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Article

Tribological Characteristics of Biolubricant Obtained by Transesterification of Grape Seed Oil

by
Thawan Fonseca Silva
1,
Maria Marliete Fernandes de Melo Neta
2,
Paulo Roberto Campos Flexa Ribeiro Filho
1,*,
Francisco Murilo Tavares de Luna
2 and
Célio Loureiro Cavalcante, Jr.
2,*
1
Departamento de Engenharia Mecânica, Universidade Estadual do Maranhão, São Luís 65055310, Brazil
2
Grupo de Pesquisa em Separações por Adsorção (GPSA), Departamento de Engenharia Química, Campus do Pici, Universidade Federal do Ceará, Fortaleza 60455760, Brazil
*
Authors to whom correspondence should be addressed.
Lubricants 2024, 12(12), 459; https://doi.org/10.3390/lubricants12120459
Submission received: 16 November 2024 / Revised: 16 December 2024 / Accepted: 17 December 2024 / Published: 20 December 2024
(This article belongs to the Special Issue Tribological Properties of Biolubricants)
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Abstract

Research on and the development of bio-based lubricants as alternatives to mineral-based lubricants have been encouraged worldwide owing to environmental concerns and the possible depletion of oil reserves. This study explored the use of grape seed oil (GSO), a byproduct of wine production, as a raw material for biolubricant synthesis. GSO contains a triglyceride molecule rich in unsaturated fatty acids, which is ideal for obtaining biolubricants. This study addresses the technical challenges of converting GSO into a lubricant by synthesizing methyl esters (FAME) via transesterification with 2-ethylhexanol to produce a biolubricant (BL) sample. The obtained products were characterized using Fourier-transform infrared spectroscopy and nuclear magnetic resonance spectroscopy to confirm the conversion of the molecules. The density, kinematic viscosity, and viscosity index were determined using the parameters established by ASTM. The tribological characteristics of BL were evaluated using a four-ball tribometer configuration. BL exhibited physicochemical characteristics comparable with those of an ISO VG 10 lubricant, a friction coefficient (FC) 40.82% lower than that of a hydrotreated mineral oil sample, and a smoother wear surface. These results indicate that the polarity of the ester functional group was efficient in producing a protective film on metal surfaces.

