Synthesis and physicochemical properties of novel lauric acid capped estolide esters and amides made from oleic acid and their evaluations for biolubricant basestock

Industrial Crops & Products 140 (2019) 111653

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Synthesis and physicochemical properties of novel lauric acid capped estolide esters and amides made from oleic acid and their evaluations for biolubricant basestock

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Seng Soi Hoong , Mohd Zan Arniza, Nek Mat Din Nik Siti Mariam, Abu Hassan Noor Armylisas, Shoot Kian Yeong Malaysian Palm Oil Board, 6 Persiaran Institusi, Bandar Baru Bangi, 43000, Kajang, Selangor, Malaysia

A R T I C LE I N FO

A B S T R A C T

Keywords: Fatty acid Estolide Biolubricant Pour point Oxidative stability Anti-wear

Vegetable oils have been used as environmentally friendly biolubricants because of their inherent biodegradability, good lubricity, higher viscosity index and low evaporative loss. However, their use is limited due to significantly poor cold flow properties and inferior oxidative stability. This paper presents an approach to modify vegetable oil derivatives namely oleic acid to yield biolubricant with good cold flow properties and oxidation stability. Oleic acid was converted to polyhydroxy estolide through reaction with only hydrogen peroxide, which is considered a ‘green’ oxidant. The synthesized polyhydroxy estolide was further reacted with appropriate amount of lauric acid to end-cap its hydroxy groups, which yielded lauric acid capped estolide as the product. Subsequently, the prepared lauric acid capped estolide was reacted with branched and straight chain alcohols as well as secondary amines to afford estolide esters and amides. In general, most of the prepared samples showed improved cold flow properties as compared to vegetable oil-based lubricants, where the best pour point (−41 °C) was achieved by 4-methyl-2-pentyl estolide ester. Meanwhile, estolide amides exhibited the best oxidative stability among samples evaluated with an oxidation onset temperature of 205 °C, which is significantly higher than vegetable oil-based lubricants. Generally, the oxidative stability, viscosity index and anti-wear properties of prepared estolide esters and amides were found to be comparable to the properties of commercial samples used as benchmark.

1. Introduction Majority of lubricant base oils used in our daily activities are typically made from non-sustainable petrochemicals, which are toxic to the environment and have poor biodegradability, therefore difficult to discard after their service life. In addition, studies have shown that most of lubricants used were accidentally or deliberately released to the environment (Mannekote and Kailas, 2016; Horner, 2002), which is a major problem for the ecosystem. This undesirable situation has encouraged lubricant scientists to develop sustainable and biodegradable biolubricants as alternative to mainstream fossil fuel-based lubricants. One of the major sustainable feedstocks for making biolubricant is vegetable oils. Vegetable oil-based lubricants are known to be biodegradable and non-toxic to the environment (Schneider, 2006). In addition, they also exhibit higher viscosity index, lubricity, flash point and lower evaporative losses as compared to petrochemical-based lubricants (Erhan and Asadauskas,

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2000; Jayadas et al., 2007). These desirable properties of vegetable oilbased lubricants originated from the polar ester groups and high molecular weight of vegetable oils (Adhvaryu et al., 2005). However, vegetable oil-based lubricants are not without disadvantages such as inferior oxidative stability due to the presence of allylic protons that are susceptible to radical oxidation, which lead to oxidative degradation (Soni and Agarwal, 2014; Becker and Knorr, 1996). In addition, vegetable oil-based lubricants also exhibit poor cold flow properties due to the presence of saturated aliphatic fatty acid moiety in the vegetable oil-based lubricants that crystallize at higher temperature than mineral oil lubricants (Kassfeldt and Goran, 1997). Therefore, in order to overcome these weaknesses, lubricant scientists have manipulated the chemical structure of vegetable oils through chemical (Salimon et al., 2010) and enzymatic (Greco-Duarte et al., 2017) methods. Chemical modification of alkene group of vegetable oils through epoxidation is a common method employed to improve the oxidative

Corresponding author. E-mail address: [email protected] (S.S. Hoong).

https://doi.org/10.1016/j.indcrop.2019.111653 Received 6 April 2019; Received in revised form 2 July 2019; Accepted 7 August 2019 Available online 21 August 2019 0926-6690/ © 2019 Elsevier B.V. All rights reserved.

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impart desirable properties to biolubricants. Another approach to improve the lubricant properties of vegetable oils is through the formation of estolide from unsaturated fatty acids such as oleic acid (Cermak et al., 2007) or from hydroxy fatty acid such as ricinoleic acid (Sammaiah et al., 2016) and lesquerolic acid through chemical and enzymatic methods (Isbell, 2011). In general, estolides are known to be readily biodegradable in the environment and they exhibited superior cold flow and oxidative stability properties than vegetable oils (Bredsguard et al., 2016). The preparation of estolide from oleic acid involved the use of strong acid such as sulfuric acid (H2SO4), methanesulfonic acid (CH3SO3H) and perchloric acid (HClO4) as the catalyst to activate the alkene group of oleic acid (Haro et al., 2018; Cermak and Isbell, 2001). The most effective acid catalyst for the reaction was found to be perchloric acid. However, the main disadvantage of this method is the safety hazards pose by perchloric acid, which is known to be a strong corrosive acid and readily forms potentially explosive mixtures (Everett and Graf, 1971). Furthermore, the prepared estolide still contain significant amount of alkene group that is susceptible to oxidative degradation. Therefore, it would be ideal to prepare estolide from oleic acid using less hazardous chemicals than perchloric acid that could completely convert all alkene group to other functional groups. On the other hand, estolides made from naturally occurring hydroxy acids i.e. ricinoleic acid and lesquerolic acid have alkene groups that are susceptible to oxidative degradation and this necessitate further chemical treatment to eliminate the alkene groups of prepared estolides, which will increase the production cost. Furthermore, they are also costly and not produced in significant quantity for economical commercial production of biolubricants as compared to commodity fatty acids such as oleic acid (Sammaiah et al., 2016; Patel et al., 2016). Therefore, preparation of biolubricants from abundantly available oleic acid would be ideal for commercial production of biolubricants. To the best of authors knowledge, very few studies have investigated a direct one-pot method to synthesize polyhydroxy estolides from oleic acid by using less hazardous chemicals than perchloric acid. Thus, this paper reports the synthesis of estolide esters and amides from oleic acid that involved the preparation of polyhydroxy estolide as an intermediate compound. The synthesis of polyhydroxy estolide from oleic acid employed only hydrogen peroxide as the reactant, which is a relatively safe and environmentally friendly ‘green’ oxidant (Ciriminna et al., 2016). Subsequently, the hydroxy and carboxylic acid groups of the prepared polyhydroxy estolide were converted to ester and amide groups to afford biolubricants that showed good cold flow and oxidative stability properties.

