(215e) High Pressure Thermodynamics and Viscosity of Group IV Base Oil Lubricants Modified with Polyisobutylene Dispersants and Their Modeling with Free Volume and Density-Scaling

Group IV base oils are synthetic oils often used as the base oil in transmission fluids and are composed solely of parrafins. In order to improve the performance of a base oil, additives such as dispersants or viscosity index modifiers are often added to the base oil. This study examines the effects of additives on viscosity and thermodynamic properties at high pressure (10-40 MPa) and high temperature (298-373K) conditions.

The viscosity is measured at high pressure and temperature conditions with varying shear rates using a uniquely designed rotational viscometer. This viscometer consists of a rotating cylindrical shaft with jewel bearings to reduce the friction. A magnet is embedded in the shaft on the top to allow magnetic coupling of the shaft to an outside torque transducer which allows the rotational speed to be controlled without compromising the sealing arrangement of the system.

The density is determined under high pressure and temperature conditions using a variable volume view cell. The density is fit to the Sanchez-Lacombe (SL) equation of state along isotherms ranging from 25ËšC to 125ËšC across pressure scans from 10-40 MPa. The SL descriptions of density are then used to evaluate the derived thermodynamic properties such as the isothermal compressibility, isobaric expansivity, and internal pressure.

Viscosity is then modeled with density using both the free-volume and density scaling formalisms. Both models 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.