Diffusion pump silicone oil refers to a liquid polydimethylphenylsiloxane with high phenyl content. It is colorless and transparent, chemically stable, capable of achieving high vacuum levels, and thus serves as an ideal liquid medium material. 
Basic Information
Silicone oil, high temperature; 
Chinese alternative names: methylphenyl silicone oil; poly(methylphenyl) organosilicone resin; 
CAS number 63148-58-3; 
Molecular formula: C11H16OSi, molecular weight: 192.33000; 
PSA: 9.23000; LogP: 1.73620; 
Colorless to pale yellow liquid, density 1.05, boiling point >140°C at 0.002 mm Hg (lit.), flash point 620°F, refractive index n20/D 1.5365 (lit.). 
Characteristics 
Colorless, transparent, oil-like substance with low saturated vapor pressure, offering excellent heat and cold resistance, oxidation and radiation resistance, chemical stability, and high vacuum capability, making it an ideal liquid medium material. 
Property indicators 
Appearance: Colorless or slightly yellow transparent liquid; 
Color: ≤40 degrees; 
Kinematic viscosity: (25°C) 165–185 cSt; 
Refractive index (nD25): 1.5765–1.5785; 
Flash point: (open cup method) ≥243°C; 
Pour point: ≤ -14°C. 
Purpose 
Primarily used in industries such as electronics, metallurgy, and instrumentation, it serves as high-vacuum diffusion pump oil, high-temperature heat transfer medium, and transmission fluid in instruments. 
Ultra-high vacuum diffusion pump silicone oil 
Ultra-high vacuum diffusion pump silicone oil (Type 275), also known as high-temperature silicone oil in Chinese, has the English name Silicone Oil and CAS number 63148-58-3. Its molecular formula is C11H16OSi, and it is a chemical intermediate. 
Synthesis Method 
Method 1: In a four-necked reaction flask equipped with a mechanical stirrer, thermometer, dropping funnel, and reflux condenser, add 0.5 mol of methylphenyl dimethoxysilane, 0.0125 mol of tetramethyl disiloxane (based on theoretical molar mass of 5600 g/mol), and 25 g of xylene. Stir thoroughly, then dissolve the catalyst in 0.85–1.1 mol of pure water and slowly drip the aqueous solution of the catalyst (hydrochloric acid or potassium hydroxide) into the mixture. Heat to 72°C and carry out hydrolysis for 2–4 hours. Then increase the temperature to 80–100°C, distill under atmospheric pressure, and allow the condensation reaction to proceed for 3–7 hours before stopping the reaction. Finally, remove water, filter, and under reduced pressure remove the solvent and low-molecular-weight byproducts to obtain a clear, viscous product—ultra-high vacuum diffusion pump silicone oil (Type 275)—with a yield of 85%. 
Method 2: Mix polydimethylsiloxane with fumed silica and roll-mill into a silicone grease, then emulsify by adding polyoxyethylene alcohol, Tween 80, and deionized water to produce ultra-high vacuum diffusion pump silicone oil (Type 275). 
Method 3: Octamethylcyclotetrasiloxane is obtained from dimethyldichlorosilane, water, and solvent through hydrolysis, neutralization, and distillation. Under the action of a catalyst, octamethylcyclotetrasiloxane undergoes polymerization to produce ultra-high vacuum diffusion pump silicone oil (Type 275). 
Method 4: There are mainly two approaches—acid-balanced and alkali-balanced methods. The acid-balanced method is suitable for producing low-viscosity silicone oils with kinematic viscosities below 500 mm²/s. This method typically uses concentrated sulfuric acid as a catalyst, conducts the reaction at 50–60°C, followed by water washing to remove acid, removal of low-boiling substances, decolorization using activated carbon, and filtration to obtain the final product. In contrast, the alkali-balanced method is appropriate for manufacturing medium- to high-viscosity silicone oils with kinematic viscosities above 100 mm²/s. Additionally, ultra-high vacuum diffusion pump silicone oil (Type 275) can be synthesized using dimethyl mixed cyclic siloxanes and decamethyltetrasiloxane as raw materials, tetramethylammonium hydroxide as a catalyst, and temperature programmed control between 90–120°C, increasing the conversion rate from an average of 65% to over 80%. 
Contributors to this entry: 
Shi Jiying - Associate Professor - Tianjin University