Silicone
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Silicones, or polysiloxanes, are inorganic polymers consisting of a silicon-oxygen backbone (...-Si-O-Si-O-Si-O-...) with side groups attached to the silicon atoms. Certain 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 into a wide variety of materials. They can vary in consistency from liquid to gel to rubber to hard plastic. The most common type is linear polydimethylsiloxane or PDMS. The second largest group of silicone materials is based on silicone resins, which are formed by branched and cage-like oligosiloxanes.
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Properties
Silicones are odorless, colorless, water resistant, chemical resistant, oxidation resistant, stable at high temperature, and do not conduct electricity. They have many uses, such as lubricants, adhesives, sealants, gaskets, breast implants, pressure compensating diaphragms for drip irrigation emitters, dishware, Silly Putty, and many other products. Due to their thermal stability and relatively high melting and boiling points, silicones are often used where organic polymers are not applicable. Their unreactivity generally makes them non-toxic (see below).
A controversy developed during the 1990s around allegations that silicone in breast implants was responsible for several diseases. Health concerns included pain, deformity and the less obviously related connective tissue disorders (eg. scleroderma, arthritis) and chronic fatigue syndrome. Leakage of silicone from implants could be demonstrated easily but proof of its safety or otherwise was lacking. The Dow Corning corporation declared bankruptcy and settled several class actions globally; later, good evidence emerged clearing silicone of causing connective tissue disease. Silicone implants have been removed from the market in some countries (notably the US) because of the silicone controversy but are extensively used elsewhere.
Simethicone, a silicone-based anti-foaming agent, has remained available as an over-the-counter substance and food additive.
Chemical terminology
Image:Silicone.png Silicone is often mistakenly referred to as "silicon". Although silicones contain silicon atoms, they are not made up exclusively of silicon, and have completely different physical characteristics from elemental silicon.
The word "silicone" is derived from ketone, and is technically not the correct term for the polymers this article describes. A true silicone group has a double bond between oxygen and silicon (see figure), like a ketone group with Si in place of C (the same terminology is used for compounds such as silane, which is an analogue of methane). Polysiloxanes are called "silicone" due to an early mistaken assumption about their structure, but it has since been shown that they contain no silicone groups at all. For an idea of what a genuine polysilicone molecule would look like, see polyketone.
External links
- Silicone Polymers (Virtual Chembook, Elmhurst College)
- Science of Silicone Polymers (Silicone Science On-line, Centre Européen des Silicones - CES)
- Silicon Chemistry (Silicon Chemistry Basics, Dow Corning)
- FDA: A Status Report on Breast Implant Safety (U.S. Food & Drug Administration)
- Safety of Silicone Breast Implants (Publication by Editors, Committee on the Safety of Silicone Breast Implants, Institute of Medicine (viewable online))
- Silicone and Elastomerics materials - Stockwell Elastomerics
- Technical Data and Commercial Availability - Ja-Bar Silicone Corporationar:سليكون
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