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Synthesis and physical properties of new coco-oleic estolide branched esters
Industrial Crops and Products 74 (2015) 171–177
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Synthesis and physical properties of new coco-oleic estolide branched esters夽 Steven C. Cermak a,∗ , Jakob W. Bredsguard b , Katie L. Roth b , Travis Thompson b , Kati A. Feken b , Terry A. Isbell a , Rex E. Murray a a Bio-Oils Research Unit, National Center for Agricultural Utilization Research, USDA, Agricultural Research Service, 1815 N. University St., Peoria, IL 61604, USA b Biosynthetic Technologies, 2 Park Plaza, Suite 200, Irvine, CA 92614, USA
a r t i c l e
i n f o
Article history: Received 26 January 2015 Received in revised form 21 April 2015 Accepted 8 May 2015 Keywords: Biodegradable oils Esters Estolides Physical properties Pour points Viscosities
a b s t r a c t 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: cocooleic 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 27.5–51.7 cSt @ 40 ◦ C and 3.0–9.5 cSt @ 100 ◦ C, whereas the coco-oleic trimer plus estolide esters ranged 120.8–227.7 cSt @ 40 ◦ C and 17.9–29.4 cSt 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 417.3 cSt @ 40 ◦ C and 38.9 cSt @ 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. Published by Elsevier B.V.
1. Introduction Due to increasing petroleum prices and the desire for consumers and producers to find and use environmentally friendly products, interest in a class of esters based on vegetable oils called estolides (Isbell and Kleiman, 1994) has continued to increase. Estolides are formed when the carboxylic acid functionality of one fatty acid (FA) reacts at the site of unsaturation of another FA to form an ester linkage. A number of these linkages are used to help characterize the structure of the estolide since the estolide number (EN) is defined as the average number of FAs added to a base FA (Fig. 1, EN = n + 1 where n is an integer). Thus, if n = 0, the EN would be one which
夽 Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer. ∗ Corresponding author. Tel.: +1 309 681 6233; fax: +1 309 681 6524. E-mail address: [email protected] (S.C. Cermak). http://dx.doi.org/10.1016/j.indcrop.2015.05.011 0926-6690/Published by Elsevier B.V.
would represent a dimer estolide (III), two FA molecules linked together. An EN value of two or higher would indicate multiple ester linkages and is termed a trimer plus estolide (IV) as shown in Fig. 1. The secondary ester linkages of the estolide are more resistant to hydrolysis than those of triglycerides (Cermak and Isbell, 2003a; Potula et al., 2009), and the unique structure of the estolide results in materials that have far superior physical properties for certain applications than vegetable and mineral oils (Cermak and Isbell, 2002a). There have been numerous reports on the physical properties of two different types of estolide esters. First, the oleic-based estolide esters which were originally discovered by Isbell et al. (2000); and second, the saturated capped oleic-based estolide esters discovered by Cermak and Isbell (2001a,b); Cermak and Isbell (2001a,b). The term “saturated capped” refers to estolides that have a saturated FA, coconut or coco, as the last FA to add across the double bond during the oligomerization reaction. In general, both types of these estolides are usually esters of 2-ethylhexyl (2-EH) alcohol. The extra branching (of the 2-EH ester) and chain length located in the ester
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Fig. 1. Coco-Oleic estolide synthesis and separation. *The estolide position was distributed from positions 5–13 with the original 9 and 10 positions having the greatest abundances. Table 1 Comparison of low temperature properties of estolide 2-ethylhexyl esters and commercial lubricants. Pour point (◦ C)
Lubricant a
Commercial petroleum oil Commercial synthetic oila Commercial soy-based oila Commercial hydraulic fluida Commercial aviation oila Oleic 2-EH estolide ester Coco-Oleic 2-EH estolide ester a b
−21 to −27b −21 to −33b −12 to −18b −24 to −33b −30 to −33b −30 −33
Fully formulated materials from local vendors. Range of a sampling of a number of different manufacturer samples.
portion are also responsible for their improved low temperature properties. The saturated capped estolide esters have become very versatile and desirable in the formulation of bio-based fluids. The saturated estolides have increased oxidative stability (Cermak et al., 2008; Cermak and Isbell, 2003a) and improved low temperature properties (Cermak and Isbell, 2003b) over the traditional oleic based estolide esters. In general, both types of estolide 2-EH esters (oleic and saturated capped) compared well to other fully formulated petroleum- and bio-based fluids, and even exceeded them in terms of pour point (PP, Table 1). When compared to finished motor oils, the cloud points (CP) of the estolide 2-EH esters were far superior to all samples tested (Cermak, 2006). The estolides do not have the normal disadvantages that traditional vegetable oils
have, such as poor thermal oxidative stability (Becker and Knorr, 1996; Cermak and Isbell, 2003a), low hydrolytic stability (Herdan, 1999) and poor low temperature properties (Asadauskas and Erhan, 1999). Estolides and estolide esters have been known in the literature for some time and current research continues to gain a better understanding of the structural functions of estolides in fluids. To date, various types of estolides and estolide esters have been tested and developed (Bredsguard, 2011a,b; Bredsguard et al., 2011; Greaves and Khelidj, 2012; Vinci and Beckerdite, 2011); additionally, estolides from new crop sources are being evaluated (Cermak et al., 2006; Cermak and Isbell, 2004; Cermak et al., 2011). New applications are being evaluated in a host of different applications ranging from biodiesel (Moser et al., 2008) to bio-based products (Bredsguard et al., 2012; Cermak and Isbell, 2009; Cermak et al., 2007) Cermak et al. (2013) have recently synthesized different esters of the oleic estolide series and explored the effects of the various chain lengths and branching on the physical properties of estolides. The branched esters out-performed the linear esters in terms of low temperature properties. The branched alcohols 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, Fig. 2) sample, a 16 carbon-chained branched material, and 2-octyldodecanol (Jarcol I20, Fig. 2), a 20 carbon branched material, with a PP at −39 ◦ C. All the estolides presented by Cermak et al. (2013) were estolide ester
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Fig. 2. Structures, melting points, and commercial names for complex branched alcohols.
mixtures (estolides that contain both dimer and trimer plus estolide esters). There have been no reports on the lubrication properties of isolated individual oligomeric estolides from oleic sources. With the fact that the saturated oleic estolide esters have better stability and low temperature properties, the question becomes what part(s) of the estolide ester mixture are most responsible for controlling these enhanced properties. There have been no reports in the literature on the separation or isolation of an oleic based estolide on molecular size. Given the ability to measure properties of the dimer estolides, EN = 1, or the trimer plus estolides, EN > 2, an understanding of molecular estolide interactions might be possible. When this theory is combined with the best performing estolide, saturated capped or coco-oleic estolide, new advanced materials could be discovered. Cermak and Isbell (2004) reported that by varying the capping material on the estolide, the crystal lattice structure of the material was disrupted as it approached its PP, which led to estolide esters with excellent low temperature properties, PPs of −36 ◦ C and CPs of −41 ◦ C. Finally, when combined with the best performing ester groups as defined by Cermak et al. (2013) with the oleic based estolides, the new series of separated dimer and trimer plus estolide esters could display unique physical properties. This paper evaluates physical properties (PP, CP, and viscosity) of a series of new and different estolide esters. As shown in Fig. 1, the general coco-oleic estolide mixture (I) underwent distillation to remove the monomer and the resulting residue, coco-oleic estolide (II), was distilled further using a molecular distillation apparatus to separate coco-oleic dimer estolide (III) from the coco-oleic trimer plus estolide (IV). Once separated, each estolide was esterified with a range of branched alcohols under similar conditions to generate a series of coco-oleic estolide esters (V and VI, Fig. 3). The cold temperature properties of these new estolide esters should surpass those of commercial products already available such as petroleumbased hydraulic fluids, soy-based fluids, and petroleum oils, while other physical properties should compete auspiciously with the current lubricants.
