Synthesis of levulinic acid-based polyol ester and its influence on tribological behavior as a potential lubricant

Abstract

Levulinic acid (LA) has been identified by the US Department of Energy as a top platform chemical and regarded as one of the twelve most promising molecules derived from biomass. In this study, it was used to prepare lubricant base stocks by esterification with three different polyols including neopentyl glycol (NPG), trimethylolpropane (TMP), and pentaerythritol (PE) in the presence of sulfuric acid. The crude product was distilled to obtain the target product with a purity above 95%. The products of the polyol ester were characterized using ¹H NMR and mass spectrometry techniques. The lubricant properties, such as kinematic viscosity, viscosity index, and pour and flash points, were evaluated using standard ASTM methods. Among the base stocks prepared, TMP-tri-LA ester exhibited superior lubricant properties like good kinematic viscosity at 40 °C (87.28) and at 100 °C (8.42), a viscosity index of 48, a low pour point (−25 °C), a high flash point (238 °C), a low cloud point (−4 °C), a low copper strip corrosion class (1a), and low volatility at 120 °C (0.41%). Wear prevention characteristics of this ester were tested and produced a low coefficient of friction and small wear scar diameters. It was found that this kind of polyol ester is suitable for lubricant base stocks.

---

1. Introduction

Biodegradable polyol esters, which serve as biolubricant components, have aroused increasing interest because of their excellent lubricity, biodegradability, viscosity–temperature characteristics, and low volatility.¹ However, natural triglycerides are thermally sensitive at high temperature. The main reason for this phenomenon is the presence of hydrogen atoms in the β position of the glycerol molecular backbone,² since this structural feature is prone to cause partial fragmentation of the molecule and formation of unsaturated compounds. The compounds thus formed by polymerization led to an increase in viscosity of the liquid and resulted in the formation of precipitate particles.³ Thus, natural triglycerides are generally not appropriate for the lubricant applications that require heat resistance. This problem could be easily solved by replacing glycerol with another special polyhydric alcohol which does not contain β-hydrogen atoms.^4,5 In previous literature, researchers used neopentyl polyols such as neopentyl glycol (NPG), trimethylolpropane (TMP) and pentaerythritol (PE) for this purpose.^6–8 So far, the investigation of polyol esters biolubricant preparation has been conducted using polyols with various fatty acids by transesterification or esterification. Most of the fatty acids reported in the literature are generally obtained from animal oils such as beef tallow, lard, mutton tallow as well as vegetable oils, such as soybean oil, sunflower oil, rapeseed oil, castor oil, olive oil, palm kernel oil and waste cooking oil.^9–15 These high monounsaturated oils have optimal performance as raw materials to generate environmentally friendly lubricants with high performance. However, part of these oils are edible, and their use for the production of lubricants will indirectly influence the food demand. Hence, more attention is being paid to the preparation of non-edible oil-based polyol esters containing any unusual fatty acids.

Levulinic acid (LA), having five carbon atoms with a carboxyl group and ketone functional groups, can be easily produced from glucose, fructose, starch and lignocellulosic residues.¹⁶ It has been identified by the US Department of Energy as a top platform chemical and regarded as one of the twelve most promising molecules derived from biomass, because it can be transformed into a variety of other important compounds in the chemical industry.^17,18 In the 1990s, there were some patents about the production of LA by conversion of lignocellulose by acid degradation at an elevated temperature and the theoretical yield of LA could reach 55% (molar yield based on the hexose polymer content of wood feed) under the optimal condition.^19,20 Recent research demonstrated that LA could be obtained from cellulose in the presence of a catalyst such as functionalized ionic liquids,²¹ dilute acid,²² alkaline-treated zeolite,²³ and the Fe/HY catalyst.²⁴ LA produced from cellulose has attracted considerable attention and become one of the key steps for biomass refining.²¹ Meanwhile, the esterification of levulinic acid with polyols could be easily implemented and the polyol esters based on levulinic acid are expected to have similar lubricant properties compared to other polyol esters. Therefore, there may be broader application prospects for LA as a means to produce biolubricants compared to other vegetable oils.

Hence, this study was aimed at employing levulinic acid and polyols such as NPG (dihydric), TMP (trihydric) and PE (tetrahydric) for the synthesis of polyol esters. The potential of the synthesized esters as base stocks for biodegradable lubricants was also evaluated.

