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 .