(6f) The Effects of Polymeric Additives on the High-Pressure Thermodynamic and Rheological Properties of Mineral Lubricant Base Oils

Mineral oils derived from petroleum are common base oil lubricants used in automatic transmission fluids and engine oils. In order to improve their performance, they are modified with additives such as viscosity index modifiers. This study examines the impact of two types of polymeric viscosity index modifiers, polymethacrylates and ethylene-propylene copolymers, on the properties of a Group III mineral base oil. In this presentation, we will discuss the effect of concentration and functionalities of these polymeric additives on the thermodynamic and rheological properties of the base oil at high temperature (298-398 K) and high pressure (10-45 MPa) conditions.

The densities of the oils were determined using a variable-volume view-cell. The data were correlated with the Sanchez-Lacombe Equation of State and then used to evaluate the derived thermodynamic properties, namely isothermal compressibility, isobaric expansivity, and internal pressure. Knowing these properties is of practical value in providing a rational approach to an improved understanding of packing and film formation of the lubricant under high pressure and temperature conditions.

Viscosity determinations were carried out using a high-pressure rotational viscometer. This viscometer consists of a rotating cylindrical shaft with jewel bearings to reduce friction. A magnet is embedded in the rotating inner shaft on the top to allow magnetic coupling of the shaft to an outside torque transducer. Magnetic coupling allows the rotational speed of the inner shaft to be controlled without compromising the sealing arrangement of the system.

Viscosity data were modeled with density using both the free-volume and density scaling formalisms. Both models were found to correlate the viscosity well. The density scaling approach reduces all the data generated at different pressures and temperatures into a master curve if the data is plotted as a function of (ρ^γ/T) where the exponent γ is the scaling parameter. This parameter provides insight into the relative sensitivity of the viscosity of the lubricant to changes in density and temperature.