Evolution of ethylene propylene copolymers properties during ageing

Ageing of Ethylene Propylene copolymer used as joints for electrical materiels in nuclear power plants has been studied in EDF (Electricite de France). The objective was to determine the elastomer service lifetime in normal and accidental conditions. This material is thermally stable up to 130 degrees C for long term service and it can be used until a radiation dose of at least 60 kGy. If the probability to have no more waterproofness in normal conditions during 30-50 years at 40 degrees C and at 0.01 Gy/h is low, it could be quite different in accidental conditions where the temperature could be higher than 150 degrees C. That is why we first studied the thermal ageing of this material at constant temperatures from 90 degrees C to 150 degrees C. During ethylene propylene copolymer ageing, we examined the evolution of mechanical properties and morphology. Mechanical properties such as elongation at break and tensile strengh allowed us to appreciate the material service lifetime according to IEC 544-2 (Guide for determining the effects of ionizing radiation on insulating materials). Besides, the evolution of morphology including density and oxidation was examined in order to determine whether crosslinking or chain scission was predominant at high temperatures. The lifetime prediction of ethylene propylene in accidental thermal conditions was estimated by applying a kinetic model where the constant depends on the Arrhenius law. In a second step, we proceeded to a radiation ageing on non-aged ethylene propylene copolymer samples according to normal and accidental conditions. The preliminary results obtained under gamma radiation are presented.