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The Basics of Silicon Chemistry
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Basic Silicon Production and Siloxane Polymerization
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| We previously discussed the origins of silicones and the "direct process." Now let's look at the chemistry in more detail.
The basic raw material for the silicone industry is silica - a plentiful, naturally occurring resource in the form of sand. At Dow Corning, silica is reduced to elemental silicon in a carbo-thermic smelting process: SiO2 + 2C -> Si + 2CO.
Polymerization
The direct reaction between silicon metal and methyl chloride in a fluid bed reactor yields a complex mixture of methyl chlorosilanes with the following distribution of products: |  |
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| The chlorosilanes are distilled or purified, and the primary product - dimethyldichlorosilane, (CH3)2SiCl2 - is reacted with water (hydrolysis) to give poly(dimethylsiloxane) oligomers: [Me2SiO]n. The whole process is self-contained: the chloride ion is recycled with the regeneration of methyl chloride via the reaction of methanol and HCl. Thus, only readily available raw materials are used and useful products emerge from this cycle, as shown here: |
| | Linear and cyclic siloxane oligomers are both formed simultaneously in the hydrolysis reaction. These are the basic raw materials (monomers) from which a host of useful silicone products can be made.
The process of conversion is called polymerization. It occurs by ring opening and/or condensation in the presence of an acid or base catalyst to form silicone fluids. During this reaction, which is commonly known as equilibration, the SiO bonds are constantly cleaved and reformed in a series of competing reactions involving both cyclic and linear species - building up molecular structure until an equilibrium distribution is reached.
Molecular weight control is obtained by the use of "end-blockers," such as hexamethyldisiloxane derivatives, which act as chain-termination agents. This is most simply represented by the equation: [Me2SiO]n + Me3SiOSiMe3 -> Me3SiO[Me2SiO]n SiMe3 siloxane fluid.
Molecular weight and many of the ultimate properties, such as viscosity, are determined by the ratio of trimethyl (Me3) to dimethyl (Me2) siloxanes in the reaction. Simple silicone fluids may undergo further processing with other monomers and fillers to make cross-linked-network resins and a wide variety of elastomeric products. These complex ring-chain equilibrium processes have been widely studied and the fundamentals of siloxane polymerization are well known. Several comprehensive references are available. |
Common Forms
Siloxane polymers occur in a variety of common forms - fluids, gels, elastomers, and resins - for a wide variety of applications.
Silicone fluids are usually straight chains of poly(dimethylsiloxane), or PDMS, which are terminated with a trimethylsilyl group (or groups). PDMS fluids come in all viscosities - from water-like liquids to intractable fluids. All of these are essentially water insoluble. PDMS fluids may be further modified with the addition of organofunctional groups at any point in the polymer chain. |  |
| Silicone gels are lightly cross-linked PDMS fluids, where the cross-link is introduced either through a trifunctional silane - such as CH3SiCl3 giving a "T-branched" silicone structure - or through a chemical reaction between a Si- vinyl group on one polymer chain with a hydrogen bonded to silicon in another. This chemical "tying" of siloxane chains produces a three-dimensional network that can be swollen with PDMS fluids to give a sticky, cohesive mass without form.
Silicone elastomers are cross-linked fluids whose three-dimensional structure is much more intricate than a gel. In addition, there is very little free fluid in the matrix. Fillers, such as amorphous silica, are frequently added to the matrix to give greater reinforcement to the network and thereby increase the strength of the product.
Silicone resins are more heavily cross-linked polymer networks that are formed by the introduction of a predominance of tri- and tetra-functional monomers, such as CH3SiCl3. The physical properties of the finished silicone resin can be tailored to suit many applications by varying the ratio of branched and linear siloxanes and also the functional groups attached to the silicon. |
Learn more about:
Sources:
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W. Noll, Chemistry and Technology of Silicones, Academic Press, New
York (1968).
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T.C. Kendrick, B. Parbhoo, J.W. White, “Siloxane Polymers and Copolymers,”
in The Chemistry of Organic Silicon Compounds Pt2 (edited by S. Patai
and Z. Rappoport), 21, p1289-1361, John Wiley, Chichester (1989).
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S.J. Clarson, J.A. Semlyen, Siloxane Polymers, Prentice Hall, New
Jersey (1993).
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J.W. White, R.C. Treadgold, “Organofunctional Siloxanes,” in Siloxane
Polymers (edited by S.J. Clarson and J.A. Semlyen), 4, p193-215, Prentice
Hall, New Jersey (1993).
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W. Gardiner, J.W. White, “Specialty Silicones as Building Blocks for
Organic Polymer Modification,” in High Value Polymers (edited by A.H.
Fawcett), Royal Society of Chemistry, Cambridge (1990).
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M. Brook, Silicon in Organic, Organometallic and Polymer Chemistry,
John Wiley and Sons, New York (2000).
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