Comparison of a New Estolide Oxidative Stability Package

The rotating pressurized vessel oxidation test (RPVOT) was used in the analysis and determination of a new oxidative stability package (OSP) for a series of estolide based materials. Three antioxidants (BHT, two different alkylated diphenyl amines) were used in either 0.5 or 1.0 wt/wt%, in different ratios, and in conjunction with one another (hindered phenol/alkylated diphenyl amines or hindered phenol/mixed alkylated diphenyl amines). The estolide-based samples analyzed for their resistance to oxidation were two pure (distilled) estolides (oleic estolide 2-EH esters and coco-oleic estolide 2-EH esters), an estolide mixture that was analyzed straight from the reaction (coco-oleic estolide 2-EH esters with coco 2-EH esters) and finally the ester fraction from the estolide mixture (coco 2-EH esters). The coco estolide mixture and coco 2-EH esters had the best overall RPVOT times with 1.0% of the alkylated diphenyl amine, coco estolide mixture, 326 min, and coco 2-EH esters, 310 min. Coco estolides were expected to have an advantage over the simple oleic estolides due to the increase in saturation in the estolide. Unexpectedly, the two distilled estolides (oleic and coco) had very similar RPVOT max times with all the antioxidants, and were much higher than the other oxidative packages tested to date. In general, the alkylated diphenyl amine outperformed mixed alkylated diphenyl amines in the majority of the individual samples tested specially the coco 2-EH esters and distilled coco-oleic estolide 2-EH esters material at 1% OSP. Overall, a series of new antioxidants were tested and compared to other commercial products. A variety of physical properties of the four estolide based material were collected and compared to commercially acceptable material. Coco-oleic estolide 2-EH esters were formulated to have excellent pour points (−36 °C), were both oxidatively and hydrolytically stable (RPVOT, 310 min), with expected good biodegradability which should help commercialization. Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by USDA implies no approval of the product to the exclusion of others that may also be suitable.     

Cross‐Linking of Polyesters Based on Fatty Acids

This study aims at obtaining cross‐linked polymeric materials of biomass origin. For this purpose, one‐pot polyesterification of methyl ricinoleate and methyl 12‐hydroxystearate using titanium isopropoxide as a catalyst is performed leading to polyesters known as estolides. The obtained estolides are successfully cross‐linked using dicumyl peroxide or a sulfur vulcanization system. The so‐formed bio‐based elastomers appear to exhibit promising properties. The latter are analyzed by mechanical tensile tests and thermal techniques (TGA, DSC, DMA) and show high thermal stability (T_5% = 205–318 °C), tailored physico‐mechanical properties (low glass transition temperature in the range from ?69 to ?54 °C), and good tensile strength (0.11–0.40 MPa). Networks prepared from high molecular weight estolides appear to be promising bio‐based elastomers. Practical Applications: The vegetable oil‐based estolides described in this contribution are new fully bio‐based precursors for further elastomers synthesis. The resulting estolide networks (obtained by peroxide or sulfur cross‐linking) exhibit tailored thermo‐mechanical properties.     

Synthesis and physical properties of cuphea-oleic estolides and esters

Cuphea-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).     

Synthesis and Characterization of Polyethylene Glycol Diesters from Estolides Containing Epoxides and Diols

Estolides from oleic acid, 12-hydroxystearic acid, and methyl ricinoleate were synthesized and converted to polyethylene glycol (PEG) diesters. Oleic estolide was synthesized from oleic acid as a homo-oligomeric material using perchloric acid in 68.8% yield and an estolide number (EN) value of 1.29. Estolides from 12-hydroxystearic acid were homo-oligomers made by heating under vacuum at 150 °C for 24 hours to give a quantitative yield of estolide with an EN value of 2.55. Oleic acid-based estolides and 12-hydroxystearic acid-based estolide were esterified with PEG-200 diol to form PEG 200 diesters. Ricinoleate estolides was capped with lauric acid or 12-hydroxystearic estolide by reacting methyl ricinoleate with the corresponding fatty acids at 150 °C using tin(II) octoate as a catalyst. The corresponding estolides were transesterified with PEG-200 diol to form the diesters. The residual olefin of ricinoleate was then epoxidized and underwent ring opening hydrolysis to form the corresponding diol. NMR spectroscopy (¹H, ¹³C, distortionless enhancement by polarization transfer, heteronuclear single quantum correlation, heteronuclear multiple bond correlation, and correlated spectroscopy) was used to characterize the products.     

