Key words

biolubricants; triacylglycerols; fatty acid esters; oxidative stability; low temperature fluidity; viscosity; high oleic oils; chemical modifications

17.1 Introduction

Consumers around the world are more receptive to greener products made from renewable resources and which offer societal benefits such as lower CO₂ emissions, reduction of waste to landfills, and reduced reliance on fossil resources. Globally, studies have shown that biobased products can reduce CO₂ emissions by using less fossil fuels and emitting fewer greenhouse gases than traditional petroleum-based alternatives. In the United States, approximately 10% of crude oil imports are used to produce chemicals, lubricants and plastics. Replacing petroleum-based products with products derived from renewable sources will directly reduce the dependence on crude oil. Development of biobased products of added value is an essential component to make the integrated biorefineries economically competitive. Although biobased products are much lower volume than fuels, they add disproportionate value. With new developments occurring in biotechnology and research and development in the production of fats and oils through crops, algae and fermentation and their subsequent chemical transformations, biobased products can gain market traction resulting in manufacturing economies-of-scale. Therefore, biobased products will increasingly achieve cost parity with traditional petroleum-based products in the next decade (Biotechnology Industry Organization, 2010).

Even before the invention of the wheel, early civilizations used natural fats and oils also known as triacylglycerols (TAG), for illumination, heating and lubrication. In the late nineteenth and early twentieth centuries, the availability of cheap and abundant supply of petroleum products rapidly displaced fats and oils in industrial applications. However, in the past three decades awareness and concern about the usage of petroleum-based products and their impact on the environment has created an opportunity to produce environmentally acceptable products from agricultural feedstock. The benefits of these products in comparison to petroleum derivatives include lower pollution (air, water and soil), minimal health and safety risks, and easy disposal due to their non-toxicity and facile biodegradability. Additionally, plant-derived products are sustainable, human-compatible and will not change the natural balance of our ecosystem (Kodali, 1996; Willing, 2001).

Biolubricants are broader in scope, contain base oils derived from renewable materials including plants and algae and possess desirable environmental qualities such as ready biodegradability, low toxicity and human compatibility with minimal safety and health risks. Biolubricants are also considered as environmentally friendly lubricants (EFL). A recent Environment Protection Agency (EPA) document on environmentally acceptable lubricants (EAL) describes EAL fluids as meeting standards for biodegradability, toxicity and bioaccumulation potential that minimize their likely adverse consequences in the aquatic environment, compared to conventional lubricants (EPA, 2011). The base oils used in EAL must be biodegradable, and the most common categories of base oils used in EAL fluids are vegetable oils, synthetic esters and polyalkylene glycols. However, currently there are no regulatory standards for EAL fluids.

The recent heightened interest in biolubricants derived from renewable resources such as vegetable oils is due to their ready biodegradability, low/no toxicity, and environmentally benign nature. The realization that the petroleum-based economy cannot be sustained emphasizes the need to use renewable materials and practices to replace the petroleum products to a larger extent in the near future. The natural abundance, cost effectiveness, and inherent lubricity of vegetable oils and their ester derivatives are making inroads into various lubrication formulations such as metalworking fluids, hydraulic fluids, fuels, petroleum fuel additives and electrical insulation fluids (Hwang and Erhan, 2002; Erhan, 2005).

The commodity crops in the US produce a surplus and are partially sustained by subsidies. Global competitiveness in the production of fats and oils led to overproduction, beyond that required for food needs. Currently more than 20% of the world’s vegetable oils production is being used for non-food applications (Gunstone, 2011). The total production of fats/oils over time is approximately doubling in volume every 20–25 years. For example, the total annual production of fats and oils around the world grew from about 50 million metric tons (MMT) in 1980 to more than 100 MMT in 2000 and is expected to reach close to 200 MMT by 2020. The recent prices of major types of vegetable oils are almost comparable to those of mineral oils (Gunstone, 2011). These factors strongly support the development of alternative applications for vegetable oils that provide new and value added markets.

The global demand for finished lubricants was 37.4 million tons in 2004 and the recent estimates for 2011 were very similar, with very little to no growth in the recent past. Of this, the United States accounts for 25%. The US finished lubricants are valued at about $10 billion. The major segments are about 50% for automotive lubricants and about 30% for industrial lubricants. More than 80% of the automotive fluids are engine oils, which require demanding performance. A large segment of the industrial fluids, about 40%, are hydraulic fluids, which require high performance but to a considerably lesser degree than engine oils. Other large segments of industrial lubricants include about 16% for metalworking fluids, 9% for greases and various other applications. Whatever the precise figures, approximately 50% of all lubricants used worldwide end up in the environment via total-loss applications, evaporation, spillage or accidents (Schneider, 2006). Estimates for the loss of hydraulic fluids are even higher, about 70–80% (Carnes, 2004; Miller et al., 2000). Currently, over 95% of the materials used in lubricant products are based on mineral oils or synthetic oils. Mineral oils are toxic for mammals, fish and bacteria.

The important lubricant functional properties are viscosity, viscosity–temperature behavior (viscosity index, VI), reduced coefficient of friction to minimize wear of moving parts, oxidation stability, low-temperature fluidity and less volatility (low evaporation losses) and compatibility with seals (elastomers). The physical and functional properties of vegetable oils compared to mineral oils differ considerably and are related to their respective chemical structures (Kodali, 2002). The representative chemical structures of TAG oils and mineral oils are shown in Fig. 17.1. Petroleum-based mineral oils comprise straight or branched-chain paraffins, alicyclic and aromatic hydrocarbons and combinations thereof. This structural variation provides broad viscosity ranges and low temperature properties for formulation of different products from the base stocks. Also the structural heterogeneity, lack of polarity and chemically reduced state offer good low temperature fluidity and oxidative stability. However, the mineral oils by themselves are not good lubricants as they are inert and do not deliver the desired lubrication properties.

As shown in Table 17.1, mineral oils have significantly higher coefficient of friction (μ) compared to ester-based fluids. The additives technologies developed since 1950 made it possible that the mineral oils can be formulated into almost all the lubrication applications. Due to lack of chemical functionality, the mineral oils lack boundary lubrication, degrade slowly, possess lower viscosity index and higher volatility. The molecular heterogeneity of mineral oil-both in structure and molecular weight contribute to higher volatility. The evaporation losses are directly proportional to the volatility of the material. The evaporation losses of mineral oil-based formulations at 250°C for 1 hr (Noack volatility) can vary from 7 to 20%, whereas the fatty acid esters are < 5%, usually 1–3% under the same conditions. This point in general is illustrated in Fig. 17.2, where the weight loss is plotted against the temperature for mineral oils vs. fatty acid esters by thermogravimetric analysis (TGA).

Comparatively, vegetable oils possess the desirable characteristics of high viscosity index, high flash point, and reasonable pour points. Vegetable oils provide better boundary lubrication and load carrying capacity due to their inherent chemical structure. The polar ester region of the vegetable oil orients itself toward the metal surface, while the nonpolar hydrocarbon region orients away from the metal, providing a stable boundary layer that reduces wear and enhances the load and boundary lubrication properties as evidenced from Table 17.1. In addition, they are nontoxic, readily biodegradable, and a renewable resource. However, the oxidative stability and low temperature properties are a major limitation for lubrication applications. Both these properties are interdependent due to the chemical structure of triacylglycerols (TAG). Oxidative stability is related to polyunsaturated content of the oil, whereas the low temperature flow properties are related to the amount of saturation. Some conventional vegetable oils with high monounsaturated content, such as rapeseed and high oleic sunflower, are suitable for use in less severe lubrication applications. However, many high monounsaturated oils such as olive oil have limited application as lubricants mainly due to prohibitive cost and limited availability.

Many equipment and lubrication manufacturers are showing greater interest in environmentally acceptable lubricants due to regulations and the need to provide a green alternative to their customers (Battersby, 2000). The cost of vegetable oils is comparable to mineral oils, the regular feedstock used for industrial applications. Compared to synthetic esters, the high end ingredient for lubricant formulations, vegetable oils offer considerable cost advantage. The industry is looking for sustainable biobased renewable products like vegetable oils and their derivatives to replace petroleum-based products. The vegetable oil varieties with modified fatty acid profile and/or their chemically modified derivatives having good oxidative stability and low temperature fluidity can fill this new market niche.

