Rubber modified vinyl ester resins of different molecular weights

Abstract

The morphology, as well as the related fracture and mechanical behavior of vinyl ester resins (DVER) of different molecular weights cured with styrene (S) and modified with two different liquid rubbers are presented and discussed. The liquid rubbers are: carboxyl terminated poly(butadiene-co-acrylonitrile) (CTBN), a common toughening agent for epoxy resins, and an almost unreactive rubber with the DVER and S comonomers, and a reactive rubber (vinyl terminated poly(butadiene-co-acrylonitrile), (VTBN). The initial miscibility of the modified systems and the reactivity of the rubber determine the final morphology of the material. This morphology will correspond to a continuous main phase (rich in the DVER-S copolymer) with simple rubber rich inclusions (as in the epoxy-rubber systems) or with inclusions with a complex internal structure, where phase separation occurs as in the low profile modified unsaturated polyester resins. The morphologies developed are strongly dependent on the resin molecular weight as well as on the elastomer added. In spite of the initially higher compatibility of the S-DVER-CTBN system with respect to the S-DVER-VTBN system, the reactivity of the vinyl-ended elastomer leads to a much finer distribution of the elastomeric phase. In particular, the low molecular weight resin cured with S and modified with 10% of CTBN leads to a cocontinuous structure with microvoids that generates a material of low density and poor mechanical and fracture properties. On the other hand, the use of VTBN as additive leads to a more compact morphology, with gradual reduction of the mechanical performance of the modified resins and improved fracture behavior.