Biolubricants: Raw materials, chemical modifications and environmental benefits

Authors

  • Jumat Salimon,

    Corresponding author
    1. School of Chemical Sciences and Food Technology, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, Bangi, Selangor, Malaysia
    • School of Chemical Sciences & Food Technology, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia Fax: +60 3 8921 5410.
    Search for more papers by this author
  • Nadia Salih,

    1. School of Chemical Sciences and Food Technology, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, Bangi, Selangor, Malaysia
    Search for more papers by this author
  • Emad Yousif

    1. Department of Chemistry, College of Science, Al-Nahrain University, Baghdad, Iraq
    Search for more papers by this author

Abstract

The depletion of the world's crude oil reserve, increasing crude oil prices, and issues related to conservation have brought about renewed interest in the use of bio-based materials. Emphasis on the development of renewable, biodegradable, and environmentally friendly industrial fluids, such as lubricants, has resulted in the widespread use of natural oils and fats for non-edible purposes. In this study, we have reviewed the available literature and recently published data related to bio-based raw materials and the chemical modifications of raw materials. Additionally, we have analyzed the impacts and benefits of the use of bio-based raw materials as functional fluids or biolubricants. The term biolubricants applies to all lubricants, which are both rapidly biodegradable and non-toxic to humans and other living organisms, especially in aquatic environments. Biodegradability provides an indication of the persistence of the substance in the environment and is the yardstick for assessing the eco-friendliness of substances. Scientists are discovering economical and safe ways to improve the properties of biolubricants, such as increasing their poor oxidative stability and decreasing high pour points. “Green” biolubricants must be used for all applications where there is an environmental risk.

Abbreviations:

PE, pentaerythritol; PP, pour point; PTSA,p-toluenesulfonic acid; TMP, trimethylolpropan.

Introduction

The use of natural fats and oils by man dates back to antiquity. The chemical composition of fats and oils and their specific properties have allowed them to be used as foods, fuels and lubricants. The sources of natural fats and oils are numerous and encompass vegetable, animal and marine sources. The usefulness of fats and oils are determined by their chemical nature, and these compounds have common characteristics. Fats and oils are naturally occurring substances that consist – predominantly – of mixtures of fatty acid esters derived glycerol 1.

The present emphasis on conservation has brought about renewed interest in the use of these “natural oils” for non-edible purposes. The sources of natural oils and fats come from various plants and animals-based raw materials (e.g., soy bean, palm, tallow, lard) 2. Plant oils are superior in terms of biodegradability, especially when compared to mineral oils. Attention has been focused on technologies that incorporate plant oils as biofuels and industrial lubricants 3, due to the fact that they are renewable and non-toxic.

The application of plant oils and animal fats for industrial purposes, specifically as lubricats, has been in practice for many years. Environmental and economic reasons lead to the utilization of plant oils and animal fats, or used oils and fats after their appropriate chemical modification. Plant oil-based lubricants and derivatives have excellent lubricity and biodegradability, for which they are being investigated as a base stock for lubricants and functional fluids 4.

Biodegradation is the process by which organic substances are broken down by the enzymes produced by living organisms. The term is often used in relation to ecology, waste management, and environmental remediation (bioremediation). Organic material can be degraded aerobically, with oxygen or anaerobically, without oxygen. A term related to biodegradation is biomineralization, in which organic matter is converted into minerals. By definition, biodegradation is the chemical transformation of a substance by organisms or their enzymes 5.

Plant oils have different unique properties compared to mineral oils, due to their unique chemical structure. Plant oils have a greater ability to lubricate and higher viscosity indices. Superior anticorrosion properties are observed in vegetable oils and are induced by a greater affinity for metal surfaces. High flash points over 300°C classify vegetable oils as non-flammable liquids. However, the applicability of vegetable oils in lubrication is partly limited, as these oils tend to show low oxidative stability and higher melting points. Chemical modification of vegetable oils is an attractive way of solving these problems 6. Biolubricants formulated from plant oils should have the following advantages derived from the chemistry of the base stock:

  • (i)Higher lubricity leading to lower friction losses, yielding more power, and better fuel economy.
  • (ii)Lower volatility resulting in decreased exhaust emissions.
  • (iii)Higher viscosity indices.
  • (iv)Higher shear stability.
  • (v)Higher detergency eliminating the need for detergent additives.
  • (vi)Higher dispersancy.
  • (vii)Rapid biodegradation and hence decreased environmental/toxicological hazards.

