Fire retarded fibre-reinforced composites: Effect of heat and fire on carbon fibre oxidation and resultant properties

This study has investigated the impact of fire retardants in carbon fibre-reinforced epoxy composites (CFRC) on physico-mechanical and oxidative properties of carbon fibres after exposure of CFRCs to high temperatures and fire. Three fire retardants were chosen based on their activity in condensed phase (ammonium polyphosphate, APP) and/or vapor phase (9,10-Dihydro-9-oxa-10-phosphaphenanthrene 10-oxide, DOPO, and resorcinol bis-(diphenyl phosphate), RDP). The composites were subjected to high heat fluxes (75–116 kWm -2 ) and fire using a cone calorimeter and propane burner. Post-exposure, the carbon fibres extracted from different plies were analysed for surface oxidation, mass loss, diameter reduction, and changes in tensile and electrical properties. Carbon fibres exhibited differing degrees of oxidation across the plies, with surface ply fibres showing greater oxidation and diameter reductions, while underlying plies experienced limited oxidation due to restricted oxygen access. The charred residues from fire-retarded samples (residue levels: APP>RDP>DOPO> control) adhered to the fibres, reducing oxidation and preserving tensile properties. However, the charred residues increased the electrical conductivity of the carbon fibres. This analysis has enabled the evaluation of each retardant’s effectiveness. • FR (APP, DOPO, RDP)–epoxy resin–carbon fibre (C) interaction in CFRCs investigated. • P-rich char residues enhanced thermal stability and oxidation resistance of C. • APP was most effective in protecting fibres from oxidation during cone exposure. • RDP reduced burning time, preserving fibre properties in propane burner test. • FRs generally increased electrical conductivity of carbon fibres post-exposure.