Flame retardancy of lignin-based epoxy resins with a high amount of lignin prepared via copolymerization in solvent

To enhance the flame retardancy of lignin-based epoxy resins and simultaneously achieve a higher percentage of utilizing lignin, lignin-based epoxy resin with a high amount of lignin was prepared from pulping black liquor lignin and epoxy resin of E51 using xylene as the solvent via one-step copolymerization, then directly used in situ to produce lignin-based epoxy resin composites. The effects of the amount of sulfuric acid treated black liquor lignin on the flame retardancy of lignin-based epoxy resin composites were investigated. The synthesis of the lignin-based epoxy resin was confirmed. The results of two control lignin-based epoxy resins indicated that the additive xylene reduced the crosslinking density of epoxy resin. The prepared lignin-based epoxy resins were completely dissolved in xylene because of the reduced crosslinking density, which was caused by the decreased number of epoxy groups reacted with lignin. It was found that flame retardancy of lignin-based epoxy resin composites were increased with the amount of lignin, and exhibited a higher flame retardancy than other lignin-based epoxy resin composites without flame retardants. The analysis of char residue indicated that a novel chemical structure char layer was composed of char residue, sodium carbonate, and sodium monohydrate carbonate, which not only prevented the inward diffusion of heat and oxygen but also prevented the outward release of hazardous gases and smoke. The lignin-based epoxy resin with a high amount of lignin presented a novel material for the high-value utilization of lignin and a significantly increased utilization percent of lignin.