Viscometric Properties of Polyethylene Glycol di-Esters of Estolides

Polyethylene glycol (PEG) diesters from estolides of oleic acid, ricinoleic acid, and 12-hydroxy stearic acid were used up to 5 wt% additive in petroleum-derived base oils: 100 N, 220 N, 600 N, PAO 2cSt, PAO 4cst, and PAO 8cSt. The viscosity indices of the petroleum base oils were the lowest (VI = 104–108); the polyalpholefin (PAO) synthetic petroleum-derived base oils were better (VI = 124–136) and the vegetable-derived oils were the best (VI = 111–205). 12-Hydroxystearic estolide PEG 400 diester gave the largest increase in viscosity index in 5% w/w admixtures with PAO 2cSt, 4cSt, 220 N, and 600 N base oils with a 12.3–16.3% increase in viscosity index. Single fatty chain esters had the least impact on viscosity index. As the molecule bulk increased, the viscosity index also increased. This viscosity index effect was demonstrated for the series of monomer to oleic estolide to oleic estolide PEG-200 diester, which gave viscosity indices of 110, 111–122, respectively, in 5% admixtures with 100 N base oil. The larger the size of the molecule coupled with the polar PEG moiety gave the largest impact on viscosity index improvement where a 5 wt% admixture of 12-hydroxystearate estolide PEG-200 diester gave a 25.8% increase in 100 °C viscosity in PAO 2cSt. Incorporation of a dihydroxyl moiety into the molecule at a central side chain location in laurate-capped ricinoleate estolide PEG-200 diester did not increase viscosity index of the base oil but greatly reduced its solubility to less than 0.5 wt%.     

Fatty Acid Estolides: A Review

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.     

Synthesis of estolides from oleic and saturated fatty acids

Oleic acid and various 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. Yields varied between 45 and 65% after Kugelrohr distillation. The estolide number (EN), i.e., the average number of fatty acid units added to a base fatty acid, varied as a function of temperature and saturated fatty acid. The shorter-chain saturated fatty acids, i.e., butyric and hexanoic, provided material with higher degrees of oligomerization (EN=3.31) than stearic acid (EN=1.36). The individual, saturated fatty acid estolides each have very different characteristics, such as color and type of by-products. The higher-temperature reactions occurred at faster rates at the expense of yield, and lactones were the predominant side products. At 55°C, lactone yields increased, but the δ-γ-lactone ratio decreased; this led to lower estolide yields. The opposite trend was observed for the 45°C reaction. The saturate-capped, oleic estolides were then esterified with 2-ethylhexyl alcohol, and the chemical composition of these new estolides remained consistent throughout the course of the reaction.     

Studies in estolides. I. Kinetics of estolide formation and decomposition

A study has been made of the rate of formation of estolides from castor oil fatty acids and of their decomposition at different temps. Estolide formation appears to be optimum at the end of 5 hr at about 220C, beyond which estolides start decomposing, giving rise to DCO fatty acids. At about 300C, estolide decomposition is complete within an hour. During estolide formation there is a linear increase in optical rotation which again decreases linearly, although at a different rate, during estolide decomposition. At optimum estolide formation, the optical rotation is as high as 25.5. Part of the thesis submitted by S. N. Modak to the University of Bombay for the degree of Ph.D.     

