Shedding some light on UV curable coatings.

European finishing methods are gaining popularity in the U.S.

A quiet revolution is taking place in the U.S. wood finishing industry. Stricter federal regulation of volatile organic compound emissions is leading to an increase in the use of ultraviolet light-curable coatings and automated finishing lines similar to the European style of wood finishing.

While not explosive, the overall growth rate in the U.S. for sales of UV coatings and equipment is more than twice that for the general coatings industry. UV coating sales are predicted to maintain a healthy 10 to 15 percent growth rate. Growth is expected to increase as wood finishers become familiar with the advantages and application characteristics of non-thermal curing technologies.

Light-Induced Curing

UV-curable coatings are high-solids, cross-linked coatings cured by brief exposure to intense ultraviolet light. The cure schedules for these types of coatings are incredibly fast - usually less than five seconds - allowing finishing lines to be shortened, line speeds increased, and products immediately handled and packaged. Also, since these products are high solids with little or no VOCs, compliance to stricter VOC regulations is made easier. Other advantages of UV coatings include increased superior hardness and durability compared to conventional coatings.

The chemical reaction, or polymerization, that occurs in UV coatings involves two major constituents: monomers and oligomers. While reaction time is dependent on radiant intensity levels of the energy source, these two components are capable of reacting at extremely high rates. Oligomers are polymers that vary in chemical composition and define a coating's physical properties. There are four types of oligomers currently used in the majority of UV-wood coatings. They include urethane, epoxy and polyester acrylates, and unsaturated polyesters.

With increasing regulation on solvent usage, significant advances have been made in waterborne oligomers for UV coatings. Existing waterborne UV-curable coatings provide films with lower solids, but achieve VOC emissions as low as 0.2 pounds per gallon. However, as is the case with conventional waterbornes, additional time and energy are required to remove the water from these coatings, and the finishes are more susceptible to flaws created by improper application and curing.

While the oligomer resin is the backbone structure, there are many other components that complete the UV-coating formulation. Multifunctional monomers are the "link" to oligomers that form the molecular network during the exposure of radiant energy. As the number of reactive groups increase, the higher the cross-linked density and the tighter the coating film. Although increasing cross-linked density is the primary function of a multifunctional monomer, a majority of these co-reactants are relatively low in viscosity and also serve a dual function as a diluent in reducing viscosity. Other components of UV coatings include additives, pigments and fillers, similar to other wood coatings, which are designed to obtain desirable flow, leveling, curing and color characteristics.

While not new - UV coatings have been in existence for more than 25 years and are used extensively in Europe - they have been slow to catch on domestically. One reason for this is that, traditionally, most of the U.S. furniture industry has finished products manually using overhang lines and track-on-ceiling or tow-cart conveyor systems. In addition, wood products produced in the U.S. are typically assembled first and then finished, while in Europe wood furniture is often finished knocked-down, then assembled after finishing.

Consumer taste and technology advances, too, have played a role in the reluctance of U.S. wood manufacturers to implement the technology. The graceful, curing styles and intricate details typical of the U.S. residential furniture market are not conducive to flat-line finishing with highly-viscous UV coatings, white the more boxy, squared-off furniture popular in Europe is easily flat-line finished. Similarly, the wide variety of color options U.S. producers must carry to satisfy consumer preferences has prevented UV technology from being an economic alternative.

Due to recent advances in spray technology these systems are now being employed at cabinet shops and furniture manufacturers across the country.

Since UV coatings are generally formulated with little or no use of the aromatics, esters and ketones (solvents required to keep liquid coatings in solution until the finish has cured in oil- or gas-fired convection ovens) VOC emissions are typically negligible. Curing is a free-radical, photoinitiating process, in which the coating is exposed to UV light. Photoinitiator chemicals in UV coatings quickly absorb the ultraviolet light and start a rapid chemical reaction which is often completed in as little as five seconds. Because the liquid coating is entirely converted into a solid, there are virtually no emissions.

Equipment Advances

While UV coatings can be applied by airless or conventional spray guns, reverse roll coaters or curtain coaters, they are always applied using automated processes. Spray lines typically provide about 50 to 75 percent transfer efficiency, while roll coater lines can achieve 100 percent transfer efficiency. According to equipment manufacturers, roller and curtain coater systems are becoming popular for flat-panel production, while spray systems are being used for shaped and three-dimensional pieces.

New automatic, computer-controlled, carousel-type robotic spraying units with a photo eye "read" the substrate and fire the precise amount of coating to an exact configuration. Also providing exceptional finishing capabilities are reciprocating sprayers which use separate heads that traverse the conveyor and employ the photoeye technology within an enclosed structure.

The new application technologies make it increasingly likely that manufacturers who already make limited use of UV coatings will expand the use of these systems to more intricate or detailed furniture. Kitchen cabinet manufacturers, for example, who already use UV coatings on end panels are likely to find that equipment advances now make it practical for them to apply UV coatings to doors.

Costs/Benefits

While the investment required in UV application equipment is steep, the potential benefits are tremendous. Costs to outfit a UV coating line typically range from $50,000 at the very low end to $1 million, depending on the sophistication of the system. It is not unusual for a manufacturer to recoup that investment in a relatively short time from production benefits.

For example, a 500-foot finishing line, with 25 employees using conventional spray-applied coatings, could be shortened to 90 feet and would require only two or three employees if redesigned for UV coatings. Production time in such a scenario would be reduced from 1.5 hours to 10 to 20 minutes or less. In addition, energy costs can be lowered and virtually all VOCs eliminated.

Chuck Susnis is the chemical coatings market development manager for the Sherwin-Williams Co. of Cleveland, OH.