Synthesis and properties of some monolithic silica carbogels produced from polyethoxydisiloxanes dissolved in ethylacetoacetate (etac) and acid catalysis

The aim of the present work described in this paper was to synthesize transparent and superinsulating monolithic carbogels in order to use them as transparent double window insulation spacers at the laboratory scale. The monolithic silica carbogels were made by sol-gel technique using mixtures of a new family of precursors noted PEDS-Px (or polyethoxydisiloxanes which are prepolymers of silica differing by their substoichiometric hydrolysis degrees) [G.M. Pajonk et al., J. Non-Cryst. Solids 186 (1995) 1] with organic solvents in the presence of hydrofluoric acid with no excess of water added, and subsequent drying in the supercritical conditions with respect to CO2 [P.H. Tewari et al., in: L.L. Hench, D.R. Ulrich (Ed.), Science of Ceramic Chemical Processing, Wiley, New York, 1986, p. 123]. Our research was focused at finding the best conditions of preparation of the monolithic silica carbogels in terms of optical and thermal insulation properties. A new solvent ethylacetoacetate (etac) was found to be much more soluble in liquid CO2 than common alcohols and acetone, and the so obtained carbogels, with etac, present very interesting optical visible transmission and thermal properties respectively. The best samples were prepared from mixtures composed of 60% of a prepolymer prepared with 1.8 moles of water per TEOS mole (noted P900) and 40% of etac (on a volumetric basis), and presented very good transparency close to the one shown by a flat glass pane together with a very small thermal conductivity (unequalled, to the best of our knowledge, by any other solid material at the present time).