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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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Preface
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
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Contents
Chapter 1
Chapter 2
Chapter 3
Chapter 4
Chapter 5
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
iii
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 . .
14
Vegetable Oil-Based Engine Oils: Are They Practical? Joseph M. Perez . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
31
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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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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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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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
— — — — — <9.30 <12.5 <16.3 <21.9
— — — — — 2.6 2.9 2.9 3.7
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TABLE 3.3 Military Requirements Requirement Pumping viscosity cP max at (°C) cSt Viscosity at 100°C min/max VI, min HTHS, cP, min Pour point, °Cmax Flash point, °C COC, min Evaporation, %max aAbbreviations:
10W
30
40
5W30
10W30
30,000 at (–25) 4.1/<7.4
— — 9.3/<12.5
— — 12.5/<16.3
30,000 at (–30) 9.3/<12.5
30,000 at (–25) 9.3/<12.5
— 2.9 –30 205
80 — — 220
80 — — 225
— 2.9 –35 200
— 2.9 –30 205
18
—
—
20
17
VI, viscosity index; HTHS, high temperature high shear; COC, Cleveland Open Cup.
To be certified requires a number of fired engine tests and bench tests. The requirements for gasoline engine oils are discussed elsewhere (18). For diesel engines, the current API CI-4 engine oil requires eight fired engine tests and seven new and used oil bench tests found in Tables 3.4 and 3.5. The number of engine tests for certification has increased from two (CD Oil) to eight (CI-4 oil) as requirements for improved quality have increased in the past 50 years. Conducting development and certification tests on crankcase oils exceeds 1000K. The time for recovery of investment on these costs is now ~5 years before the next generation of oil must be certified. The target for PC-10, the next generation of diesel engine oils is 2007. CI-4 certifications started in 2002. Vegetable Oil Properties The advantages and weaknesses of vegetable oils for lubricant applications are well known (Table 3.6). Vegetable oils have improved lubricity and natural viscosityTABLE 3.4 Fired Engine Test for American Petroleum Institute (API) CI-4a Performance requirement
Engine test
Ring, liner, bearing wear and oil consumption Ring, valve train wear, filter pressure Differential and sludge content Roller-follower wear Piston deposits and oil consumption, two-piece piston Piston deposits and oil consumption, aluminum piston Engine oil viscosity control due to soot Oil aeration control Oil oxidation
Mack T-10 (EGR) Cummins M11-EGR
aAbbreviations:
GM 6.5 Liter Caterpillar 1R SCTE Caterpillar 1K or 1N SCTE Mack T-8E International 7.3 L GM 3.8 Liter Sequence IIIF
EGR, exhaust gas recirculation; SCTE, single cylinder test engine.
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TABLE 3.5 Bench Tests for American Petroleum Institute (API) CI-4 Performance characteristics
Bench test
Shear stability High temperature high shear (HTHS) Volatility Low-temperature pumpability
Bosch injector – ASTMa D3945 ASTM D4683 NOACK–ASTM D5800 Used oil containing 5% soot from Mack T10–MRV TP-1, ASTM D4684 ASTM D471 ASTM D5968 ASTM D892
Elastomer compatibility Corrosion control Foaming aASTM,
American Society of Testing Materials.
temperature properties compared with petroleum-based mineral oils. However, serious negative characteristics of vegetable oils are poor oxidation stability and low-temperature properties. Also of concern are corrosion and hydrolytic stability. Research in the 1990s by a number of organizations including ADM, Cargill, Dow, Dupont, Lubrizol, Penn State, Renewable Lubricants, the USDA (Food & Industrial Oil Research Unit, Peoria, IL), and others was aimed at improving these weak links. Research focused on using additive technology, chemical modifications, genetic engineering, and blending with other biodegradable synthetic fluids. Oxidation Stability. Basically, the key to oxidation stability is chemical structure. The more oleic double bonds in the fatty acid chains of the triglyceride, the better the oxidation stability. The more conjugated double bonds in the fatty acid chain the poorer the oxidation stability. Typical vegetable fatty acids are oleic, linoleic and linolenic, Figure 3.1. The relative rates of reactivity are found in Figures 3.2 and 3.3. The composition varies depending on the source, Table 3.7. Studies have shown that oxidation stability can be improved by the use of selected additives, TABLE 3.6 Vegetable Oils Advantages Environmental Biodegradable Renewable Nontoxic Disadvantages or unresolved issues Environmental Oxidation issues
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Properties Low volatility Viscosity-Temperature (High viscosity index) Friction and wear
Properties
Other
Low temperature Oxidation stability Hydrolysis
Cost Volume Quality
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Fig. 3.1. Vegetable oil structures.
genetic engineering and chemical modifications. However, some of these improvements result in poor low-temperature properties. These properties can be improved by blending the vegetable oils with low pour point basestocks that have acceptable biodegradability. In some cases, chemical modification, such as alkylation and esterification with selected polyol alcohols, can also improve low-temperature properties. Engine Oil Status Currently, a few small companies are marketing 2- or 4-cycle vegetable-based engine oils on a limited basis, and some major companies have active research in progress in this area. Companies in the United States that are marketing 4-cycle vegetable oil-based engine oils include Agro Management Group of Colorado DOUBLE ALLYLIC >
Fig. 3.2. Relative reactivity of hydrogens in vegetable oils.
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Fig. 3.3. Order of reactivity of hydrogens: ALLYLIC >> TERTIARY > SECONDARY >
PRIMARY.
Springs, CO (AGM) and Renewable Lubricants (RLI) of Hartsville, OH. Others such as Penn State and the USDA Food & Industrial Oil Research Unit are conducting research aimed at understanding and improving the performance of these oils. Lubrizol was one of the early leaders and focused on additive chemistry. Other major lubricant and additive suppliers have active programs. Valvoline, a Division of Ashland Chemical, is one of the leaders in this area and has shared some of the progress and problems in the development of vegetable-based engine oils (19). The Agro Management Group developed a canola-based 4-cycle engine oil in the early 1990s. The oil (AGM 2000) was evaluated in various vehicles by AGM including tests on ~75 vehicles operated by the U.S. Postal Service in the state of Michigan. Results of these tests were reported earlier (18). According to the authors, Rhodes and Johnson, vegetable-based oils performed well in well-managed fleets, resulting in a significant reduction in emissions and improved engine performance. RLI collaborated with the Tribology Group, Chemical Engineering Department of Penn State in the early 1990s to develop vegetable-based engine oils. Various vegetable oils were evaluated including genetically modified high-oleic vegetable oils (20–23). An understanding was developed regarding the mechanism of oxidation and TABLE 3.7 Composition of Soybean Oils
Composition 18:0 18:1 18:2 18:3 TOTAL-C=C- per 100 molecules aTMP,
trimethylolpropane.
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Normal soybean (%)
Genetically modified (%)
Epoxidized (%)
TMPa (Oleic) (%)
15 23 53 7 150
11 83 3 2 95
(100) 0 0 0 0
0 100 0 0 100
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the formation of deposits. A high-oleic basestock containing a unique additive package was tested in several RLI vehicles for several years. Vehicles included a 3.8 L Oldsmobile engine, a flex-fuel Ford Taurus, a 3500 HP ethanol drag racing funny car and a three-quarter ton Ford pickup diesel truck. The vehicles accumulated >200,000 miles, and some test results were reported previously (24). This resulted in one of the first U.S. patented 4-cycle vegetable-based engine oils (25) and subsequently resulted in several additional biobased lubricant patents by RLI (26,27). Research by RLI has continued, and currently a fleet of 17 vehicles are being tested. RLI has an excellent track record with their fluids and is one of the leaders in vegetable oil technology (www.renewablelube.com). The Food & Industrial Oil Research Unit, National Center for Agricultural Research Unit, Agricultural Research Service, USDA in Peoria, IL and Penn State University have collaborated in studying industrial uses of vegetable oils since the late 1990s. This research resulted in various products and one patent but more importantly in an understanding of the chemistry affecting oxidation stability and low-temperature properties (28–32). Valvoline, a division of Ashland Chemicals, is aggressively pursuing the use of vegetable oils in engine oils. They are currently testing mid-oleic soybean oil for use in crank formulations. The project involves full bench test screening of formulas containing soybean oil and other basestocks. The formulations contain additives used in the latest engine oil categories. One of the benefits seen is cost reduction for new oils that would come from a reduction in the use of expensive Group III and II+ petroleum basestocks required to meet NOACK volatility requirements. Cold crank properties, high viscosity index (VI; temperature-viscosity properties), and friction and wear benefits are drivers in this ongoing study (19).
Summary Can vegetable oils make good engine oil basestocks? The research conducted to date indicates that chemically and genetically modified vegetable oils have excellent potential to perform adequately in engine oils. Some technical and logistic concerns remain regarding the ability to maintain consistent quality oils that would meet oil property and performance specifications. Limited data exist on the blending of vegetable oils; to produce adequate volumes, “refineries” using different vegetable oils and producing various products will be required. Currently, the publication, Physical and Chemical Characteristics of Oils, Fats and Waxes (33), lists 350 oils that could be considered for use as engine oil basestocks. The bottom line is that vegetable oils have shown acceptable property and performance characteristics. However, to make a major penetration of the engine oil market in the next 10 years as a major competitor for petroleum-based engine oils, certification must be obtained by passing the current automotive engine performance tests. These are very expensive and must be justified by a large market. Therefore, at the present time, the chicken and egg syndrome persists.
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References 1. Dowsan, D., History of Tribology, Longman Group, Ltd., New York, 1979. 2. United Soybean Board, Soy Products Guide, www.unitedsoybean.org, 2005. 3. Campen, J., “USB Overview,” Lubricants Technical Advisory Board Presentation, Chicago, IL, Sept. 22, 2004, www.unitedsoybean.org. 4. Erhan, S.Z., The Use of Vegetable Oils as Lubricants, in Bailey’s Industrial Oil and Fat Products, 6th ed., John Wiley & Sons, New York, 2004. 5. Lubricants Industry Sourcebook, Lubes’n’Greases, 2004–2005. 6. Transportation Energy Book: Edition 21,DOE/ORNL-6966. 7. Energy Information Administration Annual Energy Outlook 2002, DOE/EIA-0383 (2002), December, 2001. 8.
[email protected]. 9. McFall, D., Almost 1.5 Billion Gallons, Lubes’n’Greases 10: 6 (November 2004). 10. Boehman, A.L., W.H. Swain, D.E. Weller, and J.M.Perez, Use of Vegetable Oil Lubricant in a Low Heat Rejection Engine to Reduce Particulate Emissions, SAE Technical Paper 980887, Detroit, MI (February 1998). 11. Weller, D.E., W.H. Swain, H. Hess, A.L. Boehman, and J.M. Perez, Changes in Particulate Composition and Morphology When Using Vegetable Oil Lubricant in a Low Heat Rejection Engine, SAE Technical Paper No. 1999-01-0975. 12. Perez, J.M., and A.L. Boehman, Environmentally Friendly Fuels and Lubricants, in Biobased Industrial Fluids and Lubricants, edited by S.Z. Erhan and J.M. Perez, Ch. 6, AOCS Press, Champaign, IL, 2002. 13. SAE Recommended Practice J300, SAE Handbook, Society of Automotive Engineers, Warrendale, MI. 14. ASTM D4684, Standard Test Method for the Determination of Yield Stress and Apparent Viscosity in Engine Oils at Low Temperatures (1994), ASTM, West Conshohocken, PA 19428-2959. 15. ASTM 5481, Standard Test Method for Measuring Apparent Viscosity at High Shear Rate by Multicell Capillary Viscometer (1994), ASTM, West Conshohocken, PA 19428-2959. 16. ASTM D445, Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids—the Calculation of Dynamic Viscosity (1988), ASTM, West Conshohocken, PA 19428-2959. 17. Cheenkachorn, K., A Study of Environmentally Friendly Lubricants for Automotive Engine Oils, M.S. Thesis, The Pennsylvania State University, May 1999. 18. Rhodes, B.N., and D. Johnson, Vegetable-Based Motor Oils, in Biobased Industrial Fluids and Lubricants, edited by S.Z. Erhan and J.M. Perez, Ch. 8, AOCS Press, Champaign, IL, 2002. 19. McCoy, S., Valvoline Engine Oil Development, Proceedings of the 9th United Soybean Board Lubricants and Fluids Technical Advisory Panel Meeting, Ch. 8, Chicago, IL, September 22, 2004. 20. Asadauskas, S., J.M. Perez, and J.L. Duda, Suitability of Basestocks for Biodegradable Lubricants, Petroleum Division Preprints, Vol. 42, No. 1, Feb. 1997, presented at 213th ACS Meeting, San Francisco, CA, April 15, 1997. 21. Asadauskas, S., J.M. Perez, and J.L. Duda, Oxidative Stability and Antiwear Properties of High Oleic Vegetable Oils, Lubr. Eng. 52: 877–882 (December 1996).
