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Silicone caulk can be used as a basic sealant against water and air penetration.

In organosilicon and polymer chemistry, a silicone or polysiloxane is a polymer composed of repeating units of siloxane (−O−R2Si−O−SiR2−, where R = organic group). They are typically colorless oils or rubber-like substances. Silicones are used in sealants, adhesives, lubricants, medicine, cooking utensils, thermal insulation, and electrical insulation. Some common forms include silicone oil, grease, rubber, resin, and caulk.[1][2]

Silicone is often confused with one of its constituent elements, silicon, but they are distinct substances. Silicon is a pure chemical element, a metalloid, which forms a dark-grey semiconducting crystalline solid. In its crystalline form it is used to make integrated circuits ("electronic chips") and solar cells. In contrast, silicone is formed by the polymerization 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 rubbery resin.

History

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F. S. Kipping coined the word silicone in 1901 to describe the formula of polydiphenylsiloxane, Ph2SiO (Ph = phenyl, C6H5), by analogy with the formula of the ketone benzophenone, Ph2CO (his term was originally silicoketone). Kipping was well aware that polydiphenylsiloxane is polymeric[3] whereas benzophenone is monomeric and noted the contrasting properties of Ph2SiO and Ph2CO.[4][5] The discovery of the structural differences between Kipping's molecules and the ketones means that silicone is no longer the correct term (though it remains in common usage) and that the term siloxane 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 making fused silica, a high-quality glass later used in aeronautics, advanced telecommunications, and computer chips. His work led to the formation of Dow Corning, an alliance between the Dow Chemical Company and Corning Glass Works that was specifically created to produce silicone products.

Chemistry

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Main article: Organosilicon compound
Chemical structure of the silicone polydimethylsiloxane (PDMS)

Alfred Stock and Carl Somiesky examined the hydrolysis of dichlorosilane, a reaction that was proposed to initially give the monomer H2SiO:

SiH2Cl2 + H2O → H2SiO + 2 HCl

When the hydrolysis is performed by treating a solution of H2SiCl2 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. All polymerized siloxanes or polysiloxanes, silicones consist of an inorganic silicon–oxygen backbone 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, 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.[citation needed] The second-largest group of silicone materials is based on silicone resins, which are formed by branched and cage-like oligosiloxanes.[citation needed]

Synthesis

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Most common are materials based on polydimethylsiloxane, which is derived by hydrolysis of dimethyldichlorosilane. This dichloride reacts with water as follows:

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

The polymerization typically produces linear chains capped with Si−Cl or Si−OH (silanol) groups. Under different conditions, the polymer is a cyclic, not a chain.[1]

For consumer applications such as caulks, silyl acetates are used instead of silyl chlorides. The hydrolysis of the acetates produces the less dangerous acetic acid (the acid found in vinegar) as the reaction product of a much slower curing process. This chemistry is used in many consumer applications, such as silicone caulk and adhesives.

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 and methyltrimethoxysilane. 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.

Combustion

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When silicone is burned in air or oxygen, it forms solid silica (silicon dioxide, SiO2) as a white powder, char, and various gases. The readily dispersed powder is sometimes called silica fume. The pyrolysis of certain polysiloxanes under an inert atmosphere is a valuable pathway towards the production of amorphous silicon oxycarbide ceramics, also known as polymer derived ceramics. Polysiloxanes terminated with functional ligands such as vinyl, mercapto or acrylate groups have been cross linked to yield preceramic polymers, which can be photopolymerised for the additive manufacturing of polymer derived ceramics by stereolithography techniques.[8]

Properties

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This silicone rubber folding chessboard resists creasing and wrinkling.

Silicones exhibit many useful characteristics, including:[1]

  • Low thermal conductivity
  • Low chemical reactivity
  • Low toxicity
  • Thermal stability (constancy of properties over a wide temperature range of −100 to 250 °C)
  • The ability to repel water and form watertight seals.
  • Does not stick to many substrates, but adheres very well to others, e.g. glass
  • Does not support microbiological growth
  • Resistance to creasing and wrinkling
  • Resistance to oxygen, ozone, and ultraviolet (UV) light. This property has led to the widespread use of silicones in the construction industry (e.g. coatings, fire protection, glazing seals) and the automotive 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.
  • High gas permeability: at room temperature (25 °C), the permeability of silicone rubber for such gases as oxygen is approximately 400 times[9] that of butyl 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.

Applications

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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]

Automotive

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Silicone caulks and rubber components are often used in automotive applications

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. DOT 5 brake 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 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 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 for airbags; 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]

Aerospace

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Silicone is often used to seal maintenance access openings in aerospace equipment

Silicone is a widely used material in the aerospace 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 ensuring gaskets 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.

Solid propellant

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Polydimethylsiloxane (PDMS) based binders along with ammonium perchlorate (NH4ClO4) are used as fast burning solid propellants in rockets.[11]

Building construction

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The strength and reliability of silicone rubber are 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 simplifies installation. In plumbing, silicone grease is typically applied to O-rings in brass taps and valves, preventing lime from sticking to the metal.

Structural silicone has also been used in curtain wall building façades since 1974 when the Art 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]

3D printing

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Silicone 3D print with support material
Silicone 3D print with support material

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 the pot 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.

Coatings

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Silicone films can be applied to such silica-based substrates as glass to form a covalently bonded hydrophobic coating. Such coatings were developed for use on aircraft windshields to repel water and to preserve visibility, without requiring mechanical windshield wipers which are impractical at supersonic speeds. Similar treatments were eventually adapted to the automotive market in products marketed by Rain-X and others.

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

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.

Cookware

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Soup ladle and pasta ladle made of silicone
A silicone food steamer to be placed inside a pot of boiling water
Flexible ice cube trays made of silicone allow easy extraction of ice
Silicone 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 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 gloves are able to withstand temperatures up to 260 °C (500 °F), making it possible to reach into boiling water.

Other products include molds for chocolate, ice, cookies, muffins, and various other foods; non-stick bakeware and reusable mats used on baking sheets; steamers, egg boilers or poachers; cookware lids, pot holders, trivets, and kitchen mats.

Defoaming

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

Dry cleaning

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Liquid silicone can be used as a dry cleaning solvent, providing an alternative to the traditional chlorine-containing perchloroethylene (perc) solvent. The use of silicones in dry cleaning reduces the environmental effect of a typically high-polluting industry.[citation needed]

Electronics

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Silicone rubber keypad

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 extreme environmental 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.

Silicones are often components of thermal pastes used to improve heat transfer from power-dissipating electronic components to heat 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.

Firestops

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Red-colored silicone firestopping

Silicone foam has been used in North American buildings in an attempt to firestop 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 high dielectric 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 of nuclear power plants) of the Nuclear Regulatory Commission (NRC).[citation needed]

Silicone firestops are also used in aircraft.

Jewelry

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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]

Lubricants

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Silicone grease is often used with laboratory glassware to prevent seizing

Silicone greases are used for many purposes, such as bicycle chains, airsoft gun parts, and a wide range of other mechanisms. 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 an oil-based or other traditional "wet" lubricant.

