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100 percent solids ultraviolet (UV)-curable coatings have long held the promise of lower emissions. With the advent of nanotechnology, enhanced performance is possible as well. This new generation of coatings technologies offers improvements in scratch resistance, abrasion resistance and solvent resistance—a promising set of characteristics for OEM manufacturers.
Today’s challenge lies in convincing finishers to convert promise to reality by implementing this new technology. The industry must first overcome many barriers to change. New technologies require capital expenditures. Employees must be trained in new techniques. New coatings must be qualified with customers. However, the biggest barrier to change may be the inertia of the industry itself.
The impetus needed to overcome that inertia may be found in the current energy crunch. As the price of natural gas soars, operating costs for curing ovens are rising. As a result, the cost of curing powders, conventional liquid coatings and many waterborne coatings continues to increase.
For companies that outsource parts to finishing shops, the rising cost of transportation may present an even more severe challenge. In many cases, the cost of shipping a part already exceeds the price for coating it. That cost continues to rise, with no end in sight. The industry is approaching a point where the capital expenditure needed to bring finishing operations in-house is lower than the cost of shipping parts to a finishing shop. When that energy cost tipping point is reached, the change to newer technologies will become a practical or even more attractive option.
Companies considering bringing previously outsourced parts in-house must weigh several factors. One is space. Obviously, it is most economical to use available space rather than to expand. New equipment should have the smallest possible footprint, a need that UV curing can fulfill. A drying oven for a conventional web coating line typically takes up 500–1,000 sq ft of floor space. UV curing only requires space for lights, power supplies and air for cooling and ventilation—probably an order of magnitude less space.
Another factor in the economic equation is time. Using energy-intensive thermal systems, curing can take 20 minutes or longer. In comparison, a UV cure can often be accomplished in a few seconds. Jobs that might require three shifts using a thermal system can be completed within a single shift. The savings in labor, benefits and insurance costs in such cases are considerable.
Another factor in deciding whether or not to bring in finishing of previously outsourced parts is the components themselves. Parts that feature combinations of materials, such as metal tanks with rubber seals, dampers with rubber inserts and metal with plastic attachments, can all be finished without disassembly, reducing the number of steps and thus further increasing productivity. Our company’s UV-curable products include coatings that can be used on metals and many plastics. The use of such a functional coating may eliminate the need for masking, further increasing savings.
The last economic factor can be found in every newspaper and every newscast—the rising price of fossil fuels. Because of the short duration of cure, UV curing can reduce energy costs up to 80%. When combined with the other cost considerations, this may tip the balance point toward a cleaner, more efficient future.
Making the Switch
For companies that are already finishing parts in-house, a complete process overhaul is not necessary to start benefiting from the advantages of 100% solids UV-curable coatings. For example, the viscosity of our company’s coatings materials is low enough to allow the products to be applied with traditional equipment. The combination of HVLP spray guns with a turbine can provide smooth and efficient application. Clean, warm air may also be helpful for viscosity control.
Satisfactory results can also be obtained with other types of application systems. Many of our products can be applied by electrostatic spraying. These systems are appropriate for both clear and pigmented coatings. Conventional air spray, generally between 40 and 50 psi, can also be used, as can air-assisted airless spray. Thus, a variety of application methods, most of which are likely already available in-house, can be used for these next-generation 100%-solids UV-curable coatings.
A more important process step is curing. Both the power and the spectral output of the UV lights must be matched to the coatings used. Curing can only be accomplished when all areas of a coated surface are sufficiently exposed to the required dosage of light. For this reason, parts with hidden areas are not a good choice for UV cure. However, creative placement of lights, combined with appropriate reflectors, enable effective curing of many types and shapes of parts. Parts with radial symmetry, such as tanks, air filters, dampers and brake rotors, are particularly conducive to this type of process.
While microwave-powered UV lights provide excellent results, not every shop can accommodate the combination of lights and hefty power supplies that such systems require. Many such lights require three-phase power or considerable airflow for cooling, two factors that might not be readily accommodated in some plants. In some cases, extra ventilation is necessary to prevent ozone build-up. Finally, the combination of light and power supply can also prove heavy and unwieldy, especially for smaller workers.
|COMPARATIVE COSTS AT A GLANCE|
||Curing Time||Electricity||Natural Gas||Floor Space|
|UV||As little as 3 sec||15–20 kWh||-0-||400 sq ft|
|Powder||20-40 min||90 kWh||1,200 cf/hr||3,000 sq ft|
|Air drying: hours||-0-||-0-||3,000 sq ft|
|Heated flash-off tunnel: minutes||90 kWh||-0-||3,000 sq ft|
Fortunately, as coatings have been evolving, UV lights have advanced as well. Semiconductor light matrix (SLM) technology provides lights that can be plugged into simple 110-v outlets. The systems are small and light, and can be controlled by ordinary laptop computer. SLM lights can also be tuned to precise wavelengths, allowing for a very effective match between coating and light.
