Effect of co-promoter and secondary monomer on shrinkage control of unsaturated polyester (UP)/styrene (St)/low-profile additive (LPA) systems cured at low temperatures

The shrinkage of unsaturated polyester (UP)/styrene (St) resins cured at lo w temperatures can be reduced by the presence of low-profile additives (LPA s). It is believed that the reaction-induced phase separation and the polym erization shrinkage in both the LPA-rich and UP-rich phases result in the f ormation of microvoids, which partially compensates the resin shrinkage. Th e relative reaction rate in the two phases plays an important role in shrin kage control. In this study, secondary monomers [such as divinylbenzene (DV B) and trimethylopropane trimethacrylate (TMPTMA)] and a co-promoter, 2,4-p entandione (2,4-P), were added into the UP/St/LPA resin systems to investig ate their effect on the shrinkage control of resins cured at low temperatur es. Dilatometery results showed that the addition of both TMPTMA and 2,4-P resulted in an earlier volume expansion during curing and better shrinkage control. The phase separation, reaction kinetics, and viscosity changes in the LPA-rich and UP-rich phases during curing were also investigated. The r esults confirmed that the increased reaction rate in the LPA-rich phase led to an earlier formation of microvoids and, consequently, less volume shrin kage of the cured resin. (C) 2001 John Wiley & Sons, Inc.