Relating laboratory and outdoor exposure of coatings: IV. Mode and mechanism for hydrolytic degradation of acrylic-melamine coatings exposed to water vapor in the absence of UV light

Abstract

Acrylic-melamine coatings are known to be susceptible to hydrolysis when exposed to water or humid environments. The mode and specific pathways for hydrolytic degradation of acrylic-melamine coatings exposed to water vapor in the absence of ultraviolet light are presented. Samples of a partially methylated melamine-acrylic coating applied to CaF2 substrates were subjected to five different relative humidity levels ranging from approximately 0 to 90% at 50°C. Coating degradation was measured with transmission Fourier transform infrared spectroscopy (FTIR) and tapping mode atomic force microscopy (AFM). In humid environments, partially methylated melamine-acrylic coatings undergo hydrolysis readily, causing considerable material loss and formation of mainly primary amines and carboxylic acids. The rate of hydrolysis increases with increasing RH. Hydrolytic degradation of acrylic-melamine coatings is an inhomogeneous process in which pits form, deepen, and enlarge with exposure. Such localized degradation mode suggests that hydrolysis of this material is an autocatalytic progression where acidic degradation products formed in the pits catalyze and accelerate the hydrolysis reactions.