Shrinkage control of low-profile unsaturated polyester resins cured at low temperature

With the development of new manufacturing processes, such as low-pressure compression moulding of sheet moulding compound (SMC), resin transfer moulding (RTM) and vacuum infusion liquid composite moulding (e.g. SCRIMP) processes, low-shrinkage moulding compounds which can be processed at low temperatures have attracted considerable interest from the composite industry. In this paper, an integrated rheology-kinetics-morphology-dilatometry study on an unsaturated polyester resin (UPE) mixed with different low-profile additives (LPAs) was carried out to investigate the shrinkage control mechanism of LPA under low-temperature cure. The reaction rate was determined by a differential scanning calorimeter, while a scanning electron microscope and a Rheometrics dynamic analyser were employed to follow the morphological and rheological changes respectively. The volume change of the resin mixture during the curing process was measured by a dilatometer. It was found that the shrinkage behaviour of the resin mixture strongly depends on the competition of the shrinkage induced by the resin polymerization and the expansion induced by microvoid formation. The results also showed that LPAs with higher molecular weight and lower LPA content seem to work better under low-temperature cure. Several moulding experiments were conducted to verify the dilatometry results.