Exceptional flame-retardant cellulosic foams modified with phosphorus-hybridized graphene nanosheets

Lightweight bio-based foams from renewable cellulose nanofibers (CNF) and phosphorus-hybridized graphene nanosheets (PGN) were prepared by means of a simple freeze-drying process. By comparison, the CNF–Graphite, CNF–chemically reduced graphene nanosheets (CRG), CNF–red phosphorus (RP) composite foams were also fabricated. The CNF–PGN composite foam showed porous structures and the PGNs were uniformly distributed in the pore wall of the cellulose foams. The resultant CNF composite foams exhibited a thermal conductivity in the range of 0.0299–0.0310 W/(m K). The CNF–PGN composite foam exhibited a high char residue (25.6 wt%) at 800 °C, which was 205% higher than the calculated value, suggesting the excellent char formation ability. In the vertical burning tests, the CNF foam burnt out with rapid flame propagation, while the CNF–Graphite, CNF-RP and CNF–CRG composite foams cannot stop the smoldering behaviors. In contrast to these composite foams, the CNF–PGN composite foam displayed self-extinguishing behaviours and the flame spread was suppressed completely. Cone calorimeter measurements further manifested excellent fire retardancy of the CNF–PGN composite foam, which showed that the peak heat release rate was reduced to 14.6 kW/m², lower than most of the reported by state-of-the-art flame retardant polymeric foams.