2. Laboratory-Based Predictions of Weathering in Outdoor Environments over the Entire Degradation Pathway

A useful estimate of outdoor service life for a material or product based on laboratory weathering experiments requires a careful assessment of the degradation pathways that result from exposure. Furthermore, converting real-world conditions into parameters that serve as inputs to models based on the accelerated weathering stresses of radiation, heat, and moisture is not trivial. In an effort to study these relationships, a model material was weathered under accelerated conditions in the laboratory, from which mathematical formulas were derived to describe the resultant photodegradation rate as a function of irradiance and temperature. Calculations for a specific geographical location yielded degradation as a function of time that exhibited excellent agreement with actual outdoor weathering results over the entire degradation period. Variations on the method of calculation proved the mathematical model to be robust. Investigation of chemical degradation in the model material revealed the possibility of more than one reaction pathway. Such behavior is readily apparent in other polymer systems we have studied, wherein the exposure conditions employed can lead to a lack of synchronization of changes in the material or can produce significantly different degradation pathways, both of which affect lifetime estimates.