Industrial Uses of Vegetable Oil [1 ed.] 1893997847, 9781893997844

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Industrial Uses of Vegetable Oils

Editor Sevim Z. Erhan Food and Industrial Oil Unit National Center for Agricultural Utilization Research Agricultural Research Service United States Department of Agriculture Peoria, IL 61804

Champaign, Illinois

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AOCS Mission Statement

To be the global forum for professionals interested in lipids and related materials through the exchange of ideas, information science, and technology. AOCS Books and Special Publications Committee M. Mossoba, Chairperson, U.S. Food and Drug Administration, College Park, Maryland R. Adlof, USDA, ARS, NCAUR, Peoria, Illinois P. Dutta, Swedish University of Agricultural Sciences, Uppsala, Sweden T. Foglia, ARS, USDA, ERRC, Wyndmoor, Pennsylvania V. Huang, Abbott Labs, Columbus, Ohio L. Johnson, Iowa State University, Ames, Iowa H. Knapp, Deanconess Billings Clinic, Billings, Montana D. Kodali, Global Agritech, Inc., Plymouth, Minnesota T. McKeon, USDA, ARS, WRRC, Albany, California R. Moreau, USDA, ARS, ERRC, Wyndoor, Pennsylvania A. Sinclair, RMIT University, Melbourne, Victoria, Australia P. White, Iowa State University, Ames, Iowa R. Wilson, USDA, REE, ARS, NPS, CPPVS, Beltsville, Maryland Copyright (c) 2005 by AOCS Press. All rights reserved. No part of this book may be reproduced or transmitted in any form or by any means without written permission of the publisher. The paper used in this book is acid-free and falls within the guidelines established to ensure permanence and durability.

Library of Congress Cataloging-in-Publication Data Industrial uses of vegetable oils / editor, Sevim Z. Erhan. p. cm. Includes index. ISBN 1-893997-84-7 1. Vegetable oils--Industrial applications. I. Erhan, Sevim Z. TP680.I555 2005 665'.384--dc22 Printed in the United States of America. 08 07 06 05 04 5 4 3 2 1

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Preface

Vegetable oils are used in various industrial applications such as emulsifiers, lubricants, plasticizers, surfactants, plastics, solvents and resins. Research and development approaches take advantage of the natural properties of these oils. Vegetable oils have superb environmental credentials, such as being inherently biodegradable, having low ecotoxicity and low toxicity towards humans, being derived from renewable resources, and contributing no volatile organic chemicals. United States agriculture produces over 25 billion pounds of vegetable oils annually. These domestic oils are extracted from the seeds of soybean, corn, cotton, sunflower, flax, and rape. Although a major part of these oils are used for food products such as shortenings, salad and cooking oils and margarines, large quantities serve feed and industrial applications. Other vegetable oils widely used industrially include palm, palm kernel, coconut, castor, and tung. However, these are not of domestic origin. The three domestic oils most widely used industrially are soybean, linseed from flax, and rapeseed. Nonfood uses of vegetable oils have grown little during the past 40 years. Although some markets have expanded or new ones added, other markets have been lost to competitive petroleum products. Development of new industrial products or commercial processes is the objective of continued research in both public and private interests. The following selected examples illustrate progress in identifying and developing new technologies based on vegetable oils. Great progress has been made in understanding of the biochemical basis for biosynthesis of oils containing fatty acids. This biochemical information is in turn used to identify and isolate genes that are needed to make these oils. By genetically engineering the introduction and expression of these genes, domesticated crops that can produce these potentially useful fatty acids have been engineered and are continuing to be developed to produce an ever wider range of novel oils. Chapter 1 explains the biochemical changes that can be introduced to alter fatty acid composition. It also discusses industrial oils that have been developed through genetic engineering, as well as some that have been developed on the laboratory scale, but have not yet been introduced commercially. Recent environmental awareness and depletion of world fossil fuel reserves have forced to look a substitute for mineral oils with the biodegradable fluids such as vegetable base oils and certain synthetic fluids in grease formulations. The nontoxic and readily biodegradable characteristics of vegetable oil based greases pose less danger to soil, water, flora, and fauna in case of accidental spillage or during disposal. Biodegradable greases are particularly useful in open lubrication systems where the lubricant is in direct contact with environment, and total loss lubricants like railroads, where immediate contact with the environment is anticipated. Chapter 2 discusses the various components (base oils, thickeners and additives), functional properties, and characteristics of biodegradable greases. The base oils included synthetic esters, castor, rapeseed, and soybean oil. iii

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Chapter 3 reviews some of the advantages and disadvantages of using vegetable oil lubricants and their availability. Some of the history in the development of vegetable-based engine oils and their current status is described. The requirements for further development and penetration of the petroleum based engine oil market are discussed. Besides transesterification to alkyl esters, three other approaches—dilution with conventional, petroleum-based diesel fuel, microemulsions (co-solvent blending), and pyrolysis—have been explored for utilizing vegetable oils as fuel. However, as the mono-alkyl esters of vegetable oils and animal fats—biodiesel—are the only approach that has found widespread use (and, accordingly, the vast majority of research papers deal with this approach), Chapter 4 focuses on such mono-alkyl esters in terms of use, properties, economies, and regulatory issues. Chapter 5 presents a background on home heating systems and highlights recent research to develop renewable biofuels for home heating applications. Petroleumbased liquid home heating oil is used to heat over 8 million homes in the U.S., predominantly in the northeastern U.S. This comprises approximately 6.6 billion gallons of fuel oil annually. With recent rises in petroleum prices to over $50 per barrel and anticipated future price increases as petroleum resources become less available, many applications that depend on petroleum are searching for alternatives. Additional concerns over environmental issues involving sulfur and nitrogen oxide emissions from oil-based home heating systems have sparked a search for alternative fuels to supply this market. Polyurethanes are the most versatile group of polymers which can be used in the form of foams, cast resins, coatings, adhesives and sealants. Polyols used in the polyurethane industry currently exceed 2.4 million tons/year in the U.S. To use natural oils as raw materials for polyurethane production, multiple hydroxyl functionality is required. Castor oil has hydroxyl functionality naturally built in, thus it has received extensive exploration as polyurethane building blocks, such as casting resins, elastomers, urethane foams, and interpenetrating networks. Hydroxyl functionality can be introduced synthetically in other natural oils. This process involves a number of approaches and has been studied extensively by scientists around the world, but commercial production of oil-based polyols has been scarce. Chapter 6 discusses the four main approaches for the hydroxylation of vegetable oils. In Chapter 7, the authors summarize the type of natural composites reinforced with different fibers along with different composite molding methods. The Solid Freeform Fabrication Method and its advantages are included in the discussion. Technologies that have improved the use of oils in coatings are highlighted in Chapter 8. The petroleum shortage in the 1970s stimulated research on vegetable oil-based inks as a substitute for petroleum based products. Vegetable oils are mainly used in paste inks; therefore the role of vegetable oils in the paste ink formulations and their environmental properties are the main subject of Chapter 9. Chapter 10 explains that vegetable oils provide a renewable source of fatty acids that can serve as raw materials for the production of numerous surfactant compounds. Structural modification of the fatty acids can impart unique physical prop-

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erties that alter the performance of the product in a predictable manner. Chemical functionality can be introduced at the carbonyl carbon or along the carbon chain by appropriate selection of reactants, catalysts, and reaction conditions. A tremendous diversity of products is available with these oleochemical substrates. In addition, vegetable oils provide a favorable alternative to petrochemical feedstocks. The editor of this timely publication thanks the authors and their organizations for their technical contributions in the chapters of this book. A special thanks goes to Brittney Mernick for her assistance in the preparation of chapters for publication. Sevim Z. Erhan February 14, 2005

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Contents

vii

Contents

Chapter 1

Chapter 2

Chapter 3

Chapter 4

Chapter 5

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Genetic Modification of Seed Oils for Industrial Applications Thomas A. McKeon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1

Current Developments of Biodegradable Grease Atanu Adhvaryu, Brajendra K. Sharma, and Sevim Z. Erhan . .

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Vegetable Oil-Based Engine Oils: Are They Practical? Joseph M. Perez . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Biodiesel: An Alternative Diesel Fuel from Vegetable Oils or Animal Fats Gerhard Knothe and Robert O. Dunn . . . . . . . . . . . . . . . . . . .

42

Biofuels for Home Heating Oils Bernard Y. Tao . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

90

Chapter 6

Vegetable Oils-Based Polyols Andrew Guo and Zoran Petrovic . . . . . . . . . . . . . . . . . . . . . . . . 110

Chapter 7

Development of Soy Composites by Direct Deposition Zengshe S. Liu and Sevim Z. Erhan . . . . . . . . . . . . . . . . . . . . . 131

Chapter 8

Vegetable Oils in Paint and Coatings Michael R. Van De Mark and Kathryn Sandefur . . . . . . . . . . . 143

Chapter 9

Printing Inks Sevim Z. Erhan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163

Chapter 10

Synthesis of Surfactants from Vegetable Oil Feedstocks Ronald A. Holser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170 ...

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179

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Copyright © 2005 AOCS Press

Chapter 1

Genetic Modification of Seed Oils for Industrial Applications Thomas A. McKeon USDA, ARS, WRRC, Albany, CA 94710

Introduction While most vegetable oils are produced for food and feed uses, up to 15% of soy (as well as other food oils) and up to 100% of certain commodity oils are used for industrial purposes. Most food oils, such as soybean or canola, are composed primarily of five fatty acids (FA): palmitic, stearic, oleic, linoleic, and linolenic; these oils are used to produce surfactants, lubricants, inks, coatings, and polymers. Commodity oils containing uncommon FA, such as castor (90% 12-hydroxyoleate) and tung (up to 80% conjugated FA), have no nutritive value, but due to the unusual properties of the FA, they prove very useful for industrial applications. It is the chemical functionality of a vegetable oil that can make it useful to industry; chemical functionality can alter physical properties or allow chemical precursors or useful derivatives to be made. For example, ricinoleate, the FA from castor oil, has a mid-chain hydroxyl group that enhances its viscous properties for use as grease and also enables production of an extensive range of chemical derivatives (1). Coconut oil contains laurate (12:0) which has excellent foaming properties and is used to make anionic surfactants. Hydroformylation of petroleum provides an equivalent surfactant (2). The possibility of replacing such petroleum products with plant-derived FA is a major goal of seed oil utilization research. There are hundreds of FA with unusual functionalities, at least some of which would have immediate application if readily available from a suitable crop. To the extent that uncommon FA are produced in a given plant, these are a result of evolution, perhaps providing selective advantage as a result of toxic or other protective effects of the FA on pathogens. Though it operates on a long time scale, evolution has provided an unusual array of genetic material for production of useful FA. However, many of these FA are produced in plants that are unsuitable as crops. Traditional breeding techniques can alter levels of FA present in the oil and, with suitable germ plasm, can reduce or eliminate one or more of the FA normally present, as was the case in the development of canola (low-erucic acid rapeseed) (3,4). Breeding has been used to develop plant selections with a high proportion of a single component, e.g., such as high oleic safflower. High enrichment of a single component such as oleate represents another industrially useful feature, as it 1

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reduces the expense of purifying the desired component. But breeding cannot be used to introduce a FA not already present in one of the crossed plants. Random mutagenesis using chemical or radiation agents to alter the genome followed by screening and breeding has also produced varieties with altered FA composition in oil (5). Genetic identification and chemical characterization of FA biosynthetic mutants in mutated Arabidopsis thaliana has provided an extensive genetic map of FA and lipid biosynthetic steps during plant growth and development (6), in many cases providing null mutants lacking a specific enzymatic activity. Since the mutagenic approach is geared toward eliminating genes, this approach has been used as part of breeding programs to reduce levels of undesirable FA components such as high polyunsaturates from linseed oil (7) or to increase levels of a desired FA, e.g., oleate in sunflower by eliminating the enzyme that normally converts it to linoleate (8). A recent innovation in this approach is TILLING (Targeting Induced Local Lesions IN Genomes), which uses a mutagenic approach, but introduces high-throughput screening of the M2 generation (the second generation of self-pollinated, mutated lines) in order to identify specific genes that have been altered or inactivated by mutagenic events (9). Plant selections carrying these mutated genes can then be screened directly for desired characteristics. The TILLING process thus moves most of the screening effort into the laboratory, considerably reducing the population that would otherwise have to be grown in the field for phenotypic screening. With the advent of genetic engineering, the technology needed to introduce novel traits became available to breeders. A driving force behind development of genetically engineered oils is the perennial surplus of oils produced. The unused inventory of soybean oil may reach nearly two billion pounds in any year. Crops with altered oil composition hold the promise of reducing or preventing annual inventory carryover, thus stabilizing or improving farm income. This chapter will explain the biochemistry underlying the alteration of FA composition, briefly describe some oils that have been developed through genetic engineering and mention some of the “target” FA of interest for production in transgenic oilseed crops.

