Synthesis of organophosphate-functionalized graphene oxide for enhancing the flame retardancy and smoke suppression properties of transparent fire-retardant coatings

A series of organophosphate-functionalized graphene oxide flame retardants (GPPBs) were successfully synthesized by grafting flexible phosphate ester (PPB) on the surface of graphene oxide (GO), and well characterized by Fourier transform infrared (FTIR) spectroscopy and 1H nuclear magnetic resonance (1H NMR) spectroscopy. The obtained GPPBs were then incorporated into amino resin to produce transparent fire-retardant coatings for reducing the fire hazard of wood. The transparency analysis shows that the GPPBs endow the resulting coatings with a high degree of transparency even at a relatively high GO contents due to the uniform dispersion and completely exfoliated states of GO in amino matrix. The evaluation of combustion behavior reveals that the introduction of GO greatly reduces the weight loss, char index, flame spread rating (FSR), smoke production and heat release of the coatings concomitant with an increase in the insulation property, for example, 60.7% reduction in smoke density rating (SDR) and 33.3% reduction in total heat release (THR) are observed in the case of MGPPB3 coating obtained from GPPB3 with 0.07 wt% GO in comparison to MGPPB0 coating obtained from PPB. Thermo-gravimetric analysis shows that the thermal stability and residual weight of the coatings are improved after introduction of GO, and MGPPB3 exhibits the highest residual weight of 34.5% at 700 °C. The main mechanism of the GPPBs for enhancing the fire safety of transparent fire-retardant coatings is ascribed to the formation of high-quality intumescent char with excellent compactness, anti-oxidation ability and barrier effect against fire.