Industrial development and applications of plant oils and their biobased oleochemicals

4.1 Oleochemicals for polymer applications

Oleochemicals as polymer materials represent a relatively small market, but are well established. We have to keep in mind that before crude oil was explored the only possibility to do chemistry was by using renewable resources. One example is linseed oil, which is used to produce linoleum. Here, the demand has increased from 10 000 tons in 1975 to 50 000 tons in 1998 (coming from 120 000 tons in 1960!) (Yap et al., 2010). Another example: epoxidized soybean oil (ESO) as a plastic and coating additive has a relatively stable market of approximately 100 000 tons/year (Sharma et al., 2006a). It is worth mentioning that the dicarboxylic acids are industrially produced either via ozonolysis of oleic acid to produce azelaic acid (one of the few examples of large-scale industrial ozonolysis) or by dimerization of linoleic acid and oleic acid to obtain complex mixtures of high molecular weight diacids (e.g., EMPOL® types), originally introduced in the 1950s by General Mills Chemicals and Emery (both now Cognis Corp.) (Höfer, 1996).

In a variety of polymer applications, such as coatings, large amount of solvents has to be used, including hydrocarbon and chlorinated solvents. Here, a clear trend is seen toward the so-called “green solvents”, such as ester solvents, typically produced from naturally based fatty acids and/or fatty alcohols, representing the largest group (Folić et al., 2008).

4.2 Plant oils and their oleochemicals as alternative diesel fuels

The use of plant oils in diesel engines is nearly as old as the diesel engine itself. The inventor of the diesel engine, Rudolf Diesel, reportedly used groundnut (peanut) oil as a fuel for demonstration purposes in 1900 (Kirakosyan and Kaufman, 2009). Some other work was carried out on the use of plant oils in diesel engines in the 1930’s and 1940’s. The fuel and energy crises of the late 1970’s and early 1980’s as well as the accompanying concerns about the depletion of the world’s non-renewable resources provided the incentives to seek alternatives to conventional, petroleum-based fuels. In this context, plant oils as fuel for diesel engines were remembered. They now occupy a prominent position in the development of alternative fuels. Hundreds of scientific articles and various other reports from around the world dealing with plant oil-based alternative diesel fuels (“biodiesel”) have appeared in print. They have advanced from being purely experimental fuels to initial stages of commercialization. Nevertheless, various technical and economic aspects require further improvement of these fuels.

Numerous different plant oils have been tested as biodiesel. Often the plant oils investigated for their suitability as biodiesel are those which occur abundantly in the country of testing. Therefore, soybean oil is of primary interest as biodiesel source in the United States while many European countries are concerned with rapeseed oil, and countries with tropical climate prefer to utilize coconut oil or palm oil. Other plant oils, including sunflower, and safflower, have also been investigated. Furthermore, other sources of biodiesel studied include animal fats and used or waste cooking oils. Sources of biodiesel with some emphasis on developing countries have been discussed (Shay, 1993).

However, the direct use of plant oils in fuel engines is problematic. Due to their high viscosity (about 11–17 times higher than diesel fuel) and low volatility, they do not burn completely and form deposits in the fuel injector of diesel engines (Demirbas, 2003). Different ways have been considered to reduce the high viscosity of plant oils:

1. Dilution of 25 parts of plant oil with 75 parts of diesel fuel,

2. Microemulsions with short chain alcohols such as ethanol or methanol,

3. Thermal decomposition, which produces alkanes, alkenes, carboxylic acids and aromatic compounds,

4. Catalytic cracking, which produces alkanes, cycloalkanes and alkylbenzenes, and

5. Transesterification with ethanol or methanol.

Dilution of oils with solvents and microemulsions of plant oils lowers the viscosity; some engine performance problems, such as injector coking and more carbon deposits, still exist. Among all these alternatives, the transesterification seems to be the best choice, as the physical characteristics of fatty acid esters (biodiesel) are very close to those of diesel fuel and the process is relatively simple. Furthermore, the methyl or ethyl esters of fatty acids can be burned directly in unmodified diesel engines, with very low deposit formation.

4.3 Surfactants derived from plant oil-based fatty alcohols and fatty acids

Surfactants are molecules which affect the surface or interfacial energy of surfaces. Their fundamental properties make their applications almost limitless. They are used in soaps, detergents, foaming agents, herbicides, de-foaming agents, wetting agents, emulsifiers, cosmetics, fabric softeners, and virtually any other application where two dissimilar types of compounds are brought together. Surfactants are generally made up of a hydrophilic head group and a hydrophobic tail.

The hydrophobic end group is usually a saturated hydrocarbon chain, which can be either petrochemically based such as a long chain alcohol or from an oleochemical, such as a fatty acid chain. The head group can also be from either source. Some commonly used biobased hydrophiles include amino acids (Reznik et al., 2010; Infante et al., 2004), polyglycerides, polysaccharides and the carboxylate groups on fatty acid chains. For a surfactant to be biobased, either the hydrophobe or the hydrophile should be made from natural sources. A class of useful surfactants has been made using oleochemically based fatty esters as the hydrophobe and combining them with ethylene oxide as the hydrophile (Abd Maurad et al., 2006; Deleu and Paquot, 2004; Maneerat, 2005; Warwel et al., 2001). They have even (Johansson and Svensson, 2001) been able to create a mild surfactant by using their catalyst on a plant oil.

