Synergistic effect of bismuth oxide and mono-component intumescent flame retardant on the flammability and smoke suppression properties of epoxy resins

A novel mono-component intumescent flame retardant named pentaerythritol phosphate melamine salt (PPMS)-hybrid bismuth oxide (PPMS-Bi2O3) was synthesized and carefully characterized by FTIR, H-1 NMR, P-31 NMR, SEM-EDS, and TG analyses. Then, PPMS-Bi2O3 was utilized as flame retardant for epoxy resins (EPs), and the thermal stability, flame retardancy, and smoke suppression properties of EP composites were investigated. TG results show that PPMS-Bi2O3 is more conducive to enhance the thermal stability and char forming ability of EP composites compared with the same addition of PPMS or the mixture of PPMS and Bi2O3, and this positive effect is enhanced with the increasing Bi2O3 content. Cone calorimeter test reveals that the PPMS-Bi2O3 can effectively reduce the heat release and smoke production in comparison with PPMS or the mixture of PPMS and Bi2O3 due to the formation of a more compact and intumescent char against fire, as judged by digital photographs and SEM images. EDS analysis indicates that the combination PPMS and Bi2O3 by hydrogen bonds promotes to generate more phosphorus-rich and aromatization structures in the condensed phase that enhance the barrier effect and anti-oxidation ability of the char, thus imparting higher flame retardant and smoke suppression efficiencies to EP composites.