Conventional infrared spectroscopy is used to investigate the degradation region after electrical treeing in polymers. However, the dimension of the infrared light spot is too large compared with the tree channel, and the position of the light spot is imprecise. Consequently, the micro-infrared spectroscopy is proposed in this paper, which is aim to do chemical analysis for electrical tree area in polyethylene at a micrometer scale. A ramped ac voltage was applied for electrical tree inception in polyethylene, utilizing a wire-plane electrode structure. Slices of samples with electrical trees were then analyzed through micro-infrared spectroscopy. The characteristic group distribution within a selected treeing area was provided through micro-infrared mapping. The experiment results indicated the existence of C=O, C-OH and C-O-C groups, and the content of C-OH and C-O-C groups was more than C=O group. The appearance of C=O group was accompanied with the C-OH group, and the latter absorbance is higher, so it is inferred the C=O group tend to degrade further according to the Norrish type II regime. In addition, not all the electrical tree area had the oxygen containing groups. The material degradation during electrical treeing was discussed with and without oxygen.

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