissivity, a dimensionless number ranging from one to zero, is defined as the ratio of the radiant exitance of an object’s surface (OS) to the radiant exitance of a blackbody (BB) with an emissivity of one at the same wavelength and temperature at thermal equilibrium, as shown in equation (4).³³ Emissivity is the only contributor to the radiant exitance that can be altered.

(4)

Lowering the emissivity of a material reduces the apparent temperature of an object by reducing the quantity of radiation emitted; however, it does not change the peak wavelength of the thermal emission or its true physical temperature. The spectral radiance in the TIR wavebands of an object at 700 K, for example, is shown to depend strongly on the value of the surface emissivity (Figure 5). By lowering the emissivity, the spectral radiance curves can be altered to appear similar to the spectral radiance of cooler objects; for example, objects at 600 and 500 K.

Conventional coatings currently used on military equipment are formulated with materials that display an emissivity approximating 0.95 (approaching that of a BB)³⁴ and, therefore, emit a near theoretical TIR maximum. It is possible to formulate LE coatings that have visible camouflage properties similar to conventional coatings while simultaneously suppressing thermal emissions. This allows reduction in the detection range (determined by the inverse square law) and better camouflage of the platform in its operating environment when viewed in the TIR.

LE materials include conductive materials (those that contain mobile electrons), such as gold, chromium, zinc, copper, silver, and aluminum,³⁵ and semiconductors (with low-valence band energies), such as silicon or lead compounds. When fabricated as pigments, the LE materials can be dispersed into binders to produce coatings with a range of emissivity properties. When tinted with conventional pigments, the desired visual camouflage color can be achieved. The physical advantages of using LE coatings are principally the reduction of thermal emissions in Bands I to III from operational platforms. It must be noted that the LE coating would only be applied to the hotspots of a military platform. Due to the law of energy conservation, it is desired that TIR radiation not emitted from the surface be redirected to noncritical directions that have a lower probability of detection. This is to prevent a thermal insulation effect that would raise the temperature of the entire platform if coated in its entirety with LE material.

Strategic advantages in formulating LE coatings in-house include provision and control of sovereign technological capability in thermal suppression, which enables rapid LE formulation changes in required colors and emissivity levels as dictated by operating environments and mission needs.

Defence Specifications

Commercially available LE coatings are generally not formulated to colors used by the RAAF, RAN, or Army, and none have been fully qualified to current ADF specifications. The main topcoat specifications used by the ADF are listed in Table 4.

The current specification used for qualification of most RAAF topcoats is U.S. Military Specification MIL-PRF-85285E, “85285E”, released in 2012. Prior to this, coating specification DEF(AUST) 9001A, “9001A”, released in 2009 by the Commonwealth of Australia, governed the use of both primer (qualified to MIL-PRF-23377)⁴⁰ and topcoat as a total coating system for operational RAAF aircraft. Specification 9001A contained many similar tests to MIL-PRF-85285D⁴¹ but incorporated more demanding accelerated UVA weathering and corrosion requirements suited for Australian conditions. Also, 9001A removed restrictions on volatile organic compounds (VOCs) and relaxed the requirement for coating flexibility. It was subsequently withdrawn from use in September 2013 due to the inclusion of a Type IV coating category into the 85285E specification, i.e., aircraft application with extended weatherability. Prior to the release of 85285E, many military coatings developed in Australia, including the LE coatings, were formulated to conform to the requirements of 9001A.

Existing ADF topcoat specifications are used as a requirement for expected performance of coatings used for Defence applications. The formulated LE coatings have their own unique properties, such as TIR emission reduction and, therefore, it may be difficult for LE coatings to pass all requirements. This will not preclude the use of LE coatings on ADF equipment due to the unique properties these coatings provide. The information gathered by testing LE coatings against the requirements of the specifications is important if critical coating properties, such as adhesion, are to be deemed acceptable for the intended application. Due to the wide operational use of the AMSS 36375 color by the RAAF, an LE variant was formulated to this color and tested against the 9001A and 85285E aircraft topcoat specifications.

Preliminary work to obtain an emissivity of less than 0.5 in Bands I, II, and III while maintaining general coating integrity had been previously conducted. However, tinting of these prototype coatings to camouflage colors was not achieved, and testing conformance against a coating specification was not attempted.

Experimental

Substrate Preparation

Plain aluminum panels with a thickness of 1.2 mm were cleaned by scrubbing with a 3M Scotch-Brite 7447+ pad soaked in a 33% aqueous solution of Bonderite C-IC 624 Acid Cleaner (Henkel), then rinsed with tap water. The process was repeated until a water break-free surface was obtained. For aircraft-related coating tests, panels were pretreated by immersion in a chromate solution at 23 ± 2°C for 35 s. The chromate solution was prepared by dissolving 8.0 g of Bonderite M-CR 1200S Aero in one liter of water, adjusted to a pH less than 2.0 with aqueous nitric acid (if required), and used within 24 h. Following removal from the chromate solution, the test panels were rinsed with tap water and allowed to dry for 24 h at ambient temperature before application of primer. For filiform and salt fog corrosion testing, coupons composed of aluminum alloy clad 2024-T3 with a thickness of 1.2 mm were employed and prepared in the same manner as the plain aluminum panels. For impact and aged impact testing, aluminum alloy clad 2024-0 coupons with a thickness of 0.5 mm were employed as the substrate.

For cold flexibility tests, 0.3 mm tinplate was employed as the substrate. The tinplate was prepared by abrading with P360-grit emery paper and then cleaned by wiping with a cloth soaked with methyl ethyl ketone (MEK).

Commercial Coatings

For all the panels tested, the primers used are shown in Table 5, where Component A (base component) and Component B (curing agent) were mixed together at the suppliers’ recommended volume ratios. Component C was then added to provide a viscosity suitable for spray application (20–30 s through a Ford 4 flow cup). The liquid coating was allowed to stand at 23±2ºC for 20 min before being transferred to the spray gun. A commercial polyurethane AMSS 36375 topcoat conforming to specification MIL-PRF-85285E, Type I, Class H was applied over the CA7255 primer and used as a control for comparison with the experimental LE AMSS 36375 coating.

LE Coatings

LE coatings were manufactured at the DST Group laboratory by blending hydroxyl functional polyester resins, additives, and organic solvents to form a homogenous mixture. The TIR-suppressing pigments were dispersed into this mixture using a high-speed disperser at a speed not greater than 500 rpm. The visual camouflage colors listed in Table 6 were then achieved by stirring in solid-color pigment concentrates formulated at DST Group. Addition of aliphatic isocyanate hardeners then completed the formulation.

Coating Application

Spray application was performed with a gravity-fed Anest Iwata W-400-132G LV-2 spray gun with a pressure of 241 kPa (35 psi), maximum fan (aperture), and full fluid flow. All coatings were filtered through a 190 mm fine nylon filter cone before application. Application conditions were 23 ± 2°C and 30–70% relative humidity (RH). Wet and mist coats were applied moving the spray gun horizontally at a speed of 0.2–0.4 m s^-1 at an application distance of 0.15 m from the panel with sufficient overlap of passes to obtain the required dry film thickness (DFT). Solvent flash-off from each coat was required to be complete before commencing the next pass. Topcoats were applied 4–24 h after application of primer. The topcoat colors and coating DFTs are listed in Table 7.

Coating Assessment

Coatings were allowed to cure for one week under ambient conditions for gloss and color measurements or two weeks under ambient conditions for all other coating tests. American Standard Test Methods (ASTM), ISO Standards, and the Australian Standards 1580 series quoted in the testing phase can be found at the ASTM International website and Standards Australia.^43,44

Color Measurement

CIELAB (CIE 1976 L*a*b* color space)⁴⁴ color measurements were conducted using a Konica Minolta CM-2500d spectrophotometer. Calibration was completed with a Spectralon white tile and a Konica Minolta CM-A32 Zero Calibration Box. Color measurement conditions were observer 10° and illuminant D65. An average of three scans was taken. Specular component-included (SCI) values are reported. Total color difference (DE*_ab) was calculated using equation (5):

(5)

where DL* is the lightness difference on the L* axis, Da* is the red-green color difference on the a* axis, and Db* is the yellow-blue color difference on the b* axis.

AMSS series color standards⁴⁵ were stored in a dark refrigerator when not in use to minimize color drift between uses. Standards were allowed to equilibrate to room temperature for 30 min before being used for color measurements.

Gloss Measurements

Specular gloss measurements of coatings were made using an Elcometer 402 NOVO-GLOSS Statistical Glossmeter. Calibration was conducted using a highly polished reference black glass standard with a defined refractive index, having a specular reflection of 100 gloss units (GU) at the specified angle. The lower end point was established at 0 GU using a near-perfect black matte surface. Gloss measurements were recorded simultaneously at three specular directions to the normal (20°, 60°, and 85°). Three measurements were made at different points on the coating, and the values reported for each angle were the average of these.

Accelerated Weathering

Accelerated weathering of coatings was undertaken using a Q-Lab Products Q-Sun Xe-1 Xenon test chamber in combination with Q-Labs 1800 W, 800 V Xenon Lamp, and a Q-Sun Daylight Q filter. The conditions used (following ASTM G155)⁴⁶ were a constant spectral irradiance of 0.70 W/m²/nm at 340 nm and a black tile temperature of 63°C for 102 min alternating with irradiance in combination with water spray for 18 min (air temperature not controlled).

Coatings removed from the Q-Sun test chamber were then allowed to dry overnight at ambient temperature to remove, by evaporation, any sprayed water from the coating before measurements of color, gloss, and weight were conducted.

Exterior Exposure

Test panels were placed on an exposure rack at a seasonally adjusted angle (November–April 20°N, May–July 55°N, August–October 37.5°N) at Monegeetta, Victoria, Australia (latitude -37.93°S and longitude 144.77°E). Coatings placed for exterior exposure were measured for changes in color, gloss, chalking, and weight biannually. After each exposure period, the weight of the coating was measured, and the entire coating was rinsed under a gentle stream of water to remove loose dirt. The right side of the coated panel was gently wiped up and down 10 times with a water-soaked cotton wool swab. The left side was kept unwiped to estimate dirt pick-up. The coupon was then allowed to dry overnight at ambient temperature. Color and gloss changes on the wiped and unwiped sides of the coating were measured at the same three distinct locations, and the panel was re-weighed. The degree of chalking was rated by the tape test conducted on the wiped and unwiped sides using the method of AS/NZS 1580.481.1.11:1998⁴⁷ with 3M Scotch 600 Transparent Tape. Exposure panel weights were recorded using a Sartorius 1702 electronic balance.

Infrared Reflectivity Spectrum Analysis

Emissivity measurements of coatings were conducted using a Nicolet 5700 Fourier transform infrared spectrophotometer with a 75 mm diameter OpTec gold-coated integrating sphere, Model A562, over the 2–25 mm wavelength range. The total spectral reflectivity, r, (both specular and diffuse components) was measured by placing a gold-coated port plug (reference material) into the bottom sampling port and the sample on the top port. The beam was directed onto the diffuse gold reference to record a reference measurement. The beam steering mirror was then rotated to direct the beam onto the sample to collect the TIR spectrum. The following parameters were used to collect the TIR spectra: resolution, 8 cm^-1; number of scans coaveraged, 500; scan velocity, 0.3165 cm s^-1; acquisition mode, double sides, forwardbackward, apodization, Happ-Genzel; phase correlation mode, Mertz, zero; filling: none. A deuterated lanthanum alpha-alanine-doped triglycine sulphate detector was used for the measurement and the diffuse gold plug (Infragold) as the reference material. The sampling area was approximately 10 mm in diameter.

Using Kirchhoff’s Law and Helmholtz’s reciprocity theorem,⁴⁸ the emissivity (e) for an opaque surface was calculated using equation (6).

(6)

Dry Film Thickness

Dry film thickness (DFT) reported for primers and topcoats was obtained with an Elcometer 355 Coating Thickness Gauge.

Thermal Degradation

Test panels were primed and topcoated with the conventional coating or the LE variant and cured for two weeks under ambient conditions. Coatings were placed in an oven set to 200°C and held at this temperature for 48 h. Color measurements were made on both control and thermally treated samples, and the color differences were calculated using equation (5).

Cleaning Efficiency

Cleaning efficiency (CE) for both the LE and conventional coatings was undertaken using the method described in 85285E. Artificial soil was produced by dispersing 50 g of Vulcan XC72 carbon black pigment (Cabot Corporation) in 500 g of Royco 782 hydraulic fluid (conforming to MIL-PRF-83283)⁴⁹ with a high-speed disperser at 2500 rpm for 15 min. The artificial soil was applied by brush onto clean coatings where the value of L* under SCI conditions, as previously described, had been measured (value A). Excess soil was then removed with a paper towel pressed down by a 2.5 kg rubber roller. The soiled surface was subsequently brushed to provide an even dark surface. Coatings were then baked at 105 ± 2°C for one hour, cooled, and the L* values again measured (value B). Coatings were then gently washed twice with paper towels dipped in a 20% w/w aqueous mixture of Calla 855 (an alkaline cleaning solution conforming to MIL-PRF-85570⁵⁰). The coatings were then rinsed with tap water, dried, and L* was measured again (value C). Cleaning efficiency was calculated using equation (7):

(7)

Thermal Measurements

Thermal images of low-emissivity and conventional coatings were compared in the wavelength range of 7–13 mm using an FLIR^® Systems ThermaCAM P60 camera. Coated panels were placed on a preheated hotplate for 15 min to obtain thermal equilibrium. False color images were displayed with thermal ranges, and temperature was automatically generated by the ThermaCAM. ThermaCAM settings were e = 1.00, observation distance 1.0 m, humidity 50%, and relative temperature 24°C.

