Low-Profile mechanisms in a PVAc/polyester blend
The different steps associated with the curing of a PVAc/polyester blend are identified and correlated to the mechanism of shrinkage control in the presence of a low-profile additive (LPA). Poly(vinyl acetate) (PVAc) is used as a LPA and is shown to induce a phase separation upon curing that leads to an interconnected globule morphology. This morphology strongly modifies the rheokinetics of the blend compared to that of the neat polyester resin. In particular, the presence of PVAc delays the cure kinetics and the gel time. A comparison between these delays, called shift times, demonstrates an increase in the gel conversion of polyester in the presence of PVAc. This, coupled to the thermal expansion of PVAc at the early stages of curing, contributes to the low-profile effect. Microvoids in the LPA-rich phase, which are believed to play a key role in the mechanism of shrinkage control, are efficient at the later stages of curing and during cooling and complete the low-profile effect. However, it is also shown that the formation of microvoids may indirectly induce macroscopic voids that could be at the origin of pinholes at the surface of the parts molded with this material.