Comparative evaluation of lubricant properties of jatropha and jojoba methyl ester

Abstract

It is well known that lubricating oils can reduce the coefficient of friction between two contacting surfaces. Owing to their poor biodegradability and toxicity, petroleum lubricants are typically deemed unacceptably harmful to the environment. These oils have a significant negative impact on both human and plant life and contaminate air, soil, and drinking water. Consequently, the public’s concerns about a pollution-free environment are growing along with the demand for ecologically friendly lubricants. Because of their superior lubricity, biodegradability, viscosity-temperature properties, and low volatility, plant oils hold promise as basis fluids for lubricants. In the current work, jatropha and jojoba oil were converted into bio-lubricants by chemical modification processes such as transesterification and epoxidation using H₂SO₄ and HCl catalysts. The kinematic viscosity of jatropha ester increases by 12.93 and 123.22%, and that of jojoba ester increases by 15.91 and 104.24% at 32 and 90 °C, respectively, when the concentration of the catalyst is increased from 0.3 to 0.9 ml for H₂SO₄ catalyst. Similarly, for the HCl catalyst, the kinematic viscosity values of jatropha are increased by 5.43 and 30.25%, and for jojoba, 20.84 and 50.96% at 32 and 90 °C, respectively. The epoxidized jatropha had greater experimental flash and fire point values than the epoxidized jojoba. The anti-wear and friction-reduction qualities were tested using an Anton Paar TRB³ ball on a disk. In comparing the 0.9 ml concentration of jatropha bio-lubricant samples to the 0.3 ml concentrations, the percentage reduction in wear was 31.68% for epoxidized jatropha—HCl catalyst, and 33.95% for epoxidized jatropha—H₂SO₄ catalyst.

Keywords:

• Lubricants
• transesterification
• epoxidation
• kinematic viscosity
• flash and fire points
• ball on disk tribometer

Reviewing Editor:

• Dr. Ian Phillip Jones, University of Birmingham, Birmingham, United Kingdom of Great Britain and Northern Ireland

Subjects:

• Tribology
• Clean Tech
• Transport & Vehicle Engineering

1. Introduction

A lubricant is a substance inserted between two moving surfaces to reduce wear, disperse heat, and increase efficiency. One of the most effective techniques to reduce the wear and friction between two contacting surfaces that are moving relative to one another is lubrication (Ingole et al., 2013). Engine oils (petrol and diesel engine oils), automatic transmission fluids, gearbox fluids, braking fluids, and hydraulic fluids are the most widely used liquid lubricants in the automotive industry (Woma et al., 2019). Mineral-based and synthetic lubricants are the most common types of lubricants available on the market. Although these lubricating oils work well, they are not eco-friendly. Mutations and carcinogenesis are caused by a complex variety of chemicals found in mineral oil. This can cause infant mortality rates to rise, and children may become malnourished (IMR) (Salimon et al., 2010). Additionally, the world’s reserves of mineral oil are running out, and environmental concerns regarding the negative effects of mineral oil are intensifying. A cutting-edge area of study in the lubricant industry is the search for environmentally benign alternatives to mineral oils used as base oils in lubricants (Bongfa et al., 2015). If used in internal combustion (IC) engines and not completely miscible with basic fuels, such as diesel, their leakage into the combustion chamber will cause them to burn partially, which will result in the creation of pollutants. Therefore, the use of mineral oils results in higher engine emissions of smoke and unburned hydrocarbons (UBHC). Because of the severe environmental damage caused by the contamination of combustion products and the damage to the cylinder surface caused by the enhanced rubbing action of metallic additives added to the lubricant, mineral lubricants are no longer recommended for use (Syahrullail et al., 2013). In comparison to mineral lubricating oils, synthetic lubricating oils have some benefits, including improved low-temperature fluidity, oxidation stability, and thermal stability. In addition to the aforementioned benefits, they have a few drawbacks that restrict their use as lubricants. The increased price of synthetic oil is likely the most obvious drawback. Synthetic oil is two to four times more expensive than regular oil. For synthetic oils, cold storage may increase the risk of additive precipitation (do Valle et al., 2018). New alternatives have been studied to replace mineral-based lubricants and synthetic bases with renewable ones that are less hazardous to the environment (Trajano et al., 2014). Plant-based oils are structurally similar to the long-chain hydrocarbons in mineral oils and have the advantages of being renewable, non-toxic, economical, and environmentally friendly; they have great potential and are highly attractive candidates to replace conventional mineral oils for use in lubricant production (Nagendramma and Kaul, 2012). In general, both solvent and pressing methods are used to extract vegetable oils from plants (Jayadas and Nair, 2006). In comparison to mineral-based lubricating oils, vegetable-based bio-lubricating oils are over 95% biodegradable and decompose 2–30% more quickly (Jeevan and Jayaram, 2018). Based on the prevalence of fatty acids, vegetable oils are divided into four major types. These include ricinoleic acid, oleic-linoleic acid, lauric acid, and erucic acid (Liaquat et al., 2013). Oleochemical esters are a class of compounds that meet the fundamental criteria for lubricant base stocks and enhance the thermal and cold flow instability of neat vegetable oils (Srivastava and Sahai, 2013). The lubricating capabilities of biolubricants, which are esters of heavy alcohols generated from feedstock based on vegetable oils, are comparable to those of mineral oil-based lubricants (Heikal et al., 2017). Biolubricants have been created from crude plant oils to function better than mineral oil lubricants by chemically altering their properties (Panchal et al., 2017). Compared to petroleum-based lubricants, bio-based lubricants have a viscosity index (VI) that is quite high. This makes it possible for lubricants to function over a broader temperature range, which is necessary for some applications. The greater flash point (>200 °C) of all bio-based lubricants made from vegetable oils further suggests that these substances are non-volatile and can be attributed to the presence of fatty acids in these substances (Lowenstein and Vollertsen, 1915).