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

Medicine and cosmetic surgery

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Silicone is used in microfluidics, seals, gaskets, shrouds, and other applications requiring high biocompatibility. 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.

Scar treatment sheets are often made of medical grade silicone due to its durability and biocompatibility. Polydimethylsiloxane (PDMS) is often used for this purpose, since its specific crosslinking results in a flexible and soft silicone with high durability and tack. It has also been used as the hydrophobic block of amphiphilic synthetic block copolymers used to form the vesicle membrane of polymersomes.

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 the vitreous 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 external tamponade in tractional retinal detachment, and anteriorly-located break in rhegmatogenous retinal detachment.

Addition and condensation (e.g. polyvinyl siloxane) silicones find wide application as a dental impression material due to its hydrophobic property and thermal stability.[19][20][21]

Moldmaking

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Silicone mold used to reproduce an architectural detail

Two-part silicone systems are used as rubber molds to cast resins, 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, common vegetable cooking oils or petroleum jelly can be used on mating surfaces as a mold-release agent.[22]

Silicone cooking molds used as bakeware 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.

Personal care

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Silicone rubber earplugs for hearing protection

Silicones are ingredients widely used in skincare, color cosmetic and hair care applications. Some silicones, notably the amine 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 in baby bottle nipples (teats) for its cleanliness, aesthetic appearance, and low extractable content.

Silicones are used in shaving products and personal lubricants.[24]

Toys and hobbies

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Baby toys made of nontoxic silicone rubber

Silly Putty and similar materials are composed of silicones dimethyl siloxane, polydimethylsiloxane, and decamethyl 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 as Sugru) 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 for Sugru.[26]

In making aquariums, 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]

Special effects

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Silicone is used in special effects as a material for simulating realistic skin, either for prosthetic makeup, prop body parts, or rubber 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 for foam latex prosthetics.

Marketing

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The leading global manufacturers of silicone base materials belong to three regional organizations: the European Silicone Center (CES) in Brussels, Belgium; the Silicones Environmental, Health, and Safety Center (SEHSC) in Herndon, Virginia, US; and the Silicone Industry Association of Japan (SIAJ) in Tokyo, 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 the Registration, 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.

Safety and environmental considerations

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Silicone compounds are pervasive in the environment. Particular silicone compounds, cyclic siloxanes D4 and D5, 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 of silanols during biodegradation in mammals.[clarification needed][34] The resulting silanediols and silanetriols are capable of inhibiting hydrolytic enzymes such as thermolysin, acetylcholinesterase. However, the doses required for inhibition are by orders of magnitude higher than the ones resulting from the accumulated exposure to consumer products containing cyclomethicone.[35][36]

At around 200 °C (392 °F) in an oxygen-containing atmosphere, polydimethylsiloxane releases traces of formaldehyde (but lesser amounts than other common materials such as polyethylene[37][38]). At this temperature, silicones were found to have lower formaldehyde generation than mineral oil and plastics (less than 3 to 48 μg CH2O/(g·hr) for a high consistency silicone 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]

Similar substances

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Compounds containing silicon–oxygen double bonds, now called silanones, but which could deserve the name "silicone", have long been identified as intermediates in gas-phase processes such as chemical vapor deposition in microelectronics production, and in the formation of ceramics by combustion.[45] However, they have a strong tendency to polymerize into siloxanes. The first stable silanone was obtained in 2014 by A. Filippou and others.[46]

See also

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References

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Silicone

View on Grokipedia
from Grokipedia
Silicones are a class of synthetic composed of repeating units, featuring a backbone of alternating and oxygen atoms with organic groups, typically methyl or phenyl, attached to the . This structure imparts distinctive properties including high thermal stability over a wide temperature range, low chemical reactivity, flexibility, and resistance to moisture, oxidation, and . Silicones find extensive use in sealants, adhesives, lubricants, electrical insulation, medical implants, cookware, and automotive components due to these attributes, which stem from the strong Si-O bonds and the hybrid inorganic-organic composition that differs fundamentally from carbon-based . The term "silicone" was coined in the early by Frederic Stanley Kipping during studies of organosilicon compounds, though commercial development and widespread application occurred in the , driven by advancements in techniques.

Chemical Composition

Molecular Structure

Silicones, also known as polysiloxanes, are organosilicon polymers featuring a backbone of repeating units, consisting of alternating and oxygen atoms linked by strong Si-O bonds, with each atom bonded to two organic groups, typically alkyl or aryl substituents such as methyl (-CH₃) or phenyl (-C₆H₅). This structure is represented generally as [-O-SiR₂-]ₙ, where R denotes the organic side groups and n is the . The most prevalent variant, (PDMS), incorporates methyl groups on each silicon, yielding the formula CH₃[Si(CH₃)₂O]ₙSi(CH₃)₃. The Si-O bond dissociation energy measures approximately 452 kJ/mol, exceeding the 346 kJ/mol for typical C-C bonds in organic polymers, which underpins the superior thermal stability and flexibility of chains due to wider bond angles (around 130-150°) and lower intermolecular forces compared to carbon-oxygen analogs. These chains can adopt linear, cyclic, or branched configurations, with chain length influencing molecular weight and resultant —from low-molecular-weight fluids to high-viscosity gums—while pendant group variations modulate hydrophobicity and . Crosslinking via multifunctional silanes introduces network structures, transforming linear polysiloxanes into elastomers or resins, where the extent of crosslinking density governs mechanical properties like elasticity and hardness without altering the fundamental backbone. Functionalization of side groups, such as introducing vinyl or moieties, enables further tailoring of reactivity while preserving the inert Si-O-Si core.

Types and Variants

Silicones, primarily polysiloxanes, are classified by molecular architecture into linear, cyclic, and branched structures. Linear variants, exemplified by (PDMS), consist of unbranched chains of repeating dimethyl units terminated by trimethylsilyl groups. Cyclic silicones form closed-ring oligomers, such as those generated as byproducts during linear preparation. Branched structures incorporate trifunctional or multifunctional siloxane units, fostering three-dimensional configurations. These polymers manifest in distinct physical forms dictated by molecular weight and crosslinking extent: fluids, gels, elastomers, and resins. Silicone fluids derive from short linear PDMS chains, yielding low-viscosity liquids. Gels arise from lightly crosslinked PDMS networks, often via reactions with trifunctional silanes or hydrosilylation. Elastomers feature densely crosslinked linear polysiloxanes, categorized as room-temperature vulcanizing (RTV) types that cure via tin or platinum catalysts, or high-temperature vulcanizing (HTV) types that require peroxide initiators and elevated temperatures. Resins exhibit extensive branching and crosslinking for rigid, film-forming applications. Specialized variants incorporate modifications to the siloxane backbone. Fluorosilicones, or fluorosilicones (FVMQ), integrate groups like 3,3,3-trifluoropropyl onto PDMS, preserving thermal stability while imparting resistance to hydrocarbons and solvents. Medical-grade silicones undergo purification to minimize extractables and volatiles, ensuring for implants and prosthetics per standards such as USP Class VI.

