On the Steady-State Flow and Yielding Behaviour of Lubricating Greases

Abstract

Practical steady-state flow curves were obtained from different rheological tests and protocols for five lubricating greases, containing thickeners of a rather different nature, i.e., aluminum complex, lithium, lithium complex, and calcium complex soaps and polyurea. The experimental results demonstrated the difficulty to reach “real” steady-state flow conditions for these colloidal suspensions as a consequence of the strong time dependence and marked yielding behavior in a wide range of shear rates, resulting in flow instabilities such as shear banding and fracture. In order to better understand these phenomena, transient flow experiments, at constant shear rates, and creep tests, at constant shear stresses, were also carried out using controlled-strain and controlled-stress rheometers, respectively. The main objective of this work was to study the steady-state flow behaviour of lubricating greases, analyzing how the microstructural characteristics may affect the yielding flow behaviour.

Cellulose Pulp- and Castor Oil-Based Polyurethanes for Lubricating Applications: Influence of Streptomyces Action on Barley and Wheat Straws

“…Regarding the viscous flow behavior of these polyurethane oleogels, a markedly shear-thinning response was obtained in all cases, as shown in Figure 5, with a viscosity decay of several decades with the increasing shear rate, which is also a distinctive characteristic of traditional lubricating greases [49]. This power-law evolution, generally associated with very low values of the flow index, is, however, part of a more general and complex viscous flow behavior of this type of material (see, for instance, [18]). In fact, at very low shear rates, a tendency to achieve constant high viscosity values should be observed, although the controversy about the existence of an apparent yield stress value is still open, whereas at very high shear rates, again, constant values of limiting viscosity must be reached.…”

“…The values of these fitting parameters are listed in the insets of Figure 5. As can be seen, extremely low values of the flow index, n, were always obtained, especially for the most structured systems, as discussed above, which is representative of the yielding behavior typically exhibited by lubricating greases [18]. Moreover, the k values of these oleogels are also close to those shown by commercial calcium and lithium lubricating greases and model polyolefin-thickened lithium greases, which exhibited values in the range of 600-1500 Pa•s n [19,49], again highlighting the rheological similarity between the traditional products and the obtained ones.…”

“…As the oil medium was the same in all oleogel samples, these differences must be attributed to the thickener. Either the cellulose pulps containing higher cellulose pulp/lignin ratios-presumably yielding more crosslinked networks and also generally having higher DP values-favor oil entrapment, thus preventing its release, or simply the whole oleogel sample, including the thickener, can penetrate in the lubricating contact, and the highest friction is just a consequence of the higher viscosity [18]. Since the wear scars associated with the friction experiments were generally reduced when fermented-straw-derived cellulose pulps were used as a thickener agent, the second assumption seems to be more feasible.…”

“…The development of bio-based oleogels may represent a new market niche in the lubricant industry with interesting perspectives for the future, since metallic soap-based thickening agents and mineral oils, the main components in lubricating grease formulations, may have to be replaced soon as a consequence of more restrictive politics [16,17]. At the Chemical Process and Product Technology Research Center (Pro2TecS, Universidad de Huelva), we have dedicated our efforts over the last two decades to the development of eco-friendly oleogels, attempting to provide them with outstanding functional properties so they can be proposed as efficient alternatives to traditional lubricating greases, as well as to the comprehensive characterization of the rheological and tribological behaviors of both commercial and bio-based lubricating greases [18,19]. More specifically, this research has been focused on the finding of suitable and sustainable gelling or thickening agents in oily media by following diverse approaches.…”

Editorial for Special Issue “Advances in Experimental and Computational Rheology”

“…The special issue "Advances in Experimental and Computational Rheology" joins fifteen works covering some of the latest advances in the fields of experimental and computational rheology applied to a diverse class of materials and processes, which can be grouped into four main topics: rheology [1][2][3][4][5], effect of process variables [6][7][8][9], rheometry and processing [10,11], and theoretical modeling [12][13][14][15] The characterization of rheological behavior is the main topic of five contributions, covering the following material systems: lubricating greases (Delgado et al [1]), Carbopol ® dispersion in water and in water/glycerol solutions (Varges et al [2]), natural hydraulic lime grouts (Baltazar et al [3]), fresh cement pastes (Rubio-Hernández [4]), and legume-protein-stabilized emulsions (Félix et al [5]).…”