1. Introduction

Lubrication is used to reduce friction between surfaces in relative motion by adding a lubricant that forms a protective film when applied, reducing wear caused by metal-to-metal contact, and consequently extending the useful life of parts [1,2,3]. Lubricants are produced from base oils, which constitute 70–95% of the final formulation and are categorized as mineral, synthetic, or biological bases. Currently, mineral oil obtained from petroleum represents the largest portion of lubricating oils on the market. However, these reserves are nonrenewable resources and will therefore become increasingly scarce over time, driving the trend towards replacement with oils obtained from renewable sources. Additionally, as mineral oils are harmful to the environment, growing concerns regarding environmental issues have encouraged research into the development of more biodegradable and less toxic lubricants [4,5,6].
Thus, biolubricants have gained prominence because, in addition to their high biodegradability, they meet other technical requirements such as high flash points, high viscosity indices, and good lubricity [7,8,9]. As a result, the global lubricant market has grown at a compound annual growth rate of 4.2%, driven mainly by the need to serve environmentally friendly industries such as offshore wind farms, which require ecological lubricants in their transformers. In addition, technological advancements, such as the increased use of electric vehicles, have led to the adaptation of lubricant formulations to meet the specific friction reduction requirements of electric transmission systems, as well as the demand for high-performance lubricants suitable for diverse equipment in industries focused on environmental sustainability [10].
Biolubricants are obtained from vegetable oils and animal fats; therefore, they are biodegradable and less toxic to the environment. Despite their ecological advantages, biolubricants occupy approximately 2% of the global lubricant market, mainly because of the lack of incentives for the research and development of lubricants based on vegetable oils and animal fats [11]. Among vegetable oils, those that do not have food applications stand out to avoid competition with edible oils and the deforestation of large areas for planting. Priority is given to obtaining biolubricants from non-edible bases, such as waste from production processes [12,13].
Grape seed oil (GSO) is a promising lubricant base with both advantages and disadvantages. Its main benefit is that it is a winemaking byproduct, representing approximately 12% of the substrate generated during wine production. Additionally, its use promotes sustainability without competing with edible oils such as soybean and palm oils. Using this residue reduces production chain bottlenecks and adds value to materials that would otherwise be discarded [14,15]. Despite being a residue from wine production, GSO is rich in unsaturated fatty acids, with 47–60% linoleic acid and 9–17% oleic acid. It can be used as a base to produce biolubricants with improved anti-friction and anti-wear capacities by converting natural esters (triglycerides) into synthetic esters through the chemical modification of unsaturated oils. However, its production may be limited, and its application is more prone to oxidation, which may require additives to enhance oxidative stability. Conversely, oils such as palm oil, which contain higher percentages of saturated fatty acids, have greater natural oxidative stability and are better suited for large-scale production [16,17]. Thus, the use of GSO depends on factors related to its application and availability. In this regard, chemical processes for synthesizing synthetic esters with better performance are necessary, as the unsaturated bonds in natural esters impair their thermo-oxidative stability and fluidity at low temperatures, making their direct application as lubricants difficult [18,19,20].
Chemical processing has been used to improve natural oils’ chemical and physicochemical properties. Sanjurjo et al. highlighted that chemical modification via esterification, transesterification, epoxidation, oxirane ring opening, and estolide formation improve the physicochemical properties of natural oils [21]. Chemical esterification reactions occur when an alcohol reacts with a sample of free fatty acids to produce an ester, which, in turn, can react with the alcohol through transesterification, causing changes in the alkoxy group of the ester and producing biolubricants with good thermo-oxidative stability and excellent high-temperature properties [22,23]. However, the physicochemical properties of synthetic esters depend on the acids, alcohols, and base oils used in the transesterification reactions.
Some oils obtained from seeds have already been used to obtain biolubricants, including mustard seed oil, in which the lubricating ester is obtained through a two-step alkaline transesterification process. The samples are reacted with ethanol to form ethyl esters and then with 2-ethylhexanol, thus achieving promising results in terms of physical properties such as density, kinematic viscosity, and cold flow [24]. Regarding tribological evaluations, biolubricants have been evaluated mainly using four-ball tests, which present several advantages, including the ability to simulate specific pressure and temperature conditions common in real lubrication applications, as well as a lower cost compared to other tribological evaluation methods [25]. In the four-ball test, the anti-wear and anti-friction capacities are evaluated by measuring the wear scar diameter (WSD) and coefficient of friction (FC) [26]. In general, biolubricants obtained through transesterification exhibit promising anti-wear and anti-friction characteristics, indicating that a greater amount of ester results in less wear and a lower FC. However, the physicochemical properties of the developed products affect their tribological behavior depending on the type of base used to obtain the biolubricants [27,28,29].
While previous research demonstrates improvements in the lubrication properties of samples produced via transesterification, there are no studies in the literature regarding the tribological characteristics of biolubricants obtained from GSO. Therefore, in this study, GSO, a non-edible vegetable base, was used to obtain a biolubricant sample. Methyl esters were produced from GSO through a transesterification reaction and then reacted with 2-ethylhexanol to produce biolubricants. Subsequently, the chemical and physicochemical characteristics of the biolubricants were determined, and their tribological behavior was investigated using a four-ball tribometer.

2. Materials and Methods

2.1. Materials

GSO from the vitae Vitis vinifera was purchased from Mundo dos Óleos (Brazil). The samples comprised a mixture of linoleic acid (58–78% by weight), oleic acid (12–28% by weight), palmitic acid (5.5–11% by weight), and stearic acid (3–6% by weight). The reagents methanol (CH3OH), potassium hydroxide (KOH), and 2-ethylhexanol used in the synthesis to obtain FAME (GSO-based methyl esters) and BL (Biolubricant obtained by transesterification of FAME) were purchased from Sigma-Aldrich (USA). Glycerol (25% by weight) used in the reactions was recycled. The balls used in the tribological tests are made of chromium alloy steel (AISI52100) with a hardness of 64 HRC, a diameter of 12.7 mm, and an initial surface roughness of 0.015 μm. Samples of hydrotreated mineral oil (HMO), derived from a naphthenic base, were obtained from Petrobras (Brazil). Table 1 summarizes the key physicochemical properties of the HMO.

