Vegetable oils are considered to be potential candidates for the preparation of biolubricant base stocks to replace conventional mineral oil-based lubricating oils and synthetic esters. Vegetable oil based lubricants are attractive alternatives to mineral oil-based lubricants due to their enhanced biodegradability, lower toxicity and several other properties.
Estolides are a class of esters, based on vegetable oils that form when the carboxylic acid functionality of one fatty acid reacts at the site of unsaturation or double bond, of another fatty acid to form an ester linkage, and the product is popularly known as estolides. The secondary linkages of the estolides are more resistant to hydrolysis than triglycerides, and the unique structure of the estolide results in materials that have physical properties far superior to those of vegetable and mineral oils for certain applications [Inform, 15, p. 515 (2004)].
Esters of estolides derived from oleic acids and C6 to C14 saturated fatty acids, characterized by superior properties for use as lubricant base stocks have been described in Industrial Crops and Products (2001), 13(1), 11-20. In another study saturate-capped, oleic estolides were esterified with 2-ethylhexanol to obtain the corresponding ester [Industrial Crops and Products, 18, p. 183 (2003); U.S. Pat. No. 6,018,063 (2002) & U.S. Pat. No. 6,316,649 (2001)]. These coco-oleic estolide esters displayed superior low temperature properties (−36° C.) and more suitable as a base stock for biodegradable lubricants and functional fluids than current vegetable oil-based commercial materials.
In the another type of estolides, hydroxy fatty acids of castor oil can be readily converted into estolides by homopolymerization [J. Am. Oil Chem. Soc. 42; p. 428 (1965). Unlike normal estolides, that are formed when the carboxylic acid functionality of one fatty acid links to the site of unsaturation of another fatty acid to form oligomeric esters, the fatty acids of castor have a hydroxy functionality that provides a site for esterification to take place to produce estolides.
Castor oil is an attractive industrial raw material for the preparation of several functional compounds including lubricants [J. Am. Oil Chem. Soc. 51, p. 65 (1974), J. Am. Oil Chem. Soc. 48, p. 758 (1971)]. The presence of ricinoleic acid (hydroxyl fatty acid (12-hydroxy 9 cis-octadecenoic acid)) up to 85-90% projects castor oil as an attractive lubricant base stock.
Secondary alcohol esters of hydroxy acids, e.g., ricinoleate esters of secondary alcohols, are prepared by reacting an ester of a hydroxy acid with a secondary alcohol, in the presence of a organo metallic transesterification catalyst have been reported in U.S. Pat. No. 6,407,272 (2002), which are useful as a lubricity agent.
Estolides of lesquerella and castor triglycerides with oleic acid have been reported in Journal of the American Oil Chemists' Society, 79, p. 1227 (2002). Synthesis and physical properties of lesquerella and castor hydroxy triglycerides was reported in Industrial Crops and Products, 23, p. 256 (2006). Lesquerella and castor oils were converted to their corresponding estolides by reacting with saturated fatty acids (C2-C18) in the presence of a tin 2-ethylhexanoate catalyst (0.1 wt. %) and utilizing the condensation of hydroxy with corresponding anhydride or heating under vacuum at 200° C.
Estolides of lesquerella and castor fatty acids with different types of saturated, unsaturated and branched fatty acids have been reported in Industrial Corps and Products, 23, p 54 (2006). Castor and lesquerella estolide esters had the best cold temperature properties when capped with oleic or a branched fatty acid. As the saturation in the estolide increased pour and cloud points also increased