Synthesis and physical properties of iso-oleic estolides

Abstract: One of the drawbacks of using vegetable oils directly as lubricants is their poor performance at low temperature [pour point (PP) and cloud point (CP)]. Various approaches have been used over the years to overcome this disadvantage including the use of expensive pour point depressant additives. Chemical modification of vegetable oils has also been used to lower PP and CP. However, such products still require blending PPD additives to meet PP and CP specifications for some high-volume applications, such as engine oils, gear oils, and hydraulics. These and other problems continue to depress the global market share of bio-based lubricants to under 1%. Oleic estolides were developed to improve these properties as well as increase the oxidative stability and improve the low temperature properties of these new oils. Estolides are biobased synthetic esters obtained from the reaction of a fatty acid with a double bond (e.g., oleic acid), hydroxyl group, or epoxide on its chain, with a second fatty acid of similar or different structure, in the presence of a catalyst. A new series of iso-oleic estolide free acids and 2-ethylhexyl (2-EH) esters capped with various fatty acids had superior physical properties and cold flow properties, compared to traditional oleic based estolides. The average PP of the iso-oleic estolide 2-EH esters was -41^oC, compared to -23^oC of iso-oleic estolide free acids. The iso-­oleic-octanoic and iso-oleic-decanoic estolide 2-EH esters had excellent PPs of -60^oC. The cold flow properties of iso-oleic estolides decrease with increasing the carbon number of the saturated capping fatty acid, while the viscosity index and oxidation stability were only slightly improved. lso-oleic-coconut estolide 2-EH esters exhibited good physical properties, with PPs as low as -45^oC. The iso-oleic estolide free acids and 2-EH esters are a new kind of low-cost materials for bio-lubricants with excellent cold flow properties.