Insulation performance of polyimide/epoxy resin interface modified by silane coupling agent

High-frequency transformers use a composite insulation of epoxy resin (EP) bulk and polyimide (PI) inter-turn coating. The PI-EP interface is a weak point. To enhance interfacial stability, KH550 silane coupling agent is employed as a molecular bridge to form Si-O-C chemical bonds between PI and EP. The silane loading is controlled by adjusting the treatment time of the PI film. Peel tests and partial discharge experiments show that a 6-min treatment significantly improves interfacial bonding. Compared with the acid-alkali treated sample without silane (SI-0), the silane-modified sample exhibits an additional improvement in partial discharge inception voltage and breakdown field strength, attributable exclusively to the KH550-induced interfacial cross-linking. Electrothermal aging tests reveal an 85% longer service life and an 84.8% reduction in average discharge magnitude. Simulation analysis further investigates the electric field distribution and breakdown behavior in the presence of voids at the PI-EP interface. These results demonstrate that silane coupling agent modification effectively stabilizes the PI-EP interface and enhances insulation performance.