Degradation mechanisms of polyolefins by radiation and thermal ageing

Seguchi, Tadao*; Tamura, Kiyotoshi*; Kudo, Hisaaki*; Shimada, Akihiko; Sugimoto, Masaki

The degradation of ethylene propylene rubber (EPR) as the cable insulation material for nuclear power plants (NPP) was studied by the accelerated thermal ageing, radiation ageing and thermal - radiation combined ageing. The oxidation of EPR proceeded with both thermal and radiation and the decay of mechanical property was closely related to the degree of oxidation. The thermal oxidation was much affected by the content of antioxidant in EPR as a stabilizer, that is, the oxidation rate was almost same above a critical content of antioxidant but increased much by the decay less than the critical value. On the other hand, the radiation oxidation rate was scarcely affected by the content of antioxidant. As the antioxidant in EPR evaporates from the material surface during thermal ageing and decomposes with dose by the radiation ageing, so the oxidation distribution in the depth of EPR sheet specimen was changed with reflecting the content of antioxidant above or less than the critical value. In the case of combined oxidation of thermal and radiation ageing, the yield and distribution of oxidation in EPR specimen were different by the sequence of ageing combination. The mechanical properties were well reflected by the yield and distribution of oxidation products. As far as the antioxidant content was held above the critical value, the degradation was almost same among the sequence of thermal and radiation ageing, but when the antioxidant was decayed less than the critical value the degradation was much increased in the reverse sequential ageing. For the qualification testing method of the NPP cables, the reverse sequential combination, radiation then thermal ageing, could be recommended.