2. Materials and methods

2.1. Chemicals

NPG, TMP and PE were obtained from Fuchen Chemical Co. Ltd. (Tianjin, China); levulinic acid (LA) with a purity of 99% was purchased from Adamas Reagent Co., Ltd.; sulfuric acid with a purity of 98% was procured from Beijing Chemical Factory. Other solvents of analytical grade were obtained from Beijing Chemical Factory. Mineral oil with similar viscosity to LA-based TMP ester was provided kindly by SINOPEC in Beijing.

2.2. Analysis

¹H NMR spectra were obtained using a Bruker AR X 400 Spectrometer (400, 200 MHz). Mass spectra were recorded on a VG Auto Spec-M (Manchester, UK) and the compounds including LA, mono-, di- and trisubstituted esters in the reaction mixture were quantified using a GC-2010 gas chromatograph (Shimadzu, Japan) equipped with a DB-1ht capillary column (30 m × 0.25 mm × 0.1 μm; J&W Scientific, USA) and a flame ionizing detector (FID). The column temperature was held at 110 °C, then heated to 132 °C at 12 °C min⁻¹, and to 180 °C at 30 °C min⁻¹ and finally to 300 °C at 20 °C min and then maintained for 5 min. The temperatures of the injector and detector were both set at 300 °C.

2.3. Viscosity

Viscosity measurements were conducted at 40 °C and 100 °C using Cannon Fenske viscometer tubes in a Cannon Constant Temperature Viscosity Bath (Hunan Petrochemical Instrument CO., LTD.). Viscosity and the viscosity index were calculated using ASTM methods D445 and D2270, respectively. All viscosity measurements were carried out in triplicate, and the average value was reported.

2.4. Pour point

Pour points were determined by the ASTM method D97 with an accuracy of ±3 °C using an Automatic Pour Point Tester, manufactured by Hunan Petrochemical Instrument CO., LTD. All runs were carried out in duplicate. Sample temperature was measured in 3 °C increments at the top of the sample until it stopped pouring.

2.5. Flash point

Flash points of the products were determined using a Flash Point Tester (Hunan Petrochemical Instrument CO., LTD.) according to the ASTM D93 method. The lowest temperature for application of the test flame, which caused the vapor above the surface of the liquid to ignite, was taken as the flash point of the product at ambient barometric pressure.

2.6. Cloud point

Cloud points were determined by the standard ASTM D2500 using a Cloud Point Apparatus (Beijing Timepower Measure and Control equipment CO., LTD.).

2.7. Copper strip corrosion

Copper strip corrosion was determined using a Copper Strip Corrosion Tester (Beijing Timepower Measure and Control equipment CO., LTD.). The appearance of the Cu strip was assessed by comparison with Copper Strip Corrosion Standards (ASTM D130).

2.8. The volatility determination

The volatility was determined in agreement with the ASTM method D6184 in an electrical stove using glass pans that were 4 cm in diameter.

2.9. Thermogravimetric analysis (TGA)

Thermogravimetric analysis (TGA) is a measure to evaluate the thermal stability of materials. A higher TGA onset temperature represents a higher thermal stability of a material. Thermal behavior of the LA-based polyol esters were studied using a PerkinElmer thermogravimetric differential thermal analyzer (TG/DTA) under a flow of nitrogen (flow rate of 40 ml min⁻¹) at a constant heating rate of 10 °C min⁻¹.

2.10. HFRR (high frequency reciprocating rig) configuration

The HFRR wear test was used to investigate the effect of the ester under fluid film lubrication as shown in Fig. 1. The HFRR test (ASTM D6079) used in this study included a weighted cast iron pin cylinder (6 mm length) and a stationary cast iron plate (15 mm × 15 mm), which was completely submerged in 10 ml of the sample. The ball and disk were tested at room temperature and brought into contact with each other and the entire apparatus was vibrating at 50 Hz for 75 min with a load of 200 g. The sliding stroke was maintained at 2 mm. Several blends of LA-based TMP ester including 1%, 3%, 7%, 10%, 20% and 100%, were tested and analyzed. Each test was carried out three times repeatedly to observe any errors which needed to be analyzed.

Fig. 1 The schematic diagram of the HFRR wear test.

