The effect of the combined application of solid- and gas-phase flame retardants in epoxy gelcoats on the thermal stability, fire performance and adhesion of coated carbon fibre–reinforced epoxy composites

Abstract

We explored the combined effect of two flame retardants (FR) containing phosphorus—ammonium polyphosphate (APP) and resorcinol bis(diphenyl phosphate) (RDP)—on the fire performance of coated carbon fibre–reinforced composites. We prepared epoxy gelcoats with 10% total phosphorus content with different ratios of APP and RDP and characterised by DSC, TGA, rheometry, limiting oxygen index, UL-94 test and mass loss calorimetry (MLC). The increasing ratio of liquid RDP led to a significant reduction in viscosity: gelcoats over 3%P RDP content became sprayable without any diluents. The gelcoat with 7%P APP and 3%P RDP had the best fire performance (pHRR of 141 kW m−2), followed by the gelcoat with 6%P APP and 4%P RDP (pHRR of 171 kW m−2). We prepared flame-retarded and reference epoxy/carbon fibre–reinforced composites by hand lamination, wet compression and vacuum infusion. The flammability of the laminates decreased with increasing fibre content due to the reduction in resin content; therefore, the vacuum-infused sample performed the best. However, due to the more compact structure of the vacuum-infused sample, the reduction in total heat release was greater than expected from the fibre content alone. The MLC results of the coated composites indicated that it was not necessary to incorporate 3%P RDP in the composite matrix of the coated composites, as FRs in the coating alone are sufficient to protect the composite. The adhesion between the composite and the gelcoat was influenced by the presence of the FR additives in the resins, and adhesion was stronger when the composite matrix and the gelcoat contained the same FR.