High-Pressure Characterization of Dynamic Viscosity and Derived Properties for Squalane and Two Pentaerythritol Ester Lubricants: Pentaerythritol Tetra-2-ethylhexanoate and Pentaerythritol Tetranonanoate A. S. Pensado, M. J. P. Comun ˜ as, L. Lugo, and J. Ferna ´ ndez* Laboratorio de Propiedades Termofı ´sicas, Dpto. de Fı ´sica Aplicada, Facultade de Fı ´sica, UniVersidade de Santiago de Compostela, E-15782 Santiago de Compostela, Spain The experimental measurements of dynamic viscosity for squalane, pentaerythritol tetra-2-ethylhexanoate (PEB8), and pentaerythritol tetranonanoate (PEC9) have been performed using a Ruska rolling-ball viscometer over the temperature range of 303.15-353.15 K and a pressure range of 0.1-60 MPa. A total of 2016 experimental measurements of the rolling time have been obtained for the determination of 252 dynamic viscosity data. The available literature viscosity data for squalane at high pressures have been compared with the new measurements, and an average deviation of 1.5% has been obtained, which is within the experimental uncertainty ((3%). The higher viscosity values are reached for PEB8, followed by PEC9. For the pentaerythritol esters, it has been observed that the dynamic viscosity increases with the branching degree of the molecule. The relative change in viscosity with temperature is biggest for PEB8 and, consequently, the poorer viscosity index (VI) values and higher temperature coefficients have been obtained for this fluid. The lowest pressure- viscosity coefficient and the highest VI have been obtained for PEC9. Introduction Environmentally adapted lubricants have received growing attention in recent years in several industrial applications, such as refrigeration, air conditioning, and hydraulic fluids for mobile hydraulics. To meet the market demand, and to be able to develop new and highly efficient environmentally adapted fluids, the physical properties of the base fluids that affect lubrication should be reliably determined. These properties are needed, for example, in the determination of the minimum oil film thickness and the associated friction losses. 1 One of the most important physical properties of lubricants, which affect the processes of heat and mass transfer, is the dynamic viscosity. Viscosity is, by far, the most significant property for establishing the thickness of an oil film in hydrodynamic lubrication and the pressure and temperature conditions at which elastohydrodynamic lubrication occurs. Viscosity is also a significant factor in predicting the perfor- mance and fatigue life of rolling-element bearings and gears. 2 Thus, the energy requirement of an appliance compressor can be reduced by diminishing the lubricant viscosity. However, if the lubricant viscosity is reduced too far, it is no longer high enough to ensure that a full fluid film is entrained: metal-to- metal contact occurs, so some wear is unavoidable. 3 Besides, two properties now recognized as being important in engine oil design are the elastohydrodynamic traction and the pressure- viscosity coefficient (R), which characterizes the variation of viscosity with pressure and is involved in the determination of the oil film thickness in both hydrodynamic and elastohydro- dynamic regimes. In the hydrodynamic lubrication, the film thickness of fluid that protects the mechanical device from high friction and premature wear is also dependent on R. 4 Fluids with a higher R value produce thicker lubricant films, so rolling- element bearings, gears, and rotors are better protected at high pressures. However, in the formulation of many fuel-economy- enhancing “energy-conserving” engine oils, gear lubricants, and transmission fluids, a less-viscous film is desired because mechanical energy can be wasted if the film is too robust. In other applications such as continuously variable transmissions, where the fluid must provide high levels of traction, a very high R value is needed. 5,6 Another important property is the viscosity index (VI), which characterizes the temperature dependence of the lubricant’s kinematic viscosity. In selecting the best oil viscosity for any application, it is the viscosity, at the usual operating temperature, that is most important. However, if the equipment will need to make a cold start, it is also important that the viscosity at the starting temperature is low enough for the machine to be started easily. 7 A temperature increase of a few degrees can induce an important viscosity decrease, which significantly changes the film thickness. It follows that it is important to know how much the viscosity will change with temperature. The ideal lubricant would be one whose viscosity is minimally affected by the range of operating temperatures. 8 Although the temperature dependence of viscosity at atmo- spheric pressure is usually well-known for numerous fluids, the viscosity dependence on pressure under isothermal conditions shows a lack of experimental values. 9 To develop better formulations of lubricants, it is necessary to extend the database of viscosities for different base lubricant fluids over wide temperature and pressure ranges. The growing demand to save energy and reduce pollution levels has made the designing of new lubricants with better efficiency an important area of active research. A first step in designing better lubricants is to understand the structural effects of molecular structure on the viscosity and its pressure and temperature derivatives. Following our current research project on environmentally adapted oils, 10 two pentaerythritol ester lubricants are the object of study in this work. One of them, pentaerythritol tetra-2- ethylhexanoate (PEB8), has four branched chains and the other, pentaerythritol tetranonanoate (PEC9), four linear chains. Thus, new dynamic viscosity measurements of PEB8 and PEC9, up to 60 MPa at six temperatures in the range of 303.15-353.15 K are reported together with new dynamic viscosity data for * To whom correspondence should be addressed. Tel.: 34981563100, ext. 14046. Fax: 34981520676. E-mail: fajferna@usc.es). 2394 Ind. Eng. Chem. Res. 2006, 45, 2394-2404 10.1021/ie051275w CCC: $33.50 © 2006 American Chemical Society Published on Web 03/03/2006