Toward High Glass-Transition Temperatures in Epoxy Powder Coatings

By Vinay Mishra , Kevin Biller, Jeff Dimmit , Nikola Bilic

Epoxy-based products are used in many applications that face aggressive operating environments. Some examples are electrically insulative encapsulants or powder coatings (e.g., inside electrical motors), chemical-resistant pipe linings, adhesives, and composite parts. Under extreme service conditions (e.g., regarding heat, chemical exposure, and mechanical stresses, often in combination), many traditional epoxy formulations fail because they suffer from a loss of integrity over time. Traditional solutions require switching from epoxy to alternate chemistries such as cyanate ester, bismaleimide, and polyimides, which, while suitable, can add complexity to the process and increase cost. Aromatic dianhydrides such as BTDA® (3,3’,4,4’-benzophenone tetracarboxylic dianhydride) have been known to impart high crosslinking densities to epoxy formulations. The resulting dense crosslinking, in combination with the structure of the BTDA linkages, leads to epoxy powder coatings with high glass-transition temperatures (Tg) and heat resistance. These formulations also offer superior dielectric properties, mechanical properties, and chemical resistance. As a result, BTDA-based powder coatings find uses in aggressive environments such as those that are high temperature, involve chemical exposure, or are for long-term electrical applications. It is noteworthy that such successes are achieved using simple, bisphenol-A based solid epoxy resins. Specialized resins such as epoxy novolacs and other multifunctional resins can certainly raise the performance but are not necessary when using a dianhydride curing agent.