RELAXATIONS IN THERMOSETS .24. THEORY AND EXPERIMENTS ON NONISOTHERMAL CURING OF THERMOSET POLYMERS

A generalized formalism for the dielectric behavior of thermosets during their curing under isothermal and nonisothermal conditions has been developed. It is based on the theory that the relaxation time of a thermoset is related to the extent of chemical reaction. Changes in both the dc conductivity and dipolar orientation polarization have been considered as gelation and vitrification for a given heating rate occur. It is shown that the plots of the normalized complex permittivity measured for a fixed frequency during the curing on ramp heating are similar to those observed under isothermal conditions for which the curing parameter remains constant. The validity of the formalism has been tested by using data obtained for the curing of the diglycidyl ether of Bisphenol-A with ethylenediamine at two heating rates. The formalism is in satisfactory agreement with the measurement.