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22. Asadauskas, S., J.M. Perez, and J.L. Duda, Lubrication Properties of Castor Oil—Potential Basestock for Biodegradable Lubricants, Lubr. Eng. 53: 35–40 (December 1997). 23. Perez, J.M., S. Asadauskas, and W.W. Garmier, Vegetable Based Engine Oils, presented at 52nd Annual STLE Meeting, Kansas City, MO, May 29, 1997. 24. Garmier,W.W., Vegetable Oil Stabilization, Proceedings of the 4th United Soybean Board Lubricants and Fluids Technical Advisory Panel Meeting, Chicago, IL, September 1999. 25. Garmier,W.W., U.S. Patent No. 5,736,493 (1998). 26. Garmier,W.W., U.S. Patent No. 5,863,872 (1999). 27. Garmier,W.W., U.S. Patent No. 5,990,055 (1999). 28. Adhvaryu, A., S.Z. Erhan, and J.M. Perez, Oxidation Kinetic Studies of Unmodified and Genetically Modified Vegetables Using PDSC and NMR Spectroscopy, Thermochim. Acta 364: 87–97 (2000). 29. Asadauskas, S., and S.Z. Erhan, Depression of Pour Points of Vegetable Oils by Blending with Diluents Used for Biodegradable Lubriants, J. Am. Oil Chem. Soc. 76: 313–316. 30. Weller, D.E., Jr., and J.M. Perez, The Effect of Chemical Structure on Friction and Wear, Lubr. Eng. 56: 39–44 (November 2000).(AQ3) 31. Weller, D.E., Jr., and J.M. Perez, A Study of the Effect of Chemical Structure on Friction and Wear: Part 2 Vegetable Oils and Esters, Lubr. Eng. 57: 20–26 (May 2001). 32. Cheenkachorn, K., Ph.D. Thesis, The Pennsylvania State University, May 2003 33. Firestone, D., ed., Physical and Chemical Characteristics of Oils, Fats and Waxes, AOCS Press, Champaign, IL (1999).
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Chapter 4
Biodiesel: An Alternative Diesel Fuel from Vegetable Oils or Animal Fats Gerhard Knothe and Robert O. Dunn USDA, Agricultural Research Service, 1815 N. University Street, Peoria, IL 61604, USA
Introduction Biodiesel is defined as “a fuel comprised of the mono-alkyl ester of long chain fatty acids derived from vegetable oils or animal fats” (1). Accordingly, biodiesel is derived from vegetable oils or animal fats by a transesterification reaction (Scheme 4.1), in which the oil or fat is reacted with a monohydric alcohol in the presence of a catalyst. Methanol is the alcohol used most commonly to produce biodiesel as it is the least expensive alcohol in many countries. Besides transesterification to alkyl esters, three other approaches—dilution with conventional, petroleum-based diesel fuel (DF), microemulsions (co-solvent blending), and pyrolysis—have been explored for utilizing vegetable oils as fuel (2). 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), this article will focus on such mono-alkyl esters.
History and Overview The use of vegetable oils as a DF is a concept nearly as old as the diesel engine itself. At the Paris World Exhibition in 1900, at the request of the French government, a diesel engine was demonstrated using a vegetable oil (peanut oil) as fuel
Scheme 4.1. The transesterification reaction. 42
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(3) and was described in reports by Rudolf Diesel (4,5), the inventor of the engine that bears his name. Numerous papers were published until the late 1940s regarding the use of vegetable oils as fuel (3). The background in most cases was a rather modern one—namely, to provide an alternative source of energy. This was the case also in European countries with African colonies. Among those numerous reports are several articles (6–8) from Belgium and its former colony, the Belgian Congo, which are presumably the first to describe the use of what is today termed biodiesel as fuel (3). In that work, ethyl esters of palm oil were utilized, however, other oils and esters were described as suitable. During the age of inexpensive and easily accessible petroleum from approximately 1950 until the 1970s, hardly any reports on vegetable oilbased fuels were published. As a result of the energy crises of 1973 and later years, the interest in alternative fuels, among them vegetable oils was renewed. In 1980, the use of esters of sunflower oil as DF was reported (9). These authors explicitly pointed out that viscosity of the esters is significantly decreased compared to the parent vegetable oil, now being close to that of conventional DF. The lower viscosity nearly eliminated engine problems such as injector coking and deposits which are a problem with neat vegetable oils. Besides the energy security aspect, environmental concerns (with regulations and legislation arising therefrom) as well as utilization of excess agricultural commodities have become significant driving forces for the use of alternative fuels such as those derived from vegetable oils. Commercialization has occurred in numerous countries around the world with varying economic and legislative conditions. Standards have been or are being developed in Europe, the U.S. and elsewhere. Countless research papers on biodiesel as well as numerous reviews, book chapters, and general interest articles have been published. Sources of Fuels Biodiesel is obtained from various sources depending largely on the country and its climate. Accordingly, soybean oil is the vegetable oil most commonly used for biodiesel production in the U.S. Rapeseed oil (in low-erucic form canola oil) is the major feedstock in Europe, although it is also used in the Pacific Northwest of the U.S. Countries with tropical climate utilize tropical oils such as coconut and palm oil. Other common oils such as cottonseed, peanut (groundnut) and sunflower oils have been studied. Potential sources of biodiesel with some emphasis on developing countries were discussed in more detail (10). Less common or less studied vegetable oils that have been investigated as sources of biodiesel include Cynara cardunculus (Castillian thistle) (11), rubber seed oil (12), karanja (Pongamia glabra) and nahor (Mesua ferrea L.) oils (13), Cuphea viscosissima (14), and Jatropha curcas L. (15), tobacco seed oil (16) as well as other non-conventional oils (17). Various feedstocks for biodiesel were also explored in earlier times (3). Animal fats such as tallow have also been investigated. More recently, increasing emphasis has been placed on low-cost feedstocks such as used frying oils and soapstock. Microalgae were also suggested as a source of biodiesel (18).
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Economics, Regulatory Issues, and Utilization A major obstacle facing biodiesel commercialization is economics. The higher price of the feedstock compared to petroleum translates into a higher price for the resulting fuel. In the U.S. the price of a gallon (about 3.8 liters) of soybean oil is approximately $1.40–$1.50. Other feedstocks are even more expensive. After completion of fuel production, the price of a gallon of methyl soyate is even higher, approximately three to four times that of conventional DF. On the other hand, glycerol, which has numerous uses as a by-product of the transesterification process, may help offset some of the costs. Biodiesel with accompanying glycerol production has had a significant influence on glycerol prices in recent years (19). Incentive-providing legislation and regulations regarding taxes, the environment and energy security offset the inherent economic disadvantage of biodiesel toward conventional DF. In some European countries such as Germany, biodiesel is not subject to the excise tax levied on petroleum-derived fuels. Since the total taxes on conventional fuels are so high, biodiesel becomes economically competitive with conventional DF if it is not subject to these taxes, causing biodiesel to be fairly widely available at filling stations for general use. However, beginning in 2003, tax incentives proportional to the level of biodiesel were applied in Germany to blends of biodiesel with conventional DF. In France, biodiesel is being promoted by approval of a 5% blend of biodiesel in conventional DF available at the pump and as 30% blend available to fleets of “urban vehicles.” In the U.S., environment and energy security-related issues have been the major driving forces apply, expressed in the Clean Air Act Amendments of 1990 and the Energy Policy Act (EPAct) of 1992 as well as the Energy Conservation Reauthorization Act of 1998. Both neat biodiesel and “B20” (a blend of 80 vol% conventional DF with 20 vol% biodiesel) are recognized as alternative fuels in the U.S. under EPAct criteria. Regulated fleets (government and other public fleets) can earn credits for using a certain amount of both neat biodiesel and B20. Therefore, in the U.S. biodiesel has been mainly promoted for use in regulations-affected and niche markets (regulated fleets, urban bus fleets, marine and mining markets). In developing countries, especially those that produce significant amounts of tropical oils, the main incentive for using biodiesel is to become independent of petroleum imports. A major development which will likely serve to additionally promote the use of biodiesel is upcoming mandates prescribing the use of low-sulfur conventional DF. Removal of the sulfur-containing components from conventional DF causes the lubricity of the conventional DF to be significantly reduced or even eliminated. Inherent lubricity of the fuel is important for the functioning of engine components such as fuel pumps and injectors. Adding biodiesel in low amounts (1–2%) to lowsulfur conventional DF restores the lubricity. An advantage of biodiesel in that case is its inherent fuel value, which is not necessarily the case with lubricity additives. Agricultural policies play a role in the production of biodiesel. In the U.S., the goal is to find a significant use of excess soybean oil and provide price support for
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that commodity. Increased demand for soybean oil as a result of biodiesel production could increase soybean oil prices by up to 14% (20). The 2002 Farm Bill contains an energy title which provides for payments to biofuels producers who purchase agricultural commodities for that purpose. In countries of the European Community, set-aside land taken out of crop production for food use may be used for growing crops for nonfood uses. Affected farmers would still receive the general set-aside premium, providing an additional incentive to grow crops for nonfood uses. Biodiesel production has increased significantly over the past few years and is likely to increase further (21, 22). According to some estimates, the capacity for biodiesel production in Europe in 2003 was approximately as high as 2.5 to 2.7 million metric tons (22). More than 90% of this capacity is in Europe with the U.S. accounting for the rest. Numerous biodiesel plants are under construction or in planning stages with accompanying significant increases in production. According to a life-cycle analysis of biodiesel (23) in the U.S., biodiesel is competitive with other alternative fuels such as compressed natural gas (CNG) and methanol in the urban transit bus market. An advantage of biodiesel is its near full compatibility with existing fuel distribution and use infrastructure. No engine modifications (with the exception of some seals and gaskets) are required to commercial diesel engines and storage is similar to conventional DF. This advantage can offset the higher price of the biodiesel fuel in comparison to other alternative fuels which may require significant and expensive infrastructure changes. On the other hand, life-cycle assessments should be carried out individually for each source as the results can vary significantly and the methods used also have great influence (24). With the exception of nitrogen oxides (NOx), the use of biodiesel reduces exhaust emissions compared to conventional petroleum-based DF. Due to its lack of sulfur, biodiesel does not cause SO2 emissions. The lower emissions make biodiesel attractive for use in urban bus fleets and other niche markets such as mining and marine engines. Besides environmental and health reasons with accompanying regulations, focusing on the use of biodiesel in niche markets is additionally attractive because not enough feedstocks are available to supply the whole diesel market with biodiesel. Cetane Numbers and the Suitability of Fatty Compounds as Diesel Fuel A scale conceptually similar to the familiar octane scale used for gasoline (petrol), the cetane number (CN), exists for describing the ignition quality of conventional DF or its components. The CN is determined by standards such as ASTM D613. The CN of a DF is determined by the ignition delay time, that is, the time that passes between injection of the fuel into the cylinder and onset of ignition. The shorter the ignition delay time, the higher the CN and vice versa. Generally, a com-
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pound that has a high octane number tends to have a low CN and vice versa. Thus, 2,2,4-trimethylpentane (iso-octane), a short, branched alkane, is the high-quality standard (a primary reference fuel; PRF) for the octane scale (and also gives it its name) of gasoline with an octane number of 100 while n-heptane is the low-quality PRF with an octane number of 0 (25). For the cetane scale, a long straight-chain hydrocarbon, hexadecane (C16H34; trivial name cetane, giving the cetane scale its name) is the high-quality standard and has been assigned a CN of 100. At the other end of the scale, a highly branched compound, 2,2,4,4,6,8,8,-heptamethylnonane (HMN, also C16H34), a compound with poor ignition quality in a diesel engine, has been assigned a CN of 15. Thus branching and chain length influence CN with CN decreasing with decreasing chain length and increasing branching. Aromatic compounds occur in significant amounts in conventional DF and have low CN but their CN increase with increasing size of n-alkyl side chains (26,27). The cetane scale is arbitrary and compounds with CN >100 (although the cetane scale does not provide for compounds with CN >100) or CN <15 have been identified. The cetane scale demonstrates the suitability fatty compounds as alternative DF. The structure of the long, unbranched chains of fatty acids (FA) is similar to those of n-alkanes which yield good conventional DF. CN of fatty compounds and alkyl esters of vegetable oils and animal fats are presented in Tables 4.1 and 4.2. However, besides ignition quality as expressed by the cetane scale, several other properties determine the overall quality of any DF. Especially heat of combustion, pour point (PP), cloud point (CP), viscosity, oxidative stability and lubricity are of importance.
