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Water-based UV finishes have only been around for about 10 years, but they are starting to make a real impact in the wood finishes market. The obvious advantage of water-based UV finishes is the coating is very easy to apply. The viscosity and film build can easily be adjusted by the addition of water to the system.
When one thinks of "100% solids," you might think of something like a brick or a rock or something without any liquid in it. When finishing people talk about "100% solids," they are talking about a coating or finish that is liquid at room temperature, but when it is applied to the substrate and cured, it will not emit solvent or water. The finish has no solvent and is essentially zero VOC. The finish reacts on the surface of the part to create a solid coating on the surface of the substrate with little or no loss of weight.
It is slightly inaccurate in talking about these coatings as "100% solids UV" since there is some volatile content in the coating from additives and impurities; these potentially volatile contents may amount to about 0.1 to 0.4 lbs. per gal. of VOC.
The wood cabinet and moulding industries have long had to compromise the "look" of UV finishes vs. the VOC benefits. The coatings industry has focused on adding solvent to the UV clear coating to give a "lacquer look" — and some of these finishes had from 34 to 65 percent solvent in them. These solvent-based UV, or dual-cure finishes as some companies called them, did little to help the wood finishing industry other than helping it meet compliance for a short while. The VOC range of these solvent-diluted UV products could range from 2.8 lbs. per gal. to over 5.0 lbs. per gal. This may have been an improvement over some lacquers and conversion varnishes, but it did not solve the problem of VOC compliance for most manufacturers in the long run.
These coatings also didn't thrill insurance companies that were worried about the potential for a catastrophic loss at the facilities in the event of a solvent-related fire.
One potential solution to VOC problems, says Craig Martin, vice president of Mid-America Protective Coatings, a UV and water-based coatings manufacturer, is the new water-based UV coatings.
"Water-based UV coatings provide the cabinet and furniture manufacturer with a near zero VOC coating that gives the look of a conversion varnish or dual-cure UV — without all the hazards of the solvent-based technologies," Martin says. "Many of our customers can use water-based UV topcoats and immediately reduce their VOCs without sacrificing performance properties."
When asked to compare water-based UV vs. 100% solids UV, Martin suggests the best use of UV is to use both water-based UV and 100% UV in combination with one another to get the best of both worlds.
"Using 100% solids UV sanding sealers and water-based UV topcoats is the best way to get maximum performance out of UV," Martin says. "100% solids UV sealers are great for sealing the wood, and water-based UV gives tenacious adhesion to the sealer. The chemical resistance of a well-formulated water-based UV will out-perform precatalyzed or post-catalyzed lacquer and still have the feel of a traditional solvent-based finish."
Martin notes that many of his customers are already moving to a 100% solids UV sanding sealer, followed by a water-based UV topcoat.
According to Robert Wade, an associate scientist with Bayer Material Science, a UV raw material manufacturer, water-based UV is the best fit for the kitchen cabinetry industry.
"The cabinet industry can easily switch over to water-based UV finishes using existing manufacturing lines," Wade says. "All that is usually needed to run water UV online is a UV curing station that will take up less than 6 ft. of line space after the dryers."
Unfortunately, both the solvent-based UV and the water-based UV have the downside of having to dry or flash off the solvent or water. The solvent-based UV is also flammable and may require permitting depending upon the type of solvent used and the area in which the facility is located. Water-based UV needs to be "dried" before UV curing. This can typically be done in about five to seven minutes using conventional drying technology.
The water-based UV systems can give excellent block resistance and thus can be stacked immediately after cooling. Kyle Sass, UV technology scientist at BASF suggests that water-based UV can be cured with less UV energy when compared to 100% solids UV. Sass notes that lower glosses are also easier to achieve with water-based UV.
100% solids UV coatings without the plastic look
Most 100% solids UV coatings have not been able to give the "lacquer look" because they would bridge the soft grain of the oak leaving a "plastic look." This "plastic look" has long been the reason that 100% solids UV could not be used in high-end applications that required a thinner coating appearance.
