Correlating viscosity of 2-ethylhexyl oleic estolide esters to their molecular weight

Abstract

Estolides are a relatively new class of biobased materials, and their properties are not thoroughly investigated. Since they have been recently commercialized as lubricants, a deeper investigation of their viscosity was warranted. Five 3-parameter viscosity models with theoretical derivations were found in the literature. Additionally, several empirical 2- and 3-parameter models, including one empirical 4-parameter model, used to describe the viscosity of lipids, were selected for investigation. These models were used to fit the experimental viscosity of 13 oleic estolide ester samples. The viscosity was measured at six temperatures between 20 and 100 °C. The jackknife procedure was used to estimate the uncertainty of the fitted parameters. For the theory-based models, correlations were derived between the fitted coefficients and the molecular weight of the sample. The results allowed for simple prediction of the viscosity of estolide esters as a function of the temperature, based only on number-average molecular weight (or on the estolide number). The best model required additional knowledge of the weight-average molecular weight.