A Brief Discussion on Lubricating Oil Antifoaming Agent
Modern lubricants, whether engine lubricants, industrial lubricants or transmission lubricants, all contain a certain amount of additives. During the use of the oil, due to the transportation of the oil or the violent vibration of the oil and machine parts, the oil will tend to generate foam, and the formation of stable foam will cause extremely adverse consequences, not only causing waste and loss of lubricants, but also reducing the performance of the oil and causing wear of the machine. In order to prevent the lubricant from generating foam or eliminate the generated foam as soon as possible, anti-foam additives are used in the lubricant. This article briefly introduces the mechanism, types and selection of anti-foam additives used in lubricants.
1. Defoaming mechanism of antifoaming agent
Oil foaming can be divided into two aspects. One is surface foaming, which can usually be controlled by antifoaming agents. The other is the foam inside the oil. The internal foam is not easy to be improved by antifoaming agents, and the antifoaming agents that are effective on surface foam may make the foam inside the oil more stable. For oils that require excellent defoaming performance, base oils and additives need to be selected for blending. The mechanism of action of antifoaming agents is relatively complex, and there are different opinions. The representative views are reducing partial surface tension, expansion and penetration.
①Reduce surface tension
This mechanism of action is achieved by reducing surface tension. This view holds that the surface tension of the anti-foaming agent is smaller than that of the foaming liquid. When the anti-foaming agent comes into contact with the bubble film, the surface tension of the bubble film is partially reduced while the rest remains unchanged. The strong tension of the bubble film pulls the weaker tension part, thus causing the foam to burst. However, this mechanism is limited to a local position. Because this type of substance is insoluble in water, its diffusion force is poor and it does not have much effect on the surrounding foam. When the local tension on the foam surface is reduced, it slowly spreads to the surroundings under the action of this force and finally breaks.
②Expansion
This view holds that the antifoaming agent droplets penetrate into the bubble membrane and become part of the membrane, and then expand on the membrane, which can effectively inhibit the diffusion of the active agent, making it difficult for it to exert its elastic ability, thereby eliminating surface foam.
③Penetration
This view holds that the role of antifoaming agents is to increase the permeability of the bubble wall to air, thereby accelerating the merging of foam, reducing the strength and elasticity of the bubble membrane wall, and achieving the purpose of breaking the bubble.
2、Common types of antifoaming agents
In the process of industrial production, many harmful foams will be generated, and antifoaming agents need to be added. There are many types of antifoaming agents, such as organic siloxane, polyether, silicon and ether grafting, amine, imine and amide, which have the characteristics of faster defoaming speed, longer defoaming time, wider range of applicable media, and even harsh media environments such as high temperature, strong acid and strong alkali.
①Silicon
The most commonly used is polydimethylsiloxane, also known as dimethyl silicone oil. Pure polydimethylsiloxane is difficult to be used as an antifoaming agent without dispersion treatment. It has the characteristics of low surface energy and low surface tension, so it can effectively inhibit the generation of foam, reduce machine wear and extend the service life of oil products. Silicone polymer antifoaming agent does not dissolve in oil. It is distributed in the oil through a highly dispersed and stable colloid state. For example, in acidic oil products, as time goes by, silicone-type antifoaming agents will become unstable and settle, and accumulation will cause the defoaming performance to fail. The mechanism of action of silicone polymer antifoaming agents is to change the surface tension between liquid and air, making it difficult to generate bubbles. Therefore, oil products containing silicone polymer antifoaming agents have poor air release. The amount of silicone-type antifoaming agent added is usually 2-20ppm.
②Non-silicone polymers
In acidic oils such as turbine oil and hydraulic oil, silicone polymers will lose their defoaming properties after long-term use. Non-silicone polymer antifoaming agents can be used in such situations. The most widely used non-silicone polymer lubricant antifoaming agents are acrylates and alkyl acrylic acid copolymers. They have good solubility in mineral oil, are not sensitive to blending technology, and are used in very small amounts of 0.001%~0.05% (mass fraction). The antifoaming properties of this type of antifoaming agent are greatly affected by the existing surfactants in the system. The effect of the antifoaming agent depends on the entire oil system. Therefore, the same surfactant has the ability to reduce foam in one oil system, but may have no antifoaming properties in another case. In some cases, not only does it have no antifoaming properties, but it promotes the generation of foam in the oil. Special attention should be paid to this situation when detergents are present in the lubricating oil system. Compared with silicone-type antifoaming agents, this type of antifoaming agent can effectively improve the air release of oil. Because the mechanism of action of this type of non-silicone polymer antifoaming agent is to partially replace the surfactant in the original bubble film, thereby changing the intermolecular attraction of the film layer, reducing the film strength, or reducing the toughness to reduce the stability of the foam, thereby achieving the purpose of defoaming. Therefore, the air in the oil containing this type of non-silicone polymer antifoaming agent can be released in the form of bubbles, and it has good air release properties.
③Composite antifoaming agent
Due to the advantages and disadvantages of the above two types of antifoaming agents, in some cases, using them alone cannot meet the performance requirements of oil products. Therefore, composite antifoaming agents have been developed to increase the antifoaming effect and improve the stability of antifoaming agents through compounding. Most composite antifoaming agents are a combination of the above two types of antifoaming agents to give full play to their respective advantages and meet the performance requirements of oil products. In addition, there are also composites between non-silicon antifoaming agents.
3、Selection of antifoaming agents
The principle of selecting the type of antifoaming agent depends on the composition of the lubricant and the way the oil is used. The additives used in lubricants are mostly surfactants. A surfactant can have several functions. Therefore, in order to reduce and inhibit the generation of foam in lubricants, the types of other functional additives used in the lubricant formula should be considered. The interaction mechanism between surfactants in oil products is complex. In many cases, theoretical reasoning cannot solve the problem. In order to make the application of lubricant antifoaming agents have a good effect, it is necessary to conduct multi-faceted tests on the actual oil system. In addition, the study and use of multi-effect additives is the key to improving economic benefits and improving the quality of oil products, because the use of multi-effect additives can reduce the number of functional additives used while meeting the function of oil products. This can reduce the complexity of the lubricant system containing multiple surfactants and reduce the impact of uncertain factors on the stability of oil products. The stability of oil products is the key to determining their quality.
