UV technology is considered by many to be the “up-and-coming” technology for curing industrial coatings. Though it may be new to many in the industrial and automotive coatings industry, it has been around for more than three decades in other industries. People walk on UV-coated vinyl flooring products every day, and many of us have them in our homes. UV curing technology also plays a major role in the consumer electronics industry. For instance, in the case of cell phones, UV technology is used in the coating of plastic housings, coatings to protect internal electronics, UV adhesive bonded components and even in the production of the color screens found on some phones. Similarly, the optical fiber and DVD/CD industries use UV coatings and adhesives exclusively and would not exist as we know them today if UV technology had not enabled their development.
So what is UV curing? Most simply, it is a process to cross-link (cure) coatings by a chemical process initiated and sustained by UV energy. In less than a minute the coating is converted from a liquid to a solid. There are fundamental differences in some of the raw materials and the functionality on the resins in the coating, but these are transparent to the coating user.
Conventional application equipment such as air-atomized spray guns, HVLP, rotary bells, flow coating, roll coating and other equipment apply UV coatings. However, instead of going into a thermal oven after coating application and solvent flash, the coating is cured with UV energy generated by UV lamp systems organized in a manner that illuminates the coating with the minimum amount of energy required to achieve cure.
Companies and industries that exploit the attributes of UV technology have delivered extraordinary value by providing superior production efficiencies and a superior end product while improving profits.
Exploiting UV’s Attributes
What are the key attributes that can be exploited? First, as mentioned previously, curing is very fast and can be done at room temperature. This allows efficient curing of heat-sensitive substrates, and all coatings can be cured very quickly. UV curing is a key to productivity if the constraint (bottle-neck) in your process is a long cure time. Also, the speed allows a process with a much smaller footprint. For comparison, a conventional coating requiring a 30-minute bake at a line speed of 15 fpm requires 450 ft of conveyor in the oven, while a UV cured coating may require only 25 ft (or less) of conveyor.
The UV cross-linking reaction can result in a coating with vastly superior physical durability. Though coatings can be formulated to be hard for applications such as flooring, they can also be made to be very flexible. Both types of coatings, hard and flexible, are used in automotive applications.
These attributes are the drivers for the continued development and penetration of UV technology for automotive coatings. Of course, there are challenges associated with UV curing of industrial coatings. The primary concern to the process owner is the ability to expose all areas of complex parts to UV energy. The complete surface of the coating must be exposed to the minimum UV energy required to cure the coating. This requires a careful analysis of the part, racking of parts, and the arrangement of lamps to eliminate shadow areas. However, there have been significant improvements in lamps, raw materials and formulated products that overcome most of these constraints.
Automotive Forward Lighting
The specific automotive application where UV has become the standard technology is in the automotive forward lighting industry, where UV coatings have been used for more than 15 years and now command 80% of the market. Headlamps are composed of two primary components that need to be coated -- the polycarbonate lens and the reflector housing. The lens requires a very hard, scratch-resistant coating to protect the polycarbonate from the elements and physical abuse. The reflector housing has a UV basecoat (primer) that seals the substrate and provides an ultra-smooth surface for metallization. The reflector basecoat market is now essentially 100% UV cured. The primary reasons for adoption have been improved productivity, small process footprint and superior coating-performance properties.
Though the coatings used are UV cured, they do contain solvent. However, most of the overspray is reclaimed and recycled back into the process, achieving close to 100% transfer efficiency. The focus for future development is to increase the solids to 100% and eliminate the need for an oxidizer.
Exterior Plastic Parts
One of the lesser known applications is the use of a UV curable clearcoat over molded-in-color body side moldings. Initially, this coating was developed to decrease the yellowing on exterior exposure of vinyl body side moldings. The coating had to be very tough and flexible to maintain adhesion without cracking from objects striking the molding. The drivers for the use of UV coatings in this application are the speed of cure (small process footprint) and superior performance properties.