Results

Initial Work

Before commencing testing against the aircraft coating specifications, initial work involving accelerated weathering on the LE coatings was conducted to determine if sufficient durability had been achieved. Durability was assessed by examination of color and gloss changes of the LE variants exposed to UVA under accelerated weathering conditions for 2000 h. The 2000-h duration was chosen as that length of time that exceeded the requirements of all specifications used by the ADF for qualification of polyurethane-based coatings. The test was also completed before the inclusion of a Type IV coating in the 85285E specification that requires a 3000-h exposure under these conditions.

The accelerated durability results of the most color-stable LE coating variant for each camouflage color developed are shown in Figure 6. None of the preferred formulated LE coatings exceeded a nominal DE*_ab value of 0.8 after 1000 h of exposure, the most stringent DE*_ab limit of all the ADF specifications (found in APS-0501 specification for Type I polyurethane coatings). LE AMSS 35237 marginally passed this value at 2000 h of exposure. Gloss change was a maximum of 0.5 GU at 60° and 1.0 GU at 85° for these LE coatings. These results are a reliable indication that durable coatings had been formulated.

Although the LE AMSS 36375 coating was formulated to present a color appearance similar to the conventional AMSS 36375 coating, when the two panels were heated together at 100°C and then viewed through a thermal camera (Figure 7), the conventional AMSS 36375 coating appeared hot (red) while the LE AMSS 36375 coating appeared much cooler (green). The LE properties of each coating were confirmed by emissivity measurements (Figure 8).

The quality of the topcoat and the ease of application by spray of the LE coatings was demonstrated when applied over large areas (Figure 9). The resultant cured coatings showed a consistent uniform appearance free from grit and with no indication of banding. Further spray application tests were conducted on 1.2 m x 1.2 m sized panels with similarly successful results.

Specification Testing

Coating LE AMSS 36375 was tested against the performance requirements listed in the 85285E and 9001A aircraft topcoat specifications, and results are listed in Tables 8-12. Three additional DST Group tests specific for LE coatings were also undertaken and are denoted by “LE property” in Tables 10-11.

The results of testing the other LE coatings against their respective topcoat specifications will not be reported in this article.

Composition Properties

The composition tests for the LE AMSS 36375 coating indicated that all tests in this category conformed to the requirements of 85285E except for the solvent content (Table 8) that did, however, conform to the requirements of 9001A, Type I coatings. A solvent content exceeding 420 g/L was required to assist in reducing the viscosity, imparted by the high molecular weight polyol used in Component A and to ensure good flow and laydown of the TIR-suppressing pigments so that a finish suitable for military equipment could be obtained while providing the required LE properties.

Liquid Properties

The solvency of Component A of LE AMSS 36375 was selected to assist in giving adequate flow while ensuring good drying times for the admixture. This enabled rapid equipment turn-around times during application or repairs. The particle size of the TIR-suppressing pigment was also found to be an important consideration when evaluating the flow of the admixed coating and the quality of the cured films (Table 9). A balance between the particle-size distribution of the TIR-suppressing pigments and coating properties normally expected for a conventional coating is required when formulating LE coatings. Pigment particles too small to provide large changes in LE properties increase the pigment volume concentration (PVC), which can lead to mechanical failure of the coating. Pigment particles too large, while providing better TIR suppression properties, can cause issues during filtering and spray application. A compromise between LE properties and coating integrity was found by formulating with TIR-suppressing pigments that have a particle size distribution containing a fraction larger than 45 mm that are subsequently collected by a #325 sieve. The performance of low-emissivity coatings requires the use of large TIR-suppressing pigments (when compared with conventional colored pigments) that would not normally be used for conventional coatings. Thus, the coarse particle test described in 85285E is not applicable for low-emissivity coatings.

Cured Coating Properties

The cured LE AMSS 36375 coating showed acceptable film properties, such as gloss levels, appropriate for military applications and acceptable wet and dry adhesion to the test substrate (Table 10) in conjunction with suitable LE properties. Properties relating to the flexibility of the coating did not conform to the requirements of 85285E or 9001A. The coating flexibility performance is attributed to the large-sized TIR-suppressing pigments that have large interfaces within the polymer network. These act as weak spots within the film when stress is applied. Evidence to support this assertion was that the Army LE coatings that contain more resin did not show cracking at 20% elongation during impact flexibility testing. Also, the unpigmented cured polymer used for the LE coatings was found to be flexible.

Resistance Properties

Excellent results were obtained for resistance to hydrocarbon-based liquids such as hydraulic fluid, lubricating oil, and aviation fuel. Resistance to sources of thermal and UV radiation was also demonstrated (Table 11). Some coating damage was noted on interaction with MEK. This did not affect the LE performance of the coating.

Working Properties

The admixed LE AMSS 36375 liquid showed mixing and spraying properties suitable for application by a gravity-fed spray gun (Table 12) as all results in this category conform to 85285E.

Importantly, the cured LE AMSS 36375 coating displayed excellent cleanability. The utility of LE coatings is in the suppression of TIR. If the LE coating were to be permanently contaminated by high-emissivity materials during use, it would lose the attribute to deliver LE properties. Therefore, the CE of the LE coating is critically important. The conventional AMSS 36375 and LE AMSS 36375 were found to have a CE of 95% and 99%, respectively. This CE was confirmed by examining the emissivity of the coating before and after staining. The high CE performance of LE AMSS 36375 is shown in Figure 10. This cleanability result was maintained when tested after three consecutive soiling/cleaning cycles.

LE-Specific Testing

The exterior weathering performance of the LE AMSS 36375 topcoat was compared with that of a conventional aerospace coating. Exterior weathering over two years indicated that the LE AMSS 36375 coating had good performance (Figure 11). After two years of exposure at Monegeetta, the wiped side of the panel showed a DE*_ab value of 1.6 units and a gloss change of only 0.5 GU at 60°. The major contributor to DE*_ab was a loss of blue with a Db* of 1.5 units. By this time, the DE*_ab of the conventional AMSS 36375 was 0.8. For both coatings, practically no chalking was observed.

The thermal stability of LE AMSS 36375 was shown to be better than the conventional AMSS 36375. When subjected to the long-term heat test (Figure 12), LE AMSS 36375 showed some yellowing with a DE*_ab of 8.3, while the conventional AMSS 36375 had changed to a brown color with a DE*_ab of 19.8.

Discussion

Results Conforming to the Requirements 85285E and 9001A

An evaluation of the LE AMSS 36375 topcoat against 85285E and 9001A found that most of the specification requirements were met. This was an excellent result for a coating that was formulated with the primary purpose of suppressing TIR emissions from hotspots on military equipment.

Much of this success is attributed to the polyurethane (PU) system chosen for the LE coatings. The UVA-accelerated weathering performance demonstrated that the polyester resin and aliphatic isocyanate combination chosen for LE coatings produced a durable PU coating. To extend the weathering durability of a coating, one formulating approach could be to use a fluorinated PU or polysiloxane resin. These resins are proven to have better exterior durability than two-pack PUs employing polyesters or conventional acrylics.^51,52 When formulating LE coatings, the absorption of TIR by the binder, pigments, and additives in the bands of interest must be minimized. Both fluorinated PU and polysiloxane resins will adversely affect performance of LE coatings in Band III when compared with the chosen PU system due to the presence of either carbon-fluorine or silicon-oxygen bonds in its polymer backbones. These functionalities produce unwanted absorption bands from 8–10 mm (Figure 13). PUs based on polyesters tend to have more adsorption at wavelengths from 5–7 mm that are located in the opaque waveband between Bands II and III where water and carbon dioxide absorb,⁵³ which is a positive attribute for low-emissivity coatings operating at temperatures <250°C. In this respect, the operational performance as an LE coating superseded the consideration for extended durability.

The use of a PU binder limits the operational performance of these LE coatings to a maximum temperature of 250°C before polymer degradation and coating breakdown occurs. This restricts the application of these formulated LE coatings over hotspots, where the majority of the TIR emissions in Bands II and III are generated. LE coatings tailored for Band I suppression could be formulated by using a high-temperature-resistant silicone resin not presented in this article.

Results Not Conforming to the Requirements 85285E and 9001A

An evaluation of the LE AMSS 36375 topcoat against the 85285E and 9001A specifications found that some requirements were not met. An option would be to write a new specification for LE coatings that reduces the requirements to match their properties. While this is possible, the nonconformances against the specifications may point towards formulation changes that can be investigated in future work.

In formulating the LE coating, the VOC limit of 420 g/L specified in 85285E was exceeded but passed the 9001A requirement when classified as a Class 1 conventional coating.

The need for a high-solids LE coating was not the primary target of this work. LE coatings are required for application over hotspots on defense platforms. These areas are generally small in size; therefore, the overall amount of VOC release will be small compared with a conventional coating applied onto large areas. Should a low-VOC formulation of an LE coating be required in the future, a number of options could be explored. These include, in order of difficulty, a replacement for the current organic solvents with resin-compatible VOC-exempt solvents; a change in use of the current polyol to a lower molecular weight version with lower viscosity; or move to waterborne technology that uses minimal organic solvent.

An irradiance of 0.70 W/m²/nm at 340 nm was used in the extended UVA durability testing for the LE AMSS 36375 coating. This is not the irradiance described in the requirements of the 85285E (0.35–0.50 W/m²/nm at 340 nm) or 9001A (0.35 W/m²/nm at 340 nm) specifications. The irradiance of 0.35 W/m²/nm at 340 nm and duration of exposure described in 9001A was determined not to be demanding enough to simulate the severe Australian climatic conditions experienced by coatings in ADF service. Before the inclusion in 2014 of a Type IV coating in 85285E, the 3000-h requirement for extended durability for Type IV coatings (at an irradiance of 0.35-0.50 W/m² at 340 nm) was not required. By that time, the accelerated weathering for LE AMSS 36375 over a 2000-h period had been completed at an irradiance of 0.70 W/m² at 340 nm. It could be argued that 2000 h of exposure at an irradiance of 0.70 W/m² (a total energy input of 5.04 MJ at 340 nm) exceeds that of a 3000-h exposure at 0.35 W/m² (the lower limit of the 85285E test requirement) with a total energy input of 3.78 MJ at 340 nm). A counter argument is that the exposure has a temporal requirement, not just total energy adsorbed by the coating, and the test must be repeated for the full 3000 h at the correct irradiance. Until that is completed, the LE coating will be tentatively classified as conforming to 9001A, but only as a conformance against a Type I coating described in 85285E.

The 40% elongation requirement in 85285E is difficult to meet. The achieved coating flexibility performance of 10% elongation for the LE AMSS 36375 coating is attributed to the large-sized TIR-suppressing pigments that have large interfaces and act as weak spots within the film when stress is applied. A 10% elongation is considered reasonable for a coating containing both TIR-suppressing pigments that provide the required LE properties and a polymer system with chemical resistance against hydrocarbon fluids. Therefore, there should be no hesitation in using these coatings on military platforms. Conventional coatings based on both solvent and waterborne systems tested for impact flexibility only achieved 20% elongation while displaying chemical-resistance properties. Those that did pass the 40% elongation requirement showed lower chemical resistance performance.⁵⁴ A more flexible polyester resin could be investigated during future work. For current purposes, the use of LE coatings on areas of military platforms where extreme flexibility is required should be avoided to prevent water ingress that leads to corrosion if a crack in the coating were to occur.

LE-Specific Properties

The emissivity of the LE coatings was formulated to be ≤ 0.5 in Bands I, II, and III to demonstrate the utility of these coatings to suppress TIR emissions when compared with a conventional coating. The emissivity of these coatings can be adjusted to suit the needs of the operating environment. Coating opacity with a CR of 0.99 was achieved throughout the TIR wavelength ranges, and this ensured that full TIR radiation suppression by the LE coating was obtained.

The extended heat resistance testing of both the LE AMSS 36375 and conventional AMSS 36375 coatings showed that the LE AMSS 36375 coating exhibited less color change than the conventional AMSS 36375 coating. This provides evidence that the PU selected for the LE coatings either develops less colored chromophores on heating, or that the color pigments selected for the LE coatings are more thermally stable and do not change color on heating when compared with the conventional AMSS 36375. If required, the thermal stability of LE AMSS 36375 could be further improved by the addition of an antioxidant, but this would decrease the LE performance due to its high-emissivity composition.