Physical and Chemical Properties

Mechanical and Thermal Properties

Silicone elastomers demonstrate notable mechanical resilience, with tensile strength typically ranging from 5 to 10 MPa and elongation at break between 300% and 800%, enabling high flexibility and elastic recovery under deformation. These characteristics arise from the polymer's crosslinked network structure, which provides low modulus values, often 1 to 3 MPa for in standard (PDMS) formulations. Mechanical performance is assessed via standardized , such as ASTM D412, which measures stress-strain behavior to ensure consistency across formulations.
PropertyTypical Range/ValueNotes/Standard
Tensile Strength5–10 MPaVaries by curing and fillers; ASTM D412
Elongation at Break300–800%Indicates ; higher in unfilled types
Young's Modulus1–3 MPaLow for compliance; PDMS-specific
Thermally, silicone rubbers exhibit broad operational stability, supporting continuous service from -100°C to 250°C, with specialized grades withstanding intermittent exposure up to 350°C without significant degradation. This range stems from the backbone's resistance to , maintaining flexibility at cryogenic lows and integrity at elevated temperatures, as quantified by retention of elongation above 50% after prolonged heating per JIS K 6261 equivalents. The linear coefficient of measures approximately 300 × 10^{-6} /°C, lower than many organic elastomers, minimizing dimensional changes under . Silicone's inherent hydrophobicity, characterized by angles of 110° to 130°, reduces surface wettability and aids in moisture repulsion, while properties feature a constant of ~3.0 and breakdown strength exceeding 20 kV/mm, supporting insulation efficacy per ASTM evaluations. These attributes, derived from methyl-substituted chains, enhance durability in variable environments without relying on additives.

Chemical Stability and Reactivity

Silicones, particularly (PDMS), exhibit high chemical stability arising from the robust Si-O-Si backbone, where the bond dissociation energy of approximately 433 kJ/mol exceeds that of C-O (360 kJ/mol) and C-C bonds, rendering it resistant to cleavage under ambient conditions. This strength, combined with steric hindrance from methyl groups that shield the siloxane chain, minimizes nucleophilic or electrophilic attacks, contributing to overall inertness. The partial polarity of the Si-O bond, with silicon bearing a positive charge, theoretically facilitates , but kinetic barriers and the low of silicones limit reactivity in neutral to mildly acidic or basic environments. PDMS demonstrates exceptional resistance to water, dilute acids, and bases across pH 1-14 at room temperature, with negligible degradation observed in prolonged exposure to aqueous solutions. Oxidation resistance stems from the absence of easily oxidizable C-H bonds in the backbone, unlike organic polymers, allowing stability in oxidative atmospheres without significant chain scission. However, swelling occurs in non-polar organic solvents such as toluene or hexane due to favorable solubility parameter matching (PDMS δ ≈ 15.4 MPa^{1/2}), though equilibrium swelling ratios remain moderate (e.g., <100% volume increase in many cases) compared to highly solvated elastomers. Superior UV resistance relative to hydrocarbon rubbers results from the inorganic siloxane structure lacking conjugated double bonds prone to photodegradation; PDMS retains over 80% of tensile strength after 1000 hours of QUV exposure, versus rapid embrittlement in natural rubber. Ozone resistance is similarly enhanced, with no cracking observed at 50 pphm exposure for thousands of hours, attributable to the saturated backbone impervious to ozonolysis, outperforming diene-based rubbers that fissure under similar conditions. Limited reactivity manifests primarily under extreme conditions, such as in concentrated acids or bases at elevated temperatures. For instance, in alkaline solutions ( >13), hydrolysis proceeds via nucleophilic attack on , with rate constants around 0.28 mg Si/L/day at 24°C, yielding intermediates that can repolymerize or degrade further. In strong acids, rates are slower (≈0.07 mg Si/L/day), but combined with heat (>100°C), chain scission accelerates, though practical stability persists below these thresholds. These reactions underscore that while silicones are for most applications, extreme and thermal stress can induce quantifiable .

Combustion and Flammability

Silicone polymers demonstrate high ignition resistance, with flash points exceeding 300°C and autoignition temperatures around 450°C for typical vulcanizates. These properties arise from the strong Si-O bonds in their backbone, which require sustained elevated temperatures for decomposition, contrasting with lower ignition thresholds in hydrocarbon-based materials. Commercial silicone rubbers, particularly flame-retardant grades, often attain V-0 ratings, signifying vertical flame testing where specimens self-extinguish within 10 seconds, exhibit no flaming drips, and leave minimal afterflame or glow. During , silicones form a cohesive silica (SiO₂) residue that acts as an barrier, restricting oxygen access and conductive to underlying material, thereby suppressing flame spread and promoting rapid self-extinguishment. This char formation yields lower peak heat release rates—typically under 20 kW/m² in modified formulations—relative to hydrocarbons exceeding 50 kW/m², reducing overall intensity. Empirical combustion studies reveal silicones produce dense smoke but limited toxic effluents, primarily , , water vapor, and siloxanes, with negligible or other absent in their composition. In contrast to PVC, which decomposes to release corrosive HCl alongside CO, silicones exhibit lower in fire effluent profiles per standardized assays. ASTM E84 surface burning tests on silicone insulation materials confirm low spread indices (often <25) and controlled smoke development, supporting their use in fire-sensitive applications despite inherent combustibility. These traits underscore silicones' utility in mitigating propagation risks, though additive enhancements are common for optimized performance.

Synthesis and Manufacturing

Industrial Production Processes

The primary industrial route to silicones begins with the direct process, or Müller-Rochow synthesis, in which powdered silicon metal reacts with methyl chloride gas over a copper catalyst at temperatures of 250–300°C and pressures around 2–3 atm, yielding dimethyldichlorosilane ((CH₃)₂SiCl₂) as the dominant product alongside minor chlorosilanes. This exothermic, continuous fluidized-bed process operates with high silicon utilization, typically achieving selectivities exceeding 85% to (CH₃)₂SiCl₂ after distillation purification, enabling annual global production capacities in the multimillion-ton scale for silicone precursors. Subsequent hydrolysis of purified dimethyldichlorosilane occurs by controlled addition to excess water or aqueous mixtures, often in the presence of solvents to manage the rapid HCl byproduct evolution and prevent gelation, producing a mixture of linear oligomeric siloxanes, cyclic siloxanes (predominantly D₃, D₄, and D₅ species), and silanol-terminated chains. Yields of hydrolyzable siloxanes approach 95% under optimized conditions, with cyclic fractions (up to 20–30% D₄) separated via distillation for further use. Polydimethylsiloxanes (PDMS), the backbone of most silicones, are then synthesized via equilibration or ring-opening polymerization of these cyclic siloxanes, such as octamethylcyclotetrasiloxane (D₄), under anionic (e.g., KOH) or cationic (e.g., triflic acid) catalysis at 100–150°C, yielding high-molecular-weight linear polymers with controlled end-groups for fluids, resins, or gums. This step achieves near-quantitative monomer conversion (>99%) in large-scale reactors, though 10–15% cyclic byproducts necessitate devolatilization for purity. For silicone elastomers, crosslinks the PDMS chains: methods employ (e.g., ) at 150–200°C to generate radicals that abstract hydrogens and bridge siloxy units, suitable for high-strength industrial applications with cure times of minutes; alternatively, hydrosilylation uses catalysts (e.g., Karstedt's complex) to add Si-H crosslinkers to vinyl-substituted PDMS at to 150°C, offering void-free cures and no volatile byproducts for precision molding. These techniques scale efficiently, with hydrosilylation favored for yields >98% in cleanroom-compatible silicone rubber (LSR) injection molding.