2. Materials and methods 2.1. Materials The fatty acid methyl ester (FAME) standard mixtures were obtained from Alltech Associates, Inc. (Deerfield, IL) and Nu-Chek Prep, Inc. (Elysian, MN). Methanol, ethanol, isobutanol, and 2ethylhexanol were from Sigma–Aldrich Corporation (St. Louis, MO). 2-Propylheptanol was from the BASF Chemical Company (Triangle Park, NC). The hydrogenated isobutanol was from Gevo, Inc. (Englewood, Co.). The 2-butyloctanol (Jarcol I-12), 2-hexyldecanol (Jarcol I-16) and 2-octyldodecanol (Jarcol I-20) were obtained as free sam-
ples from Jarchem Industries, Inc. (Newark, NJ). Iso-stearyl alcohol (Oxocol 180) was obtained as free samples from Nissan Chemical Industries, Ltd. (Houston, TX). Magnesol®XL was obtained as a free sample from the Dallas Group of America, Inc. (Whitehouse, NJ). 2.2. Equipment and procedures 2.2.1. Gas chromatography (GC) A Hewlett-Packard 6890N Series gas chromatograph (Palo Alto, CA) equipped with a flame ionization detector and an autosampler/injector was used for GC analysis. Analyses were conducted on an SP-2380 30 m × 0.25 mm i.d. column (Supelco, Inc., Bellefonte, PA). Saturated C8–C30 FAMEs provided standards for making FA and by-product assignments. Parameters for SP-2380 analysis were: column flow 1.4 mL/min with a helium head pressure of 136 kPa; split ratio 50:1; programmed ramp 120–135 ◦ C at 10 ◦ C/min, 135–175 ◦ C at 3 ◦ C/min, 175–265 ◦ C at 10 ◦ C/min, hold 5 min at 265 ◦ C; injector and detector temperatures set at 250 ◦ C. 2.2.2. GC analysis of hydroxy and non-hydroxy FAs for EN Analytical hydroxy and non-hydroxy samples for GC were prepared using procedures described by Cermak and Isbell (2001b). 2.2.3. High performance liquid chromatography Reverse phase HPLC, analyses were performed on a Thermo Separations Spectra System AS1000 autosampler/injector (Fremont, CA) with a P2000 binary gradient pump from Thermo Separation Products (Fremont, CA) coupled to a Alltech ELSD 500 evaporative light scattering detector (Alltech Associates). A C-8 reverse phase analysis used to separate reaction mixtures was carried out with a Dynamax column (250 mm × 4.5 mm, 5- particle size) from Rainin Instrument Co. (Woburn, MA). Two methods for reverse phase analysis were used to separate the reaction mixtures. Method A (16 min run time) was used to follow the reaction and distillation operations. It provided information on the overall progress of the reaction as well was verified the removal of oleic acid and esters during the distillation of the estolide mixture (I) to yield coco-oleic estolide (II). Method B (35 min run time) produced a more detailed separation of the reaction mixture, in particular a separation of estolide esters and fatty esters. The same general trend is observed as reported by Isbell and Kleiman (1994). Operating parameters for method A were: flow rate of 1 mL/min; 0–4 min 80% acetonitrile, 20% acetone; 6–10 min 100% acetone; 11–16 min 80% acetonitrile, 20% acetone. The ELSD drift tube was set at 55 ◦ C with the nebulizer set at 138 kPa N2 , providing a flow rate of 2.0 standard liters per minute (SLPM). Retention times for eluted peaks: estolides, 9.8–13.0 min; methyl oleate, 6.3 min; and oleic acid, 5.1 min.
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2.2.9.2. Coco-oleic
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estolide
esters
(V
and
VI).
Fig. 3. Coco-Oleic Dimer and Trimer plus estolide ester synthesis.
Operating parameters for method B were: flow rate of 1 mL/min; 0–2 min 60% acetonitrile 40% acetone; 20–25 min 100% acetone; 30–35 min 60% acetonitrile 40% acetone. The ELSD drift tube was set at 55 ◦ C with the nebulizer set at 138 kPa N2 , providing a flow rate of 2.0 SLPM. Retention times for eluted peaks: estolide esters, 9.0–25.6 min and methyl oleate, 5.5 min. 2.2.4. Acid value (AV) The 751 GPD Titrino from Metrohm Ltd. (Herisau, Switzerland) was used for measurements. AVs were determined by the official AOCS Method Te 2a-64 (Firestone, 1994) with ethanol substituted for methanol to increase the solubility of the estolide esters during the titration. All AVs were run in duplicate and average values were reported. 2.2.5. Gardner color A Lovibond 3-Field Comparator from Tintometer Ltd. (Salisbury, England) using the official AOCS method Td 1a-64 (Firestone, 1994) was used for Gardner color measurements. The “+” and “−” notation was employed to designate samples that did not match one particular color. 2.2.6. Pour point (PP) The official ASTM Method D 97–96a was used to measure PPs to an accuracy of ±3 ◦ C. The PPs were determined by placing a test jar with 50 mL of the sample into a cylinder submerged in a cooling medium. The sample temperature was measured in 3 ◦ C increments at the top of the sample until the material stopped pouring. This point was determined when the material in the test jar did not flow when held in a horizontal position for 5 s. The temperature of the cooling medium was chosen based on the expected PP of the material. Samples with PPs that ranged from +9 to −6, −6 to −24, and −24 to −42 ◦ C were placed in baths with temperatures at −18, −33, and −51 ◦ C, respectively. The PP was defined as the coldest temperature at which the sample still poured. All PP measurements were conducted in duplicate and average values reported. 2.2.7. Cloud point (CP) The official ASTM Method D 2500-99 was used to measure CPs to an accuracy of ±1 ◦ C. The CPs were determined by placing a test jar with 50 mL of the sample into a cylinder submerged into a cooling medium. The sample temperature was measured in 1 ◦ C increments
at the bottom of the sample until any cloudiness was observed at the bottom of the test jar. The temperature of the cooling medium was chosen based on the expected CP of the material. Samples with CPs that ranged from room temperature to 10, 9 to −6, −6 to −24, and −24 to −42 ◦ C were placed in baths with temperatures at 0, −18, −33, and −51 ◦ C, respectively. The CP was defined as the temperature at which a haze or cloud is first observed at the bottom of the test jar. All CPs were measured in duplicate and average values reported. 2.2.8. Viscosity and viscosity index Calibrated Cannon-Fenske viscometer tubes obtained from Cannon Instrument Co. (State College, PA) were used to measure viscosity. Measurements were run in a Temp-Trol (Precision Scientific, Chicago, IL) viscometer bath set at 40.0 and 100.0 ◦ C. Viscosity and viscosity index were calculated using the official ASTM Methods D 445-97 and ASTM D 2270-93, respectively. Duplicate measurements were made and average values reported. 2.2.9. Synthesis 2.2.9.1. Coco-oleic dimer estolide (III) and coco-oleic trimer plus estolide (IV). Fig. 1 showing the synthesis of estolides, gives the structures, names, and abbreviations of the various estolides and FAs presented in this study. Under conditions previously reported by Cermak and Isbell (2003b), the coco-oleic estolide mixture (I) was produced on the pilot plant scale (35 gal). The coco-oleic estolide mixture (I) was separated using the Myers Pilot 15 Molecular Distillation Unit (Kittanning, PA) into monomers and the coco-oleic estolide (II) fractions at TMC Industries (Waconia, MN) using conditions similar as those prepared and supplied by Cermak and Isbell (2002b). The monomer fraction contained coconut and oleic-based FAs while the coco-oleic estolide (II) fraction contained a mixture of coco-oleic dimer estolide (EN = 1) and oleic trimer plus estolides (EN ≥ 2). The coco-oleic estolide (II) was further separated using the Myers Pilot 15 Molecular Distillation Unit into the coco-oleic dimer estolide (III) and oleic trimer plus estolides (IV) fractions at TMC Industries using conditions similar and supplied by Isbell and Cermak (2004). The physical properties of the coco-oleic estolide (II), coco-oleic dimer estolide (III) and coco-oleic trimer plus estolides (IV) were measured independently and recorded in Table 2.
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Table 2 Physical properties of starting estolides. Alcohola
Pour point (◦ C)
Coco-Oleic Estolide (II) Coco-Oleic Dimer Estolide (III) Coco-Oleic Trimer Plus Estolide (IV)
−27 −26 −24
a b
Cloud point (◦ C) −29 −25 NAb
ENa
Vis. @ 40 ◦ C (cSt)
Vis. @ 100 ◦ C (cSt)
Vis. index
1.80 1.25 2.50
317.7 112.4 824.4
33.0 15.0 65.0
145 139 146
Estolide number. NA-too dark to determine.