FA Biosynthesis FA biosynthesis in plants proceeds from acetylCoA, which initiates a set of condensation reactions with malonyl-ACP through six or seven additional condensations with malonyl-ACP. This yields the saturated FA palmitate or stearate, respectively, as depicted in Figure 1.1, which depicts the pathway of FA biosynthesis to linoleic acid, with the reactions leading to palmitate, stearate, and oleate occurring in the plastid, separate from reactions leading to oil biosynthesis. Given the dependence of FA production on malonyl-CoA production (to provide malonyl-ACP), the acetyl-CoA carboxylase (ACCase) is generally thought to play a regulatory role in FA production and oil biosynthesis (10). This hypothesis is supported by research in which ACCase from Arabidopsis was overexpressed in potato, leading to an increase in FA production and a fivefold increase in triacylglycerol levels in the tuber (11).

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Genetic Modification of Seed Oils

Fig. 1.1. The pathway of fatty acid biosynthesis to linoleic acid.

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Medium Chain-Length FA Biosynthesis In seeds of certain plants such as coconut, palm kernel, bay laurel, and cuphea, the flow of carbon to the long-chain saturated FA is disrupted, and this occurs as the result of an acyl-ACP thioesterase (product of the FAT B gene), which removes the ACP from the elongating FA chain prior to achieving full length. This produces a medium chain-length FA which is transported from the plastid and enters the oil biosynthetic pathway. This approach copied from nature led to the development of the first transgenic oilseed modified to produce an industrial oil product, namely Laurate Canola (12). By inserting into Canola the cDNA for a medium-chain specific acyl-ACP thioesterase (13) from California bay laurel, a plant which produces seeds containing >60% laurate(dodecanoate) in its oil, plastidial FA synthesis was diverted to the production of laurate, which was incorporated into the seed oil (14). Although this achievement was a key early success in the contribution of genetic engineering to agriculture, the underlying science also pointed to a number of technical problems that have since been widely recognized. The production of a FA not normally produced by the seed may trigger a “counter-reaction.” In the case of laurate, considerable amounts of the laurate were β-oxidized, since the cytoplasmic lauroyl-CoA used to acylate glycerolipid is also an intermediate in β-oxidation (15,16). While increased carbon flux through the FA biosynthetic pathway enhanced laurate production, the overall outcome was a canola cultivar with reduced oil yield, since some of the carbon incorporated into laurate production was oxidized through the futile cycle. The laurate canola oil produced also lacked laurate in the sn-2 position of the triacylglycerol (TAG) (17). The canola seed lacked a lyso-phosphatidic acid acyltransferase (LPAAT) that could use lauroyl-CoA as an acyl donor for the sn-2 position of glycerolipid. Researchers at Calgene solved this problem by crossing a canola plant containing an LPAAT gene from coconut (17), with a laurate canola plant (18). The resulting plant produced an oilseed in which laurate is distributed among all three positions of the TG. The resulting “High-Laurate Canola” had a laurate content of up to 70%. The successful design of a novel, temperate-climate industrial crop provided a great impetus to follow this approach for other industrially useful products, especially oils. It also provided a foreshadowing of the difficulties to be encountered in engineering production of uncommon FA in oilseeds. Monounsaturated FA Biosynthesis In general, once saturated FA are released from acyl-ACP, they are incorporated into oil without any apparent modification except, to a minor extent, elongation. In the plastid, though, the saturated fatty acyl-ACP can be desaturated by the ∆9desaturase, a class of soluble enzymes (as opposed to membrane-bound) formerly identified as the stearoyl-ACP desaturase, which is the type present in most oilseeds. These enzymes share a considerable degree of amino acid sequence homology and the same type of active site in which the desaturation is carried out.

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In most plants, the ∆9-desaturase produces oleate which, for purposes of oil biosynthesis, is transported from the plastid to the endoplasmic reticulum and incorporated into CoA, phospholipid, and acylipid. Oils high in oleic acid content have been considered desirable both for food and nonfood uses. A high-oleate soybean oil containing greater than 80% oleic acid was developed by suppressing expression of the desaturase enzyme that converts oleate to linoleate in soybean. This oilseed has been commercialized and for industrial purposes find applications as a stable, biodegradeable hydraulic oil and is likely useful for developing other bio-based lubricant applications (19). Some plants produce monounsaturated FA of differing chain-length or with the double-bond in a different position on the carbon chain, or both. Many such desaturases have been cloned, the crystal structure of the soluble desaturase from castor (Ricinus communis) has been determined, and considerable insight on factors involved in chain-length and positional specificity of the desaturase reaction have been revealed (20,21). The ability to engineer this type of enzyme to introduce a cis-double bond at a specific position on a selected chain-length represents a bench chemist’s dream for saturated hydrocarbon chemistry. However, despite the apparent similarity of some products to oleate, e.g., 18:1 ∆6 (petroselenate), their production can differ from that of oleate, resulting in limited amounts of the product when introduced into a transgenic plant (10). It has been shown that, in some cases, co-factors such as ferredoxin and ACP isoforms that interact specifically with the enzyme are required. Moreover, the FA may also require altered lipid metabolism to be suitably incorporated into TAG (10). Thus, further understanding of lipid biochemistry leading to TAG production will underly successful attempts to engineer oil composition. Modification of Oleate In most temperate climate oilseeds, the oleate may be further desaturated to linoleate and α-linolenate. In rapeseed, crambe and na-sturtium, the oleate may be elongated to erucic acid by the action of an acylCoA based elongation reaction, mediated in part and possibly regulated by expression of a keto-acyl synthase (KAS) specific to elongation of long-chain FA. The products of elongation, usually 20:1 ∆11 and 22:1 ∆13 are incorporated into the TAG fraction (oil). In some plants, the oleate is oxidized to uncommon FA. For example, in Vernonia, 18:1 ∆9, 12-13 epoxy (vernolate) is formed and then incorporated into TAG (21,22). The possibilities resulting from oleate production provided the basis for the original concept of oleate as the central substrate in plant FA biosynthesis (23). The set of modification reactions that can alter oleate is unusual, in that it comprises a family of homologous enzymes that have evolved from the FAD2 genes, which encode the oleoyl desaturase in oilseeds. Enzymes that have evolved from the FAD2 have been found to carry out an unusual array of conversions, using an oleoyl-phosphocholine (oleoylPC)-based substrate. These reactions include hydroxylation, epoxi-

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dation, desaturation-conjugation and desaturation to a triple bond (22,24,25). In the case of hydroxylation and desaturation, changes in as few as 4–6 amino acid residues result in an interchange of the two types of activity (24,26). In fact, the oleate 12-hydroxylase from Lesquerella has mixed functionality, and can introduce a hydroxyl group or double bond (27). Interestingly, as with some other uncommon FA described in previous sections, introduction of genes with suitable production of these uncommon FA in the oil of a transgenic plant has also proven difficult. The next section will elaborate on this theme by describing the biochemistry of castor oil, an important commodity oil with numerous applications (1). Castor Oil Biosynthesis Castor oil is a product of great interest to plant lipid scientists. It is an established commercial product with a significant market and a cost of 45–50 cents per pound versus soybean oil at 15–25 cents, yet is entirely imported by most industrialized nations. Because castor seed contains noxious proteins, it is problematic as a crop. Therefore, producing castor oil transgenically represents an enticing target and a long-term challenge. Understanding the basis for the regulation of seed oil yield is also a major research goal and castor, at 60% oil, has served as a benchmark for high oil content. Interest in castor oil biochemistry precedes the genetic engineering revolution. In the 1960s, both the Stumpf research group at University of California, Davis, and the Morris group at Unilever Research in Great Britain, carried out basic research investigating the hydroxylation reaction that converts oleate to ricinoleate (28,29). These early biochemical developments were followed by the research groups of Stymne at Uppsala and Somerville and colleagues from MSU. These groups contributed greatly to current understanding of ricinoleate production, and the latter two groups elucidated the genetic basis for castor oil production by identifying and cloning two of the key genes (30,31) The oleoyl-12-hydroxylase enzyme proved challenging to purify (32–36). Although the enzyme has not been purified to date, the cDNA for its gene was cloned by a genomics approach (30). Based on the hypothesis that the hydroxylation reaction is analogous to, or the first step in, the desaturation reaction, this research group proposed that the hydroxylase would share sequence elements in common with FA desaturases. Using this approach, hundreds of cDNAs from developing castor seed were sequenced, prospective hydroxylase cDNAs expressed in tobacco seed, and the seed oil assayed for hydroxy FA. Although ricinoleate production was low, 0.1%, it was sufficient to show that the hydroxylase had been cloned and successfully expressed in a transgenic plant. However, to date, oilseeds transformed to express the gene for oleoyl-12-hydroxylase produce much less than the 90% present in castor oil, with most transgenes producing less than 20% hydroxy FA content in oil (37). It has been hypothesized that the ricinoleate incorporated in lipid inhibits membrane function in most plants, so it may

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be eliminated from the membrane by endogenous phospholipases (38) and betaoxidized (39) by analogy to laurate. On the other hand, castor has evolved biochemically to produce and incorporate ricinoleate into oil. This led to the approach of identifying additional enzyme components in castor that enable it to produce an oil with 90% ricinoleate. Based on a considerable body of research (31,33,38,40–43), a number of enzymes have been identified that appear to be involved in high ricinoleate production, ricinoleate incorporation into oil, or maximizing oleate conversion to ricinoleate (44). The latter role is clearly fundamental, since the final content of oleate in castor oil is less than 4%, and the castor oil biosynthetic pathway is 96% efficient in converting oleate. This research has been aided by development of methods for “metabolic profiling” castor oil biosynthesis. In an effort to develop an alternative oilseed that could produce castor oil, a microsomal system that carries out the biosynthesis of castor oil in microsomes prepared from immature castor seed endosperm and embryo has been developed (36,42). The microsomal system is effective in synthesizing the TAG produced by the intact seed and provides a realistic model system for investigating castor oil biosynthesis. Using this system and analysis of lipid metabolites by high-performance liquid chromatography with selected columns and solvent conditions, intermediates that accumulate during castor oil biosynthesis can be separated and identified (44). This approach has enabled the identification of additional enzymes that provide the unique basis for biosynthesis of castor oil, since the gene for FA hydroxylation by itself is not sufficient to produce high levels of ricinoleate in other oilseeds (37). Based on these research results and other published research, the pathway in Figure 1.2 has been proposed. The following narrative of the pathway summarizes these findings, with key reactions and their role described briefly: (i) The lyso-phosphatidylcholine acyltransferase (LPCAT) transfers the oleoylmoiety from oleoyl CoA into the sn-2 position of PC for hydroxylation. (ii) The oleoyl-12-hydroxylase hydroxylates the sn-2 oleate to form sn-2 ricinoleoyl-PC. (iii) The phospholipase A2 preferentially removes ricinoleate from the sn-2 position of PC and releases lyso-PC for reincorporation of oleate by LPCAT. (iv) The free ricinoleate is preferentially incorporated into ricinoleoyl-containing diacylglycerols by the diacylglycerol acyltransferase (DGAT) to form diricinoleins and triricinolein, which make up castor oil. (v) The phospholipid-diacylglycerol acyltransferase (PDAT) incorporates the sn-2 ricinoleate directly from the ricinoleoyl-PC product of the hydroxylase reaction into the TAG end product. The final step in oil biosynthesis (Fig. 1.2) shows a high degree of selectivity for incorporating ricinoleate preferentially. Based on in vitro results, both the DGAT and PDAT (45) appear to be active in carrying out the incorporation of ricinoleate

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Fig. 1.2. Castor oil pathway.

into castor oil. The DGAT cloned from castor shows a preference for using diricinolein as a substrate in comparison to the DGAT from Arabidopsis, a plant that does not produce hydroxy FA in its seed oil (46).