This paper gives a short overview of biolubricants and provides specific examples of the chemical modifications of plant oil-based materials for biolubricants. Additionally, a discussion on the contribution of biolubricants to the reduction of environmental hazards is included.

Oleochemicals as raw materials

Oleochemicals have been used in polymers for a long period of time. One can differentiate between the use of oleochemicals as polymer materials and lubricants. Traditionally, oleochemicals such as linseed oil and soybean oil have been used as drying oils in the polymer industry. Oleochemicals have also been used as polymer additives, such as epoxidized soybean oil, which is used as a plasticizer. Oleochemicals are used as monomers for polymer, such as dicarboxylic acids, which are used in polyesters or polyamides (Table 1) 7. Naturally derived materials continue to find use in polymer-related and lubricant-based applications, especially in niche markets 8, 9.

Table 1. Oleochemicals for selected applications
 Product/useSource
Polymer materials
Polymerized soybean oil, castor oilDrying oilsSoybean oil, castor oil
Polymerized linseed oilLinoleumLinseed oil
Additives
EpoxidesStabilizers, plasicizersSoybean oil
Soaps (Ba/Cd, Ca/Zn)Stabilizersstearic acid
Fatty acid esters,- amidesLubricantsRapeseed oil
Building blocks
Dicarboxylic acidsPolyamides, polyesters, alkyl resins, lubricantsTall oil, soybean oil, castor oil
Ether-/ester polyolsPolyurethanes, lubricantsSunflower oil, linseed oil, oleic acid

Oleochemical-based monomers, such as epoxidized oils, maleinated oils, and amidated fats are under investigation 10. Oleochemical-based dicarboxylic acids, such as azelaic, sebacic, and dimer acid (Fig. 1) amount to ∼100 000 tones /year are used as base materials for polymers and lubricants. Oleochemical dicarboxylic acids approximate is about 0.5% of the total dicarboxylic acid market for monomers, where phthalic and terephthalic acids represent 87% 11, 12.

Figure 1.

Building blocks for oleochemicals based on natural oils.

The chemical nature of these oleochemical-derived dicarboxylic acids tends to alter or modify condensation polymers, and will therefore remain in a special niche market. Some desirable properties related to oleochemical derived dicarboxylic acids are elasticity, flexibility, high impact strength, hydrolytic stability, hydrophobicity, lower glass transition temperatures, and flexibility 13. The critical reaction in the development of oleochemicals as building blocks in polymeric materials, coating, and lubricants, is conducted on the double bond of unsaturated triacylglycerol or fatty acids. These reactions such as caustic oxidation, ozonolysis, dimerization, autooxidation, epoxidation, and epoxide ring opening are important processes (Fig. 2) 14. In the following sections recent developments in the field of biolubricants will be presented in greater detail.

Figure 2.

Dimerization of unsaturated fatty acids.

Biolubricants and the environment

Strong environmental concerns and growing regulations over contamination and pollution in the environment have increased the need for renewable and biodegradable lubricants. Accelerating research and development in this area has also been driven by public demand, industrial concern, and government agencies. Better ways to protect the ecosystem or reduce, or reduce the negative impact of spills or leakage of lubricants must be outlined 15. Many terms are used for the classification of lubricants and include products that are environmentally friendly, environmentally acceptable, biodegradable, non-toxic, etc. Approximately 1% of the total mineral oil consumption is used to formulate lubricants.

