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Fluids

Silicone fluids are linear polydimethylsiloxanes whose chains contain between 2 and well over 1,000 silicon atoms and are obtained through condensation of dimethyldichlorosilane.

The condensation to higher molecular units as well as the ring opening of the cyclic by-products is done with acidic or alkaline catalysts. Adding monofunctional trimethylchlorosilane generally ends the growth of the molecular chain. The resulting trimethylsilyl-terminated silicone fluids are henceforth chemically inert.

Modified silicone fluids (eg alkyl-, vinyl- or aminofunctional polysiloxanes) can be produced via the hydrolysis of organofunctional silanes. The implementation of ethylene oxide (-CH2-CH2O) or propylene oxide units generates copolymer systems that are distinguished by their specific hydrophilic character or their properties as surfactants.

Typical applications

Polydimethylsiloxanes make ideal hydraulic fluids, damping liquids, diffusion pump oils, thermostable lubricants, dielectrics, defoamers and release agents. Special types serve as impregnating agents for textiles and leather. They are also added in tiny amounts to surface coatings.

Other major areas of application include the cosmetics, pharmaceuticals and healthcare industries.

Properties

Silicone fluids are mostly clear, inert, hydrophobic, colourless and odourless liquids. Depending on the chain length, they have a molecular weight from 176 to 10,000 and a viscosity of in the extreme case only 0.65 to 1,000,000 mPA.s. They resist temperatures between -60°C and +300°C, and have extremely low volatility, excellent shear stability, a low surface tension and high water repellency. Their electrical characteristics are also insensitive to a wide range of temperatures.

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Physical properties of silicone fluid (polydimethylsiloxane) of viscosity 350 mPa s
Molecular weight (number average) Ca. 10 000
Flash point > 300°C
Solidifying point -50°C
Thermostability <= 200°C (in air)
Ignition point Around 500°C
Thermal conductivity at 50°C 0.15 W/K/m
Dielectric strength 14 kV/mm
Resistivity 6 X 1015 rcm
Surface tension 21mN/m