EPR study of the ageing process in epoxy resin

The authors describe a study in which epoxy resin specimens were stressed electrically and/or thermally and the evolution of the EPR spectra was measured. The electrical stresses used caused significant internal discharges in some samples prior to failure. Furthermore, electrical pulses at the terminals can be expected in concert with the channel development observed. For this reason, discharge detection measurements were conducted to provide a basis for correlating the EPR signal with the observed electricals behavior. In regimes where internal discharges are not observable, conduction properties become the only non-destructive means to characterize the samples electrically. On the basis that EPR centers are associated with bulk states in the material, it is important to try to examine the nature of the bound and free charge. To this end, DC absorption tests and thermally stimulated current measurements were also conducted