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From Wikipedia, the free encyclopedia

A mobile phone case made from silicone.

Silicones are largely-inert, man-made compounds with a wide variety of forms and uses. Typically heat-resistant, nonstick, and rubber-like, they are commonly used in cookware, medical applications, sealants, adhesives, lubricants, insulation, and breast implants.

Silicones are polymers that include silicon together with carbon, hydrogen, oxygen, and sometimes other chemical elements. Some common forms include silicone oil, silicone grease, silicone rubber, and silicone resin.

Contents

Properties

Some of the most useful properties of silicone include:

  1. Good electrical insulation. Because silicone can be formulated to be electrically insulative or conductive, it is suitable for a wide range of electrical applications.
  2. Thermal stability (constancy of properties over a wide operating range of −100 to 250 °C).
  3. Though not a hydrophobe, the ability to repel water and form watertight seals.
  4. Excellent resistance to oxygen, ozone and UV light (sunlight). This has led to widespread use in the construction industry (e.g. coatings, fire protection, glazing seals), and automotive industry (external gaskets, external trim).
  5. Does not stick.
  6. Low chemical reactivity.
  7. Low toxicity, but does not support microbiological growth.
  8. High gas permeability: at room temperature (25 °C) the permeability of silicone rubber for gases like oxygen is approximately 400 times that of butyl rubber, making silicone useful for medical applications (though precluding it from applications where gas-tight seals are necessary).

History

Frederick Kipping was the chemist who pioneered the study of the organic compounds of silicon (organosilicons), and coined the term silicone.[1]

Technical details

More precisely called polymerized siloxanes or polysiloxanes, silicones are mixed inorganic-organic polymers with the chemical formula [R2SiO]n, where R is an organic group such as methyl, ethyl, or phenyl. These materials consist of an inorganic silicon-oxygen backbone (…-Si-O-Si-O-Si-O-…) with organic side groups attached to the silicon atoms, which are four-coordinate.

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.

Synthesis

Silicones are synthesized from chlorosilanes, tetraethoxysilane, and related compounds. In the case of PDMS, the starting material is dimethyldichlorosilane, which reacts with water as follows:

n Si(CH3)2Cl2 + n H2O → [Si(CH3)2O]n + 2n HCl

During polymerization, this reaction evolves hazardous hydrogen chloride gas. For medical uses, a process was developed where the chlorine atoms in the silane precursor were replaced with acetate groups, so that the reaction product of the final curing process is nontoxic acetic acid (vinegar). As a side effect, the curing process is also much slower in this case. This is the chemistry used in many consumer applications, such as silicone caulk and adhesives.

Silane precursors with more acid-forming groups and fewer methyl groups, such as methyltrichlorosilane, can be used to introduce branches or cross-links in the polymer chain. Ideally, each molecule of such a compound becomes a branch point. This can be used to produce hard silicone resins. Similarly, precursors with three methyl groups can be used to limit molecular weight, since each such molecule has only one reactive site and so forms the end of a siloxane chain.

Modern silicone resins are made with tetraethoxysilane, which reacts in a more mild and controllable manner than chlorosilanes.

Chemical terminology

Silicone is often mistakenly referred to as "silicon." Although silicones contain silicon atoms, they are not made up exclusively of silicon, and have completely different physical characteristics from elemental silicon.

F. S. Kipping coined the word "silicone" in 1901 to describe polydiphenylsiloxane by analogy of its formula, Ph2SiO, with the formula of the ketone benzophenone, Ph2CO (Ph stands for phenyl, C6H5). Kipping was well aware that polydiphenylsiloxane is polymeric whereas benzophenone is monomeric and noted that Ph2SiO and Ph2CO had very different chemistry.[2][3]

A true silicone group with a double bond between oxygen and silicon (see figure) does not commonly exist in nature; chemists find that the silicon atom much prefers a single bond with each of two oxygen atoms, rather than a double bond to a single atom. Polysiloxanes are still more commonly known as "silicones".

Molecules containing silicon-oxygen double bonds do exist and are called silanones. Several silanones have been studied in argon matrices[4] and in the gas phase, but they are highly reactive.[5] Despite their reactivity, silanones are important as intermediates in gas-phase processes such as chemical vapor deposition in microelectronics production, in the formation of ceramics by combustion,[6] and in astrochemistry.[7]

Uses

Aquarium joints

Glass aquarium manufacturers have used 100% silicone sealant exclusively from its inception in order to join glass plates, making aquariums of every size and shape. Glass joints made with silicone sealant can withstand a great deal of pressure, making obsolete the original aquarium construction method using angle-iron and putty. This same silicone is also used to make hinges in aquarium lids or even for minor repairs. Not all commercial silicones are safe for aquarium manufacture, nor is silicone used for the manufacture of acrylic aquariums as silicones do not have long-term adhesion to plastics.[8]

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.