2.11. Synthesis methods

2.11.1. Preparation of the NPG diester of LA (NPG-di-LA). Synthesis of NPG-di-LA was carried out according to Fig. 2(A), and the catalyst was sulfuric acid. The reaction was performed in a temperature controlled reactor with mechanical stirring. The reaction mixture contained 500.00 g (4.30 mol) LA and 149.15 g (1.43 mol) NPG. It was incubated for 2 h at 95 °C in the presence of 6.49 g sulfuric acid (1% of the total mixture). The process of the reaction was monitored by GC, and after completion of the reaction, the crude product was distilled by Rotary Film Molecular Distillation (VTA GMBH & Co. KG, Germany) at 140 °C and 1 mbar to remove sulfuric acid and excess LA. After that, the obtained product from the first distillation was distilled once again at 170 °C and 0.3 mbar to get the final NPG-di-LA product with a purity of 96.21%. The structure of the target product was characterized by ¹H NMR and mass spectrometry studies.

Fig. 2 Schematic representation of the esterification reaction between LA and NPG, TMP, or PE.

¹H NMR (CDCl₃, δ ppm): 0.97 [s, 6H, 2 × (–CH₃)], 2.21 [s, 6H, 2 × (–CO–CH₃)], 2.59–2.62 [m, 4H, 2 × (–CO–CH₂–)], 2.76–2.79 [s, 4H, 2 × (–CH₂–CO–)], 3.90 [s, 4H, 2 × (–CO–CH₂–)]; ESI MS: m/z 323.15 [M + Na]⁺.

2.11.2. Preparation of the TMP triester of LA (TMP-tri-LA). Synthesis of TMP-tri-LA was carried out according to Fig. 2(B), and the catalyst was sulfuric acid. The reaction was performed in a temperature controlled reactor with mechanical stirring. The reaction mixture included 500.00 g (4.30 mol) LA and 144.43 g (1.08 mol) TMP. It was incubated for 2 h at 95 °C in the presence of 6.44 g sulfuric acid (1% of the total mixture). The process of the reaction was monitored by GC, and after completion of the reaction, the crude product was distilled by Rotary Film Molecular Distillation (VTA GMBH & Co. KG, Germany) at 140 °C and 1 mbar to remove sulfuric acid and excess LA. Thereafter, the obtained product from the first distillation was distilled once again at 230 °C and 0.009 mbar to get the final TMP-tri-LA product with a purity of 95.82%. The structure of the target product was characterized by ¹H NMR and mass spectrometry studies.

¹H NMR (CDCl₃, δ ppm): 0.89–0.92 [s, 3H, (–CH₃)], 1.48–1.50 [s, 2H, (–CH₂–CH₃)], 2.20 [s, 9H, 3 × (–CO–CH₃)], 2.58–2.60 [s, 6H, 3 × (–CO–CH₂–)], 2.76–2.79 [s, 6H, 3 × (–CH₂–CO–)], 4.03 [s, 6H, 3 × (–CO–CH₂–)]; ESI MS: m/z 451.20 [M + Na]⁺.

2.11.3. Preparation of the PE tetraester of LA (PE-tetra-LA). Synthesis of PE-tetra-LA was carried out according to Fig. 2(C), and the catalyst was sulfuric acid. The reaction was performed in a temperature controlled reactor with mechanical stirring. The reaction mixture contained 500.00 g (4.30 mol) LA and 117.24 g (0.86 mol) TMP. It was incubated for 12 h at 115 °C in the presence of 6.17 g sulfuric acid (1% of the total mixture). The process of the reaction was monitored by GC, and after completion of the reaction, the crude product was distilled by Rotary Film Molecular Distillation (VTA GMBH & Co. KG, Germany) at 140 °C and 1 mbar to remove sulfuric acid and excess LA. Afterward, the obtained product from the first distillation was distilled once again at 290 °C and 0.0085 mbar to get the final TMP-tri-LA product with a purity of 95.01%. The structure of the target product was characterized by ¹H NMR and mass spectrometry studies.

¹H NMR (CDCl₃, δ ppm): 2.19 [s, 12H, 4 × (–CO–CH₃)], 2.57–2.60 [s, 8H, 4 × (–CO–CH₂–)], 2.75–2.78 [s, 8H, 4 × (–CH₂–CO–)], 4.12 [s, 8H, 4 × (–CO–CH₂–)]; ESI MS: m/z 551.21 [M + Na]⁺.

