Introduction

Most lubricants on today’s global market are petroleum-based, and the remaining are formulated from chemicals or crude oil. High prices of unrefined petroleum products will be seen after 2050 because of depletion of reservoirs. Sustainable energy sources like alkyl ester and green lubricants might be complementary for the smooth running of vehicle engines [1]. A large extent of lubricants used in the industries are lost in the environment, polluting both water and soil [2]; air is also influenced by fossil-based lubricants [3]. Currently, environmental health and emission concerns are becoming increasingly critical for the public. To overcome these issues, the most practical decision is to use eco-friendly, biodegradable and prudent green lubricants instead of today’s petroleum-based lubricants [4].

There are various methods for producing green lubricants by using different polyols (such as neopentylglycol, trimethylolpropane, pentaerythritol, and dipentaerythritol) [5]. These polyols do not have β-hydrogen atoms, but TMP based-transesterification is the most convenient method for adressing that lack. TMP-based green lubricants can be used as hydraulic fluids, automobile engine oil, jet engine lubricant, and refrigeration lubricants [6].

Green lubricants have recently been getting attention as alternatives to standard petroleum fuels because of their energy independence. Bio-based lubricants are esters of substantial alcohols of crop seed oils, and their lubricating properties are comparable with petroleum-based lubricants [7]. Costs of green lubricants are high compared to conventional petroleum lubricants, so industries are putting resources into R&D and genetic engineering to extract maximum oil from seeds with minimum cost [8]. A financial examination demonstrated that the high cost of green lubricants, in contrast with petroleum fuel, is because of the cost of virgin crop oil [9].

In the US, soybean oil is considered as the main feedstock for green lubricants. However, there is a competition for soy between biofuel and food, which in returns increases the price of biofuels. So in order to reduce production cost for green lubricants, efforts should be made for alternative feedstock [10]. This issue can be resolved by getting base oil from indigenous non-edible crops that could be cultivated on marginal land [11]. Malaysia and Indonesia have an abundance of the indigenous palm tree; more than 80% of global cooking oil demand is fulfilled by palm oil. Their palm oil production is increasing continuously because its utilization as a feedstock is also expanding in the biofuel market. Europe utilizes rapeseed (canola) and sunflower oils from its native crops, while the USA has high production of native soybean oil to formulate bio-based lubricants. Various research papers show that the USA, Europe, and South East Asia have been producing green lubricants from their indigenous crops and using thm in vehicles for the last few decades [12, 13]. This diversity in the utilization of crop oil from indigenous crops plays a significant role in improving the market economy of a country.

Pakistan imports an extensive amount of petroleum products from Middle East nations to fulfill energy needs. Unrefined petroleum imports were 5.9 million tons (27.1 million barrels) with an estimation of US $ 1.84 billion in July-March FY 2017, while about 24.2 million barrels of crude oil were produced domestically during July-March FY 2017 [14]. Pakistan’s car industry has expanded about 25%in the past 3 years. Looking to the future, it is expected that the energy demands for transportation and power will increase up to 53% by the year 2030. This significantly increases fuel demand and purchases. Six out of 11 members of OPEC oil had depleted reservoirs, causing high costs for crude oil. The economy and prosperity of Pakistan is contingent upon the availability of affordable fuel, so it is essential to look for alternative resources which can be extracted from indigenous feedstock [15]. The PCRET and AEDB departments are already working in Pakistan to facilitate renewable energy (RE) resource projects [16, 17], while the Ministry of Petroleum & Natural Resources is working on biofuel standards for biodiesel (B-100) and its blending mandates up to B-20 [18].

At present, it is essential to discuss the valuable role of indigenous oil-yielding crops as sustainable sources of energy for green-lubricant production in Pakistan. This approach has the advantage of providing more employment to farmers [15, 19]. Alternate sources of energy will thus appear as a significant player in improving the socio-economics of Pakistan by diversifying the use of crop oils [20]. After extracting crop seed oils, the remaining oil cake can be utilized as feed for livestock. Oilseed residues can be decomposed into N (nitrogen), P (phosphorus) and K (potassium) rich biofertilizers, or it can be utilized to produce methane gas for power generation [16]. Biodiesel and green-lubricant synthesis require alcohol (methanol or ethanol), which are easily accessible from Pakistan’s sugar industries.

