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The silicone introduction
- Dec 09, 2017 -

Chemistry

More precisely called polymerized siloxanes or polysiloxanes, silicones consist of an inorganic silicon-oxygen backbone chain (⋯-Si-O-Si-O-Si-O-⋯) with organic side groups attached to the silicon atoms. These silicon atoms are tetravalent. So, silicones are polymers constructed from inorganic-organic monomers. Silicones have in general the chemical formula [R2SiO]n, where R is an organic group such as an alkyl (methyl, ethyl) or phenyl group.

 

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 to gel to rubber to hard plastic. The most common siloxane is linear polydimethylsiloxane(PDMS), a silicone oil. The second largest group of silicone materials is based on silicone resins, which are formed by branched and cage-like oligosiloxanes.

 

Terminology and history

F. S. Kipping coined the word silicone in 1901 to describe polydiphenylsiloxane by analogy of its formula, Ph2SiO (Ph stands for phenyl, C6H5), with the formula of the ketone benzophenone, Ph2CO (his term was originally silicoketone). Kipping was well aware that polydiphenylsiloxane is polymeric whereas benzophenone is monomeric and noted that Ph2SiO and Ph2CO had very different chemistry.[2][3] The discovery of the structural differences between Kippings' molecules and the ketones means that silicone is no longer the correct term (though it remains in common usage) and that the term siloxanes is correct according to the nomenclature of modern chemistry.

Silicone is sometimes mistakenly referred to as silicon. The chemical element silicon is a crystalline metalloid widely used in computers and other electronic equipment. Although silicones contain silicon atoms, they also include carbon, hydrogen, oxygen, and perhaps other kinds of atoms as well, and have physical and chemical properties that are very different from elemental silicon.

A true silicone group with a double bond between oxygen and silicon does not commonly exist in nature; chemists find that the silicon atom usually forms single bonds with each of two oxygen atoms, rather than a double bond to a single atom. Polysiloxanes are among the many substances commonly known as "silicones".

 

Molecules containing silicon-oxygen double bonds do exist and are called silanones but they are very reactive. Despite this, silanones are important as intermediates in gas-phase processes such as chemical vapor deposition in microelectronics production, and in the formation of ceramics by combustion.

Uses

Silicones are used in many products. Ullmann's Encyclopedia of Industrial Chemistry lists the following major categories of application: Electrical (e.g., insulation), electronics (e.g., coatings), household (e.g., sealants for cooking apparatus), automobile (e.g., gaskets), aeroplane (e.g., seals), office machines (e.g., keyboard pads), medicine and dentistry (e.g., tooth impression molds), textiles and paper (e.g., coatings). For these applications, an estimated 400,000 tons of silicones were produced in 1991. Specific examples, both large and small are presented below.

Automotive

In the automotive field, silicone grease is typically used as a lubricant for brake components since it is stable at high temperatures, is not water-soluble, and is far less likely than other lubricants to foul. It is also used as DOT 5 brake fluid.

Automotive spark plug wires are insulated by multiple layers of silicone to prevent sparks from jumping to adjacent wires, causing misfires. Silicone tubing is sometimes used in automotive intake systems (especially for engines with forced induction).

Sheet silicone is used to manufacture gaskets used in automotive engines, transmissions, and other applications.

Automotive body manufacturing plants and paint shops avoid silicones, as they may cause "fish eyes", small, circular craters in the finish.

Additionally, silicone compounds such as silicone rubber are used as coatings and sealants for airbags; the high strength of silicone rubber makes it an optimal adhesive and sealant for high impact airbags. Recent technological advancements allow convenient use of silicone in combination with thermoplastics to provide improvements in scratch and mar resistance and lowered coefficient of friction.

Coatings

Silicone films can be applied to such silica-based substrates as glass to form a covalently bonded hydrophobic coating.

Many fabrics can be coated or impregnated with silicone to form a strong, waterproof composite such as silnylon.

Cookware

Soup ladle and pasta ladle made of silicone.

A silicone food steamer to be placed inside a pot of boiling water.

Ice cube trays made of silicone.

  • As a low-taint, non-toxic material, silicone can be used where contact with food is required. Silicone is becoming an important product in the cookware industry, particularly bakeware and kitchen utensils.

  • Silicone is used as an insulator in heat-resistant potholders and similar items; however, it is more conductive of heat than similar less dense fiber-based products. Silicone oven mitts are able to withstand temperatures up to 260 °C (500 °F), allowing reaching into boiling water.

  • Molds for chocolate, ice, cookies, muffins and various other foods.

  • Non-stick bakeware and reusable mats used on baking sheets.

  • Other products such as steamers, egg boilers or poachers, cookware lids, pot holders, trivets, and kitchen mats.

Defoaming

Silicones are used as active compound in defoamers due to their low water solubility and good spreading properties.

Dry cleaning

Liquid silicone can be used as a dry cleaning solvent, providing an alternative to the traditional chlorine-containing perchloroethylene (perc) solvent. Use of silicones in dry cleaning reduces the environmental impact of a typically high-polluting industry.

Electronics

Electronic components are sometimes encased in silicone to increase stability against mechanical and electrical shock, radiation and vibration, a process called "potting".

Silicones are used where durability and high performance are demanded of components under hard conditions, such as in space (satellite technology). They are selected over polyurethane or epoxy encapsulation when a wide operating temperature range is required (−65 to 315 °C). Silicones also have the advantage of little exothermic heat rise during cure, low toxicity, good electrical properties and high purity.

The use of silicones in electronics is not without problems, however. Silicones are relatively expensive and can be attacked by solvents. Silicone easily migrates as either a liquid or vapor onto other components.

Silicone contamination of electrical switch contacts can lead to failures by causing an increase in contact resistance, often late in the life of the contact, well after any testing is completed.Use of silicone-based spray products in electronic devices during maintenance or repairs can cause later failures.

Firestops


Silicone-foam firestops have been the subject of controversy and press attention due to smoke development from pyrolysis of combustible components within the foam, hydrogen gas escape, shrinkage and cracking. These problems have led to reportable events among licensees (operators of nuclear power plants) of the Nuclear Regulatory Commission (NRC).Silicone foam has been used in North American buildings in an attempt to firestop openings within fire-resistance-rated wall and floor assemblies to prevent the spread of flames and smoke from one room to another. When properly installed, silicone-foam firestops can be fabricated for building code compliance. Advantages include flexibility and high dielectric strength. Disadvantages include combustibility (hard to extinguish) and significant smoke development.

 

Silicone firestops are also used in aircraft.