Overview of accelerated aging and polymer degradation kinetics for combined radiation-thermal environments

Abstract

Polymer aging under combined radiation-thermal oxidative conditions is intrinsically more convoluted than traditional thermal degradation. Accelerated aging methods for predictive purposes have to include thermal as well as radiative degradation pathways that initially may be regarded as independent parallel processes without additional synergism. Material aging is therefore represented as the sum of a thermal and radiative contribution. Data from accelerated aging may be available as dose to equivalent damage (DED) or degradation rates, yet they require different analytical approaches to yield the underlying temperature dependence and its activation energy. Further, kinetic models that embrace combined pathways can offer guidance for extrapolation of accelerated to ambient conditions, enabling the prediction of material aging behavior or remaining performance margins for requalification purposes. The existing theoretical approaches, their implications and an alternative option for globally fitting experimental data sets to kinetic aging models for combined environments are reviewed. This overview offers a pragmatic approach towards an expanded interpretation of oxidation rate and aging data properties for combined environments, all the way to time-dependence for rates and synergistic contributions. Further evidence is provided that for some material behaviors an additional Ea for the radiative term under high dose rate conditions could be beneficial, as similarly expressed by increases in a synergistic interaction parameter. Improved kinetic aging models are derived and applied to a comprehensive set of experimental oxidation rates for a chlorosulfonated polyethylene material. Emphasized is also the issue that initial oxidation rates versus superposition of oxidation levels (integrated rates) may result in slightly different thermal Ea values through added time dependency. Constant oxidation rates relate to an exponential decay in elongation at break data. Aging predictions can be improved through measured oxidation rates, a systematic understanding of material behavior over a large dose rate - temperature regime, and application of an appropriate aging model. The most general aging model will contain a radiative Ea, time-dependence of rate, and added synergism that may grow with temperature.