Highly efficient flame-retardant epoxy resin with a novel DOPO-based triazole compound: Thermal stability, flame retardancy and mechanism

A novel 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO)-based triazole compound (DTA) was prepared by reacting DOPO with imine obtained from the condensation of terephthalaldehyde with 3-amino-1H-1,2,4-triazole. Its chemical structure was characterized by FTIR, 1H NMR, 13C NMR, 31P NMR and elemental analysis. DTA was used as co-curing agent to improve the flame retardancy of epoxy resin. The thermal stabilities and flame-retardant properties of epoxy thermosets were evaluated by thermogravimetric analysis (TGA), limiting oxygen index (LOI) and UL-94 test. Moreover, the flame-retardant mechanism was investigated by FTIR, Py-GC/MS, SEM, elemental analysis and Raman spectroscopy. The results revealed that the addition of DTA resulted in the decrease of 5 wt% mass loss temperature of epoxy thermoset. With triazole ring and DOPO group in the molecular structure, DTA possessed superior flame-retardant efficiency. The thermoset modified with 4 wt% DTA achieved an LOI value of 34.8% and V-0 rating in UL-94 test, where the phosphorus content was only 0.34 wt%. Impressively, all modified thermosets showed blowing-out effect during combustion, which resulted from the concentrated release of pyrolytic gases containing phosphorus-based radicals and nonflammable gases. The flame-retardant mechanism was ascribed to the barrier effect of phosphorus-rich char layers with polyaromatic structures in the condensed phase, and quenching effect of phosphorus-based radicals and diluting effect of nonflammable gases in the gas phase.