3. Results and discussion

3.1. Characterization and physico-chemical properties of esters

Polyol esters, derived from esterification or transesterification using plant oils and animal oils with neopentyl polyols without hydrogen atoms on the β carbon, have been widely used as lubricants. Due to their superior physico-chemical properties and performances compared with mineral oils, their applications have expanded largely. The present study was focused on the use of non-edible levulinic acid, a promising platform chemical derived from biomass, combined with dihydric, trihydric and tetrahydric alcohols named NPG, TMP and PE respectively to produce LA-based polyol esters. The polyol esters based on levulinic acid were expected to have similar lubricant properties compared with other polyol esters.

The purity of the levulinic acid-based polyol esters was determined by GC analysis and was found to exceed 95%. The structures of the polyol esters were confirmed using ¹H NMR (Fig. 4) and ESI-MS (Fig. 3) spectral studies. The molecular ions 323.15 [M + Na]⁺, 451.20 [M + Na]⁺, and 551.21 [M + Na]⁺ correspond to compounds (A) NPG-di-LA, (B) TMP-tri-LA, and (C) PE-tetra-LA, respectively. All the products were characterized for their physico-chemical properties, such as viscosity at 40 °C and 100 °C, viscosity index (VI), pour point, flash point, cloud point and copper strip corrosion (Table 1).

Fig. 3 Mass spectra of levulinic acid-based polyol esters: (A) NPG-di-LA, (B) TMP-tri-LA, and (C) PE-tetra-LA.

Fig. 4 ¹H NMR spectra of levulinic acid-based polyol esters: (A) NPG-di-LA, (B) TMP-tri-LA, and (C) PE-tetra-LA.

Table 1 Physico-chemical properties of the LA-based polyol esters

Property	NPG-di-LA	TMP-tri-LA	PE-tetra-LA
Viscosity at 40 °C, (mm^2 s^−1)	17.29	87.28	417.45
Viscosity at 100 °C, (mm^2 s^−1)	3.35	8.42	19.14
Viscosity index	30	48	15
Pour point (°C)	−34	−25	−8
Flash point (°C)	185	228	210
Cloud point (°C)	−11	−4	7
Copper strip corrosion	1a	1a	1a
Volatility at 120 °C (%)	0.57	0.41	0.33

---

Kinematic viscosity is an important characteristic to exhibit the relationship between viscosity and temperature. The property of viscosity–temperature directly represents the lubricant performance and determines its application. Table 1 shows the kinematic viscosities of the different ester products. NPG-di-LA, TMP-tri-LA and PE-tetra-LA exhibited kinematic viscosity at 40 °C in the range of ISO VG10, reaching 17.29, 87.28 and 417.45, respectively. At 100 °C their kinematic viscosities were 3.35, 8.42 and 19.14 cSt. Viscosity of the polyol ester base fluids generally increases with the increase of the number of acyl functionalities present.²⁵ In this study, the LA based esters prepared with NPG, TMP and PE exhibited a similar trend as the result of Gryglewicz’s and Padmaja’s in terms of the relationship between the pour point and the number of branches in the molecule.^25,26

The viscosity index (VI) is an another parameter generally used for evaluating the performance of a potential lubricant. By detecting the value of the viscosity index, variation in the kinematic viscosity with temperature can be determined. Generally, the higher its value, the lower variation in viscosity with temperature, and thus a better performance of the lubricant. Table 1 also shows the viscosity index of NPG-di-LA, TMP-tri-LA, and PE-tetra-LA as 30, 48 and 15, respectively. As a high viscosity index means there is little change over a wide temperature range, the performance of the three synthesized esters needs further improvement for use as lubricants to increase the viscosity index, which is a desirable property for a lubricant, by adding additives in the formulated lubricants. The low viscosity index can also be overcome by increasing the molecular weight of the products and altering the structure of their molecules, especially through introduction of a double bond in their composition.^27,28 The reaction between LA and furfural, which is also a platform chemical from lignocellulosic residues, can be easily implemented in aq. NaOH.²⁹ The product, 6-furfurylidene levulinic acid, can be used to prepare biolubricant base stocks by esterification with polyols. This kind of polyol esters contains a double bond and the molecular weight is higher than that of the levulinic acid-based polyol esters. So the viscosity index could be clearly improved.

