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 degreesC). 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.