Fall 2001 Vol. 3, No. 1

Use of powder coatings is growing rapidly. However, surface defects such as orange peel and pinholing prevent the expansion of powder coatings into some markets. For the prevention of pinholes due to degassing, benzoin has been the favorite material because of its high efficiency and low cost. In fact, benzoin is used in virtually all powder coating systems worldwide. The prospect of developing an improved air-release agent has stimulated research on benzoin's role in degassing in powder coatings. In some cases, alternative air-release agents have been developed, but these alternatives generally fall short either in performance or in terms of economics.

In 1992, Jan Schippers at DSM filed a patent application claiming that crystalline waxes could be used to prevent pinholes without yellowing in Primid-based systems (3). This application claims that some crystalline waxes will prevent pinholes while avoiding the yellowing seen during curing in Primid systems with benzoin, especially under overbake conditions. Also in 1992, a patent by Stanislaw Skora claims a bisphenoxy propanol compound that can completely prevent pinholes in thermosetting powders when used at levels of 0.5 to 3% by weight (4). In a paper presented at the Waterborne, Higher-Solids and Powder Coating Symposium in 1993, William Arendt of Velsicol claims that 1,4-cyclohexane dimethanol dibenzoate (CHDM Dibenzoate) gave excellent coatings with less yellowing at levels three to nine times higher than that of benzoin in epoxy-hybrid systems (5). In a European patent application filed in 1993, Wilkinson and Phillips claim a solid thermoplastic polymer can be used in place of benzoin in Primid-based powder coatings to give superior color. The use of these solid thermoplastic degassing agents in other powder systems is mentioned with statement to the effect that differences in color are noted in other systems. This application is the first to attempt to explain how degassing or air-release agents work. The authors claim degassing agents have a surface tension effect that "holds the surface open" during cure (6). However, no data is offered to support the claim.