Silicone is often confused with one of its constituent elements,silicon, but they are distinct substances. Silicon is a purechemical element, ametalloid, which forms a dark-greysemiconductingcrystalline solid. In itscrystalline form it is used to makeintegrated circuits ("electronic chips") andsolar cells. In contrast, silicone is formed by thepolymerization of the siloxane molecule, which itself is made up of a variable combination of carbon, hydrogen, oxygen and silicon atoms. Depending on the chemical makeup and polymer structure of a particular silicone product, it can possess a variety of physical properties, ranging from an oily liquid to a rubberyresin.
F. S. Kipping coined the wordsilicone in 1901 to describe the formula of polydiphenylsiloxane,Ph2SiO (Ph =phenyl,C6H5), by analogy with the formula of theketonebenzophenone,Ph2CO (his term was originallysilicoketone). Kipping was well aware that polydiphenylsiloxane ispolymeric[3] whereas benzophenone is monomeric and noted the contrasting properties ofPh2SiO andPh2CO.[4][5] The discovery of the structural differences between Kipping's molecules and the ketones means thatsilicone is no longer the correct term (though it remains in common usage) and that the termsiloxane is preferred according to the nomenclature of modern chemistry.[6]
James Franklin Hyde was an American chemist and inventor. He has been called the "Father of Silicones" and is credited with the launch of the silicone industry in the 1930s. His most notable contributions include his creation of silicone from silicon compounds and his method of makingfused silica, a high-quality glass later used in aeronautics, advanced telecommunications, and computer chips. His work led to the formation ofDow Corning, an alliance between theDow Chemical Company andCorning Glass Works that was specifically created to produce silicone products.
When the hydrolysis is performed by treating a solution ofH2SiCl2 in benzene with water, the product was determined to have the approximate formula[H2SiO]6. Higher polymers were proposed to form with time.[7]
Most polysiloxanes feature organic substituents, e.g.,[(CH3)2SiO]n and[(C6H5)2SiO)]n. Allpolymerizedsiloxanes or polysiloxanes, silicones consist of an inorganic silicon–oxygenbackbone chain (···−Si−O−Si−O−Si−O−···) with two groups attached to each silicon center. The materials can be cyclic or polymeric. By varying the−Si−O− chain lengths, side groups, andcrosslinking, 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 linearpolydimethylsiloxane (PDMS), asilicone oil.[citation needed] The second-largest group of silicone materials is based onsilicone resins, which are formed by branched and cage-like oligosiloxanes.[citation needed]
The polymerization typically produces linear chains capped withSi−Cl orSi−OH (silanol) groups. Under different conditions, the polymer is a cyclic, not a chain.[1]
For consumer applications such as caulks, silylacetates are used instead of silyl chlorides. The hydrolysis of the acetates produces the less dangerousacetic acid (the acid found invinegar) as the reaction product of a much slower curing process. This chemistry is used in many consumer applications, such as siliconecaulk andadhesives.
n Si(CH3)2(CH3COO)2 +n H2O → [Si(CH3)2O]n + 2n CH3COOH
Branches or crosslinks in the polymer chain can be introduced by using organosilicone precursors with fewer alkyl groups, such as methyl trichlorosilane andmethyltrimethoxysilane. Ideally, each molecule of such a compound becomes a branch point. This process 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.
Resistance to oxygen, ozone, andultraviolet (UV) light. This property has led to the widespread use of silicones in the construction industry (e.g. coatings, fire protection, glazing seals) and theautomotive industry (external gaskets, external trim).
Electrical insulation properties. Because silicone can be formulated to be electrically insulative or conductive, it is suitable for a wide range of electrical applications.
Highgas permeability: at room temperature (25 °C), the permeability ofsilicone rubber for such gases as oxygen is approximately 400 times[9] that ofbutyl rubber, making silicone useful for medical applications in which increased aeration is desired. Conversely, silicone rubbers cannot be used where gas-tight seals are necessary such as seals for high-pressure gasses or high vacuum.
Silicone can be developed into rubber sheeting, where it has other properties, such as being FDA compliant. This extends the uses of silicone sheeting to industries that demand hygiene, for example, food and beverage, and pharmaceuticals.
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 and cooking utensils), automobile (e.g. gaskets), airplane (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 tonnes of silicones were produced in 1991.[clarification needed] Specific examples, both large and small are presented below.[1]
Silicone caulks and rubber components are often used in automotive applications
In theautomotive field,silicone grease is typically used as a lubricant forbrake components since it is stable at high temperatures, is not water-soluble, and is far less likely than other lubricants to foul.DOT 5brake fluids are based on liquid silicones.
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 withforced induction).
