Rigid-flexible double attributed thermal insulated nanofibrillated cellulose aerogel with outstanding flame retardancy and mechanical performance

Developing sustainable cellulose aerogels that simultaneously provide exceptional flame retardancy and high mechanical robustness is crucial for energy-efficient buildings, however, it remains a significant challenge. Herein, mullite fibers frameworked porous multi-ridged sandwich-structured nanofibrillated cellulose (NFC) aerogel has been fabricated via an "interface bonding strategy" as thermal insulation material for construction that integrates fire resistance and high strength. The middle flexible NFC aerogel layer serves as a substrate to ensure effective thermal insulation and shape stability, while the outer rigid mullite fibers layer (MFL) creates a framework to retard flame propagation and mechanical damage. Notably, sodium alginate enhances interlayer adhesion through hydrogen bonding and ionic interactions, which fundamentally mitigates interlayer delamination and endows the as-prepared composite with combined rigid-flexible characteristics. The optimal sample, NFCA@M3, exhibits a remarkable performance with an ultra-low thermal conductivity of 0.032 W·m-1·K-1, a high limit oxygen index of 47.13%, and an exceptionally low peak heat release rate of 16.82 kW·m-2. Moreover, NFCA@M3 also possesses superior lightweight properties and ultra-high mechanical strength of 5.53 MPa. Such remarkable comprehensive properties stem from its synergistic suppression of radiative, conductive, and convective heat transfer, as well as the cooperative physical barrier and chemical intumescent flame-retardant mechanisms. This work offers a feasible paradigm for designing the next generation of fireproof and thermal insulation materials, with significant potential applications in building materials and fire-protection protective gear.