PHOTO-INITIATION RATE BEHAVIOR OF WEATHERED COATINGS - ESR-NITROXIDE DECAY ASSAY

A procedure is described to extend the nitroxide decay assay measurement of coating photo-initiation rates to weathered coatings. The procedure involves post-cure doping of coatings with 4-hydroxy-2,2,6,6-tetramethyl-piperidinyloxy free radical in the presence of CH//2Cl//2/CH//3OH vapor at 60 degree C. The photo-initiation rates of three acrylic-melamine coatings have been followed as a function of 'QUV-light only' accelerated exposure. For each of these coatings, the photo-initiation rate drops during the first 10-20 h of exposure and then remains constant at a level indicative of their weatherability for the duration of the exposure. No evidence is found for autoxidation in these coatings.