General Comparison of Fuels from Vegetable Oils and Animal Fats Some relevant properties of the most common FA occurring in vegetable oils and animal fats as well as some of their esters are listed in Table 4.1. Besides these FA, numerous other FA occur in vegetable oils and animal fats, but their amounts usually are considerably lower. Properties of esters of oils and fats are given in Table 4.2. The most common derivatives of TG (or FA) for fuels are methyl esters. These are formed by transesterification of the TG with methanol in the presence of usually a basic catalyst to give the methyl ester and glycerol (see Scheme 4.1). Other alcohols have been used to generate esters such as the ethyl, propyl, and butyl esters. As discussed above, the suitability of fats and oils as DF results from their molecular structure and high energy content. Long, saturated, unbranched hydrocarbon chains as they are found in fatty compounds are especially suitable for conventional DF as shown by the CN scale. CN generally increase with increasing chain length (28). Other observations (29) are (i) that double bonds decrease CN (therefore, the number of double bonds should be small rather than large), (ii) that a double bond, if present, should be positioned near either end of the molecule, and (iii) no aromatic compounds should be present. However, branching in the ester
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moiety (iso-propyl esters, etc.) does not negatively influence CN of fatty compounds (30). The combustion of the glyceryl moiety of triacylglycerols could lead to the formation of acrolein and this in turn to the formation of aromatics (29), although no acrolein was found in precombustion of triacylglycerols (31). This may be one reason why fatty esters of vegetable oils perform better in a diesel engine than oils containing the TG (29). The above statements on CN correlate with the values given in Tables 4.1 and 4.2. The CN of mixtures are influenced by the nature of their components. Fuel components with higher CN impart these higher CN to the biodiesel fuel. It is occasionally emphasized that biodiesel is an oxygenated fuel, implying its oxygen content plays a role in making fatty compounds suitable as DF by “cleaner” burning. The responsibility for this suitability rests, however, mainly with the hydrocarbon portion which is similar to conventional DF. Furthermore, the oxygen in fatty compounds may be removed from the combustion process by decarboxylation, which yields incombustible carbon dioxide (CO2), as precombustion (31), pyrolysis and thermal decomposition studies discussed below imply. Also, pure unoxygenated hydrocarbons, like cetane, have CN higher than biodiesel. The CN of esters correlate well with boiling points (28). Quantitative correlations and comparisons to numerous other physical properties of fatty esters confirmed that the boiling point gives the best approximation of CN (32). ASTM D613 is used in determining CN. For vegetable oil-derived materials, an alternative scale utilized a constant-volume combustion apparatus (CVCA) (33). The amount of material needed for CN determination was reduced significantly with this bomb and it also allows studying materials with high melting points (mp) that cannot be measured by ASTM D613. Estimated cetane numbers (ECN) were determined on a revised scale permitting values greater than 100. In this case, the ECN of methyl stearate is 159 and that of methyl arachidate (20:0) is 196 (33). The ECN of other esters were methyl laurate 54, methyl myristate 72, methyl palmitate 91, and methyl oleate 80. The ECN of the TG trilaurin and trimyristin exceeded 100, while in another series of experiments the ECN of tripalmitin was 89, tristearin 95, triolein 45, trilinolein 32, and trilinolenin 23. The term “Lipid Combustion Quality Number” with an accompanying scale was suggested instead of CN to provide for values in excess of CN 100 (33). Often the “cetane index” of a fuel is published and should not be confused with CN. This is an ASTM-approved alternative method for a “non-engine” predictive equation of CN for petroleum distillates (34). Equations for predicting CN are usually not applicable to nonconventional DF such as biodiesel or other lipid materials (35). Cetane indices are not given here. Methods for estimating the cetane indices of esters of FA and vegetable oils were presented (36,37). Besides CN, gross heat of combustion (HG) is another property of fatty compounds that is essential in proving the suitability of these materials as DF as the heat content of vegetable oils is nearly 90% that of DF2 (38–41). The heats of
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m.p. (°C)
b.p. (°C)
Myristic (Tetradecanoic); 14:0 Methyl ester Ethyl ester
58 18.5 12.3
250.5100 295751 295
Palmitic (Hexadecanoic); 16:0 Methyl ester Ethyl ester Propyl ester Iso-propyl ester
63 30.5 19.3/24 20.4 13–14
350 415–4418747 19110 19012 1602
Stearic (Octadecanoic); 18:0 Methyl ester Ethyl ester Propyl ester Iso-propyl ester
71 39 31–33.4
360d 442–3747 19910
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61.4 (99.1)
1.95c; 2.38 2.88c
1940 2098
66.2 (96.5) 66.9 (99.3)
2.69c, 3.23
2254 2406
74.5 (93.6) 85.9 93.1 85.0 82.6
3.60c; 4.32
2550 2717
4.74c, 5.61
2859 3012
61.7 86.9 (92.1) 101 76.8; 97.7 69.9; 90.9 96.5
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1311 266766 16325
HG (kg-cal/mole)
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Kinematic viscosity (40°C; mm2/s = cSt)
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Lauric (Dodecanoic); 12:0 Methyl ester Ethyl ester
Cetane no.
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TABLE 4.1 Properties of Fatty Acids and Esters of Relevance to Biodiesela
51.0
Linoleic (9Z,12Z-Octadecadienoic); 18:2 Methyl ester Ethyl ester Propyl ester
–5 –35
229–23016 21520 270–275180
31.4 42.2; 38.2 37.1; 39.6 40.6; 44.0
Linolenic (9Z,12Z,15Z-Octadecatrienoic); 18:3 Methyl ester Ethyl ester Propyl ester
–11 –57/–52
230–23217 1090.018 1742.5
20.4 20.6; 22.7 26.7 26.8
Erucic (13Z-Docosenoic); 22:1 Methyl ester Ethyl ester
33–34
26515 22–2225 229–2305
aMelting
3.73i; 4.45 5.50 (25°)c
3.05c; 3.654
2794
2.65c; 3.27
2750
5.91c; 7.21i
3454
points (m.p.) and boiling points (b.p.) obtained from References 42 and 309. Superscripts denote pressure (mm HG) at which the boiling point was determined. Heats of combustion obtained from References 40 and 42. Cetane numbers obtained from References 28, 30, and 180. Number in parentheses indicates purity (%) of the material used in CN determinations in Reference 28. Kinematic viscosity values from Reference 310. Dynamic viscosity values from Reference 313. bThe numbers denote the number of carbons and double bonds. For example, in oleic acid, 18:1 stands for eighteen carbons and one double bond. cDynamic viscosity values, see footnote a.
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46.1 55; 59.3 53.9; 67.8 55.7; 58.8 86.6
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Oleic (9(Z)-Octadecenoic); 18:1 Methyl ester Ethyl ester Propyl ester Iso-propyl ester
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HG high/low (kJ/kg)
Kinematic viscosity (mm2/s)
Cloud point (°C)
Pour point (°C)
Flash pointa (°C)
Reference
67.4
38158
3.08
5
–3
190
223
Corn Methyl
65
38480 (lower)
4.52
–3.4
–3
111
312
Cottonseed Methyl
51.2
—
6.8 (21°)
—
–4
110
313
Palm Ethyl
56.2
39070 (h)
4.50 (37.8°C)
8
—
314
Rapeseed (low-erucic; canola) Methyl Methyl Ethyl
53.7 47.9 67.4
38850 (h/l) 39870 (h) 40663
4.96 4.76 6.02
CFPP: –6 –3 1
166 170
315 314 223
Safflower Methyl
49.8
40060
—
—
180/149
316
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6
–9 –12
–6
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Coconut Ethyl
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Cetane number
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Oil or fat; ester
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TABLE 4.2 Fuel-Related Physical Properties of Esters of Oils and Fats
38472
4.39
61.8
39961/37531
4.99 (40°C) 7.10
Hydrogenated soybean Ethyl
65.1
40093
Yellow grease Methyl
62.6
39817/37144
Tallow Methyl iso-Propyl
aSome
–3.9 0 –12
1.5
3
15.6 8
12.8 0
5.54
7
6
5.16
—
—
flash points are very low. These may be typographical errors in the references or the materials may have contained residual alcohols. bDynamic viscosity.
190.6 185
317 223 218
110
312
187.8
317 220
174
223
—
318
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Sunflower Methyl
–1.1 1 –9
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Soybean Methyl Methyl iso-Propyl
51
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combustion of fatty esters and triacylglycerols (40) as well as fatty alcohols (41) were determined and shown to be within the same range. Heats of combustion range from about 1,700 kg-cal/mole to about 3,500 kg-cal/mole for C12–C22 FA and esters, increasing with chain length (40,42). For purposes of comparison, the literature value (42) for the heat of combustion of hexadecane (cetane), the high CN standard for conventional DF, is 2559.1 kg-cal/mole (at 20°C), which is in the same range. Other important fuel properties are viscosity and those related to low temperatures. This is shown by the data in Tables 4.1 and 4.2, which list the mp (and boiling points) of neat fatty compounds as well as the viscosities of neat fatty compounds and methyl esters of some oils and fats. The viscosity of vegetable oils is approximately one order of magnitude greater than that of conventional DF. The high viscosity with resulting poor atomization in the combustion chamber was identified early as a major cause of engine problems such as nozzle coking, deposits, etc. (9,40,43–46). Therefore, neat vegetable oils have been largely abandoned as alternative DF. However, as the data show, the viscosity of the alkyl esters is close to that of conventional DF. Accordingly, the ranges for kinematic viscosity in biodiesel standards are 1.9–6.0 mm2/s (ASTM D6751) and 3.5–5.0 mm2/s (EN 14214). FA methyl esters have higher CP and PP than their parent oils and fats and conventional DF. This is important for engine operation in cooler environments. The CP is defined as the temperature at which the fuel becomes cloudy due to formation of crystals which can clog fuel filters and supply lines. The PP is the lowest temperature at which the fuel will flow. It is recommended by engine manufacturers that the CP be below the temperature of use and not more than 6°C above the PP. A more detailed discussion of low temperature of esters from vegetable oils and animal fats can be found below. Numerous reports exist showing that fuel economies of certain biodiesel blends and conventional DF are virtually identical. In numerous on-the-road tests, primarily with urban bus fleets, vehicles running on blends of biodiesel with conventional DF (usually 80 vol% conventional DF and 20% biodiesel) required only about 2–5% more of the blended fuel than of the conventional fuel. No significant engine problems were reported. The methyl and ethyl esters of soybean oil generally compared well with DF2 with the exception of gum formation which leads to problems with fuel filter plugging (45). Another study reports that methyl esters of rapeseed and high-linoleic safflower oils formed equal and lesser amounts of deposits than a DF standard while the methyl ester of high-oleic safflower oil formed more deposits (47). Soybean methyl and ethyl esters were evaluated by 200-h EMA (Engine Manufacturers Association) engine tests and compared to DF2. Engine performance with soybean esters differed little from that with DF. In that work, also a slight power loss was observed, together with an increase in fuel consumption due to the lower heating values of the esters. The emissions for the two fuels were sim-
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ilar, with the exception of NOx which are higher for the esters as discussed above. Engine wear and fuel-injection system tests showed no abnormal characteristics for any of the fuels. Although deposit amounts in the engine were comparable, the methyl ester showed greater varnish and carbon deposits on the pistons. Operating DI engines with neat soybean oil esters under certain conditions produced lubricating oil dilution which was not observed with an IDI engine (50). Vegetable oil-based fuels possess inherent lubricity. This is significant because environmental concerns and resulting future regulations dictate the use of conventional DF with low sulfur levels. Conventional DF serves as its own lubricant within the fuel system. At low sulfur levels, that ability is lost. Even at low blend levels (additive levels, approximately ≤2 wt%), biodiesel could serve not only as a fuel component but as a lubricity-improving additive. Methyl soyate was a more effective lubricant than soybean oil at equal treatment rates but a soybean oil-based additive was even more effective (49). For the conventional DF with poorest inherent lubricity, 1% or slightly higher treatments rates with soybean oil were necessary to meet specifications. Biodiesel is readily biodegradable, which contrasts with conventional DF. This was shown by the CO 2 evolution method in an aquatic system (50). Conventional DF degraded faster in the presence of biodiesel. Another advantage of esters from vegetable oils and animal fats as fuels is their higher flashpoint which makes them safer to handle and store than conventional DF. Due to its solvent properties (see above), ester fuels are not compatible with some polymers used in fuel system components such as seals and gaskets of conventional DF. Accordingly, it has been reported that Teflon® and other fluoroelastomers are least affected by biodiesel and its blends with conventional DF while nitrile rubber, nylon 6/6 and high-density polypropylene showed less resistance (51). It was reported in the same study that copper-containing metals are severely corroded by biodiesel and its blends and also showed gum formation. Steel and aluminum did not show gum formation but caused high acid numbers which could cause corrosion. Storage stability is discussed below.