The viscosity or viscosity index of a lubricant is crucial for reducing collisions and rubbing between mechanical components while they are in motion and increasing the efficiency of the mechanical device. Methyl esters were derived from the transesterification of vegetable oil by Rodrigues et al. (2006), and a viscosity index greater than 200 was regarded as remarkable and highly favorable. When there is a double bond, the viscosity of the lubricant increases. However, when there are two or more double bonds, the viscosity decreases. According to Shahabuddin et al. (2013), fluids are frequently evaluated based on their viscosity index to avoid friction differences caused by temperature swings. The viscosity is less affected by temperature if the viscosity index is high. Trimethylolpropane esters synthesized by transesterification process from methyl esters based on palm oil and jatropha oil showed good potential and were tested for kinematic viscosity by Heikal et al. (Heikal et al., 2017) in comparison with mineral oil, which showed both a low pour point temperature (3 °C) and a high viscosity index (140). To increase the viscosity range of high-oleic sunflower oil, Shahabuddin et al. (2012) investigated a unique, environment-friendly lubricant formulation with improved viscosities employing ethylene-vinyl acetate and styrene-butadiene-styrene copolymers. The maximum kinematic viscosities were, respectively, between 25 and 35 cSt and between 25 and 240 cSt at 40 °C and 100 °C. According to the experimental findings of Asadauskas et al. (1997), who used Cannon-Fenske viscometers to evaluate a biodegradable biolubricant, vegetable oils exhibited a higher viscosity index than mineral oils. In an experimental study on an IC engine employing a bio-lubricant (Pongamia oil), Bekal and Bhat (2012) conducted a transesterification process and concluded that the Pongamia oil lubricant achieved maximum braking thermal efficiency with lower viscosity. Experimental work was conducted on epoxidized soybean oil by Ting and Chen (2011), who examined the viscosity and effectiveness of biolubricants based on soybean oil and found that the viscosity of epoxidized soybean oil was greater. The transesterification process was carried out on non-edible mandarin seed oil by Azad (2017) and was assessed as a viable replacement and second-generation fuel for transportation. Kinematic viscosity at 40 °C was measured using an ARES rheometer. Additionally, the study discovered that the viscosity of oil is higher at low temperatures and lower at high temperatures than that of diesel. Reeves et al. (2015) studied the characteristics of biolubricants made from edible and non-edible oils as well as their friction and wear characteristics. Viscosity and temperature were analyzed using a digital paddle vibrational viscometer, to determine the correlations between them. Natural oils ranged in viscosity from 54 cP for corn oil to 71 cP for peanut oil at room temperature (21 °C). Using a redwood viscometer following ASTM D2270, Rani et al. (2015) assessed the viscosity of rice bran oil. Compared to SAE20W40, vegetable oil showed a relatively low viscosity range.

