Variation in dielectric properties of an epoxy-novolac molding compound during dynamic cure

The variation in dielectric characteristics of an epoxy-novolac molding compound during dynamic cure was studied by means of dielectrometry (DE). Dielectric parameters such as permittivity (epsilon'), loss factor (epsilon'), and dissipation (tan delta) were observed to depend on various factors including temperature, frequency, the extent of cure (alpha, measured previously by using differential scanning calorimetry, DSC), as well as contributions from ionic conductivity (sigma) and electrode polarization. The characteristic relaxation time (tau) and the relaxed permittivity (epsilon(r)), suggested in the Literature as possible parameters for cure monitoring purposes, was found difficult to determine because of interference from ionic conduction and electrode polarization. In comparison, sigma could be measured throughout the entire cure process and was observed to depend only on temperature and alpha. In combination with our previous DSC results, an empirical function relating sigma to temperature and alpha was constructed. Our analysis also indicated that the strong maximum in the epsilon' and the epsilon" curves during the course of dynamic cure was a direct result of the alpha- and the temperature-dependence of sigma; the strong maximum of epsilon" is directly related neither to gelation nor to vitrification.