Morphological evolution of polytetrafluoroethylene (PTFE) during thermal-oxidative ageing above and below the melting temperature

The organic electric insulator polytetrafluoroethylene (PTFE) is used in aerospace industry under extreme conditions of temperature and electric field. The melting temperature of PTFE is about 327°C. In our days operating temperature of this kind of insulators could punctually reach 300°C and 350°C for the new generation machines. This last temperature is above the melting temperature of the material. These severe operating conditions can be factors of ageing acceleration and/or degradation of the insulators. In general, when a polymer is exposed to an important thermal stress, chemical degradation reactions may take place, such as the macromolecular chain scissions and the formation of new chemical groups. This may affect significantly the molecular weight, chains arrangement or the size distribution of macromolecules. Hence, all this processes may cause modifications of theirs properties. Our present work is focused on the organic insulator behaviour above and below the melting temperature in order to understand the different mechanisms of thermal-oxidative ageing and degradation in the molten and the solid state. Thin films in PTFE were aged by accelerated method under oxidizing environment (in air) and severe thermal stresses between 250 and 450°C with exposure times from 2 to 300 hours. Before and after each ageing cycle, samples were characterized by weight loss evolution (performed by a microbalance), Fourier transform infrared spectroscopy (FTIR), and Differential scanning calorimetry (DSC).