Figure 3 reveals the volume of the worldwide lubricant market, showing that about one third of all lubricants are consumed in Europe, America, and Asia. Between 13% (EC countries) and 32% (USA) of all used lubricants return to the environment with altered physical properties and appearances 16. These lubricants included those used in frictional loss lubrication and total ∼40 000 tons annually in Germany. Lubricants that remain in the environment also include those used in circulation systems, which are not collected and disposed. In addition, leaked lubricants and those remaining in filters or containers have to be taken into account. Altogether, the environment in Germany is exposed to about 150 000 tons annually. This value is based on the on the static mentioned above and represent the lubricant volume that returns to the environment 17. A calculation based on the actual lubricants consumption in Germany and the disposal rates for different types of lubricants results in about 250 000 tons annually. Once the volume representing lost lubricants and undefined lubricants is accounted for the total volume of lubricants in Germany returning to the environment may be on the order at least 300 000 t/a 18.

Figure 3.

Worldwide consumption of lubricants.

The production, application, and disposal of lubricants have to meet the requirements for the best possible protection of the environment and of living beings in particular. Most often, health hazards to humans are derived from indirect routes through the environment. For all cases of direct contact between lubricants and human beings, compatibility has to be verified. All measures have to be taken to keep the impairment of the environment at the lowest possible level. In evaluating acceptable detrimental effects upon the environment, the benefit of lubricants, such as their performance or economic properties, must be considered and weighed against the risks associated with these lubricants 19.

Environmental definition aspects of biolubricants

A lubricant is a substance (often a liquid) introduced between two moving surfaces to reduce the friction between them, improving efficiency and reducing wear. Lubricants dissolve or transport foreign particles and distribute heat. Some biolubricants also contain small amounts of additives. Plant oils or synthetic liquids such as hydrogenated polyolefins, esters, silicone, and fluorocarbons are used as base oils. Additives deliver reduced friction and wear, increased viscosity, improved viscosity index, resistance to corrosion and oxidation, aging or contamination, etc. 24. In addition to industrial applications, lubricants are used for many other purposes. Other uses of lubricants include bio-medical applications (e.g., lubricants for artificial joints) and the use of personal lubricant for sexual purposes. In an attempt to classify lubricants according environmental risk, many different terms have been established (Table 2).

Table 2. Terms regarding lubricants and the environment
Environmentally positive – non-injurious
Environmentally friendly – eco-friendly
Environmentally sociable – socially friendly
Environmentally justified – environmentally favorable
Environmentally careful – environmentally conservative
Environmentally neutral – harmless to the environment
Environmentally protective – preserves the environment
Environmentally conformable – respectful of the environment

It is useful to rank the terms related to environment risk, as in Table 3. According to this catergorization, no lubricant can be regarded as environmentally friendly, because this term implies an improvement to the environmental conditions. One has to be content with the fact that the lubricant is environmentally acceptable and that it affects the environment to a less pronounced degree 21.

Table 3. Common terms related to the environment
Friendly – improves the environment
Neutral – unimportant, harmless
Threshold of Perception
Sociable – low, unsuitable
Start of Legal Regulations
Annoying – disagreeable, unpleasant, impairing
Irksome – troublesome, inconvenient
Limit of Burdening
Endangering – excessive, unimputable
Harmful – dangerous, irreversible effects

The role, functions, and requirements of a lubricant

A lubricant is a material used to facilitate the relative motion of solid bodies by minimizing friction and wear between interacting surfaces. In addition to the primary purposes of reducing friction and wear, lubricating oils are also required to carry out a range of other functions, including the removal of heat, corrosion prevention, and the transfer of power. Additionally, lubricants must provide a liquid seal at moving contacts and remove of wear particles. In order to perform these roles, lubricating oils must have specific physical and chemical characteristics. Perhaps the fundamental requirement of lubricants is that the oil should remain a liquid over a broad range of temperatures. In practice, the usable liquid range is limited by the pour point (PP) at low temperatures and the flash point at high temperatures. The PP should be low to ensure that the lubricant is pump-able when the equipment is started from extremely low temperatures 22. The flash point should be high to allow the safe operation and minimum volatilization at the maximum operating temperature. For the most demanding applications, such as aviation jet engine lubricants, an effective liquid range over 300°C may be required.23 The efficiency of the lubricant in reducing friction and wear is greatly influenced by its viscosity. The relationship between speed, viscosity, load, oil film thickness, and friction is illustrated by the Stribeck diagram (Fig. 4) 24.

Figure 4.