In the past, research has been conducted on various aspects of plants and crop production for biodiesel synthesis in Pakistan [21], but no one has reviewed and studied green-lubricant prospects as an alternative energy source, or its synthesis and applications in Pakistan.

Considering the upcoming scarcity of petroleum sources, the present research work reviewed fatty acid composition of crop seed oils feedstock and its effect on physio-chemical properties of lubricants along with measuring methods and standards. This paper also gives comprehensive details on crop oil potential for green-lubricant synthesis from appropriate standardized advance methods to overcome the limitations of green lubricants in global automobile applications.

Research Methodology

This section describes the three-stage analysis framework adopted for this review.

In the first stage, a systematic literature search was conducted to collect information related to

  • Current status of global and Pakistani green-lubricant markets according to region and applications

  • Doctrinal methods were employed to find Pakistani crop oils’ potentials as a feedstock for green-lubricant production and recent developments in green-lubricant policy in Pakistan. Data were retrieved from primary sources such as national blending mandates, alternate energy development board (AEDB), Pakistan lubricant manufacturers association (APLMA), Pakistan council of renewable energy technologies (PCRET) and Organization of Petroleum Exporting Countries (OPEC) departments

  • All available feedstock for green-lubricant synthesis along with their fatty acid compositions

  • Importance of transesterification as a chemical modification method and advances in this method

  • Factors involved in transesterification and their effect on yield

  • Effect of fatty acid composition and chemical modification on physicochemical, lubricity and tribological properties

  • Advantages, demerits, and constraints of green-lubricant utilization and policy barriers in commercializing the green lubricants

  • Measuring Standards and recent amendments in the laws and regulations for green lubricants made by different countries

All information is accessed from government documents, official reports, news reports, books, Google Scholar, Scopus, Web of Science, Springer, ScienceDirect, Wiley, Emerald, and Hindawi database.

The second stage involved organization, screening, and selection of relevant data by qualitative assessment and cross-referencing. Previously published reports, government documents, articles and review papers are examined and compared with the recent ones to select the most relevant literature for analysis. Findings of the articles were considered as a guideline for critical analysis.

The third stage involved the content analysis to make conclusions. Critical analysis of collected literature shows that most of the green-lubricant feedstock is obtained from edible sources (like soybean, rapeseed, corn, palm, sunflower, canola, olive, coconut, etc), which creates competition with food and increases its prices in the global market. So to find the potential of new alternative non-edible sources of green lubricant feedstock, it is necessary to investigate the effect of fatty acids composition of crop oils on their physicochemical, lubricity and tribological properties.

Moreover, Pure crop seed oils have very low thermo-oxidative stability and poor pour points. Most of the research papers focus on production processes and the use of additives to enhance these limitations. No one has critically reviewed the significance of the fatty acid structural compositions of crop oils. The aim of this paper is also to highlight the importance of laws, regulations and measuring standards. In general, the reports and research articles cited in this review paper not only cover the span of 2007 to present (2019), but also systematically communicate limitations, modifications, development, and effectiveness of green lubricants.

Current Status of Global and Pakistani Green Lubricant Markets

According to a recent report, the world’s green lubricant market was valued from about USD 2.24 billion in 2017. This revenue is expected to increase up to 6.2% with a value of 3.41 billion USD in 2024 with, automotive applications dominating [22]. The global green lubricant market segment and its utilization by region and applications are shown in Figs. 1 and 2, respectively. Figure 2b also shows that maximum utilization of green lubricants is in automotive and transport.

Fig. 1
figure 1

Green lubricant Market Segment

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Fig. 2
figure 2

FY 2017: Global Green lubricant utilization a by regions and b by applications [22]

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According to APLMA, Pakistan’s annual lubricant demand is about 400,000 tons. Figure 3 presents automotive growth and lubricant consumption by automobiles in Pakistan, which is increasing continuously. Mineral and synthetic-based lubricants are used in Pakistan at a large extent, while green lubricants share is less than 1%. Most of lubricant market share is fulfilled by national (local) brands of Pakistan, with only 22% of lubricant being imported, as shown in Fig. 4. Therefore, local brands of Pakistanhave a role to play in replacing petroleum lubricants with green lubricants.