The pour point is an another key parameter to assess the performance and application of a potential lubricant. It is the temperature at which a lubricant can’t flow when a jar is tilted within a cooling environment. Lubricants with a low pour point are conducive to operation in an engine in cold temperature climates. The pour point of the ester products in this study was measured by ASTM D97. Generally, pour points of polyol esters prepared from unsaturated fatty acids are very low, and a similar trend was observed in term of the pour points of NPG-di-LA (−34 °C), TMP-tri-LA (−25 °C) and PE-tetra-LA (−8 °C). It was found that the pour point of NPG-di-LA with fewer numbers of acyl chains was lower compared to other polyols, which could be due to more effective disruption of the molecular packing compared to the PE and TMP esters. However, the pour point of PE-tetra-LA was high. This suggested that the molecular configuration of the NPG and TMP esters was more effective at disrupting the molecular packing compared to the PE ester. Such low pour points are useful for application in machine tools and hydraulic systems.³⁰ Compared with recent research which reported the pour points of other fatty acid-based polyol esters, the NPG-di-La and TMP-tri-LA ester exhibited competitive characteristics at lower temperature. Their pour points are lower than products (pour point −27 °C) from castor oil and TMP¹² and that of the polyol ester (pour point −18 °C) from rapeseed oil and TMP,³² and the polyol ester (pour point −6 °C) from rubber seed oil and TMP.³³

The flash point is defined as the minimum temperature at which a liquid produces a sufficient concentration of vapor above it, forming an ignitable mixture with air. The lower the flash point, the greater the fire hazard is. The TMP-tri-LA ester possessed excellent thermal stability with a flash point of 228 °C followed by the PE-tetra-LA ester at 210 °C and NPG-di-LA at 185 °C. The TMP-tri-LA ester had higher thermal stability followed by the NPG-di-LA and PE-tetra-LA esters. This trend was different from other synthetic esters.³¹ Flash points of all the polyol esters were found to be very high within the range of 185–228 °C, and the esters with a flash point ≥165 °C lie well within the range of hydraulic oils.³⁴

The cloud point is the temperature at which a liquid becomes cloudy in appearance. A high cloud point could lead to filter plugging and poor pumpability in cold weather applications. Among the three new kinds of esters in this paper, NPG-di-LA had the lowest cloud point, which reached −11 °C and even lower. The cloud point of TMP-tri-LA was −4 °C followed by 7 °C for PE-tetra-LA.

The copper corrosion test was also conducted in order to measure the tendency to cause metal deterioration. The result indicated that the three kinds of levulinic acid-based polyol esters were class 1a (slight tarnish). The term “class” represents the property of copper corrosion, and it is judged according to the color of the copper strip. The lower the copper corrosion property the lower the “class” of the oil. 1a was the lowest class of the Copper Strip Corrosion Standards (ASTM D130). Thus, all the esters in this study had a low corrosion effect on metal materials.

The volatility relates to the potential loss of lubricant and to the environmental impact of the volatile components of the fluid. Both European and U.S. OEM and industry organizations include volatility in their specifications.³⁵ In this study, NPG-di-LA, TMP-tri-LA and PE-tetra-LA exhibited very low volatility with only 0.57%, 0.41% and 0.33% of weight loss at 120 °C, respectively. The weight loss of the three kinds of levulinic acid-based polyol esters due to high-temperature volatility was found to decrease steadily with an increase in carbon numbers. Therefore, in levulinic acid-based polyol esters formulations, volatility is not a problem, which is due to higher molecular weights associated with the structure of glycerol analogues.

TGA is a measure to evaluate the thermal stability of materials. Generally, the higher T_onset of a material, the higher its thermal stability. In previous literature, some researchers have used this method to measure the thermo-oxidative stability of oils.^36,37 According to the TGA analytical results as shown in Fig. 5, the TGA curves of all samples had similar behavior. The accurate analysis of the TGA curves are summarized in Table 2. It was also found that the three prepared levulinic acid-based polyol esters possessed good thermal stability. NPG-di-LA, TMP-tri-LA, and PE-tetra-LA were thermally stable with less than 4% mass weight loss below the temperature of 189 °C, 255 °C and 222 °C, respectively. In addition, about 9–10% of weight loss was observed for each sample within the temperature range of 203–205 °C, 281–286 °C and 252–259 °C, respectively. The weight loss of NPG-di-LA, TMP-tri-LA, and PE-tetra-LA reached 97% when the temperature reached 579, 487 and 684 °C, respectively. Then, there was no further loss of weight with any further increase in temperature. It is known that levulinic acid-based polyol esters are not susceptible to undergoing autoxidation reactions. While these TGA measurements provided a preliminary estimate of the stability of a lubricant, further chemical studies would be necessary to check for the stability and functions of these lubricants.

