Role of lignin on microstructure, mechanical properties and flame retardancy of nanocellulose-based composite hydrogels

Flame-retardant composite hydrogels provide significant advantages over conventional fire suppressants due to their high water retention, char-forming ability, and mechanical adaptability. However, the current formulations mostly rely on synthetic polymers or nanoparticles which restricts their large-scale application and reduces sustainability. In this work, we developed lignin-incorporated dynamically crosslinked cellulose nanocrystal (CNC) hydrogels and systematically investigated the interrelation between microstructure, rheological behavior, thermal properties, and flame-retardant performance. The incorporation of lignin at moderate concentrations enhanced hydrogen bonding, resulting in a denser and more homogeneous hydrogel network with reduced mesh size. The resulting elastic network enhanced water retention during burning, promoting flame retardancy via substrate cooling and fuel dilution effects. Additionally, lignin facilitated the formation of a compact glassy char layer, effectively serving as a heat and oxygen barrier. Thermal decomposition of dried CNC films resulted in 8.6 % residue at 700 °C, whereas CNC-lignin-borax composites exhibited a significant increase in char yield, reaching 70.5 %. The optimal lignin composition extended burn-through time of wood to 12 min-71 % and 33 % longer than uncoated and neat CNC samples. Our findings highlight the potential of CNC-borax-lignin hydrogels as biorenewable environmentally friendly coatings with superior flame-retardant properties, offering a sustainable approach to fire prevention in wood-based materials.