Toughness behaviour of vinylester/epoxy thermosets with interpenetrating network structure

Abstract

Vinylester/epoxy-based (VE/EP) thermosets of interpenetrating network (IPN) structure were produced by using a VE resin (bismethacryloxy derivative of a bisphenol-A type EP resin) and EP resins of aliphatic (Al-EP) and cycloaliphatic (Cal-EP) nature. Curing of the EP resins occurred either by an aliphatic (Al-Am) or a cycloaliphatic (Cal-Am) diamine compound. Dynamic mechanical thermal analysis (DMTA) and atomic force microscopy (AFM) suggested the presence of an interpenetrating network (IPN) in the resulting thermosets. AFM scans taken on the ion-etched surface of EP showed a featureless homogeneous structure. On the other hand, VE exhibited a two-phase microgel, whereas VE/EP a two-phase interpenetrating network (IPN) structure. Toughness was characterised by parameters of the linear elastic fracture mechanics, viz. fracture toughness (Kc) and fracture energy (Gc). Unexpected high K c and Gc data were found for the systems containing cyclohexylene units in the EP network. This was attributed to beneficial effects of the conformational changes along the cyclohexylene linkages (chair/boat). The failure mode of the VE/EP thermoset combinations was studied by scanning electron microscopy (SEM) and discussed.