Deep Eutectic Solvents as Green and Novel Lubricant Additives for Castor Oil with High Tribological Performance

Abstract

This research reveals the anti-friction and anti-wear performance of lubricants using a castor oil base and a deep eutectic solvent (DES1) as an additive. To this end, DES1 was synthesized in a successful manner using DL-menthol and dodecanoic acid as components. Mass concentrations from 0.1 wt% up to 5 wt% of DES1 additives were chosen to formulate the lubricants. Friction experiments were conducted, yielding friction enhancements up to 4% compared to the castor oil base. Notably the greatest reduction was achieved for the lubricant with 0.1 wt% of DES1. In terms of the wear generated, the best anti-wear performance was achieved for the 0.5 wt% DES1 lubricant (with a wear reduction of 17%). Furthermore, by means of the profilometry of worn surfaces, it can be observed that the tribofilm formation of DES1 on steel surfaces is a potential lubrication mechanism.

1. Introduction

In the contemporary context, energy conservation has emerged as a pivotal concern, and friction and wear are the predominant sources of energy dissipation in mechanical components. The implementation of lubrication techniques has been demonstrated to mitigate these losses. It is noteworthy that approximately 90% of the components in earthmoving machinery are lubricated [1], with mineral lubricants comprising approximately 95% of the utilized lubricants [2]. However, it is crucial to acknowledge that this category of lubricants is associated with numerous challenges and the potential for hazardous implications. The mineral lubricants in question are derived from crude oil via a conventional purifying process and are formed by a multitude of chemical elements, which renders them greatly ecotoxic and weakly biodegradable. Consequently, their release into the environment constitutes a considerable threat [3]. Considering these concerns, environmentally friendly lubricants have witnessed a marked increase in their acceptance for mechanized applications. Evidence suggests that bio-lubricants represent a promising alternative to mineral lubricants, as they can circumvent the harmful effects of the latter. In addition to their environmental benefits, bio-lubricants have been demonstrated to be cost-effective and non-toxic. The biodegradable nature of vegetable oils renders them particularly well-suited as base oils for bio-lubricants. Vegetable oils are generally considered to be amphiphilic due to their long fatty acid and natural ester chains, which provide excellent lubricating properties by interacting with metals and polar functional groups. Notwithstanding the lubrication properties of vegetable oils, they necessitate the incorporation of specific additives, such as anti-wear additives, to enhance their lubrication characteristics. In this situation, nanomaterials, ionic liquids (ILs), and organometallic and organic compounds were examined as potential additives to lubricants with the objective of minimizing friction and wear [4,5,6]. Nevertheless, with the mounting global environmental consciousness, the difficult synthesis procedures, elevated economic price of precursors, difficult purification procedures, elevated toxicity of ILs, and huge problems with the sedimentation of NPs are significant impediments and make difficult their possible lubricant applications [7].

Thus, deep eutectic solvents (DESs) have been identified as a promising solution, serving as a green and economical alternative to ILs. These compounds are formed by hydrogen bond donors (HBDs) and hydrogen bond acceptors (HBAs), resulting in a complexation that exhibits insignificant volatility. This characteristic is crucial for ensuring environmental and economic viability, facilitating the simplicity of preparation, and demonstrating outstanding attraction towards engineering surfaces. Furthermore, DESs are distinguished by tunable physicochemical properties [8,9], which serves to further enhance their appeal as hopeful applicants for novel lubricants and lubricant additives. Choline chloride (ChCl)-based DESs were initially documented as lubricants, exhibiting diminished friction in steel–steel contacts in comparison to traditional mineral oils [10]. Furthermore, Li et al. [11] synthetized different natural deep eutectic solvents (NADESs) and compared their tribological performance with G1860 synthetic base oil, finding that the friction and wear rate of the greatest NADES are only 70% and 54% of the commercial G13830 ester oil. Nevertheless, the extensive utilization of DESs as lubricants may not always be economically viable for particular applications due to cost implications. However, the utilization of these substances as additives to base oils has yielded encouraging outcomes, although their development is still at its infancy stage. Hence, Li et al. [12] incorporated 0.5 wt% of an oil-miscible DES (DL-menthol and dodecanoic acid) into PEG 200 and PAO 40 observing friction and wear reductions up to 40% and 60%, respectively, attributable to the adsorption of DES molecules onto friction surfaces, forming a protective film. In a similar way, Khan et al. [13] studied the tribological performance of SN150 base oil using aminoguanidine salt-based DESs, finding friction reductions of 30% and wear volume reductions up to 90%. Furthermore, Li et al. [14] studied the tribological response of a Mobil DTE base oil additivated with a blend of DES/MXene at different mass concentrations, revealing great reductions in the friction and wear rate, by 47 and 57% for 0.1 wt% DES/MXene. Finally, Nagendramma et al. [15] synthesized a novel polyol-based deep eutectic solvent (PDES) and they studied its tribological response as additive for cotton seed oil. These authors observed maximum reductions in friction and wear around 56% and 89%, respectively, at a 5 wt% dose of the additive. Considering the promising performance of DESs as oil additives and the insufficient experimental works in the literature, it is evident that further systematic studies are needed in order to justify and enhance DES-based formulations, seeing also the novel idea of combining biodegradable vegetable oils with DESs as additives and taking into account the scarcity of research of this nature. Consequently, the present study successfully synthesized a DES consisting of green DL-menthol and dodecanoic acid by a straightforward one-step method. The prepared DES (DES1) was tribologically characterized as an additive for a vegetable base oil, castor oil, observing good lubrication performance with friction and wear reductions in comparison to the base oil.

