Flame retardant effect and mechanism of a novel DOPO based tetrazole derivative on epoxy resin

A novel highly efficient flame retardant with phosphaphenanthrene group and tetrazole ring, 6-(((1H-tetrazol-5-yl)amino)(4-hydroxyphenyl)methyl)dibenzo[c,e][1,2]oxaphosphinine 6-oxide (ATZ), was synthesized and incorporated into epoxy matrix as co-curing agent to remarkably improve the flame retardancy of epoxy resin. The curing behaviors of epoxy systems were investigated by non-isothermal differential scanning calorimetry (DSC) tests. The thermal and flame retardant properties of epoxy thermosets were studied by thermogravimetric analysis (TGA), DSC analysis, limiting oxygen index (LOI), UL-94 and cone calorimeter test. Moreover, the flame retardant mechanism was comprehensively investigated by analyzing the thermo-oxidative degradation and pyrolysis behaviors, along with the morphology and structure of residual char of epoxy thermosets. It showed that ATZ could accelerate the crosslinking reaction of epoxy system. The incorporation of ATZ resulted in the decrease of glass transition temperature (T-g) and initial degradation temperature, while the increase of char yield at high temperature of epoxy thermoset. With the incorporation of 6 wt% ATZ, the thermoset achieved an LOI value of 33.7% and V-0 rating in UL-94 test. Moreover, the peak heat release rate (PHRR) in cone calorimetry test decreased by 26% in comparison with that of neat EP, accompanied with lower total heat release (THR) and total smoke production (TSP). The flame retardant mechanism was ascribed to the barrier effect of intumescent and compact phosphorus-rich char layers with honeycombed cavity structure inside in the condensed phase, and quenching effect of phosphorous radicals and diluting effect of nonflammable gases in the gas phase. The interesting blowing out effect observed during UL-94 test arose from the intensive release of pyrolytic gases with considerable phosphorous radicals and nonflammable gases from the surface of sealed char layers.