Physicochemical Properties of Coconut and Waste Cooking Oils for Biofuel Production and Lubrication

Abstract

Vegetable oils are an important alternative to the massive use of fuels and lubricants from non-renewable energy sources. In this study, the physicochemical properties of coconut oil and waste cooking oil are investigated for biofuels and biolubricant applications. A transesterification of both oils was reached, and the transesterified oils were characterized by infrared analysis and gas chromatography. The lubricant performances of these oils have been evaluated using a ball-on-plane tribometer under an ambient atmosphere. Different formulations were developed using graphite particles as solid additive. Each initial and modified oil has been investigated as a base oil and as a liquid additive lubricant. The best friction reduction findings have been obtained for both initial oils as liquid additives, highlighting the key role of triglycerides in influencing tribological performances.

1. Introduction

In the atmosphere, overwhelming evidence has shown that human activity is the main driver of climate change, and that its consequences are having an impact on food production, migration patterns, the economic and political situation. [1]. One of the factors contributing to global warming is the excessive use of petroleum-based fuels, which leads to the emission of greenhouse gases [2]. At the same time, climate change is exacerbating environmental degradation, slowing down the economic process [3,4]. It is clear that the energy systems of most countries, whether developed or developing, are based on fossil fuels [5,6]. Despite their qualities, such as good operational control in thermal power plants, their use poses numerous problems, such as environmental impacts, scarcity, supply risk, and unstable market prices, placing fossil fuels at the center of the transition to energy-efficient energies [5,7]. Given the negative impact of fossil fuels on the environment, a new green fuel concept has been developed: biodiesel. Biodiesel is diesel fuel produced from natural oils and fats obtained from plants and animals. A mixture of long-chain fatty acid monoalkyl esters obtained by converting vegetable oils or animal fats [8,9,10]. It is environmentally friendly and reduces the use of fossil fuels [11]. Vegetable oils are biodegradable (70 to 100%), non-toxic [12], with low toxicity, high flash point, and high viscosity [13]. Waste cooking oil and coconut oil can be used to make environmentally friendly biodiesel. Several studies have been carried out on different vegetable oils as base oils for biodiesel, in particular rapeseed oil, coconut oil, etc. [14,15]. The physical properties of these fatty acid esters, known as biodiesel, have been shown to be very similar to those of fossil diesel [10,16,17]. The biodiesel presents lower volatility, poor cold flow properties, and polyunsaturated behavior, compared with conventional diesel fuel. These properties limited its use in diesel engines or as an alternative diesel fuel [8,18,19,20]. In order to use vegetable oils as biodiesel, numerous parameters need to be evaluated, such as viscosity, flammability, stability, temperature resistance, etc. Most of these oils precipitate and solidify when exposed to very low temperatures over an extended period [11]. Transformations are useful for modifying the structure of these oils to optimize their capabilities as biolubricants. There are four main methods of producing biodiesel, such as direct use and blending, microemulsions, thermal cracking (pyrolysis), and transesterification [21]. The most common method is transesterification, since the biodiesel obtained from transesterification can be used directly or blended with diesel fuel in an engine [22,23]. In the present study, a transesterification process is carried out to chemically modify the oils’ structures, as it is a rapid reaction that allows high yields. Transesterification is the transformation of triglycerides into alcohol monoesters with lower boiling points. The products obtained in this reaction are alcoholic esters of vegetable oils and glycerin as a by-product [24]. The alcohols used are methanol, ethanol, propanol, and butanol. Methanol and ethanol are the most widely used and offer a number of advantages. Ethanol is easier to obtain in our archipelago (due to the sugar cane production) and is less polluting. Methanol has several advantages, such as its low cost, and it has physical and chemical advantages, as it reacts easily with fatty acids and dissolves rapidly with soda ash. This transesterification reaction is reversible, so it requires an excess of alcohol to shift the equilibrium towards the products. The reaction can take place in basic, acidic, or enzymatic media. In a basic environment, fatty acids and alcohols must be substantially anhydrous in order to avoid saponification [25]. The most commonly used bases are sodium hydroxide (soda), potassium hydroxide (potash), and carbonates. High temperatures reduce reaction time and speed up the reaction. This article deals with the valorization of waste cooking oil and coconut oil.

