Esters, silicate condensed

Extensive research on sol—gel processing of silicic acid esters, eg, tetraethoxysilane (TEOS), Si(OC2H5)4, in alcohol—water mixtures has ducidated silica polymerization in nonaqueous solvents (54,55). The relative rates of hydrolysis and condensation depend on the substrate, water, and catalyst (acid or base) concentrations these rates determine the polymer structure. For example, acid-catalyzed hydrolysis of TEOS at low water concentrations produces linear polymers. These solutions yield fibers upon spinning. Conversely, high H20 /TEOS ratios favor the formation of highly cross-linked polymers under... 

In practice the ester is mixed with the required amount of water and a suitable solvent (e.g., alcohols, esters, ketones). To obtain 100 Vo hydrolysis of 1 mol of an orthosilicate, 2 mol of water must be added. By maintaining a constant pH (for ethyl silicate ca. 2) the condensation reaction can be suppressed to such an extent that hydrolyzates of ethyl silicate containing 20% SiOj and with a degree of hydrolysis of 80-90% remain stable for about one year. If this equilibrium is disturbed (e.g., by shifting the pH value or evaporating the solvent), condensation commences and the solution solidifies after a short time. This mechanism is used in two-pack zinc dust paints. In the simplest case the binder is a hydrolyzate in alcoholic solution. Zinc dust and, if necessary, extenders, antisettling agents, colorants, etc. are then added. Addition of zinc consumes acid and the pH value increases. 

On application, the paint layer hardens within a short time due to evaporation of the solvent, progressive hydrolysis of ester groups after uptake of atmospheric moisture, reaction of silanol groups with the zinc and substrate, and condensation of silanol groups to form sparingly soluble, cross-linked silica gel [2.186]. Conditions are generally chosen such that the film is hard enough to allow transportation after 4 h and a topcoat after 24 h. Alkyl silicate zinc dust paints are particularly suitable... 

The sol-gel process can be viewed most simply as a series of two reactions (usually hydrolysis and then condensation) where at first the reactants form small particles in a solvent and then condense fiirther to form a gel. Simple alkylsilanes and especially silicic acid esters are often crucial in these reactions as they are the raw materials in this process. Organofiinctional silanes have the additional feature in that they can interconnect organic resins with the inorganic part of these... 

Silicates are the salts or esters of orthosilicic acid (1 48104) and of its condensation products. The silicates are categorized according to the arrangement of the [Si04] tetrahedra in their crystal structure. Figure 3.2-1 shows some simple, planar arrangements. In... 

Silica has been introduced as ethyl silicate in the monomer precursor mix of polyester resins (579). The water liberated during condensation polymerization, for example, in making poly(ethylene succinate), hydrolyzes the ester, liberating silica essentially as a copolymer. This should give extremely fine pores if the silica were removed. 

The so-called RTV-2 method — room temperature vulcanization of a two component system — adds a crosslinking agent such as tri-or tetraalkoxysUane and a metallic salt catalyst to hydroxyl terminated polysiloxane precursors. The hydroxyl end groups react with the silicic ester, e.g., tetraethyl silicate, in a condensation reaction and ethanol is liberated. This condensation reaction is catalyzed by stannous-based catalysts such as dibutyltindilaurate. 

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