The emergence and impact of foam
Foaming of lubricants is a highly undesirable phenomenon. The strong mixing of the foam with the air enhances the oxidation reaction, causing cavitation and insufficient oil delivery in the circulation system, even leading to unlubrication. Such as internal combustion engine oil, hydraulic transmission oil, hydraulic transmission oil, gear oil tank and other circulation systems, because of the intense agitation, it is easy to produce foam. In addition to causing overflow or venting loss, the oil pressure is also reduced to affect lubrication. Especially when the oil level is low, a large amount of foam enters the oil passage, so that the supply of lubricating oil is reduced, the oil passage is blocked, the lubrication is interrupted, and the machine parts are burned out. When the hydraulic oil is foamed, the pressure of the hydraulic system will be unstable, and even the air resistance will stop working, and the precision of the machined parts will be affected on the machine tool. When the turbine oil is foamed, the oil circulation is not smooth, the oil supply is insufficient, and the lubricating oil is accelerated to deteriorate.
The possibility of foaming of the lubricating oil depends entirely on the lubricant itself, and is also subject to the impact of surface tension of the base oil, especially the surface active substances such as added antioxidants, detergents, extreme pressure agents, corrosion inhibitors and other ionic compounds etc. The foaming of the lubricating oil is also caused by the contamination of the lubricating oil, deterioration which make the surface tension lowered. Especially when the oil temperature is low, the viscosity is large, or it has been oxidized and decomposed, it is easier to foam. Lubricating oils with antioxidants or detergents, multi-effect additives, extreme pressure agents are more prone to foaming, while foams with elasticity and plasticity are particularly stable.
Identify surface foam and internal foam
The distinction between the so-called surface foam and the internal foam is very important for understanding the foaming effect.
The surface foam can be controlled with a defoamer. Effective defoamers have a surface tension lower than that of the lubricant base oil and are generally insoluble in the base oil. Therefore, in order to make it sufficiently stable even after long-term storage or use, the defoamer should be in a finely dispersed state. The particle size of the dispersed defoamer should be less than 100 μm, or even less than 10 μm.
Internal foam refers to finely divided bubbles within the lubricant that form a very stable dispersion. Ordinary antifoams are only used to control the surface foam and stabilize the tendency of the internal foam. Additives generally do not improve the air release properties of the lubricant. Conversely, multiple additives may also have a negative impact. Lubricants with excellent venting properties, such as turbine lubricants, should be formulated with specially selected base oils and additives.
Foam control and elimination
In order to prevent foaming, it is generally possible to add 1-100 ppm (0.0001 to 0.01%) of benzyl silicone oil or methyl silicone oil. The action form is that the silicone oil is adsorbed on the surface of the oil to prevent the occurrence of foam or to immerse the foam film to break the foam. Dimethyl silicone oil, dimethyl siloxane or tolyl silicone oil having a relatively large molecular weight is often used. Generally, the viscosity at 25 ° C is effective above 100 centistokes. If the viscosity is too high, it is not conducive to the dispersion of the defoamer. Methyl silicone oil of about 12500 centistokes is usually used. The surface tension needs to be defoamed below 21 dynes/cm (the surface tension of water is 72 dynes/cm, and the surface tension of petroleum lubricating oil is 30 to 50 dynes/cm). Silicone oil, which is small and easily soluble in petroleum, not only does not eliminate foaming, but will become a stabilizer for foam, so it is not suitable for use. The low-viscosity silicone oil tends to diffuse in the oil to cause defoaming, but its solubility is large and the defoaming durability is short. High-viscosity silicone oil has poor defoaming effect, but has good durability. In order to make up for short, it is common to mix two kinds of high-low viscosity silicone oil.
Since silicone oil is a non-polar oil, it is hardly soluble in petroleum or has a low solubility, and it can only defoam when it is in a diffused state. It must be made to form very fine particles (1 to 4 microns in diameter) dispersed in the oil to prevent settling failure. The smaller the particles, the better the diffusion, the less the sedimentation, and the better the defoaming effect. Generally, the silicone oil is first dispersed in kerosene to prepare a 1% strength silicone oil solution, and stirred and mixed at 93 ° C and a speed of 4800 rpm. The amount of the defoamer to be added needs to be defoamed when the oil is dissolved in the silicone oil. When it is less than the amount of dissolution, not only does it not defoam but it acts as a foaming action. Therefore, the amount added depends on the viscosity of the silicone oil and the solubility in the oil. It is preferable to use a high-viscosity silicone oil for a light lubricating oil, and a silicone oil having a low viscosity in a lubricating oil having a very low solubility to a silicone oil.
Post time: Nov-27-2019