Core-shell flame retardant/graphene oxide hybrid： a self-assembly strategy towards reducing fire hazard and improving toughness of polylactic acid

Biobased flame retardant/graphene oxide hybrid (GOH) as a multifunctional flame retardant for polylactic acid (PLA) was synthesized through organic-solvent-free self-assembly. The electrostatic interactions deposited the polyethylenimine (PEI) and biobased polyelectrolyte (BPE) coating on the surface of ammonium polyphosphate (APP) in water, which gave the negatively charged core-shell flame retardant. Then, GOH was obtained via aqueous self-assembling between the positively charged graphene oxide (pGO) obtained by grafting pristine GO with PEI and the core-shell flame retardant. Subsequently, GOH was employed as multifunctional flame retardant to PLA, aiming to enhance both flame retardancy and toughness. Based on the investigation via LOI, UL94 test and the cone calorimetry, it clearly showed that GOH endowed PLA significantly enhanced flame retardancy. The flame retardancy of GOH in PLA was performed in both of the gas-phase and condensed-phase mechanisms according to the analysis of the volatile gases and the residues. As for the mechanical properties of PLA/GOH composites, an over 6 folds increment in elongation at break (52.4%) and 86.7% increase in notched impact strength (5.6 kJ/m2) were achieved for PLA/10%GOH, compared with that of the neat PLA. It meant the introduction of GOH remarkably improved toughness of PLA.