Lignin-derived bio-based flame retardants toward high-performance sustainable polymeric materials

Bio-based flame retardants represent one of the most promising directions for next-generation flame retardants due to their sustainability, environmental benefits and comparable efficiency to current non-bio-based counterparts. Because of lignin's unique aromatic structure and high charring capability, recent years have witnessed the great flame-retardancy potential of pristine lignin and its derivatives in a wide range of polymeric materials. Unfortunately, to date there has been no critical review on the preparation, modification and application of lignin-derived flame retardants for polymeric materials. This review focuses on the flame-retardancy effects of pristine lignin and lignin chemically modified by introducing the elements phosphorus and/or nitrogen, as well as their synergistic effects with existing flame-retardant additives. This review also comprehensively compares the flame-retardancy performances of different lignin-derived flame retardants in various polymeric matrices, such as polylactic acid, polypropylene and polyamide 11. Following the conclusions, future perspectives are also presented along with new opportunities for the development of more efficient and effective lignin-based flame retardants for polymeric materials.