Silikon

Silikon (tudi polisiloksan) je polimer siloksana (−R₂Si−O−SiR₂−, kjer R predstavlja organsko stransko skupino). Silikonski materiali (silikoni) se pojavljajo v obliki brezbarvnih olj ali gumi podobnih snovi ter kot masti in smole. Najpogosteje se uporabljajo v vlogi tesnil, materialov za lepljenje, maziv, medicinskih in kuhinjskih pripomočkov ter toplotnih in električnih izolatorjev.<ref name=Ullmann/><ref>Template:Cite book</ref>

Kemija

Stock in Somieski sta med preučevanjem reakcije hidrolize diklorosilana ugotovila, da se pri reakciji tvori monomer siloksana H₂SiO:

SiH₂Cl₂ + H₂O -> H₂SiO + 2 HCl.

Hidrolizo lahko izvajamo tako, da raztopini diklorosilana v benzenu dodajamo vodo. V tem primeru je večinski produkt polimer s približno formulo [H₂SiO]₆, verjetno pa se pri daljšem času izvajanja reakcije tvorijo tudi daljši polimeri.<ref name="Seyferth">Seyferth, D., Prud'Homme, C., Wiseman, G., Cyclic Polysiloxanes from the Hydrolysis of Dichlorosilane, Inorganic Chemistry, 22, 2163-2167</ref>

Osnova vseh polisiloksanov je veriga, v kateri se izmenjujeta silicijev in kisikov atom (…–Si–O–Si–O–Si–O–…) in je včasih sklenjena v krog. Na vsakega izmed silicijevih atomov sta vezani dve stranski skupini, ki sta največkrat organska substituenta – primer takšnih polimerov sta [(CH₃)₂SiO]_n in [(C₆H₅)₂SiO)]_n. Med proizvodnjo silikonov lahko s spreminjanjem dolžine polimerov, stranskih skupin in stopnje zamreženosti sintetiziramo snovi z izjemno raznolikimi lastnostmi, o d tekočin in gelov do mehke gume in trdnih materialov s plastičnimi lastnostmi. Najosnovnejši polisiloksan je linearni polidimetilsiloksan (PDMS), ki je pri sobnih pogojih v tekočem agregatnem stanju. Zelo velika skupina silikonskih materialov so silikonske smole, ki so sestavljene iz zamreženih polimerov, katerih struktura spominja na kletko.

Terminologija in zgodovina

Beseda silikon (angl. silicone), ki jo je leta 1901 vpeljal Frederic Stanley Kipping, je nastala kot krajše ime za polidifenilsiloksan ([Ph₂SiO]_n). Pri tem se je zgledoval po ketonu benzofenonu (Ph₂CO), katerega je poimenoval silikoketon (angl. silicoketone). Kljub takšni analogiji pa je Kipping s preučevanjem lastnosti obeh spojin pokazal, da je benzofenon monomerna molekula, polidifenilsiloksan pa polimer, kar pomeni, da silikon ni popolnoma ustrezno poimenovanje.<ref>Template:Greenwood&Earnshaw2nd</ref><ref>Template:Cite journal</ref> Ker je bilo dokazano, da je strukturno polidifenilsiloksan bistveno drugačen od benzofenona, bi bilo glede na kemijsko nomenklaturo namesto silikon bolj ustrezno uporabljati poimenovanje siloksan, vendar se ta sprememba nikoli ni uveljavila, zato je izraz silikon v uporabi še danes. <ref name="CincinnatiUniversity2005">Template:Cite book</ref>

F. S. Kipping coined the word silicone in 1901 to describe the formula of polydiphenylsiloxane, Ph₂SiO (Ph denoting phenyl, C₆H₅), by analogy with the formula of the ketone benzophenone, Ph₂CO (his term was originally silicoketone). Kipping was well aware that polydiphenylsiloxane is polymericTemplate:Citation needed whereas benzophenone is monomeric and noted the contrasting properties of Ph₂SiO and Ph₂CO.<ref>Template:Greenwood&Earnshaw2nd</ref><ref>Template:Cite journal</ref> 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.<ref name="CincinnatiUniversity2005">Template:Cite book</ref>

James Franklin Hyde (born 11 March 1903) 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.

Silicone is often confused with silicon, but they are distinct substances. Silicon is a chemical element, a hard dark-grey semiconducting metalloid, which in its crystalline form is used to make integrated circuits ("electronic chips") and solar cells. Silicones are compounds that contain silicon, carbon, hydrogen, oxygen, and perhaps other kinds of atoms as well, and have many very different physical and chemical properties.

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.<ref>Template:Cite journal</ref> However, they have a strong tendency to polymerize into siloxanes. The first stable silanone was obtained in 2014 by A. Filippou and others.<ref name=filippou>Alexander C. Filippou, Bernhard Baars, Yury N. Lebedev, and Gregor Schnakenburg (2014): "Silicon–Oxygen Double Bonds: A Stable Silanone with a Trigonal‐Planar Coordinated Silicon Center". Angewandte Chemie International Edition, volume 53, issue 2, pages 565–570. Template:Doi.</ref>

Synthesis

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.<ref name=Ullmann/>

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

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.<ref>Additive manufacturing of ceramics from preceramic polymers: A versatile stereolithographic approach assisted by thiol-ene click chemistry. Additive Manufacturing, (2019) volume 27, pp. 80–90.</ref>