(21c) High-Pressure Thermodynamic and Rheological Properties of Lubricant Mineral Base Oils and the Effects of Viscosity Index Modifiers with Different Architectures

Lubricant systems are generally composed of a petroleum-derived mineral base oil in which additives are mixed to improve the lubricant performance. A commonly used additive is a viscosity index modifier, which is used to mitigate the lubricant’s decrease in viscosity at higher temperatures. This study examines the influence of several types of viscosity index modifiers on the high pressure (10-45 MPa) and high temperature (298-398 K) thermodynamic and rheological properties of a typical lubricant mineral base oil. The viscosity index modifiers that were compared included a linear olefin copolymer, a branched polymethacrylate copolymer, and a star-styrene butadiene copolymer.

High-pressure density values are tested using a variable volume view cell along isotherms ranging from 298 K to 398 K across pressure scans from 10-35 MPa. The results are correlated to the Sanchez-Lacombe equation of state which can then be used to evaluate the derived thermodynamic properties of isothermal compressibility, isobaric expansivity, and internal pressure. These properties are used to further interpret the molecular packing and film formation of lubricants at high temperature and pressure conditions.

Viscosity is determined using a uniquely designed high-pressure rotational viscometer. This viscometer consists of a rotating shaft in which the tips rotate within jewel bearings to reduce friction. A magnet is embedded on the top of the rotating shaft 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.

Viscosity is then modeled with density using both the free-volume and density scaling formalisms. The density scaling approach reduces all data 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. This parameter provides insight into the relative sensitivity of the viscosity of the lubricant system to density and temperature changes.