Automotive body manufacturing plants and paint shops avoid silicones, as trace contamination may cause "fish eyes", which are small, circular craters which mar a smooth finish.[citation needed]
Additionally, silicone compounds such as silicone rubber are used as coatings and sealants forairbags; the high strength of silicone rubber makes it an optimal adhesive and sealant for high impact airbags.[citation needed] Silicones in combination with thermoplastics provide improvements in scratch and mar resistance and lowered coefficient of friction.[citation needed]
Silicone is often used to seal maintenance access openings in aerospace equipment
Silicone is a widely used material in theaerospace industry due to its sealing properties, stability across an extreme temperature range, durability, sound dampening and anti-vibration qualities, and naturally flame retardant properties. Maintaining extreme functionality is paramount for passenger safety in the aerospace industry, so each component on an aircraft requires high-performance materials.
Specially developed aerospace grades of silicone are stable from−70 to 220 °C,[10] these grades can be used in the construction of gaskets for windows and cabin doors. During operation, aircraft go through large temperature fluctuations in a relatively short period of time; from the ambient temperatures when on the ground in hot countries to sub-zero temperatures when flying at high altitude.Silicone rubber can be molded with tight tolerances ensuringgaskets form airtight seals both on the ground and in the air, where atmospheric pressure decreases.
Silicone rubber's resistance to heat corrosion enables it to be used for gaskets in aircraft engines where it will outlast other types of rubber, both improving aircraft safety and reducing maintenance costs. The silicone acts to seal instrument panels and other electrical systems in the cockpit, protecting printed circuit boards from the risks of extreme altitude such as moisture and extremely low temperature. Silicone can be used as a sheath to protect wires and electrical components from any dust or ice that may creep into a plane's inner workings.
As the nature of air travel results in much noise and vibration, powerful engines, landings, and high speeds all need to be considered to ensure passenger comfort and safe operation of the aircraft. As silicone rubber has exceptional noise reduction and anti-vibration properties, it can be formed into small components and fitted into small gaps ensuring all equipment can be protected from unwanted vibration such as overhead lockers, vent ducts, hatches, entertainment system seals, and LED lighting systems.
The strength and reliability ofsilicone rubber are widely acknowledged in the construction industry. One-part siliconesealants and caulks are in common use to seal gaps, joints and crevices in buildings. One-part silicones cure by absorbing atmospheric moisture, which simplifies installation. In plumbing, silicone grease is typically applied to O-rings inbrass taps and valves, preventinglime from sticking to the metal.
Structural silicone has also been used incurtain wall building façades since 1974 when theArt Institute of Chicago became the first building to receive exterior glass fixed only with the material.[citation needed] Silicone membranes have been used to cover and restore industrial roofs, thanks to its extreme UV resistance, and ability to keep their waterproof performance for decades.[citation needed]
Silicone rubber can be 3D printed (liquid deposition modelling, LDM) using pump-nozzle extrusion systems. Standard silicone formulations are optimized to be used by extrusion and injection moulding machines and are not applicable in LDM-based 3D printing. The rheological behavior and thepot life need to be adjusted for use with LDM.[12]
3D printing also requires the use of a removable support material that is compatible with the silicone rubber.
Silicone films can be applied to such silica-based substrates as glass to form acovalently bondedhydrophobic coating. Such coatings were developed for use on aircraftwindshields to repel water and to preserve visibility, without requiring mechanicalwindshield wipers which are impractical at supersonic speeds. Similar treatments were eventually adapted to the automotive market in products marketed byRain-X and others.
Many fabrics can be coated or impregnated with silicone to form a strong, waterproof composite such assilnylon.
A silicone polymer can be suspended in water by using stabilizing surfactants. This allows water-based formulations to be used to deliver many ingredients that would otherwise require a stronger solvent, or be too viscous to use effectively. For example, a waterborne formulation using a silane's reactivity and penetration ability into a mineral-based surface can be combined with water-beading properties from a siloxane to produce a more-useful surface protection product.
Soup ladle and pasta ladle made of siliconeA silicone foodsteamer to be placed inside a pot of boiling waterFlexible ice cube trays made of silicone allow easy extraction of iceSilicone brush used for basting and applying flavoring liquids
As a low-taint, non-toxic material, silicone can be used where contact with food is required. Silicone is becoming an important product in thecookware industry, particularlybakeware andkitchen 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 gloves are able to withstand temperatures up to 260 °C (500 °F), making it possible to reach into boiling water.
Other products includemolds for chocolate, ice, cookies, muffins, and various other foods; non-stick bakeware and reusable mats used on baking sheets;steamers, egg boilers orpoachers; cookware lids,pot holders,trivets, and kitchen mats.