An extensive literature survey on the application of lubricants indicated that raw vegetable oils, both edible and non-edible, cannot be directly used as bio-lubricants. Vegetable oils must be subjected to chemical alterations to change their molecular structures and chemical properties. Studies related to chemical modification techniques, such as epoxidation, and the factors influencing the process have not been thoroughly investigated. Moreover, the use of jojoba and jatropha oil as biolubricants has not been explored in previous studies. It has been suggested that jojoba (Simmondsia chinensis) and jatropha (Jatropha curcas) oils would make good substitutes for biolubricants. In addition, the kind of catalyst used, which could vary depending on the kind of raw oil chosen also influence it. To overcome these gaps, in the present study, the raw oils of jatropha and jojoba were subjected to transesterification and epoxidation processes, and the prepared samples of biolubricants were tested for various physicochemical properties. Table 1 displays the physicochemical characteristics of the raw jojoba and jatropha. Table 2 shows the fatty acid compositions of jatropha and jojoba oil. There is a lack of knowledge regarding the characterisation of vegetable oils as biolubricants in the literature. In an effort to address most of the shortcomings of mineral oils, vegetable oils can be viewed as an alternate lubricating agent that can be used in a variety of operating circumstances, such as greater temperatures and pressures occurring in engine lubrication. This is the novelty of the current study.

Table 1. Physicochemical Properties of Jatropha and Jojoba oil (Adamu & Adem 2020; Al-Widyan & Mu’taz 2010; Cecilia et al., 2020; Mushtaq & Hanief, 2021; Sukkar et al., 2019).

Sl.no	Properties	SAE 15W-40	Jatropha oil	Jatropha bio-lubricant	Jojoba oil	Jojoba bio-lubricant
1	Density (kg/m^3)	904	895	918	864	872
2	Viscosity at 40 °C (cSt)	109.84	35.4	43.9	22.507	27.384
3	Viscosity at 100 °C (cSt)	14.5	7.9	8.71	6.163	7.116
4	Viscosity index	135	205	180	247	242
5	Pour point (°C)	−27	+8	−6	+9	+5
6	Flash point (°C)	220	186	225	150	238
7	Calorific value (MJ/kg)	–	39.65	–	42.1	–

Table 2. Fatty acid composition of jatropha and jojoba oil (Aransiola et al., 2012; Nayak & Mishra, 2016).

Fatty acid	Structure	Jatropha oil	Jojoba oil
Oleic acid	C18:1	44.7	42.84
Linoleic acid	C18:2	32.8	31.52
Stearic acid	C18:0	7.0	4.14
Palmitic acid	C16:0	14.2	1.59
Palmitoleic acid	C16:1	0.7	0.11
Myristic acid	C14:0	0.1	0.09

2. Methodology

The raw, non-edible oils, such as jatropha and jojoba, were purchased from vendors. Due to their low thermal and oxidation stability, raw vegetable oils were not used directly as lubricants. These disadvantages were mitigated through chemical modifications such as transesterification and epoxidation. H₂SO₄ and HCl were the two catalysts used for the study.

2.1. Transesterification of vegetable oils

Initially, raw jatropha and jojoba oils were procured from the vendor and subjected to transesterification. Table 1 shows the physicochemical properties of the raw jatropha and jojoba oils. The jatropha oil was collected in a 3-neck bottom flask, heated at the required temperature of 65 °C for 90 min, and magnetically stirred. Methanol and sodium hydroxide pellets were mixed with a magnetic stirrer until the pellets were dissolved. After dissolving the pellets, the mixture was poured into a separating flask for glycerol and biodiesel separation. Finally, the mixture was again washed. The trans-esterified oil was washed several times, and then methyl ester was added to a beaker. The final product was then heated and cooled. A similar procedure was used for the transesterification of jojoba oil.