Stribeck diagram. The coefficient of friction for bearing is plotted against the dimensionless duty parameter µN/σ, where µ is the dynamic viscosity of the lubricant, N the rotational speed of the shaft, and σ is the loading force per unit area 24.

Furthermore, biodegradability is the most important aspect with regard to the environmental fate of a substance. Primary degradation is the first step in the breakdown of a substance and involves the disappearance of the original molecule. However, the determination of the ultimate degradability or the mineralization of substances to CO2, H2O, and the formation of biomass is important. Ultimate biodegradability guarantees the safe reintegration of the organic material in the natural carbon cycle and is important for its environmental classification. Biodegradability depends more on the chemical structure of the lubricant than on its water solubility.

Basic properties and benefits of biolubricant

The term biolubricants applies to all lubricants that are both rapidly biodegradable and non-toxic to humans and aquatic environments. A biolubricant may be plant oil-based (e.g., rapeseed oil) or derived from synthetic esters manufactured from modified renewable oils or from mineral oil-based products 18, 20. Table 4 summarizes some of the benefits of biolubricants.

Table 4. Benefits of biodegradable lubricants
Less emission – due to the higher boiling temperatures of esters. Native triacylglycerol leads to partly gummy structures at high temperature and can accumulate acroleins, which are irritating.
Totally free of aromatics – over 90% biodegradable oils, non-water polluting.
Oil mist and oil vapor reduction-leads to less inhalation of oil mist into the lungs.
Better skin compatibility – less dermatological effects.
High cleanliness at the work place.
Equal and often higher tool life – due to a higher wetting tendency of polar esters, which leads to a reduction in friction.
Higher viscosity index – viscosity does not vary with temperature as much as mineral oil. This can be an advantage when designing lubricants for use over a wide temperature range. This can also result in lower viscosity classes for the same applications combined with easier heat transfer.
Higher safety on shop floor – higher flashpoints at the same viscosity.
Cost savings on account of less maintenance, man power, storage, and disposal costs.

Biolubricants laws and regulations

In several European countries regulations and policies exist in favor of biolubricants 25:

  • (i)In Germany, Austria, and Switzerland regulations are in place that forbid the use of mineral oil-based lubricants around inland waterways and in forest areas. In addition, the German federal government has introduced a program called “Market Introduction Program (MIP) Biolubricants and Biofuels” for the reimbursement of costs associated with substituting mineral oil-based lubricants for lubricants based on renewable resources with a mass content greater than 50%. This program, which is managed by the German Agency of Renewable Resources, is a success especially for hydraulic fluids.
  • (ii)The Swedish City of Gothenburg has set up an advice and technology program for lubricant products, which has encouraged the manufacturing industry to switch to biolubricants. This so-called “Ren Smörja” (Clean Lubricants) project was a co-operation between municipal authorities, consultants, and industries and has resulted in environmental criteria for lubricating greases and hydraulic fluids. These criteria have been identified and are now a part of Swedish Standards. In addition, in the Scandinavian countries a tax exemption on biolubricants is in place.
  • (iii)In Italy, there is a tax on mineral oils and products that contain them.
  • (iv)In 1991, Portugal introduced a regulation mandating the use of biolubricant two-stroke engine oils in outboard boat engines.
  • (v)Belgium has enacted legislation that requires biolubricants to be used in all operations that take place near non-navigable waters.
  • (vi)In the Netherlands, the Dutch Ministry of Spatial Planning, Housing and the Environment issued a policy and action program in favor of biolubricants in 1996. Tax incentives affecting biolubricants are operated under the Dutch VAMIL, which allows for the accelerated depreciation of environmental investments.
  • (vii)Within the USA, the Department of Agriculture is proposing the establishment of guidelines for the designation of items made from bio-based products (including plant-based lubricants) that would be given federal procurement preference, as required under the Farm Security and Rural Investment Act of 2002.