Raw Materials and Precursors

, the foundational element in silicone polymers, is obtained from (SiO₂), a crystalline form of silica abundant in the , where constitutes approximately 27.7% by mass. is typically sourced from high-purity deposits such as or , ensuring minimal impurities that could affect downstream quality. The extraction begins with carbothermic reduction, where reacts with carbon (from sources like coke or ) in furnaces at temperatures exceeding 1900°C: SiO₂ + 2C → Si + 2CO. This metallurgical process yields metallurgical-grade (purity around 98-99%), but it is energy-intensive, requiring 10-14 kWh per of due to the high thermal and electrical demands of sustaining the reaction and separating the silicon from slag. Organochlorosilane , essential for chains, are derived from this via the Müller-Rochow direct , involving the reaction of powdered with methyl chloride (CH₃Cl) gas in the presence of a catalyst at 250-350°C. The primary product is dichlorodimethylsilane (Me₂SiCl₂), which achieves selectivities of 85-95% in industrial settings, serving as the key building block for polydimethylsiloxanes. Methyl chloride itself is produced from (derived from or ) and recycled , closing a loop in integrated facilities. Minor include (MeSiCl₃) and trimethylchlorosilane (Me₃SiCl), which enable branched or end-capped structures but constitute less than 10% of the output. Byproduct management in precursor production emphasizes to counter the process's demands. Side reactions yield unwanted chlorosilanes and disilanes, separated via under reduced pressure to recover valuable fractions. , generated during the of chlorosilanes in later stages, is captured and reacted with to regenerate methyl chloride, reducing net HCl emissions and feedstock needs by up to 90% in closed-loop systems. residues from are also recovered and reused, mitigating metal losses in the silicon- formed. These practices underscore the causal trade-offs of high-purity inputs against and waste costs inherent to silicone's silicon-oxygen backbone.

Historical Development

Early Discoveries and Research

The synthesis of the first organosilicon compounds occurred in 1863 when French chemists and American chemist Crafts reacted diethylzinc with to produce tetraethylsilane (Si(CH₂CH₃)₄), marking the initial formation of a silicon-carbon bond. In the same year, of alkoxysilanes in their experiments yielded a "polysilicic acid ether," recognized retrospectively as an early , though its polymeric nature was not fully appreciated at the time. Subsequent 19th-century efforts built on this foundation; German chemist Albert Ladenburg isolated the first cyclic , , in 1871 through similar zinc-mediated reactions, providing empirical evidence of siloxane ring structures but without scalable purification methods. Systematic organosilicon research advanced in the early 20th century under British chemist Frederic Stanley Kipping at the , who from 1900 onward investigated alkylchlorosilanes and their products, producing a range of including polydimethylsiloxanes via Grignard reactions on . Kipping coined the term "silicone" around 1904, deriving it from an erroneous analogy to "silicon ketone," presuming a silicon-oxygen double bond (Si=O) in the polymers, whereas X-ray and spectroscopic data later confirmed single Si-O-Si linkages. By the 1920s and into the 1930s, Kipping's group characterized alternating silicon-oxygen polymers but encountered persistent challenges: products were predominantly low-molecular-weight cyclic oligomers or oily liquids rather than stable high polymers, with purification hindered by moisture sensitivity, thermal decomposition, and contamination yielding brittle or unstable materials unsuitable for practical use. These empirical limitations, documented in Kipping's 51 papers concluding around 1940, underscored the need for refined synthetic controls absent in pre-commercial efforts.

Commercialization and Key Milestones

was formed in 1943 as a between and Corning Glass Works to commercialize silicone polymers, leveraging General Electric's foundational research on organosilicon compounds. This partnership enabled the first large-scale production of silicones, initially driven by demands for materials resilient to extreme conditions; gaskets and seals were deployed in critical applications, including searchlights and superchargers for B-29 bombers as early as 1943. The saw process optimizations, such as enhancements to the Müller-Rochow direct synthesis for methylchlorosilanes, which lowered costs and expanded output for diverse applications. Silicone's prompted initial adoptions, including rubber formulations for prosthetics and urological replacements like urethral prostheses by the late . Consumer-oriented innovations emerged, exemplified by General Electric's 1943 filing (granted 1947) for boric oxide-treated dimethyl silicone polymers, which influenced versatile elastomers for household uses. From the to the , silicone commercialization accelerated into consumer markets, with patents for compounding and curing methods enabling of sealants, utensils, and personal care items. This era's growth reflected silicones' stability and flexibility, transitioning from niche industrial roles to widespread household integration, supported by firms like and GE's expanded silicone divisions.

Post-2000 Advancements

In the , researchers developed nanocomposites incorporating fillers to achieve superior mechanical reinforcement, with studies demonstrating improved tensile strength and modulus through uniform dispersion of graphene nanosheets via functionalization. These materials exhibited enhanced thermal conductivity and electrical properties, enabling applications requiring durability under stress, as evidenced by controlled spatial distribution of graphene within the silicone matrix. Parallel efforts focused on bio-based silicone precursors to mitigate reliance on petroleum-derived siloxanes, with industrial-scale production of silicone rubber compounds from biomethanol—sourced from plant-based feedstocks—announced in 2022 by , yielding products with equivalent performance to conventional variants but lower carbon footprints. By 2024, bio-organic silicone additives derived from or plant alpha-olefins further expanded this shift, offering non-petroleum alternatives for fluids and elastomers while maintaining hydrolytic stability. Advancements in optical silicones included phenyl-modified encapsulants with refractive indices exceeding 1.50, optimized for high-power LEDs to boost light extraction efficiency by up to 7% compared to standard methylsilicones, as commercialized in materials like those from Shin-Etsu since the mid-2010s. Sustainable vulcanization innovations progressed with alternatives to platinum catalysts in hydrosilylation curing, including sulfur-mediated dynamic cross-linking for vitrimeric silicones in 2024, which enabled recyclability without compromising elasticity, addressing platinum scarcity and cost volatility. Recycling of waste vulcanized silicone into functional fillers also gained traction, enhancing flame retardancy in new composites via pyrolyzed silica recovery. These developments, driven by market demands for lower environmental impact, reflect empirical progress in reducing resource intensity while preserving silicone's inherent stability.

