LIFETIME PREDICTION IN THE CASE OF RADIOOXIDATIVE AGING OF A ETHYLENE-PROPYLENE RUBBER USED FOR ELECTRICAL INSULATION

A new approach to lifetime prediction in radiochemical ageing of hydrocarbon polymers is proposed. It consists essentially of building a graph (temperature-dose rate) in which four domains are distinguished: (I) predominating thermal oxidation, (II) oxidation initiated by both thermolysis of hydroperoxides and polymer radiolysis, (III) predominating radiolytically-initiated oxidation in a non-diffusion-controlled regime and (IV) diffusion-controlled radiochemical oxidation. The corresponding boundaries can be drawn from experimental data on thermal oxidation at moderate temperatures (typical time scale, one week-one year) and on radiochemical oxidation in a wide dose-rate range (typically 10-2500 Gy h-1). This approach is illustrated by the example of an EPR industrial rubber for which the decrease of tensile ultimate properties has been studied at 40-degrees-C for dose rates of 10, 100, 500, 1000 and 2500 Gy h-1, at 50 and 90-degrees-C for a dose rate of 100 Gy h-1, and at 70-135-degrees-C without irradiation.