Fatty acid estolides: A review

Authors: CHEN, YUNZHI - South China University Of Technology; Biresaw, Girma; Cermak, Steven - Steve; Isbell, Terry; Ngo, Helen; CHEN, LI - South China University Of Technology; Durham, Amber

Submitted to: Journal of the American Oil Chemists' Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/11/2019
Publication Date: 1/23/2020
Citation: Chen, Y., Biresaw, G., Cermak, S.C., Isbell, T., Ngo, H.L., Chen, L., Durham, A.L. 2020. Fatty acid estolides: A review. Journal of the American Oil Chemists' Society. 97(3)231-241. https://doi.org/10.1002/aocs.12323.
DOI: https://doi.org/10.1002/aocs.12323

Interpretive Summary: Estolides are biobased base oils currently in the early stage of commercialization. Estolides have several properties that make them very desirable for application in lubricants and cosmetics. Estolides can be produced in a wide range of viscosity grades and generally have superior cold flow and viscosity index properties. Another major strength of estolide is that they can be manufactured using a wide range of raw materials, catalysts, and reaction conditions (time and temperature). The range of raw materials include: purified fatty acids, fatty acid mixtures from various vegetable oils, fatty acids with double bonds, fatty acids with hydroxy groups, vegetable oils (triglycerides), and various combinations of the above. Previous work has shown that by carefully selecting the combinations of raw materials, catalysts, and reaction conditions, one can produce estolides with the desired viscosity grade and other critical properties for the intended application. In this review, the synthesis of estolides using different catalysts, fatty acids and/or triglycrides is summarized. In addition, the effect of changing fatty acid structures (degree of unsaturation, chain length, branching) on critical physical properties are reviewed. Differences in properties of estolide free acids vs. estolide 2-ethylhexyl esters is also highlighted. This review will be useful to scientists and engineers entering the estolide field or engaged in developing new commercial applications for estolides. This in turn will increase the utilization of seed oils used in estolide production, increase demand for farm products and strengthen the rural economy.

Technical Abstract: Estolides are bio-based oils synthesized from fatty acids or from the reaction of fatty acids with vegetable oils. Estolides have many advantages as lubricant base oils, including excellent biodegradability and cold flow properties. Promising applications for estolides include bio-lubricant base oils and in cosmetics. In this review, the synthesis of estolides from fatty acids using four different types of catalysts, namely, mineral acids, solid acids, lipases, and ionic liquids, is summarized. The summary includes the yield of estolide obtained from varying synthetic conditions (time, temperature, catalyst). Also reviewed are studies comparing the physical properties of estolides synthesized from refined fatty acids against those synthesized from fatty acid mixtures obtained from vegetable oils such as coconut, castor, Physaria, etc. By varying the structure of the fatty acids, estolides with a wide range of pour point, cloud point, and viscosity are synthesized to meet a wide range of application requirements. Currently, estolide products are being commercialized for personal care and lubricant base oils for automotive, industrial, and marine applications. The application areas and the demand for estolides is expected to grow as the drive for switching from petroleum to bio-based products keeps growing.