Nitrogen/sulfur-containing DOPO based oligomer for highly efficient flame-retardant epoxy resin

Abstract Improving the flame retardancy of epoxy resin will greatly expand its application in the fields requiring flame retardancy. Herein, a nitrogen/sulfur-containing DOPO based oligomer (SFG) for highly efficient flame-retardant epoxy resin was successfully synthesized via a facile one-pot method with DOPO, sulfaguanidine and p-phthalaldehyde. The incorporation of SFG accelerated the cross-linking reaction of epoxy system and improved the tensile strength, flame retardancy and fire safety of epoxy thermoset. SFG showed high flame-retardant efficiency due to the synergism among nitrogen, sulfur and phosphorus. With only 0.38 wt% phosphorus content, the corresponding epoxy thermoset achieved an LOI value of 32.8%, passed UL-94 V-0 rating and possessed lower fire growth rate and total heat release in cone calorimeter test in comparison with neat thermoset. SFG exerted dual flame-retardant effect not only by diluting oxygen and combustible volatiles and quenching active radicals, but also by promoting the formation of thermal insulating char layer with honeycombed cavity inside. Moreover, the appearance of blowing out phenomenon during combustion was disclosed that the intensive release of gases blew out the flame based on the dilution and quenching effects when the inner gas pressure was sufficient to break through the char layer.