Mechanical and Dielectric Properties of Glass-Mica-Epoxy Composites 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. Long-term electrical insulation may be affected by the occurrence of partial discharges. A specific aging arrangement has been set-up allowing accelerated thermo-oxidative aging thanks to the application of a pressure up to 10 bars on samples submitted to controlled atmospheres, especially dry O2, at temperatures from ambient to 200°C. Mechanical and dielectric properties of samples have been followed along aging, using dynamic mechanical analysis (DMA) and dielectric analysis (DEA). The effects of the oxidation on polymer network, i.e. crosslinking and elimination reactions, were observed thanks to the evolution of the glass transition temperature (Tg). The effect of 50-180°C temperature cycles has been compared with thermal oxidation at constant temperature. Correlations with weight losses are also underlined.