Automotive spark plug wires are often 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 must avoid the presence of all silicones, as they may cause "fish eyes," small, circular craters that appear in the finish.

Coatings

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

Fabrics may be coated or impregnated with silicone to form a strong, waterproof composite such as silnylon.

Cookware

  • 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.
  • It is used as an insulator in heat resistant potholders and similar, however it is more conductive of heat than the less dense fiber-based ones. Silicone oven mitts are able to withstand temperatures up to 675 °F (357 °C), and allow reaching into boiling water.[9]
  • Molds for chocolate, ice, cookies, muffins, etc.
  • Some novel designs are steamer, egg boiler, vegetables cooker, cooking lids, pot handle, kitchen mats, etc.

Defoaming

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

Dry cleaning

Liquid silicone can be used as a dry cleaning solvent. Touted as an "environmentally friendly" alternative to the traditional perchloroethylene (or perc) solvent, the decamethylpentacyclosiloxane (D5) process has been patented by the company GreenEarth Cleaning.

The solvent degrades into silica and trace amounts of water and CO2, and while the silica waste produced is hazardous if inhaled,[10] there is not currently any reason to think it is any environmentally worse than the silica present in regular beach sand. This significantly reduces the environmental impact of a typically high-polluting industry.

Additionally, liquid silicone is chemically inert, meaning it does not react with fabrics or dyes during the cleaning process. This reduces the amount of fading and shrinking that most dry-cleaned garments experience.

Electronics

Electronic components are sometimes encased in silicone to increase stability against mechanical and electrical shock, radiation and vibration. This is often 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.[11] 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.[12][13] Use of silicone-based spray products in electronic devices during maintenance or repairs can cause later failures.

Firestops

Silicone foams have 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.

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 been exposed by whistleblower Gerald W. Brown and have led to a large number of reportable events among licensees (operators of nuclear power plants) of the Nuclear Regulatory Commission (NRC).

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 can also be found in aircraft technology.

Lubricants

Silicone greases are used for many purposes, such as bicycle chains. A dry-set lubricant is delivered with a solvent carrier to penetrate the chain. The solvent evaporates, leaving a clear film that lubricates but does not attract dirt and grit as much as a traditional "wet" lubricant.

Silicone personal lubricants are also available, for use in medical procedures or sexual activity.

Medicine

Silicone, particularly the gel form, is used in bandages and dressings, in breast implants and a variety of other medical uses.

Polydimethylsiloxane (PDMS) has been used as the hydrophobic block of amphiphilic synthetic block copolymers used to form the vesicle membrane of polymersomes.

Moldmaking

Two-part silicone systems are used to create rubber molds which can be used for production casting of resins, foams, rubber and low-temp alloys.

A mold made of silicone generally requires little or no mold release or surface preparation as most materials do not adhere to moldmaking silicone.

For experimental uses, ordinary one-part silicone can also be used, either to make molds, or to mold into shapes. Common vegetable cooking oils and petroleum jelly can be used on mating surfaces as a mold release agent.[14]

Personal care

Silicones are ingredients in many hair conditioner, shampoo, and hair gel products. Some silicones, notably the amine functionalized amodimethicones, are excellent conditioners. They improve combability, feel, and softness, and also lessen frizz. Another silicone family, the phenyltrimethicones, are used in reflection-enhancing and color-correcting hair products, where they increase shine and glossiness (and possibly effect subtle color changes). Phenyltrimethicones, unlike the conditioning amodimethicones, have refractive indices (typically 1.46) close to that of human hair (1.54). It should be noted that achieving both high-shine and excellent conditioning in one hair care product is much more difficult than simply adding two different silicones to the formulation, because amodimethicone and phenyltrimethicone interact with and dilute each other. This is one reason why modern hair care products, and cosmetics generally, are among the most highly engineered consumer products.

Silicones are also used in some shaving products and personal lubricants. Menstrual cups are often made of medical grade silicone for its durability, reusability, and biocompatibility. Silicone is also material of choice for soft sex toys, due to its durability, cleanability and lack of phthalates, chemicals suspected of having carcinogenic and mutagenic effects on the skin and mucous membranes.[15][16][17]

Specific grades of silicone rubber are used widely in the production of baby bottle teats due to their cleanliness, aesthetic appearance, and low extractable content.

Plumbing and building construction

The strength and reliability of silicone rubber is widely acknowledged in the construction industry.

One-part silicone sealants and caulks are in common use to seal gaps, joints and crevices in buildings. One-part silicones cure by absorbing atmospheric moisture, which helps in the professional installation.

In plumbing, silicone grease is typically applied to O-rings in faucets and valves. Whilst the film is extant it prevents lime from sticking to the brasswork.

Toys

Silicone balls have become a juggler's favorite due to the high bounce back, and are used as a response system in many low response yo-yos.[18] Silicone has the potential of replacing plastic in creating many forms of toys.

