Effects of Hygrothermal Ageing on Breakdown Performance of Polyesterimide Nanocomposites

Polyesterimide and its nanocomposites are widely applied in motors and generators. However, the electrical properties of them deteriorated under the hygrothermal environment, which is a critical problem for safe operation of motors and generators in power systems. Therefore, it is urgent to study the mechanism of electrical properties during hygrothermal ageing. In this paper, neat polyesterimide and polyesterimide nanocomposites were prepared. The specimens were exposed in an oven with 100% relative humidity and 80°C for two weeks. The changes of specimen mass were measured to acquire moisture absorption. The breakdown experiment and the thermally stimulated current experiment were conducted. It is found that the trap level decreases after hygrothermal ageing process, which leads to the promoted carrier migration and the deceased breakdown strength. In addition, as the ageing time increases, the reduction proportion of breakdown strength of polyesterimide nanocomposites is smaller than that of neat polyesterimide. It is inferred that the interaction zone induced by nanoparticles, on one hand, increases the deep trap level. On the other hand, the bonding between nanoparticles and the matrix is enhanced by the surface treatment on nanoparticles, resulting in improved property stability of nanocomposites in the hygrothermal ageing.