Effects of Polyisobutylene Additives on the Thermodynamic Properties of a Poly(alpha olefin) Base Oil

Automotive transmission fluids are often composed of a synthetic poly(alpha olefin) base oil modified with additives. The additives serve different functions and improve the performance of the base oils. Dispersants are a type of additive that suspend harmful particles within the oil, preventing their agglomeration. It is however also important to know if addition of the dispersant agent alters other properties of the base oils that may influence their performance as lubricants. Among common dispersants are polyisobutylene (PIB)-based polymers.

In this study, we have explored the effect of PIB (MW = 1986 g/mol) on the density and the related derived thermodynamic properties of a poly(alpha olefin) base oil, PAO 4 (MW = 489 g/mol). We have studied the effect of PIB concentration (1, 2, 5 wt %) at 298, 323, 348, 373, and 398 K over a pressure range from 10 to 40 MPa.

The density determinations were made using a variable-volume view-cell. The cell houses a moveable piston. The position of the piston is recorded with a linear variable differential transformer which allows assessment of the internal volume of the cell under all pressure and temperature conditions. A pressure generator alters the pressure on the back side of the piston, causing the piston to move and therefore changing the internal volume of the cell. A known mass loading allows the calculation of densities at different operating conditions. The view-cell also includes a platinum RTD 150 temperature sensor that is in direct contact with the fluid and a Dynisco pressure transducer to accurately measure the temperature and pressure of the system, respectively.

The measured density values were then correlated with the Sanchez-Lacombe Equation of State (EOS) and the derived thermodynamic properties, namely, isothermal compressibility, isobaric expansivity, and internal pressure, were then evaluated using these correlations [1].

The results show that the base oil modified with 5 wt % PIB displays a distinct decrease in the density while 1 and 2 wt % addition of PIB did not have a significant impact on density. Isothermal compressibility was observed to increase, with a higher increase being displayed for the 1 wt % addition case. Isobaric expansivities were observed to increase, but only slightly, at all addition levels. Internal pressure was lowered at 1 wt % addition case but was observed to increase over that of the base oil for the 2 and 5 wt % PIB.