Degradation mechanisms of polyolefins by radiation and thermal ageing

熱及び放射線劣化におけるポリオレフィンの分解メカニズム

瀬口 忠男*; 田村 清俊*; 工藤 久明*; 島田 明彦; 杉本 雅樹

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

原子力発電所のケーブル絶縁材として用いられているエチレンプロピレンゴムと架橋ポリエチレンについて厚さ1-2mmのシート状試料を用いて、絶縁材に含まれている酸化防止剤が、熱、あるいは放射線劣化に及ぼす影響、並びに熱・放射線複合劣化における順序効果について調べた。熱劣化については、絶縁材に酸化防止剤が限度濃度(0.04%程度)以上含まれる場合は、劣化時間に比例して酸化が進行するが、酸化防止剤が限度濃度以下になると劣化が急激に進行する。一方、放射線劣化は線量に比例して進行するが、酸化防止剤は放射線劣化にはあまり影響を及ぼさない。熱と放射線の複合劣化では、酸化の度合いは劣化の順序により異なる。逐次法(熱劣化後に放射線劣化させる方法)に比べて逆逐次法(放射線劣化後に熱劣化させる方法)の方が、劣化が著しい。これは、放射線劣化により酸化防止剤が分解されることによりその後の熱劣化において劣化が急激に進行するためである。熱と放射線を同時に与える方法では、これらの2つの劣化の中間の挙動を示した。

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.