Synergistic charring effect of triazinetrione-alkyl-phosphinate and phosphaphenanthrene derivatives in epoxy thermosets

A flame retardant tri-3-(aluminum phosphinate)-propyl-1-triazine-trione (TAHP) was synthesized via the addition reaction between triallyl isocyanurate (TAIC) and hypophosphorous acid, and the molecular structure and thermal stability of TAHP were characterized. Then, TAHP was applied to the diglycidyl ether of bisphenol A (EP) cured using 4,4'-diamino-diphenylmethane with another DOPO derivative TAD, and the synergistic flame-retardant behavior of TAHP/TAD and the mechanism of action were also investigated. Compared with 4% TAHP/EP and 4% TAD/EP samples, the sample with 1 wt% TAHP and 3 wt% TAD obtained an LOI value of 36.0%, passed the UL94 V-0 rating test and exhibited a decreased peak value of the heat release rate. The interaction of TAHP and TAD locked more phosphorus-containing components in the residue and formed a phosphorus-rich char layer mixed with aluminum phosphate, which generated a synergistic charring effect between TAHP and TAD and brought a better barrier effect to the epoxy thermosets during combustion. The interaction between TAHP and TAD led to a more balanced flame retardancy in the gaseous phase and condensed phase. Therefore, the TAHP/TAD system was able to endow epoxy thermosets with better flame retardancy than them used alone.