Degradation processes in polyolefins with phenolic stabilizers subjected to ionizing or non-ionizing radiation

Abstract

We investigated degradation processes in polymer plaques made of polyolefin (HDPE or UHMWPE or COC) prepared by melt-mixing with or without phenolic stabilizer (natural α-tocopherol or synthetic Irganox®1010) and spin trapping agent (TTBNB; 2,4,6-Tri‑tert-butylnitrosobenzene). The degradation was initiated either by low-energy, non-ionizing radiation (wavelengths corresponding to terrestrial range of solar UV radiation) or high-energy, ionizing radiation (electrons accelerated at 1.5 MeV). The motivation for the study was our recent finding that some phenolic stabilizers exhibited a surprising pro-oxidant activity in polyolefins subjected to non-ionizing radiation. Consequently, we asked if the phenolic stabilizers in polyolefins behave in the same way during long-term, low-energy, non-ionizing UV irradiation and after short-term, high-energy, ionizing e-beam irradiation. All samples were characterized thoroughly by means of electron spin resonance (ESR; the information about radiation-induced radicals), infrared microspectroscopy (IR; the detection of oxidation products and structural changes), instrumented microindentation (MHI; local mechanical properties) and light and electron microscopy (LM and SEM; surface morphology). The results proved that (i) the degradation processes in polyolefins subjected to non-ionizing or ionizing radiation were different, (ii) the phenolic stabilizers exhibited mostly their expected antioxidant activity, while pro-oxidant activity was detected only for specific conditions and/or systems subjected to non-ionizing radiation, and (iii) the selected spin trapping agent, TTBNB, was stable enough to survive standard sample preparation by melt-mixing and catch short-living unstable radicals.