Review of Some Important Issues and Resolutions When Lifetime Predictions of Elastomers Are Made Using Accelerated Aging Techniques

Elastomeric materials are often exposed to air or high humidity environments.  For materials expected to last for extended periods of time (years to decades), lifetime estimates normally involve extrapolating results from accelerated aging studies made at higher than ambient environments.  This talk will review several of the potential problems associated with such studies, outline experimental and theoretical methods to determine the importance of these potential problems and offer ways around such problems.  In particular we will first emphasize the importance of verifying time-temperature superposition of degradation data as evidence that the chemistry underlying degradation does not change as the accelerated temperature changes.  We will then describe in detail the confounding effects that occur when diffusion-limited oxidation (DLO) enters under accelerated conditions and show that its importance can be modeled for a material by measurements or estimates of its oxygen permeability coefficient (P_Ox) and its oxygen consumption rate (f).  It turns out that measurements of P_Ox and f can be influenced by DLO and we will describe how confident values can be derived.  In addition, we will describe several experimental profiling techniques that screen for DLO effects.  We then will show how values of f taken from high temperature to temperatures approaching ambient can be used to more confidently extrapolate accelerated aging results for air-aged materials and that many studies now show that Arrhenius extrapolations bend to lower activation energies as the aging temperature is lowered.  Finally we will describe the best approach for accelerated aging extrapolations of humidity-exposed materials.