In modern lubricating oils, whether engine lubricating oils, industrial lubricating oils or transmission lubricating oils all contain a certain amount of additives. During the use of oil products, due to reasons such as the transportation of oil products or the intense vibration of oil and machine parts, oil products tend to generate foam. The formation of stable foam will cause extremely adverse consequences. Not only will it cause waste and loss of lubricating oil, but also reduce the performance of oil products and cause wear and tear of machines. In order to prevent lubricating oil from generating foam or eliminate the generated foam as soon as possible, anti-foam additives are used in lubricating oil. This article briefly introduces the action mechanism, types and selection of anti-foam additives used in lubricating oil.
The defoaming mechanism of defoamers
Oil foaming can be divided into two aspects. One is surface foaming, which can usually be controlled by anti-foaming agents; the other is internal foam of oil. Internal foam is not easily improved by anti-foaming agents, and anti-foaming agents that are effective for surface foam may instead make the internal foam of oil more stable. For oils that require excellent defoaming performance, base oils and additives need to be selected and blended. The action mechanism of anti-foaming agents is relatively complex and there are different opinions. The representative views include three viewpoints: reducing partial surface tension, expansion and penetration.

1. Reduce surface tension
   This action mechanism is achieved by reducing surface tension. This view holds that the surface tension of anti-foaming agents is smaller than that of foaming liquids. When anti-foaming agents come into contact with foam films, the surface tension of the foam films is locally reduced while the rest remains unchanged. The stronger tension of the foam film pulls the weaker tension part, thereby causing the foam to burst. However, this mechanism is limited to a local position. Because such substances are insoluble in water, their diffusivity is poor and they do not have much effect on the surrounding foam. When the local surface tension of the foam is reduced, under the action of this force, it slowly spreads to the surroundings and finally forms a rupture.
2. Expansion
   This view holds that the droplets of anti-foaming agents invade into the foam film and make it a part of the film, and then expand on the film, which can effectively inhibit the diffusion of active agents and make it difficult for them to exert their elastic ability, thereby eliminating surface foam.
3. Penetration
   This view holds that the function of anti-foaming agents is to increase the air permeability of the bubble wall, thereby accelerating the merging of bubbles and reducing the strength and elasticity of the foam wall to achieve the purpose of breaking bubbles.