Evolution of structural properties of glass-mica-epoxy insulations along accelerated thermo-oxidative aging

The stator windings of large air-cooled rotating machines are insulated with mica-glass fabric impregnated with epoxy. These composites are submitted to thermal, mechanical and electrical stresses. A specific aging arrangement has been setup allowing artificial thermo-oxidative aging of dielectric samples thanks to the application of a dry O₂ pressure at a controlled temperature. Two different materials are considered: a class H material, designed to sustain temperatures up to 180degC, and a high conductivity (HC) tape material, which is a class F insulation. The samples were followed in mass and DMA (dynamic mechanical analysis). Similar profiles of mass evolution with various temperature and oxygen pressure contribute to validate the artificial aging procedure. DMA allowed determining the glass transition temperature (Tg) of the epoxy resin and observing the evolution of the network density of the polymeric matrix along aging. Delaminations of a few tens mum thick were observed within the samples after aging. These characterizations allowed the comparison between both materials.