Estolide Esters Research Articles

Synthesis of 12-hydroxystearic estolide and its esters to study the effect of molecular structure on physicochemical properties

The global point of view associated with bio-lubricants has dramatically changed due to depleting petroleum supplies, volatile prices, growing environmental concerns, and strict government regulations. Vegetable oils are considered a potential replacement for petroleum-based lubricants; however, they are constrained by poor cold-flow characteristics, low hydrolytic stability, and low resistance to oxidative deterioration. Increased oxidative and hydrolytic stability and enhanced cold flow properties result from a chemical alteration of vegetable oil by esterification or estolide formation. This work prepares a 12-hydroxystearic acid estolide (HSAE) series, followed by esterification with alcohols (i.e., 2-ethyl hexanol, trimethylolpropane, and pentaerythritol) to obtain estolide esters. The effect of reaction time on the degree of oligomerization of 12-hydroxystearic acid estolide has been studied. Further, the comparative analysis of the physicochemical properties (i.e., kinematic viscosity at 40 °C & 100 °C, viscosity index, and pour point) of estolide esters was also evaluated to study the effect of oligomerization and molecular structure of alcohol on kinematic viscosity, viscosity index, and pour point.

Correlating viscosity of 2-ethylhexyl oleic estolide esters to their molecular weight

Estolides are a relatively new class of biobased materials, and their properties are not thoroughly investigated. Since they have been recently commercialized as lubricants, a deeper investigation of their viscosity was warranted. Five 3-parameter viscosity models with theoretical derivations were found in the literature. Additionally, several empirical 2- and 3-parameter models, including one empirical 4-parameter model, used to describe the viscosity of lipids, were selected for investigation. These models were used to fit the experimental viscosity of 13 oleic estolide ester samples. The viscosity was measured at six temperatures between 20 and 100 °C. The jackknife procedure was used to estimate the uncertainty of the fitted parameters. For the theory-based models, correlations were derived between the fitted coefficients and the molecular weight of the sample. The results allowed for simple prediction of the viscosity of estolide esters as a function of the temperature, based only on number-average molecular weight (or on the estolide number). The best model required additional knowledge of the weight-average molecular weight.

Synthesis of Estolide Ester and Amide from Acetylated Polyhydroxy Estolide for Lubricant Base Oil

AbstractPlant oil‐based lubricants are used as alternative for mainstream petroleum‐based lubricants mainly because they are known to be environmentally friendly. However, their market acceptance is limited by their high pour point and poor oxidation stability. This study presents an approach to make biodegradable lubricant from oleic acid that shows low pour point and satisfactory oxidation stability. A mixture of estolides with acetyl and hydroxy functionalities is prepared from reaction between oleic acid, acetic acid, and hydrogen peroxide. Subsequently, the hydroxy groups of estolide are end‐capped with lauric acid to improve oxidation stability. Further reaction with alcohol and amine yielded estolide ester and amide, respectively. Physicochemical properties evaluation of prepared estolide ester and amide reveal that they have properties comparable to commercial samples in terms of pour point, oxidation stability, viscosity index, and antiwear. Furthermore, both estolide ester and amide are found to be readily biodegradable, which support their use as environmental friendly lubricant. Therefore, the inferior properties of plant oil‐based lubricants can be resolved by chemical modifications that yield specific estolide ester and amide with excellent properties suitable for lubricants.Practical Applications: The prepared estolide ester and amide have good potential to be used as lubricant base oil for environmentally acceptable lubricants due to their inherent readily biodegradability nature and excellent lubricant properties.

Synthesis and physicochemical properties of novel lauric acid capped estolide esters and amides made from oleic acid and their evaluations for biolubricant basestock

Vegetable oils have been used as environmentally friendly biolubricants because of their inherent biodegradability, good lubricity, higher viscosity index and low evaporative loss. However, their use is limited due to significantly poor cold flow properties and inferior oxidative stability. This paper presents an approach to modify vegetable oil derivatives namely oleic acid to yield biolubricant with good cold flow properties and oxidation stability. Oleic acid was converted to polyhydroxy estolide through reaction with only hydrogen peroxide, which is considered a ‘green’ oxidant. The synthesized polyhydroxy estolide was further reacted with appropriate amount of lauric acid to end-cap its hydroxy groups, which yielded lauric acid capped estolide as the product. Subsequently, the prepared lauric acid capped estolide was reacted with branched and straight chain alcohols as well as secondary amines to afford estolide esters and amides. In general, most of the prepared samples showed improved cold flow properties as compared to vegetable oil-based lubricants, where the best pour point (−41 °C) was achieved by 4-methyl-2-pentyl estolide ester. Meanwhile, estolide amides exhibited the best oxidative stability among samples evaluated with an oxidation onset temperature of 205 °C, which is significantly higher than vegetable oil-based lubricants. Generally, the oxidative stability, viscosity index and anti-wear properties of prepared estolide esters and amides were found to be comparable to the properties of commercial samples used as benchmark.

