# Silicones

Pasty silicone, as it is often used as a sealant in the construction sector.

Silicones (also silicones ; singular silicone or silicone ), chemically more precisely poly (organo) siloxanes , is a name for a group of synthetic polymers in which silicon atoms are linked via oxygen atoms . The term “silicone” was introduced at the beginning of the 20th century by the English chemist Frederic Stanley Kipping (1863–1949).

Silicone ( English silicone ) must not with silicon (engl. Silicone ) to be confused. The similar spelling in English often leads to incorrect translations (see False Friend ). Crystalline silicon is a raw material for the manufacture of semiconductors.

It can molecular chains and / or networks occur. The remaining free valence electrons of silicon are saturated by hydrocarbon residues (mostly methyl groups ). Silicones belong to the group of organosilicon compounds .

Because of their typically inorganic structure on the one hand and the organic residues on the other, silicones occupy an intermediate position between inorganic and organic compounds, in particular between silicates and organic polymers . In a certain sense they are hybrids and have a unique range of properties that cannot be matched by any other plastic .

In nature there are only inorganic silicon compounds, namely silicon dioxide , silicates and silicic acid . All other silicon compounds, including silicones, are of synthetic origin. Natural silicones are therefore only available in the language of advertising .

## history

VEB Chemiewerk Nünchritz

At the beginning of the 20th century, the English chemist Frederic Stanley Kipping experimented with silicon and its compounds. He first produced a large number of silicon-carbon compounds and discovered resin-like products, which he called "silicon ketones". In 1940, the American chemist Eugene G. Rochow and the German chemist Richard Müller found a possibility for the large-scale production of chloromethylsilanes , the most important precursors for the production of silicones, almost simultaneously . The process is known today as the Müller-Rochow synthesis .

Richard Müller conducted his experiments in the Radebeul chemical factory v. Heyden , the later drug factory in Dresden.

“However, this discovery was more of a coincidence. In 1932, I had the idea of ​​inventing an artificial fog to envelop entire cities in case there was ever another war. At that time there was no radar. But only a snow-white gas came out. After years of experimentation - it was now 1941 - I continued my research in a different direction. Then I finally discovered a viscous white mass - the silicone. "

- Richard Mueller

Despite major problems in the Soviet zone of occupation at the time and in the GDR , Müller succeeded in setting up silicone production in the "Heyden AG heavy chemicals plant" in Nünchritz and in initiating the establishment of an "Institute for Silicone Chemistry ". Finally, the GDR leadership recognized the importance of silicone and thus Richard Müller. In 1951 he was awarded the national prize. Later the official name of the factory was VEB Chemiewerk Nünchritz , from which the trade name for the Nünchritzer silicone products was derived, among them the most famous Cenusil , an adhesive and sealant, and Cenupaste , a separating and lubricating agent.

Today the Nünchritz plant belongs to Wacker Chemie AG . After the facilities have been completely rebuilt, silicones and other products will be produced again.

The world production of silicone is constantly increasing. In 1974 about 130,000 t were produced, in 1986 it was about 430,000 t. The greater part of the amount produced is accounted for by silicone elastomers. The other set is silicone fluids and greases, as well as silicone resins and specialty products. More recent developments concern the use of new monomers with organofunctional groups (chlorinated aromatics, esters, epoxides, vinyl, allyl, amino, carboxy or alkoxy groups), the inclusion of silazanes, borates, carboranes, of silanes with easily hydrolyzable groups (alkoxy) for adhesion promoters, sulfonic acid groups for silicone surfactants, etc.

## structure

Silicones consist of individual siloxane units . The silicon atoms, which do not reach their octet (electron shell) due to the formation of bonds with oxygen , are saturated with organic residues.

The composition of the siloxane unit results from the fact that each oxygen atom is a bridge between two silicon atoms: R n SiO (4-n) / 2 (n = 0, 1, 2, 3), i. This means that a siloxane unit can have one to four further substituents, depending on the number of valences remaining free on the oxygen. Siloxane units can therefore be mono-, di-, tri- and tetrafunctional . In symbolic notation this is represented by the letters M (mono), D (di), T (tri) and Q (quatro): [M] = R 3 SiO 1/2 , [D] = R 2 SiO 2 / 2 , [T] = RSiO 3/2 and [Q] = SiO 4/2 . A network made up of Q units would correspond to quartz glass .

