The synergistic effect of expandable graphite and zinc oxide as flame retardants on the properties of epoxy resin

In this study, zinc oxide (ZnO) nanoparticles and expandable graphite (EG) were used as flame retardants to reduce the flammability of epoxy resin. The flame retardants increased the flexural modulus of the epoxy matrix by up to 50%. The effective heat of combustion of the samples containing both ZnO and EG was reduced to more than one-third of the reference value, while the maximum average rate of heat emission of the samples showed a reduction of almost 60% compared to pure epoxy. During the burning of the epoxy resin, ZnO and EG reduced the emission of gas-phase compounds containing C-H and N-H bonds. The incorporation of ZnO into the epoxy matrix resulted in a "puff pastry-like" structure that facilitated the release of evolved gases during combustion, thereby promoting foaming in the intumescent char formation process. Moreover, ZnO contributed to the enhanced structural stability of the EG residue.