Liquid silicone can be used as adry cleaningsolvent, providing an alternative to the traditionalchlorine-containingperchloroethylene (perc) solvent. The use of silicones in dry cleaning reduces the environmental effect of a typically high-polluting industry.[citation needed]
Electronic components are sometimesencased 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 extreme environmental conditions, such as in space (satellite technology). They are selected overpolyurethane orepoxy encapsulation when a wideoperating 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.
Silicones are often components ofthermal pastes used to improve heat transfer from power-dissipating electronic components toheat sinks.
The use of silicones in electronics is not without problems, however. Silicones are relatively expensive and can be attacked by certain 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.[13][14] Use of silicone-based spray products in electronic devices during maintenance or repairs can cause later failures.
Silicone foam has been used in North American buildings in an attempt tofirestop openings within the 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 highdielectric strength. Disadvantages include combustibility (hard to extinguish) and significant smoke development.
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 ofnuclear power plants) of theNuclear Regulatory Commission (NRC).[citation needed]
Silicone is a popular alternative to traditional metals (such as silver and gold) with jewelry, specifically rings. Silicone rings are commonly worn in professions where metal rings can lead to injuries, such as electrical conduction and ring avulsions.[15][16] During the mid-2010's, some professional athletes began wearing silicone rings as an alternative during games.[17]
Silicone grease is often used with laboratory glassware to prevent seizing
Siliconegreases are used for many purposes, such asbicycle chains,airsoft gun parts, and a wide range of othermechanisms. Typically, a dry-set lubricant is delivered with a solvent carrier to penetrate the mechanism. The solvent then evaporates, leaving a clear film that lubricates but does not attract dirt and grit as much as anoil-based or other traditional "wet" lubricant.
Siliconepersonal lubricants are also available for use in medical procedures or sexual activity.
Silicone is used inmicrofluidics, seals, gaskets, shrouds, and other applications requiring highbiocompatibility. Additionally, the gel form is used in bandages and dressings,breast implants, testicle implants, pectoral implants,contact lenses, and a variety of other medical uses.
Illicit cosmetic silicone injections may induce chronic and definitive silicone blood diffusion with dermatologic complications.[18]
Ophthalmology uses many products such as silicone oil used to replace thevitreous humor following vitrectomy, silicone intraocular lenses following cataract extraction, silicone tubes to keep a nasolacrimal passage open following dacryocystorhinostomy, canalicular stents for canalicular stenosis, punctal plugs for punctal occlusion in dry eyes, silicone rubber and bands as an externaltamponade in tractional retinal detachment, and anteriorly-located break in rhegmatogenous retinal detachment.
Silicone mold used to reproduce an architectural detail
Two-part silicone systems are used as rubber molds to castresins, foams, rubber, and low-temperature alloys. A silicone mold generally requires little or no mold-release or surface preparation, as most materials do not adhere to silicone. For experimental uses, ordinary one-part silicone can be used to make molds or to mold into shapes. If needed, commonvegetable cooking oils orpetroleum jelly can be used on mating surfaces as a mold-release agent.[22]
Silicone cooking molds used asbakeware do not require coating with cooking oil; in addition, the flexibility of the rubber allows the baked food to be easily removed from the mold after cooking.
Silicones are ingredients widely used in skincare, color cosmetic and hair care applications. Some silicones, notably theamine functionalized amodimethicones, are excellent hair conditioners, providing improved compatibility, feel, and softness, and lessening frizz. The phenyl dimethicones, in another silicone family, are used in reflection-enhancing and color-correcting hair products, where they increase shine and glossiness (and possibly impart subtle color changes). Phenyltrimethicones, unlike the conditioning amodimethicones, have refractive indices (typically 1.46) close to that of a human hair (1.54). However, if included in the same formulation, amodimethicone and phenyltrimethicone interact and dilute each other, making it difficult to achieve both high shine and excellent conditioning in the same product.[23]
Silicone rubber is commonly used inbaby bottle nipples (teats) for its cleanliness, aesthetic appearance, and low extractable content.
Silly Putty and similar materials are composed of siliconesdimethyl siloxane,polydimethylsiloxane, anddecamethyl cyclopentasiloxane, with other ingredients. This substance is noted for its unusual characteristics, e.g., that it bounces, but breaks when given a sharp blow; it will also flow like a liquid and form a puddle given enough time.
Silicone "rubber bands" are a long-lasting popular replacement refill for real rubber bands in the 2013 fad "rubber band loom" toys at two to four times the price (in 2014). Silicone bands also come in bracelet sizes that can be custom embossed with a name or message. Large silicone bands are also sold as utility tie-downs.
