Flame-retardant effect of a novel phosphaphenanthrene/triazine-trione bi-group compound on an epoxy thermoset and its pyrolysis behaviour

A novel phosphaphenanthrene/triazine-trione bi-group flame retardant TOD containing two different chemical bridge bonds between phosphaphenanthrene and triazine-trione groups was synthesized through the addition reaction of 1,3,5-triglycidyl isocyanurate (TGIC), 9,10-dihydro-9- oxa-10-phosphaphenanthrene-10-oxide (DOPO) and 10-(2,5-dihydroxyphenyl)-10-H-9-oxa-10-phosphaphenanthrene- 10-oxide (ODOPB) three raw materials in turn. TOD was then applied to prepare the flame-retardant epoxy thermoset in the diglycidyl ether of bisphenol-A cured with 4,4'-diamino- diphenyl methane (EP). By a limited oxygen index (LOI) measurement, UL94 vertical burning test and cone calorimeter test, TOD was observed to efficiently endow the EP thermoset with a higher LOI value, higher UL94 rating, and reduced total heat release in combustion. The introduction of 4 wt% TOD endowed the EP thermoset with a LOI value of 35.9%, UL94 V-0 rating, 42.4% decreased peak of heat release rate, 46.5% decreased total heat release, and slightly elevated char yields. The analyses of the thermal and combustion behaviors of the epoxy thermoset as well as the pyrolysis behavior of TOD together revealed that the free radical quenching effect, inert volatile diluting effect, charring effect, and char layer barrier jointly caused the high-efficiency flame retardant mechanism of TOD in the epoxy thermoset. This suggests a possibility to further improve the flame retardant efficiency of bi- or multi-group compounds by selecting proper bridge-linking between functional groups.