Ester Hydroxy Derivatives of Methyl Oleate: Tribological, Oxidation and Low Temperature Properties Q

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Bioresource Technology 99 (2008) 7333–7340

Ester hydroxy derivatives of methyl oleate: Tribological, oxidation and low temperature properties q

Brajendra K. Sharma a,b, Kenneth M. Doll a,*, Sevim Z. Erhan a

a USDA/NCAUR/ARS, Food and Industrial Oil Research, 1815 North University Street, Peoria, IL 61604, USA b Department of Chemical Engineering, Pennsylvania State University, University Park, PA 16802, USA

Received 26 January 2007; received in revised form 6 December 2007; accepted 6 December 2007 Available online 31 January 2008

Abstract

Five branched oleochemicals were prepared from commercially available methyl oleate and common organic acids; and their lubricant properties were determined. These branched oleochemicals are characterized as 9(10)-hydroxy-10(9)-ester derivatives of methyl oleate. These derivatives show improved low temperature properties, over olefinic oleochemicals, as determined by pour point and cloud point measurements. The derivatization also increased thermo-oxidative stability, measured using both pressurized differential scanning calorimetry (PDSC) and thin film micro oxidation (TFMO) methods. Branched oleochemicals were used as additives both in soybean oil and in polyalphaolefin. Their lubrication enhancement was evaluated by both four-ball and ball-on-disk wear determinations. These derivatives have good anti-wear and friction-reducing properties at relatively low concentrations, under all test loads. Their surface tensions were also determined and a trend was observed. The materials with larger side chain branches had lower surface tension than those containing smaller side chain branches. An exception to this trend was found when studying the compound with the carbonyl containing levulinic acid side chain, which had the highest surface tension of the branched oleochemicals studied. Overall, the data indicate that some of these derivatives have significant potential as a lubricating oil or fuel additives. Published by Elsevier Ltd.

Keywords: Epoxidized methyl oleate; Hydroxy ester; Lubricant additive; Lubricant; PDSC

1. Introduction 40% (Gawrilow, 2004; Sharma et al., 2006) of a lubricant can be lost to the environment. The inherent biodegrad- The use of vegetable oils, or their derivatives, is a good ability of vegetable oils serves to reduce their environmen- alternative to using petroleum oils as lubricants or lubri- tal impact. Perhaps due to environmental concerns, the cant additives in environmentally sensitive industrial appli- demand of biobased lubricants is increasing, as seen from cations. These include hydraulic equipment used in ports, 16.3% growth of biobased lubricants in European market. waterways and agriculture; ski slope machinery, wire ropes This contrasts with the essentially flat rate of consumption to control dams, chainsaws, two-cycle marine outboard for lubricants as a whole and the modest 2.2% growth for engine oils, and other areas where there is high risk of envi- synthetic lubricants (Tocci, 2006). ronmental contamination. In many industries, as much as Vegetable oils, especially soybean oil and palm oil, are relatively inexpensive. In the United States, the major q The use of trade, firm, or corporation names in this publication is for source of vegetable oil for industrial application is soybean the information and convenience of the reader. Such use does not oil. Development of economically feasible new industrial constitute an official endorsement or approval by the United States products or commercial processes using soybean oil is Department of Agriculture or the Agricultural Research Service of any highly desirable. At the same time, petroleum prices are product or service to the exclusion of others that may be suitable. * Corresponding author. Tel.: +1 309 681 6103; fax: +1 309 681 6340. high, and not likely to significantly decline in the near E-mail address: [email protected] (K.M. Doll). future (de Guzman, 2005).

0960-8524/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.biortech.2007.12.057 7334 B.K. Sharma et al. / Bioresource Technology 99 (2008) 7333–7340