A new class of biobased esters, derived from vegetable oils, with excellent low-temperature flow properties and oxidative stability can be produced. One of the major advantages of these biobased esters is better performance at a lower cost compared to petroleum-based synthetic esters. This is possible owing to recent advances in the biotechnology of vegetable oils, and their chemical modifications to convert these natural esters into highperformance biolubricants.

The scope of this chapter is to provide the fundamental understanding of the structural features of TAG oils and fatty acid ester derivatives that make them (un)suitable for lubrication. Further, the structure and functional property relationships and comparison of biobased materials and petroleum products will be presented. The TAG oils production, markets, fatty acid composition and how the newer oils created by biotechnology differ from the regular oils will be provided. The generally required functional properties of lubricants will be discussed followed by how the biobased lubricants can meet these required functional properties. The TAG oils’ inherent structural features, modified by chemical transformations resulting in products providing better functionality suitable for lubrication, will be described from the recent developments. A number of new technologies that have potential for commercial applications, their salient features and advantages will be discussed followed by future trends. This chapter is geared mainly towards understanding and development of high-performance biobased ester lubricants.

17.2 Markets for lubricants

The total lubricants consumed in 2004 amounted to 37.4 million tons worldwide of which 53% were automotive lubricants, 32% industrial lubricants, 5% marine oils, and 10% process oils. Of total industrial lubricants, 37% were hydraulic oils, 7% industrial gear oils, 31% other industrial oils, 16% metalworking fluids, and 9% greases (Mang, 2007). The total value of lubricants is about $40 billion annually. The current demand for lubricating fluids in the US is about 10 million metric tons. Engine oils and hydraulic fluids represent a substantial volume of this demand. Bremmer and Plonsker (2008) provided a comprehensive study on the biobased lubricants markets, volumes and value with emphasis on US markets.

Most of the lubricant base stocks are petroleum-derived mineral oils (Groups I, II and III), polyalphaolefins (PAO), synthetic esters and poly-alkyleneglycols (PAG). The approximate price comparisons of petroleum base oils, synthetics and vegetable oils are provided in Table 17.2. Synthetic oils are high performance and value materials that are of interest in comparing biolubricants in terms of cost/performance in various applications. Synthetic oils by industry definition include the very high viscosity index (VHVI) group III oils, polyalphaolefins (PAO), polyalkyleneglycols (PAG), saturated and unsaturated synthetic esters and other modified materials used for lubrication. The VHVI group III oils are produced by cracking and severe hydrogenation of mineral oil and contain > 90% saturates and offer good oxidative stability. They are mainly used in automotive crankcase, transmission and power steering fluids. They are a cheaper alternative to PAO in the high performance synthetic lubricants market.

Polyalphaolefins (PAO) are saturated oligomers made from long-chain alpha olefins of C8 to C12. They are relatively homogeneous materials and offer many functional advantages like higher oxidative stability, multiple viscosity grades, good thermal stability, low corrosivity and compatibility with mineral oils. The PAO markets are mainly in automotive applications with a minor fraction in industrial applications and a very small segment as aviation fluids. Most high end synthetic lubricants are formulated with PAO. However, they may require esters to compensate for or enhance the performance properties. Polyalkyleneglycols (PAG) are unique base fluids that are miscible with water. They are mainly used in fire resistant applications. The main market segments are hydraulic fluids, heat transfer fluids, and metal working fluids among others.

Synthetic esters offer many functional performance advantages. The saturated esters offer excellent thermal and oxidative stability. They offer good boundary lubrication, low wear, high viscosity index, low volatility, good additive solubility and are compatible with mineral oils and PAO. Additionally they are readily biodegradable and less toxic. There are three types of synthetic esters: diesters, polyolesters and complex esters. The polyolesters and complex esters are higher molecular weight materials and offer better oxidative stability and withstand higher operational temperatures. The excellent lubrication properties of esters combined with higher temperature performance make them the fluids of choice in jet engine lubrication. They are mainly used in industrial applications, aviation fluids and in the automotive industry. The major applications of esters are jet engine oils, refrigeration lubricants, crankcase motor oils, and compressor lubricants.

In almost all applications, the primary criterion to switch from petroleum-based fluids to biolubricants is cost/performance ratio with a rare exception of enforced regulations. The estimated base oil prices provided in Table 17.2 give a comparison of cost of base oil. The formulated product prices roughly increase 50–100% from the base oil. The high performance (operational longevity) is a very important criterion for lubricants. This is helping the synthetic lubricants to increase their market share at the expense of lower performing petroleum-based materials. The same high performance is also responsible for flat or even negative growth rate in mature markets. The new technologies in base stock production and synthetic lubricants production are increasing the performance standards of conventional products as well. For example, in the US, the GF-4 passenger car engine oil standards require higher viscosity index and lower volatility. This necessitated a switch to a higher performing Group III and PAO base stocks. Developments in this direction will also encourage use of biolubricants if the performance and cost to performance ratio of biolubricants can be improved. The biolubricants industry believes that eventually the biolubricants have the potential to replace 90% of petroleum-based products used as lubricants (Mang, 1997).

Overall public awareness, regulations and incentives for environmentally friendly fluids in the EU, North America and other developed countries (Japan, Australia, NZ, etc.) are helping to develop the biolubricants market. Biolubricants represent a very small fraction of the total lubricants market. The EU is the leading producer and consumer of biolubricants. The estimated production of biolubricants in Europe for 2006 was about 127,000 tons/year with an estimated growth of 3.7%/year (Bremmer and Plonsker, 2008). The current estimated worldwide biolubricants market is smaller than 400,000 tons/year.

In certain applications, lubrication fluids are lost into the environment due to the type of application, e.g., two-stroke oils, chain saw oils, mold release agents, drilling oils. In these instances there is a greater need to replace petroleum-based products with biorenewable materials like vegetable oils and their derivatives, especially in Europe and to a certain extent in North America. Total loss lubricant fluid applications include chain saw oils, mold release oils, two stroke engine oils and certain grease applications used for chassis and wheel flange applications. Use of lubricants in environmentally sensitive areas such as waterways, forestry, and mining also heighten the need for biocompatible, environmentally friendly, readily biodegradable and nontoxic materials. A recent study reported an analysis of marine oil losses due to vessel operational discharges and leaks of 61 million liters annually worldwide, the equivalent of one and a half to two times that of the Exxon Valdez-sized spills. The total annual estimated response and damage costs for these leaks and operational discharges are estimated to be about $322 million worldwide. About 70–80% of hydraulic fluids are lost into the environment. The recovery of lubricants used in all lubrication applications taken together is at best 50% of the total volume and the rest is lost into the environment (Bremmer and Plonsker, 2008).

In some EU countries, ready biodegradability and low toxicity are a requirement for certain environmentally sensitive applications like total loss lubricants. Consumer demand is helping the use of biolubricants due to their environmentally benign image. Regulatory issues also usually help the biolubricants. In the EU, many countries have ecofriendly labels that differentiate and support biolubricants compared to conventional products. Many eco-label standards require biodegradability, aquatic toxicity and renewability on product labels. Most of the applications that require eco-labels are total loss or high risk lubricants, e.g. chain saw lubricants, marine two stroke engine oils, forestry and mining applications. Nordic environmental label requirements for raw materials and additives were discussed with examples by Laemsae (2002). Many countries that issue eco-labels are in the EU: Blue Angel in Germany; Nordic Swan in Sweden and the Nordic Countries; and other labels in The Netherlands, Austria and France. In the United States there is some differentiation of biolubricants by some OEM manufacturers, e.g. Caterpillar’s BF1 fluids standard.

Even though the annual consumption of lubricants is very large, the market share of biolubricants is a very small percentage of this volume. There are a number of factors that are responsible for this lack of market penetration. Some of the factors are related to performance and cost, whereas others are related to barriers created by historical use and the interests of the petroleum industry. For example, the established test methods and specifications are based on material properties (of petroleum derivatives) rather than application requirements. The efforts required to go through a battery of tests and protocols for new materials to prove their suitability in a given application like engine oil are very time consuming and can be prohibitively expensive (Bremmer and Plonsker, 2008).

