Abstract
The photo-initiation rate of free radicals in cured coatings can be measured using a free radical scavenger technique termed the nitroxide decay assay. Photo-initiation rate measurements on a series of weathered acrylic melamine coatings revealed that photo-initiation rate values drop rapidly from their initial value, at the onset of weathering, to lower values which remain constant, indicating non-autocatalytic photo-oxidation (Gerlock et al., Poly. Deg. and Stab., 20 (1988) 123). The sustained values reflect long term coating weatherability performance. In this paper, photo-initiation rates have been measured for a series of proprietary polyester urethane coatings. In contrast to the acrylic melamine results, photo-initiation rate values increase with weathering time, indicating that autocatalytic photo-oxidation occurs. The rate of increase varies with coating composition and weathering conditions. Small amounts of ultraviolet light at wavelengths shorter than 295 nm accelerate the onset of autocatalytic oxidation. Photo-initiation rate values that accurately reflect photo-oxidation processes occurring outdoors can be obtained with filtered xenon arc light.
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