Silicone resin

Polymers composed of alternating atoms of silicon and oxygen with organic substituents attached to the silicon atoms, as shown in the formula below.

Silicones, also called organopolysiloxanes, may exist as liquids, greases, resins, or rubbers. Silicone polymers have good resistance to water and oxidation, stability at high and low temperatures, and lubricity.

Silicones are obtained by the condensation of hydroxy organosilicon compounds formed by the hydrolysis of organosilicon halides. The first products are usually low in molecular weight (n = 2 to 7), and usually consist of a mixture of linear and cyclic species, especially the tetramer. Fluids having a wide range of viscosity are prepared by polymerizing further, using a monofunctional trichlorosilane to limit molecular weights to the value desired. Elastomers are made by polymerization of the purified tetramer using an alkaline catalyst at 100–150°C (212–302°F). Properties can be varied by partial replacement of some of the methyl groups by other substituents and by the use of reinforcing fillers.

The wide range of structural variations makes it possible to tailor compositions for many kinds of applications. Low-molecular-weight silanes containing amino or other functional groups are used as treating or coupling agents for glass fiber and other reinforcements in order to cause unsaturated polyesters and other resins to adhere better.

The liquids, generally dimethyl silicones of relatively low molecular weight, have low surface tension, great wetting power and lubricity for metals, and very small change in viscosity with temperature. They are used as hydraulic fluids, as antifoaming agents, as treating and waterproofing agents for leather, textiles, and masonry, and in cosmetic preparations. The greases are particularly desired for applications requiring effective lubrication at very high and at very low temperatures.

Silicone resins are used for coating applications in which thermal stability in the range 300–500°C (570–930°F) is required. The dielectric properties of the polymers make them suitable for many electrical applications, particularly in electrical insulation that is exposed to high temperatures and as encapsulating materials for electronic devices.

Silicone rubbers are compositions containing high-molecular-weight dimethyl silicone linear polymer, finely divided silicon dioxide as the filler, and a peroxidic curing agent. The silicone rubbers have the remarkable ability of remaining flexible at very low temperatures and stable at high temperatures. See also Inorganic polymer; Plastics processing; Rubber; Silicon.

The noun has one meaning:

Meaning #1: a polymeric silicone compound

Silicone resins are a type of silicone material which is formed by branched, cage-like oligosiloxanes with the general formula of R_nSiX_mO_y, where R is a non reactive substituent, usually Me or Ph, and X is a functional group H, OH, Cl or OR. These groups are further condensed in many applications, to give highly crosslinked, insoluble polysiloxane networks.^[1]

When R is methyl, the four possible functional siloxane monomeric units are described as follows^[2]:

• "M" stands for Me₃SiO,
• "D" for Me₂SiO₂,
• "T" for MeSiO₃ and
• "Q" for SiO₄.

Note that a network of only Q groups becomes fused quartz.

The most abundant silicone resins are built of D and T units (DT resins) or from M and Q units (MQ resins), however many other combinations (MDT, MTQ, QDT) are also used in industry.

Silicone resins represent a broad range of products. Materials of molecular weight in the range of 1000-10 000 are very useful in pressure sensitive adhesives, silicone rubbers, coatings and additives.^[3][4]

Silicone resins are prepared by hydrolytic condensation of various silicone precursors. In early processes of preparation of silicone resins sodium silicate and various chlorosilanes were used as starting materials. Although the starting materials were the least expensive ones (something typical for industry), structural control of the product was very difficult. More recently, a less reactive tetraethoxysilane - (TEOS) or ethyl polysilicate and various disiloxanes are used as starting materials.^[1]

References

1. ^ ^{a b} S.J. Clarson, J.A. Semlyen, Siloxane Polymers, Prentice Hall, New Jersey (1993).
2. ^ Dow Corning nomenclature of silicon resins
3. ^ List of silicone resins produced by Dow Corning
4. ^ Database of silicon resins produced by Momentive Performance Materials (formerly GE Silicones)

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