Since dimethyl silicone oil can well represent the general characteristics of silicone oil, this article takes dimethyl silicone oil as an example.

1. Volatility and vapor pressure of silicone oil
Boiling point and molecular weight:
The boiling point of dimethyl silicone oil is related to its viscosity or molar mass. Low-viscosity silicone oil can be distilled and the boiling point can be determined at normal pressure, while high-viscosity silicone oil is not easy to boil even at low pressure and high temperature.

Vapor pressure curve:
For dimethyl silicone oil with lower viscosity, studies have determined its vapor pressure curve. The vapor pressure of these silicone oils at 25°C decreases with increasing viscosity.

Vapor pressure of methylphenyl silicone oil:
Methylphenyl silicone oils with low molar mass, such as tetramethyltetraphenyltrisiloxane and trimethylpentaphenyltrisiloxane, have very low vapor pressures at room temperature, 10-6Pa and 10-8Pa respectively, and are suitable as diffusion pump oils, and can achieve a high vacuum of 10-8Pa.

Volatility comparison:
Compared with general organic oils, silicone oil has very low volatility. Except for low-viscosity varieties, silicone oil without low-molecular-weight siloxane has almost no volatility.

Relationship between volatility and viscosity:
In the lower viscosity range, the volatility of dimethyl silicone oil decreases rapidly with the increase of viscosity. When the viscosity exceeds 100 mm²/s (25°C), the volatility remains basically constant.

Volatility and thermal oxidation stability:
Above a certain temperature, the volatility of silicone oil is not only affected by vapor pressure or evaporation, but also related to thermal oxidation stability.

2. Flash point and ignition point of silicone oil
Flash point:
Except for low-viscosity varieties, the flash point of dimethyl silicone oil is usually above 300°C. This means that at this temperature, silicone oil may flash instantly, but once the heat source is removed, the combustion will stop.

Autoignition point: The autoignition point of dimethyl silicone oil is about 450℃, that is, in the absence of an external fire source, silicone oil will burn automatically when it reaches this temperature.

3. Light transmittance of silicone oil
Dimethyl silicone oil with different viscosity grades, such as varieties with a viscosity of 50mm²/s and 500mm²/s at 25℃, can reach 100% light transmittance for visible light. This shows that dimethyl silicone oil has high transparency in optical applications.

4. Sound transmission characteristics of silicone oil

Sound velocity: The propagation speed of sound in dimethyl silicone oil (sound velocity) is similar to that of general organic liquids, but because its viscosity temperature changes less, it can maintain stable sound propagation characteristics over a wide temperature range.

Relationship between sound velocity and viscosity:
The sound velocity increases with the increase of dimethyl silicone oil viscosity. For example, the sound velocity of dimethyl silicone oil with a viscosity of 1000mm²/s at 25℃ is about 1km/s.