2.2. Synthetic Procedure

The products were obtained following a methodology adapted from [24]. FAME was synthesized using GSO (triglyceride), methanol, and recycled glycerol, with KOH used as the catalyst (Figure 1).
The reaction occurred under reflux (Figure 2) at 35 °C and atmospheric pressure, with a methanol/oil molar ratio of 8:1 and 1.1% by weight of KOH, for 50 min. After 30 min, recycled glycerol (25% by weight) was added, followed by vigorous stirring for 5 min, cooling to room temperature, and stirring at 250 rpm. Excess methanol from the reaction mixture was removed using a rotary evaporator at 70 °C and 180 mbar for 1.5 h. Esters obtained were not purified, except for biodiesel, where drying with adsorbent was necessary.
Transesterification was performed to obtain the biolubricant sample (BL) using FAME, 2-ethylhexanol, and KOH (catalyst) (Figure 3). The reaction occurred at 70 °C under vacuum, with an alcohol/oil molar ratio of 6:1, using 2% by weight of KOH as the catalyst. The chemical reactions were conducted for 65 min.

2.3. Physicochemical Characterization

Fourier-transform infrared spectroscopy (FTIR) was used to identify the functional groups and molecular changes caused by the chemical reactions to obtain FAME and BL. Shimadzu IRTracer-100 (Shimadzu, Kyoto, Japan) equipment was used in the range of 400–4000 cm−1 with a resolution of 4 cm−1, 32 scans, and potassium bromide (KBr) tablets. The tablets were prepared by pressing at a force of 8 kN.
The chemically synthesized samples were analyzed using one-dimensional proton nuclear magnetic resonance spectroscopy (1H NMR; Bruker Avance DRX-500, Billerica, MA, USA) at 500 MHz with deuterated chloroform as the solvent.
Density at 20 °C and kinematic viscosity at 40 °C and 100 °C of all samples were determined following [31,36] methods, respectively, in an Anton Paar SVM 3000 instrument (Graz, Austria). The method yields a viscosity index (VI) [32].

2.4. Tribological Evaluation

Tribological evaluation was conducted in a steady state, where the loading force, sliding speed, and temperature remained constant throughout the test. The tests were conducted in triplicate according to the methodology of Ruggiero et al. (2019) [37]. The FAME and BL samples were tested using a 4-ball tribology accessory (Figure 4) coupled to a DHR-3 rheometer (TA Instruments, New Castle, DE, USA). This accessory enables the measurement of the coefficient of friction (FC) between two solid surfaces under both dry and lubricated conditions. Its design ensures consistent solid–solid contact and even axial force distribution, enabling precise control of the rotational speed and temperature across a broad range of friction measurements. The FC in the four-ball experiment is calculated using the following equation:
F C = T 6 3 r W ,
where FC = coefficient of friction (dimensionless), T = frictional torque (Nmm), r = distance from center of contact surface on lower balls to axis of rotation (mm), and W = applied load (N). The value of r was determined to be 3.67 mm in most studies.
Data were collected using TRIOS software version 5.8 with monitored variables, including the FC, load force, and friction force. The details of the tribological tester used are described in [38]. The test involved the application of a loading force under a rotating ball against three fixed balls on a mandrel bathed in the lubricant to be evaluated. The parameters used in the test were a duration of 1 h, a loading force of 55 N, a temperature of 75 °C, and a sliding speed of 460 mm/s. Prior to the tests, the balls were cleaned with acetone and dried at room temperature. The wear scar diameters (WSDs) and wear morphologies of the balls were evaluated using an optical microscope (Zeiss, Oberkochen, Germany). New balls were used in each test. The HMO sample was used as a reference for evaluating the biolubricants. FC and WSD were measured during the test. The arithmetic means of the FC and WSD values were calculated for comparison. For the WSD values, the OriginPro software (version 8.5) was used for statistical evaluation, with the statistical significance level set at 5% (p < 0.05).