Soybean Oil Fatty Acid Ester Estolides as Potential Plasticizers

Fatty acid ester estolides were synthesized from soybean oil and evaluated for plasticizer functionality in poly(vinyl chloride) (PVC). The plasticization ability of the fatty acid ester estolides depends upon the molecular features such as polarity, molecular weight, and branching. The structure of the fatty acid derivatives was modified at the ester head group with various alcohols and the estolide branch was created at the site of unsaturation. Soy fatty acid esters of methanol, iso-butanol, 2-ethylhexanol, and glycerol were prepared to vary the size and polarity at the ester head group. Estolides of these fatty acid esters were prepared using two synthetic routes. In the first route, the fatty acid ester was condensed with an aliphatic acid at the site of unsaturation in the presence of a strong mineral acid. In the second route, the fatty acid ester double bonds were converted to epoxy groups, which were ring opened and acetylated to form acetate estolides. The first synthetic route resulted in low-average estolide content per fatty acid chain while the second route resulted in a higher estolide content per fatty acid chain. The fatty acid ester estolides compounded with PVC showed good plasticizer properties as evidenced by the rheological properties and reduction in glass transition temperature. The fatty acid ester estolides with a higher estolide content had better plasticizer functionality, comparable to commercial controls.     

Mineral acid-catalyzed condensation of meadowfoam fatty acids into estolides

Meadowfoam fatty acids (83% monoenoic fatty acid), reacted with 0.01–0.1 mole equivalents of perchloric acid, gave 33–71% yield of estolide, an oligomeric 2° ester, resulting from self condensation. Equimolar amounts of perchloric acid to fatty acid failed to produce estolide but converted the fatty acids to a mixture of lactones, mainly γ-eicosanolactone. Temperature plays a critical role; higher temperatures (75–100°C), at the same acid concentration, provide lactones while lower temperatures (20–65°C) yield estolides. Lower acid levels (<0.1 mole equivalents) gave the best yields (≈70%) at 65°C. The estolide and monomer were characterized by nuclear magnetic resonance, infrared high-pressure liquid chromatography, gas chromatography, gas chromatography/mass spectrometry. The estolide is a mixture of oligomers with an average distribution near 1.65 ester units. The ester linkages are located mainly at the original double bond positions but have some positional isomerization to adjacent sites in accord with carbocation migration along the alkyl chain. The residual double bond of the estolide was extensively isomerized fromcis totrans and positionally along the chain. The distilled monomer is similar in structure to the unsaturated portion of the estolide with geometrical and positional double bond isomerization. In addition, a significant amount of cyclization of the fatty acids to lactone (≈30%) had occurred.     

Viscous,thermal and tribological characterization of oleic and ricinoleic acids-derived estolides and their blends with vegetable oils

This work deals with the viscous, thermal and tribological characterization of a variety of estolides, obtained from both oleic and ricinoleic acids, using different acid-catalysed synthesis protocols, and their blends with vegetable (high-oleic sunflower, HOSO, and castor, CO) oils. Estolides with molecular weights between 4.4 and 6.9 times higher than the originating fatty acids were obtained. Polymerization degree was larger when using the sulphuric acid-catalysed synthesis protocol. Estolides obtained from oleic acid displayed higher freezing temperatures than the fatty acid, whereas the crystallization process was delayed in estolides obtained from ricinoleic acid, yielding improved low-temperature properties. Ricinoleic acid-derived estolides showed much higher viscosity values than those prepared from the oleic acid, with values of kinematic viscosity up to around 6700 mm²/s. In general, viscosities were related to estolide molecular weight. Significant increments in HOSO and CO viscosities were found when they were blended with estolides, especially those prepared from the ricinoleic acid using the sulphuric and p-toluensulphonic acids-catalyzed methods. Relative increments in kinematic viscosities up to 1500% and 700% were obtained for HOSO and CO, respectively. HOSO's viscosity-temperature dependence was significantly improved when it was blended with different estolides, whereas CO/oleic acid-derived estolides blends showed a more moderate improvement of CO thermal dependence. The sulphuric acid-catalysed method influences friction and wear in the ball-on-plates contact lubricated with estolides. The addition of the different estolides to HOSO or CO does not modify their frictional behavior, resulting in just one single Stribeck curve for all samples, and significantly reduces wear.     

