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This paper describes the high-pressure volumetric properties of three commonly used poly(α-olefin) base oilsPAO 2, PAO 4, and PAO 8with nominal 100 °C kinematic viscosities of 2, 4, and 8 cSt. Using a special variable-volume view cell that permits continuous pressure scan and volume measurements, densities were determined at 298, 323, 348, 373, and 398 K at pressures up to 40 MPa. Isothermal compressibilities, isobaric thermal expansivities, thermal pressure coefficients, and internal pressures were then evaluated. At each temperature, density–pressure correlations were developed for the range from 10 to 40 MPa. Density–temperature correlations were developed at 10, 20, 30, and 40 MPa. It is shown that the densities of PAO 2 (which were in the range from 0.7364 to 0.8214 g/cm3) are lower than those of PAO 4 (in the range from 0.7663 to 0.8470 g/cm3) or PAO 8 (in the range from 0.7664 to 0.8498 g/cm3). The densities of PAO 4 and 8 were essentially the same. Isothermal compressibilities of each base oil were of the same order of magnitude at each temperature (with the range being from 6.8 to 10.2 × 10–4 MPa–1), with PAO 2 showing slightly higher values. Isobaric thermal expansivities were in the range (7.2–9.5) × 10–4 K–1. PAO 2 showed higher expansivity values at all pressures. Thermal pressure coefficients were in the range 0.8–1.2 MPa K–1. The values were higher for PAO 2 at 323 K and higher temperatures. PAO 8, while showing the highest thermal pressure coefficient at 298 K, became lower than that of PAO 2 at 323 K and lower than that of PAO 4 at 348 K and higher temperatures. At 398 K, PAO 4 shows the highest thermal pressure coefficients. The trends observed with the thermal pressure coefficients were also reflected in the internal pressures for the respective oils. The internal pressure values were in the range from 260 to 370 MPa. The internal pressure value in each system was observed to decrease with pressure. These observations were interpreted in terms of the differences in the carbon chain lengths, ease of packing, and relative significance of attractive versus repulsive forces that develop as a function of the pressure or temperature.

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Temperature and pressure effects on density and viscosity are reported for six base oils consisting of four mineral oils and two synthetic oils composed of poly­(α-olefins). Using a variable-volume view cell, density data were collected at 298, 323, 348, 373, and 398 K from 10 to 40 MPa. The data were then fit to the Sanchez–Lacombe equation of state and used to determine isothermal compressibility, isobaric thermal expansion coefficient, and internal pressure. Compressibility and internal pressure were found to vary based on composition, specifically with cycloalkane content. Viscosity data were collected as a function of temperature, pressure, and rotational speed using a custom-built high pressure rotational viscometer. Data were collected at 298, 323, 348, and 373 K from 10 to 40 MPa at rotational speeds of 300–800 rpm. A free-volume model was used to model the viscosity and relate the viscous effects to density.