New and Improved Crops The production of industrially useful FA in transgenic crops is complicated by the need for greater understanding of how such FA are efficiently made in the plants that make them, and how their incorporation into oil is directed. Table 1.1 lists a number of FA and related products that are of interest to researchers seeking to expand the role of seed oils in the “hydrocarbon economy.” The plants developed would be renewable resources, enhance opportunities for rural development, and contribute to the improvement of the environment. Current research efforts are on the appropriate control of gene expression, elucidating the synthesis of the FA, and controlling its “destiny”—assuring its incorporation in oil and preventing it from being further metabolized. Another application of transgenic technology is the development of oilseeds with improved agronomic characteristics. In fact, this has been the primary goal of agricultural chemical producers that have initiated programs to produce GM crops. Currently, the four genetically engineered crops that have been adopted are all oilseed crops: soy, corn, cotton and canola. They account for 99% of transgenic crops planted

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TABLE 1.1 Industrially Useful Fatty Acids for Transgenic Plant Production Fatty acid

Functionality

Source

Use

Eleostearic Octadeca-9c,11t,13t-trienoic

Conjugated double bonds

Tung, bitter melon

Drying oil

Erucic Docosa-13c-enoic

Very long-chain (VLC)

Rapeseed, crambe

Lubricants, antislip agent

γ-Linolenic Octadeca-6c,9c,12c-trienoic

Polyunsaturate

Borage, blackberry

Nutraceutical

Caproic to Myristate 6 to 14 carbons

Medium chain-length

Cuphea, coconut, bay laurel

Detergents

Oleic Octadeca-9c-enoic

Monounsaturate

Many

Hydraulic oil, oleochemicals

Petroselenic Octadeca-6c-enoic

Monounsaturate isomer

Coriander

Nylon 6,6

Ricinoleic Octadeca-9c,12-OH-enoic

Hydroxylated

Castor

Lubricants, polymers

Vernolic Octadeca-9c, 12,13-O-enoic

Epoxy

Vernonia, Euphorbia lagascae

Coatings, plasticizer

Docosahexaenoic

VLC polyunsaturated

Algae

Nutraceutical

Nervonyl Erucate

VLC wax ester

Jojoba

High-temperature lubricant

worldwide. Over 70% of the soy grown in the U.S, 50% of the corn and 70% of the cotton are genetically engineered. Most of the canola grown in Canada, a leading producer, is transgenic. An increasing number of countries have adopted the technology. The U.S., Argentina, Canada, Brazil, China, and South Africa account for 99% of the transgenic crops produced, with an additional 12 countries adopting the technology (47). The growth in planting of transgenic crops is remarkable in that it has all occurred in the last eight years, from the time the first transgenic crops were introduced in 1996. At this time, each of these crops has been modified for “input” traits, reducing or eliminating the need for chemical applications by the introduction of genes encoding herbicide tolerance (soy, canola), insect resistance (corn, cotton), or both (cotton). As plant genomics and proteomics programs identify other agronomically useful genes, other transgenic traits will also be incorporated. These can range from elimination of noxious components (48) to introduction of dwarfing genes for greater plant efficiency. Small volume crops, such as papaya and squash, have already been genetically modified for viral resistance. Crop genetic engineering holds great promise as a means for developing oilseed crops with unique characteristics that add both commercial and nutritive value, increase utilization, and benefit the environment.

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Summary Oilseeds are an important source of chemicals for industry. Most temperate climate oilseeds produce oils containing the same five FA (palmitate, stearate, oleate, linoleate, and α-linolenate) in different proportions. In addition to nutritive uses, these FA are used to produce soaps and detergents, coatings, lubricants, cosmetics, plastics, plasticizers, and numerous chemical derivatives. For specific uses, certain FA are more desirable. For example, the conjugated double bond system present in FA of tung oil gives it excellent properties as a drying oil. Lauric acid from coconut provides a chemical feedstock for producing detergents. Laurate canola was the first commercial crop that was genetically designed to produce an industrial FA. The ability to manipulate FA composition in oilseeds resulted from a combination of three approaches. First, biochemical characterisation has identified most of the steps in FA biosynthesis. Secondly, genetic identification and chemical characterization of Arabidopsis thaliana mutants has provided an extensive genetic map of FA and lipid biosynthetic steps during plant growth and development. Finally, the additional information needed to broaden the spectrum of FA available from oilseeds has been provided by the identification, characterization, and cloning of unusual enzyme activities from plants that produce uncommon, industrially useful FA. Hundreds of uncommon FA, with unusual chemical functionalities, are produced by one or more oilseed plants. A considerable amount of research has gone into elucidating the biosynthetic process by which such FA are made; much of the enzymology underlying the introduction of unsaturation, conjugated unsaturation, and hydroxyl, acetylenic, and epoxy functionality is now understood. As knowledge of the mechanistic and structural knowledge of these enzymes expands, there is potential for engineering production of FA that are not yet known. The specificity of the chemistry carried out on what is essentially a straight hydrocarbon chain is unprecedented for the bench chemist, and presents the possibility of “green” chemistry carried out in green plants to produce a wide array of chemicals designed for industrial applications. References 1. Caupin, H.J., Products from Castor Oil: Past, Present, and Future, in Lipid Technologies and Applications (Gunstone, F.D. and Padley, F.B., eds.), Marcel Dekker Inc., New York, 1997, pp. 787–795. 2. Porter, M.R., Anionic Detergents, in Lipid Technologies and Applications (Gunstone, F.D. and Padley, F.B., eds.), New York: Marcel Dekker Inc., 1997, pp. 579–608. 3. Stefansson, B.R., F.W. Hougen, and R.K. Downey, Note on the Isolation of Rape Plants with Seed Oil Free from Erucic Acid, Can. J. Plant Sci. 41: 218–219 (1961). 4. Downey, R.K., A Selection of Brassica campestris L. Containing No Erucic Acid in its Seed Oil, Can. J. Plant Sci. 44: 295 (1964). 5. Knowles, P.F., Genetics and Breeding of Oil Crops, in Oil Crops of the World, (Robbelen, G., Downey, R.K., and Ashri, A., eds.), McGraw-Hill, New York, pp. 260– 282 (1985).

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6. Browse, J., and C. Somerville, Glycerolipid Synthesis: Biochemistry and Regulation, Ann. Rev. Plant Physiol. Plant Mol. Biol. 42: 467–506 (1991). 7. Green, A.G., A Mutant Genotype of Flax (Linum usitatissimum L.) Containing Very Low Levels of Linolenic Acid in its Seed Oil, Can. J. Plant Sci. 66: 499–503 (1986). 8. Velasco, L., and J.M. Fernandez-Martinez, Breeding Oilseed Crops for Improved Oil Quality, J. Crop Prod. 5: 309–344 (2002). 9. Henikoff, S., and L. Comai, Single-Nucleotide Mutations for Plant Functional Genomics, Annu. Rev. Plant Biol. 54: 375–401 (2003). 10. Thelen, J.J., and J.B. Ohlrogge, Metabolic Engineering of Fatty Acid Biosynthesis in Plants, Metabolic Engineering 4: 12–21 (2002). 11. Klaus, D., J.B. Ohlrogge, H.E. Neuhaus, and P. Dormann, Increased Fatty Acid Production in Potato by Engineering of Acetyl-CoA carboxylase, Planta 219: 389–396 (2004). 12. Del Vecchio, A.J., High-Laurate Canola, inform 7: 230–243 (1996). 13. Pollard, M.R., L. Anderson, C. Fan, D.J. Hawkins, and H.M. Davies, A Specific AcylAcp Thioesterase Implicated in Medium-Chain Fatty Acid Production in Immature Cotyledons of Umbellularia californica, Arch. Biochem. Biophys. 284: 306–312 (1991). 14. Voelker, T.A., A.C. Worrell, L. Anderson, J. Bleibaum, C. Fan, D.J. Hawkins, S.E. Radke, and H.M. Davies, Fatty Acid Biosynthesis Redirected to Medium Chains in Transgenic Oilseed Plants, Science 257: 72–74 (1992). 15. Voelker, T.A., T.R. Hayes, A.M. Cranmer, J.C. Turner, and H.M. Davies, Genetic Engineering of a Quantitative Trait: Metabolic and Genetic Parameters Influencing the Accumulation of Laurate in Rapeseed, Plant Journal 9: 229–241 (1996). 16. Eccleston, V., and J.B. Ohlrogge, Expression of Lauroyl-Acp Thioesterase in Brassica napus Seeds Induces Pathways for Both Fatty Acid Oxidation and Biosynthesis and Implies a Set Point for Triacylglycerol Accumulation, Plant Cell 10: 613–621 (1998). 17. Davies, M.H., D.J. Hawkins, and J.S. Nelson, Lysophosphatidic Acid Acyltransferase from Immature Coconut Endosperm Having Medium Chain Length Substrate Specificity, Phytochem. 39: 989–996 (1995). 18. Knutzon, D.S., T.R. Hayes, A. Wyrick, H. Xiong, H.M. Davies, and T.A. Voelker, Lysophosphatidic Acid Acyltransferase from Coconut Endosperm Mediates the Insertion of Laurate at the sn-2 Position of Triacylglycerols In Lauric Rapeseed Oil and Can Increase Total Laurate Levels, Plant Physiol 120: 739–746 (1999). 19. Kinney, A.J., Perspectives on the Production of Industrial Oils Genetically Engineered Oilseeds, in Lipid Biotechnology (Kuo, T.M., and Gardner, H.W., eds.), Marcel Dekker Inc., New York, 2002, pp. 85–93. 20. Lindqvist, Y., W. Huang, G. Schneider, and J. Shanklin, Crystal Structure of ∆ 9 Stearoyl-Acyl Carrier Protein Desaturase from Castor Seed and its Relationship to Other Di-iron Proteins, EMBO J. 15: 4081–4092 (1996). 21. Voelker, T., and A.J. Kinney, Variations in the Biosynthesis of Seed-Storage Lipids, Annu. Rev. Plant Physiol. Mol. Biol. 52: 335–3361 (2001). 22. Lee, M., M. Lenman, A. Banas, M. Bafor, S. Singh, M. Schweizer, R. Nilsson, C. Liljenberg, A. Dahlqvist, P.-O. Gummeson, S. Sjodahl, A. Green, and S. Stymne, Identification of Non-heme Diiron Proteins that Catalyze Triple Bond and Epoxy Group Formation, Science 280: 915–918 (1998). 23. Stumpf, P.K., D.N. Kuhn, D.J. Murphy, M.R. Pollard, T. McKeon, and J.J. MacCarthy, Oleic Acid—the Central Substrate, in Biogenesis and Function of Plant Lipids (Mazliak,

