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

The aim of the present work described in this paper was to synthesize trans parent and superinsulating monolithic carbogels in order to use them as tra nsparent double window insulation spacers at the laboratory scale. The mono lithic 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 degre es) [G.M. Pajonk et al., J. Non-Cryst. Solids 186 (1995) 1] with organic so lvents 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 C hemical processing, Wiley, New York, 1986, p. 123], Our research was focuse d at finding the best conditions of preparation of the monolithic silica ca rbogels in terms of optical and thermal insulation properties. A new solven t 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 properti es respectively. The best samples were prepared from mixtures composed of 6 0% of a prepolymer prepared with 1.8 moles of water per TEOS mole (noted P- 900) and 40% of etac ton a volumetric basis), and presented very good trans parency close to the one shown by a flat glass pane together with a very sm all thermal conductivity (unequalled, to the best of our knowledge, by any other solid material at the present time). (C) 1999 Elsevier Science S.A. A ll rights reserved.