The use of carbohydrates as the hydrophilic part of a surfactant has been known for half of a century (Bazin et al., 1998; Gouéth et al., 1995). The carbohydrates range from lignin and lecithin to sugars (Bazito and El Seoud, 2001), such as sorbitol. Sorbitol-based surfactants have enjoyed great success and are currently commercially available from multiple sources, most commonly as the Span™ series (Fig. 2). These surfactants are considered to have excellent properties for emollients, or emulsifiers in the food and personal-care industries. An additional benefit is that in some of these surfactants, such as Span™ 60, both the hydrophilic part and the hydrophobic part of the surfactant are biobased, a sugar and fatty acid, respectively.

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Figure 2

The structure of a sugar surfactant (sorbitan mono-oleate). The most common isomer at the five sugar position is shown, but substitution at the two and six positions is also common.

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Biobased glycerol, produced in excess from the biodiesel process, is another low cost source for a biobased surfactant precursor. Polymers of glycerol (polyglycerides) have been shown to be good hydrophobes. Polyglyceryl laurates have been shown to have similar or even superior detergency to alcohol ethoxylates commonly used in laundry detergents (Yangxin et al., 2008). Polyglycerol surfactants can be made from epoxide-containing compounds like glycidol (Istratov et al., 2003), or from glycerol polymerization (Mendrek et al., 2010). Recently, another class of polyglyceride surfactants with potential for introduction of branching points was made from glycerol and methyl oleate (Doll and Erhan, 2006) using only sodium hydroxide as a catalyst (Fig. 3). This surfactant also demonstrated similar surfactant properties to those of alcohol ethoxylates. Additionally, the potential for introducing branches to the surfactant makes it of even more interest.

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Figure 3

The synthesis of oleochemical- and glyceride-based surfactants. Only the major product is shown.

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Surfactants are used in a wide range of fields. By far, the most important field of application is the washing and cleansing sector as well as textile treatment and cosmetics; these use more than 50% of the total amount of surfactants. Surfactants are also used in food sector, crop protection, mining, and production of paints, coatings, inks, and adhesives. The basic manufacturing routes to important surfactants are laid out in Fig. 4. It is true that the most important surfactant from the amount produced apart from soap is still the petrochemical-based alkyl benzene sulfonate; however, a continuous trend toward surfactants based on renewable resources has become apparent (Falbe, 1987).

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Figure 4

Production of surfactants and examples of products.

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4.4 Emollients

The physicochemical nature of the oil-phase components in a cosmetic emulsion, the emollients, determines their hair- and skin-care effects, such as smoothing, spreading, and sensorial appearance (Keng et al., 2009). Test methods have been developed to characterize and classify the numerous emollients available on the market, such as silicones, paraffin, and oleochemical-based products. The latter include glycerides, esters, alcohols, ethers, and carbonates with tailor-made structures depending on the performance needed (Table 2). However, especially with regard to additional effects, there is still a demand for new products and product combinations with unique performance properties.

For example, a compound based on dioctyl ether (INCI: dicaprylyl ether) from coconut- or palm kernel-oil-based octanol (e.g., Cetiol® LDO) allows the formulation of silicon oil-free hair-care products, particularly for the use as hair cleansing preparations in order to improve the tactile hair feeling and hair gloss. In combination with wax esters and cationic polymers, additional benefits like improved sensorial feeling are achieved (Lodén and Sc, 2003).

One example of a new product type for skin care is dioctyl carbonate (INCI: dicaprylyl carbonate, e.g., Cetiol® CC). The product is synthesized by the trans-esterification reaction of octanol and dimethyl carbonate in the presence of alkali catalyst. Dioctyl carbonate is a dry emollient with excellent dermatological compatibility and a comprehensive performance profile, such as solubilizing and dispersing ability for sun-care filters, and a unique sensorial feeling.

R. C. Guerbet in 1899 discovered the self-condensation reaction of alcohols, which, via the aldehyde as an intermediate, leads to branched structures (2-alkyl alcohols)–the Guerbet alcohols. Starting from fatty alcohols from plant sources, such as octanol and decanol, the corresponding C16 and C20 alcohols, 2-hexyl decanol and 2-octyl decanol, are produced. The reaction is carried out under alkali catalysis and high temperatures (>200 °C). Over the years, both products have proven to be efficient emollients in skin-care product formulations, but are also used for other applications, such as plasticizers.

