- Also called:
- polysiloxane
- Key People:
- Frederic Stanley Kipping
silicone
-
What is silicone?
Silicones are a diverse class of synthetic polymers composed of chains of alternating silicon and oxygen atoms. Most silicones also have two organic groups attached to each silicon atom.
These substances are known for their chemical inertness, resistance to water and oxidation, and stability across a wide range of temperatures, making them useful in many commercial applications. They can be manufactured as fluids, resins, or elastomers.
Silicones are used as lubricants, electrical insulators, and in medical implants. The most common silicone polymer is polydimethylsiloxane, derived from silica sand.
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What elements make up silicone?
Silicone is a diverse class of fluids, resins, or elastomers based on polymerized siloxanes. These substances have molecules consisting of chains made of alternating silicon and oxygen atoms.
Most industrial polymers contain carbon in the chains of linked atoms that make up the backbones of their molecules; however, silicones do not. Two organic groups, such as vinyl ((CH_2)), methyl ((CH_3)), or phenyl ((C_6H_5)), are attached to each silicon atom in most members of the silicone class. A general formula for silicones is ((R_2SiO)_x), where R can be any one of a variety of organic groups.
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What are some common uses of silicone in daily life?
Silicones are a diverse group of synthetic polymers with a wide array of applications in daily life due to their unique properties, such as thermal stability and chemical inertness.
Common uses of silicones:
* Lubricants and Hydraulic Fluids Because their viscosity remains stable across a wide range of temperatures, silicones are useful as lubricants in extreme conditions.
* Electrical Insulation Silicones are employed as insulators for electrical wiring because of their stability and resistance to heat.
* Water Repellents Silicones can create water-repellent films on fabrics like paper, wool, and silk.
* Medical Implants The chemical inertness of silicones makes them suitable for biomedical implants such as breast implants.
* Seals and Gaskets Silicone rubber is used in O-rings, heat-resistant seals, caulks, and gaskets.4 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
How is silicone different from the element silicon?
Silicones and silicon differ significantly in composition, structure, and applications. Silicon (Si) is a nonmetallic chemical element, the second most abundant in Earth's crust. In contrast, silicones are synthetic polymers composed of siloxanes, which are chains of alternating silicon and oxygen atoms. The general formula for silicones is (R₂SiO)ₓ, where R represents organic groups.
The absence of carbon in the polymer backbones of silicones distinguishes them from most industrial polymers. Silicones are known for their stability, chemical inertness, and resistance to water and oxidation. Silicon is a crucial semiconductor in electronics, used in computer chips and solar cells. Silicones are used in lubricants, hydraulic fluids, and biomedical implants.
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Why is silicone popular in products that need to be heat resistant or flexible?
Silicone's popularity in heat-resistant and flexible products stems from its unique molecular structure. The backbone of silicone molecules consists of chains of alternating silicon and oxygen atoms (polysiloxanes), unlike most industrial polymers that contain carbon.
Silicone is remarkably stable because the siloxane molecules rotate freely around the Si-O bond, making the molecule highly flexible, even with vinyl, methyl, or phenyl groups attached to the silicon atoms. The Si-O bond is also highly heat-resistant and not readily attacked by oxygen or ozone. These properties give silicone products excellent chemical inertness, stability, and resistance to water and oxidation across a broad temperature range.
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How is silicone manufactured and processed for various applications?
Silicones, a class of compounds based on polymerized siloxanes, are manufactured as fluids, resins, or elastomers, depending on the polymer's molecular weight and the degree to which the polymer chains are interlinked.
Linear and cyclic silicones are produced through the reaction of water with organochlorosilanes, followed by polymerization that eliminates water molecules. The most common siloxane polymer, polydimethylsiloxane, forms when chlorine atoms of the monomer are replaced by hydroxyl groups through hydrolysis. The resultant unstable compound condenses to form the polymer, with water loss.
These materials are processed into various forms, including room-temperature-vulcanizing (RTV) elastomers and high-temperature-vulcanizing (HTV) elastomers. RTV elastomers are cast or molded and then interlinked at room temperature, while HTV elastomers are mixed and processed like other elastomers. Fillers like silica are often added to strengthen silicone rubbers.
