By Kristy Wagner, Red Spot Paint and Varnish
Sustainability encompasses a complex array of environmental, economic and social considerations in modern manufacturing. While organizations implement diverse sustainability frameworks, the industrial coating sector consistently has recognized ultraviolet (UV) cured coatings as a more environmentally responsible alternative to conventional thermally cured systems. This distinction remains well-supported by empirical evidence and industry benchmarks. This paper presents a comprehensive analysis of UV-curable coating systems’ sustainability advantages and demonstrates their superior performance metrics when compared to thermal curing technologies in automotive applications.
An Introduction to Sustainability
Sustainability: It is strived for in almost all industries, but what does it mean? An internet search can reveal over 20 definitions, but most definitions can be summarized as the three Ps: People, Planet and Profit. To apply this to coatings, one can define a sustainable coating as a product that is designed to minimize environmental impact while improving product performance. Follow that by looking up a definition for a UV-curable coating, and one might just find the same definition.
People
From a People perspective, UV-curable coatings can be safer for users than two-component (2K) isocyanate thermally cured coatings. 2K coatings that contain isocyanates can cause skin and respiratory sensitization, asthma and other lung problems. They also can irritate the eyes, nose, throat and skin. Since UV-curable formulations do not contain isocyanates, they do not have these same issues. Though acrylate chemistries may cause skin sensitivity in some individuals, proper industrial hygiene practices effectively mitigate these concerns.
Planet
The benefits for the second “P,” Planet, can be seen in the differences in processing between the two chemistries. Thermally cured coatings generally will require 20- to 40-minute bake times at 180° F (82° C) or higher. This dictates a long tunnel that not only consumes energy but also takes up floor space and slows production. A typical UV process would entail application of the coating; ambient/heated flash of one to five minutes at 100° to 160° F (38° to 71° C) followed by a UV cure. If an infrared (IR) oven is used instead of a convection oven, the time almost can be cut in half. IR ovens can use 50 to 65% less energy than their counterparts. Some 2K coatings can use IR ovens, but many cannot. The IR will penetrate the coating from the top down; heat causes the coating to cure, so if an uneven cure is seen, performance and cosmetic defects will be present. Since UV coatings use heat to help with flow and leveling, no crosslinking occurs. IR does not have the same issues with UV technology.
UV coatings also can be higher solids than their thermal counterparts, which lowers their Volatile Organic Content (VOC) and, consequently, helps prevent air pollution. If the coatings are sprayed, some solvent is beneficial to efficiently lower the viscosity and to widen the processing window, thus making the coatings non-VOC free. However, UV spray-applied coatings can be up to 80 to 90% solids without sacrificing processability windows or negatively effecting scrap rate.
For UV-curable coatings, there are several oligomer and reactive diluent selections that contain plant-based, bio-renewable content. This amount can range from 10% to 85% content based on the raw material. Not all the available materials can produce superior exterior durable coatings, but they offer opportunities to enhance sustainability metrics where appropriate.
To help reduce waste that eventually could end up in landfills or be incinerated, overspray of UV-curable coatings can be collected, reconstituted (if originally containing solvent) and re-applied. This reclaimability aspect of UV coatings will reduce coating waste and help with the third “P,” Profitability.
Another benefit of UV-curable coatings is their excellent performance properties. By improving scratch, abrasion and chemical resistance, the finished product will have more durability and will not have to be replaced as often. These key characteristics will reduce the number of parts that end up in the landfill, further increasing their environmental benefits.
UV-curable coatings that can be used with a Physical Vapor Deposition (PVD) process can be an excellent environmentally friendly choice when compared to chrome plating. Decorative chrome plating can vary in its robustness, depending on how many layers of copper, nickel and chrome are applied; however, it cannot be denied that the process is very environmentally unfriendly. All the waste products (including rinse water) are regulated and must be disposed of as hazardous waste. In addition, chrome plating is accomplished with hexavalent chrome. This compound is an extreme health hazard. Without proper safeguards, tiny amounts can leach into the ground water and contaminate large areas very quickly.
PVD is the deposition of a metal onto a substrate through changes in the physical state of the metal (solid to gas to solid). A very thin layer of metal, approximately 600 to 1,000 angstroms, is deposited onto the basecoat layer. A wide variety of metals can be deposited, including aluminum, chrome, titanium, stainless steel, nickel chrome, tin, etc. Because this process does not use hexavalent chrome, it is more environmentally friendly. When UV-curable coatings are used as a basecoat and/or topcoat, the advantages of this process become even greater. Automotive interior and exterior trim parts are excellent places to use this technology. By changing the topcoat, a variety of appearances – from high gloss and satin to tinted rose gold, to name a few – can achieve visually pleasing and extremely durable additions to vehicles. Increased throughput, a wider range of plastic substrates, a wider range of appearances and design flexibility, and lower capital investment all can be expected – again, meeting all three “Ps” of sustainability.
Profitability
Almost all the advantages discussed as being beneficial to the Planet also can contribute to Profitability: decreased energy costs, increased throughput, and decreased scrap and waste. In addition to these aspects, UV coatings can be applied at lower film builds and still match or exceed thermally cured coatings. To get an aesthetically acceptable appearance and still achieve all the physical properties, UV coatings can be applied at 0.4 to 0.7 mils. For a typical thermally cured 2K clear coat, film builds of 1.5 mils or greater are needed. In fact, if one were to apply most UV coatings at thermal film builds, one would see a decrease in performance and processing.
