Deterioration of highly filled EPDM rubber by thermal ageing in air: Kinetics and non-destructive monitoring

The effects of air ageing at different temperatures between 110 and 170 °C on cable transit seals based on highly filled EPDM rubber used in nuclear power plants were studied. The changes of the macroscopic mechanical properties (Young's modulus, indentation modulus and strain-at-break) were in accordance with the Arrhenius equation with an activation energy of 110 kJ mol−1. Profiling to assess the structure and property gradients within aged blocks was performed via IR spectroscopy, micro-indentation, gravimetric analysis of n-heptane-extracted samples and non-invasive portable NMR spectroscopy. A previously developed methodology was used to separate the deterioration into three different processes: polymer oxidation that was diffusion-limited at all temperatures, migration of low-molar-mass species to the surrounding media and anaerobic changes to the polymer network. The methodology allowed the assessment of the kinetics (rate as a function of time and temperature) of the different processes. It was noticed that polymer oxidation yielded more crosslinking at higher temperatures than at lower temperatures. The data obtained by both the portable NMR (a non-invasive method) and the indentation modulus profiling showed correlations with strain-at-break data, indicating their usefulness as condition monitoring methods.