Chemical structure of plant, mineral, and synthetic oils

Plant oils are composed mostly of triacylglycerol (98%) and contain of different fatty acids attached to a single molecule of glycerol. They also contain minor amounts of mono- and di-glycerols (0.5%), free fatty acids (0.1%), sterols (0.3%), and tocopherols (0.1%) 25. Fatty acids are mainly long chain (C18[BOND]C24) unbranched aliphatic acids, with hydrogen atoms attached to carbons and other groups, and a carboxylic acid terminating the chain. The shortest non-branched fatty acid chains (C6) is water-soluble due to the presence of the polar [BOND]COOH group. By increasing the length of the chain, the fatty acid takes on oily or fatty characteristics and becomes increasingly less water-soluble. The carbon chain of a fully saturated fatty acid is straight. When hydrogen atoms are missing from adjacent carbon atoms, the carbons share a double bond instead of a single bond. This type of fatty acid is called an unsaturated fatty acids. These acids have lower melting points than saturated fatty acids. The fatty acid is polyunsaturated if double bonds occur at multiple sites. Therefore fatty acids can be classified as being saturated, mono-, di-, tri-unsaturated, etc. 26.

Mineral oils, on the other hand, are extremely complex mixtures of C20[BOND]C50 hydrocarbons containing a range of linear alkanes (waxes), branched alkanes (paraffinics), alicyclic (naphthenic), olefinic, and aromatic species. They also contain significant amounts of heteroatoms, mainly sulfur. Mineral oils are more stable, cheaper, and more readily available than natural oils, and are also available in a wider range of viscosities. One issue regarding mineral oils is that oils derived from different fields have different characteristics. Another issue regarding mineral oils is the volatilization of low molecular weight components, which leads to a tendency to thicken during use. The presence of low molecular weight components also reduce the flash point of mineral oils compared to natural oils of the same viscosity 27.

There are some applications where performance requirements cannot be met by mineral oils, and it is necessary to chemically synthesize base lubricants with superior properties. There are a range of synthetic lubricants such as poly(α-olefins) (PAOs). These lubricants have characteristics similar to highly refined paraffinic mineral oils, but with a more narrowly defined molecular weight distribution. Alkylbenzenes are a class of synthetic hydrocarbons, although the lubricant industry now almost exclusively uses branched alkylbenzenes, as they have better low temperature fluidity 28. Synthetic organic esters are another class of widely used lubricants. Examples include esters derived from C8[BOND]C13 mono-alcohols and diacids such as adipic acid (diesters) and esters of C5[BOND]C18 monoacids with neopentyl polyols, such as pentaerythritol (PE, polyol esters). The presence of the ester group confers low temperature fluidity and reduces volatility at high temperatures. It also provides an affinity for metal surfaces. Esters were originally developed for the lubrication of aircraft jet engines but have subsequently found wide-spread use, particularly in applications where biodegradability is required 29. For applications where chemical stability is an overriding requirement, non-hydrocarbon-based fluids such as poly(dimethyl siloxanes) and perfluoroalkyl ethers may be used. However, the use of these non-hydrocarbon-based lubricants is restricted by their relatively high cost and their incompatibility with other lubricants and standard additives. All synthetic lubricants are normally used as formulations containing the same types of functional additives as are used in mineral oils 30. Table 5 shows further informations regarding synthetic lubricants available on the market.

Table 5. Type and application of synthetic lubricants
ClassTypeOperating temp. (°C)ApplicationsAdvantages vs. mineral oilLimiting properties
Synthesized fluids hydrocarbons (SFHs)Polyalphaolefins, alkylated aromatics, polybutenes, cycloaliphatic155 to −45Machine tool spindles, freezer plants-motors, conveyors, bearingsHigh temperature stability, long life, low temperature fluidity, high viscosity index,Solvency/detergency, seal compatibility
    Improved wear protection, 
    Low volatility, oil economy 
Organic estersDibasic acid ester, polyol ester204 to −35Commercial manual transmissionNo wax, high temperature stability, long life, low temperature fluidity, solvency/detergencySeal compatibility, mineral oil compatibility, antiwear and extreme pressure, hydrolytic stability, paint compatibility
Phosphate esters (phosphoric acid esters)Triaryl phosphate ester, trialkyl phosphate ester, mixed alkylaryl phosphate esters180 to -18Hydraulic systemsFiber resistance, lubricating abilitySeal compatibility, low viscosity index, paint compatibility, metal corrosion, hydrolytic stability
PolyglycolsPolyalkylene, polyoxyalklylene, polyethers, glycols245 to −20Gas turbinesWater versatility, high viscosity index, low temperature fluidity, antirust, no waxMineral oil compatibility, paint compatibility, oxidation stability