Fig. 3
figure 3

Car growth and lubricant consumption by automobiles in Pakistan [23, 24]

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Fig. 4
figure 4

FY 2018: Lubricant market share by different brands in Pakistan [25]

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Feedstock Considerations for Green Lubricants

Bio-based lubricants e(specially oleochemicals) are usually, but not essentially, based on various crop seed oils (CSOs). Crop oils can easily solubilize polar type additive molecules, so desired properties can easily be achieved for particular applications. Green lubricants can also be synthetic esters that are partly extracted from bioresources, or they can be a combination of crop seed oils and other chemicals. Green lubricants are better than mineral oil-based lubricants because of renewability, low toxicity, biodegradability, high viscosity, high lubricity, high flash points (low volatility) and less friction and wear. However, these have poor pour points and oxidative stability, which limit their use in those applications where the green lubricant is utilized “once-through”, and where very less toxicity is needed; examples include mold release, chain drive or sawmill blades, transmission fluids, cutting fluids and some two-stroke engine lubricants [26,27,28]. Most green lubricants are extracted from edible and non-edible crop seed oils or some other sources as mentioned in Table 1.

Table 1 General classification of green-lubricant feedstocks
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Non-edible crops are attractive for cultivation because of having no competition with edible agricultural sources. Non-edible crop cultivation for green lubricant production has very little or no effect on cultivation or prices of the world’s food. Such crops often can be cultivated on less fertile land [12]. Pakistan, being an agricultural country, has a great potential for increasing the cultivation of oil-yielding crops, which will also help in improving exports and economic growth [29]. Depending upon climate conditions, production, cultivation, and harvesting of oilseed crops in Pakistan can be classified into the categories mentioned in Table 2.

Table 2 The potential of oil yielding crop seeds in Pakistan [30]
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Crop seed oils contain 98% of triacylglycerides (triglycerides) which have saturated or mono-, di-, tri- or poly-unsaturated chains of various fatty acids and glycerol, as shown in Fig. 5.

Fig. 5
figure 5

Triglyceride structure of crop seed oils

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Carboxyl groups of fatty acids attach with three hydroxyl groups of glycerol via ester linkages. Other than triacylglycerides, very small amounts of free fatty acids (0.1%), sterols (0.3%), tocopherols (0.1%) and diglycerides (0.5%) are also the part of vegetable oils [31].

About 350 indigenous oil-yielding crops are available throughout the world for extracting crop seed oils as feedstock for green lubricants. The popular feedstocks of the global market and their prospects in Pakistan, with their fatty acids compositions, are given in Tables 3 and 4.

Table 3 Globally used common green-lubricant feedstocks with fatty acids composition [32,33,34,35]
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Table 4 Major feedstock availability in Pakistan with fatty acids composition
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Physicochemical characteristics of the different feedstock are influenced by the structural compositions of the triacylglycerides, i.e. the number of double bonds and carbon chain lengths in the fatty acids. In crop oils, saturated or unsaturated fatty acid carbon chains vary from C-06 to C-24. Minimum carbon chain length required for successful lubrication is C-9 [43] and lubricity is evaluated by Gibbs free energy of adsorption [44]. Poor pour point and high viscosity of crop seed oils are because of the high molecular weight (longer carbon chain) of triglyceride, which also helps in maintaining their thermal and structural stability [8, 26, 45]. Researchers reported pure crop oils are effective as boundary lubricant because the polar nature of ester group of triglyceride developed strong interaction with lubricating surfaces. In addition, these also have the capability to solubilize additives and contaminants. Increase in double bonds (high degree of unsaturation) improves the lubricity [44] and low-temperature properties but decreases oxidative-thermal stability and viscosity at high temperatures and loads [46]. Sevim Z. Erhan et al. reported that required thermo-oxidative stability at high temperatures can also be achieved by blending high-oleic crop seed oils with polyalphaolefin (PAO) as a diluent, while employing zinc diamyl dithiocarbamate (ZDDC) and antimony dialkyldithiocarbamate (ADDC) as chemical additives [47].

Pure crop seed oils cannot be used directly in industry and IC engine applications for prolonged times due to lower thermo-oxidative stability. However, chemical modification of crop seed oils containing more monounsaturated fatty acids, like high oleic or palmitoleic acid, have a good balance for thermo-oxidative stability, low pour point and viscosity for high-temperature applications [4, 45, 47, 48]. Transesterification, epoxidation or hydrogenation are some recognized chemical modification methods for converting crop oil into effective green lubricant [32, 33, 49]. So it is important to grow crop’s seed, having a high percentage of oleic (C18:1) and palmitoleic (C16:1) acids, by genetic modification. Varieties of soybean, palm, sunflower, rapeseed, and camelina have been produced by genetic engineering to get the desired fatty acid compositions [50,51,52,53]. Genetically and chemically modified crop seed oils, having high thermo-oxidative stability, are currently being utilized in a few fields [54].

