Foam Challenges in Lubricating Oils
Industry Standards for Measuring Foam - ASTM Methods D892 and D6082
Savant Labs Author: Jennifer Richter, Laboratory Instrument Technician
Reducing foaming tendency is an important aspect of lubricating oil formulation and is a requirement in most modern engine oil specifications. Foam testing can also be an important tool in condition monitoring of in-service lubricants. When foam appears in lubricating oil, there is a good chance of contamination in the system. Lubricating oils always have some air content, and it is common to have up to about 12 percent of its volume attributed to dissolved air. In this state, there will be no visual presence of air and the oil will appear transparent. It does not become problematic until the air is no longer dissolved in the oil as it is heated quickly or there is a sudden drop in pressure. Once the bubbles are in the oil, they can cause damage in the form of oxidation, impaired cooling abilities, shortened oil life, reduced carrying capacity of the oil film, oil spills, decreased pump capacity, lack of lubrication, cavitation, micro dieseling, and additive depletion.
Contaminants Causing Foam in Lubricating Oil
Contaminants may include solids, oxidizing agents, water contamination, or solvents. These contaminants alter the speed at which air bubbles can travel through oil and arrive on the surface. The foam produced from the introduction of contaminants can cause an aging effect on the oil, which allows polar products to form, the viscosity to increase, and it can result in antifoaming properties to be filtered out. Some other sources of foam include the depletion of or too much antifoaming agents, mechanical issues, cross-contamination of lubricants, and grease contamination.
Effects of Foam in Your System
The foam acts as an insulator and reduces a lubricant's cooling abilities, resulting in overheating. Foam can also lead to poor performance, mechanical failure, costly repairs, and a shortened operability lifetime due to an inability to lubricate effectively or undergo cavitation. Therefore, for oil to have optimal performance, it must have the ability to separate the air that becomes suspended in it. Oils that can separate entrained air have improved performance and can be used in smaller systems where the oil circulates through much quicker.
Testing Foaming Characteristics
There are two common methods for measuring foaming characteristics. ASTM D892 Foaming covers sequences I-III and tests foaming tendency and stability at 24°C and 93.5°C. Both sequences I and III are completed at 24°C. Sequence III utilizes the cooled sample of oil that was tested in Sequence II at 93.5°C, whereas Sequence I uses a fresh sample. ASTM D6082 Foaming covers sequence IV and determines the foaming characteristics of lubricating oils at 150°C. ASTM D6082 was designed to reflect operations in high-speed gearing, high-volume pumping, and splash lubrication. These tests measure both the volume of foam formation and the stability (amount of foam left as a function of settling time).
If option A is selected for either method, a sample will be blended in a high-speed blender for one minute before testing. This option is intended for stored samples to ensure antifoam agents are of the intended particle size and uniformly dispersed. Both methods utilize a diffuser stone that is submerged in the test oil. Before introducing air into the sample, the initial volume of oil is recorded. Once testing commences, air will be forced through the diffusers at a constant rate. For ASTM D892 the rate is regulated at 94ml/min whereas for ASTM D6082 it is at 200ml/min. After allowing the air to flow through the diffusers for five minutes, the total volume and the volume of the top of the oil or kinetic foam is recorded. The difference between the total volume and the initial volume gives a value of the total increase in volume. The difference between the total volume and the top of the oil/kinetic foam gives the foaming tendency. The volume of the kinetic foam is recorded only for ASTM D6082. The sample is then observed for foam collapse for up to 10 minutes and testing is completed.
Savant also has available an additional article entitled “Why is ASTM D3427 Air Release different from ASTM D892 Foaming Characteristicstesting?” under Testing Highlights.
Kinetic Foam Versus Static Foam
Static foam is the foam that sits on the surface of the oil and is the type with which most people are familiar. Demonstrated in the left cylinder of this image is an oil with a high foaming tendency. As you can see, static foam constitutes nearly all of its volume. In comparison, the oil on the right has very little static foam. Kinetic foam is a little more difficult to observe. To observe kinetic foam, you have to see below the surface of the oil. Kinetic foam is the entrained air that has been created by the passage of air through the diffuser during the test; the visible layer of bubbles accumulated below the surface of the oil still migrating through the oil up to the surface. The right cylinder in this image demonstrates kinetic foam where there is air dispersed within the oil that has not yet made it to the surface. Kinetic foam is significant as it estimates the amount of air still trapped in the oil that will come up to the surface to become static foam.
Additional Test Methods
In addition to the foam testing mentioned above, Savant can
recommend additional testing when high foam results are found. For
example, ASTM D5185 Determination of Additive Elements, Wear Metals, and
Contaminants can help identify changes in additive concentrations or
elemental contaminants like iron or silicon. ISO 4406 Particle Count
may be useful in identifying suspended solids. ASTM D7414 FTIR Analysis
could be used to determine the oxidation levels and ASTM D6304 Water by
Karl Fischer can be used to determine water levels. It is also useful
to run ASTM D445 Kinematic Viscosity to assure it aligns with original
specified values for the lubricant.
Foam can have a detrimental effect on the life of the equipment. The test methods mentioned above are the industry standard for measuring foam and Savant's team of dedicated chemists and technicians will recommend the appropriate testing. In addition, the team is here to interpret and provide insights into the results. Contact us for your testing needs.
Because Quality Matters
Savant is positioned to offer some of the most precise data found in the industry to date. Regular instrument calibration and maintenance to meticulous reference values and control chart monitoring is part of the quality regimen. In addition, all test results undergo a stringent quality review process, data is delivered on time and reported via email in a logical format designed for easy interpretation.