Chemical modification of vegetable oils for the production of biolubricants using trimethylolpropane: A review

Egyptian Journal of Petroleum xxx (xxxx) xxx

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Review

Chemical modification of vegetable oils for the production of biolubricants using trimethylolpropane: A review F.J. Owuna a,⇑, M.U. Dabai a, M.A. Sokoto a, S.M. Dangoggo a, B.U. Bagudo a, U.A. Birnin-Yauri a, L.G. Hassan a, I. Sada b, A.L. Abubakar c, M.S. Jibrin d a

Department of Pure and Applied Chemistry, Faculty of Science, Usmanu Danfodiyo University, Sokoto, Nigeria Department of Pure and Industrial Chemistry, Faculty of Natural and Applied Science, Umaru Musa Yaráduwa University, Katsina, Nigeria Department of Biochemistry, Faculty of Science, Usmanu Danfodiyo University, Sokoto, Nigeria d Department of Pure and Industrial Chemistry, Faculty of Physical Sciences, Bayero University, Kano, Nigeria b c

a r t i c l e

i n f o

Article history: Received 21 January 2019 Revised 17 October 2019 Accepted 21 November 2019 Available online xxxx Keywords: Chemical modification Biolubricant Lubricating oils Vegetable oils Trimethylolpropane

a b s t r a c t Lubricating oil producers are shifting attention toward the use of renewable and biodegradable energy sources for the production of lubricating oils. This is necessitated by the depleting mineral based energy sources and the negative impact of continuous usage of engine oils from fossil sources. Biomass sources are cheap, environmentally friendly, and offer a good alternative to the conventional mineral oil sources. Biolubricants provide lubricity for two moving-surfaces in contact. They are essential for heat transfers, power transmissions, lubrication, and corrosion inhibition in machinery. However, the use of biolubricating oils are associated with challenges such as poorer low temperature properties and poor oxidative stability during usage. Chemical modification of vegetable oils with polyols has been explored as a potential source for biolubricant synthesis and production. This paper provides a concise review of the use of trimethylolpropane (TMP) as the polyol used for chemically-modified biolubricants using vegetable oils as base stocks. TMP improves the physicochemical properties of biolubricants and enhances the thermooxidative stability of the biolubricants. Ó 2019 Egyptian Petroleum Research Institute. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Contents 1. 2.

3. 4. 5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Lubricating oil compositions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Mineral oil base stocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Re-refined oil base stocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Synthetic oil base stocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Biomass base stocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5. Lubricating oil additives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Merits of using biolubricants. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Demerits of using biolubricants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Physicochemical properties of biolubricants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Viscosity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

00 00 00 00 00 00 00 00 00 00 00

Abbreviations: AV, Acid Value; A-W, Anti-Wear; CP, Cloud Point; D, Density; DBP, Dibutyl 3,5-di-t-butyl 4-hydroxy Benzyl Phosphate; EP, Extreme Pressure; FM, Friction Modifiers; FP, Flash Point; ISO, International Organization for Standardization; IV, Iodine Value; KV, Kinematic Viscosity; OS, Oxidative Stability; PAG, Polyalkylene Glycols; PAMA, Poly Alkylmethacrylate; PAOs, Poly Alpha Olefins; PKO, Palm Kernel Oil; PKTMP, Palm Kernel Trimethylolpropane; PP, Pour Point; PPD, Pour Point Depressants; SAE, Society of Automotive Engineers; SOA, Sulfurized Octadecanoic Acid; SV, Saponification Value; TAG, Triacylglyceride; TE, Triesters; TETA, Triethylenetetramine; TMP, Trimethylolpropane; VG, Viscosity Grade; VI, Viscosity Index; WCO, Waste Cooking Oil; ZDDP, Zinc Dialkyldithiophosphate. Peer review under responsibility of Egyptian Petroleum Research Institute. ⇑ Corresponding author. E-mail address: [email protected] (F.J. Owuna). https://doi.org/10.1016/j.ejpe.2019.11.004 1110-0621/Ó 2019 Egyptian Petroleum Research Institute. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Please cite this article as: F. J. Owuna, M. U. Dabai, M. A. Sokoto et al., Chemical modification of vegetable oils for the production of biolubricants using trimethylolpropane: A review, Egyptian Journal of Petroleum, https://doi.org/10.1016/j.ejpe.2019.11.004

