Correlation of Elastohydrodynamic Friction with Molecular Structure of Highly Refined Hydrocarbon Base Oils

Abstract

The molecular compositions of a range of low viscosity hydrocarbon base oils spanning API Groups II to IV have been quantified using 13C NMR and correlated with base oil elastohydrodynamic (EHD) friction. A strong correlation has been found between the proportions of paraffin, linear and branched carbons and EHD friction, with a high proportion of linear and paraffinic carbon atoms contributing to low-EHD friction but branched carbons contributing to high-EHD friction. Correlation equations have been developed to predict EHD friction based on base oil composition. At very high temperature and low pressure, this correlation breaks down as the lubricant in the contact does not reach sufficiently high shear stress for shear thinning to occur. For Group IV polyalphaolefin, the correlation must be extended to account for the very high proportion of linear carbons originating from linear alkene oligomerization. The correlations developed in this study can be used to guide the design of low-EHD friction base oils.