Improvement of pour point and oxidative stability of synthetic ester basestocks for biolubricant applications

2.1 Materials

Formic acid (88%) was obtained from Fisher Scientific (Pittsburgh, PA) and oleic acid (99%) from Nu-Chek Prep, Inc. (Elysian, MN). All other chemicals and reagents were obtained from Aldrich Chemical (Milwaukee, WI). All materials were used without further purification. All organic extracts were dried using anhydrous magnesium sulfate (Aldrich Chemical).

2.2 Characterization

The percentage compositions of the elements (CHNS) for the compounds were determined using an elemental analyzer CHNS Model Fison EA 1108. ¹H and ¹³C NMR spectra were recorded using a JEOL JNM-ECP 400 spectrometer operating at a frequency of 400.13 and 100.77 MHz, respectively, using a 5 mm broadband inverse Z-gradient probe in DMSO-d₆ (Cambridge Isotope Laboratories, Andover, MA) as solvent. Each spectrum was Fourier-transformed, phase-corrected, and integrated using MestRe-C 2.3a (Magnetic Resonance Companion, Santiago de Compostela, Spain) software. FTIR spectra were recorded neatly on a Thermo Nicolet Nexus 470 FTIR system (Madison, WI) with a Smart ARK accessory containing a 45 ZeSe trough in a scanning range of 650–4,000 cm^–1 for 32 scans at a spectral resolution of 4 cm¹.

2.3 Low temperature operability

The pour point is defined as the lowest temperature at which the sample still pours from a tilted jar. This method is routinely used to determine the low temperature flow properties of fluids. Pour point values were measured according to the ASTM D5949 method using a phase Technology Analyzer, Model PSA-70 S (Hammersmith Gate, Richmond, B.C., Canada). Each sample was run in triplicate and average values rounded to the nearest whole degree are reported. For a greater degree of accuracy, PP measurements were done with a resolution of 1 °C instead of the specified 3 °C increment. Generally, materials with lower PP exhibit improved fluidity at low temperatures than those with higher PP.

2.4 Flash point values

The flash point is defined as the minimum temperature at which the liquid produces a sufficient concentration of vapor above it that it forms an ignitable mixture with air. The lower the flash point is, the greater the fire hazard is. Flash point determination was run according to the American National Standard Method using a Tag Closed Tester (ASTM D 56-79). Each sample was run in triplicate and the average values rounded to the nearest whole degree are reported.

2.5 Viscosity index measurements

Automated multi range viscometer tubes HV M472 obtained from Walter Herzog (Germany) were used to measure viscosity. Measurements were run in a Temp-Trol (Precision Scientific, Chicago, IL, USA) viscometer bath set at 40.0 and 100.0 °C. The viscosity and viscosity index were calculated using ASTM methods D 445-97 and ASTM D 2270-93, respectively. Triplicate measurements were made and the average values were reported.

2.6 Oxidation stability

Pressurized DSC (PDSC) experiments were accomplished using a DSC 2910 thermal analyzer from TA Instruments (Newcastle, DE). Typically, a 2-μL sample, resulting in a film thickness of <1 mm, was placed in an aluminum pan hermetically sealed with a pinhole lid and oxidized in the presence of dry air (Gateway Airgas, St Louis, MO), which was pressurized in the module at a constant pressure of 1,378.95 kPa (200 psi). A 10 °C min^–1 heating rate from 50 to 350 °C was used during each experiment. The oxidation onset (OT, °C) and signal maximum temperatures (SMT, °C) were calculated from a plot of heat flow (W/g) versus temperature for each experiment. Each sample was run in triplicate and average values rounded to the nearest whole degree are reported (Table 2).

2.7 Lubricity determination

Lubricity determinations were performed at 60 °C, according to ASTM methods D6079 using a high-frequency reciprocating rig (HFRR) lubricity tester (PCS Instruments, London, UK) via Laser Scientific (Granger, IN, USA). The average wear scan (μm) diameter of each replicates was determined by calculating the average of the x- and y-axis wear scar length. Each experiment was conducted in triplicate and the data are reported as mean ± SD of triplicate determinations.

2.8 Synthesis

2.8.4

2.8.4 2-Ethylhexyl 9-(octanoyloxy)-10-(acyloxy)octadecanoate (EHOAOD, 16–22)

The reaction scheme of triesters formation is shown in Fig. 1. Appropriate amounts of EHHYAOD, pyridine and CCl₄ were weighed into the 500 mL three-neck flask equipped with a cooler, dropping funnel and thermometer. The mixture was heated to 50 °C, with suitable aliquots of octanoyl chloride added during 1 h, and the reaction mixture was subsequently refluxed for 4 h. On completion, the mixture stood overnight at ambient temperature. After washing with water, the solvent extract was dried over anhydrous sodium sulfate, further filtered and vacuum distilled to remove solvent.

