Synthetic biolubricant basestocks based on environmentally friendly raw materials

1 Introduction

Synthetic biolubricants are used for two major reasons: when an equipment demands specific performance features that cannot be met with conventional mineral oil-based lubricants, examples are extreme high or low operating temperature, stability under extreme conditions and long service life and when synthetic biolubricants can offer economic benefits for overall operation, such as reduced energy consumption, reduced maintenance and increased power output (Bartz, 2006).

Conventional lubricants are formulated based on mineral oils derived from petroleum. Mineral oil contains many classes of chemical components, including paraffins, naphthenes, aromatics, hetero-atom species, etc. Its compositions are pre-determined by the crude source. Modern oil refining processes remove and/or modify the molecular structures to improve the lubricant properties, but are limited in their ability to substantially alter the initial oil composition to fully optimize the hydrocarbon structures and composition (Kodali and Nivens, 2001). Mineral oils of such complex compositions are good for general-purpose lubrication, but are not optimized for any specific performance feature. The major advantages for mineral oils are their low cost, long history and user’s familiarity. But this paradigm is now changing (Puşcaş et al., 2006).

The trend with modern machines equipment is to operate under increasingly more severe conditions, to last longer, to require less maintenance and to improve energy efficiency. In order to maximize machine performance, there is a need for optimized and higher performance lubricants. Synthetic biolubricants are designed to maximize lubricant performance to match the high demands of modern machines equipment and to offer tangible performance and economic benefits (Schuchart et al., 1998).

Synthetic biolubricants differ from conventional lubricants in the type of components used in the formulation. The major component in a synthetic biolubricant is the synthetic base stock. Synthetic base stocks are produced from carefully-chosen and well-defined chemical compounds and by specific chemical reactions. The final base stocks are designed to have optimized properties and significantly improved performance features meeting specific equipment demands (Guzman, 2002). The most commonly optimized properties are

• Viscosity index (VI). VI is a number used to gauge oil’s viscosity change as a function of temperature. Higher VI indicates less viscosity change as oil temperature changes – a more desirable property.

• Pour point and low temperature properties. Many synthetic base stocks have low PP, −30 to −70 °C, and superior low-temperature properties. Combination of low pour point and superior low-temperature properties ensures oil flow to critical engine parts during cold starting, thus, offering better lubrication and protection. Conventional mineral oils typically have pour points in the range of 0 to −20 °C. Below these temperatures, wax crystallization and oil gelation can occur, which prevent the flow of lubricant to critical machine parts.

• Thermal/oxidative stability. When oil oxidation occurs during service, oil viscosity and acid content increase dramatically, possibly corroding metal parts, generating sludge and reducing efficiency. These changes can also exacerbate wear by preventing adequate oil flow to critical parts. Although oil oxidation can be controlled by adding antioxidants, in the long term service and after the depletion of antioxidant, the intrinsic oxidative stability of a base stock is an important factor in preventing oil degradation and ensuring proper lubrication (Fox and Stachowiak, 2007). Many synthetic base stocks are designed to have improved thermal oxidative stability, to respond well to antioxidants and to resist aging processes better than mineral oil.

The aim of this study was to present a novel synthetic approach for the chemical modification of oleic acid to yield oleic acid-based triester derivatives with improved physicochemical properties.

2 Experimental

2.7

2.7 Synthesis

2.7.1

2.7.1 Epoxidized oleic acid (EOA, 1)

Hydrogen peroxide solution (30% in H₂O, 8.0 mL) was added slowly into a stirred solution of oleic acid (OA) (90%, 15 g) in formic acid (88%, 14 mL) at 4 °C (ice bath). The reaction proceeded at room temperature with vigorous stirring (900 rpm) until the formation of a powdery solid was noticed in the reaction vessel 2–5 h. The solid was collected via vacuum filtration, washed with H₂O (chilled, 3 × 10 mL), and placed under high vacuum for 12 h, yielding epoxidized oleic acid (EOA) as a colorless, powdery solid.

2.7.2

2.7.2 9-Hydroxy-10-acyloxyoctadecanoic acid (HYAODA, 2–8)

To a mixture of EOA (31 g) was added 5 g of p-toluenesulfonic acid (PTSA), toluene (70 mL) and fatty acids (6 g) over 1.5 h in order to keep the reaction mixture temperature under 70–80 °C. The reaction mixture was subsequently heated to 90–100 °C and refluxed for 3 h. After reaction termination, the heating was stopped and the mixture was left to stand overnight at ambient room temperature. The mixture was washed with water the next day. The organic layer was dried over anhydrous magnesium sulfate and the solvent was removed using a vacuum evaporator.

