Solvent Selection and the Control of Sol-Gel Reactions
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SOLVENT SELECTION AND THE CONTROL OF SOL-GEL REACTIONS K JONES, J MBOULTON AND H G EMBLEM, Department of Chemistry, University of Manchester Institute of Science &Technology, UMIST, Manchester M60 1QD, United Kingdom. ABSTRACT The hydrolysis and gelation sequence occurring inthe formation of a filament or a rigid coherent gel from an organic silicate requires a mutual solvent for the organic silicate and water ifit is to proceed successfully. The solvent is usually a water-miscible alcohol. This alcohol can also be a reaction product, which makes the sequence very solvent dependent. The best control of both hydrolysis and gelation is obtained when the alcohol solvent and the organic silicate each contain the same alkoxy group. The gel resulting is also the best binder for refractory grain. Suitable systems are ethyl silicate and ethanol also isopropyl silicate and isopropanol. The formation of rigid coherent gels, or filaments which on firing convert to ceramic fibres, from hydrolysates of ethyl silicate and aluminium chlorhydrate species is also very solvent dependent. With glycol solvents, aluminium chlorhydrates do not form filaments nor ceramic fibres. With ethanol as solvent, aluminium chlorhydrate-polyol complexes readily form rigid coherent gels and filaments, hence ceramic fibres, because the polyol complex is formed by displacing coordinated water inthe aluminium chlorhydrate. This reduces the amount of water available for reaction, which optimises filament and fibre formation. INTRODUCTION The hydrolysis and gelation sequence occurring inthe formation of a filament or a rigid coherent gel from an organic silicate requires a mutual solvent for the organic silicate, water and any catalyst that might be desirable. The solvent is usually a watermiscible alcohol. However, because alcohols are frequently products of the resulting hydrolysis, the overall reactions can be very solvent dependent. The formation of hydrolysates from technical ethyl silicate and aluminium chlorhydrate species capable offorming rigid coherent gels and filaments which on firing convert to ceramic fibres is also very solvent dependent. ETHYL SILICATE &TECHNICAL ETHYL SILICATE PREPARATION The traditional procedure is to treat tetrachlorosilane with ethanol. Absolute ethanol gives tetraethoxysilane, also known as ethyl orthosilicate, having a silica equivalent of 28% w/w. SiC14 + 4EtOH --- > Si(OEt) 4 + 4HCI
Aueous ethanol gives [1] a mixture of tetraethoxysilane and ethoxypolysiloxanes (hyl polysilicates) formed by the condensation-polymerisation reactions resultant on water being present and catalysed by the by-product hydrogen chloride. Reaction conditions are chosen to give a product, ethyl silicate-40 (technical ethyl silicate), with a silica equivalent ca 40% w/w. A more recent procedure is the direct reaction of silicon and ethanol inthe presence of metal alkoxides [2] to give tetraethoxysilane, which can be converted by controlled hydrolysis to ethyl silicate-40. ALKYL SILICATE HYDROLYSIS PROCEDURES A mutual solvent for water and
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