2.2. Epoxidation of vegetable oils

Another process required to convert raw vegetable oil into a biolubricant is epoxidation. It involves the removal of double bonds between two carbons via an oxygen atom, which results in an epoxide functional group, a three-atom ring composed of two carbon atoms, and an oxygen atom. This method was used to improve the oxidative stability, lubricity, viscosity, viscosity index, and pour point. For the epoxidation procedure, a three-necked round-bottom flask equipped with a magnetic stirrer was employed. One neck of the flask contained a temperature sensor, while the other neck was connected to the reflux condenser. The required amount of esterified jatropha (250 ml) was collected in a flask with 10% weight of the oil of peracetic acid and 0.1% weight of the oil of sulfuric acid as a catalyst. The mixture was agitated at room temperature and stirred to achieve a uniform mixture. 25% of the oil of hydrogen peroxide was added to the mixture, and the solution was heated for four hours at 500 rpm. Subsequently, the aqueous layer formed at the bottom of the flask was drained. Cold and warm water were used to wash the solution. In the final stage, the solution was treated with anhydrous sodium sulfate. The solution was filtered and stored in a beaker. A similar procedure was followed for the epoxidation of jojoba oil.

Based on the yield obtained by varying catalyst concentrations, the prepared bio-lubricant samples are subjected to tribological tests. Beilsten’s test was conducted to determine the content of halogen in the bio-lubricant sample. A viscosity test was performed using Anton Par MCR 92 Rheometer for all the samples. Flash and fire points were conducted on Cleveland’s open cup apparatus. The anti-wear and friction-reduction performances of Anton Paar’s TRB³ ball on a disk were evaluated. All the trials were repeated thrice to satisfy the aspect of repeatability.

3. Results and discussion

As the altered chemical properties are dependent on the process variables, such as the type of catalyst and its concentrations, 12 samples were prepared using jatropha and jojoba oil with H₂SO₄ and HCl as catalysts at concentrations of 0.3, 0.6 and 0.9 ml, respectively.

3.1. Calculation of yield

Epoxidized jatropha (EJA) and epoxidized jojoba (EJO) biolubricant samples were prepared with various catalyst concentrations (0.3, 0.6 and 0.9 ml for H₂SO₄ and HCl, respectively. After epoxidation, the bio-lubricant yield for the different 12 combinations for 250 ml of esterified oil is noted and is presented in Figure 1. From experimentation, we observed that the yield for epoxidized jatropha was highest for both 0.6 ml H₂SO₄ and 0.6 ml HCl concentrations. In case of epoxidized jojoba, highest yield was obtained at 0.3 ml H₂SO₄ and 0.9 ml HCl concentrations. The percentage of yield (ml) was calculated using the following equation, and the corresponding yield values are listed in Table 3. (1) $$\mathrm{\text{Yield}}\left(\mathrm{\%}\right)=\frac{\text{Epoxidied oil yield}\left(\mathrm{\text{ml}}\right)}{\text{Oil sample taken}\left(\mathrm{\text{ml}}\right)}\text{*}100.$$(1)

Figure 1. Variation of yield with different epoxidized oils.

Table 3. Yield (%) for the different epoxidized oil samples.

Sl.no	Samples	Catalyst	Catalyst concentration (ml)	Epoxidized yield (ml)	Yield (%)
1	EJA	H2SO4	0.3	229	91.6
			0.6	235	94
			0.9	224	89.6
2	EJA	HCl	0.3	225	90
			0.6	230	92
			0.9	218	87.2
3	EJO	H2SO4	0.3	215	86
			0.6	208	83.2
			0.9	201	80.4
4	EJO	HCl	0.3	210	84
			0.6	219	87.6
			0.9	225	90

3.2. Beilstein’s test

The Beilstein test is used to determine whether an organic compound contains a halogen (iodine, bromine or chlorine). The test substance is placed on a piece of copper wire or gauze that has been pre-heated in a Bunsen burner’s oxidising flame until the flame is no longer green. The wire or gauze is then re-heated. A halogen is present when there is a green flame. With our experimental study, the Beilstein test is done for all samples of epoxidized oils, to determine the breakage of carbon bonds. After igniting, if the fumes come out of copper pipe, then it is an indication of unsaturation (bonds are not broken). From our experimental test, there were no fumes after igniting.

3.3. Viscosity

Viscosity is a significant characteristic that determines the efficiency of a lubricant. The kinematic viscosity and absolute viscosity were determined for 12 samples at various temperatures ranging from 32 °C to 90 °C using Anton Par MCR 92 Rheometer. Sample values for the epoxidized jatropha 0.3 ml H₂SO₄ catalyst are tabulated in Table 4.

