Permeability and additive sorption and extraction in lubricating greases

The effect of the area and type of surface of ten silica thickeners and aluminum oxide on grease permeability, on the sorption of PAN [Phenyl-α-naphthylamine] by the grease, and on subsequent extraction were studied. For grease containing 6% per cent of thickener, permeability coefficients varied from 20.1 x 10⁻¹¹ cm² to 168 x 10⁻¹¹ cm². For each silica type they decreased with increase in surface area of the thickener. The level of the permeability coefficient is generally related to the worked penetration. In sorption measurements using permeating oils of varying PAN concentration, the amount of PAN associated with the thickener surface increased with PAN concentration. PAN concentration after equilibrium was reached was independent of the thickness of the grease cake. For each type of the silica thickener studied, the equilibrium PAN concentration increased with thickener surface area. Extraction experiments were performed with oil containing no additive on the same grease samples on which sorption experiments were run. The final PAN concentrations were small. The differences among the final equilibrium concentrations of additive in the greases were a function of the type of silica thickener. No significant surface area trend was observed. Initial rate constants for the additive exchange were the same for both sorption and extraction suggesting that the rate limiting step for replacement of the oil in the grease or for additive transfer between permeating oil and the oil in the grease is the same for the sorption and extraction experiments --Abstract, page x-xi