Full conversion of oleic acid to estolides esters, biodiesel and choline carboxylates in three easy steps

A new three step process has been developed and optimized in order to demonstrate that is possible to get complete conversion of free fatty acids (FFA) into estolide esters (biolubricant), biodiesel and choline carboxylates (biosurfactants). This new approach consists in the utilization of HClO4, a cheap and industrially available strong acid, to generate the estolides followed by an esterification process with methanol in presence of a cheap ionic liquid to transform them into estolide esters and the unreacted FFA into biodiesel. A final neutralization step with choline hydroxide allowed to obtain biosurfactants. Finally, estolide esters and biodiesel were optimally split by means of molecular distillation. Both products were found to have excellent properties, fulfilling the requirements of international standards SAE J306 and EN 14214 for lubricants and biodiesel, respectively and demonstrating the industrial application of FFA as raw material to produce higher added value products.

Derivatization of castor oil based estolide esters: Preparation of epoxides and cyclic carbonates

Estolides that are based on castor oil and oleic acid are versatile starting points for the production of industrial fluids with new properties. A variety of unsaturated estolides were derivatized by epoxidation with hydrogen peroxide. The epoxidized estolides were further modified using supercritical carbon dioxide and tetrabutylammonium bromide to chemically incorporate carbon dioxide into the material yielding a 5-membered cyclic carbonate structure. These new epoxides and cyclic carbonates exhibited higher pour points, oxidation onset temperatures, and viscosities, compared to the corresponding unsaturated precursors. One derivative had a dynamic viscosity of ∼9000mPa s at 40°C, demonstrating potential for use in industrial applications.

Synthesis and Characterization of Estolide Esters Containing Epoxy and Cyclic Carbonate Groups

AbstractThe unsaturated sites in oleic 2‐ethylhexyl estolide esters (containing 35 % monoenic fatty acids) were converted into epoxide and five‐membered cyclic carbonate groups and the products characterized by Fourier transform infrared spectra (FTIR), 1H, and 13C nuclear magnetic resonance (NMR) spectroscopies. Epoxidation of the alkene bonds was accomplished using performic acid generated in situ from formic acid and hydrogen peroxide. Greater than 90 % alkenes were converted into their corresponding epoxide groups as determined by oxirane values and the epoxide ring structure was confirmed by 1H and 13C NMR. The estolide ester epoxide material was subsequently reacted with supercritical carbon dioxide in the presence of tetrabutylammonium bromide catalyst to produce the corresponding estolide ester containing the cyclic carbonate group. The signals at 1,807 cm−1 and δ 82 ppm in the FTIR and 13C‐NMR spectra, respectively, confirmed the desired cyclic carbonate was produced. The carbonated estolide ester exhibited a dynamic viscosity, at 25 °C, of 172 mPa·s as compared to 155 mPa·s for the estolide ester starting material. The estolide ester structure of these new derivatives was shown to be consistent throughout their synthesis.

Synthesis and physical properties of novel estolides from dicarboxylic acids and methyl ricinoleate

Castor oil is an attractive feedstock for the preparation of a variety of industrially important products due to the presence of ricinoleic acid with hydroxyl functionality up to 85–90%. Methyl ricinoleate was esterified with different dicarboxylic acids to prepare a series of a new class of estolides under solvent and catalyst‐free conditions. The products were evaluated for their physico‐chemical and lubricant properties. The estolide esters have a wide viscosity range varying from ISO VG 15 to 46. All the products exhibited excellent pour points (−48 to <−60°C) outperforming the reported estolide products. Among the estolide esters, 2‐ethyl‐1‐hexyl esters of monoacid estolides exhibited higher oxidative onset values (199°C). Dimer estolides and 2‐ethylhexyl esters of monoacid estolides exhibited better thermal stability compared to all other estolide esters. The products having low pour points can be potential base oils for low temperature lubricant applications.Practical applications: Synthesis of potential lubricant basestocks from indigenous non‐edible oil under solvent and catalyst‐free conditions. Prepared biolubricants with very low pour points, which is difficult to achieve with vegetable oil derivatives. The products are potential candidates for low temperature applications.Estolides with low pour points.

