Comparison of radical processes in non-aged and radiation-aged polyethylene unprotected or protected by antioxidants

Abstract Silane cross-linked polyethylene was examined by electron paramagnetic resonance (EPR) spectroscopy before and after gamma-irradiation at a dose rate of 77.7 Gy/h to a dose of 374 kGy, which corresponds to approximately 40 years of radiation aging in nuclear power plants (NPP). Radicals generated after post-aging by gamma radiation at 77 K (10 kGy, 2.2 kGy/h) were identified in a polymer unprotected, doped with phenolic antioxidant, thioether antioxidant or both. The radical processes in non-aged and long-term aged polyethylene differed significantly. In the second case, peroxyl radicals dominated at lowest temperatures, while the content of secondary alkyl radicals increased gradually during thermal annealing to 220 K, whereas above ambient temperature tertiary or primary alkyl radicals appeared. The stability of alkyl radicals was related to the rapid consumption of oxygen and, consequently, its depletion in earlier stages of radical processes. Phenolic antioxidant suppressed the above effects and protected the cross-linked structure of the polymer matrix.