High-efficiency flame retardant behavior of bi-DOPO compound with hydroxyl group on epoxy resin

To explore the more effective non-halogen flame retardants in epoxy resins, a flame retardant named ABD owning relatively short bridged bonds and a hydroxyl group was synthesized via addition reaction between acrolein and 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO). Then, ABD was applied into flame retardant epoxy resin, diglycidyl ether of bisphenol-A, cured by 4,4′-diamino-diphenylmethane. When the mass fraction of ABD in epoxy resin was only 3 wt%, the thermoset acquired a higher limited oxygen index (LOI) value 36.2% and passed UL94 V-0 rating. ABD also reduced all the values of peak of heat release rate (pk-HRR), total heat release (THR), average effective heat of combustion (av-EHC), total mass loss (TML) and average CO2 yield, and increased the residue yield. In contrast to DOPO, ABD can impose the higher LOI value, the higher UL94 rating and the lower peak value of HRR to epoxy thermosets. Further, the results from the cone calorimeter test reveal that the better flame retardant performance of ABD was caused by quenching effect from phosphaphenanthrene groups in gas phase and the charring effect from the joint action of phosphaphenanthrene group and hydroxyl group in condensed phase simultaneously. The excellent flame retardant performance of ABD is from its higher phosphorus contents and its special hydroxyl group.