From raw material to silicones

From Raw Material to Silicones

The Element Silicon

The following pie diagram shows the frequency of the elements in the earth's crust:

Silicon makes up 26 % of the earth’s crust where it is the second most abundant element. Although it is much more abundant than carbon (which makes up 0.087 % of the earth’s crust), it does not play any major role in the animate world of biomolecules. It could be said, however, that silicon is the base material of the inanimate world. It almost exclusively occurs in combination with the most abundant element (oxygen) in the form of silica and silicates. Aside from the various silicates (salts of silicic acid) with magnesium, calcium or iron fractions, silica makes up a sizable fraction of the earth's crust in its sand, quartz and pebble variants. While sands and clay minerals have been processed into cultural objects since time immemorial, it was not until 1824 that elemental silicon in amorphous form was isolated by Berzelius. All the silicon compounds mentioned so far have a tetrahedral structural unit consisting of a silicon bonded to four oxygen atoms.

Structure of silicon dioxide (quartz)

Although silicon, like carbon, is in the fourth main group of the periodic table, the two elements have vastly different chemical properties. Carbon forms a nearly inexhaustible range of compounds but there is no analogous range of silicon compounds. This applies both to natural and synthetic compounds. The differences between the two elements (carbon and silicon) are briefly illustrated below.

Comparison of the Elements Carbon and Silicon:

1. Electronegativity

Carbon has a higher electronegativity than silicon (C: 2.5; Si: 1.8).

2. Atomic radii

C: 0.77 Å; Si: 1.15 Å

3. Electronic configuration

C: 1s² 2s² 2p²; Si: 1s² 2s² 2p⁶ 3s² 3p²

4. Bonds Formed with Oxygen

Carbon tends to form double bonds with oxygen, whereas silicon forms very stable single bonds. Silicon only forms double bonds in the case of some unstable silane compounds. This behavior can be understood with the help of the following molecular orbital diagram. It is reflected in the lower energy required for transitions of alkenes and disilanes: for C, approx. 6 eV, for Si, approx. 3 eV .
Aside from its tetravalency, silicon is also capable of forming compounds with higher or lower coordination numbers:

Rough Diagram of Industrial Silicon Synthesis

		As the diagram opposite of a section of a linear silicone molecule shows, silicones are essentially organically modified quartz. The raw materials needed for synthesizing silicones are sand, coal, natural gas/petroleum, rock salt, air and water. Raw silicon is obtained from sand and coal and is continually converted into the desired silicones. Natural gas or petroleum serves to produce methanol (synthesis gas), which is another starting material in the manufacture of silicones. Electrolysis of rock salt solutions yields chlorine, which is converted into HCl and fed to the synthesis. The lower diagram shows a schematic diagram of how silicones are produced: The first step is to convert methanol and HCl into chloromethane (chloromethane synthesis). The chloromethane is then made to react with silicon to form a mixture of raw silanes (chlorosilane synthesis). These are separated by distillation. Dichlorodimethylsilane (CH3)2SiCl2 is then converted into polydimethylsiloxane by hydrolysis.

The other silanes are converted with alcohol to yield resins, masonry protection agents or specialty silicones. The interesting point about this approach is that the hydrogen chloride gas is recycled in a continuous loop. This not only increases cost-effectiveness but also conserves resources and considerably eases the burden on the environment.