As with organic polymers, the multitude of possible compounds is based on the fact that different siloxane units can be linked to one another in the molecule. Based on the systematics of organic polymers, one can differentiate between the following groups:

Poly (dimethylsiloxane)
• Cyclic polysiloxanes are built up in a ring shape from difunctional siloxane units. Design [D n ].
• Linear polysiloxanes with the design [MD n M] or R 3 SiO [R 2 SiO] n SiR 3 ( e.g. poly (dimethylsiloxane))
• Crosslinked polysiloxanes in this group are chain or ring-shaped molecules linked to planar or three-dimensional networks with the aid of tri- and tetrafunctional siloxane units. Chain formation and cross-linking are the dominant principles for the structure of high molecular weight silicones.
• Branched polysiloxanes which have trifunctional or tetrafunctional siloxane units as branching elements. Design [M n D m T n ]. The branch point (s) is / are either built into a chain or a ring.

Silicones can be further subdivided according to the substituents attached to silicon. The siloxane structure can contain various hydrocarbons, silicon-functional and organofunctional groups can be present. A subdivision into non-, silicon- or organofunctional is therefore appropriate.

## Manufacturing

The starting materials for production are dust-fine ground silicon (Si) and methyl chloride (CH 3 Cl). These are converted into chloromethylsilanes using copper as a catalyst at approx. 300 ° C in fluidized bed reactors ( Müller-Rochow synthesis ). By fractional distillation , the chloromethylsilanes are separated into:

The synthesis of chlorophenylsilanes (phenylchlorosilanes) from silicon and chlorobenzene in the presence of copper or silver proceeds in the same way .

By hydrolysis of the organochlorosilanes, silanols are formed , which are directly polycondensed at elevated temperature and in the presence of catalysts or, after being converted into cyclosiloxanes, are polymerized to give the desired end product. For example, polydimethylsiloxane is synthesized from dimethyldichlorosilane and trimethylchlorosilane as terminal groups:

${\ displaystyle \ mathrm {(CH_ {3}) _ {3} SiCl + n \ (CH_ {3}) _ {2} SiCl_ {2} + ClSi (CH_ {3}) _ {3} + (n + 1) \ H_ {2} O \ longrightarrow}}$ ${\ displaystyle \ mathrm {(CH_ {3}) _ {3} Si {-} O {-} [Si (CH_ {3}) _ {2} {-} O] _ {n} {-} Si ( CH_ {3}) _ {3} + (2n + 2) \ HCl}}$

The addition of silanes or siloxanes with Si – H bonds to unsaturated hydrocarbons (hydrosilylation), the substitution of chlorine atoms with Grignard reagents or other metal organyls and the substitution of silicon-bonded hydrogen atoms are also used for the production of special silicones.

Silicone resins are produced by hydrolytic condensation of various silicon precursors. In the first production processes, sodium silicate and various chlorosilanes were used as starting materials. Although these substances are very cheap, the control of the reaction products has proven to be very difficult. Newer processes use less reactive tetraethyl orthosilicate (TEOS) or ethyl polysilicates and various disiloxanes as starting materials.

Casting resins can be produced by radical graft copolymerization of silicones with styrene , acrylonitrile , vinyl acetate and other olefins .

The largest silicone producers worldwide are:

## nomenclature

The term silicone is derived from the English "silicon ketone". Simple linear silicones are structured according to the scheme (R 2 SiO) n , and R 2 SiO corresponds to the general formula for ketones R 2 C = O. Since Si – O double bonds are not stable, this derivation is purely formal. Si – O – Si bonds, on which the silicones are based, are called siloxane bonds. Silicones are therefore polyorganosiloxanes.

A polymer with the general formula (CH 3 ) 3 - Si - O - [(CH 3 ) 2 Si - O] n - Si - (CH 3 ) 3 is referred to as poly (dimethylsiloxane). According to the IUPAC rules for naming linear organic polymers, the compound should actually be called poly [oxy (dimethylsilylene)], according to the provisional rules for inorganic macromolecules, catena-poly [(dimethylsilicon) -m-oxo]. The international non-proprietary name assigned by the World Health Organization (WHO) for the connection to medicinal use is Dimeticon .

