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Silanes

Silanes are the homologues of saturated carbon-hydrogen compounds, that is, of alkanes. They all share the general formula SinH2n+2.

Unsubstituted silanes, made of silicon and hydrogen only, are very unstable and can only be produced in the absence of oxygen. Much more important are the methylsilanes, where some or all hydrogen atoms in the silanes are formally replaced by the methyl groups. If the hydrogen atoms are partially replaced by chlorine atoms and other hydrogen atoms by methyl groups, the important class of methylchlorosilanes results, often abbreviated as chlorosilanes. Methylchlorosilanes form the basis for all silicone chemistry.

Methylsilanes

Methylsilanes are the raw materials for manufacturing silicones. They are produced by direct reaction between silicon and methyl chloride in the Mueller Rochow synthesis. They are extremely mobile, colourless, liquids that are soluble in organic solvents and, in some cases, in anhydrous alcohol. Silanes have low molecular weights and are thus highly volatile.

Industrial silicone production has its commercial basis in the direct synthesis of methylchlorosilanes from silicon and methylchloride via a process called the Mueller-Rochow synthesis. This technique, developed independently in 1940/41 by professors R Mueller in Germany and E G Rochow in the United States, takes place in the presence of a copper catalyst at approximately 280°C. Finely ground and well mixed Si and Cu are brought together in a fluid bed reactor with methyl chloride in gaseous form. This produces a silane mixture from which the most important organochlorosilanes are derived.

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Decisive for the commercial production process is to gain dimethyldichlorosilane (CH3)2SiCl2, the most important organochlorosilane. Optimal production depends on the selectivity and activity of the copper catalyst as well as on the homogeneity of the Si/CH3Cl mixture and an even temperature distribution in the reactor (avoiding 'hot spots'). Contamination of the copper catalyst, for example by lead, has a drastic negative effect while antimony additives, on the other hand, can promote the yield.

Modern fluid bed reactors have a capacity of approximately 40,000 tonnes of raw silanes per year and more. Today European manufacturers no longer consider plants producing less than 60,000 tonnes per year as commercially viable.

During the process, the solid elements of the reaction mixture, such as unconverted elemental silicon and the copper catalyst are separated from the gases. The solids are fed back into the reaction process and the gases, after condensation, separated into liquid crude mixture and gaseous methylchloride. The latter is then also fed back.

Product Distribution in the Muller-Rochow Synthesis

Product Distribution in the Muller-Rochow Synthesis

 

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Due to the sometimes very small differences between the boiling points of silanes (e.g. methyltrichlorosilane at 66°C, dimethyldichlorosilane at 70°C) distillation units have to fractionate them in several stages to obtain the individual silanes. The distillation columns therefore have many plates and thus high separation efficiency. Even small amounts of contaminants (e.g. CH3SiCl3 in (CH3)2SiCl2), in the parts per million (ppm) range, interfere with the further processing of the organochlorosilanes to silicones.

Organochlorosilanes are very sensitive to hydrolysis, that is, they react readily with water and vigorously give off hydrochloric acid. For safety reasons in case of leaks, distillation columns are usually not cooled with water but with air.

Special Silane Syntheses

Manufacturers use other methods to produce chlorosilanes to laboratory standard or to manufacture special silanes, such as those containing phenyl groups.

The following syntheses still play an important role in addition to the Mueller-Rochow direct synthesis of silanes; Addition reaction (hydrosilylation); Nucleophilic substitution; Grignard reactions.

The purpose of all these syntheses is to incorporate organic functional groups.

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Typical Reactions of the Chlorosilanes

The hallmark of chlorosilane chemistry is their pronounced tendency to polycondensation. Organochlorosilanes react violently with water releasing hydrochloric acid. Care is needed, since typically nearly 250 l HCl are released per kg of dimethyldichlorosilane. In silicone production the hydrochloric acid formed is returned to the process and reacted with methanol to produce the feedstock material methylchloride.

Silanols

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The reaction of the monomer organochlorosilanes with water (hydrolysis) or methanol (methanolysis) produces silanols, which, using HCl catalysis, lead directly to further reacted oligomers or polymer siloxanes ('sil' represents silicon 'ox' stands for oxygen and 'ane' describes the saturated nature of the bond). The description 'silicone' for the whole class of polysiloxanes is an imitation of the oxygen-carbon bonds of carbon chemistry which are known as 'ketones'. The latter however, because of their particular characteristics, form double bonds instead of single bonds.

 

 

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Mono-, di-, tri- or tetrafunctional siloxane units with Si-O bonds arise from polycondensation according to the number of chlorine atoms of the basic silane molecule.

The diverse halogenated silanes serve as building blocks for the synthesis of the various product types of silicones such as fluids or resins.

Dimethyldichlorosilane enables the formation of long Si-O chains. At first the hydrolysis or methanolysis of dimethyldichlorosilane gives a mixture of short chained, difunctional and therefore linear siloxanes with OH and groups as well as cyclic siloxanes having normally between three and six chain units. The linear siloxanes show a helix structure with the methyl groups being freely able to rotate. All silicone fluids, emulsions and rubbers are based on dimethyldichlorosilane. This is therefore the decisive base product.

If the trichloro compounds are used, a cross-linking between the linear chains is produced as a result of the three reactive sites of silicon. A three dimensional network is the consequence. This process is crucial in the formation of silicone resins. Monochlorosilanes, on the other hand, because of their single reactive site, can only be used for the terminating of the chain growth by polycondensation. They react as a sort of 'capping agent' for the growing silicone chain.

Chloromethane (or methyl chloride) is made both in nature and industrially. The manufacturing process is based on either chlorination of methane or more commonly by reaction of methanol with hydrogen chloride.