Ambient pressure drying of silica aerogels after hydrophobization with mono-, di- and tri-functional silanes and mixtures thereof

摘要

Aerogels are mesoporous materials with a rapidly growing thermal insulation market interest, partly because ambient pressure drying (APD) reduces cost. Silica gels must be hydrophobized to prevent condensation reactions and irreversible shrinkage during APD. Here, we study how hydrophobization with mono-, di-, and trifunctional organoalkoxysilanes (M, D, T) and their mixtures, affects spring-back, density, surface area, pore size and hydrophobicity. All aerogels were synthesized from a polyethoxydisiloxane precursor prepared from a tetraalkoxysilane (tetraethoxysilane, TEOS). Aged gels, hydrophobized with trimethylethoxysilane (M100), display the highest spring-back and lowest density (0.100 g/cm(3)) upon APD drying. Hydrophobization with pure dimethyldiethoxysilane (D100) or methyltriethoxysilane (T100) leads to excessive shrinkage and respective densities of 0.440 and 1.450 g/cm(3) due to the self-polymerization from the di- and triftinctional silanes: in the D100 material, the formation of polydimethyldisiloxane (PDMS) is evident in the Si-29 NMR spectra, whereas 3D silica structures (beads) are apparent for the T100 sample. Aerogels hydrophobized with binary and ternary mixtures of M, D and T display systematic variations in density: nearly constant (< 0.12 g/cm(3)) values are obtained after replacing up to 20% of M, but the density increases rapidly upon further substitution. In contrast to the strong effects on density, the humidity uptake remains below 2 wt% for most samples and conditions. The possibility to use mixtures of hydrophobization agents, rather than pure compounds, may decrease the production costs of silica aerogels, and can be readily carried out for up to 20%, but would require further process optimization for higher degrees of substitution.