Applications

Automotive and Aerospace

Silicone rubber is widely used in automotive and aerospace applications for seals, gaskets, hoses, and O-rings due to its ability to maintain flexibility and sealing integrity across extreme temperatures, typically from -60°C to +230°C. This range exceeds that of many organic rubbers, reducing leakage risks in engine compartments and fuel systems where thermal cycling causes material degradation. In automotive contexts, silicone hoses withstand continuous exposure up to 150-200°C without hardening or cracking, outperforming nitrile rubber in high-heat scenarios like turbocharger connections. In aerospace, specifications incorporate silicone sealants such as RTV106 for high-temperature sealing, meeting standards like BMS for material performance under vibration and pressure variations. Silicone O-rings, as in AS3582-007, provide reliable static sealing in , with low ensuring long-term integrity against differential expansion rates that could otherwise lead to joint failures. Its viscous-elastic properties enable effective vibration damping in mounts and bushings, dissipating energy to minimize structural fatigue in both sectors. For rocket applications, silicone-based ablatives like Dow 93-104 protect nozzles and launch pads from gases and hypersonic heating by charring and eroding sacrificially while insulating underlying structures. evaluations confirm these materials' efficacy in missions, with formulations like silicone-impregnated ceramics (SIRCA) offering lightweight thermal protection for re-entry vehicles. Compared to traditional rubbers, silicone components exhibit superior in oxidative environments, with degradation rates low enough to justify higher initial costs through extended service intervals—often exceeding organic alternatives by factors of 2-5 in accelerated aging tests under SAE-relevant conditions. Empirical data from exposure simulations indicate failure thresholds beyond typical mission durations, though abrasion sensitivity necessitates careful dynamic application selection.

Electronics and Coatings

Silicone potting compounds serve as encapsulants for printed circuit boards (PCBs) in electronics, delivering superior electrical insulation to avert short circuits and arcing while shielding components from moisture, vibration, and thermal stress. These materials exhibit dielectric strengths typically exceeding 20 kV/mm and maintain efficacy across temperature ranges from -65°C to over 200°C, with silicone formulations prized for their flexibility and thermal conductivity up to 1.5 W/m·K, outperforming rigid epoxies in dynamic applications. Conformal silicone coatings, applied as thin films (25-250 micrometers thick), conform to PCB surfaces to inhibit by forming impermeable barriers against humidity, salts, and contaminants, in compliance with IPC-CC-830 qualification for resistance and IPC-A-610 criteria for uniform coverage without voids or pinholes. These coatings demonstrate prevention efficacy in accelerated tests, reducing surface insulation resistance degradation by over 90% in humid environments compared to uncoated boards, though failures can occur if surface contaminants exceed 5% coverage prior to application. High-refractive-index optical silicones, with indices above 1.5, are integrated into display technologies such as and micro-LED encapsulants to minimize light loss and boost efficiency by up to 20% through better extraction, amid market expansion from $1.62 billion in 2024 to a projected $4.03 billion by 2035 driven by formulation advances in transparency and . In vacuum-dependent electronics like systems, silicones offer low rates below 1% total mass loss per ASTM E595 standards, preserving optical clarity and preventing ; however, unmitigated oil migration in high-humidity settings (>80% RH) can lead to weakening, addressable via low-volatility additives that reduce extractables by 50-70%.

Medical and Personal Care

Silicone polymers, particularly (PDMS), are widely employed in medical devices due to their , flexibility, and inertness, which minimize tissue rejection and . In applications such as catheters and tubing, PDMS provides a hydrophobic, transparent barrier that resists bacterial while maintaining structural under physiological conditions. Medical-grade silicones undergo rigorous testing under standards like to ensure low and hemocompatibility, enabling uses in implants like urinary stents and drains where long-term implantation is required. In personal care, silicones function primarily as emollients in , forming a breathable film on the skin that enhances spreadability and reduces without deep penetration. Topical formulations, including lotions and serums, leverage cyclomethicone and dimethicone for their non-comedogenic properties, supported by dermatological evaluations confirming safety for prolonged contact. Silicone gel sheets and adhesives have demonstrated efficacy in management, reducing hypertrophic and formation through occlusion and hydration mechanisms, with clinical trials showing significant improvements in scar pliability and pigmentation after 6-12 months of use. For consumer products like cookware, bakeware, and infant toys, FDA-compliant silicones exhibit minimal chemical migration under heat and acidic conditions, with migration tests verifying levels below 10 mg/kg for total substances, ensuring negligible leaching into or . While rare reactions, such as , have been reported in isolated cases involving medical devices, empirical studies indicate an incidence below 1%, often attributable to additives rather than the itself, underscoring silicone's overall low allergenicity profile.

Construction and Consumer Products

Silicone sealants are widely used in for weatherproofing joints in facades, windows, and roofs, providing durable and flexibility to accommodate building movements. High-performance structural silicone glazing systems have demonstrated service lives exceeding 50 years under real-world conditions, as evidenced by long-term studies on facade sealants exposed for over three decades. These materials meet ASTM standards such as those outlined in STP1583 for evaluating factors including retention after accelerated and cyclic movement testing. Class 50-rated silicone sealants maintain permanent flexibility and adhesion, outperforming alternatives like in 40-year outdoor exposure trials where silicones showed superior resistance to cracking and chalking. In building applications, silicones contribute to energy efficiency by forming airtight seals that minimize air leakage and enhance insulation performance, reducing primary energy demand for heating and cooling. Silicone-based coatings reflect and seal leaks, lowering cooling loads in commercial structures, while their use in HVAC systems provides and protects against temperature extremes, supporting overall building energy savings. These properties facilitate compliance with sustainable building practices, with silicone sealants enabling reduced through improved envelope integrity. However, exposure to (UV) radiation can cause yellowing in clear silicone formulations due to oxidative degradation, though this is mitigated by incorporating UV stabilizers and light-blocking additives that extend color stability and weather resistance. For consumer products, food-grade silicones are employed in cookware, bakeware, and utensils due to their non-stick surfaces, flexibility, and heat resistance up to 450°F without degradation or leaching harmful substances; they are chemically inert, preventing leaching or reaction with acidic or fatty foods, feature non-porous surfaces that resist odor and bacteria absorption, and are dishwasher-safe, while high-quality variants avoid migration issues from fillers in low-quality products. The U.S. (FDA) approves these silicones for direct food contact under regulations like 21 CFR 173.340 for defoaming agents and general food-grade standards, confirming their inertness and safety in applications such as non-stick spatulas and steamers. In 2024, trends toward sustainable incorporated low-VOC, recyclable silicone sealants, aligning with urbanization-driven demands for durable, eco-friendly building materials that reduce replacement frequency and waste. Market analyses project the silicone sealants sector to grow from USD 4.22 billion in 2024 to USD 6.15 billion by 2032, driven by these enhancements and initiatives.