Table 4. Epoxidized jatropha 0.3 ml H₂SO₄ catalyst.

Sl. No	Temp, of oil (°C)	Weight of 60 cc empty flask	Weight of flask + oil (g)	Weight of 60 cc oil (g)	Time for 60 cc of oil flow t (s)	Density of oil (g/cc)	Kinematic viscosity (Stokes)	Absolute viscosity (Poise)
1	32	45.6	98.2	52.6	61	0.8767	1.059	0.9284
2	40	45.6	97.8	52.2	55	0.8700	0.888	0.7726
3	50	45.6	97.4	51.8	51	0.8633	0.77	0.6647
4	60	45.6	96.9	51.3	47	0.8550	0.647	0.5532
5	70	45.6	96.5	50.9	41	0.8483	0.451	0.3826
6	80	45.6	96.1	50.5	37	0.8417	0.309	0.2601
7	90	45.6	95.8	50.2	33	0.8367	0.155	0.1297

Sample 1

High viscosity encourages high flow resistance, thicker lubricant films, and greater power consumption. Whereas low viscosity results in low flow resistance, thinner lubrication films, and lower power consumption. Due to the degree of random intermolecular interactions growing with the length of the fatty acid chain, bio-oils become more viscous. Because of this, oils containing long-chain fatty acids typically have a high viscosity, whereas oils containing short-chain fatty acids have a low viscosity. Moreover, the degree of unsaturation has a significant role in determining the viscosity of oil; the viscosity of a single double bond is higher than that of two or three double bonds. One possible explanation for the viscosity increase of the bio-lubricants made from jatropha and jojoba due to the chemical alteration process is the creation of high-molecular-weight molecules, specifically hydroperoxides, aldehydes and ketones. Since it has been discovered that saturated fatty acids like arachidic and behenic can generate a significant rise in viscosity in vegetable oils through thermal aging or storage, this could also be the explanation for the large viscosity increase in jatropha and jojoba bio-lubricants (Farfan-Cabrera et al., 2022). Since the carbon chains of the triacylglyceride molecules include double bonds and free fatty acids, jatropha and jojoba bio-lubricants are prone to rapid oxidative breakdown. Natural oils must have a low proportion of polyunsaturated fatty acids, such as linoleic acid, to be thermally stable. Therefore, as the quantity of polyunsaturated fatty acids decreases, oxidative stability rises. Additionally, monounsaturated fatty acids with a single double bond, like oleic acid, have better tribological and low-temperature capabilities in addition to improving oxidative stability. These are the factors responsible for the change in viscosity as well as tribological properties due to the chemical alteration of vegetable oils.

Figure 2(a) and (b) show the variation in kinematic viscosity (KV) and absolute viscosity (AV) with temperature for epoxidized jatropha oil for different concentrations of H₂SO₄ and HCl catalysts. It is observed that it follows the general trend of a decrease in KV and AV with temperature for all tested cases. When H₂SO₄ concentrations were increased, both KV and AV increased. This is due to the fact that as the catalyst concentrations are increased, the viscosity of the modified oil in comparison to the raw oil increases due to the addition of trimethylolpropane. When HCl was used as a catalyst, the trend variation with temperature was found to be similar to that of the H₂SO₄ catalyst, but the percentage increase with the concentration was lower than that of H₂SO₄. The increase in viscosity at higher HCl concentrations may be due to the formation of long-chain molecular structures. The increase in the catalyst concentration promoted agglomeration of the catalyst on the surfaces, which promoted molecular collisions, resulting in increased viscosity. The kinematic viscosity of jatropha oil increased by 12.93 and 123.22% at 32 and 90 °C, respectively, for H₂SO₄ catalyst when the catalyst concentration was increased from 0.3 to 0.9 ml. Kinematic viscosity values of HCl catalyst also increased by 5.43 and 30.25% at 32 and 90 °C, respectively. The absolute viscosity of jatropha oil increased by 12.93 and 123.30% at 32 and 90 °C, respectively, using H₂SO₄ catalyst when the catalyst concentration was increased from 0.3 to 0.9 ml. In the same manner, the absolute viscosity values for the HCl catalyst increased by 5.43 and 30.28% at 32 and 90 °C, respectively.

Figure 2. (a) and (b) Variation of KV and AV with temperature for epoxidized jatropha H₂SO₄ and HCl catalyst, respectively.