## Product classes of silicone

### Silicone liquid and silicone grease

Silicone liquids are clear, colorless, neutral, odorless, hydrophobic liquids with a molecular weight of (162 to 150,000) g / mol, a density of (0.76 to 1.07) g / cm³ and viscosities of (0.6 to 1,000. 000) m Pa · s. Silicone fluids have a low surface tension of 21.5 mN / m (at 25 ° C) or less. They are permanently heat-resistant up to approx. 180 ° C in air. Depending on the viscosity, their pour point is (−80 to −40) ° C. Silicone fluids have lubricating properties between −60 ° C and up to 200 ° C. The lubricity is lower than that of mineral oils and other lubricants. They do not tend to gum up. Silicone fluids are soluble in benzene , toluene , aliphatics and chlorinated hydrocarbons. They are not very resistant to strong inorganic acids and bases. Like all silicones, they are very gas-permeable.

Silicone fluids are used for defoamer formulations ( e.g. as additives in diesel fuels ), as hydraulic fluids, as mold release agents, as an ingredient for special printing inks, for hydrophobing glass (e.g. in pharmaceuticals), ceramics, textiles, leather, etc., as lubricants for the Plastics processing, lubricants in plastic gears, polishing agent additive for car paints, leather and furniture, used to prevent pigments from floating in pigmented paints, used as a pressure gauge fluid , part of metal cleaning agents , collectors in flotation processes , etc.

In vehicle construction, silicone fluids of higher viscosity are used than fluids for torque transmission with automatic speed compensation in viscous clutches as axle and / or (in connection with a) central differential.

Silicone fluids play an important role as electrical insulating materials ( dielectrics ) (e.g. in transformers), as diffusion pump oils, heating fluids and damping agents.

In medicine and cosmetics, silicone fluids are used as a component of cosmetics, skin protection ointments, ointment bases, implants, for hairstyle stabilization, as a fixative for fragrances and the like. a.

Other uses include massage oils as well as lubricants and treatments for condoms and latex clothing .

By adding consistency regulators and fillers, silicone pastes or silicone greases can be produced from the silicone liquids. Silicone pastes are used as protective and sealing pastes for sensitive metal and apparatus parts, silicone greases as lubricants at low, high or strongly fluctuating temperatures, those based on polymethylphenylsiloxanes, for example in the range from −70 ° C to 230 ° C.

### Silicone rubber and silicone elastomers

Silicone baking mold (blue)
Ice cube molds made of silicone
Muffin baking molds made of silicone
Silicone roller from the fuser of a laser printer

Silicone rubbers are compounds which can be converted into the rubber-elastic state and which contain poly (organo) siloxanes which have groups accessible for crosslinking reactions. Hydrogen atoms, hydroxyl groups and vinyl groups , which are located at the chain ends, but can also be incorporated into the chain, are predominantly suitable as such . Silicone rubbers contain reinforcing substances and fillers, the type and quantity of which have a significant influence on the mechanical and chemical behavior of the silicone elastomers created by the crosslinking. Silicone rubbers can be colored with suitable pigments.

A distinction is made between cold (RTV) and hot crosslinking (HTV) silicone rubbers (RTV = room temperature crosslinking, HTV = high temperature crosslinking) according to the necessary crosslinking temperature. HTV silicone rubbers are plastically deformable materials. They very often contain organic peroxides for crosslinking. The elastomers made from it by crosslinking at high temperatures are heat-resistant products that are elastic between −40 and 250 ° C. B. can be used as high-quality sealing, damping, electrical insulating components, cable sheathing and the like.

Silicone-based coatings also serve to make nylon fabrics impermeable to air and more UV- resistant.

Another crosslinking mechanism consists in an addition, usually catalyzed by noble metal compounds, of Si – H groups to silicon-bonded vinyl groups , both of which are built into the polymer chains or at their ends. The liquid rubber technology (LSR = Liquid Silicone Rubber ) based on this has been established since 1980 . The silicone rubber components, which, in contrast to the HTV rubbers described above, have a lower viscosity and are therefore pumpable, are dosed and mixed with suitable mixing and metering machines and usually processed in automatic injection molding machines. This technology allows high cycle rates due to the short cross-linking time of the rubbers.

A very common application of this technology is the manufacture of the mouth parts of all types of baby teats ( pacifiers ). Elastic baking molds are a newer application of the heat-resistant silicone elastomers.

