Study on melting characteristics of crystals in thermal aged XLPE cable insulation at elevated temperature

As a typical semi-crystalline polymer, the macroscopic properties of crosslinked polyethylene (XLPE) are closely related to the crystal structure. During service, thermal aging caused by overheating of the cable will cause the deterioration of the crystal structure, which in turn will cause the deterioration of macroscopic properties of XLPE. In this paper, 110 kV XLPE cable insulation was thermally aged at 160 °C. The melting characteristics of crystals in different aged samples and the influence of cooling rate after aging were investigated by Differential Scanning Calorimeter (DSC) Technology. The results show that the melting temperature (Tm) and the crystallinity of XLPE samples aged at 160 °C are basically unchanged and then decrease rapidly with the further increase of aging time. A new melting peak can be observed around 50 °C after 96 h. This peak moves toward lower temperature and the area gradually increases with the aging time. The appearance of this peak is mainly due to the destruction of the crystal structure by the oxidation reaction not the cooling process of the sample after aging, which can be confirmed by results of Oxidative Induction Time measurement and DSC curves of aged samples with different cooling methods. It is deduced that the oxidation reaction at elevated temperature can lead to drastic chains scission after antioxidants in XLPE are totally consumed. When the aged samples were taken out of the oven and then recrystallized, the number of molecular chains which can be folded to form lamellas decreases and the regularity of the crystals formed again becomes worse, resulting in the decrease in Tm and the crystallinity.