Article pubs.acs.org/IECR
Lipoate Ester Multifunctional Lubricant Additives Girma Biresaw,*,† David Compton,‡ Kervin Evans,‡ and Grigor B. Bantchev† †
Bio-Oils Research Unit and ‡Renewable Product Technology Research Unit, National Center for Agricultural Utilization Research, Agricultural Research Service, United States Department of Agriculture, 1815 N. University Street, Peoria, Illinois 61604, United States ABSTRACT: Seven lipoate esters were synthesized by esterification of lipoic acid with different structures of alcohols in the presence of a solid acid catalyst and without solvent. The esters were obtained in good yield, characterized using 1H NMR and GPC; and their physical properties investigated. Four of the seven lipoate esters, with good solubility in high oleic sunflower oil (HOSuO) and polyalphaolefin (PAO6) base oils, were further investigated for their lubricant additive property. Additive properties were investigated in HOSuO base oil by varying the additive concentrations in the range 0−20 (w/w %). Properties investigated include: density; kinematic and dynamic viscosity; viscosity index (VI); oxidation stability from onset and peak oxidation temperatures on a PDSC; 4-ball antiwear (AW); and 4-ball extreme pressure (EP). Neat lipoate esters displayed high oxidation stability and high VI. As additives to HOSuO base oil, lipoate esters displayed good viscosity index improver (VII), good antioxidant, good extreme pressure (EP), but poor AW properties. The VI and VII properties were attributed to the presence of low concentrations of polymeric byproducts in the ester product mixtures. This work demonstrates that lipoate esters can be developed into effective multifunctional biobased lubricant additives with excellent VII, AO and EP properties.
1.0. INTRODUCTION
So, there are many incentives for developing and using biobased lubricants in the modern economy. Achieving this goal requires developing biobased base oils and biobased additives that go into lubricant formulations. Such products must successfully compete against current petroleum based products in performance and cost. Advances have been made in the development of biobased base oils from vegetable oils,5,6 derivatives of vegetable oils and fats,7−9 as well as from sugars.10 Some of these products have displayed competitive performance relative to petroleum based base oils but more work is needed to make them cost competitive. Some progress has also been made with the development of biobased additives.11−14 However, this progress is limited to a small fraction of the vast number of additive chemistries used in lubrication formulations.15 As a result, almost all current biobased lubricants in the market are formulated using commercial petroleum based additives. Our group in USDA has been engaged in the development of sulfur and phosphorus containing antiwear and extreme pressure (EP) additives.9,16−18 Recently, we have extended our investigations to biobased additives with disulfide moiety in their structure. These materials were synthesized by enzymatic transesterification reaction involving high oleic sunflower oil (HOSuO) and lipoic acid. The transesterified glyceride displayed EP properties comparable to commercial polysulfide EP additives.19 In the current work, we have expanded our investigation of lipoic based disulfide structures by synthesizing and evaluating a wide range of lipoate ester structures. These products were obtained by solid acid catalyzed esterification of lipoic acid with normal and branched C4 to C18 alcohols. In