Synthesis and physical properties of new coco-oleic estolide branched esters

Oils derived from vegetable oils tend to not meet the standards for industrial lubricants because of unacceptable low temperature properties, pour point (PP) and/or cloud point (CP). However, a catalytic amount of perchloric acid with oleic and coconut (coco) fatty acids produced a coco-oleic estolide. The resulting coco-oleic estolide was separated into two components based on the extent of oligomerization: coco-oleic dimer estolide and coco-oleic trimer plus estolide. These two estolides were then esterified with a series of different branched alcohols; the coco-oleic dimer estolide esters had the lowest PP=−45°C when with esterified 2-hexyldecanol and PP=−39°C with 2-octyldodecanol. The best CP performer from the same series was the 2-octyldodecanol ester, CP=−37°C. The coco-oleic trimer plus estolide esters had slightly higher PPs (−24 to −39°C) with the same alcohols. The viscosities and viscosity indexes were as expected in terms of trends. The coco-oleic dimer estolide esters ranged 27.5–51.7cSt @ 40°C and 3.0–9.5 cSt @ 100°C, whereas the coco-oleic trimer plus estolide esters ranged 120.8–227.7cSt @ 40°C and 17.9–29.4cSt at 100°C for the same series as the dimer esters. Outside the series tested, an iso-stearyl trimer plus ester had the highest reported viscosity of 417.3cSt @ 40°C and 38.9cSt @ 100°C. Because these new branched estolide esters have excellent viscosity and low temperature physical properties without the addition of other chemicals, they minimize the effect on the environment while replacing nonrenewable products.

Synthesis and physical properties of pennycress estolides and esters

A new series of pennycress (Thlaspi arvense L.) based free-acid estolides was synthesized by an acid-catalyzed condensation reaction, followed by an esterification reaction to produce the 2-ethylhexyl (2-EH) esters of the initial estolides. The physical properties of the estolides are highly affected by the length and unsaturation level of the capping fatty acid, the base fatty acid unit, and estolide linkage position. Both the free-acid estolides and the estolide 2-EH esters produced proved to have marked viscosity increases over previously synthesized estolides. Kinematic viscosities of the free-acid estolides were higher than the corresponding estolide 2-EH esters, ranging from 494.4cSt to 870.5cSt at 40°C and 53.6 to 75.3cSt at 100°C with viscosity indices (VI) from 134 to 163. Viscosities of the estolide esters ranged from 116.3 to 245.75cSt at 40°C and 18.2 to 33.6cSt at 100°C with viscosity indices (VI) from 169 to 183. The highest viscosity values belonged to the pennycress estolides made with pennycress fatty acids in the reaction acting as the base material, as well as the capping material. The oleic capped pennycress estolide 2-EH esters had the best low-temperature properties with a pour point (PP) of −33°C and cloud point (CP) of −31°C. With the exception of palmitic acid, there was very little variation in the low-temperature properties of the remaining saturate capped estolide 2-EH esters, PP ranging from −18 to −21°C. Pennycress is currently being developed for green diesel applications; the development of other industrial applications is a necessity for the success of an alternative crop. The combination of high viscosity and modest cold temperature properties of these pennycress estolides could fill a specialty niche as a high viscosity industrial lubricant.

Physical Properties of Low Viscosity Estolide 2‐Ethylhexyl Esters

AbstractAcetic‐ and butyric‐capped oleic estolide 2‐ethylhexyl (2‐EH) esters were synthesized in a perchloric acid catalyzed (0.05 equiv) one‐pot process from industrial 90 % oleic acid and either acetic or butyric fatty acids at two different ratios. This was directly followed by the esterification process incorporated into an in‐situ second step to provide a low viscosity estolide ester functional fluid. The monoestolide and polyestolides were separated via vacuum distillation (6–13 Pa) at 240–250 °C. The physical properties of these materials were followed throughout the synthetic process and are reported. The final low viscosity acetic‐ and butyric‐capped monoestolide 2‐EH esters had viscosities of 19.9 and 24.2 cSt at 40 °C and 4.8 and 5.5 cSt at 100 °C with viscosity indexes (VI) of 161 and 163, respectively. Both monoestolide esters displayed excellent pour points (PPs). The PPs of the two were as follows: acetic‐capped estolide 2‐EH ester PP = −45 °C and butyric‐capped estolide 2‐EH ester PP = −27 °C. The biodegradable short‐capped oleic estolide 2‐EH esters had excellent low temperature properties and should perform well in low viscosity applications.

