Table 7. Surface Tensions of Organic Solvents and Reactive Diluents Solvent / Reactive Diluent
Surface Tension (dynes / cm2)
Isopropyl Alcohol1
21.3
t-Butyl acetate
22.4
1
Butyl Acetate1 Methyl Amyl Ketone
25.1 1
Dimethyl Carbonate
26.1 22.3
Acetone1 1
28.5 29.5
IBOA
2
TPGDA2
32
HDDA
34.5
2
TMPTA2
35
PETA2
38
Eastman Chemical Company Solvent Selector Chart, 2016 2 Eternal Chemical UV Curable Materials Monomers and Oligomers, 2013 1
Table 8. HAP Listed Solvents Benzene
Methyl Ethyl Ketone
Diethanolamine
Methyl Isobutyl Ketone
Ethylene Glycol
Toluene
Hexane
Xylene
Methanol Clean Air Act Amendment of 1990
scrap rates. In addition, for low-gloss coatings, as the solvent evaporates, it carries the silica matting additives to the surface and improves the appearance of matte finishes. As the name suggests, organic solvents also are beneficial when needing to dissolve solid raw materials, such as photoiniators and UVA/HALs. Although organic solvents offer multiple benefits, they are considered destructive to the environment and may require cumbersome permitting and regulations. In addition, there are some solvents that are considered Hazardous Air Pollutants (HAPs). These are solvents that are known to cause cancer and other serious health impacts. Even stricter regulations are required for these materials. A list of some common HAP solvents is in Table 8. Like aggressive reactive diluents, certain solvents can attack sensitive substrates. This likelihood increases as flash time/ temperature increases. Slower evaporating solvents will greatly improve flow and leveling; however, the slower the evaporation, the longer the dwell time in the flash oven – which, consequently, lengthens processing time and can potentially lead to substrate attack. uvebtechnology.com + radtech.org
Weatherable, scratch-resistant clear coat on polycarbonate headlamp lens is UV cured. Photo courtesy of Red Spot Paint and Varnish.
Safety Considerations In general, UV-curable materials have low volatility; however, during spray applications, aerosols are generated that may cause issues if not properly addressed. When at all possible, engineering controls, such as mechanized spraying, should be utilized to minimize human exposure to aerosol mists. Always wear personal protective equipment, as listed in the safety data sheet of each coating. Spray booth equipment must be properly bonded and grounded. If solvents are used to reduce coating viscosity, the area must be explosion-proof as well. Local exhaust/ventilation is imperative to minimize worker exposure to generated vapors. Since UV-curable materials will not cure unless exposed to UV, coating overspray also will need to be addressed. The spray booth can contain either filter media or water. Either system must follow state and local disposal regulations. Another option is to collect overspray to re-use in subsequent applications. If it has been reduced with solvent, solids of the collected overspray will need to be measured and adjusted accordingly prior to re-spraying. Because all coating compositions vary, it always is advised to contact the coating manufacturer for its recommendations on proper engineering controls and to follow state and local regulations. Water Water-reducible UV coatings also can address several of these issues. Water is not under regulation by the Environmental Protection Agency and is not a skin or eye irritant. Many finishers are finding success with aqueous UV-curable coatings. As with all options, there are drawbacks. If the substrate is nonporous, flash times can be longer to ensure evacuation of all the water. Additionally, some co-solvents, which can be subject to regulations, still may be necessary to assist in water evacuation. Although improving, water-reducible oligomer choices are limited, especially if exterior durability is needed. In addition, a limited number of photoiniators and additives are compatible page 54 UV+EB Technology • Issue 2, 2019 | 53