Production Technology of Green Lubricants

Pour point and thermal stability of crop seed oils can further be enhanced by transesterification of polyol (NPG/TMP/PE) and methyl ester [52, 55, 56], by epoxidation [28, 57,58,59], which converts C=C bonds into oxirane rings and by selective hydrogenation [60,61,62], which eliminates polyunsaturated C=C bonds. Transesterification is considered suitable when crop seed oils contain high oleic acid (C18:1) content and their acidic value is less than or equal to 1 (FFA < 0.5%) [63]. As we know FFA = 0.5% * AV.

Transesterification with alkaline catalysts produces soaps when reacting with excess FFA. Presence of soap causes difficulty in separating ester and glycerin. Hence, FFA (AV) should be low. The acid value of vegetable oils can also be reduced by acidic catalyzed esterification [64]. Here we discuss only the transesterification, because it is most suitable for genetically improved crops rich in oleic acid. Many green lubricants produced by transesterification use NPG, PE and TMP polyol. However, TMP is most preferable because of its low melting point. This method not only improves thermo-oxidative stability [65], but also reduces wear and friction at extreme pressure by eliminating H-molecule at the beta C-position of a triglyceride of the crop seed oil substrate. TMP ester (green lubricant) made by the transesterification process is suitable for high temperature and pressure applications, such as automotive engines [66].

Chemical Modification of Crop Oils into TMP Based CSOME

Synthesis of chemically modified green lubricant from methyl ester as a starting material is the most efficient method, and it also expands the methyl ester business in the eastern European market [67]. Chemical modification of crop seed oils into TMP-CSOME (green lubricant) by transesterification involves the following steps.

Pre-Transesterification Treatment to Reduce FFA

Initially, it is necessary to determine the acid value (mg KOH/g) of crop seed oils, which indicates the amount of FFA. If the acidic value is greater than 1%, then an esterification process (pre-transesterification treatment) should be performed [68]. During this process, crop seed oils (triacylglycerol) are treated with alcohols (methanol/ ethanol) in the presence of acidic catalysts such as H2SO4, HCl, H3PO4 and sulfonic acid [69]. Usually, the crude crop seed oil is preheated to about 45-50 °C in a flask and then mixed with methanol and stirred for a few minutes. After that, acid is poured into it at atmospheric pressure and reaction continues for at least 30 min with constant stirring at 50 °C. On completion, product is transferred into a separating funnel and left for 10-24 h, then two layers appear. The upper layer has excess alcohol along with acid and impurities. A lower layer having CSO with very low FFA is separated and used for further transesterification processing.

Transesterification of Crop Oil into Methyl Ester

During the production of CSOME, the low FFA triglyceride of crop seed oil (CSO) reacts with alcohol to yield fatty acid alkyl esters. In methanolysis, methanol, as alcohol, reacts with triglyceride. Stoichiometrically, one mole of CSO (triglyceride) requires three moles of methanol, as shown by Eq. (1). Transesterification is a reversible reaction, but catalyst (alkaline) and an excess amount of methanol accelerate the conversion of reactants into methyl esters.

(1)

In conventional transesterification, CSO is pre-heated at 100 °C to ensure the evaporation of any water content, then it is transferred to a batch glass reactor and the temperature is sustained below the boiling point of methanol (65 °C). The optimized quantity of catalyst dissolves in methanol and is poured into the glass reactor. The reaction proceeds for 1-2 h with continuous stirring. Upon completion, the mixture is transferred to a separating funnel at room temperature for 10-24 h. After the prescribed time two layers appear, methyl ester, along with excess alcohol and unreacted triglyceride, remains in the top layer, and glycerol remains in the bottom layer. Both layers are removed from the funnel by sedimentation. The obtained top layer of fatty acid methyl ester is heated at 80 – 100 °C to remove methanol. It is then washed with hot water (80 °C) to remove glycerol, methanol, and catalyst. Anhydrous Na2SO4 is used to dry the remaining water and, finally, purified CSOME is obtained after filtration [70]. The percent yield of CSOME is calculated by using Eq. (2).