Soybean oil has a triacylglycerol structure. The fatty oxide (Sigma–Aldrich, St. Louis, MO, A.C.S. Reagent, chains comprising it vary from 14 to 22 carbons in length, 30% Solution); formic acid (Sigma–Aldrich, 96%, A.C.S. with 0–3 double bonds per fatty chain. The average num- reagent); hexanes (Sigma–Aldrich, St. Louis, MO, >95%, ber of carbon–carbon double bonds per molecule of refined HPLC grade); sodium chloride (Fisher, Fairlawn, NJ, soybean oil of North American origin is around 4.6. The A.C.S. Reagent); sodium bicarbonate (Fisher, Fairlawn, advantages of using soybean oil or oleochemicals as lubri- NJ, A.C.S. Reagent); acetone (Sigma–Aldrich, St. Louis, cants are well known. For example, the superior lubricity MO, Chromasolve for HPLC, 99.9%); sodium hydroxide of biodiesel (Bhatnagar et al., 2006; Knothe, 2005), com- (Fisher, Fairlawn, NJ, A.C.S. Reagent); sodium sulfate pared to petroleum based diesel, is often given as one of (Sigma–Aldrich, St. Louis, MO, 99+%, A.C.S. Reagent); the major advantages of the biofuel. However, vegetable n-butanol (Fisher, Fairlawn, NJ, A.C.S. Reagent); propi- oil based lubricants have a lower oxidative stability (Cosci- onic acid (Sigma–Aldrich, St. Louis, MO, 99.5%); levulinic one and Artz, 2005; Dunn, 2005; Erhan et al., 2006) and acid (Sigma–Aldrich, St. Louis, MO, 98%); hexanoic acid poor cold flow properties at low temperatures (Hwang (Sigma–Aldrich, St. Louis, MO, 99.5%); octanoic acid and Erhan, 2001; Li et al., 2004). One potential way to (Sigma–Aldrich, St. Louis, MO, 99%); 2-ethylhexanoic improve oxidation and low temperature properties is to acid (Sigma–Aldrich, St. Louis, MO, 99%); tetrabutylam- attach some functional groups to vegetable oils or fatty monium bromide (Sigma–Aldrich, St. Louis, MO, 99%); acids, at their sites of unsaturation, through chemical mod- lithium bromide (Sigma–Aldrich, St. Louis, MO, 99+%); ifications. Hydrogenation (King et al., 2001), esterification potassium bromide (Buck Scientific, E. Norwalk, CT); (Doll et al., 2007b), epoxidation (Campanella et al., 2004; polyalphaolefin (PAO4, BP Amoco, Naperville, IL); and Carlson et al., 1994; Findley et al., 1945), metathesis hexadecane (Aldrich, St. Louis, MO, 99% anhydrous) were (Erhan et al., 1997; Holser et al., 2006; Verkuijlen et al., all used as received. 1977), alkylation, acylation (Sharma et al., 2006), or a com- bination of chemistries, have all been used in order to syn- 2.2. Synthesis thesize an improved product. Chemical modification of the fatty chains of soybean oil The first step in the synthesis is an epoxidation reaction has been shown to effect its lubricant properties as well as (Doll and Erhan, 2005) of methyl oleate, as shown in its physical properties, such as pour point and cloud point Scheme 1a, using formic acid and hydrogen peroxide to (Erhan et al., 2003). The epoxidation of the unsaturated give epoxidized methyl oleate (EMO). This was followed methyl esters of soybean oil resulted in a lower coefficient by a ring opening reaction of EMO using propionic, levu- of friction, compared to their olefinic counterparts (Kurth linic, hexanoic, octanoic, or 2-ethylhexanoic acids to give et al., 2007), when they were studied as lubricant additives respective 9(10)-hydroxy-10(9)-ester derivatives of methyl in hexadecane solution. Additionally, thermal analysis of oleate (PMO, LMO, HMO, OMO, EHMO). The detailed epoxidized methyl esters of soybean oil showed an synthesis and product characterized using GC, GC–MS, increased stability over their olefinic counterparts, under FT-IR, 1H NMR and 13C NMR spectroscopies are dis- nitrogen atmosphere (Doll and Erhan, 2005). These cussed in a previous paper (Doll et al., 2007b). changes in the alkyl chain of fatty acid esters can significantly increase the performance of a surfactant or lubricant material. Oleochemical epoxides have been used to synthesize a ser- O ies of branched fatty esters utilizing carboxylic acids. They CH3 are: the propionic ester of methyl hydroxy-oleate (PMO), O the levulinic ester of methyl hydroxy-oleate (LMO), the hex- Methyl oleate (MO)

+ anoic ester of methyl hydroxy-oleate (HMO), the octanoic (a) Epoxidation [H ] H2O2 / HCOOH ester of methyl hydroxy-oleate (OMO), and the 2-ethylhexyl ester of methyl hydroxy-oleate (EHMO). Their synthesis, O O kinetics, and product characterization have been reported CH3 O previously (Doll et al., 2007b). In this study, we report the viscosity, thermo-oxidative stability, cold ﬂow properties, Epoxidized methyl oleate (EMO) and tribological behavior of these compounds: PMO, (b) Ring opening acylation RCOOH LMO, HMO, OMO, and EHMO. O HO OCOR

CH3 2. Methods O

2.1. Materials Scheme 1. Epoxidation of methyl oleate (a), followed by ring opening acylation of EMO using organic acid (b). RCOOH is propionic, levulinic, octanoic, hexanoic, or 2-ethylhexanoic acid for PMO, LMO, OMO, Methyl oleate (Sigma–Aldrich, St. Louis, MO, Tech HMO, and EHMO, respectively. Of note is that levulinic acid contains a 70%; Nu check Prep, Elsyian, MN, >99%); hydrogen per- ketone functionality. B.K. Sharma et al. / Bioresource Technology 99 (2008) 7333–7340 7335

