What is silicone? (Note the E)
Simply put, Polysiloxane, (the proper name for Silicones), are stable synthetic compounds (polymers), with lubricating properties that mimic organic, carbon based compounds, such as petroleum, animal fats and vegetable oil.
Silicone is a term much like "plastic" - it covers a rather wide range of materials and properties. The chemistry of a particular polymer determines its characteristics, from hard and brittle varnishes, to soft and flexible rubbers. The initial work on polysiloxane chemistry dates back almost 60 years and it continues today.
Silicones are used for a lot of things. They can be elastomers and lubricating oils. The caulking in your bathroom is probably made of a silicone. Among other things, silicones are also used to make the heat resistant tiles on the bottom of the space shuttle, breast and other implants, and hair conditioners that don't cause buildup.
Dictionary.com defines Silicone as:
" 1) Any of a group of semi-inorganic polymers based on the structural unit R2SiO, where R is an organic group, characterized by wide-range thermal stability, high lubricity, extreme water repellence, and physiological inertness and used in adhesives, lubricants, protective coatings, paints, electrical insulation, synthetic rubber, and prosthetic replacements for body parts."
" 2) Any of a large class of siloxanes that are unusually stable over a wide range of temperatures; used in lubricants and adhesives and coatings and synthetic rubber and electrical insulation [syn: silicone polymer]"
Silicone is different from Silicon and Silica, though it contains a relatively high proportion of silicon. Silicon, which is found in rocks and sand, is the second most common element in the crust of our earth. It is not found in its elemental form, but occurs mainly as oxides and silicates. Silica is a three dimensional network of silicon dioxide, most commonly encountered as sand. Silica exists in crystalline and amorphous forms, is chemically resistant at ordinary temperatures, and can undergo a variety of transformations at high temperatures (greater than 500ºC, 950ºF) and pressures.
The basic difference between silicone polymers and "organic" polymers is in the molecular make-up. Silicone or Dimethyl Polysiloxane, is made up of silicone/oxygen linkages, the same found in high temperature materials such as quartz, glass, and sand. Natural rubber, or organic polymers, are made up of carbon/carbon linkages.
Through altering the chemical make-up of the silicones by adding phenyls, vinyls, and fluorines, significant variations in physical properties can be achieved. But these alterations are often responsible for creating toxicity.
Basically, we haven't a clue what is in our particular tyres and jonny is right about the potential dangers.