Air is one of a lubricant’s biggest enemies, yet formulators are faced with the harsh fact that the sliding, pumping and spinning motion of the parts that their fluids are designed to protect enhances the chance of air being incorporated into the fluid. Add to that the aerating effects of spraying or splashing that is designed to get lubricant where it is required, and the job of the formulator can be very difficult. Trevor Gauntlett looks at some recent changes in our understanding of some of the fundamental properties of air in lubricants.
Gas is usually bad news for a lubricant. It doesn’t always have to be air (think gas compressors, for example), but for most lubrication applications the ambient gas is air. Therefore, air will be a convenient shorthand in this article. Air has three significant interactions with a lubricant. It can form foam on the surface, be entrained in (usually smaller) air bubbles in the bulk, or it can dissolve. Most of the time it does all three. The effects can then be split into physical and chemical (or biochemical).
As air has negligible viscosity, Sir Isaac Newton explained many years ago that it cannot hold two sliding surfaces apart. Therefore, if bubbles (foam or entrained air) migrate into a contact zone in any load-bearing application, the two surfaces are more likely to come into contact. If the application is a gearbox or a turbine, then that could result in a catastrophic and very expensive failure. In metalworking fluids, foam can prevent wetting of the surface by the fluid leading to inadequate cooling of the piece, which then leads to poor surface finish and reduced tool life.
Entrained air can enhance cavitation, which can also lead to catastrophic failures. Entrained air bubbles act as nuclei for dissolved air to come out of solution, forming a larger air bubble, which then collapses rapidly, and the resultant shock wave can damage metal parts. This is most prevalent in applications where the pressure changes rapidly, such as hydraulic pumps or gas compressors. (Note that in some applications, the more volatile components of the lubricant can form vapor bubbles that cause similar damage.)
When oxygen from the air dissolves in a lubricant, it enhances chemical (oxidation) or biological (microbial growth) degradation. Hot applications, such as crankcase lubricants, therefore, suffer from oxidative attack, while dissolved air in metalworking fluids facilitates microbial growth as soon as any water is present. Entrained air or foam bubbles can be a conduit for dissolving air, as they increase the surface area of the air/liquid interface.
“Foaming introduces a number of challenges for formulators and operators,” said Nicole Clarkson, global segment lead of metalworking fluids at Angus Chemical Company of Buffalo Grove, Illinois. Lubricants contain many surfactants that can stabilize foam bubbles, in addition to their primary function. Examples include the molecules that stabilize the base-carrying micelles in crankcase detergents or stabilize the oil emulsions in cutting fluids.
Foaming in aqueous systems, where it is often desired, has been thoroughly studied, but this is not the case for lubricants, where foaming is definitely not desired. A joint research project between Shell Global Solutions in Houston, Texas, and the Department of Chemical Engineering of Stanford University, California, recently published some findings in the Proceedings of the National Academy of Sciences that took a small step toward redressing those imbalances.
The technique deployed was dynamic fluid-film interferometry, or DFI, which studies single bubbles. The researchers first demonstrated for the fluids under consideration that the foam stability results correlated with frequently used bulk foaming tests, such as ASTM D892. Then, from the DFI observations, they obtained an unexpected description of how lubricant foam bubbles are stabilized in multicomponent lubricants. As the more volatile components evaporate, so the less volatile and slightly higher surface tension components are drawn to the bubble surface and stabilize the bubble.
The Stanford-Shell work was published in 2018. “This revealed new information about bubble stabilization, which gave hints to the reason why fluids such as PAOs have relatively good foaming performance,” said Ernest Galgoci, IFL technology director of the U.S. division of Münzing Chemie, a German company with a large portfolio of anti-foam additives.
Getting Rid of Foam
The stability of a foam is dependent on the surface tension of the fluid, so the means of preventing (antifoam) or collapsing (defoamer) foams has been traditionally to use fluids with lower surface tension than the base fluid.
“The options open to a lubricants formulator are very limited. Mineral oil can be effective as an antifoam in aqueous systems, but the surface tension of the base fluid in a lubricant is lower, so the formulator is left with polydimethylsiloxanes [PDMS] or polyacrylates,” Galgoci said.
Clarkson explained the compromises that often must be made when trying to combat foam. Formulators may change the emulsifier package or even the base oil content used in the metalworking fluid, but “these changes often create a larger impact to overall fluid performance with regard to corrosion control and staining, fluid longevity, lubrication and others,” she said.
According to Galgoci, “PDMS is the more effective antifoaming chemistry, but it is not suitable for all applications, so polyacrylates are sometimes used.” The applications include anything where silicon is not allowed, such as some engine oils or the nuclear industry. “Silicones can enhance the ability of a base oil to entrain and dissolve air,” said Galgoci.
This gives formulators a problem.
The Air Release Issue
Whereas foaming in a lubricant can be controlled or at least reduced by use of a very small chemistry set, it has long been thought that formulators can do little to affect air release positively. And when your regular antifoam additive can make air release worse, there is a problem.
Good air release is important in hydraulic fluids, as original equipment manufacturers are tending to decrease reservoir sizes. Consequently, there is very little time in the reservoir for the entrained air bubbles to rise to the surface before the fluid returns to being churned in the pump.
Another fundamental study by Shell – this time with the Milwaukee School of Engineering – examined the characteristics of a fluid that influence air release. The team aerated hydraulic fluids before they entered or returned to the reservoir of a Bosch-Rexroth A10VSO axial piston pump, which is commonly used in off-highway, construction and material handling equipment. They found that hydraulic fluids based on API Group I and Group II base oils released air more slowly, stabilized small air bubbles, produced much more noise in the pump and caused the pump to be less efficient than fluids based on Shell gas-to-liquids base stock or polyalphaolefin. This property was linked by the authors to the narrower distribution of molecular sizes in GTL and PAO.
Both Shell teams approaching similar but related problems found that bubble stabilization due to a wide range of molecular sizes contributed to poor performance in both foaming and air release.
Forever Blowing Bubbles
Are there finally opportunities for formulators to improve both foaming and air release in their formulations? Perhaps.
The patent literature contains claims that simple linear paraffins or PAO can improve the air release of fully formulated lubricants based on Group I or Group II base stocks, and that GTL contributes to superior air release. These offer the possibility of additional formulating space to address foaming by use of traditional antifoams. But there may also be another way forward.
In a presentation last year at the annual STLE meeting, Galgoci and colleagues showed that hybrid antifoams can disrupt foam bubbles without stabilizing entrained air bubbles in the bulk.
“Hybrid antifoam technologies based on combinations of polyacrylates and silicones are the way forward for applications where both air release and low foaming are required,” said Galgoci. He also drew attention to the synergistic effects. “It’s all about achieving balance, but it is possible to formulate combinations of silicones and polyacrylates that perform better in both air release and foaming tests than the individual components.”