A novel bio‐based phthalonitrile resin derived from catechin: synthesis and comparison of curing behavior with petroleum‐based counterpart

The development of bio‐based thermosetting resins with good thermal stability can potentially afford sustainable polymers as replacements for petroleum‐based polymers. We report a practical route to a novel catechin‐based phthalonitrile resin precursor (CA‐Ph), which contains free phenolic hydroxyl groups that result in ‘self‐curing’ at elevated temperatures to afford a thermostable polymer. Comparison of the performance of this CA‐Ph resin with that of a conventional petroleum‐based bisphenol A phthalonitrile resin (BPA‐Ph; containing 5 wt% of the curing agent 4,4′‐diaminodiphenylsulfone) revealed that CA‐Ph exhibits a lower melting point and curing temperature. Cured CA‐Ph resin retains 95% of its weight at 520 °C under a nitrogen atmosphere, which compares favorably with results obtained for BPA‐Ph resin that retains 95% of its weight at a lower temperature of 484 °C. Kinetic results indicated that the curing reactions of both CA‐Ph and BPA‐Ph systems follow an autocatalytic mechanism. These results suggest that catechin is a useful bio‐based feedstock for the preparation of self‐curing and thermally stable phthalonitrile resins for advanced technological applications. © 2017 Society of Chemical Industry     

Lipase-catalyzed synthesis and properties of estolides and their esters

Eight lipases were screened for their ability to synthesize estolides from a mixture that contained lesquerolic (14-hydroxy-11-eicosenoic) acid and octadecenoic acid. With the exception ofAspergillus niger lipase, all 1,3-specific enzymes (fromRhizopus arrhizus andRhizomucor miehei lipases) were unable to synthesize estolides.Candida rugosa andGeotrichum lipases catalyzed estolide formation at >40% yield, with >80% of the estolide formed being monoestolide from one lesquerolic and one octadecenoic acyl group:Pseudomonas sp. lipase synthesized estolides at 62% yield, but the product mixture contained significant amounts of monoestolide with two lesquerolic acyl groups as well as diestolide. ImmobilizedR. miehei lipase was chosen to catalyze the esterification of mono-and polyestolide, derived synthetically from oleic acid, with fatty alcohols or α,ω-diols. Yields were >95% for fatty alcohol reactions and >60% for diol reactions. In addition, the estolide linkage remained intact through the course of the esterification process. Esterification of estolides improved the estolide’s properties—for example, lower viscosity and higher viscosity index—but slightly raised the melting point. Estolides and, particularly, estolide esters may be suitable as lubricants or lubricant additives.     

From phenolics to phenolipids: Optimizing antioxidants in lipid dispersions

Natural non‐polymeric phenolics are known to be powerful antioxidants, but their use for preventing lipid oxidation in unsaturated fat‐based products, such as foodstuffs and cosmetics, is limited owing to their hydrophilic character. A promising technique – referred to as lipophilization – consists of covalently grafting a lipid moiety to the phenolic part to improve both surface activity and oil solubility of a given phenolic antioxidant. After being based on empiricism for decades, the lipophilization of phenolics is today more rationalized, as recent advances in the field have shown that the grafting of a medium chain‐length is the best strategy to design powerful, custom‐made phenolic antioxidants to protect lipid dispersion against oxidation. To illustrate, we report here the main strategies in the lipophilization of phenolic acids, the effect of such modification on the antioxidant properties, and current and potential applications of the resulting phenolipids.     

Synthesis and Evaluation of Soy Fatty Acid Ester Estolides as Bioplasticizers in Poly(Vinyl Chloride)

Plasticizers are nonvolatile organic liquids that impart flexibility to polymers. Due to environmental, health, and safety reasons, the industry is looking for bioplasticizers to replace petroleum-derived phthalates. To fulfill this need, soy fatty acid ester estolides were synthesized, characterized, and evaluated as phthalate replacements. Soybean oil was transesterified with methanol or glycerol to form lower molecular weight fatty acid esters that were epoxidized and ring opened with acetic acid and acetylated to give the final products. Ring opening and acetylation of the epoxidized oleic acid esters gave acyclic acetate fatty acid ester estolides, whereas the polyunsaturated fatty acid esters, linoleate, and linolenate gave cyclic tetrahydrofuran derivatives and cross-linked higher molecular weight materials. The cyclization mechanism to form the tetrahydrofuran derivatives was postulated. Soy fatty acid ester estolides were compounded with formulated poly(vinyl chloride), (PVC) and tested for their functional properties. The physical and functional properties of the new bioplasticizers were compared with commercial plasticizers. The elasticity of PVC compounded with experimental plasticizers and commercial phthalates was comparable. PVC compounded with fatty acid methyl ester estolide showed lower glass transition temperature and similar tensile properties compared to PVC compounded with the commercial phthalate. PVC compounded with the glyceryl fatty acid ester estolide showed a higher glass transition temperature, higher tensile properties compared to PVC compounded with the commercial phthalate.     

