New method for predicting lifetime of seals from compression stress relaxation experiments

Interpretation of compression stress-relaxation (CSR) experiments for elast omers in air is complicated by (1) the presence of both physical and chemic al relaxation and (2) anomalous diffusion-limited oxidation (DLO) effects. For a butyl material, we first use shear relaxation data to indicate that p hysical relaxation effects are negligible during typical high temperature C SR experiments. We then show that experiments on standard CSR samples (simi lar to 15 mm diameter when compressed) lead to complex non-Arrhenius behavi or. By combining reaction kinetics based on the historic basic autoxidation scheme with a diffusion equation appropriate to disk-shaped samples, we de rive a theoretical DLO model appropriate to CSR experiments. Using oxygen c onsumption and permeation rate measurements, the theory shows that importan t DLO effects are responsible for the observed non-Arrhenius behavior. To m inimize DLO effects, we introduce a new CSR methodology based on the use of numerous small disk samples strained in parallel. Results from these par a llel, mini-disk experiments lead to Arrhenius behavior with activation ener gy consistent with values commonly observed for elastomers, allowing more c onfident extrapolated predictions. In addition, excellent correlation is no ted between the CSR force decay and the oxygen consumption rate, consistent with the expectation that oxidative scission processes dominate the CSR re sults.