The dielectric properties of glass-fibre-reinforced epoxy resin during polymerisation

The change in dielectric properties of thermosets between 10 Hz and 1 MHz allows us to monitor the cure of these materials. During the first heating phase at 80°C the resin conductivity (which is correlated with the Newtonian viscosity by a power law) decreases by several decades. In the subsequent heating phase at 140°C, the glass transition process is observed as a broad maximum of the dielectric loss factor at 30 kHz. The shift of the loss factor peak to lower frequencies with cure time marks the increasing relaxation time of the curing system. The relaxation time and the conductivity are indicators for cure monitoring that do not depend on frequency. So they are suitable for a comparison with other methods like DSC, FTIR and rheometry. The glass transition is analysed in the complex plane with a skewed arc function proposed by Havriliak and Negami (Havriliak S, Negami S. A complex plane analuysis of α-dispersions in some polymer systems. J Pol Sci C 1966;14:99–117). There is no significant change in the shape of the dielectric complex-plane plot during curing. The cure-monitoring parameters conductivity and relaxation time are not much affected by glass-fibre reinforcement. Only the relaxation strength and the maximum value of the loss factor are reduced as a result of the low loss factor of the glass fibres.