Thermo-oxidation, friction-reducing and physicochemical properties of ricinoleic acid based-diester biolubricants

2.1 Materials

Ricinoleic acid (95%) was obtained from Nu-Chek Prep, Inc. (Elysian, MN, USA). Formic acid (88%) and hydrogen peroxide (30% solution) were obtained from Fisher Scientific (Pittsburgh, PA). 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 Methods

2.3 Synthesis

2.3.3

2.3.3 Octyl 10,12-dihydroxy-9-acyloxystearate (ODHAS) (10–16)

The reaction scheme for the formation of the diesters is provided in Fig. 1. Sulfuric acid (conc. H₂SO₄, 10 mol-%) was added to a stirred suspension of 10,12-dihydroxy-9-acyloxystearic acid 3–9 (3.35 mmol) in octanol (3.35 mL). The suspension was stirred and heated at 60 °C for 10 h. Then, hexane (5 mL) was added and the solution was washed once with saturated aqueous NaHCO₃ (0.5 mL) and brine (2 × 1 mL), dried (MgSO₄), filtered, and concentrated under vacuum for 6 h to yield octyl 10,12-dihydroxy-9-acyloxystearate derivatives.

Close

Figure 1

The reaction scheme for the formation of diesters.

Export to PPT

3.1 Synthesis

Generally, epoxidized plant oils provide promising intermediates for the utilization of plant oils as biolubricant basestocks because the epoxide, or oxirane, group can easily be functionalized and chemically modified. In this work, the first step, as shown in Fig. 1, was the epoxidation of ricinoleic acid 1 using formic acid and hydrogen peroxide to yield epoxidized ricinoleic acid 2. The second step was the opening of the oxirane ring in the epoxidized oil by attacking it with octanoic, nonanoic, lauric, myristic, palmitic, stearic and behenic acids to yield the respective 9(10)-hydroxy-10(9)-ester derivatives of ricinoleic acid 3–9. The diester-derivatives of 10,12-dihydroxy-9-acyloxystearate 10–16 were prepared by the esterification of the seven corresponding acyloxy-derivatives using octanol and sulfuric acid.

3.2 Characterization

All synthesized compounds were characterized via ¹H, ¹³C NMR and FTIR spectroscopy. Significant signals found in the ¹H spectrum of the epoxidized ricinoleic acid 2 at ∼2.2 and 2.4 ppm correspond to protons on the quaternary carbons of the oxirane ring, whereas a double in the ¹³C spectrum at ∼56.82 and 56.86 ppm corresponds to the carbons of the oxirane ring. Furthermore, the ¹H spectrum of the epoxidized ricinoleic acid 2 exhibited singlet signals at ∼9.15 and ∼9.32 ppm, which represent the protons of the –OH groups (Campanella et al., 2010). In contrast, a singlet at ∼9.12–9.29 ppm represents the –OH protons, and the bands at ∼2.03–3.64 ppm correspond to –CH₂– groups in the ¹H spectra of the 10,12-dihydroxy-9-acyloxystearic acid 3–9 derivatives. The ¹H spectra of octyl 10,12-dihydroxy-9-acyloxystearate 10–16 consist of multiplet signals at ∼9.18–9.35 ppm due to – the –OH protons and at ∼1.42–3.24 ppm due the –CH₂–, –CH(OH) and –CH(OCOR) protons, respectively. Furthermore, the signals at ∼173.1–176.7 ppm in the ¹³C NMR spectra are attributed to the ester carbonyl groups (Salimon et al., 2012a). These signals are in agreement with the proposed structures.

The structures of the ester functional groups were also confirmed via IR spectral analysis. Bands representing the ester C⚌O group (∼1740 cm⁻¹), the CH₃ group (∼1376 cm⁻¹), the OH group (∼3478–3443 cm⁻¹) and the C–O–C functionality (∼1000–1100 cm⁻¹) are clearly visible in the spectra (Sliverstein et al., 2005).

