Thermal degradation and endurance of polyimide films

The thermal degradation and endurance of polyimide films were investigated in an attempt to develop electrical insulation systems operable at high temperatures. Two kinds of polyimide films were investigated: a widely used polyimide with diphenyl-ether and a recently introduced type without diphenyl-ether. Film samples were thermally aged at temperatures up to 400°C for periods of 5000 h max. Variation in the tensile strength and electric properties were measured during the aging process. This test was carried out not only in air but also in He gas in order to investigate the influence of oxidation. Thermal life curves, obtained from the tensile strength, suggest that the two polyimide films can be used in electrical insulation systems which operate at temperatures up to 350°C in inert gas. Only one of the two, the polyimide without the diphenyl-ether, is usable at 300°C in air. It is also shown that the diphenyl-ether part is vulnerable to thermal aging in air, that degradation in air is accelerated by oxidation, and that the polyimide without diphenyl-ether shows little structural change during aging