A silicone or polysiloxane is a polymer composed of repeating units of siloxane (−O−SiR2−O−SiR2−, 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, 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 whereas benzophenone is monomeric and noted the contrasting properties of Ph2SiO and Ph2CO. 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 H2SiO:
SiH2Cl2 + H2O → H2SiO + 2 HCl The reaction occurs below 0 °C. The reaction produces a mixture of cyclic siloxanes with the pentamer [H2SiO]5 and hexamer [H2SiO]6 predominating. The hydrolysis of Si(CH3)2Cl2, 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:
n Si(CH3)2Cl2 + n H2O → [Si(CH3)2O]n + 2n HCl The polymerization typically produces linear chains terminated with Si−Cl or Si−OH (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:
n[Si(CH3)2O]3 → [Si(CH3)2O]3n 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.
n Si(CH3)2(CH3COO)2 + n H2O → [Si(CH3)2O]n + 2n CH3COOH
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 (···−Si−O−Si−O−Si−O−···) with two groups attached to each silicon center. The silicon center is tetrahedral. The oxygen atoms adopt a bent ether-like geometry. The R2Si centers are well separated from each other, which means that the chain is particularly flexible. By varying the −Si−O− 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, 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.
Properties
Silicones exhibit many useful characteristics, including:
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