Investigation of dielectric properties of polyethylene terephthalate under different aging temperatures

The polyethylene terephthalate (PET) is widely used in dry-type insulation application because of its excellent insulation properties. In practice, thermal aging of PET is often encountered, but the related mechanisms and effect of thermal aging on the PET dielectric properties still remain unclear. In the present paper, we presented a systematic and comparative study on the effect of thermal aging on surface microscopic morphological characteristics, DC breakdown strength, trap depths and the apparent mobility. The results indicate that when aged at 130°C, the surface topography becomes coarser than that aged at 80°C. The polarity reversal at the PET/PET interface during the polarization indicates that the injection abilities of Al and Sc electrodes are gradually enhanced when aged at 80°C and 130°C, respectively. Moreover, the measured isothermal discharge currents during depolarization show that the trap depth corresponding to the maximum trap density increases significantly in the beginning of aging, and then it decreases gradually when aged at 80°C, but it increases evidently when aged at 130°C with increasing the aging degree. It also has been found that the apparent trap-controlled charge mobility is greatly enhanced when aged at 80°C with the increase in thermal aging degree. This enhancement is proved to be associated with the decrease in trap energy level at the PET/PET interface. Also, it can be preliminary concluded that the enhancement of DC breakdown strength (at 1500 h) is closely related to the increase in electron type trap energy and its density when aged at 130°C.