Physical Properties of Low Viscosity Estolide 2-Ethylhexyl Esters

Acetic‐ 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.     

Methods for increasing estolide yields in a batch reactor

Estolides are formed when the carboxylic acid group of one fatty acid forms an ester link at the site of unsaturation of another fatty acid. These compounds have the potential to be used in a variety of applications, such as lubricants, greases, plastics, inks, cosmetics, and surfactants. By manipulating the reaction equilibrium, yields of 20% estolide in clay-catalyzed estolide reactions have been increased to 30%. Reactions conducted at 180°C, where water was vented out of the reactor at specific times, not only gave dimer-free estolides but also yields up to 30%. Steam has also been used instead of water with similar results. Estolides were quite stable at temperatures up to 250°C, even when they were exposed to air.     

Estolides: From structure and function to structured and functionalized

Oligomeric fatty acid esters, known as estolides, have been studied for close to a century. It has only been in the last few years, however, that efficient methods for preparing estolides have been developed that also allow some control over their molecular structures. By varying the bio‐based feedstocks and reaction conditions, the functional properties, particularly those geared toward lubrication, can be modified. More recently, synthetic advances have permitted stepwise construction of estolides with functional groups (hydrophilic or reactive) positioned at specific sites within the chain.     

Sophorolipid-Derived Unsaturated and Epoxy Fatty Acid Estolides as Plasticizers for Poly(3-Hydroxybutyrate)

Unsaturated and epoxy fatty acid estolides were synthesized from the ω and ω‐1 hydroxy fatty acids derived from sophorolipids (SL). These estolides were utilized as additives in solution‐cast poly(3‐hydroxybutyrate) (P3HB) films and their plasticizing effects reported. Estolides in the P3HB film matrix resulted in slight reductions of melting and glass transition temperatures while the crystallinity remained relatively constant (±2.0 %). Scanning electron microscopy revealed irregular film surfaces in the presence of both estolides and the formation of pores within the P3HB film matrix. These irregularities influenced the tensile properties of the films by incrementally decreasing the tensile strength and moduli of the P3HB films and increasing their elongation. Curing the films for 3 months in the presence of estolide triggered an enhanced tensile strength and modulus when compared to the initial films. Irradiation was implemented in an attempt to crosslink the estolides. Results showed that radiation did not result in crosslinking but rather instigated chain scission and reduced the molecular weights by up to 81 % which further reduced the tensile strength, elongation and modulus of the P3HB films. These tensile property variations showed that while the tensile strength of the P3HB films declined in the presence of both estolides, the plasticity and elongation improved validating that these unique SL‐derived estolides can be effectively used as plasticizers in P3HB films.     

LDPE‐based blends and films stabilized with nonreleasing polymeric antioxidants for safer food packaging

Several novel random copolymers of ethylene and 1‐olefin counits bearing a highly efficient phenolic antioxidant moiety placed at different distances from the polymerizable double bond were prepared in the presence of a metallocene catalyst. These copolymers were melt‐blended with an antioxidant‐free LDPE in an internal batch mixer to obtain innovative materials containing nonreleasing polymeric antioxidants suitable for safer food packaging applications. Blends and films, obtained by compression molding, were tested for their thermal and thermo‐oxidative stability by thermogravimetric analysis both in dynamic and isothermal conditions. Films containing the macromolecular antioxidants showed a longer induction time before O₂ uptake starts and, consequently, a higher degradation temperature than neat LDPE or LDPE containing a low molecular weight commercial additive. Aging tests demonstrated that the new polymeric antioxidants also exert a valid protection against photo‐oxidation. Eventually, migration tests demonstrated the absence of any trace of products containing the antioxidant moiety when the films were kept in contact with a food simulant. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012