2. Materials and Methods

2.1. Base Oil and Additives

The vegetable base oil used in this research, castor oil, was provided by Merck (Darmstadt, Germany). Castor oil has a viscosity index of 92, a dynamic viscosity of 236.6 mPa s, and a density of 0.9462 g·cm⁻³ at 313.15 K [5]. Regarding lubricant additives, the DES1 was synthesized using the following reagents: DL-menthol (98% purity) and dodecanoic acid (99% purity), both acquired from Merck (Darmstadt, Germany).

2.2. Lubricant Formulation

To prepare the DES1 additives, the procedure carried out by Li et al. [12] was followed. Thus, DL-menthol and dodecanoic acid were mixed and stirred in a molar ratio of 2:1, heating at 70 °C for 2 h and obtaining a homogeneous and transparent liquid, the desired DES1 (Figure 1a). Before the synthesis, both reagents were solid in powder form. It should be noted that different conditions of molar ratio (1:2, 1:1, and 2:1), temperature (50, 60, and 70 °C) and time (2 h, 3 h, and 4 h) were tested to obtain DES1 in liquid form, since sometimes this DES appears as a white solid after the synthesis procedure. The molar ratio 2:1 was previously also utilized by Cañadas et al. [16]. After the synthesis, the DES1 was cooled to 20 °C, maintaining a transparent liquid appearance. The castor base oil was used as base lubricant and the synthesized DES1 additives were added into castor base oil with weight concentrations of 0.1 wt%, 0.5 wt%, 1 wt%, and 5 wt%. These mass concentrations were selected based on previous studies of DESs as lubricant additives [12,14]. Regarding the visual appearance of lubricants and based on visual observation, the formulated DES1 lubricants have a transparent appearance with the same color as the castor oil base (Figure 1b).

2.3. Tribological Tests: Pure Sliding

Friction tests were conducted under conditions of pure sliding with an MTL/10/AT tribometer from MICROTEST (Madrid, Spain). In this research, a pin-on-disk test configuration was utilized, wherein the ball is placed on a fixed ball holder shaft pressed against a steel plate in reciprocating motion. Therefore, chrome steel balls AISI 52100/535A99 (diameter: 6 mm; roughness < 0.05 μm) were run against AISI304 rectangular stainless-steel plates (surface finish < 0.02 μm Ra). It is imperative to acknowledge that a preliminary step was undertaken prior to commencing the friction test. This entailed the meticulous cleaning of both balls and plates using acetone. Then, between the higher specimen (ball) and the lower specimen (plate) four drops of the tested lubricant were set to lubricate both components during the tribological test. Friction experiments were run under a load of 5 N (a 1.5 GPa contact pressure), with a sliding speed of 0.10 m·s⁻¹ and a sliding distance of 350 m. Furthermore, three replications were made for each lubricant sample to ensure good reproducibility, as is common in this type of tribological test [12,17,18]. Subsequent to the tribological tests, the wear produced on the plates was analyzed using a Nikon Eclipse L150 microscope determining the wear track width (WTW) and the wear track depth (WTD) with a 20× objective. These parameters were determined in four different parts of the worn tracks to ensure good reproducibility.

3. Results

3.1. Characterization of Lubricant Components: Base Oil and Deep Eutectic Solvent

The castor oil base and deep eutectic solvent DES1 were characterized through infrared spectroscopy, FTIR, (VARIAN 670-IR spectrometer from Agilent, Santa Clara, CA, USA) to observe the main functional groups and in the case of DES1, also that the synthesis was performed succesfully.