Modification of plant oils for biolubricants

Direct applications of plant oils as lubricants are less favorable due to a variety of factors. Plant oils have poor oxidative and thermal stability, which is due to the presence of acyl groups. The presence of the glycerol backbone in oil gives tertiary β-hydrogen, which is thermally unstable. The chemical modification of plant oils by addition reactions to the double bonds constitutes a promising manner of obtaining valuable commercial products from renewable raw materials. In order to use plant oil-based lubricants with special additives, chemical modifications, de novo synthesis, breeding, and/or biotechnology play an important role. These methods improve the performance and stability of base oils in lubricating formulations. They also provide a sufficient capacity of plant-based oil substrates for green engineering.

Antioxidants and other additives

As outlined above, lubricants and hydraulic fluids are materials that are composed of a base fluid and additives. In hydraulic fluids additives account for only 1–2% of the formulation and ∼10% of the total volume in motor lubricants. In transmission oils, additives constitute about 30% of the formulation. Typically, additives are used as antioxidants, rust (corrosion) inhibitors, de-emulsifiers, wear reducers, PP depressors, and hydrolysis inhibitors. Most additives are common in mineral and plant oils, but the toxicity of currently used additives require research on the development and use of alternatives bio-based additives.

Naturally occurring antioxidants such as tocopherol (vitamin E), L-ascorbic acid (vitamin C), esters of gallic acid, citric acid derivatives, or lipid-modified EDTA derivatives serve as synthetic metal scavengers and may be investigated as alternatives to the currently used toxic antioxidants 31.

Chemical modifications

Chemical modifications such as epoxidation, estolides formation, and tranesterification of plant oils with polyols have been shown to improve the oxidative stability of plant oil-based lubricants and to achieve optimal characteristics for extreme applications 13.

Epoxidation of the carbon–carbon double bond

Plant oils and animal fats are increasingly used as green raw materials in various areas of industry. In the field of lubricants, environmental, and economic reasons lead to the utilization of plant oils and animal fats, or used oils and fats after appropriate chemical modifications. The temperature flow property of pant oils is extremely poor, and this limits their use at low operating temperatures, especially in automotive and industrial fluids. Plant oils have a tendency to form macrocrystalline structures at low temperatures through uniform stacking of the “bend” in the triacylglycerol backbone. Such macrocrystals restrict flow due to the loss of kinetic energy of individual molecules during self-stacking. Several diester compounds have been synthesized from commercially available oleic acid and common fatty acids 32–35. The key steps in the three step synthesis of oleochemical diesters includes epoxidation and ring opening of epoxidized oleic acid with different fatty acids (octanoic, non-anoic, lauric, myristic, palmitic, stearic, and behenic acids) using p-toluenesulfonic acid (PTSA) as a catalyst to yield mono-ester compounds. The esterification reaction of these compounds with butanol, isobutanol, octanol, and 2-ethylhexanol was further carried out in the presence of 10 mol% H2SO4, producing the desired diester compounds (Fig. 5) 32–35.

Figure 5.

Epoxidation of oleic acid, followed by ring opening acylation of EOA using PTSA as a catalyst. RCOOH is octanoic, non-anoic, lauric, myristic, palmatic, stearic, and behenic acid. The esterification reaction was carried out using alcohol (butanol, isobutanol, 2-ethylhexanol, octanol) and H2SO4 as a catalyst.

Not surprisingly, as the length of the mid-chain increase, a corresponding improvement in low temperature behavior is observed. This phenomenon is due to the increased ability of the long chain esters to disrupt macrocrystalline formation at low temperatures. Another observation is the positive effect of branching at the chain end on the low temperature performance of the resultant products, which leads to the formation of microcrystalline structures rather than macrocrystalline structures.