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Figure 1

Triester formation.

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3.1 Epoxidation, esterification and acetylation

Preparation of octyl 9-(lauroyloxy)-10-(acyloxy)octadecanoate 16–22 from epoxidized oleic acid is 1 an effective way of introducing branching on the fatty acid chain of plant oils. This reaction is carried out in four stages and the final products have significantly improved oxidative stability and low temperature property compared with the starting materials (Fig. 1). The straightforward epoxidation of oleic acid was closely monitored to avoid the synthesis of the undesired 9,10-dihydroxyoctadecanoate, which will form if the reaction temperature is elevated or the reaction is allowed to progress for too long.

The removal of unsaturation in the oleic acid by converting them into epoxy-groups 1 improves the oxidative stability. It has already been established that the presence of multiple double bonds in the plant oil FA chains accelerates oxidative degradation. However, the low temperature fluidity of 1 is poor and found to solidify at 0 °C (see Table 1). This would limit the application of plant oil at low operating temperature especially as automotive and industrial fluids. A suitable approach to improve the low temperature flow behavior of 1 is to attach branching sites at the epoxy carbons. This was achieved by careful ring opening to obtain the 9-hydroxy-10-acyloxyoctadecanoic acid oils 2–8. Then, esterification of these products was carried out using 2-ethylhexanol and sulfuric acid as catalyst to yield 2-ethylhexyl 9-hydroxy-10-acyloxyoctadecanoate 9–15. The seven prepared octyl esters were used as precursors for the synthesis of modified triester-derivatives by acetylation with octanoyl chloride in an aprotic solvent.

3.2 Characterization

In the FTIR spectra of oils 2–22, the absorption due to the epoxy group (820 and 843 cm⁻¹) is not observed. This fact suggests that 1 undergoes complete ring opening under the reaction condition. Bands representing C⚌O groups (724, 1740 cm⁻¹), CH₃ groups (1370–1463 cm⁻¹), OH groups (3473–3444 cm⁻¹) and also C–O–C bands in esters (999–1105 cm⁻¹) are clearly visible in the spectra (Salimon and Salih, 2010).

All synthesized oils were verified by ¹H and ¹³C NMR spectroscopy. Significant signals in the ¹H spectrum of epoxidized oleic acid 1 between 2.6 and 2.8 ppm correspond to quaternary carbons of the oxiran ring and the doublet in the ¹³C spectrum between 56.86 and 56.91 ppm correspond to carbons of the oxirane ring. Furthermore, ¹H spectrum of epoxidized oleic acid showed singlet signal at 9.23 ppm due to OH group. A signal in the area around 9.17–9.30 ppm, representing an OH group, and the bands at 2.05–3.68 ppm, corresponding to –CH₂− groups, are present in the ¹H spectra of monoesters, 9-hydroxy-10-acyloxyoctadecanoic acid 2–8. The ¹H spectra of synthesized diesters, 2-ethylhexyl 9-hydroxy-10-acyloxyoctadecanoate 9–15, and triesters, 2-ethylhexyl 9-(octanoyloxy)-10-(acyloxy)octadecanoate 16–22, consist of signals of low intensity at about 9.22–9.40 ppm and 2.10–3.65 ppm. Broad lines at 1.41–1.77 ppm represent the CH₂ groups’ hydrogen peaks. In the ¹³C NMR spectra significant bands at about 176 ppm are present, which exhibit the characteristic signals attributed to ester groups (Sliverstien et al., 2005). Furthermore, elemental analysis was done and the obtained data are in agreement with the proposed structures (Table 1).

3.3 Products parameters

The ability of a substance to remain liquid at low temperatures is an important attribute for a number of industrial materials, such as biolubricants, surfactants and fuels. The cold flow property of plant oils is extremely poor and this limits their use at low operating temperature especially as automotive and industrial fluids. Plant oils have a tendency to form macrocrystalline structures at low temperature through uniform stacking of the ‘bend’ triglyceride backbone (Kleinová et al., 2008). Such macrocrystals restrict the easy flow of the system due to loss of kinetic energy of individual molecules during self-stacking. Cold flow properties of these samples were determined using their pour points. In practice, the usable liquid range is limited by the pour point (PP) at low temperatures and the flash point at high temperatures. The PP should be low to ensure that the lubricant is pump-able when the equipment is started from extremely low temperatures. The flash point should be high to allow the safe operation and minimum volatilization at the maximum operating temperature. For the most demanding applications, such as aviation jet engine lubricants, an effective liquid range over 300 °C may be required (Salimon et al., 2010). The epoxidized oleic acid, synthetic mono-, di- and triesters described above were screened for low temperature behavior through determination of their pour point (PP).