Lubricants play numerous critical roles in many aspects of the modern society. An example of such a critical role is increasing the efficiency of manufacturing and transportation systems, resulting in a huge reduction of both energy consumption and release of greenhouse gases (GHG). Recent reports indicate that much more efficiency and, hence, more dramatic reductions of fuel consumption and GHG emissions can be attained with further advances in lubrication and lubricants.1,2 While lubricants contribute tremendously to a reduction in the generation of GHG and other pollutants, they can also be a source for such products. This happens when lubricants leak into the environment accidentally, intentionally or due to negligence. Leaked lubricants can contaminate land, waterways, and also evaporate and enter the atmosphere as GHGs. The damage from leaked lubricants depends on the nature of the lubricants. Lubricants currently in the market are almost exclusively petroleum based and not biodegradable. Leaks of petroleum based lubricants will have a severe effect on the environment and will also be extremely expensive to clean and remediate.3 One way of countering the negative impact of lubricants on the environment is to use biobased, and, hence, biodegradable ingredients to formulate biobased lubricants. Biobased lubricant ingredients are obtained from plants and animals which, in addition to being biodegradable, are also renewable and sustainable. Also, the production and application of biobased lubricants results in a net negative GHG emission.4 On the other hand, petroleum based lubricants, in addition to being nonbiodegradable, are also nonrenewable, i.e., they are depleting and also net GHG emitters. In addition to environmental benefits, the use of biobased ingredients in lubrication will also produce economic benefits to farmers and the rural economy. This article not subject to U.S. Copyright. Published XXXX by the American Chemical Society
Received: October 2, 2015 Revised: December 17, 2015 Accepted: December 18, 2015
A
DOI: 10.1021/acs.iecr.5b03697 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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mol), ∼5-fold excess alcohol (mol:mol), molecular sieves (16.0 g), and Amberlyst-15 (15.0 g). The reaction mixture was stirred under nitrogen at 70 °C for 96 h. The resultant, very viscous residue was cooled to room temperature and diluted with ethyl acetate (200 mL). The diluted reaction mixture was filtered through a bed of Celite. The reaction flask was rinsed with ethyl acetate (200 mL) and the washings filtered and combined with the filtrate. The solvent was removed from the filtrate by rotoevaporation and the crude reaction mixture separated into ∼8 mL aliquots and stored at −4 °C for purification. The crude lipoate esters were purified using a Teledyne Isco, Inc. (Lincoln, NE,) CombiFlash Rf 200i flash chromatography system. Product elution was monitored by an evaporative light scattering detector (ELSD) (nitrogen) set to 4 min peak width, 0.1 V collection threshold, 40 °C spray temperature, 70 °C drift temperature, and ultraviolet violet spectroscopy (340 nm). Eluent peaks were collected in 25 × 150 mm test tubes (50 mL maximum volume). A crude lipoate ester aliquot (∼8 mL) was thawed, diluted with additional ethyl acetate (2.0 mL), and loaded by syringe onto a 220 g RediSep Rf High Performance Gold silica column (1 column volume, CV, equaled 162 mL at 100 mL/min flow rate). The RediSep silica column was aspirated to dryness for 1 h. The sample loaded RediSep silica column was developed with a hexane/ethyl acetate gradient at 100 mL/min: 0% ethyl acetate for 1 CV, increased to 10% ethyl acetate over 2 CV, held at 10% ethyl acetate for 3 CV, increased to 100% ethyl acetate over 1 CV, held at 100% ethyl acetate for 2 CV, decreased to 0% ethyl acetate over 1 CV, held at 0% ethyl acetate for 2 CV, followed by a 4 min air purge of the column. The lipoate