Hexachlorocyclotriphosphazene functionalized lignin as a sustainable and effective flame retardant for epoxy resins

Lignin, a low-cost and abundant polyphenol biomass, has considerable potential to develop sustainable flame retardants for polymers due to its aromatic skeleton and high char-forming capability after combustion. In order to integrate phosphorous and nitrogen elements, herein lignin was facilely modified by hexachlorocyclotriphosphazene through a one-step nucleophilic substitution reaction. The functionalized lignin (Lig-HCCP) was used as an additive to improve the flame retardancy of epoxy resins. The effects of Lig-HCCP with different loading levels on the curing behaviors, rheological characteristics, as well as thermal, mechanical, and flame-retardant properties of the resulting epoxy composites were studied in detail. Notably, the epoxy composite containing 20 wt% Lig-HCCP showed the highest limited oxygen index (LOI) of 28.2% and achieved a UL-94 V-0 rating in the vertical burning test. Furthermore, the results of cone calorimeter tests indicated that the peak heat release rate, heat release rate, and total smoke production of the epoxy composite were decreased by 58.9%, 38.5%, and 63.1%, respectively, compared with the control resin. The flame retardant mechanisms could be ascribed to the P-N synergy and abundant aromatic structures in Lig-HCCP, which contributed to the formation of intumescent and compact char layers. This work demonstrates a simple and promising approach to utilize lignin feedstocks for value-added flame retardants.