LOW-DENSITY, HYDROPHOBIC AEROGELS

Abstract

Low-density, hydrophobic aerogels synthesis consisting of a two-step synthesis, supercritical drying with liquid carbon dioxide and surface modification by vapor-phase methoxylation is described. No alcohol was added in the first stage of oligomer synthesis. Wet gels were produced from tetramethoxy silane or tetraethoxysilane under basic conditions by adding water and NH4OH. Non-alcoholic diluents were mixed to control the density of aerogels. The gelation time decreased with decreasing amount of diluents. Wet gels were extracted with liquid CO2 followed by supercritical drying at 40 degrees C and 100 atm. Aerogel densities of 0.034-0.38 g/cm(3) were produced without cracks. Hydrophilic surfaces of aerogels were modified with methanol vapor. Methoxylation was carried out at 240 degrees C for 10 h, The density of the hydrophobic aerogel was unchanged for 40 days under a humid, open atmosphere. The Brunauer, Emmett and Teller surface area was in the range of 750-870 m(2)/g for aerogel densities of 0.05-0.2 g/cm(3).