esters eluted at 10% ethyl acetate during the gradient. The procedure was repeated for the remaining aliquots. The 220 g RediSep Rf High Performance Gold silica column was reused for nine separations before being discarded. The contents of the collection tubes containing the lipoate ester were combined and the solvent removed by rotoevaporation, and the resultant oily residue was dried under vacuum for 24 h at 50 °C. 1 H NMR. Spectra were obtained on a Bruker Avance 500 spectrometer (500 MHz 1H) using a 5 mm BBI probe (Bruker, Billerica, MA, USA). All samples were dissolved in CDCl3, and all spectra were acquired at 27 °C within 2 min of dilution in CDCl3. Chemical shifts are reported as ppm from tetramethylsilane calculated from the lock signal (ΞD = 15.350 609%). GPC. GPC analysis was conducted on Agilent Series 1100 HPLC system (Santa Clara, CA) with a refractive index detector, using a Phenomenex (Torrance, CA) 5 μm Linear (2) column (300 × 7.8 mm) maintained at 35 °C. Samples were incubated at 35 °C in the autosampler tray prior to injection. The injection volume was 15 μL for the test samples and 10 μL for the standards. Tests were conducted at solvent flow rate of 1.0 mL/min and run time of 20 min. Molecular weight calculations were based on the peak retention times of the samples relative to that of the standards. Poly(methyl methacrylate) (PMMA) with molecular weights from 2 kDa to 1500 kDa, obtained from American Polymer Standards Corporation (Mentor, OH), were used as the standards. Lipoate ester solutions for the test were prepared by dissolving 50 mg of the esters in 10 mL of tetrahydrofuran (THF) to make 0.5% solutions. PMMA solutions for the test were prepared by dissolving 25 mg of the standard in 5 mL of THF to make 0.5% solutions. 2.2.2. Synthetic Yield and 1H NMR Absorbances of Lipoate Esters. Ethyl Lipoate, 3a. Yield: 0.62 g (49.2% based on lipoic
this manuscript, we provide details of the synthesis and identification of these products along with the investigation of tribological and related properties of selected lipoate ester structures. α-Lipoic acid (1 in Scheme 1) is cyclic disulfide of octanoic acid that occurs naturally in plants and animals and that has Scheme 1. Condensation Reaction of Lipoic Acid and Alcohols To Form Lipoate Esters
been intensely studied and reviewed.20 The bioavalability of lipoic acid is primarily from liver, heart and kidney.20 Plant sources of lipoic acid include green vegetables, such as spinach, broccoli and brussel sprouts, as well as potatoes, rice bran and tomatoes.20−22 The pharmacological benefits of lipoic acid as a free radical scavenger, metal chelator, diabetic and neuropathologic aid, and anticancer agent have been investigated.23−27 The natural occurrence of lipoic acid in the environment makes it an excellent candidate for development into biobased industrial chemicals and products.28,29 Lipoic acid is currently manufactured and sold as a dietary/nutritional supplement.
2.0. EXPERIMENTAL SECTION 2.1. Materials. (±)-α-Lipoic acid (lipoic acid), Amberlyst15 hydrogen form, 2-Ethylhexanol, 1-octanol (anhydrous), 1dodecanol, 1-decanol, ethanol, isobutanol, LC grade hexanes and ethyl acetate were purchased from Sigma-Aldrich (St. Louis, MO). Isopropyl alcohol (99.9%) was obtained from Thermo Fisher Scientific, Inc. (Pittsburgh, PA). Isostearyl alcohol (Fine Oxocol-180) was a free sample from Nissan Chemical American Corp. (Houston, TX). High-oleic sunflower oil (81% oleic acid) was purchased from Columbus Foods Company (Des Plaines, IL). Polyalpha olefin with a viscosity of 6 cSt at 100 °C (PAO6), sold under the trade name Durasyn 166, was a free sample from Ineos Oligomers (League City, TX). Di-tert-dodecyl polysulfide, known under the trade name