Silicones are largely inert compounds with a wide variety of forms and uses. Typically heat-resistant, nonstick, and rubberlike, they are frequently used in cookware, medical applications, sealants, lubricants, and insulation. Silicones are polymers that include silicon together with carbon, hydrogen, oxygen, and sometimes other chemical elements. One of the most common uses is holding glass together, e.g. in aquariums. Silicone can also be found in rocks such as tar.

Properties

Some of the most useful properties of silicone include:

1. Thermal stability (constancy of properties over a wide operating range of −100 to 250 °C).
2. Though not Hydrophobic, the ability to repel water and form watertight seals.
3. Excellent resistance to oxygen, ozone and sunlight.
4. Flexibility.
5. Good electrical insulation.
6. Nonstick.
7. Low chemical reactivity.
8. Low toxicity.
9. High gas permeability: at room temperature (25 °C) the permeability of silicone rubber for gases like oxygen is approximately 400 times that of butyl rubber, making silicone useful for medical applications (though precluding it from applications where gas-tight seals are necessary).

Technical details

More precisely called polymerized siloxanes or polysiloxanes, silicones are mixed inorganic-organic polymers with the chemical formula 1_n, where R = organic groups such as methyl, ethyl, and phenyl. These materials consist of an inorganic silicon-oxygen backbone (…-Si-O-Si-O-Si-O-…) with organic side groups attached to the silicon atoms, which are four-coordinate.

In some cases organic side groups can be used to link two or more of these -Si-O- backbones together. By varying the -Si-O- chain lengths, side groups, and crosslinking, silicones can be synthesized with a wide variety of properties and compositions. They can vary in consistency from liquid to gel to rubber to hard plastic. The most common siloxane is linear polydimethylsiloxane (PDMS), a silicone oil. The second largest group of silicone materials is based on silicone resins, which are formed by branched and cage-like oligosiloxanes.

Synthesis

Silicones are synthesized from chlorosilanes, tetraethoxysilane, and related compounds. In the case of PDMS, the starting material is dimethylchlorosilane, which reacts with water as follows:

n 2 + n 3 → 4_n + 2n HCl