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P., Benveniste, P., Costes, C. and Douce, R., eds.) Elsevier, North Holland, 1980, pp. 3–10. Broun, P., J. Shanklin, E. Whittle, and C. Somerville, Catalytic Plasticity of Fatty Acid Modification Enzymes Underlying Chemical Diversity of Plant Lipids, Science 282: 1315–1317 (1998). Cahoon, E.B., T.J. Carlson, K.G. Ripp, B.J. Schweiger, G.A. Cook, S.E. Hall, and A.J. Kinney, Biosynthetic Origin of Conjugated Double Bonds: Production of Fatty Acid Components of High-Value Drying Oils in Transgenic Soybean Embryos, Proc. Nat. Acad. Sci. (USA) 96: 12935–12940 (1999). Broadwater, J.A., E. Whittle, and J. Shanklin, Desaturation and Hydroxylation, Residues 148 and 324 of Arabidopsis FAD2, in Addition to Substrate Chain Length, Exert a Major Influence in Partitioning of Catalytic Specificity, J. Biol. Chem. 277: 15613–15620 (2002). Broun, P., S. Boddupalli, and C. Somerville,, A Bifunctional Oleate 12-Hydroxylase: Desaturase from Lesquerella fendleri, Plant Journal 13: 201–210 (1998). Galliard, T., and P.K. Stumpf, Fat Metabolism in Higher Plants, 30: Enzymatic Synthesis of Ricinoleic Acid by a Microsomal Preparation from Developing Ricinus communis Seeds, J. Biol. Chem. 241: 5806–5812 (1966). Morris, L.J., The Mechanism of Ricinoleic Acid Biosynthesis in Ricinus communis Seeds, Biochem. Biophys. Res. Commun. 29: 311–315 (1967). Van de Loo, F.J., P. Broun, S. Turner, and C. Somerville, An Oleate 12-Hydroxylase from Ricinus communis L. is a Fatty Acyl Desaturase Homolog, Proc. Natl. Acad. Sci. USA 92: 6743–6747 (1995). Banas, A., A. Dahlqvist, U. Stahl, M. Lenman, and S. Stymne, The Involvement of Phospholipid:Diacylglycerol Acyltransferases in Triacylglycerol Production, Biochemical Society Transactions 28: 703–705 (2000). Moreau, R.A., and P.K. Stumpf, Recent Studies of the Enzymic Synthesis of Ricinoleic Acid by Developing Castor Beans, Plant Physiol 67: 672–676 (1981). Bafor, M., M.A. Smith, L. Jonsson, K. Stobart, and S. Stymne, Ricinoleic Acid Biosynthesis and Triacylglycerol Assembly in Microsomal Preparations from Developing Castor-Bean (Ricinus communis) Endosperm, Biochem J. 280: 507–514 (1991). Richards, D.E., R.D. Taylor, and D.J. Murphy, Localization and Possible Substrate Requirement of the Oleate-12-hydroxylase of Developing Ricinus communis Seeds, Plant Physiol Biochem 31: 89–94 (1993). Lin, J.T., T.A. McKeon, M. Goodrich-Tanrikulu, and A.E. Stafford, Characterization of Oleoyl-12-hydroxylase in Castor Microsomes Using the Putative Substrate, 1-acyl-2oleoyl-sn-glycero-3-phosphocholine, Lipids 31: 571–577 (1996). McKeon, T.A., J.T. Lin, M. Goodrich-Tanrikulu, and A.E. Stafford, Ricinoleate Biosynthesis in Castor Microsomes, Industrial Crops and Products 6: 383–389 (1997). Broun, P., and C. Somerville, Accumulation of Ricinoleic, Lesquerolic, and Densipolic Acids in Seeds of Transgenic Arabidopsis Plants that Express a Fatty Acyl Hydroxylase cDNA from Castor Bean, Plant Physiol. 113: 933–942 (1997). Banas, A., I. Johansson, and S. Stymne, Plant Microsomal Phospholipases Exhibit Preference for Phosphatidylcholine with Oxygenated Acyl Groups, Plant Science 84: 137–144 (1992). Moire, L., E. Rezzonico, S. Goepfert, and Y. Poirier, Impact of Unusual Fatty Acid Synthesis on Futile Cycling Through β-oxidation and on Gene Expression in Transgenic Plants, Plant Physiol. 134: 432–442 (2004).

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40. Smith, M.A., L. Jonsson, S. Stymne, and K. Stobart, Evidence of Cytochrome b5 as an Electron Donor in Ricinoleic Biosynthesis in Microsomal Preparations from Developing Castor Bean (Ricinus communis L.), Biochem. J. 287: 141–144 (1992). 41. Vogel, G., and J. Browse, Cholinephosphotransferase and Diacylglycerol Acyltransferase, Plant Physiol. 110: 923–931 (1996). 42. Lin, J.T., C.L. Woodruff, O.J. Lagouche, T.A. McKeon, A.E. Stafford, M. GoodrichTanrikulu, J.A. Singleton, and C.A. Haney, Biosynthesis of Triacylglycerols Containing Ricinoleate in Castor Microsomes Using 1-acyl-2-oleoyl-sn-glycerol-3-phosphocholine as the Substrate of Oleoyl-12-hydroxylase, Lipids 33: 59–69 (1998). 43. Lin, J.T., J.M. Chen, L.P. Liao, and T.A. McKeon, Molecular Species of Acylglycerols Incorporating Radiolabeled Fatty Acids from Castor (Ricinus communis L.) Microsomal Incubations, J. Ag. Food Chem. 50: 5077–5081 (2002). 44. McKeon, T.A., and J.T. Lin, Biosynthesis of Ricinoleic Acid for Castor Oil Production, in Lipid Biotechnology (Kuo, T.M., and Gardner, H.W., eds.), Marcel Dekker, Inc., New York, 2002, p. 129–139. 45. Dahlqvist, A., U. Stahl, M. Lenman, A. Banas, M. Lee, L. Sandager, H. Ronne, and S. Stymne, Phospholipid:Diacylglycerol Acyltransferase: An Enzyme that Catalyzes the Acyl-CoA-independent Formation of Triacylglycerol in Yeast and Plants, Proc. Natl. Acad. Sci. USA 97: 6487–6492 (2000). 46. He, X., C. Turner, G.Q. Chen, J.T. Lin, and T.A. McKeon, Cloning and Characterization of a cDNA Encoding Diacylglycerol Acyltransferase from Castor Bean, Lipids 39: 311–318 (2004). 47. James, C., Global Status of Commercialized Transgenic Crops: 2003 Executive Summary International Service for the Acquisition of Agri-Biotech Applications (http://www.isaaa.org, accessed Jan. 2005) p. 1–7 (2004). 48. McKeon, T.A., J.T. Lin, and G. Chen, Developing a Safe Source of Castor Oil, inform 13: 381–385 (2002).

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Chapter 2

Current Developments of Biodegradable Grease Atanu Adhvaryua,b, Brajendra K. Sharmaa,b, and Sevim Z. Erhanb aDepartment

of Chemical Engineering, Pennsylvania State University, University Park, PA 16802, USA; bUSDA, Food and Industrial Oil Research, 1815 N. University Street, Peoria, IL 61604, USA

Introduction The modern definition of lubricating grease, according to the American Society for Testing and Materials (ASTM), is a solid or semi-solid product obtained by the dispersion of a thickening agent in a liquid lubricant. This system may also include other ingredients that impart special properties (see the American Society for Testing and Materials, Standard Definition of Terms Relating to Petroleum Products, 2000). This definition was further extended by the National Lubricating Grease Institute (NLGI): “The material we disperse in a liquid lubricant is usually a solid. The dispersion . . . will not settle out when left standing. In order to develop thickening, the solid and the lubricating liquid had best have some affinity for each other. This affinity also helps keep the dispersion stable” (1). Lubricating greases are semi-solid colloidal dispersions of a thickening agent in a liquid lubricant matrix. They owe their consistency to a gel-forming network where the thickening agent is dispersed in the lubricating base fluid. The fluid lubricant that performs the actual lubrication can be petroleum (mineral) oil, synthetic oil, or vegetable oil. The thickener gives grease its characteristic consistency (hardness) that is sometimes thought of as a “three-dimensional fibrous network” or “sponge” that holds the oil in place. Therefore, the base fluid imparts lubricating properties to the grease while the thickener, essentially the gelling agent, holds the matrix together. This is a two-stage process. First, the absorption and adhesion of base oil in the soap structure results, and secondly, there is a swelling of the soap structure when the remaining oil is added to the reaction mixture. A typical grease composition contains 60–95% base fluid (mineral, synthetic, or vegetable oil), 5–25% thickener (common thickeners are fatty acid soaps and organic or inorganic non-soap thickeners), and 0–10% additives (antioxidants, corrosion inhibitors, anti-wear/extreme pressure, antifoam, tackiness agents, etc.) (2) (Fig. 2.1). Additives enhance performance and protect the grease and lubricated surfaces (3). Grease has been described as a temperature-regulated feeding device: when the lubricant film between wearing surfaces thins, the resulting heat softens the adjacent grease, which expands and releases oil to restore film thickness. The 14

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Fig. 2.1. Grease composition.

semi-solid nature of lubricating grease has several advantages over lubricating oils. Oxidative stability and consistency of the grease matrix controls a wide variety of performance properties in grease lubrication. Some of these properties are the ability to flow under force and subsequently lubricate hard-to-reach points; lower friction coefficient through adhesion on surface (4); wide temperature range effectiveness; water stability; the ability to seal out contaminants as a physical barrier; decreased dripping, spattering, and frequency of relubrication (act as sink for lubricating oils). It is important to note at this point that grease structure and composition undergoes significant modification while working by shearing and oxidation. The usefulness of grease in a particular application is controlled to a large extent by the ability of the grease to sustain change in temperature, pressure, operating environment, and shearing force. Liquid lubricants possess certain shortcomings and are not able to cope with an exponential rise in performance requirements in automotive and industrial sectors. Technology is constantly being challenged to develop multifunctional lubricants to operate at higher temperatures, higher pressures, and with a variety of contact surfaces to minimize friction and increase system efficiency. This has triggered a steady rise in the development and application of greases in elastohydrodynamic regimes. Thickness and stability of lubricant film is largely dependent on the unique chemistry and composition delivered by greases. The function of grease is to remain in contact with and lubricate moving surfaces without leaking out under gravity or centrifugal action, or be squeezed out under applied pressure. Development of vegetable oil-based greases has been an area of active research for several decades (5,6). Technical progress taking place in industry and

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agriculture has caused an intensive exploitation of natural resources like mineral oil. The search for environmentally friendly materials to replace mineral oil is currently being considered a top priority research in the fuel and energy sector. This emphasis is largely due to the rapid depletion of world fossil fuel reserves and increasing concern for environmental pollution from excessive mineral oil use and disposal. Renewable resources like seed oils and their derivatives are being considered as potential replacements for mineral oil base stocks in certain lubricant applications, where immediate contact with the environment is anticipated. The nontoxic and readily biodegradable characteristics of vegetable oil based lubricants pose less danger to soil, water, flora and fauna in case of accidental spillage or during disposal (7). Environmentally friendly lubricants and greases are already in market (8). These products are highly desired in total loss lubricants like railroads, as their accidental spillage doesn’t invoke alarm and cause any harm to environment. Dwivedi et al. described the preparation of total vegetable oil-based grease using castor oil (9). Florea et al. have studied the effect of different base fluids on the properties of biodegradable greases (10). A suitable composition of grease is desired with good performance properties capable of use in multifunctional products. Despite the overwhelming importance of biodegradable greases, very little is known about the relationship between their composition and performance properties.

