Relationship Among Microstructure, Rheology and Processing of a Lithium Lubricating Grease

The overall objective of this work was to evaluate the evolution of the microstructure and rheology of a lubricating grease during its manufacturing process. With this aim, lithium lubricating greases were prepared by inducing the saponification reaction between 12-hydroxystearic acid and hydrated lithium hydroxide within a naphtenic lubricating oil medium, applying a controlled-temperature programme. The manufacturing process was followed through the mixing rheometry technique by measuring the evolution of torque with processing time. Samples of incipient and finished grease were taken from the stirred tank at different processing times. Rheological (linear viscoelasticity and viscous flow) and mechanical stability tests, as well as morphological studies, were carried out on each sample. The changes found in the microstructure and rheology of a lubricating grease during its processing have been related to the soap concentration in the reacting mixture, the waxy soap transition at high temperatures and the further cooling steps. From the experimental results obtained, we may conclude that a suitable lithium lubricating grease is a highly structured system, based on the development of a three-dimensional network due to the presence of metallic soap crystallites.