The main innovation of the present protocol of transesterification lies in the fact that starting oils are produced locally. Importantly, the waste cooking oil elimination remains an important economic problem for the Caribbean Islands.

The different classic transesterification protocols were presented. The properties of the different transesterified oils were studied with a view to their use as diesel fuel.

Tribology is a field of study that investigates the sliding motion between two sliding surfaces, including friction, wear, and lubrication. Wear is defined as the deformation or removal of surface materials resulting from the interaction between two or more surfaces. Lubrication is the process or technique to reduce the friction and wear between two relative moving surfaces by using a lubricant. Tribological performances of the lubricant blends were evaluated using a ball-on-plane tribometer under ambient atmosphere. Bio-lubricant can replace mineral oil due to its environmental friendliness and helps mitigate the decline in fossil energy. The biodegradability of vegetable oil and ester oils suggests that they have high potential to be used as lubricating oils [26,27,28]. The physicochemical adsorption film between friction steel surfaces is the key factor to obtain friction reduction and antiwear properties [29,30]. Waste cooking oil, coconut oil, and their transesterified oils were investigated for lubricant applications. In this work, the tribological performances of initial oils and transesterified oils were studied as base oil lubricants and as liquid additives in a mineral base oil lubricant, dodecane [31]. Graphite particles were the solid additive used in this study due to their excellent friction properties [32,33]. The findings highlight the key role of triglycerides in influencing tribological performance.

2. Materials and Methods

2.1. Biofuel Synthesis and Characterization

The waste cooking oil recovered from the fast food restaurant “Bayron Burger” (Guadeloupe, France) was vacuum-filtered before use. The coconut was collected in our gardens and then grated. An extraction with acetone (Sigma Aldrich^®, St. Louis, MO, USA) through a Soxhlet device allowed us to obtain the coconut oil.

Waste cooking oils (WCOs)

Methyl ester of WCOs (WCOME): The reagent used for the reaction is sodium methoxide (MeONa). Dissolve 1 g of sodium hydroxide (NaOH, (Sigma Aldrich^®)) in 100 mL of methanol (MeOH, Carlo Erba^®, Cornaredo, Italy) (dissolution may take 1 h). In a 100 mL flask equipped with a magnetic stirrer, 10 mL of WCOs and 20 mL of MeONa are added. The mixture is refluxed for 1 h at a temperature of 150 °C. At the end of the reaction, the reaction medium was cooled to room temperature and then neutralized with a 10% HCl solution until pH 7. Then, the solution was released with a saturated NaCl solution, and then it was extracted with hexane. The organic phase is dried with sodium sulphate anhydrous (Na₂SO₄₎ and evaporated under vacuum at 90 °C. Finally, a yellow oil was obtained with a yield of 90%. No color changes were observed after transesterification (Figure S1).

Ethyl ester of WCOs (WCOEE): The reagent used for the reaction is sodium ethoxide (EtONa); 1 g of sodium hydroxide is dissolved in 100 mL of ethanol (Honeywell^®, Charlotte, NC, USA). The same protocol as in the case of methyl ester of WCOs was adopted, but without release. A yellow-colored oil was obtained with a yield of 98%. No color changes were observed after transesterification (Figure S1).

Coconut oil (CO)

Coconut oil methyl ester (COME): The reagent used for the reaction is sodium methoxide (MeONa). Dissolve 1 g of sodium hydroxide (NaOH, (Sigma Aldrich^®)) in 100 mL of methanol (MeOH, Carlo Erba^®) (dissolution may take 1 h). In a 100 mL flask equipped with a magnetic stirrer, 10 mL of COs and 20 mL of MeONa are introduced. The mixture is refluxed for 1 h at a temperature of 150 °C. At the end of the reaction, the reaction medium is cooled to room temperature and then neutralized with a 10% HCl solution until pH 7. Then, a release with a saturated NaCl solution was performed, and the resulting solution was extracted with hexane. The organic phase is dried over Na₂SO₄ and evaporated under vacuum at 90 °C. The product obtained is a white-colored oil with a yield of 50%. No color changes were observed after transesterification (Figure S1).