stability of biolubricants. For example, a study by Sharma et al. (2006) reported the chemical modification of soybean oil by epoxidation of alkene group followed by epoxide ring-opening with water to generate dihydroxy derivative of soybean oil. Subsequently, the dihydroxy derivative was reacted with various acid anhydrides to yield diester derivatives of soybean oil. The chemical modifications performed on soybean oil managed to improve marginally the oxidative stability of the resultant product. Nevertheless, the modified soybean oil still exhibited pour point that is not acceptable for low temperature lubricant applications. This could be due to the triglycerides structure of soybean oil that facilitated crystallization even though it was modified. Therefore, chemical modification of vegetable oils should be focused on fatty acids in order to achieve desirable lubricant properties. An alternative method for making biolubricants utilizes fatty acids and polyols to synthesize polyol esters. For example, Cavalcante et al. (2019) reported the use of undecylenic acid that originated from castor oil as biobased component for esterification reaction with polyols such as trimethyloltoluene (TMT) and trimethylolpropane (TMP) to form polyol esters. The esterification process was conducted at 150 °C for 8 h in the presence of a tin catalyst and xylene to yield triester of polyols. The synthesized trimethylolpropane triundecylenate showed only marginal improvement in terms cold flow properties as compared to vegetable oils. On the other hand, the triesters of TMT exhibited significantly improved cold flow properties. Nevertheless, these synthesized polyol esters still have terminal alkene groups that are vulnerable to oxidative degradation and necessitates further steps to improve their stability. In addition, TMT and undecylenic acid are relatively expensive and not available in large quantity in comparison to commodity fatty acid such as oleic acid. One of the reasons for the relatively small market share of biolubricants is due to higher cost of biolubricants as compared to mineral oil-based lubricants. Therefore, it is important to produce biolubricant using lower cost starting materials. Fatty acids and fatty acid methyl esters are alternative sources of biobased feedstock from vegetable oils that can be used to make biolubricants. There are numerous studies conducted on chemical modification of fatty acids and fatty acid methyl esters to make biolubricants (McNutt and He, 2016). As an example, a study by Salih et al. (2013) reported that ricinoleic acid derived from castor oil was chemically manipulated through several reaction steps to yield biolubricant with improved oxidative stability, better cold flow and antiwear properties as compared to castor oil. In their study, the alkene group of ricinoleic acid was epoxidized and then the epoxide group was ring-opened with fatty acid to afford hydroxy acid derivative. Further conversion of hydroxy and carboxylic acid groups of the synthesized hydroxy acid derivative to ester groups contributed to the excellent biolubricant properties as mentioned above. The main drawback of this approach is the multiple reaction steps required for making biolubricant from costly ricinoleic acid, which contributes to higher production cost. Hence, it is worthwhile to prepare biolubricant with less reaction step that has competitive production cost. Similarly, a study reported by Borugadda and Dalai (2018) employed epoxidized methyl oleate originated from canola biodiesel as the starting material for making biolubricant. The epoxide group of epoxidized methyl oleate was ring-opened with oleic acid in the presence of a aluminosilicates solid acid catalyst that yielded oleic acid-capped hydroxy estolide. Subsequently, the generated hydroxy group was further end-capped with excess oleic acid, catalyzed by the same solid catalyst to afford oleic acid-capped estolide, which showed improved cold flow, rheological and lubricity properties. However, like polyols esters reported by Cavalcante et al. (2019), the synthesized oleic acidcapped estolide has alkene groups that requires further chemical modification to achieve better oxidative stability, which will increase the production cost. In addition, the cost of epoxidized methyl oleate as starting material is significantly higher than basic commodities fatty acids such as oleic acid. Therefore, ideally biolubricants should be made from low cost commodities fatty acids through simple processes that

2. Materials and methods 2.1. Materials Oleic acid (75%, Palmac 750) was obtained from IOI Oleochemical Industries Bhd (Penang, Malaysia). Hydrogen peroxide (50% in H2O), lauric acid (98%), 2-ethylhexanol (99%), 1-dodecanol (98%), 1-octanol (99%), isobutanol (99%), isoamyl alcohol (98%), 4-methyl-2-pentanol (98%), dibutylamine (99%) and diisobutylamine (98.5%) were purchased from Sigma Aldrich (St Louis, MO, USA). Bis(2-ethylhexyl) dimerate ester (Radialube 7121) and refined paraffinic mineral oil (TUDALEN 13) were obtained from commercial sources. All chemicals were used as received without further purification. 2.2. General procedures for synthesis of polyhydroxy estolide from oleic acid Oleic acid (500 g, 1.77 mol) and hydrogen peroxide (600 mL, 8.8 mol) (mole ratio between oleic acid: hydrogen peroxide, 1 : 5) were charged into a round bottom flask equipped with a magnetic stirrer and a condenser. The mixture was stirred vigorously (500 rpm) and heated 2

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2.5. Characterization

at 80 °C for 24 h. Then, the mixture was poured into a separating funnel. Ethyl acetate (300 ml) and deionized water (300 ml) were added to the mixture in the separating funnel. The organic layer was separated from the aqueous layer. The organic layer was washed two more times with deionized water (300 ml). The organic layer was dried over anhydrous MgSO4 and ethyl acetate was removed in vacuo to yield a viscous colorless liquid labeled as polyhydroxy estolide (540 g, 77% yield). νmax/ cm−1 3405 (OH) 2923, 2854 (C–H) 1731, 1708 (C = O) 1241, 1180 (C–O) 723 (CH2); 1H NMR (600 MHz, CDCl3): δH = 4.81-4.78 (1H, m, HCOC = O), 3.57-3.52 (1H, m, HCO) 3.37-3.33 (2H, m, HCO), 2.28 (4H, t, J = 7.1 Hz, O = CCH2CH2), 1.63-1.54 (4H, m, O = CCH2CH2CH2), 1.45-1.33 (8H, m, CH2COH), 1.32-1.14 (40H, m, CH2CH2CH2), 0.83 (6H, t, J=6.0 Hz, CH2CH3); 13C NMR (150 MHz CDCl3): δC = 179.0, 173.9 (C = O), 76.9 (HCOC = O), 75.6, 72.5 (HCO), 34.6 (O = CCH2), 24.7 (O = CCH2CH2CH2), 31.8, 29.1, 24.6, 22.6 (CH2CH2CH2), 14.6 (CH2CH3); m/z (ES+) [M + Na+] Required 637.5014, Found 637.5018.