An improvement in the cold flow behavior of diesters, 2-ethylhexyl 9-hydroxy-10-acyloxyoctadecanoate, and triesters, 2-ethylhexyl 9-hydroxy-10-acyloxyoctadecanoate, was obtained over that of their monoester precursor’s 9-hydroxy-10-acyloxyoctadecanoic acid. Actually there are two reasons for this behavior. The first reason is that the presence of a side chain attached to the FA backbone does not allow individual molecules to come close for easy stacking due to steric interactions. This results in the formation of microcrystalline structures rather than macro structures. At lower temperatures, such microcrystalline structures can easily tumble and glide over one another resulting in better fluidity of the total matrix. Secondly, the lack of one hydroxyl group in diesters and then absence of it in triester structures mean the number of hydrogen bonds decrease, which could cause the molecules to stack together.

The efficiency of the biolubricant in reducing friction and wear is greatly influenced by its viscosity. Generally, viscosity-temperature charts are available, making a good choice of a biolubricant operation temperature. The viscosity of a biolubricant is its tendency to resist flow. A biolubricant oil of high viscosity flows very slowly. The viscosity must always be high enough to keep good oil film between the moving parts. Otherwise, friction will increase, resulting in power loss and rapid wear on the parts. The viscosity index, commonly designated VI, is an arbitrary numbering scale that indicates the changes in oil viscosity with changes in temperature. A low index means a steep slope of the curve, or a great variation of viscosity with a change in temperature; high index means a flatter slope, or lesser variation of viscosity with the same changes in temperature. Increased viscosity index (VI) of triesters is the result of their higher molar weight, and especially the altered structure of their molecules. The VI values are high, characteristic for oils of ester type (Table 2).

The ability of a substance to resist oxidative degradation is another important property of biolubricants. Therefore, epoxidized oleic acid, 9-hydroxy-10-acyloxyoctadecanoic acid, 2-ethylhexyl 9-hydroxy-10-acyloxyoctadecanoate and 2-ethylhexyl 9-(octanoyloxy)-10-(acyloxy)octadecanoate were screened for oxidation stability using PDSC through determination of OT and SMT. PDSC is an effective method for measuring oxidation stability of oleochemicals in an accelerated mode (Du et al., 2002). The OT is the temperature at which a rapid increase in the rate of oxidation is observed at a constant, high pressure (200 psi). A high OT would suggest high oxidation stability of the material. The SMT is the temperature at which maximum heat output is noted from the sample during oxidative degradation. A higher SMT does not necessarily correlate with improved oxidation stability. Both OT and SMT were calculated from a plot of heat flow (W/g) versus temperature that was generated by the sample upon degradation and, by definition, SMT > OT.

In the present study, as the chain length of the mid-chain ester is decreased, a corresponding improvement in oxidation stability was observed, which is because longer chains are more susceptible to oxidative cleavage than shorter chains. These results are in agreement with other studies on synthetic esters (Randals, 1999). For example, when comparing 9-hydroxy-10-acyloxyoctadecanoic acid, 2-ethylhexyl 9-hydroxy-10-acyloxyoctadecanoate and 2-ethylhexyl 9-(octanoyloxy)-10-(acyloxy)octadecanoate, an improvement in OT was noticed as the mid-chain ester length (R) was decreased (Table 2).

An important property of lubricants is their ability to maintain a stable lubricating film at the metal contact zone. Triacylglycerols of plant oils are known to provide excellent lubricity due to their ester functionality. This is because, the polar head of the triacylglycerol molecule, i.e. glycerol end attaches to metal surfaces and allows a monolayer film formation with the non-polar end of fatty acid chains sticking away from the metal surface. This prevents the metal-to-metal direct contact by providing a sliding surface. Without a good sliding surface, the two metals at the contact zones of moving parts come in direct contact with each other and results in increase in temperature causing adhesion, scuffing or even welding. The ester structures in triacylglycerols offer active oxygen sites that trigger binding on the metal surface forming a protective film. This protective film builds further with time to reduce friction.

In this work, the antiwear and friction reducing properties of synthetic ester oil basestocks were evaluated using high-frequency reciprocating rig (HFRR) lubricity tester. The HFRR method determines the lubricity or ability of a fluid to affect friction and wear between the surfaces in relative motion under load. The average ball scar diameter, width of wear track on disk at x-axis, film percentage, and coefficient of friction (CoF) for synthetic ester oils are shown in Table 3.

The high ball wear scan diameter for EHOBOD oil may be due to low amount of free fatty acids (FFA) present in it. The HYOODA oil provided the lowest ball wear scar diameter, which may be due to presence of higher amount of FFA present in it. The disk x-scar results were also similar, with biggest scar width for EHOBOD oil. The average CoF is higher (0.097) for EHOBOD, followed by EHHYNOD (0.096) and least for HYBODA (0.095). The lower CoF in EHOSOD, EHHYLOD and HYLODA may be due to their higher viscosity compared to other oils. These results showed that lubricity properties of synthetic ester oils are on par with other plant oils (Salimon et al., 2010).