Ionic liquids: Part 2

Don Smolenski | TLT Machinery August 2021

In recent studies, ionic liquids show promise for industrial applications.
 

The thrust force, when the milling tool initially enters the workpiece, was not significantly improved with ILs. The overall average cutting force, however, was improved by one of the ILs compared to all other conditions and was similar to flood cooling.
As I discussed in the June 2021 TLT, ionic liquids (ILs) are a wide departure from typical lubricant additives and are comprised of organic cations such as ammonium-, phosphonium-, imidazolium- or pyridinium-based, with a large variety of anions. Oil-soluble ILs are being used as oil additives; my previous column discussed their application in engine oils. This column will investigate their use in industrial applications, such as hydraulic and cutting fluids.

One study1 reported on three ILs based on the 1-methyl 3-butylimidazolium cation applied as additives to vegetable oil in minimum quantity lubrication (MQL) milling machining of steel. In MQL machining, a very small amount of lubricant is applied to the cutting zone in atomized form, usually with a gas stream, and the lubricant is consumed. The experiments were conducted under different conditions, including dry cutting, dry cutting with an air jet, conventional flood cooling and MQL. In MQL machining, vegetable (canola) oil was used. The cutting tests were conducted with neat vegetable oil as the MQL fluid, and then repeated with three different ILs added to the vegetable oil at 1 wt. %.

The results show that the presence of even minute quantities of ILs can significantly affect the machining process through reduction in cutting forces and surface roughness of the machined workpiece, as compared to dry cutting, conventional flood-cooled cutting or MQL machining with neat vegetable oil. The thrust force, when the milling tool initially enters the workpiece, was not significantly improved with ILs. The overall average cutting force, however, was improved by one of the ILs compared to all other conditions and was similar to flood cooling. Surface roughness was better with all three ILs than other MQL conditions, and as good or better than with flood coolant.

In another study2 on the machining of aluminum, results showed friction reductions of more than 70%, improved surface finish and potential of extended tool life when ILs were used in water-based fluids in rig tests, with no actual machining performed. Overall, though, ILs showed enough promise to justify further study as metal-removal fluid additives.

There also has been some fundamental work done on the potential of ILs used neat as hydraulic fluids.3 Basic properties evaluated included flash/fire point, density, kinematic viscosity, viscosity index, corrosion resistance, corrosiveness to copper, demulsibility, foaming tendency, wear resistance, compressibility, foaming tendency, filterability, materials compatibility and others.

The results of the fundamental work (no actual testing in hydraulic applications was performed) suggested that ILs show promise as hydraulic fluids with some limitations. Corrosivity in the presence of moisture is a concern. Some ILs showed significant promise for use as hydraulic fluids owing to better properties than mineral oils. Compatibility with conventional paints and filter fibers are concerns, however. Probably the greatest limitation is the extremely high cost of ILs. More extensive testing would be required.

REFERENCES
1. Goindi, G., Chavan, S., Mandal, D., Sarkar, P., and Jayal, A. (2015), “Investigation of ionic liquids as novel metalworking fluids during minimum quantity lubrication machining of a plain carbon steel,” Procedia CIRP, 26. DOI: 10.1016/j.procir.2014.09.002.
2. Del Sol, I., Gámez, A.J., Rivero, A. and Iglesias, P. (2019), “Tribological performance of ionic liquids as additives of water-based cutting fluids,” Wear, 426-427, pp. 845-852. DOI: 10.1016/j.wear.2019.01.109.
3. Lovrec, D., Kalb, R. and Tic, V. (2020), “Basic aspects when using ionic liquids as a hydraulic fluid,” 12th International Fluid Power Conference, Dresden.
 
Don Smolenski is president of his own consultancy, Strategic Management of Oil, LLC, in St. Clair Shores, Mich. You can reach him at donald.smolenski@gmail.com.