Phosphorus-Free Vanillin-Derived Intrinsically Flame-Retardant Epoxy Thermoset with Extremely Low Heat Release Rate and Smoke Emission

Recently, the development of bio-based intrinsically flame-retardant epoxy thermosets has attracted increasing interest for the replacement of petroleum-based epoxy thermosets in fields that demand high anti-flammability. In this work, we synthesized a Schiff base triazole-containing epoxy monomer (Triazole-VA-EP) using vanillin (from a renewable source) and 3,5-diamino-1,2,4-triazole. The molecular structure of Triazole-VA-EP was confirmed by the proton and carbon nuclear magnetic resonance. The Triazole-VA-EP monomer was cured by 4,4′-diaminodiphenylmethane (DDM) to obtain a bio-based epoxy thermoset, with a petroleum-based diglycidyl ether of bisphenol A (DGEBA) cured by DDM as a contrastive sample. The cured Triazole-VA-EP/DDM product displayed higher tensile strength and glass transition temperature than the cured DGEBA/DDM system. The cured Triazole-VA-EP/DDM product showed an outstanding intrinsic flame resistance, with a relatively high limited oxygen index (LOI) value of 39.5% and UL-94 V-0 rating, whereas the cured DGEBA/DDM system displayed a low LOI value of 23.5% and no rating in the UL-94 vertical burning measurement. Furthermore, the cured Triazole-VA-EP/DDM system exhibited 82.3, 52.8, and 71.7% decline in peak heat release rate (199.6 versus 1125.2 kW/m2), total heat release (47.04 versus 99.73 MJ/m2), and total smoke production (5.70 versus 20.12 m2), respectively, compared to the cured DGEBA/DDM system. The super anti-flammability of the cured Triazole-VA-EP/DDM system was attributed to its excellent charring ability, which not only cut off the fuel supply by reducing the amount of thermal decomposition volatiles but also served as a barrier to retard the heat release and smoke emission. This work provides a new strategy to synthesize a bio-based fire-safe epoxy monomer by Schiff base reaction without using phosphorus-containing compounds.