Versatile applications

Silicone Industry Trade Organizations

The leading global manufacturers of silicone base materials belong to three regional organizations: the European Silicone Center(CES) in Brussels, Belgium; the Silicone Environment Health and Safety Council(SEHSC) in Washington, USA; and the Silicone Industry Association of Japan (SIAJ) in Tokyo, Japan. A fourth organization, the Global Silicone Council (GSC) acts as an umbrella structure over the regional organizations. All four are nonprofit making and have no commercial role. Their primary mission is to promote the safety of silicones from a health, safety and environmental perspective. As the European chemical industry is getting prepared to implement the REACH legislation, CES is leading the formation of a consortium[19] of silicones, silanes and siloxanes producers and importers to facilitate data and cost sharing.

See also

References

  1. ^ K.L. Mittal, A. Pizzi (2009). Handbook of Sealant Technology. CRC Press. p. 27. ISBN 0849391628. http://books.google.com/books?id=0VJD6MzH9D8C&pg=PT27. 
  2. ^ Greenwood, Norman N.; Earnshaw, A. (1997), Chemistry of the Elements (2nd ed.), Oxford: Butterworth-Heinemann, p. 362, ISBN 0-7506-3365-4 
  3. ^ Frederick Kipping, L. L. Lloyd (1901). "XLVII.?Organic derivatives of silicon. Triphenylsilicol and alkyloxysilicon chlorides". J. Chem. Soc., Trans. 79: 449–459. doi:10.1039/CT9017900449. 
  4. ^ R. Withnall, L. Andrews (1986). "Infrared spectroscopic evidence for silicon-oxygen double bonds: methyl- and dimethylsilanones in solid argon". J. Am. Chem. Soc. 108 (25): 8118–8119. doi:10.1021/ja00285a054. 
  5. ^ M. Bogey, B. Delcroix, A. Walters, J-C Guillemin (1996). "Experimentally Determined Structure of H2SiO by Rotational Spectroscopy and Isotopic Substitution". J. Mol. Spectrosc. 175 (2): 421–428. doi:10.1006/jmsp.1996.0048. 
  6. ^ V. N. Khabashesku, Z. A. Kerzina, K. N. Kudin, O. M. Nefedov (1998). "Matrix isolation infrared and density functional theoretical studies of organic silanones, (CH3O)2Si=O and (C6H5)2Si=O". J. Organomet. Chem. 566 (1-2): 45–59. doi:10.1016/S0022-328X(98)00726-8. 
  7. ^ J. L. Turner, A. Dalgarno (1977). "The chemistry of silicon in interstellar clouds". Astrophysical Journal 213: 386–389. doi:10.1086/155167. 
  8. ^ Aquarium Silicone Applications
  9. ^ Silicone oven mitts
  10. ^ Antonini, J. M. (2000). "Effect of Silica Inhalation on the Pulmonary Clearance of a Bacterial Pathogen in Fischer 344 Rats". Lung 178: 341. doi:10.1007/s004080000038. 
  11. ^ See Resin dispensing for how silicones can be dispensed in circuit board production.
  12. ^ Paul G. Slade (1999). "16.4.1". Electrical Contacts: Principles and Applications. CRC Press. p. 823. ISBN 0824719344. http://books.google.com/books?id=EkStW7v8VPkC&pg=PA823. 
  13. ^ "A Comparison for the Effects of Various Forms of Silicon Contamination on Contact Performance" by Witter, G. (Published in "Components, Hybrids, and Manufacturing Technology, IEEE Transactions on" Mar 1979).
  14. ^ Chapter 7 - Silicone Caulk Molds
  15. ^ W.M. Kluwe (1986). "Carcinogenic potential of phthalic acid esters and related compounds: structure-activity relationships" (free text). Environmental Health Perspectives 65: 271–278. doi:10.2307/3430194. PMID 3709453.& PMC 1474699. http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1474699. 
  16. ^ Norbert H. Kleinsasser, Ernst R. Kastenbauer, Herbert Weissacher, Ruth K. Muenzenrieder, Ulrich A. Harréus (2000). "Phthalates demonstrate genotoxicity on human mucosa of the upper aerodigestive tract". Environmental and Molecular Mutagenesis 35 (1): 9–12. doi:<9::AID-EM2>3.0.CO;2-1 10.1002/(SICI)1098-2280(2000)35:1<9::AID-EM2>3.0.CO;2-1. PMID 10692222. http://www3.interscience.wiley.com/cgi-bin/abstract/69504565/ABSTRACT?CRETRY=1&SRETRY=0. 
  17. ^ Walter J. Kozumbo, Rosanna Kroll, Robert J. Rubin (1982). "Assessment of the Mutagenicity of Phthalate Esters". Environmental Health Perspectives 45: 103–109. http://www.jstor.org/stable/3429391. 
  18. ^ "Silicone as a response". http://yoyowiki.org/wiki/Response_Systems#Silicone. 
  19. ^ REACH consortium

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