Conclusion

DST Group LE coatings with five different visual camouflage colors designed to have an emissivity on average of ≤ 0.5 in Bands I, II, and III were formulated to suppress TIR emissions from military platforms. These coatings showed excellent color and gloss retention under accelerated UVA weathering conditions. Qualification testing of the LE AMSS 36375 coating to the requirements of the military topcoat specifications MIL-PRF-85285E and DEF(AUST) 9001A found that the majority of the results conform to the requirements of these specifications. The LE AMSS 36375 coating was found to have comparable exterior weathering performance to that of a conventional AMSS 36375 coating after two years in a temperate climate. The cleanability of the LE AMSS 36375 coating against soiling, which is critical to LE performance, was excellent.

Acknowledgments

The author wishes to thank Dr. Christopher J. Lyons, Mr. Stefan K. Danek and Mr. Gary Mathys, all of DST Group, for assistance with this project.

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CoatingsTech | Vol. 17, No. 4 | April 2020

Powder coating technology is viewed as a sustainable option in many industrial coating applications. It is most widely used to coat metal substrates, but efforts have focused on developing curing solutions that enable the application of powder coatings to temperature-sensitive materials such as wood and plastics. Growth in manufacturing and construction markets combined with growing demand for more sustainable solutions is creating new opportunities for powder coatings. Raw material suppliers and coating formulators are responding with the development of new powder coating technologies that can be produced and applied more efficiently, exhibit improved performance attributes, and have a greater range of potential end uses.

CoatingsTech surveyed resin, pigment, and additives suppliers and coating manufacturers regarding the current drivers of powder coating technology, recent developments, and what might be expected in the future. Their insights are presented below.

Participants in the discussion included:

Daniela Vlad—managing director, AkzoNobel Powder Coatings;

Robert Watson—research and development manager, allnex;

Cindy Fruth—sales and market development manager for Powder Coatings, Arkema Coating Resins;

Josh Gingras—North American Coatings business manager for Technical Polymers, Arkema Inc.;

Thomas Czeczatka—global end use manager for Powder Coatings, BYK;

Robert K. Roop—vice president of Global Refinish and Industrial Technology, Axalta Coating Systems;

Romesh Kumar—senior technical sales manager for North America, Clariant Plastics & Coatings USA Inc.;

Marten Houweling—global metal program director and product manager for Powder Coating Resins, DSM;

Brian Coutts—president, Erie Powder Coatings Inc./EPC Powder Mfg.;

Beth Ann Pearson—director of Marketing and Business Development, Estron Chemical; and

Kevin Biller—president, Powder Coating Research Group.

Q. What are the major drivers of new technology development for powder coatings? How have these drivers changed in the last 5–10 years? Do you anticipate any major shifts going forward?

Biller, The Powder Coating Research Group: Powder coatings’ pillars of efficiency, economy, excellent performance, and environmental compliance have compelled finishers to consider it as an alternative to solventborne coatings for decades. Interest continues for non-traditional substrates such as engineered boards, plastics, and composites. There have always been barriers to entry, with most centered around fear of radical change and the potential for failure. In the last 5–10 years, there has been a resurgence of interest in powder for novel applications as part of a rebound from the inertia experienced as the result of the economic downturn and industry’s subsequent reluctance to invest in capital equipment. My hope is for a major shift in powder displacing VOC-emitting liquid paint technology, but realistically the changes will be slow, deliberate, and incremental.

Pearson, Estron Chemical: For the past several years, the powder coatings market has been, and continues to be, in a mode of growth and innovation throughout multiple market spaces. The emergence of technologies that meet the ever-present challenges for cost and manufacturing efficiencies drive these needs. One example is the growth and expansion of compact process systems. Compact systems, known as short systems in the EU, are systems in which there is a chance to lower the overall processing costs; for instance, lower cure technologies for metal and plastic, faster curing times, coatings with dual-layer functionality, and dry-on-dry cure. The industry has also developed advanced technologies enabling the use of powder on wood and medium density fiberboard (MDF) substrates and offering enhanced corrosion protection on metal.

Watson, allnex: In comparison with some years ago, today there is a dual approach to new technology development. One aspect involves improvement of numerous coating performance targets, such as flow, edge coverage, and corrosion resistance, to achieve performance properties better aligned with those of liquid technologies. Improvement of the mechanical properties of durable and superdurable coatings is also important. The second approach relates to the need for affordable and reliable products without sacrificing overall technical performance.

Vlad, AkzoNobel Powder Coatings: The main drivers for technology development are the increasing demand for sustainable solutions and meeting customer demands for enhanced performance. With an ambition to reduce environmental impact, customers continuously aim to improve process efficiencies, such as removing coatings steps or better utilizing coating materials. A reduction in baking temperatures means powders can be applied on more and more substrates, resulting in lower energy consumption, no VOCs, and therefore a more sustainable alternative to liquid coatings and anodizing.

Recent powder coating technology developments include creating a chrome look, hammered effects, textures, ultra matt, etc. There are also strong needs around durability, corrosion-protection, and lifelong aesthetics of coated products. We also expect the demand to increase for less uniform finishes so that they look more natural, such as stone or wood effect. In addition, wider color options will reinforce powder as a strong substitute for liquid coatings. Our ambition is to make powder coatings available to as many markets as possible, as it is a very sustainable technology.

Roop, Axalta Coating Systems: In the last 5–10 years, the focus has been on two-layer systems with the primer being formulated for corrosion, chemical, and edge coverage, and the topcoat formulated for weatherability and aesthetic appeal (e.g., metallics that resemble automotive finishes). A few current drivers for new technology development in the industry are high-performance direct-to-metal weatherable topcoats that do not require a primer, and enhanced corrosion resistance, edge coverage performance, and superior performance for heavy-duty industrial applications. Other notable drivers include self-cleaning technology, improved mar resistance, and low gloss finishes. In addition, environmental regulations will continue to make an impact (e.g., REACH).

Fruth, Arkema Coating Resins: Currently, Arkema sees three primary drivers in powder coating product development—improved durability, better cost in use, and the development of more products for use on non-metal surfaces. Durability has always been important, but as powder coatings gain new market share in areas like North American architectural coatings, they will need to consistently meet AAMA (American Architectural Manufacturers Association) 2604 and AAMA 2605 specifications. In addition, we have seen more requests for superdurable products for all markets for low temperature cure and improved corrosion resistant coatings. The shift to superdurable products versus standard solutions is going to continue for the foreseeable future. Superdurability will become the baseline in the industry.

Kumar, Clariant Plastics & Coatings USA: Drivers are higher durability (color and structural improvement) and bright hues (opaque yellow, red, and orange shades). Development of new and improved (more weatherfast) resins that require higher performance and expectations for non-lead pigments are ongoing trends. New applications include the use of powder coatings on wood (e.g., kitchen cabinets) and on metal substrates to offset coil coatings among others.

Coutts, Erie Powder Coatings: In the end, customers using powder coatings are the main drivers of technological development. Customers need something or need a powder to do something that they currently can’t, and we, along with our suppliers, develop the answers. I don’t think this has ever changed or will ever change.

Q. What do you identify as the most important recent advances in powder coating technology over the past few years?

Roop, Axalta Coating Systems: There are several important advances. Improved hardness and chemical resistance utilizing high crosslinked resin technology; polyester HAA and TGIC primer technology with anticorrosive additives, barrier extenders, and hydrophobic properties; dry-on-dry technology for improved productivity and reduced energy consumption; bonded metallic coatings that deliver a unique, quality appearance; sprayable thermoplastic coatings that enable the application of ultra-durable thermoplastic to a much wider audience of coaters; cool coatings that help lower energy consumption; and coatings designed to speed coating and curing for faster line speeds. These technologies are benefiting architectural, agriculture, construction, and earthmoving (ACE), heavy-duty truck, transportation, and spec-driven general industrial applications.

Czeczatka, BYK: Powder coatings have achieved more widespread access in new applications. It is, specifically, lower baking temperatures that have permitted access to new substrates like plastic, MDF, and wood. The technology is opening up new possibilities and new markets. This trend is highly supported by improvements in application equipment and process control, which offer a wider range of application fields like dry-on-dry systems.

Fruth, Arkema Coating Resins: The introduction of powder-on-powder application (applying primer and topcoat with one cure step) opened new application areas and potential for powder coatings. Being able to apply to non-metal surfaces has and will drive new powder coatings growth, as current markets have matured to near full potential. Short term, we see more potential in application to MDF, as there are still many obstacles to overcome in applying onto wood substrates. These products offer a wide range of attributes based on formulation, but the advantages most customers see include a reduced carbon footprint, reduced emissions, and improved operational efficiency.

Coutts, Erie Powder Coatings: For Erie, the most interesting advances by far are in corrosion control coatings. This has been an area where all the stars align; there is a huge customer need for better corrosion control in the market place. It is also an area where great advances can still be made, and the suppliers of raw materials are interested in helping to develop new strategies and products. Corrosion is a huge market, costing customers and the economy in general billions of dollars. There have been some advances, such as our easy-to-coat primers that haven’t been available on the market until now, but there are a number of new corrosion control advances that are just getting to the market or are still in testing.

Vlad, AkzoNobel Powder Coatings: The ability to make weather-resistant, ultra-matt powder coatings (gloss levels <10) is an important advance because these coatings can mimic the sort of anodizing finishes that are currently very popular on commercial buildings, while avoiding the problems that often come with anodizing. Powder coatings can be applied to more than just aluminum, have greater color consistency between parts, and can be repaired. Specifiers of architectural coatings now have access to more sustainable alternatives to liquid or anodized ultra-matt finishes, backed by long-term warranties and industry certification.

Powder coatings can also combine increased functionality, beyond the already appreciated aesthetic and substrate protection qualities, with attributes such as easy clean for outdoor furniture and high-temperature resistance coatings such as on vehicle exhaust systems.

Pearson, Estron Chemical: There have been significant advances in flow control agents (FCAs) for compatibility and functionality. The advancement of these FCAs offer multiple advantages to the powder coating manufacturer, as they are designed to reduce cycle time, increase scheduling flexibility, and decrease part rejection rate with multi-technology use. These agents are invaluable for designing more functionality into one solution that “protects” multi-variant systems—meaning that an intended modification for improvement will only affect that single variable rather than multiple variables.

Biller, The Powder Coating Research Group: Application equipment makers continue to optimize and refine powder delivery technology and color changing capability. Dense phase feed systems and quick color change modules are improving powder transfer efficiency and application system uptime. Fascinating bio-based resin technology is emerging that could alter the feedstock components of the supply chain. United Soybean Board funding of a Battelle Memorial Institute project has generated a low temperature cure resin system based on soybean oil that exhibits excellent UV durability and mechanical flexibility. The novel bio-based resin technology may offer an alternative to super- and hyper-durable powder technology that conforms to AAMA 2604 and 2605 architectural specifications.

Q. What advances have been made in the area of smart powder coatings?

Watson, allnex: The concept of smart coatings, which are intended as finishing materials that can dynamically adapt their properties to an external stimulus, has started to impact powder coatings as well all the other coating technologies. In summary, easy-to-clean, improved corrosion, and anti-microbial coatings are capturing interest in the market.

Vlad, AkzoNobel Powder Coatings: Smart coatings can be defined as those that respond in a controlled manner to a specific external stimulus, and are increasingly of interest. There have been several advances in powder coatings that add unique performance features, albeit in a passive form. Examples include: active corrosion protection primers where the coating interacts chemically to disrupt the electrochemical corrosion mechanism, thereby reducing corrosion; anti-microbial coatings that protect against degradation of the coating by bacteria; and thermochromic coatings that change color on exposure to heat. Low solar absorption coatings contain a reflective pigment that deflects infrared light, and thus the sun’s heat, from any substrate that it coats, helping to keep interior spaces cool and reduce energy consumption (air conditioning).

Houweling, DSM: Typically the definition of smart coatings is related to new functionalities outside the decorative and protective field. For powder coatings there are numerous examples where new functionalities are formulated into powder coatings: antibacterial, easy-clean, self-healing, anti-static, conductive, EMS shielding, electrochroming, and also sensory, soft feel, and isolation functionalities are possible.

Roop, Axalta Coating Systems: There have been many major breakthroughs in smart powder coatings recently. Most smart powder coatings are specifically formulated for the end use of the product and its functionality. For instance, anti-graffiti coatings feature easy-to-clean properties to protect surfaces, such as signs, lockers, indoor and outdoor recreation equipment, public areas, and transportation terminals, from the permanent effects of spray paint and markers. Another interesting advancement is a nanocomposite coating based on compounds specifically designed to only react to liquid hydrocarbons.

Pearson, Estron Chemical: Smart coatings are those that are perceived as being passive, but are actually active, and vary based on the trigger mechanism. A prime example is coatings with flow control agents that manipulate the surface tension of a coating to give a resultant smoother surface finish, making it multi-functional.