Table 3 Physical properties of the branched coco-oleic dimer estolide branched ester (V). Alcohola
Pre-acid valueb
Post-acid valuec
Pour point (◦ C)
Cloud point (◦ C)
Vis. @ 40 ◦ C (cSt)
Vis. @ 100 ◦ C (cSt)
Vis. index
Gardner color
Methanol Isobutanol H-Isobutanold 2-Butyloctanol 2-Hexyldecanol 2-Octyldodecanol
1.99 0.21 0.02 0.99 0.99 0.69
0.09 0.01 0.02 0.08 0.04 0.04
−27 −30 −15 −39 −45 −39
−23 −25 −7 −14 −11 −37
27.5 31.3 33.2 47.5 46.8 51.7
3.0 6.6 6.9 8.4 8.9 9.5
175 175 172 162 173 171
4 7 NA 6 6− 4
Vis. @ 40 ◦ C (cSt)
Vis. @ 100 ◦ C (cSt)
Vis. index
Gardner color
120.8 140.0 174.2 190.0 205.3 417.3 227.7
17.9 19.8 23.2 25.1 25.9 38.9 29.4
165 163 162 164 159 140 169
16+ 17 16+ 17+ 17+ 18 17
a b c d
IUP names found in Fig. 2. Pre Magnesol-XL treatment (mg KOH/g Oil). Post Magnesol-XL treatment (mg KOH/g Oil). Hydrogenated.
Table 4 Physical properties of the trimer plus estolide branched esters (VI). Alcohola Methanol Isobutanol 2-Propylheptanol 2-Butyloctanol 2-Hexyldecanol Iso-stearyl Alcohol 2-Octyldodecanol a b c d
Pre-acid valueb 0.11 0.38 3.71 0.41 0.43 0.57 1.06
Post-acid valuec
Pour point (◦ C)
0.10 0.09 0.08 0.09 0.09 0.08 0.07
−30 −27 −42 −39 −24 −18 −33
Cloud point (◦ C) d
NA NA NA NA NA NA NA
IUP names found in Fig. 2. Pre Magnesol-XL treatment (mg KOH/g Oil). Post Magnesol-XL treatment (mg KOH/g Oil). NA – too dark to determine.
The coco-oleic dimer estolide (III) or coco-oleic trimer plus estolide (IV) was combined with a 0.5 M BF3 /corresponding alcohol (as listed in Tables 3 and 4) solution [3 × estolide wt, (g) = mLs of 0.5 M BF3 /corresponding alcohol solution] in a 500-mL roundbottomed flask. The reactions were conducted under vacuum at either 60 or 80 ◦ C depending on the vapor pressure of the alcohol with a magnetic stirrer. Esterification reactions were run until they were >99% complete, then transferred to a separatory funnel and washed with saturated NaCl (2 × 75 mL). The pH of the organic layer was adjusted to 5.3–6.0 with the aid of pH 5 buffer (NaH2 PO4 , 519 g in 4 L H2 O) (2 × 50 mL). The oil was dried over sodium sulfate and filtered. All reactions were concentrated in vacuo, then Kugelrohr-distilled at 100–120 ◦ C at 0.3–66 Pa to remove excess alcohol to yield the corresponding cocooleic dimer estolide ester (V) or coco-oleic trimer plus estolide ester (VI). 2.2.9.3. AV reductions of coco-oleic estolide esters (V and VI). Prior to AV reduction techniques, the AV of the coco-oleic dimer estolide ester (V) and coco-oleic trimer plus estolide ester (VI) were measured and recorded (pre-acid value) in Tables 3 and 4. Coco-oleic estolide esters (V and VI) were independently heated (30.0 g) to 85 ◦ C and mixed with a stir bar under house vacuum (∼2 kPa), followed by the addition of Magnesol® XL (1.5 g) and mixed under house vacuum for an additional 10 min. The mixture was then cooled to room temperature under vac. The cooled material was filtered to remove the Magnesol® XL material followed by a second AV determination with the results recorded (post-acid value) in Tables 3 and 4.
3. Results and discussion The coco-oleic estolide (II, Fig. 1) is formed from the cationic homo-oligomerization of unsaturated FAs resulting from the addition of a saturated or unsaturated FA carboxyl across the olefin. This polymerization type reaction can continue, resulting in oligomeric compounds where the average degree of oligomerization is defined as the EN (n + 1, Fig. 1) (Cermak and Isbell, 2001a,b). The cocooleic estolide mixture (I) then underwent vacuum distillation (Cermak and Isbell, 2002b) to remove the monomers, which provided a nearly pure estolide fraction of coco-oleic estolide (II) that demostrated better low temperature properties. The HPLC data showed that all the monomer had been removed. The presence of monomer in the estolide has already been proven to degrade the low temperature properties of the material (Cermak and Isbell, 2002a). The separation of the coco-oleic estolide mixture (II), as shown in Fig. 1, into two different fractions: coco-oleic dimer estolide (III) and coco-oleic trimer plus estolide (IV), was separated via a molecular distillation apparatus. The HPLC showed that the cocooleic dimer estolide (III) is primarily dimer estolide with some trimer estolide. The EN was used as a way to measure the average size of each estolide fraction. Table 2 shows the EN of the starting coco-oleic estolide (II) as well as both resulting fractions, the dimer (III) and trimer plus (IV) estolides, where the EN either decreased or increased as expected. The Myers Pilot 15 Molecular Distillation Unit has one theoretical plate and, along with the heavy weight of the different estolide units, leads to a difficult clean separation. The distillation of the coco-oleic estolides more closely
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resembles an enrichment process rather than a clean singlecomponent separation, column chromatography, usually found in the organic chemistry literature. The physical properties of two different estolide fractions, dimer (III) and trimer plus (IV), were recorded in Table 2. The coco-oleic estolides (III and IV) as free acid estolide are known to have higher viscosities than estolide esters due to hydrogen bonding interactions. As expected, the larger coco-oleic trimer plus estolide (IV) was more viscous with viscosities of 824.4 and 65.0 cSt at 40 and 100 ◦ C. The coco-oleic dimer estolide was significantly less viscous than the combined fractions with viscosities of 112.4 and 15.0 cSt at 40 and 100 ◦ C. In terms of low temperature properties, the three estolide samples in Table 2 show no real difference between them as the free acid estolide. Isbell et al. (2001) have already demonstrated that the introduction of branching in the alcohol portion of the ester has a positive impact on the low temperature properties of estolide esters. Previously, a series of new oleic estolide esters were synthesized from both branched and linear-chained alcohols under standard esterification conditions (Cermak et al., 2013). In this example, Cermak and coworkers started with an estolide that was a mixture of dimer and trimer plus estolides which were examined as effective esters with better low temperature properties. Overall, the branched based estolide esters displayed the best low temperature properties. Thus, applying this branch ester approach on estolides separated by the extent of oligomerization could aid in understanding the structureproperty relationships. Under standard esterification conditions with a series of branched alcohols (Fig. 2), two new categories of coco-oleic estolide esters were synthesized (coco-oleic dimer estolide ester (V) and coco-oleic trimer plus estolide ester (VI), Fig. 3). These branched longer-chain alcohols are typically synthetic materials having trade names associated with them to help designate purity and possible isomers. Table 1 shows the general range for different commercial materials as well as past synthetic estolide esters (oleic and coco-oleic based estolide esters). The soy-based materials were considered acceptable (since they have been sold for commercial applications) so it would be concluded that all materials with PP colder than −18 ◦ C would be commercially acceptable. The effectiveness of coco-oleic estolide esters as prospective lubricants in cold weather applications was assessed (Table 3 and 4). Table 3 examines the series of esterification reactions of cocooleic dimer estolide (III) with the branched alcohols and their effect on the physical properties of the corresponding coco-oleic dimer estolide esters (V). The alcohols ranged from 1 to 20 carbons in length where the 2-hexyldecanol ester yielded the best PP at −45 ◦ C. The 2-butyloctanol and 2-octyldodecanol esters each had a pour