Figure 3(a) and (b) show the variation in kinematic viscosity (KV) and absolute viscosity (AV) with temperature for epoxidized jojoba oil for different concentrations of H₂SO₄ and HCl catalysts. The absolute values of the viscosities at all temperatures were lower than those of jatropha oil. This may be due to the varied compositions of the oils. Although jojoba has more lignin than jatropha, the latter has more cellulose and hemicellulose. Jatropha typically experiences a large increase in viscosity as a result of dimerization in addition to an increase in molecular weight. In the case of epoxidized jatropha, the functionalization of double bonds with epoxy groups results in a notable increase in viscosity. Similarly, in the case of AV, as the catalyst concentration is increased, the viscosity increases, and the amount of increase is considerably higher than the increase in the corresponding values of jatropha oil. This is because these esters have a high natural viscosity owing to their triglyceride structure, which also accounts for their structural stability across reasonable working temperature ranges. The kinematic viscosity of jojoba oil increased by 15.91 and 104.24% at 32 and 90 °C, respectively, for the H₂SO₄ catalyst when the concentration of the catalyst was increased from 0.3 to 0.9 ml. Kinematic viscosity values of the HCl catalyst also increased by 20.84 and 50.96%, at 32 and 90 °C, respectively. The absolute viscosity of jojoba oil increased by 15.90 and 104.27% at 32 and 90 °C, respectively, for the H₂SO₄ catalyst when the catalyst concentration was increased from 0.3 to 0.9 ml. In the same way, the absolute viscosity values for the HCl catalyst increased by 20.84 and 51%, at 32 and 90 °C, respectively.

Figure 3. (a) and (b) Variation of KV and AV with temperature for epoxidized jojoba H₂SO₄ and HCl catalyst respectively.

3.4. Flash and fire points

The lowest temperature of a liquid, corrected to 101.3 kPa, at which, under particular test conditions, the sample’s vapor ignites in the presence of an ignition source and the flame spreads across the sample’s surface is known as the flash point temperature. The lowest temperature at which a material’s vapors will ignite and burn always, even after the ignition source is removed, is known as the fire point. Because there aren’t enough vapors generated at the flash point to ignite the fuel, the fire point is higher than the flash point. In the case of lubricant leaking, the high flash and fire point significantly lower the risk of fire and offer safety. From Figure 4(a) and (b), the flash point and fire point of both epoxidized jatropha and jojoba oil increases with an increase in the concentration of both catalysts. Epoxidized jojoba has higher flash and fire points than epoxidized jatropha. Because most jojoba oil is made up of alcohol esters, primarily eicosenoic and docosenoic acids, epoxidized jojoba oils include long-chain fatty acids. These compounds have relatively large molecular weights and are less volatile than the alcohols and shorter-chain fatty acids found in other oils. Epoxidized jojoba oil contains antioxidants called tocopherols, or vitamin E, which can help stabilize the oil and prevent it from deteriorating at high temperatures.

Figure 4. (a) and (b) Variation of H₂SO₄ and HCl concentration for flash and fire point.

3.5. Wear and coefficient of friction (COF)

The anti-wear and friction-reduction performances of Anton Paar’s TRB³ ball on a disk were evaluated. The Al4032 alloy material was used for testing at room temperature (298 K) with a 20 N applied load. A wire EDM machine was used to cut the wear test samples of Al4032 alloy. The working concept for Anton Paar’s TRB³ ball-on-disk involves applying regulated pressure to a flat, rotating disk using a spherical ball made of steel. The wear rates and frictional forces were recorded as the ball rolled across the disk surface. Comparative evaluations of wear and COF were conducted for two distinct biolubricants: jojoba and jatropha. The COF and wear rate of the commercial lubricants and prepared bio-lubricant samples are shown in Tables 5 and 6, respectively.

Table 5. COF and wear rate of the commercial lubricants.

S. No.	Samples	COF	Wear scar diameter (mm)	References
1	SAE 20 W-40	0.106	0.575	Aravind et al. (2015)
2	SAE 20 W-50	0.080	0.360	Jayadas et al. (2007)
3	SAE 40	0.068	0.491	Shahabuddin et al. (2013)
4	SAE 15 W-40	0.080	0.700	Kalam et al. (2017)

Table 6. COF and Wear rate of the prepared bio-lubricant samples.

