Steady flow and viscoelastic properties of lubricating grease containing various thickener concentrations

The flow and viscoelastic properties of a lubricating grease formed from a thickener composed of lithium hydroxystearate and a high-boiling-point mineral oil were investigated as a function of thickener concentration. The flow properties of grease were measured using continuous shear rheometry, while the viscoelastic properties were measured using oscillatory shear measurements. The flow properties show that grease is a shear-thinning fluid with a yield stress that increases with thickener concentration. At concentrations of lithium hydroxystearate greater than 5% by volume, the storage modulus, G', was found to be greater than the loss modulus, G", with both moduli increasing with increasing thickener concentration, below this critical concentration G" was greater than G'. Slip at the wall of the measuring platens was a major problem encountered during the rheological measurement of grease, this is hardly surprising, and greases are designed to slip in their lubricating functions. Therefore the measuring platens were roughened by sandblasting and significantly higher yield values were recorded with the roughened geometries. Creep experiments were also performed. In the creep test, yield stresses of greases could be obtained. Zero shear viscosity was also calculated from the creep experiment and as a result viscosities over nine orders of magnitude were obtained. The power law index of the scaling law of the elastic modulus and yield stress with increasing volume fraction was found to be 4.7+/-0.2 suggesting that the flocculation of the particles that compose the grease is likely to be of the chemically limited aggregation variety.