Phase separation behavior in a rubber-toughened epoxy system

Four series of epoxy resins were cured in the presence of a carboxyl-terminated butadiene acrylonitrile (CTBN). The first two series consisted of 7 components, in which the epoxy, hardener, and rubber form one network and two methacrylates form a second interpenetrating, interconnected network. The variables explored are the initial composition of rubber and the ratio of the two methacrylates. The last two series consist of only 3 components, epoxy, hardener, and rubber. The only variable in each is the rubber concentration. The cure schedule for all the samples was the same. The final size of the rubber particles in each sample were measured using optical microscopy (OM) and scanning electron microscopy (SEM). The impact strength of the seven component samples was measured using an Izod impact tester. Particles were not seen in OM for the first two series and only one rubber concentration value in the second two. SEM on etched samples confirmed the presence of particles formed by nucleation and growth in all the samples. The SEM confirmed the OM results of 8.5 and 9 μm domain sizes for the two samples that showed optically. Phase separation was seen in the 7 component series up to 16% rubber in the first and 10% in the second and rubber particles were measured at 0.75-3 μm in size. The Izod results showed an apparent increase in toughness with increasing rubber concentration and no clear dependence on particle size or size distribution, though the error was so large that the true effect of the presence of particles cannot be confirmed.