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F.J. Owuna et al. / Egyptian Journal of Petroleum xxx (xxxx) xxx

5.2. Viscosity index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Cloud point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4. Pour point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5. Flash point and fire point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6. Acid value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.7. Base value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.8. Oxidative stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.9. Iodine value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Methods of improving vegetable oils for the production of biolubricants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. Chemical modification of vegetable oils using TMP. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8. Properties of TMP based biolubricants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9. Applications of biolubricants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Compliance with ethics requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Declaration of Competing Interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1. Introduction

2. Lubricating oil compositions

Biolubricants act as antifriction agents which reduce the risks associated with machine failures and maintains optimum operations. They are essential for heat transfer, power transmission, lubrication, and corrosion inhibition in machinery [1].The main purposes of biolubrication are to protect the surfaces from corrosion, reduce oxidation, reduce wear due to contact, prevent heat loss from the surfaces in contact, act as insulator in transformer applications, as sealing agents (against dust, dirt and water), biodegradable, and improve efficiency of machines [1–4]. The major property of any biolubricant is viscosity, which is responsible for preventing friction between two surfaces in contact [1]. Other important qualities of biolubricants include temperature stability, prices, availability, environmental friendliness, toxicity, chemical stability, corrosiveness, flammability, and compatibility [4,5].The common chemical-pretreatment (Fig. 1) for the conversion of vegetable oils to biolubricants is esterification of the free fatty acids with methanol in the presence of acidic catalysts, and subsequent transesterification of the produced methyl esters to biolubricants using TMP [6–9]. Jeevan and Jayaram [10] observed that chemical modification of vegetable oils produced better lubricating effects and improved the lubricating oils’ affinity between metal surfaces in contact. Vegetable oils have been found useful in biolubricating processes to produce tailor-made products [11–14]. Muhammad et al. [15] reported that vegetable oil producing plants take more carbon dioxide from the atmosphere through the process of photosynthesis than the amount of carbon dioxide that is added to the atmosphere during burning. Vegetable based bioluricating oils are over 95.00% biodegradable and 20.00 – 30.00% degrade faster compared to mineral based lubricating oils [5]. Biolubricants have been synthesised from crude plant oils by chemical modification of their porperties in order to achieve better performances than mineral oil lubricants [3,16–18]. Chemical modification of fatty acids (Table 1) in vegetable oils enhances their thermal as well as oxidative stability, and enablesthe biolubricants to withstand wide range of operating conditions [3,10,17]. Trimethylolpropane is a colourless triol with molecular formular CH3CH2C(CH2OH)3, and it is a good substitute for triacylglyceride (TAG) in the production of triesters (TE) as biolubricants [3,19–23]. The use of TMP for the formulation of biolubricants from vegetable oils has generated interests among researchers. This paper is aimed at reviewing the chemical modification of vegetable oils for the formulation/production of biolubricants using TMP as the polyol.

Pre-defined properties of base stocks, such as, low volatility, ideal cleanliness, high biodegradibility, high solvency for lubricant additives, negligible effects on seals and elastomers, are majorly responsible for determining oxidative stability, low temperature properties, hydrolytic stability, deposit forming tendencies and viscometric parameters of lubricants [3,17,32–34]. Lubricating oils are composed of basestocks and additives formulated to enhance their performance.

2.1. Mineral oil base stocks Mineral oil base stocks are obtained from petroleum mineral oil [18,35]. They are available and cheap because they are obtained as lube fractions (distillates from the vacuum distillation) during refining of crude oil [36,37]. Their utilisation do not constitute food crises, however, they are non-renewable and toxic to the environment [38,39].

2.2. Re-refined oil base stocks Re-refined oils are used oils derived from petroleum that have been refined and purified for the removal of contaminants (and impurities). They are processed via treatment to remove volatile and insoluble components and additives through acid/clay treatment, solvent extraction, flash/vacuum distillation, and demetallization and hydroprocessing catalysts [40,41]. Hence, products with equal characteristics with mineral base oils are obtained. Reactivation of used oils protects the environment from negative effects of improper disposition, reduces heavy metals and greenhouse gas emission compared to burning the exhausted oils as fuel [41].

