Dielectric analysis of selected polyimide films and derived epoxy-based sandwich composites

Composites with the epoxy matrix are still the most common composites used in high voltage electrical insulation systems. Polyimides are materials with excellent heat resistance and high electrical strength for application, where high overall endurance and reliability even under harsh operating conditions is needed. Polymer composites with very good dielectric parameters may be composed of base polymer composite and additional dielectric barrier. The combination of the polyimide film and the epoxy-based composite, e.g., in the form of a prepreg, leading to the formation of a multicomponent sandwich composite, is relatively simple and the resulting material properties can be very advantageous. However, the detailed analysis of polyimide films is necessary for a suitable oversizing of the mentioned composites. The study of selected dielectric properties, e.g., volume resistivity, permittivity, and dissipation factor, of several types of polyimides, are presented in this contribution in the first step. Epoxy resin without filler was also analyzed as a base material for the tested composite. Two-component composites (epoxy resin with polyimide film) as a fundamental variant of the mentioned multicomponent sandwich composite were tested in the second step. The results show the impact of polyimide type on the final composite properties from the dielectric point of view. The polyimides incorporated in the composite generally decrease the volume resistivity but increase the dielectric strength. The change of dissipation factor of tested composites related to the epoxy resin depends on the type of used polyimide film.