New understanding on rapid irradiation oxidation of polyethylene

To explore the ionizing radiation induced oxidation, three types of polyethylene (LDPE, UHMWPE and HDPE) were irradiated by gamma rays in Ar or air, and subsequently exposed to air. The evolution of carbonyl group and free radicals were studied by Fourier transform infrared microscopy measurement (FTIR) and electron paramagnetic resonance spectroscopy (EPR) techniques, respectively. It was found that all types of PE were rapidly oxidized during irradiation in air (i.e, direct irradiation oxidation) compared with storage in air after irradiation (i.e., post-irradiation oxidation). The oxidation product of carbonyl group on UHMWPE and HDPE for a 17-hour direct irradiation oxidation exceeded that for a 52-day post-irradiation oxidation. The carbonyl group on HDPE and UHMWPE increased gradually with time in the post-irradiation oxidation, while there was almost no such product formation on LDPE. EPR testing showed that the free radical decay on irradiated LDPE was much faster than that on HDPE and UHMWPE, indicating that the post irradiation oxidation was related to free radical oxidation. Additionally, ozone oxidation experiment and shielding irradiation oxidation experiment were designed and certified that the direct irradiation oxidation of PEs should be attributed to the highly active oxidative species induced in situ by gamma irradiation. Based on these findings, it was proposed that the rapid direct irradiation oxidation was a three-pathway oxidation reaction. This work would provide new insight to understand the direct irradiation oxidation of other polymers. ​