Proton (1H) and carbon (13C) nuclear magnetic resonance (NMR) spectroscopy were performed using JOEL JNM-ECZ600R at 600 MHz and 150 MHz respectively with approximately 10% w/v sample dissolved in deuterated chloroform solvent. NMR spectroscopy is used to determine the molecular structure and functional groups of an organic sample. NMR analysis was conducted on all synthesized products namely polyhydroxy estolide, lauric acid capped estolide as well as lauric acid capped estolide esters and amides. FTIR spectra were recorded on Perkin-Elmer FTIR Spectrum100 spectrometer equipped with an attenuated total reflectance (ATR) top plate. Samples were analyzed as thin film over the ATR and spectra were recorded in the range of 650 – 4000 cm−1. The average value of 8 scans with 4 cm−1 resolution was recorded for each sample. FTIR spectroscopy is used to identify functional groups of an organic sample. FTIR analysis was conducted on all synthesized products. Low resolution mass spectrometry was performed using Agilent 6120B LC/MS with electrospray ionization (ESI) system while high resolution mass spectrometry was conducted using Bruker MicroTOFQIII with ESI-TOF system. Sample was directly injected into the mass spectrometry system without going through a liquid chromatography column. Mass spectrometry is used to determine the molecular mass of sample. Mass spectrometry was conducted on polyhydroxy estolide. Gel permeation chromatography (GPC) was performed on a Varian PL-GPC 50 Plus equipped with an autosampler and a differential refractive index (DRI)/viscometer combined detector. Sample was dissolved in tetrahydrofuran with a concentration of 2 mg/mL. Sample was analysed using a PLgel Mixed D column with tetrahydrofuran as the eluent at a flow rate of 1 mL/minute. Molecular weights were obtained based on universal calibration curve generated using polystyrene standards with range from 162 to 1 × 105 Da. Data were analyzed using Cirrus GPC/SEC software. GPC is used to determine the average molecular weight and molecular weight distribution of a sample. GPC analysis was conducted on all synthesized products. Wet chemistry analyses were conducted according to AOCS official methods (American Oil Chemists’ Society (AOCS, 2009):- Acid value (AV) and hydroxy value (OHV) of sample were determined according to AOCS Official Method Te 2a-64 and Cd 13–60, respectively. AV and OHV were conducted on all synthesized products. Iodine value analysis is used to quantify alkene group of a sample and it was conducted on polyhydroxy estolide. Iodine value was determined according to AOCS Official Method Cd 1d-92. Saponification analysis (SapV) is used to quantify ester group of a sample and it was performed according to AOCS Official Method Cd 3-25. SapV was conducted on all synthesized products. The oxirane oxygen content (OOC) analysis is used to measure the epoxide content of a sample and it was conducted according to AOCS Official Method Cd 9-57. OOC was conducted on polyhydroxy estolide. Kinematic viscosity is defined as the ratio of the viscosity to the density of the fluid. Kinematic viscosity measurements were carried out according to ASTM D445 standard method at 40.0 °C and 100.0 °C using calibrated Ubbellohde viscometer tubes and a Normalab NVB Classic viscosity bath with constant temperature. Kinematic viscosity analysis was conducted on lauric acid capped estolide esters and amides. Viscosity index for lauric acid capped estolide esters and amides samples were calculated according to standard method ASTM D2270-93. The pour point of a liquid is the temperature below which the liquid is unable to flow. Cloud point of a liquid is the temperature below which the liquid forms cloudy appearance. Pour point and cloud point measurements for lauric acid capped estolide esters and amides samples were performed according to standard method ASTM D97 and ASTM D2500, respectively. Measurements were done using Normalab CPP Classic test cabinet with integrated cooling. The oxidation onset temperature (OOT) analysis is used to measure the oxidative stability of a sample. The OOT analysis was carried out

2.3. General procedure for synthesis of lauric acid capped estolide Polyhydroxy estolide (200 g, 0.61 mol of −OH based on hydroxy value of 171 mgKOH/g) and lauric acid (100 g, 0.5 mol, 50% w/w of polyhydroxy estolide) were weighed into a round bottom flask equipped with a magnetic stirrer and distillation apparatus. The mixture was stirred vigorously (500 rpm) and heated to 210 °C for 8 h. After the mixture was cooled to room temperature, ethyl acetate (100 ml) was added the reaction mixture and dried over anhydrous MgSO4. Ethyl acetate was removed in vacuo to yield a viscous yellowish liquid labeled as lauric acid capped estolide (284 g, 94% yield). νmax/cm−12923, 2854 (C–H) 1736, 1708 (C = O) 1237, 1156 (C–O) 723 (CH2); 1H NMR (600 MHz, CDCl3): δH = 4.98-4.92 (4H, m, HCOC = O), 2.29 (2H, t, J = 7.5 Hz, O = CCH2CH2), 2.24 (8H, t, J = 7.2 Hz, O = CCH2CH2), 1.62-1.53 (10H, m, O = CCH2CH2CH2), 1.52-1.42 (8H, m, CH2CHO), 1.34-1.14 (88H, m, CH2CH2CH2), 0.84 (15H, t, J=6.9 Hz, CH2CH3); 13C NMR (150 MHz CDCl3): δC = 179.9, 173.4 (C = O), 73.9 (HCOC = O), 34.4, 34.1 (O = CCH2), 25.5 (O = CCH2CH2CH2), 31.9, 29.3, 24.7, 22.7 (CH2CH2CH2), 14.1 (CH2CH3).

