Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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application methods ULTRAVIOLET (UV) CURED COATINGS BY ANDREW A. SOKOL UV COATINGS LTD., CLEVELAND The sheer number of available alloys makes metal coating a complicated feat. Selecting and implementing an appropriate coating that will provide perfor- mance while minimizing cost and production problems is more art than sci- ence. Accomplished properly the coating of metal components should seem sim- ple and effortless providing years of trouble-free service. Simplicity is the prod- uct of a practiced professional whose job is just that — to produce a trouble- free product. Professionalism means continuous awareness of new processes and finishing methods with which comes continuous improvement in production and product quality. An exciting development in metal finishing is the use of UV-curable coatings for metal substrates. Recent developments in UV-curable coatings for metal re- sulted in a "lock-and-key fit" of protective and aesthetic properties for metal finishing. This combinatorial coating system combines the simplicity of applying a liquid coating and the best protective properties of powder and liquid coat- ings, hence the designation "Liquid Powder."1 This article shall provide an overview of UV technology, history, chemistry, and its safety record. A description of UV processing and its underpinnings will lead into a discussion of the chemistry of coating formulations and how they can be engineered to achieve a "lock-and-key fit" of coating properties. HISTORICAL OVERVIEW OF UV TECHNOLOGY Perceived to be a radical new technology, UV, or more accurately photopolymer chemistry, was born of wartime efforts to find a synthetic alternative to natural rubber. Those efforts laid the foundation for the commercial implementation of UV processing in the late 1950s and early 1960s. Early-on experiences with UV resins caused UV processing to be carefully scrutinized, which has result- ed in a remarkably detailed, understood, and favorable track record in long-term health and safety issues. Today's UV technology makes UV processing, if not the safest, one of the safest, and more efficient and profitable industrial process technologies available today. UV processing first found commercial viability in printing applications such as screen printing. "First generation" UV curable inks were very viscous, pastelike materials that could only be applied by means of physically transferring it onto a substrate. These primitive UV materials had associated with them severe skin, eye, and respiratory problems. These UV inks were observed to undergo a viscosity change with increasing tem- perature. It was an easy leap to guess that simply by heating the paste it may be possible to lower its viscosity enough to allow spraying it. Attempts to spray first-generation UV formulations resulted in widespread health problems — any- one exposed to the spray cloud would never want to spray the material again! The health issues were so widespread that even as late as the early 1990s the market held it as unthinkable to spray UV-curable products. Because of this deep-set negative market experience, coupled with the potential litigation exposure, coat- ing manufacturers halted or never implemented programs to investigate or de- 172

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