Preparation and characterization of flame-retardant and thermal insulating bio-based composite aerogels

Biomass aerogels have gotten a lot of attention because of their remarkable characteristics such as high porosity, low thermal conductivity, environmental friendliness, and sustainability. However, the general flammability of biomass raw materials reduces their fire safety. In this study, flame-retardant bio-based composite aerogels are proposed for building energy saving. Alginate is chosen as the basic skeleton. Noncombustible nano calcium carbonate enhances the flame retardancy of the aerogels by serving as a nano-filler. It is also a source of calcium ions to further promote the char formation of the cross-linked alginate network during thermal degradation. Moreover, boric acid is introduced into the cross-linked network to improve thermal stability and char yield. Therefore, the limiting oxygen index of the alginate–calcium carbonate (Alg-CaCO3) composite aerogels reaches up to 39.5 %. The results of thermogravimetric analysis, thermogravimetry-infrared spectrometry, and in-situ diffuse reflectance infrared Fourier transform spectroscopy demonstrate that the Alg-CaCO3 composite aerogels have a significant tendency to form char during thermal degradation. In addition, the Alg-CaCO3 composite aerogels also show high porosity (up to 92.40 %), excellent mechanical properties (compression modulus up to 0.936 MPa), and outstanding heat insulation (thermal conductivity as low as 0.031 W m−1 K−1). Furthermore, natural shells, pearls, and eggshells are adopted to substitute nano-CaCO3 for preparing fully bio-based composite aerogels, which also reflect the promotion effect of fire resistance. Such ultralight flame-retardant thermal insulation material derived from biomass has the potential to be a significant green building material.