2.3. Viscosity measurements was designed to eliminate any gas diffusion limitation. After oxidation, the pan containing the oxidized oil sample Brookfield viscosity: The dynamic viscosity at 25 °C was was removed from the oxidation chamber, cooled rapidly measured on a Brookfield (Middleboro, MA), DV-III pro- under a steady flow of dry nitrogen, and transferred to a grammable Rheometer controlled by Rheocalc 2.4 soft- desiccator for temperature equilibration. After 2 h, the ware. It was equipped with a CP-40 spindle and pan containing the oxidized oil was weighed to determine programmed to vary the sheer rate from 0.5 to 10 rpm. the volatile loss (or gain) due to oxidation and then soaked The software used rheometer data to calculate the relation- (30 min) with tetrahydrofuran (THF) to dissolve the solu- ship of the shear stress to the applied shear rate. This linear ble portion of the oxidized oil. After dissolving the soluble relationship, know as the Bingham model, gives viscosity portion of the oxidized oil, the pan was dried and weighed as the slope, and yield stress as the intercept. The temper- to determine the remaining insoluble deposits. ature of the system was controlled by a Brookfield (Middle- boro, MA) TC-602 water bath. 2.5. Pour point and cloud point Kinematic viscosity: The kinematic viscosity of various hydroxy ester derivatives of methyl oleate was measured The pour point is defined as the temperature when the using Cannon–Fenske calibrated viscometers (Cannon sample still pours when the jar is tilted. Statistically the Instrument Co., State College, PA) in a Cannon tempera- method has shown quite good consistency for determining ture bath (CT-1000) at 40 and 100 °C, as per the ASTM low temperature flow property of fluids. Pour points and D445-95 method. The viscosities obtained are average val- cloud points were measured as per the ASTM D-5949 ues of 2–3 determinations and the precision is within the and ASTM D-5773 methods respectively using the Phase limits of ASTM specification. Technology Analyzer, Model PSA-70 S (Phase Technol- ogy, Hammersmith Gate, Richmond, B.C., Canada). 2.4. Oxidative stability 2.6. Tribological behavior Pressure differential scanning calorimetry (PDSC): The experiments were carried out using a PC controlled DSC Ball-on-disk: Boundary lubrication properties of 2910 thermal analyzer from TA Instruments (New Castle, hydroxy ester derivatives of methyl oleate were studied DE). A small amount of analyte (typically 1.5–2.0 mg) using a ball-on-disk configuration on a multi-specimen fric- was placed in a hermetically sealed aluminum pan with a tion measurement apparatus from FALEX (Sugar Grove, pinhole lid for interaction of the sample with the reactant IL), at ambient temperature. Ball-on-disk experiments gas (extra dry air). The controlled diffusion of the gas (1018 steel disk, Rc 15–25) were carried out for 30 min at through the hole greatly restricts the volatilization of the 6.22 mm/s (5 rpm) and a load of 181.44 kg (1779 N) at oil while still allowing for saturation of the liquid phase 25 °C. The concentration of the branched oleochemicals with air. A film thickness of less than 1 mm was required was 0.01 M in hexadecane or polyalphaolefin (PAO). Mea- to ensure proper oil–air interaction and to eliminate any surements of coefficient of friction (CoF) and torque were discrepancy in the result due to gas diffusion limitations. made in each case. The CoF values reported are averages The module was first calibrated for baseline, and then for of two or three independent experiments and the standard temperature using the melting point of indium metal deviation observed was ±0.02. (156.6 °C) at 10 °C/min heating rate. Extra dry air was Four-ball: The experiment is designed to study the pressurized in the module at a constant pressure of anti-wear properties of samples under sliding contact by 1379 kPa (200 psi) and a scanning rate of 10 °C/min was four-ball test geometry using a Falex apparatus (Model used throughout the experiment. The onset temperature Multi-Specimen, FalexÒ Corporation, Sugar Grove, IL), (OT) of oxidation was calculated from the exotherm in at ambient temperature. The balls (52100 steel, 12.7 mm each case. diameter, 64–66 Rc hardness and extreme polish) were Thin film micro oxidation (TFMO): TFMO experiments thoroughly cleaned with dichloromethane and hexane were carried out using a Walco model 51000 (State College, before each experiment. Test fluid, 15 mL of 5 wt% samples PA) instrument. In this experiment, a thin film of oil (25 ll) in base oil (soybean oil (SBO), polyalphaolefin (PAO), or was oxidized on the catalytic, high-carbon steel pan surface hexadecane) was poured in the test cup to cover the sta- with a steady flow (20 mL/min) of dry air. These pans were tionary balls. For experiments in PAO solution, a set placed on a heated aluminum slab lying on a hot plate. The rpm of 1200 and a normal load of 40 kg (392 N) was temperature was maintained within ±1 °C. This arrange- applied at room temperature for 15 min. For SBO and ment eliminated the possibility of a temperature gradient hexadecane solutions, the test sequence was same except across the aluminum surface and transferred uniform heat for the speed was increased to 2400 rpm. Temperature of to the pans placed on the slab. The pans were covered by a the test fluid was 22 °C which increased to 27–28 °Cat bottomless glass reactor. Oxidation tests were done at 150, the end of the 15 min run. The wear scar