Functionalization of graphene with grafted polyphosphamide for flame retardant epoxy composites: synthesis, flammability and mechanism

A polyphosphamide (PPA) was synthesized and covalently grafted onto the surface of graphene nanosheets (GNSs) to obtain a novel flame retardant, PPA-g-GNS, and subsequently PPA-g-GNS was incorporated into epoxy resins (EPs) to enhance the fire resistance. The chemical structures and morphology of the precursors and target product were confirmed using 1H-NMR spectroscopy, Fourier transform infrared spectroscopy and atomic force microscopy. The tensile results showed that the mechanical strength and modulus of the PPA-g-GNS/EP composite were higher than those of pure EP and PPA/EP, owing to the outstanding reinforced effect of graphene. The evaluation of the thermal properties demonstrated that the addition of PPA or PPA-g-GNS to epoxy had a thermal destabilization effect below 400 °C, but led to a higher char yield at higher temperatures. Furthermore, the PPA-g-GNS/EP composite exhibited superior fire resistant performance, such as higher LOI values and reduced PHRR and FIGRA values, compared to pure EP and PPA/EP, which was probably attributed to the higher char yield during combustion. A possible flame retardant mechanism was speculated according to the direct pyrolysis-mass spectrometry results: the phosphate species degraded from PPA catalyzed the decomposition of the PPA-g-GNS/EP composites to generate various pyrolysis products; the pyrolysis products were absorbed and propagated on the graphene which served as a template of micro-char, and thus a continuous and compact char layer was formed; such a char layer provided effective shields to protect the underlying polymers against flame.