Biodegradable Grease Base Oils Base fluids make up to 75 to 95% of the total composition of grease. Generally, the base oils can be divided into two main categories: (i) water miscible, and (ii) nonwater miscible. Glycols are exclusively water soluble; the most frequently used are monopropylene glycol or polyethylene glycol with an average molecular weight of 200–1500. The advantages of these compounds lies in their resistance to aging and hydrolysis, while the major disadvantages are solubility in water and incompatibility with mineral base oils. Non-water soluble base oils can be subdivided into two groups: (i) vegetable oils, and (ii) synthetic esters (11,12). This class of compounds basically has the same structure, and therefore, similar physical and chemical properties. The search for bio-based material as industrial and automotive lubricants has accelerated in recent years. This trend is primarily due to the nontoxic and biodegradable characteristics of seed oils and esters (13) that can substitute mineral oil as base fluid in grease making. The performance properties of grease are primarily dependent on their ability to provide lubrication to mechanically operating moving parts by supplying base oil as a thin film separating the metallic surfaces, and also removing heat and wear debris from the friction zone. Today, greases are expected to work under extreme operating conditions, including shock load, wide temperature range, varying pressure, surface material and environment. As

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mechanical systems become more complex in operation, eco-friendly base oils are used that can deliver performance properties similar to mineral base fluids and yet are nontoxic to the environment. Synthetic esters are generally obtained from branched alcohols and long-chain fatty acids (e.g., oleic acids) for better properties. Long-chain esters with several branching sites exhibit good low-temperature properties and resistance to hydrolytic degradation (14). Among the esters used for grease making are trimethylolpropane, pentaerythritol, and neopentylpolyol. Compared to vegetable oils, these fluids deliver good thermal stability, solvency, low temperature fluidity, sub-ambient storage stability, lubricity, compatibility with mineral oil, biodegradability, and longer service life. Diesters of a number of fatty acids like oleic and stearic acid or dibasic acids like adipic, azelaic, phthalic and sebacaic acids are widely used for grease making. A real need exists for research and development of new technologies for production of lubricants according to the most advanced, “ecological” trends. The best approach seems to focus on alternative, renewable, widely available, natural resources, such as vegetable oils. They are naturally occurring triacylglycerols that are formed by the reaction of one mole of glycerol with three moles of fatty acids or a mixture of fatty acids (Fig. 2.2). Preferably the fatty acids are oleic acid, linoleic acid and linolenic acid or mixtures thereof. Vegetable oils are a potential source of environmentally friendly base oils that have the additional advantage of not disturbing the global carbon dioxide equilibrium. They exhibit excellent lubrication properties due to unbalanced electrical charges which make them attach to metal surfaces. Vegetable oils that are extensively used for biodegradable grease preparations are soybean, rapeseed, sunflower, and castor oil. Other vegetable oils used are olive, peanut, palm, corn, cottonseed, safflower, lesquerella, coconut and linseed. Genetically modified vegetable oils typically contain higher than normal oleic acid content. For example, normal sunflower oil has an oleic acid content of 20–30% which can be up to 60–90% in genetically modified high oleic sunflower

Fig. 2.2. Typical vegetable oil structure.

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oil. It may be noted that genetically modified vegetable oils have a high oleic acid content at the expense of the di- and tri-unsaturated acids. The presence of a polar group with a long hydrocarbon chain makes vegetable oil amphiphilic in nature, allowing it to be used as a boundary lubricant. The molecules have strong affinity for and interact strongly with metal surfaces. The long hydrocarbon chain is oriented away from the metal surface to form a monomolecular layer with excellent boundary lubrication properties. When the molecule is adsorbed on the metal surface, two types of interactions occur. The adhesive interaction between the ester group and metal is very sensitive to the type and number of functional groups. The lateral interaction caused by dipole-dipole and dispersive interaction between the hydrocarbon chains is sensitive to structural properties including chain length, unsaturation, and stereochemistry (15). Castor Oil. Castor oil consists of triacylglycerols with the major fatty acid component being ricinoleic acid (~89 wt%) (16). It is a nondrying oil with high viscosity and is quite suitable for various lubricant applications. It can be mixed with other vegetable oils to obtain various viscosity grades (17) and offer excellent viscositytemperature characteristics. Phoronic acid (having shorter chain length as compared to 12-hydroxystearic acid) derived from castor oil is superior in making greases since it has a higher metal content, delivering long grease life at higher temperatures. The shorter chain of phoronic acid is less subject to shear degradation when used in a grease matrix (18). Castor oil has also been used to prepare total vegetable oil based grease with sodium and lithium gallants. Vegetable oil, alcohol, and alkali are taken in such a ratio as to give a predetermined ratio of soap and ester in the product. The alkali is selected based on the type of grease to be formed (Li, Na or Ca) and alcohol selection controls the viscosity of the lubricant. Higher carbon number and molecular weight of the alcohol produces lubricants with higher viscosity (19). The residual hydroxyl group in the ricinoleic acid chain offers an active site for adherence to metal surfaces. It is therefore expected that greases prepared from castor oil will have better extreme pressure characteristics. Rapeseed Oil. Rapeseed oil has a high viscosity and is often used as a lubricant base oil mixed with other seed and mineral oils. Lithium greases prepared with soap made from rapeseed oil and lithium hydroxide had better mechanical stability if some calcium hydroxide was used in the mixture (20). Soybean Oil. Soybean is the second highest value cash crop in the United States. The farm value of soybean production in the crop year 2000 was $13 billion. The 3.1 billion gallons of soybean oil produced in the United States is half of the 6.2 billion gallons produced worldwide. Soy oil (typically 18% of the weight of the soybean) can be used in its raw or refined form in a variety of industrial products (fuels, inks, paints, industrial fluids, etc.). This oil is a good source if a high unsaturation in the triacylglycerol is desired for grease formulation. Current develop-

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ments on bioengineered (high oleic and/or low linoleic) soybean oil may provide highly desirable improvements for fuels and other industrial products. Unlike petroleum-based grease that takes 12 to 18 months to decompose, soy-based products are less toxic than traditional products and are less likely to catch fire. The use of oils from genetically modified seeds has opened up several possibilities in the field of nonfood uses of vegetable oils. DuPont has developed a genetically modified soybean that would produce soy oil with enhanced stability for a variety of industrial uses including application in grease making (21). Soap Thickeners Vegetable oil-based greases are semi-solid colloidal dispersions of a thickening agent (a metal soap), in a liquid lubricant matrix (vegetable oil). The thickener is a reaction product of a metal (alkali or alkaline earth metal) based material (oxide, hydroxide, carbonate or bicarbonate) and carboxylic acid or its ester. Acids can be derived from animal fat such as beef tallow, lard, butter, fish oil, or from vegetable fat such as olive, castor, soybean, or peanut oils. The most common alkalies used are the hydroxides from earth metals such as aluminum, calcium, lithium, sodium, and titanium. Soap is created when a long-carbon-chain fatty acid reacts with the metal hydroxide. This reaction often produces some amount of water. For certain types of grease, the water assists in forming the soap structure. The metal is incorporated into the carbon chain and the resultant compound develops a polarity. The polar molecules form a fibrous network that holds a certain amount of base fluid by interaction forces. The soap structure is very important to the performance of the grease and will vary in thickness, length and oil solubility, depending on the type of metal hydroxide used. These variations are ultimately displayed in the final properties of the grease. Listed in Table 2.1 are some of the important physical properties of grease affected by the structure of fatty acids. Vegetable oil-based grease thickened with polyurea is environmentally friendly and biodegradable in nature (22). Polyurea is the most important organic nonsoap thickener and has excellent oxidation resistance due to the absence of metal soaps (which tend to initiate oxidation). It effectively lubricates over a wide temperature range (–20 to 177°C) and has a long service life that makes it suitable in sealed-forlife bearing applications. Polyurea complex grease is produced when a complexing agent, most commonly calcium acetate or calcium phosphate, is incorporated into the polymer chain. Such greases showed good shear stability when subjected to the roll stability test. Organic clay, though readily biodegradable, is a naturally occurring nontoxic material, so its carbon content is not counted in the determination of ready biodegradability (23). Thickeners based on organic clay pose the least manufacturing challenges for biodegradable greases. When vegetable oil is used, the required concentration of organo-clay is typically 14%, which may be higher for NLGI No. 2 consistency. Organo-clay thickeners have amorphous gel-like structure rather than

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TABLE 2.1 Fatty Acid Structures and Grease Properties Fatty acid structure

Grease property

Distribution of chain length Amount of unsaturation Degree of branching Polar groups in the fatty acid structure

Variations in grease hardness Variations in dropping point Non-uniform crystalline structure Positive effects on intermolecular interactions

the fibrous, crystalline structures of soap thickeners. This grease has excellent heat-resistance since clay does not melt and can effectively lubricate up to 260°C. The high temperature application of modern machinery has lead to the development of “complex” soap greases. A complex soap is formed by the reaction of a fatty acid and alkali (soap), and the simultaneous reaction of the alkali with a short-chain organic or inorganic acid to form a metallic salt (the complexing agent). Basically, complex grease is made when a complex soap is formed in the presence of base oil. Common organic acids are acetic or lactic, and common inorganic acids are carbonates or chlorides. The dropping point of complex grease is at least 38°C (100°F) higher than its normal soap-thickened counterpart, and its maximum usable temperature is around 177°C (350°F). Generally, complex greases have good all-around properties and can be used in multipurpose applications. Grives has discussed commercial methods of biodegradable grease preparation using different thickener systems (24). Although it is known that the general structure of the soap phase in grease consists of crystallites, which take the form of fibers, this does not clearly explain why a small amount of a solid (soap) could immobilize a large volume of the base oil in grease. These fiber structures form a complex network that traps the base oil molecules in two ways: (i) by direct sorption of the oil by polar ends of soap molecule, and (ii) penetration of base oil in the interlacing structure of soap fiber. The oil-retaining property of grease may be due to the attractive influence of soap fibers extending through many layers of the base oil molecule and not to the swelling of the fibers (25). Therefore, the physical and chemical behavior of grease is largely controlled by the consistency or hardness, which is dependent upon the microstructure of soap fibers. Thus, a somewhat rigid gel-like material “grease” is developed. Base oil and composition of the thickening agent plays an important role in grease consistency. For low and high temperature applications, regulating the base oil quantity and fatty acid composition can be used to control grease hardness. Therefore, preparation of lubricating grease is a complicated trial-and-error process in which the optimization of the reactants and the reaction protocol are critical to achieve the desired grease consistency. The chemistry of the fatty acid soap structure is responsible for certain performance characteristics of grease including rust/corrosion inhibition, friction, and wear resistance (26). Polar components in grease are surface active and therefore

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have a strong affinity for metal surfaces while the hydrocarbon tail is directed away resulting in the prevention of oxygen and water (rust and corrosion agents), and dust particles from coming in direct contact with metal surfaces. Also, the tightly adsorbed grease layer on metal is highly effective in lowering metal-tometal friction (27). Therefore, the application of bio-based grease is particularly useful in open lubrication systems where the lubricant is in direct contact with environment. Grease is the preferred form of lubrication in hard-to-reach places in a mechanically rubbing or dynamic system. Much of the functional properties of grease are dependent on their ability to flow under force, have shear stability, resist viscosity changes with temperature and pressure, maintain water stability, seal out contaminants, and decrease dripping and spattering. The dependability of lubricating grease relies on physical properties that are structurally related, which are obtained by the proper selection of ingredients and processing. Thus, it is pertinent to understand the grease microstructure as it contributes significantly to various functional properties of grease. Grease consistency [or National Lubricating Grease Institute (NLGI) hardness] (28) is largely dependent upon the thickener fiber structure and its distribution in the grease medium. Grease hardness depends primarily on a metal soap thickener microstructure and experimental data show that the fatty acid chain length and C-C unsaturation influences soap fiber structure/networking mechanisms. An understanding of fiber growth and their network structure in a grease matrix is required to relate base oil holding capacity and oil release by shear degradation of soap thickener during operation to additive compatibility, bleed resistance, viscosity, thermal stability, texture, and appearance. Critical physiochemical properties are, therefore, dependent on the consistency of grease and their behavior in the mechanical system. Controlling the growth and distribution of soap fiber during grease manufacturing processes can result in products with the desired physical, chemical, and performance properties. The soap fibers derived from short-chain fatty acids are not well developed and sufficiently elaborate to create a strong interaction with the base oil. Increasing the fatty acid chain length (Cn; n = number of carbon atoms) in soap resulted in stronger bonding interaction and a harder grease matrix. Beginning with a C13 fatty acid chain length, there is a significant increase in grease hardness up to C15, with an optimum at C17 resulting in NLGI grade 2 grease (using a 1:1 equivalent ratio of metal to fatty acid and 75 wt% of soy oil in the grease mixture) (Fig. 2.3). In a study using transmission electron microscopy (TEM), formation of dispersiod structure with compact network with an increase in the chain length of the fatty acid in lithium soap was observed. With more interlocking resulting from the long-chain fiber structure, increased interactions with base oil in the matrix can be achieved. Grease developed under such conditions shows high consistency resulting in higher hardness. The TEM of palmitic [CH3(CH2)14COOH] and stearic [CH3(CH2)16COOH] acids used in the lithium soap to develop soybean oil-based grease are shown in Figures 2.4a and 2.4b, respectively.