2.4. General procedures for synthesis of lauric acid capped estolide esters and amides 2.4.1. Synthesis of 2-ethylhexyl ester of lauric acid capped estolide as an example Lauric acid capped estolide (200 g; 0.38 mol of −CO2H based on acid value of 107 mgKOH/g) and 2-ethylhexanol (108 g, 0.83 mol) were charged into a round bottom flask equipped with a magnetic stirrer and Dean-Stark apparatus. The mole ratio between CO2H:OH was about 1:2. The mixture was stirred vigorously (500 rpm) and heated to 210 °C for 8 h. Excess 2-ethylhexanol was Kugelrohr-distilled from the reaction mixture at 210 °C under vacuum of 20 mbar. After the mixture was cooled to room temperature, ethyl acetate (100 ml) was added the reaction mixture and dried over anhydrous MgSO4. Ethyl acetate was removed in vacuo to yield a viscous yellowish liquid labeled as 2ethylhexyl estolide ester (234 g, 96% yield). νmax/cm−12924, 2855 (C–H) 1736 (C = O) 1463 (CH2) 1378 (CH3) 1170 (C–O) 723 (CH2); 1H NMR (600 MHz, CDCl3): δH = 4.98-4.92 (4H, m, HCOC = O), 3.97-3.9 (2H, m, H2CO) 2.24 (10H, t, J = 7.1 Hz, O = CCH2CH2), 1.61-1.54 (10H, m, O = CCH2CH2CH2), 1.53-1.48 (1H, m, CHCH2O), 1.47-1.42 (8H, m, CH2CHOC = O), 1.33-1.14 (96H, m, CH2CH2CH2), 0.89 (21H, t, J = 7.5 Hz, CH2CH3); 13C NMR (150 MHz CDCl3): δC = 174.0 (C = O), 73.5 (HCOC = O), 66.6 (H2C-O), 38.8 (CHCH2O), 34.4 (O = CCH2), 30.5 (CH2CHOC = O), 25.1 (O = CCH2CH2CH2), 31.9, 29.6, 29.3, 23.8, 22.9 (CH2CH2CH2), 14.0, 11.0 (CH2CH3).

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Alternatively, the epoxide group of epoxidized oleic acid can also be ring-opened by water in the reaction system, which produced 9,10-dihydroxystearic acid. The 9,10-dihydroxystearic acid is in fact the monomer of polyhydroxy estolide. However, due to carboxylate ion of oleic acid being the stronger nucleophile than water, the reaction predominantly produced polyhydroxy estolides. Several reaction parameters such as reaction time and mole ratio were studied in order to optimize the formation of polyhydroxy estolide. Table 1 shows the properties of reaction products and the conversion percentage of alkene group. Referring to Table 1, iodine value (IV) and oxirane oxygen content (OOC) are measurements of alkene group and epoxide group, respectively, for oils and fats products. The original IV of oleic acid was about 89 g I2/100 g sample and after 24 h reaction with hydrogen peroxide, the value was reduced to 12.2 (Entry 3). The lowest IV achieved was 2.7 (Entry 6) when larger amount of hydrogen peroxide was employed. This indicated that the alkene group of oleic acid was converted to other functional groups through epoxidation and epoxide ring opening steps. At the same time, the OOC wet analysis showed that the alkene group of oleic acid was converted to epoxide group and the highest value recorded was 1.05% (Entry 4) after 8 h of reaction. The OOC was not able to achieve higher value due to concurrent occurrence of epoxidation and epoxide ring opening steps in the reaction. Subsequently, with longer reaction duration, the epoxide group was ring-opened to yield product with low IV and OOC values (Entry 6). This study showed that oleic acid was able to “self-epoxidize” in the presence of hydrogen peroxide without any catalyst and the newly formed epoxide group was ring-opened by carboxylic acid group to form polyhydroxy estolide. The product from Entry 6 was also analyzed by using NMR spectroscopy, which revealed the presence of ester functionality represented by a peak observed at 4.80 ppm of the proton NMR spectrum (Fig. 5 A). Hydroxy functionality was also observed at 3.40 ppm of the spectrum. The NMR analysis also revealed that the alkene group of oleic acid was not detected, which indicated nearly all the alkene group was converted to other functional groups. Similar proton NMR spectrum was reported by Awang et al. (2007) for their work on 9,10-dihydroxystearic acidbased estolide. The FTIR spectrum of polyhydroxy estolide shows peaks associated with ester and carboxylic acid groups at 1730 cm−1 and 1708 cm−1, respectively (Fig. 4A). The acid, saponification and hydroxy value of the product (Entry 6) were 120 mgKOH/g, 243 mgKOH/g and 155 mgKOH/g, respectively, which were quantified through wet analysis. The acid value of starting material (oleic acid) was reduced from 195 to 120 mgKOH/g, which

using pressure differential scanning calorimetry (PDSC) technique. The OOT analysis for lauric acid capped estolide esters and amides samples were conducted using a DSC Q20 P thermal analyzer equipped with Tzero pressure DSC cell acquired from TA Instruments. The OOT analysis was performed according to standard method ASTM E2009. The anti-wear properties for lauric acid capped estolide esters and amides samples were evaluated according to standard method ASTM 4172 (Four-ball method). The test was conducted using a four-ball tester TR-30 l made by DUCOM Instrument, India. The test balls with product code RB-12.7/G20W were purchased from SKF Malaysia Sdn. Bhd. 3. Result and discussion 3.1. Synthesis of polyhydroxy estolide The first step in the preparation of biolubricant from oleic acid involved the conversion of alkene group of oleic acid to other functionalities. This is to ensure that the oleic acid-based biolubricant will have better oxidative stability properties than the starting material itself. In this study, oleic acid was reacted with only ‘green’ oxidant hydrogen peroxide to convert the alkene group of oleic acid to ester group and hydroxy group. The reaction was conducted at 80 °C for 24 h with proposed reaction mechanism to yield polyhydroxy estolide as shown in Fig. 1. The reaction between carboxylic group of oleic acid and hydrogen peroxide generated peroxy acid namely peroleic acid. The peroleic acid subsequently epoxidized the alkene group of another oleic acid, which yielded epoxidized oleic acid and the peroleic acid was converted back to oleic acid. The newly formed epoxidized oleic acid was ring-opened by another oleic acid to form hydroxy estolide. The synthesized hydroxy estolide had alkene group that was subjected to the same epoxidation and epoxide ring-opening steps, which consecutively generated polyhydroxy estolide. It is noteworthy to mention that the epoxidation and epoxide ring opening steps occurred concurrently as an overall reaction that converted oleic acid to polyhydroxy estolide without any catalyst. Similar epoxidation of oleic acid that utilized acetic acid to generate peroxy acid was reported by Aguilera et al. (2016). Subsequently, the peroxy acid epoxidized the oleic acid and reverted to acetic acid. A side reaction that occurred was epoxide ring opening by acetic acid. This literature indicated that oleic acid can accomplish similar self-epoxidation and epoxide ring opening reactions to generate polyhydroxy estolide albeit longer reaction time and higher reaction temperature employed in our study.

