A novel strategy utilizing oxidation states of phosphorus for designing efficient phosphorus-containing flame retardants and its performance in epoxy resins

The oxidation state of phosphorus has a great influence on the flame retardant action of its flame retardants. In this study, a reactive phosphorus-containing flame retardant (POG-DOPO) with both low and high phosphorus oxidation states is synthesised and incorporated into epoxy resin. The results show that the part in high oxidation state act mainly in the condensed phase and the part in low oxidation state act mainly in the gas phase. Furthermore, the flame retardant exhibits a synergistic flame retardant effect, resulting in a higher flame retardant efficiency than that of flame retardants with a single phosphorus oxidation state. Compared with unmodified epoxy resin, the peak heat release rate, total heat release rate and total smoke release of POG-DOPO modified epoxy resin with only 3wt% phosphorus are reduced by 70.9%, 51.2% and 55.7%, respectively. Its LOI increases to 31.8% and vertical combustion test achieves UL-94 V-0 rating. This study provides a strategy for the structural design of efficient phosphorus-based flame retardants.