A phosphorus/sulfur-containing compound toward simultaneously endowing epoxy resin with good flame retardancy and high transparency

A phosphorus/sulfur-containing compound (sulfonylbis(3,1-phenylene) bis(diphenyl-phosphinate), named as SBPDPP) is designed and synthesized by nucleophilic substitution reaction between diphenylphosphinyl chloride and 4,4-dihydroxydiphenylsulfone, and its chemical structure is verified by Fourier transform infrared, 1 H and 31 P nuclear magnetic resonance. The incorporation of SBPDPP accelerates the curing process but decreases the glass transition temperature and the crosslinking density of the epoxy resin (EP). The cured EP with 6 wt% SBPDPP passes UL-94 V0 rating and gives limited oxygen index value of 38.8%, meanwhile maintaining 90% of the transmittance of the pure EP. Its peak heat release rate, total heat release, total smoke production, and total smoke rate are reduced by 40.8%, 42.0%, 26.3%, and 21.9% in comparison with the pure EP, demonstrating that the SBPDPP improved simultaneously the fire safety and smoke suppression. The comprehensive analysis of the char residues after cone calorimeter test and the volatile gasses during the pyrolysis demonstrates that the SBPDPP exerted dual flame retardant effect in the gas and condensed phase. The cured FREPs have promising applications that requiring flame retardancy and maintaining transparency.