Kinematic viscosity of fatty acid methyl esters: Prediction, calculated viscosity contribution of esters with unavailable data, and carbon–oxygen equivalents

Many properties of biodiesel, the mono-alkyl esters of vegetable oils, animal fats or other triacylglycerol-containing feedstocks, are largely determined by its major components, the fatty acid alkyl esters. Therefore, information on the properties of individual components and their interaction is essential to understanding and predicting the properties of biodiesel fuels. Viscosity, which affects flow and combustion of a fuel, is such a property. In previous literature, the effect of the structure of fatty esters on viscosity was discussed. However, these data are largely confined to esters with an even number of carbon atoms in the chain and that are liquid at 40 °C. To gain a better understanding of kinematic viscosity, this work additionally reports data on esters with an odd number of carbons in the fatty acid chain and some unsaturated fatty esters. Furthermore, the kinematic viscosity of some biodiesel fuels is affected by components that are solids at 40 °C. A method based on polynomial regression for determining the calculated viscosity contribution (CVC) of esters that are solid at 40 °C (saturated esters in the C₂₀–C₂₄ range) or esters that are liquids but not available in pure form is presented as these values are essential for predicting the kinematic viscosity of mixtures containing such esters. The kinematic viscosity data of esters are compared to those of aliphatic hydrocarbons in the C₆–C₁₈ range and those of dimethyl diesters. The increase of kinematic viscosity with increasing number of CH₂ groups in the chain is non-linear and depends on the terminal functional groups, chain length and double bonds. To illustrate this effect, carbon–oxygen equivalents (COE) are used in which the numbers of carbon and oxygen atoms are added. A straightforward equation, taking into account only the amounts and kinematic viscosity values of the individual neat components, suffices to predict the viscosity of mixtures of fatty esters (biodiesel) at a given temperature.