Thus, Figure 2 exposes the castor oil IR spectrum with adsorption peaks at 3500 cm^–1 that correspond to the -O-H stretching vibration. A minor stretching vibration peak at 3015 cm^–1 is associated with the double -C=C bond. Also, deformation vibrations of -CH₃ happen at about 1460 cm^–1 and 1370 cm^–1. Likewise, two intense peaks at 2935 cm^–1 and 2845 cm^–1 are linked to the stretching vibrations (symmetric and asymmetric) of CH₂, correspondingly. Additionally, an intense and thin band at 1745 cm^–1 is connected to the double -C=O bond of esters. Moreover, the bands at 1245 cm^–1 and 1165 cm^–1 are correlated with the stretching vibration for the -C-O of the ester group. In addition, a little peak at 850 cm^–1 is assigned to the bending vibration of =C-H and the strong band at 720 cm^–1 is typical of -CH for deformation vibration [19,20].

Concerning the FTIR spectrum of DES1 and their individual components, Figure 3 displays the following characteristic peaks: the peaks observed at 1390, 1270, 920, and 710 cm⁻¹ are ascribed to the stretching/bending vibrations of the O-H, C-H, C=O, and C-O groups of dodecanoic acid, repectively [21]. Concerning the typical DL-menthol peaks, the spectrum show peaks around 1365 cm⁻¹ (methyl groups), 990–1230 cm⁻¹ (C-O stretching vibration), 1020–1050 cm⁻¹ (C-O stretching vibrations), 850–980 cm⁻¹ (hexagonal ring), and at 495–670 cm⁻¹ (skeletal mode vibration) [22]. Moreover, the distinctive peak of dodecanoic acid at 1700 cm⁻¹ corresponds to the carbonyl (C=O) stretching vibration, in this DES1 spectrum this is shifted to 1715 cm⁻¹ [23]. Furthermore, the typical OH- group of DL-menthol at 3265 cm⁻¹ is shifted to 3400 cm⁻¹ in the DES1 spectrum [24]. These alterations denote hydrogen bonding connections among DL-menthol and dodecanoic acid, proving the effective synthesis of DES1.

3.2. Friction and Wear Results in Tribological Tests

The average friction coefficients that were empirically attained for the castor oil base and the additivated DES1 lubricants are exposed in Figure 4 and

Table 1. Friction coefficients of μ and parameters of wear—width (WTW), depth (WTD), and volume—with their standard deviations for the tested lubricants based on castor oil.

Lubricant	COF	σ	WTW/μm	σ/μm	WTD/μm	σ/μm	Volume/103 μm3	σ/103 μm3
Castor Oil	0.1497	0.0035	687.6	16	60.5	3.5	207,999	20
+0.1 wt% DES1	0.1434	0.0018	633.2	12	58.6	2.3	182,559	14
+0.5 wt% DES1	0.1495	0.0019	572.2	11	47.5	2.4	133,451	11
+1 wt% DES1	0.1481	0.0033	621.1	13	59.2	2.8	183,110	15
+5 wt% DES1	0.1485	0.0037	638.3	14	59.6	1.8	188,539	17

. As can be observed, for all the DES1 lubricants the obtained friction coefficientes are slightly enhanced compared to the castor oil base. Specifically, the best antifriction performance was detected for the castor oil + 0.1 wt% DES1 lubricant with an average friction coefficient of 0.1434, while the one of the castor oil base is 0.1497, which involves a friction reduction of 4.2%. Consequently, the incorporation of a deep eutectic solvent (DES1) as an additive to the castor oil base yields an antifriction capability that ensures superior tribological performance in comparison to the base oil devoid of additives. The aforementioned phenomenon can be attributed to the polar functionalities of DES1, which facilitate lubrication and the interactions with steel contact interfaces, thereby enhancing the tribological performance of castor oil [25].

Concering the wear generated in the plates (lower specimen) after friction tests, Figure 5 and Table 1 reveal the wear track width values obtained for all the tested lubricants, observing a better antiwear performance for DES1 lubricants in comparison to non-additivated castor oil. It is noteworthy that the castor oil + 0.5 wt% DES1 lubricant exhibited the most significant antiwear performance (Table 1). This lubricant achieved reductions up to 17% in width, 21% in depth, and 36% in worn volume, compared to the castor oil base. Therefore, it has been determined that the incorporation of a deep eutectic solvent into a castor oil base results in the substantial enhancement of its antiwear performance. This enhancement has been observed to range from 7 to 17% in terms of WTW, thereby demonstrating a clear improvement in wear resistance. In consideration of the impact of mass concentration of DES1 additives, it is observed that the greatest antiwear performance was obtained for an intermediate concentration, whereas for the lowest and highest mass concetrations, the wear improvements were somewhat lower (8 and 7%, respectively, for WTW). The fact that, at low concentrations of DES1, the tribological performance is a bit worse could be due to there not being enough additive to protect the steel surface. This behavior was also observed by other authors for low mass concentrations of DESs as lubricant additives [12].