4.5 Plant oils and their oleochemicals in pesticide formulations

Oils and fats, particularly plant based, are important raw materials for the chemical industry worldwide. Plant oil and their oleochemicals have major advantages over mineral oils in that they are renewable and readily available all around the world, often as a result of by-products from other industrial activities. Also, they are generally biodegradable, non-flammable and cause fewer medical problems and allergies to the end users (Fukuyama et al., 2009). Baumann and Biermann (1994) showed that oils and fats are biological raw materials which are constantly renewed. This will be reflected, at least in the long term as an economic advantage for chemical products from oils and fats, compared with those from mineral oil. Favorable ecological characteristics can be expected from linear fatty acid molecules which have no branches or ring structures.

Plant oils such as coconut, soybean, corn, palm, palm kernel oils and their oleochemicals are used to produce inert ingredients such as surfactants, wetting agents, dispersing agents, emulsifying agents, adjuvants, solvents and carrier/diluents. for pesticide formulations. In general, preparation of pesticide formulations will include two main components, i.e., the active ingredients and inert ingredients. Active ingredients mean the chemical or chemicals in a product responsible for the desired effect, which are capable of preventing, destroying, repelling or mitigating insects, fungi, weeds, rodents, or other pests. Meanwhile, inert ingredients also called supplements are inactive ingredients which have no pesticidal action (Abhilash and Singh, 2009). There are various types of inert ingredients normally found in pesticide formulation, for example, emulsifiers, organic solvents, dispersing agents, and wetting agents.

4.6 Biolubricants from chemically modified plant oils

There has been a constant demand for environmentally friendly or “green” lubricants. A significant lubricant market of some nine million metric tons per year of industrial and automotive lubricants exists. Currently, plant oils (VO) provide only a fraction of the lubricant market. VOs are already in use as lubricants due to their superior lubricity, good anticorrosion, better viscosity-temperature characteristics and low evaporation loss in industrial applications such as rolling, cutting, drawing, quenching operations, and greases either alone or in combination with mineral oils (Campanella et al., 2010; Erhan and Asadauskas, 2000). Their volatility is low due to the high molecular weight of the triglyceride molecule and they have a narrow range of viscosity changes with temperature. Polar ester groups are able to adhere to metal surfaces and, therefore, possess good boundary lubrication properties. In addition, VOs have high solubilizing power for polar contaminants and additive molecules. From the environmental point of view their importance is evident especially in areas of total loss lubrication, military applications, and in outdoor activities such as forestry, mining, railroads, dredging, fishing and agriculture hydraulic systems. However, their use is still restricted due to low thermo-oxidative stability and poor cold flow behavior. There are various ways to improve the oxidation and the cold flow behavior of VOs. These include use of heat-, chemically, and genetically modified plant oils and additives.

Because the most active site for various oxidation reactions of a triacylglycerol is at allylic site of the carbon chain, elimination of the double bonds in the triacylglycerol is an important step in increasing the oxidative stability of the oil. The double bonds in triacylglycerol can be eliminated by polymerization using heat modification method (Fox and Stachowiak, 2007; Erhan et al., 2006), resulting in lubricating base oils with different viscosities. Heat modification is usually carried out at a temperature range of 232–330 °C in an inert atmosphere. At this high temperature the double bond migrates and conjugated dienes are formed, which readily add to active double bonds to form a six-membered ring containing one double bond. This polymerization process often leads to an increase in oil viscosity. Microwave-irradiation also provides higher viscosity oils like heat-modified oils (Biswas et al., 2007). These heat-modified VOs, which have improved oxidation properties, are successfully used in a formulation of hydraulic elevator fluid for the Statue of Liberty elevator and for the metalworking fluid industry.

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 (Wagner et al., 2001). 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 (Salimon and Salih, 2009a,b). The key steps in the three-step synthesis of oleochemical diesters include epoxidation and ring opening of epoxidized oleic acid with different fatty acids (octanoic, nonanoic, 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) (Salimon and Salih, 2009c,d).

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Figure 5

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

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Not surprisingly, as the length of the mid-chain increases, 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.

Oleic acids are more thermally stable than polyunsaturated fats, and therefore are highly desired components in VOs for lubricant applications. Successes in genetic modification have helped to develop high oleic VOs, which are now available for commercial uses. The combination of high-oleic VOs and chemical additives offers the best option for achieving the ultimate goal. The lubricants formulated using this approach exhibit similar or better oxidative stability and low temperature properties compared to petroleum-based lubricants (Sharma et al., 2006b). Greases are solid or semi-solid products obtained by the dispersion of a thickening agent in a liquid lubricant. Development of VO-based greases has been an area of active research for several decades (Awawdeha et al., 2009). It has been found that the thermo-oxidative and tribochemical behavior of soybean oil-based greases is affected by the composition of the metal soap thickener (Sharma et al., 2005). Biobased grease with high oxidative stability can be prepared using epoxy VOs for industrial, agriculture/ farming equipment, and forestry applications (Sharma et al., 2006b). These greases deliver at par or better performance properties (effective lubrication, wear protection, corrosion resistance, friction reduction, heat removal, etc.) than existing mineral oil-based grease currently used in similar trade.