Thermal properties and space charge behavior of thermally aged polypropylene/elastomer blends nanocomposite

Polypropylene (PP)-based composites including PP/thermoplastic polyolefin (TPO) elastomer blends and PP/TPO-based nanocomposite are rather promising insulation materials for HVDC cables. Aging is an inevitable issue in practical operation and needs to be investigated thoroughly. In this paper, PP/TPO blends and PP/TPO/ZnO nanocomposite were prepared and thermally aged at 130 °C for various periods (28 days at longest). The micro-morphology, thermal properties and high-temperature space charge behavior were investigated. The results show that PP and TPO still exhibited excellent compatibility without phase separation after thermally aging. The crystallinity was not influenced after thermally aging, but a new endothermic peak at about 140 °C appeared in the melting process of thermally aged composites. Space charges accumulated in the two thermally aged composites, but dissipated slowly in PP/TPO, while dissipated rapidly first and then reduced in PP/TPO/ZnO nanocomposite. The calculated apparent charge mobility and trap level distribution based on space charge dissipation show that deep traps increased in aged PP/TPO, but decreased in aged PP/TPO/ZnO nanocomposite, which may be correlated with the chemical variation of thermally aged polymer matrix. The work may give a reference on the aging research of PP-based composite material for recyclable HVDC cable insulation.