point of −39 ◦ C, while methanol and hydrogenated isobutanol also yielded acceptable low temperature properties. Table 4 shows the physical properties of the series of cocooleic trimer plus estolide esters (VI) esterified with seven different alcohols. The highly branched iso-stearyl alcohol yielded a PP of −18 ◦ C, which is at the upper limit of the acceptable range. All other samples met the industry standards as a bio-based material when compared to commercial materials listed in Table 1. Some of the best performers from Table 4 were 2-propylheptanol (−42 ◦ C), followed by 2-butyloctanol (−39 ◦ C), and 2-octyldodecanol (−33 ◦ C), (Fig. 2), both with PPs better than −30 ◦ C. In general, both the cocooleic dimer estolide branched esters (V) and coco-oleic trimer plus estolide branched esters (VI) surpassed the standard oleic estolide 2-EH estolide ester (Cermak and Isbell, 2002a) in terms of PPs. However the coco-oleic dimer estolide branched esters (V) produced the lowest PPs while the coco-oleic trimer plus estolide branched esters (VI) were slightly higher. As the estolides ENs increase, they have more steric interactions and limited rotational degrees of freedom which leads to better packing and less desirable low
temperature properties. In terms of CP, the coco-oleic dimer estolide 2-octyldodecanol ester had the lowest CP of −37 ◦ C. The coco-oleic trimer plus estolide branched ester materials were all too dark for CP values to be accurately obtained. The poor color of this fraction resulted from all the impurities carried into the residue fraction from the distillation. Additionally, the long-chain complex alcohols used to esterify the estolides also had some color impurities which caused dark Gardner Color values. The viscosities of the branched dimer and trimer estolide ester (V and VI) materials were consistent with what was expected: as the chain length gets longer and more complex, the viscosities increased due to inter- and intra-molecular tangling. The cocooleic dimer estolide esters (V) ranged from 27.5 to 51.7 cSt @ 40 ◦ C and from 3.0 to 9.5 cSt @ 100 ◦ C. Previously, Cermak and coworkers reported viscosities of the same branched materials on an oleic estolide as a mixture of dimer and trimer plus that ranged from 92.6 to 151.1 cSt @ 40 ◦ C and from 15.2 to 21.4 cSt @ 100 ◦ C (Cermak et al., 2013). Samples that contain just the dimer estolide show a striking decrease in sample viscosities. The coco-oleic trimer plus estolide esters (VI) ranged from 120.8 to 227.7 cSt @ 40 ◦ C and from 17.9 to 29.4 cSt @ 100 ◦ C for the same series as the dimer esters. The highly branched iso-stearyl alcohol ester was tested with the trimer plus estolides to measure the upper viscosity limits of these samples which yielded viscosities of 417.3 cSt @ 40 ◦ C and 38.9 cSt @ 100 ◦ C. Finally, the viscosity index of all the coco-oleic estolide esters (V and VI) were excellent, showing results similar to other commercial oils (Cermak, 2006). The esterification process rarely achieves 100% conversion, thus AV will never be zero without extreme and expensive separation techniques. As a comparison, AV requirements for biodiesel are less than 0.5 mg KOH/g of oil (Moser, 2012). The desired AV was set at 0.1 mg of KOH/g of oil sample. Initial AV readings of both dimer and trimer estolides were higher than desired, thus a lower threshhold was desired in an effort to diminish the varying effect AVs would have on the physical properties. Thus, the materials underwent acid reduction techniques to lower the AV of both types of estolide esters (V and VI) to their purest possible state. In all cases (Tables 3 and 4), the Magnesol-treated materials had their AVs lowered to levels of 0.10 mg of KOH/g of oil or less. While slightly elevated AVs do not have negative effects on low temperature properties, the viscosities of the estolide esters (V and VI) may be impacted as a result of hydrogen bonding in the samples.
4. Conclusions A series of branched alcohols underwent an esterification process with either coco-oleic dimer or trimer plus estolide (III or IV) to produce 10 new coco-oleic dimer or trimer plus estolide esters (V or VI) followed by an evaluation of their physical properties. The coco-oleic estolide (II) was separated into two components based on the extent of oligomerization: coco-oleic dimer estolide (III) and coco-oleic trimer plus estolide (IV). The two different estolides were esterified with a series of branched alcohols. In general, the branched alcohols produced esters with better cold temperature properties than current commercially available materials. The coco-oleic dimer estolide esters (V) had the lowest PP where the 2-hexyldecanol ester, a 16 carbon-chain branched material, had a PP of −45 ◦ C and 2-octyldodecanol ester, a 20 carbon branched material, had a PP of −39 ◦ C. The best CP performer from the same series was 2-octyldodecanol ester, with a CP of −37, followed by hydrogenated isobutanol and a linear chain alcohol, methanol, at −25 and 923 ◦ C, respectively. The coco-oleic trimer plus estolide esters (VI) were too dark to obtain CP, but their PP ranged from −18 to −42 ◦ C which was slightly higher than the dimer series.
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The viscosities and viscosity indexes were as expected in terms of trends and ranges. The iso-stearyl alcohol ester had the most interesting viscosity of 417.3 cSt @ 40 ◦ C and 38.9 cSt @ 100 ◦ C, which was higher than all other oleic based estolides reported to date. The coco-oleic dimer estolide esters (V) ranged from 27.5 to 51.7 cSt @ 40 ◦ C and from 3.0 to 9.5 cSt @ 100 ◦ C, whereas the cocooleic trimer plus estolide esters (VI) ranged from 120.8 to 227.7 cSt @ 40 ◦ C and 17.9 to 29.4 cSt @ 100 ◦ C for the same series as the dimer esters. Finally, the ability to separate the estolides into different fractions based on EN or size has shown their potential usefulness as commercial lubricants. Many industrial crops that are high in oleic acid would have an advantage in supplying this demand. Having the ability to choose different viscosity ranges while still maintaining/exceeding low temperature properties is very desirable. Whatever the choice of ester, the development of these novel estolide esters will have a significant impact on industrial oils in the future as these estolide esters (V and VI) require no additives to obtain the improved low temperature and viscosity performance, thus limiting our impact on the environment and replacing fluids that are based on nonrenewable resources. Acknowledgments The authors are extremely grateful to Amber L. Durham, Katelyn N. Isbell, Billee L. John, Justin S. McCalvin, Kristy Thompson, and Melissa Becker for their assistance of estolide ester production, purification, and property testing. References Asadauskas, S., Erhan, S.Z., 1999. Depression of pour points of vegetable oils by blending with diluents used for biodegradable lubricants. J. Am. Oil Chem. Soc. 76, 313–316. Becker, R., Knorr, A., 1996. An evaluation of antioxidants for vegetable oils at elevated temperatures. Lubr. Sci. 8, 95–117. Bredsguard J., 2011. Acetic acid-capped estolide base oils and methods of making the same. U.S. Patent App. US 2012/0083435 A1. Bredsguard, J., 2011. High-and low-viscosity estolide base oils and lubricants. U.S. Patent App. US 2012/0178660 A1. Bredsguard, J., Forest, J., Thompson, T., 2011. Catalytic processes for preparing estolide base oils. U.S. Patent App. US 2012/0172609 A1. Bredsguard J., Forest J., Thompson T., 2012. Refrigerating fluid compositions comprising estolide compounds. US. Patent 8,236, 194. Cermak, S.C., Isbell, T.A., 2001. Biodegradable oleic estolide ester having saturated fatty acid end group useful as lubricant base stock, U.S. Patent 6,316,649 B1.