The suitable markets for unmodified vegetable oils and their formulated products are less demanding applications like hydraulic fluids, two stroke engine oils, mold release agents, transformer fluids, cutting fluids, etc. This is partly due to performance deficiencies of TAG oils. The major advantages of using vegetable oils for lubrication applications are a renewable and environmentally friendly, nontoxic alternative at a comparable cost.

Global industry analysts forecast that the North American biolubricants market is set to reach about 250,000 tonnes by the year 2017, spurred by legislative initiatives, sustained demand from hydraulic fluids market and growth potential of product types, bio 2 cycle engine oils, greases and concrete release agents in the long term. Currently, use of biolubricants is primarily restricted to the developed markets of North America and Europe, with a lack of awareness and high pricing posing major hurdles to widespread appeal and usage in other parts of the world. In the future, increasing environmental concerns and emphasis on a shift from non-biodegradable lubricants to the environmentally safe and ‘green’ biolubricants may drive growth (P.R. Web, 2011).

The biolubricants market currently constitutes only a small portion of the worldwide lubricant market. Widespread usage of biolubricants is currently limited on account of its high pricing, estimated to be at least 2–3 times more expensive than the standard lubricant. Future growth in different markets worldwide will be a gradual process based on slow replacement and specific legislative norms of each country. Industrial lubricants primarily comprise non-biodegradable materials including petroleum derivatives or synthetic oils. Biodegradable lubricants, generally based on vegetable oils and ester derivatives, represent a new technological trend in the lubricants industry. These lubricants are being increasingly preferred over conventional lubricants. Although petroleum-based lubricants are less expensive, offer a high degree of stability and are universally accepted, they are non-biodegradable, and tainted with environmental hazards. Since petrochemical derivatives used as lubricants are not environmentally friendly, there is a need to develop lubricants that are high on performance, biodegradable, as well as nontoxic. The increasing number of automobiles and the growing need for oil change remain some of the main growth drivers.

According to Global industry analysts, North America and Europe represent two of the largest markets worldwide, as stated by the new research report on biolubricants. Rapid initiatives and national level legislation are in vogue in the European countries of France, Portugal, Germany, Austria, Switzerland and Sweden. Increasing activity in the manufacturing and automotive industries is a major growth impetus for the industrial lubes market, translating into higher demand for segments and applications. The hydraulic fluids segment constitutes the most important product line in both regions. Environmental regulations and fiscal incentives drive the market, with strong growth indications for the hydraulic fluids and cutting oil segments. The bio 2 cycle engine oils segment is poised to deliver robust growth of more than 12% through 2017 in the North American biolubricants market (P.R. Web, 2011).

The use of biodegradable replacements like high oleic oils and their derivatives for mineral oils in some application areas, such as chain saw oil, gearbox oils, hydraulic oils and lubricants for crude oil production, is already well established. The new developments use tailor-made fatty acid esters with specific lubricant properties. In Europe, the long-term potential for such tailor-made esters is estimated to be 10–20% of the total market (500, 000–1,000, 000 tonnes/year). The major vegetable oils and their annual production volumes are shown in Table 17.3. Of the 146 million tons of vegetable oils produced annually, oleochemicals currently use about 20 million tons per year. Oleochemical applications are a very broad range of products where mostly the split fatty acids from TAG oils are used in surfactants, functional fluids, plastics and other specialty chemicals. In lubricants, oleic acid-based esters are used as synthetic esters. The oleic acid-rich oil fraction (triacylglycerols or TAG) are hydrolyzed to make oleic acid which is chemically modified to make key intermediates like methyl oleate, ethylhexyloleate and other polyol esters like NPG, TMP, and PE oleates.

As shown in Table 17.3, historically of the major vegetable oils, palm oil tends to be cheaper whereas sunflower oil is more expensive than other oils. This is partly due to cost of production. The palm is a perennial crop and the oil is derived from the pericarp of palm fruit, which yields 5 to 10 times higher volume per acre than other oils. A small portion of these regular vegetable oils are useful for lubrication applications where the applications are not too demanding as they do not possess high performance characteristics. In the last 20 years the fatty acid composition of the major vegetable oils has been modified through plant breeding and genetics. The high oleic oils and their derivatives thus produced have higher performance as lubricants. The fatty acid compositions of high oleic oils, their chemical modifications and their performance are discussed later.

17.3 Biolubricant performance requirements

The primary function of a lubricant is to reduce friction, minimize wear and dissipate heat generated by moving parts. It should also disperse deposits and sludge generated through use and contamination, inhibit corrosion and provide a seal at critical contact joints. The major constituent of a lubricating fluid is base oil (base stock) formulated with small amounts of additives. The base oil provides the primary lubrication functionality and performance. The additives enhance the performance of the base oil and also provide additional advantages and/or diminish the shortcomings of the base oil. The amount and type of additives used depend upon the severity of the application; usually the additives are from 1 to 30% of the total formulation depending on the base oil and the application requirements. Recently an excellent treatise covering all aspects of lubricants and lubrication has been edited by Mang and Dresel (2007). The important chemical and physical characteristics required for a good functional lubricant are appropriate viscosity, high viscosity index, low pour point, high stability (oxidative, hydrolytic, thermal), corrosion prevention, compatibility with additives and seals, high flash point, low volatility and good environmental acceptability, like low toxicity and high biodegradability (Odi-Owei, 1989; Kodali, 1997; Willing, 2001).

The bulk of lubrication fluids can be characterized into mineral oils and synthetics. Mineral oils are hydrocarbons derived from petroleum having different viscometrics based on molecular weight and chemical nature, having normal, branched, cyclic and aromatic structures. Mineral oil products are lower in cost as they are refined (not chemically modified) petroleum fractions, and formulated to suit an application. On the other hand, synthetics are derived from petroleum or biobased materials and offer better performance at a higher cost. Broadly the synthetic fluids encompass hydrocarbon fluids like PAO and ethers like PAG and synthetic esters. By legal definition, the hydrocracked mineral oils (Group III) having very high viscosity index (VHVI) are also classified as synthetics. Synthetic esters are condensation products of fatty acids and alcohols and usually contain two or more ester groups. A large number of carboxylate groups in the ester molecule improves the thermal properties, and provides boundary lubrication where a film can be maintained in highly loaded and high-slip contacts. Synthetic esters form a broad range of base fluids with properties varying greatly depending on the chemical structure. In principle, ester properties can be tailor-made to fit a given application. This requires understanding of structure–property relationships. For a lubricant, the molecular structure is closely related to its properties.

Viscosity and viscosity index are vital parameters for a lubricant as they determine the usefulness of a fluid for a given application. Viscosity is a measurement of internal friction of a liquid as the molecules pass each other to affect the flow. The viscosity at 40°C is a common measure of usefulness of a fluid for a specific application. The viscosity increases with molecular weight of the composition. The increase in chain length, branching, cyclic structures, and polarity (number of ester groups) will increase the viscosity of a composition. The viscosity index (VI) is a dimensionless number calculated from the variation of viscosity with temperature. The lower the variation of viscosity with temperature, the higher the viscosity index and more useful is the fluid in a given application. The viscometrics, viscosity and VI influence the ability of a fluid to form a lubricating film that reduces the friction and wear and determine its effectiveness in a given application. Both these properties are related to the molecular weight and polarity.

Vegetable oils and fatty acid esters exhibit higher viscosity index compared to mineral oils. Viscosity, VI and low temperature flow properties of various ester fluids for comparison are given in Table 17.4 (Mang and Dresel, 2007). Branching is beneficial for low-temperature properties, but decreases VI. This indicates under certain structural conditions VI and pour point have a trade-off. Recently Yao and Hammond (2006) presented the melting properties of methyl and isopropyl esters of iso- and anteiso-branched fatty acids isolated from lanolin. The branched chain fatty acid esters showed considerably lower melting temperatures and heats of fusion compared to the normal chain counterparts. Later Yao et al. (2008) extended this study to show the effect of structure on melting points and viscosities of oleate esters with alcohols of various chain length and branching. The cyclic structures in the acyl chain can lower the VI more than branching. The increased fatty acid chain length of TMP oleate as compared to TMP C8-C10 increases viscosity and VI, but pour point is unaffected because of the presence of double bonds in the acyl chain that prevent crystallization. The viscosity, VI and the cold flow properties are dependent upon the chemical structure, molecular weight, branching and polarity. To illustrate the point, the properties of mono- and polyolesters are given in Table 17.4.

