THE ROLE OF HUMIDITY IN THE PHOTOOXIDATION OF ACRYLIC MELAMINE COATINGS

Predicting the weatherability of acrylic melamine coatings commonly us ed as enamel clearcoats requires a detailed understanding of each of t he factors that influence photooxidation kinetics. Previous work1 has shown that the photo-oxidation rate in coatings can be written as the following function of hydroperoxide concentration: photooxidation rate = K[YOOH] + M. The existence of a measurable photooxidation rate in t he absence of hydroperoxide (i.e. a non-zero value of the intercept, M ) has been observed only in melamine crosslinked coatings. It has also been observed that the photooxidation rate in acrylic melamine coatin gs increases with increasing humidity. In contrast, for urethane cross linked coatings the value of M is zero, and the photooxidation rate is independent of humidity. In this paper, infrared spectroscopic measur ements of functional group changes (e.g. carbonyl growth and crosslink scission) are used to measure photooxidation rates in acrylic melamin e coatings during UV exposures at different humidities. Comparisons of these rates to measured hydroperoxide concentrations for the same coa tings and exposures reveal that the increase in photooxidation rate wi th humidity is due to the fact that the intercept M increases with inc reasing humidity. Since the intercept is zero under dry conditions, th e chemical reactions responsible for the intercept in melamine crossli nked coatings must involve both UV light and moisture. These results c onfirm the importance of accurately controlling the humidity during UV exposure for predicting the weatherability of melamine crosslinked co atings.