The electrical properties of anhydride-cured epoxy resin insulation materials

Anhydride-cured epoxy resins are widely used to insulate electrical equipment (e.g., motors, generators, transformers, circuit breakers, etc.). When high voltage insulation (i.e., above 7 kV) capability is required in a.c. electrical equipment, the electrical properties, particularly dielectric constant and dissipation factor, of the cured resins have to conform to certain stringent requirements to be acceptable. This paper describes various "screening" tests which are used to evaluate the electrical properties of epoxy-anhydride resins. In particular, dielectric constant and dissipation factor electrical data, obtained in the range 25–150°C and 60 Hz, are presented for epoxy-anhydride resins containing various types of latent accelerators such as quaternary phosphonium and quaternary ammonium compounds, Boron-Lewis Acid complexes, metal acetates and metal acetylacetonates. Details of sample preparation, electrical property measurements and data interpretation are presented. Possible "pitfalls" in the technique leading to erroneous results and the possible misinterpretation of the data are mentioned. The reasons for the apparent high dielectric constant and dissipation factor values obtained with some of the latent accelerator additives in the epoxy-anhydride resin are discussed and the possible limitations of some of these materials for high voltage insulation applications are reviewed.