Emerging and Specialized Uses

In additive manufacturing, silicone materials have seen significant innovations enabling direct of flexible components, bypassing traditional molding processes. In 2023, Formlabs launched Silicone 40A Resin, the first commercially accessible 100% pure silicone formulation for vat photopolymerization printers, achieving Shore 40A hardness and suitable for prototyping and medical devices. By 2024, Filament2 introduced the world's first silicone filament for fused deposition modeling at Formnext, incorporating supports to manage silicone's cohesion during layering and enabling production of elastic parts with controlled tensile properties up to 500%. In July 2025, partnered with Shin-Etsu to release P3 Silicone 25A for PolyJet printing, offering enhanced tear resistance and elongation at break exceeding 600%, targeted at custom and seals. Integration of sensors into silicone matrices represents a 2025 advancement in , where embedded conductive elements enable real-time monitoring in wearable and industrial applications. Custom silicone designs now incorporate piezoresistive sensors for strain detection, with processes achieving response times under 10 ms and beyond 100,000 cycles, as demonstrated in prototypes for health monitoring patches. These developments leverage silicone's inherent flexibility and properties to encase sensors without compromising , contrasting with rigid alternatives that fail under deformation. In maternal and infant products, silicone formulations have evolved with antimicrobial additives introduced in 2023, reducing bacterial by up to 99% in items like pacifiers and feeding nipples through silver-ion doping or quaternary ammonium compounds. By 2025, innovations such as Silcotech's mōmi nipples utilize ultra-soft, platinum-cured silicone to mimic dynamics, minimizing transition stress with durometers as low as 10 Shore A and validated in clinical trials showing 20% faster rates. These specialized uses prioritize empirical data over unsubstantiated claims, with tests confirming negligible migration of oligomers under simulated oral conditions. For special effects prosthetics, recent silicone compounds enhance realism and actor comfort via low-viscosity, skin-like formulations with tunable translucency and sweat permeability. In 2025 applications, platinum silicones blended with thinners achieve application thicknesses under 1 mm while maintaining for 12+ hours, as used in high-fidelity simulations that replicate subsurface vascularity through pigment layering. This contrasts with earlier latex-based methods prone to allergic reactions, with silicone's profile supported by dermatological patch tests showing zero in 500+ subjects.

Economic and Market Dynamics

Global Production and Trade

Global silicone production capacity reached approximately 4.5 million metric tons per year as of 2024, with commanding about 76% of this total through its expansive domestic facilities. This dominance stems from rapid capacity expansions in the 2010s, shifting the industry's center from and the to , where lower energy costs and integrated supply chains favor large-scale output. Major multinational producers include Dow Inc., Wacker Chemie AG, and Shin-Etsu Chemical Co., which together account for a significant portion of non-Chinese capacity, focusing on high-value specialty silicones, while Chinese firms like Hoshine Silicon Industry dominate bulk production of commodity grades. The upstream supply chain relies on metal as the primary feedstock, produced via carbothermic reduction of (SiO₂) in submerged furnaces using coke as the reductant, a process parallel to ferrosilicon manufacturing but refined for higher purity (typically 98-99% ) suitable for organosilicon synthesis. Global metal output exceeded 8 million metric tons in recent years, with producing over 80%, ensuring feedstock availability but exposing downstream silicone production to raw material price volatility tied to and markets. From metal, the process advances through reaction with to form chlorosilanes (e.g., dichlorodimethylsilane), followed by and to yield and other siloxanes. International trade in silicones, valued in primary forms and intermediates, is export-led from , with as the top supplier to markets in , , and . In 2025, the maintained a trade surplus in silicones, exporting $110 million while importing $56.4 million in July alone, though volumes reflect processed products rather than raw capacity. Post-2018 U.S.- trade measures imposed 25% tariffs on certain silicone precursors like siloxanes and catalysts originating from , elevating import costs by up to 20-30% for U.S. manufacturers and prompting some supply chain diversification to and domestic expansion. These tariffs, extended and adjusted through 2025, have not significantly curtailed overall flows but have incentivized tariffs on alternative sources, including ferro-silicon inputs, amid broader Section 232 investigations. The global silicone market reached a value of USD 24.5 billion in 2024, driven primarily by demand in elastomers, which accounted for nearly 50% of due to applications in encapsulation and sealants. This segment is projected to grow at a CAGR of 5.2% through 2030, reaching USD 33.2 billion overall, with key causal factors including rising production of requiring thermal-stable materials and infrastructure development in emerging economies boosting usage. Alternative forecasts indicate a slightly higher , with the market expanding from USD 17.4 billion in 2024 to USD 23.3 billion by 2029 at a 6.0% CAGR, reflecting robust industrial adoption despite varying estimates across analysts. Recent trends emphasize supply chain diversification following disruptions, where initial shortages in raw silicones prompted manufacturers to invest in regional sourcing and inventory buffering to enhance resilience against geopolitical and logistical risks. initiatives are gaining traction, with formulations incorporating bio-derived siloxanes introduced in 2024-2025 to reduce reliance on petroleum-based feedstocks, though these remain niche due to higher costs and performance trade-offs compared to conventional silicones. Competition from organic polymers is limited by silicones' superior heat resistance and durability, sustaining market dominance in high-specification sectors like components. Market expansion has spurred job creation in silicone manufacturing, particularly in hubs where production capacity grew by over 5% annually post-2023, supporting roles in and processes amid electronics boom. Projections to 2029 anticipate sustained growth from electric vehicle battery seals and infrastructure, offsetting potential slowdowns in mature markets through innovation in low-volatility grades.

Safety, Health, and Toxicology

Human Health Effects and Empirical Data

Silicone polymers, primarily (PDMS), exhibit low via oral and dermal routes, with median lethal doses (LD50) exceeding 5,000 mg/kg in rats for oral exposure and over 2,000 mg/kg for dermal exposure in rabbits, indicating minimal risk from accidental ingestion or skin contact under typical conditions. These values derive from standardized assays, where no significant adverse effects were observed at doses far above environmental or occupational exposures, attributable to the chemical inertness and large molecular weight of PDMS limiting gastrointestinal absorption and systemic distribution. Inhalation exposure to vapors or aerosols can cause mild respiratory irritation at high concentrations, with LC50 values exceeding 20 mg/L in 4-hour studies, but empirical workplace monitoring shows effective mitigation through established limits such as 5 ppm time-weighted average () for related siloxanes, preventing clinical effects in controlled settings. Mechanistically, stems from physical solvent-like properties rather than reactive metabolites, as PDMS does not undergo significant metabolic breakdown . In applications involving heating, such as cookware, silicone safety depends on quality; high-quality food-grade variants show minimal leaching at cooking temperatures, while lower-quality products with fillers are more prone to releasing siloxanes or other substances, with migration rates up to eight times higher above 200°C. Chronic exposure studies, including 90-day subchronic oral dosing in up to 1,000 mg/kg body weight daily, reveal no evidence of in Ames bacterial assays, mammalian cell gene mutation tests, or chromosomal aberration studies, nor carcinogenicity in long-term bioassays where tumor incidences remained comparable to controls. The absence of DNA reactivity aligns with PDMS's stable backbone, which resists or oxidation under physiological conditions, precluding initiation of mutagenic or neoplastic pathways. PDMS demonstrates negligible bioaccumulation potential, with bioconcentration factors (BCF) below 100 in aquatic organisms per OECD guideline simulations, due to low water solubility (log Kow >6 for oligomers) and rapid fecal elimination without tissue retention in mammals. Human epidemiological data from occupational cohorts exposed to silicone fluids over decades corroborate these findings, showing no elevated incidence of systemic toxicity or organ-specific effects beyond localized irritation.

