Lifetime prediction in the case of radiooxidative ageing of an ethylene-propylene rubber used for electrical insulation

A new approach of the 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 radiolitically initiated oxidation in non diffusion controlled regime and (IV) diffusion controlled radiochemical oxidation. The corresponding boundaries can be drafted from experimental data on thermal oxidation at moderate temperatures (typical scale time 1 week-1 year), and on radiochemical oxidation in a wide dose rate range (typically 10-1000 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 110, 125 and 135 degrees C without irradiation