Fig. 5 The TGA curves of the LA-based polyol esters.

Table 2 Thermal stability data of the ester products

Product	Tonset (°C)	9–10% wt loss (°C)	97% wt loss (°C)
NPG-di-LA	189	203–205	579
TMP-tri-LA	255	281–286	487
PE-tetra-LA	222	252–259	684

---

Among the three levulinic acid-based polyol esters, the TMP ester exhibited superior properties such as its pour point, viscosity index and oxidation stability which were similar to that of TMP trioleate.³⁸ Due to its high viscosity index, high thermal stability and low pour point, TMP-tri-LA ester could be considered as a potential biodegradable lubricant base stock.

3.2. Tribological properties of TMP-tri-LA

3.2.1. Coefficient of friction (COF). Fig. 6 shows the friction coefficient plotted against the sliding time for various percentages of TMP-tri-LA biolubricants. The results of the figure depict that the value of the friction coefficient ranged from 0.007 to 0.17. For ordinary mineral oil (0% ester), it can be seen that the coefficient of friction, which is 0.15 on average, is the highest within the operation time. For samples of 3%, 7%, 10%, and 20% TMP-tri-LA in mineral oil, the coefficient of friction was higher at the beginning and then fell down rapidly. 100% TMP-tri-LA kept a constantly low coefficient of friction, which was 0.09 on average, within the operation time. A considerable improvement of the friction coefficient was observed with the addition of the TMP-tri-LA ester in ordinary mineral oil. This phenomenon was consistent with Madankara’s work which claimed that the friction coefficient was reduced significantly by adding a hydroxy ester product to diesel fuel.³⁹ The TMP-tri-LA component of biolubricants formed multi and mono layers on the surface of the rubbing zone and make a stable film to prevent contact between the surfaces.

Fig. 6 Variation in the coefficient of friction with different percentages of TMP-tri-LA in ordinary mineral oil.

3.2.2. Wear scar diameter (WSD). The frictional wear results following the addition of different percentages of the TMP-tri-LA ester to mineral oil were studied using a HFRR apparatus. Microscopy images of the wear scar generated on the surface of a test sample are showed in Fig. 7 when different percentages of TMP-tri-LA in ordinary mineral oil were used as lubricant contact fluids. It is evident in Fig. 8 that the highest wear scar diameter of 300 μm was generated with 1% ester in ordinary mineral oil. The ordinary mineral oil (0% ester) has generated a wear scar diameter of 291 μm. There was a significant reduction in the wear scar diameter with higher percentages of TMP-tri-LA in ordinary mineral oil. The significance improvement in the wear scar diameter to 132 μm was found for the 100% TMP-tri-LA product. It was reported in previous research that the TMP esters of palm and palm kernel (E. guineensis) oil had resulted in wear scar diameters of 200–350 μm and 200–400 μm, respectively.⁴⁰ Compared with other biolubricants, the TMP-tri-LA ester exhibited superior lubricity performance.

Fig. 7 Microscopy images of the wear scar with different percentages of TMP-tri-LA in ordinary mineral oil.

Fig. 8 Wear scar diameters of different percentages of TMP-tri-LA in ordinary mineral oil.

