24th International Colloquium Tribology - January 2024 185 Panta Rei: Everything Flows But not Everything Flows the Same René Westbroek 1* , Ben Habgood 2 , Daniel Williams 2 1 Axel Christiernsson International AB, Nol, Sweden 2 Centre for Industrial Rheology, Hampshire, UK * Corresponding author: rene.westbroek@axelch.com 1. Introduction Around 500 BC the famous Greek philosopher Heraclitus allegedly said that everything flows. His term Panta Rhei has generally been adopted by the rheology world, but although studies on the flow behaviour of lubricating grease date back to the mid 20 th Century [1] there still is a lot of confusion on what flow of grease actually means! Experience from the field has taught us that two greases with the same NLGI grade can behave very differently in for example central lubrication systems [2]. Previous studies have shown that not only the NLGI grade of the grease affects the flow behaviour of the grease [3], but also the grease thickener type can play a significant role in the pumpability of the grease [4]. Besides pumpability in for example central lubrication systems, the flow behaviour of the grease will also play a role inside the bearing, during for example the churning phase [5]. This manuscript aims to study the underlying role the thickener plays in the flow behaviour of lubricating greases. 2. Material For this study several greases were manufactured on the lab aiming to keep the variation between the greases at a minimum. All greases are adjusted to an NLGI 2 consistency with PAO-6 as the main oil and neither of the greases contain any additives. For the grease thickened with polypropylene (PP) a small amount of oil soluble polyalkylene glycol (OSP) is added, while the other greases contain a similar amount of an alkylated naphthalene (AN). As the polar oil only makes up a tiny amount of the total oil composition, the effects that this small deviation in the composition could bring are judged to be negligible. Table 1 presents the details of the greases. 3. Method Description The rheological properties of the greases were tested in three different experiments on an Anton Paar MCR 301 at 0-°C, 20- °C and 40- °C. All experiments were performed with a 25 mm plate-plate geometry and the temperature was controlled with a Peltier hood. The gap size for all experiments was set to 1000 µm. Table 1: Grease Properties Grease Thickener Oil Composition Worked Penetration 60-strokes LiX Lithium Complex 95% PAO6 5% AN 276 PP Polypropylene 95% PAO6 5% OSP 281 PU MDI-Polyurea 95% PAO6 5% AN 284 In the first experiment the greases were exposed to a strain sweep, increasing the strain from 0,01 to 1000%. From these experiments both the yield stress (σ y ) and the stress in the crossover point (σ f ), where G’=G’’, were determined. To determine the thixotropic behaviour of the greases, a socalled hysteresis test was performed. In this experiment first the shear rate is increased from 2 to 100 s -1 . The shear rate was then maintained at 100 s -1 for 50 seconds, after which it was decreased back to 2 s -1 . A larger difference of the apparent viscosity for the increaseand decrease step indicates a larger thixotropic behaviour of the grease. Lastly, flow curves were determined at the three test temperatures by increasing the shear stress from 100 to 3000 Pa. The obtained results from these experiments were fitted with the Herchel-Bulkely model: As well as the Casseau-Yasuda model: From these models the flow index (n) and the zero shear viscosity (h 0 ) can be determined for each grease at the three test temperatures. To get more insight in how the different thickener structures behave, rheo-microscopy analysis of the greases was performed using a TA Instruments Discovery HR10 fitted with a TA Instruments Modular Microscope Accessory capturing images in cross polarisation mode with a Nikon MRH08430 40X (NA 0.60) objective and a THORLABS M470L5 470 nm 186 24th International Colloquium Tribology - January 2024 Panta Rei: Everything Flows light source. Temperature was controlled using a TA Instruments Upper Peltier Plate and was set to 25-°C throughout testing. The rheometer was equipped with a 40 mm diameter flat plate geometry with a glass lower plate. The gap size was 200 µm. Each grease was subjected to subsequent shear rate peak holds (0.1/ 1/ 10/ 100/ 500/ 1000 s -1 ) for 30 s with data sampled at an interval of 1 s pt -1 and a 30 s equilibration period (0 s -1 ) allowed between each peak hold. 4. Results and Discussion Combining all the results in a spiderweb diagram makes it easy to visualize the difference between the different greases. Figure 1 shows the comparison for the experiments at 20-°C. The results show clear differences between the different greases, indicating that the thickener plays a major role in the flow behavior of the greases as both the NLGI grade and oil composition are almost identical. Figure 1: Comparison of rheological test results at 20-°C. Experiments with the rheo-micrsocope have shed some light on the different nature of the thickeners as can be seen in the example in figure 2, which compares a lithium complex greases (left) with a polypropylene grease (right). Figure 2: Comparison of thickener structures from lithium complex (left) and polypropylene (right) greases. 5. Conclusion This study has presented controlled laboratory experiments which show that the thickener in a lubricating grease can play a significant role in the flow behavior of the grease. These results support the experience from the field which has shown large differences in the pumpability of NLGI 2 greases with different thickener types. Acknowledgement The authors would like to thank Anurag Singh at Axel Christiernsson International for performing the rheological testing on the greases. References [1] Sisko, A.W. The Flow of Lubricating Greases. Ind. Eng. Chem., 1958, 50, 1789-1792 (https: / / doi.org/ 10.1021/ ie50588a042). [2] Personal communication, Roger Persson, Volvo Group. [3] Westerberg, L.G.; Lundström, T.S.; Höglund, E.; Lugt, P.M. Investigation of Grease Flow in a Rectangular Channel Including Wall Slip Effects Using Microparticle Image Velocimetry. Tribology Transactions, 2010, 53: 4, 600-609. (https: / / doi.org/ 10.1080/ 10402001003 605566). [4] Farré-Lladós, J.; Westerberg, L.G.; Casals-Terré, J.; Leckner, J.; Westbroek, R. On the Flow Dynamics of Polymer Greases. Lubricants, 2022, 10, 66 (https: / / doi. org/ 10.3390/ lubricants10040066). [5] Lugt, P. M., Velickov, S., Tripp, J. H. (2009), On the Chaotic Behaviour of Grease Lubrication in Rolling Bearings, Tribology Transactions, 2009, 52, 581-590. (https: / / doi.org/ 10.1080/ 10402000902825713).