Grease incompatibility — the potential for a mixture of greases to deteriorate compared to the performance of the individual products — has been an issue in the industry for decades. Incidence tends to increase, though, when end users are shifting the types that they use, since mixing occurs mostly when operators change the type used in a piece of equipment.
The issue is gaining importance, then, during this time when the industry is trending away from its most popular thickening agent, lithium. To investigate issues that end users might encounter, grease producer Eldon’s S.A., of Athens, Greece, conducted a study analyzing the compatibility with three types of potential replacement for lithium greases.
They did indeed find a significant amount of incompatibility, although results depended on the definition of compatibility consulted. The researchers concluded the industry should work to bolster its methodology for assessing compatibility.
When Greases Mix
Compared to the automotive segment, where performance specifications are usually upgraded every few years, users on the industrial side of the industry change products much less often.
“When it comes to lubrication in industrial plants, the use of the same products over a long period of time allows for smooth operation of equipment,” Eldon’s Chemical Engineer George S. Dodos said last January at the International Colloquium on Tribology in Esslingen, Germany. “Sometimes, though, conditions dictate a change from one lubricant to another.”
When greases are switched, it is difficult to do so without mixing, to some extent, the old with the new. “In the case of lubricating oils, it is often possible to change the oil completely,” Dodos said. “In grease-lubricated applications, on the other hand, it is usually impossible to completely remove the previous grease from the application when changing products. Grease and equipment manufacturers alike recognize … grease mixing will occur and at times cannot be avoided, despite all warnings to the contrary.”
What happens in such cases can be difficult to predict.
“Grease compatibility is a multi-complex issue that can occur due to interactions between the thickeners, the base oil or the additives,” Dodos said. He also noted that incompatibility can cause a variety of problems. The grease could harden or it could soften, in either case leading to inadequate lubrication. The oil can separate out of the grease, leaving the thickener to dry and harden and possibly then to inadequate lubrication and component failure.
“Such events will gradually give rise to loss of performance, increased wear, more frequent relubrication and maintenance and could even lead to catastrophic lubrication failures,” he said.
That’s not to say that users should not switch greases, just that they should be careful and consider the products’ compatibility. Dodos said the risk of problems can be reduced through practices such as transitioning the change gradually and testing the mixed grease during the transition. Still, he maintained it is best to know the compatibility of a grease being considered as a replacement, to assess if problems are likely or if different replacements should be considered.
It stands to reason that incidents of grease switching would rise as the popularity of lithium greases declines. That popularity peaked late last decade when products made with simple lithium and lithium complex thickeners together accounted for three-quarters of global production. Then lithium costs spiked because of competing demand from batteries used in electric vehicles, and end users and producers began looking for alternative thickeners. By 2024, the portion of global grease production containing either of the two lithium thickener systems fell to just 57%, according to the NLGI Lubricating Grease Production Survey of 2025. (See Fig. 1.)
Figure 1. Global Grease Production by Thickener Type
Source: 2024 NLGI Lubricating Grease Production Survey
* Undercounted because this category not broken out for China
Now the European Chemicals Agency has proposed declaring lithium a category 1A reproductive toxin — an act that would require hazardous chemical labeling for products containing more than threshold concentrations of the mineral, which could deter its use in at least some applications. Lithium also loses points with some grease producers and end users for its sustainability profile. Some lithium extraction methods use quite large amounts of water, and lithium has a much higher carbon footprint than some alternative thickener minerals, such as calcium.
The industry does have charts from a number of sources listing the compatibility or non-compatibility of various combinations of grease. Dodos, however, questioned the reliability of those charts.
“Since these charts will not report on the methodology used to create their ratings, end users are not aware of their limitations and the fact that they have been developed for general guidance only and not for any specific application,” he said.
Given the issues surrounding lithium, Dodos and former Eldon’s product certification and safety compliance specialist Nora Kaframani undertook to investigate the compatibility of lithium greases with potential replacements. They identified three types of thickeners as good replacement candidates:
• Calcium sulfonate greases, which provide outstanding water resistance and load carrying capacity, good protection from corrosion and high dropping points;
• Polyurea greases, which have oxidation stability, long service life, and performance in high-temperature environments;
• Aluminum complex greases, which have excellent resistance to water washout, good mechanical stability and moderate high-temperature performance.
To conduct the study, the researchers followed the methods of ASTM D6185, mixing a lithium complex grease with an example of each of the other types in concentrations of 10%, 25% and 50% (for the lithium grease), then testing the dropping point, roll stability and high-temperature storage stability of each mixture. (The roll stability test represented a modification to D6185, replacing a worked penetration test to evaluate shear stability.) The researchers extended the study by conducting two secondary tests: the ASTM D8602 rapid small-scale oxidation test; and the ASTM D2266 test for wear preventative characteristics.
To add perspective, the research team compared test results not only to limits prescribed by D6185 but also to other limits elsewhere in industry literature. (See Fig. 2.) The latter tended to be more lenient. Both the ASTM standard and literature define limits for compatibility, borderline compatibility and incompatibility.
Hinting at the potential for incompatibility, Dodos noted that the greases used for the study differed significantly in content and performance characteristics. The lithium complex, polyurea and calcium sulfonate grease were made with mineral oil base stocks, while the aluminum complex candidate blended mineral oil with polyalphaolefin. (See Fig. 3.) The base oil viscosity grade of the aluminum complex grease was 460, while the other three were 220. The thickener concentration of the lithium and aluminum greases was 12% compared to 10% for the polyurea and 30% for the calcium sulfonate.
