Microvoids in unsaturated polyester resins containing poly(vinyl acetate) and composites with calcium carbonate and glass fibers

When thermoplastics is added as a low-profile additive (LPA) to an unsaturated polyester (UP) resin, microvoids are introduced and compensate for the high volume shrinkage of the resin. The characteristics and mechanisms involved in the microvoid formation of a low-shrinkage UP resin system with poly(vinyl acetate) (PVAc) as LPA were investigated in this work. Both PVAc and styrene were found to play vital roles in controlling the microvoid formation. The voids were developed from an isolated type to an interconnected type by increasing PVAc content to reach a co-continuous phase structure of UP resin and PVAc, and their sizes were significantly enlarged by increasing styrene content. The microvoid formation was also affected by the curing conditions and compositions. It was enhanced by high curing temperature, but substantially restrained by an applied external pressure. When a filler was introduced to the PVAc-containing resin or when the resin was cured with t-butyl perbenzoate (TBPB), the number of voids increased. A foaming process, i.e. the surplus-styrene nucleation and expansion in the PVAc phase during the curing process, is considered to be the major mechanism for the microvoid formation.