Oxidation of cross-linked polyethylene due to radiation-thermal deterioration

Depth profiles of the oxidation degrees of cross-linked polyethylene (XLPE) cable insulation irradiated by gamma rays at dose rates of 3, 17, and 100 Gy/h at 80°C were measured with a micro FT-IR, and the relationship between the oxidation degree and the elongation at break was examined. The oxidation degree was defined as the ratio of the IR absorption peak for the C=O group to that for the C-H group, and after full deterioration the oxidation degree was distributed almost uniformly with depth for the different irradiation rates of 3, 17, and 100 Gy/h, although near the initial stage of deterioration for the cases of 3 and 17 Gy/h, the oxidation degree showed the largest value at the inner and/or outer surface and was distributed non-uniformly with the depth. The elongation at break decreased with an increase in the average oxidation degree, defined as the mean value of the oxidation degrees at four different depths. The relationship between the average oxidation degree and the elongation at break may be expressed by a single curve even when the dose rates were different, implying that the same mechanism was responsible for the decrease in the elongation at break due to oxidation at different dose rates from 3 to 100 Gy/h. Experimental data shown in this study may be useful in considering the relationship between the deterioration mechanism under accelerated-deterioration environments and that under the actual environments.