(246g) Density and Viscosity of Lubricant Base Oils Modified with Polymeric Additives and Their Correlations

Lubricant oils reduce friction and wear and are critical to fuel economy in cars. These oils are base oils modified with a variety of additives such as viscosity index modifiers to improve their performance. This study examines the effects of viscosity index modifiers on the thermodynamic properties and viscosity of base oils at high pressure (10-45 MPa) and high temperature (298-398 K) conditions.

Assessment of the thermodynamic properties rely on the determination of the density and its variation with pressure at selected temperatures. The experimental density data are generated using a variable volume view cell and then correlated with the Sanchez-Lacombe equation of state. These correlations are then used to determine the derived thermodynamic properties, such as the isothermal compressibility, isobaric expansivity, and internal pressure. These thermodynamic properties provide a rational approach for development of possible strategies that can help compare the performance of oils with respect to their ability to form sufficient film thickness without causing high viscosity.

Viscosity determinations are carried out using a high-pressure rotational viscometer. This viscometer consists of a rotating cylindrical shaft with jewel bearings to reduce the 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.

The viscosity data are generated as a function of pressure at selected temperatures The data is then correlated with the free-volume and density scaling models. These correlations require the knowledge of density at each pressure and temperature which are taken form the Sanchez-Lacombe correlations of density. The free volume-based correlations of viscosity describe the variation of viscosity with pressure at each temperature. In the density scaling approach, viscosity is expressed as a mixed function of density (ρ) and temperature (T) given by (ρ^γ/T) where the exponent γ is the scaling parameter. This approach unifies all the viscosity data at different temperatures and pressures into a single curve. This provides a rational approach for comparative assessment of different oils with respect to their sensitivity and the relative importance of packing density versus temperature.

In this talk we will present recent data on how polymeric additives such as polymethacrylate alter the properties of base oils with respect their thermodynamic properties and viscosity.