$$ \mathrm{CSOME}\%\mathrm{yield}=\left[\mathrm{weight}\ \left(\mathrm{g}\right)\ \mathrm{of}\ \mathrm{CSOME}/\mathrm{weight}\ \left(\mathrm{g}\right)\ \mathrm{of}\ \mathrm{oil}\ \mathrm{in}\ \mathrm{the}\ \mathrm{sample}\right]\times 100\% $$
(2)

New advanced microwave-assisted transesterification (MAT) can also be utilized for ester production. It activates the alcohol molecules and ions by a magnetic field that changes continuously, increasing the reaction rate and yield within a short time [71]. In MAT, the microwave unit has a magnetic stirrer bar and non-contacting infrared constant temperature system. CSO is preheated in the microwave up to the required temperature. Then the mixture of methanol and catalyst is fed into the flask through the condenser. The reaction mixture is irradiated under reflux at optimal reaction times, ranging from 2 to 7 min [72].

Transesterification of TMP with CSOME

Transesterification of TMP with CSOME is carried out in a rotary vacuum evaporator with a round flask to produce CSO-TMP ester, as shown in Eq. (3). At least 20 mbar of vacuum pressure is required for maximum yield of the product.

(3)

Green lubricant is prepared by transesterification of TMP with CSOME as summarized in Fig. 6.

Fig. 6
figure 6

Synthesis Process of CSO based TMP ester (Green lubricant)

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Factors that Affect the Transesterification

The following key factors should be considered carefully to obtain the maximum yield of esters by the transesterification process.

Effect of Molar Ratios of Reactants

The alcohol-to-crop-seed-oil (triglyceride) molar ratio affects the conversion efficiency and production cost. Theoretically, alcohol-to-triglyceride molar ratio is 3:1. However, in most cases, the molar ratio is greater than the stoichiometric ratio to stimulate the reaction towards completion, so its optimum value should be determined to minimize the production cost. Further increase in molar ratio beyond the optimum value yields little improvement in the conversion efficiency and makes ester recovery difficult. Moreover, it also decreases ester yield and viscosity due to its dilution effect [70].

Effect of Catalysts

Correct catalyst type and optimum concentration accelerate the reaction. The concentration of acid catalyst (like H2SO4, HCl, H3PO4 and sulfonic acid) in esterification process usually varies between the ranges of 0.25–2% by volume fractions of the crop seed oil used in the reaction. Addition of acid above the optimum value darkens the color of the product.

Homogeneous alkali catalysts (such as NaOH, KOH, and sodium methoxide) in the transesterification process were varied from 0.5–1.5% by weight of the crop seed oil. The reaction did not accelerate with the insufficient concentration of catalyst, while an excess amount of catalyst beyond the optimum value caused emulsion (gel) formation and ultimately increased the viscosity [73]. Gel formation increases the difficulty in glycerol separation and ester and decreases the ester’s yield.

Heterogeneous base catalysts are getting more attention than homogeneous ones for economic reasons. They are reusable and can easily be modified to include desired characteristics in such a way that even water content and FFA do not adversely affect the transesterification process. Different types of heterogeneous catalysts such as porous zirconia, titania, KOH with MgO or CaO [74, 75] alumina/silica-supported K2CO3 silica [76] zeolites [77] can be efficiently used in esterification and transesterification reactions [78].

The transesterification of TMP with CSOME is a relatively slow process, so sodium methoxide (homogeneous) and calcium methoxide (heterogeneous) catalysts are used to speed up this reaction [79].

Effect of Reaction Temperature

Temperature also affects the conversion efficiency. At room temperature, efficiency is very low, but it increases rapidly as with the rise in temperature up to the optimal value. Overly-high temperature during acidic esterification, increases the production cost and darkens the color of the product. During alkali transesterification of CSO, the temperature should be below 65 °C to avoid evaporation of methanol and saponification of the FFA of glycerides with alkali catalyst. For TMP, methyl ester reaction temperature varies from 100 to 250 °C.

Effect of Reaction Duration

During the reaction, constant and continuous stirring accomplish perfect contact between the crop seed oil and the reactant. Ester production rate slightly increases with the increase in reaction time. Usually, 30-180 min are adequate for esterification and conventional transesterification processes, and 2-7 min are required in microwave-assisted transesterification.

