Structure–property relationships of void-free phenolic–epoxy matrix materials

Abstract The structure–property relationships of phenolic–epoxy networks have been investigated by several methods. Network densities have been explored by measuring the moduli in the rubbery regions and these experimental values were compared with those predicted from stoichiometry. The T g s decreased, and toughness increased, as the phenolic Novolac content in the network was increased. Both results could be correlated to the decrease in network densities along this series. Analysis of the cooperativity of the networks suggests a crossover in properties from two competing factors, network density and intermolecular forces (hydrogen bonding). Measured fracture toughness values exceed those of typical untoughened epoxy networks and far exceed existing commercial phenolic networks. In addition, an increase in Novolac content improves the flame retardance rather dramatically. Thus, by controlling the Novolac content to reach an appropriate phenol to epoxy ratio, a void-free system with both favorable mechanical properties and flame retardance can be achieved.