Fig. 1. Synthesis of polyhydroxy estolide from oleic acid with proposed reaction mechanism. 4

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Table 1 Properties of reaction products and the conversion percentage of alkene group. Entry

Mole ratio

Time (hours)

IV (g I2/100 g sample)

OOC (%)

OHV (mgKOH/g)

Mn (Da)

Conversion (%)

1 2 3 4 5 6

1 : 2.5

8 16 24 8 16 24

56.8 29.7 12.2 55.6 27.5 2.7

0.43 0.47 0.18 1.05 1.01 0.17

93 142 154 74 130 171

404 491 551 384 462 690

36 67 86 38 69 97

1:5

Mole ratio = oleic acid : hydrogen peroxide; Time = reaction duration; IV = iodine value; OOC = oxirane oxygen content; OHV = hydroxy value; Mn = number average molecular weight; Conversion = conversion of alkene group.

optimum amount of lauric acid needed to fully cap the entire hydroxy group. The following Table 2 shows the analysis results of reaction products obtained from self-esterification of polyhydroxy estolide in combination with lauric acid. The polyhydroxy estolide and 9,10-dihydroxystearic acid have both hydroxy and carboxylic acid groups in their molecular structure, which enable them to perform intermolecular self-esterification reaction as reported by Awang et al. (2007). Direct esterification reactions without catalyst at high temperature have been reported in literature (He et al., 2014). Referring to Table 2, self-esterification of polyhydroxy estolide without lauric acid generated estolide (Entry 7) with high viscosity, which indicated the formation of high molecular weight compound from self-oligomerization of polyhydroxy estolide through intermolecular reaction between hydroxy and carboxylic groups. In addition, the prepared estolide also had relatively high hydroxy value, which showed that self-esterification of polyhydroxy estolide without lauric acid produced high molecular weight estolide with significant amount hydroxy groups that are susceptible to oxidative degradation at high temperature. Conversely, when self-esterification of polyhydroxy estolides was conducted in the presence of 30 wt% of lauric acid (Entry 8), the resultant compound had significant lower hydroxy value, molecular weight and viscosity than compound from Entry 7. This revealed that the addition of lauric acid into the reaction mixture facilitated the conversion of some hydroxy group of polyhydroxy estolide to ester functionality and excluded these hydroxy groups from further esterification reaction. This prevented the build-up of molecular weight and lowered the hydroxy value of reaction product. Furthermore, when the amount of lauric acid was increased to 50 wt % (Entry 9), the self-esterification of polyhydroxy estolide in the presence of lauric acid yielded product with hydroxy value lower than 10 mgKOH/g, which showed that almost all of the hydroxy groups of polyhydroxy estolide were ‘capped’ with lauric acid and the product also exhibited lower viscosity and molecular weight as compared to product from Entry 8. This is due to higher amount of lauric acid made available in the reaction mixture to ‘end-cap’ hydroxy group and prevented polyhydroxy estolide from reacting with each other. The selfesterification of polyhydroxy estolide in the presence of lauric acid can be illustrated by Fig. 3. Further increment of lauric acid in the reaction mixture (Entry 10) yielded product with similar low hydroxy value and molecular weight. However, the product was a semi-solid at room temperature and the acid value of the product was relatively higher than other prepared samples. These results indicated that the amount of lauric acid incorporated into the reaction mixture at 70 wt% was in excess and the unreacted lauric acid formed the semi-solid layer observed in the final product. The wet analysis (OHV) results are supported by analysis using FTIR spectroscopy, which revealed that the spectrum of product from Entry 7 showed a significant peak associated with hydroxy group. On the contrary, the FTIR spectrum of product from Entry 9 showed significant appearance of a peak associated with ester group while no peak associated with hydroxy moiety was observed. Fig. 4 shows the overlaid

showed the conversion of carboxylic acid group to ester functionality through epoxide ring opening that concurrently generated the hydroxy functionality. This finding is supported by the saponification value of the product, which is higher than the acid value and this indicated the formation of ester functionality. Furthermore, gel permeation chromatography (GPC) analysis of the product also showed the formation of estolide as the measured number average molecular weight of the product was about 690 Da, which correspond with a dimer of 9,10-dihydroxystearic acid linked by an ester group that can be illustrated by Fig. 2(a). The GPC analysis also revealed that the oligomeric content of the product was about 77% with the other 23% consists of monomer 9,10-dihydroxystearic acid. In addition, the GPC analysis result is supported by analysis result of mass spectrometry conducted on polyhydroxy estolide, which detected a peak at 637 Da that corresponded to a sodium ion adduct of a polyhydroxy estolide structure as shown in Fig. 2 (b).

3.2. Synthesis of lauric acid capped estolide Literature reported that lubricant base oils with hydroxy functionality have inferior oxidative stability as compared to those without (Salih et al., 2013). The synthesized polyhydroxy estolide has hydroxy groups that are susceptible to oxidative degradation at high temperature; therefore, they need to be converted to other functional group such as ester in order to enhance the oxidative stability of estolidebased biolubricant. Thus, the prepared polyhydroxy estolide was subjected to self-esterification reaction without catalyst at 210 °C in combination with lauric acid to convert all the hydroxy groups to ester groups that are more stable towards oxidative degradation. Lauric acid was selected as the capping acid because it was reported to impart good cold flow properties (Cermak and Isbell, 2009) in addition to its significant lower cost as compared to other fatty acids such as octanoic and decanoic. The amount of lauric acid that was used to react with polyhydroxy estolide was varied from 0% to 75% based on the weight of polyhydroxy estolide. The intention of this step is to identify the optimum amount of lauric acid needed to cap or fully convert all the hydroxy group of polyhydroxy estolides to ester group. Hydroxy value wet analysis; infra-red and NMR spectroscopy were used to determine the

