Highly-Flame-Retardant Performance and Sustainable Polyurethane Foams from Industrial Kraft Lignin via Exploiting Lignin Demethylation

Achieving the enhancement of thermal stability in polyurethane (PU) foams and promoting their green and harmless development have long been both hotspots and challenges in this field. Here, the molecular integration of demethylated lignin and 2NH2DOPO was achieved through the Mannich reaction, which were successfully incorporated into the macromolecular network of polyurethane foam. The unique molecular structure and functional groups synergistically enhance both its condensed-phase and gas-phase flame-retardant performances, which endows the PU foam with an astonishing limiting oxygen index (LOI) value as high as 34.5% and enables it to meet the combustion standard of UL-94 V-0. In addition, the incorporation of lignin enhances the rigidity of the polymer network structure and establishes a chemically reversible dynamic network, thereby endowing the foam composites with improved compressive strength. This work provides an innovative approach for coordinating the development of sustainable and flame-retardant PU foams and paves the way for the high-value utilization of lignin.