Antiwear Additive Derived from Soybean Oil and Boron Utilized in a

Aug 26, 2012 - These additives were also tested in a gear oil blend, and shown to reduce both wear and oxidation. They were also compatible with a pop...
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Antiwear Additive Derived from Soybean Oil and Boron Utilized in a Gear Oil Formulation Brajendra K. Sharma,§ Kenneth M. Doll,*,† Glenn L. Heise,§ Malgorzata Myslinska,§ and Sevim Z. Erhan∥ †

Illinois Sustainable Technology Center, University of Illinois, 1 Hazelwood Drive, Champaign, Illinois 61820, United States USDA/NCAUR/ARS, Bio-Oils Research Unit, 1815 North University Street, Peoria, Illinois 61604, United States § Anderson Development Company, 1415 East Michigan St. Adrian, Michigan 49221, United States ∥ USDA/NCAUR/ARS, 600 East Mermaid Lane, Wyndmoor, Pennsylvania 19038, United States †

ABSTRACT: The synthesis of lubricant additives based on boron and epoxidized soybean oil are presented. These additives are made from a simple patent pending method involving a ring-opening reaction of the epoxidized oil. A couple of these borates were tested in soybean oil, polyalpha olefin basestock, group III basestock, and hexadecane. An aromatic additive was able to increase the oxidation onset of the basestocks by 14, 52, 48, and 49 °C, respectively, when used at 2% wt. The other additive was shown to reduce the wear scar diameter in a friction test when used in soybean oil basestock, from 0.61 mm down to 0.41 mm. These additives were also tested in a gear oil blend, and shown to reduce both wear and oxidation. They were also compatible with a popular additive, additive zinc dialkyl dithiophosphate.

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INTRODUCTION The area of the synthesis of biobased materials has been high over the past decade, especially concerning polymers,1−4 platform chemicals,5−11 surfactants,12−14 and of course fuels. Lubricants have also seen successful development15 from both theoretical16,17 and practical standpoints.18 As an additional driver, the United States Department of Agriculture has a program establishing guidelines for procurement preference.19 The synthesis of lubricant additives for biobased products has been a rich area20−26 for multiple reasons. First, vegetable oil based lubricants are poor performers and many available additives are not compatible or effective. Second, to meet biopreferred criteria, a percentage of the carbon in the material must be biobased. If the additives are biobased, there is additional room in the formulation for nonbiobased components- such as extreme pressure additives. This combination of economic factors and compatibility with vegetable based lubrication fluids along, with the many reactive locations for modification, make this an interesting area. Although there are numerous additives containing phosphorus, sulfur, oxygen, and nitrogen, there is only a single recent report on boron based lubricant additives,27 despite its favorable molecular properties. This report increases the number, and reports a versatile platform in which it is possible to make a variety of materials with antioxidant or antiwear properties.28 These additives, with their high biobased carbon content, may be valuable in formulations ranging from metalworking emulsion, to hydraulic oils, to the blends studied herein, gear oils.

San Ramon, CA), and hexadecane (Aldrich, 99% anhydrous, St. Louis, MO) were all used as received. The thermally modified soybean oil used in the gear oil formulation was prepared according to literature methods.29,30 The method involves heating the oil to 330 °C in a three neck flask under a nitrogen atmosphere. In this work, a reaction time of 240 min was used. Synthetic ester (Esterex TM 101, triisoocyl trimellitate, Exxon Mobil, Houston, TX), the pour point depressant (LZ 7671A, Lubrizol, Wickliffe, OH), and zinc dialkyl dithiophosphate (Additin RC 3180, Rhein Chemie, Mannheim, Germany) were used as received. Viscosities. The viscosity of the neat additives were measured on a Model DV-III (Brookfield, Middleboro, MA) programmable Rheometer ran by Rheocalc version 2.4 software. A CP-40 spindle was used, and the resulting sheer stress versus sheer rate plot was analyzed by the Bingham model. The gear oil viscosities were measured by an automated viscometer (Stabinger SVM 3000, Anton Paar, Ashland, VA) according to ASTM D445. Viscosity index was calculated by an online calculator, http://oil-additives.evonik.com/product/oiladditives/en/calculation-tools/viscosity-index/Pages/calculate. aspx, from the 40 and 100 °C viscosities which has been shown to give identical results to ASTM D2270. Friction and Wear. A multispecimen friction and wear tester (Falex Corporation, Sugar Grove, IL), was used in a 4ball configuration according to ASTM 4170-94. In each test, 15 mL of lubricant was used with an applied load of 40 kg. The apparatus was heated to 75 °C, with the top ball rotating at 1200 rpm for 60 min. The wear scar of the three stationary balls was measured using a microscope (6SD with L2 fiber-optic light, Leica, Bannockburn, IL) equipped with a digital camera