Debunked Myths and Misconceptions

A prevalent misconception holds that silicones, such as dimethicone in skincare products, clog pores and suffocate the skin, leading to or . Empirical from dermatological assessments demonstrate that silicones are non-comedogenic, as their high molecular weight prevents penetration into pores, and they form a breathable rather than an occlusive barrier that traps sebum or . Another falsehood suggests that silicones cause toxic buildup on or , implying irreversible accumulation that harms over time. Formulation science reveals that most cosmetic silicones are designed to be rinsable, with skin shedding and cleansing routines preventing any permanent residue, while water-soluble variants avoid buildup entirely on hair shafts. No clinical evidence links routine silicone use to systemic from such purported accumulation. Claims portraying silicones as inherent endocrine disruptors often extrapolate from isolated product tests to the material broadly. A 2023 study examining 42 food-contact silicone items sourced exclusively from the Chinese market reported endocrine activity in 84% via assays, yet this reflects potential contaminants or additives in unregulated samples rather than silicone polymers themselves, which exhibit chemical inertness and lack hormone-mimicking structures in pure form. Global regulatory evaluations, including those by the FDA, affirm silicones' safety profile absent such impurities, underscoring that the findings are not generalizable to high-quality, compliant products.

Medical Implant Controversies

In the early 1990s, silicone gel-filled breast implants faced intense scrutiny amid reports of local complications such as rupture and capsular contracture, as well as unverified claims of systemic illnesses including autoimmune disorders. This led to a U.S. Food and Drug Administration (FDA) moratorium in January 1992 restricting their use for cosmetic augmentation, though reconstructive applications continued under investigational protocols. Concurrently, thousands of lawsuits alleged links to connective tissue diseases, culminating in massive litigation; Dow Corning, a major silicone supplier, filed for bankruptcy in 1995 after over 400,000 claims, with a 1994 class-action settlement allocating $4.25 billion for damages despite limited epidemiological evidence supporting causation at the time. The FDA lifted the moratorium in November 2006 following review of premarket studies demonstrating device durability and absence of strong systemic risks, approving silicone implants from manufacturers like and Mentor with requirements for post-approval surveillance, including MRI screening recommendations for rupture detection. Post-market data from FDA-mandated studies, tracking over 40,000 patients, confirmed low rupture rates—approximately 0.8% annually for primary augmentations after 10 years—and no elevated incidence of , with some cohorts showing reduced risk (relative risk 0.67). Rare associations with anaplastic large cell (BIA-ALCL) and have emerged, primarily linked to textured implants and , but incidence remains under 1 in 3,000, prompting targeted recalls rather than broad silicone rejection. Claims of autoimmune/inflammatory syndrome induced by adjuvants (ASIA), proposed in 2011 and associating silicone with symptoms like fatigue and arthralgia, rely on case series and self-reported data, but large-scale reviews and meta-analyses find no causal link, attributing patterns to reporting bias and nocebo effects rather than adjuvant-driven autoimmunity. A 2018 MD Anderson study of nearly 100,000 women noted statistical associations with Sjögren's syndrome and sarcoidosis (adjusted rate ratios around 8-9), yet emphasized rarity (fewer than 20 excess cases per 10,000) and lack of dose-response or biological plausibility beyond local inflammation. Explantations yield symptom improvement in subsets, often via placebo or resolution of local issues, but controlled trials show no excess autoimmune incidence compared to unimplanted populations. Litigation narratives amplified early fears, with settlements exceeding $3 billion by the late despite subsequent data refuting widespread harm, highlighting how adversarial incentives can outpace empirical validation. FDA post-market surveillance, including the National Registry, continues to affirm that while local complications necessitate monitoring, systemic risks do not outweigh benefits for reconstruction—where psychological improvements in reduce rates—or select augmentations, per quality-of-life metrics from longitudinal cohorts. Ongoing debates persist among groups favoring restrictions, yet regulatory bodies prioritize device-specific evidence over anecdotal clusters, underscoring silicone's inert profile .

Environmental Considerations

Persistence and Ecological Impact

Silicones, primarily (PDMS), demonstrate high environmental persistence due to their stable backbone, which resists and under most natural conditions. In aerobic soils, microbial degradation can occur, yielding half-lives of 1-2 months through volatilization and breakdown, but in anaerobic sediments, persistence extends to decades as adsorption to particulate matter limits further transformation. PDMS's low , typically below 100 ppb, restricts aqueous and promotes partitioning to sediments or surfaces rather than dissolution in bodies. Ecological impacts remain limited despite this persistence, as PDMS exhibits minimal . Although log Kow values around 6 for lower-molecular-weight fractions suggest hydrophobic accumulation potential, the polymer's large size and surface properties hinder uptake across biological membranes, resulting in low factors in and field exposures. Field monitoring in aquatic ecosystems has shown no trophic magnification of PDMS, with concentrations in higher trophic levels remaining below detection or equilibrium with sediments, contrasting with biomagnifying cyclic siloxanes like D5. Releases into the environment occur mainly via from products such as shampoos and sealants, where concentrations peak in industrial effluents but dilute rapidly in municipal systems and receiving waters to levels below 1 μg/L. A 2023 Raman study quantified micro- and nanoplastic emissions from degrading silicone sealants in simulated kitchen conditions, estimating releases of up to thousands of particles per square centimeter over time; however, these contributions represent a minor fraction of total microplastic loads from diverse sources, and empirical tests indicate negligible adverse effects on organisms at detected environmental concentrations. Overall, while persistent, silicones' low , poor , and absence of profiles contribute to constrained ecological risks.

Sustainability and Recycling Challenges

Silicone materials, particularly crosslinked elastomers, pose recycling challenges due to their stable networks formed via curing processes like hydrosilylation or peroxides, which resist mechanical breakdown without degrading properties. Traditional mechanical grinding yields fillers but not reusable polymers, while through recovers heat yet incurs high energy costs and emits CO2 without material circularity. Devulcanization analogs, such as oxidative or mechanochemical treatments, selectively cleave crosslinks to restore processability, though remains limited by demands and incomplete bond reversal. Recent advances emphasize chemical to depolymerize silicones into monomers or chlorosilanes, enabling high-purity ; for instance, in April 2025, processes were reported to convert waste silicones (e.g., sealants, gels) back to building blocks like via and chlorination, bypassing sorting complexities. Similarly, mechanical pathways for hydrosilylation-cured rubbers, developed by September 2025, allow reincorporation of recycled high-consistency rubber (HCR) waste into new compounds, reducing virgin material needs by up to 20% in formulations. These methods, while promising, demand precise control to avoid side reactions and are energy-intensive compared to , with commercial viability hinging on cost reductions projected for 2026 onward. Lifecycle assessments highlight silicones' advantages in durability, extending product lifespans (e.g., seals lasting 20-50 years versus plastic alternatives' 5-10 years), thereby lowering cumulative CO2 emissions from production and disposal despite higher upfront energy for silicon derivation (approximately 10-15 kg CO2eq per kg for base silicones). This offsets frequent replacements required for less resilient materials, yielding net reductions in embodied emissions for applications like insulation or gaskets. Regulatory frameworks, such as EU REACH, impose scrutiny on cyclic siloxanes (e.g., D4-D6) for potential, resulting in bans above 0.1% in wash-off from June 2026, yet empirical data and low release volumes support exemptions for durable, non-leaching uses where performance benefits—such as energy efficiency in seals—outweigh persistence risks. No broad prohibitions have emerged, as lifecycle evidence affirms silicones' role in reducing overall environmental footprints through substitution of higher-emission alternatives.

