THE THERMOCHEMICAL AND DIELECTRIC BEHAVIOR OF UNSATURATED POLYESTER RESIN DURING CURE

The dielectric behavior of an unsaturated polyester resin as a model r esin was studied in two stages. During the first stage, temperature an d conversion profiles during hot curing of the resin were studied. It was found, unexpectedly, that the reaction exotherm was greatest tn th e outer layer close to the hot mold surface and least in the center co re. The opposite was true for room temperature curing with a different initiator system. Comparison of the experimental results with heat tr ansfer calculations has lead to the conclusion that the kinetic charac ter of cure across the polyester slab thickness was location dependent . The variation with location was interpreted in terms of diffusion of the initiator components from the core towards the surface due to the ir faster depletion close to the hot surface and the imposition of a h ot surface and the imposition of a temperature profile. During the sec ond stage the effect of hot curing of unsaturated polyester resin on t he dynamic relaxation time was studied using dielectric measurements a long with two dynamic mechanical measurement methods. It was found tha t the dynamic response during cure was a material frequency dependent property and did not depend on the measurement method. All relaxation times, measured during cure, by all three measurement methods used, co nverged to a single equation: tau(t)(av) = at(b) where t - curing time , a,b-constants. The crosslinking density was found to increase slowly with conversion, while the relaxation time increased exponentially. T hese two different modes of behavior during cure explain the high reso lution of dynamic measurements as a cure monitoring tool, which can ea sily detect small curing changes. The dielectric relaxation of unsatur ated polyester resin during cure was modeled using a modified Williams - Watts decay function: Phi(t) = exp(-(tau/t)(beta)) where tau - rela xation time and beta - empirical parameter are time dependent. It was found that the variation of the relaxation time during cure is a WLF p rocess using Tg(alpha) (alpha - conversion) as the varied temperature. Relaxation times above and below the dielectric measurement system ca pability were computed thus, demonstrating the capability of yielding the entire relaxation times spectrum during cure, out of a single limi ted frequency dielectric measurement.