3.3 Low-temperature properties and the viscosity index

In this work, seven fatty acids were chosen for investigating the effects of various alkyl groups on the low-temperature properties of the synthesized products (Table 1). These fatty acids were chosen based on the length of their carbon-chain backbone, which was medium (C₈–C₁₂) to long (C₁₄–C₂₂). The presence of long saturated fatty acids in the compounds does not allow easy stacking of the individual molecules due to disruptions in the molecular symmetry. As a result, microcrystalline structures form instead of macro structures. At low temperatures, such microcrystalline structures can easily tumble and glide over one another enhancing the fluidity of the liquid, i.e., the attachment of an ester side chain of optimum length at the 9 or 10 position of the fatty acid chain can significantly improve its low-temperature properties (Hawang and Erhan, 2001). While epoxidized ricinoleic acid 2 has a pour point of 9.16 °C and a cloud point of 12.31 °C; the synthesized esters 3–16 all have significantly improved pour points ranging from −10.22 to −53.26 °C, and cloud points in the range −7.58–−49.75 °C.

In practice, a biolubricant should be useful over a broad temperature range; in cold weather, the range of lubricant low-temperature performance is measured through the pour point (PP) and the cloud point (CP), which represent the primary drawbacks of plant oil biolubricants (Salimon et al., 2011b). Plant oils are complex molecular substances in which the transition from liquid to solid does not occur at a unique temperature, but over a relatively wide range and involves several polymorphic crystalline shapes (α, β′, β) that depend on the unsaturated/saturated fatty acid ratio (Sato, 2001; Adhvaryu et al., 2003; Govindapillai et al., 2009). The pour point (PP) and cloud point (CP) of a biolubricant are good indicators of its low-temperature fluidity (Salimon et al., 2012b).

The viscosity is the most important physical property of a lubricant. The viscosity, which may be defined as a liquid’s resistance to flow, is the characteristic most frequently stipulated by equipment manufacturers when making lubricant recommendations. The viscosity index, commonly designated VI, is an arbitrary numbering scale that indicates the variation of oil’s viscosity with changes in temperature. A low viscosity index indicates that the viscosity changes greatly with temperature. Therefore, a fluid that has a high viscosity index is expected to undergo little change in viscosity with temperature extremes and is considered to have a stable viscosity. A fluid with a low viscosity index is expected to undergo significant changes in viscosity as the temperature fluctuates. Furthermore, the viscosity of a lubricant is closely related to its ability to reduce friction (Sharma et al., 2009; Salimon et al., 2011a).

The results (Table 1) indicate that an overall increase in the molecular weights of the products resulting from the increased chain length of the ester functionality in longer mid-chain groups increases the viscosity index.

3.4 Thermo-oxidative stability

Epoxidized ricinoleic acid 2, 10,12-dihydroxy-9-acyloxystearic acid 3–9, and octyl 10,12-dihydroxy-9-acyloxystearate 10–16 were screened for their oxidation stability using pressurized differential scanning calorimetry to determine the onset temperature (OT) and the signal maximum temperature (SMT). The oxidation onset temperature is a relative measure of the degree of oxidative stability in a material evaluated for a given heating rate and oxidation environment. For example, in oxygen, a higher OT value indicates a more oxidatively stable material. The signal maximum temperature is the temperature at which maximum heat output is observed in the sample during oxidative degradation. A higher SMT value does not necessarily correlate with improved oxidative stability.

The ability of a material to resist oxidation is represented by its oxidative stability, which plays a major role in its aging, shelf-time and other performance characteristics. To avoid these problems, biolubricants must possess superior oxidative stability. The results in Table 2 indicate that a general improvement in the oxidative stability was observed in the synthesized compounds 3–16 as the chain length of the mid-chain ester decreased because longer side chains have additional accessible sites that can easily oxidized making them more susceptible to cleavage than short chains (Salih et al., 2011).

