Rheology of Lubricating Grease
Jinxia Li, Ph.D.,(1) Erik Höglund, Prof.,(1)
Boris Zhmud, Ph.D., Assoc. Prof.(2) (1)
Lars G. Westerberg, Ph.D., Assoc. Prof.,(1)
Department of Engineering Sciences and Mathematics, Luleå University of Technology, Sweden; (2) Applied Nano Surfaces Sweden AB, Knivstagatan 12, SE-75323 Uppsala, Sweden
Lubricating oils and greases are the most common technical lubricants. Greases are preferred in situations where lubricant leakage from lubricated joints under the action of gravity or capillarity needs to be avoided. In particular, greases are extensively used as a component of distributed lubricating systems in modern cars and construction vehicles which are supposed to work well in any climatic conditions. As known, low temperatures cause grease thickening which may disturb operation of distributed lubricating systems in cold climate.
The type of base oil used in grease production is one of the key determinants of grease performance at various temperatures. Mineral oils used by grease manufacturers can be divided into two major groups: naphthenic oils and paraffinic oils. Each of these groups of base oils has their own advantages and disadvantages. One major advantage of the naphthenic oils over the paraffinic oils with similar aromatic content is better low-temperature pumpability of the formers. Another possible advantage is that naphthenes have better solvency as compared to paraffins. When the base oil in grease has good solvency towards the thickener, less thickener is needed to achieve a certain consistency of the finished product, and higher treat levels can be maintained. On the other hand, the higher soap content in greases produced from paraffinic oils is partly responsible for their better mechanical stability, which may be beneficial at high temperatures.
The typical grease formulations contain up to 90 wt% of base oil, and therefore, it is the thickener that turns a viscous material into a viscoelastic one. A physical model explaining some of the grease flow properties is a sponge impregnated by oil [1]. When a stress is applied to grease, the thickener fibres will resist deformation due to elastic forces, whereas the oil contained in it will resist deformation due to viscous forces. In rheological terms, the elastic response to deformation is described by the storage modulus, and the viscous response is described by the loss modulus. With increasing the thickener concentration, both the storage and the loss moduli will in general increase, but at a certain concentration, called the gel point, the storage modulus demonstrates a very rapid increase by a few orders of magnitude. At this point, disjoined thickener fibres and their aggregates come into contact with each other, forming a continuous three- dimensional network, and as a result, the grease loses its ability to flow freely. However, when the applied stress exceeds the yield stress, the internal structure of grease is broken and the grease acquires fluidity. After the stress is removed, the flow stops but
it may take quite some time - called the relaxation time - for the internal structure to get restored back.
If the grease is exploited in real-life applications, it may experience extreme pressures and shear rates in tribological contacts, and therefore, the thickener fibre structure itself can be damaged. This can result in fibre compaction and oil bleeding.
Grease rheology
Rheology is the science of deformation and flow of materials. From our everyday experience we know that materials can be classified as elastic (most solids) and viscous (most liquids). When stressed, an elastic material stores all the deformation energy and recoil to its original shape after the stress is removed; the force opposing deformation only depends on the amplitude of the deformation but not on its rate. A viscous material, on the contrary, is not able to store the deformation energy and it starts to flow if a stress is applied to it; the flow continues as long as the stress is present. For a viscous material, the force opposing deformation only depends on the rate of deformation but not on its amplitude.
From a rheological viewpoint, greases are classified as viscoelastic materials. The elasticity is essential for prevention of grease leakage from lubricated joints, whereas the fluidity is essential both for the grease lubricating efficiency and for grease pumpability. Both the storage modulus (elasticity) and the loss modulus (viscosity) of greases increase with increasing thickener concentration. At a thickener concentration of around 5 vol.%, the rheology cross-over point is achieved, where the grease rheology changes from predominantly viscous to predominantly elastic as a result of rapidly increasing connectivity of the thickener network [2,3].
(i) Amplitude sweep experiments
Amplitude sweep experiments are carried out in order to locate borders of the linear viscoelasticity (LVE) region [2,3]. The approximate borders of the LVE region can be concluded from the strain amplitude dependence of the storage G’ and the loss G’’ moduli as shown in Figure 1. It is only within the LVE that the stress is proportional to the strain, and the properties of the material are unaffected by the measurements. In other words, the grease microstructure remains close to its equilibrium state, which is a natural reference point. Outside of LVE, the internal structure relaxation time becomes an essential parameter.
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LUBE MAGAZINE NO.126 APRIL 2015
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