The role of thermo-oxidative aging at different temperatures on the crystal structure of crosslinked polyethylene

In order to investigate the role of thermo-oxidative aging on the crystal structure of crosslinked polyethylene (XLPE) at temperatures below and above the melting point, thermo-oxidative aging was conducted on XLPE cable insulation at 100, 120, 140 and 160 °C respectively. The crystal structure of the fresh sample and aged samples was characterized by X-ray diffraction, differential scanning calorimetry (DSC) and scanning electron microscopy tests. The concentration of antioxidants in XLPE samples was measured by the DSC method. It was deduced by the obtained results that the thermo-oxidative aging process could be divided into the physical aging stage and the chemical aging stage depending on the concentration of antioxidants in XLPE. In the physical aging stage, re-crystallization process led to the improvement of crystal structure and the increase in crystallinity. In the chemical aging stage, the breakage of intramolecular and intermolecular bonds led to drastic chains scission, resulting in the decrease in crystallinity. At 100 °C, the thermo-oxidative aging mainly affected the amorphous regions and had a limited effect on the crystalline regions. At 120, 140 and 160 °C, the thermo-oxidative aging affected the molten spherulites, resulting in decreased lamellae, increased lamellar spacing and the destruction of spherulites.