Comparative Analysis with Alternatives

Silicones demonstrate advantages over common alternatives like PVC in environmental persistence, as they resist breakdown into under UV exposure, weathering, and mechanical stress, unlike PVC which fragments and contributes to long-term aquatic and . This stability stems from silicones' inorganic Si-O backbone, which maintains without leaching additives or degrading into persistent pollutants, reducing secondary environmental burdens over the material's lifecycle. Compared to , silicones offer extended longevity in outdoor and high-stress applications, with resistance to and UV degradation enabling service lives of 10-50 years in formulations for seals and , versus 's typical 5-10 years under similar conditions. This durability translates to lower material replacement rates, decreasing total waste volume by factors of up to 5 times for seals, as fewer units are discarded over equivalent functional periods. In contrast, EPDM , while recyclable via conventional methods, underperforms silicones in extreme temperature cycling, leading to higher failure rates and increased inputs in demanding uses. Production trade-offs include elevated energy demands for silicones, driven by high-temperature reduction of to elemental (requiring approximately 10-15 kWh/kg Si), compared to lower inputs for hydrocarbon-based polymers like PVC or EPDM. However, lifecycle analyses indicate these upfront costs are mitigated by silicones' efficiency in enabling energy savings during use—such as in insulation reducing building energy loss by 20-30%—and by curtailing waste through longevity, yielding a net positive "Si-chemistry carbon balance" where enabled GHG abatements exceed production emissions by ratios up to 6:1 across applications. Advocates for biodegradables, including , prioritize rapid decomposition to avoid persistence, yet empirical data on total system impacts reveal silicones' lower volumetric waste footprint, as short-lived alternatives necessitate higher production throughput and associated resource extraction. While silicones pose challenges due to cross-linked structures resistant to mechanical reprocessing, their inertness prevents the toxic leachates seen in degrading PVC or rubbers, supporting a case for superiority in closed-loop potential via emerging chemical methods.

Organosilicon Compounds

Organosilicon compounds comprise a diverse class of molecules featuring at least one carbon-silicon bond, distinguishing them from purely inorganic silicon derivatives like silicates while encompassing monomeric and oligomeric species that serve as precursors to advanced materials. These compounds exhibit reactivities influenced by silicon's electropositive nature compared to carbon, enabling applications in catalysis, surface modification, and synthesis routes that differ from the elastomeric or viscous properties of silicone polymers. Unlike silicones, which are defined by repeating siloxane (Si-O-Si) units with organic substituents forming high-molecular-weight chains, organosilicon compounds often include volatile, low-molecular-weight entities such as silanes and their derivatives, prioritizing reactivity over polymeric stability. Silanes represent foundational organosilicon building blocks, analogous to hydrocarbons but with replacing carbon, typically featuring formulas like R₄Si or R₃SiX where R is an organic group and X a or alkoxy. For instance, chlorosilanes such as hydrolyze readily to form linkages, acting as direct precursors in material synthesis without forming the extended networks characteristic of silicones. These compounds' Si-C bonds confer hydrolytic sensitivity and volatility, contrasting with the thermal stability and elasticity of , and they enable precise control in reactions like hydrosilylation for coupling organic functionalities to scaffolds. Alkoxysilanes, including tetraethylorthosilicate (, Si(OC₂H₅)₄), function as precursors in sol-gel processes, undergoing and to yield silica networks rather than organic-inorganic hybrid polymers. , synthesized via ethanolysis of , hydrolyzes under acidic or basic conditions—e.g., with HCl or NH₃ catalysts—to form SiO₂ gels, as evidenced by ²⁹Si-NMR studies tracking the progression from monomeric to oligomeric species. This route produces amorphous silica for applications like thin films or aerogels, with reaction kinetics showing first-order rates dependent on , yielding products distinct in rigidity from flexible silicone elastomers. Related non-polymeric organosilicon variants include silazanes, featuring Si-N bonds (e.g., perhydropolysilazanes with repeating [SiH₂-NH] units), which pyrolyze at approximately 1000°C under inert atmospheres to amorphous , offering oxidation resistance up to 1400°C superior to oxide-based alternatives. These precursors enable fiber-reinforced composites via melt-spinning or infiltration, with ceramic yields exceeding 70% by , but their nitrogen-rich structures impart and volatility during processing, setting them apart from the hydrophobic, chain-like silicones used in sealants or coatings. Overlaps exist in synthesis, as organosilicon monomers like silanes can initiate both ceramic and pathways, yet their discrete molecular nature underscores reactivity profiles—e.g., facile bond cleavage—unmatched by the crosslinked, high-viscosity silicones.

Siloxanes and Polymers

Siloxanes are organosilicon compounds characterized by the presence of one or more Si-O-Si linkages, forming the backbone of silicone materials. Polysiloxanes, commonly referred to as silicones when polymerized, consist of repeating units with the general formula [-Si(R)(R')O-]n, where R and R' are typically organic groups such as methyl or phenyl, conferring specific properties like flexibility and stability. The silicon-oxygen (approximately 1.64 ) and bond (around 140-150°) enable a wide helical conformation, contributing to the low temperatures observed in these polymers, often below -100°C for linear variants. The predominant polysiloxane is (PDMS), with the repeating unit [-Si(CH3)2O-]n, synthesized primarily through (ROP) of cyclic siloxanes such as (D4) using cationic or anionic initiators. This method, which has largely supplanted early hydrolysis-condensation of since the mid-20th century, allows control over molecular weight (from oligomers at ~500 g/mol to high polymers exceeding 106 g/mol) and polydispersity. Alternative routes include hydrosilylation of silanes or copolymerization with other monomers to introduce functional groups, enabling tailored architectures like block copolymers with polyimides for enhanced mechanical properties. Polysiloxanes exhibit diverse morphologies: linear chains form fluids and gums with viscosities ranging from 0.65 cSt (for ) to over 106 cSt; branched or cross-linked variants yield elastomers and resins via curing or hydrosilylation, achieving tensile strengths up to 10 MPa and elongation at break over 500%. Cyclic siloxanes, such as D3 (hexamethylcyclotrisiloxane) and D4, serve as monomers or low-molecular-weight volatiles, while networked structures, including ladder and cage polymers like polyhedral oligomeric silsesquioxanes (POSS), provide rigidity for high-temperature applications up to 400°C in inert atmospheres. These properties stem from the weak intermolecular forces and high Si-O (approximately 452 kJ/mol), rendering polysiloxanes hydrophobic, non-toxic in bulk form, and resistant to oxidation under ambient conditions.

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