Accelerated thermal aging of polyester film: mechanisms affecting thermal endurance

Accelerated thermal aging tests have been used to determine long term reliability of polyester films used as ground and phase insulation in electric motors. Consideration must be given not only to the temperatures used in this accelerated aging, but also to the environment in which the insulation is to be functional, such as hermetic applications in various refrigerant systems. Past studies have indicated that dielectric properties are unaffected as long as the films maintain mechanical integrity. Hence, these studies have focused on the tensile strength and elongation retention upon exposure to heat in different environments (sealed in refrigerant systems or oxygen-free atmosphere, and open air). When exposed to temperatures of 140°C, these studies have indicated that mechanical degradation mechanisms do not result exclusively from molecular weight degradation from hydrolysis or oxidation, but occur as a result of thermally induced crystallization in the film. Therefore, in addition to initial molecular weight, insulation thermal endurance also depends on film anisotropy and heat set. This investigation indicates that the direction of lowest molecular orientation (highest tensile elongation) appears to have the highest rate of embrittlement. While limited in predicting actual reliability performance of specific film types in motor applications, these results provide a means of optimizing film reliability, as, for instance, selection of a film product, the orientation of the film in a specific motor design, and/or considerations given to motor operating temperature and environment