A possible mechanism for the reduction of friction was that the uniform molecular structure of the TMP-tri-LA ester formed a monolayer film in the contact area with the addition of different percentages of ester to mineral oil. Basically, polyol esters are well known for providing good lubricity because of their uniform molecular structure.^41,42 Fatty acids, used to synthesize polyol esters, have longer branched chains than those in ordinary mineral oil. This kind of molecular structure allows for the formation of a good monolayer film on a sliding surface. The second possible mechanism for lubrication is the polarity of the TMP-tri-LA ester molecule contributing to improved lubricity characteristics. The molecular structure of the polyol ester is polar at one end and non-polar at the other end. The reason that friction was reduced as the percentage of ester to mineral oil increased is that molecular adsorption on the surface of the metal was enhanced. One end of the molecule was attracted strongly to the metal surface due to its polarity and the other end was extended out. This distribution of the molecules in the mixture samples produced a strong barrier to avoid direct friction between the two surfaces.^43–45 So, there is a significant reduction in the wear scar diameter with the addition of the TMP-tri-LA ester in ordinary mineral oil. This phenomenon was consistent with other results where the wear scar diameter became smaller and friction was clearly reduced when the carboxylate group in the ester and the polarity increased in ordinary mineral oil.^46,47

4. Conclusions

LA is a promising derivative derived from biomass. In this study, new kinds of levulinic acid-based polyol esters were prepared by esterification of levulinic acid with three different kinds of polyol. The crude product was distilled to obtain the target product with a purity above 95%. Among the three polyol esters obtained in this study, the TMP-tri-LA ester exhibited superior lubricant properties like a good viscosity index, low pour point, high flash point, low cloud point and copper strip corrosion, and low volatility. The biolubricity of the TMP-tri-LA ester was tested on a High Frequency Reciprocating Rig (HFRR) apparatus. The result exhibited that this kind of biolubricant had a low coefficient of friction and wear scar diameters. It suggested that this kind of polyol ester could be regarded as potential base stocks for production of lubricants with high performance.

Acknowledgements

The authors wish to express thanks for support from the National High Technology Research and Development 863 Program of China (grant no. 2013AA050702, 2012AA022304), the Fundamental Research Funds for the Central Universities (YS1407) and the Second Generation Biofuel Feedstock System of China Petroleum Funding, this project also supported by the State Key Laboratory of Motor Vehicle Biofuel Technology (KFKT2014004).

