Synthesis of a DOPO-containing imidazole curing agent and its application in reactive flame retarded epoxy resin

A novel DOPO-containing imidazole curing agent (DIB) was successfully synthesized, and served as a co-curing agent of 4,4‘-diaminediphenyl sulfone (DDS) to prepare a reactive flame-retarded epoxy thermoset (EP/DDS/DIB). The DSC results showed that the curing temperature of EP/DDS/DIB mixture was much lower than that of EP/DDS mixture due to the catalytic curing effect of imidazole group. Although the Tg values of EP/DDS/DIB thermosets decreased in some degree, they were still obviously higher than that of EP/DDS/DOPO thermoset. The combustion tests indicated that DIB was a high-efficiency flame retardant for epoxy resin. The LOI value and UL94 rating of EP/DDS/DIB-0.75 sample reached 36.8% and V-0, respectively. In addition, the pk-HRR, av-HRR and THR of EP/DDS/DIB-1.0 thermoset were decreased by 57.2%, 37.9% and 36.0%, respectively, compared with those of EP/DDS thermoset. The studies concerning decomposition products in both condensed and gaseous phases revealed that the introduction of DIB brought about the formation of a protective intumescent char layer and the release of phosphorus-containing radicals and noncombustible nitrogen-containing fragments during combustion. DIB exerted bi-phase flame retardant effect on epoxy resin.