diameter 175, 200, and 225 °C for 120 min. The constant air flow (WSD) on balls were measured using a digital optical ensured removal of volatile oxidation products. The test microscope. Two measurements, perpendicular to each 7336 B.K. Sharma et al. / Bioresource Technology 99 (2008) 7333–7340 other, were recorded for each scar on a ball and the average 3.2. Low temperature fluidity of six measurements, for three balls, was taken in each case. The scar diameter is reported in millimeters. Duplicate tests The pour point (PP) and cloud point (CP) of a lubricant were always done with new set of balls and the standard are good indicators of its low temperature fluidity. The deviation of six measurements was less than 0.04 in all cold flow properties of unmodified vegetable oils are poor the experiments. and this limits their use in subzero temperatures. At low temperatures, vegetable oils have a tendency to form macro 2.7. Surface tensions crystalline structures through uniform stacking. Branching on the fatty acid chains may disrupt this stacking process Dynamic surface tension measurements were taken with and results in improved low temperature properties. This a Sita t60 bubble pressure tensiometer (Desden, Germany) approach is used here to improve the low temperature flow using Sita online V2.1 software. Temperatures, 23–60 °C, behavior of fatty acid esters by attaching ester branching at were controlled by an Ingenieurbu¨ro (Staufen, Germany) the double bond sites on the fatty acid chains as shown in Cat M 26 stir plate. Scheme 1. This structure was confirmed by both 1Hand 13C NMR spectroscopies as well as FT-IR spectroscopies 3. Results and discussion (Doll et al., 2007b). Attachment of an ester side chain, with optimum length, at the 9–10 position of the fatty acid chain The one-pot synthesis of hydroxy ester derivatives from improves the PP significantly. While EMO has PP of 0 °C EMO, shown in Scheme 1, is an effective way to introduce and CP of 3.9 °C; the branched hydroxy esters, with the branching on the fatty acid (FA) chain. The branched exception of LMO, have much better PPs, in the range of products have significantly improved low temperature flow À15 to À33 °C, and CPs, in the range À8toÀ31 °C. characteristics, higher thermo-oxidative stability, and bet- Attachment of octanoic acid to produce OMO was the ter friction wear properties compared with EMO or olefinic most effective decreasing the pour and cloud points to analogs. À33 and À31°C respectively. It can be assumed that the presence of a large branching point on the fatty acid ester 3.1. Viscosity creates a steric barrier around the individual molecules and inhibits crystallization, resulting in lower pour and cloud The physicochemical properties of EMO, PMO, LMO, point. The introduction of levulinic acid side chain did OMO, EHMO and HMO are presented in Table 1. Mea- not improve the cold flow behavior. This is possibly due surements of both the dynamic viscosity of the products, to the polar structure of LMO, also shown by its increased measured by a Brookfield rheometer at 25 °C, and the viscosity and surface tension. kinematic viscosity of the products, measured by Can- non–Fenske method at 40 and 100 °C, were taken. Both 3.3. Friction and wear properties are significantly larger for LMO, than the other products. This may be due to the more polar structure of LMO, com- An important property of lubricants is their ability to pared with HMO, OMO and EHMO, which results in maintain a stable lubricating film at the metal contact zone. stronger intermolecular interaction. The viscosities of the Vegetable oils and fatty acid esters are known to provide compounds which do not include a carbonyl group in their excellent lubricity due to their ester functionality. The ester side chain, increase in the order EMO < PMO < OMO < ends of the fatty acid chain adsorb to metal surfaces, thus EHMO. This can be rationalized by an overall increase permitting monolayer film formation with the hydrocarbon in the molecular weights of the products with increasing end of fatty acids oriented away from the metal surface. chain length of newly added ester group. The fatty acid chain thus offers a sliding surface that pre- vents the direct metal-to-metal contact. If the film is not formed, direct metal contact may result in high-temperatures at the contact zones of moving parts causing adhe- Table 1 sion, scuffing, or even metal-to-metal welding. Under Viscosity and low temperature properties of hydroxy ester products lubricated conditions, the hydroxy and ester group of the Samples Dynamic Kinematic Kinematic Pour Cloud products offers active oxygen sites that bind to the metal viscosity viscosity viscosity point point surface. The friction reducing property of hydroxy ester 25 °C 40 °C 100 °C (°C) (°C) products as additives in hexadecane is demonstrated by (mPa s) (mm2 sÀ1) (mm2 sÀ1) the coefficient of friction (CoF) obtained using the ball- MO À21 À15 on-disk experiment. The CoF values reported in Fig. 1 EMO 8.0 2.5 0 4 PMO 38.2 21.1 4.0 À15 À9 are averages of two or three independent experiments and LMO 132.6 57.9 7.5 À5 À5 the standard deviation observed was ±0.02. Under a high OMO 46.5 26.9 4.8 À33 À31 load of 181.44 kg and a low speed of 6.22 mm sÀ1 EHMO 52.5 27.7 4.7 À24 À19 (5 rpm), all of the hydroxy ester products show excellent HMO 48.5 À18 À11 reduction in CoF at 0.01 M concentration. The CoF values B.K. Sharma et al. / Bioresource Technology 99 (2008) 7333–7340 7337