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Cermak, S.C., Isbell, T.A., 2001b. Synthesis of estolides from oleic and saturated fatty acids. J. Am. Oil Chem. Soc. 78, 557–565. Cermak, S.C., Isbell, T.A., 2002a. Physical properties of saturated estolides and their 2-ethylhexyl esters. Ind. Crops Prod. 16, 119–127. Cermak, S.C., Isbell, T.A., 2002b. Pilot-plant distillation of meadowfoam fatty acids. Ind. Crops Prod. 15, 145–154. Cermak, S.C., Isbell, T.A., 2003a. Improved oxidative stability of estolide esters. Ind. Crops Prod. 18, 223–230. Cermak, S.C., Isbell, T.A., 2003. Synthesis and physical properties of estolide-based functional fluids. Ind. Crops Prod. 18, 183–196. Cermak, S.C., Isbell, T.A., 2004. Synthesis and physical properties of cuphea-oleic estolides and esters. J. Am. Oil Chem. Soc. 81, 297–303. Cermak, S.C., 2006. Biobased crankcase lube oils. Ind. Bioprocess. 28, 3. Cermak, S.C., Brandon, K.B., Isbell, T.A., 2006. Synthesis and physical properties of estolides from lesquerella and castor fatty acid esters. Ind. Crops Prod. 23, 54–64. Cermak, S.C., Skender, A.L., Deppe, A.B., Isbell, T.A., 2007. Synthesis and physical properties of tallow-oleic estolide 2-ethylhexyl esters. J. Am. Oil Chem. Soc. 84, 449–456. Cermak, S.C., Biresaw, G., Isbell, T.A., 2008. Comparison of a new Estolide oxidative stability package. J. Am. Oil Chem. Soc. 85, 879–885. Cermak, S.C., Isbell, T.A., 2009. Synthesis and physical properties of mono-estolides with varying chain lengths. Ind. Crops Prod. 29, 205–213. Cermak, S.C., Isbell, T.A., Evangelista, R.L., Johnson, B.L., 2011. Synthesis and physical properties of petroselinic based estolide esters. Ind. Crops Prod. 33, 132–139. Cermak, S.C., Bredsguard, J.W., John, B.L., McCalvin, J.S., Thompson, T., Isbell, K.N., Feken, K.A., Isbell, T.A., Murray, R.E., 2013. Synthesis and physical properties of new estolide esters. Ind. Crops Prod. 46, 386–391. Firestone, D. (Ed.), 1994, fourth ed. AOCS, Champaign, IL. Greaves, M.R., Khelidj, N., 2012. Estolide compositions having excellent low temperature properties, U.S. Patent App. US 2011/0282084. Herdan, J.M., 1999. Rolling fluids based on vegetable oils. J. Synth. Lubr. 16, 201–210. Isbell, T.A., Kleiman, R., 1994. Characterization of estolides produced from the acid-catalyzed condensation of oleic acid. J. Am. Oil Chem. Soc. 71, 379–383. Isbell, T.A., Abbott, T.P., Asadauskas, S., Lohr, J.E., Jr., 2000 Biodegradable oleic estolide ester base stocks and lubricants, U.S. Patent 6,018,603. Isbell, T.A., Edgcomb, M.R., Lowery, B.A., 2001. Physical properties of estolides and their ester derivatives. Ind. Crops Prod. 13, 11–20. Isbell, T.A., Cermak, S.C., 2004. Purification of meadowfoam monoestolide from polyestolide. Ind. Crops Prod. 19, 113–118. Moser, B.R., Cermak, S.C., Isbell, T.A., 2008. Evaluation of Castor and Lesquerella oil derivatives as additives in biodiesel and ultralow sulfur diesel fuels. Energy Fuels 22, 1349–1352. Moser, B.R., 2012. Biodiesel from alternative oilseed feedstocks: camelina and field pennycress. Biofuels 3, 193–209. Potula, S.B., Korlipara, V.P., Bhamidipati, V.S.K.R., Krishnasamy, S., Rachapudi, B.N.P., 2009. Castor oil fatty acid based estolide esters and their derivatives as potential lubricant base stocks. U.S. Patent App. US 2011/0282084. Vinci, D., Beckerdite, J., 2011.Estolide derivatives useful as biolubricants. U.S. Patent App. US 2011/0213170 A1.
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Industrial Crops and Products journal homepage: www.elsevier.com/locate/indcrop
Synthesis and physical properties of new coco-oleic estolide branched esters夽 Steven C. Cermak a,∗ , Jakob W. Bredsguard b , Katie L. Roth b , Travis Thompson b , Kati A. Feken b , Terry A. Isbell a , Rex E. Murray a a Bio-Oils Research Unit, National Center for Agricultural Utilization Research, USDA, Agricultural Research Service, 1815 N. University St., Peoria, IL 61604, USA b Biosynthetic Technologies, 2 Park Plaza, Suite 200, Irvine, CA 92614, USA
a r t i c l e
i n f o
Article history: Received 26 January 2015 Received in revised form 21 April 2015 Accepted 8 May 2015 Keywords: Biodegradable oils Esters Estolides Physical properties Pour points Viscosities
a b s t r a c t 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: cocooleic 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 27.5–51.7 cSt @ 40 ◦ C and 3.0–9.5 cSt @ 100 ◦ C, whereas the coco-oleic trimer plus estolide esters ranged 120.8–227.7 cSt @ 40 ◦ C and 17.9–29.4 cSt 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 417.3 cSt @ 40 ◦ C and 38.9 cSt @ 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. Published by Elsevier B.V.
1. Introduction Due to increasing petroleum prices and the desire for consumers and producers to find and use environmentally friendly products, interest in a class of esters based on vegetable oils called estolides (Isbell and Kleiman, 1994) has continued to increase. Estolides are formed when the carboxylic acid functionality of one fatty acid (FA) reacts at the site of unsaturation of another FA to form an ester linkage. A number of these linkages are used to help characterize the structure of the estolide since the estolide number (EN) is defined as the average number of FAs added to a base FA (Fig. 1, EN = n + 1 where n is an integer). Thus, if n = 0, the EN would be one which
夽 Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer. ∗ Corresponding author. Tel.: +1 309 681 6233; fax: +1 309 681 6524. E-mail address: [email protected] (S.C. Cermak). http://dx.doi.org/10.1016/j.indcrop.2015.05.011 0926-6690/Published by Elsevier B.V.
would represent a dimer estolide (III), two FA molecules linked together. An EN value of two or higher would indicate multiple ester linkages and is termed a trimer plus estolide (IV) as shown in Fig. 1. The secondary ester linkages of the estolide are more resistant to hydrolysis than those of triglycerides (Cermak and Isbell, 2003a; Potula et al., 2009), and the unique structure of the estolide results in materials that have far superior physical properties for certain applications than vegetable and mineral oils (Cermak and Isbell, 2002a). There have been numerous reports on the physical properties of two different types of estolide esters. First, the oleic-based estolide esters which were originally discovered by Isbell et al. (2000); and second, the saturated capped oleic-based estolide esters discovered by Cermak and Isbell (2001a,b); Cermak and Isbell (2001a,b). The term “saturated capped” refers to estolides that have a saturated FA, coconut or coco, as the last FA to add across the double bond during the oligomerization reaction. In general, both types of these estolides are usually esters of 2-ethylhexyl (2-EH) alcohol. The extra branching (of the 2-EH ester) and chain length located in the ester
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Fig. 1. Coco-Oleic estolide synthesis and separation. *The estolide position was distributed from positions 5–13 with the original 9 and 10 positions having the greatest abundances. Table 1 Comparison of low temperature properties of estolide 2-ethylhexyl esters and commercial lubricants. Pour point (◦ C)
Lubricant a
Commercial petroleum oil Commercial synthetic oila Commercial soy-based oila Commercial hydraulic fluida Commercial aviation oila Oleic 2-EH estolide ester Coco-Oleic 2-EH estolide ester a b
−21 to −27b −21 to −33b −12 to −18b −24 to −33b −30 to −33b −30 −33
Fully formulated materials from local vendors. Range of a sampling of a number of different manufacturer samples.
portion are also responsible for their improved low temperature properties. The saturated capped estolide esters have become very versatile and desirable in the formulation of bio-based fluids. The saturated estolides have increased oxidative stability (Cermak et al., 2008; Cermak and Isbell, 2003a) and improved low temperature properties (Cermak and Isbell, 2003b) over the traditional oleic based estolide esters. In general, both types of estolide 2-EH esters (oleic and saturated capped) compared well to other fully formulated petroleum- and bio-based fluids, and even exceeded them in terms of pour point (PP, Table 1). When compared to finished motor oils, the cloud points (CP) of the estolide 2-EH esters were far superior to all samples tested (Cermak, 2006). The estolides do not have the normal disadvantages that traditional vegetable oils
have, such as poor thermal oxidative stability (Becker and Knorr, 1996; Cermak and Isbell, 2003a), low hydrolytic stability (Herdan, 1999) and poor low temperature properties (Asadauskas and Erhan, 1999). Estolides and estolide esters have been known in the literature for some time and current research continues to gain a better understanding of the structural functions of estolides in fluids. To date, various types of estolides and estolide esters have been tested and developed (Bredsguard, 2011a,b; Bredsguard et al., 2011; Greaves and Khelidj, 2012; Vinci and Beckerdite, 2011); additionally, estolides from new crop sources are being evaluated (Cermak et al., 2006; Cermak and Isbell, 2004; Cermak et al., 2011). New applications are being evaluated in a host of different applications ranging from biodiesel (Moser et al., 2008) to bio-based products (Bredsguard et al., 2012; Cermak and Isbell, 2009; Cermak et al., 2007) Cermak et al. (2013) have recently synthesized different esters of the oleic estolide series and explored the effects of the various chain lengths and branching on the physical properties of estolides. The branched esters out-performed the linear esters in terms of low temperature properties. The branched alcohols 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, Fig. 2) sample, a 16 carbon-chained branched material, and 2-octyldodecanol (Jarcol I20, Fig. 2), a 20 carbon branched material, with a PP at −39 ◦ C. All the estolides presented by Cermak et al. (2013) were estolide ester
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Fig. 2. Structures, melting points, and commercial names for complex branched alcohols.