Sl.no	Samples	Catalyst	Catalyst concentration (ml)	COF	Wear rate (mm^3/Nm)
1	EJA	H2SO4	0.3	0.052	5.782 * 10^−6
			0.6	0.049	4.432 * 10^−6
			0.9	0.045	3.819 * 10^−6
2	EJA	HCl	0.3	0.062	7.951 * 10^−6
			0.6	0.058	6.516 * 10^−6
			0.9	0.053	5.432 * 10^−6
3	EJO	H2SO4	0.3	0.065	8.483 * 10^−6
			0.6	0.061	7.152 * 10^−6
			0.9	0.057	6.051 * 10^−6
4	EJO	HCl	0.3	0.056	6.781 * 10^−6
			0.6	0.053	5.923 * 10^−6
			0.9	0.051	4.495 * 10^−6

The main factors affecting the lubricity (COF and wear rate) of bio-lubricants are the unsaturation of fatty acids, chain length, and branching. The carbon chain length features are linear (saturated and monounsaturated fatty acids) and lengthy (n ≥ 9). High polarity and low unsaturation degree of fatty acids help bio-lubricants achieve low wear and coefficient of friction. As mentioned in Table 2 in the manuscript, jatropha and jojoba oils contains several fatty acids such as oleic acid, linoleic acid, stearic acid, palmitic acid, palmitoleic acid and myristic acid. Since longer hydrocarbon chains of fatty acids result in stronger molecular bonds, the coefficient of friction is expected to decrease as the length of the carbon chain increases. The primary constituents of bio-lubricants that satisfy international requirements for lubricity are monoglycerides and methyl esters. Though not as much as monoglycerides, free fatty acids and diglycerides and triglycerides also have an impact on the lubricity of bio-lubricants. By altering chemical properties of vegetable oils, long polar fatty acid chains create very effective lubricant coatings that have a strong interaction with metallic surfaces. This film can thereby lessen wear and friction. These are the reasons which impact lubricity in terms of fatty acids (Farfan-Cabrera et al., 2019, 2022).

Low friction losses in the cylinder liners are anticipated for low friction coefficients. The material will wear more quickly at greater wear rates (Farfan-Cabrera et al., 2020). From Tables 5 and 6, the wear rates and friction coefficients obtained by using the commercial lubricants and bio-lubricants are shown. When it came to the coefficient of friction, commercial lubricants demonstrated the greatest coefficients, whilst jojoba and jatropha bio-lubricants demonstrated the lowest. However, the considerable rise in viscosity brought on by oxidation may be the reason for the decrease in wear rate with jatropha and jojoba bio-lubricants when compared to commercial lubricants.

Of the different samples tested, epoxidized jatropha and jojoba bio-lubricants showed the maximum wear rate for 0.3 ml catalytic concentration, as shown in Figure 5. By the addition of catalytic concentrations by 0.3 ml, the wear rate was considerably reduced for all tested samples. The minimum wear rate was obtained for 0.9 ml catalytic concentration for all samples. The percentage reduction in wear for jatropha biolubricant samples for 0.9 ml concentration in comparison to the 0.3 ml concentrations was 33.95 and 31.68% for EJA-H₂SO₄ and EJA-HCl, respectively. Similarly, for jojoba bio-lubricant samples i.e., EJO-H₂SO₄ and EJO-HCl for 0.9 ml concentration, the reduction in wear rate was found to be 28.66 and 33.71%, respectively. It was noted that higher catalyst concentrations had a faster reaction rate than lower catalyst concentrations. From Figure 5, it can be seen that higher concentrations showed a better wear rate as it resulted in the formation of protective coatings on moving parts surfaces. By serving as a barrier, these films can reduce wear by preventing direct metal-to-metal contact. From Figure 6, it is also evident that the COF decreased with an increase in catalytic concentration levels as compared to the low-concentration samples. The COF of the EJA-H₂SO₄, EJA-HCl, EJO-H₂SO₄ and EJO-HCl samples were found to be 0.045, 0.053, 0.057, and 0.051 respectively. In summary, adding appropriate concentrations of catalysts to bio lubricants significantly reduces the wear rate and hence improves the lubricating properties of bio lubricants.

Figure 5. Wear rate of jatropha and jojoba bio-lubricant with various catalysts and their catalytic concentrations.

