THE ROLE OF HYDROPEROXIDES IN THE PHOTOOXIDATION OF CROSS-LINKED POLYMER-COATINGS

Iodometric titration has been used to measure hydroperoxide concentrations in a series of unstabilized acrylic melamine and acrylic urethane coatings as a function of laboratory weathering time. Hydroperoxide concentrations are found to be a function of the initial free radical formation rates, the exposure conditions used, and the crosslinker type. For coatings with high initial free radical formation rates, the hydroperoxide level rises rapidly to a maximum and then decreases. For a given copolymer, the hydroperoxide level in the melamine crosslinked coating is always significantly lower than that in the urethane crosslinked coating. For coatings with low initial radical formation rates, the initial rise in hydroperoxide concentration is small. The hydroperoxide concentration in the melamine coating remains small, while that in the urethane coating slowly increases suggesting autocatalytic oxidation. Hydroperoxides contribute significantly to free radical formation at long exposure times, especially in the urethane coatings. The lower level of hydroperoxide in the melamine coatings is attributed to the ability of the melamine crosslinker to decompose hydroperoxides.