Silicones are polymers that include silicon together with carbon, hydrogen, oxygen, and sometimes other chemical elements.

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 a hydrophobe, the ability to repel water and form watertight seals.
3. Excellent resistance to oxygen, ozone and UV (sunlight). This has led to widespread use in the construction industry (e.g. coatings, fire protection, glazing seals), and automotive industry (external gaskets, external trim).
4. Good electrical insulation. Because silicone can be formulated to be electrically insulative or conductive, it is suitable for a wide range of electrical applications.
5. Non-stick.
6. Low chemical reactivity.
7. Low toxicity, but does not support microbiological growth.
8. 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).

History

Frederick Kipping was the chemist who pioneered the study of the organic compounds of silicon and invented the term silicone.

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 dimethyldichlorosilane, which reacts with water as follows:

n 2 + n 3 → 4_n + 2n HCl

During polymerization, this reaction evolves potentially hazardous hydrogen chloride gas. For medical uses, a process was developed where the chlorine atoms in the silane precursor were replaced with acetate groups, so that the reaction product of the final curing process is nontoxic acetic acid (vinegar). As a side effect, the curing process is also much slower in this case. This is the chemistry used in many consumer applications, such as silicone caulk and adhesives.