Radiochemical aging of polyethylene: a simplified approach for lifetime prediction

A simplified method is proposed to estimate the lifetime of polyethylene (PE) exposed to low dose rates at low temperatures, typically close to the ambient temperature, in air. The point of departure is the observation that, at a given temperature, the "ceiling lifetime" is the lifetime due to thermal oxidation. Thermal oxidation predominates at very low dose rates (for I < 2 × 10^-3 Gy s^-1). Oxidation is initiated by the thermal decomposition of hydroperoxide species and characterised by very long induction times, typically 15 years for unstabilised PE at ambient temperature. But at the end of the induction period, oxidation accelerates sharply and the lifetime criterion (linked to polymer embrittlement) is rapidly reached, so that the lifetime is t_FT ≈ 15 years. Above a critical dose rate (for I > 5 × 10^-1 Gy s^-1), polymer radiolysis becomes the main source of radicals and the lifetime is proportional to the reciprocal dose rate I: t_FR ≈ 3.6 × 10⁵ I^-1.1.