In situ preparation of reduced graphene oxide/DOPO-based phosphonamidate hybrids towards high-performance epoxy nanocomposites

The work reports a strategy based on piperazine-reduced graphene oxide (rGO)/piperazine-based DOPO-phosphonamidate (PiP-DOPO) to overcome the challenge of the dispersion of graphene and mechanical deterioration of epoxy resin (EP) matrix with additive-type flame retardants. Graphene oxide (GO) was functionalized and reduced by piperazine simultaneously, and then incorporated into PiP-DOPO through in situ reaction, resulting in the formation of the hybrids (PD-rGO). Subsequently, the PD-rGO was incorporated into epoxy resin (EP) to fabricate nanocomposite. The structure and thermal properties of PD-rGO were well characterized. The presence of PD-rGO in EP improved the char yields at 700 °C and reduced the maximum mass loss rate under nitrogen, indicating the improved thermal stability at elevated temperature. The evaluation of combustion behavior demonstrated that the PHRR and THR values were decreased significantly by 43.0% and 30.2% by the addition of 4 wt% PD-rGO10 (10 wt% rGO in hybrid) in EP respectively, in comparison to neat EP. The epoxy composite with 4 wt% PD-rGO5 (5 wt% rGO in hybrid) could pass UL-94 V0 rating and the LOI was 28.0%. The flame retardant mechanism could be attributed to the synergism between two compositions in PD-rGO hybrid: the PiP-DOPO was favorable to flame spread inhibition in UL-94 burning test, while the barrier effect of graphene was dominant in terms of heat release rate suppression. Moreover, the addition of PD-rGO hybrid led to slightly improved storage modulus and tensile strength, due to the high stiffness of graphene in hybrids. The PD-rGO hybrid combines the outstanding mechanical behavior of graphene with the good flame retardant effect of DOPO-based compounds, which provides a promising solution to high performance epoxy nanocomposites with improved flame retardant and mechanical properties simultaneously.