Influence of Liquid Media on Lifetime Predictions of Nitrile Rubber

Evaluating a material's suitability for an application includes determination of its expected service lifetime. For alternative fuels, this entails assessing, inter alia, their effect on the durability of polymeric engine components, e.g., seals, gaskets, and O-rings. When this is governed by thermally activated chemical deterioration, the conventional approach to characterizing aging is laboratory measurements of property changes of the polymer subjected to accelerated conditions (usually higher temperatures), with the data analyzed by an Arrhenius analysis. However, this method is inefficient and time-consuming when the number of candidate alternative fuels is large. Herein, we test the hypothesis that the activation energy governing thermal oxidation of elastomeric engine components is independent of the fuel; thus, while the aging rate may vary, the effect of temperature is independent of the contacting liquid. Accelerated testing of the thermal oxidation of nitrile rubber O-rings were carried out in three liquids, including a fossil fuel and a bio-fuel. The activation energy obtained from changes in crosslink density, 582 kJ/mol, was the same for all liquids and consistent with the broad range of literature values for similar compounds aged in air. This result suggests the possibility that estimates of the lifetime of polymeric engine components require only a single accelerated aging test, with the known activation energy used to predict the durability at the service temperature. This would represent at least an order of magnitude reduction in testing requirements. The extension of the approach to the general aging of polymers exposed to different environments is obvious. V C 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40296.