As we all know, air bubbles and foam in lubricating oils can have adverse effects on both the oil itself and the equipment. However, during production, storage, transportation, and use, it is unavoidable that lubricating oils will contain some entrained air. Some of this air remains dissolved in the oil and does not manifest visibly, so no foam or bubbles are apparent. Other air, however, does not dissolve and instead exists as free gas, giving rise to the bubbles and foam that are readily visible to the naked eye. Air dissolved in the lubricating oil generally poses little risk; the primary concern stems from the free air—i.e., the bubbles and foam—that is present in the oil.

Table 1: Air in Lubricating Oil and Its Hazards

Air dissolved in lubricating oil:	Foam and air bubbles in lubricating oil:
Note: Lubricating oils can dissolve a certain amount of air; under normal conditions, mineral lubricating oils typically contain about 9% dissolved air.	Note: Air that has not been dissolved in the lubricant, i.e., free air.
Appearance: It dissolves in the lubricant and exhibits no visible appearance.	Appearance: Foam floats on the surface of the lubricant, while some remains trapped within the lubricant as air bubbles.
Causes:	Causes:
When lubricating oil comes into contact with ambient air, air naturally dissolves into the oil. The amount of air that can dissolve in the lubricating oil depends on temperature, pressure, and the oil’s intrinsic solubility for air.	This is generally caused by air entrainment in the lubricating oil, leading to bubble formation. Examples include agitation during machine operation, air being drawn into the suction line, leaks in the oil circuit or seals, and turbulent flow in the return line that stirs the oil and entrains air, among other factors.
Impact:	Impact:
Under normal circumstances, no adverse effects will occur. When temperature and pressure undergo significant changes, dissolved air escapes, causing bubbles to form in oil that was previously free of them.	Foam and bubbles can cause lubrication issues as well as equipment problems. Harm to lubricating oils: accelerates oxidative degradation; compromises the integrity of the lubricant film, leading to inadequate lubrication; increases the compressibility of the oil—e.g., in hydraulic fluids used for power transmission, increased compressibility can cause operational problems; bubbles in high-pressure zones are compressed, generating high temperatures that lead to thermal degradation of the oil (micro-dieseling); causes lubricant overflow; induces cavitation in equipment; reduces the cooling capacity of the lubricant; and impairs the stability of equipment operation, among other effects.

1. Hazards of Lubricant Foam:

Because foam can cause numerous adverse effects, the lubricating oil reservoir not only serves to store oil and dissipate heat, but also to release entrained air bubbles. A larger reservoir volume facilitates bubble release; moreover, sufficient spacing between the return line and the suction line, coupled with the installation of a baffle to separate the two, further promotes bubble release and the settling of contaminants. When necessary, a filter screen can also be installed between the return and suction zones to aid in bubble release. If air bubbles become entrained in the lubricating oil, they should rise to the oil surface as quickly as possible; otherwise, slow bubble release will lead to increased foaming. The factors that influence the rate of air release from the lubricating oil include the following:

Bubble size: Larger bubbles tend to escape easily, whereas small bubbles trapped within the lubricant are more difficult to release and can cause significant damage.

Lubricant viscosity: Lubricants with higher viscosity exhibit a slower air-release rate than those with lower viscosity.

Temperature: Low temperatures increase the viscosity of lubricating oil, thereby reducing the rate of bubble release.

Lubricant oxidation: Oxidative degradation of the lubricant involves the deterioration of the base oil and the depletion of additives, both of which can adversely affect the lubricant’s anti-foaming properties.

Lubricant contamination: A common example is water ingress, where water mixes into the lubricant. This reduces the lubricant’s surface tension, preventing large air bubbles from rising to the surface; instead, the bubbles break up into fine droplets that remain suspended within the lubricant. In addition to water, the presence of other oils or solvents in the lubricant can also impair its air-release properties.

2. Anti-foaming and air-release properties of lubricating oil:

Air entrained in lubricating oil exists in two forms: bubbles that remain dispersed within the oil and foam that floats on the oil surface. When bubbles rise to the surface, they coalesce to form foam. The time it takes for bubbles to reach the oil surface is referred to as the air-release property of the lubricant. For foam floating on the oil surface, the time required for the foam to rupture and dissipate is known as the foam characteristics, or anti-foaming property. The air-release property of a lubricant cannot be improved by anti-foaming additives, whereas the anti-foaming property can be enhanced through additive formulation. Consequently, silicone-based anti-foam additives or oil-soluble PAGs used for defoaming do not improve the air-release property of lubricants. On the contrary, excessive addition of anti-foaming agents can significantly reduce the air-release property, making it more difficult for entrained air bubbles to escape from the lubricant.

Table 2: Comparison of Anti-foaming and Air Release Properties of Lubricating Oils

Air Release Properties of Lubricating Oils	Antifoaming Properties of Lubricating Oils
Description: The time required for air bubbles within the lubricating oil to rise to the oil surface.	Note: The time required for the foam on the oil surface to rupture and disappear.
Design the oil tank’s circulation ratio or its dimensions appropriately so that the lubricating oil remains in the tank for longer than the time required for air to be released from the oil, thereby allowing sufficient time for air bubbles to escape and be discharged.	Foam should rupture and dissipate rapidly to prevent prolonged stability, which could lead to excessive foam accumulation, resulting in oil spillage and impaired heat dissipation.
Large bubbles rise quickly and are more likely to be released.	Small bubbles form a large bubble and then burst.
Bubbles rise more slowly in high-viscosity oil.	For lubricants with the same formulation, higher-viscosity oils are more likely to form stable foam, meaning the foam is more difficult to dissipate.
The use of additives does not improve air release.	Foam resistance can be improved by using antifoaming agents.
Hazards: Increased compressibility of the hydraulic fluid; under high pressure, the rapid compression of air bubbles generates high temperatures, leading to degradation of the lubricating oil; cavitation damage to components; and inadequate lubrication.	Hazard: Impairs heat dissipation and accelerates oxidation and degradation of the lubricating oil.

