Predicting Polymer Degradation and Mechanical Property Changes for Combined Aging Environments

Accelerated aging models are needed when two environments impact degradation and the current paper considers combined radiation-temperature environments (CRTE). A viable model described in the past is the dose to equivalent damage (DED) model which assumes that in a CRTE, by accelerating the thermal-initiation rate by a factor x (from Arrhenius T-only analysis) and raising the radiation dose rate by x leads to a factor x increase in the combined degradation rate. We critically review the DED model, using a 2-D plot that describes the model’s underlying framework and how its assumptions can be tested using so-called matched accelerated conditions (MAC) which reflect the simple acceleration factor assumption above. Historical data on several elastomers not only confirms the model assumptions, but shows that substantial degradation chemistry changes occur as one transitions from thermo-oxidative dominated to radiation-dominated degradation. This latter, but not unexpected observation, handled appropriately in the DED approach, conflicts with the primary assumption used in an alternative but highly-empirical past model and therefore eliminates this alternate model from consideration.

Given the chemical changes that depend upon the particular mix of radiation plus temperature, accelerated aging simulations should ideally choose accelerated conditions along the MAC line that intersects the ambient conditions, leading to more confident lifetime predictions. Finally we show that these same concepts lead to an approach that allows remaining lifetimes of ambiently aged samples to be derived using a 2-D analogy to the 1-D Wear-out approach developed previously for thermo-oxidative aging.