Biller, The Powder Coating Research Group: The smartest powder technology revolves around self-healing formulation schemes. Novel core-shell technology has been pioneered by Autonomic Materials Inc. that could be a game-changer. These materials repair breaches in the coating without the use of heavy metals or phosphates. Fun technologies such as mosquito-repellent powder coatings and pollution absorbing formulas have debuted. Market acceptance is still unknown as these niches are rather narrow. Anti-microbial technology has advanced beyond the common silver ion technique and is being evaluated for an array of microbe killing performance.

Czeczatka, BYK: In addition to involving further developments in technology, smart coatings are also a marketing trend to promote powder coatings in more specialized application fields and niche areas away from commodity applications. In addition, smart powder coating technology also demonstrates that powder coatings nowadays are used in more specialized areas than in the past.

Q. Have there been any notable developments in hyper-durable powder coatings?

Watson, allnex: The possibility of going beyond superdurable with respect to weathering resistance with powder coating is not so new. Let’s consider for instance acrylic technology or fluoropolymer based chemistry, as both can be considered as hyper-durable technologies. The point is eventually to meet advanced weathering requirements without the limitations of those chemistries, including their intrinsically high cost and limited surface finishing effects.

Vlad, AkzoNobel Powder Coatings: Hyper-durable powder coatings are the most weather-resistant coatings available, using similar chemistry to liquid polyvinylidene difluoride (PVDF) coatings to give 10-year outdoor performance in Florida weathering tests. They meet the most exacting standard for coatings—AAMA2605 in the United States and Qualicoat class 3 in the rest of the world. Hyper-durable powder coatings are increasingly recognized as a relevant alternative to liquid PVDF, as demonstrated by the recent specification on monumental buildings, for instance the Hudson Yards development in New York.

Houweling, DSM: The hyper-durable market is dominated by liquid systems but the powder coating market share is growing based on properties, economics, and carbon footprint.

Roop, Axalta Coating Systems: Fluoropolymer technology is mainly used in architectural applications to promote weatherability; however, more ACE OEMs are requiring extended weathering performance. This technology has the capability to add 6000 hours of weather resistance under accelerated test conditions.

Fruth, Arkema Coating Resins: New applications in the ACE markets are driving demand for improved durability on superdurable products. Our customers are experimenting with alternative chemistries looking for the right mix of improved durability and cost effectiveness.

Pearson, Estron Chemical: Hyper-durable powder coatings are desired in the market as they are lower cost, easier to process, and demonstrate high durability. These applications require an advanced understanding of resin design and extensive weathering testing. The current technologies have a fluorocarbon base, which, in combination with a stabilized pigment system, make them extremely stable against degradation (both of the polymer and due to visual color loss). However, there are trade-offs, as this higher degree of crosslink density also results in a system that is more brittle and thus not recommended for applications involving high mechanical stress. The choice of pigment is also limited due to the stringent weathering requirements.

Q. What are the latest advances in ultraviolet (UV) and near-infrared (NIR) curable powder coating technologies?

Coutts, Erie Powder Coatings: UV coatings showed so much promise, but appear to have gone nowhere except for some very specialty applications. This appears to be due to safety issues with the chemicals, pigments blocking the UV light, and a number of other reasons including simple market inertia.

Biller, The Powder Coating Research Group: UV-curable powder coatings are a conundrum. They are beyond a chicken or an egg proposition. Resin companies and formulators intensely pursued the development of this technology in the 1990s. A smattering of new applications arose, including fully assembled electric motors, MDF cabinetry, vinyl flooring, and automotive radiators. Most eventually fell by the wayside due to performance issues (mainly process related) and a lack of strong technical support. A few brave souls continued their quest in spite of economic uncertainty and a general disinterest in the industry. Recently, new opportunities have sprouted that are a good fit for UV-curable powder, including hardwood and composite applications. Technologists are seriously revisiting UV-curable approaches to meet these specifications. A big question remains if the major resin suppliers will be willing to support these new applications.

Pearson, Estron Chemical: Neither of these technologies have really gained a strong foothold, largely because of the modifications that would need to be made to coaters’ lines, as well as recognized challenges for use. In an ideal world, UV-cured powder coatings should offer advantages such as faster cure cycles with lower cure temperatures, and can be used for substrates that are both heat sensitive and metallic. There are limitations in that there may be issues with some colors that can be cured due to interference with pigment choices, and the cure may not be as efficient with parts having a complex shape. NIR curing allows for selective heating of a coating with extremely high cure rates.

Houweling, DSM: We see both UV and NIR as promising technologies that fit well with the high-growth trend to develop powder coatings for heat-sensitive substrates.

Czeczatka, BYK: These are systems for low bake applications such as powder coatings on wood. They require special binders and special application equipment—mainly different ovens. Further improvements in equipment along the complete processing line combined with further developments in raw materials and formulations will help these products enter new markets.

Watson, allnex: These are two competitive technologies that can be considered valid options for thermo-sensitive substrates. Recently, it seems that the market is more oriented to NIR in conjunction with thermosetting technology in competition with UV-curable systems.

Kumar, Clariant Plastics & Coatings USA: These powder coatings could be used for wood, glass, plastics (recycled), and other non-metal substrates—even automotive interiors to replace soft touch paints. When good resins (UV and NIR curable) are available, this technology will grow, but for now their poor gloss and orange peel performance remain a challenge.

[Note that Biller asserts that the gloss of UV/NIR coatings can in fact be very high and equal to that of liquids, while the orange peel is significantly less than conventional powder coatings.]

Q. Have any noteworthy developments in functional powder coatings been made recently?

Pearson, Estron Chemical: Functional powder coatings typically refer to those specifically made with fusion bonded epoxy (FBE) used to protect steel pipe, rebar, and metal wire from corrosion. Rapid or snap cure and excellent flow are commonly sought attributes. The powder must be able to deposit on a moving substrate with a resultant smooth and contiguous layer because of what is being protected. Targeted applications utilize these coatings because not only is the coating functional in nature, but the substrate or part being coated is also functional in its usage.

Kumar, Clariant Plastics & Coatings USA: These low-priced, high-volume coatings remain based on epoxy resins. Epoxy resin prices have been on the upswing, however, and there is an opportunity for other resins to take away some market share in this high-volume business. The growing infrastructure market is also having a positive impact on the demand for functional powder coatings.

Czeczatka, BYK: Functional powder coatings must fulfill high anti-corrosive requirements, be chip- and chemical-resistant, and flexible. When we consider all recent and ongoing changes in regulatory affairs, especially for solventborne coating systems, we can expect a further move to powder coating technology. Improved powder coating systems might even replace a certain part of the market share held by waterborne systems. This is a trend in all global regions and particularly strongly driven by China.

New opportunities have sprouted that are a good fit for UV-curable powder, including hardwood and composite applications.

Vlad, AkzoNobel Powder Coatings: In our business, the term “functional powder coatings” refers to the market segment in which we supply our Resicoat range of functional powder coatings, which are totally different than other powders as most are hot-applied and in higher thickness, up to 1000 µm (1 mm). They are used for heavy-duty corrosion protection of cast iron valves and fittings, pipelines, and rebar, as well as for insulation and corrosion protection on lamination stacks, bus bars, and electronic components. Advances include higher T_g-powders to extend in-service lifetimes and the ability to operate in more aggressive environments. Powder coatings with preheating temperatures approximately 50–60°C lower (from 230°C to 160–170°C) offer the applicator significant energy savings and improved productivity while also reducing both the carbon footprint and the manufacturing costs associated with the paint application process.

Roop, Axalta Coating Systems: Axalta recently developed a pipe-in-pipe product technology for the oil and gas industry that creates a durable internal vessel able to extend the usable life of damaged pipes. Functional powder coatings also protect valves and fittings for fluid and gas handling systems, fire hydrants, and even large storage tanks. The term can also include products for wire encapsulation and electrical insulation.

Gingras, Arkema Coating Resins: As functional powder coatings become more attractive vs solventborne liquids for corrosion protection applications, products made completely from renewable resources such as castor oil may attract more attention. These powders are used to protect metal in industries like automotive, oil and gas, medical, and more.

Q. What gaps remain to be addressed by advances in powder coating technology, and what actions are being taken to do so?

Vlad, AkzoNobel Powder Coatings: Everything we do is driven by market needs and providing more sustainable solutions for our customers around the world—whether that’s reducing baking temperatures, removing process steps through dry-on dry-application, improved material usage through lower applied film builds, improving the longevity of the coated article through improved UV durability/corrosion-protection performance, or adding more functionality to the coatings. Providing the ultimate powder solutions and helping our customers to reach their sustainability goals are at the core of our work. For example, AkzoNobel recognized the desire for a low gloss finish several years ago, when matt and textured surfaces started to become more popular,  and were the first to bring to market a range of ultra matt coatings with high scratch resistance.

Fruth, Arkema Coating Resins: There is still some uncertainty around the future of triglycidyl isocyanurate (TGIC) crosslinker technology outside of Europe, where it has already been regulated out. Enhancing the performance of hydroxyalkyl amide (HAA) crosslinkers remains a topic of interest for global paint companies. Our customers have expressed a desire for a “global” crosslinker technology that performs as well as or better than the existing systems.

Gingras, Arkema Coating Resins: In order for powder coatings technology to advance, there must be acceptance from the entire value chain. The end customers must appreciate the value high-performance powder coatings provide, while the powder coating supplier must invest resources to develop and promote the new technology. To this end, in the past nine months, Arkema announced capital investments in polyamide 11 chemistry to demonstrate its commitment to the market.

Functional powder coatings also protect valves and fittings for fluid and gas handling systems, fire hydrants, and even large storage tanks

Roop, Axalta Coating Systems: A gap Axalta is addressing is coating performance on blasted steel substrates vs hot or cold rolled steel or smooth/polished aluminum. Currently, we have to formulate differently depending on the surface type particularly for grit blasted profiles. Customers are coating complex parts, and in some cases, will have both preparations on a finished part. In addition, Axalta is developing a new generation of FBEs to protect the world’s pipelines. As oil and gas producers drill into deeper reservoirs, pipeline operators must raise temperatures to facilitate the movement of this thicker, more viscous crude. Traditionally, FBEs formulated for high-temperature service are more brittle, less flexible, and have less adhesion to the substrate. Axalta’s new generation of FBEs are changing that. With respect to quality gaps, Axalta has created an approved applicator program for architects working on global projects that involves evaluation and approval of the consistency of all applicators of Axalta powder products globally. We’ve found that this grants our customers peace of mind when choosing Axalta for their aluminum façade and architectural projects.

Czeczatka, BYK: The drawback of powder coatings continues to be their optical surface appearance, which differs from the visual properties of liquid systems. The finish and surface quality are a little lower than wet-look systems. The visual effects, for instance of metallic finishes, is slightly less brilliant in powder coatings, than in liquid coatings. All of these drawbacks are known, but new and further developments in raw materials and processing of powder coatings will tackle some of them.

BYK as an additive supplier actively works in areas where our products can overcome existing gaps, with adjustments to local requirements in different regions from a global perspective. In addition, we are working closely together with our customers to support their new development activities.

Kumar, Clariant Plastics & Coatings USA: Achieving high gloss like that obtained for liquid solvent-based coatings is still a challenge for powder systems. Cool coatings with IR reflective pigments (duller shades only; bright ones are too expensive) are also needed. Clariant is also focused on the development of unique pigment combinations based on pigments with high opacity, high chroma, high gloss, and high durability. Metallic shades remain far poorer in appearance than those achieved with liquid coatings, which is another issue we are looking to address.

Houweling, DSM: We keep improving the sustainability of our powder coating solutions because we see this attribute as the key success factor for powder in combination with lower curing temperatures (heat-sensitive substrates and heavy mass), improved appearance, improved corrosion resistance, and epoxy replacement.

Coutts, Erie Powder Coatings: Our focus has been on corrosion control. We see many gaps and potential for advances in this area. Our main focus has been on solving problems such as the re-coatability inter-coat adhesion issues that were common with existing products, and developing new technologies with new chemicals, pigments, and smart technologies.

Pearson, Estron Chemical: Challenges continue to exist with increasingly stringent specifications for degree of cure without sacrificing appearance or performance. Estron is focused on overcoming performance deficiencies associated with manipulation of coating components—the physical mobility of the resin reactive groups finding each other during the cure process at a pre-determined time and manner—and how to determine the most effective method to deliver a product that aligns with the needs of the individual markets.

Estron has developed a proprietary manufacturing process resulting in the uniform dispersion of additives while minimizing the potential for reaction between resin functionalities and additives. The process has also been proven effective for dispersion of additives into coatings that are challenging to work with using conventional methods.

Biller, The Powder Coating Research Group: PCR Group is working on a wide array of new technologies, including improvements in corrosion resistance for both primers and polyester topcoats; coating technology for composites, MDF, and hardwood; and sustainable resin systems based on plant material feedstocks. Significant advancements are being realized with formulating techniques, new materials, and corresponding processes such as infrared and UV curing.