One-component and two-component systems can be distinguished for RTV silicone rubbers. The first group (RTV-1) crosslinks at room temperature under the influence of atmospheric humidity, the crosslinking taking place through condensation of SiOH groups with the formation of Si – O bonds. The SiOH groups are formed by hydrolysis of SiX groups of an intermediate species formed from a polymer with terminal OH groups and a so-called crosslinker R-SiX3 ( acetate : X = -O-CO-CH3, amine : X = -NHR) . During curing, the known acetate systems split off acetic acid , the amine systems split off amine compounds that smell fish-like. In the case of two-component rubbers (RTV-2), z. B. Mixtures of silicic acid esters (e.g. ethyl silicate) and organotin compounds are used, with the formation of a Si – O – Si bridge from Si – OR and Si – OH as a crosslinking reaction by splitting off alcohol.

Typical silicone sealant. If the silica content is lower, this substance can also be made liquid.

A widespread use of silicone elastomers can be found in the construction industry as a sealant for filling joints (see also: silicone joint , bulkheads ). But there they are also used for the production of molding and potting compounds and as coating compounds for fabric.

In art casting , silicone rubber is used to produce a negative mold for later casting of sculptures and reliefs . It is characterized by the accuracy of drawing surface details and is practically shrink-free when the cast form hardens. In the context of rapid prototyping technologies, silicones are used to manufacture molds. Prototypes or small series parts are cast in these molds.

In dentistry , addition (A-silicones) and condensation-curing (K-silicones) are essential for the production of precision models for the exact molding of the rows of teeth and the jaw.

In orthopedic technology, silicones are used to manufacture liners or prosthetic inner shafts, but also for the manufacture of exoprostheses such as breast prostheses .

Depending on the components of the rubber mixture, the following abbreviations are used:

### Silicone resin

Silicone resin with R = CH 3 , H or OH

The silicone resins commonly used in the art are crosslinked polymethylsiloxanes or polymethylphenylsiloxanes, whose elasticity and heat resistance increase with the content of phenyl groups . Pure methyl silicone resins are relatively brittle and moderately heat resistant.

The long-term heat resistance of silicone resins is high (180 to 200 ° C), the favorable dielectric values are largely independent of temperature up to 300 ° C. A methylphenyl silicone resin can be used for 10,000 hours, while an epoxy or alkyd resin can only be used for a few hours at 200 ° C.

Silicone resins are usually sold in precondensed form. If they are processed into paints, they are dissolved in organic solvents.

Some of them are also made with organic resins, such as. B. alkyd, epoxy, melamine , phenolic and polyester resins , combined to improve gloss, surface hardness, hiding power, pigment compatibility and heat and chemical resistance.

Copolymers of low molecular weight, hydroxyfunctional silicones with polyesters , alkyd and acrylic resins , which are processed into so-called silicone enamel, a decorative, heat-resistant coating for kitchen appliances, etc., are also known as silicone combination resins .

Silicone resin stoving enamels are made from polysiloxanes , which are cured at temperatures of around 250 ° C and serve as binders for lacquer paints and heat-resistant glass fiber insulation materials. Toluene and xylene serve as solvents . Due to their good insulating properties, they are used as insulating varnishes for electrical coils as well as in the ceramics industry and as stoving resins for potting and encapsulating electronic components.

Vinyltrichlorosilicones and vinyltriethoxysilicones serve as adhesion promoters between glass fibers and organic resins in the manufacture of glass fiber reinforced plastics, since silicones form a close bond with both the glass and the organic resins.

Alkaline alkyl siliconates as precursors of silicone resins serve as building protection agents for impregnating bricks , concrete and masonry in order to limit moisture penetration and efflorescence.

To produce molding compounds and laminates, silicone resins are mixed with suitable fillers such as glass fibers , quartz powder , mica , etc., and possibly also color pigments .

Since silicone resins generally have to be condensed (hardened) with the aid of condensation catalysts and at elevated temperatures, they can be classified as stoving resins. At temperatures between 250 ° C and 600 ° C, the silicone resin decomposes with the formation of silica. With the added pigments (tin, aluminum), this gives a permanent, anti-corrosive surface film, e.g. B. to protect against rust in exhaust pipes.

Walls are made water-repellent with dissolved or powdered silicone resins or siliconeates such as sodium methyl siliconate [H 3 C – Si (OH) 2 ONa] without reducing the water vapor permeability . When exposed to acids (such as atmospheric carbon dioxide ), sodium methyl siliconate forms a cross-linked silicone (and sodium carbonate).

A special application is the preservation process called plastination , whereby the water in the cells of biological objects is replaced by silicone resin, which is then hardened with a "gas hardener".

Furthermore, silicone resins are used as additives in defoamers .