Ready biodegradability is a major structural feature of vegetable oils and their derivatives including fatty acid esters. The biodegradability of a compound greatly depends upon the microorganisms’ ability to breakdown and metabolize. The compounds containing ester functionality can be broken down readily due to lipase enzymes that hydrolyze the ester function and metabolize the resulting fatty acids by β-oxidation. Lubricants can be categorized into readily biodegradable (> 60% in 28 days), inherently biodegradable (30–59% in 28 days) or not biodegradable (< 30% in 28 days) as measured by the ASTM D5864 biodegradability test. Mineral oils, hydrocracked oils, polyethylene glycols and polyalphaolefins biodegrade < 60% in 28 days, making them fall into inherently biodegradable or not biodegradable categories, whereas almost all vegetable oils and fatty acid esters fall into the readily biodegradable category.

Lubricants are used in open and closed applications. Lubricants used in open applications are called loss lubricants (two-stroke oils, chain saw oils, mold release agents, drilling oils, etc.). They are often, if not always, used in machinery outdoors and are by definition directly emitted into the surroundings. The extent, duration and frequency of emissions depend upon the equipment, the use of the equipment and the hygiene. Loss lubrication fluids amount to 8% of all lubricants. Two-stroke engines make up the biggest part of the loss lubricant market, accounting for 30%. Other examples of loss lubricants are chain saw oils (8% of the total loss lubricant market), protective oils (10%), mold release agents (25%) and various greases (17%).

Closed systems confine the lubricant by their design. The losses in closed systems may occur due to leakages during normal operation or by accident, e.g. a broken pipe or tube rupture. Hydraulic fluids, which make up 15% of the total lubricant market in Europe, are especially susceptible to accidental spillage. The environmental damage caused by lubricants is largely associated with the approximately 50%, accounting for about 3 million tonnes/year, of predominantly mineral oil-based lubricants which are lost during use and not properly disposed of.

Most of the lubricant functional properties are provided by the base oils. However, the base oils need to be formulated with various additives to provide or enhance the required functionality. Historically, the additive formulations were based on mineral oils as they were the mainstay of the industry for a long time. The chemistry and applications of various additives differ greatly as they provide different functionality; a comprehensive account of additive chemistry is provided by Rudnick (2009). Due to structural differences, the additives requirement and the function differ from petroleum-based mineral oil and biobased lubricant formulations. Enough consideration should be taken in the formulation of biolubricants as some of the additive components (metals and chemicals) can alter the toxicity and biodegradability of the formulation. In formulating the biolubricants, the additives should not alter the ecological criteria such as toxicity, biodegradability, water pollution and waste management. A detailed account of additives for biodegradable lubricants was presented recently (Miller, 2009). Base oils on average account for 95% of the formulation. The amount of additives in a formulation is dictated by the base oil and the application requirements, e.g., hydraulic and compressor oils require only a few percent (1–3%) additves whereas metalworking fluids and gear lubricants may require larger quantities (20–30%).

In general, various types of additives, functionality and levels of addition are presented below. Pour point depressants improve the low temperature flow properties. They modify the crystallization characteristics of the base oils by changing the kinetics, crystal size and their agglomeration, thereby preventing gelation at low temperatures. Even though pour point measurement by ASTM method is the industry standard for lubricants, in the case of biolubricants, due to their molecular size, the kinetics of gelation or solidification are slower and hence the test method usually followed to measure the low temperature flow properties is to incubate the oil at a given low temperature, say − 20 or − 30°C, for 24 hours, and reporting the change in flow is a more appropriate method. Increasing the heterogeneity or the addition of synthetic polyolesters with low pour point can improve the pour points of biolubricants. Polymers such as styrene esters or polymethacrylates are used as pour point depressants at 0.1–2% of the formulation.

Antifoaming agents prevent foaming. Silicon alkylates are effective antifoam agents at concentrations of 0.05–0.1%. Detergents and dispersants prevent sludge formation and keep oil-insoluble combustion products in suspension. Various surfactants, sulfonates, phosphates and phosphate esters are used at 1–5% of the formulation.

Antioxidants prevent oxidation and increase the use life of the lubricant formulation. Oxidation creates many unwanted changes such as discoloration, breakdown products, increasing polar compounds and decreasing viscosity. In the case of biolubricants where the base oil has considerable unsaturation the oxidation can lead to polymerization (thickening, increased viscosity) and to deposits forming on the surfaces. Many natural and synthetic antioxidants such as phenols like tocopherols, butylatedhydroxytoluene (BHT), alkylated phenylamines and sulfur compounds, are used at 0.1–3%. Recently, various antioxidants have been evaluated in biolubricant formulations. The natural antioxidant propyl gallate and synthetic antioxidant 4,4'-methylenebis(2,6-di-tert-butylphenol) were found to be most effective (Quinchia et al., 2011).

Antiwear and extreme pressure (EP) additives reduce wear, extreme stress and friction. The concentration of EP additives is < 5%, usually 0.20–2.0%, based on the application. Corrosion and rust inhibitors prevent metal corrosion and oxidation by acids and oxygen. Corrosion inhibitors in a formulation are in the range of 0.01–2%. Biocides are typically used in metalworking fluids and hydraulic fluids and usually in small quantities, 0.1%–0.5%.

Viscosity modifiers, are used to balance changes in viscosity of the base fluid and thickener owing to temperature changes and aging. Demulsifiers and emulsifiers prevent the formation of water-in-oil emulsions. They are all surfactants used in small concentrations depending on the application. Friction modifiers prevent stick-slip oscillations and noises by reducing frictional forces. The total additives in a formulation varies based on the application. In a hydraulic fluid they may be as low as 1%, but in engine oils they could be as high as 15–25 wt%.

17.4 Applications of biolubricants

The factors that determine the market penetration of a biolubricant depend upon its performance, price, safety and environmental benefits in use. The driving forces for vegetable oil use in lubricants are because the application is considered a loss lubricant and a threat to environmentally sensitive areas. The largest potential markets for vegetable oil base fluids are mining, agriculture, food machinery, outboard engine and marine applications. However, because of the performance limitations, biolubricants are used only in certain applications. Some of the promising application areas for biolubricants and the issues for each application are discussed.

Automotive applications are the biggest market for lubricants. The environmentally friendly automotive lubricants present a huge market opportunity, but tough performance requirements and the low price of petroleum alternatives make this a difficult market to enter. Automotive lubricants are not perceived as loss or high risk lubricants. The environmental issues of automotive lubricants are their impact on the fuel consumption and other issues related to the proper collection and recycling of used oil. So at best the fatty acid ester derivatives including high stability vegetable oils can be used in engine oil formulation either as an additive or as part of base fluid. This can only happen when the high stability oil or chemically modified ester derivative enhances the performance and meets the oxidative stability and low temperature flow properties while remaining cost effective.

Non-engine lubricants have lesser technical and performance demands in lubrication properties and therefore represent the best potential opportunities for vegetable oil-based ester fluids. The key applications are: two-cycle engine oils, antiwear hydraulic fluids, chain bar lubricants, gear oils, metalworking fluids, food machinery lubricants, textile lubricants, and greases.

Two-stroke oils are used in engines on boats, snowmobiles and other vehicles like two-stroke engine mopeds and personal watercrafts often used in environmentally sensitive areas. Two-cycle engines by design emit part of their fuel and lubricant unburned. An estimated 30% of the mixture of fuel and lubricant used in two-stroke engines ends up in the environment. Two-stroke oils are a complex product requiring a high amount of additives making it very difficult to formulate an environmental friendly product. Outboard motors are particularly problematic due to direct discharge into the water and use of vegetable oil-based lubricants is already mandated in parts of the world such as Europe. Vegetable oil-based systems could offer high performance and considerable savings over other ester-based lubricants. The market growth will accelerate when government legislation is enacted. The vegetable oil-based formulations need to have good low-temperature properties, good oxidative stability, and miscibility with gasoline.