Recent advances in coatings formulations, application technology and curing technology have allowed the 100% solids UV coating to look more like the old low solids technology. These developments could be very significant for an industry that is being harassed by environmental regulators across North America. The major advantages of 100% solids spray UV, according to Chuck Gambino, R&D specialist for Bayer Material Science, "Is that there is no long flash time needed to dry out water or solvent. This also means that the foot-print of a 100% solids UV spray can be 100 to 300 ft. shorter than a comparable solvent-based or water-based line. The space savings can be quite significant."
What has changed?
During the last few years raw materials have been developed that can be formulated into a coating with a low enough viscosity that they can be sprayed without adding solvent. Many coatings suppliers still tell their customers they cannot spray 100% solids UV clear coatings because they are unwilling to reformulate the coatings they are already selling.
Many large coatings manufacturers have failed to bring this technology to the market because the solvent-based UV products are more difficult to spray without the proper air-assisted airless guns. Some do not want to introduce a new technology to a customer who is not complaining loud enough about the current technology.
"Many vendors will not think outside of the box to customize the VOC solution to the individual cabinet manufacturer's line," Martin, Mid-America Protective Coatings, suggests. "100% solids UV and water-based UV are many times part of the answer to VOC problems."
100% solids UV spray today
Viscosities of 20 seconds or less on a No. 2 Zahn Cup are achievable and have helped to give the flow and leveling necessary to eliminate the "orange peel" that was common with old UV technology. Raw material advances have made "well-formulated" UV coatings easier to sand than in the past. 100% solids UV sealers can be made to sand much like traditional solvent-based sealers.
The 100% solids UV topcoats that are now available are mar- and scratch-resistant, and will easily exceed KCMA stain testing. These coatings are recyclable when used with the new flat line spray systems. Another advantage of 100% solids UV spray finishes is that they are not flammable. This is an advantage that your insurance person will love, and you won't have to worry about someone making a mistake that will result in a major plant fire. Most 100% solids UV coatings have flash points above 205 degrees F.
The new spray technology
The flat line units are now extremely efficient and can recycle 75 to 90 percent of overspray. There are three types of spray systems currently in use: reciprocating, rotary and misting.
These units usually use four to eight conventional air spray guns or air-assisted airless spray gun technology. Both types of guns are able to atomize the thinner viscosity coatings better. If you are into high-tech, the new electrostatic rotary atomizing guns are also good for breaking up the 100% solids UV coatings into a fine mist. Electrostatic technology has long been used in the automotive industry for getting great looking finishes and high-transfer efficiency.
All three types of guns allow the applicator more latitude in applying thin films or thicker films, depending upon the look that is desired. Mist coaters allow for the "open grain look." This look requires thinner films in 100% solids UV, some as low as 0.6 to 1.0 mil that can be achieved with mist-type spray coaters.
The UV curing technology
In the past, people would call the "UV Curing Unit" a "UV Oven." This term was not without significance. The UV curing units of the past were "ovens" in that they provided far too much thermal energy to the wood. Wood parts coming out of the UV curing units with temperatures exceeding 200 degrees F are not uncommon.
Traditional UV curing systems mounted lamps directly above the substrate. This is just fine for flat parts, but in the cabinet industry it is necessary to deliver uniform UV energy to the leading, trailing and sides of raised panel doors. Traditional systems delivered an inordinate amount of UV energy to the top of the substrate and just enough energy to the sides of the door.
Summary
Advancements in both equipment design and coating chemistry have developed together to bring the use of water-based UV spray and 100% solids UV spray to the forefront of the cabinet industry. New chemistry from raw material suppliers produced water-based UV spray and 100% solids UV spray formulations. They require much less UV energy to cure and still exhibit features necessary for the cabinet industry.
Properties now attainable are low-gloss sheen, open grain look, superior adhesion, and chemical and mar resistance. The UV curing systems have been re-engineered to deliver more uniform UV energy with a lot less heat. The upside is tremendous: Cabinet manufacturers will have the ability to finish cabinet doors faster, more efficiently, with less energy, lower VOC and in a more compact line. The future looks bright for water-based UV spray and 100% solids UV spray in the cabinet industry.
Ed. note: Donald Hart is currently the technical service manager for Mid-America Protective Coatings Inc. He is a veteran of the coatings industry for the past 24 years in both factory service and coatings formulation. Hart holds seven U.S. Patents in the area of coatings application and formulation. Questions? Please feel free to e-mail Donald Hart at donphart@aol.com.
author: By Donald Hart