3. Causes of excessive foaming in lubricating oil:

The causes of excessive foaming in lubricating oil can be categorized into two types: equipment-related factors and intrinsic properties of the lubricant itself.

 

Lubricant-related causes include: high oil viscosity; reduced oil temperature (which also increases lubricant viscosity); contamination of the lubricant (by water, other oils, liquids, chemicals, or impurities); oxidative degradation of the lubricant (as the oil deteriorates with use); and depletion of additives—contamination can lead to premature additive consumption, and there is also the possibility that anti-foaming agents are removed by the filter element.

Causes attributable to the equipment include: air leaks, inadequate sealing, air entrainment in oil lines, air being stirred into the oil during mechanical operation, improper tank design, insufficient residence time of the oil in the tank, and excessively high splash velocity of the lubricant, among others.

In practical applications, excessive foaming is often caused by a combination of factors, necessitating a comprehensive analysis. When addressing excessive foaming in lubricating oils, the correct approach is to identify the root cause and eliminate it at the source, rather than blindly adding antifoam additives.

4. Air-release property test standard for lubricating oils:

The commonly used standards for the air-release property of lubricating oils include ASTM D3427, ISO 9120, and IP 313. These standards all employ the same test method—the impinger method. In the test, a specified volume of oil sample is subjected to a defined pressure and temperature while air is continuously bubbled through it, thereby saturating the lubricant with entrained air. The bubbling is then allowed to proceed until the air bubbles begin to dissipate; timing is stopped when the bubble volume stabilizes and no further dissipation occurs. The air-release property of the lubricant is defined as the time (in minutes) required for the bubble volume in the oil sample to decrease to 0.2% of its initial value—i.e., the time needed under specified conditions for the lubricant to release 99.8% of the entrained air. A shorter release time indicates better air-release performance.

Table 3: Air Release Requirements for Common Lubricating Oils

Types of Lubricating Oil	Lubricant Viscosity (ISO VG)
	32	46	68	100	150	Above 320
Turbine oil/turbine lubricating oil (including steam turbine oil, gas turbine oil, and hydro turbine oil) (as specified in DIN 51515 and ISO 8068).	5	5	6	-	-	-
Hydraulic Oil HLP/HM (According to DIN 51524/2 and ISO 11158)	5	10	13	21	32	-
Gear oil, circulating system oil CKC/CLP (According to DIN 51517/3 and ISO 12925/1)	-	-	-	-	-

The table above shows international requirements for the air-release properties of various lubricating oils. The air-release values listed in the table represent the minimum requirements; in practical applications, it is important to ensure that the lubricating oil’s air-release performance meets or exceeds these minimum standards.

       

Different types of equipment have varying requirements for the air-release properties of lubricating oils; some, such as hydraulic oils and steam-turbine oils, demand particularly high air-release performance, and international standards specify clear requirements for these oils. Once in service, air release is also an important test parameter. When testing the air-release properties of high-viscosity lubricants, the temperature is set at 75°C, which is higher than the standard testing temperature for most lubricants.

5. Test standard for the air-release property of lubricating oils:

Common standards for testing the anti-foaming properties of lubricating oils (i.e., the foaming characteristics of lubricating oils) include ASTM D892, ISO 6247, and IP 146. Under specified temperature and pressure conditions, air is drawn into the oil sample to saturate the lubricant with bubbles. As these bubbles rise to the oil surface, they form foam. After 5 minutes, the aeration is stopped, and the volume of foam formed is immediately recorded upon shutting off the air pump. Ten minutes after the pump is turned off, the foam volume is recorded again. The two recorded volumes are reported as the test results, which therefore consist of two numerical values representing the foam volumes. This test involves determining the anti-foaming properties of the lubricating oil at three different temperatures: first at 24°C, then at 93.5°C, and finally again at 24°C. Each lubricant’s anti-foaming test comprises these three sets of experiments, with identical procedures for each set, all conducted in accordance with the aforementioned test methods. Each set of experiments yields a result expressed as two numbers, corresponding to the foam volumes generated during the test. In Table 4, I, II, and III represent the foam characteristic values for the same lubricant measured at the three different temperatures, respectively.

Table 4: Basic Requirements for Antifoaming Properties of Common Industrial Lubricating Oils

Types of Lubricating Oil	Three sets of experiments for foam resistance testing	Foam Resistance Requirements for Lubricating Oils of Different Viscosities (ISO VG Viscosity Grades)
Turbine oil/turbine lubricating oil (including steam turbine oil, gas turbine oil, and hydro turbine oil) (as specified in DIN 51515 and ISO 8068).		32	46	68	100	150	>320
	I	450/0				-	-
	II	50/0
	III	450/0
Hydraulic Oil HLP/HM (According to DIN 51524/2 and ISO 11158)	I	150/0 75/0 150/0
	II
	III
Industrial gear oil; circulating system oil CKC/CLP (According to DIN 51517/3 and ISO 12925/1)	I	100/0; 100/10 100/0; 100/10 100/0; 100/10						150/60
	II
	III

The data in the table represent the basic requirements; the air-release property of industrial lubricating oils shall meet or exceed the values specified in the table.

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