Effect of Dimeric Methyl Ester Concentration on Kinematic Viscosity of Biodiesel Fuel

During cooking, oligomers such as dimers and trimers of triglycerides are produced when vegetable oils are heated in air at high temperatures. When oils containing these components are used for methyl esterification, they are converted to dimeric methyl esters. Their molecular weight is approximately twice that of the monomer esters, and these materials are known to affect the properties of biodiesel fuels. In some countries, used cooking oils serve as raw materials for producing biodiesel fuels; therefore, it is important to know how these components quantitatively affect fuel properties. In this paper, we report the relationship between the concentration of dimeric methyl esters and the kinematic viscosity of biodiesel fuels. Depending on the type of vegetable oil, the kinematic viscosity was more than 5 mm(2)/s when the concentration of dimeric methyl esters in biodiesel was above approximately 4 wt%. The biodiesel containing dimeric methyl esters in this concentration range were produced from vegetable oils containing approximately 9 wt% dimers.