TPS-32, was a free sample from Arkema Canada Inc. (Burlington, ON, Canada). Davison Molecular Sieves (4 Å, 14−30 mesh) were purchased from Fischer Scientific (Pittsburgh, PA), dried at 110 °C under vacuum, and stored under nitrogen until used. Celite was purchased from Fischer Scientific (Pittsburgh, PA). RediSep Rf High Performance Gold silica columns (220 g, 23 × 5.5 cm) was purchased from Teledyne Isco, Inc. (Lincoln, NE). All chemicals were used as supplied unless noted above. Steel balls used in 4-ball tribological experiments were obtained from Falex Corporation (Aurora, IL) and were cleaned prior to use by consecutive 10 min sonications in isopropyl alcohol and hexane solvents. The steel balls have the following specifications: material, chrome− steel alloy made from AISI E52100 standard steel; hardness, 64−66 Rc; diameter, 12.7 mm; finish, grade 25 extra polish. 2.2. Synthesis. 2.2.1. General Procedure for Synthesis and 1 H NMR Identification of Lipoate Esters. The solid acidcatalyzed esterification of lipoic acid (0.4 and 40 g scales) with various alcohols was conducted using the following general method. A 1L Schlenk flask containing a stirbar and fitted with a rubber septum was charged with lipoic acid (40.0 g, 0.19 B
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and 100 °C following the procedure outlined in ASTM D 227093.31 2.3.3. Pressurized Differential Scanning Calorimetry (PDSC). PDSC tests were conducted according to ASTM D618632 on a Q20P pressure differential scanning calorimeter (TA Instruments - Waters LLC, New Castle, DE) fitted with a computer and appropriate software to allow for data acquisition and analysis. All tests were conducted with the cell pressurized with pure oxygen to 500 ± 25 psig in dynamic mode, i.e., with a positive oxygen flow rate of 100 ± 10 mL/min. Details of the test procedure have been given before.19 Duplicate runs were conducted, and average values of onset temperature (OT) and peak temperatures (PT) are reported. 2.3.4. Four-Ball (4-Ball) Tribological Tests. Tests were conducted on a model KTR-30L 4-ball tribometer equipped with TriboDATA software (Koehler Instrument Company, Bohemia, NY). Details of the instrument hardware and software have been given before.18 2.3.5. 4-Ball Antiwear (AW) Test. 4-ball AW tests were conducted according to the ASTM D4172-94 procedure.33 The coefficient of friction (COF) from each test was calculated from the corresponding torque and load data in accordance with ASTM D5183 procedure.34 Wear scar diameters, along and across the wear direction of the three top balls after each test, were measured using a wear scar measurement system comprising hardware and ScarView software (Koehler Instrument Company, Inc., Bohemia, NY). Each test lubricant was used in at least two AW measurements and average COF and wear scar diameter (WSD) values were reported. 2.3.6. 4-Ball Extreme Pressure (EP) Test. A 4-ball EP test was conducted according to the ASTM D2783 procedure.35 The method involves a series of 10 s tests conducted with increasing loads until welding of the four balls is observed. The load at which welding occurs is reported as the weld point of the lubricant tested. Weld point is an inherent property of lubricants and higher weld point corresponds to superior EP property 2.4. Data Analysis. Data analysis was conducted with IgorPro Version 5.0.3.0 software (WaveMetrics, Inc., Lake Oswego, OR).