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Fig. 2.3. Lithium soap fatty

acid chain length effect on soy-grease NLGI hardness.

It appears that such compact mesh structure can hold relatively larger amounts of base oil in the soap matrix due to the excellent interaction. This increases the ability of the grease to resist deformation with increasing fiber length, because a long fiber can make more contacts with neighboring fibers than a short fiber with the same diameter. It may be noted at this stage that during extreme shear stress, when a fiber breaks into smaller fragments, the consistency will decrease, whereas when they split into thinner fragments, the consistency will increase. Therefore, the hardness of grease as a result of soap structure can affect oxidation stability, water washout, oil bleeding at higher temperature, and lubricity (29,30). Unsaturation in the fatty acid structure of soap molecules has significant impact on the grease fiber structure. Linoleic acid (C18:2) (Fig. 2.5b) with two sites of C-C unsaturation in the chain shows a much thinner and compact fiber network

Fig. 2.4. TEM of (a) palmitic and (b) stearic acid used to develop soybean oil-based

grease.

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than oleic acid (C18:1) (Fig. 2.5a) in the soap composition. Excessive thinning of the fiber strand may result in softer grease due to a weak mesh structure that is unable to hold the base oil in the grease matrix. Furthermore, the presence or absence of C-C unsaturation with the same chain length acids (Fig. 2.4b and 2.5b) in the soap structure results in a distinct difference in the shape and distribution of the fibers. With process parameters and composition remaining the same, and with a decrease in the soap fiber length, there is a tendency to form softer grease (31). Because the growth of soap fibers in the grease matrix is a result of fusion and solidification of adjoining short fibers, this phenomenon is also controlled to a large extent by the procedure used to manufacture grease (32). Soap molecules with oleic acid show a comparatively larger fiber structure than linoleic acid. It has been observed that decreasing the soap concentration or lowering the cooling rate could produce long-fiber grease. Moreover, fiber length of grease increases with an increase in the heat-retention time. Especially the addition of soap powder to grease in the heat retention process resulted in gigantic fibers. In contrast, short-fiber grease could be produced by increasing the soap concentration or raising the cooling rate. A very high cooling rate results in lowering the ratio of fiber length L to width D, L/D, resulting in a softer grease. The soap fibers in grease are considered to grow when the grease is maintained near the melting point of soap (33). The fluctuation in temperature leads to the fusion and solidification of the soap fiber, leading to the disappearance of short fibers and the growth of long fibers (34). Additive Effects Additives are usually introduced during the cooling phase of grease making and remain dispersed in the matrix. These additives are found to enhance some of the functional properties of the base oil in the grease such as oxidation, load-bearing, anti-wear, anti-corrosion and anti-rust (7,35–37). Due to the presence of additives, the soap thickener structure can be influenced to a large extent by either changing the solubility of the soap in the base oil or influencing its crystallization. Similarly,

Fig. 2.5. TEM of (a) oleic acid and (b) linoleic acid.

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the optimum condition for fiber growth through crystallization may vary with different additives. It is believed that the additive molecules are first bound to the soap fibers and the chains attached to additives hold the oil. The TEM (Figs. 2.6a and 2.6b) show the effect of antimony dithiocarbamate additive on Li-stearate soap structure. Under magnification (1.81 µm), the additive-doped grease has a looser network structure with larger fibers than the nonadditive-doped grease with a similar metal, fatty acid, and base oil composition. It may be noted, however, that due to the presence of additive molecules, grease hardness is not altered significantly as a result of changes in the soap fiber length and their distribution in the matrix. There are various reports in the literature where researchers have investigated additive effects on grease performance. Kato et al. (38) have reported the effectiveness of antioxidants in obtaining long lubricant life of grease with a rapeseed oil base. It was observed that the main causes of a reduced grease life are chemical deterioration due to oxidation and polymerization of the base oil. The antioxidants can delay such processes, but as soon as they are consumed, the degradation starts rapidly. The performance properties of grease are primarily dependent on their ability to provide lubrication to mechanically moving parts by supplying base oil as a thin film to separate the metallic surfaces, and also by removing heat and wear debris from the friction zone. Similarly, the nature of the fatty acid in the soap structure of grease has a significant influence on the physical and chemical properties. Soaps can lubricate and are considered to be more important than the lubricating oil because they can improve the lubricating ability of the oil. Elliott (39) found that the chain length of the fatty acid was an important factor in determining grease characteristics. The starting and running torques are less for the grease than for the oil itself. Using the Four-Ball Tester, Jiang (40) showed that a lithium grease could prevent seizure at a load of 35 Kgs., whereas the oil film broke and failed. Physical and chemical degradation of grease during use (41,42) and failure of various

Fig. 2.6. TEM showing the effect of antimony dithiocarbamate additive on Li-stearate

soap structure.

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mechanical parts due to inadequate lubrication (43) have been reported. Several mechanisms have been proposed on timed lubricant release and replenishment of starved lubricant sites during operation. Laboratory simulations range from simple thermal stability tests to more complex lubrication measurements (44–46). Functional Properties Grease is used when it is not practical or convenient to use oil in a dynamic system. The use of grease is largely dictated by the design of the machinery and operating conditions that are suitable for a desired lubricant characteristic. Grease functions as a sealant (based on consistency) to minimize leakage, keep out contaminants and prevent entrance of corrosive foreign materials into the system. Grease, unlike oils, by virtue of its rigidity, is easily confined with simplified, less costly retention devices. The major practical requirement of grease is to retain its functional properties under shear at all temperatures it is subjected to during use. They have the ability to hold finely ground solid lubricants and additives in a stable dispersed condition and are able to deliver them at the point of metal contact for better lubrication. Grease maintains thicker films in clearances enlarged by wear and can extend the life of worn parts that were previously oil lubricated. Thicker grease films also provide noise insulation. Therefore, grease is mainly applied in equipment that is seldom used or is in storage for an extended period of time. High quality greases are also used in areas that are inaccessible to frequent relubrication and sealed-for-life type devices (e.g., motors and gear boxes). They also find use in applications involving extreme temperature, pressure, shear stress, shock loads, etc. Under these circumstances, grease provides thicker film cushions to protect and deliver adequate lubrication, where oil films can fail due to thinning. Grease Characteristics Consistency. This is an important parameter of grease that controls most of its physicochemical characteristics. Hard grease will not lubricate properly while very fluid grease may leak out of the system. Grease consistency depends on the type and amount of thickener used and the viscosity of its base oil. Grease’s consistency is its resistance to deformation by an applied force and is generally measured by ASTM D 217 and D 1403 methods (47). Corrosion and Rust Resistance. This denotes the ability of grease to protect metal parts from chemical attack. The thickener type provides most of the natural resistance of grease; however corrosion and rust inhibitors are often used in actual formulations. Oxidation Stability. Oxidation is the most important chemical property of grease that results in insoluble gum, sludge, deposits and therefore leads to sluggish operation, reduced metal wetability, decreased wear protection and increased corrosion,

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among others. Oxidation is also associated with evaporation which causes grease to harden due to increased thickener concentration. Excessive temperatures result in accelerated oxidation or even carbonization where grease hardens or forms a crust. Therefore, higher evaporation rates require more frequent relubrication. A number of studies have been reported on various aspects of thermo-oxidative stability (42,48,49) and oxidative degradation using chromatographic and spectroscopic techniques (41). Bleeding. This is a condition where long storage periods and high temperatures induce a liquid lubricant to separate from the grease thickener. When the oil separates from the grease, thickener concentration increases resulting in grease plugging. Under certain circumstances, when two greases are in contact, the oil may migrate from one grease to the other, changing the structure of either grease. Grease, when subjected to high temperature for an extended length of time, loses its consistency and becomes fluid enough to drip. The dropping point indicates the upper temperature limit at which the grease retains its structure. However, a few greases have the ability to regain their original fiber structure after cooling down from the dropping point. Low Temperature Stability. Grease hardens at low temperatures, leading to poor pumpability and rheological properties. Typically the base oil’s pour point is considered the low-temperature limit of grease. Biodegradability In 2002, around 57 million tons of lubricant was used worldwide, and it is estimated that as much as 35% finds its way back into the environment unchanged. Some of this will degrade but there are potential dangers to the environment such as bioaccumulation and biocidal effects. During biodegradation, the material is gradually broken down through the metabolic action of such living organisms as bacteria, fungi, yeast and algae. Naturally, this process is not entirely predictable and can be influenced by the mix of living organisms present, the ambient temperature, and the humidity. Sometimes a material that may easily degrade under one set of circumstances may not readily degrade under others. The minimum basic requirements are sufficient bacteria population, correct oxygen levels and a suitable temperature range. The rate of biodegradation is also affected by parameters such as fluid viscosity, pH levels, sunlight, mineral salt content, nitrogen availability, solubility and the ability of the bacteria to adapt to the source of oil nutrient. Ideally, in due course of time, the lubricant should be reduced to its simplest natural form while leaving no harmful by-products that could have a detrimental and long-term effect on the local environment. Biodegradability and renewability are becoming increasingly important to formulators as new federal environmental regulations go into effect. Environmentally

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sensitive application areas typically involve exposure to the elements, where there is a definite potential for the grease to contaminate the ecosystem through washout or accidental leak. Further, it is almost impossible to prevent accidental breaking of one of the small grease lines, the consequence of which is that grease can find its way into the water or at the very least into landfills. Currently, many greases are used in loss-lubrication systems, where a certain amount of grease ends up in the environment. Such applications include but are not limited to: forestry (chainsaw/ grapples); agriculture (tractors, harvesters); lubricants (marine, boat trailer bearings); railway (curve/axle greases); mining (conveyor greases); manufacturing (hot and cold rolling mills); construction (waterways, bridges, locks, dams). Accordingly, biodegradability has become vital. It is well known that mineral oils are not readily degradable under normal environmental conditions. Therefore, they have a high potential to accumulate in the environment. Mineral oils are also known to taint water and fish, making them unsuitable for consumption. Vegetable oils are not toxic to aquatic organisms and biodegrade relatively fast and completely. Soybean grease, for instance, decomposes within weeks; petroleum-based grease, however, takes from a year to 18 months to decompose. Metal soaps used in greases are commonly based on stearates, the main component of a natural soap, and are biodegradable to a large extent with the only exception of the type of metal present. Inorganic thickeners such as clay-like materials, that are found abundant in nature, however, are not entirely biodegradable, but are also not toxic to aquatic organisms. The additives constitute a very diverse range of chemicals and are often present in small quantities, possessing a wide range of biodegradability and aquatic toxicity. Some additives present in smaller quantity in the grease, and sparingly soluble in water, may increase several-fold due to the presence of other solubilityenhancing materials. Therefore, its effect is much higher than could be expected based on the small amount present in the grease. Therefore, it is difficult to make any generalized statement on additive biodegradation and aquatic toxicity (50). One of the most important methods used to determine biodegradability is also the only one available for testing products immiscible with water: CEC-L-33-A-93 (formerly CEC-L-33-T-82). All of the test methods were initially designed mainly for use with single chemical species that have demonstrated water solubility. In a biodegradation test the microbe feeds on the substrate (compound to be tested) and degrades it. In general, this process is monitored by measuring oxygen consumption, carbon dioxide production, and the drop in dissolved organic carbon. According to CEC test methods, the biodegradability of a lubricant (i.e., grease) is plotted over a period of 21 days in comparison with white oil (20–30% biodegradability) and di-iso-tridecyladipate (100% biodegradability). The results are evaluated by measuring the fluctuations in the CH3–CH2 bands at the 2930 cm–1 using an infrared spectrometer. According to CECL-33-T-82, if the product is more than 80% biodegradable the German “Blue Angle” criteria makes it readily biodegradable (51,52). Biodegradability of used lubricants can be altered by contamination and can be as much as 15%. A lubricant that is 90% degradable when fresh may only be 75% degradable when used.