Hydraulic oils make up the biggest part of industrial oils and are the second most important group of lubricants after automotive lubricants accounting for about 15% of the total lubricant consumption. Hydraulic oils are typical high risk lubricants, used in a wide range of applications both in stationery and mobile equipment in the open air (hydraulic elevators, sweepers, garage trucks, fork lifts, motor graders, front end loaders). It is important from an environmental point of view that hydraulic oils have a high degree of biodegradability and low toxicity. Hydraulic fluids represent the largest market segment growth for vegetable oils. The immediate markets are in Canada and Europe, with the US market to trail development and usage. A key aspect for growth in this area is to demonstrate higher performance and value as compared to other commodity oils. The threat of regulation is always hovering with spills and waste disposal. For this application, benefits are good biodegrability and low toxicity, anti-wear properties, protection against rust and copper corrosion, good filterability, pour point approximately − 20 °F, compatible with conventional hydraulic seals, miscible with mineral oils and synthetic esters. The major concerns are low pour points and poor thermal oxidative stability. Environmental awareness has already forced the conversion to more environmentally acceptable hydraulic fluids in sensitive areas such as waterways, farms and forests, and a lot of lubricant manufacturers market eco-friendly hydraulic fluids. Eco-hydraulic oils currently marketed in the EU make up the largest market of eco-lubricants and are either rapeseed-based lubricants or synthetic fluids that have been used successfully for a decade. Caterpillar, Inc. estimates this constitutes 12% of the European hydraulics market.

Chain saw oils make up 8% of the total loss lubricants market but only a very small percentage of the total lubricant market. Chain saw oils marketed as biolubricants are usually based on rapeseed oil and were launched in Europe already in the 1980s. They are low technology products, relatively low priced compared to other biolubricants. Chain saw oils of the biolubricant type are mainly used in forestry in countries with high environmental awareness: Scandinavian countries, Germany, Austria and Switzerland.

Compressor oils are used in confined systems in both industrial and mobile equipment. Compressor oils are not susceptible to accidental leakages. Gear oils may be used in open gears. For these applications, gear oils are to be considered as loss lubricants. Greases marketed as biolubricants represent a small portion of the market. Many applications of greases involve open systems like railway equipment, switch plates and others. There are products marketed today as environmentally friendly greases.

Metalworking fluids are used in the metalworking industries at the forming and cutting of metal parts. Metalworking fluids contain complex mixtures of chemicals. Metalworking fluids are no loss and high risk lubricants. Biolubricants have been introduced in this market segment in order to reduce the health and safety risks associated with the mineral oil-based products. Metalworking fluids are governed by entirely different issues than loss and high risk lubricants. These issues include occupational health issues (a potential risk for the development of allergic contact dermatitis) and environmental issues related to releases of volatile compounds in the atmosphere, generation of waste and wastewater treatment.

Concrete release agents are a typical example of loss lubricants. They make up 25% of the total loss lubricants market. Concrete release agents are usually based on rapeseed and soybean oils.

The transformer oils market in the US is close to 150,000 tons. Until recently, this market has been served by high molecular weight hydrocarbons (naphthenic oils), synthetic esters and silicone fluids. In the past decade, vegetable oil-based readily biodegradable, environmentally friendly fluids were introduced into this market. Currently in the US, there are two vegetable oil-based commercial products that are sold in this market segment: EnviroTemp FR-3 made by Cargill and Cooper and BioTemp by ABB. The EnviroTemp FR-3 fluid uses less expensive and abundant soybean oil, whereas ABB uses high oleic oils as base stock. These oils are treated with clay to remove the polar conducting materials and formulated with additives (antioxidants, metal deactivators and pour point depressants). The use of high oleic oils in transformer fluid applications has the following advantages and limitations. Advantages include renewable raw material; good biodegradability; high dielectric strength; high flash and fire point; low toxic/spill risk; and high water absorption – prolonged transformer life. Limitations include moderate oxidative stability; high viscosity; high pour point, poor low temperature properties (Biermann and Metzger, 2007).

The vegetable oil-based transformer oils gained market share over the past several years. This is mainly because of their environmentally friendly image and less the remediation risk in case of a spill. Functionally the vegetable oils are superior to mineral oils as they are more hygroscopic compared to mineral oils, thereby keeping the moisture sequestered away from the insulation paper and thus extending the transformer fluid life. The other functional properties like high fire point (necessary to get certified as fire resistant fluid) and flash point are inherent to vegetable oils. The strict controls in the refining process to remove the polar compounds like fatty acids, and maintaining the specifications for moisture after treating with clay are important steps in maintaining the quality of the transformer fluids. The oils thus prepared will meet most of the transformer fluid specifications of dielectric strength, power factor (dissipation factor), interfacial tension, acid value and moisture content. Most of the transformers used in the US are closed loop, unlike in Europe where the transformer fluid is exposed to external air (oxygen). This closed architecture of transformers made it possible to use vegetable oils like soybean oil which has marginal oxidative stability.

17.5 Feedstocks for biolubricants: key properties

Vegetable oils are becoming an important alternative to synthetic esters on a cost/performance ratio. There are a number of environmentally acceptable lubricant products commercially available both in Europe and North America (Bartz, 1997; Stempfel 1998). Most of these products are formulated from regular vegetable oils and perform poorly. They are used mostly in less stringent applications like hydraulic fluids and chain saw lubricants. Before vegetable oils can be considered as lubricant base stocks for severe applications like motor oils, two major limitations need to be addressed: oxidative stability and low temperature behavior (Asadauskas and Erhan, 1999; Kodali, 2002; Adhvaryu et al., 2003).The basic understanding and molecular origins of these two properties will enhance the future chemical modifications that will provide the cost-effective high performance lubricants from TAG oils.

The physical and functional properties of vegetable oils and mineral oils differ considerably and are related to their respective chemical structures. The general molecular structural features differentiating mineral oils and vegetable oils are shown in Fig. 17.1 and discussed in the introduction. In brief, mineral oils are inert hydrocarbons having different molecular weights with structural heterogeneity. The structural variation comes from normal branched, cyclic, acyclic and aromatic. The heterogeneous structural features with varying molecular weights will have different boiling points, making different cuts having different compositions and properties. Based on the functionality, the different viscosity grades are formulated to suit different applications. However, the structural features of petroleum-derived materials do not particularly offer lubrication function but give the desirable low temperature fluidity and higher oxidative stability. Compared to petroleum products, vegetable oils possess the desirable characteristics of high viscosity index, high flash point, and reasonable pour points. Vegetable oils and their ester derivatives provide better boundary lubrication and load-carrying capacity due to their inherent chemical structure. Thus the friction coefficient (μ) of ester fluids is much lower than mineral oils (Table 17.1).

The polar ester region of the vegetable oil orients itself toward the metal surface and the nonpolar hydrocarbon region away from the metal, providing a stable interfacial layer thus reducing the friction coefficient. The favorable friction and wear behavior of esters compared to mineral oils reduce the need for additives and enhance the environmental acceptability. In addition, they are nontoxic, readily biodegradable, and a renewable resource. However, their oxidative stability and low temperature properties are a major limitation for lubrication applications. These properties are interdependent due to the chemical structure of triacylglycerols (TAG). Oxidative stability is related to polyunsaturated content of the oil, whereas the low temperature properties are related to the amount of saturation. In this section, an in-depth understanding of structural features of vegetable oils and fatty acid esters that are responsible for the functional shortcomings as lubricants will be discussed.

17.5.1 Functionality of TAG oils

The molecular structure composition of refined fats and oils is 99 + % TAG, where a glycerol is esterified to three different or the same fatty acids. The TAG oil properties and their use depend upon the fatty acid composition. The industrial applications (excluding paints and coatings) require two main functional properties. They are oxidative stability and low temperature fluidity. Because of the higher importance of these properties in almost all applications, understanding their molecular origins is important. They are further explained below.

The oxidative stability of oil depends on three factors: glycerol structure, fatty acid composition and the presence of natural antioxidants like tocopherols. Almost all vegetable oils inherently contain antioxidants to protect them from oxidative deterioration. The natural antioxidants enhance the oxidative stability by reducing the rate of oxidation by scavenging the free radicals formed during the initiation, but cannot stop the oxidation process. The effectiveness of various antioxidants depends upon the type and concentration. The nature and concentration of various antioxidants present in a given oil depend upon oil type and the species’ genetic disposition.With the recent advancements in molecular genetics, antioxidants are one of the traits targeted to increase the concentration and right type and ratio of antioxidants. For example, it is known that the higher total concentration of tocopherols and also higher ratio of γ-isomer compared to other isomers will provide better oxidative protection.

