Interpreting weathering acceleration factors for automotive coatings using exposure models

A number of exposure models have been developed to account for the variation in photo-oxidation rate with light intensity, wavelength distribution, temperature and humidity. These models have proven useful in accounting for the different observed rates of photo-oxidation and other weathering phenomena in different outdoor locations. In this paper, the same models are applied to calculate acceleration factors of various accelerated weathering :tests relative to standard outdoor Florida weathering. The predicted acceleration factors are in reasonable agreement with the ranges commonly observed for various accelerated tests. The models also can be used to assess the sensitivity of the acceleration factors to material differences and to variations in environmental conditions within a given chamber and from chamber-to-chamber. The critical importance of variability to the reproducibility and reliability of accelerated test results are discussed. Difficulties of comparing measurements made outdoors with values for accelerated tests are also discussed. Of particular note is the need for all exposures to measure and report actual sample temperatures rather than air or black panel temperatures. (C) 2000 Elsevier Science Ltd. All rights reserved.