mixtures (estolides that contain both dimer and trimer plus estolide esters). There have been no reports on the lubrication properties of isolated individual oligomeric estolides from oleic sources. With the fact that the saturated oleic estolide esters have better stability and low temperature properties, the question becomes what part(s) of the estolide ester mixture are most responsible for controlling these enhanced properties. There have been no reports in the literature on the separation or isolation of an oleic based estolide on molecular size. Given the ability to measure properties of the dimer estolides, EN = 1, or the trimer plus estolides, EN > 2, an understanding of molecular estolide interactions might be possible. When this theory is combined with the best performing estolide, saturated capped or coco-oleic estolide, new advanced materials could be discovered. Cermak and Isbell (2004) reported that by varying the capping material on the estolide, the crystal lattice structure of the material was disrupted as it approached its PP, which led to estolide esters with excellent low temperature properties, PPs of −36 ◦ C and CPs of −41 ◦ C. Finally, when combined with the best performing ester groups as defined by Cermak et al. (2013) with the oleic based estolides, the new series of separated dimer and trimer plus estolide esters could display unique physical properties. This paper evaluates physical properties (PP, CP, and viscosity) of a series of new and different estolide esters. As shown in Fig. 1, the general coco-oleic estolide mixture (I) underwent distillation to remove the monomer and the resulting residue, coco-oleic estolide (II), was distilled further using a molecular distillation apparatus to separate coco-oleic dimer estolide (III) from the coco-oleic trimer plus estolide (IV). Once separated, each estolide was esterified with a range of branched alcohols under similar conditions to generate a series of coco-oleic estolide esters (V and VI, Fig. 3). The cold temperature properties of these new estolide esters should surpass those of commercial products already available such as petroleumbased hydraulic fluids, soy-based fluids, and petroleum oils, while other physical properties should compete auspiciously with the current lubricants.
2. Materials and methods 2.1. Materials The fatty acid methyl ester (FAME) standard mixtures were obtained from Alltech Associates, Inc. (Deerfield, IL) and Nu-Chek Prep, Inc. (Elysian, MN). Methanol, ethanol, isobutanol, and 2ethylhexanol were from Sigma–Aldrich Corporation (St. Louis, MO). 2-Propylheptanol was from the BASF Chemical Company (Triangle Park, NC). The hydrogenated isobutanol was from Gevo, Inc. (Englewood, Co.). The 2-butyloctanol (Jarcol I-12), 2-hexyldecanol (Jarcol I-16) and 2-octyldodecanol (Jarcol I-20) were obtained as free sam-
ples from Jarchem Industries, Inc. (Newark, NJ). Iso-stearyl alcohol (Oxocol 180) was obtained as free samples from Nissan Chemical Industries, Ltd. (Houston, TX). Magnesol®XL was obtained as a free sample from the Dallas Group of America, Inc. (Whitehouse, NJ). 2.2. Equipment and procedures 2.2.1. Gas chromatography (GC) A Hewlett-Packard 6890N Series gas chromatograph (Palo Alto, CA) equipped with a flame ionization detector and an autosampler/injector was used for GC analysis. Analyses were conducted on an SP-2380 30 m × 0.25 mm i.d. column (Supelco, Inc., Bellefonte, PA). Saturated C8–C30 FAMEs provided standards for making FA and by-product assignments. Parameters for SP-2380 analysis were: column flow 1.4 mL/min with a helium head pressure of 136 kPa; split ratio 50:1; programmed ramp 120–135 ◦ C at 10 ◦ C/min, 135–175 ◦ C at 3 ◦ C/min, 175–265 ◦ C at 10 ◦ C/min, hold 5 min at 265 ◦ C; injector and detector temperatures set at 250 ◦ C. 2.2.2. GC analysis of hydroxy and non-hydroxy FAs for EN Analytical hydroxy and non-hydroxy samples for GC were prepared using procedures described by Cermak and Isbell (2001b). 2.2.3. High performance liquid chromatography Reverse phase HPLC, analyses were performed on a Thermo Separations Spectra System AS1000 autosampler/injector (Fremont, CA) with a P2000 binary gradient pump from Thermo Separation Products (Fremont, CA) coupled to a Alltech ELSD 500 evaporative light scattering detector (Alltech Associates). A C-8 reverse phase analysis used to separate reaction mixtures was carried out with a Dynamax column (250 mm × 4.5 mm, 5- particle size) from Rainin Instrument Co. (Woburn, MA). Two methods for reverse phase analysis were used to separate the reaction mixtures. Method A (16 min run time) was used to follow the reaction and distillation operations. It provided information on the overall progress of the reaction as well was verified the removal of oleic acid and esters during the distillation of the estolide mixture (I) to yield coco-oleic estolide (II). Method B (35 min run time) produced a more detailed separation of the reaction mixture, in particular a separation of estolide esters and fatty esters. The same general trend is observed as reported by Isbell and Kleiman (1994). Operating parameters for method A were: flow rate of 1 mL/min; 0–4 min 80% acetonitrile, 20% acetone; 6–10 min 100% acetone; 11–16 min 80% acetonitrile, 20% acetone. The ELSD drift tube was set at 55 ◦ C with the nebulizer set at 138 kPa N2 , providing a flow rate of 2.0 standard liters per minute (SLPM). Retention times for eluted peaks: estolides, 9.8–13.0 min; methyl oleate, 6.3 min; and oleic acid, 5.1 min.
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2.2.9.2. Coco-oleic
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estolide
esters
(V
and
VI).
Fig. 3. Coco-Oleic Dimer and Trimer plus estolide ester synthesis.
Operating parameters for method B were: flow rate of 1 mL/min; 0–2 min 60% acetonitrile 40% acetone; 20–25 min 100% acetone; 30–35 min 60% acetonitrile 40% acetone. The ELSD drift tube was set at 55 ◦ C with the nebulizer set at 138 kPa N2 , providing a flow rate of 2.0 SLPM. Retention times for eluted peaks: estolide esters, 9.0–25.6 min and methyl oleate, 5.5 min. 2.2.4. Acid value (AV) The 751 GPD Titrino from Metrohm Ltd. (Herisau, Switzerland) was used for measurements. AVs were determined by the official AOCS Method Te 2a-64 (Firestone, 1994) with ethanol substituted for methanol to increase the solubility of the estolide esters during the titration. All AVs were run in duplicate and average values were reported. 2.2.5. Gardner color A Lovibond 3-Field Comparator from Tintometer Ltd. (Salisbury, England) using the official AOCS method Td 1a-64 (Firestone, 1994) was used for Gardner color measurements. The “+” and “−” notation was employed to designate samples that did not match one particular color. 2.2.6. Pour point (PP) The official ASTM Method D 97–96a was used to measure PPs to an accuracy of ±3 ◦ C. The PPs were determined by placing a test jar with 50 mL of the sample into a cylinder submerged in a cooling medium. The sample temperature was measured in 3 ◦ C increments at the top of the sample until the material stopped pouring. This point was determined when the material in the test jar did not flow when held in a horizontal position for 5 s. The temperature of the cooling medium was chosen based on the expected PP of the material. Samples with PPs that ranged from +9 to −6, −6 to −24, and −24 to −42 ◦ C were placed in baths with temperatures at −18, −33, and −51 ◦ C, respectively. The PP was defined as the coldest temperature at which the sample still poured. All PP measurements were conducted in duplicate and average values reported. 2.2.7. Cloud point (CP) The official ASTM Method D 2500-99 was used to measure CPs to an accuracy of ±1 ◦ C. The CPs were determined by placing a test jar with 50 mL of the sample into a cylinder submerged into a cooling medium. The sample temperature was measured in 1 ◦ C increments
at the bottom of the sample until any cloudiness was observed at the bottom of the test jar. The temperature of the cooling medium was chosen based on the expected CP of the material. Samples with CPs that ranged from room temperature to 10, 9 to −6, −6 to −24, and −24 to −42 ◦ C were placed in baths with temperatures at 0, −18, −33, and −51 ◦ C, respectively. The CP was defined as the temperature at which a haze or cloud is first observed at the bottom of the test jar. All CPs were measured in duplicate and average values reported. 2.2.8. Viscosity and viscosity index Calibrated Cannon-Fenske viscometer tubes obtained from Cannon Instrument Co. (State College, PA) were used to measure viscosity. Measurements were run in a Temp-Trol (Precision Scientific, Chicago, IL) viscometer bath set at 40.0 and 100.0 ◦ C. Viscosity and viscosity index were calculated using the official ASTM Methods D 445-97 and ASTM D 2270-93, respectively. Duplicate measurements were made and average values reported. 2.2.9. Synthesis 2.2.9.1. Coco-oleic dimer estolide (III) and coco-oleic trimer plus estolide (IV). Fig. 1 showing the synthesis of estolides, gives the structures, names, and abbreviations of the various estolides and FAs presented in this study. Under conditions previously reported by Cermak and Isbell (2003b), the coco-oleic estolide mixture (I) was produced on the pilot plant scale (35 gal). The coco-oleic estolide mixture (I) was separated using the Myers Pilot 15 Molecular Distillation Unit (Kittanning, PA) into monomers and the coco-oleic estolide (II) fractions at TMC Industries (Waconia, MN) using conditions similar as those prepared and supplied by Cermak and Isbell (2002b). The monomer fraction contained coconut and oleic-based FAs while the coco-oleic estolide (II) fraction contained a mixture of coco-oleic dimer estolide (EN = 1) and oleic trimer plus estolides (EN ≥ 2). The coco-oleic estolide (II) was further separated using the Myers Pilot 15 Molecular Distillation Unit into the coco-oleic dimer estolide (III) and oleic trimer plus estolides (IV) fractions at TMC Industries using conditions similar and supplied by Isbell and Cermak (2004). The physical properties of the coco-oleic estolide (II), coco-oleic dimer estolide (III) and coco-oleic trimer plus estolides (IV) were measured independently and recorded in Table 2.