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MATERIALS AND METHODS Reagents. The base oils used for additive evaluation: Soybean oil (RBD grade, KIC Chemicals, New Paltz, NY), poly alpha olefin (Synfluid Polyalphaolefin, PAO 8, ChevronPhillips, Woodlands, TX), group III (UCBO 7R, Chevron, © 2012 American Chemical Society

Received: Revised: Accepted: Published: 11941

June 12, 2012 August 17, 2012 August 26, 2012 August 26, 2012 dx.doi.org/10.1021/ie301519r | Ind. Eng. Chem. Res. 2012, 51, 11941−11945

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(Pax-it 6.4 software, PAXcam, Villa Park, IL). For each experiment, two measurements were taken of the scar on 2 on each ball for a total of six values. Additionally, replicate experiments were done in all of the soybean oil, polyalpha olefin, and group III mineral oil systems. FTIR-Spectroscopy. The FTIR spectroscopy was recorded on a Thermonicolet (Madison, WI) Nexus 470 FTIR with a Continuum microscope in reflectance mode. Data was processed on a Windows 2000 equipped Dell Optiplex GX260 pentium 4, 2.46 GHz computer running Omnic 6.2 software. Oxidation. The oxidation of samples was measured on a pressurized differential scanning calorimeter (PDSC, Q10, TA Instruments, New Castle, DE). A hermetically sealed aluminum pan with a pinhole lids were used to analyze the ∼2 mg samples. Air pressure of 1379 kPa was kept over the sample throughout the run and the temperature was ramped at 10 °C min−1. The onset of oxidation was determined by the start of the exothermic reaction peak.

Scheme 2. Synthesis of Additive II

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RESULTS AND DISCUSSION Synthesis. The additives were synthesized in a straightforward manner (Schemes 1 and 2). In the formation of additive I Scheme 1. Synthesis of Additive Ia

Epoxidized soybean oil has ∼4.2 epoxy moieties per molecule which allows crosslinking and multiple borates or acetate groups per triacylglycerol.

a

Figure 1. Wear scar diameter observed from a 4-ball friction and wear evaluation of the additives in soybean oil base. The additives are shown by filled symbols (additive I), open symbols (additive II), and halffilled symbols (an equal mixture of the additives by mass).

(Scheme 1), epoxidized soybean oil was reacted with a difunctional borate. Titanium isopropoxide was used as a Lewis acid catalyst to accomplish the ring-opening. A capping borate was then used to react some of the residual hydroxyl groups with the addition of acetic anhydride to any remaining groups. Additive II (Scheme 2) was prepared in a similar manner, except the original borate was monofunctional, with the capping borate already in place, and it contained aromatic

moieties. In this additive, acetic anhydride was not added leaving free hydroxyl groups in the molecule. One difference between these two syntheses is that the difunctional borate can cause cross-linking of the material. This different molecular weight affects physical properties of the material, especially viscosity. 11942

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Figure 2. Wear scar diameter observed from a 4-ball friction and wear evaluation of the additives in hydrocarbon bases. The three different bases are shown by circles (polyalpha olefin), (diamonds) group III mineral oil, and triangles (hexadecane). The additives are shown by filled symbols (additive I) and open symbols (additive II).

Figure 5. Observed wear scar observed in a 4-ball friction and wear test. The formulated oils were on par with a popular commercially available gear oil in the same test.

loading, neither additive modified the viscosity of soybean oil to a measurable extent. Selection of Basestocks. The additive performance was evaluated in four separate basestocks to evaluate the overall potential. As a biobased lubricant base, soybean oil was selected, as well as a synthetic polyalpha olefin fluid, a group III mineral oil, and hexadecane. These selected oils give a broad range, from pure hydrocarbons to much more oxygenated stocks. In the soybean oil, each additive was tested at multiple concentrations whereas a level of 2 wt % was used in the other stocks. Evaluation in Soybean Oil. The wear scar measurements (Figure 1) in the systems show a fairly straightforward trend where addition of either additive, or of a mixture of the additives, reduce the scar. Additive I was significantly more effective, especially at concentrations less than 1 wt %. Evaluation in Hydrocarbon Oils. The trend was completely different in hydrocarbon oils (Figure 2), where the additives did not reduce the wear scar in any of the systems. In other words, these materials actually promoted wear. Rationalization for this behavior can be surmised by looking at the structures. In both additives, the triacylglycerol remains intact and helps compatiblize the soybean oil systems but not the hydrocarbon oils. If this compatibility is achieved, the additive is effective at delivering the boron antiwear properties to the metal surface for a protective effect. Additive II, in addition to the polar borates, also contains free hydroxyl groups which exacerbate problems in hydrocarbon rich oils. FTIR Study. A study by FTIR spectroscopy, on the system where the additive was beneficial, was illustrative. In that study (Figure 3), the addition of 2% of additive I to the oil did not significantly change the spectra of the material on the untested specimen. However, after the specimen was subject to the

Figure 3. Spectra of oil films on friction specimen balls taken by an FTIR-microscope. Ordinary soybean oil (bottom), and a 2% solution of additive I in soybean oil are shown before the test was run. The spectrum of the scarred ball (top).

