Theoretic And Experimental Study Of Shrinkage Control Mechanism Of Low Profile Unsaturated Polyester Resins During Curing At Low-medium Temperature

Low profile additives have been found highly effective in eliminating the polymerization shrinkage of unsaturated polyester resins (UPR) in high temperature molding processes such as compression molding of SMC and injection molding of BMC. Recently, because of the growing interest of new manufacturing processes such as low pressure/low temperature SMC molding, RTM and SCRIMP, low shrinkage molding compounds which can be processed at low temperature and low pressure have attracted considerable interest in the composite industry. However, the detailed LPA mechanism, especially the low-medium temperature LPA mechanism, is still not well known.In our work, many methods were used to study the shrinkage control mechanism of unsaturated polyester resins with low profile additives cured at low-medium temperatures, including a dilatometer for volume change which was designed and developed in our laboratory, and SEM for morphology. In the volume change study, we found appropriate temperature is helpful for shrinkage control, and different LPAs have different shrinkage control effects due to their dipole moment, Tg, miscibility with UPR and morphology after blending. There existed an optimal concentration range of poly(vinyl acetate) based LPAs for shrinkage control. LPA was effective for shrinkage compensation in this range, on the other hand, LPA only acted as filler and the low profile effect vanished out of this range, however, for polystyrene based LPA, the more concentration the better shrinkage control. For poly(vinyl acetate) based LPAs, It seemed that LPAs with higher molecular weights worked better than that with the lower molecular weights and LPA with low molecular weights need high concentration for good shrinkage control.A detailed study of the morphology of low-profile unsaturated polyester resins after cure at low-medium temperature was carried out. The experimental results show that the influence of temperature on morphology of cured sample was smal 1 for poly(vinyl acetate) based LPAs but large for polystyrene basedLPA. Polar polymers tend to separated from UPR and make system form co-continuous structure, which make for shrinkage compensation. Big difference of Glass transition temperature(Tg) between UPR and LPA and a Tg below the cure temperature is preferred, because there is more time and more efficient for microvoid formation of cured sample. For the sample with poly(vinyl acetate) based LPAs, the morphology exists two transition points with the increase of LPA concentration. The LPA-rich phase and the UP-rich phase became co-continuous at the first transition point and the paniculate structure came to dominate after the second transition point with continuous UP-rich phase vanishing. LPA with higher molecular weights needs lower concentration to obtain co-continuous structure. However, for the sample with polystyrene based LPA, the morphology had little change with the increase of LPA concentration. We also found that the opacity was the symbol of the the formation of microvoid which advanced the shrinkage control effect of sample.Based on the analysis of the experiment results, a low profile mechanism at low-medium temperature cure was proposed. And the following conclusions were drawn. First, the low profile performance of UP/LPA/styrene system is governed by two critical steps: the phase separation and the microvoid formation. Second, the cure of the low profile unsaturated polyester resin can be divided into four periods: induction, phase separation (including spinodal decomposition and coalescence and growth), gelation and microvoid formation.