Cresol novolac–epoxy networks: properties and processability

Linear controlled molecular weight ortho-cresol novolac oligomers were crosslinked with epoxies to form tough, flame retardant networks with enhanced processability and reduced moisture uptake. At the networksʹ optimum stoichiometry, the fracture toughness, assessed by the critical stress intensity factor (KIC), was slightly higher for the cresol novolac–epoxy networks (∼1.1 MPa m1/2) than for similar phenolic novolac–epoxy networks (∼0.85 MPa m1/2). Cresol novolac–epoxy networks exhibited improved processability under melt conditions relative to phenolic novolac–epoxy materials, and this was attributed to the presence of a methyl group adjacent to the hydroxyl group on cresol repeat units, which reduced the rate of reaction between phenol and epoxy. The use of cresol novolac oligomers also reduced the network equilibrium water absorption, and this was likely due to the presence of the additional methyl on each repeat unit. The equilibrium water absorption for all cresol novolac containing compositions was between 1.7 and 2.1 wt%, comparable to epoxy networks. Cone calorimetry showed that the flame retardance of cresol novolac–epoxy networks was lower than that for the phenolic novolac–epoxy materials, but was far superior to the control epoxy networks.