0.5 2.3 2.2 0.4 2.1

0.3 2 1.9 CoF 0.2

WSD (mm) 1.8

0.1 1.7 1.6 0 HD EMOPMO LMO EHMO OMO HD PMO LMO OMO EHMO HMO Fig. 3. Four-ball wear scar diameters (WSD) on 5% solutions of hydroxy Fig. 1. Coeﬃcient of friction data on 0.01 M concentrations of hydroxy ester products in hexadecane (HD): load 88 lb, 2400 rpm, room temper- ester products in hexadecane (HD) obtained using ball-on-disk conﬁgu- ature, 15 min. ration: load 400 lb, 5 rpm, ambient temperature, 30 min.

0.9 of all products are less than 0.15, a considerable improve- ment over the value of 0.4 observed for neat hexadecane 0.8

(HD). The esters are more eﬀective than either methyl ole- 0.7 ate, 0.19 (Kurth et al., 2007) or EMO, 0.22 (Sharma et al.,

2007) as additives in hexadecane at the same WSD, mm 0.6 concentrations. 0.5 The four-ball tests were done on solution of hexadecane 0482610 (HD), soybean oil (SBO), and polyalphaolefin (PAO), to Branching chain length demonstrate their effectiveness in petroleum, bio and synthetic base oils. In order to simplify results, the base oils Fig. 4. Four-ball wear scar diameters (WSD) on 5% solutions of hydroxy used did not contain any other additives. Hydroxy ester ester products in SBO as a function of branching chain length: load 88 lb, products (PMO, LMO, HMO, OMO, EHMO) were intro- 2400 rpm, room temperature, 15 min. duced into base oils at 5% concentration. At 1200 rpm, a normal four-ball test speed, the addition of the hydroxy The improved tribological properties of hydroxy ester ester products as additives in PAO lowered the wear scar products may be caused by the two extra polar functional diameter by more than 0.1 mm in all cases, compared to groups apart from the ester group of fatty acid ester. Oxy- PAO alone (Fig. 2). Even at the high speed of 2400 rpm, gen moieties, like the hydroxy and ester functionalities at 9, the ester hydroxy products at 5% reduced the wear scar 10 position on the fatty acid, help the compounds adhere to diameter of hexadecane (Fig. 3). Overall, the addition of the metal surface and reduce friction, especially under any of the hydroxy ester products causes a considerable excessive load. This mechanism has been demonstrated reduction in wear in either PAO or hexadecane lubrication for the epoxy moieties (Adhvaryu and Erhan, 2002), and fluid. However, the additive’s effect in SBO was much less could be functioning in the hydroxy ester compounds as evident since SBO is a good lubricant in its natural state. well. The active groups of these compounds start function- The overall trend, is that as the branching chain length ing during the metal rubbing process. During this time, of the additive increases, the esters become more effective, these molecules undergo transformation at the metal con- as shown for 5% solutions in SBO in Fig. 4. One possible tact zone and develop a stable tribochemical film to protect reason is increased viscosity due to an increase in molecular further wear of the metal. The branching structure also weight as a result of long branches. changes may be increasing the strength of this film, thereby reducing friction and wear. Because of these phenomena, these compounds demonstrate excellent antifriction and 0.90 antiwear properties on both the ball-on-disk and four-ball tests (Biresaw et al., 2003; Kurth et al., 2005). 0.80

