Silicon is the second most abundant element on Earth. However, silicon is never found in nature as the raw element, but rather in combination with oxygen to yield various forms of silicas, such as sand, quartz, minerals and glasses. Silicon in combination with oxygen makes up 75% of the Earth's crust. Utilitarian application of the naturally occurring forms of silicon in tools, weapons, glasses, and gems predates recorded history.
Silicon is found in the same element group on the periodic chart as carbon. In the past, this close family relationship to carbon has led many scientists to speculate that a realm of chemistry awaited discovery wherein silicon would freely replace carbon. A new branch of chemistry was at hand; however silicon was not a replacement for carbon.
In the early 1940s, Eugene G. Rochow, professor emeritus of Harvard University, pioneered the large scale manufacture of commercial silicones. This was done via the direct reaction of methyl chloride with silicon, in the presence of copper and other catalysts. Thousands of papers have since been published demonstrating how this process can lead to numerous organosilicon compounds for thousands of industrial applications.
Silicas
The term silica refers to the compound silicon dioxide, SiO2. Silica is a ubiquitous chemical substance with much chemical, geological and commercial importance. It is the sole source of elemental silicon, for commercial use. It is used in large quantities as a constituent of building materials, ceramics, concretes and glasses. In its various amorphous forms, it is used as a desiccant, absorbent, reinforcing agent, binder, builder for detergents and as a catalyst component for support.
Amorphous silica has the same basic atomic structure as crystalline silica but lacks a highly ordered geometry. Fumed silicas, which are amorphous, are generated by burning silanes and are used as reinforcing agents in many elastomeric and rubber silicone products.
(Silane= a discrete molecule whose structure is SiH4; also derivatives of this).
Silicone
Is the generic description for an entirely synthetic polymer containing a repeating Si-O backbone. The organic groups attached to the silicon atom via silicon-carbon bonds define the class of silicone. The most common example is poly-dimethylsiloxane or PDMS. This synthetic polymer has a repeating [(CH3)2SiO] unit. This is the basic building block of silicones. Depending upon the number of repeat units in the chain and the amount of tying the chains together, six classes of commercially important products can be produced. They are: fluids, emulsions, compounds, lubricants, resins, and elastomers or rubbers.
Silicone Fluids
Fluids are usually straight chains of PDMS, which are terminated with trimethylsilyl groups. PDMS fluids come in all viscosities from water-like liquids to non-pourable fluids, and all of them are essentially water insoluble.
Gels
Silicone gels are lightly cross-linked PDMS where the crosslink is achieved through either a "T" silicone structure or the chemical reaction between a vinyl group on one of the silicon atoms, with a hydrogen bonded to a silicon in a different part of the polymer. This chemical tying of siloxane chains produces a 3-dimensional network which can be swollen with PDMS fluids to give a sticky, cohesive mass without form. Its physical appearance is dictated by the amount of crosslinking and how much fluid is added to the network.
Elastomers
Silicone elastomers are crosslinked fluids whose three-dimensional structure is much more intricate than a gel. There is very little free, non-crosslinked fluid in the matrix. Amorphous silica is frequently added to the matrix to give greater reinforcement of the network and thereby increase the strength of the article.
Quote about bioreactivity:
"Polydimethylsiloxane (PDMS) fluids with more than 6-8 silicon atoms have no known toxicology or relevant biological effects.
--"The Pharmacology of Silanes and Siloxanes," from Biochemistry of Silicon and Related Problems, Bendz and Lindquist, Plenum Publishing Corp., 1978.
How much Silicon is in a human?
Silicon plays an active role in the development of plants and animals. Wannagat discovered that the silicon content of living organisms decreases as the complexity of the organism rises. The ratio of silicon to carbon is 250:1 in the earth's crust, 15:1 in humus soil, 1:1 in plankton, 1:100 in ferns, and 1:5000 in mammals.
In the human body there is usually only 5 to 10 grams of silicon, which mostly likely is acquired from the environment. There is dissolved silicic acid in drinking water and silicate dust in the air we breathe. Silicon plays a key, but not fully understood role in the growth of hair, nails, bones and feathers. Interestingly, at the site of a bone fracture silicon content increases 50-fold in the collagen web.
Excerpted from:
"Silica, Silicon, and Silicones....Unraveling the Mystery" paper by T.H. Lane, Ph.D. of Dow Corning Corporation. (1995)
Silicon and Silicones: About Stone-age Tools, Antique Pottery, Modern Ceramics, Computers, Space Materials and How They All Got That Way, by Eugene G. Rochow, Springer-Verlag Berlin Heidelberg(1987)
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