Additionally, in order to better characterize the wear produced during friction tests, the worn volumes were calculated using the ASTM G133-05, which involves laboratory procedures for calculating the sliding wear in a linear reciprocating ball-on-flat test configuration.

To obtain the worn volume of the flat specimen, the mean cross-sectional worn area (Wq) is multiplied by the sliding stroke (s), as presented in Equation (1):

$$Wvol=Warea \cdot s$$

(1)

To calculate the average cross-sectional worn area, four different zones (Figure 6) in the worn track were considered to ensure good reproducibility. This approach disregards the rounded worn zones at ends of strokes that are associated with the ends of the strokes corresponding to the reversion of the sliding direction. This method is deemed appropriate for moderately long stroke length tests (in our study the distances were around 5 mm). There are other worn volume calculations, such as the ASTM D7755-11 standard, that consider the role of both round edges; nevertheless, according to the authors of [26], this method is suitable for stroke lengths under 2.5 mm. The worn volumes calculated with the ASTM D7755-11 standard are presented in Table 1. As can be observed for all the DES1-formulated lubricants, lower worn volumes are observed in comparison to the castor oil base. Specifically, volume reductions from 9% to 36% are observed, these being the biggest reductions achieved with the 0.5 wt% DES1 lubricant, which thus exhibited the greatest antiwear behavior.

The tribological performance enhancements can clearly be seen in the microscopy images of the worn tracks exhibited in Figure 7. It is clear that the wear track width considerably decreases for castor oil after mixing with DES additive. Also, blending DES with the castor oil base results in inferior interfacial roughness, a narrower wear width, and a shallower wear track. Consequently, in the context of castor oil lubrication, the presence of deep grooves in the friction direction is indicative of abrasive wear, as evidenced by the formation of deep furrows. Nevertheless, when DES are added to the castor oil base, specially the 0.5 wt% DES1 lubricant, the friction surfaces are notably smoother, with reduced furrows, which are narrower and shallower than those for the castor oil base. Therefore, it can be suggested that the DES1 additives form a protective tribofilm that cover the steel surfaces and result in improved antiwear performance. Therefore, the attraction of DESs toward engineering surfaces and their layered structure play significant role in the tribological properties [27], generating DES1, which is a suitable layering structure for tribo interfaces, faciliting sliding actions and decreasing friction and wear. The arrangement of DES1 layers over steel surfaces is influeced by the charge distribution in the salt (HBA), the interaction with hydrogen, the chemical composition of HBA and HBD, and the charge on the engineered surfaces. A similar behavior was also observed by Li et al. [12] who studied the same DES1 as additive for PAO40 and PEG200 oils. The favorable tribological performance was ascribed to the tribo-chemisorption of DES with steel surfaces. Furthermore, the synthesis of oil-soluble DESs from materials such as menthol and thymol has been identified as a novel approach to developing high-performance lubricant additives. These DESs have been demonstrated to effectively modify existing lubrication systems, providing enhanced friction characteristics and enabling the formation of tribofilms that significantly mitigate wear [14].

4. Conclusions

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The deep eutectic solvent (DES1), formed by DL-menthol as an HBA and dodecanoic acid as an HBD, was successfully prepared by a one-step route, forming a halogen-free, eco-friendly, and economical lubricant additive.

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The formulation of lubricants composed of a castor oil base and DES1 as an additive was completed observing good miscibility.

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DES1 lubricants have better antifriction performance than castor oil, with the maximum friction reduction observed for the 0.1 wt% DES1 lubricant (4%).

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DES1 lubricants have better antiwear performance than castor oil. In particular, the castor oil + 0.5 wt% DES1 lubricant achieved the best antiwear performance with reductions up to 17% in wear track width.

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The ASTM D7755-11 standard was utilized to obtain the worn volumes, observing volume reductions up to 36% for the castor oil + 0.5 wt% DES1 lubricant.

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Steel surfaces are particularly smooth with reduced furrows when DESs are added to a base oil, indicating a clear tribological improvement in comparison with steel surfaces lubricated with castor oil.