0.70 3.4. Thermo-oxidative stability

0.60 Diﬀerential scanning calorimetry (DSC) is one of the WSD (mm) 0.50 useful techniques in evaluating the eﬀect of temperature on properties of materials. These changes are represented 0.40 as exothermic or endothermic peaks as a function of tem- PAO4 PMO LMOHMO EHMO OMO perature. In general, chemical decomposition and oxida- Fig. 2. Four-ball wear scar diameters (WSD) on 5% solutions of hydroxy tion cause exothermic peaks, while physical properties ester products in PAO: load 88 lb, 1200 rpm, room temperature, 15 min. such as melting and boiling cause endothermic peaks. 7338 B.K. Sharma et al. / Bioresource Technology 99 (2008) 7333–7340

Pressurized DSC (PDSC) is an effective way of measur- 100 ing the oxidative tendency of lubricant base oils, vegetable 80 oils, and oleochemicals, in an accelerated mode (Adhvaryu et al., 2006; Dunn, 2005; Zhang et al., 2004). At high air 60 pressure (1379 kPa), the concentration of oxygen is in 40 excess and at equilibrium with the sample. Thus, any 20 inconsistency due to the access rate of oxygen and egress % Volatile loss rate of volatile degradation product is effectively elimi- 0 nated. The onset temperature (OT) is the temperature at 125 150 175 200 225 250 which a rapid increase in the rate of oxidation is observed. Temperature, °C It is obtained by extrapolating the tangent drawn on the steepest slope of reaction exotherm to the baseline. A high EMO PMO LMO HMO OMO EHMO OT would suggest a high oxidation stability of the oil. Fig. 5. Volatile loss for EMO and its hydroxy ester derivatives, obtained Here, we compared the stabilities of the hydroxy ester using TFMO at various temperatures. products. The OT for PMO is highest (175 °C) among this series of hydroxy ester products, followed by EHMO (166 °C), LMO (162 °C), and OMO (160 °C). The data 25 shows that oxidative stability decreases with increases in chain length of the ester side chain. This may be because 20 the longer side chains have more easily accessible sites for 15 oxidation leaving them more susceptible to cleavage than 10 short ones. The results are corroborated by other studies 5 on chemically modified vegetable oils (Sharma et al., 0 2006) and synthetic esters (Randals, 1999). In those studies, % Insoluble Deposit 125 150 175 200 225 250 the authors mention that short chain acids are more stable Temperature, °C than long chain acids. EMO PMO LMO HMO OMO EHMO Another oxidation test was conducted using thin film micro oxidation (TFMO) to study the volatility and Fig. 6. TFMO insoluble deposit for EMO and its hydroxy ester deposit forming tendencies of hydroxy ester products. In derivatives at various temperatures. most of the applications, a lubricant functions as a thin film, so the TFMO test is considered the test of choice to dation process of the ester products, indicating stability simulate the actual conditions (Adhvaryu et al., 2006). against polymerization. After 200 °C, an increase in the TFMO is suitable for quantitative evaluation of a lubri- deposit formation (4–23%) is noted for the hydroxy ester cant’s thermal and oxidative stability, and correlates well products. This suggests that after 200 °C, oxidative poly- with the PDSC method (Dunn, 2005; Sharma et al., merization is occurring, perhaps through the generation 2007) and the time consuming rotary bomb oxidation test. of reactive oxygen containing radicals. From the volatile During the oxidation process, some small primary oxida- loss data, the larger side chains have more sites for reactive tion products are measured as volatile loss, while others, cleavage. It follows that they may also have more sites for in presence of excess oxygen, undergo oxy-polymerization possible polymerization reactions. When the species are to form oil insoluble deposits. Unsaturated oleochemicals chemically different, as in the case of LMO, this is espe- have a higher tendency to form such deposits, which limits cially evident. Because LMO’s carbonyl functionality their use in high-temperature lubricants. Using the volatile allows the formation of different polymer types, it displays loss and insoluble deposit data obtained from TFMO tests, a sharp increase in the deposit formation (24%). The per- the amount of oil left for lubrication can be predicted, giv- cent insoluble deposit for EMO is greater than for the ing an indication of useful lubricant lifetime. hydroxy esters below 200 °C. TFMO was carried out on EMO and various hydroxy ester products at different temperatures. Fig. 5 presents the volatile loss obtained at different temperatures. The 3.5. Surface tension results volatile loss increases as the temperature was increased from 150 to 225 °Cin25°C increments. This indicates that Surface tension has been shown to be one of the deter- breakdown of hydroxy ester products, as a result of oxida- mining factors in film formation. The surface tensions of tive degradation, increases with temperature. EMO showed oleochemicals have been studied recently, to determine much faster reaction. At 150 °C, volatile loss for EMO their flow and atomization characteristics (Allen et al., (50%) is 2–2.5 times higher than hydroxy ester products. 1999; Cochran et al., 1987; Doll et al., 2007a). We mea- The insoluble deposits obtained during TFMO at vari- sured the surface tensions of the hydroxy ester products ous temperatures are shown in Fig. 6. Negligible amounts as a function of temperature from 25 °Cto60°C of insoluble deposits are observed up to 200 °C during oxi- (Fig. 7). The values were near the expected range (25– B.K. Sharma et al. / Bioresource Technology 99 (2008) 7333–7340 7339