Biodiesel Standards As mentioned above, some biodiesel standards have been established, including in the U.S. and Europe. While similar in many aspects, they also contain some notable differences as briefly discussed here and elsewhere in this chapter. The iodine value (IV) has been included in the European standard and is based on rapeseed oil as biodiesel feedstock. The maximum IV in EN 14214 is 120, which would largely exclude soybean oil and some other common vegetable oils (neat vegetable oils and their methyl esters have nearly identical IV) as biodiesel feedstock. However, the use of the IV is not without problems (52), including the fact that it does not adequately reflect that a specific IV can arise from a nearly infinite number of FA profiles. Biodiesel from vegetable oils with high amounts of
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saturates (low IV) will have a higher CN while the low-temperature properties are poor. Biodiesel from vegetable oils with high amounts of unsaturates (high IV) will have low CN while the low-temperature properties are better. Thus, CN and low-temperature properties run counter to each other and this must affect IV for biodiesel standards. The IV also does not take into consideration structural factors of fatty compounds as discussed above where the CN depend on double bond position, etc. It was suggested that it appears better to limit the amount of higher unsaturated FA (e.g., linolenic acid) than to limit the degree of unsaturation by means of the IV (53). In the standard EN 14214, the amount of linolenic acid is now limited to 12%, which may be difficult to reconcile with an IV limit of 120. Soybean oil, rapeseed oil, and canola oil (low-erucic rapeseed oil) have very similar 18:3 FA content, which is the most problematic in the formation of engine deposits through polymerization. However, linseed oil methyl ester (high 18:3 content and IV) satisfactorily completed 1000 h of testing in a DI engine while neat linseed oil caused the engine to fail (54). In a study on prepared ester fuels of medium to high iodine values, no significant differences in engine performance and deposits were observed and no limit for the IV could be given (55). EN 14214 calls for a maximum of 12% C18:3 in biodiesel, which would permit soybean oil as feedstock. A major difference therefore affecting feedstocks and their IV is the C18:2 content. Additionally, EN 14214 limits the amount of FA with ≥4 double bonds to 1%, which would mainly appear to affect biodiesel from animal sources (fish oils). Since most esters have higher CN than neat vegetable oils and conventional DF, the esters could accommodate higher CN than the minimum of 40 given in the ASTM standard for conventional DF. For example, the lowest reported CN for methyl soyate is 46.2. The minimum CN in the European standard is higher. The European biodiesel standard EN 14214 includes the cold-filter plugging point (CFPP) that pertains to the low-temperature flow properties of biodiesel. This low-temperature property test is used in Europe, South America, and the Pacific rim. Each European country can determine the limit for CFPP individually depending on the time of year. In North America, a more stringent test, the LowTemperature Flow Test (LTFT), is used and specified by ASTM D4539. Although the LTFT is more useful in evaluating low-temperature flow properties, ASTM D6751 requires only specification of CP for certification without a limit being given in the standard due to varying climate conditions in the U.S. Combustion and Emissions Conventional DF and vegetable oil-derived fuels generate similar types of compounds in exhaust emissions. This is another indication of the suitability of fatty compounds as DF because there presumably exist similarities in their combustion behavior. Emissions from any engine are the result of the preceding combustion within in the engine. The combustion process, in relation to the properties of the fuel, and its (in)completeness are responsible for any problems associated with the use of
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any fuel, such as formation of deposits. Ideally, the products of complete combustion of hydrocarbons are CO2 and water. Combustion in a diesel engine occurs mainly through a diffusion flame and thus is incomplete (56). This causes the formation of partially oxidized materials such as carbon monoxide (CO), other oxygenated species (aldehydes, etc.), and hydrocarbons. In the case of biodiesel, liberation of CO2 (decarboxylation), as discussed above, can occur besides combustion formation of CO2 from the hydrocarbon portions of biodiesel. The formation of incombustible CO2 shows that oxygenated compounds as combustion enhancers need to be chosen judiciously since the combustion-enhancing properties depend on the nature of the oxygen (bonding, etc.) in these compounds. Therefore, the higher oxygen content of biodiesel does not necessarily imply improved combustion compared to conventional DF as oxygen may be removed from the combustion process by decarboxylation. CO2, however, may contribute to combustion in other ways. Exhaust emissions observed in the combustion of conventional DF and biodiesel are smoke, which is a mixture of fuel and lubricating oil particles in unburned, partially burned or cracked states (57), particulates [particulate matter (PM)], polyaromatic hydrocarbons (PAH), hydrocarbons, CO, and oxides of nitrogen (NOx, also referred to as nitrous oxides, or nitrogen oxides). Sulfur-containing emissions are not formed from neat biodiesel due to its lack of sulfur. Rapeseed contains low amounts of sulfur but in variations such as canola, erucic acid and sulfur content are further reduced (58). The rapeseed oil mentioned in European publications on alternative fuels usually refers to canola-type oil. The composition of particulate matter has been studied for conventional DF. Particulates are soot that has collected high molecular weight hydrocarbons (and sulfates in the case of conventional DF) (57,59). Thus, particulates from conventional DF have a high carbon to hydrogen ratio. Soot particles consist of spherules (somewhat spherical species) arranged in irregular clusters or chains (57). The size of particulates is of concern because smaller species (diameters less than 10 µm) can be inhaled deeply into the lungs. Therefore, the size distribution of particulate matter may be of greater significance than its mass. PAH are compounds composed of fused aromatic rings that may carry alkyl substituents such as a methyl group. They are of concern because many of them are known carcinogens. Hydrocarbons represent a broad category of compounds including hydrocarbons and oxygenated species such as aldehydes, ketones, ethers, etc. Although some of these emissions such as aldehydes are unregulated (not limited by legislation), many of these species such as formaldehyde are ozone precursors. As precursors of ozone, which in turn is a major component of urban smog, NOx exhaust emissions are of particular concern. Accordingly, both NOx exhaust emissions and ozone in ambient air are subject to environmental regulations. NOx species arise by the reaction of nitrogen and oxygen from air at an early stage in the combustion process (56,60). NOx emissions are difficult to control because
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such techniques may increase other emissions or fuel consumption. Soot can be reduced by higher injection pressures and higher air swirl levels but this enhanced combustion increases NOx (59). The trade-off between particulates and NOx as a result of changes in engine design or operational parameters is a general problem (59). Neat vegetable oils give satisfactory engine performance and power output, often equal to or even slightly better than conventional DF. However, vegetable oils cause engine problems. This was recognized in the early stages of renewed interest in vegetable oil-based alternative DF. Studies on sunflower oil as fuel noted coking of injector nozzles, sticking piston rings, crankcase oil dilution, lubricating oil contamination, and other problems (9,44). These problems were confirmed and studied by other authors (61–68). A test for external detection of coking tendencies of vegetable oils was reported (69). The causes of these problems were attributed to the polymerization of TG via their double bonds which leads to formation of engine deposits as well as low volatility and high viscosity with resulting poor atomization patterns. For blends of vegetable oils and conventional DF, it was postulated that carbon buildup was a result of a polymerization growth process on preferred metallic surfaces (70). The engine problems have caused neat vegetable oils to be largely abandoned as alternative DF and have led to the research on the aforementioned four solutions (2).
The Transesterification Process Several reviews on transesterification are available (71–74). The most commonly prepared esters are methyl esters, which is largely a result of methanol being the least expensive alcohol. Alkali catalysis (sodium or potassium hydroxide or alkoxides) is a much more rapid process than acid catalysis in the transesterification reaction (75–77). For the transesterification to give maximum yield, the alcohol should be free of moisture and the free FA content of the vegetable should be less than 0.5% (76). At 32°C, transesterification was 99% complete in 4 h when using an alkaline catalyst (NaOH or NaOMe). At 60°C and an alcohol:oil molar ratio of at least 6:1 and with fully refined oils, the reaction was complete in 1 h to give methyl, ethyl, or butyl esters. The reaction parameters investigated were molar ratio of alcohol to vegetable oil, type of catalyst (alkaline vs. acidic), temperature, reaction time, degree of refinement of the vegetable oil, and effect of the presence of moisture and free FA. Although the crude oils could be transesterified, ester yields were reduced because of gums and extraneous material present in the crude oils. Another paper reports on the use of both NaOH and KOH in the transesterification of rapeseed oil (78). Recent work on producing biodiesel from waste frying oils employed KOH. With the reaction conducted at ambient pressure and temperature, conversion rates of 80 to 90% were achieved within 5 min, even when stoichiometric amounts of methanol were employed (79). In two steps, the ester yields
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are 99%. It was concluded that even a free FA content of up to 3% in the feedstock did not affect the process negatively and phosphatides up to 300 ppm phosphorus were acceptable. The product met the quality requirements for Austrian and European biodiesel without further treatment. In a study similar to previous work on the transesterification of soybean oil (75–76), it was concluded that KOH is preferable to NaOH in the transesterification of safflower oil of Turkish origin (80). The optimal conditions were given as 1 wt% KOH at 69±1°C with a 7:1 alcohol:vegetable oil molar ratio to give 97.7% methyl ester yield in 18 min. Transesterification is a reversible reaction. The transesterification of soybean oil with methanol or 1-butanol was reported to proceed with pseudo-first-order or second-order kinetics, depending on the molar ratio of alcohol to soybean oil (30:1 pseudo-first order, 6:1 second order; NaOBu catalyst) while the reverse reaction was second order (81). The methanolysis of sunflower oil at a molar ratio of methanol:sunflower oil = 3:1 was reported to begin with second-order kinetics but then the rate decreased to formation of glycerol (82). The originally reported kinetics (81) were reinvestigated (82–85) and differences were found. A shunt reaction originally proposed (81) as part of the forward reaction was shown to be unlikely, that second-order kinetics are not followed and that miscibility phenomena play a significant role (82–85). The addition of co-solvents such as tetrahydrofuran (THF) or methyl tert-butyl ether (MTBE) to the methanolysis reaction was reported to accelerate the methanolysis of vegetable oils as a result of solubilizing methanol in the oil and to a rate comparable to that of the faster butanolysis (83,85). Other possibilities for accelerating the transesterification appear to be microwave (86) and ultrasonic (87) irradiation. Factorial experiment design and surface response methodology have been applied to different production systems (88). A continuous pilot plant-scale process for producing methyl esters with conversion rates greater than 98% was reported (84) as well as a discontinuous two-stage process with a total methanol:acyl ratio of 4:3 (89). The kinetics of noncatalyzed alcoholysis of soybean oil were also investigated (90). The transesterification of beef tallow was studied with regard to effects of mixing (91), catalyst, free FA and water as well as solubilities of different alcohols in the fat (92). Water had the greatest undesirable effect (93). With increasing emphasis on utilizing low-cost sources of biodiesel, the question of the quality of these sources is of utmost importance. The low-cost sources such as restaurant greases and soapstock are of lower quality than refined vegetable oils. A major problem associated with them is the high content of free FA, which, as indicated above, the feedstock should contain a minimum of. Thus the processing of high-free FA feedstocks requires some changes to the overall production process. A two-step alkali-catalyzed transesterification was reported for high-free FA feedstocks (94). Pretreatment of the free FA by acid-catalyzed esterification prior to converting the triacylglycerols by alkali-catalyzed transesterification is an effective method for producing biodiesel from high-free FA feedstocks (95). A pilot plant based on this process was described (96). For the production of
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biodiesel from soapstock, all ester bonds were hydrolyzed by alkali catalysis and the resulting FA sodium salts converted to methyl esters by acid catalysis (97). This procedure was also taken to the small pilot scale (98). Relatedly, acid oil was converted to biodiesel (99). More aspects are given below. Other Transesterification Processes Besides the methods discussed here, other catalysts have been applied in transesterification reactions (100). Some recently studied variations of the above methods as applied to biodiesel preparation are briefly discussed here. Enzymatic transesterification methods are receiving attention for producing esters suitable as biodiesel. Advantages of enzymatic reactions can be specificity, mild reaction conditions, reduced product isolation problems and reduced waste (101), although they are more expensive. Lipases from Pseudomonas fluorescens as well as two immobilized enzymes from Mucor miehei and a Candida sp. with petroleum ether as solvent yielded methyl and ethyl esters of sunflower oil (102). The lipase from Mucor miehei was most efficient in yielding esters of primary alcohols while the lipase from Candida antarctica was most efficient for yielding branched esters from secondary alcohols (103). Some other reports on enzymatic production of esters mainly for fuel purposes include ethanolysis of sunflower oil with a solvent-free, immobilized 1,3-specific Mucor miehei lipase (104); a variety of enzymes used for producing different materials (105) with dependence on the presence of solvent (106) as well as stepwise addition of methanol (106,107); the synthesis of esters of restaurant greases (108–110); stepwise use of immobilized Candida antarctica lipase (111) modified later for continuous use (112), methyl acetate as an acyl acceptor (113); use of Rhizopus oryzae lipase in a water-containing system without an organic solvent (114); and in the methanolysis of vegetable oils contained in waste activated bleaching earth (115). Supercritical technology has also been employed for transesterification (116– 119). Alkylguanidines attached to modified polystyrene or siliceous MCM-41, encapsulated in the supercages of zeolite Y or entrapped in SiO2 sol-gel matrices were used as transesterification catalysts (120). Various alkaline-earth metal compounds such as calcium oxide, calcium methoxide and barium hydroxide were used as heterogeneous catalysts for producing rapeseed oil methyl esters (121) as was calcium carbonate (122). Diorganotin (IV) compounds were studied as catalysts for the methanolysis of tripalmitin (123). Methyl and ethyl esters of palm and coconut oils were produced by alcoholysis of raw or refined oils using boiler ashes, H2SO4 and KOH as catalysts (124). Fuel yields >90% were obtained using alcohols with low moisture content and EtOH-H2O azeotrope. One-step in situ processes in which the alcohol acts as extraction solvent for the oil-containing material and as esterifying reagent have been reported. Sunflower seed oils were transesterified in situ using macerated seeds with methanol in the pres-