SMC Body Panels
Sheet molding compound (SMC) is a composite material that has been used as an alternative to steel for more than 30 years. SMC consists of a glass-fiber-filled polyester resin that has been cast into sheets. These sheets are then placed in a compression mold and formed into body panels. SMC can be chosen because it lowers tooling costs for small production runs, reduces weight, provides dent and corrosion resistance, and gives greater latitude to stylists. However, one of the challenges in using SMC is the finishing of the part in the assembly plant. SMC is a porous substrate. When the body panel, now on a vehicle, goes through the clearcoat paint oven, a paint defect known as a “porosity pop” can occur. This will require at least a spot repair, or if there are enough “pops,” a full repaint of the body shell.
Three years ago, in an effort to eliminate this defect, BASF Coatings commercialized a UV/thermal hybrid sealer. The reason for using a hybrid cure is that the overspray will be cured on non-critical surfaces. The key step to eliminate the “porosity pops” is the exposure to UV energy, significantly increasing the cross-link density of the exposed coating on the critical surfaces. If the sealer does not receive the minimum UV energy, the coating still passes all other performance requirements.
The use of dual-cure technology in this instance provides new coating properties by utilizing UV curing while providing a safety factor for the coating in a high-value application. This application not only demonstrates how UV technology can provide unique coating properties, it also shows that a UV-cured coating system is viable on high-value, high-volume, large and complex automotive parts. This coating has been used on approximately one million body panels.
Arguably, the UV technology market segment with the highest visibility is the automotive exterior body panel Class A coatings. Ford Motor Company exhibited UV technology on a prototype vehicle, the Concept U car, at the North American International Auto Show in 2003. The coating technology demonstrated was a UV-cured clearcoat, formulated and supplied by Akzo Nobel Coatings. This coating was applied and cured over individual body panels made from various materials.
At Surcar, the premier global automotive coatings conference held every other year in France, both DuPont Performance Coatings and BASF gave presentations in 2001 and 2003 on UV-curing technology for automotive clearcoats. The driver for this development is to improve a primary customer satisfaction issue for paint—scratch and mar resistance. Both companies have developed hybrid-cure (UV & thermal) coatings. The purpose of pursuing the hybrid technology path is to minimize the UV curing system complexity while achieving the target performance properties.
Both DuPont and BASF have installed pilot lines at their facilities. The DuPont line in Wuppertal has the ability to cure full bodies. Not only do the coating companies have to show good coating performance, they also have to demonstrate a paint-line solution. One of the other benefits of UV/thermal curing cited by DuPont is that the length of the clearcoat portion of the finishing line can be reduced by 50% simply by reducing the length of the thermal oven.
From the engineering side, Dürr System GmbH gave a presentation on an assembly plant concept for UV curing. One of the key variables in these concepts was the location of the UV curing process in the finishing line. Engineered solutions included locating UV lamps before, inside or after the thermal oven. Dürr feels that there are engineering solutions for most of the process options involving current formulations under development. Fusion UV Systems also presented a new tool -- a computer simulation of the UV-curing process for automotive bodies. This development was undertaken to support and accelerate adoption of UV-curing technology in assembly plants.
Development work continues for plastic coatings used on automotive interiors, coatings for alloy wheels and wheel covers, clearcoats over large molded-in-color parts and for under-hood parts. The UV process continues to be validated as a stable curing platform. All that is really changing is that UV coatings are moving up to more complex, higher-value parts. The stability and long term viability of the process have been demonstrated with the forward lighting application. It started over 20 years ago and is now the industry standard.
Though UV technology has what some consider a “cool” factor, what the industry wants to do with this technology is to provide the best solutions to finishers’ problems. No one uses a technology for technology’s sake. It has to deliver value. The value can come in the form of improved productivity related to the speed of cure. Or it can come from improved or new properties that you have not been able to achieve with the current technologies. It can come from higher first-time-quality because the coating is open to dirt for less time. It may provide a means to reduce or eliminate VOC at your facility. The technology can deliver value. The UV industry and finishers need to continue to work together to craft solutions that improve the finisher’s bottom line.