Thermal Insulation and Flame Retardancy of the Hydroxyapatite Nanorods/Sodium Alginate Composite Aerogel with a Double-Crosslinked Structure

As advanced thermal management materials, aerogels have great research value in the fields of engineering insulation, pipeline transportation, and packaging insulation. The composite interaction of the two-phase interface and the construction of a porous structure have an important impact on the thermal properties. Herein, a novel HANRs/SAB composite aerogel was prepared using sodium alginate (SA) with hydroxyapatite nanorods (HANRs), combined with boric acid crosslinking and freeze drying. In the prepared sample, the calcium ions in HANRs and SA formed the first layer of binding force and the chemical crosslinking of sodium alginate with boric acid formed the second layer of strong binding force, which effectively supported the skeleton of the aerogel and enhanced the overall mechanical properties. The modulus and maximum compressive strength of the obtained HANRs/SAB aerogel were 2.39 and 0.75 MPa, respectively, while the bulk density was 0.038-0.068 g·cm^-3. Based on the prominent physical structure, the as-prepared HANRs/SAB aerogel exhibited good thermal insulation (∼35.15 mW·m^-1·K^-1) and outstanding flame retardant performance. Flame-retardant boric acid and high-thermal stability HANRs could effectively prevent heat transfer and organic combustion, thus resulting in an extremely low smoke gas release (11.3 m² m^-2). Therefore, the low-cost biopolymer composite aerogel based on a crosslinking strategy has broad application prospects in the field of thermal insulation and flame retardancy.