Coconut oil ethyl ester (COEE): The reagent used for the reaction is sodium ethoxide (EtONa); 1 g of sodium hydroxide is dissolved in 100 mL of ethanol (Honeywell^®). The same protocol as in the case of methyl ester of WCOs was adopted, but without release. The product obtained is a white-colored oil with a yield of 86%. No color changes were observed after transesterification (Figure S1).

Fourier transform infrared (FTIR) spectroscopy

Analyses were performed to identify functional groups using a PerkiElmer Spectrum Two spectrometer^®, made in Llantrisant, UK, with a range of 4000 to 700 ppm.

The infrared spectra of samples obtained from the 6 samples (WCO, CO, WCOME, WCOEE, COME, COEE) were recorded at room temperature in the wave-number range of 700–4000 cm⁻¹ using a PerkiElmer Spectrum Two spectrometer^®. As stated, only structurally important peaks (υ) are presented in cm⁻¹.

Gas chromatography-mass spectrometry (GC-MS)

Analyses were also performed to identify the composition of the different oils. The instrument is Agilent 7820, Mass Spectrometer 5977E Injection 1 μL, Column HP-5MS Phenyl methoxy 30 m × 250 μm × 0.25 μm Split 10 Carrier gas He, 70 °C 3 min 7.5°C/min to 250 °C; then, 1 min to 250 °C.

All the peaks are compared with the NIST data base. A correlation of over 85% is considered correct. For all calculations, it was assumed that all the ethyl or methyl fatty esters had more or less the same response factors (following the Agilent recommended procedures available on Agilent website).

Viscosity analyses

The viscosity analysis were performed using a Kinexus Pro+ instrument (Malvern Panalytical). Variation of the shear viscosity was measured with a plane of 20 mm with a gap of 0.1 mm and temperature of 30 °C. The analysis time was 8 min. The variation in stress was from 0 to 500 steps.

The overall appearance of the variation in viscosity or shear rate as a function of stress is very similar for all samples (WCOME, WCOEE, COME, and COEE). All appear to have Newtonian rheological behavior after threshold flow for 2 of them (COEE and WCOEE). For the other 2, the decoupling threshold can be considered non-existent. Here, too, the value of the viscosity plateau differentiates the four samples in the same way. The values of viscosity for COME-WCOME present a maximum of 16 mPas/s, compared with the values for COEE-WCOEE, whose maximum viscosity values are 30 and 40 mPas/s, respectively.

2.2. Lubrication Applications

Different lubricant mixtures were tested via a reciprocating ball-on-plane tribometer. Each pure oil was tested. The tribological properties of each oil were investigated as lubricant base and as liquid additive in mineral base oil lubricant.

Exfoliated graphite particles were used as solid friction reduction additive (Timcal Society). Graphite particle thickness is about 100 nm with an average size of 40 µm. The ratio between size and thickness is about 400. In the first part, lubricant composition was prepared by adding 1 wt% of graphite to each of the oils studied. Dodecane, Reagent Plus 99% used in this study as mineral base oil was provided by Sigma-Aldrich. The mixture preparation consists in simply weighing with a precision of 0.01 mg. In each case, three lubricant compositions were prepared by adding 1 wt% of graphite in base oil composed of 1, 2, and 3 wt% of the oil studied in dodecane by the same weighing technique. The dispersion of the different blends was obtained in ultrasonic bath for 5 min.

