Polyester-imide solventless impregnating resin and its nano-silica modified varnishes with excellent corona resistance and thermal stability

A novel polyester-imide solventless impregnating resin, coded as PEI, has been fabricated using a propoxylated neopentyl glycol dimethyl acrylic ester as the reactive crosslinking monomer. It is shown that the PEI resin is nontoxic and has a volatile content of 0.8%, far lower than 24.6 and 7.5% of the traditional unsaturated polyester impregnating resins containing styrene and vinyl toluene as the reactive diluents. The investigations on its curing behavior indicate that the PEI resin exhibits a low curing temperature at about 130 °C and has excellent storage stability. Furthermore, nanosilica has been dispersed into the PEI resin to prepare nano-modified impregnating varnishes. The obtained nano-PEI resins not only have excellent storage stability and show no obvious changes after having been applied in practice for one year, but also exhibit improved corona resistance, bond strength, thermal stability, and electrical insulating properties. The corona resistant life and breakdown voltage strength of the nano-PEI resin containing 12% of nano-silica are 1608 min and 28.1 kV/mm, about 45.9 and 1.2 times of that of the PEI resin, respectively. The excellent electrical insulation properties, processing characteristics, and storage stability make the nano- PEI resins promise for use as the impregnating varnishes for variable frequency motors and high voltage motors.