2.3. Synthetic oil base stocks Synthetic base oils are obtained by chemical modification of petroleum oil or petrochemical feedstocks. They are generally more stable to heat and oxidation than mineral basestocks. They are also available with superior viscosity index, and are more biodegradable in comparison to mineral oils. Synthetic base stocks are ecofriendly and are end products of reactions that are tailored for specific operations. Examples of these base oils are synthetic esters, poly alpha olefins (PAOs), polyalkylene glycols (PAG) [41], and silicons.

Please cite this article as: F. J. Owuna, M. U. Dabai, M. A. Sokoto et al., Chemical modification of vegetable oils for the production of biolubricants using trimethylolpropane: A review, Egyptian Journal of Petroleum, https://doi.org/10.1016/j.ejpe.2019.11.004

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F.J. Owuna et al. / Egyptian Journal of Petroleum xxx (xxxx) xxx

Fig. 1. Synthesis of polyol ester from vegetable oils using trimethylolpropane. Where R1, R2 and R3 are different alky groups.

Table 1 Percentage Fatty Acids Composition of some Vegetable Oils. Vegetable oils

Palmitic acid (16:0)

Stearic Acid (18:0)

Oleic Acid (18:1)

Linoleic Acid (18:2)

Linolenic (18:3)

Reference

Calabash oil Calabash oil Calabash oil Palm oil Palm oil WCO Moringa oil

12.11 2.11 16.32 39.32 41.50 28.91 5.50

8.49 2.54 7.86 4.36 2.70 0.93 5.70

17.86 20.20 36.23 42.52 40.60 26.51 73.20

60.15 58.20 1.74 11.35 11.90 27.44 1.00

0.12 1.70 – – 0.30 4.60 –

[24] [25] [26] [23] [27] [8] [28]

2.4. Biomass base stocks Biomass base stocks are obtained from plants and animal sources and are used for the production and or synthesis of biolubricants. Biomass sources include protein, free leaves, vegetable oils, coffee pulp, seaweeds, paper mill sludge, lignocellulose, and various agro-residues [42–47]. 2.5. Lubricating oil additives Additives are formulated with lubricating oils to enhance their physichochemical properties during operations. Lubricating oil

additives that are produced from mineral based resources introduce harmful materials, such as heavy metals and sulphur compounds, into the environment [48]. Some common additives used for lubricating oil formulations include dibutyl 3,5-di-t-butyl 4hydroxy benzyl phosphate (DBP), triphenyl phosphorothionate, sulfurized octadecanoic acid (SOA), sulfurized docosanoic acid (SDA), triethylenetetramine (TETA), zinc dialkyldithiophosphate (ZDDP), and poly alkylmethacrylate (PAMA) [48–50]. In recent times, producers of lubricating oil additives have started reformulating and redesigning additives, such as fatty amides, fatty amines, fatty alcohols, propyl gallate, tailored for biolubricants with great successes [17,48]. These additives func-

Please cite this article as: F. J. Owuna, M. U. Dabai, M. A. Sokoto et al., Chemical modification of vegetable oils for the production of biolubricants using trimethylolpropane: A review, Egyptian Journal of Petroleum, https://doi.org/10.1016/j.ejpe.2019.11.004

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F.J. Owuna et al. / Egyptian Journal of Petroleum xxx (xxxx) xxx

Table 2 Physicochemical Properties of some Vegetable Oils. Oils Jatropha[18] Calabash Seed oil [25] Calabash Seed oil[29] Palm oil[23] Palm oilester[23] WCO lub. [30] Palm kernel[31]

KV @100 °C (cSt)

KV @40 °C (cSt)

VI

14.24 – – – 10.96 8.500 7.800

66.74 – – 52.13 50.33 36.70 34.90

220 – – – 214 220 210

D @15 °C (g cm 3)

FP (oC)

0.92 – – 0.91 – – 0.92

– – – – 253 – 322

PP (oC)