34 (4) Overall, the hydroxy ester products have good PMO thermo-oxidative stability shown by low volatile loss EHMO and insoluble deposit compared with epoxy methyl

32 OMO oleate. LMO (5) For this series of hydroxy ester products, the viscosity and antiwear properties increase, and pour point and

) EMO -1 cloud point decrease with increase in chain length of 30 branching. (6) The surface tension of the branched materials is reduced by longer side chain branches, except for 28 the case of LMO, where the extra polar interactions cause an overall increase in surface tension.

Sufrace Tension (mN m 26 These hydroxy ester products have shown the dual function of improving low temperature ﬂuidity as well as an eﬀective antifriction and antiwear additive in various lubrication system. 24 20 30 40 50 60 70 Acknowledgements Temp °C

Fig. 7. The surface tensions of PMO, EHMO, OMO, LMO, and EMO at The authors gratefully acknowledge Mr. Richard Henz various temperatures. All show the expected lower surface tension at for acquiring the TFMO, CoF and wear scar data. They higher temperatures. The hydroxy ester materials show a trend of lower also acknowledge Ms. Donna I. Thomas for synthetic work. surface tension for compounds with larger branching. References

32 mN mÀ1) for natural oleochemicals, (Gros and Feuge, Adhvaryu, A., Erhan, S.Z., 2002. Epoxidized soybean oil as a potential 1952; Nevin et al., 1951) and showed the expected decrease source of high-temperature lubricants. Ind. Crops Prod. 15, 247–254. with increasing temperature. However, an interesting trend Adhvaryu, A., Sharma, B.K., Hwang, H.S., Erhan, S.Z., Perez, J.M., 2006. Development of biobased synthetic fluids: application of was uncovered. All of the hydroxy esters exhibited signifi- molecular modeling to structure-physical property relationship. Ind. cantly higher surface tensions than EMO. Within the Eng. Chem. Res. 45, 928–933. hydroxy esters, increasing side chain length served to Allen, C.A.W., Watts, K.C., Ackman, R.G., 1999. Predicting the surface decrease surface tension significantly. This may be caused tension of biodiesel fuels from their fatty acid composition. J. Am. Oil by the larger branches inhibition to close packing. LMO, Chem. Soc. 76, 317–323. Bhatnagar, A.K., Kaul, S., Chhibber, V.K., Gupta, A.K., 2006. HFRR and its polar side chain displayed the highest surface ten- studies on methyl esters of nonedible vegetable oils. Energy Fuels 20, sion as expected, possible due to dipolar interactions. 1341–1344. Biresaw, G., Adhvaryu, A., Erhan, S.Z., 2003. Friction properties of 4. Conclusions vegetable oils. J. Am. Oil Chem. Soc. 80, 697–704. Campanella, A., Baltana´s, M.A., Capel-Sańchez, M.C., Campos-Martıń, J.M., Fierro, J.L.G., 2004. Soybean oil epoxidation with hydrogen The results show that under some conditions, hydroxy peroxide using an amorphous Ti/SiO2 catalyst. Green Chem. 6, 330–334. ester products have better lubricant properties which Carlson, K.D., Kleiman, R., Bagby, M.O., 1994. Epoxidation of enable them to outperform their methyl ester analogs. lesquerella and limnanthes (meadowfoam) oils. J. Am. Oil Chem. Additionally, they maintain the advantages inherent from Soc. 71, 175–182. their biobased nature. These products remain a viable Cochran, D.L., Threadgill, E.D., Law, S.E., 1987. Physical properties of three oils and oil–insecticide formulations used in agriculture. Trans. option for application in the lubricant industry. The fol- ASAE 30, 1338–1342. lowing conclusions can be drawn: Coscione, A.R., Artz, W.E., 2005. Vegetable oil stability at elevated temperatures in the presence of ferric stearate and ferrous octanoate. J. (1) Hydroxy ester products as additives in hexadecane Agric. Food Chem. 53, 2088–2094. possess good antifriction properties demonstrated de Guzman, D., 2005. Fats and oils prices to weaken in 2005. Chem. Market Reporter 267, 15. using ball-on-disk configuration. Doll, K.M., Erhan, S.Z., 2005. Synthesis of carbonated fatty methyl esters (2) These products show good antiwear properties com- using supercritical carbon dioxide. J. Agric. Food Chem. 53, 9608– pared with various neat base oils such as PAO, hexa- 9614. decane, and soybean oil. Doll, K.M., Moser, B.R., Erhan, S.Z., 2007a. Surface tension studies of (3) One of the hydroxy ester products, OMO, demon- alkyl esters and epoxidized alkyl esters relevant to oleochemically based fuel additives. Energy Fuels 21, 3044–3048. strates excellent low temperature properties, while Doll, K.M., Sharma, B.K., Erhan, S.Z., 2007b. Synthesis of branched others show good low temperature fluidity compared methyl hydroxy stearates including an ester from bio-based levulinic to their epoxidized ester analogs. acid. Ind. Eng. Chem. Res. 46, 3513–3519. 7340 B.K. Sharma et al. / Bioresource Technology 99 (2008) 7333–7340