Table 5 gives details of feedstock for green-lubricant production along with influencing factors and operating conditions.

Table 5 Production methods for green-lubricant synthesis
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Microwave assisted transesterification appears to be a good upgrade to speed up the production method.

Physicochemical and Lubricity Properties of Green Lubricants

Acid Value (AV)

Acid value (mg KOH/g) of a crop seed oil indicates the quality and amount of FFA in the oil. A low acid value is required to protect the metal parts of the machine or engine from corrosion. The acid value of anhydrous crop seed oil should be less than 1 for alkali-catalyzed transesterification to avoid soap formation [88]. The acid value of high FFA-containing crop seed oil can be reduced by the esterification process. During esterification, crop seed oils are treated with alcohols in the presence of acidic catalysts [69]. AV can be determined using the AOCS Cd 3d-63 method or by the titrimetric method under ASTM D 664.

Viscosity & Viscosity Index

Oil-resistance against flow/ fluidity is called viscosity. High viscosity means long fatty acid C-chain length and establishment of thicker lubricant film. High viscosity plays a very vital role in increasing the lubricity of green lubricants [89]. For industrial applications, the viscosity range should be 5–15 cSt at 100 °C. Generally, kinematic viscosities of polyol esters range between 5-225 cSt @ 40°C and 2-20 cSt @ 100°C. Polypropylene can be used as a viscosity enhancer

The viscosity index (VI) reflects variation in viscosity because of temperature changes. Low VI indicates that viscosity will change as temperature varies, and vice versa. High viscosity is required in a good lubricant, as it can maintain oil film thickness over a wide range of temperatures. Lubricants with low VI become less stable at high temperature as oil film gets thinner and less viscous. The VI also depends on the molecular structure length of the fatty acids. It increases as linear C-chain length and un-saturation level increase, but branching in the structure decreases [90] the VI [91]. For instance, castor oil has higher viscosity and VI than highly refined mineral oil because of the hydrogen-bond of the hydroxyl monounsaturated triglycerides [19].

Some additives, like decyl acrylate polymers and isooctyl acrylate polymers, can be used to enhance the VI of green lubricants. Polyol esters (NPG, TMP, and PE-based chemically structured green lubricants) shows high viscosity. Polyol esters are recommended for drilling fluids [90], hydraulic fluids, catapult oil and engine oils. Usually, viscosity and VI are measured at 40 °C and at 100 °C by viscometer under kinematic viscosity methods ASTM D445 and ASTM D2270, respectively.

Flashpoint/ Fire Point

Flashpoint is the minimum temperature required to heat the green lubricant before it vaporizes. It demonstrates the instability range and safe working temperature of the oil. Fire pointreflects the minimum temperature at which the green lubricant catches fire from an external flame. Flash and fire points should be high in order to minimize fire risk during lubricant leakage. Flashpoints of crop seed oils are high, correlating with very low volatility and vapor pressure, thus reducing hazards during practical usage [53]. Both can be measured by Cleveland open cup apparatus, as per ASTMD 92 / ASTM D93, or by using a tag closed tester under ASTM D5679.

Thermal / Oxidative Stability

Thermal stability gives information about the degradation rate of the green lubricant at elevated temperatures. It should be superior or similar to the maximum limit of temperature in any particular application [92], and is measured by thermogravimetric analysis.

Oxidation causes polymerization and increases the viscosity and acid content of the green lubricant. Consequently, metal parts become corroded, sludge is produced and decreases the lubricant’s overall efficiency. Oxidation starts because of the double bonds present at the beta carbon-position in the structure of fatty acids, as shown in Fig. 7.

Fig. 7
figure 7

Critical oxidation sites in triglyceride structure of crop oils

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Thermal-oxidative stability of crop oils is strongly influenced by fatty acid composition and its structure. Shorter FFA chains and a high proportion of saturated fatty acids in a crop oil increases its oxidative stability, but is not favorable for low-temperature behavior. Coconut oil has high oxidative stability because it contains 90% saturated fatty acids [93]. While poly-unsaturated crop seed oils readily oxidize at room temperature, only crop oils containing mono-saturated fatty acids (such as oleic acids, C18:1) have comparatively good oxidative stability at high temperature.

