Global exposure models for automotive coating photo-oxidation

Successful prediction of long-term weatherability performance of organic coatings in different markets requires an understanding of the relationship between the rate of weathering in specific outdoor and accelerated tests with those observed in the field within that market. Since the rate of weathering is, to a large extent, controlled by the rate of coating photo-oxidation, the development of such relationships requires the following information: (1) identification of the critical environmental variables that control photo-oxidation rate; (2) determination of the dependence of the rate of photo-oxidation for coatings of interest on those variables; (3) determination of the distribution of the values of the critical variables in the given market relative to those for the controlled exposure. For general polymer photo-oxidation, the critical environmental variables are well known: light intensity, wavelength distribution, and temperature. For some polymeric materials, including those widely used in automotive coatings, humidity and moisture also play an important role in weathering. In this paper, three specific models for the dependences of coating photo-oxidation on these variables are developed. Using these models and measured or calculated distributions of light intensity, wavelength, temperature and humidity in specific automotive markets, predictions are made of the length of time required for standard outdoor exposures to insure performance in those markets.