Mechanism of Shrinkage-Reduction Behaviors for Unsaturated Polyester Resins

摘要

For unsaturated polyester resins combined with thermoplastic resins, microstructure nd volume changes were observed both before and after curing. The shrinkage-reduction (so-called low profile mechanism) of unsaturated polyester resins was studied in relation to miscible single-phase and immiscible two-phase systems.In the case of a two-phase system, shrinkage-reduction is due to formation of voids in the dispersed phase. These voids are generated by both the volume expansion of the dispersed phase and the difference in thermal contraction between the matrix phase and the dispersed phase. On the other hand, shrinkage-reduction of a single-phase system is due to the generation of continuous voids among granulated cross-linked polymers. The granulation is caused by incompatibility between the cross-linked polymer and shrinkage-reducing additives. The size of granulated resins is one of the important factors for shrinkage-reduction. A single-phase system has optimum curing temperature for low profile, and does not always require high reactivity for unsaturated polyester resins. It is suggested that the shrinkage-reducing effect depends on both the glass transition temperature and the difference in solubility parameter between unsaturated polyester resins and shrinkage-reducing additives for various thermoplastics. Therefore, the shrinkage ratio can be approximately estimated by using the expansion coefficient of shrinkage-reducing additives and a parameter determining the ratio of the two phases.