acid). 1H NMR (500 MHz, CDCl3): δ 4.09 (2 H, q, −C(O)O− CH2−CH3), 3.62 (1 H, m, −CH2−CH(S)−CH2−), 3.17 (2 H, dm, J = 39.7 Hz, −S−CH2 −CH2−), 2.50 (1 H, sextet, −S− CH2−CH2−CH−), 2.31 (2 H, t, −CH2−C(O)−O−), 1.92 (1 H, sextet, −S−CH2−CH2−CH−), 1.75 (1 H, m, −CH2−), 1.65 (3 H, −CH2−), 1.47 (2 H, m, −CH2−), 1.22 (3 H, t, −CH3). Isobutyl Lipoate, 3b. Yield: 0.57 g (44.7% based on lipoic acid). 1H NMR (500 MHz, CDCl3): δ 3.87 (2 H, d, −C(O)O− CH2−CH−), 3.59 (1 H, m, −CH2−CH(S)−CH2−), 3.17 (2 H, dm, J = 35.0 Hz, −S−CH2 −CH2−), 2.48 (1 H, sextet, −S− CH2−CH2−CH−), 2.35 (2 H, t, −CH2−C(O)−O−), 1.94 (1 H, sextet, −S−CH2−CH2−CH−), 1.70 (5 H, −CH2− and −C(O)−CH2−CH−), 1.49 (2 H, m, −CH2−), 1.22 (6 H, d, −CH3). Octyl Lipoate, 3c. Yield: 52.0 g (84.3% based on lipoic acid). 1 H NMR (500 MHz, CDCl3): δ 4.07 (2 H, t, −C(O)O− CH2−), 3.58 (1 H, m, −CH2−CH(S)−CH2−), 3.16 (2 H, dm, J = 31.3 Hz, −S−CH2 −CH2−), 2.47 (1 H, sextet, −S−CH2− CH2−CH−), 2.33 (2 H, t, −CH2−C(O)−O−), 1.92 (1 H, sextet, −S−CH2−CH2−CH−), 1.80−1.20 (18 H, broad multiplets, −CH2−), 0.90 (3 H, t, −CH3). 2-Ethylhexyl Lipoate, 3d. Yield: 49.9 g (81.0% based on lipoic acid). 1H NMR (500 MHz, CDCl3): δ 4.00 (2 H, m, −C(O)O−CH2−), 3.58 (1 H, m, −CH2−CH(S)−CH2−), 3.16 (2 H, dm, J = 32.6 Hz, −S−CH2 −CH2−), 2.47 (1 H, sextet, −S−CH2−CH2−CH−), 2.33 (2 H, t, −CH2−C(O)− O−), 1.92 (1 H, sextet, −S−CH2−CH2−CH−), 1.75−1.20 (15 H, broad multiplets, −CH2− and −CH2−CH(CH2)−CH2−), 0.90 (6 H, t, −CH3). Decyl Lipoate, 3e. Yield: 0.54 g (31.8% based on lipoic acid). 1H NMR (500 MHz, CDCl3): δ 4.07 (2 H, q, −C(O)O− CH2−), 3.58 (1 H, m, −CH2−CH(S)−CH2−), 3.16 (2 H, dm, J = 30.9 Hz, −S−CH2 −CH2−), 2.48 (1 H, sextet, −S−CH2− CH2−CH−), 2.33 (2 H, t, −CH2−C(O)−O−), 1.92 (1 H, sextet, −S−CH2−CH2−CH−), 1.80−1.20 (22 H, broad multiplets, −CH2−), 0.89 (3 H, t, −CH3). Dodecyl Lipoate, 3f. Yield: 36.3 g (49.9% based on lipoic acid). 1H NMR (500 MHz, CDCl3): δ 4.07 (2 H, t, −C(O)O− CH2−), 3.58 (1 H, m, −CH2−CH(S)−CH2−), 3.15 (2 H, dm, J = 30.9 Hz, −S−CH2 −CH2−), 2.47 (1 H, sextet, −S−CH2− CH2−CH−), 2.32 (2 H, t, −CH2−C(O)−O−), 1.92 (1 H, sextet, −S−CH2−CH2−CH−), 1.80−1.20 (26 H, multiplets, −CH2−), 0.89 (3 H, t, −CH3). Isostearyl Lipoate, 3g. Yield: 54.1 g (61.1% based on lipoic acid). 1H NMR (500 MHz, CDCl3): δ 4.07 (2 H, m, −C(O)O−CH2−), 3.58 (1 H, m, −CH2−CH(S)−CH2−), 3.15 (2 H, dm, J = 30.9 Hz, −S−CH2 −CH2−), 2.47 (1 H, sextet, −S−CH2−CH2−CH−), 2.32 (2 H, t, −CH2−C(O)− O−), 1.92 (1 H, sextet, −S−CH2−CH2−CH−), 1.80−1.00 (17 H, multiplets, −CH2−, −CH(CH3)−CH2− and −CH(CH2)− CH2−), 0.89 (24 H, t, −CH3). 2.3. Characterization of Physical and Tribological Properties. 2.3.1. Solubility. Solubility (% w/w) of lipoates esters at room temperature in polyalpha olefin (PAO6) and high oleic sunflower oil (HOSuO) base oils was determined gravimetrically by visual inspection for any changes in oil transparency with increased ester concentration. 2.3.2. Density, Dynamic/Kinematic Viscosity and Viscosity Index (VI). Density and kinematic viscosity at 40 and 100 °C were measured on a Stabinger SVM3000/G2 viscometer (Anton Paar GmbH, Graz, Austria) according to ASTM D 7042.30 VI was calculated from kinematic viscosity data at 40
3.0. RESULTS AND DISCUSSION 3.1. Synthesis and Spectroscopic Identification. A series of lipoic acid esters, referred to as lipoates, were synthesized by the condensation of lipoic acid with a series of alcohols (Scheme 1). Straight chain lipoic acid esters have been synthesized from the HCl catalyzed reaction of lipoic acid with C6 to C13 n-alcohols.36 These previous syntheses were limited to a 5 g scale and required the use of the solvents ethyl chloroformate, triethylamine, and THF. The purification of the lipoate esters required neutralization of the reaction mixture, phase separation, a saline wash, and drying over sodium sulfate. The authors reported issues with lipoic acid polymerization and did not report yields. Amberlyst-15 has previously been reported to esterify ω-sulfhydryl fatty acids with alcohols at 70 °C without promoting the formation of thoiesters, disulfides, or thioethers.37 Attempts to conduct these esterifications with stronger acids of pKa < 1.0 (e.g., p-toluenesulfonic acid, sulfuric acid) resulted in thioether and disulfide formation. Thus, Amberlyst-15 was used to catalyze the syntheses of the lipoate esters, which improved the esterification compared to previous reported syntheses and was expanded to include highly branched alcohols (Figure 1).36 While requiring longer reaction C
