Influence of Water Absorption and Temperature on Charge Transport and Electrical Degradation in Epoxy Resins

The work aims to understand bulk transport mechanisms in epoxy resins and their influence on partial discharge and electrical treeing phenomena. The charge transport processes in two bisphenol-A epoxy resin systems were studied using dielectric spectroscopy and related to moisture uptake. In both resins, absorbed moisture was implicated in the formation of a bulk quasi-dc (QDC) charge transport mechanism above the glass transition temperature. Complementary investigations of the electrical degradation process in epoxy resins, principally electrical treeing, were also found to be dependent on temperature and moisture absorbed by the samples. A model for electrical tree growth in flexible epoxy resins has been proposed. The novelty of the model is that it considers a QDC charge transport through the bulk polymer, where proton hopping is suggested as a long range transport mechanism. The time needed for the neutralisation of the space charge accumulated ahead of a growing tree structure by charges involved in the QDC process is suggested as a governing parameter of the tree growth rate. Electrical tree growth is related to changes in the time constant of the QDC process and hence the percolation distance that the protons can travel. At high levels of moisture and/or temperature, multiple percolation paths can be established thus increasing the effective ionic conductivity of the material leading to thermal breakdown when tanδ is greater than 1.