Dielectric breakdown analysis of composites consisting of epoxy resin and polyimide film

The breakdown phenomena and determination of dielectric strength is one of the essential topics in the field of insulating materials for high-voltage applications. In the case of basic thermosetting plastics, including epoxy resins, dielectric breakdown analysis has been performed many times, and the mechanism of conducting channel formation is already well known. However, the situation becomes more complicated if other material components are combined with the base polymer. The application of composites in electrical insulation systems is currently becoming more common. Of all available electrical insulating types of composites, the attention is paid to sandwich composite in this contribution. Specifically, a sandwich structure, which is an interconnection of a heat-resistant thermoplastic (polyimide film) and a conventional thermoset for high-voltage applications (epoxy resin layer), is analyzed. Tested structures were prepared as the composition of each one of the three different types of polyimide films and always the same conventional epoxy resin which is cured at elevated temperature after the amine curing agent addition. In the case of performed testing of sandwich composites, the conductive channel is formed rapidly through the two polymer layers, which are very different in chemical structure. In addition, an interface is formed between the material layers when these layers are interconnected during the epoxy resin curing process, and the effect of this interface must also be considered in the analysis of dielectric breakdown. This contribution is focused primarily on visual description of differences in a breakdown that occurs when different polyimide films without or even with a certain surface treatment are tested. The values of dielectric strength determined by the dynamic method for tested structures are also mentioned. It appears that the choice of a suitable polyimide film can be quite significant in the sandwich structure of the electrical insulation system in terms of dielectric strength.