Submitted to: Industrial Crops and Products
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/4/2005
Publication Date: 5/16/2006
Citation: Isbell, T.A., Lowery, B.A., Dekeyser, S.S., Winchell, M.L., Cermak, S.C. 2006. Physical properties of triglyceride estolides from lesquerella and castor oils. Industrial Crops and Products. 23:256-263.
Interpretive Summary: Lesquerella is a developing new rotation crop for the desert Southwestern United States that produces a seed containing 30% oil rich in hydroxy fatty acids. Hydroxy fatty acids are currently imported into the United States principally from India as castor oil. The castor plant has been grown in the U.S., but has received limited interest because the plant contains a very potent toxin which lesquerella does not. Therefore, development of a domestic source of hydroxy fatty acids is of vital interest. In an attempt to develop markets for lesquerella oil, new compounds must be made that improves the chemical parts of the molecule that are detrimental to its performance, particularly in industrial applications such as in lubricants. We made a number of new compounds called estolides that improved the performance of lesquerella making it suitable for many lubricant applications. We also compared these estolides to similar estolides made from castor oil. The lesquerella and castor estolides had very similar properties for most all cases studied, indicating that lesquerella will be suitable in many applications where castor oil is currently used. As a result, increased acreage for lesquerella production may be needed to supply the markets that these materials will enter.
Technical Abstract: Lesquerella is a developing hydroxy oilseed crop suitable for rotation in the arid Southwestern United States. The hydroxy oil of lesquerella makes it suitable for esterification into triglyceride estolides. The estolide functionality imparts unique physical properties that make this class of materials suitable for functional fluid applications. Lesquerella and castor hydroxy triglycerides were converted to their corresponding estolides by reacting the oils with saturated fatty acids (C2 to C18) in the presence of a tin 2-ethylhexanoate catalyst (0.1 wt%), utilizing the condensation of hydroxy with corresponding anhydride, or heating under vacuum at 200 deg C. Two homologous series of estolides for each triglyceride were synthesized for comparison; mono capped (one hydroxy functionality per triglyceride molecule) and fully capped (all hydroxy functionalities per triglyceride molecule). Physical properties (pour point, cloud point, viscosity and oxidative stability) were compared for this estolide series. Longer chain saturate capped estolides (C14 - C18) gave the highest pour points for both mono-capped (9 deg C, C18:0) and fully-capped (24 deg C, C18:0) lesquerella estolides. Castor mono-capped (9 deg C) and fully-capped (18 deg C) triglyceride estolides gave similar results. However, pour points improved linearly as shorter saturated fatty acid capping chain lengths were esterified with the hydroxy triglycerides. Lesquerella capped with a C6:0 fatty acid gave pour points of -33 deg C for mono-capped and -36 deg C for fully capped where castor gave -36 deg C and -45 deg C respectively. Oxidative stability of the estolides were compared for oleic, lauric and lauric-hydrogenated mono- and full-capped materials by RBOT. RBOT times for oleic and lauric capped estolides were low and similar with times centered around 15 minutes. However, when anti-oxidant (4 wt%) was added RBOT times increased to 688 minutes for the hydrogenated full-capped lesquerella lauric estolide. The anti-oxidant had little effect on observed RBOT times when 2 wt% or less anti-oxidant was added for all the estolides except those that were hydrogenated. The hydrogenated estolides showed improvements in oxidative stability at all concentrations of anti-oxidant tested. Viscosity index ranged from 130 to 202 for all estolides with the shorter chain length capped estolides gave the lower viscosity index values. Viscosity at 100 deg C ranged from 13.9 to 26.6 cSt and the 40 deg C viscosity ranged from 74.7 to 260.4 cSt where the longer chain length capped estolides gave the highest viscosities.