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Table 2 Physical properties of starting estolides. Alcohola
Pour point (◦ C)
Coco-Oleic Estolide (II) Coco-Oleic Dimer Estolide (III) Coco-Oleic Trimer Plus Estolide (IV)
−27 −26 −24
a b
Cloud point (◦ C) −29 −25 NAb
ENa
Vis. @ 40 ◦ C (cSt)
Vis. @ 100 ◦ C (cSt)
Vis. index
1.80 1.25 2.50
317.7 112.4 824.4
33.0 15.0 65.0
145 139 146
Estolide number. NA-too dark to determine.
Table 3 Physical properties of the branched coco-oleic dimer estolide branched ester (V). Alcohola
Pre-acid valueb
Post-acid valuec
Pour point (◦ C)
Cloud point (◦ C)
Vis. @ 40 ◦ C (cSt)
Vis. @ 100 ◦ C (cSt)
Vis. index
Gardner color
Methanol Isobutanol H-Isobutanold 2-Butyloctanol 2-Hexyldecanol 2-Octyldodecanol
1.99 0.21 0.02 0.99 0.99 0.69
0.09 0.01 0.02 0.08 0.04 0.04
−27 −30 −15 −39 −45 −39
−23 −25 −7 −14 −11 −37
27.5 31.3 33.2 47.5 46.8 51.7
3.0 6.6 6.9 8.4 8.9 9.5
175 175 172 162 173 171
4 7 NA 6 6− 4
Vis. @ 40 ◦ C (cSt)
Vis. @ 100 ◦ C (cSt)
Vis. index
Gardner color
120.8 140.0 174.2 190.0 205.3 417.3 227.7
17.9 19.8 23.2 25.1 25.9 38.9 29.4
165 163 162 164 159 140 169
16+ 17 16+ 17+ 17+ 18 17
a b c d
IUP names found in Fig. 2. Pre Magnesol-XL treatment (mg KOH/g Oil). Post Magnesol-XL treatment (mg KOH/g Oil). Hydrogenated.
Table 4 Physical properties of the trimer plus estolide branched esters (VI). Alcohola Methanol Isobutanol 2-Propylheptanol 2-Butyloctanol 2-Hexyldecanol Iso-stearyl Alcohol 2-Octyldodecanol a b c d
Pre-acid valueb 0.11 0.38 3.71 0.41 0.43 0.57 1.06
Post-acid valuec
Pour point (◦ C)
0.10 0.09 0.08 0.09 0.09 0.08 0.07
−30 −27 −42 −39 −24 −18 −33
Cloud point (◦ C) d
NA NA NA NA NA NA NA
IUP names found in Fig. 2. Pre Magnesol-XL treatment (mg KOH/g Oil). Post Magnesol-XL treatment (mg KOH/g Oil). NA – too dark to determine.
The coco-oleic dimer estolide (III) or coco-oleic trimer plus estolide (IV) was combined with a 0.5 M BF3 /corresponding alcohol (as listed in Tables 3 and 4) solution [3 × estolide wt, (g) = mLs of 0.5 M BF3 /corresponding alcohol solution] in a 500-mL roundbottomed flask. The reactions were conducted under vacuum at either 60 or 80 ◦ C depending on the vapor pressure of the alcohol with a magnetic stirrer. Esterification reactions were run until they were >99% complete, then transferred to a separatory funnel and washed with saturated NaCl (2 × 75 mL). The pH of the organic layer was adjusted to 5.3–6.0 with the aid of pH 5 buffer (NaH2 PO4 , 519 g in 4 L H2 O) (2 × 50 mL). The oil was dried over sodium sulfate and filtered. All reactions were concentrated in vacuo, then Kugelrohr-distilled at 100–120 ◦ C at 0.3–66 Pa to remove excess alcohol to yield the corresponding cocooleic dimer estolide ester (V) or coco-oleic trimer plus estolide ester (VI). 2.2.9.3. AV reductions of coco-oleic estolide esters (V and VI). Prior to AV reduction techniques, the AV of the coco-oleic dimer estolide ester (V) and coco-oleic trimer plus estolide ester (VI) were measured and recorded (pre-acid value) in Tables 3 and 4. Coco-oleic estolide esters (V and VI) were independently heated (30.0 g) to 85 ◦ C and mixed with a stir bar under house vacuum (∼2 kPa), followed by the addition of Magnesol® XL (1.5 g) and mixed under house vacuum for an additional 10 min. The mixture was then cooled to room temperature under vac. The cooled material was filtered to remove the Magnesol® XL material followed by a second AV determination with the results recorded (post-acid value) in Tables 3 and 4.
3. Results and discussion The coco-oleic estolide (II, Fig. 1) is formed from the cationic homo-oligomerization of unsaturated FAs resulting from the addition of a saturated or unsaturated FA carboxyl across the olefin. This polymerization type reaction can continue, resulting in oligomeric compounds where the average degree of oligomerization is defined as the EN (n + 1, Fig. 1) (Cermak and Isbell, 2001a,b). The cocooleic estolide mixture (I) then underwent vacuum distillation (Cermak and Isbell, 2002b) to remove the monomers, which provided a nearly pure estolide fraction of coco-oleic estolide (II) that demostrated better low temperature properties. The HPLC data showed that all the monomer had been removed. The presence of monomer in the estolide has already been proven to degrade the low temperature properties of the material (Cermak and Isbell, 2002a). The separation of the coco-oleic estolide mixture (II), as shown in Fig. 1, into two different fractions: coco-oleic dimer estolide (III) and coco-oleic trimer plus estolide (IV), was separated via a molecular distillation apparatus. The HPLC showed that the cocooleic dimer estolide (III) is primarily dimer estolide with some trimer estolide. The EN was used as a way to measure the average size of each estolide fraction. Table 2 shows the EN of the starting coco-oleic estolide (II) as well as both resulting fractions, the dimer (III) and trimer plus (IV) estolides, where the EN either decreased or increased as expected. The Myers Pilot 15 Molecular Distillation Unit has one theoretical plate and, along with the heavy weight of the different estolide units, leads to a difficult clean separation. The distillation of the coco-oleic estolides more closely
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resembles an enrichment process rather than a clean singlecomponent separation, column chromatography, usually found in the organic chemistry literature. The physical properties of two different estolide fractions, dimer (III) and trimer plus (IV), were recorded in Table 2. The coco-oleic estolides (III and IV) as free acid estolide are known to have higher viscosities than estolide esters due to hydrogen bonding interactions. As expected, the larger coco-oleic trimer plus estolide (IV) was more viscous with viscosities of 824.4 and 65.0 cSt at 40 and 100 ◦ C. The coco-oleic dimer estolide was significantly less viscous than the combined fractions with viscosities of 112.4 and 15.0 cSt at 40 and 100 ◦ C. In terms of low temperature properties, the three estolide samples in Table 2 show no real difference between them as the free acid estolide. Isbell et al. (2001) have already demonstrated that the introduction of branching in the alcohol portion of the ester has a positive impact on the low temperature properties of estolide esters. Previously, a series of new oleic estolide esters were synthesized from both branched and linear-chained alcohols under standard esterification conditions (Cermak et al., 2013). In this example, Cermak and coworkers started with an estolide that was a mixture of dimer and trimer plus estolides which were examined as effective esters with better low temperature properties. Overall, the branched based estolide esters displayed the best low temperature properties. Thus, applying this branch ester approach on estolides separated by the extent of oligomerization could aid in understanding the structureproperty relationships. Under standard esterification conditions with a series of branched alcohols (Fig. 2), two new categories of coco-oleic estolide esters were synthesized (coco-oleic dimer estolide ester (V) and coco-oleic trimer plus estolide ester (VI), Fig. 3). These branched longer-chain alcohols are typically synthetic materials having trade names associated with them to help designate purity and possible isomers. Table 1 shows the general range for different commercial materials as well as past synthetic estolide esters (oleic and coco-oleic based estolide esters). The soy-based materials were considered acceptable (since they have been sold for commercial applications) so it would be concluded that all materials with PP colder than −18 ◦ C would be commercially acceptable. The effectiveness of coco-oleic estolide esters as prospective lubricants in cold weather applications was assessed (Table 3 and 4). Table 3 examines the series of esterification reactions of cocooleic dimer estolide (III) with the branched alcohols and their effect on the physical properties of the corresponding coco-oleic dimer estolide