A silicone or polysiloxane is a polymer composed of repeating units of siloxane (, where "R" stands for an 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. From the chemical perspective, silicones are unusual because they feature inorganic backbones, composed only of Si and O, but they have properties of organic polymers. They represent one of the main applications of organosilicon chemistry.
Silicone is often confused with one of its constituent elements, silicon. Silicon, a hard gray solid, is used to make integrated circuits ("electronic chips") and solar cells. In contrast, silicones, which tend to be electrical insulators, are often colorless oils or rubbery resin.
History F. S. Kipping coined the word silicone in 1901 to describe the formula of polydiphenylsiloxane, (Ph = phenyl, ), by analogy with the formula of the ketone benzophenone, (his term was originally silicoketone). Kipping was well aware that polydiphenylsiloxane is polymeric whereas benzophenone is monomeric and noted the contrasting properties of and . 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.
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.
Alfred Stock and Carl Somiesky examined the hydrolysis of dichlorosilane, a reaction that was proposed to initially give the monomer :
The reaction occurs below 0 °C. The reaction produces a mixture of cyclic siloxanes with the pentamer and hexamer predominating. The hydrolysis of , relevant to modern siloxane technology, proceeds more slowly.
Synthesis
The most common silicones are composed mostly of polydimethylsiloxane (PDMS). PDMS is derived by hydrolysis of dimethyldichlorosilane according to the following idealized equation:
The polymerization typically produces linear chains terminated with or (silanol) groups. Depending on conditions, the polymer can be cyclic, not a chain. Commercial routes to PDMS usually involve ring-opening polymerization of cyclic siloxanes. A representative reaction would start with hexamethyltrisiloxane:
In reality, a series of ring sizes comprise the precursor cyclic siloxanes. The reaction is typically catalyzed by base, such as an alkali metal oxide. The base must be removed when the polymerization is complete.
For some 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.
Crosslinking Many silicones are crosslinked, which confers a rubbery or even hard texture. Crosslinking is achieved by the cohydrolysis of methyl trichlorosilane and dimethyldichlorosilane.
Structure
All polymerized siloxanes or polysiloxanes, silicones consist of an inorganic silicon–oxygen backbone chain () with two groups attached to each silicon center. The silicon center is tetrahedral. The oxygen atoms adopt a bent ether-like geometry. The centers are well separated from each other, which means that the chain is particularly flexible.
By varying the chain lengths, organic substituents attached to silicon, and crosslinking, silicones can be synthesized with a wide variety of properties and compositions. They can vary in consistency from liquid to gel to rubber to hard plastic. The most common siloxane is linear polydimethylsiloxane (PDMS), a silicone oil. The second-largest group of silicone materials is based on silicone resins, which are formed by branched and cage-like oligosiloxanes.
Combustion
When silicone is burned in air or oxygen, it forms solid silica (silicon dioxide, ) 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.
Properties
Silicones exhibit many useful characteristics, including:
Low thermal conductivity Low chemical reactivity Low toxicity Thermal stability (constancy of properties over a wide temperature range of ) 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 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 gases 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
Silicones are used in many products. Ullmann's Encyclopedia of Industrial Chemistry lists the following major categories of application: Electrical (such as insulation), electronics (such as coatings), household (such as sealants and cooking utensils), automobile (such as gaskets), airplane (such as seals), office machines (such as keyboard pads), medicine and dentistry (such as tooth impression molds), and textiles and paper (such as coatings). For these applications, an estimated 400,000 tonnes of silicones were produced in 1991. Specific examples, both large and small are presented below.
Automotive
In the automotive field, silicone grease is typically used as a lubricant for brake components since it is stable at high temperatures, is not water-soluble, and is far less likely than other lubricants to foul. 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.
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. Silicones in combination with thermoplastics provide improvements in scratch and mar resistance and lowered coefficient of friction.
Aerospace
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.
Specially developed aerospace grades of silicone are stable from , 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 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 Polydimethylsiloxane (PDMS) based binders along with ammonium perchlorate () are used as fast burning solid propellants in rockets.
Building construction 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. 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.
3D printing
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.
3D printing also requires the use of a removable support material that is compatible with the silicone rubber.
Coatings 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.