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ence of H2SO4 (125). Higher yields were reported than from transesterification of the extracted oils although seed moisture reduced the yield of methyl esters. The CP of the in situ prepared esters appeared slightly lower than those prepared by conventional methods. In a related study, best yields were achieved with a 300:1 molar ratio methanol: oil (126). Similarly, macerated soybeans were treated with methanol, ethanol, n-propanol and n-butanol to give the corresponding esters (127), although due to the insolubility of soybean oil in methanol, conversion was low in that case. The synthesis of methyl or ethyl esters with 90% yield by reacting palm and coconut oil from the press cake and oil mill and refinery waste with methanol or ethanol in the presence of easily available catalysts such as ashes of the waste of these two oilseeds (fibers, shell, and husk), lime, zeolites, etc., was reported (128). Similarly, the methanolysis of vegetable oils was catalyzed by ashes from the combustion of plant wastes such as coconut shells or fibers of a palm tree that contain K2CO3 or Na2CO3 as catalyst (129). Thus the methanolysis of palm oil by refluxing 2 h with MeOH in the presence of coconut shell ash gave 96–98% methyl esters containing only 0.8–1.0% soap. The ethanolysis of vegetable oils over the readily accessible ash catalysts gave lower yields and less pure esters than the methanolysis. Several catalysts (CaO, K2CO3, Na2CO3, Fe2O3, NaOMe, NaAlO2, Zn, Cu, Sn, Pb, ZnO, and an anion exchange resin) were tested for catalytic activity in the reaction of low-erucic rapeseed oil with MeOH (130). The best catalyst was CaO on MgO. At 200°C and 68 atm, the anion exchange resin produced substantial amounts of fatty methyl esters and straight-chain hydrocarbons. The transesterification reaction is also the subject of numerous patents. A transesterification process permitting the recovery of all by-products such as glycerol and FA has been described (131). Some patent procedures were briefly reviewed (132). The procedures used in the proprietary literature generally resemble those published in journals. Analysis of Transesterification Products Potential contaminants of biodiesel include unreacted triacylglycerides; residual alcohol and catalyst may be present as well as intermediate mono- and diacylglycerides and glycerol co-product. Various methods have been investigated for analyzing biodiesel accordingly (133). Glycerol mixtures were analyzed by TLC-FID (thin-layer chromatography/ flame ionization detection) (134), which was also used in the studies on the variables affecting the yields of fatty esters from transesterified vegetable oils (76). The TLC- FID method has been abandoned (134–135). Gas chromatography (GC) is the most commonly used method for detailed analysis of transesterification and biodiesel. Analysis of reaction mixtures by capillary GC determining esters, mono-, di- and triacylglycerols was carried out in one run (136). Free glycerol was deter-
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mined in transesterified vegetable oils (137–138). Besides analyzing esters for sterols (139–141), which are often minor components in vegetable oils, and different glycerides (142–143), the previous GC method (136) was extended to include analysis of glycerol (144). In both papers (136,144), the hydroxy groups were derivatized by silylation with N-methyl-N-trimethylsilyltrifluoroacetamide. The GC method simultaneously analyzing for glycerol, methyl esters, and the various acylglycerols (144) forms the basis of standards such as ASTM D6584 and EN 14105. Simultaneous analysis of methanol and glycerol was also described (145) as was a sole determination of methanol (146). Flame-ionization detectors (FID) were usually employed although use of a mass selective detector was reported (138,145). Other authors, using GC to determine the conversion of TG to methyl esters, gave a correlation between the bound glycerol content determined by TLC/FID and the acyl conversion determined by GC (135). Glycerol was also analyzed by high-performance liquid chromatography (HPLC) using pulsed amperometric detection, which offers the advantage of higher sensitivity compared to refractometry and being suitable for detection of small amounts for which GC may not be suitable (147). An enzymatic method for glycerol analysis was reported (148). An enyzme-based analysis method for glycerol is now commercially available (149). Several other studies have been performed using HPLC analyses. The first HPLC-related report (150) used density detection to analyze for the various glycerides and methyl esters as classes of compounds in order to determine the conversion of transesterification reactions. An evaporative light scattering detector (ELSD) was employed for analyzing the product, glycerides, and free FA from an enzymatic transesterification (151). Atmospheric pressure chemical ionization mass spectrometry was more suitable as a detection method for reversed-phase HPLC than ELSD and UV (152). Several LC-GC methods were also reported including methods for analyzing for sterols (140,141,153). High-performance size exclusion chromatography was applied in the ethanolysis of rapeseed oil (154) or methanoloysis of palm oil (155). Other analyses include the determination of biodiesel in conventional DF by silica cartridge chromatography (156). Spectroscopic methods applied to the analysis of transesterification and biodiesel are nuclear magnetic resonance (NMR) and near-infrared (NIR) spectroscopies. In 1H-NMR spectrometry, the protons of the methylene group adjacent to the ester moiety in triacylglycerols and the protons in the alcohol moiety of the methyl esters were used to monitor the methanolysis of rapeseed oil (157). For determining conversion and kinetics by 13C-NMR, the unchanging signal of the terminal CH3 groups as well as the signals of the glyceridic moieties in the triacylglycerols (158) were used. NIR peaks at 4428 cm–1 and especially 6005 cm–1 which distinguish methyl esters and triacylglycerols can be used for monitoring transesterification and assessing biodiesel fuel quality by an inductive method (159). The use of a fiber-optic probe for acquiring the spectra renders the method rapid and easy. NIR and 1H-NMR results can be correlated (160).
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Viscosity was used for determining the methyl ester content based of a transesterification mixture (161). The results agreed well with GC. The method is reportedly more rapid than GC and therefore especially suitable for process control. IR spectroscopy (carbonyl absorption at 1750 cm–1) with (162) and without (163) a fiber-optic probe was used to assess the amount of biodiesel in lubricating oil. The problem is important because biodiesel can cause dilution of lubricating oil, ultimately resulting in engine failure. With the increased use of biodiesel blends, the determination of blend levels of biodiesel has recently received more attention. Methods that have been employed include NIR and NMR spectroscopy (164), ester number (165), IR spectroscopy (165) and silica cartridge chromatography (166). A European standard, EN 14078, utilizing the carbonyl absorption in the IR range is being developed. An interesting problem involves the in-vehicle detection of biodiesel blends in order to adjust relevant engine functions to the blend level. For this purpose, sensors have been developed (167,168). Emissions from Ester Fuels (Biodiesel) Generally, most exhaust emissions observed when using conventional DF are reduced when using biodiesel. NOx emissions, however, are an exception. In an early paper reporting emissions with methyl and ethyl soyate as fuel (45), it was found that CO and hydrocarbons were reduced but NOx emissions were produced consistently at a higher level than with the conventional DF. The differences in exhaust gas temperatures corresponded with the differences in NOx levels. Similar results were obtained from a study on the emissions of rapeseed oil methyl ester (169). NOx emissions were slightly increased, while hydrocarbon, CO, particulate and PAH emissions were in ranges similar to the DF reference. Alkyl esters emitted less aldehydes than the corresponding neat rapeseed oil. Unrefined rapeseed oil methyl ester emitted slightly more aldehydes than the refined ester, while the opposite held for PAH emissions. A 31% increase in aldehyde and ketone emissions was reported when using rapeseed oil methyl ester as fuel, mainly due to increased acrolein and formaldehyde, while hydrocarbons and PAH were significantly reduced, NOx increased slightly, and CO was nearly unchanged (170). Another study on agricultural tractors found that aldehydes increased by 20% with CO and NOx similar to those from conventional DF (171). The general trend on reduced emissions except NOx was confirmed by later studies (172), although some studies report little changes in NOx (173,174). Little differences compared to conventional DF were also reported for formaldehyde emissions when using soy methyl ester (174). It was suggested that the average slight rise in NOx exhaust emissions when using biodiesel can be at least partially traced to isentropic bulk modulus and speed of sound of fatty esters, which are higher than for conventional DF (175). This means that the fuel is less compressible (due to the larger mole-
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cules) which in turn causes early injection timing leading to higher combustion temperatures and pressures responsible for the rise in NOx exhaust emissions. High temperatures are known to increase the formation of NOx in fuel combustion (60). However, the structure of the FA chains was found to have a significant impact on emissions. Increasing the number of double bonds increased NOx exhaust emissions (176). This could not be explained by the known trade-off between NOx and particulate emissions, also indicating that NOx formation when using biodiesel is not driven thermally. Particulate emissions were essentially constant if CN >45 and density <0.89 (176). NOx exhaust emissons can be reduced by retarded injection timing (177) and comparing at the same start of combustion led to lower NOx exhaust emissions than from conventional DF. Various compounds such as alkyl nitrates and peroxides are used as cetaneenhancing additives in conventional DF (27). NOx exhaust emissions reportedly are lowered by increasing the CN of conventional DF (178). Thus, an approach utilizing cetane improvers with biodiesel to lower NOx exhaust emissions appears feasible. It was reported (179) that in a turbulence combustion chamber and at an intake air temperature of 105°C, 8% hexyl nitrate in vegetable oils (cottonseed, rape, palm) was necessary to exhibit the same ignition delay as conventional DF. Recently, some oxygenated compounds were identified as cetane improvers for fatty esters (180) which apparently had selective effects on fatty compounds depending on the kind of alkyl ester and nature of the FA chain, This may offer the possibility of tailoring the cetane improver to the predominant fatty compound in a specific biodiesel fuel. In a comparative study (181) of exhaust emissions at various injection timings of conventional DF, B20, B20 with a peroxide cetane improver and other DF, it was observed that retarded timing reduced NOx and increased particulate matter for all settings. The fuel blend with cetane improver had the lowest PM emissions of all fuels. NOx emissions for the additized blend were slightly lower than for the unadditized blend. Furthermore, cetane improvers derived from fatty compounds have been reported (182–185). A study on PAH emissions (186), in which the influence of various engine parameters was also explored, found that these emissions from sunflower ethyl ester were situated between DF and the corresponding neat vegetable oil. Reduced PAH emissions may correlate with the reduced carcinogenicity of particulates when using rapeseed methyl ester as fuel (187). With the same ester fuel in DI engines, particulate matter showed large amounts of volatile and extractable compounds adsorbed on the soot, which caused the PM emissions to be higher than with conventional DF, although the soot itself was reduced (171). While total particulate matter was lower for methyl soyate than for conventional DF, an oxidation catalytic converter (OCC) reduced those emissions by 50–80% (188). Greater reductions in mutagenicity were observed for the biodiesel fuel when using the OCC. Other authors (189) reported similar results when using a catalytic converter. However, conflicting results were obtained when using a ceramic trap. These and additional authors (190) also found that exhaust emissions from biodiesel fuel,
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both rapeseed and soybean oil methyl esters, had reduced environmental impact and mutagenicity compared to conventional DF. Emissions from low-sulfur conventional DF was also reported to be in the range of those from biodiesel, thus high sulfur content as well as high engine speeds and loads were associated with increased mutagenicity of diesel exhaust particles (190). In another study (191), the emitted mass of PM from rapeseed methyl ester fuel was higher compared to that from conventional DF but mutagenicity again was reduced. In the U.S., biodiesel is the only alternative fuel that has completed Tier 2 Health Effects Testing required by the Clean Air Act Amendments of 1990 (192). In a DI engine, sunflower methyl ester produced equal hydrocarbon emissions but less smoke than a 75:25 blend of sunflower oil with DF (193). A diesel oxidation catalyst (DOC) in conjunction with soy methyl ester was reported to be a possible emissions reduction technology for underground mines (194). Soy methyl esters reportedly were more sensitive toward changes in engine parameters than conventional DF (195). Precombustion phase studies of methyl, ethyl, n-propyl, and n-butyl fatty esters in a reactor simulating conditions in a diesel engine showed that various species such as branched and straight-chain alkanes, alkenes, cyclic hydrocarbons, aldehydes, ketones, esters, substituted benzenes as well as other compounds can arise at this stage (196). Aromatic compounds, which possess low CN, were observed more frequently for unsaturated fatty compounds. This observation may constitute a possible partial explanation for the differing CN of fatty compounds and may correlate with emissions studies.