Fig. 2. (a) Dimer of 9,10-dihydroxystearic acid linked by an ester group, (b) sodium ion adduct of dimer of 9,10-dihydroxystearic acid. 5

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Table 2 Analysis results of reaction products obtained from self-esterification of polyhydroxy estolide in combination with lauric acid. Entry

Lauric acid (wt %)

Conversion (%)

AV (mgKOH/g)

OHV (mgKOH/g)

SV (mgKOH/g)

Viscosity (mPa)

Mn (Da)

7 8 9 10

0 30 50 75

36 78 95 98

64 72 107 142

110 37 8 3

255 286 292 298

58600 8600 1000 solid

4300 2200 2100 1200

Lauric acid = weight % of lauric acid based on weight of polyhydroxy estolide; Conversion = conversion of hydroxy group; AV = acid value; OHV = hydroxy value; SV = saponification value; viscosity = viscosity of product at 25 °C; Mn = number average molecular weight.

210 °C forabout 8 h (Fig. 6), which yielded estolide esters and amides, respectively. Direct amidation of carboxylic acid by secondary amines without catalyst at high temperature have been reported in literature (Goossen et al., 2009). The alcohols and amines were used in excess to ensure that all the carboxylic group of lauric capped estolide was converted to ester and amide groups, respectively. The targeted acid value of reaction products is below 5 mgKOH/g. This is to ensure that the prepared estolide-based lubricant base oils have minimal free carboxylic acid group, which otherwise may cause corrosion to the metallic surface they intended to protect. Alcohols used for the preparation of estolide esters were straight chain fatty alcohols namely octanol and lauryl alcohol, which were selected with the intention to maximize the biobased content of the final product. Meanwhile, selected branched alcohols were 2-ethylhexanol, isobutanol, isoamyl alcohol and 4-methyl-2-pentanol, which are commercially available in bulk quantity. Branched alcohols were included in this study because they were reported to impart good cold flow property to the biolubricant (Cermak et al., 2013). On the other hand, two secondary amines namely dibutylamine and diisobutylamine were included in this study in order to make estolide amides and evaluate the effect of amide bond on lubricant properties. Synthesized estolide esters and amides were analyzed using FTIR and NMR spectroscopy to determine the functional groups and chemical structure of the products. FTIR analysis revealed that the acid group of lauric acid capped estolide was fully converted to either ester or amide functional groups as represented by peak at 1736 cm−1and 1647 cm−1, respectively (Fig. 7). Furthermore, 1H NMR analysis conducted on lauric acid capped estolide ester and amides products revealed more information on the structure of these products (Fig. 8). For example, the 1H NMR of estolide ester (4) exhibited a peak at 3.9 ppm

FTIR spectra of polyhydroxy estolide, product from Entry 7 and product from Entry 9 (lauric acid capped estolide). In addition, NMR spectroscopy analysis conducted on prepared samples also revealed that the 1H NMR spectrum of product from Entry 7 showed a peak at 3.55 ppm that corresponds with methine proton attached to hydroxy group. In comparison, the same peak was not observed in the 1H NMR spectrum of product from Entry 9, which indicated that all the hydroxy groups were converted to ester functionality Meanwhile, another peak at 4.96 ppm that corresponds with ester group was observed in the same spectrum. Fig. 5 shows the overlaid proton NMR spectra of polyhydroxy estolide, product from Entry 7 and product from Entry 9 (lauric acid capped estolide). Based on all the analysis results, the optimum amount of lauric acid to be used in the self-esterification of polyhydroxy estolide was determined to be 50 wt % of polyhydroxy estolide. The product from this particular reaction exhibited low hydroxy value (Entry 9), which indicated that most of the hydroxy groups of polyhydroxy estolide were converted to ester functionality and this will warrant the product to have good oxidative stability. GPC analysis revealed that the number average molecular weight of the product was 1850 Da and the purity of the product was about 95% with lauric acid constitute the remaining 5%.

3.3. Synthesis of lauric acid capped estolide esters and amides The synthesized lauric acid capped estolide from Entry 9 was used as starting material for the preparation of estolide esters and amides through esterification and amidation reactions, respectively. The carboxylic group of lauric acid capped estolide was reacted with various alcohols and amines without catalyst at temperature between 180 °C to

Fig. 3. Self-esterification of polyhydroxy estolide in combination with lauric acid. 6

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Fig. 4. Overlaid FTIR spectra of (A)polyhydroxy estolide, (B)products from Entry 7 and (C)product of Entry 9, respectively.

Fig. 5. Overlaid 1H NMR spectra of (A)polyhydroxy estolide, (B)products from Entry 7 and (C)product of Entry 9, respectively.

associated with methyl groups of dibutylamine were observed in the spectrum at 0.91 ppm and 0.87 ppm. Furthermore, a peak at 2.59 ppm that corresponded to methylene protons adjacent to amine group were not observed in the spectrum, which suggested complete conversion of amine group to amide functionality. Similar to the 1H NMR spectrum of estolide ester, the 1H NMR spectrum of estolide amide also did not show the presence of a peak associated with α-methylene protons next to the carboxylic acid group and this indicated complete conversion of

corresponding to methylene protons of alcohol adjacent to the ester moiety. At the same time, the peak associated with α-methylene protons next to the carboxylic acid group was not present in the spectrum and this indicated complete conversion of the carboxylic group to ester functionality. On the other hand, the 1H NMR spectrum of estolide amide (5) showed two peaks at 3.26 ppm and 3.17 ppm, which corresponded with methylene protons adjacent to amide group. In addition, peaks 7

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Fig. 6. Synthesis of lauric acid capped estolide esters and amides.