Glycerol, a three carbon tri-hydroxy compound, is integral to all triacylglycerol oils where the hydroxyls are esterified to fatty acids. It is known that the glycerol β-hydrogen present on the second carbon is labile, especially at high temperatures leading to degradation. The ester functionality present on the second carbon decreases the stability of secondary hydrogen and increases the thermal instability. Even though it is not proven conclusively, a possible mechanism of glycerol instability and the formation of decomposition products at high temperatures is shown in Fig. 17.3. The facile six membered ring transition formed by the carbonyl of one of the primary esters of glycerol abstracts the β-hydrogen leading to the formation of breakdown products.

The fatty acid composition of vegetable oil is the signature of a given crop and its genetic disposition. The saturated fatty acids offer the best oxidative stability; however, their concentration in any appreciable quantities leads to higher melting temperatures, making them undesirable for lubrication fluid applications. Among the unsaturated fatty acids, the oxidative stability is inversely proportional to the concentration of allylic and bis-allylic methylenes, the latter being the least desirable. The allylic methylenes are the methylenes adjacent to a double bond and the bis-allylic methylenes are the ones that are between two double bonds. For instance, as shown in Fig. 17.4, oleic acid C18:1, has two allylic methylenes and no bis-allylic methylene; linoleic acid, C18:2 has two allylic methylenes and one bis-allylic methylene; linolenic acid C18:3 has two allylic and two bis-allylic methylenes. The carbon–hydrogen bond strength of a regular methylene, allylic methylene and bis-allylic methylene are approximately 97, 88 and 78 kcal/mol, respectively. Oxidation is initiated by abstraction of hydrogen from the highly susceptible (due to lower bond energy) allylic and bis-allylic methylenes of the lipid molecule. Due to the presence of allylic and bis-allylic methylenes, the relative oxidation rates of oleic (C18:1), linoleic (C18:2) and linolenic (C18:3) acids are approximately 1:10:20 respectively.

Oxidation is the major factor limiting the use of vegetable oil as lubricating fluids. Oxidation leads to polymerization and degradation. Polymerization increases the viscosity and reduces lubrication functionality. Degradation leads to breakdown products that are volatile, corrosive and detrimental to lubricant properties. The ease of oxidation depends upon the fatty acid composition of the vegetable oil. Unsaturated fatty acyl chains react with molecular oxygen to form free radicals that lead to polymerization and fragmentation products. The rate of oxidation depends upon the type and degree of unsaturation of a fatty acyl chain as shown in Fig. 17.4. The increased oxidative susceptibility of the polyunsaturated fatty acids is due mainly to the bis-allylic methylenes between the double bonds (Kodali, 2003).

The liquidity or low temperature fluidity of oils is an important characteristic for both food and industrial applications. The low temperature flow properties of oils are mainly determined by the efficiency of molecular packing, intermolecular interactions and molecular weight. The major structural component of oils, TAGs have high molecular weight between 800 and 900 Daltons with slightly polar ester functionality. In spite of the high molecular weight, the ‘cis’ double bonds present in fatty acids inhibit intermolecular interactions (packing) necessary for crystal formation and growth. For this reason, the oils keep the fluidity at room temperature. The level of unsaturation influences the low temperature behavior in TAGs. For example, the melting points of tristearin (saturated C18), triolein (monounsaturated C18), tilinolein (diunsaturated C18), and trilinolenin (triunsaturated C18) are, respectively, 74°C, 5°C, − 11°C, and − 24°C. The decreased melting temperatures in these compounds are a result of disorganization that lowers the ability to pack into a crystalline structure due to the presence of cis-double bonds. Thus the increase in saturated fatty acids leads to crystal formation that melts at higher temperature, while the monounsaturated fatty acid improves the low temperature flow properties.

The presence of polyunsaturated fatty acids like C18:2 and C18:3 further improves the low temperature fluidity (better than C18:1), but at the expense of oxidative stability of the oil as discussed above. So, the monounsaturates prove to be optimal in achieving the better low temperature fluidity at a reasonable loss of oxidative stability. However, the oils having very high concentrations of oleic acid will not provide the best low temperature fluidity. This is because the higher concentration of oleic acid leads to pure oleic TAG (triolein, OOO) that phase separates and crystallizes below its melting temperature. For instance, high oleic oils having > 90% oleic acid may contain > 70% triolein (a pure TAG) that can separate out from the other mixed fatty acid TAG species that solidify below 5°C, the melting temperature of OOO. This demonstrates the difficulty of simultaneously achieving both low temperature fluidity and good oxidative stability in a given oil. Still, the best balance of oxidative stability and low temperature fluidity properties is achieved with vegetable oils containing high monounsaturated fatty acids. Further improvements in low temperature flow properties of high monounsaturated oils can be achieved through different chain lengths in monounsaturated fatty acids that create heterogeneous TAG (Kodali et al., 2001).

Additionally, the double bonds in polyunsaturated fatty acids isomerize under thermal and catalytic conditions to form conjugated fatty acids that polymerize by Diels-Alder and radical mechanisms. Polymerization increases the molecular weight, leading to increased viscosity, gelling and loss of functionality. For this reason, the recommended operating temperatures for vegetable oil-based lubricants are lower than for mineral oils. The elimination of allylic methylenes by hydrogenation will increase oxidative stability, but the low temperature properties will be degraded. For example, hydrogenation converts unsaturated oils to trans and saturated fats and improves oxidation stability dramatically, but this will destroy the low temperature flow properties by producing a high melting solid fat. Altering the fatty acid profiles through advanced plant breeding and genetic engineering can create the high monounsaturated fatty acid containing oils. In high oleic oils the polyunsaturated fatty acid content has been dramatically reduced and the mononunsaturated fatty acids increased. The oxidative stability of such high monounsaturated oils is 3–10 times greater than conventional oils as determined by pressure differential scanning calorimetry (Kodali, 2005). A vegetable oil thus produced will provide both high stability and good low temperature properties. The functional properties of such ideal vegetable oils will be superior to mineral oils and comparable to the expensive synthetic oils. Some of the commercially produced high oleic vegetable oils and their fatty acid composition is provided in Table 17.5. The fatty acid composition reveals the increase in high oleic oils at the expense of polyunsaturated fatty acids and especially linolenic acid (C18:3) in certain oils is noteworthy as it provides the best stability.

Table 17.5

Fatty acid composition of high oleic vegetable oils

Oil source	C18:1	C18:2	C18:3	Total polyunsaturated	C16:0	C18:0	Total saturated
HO canola	70–85	6–11	3	9–14	2	4	6–8
HO sunflower	80–92	3–10	0	3–10	3	4	7–8
HO soy	70–85	3–8	2-5	5–13	6–10	4	10–15
HO safflower	75–80	14–16	0	14–16	5	1	7–8
Olive	73–78	9–11	1	10–12	10	3–5	13–16
HO palm	59	16	0	16	18	7	25–26

17.6 Chemical modifications of biolubricant feedstocks

The use of vegetable oils in the industrial fluid applications is limited by major functional properties:

The other factors that weigh in positively are: use of renewable materials (sustainability), ready biodegradability, nontoxic to environment (environmentally friendly), and human compatible (health and safety) materials. Above all, the products made from vegetable oils can be cost competitive compared to the commercial products. The structural features of TAG responsible for the above shortcomings arise from head group region and fatty acid chain. So the structural changes that can be achieved through chemical transformation in these two regions will be discussed in this section.

