Silicone is a plastic that we encounter in everyday life in a wide variety of products.
It is physiologically harmless, which is why it is a popular plastic in medical technology, where it is used to make implants, artificial heart valves or pacemakers. It is also found in everyday products such as hair shampoos, gels, cosmetics and the popular silicone baking pans.
The automotive industry uses silicone-based oils, the construction industry uses silicone as a sealing material, the textile industry for impregnation and the electronics industry for cable sheathing.
Silicone is a plastic that we encounter in everyday life in a variety of products. The name silicone or silicones refers to a family of plastics characterized by the same basic structure. The basic structure is formed by silicon atoms, which are linked by oxygen atoms to form molecular chains, either linear or in the form of a network. The remaining two free places on the silicon atom are occupied by organic hydrocarbon groups, mostly methyl groups. [ Image: https://upload.wikimedia.org/wikipedia/commons/8/8c/Dimethylpolysiloxan.png]
Instead of the term silicones, the exact designation poly(organo)siloxanes, polysiloxanes or simply siloxanes can also be found, especially in scientific literature.
Silicones were discovered by the British chemist Frederick Stanley Kipping at the beginning of the twentieth century. The name "silicone" is derived from silicon-based ketones.
This was intended to refer to the analogy with the corresponding oxygen compounds of the chemically related element carbon, the polyketones.
Silicone plastics are very stable and are characterized by the following properties:
Silicone has different mechanical properties, depending on the degree of crosslinking. The degree of crosslinking describes how much the linear Si-O-Si chains are linked together. If they are interconnected via every silicon atom, the result is a very strongly cross-linked plastic that can be imagined as a dense, close-meshed lattice. This plastic made of silicone is very hard. In silicone rubber or silicone resins, on the other hand, the chains are not linked together via each silicon atom, so that the structure can be imagined more as a wide-meshed network. This plastic is soft and flexible.
Silicone is a very heat resistant plastic due to its strong Si-O-Si bond. This can only be separated by very high temperatures. Thus, silicones are stable in the temperature range from -5°C to 200°C and retain their flexibility. Some silicone plastics can withstand temperatures up to +300°C, while others can still be used at low temperatures as low as -100°C.
Silicones and plastics made of silicone are physiologically harmless.
The basic structure of silicones offers several linkage possibilities, depending on which basic building block is used.
Linear and cyclic polysiloxanes are formed from the difunctional siloxane units. Here, each individual silicone atom is linked to the next via two oxygen atoms, forming long polymer chains.
Linear silicones are liquid and are marketed, for example, as silicone oils.
Difunctional siloxane units can also form rings. These cyclosiloxanes are further processed and are starting materials for subsequent polymerization reactions.
Trifunctional polysiloxanes can be linked at three points. They form branched structures and are the basis for silicone resins.
Tetrafunctional basic units react with each other at four points and a very dense molecular network is formed. These silicone plastics are very hard and comparable to silicates. The best known silicate is quartz.
Silicone oils are liquid. They are
Silicone pastes are obtained from silicone oils by incorporating so-called inorganic thickeners, for example highly disperse (i.e. very finely dispersed) silicas. Another possibility is the incorporation of metal soaps into silicone oils. These products are known as silicone greases.
Silicone rubber is composed of linear polysiloxane units and forms the basis for a large number of rubber products (= silicone elastomers) . Silicone rubber contains functional groups (vinyl or hydroxyl groups) through which the crosslinking of the linear polysiloxane units takes place. The crosslinking reaction can take place at high or low temperatures.
A distinction is therefore made between hot-vulcanizing silicone rubbers and cold-vulcanizing silicone rubbers.
Silicone can also exist in solid form as solid silicone rubber (HCR) or as liquid silicone (LSR).
Finally, a distinction is made between the type of vulcanization, i.e. the process by which the silicone rubber is transformed into the final silicone rubber.