The tribological performances of the different lubricants were measured under ambient atmosphere (25 °C) using a home-built reciprocating ball-on-plane tribometer consisting of an AISI 52100 steel ball rubbing against a static AISI 52100 steel plane (Figure 1). The ball with a diameter of 1 cm was brought in contact with the plane with a normal load of 10 N, and the alternative motion of the ball was performed with a sliding speed of 2–3 mm·s⁻¹. The frequency is 1 Hz. The tangential force F_T was estimated with a computer-based data acquisition system. The friction coefficient value was calculated as $\mu ={\displaystyle \raisebox{1ex}{$F_T$}\!\left/ \!\raisebox{-1ex}{$F_N$}\right.}$. Subsequently, 2000 friction cycles were performed, a cycle corresponding to an alternative motion of the ball. According to Hertz’s theory, such tribological conditions lead to a maximum contact pressure of 1 GPa and a contact diameter of 140 µm [34]. The generation of multidirectional stripes favors the adherence of graphite particles on the sliding surfaces. For all experiments, the initial roughness of the steel ball is about 50 µm. Before friction experiments, both steel materials were successively cleaned in ultrasonic acetone and ethanol baths. A drop of the selected mixture was deposited on the plane before the friction experiment. The blends with graphite and oil studied are denoted as ${\mu }_{1wt\% graphite+oil studied}$. For mixtures containing graphite and oils as additives in dodecane, the notation is ${\mu }_{Graphite+wt\% oil studied+dodecane}$. Wear diameter of the ball was measured after experiments via a light microscope (Zeiss Axiskop 2). Steel planes were analyzed using 3D profilometry (Altisurf 500).

3. Results and Discussion

3.1. Physicochemical Properties of Transesterified Oils

Infrared and Gas Chromatography Analysis of Transesterified Waste Cooking Oils (WCOs)

Figure 2 shows the FTIR spectrum of waste cooking oil methyl ester with the attribution of the different signals. Thus, signals around 1173 ppm (two peaks of average intensity C-O-C), 1742 cm⁻¹ (a sharp band of high intensity C=O), 2852 cm⁻¹, and 2924 cm⁻¹ (aliphatic C-H) were observed, and regarding FTIR analysis of the ethyl ester, they were observed around 1181 cm⁻¹ (two peaks of average C-O-C intensities), 1737 cm⁻¹ (a sharp band of high C=O intensity), 2854–2924 cm⁻¹ (aliphatic C-H). GC-MS analysis shows that the methyl ester is mainly composed of C19, as shown in

Table 1. Composition of transesterified waste cooking oils.

Ethyl Ester of Waste Cooking Oils (WCOEE)			Methyl Ester of Waste Cooking Oils (WCOME)
Composition		%	Composition		%
E-11-Hexadecenoic Hexadecanoic Linoleic Ethyloleate Octadecanoic	C18 C20 C20 C20 C20	0.16 7.99 50.21 34.16 4.47	Hexadecanoic 9,12-Octadecadienoic 8-Octadecenoic Methylstearate Cis-11-Eicosenioc Methyl-18-methylnonadecanoate	C17 C19 C19 C19 C21 C21	7.73 52 32.39 4.67 0.25 0.34

, with methyl 9Z,12Z octadecadienoate (52%) and methyl 8-octadecenoate (32.39%). The FTIR spectrum of cooking oil methyl ester gives an intense peak around 1742 cm⁻¹ (C=O) and the peak around 1173 cm⁻¹ (O-CH₃), which confirms that the cooking oil has been transesterified [35]. The same applies to ethanol-transesterified cooking oil, which shows a peak around 1737 cm⁻¹ (C=O) and the peak at 1181 cm⁻¹ (O-CH₂CH₃), as shown by Ortega et al. [36].

On the other hand, the GC-MS analysis of the ethyl ester shows a predominantly C20 composition as shown in Table 1 with ethyl linoleate (50.21%) and ethyl oleate (34.16%). On the basis of environmental and yield the ethyl ester of waste cooking oils (WCOs) is desirable than the methyl ester. The ethyl ester comes from even-chained fatty acids while the methyl ester comes from odd-chained fatty acids.

The viscosity of methyl and ethyl esters of coconut and waste cooking oils is significantly lower than that of coconut and waste cooking oils (Figure 3). It shows that transesterified oils are more likely to be used as biodiesel than non-transesterified oils because the lower the viscosity, the easier the flow.

