Synergistic flame-retardant behavior and mechanisms of aluminum poly-hexamethylenephosphinate and phosphaphenanthrene in epoxy resin

The flame retardants aluminum poly-hexamethylenephosphinate (APHP) and 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide(DOPO) were incorporated into diglycidyl ether of bisphenol A (EP) thermoset, and then the synergistic flame-retardant behavior and mechanism of APHP/DOPO were investigated. Comparing with the thermosets with 6%APHP and 6%DOPO alone, 2%APHP/4%DOPO/EP thermosets obtained the higher limited oxygen index, higher UL94 rating, decreased peak of heat release rate and less total heat release from cone calorimeter test. The results reveal a synergistic effect between APHP and DOPO. The synergistic effect of APHP/DOPO in gaseous phase obviously reduced effective heat of combustion, which implies the better flame inhibition effect through quenching free radical chain reaction of combustion. The synergistic charring effect in condensed phase led to the higher char yield, which locked more carbonaceous contents in residue and form more barrier to heat spreading. All the results were caused by the early decomposed DOPO that interacted with the later decomposed APHP to produce more char and decease release of the inflammable gas. Therefore, the burning intensity of APHP/DOPO thermosets obviously was weakened by the synergistic effect of APHP/DOPO.