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Bursting Bubbles


Bubbles, froth or foam are fine in a steaming bathtub or frosted mug, but not in metalworking fluids or other lubricants. Foam is formed when gas is trapped in pockets in a liquid or solid. It is easy to recognize foam, but it can be difficult to prevent its formation and knock it down in metalworking fluids and other lubricants in the field.

Many essential raw materials in metalworking fluids, including emulsifiers, detergents and corrosion inhibitors, can contribute to foaming, as can typical operating conditions like agitation and shear. Antifoam additives or defoamer can be effective, but performance depends on the defoamers chemistry, concentration and interaction with other raw materials and contaminants; base oil chemistry; operating conditions and even the method of incorporating a defoamer when blending lubricants.

How can formulators and engineers make sense of a confusing welter of laboratory test data and chemical details? Several technical presentations addressed this question at the Society of Tribologists and Lubrication Engineers annual meeting in Atlanta in May.

Foam Foiling Fundamentals

Kalman Koczo and Mark Leatherman of Momentive Performance Materials in Tarrytown, New York, and Kevin Hughes and Don Knobloch of Wickliffe, Ohio-based Lubrizol gave a pair of joint presentations during the May gathering, on some of the fundamentals of foaming and mechanisms of antifoaming action.

They explained that the lubricants industrys shift from API Group I to more highly refined and synthetic base oils has motivated modification of antifoam additives to suit newer base oils. Hotter and faster operating conditions, as well as smaller lubricant volumes used in applications, have necessitated improvements in antifoam durability. The presenters researched the fundamentals of foam formation and stability in order to gain insights to support development of antifoam additives.

Effective antifoam additives break thin liquid films around gas bubbles. The critical conditions for antifoaming performance are that the antifoam additive has lower surface tension than and limited solubility in the base fluid. This limited solubility enables the formation of small antifoam droplets. The presenters compared their observations of foam in oil based formulations with established knowledge of foam in water based systems. They attributed foaming behavior differences between the two types of fluids to two key factors: The bulk viscosity of typical lubricating oils tends to be much higher than water, and the surface tension of typical base oils tends to be lower than water.

One of their most important findings revealed that the method of blending an antifoam additive into a lubricant batch affects defoaming performance. Data from ASTM D892 Sequence II (Standard Test Method for Foaming Characteristics of Lubricating Oils) for one antifoam additive revealed a correlation between smaller size distribution of antifoam droplets-from higher shear mixing-and more effective defoaming.

Testing Performance

In another presentation, Justin D. Mykietyn, manager of industrial fluid applications at Munzing North America in Bloomfield, New Jersey, reminded his audience that excess foam during operations using aqueous metal removal fluids can lead to unsafe working conditions, loss of fluid, insufficient lubrication and cooling, and shorter tool life. He strongly recommended performing foam tests in the laboratory before evaluating any metalworking fluid formulation in field trials.

There is no objective standard for the acceptable foam tendency of an aqueous metal removal fluid, as every operation has different needs, Mykietyn continued. For this reason, it is important to always include a point of reference in foam testing for relative comparisons.

In principle, tests for foaming tendency should accurately model end-use applications. The four most widely used methods are lab scale shake, blender, air sparge and recirculation tests-in order of increasing accuracy relative to field applications but decreasing convenience, ease of use and conservation of test material.

Mykietyn cautioned that these tests measure different aspects of foaming tendency.

In shake and blender tests, samples briefly undergo agitation at relatively low or high mechanical shear, respectively. After a specified time interval of relaxation, foam volume is measured visually. Data provide insight into how fast foam collapses after formation.

Mykietyn tested four metalworking fluids. He reported, The sample which provides the fastest foam break in shake testing is not necessarily the sample that provides the lowest foam profile [foam height or volume versus time] in the field.

In the field, fluid gets recirculated and reused, he explained. It is very important that the foam tendency of the fluid be low at the beginning of operations and stay low over a long period of time. Persistence is the term often used to refer to the foam profile over prolonged exposure to shear.

In sparge tests, air is forced through a porous diffuser immersed in the sample. Prolonged air sparge tests provide some measure of persistence. However, Mykietyn observed that bubbles were larger in sparge tests than shake and blender tests. The mechanism for bubble formation could influence test results.

To measure foam persistence, Mykietyn recommended recirculation testing. Fish tank, CNOMO (Afnor NFT 60-185) and small-scale peristaltic tests use a pump to circulate a fluid sample through a loop of tubing and a graduated cylinder or other container. Sample flow rate, nozzle type, in-line filtration and temperature can be controlled. Options for components and duration of recirculation tests can be an advantage, but the equipment for these tests is more costly than shake, blender and sparge tests.