3. Results and Discussion

Physicochemical Characterization

Table 2 lists the density, kinematic viscosity at 40 °C and 100 °C, and viscosity index (VI) values of grape seed oil (GSO), methyl ester (FAME), and biolubricant (BL). The densities of FAME and BL varied little compared to the GSO sample and were within the acceptable range for lubricants (0.7–0.95 g/cm3) [39,40,41,42]. Density is a key property used to ensure the quality standard of a lubricant and is also widely used to evaluate lubricant contamination during operations. An increase in the density of a lubricant during operation may indicate contamination with water-insoluble or higher-density products. Conversely, a decrease in density may indicate contamination with lower-density products and fuels [43]. The kinematic viscosity of the lubricant is a fundamental property as it determines the most suitable mechanical system for its application [44]. This is because low-viscosity lubricants are more suitable for systems that require greater fluidity and thinner lubricating films, while high-viscosity lubricants are recommended for applications where a thicker lubricating film is required to generate greater protection for the tribological pair. The products obtained, FAME and BL, presented kinematic viscosities of 8.930 and 9.473, respectively, at 40 °C. The ISO standard, which classifies industrial lubricants using the acronym VG (viscosity grade) followed by the number representing the kinematic viscosity at 40 °C, classifies the BL sample as ISO VG 10, a lubricant grade typically used in machine tool spindles and some pneumatic tools [45]. The viscosity index (VI) measures the influence of temperature changes on viscosity; the higher the VI, the smaller the viscosity variation. VI values for the FAME and BL samples were 206 and 164.4, respectively. According to API 1509, BL is classified as Group V (synthetic esters) [46]. The high VI of the BL (168) sample indicates that the sample tends to have high oxidative stability because its viscosity becomes more stable as the operating temperature varies [47,48].
The FTIR spectra of GSO, FAME, and BL are shown in Figure 5. The FTIR spectrum of GSO shows bands with greater intensity in the region between 3050 and 2800 cm−1, corresponding to the axial deformation vibrations of the C-H bonds in methyl (CH3) and methylene (CH2) groups and double bonds (=C-H). The band at approximately 1741 cm−1 corresponds to the axial deformation vibrations of the carbonyl group (C=O) that constitute the ester groups in the GSO triglyceride molecules. The GSO bands between 1380 and 1080 cm−1 are attributed to the axial deformation vibrations of the C-O bonds in the ester groups of triglyceride molecules [49]. The FAME spectrum displayed stretching vibration bands for CH3, CH2, and CH at 2853, 2928, and 3008 cm−1, respectively. The high-intensity bands at 1436 and 1731 cm−1 can be attributed to carbonyl stretching vibrations (-C=O), whereas the band at 1168 cm−1 corresponds to the antisymmetric and asymmetric axial stretching vibrations of the C-O bond [50]. Finally, the BL spectrum presented a band at approximately 1736 cm−1, which corresponded to esters resulting from transesterification. The absence of a broad and prominent band at approximately 3500 cm−1 indicates the absence of hydroxyl groups in the chemical structure of the sample, confirming the conversion of -OH groups and the absence of excess 2-ethylhexanol [51].
The 1H NMR spectra of the GSO, FAME, and BL samples are presented in Figure 6. The peaks (I) in the GSO spectrum between 5.2 and 5.5 ppm indicate the unsaturation of the fatty acid chain of the triglyceride molecule. The FAME spectrum shows a methoxide proton peak (II) at 3.67 ppm, confirming the transesterification reaction of GSO. This peak is absent in the GSO spectrum. Meanwhile, peak (III) represents the unsaturation of the FAME fatty acid chain [49,50,51,52]. The formation of BL molecules can be confirmed by the presence of a peak (IV) at 4.02 ppm in the BL spectrum, which is absent in the FAME spectrum. This peak confirms the substitution of the hydroxyl group (-OH) by the fatty acid chain [49,53,54]. As observed in the FAME and BL spectra, a peak (V) at approximately 5.3 ppm was observed, which indicates the unsaturation of the fatty acid chain of GSO.
The FC and WSD values of FAME, BL, and HMO are shown in Figure 5. FAME displayed lower mean FC (0.01) and WSD (274.75 µm) values than BL (0.05 and 286.1 µm). The wear scar data were statistically analyzed to evaluate the differences between the results. The ANOVA test indicated a significant variance (F (2, 6) = 80.62; p < 0.0001). Tukey’s test demonstrated significant differences between HMO and BL and between HMO and FAME and demonstrated non-significant differences between BL and FAME (Figure 7).
