RHEOLOGY OF COLLOIDAL SUSPENSIONS - CASE OF LUBRICATING GREASES

Lubricating greases are two-phase colloidal suspensions consisting essentially of a mineral oil and a metal-soap based thickener forming a three-dimensional gelling network. This study examines the rheology of two greases, based on lithium soap and calcium complexes, under steady, transient and dynamic flow conditions, with homogeneous shear being applied in a cone-plate geometry. Controlled speed and controlled torque rheometers were used in this study. Various precautions need to be taken in order to obtain reliable results. These include checking and eliminating slip at the walls and fracture, correct choice of apparatus, and suitable measurement procedures. It is shown that discrepancies may arise as a result of choosing the wrong test procedures. The lithium grease is characterized by a yield stress and shear-thinning behavior, whereas the calcium complex has a minimum in the flow curve, for which controlled torque rheometer measurements are not the most suitable. Original test procedures are adopted for studying viscoelasticity and thixotropy, the importance of which depends on the type of constituents involved.