Alkyl silicates

Alkyl silicates may be regarded as esters of the hypothetical silicic acid, which may exist in monomeric or oligomeric form. They are also referred to as silicate esters or 

Hydrolysis of alkyl silicates produces silicic acids with varying degrees of condensation and ultimately silicon dioxide. The reaction time increases with increasing size of the alkyl group. Elevated temperature and high moisture content accelerate hydrolysis and condensation. 

Composition and Properties. The binders are produced from orthosilicate esters Si(0R)4 or industrial mixtures of polysilicate esters. Although the methyl esters are readily hydrolyzed and have a very high silicon content (generally given as SiOj content), the ethyl esters are normally used for physiological and toxicological reasons. The less readily hydrolyzable esters of longer-chain alcohols are employed in special applications. 

Various polymeric silica distributions are obtained depending on the production process, Although this distribution may possibly affect hardening behavior, investigations have shown that this is insignificant in practical applications. 

The use of alkyl silicates in coatings is based on the following reaction  

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A general method for preparing acetals by treating aldehydes or ketones with the appropriate tetra-alkyl silicate, using dry hydrogen chloride as the catalyst. Helferich and Hausen, Ber. 57, 795 (1924). 

A new route to alkyl silicates and organosiloxanes is described. This route has three steps. These are (1) the protonation of a silicate (obtained by collection, mining, or laboratory or commercial synthesis), (2) the esterification of the silicic acid formed by the protonation, and (3) the organodealkoxylation of the alkyl silicate resulting from the esterification. An important feature of this route is that it does not depend on elemental silicon. The route is illustrated with the synthesis of (EtO)4Si from 7-Ca2SiC>4,... 

Some of the many silicates that are readily available have silicate ions with frameworks that are similar to or the same as those present in common alkyl silicates and common organosiloxanes (5,7). In light of this, a synthetic approach to alkyl silicates and organosiloxanes based on substitution reactions becomes conceivable. 

The complete process for synthesizing such species using this approach would entail the acquisition of an appropriate natural silicate or the preparation of an appropriate synthetic silicate and then the conversion of this silicate into the alkyl silicate or organosiloxane by suitable substitution reactions. In terms of bond cleavage, this process could entail no destruction and reformation of framework silicon-oxygen bonds, and, in terms of oxidation number, it would entail no reduction and reoxidation of the silicon. 

This process contrasts with the elemental-silicon processes sometimes used for alkyl silicates (8) and the elemental-silicon processes generally used for oligomeric and polymeric organosi-loxanes ( ,7) Since the silicon in these processes is obtained from quartz, these processes entail, in terms of bond cleavage, the destruction of four silicon-oxygen bonds per silicon and the subsequent reformation of the required number of such bonds. In terms of oxidation number, they entail the reduction of the silicon from four to zero and then its reoxidation back to four, Figures 2 and 3. 

In the present paper, a route to alkyl silicates and organosiloxanes which has elements in common with these pieces of work and which is based on the substitution approach is illustrated and discussed. 

In the route used in this work, a monomeric or oligomeric silicate is protonated, the resulting silicic acid is esterified, and the alkyl silicate is organodealkoxylated. In each step, the siloxane framework is fully or substantially preserved. For the case starting with a metal disilicate the reactions can be represented as ... 

A novel feature of the route is that it leads to monomeric and oligomeric alkyl silicates from metal silicates in good yield and with full or substantial siloxane framework preservation. Further, it leads to oligomeric organosiloxanes of intermediate molecular weight from alkyl silicates in good yield and with siloxane framework preservation. 

From the results of several experiments it has been learned that isopropanol and n-butanol can be used to make alkyl silicates... 

The fact that the alkyl silicates produced by the route retain the siloxane framework of the parent silicates fully or substantially, Figure 4, shows that the framework is at least largely retained in the first step. A number of experiments indicate one factor aiding structure retention in this step is the use of an amount of HC1 that is just slightly above that which is stoichiometrically required (105-110%). Other experiments indicate that in some instances the use of a low temperature (—-10 °C) likewise helps with structure retention. 

Also pertinent to structure retention in this step are results from experiments showing that the route yields alkyl silicates having poor structure retention when the intermediate silicic acid concentration is high and good retention when it is low. These results suggest that a low silicic acid concentration (-0.04M) aids structure retention in the first step. If so, this is easily understandable in terms of the ease with which silicic acids react with themselves. 

The fact that the alkyl silicates produced by the route fully or substantially retain the original siloxane framework also shows that the framework is at least largely retained in the second step. The results pertaining to silicic acid concentration already mentioned lead to the conclusion that a low silicic acid concentration aids this structure retention. Also aiding it, no doubt, is the ability of the alcohol to sheath and protect the silicic acid. 

Figure 4. Structures of alkyl silicates produced from...

The route also provides a satisfactory path to some alkyl silicates for which no other satisfactory routes are available. Some of these alkyl silicates may be of interest in the synthesis of ceramics by the sol-gel technique. 

Stober A process for making metal oxides in the form of small spheres of uniform diameter by the controlled hydrolysis of metal alkoxides. First used in 1968 to make silica spheres from alkyl silicates. The products can be used to make high quality oxide ceramics. See also Sol-Gel. 

Synthesis of Other Silicon Compounds. The substitution route to organosilicon compounds obviously also opens up new avenues to alkyl silicates. For example, as indicated previously, the substitution route is a good avenue to the isomeric alkyl silicates [5.5.1] ( 2H50)ioSie07 and [5.3.3] ( 2H50)ioSie07. The availability of this substitution route for alkyl silicates is of considerable importance, because some of the routes that have been used to make complex alkyl silicates are not good in terms of yield or convenience (22). 

Both one-component and two-component silicone sealants contain polydimethyl siloxane as the base polymer along with fillers such as calcium carbonate and/or fumed silica fillers, plasticisers (silicone oil) and a variety of cross-linking agents and adhesion promoters. Two-component sealants utilise a catalyst such as dibutyl tin dilaurate, alkyl silicate esters and metallic salts (Maslow, 1982). 

Introduction of an alkyl silicate component into a sol-gel mixture creates an extremely complicated kinetic situation (192, 193). Typically, these network modifiers are diluted with an orthosilicate species such as TMOS (network formers) to afford a porous material. Condensation of pure aUcyltriaUcoxysilanes tends to result in nonporous materials, as is apparent from the plot of surface area versus silylated metal complex TEOS ratio shown below (Fig. 19). 

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... 

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