There are several different forms of silicone, which vary in their degree of crosslinking. The degree of crosslinking describes how interconnected the silicone chains are, with higher values resulting in a more rigid silicone material. This variable alters properties such as the strength of the polymer and its melting point.

The forms of silicone, as well as some of their applications, include:

• Silicone fluids, also called silicone oils, consist of straight chains of the silicone polymer with no crosslinking. These fluids have found use as lubricants, paint additives, and ingredients in cosmetics.
• Silicone gels have few crosslinks between the polymer chains. These gels have been used in cosmetics and as a topical formulation for scar tissue, since silicone forms a barrier that helps the skin stay hydrated. Silicone gels are also used as as materials for breast implants and the soft part of some shoe insoles.
• Silicone elastomers, also called silicone rubbers, include even more crosslinks, yielding a rubberlike material. These rubbers have found use as insulators in the electronics industry, seals in aerospace vehicles, and oven mitts for baking.
• Silicone resins are a rigid form of silicone and with a high crosslinking density. These resins have found use in heat-resistant coatings and as weather-resistant materials for protecting buildings.

Silicone Toxicity

Because silicone is chemically inert and more stable than other polymers, it is not expected to react with parts of the body. However, toxicity depends upon factors such as exposure time, chemical composition, dose levels, type of exposure, absorption of the chemical, and the individual response. 

Researchers have examined the potential toxicity of silicone by looking for effects such as skin irritation, changes in the reproductive system, and mutations. Although a few types of silicone showed potential to irritate human skin, studies have shown that exposure to standard quantities of silicone typically produce few to no adverse effects.

Key Points

• Silicone is a type of synthetic polymer. It has a silicon-oxygen backbone, with “sidechains” consisting of hydrogen and/or hydrocarbon groups attached to the silicon atoms.
• The silicon-oxygen backbone makes silicone more stable than the polymers that have carbon-carbon backbones. 
• Silicone is durable, stable, and easy to manufacture. For these reasons, it has been widely commercialized and is found in many everyday items. 
• Silicone contains silicon, which is a naturally-occurring chemical element.
• The properties of silicone change as the degree of crosslinking increases. Silicone fluids, which have no crosslinking, are the least rigid. Silicone resins, which have a high level of crosslinking, are the most rigid. 

Sources

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New types of silicone resin open up wider fields of application, Marco Heuer, Paint & Coatings Industry.

“Silicone toxicology.” In Safety of Silicone Breast Implants, ed. Bondurant, S., Ernster, V., and Herdman, R. National Academies Press, 1999.

"Silicones." The Essential Chemistry Industry.

Shukla, B., and Kulkarni, R. "Silicone polymers: history & chemistry."

“The Technic explores silicones.” The Michigan Technic, vol. 63-64, 1945, pp. 17.

Wacker. Silicones: Compounds and properties.