Curing behaviour of IPNs formed from model VERs and epoxy systems: Part II. Imidazole-cured epoxy

The polymerisation kinetics for IPNs formed from a model vinyl ester resin (VER) and an imidazole-cured epoxy resin (DGEBA) has been studied by scanning DSC and isothermal FTIR. The chemical interactions between the VER initiating system (either cumene hydroperoxide, CHP; methyl ethyl ketone peroxide, MEKP; benzoyl peroxide, BPO; or azobisisobutyronitrile, AIBN) and the epoxy curative (1-methyl imidazole, 1-MeI) have been examined. IPNs containing AIBN showed minimal initiator interactions but a strong accelerating effect was observed by the imidazole curing agent on the peroxide initiator systems (at least in the early stages) due to enhanced radical production apparently via a redox reaction. For IPNs containing MEKP and BPO, the imidazole also appeared to cause a significant loss of initiator activity, perhaps by non-radical redox side reactions. During the isothermal cure of the IPN, the DGEBA monomer acted as a diluent on the polymerisation of the azo-initiated vinyl ester, resulting in a slower initial cure rate than in the neat VER. Due to the slower cure of the 1-MeI/DGEBA component, the DGEBA monomer acted as a plasticiser in the later stages of the isothermal VER cure, thus enabling higher final levels of vinyl cure prior to vitrification. As a consequence, the higher level of crosslinking of the VER component in the IPN reduced the maximum isothermal conversion of the epoxy component during its isothermal cure.