17.6.1 Modification of synthetic esters

As discussed earlier, the glycerol head group is not the ideal structure for optimal oxidative stability and low temperature characteristics of vegetable oils due to the presence of β-hydrogen on the glycerol carbon-2. Glycerol is a natural polyol and available in large quantities as a byproduct of biodiesel production. Recently, to overcome the shortcomings of glycerol in lubricants, 2-alkylglycerol ethers were synthesized to use as polyol to make synthetic esters (Kodali, 2011). Unlike glycerol, which is a highly viscous material that melts at 18°C, the 2-alkylglcyerols and its 2-ethylhexanoate ester have low viscosity and do not crystallize or melt up to − 45°C as shown in Table 17.6. The steric hindrance and reduced hydrogen bonding created by the 2-alkyl group on the glycerol is responsible for the reduced viscosity and improved low temperature properties. Also due to the ether substitution instead of ester at the glycerol 2-carbon, the thermal instability of β-hydrogen is reduced in these glycerol ether derivatives. Another advantage of 2-alkylglycerols is that the hydroxyls at carbon 1 and 3 are primary hydroxyls and can be easily esterified similar to other polyols, NPG and TMP.

Table 17.6

Thermal behavior of glycerol, 2-alkylglycerols and 2-alkylglycerol ester

Compound	DSC crystallization	DSC melting
Glycerol	Crystallizes below 0°C	18°C
2-Methylglycerol	No crystallization until − 45°C	No melting up to − 45°C
2-Ethylglycerol	No crystallization until − 45°C	No melting up to − 45°C
2-Propylglycerol	No crystallization until − 45°C	No melting up to − 45°C
2-Methylglycerol-1,3-di-2-ethylhexanoate	No crystallization until − 45°C	No melting up to − 45°C

Source: Kodali (2011).

Other polyols that are used frequently to make synthetic esters useful in lubrication applications are NPG, TMP and PE. These three polyols are petroleum derived and have 2, 3 and 4 primary hydroxyl groups without any β-hydrogen. The structural features of synthetic esters made from these polyols along with 2-glycerolether are shown in Fig. 17.5. The polyol esters are prepared by condensation of polyol with an appropriate fatty acid. Most of the long-chain fatty acids are derived from renewable materials by splitting the TAG. Some short-chain and branched-chain fatty acids like 2-ethylhexanoic acid are petroleum derived. The synthesis of these polyol fatty acid esters can be affected by various chemical and enzymatic processes. There are a number of condensation procedures in the literature that utilize either homogeneous or heterogeneous catalysis. The molecular weight and viscosity of these polyol esters for a given fatty acid increase with number of hydroxyls. Based on the functional requirement of an application, the right polyol and fatty acid can be determined to suit the need. In addition to these esters, complex polyol esters with higher molecular weight and viscosity are produced by using dibasic acids that crosslink the polyols, thereby increasing the molecular size.

Synthetic esters are readily biodegradable with low or no toxicity. Due to ester function they exhibit low volatility and high flash point. They are structurally homogeneous and usually contain saturated or monounsaturated fatty acids and hence exhibit higher oxidative and thermal stability. They also exhibit higher solvency, lubricity and hydrolytic stability. Most of the polyol esters exhibit excellent low temperature flow properties due to the presence of steric hindrance at the head group.

17.6.2 Heterogeneity through transesterification

An alternative approach to improve the low temperature properties is to introduce branching to disrupt the packing of the hydrocarbon chains or at the head group. Some microorganisms that live in low temperature environments maintain membrane fluidity with the help of a branched acyl chain containing phospholipids. Other methods for disrupting packing and crystallization of vegetable oils include use of different chain lengths and unsaturated fatty acids with the double bond(s) at different positions in the chain. Recently the above strategy of creating molecular asymmetry to improve the low temperature properties of oils has been successfully exploited through biotechnology and chemical modification. High-performance esters from natural oils as environmentally acceptable lubricants were created by simply transesterifying the high oleic oils with branched-chain fatty acid (2-ethyl hexanoic acid) esters of polyols like TMP or NPG. The structural features of transesterified oil are shown in Fig. 17.6.

The transesterified oils were produced by Cargill and commercialized under the trade name ‘Agri-Pure’ and reviewed (Kodali, 2002; Kodali and Nivens, 2002). This technology made it possible to introduce heterogeneity at the head group region and at the fatty alkyl region. These esters showed particularly high performance (very high oxidative stability and low temperature fluidity) while containing very low unsaturation (iodine value). One of the major advantages of biobased synthetic esters is better performance at a lower cost compared to synthetic esters. Various polyol esters that can be used are diesters, triesters, tetraesters of various polyols with different head group structure. The transesterification process can be accomplished in a short time with a very small amount of inexpensive catalyst and an easy workup. The manufacturing cost of the product varies with the volume, with a range anywhere between 10 to 20 cents/pound above the raw material cost. The product physical properties and functionality depend upon the reactant’s structure and stoichiometry.

The molecular heterogeneity in the products created by transesterification of high oleic TAG oil with TMP tri-2-ethylhexanoate is shown in Fig. 17.7. As a result of exchanging the fatty acid ester groups present in the two types of head groups, structural modification occurs both in the head group due to the incorporation of TMP along with glycerol and the ester composition on a given head group. This molecular heterogeneity creates numerous asymmetric molecular structures that show excellent low temperature flow properties. The saturated fatty acid content of the composition can be increased to > 50% without sacrificing the low temperature properties, thereby producing products having excellent oxidative stability.

The salient features of transesterified polyol esters technology are that the high oleic oils can be used as is and the properties of the resulting products include:

• excellent low temperature fluidity (− 25°C)

• excellent oxidative stability (comparable to commercial products)

• very high viscosity index

• range of viscosities 20–50 cSt. @40°C

• low volatility

• good boundary lubrication and ready biodegradability

• most of all cost effective.

17.6.3 Estolide esters

Isbell and Cermak at USDA, NRR labs developed a functional fluids technology called estolides based on oleic acid, saturated fatty acid and alcohol (Isbell et al., 1997, Isbell and Cermak, 2002). The synthesis and structural features of estolides are shown in Fig. 17.8. The oleic acid is reacted with saturated fatty acid in the presence of perchloric acid. The carbocation formed at the site of unsaturation undergoes nucleophilic addition of another fatty acid to form an ester linkage on the acyl chain. If the fatty acid that reacted with carbocation is a saturated fatty acid, it becomes a cap as it is unable to react further. If it is an oleic acid, it will continue reacting to form the backbone. The number of fatty acids in the backbone determines the estolide number and the average molecular weight. The resulting estolide acid is further esterified with a saturated alcohol, usually 2-ethylhexanol to form the head group. In short, the estolide structure as shown in Fig. 17.8, contains three structural features: backbone, head group and a cap. The backbone part is made from oleic acid derived from high oleic oils, whereas the head is a branched-chain petroleum-derived alcohol and the cap is a saturated fatty acid. In this technology, the high oleic oils are not used as is, but the oleic acid derived from high oleic oils is being used.

Recently estolide lubricants technology has been reviewed by Cermak (2011). The estolide technology addresses three characteristics: oxidative stability, low temperature flow properties and viscosity characteristics (inherent viscosity and viscosity index). The low temperature flow properties of estolides are excellent due to molecular asymmetry and heterogeniety that makes close packing (solidification) very difficult. The estolides do not have excellent oxidative stability properties but respond to commercial antioxidants very favorably, producing materials that can meet most of the stringent applications like crank-case oils. The viscosity characteristics mostly depend upon the molecular weight and nature of the estolides. The viscosity indices of the estolides are very good and comparable to synthetic lubricants. The limitations of the technology include the reaction yields and processing: the estolide yields with oleic acid (90%) as starting materials at best are 70–80%. The amount of oleic acid in the final estolide product is about 50% as it contributes mainly to the backbone of the product; the cap and head group come from other materials. The processing requires long reaction times (24 hrs) and multiple steps increasing the cost of production. Use of strong acids like perchloric acid in the reaction creates color fixation making it difficult to achieve low color materials. Purity of the end product with low acid value and hydroxyl value is required for low volatility and prolonged use-life. The distillation used to achieve the purity of the end product is expensive and throughput limiting.

17.6.4 Cyclopropanated oils

Another successfully employed technique was to create asymmetry and steric hindrance in the acyl chain similar to double bond geometry through cyclopropanation by inserting a carbene into the double bond. The unsaturated esters including high oleic oils are treated with methylene halide in the presence of zinc-copper reagent in a Simon-Smith type reaction. The carbine produced in the reaction was inserted into the double bond to transform it into a cyclopropane group. This procedure eliminated the unsaturation while creating asymmetry. The synthesis of cyclopropanated oils from an unsaturated TAG is shown in Fig. 17.9. The cyclopropanated oils showed excellent flow properties, being liquid at − 40°C along with very high oxidative stability (Kodali and Li, 2000, 2001). However, the cyclopropanated oils were very expensive to make and were not commercialized. This technology is useful for some niche applications. The cyclopropanated oils are produced by certain plant species and it is quite possible in future to produce cyclopropanated oils cost effectively through biotechnology (Bao et al., 2003).

