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Introduction
The microstructure of grease with soap thickeners can be compared to that of a sponge soaked with lubricating oil. It constitutes a three-dimensional cohesive structure of cross-linked particles referred to as fibres, floccules or ribbons. From the chemical point of view, the particles are associated molecules. The stability of the cross-linked microstructure mainly depends on [18], [19] Kuhn (1995, 1998) and [14] Frojsteter et al. (1980):
- the energy potential that is stored in the microstructure (called energy density);
- the forces of mutual attraction between individual particles at the phase boundary;
- the time of contact between individual particles in a unit volume;
- the association intensity limit value; and
- several other physicochemical factors.
The shape of soap thickener particles, their anisometry, dispersivity and percentage in the total grease volume as well as the kind of base oil viscosity and solid additives have a significant influence on the physical properties of the final lubricating compound (e.g. its structural viscosity and flow resistance (pumpability and deformability)) ([12] Delgado et al. , 2006; [35] Yeong et al. , 2004; [8], [9] Czarny and Paszkowski, 2006, 2007; [21], [20] Martín-Alfonso et al. , 2007, 2009). All these factors determine the ease with which various physicochemical bonds form between grease microstructure components.
Because of their semi-solid structure, greases show thixotropic properties. This phenomenon is defined as an isothermal decrease in the structural (apparent) viscosity of a substance during shearing (at both a constant and variable shear rate), followed by an increase in the viscosity and the resolidification of the substance once shearing ends ([31] Paszkowski, 2008). The increase in viscosity is caused by physicochemical interactions resulting in the formation of a spatial cross-linked structure during the relaxation of the substance. The thixotropy of lubricating greases is described in detail in [30], [31] Paszkowski (2007, 2008) and [4], [5] Czarny (1989, 1992).
The thixotropy of lubricating grease thickener microstructure may significantly affect the operation of lubrication systems. The safety and a higher efficiency of lubrication of tribological nodes, and so a lower probability of failure of the system being lubricated, can be reached by reducing lubricating pump pumping resistance. The performance of a grease under load and changes in its consistency in different operating conditions can be...