Combination of expansible graphite and bismuth oxide for simultaneously improving the thermal behavior, fire performance and mechanical property of intumescent flame-retardant epoxy resins

For effectively balancing the mechanical properties and flame resistance of intumescent flame-retardant epoxy resin (EP), the flame-retardant EPs were prepared to explore the synergistic effect of expansible graphite (EG) and bismuth oxide (Bi2O3) on traditional intumescent flame retardant (IFR). The results demonstrate that IFR-EG-Bi2O3 system has better flame resistance effect than IFR-EG or IFR-Bi2O3 system. Especially, 27 wt% IFR, 1.5 wt% EG and 1.5 wt% Bi2O3 endows EP with a limiting oxygen index (LOI) value of 28.7% and a UL94 V-0 rating, while 30 wt% IFR only imparts a LOI of 26.1% and a UL94 V-2 rating to EP. Moreover, the total heat release (THR) and total smoke release (TSR) of EP/IFR-EG-Bi2O3 composite are decreased by 18.3% and 45.0% compared to EP/IFR system. The enhanced thermal stability of EP/IFR-EG-Bi2O3 is attributed to the combination of EG and Bi2O3 that favors the retention of large amounts of aromatic structures in condensed phase. Meanwhile, replacing 3.0 wt% IFR with EG and Bi2O3 can effectively reduce the negative impact on EP matrix and achieve a better balance between mechanical properties and flame retardancy of intumescent flame-retardant EP due to the good interfacial adhesion between EG and EP and the crosslinked points formed by Bi2O3 and EP molecule.