Testing the stability – ionic liquids in new anti-wear grease
Ionic liquids are gaining popularity as anti-wear additives in lubricants and greases, and their properties are proving particularly suitable for modern electric motors. But, what about their impact on the oxidation stability of the grease? Nynas’ technical advisor for lubrication, Jinxia Li, decided to find out.
Dr. Jinxia Li
Technical Advisor Lubricants, Nynas AB.
“The oxidation stability of greases with ionic liquids has not been tested so far, and therefore very little is known about if and how ionic liquids might affect the oxidation stability. So, this is what we wanted to investigate,” says Nynas Technical Advisor Dr. Jinxia Li.
The anti-wear and extreme pressure (AW/EP) performance of ionic liquids is well-established. Due to their miscibility in nonpolar hydrocarbon oils, and their ability to form tribofilms in metal-metal contact, ionic liquids can be utilised as a lubricant additive. The phosphorus-based ionic liquid additive can reduce wear between moving parts, and there is a growing interest in using them as additives also in lubricating grease.
In addition to their proven AW/EP properties, these ionic liquids demonstrate desirable properties such as high thermal stability and the ability to improve electrical conductivity. Importantly, ionic liquids also meet criteria for electric vehicle engines, such as being metal-/ halide-free.
Although the properties of ionic liquids have been known for some time, high costs have so far proved a prohibitive factor for their use. However, the current market shift towards electric vehicles has ignited a newfound interest in ionic liquids as additives in grease, particularly for lubricating electric vehicle motors and transmissions.
“Whereas recent studies have focused on certain parameters, we found that the oxidation stability has been a somewhat overlooked aspect,” says Jinxia Li.
In her lab, Jinxia Li tested the oxidation stability of a standard lithium complex grease based on the combination of one naphthenic and one synthetic base oil, to which 5% of two different phosphorus-based ionic liquids had been added, comparing these samples with the fresh additive-free grease.
“The results clearly show that at least these two ionic liquids do affect the oxidation stability of the grease in a negative way. The addition of the ionic liquids decreased the oxidation stability of the grease.”
To counteract this instability, a specially designed Anti-Oxidant (AO) package was added to the grease formulations.
“The AO package improved the oxidation stability, and it shows that grease formulations with ionic liquids can be tweaked for improved oxidation performance,” explains Jinxia Li.
Further studies need to be conducted, but what this study shows is that the addition of ionic liquid to grease can have a negative effect on its oxidation stability, and that this is a fact to keep in mind when formulating new anti-wear greases suitable for an electric future.
Standard system set-up
To test the oxidation stability of the grease samples, Jinxia Li used a highly specialised instrument called Quantum Oxidation tester, Figure 1. For solids such as grease, a special sample holder is utilised, Figure 4. The standard test method for grease, ASTM D942, describes the test parameter.
Grease samples, each weighing 4g, were placed on glass plates, positioned in the special sample holder, and then inserted into the test chamber, Figure 4. The test temperature was 99°C and the oxygen pressure measurement started after 2 hours, when the pressure had stabilised at 110 psi.
The time before the pressure drop occurs indicates the oxidation stability of the sample, see Figure 2. For the fresh grease, the pressure drop started around 12,000 minutes. When 5% of ionic liquid (IL 1) was added to the grease, the pressure dropped much earlier, well before 2,000 minutes. For the other sample with 5% of IL 2, the pressure dropped gradually after 2,000 minutes, but more slowly compared to the sample with IL 1. So clearly, the addition of either of the two ionic liquids has a negative influence on the oxidation stability properties of the grease.
Fortunately, the addition of the specially designed Anti-Oxidant package (1% AO) to the grease samples helped to counterbalance the oxidation instability brought by the ionic liquid. In Figure 3, the light blue line represents the fresh grease, and the three other lines show fresh grease with Anti-Oxidant, and fresh grease with both Anti-Oxidant and either of the two ionic liquids.
The stabilising effect of the AO package was also clearly demonstrated as the testing time was extended beyond 6,000 minutes (100 hours). After 12,000 minutes (200 hours), the drop in pressure, which indicates the progress of oxidation in the samples, was significantly smaller in the samples with added ionic liquid and AO package than in the fresh additive- free grease sample.