In this work, we used balls with the same roughness pattern to isolate the lubricant behavior. The results show that the lubricating film of FAME and BL, due to the presence of ester (polar) functional groups, presents greater chemical affinity with the metallic surfaces of the balls, forming an adherent tribofilm. This ability of FAME and BL reduces direct contact between asperities, which increases the uniformity of the real contact area (RCA). At the same time, in balls lubricated with HMO (nonpolar), the tribofilm depends only on rheological properties such as viscosity, which results in a lower RCA due to the more localized and direct interaction between the asperities, but it favors the emergence of abrasion and adhesion wear mechanisms [55,56].
The polarity of biolubricants affects their interaction with metal surfaces; the greater the polarity of a molecule, the better its anti-friction and anti-wear properties [26]. An analysis of the samples in this study suggests that the methyl alcohol group present in FAME increases the polarity of the molecule because it is less hydrophobic than the 2-ethylhexyl group in BL. The longer 2-ethylhexyl chain in BL creates a more hydrophobic region, diminishing the effect of its polar ester group. Although BL contains a polar ester group, its impact is reduced by the extended nonpolar chain, leading to weaker interactions with metal surfaces. This results in a higher FC and an equivalent level of protection against wear compared with FAME [26,57,58,59,60].
The insignificant difference between BL and FAME can be attributed to the reduced hydrophobicity of FAME, which balances its molecular polarity and results in a comparable performance between BL and FAME.
Subsequently, the results of bio-based samples (FAME and BL) were compared with those of HMO, which has a mean FC of 0.08 and a mean WSD of 195 µm. Compared to HMO, FAME and BL exhibited superior anti-friction behavior. The ester functional groups in both molecules provide a sufficient adhesive force to enhance the sliding of the tribofilm [26,61,62,63]. However, the anti-wear capacities of FAME and BL were lower than that of HMO. This is because with increased energy at the contact interface, the carbon–carbon double bonds in FAME and BL tend to break, reducing the number of carbon atoms in the molecules and resulting in a thinner film that is less capable of separating the metal surfaces of the balls [64,65,66].
HMO, a predominantly nonpolar molecule without unsaturation, forms a more stable tribofilm with a greater ability to separate tribological pairs, thus presenting the lowest WSD values observed [67].
The wear morphologies of the balls lubricated with FAME, BL, and HMO are listed in Table 3. The observed wear mechanisms vary in extent and severity depending on the lubricant type. In FAME and BL, the formation of a polar film reduces wear severity by minimizing direct surface contact, but it increases wear extent due to more uniform stress distribution, resulting in larger wear scar diameters (WSDs) and smoother surfaces. Conversely, HMO, lacking chemical affinity, produces more localized wear with lesser extent but greater severity at direct contact areas, evidenced by smaller yet rougher wear scars associated with abrasive and adhesive mechanisms. Specifically, HMO-lubricated balls exhibited deeper grooves, micro-cuts, and irregular surfaces, indicating abrasion and adhesion wear [64,68,69]. In contrast, FAME- and BL-lubricated balls showed smoother surfaces with regular edges and fewer deep grooves, characteristic of abrasion wear, with BL exhibiting smoother surfaces than FAME [64,70,71,72].
The lubrication mechanisms of FAME, BL, and HMO are illustrated in Figure 8. Figure 8a shows that FAME and BL contain an ester group (RCOOR) in their molecular structures, which has a high affinity for the metal surface of the balls; thus, this functional group adheres easily to the metal surface, forming a friction-reducing layer [72,73]. The RCOOR groups in FAME and BL (bio-based lubricants) provide sufficient adhesiveness to form a monolayer with improved anti-friction capacity. As a result, FAME and BL displayed FC values of 0.01 and 0.05, respectively, which are lower than that observed for HMO (0.08). However, when evaluating the anti-wear capacity of the bio-based samples, the WSD values for FAME and BL were 274.75 µm and 286.1 µm, respectively. These results indicate that the RCOOR group does not improve their anti-wear capacity owing to its moderate adhesiveness and polarity [57,74].
Furthermore, as shown in Figure 8a, the carbon chain of the fatty acids initially forms a protective barrier for the balls. However, as the energy in the contact region increases, as shown in Figure 8b, the unsaturated bonds between the carbon atoms become more susceptible to rupture. This reduces the length of hydrocarbon chains in fatty acids, consequently decreasing the protection provided by the lubricating film of FAME and BL against wear. This results in higher WSD values in FAME and BL but with apparently shallower depths and smoother aspects.
As shown in Figure 8c, the lubricating film formed by HMO does not easily orient itself to the surface of the spheres, unlike FAME and BL. It has less affinity for the surfaces of the balls because HMO predominantly contains nonpolar hydrocarbon molecules. Consequently, HMO exhibits the highest FC. However, due to the absence of unsaturation in its hydrocarbon chain, it presents the lowest WSD, indicating its high resistance to the shear forces that generate wear [75].

