Evaporation and flow properties of several hydrocarbon oils

Abstract

Two properties that delimit the useful temperature range of synthetic lubricating oils are the vapor pressure and viscosity. This study investigates the ability of fluid theory to model these two properties for diesters with different diacid chain length and alcohol branching, triesters consisting of triglycerides and a trimethylol propane ester, and nonpolar oils including aromatic hydrocarbons, a polyalphaolefin, and two branched alkanes. Within each type of oil there are structural isomers. Isomerism influences the vapor pressure and viscosity, in addition to the well-known dependence on molecular weight. The expressions for the vapor pressure and viscosity are combined using the absolute rate theory of Eyring and equilibrium thermodynamics. A modification of the rate theory allowing for a rotational contribution to the flow-activation entropy was needed to simultaneously fit the vapor pressure and viscosity data between −20 and 100°C. The flow-activation rotational entropy, calculated from the combined vapor pressure and viscosity data, is a significant contribution to the viscosity. This analysis provides the background for molecular dynamics simulation to assist in designing new low viscosity and vapor pressure synthetic oils. © 2000 Taylor & Francis Group, LLC.