Low temperature cure of unsaturated polyester resins with thermoplastic additives II. Structure formation and shrinkage control mechanism

An important feature in the cure of unsaturated polyester (UP)/styrene/ther moplastics system is the formation of a two-phase structure. Its final morp hology is primarily determined by the phase separation process and the gela tion resulting from the polymerization. In this study, the phase separation process during the cure of UP resins with thermoplastic additives was inve stigated by optical microscopy. It was found that depending on the system m iscibility and reaction kinetics, the formation of sample structure follows the same route, but may end at different stages with different types of st ructure. Two key factors-the volume fraction of the thermoplastic-rich phas e and the phase separation period i.e. the time period between the onset of phase separation and the gelation-determine the final structure of the sam ple. Based on experimental results, a shrinkage control mechanism at low te mperature cure was proposed. (C) 1999 Elsevier Science Ltd. All rights rese rved.