3.2. Infrared and Gas Chromatography Analysis of Transesterified Coconut Oil (CO)

The FTIR technique allowed us to determine the peaks of the coconut oil methyl ester. Around 1171 cm⁻¹ (two peaks of average C-O-C intensities), 1742 cm⁻¹ (a sharp band of high C=O intensity), 2853 cm⁻¹, and 2923 cm⁻¹ (aliphatic C-H) are observed as shown in Figure 4. For coconut oil ethyl ester, spectra show us around 1177 ppm (two peaks of average C-O-C intensities), 1738 cm⁻¹ (a sharp band of high C=O intensity), 2854 cm⁻¹, and 2923 cm⁻¹ (aliphatic C-H). The FTIR results for transesterified coconut oil show two main peaks around 1742 cm⁻¹ (C=O) and 1173 cm⁻¹ (O-CH₃). The FTIR spectrum of ethanol-transesterified coconut oil shows the peak at 1738 cm⁻¹ (C=O) and another peak at 1177 cm⁻¹ (O-CH₂CH₃), corresponding to the alcoholxy group, as confirmed by Ekeoma et al. [37].

Gas chromatography-mass spectrometry (GC-MS) was used to analyze the methyl ester content of coconut oil. Indeed, the transesterification carried out with coconut oil in the presence of methanol contains a large amount of methyl esters of C1₃ and C1₅ fatty acids, as shown in

Table 2. Composition of transesterified coconut oils.

Coconut Oil Ethyl Ester (COEE)			Coconut Oil Methyl Ester (COME)
Composition		%	Composition		%
Hexanoic Octanoic Decanoic Dodecanoic Tetradecanoic Hexadecanoic Octadecanoic Linoleic Ethyloleate	C8 C10 C12 C14 C16 C18 C20 C20 C20	0.50 7.74 7.03 39.83 19.37 9.71 4.40 1.05 5.78	Octanoic Decanoic Dodecanoic Methyltetradecanoate 14-Methylpentadecanoic 10,13-Octadecadienoic 9-Octadecenoic Methylstearate	C9 C11 C13 C15 C17 C19 C19 C19	7.54 7.20 39.72 20.27 10.68 1.19 6.72 4.74

. The amount of dodecanoic acid methyl ester (39.72%) is the highest, followed by tetradecanoic acid ester (20.27%). The coconut oil ethyl ester has an amount of dodecanoic acid ester (39.83%) substantially equal to that of the methyl ester and has 19.37% tetradecanoic acid ester according to Table 2. In view of the yield and environmental protection, it would be better to transesterify coconut oil with ethanol than with methanol.

Based on work carried out by Miao, the temperature was set at 150 °C, as temperature plays an important role in biodiesel production [15]. A higher temperature could produce biodiesel in a shorter time. The molar ratio of methanol or ethanol to oil is also a factor in this type of reaction. In our case, a molar ratio of sodium methylate and sodium ethylate solution to oil of 2:1 was used with very good yields, as the presence of excess methanol or ethanol in the transesterification reaction is important, as they are responsible for breaking glycerol–fatty acid bonds [13]. However, an excess of methanol or ethanol in very large quantities slows down the separation of the phases produced [38]. Sodium ethylate or methylate solution was employed, as the reaction must take place in an anhydrous medium. Dissolving soda ash in water using methanol or ethanol as a solvent leads to a saponification reaction. Infrared spectroscopy of coconut oil shows the presence of an OH band, indicating the presence of diglyceride and monoglyceride in coconut oil. This band is not observed in the methyl and ethyl esters of coconut and cooking oils, which could mean that our oils have been totally transesterified. These various analyses are confirmed by GC-MS. In fact, the structures of the molecules obtained prove that our oils have been transesterified. According to Knothe et al., the fatty acid esters contained in biodiesel were hexadecanoic acid, octadecanoic acid, 9-octadecenoic acid, 9,12-octadecadienoic acid, and 9,12,15-octadecatrienoic acid [39]. In line with Knothe’s results, the methyl ester waste cooking oil is an oil that could be used as biodiesel, as it consists of 52% 9,12-octadecadienoic acid. In addition, it is advantageous for biodiesel to be made from a single major component, which is the case with methyl ester of waste cooking oil. According to several processes, methanol is the most commonly used worldwide due to its physical and chemical advantages [40]. Methanol has a short chain length, which facilitates the separation process between glycerine and esters [38].