Figure 6. COF of jatropha and jojoba bio-lubricant with various catalysts and their catalytic concentrations.

4. Conclusions

From the current study on the synthesis and characterization of biolubricants, the following conclusions were drawn.

• For epoxidized jatropha, 0.6 ml of H₂SO₄ and HCl catalyst concentration was found to be preferable because it assisted in obtaining the greatest yield. Similar to this, the maximum yield for epoxidized jojoba was achieved at 0.3 ml H₂SO₄ and 0.9 ml HCl catalyst concentration.

• The kinematic viscosity of jatropha oil increased by 12.93 and 123.22% at 32 and 90 °C, respectively, for H₂SO₄ catalyst when the catalyst concentration was increased from 0.3 to 0.9 ml. kinematic viscosity values of HCl catalyst also increased by 5.43 and 30.25% at 32 and 90 °C, respectively

• The absolute viscosity of jojoba oil increased by 15.90 and 104.27% at 32 and 90 °C, respectively, for H₂SO₄ catalyst when the catalyst concentration was increased from 0.3 to 0.9 ml. In the same way, the absolute viscosity values for the HCl catalyst increased by 20.84 and 51%, at 32 and 90 °C, respectively.

• The flash and fire points of epoxidized jojoba values are higher than epoxidized jatropha. As epoxidized jojoba oils contain long-chain fatty acid and alcohol esters, principally eicosenoic and docosenoic acids, make up the majority of jojoba oil.

• The percentage reduction in wear for jatropha bio-lubricant samples for 0.9ml concentration in comparison the 0.3 ml concentrations was 33.95 and 31.68% for EJA-H₂SO₄ and EJA-HCl, respectively.

• With catalyst concentration of 0.9ml for EJO-H₂SO₄ and EJO-HCl bio-lubricant samples, the percentage reduction in wear rate was found to be 28.66 and 33.71%, respectively as compared to 0.3 ml catalyst concentration.

• The COF values for the EJA-H₂SO₄, EJA-HCl, EJO-H₂SO₄ and EJO-HCl samples are 0.045, 0.053, 0.057 and 0.051, respectively.

After careful modification of the raw vegetable oils, their properties can be altered, and the prepared lubricants certainly offer important benefits, including excellent biodegradability, less reliance on petroleum, fewer negative effects on human health, and minimal environmental harm. This will reduce the dependency on mineral-based lubricants and help save the environment.

Author contributions

Rajendra Uppar: conceptualization, investigation, formal analysis, writing-original. Shiva Kumar: supervision, visualization, writing-review and editing. P. Dinesha: Supervision, Writing-review and editing.

Disclosure statement

No potential competing interest was reported by the authors.

Data availability statement

Data can be made available on a reasonable request.

Additional information

Funding

The authors would like to acknowledge the Manipal Academy of Higher Education, Manipal, for providing financial assistance through the Intra Mural Fund (Ref: MAHE/CDS/PHD/IMF/2019) for the Ph.D. work.

Notes on contributors

Rajendra Uppar

Rajendra Uppar is a Research Scholar in the Department of Mechanical and Industrial Engineering, at Manipal Institute of Technology, Manipal Academy of Higher Education, Manipal, India. He is pursuing his doctoral degree in the area of bio-lubricants with nanoparticle additives for diesel engines. He has published his research papers in reputed international journals and conferences. His research interests include renewable energy, engine combustion, alternative fuels and bio-lubricants usage in CI engines, combustion emissions reduction etc.

Shiva Kumar

Shiva Kumar is a Professor in the Department of Mechanical and Industrial Engineering, at Manipal Institute of Technology, Manipal Academy of Higher Education, Manipal, India. His research interests include refrigeration and air conditioning, bio-lubricants, desiccant-based dehumidification and humidification, Engine combustion, Alternative fuels, and pollution control. Professor Kumar has published more than 95 papers in reputed journals and conference proceedings.

P. Dinesha

P. Dinesha is a Professor in the Department of Mechanical and Industrial Engineering, at Manipal Institute of Technology, Manipal Academy of Higher Education, Manipal, India. His research interests include waste to energy conversion, bio-lubricants, CO₂ capture, Engine combustion, Alternative fuels, and pollution control. Professor Dinesha has published more than 75 papers in reputed international journals and conference proceedings.