High strain recovery with improved mechanical properties of gelatin–silica aerogel composites post-binding treatment

Abstract

Silica aerogels are very light and highly porous materials that are intriguingly and complexly networked with large internal surface area, high hydrophobicity with extremely low density and thermal conductivity. These features make them ideal choice for applications as thermal and acoustics insulators or as optical, electrical, and energy storing devices. However, their exploitation for structural applications is primarily inhibited by their brittleness. The brittleness of the silica aerogels makes their processing and handling difficult. Volumetric shrinkage occurs, which becomes more apparent at elevated temperatures. While there are hybrid silica aerogels doped with materials such as polymer, ceramics, metals in the market, the improvements in the mechanical properties are compromised with tremendous increase in density and reduction in the insulation performance. Post-synthesis binding treatment of silica aerogels composites are not extensively explored due to the chemically inert trimethylsilyl (TMS) terminal groups on the surface of the hydrophobic silica aerogels. This paper discusses a unique fabrication method of developing a ductile silica aerogel composite solid via post-synthesis binding treatment. Gelatin–silica aerogel (GSA) and GSA–sodium dodecyl sulfate (SDS) composite blocks were produced by mixing hydrophobic aerogel granulates in a gelatin–SDS foamed solution by frothing method. The entire fabrication process and grounds for using a controlled % of gelatin as the main binder and SDS as an additive are explained. The compression testing of the blocks is presented. The associated strain recovery—an unusual phenomenon with brittle silica aerogels, observed upon unloading is highlighted and studied. The microstructure and surface characterization of these composites was examined via FESEM/EDX and XPS/ESCA, respectively. The dependency of process variables involved were analyzed through analysis of variance (ANOVA) model. Empirical models that relate the composition of gelatin, aerogel, and SDS to achieve the optimal strain recovery with the associated compressive modulus and strength and density are established. The transition from brittleness to ductility is measured in terms of compressive stress versus strain behavior for various mass fractions of gelatin and SDS. The test data presented indicate analogous behavior of these to creep-like behavior of a material typically identified as the primary, secondary, and tertiary stages. The rationale and mechanisms behind such creep-like three stages are explained using schematic diagrams.

Appendix

See Tables 5 and 6

Table 5 Experimental data for density GSA and GSA–SDS composite blocks

Table 6 Experimental data for strain recovery, compressive strength, and compressive modulus GSA and GSA–SDS composite blocks