Lignin-Derived Flame-Retardant Cross-linker toward High-Strength and Multifunctional Cellulose Aerogel

Cellulose aerogel shows a promising application prospect in the building insulation field because of its environmentally friendly nature and superior thermal insulation performance, whereas it still confronts the inherent drawbacks of unsatisfactory strength, high flammability, and inferior UV-shielding capacity. In the present study, we synthesized a lignin-derived water-soluble flame-retardant cross-linker (LG). Subsequently, we constructed high-strength and multifunctional cellulose composite aerogels based on TEMPO-oxidized cellulose nanofiber and LG through a freeze-drying technique followed by a baking-cross-linking process. The composite aerogel (LG70) achieved outstanding flame retardancy with an LOI value of 30.1% and a UL-94 rating of V-0. Moreover, it self-extinguished and maintained its structural integrity when sprayed with a butane torch. Its specific modulus reached as high as 30.7 MPa/(g/cm3) while maintaining a lightweight feature (15.98 mg/cm3). LG70 exhibited a low thermal conductivity of 23.8 mW/(m·K) comparable to that of commercial polyurethane foam, showing high application potential in the building insulation field. Moreover, it possessed a high DPPH· scavenging rate (70.5%) and an ultrahigh ultraviolet protection factor (6354.5). This work provides not only a new idea for the synthesis and application of lignin-derived flame retardants with high water solubility and reactivity but also a new insight into addressing the inherent drawbacks of cellulose aerogel using biomass resources.