Dunn, R.O., 2005. Effect of antioxidants on the oxidative stability of Kurth, T.L., Biresaw, G., Adhvaryu, A., 2005. Cooperative adsorption methyl soyate (biodiesel). Fuel Process. Technol. 86, 1071–1085. behavior of fatty acid methyl esters from hexadecane via coefficient of Erhan, S.Z., Bagby, M.O., Nelsen, T.C., 1997. Drying properties of friction measurements. J. Am. Oil Chem. Soc. 82, 293–299. metathesized soybean oil. J. Am. Oil Chem. Soc. 74, 703–706. Kurth, T.L., Sharma, B.K., Doll, K.M., Erhan, S.Z., 2007. Adsorption Erhan, S.Z., Adhvaryu, A., Liu, Z. Chemically Modified Vegetable Oil- behavior of epoxidized fatty esters via boundary lubrication coefficient based Industrial Fluid. US 6583302 B1, June 24, 2003. of friction measurements. Chem. Eng. Commun. 194, 1065–1077. Erhan, S.Z., Sharma, B.K., Perez, J.M., 2006. Oxidation and low Li, S., Blackmon, J., Demange, A., Jao, T.-C., 2004. Linear sulphonate temperature stability of vegetable oil-based lubricants. Ind. Crops detergents as pour point depressants. Lubr. Sci. 16, 127–137. Prod. 24, 292–299. Nevin, C.S., Althouse, P.M., Triebold, H.O., 1951. Surface tension Findley, T.W., Swern, D., Scanlan, J.T., 1945. Epoxidation of unsaturated determinations of some saturated fat acid methyl esters. J. Am. Oil fatty materials with peracetic acid in glacial acetic acid solution. J. Am. Chem. Soc 28, 325–327. Chem. Soc. 67, 412–414. Randals, S.J., 1999. Esters. In: Rudnick, L.R., Shubkin, R.L. (Eds.), Gawrilow, I., 2004. Vegetable oil usage in lubricants. INFORM 15, 702–705. Synthetic Lubricants and High Performance Functional Fluids. Gros, A.T., Feuge, R.O., 1952. Surface and interfacial tensions, viscos- Marcel Dekker, New York, pp. 63–102. ities, and other physical properties of some n-aliphatic acids and their Sharma, B.K., Adhvaryu, A., Liu, Z., Erhan, S.Z., 2006. Chemical methyl and ethyl esters. J. Am. Oil Chem. Soc 29, 313–317. modification of vegetable oils for lubricant applications. J. Am. Oil Holser, R.A., Doll, K.M., Erhan, S.Z., 2006. Metathesis of methyl soyate Chem. Soc. 83, 129–136. with ruthenium catalysts. Fuel 85, 393–395. Sharma, B.K., Doll, K.M., Erhan, S.Z., 2007. Oxidation, friction Hwang, H.-S., Erhan, S.Z., 2001. Modification of epoxidized soybean oil reducing, and low temperature properties of epoxy fatty acid methyl for lubricant formulations with improved oxidative stability and low esters. Green Chem. 9, 469–474. pour point. J. Am. Oil Chem. Soc. 78, 1179–1184. Tocci, L., 2006. Rethinking hydraulic fluids. Lubes’N’ Greases 12, 26. King, J.W., Holliday, R.L., List, G.R., Snyder, J.M., 2001. Hydrogena- Verkuijlen, E., Kapteijn, F., Mol, J., Boelhouwer, C., 1977. Heteroge- tion of vegetable oils using mixtures of supercritical carbon dioxide neous metathesis of unsaturated fatty acid esters. Chem. Commun. and hydrogen. J. Am. Oil Chem. Soc. 78, 107–113. 1997, 198–199. Knothe, G., 2005. The history of vegetable oil-based diesel fuels. In: Zhang, Y.Y., Ren, T.H., Wang, H.D., Yi, M.R., 2004. A comparative Knothe, G. (Ed.), The Biodiesel Handbook. AOCS Press, Champaign, study of phenol-type antioxidants in methyl oleate with quantum pp. 4–16. calculations and experiments. Lubr. Sci. 16, 385–392.