Thermal properties and flame retardancy of an intumescent flame-retarded epoxy system containing phosphaphenanthrene, triazine-trione and piperidine

A phosphaphenanthrene/piperidine-containing phenolic derivative (PHTD) was synthesized through the one-pot reaction between triacetonediamine, p-hydroxybenzaldehyde and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide. The intumescent reactive flame-retardant epoxy thermosets (P-0.25, P-0.5, P-0.75 and P-1.0) were composed of diglycidyl ether of bisphenol-A, 1,3,5-triglycidyl isocyanurate (TGIC), 4,4 '-diamino-diphenyl sulfone (DDS) and PHTD. Due to the incorporation of PHTD and TGIC, the flame retardancy of EP samples was remarkably improved. For instance, the LOI value and UL94 rating for P-0.75 sample with only 0.75 mass% phosphorus reached 36.6% and V-0. The average of heat release rate, peak of heat release rate and total heat release for P-1.0 system were decreased by 35.6%, 52.5% and 36.4% compared with those for EP/DDS system. During combustion, plenty of PO radicals, hindered amine and isocyanate fragments were generated by the pyrolysis of PHTD and TGIC in gaseous phase, which suppressed the combustion reaction and diluted the fuels. In condensed phase, the pyrolytic products of phosphaphenanthrene and triazine-trione groups jointly promoted the formation of a compact and intumescent char layer to isolate heat and oxygen. The remarkable improvement of flame retardancy of EP thermoset was ascribed to the flame-retardant effects of PHTD and TGIC in gaseous and condensed phases.