There are, of course, drawbacks to such systems. Cure time is longer, by about an order of magnitude, and as yet the curing area is very small, about a square inch in size. Even accounting for these shortcomings, curing is accomplished in about one-twentieth the time needed for conventional technologies. For small, temperature-sensitive parts, SLM is certainly a curing technology to explore.
Larger parts typically require a substantial number of UV lights to cure, a requisite that can rapidly price the systems out of the market for many companies. A more cost-effective emerging technology utilizes banks of lights with curing areas of eight sq ft or more. Thus, as UV coatings are coming of age, more practical curing options are become available as well.
Aside from cost cutting, what advantages can a finisher derive from pursuing 100% solids UV-curable technology? There are considerable environmental advantages that also translate into nice efficiencies and economies in the face of increasing regulatory pressures.
A primary target of federal and state-level environmental agencies, volatile organic compounds (VOCs) are nearly non-existent in the 100% solids formulations; thus, reporting burdens are significantly diminished as are attendant emission costs. Because the coatings contain no carrier such as an organic solvent or water, they are also almost 100% reclaimable. Overspray can be captured, filtered, and re-used without modification. In addition to increasing material efficiencies, reclaiming reduces the production of waste. Since no flammable solvents are used, the small amount of waste generated is usually considered non-hazardous, further easing regulatory issues associated with
Offering a final practical advantage to finishers, 100% solids UV-curable coatings can also exhibit performance characteristics equal or superior to those of powder coats and conventional liquid coatings. Marking a crucial step in that direction, our company produced the first non-powder coat technology to pass Toyota’s rigorous long-term abrasion testing. That coating has since been licensed to DuPont, a solid indicator of the economic advantage inherent in a technology exhibiting those combined application and performance characteristics. Additional performance results for that coating at thicknesses of 1.1–1.55 mils include:
Initial Ford Adhesion: Pass
Cleveland Humidity, 110°F, 10 day: Pass
Chip Resistance -40°C: 1A
Gasoline Resistance: Good
65°C 10% Sulfuric Acid Etch: First mark 12 min, first etch 16 min
Boiling water crack resistance: Pass.
Coating performance can be even more impressive when nanoparticles are employed. Nanoparticles can be used in 100% solids-UV curable coatings to improve rheological properties, resulting in clear improvements in leveling and edge coverage when finishing metal parts.
Nanoparticle additions can also improve hiding power. Incorporation of nanoparticles resulted in hiding power at 1.2 mils equivalent to that of a similar black, pigmented coating 1.5 mils thick.
Corrosion resistance is an important issue in metal finishing, and, with regulations regarding the use of chromium recently tightened, manufacturers and finishers are increasingly pursuing alternative methods. When corrosion takes place, a part is essentially part of a galvanic cell. A sacrificial pigment corrodes instead of the part itself. Nanoparticles do not act in a sacrificial manner. Rather, they create a barrier to both water and air, preventing the formation of such a cell.
This is particularly apparent in coatings which contain both nanoparticles and those in the micron-sized range. The intermingling of particles effectively fills space, blocking entry by water. Dense applications may block air as well. Nanoparticles also fill tiny flaws, making the barrier even more effective.
A sufficient loading of nanoparticles will also further increase abrasion resistance in 100% solids UV-curable metal coatings. These types of improvements are most easily seen in resistance to falling sand. Metal finished with such a coating can easily endure 60 liters of sand without wear-through, almost quadruple the performance of nanoparticle-free equivalents.
Nanoparticles can also enhance abrasion resistance in clearcoats for plastic without sacrificing clarity, and can maintain clear-coat flexibility. The following results were achieved on polycarbonate film:
Thickness: 3.668–28.8643 microns
Light Transmission: 91.8–92.6
Adhesion, #610 tape crosshatch: Pass
Wrapping on 5/8-inch rod: No cracking
Refractive Index: 1.4988–1.5177
60 double rubs 0000 steel wool: Pass
Radcat (adhesion after humidity): Pass
Nanoparticles can be used in both high-gloss and flat coatings. Our company has developed techniques to disperse nanoparticles in monomer components in such a way that we have produced liquid components for the matting of 100% solids UV-curable coatings. When used in an appropriate formula, those coatings can achieve a 60° gloss of 11, an especially useful characteristic for achieving a natural look on wood. There is almost no increase in the viscosity of these matted coatings, so special application methods are unnecessary. Curing is much the same as with the corresponding glossy coating.
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