Conventional fluids are the well-known general purpose silicones described in chemical notation as polydimethylsiloxanes. They are commercially produced in viscosities ranging from 0.65 to 20,000,000 cSt. Conventional silicone fluids are composed of polymer chains with unique flexibility. Polydimethylsiloxane has virtually no energy barrier for rotation. This results in one of the lowest glass transition temperatures of any polymer. The liquid surface tension of polydimethylsiloxane is lower than the critical surface tension of wetting (24 dynes/cm). This causes polymers to spread over their own adsorbed films. An important consequence of the low intermolecular forces in polysiloxanes is the highest permeability coefficients of any polymer for oxygen and nitrogen. The fluids are thermally stable indefinitely at 150 °C in air. Fluids with viscosities ≥ 50 cSt have negligible vapor pressure.
At viscosities > 1,000 cSt (correlating to molecular weights > 30,000), polymer chain entanglement occurs which results in leveling of physical property change vs. viscosity. Refractive index, surface tension, density, and viscosity-temperature coefficients are strikingly flat.
Polydimethylsiloxanes, trimethylsiloxy-terminated: Properties
|Product code||Viscosity (cSt)||Viscosity-Temperature Coeffecient||Pourpoint (°C)||Density||Refractive Index||Coefficient of Thermal Expansion x 104||Thermal Conductivity (cal/cm.sec x 104 °C)||Surface Tension||Dielectric Constant||Dielectric Strength||Flashpoint (°C)||Molecular Weight|
|Fluid Viscosity (cSt)||Velocity of sound (m/s) at 30 °C||Velocity of sound (m/s) at 50.7 °C|
|Dielectric Constant 102-106 Hz, 20 °C||2.44-2-2.76|
|Volume Resistivity||1x1015 ohm-cm at 20 °C|
|Pressure (psi)||Volume reduction of 100 cSt fluid|
Note: The straight portion of the slope corresponds to A.J. Barry’s relationship on molecular weights > 2,500: log μcSt = 1.00 + 0.0123M0.5
|Refractive index, 25 °C||1.397-1.404|
|Verdet constant of magnetic rotary power||16.2-16.9 x 10-3 mm/gm/cm|
While they exhibit low reactivity under many conditions, certain environments are destructive to silicone fluids. For example, hydrogen fluoride attacks the silicon-oxygen bond to produce dimethylsilyl fluorides and water, which generate corrosive gases. Strong bases such as methanolic potassium hydroxide destroy silicone fluids and create resinous byproducts.
Thermal degradation at elevated temperatures causes rearrangement of the silicon-oxygen bonds to product volatile byproducts. Free-radical reaction of the methyl groups to form crosslinked materials by oxidation with peroxy compounds increases fluid viscosity and causes the fluid to gel.
Methylene chloride, chlorofluorocarbons, ethyl ether, xylene, and methylethyl ketone are typical solvents for dimethylsiloxanes. Low viscosity polymers are also soluble in acetone, ethanol, dioxane, and dihexyladipate. They are insoluble in methanol, cyclohexanol, and ethylene glycol. The solubility parameter for 100 cSt fluid is 7.4.
The equilibrium water absorption of silicones is 100-200 ppm at 50-85% relative humidity. Drying of fluids is recommended for maximum performance in electrical applications. A typical drying protocol is to apply 1 mm vacuum for 1 hour, which typically reduces water levels below 25 ppm.
|Gas||mL gas/mL liquid at 25 °C|
|Gas||P* x 109|
values adjusted from filled silicone membranes
|Specific heat||0.35-0.37 cal/gm/°C|
|Heat of formation||-2.41 kcal/gm|
|Heat of combustion (>50 cSt)||6.13 kcal/gm|
|Glass transition temperature||-128 °C|
|Gel time, 150 °C||indefinite|
|Gel time for intermediate viscosity fluids, 200 °C||200 hours|
|Gel time for high viscosity fluids, 200 °C||100 hours|
|Autoignition temperature for fluids >10 cSt||> 460 °C|
At shear rates commonly encountered (≤ 104 s-1) polydimethylsiloxanes behave, at viscosities up to 1,000 cSt, like Newtonian fluids. Viscosity is constant and independent of the velocity gradient. Apparent viscosity is identical with viscosity extrapolated to zero velocity gradient.
For oils with viscosity > 1,000 cSt, this ratio is only constant for velocity gradients below a certain value. Beyond this value, becoming lower as the product becomes more viscous—the ratio is no longer constant: apparent viscosity falls below real viscosity (extrapolated for a zero velocity gradient) and the behavior is then known as “pseudoplastic.” This change is perfectly reversible, and behavior again becomes Newtonian when the velocity gradient falls once more below the critical value. Viscosity returns to its initial level even after intense shearing of long duration.
|Viscosity (cSt)||Critical velocity gradient (s-1)||Apparent viscosity for a velocity of 10,000 s-1 (in cSt)|
As a guide, the table indicates the “critical” velocity gradients for polydimethylsiloxanes (where change of rheological behavior occurs) as well as apparent viscosity measured at velocity gradient equal to 10,000 s-1
Volatile, low molecular weight components are present in polydimethylsiloxanes as a consequence of the equilibrium polymerization utilized in their manufacture. Typically, silicones with viscosities < 50 cSt have > 10% volatiles, while those with viscosities > 50 cSt have 0.5-4.0% volatiles. Low molecular weight components can impart undesirable effects in certain critical applications. These can cause outgassing, migration, bleed, plasticization, and stress-cracking in contact with certain plastics and rubbers. Devolatilized silicones are offered in two classes. Reduced volatility silicones have > 90% low molecular weight components removed and are generally acceptable for polymer contacting applications.
Extreme low volatility silicones have virtually zero volatiles and are suitable for extreme vacuum applications including systems deployed in space exploration and communication. An example of a space application is as a damping fluid for solar panels.
Low molecular weight silicones that possess a cyclic structure rather than a chain structure serve as volatile carriers for a variety of formulations. Low heats of vaporization and the ability to select a desired vapor pressure has led to their use as cosmetic vehicles. While most display a broad range of liquid behavior, the most volatile cyclic dimethylsiloxane (D3) is a solid at room temperature.
Volatile Cyclic Dimethylsiloxanes (Cyclomethicones)
|Product Code||Name||Viscosity (cSt)||Boiling point (°C)||Vapor pressure (25 °C, mm)||Heat of vaporization (Kcal/mole)||Density||Refractive index||Molecular weight|
|SIH6105.0||D3||solid, m.p. 65 °C||134||10||9.5||1.02||--||222.46|
Silicone emulsions are easy-to-use, water-dilutable, fine particle dispersions of conventional polydimethylsiloxane fluids. They are employed as release agents and lubricants in a variety of rubber and plastic applications including molding of mechanical rubber parts such as O-rings and footwear, producing shell molds and cores for metal casting, wire and cable extrusion, and conveyance devices in high-speed printing. They are usually diluted with water to a final solids concentration of 0.1-3.5% at the point of application.
Very high viscosity silicone fluids are difficult to apply as thin films. Solutions in volatile low viscosity silicones are easy to handle and facilitate film spread.
Dyes in silicone fluids provide coloration without compromising transparency. The fluids may be used directly in applications such as gauge fluids or as tint additives for silicone fluids and elastomers.