Estolides of oleic acid and saturated fatty acids

Estolides are a class of esters-based on plant oils and are synthesized by the formation of a carbocation at the site of unsaturation. This carbocation can undergo nucleophilic attack by other fatty acids, with or without carbocation migration along the length of the chain, to form an ester linkage. Estolides were developed to overcome some of the short-falls associated with plant oils, such as poor thermal oxidative stability 36 and poor low temperature properties 29. Some deficiencies can be improved with the use of additives but usually at the expence of biodegradability, toxicity, and cost. Cermak and Isbell 37 synthesized saturated mono-estolide esters and enriched saturated mono-estolide 2-ethyl hexyl esters from oleic and saturated fatty acids using three different synthetic routes (Fig. 6). The estolide numbers (ENs), the average number of fatty acid units added to a base fatty acid, varied and was dependent on synthetic conditions. The physical property data indicated that both chain length and EN affect low temperature properties.

Figure 6.

Schematics of the synthesis of oleic estolide esters.

Tallow is a fat that is derived primarily from cattle, but can also be obtained from sheep and goats. Some researchers have used tallow as source of animal fats because the demand for tallow in the global food market has gradually decreased due to health concerns and competition from other fats and oils. Value-added products, such as nutraceuticals, cleaning solvents, and biofuels are developed from tallow to improve its commercial value 38. Tallow is currently in use as an edible oil but is slowly being replaced with healthier oils. With the slow trend away from edible applications to industrial applications, tallow is found in a wide range of products from plastics 39 to lotions and softeners 40, soaps and detergents 41, tires 42, candles 43, paints and varnishes 44, lubricants and fuels 45, and even pharmaceuticals 46.

Foglia et al. 47 determined the low-temperature properties of alkyl esters derived from tallow and recycled greases in neat esters and 20% ester blends in No. 2 low-sulfur diesel fuel. The properties studied included cloud point (CP), PP, cold filter plugging point (CEPP), low-temperature flow test (LTFT), crystallization onset temperature (Tc), and kinematic viscosity. The compositional properties of the alkyl esters determined included water, residual free fatty acids, and free glycerol content. In general, secondary alkyl esters of tallow showed significantly improved cold-temperature properties over normal tallow alkyl ester derivatives. Wu et al. 48 prepared three monoalkyl fatty acid esters derived from tallow and grease by lipase-catalyzed transesterification and evaluated the esters as prospective diesel engine fuels. The low temperature properties of the esters, both neat and as 20% blends in No. 2 diesel fuels were evaluated. The properties investigated included CP, PP, CEPP, LTFT and crystallization onset temperature. Other properties of the esters, such as kinematic viscosity, heating value, and calculated cetane number, were also determined. All three esters had acceptable physical and low-temperature properties as well as acceptable fuel properties, when used as a 20% blend in diesel. Tallow-oleic estolide 2-ethylhexyl (2-EH) esters were synthesized in a one-pot perchloric acid-catalyzed process from 90% industrial oleic and tallow fatty acids with varying ratios. Varying the ratio of tallow and oleic fatty acids, along with the esterification process, provided a functional fluid that may be used as a lubricant 49 (Fig. 7):

Figure 7.

Reaction scheme for the formation of tallow-oleic estolide 2-ethylhexyl ester.

Organic polyesters synthesis

Biodegradable organic polyesters derived from the transesterification/esterification of plant oils and branched neopolyols such as trimethylolpropan (TMP) and PE have been developed for various applications (Fig. 8). Uosukainen et al. 50 described the synthesis of biodegradable TMP [2-ethyl-2-(hydroxymethyl)-1,3-propanediol esters of rapeseed oil fatty acids via enzymetic and chemical methods. Sodium methylate (0.5% w/w) was employed as catalyst, and the reaction mixture was refluxed under a reduced pressure of 3.3 kPa. Approximately 99% conversion was achieved at 110–120°C in 10 h. Using 40% w/w Candida rugosa lipase, only 64% of the TMP was converted to triesters in 24 h at 5.3 kPa at 47°C. With immobilized Rhizomucor miehei (50% w/w), the highest conversion to the TMP triester was 90% and was achieved in 66 h.

Figure 8.

TMP transesterification.