3.3. Tribological Performances

Figure 5a presents the friction coefficient values of pure oil studied obtained after 2000 cycles. In our experimental conditions, no matter the chemical composition of the different oils tested, the friction value was quite similar: ${\mu }_{pure oil}=0.1 \pm 0.05$. In the case of pure graphite particles, the friction coefficient value is stable after an induction period of about 30 cycles (Figure 5b). This period is characterized by a high friction value and is associated with the formation of the tribofilm. Then, friction coefficient decreases to reach an asymptotic value ${\mu }_{graphite}=0.09\pm 0.01$.

3.3.1. Oils as Base Oil Lubricant

In this part, the tribological properties of these oils as lubricant base using graphite as solid additive were evaluated. In order to compare with previous studies with dodecane, all blends were made with 1 wt% of particle. It is important to note that the friction coefficient of pure dodecane is high; ${\mu }_{pure dodecane }=0.22\pm 0.02$. In previous studies, the influence of the percentage of graphite in the presence of dodecane was demonstrated [41]. Indeed, ${\mu }_{0.5wt\% graphite+dodecane }=0.09\pm 0.005$ while ${\mu }_{1wt\% graphite+dodecane }=0.06\pm 0.005$. Figure 5b presents the friction values obtained for the mixtures. No modification of the friction coefficient was noted, regardless of the oil tested. Pure oils and blends with graphite have similar friction values: ${\mu }_{pure oils}={\mu }_{1wt\% graphite+oil}=0.1 \pm 0.05$. The lowest value is observed in the presence of dodecane (gray line). Indeed, a reduction of approximately 75% is observed by using 1wt% of graphite. These results are consistent with the influence of the viscosity parameter on the friction performances of particles, as shown in previous studies [8,42]. In the presence of a liquid with high viscosity, the friction reduction in mixtures is higher than that of a liquid with weak viscosity. Dodecane has a lower viscosity than vegetable oil [41]. ${\upsilon }_{pure dodecane}=1.383 mPa.s$, while ${\upsilon }_{pure CO}\approx 40 mPa.s$ and ${\upsilon }_{pure WCOs}\approx 50 mPa.s$.

Figure 6a presents a scar diameter microphotograph measured on a ball. Friction traces are observed, characterizing the sliding direction. The horizontal diameter values measured are recapitulated in Figure 6b. Pure dodecane has the highest wear scar, in agreement with a higher friction coefficient value, allowing us to suppose an important pressure during the sliding experiment. In the case of pure graphite, the scar diameter value is more important than in the presence of dodecane or oils. Indeed, Nomede-Martyr et al. have demonstrated that in the presence of liquid in the sliding contact, the stress undergone by the solid particles (graphite) is lower [43]. The difference between the latter is explained by the friction coefficient values.

3.3.2. Oils as Liquid Additive Lubricant

The tribological performances of lubricant formulas composed of graphite and oil as additives in dodecane were investigated. The percentage of particles is 1wt%, and for each oil tested, mixtures with 1, 2, or 3 wt% were made. The friction behavior between mixtures with initial and transesterified oils is different. Regardless of the percentage of the modified oils added, no modification of the friction coefficient value was observed: ${\mu }_{Graphite+wt\% transesterified oils+dodecane}\approx 0.1$. This friction value is similar to the coefficient of pure oil values. Meanwhile, in the presence of initial oils, a reduction in friction values was obtained (Figure 7). No influence of the weak amount of vegetable oils is noted. Regardless of the percentage, the stabilized friction values of the mixtures were similar: ${\mu }_{Graphite+wt\% CO+dodecane}\approx {\mu }_{Graphite+wt\% FO+dodecane}\approx 0.075\pm 0.005$, except in the presence of 1 wt% of CO. At the beginning of the test, the friction coefficient is weak, then increases up to 0.13, suggesting a desorption of the fatty acid molecules during the sliding.

