Two‐step fabrication of lignin‐based flame retardant for enhancing the thermal and fire retardancy properties of epoxy resin composites

Lignin is a biomass material that attracts more and more attention because of its extremely low utilization rate. In this paper, we have fabricated a lignin‐based flame retardant chemically grafted with PEI followed with polyphosphoric acid by a mild two‐step method. It is found that the nitrogen/phosphorus modified lignin exhibited a lower char forming temperature and enhanced thermal stability. After adding 7 wt% functionalized lignin, the maximum decomposition temperature of epoxy resin (EP) composite was reduced from 387°C to 364°C, while UL‐94 grade and limiting oxygen index was up to V‐0 grade and 31.4%, respectively, with excellent flame retardant effect. Moreover, the results of the characterization show that the carbon residue becomes much denser with the addition of multifunctional lignin‐based flame retardant. It is proposed that the phosphorus element acts to promote char formation in the EP composite, endowing the resin matrix with a dense char layer. And the dense char layer with released ammonia from the nitrogen element to block the entry of air during combustion achieves the purpose of high‐efficiency flame retardancy. This gentle and efficient multifunctional process enhances the driving force for a wide range of applications of lignin‐based flame retardants.