4. Conclusions

In this study, GSO, a byproduct of wine production, was used to prepare biolubricants. First, methyl esters (FAME) were obtained from GSO. Subsequently, lubricating esters were obtained by reacting FAME with 2-ethylhexanol through a transesterification reaction. The density, kinematic viscosity at 40 °C and 100 °C, and viscosity index of the obtained products were determined, highlighting that the BL sample could be classified as ISO VG 10, with a viscosity index compatible with group V of the API classification (synthetic esters). The FTIR and NMR spectroscopy results prove that the triglyceride molecules of GSO were first converted into FAME and subsequently into a biolubricant (BL) using 2-ethylhexanol to obtain branching. The tribological results demonstrate that the bio-based samples (FAME and BL) were capable of forming a lubricating film with improved adhesive capacity, resulting in an anti-friction behavior superior to that observed using a mineral lubricant (HMO). However, the anti-wear capacity is impaired by the presence of double bonds between carbons, which are fragile when the energy in the contact region increases. This leads to the rupture of the lubricant molecule and involuntary reduction in the carbon chain, leading to the formation of larger wear scar diameters (WSDs). In addition, the balls lubricated with FAME and BL exhibited abrasion wear characteristics, whereas those lubricated with HMO exhibited abrasion and adhesion wear characteristics. The sketches of the wear mechanisms proposed in this study show that the bio-based samples (FAME and BL) have a greater affinity for the metallic surfaces of the balls owing to the presence of polar ester functional groups (RCOOR) in their molecular structures. However, the carbon chain, which is the nonpolar region of these molecules, is very vulnerable to an increase in the intensity of the operational conditions. This indicates that the reduction in unsaturation by other chemical processes can contribute to improving the anti-wear capacity of FAME and BL. The results obtained for the lubricants derived from GSO are promising and encourage the development of new molecules from other chemical processes to expand our knowledge of their physicochemical and tribological properties. This study contributes to the literature on the tribological behavior of lubricants derived from a vegetable base not used in food, thereby expanding the range of raw materials available for biolubricant production. However, further research is necessary because this study was limited to obtaining biolubricants through a synthetic process, so it does not represent all potential methods for obtaining biolubricants from GSO. Additionally, tests under operational conditions are required to validate the obtained results.

Author Contributions

Conceptualization, T.F.S., M.M.F.d.M.N. and P.R.C.F.R.F.; methodology, T.F.S., M.M.F.d.M.N. and P.R.C.F.R.F.; validation, T.F.S., M.M.F.d.M.N. and P.R.C.F.R.F.; formal analysis, P.R.C.F.R.F., F.M.T.d.L. and C.L.C.J.; investigation, P.R.C.F.R.F.; resources P.R.C.F.R.F., F.M.T.d.L. and C.L.C.J.; data curation, P.R.C.F.R.F.; writing—original draft preparation, P.R.C.F.R.F., F.M.T.d.L. and C.L.C.J.; writing—review and editing, P.R.C.F.R.F., F.M.T.d.L. and C.L.C.J.; supervision, P.R.C.F.R.F., F.M.T.d.L. and C.L.C.J.; project administration, P.R.C.F.R.F., F.M.T.d.L. and C.L.C.J.; funding acquisition, P.R.C.F.R.F., F.M.T.d.L. and C.L.C.J. All authors have read and agreed to the published version of the manuscript.

Funding

We thank the CNPq (Conselho Nacional de Pesquisa e Desenvolvimento Científico) Project: 404427/2023-5 (Campos Flexa Ribeiro Filho, P.R.).

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.

Acknowledgments

We thank CNPq (Conselho Nacional de Pesquisa e Desenvolvimento Científico) for providing financial support.

Conflicts of Interest

The authors declare no conflicts of interest.

