| Low profile additives have been found highly effective in eliminating the polymerization shrinkage of unsaturated polyester resins in high temperature molding processes such as compression molding of SMC and injecting 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 with the ability to be processed at low temperature and low pressure have attracted considerable interest from the composite industry. However, the detailed LPA mechanism, especially the low temperature LPA mechanism, is still not well known.; In this work, an integrated analysis was carried out to study the shrinkage control mechanism of unsaturated polyester resins with low profile additives cured at low temperatures. Different experimental techniques, including DSC and FTIR spectroscopy for reaction kinetics, RDA for viscosity change, dilatometry for volume change, SEM and optical microscopy for morphology, and BET technique for internal surface area, were employed for this purpose. In the dilatometry study of volume change, we found that a volume expansion occurred for samples with shrinkage control. The volume expansion was associated with the change of the sample opacity, suggesting the occurrence of microcracking and the formation of microvoids. The high internal surface area measured by BET confirmed that microvoid formation is the main source of shrinkage compensation at low temperature cure. The measured resin shrinkage, sample opacity and the internal surface area were well correlated. A detailed study of the relationship between the volume change and the LPA concentration showed that there were two transition points in both volume and morphological changes. LPAs started to be effective at the first transition point when the LPA-rich phase and the UP-rich phase became co-continuous. The low profile effect vanished at the second transition point when the fusion among the particulate structure was severe. The study of the dynamics of phase separation around the first transition point by using an optical microscope showed that, depending on the system miscibility and the reaction kinetics, the phase separation process followed the same route, but ended at different stages with different type of structure. Two key factors, the volume fraction of the LPA-rich phase and the phase separation period, determined the end point of the phase separation process.; The low profile performance of UP/LPA/styrene system is governed by two critical steps: the phase separation and the microvoid formation. A conceptual model for the microvoid formation was proposed in this work. A low profile mechanism at low temperature cure was also proposed. The cure of the low profile unsaturated polyester resin can be divided into six steps: induction, spinodal decomposition, coarsening, coalescence and growth, gelation and microvoid formation.; The application of this model for improved material design was also explored. Polyvinyl acetate, a well-known low profile additive, was synthesized and modified to improve the shrinkage control efficiency. The results support the model prediction. |
