High-performance flame retardant epoxy resin based on a bi-group molecule containing phosphaphenanthrene and borate groups

A flame retardant named ODOPB-Borate constructed by phosphaphenanthrene (DOPO) and borate groups was synthesized via esterification reaction between boric acid and 10-(2,5-dihydroxyphenyl)-10-H-9-oxa-10-phosphaphenanthrene-10-oxide (ODOPB). The molecular structure of ODOPB-Borate was characterized and verified. To research the group synergistic effect of phosphaphenanthrene and borate groups, ODOPB-Borate was incorporated into epoxy resin, diglycidyl ether of bisphenol-A (DGEBA), cured by 4,4-diamino-diphenyl sulfone (DDS). ODOPB-Borate increased the LOI and particularly improved the UL94 classification of epoxy resin thermosets, and also reduced the heat release of epoxy thermosets. Further, the decrease of total mass loss value of thermoset containing ODOPB-Borate revealed that ODOPB-Borate facilitated the EP composites to form more residue. The analyzed pyrolysis route of ODOPB-Borate revealed that ODOPB part mainly exerted quenching effect in gas phase and the borate part mostly remained in condensed phase and promoted charring effect and smoke suppressing effect. The synergistic effect from phosphaphenanthrene and borate groups endowed epoxy resins with flame retardancy. Moreover, ODOPB-Borate also increased the glass transition temperature and impact strength of epoxy resin.