Effects of hydrothermal aging on the dielectric properties of epoxy composites

Epoxy/glass/mica composites used for high power generators were aged in water for various amounts of time (up to one month) and at different temperatures (up to 110°C). Water sorption follows a Fickian law (activation energy of 11.5 kcal/mol) and seemed to be controlled by the epoxy matrix. There is an abrupt decrease in the bulk resistivity during the first stage of water absorption (by about 7 decades when the water uptake varies from 0 to 0.4%). For water absorption higher than 0.4% we observe a drastic increase in the dielectric constant and loss at low frequencies (<100 Hz). Changes in the complex permittivity can be explained by three relaxation mechanisms (a conduction term, an interfacial polarization mechanism, and a polar relaxation related to the epoxy network) whose characteristics vary with water absorption.