AV (mg KOH g

5 – – – 5

29.06 5.92 2.02 6.35 – 1.56 0.05

2 15

1

)

IV (mg I2 g

1

)

– 4.02 0.75 – – – 0.89

Keys: KV (Kinematic Viscosity); VI (Viscosity Index); D (Density); FP (Flash Point); PP (Pour Point); AV (Acid Value); IV (Iodine Value).

tion are viscosity index improvers (VII), pour point depressants (PPD), detergents, dispersants, anti-wear (AW) agents, extreme pressure (EP) additives, anti-oxidants, friction modifiers (FM), anti-foams, metal deactivators, and rust and corrosion inhibitors [17]. Additives and or impurities dependent characteristics of lubricants include demulsibility, water rejection, colour, foaming, ash content, acidity, load carrying capacity, corrosion inhibition, antiwear protection and lubricity [17,32,48]. 3. Merits of using biolubricants Biolubricants have excellent lubricity, higher viscocity index, have been reported to produce fewer emission due to higher boiling temperature ranges of esters, lower volatility, higher flash/fire points, less dermatological problems both to humans and animals, and are biodegradable [3,13,17,18,51]. 4. Demerits of using biolubricants Previous researches revealed that the major concerns with the use of biolubricants are their poor resistance to thermal and oxidative degradation owing to the presence of acyl group in their molecules (glycol backbone in oil give rise to a tertiary b-hydrogen that is thermally unstable) [3,10,12,17,51–53]. It was also reported that low thermal and oxidative property of vegetable oils is caused by methylene interrupted poly unsaturation [4,10,32]. Biolubricant also have poor low temperature properties due to the formation of macro crystalline structures (at low temperatures through uniform stacking of triglycerides’ back bones) which limit the easy flow of the fluids due to loss of kinetic energy of the molecules during the self-stacking [30]. The presence of ester group was also found to be responsible for low-temperature fluidity and reduced volatility at high temperatures in biolubricants [13]. Further reviews confirmed that for suitability as biolubricant, a fluid should contain properties such as biodegradability, cleanliness (particle count), less water content, poor acidity, high viscosity, high viscosity index, homogeneity, low volatility, low pour point, high oxidative stability, low iodine value, and elastomer compatibility [3,54]. 5. Physicochemical properties of biolubricants Biolubricating oils have some properties that enhance their performances and suitability for various applications. Some of these properties, which are described in this section, are also presented in Table 2 and Table 4. 5.1. Viscosity Viscosity is a measure of biolubricant’s thickness, or resistance to flow. Quantitative measure of fatty acids’ and vegetable oils’

resistance to flow is called viscosity [4,13]. The higher the biolubricant’s viscosity, the thicker it will be and more energy will be needed to move an object through it [4,13].Viscosity is a major factor which determines biolubricant’s application. Low viscosity stocks can be used for automotive transmission oils, while higher viscosity stocks are used in diesel engine oils. In metal forming application, the effectiveness of a firm in separating the workpiece from the tool (in order to control friction and wear) is determined by the viscosity of the biolubricant [13]. The kinematic viscosity of biolubricants are measured at 40 and 100 °C, and the viscosity compared with an empirical reference scale. 5.2. Viscosity index Viscosity index (VI) of a biolubricant is the measure of the change of the biolubricant’s viscosity with change in its temperature. Viscosity of a biolubricant is inversely proportional to its temperature; therefore a machine that operates over a wide temperature range will require a lubricant with higher viscosity index. The higher the viscosity index, the lower the effect of temperature on the viscosity of lubricating product [4]. 5.3. Cloud point Cloud point (CP) is the temperature at which the first sign of wax formation for biolubricant can be detected [13]. It is the temperature at which first sign of haziness is observed. The wax crystals formed can clog filters and openings, thereby leaving deposits on surfaces such as a heat exchanger, and increase the viscosity of the biolubricating oil. 5.4. Pour point At low temperature, the viscosity of the biolubricant will be very high, causing it to resist flow. The lowest point at which biolubricant sample can flow by gravity alone is known as the pour point of the oil. It is also the last temperature before movement ceases, and not the temperature at which solidification occurs [13]. Pour point (PP) is an important characteristic for an equipment that operates in a cold environment or handles cold fluids. Highly viscous oils may cease to flow at low temperatures because their viscosity become too high due to wax formation. In such case, the pour point will be higher than the cloud point. 5.5. Flash point and fire point Flash point (FP) of a biolubricant is the lowest temperature at which the vapourised oil can be ignited by an external source, while fire point is the temperature at which biolubricant combustion would be sustained for, at least, five minutes after the ignition source has been removed [16,55]. The flash point and fire point are used in determining biolubricant’s volatility and fire resistance.

