HNTs Improve Flame\nRetardant and Thermal Insulation\nof the PVA/CA Composite Aerogel

Porous materials are widely used in construction, batteries,\nelectrical\nappliances, and other fields. In order to meet the demand for flame-retardant\nand thermal insulation properties of organic porous materials, in\nthis work, poly(vinyl alcohol)/calcium alginate/halloysite nanotube\n(PVA/CA/HNTs) aerogels with a hierarchical pore structure at micrometer–nanometer\nscales were prepared through freeze-drying using PVA as the substrate.\nThe cross-linking reactions of PVA with H<sub>3</sub>BO<sub>3</sub> and sodium alginate (SA) with CaCl<sub>2</sub> constructed a double\ncross-linking network structure within the aerogel. And the HNTs were\nincorporated as reinforcing agents. The experimental results showed\nthat the PVA/CA/HNTs aerogels had excellent flame-retardant and thermal\ninsulation properties, and the heat release rate (HRR) and total heat\nrelease (THR) were effectively reduced compared to the PVA/CA aerogel.\nIn addition, PVA/CA/HNTs aerogels had a high limiting oxygen index\n(LOI 60%) and low thermal conductivity (0.040 W/m·K). While their\nsurface was subjected to a flame (800–1000 °C) for 25\nmin, the temperatures of the back surface were still lower than 80\n°C. The low thermal conductivity of HNTs with hollow nanotube-like\nstructures and the excellent flame-retardant properties of CA contributed\nto this phenomenon. The presence of HNTs and CA facilitated the formation\nof a dense carbon layer during combustion, enhancing the flame retardancy\nfor PVA. In addition, the interpenetrating cross-linking network and\nthe unique nanopores of HNTs collectively established a hierarchical\npore structure within the gel, effectively impeding substance and\nheat exchange between the substrate and external environment. As the\nflame-retardant and thermal insulating material, PVA/CA/HNTs aerogels\nhave a promising development prospect and potential in the fields\nof construction, transportation, electronics, and electrical appliances.