Silicone Oils

organosiloxane oligomers or polymers of moderate molecular weight that retain their fluidity throughout a wide temperature range. The most widely used silicone oils are macromolecules with linear (I) or branched (II) structures and capped chain ends, most frequently polydimethylsiloxanes (R = R’ = CH₃), polydiethylsiloxanes (R = R’ = C₂H₅), and polymethylphenylsiloxanes (R = CH₃, R’ = C₆H₅) with molecular weights from several hundred to 30,000.

Silicone oils resemble mineral oils in appearance. They have very valuable properties: hydrophobicity, high compressibility, physical and chemical inertness, relatively small changes in viscosity with temperature, and resistance to high temperatures, even in oxidative mediums.

The coefficient of adiabatic compressibility at 30°C for polydimethylsiloxanes with viscosities of 0.65 and 50 mm²/ sec, or centistokes, are 1.74 × 10^-9 sq m per newton (m²/N), or 1.74 × 10^-10 cm²/dyne, and 1.09 × 10^-9 m²/N, or 1.09X10^-10 cm²/dyne, respectively. For ethylene glycol the value is 0.33 × 10^-9 m²/N, or 0.33 × 10^-10 cm²/dyne.) The viscosity of silicone oils increases appreciably upon compression. They have excellent dielectric properties.

Polydimethylsiloxane oils do not change noticeably upon heating in air up to 175°C; oxidation starts at 200°C. Certain elements (copper, lead, selenium, and tellurium) catalyze the splitting up of the siloxane. Thermal degradation in an inert atmosphere becomes noticeable only above 250°C. Polymethyl-phenylsiloxanes begin to decompose at 250°C in air but only at 300°C in an inert atmosphere.

Silicone oils are synthesized by the same methods as the other polyorganosiloxanes.

Silicone oils are frequently used for water-repellency treatment of glass, ceramics, fabrics, paper, and other materials. They are also used in hydraulic drives and hydraulic clutches; the low viscosity of polydimethylsiloxanes makes possible reduction of the total weight of the hydraulic system by one-half and a decrease in the hydraulic line diameter. High-viscosity silicone oils are used in various damping devices. The high compressibility of organosiloxanes makes possible the production of “liquid springs.” Many silicone oils serve as lubricants or the bases for greases, frequently in combination with petroleum lubricants or synthetic organic lubricating oils. Such lubricants are superior to petroleum lubricants in the stability of their rheological properties over a wide temperature interval.

Silicon oils are often used as liquid dielectrics in transformers, capacitors, and certain radio-electronic parts. They may also be used as foam fire-extinguishing agents, as antiadhesive molding oils, and as fluids for high-vacuum diffusion pumps. They are also used in creams, lotions, and ointments.