Effect of physical aging on dielectric, thermal and mechanical properties of cast-epoxy insulators

Samples of cast bisphenol-A and cycloaliphatic epoxy insulation were quenched from a temperature above T_g (170°C) to room temperature. Variations in dielectric, thermal and mechanical properties were examined during subsequent sub-T_g annealing (physical aging). It was found that the impact strength of the epoxy insulation decreased considerably with physical aging. Moisture intake and subsequent dielectric loss at room temperature were also found to decrease with physical aging. Glass transition temperature and excess enthalpy measured by DSC demonstrated significant increase with increased sub-T_g annealing. On the other hand, dielectric properties showed different changing patterns during sub-T_g annealing in different epoxy systems. It is concluded that epoxy insulating materials should be properly annealed before assessing their dielectric and other physical properties because of the possibility of unknown rapid cooling histories