Linking Accelerated Laboratory Test with Outdoor Performance Results for a Model Epoxy Coating System

Laboratory and field exposure results have been successfully linked for a model epoxy coating system. The mathematical model used in making this linkage only assumed that the total effective dosage, additivity law, and reciprocity laws wet-e valid. In this study, accurate and time-based measurements on both exposure environments and degradation properties for polymer specimens exposed to accelerated laboratory weathering device and outdoor environments have been carried out. Laboratory weathering tests were, conducted in the extremely well-controlled NIST SPHERE. A factorial design consisting of four temperatures, four relative humidities (RH), four ultraviolet (UV) spectral wavelengths, and four UV spectral intensities was selected for the SPHERE exposure conditions. Based on SPHERE exposures, effects of-critical environmental conditions on chemical degradation of the UV-exposed epoxy materials have been assessed. The outdoor exposure experiments were carried out on the roof of a NIST laboratory, located in Gaithersburg, MD. The temperature and RH of the outdoor exposure were continuously recorded every minute and the solar spectrum was recorded every 12 minutes. The chemical degradation for specimens exposed to the SPHERE and outdoor environments was quantified by transmission FTIR and UV-visible spectroscopies. It is found that the mechanism of chemical degradation for samples exposed to outdoor environments is similar to those exposed to SPHERE. Three approaches, chemical ratios as a metric, a model-free heuristic approach, and a mathematical predictive model, have been used to combine the chemical degradation data from the SPHERE and the outdoor exposures. Successful link-ages have been made via all three approaches. It has been shown that the reliability-based methodologi, is capable of linking laboratory and field exposure data and predicting the service life of polymeric materials.