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Conclusions Lubricants based on renewable materials have been around for a long time and have only recently been extensively researched for nonfood industrial applications to be more competitive with petroleum-based products. Pollution from petroleum products has been a big concern for the environment. Of the 2.5 billion gallons of lubricant sold in the U.S. market, 30 to 40% escape into the environment through accidental spillage, leaks and evaporation. Since these lubricants are directly released on the ground or into the water, it is important they are biodegradable and will not persist in the environment for too long. Lubricants based on fossil fuels are persistent, and therefore not suitable in these applications. Vegetable oil-based lubricants in the form of greases are biodegradable and can be formulated to meet or exceed industry specifications. This will increase the use of agricultural products such as vegetable oils in industrial (nonfood) applications and therefore increase the agricultural land use and work. Further, the environment is protected by the introduction of an environmentally friendly lubricant in an industrial application, and finally, the dependence on importoriented mineral oil is largely averted. References 1. National Lubricating Grease Institute, Lubricating Grease Guide, Kansas City, MO, 1984, p. 1.2. 2. Stempfel, E.M., L.A. Schmid, Biodegradable Lubricating Greases, NLGI Spokesman 55: 25–33 (1991). 3. Couronne, I., P. Vergne, L. Ponsonnet, N. Truong-Dinh, D. Girodin, Influence of Grease Composition on its Structure and its Rheological Behavior, Thinning Films and Tribological Interfaces, Downson, D., ed., Elsevier Science Ltd., 425–432 (2000). 4. Odi-Owei, S., Tribological Properties of Some Vegetable Oils and Fats, Lubr. Eng. 11: 685–690 (1989). 5. Dresel, W.H., Biologically Degradable Lubricating Greases Based on Industrial Crops, Ind. Crops Prod. 2: 281–288 (1994). 6. Hissa, R., J.C. Monterio, Manufacture and Evaluation of Li-Greases Made from Alternate Base Oils, NLGI Spokesman 3: 426–432 (1983). 7. Stempfel, E.M., Practical Experience with Highly Biodegradable Lubricants, Especially Hydraulic Oils and Lubricating Greases, NLGI Spokesman 62(1): 8–23 (1998). 8. Sullivan, T., Soy Grease on Track for Sales Boom, Lube Report, July 22, 2003. 9. Dwivedi, M.C., S. Sapre, Total Vegetable Oil-Based Greases Prepared from Castor Oil, J. Synthetic Lubrication 19: 229 (2002). 10. Florea, O., M. Luca, A. Constantinescu, D. Florescu, The Influence of Lubricating Fluid Type on the Properties of Biodegradable Greases, J. Synthetic Lubrication 19: 303 (2003). 11. Roehrs, I., T. RoBrucker, Performance and Ecology—Two Aspects for Modern Greases, NLGI Spokesman 58(12): 8474–8483 (1995). 12. Mang, T., Environmentally Harmless Lubricants, NLGI Spokesman 57: 233–239 (1993). 13. Ortansa, F., L. Marcel, C. Anea, F. Danilian, The Influence of Lubricating Fluid Type on the Properties of Biodegradable Grease, J. Synthetic Lubrication 19(4): 303–313 (2003).

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14. Korff, J., A. Fessenbecker, Additives for Biodegradable Lubricants, NLGI Spokesman 57(3): 102–112 (1993). 15. Jahanmir, S., M. Beltzer, An Adsorption Model for Friction in Boundary Lubrication, ASLE Trans. 29: 423–430 (1986). 16. Hurd, P.W., The Chemistry of Castor Oil and Its Derivatives, NLGI Spokesman 60(1): 14–23 (1996). 17. Kuliev, R.Sh., F.R. Shirinov, F.A. Kuliev, Vegetable Oils as Component of Lubricant Base Stocks, Chemistry and Technology of Fuels and Oils 31(3): 9–10 (1995). 18. Morway, A.J., and Wellman, W.E., US Patent 3,278,431 (1966). 19. Dwivedi, M.C., S. Sapre, Total Vegetable Oil Based Grease Prepared from Castor Oil, J. Synthetic Lubrication 19: 229–241 (2002). 20. Evans, I.S., The Development of Commercial Lubricating Grease Using Rapeseed Oil, NLGI Spokesman 26(5): 146–149 (1962). 21. Naegley, P.C., Environmentally Friendly Acceptable Lubricants, Lubrizol Corporation, Wickliffe, OH (1992). 22. Hissa, R., A Biodegradable Vegetable Polyurea Grease, NLGI Spokesman 57(5): 188–191 (1993). 23. Environmental Protection Agency (EPA), Chemical Fate Testing Guidelines Aerobic Aquatic Biodegradation Method CS-2000, EPS 560/6-82-003 (1982). 24. Grives, P.R., The Manufacture of Biodegradable Nontoxic Lubricating Greases, NLGI Spokesman 63: 25–29 (2000). 25. Browning, G.V., A New Approach to Lubricating Grease Structure, NLGI Spokesman 14(1): 10–15 (1950). 26. Honary, L., A.T. Field Test Results of Soybean Based Greases Developed by UNIABIL Research Program, NLGI Spokesman 64(7): 22–28 (2000). 27. Hurley, S., P.M. Cann, Infrared Spectroscopic Characterization of Grease Lubricant Films on Metal Surfaces, NLGI Spokesman 64(7): 13–21 (2000). 28. Annual Book of American Society for Testing and Materials, ASTM D-217, Vol. 05.01 (2000). 29. Kernizan, C.F., D.A. Pierman, Tribological Comparison of Base Greases and Their Fully Blended Counterparts, NLGI Spokesman 62(2): 12–28 (1998). 30. Hurley, S., P.M. Cann, Grease Composition and Film Thickness in Rolling Contacts, NLGI Spokesman 63(4): 12–22 (1999). 31. Boner, C.J., Manufacture and Application of Lubricating Grease, Reinhold Publishing Corp., NY (1954). 32. Yamamoto, Y., S. Gondo, T. Kita, Friction Characteristics and Soap Fiber Structure of Lithium Soap Grease Under Boundary Lubrication Conditions, Tribologist 42(6): 462–469 (1997). 33. Kita, T., Y. Yamamoto, Manufacturing Condition and Soap Structure of 12-HydroxyStearate Lithium Soap Grease, Tribologist 40: 2 (1995). 34. Kimura, H., Y. Imai, Y. Yamamoto, Study on Fiber Length Control for Ester Based Lithium Soap Grease, STLE Preprint No. 01-AM-9, pp. 1–6 (2001). 35. Mittal, B.D., E. Sayanna, K.P. Naithani, M.M. Rai, A.K. Bhatnagar, Effect of Metallic Thiophosphates on Dropping Point and Penetration Properties of Some Greases, Lubr. Sci. 10(2): 171–176 (1998). 36. Fish, G., Constant Velocity Joint Grease, NLGI Spokesman 63(9): 14–29 (1999). 37. Hunter, M.E., R.F. Baker, The Effect of Rust Inhibitors on Grease Properties, NLGI Spokesman 63(12): 14–21 (2000).

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38. Kato, N., H. Komiya, A. Kimura, H. Kimura, Lubrication Life of Biodegradable Grease with Rapeseed Oil Base, STLE Tech. Paper, pp. 19–25 (1998). 39. Elliott, S.B., Metallic Soaps for Greases, Oil Gas J. 46: 26,63 (1947). 40. Jiang, R.H., Effects of the Composition and Fibrous Texture of Lithium Soap Grease on Wear and Friction, Tribology International 18: 2, 121–124 (1958). 41. Carré, D.J., R. Bauer, P.D. Fleischauer, Chemical Analysis of Hydrocarbon Grease from Spring Bearing Tests, ASLE Trans. 26: 475–480 (1983). 42. Araki, C., H. Kanzaki, T. Taguchi, A Study on the Thermal Degradation of Lubricating Greases, NLGI Spokesman 59: 15–23 (1995). 43. Cann, P.M., A.A. Lubrecht, Analysis of Grease Lubrication in Rolling Element Bearings, Lubr. Sci. 11: 227–245 (1999). 44. Aihara, S., D. Dowson, A Study of Film Thickness in Grease Lubricated ElastoHydrodynamic Contacts, Proc. 5th Leeds-Lyon Symposium in Tribology, Paper III, pp. 104–115 (1978). 45. Zhu, W.S., Y.T. Neng, A Theoretical and Experimental Study of EHL Lubricated with Grease, ASME Trans., J. Trib. 110: 38–43 (1988). 46. Williamson, B.P., An Optical Study Of Grease Rheology in an Elastohydrodynamic Point Contact Under Fully Flooded and Starvation Conditions, Proc. I Mech. Eng., J. Eng. Trib. Part J. 209: 63–74 (1995). 47. Annual Book of American Society for Testing and Materials, ASTM D-1403, Vol. 05.01 (2000). 48. Harris, J. W., Relative Rates of Grease Oxidation in a Penn State Microoxidation Apparatus on Glass and on Steel Sample Pans, NLGI Spokesman 65(11): 18 (2002). 49. Honary, L.A.T., Performance Characteristics of Soybean-Based Grease Thickened with Clay, Aluminum Complex and Lithium, NLGI Spokesman 65(8): 18–27 (2001). 50. Rhee, I.S., 21st Century Military Biodegradable Greases, NLGI Spokesman 64(1): 8–17 (2000). 51. Mang, T., Lubricants with Environmentally Harmless Base Oils, Proc. Intl. Conf. on Petroleum Refining and Petrochemical Processing, Vol. 1, Interpec Beijing, China (1991). 52. Mang, T., Legislative Influence on the Development, Manufacture, Sale and Application of Lubricants in Federal Republic of Germany, IIIrd CEC Symp., Paris, France (1989).

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Chapter 3

Vegetable Oil-Based Engine Oils: Are They Practical? Joseph M. Perez Tribology Group, Chemical Engineering Department, The Pennsylvania State University, University Park, PA 16802

Introduction Vegetable oils were the primary lubricants for machinery and transportation vehicles for thousands of years until the discovery of petroleum. Petroleum, primarily on the bases of lower cost and improved performance, quickly replaced vegetable oils as the lubricant (1). Now, with increased petroleum costs, decreased petroleum reserves, and environmental concerns as major factors, vegetable-based oils for lubricants are making a slow but steady comeback. In the past decade, the initial applications have been niche markets such as chain saws, track lubricants, and other total loss lubricants. In Europe, legislation has helped to expand the use of vegetable-based lubricants to the hydraulic fluid market, a potentially large market for biodegradable vegetable oils and synthetic fluids. Looking ahead, the engine oil market is a larger market, one in which vegetable-based lubricants might achieve penetration. However, are vegetable oil-based engine oils practical? The 2004 Soy Products Guide (2), a listing of commercially available industrial products made from soybeans, lists only three companies selling hydraulic fluids and six selling engine oils containing soybean oil. There are a number of companies selling “biodegradable” or “environmentally friendly” hydraulic fluids but these contain other oils such as rapeseed, canola, or synthetic oils. Of the six companies selling engine oils, three sell the same product. There is at least one additional company selling engine oil based on sunflower oil. None of the products have undergone the Society of Automotive Engineers (SAE)/American Society of Testing Materials (ASTM) engine oil test series required to receive American Petroleum Institute (API) certification. Of the vegetable oils on the market, limited field test data are available. There are a number of factors that must be considered to determine whether vegetable oils are practical, including whether they can match the performance required to displace petroleum-based engine oils. Available Vegetable Oil. In the United States, the major source of in-house vegetable oil for lubricant applications is soybean oil. The total estimated supply of soybeans in 2004 was 2.9 billion bushels; of these, 1.6 billion bushels were crushed to supply 18 billion pounds (2.3 billion gallons) of oil (3,4). Most of this oil enters the food chain and only ~10% is available for use in plastics, solvents, 31

