Synthesis and application of a novel phosphoryl thiourea-containing flame retardant for epoxy resin

A novel phosphoryl thiourea-containing flame retardant (PSN) was synthesized and fully characterized. The influences of PSN with different phosphorus content on diglycidyl ether of bisphenol-A epoxy resin (DGEBA) cured by 4,4 '-diaminodiphenylsulfone (DDS) were comprehensively evaluated via the measurement of flame retardancy, thermal performance, and mechanical property. The results showed that PSN-modified DGEBA/DDS thermosets exhibited moderate changes in glass transition temperature and thermal stability, and PSN could catalyze DGEBA/DDS to produce more char residues under both nitrogen and air atmosphere. When the phosphorus content reached only 1.0 wt%, the PSN-modified DGEBA/DDS thermoset obtained limited oxygen index (LOI) value of 29.2%, and reached vertical burning (UL-94) rating of V-0, which exhibited more excellent flame retardancy than that of the neat DGEBA/DDS thermoset with LOI value of 22.5% and no UL-94 rating. Moreover, compared with DGEBA/DDS, PSN-1.5/DGEBA/DDS thermoset with phosphorus content of 1.5 wt% presented lower combustion parameters of peak heat release rate (PHRR), total heat release (THR), and total smoke release (TSR), which further showed that PSN could effectively enhance the flame retardancy of DGEBA/DDS epoxy thermosets. The investigation of flame-retardant mechanism revealed that a P/N/S synergistic effect was presented by PSN, which was reflected in the condensed phase via the increase in the phosphorus-containing char yield and the gas phase via the release of non-flammable gas such as NH3, SO2, and H2O. Additionally, PSN-modified DGEBA/DDS thermosets could maintain above 83% of Izod impact strength for DGEBA/DDS. The high flame-retardant efficiency, good smoke suppression performance, and less negative effect on Izod impact strength of DGEBA/DDS epoxy thermoset promise that PSN would be a potential flame retardant for DGEBA/DDS epoxy system.