Q. Is there anything else about advances in powder coating technology you would like to mention?

Czeczatka, BYK: Powder coatings have clear environmental benefits. Their VOC content and carbon footprint are often far superior to those of liquid, i.e., solventborne and even waterborne, systems.

Watson, allnex: Recent regulatory reclassification for some crosslinking agents will drive reformulation. New architectural standards for the Chinese market will also drive development to obtain the specified performance. In addition, there is focus on identifying the use of sustainable raw materials and eliminating materials of concern, which will further underpin the green credentials of powder coatings.

Pearson, Estron Chemical: There are areas that readers may be interested in with respect to architectural powder coatings and their advancement in Europe vs North America. The acceptance of powder in the architectural market in Europe is greater than in the United States, with a recognized barrier to entry being color and gloss variations. Europe is more accepting of the low gloss that is achieved with technology blends, but this technique results in a sparkle effect that has not been widely adopted or accepted in North America.

[Note that Biller disagrees. He states that the difference in consumption is due to different requirements for UV protection. Europe requires coatings with significantly less UV resistance. The continental United States is exposed to significantly higher doses of UV, hence the need for higher performance. Fairly standard resin technology meets most European architectural standards. In the United States, fluoropolymers are required to meet commercial architectural specifications, creating the higher barrier to market entry.]

Vlad, AkzoNobel Powder Coatings: Customers want to be able to make faster and better-informed decisions. The growth of digital innovation in the powder coatings industry is bringing powder coatings closer to the end users, making the decision process easier. Color digitalization and digital tools are a key area of development; AkzoNobel has rolled out a number of digital apps across different market segments supported by Instamatch, a highly accurate color-measurement tool that pairs with mobile software to allow fast and accurate color selection while on the go.

Apps enable easy and convenient research and in some cases, like AkzoNobel Design, can even be used to develop a bespoke technical specification in a matter of minutes, by offering a choice of filters at each stage of the decision process. Filters might range from type of substrate/construction material, environment, geographical location, durability requirements, color choices, finished look, etc.

Fruth, Arkema Coating Resins: Not much has changed for powder coatings in the past 40 years. Most innovation seems to come as technology transfer from the plastics engineering sector. Until there is a significant breakthrough in polymer design and how coatings are manufactured, powder will continue to occupy a niche position in the coating market. That said, Arkema continues to look for new ways to serve and support our customers in this market sector.

CoatingsTech | Vol. 15, No. 8 | August 2018

Solvent-based varnishes and lacquers have been the coatings of choice for industrial wood applications for many years. These coatings can provide an attractive durable finish that is cost effective. Kitchen cabinet and furniture manufacturers choose these coatings because they are fast drying, they are easily repaired, they tolerate climate differences well, and they are extremely forgiving. Some of these coatings have good chemical and water resistance as well as good wear resistance.  The disadvantage of these chemistries is the high volatile organic compounds (VOC), the extreme flammability, the odor, which causes poor indoor air quality, the formaldehyde emissions, and the pot life incurred when the conversion varnish is catalyzed with an acid catalyst.

Due to increasing regulations, more environmentally friendly alternatives are now being considered. Waterborne (WB) acrylics, polyurethane dispersions (PUDs), and WB UV coatings are becoming more common for use in industrial wood applications because they have excellent resistance and mechanical properties, excellent application properties, and very low solvent emissions. Self-crosslinking acrylics have very good durability and moderately fast drying times. PUDs have very good abrasion and wear resistance. WB UV chemistry is gaining market share over traditional solvent-based chemistry because it enables the end user to increase production efficiency and maintain a smaller manufacturing footprint. WB acrylics, PUDs, and WB UV coatings can be formulated to pass Kitchen Cabinet Manufacturers Association (KCMA) and Architectural Woodworking Standards (AWS) specifications. WB chemistries can provide the appearance and resistance properties of solvent-based coatings with lower VOCs, lower flammability, and decreased toxicity.

Three types of solvent-based coatings are commonly used in industrial wood applications—nitrocellulose lacquers, pre-catalyzed lacquers, and conversion varnishes. Nitrocellulose lacquer is typically a low solids blend of nitrocellulose and oils or oil-based alkyds. These coatings are fast drying and have high gloss potential. They are typically used in residential furniture applications. Disadvantages include yellowing with time, becoming brittle, and poor chemical resistance. Nitrocellulose lacquers have very high VOCs—usually at 500 g/L or higher.

Pre-catalyzed lacquers are blends of nitrocellulose, oils or oil-based alkyds, plasticizers, and urea-formaldehyde. They use a weak acid catalyst such as butyl acid phosphate. These coatings have a shelf life of approximately four months. They are used in office, institutional, and residential furniture. Pre-catalyzed lacquers have better chemical resistances than nitrocellulose lacquers. They also have very high VOCs.

Conversion varnishes are blends of oil-based alkyds, urea formaldehyde, and melamine. They use a strong acid catalyst such as p-toluene sulfonic acid and have a pot life of 24 to 48 h. They are used in kitchen cabinet, office furniture, and residential furniture applications. Conversion varnishes have the best properties of the three types of solvent-based coatings typically used for industrial wood. However, they also have very high VOCs and formaldehyde emissions.

Water-based self-crosslinking acrylic emulsions and polyurethane dispersions can be excellent alternatives to solvent-based products for industrial wood applications. Acrylic emulsions offer very good chemical and block resistance, superior hardness values, outstanding durability and weatherability, and improved adhesion to nonporous surfaces. They have fast dry times enabling the cabinet or furniture manufacturer to handle the parts soon after application. PUDs offer excellent abrasion resistance, flexibility, and scratch and mar resistance. They are good blending partners with acrylic emulsions to improve mechanical properties. Both acrylic emulsions and PUDs can react with crosslinking chemistries such as polyisocyanates, polyaziridine, or carbodiimides to form 2K coatings with improved properties.

Waterborne UV-curable coatings have become popular choices for industrial wood applications. Kitchen cabinet and furniture manufacturers choose these coatings because they have excellent resistance and mechanical properties, excellent application properties, and very low solvent emissions. WB UV coatings have excellent block resistance immediately after cure, which allows the coated parts to be stacked, packaged, and shipped right off the production line with no dwell time for hardness development. The hardness development in the WB UV coating is dramatic and occurs in seconds. The chemical and stain resistance of WB UV coatings is superior to that of solvent-based conversion varnishes.

Experimental: Waterborne UV Coatings

A study was conducted to compare the properties of three WB UV coatings with commercially available solvent-based conversion varnish, water-based conversion varnish, and water-based pre-catalyzed lacquer. The project plan was to develop high performance WB UV resins and investigate their performance for industrial wood applications. These coatings were tested according to KCMA, AWS, and individual furniture manufacturer’s specifications.

Panel Preparation

UV Coatings

Approximately 3 wet mils of coating were sprayed over 18×18 in. stained birch plywood panel, air dried for 10 min, and force dried for 10 min at 50°C. The coating was cured with mercury bulb at 500 mJ/cm² and sanded with 3M Superfine Sanding Sponge. A second coat was applied at approximately 3 wet mils, air dried for 10 min, and then force dried for 10 min at 50°C. The coating was cured with mercury bulb at 500 mJ/cm². The coating was tested after 7 days. For edge soak, all sides of a 4×4 in. solid oak panel were coated and cured.

Other Coatings

Approximately 3 wet mils of coating were sprayed over 18×18 in. stained birch plywood panel, air dried for 10 min, and force dried for 30 min at 50°C. A second coat was applied at approximately 3 wet mils, air dried for 10 min, and then force dried for 30 min at 50°C. The coating was tested after 7 days. For edge soak, all sides of a 4×4 in. solid oak panel were coated.

Test Methods

Chemical/Stain Resistance

Enough chemical/stain was applied to create a 0.25 to 0.5-in. diameter spot on the test panel and covered with watch glass. After 16–24 h, the chemical/stain was removed, and the surface of the panel was washed with water. Each chemical/stain was rated on a scale of zero to five, with zero being complete destruction of the film and five being no effect on the film (see Figures 1 and 2).

Scrape Adhesion

A 4×4 in. piece was cut from each test panel. Adhesion was tested using a BYK Balanced Beam Scrape Adhesion and Mar Tester with 5000 g weight using the loop stylus. Adhesion was rated on a scale of zero to five, with zero being complete removal of the film and five being no effect on the film (see Figure 3).

Ball Point Pen Indentation

A 4×4 in. piece was cut from each test panel. Ball point pen indentation was tested with a BYK Balanced Beam Scrape Adhesion and Mar Tester with 300 g weight using the small pen #5785. The panel was tested after 1 h and rated on a scale of zero to five, with zero being complete removal of the film and five being no effect on the film (see Figure 3).

Plasticizer Resistance

A 2-in. square piece of red vinyl was applied to the test panel. A force of 0.5 lb/in.² was applied. The specimen was placed in an oven at 50°C for 72 h. After cooling at room temperature for 1 h, the vinyl square was removed and evaluated for softening and blistering. Results are shown in Figure 4.

Green Print Resistance

After the test panel cured for 1 h, a 2-in. square piece of # 10 cotton duck cloth was applied to the finish. A force of 2 lb/in.² was applied directly to the duck cloth. After 24 h, the cotton duck cloth was removed and evaluated for printing. See Figure 4 for results.

Hot Print Resistance

After the test panel cured for 14 days, a 2-in. square piece of # 10 cotton duck cloth was applied to the finish. A force of 1 lb/in.² was applied directly to the duck cloth. The specimen was placed in an oven at 60°C for 24 h. The duck cloth was removed, and the specimen was allowed to cool for 1 h and evaluated for printing (see Figure 4).

Boiling Water Resistance

Approximately 10 ml of boiling water was applied to the test panel. A ceramic coffee cup full of boiling water was placed on top of the 10 ml of water. After 1 h, the cup was removed and wiped with paper towel. After 24 h, whitening was evaluated. The results are depicted in Figure 5.

Hot and Cold Check Resistance

A 4×4 in. piece was cut from each panel. The panel was placed in humidity cabinet at 50°C and 70% humidity for 1 h. The panel was allowed to reach original room temperature and humidity. After 30 min, the panel was placed in a freezer at -10°C for 1 h and then removed and allowed to reach original room temperature and humidity. This cycle was repeated five times. Results are shown in Figure 6.

Edge Soak

A cellulose sponge was placed in a plastic container. The container was leveled and filled with detergent solution (1% Dawn® dish soap by weight in water) to 0.5 in. below top level of sponge. The panel was placed on sponge, cut side down, and permitted to stand for 24 h (see Figure 6).

Formulations

The formulations used for the WB UV coatings are presented in Table 1. Table 2 shows the data for the WB UV coatings.

Testing

Test results for WB UV coating formulations are presented in Figures 1-6.

Results

All of the WB UV coatings exhibited excellent chemical resistance. WB conversion varnish and SB conversion varnish had very good chemical resistance. WB pre-catalyzed lacquer had adequate chemical resistance for KCMA coatings. WB UV 2, WB conversion varnish, and SB conversion varnish had the best scrape adhesion. All of the coatings had excellent ball point pen indentation, plasticizer resistance, hot print and green print resistance, hot and cold check resistance, and edge soak. All of the WB UV coatings had superior boiling water resistance.

Experimental: Acrylic Emulsion

A multiphase self-crosslinking acrylic emulsion was developed and evaluated for use as both a one-component (1K) and two-component (2K) industrial wood coating. The performance was benchmarked against a competitive self-crosslinking acrylic that is promoted for KCMA/furniture finishes. Two-component formulations were crosslinked with 6% carbodiimide (by weight).

Formulations

The formulations used for the acrylic emulsion coatings are presented in Table 3.

Panel Preparation

Birch Plywood

Two coats at 200 microns were applied to birch plywood panels. The first coat was air dried for 1 h and sanded. The second coat air dried for 7 days and then tested.

Glass

A 200-micron drawdown was performed on glass panels. The panels were air dried for 7 days and were tested after 14 days.

Testing

Test results for the acrylic emulsion formulation are presented in Figures 7–13.

Results

The ABI emulsion is a viable product for industrial wood finishes, especially for lower VOC formulations. The ABI emulsion offered better processing, higher film build, and better wet clarity when compared with a competitive emulsion (see Figure 14). Carbodiimide crosslinking offers improved performance and is a potential alternative to pre-catalyzed lacquers.

Experimental: Polyurethane Dispersion

An amine-free PUD was developed and evaluated for use as a clear self-sealing topcoat in KCMA/furniture applications. Performance was benchmarked against a traditional PUD. Two-component products were crosslinked with 6% carbodiimide.

Formulations

The formulations used for the polyurethane dispersion testing are presented in Table 4.

Panel Preparation

3BH Leneta Cards

For testing using Leneta cards, 1.5 Bird drawdowns were conducted. The cards were air dried for 15 min, force dried for 15 min at 50°C, and aged for 7 days before testing.

