Chemistry
Siloxanes
Siloxanes are fundamental members of the broad family of silicone materials.
What are cyclic siloxanes?
Cyclic siloxanes (cyclosiloxanes) are basic members of the broad family of silicone materials and are used as building blocks for the production of a diverse array of silicone polymers. A common denominator for cyclosiloxanes is that they contain repeating units of silicone (Si) and oxygen (O) atoms in a closed loop, giving it a “cyclic” structure.
This also gives them their unique properties as hybrid inorganic-organic substances. D4, D5, D6 contain 4, 5 and 6 repeating units respectively. They are the three main cyclosiloxanes in commercial production and several decades of research have proven that they are safe for human health and the environment.
Where is it used?
Octamethylcyclotetrasiloxane is an odourless, colourless liquid mostly used as an intermediate or basic raw material in the production of silicone rubbers, gels and resins. When used as an intermediate during the manufacturing process, virtually all D4 is consumed with only a tiny amount remaining in final products.
Where is it used?
Decamethylcyclopentasiloxane is an odourless, colourless liquid mostly used as an intermediate or basic raw material in the production of silicone rubbers, gels and resins. When used as an intermediate during the manufacturing process, virtually all D5 is consumed with only a tiny amount remaining in final products. D5 can be added as an ingredient in silicone mixtures used in cosmetic applications such as skin creams and sunscreen, where it may be labelled “cyclomethicone” or “cyclopentasiloxane”. D5 can also be used as a dry-cleaning solvent in closed systems, which significantly limits exposure to workers, consumers and the environment.
Where is it used?
Dodecamethylcyclohexasiloxane is also an odourless, colourless liquid mostly used as an intermediate or basic raw material in the production silicone rubbers, gels and resins. When used as an intermediate during the manufacturing process, virtually all D6 is consumed with only a tiny amount remaining in final products. D6 can also be used as an ingredient in silicone mixtures used in cosmetic applications such as skin creams and sunscreen, where it may be labelled “cyclomethicone” or “cyclohexasiloxane”.
Properties of D4, D5 and D6
| Properties | D4 | D5 | D6 |
| Melting Point, °C | 17.7 | -38 | -3 |
| Boiling Point, °C | 175 | 211 | 245 |
| Density, g/cm3 at 25°C | 0.95 | 0.954 | 0.963 |
| Vapour Pressure, Pa at 25°C | 132 | 33.2 | 4.6 |
| Water Solubility, mg/L at 23°C | 0.056 | 0.017 | 0.053 |
| Henry’s Law Constant, Pa m3/mol at 25°C | 1,214,000 | 3,342,000 | 14,667 |
| Heat of Evaporation, kJ/mol | 44 | 51.4 | – |
What are linear siloxanes?
Linear methyl-siloxanes (linears) are also basic members of the silicone family. L2, L3, L4 and L5 are part of this group. Linears contain repeating units of silicon (Si) and oxygen (O) in a repeating dimethyl-siloxane chain (Me2Si-O-) and as an endgroup e. g. a trimethyl-siloxane unit (Me3Si-O-). E. g. L2 is frequently used as critical building block (endgroup) of the production of a broad range of silicone polymers.
Where is it used?
Hexamethyldisiloxane is a colourless liquid and used in liquid bandages to protect damaged skin from irritation from other body fluids. It is also used in applications for the semiconductor industry.
Where is it used?
Octamethyltrisiloxane is a clear, transparent viscous liquid and is used in adhesives and conductive materials. It is also used in the cosmetics and apparel industries.
Where is it used?
Decamethyltetrasiloxane is a colourless liquid used as an intermediate in the manufacture of other chemical compounds and in cleaning products. It is also used in colorants and in personal care products.
Where is it used?
Dodecamethylpentasiloxane is a colourless liquid and is used to make silicone oils and as a petroleum lubricating oil ingredient.
Beyond Chemistry
The silicones industry is at the forefront of research and leading the way in ensuring that different uses of siloxanes are safe and beneficial to the society and the environment
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