17.6.5 Other esters

Epoxidized soybean oil is ring opened with acid anhydride in the presence of a catalyst to transform it into diesters (Erhan et al., 2003). This technology was patented by Erhan and her coworkers from USDA, who demonstrated the usefulness of the resulting oils by utilizing them in hydraulic applications. In general, the vegetable oils having unsaturated fatty acid substituents are modified to convert sites of unsaturation to C-2–C-10 diesters. The resulting derivatives have good thermal and oxidative stability, low temperature performance properties and are environmentally friendly. They have utility as hydraulic fluids, lubricants, metalworking fluids and other industrial fluids. The TAG oils are most easily prepared via epoxidized vegetable oils which are then converted to the diesters in either a one- or two-step reaction.

Similar to the above approach, Salimon and Salih reported synthesis and evaluation of a number of potential biolubricant molecules by ring opening of epoxidized oleic acid with different fatty acids and the subsequent esterification provided di and tri-ester derivatives of C18 fatty acid having excellent low temperature fluidity and oxidative stability (Salimon and Salih, 2010). The epoxidized oils were also used to synthesize ether esters by ring opening with an alcohol followed by acylation (Lathi and Mattiasson, 2007).

Ricinoleic acid (12-hydroxy-9-cis-octadecenoic acid), the primary fatty acid constituent (> 85%) of castor oil, even though generally more expensive than other oils, contains hydroxyl and unsaturation functionalities suitable for various chemical modifications. Recently Yao et al. (2010) synthesized ricinoleate and 12-hydroxystearate esters and their estolide derivatives as biolubricants. The branched ester derivatives of saturated and unsaturated fatty acid esters showed excellent low temperature flow properties and viscosity behavior suitable for lubrication applications.

The alkyl branched fatty compounds were synthesized by applying modern synthetic methods through selective functionalization of the alkyl chain (Knothe and Derksen, 1999). Carbon–carbon bond forming addition reactions afford new branched chain or elongated fatty ester compounds with interesting physical properties (Biermann et al., 2000; Biermann and Metzger, 2004).

Recently, high oleic sunflower oil formulations have been tested as hydraulic fluids in agricultural applications (Mendoza et al., 2011). The fatty acid composition of the base oils is not provided but the lab tests reveal good performance and the formulated product is being field tested.

Another interesting fatty acid ester chemical modification technology that is worth noting is the isomerization of unsaturated linear chain fatty acids into saturated branched-chain fatty acid isomers under different catalytic conditions. This technology has been patented by a number of researchers with different catalysts and conditions (Foglia et al., 1983; Zhang et al., 2005). Recently Ngo and her coworkers reported skeletal isomerization of oleic acid by a solid modified H-Ferrierite zeolite catalyst to produce predominantly saturated branched-chain fatty acids and small amounts of dimer coproducts (Ngo et al., 2007, 2011). The structural features of saturated branched-chain fatty acid methyl ester derivatives are shown in Fig. 17.10. The products have excellent low temperature flow and thermal and oxidative stability comparable to commercial products. Very recently Manurung et al. (2012) reported chemical modification of sterculia oil and its fatty acid methyl esters containing cyclopropene functionality to branched ester derivatives. The chemical conversions of cyclopropene to branched-chain esters were excellent and carried out under mild conditions. The cold flow properties of the resulting branched esters are similar to the starting materials.

17.7 Future trends

The current and continuing future drivers for the biolubricants market segment are performance, cost, environmental acceptability, and health and safety. However, in certain applications that might affect pristine or precarious environments, regulations or legislation may force the producer and end users to develop and use environmentally acceptable biolubricants exclusively.

The use of biolubricants will continue to increase due to sustainability (derived from renewable resources) and their reduced environmental impact (ready biodegradability) in all applications as long as their cost performance ratio and availability are not a hindrance. In other words, economic and environmental factors will lead the biolubricants industry, which has a greater emphasis on saving resources, energy and reducing emissions, to gain ground along with cost and performance.The improvements in functionality in biobased natural and modified esters to match the designer synthetic esters will continue until they meet the most stringent lubrication applications. The higher performance will also include longer service life of a lubricant. Many of these improvements are going to come from a combination of chemical modification, additive technologies and biotechnology.

TAG structural modifications in the form of more stable liquids having high levels of monounsaturated oils will keep increasing in conventional crops for industrial applications and also in developing industrial crops like jatropha. Other unconventional technologies of producing TAG oils by algae and fermentation will become mainstream. As their production cycle intervals are much shorter and can be controlled, these new technologies will provide large-scale production, refining (clean-up) and restructuring through chemical modifications (e.g., ester splitting and re-synthesis). The scale of economy and finding value for the byproducts will enhance the cost effectiveness of biolubricants.

As progress in materials sciences advances, the use of various polymers as moving parts at least in ambient or low temperature applications will start increasing. This will necessitate a new type of lubricant that may use an aqueous medium. In this regard, a recent review discusses an extremely efficient synovial joint lubrication that has to last for a lifetime (Dedinaite, 2012). Lubrication of synovial joints is the most efficient and sophisticated solution to friction control in aqueous media with friction coefficients of 0.001-0.01 under high and low loads. Its efficiency is due to the complex structure of cartilage combined with the synergetic actions of self-assembled structures formed by phospholipids and biomacromolecules. Another report describes the use of polysaccharide as a superior biolubricant (Arad et al., 2006). In this study the rheological properties of a natural polymer, sulfated polysaccharide derived from red microalga indicate that it is an excellent lubricant under aqueous conditions. These two studies demonstrate that some of the future lubrication applications may rely on aqueous media and may use natural monomers and polymers that can self-assemble to interact with the moving surfaces and provide extremely efficient lubrication. The type of molecules that work in aqueous media will provide the best protection to the environment and prove to be the ultimate biolubricants.

17.8 Conclusion

Biolubricants are derived from renewable raw materials, are benign to the environment and possess the required functional properties for lubrication. Recent public awareness and concern about the usage of non-sustainable petroleum-based products and their impact on the environment has created an opportunity to produce environmentally acceptable products from agricultural feedstock that can lower pollution (air, water and soil), are sustainable, are human compatible with minimal health and safety risks, and are easy to dispose of due to their nontoxicity and facile biodegradability. Due to their natural abundance, cost effectiveness, inherent lubricity and the recently developed high monounsaturated oils with high stability characteristics, vegetable oils can make inroads into various lubrication applications.

Understanding the structure–functionality relationship is the key to overcoming the shortcomings of a base stock and creating high performance biolubricants. Compared to mineral oils, vegetable oils possess the desirable lubrication characteristics of high viscosity index, high flash point, low volatility, reasonable pour points, and better boundary lubrication with high load-carrying capacity and low wear characteristics. Oxidative stability and low temperature fluidity are the major limitations of vegetable oils. The high monounsaturated oils like high oleic oils created through the advancements in biotechnology or breeding address some of the limitations. Further improvements of TAG natural esters can be achieved by chemically modifying the oils. The chemical modifications include the synthesis of molecular structures that modify the head group region and the acyl chain. The fatty acids derived from vegetable oils or petroleum can be used to make synthetic esters by condensation of polyols with the fatty acids.

Synthetic esters are versatile materials that can have uniform molecular structure with well-defined properties that can be tailored to specific applications. The knowledge of the structure–function relationship of ester fluids can be useful to design the right structure and raw materials to suit the requisite functionality and application. Ester base fluids provide both hydrodynamic and boundary lubrication. They can be used as base stocks and additives.

New technologies that are pertinent to biolubricants, that have been developed in the last two decades and have potential commercial applications are presented. These include transesterified oils, cyclopropanated oils, estolides and branched-chain esters. The structure–functionality of these new esters and their advantages and limitations are also presented.

17.9 Acknowledgements

The technical assistance of Lucas J. Stolp to create the figures is gratefully acknowledged.