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coatings, printing inks, adhesives, and some lubricants. Another competitor for this oil is the growing biodiesel fuel market. Even by blending vegetable oil with synthetics, to control low temperature characteristics of the basestocks, other oils such as sunflower, canola, corn, or palm oil may be needed to have a significant effect on the market. This could result in a continued dependence on foreign oil, but from different sources such as Canada, Brazil, and Indonesia. Engine Oil Market Size. In 2002, 46% of the 2.4 billion gallons of lubricants sold in the U.S. market were crankcase oils; 13% were transmission, hydraulic fluids, and other automotive oils (5). Various industrial oils make up the remaining 41% of the lubricant market. With current farm production, only ~10% of this market could be supplied by vegetable oil products. Foreign Oil Replacement. One frequently stated advantage of using renewable lubricants is the replacement of petroleum-based products. Any displacement of petroleum oil affects the balance of payments. Use of renewable lubricants in crankcase oils would result in a small but positive displacement. The total use of petroleum in the U.S. in 2005 was projected to be ~23 million barrels/d. The U.S. transportation demands were projected to use ~55% of the total or >13 million barrels of oil/d (6,7). Basestocks for use in lubricants comprise a small but significant quantity. From each 42-gallon barrel of oil processed, 1.2% is used for lubricants. However, in 2002, the automotive lubricant market alone was >2.4 billion gallons (57 million bbls). Basestock Cost. Economics is a major factor in the market growth of new products. The cost of vegetable base oils exceeds the current petroleum base oil price of $1.5/gallon by at least 50%. As the price per barrel of oil increases the difference becomes smaller. However, it will take years to significantly reduce the cost differences. If the size of the market increases, the cost of the vegetable base oil will decrease. However, the engine oil market will not increase significantly without API certification, and running the required bench and engine tests will not occur until the market increases. The drivers to produce vegetable-based engine oil lubricants include environmental concerns, improved performance, increased value for farm products, jobs, and the world’s disappearing petroleum reserves. Environmental Concerns. Currently, the major driver in many countries of the world is the concern over air, water, and soil contamination by petroleum products. In many European countries, these concerns have resulted in regulations that require the use of biodegradable lubricants in hydraulic systems of equipment used in forestry and waterways projects. In the United States until recently, no such federal regulations existed outside of Presidential Directives. Some states are requiring the use of low levels of biodiesel, but no hydraulic or engine oil regulations currently exist. However, the 2002 Farm Bill (Farm Security and Rural Investment

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Act, published January 11, Federal Register), Section 9002 includes language directing all federal government agencies to give preference to “biobased” products, unless it is unreasonable to do so, based on price, availability, or performance. Lubricants are one category specified in the guidelines to be published. The USDA’s Office of Energy Policy and New Uses was delegated the authority to implement Section 9002 (8). Lubricant loss to the environment is another concern. Reportedly, this is the fate of >60% of all lubricants (9). Some obvious areas for increased use of biodegradable hydraulic fluids and engine oils are construction, forestry, farming, and waterways where losses directly affect the environment. Spill cleanup costs in some states are significant. The use of biodegradable lubricants can reduce these costs. Reduction in cleanup costs neutralizes the increased cost of using biodegradable hydraulic fluids made from renewable resources. For waterways, biodegradable 2-cycle or 4-cycle engine oil lubricants are currently available for use in motorboat engines used on our many lakes and rivers. The 2-cycle oil market is ~2% of the total lubricant market. Environmentally, biodiesel is a separate issue but it does affect the availability of vegetable oils for lubricants. Diesel engines are more efficient than gasoline engines. This results in a reduction in greenhouse gases. The need to reduce diesel particulate emissions has led to the widespread use of biodiesel. Use of B20 and vegetable-based lubricant results in reduced particulates and possibly a change in the morphology of the particulates. This is discussed elsewhere in detail in the literature (10–12). Performance Requirements The major concern of original equipment manufacturers (OEM) is whether the lubricants made with vegetable-based oils are going to give equivalent performance to petroleum products currently in use. This affects engine and equipment durability and warranty costs. Some of the desired fluid properties for hydraulic fluids and engine oils are found in Table 3.1. Of these, oxidation stability and low temperature fluidity are known weak links for vegetable-based stocks. Engine oil performance requirements are more severe than hydraulic fluid requirements due to differences in the operating conditions. In hydraulic system applications, fluid compressibility, hydrolytic stability, foam, and air entrainment requirements are the more important properties. In an engine, oil oxidation stability, deposit formation, friction, and wear are the major concerns. Low temperature fluidity is critical to both applications. Engine Oil Specifications The requirements for engine oils are well defined in ASTM and SAE specifications (13–16). SAE J300 defines the viscosity requirements for the various viscosity grades (Table 3.2). ASTM Method D455 is used to define the kinematic viscosity.

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J.M. Perez

TABLE 3.1 Desired Fluid Properties Automotive lubricants

Hydraulic fluids

High temperature oxidation and thermal stability Temperature-Viscosity (VI) Low temperature fluidity Control friction and wear Suspend contaminants/cleanliness Acidity/rust Foam control Component compatibility Fuel compatibility Volatility Environment, safety, and health Cost

Oxidation stability Viscosity (ISO 32/42) Low temperature fluidity Control friction & wear Control contaminants/cleanliness Hydrolytic stability Foam/air entrainment control Component compatibility Fluid compatibility Volatility Environment, safety, and health Cost

aVI,

viscosity index; ISO, International Organization for Standardization.

To simulate viscosity of an operating engine, ASTM High Temperature High Shear Method D4624 is used. ASTM D4684 Mini-Rotary Viscometer Method defines low-temperature pumping properties. Some military requirements are found in Table 3.3. There are vegetable-based engine oils available on the current market. The basestock of some oils is essentially all vegetable oils and others are blends of vegetable oils and petroleum or synthetic basestocks. In an earlier study by Cheenkachorn at Penn State (17), a number of the oils obtained did not meet their stated viscosity grade as specified in SAE J300. It is more significant that none were API certified. TABLE 3.2 Society of Automotive Engineers (SAE) Viscosity Requirements-J300/95

SAE viscosity grade

Pumping Max cP at (°C)

5W 10W 15W 20W 25W 20 30 40 50

60,000 at (–35) 60,000 at (–30) 60,000 at (–25) 60,000 at (–20) 60,000 at (–15) — — — —

aHTHS,

high temperature high shear.

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Kinematic viscosity cSt at 100°C min

max

HTHSa (min) cSt at 150°C and 106 s–1

3.8 4.1 5.6 5.6 9.3 5.6 9.3 12.5 16.3

— — — — — 45 and density $400 million in 2000. The formulation and mechanism of cure are discussed in the following section. Oils are classified as drying if the iodine number is >130, semi-drying if between 115 and 130, and nondrying if 172

14 10 8 5

124

54

17

8

2

92

Although oleic acid is unsaturated, it is not a drying acid.

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Curing Mechanism and Catalysts The curing mechanism for oil-based products, including alkyds, follows an air oxidation mechanism (5). Oxygen diffuses into the film and reacts with the diallylic hydrogens to produce a hydroperoxide (Fig. 8.1). The hydroperoxide is formed at a relative rate of 1:120:330 for the trioleate:trilinoleate:trilinolenate (3). The curing mechanism then involves the decomposition of the hydroperoxide by a redox catalyst. The most popular catalysts for the decomposition of the hydroperoxides are cobalt and manganese naphthenate salts. These add color to the coating and thus are kept at a low concentration. Other additives are used to improve the drying characteristics. These include calcium and zirconia salts as driers and 9,10-phenanthrolene, a complexing agent that accelerates cure. To retard the curing process, a cobalt complexing agent, called an antiskinning agent, ties up the cobalt in the can but evaporates and thus activates the cobalt after application. The skinning of oilbased products in the can has been a problem since they were first used. Alkyds Alkyds comprise a large class of coatings. It includes the three normal alkyd classes, which are based upon the amount of unsaturated oils used in the manufacture of the alkyd resin: (i) Long Oil Alkyds are used typically for architectural paints. They contain >60% oil, are soluble in mineral spirits, and are slow drying and soft. (ii) Medium Oil Alkyds are used typically for architectural paints and as a co-resin in some original equipment manufacturers (OEM) coatings. Their oil content is between 40 and 60%; they are soluble in mineral spirits and aromatic solvents and are slow to dry but faster than long oil alkyds and slightly harder. (iii) Short Oil Alkyds are used typically for OEM and other rapid dry applications. They contain 100,000 h. The coating is also relatively stain resistant and nonstick. The dimethyl siloxanes are very flexible even at very low temperatures, whereas the diphenyl siloxanes are hard. Due to their high cost, siloxanes are often used to modify the properties of other resins such as acrylics or alkyds where they can be used cost effectively. New Oils and Their Use Several new oils have been studied for their potential use in coatings. Today the oil market is not local but global. Soybean oil is produced around the world and its production in Brazil or elsewhere will affect its price and use in all parts of the world. Oils formerly produced only in a remote area but found to be of value can be converted into a new agricultural product. Lesquerella oil (LO) and the dehydrated lesquerella oil (DLO) were studied for use in alkyd-type coatings. Their performance was found to be comparable to that of castor oil and dehydrated castor oil, respectively. The lesquerella oil resins were generally found to perform better in drying time, flexibility, and corrosion resistance (8). Another interesting oil derived from Euphorbia lagascae and Vernonia galamensis is 9c,12,13 epoxy-octadecenoic acid (vernolic acid) (9). This epoxy acid and its

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Fig. 8.5. Urethane modified alkyd.

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Alkyd-SiMe2-O-(SiMe2)n-OMe Typically, n = 5-10

Fig. 8.6. Siloxane.

esters can function as a reactive diluent in many solvent and water-borne coatings. Its use in UV cure systems will be discussed later (10,11). This epoxy system can be polymerized through the action of any carboxylic or mineral acid. Thus, in a conventional baking system, these epoxy groups could be used to cross-link an acid-rich resin. A rapid cure oil derived from Calendula officinalis, “Marigold,” was found to comprise >63% of the C18 triene 8t,10t,12c-octadecatrienoic acid, calendic acid. This acid is analogous to the well-known drying oil, tung oil (11). Conventional drying oils can be modified through a conjugation of the double bonds. Various catalysts have been employed to put the double bonds into conjugation, including bases and metal catalysts. Here the new diene or triene is much more reactive and curing is more rapid. The future of oils will continue through the use of genetic engineering. The development of new soybean varieties as well as other oil-producing plants that can produce highly specific fats will continue to increase. Today soybeans, which are high in oleic acid, are available, and work is underway to produce high triene content soybean oil. Having plants custom-make our chemicals will increase the performance of these natural products. The high cost and lack of availability of tung oil makes the high triene soybean oil an attractive technological breakthrough. The scientific and social acceptance of this technology as well as the full evaluation as to the safety of the use and production of such products through genetic engineering must first be accomplished. Once proven safe, these new plant-based chemical producers will reduce our need for petroleum-based products in coatings. Water-Borne Coatings Water-borne coatings can fit into any of the following types: water-soluble, emulsions, dispersions, latex, and water-reducible resins. Water-soluble resins are the least important and are rare because most resins derived from oils are insoluble in water. The true emulsions are based upon the emulsification of the oil or alkyd through either the action of a surfactant or a resin that has a surfactant-like character; these are oil-inwater emulsions. In this system, the resin must be a liquid emulsified in water. A few systems utilize water dispersed in an oil or alkyd. This latter system is termed a waterin-oil emulsion. If the resin is a solid and is dispersed in water, it is termed a dispersion. The last class is the latex. Here the resin is usually vinyl acetate, styrene, acrylates, or methacrylates radically copolymerized in a micelle to form particles 0.1 µm in diameter. Water-reducible resins are similar to the dispersions in that they are particles; however the particle size is generally