Cold Flow Properties All DF are susceptible to start-up and operability problems when subjected to cold temperatures. As ambient temperature decreases below saturation temperature(s) of high-molecular weight paraffins (C18–C30 n-alkanes) present in conventional DF, these paraffins nucleate to form crystals suspended in a liquid phase composed of shorter-chain n-alkanes and aromatics (197–201). Leaving the fuel unattended in cold temperatures overnight can cause start-up and operability problems the next morning. The tendency of DF to solidify at low temperatures may be quantified by the following parameters: CP, PP, cold filter plugging point (CFPP) and low temperature flow test (LTFT). The CP is defined as the temperature where crystalline growth becomes visible in the form of a hazy or cloudy suspension of small solid crystals ~0.5 µm in diameter (197,198,202–204). As temperatures decrease below the CP larger crystals interlock and form agglomerates that restrict or cut off flow through fuel lines and filters (197,198,200,201,204–209). The temperature where sufficient agglomeration prevents free pouring of fluid is determined by measuring PP (199,202–204). In terms of predicting tendency of crystals to cause start-up or operability problems after cooling overnight, the CP tends to be over-cautious and
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the PP tends to be over-optimistic (197,199,205). Therefore, low temperature filterability tests (CFPP and LTFT) were developed based on results from cold weather operability field studies (197,199,201,205,207–210). Both parameters are measured by cooling a sample under controlled conditions then drawing a specified volume through a wire mesh filter screen within 60 sec. Conditions for measuring CFPP are volume = 20 mL, vacuum force = 0.0194 atm and mesh size = 45 mm. Corresponding conditions for LTFT are 180 mL, 0.197 atm and 17 µm (197, 199,202,203,211). The CFPP serves as the standard low temperature operability parameter for most of the world while the more stringent and less user-friendly LTFT is the standard for North America. Thermal analytical methods such as sub-ambient differential scanning calorimetry (DSC) have also been applied in analysis of conventional DF (206,212–216). DSC has advantages including relatively small sample sizes (<20 mg) and applicability to samples that are solid at room temperature. Most importantly, cold flow properties may be directly correlated to crystallization onset temperature (TCO) and mp. Biodiesel made from feedstocks containing larger amounts of high-mp longchain saturated FA tends to have relatively poor cold flow properties (see Tables 4.1 and 4.2). For example, tallow contains 23.7–27.6 wt% palmitic acid (C16) and 18.4–25.0 % stearic acid (C18) (217,218), compounds whose corresponding methyl esters have mp = 30 and 39.1°C, respectively (219). As a consequence, tallow methyl esters have a relatively high CP (15–17°C) (220). In contrast, feedstocks with relatively low concentrations of saturated long-chain FA generally yield biodiesel with much lower CP and PP. Thus, linseed, olive, rapeseed, safflower and soybean oils are examples whose biodiesel product has CP near or below 0°C (47,80,221–223). Nevertheless, comparison with No. 2 conventional DF (CP = –16°C; PP = –27°C (224) suggests that biodiesel from most common feedstocks will have less reliable operability during cold weather. Previous studies (224–226) reported a nearly linear correlation between lowtemperature filterability (CFPP and LTFT) and CP of soybean oil methyl esters and its blends with conventional DF. Results showed that a 1°C decrease in CP reduced CFPP or LTFT by 1°C (224). This work concluded that efforts to improve cold flow properties of biodiesel should emphasize development of approaches that significantly decrease CP. This conclusion was applicable to neat biodiesel and its blends with conventional DF and was later reported to apply to blends treated with commercial DF cold flow improver additives (226). Analyses of methyl esters of soybean oil, tallow, admixtures thereof and winterized soybean oil methyl esters showed that CP, PP and other cold flow properties could be accurately correlated to parameters inferred from sub-ambient DSC analyses (227). The temperature of maximal heat flow for freezing peaks yielded the most accurate correlations with respect to CP, PP and CFPP; LTFT was most accurately correlated to freezing point. Onset temperature (TCO) yielded good correlations for predicting PP, CFPP and LTFT. Although parameters from analysis of
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cooling curves were found to be more reliable than those from heating curves, other studies (217,218,228) successfully correlated CP of the soybean oil, tallow and waste grease alkyl esters to results from analysis of DSC heating curves. Approaches investigated or under investigation to address improvement of cold flow properties of biodiesel include the following: blending with conventional DF, cold flow improver additives, branched-chain esters and fractionation/winterization. Blending of esters is the simplest method for improving cold flow operability and will be discussed in the next section. Studies of commercial DF cold flow improver additives as modifiers for biodiesel revealed these additives were very effective in reducing PP and CFPP (225,226,229–232). These additives depressed PP by up to 18°C for 20 vol% blends of soybean oil methyl esters in No. 2 conventional DF (226). Increasing additive loading (concentration) resulted in nearly linear reductions in PP, though some additives were more effective than others (225,226). These results demonstrated that additives may be used to ease biodiesel pumping operations during cooler weather. Some additives depressed CFPP, demonstrating they had greater selectivity for modifying wax crystallization of biodiesel than simple PP depressants (197,199–201,207,208,233,234). None of the additives demonstrated sufficient selectivity to significantly affect nucleation in neat or blended biodiesel, resulting in little effect on CP or LTFT (224–226). Thus, additives designed primarily to modify wax crystallization in conventional DF also demonstrated some degree of selectivity for modifying crystal nucleation mechanisms prevalent in biodiesel. The use of fatty compound-derived materials with bulky moieties in the chain at additive levels is another approach (235). The background is that the bulky moieties would destroy the harmony of the solids which are usually oriented in one direction. However, these materials had only slight influence on the CP and PP. Efforts to employ glycerol yielded as co-product from biodiesel production in synthesis of agents that effectively improve cold flow properties of biodiesel were also studied (236). Glycerol ether derivatives from reaction with isobutylene or isoamylene in the presence of strong acid catalyst were shown to improve cold flow properties of in blends with biodiesel. Transesterification of oils or fats with medium chain-length (C3–C8) or branchedchain alkyl alcohols is known to improve the cold flow properties of biodiesel. Large or bulky headgroups disrupt spacing between individual molecules in the crystal lamellae causing rotational disorder in the hydrocarbon tailgroup chains, resulting in formation of crystal nuclei with less stable chain packing. Transition to a more stable form eventuates at lower temperatures (237). Comparison of data in Table 4.1 illustrates many examples of the effects of chain-length and branching in the alkyl moiety of the ester “headgroup” on the melting properties of esters. Consistent with the first part of this comparison, biodiesel made from canola, linseed and soybean oil shows decreases in both CP and PP with increasing alkyl chain-length from methyl to n-butyl (220,222,224,238). Similar
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decreases in CFPP and LTFT for tallow alkyl esters were also reported (220). Consistent with the second part of the comparison of data in Table 4.1, biodiesel made from soybean oil showed decreases in TCO (measured by DSC) of 7–11°C for isopropyl esters and 12–14°C for 2-butyl esters relative to methyl esters (218). Other studies (224,239) showed significant decreases in CP and PP for branched-chain alkyl esters of soybean oil. Although decreases in CP and PP were reported for isopropyl and isobutyl esters of tallow, CFPP and LTFT only decreased slightly (220). Crystallization fractionation is the separation of components of fatty derivatives based on differences in MP (240–247). The physical nature of biodiesel suggests that this technique may be a useful for improving cold flow properties by reducing total concentration of high-mp saturated esters. Traditional crystallization fractionation features two process stages—nucleation and crystalline growth under carefully controlled conditions followed by separation of fractions by filtration or centrifugation. Cooling rate and agitation in the crystallizer must be controlled to reduce the rate of entrainment of liquid inside crystal agglomerates (240–245, 247,248). Efficient separation of solid from low-mp unsaturated esters (liquid) yields fractions with significantly altered physical properties (242–244). Dry fractionation, sometimes referred to as winterization, is the simplest and least costly technique for separating high- and low-mp fatty derivatives (240–244). Bench-scale dry fractionation studies on soybean oil methyl esters indicated that CP and PP may be decreased to –16°C, a temperature close to LTFT of No. 2 conventional DF (226,228). Although fractionation reduced total concentration of saturated methyl ester (C16 and C18) to <6 wt%, yields of liquid fractions were very low at only 25–30% relative to starting material. Cooling times were typically overnight (~16 h) and multiple crystallization steps were necessary to significantly impact CP. Similar fractionation of tallow methyl esters (IV = 41; CP = 11°C) resulted in yields of 60–65% liquid fraction characterized by IV = 60 and CP = –1°C (244). Fractionation of waste cooling oil methyl esters was reported to decrease CFPP by 2–4°C (249). Addition of commercial DF cold flow improver additives improved separation efficiency of to 80–87% liquid fraction but still required multiple crystallization steps to reduce CP to –10°C (250). Solvent fractionation was also applied to separation of alkyl esters. Soybean oil methyl esters have been successfully fractionated from acetone, isopropanol and hexane solvents. In isopropanol, one 5-h crystallization step at –15°C was necessary to reduce CP to –8°C (250). Liquid fraction yields were in the range improved to 77–86% (226,250). Other solvents that have been applied to partial separation of alkyl esters include methanol, Skellysolve B and ether (246). Trace contaminants of biodiesel as they could remain after refining and transesterification affected the cold flow properties of methyl soyate and blends thereof with No. 1 conventional DF (251). Mono- and diglycerides did not influence the PP of the esters, but the CP increased with increasing amounts of saturated monoor diglycerides. Even low concentrations of 0.1 wt% saturated mono- or diglycerides raised the CP. Monoolein did not affect CP and PP. Unsaponifiable material
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at concentrations as low as 3% raised the TCO, CP and PP of soybean oil methyl esters but had virtually no effect on a 20% blend in No. 1 conventional DF.
