MECHANISM OF SHRINKAGE CONTROL IN POLYESTER RESINS CONTAINING LOW-PROFILE ADDITIVES

A commercial low-profile additive containing acid-terminated poly(vinyl acetate) (PVAc) was added, at concentrations of 0-16 wt%, to a solution of unsaturated polyester resin in styrene. The blends were cured under standard conditions. In all cases, the linear shrinkage during cure was 3.2 +/- 0.2%, independent of PVAc content. In the absence of PVAc, addition of up to 60 wt% of CaCO3 simply reduced the shrinkage of the resin in proportion to the volume fraction of filler. However, a combination of CaCO3 with 16% PVAc gave a synergistic effect: the resulting shrinkage was substantially smaller than with CaCO3 alone. On the basis of microscopy and other evidence, it is concluded that low-profile modifiers work by providing weak co-continuous regions in the resin, which can cavitate in response to tensile stresses arising from thermal- and cure-contraction in the presence of mechanical constraints. These constraints may be imposed internally, by mineral fillers or glass fibres, or externally, by forces acting on the surface of the resin. Optical microscopy provides evidence to support this interpretation.