Dielectric breakdown and morphological evolution of PTFE during thermal-oxidative ageing at temperatures lower and higher than the melting temperature

The organic electric insulator in polytetrafluoroethylene (PTFE) is widely used under extreme conditions of temperature and electric field. Nowadays, operating temperature of this kind of insulators can reach about 350 °C for new generations of machines, which is above than the melting temperature of the material (327 °C). These severe operating conditions can be factors of ageing acceleration and/or degradation of the insulators. Currently, there are very few specifications on the insulators for high temperature and/or high voltage applications, furthermore their ageing and degradation patterns are not well known. Our present work is focused on the organic insulator behavior at high temperature under oxidizing atmosphere in order to characterize the mechanisms of thermal-oxidative ageing and degradation as well as the impact on the dielectric breakdown strength. Thin films in PTFE were aged by severe thermal stresses between 250 and 450 °C under oxidizing environment were characterized by using weight loss measurements, FTIR as well as dielectric breakdown under AC voltage. The experimental results show that irreversible chemical ageing occurs at about 100 °C higher than melting temperature. PTFE losses it´s thermal stability and electrical performance at 400 °C; no noticeable thermal decomposition was revealed at 340 °C until 300 h of ageing. The degree of crystallinity has tendency to increase at early stage of ageing (340 °C), then it decrease for samples aged at very high temperature during long time (400 °C during 300 h and 450 °C during 50 h).