Figure 3. Different Compatibility Methods, Different Conclusions
ASTM Methods | Lubricant Industry Literature |
Dropping Point | |
Critical parameter: ASTM D2265 repeatability | Critical parameter: Binary mixture predicted DP value |
Compatible: dropping point of mixture is at least equal to that of either grease | Compatible: dropping point no more than 40 C less than predicted value |
Borderline: dropping point less than lower of constituent greases by no more than repeatability of test method | Borderline: dropping point of mixture is 40-60 C less than predicted value |
Incompatible: dropping point less than lower of constituent greases by more than repeatability of test method | Incompatible: dropping point more than 60 C less than precited value |
Shear (Roll) Stability | |
Critical parameter: ASTM D1831 repeatability (1/10 mm): 11 | Critical parameter: Binary mixture predicted RS value |
Compatible: penetration of mixture equal to or between than values of constituents | Compatible: penetration of mixture no more than 30 points different than predicted |
Borderline: penetration of mixture less than lower of constituents or greater than the higher by no more than repeatability of test method | Borderline: penetration of mixture 31-45 points different than predicted |
Incompatible: penetration of mixture less than lower or greater than the higher by more than repeatability of test method | Incompatible: penetration of mixture more than 45 points different than predicted |
High-temperature Storage Stability | |
Critical parameter: ASTM D217 repeatability (1/10 mm): 7 | Critical parameter: Binary mixture predicted HTSS value |
Compatible: 60-stroke penetration change of mixture equal to or between those of constituents | Compatible: penetration of mixture 30 points or less different than predicted |
Borderline: 60-stroke penetration of mixture less than lower of constituents or greater than the higher by no more than repeatability of test method | Borderline: penetration of mixture 31-45 points different than predicted |
Incompatible: 60-stroke penetration of mixture less than lower of constituents or greater than the higher by more than repeatability of test method | Incompatible: penetration of mixture more than 45 points different than predicted |
Secondary Testing | |
Rapid small-scale oxidation testCritical parameter: ASTM D8602 repeatability | Wear prevention characteristicsCritical parameter: ASTM D2266 repeatability (mm): 0.12 |
Compatible: induction period of mixture equal to or between values for constituents | Compatible: wear scar of mixture equal to or between those of constituents |
Borderline: induction period of mixture greater than higher of constituents or less than lower by no more than repeatability of test method | Borderline: wear scar of mixture greater than higher of constituents or less than lower by no more than repeatability of test method |
Incompatible: induction period of mixture greater than higher of constituents or less than lower by more than repeatability of test method | Incompatible: wear scar of mixture greater than higher of constituents or less than lower by more than repeatability of test method |
The dropping point of the constituents ranged from 260 degrees C for the polyurea grease to more than 316 C for the calcium sulfonate. Oxidation stability, gauged in the RSSOT, ranged from 548 minutes for the lithium grease to 1,539 minutes for the one with aluminum complex thickener. High-temperature stability ranged from 22 millimeters for the polyurea to 345 for the calcium sulfonate. The calcium sulfonate grease showed the worst antiwear performance (.41 mm) and the aluminum complex the best (.58 mm).
Results illustrated how performance of mixtures can be difficult to predict just based on performance of each constituent. The 10:90 mixture with the aluminum complex grease scored close to a line between the scores of the constituent greases. Scores for the 25:75 and 50:50 mixtures, however, fell significantly below than that line. Both of the latter fell outside the ASTM borderline limits meaning they were incompatible for dropping point at that mixture. The 25:75 mixture scored below the literature compatibility limit but within the corresponding borderline limit, rating it borderline according to the literature.
The mixtures with polyurea grease, which was the closest to lithium grease for dropping point performance, were generally closer to the line between the constituents, but the 25:75 and 50:50 mixtures still rated incompatible according to the ASTM standard. All were compatible based on the literature standard. Mixtures with calcium sulfonate, which has the largest difference from lithium’s dropping point, showed by far the greatest variance from the predicted dropping points, and all three mixtures rated incompatible according to both standards.
“Based on the data generated, the dropping point appears to be the most challenging [parameter] among the tested ones,” Dodos said.
Overall the results showed quite a lot of incompatibility according to ASTM D6185 but more compatibility based on literature limits. ASTM defines a binary blend as incompatible if it fails at least one primary test (borderline counts as passing), and the 10% blend with the polyurea grease was the only mixture in the study to clear that bar. It also was merely borderline compatible for all three primary tests. In contrast, only the calcium sulfonate greases were incompatible based on literature limits.
Dodos said the study illustrates that switching from lithium complex greases may lead to problems if the lithium grease mixes with its replacement.
“Alternatives to lithium complex greases can be applied, but caution should be exerted when switching from one chemistry to another, and extra attention shouldç be given even to those types that are frequently advertised as being compatible.”
He did clarify that the study’s findings about compatibility and incompatibility apply only to the greases used in the study; results could differ for other products of the same thickener types, for example if they contain different base stocks or additives.
Dodos concluded that further work is needed to refine methods for determining the compatibility of different greases.
“Due to the complex nature of grease compatibility, an update of the compatibility methodology appears to be reasonable and should encompass a more dynamic approach by designating specific secondary protocols and taking into consideration the visual observation evaluation as well.”
Tim Sullivan is executive editor for Lubes’n’Greases. Contact him at Tim@LubesnGreases.com.