For vulcanization at higher temperatures, solid silicone rubber (HCR) can be used as a starting material. This is plastically deformable. Crosslinking takes place by means of so-called peroxides or platinum catalysts.
The resulting silicone elastomers have the following properties:
Liquid silicone, also known as liquid silicone rubber (LSR), is also one of the hot-vulcanizing silicone rubbers. It has a low viscosity and consists of shorter molecular chains than solid silicone rubber. Liquid silicone has the advantage that it can be poured or sprayed into molds. It consists of two components that are mixed shortly before processing. Crosslinking usually takes place very quickly with the aid of platinum catalysts.
RTV silicone rubbers crosslink already at room temperature. They are available as liquid, pourable or more solid, kneadable compounds. RTV silicone rubbers consist of either one- or two-component systems..
One-component systems cure in air. They are used as sealing or jointing compounds and are characterized by good weathering and aging resistance.
In two-component systems, one component has unsaturated groups at which the crosslinking takes place and a catalyst. The second component contains the crosslinker. If both components are added together, the reaction can take place: the plastic cures.
Silicone rubbers are additionally stabilized by fillers that are incorporated into the network of the silicone rubber. In this way, they strengthen it and make it more stable. Typical fillers are special carbon black or fumed silica (solid "silica lumps"). In addition, silicone elastomers can be easily colored.
Silicone resins are highly crosslinked polymethylsiloxanes or poly(methylphenyl)siloxanes with the following properties: They are resistant to:
The number of phenyl groups increases the heat resistance and elasticity of the material.
Silicone is produced on an industrial scale by the Müller-Rochow synthesis. In this process, finely ground silicon dust is reacted with methyl chloride vapor at a temperature of 300°C and using a copper catalyst.
The reaction product is a mixture of differently substituted methylchlorosilanes. For example, four methyl groups are bonded to one silicon atom, three methyl groups and one chlorine atom, and so on. In total, the following reaction products are formed:
These are separated by fractional distillation. In a subsequent acid-catalyzed hydrolysis step, the chlorine atoms are replaced by hydroxy groups and reactive silanols are formed. These react directly to form the desired silicone, for example poly(dimethyl)siloxane.
Silicone is found in a variety of products in its different forms as silicone oil, silicone rubber or silicone resin. Since it is physiologically harmless, it can be used in almost every area of daily life.
Silicone oils are often used as release agents or lubricants due to their hydrophobic properties. Here they can be found as:
Silicone oils have an antifoaming effect. As defoamers, they are used wherever foam formation is to be prevented. These can be fermenter or distillation plants.
Silicone rubber is very popular in the medical, pharmaceutical and food industries because, in addition to its elasticity and heat resistance, it is above all physiologically harmless. Silicone serves as a basis for the production of
Dentistry uses silicone-based liquid rubbers to make impressions and molds. They serve as implants or as fillings.<br/
Silicone oils form the basis for a wide range of ointments in cosmetics and medicine and are a component of skin protection and hair care products. They are also used as a carrier substance for deodorants and other personal care products.
Liquid rubber is widely used in the construction industry as a jointing and sealing agent. Silicone resins are components of many facade coatings and paints. In addition, silicone is used as protection for a wide variety of building and house parts.
Cables with sheaths made of silicone rubber are used in conductor wires of medium- and high-voltage cables. Other applications are heating wires or motor cables.
Finally, silicone gels or silicone rubbers are applied to computer chips. Here they serve to protect the circuits from moisture or other external influences.
Silicone-based products are hydrophobic, i.e. water-repellent. The textile industry uses this property to impregnate its fabrics. The paper industry, in turn, makes use of this advantage to produce release labels, for example.
Finally, silicone is found as a component of seals and damping materials.
As Managing Director of Sattler GmbH, it is a constant concern of mine to meet our customers not only as a trading partner, but also as a competent procurement service provider for applications in the plastics and rubber sector. I am an engineer by heart and most passionate when I can solve detailed technical issues to create sophisticated, customized solutions.
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