Surface Discharge Effects on Dielectric Properties of Epoxy Resins

The effects of surface discharges on dielectric properties of filled and unfilled epoxy resins have been studied. By means of parallel-plate electrodes, sheet samples have been aged through direct and indirect exposure to electric discharges at 15 kV ac 50 Hz voltage in static dry air for time intervals up to 300 hours. The samples have been examined with regard to changes in weight and dielectric strength; then permittivity and loss tangent measurements have been performed at several frequencies as functions of temperature. A good agreement between the behavior of dielectric properties and of dilelectric strength has been found for the materials examined, provided the strength is measured on samples from which the damaged surface has been removed by peeling the samples to a depth of about 0.5 mm.