Vegetable oils contain non-polar hydrocarbon chains and polar ester groups (and possibly also other functional groups such as hydroxyl groups in castor oil). The presence of polar and non-polar groups within the same molecule gives vegetable oil amphiphilic character. The density, refractive index, viscosity, pressure-viscosity coefficient, elastohydrodynamic film thickness of neat oils, and binary blends of vegetable oils or estolides with synthetic esters, polyalphaolefins and polyglycols is discussed. Several literature models for predicting blend properties from neat oil properties are described and compared with experimental data. The effect of vegetable oils amphiphilicity and aging on elastohydrodynamic film thickness of lubricating blends is discussed. The effect is most pronounced in the ultralow film thickness (below 20 nm) regime. This lubrication regime is very critical since wear rate starts increasing rapidly with decreasing the film thickness.
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