### Fluorosilicones

Fluorosilicones are temperature- and oxidation-resistant silicones in which the methyl groups have been replaced by fluoroalkyl groups. The fluorosilicones have an even higher oxidation u. Chemical resistance than the silicones, are insoluble in water, hydrocarbons and chlorinated hydrocarbons, resistant between −60 and +290 ° C, available in the form of oils, fats, pastes and the like. They are mainly used as lubricants for extreme temperatures, defoamers, compressor oils, hydraulic oils and damping media.

### Highly transparent silicone

Highly transparent silicones are a special type of silicone elastomer that is mainly used in the optical sector. They belong to the LSR (liquid silicone rubber) materials, which are mainly characterized by their low viscosity and the associated possibility of processing the silicone in injection molding. In addition to the types that can be injection molded, there are also types that are suitable for encapsulation. They again have a lower viscosity.

The silicone offers the greatest advantage for optical applications in terms of its resistance behavior. In this way it remains stable in a wide temperature range (−40 ° C to +150 ° C) on the one hand in its mechanical behavior, on the other hand in its optical, i.e. H. Compared to other plastics, the silicone does not show any yellowing over time. Yellowing must be avoided at all costs, especially in optical applications, as this leads to severe functional impairment and even functional failure.

Another advantage of silicone over other materials is its elasticity. This can be used in headlights, for example, to influence the light conduction through deformation and thus to generate dynamic cornering lights, among other things.

Furthermore, optical components made of silicone are significantly lighter than their glass counterparts due to their lower density. Silicone also offers advantages in terms of manufacturing technology. The processes are easier to handle, short cycle times can be achieved and production within very tight tolerances. In addition, the flow behavior of the material allows very complex geometries to be molded. Injection molding machines for silicone are also semi-automatic, so that the need for personnel is low. One employee can therefore look after several systems. Another property of silicones is their electrical insulation properties.

Highly transparent silicones are used for optical applications. On the one hand, they can serve as a protective layer for LEDs, for example, or they can be processed in 2-component injection molding in order to be able to manufacture geometrically complex molded parts tailored to the application.

There is the possibility of guiding the light by means of the molded parts, of shaping the light beam or of making an optical coupling.

## Environment and health

Silicones do not occur in nature. According to Öko-Test , silicones are difficult to break down, but non-toxic. According to the Federal Environment Agency , long-chain silicone oils (especially polydimethylsiloxanes , PDMS for short) degrade so slowly in moist sediments that they can serve as a marker for anthropogenic activities even after decades . The degradation is catalyzed by clay minerals and provides the water-soluble monomer dimethylsilanediol , which is photolytically degraded in the atmosphere and biologically by microbes in the soil to form silicon dioxide and carbon dioxide.

## Silicone nanofilaments

In 2004, the chemist Stefan Seeger developed a new type of supramolecular silicon structure. These are silicone nanofilaments that arise from the gas phase or organic solvents on surfaces, provided that suitable concentrations of trichlorosilanes and water are set. These nanofilaments have a thickness in the range from 10 to 20 nm and a length from 50 to approx. 150 nm. Silicone nanofilaments completely change the surface properties. So these surfaces have super hydrophobic properties and, if they are then fluorinated , at the same time super oleophobic properties, i. In other words, they are extremely water and oil repellent at the same time.

## Individual evidence

1. ^ Matthias Petran: Exxon Chemical. (PDF) ( Memento of the original from August 27, 2016 in the Internet Archive ) Info: The  archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. portal-schule-wirtschaft.de, accessed on March 30, 2014.
2. Designations according to ISO 1629, see Richard B. Simpson: Rubber Basics , p. 97, iSmithers Rapra Publishing, 2002.
3. Designations according to ISO 1629, see Richard B. Simpson: Rubber Basics , p. 96, iSmithers Rapra Publishing, 2002.
4. ^ Encyclopedia entry paints and polysiloxanes , Internet presence of the journal 'Spectrum of Science'.
5. Hair Conditioners - Silicone Valley , Öko-Test, 2014.
6. Christoph Rücker, Klaus Kümmerer: Environmental Chemistry of Organosiloxanes . In: Chemical Reviews . tape 115 , no. 1 , January 14, 2015, p. 466-524 , doi : 10.1021 / cr500319v .
7. Examination of the effects of silicone oils (polydimethylsiloxanes) used in antifouling / foul-release products on the marine environment , Federal Environment Agency, 2007.