Abstract: In order to design and develop EP flame retardants with green, environmentally friendly high flame retardant efficiency, two salicylaldehyde-based Schiff alkali metal compounds, SAPO-Fe and SAPO-Bi, were designed and synthesised using SAPO and Fe(NO3)3·9H2O and Bi(NO3)3·5H2O as raw materials. The effects of these compounds on the thermal stability and flame retardancy properties of EP composites were investigated and the related flame retardant mechanisms were probed. The results showed that the vertical combustion test of EP/6% SAPO-Fe and EP/6% SAPO-Bi passed UL-94 V1 rating; The LOI values were improved by 20.6% and 16.2%, respectively; the pHRR values were reduced by 28.6% and 34.8%; The THR values were reduced by 30.8% and 14.2%， and the amount of residual carbon was increased compared with that of EP by 74.1% and 112.9%; TSP values decreased by 40.4% and 38.9%; PCOP values decreased by 44.4% and 50%. The significant improvement in flame retardant performance is mainly due to the synergistic effect of P and N elements and metal ions Fe3+ and Bi3+. The significant increase in the amount of residual carbon and the effective reduction of smoke release are attributed to the excellent catalytic carbon formation ability of these metal ions, which forms a dense carbon layer to enhance the physical barrier effect, thus effectively preventing the generation of smoke, the escape of combustible gases, the entry of oxygen and the release of heat.

Key words: epoxy resin, DOPO, flame retardant properties, Schiff base metal complexes, P/N synergistic flame retardant