Please cite this article as: F. J. Owuna, M. U. Dabai, M. A. Sokoto et al., Chemical modification of vegetable oils for the production of biolubricants using trimethylolpropane: A review, Egyptian Journal of Petroleum, https://doi.org/10.1016/j.ejpe.2019.11.004

F.J. Owuna et al. / Egyptian Journal of Petroleum xxx (xxxx) xxx

Transportation and storage requirements of biolubricant are determined by the biolubricant’s flash point. Products with a flash point less than 38 °C (100°F) will normally require cautions for safe handling. Thus, flammability hazard of lubricating oils are determined by the flash point and fire point [13,74]. Mahmud et al. [76] reported that the number of carbon atoms in the tryglycerides determine the flash point of a biolubricant. 5.6. Acid value When the concentration of acidic compounds in a biolubricant is high, it can lead to corrosion of machine parts and clogged oil filters due to the formation of varnish and sludge. Total acid value is a measure of acid concentration present in a biolubricant. The acid concentration of any biolubricant depends on the presence of additive package, acidic contamination, and oxidation by-products [49,56]. 5.7. Base value Lubricants are formulated with alkaline additives in order to combat the build-up of acids as they break down. Total base value is a measure of alkaline concentration present in a biolubricant. Gasoline oils are typically formulated with total base value of about 5–10 mg KOH g 1 whereas diesel engine oils, due to more severe operating conditions, are higher (15–30 mg KOH g 1). The base value is depleted as the oil remains in service. Depletion of base value beyond certain limit puts the engine at risk of corrosion, sludge, and varnish formations. At such point, it is necessary to change the lubricant or top-off [56]. 5.8. Oxidative stability Reaction of biolubricants resulting in corrosion, acidity, volatility and viscosity are determined by their oxidative properties. Oxidative stability is frequently used to predict lubricants’ service life in conditions of higher temperatures and other extreme applications [13,23]. Saturated fatty acids have relatively high oxidation stability, which decrease with increasing unsaturation in the molecules [5]. Vegetable oils have poor oxidative and thermal stability owing to the presence of acyl group in their molecules. Glycol backbone in an oil give rise to a tertiary b-hydrogen, which is thermally unstable [13]. Chemical modification of vegetable oils via reactions such as esterification, acetylation across double bonds and epoxidation, are promising methods for obtaining valuable commercial products from biodegradable raw materials. The unsaturated structures of vegetable oils make them less stable to oxidation than mineral oils. Antioxidants of 0.10–0.20% are effective in mineral oil formulation, but vegetable oils may require a larger amount of such antioxidants (1.00 – 5.00%). A low oxidative stability indicates that oil oxidizes rapidly during its use, becoming thick and polymerizing to a plastic-like structure [41]. 5.9. Iodine value Iodine value is a measure of the amount of double bonds present in the molecules of a given sample of biolubricant. It reflects the biolubricant’s susceptibility to oxidation reactions. Unsaturated biolubricants take up iodine while saturated ones do not and therefore, the later have zero iodine values [57]. Iodine value is a unique property of seed oils [58] making them good starting materials for soaps, foods, lubricants, pharmaceutical, and cosmetics industries [58,59].