Due to their stability, chemical inertness, and resistance to water and oxidation, silicones are used in lubricants, hydraulic fluids, electrical insulators, and biomedical implants.
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silicone, any of a diverse class of fluids, resins, or elastomers based on polymerized siloxanes, substances whose molecules consist of chains made of alternating silicon and oxygen atoms. Their chemical inertness, resistance to water and oxidation, and stability at both high and low temperatures have led to a wide range of commercial applications, from lubricating greases to electrical-wire insulation and biomedical implants (such as breast implants).
Composition, structure, and properties
The silicones differ from most industrial polymers in that the chains of linked atoms that make up the backbones of their molecules do not contain carbon, the characteristic element of organic compounds. This lack of carbon in the polymer backbones makes polysiloxanes into unusual “inorganic” polymers—though in most members of the class two organic groups, usually vinyl (CH2), methyl (CH3), or phenyl (C6H5), are attached to each silicon atom. A general formula for silicones is (R2SiO)x, where R can be any one of a variety of organic groups.
The most common silicone compound, poly-dimethylsiloxane, can illustrate the central characteristics of the class. The starting material is metallic silicon, which is obtained from silica sand. Silicon is reacted with methyl chloride (CH3Cl) over a copper catalyst, forming dimethyldichlorosilane ([CH3]2Si[Cl]2). By reacting this compound with water, the chlorine atoms are replaced by hydroxyl (OH) groups. The resultant unstable compound, silanol ([CH3]2Si[OH]2), polymerizes in a condensation reaction, the single-unit molecules linking together to form poly-dimethylsiloxane with concomitant loss of water. The dimethylsiloxane repeating unit of the polymer has the following structure: 
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Siloxane molecules rotate freely around the Si-O bond, so, even with vinyl, methyl, or phenyl groups attached to the silicon atoms, the molecule is highly flexible. In addition, the Si-O bond is highly heat-resistant and is not readily attacked by oxygen or ozone. As a result, silicones are remarkably stable, and they have the lowest glass-transition temperature (the temperature below which the molecules are locked in a rigid, glassy state) and the highest permeability to gases of any polymer. On the other hand, the Si-O bond is susceptible to hydrolysis and attack by acids and bases, so silicone plastics and rubbers are relatively weak and readily swollen by hydrocarbon oils.
Applications
Polysiloxanes are manufactured as fluids, resins, or elastomers, depending on the molecular weight of the polymers and the degree to which the polymer chains are interlinked. Nonvulcanized, low-molecular-weight polysiloxane fluids are exceptionally stable to decomposition by heat, water, or oxidizing agents and are good electrical insulators. They make excellent lubricants and hydraulic fluids, as well as emulsions for imparting water repellency to textiles, paper, and other materials. Silicone resins are used in protective coatings and electrically insulating varnishes and for laminating glass cloth.
Vulcanized silicone rubber is prepared in two principal forms: (1) as room-temperature-vulcanizing (RTV) elastomers, which are low-molecular-weight liquids that are cast or molded into desired shapes and then interlinked at room temperature, and (2) high-temperature-vulcanizing (HTV) elastomers, which are higher-molecular-weight gums that are mixed and processed like other elastomers. Silicone rubbers are usually strengthened by fillers such as silica; other fillers are mixed in to add bulk and colour. Valued for their electrical-insulating properties, chemical stability, and the wide temperature range over which they retain resiliency, silicone rubbers are used mainly in O-rings, heat-resistant seals, caulks, gaskets, electrical insulators, flexible molds, and (owing to their chemical inertness) surgical implants.
History
Siloxanes were first characterized as polymers by the English chemist Frederic Stanley Kipping in 1927. Because Kipping thought that the structure of the repeating unit was essentially that of a ketone (the polymer chains formed by silicon atoms, with oxygen atoms attached by double bonds), he incorrectly called them silicones, a name that has persisted. In 1940 American chemist Eugene George Rochow at the General Electric Company laboratories in Schenectady, N.Y., U.S., prepared methyl siloxanes by the process that remains the basis of modern polymerization methods. Meanwhile, researchers at Corning Glass were exploring the production of silicones, and in 1943 Corning and the Dow Chemical Company formed the Dow Corning Corporation to produce silicone products.
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