Study On Low Shrinkage Control Mechanism And Curing Stress Of Low Profile Unsaturated Polyester Resin

In recent years, Liquid Composite Moulding (LCM) process has been focused widely with the development of advanced composites with high quality and low cost. However, the heat-expansion mechanism of low profile additives (LPA) at high temperature is not fit for molding processes at low temperature, which means that the detailed mechanism at low temperature still need to be studied. According to the development of LCM, the shrinkage control mechanism of Unsaturated Polyester (UP) resin system with LPA during curing at low-medium temperature was investigated systemically in this paper. Based on phase separation, structure formation and curing shrinkage stresses in UP/ LPA/styrene system, the low shrinkage mechanism was proposed to break through the key techniques difficulties in LCM application.One of the important features during the cure of UP/ LPA/styrene system is the formation of a two-phase structure (LPA-rich phase and UP-rich phase). In this study, the phase separation process during the cure of UP resins with LPA was investigated. It is found that the structure formation followed the same route in terms of the system miscibility and reaction kinetics, while could end at different stages which resulted in different structures. Two key factors, the volume fraction of the LPA-rich phase and the phase separation period, i.e. the time period between the onset of phase separation and the gelation, determined the final structure and the degree of the microstructure formation, which was effected by the change of LPA concentration.The development of stress under one-dimensional (1-D) shrinkage constraint occurring after the gel point of the low profile UP and the effect of low shrinkage on curing stresses were also investigated. It was found that LPA could obviously reduce the curing stresses hi UP resins. And the concentration and type of LPA together with the temperature had evident effects on the beginning and development of curing stresses. It was also found that the volume shrinkage was non-linear with curing stresses.Based on the above experimental results, a shrinkage control mechanismat low-medium temperature was proposed. The performance of low profile in the UP/LPA/styrene system was governed by two critical steps: phase separation and microvoid formation. The whole reaction process could be divided into six steps: the induction, spinodal decomposition, coarsening, coalescence and growth gelation and microvoid formation.The understanding of the LPA low shrinkage control mechanism not only makes LCM not rely on the trial-and-error method, but also provide guidance of material design for the synthesis and preparation the new type LPA. This study will also establish the theoretic foundation for the LCM technique and has significant influence on prompting the wide applications of composites with high quality and low cost.