Lubricity of Silicone Oil

Sep 8, 2022

Silicone oil has very good hydrophobicity. The so-called hydrophobicity means that the water in the liquid phase only wets the silicone oil/air interface to a lesser extent. The size of the contact angle formed by the water droplets placed on the surface with the surface is a measure of the hydrophobicity of the surface. If the contact angle θ is greater than 90°, the hydrophobicity is better, and the contact angle θ is less than 90°, which indicates that the water is poor.

The surface hydrophobicity of glass and ceramics treated with simethicone with a viscosity (25°C) of 100mm2·S-1 can reach the level of paraffin wax, and the contact angle is between 90° and 100°. The treatment method is to immerse the degreased glass in a chloroform solution with a silicone oil mass fraction of 0.5%, take it out and let the solvent volatilize, and then heat it. When the heat treatment temperature is 100°C, the contact angle of the water droplets is about 60°; when the heat treatment temperature is 400°C, the maximum contact angle is 100°~110°; when the heat treatment temperature exceeds 400°, it will be decomposed by polydimethylsiloxane , The contact angle becomes smaller.
The hydrophobic treatment mechanism of simethicone is to make the siloxane couple point to the phase interface through heat treatment (350~400℃), and the free surface is covered by the methyl group in a tightly packed manner.

The property closely related to hydrophobicity is that simethicone has very good anti-sticking or demoulding properties for viscous materials. Like hydrophobicity, this property is a manifestation of the basic chemical properties of polyorganosiloxanes (such as paraffin-like nature, molecular saturation characteristics and corresponding small Van der Waals forces).

The foaming of dimethyl silicone oil is very low, and methyl phenyl silicone oil shows varying degrees of foaming.
Silicone oil causes a decrease in the surface tension of the solution in an organic solvent, which can enrich it on the surface of the liquid phase that can dissolve them, indicating that the silicone oil has surface activity. Silicone oil can emulsify in water, causing the surface tension of the water to decrease.
This is the reason why silicone oil can be used as a defoamer. Their mode of action depends on the concentration; if the concentration of silicone oil increases in an aqueous system, the defoaming effect is more pronounced.
According to theory, the defoaming agent must be as insoluble as possible in the defoaming medium; because when the total volume of the liquid absorbs less material, it is easier to reach a high surface concentration. In addition, the spreading pressure must be Positive value, in order to quickly form a closed film on the surface of the foam thin layer; and the foam inhibitor and the foaming agent in the surface film must not form a mixture. In an aqueous system, simethicone can meet all these requirements, because they have the smallest volume solubility in water, can spread, and have the characteristics of low miscibility of many organic and most inorganic substances.

Lubricity of silicone oil
The force between simethicone molecules is small, so the load performance of its oil film is low. In the boundary friction area, dimethyl silicone oil has no lubricity for the combination of steel and steel; but it has good lubricity for light-loaded rolling bearings. For example, it has the best lubrication performance for plastic bearings such as polyamide, polystyrene or phenolic resin; in the range of metal materials, it can also be obtained for the combination of steel and bronze, chromium, zinc or cadmium, or the combination of chromium and bronze. Very good result.
The introduction of phenyl groups can improve lubricity. Methyl phenyl silicone oil is suitable as a lubricant for bearings whose pressure is not too high. Further improvement in lubricity can be achieved by halogenating phenyl groups or aliphatic groups.
The lubricity of simethicone can be improved by various additives, for example, the addition of methyl ricinoleate can greatly improve the lubricity.