Mechanically robust, flame-retardant, and waterproof kapok fiber/sodium alginate composite aerogel for building thermal insulation.

The construction sector significantly contributes to global energy use, particularly through heating/cooling systems. Amid growing energy and environmental concerns, developing efficient insulation materials has become crucial. While petroleum-based foams currently dominate insulation markets, biomass-derived aerogels show promise as sustainable alternatives due to their lightweight nature and eco-friendly properties. However, their practical application is limited by the low compressive strength, flammability, and poor performance in humid environments. This study proposes a novel biomass aerogel that addresses challenges through multi-scale structural design. Using natural kapok fibers (KF) as the heat-insulating skeleton and sodium alginate-phytic acid (SA-PA) supramolecular network as the reinforcing matrix, combined with chemical vapor deposition (CVD) to construct a siloxane nano-hydrophobic and flame-retardant layer, a fully biomass-based aerogel was successfully prepared. The synergistic effect of the crosslinked network of biomolecules and the siloxane barrier gives the aerogel an excellent compression modulus (8.1 ± 1.3 MPa) at ultra-low densities (0.047-0.066 g/cm), as well as excellent thermal insulating capacity, water resistance (145.7 ± 1.1°) and self-extinguishing properties (limiting oxygen index 34.6 %). This work demonstrates a green strategy leveraging biomacromolecular interactions for developing multifunctional insulation materials, providing new opportunities for sustainable construction and beyond.