Transesterification cure for coatings: catalysis by epoxy and nucleophiles

Abstract

Transesterification cure in coatings can be catalyzed by an alkoxide which is formed during bake by the combination of an epoxide with a nucleophile. Support for transesterification cure in the presence of epoxides and nucleophiles has been provided by FTIR evolved gas analysis (EGA/FTIR) and thermal gravimetric analysis (TGA). Effective nucleophiles include tertiary amines, certain inorganic nucleophiles like cyanide, hydroxide and azide, and quaternary ammonium salts. Quaternary ammonium carboxylates are particularly effective nucleophile sources for this cure chemistry, especially those that have better thermal stability. EGA/FTIR was used to demonstrate differences in the thermal stability of quaternary ammonium salts, and these differences were correlated with differences in cure at higher bake temperatures. Transesterification cure of hydroxyl functional acrylics can provide greatly improved impact resistance if the acrylics are blended with polyesters. Dynamic mechanical analysis (DMA) has shown differences between acrylic and acrylic-polyester blends which may help to explain the differences in impact resistance.