Storage Stability In terms of handling and storage safety, biodiesel compares favorably with conventional DF. The reason is the significantly higher flashpoint of vegetable oils and their methyl esters. However, due to the content of unsaturated fatty compounds, storage stability can be an issue when using biodiesel. The more conjugated or methylene-interrupted double bonds in a fatty molecule, the more susceptible the material is to oxidation and degradation. Generally, storage stability is defined by the relative resistance of liquid fuels to physical and chemical changes resulting from interaction with the environment (202). Stability takes into account interactions of olefins, dienes and nitrogen-, sulfur- and oxygen-containing compounds that can lead to sediment formation and changes in color that depend on type and quantity of unstable materials present. Cleanliness of the fuel with respect to the presence of water, particulate solids, fuel degradation products and microbial slimes can also influence stability (252). Degradation in fuel properties during long-term storage occurs primarily by the following mechanisms: (i) oxidation (autoxidation) from contact with air; (ii) thermal or thermal-oxidative decomposition from excess heat; (iii) hydrolysis from contact with water or moisture in tanks and fuel lines; and (iv) microbial contamination from the migration of dust particles or water droplets containing bacteria or fungi into the fuel (202,252). A book on oxidation of fatty compounds has been published (253). Numerous studies usually relating to the first two aforementioned mechanisms have been carried out regarding stability of biodiesel (254–272). Summarily, these studies have shown that storage stability of biodiesel is affected by factors such as presence of air, heat, traces of metal (including influence of metal storage containers), peroxides, light, structural features of the compounds themselves as well as degree of refining. High temperature, presence of light, presence of air and presence or metals, copper being especially effective (257), usually accelerate oxidation of fatty compounds and their ultimate degradation. Antioxidants such as tocopherols (which occur naturally in vegetable oils), TBHQ (tert-butyl hydroquinone), BHA (butylated hydroxyanisole), BHT (butylated hydroxytoluene), pyrogallol and propyl gallate were investigated and shown to often have beneficial effects of increasing storage stability. Methods that were used for investigating storage stability include peroxide value, acid value, viscometry, oxidative stability index (OSI) which is closely related to the Rancimat, and other procedures. Acid value and viscosity were shown to be especially appropriate methods (258). Methods such as ASTM D2274 for testing the oxidative stability of conventional DF were found to be inappropriate for biodiesel (265). Sometimes contradictory results are reported, also in regard to the effectiveness of specific additives affecting oxida-
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tion. They may be due to different reaction conditions and other parameters, including feedstock quality, employed by different researchers. An oxidative stability parameter has not been included in the ASTM standard D6751. Several properties in the European standards EN 14214 and EN 14213 are a result of concerns regarding oxidative stability. In the years 1998–2002, the BIOSTAB project in Europe involving research groups in several countries carried out related experiments. Some results were summarized in several publications (254–256,261). In the standards EN 14213 and EN 14214, a parameter on oxidative stability was added as a result. This parameter prescribes the use of the standard EN 14112 (Rancimat) for determining the oxidative stability. At 110°C, the minimum induction time for biodiesel as transportation fuel (EN 14214) is prescribed as 6 h and for heating purposes the time is 4 h. Other studies (257–258,260) evaluated oxidation induction periods by running OSI analyses in accordance with standard methods such as AOCS Cd 12b-92 modified for lower temperatures. Milder reaction conditions were necessary because many biodiesel samples contained high concentrations of more readily oxidizable polyunsaturated FA esters. Thermal analytical techniques such as thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) have been applied in the analysis of oxidation of petroleum-based and synthetic lubricants (273–279). These studies generally showed that DSC analysis under pressure (P-DSC) increases the total number of moles of oxygen present in the cell accelerating oxidation at lower temperatures. Cross (280) and Hassel (281) were among the first to apply DSC and P-DSC in analysis of edible fats and oils. Results from these studies together with later work (282) showed good correlations between isothermal induction periods measured by P-DSC (in minutes) and OSI (in hours or days). Perhaps the first study to examine oxidation of FA methyl esters was performed by Raemy et al. (283). Results from that study showed that increasing temperature or degree of unsaturation decreased induction period as measured by conventional DSC. Results showed a direct correlation with those from the Rancimat test, with respect to 6–12% variation. P-DSC analysis was also identified as an efficient means for screening the activity of antioxidants with respect to type and concentration in biodiesel (284). Although most of the DSC and P-DSC studies cited above featured isothermal analysis of oxidation induction period, non-isothermal (heat-ramping) thermal analyses may provide a more rapid means for evaluating resistance to oxidation. Several studies have reported on the use of non-isothermal DSC and P-DSC in analysis of fatty derivatives. With respect to biodiesel, Litwinienko and co-workers (285–287) studied oxidation kinetics of unsaturated C18 FA and their ethyl esters. Non-isothermal DSC and P-DSC analyses were applied in screening phenolic antioxidants in methyl soyate (259) and linolenic acid (288). Finally, induction period results showed a good correlation between non-isothermal P-DSC scans and those from analysis of OSI (259).
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Other parameters that can be seen as relating to oxidative stability include content of FA ≥3 double bonds limited to 1% in both EN 14213 or EN 14214. Contained only in EN 14214 and not in EN 14213 is a provision that linolenic acid be limited to a maximum of 12%. The IV is limited in EN 14213 to a maximum of 130 and in EN 14214 to a maximum of 120. However, structure indices termed “bis-allylic position equivalents” and “allylic position equivalents” probably better reflect the tendency of a fatty compound to oxidize than the iodine value (257, 289).
Blending of Esters The most common application of esters (usually methyl esters; “biodiesel”) of vegetable oil in the U.S. is blends of conventional DF with these esters. The most common ratio is 80% vol% conventional DF and 20% vegetable oil ester (also termed “B20,” indicating the 20% level of biodiesel). There have been numerous reports that significant emission reductions are achieved with these blends. Methyl soyate can also be blended with jet fuels JP-5 and JP-8 (290). Winterization or use of additives improves the low-temperature properties of those blends. No engine problems were reported in larger-scale tests with, for example, urban bus fleets running on B20. Fuel economy was comparable to DF2, with the consumption of biodiesel blend being only 2–5% higher than that of conventional DF. Another advantage of biodiesel blends is the simplicity of fuel preparation which only requires mixing of the components. Ester blends were reported to be stable. For example, a blend of 20 vol% peanut oil with 80% DF did not separate at room temperature over a period of 3 mon (291). A few examples from the literature demonstrate the suitability of blends of esters with conventional DF in terms of fuel properties. In transient emission tests on an IDI engine for mining applications (194), the soybean methyl ester used had a CN of 54.7, viscosity 3.05 mm2/s at 40°C, and a CP of –2°C. The DF2 used had CN 43.2, viscosity 2.37 mm2/s at 40°C and a CP of –21°C. A 70: 30 DF2: soybean methyl ester blend had CN 49.1, viscosity 2.84 mm2/s at 40°C, and a CP of –17°C. The blend had 4% less power and 4% higher fuel consumption than the DF2, while the neat esters had 9% less power and 13% higher fuel consumption than DF2. Emissions of CO and hydrocarbons as well as other materials were reduced. NOx emissions were not increased here, although higher NOx emissions have been reported for blends (DI engines) (169, 173). Irregularities compared to other ester blends were observed when using blends of the isopropyl ester of soybean oil with conventional DF (239). Deposits were formed on the injector tips. This was attributed to the isopropyl ester containing 5.2 mol% monoglyceride which was difficult to separate from the isopropyl ester. However, in another report (292), the isopropyl and methyl esters of soybean oil performed well in blends with conventional DF. CO and unburned hydrocarbon emissions were reduced as were particulates by 28% when using the isopropyl
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ester (25% reduction with the methyl ester); however, NOx emissions increased a maximum of 12% for a 1:1 isopropyl soyate/DF blend. Ester Fuels from Animal Fats and Waste Oils Animal Fats. The animal fat most commonly studied for potential biodiesel use is tallow. Tallow contains a high amount of saturated FA (Table 4.2) and its mp is above ambient temperature. Blends of tallow esters (methyl, ethyl, and butyl) with conventional DF were studied (293). Smoke emissions were reduced with the esters, particularly the butyl ester. Other features such as torque, power, and thermal efficiency did not deviate from conventional DF by more than 3% in any case. Specific fuel consumption was higher for the neat esters but only 1.8% higher for a 50:50 blend of butyl tallowate with conventional DF. A study on beef tallow and an inedible yellow grease both neat and a 1:1 (weight ratio) blend of tallow with DF in shortterm engine tests with DI and IDI engines was carried out (294). The deposits were softer than those formed with reference to cottonseed oil but still excessive. In a 200h EMA test the deposits caused ring sticking and cylinder wear. Thus, animal fats, like vegetable oils, are not suitable for long-term use unless modified. Other researchers blended methyl tallowate with 35 vol% ethanol to achieve the viscosity of conventional DF and the fuel properties were closely related to that of No. 2 DF (295). In an investigation of blends of DF2 with methyl tallowate and ethanol (296), an 80:13:7 blend of DF2:methyl tallowate:ethanol reduced emissions the most without a significant drop in engine power output. The same authors determined numerous physical properties of blends of DF with methyl tallowate, methyl soyate and ethanol and found them to be similar to the pertinent properties of DF2. Waste Vegetable Oils. The use of vegetable oils as frying oils produces significant amounts of used oils which may also present a disposal problem. A major incentive for the use of waste oils is their lower price. Acid catalyzed-processes for biodiesel production from waste oils is economically competitive with the alkalicatalyzed processes applied to virgin vegetable oils (297). Used vegetable oils usually contain some degradation products of vegetable oils and foreign material. A potential operational problem of biodiesel from waste oils is their less favorable cold flow properties due to higher amounts of saturated fatty compounds. The production of biodiesel from such sources is briefly discussed above. However, in analyses of used vegetable oils it was reported (298) that the differences between used and unused fats are not very great and in most cases simple heating and removal by filtration of solid particles suffices for subsequent transesterification. The CN of a used frying oil methyl ester was given as 49 (299), thus comparing well with other materials, but little demand could be covered by this source. Biodiesel in form of esters from waste cooking oils was tested and it was reported that emissions were favorable (300). Used canola oil (only purified by filtration) was blended with DF2 (301). Fuel property tests, engine performance tests and
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exhaust emission values gave promising results. Filtered frying oil was transesterified under both acidic and basic conditions with different alcohols (methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, and 2-ethoxyethanol) (301). The formation of methyl esters with base catalysis (KOH) gave the best yields. The methyl, ethyl, and 1-butyl esters obtained here performed well in short-term engine tests on a laboratory high-speed diesel engine. However, during heating of frying oil, chemical changes such as formation of polymers can occur. In a study on heated rapeseed (canola) oil as a source of biodiesel, heating caused the amount of polymers to increase up to 15% in rapeseed oil but only 5% in the methyl esters (302). Dimeric and trimeric triacylglycerols in the starting oil were mainly converted into monomeric and dimeric FA methyl esters. The monomeric and dimeric species negatively influenced fuel properties. Thus, after 6 h heating Conradson carbon residue and after 16 h viscosity exceeded biodiesel specifications. The amount of polymers in waste oil was taken as an indicator for its suitability in biodiesel production. Viscosity and Conradson carbon residue were taken as good indicators for the existence of higher dimer levels. Waste olive oil without any further derivatization was also studied in blends with conventional DF and these blends were found to be suitable as fuel (303). Used, underivatized sunflower oil was also studied (304). Methyl esters of waste olive oil were reported to be a suitable fuel (305). Waste palm oil was transesterified with ethanol to biodiesel (306) and tested in a water-cooled furnace (307). Waste oil was thermally cracked to obtain a fuel suitable for use in a diesel engine (308). As discussed above, the FA in soapstock, a by-product of vegetable oil refining consisting mainly of water, acylglycerols, phosphoglycerols and free FA with a total FA content of 25–30%, were also converted into biodiesel and the fuel found to be competitive with biodiesel from other sources (97,98).
Summary and Outlook The use of vegetable oil-based DF, particularly in the form of esters (“biodiesel”), will probably continue to increase. Numerous reports in the popular and lay scientific press discuss this topic and new biodiesel plants are constantly being constructed. Further refinement of existing standards is likely as well as development of standards in more countries around the world. Although vegetable oil-based fuels cannot replace all petroleum-based DF, they play an important role among the alternative fuels and contribute to the goal of energy independence and security. References 1. ASTM standard D6751 “Standard Specification for Biodiesel Fuel (B100) Blend Stock for Distillate Fuels,” American Society for Testing and Materials, West Conshohocken, PA, 2003. 2. Schwab, A.W., M.O. Bagby, and B. Freedman, Preparation and Properties of Diesel Fuels from Vegetable Oils, Fuel 66: 1372–1378 (1987).
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3. Knothe, G., Historical Perspectives on Vegetable Oil-Based Diesel Fuels, inform 12: 1103–1107 (2001). 4. Diesel, R., The Diesel Oil-Engine, Engineering 93: 395–406 (1912). 5. Diesel, R., The Diesel Oil-Engine and Its Industrial Importance Particularly for Great Britain, Proc. Inst. Mech. Eng., pp. 179–280 (1912). 6. Chavanne, C.G., Belgian Patent 422,877, Aug. 31, 1937. 7. van den Abeele, M., Palm Oil as Raw Material for the Production of a Heavy Motor Fuel, Bull. Agr. Congo Belge 33: 3–90 (1942). 8. Chavanne, G., Sur un Mode d’Utilisation Possible de l’Huile de Palme à la Fabrication d’un Carburant Lourd (A Method of Possible Utilization of Palm Oil for the Manufacture of a Heavy Fuel), Bull. Soc. Chim. 10: 52–58 (1943). 9. Bruwer, J.J.; B. van D. Boshoff, F.J.C. Hugo, L.M. du Plessis, J. Fuls, C. Hawkins, A.N. van der Walt, and A. Engelbrecht, Sunflower Seed Oil as an Extender for Diesel Fuel in Agricultural Tractors, paper presented at the 1980 Symposium, South African Institute of Agricultural Engineers. 10. Shay, E.G., Diesel Fuel from Vegetable Oils: Status and Opportunity, Biomass Bioenergy, 4: 227–242 (1993). 11. Encinar, J.M., J.F. González, E. Sabio, M.J. and Ramiro, Preparation and Properties of Biodiesel from Cynara cardunculus L. Oil, Ind. Eng. Chem. Res. 38: 2927–2931 (1999). 12. Ikwuagwu, O.E., I.C. Ononogbu, and O.U. Njoku, Production of Biodiesel Using Rubber [Hevea brasiliensis (Kunth. Muell.)] Seed Oil, Industr. Crops Prod. 12: 57–62 (2000). 13. De, B.K., and D.K. Bhattacharyya, Biodiesel from Minor Vegetable Oils like Karanja Oil and Nahor Oil, Fett/Lipid 101: 404–406 (1999). 14. Geller, D.P., J.W. Goodrum, and S.J. Knapp, Fuel Properties of Oil from Genetically Altered Cuphea viscosissima, Indust. Crops Prod. 9: 85–91 (1999). 15. Foidl, N., G. Foidl, M. Sanchez, M. Mittelbach, and S. Hackel. Jatropha curcas L. as a Source for the Production of Biofuel in Nicaragua, Bioresour. Technol. 58: 77–82 (1996). 16. Giannelos, P.N., F. Zannikos, S. Stournas, E. Lois, and G. Anastopoulos, Tobacco Seed Oil as Alternative Diesel Fuel: Physical and Chemical Properties, Industr. Crops. Prods. 16: 1–9 (2002). 17. Munavu, R.M., and D. Odhiambo, Physicochemical Characterization of Nonconventional Vegetable Oils for Fuel in Kenya, Kenya J. Sci. Technol. 5: 45–52 (1984). 18. Nagle, N., and P. Lemke, Production of Methyl Ester Fuel from Microalgae, Appl. Biochem. Biotechnol. 24–25: 355–361 (1990). 19. de Guzman, D., Global Glycerine Prices Pressured on Market Fundamentals, Chemical Market Reporter February 3, 2003 (Vol. 263, No. 5), p. 12. 20. Raneses, A.R., L.K. Glaser, J.M. Price, and J. A. Duffield, Potential Biodiesel Markets and Their Economic Effects on the Agricultural Sector of the United States, Industr. Crops Prods. 9: 151–162 (1999). 21. Knothe, G., Current Perspectives on Biodiesel, inform 13: 900–903 (2002). 22. de Guzman, D., European Biodiesel Capacities Surge on EU Directives, Chemical Market Reporter June 30, 2003 (Vol. 263, No. 5), p. 12. 23. Ahouissoussi, N.B.C., and Wetzstein, M.E., in Industrial Uses of Agricultural Materials, published by the Economic Research Service, U.S. Department of Agriculture, September 1995, p. 35–41.
