Hierarchically Engineered Nanocellulose-Halloysite Composite Foams for Integrated Thermal Insulation and Intrinsic Fire Resistance.

Achieving the simultaneous integration of thermal insulation, mechanical robustness, hydrophobicity, and intrinsic flame retardancy in bio-based porous materials remains a major challenge. Here, we report a hierarchically engineered nanocomposite foam composed of nanofibrillated cellulose (NFC), alkyl ketene dimer (AKD), and halloysite nanotubes (HNT), designed via a multiscale structure-property approach. Incorporation of HNT (1-10 wt%) reinforces the fibrillar network and introduces a hierarchical macro-mesoporous architecture, while AKD imparts durable hydrophobicity with water contact angles of 133-154°. The foams exhibit low thermal conductivity (0.031-0.046 W m- 1 K- 1), high compressive modulus (∼1.1 MPa), and excellent deformability (∼80% strain). The materials display outstanding flame retardancy, with combustion velocities reduced by over an order of magnitude (∼6 to ∼0.1 cm s- 1) and clear self-extinguishing behavior. Thermal analysis reveals enhanced stability and increased char yield (∼45%), consistent with modified degradation pathways. Thermal cycling tests (10 cycles, 25°C-80°C) confirm structural stability with no pore collapse and negligible change in thermal conductivity (≤2.2%). This multifunctionality arises from the synergy between halloysite-induced barrier effects, hierarchical structural reinforcement, and controlled interfacial chemistry, providing a scalable route to sustainable, lightweight, and fire-safe insulation materials without toxic additives.