Abstract
Photooxidation of bis (2,2,6,6-tetramethylpiperidinyl-4) sebacate to nitroxide has been followed by electron paramagnetic resonance in acrylic melamine coatings under a number of ultraviolet light exposure conditions. The strong relationships observed between nitroxide kinetic behaviour, coating composition and exposure condition suggest that nitroxide kinetics analysis could be useful in probing coating photodegradation. This approach, together with infrared spectroscopic measurements of chemical changes on degradation, has suggested a novel mechanism, viz. photo-assisted hydrolysis, to explain the observation that photodegradation in acrylic melamine coatings proceeds more rapidly under more humid conditions. Photo-assisted hydrolysis of melamine ether crosslinks yields formaldehyde which can promote free radical oxidation leading to a more rapid photodegradation of these coatings under humid conditions than under dry conditions.
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