used as base oil for engine oil. Conversely, Com2 is a petrochemicalbased paraffinic mineral oil (TUDALEN 13) that is most commonly used as lubricant base oil. Viscosity is a measurement of the fluid resistance towards flow. In general, viscosity will decrease with higher temperature and the efficiency of a lubricant in reducing friction and wear is greatly influenced by its viscosity at operating temperature. Therefore, kinematic viscosity measurement will provide indication on viscosity profile of lubricant. Referring to Table 3, the kinematic viscosity at 40 °C for estolide esters made from branched alcohols decreased with increasing carbon chain length, in which 2-ethylhexyl estolide ester (4) had the lowest kinematic viscosity among estolide esters made from branched alcohols. Conversely, the isobutyl estolide esters (1) showed the highest

carboxylic acid group to amide group. Fig. 8 shows the overlaid spectra of lauric acid capped estolide, estolide ester and amide. 3.4. Evaluation of lauric acid capped estolide esters and amides as biolubricant basestock The synthesized estolide esters and amides were subjected to lubricant properties evaluation in order to ascertain the suitability of prepared samples to be used as lubricant base oil. Table 3 shows the lubricant properties of lauric acid capped estolide esters and amides. For this study, two commercial lubricants were used as benchmark for acceptable lubricant properties. Commercial biolubricant (Com1) is a 2-ethylhexyl ester of dimer fatty acid (Radialube 7121), which can be

Fig. 7. Overlaid FTIR spectra of (C)lauric acid capped estolide, (D)estolide ester and (E)estolide amide, respectively. 8

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Fig. 8. Overlaid 1H NMR spectra of (C)lauric acid capped estolide, (D)estolide ester and (E)estolide amide, respectively.

terms of VI property. Cold flow property of biolubricant is an important attribute that determine the lower limit of their operating temperature and vegetable oils are known to have poor cold flow properties. Chemical modification of vegetable oil is known to improve the cold flow properties of vegetable oil-based biolubricants (Bredsguard et al., 2016; Salih et al., 2011). The cold flow properties of prepared samples are represented by the CP and PP of each sample as shown in Table 3. The use of branched alcohols to make estolide esters greatly improves the PP of resultant biolubricants. The best PP achieved was −41 °C for estolide ester (3), which was made with the highly branched 4-methyl-2-pentanol. In addition, estolide ester made with another branched alcohol namely 2ethylhexyl ester (4) also attained good PP of −36 °C. It is notable to mention that the PP of these two samples were lower than the PP of Com1, which also employed 2-ethylhexanol as the alcohol component. Furthermore, the PP of isoamyl estolide ester (2) was found to be similar to Com1 at −33 °C. On the contrary, estolide esters made with linear alcohols exhibited poor result for PP, which were higher than commercial samples due to the long straight chain alcohols that facilitated the crystallization of estolide molecules at higher temperature.

kinematic viscosity at 40 °C. The same trend was observed for estolide esters made from linear alcohols, where lauryl estolide ester (8) had a lower kinematic viscosity at 40 °C than octyl estolide ester (7). This finding indicated that as the chain length of the alcohol moiety increases, the kinematic viscosity of estolide esters decreased due to less inter- and intra-molecular tangling similar to results reported by Hwang et al. (2003). The introduction of amide bond to estolide compounds has a significant effect on the kinematic viscosity of estolide amides, wherein both estolide amides had relatively higher kinematic viscosities as compared to their estolide esters counterpart. This could be due to the higher polarity of tertiary amide bond as compared to ester group that contribute to higher intermolecular force, which increases the kinematic viscosity of estolide amides. The kinematic viscosities for all samples at 100 °C showed similar trend to measurement conducted at 40 °C, albeit smaller differences between kinematic viscosities of samples. In terms of viscosity index (VI), which is a measurement of change of viscosity with temperature, all the prepared samples exhibited VI values higher than paraffinic mineral oil commercial sample (Com2), which indicated that most of the prepared samples are excellent in

Table 3 Lubricant properties of lauric acid capped estolide esters and amides. Sample

(1) (2) (3) (4) (5) (6) (7) (8) Com1 Com2

Carbon#

4 5 6 8 8 8 8 12 8 NA

KV 40 °C, cSt

100 °C, cSt

252 168 147 84 417 608 117 104 93 83

25 25 23 13 39 59 19 17 14 11

VI

CP (oC)

PP (oC)

OOT (oC)

WSD (mm)

129 179 184 162 140 164 180 174 148 114

NDa NDa NDa −5 6 NDa −4 16 NDa −12

−25 −33 −41 −36 −21 −15 −6 16 −33 −13

200 198 202 199 205 203 201 202 183 199

0.76 0.81 0.81 0.69 0.57 0.63 0.52 0.70 0.81 0.69

Carbon# = total number of carbons found in alcohols or amines moiety; KV = kinematic viscosity; VI = viscosity index; CP = cloud point; PP = pour point; OOT = oxidation onset temperature; WSD = four-ball wear scar diameter. NA = Not Available. Com 1 = commercial biolubricant (Radialube 7121). Com 2 = commercial mineral oil-based lubricant (TUDALEN 13). a ND = Not Detected. 9

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vegetable oil-based biolubricants can be improved through chemical modification of fatty acids to yield specific estolide esters and amides that displayed excellent properties for biolubricants.

In general, some of the prepared samples that exhibited PP lower than −18 °C are considered acceptable based on literature (Cermak et al., 2015) that reported the use of soy-based lubricant with PP of −18 °C. The cloud points (CP) of some samples were not observed up to their respective PP because their CP is lower than their respective PP, an observation similarly reported by Cermak et al. (2013). Another important aspect of biolubricant is its oxidative stability since vegetable oil-based lubricants are known to have poor oxidative stability (Becker and Knorr, 1996). This property can be evaluated by measuring the oxidation onset temperature (OOT) of biolubricant samples using pressure differential scanning calorimetry (PDSC) method. Typically, a lubricant that shows high OOT value is considered to have good oxidative stability. Referring to Table 3, the OOT results of all samples were similar or better than commercial samples, which indicated that all prepared samples show good oxidation stability. The good oxidative stability of all the prepared samples is a great improvement in comparison to vegetable oils and unsaturated fatty acids such as oleic acid. The improvement on oxidative stability can be attributed to chemical modification of alkene group of vegetable oil to ester functionality, which is less susceptible to oxidation degradation (Salih et al., 2013). It is noteworthy to mention that estolide amides exhibited the highest OOT among the prepared samples. This could be due to higher stability of amide bond as compared to ester bond that requires more energy to break, which contributed to better oxidative stability of estolide amides. One of the main functions of lubricant is to reduce friction and wear on metal surfaces it is applied by forming a stable lubricating film at the contact zone. Vegetable oils and fatty esters are known to impart excellent anti-wear properties due to their ester group. The polar ester groups have affinity for metal surfaces and attach itself to the metal surfaces forming monolayer of film on the metal surface. Meanwhile, the non-polar fatty acid hydrocarbon chain oriented away from metal surface and provides a sliding surface that prevents friction between metal surfaces (Adhvaryu and Erhan, 2002). The anti-wear properties of prepared estolide esters and amides were evaluated according to a four-ball wear test method. The four-ball wear test results in Table 3 showed that all the prepared samples exhibited satisfactory anti-wear properties comparable or better than commercial samples as indicated by respective wear scar diameter. The octyl estolide ester (7) showed the best anti-wear result with the smallest wear scar diameter. In addition, both prepared estolide amides (5 and 6) also exhibited good anti-wear properties that may be due to higher polarity of amide bond as compared to ester group, which contributes to greater affinity for metal surfaces.

