Development of a flame-retardant epoxy resin with high glass transition temperature and transparency based on a diphenylphosphine oxide derivative

To obtain high-performance flame-retardant epoxy resin (EP), diglycidyl ether of (2,5-dihydroxyphenyl) diphenyl phosphine oxide (DPO-HQ-EP) was synthesized. EP/DPO-HQ-EP samples with varying phosphorus contents were prepared by curing a mixture of DPO-HQ-EP and diglycidyl ether of bisphenol A. The incorporation of DPO-HQ-EP significantly enhanced the flame retardancy of EP without compromising its glass transition temperature. The EP/DPO-HQ-EP/0.6 exhibited a limited oxygen index of 31.7% and achieved a V-0 rating in the vertical burning test. In the cone calorimeter test, due to the incorporation of DPO-HQ-EP, the peak of heat release rate and total heat release of EP/DPO-HQ-EP/0.6 decreased by 39.4% and 15.9% compared with the values for pure EP. A detailed investigation of the flame-retardant mechanism revealed that the improved flame retardancy of EP/DPO-HQ-EP samples was attributed to the release of phosphorus-containing free radicals and non-flammable gases in the gas phase, as well as the formation of a continuous and dense char layer in the condensed phase. Moreover, the dielectric constant and dielectric loss factor of EP/DPO-HQ-EP samples were lower than those of EP/0. The water absorptivity and transparency of EP were effectively preserved with the incorporation of DPO-HQ-EP. These findings highlighted the potential of EP/DPO-HQ-EP for industrial applications in an electrical field.