Silicone
Not to be confused with Silicon.
Silicones, or polysiloxanes, are inorganic-organic polymers with the
chemical formula [R2SiO]n, where R = organic groups such as methyl, ethyl,
and phenyl, but most commonly methyl). These materials consist of an
inorganic silicon-oxygen backbone, Typical is silicone fluid:
CH3 CH3 CH3
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CH3-Si-(O-Si-)xO-Si- CH3
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CH3 CH3 CH3
Often organic side groups attached to the silicon atoms, producing many
interesting compounds;
CH3 R
CH3
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CH3-Si-(O-Si-)xO-Si- CH3
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CH3 CH3 CH3
R is:
-(CH2)3-O-(CH2CH2O)xH -(CH2)x-CH3
-(CH2)3(CF2CH3
Water soluble oil
soluble Fluoro soluble
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
(fluid) to rubber ( elastomeric) to hard plastic (resins).
Properties
Silicone oils, polymers and cured sealants are odorless and colourless,
water resistant, chemical resistant, oxidation resistant, stable at high
temperature, and have weak forces of attraction, low surface tension, low
freezing points and do not conduct electricity. Silicone caulking is
odourless only when cured. Silicones 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). Simekthicone, a silicone-based anti-foaming agent, has remained
available as an over-the-counter drug and food additive.
Chemical terminology
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. Dimethylsilicone and dimethyl
ketone (aka acetone) have analogous formulas, thus it was surmised
incorrectly that they have analogous structures. The same terminology is
used for compounds such as silane, which is an analogue of methane). A
true silicone group with a double bond between oxygen and silicon do not
exist (see figure), Polysiloxanes are called "silicone" due to early
mistaken assumptions about structure.
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