Birch Plywood

On birch plywood, a first coat was sprayed at 4–5 wet mils and air dried for 15 min. The coating was then force dried for 15 min at 50°C, allowed to cool, and then sanded. A second coat was sprayed at 4–5 wet mils and air dried for 15 min, then force dried for 15 min at 50°C and aged 7 days prior to testing.

Testing

Chemical resistance results from testing on Leneta cards are shown in Figure 15 and the birch plywood results are shown in Figure 16. Boiling water resistance results are provided in Figure 17. Scrape adhesion, edge soak, Taber abrasion, and Koenig pendulum hardness results are shown in Figures 18–21, respectively.

Results

The amine-free PUD is a viable product for industrial wood coatings. It had excellent ethanol resistance, Taber abrasion, and water resistance. It atomized well, had very good build, and good wood warmth.

Conclusions

Water-based coatings made from WB UV resins, acrylic emulsions, and polyurethane dispersions all are good candidates for industrial wood coatings. They have very good chemical resistance and mechanical properties. They can be formulated at low VOCs and have low toxicity. They are viable alternatives to solvent-based chemistries.

This article was presented at the American Coatings CONFERENCE, April 9–11, in Indianapolis, IN.

CoatingsTech | Vol. 14, No. 6 | June 2018

UV LED curing technology today offers numerous benefits in terms of sustainability, cost, and performance for certain applications—most notably graphics [primers and over print varnishes (OPVs)], wood fillers, fiber optics, electronic display coatings, and some non-graphic web coatings. The advantages being realized in these markets are generating interest from other sectors where UV LED curing technology has not reached the levels needed for commercialization. Interested end users are working closely with UV lamp/equipment manufacturers and coating formulators and their raw material (resin, additive, photoinitiator) suppliers to devise systems that will overcome current limitation and facilitate broader adoption of UV LED curing for industrial wood and other coatings, automotive/transportation, and many other applications.

Current State of the Market

Currently, UV LED curing is used most commonly for graphics applications, particularly for digital inkjet printing, screen printing, sheetfed offset printing, and flexo printing. These areas are growing rapidly, with the  fastest growth occurring in sheetfed offset printing, according to Scott Auger, global marketing manager with allnex. This is because a conventional press can be retrofitted quickly with little downtime or training while allowing flexibility to run or not run UV LED. “Sheetfed commercial printing is converting to UV LED for many reasons, but mainly for productivity, print quality improvements, and energy reduction; dry sheets of consistent quality are prepared instantly without powder, odor, or protective coatings and with brilliant, vibrant colors on any paper. In addition, spot, gloss, and strikethrough coatings are all possible with offset ‘on demand’ production,” he explains. Overall, Andrew Seecharan,market segment manager, Functional Packaging at BASF pegs the volume of UV LED inks and OPVs at approximately 5% of the market for energy curing inks and OPVs, or 3.3 kilotons, with the market growing at an estimated 4%.

UV LED curing is also used in optical fiber coating, electronics, and some wood coating and gel nail applications. “Today, most of the UV LED development for coatings has been in the graphics market (primarily OPVs and primers with some limited work in specialty), and fiber optics, wood filler coatings (not top coatings), and coatings for electronic assemblies,” says Jennifer Heathcote, global director of Business Development, Phoseon Technology. According to Dan Sweetwood, president and CEO of Allied PhotoChemical, when all applications are considered, the growth rate is estimated to be above 10% per year.

There are several applications for which UV LED curing has not yet been adopted. Few high-speed graphics applications have switched from conventional UV curing because the range of raw materials for formulating OPVs for high speed (> 700 ft/min) without high costs is limited, according to Seecharan. UV LED curing is also not used for thick-film, high-speed applications or applications where surface cure is a mandatory performance characteristic, according to Sweetwood. “Numerous industrial applications for UV coatings that require the lamps to be mounted at a distance (greater than 2 in.) or provide extreme functional or specialty photoinitiators used for these traditional UV coatings do not react in the longer UVA spectrum provided by UV LED,” she observes. Also, little work has been done with UV LED for industrial applications on parts with dramatic part profiles.

“UV LED is a new technology that is still evolving in supplier choice, capabilities, and cost.  Coating formulations need to be adjusted and tested for efficacy, especially in markets like automotive headlights that require long-term testing,” says Michela Fusco, global marketing manager at allnex. She notes that the largest installed base of UV LED lamps is in North America and Europe, and other regions have been waiting on the latest improvements to arrive before switching over.

Why UV LED Curing?

There are many good reasons to switch to UV LED curing if the technology can meet all of the requirements of an application. Like conventional UV curing with arc lamps or microwaves, LED curing allows the use of 100% solids formulations and provides fast curing with no thermal heating/ovens, increased production speeds, and a reduced footprint. Films are generated with excellent scratch and chemical resistance, and improved adhesion while waste and rejects are diminished. LEDs offer additional benefits, including a much longer lifetime over which marginal degradation of light output occurs, which is not the case for conventional lamps. LED lamps are instant on/off, reducing downtime. They consume 50–75% lower energy and provide for significantly reduced heat transfer to the substrate, allowing UV curing on heat-sensitive materials. Production speeds can be greater and floor space minimized further with LED curing. Because LED curing is based on solid-state technology, there are no moving parts and dramatically reduced maintenance requirements. The technology is also more sustainable, with no mercury-filled bulbs and no ozone that must be vented and managed. “Customers already using LED lamps have been able to validate the savings and, with lower investment costs, payback times are becoming more attractive, also in countries with lower energy costs,” observes Fusco. She adds that consistent output over time contributes to a more stable curing process improving quality delivered to market. Stable/consistent quality over a longer period also facilitates use in sensitive applications such as food packaging, where potential migration is a concern, according to Seecharan.

UV LED is strictly UVA and eliminates any safety issues related to the invisible UVB and UVC wavelengths, ozone, and mercury that must be dealt with when using conventional UV curing technologies.

In the wood coatings sector, LED curing is attractive because it may be used in conjunction with any application techniques currently in use for UV, including roll coating, vacuum coating, and molding spray, according to Brian Leonard, business development manager for Flooring with Sherwin-Williams Industrial Wood Coatings. The primary advantages over UV in the wood coatings sector are no heat transfer to the substrate; roughly half the energy consumption, thus lowering manufacturing costs; stable UV output across the width of the conveyor; no ozone emissions, which means no venting is required; and a long bulb life (approximately 20,000 h vs 1,500 h for arc lamps and 2,500 h for microwave lamps). “There is no particular wood market segment growing faster with LED, but roller coating (i.e., flat panel finishing) is the primary application,” he notes. Even if UV LED is only used to cure the successive wood fillers with the final hard top coat cured with conventional UV, the board surface temperatures can still be significantly reduced resulting in less scrap and a greater range of viable board materials, according to Heathcote.

The same regulatory issues/concerns that apply to conventional UV chemistry also apply to UV LED. However, there are sustainability advantages to UV LED curing equipment, according to Heathcote. UV LED is strictly UVA and eliminates any safety issues related to the invisible UVB and UVC wavelengths, ozone, and mercury that must be dealt with when using conventional UV curing technologies. For the automotive/aerospace industry, for instance, AkzoNobel has developed UV LED coatings that are cured only with UVA because it is considered less harmful than UVB and UVC energy conventionally used with mercury lamps, according to Emmanuelle Provost, group leader—Development Lab in the company’s Europe Specialty Coatings business. Heathcote stresses that it is up to the manufacturer to work with suppliers to ensure that the UV LED equipment is matched to the formulation and that process control is implemented to ensure sufficient and repeatable cure.

Requirement for Matched Solutions

In fact, just as conventional UV curing systems are not “one-size-fits-all,” LED curing solutions, comprising the lamps, curing equipment, ink/coating/adhesive formulations, and material handling requirements, must be designed to meet the specific needs of each application (substrate width, profile, and sensitivities; line speed; distance from the UV source) and the desired performance and functionality of the cured formulation, according to Heathcote. Photopolymerization via UV curing depends on the wavelength (nm), irradiance (Watts/cm²), and energy density (Joules/cm²) required by the formulation. Distance from the surface, the machine speed, and the chemistry of the formulation have a direct impact on these requirements. “While UV LED curing is a proven technology, not all UV LED products on the market deliver the same cure results. As a result, manufacturers should work closely with suppliers to determine what works best for their needs,” Heathcote says.

Today, the most numerous installations for UV LED curing are processes such as inkjet, fiber and display coatings, and spot-cure adhesives and sealants; however, UV LED equipment and chemistry are quickly expanding into faster-speed processes such as narrow, mid, and wide web flexo, and in some cases have exceeded 1,000 fpm. Many are designed for heat-sensitive substrates for which conventional UV is inappropriate or applications that benefit from the monochromatic energy generated by UV LED bulbs. They also tend to be flat or of shapes that can easily be positioned close to the lamps and for which formulations have been developed to cure effectively at the relevant wavelengths (365–405 nm) of UV LED lamps. “The current successes and remaining potential of UV LED curing are driving further investment in many other application areas to promote and leverage recent advances in lamp technology, resins/photoinitiator chemistries, and formulations and enable wider use of UV LED curing,” Heathcote says. “The list of viable applications continues to grow, including those for coatings,” she adds.

Manufacturers, however, are often risk averse and tasked by management to prove a strong business case before switching to UV LED, Heathcote notes. “Unfortunately, as we have seen in many cases, the numerous benefits to the bottom line are not always fully understood before implementation.  As a result, many companies that should be switching to UV LED have not done so due to the ROI calculation.  For those who have switched to UV LED, the payback has almost always been quicker than originally projected,” she asserts. Typically, companies with existing conventional UV curing equipment with years of useful life remaining are often less likely to invest in new LED systems that cost additional money and will require requalification of the curing process. For new machine purchases, however, or in cases where the conventional equipment needs to be replaced, UV LED will often present the better total solution cost. In some cases, while the technology may be feasible at slower press speeds or narrower webs, it occasionally cannot be implemented economically at the very fast production speeds and wide webs required for many coatings applications today. In either case, the technology is advancing quickly and implementation costs continue to decline, according to Heathcote.

Some of these applications will require further development and customization of the technology before they will be suitable for practical implementation. “Specific lamp designs must be engineered for the various applications and markets that incorporate wavelength, irradiance, and energy density profiles specific to the needs of the application.  The beauty of LED is that the lamp can truly be designed to deliver the exact energy needs of the application.  This makes UV LED technology incredibly efficient compared to conventional UV curing,” asserts Heathcote.

 The Problem of Surface Cure

While UV LED curing provides excellent through cure, there are some issues with surface cure that both formulators and equipment/lamp suppliers are working to address. Because UV LED curing occurs at specific wavelengths rather than a broad range of wavelengths, the photoinitiators and resins developed for conventional UV curing do not often provide cured films with the same properties using UV LED systems. “The current technology consists of a blend of photoinitiators that can

pose migration, yellowing, and formulation issues,” says Auger. He adds that there are even fewer options for water-based UV coatings and inks.

As importantly, UV LED systems on the market deliver a wider range of energy to the coating (depending on the system), and eliminate shorter wavelength UV.  The longer wavelenth energy penetrates deeper into the material for good through cure, but tends to result in increased oxygen inhibition at the surface due to the production of fewer free radicals, which can negatively impact productivity. “Formulating functional and specialty chemistry with hard surface cure across the full range of protection

qualities achieved with conventional UV still requires development, particularly for industrial applications,” Heathcote agrees. “Many industrial coating companies simply haven’t invested as much research time as the ink and graphics coatings companies and continue to promote current radiation cure chemistry. They will only begin to expand their development activities when existing and potential customers (converters, manufacturers, and large brands) start demanding UV LED alternatives,” she observes. Sweetwood adds that there is a lack of commercially viable UVC output LED systems that can overcome the issues with surface cure.

This issue is one reason why UV LED curing for wood coatings is limited to primer or filler coatings and often not used for topcoats. “There are limited raw materials available that respond to the LED wavelengths (405 nm, 395 nm, 385 nm, 365 nm) for surface cure. There are very few products that can fully cure using LEDs without discoloration; mercury lamps are often used at the end of a finishing process to ensure a full cure,” Leonard says.

UV LED equipment and chemistry are quickly expanding into faster-speed processes such as narrow, mid, and wide web flexo, and in some cases have exceeded 1,000 fpm. Many are designed for heat-sensitive substrates for which conventional UV is inappropriate.

Possible solutions to the surface cure problem include isolating the coating from oxygen in some way, increasing the amount of energy reaching the surface, increasing the photoinitiator concentration, or changing the photoinitiator/resin chemistry. However, isolating oxygen from the coating surface is very challenging and increasing the amount of energy often increases the rate of polymerization and thus negatively impacts through cure. Higher photoinitiator concentration can have a similar negative effect. As a result, efforts have been focused on tailoring photoinitiator/resin packages.

Other Hurdles to Adoption

This need to tailor the chemistry for UV LED curing has led to a slower rate of new formulation development for many of the more challenging applications and curing conditions, particularly with respect to high-speed applications, according to Seecharan. As mentioned earlier, the installed base of conventional UV curing equipment is also slowing the introduction of UV LED systems. Seecharan also notes that more operator training and growth of the knowledge base are needed with respect to implementation and use of UV LED to speed up adoption. Finally, as with any new technology, costs remain relatively high vs established UV inks, OPVs, and other systems. The business case has yet to be developed in many applications.

