Vegetable Oil-based Lubricants Research Articles

Synthesis and physical properties of estolides from lesquerella and castor fatty acid esters

Biodegradable, vegetable oil-based lubricants must have better low temperature properties as well as comparable cost to petroleum oils before they can become widely acceptable in the marketplace. The low temperature property usually measured is the pour point (pp), the minimum temperature at which the material will still pour. Viscosity and viscosity index also provide information about a fluid's properties where a high viscosity index denotes that a fluid has little viscosity change over a wide temperature range. Lesquerella oil is a good candidate for its development into a biodegradable lubricant as it is being developed as an alternative crop for the southwestern U.S. The hydroxy site on the fatty acid (FA) makes it a suitable site for esterification to yield estolides. Castor and lesquerella FA esters were combined with different types of saturated, unsaturated, and branched FAs to produce estolides. Castor and lesquerella estolide esters had the best cold temperature properties when capped with oleic (pp = −54 °C for castor and pp = −48 °C for lesquerella) or capped with a branched material, 2-ethylhexanoic acid (pp = −51 °C for castor and pp = −54 °C for lesquerella). As the saturation was increased in the estolide, pour and cloud points also increased. The increased saturation such as in stearic capped estolides allowed for sufficient alkyl stacking of these long saturated chains producing higher pour points. Oxidative stability of the estolides was compared between the oleic-castor estolide 2-ethylhexyl ester and the coco-castor estolide 2-ethylhexyl ester by the rotating bomb oxidation test (RBOT). The RBOT times for both estolides were low with a similar time of about 15 min. However, when the antioxidant package (3.5 wt.%) was added, the RBOT times increased to 403 min for the coco-castor estolide 2-ethylhexyl ester while still retaining its outstanding cold temperature properties, (pp = −36 °C and cp = −30 °C). The viscosity index ranged from 164 to 200 for these new hydroxy FA derived estolide 2-ethylhexyl esters. These oleic-castor and lesquerella estolide esters have displayed far superior low temperature properties (pp = −54 °C) than any other estolides reported to date. Due to the lack of solvent and catalysts, the cost of these estolides should be reasonable and more suitable as a base stock for biodegradable lubricants and functional fluids than current commercial materials.

Synthesis and physical properties of estolide-based functional fluids

Biodegradable, vegetable oil-based lubricants must have better low temperature properties as well as comparable cost to petroleum oil before they can become widely acceptable in the marketplace. These include low pour point temperature, low viscosity and viscosity indices that do not change over a wide temperature range. Our objective was to synthesize estolides from various sources with improved lubricating physical properties. Oleic acid and lauric acid were treated with varying equivalents of perchloric acid at 60 °C to produce complex estolides. Yields ranged between 45 and 75% after purification by Kugelrohr distillation. The estolide number (EN), the average number of fatty acid units added to a base fatty acid, varied with reaction conditions. The saturate-capped, oleic estolides were esterified with 2-ethylhexanol to obtain high yields of the corresponding ester. Coconut–oleic 2-ethylhexyl estolide esters were produced by varying the ratio of oleic and coconut fatty acids with 0.05 equivalents of HClO 4 to give estolides with excellent cold temperature properties. The amount of oligomerization (EN) played an important role on viscosity; viscosity increased with higher oligomerization. The free acid estolides were generally several hundred centistokes (cSt) more viscous than their corresponding esters. The viscosity index ranged from 141 to 170 for the free acid estolides, whereas the complex estolide 2-ethylhexyl esters had slightly higher viscosity indices, which ranged from 159 to 232. These new coco–oleic estolide esters displayed superior low temperature properties (−36 °C), were of reasonable cost, and were more suitable as a base stock for biodegradable lubricants and functional fluids than current vegetable oil-based commercial materials.

Physical properties of saturated estolides and their 2-ethylhexyl esters

Biodegradable, vegetable oil-based lubricants must have better low temperature properties before they can become widely acceptable in the marketplace. These low temperature properties are usually measured as the material's pour point, the minimum temperature at which a material will still pour. Viscosity and viscosity index also provide information about a fluid's properties where a high viscosity index denotes that a fluid has little viscosity change over a wide temperature range. Oleic acid and a series of saturated fatty acids, butyric through stearic, were treated with 0.4 equivalents of perchloric acid at either 45 or 55 °C to produce complex estolides, dimers and tetramers of fatty acids linked through the double bond and carbonyl group. Yields varied between 45 and 65% after Kugelrohr distillation. The estolide number (EN), the average number of fatty acid units added to a base fatty acid, varied with reaction temperature as well as with the change in saturated fatty acids. The saturate-capped, oleic estolides were esterified with 2-ethylhexanol to obtain high yields of the corresponding ester. As the chain length of saturate capping material increased from C-4 to C-10, the low temperature performance of the estolide 2-ethylhexyl esters, namely pour point, decreased to −39 °C. The other mid-chain, saturated estolide 2-ethylhexyl esters C-6 through C-14 also had superior low temperature properties compared with their competitors; i.e. soy-based, synthetic-based and petroleum-based oils. The amount of oligomerization (EN) had an important role with the viscosities. Viscosity increased with higher oligomerization and the free acid estolides were generally several hundred centistokes (cSt) more viscous than the corresponding esters. The viscosity index ranged from 122 to 155 for the free acids estolides while the estolide 2-ethylhexyl esters had slightly higher indices which ranged from 172 to 196. These new estolide esters displayed far superior low temperature properties, and were more suitable as a base stock for biodegradable lubricants and functional fluids than current commercial materials.

Epoxidized soybean oil as a potential source of high-temperature lubricants

Development and application of biocompatible lubricants are increasing daily as a result of stringent regulations imposed on mineral oil-based lubricants with their non-biodegradable toxic wastes. Before consumer acceptance of vegetable oil-based lubricants, they must overcome certain poor performance characteristics such as thermal and oxidative instabilities. This work demonstrates the improved performance of epoxidized soybean oil (ESBO) over soybean oil (SBO) and genetically modified high oleic soybean oil (HOSBO) in certain high temperature lubricant application. We validated the thermal and deposit forming tendencies of these oils using micro-oxidation and differential scanning calorimetry in conjunction with identification of oxidized products by infrared spectroscopy and also discussed the function of a phenolic antioxidant in these oils. Boundary lubrication properties under high load and low speed were determined and the variations explained based on the structural differences of these vegetable oils.