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Lubricants 12 00459 g001
Figure 1. Synthetic route used to obtain methyl esters (FAME) from grape seed oil (GSO).
Figure 1. Synthetic route used to obtain methyl esters (FAME) from grape seed oil (GSO).
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Figure 2. Experimental setup for synthesis of bio-based lubricants.
Figure 2. Experimental setup for synthesis of bio-based lubricants.
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Figure 3. Synthetic route to obtain biolubricants (BL) from transesterification of FAME.
Figure 3. Synthetic route to obtain biolubricants (BL) from transesterification of FAME.
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Figure 4. Four-ball test configuration.
Figure 4. Four-ball test configuration.
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Figure 5. FTIR spectra of GSO, FAME, and BL samples.
Figure 5. FTIR spectra of GSO, FAME, and BL samples.
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Figure 6. 1H NMR spectra of (a) GSO, (b) FAME, and (c) BL samples.
Figure 6. 1H NMR spectra of (a) GSO, (b) FAME, and (c) BL samples.
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Figure 7. FC and WSD values of FAME, BL, and HMO after tribological test. p < 0.05. Different letters indicate significant differences between WSDs.
Figure 7. FC and WSD values of FAME, BL, and HMO after tribological test. p < 0.05. Different letters indicate significant differences between WSDs.
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Figure 8. Sketches of the lubrication mechanisms of FAME, BL, and HMO at the friction interface. (a) The initial formation of the lubricating film of the bio-based samples (FAME and BL). (b) The rupture of unsaturated bonds of the bio-based samples (FAME and BL) and (c) HMO lubricating film.
Figure 8. Sketches of the lubrication mechanisms of FAME, BL, and HMO at the friction interface. (a) The initial formation of the lubricating film of the bio-based samples (FAME and BL). (b) The rupture of unsaturated bonds of the bio-based samples (FAME and BL) and (c) HMO lubricating film.
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Table 1. Physicochemical properties of hydrotreated mineral oil sample (HMO).
Table 1. Physicochemical properties of hydrotreated mineral oil sample (HMO).
PropertiesResultsMethods
Density at 20 °C (g/cm3)0.892[30]
Viscosity at 40 °C (cSt)10.1[31]
Viscosity at 100 °C (cSt)2.38[31]
Viscosity Index22[32]
Pour point (°C)−42[33]
Flash point (°C)162[34]
Total acid number (mg KOH/g)0.01[35]
Table 2. Physicochemical properties of GSO, FAME, and BL samples.
Table 2. Physicochemical properties of GSO, FAME, and BL samples.
PropertiesGSOFAMEBLMethods
Density at 20 °C (g/cm3)0.92900.89950.8846[36]
Viscosity at 40 °C (cSt)49.78.99.4[31]
Viscosity at 100 °C (cSt)9.912.912.86[31]
Viscosity Index191206168[32]
Table 3. Wear morphologies of balls lubricated with FAME, BL, and HMO after tribological tests.
Table 3. Wear morphologies of balls lubricated with FAME, BL, and HMO after tribological tests.
1st Ball2nd Ball3rd Ball
FAMELubricants 12 00459 i001
277.95 µm		Lubricants 12 00459 i002
268.62 µm		Lubricants 12 00459 i003
277.69 µm
BLLubricants 12 00459 i004
287.42 µm		Lubricants 12 00459 i005
281.37 µm		Lubricants 12 00459 i006
289.52 µm
HMOLubricants 12 00459 i007
178 µm		Lubricants 12 00459 i008
207 µm		Lubricants 12 00459 i009
200 µm
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Silva, T.F.; Melo Neta, M.M.F.d.; Ribeiro Filho, P.R.C.F.; Luna, F.M.T.d.; Cavalcante, C.L., Jr. Tribological Characteristics of Biolubricant Obtained by Transesterification of Grape Seed Oil. Lubricants 2024, 12, 459. https://doi.org/10.3390/lubricants12120459

AMA Style

Silva TF, Melo Neta MMFd, Ribeiro Filho PRCF, Luna FMTd, Cavalcante CL Jr. Tribological Characteristics of Biolubricant Obtained by Transesterification of Grape Seed Oil. Lubricants. 2024; 12(12):459. https://doi.org/10.3390/lubricants12120459

Chicago/Turabian Style

Silva, Thawan Fonseca, Maria Marliete Fernandes de Melo Neta, Paulo Roberto Campos Flexa Ribeiro Filho, Francisco Murilo Tavares de Luna, and Célio Loureiro Cavalcante, Jr. 2024. "Tribological Characteristics of Biolubricant Obtained by Transesterification of Grape Seed Oil" Lubricants 12, no. 12: 459. https://doi.org/10.3390/lubricants12120459

APA Style

Silva, T. F., Melo Neta, M. M. F. d., Ribeiro Filho, P. R. C. F., Luna, F. M. T. d., & Cavalcante, C. L., Jr. (2024). Tribological Characteristics of Biolubricant Obtained by Transesterification of Grape Seed Oil. Lubricants, 12(12), 459. https://doi.org/10.3390/lubricants12120459

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