The effects of reactive microgels (RM) as low-profile additives (LPA) on the volume shrinkage characteristics and internal pigmentability for low-shrink unsaturated polyester resins (UP) during the cure at 30C were investigated. These reactive LPA, which contained unsaturated C=C double bonds at the surface of the particles, were synthesized by a soapless emulsion polymerization using styrene (ST) and unsaturated polyester (UP) as comonomers. Depending on the RM composition and the type of UP employed, the reactive LPA could lead to a reduction of cyclization reaction for UP resin during the cure, and would be favorable for the decrease of intrinsic polymerization shrinkage after the cure. With the RM made from either MA-PG or MA-PA-PG type of UP, employing the MA-PG type of UP as the resin matrix would lead to a lower volume shrinkage than the MA-PA-PG system after the cure. Adding 10 wt % RM of MA-PA-PG type could result in the lowest volume shrinkage of 1.3% when the MA-PG type of UP was employed as the resin matrix, where the composition of the UP matrix was different from that of RM. The experimental results have been explained by an integrated approach of measurements for the static phase characteristics of the ST/UP/LPA system, reaction kinetics, cured sample morphology, microvoid formation, and glass transition temperatures by using differential scanning calorimetry (DSC), scanning electron microscopy (SEM), transmission electron microscopy (TEM),and the methods of thermally stimulated current (TSC).

(Key words: reactive microgel (RM), low-profile additive (LPA), unsaturated polyester resins (UP), volume shrinkage, pigmentability)