Synthesis and physical properties of new estolide esters

Vegetable oil-based oils usually fail to meet the rigorous demands of industrial lubricants by not having acceptable low temperature properties, pour point (PP) and/or cloud point (CP). Oleic estolide was produced from oleic fatty acid and a catalytic amount of perchloric acid. The oleic estolide was then esterified with a series of 16 different alcohols that were either branched or linear-chained. The new estolide esters physical properties were recorded and evaluated as a potential industrial lubricant. The linear-chain esters had higher low temperature properties (PP=−9 to −33°C) but still compete well with other commercial bio-based materials. The oleic estolide ethyl ester yielded the best PP at −33°C for the linear-chain series. The branched alcohol produced the best PP (−24 to −39°C) and CP (−30 to <−50°C) with the best PP performers being a 2-hexyldecanol (Jarcol I-16) sample, a 16 carbon-chained branched material, and 2-octyldodecanol (Jarcol I-20), a 20 carbon branched material, with a PP at −39°C. The best CP performers from the same series were the 2-octyldodecanol, with a CP lower than −50°C followed by the 2-hexyldecanol at −42°C. The viscosities (55–209cSt @ 40°C) and viscosity indexes (VIs) (169–192) performed well. The iso-stearyl alcohol (Oxocol 180) sample had the highest viscosity at 40°C of 209.3cSt which was higher than all other materials tested. Finally, these new estolide esters required no additives in order to obtain the improved performance in low temperature physical properties, thus limiting our impact on the environment and replacing fluids that are based on non renewable resources.

Elastohydrodynamic Study of Blends of Bio‐Based Esters with Polyalphaolefin in the Low Film Thickness Regime

AbstractThe film thickness in elastohydrodynamic (EHD) conditions for soybean oil (SBO), oleic estolide ester (EST) and their binary blends with polyalphaolefins (PAO2 or PAO40) were studied at 30 and 100 °C. Changes with time, for up to 200 min, were monitored. SBO and its blends with the lower viscosity PAO2 showed initially good agreement with the Hamrock–Dowson (H–D) equation down to 1–3 nm film thickness. 60 min or more after the start of the measurements, boundary layers with thickness up to 4.7 nm were observed. The blend of SBO with the more viscous PAO40 showed initially a good agreement with H–D at 100 °C. Negative deviations in film thickness were observed 15 min after the start of the measurements. At extended periods of time, up to 200 min, they were less pronounced but still detectable. EST–PAO2 blend showed initially formation of boundary layers with thickness around 2 nm. The boundary layer at 30 °C did not change for 200 min, while at 100 °C showed a decrease in thickness and/or viscosity with time. The EST and the EST–PAO40 blends showed good agreement with the H–D equation and did not display a boundary or fractionation layer within 200 min.

Synthesis and Physical Properties of Estolide Ester Using Saturated Fatty Acid and Ricinoleic Acid

A study was conveyed to produce estolide ester using ricinoleic acid as the backbone. The ricinoleic acid reacted with saturated fatty acid from C8–C18. These reactions were conducted under vacuum at 60°C for 24 h without solvent. The reaction used acid catalyst, sulphuric acid. The new saturate ricinoleic estolide esters show superior low-temperature properties (−52 ± 0.08°C) and high flash point (>300°C). The yield of the neat estolide esters ranged from 52% to 96%. The viscosity range was 51 ± 0.08 to 86 ± 0.01 cp. These new saturated estolide esters were also compared with saturated branched estolide esters.

Synthesis and physical properties of petroselinic based estolide esters

A new series of petroselinic (Coriandrum sativum L.) based estolide 2-ethylhexyl (2-EH) esters were synthesized, as the capping material varied in length and in degrees of unsaturation, in a perchloric acid catalyzed one-pot process with the esterification process incorporated into an in situ second step to provide the coriander estolide 2-EH ester. The kinematic viscosities ranged from 53 to 75 cSt at 40°C and 9.1 to 14.6 cSt at 100°C with a viscosity index (VI) ranging from 151 to 165. The caprylic (C8) capped coriander estolide 2-EH ester had the lowest low-temperature properties (pour point=−33°C and cloud point=−33°C), while the coco-coriander estolide 2-EH ester produced an estolide with modest low-temperature properties (pour point=−24°C and cloud point=−25°C). The coco-coriander estolide 2-EH ester was explored for the ability to resist oxidative degradation with the use of an biodegradable additive package added in 1.5%, 3.5%, or 7.0% units based on weight. The oxidative stability increased as the amount of stability package increased (rotating pressurized vessel oxidation test (RPVOT) times 65–273min). Along with expected good biodegradability, these coriander estolide 2-EH esters had acceptable properties that should provide a specialty niche in the U.S. as a biobased lubricant.