Acknowledgements The authors would like to thank the Director-General of Malaysian Palm Oil Board for financial funding and permission to publish this article. The authors would also like to thank the staff of Advanced Oleochemical Technology Division of Malaysian Palm Oil Board for their technical support. The authors are grateful to IOI Oleochemical Industries Bhd for providing free sample of oleic acid (PALMAC 750). References Adhvaryu, A., Erhan, S.Z., 2002. Epoxidized soybean oil as a potential source of hightemperature lubricants. Ind. Crops Prod. 15, 247–254. https://doi.org/10.1016/ S0926-6690(01)00120-0. Adhvaryu, A., Liu, Z., Erhan, S.Z., 2005. Synthesis of novel alkoxylated triacylglycerols and their lubricant base oil properties. Ind. Crops Prod. 21, 113–119. https://doi.org/ 10.1016/j.indcrop.2004.02.001. Aguilera, A.F., Tolvanen, P., Eranen, K., Leveneur, S., Salmi, T., 2016. Epoxidation of oleic acid under conventional heating and microwave radiation. Chem. Eng. 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4. Conclusion Analysis results from this study showed that polyhydroxy estolides can be synthesized from reaction between oleic acid and ‘green’ oxidant hydrogen peroxide, which converted the alkene group of oleic acid to hydroxy and ester functionalities. Subsequently, the prepared polyhydroxy estolide was reacted with suitable amount of lauric acid that converted almost all the hydroxy group to ester functionality, which increased the estolide content of the prepared lauric capped estolide. The conversion of hydroxy group to ester functionality also contributed to better oxidative stability of the product by elimination of alkene functionality. Further reaction between lauric acid capped estolide with alcohols and amines generated estolides esters and amides. In general, estolide esters made with branched alcohols have better cold flow properties than those made with straight chain alcohols. In addition, estolide amides exhibited better oxidative stability than other samples due to stronger amide bond compared to ester bond. In terms of oxidative stability, viscosity index, cold flow and anti-wear properties, some of the prepared estolide esters and amides were found to be comparable to commercial products. Results from this study showed that poor oxidative stability and cold flow properties associated with 10

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novel estolides from dicarboxylic acids and methyl ricinoleate. Eur. J. Lipid Sci. Technol. 118, 486–494. https://doi.org/10.1002/ejlt.201500109. Salih, N., Salimon, J., Yousif, E., 2011. The physicochemical and tribological properties of oleic acid based triesters biolubricants. Ind. Crops Prod. 34, 1089–1096. https://doi. org/10.1016/j.indcrop.2011.03.025. Salih, N., Salimon, J., Yousif, E., Abdullah, B.M., 2013. Biolubricant basestocks from chemically modified plant oils: ricinoleic acid based-tetraesters. Chem. Cent. J. 7, 128. https://doi.org/10.1186/1752-153X-7-128. Salimon, J., Salih, N., Yousif, E., 2010. Biolubricants: raw materials, chemical modifications and environmental benefits. Eur. J. Lipid Sci. Technol. 112, 519–530. https://doi.org/10.1002/ejlt.200900205. Schneider, M.P., 2006. Plant-oil-based lubricants and hydraulic fluids. J. Sci. Food Agric. 86, 1769–1780. https://doi.org/10.1002/jsfa.2559. Sharma, B.K., Adhvaryu, A., Liu, Z.S., Erhan, S.Z., 2006. Chemical modification of vegetable oils for lubricant applications. J. Am. Oil Chem. Soc. 83, 129–136. https:// doi.org/10.1007/s11746-006-1185-z. Soni, S., Agarwal, M., 2014. Lubricants from renewable energy resources – a review. Green Chem. Lett. Rev. 7, 359–382. https://doi.org/10.1080/17518253.2014. 959565.

Hwang, H.S., Adhvaryu, A., Erhan, S.Z., 2003. Preparation and properties of lubricant basestocks from epoxidized soybean oil and 2-ethylhexanol. J. Am. Oil Chem. Soc. 80, 811–815. https://doi.org/10.1007/s11746-003-0777-y. Isbell, T., 2011. Chemistry and physical properties of estolides. Grasas Y Aceites 62, 8–20. https://doi.org/10.3989/gya/010810. Jayadas, N.H., Nair, K., Ajithkumar, G., 2007. Tribological evaluation of coconut oil as an environment friendly lubricant. Tribol. Int. 40, 350–354. https://doi.org/10.1016/j. triboint.2005.09.021. Kassfeldt, E., Goran, D., 1997. Environmentally adapted hydraulic oils. Wear 207, 41–45. https://doi.org/10.1016/S0043-1648(96)07466-2. Mannekote, J.K., Kailas, S.V., 2016. Biodegradability and ecotoxicity evaluation of lubricants. In: Sharma, B.K., Biresaw, G. (Eds.), Environmentally Friendly and Biobased Lubricants. CRC Press, Boca Raton, FL, USA, pp. 151–165. McNutt, J., He, Q., 2016. Development of biolubricants from vegetable oils via chemical modification. J. Ind. Eng. Chem. 36, 1–12. https://doi.org/10.1016/j.jiec.2016.02. 008. Patel, V.R., Dumancas, G.G., Viswanath, L.C.K., Randall, M.R., Subong, B.J.J., 2016. Castor oil: properties, uses, and optimization of processing parameters in commercial production. Lipid Insights 2016, 9. https://doi.org/10.4137/FLPI.S40233. Sammaiah, A., Padmaja, K.V., Prasad, R.B.N., 2016. Synthesis and physical properties of

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