Superior flame retardancy and smoke suppression of epoxy-based composites with phosphorus/nitrogen co-doped graphene

Highlights: • Phosphorus/nitrogen co-doped graphene (PN-rGO) was synthesized via a scalable and green hydrothermal and microwave process. • PN-rGO was used as an additive for the first time to improve the flame retardancy and smoke suppression of epoxy resin. • Char (condense phase) and volatile (gaseous phase) analyses were measured to investigate the flame retardation mechanism. • Phosphorus/nitrogen co-doping is a viable approach to improve the efficiency of graphene as a flame retardant additive. - Abstract: Phosphorus and/or nitrogen doping is an effective method of improving the physical and chemical properties of reduced graphene oxide (rGO). In this work, phosphorus and nitrogen co-doped rGO (PN-rGO), synthesized using a scalable hydrothermal and microwave process, was used as an additive to improve the flame retardancy of epoxy resin (EP) for the first time. Chemical structure and morphology characterization confirmed that the nitrogen and phosphorus atoms were doped into the graphite lattice adopting pyrrolic-N, pyridinic-N, quaternary-N and pyrophosphate and metaphosphate forms. Doping increased the oxidization resistance of rGO and the thermal-oxidative stability of its composites' char, while also improving the catalytic charring ability of polymer. Both effects resulted in the formation of a stable char protective layer during burning and to a significant improvement in flame retardation and smoke suppression in the final composites. The peak heat release rate (PHRR), total heat release (THR) and total smoke production (TSP) for the EP-based composite (containing 5 wt% PN-rGO) decreased by 30.9%, 29.3% and 51.3%, respectively, compared to neat EP. Our work has produced a promising graphene-based flame retardant additive for the mass production of high-performance composites, also expended the application of heteroatom-doped graphene.