Pressure differential scanning calorimetry (PDSC) has several advantages: First, an oxidative stability test may make too long at atmospheric pressures to be convenient. Second, some reactions from water or methanol as a byproduct, leading to foaming in the sample, can be suppressed at higher pressures. Third, some reaction kinetics is affected by the pressure; therefore, running the reaction under controlled pressure is necessary for kinetics studies. Finally, pressure differential scanning calorimetry is an effective way to rapidly measure the oxidative tendencies of biolubricant base oils, plant oils and oleochemicals (Fox and Stachowiak, 2007).

Oxidative degradation has been identified as the most important limiting factor for lubricant application. The consequences of lubricant oxidation include increased acidity and viscosity, varnish formation, sludge and sediment formation, additive depletion, base-oil breakdown, filter plugging, rust and corrosion (Luna et al., 2011). Therefore, understanding and controlling oxidation is a major concern for improving biolubricant basestock properties (Wagner, 2001; Schneider, 2006; Sharma et al., 2007).

Another thermal-oxidative stability test was performed using thin-film micro-oxidation (TFMO) to study the volatility and the depository tendencies of the synthesized ester products. In most applications, a biolubricant functions as a thin film, so the thin-film micro-oxidation test is considered a good choice to simulate actual conditions under which oxygen diffusion is unlimited (Adhvaryu et al., 2006). Micro-oxidation is a family of thin-film oxidation tests that is widely used by the lubricant industry (Perez et al., 1987), and its results are consistent with those of pressure differential scanning calorimetry (Dunn, 2005). During the oxidation process, some small primary oxidation products are lost as volatile compounds, while others undergo oxy-polymerization to form oil-insoluble deposits in the presence of excess oxygen. The ability to form such deposits is the primary restriction to the use of unsaturated oleochemicals as high-temperature biolubricants. The volatile loss obtained from thin-film micro-oxidation tests can be used to predict the useful life of a biolubricant.

In this work, a thin-film micro-oxidation test was carried out on all synthesized products 2–16; the data from these samples are presented in Table 3. One can observe that the losses due to high-temperature volatility and insoluble deposits increased gradually as the number of carbon atoms bound to epoxy carbons in the mid-chain ester group increased (i.e., when the length of mid-chain substituents increased) due to an increased likelihood of oxidative degradation (Dunn, 2005). These results are in agreement with those obtained via pressure differential scanning calorimetry (Table 2).

3.5 Friction-reducing measurements

Table 4 shows the friction-reducing results for the synthesized compounds 2–16. The results indicate that an improvement in the friction-reducing properties (reduced wear scar diameter (WSD)) values and coefficient of frictions (μ)) occurred with longer mid-chain esters and an increased polarity in the ester functional group substituents, i.e., diester > monoesters (Table 4), due to the increased polar functional groups in the product structures, which increased the strength of the friction-reducing film at the metal contact zone and thereby increased its efficiency in reducing the friction (Biresaw et al., 2003; Kurth et al., 2005; Sharma et al., 2009). Beside this; increasing in branching points of mid-chain group which increase the polarity separation between the long non polar chain with ester functionality group resulting in increase the tendency of ester functional group attach onto metal surfaces (Salih et al., 2013).

The surface of the machinery components can appear well finished to the naked eye. When magnified, however, surface imperfections become readily apparent. These microscopic hills and valleys are called asperities (Brophy and Zisman, 1951). When dry surfaces move relative to one another, asperities may rub, lock together, and break apart. The resistance generated when these adjacent surfaces come into contact is called friction (Rabinowicz, 1995; Biresaw et al., 2002, 2003). Another consequence of friction is that the energy created by resistance is converted into heat. A lubricant is any material that is interposed between surfaces, forming a film to avoid or minimize contact between the surfaces (Salimon and Salih, 2009a). An important property of biolubricants is their ability to maintain a stable lubricating film at the metal contact zone. Plant oils and fatty esters are known to provide excellent lubricity due to their ester functionality. The ester ends of the fatty acid chain adsorb onto the metal surfaces, thus forming a monolayer film with the hydrocarbon end of the fatty acids oriented away from the metal surface (Salimon and Salih, 2009b,c; Salimon et al., 2011c, 2012b).