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initial syntheses were quite low, ca. 30−40%, and much of the loss was attributed to handling. Modification of the handling procedures detailed in the Experimental Section and refinement of the flash chromatography method led to increased yields of ca. 80%. The purified lipoate ester products were stored at −4 °C until used for the tribology tests. 1 H NMR spectroscopy was used to confirm the formation and purity of the lipoate esters. Figure 2 shows the spectra of lipoic acid, 1, octyl lipoate, 3c, ethylhexyl lipoate, 3d, dodecyl lipoate, 3f, and isostearyl lipoate, 3g. The chemical shifts of lipoic acid were unambiguous and integrated with the expected proton ratios as previously published by Kinsanuki et al, 2010.38 The straight chain lipoate esters possessed spectra with the expected proton shifts and ratios and were in agreement with those previously reported.36 The branched chain lipoate esters also showed the expected proton chemical shifts and ratios. The isobutyl, 2-ethylhexyl and isostearyl peaks of 3b, 3d and 3g were very similar to those for the structurally similar 2butyloctyl, 2-hexyldecyl and isostearyl oleic and coco estolide esters.39,40 The correct chemical shift ratios of the −CH3 protons to the lipoyl proton of carbon 1 and the lack of a broad −C(O)−OH singlet at ca. 11.5 ppm confirmed that lipoate esters isolated by flash chromatography did not conation residual alcohol or lipoic acid. Kinsanuki et al.38 has detailed the ring-opening polymerization of lipoic acid above 80 °C and characterized the resultant poly(lipoic acid) by 1H NMR spectroscopy. It was shown that upon ring-opening polymerization of the disulfide bond the three protons of carbons 1 and 3 of the disulfide ring (Figure 2) collapse into a distinct, broad, singlet shifted upfield to 2.70 ppm. The two protons of carbon 2 of the disulfide ring also collapse into a broad singlet shifted to 1.95 ppm. The proton shifts of the poly(lipoic acid) are distinct and unambiguous compared to the lipoic acid monomer.38 Esterification of the poly(lipoic acid) with methanol to form poly(methyl lipoate) did not affect the 2.70 and 1.95 ppm chemical shifts of the polymeric disulfide ring protons.38
Figure 1. Structures of lipoate esters, 3a−g.
times and higher temperatures, we eliminated the need for a solvent during the reaction (reactions are performed neat) and simplified the reaction by using a solid acid catalyst, Amberlyst15. After the catalyst was removed by filtration, the lipoate esters were isolated in 50 g quantities (ca. 80% yields) using flash chromatography. The improved method did not promote polymerization of the lipoic acid, as determined by 1H NMR, and eliminated the need of L-cysteine as a stabilizer. However, upon prolonged storage, small amounts of polymeric material did form as discussed below. Although the neat condensation reactions were straightforward, the longer chain lipoate ester products, 3c−g, were viscous at the reaction temperature of 70 °C and were very viscous and stringy at room temperature. The yields of the
Figure 2. 1H NMR (500 mHz, CDCl3) spectra of lipoic acid and lipoate esters (see Figure 1 for lipoate ester structures) containing trace poly(lipoate ester). D
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Figure 3. GPC trace of lipoate ester product mixture.
