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.
Fabrics may be coated or impregnated with silicone to form a strong, waterproof composite such as silnylon.
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.
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.
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 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.
Silicone personal lubricants are also available, for use in medical procedures or sexual activity. See below.
Polydimethylsiloxane (PDMS) has been used as the hydrophobic block of amphiphilic synthetic block copolymers used to form the vesicle membrane of polymersomes.
Cooking moulds used as bakeware do not require being coated with cooking oil, and in fact, are valued for this quality, since silicone bakeware can make it easier for the cook to remove the baked food from the mould after cooking.
Silicones are used in shaving products and personal lubricants. Menstrual cups and scar treatment sheets 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.
Silicone rubber is used in baby bottle teats for its cleanliness, aesthetic appearance, and low extractable content.
Silicones, often Polydimethylsiloxane, are used in scar treatment sheets. This specific crosslinking results in a flexible and soft silcone with high durability and tack.
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 taps and valves. Whilst the film is extant it prevents lime from sticking to the brasswork.
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.
: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. The reaction product of the final curing process is the less dangerous acetic acid (the acid found in vinegar). As a side effect, the curing process is 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.
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.
A true silicone group with a double bond between oxygen and silicon 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 and in the gas phase, but they are highly reactive. 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, and in astrochemistry.
Category:Adhesives Category:Cosmetics chemicals * Category:Thermosetting plastics
ar:سيليكون bs:Silikon bg:Силикон ca:Silicona cs:Silikon da:Silikone de:Silikone es:Silicona eo:Silikono fa:سیلیکون fr:Silicone ko:실리콘 (중합체) hi:सिलिकोन id:Silikone it:Siliconi he:סיליקון (תרכובת) lv:Silikoni nl:Silicone ja:シリコーン no:Silikoner pl:Silikony pt:Silicone ru:Силиконы sl:Silikon sr:Силикон fi:Silikoni sv:Silikon tr:Silikon uk:Силікони zh:矽氧樹脂This text is licensed under the Creative Commons CC-BY-SA License. This text was originally published on Wikipedia and was developed by the Wikipedia community.
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