INFLUENCE OF BUTANOL ON THE HYDROLYSIS CONDENSATION BEHAVIOR OF TEOS

The effect of the replacement of ethanol by n-, sec- and tert-butanol on the hydrolysis-condensation reactions of a mixture of tetra-ethoxysilane (TEOS), ethanol, and water was investigated. Hydrolysis time versus gelation time curves were recorded. Gas-liquid chromatography was used to examine the exchange reactions between ethoxy groups of TEOS and butoxy groups of the alcohol. For mixtures containing n- or sec-butanol beyond the minima of the gelation curves, a substantial increase in gelation time is observed. This is caused by a lower reactivity towards hydrolysis of the butoxysilanes formed. For tert-butanol, practically no exchange products are found. No influence on the hydrolysis time versus gelation time curves was observed when the butanols were added in a basic environment. In mixtures in which sec- or tert-butanol is applied, a lowering of the acid catalyst concentration is obtained, as a consequence of the formation of carbocations. This results in a shift of the extremes in gelation time to longer hydrolysis times. A proton is released from tert-carbocation under the formation of isobutylene or tert-butyl-ethylether (identified using gas chromatography-infrared spectroscopy). The proton is used again for the formation of a tert-carbocation and water. As a result of the generation of water, a substantial decrease of the minimum gelation time is observed for hydrolysis mixtures containing tert-butanol.