Table 1. Composition of High Molecular Weight Components in Lipoate Ester Product Mixture Estimated Using GPC and NMR lipoate ester GPC NMR
MW, kDa amount (%, w/w) amount (% mol/mol)
n-octyl
2-ethylhexyl
n-dodecyl
isostearyl
3128 5.7 5−6
3435 6.6 5−6
2253 19.8 10
97 3.1 0
Table 2. Lipoate Esters Synthesized in This Work room temperature solubility (w/w %) ester
derivative
formula
mol wt (Da)
[S], % (w/w)
HOSuO
PAO-6
3a 3b 3c 3d 3e 3f 3g
ethyl isobutyl 2-ethylhexyl n-octyl n-decyl n-dodecyl isostearyl
C10H18O2S2 C12H22O2S2 C16H30O2S2 C16H30O2S2 C18H34O2S2 C20H38O2S2 C26H50O2S2
234 262 319 319 347 375 459
27 24 20 20 19 17 14
720-fold increase. Similarly, the VI increased from 70 for the isostearyl to 567 for the dodecyl lipoate, which is a >230-fold increase. Such high variations in viscosity and VI cannot be rationalized using the structural differences of chain length and chain branching between the esters shown in Figure 1. However, the viscosity and VI differences are consistent with the GPC concentrations of high molecular weight residues in each ester which increases in the order: isostearyl < n-octyl 7% w/w. This implies that EP properties are not just functions of [S] alone and other factors must be in play as well. Previous work from our group has indicated that compounds with single S in their structures (e.g., R−S−R′) do not display EP properties;17 those with one S−S group in their structure (similar to the esters studied in this work) show some EP properties;19 and those with multiple S−S bonds (R−Sn−R′, n > 2) such as TPS-32, display excellent EP properties.45 One possible explanation for these observations is that there may be some correlation between WP and the number of consecutive S−S bonds in the structure of the EP additives. However, currently there are no sufficient data to confirm or disprove such correlations. 3.8. Structure−Property Considerations. As shown in Figure 1, the lipoate esters synthesized and investigated in this work display a wide range of structures. While they all have the same disulfide lipoate function, they differ in their molecular weight due to the different molecular weight of alcohols used in their synthesis. They also have different molecular structures, again due to the different structures of alcohols used in their synthesis. The structural differences can be described in terms of chain length (short, medium, long), presence of branching (linear, branched) and degree of branching (simple, complex). In addition, the product mixture in each ester comprise small amounts of polymeric biproducts, whose concentration is different in the different esters (Table 1). The effects of these differences on the observed properties are discussed in this section.
Table 8. 4-ball Antiwear Properties of High Oleic Sunflower Oil with Lipoate Ester Additives 4-ball antiwear properties of blends with [lipoate ester] (% w/w) of 0a
lipoate ester
a
2-ethylhexyl n-octyl n-dodecyl isostearyl
0.066 0.066 0.066 0.066
± ± ± ±
2-ethylhexyl n-octyl n-dodecyl isostearyl
0.668 0.668 0.668 0.668
± ± ± ±
1
5
coefficient of friction 0.005 0.086 ± 0.006 0.060 0.005 0.092 ± 0.005 0.066 0.005 0.089 ± 0.005 0.072 0.005 0.085 ± 0.011 0.083 wear scar diameter (mm) 0.004 0.876 ± 0.029 0.726 0.004 0.883 ± 0.018 0.677 0.004 0.917 ± 0.033 0.623 0.004 0.942 ± 0.046 0.729
± ± ± ±
0.012 0.009 0.006 0.008
± ± ± ±
0.052 0.026 0.092 0.040
Neat high oleic sunflower oil data from ref 19.
to an average value of 0.088. On the basis of examination of the standard deviations, all blends with 1% lipoate ester produce equal increases of COF. However, blends with 5% lipoate esters display lower COF than the 1% blends and equal to the value for the neat HOSuO. The exception to this is the blend with 5% isostearyl lipoate, which gave COF values similar to that of the 1% blend. Examination of the WSD data in Table 8 shows a similar result as the COF data discussed above. Thus, blends with 1% lipoate esters produced bigger WSD than the neat HOSuO. However, unlike with COF, the rise in WSD of the 1% blends were not identical but increased in the order 2-ethylhexyl < noctyl < n-dodecyl < isostearyl. On the other hand, the 5% blends displayed lower WSD than the 1% blends but similar to the values for the neat HOSuO. Lipoate esters, unlike antifriction additives, react with the friction materials and produce wear as well as rough surfaces that produce higher friction and wear than the HOSuO biobased base oil without the additives. On the other hand, HOSuO and other vegetable oils react poorly to friction materials but adsorb strongly on friction surfaces and reduce both friction and wear.43,44 3.7. Extreme Pressure (EP) Properties of Lipoate Esters in HOSuO. The 4-ball EP properties of blends of HOSuO with the various lipoate ester structures was investigated as a function of lipoate ester concentration, which was varied in the range 1−20% (w/w). The EP weld point (WP) results of the blends and neat HOSuO are summarized in Table 9. With the exception of 2-ethylhexyl lipoate, the WP of blends increased with each increase of lipoate ester concentration in the blend reaching values in the range 420−480 kgf at 20% lipoate ester composition. In blends Table 9. 4-ball EP weld point of high oleic sunflower oil with lipoate ester additives, kgf 4-ball EP weld point (kgf) of [lipoate ester] in HOSuO, % w/w
a
lipoate ester
0a
1
5
10
20
2-ethylhexyl n-octyl n-dodecyl isostearyl
120 120 120 120
220 240 200 220
320 320 260 300
320 420 440 380
320 440 480 420
Neat high oleic sunflower oil data from ref 19. H
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Figure 4. [EP Additive] in HOSuO vs weld point. Data for LGc, LGp and TPS-32 from ref 19.
