A previous thin film infrared spectroscopy study on acrylic/melamine coatings revealed that cross-link scission accelerates during exposure to ultraviolet light. Cross-link scission results in regeneration of copolymer hydroxyl consumed during cure and production of melamine methylol groups. These products suggested a hydrolytic process. Condensation of melamine methylol groups was found to yield melamine-melamine cross-links, releasing formaldehyde as a byproduct. The present work reexamines the nature of the cross-link scission process. Fourier transform infrared spectroscopy is used to follow formaldehyde release during exposure as a function of exposure conditions and coating composition. Coating doped with a hindered amine light stabilizer based nitroxide is found to release formaldehyde at a greatly reduced rate. This result indicates that the mechanism for cross-link scission during ultraviolet light exposure is free-radical in nature. Comparing formaldehyde release rate with cross-link scission rate during photolysis reveals that >75% of the formaldehyde formed as the result of melamine methylol condensation fails to escape the coating. The implications of low levels of formaldehyde on coating oxidation chemistry are discussed.

The photostabilization chemistry of a two-package acrylic/urethane coating containing a benzotriazole UV absorber and/or a hindered amine light stabilizer has been studied by using FTIR and ESR. FTIR has been used to follow the rate of chemical change on exposure of the coatings to UV light and humidity. The coatings are found to undergo photooxidation and loss of urethane cross-links during exposure. In contrast to melamine cross-linked coatings, humidity plays only a minor role in the degradation of the urethane coating. The addition of a benzotriazole UV absorber reduces the rate of degradation by reducing the rate of formation of free radicals. The addition of hindered amine greatly reduces the rate of oxidation and urethane loss. The hindered amine is significantly more effective in the urethane coating than in similar melamine coatings. ESR measurements of nitroxide concentrations in hindered amine containing coatings reflect the differences in stabilization chemistry. In addition to being more effective, the effective life of the hindered amine stabilizer is also longer in the urethane than in the melamine coating.