EFFECT OF THE SOLVENT USED ON SURFACE MODIFICATION ON THE SYNTHESIS OF AEROGELS DRIED UNDER AMBIENT PRESSURE

摘要

Aerogel is also known as solid smoke due to is low density, since more than 85% of its volume is air [R. Baetens et.al., (2011)], for this reason aerogels present a high potential as thermal and acoustic isolation materials, with the advantage over organic materials of its high stability at high temperatures and the absence of the release of toxic sub products when exposed to fire. Kistler developed the method of synthesis of SiO_2 based aerogel in 1931 [S.S. Kistler (1932)], in a process that required the use of supercritical conditions for the removal of the water after gelation of SiO_2 precursor. Main challenge to spread the use of this material in ordinary applications like isolation in construction is to lower the production cost. For this purpose the drying of the aerogel under ambient conditions has been exhaustively studied, since the collapsing of structure occurs when water is removed from the SiO_2 matrix by evaporation due to capillary forces. After wet gel is obtained, water must be substituted before drying, and to aid this process, surface hydrophilic nature is changed by means of the exchange of polar –OH groups on surface by non-polar groups, like the obtained with trimethylchlorosilane. Surface modification is made under a non-polar solvent, being hexane the most common solvent. In this work we report the results of the use of several solvents and steps on the silanol groups substitution from alcogel surface, with the aim to obtain an aerogel monolith free of cracks. Aerogels obtained presented typically as best results, a density around 0.18 g/cc, thermal conductivity of 0.04 W-m/K and surface area higher than 800 m~2/g.