5

6. Methods of improving vegetable oils for the production of biolubricants Several methods used to improve the lubricating properties of vegetable oils have been identified. These methods include partial selective hydrogenation, genetic modification, biotechnology [50], additive treatment [37], and blending [4,28,60–64]. Other methods include chemical modifciations via epoxidation, strcutural modification, and transesterification [30,65,66]. Chemical modifications and blending have been tested to improve the flash point, pour point, viscosity and oxidative stability of vegetable oil based biolubricants [2,4,5,11,63,64,66]. Weimin and Xiaobo [30] studied the chemical modification of waste cooking oil (WCO) via expoxidation using H2O2 and followed by transesterification with methanol and branched alcohols (isooctanol, isotridecanol and isooctadecanol) to produce biolubricant with improved oxidative stability and low temperature properties. It was confirmed that the synthesised biolubricants showed improved temperature flow performances due to the introduction of branched chains in the molecular structures and the oxidative stability of the WCO showed more than 10 times improvement due to the elimination of –C@C– bonds in its molecule. The tribological performances of the produced biolubricants were investigated using fourball friction and wear tester and favourable physicochemical properties and tribological performances were observed, making these oils good candidates in formulating ecofriendly lubricants. Kailas et al. [61] studied the lubrication properties of different vegetable oils, namely, soybeans oil, olive oil, almond oil, amla oil, castor oil, groundnut oil, cotton seed oil, coconut oil, mustard oil, at different temperatures. It was found that the lubricating properties of oil, such as, cloud point, pour point, flash point, fire point and % carbon residues, change with changing vegetable oil blends. In another research, Yashvir [62] studied the friction and wear characteristics of pongamia oil blended lubricant at different load and sliding distance using pin-on-disc tribometer at 3.8 m/s sliding velocity and applied load of 50, 100, 150 N. A blend of the biolubricant at a ratio of 15, 30, and 50% by volume with the base lubricant SAE 20 W-40 were carried out and it was found that the lubrication occurred regime was boundary lubrication while the main wear mechanisms were abrasive and adhensive wear. It was concluded that pongamia oil in the base lubricant acted as a very good lubricant additive that reduced the friction and wear scar diameter during the test. Also, Ozioko [28] studied the properties of blended Moringa oleifera oil with conventional SAE 40 lubricant from 10 to 40% by volume using magnetic stirrer. Aluminium pin was used against carbon steel dics to analyse the viscosity, density and rate properties of the oil and it was found that at 40 °C and 100 °C, viscosity of MOL 10 (Moringa oleifa lubricant) satisfied SAE 30 and SAE 40 grades requirements; MOL 20 satisfied SAE 30 but did not meet SAE 40 grades requirements; the densities of all the blended samples were found comparable to those of the conventional base oil; and the wear rate of all the blended samples increased with applied load. It was concluded that MOL 10 blend could be commercially viable for industrial application since it showed comparable properties with the base lubricant, SAE 40, in terms of density, viscosity and wear rate. Furthermore, Obasi et al. [37] investigated the effectiveness of additives on the performance of engine oil. The performance function selected include viscosity, density, flash point, colour as well as foaming ability/stability. The additives were B023233 (comprising of anti-oxidant, detergent, dispersant, pour point depressant, anti-corrosion and anti-rust additives) and B23333 (comprising of viscosity modifier additive). All the laboratory tests were carried out in accordance with the specification of the American Society

Please cite this article as: F. J. Owuna, M. U. Dabai, M. A. Sokoto et al., Chemical modification of vegetable oils for the production of biolubricants using trimethylolpropane: A review, Egyptian Journal of Petroleum, https://doi.org/10.1016/j.ejpe.2019.11.004

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methyl esters and trimethylolpropane to environmentally acceptable palm oil TMP ester biolubricant. Robiah et al. [31] prepared and characterized trimethylolpropane esters synthesized from palm kernel through transesterification of palm kernel oil with TMP using sodium methoxide as catalyst and the following basic properties of the non-additive palm kernel trimethylolpropane ester (PKTMP) were obtained: viscosity at 40 °C in range of 39.70–49.70 cSt, pour point of 1 to 1 °C, and viscosity index of 167–187. Furthermore, Siti et al. [72] examined batch production of trimethylolpropane ester from palm oil as lubricant base stock through transesterification reation in a mini pilot reactor. Zulkifli et al. [73] examined tribological properties of parafin oil and biolubricant with TiO2 nanoparticles as additives. The biolubricant is a TMP ester of palm oil. The friction and wear experiments were performed using four-ball machine tribotester for ten minutes under 40.00, 80.00, 120.00, and 160.00 kg load at 1200 rpm at room tempereture. The experimental result showed that nanoparticles, TiO2, added to TMP ester exhibit good frictionreduction. Other scholarly works on syntheses of biolubricants via chemical modifications of vegetable oils with trimethylolpropane are presented in Table 3 below:

