The Viscosity of Oils

We present a framework for understanding and predicting the temperature-dependent viscosity of oil-based systems, based on the Modified-VTF equation η(T) = η∞exp{D/[(T/T0)-1]}, which relates to the approach to the glass transition, even at temperatures far above it. We present data showing that this applies to oils ranging from bitumen, resid, asphaltenes, and asphalt to highly fluxed, diluted, and less viscous systems such as diesel, as well as lubricants, model compounds, and polymers in solution, which could be viscosity modifiers or additives in lubes and asphalt. The key is to describe the η(T) behavior with two parameters: T0 and η∞, and to fix the fragility (D). These two parameters are shown to vary systematically with composition (i.e., MW and aromaticity) and vary systematically with chemical changes such as oxidation. Most significantly, this parametrization results in simple linear blending rules for those parameters, which can predict the observed highly nonlinear and sometimes nonmonotonic behavior.