Yet Rapid Progress Ensues

UV LED lamp/equipment manufacturers and formulators and their raw material suppliers have been working diligently to develop solutions that will enable wider adoption of UV LED curing across more challenging applications. Lamps are increasing in power (over 20 Watts/cm) and offering lower wavelengths (as low as 340 nm from average 395 nm), according to Auger. UV LED systems are increasingly becoming more efficient, better tuned to the application, and more economical, according to Heathcote.  They are also now available in both narrow (less than 30 in.), mid (30 to 60 in.), and wide (greater than 60 in.) widths. Higher energy, air-cooled UV LED systems from Phoseon Technology, she adds, eliminate the need for a liquid circulation chiller which consumes power, takes up space, exhausts its own heat during operation, and has the potential for plumbing leaks and condensation at coolant temperatures below the dew point. While liquid cooled UV LED curing systems have their place and will continue to be offered, Phoseon is quickly responding to the market’s demand for higher powered, air-cooled alternatives, according to Heathcote.

Inerting of LED systems and the use of dual curing (LED and conventional UV) are having an impact, as is increased training opportunities through co-supplier webinars, at trade conferences, and in industry committees, according to Bryant.

Most of the recent advances are due to coating formulators better understanding how UV LED devices actually behave and emit UV output, according to Heathcote. “UV LEDs are quite different from conventional arc lamps. Simply swapping the photoinitiators or increasing the amount of photoinitiator to achieve better absorption in the UVA wavelengths is often not enough. Coatings suppliers have learned a great deal by experimenting in the lab and optimizing their formulations for what the UV LED lamps deliver. The biggest achievements in the graphics industry in the last year have resulted in increasing the line speeds on press that OPVs and primers cure to as much as 900 fpm while achieving sufficient surface cure and no yellowing. Most formulators have figured out how to prevent yellowing.  The recent focus has been on pushing the limits of press speeds,” she explains. Phoseon also continues to work on short wavelength LED devices in the UVB and UVC ranges.  “Unfortunately,” Heathcote says, “these systems are still incredibly expensive, deliver only a few hundred mW/cm², and have a very short useful life (less than 1,000 h). Short wavelength technology will continue to evolve, but the industry is several years away from practical commercial solutions.”

Raw material suppliers have introduced new photoinitiators and resins to try to address issues of cure speed, oxygen inhibition (surface cure), discoloration, and stability on press, according to Seecharan. For instance, BASF has introduced Laromer PO 9137 and Laromer PO 9139 in Europe with registrations in progress for a North American introduction, and continues development work to further improve print speed, surface cure, and reduced yellowing/discoloration of inks/OPVs. Allnex has introduced EBECRYL LED 03 booster and EBECRYL 5850 reactive resin to allow printing presses to run only UV LED lamps at top speeds. “Introducing an LED booster as an additive in existing formulations or adding quantities of more reactive resins is the right answer in most cases to achieve efficient LED curing,” Auger states.

The key for Leonard is developing LEDs that work with shorter wavelengths to help with surface cure and make LED a complete curing solution while eliminating the need for specially designed raw materials. “If LEDs could work in the range of 280–340 nm, they would offer a viable total curing solution,” he comments. He believes this area will be the focus in the next few years. “If the bulbs can advance, the coating technology will advance to where we will likely see faster growth,” Leonard adds. Sweetwood agrees that the development of UVC LEDs for UV curing is being pursued by all LED semiconductor manufacturers.

Several approaches are seen by Heathcote as having value for advancing UV LED curing in other markets. First is leveraging what has been achieved for formulations for the graphics industry (UV digital inkjet, screen, flexo, and offset) and applying it to chemistry in other markets. Second is designing the UV system to fit the needs of the chemistry and application. Third is further investigating how the chemistry reacts to UV LED light. “Conventional arc lamps are broad band and emit up to 3 Watts/cm² peak irradiance. Microwave lamps are also broad band and emit up to 5 Watts/cm². UV LED systems are commercially marketed at irradiances as high as 20 to 50 Watts/cm². Understanding how the chemistry responds to irradiance values so significantly higher than conventional UV is important. A higher irradiance is not always better and is sometimes detrimental.  Energy density and spectral output were always what formulators focused on for conventional UV.  Energy density is as equally important as irradiance.  A high irradiance may deliver a high-energy density, but in many cases UV LED products on the market that are specified at a high irradiance often don’t provide sufficient energy density to cure at fast line speeds,” she observes.

Wood Coatings

LED UV curing is attractive for industrial wood coating applications because it can be used for heat-sensitive wood substrates, including pine, fir, spruce, and mahogany. When resinous, oily woods get too hot, the resins or pitch can come to the surface and cause both discoloration and adhesion issues. There is great variation in wood composition, which makes it difficult to efficiently prepare these materials for coating. Lower-temperature UV LED curing is ideal for these woods and can potentially enable the use of lower-grade woods as well. For edge coating, UV LED makes it possible to use more company coating machines at higher speeds due to the consistent UV output. Roller coating UV LED lines are shorter and more efficient and benefit from reduced downtime and the ability to use less expensive stock. They also tend to have lower operating costs. Digital printing of the wood-grain look is also possible on many different materials. Even the texture of the grains in natural wood can be created for decorative and accent pieces.

Automotive/Transportation

As with the industrial wood sector, one of the biggest drivers for switching to UV LED curing in the transportation sector is the ability to achieve low-temperature curing. Lightweighting to boost fuel conservation is driving the use of plastics, composites, and lighter metallic alloys that can be sensitive to heat. Welding of these dissimilar materials is also not possible, leading to greater use of adhesives and sealants. UV LED curing can be ideal for this application as well, according to Heathcote.

Because UV LED curing is such a new technology in the automotive/aerospace sector, there is no clear estimate of the size of the market today, according to Provost. In the transportation sector, curing time is generally reduced by heating in ovens, although this approach is somewhat limited in the aerospace industry due to the large dimensions of many coated components. “UV LED curing is of growing interest as a means for reducing energy costs and shortening curing lines, but does represent an investment for most of our customers,” Provost says. Of particular interest is the use of inkjet for customization or fast application of complex drawings, he notes.

AkzoNobel has observed a transition from pure radical curing to hybrid systems. In response, the company is adapting its Autoclear UV system for car repair applications to meet the needs of aerospace customers for faster paint application of small pieces and airplane tails. Autoclear UV is a conventional polyurethane system that is UV catalyzed using a proprietary catalyst developed by AkzoNobel. The coating also cures without any UV light, which is important in the aerospace industry where shadow effects must be considered due to the shapes and dimensions of airplane parts. “We are now looking for customers willing to invest in UV LED curing systems. Several technologies have been proven but will not be implemented as long as customers don’t invest in this curing process,” Provost states. He adds that close collaboration with customers is important to the development of coatings that meet the requirements of their future LED curing lines.

Heathcote sees a number of potential commercial UV LED solutions being introduced to the transportation sector in the coming years, from screen-printed in-mold decorations for vehicle interiors to structural bonding of exterior components and light optically cured adhesives for electronic assemblies to printed appliqués and other markings. Longer-term, she expects to see UV LED curing of hard coats for in-mold decorating for vehicle interiors and coatings for mirrors, headlights, and the like. “Many of these applications are already achieved using conventional UV curing technology, and there is growing interest and exploration of UV LED solutions. Most of the development in this area is in fact driven by end users looking to find a way to meet their needs for lower-temperature curing using more sustainable technologies with lower operating costs,” Heathcote remarks.

Technology of the Future

UV LED curing technology has clearly been shown to offer many benefits, ranging from greater sustainability to greater productivity and quality. In many graphics markets, it is the preferred technology. In some cases, according to Heathcote, UV LED curing has been implemented where conventional UV curing could not. Even so, there remain many potential coatings applications that are not yet commercially feasible due to the need for advances in equipment and formulation chemistry designed to deliver functional cure properties at faster line speeds, greater distances, and for parts with complex profiles. All members of the UV LED value chain are tackling these issues head on and making rapid forward progress. As a result, adoption of UV LED curing will take place across a consistently expanding range of applications over the coming years. “UV LED curing technology is evolving rapidly in graphics, but all radcure applications even for consumer electronics and industrial plastics will move in this direction as the obstacles are removed,” states Auger.  Sweetwood, in fact, predicts that the development of LEDs will continue at a rapid pace, and this technology will become the curing equipment of choice within the next five to seven years.

CoatingsTech | Vol. 15, No. 5  | May 2018

Where radiation-cured coatings are employed, ultraviolet (UV)-curable systems predominate, and current development efforts are largely focused on improving waterborne systems and reducing the impacts of monomers (via migration) and photo-initiators (as residual impurities). Looking to the future, LED curing is generating significant interest and excitement, particularly as curing equipment and coating formulations improve.

In applications where their use is practical, radiation-curable coatings offer numerous advantages over thermally cured systems, including smaller manufacturing footprints, reduced energy consumption, reduced environmental impacts, and faster curing times, which often translate to faster line speeds. As recently as 2015, however, UV- and electron beam (EB)-cured coatings only accounted for 2% of the global industrial coatings market, according to IHS. The high costs of the raw materials compared to conventional coatings combined with the need to install specialized application equipment are two key hindrances to wider adoption. In addition, radiation curing of complex shapes and large objects is challenging, and thus this technology is not typically considered for these types of applications. Western Europe is the largest geographical market, followed closely by North America and China.

In 2016, 12–13 million formulated gallons worth $640 million were sold in North America, according to market research firm Nerlfi, Kusumgar & Growney (KNG). UV-cured systems represented approximately 88–90% of the total sales. Graphic arts as the largest end use by far accounted for 45–50% of the market, but is experiencing minimal growth of 1% per year. Prefinished wood products, which include wood flooring and wood furniture and fixtures, each accounted for approximately 11% of the market for radiation-cured coatings and are growing at much healthier annual rates of 5% and 6%, respectively. The other major application is release liners (e.g., labels), which accounted for 8.5% of the market; demand in this sector is growing at 11% per year. There are numerous other minor applications, including optical fibers, vinyl flooring, consumer electronics, automotive plastic components, metal packaging, and special purpose uses such as nail polish, medical implants, and field-applied floor coatings. Most radiation-cured coatings are based on acrylate resins, although cationic epoxy, epoxy silane, and unsaturated polyester formulations are used in small quantities.

State of the Market

Interest in UV-cured coatings continues to expand, particularly for heat-sensitive substrates such as plastics or wood. Kevin Joesel, director of Sales in the Americas for Heraeus Noblelight America expects growth in plastic coatings as more composites and thermoplastic materials are used for lightweighting of vehicles. He also notes that the market for UV-curable coatings on wood continues to show strength, and UV coatings for metal, while a small percentage of the market, have started to show life once again. In addition, UV technology for coatings and adhesives for electronics applications is ubiquitous, according to Joesel.  “As high-tech companies continue to move into traditional industries, they are very comfortable applying technologies they have been using for years in these legacy industries,” he notes.

Over the last decade, radiation-cured coatings have become an important technology for wood and furniture coatings, according to Marcel Krohnen, global head of End Use Wood & Furniture Coatings at BYK-Chemie GmbH. “Considering all available technologies (solventborne, waterborne, radiation cured, powder and their sub segments) and are exhibiting the strongest growth worldwide when compared to other technologies (solventborne, waterborne, powder),” he adds. Dual cure and electron-beam coatings are of interest only in very niche applications. Overall growth of 5–8 % is expected for UV-curable wood coatings for the next few years, according to Teresa M. Donahue, senior research and development project leader for Van Technologies, Inc.

In the wood sector, 100% UV-cured coatings cured using arc-lamps is standard coating technology for flooring and flat panel OEM substrates, according to Anthony Woods, Global Research, Development and Innovation, director with AkzoNobel Wood Coatings. Spray-applied technologies, on the other hand, tend to use either water-based or solventborne rather than 100% UV systems due to the environmental, health, and safety concerns associated with spraying low molecular weight oligomers, as well as the difficulties often encountered with respect to achieving effective curing. Within the building products and kitchen cabinet segments in Europe and North America in particular, AkzoNobel is seeing growth in waterborne UV technology. Electron-beam technology is not commonly used with wood coatings, although Woods does point to some segments, such as interior doors and edge-banding, where EB curing finds niche applications.

UV curing via microwave or arc-generated mercury vapor lamps is also well established for metal packaging, with approximately 40% of all printing inks being applied in this manner, according to Ian Lewis, solution lab manager–Inks with AkzoNobel Metal Coatings. “Thermally cured primers are normally still required for most can types, but the use of epoxide (cationic UV cure) and acrylate UV technologies is growing for protective OPV layers (perhaps 5%), often in-line over the printing inks, resulting in much more efficient manufacture—saving cost and reducing lead times,” he adds. At present, EB curing is not proven viable commercially, but remains interesting as a concept, particularly for the generation of primers and single coatings in coil applications, according to Lewis.