esters (V). The alcohols ranged from 1 to 20 carbons in length where the 2-hexyldecanol ester yielded the best PP at −45 ◦ C. The 2-butyloctanol and 2-octyldodecanol esters each had a pour point of −39 ◦ C, while methanol and hydrogenated isobutanol also yielded acceptable low temperature properties. Table 4 shows the physical properties of the series of cocooleic trimer plus estolide esters (VI) esterified with seven different alcohols. The highly branched iso-stearyl alcohol yielded a PP of −18 ◦ C, which is at the upper limit of the acceptable range. All other samples met the industry standards as a bio-based material when compared to commercial materials listed in Table 1. Some of the best performers from Table 4 were 2-propylheptanol (−42 ◦ C), followed by 2-butyloctanol (−39 ◦ C), and 2-octyldodecanol (−33 ◦ C), (Fig. 2), both with PPs better than −30 ◦ C. In general, both the cocooleic dimer estolide branched esters (V) and coco-oleic trimer plus estolide branched esters (VI) surpassed the standard oleic estolide 2-EH estolide ester (Cermak and Isbell, 2002a) in terms of PPs. However the coco-oleic dimer estolide branched esters (V) produced the lowest PPs while the coco-oleic trimer plus estolide branched esters (VI) were slightly higher. As the estolides ENs increase, they have more steric interactions and limited rotational degrees of freedom which leads to better packing and less desirable low
temperature properties. In terms of CP, the coco-oleic dimer estolide 2-octyldodecanol ester had the lowest CP of −37 ◦ C. The coco-oleic trimer plus estolide branched ester materials were all too dark for CP values to be accurately obtained. The poor color of this fraction resulted from all the impurities carried into the residue fraction from the distillation. Additionally, the long-chain complex alcohols used to esterify the estolides also had some color impurities which caused dark Gardner Color values. The viscosities of the branched dimer and trimer estolide ester (V and VI) materials were consistent with what was expected: as the chain length gets longer and more complex, the viscosities increased due to inter- and intra-molecular tangling. The cocooleic dimer estolide esters (V) ranged from 27.5 to 51.7 cSt @ 40 ◦ C and from 3.0 to 9.5 cSt @ 100 ◦ C. Previously, Cermak and coworkers reported viscosities of the same branched materials on an oleic estolide as a mixture of dimer and trimer plus that ranged from 92.6 to 151.1 cSt @ 40 ◦ C and from 15.2 to 21.4 cSt @ 100 ◦ C (Cermak et al., 2013). Samples that contain just the dimer estolide show a striking decrease in sample viscosities. The coco-oleic trimer plus estolide esters (VI) ranged from 120.8 to 227.7 cSt @ 40 ◦ C and from 17.9 to 29.4 cSt @ 100 ◦ C for the same series as the dimer esters. The highly branched iso-stearyl alcohol ester was tested with the trimer plus estolides to measure the upper viscosity limits of these samples which yielded viscosities of 417.3 cSt @ 40 ◦ C and 38.9 cSt @ 100 ◦ C. Finally, the viscosity index of all the coco-oleic estolide esters (V and VI) were excellent, showing results similar to other commercial oils (Cermak, 2006). The esterification process rarely achieves 100% conversion, thus AV will never be zero without extreme and expensive separation techniques. As a comparison, AV requirements for biodiesel are less than 0.5 mg KOH/g of oil (Moser, 2012). The desired AV was set at 0.1 mg of KOH/g of oil sample. Initial AV readings of both dimer and trimer estolides were higher than desired, thus a lower threshhold was desired in an effort to diminish the varying effect AVs would have on the physical properties. Thus, the materials underwent acid reduction techniques to lower the AV of both types of estolide esters (V and VI) to their purest possible state. In all cases (Tables 3 and 4), the Magnesol-treated materials had their AVs lowered to levels of 0.10 mg of KOH/g of oil or less. While slightly elevated AVs do not have negative effects on low temperature properties, the viscosities of the estolide esters (V and VI) may be impacted as a result of hydrogen bonding in the samples.
4. Conclusions A series of branched alcohols underwent an esterification process with either coco-oleic dimer or trimer plus estolide (III or IV) to produce 10 new coco-oleic dimer or trimer plus estolide esters (V or VI) followed by an evaluation of their physical properties. The coco-oleic estolide (II) was separated into two components based on the extent of oligomerization: coco-oleic dimer estolide (III) and coco-oleic trimer plus estolide (IV). The two different estolides were esterified with a series of branched alcohols. In general, the branched alcohols produced esters with better cold temperature properties than current commercially available materials. The coco-oleic dimer estolide esters (V) had the lowest PP where the 2-hexyldecanol ester, a 16 carbon-chain branched material, had a PP of −45 ◦ C and 2-octyldodecanol ester, a 20 carbon branched material, had a PP of −39 ◦ C. The best CP performer from the same series was 2-octyldodecanol ester, with a CP of −37, followed by hydrogenated isobutanol and a linear chain alcohol, methanol, at −25 and 923 ◦ C, respectively. The coco-oleic trimer plus estolide esters (VI) were too dark to obtain CP, but their PP ranged from −18 to −42 ◦ C which was slightly higher than the dimer series.
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The viscosities and viscosity indexes were as expected in terms of trends and ranges. The iso-stearyl alcohol ester had the most interesting viscosity of 417.3 cSt @ 40 ◦ C and 38.9 cSt @ 100 ◦ C, which was higher than all other oleic based estolides reported to date. The coco-oleic dimer estolide esters (V) ranged from 27.5 to 51.7 cSt @ 40 ◦ C and from 3.0 to 9.5 cSt @ 100 ◦ C, whereas the cocooleic trimer plus estolide esters (VI) ranged from 120.8 to 227.7 cSt @ 40 ◦ C and 17.9 to 29.4 cSt @ 100 ◦ C for the same series as the dimer esters. Finally, the ability to separate the estolides into different fractions based on EN or size has shown their potential usefulness as commercial lubricants. Many industrial crops that are high in oleic acid would have an advantage in supplying this demand. Having the ability to choose different viscosity ranges while still maintaining/exceeding low temperature properties is very desirable. Whatever the choice of ester, the development of these novel estolide esters will have a significant impact on industrial oils in the future as these estolide esters (V and VI) require no additives to obtain the improved low temperature and viscosity performance, thus limiting our impact on the environment and replacing fluids that are based on nonrenewable resources. Acknowledgments The authors are extremely grateful to Amber L. Durham, Katelyn N. Isbell, Billee L. John, Justin S. McCalvin, Kristy Thompson, and Melissa Becker for their assistance of estolide ester production, purification, and property testing. References Asadauskas, S., Erhan, S.Z., 1999. Depression of pour points of vegetable oils by blending with diluents used for biodegradable lubricants. J. Am. Oil Chem. Soc. 76, 313–316. Becker, R., Knorr, A., 1996. An evaluation of antioxidants for vegetable oils at elevated temperatures. Lubr. Sci. 8, 95–117. Bredsguard J., 2011. Acetic acid-capped estolide base oils and methods of making the same. U.S. Patent App. US 2012/0083435 A1. Bredsguard, J., 2011. High-and low-viscosity estolide base oils and lubricants. U.S. Patent App. US 2012/0178660 A1. Bredsguard, J., Forest, J., Thompson, T., 2011. Catalytic processes for preparing estolide base oils. U.S. Patent App. US 2012/0172609 A1. Bredsguard J., Forest J., Thompson T., 2012. Refrigerating fluid compositions comprising estolide compounds. US. Patent 8,236, 194. Cermak, S.C., Isbell, T.A., 2001. Biodegradable oleic estolide ester having saturated fatty acid end group useful as lubricant base stock, U.S. Patent 6,316,649 B1.
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