Determination of High Pressure Density for Assessment of the Derived Thermodynamic Properties of Base Oils.

This poster will describe our high-pressure variable-volume view cell that allows determination of density of fluids or fluid mixtures as a function of pressure and temperature which is then used in assessing the other thermodynamic properties such as compressibility, expansivity, and internal pressure. In this particular study, the specific system that has been evaluated is a poly(α-olefin) base oil, PAO 4. This oil forms the base for various lubricant applications, such as engine or automatic transmission oils.

The variable-volume view-cell is composed of two parts, the main cell body which is equipped with two sapphire windows for observation of the cell content, and a variable-volume part which houses a movable piston. The position of piston is monitored with an LVDT (linear variable differential transformer) sensor. The piston position and along with it the internal volume and consequently the pressure of the cell is changed with the aid of a motorized pressure generator that alters the pressure on the back side of the piston with ethanol as the pressurizing fluid. Knowing the position of the piston with the use of LVDT allows the determination of the inner volume of the cell at any pressure / temperature condition. Since the initial mass loading of the oil in the cell is known a priory, densities are readily evaluated as a function of pressure at any given temperature. The homogeneity of the oil during these measurements is verified with the sapphire windows.

The density data were generated at 298, 323, 348, 373, and 398 K over a pressure range from 10-40 MPa. The density data were then fitted to an equation of state. The equation of state used is a lattice fluid model known as the Sanchez-Lacombe Equation of state. Fitting the data to an equation of state helps not only bring consistency to the description of the pressure dependence of the density at the different temperatures, but it also facilitates the derivation of desired thermodynamic properties, specifically the isothermal compressibility, isobaric expansivity, and internal pressure which are of high value in comparing performance of base oils. The Sanchez-Lacombe Equation allows for smooth differentiation to evaluate these properties, and thus provides a complete thermodynamic description of the base oil investigated.