A very small percentage of anti-oxidants, such as pyrogallol [94], ascorbic acid, BHT (butylated hydroxytoluene), PG (propyl gallate), MTBHQ (mono-tert-butyl-hydroquinone), BHA (butylated hydroxyanisole), and naturally occurring tocopherols [45], can be used to overcome this problem. Chemically modified feedstock (polyol esters) have much better thermal-oxidative stability at high temperature and rigorous conditions. Their response is good towards antioxidants and also resists aging processes better than mineral oil [86]. The following methods can be performed to determine the oxidation stability of crop seed oils.

  1. i)

    Pressure differential scanning calorimetry (PDSC) method

In the PDSC method, oxidation stability is determined by onset temperature (OT) during oxidation induction time (OIT) of isothermal experiments. The low value of OT suggests low oxidation stability. the PDSC method consists of a computerized thermal analyzer. The OT is calculated directly from the plot (exotherm) drawn between heat flow and temperature for each case [47].

  1. ii)

    Rotary bomb oxidation tests (RBOT)

In the RBOT test, a sealed vessel containing crop seed oil and copper catalyst rotates axially in an oil bath set at 150 °C and 90-psi, as per ASTM D-2272. The pressure of the rotary bomb is also monitored and RBOT time is noted when the pressure of the rotary bomb reaches 25.4 psi [47].

  1. iii)

    Long-term oven oxidation stability test

This is the most commonly utilized test; it can be used for any type of pure, additized, blended or chemically modified vegetable oil. In this test, CSO is poured into glass vials and put into the oven at 85 °C for 28 days. Change in the weight of CSO and total acid number (TAN) are determined at 0 and 28 days to find oxidative stability. The total acid number of crop seed oils remains in the range of 0.17–0.38 mg KOH/g, but in the absence of anti-oxidant it increases to 2–3 mg KOH/g after 28 days [47].

Pour Point (Low-Temperature Properties)

Pour point indicates the lubricant’s fluidity at low temperatures. It is an important characteristic, especially in winter and for cold weather countries. A good lubricant has a low pour point so that it can provide lubrication, even at extremely low temperatures. Usually, crop oil loses fluidity below −10 °C when it remains in cold weather for a long time [33].

A long chain of FA and a high degree of unsaturation in fatty acids chains favors the low temperature and anti-wear characteristics of vegetable oils, but it is not favorable for oxidative stability [47, 95]. Rapeseed oil has the lowest pour point, at −21 °C [86]. Sevim Z. Erhan (2006) has improved the pour point (PP) of soybean oil by adding 20% PAO (polyalphaolefin) as a diluent, having PP of −57 °C. A small amount of butylated hydroxyl toluene can be used as a pour point depressant [64]. Isobutyl oleate, 2-ethylhexyl oleate, di-isodecyl adipate, trimethylolpropane trioleate, and pentaerythritol tetraoleate are also used as pour point depressants [96].

Green lubricants produced from methyl oleate, TMP triundecenoate, and canola biodiesel have pour points of −51 °C, −36 °C, and − 66 ,°C respectively [96]. Numerous chemically modified feedstock have very low pour points, −30 to −70 °C, and gives good lubrication during cold starting and protects engines parts from damage. DSC (Differential scanning calorimetry) calculates the pour point under ASTM D97.

Tribological Properties

About 15% to 20% of fuel energy is wasted in overcoming frictional forces in IC engines. A lubricant having low COF (coefficient of friction) can reduce this energy wastage [97]. The polar heads of the saturated and monounsaturated fatty acids form a monolayer film with metal surfaces, and stick away from the non-polar end. As a result, the coefficient of friction is reduced. Polyunsaturated fatty acids prevent the close packing of carbon-chains, weakening the monolayer film and increasing COF and wear rate. Avocado oil presents very good anti-frictional and anti-wear performance, better than other crop seed oils [98].

Green lubricants show better anti-wear properties than mineral oils [13, 99]. Lovell et al. discovered that boric acid powder as an additive in crop seed oil improves friction and wear performance [100]. Agarose as an additive also improves the coefficient of friction. The four-ball tester machine measures the tribological properties. The coefficient of friction is measured under ASTM D 5183-05, and wear scar diameter by ASTM D4172-94.