In addition to wood and direct-to-metal applications, Van Technologies receives numerous inquiries for a variety of other applications for UV-cured coatings, including plastic, fiberglass, and ceramic substrates. “Coating end-users are investigating the potential for the use of UV-curable coatings in their respective markets, and many of these inquiries have been directed at exterior applications,” notes Donahue.

The Growth of LED Curing

While conventional UV curing remains the leading technology, LED curing is growing rapidly and creating excitement in the market. Suppliers have developed LED systems that offer thermal management techniques to remove excess heat from the system while providing a consistent operating temperature for the diodes to function at maximum performance over their operating lifetime of greater than 20,000 hours, according to Joe Becker, a product manager with Phoseon Technology, a manufacturer of LED-curing equipment. LED-curing units also support trouble-free press operation with no warm-up time required and less down time due to the instant on/off capability. Maintenance is also minimal due to the lack of moving parts, according to Becker. “Because there is no infrared heat, converters using UV LED technology can run thin and heat-sensitive films without the need for chill rolls. In addition, UV LED systems cure at speeds 20–30% faster using up to 50% less energy and are more compact than traditional UV systems, allowing for more compact printers with shorter web paths that minimize waste and can be used in a wider range of printing environments. Furthermore, the uniformity and long-term consistency of LED lights enable the development of safer, more stable, and more reliable processes for low-migration printing,” he adds. Greater safety is an additional benefit; LEDs do not generate UV-C radiation, ozone, excessive heat, or noise. Finally, Becker notes that users report that LED light sources produce better cures and better adhesion on a wide range of substrates, including recycled materials. “They can also achieve higher speeds with black and white inks, and tough opaque whites and dense blacks are much easier to cure,” he states. These same economic and operating benefits are being applied to a wide variety of applications such as digital inkjet, display coatings, automotive coatings, wood coatings, electronics adhesives, and medical applications to name just a few.

For wood coating applications, as the cost of LED lamps goes down and their power increases, more line investments using LED lamps are being made, according to Woods. “They remain the minority at the moment, but we do believe over time the significantly longer bulb life of LEDs coupled with concerns over conventional Hg lamps will increase the usage of LED-cured systems,” he comments. Donahue agrees that LED curing is still in the early stages in regards to accommodating the wood coating market. “Wood is a naturally occurring material, and the structural dimensions can vary, resulting in challenges for LED technology,” she notes. “For example,” Donahue continues, “the lamp-to-substrate distance required for LED curing is very small, making dimensional variances difficult to accommodate. Wood finishers also typically do not coat only flat pieces of wood. They are coating multiple profiles, species, and thicknesses.” In metal packaging, LED curing has a great deal of potential if current application speeds can be met in a commercially viable way, according to Lewis.

There is no question that LEDs are going to take over a major part of the radiation curing market. It’s already happening in the printing world. When it comes to three-dimensional curing, however, there is still a lot of development that needs to take place.

The performance and power of UV-LED lamps have already increased dramatically since Phoseon first introduced UV-LED lamps to the market in 2004, and that trend will continue, according to Phoseon’s vice president Stacy Fender. “Much of this increase is due to the optics, thermal management, and patented technologies we apply at the system level. As LED efficiency increases, air-cooled lamps become more favorable versus water-cooled systems due to overall system savings,” he explains. Advanced air-cooled technologies, such as the company’s TargetCure™ and WhisperCool™ technology, enable LEDs to perform more efficiently and provide stable UV output over a longer period of time and more power while keeping sound to a minimum. In addition, UV-LED lamps are now designed specifically for applications requiring lower irradiance but higher doses, because dose is an important factor when curing coatings and in many cases more important than irradiance, according to Fender.

“There is tremendous excitement for LED curing. If you attend an industry conference, you will be shocked at the number of companies offering LED solutions,” asserts Dan Sweetwood, president and CEO of Allied PhotoChemical. He has concerns, though. “In some cases we find that customers are overly optimistic about the capabilities of LEDs. The LED manufacturers are trying very hard to differentiate themselves. I think at times they are overselling the positives and not spending enough time on issues that customers are going to care about over the long term. Customers need to be asking about maintenance, output of the lamp over time, the replacement strategy, etc.” As an example, Sweetwood notes that one of the benefits promoted by the LED industry is that LEDs run cooler than microwave or arc lamps. “While this statement is true, the light energy from a powerful LED (more than 8W) transfers a tremendous amount of heat rather quickly. Customers, however, think a cooler running light equals less heat on their substrate, which may or may not be true depending on the line speed. You can burn wood in seconds under a ‘cool,’ powerful LED,” he says.

Even so, Allied PhotoChemical is interested in LED curing. Recently, the company tested a 16W water-cooled unit in its laboratory, and Sweetwood was pleasantly surprised by the amount of energy the unit delivered. The unit operated successfully at cure speeds of greater than 100 feet per minute. “There is no question that LEDs are going to take over a major part of the radiation curing market. It’s already happening in the printing world. When it comes to three-dimensional curing, however, there is still a lot of development that needs to take place,” he concludes.

Many Drivers of Development

There are numerous drivers for growth of UV curing in different end-use market. Productivity is often the biggest, according to Joesel. “It all relates back to the speed of reaction. For free-radical polymerization, the rate of reaction is about six orders of magnitude (1,000,000 times) faster than condensation (conventional) reactions. For radiation-cured coatings, the benefits of speed, a small process footprint, and high first-time quality are all dependent on this speed of reaction,” he explains. The biggest advantages are realized, however, when UV-curable coatings enable a new technology to move into a new space, according to Joesel. “UV technology was critical to the development of optical fibers, digital recording, and personal and consumer electronics. We expect as new technologies continue to develop, the adoption of UV technology will continue,” he asserts.

One overall positive for the United States is the current on-shoring trend, according to Sweetwood. “If you are going to manufacture in the U.S., you need to be efficient. It is ultimately efficiency that drives our market,” he states. UV curing also often enables an increase in manufacturing capacity within an existing production environment, according to Woods. The ongoing shift from solventborne systems to formulations with lower VOC content is important as well, according to Krohnen. “Because the formulation of waterborne coatings can be sometimes limited due to external conditions (such as drying conditions, climate, etc.), radiation-cured systems have momentum,” he explains.

In the metal packaging industry, the desire to increase the use of radiation curing is largely explained by the vast reduction in applied costs due to production speed efficiencies, the reduced factory footprint, and increased automation for reduced labor intensity, according to Lewis. Energy savings and environmental improvements are further bonuses to change. To continue and maintain growth, however, he adds that there is a need for a larger formulating toolbox of toxicologically proven materials with assigned migration limits based on real data. “This has been hard to gain in the past due to the costs involved, but much of what is needed is now being made available as a result of the EU Registration, Evaluation, and Authorization of Chemicals (REACH) legislation,” observes Lewis.

Challenges can also create opportunities for innovation, and this situation is true for radiation-cured coatings. In the wood coatings industry, many large OEM customers are now focusing on residual photoinitiator content within coatings, which has led to the need for coating formulations that maintain effective performance and cost balances, according to Woods. He also notes that solutions are needed that provide deep color films that can be effectively cured, provide good adhesion, and are over-coatable. The trend toward deep-matte, solvent-free, UV-cured coatings for wood with gloss units below 10 is also challenging formulators, according to Krohnen, because these systems do not experience any film shrinkage, which leads to the need for a high amount of matting agents (e.g., silicas). “Doing so causes a tremendous increase in viscosity, which has a negative impact on application viscosity and makes roller-coater application almost impossible. Raw material suppliers are working to develop customized solutions, including wax additives, wetting and dispersing additives, matte resins, and specific matting agents, to meet these demands,” he observes.

For metal packaging applications, most of the biggest challenges relate to gaining proven suitability for food packaging, according to Lewis. “A great deal of the effort from the technical side goes into understanding toxicity and potential migration of components and potential non-intentionally added substances (NIAS), as is the case for many other chemistry platforms for packaging,” he comments. He adds that the Swiss ordinance covering inks and coatings for food packaging and the impending German Ink ordinance, which will likely be replaced by proposed EU central legislation, are the most notable from a food packaging safety perspective.

The global food packaging industry also seems to be driving the move away from bisphenol A-based formulations, according to Sweetwood. “This trend makes our job as formulators more difficult, but it can also create opportunities. We are always looking to improve our formulations, even those that have been on the market for many years. The BPA-free requirement forces us to think differently and creatively,” he observes. Customer specifications in other sectors (e.g., IKEA in the furniture industry) for coating materials that are free of formaldehyde, heavy metals, aromatic solvents, and organotin compounds, like stricter regulations for biocide usage, in fact make radiation-cured system of even greater interest for wood applications, according to Krohnen. It is also notable to Donahue that on a global level, particularly in China, there is a strong movement toward mandating cleaner technologies. In the United States, on the other hand, she notes that newer EPA regulations implemented under the Trump administration may see greater flexibility. Lewis also believes that legislation restricting the use of mercury lamps, which currently are covered by an exemption, could further drive the adoption of LED curing if LED technology becomes established as a viable proposition for high-speed, wide-format metal decoration.

Equipment Advances

LED technology is rapidly advancing. More power and interesting design variations are now available, such as plug-and-play bulb systems with “on-off” capability and different wavelengths. LED systems also achieve payback in less than three years, meaning that investment in LED cure is becoming a reality for wood coating manufacturers, according to Woods. “The speed of improvements to LED systems is now very exciting,” asserts Lewis. “Better cooling, higher power output, and reduction in cost are clear to see,” he adds.

Further improvements are still required, however. “We still need shorter wavelength LEDs that are affordable and more powerful,” Sweetwood says. He also predicts an eventual “market shake out” in the LED hardware space. “There are a lot of resources devoted to selling LEDs at the moment, but customer service, maintenance, replacement, and downtime avoidance are key things that will always matter. The LED manufacturers that can support the customer after the sale are the ones that are going to be successful long term,” he asserts.

While the equipment continues to adapt to high speed, industrial coating line requirements, it remains expensive, and in the wood finishing market the cost/benefit ratio is disproportionate, according to Donahue. “This ratio is currently offset to a degree via the adoption of hybrid processes consisting of combinations of LED and conventional UV-curing systems. The equipment cost and energy consumption benefits close the gap and result in a lower thermal profile to accommodate temperature-sensitive substrates,” she explains.

LED light sources produce better cures and better adhesion
on a wide range of substrates, including recycled materials.

For LED-only curing, Donahue also notes that materials need to be formulated to have increased cure responses and/or higher sensitivity to UV exposure to achieve faster cure speeds and better through-cure. “This issue is particularly relevant when curing materials that have structural variations, like wood, which render it impossible for lamps to have a very close proximity to the substrate surface. Thicker applications for both clear and opaque coatings would benefit as well,” she says. Formulators also need to be cautious in considering materials where the energy of activation is too sensitive, because package stability may be compromised, according to Donahue. As further advances in raw materials become available, this concept will become more feasible.

For conventional UV-curing applications, AkzoNobel Wood Coatings has partnered with COSTA to bring its “Inert Drying System” to market. With this approach, coatings are cured through a UV-transparent foil that reduces oxygen inhibition, allowing the curing of thicker films and reducing worries about residual photoinitiators. It also provides for a reduction in process steps and delivers an attractive final film quality, according to Woods.

New Chemistries

Raw material suppliers are continually developing new chemistries to address market needs. As is the case in many other segments of the coatings market, the switch from solvent-based to waterborne coatings is a key driver of innovation for radiation-cured systems. The growing interest in LED curing is driving the development of different photoinitiators and the use of more reactive branched amine oligomers, according to Woods. “We have also seen more reactive additives, as opposed to those that are migratory. These chemistries are very beneficial for coatings for food packaging and related industries, as well as for improving coating property longevity (e.g., longer maintenance) of slip properties or mar resistance,” Donahue says. Additive suppliers such as BYK are developing tailor-made additive packages, including wetting and dispersing additives, defoamers, and crosslinkable surface additives, as well as specialized matting agents, some of which are inorganic in nature with specific treatments and others that are based on waxes, according to Krohnen.

There is also growing interest in coating ingredients derived from renewable raw materials.  AkzoNobel recently introduced several UV coatings that contain renewable raw materials in response to this trend. Most notable, according to Woods, is the company’s collaboration with Solvay on the development renewable epichlorohydrin, a precursor to epoxies and epoxy acrylates, and other efforts that have resulted in renewable polyesters.

Regulations are having a two-fold effect, Lewis notes. “At present, many developments are quite conservative/consolidatory due to REACH and the need for global country registrations, which is likely hampering innovation. On the other hand, the movement away from BPA-containing epoxy acrylates has led to interest in more costly urethane chemistries to achieve similar properties, while regulations restricting tin are leading to the development of organic tin-free resins,” he observes.

CoatingsTech | Vol. 14, No. 4 | April 2017