Radially and Axially Oriented Ammonium Alginate Aerogels Modified with Clay/Tannic Acid and Crosslinked with Glutaraldehyde

Abstract

Lightweight materials that combine high mechanical strength, insulation, and fire resistance are of great interest to many industries. This work explores the properties of environmentally friendly alginate aerogel composites as potential sustainable alternatives to petroleum-based materials. This study analyzes the effects of two additives (tannic acid and montmorillonite clay), the orientation that results during casting, and the crosslinking of the biopolymer with glutaraldehyde on the properties of the aerogel composites. The prepared aerogels exhibited high porosities between 90% and 97% and densities in the range of 0.059-0.191 g/cm³. Crosslinking increased the density and resulted in excellent performance under loading conditions. In combination with axial orientation, Young's modulus and yield strength reached values as high as 305 MPa·cm³/g and 7 MPa·cm³/g, respectively. Moreover, the alginate-based aerogels exhibited very low thermal conductivities, ranging from 0.038 W/m·K to 0.053 W/m·K. Compared to pristine alginate, the aerogel composites' thermal degradation rate decreased substantially, enhancing thermal stability. Although glutaraldehyde promoted combustion, the non-crosslinked aerogel composites demonstrated high fire resistance. No flame was observed in these samples under cone calorimeter radiation, and a minuscule peak of heat release of 21 kW/m² was emitted as a result of their highly efficient graphitization and fire suppression. The combination of properties of these bio-based aerogels demonstrates their potential as substituents for their fossil-based counterparts.

Keywords: aerogel; ammonium alginate; crosslinking; freeze-drying; montmorillonite; tannic acid.

Figure 1

Aerogel preparation via the sol–gel method through radial and axial freeze casting (Created with Biorender.com Agrmt No. BG27599XKD).

Figure 2

(a) FTIR spectra of pristine ammonium alginate (AA) aerogel before and after its modification with TA and MMT and crosslinking with GTA; (b) XPS spectra of non-crosslinked A5 and crosslinked A5* and A5C5T2* aerogels.

Figure 3

(a) Comparison of bulk and relative density and porosity of ammonium alginate aerogel composites: N₂ adsorption/desorption isotherms and BET specific surface of (b) A5C5-R and (c) A5C5-X aerogels and (d) pore volume and pore size distribution of A5C5-R and A5C5-X aerogels.

Figure 4

Virtual µ-CT image of pristine alginate aerogel frozen in the (a) radial (A5-R) and (b) axial (A5-X) directions and (c) GTA-crosslinked alginate axial aerogel composite (A5C5T2-X*).

Figure 5

SEM images revealing the evolution of aerogel structures as the solid content increased: (a) A5-R, (b) A5C5-R, and (c) A5C5T2-R; pore alignment in the axial orientation: (d) A5-X, (e)A5C5-X, and (f) A5C5T2-X, and (h) after GTA crosslinking. (g) EDS of the dispersion of MMT clay on the A5C5T2-X* aerogel.

Figure 6

(a) Stress–strain compressive plots of alginate–clay–tannic acid aerogels; (b) specific modulus and specific yield stress of the aerogels studied.

Figure 7

(a) Correlation between the thermal conductivities and bulk densities of the AA aerogel composites, (b) Scheme of different contributions in thermal conductivity and thermography of A5C5-R and A5C5T1-X aerogels on a hot plate surface at 100 °C, and (c) Thermal conductivity and effusivity of AA composite aerogels.

Figure 8

(a) TGA weight loss; (b) derivative thermogravimetric curves of alginate composite aerogels.

Figure 9

Main representative (a) HRR, (b) THR, and (c) ARHE curves from ammonium alginate composites; (d) photograph and SEM photomicrograph and EDS elemental mapping of the char of A5C5T2 after cone calorimetry; (e) Raman spectra of A5, A5C5, and A5C5T2 aerogel ashes.

Figure 10

(a) Comparison of the compositions and properties of the aerogels evaluated in this study; (b) comparison of our ammonium alginate aerogels with other aerogel composites reported in the literature [49,50,51,52,53,54].