Facile synthesis of a reactive P/N/S-containing compound toward highly effective flame retardancy of epoxy resin with high transparency and improved mechanical strength

It is still a great challenge to develop flame-retardant epoxy resins (EP) with high transparency. Herein, a reactive P/N/S-containing compound (DOPT) was designed and synthesized by the Atherton-Todd reaction between p-toluenesulfonamide and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide. The addition of DOPT simultaneously improved the flame retardancy, fire safety and mechanical strength meanwhile kept the better transparency of the EP At 5 wt% DOPT loading, the modified EP (FREP-5) achieved a LOI value of 37.5% and UL-94 V-0 rating, with maintaining almost the same transmittance of the pure EP. Meanwhile, the FREP-5 also gave the lowest values of the peak heat release rate, total heat release, total smoke production and fire growth rate, revealing the enhanced fire safety of the flame-retardant EP. The significant improvement of flame retardancy of FREP thermosets was ascribed to the physical barrier effect of phosphorus-rich char in condensed phase, and the quenching and diluting effects of abundant phosphorus-containing free radicals and nitrogen/sulfur-containing inert gases in gaseous phase. Furthermore, the results of dynamical mechanical analysis and mechanical performance test revealed that the incorporation of DOPT improved the tensile and flexural strength of epoxy thermoset. In summary, DOPT is a novel highly efficient flame retardant for EP, which enables EP to maintain its potential application in the optical fields.