2.7.3

2.7.3 Butyl 9-hydroxy-10-acyloxyoctadecanoate (BHYAOD, 9–15)

Sulfuric acid (conc. H₂SO₄, 10 mol%) was added into a stirred suspension of HYAODA (3.35 mmol) in butanol (3.35 mL). The suspension was heated with stirring at 60 °C for 10 h. Hexanes (5 mL) was then added. The solution was washed with NaHCO₃ (sat. aq., 1 × 0.5 mL) and brine (2 × 1 mL), dried using MgSO₄, filtered, concentrated in vacuo and placed for 6 h under vacuum to yield the desired product.

2.7.4

2.7.4 Butyl 9-(decanoyloxy)-10-(acyloxy)octadecanoate (BDOAOD, 16–22)

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

Close

Figure 1

Triesters formation. RCOOH = octanoic, nonanoic, lauric, myristic, palmitic, stearic and behenic acids.

Export to PPT

3 Results and discussion

The preparation of butyl 9-(decanoyloxy)-10-(acyloxy)octadecanoate 16–22 from EOA is an effective way of introducing branching onto the fatty acid (FA) chains 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 to 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 (Hwang et al., 2003). 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 (HYAODA) products 2–8. Then, esterification of these products was carried out using butanol and sulfuric acid as catalyst to yield butyl 9-hydroxy-10-acyloxyoctadecanoate (BHYAOD) 9–15. The seven prepared butyl esters were used as feed for the synthesis of modified triester-derivatives by acetylation in an aprotic solvent. The yields are summarized in Table 1.

In the FTIR spectra of compounds 2–22, the absorption due to the epoxy group (822 and 842 cm⁻¹) is not observed. This fact suggests that 1 undergoes complete ring opening under the reaction condition. Bands representing C⚌O groups (725, 1743 cm⁻¹), CH₃ groups (1373–1461 cm⁻¹), OH groups (3475–3440 cm⁻¹) and also C–O–C bands in esters (998–1100 cm⁻¹) are clearly visible in the spectra (Salimon and Salih, 2010).

All synthesized compounds were verified by ¹H and ¹³C NMR spectroscopy. Significant signals in the ¹H spectrum of EOA 1 between 2.5 and 2.7 ppm correspond to quaternary carbons of the oxiran ring and the doublet in the ¹³C spectrum between 56.89 and 56.90 ppm corresponds to carbons of the oxirane ring. Furthermore, ¹H spectrum of EOA showed a singlet signal at 9.20 ppm due to the OH group. All signals in the area around 9.15–9.27 ppm, representing OH groups, and the bands at 2.05–3.66 ppm, corresponding to –CH₂– groups, are present in the ¹H spectra of monoesters, HYAODA 2–8. The ¹H spectra of synthesized diesters, BHYAOD 9–15, and triesters, BDOAOD 16–22, consist of signals of low intensity at about 9.22–9.40 ppm and 2.10–3.65 ppm. Broad lines at 1.40–1.78 ppm represent the CH₂ groups’ hydrogen peaks. In the ¹³C NMR spectra significant bands at about 174 ppm are present, which exhibit the characteristic signals attributed to the ester groups (Sliverstien et al., 2005).

Physicochemical properties of prepared compounds are summarized in Table 1. The ability of a substance to remain as a 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. 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 (Salimon et al., 2010). The flash point should be high to allow the safe operation and minimum volatilization at the maximum operating temperature, Table 1. For the most demanding applications, such as aviation jet engine lubricants, an effective liquid range over 300 °C may be required. The EOA, 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 BHYAOD and triesters BDOAOD was obtained over that of their monoester precursors HYAODA. 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 the absence of it in triester structures mean that 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 a 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 as 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 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 1).

The ability of a substance to resist oxidative degradation is another important aspect of biolubricants. Therefore, EOA, HYAODA, BHYAOD and BDOAOD 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 is greater than 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 HYAODA monoesters, BHYAOD diesters and BDOAOD triesters, an improvement in OT was noticed as the mid chain ester length (R) was decreased (Table 1).

4 Conclusion

The results show that under some conditions, triester products have better biolubricant properties which enable them to outperform their precursors. Additionally, they maintain the advantages inherent from their biobased nature. These products remain a viable option for application in the biolubricant industry. The following conclusions can be drawn:

• Increasing the chain length of the mid chain ester had a positive influence on the low temperature properties of the synthesized esters because they create a steric barrier around the individual molecules and inhibits crystallization, resulting in lower PP.

• The trends for PP were opposite to those of OT. For example, increasing chain length was beneficial to PP, but detrimental to OT.

• It is evident that hydrogen bonding was a critical parameter influencing the low temperature properties and oxidation stability of the synthesized esters. An increase in hydrogen bond amount will lead to an increase in PP and a decrease in the oxidation stability of these compounds.

• Removal of the unstable double bonds from fatty acid acyls, increased molar weight and change in the molecular structure result in an increased VI of the synthesized esters.