All the lubricity and tribological requirements of commercially available petroleum-based lubricants and bio-based lubricants are given in Table 6. Table 7 shows that physicochemical properties of pure crop seed oils can be improved by chemical modification.

Table 6 Reference green lubricants requirement according to applications [8, 101]
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Table 7 Physico-chemical properties of different types of green lubricants [32, 46, 52, 55, 86, 96, 102]
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Laws and Regulations for Green Lubricants

Green lubricants are receiving public attention because of environmental and health issues caused by the evaporation, loss, leakage, and spillage of mineral lubricants. Therefore it is the responsibility of legislators and regulatory authorities to take measures for protecting the ecosystem from harmful effects of lubricants and to expand the market share of environmental friendly green lubricants as mentioned in Table 8.

Table 8 Legislation and regulation’s concerns of green lubricants
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In many European countries, governments have taken strict actions in their policies for limiting the use of mineral-based lubricants. They stimulate the use of renewable, non-toxic and biodegradable resources for green-lubricant synthesis and application in sensitive areas like forestry, agriculture, mining, shipping, marine, sawmills, heavy industry, pulp and paper mills, transportation, motorcycle chains, etc. They also introduce many projects and policies to support research and development activities by giving subsidies in different areas and developing a legislative framework to expand the green-lubricant market.

Table 9 represents the laws, regulations, and policies of different countries for promoting green lubricants.

Table 9 Laws and regulations for green lubricants [103, 104]
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Conclusion and Future Recommendations

Growing demands for energy, increasing prices of mineral oil, as well as environmental and health concerns, have directed a lot of research and development in the field of lubricants. Physico-chemical property reviews showed that lubricity and tribology of lubricants depend on fatty acid composition of base oil. Increase in C-C chain length (up to C18-C22) increases the viscosity and lubricity of green lubricants. Unsaturated fatty acids play a significant role in improving lubricity with a decrease in friction and wear rate. Poly-unsaturated fatty acids are favorable for low pour points, but increase oxidative instability. Saturated FA increases oxidative stability as well as wear rate. Mono-saturated fatty acid (oleic acid C18:1 or erucic acid C22:1) content is more desirable for improving both thermal-oxidative stability, pour point and lubricity. Crop oils with high oleic acid (C18:1) content showed low COF and wear rate. Thus, crops should be cultivated according to the required fatty acid composition for any specific application.

Chemically modified green lubricants have good thermo-oxidative stability, low-temperature characteristics and improved tribological characteristics. Therefore, for high oleic acid oils, TMP-based transesterification is most suitable. Upgraded microwave-assisted transesterification increases yield and production rates of methyl ester by decreasing reaction time. High oleic transesterified TMP-based green lubricant has shown superior physio-chemical and tribological properties at extreme conditions that are comparable with commercially available lubricants. Addition of some suitable additives can also make them a competitive automotive lubricant.

Cottonseed oil has a high percentage of linoleic acids (C18:2) and oleic (C18:1) acids, but its potential as a feedstock for green lubricant production has not been explored. Its fatty acid composition shows that TMP-based transesterification process can produce a promising green lubricant from cottonseed oil for high temperature and pressure applications to replace the conventional petroleum-based lubricants. Cottonseed oil can also be utilized as an emerging green lubricant feedstock for the global market. Moreover, cottonseed oil, mustard oil, sunflower, corn and moringa oils are extracted from the indigenous crops of Pakistan, while cottonseed oil is available abundantly. Dynamic climatic conditions and huge uncultivated areas of Pakistan also support the cultivation of crops mentioned in the Table 2.

Therefore, strong collaboration between agriculture and biotechnological sectors is necessary for increasing the production rate of high oleic indigenous crop oils as green lubricant feedstock. As a result, feedstock cost could be diminished and green lubricants could prove a favorable solution for the global green lubricant market as well as for Pakistan’s crippled energy economy in the automotive sector.

Moreover, Pakistan should develop legislation, rules, and regulations for green lubricant synthesis so that the green lubricants sector can participate actively in the economic growth of the country.

Continued cooperation between government, farmers, researchers, genetic engineering, green lubricant manufacturers and the end-user is required to ensure the cultivation of cheaper and potential feedstock, according to required fatty acid composition. It is also necessary that organizations like ISO, EU, EN, and ASTM provide complete guidelines on feedstock requirements and performance criteria for environment-friendly sustainable green lubricants, along with their pros and cons.