Mykietyn was surprised to observe that some of his samples with fast foam break in shake testing had poor persistence performance-the foam lasted a long time-in recirculation tests. In one case (Figure 1, sample 3), he observed almost complete knock-down of foam only 15 seconds after a manual shake test but relatively stubborn foam during recirculation testing.

When evaluating foam tendency of aqueous metal removal fluids, he concluded, shake, air sparge and blender tests are useful screening tools or quality control tests because they give some insight into foam tendency and are quick to run.

However, to best understand the persistence of an aqueous metal removal fluid and correlate with field observations, some form of recirculation testing is important.

Automating Evaluation

Aaron Mendez, director of research and applications at Ayalytical Instruments in Chicago, has devised a way to remove human error from some foam tests.

Mendez reminded the audience in Atlanta that entrained air can reduce lubricant viscosity and contribute to oxidation reactions in addition to causing foam. Two standard tests, ASTM D892 and ASTM D6082 (Standard Test Method for High Temperature Foaming Characteristics of Lubricating Oils), use an air sparge to determine the ability of a lubricant sample to resist foam formation as well as dissipate foam at several temperatures. Both tests rely upon measurements of foam height in graduated cylinders. According to Mendez, visual observation of foam height used in these methods can be operator-biased and prone to human errors that can affect repeatability and reproducibility of data.

Mendez reported that Ayalyticals new instrument, foam digital detection imaging or FoamDDI, automatically performs ASTM D892 and D6082 procedures. The instrument uses standardized lighting, a high-resolution camera and a patent-pending vision algorithm to measure foam height. Automation is intended to reduce sample handling errors and improve throughput for product development and quality control testing.

Mendez explained that the sensitivity of the instruments optics is advantageous for observing and accurately measuring foam. Photographic images show both dynamic and stationary foam, that is, dispersed bubbles moving from the diffuser positioned at the bottom of the graduated cylinder through the liquid and the static foam layer at the sample surface, as well as the profile of the sample-atmosphere interface. FoamDDI shows potential for use in fundamental studies of bubble formation, foam kinetics, defoamer performance and related phenomena, he suggested.

Filtration Effects

Ernest C. Galgoci, Munzing North Americas industrial fluids defoamer technology director, added in-line filtration downstream after the peristaltic pump in Mykietyns recirculating test apparatus. Galgoci applied this test to compare defoamer chemistries with filtration media and pore sizes.

Galgocis results can help formulators choose defoamer technology that will perform best when filtration is used. They also provide help to formulators and end users selecting filter media that have the least impact upon defoaming performance.

First, Galgoci examined foam profiles, or graphs of the volume in graduated cylinders as a function of time during recirculation tests. He recalled that it was difficult to compare the graphs due to the large volume of data, so he developed the concept of integrated foam volume. Integrated foam volume was introduced as a means to readily compare the differences between the curves on the graphs representing foam volume measured during the course of each test, he explained. The concept is to calculate the time integral of the foam volume to obtain a value (IFV), which can be used to estimate and compare the differences in performance.

Next, he added defoamers to a fresh, semi-synthetic metalworking fluid concentrate at dosages optimized for defoaming performance (0.2-0.4 percent by weight). He prepared solutions of 5 percent concentrate in deionized water and circulated each solution for 120 minutes.

Galgoci compared three types of silicone based defoamers: 3D siloxane (three-dimensional, crosslinked siloxane polymer networks); polydimethylsiloxane linear polymers (PDMS); and organo-modified branched siloxane polymers (OMS).

For control fluids without defoamer, maximum IFV was 83.7 liter-minutes (no filtration). Three 3D siloxane defoamers and an OMS defoamer were highly effective and lowered IFV by 4 to 5 liter-minutes.

Next, Galgoci installed a filter in the test apparatus. Nylon filters of 10- and 30-micron pore sizes had minimal impact on IFV for four defoamers.

In contrast, polypropylene filters had a significantly negative effect on IFV for several defoamers. Galgoci noted that this result was consistent with anecdotes from the field about negative effects of polypropylene filters on defoaming. He hypothesized that nylon filters had less effect on defoaming performance than polypropylene filters because nylon is a more polar material than polypropylene, for which the non-polar defoamers would have greater affinity.

Galgoci reported that performance of one particular defoamer was superior in his laboratory work. In his study, filtration had less effect on this polyglycol based 3D siloxane than OMS defoamers. The top performers IFV was less than or equal to 10 liter-minutes without a filter and with nylon, polypropylene, polycarbonate and hydrophobically modified polycarbonate filters (all with 10-micron pore size).

Mary Moon, Ph.D., is a chemist with hands-on R&D and management experience formulating, testing and manufacturing lubricating oils and greases and specialty chemicals. She is skilled in industrial applications of tribology, electrochemistry and spectroscopy. Contact her at or (267) 567-7234.

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