Hydrophobic granular silica-based aerogels obtained from ambient pressure monoliths

Abstract

Aiming at obtaining granular silica-based aerogels for incorporating in superinsulating systems, strong monoliths were produced at ambient pressure by a two-step sol–gel process, subsequently crushed and selectively sieved. The key to a strong and stable hybrid network was to use two silica co-precursors, tetraethoxysilane (TEOS) and dimethyldiethoxysilane (DMDES). The role of DMDES is to contribute to a more robust initial network, able to withstand surface modification during aging. To tailor the properties of the final monoliths the DMDES:TEOS and water:precursors molar ratios were varied (0 to 1 and 4 to 10, respectively). Aging in a solution of hexamethyldisilazane in 2-PrOH was essential to improve hydrophobicity and stability to moisture, producing lighter, more open and three-dimensional pore structures, with very high specific surface areas. A well-balanced relationship between density, pore structure, strength and hydrophobicity was achieved. These properties allowed grinding the monoliths without dust release and selective sieving into granulate diameters above 2 mm, in the ranges 1–2 mm, 500 µm-1 mm, 250–500 µm, 125–250 µm and 63–125 µm, and bellow 63 µm. These granules have the advantages of being produced at ambient pressure with minimum amounts of organic co-precursor and hydrofobizing agent, which constitute a substantial fraction of the raw materials cost. Affording well defined granulate ranges opens the possibility of controlling the granulometric curve of the aerogel, and thus the mechanical properties and thermal conductivity of the final aerogel-based product. The hydrophobicity (steady water contact angles from 122–142o) envisages perspective applications not restricted to moisture-protected environments.