Non-linear adsorption modeling of fatty esters and oleic estolide esters via boundary lubrication coefficient of friction measurements

The frictional behaviors of a variety of fatty esters (methyl oleate (MO), methyl palmitate (MP), methyl laurate (ML), and 2-ethylhexyl oleate (EHO)) and oleic estolide esters (methyl oleic estolide ester (ME) and 2-ethylhexyl oleic estolide ester (EHE)) as additives in hexadecane have been examined in a boundary lubrication test regime using steel contacts. Critical additive concentrations were defined and used to perform novel and simple Langmuir analyses that provide an order of adsorption energies: EHE ≥ ME > EHO > MP > MO ≥ ML. Application of Langmuir, Temkin, and Frumkin–Fowler–Guggenheim (FFG) adsorption models via non-linear fitting demonstrates the necessary inclusion of cooperative effects in the applied model. Fits of the steady-state coefficient of friction (COF)-concentration data for EHE, ME, and EHO indicate slight cooperative adsorption. MO, MP, and ML data require larger attractive interaction terms ( α ≤ −2.3) to be adequately fit. Primary adsorption energies calculated via a general adsorption model are necessarily decreased while total adsorption energies correlate well with values obtained via critical concentration analyses. To account for multiple surface-site coverage a multiple-site model was defined. The intuitive assumption of multiple-site coverage of more massive components suggests deceptively increased calculated adsorption energies for typically applied models (e.g. FFG, Langmuir).

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 cuphea‐oleic estolides and esters

AbstractCuphea‐oleic estolides and esters were synthesized from cuphea and oleic FA with various amounts of perchloric acid (0.01 to 0.40 equiv) at 60°C. Estolide yields ranged from 30 to 65% after Kugelrohr distillation. Estolide number (EN), the average number of FA units added to a base FA, varied with reaction conditions. Cuphea‐oleic estolides were esterified with 2‐ethylhexanol to obtain high yields of the corresponding ester. A streamlined, one‐pot process was used to synthesize the estolide and its ester with 0.05 equiv of HClO4, with, esterification incorporated into an in situ second step to provide a functional fluid at a very reasonable cost. The physical properties of the cuphea‐oleic estolides and estolide esters, including their viscosities, pour points, and cloud points, were related directly to the amount of oligomerization (EN), i.e., viscosity increased with higher oligomerization. The viscosity index ranged from 132 to 166 cSt for the free‐acid estolides, whereas the complex estolide 2‐ethylhexyl esters had slightly high viscosity indices that ranged from 165 to 181 cSt. These new cuphea‐oleic estolide esters displayed good low‐temperature properties (pour point −42°C and cloud point −41°C).

Improved oxidative stability of estolide esters

Some concerns have been raised regarding the oxidative stability of vegetable-based fluids. Thus, a wide range of commercial and vegetable-based materials were evaluated for their oxidative stability by the rotating bomb oxidative test (RBOT). RBOT values ranged from 13 to 552 min. Two estolides, coconut-oleic estolide 2-ethylhexyl ester (coco) and oleic estolide 2-ethylhexyl ester (oleic), were evaluated for their oxidative stability by RBOT. As in the case with all vegetable oils, an oxidative stability package must be utilized to help decrease their rate of oxidation. A series of formulations were conducted in which the two estolides had an oxidative stability package added prior to the RBOT. In both cases, dramatic increases in the oxidative stability were observed. The coconut-oleic estolide 2-ethylhexyl ester gave the best RBOT values, 504 min with 3.5% oxidative stability package. Both estolides were formulated to meet commercial crankcase requirements (∼200 min) with as little as 1.0 and 1.5% oxidative stability package. The viscosity index ranged from 179 to 190 for the oleic estolide 2-ethylhexyl ester, whereas the coconut-oleic estolide 2-ethylhexyl ester had slightly lower viscosity indices ranging from 161 to 174. These two estolide esters have displayed far superior oxidative stability, 504 (coco) and 426 (oleic) min, are of reasonable cost, and were more suitable as a base stock for biodegradable lubricants and functional fluids than current commercial, vegetable-based 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.