Effect of thermal aging on charge-trap characteristics of polypropylene films used in capacitors and high temperature failure mechanism

Polypropylene (PP) film has been widely used in capacitors because of its good mach-inability and insulation properties. In long-term operation, the high temperature easily causes material insulation performance decline or even failure, which is the key problem to affect the stability of capacitors. In this work, the effect of the thermal aging on the charge and trap characteristics from the microscopic view were studied, and the high temperature failure mechanism was further revealed. The results indicate that the elongation at break of the 28-day aged PP film decreases to 42.73% of the as-fabricated sample's value, meeting the predefined criterion for aging-induced failure. At this time, the breakdown strength of PP film decreases by 21.40%, trap density increases by 188.41%. The energy band structure of PP molecular chain was analyzed, and it can be found that the aging process of PP is accompanied by the generation of hydroxyl and carbonyl groups. Various defects complicate the distribution of space charge in the polymer, which acts together with high temperature destruction and eventually leads to PP failure. This work provides theoretical guidance for the study of the failure mechanism and aging state assessment of high voltage capacitors.