Figure 8a presents a 3D profile of the tribological trace obtained for the lubricant formula, graphite + 3wt% of CO + dodecane. As can be seen on the profile z (Figure 8b), the friction trace width is about 200 µm, and the tribofilm has a very weak thickness. There is no significant difference between the inside and outside of the scar, which leads to the conclusion that there is no surface wear of the steel plane during the sliding.

Vegetable oil as liquid additive lubricant presents excellent performance. Indeed, for all lubricant formulations, the results show important friction reduction in the mineral base, dodecane. According to the literature, the presence of fatty acids seems to dominate the friction properties of dodecane [44]. However, no influence of the fatty acid composition on the friction coefficient of the different blends is observed. Coconut oil is mainly constituted of saturated fatty acid molecules, whereas WCOs have more polyunsaturated fatty acid molecules. Numerous studies have shown that the fatty acid composition of oils has an influence on their friction properties [45,46,47]. Oils with higher monounsaturated acid molecules (oleic) allow a significant friction reduction due to physicochemical adsorption on steel surfaces. In previous studies conducted under the same experimental conditions, better friction reduction was demonstrated using moringa oil as a liquid lubricant additive [41]. Moringa oil is mainly composed of monounsaturated fatty acid molecules. According to the percentage of moringa oil, the best lubricant formulation was obtained in the presence of 2 wt% ${\mu }_{1wt\% Graphite+2wt\% moringa oil+dodecane}=0.05\pm 0.005$. By comparing the results, the beneficial effect of the presence of unsaturated fatty acid molecules in the lubrication field was underlined.

In addition, the present results also highlight the positive and necessary action of the presence of triacylglycerides on the friction properties of vegetable oils. Regardless of the transesterification process, oils modified with ethyl or methyl esters as liquid additives, the friction results are similar to the pure oil coefficient. In boundary lubrication, the triacylglycerides of vegetable oil are more polar than petroleum-based oils; they have a higher attraction to metal surface, creating a protective thin monolayer that allows for the reduction in friction of sliding surfaces [48,49]. After transesterification, therefore, without the glycerol structure, the tribological properties of the modified oil are less effective. The results suggest that the presence of small amounts of fatty acid triacylglyceride molecules in dodecane influences the tribological properties of mixtures. Crude vegetable oils in lubricant formulations improve the friction and protective properties of metal surfaces during sliding.

4. Conclusions

Physicochemical properties of coconut and waste cooking oils were investigated for biodiesel and biolubricant applications. A comparative analysis of transesterified methyl and ethyl esters with the pure oils was performed. As a result, both transesterified oils can be proposed as potential feedstocks for biodiesel. Moreover, the methyl ester of waste cooking oil shows better results, which may be due to the fact that it doesn’t contains diglycerides or monoglycerides fatty acids. The absence of di-/mono-glycerides was demonstrated by the absence of the -O H band in the infrared spectra; this band was present in the starting waste cooking oil. GC-MS analysis showed a high percentage of C:19 fatty acids, in particular 9,12-octadecadienoic acid, in the waste cooking oil. In general, transesterification with methanol provided better results than ethanol.

In the lubrication field, initial and transesterified oils were tested as lubricant bases and as liquid additives in mineral base. Different mixtures were made using graphite particles as solid additive at 1 wt%. Dodecane was the lubricant base used to simulate the properties of a mineral lubricant. The friction coefficient values of pure oils are lower than those of pure dodecane. However, as a lubricant base, the friction coefficients obtained for the different mixtures were identical and were higher than those of the dodecane mixture. No influence of the fatty acid composition on tribological properties was observed. Lubricant mixtures of modified oils as a lubricant base or as a liquid additive all had the same friction coefficient value. The low friction values of the dodecane mixture confirmed the viscosity effect demonstrated in a previous study. As a liquid additive, the best results were obtained for mixtures containing unprocessed coconut oil and WCOs. In conclusion, this study was able to highlight the important role of triacylglycerides in vegetable oil based on their tribological performance.