Areas of Use
It seems obvious that UV-cured coatings can be environmentally friendly and profitable options, but where can this technology be used? Forward lighting (hard coat and basecoat), plastic wheel covers, rear lighting, PVD parts and high-gloss black interior coatings are just a few of the current areas using UV chemistry. Potential future uses include, but are not limited to, pillars, electric vehicle (EV) “grills,” body colors and expanded use for the interior components of a vehicle. In fact, any plastic part that needs improved scratch or chemical resistance is the perfect opportunity for a UV-curable coating.
Performance: Exterior
UV-curable hard coats for polycarbonate headlamp lenses have been on the market since 1990, when Original Equipment Manufacturers (OEMs) started replacing glass lenses with plastic. Multiple improvements have been made over the years, focusing on extending weatherability and scratch resistance. Today, lenses are expected to last five to seven years without yellowing, hazing or cracking. This duration always is being extended to ensure continuous safety without having to replace the assembly. UV-curable coatings also must withstand moisture and/or heat cycle resistance, chemicals (gasoline, oil, car washing cleaners, etc.) and abrasion resistance from cleaning processes. Using this technology as a baseline, it is easy to see where modifications of this technology can be used in other automotive exterior areas.
Side pillars are notorious for their lack of aesthetic durability. Traditionally, these parts are molded in color plastic, PC and/or PC/ABS. Because they usually are black, the plastic absorbs more heat, which accelerates the degradation of the plastic, resulting in a loss of gloss or a peeling of the substrate. UV-curable clear coats can be applied to these parts, extending their lifespan in the field. Table 1 shows the test results that can be achieved with a UV-curable clear coat for this application.
Exterior body parts could be considered a Holy Grail for UV coatings. Could they be durable enough to withstand the rigors of everyday use of a vehicle? Could they be applied and processed in a practical manner? Not all these questions have been answered, but there have been exciting developments. In a lab setting, a UV-curable black base coat with a UV-curable clear coat has shown promise.
A process that could be completed in about 15 minutes? Maximum oven temperature of 170° F (77° C)? This definitely would meet the sustainability criteria previously discussed. But what about the performance? Table 2 shows some of the test data that has been amassed. In addition to these tests, various environmental materials (acid rain, sulfuric acid, calcium sulphate, egg albumin, etc.) and plant materials (motor oil, windshield washer fluid, brake fluid, engine coolant, etc.) were tested at elevated temperatures. No signs of etching or color changes were observed. There is enough evidence here to show that this opportunity could be achievable in the future.
Performance: Interior
The performance benefits of a UV-curable coating are many, but because of the high cross-link density of this technology, scratch and abrasion resistance stand out as the number one advantage over thermally cured coatings. Table 3 compares a typical thermally cured coating to a UV-cured coating for various OEM abrasion tests. It becomes obvious that as testing harshness increases, UV-curable coatings outperform their thermal counter parts.
Chemical resistance is right behind scratch and abrasion for superior UV-curable performance. Some of the harshest tests for interior automotive coatings are air freshener and sunscreen resistance tests. Exact testing parameters vary from OEM to OEM, but Table 4 lists a comparison between the same two coatings as Table 3. Again, the UV-curable outperforms its counterpart. Better protection at lower film builds, with the same or better depth of image (DOI), is easy to achieve with UV-curable coatings.
Future Growth and Improvements
There is no debate that UV-curable coatings have environmental advantages over thermally cured coatings. The exciting growth of sustainability in energy-curable coatings can be seen in the form of curing with Light Emitting Diodes (LED) instead of traditional mercury vapor bulbs. Compared to mercury lamps used today, LED systems can use up to 80% less energy, resulting in a typical savings of 30 to 50%. LED systems run cooler than mercury systems, so a reduction in the number of cooling systems needed is a big energy savings. When mercury-containing bulbs break or reach their life span, special disposal is necessary to discard the heavy metal. LEDs do not contain mercury, so this is not a factor. Mercury bulbs will generate ozone upon start-up that must be dissipated from the work floor. Again, this is not an issue with LED units. It is estimated that by replacing a mercury system with LED, 20 tons of carbon dioxide reduction per product annually could be achieved. LED units have a longer life span (60,000 hours) than a mercury bulb (typically 1,000 to 1,500 hours). This leads to less disposal and less preventative maintenance over the life of the unit.
However, LEDs currently cannot immediately replace all mercury curing systems. LEDs emit one specific wavelength vs. multiple wavelengths in a typical mercury bulb. Oxygen inhibition and surface cure can prove to be a challenge with an LED-curable system. LEDs are an excellent choice for flat stock or where the parts can be within millimeters of the LED curing unit. As the part-to-lamp distance increases, the intensity output greatly decreases. For three-dimensional parts and for coatings fortified for exterior durability, curing with an LED unit can be problematic. However, as technology continues to improve and raw material suppliers expand their offerings for LED-curable coatings, there is an opportunity to overcome these obstacles and for this technology to become the dominant source for a sustainable, profitable coating solution.
Conclusion
For over 40 years, UV-curable coating technology has represented significant advancements in sustainable manufacturing for the automotive sector. Its demonstrated advantages in environmental impact, operational efficiency and performance characteristics position it as a key enabler for future automotive innovation. Ongoing developments in LED-curing systems and raw materials continue to expand the technology’s potential, supporting the industry’s transition toward more sustainable manufacturing practices.
Kristy Wagner earned a degree in Biochemistry at Indiana University. Wagner has been in the UV-curable industry for over 30 years. Early in her career, Wagner formulated inks and adhesives for the printing and packaging industries. For the past 25 years, she has been a formulation chemist at Red Spot Paint and Varnish in Evansville, Indiana. During this time, Wagner has developed protective and decorative coatings for various markets, including packaging, sport coatings, films, appliances and the automotive industry. For more information, email KMWagner@redspot.com or visit www.redspot.com.
