Intramolecular cooperation and biphasic flame retardant mode of action: effectiveness of hexa(1,2,4-triazol-3-ylamine) cyclotriphosphazene in epoxy resin

The substitutional structure of cyclotriphosphazene derivatives significantly influences their flame-retardant effectiveness. A cyclotriphosphazene derivative with triazole group, referred to as hexa(1,2,4-triazol-3-ylamine) cyclotriphosphazene (HATA), was utilized to improve the flame retardancy of epoxy resin (EP). Differential scanning calorimetry, thermogravimetric analysis, and dynamic mechanical analysis were employed to characterize the thermal properties of EP/HATA thermosets. HATA facilitated the curing of EP due to its triazole and secondary amine structure. EP/HATA thermosets exhibited improved char-forming ability and storage modulus, attributed to the rigid cyclophosphonitrile structure of HATA. As a result of incorporating 5% HATA with 0.73 wt% phosphorus, EP passed UL-94 V-0 level. Subsequent analysis using a cone calorimeter revealed obvious reductions in the peak heat release rate, fire growth rate, and total smoke production of EP with the addition of HATA. Simultaneously, there was a significant enhancement in the char yield of EP during combustion, indicating notable improvements in fire safety. Additional investigations, including X-ray photoelectron spectroscopy, scanning electron microscopy, TG-FTIR, and pyrolysis gas chromatography/mass spectrometry, were employed to analyze the char residue and gaseous volatiles. HATA promoted the formation of a dense, continuous, and intumescent char layer containing cyclophosphonitrile structure in EP. Moreover, the decomposition of HATA released a notable quantity of nitrogen-containing volatiles, effectively mitigating flammable gases originating from the EP matrix in gaseous phase. A biphasic flame-retardant mode of action was proposed, underscoring cooperative flame-retardant effects arising from the interaction between triazole substituents and cyclophosphonitrile structure in HATA molecular for EP.