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20 POWDER COATING, November 2016 UV-curable powder coatings, Part I—Adhesion to plastics Owing to the development and applica- tions of new materials such as plastics and composites, a key driver for the powder coating market is the ability to coat these types of substrates. Unlike metals, plastic and natural materials are unable to withstand high curing tem- peratures, thus making the develop- ment of low temperature powder coat- ings critical for these heat-sensitive sub- strates. Besides low melting tempera- ture components, ultraviolet (UV) cur- ing technologies offer many advantages, including ease of handling, fast curing compared to thermal powder coatings, environmental compliance, and lower energy consumption. Developed in the 1990s, UV-curable powder coatings contain resin, pho- toinitiators, additives, and pigments. When irradiated with UV light, pho- toinitiators are activated to induce the free radical polymerization of unsatu- rated vinyl or acrylic monomers and oligomers of the resin. Typical energy sources are a combination of low heat (about 90°C to 140°C) and infrared (IR) radiation. Typical examples are alpha-hydroxylketone in combination with bisacylphosphine oxide (BAPO) type photoinitiators. The former is acti- vated by shorter wavelength UV light, which has higher energy density, while the latter BAPO type initiators absorb at longer wavelength UV light, which exhibits deeper penetration to allow thorough cure of highly pigmented powder coatings. In addition, UV light absorbers (UVA) and hindered-amine light stabilizers (HALS) do not interfere with photoini- tiation of BAPO. Depending on pig- ment level, color, and thickness of the coating, it is critical to have the proper combination of photoinitiators, UV wavelength, and energy dose to ensure complete cure. While UV-cured powder coatings offer numerous advantages, their high crosslink density, absence of solvent, and high surface tension of certain plas- tic substrates make it more difficult to achieve adequate wettability and coat- ing-substrate adhesion. To improve this adhesion, higher surface energy sub- strates are required for UV-cured pow- ders than conventional powders. This is typically achieved by surface treatment methods such as chemical, flame, corona, and plasma treatments. Among these processes, atmospheric plasma treatments for surface modifica- tions of plastics such as polyolefins, poly(meth)acrylates, and polycarbon- ates is useful because it operates at at- mospheric pressures and requires much lower voltage levels compared to corona t r e a t m e n t s . A i r p l a s m a c o n t a i n s charged gas molecules, which pass through the surface and create polar groups such as hydroxyl, carbonyl, and carboxyl groups, thus increasing the wettability of the surface. Another alter- native is the use of microwave plasma reactors in the presence of proper gases, which leads to the formation of polar surface groups 1 . However, this process is more applicable to objects like tubing or small plastic parts. Realizing the applicability to various substrates as well as the cost, energy, and environmental benefits, the adoption of UV-cured powder coatings in combi- nation with new surface treatment tech- nologies is critical for driving new mar- kets in the powder coating industry.PC Endnote 1. Microwave Plasma Reactions of Solid Monomers with Silicone Elastomer Surfaces: A Spectroscopic Study. S.R. Gaboury and M.W. Urban, Langmuir, 1993, 9. 3225-3233. Editor's note For further reading on raw materials for powder coatings, see Powder Coating magazine's website at www.pcoating .com. Click on Article Index and search by subject category or click on Book- store. For troubleshooting or to submit a question, click on Problem Solving. Marek W. Urban is J.E. Sirrine Foundation endowed chair and pro- fessor in the Department of Materials Science and Engineering at Clemson University, 299 A Sirrine Hall, Clemson, SC 29634; 864/656- 1155; mareku@clemson.edu. Until 2012, he was a professor of polymer sci- ence and engineering in the School of Polymers and High Performance Mate- rials at the University of Southern Mis- sissippi-Hattiesburg (USM). After 13 years at North Dakota State University, he joined USM where he established and directed the National Science Foun- dation Materials Research Science Engi- neering Center (MRSEC) on Stimuli- Responsive Polymeric Films and Coatings. He is co-director of the for- mally National Science Foundation Cooperative Research Center in Coat- ings and recently renamed Interface. He received an MS in chemistry from Mar- quette University, Milwaukee, Wis., and a doctorate in chemistry and chemical engineering from Michigan Technolog- ical University, Houghton, Mich. He has also done postdoctoral work in Powder Coatings Clinic Marek W. Urban, Ph.D. Clemson University Ying Yang, Ph.D. Clemson University