for Testing and Materials (ASTM). The results of the tests obtained from the blend show that the properties such as viscosity, density and flash point, increase with increasing in additive concentration, while that of the foaming ability decreases with increasing in additive concentrations

7. Chemical modification of vegetable oils using TMP TMP is one of the polyols that is used for the modification of vegetable oils for biolubricant production [66]. Chemical modifications of seed oils via transesterification of the oils with trimethylolpropane were employed by various authors for the production of biolubricants using various conditions of temperatures and catalyst concentrations. Such seed oils include jatropha curcas [18,23,67,68], rubber seed [69], fluted pumpkin seed [70], and castor seed [71]. Robiah et al. [27] studied synthesis of palm oil and palm kernel polyol esters via transesterification of the oils with TMP using temperature column (SGE HT5) operated at 6 °C min 1 starting from 80 to 340 °C. In another investigation, Robiah et al. [19] developed optimum synthesis method for the transesterification of palm oil

Table 3 Chemical modifications of vegetable oils with TMP. Oil sources

Oil:TMP ratio

Catalyst

Reaction conditions

Yield (%)

Properties

References

Jatrophaseed Palm oil Fluted Pumpkin

3.9:1 3.9:1 6:1

1% NaOCH3 0.7% NaOCH3 Ca(OH)2

150 °C;10mba;3h 130 °C;50mba;1h 160 °C;6h

47.00 90.00 81.42

[68] [27] [70]

Rubber seed Jatropha seed Palm oil Jatropha seed Palm kernel

3.9:1 4:1 3.9:1 3.9:1 3.9:1

2% H2SO4 2% HClO4 0.9% NaOCH3 0.9% NaOCH3 0.9% NaOCH3

150 150 130 130 130

Castor seed

4:1

0.8% o-phosphoric acid

120 °C;1h(in situ)

PP(-3 °C);VI(178–183 – V@40 °C(60.78 cSt);V@100 °C(11.030 cSt);VI(1 7 6); PP(-14 °C); FP(220 °C) FP(310 °C);VI(2 8 3);PP(-40 °C) PP(–23 °C);FP(>30 °C);VI(1 5 0) PP(5 °C);FP(2 5 3);VI(2 1 4) VI(1 4 0);PP(-3 °C);FP(273 °C) V@40 °C(39.70 cSt);V@100 °C(7.700 cSt);FP(310 °C); PP(2 °C);IV(18.2;SV(223.10 V@40 °C(45.30);V@100 °C(9.200);VI(1 9 1); PP(-8 °C);FP(215 °C)

°C;5h °C;3h °C;4h:10 mmHg °C;4h °C;vaccum

79.00 70.00 97.80 98.20 98.00 96.56

[69] [67] [23] [23] [19] [71]

Keys: PP (Pour Point); VI (Viscosity Index); V (Viscosity); FP (Flash Point); IV (Iodine Value); SV (Saponification Value)

Table 4 Properties of Crude Oils and TMP Based Biolubricants. Oils

KV @100 °C (cSt)

KV @40 °C (cSt)

VI

FP (oC)

ISO VG32 ISO VG46 ISO VG68 ISO VG100 75 W-90 80 W-140 SAE20W40 Crude Jatropha Jatropha TMP Crude Palm Palm Oil TMP Crude Castor Castor Oil TMP Crude Palm Palm Oil TMP (50 mmHg) Palm Oil TMP (10 mmHg) Crude Jatropha Jatropha Oil TMP Crude PKO PKO TMP Oleate TMP Crude Jatropha Jatropha TMP

>4.100 >4.100 >4.100 >4.100 15.90 31.20 13.90 7.900 8.710 10.20 9.000 19.72 26.13 – 7.580 10.87 – 8.530 5.934 20.54 15.32 4.830 8.510

