Synergistic crosslinking of phenolic-epoxy for denser networks with enhanced thermal–mechanical properties

Phenolic polymers are widely used for fire-resistant applications but suffer from high porosity and low fracture toughness, limiting their structural performance. To address these challenges, we propose a catalyst-free approach to crosslink phenolic with epoxy, forming hybrid polymers with denser networks, reduced porosity, and enhanced thermal–mechanical properties. A systematic study examined the influence of phenolic-to-epoxy resin ratios on curing kinetics, mechanical properties, and thermal stability. The optimal formulation, comprising 75 wt.% phenolic and 25 wt.% epoxy, achieved a dramatic porosity reduction (0.9% versus 52.5% for catalyst-cured phenolic) and a char yield of 52.5%, exceeding the rule-of-mixture prediction by 23.5% and approaching pure phenolic (54.4%). This hybrid exhibited a 158% increase in initiation fracture toughness (0.169 kJ/m2) and a 17% improvement in the flexural strength (406.8 MPa) of carbon-fibre-reinforced composites at room temperature. Moreover, after exposure to 50 kW/m2 heat flux for 30 s, the composites retained 48% higher flexural strength (291.5 MPa) than those with pure phenolic (196.9 MPa). These significant improvements are attributed to the synergistic effects of reduced porosity and a stable epoxy-phenolic network, delivering superior mechanical performance without compromising flame resistance. The findings demonstrate the potential of phenolic-epoxy hybrids for high-temperature, fire resistant applications requiring robust structural integrity and low porosity.