Silicone Fluids – Heat-Resistant, Lubricating, Emulsifying

The linear molecules of silicone fluids consist of dimethylsiloxane units linked together. Silicone fluids are made by hydrolysis of dichlorodimethylsilane followed by condensation. Adding acid or base can catalyze condensation to high-molecular silicone fluids. Monofunctional chlorotrimethylsilane is always the end group of each chain. The resultant silicone fluids are largely chemically inert. They are generally clear, colorless, hydrophobic and neutral liquids. Their molecular weights range from 162 to 160000 dalton (Da) and their viscosities lie between 0.65 mPa s and 1000000 mPa s.
The following table presents the most important physical properties of a silicone fluid with a viscosity of 350 mPa s.

Molecular weight (number average)	Approx. 10,000 Da
Flash point	> 300 °C
Pour point	- 50 °C
Heat resistance	Up to 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 ·1015 W · cm
Surface tension	21 mN/m

A striking feature of silicone fluids is that their physical properties, such as thermal conductivity, viscosity, are not as temperature-dependent as those of mineral oils. The following logarithmic plot illustrates how the viscosity of different fluids varies with the temperature.

Over large ranges of molecular weights, the fluids are liquids because the forces between the individual methylsilicone chains are very weak. This is illustrated in the following table:

Viscosity [mPa s]	Molecular weight [Da]	Mean chain length w
0.65	162	0
10	750	10
100	5200	70
1000	15000	200
10000	37000	500
100000	74000	1030
1000000		2200

Other Physical Properties

Silicone fluids with a low viscosity have pour points of –50 °C. Those with a viscosity above 1,000 mPa s have an extremely low vapor pressure.
Low-viscosity silicone fluids have much lower boiling points than comparable carbon compounds of similar composition.

Silicone fluids with a viscosity of 100 mPa s and more have flash points above 300 °C and an auto-ignition temperature of more than 420 °C.

The compressibility of silicone fluids is much greater than that of mineral oils. The crucial factor, though, is that the viscosity changes much less under pressure than is the case for mineral oils.
Example: after 200000 pressure cycles lasting more than 500 hours, the viscosity of a silicone fluid will have changed by just 2 %, compared with a figure of 50 % for mineral oil.

The thermal conductivity of silicone fluids is much lower than that of aluminum, glass and water.

Silicone fluids can be chemically modified (copolymerized) to yield a large number of other silicone products. The most important product groups include: Copolymeric silicone fluids, silicone fluids with functional groups, silicone emulsions.

Copolymeric Silicone Fluids

The siloxane backbone is basically modified in two ways: Either long alkyl chains are substituted for the methyl groups or it is copolymerized with organic polymers. These methods allow the otherwise hydrophobic silicone fluids to be rendered largely hydrophilic. Polyethylene oxide (-CH₂CH₂O-)-) or polypropylene oxide units (-CH₂CH₂CH₂O-) may be used to accomplish this. This type of modification increases not only the solubility in water above a specific cloud point, but also the surfactant properties (emulsifying properties).

The following structural groups exist:

Some derivatives of cyclic siloxanes have a waxy consistency.

Silicone Fluids with Functional Groups

Siloxanes terminated with reactive end groups are called functional silicone fluids.
These include OH polymers (hydrolysate), H-siloxane, and silicone fluids with amino and epoxy groups.
They are obtained by hydrolysing the corresponding functional chlorosilanes. The aminosiloxane shown below is one of the most important functional silicone fluids.

On account of their high affinity for substrates, aminofunctional silicone fluids are mostly used in hair cosmetics, car polishes and in textile finishing. They also serve as crosslinking agents in the production of organic polymers.

Silicone fluids are frequently used in the form of aqueous emulsions. The emulsion form makes further dilution with water easier and leads to even distribution of small amounts of the substance on the substrates.

		When silicone fluids are emulsified with water, the type of emulsion is the same as that of milk: “oil in water” Stable distribution of the fluid droplets is ensured by coating their surfaces with surfactants (emulsifiers). The lipophilic ends of the emulsifier point towards the oil droplets and the hydrophilic centers confer solubility in water. This process lowers the surface tension of the oil droplet, with the emulsifier layer also serving to prevent the individual droplets from coalescing. The stability of emulsions depends heavily on the size of their particles. Emulsions are classified in terms of their particle size, as follows:

Type of emulsion	Particle size
Fine emulsion	Approx. 250 mm
Coarse emulsions	400 mm
Antifoam emulsions	4000 - 10000 mm

Finally, here is a table of the application areas of silicone fluids along with the properties needed:

Purpose	Application areas	Properties needed
Release agent	Demolding plastic parts, e.g. in the tire industry; fabrication of moldings, etc.	Heat resistance One thin coat to last for many release processes Prevents the polymers from sticking firmly to the equipment
Lubricant	Plastic bearings Sewing thread Wine corks Cutting tools Film	Reduction in surface friction Excellent slip properties Water repellency
Damping medium	Speed regulators Shock absorbing struts Fluid couplings Recording instruments Gyro compasses Nautical and aeronautical instruments	Physical properties remain virtually constant over temperatures ranging from ambient to 200 °C
Hydraulic fluid	Shock absorbers, Brake cylinders Pumps	Excellent viscosity-temperature response High compressibility and stability
Liquid dielectric	Coolants, Transformers Capacitors High-voltage tubes Space travel	Radiation resistance Electrical properties remain constant over a wide temperature range
Water-repellent agent	Glass Ceramics Coating materials Switches Insulators Textiles	Low surface tension High water repellency Not a nutrient source for fungi or bacteria
Antifoam agent	Foam prevention	High effectiveness in very small quantities Odorless and tasteless
Cosmetics	Protective skin creams Skin creams Hair-care agents Insect repellent	Non-toxicity Form a water-repellent protective film that allows the skin to breathe and does not irritate
Polishes additive	Automotive polishes Furniture polishes Shoe polishes Floor polishes	Gloss retention Water repellency Smoothing effect