Yunus et al. 51 demonstrated that palm oil TMP esters containing 98% w/w triester can be successfully synthesized in less than an hour. The chemical transesterification of TMP with palm oil methyl esters was attainable under a reduced pressure of at least 20 mbar at T = 120°C with a 3.9:1 molar ratio and the use of sodium methoxide as a catalyst. The optimum molar ratio was established as 3.9:1, and the catalyst was required at less than 1.0% w/w, a quantity far less than the lipase required for enzymatic transesterification (40–50% w/w 50, 52).

Esters of neopentylpolyols were prepared by an esterification reaction between PE and erucic acid catalyzed by 4-toluenesulfonic acid (p-PTSA) in xylenes 53. The reaction mixture was heated to 200°C under a nitrogen atmosphere. Since PE becomes the backbone of the new esters, four types of esters were obtained and included: – tetra, tri, di, and monoesters. These esters provided improved low temperature behavior. Animal fats were also used to synthesize polyol esters using calcium methoxide, but the rate of the reaction was slow. The yield of the reaction was 85–90% after 20 h 54. A two-stage low temperature crystallization process was used to improve the PPs.

Biolubricants and triboreactive materials for automotive applications

Replacing hydrocarbon-based oils with biodegradable products is one of the ways to reduce adverse effects on the ecosystem caused by the use of lubricants. The use of low or no sulfur, low ash and phosphorous (low SAP) esters- or polyglycol-based oils (intended for passenger car engine lubricants as substitutes for hydrocarbon-based oils) requires the preparation of a composition of lubricants with comparable tribological and functional properties. Igartua et al. [55 summarized the results obtained when developing biodegradable passenger car lubricants in combination with triboreactive materials. The study was focused on passenger car motor oils (PCMO) with reduced metal–organic additives. This was necessary in order to reduce the ash build-up in the treatment system and to improve its efficiency and lifetime. High fuel efficiency and long drain intervals are also necessary. Additionally, these oils have to be biodegradable and non-toxic to aqueous environments, according to the directive EC/1999/45, which is in agreement with other international standards. In a modern diesel or gasoline engine, the engine oils has to fulfill a number of functions, such as lubricating and cooling the system, protecting against wear, the handling of soot and particles with low deposit tendency, etc. The study of the biodegradability, toxicity, and tribological properties has been carried out for newly developed prototype engine bio-oils. Furthermore, plasma-sprayed triboreactive coatings have been deposited on cast iron piston rings and studied for their tribological properties. Finally, the behaviors of bio-oils and plasma-sprayed triboreactive coatings on piston rings have been screened in a real engine.

Conclusions

A tremendous demand for plant oils in the lubricant industry is expected over the next few years because plant oils are natural, renewable, non-toxic, non-polluting, and cheaper than synthetic oils. They will become an important class of base stocks for lubricant formulations due to their positive qualities. Due to growing environmental concerns, plant oils are finding their way into lubricants for industrial and transportation purposes. Plant oils, in comparison to mineral oils have different properties due to their unique chemical structures. Plant oils have better lubrication ability, viscosity indices, and superior anticorrosion properties, which are due to the higher affinity of plant oils to metal surfaces. In addition, flash points greater than 300°C classify plant oils as non-flammable liquids. To improvement characteristics such as sensitivity to hydrolysis and oxidative attacks, poor low temperature behavior, and low viscosity index coefficients, plant oils may be chemically modified. Plant oils may be used in almost all automotive and industrial applications. It will become more difficult to find a balance between the economic possibilities of biolubricants and their ecological requirements. Products with toxicological and ecological issues must be excluded from further use in lubricants, if they pose a significant health risk. However, it must be taken into account that the technological level of lubricants will decrease if unnecessary restrictions are put into place. In conclusion, plant bio-based oils are an important part of new strategies, policies, and subsidies, which aid in the reduction of the dependence on mineral oil and other non-renewable sources.

Acknowledgements

The authors acknowledge the Universiti Kebangsaan Malaysia for funding (“Code UKM-GUP-NBT-08-27-113” and “UKM-OUP-NBT-28-145/2009”), and the direct contributions of the support staff from the School of Chemical Sciences and Food Technology, the faculty of Science and Technology, Universiti Kebangsaan Malaysia.

The authors have declared no conflict of interest.

Ancillary