result is attributed to the fact that the lipoate esters participate in a tribochemical reaction with the friction surfaces producing rough surfaces and wear debris. This results in high friction and high wear of the friction surfaces. In contrast, antiwear additives adsorb onto but not participate in tribochemical reaction with friction surfaces, and effectively lower both friction and wear. Lipoate esters blended in HOSuO displayed fairly high extreme pressure (EP) weld point (WP) values that were independent of the hydrocarbon chain structures (Table 9). Thus, the EP properties are entirely due to the disulfide moiety of the lipoate esters. This observation is consistent with what is well-known about EP additives, where selected elements such as halogens, S and P in the molecule, in situ react with the friction surfaces and produce the lubricant that provides high WP. However, it is clear from previous work,17 that the presence of S in the molecular structure does not guarantee EP properties. For example, modification of soybean oil by insertion of n-butyl sulfide across the double bond did not produce EP properties,17 whereas replacement of one or more fatty acids in HOSuO by lipoic acid,19 which has a disulfide in its structure, did produce EP properties. This implies that the presence of a weak S−S bond in the structure of sulfurized EP additives is essential to producing EP properties. Furthermore, comparison of lipoate ester EP results to those of commercial sulfurized EP additive TPS-32 indicates higher WP for the commercial product which has several consecutive S−S bonds due to presence of −Sn− in its structure, where n > 2. As demonstrated in this work, the WP of the lipoate esters and those of the commercial pentasulfide EP additive TPS-32 did not correlate as a function of [S] in the molecule (Figure 4). Thus, in sulfurized EP additives, WP improvement requires the presence of S−S bonds in the structure of the molecule, and that WP will increase with increasing number of consecutive S− S bonds in the molecule. However, establishing quantitative correlations between WP and consecutive S−S bonds in the EP additive molecule will require more data on more EP additives.
Structure had a profound effect on the solubility of lipoate esters in the polar HOSuO and the nonpolar PAO6 base oils (Table 2). The results follow the general rule “like dissolves like.” The solubility data shows that, while all lipoate esters are polar, they differ in degree of polarity into high, intermediate and low polar esters. As shown in Table 2, the high polar esters (ethyl, isobutyl lipoates) have low solubilty in either base oils; the intermediate polar esters (2-ethylhexyl, n-octyl lipoates) are more soluble in HOSuO than in PAO6; and the low polar esters (n-decyl, n-dodecyl, isostearyl lipoates) are very soluble in both base oils. The effect of structure on viscosity, VI and VII properties of lipoate esters is rather complex. In general, viscosity increases with increasing intermolecular interactions (both polar and nonpolar), increasing molecular weight, and increasing branching. Application of these principles can explain some, but not all, of the observations of structural effect on the viscosity of lipoate esters summarized in Table 3. Specifically, the huge range of observed viscosity (74−53 714 cSt at 40 °C) and VI (70−567) cannot be explained by the small structural changes among the esters. However, it can be satisfactorily explained in terms of the concentrations of polymeric components in the product mixture. Thus, the lipoate esters displayed viscosity, VI and concentration of polymeric component in the product mixture that increased in the order: isosterate < n-octyl