References

1. J. R. Steven, Synthetics, mineral oils, and bio-based lubricants: chemistry and technology, CRC Press Inc., Boca Raton, 2006, pp. 47–74.
2. T. F. Bünemann, M. C. Steverink-de Zoete and R. P. van Aken, Int. Colloqu. Tribol., 2000, 15.
3. K. V. Padmajaa, B. V. S. K. Raoa, R. K. Reddya, P. S. Bhaskara, A. K. Singhb and R. B. N. Prasada, Ind. Crops Prod., 2012, 35, 237.
4. P. Nagendramma, Lubr. Sci., 2011, 23, 355.
5. S. Gryglewicz, W. Piechocki and G. Gryglewicz, Bioresour. Technol., 2003, 87, 35.
6. J. A. C. Silva, A. C. Habert and D. M. G. Freire, Lubr. Sci., 2013, 25, 53.
7. J. Oh, S. Yang, C. Kim, I. Choi, J. H. Kim and H. Lee, Appl. Catal., A, 2013, 455, 164.
8. C. O. Åkerman, Y. Gaber, N. A. Ghani, M. Lämsä and R. Hatti-Kaul, J. Mol. Catal. B: Enzym., 2011, 72, 2639.
9. C. Ting and C. Chen, Measurement, 2011, 44, 1337.
10. K. A. H. Al Mahmud, N. W. M. Zulkifli, H. H. Masjuki, M. Varman, M. A. Kalam, H. M. Mobarak, A. Imran and S. A. Shahir, Procedia Eng., 2013, 68, 550.
11. E. Kleinait, V. Jaska, B. Tvaska and I. Matijosyt, J. Cleaner Prod., 2014, 15, 40.
12. M. Hajar and F. Vahabzadeh, Ind. Crops Prod., 2014, 59, 252.
13. P. K. Sripada, R. V. Sharma and A. K. Dalai, Ind. Crops Prod., 2013, 50, 95.
14. H. A. Hamid, R. Yunus, U. Rashid and T. S. Y. Choong, Chem. Eng. J., 2012, 200–202, 532.
15. E. Wang, X. Ma, S. Tang, R. Yan, Y. Wang, W. W. Riley and M. J. T. Reaney, Biomass Bioenergy, 2014, 66, 371–378.
16. H. Chen, B. Yu and S. Jin, Bioresour. Technol., 2011, 102, 3568.
17. P. Gullón, A. Romaní, C. Vila, G. Garrote and J. C. Parajó, Biofuels Bioprod. Biorefin., 2012, 6, 219.
18. B. L. Oliveira and V. T. Silva, Catal. Today, 2014, 234, 257.
19. W. S. Fitzpatrick, US Pat.,4897497, 1990.
20. J. J. Thomas and R. G. Barile, Biomass Energy Dev., [Proc. South. Biomass Energy Res. Conf.], 3rd, 1986, 333.
21. H. Ren, Y. Zhou and L. Liu, Bioresour. Technol., 2013, 129, 616.
22. G. T. Jeong, Ind. Crops Prod., 2014, 62, 77.
23. B. Chamnankid, C. Ratanatawanate and K. Faungnawakij, Biochem. Eng. J., 2014, 258, 341.
24. N. A. S. Ramli and N. A. S. Amin, Fuel Process. Technol., 2014, 28, 490.
25. S. Gryglewicz, W. Piechocki and G. Gryglewicz, Bioresour. Technol., 2003, 87, 35.
26. K. V. Padmajaa, B. V. S. K. Raoa, R. K. Reddya, P. S. Bhaskara, A. K. Singhb and R. B. N. Prasada, Ind. Crops Prod., 2012, 35, 237.
27. G. A. C. Silva, V. F. Soares, R. Fernandez-Lafuente, A. C. Habert and D. M. G. Freire, J. Mol. Catal. B: Enzym., 2015, 122, 323.
28. J. Salimon, N. Salih and E. Yousif, Arabian J. Chem., 2012, 5, 193.
29. A. S. Amarasekara, T. B. Singh, E. Larkin, M. A. Hasan and H. J. Fan, Ind. Crops Prod., 2015, 65, 546.
30. E. R. Booser, Lubrication and lubricants, 1984, vol. 3, p. 463.
31. M. Brown, J. D. Fotheringham, T. J. Hoyes, R. M. Mortier, S. T. Orszulik, S. J. Randles and P. M. Stroud, Chem. Technol. Lubr., 2009, 54.
32. S. Gryglewicz, M. Muszyński and J. Nowicki, Ind. Crops Prod., 2013, 45, 25–29.
33. K. Kamalakar, A. Rajak, R. Prasad and M. Karuna, Ind. Crops Prod., 2013, 51, 249–257.
34. T. Paeglis, P. Karabeško, I. Mieriņa, R. Seržane, M. Strēle, V. Tupureina and M. Jure, Engineering for Rural Development, 2009, vol. 28–29, p. 171.
35. L. R. Rudnick, Biobased Ind. Fluids Lubr., 2002, 20, 20–34.
36. S. Debnath, K. S. M. S. Raghavarao and B. R. Lokesh, J. Food Eng., 2011, 105, 671.
37. C. S. Madankara, A. K. Dalaia and S. N. Naik, Ind. Crops Prod., 2013, 44, 139.
38. T. A. Mckeon, B. K. Sharma, J. T. Lin, S. Z. Erhan, J. Alander and F. D. Gunstone, Lipid Handb. (3rd Ed.), 2007, 615.
39. C. S. Madankara, A. K. Dalai and S. N. Naik, Ind. Crops Prod., 2013, 44, 139–144.
40. R. Yunus, A. Fakhru'L-Razi, T. L. Ooi, R. Omar and A. Idris, Ind. Eng. Chem. Res., 2005, 44, 8178.
41. S. Boyde, S. J. Randles and P. Gibb, Lubrication at the Frontier, 1999, pp. 799–808.
42. E. S. Forbes, Tribology, 1970, 145–152.
43. N. W. M. Zulkifli, M. A. Kalam, H. H. Masjuki, K. A. H. Al Mahmud and R. Yunus, Tribol. Trans., 2014, 57, 408–415.
44. B. K. Sharma, U. Rashid, F. Anwar and S. Z. Erhan, J. Therm. Anal. Calorim., 2009, 96, 999–1008.
45. S. A. Lawal, I. A. Choudhury and Y. Nukma, Int. J. Adv. Des. Manuf. Technol., 2013, 67, 1765–1776.
46. A. Pettersson, Tribol. Int., 2003, 36, 815–820.
47. A. Adhvaryu, G. Biresaw, B. K. Sharma and S. Z. Erhan, Ind. Eng. Chem. Res., 2006, 45, 3735–3740.

---