Figure 4. Formulation of typical biobased gear oil. In this study, both antioxidant effects and antiwear of the boron based additives were evaluated alone, and in combination with zinc dialkyl dithio phosphate (ZDDP) at levels up to 4%. Within any gear oil blend, at least 58% of the carbon must be bioderived in order for the formulation to meet USDA certified biobased product criteria.

Viscosities. The dynamic viscosity of the additive I, neat, is high, 70 000 mPas at 25 °C and 6906 at 40 °C mPas. Additive II was also viscous, but slightly less so, with viscosities of 29 800 and 4800 mPas at the respective temperatures. However, at 2% 11943

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Table 1. Oxidation Onset Temperature of Basestock Solutions with Addition of the Boron Compoundsa oxidation onset (°C) soybean oil % (wt) 0 0.5 1 2 mix 2% total

poly alpha olefin

group III basestock

hexadecane

additive I

additive II

additive I

additive II

additive I

additive II

additive I

additive II

± ± ± ± ±

174 ± 0.4 175 ± 0.7 176 ± 0.7 188 ± 0.6 169 ± 1.3

198 NA NA 192 ± 0.0 NA

198 NA NA 250 ± 0.4 NA

199 NA NA 194 ± 0.2 NA

199 NA NA 247 ± 0.7 NA

208 ± 0.7 NA NA 207 ± 0.7 NA

208 ± 0.7 NA NA 257 ± 0.2 NA

174 180 179 180 169

0.4 0.7 1.5 0.9 1.3

The oxidation onset temperature of the additives alone were respectively 204 (±2.8), 258 (±0.3), and 252 (±0.3) °C for additive I, additive II, and a 50 wt % mixture of each.

a

Table 2. Viscosity Index and Oxidation Onset Temperature of Biobased Gear Oils with Boron Additives or ZDDP additive

none

additive I

additive I with ZDDP

additive II

additive II with ZDDP

mix of additives I and II

mix with ZDDP

oxidationonset (°C) viscosity index

214 ± 0.3 166

235 ± 0.0 166

262 ± 1.1 167

233 ± 0.8 151

254 ± 0.2 164

232 ± 0.9 159

259 ± 0.4 165

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friction and wear, a noticeable change in the alkane and alkene stretching regions (2800−3080 cm−1) showed a distinct enhancement as compared to the ester group, (∼1700 cm−1). This is behavior is indicative of film formation quite possibly enhanced by the additive. Oxidation Study. The effects on the onset of oxidation as measured by PDSC were somewhat the opposite from the friction results (Table 1). In this case, additive I did not cause significant change in the observed results. However, additive II has a strong antioxidant effect and increased the onset temperature by ∼50 °C in the hydrocarbon rich stocks. Soybean oil, the most easily oxidized, was stabilized by additive II at 2% loading to a smaller but significant effect. Again these results can be explained by the structures of the additives. Additive II contains a phenyl group with at least three active hydrogen atoms per boron. Additive I does not contain this moiety, therefore no effect was observed. Evaluation in Gear Oil Formulations. Although the behavior of the additives alone and in basestocks is illustrative, an actual gear oil has many different components (Figure 2) where complex effects and synergism can dramatically alter properties of the finished product. A biobased gear oil made from a blend of thermally modified soybean oil, a synthetic ester fluid, and a pour point depressant was used in this evaluation. To this blend, the boron based additives were added, alone, mixed with each other, mixed with zinc dialkyl dithio phosphate (ZDDP), or a combination of both additives and ZDDP. The total additive loading was kept constant at 4% by weight. The results (Table 2) show that the additives do not modify the viscosity index of the oil significantly. Additive I has a slightly positive effect, whereas additive II has a slightly negative effect. The oxidation results were less obvious. Both additives increased oxidation stability of the oil slightly, but neither appears to be as effective as ZDDP. The wear scar results (Figure 3) were the opposite of those in the basestocks alone. In this system, additive II was superior indicating that in the blended oil mix of the formulation, it is able to utilize the boron more effectively. As was the case in the oxidation experiments, ZDDP was highly effective. Although the finished products are far from perfect, this data has shown the promise of boron technology in this area. It also points to the need for careful formulation of lubricant blends to achieve effectiveness for the desired application.

AUTHOR INFORMATION

Corresponding Author

*Phone: 309-681-6103. Fax: 309-681-6524. E-mail: Kenneth. [email protected]. Notes

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. The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to acknowledge Cynthia M. Ruder, Jennifer R. Koch and Richard H. Henz for the friction, wear and oxidation measurements reported herein. This work was performed under a Cooperative Research and Development Agreement between the United States Department of Agriculture and Anderson Development Company.

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