Pyrolysis route of a novel flame retardant constructed by phosphaphenanthrene and triazine-trione groups and its flame-retardant effect on epoxy resin

A novel flame retardant TGIC-DOPO, which was constructed by phosphaphenanthrene and triazine-trione groups, was synthesized via a controllable ring-opening addition reaction between 1,3,5-triglycidyl isocyanurate (TGIC) and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO). The flame-retardant effect of TGIC-DOPO on an epoxy resin, diglycidyl ether of bisphenol-A (DGEBA), cured with 4,4′-diamino-diphenyl sulfone was investigated. The results of the limited oxygen index (LOI), UL94 vertical burning test, and cone calorimeter test indicated that the TGIC-DOPO imparted flame-retardant properties to DGEBA thermosets. When the mass fraction of TGIC-DOPO reached 12wt.%, the DGEBA thermoset acquired a LOI value of 33.3%, UL94 V-0 rating, and the lower peak of heat release rate (pk-HRR) at 481 kW/m2. Specifically, the DGEBA thermoset with 6wt.% TGIC-DOPO had an LOI value of 33.3%, whereas the DGEBA thermoset with 10wt.% TGIC-DOPO had the highest LOI value of 35.2% among the specimens. Meanwhile, the time to ignition, pk-HRR, average of effective heat of combustion (av-EHC), and total heat release of the DGEBA thermoset were all negatively correlated with the mass fraction of TGIC-DOPO. Moreover, the average CO2 and CO yields exhibited a downtrend with increasing mass fraction of TGIC-DOPO from 6wt.%. The reduction of av-EHC with increase of TGIC-DOPO content in thermosets confirmed the free radical quenching effect of TGIC-DOPO in gaseous phase during combustion. The macromorphology, micromorphology and element content of the residues from the cone calorimeter test revealed the bi-phase flame-retardant effect of TGIC-DOPO. Furthermore, the pyrolysis route of TGIC-DOPO were investigated via Py-GC/MS, which disclosed that the decomposed TGIC-DOPO with double flame-retardant groups released various fragments with quenching effect on free radical chain reaction of combustion. The fragments enhanced the flame-retardant performance of DGEBA thermosets both in gaseous and condensed phases. The flame-retardant performance of TGIC-DOPO was resulted by the quenching effect of TGIC-DOPO and the synergistic effect of phosphaphenanthrene and triazine-trione groups.