>28.80 >41.40 >61.40 >90.00 120.0 310.0 105.0 35.40 43.90 52.40 47.10 220.6 287.2 52.13 38.25 50.33 36.97 51.89 26.03 48.06 80.80 17.15 39.51

>90.00 >90.00 >198 >216 140 132 132 205 180 180 176 220 119 – 171 214 – 140 185 110 200 233 204

204 220 226 246 205 210 200 – – – – 250 – – 240 ‘253 273 296 200 210 289 92 178

PP (oC) 6 6 6 6 48 36 21 6 6 5 2 27 27 – 5 5 3 3 20 8 59 7 12

OS

Reference

– – – 1670.26 – – – – 325 – 355

[66] [66] [66] [66] [66] [66] [66] [66] [66] [66] [66] [66] [66] [23] [23] [23] [23] [23] [75] [75] [76] [77] [77]

– – – – – – 189 – –

Keys: KV (Kinematic Viscosity); VI (Viscosity Index); FP (Flash Point); PP (Pour Point); OS (Oxidative Stability)

Please cite this article as: F. J. Owuna, M. U. Dabai, M. A. Sokoto et al., Chemical modification of vegetable oils for the production of biolubricants using trimethylolpropane: A review, Egyptian Journal of Petroleum, https://doi.org/10.1016/j.ejpe.2019.11.004

F.J. Owuna et al. / Egyptian Journal of Petroleum xxx (xxxx) xxx

8. Properties of TMP based biolubricants Chemically modified vegetable oils exhibit improved viscosity indices, flash/fire points, lubricities, and superior anti corrosion characteristics [13] than their crude basestocks. Furthermore, TMP modified vegetable oils have enhanced properties than their original source oils, and are comparable to standards and commercially available lubricants (Table 4). 9. Applications of biolubricants Biolubricants have been utilized for various maintenance and industrial applications [78–80]. Biolubricants have found great uses in machines that loose oil directly into the environment during operations (Total Loss Lubricants) and in machinery used in environmentally sensitive areas such as water treatment and food processing [48]. Generally, biolubricants are being used as hydraulic fluids [48,50,79] in power equipment, two-stroke engines [48,50,79], aircraft jet engine [13], metalworking fluids [79], boat engines [50], chain saw oils [48,79], drilling fluids [48] used for geological explorations, marines [48], greases [79], and railroad flanges [48]. Other applications include concrete mould release agent, dust suppressants, metalforming fluids, cutting fluids, and gear oils [79]. Ester based stocks have potentials to be used for the production of lubricants and additives [13,79], while ecofriendy TMP esters offer high performance advantages that are being exploited for their applications as engine oils, gear oils, hydraulic oils, and additives [80]. 10. Conclusion The renewed effort of producers and consumers of lubricating oils, coupled with various environmental regulations, that have resulted in researchers finding alternative sources of energy (from biomass), is yielding result. TMP has won the interest of researchers in biolubricating oil formulation due to its enhancement of the performance qualities and thermo-oxidative properties of biolubricants. Biolubricants that are produced through chemical modification of vegetable oils with TMP are eco-friendly, renewable, and useful for any operations where mineral based lubricating oils are applicable. Compliance with ethics requirements This paper does not contain studies with human/animal subjects Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement Authors acknowledge TheRoyalFamily for financial support. References [1] K.J. Amit, S. Amit, Research Approach & Prospects of Non Edible Vegetable Oil as a Potential Resource for Biolubricant - A Review, Adv. Eng. Appl. Sci.: Int. J. 1 (2012) 23–32. [2] S. Jamat, S. Nadia, Y. Emad, Biolubricants: Raw Materials, Chemical Modifications and Environmental Benefits, Eur. J. Lipid Sci. Technol. 112 (2010) 519–530.

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Please cite this article as: F. J. Owuna, M. U. Dabai, M. A. Sokoto et al., Chemical modification of vegetable oils for the production of biolubricants using trimethylolpropane: A review, Egyptian Journal of Petroleum, https://doi.org/10.1016/j.ejpe.2019.11.004