Relating product service lifetime to material composition by using standardized, artificial exposures, or even natural exposure, is notoriously difficult. An approach is reviewed here that has been developed to model degradation in polymeric coatings and may be applicable to other systems when they are exposed to natural or artificial weathering conditions. The approach focuses on the 'wear-out regime' not 'infant mortality' and breaks down the problem into units, which can be separately addressed. Random arrival, and action, of huge numbers of aggressive events, e.g., ultraviolet photons, grains of sand, etc., cause the accumulation of damage which may be modeled, in general, by Monte Carlo techniques or algebraically in simple cases, using the properties of large numbers of low probability events by the Central Limit Theorem. The model shows how degradation rate depends on physical and chemical materials characteristics, the initial state of the coating, and the parameters describing the environment. Well-known physical models for gloss and toughness provide a close match to experimental data and provide scientifically based, well-behaved functions for extrapolation. In addition, the effect of ultraviolet absorbers or anti-oxidants is modeled here in a simple way to demonstrate the flexibility of this approach.
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Acknowledgements This work was supported by the US Air Force Office of Scientific Research and the US Army Research Laboratory.
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