Formerol is a silicone rubber (marketed asSugru) used as an arts-and-crafts material, as its plasticity allows it to be molded by hand like modeling clay. It hardens at room temperature and it is adhesive to various substances including glass and aluminum.[25]
Oogoo is an inexpensive silicone clay, which can be used as a substitute forSugru.[26]
In makingaquariums, manufacturers now commonly use 100% silicone sealant to join glass plates. Glass joints made with silicone sealant can withstand great pressure, making obsolete the original aquarium construction method of angle-iron and putty. This same silicone is used to make hinges in aquarium lids or for minor repairs. However, 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.[27]
Silicone is used inspecial effects as a material for simulating realistic skin, either forprosthetic makeup, prop body parts, orrubber masks.[28] Platinum silicones are ideal for simulating flesh and skin due to their strength, firmness, and translucency, creating a convincing effect.Silicone masks have an advantage over latex masks in that because of the material properties, the mask hugs the wearers face and moves in a realistic manner with the wearer's facial expressions.[29] Silicone is often used as a hypoallergenic substitute forfoam latex prosthetics.
The leading global manufacturers of silicone base materials belong to three regional organizations: the European Silicone Center (CES) inBrussels, Belgium; the Silicones Environmental, Health, and Safety Center (SEHSC) inHerndon, Virginia, US; and the Silicone Industry Association of Japan (SIAJ) inTokyo, Japan. Dow Corning Silicones, Evonik Industries, Momentive Performance Materials, Milliken and Company (SiVance Specialty Silicones), Shin-Etsu Silicones, Wacker Chemie, Elkem Silicones, JNC Corporation, Wacker Asahikasei Silicone, and Dow Corning Toray represent the collective membership of these organizations. A fourth organization, the Global Silicone Council (GSC) acts as an umbrella structure over the regional organizations. All four are non-profit, having no commercial role; their primary missions are to promote the safety of silicones from a health, safety, and environmental perspective. As the European chemical industry is preparing to implement theRegistration, Evaluation, and Authorisation of Chemicals (REACH) legislation, CES is leading the formation of a consortium[30] of silicones, silanes, and siloxanes producers and importers to facilitate data and cost-sharing.
Silicone compounds are pervasive in the environment. Particular silicone compounds, cyclic siloxanesD4 andD5, are air and water pollutants and have negative health effects on test animals.[31] They are used in various personal care products. The European Chemicals Agency found that "D4 is a persistent, bioaccumulative and toxic (PBT) substance and D5 is a very persistent, very bioaccumulative (vPvB) substance".[32][33] Other silicones biodegrade readily, a process that is accelerated by a variety of catalysts, including clays.[1] Cyclic silicones have been shown to involve the occurrence ofsilanols during biodegradation in mammals.[clarification needed][34] The resulting silanediols and silanetriols are capable of inhibiting hydrolytic enzymes such asthermolysin,acetylcholinesterase. However, the doses required for inhibition are by orders of magnitude higher than the ones resulting from the accumulated exposure to consumer products containingcyclomethicone.[35][36]
At around 200 °C (392 °F) in an oxygen-containing atmosphere, polydimethylsiloxane releases traces offormaldehyde (but lesser amounts than other common materials such as polyethylene[37][38]). At this temperature, silicones were found to have lower formaldehyde generation thanmineral oil andplastics (less than 3 to 48 μg CH2O/(g·hr) for a high consistencysilicone rubber, versus around 400 μg CH2O/(g·hr) for plastics and mineral oil). By 250 °C (482 °F), copious amounts of formaldehyde have been found to be produced by all silicones (1,200 to 4,600 μg CH2O/(g·hr)).[38]
Some persons have been found to develop silicone allergies or extreme sensitivity,[39] particularly after prolonged exposure to certain types of silicone products such as cosmetics, medical equipment including CPAP masks[40][41] and implanted medical devices.[42][43]
Although silicones are durable polymers, they have historically been difficult to recycle. Recent research from CNRS and the University Claude Bernard Lyon 1 has reported chemical depolymerization routes that convert crosslinked silicone materials back into reusable oils or monomers with reduced environmental impacts. In industrial practice, Elkem has reported pilot and commercial initiatives for circular silicones, including recycled‑content products for the label industry and broader solutions such as mechanical re‑incorporation of elastomers and the use of bio‑based carriers and solvents.[44]
Compounds containing silicon–oxygen double bonds, now calledsilanones, but which could deserve the name "silicone", have long been identified asintermediates in gas-phase processes such aschemical vapor deposition inmicroelectronics production, and in theformation of ceramics by combustion.[45] However, they have a strong tendency to polymerize into siloxanes. The first stable silanone was obtained in 2014 byA. Filippou and others.[46]
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