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44. Bruwer, J.J., B. van D. Boshoff, F.J.C. Hugo, J. Fuls, C. Hawkins, A.N. van der Walt, A. Engelbrecht, and L.M. du Plessis, The Utilization of Sunflower Seed Oil as a Renewable Fuel for Diesel Engines, paper presented at the 1980 National Energy Symposium, American Society of Agricultural Engineers, p. 11. 45. Clark, S.J., L. Wagner, M.D. Schrock, and P.G. Piennaar, Methyl and Ethyl Soybean Esters as Renewable Fuels for Diesel Engines, J. Am. Oil Chem. Soc. 61: 1632–1638 (1984). 46. Ryan, T.W., III, L.G. Dodge, and T.J. Callahan, The Effects of Vegetable Oil Properties on Injection and Combustion in Two Different Diesel Engines, J. Am. Oil Chem. Soc. 61: 1610–1619 (1984). 47. Peterson, C.L., R.A. Korus, P.G. Mora, and J.P. Madsen, Fumigation with Propane and Transesterification Effects on Injector Coking with Vegetable Oil Fuels, Trans. ASAE 30: 28–35 (1987). 48. Siekmann, R.W., and G.H. Pischinger, Evaluation of Lubricating Oil Contaminated with Small Amounts of Soybean Oil Ester in Comparison with Normal Diesel Oil Operation, Proc. Vegetable Oil Diesel Fuel, Seminar III, Agricultural Reviews and Manuals, U.S. Department of Agriculture, Agricultural Research Service, Peoria, IL, pp. 163–168 (1983). 49. Van Gerpen, J.H., S. Soylu, and M.E. Tat, Evaluation of the Lubricity of Soybean OilBased Additives in Diesel Fuel, Proc. 1999 ASAE/CSAE-SCGR Annual Intern. Meeting, Paper No. 996134 (1999). 50. Zhang, X., C. Peterson, D. Reece, R. Haws, and G. Möller, Biodegradability of Biodiesel in the Aquatic Environment, Trans. ASAE 41: 1423–1430 (1998). 51. Bessee, G.B., and J.P. Fey, Compatibility of Elastomers and Metals in Biodiesel Fuel Blends, Soc. Automot. Eng., [Spec. Publ.], SP-1274, 221–232 (1997). 52. Knothe, G., Structure Indices in Fatty Acid Chemistry. How Relevant is the Iodine Value? J. Am. Oil Chem. Soc. 79: 847–854 (2002). . 53. Mittelbach, M., Analytical Aspects and Quality Criteria for Biodiesel Derived from Vegetable Oils, Liq. Fuels, Lubr. Addit. Biomass, Proc. Altern. Energy Conf. (Dale, B.E., ed.), American Society of Agricultural Engineers, St. Joseph, MI, pp. 151–156 (1994). 54. Andrews, A.S., and G.R. Quick, Fuel Substitution in Agriculture, Energy Agric. 3: 323–332 (1984). 55. Prankl, H., and M. Wörgetter, Influence of the Iodine Number of Biodiesel to the Engine Performance, Proc. Third Liquid Fuel Conference: Liquid Fuels and Industrial Products from Renewable Resources (J.S. Cundiff, E.E. Gavett, C. Hansen, C. Peterson, M.A. Sanderson, H. Shapouri, D.L. VanDyne, eds.), ASAE, St. Joseph, MI, pp. 191–196 (1996). 56. Lilly, L.R.C., Diesel Engine Reference Book, 1st edn, Butterworths, London, 1980. 57. Ricardo Consulting Engineers, Exhaust Smoke, Measurement and Regulation and Exhaust Emissions, in Diesel Engine Reference Book (B. Challen and R. Baranescu, eds.), 2nd edn, Society of Automotive Engineers, Warrendale, PA (1998). 58. Eskin, N.A.M., B.E. McDonald, R. Przybylski, L.J. Malcolmson, R. Scarth, T. Mag, K. Ward, and D. Adolph, Canola Oil, in Bailey’s Industrial Oil and Fat Products (Y.H. Hui, ed.), 5th edn, Vol. 5, Wiley-Interscience, New York, pp. 1–95, (1996). 59. Van Gerpen, J. and R. Reitz, Diesel Combustion and Fuels, in Diesel Engine Reference Book (B. Challen, and R. Baranescu, eds.), Society of Automotive Engineers, Warrendale, PA, pp. 89–104 (1998).
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60. Glassman, I., Combustion, 3rd edn, Academic Press, San Diego, 1996. 61. Cruz, J.M., A.S. Ogunlowo, W.J. Chancellor, and J.R. Goss, Vegetable Oils as Fuels for Diesel Engines, Resour. Conserv. 6: 69–74 (1981). 62. Bacon, D.M., F. Brear, I.D. Moncrieff, and K.L. Walker, The Use of Vegetable Oils in Straight and Modified Form as Diesel Engine Fuels, in Beyond the Energy Crisis (R.A. Fazzolare, and C.B. Smith, eds), Vol. 3, Pergamon Press, Oxford, England, pp. 1525–1533 (1981). 63. Bettis, B.L., C.L. Peterson, D.L. Auld, D.J. Driscoll, and E.D. Peterson, Fuel Characteristics of Vegetable Oil from Oilseed Crops in the Pacific Northwest, Agron. J. 74: 335–339 (1982). 64. Peterson, C.L., G.L. Wagner, and D.L. Auld, Winter Rape Oil Fuel for Diesel Engines: Recovery and Utilization, Trans. ASAE 26: 322–327, 332 (1983). 65. Pryor, R.W., M.A. Hanna, J.L. Schinstock, and L.L Bashford, Soybean Oil Fuel in a Small Diesel Engine, Trans. ASAE 26: 333–337 (1983). 66. Korus, R.A., T.L. Mousetis, and L. Lloyd, Polymerization of Vegetable Oils, ASAE Publ. 4–82 (Vegetable Oil Fuels): 218–223 (1982). 67. Darcey, C.L., W.A. LePori, C.M. Yarbrough, and C.R. Engler, Lubricating Oil Contamination from Plant Oil Fuels, Trans. ASAE, 26: 1626–1632 (1983). 68. Vellguth, G., Emissions When Using Alternative Fuels in Tractor Diesel Engines, Grundl. Landtechnik 32: 177–186 (1982). 69. Clevenger, M.D., M.O. Bagby, C.E. Goering, A.W. Schwab, and L.D. Savage, Developing an Accelerated Test of Coking Tendencies of Alternative Fuels, Trans. ASAE 31: 381–388 (1988). 70. Pestes, M.N., and J. Stanislao, Piston Ring Deposits When Using Vegetable Oil as a Fuel, J. Test. Eval. 12: 61–68 (1984). 71. Gutsche, B., Technology of Methyl Ester Production—Application for Biodiesel Production, Fett/Lipid 99: 418–427 (1997). 72. Schuchardt, U., R. Sercheli, and R.M. Vargas, Transesterification of Vegetable Oils: A Review, J. Braz. Chem. Soc. 9: 199–210 (1998). 73. Ma, F., and M. Hanna, Biodiesel Production: A Review, Bioresour. Technol. 70: 1–15 (1999). 74. H. Fukuda, A. Kondo, H. Noda, Biodiesel Fuel Production by Transesterification of Oils, J. Biosci. Bioeng. 92: 405–416 (2001). 75. Freedman, B., and E.H. Pryde, Fatty Esters from Vegetable Oils for Use as a Diesel Fuel, ASAE Publ. 4–82 (Veg. Oil Fuels): 117–122 (1982). 76. Freedman, B., E.H. Pryde, and T.L. Mounts, Variables Affecting the Yields of Fatty Esters from Transesterified Vegetable Oils, J. Am. Oil Chem. Soc. 61: 1638–1643 (1984). 77. Canakci, M., and J. Van Gerpen, Biodiesel Production via Acid Catalysis, Trans. ASAE 42: 1203–1210 (1999). 78. Mittelbach, M., M. Wörgetter, J. Pernkopf, and H. Junek, Diesel Fuel Derived from Vegetable Oils: Preparation and Use of Rape Oil Methyl Ester, Energy Agric. 2: 369–384 (1983). 79. Ahn, E., M. Koncar, M. Mittelbach, and R. Marr, A Low-Waste Process for the Production of Biodiesel, Sep. Sci. Technol. 30: 2021–2033 (1995). 80. Isigigür, A., F. Karaosmanoglu, and H.A. Aksoy, Methyl Ester from Safflower Seed Oil of Turkish Origin as a Biofuel for Diesel Engines, Appl. Biochem. Biotechnol. 45–46: 103–122 (1994).
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270. Bondioli, P., A. Gasparoli, A. Lanzani, E. Fedeli, S. Veronese, and M. Sala, Storage Stability of Biodiesel, J. Am. Oil Chem. Soc. 72: 699–702 (1995). 271. Du Plessis, L.M., J.B.M. de Villiers, and W.H. van der Walt, Stability Studies on Methyl and Ethyl Fatty Acid Esters of Sunflowerseed Oil, J. Am. Oil Chem. Soc. 62: 748–752 (1985). 272. Du Plessis, L.M., Plant Oils as Diesel Fuel Extenders: Stability Tests and Specifications on Different Grades of Sunflower Seed and Soyabean Oils, CHEMSA 8: 150–154 (1982) 273. Sharma, B.K., and A.J. Stipanovic, Development of a New Oxidation Stability Test Method for Lubricating Oils Using High-Pressure Differential Scanning Calorimetry, Thermochim. Acta 402: 1–18 (2003). 274. Gamelin, C. D., N. K. Dutta, N. Roy Choudhury, D. Kehoe, and J. Matisons, Evaluation of Kinetic Parameters of Thermal and Oxidative Decomposition of Base Oils by Conventional, Isothermal and Modulated TGA, and Pressure DSC, Thermochim. Acta 392–393: 357–369 (2002). 275. Yao, J., Evaluation of Sodium Acetylacetonate as a Synergist for Arylamine Antioxidants in Synthetic Lubricants, Tribology Int. 30: 795–799 (1997). 276. Patterson, G.H., and A.T. Riga, Factors Affecting Oxidation Properties in Differential Scanning Calorimetric Studies, Thermochim. Acta 226: 201–210 (1993). 277. Kaufman, R.E., and W.E. Rhine, Development of a Remaining Useful Life of a Lubricant Evaluation Technique, Part I: Differential Scanning Calorimetric Technique, Lubr. Eng. 44: 154–161 (1988). 278. Noel, F., Thermal Analysis of Lubrication Oils, Thermochim. Acta 4: 377–392 (1972). 279. Zeman, A., A. Sprengel, D. Niedermeier, and M. Späth, Biodegradable Lubricants– Studies on Thermo-Oxidation of Metal-Working and Hydraulic Fluids by Differential Scanning Calorimetry (DSC), Thermochim. Acta 268: 9–15 (1995). 280. Cross, C.K., Oil Stability: A DSC Alternative for the Active Oxygen Method, J. Am. Oil Chem. Soc. 47: 229–230 (1970). 281. Hassel, R.L., Thermal Analysis: An Alternative Method of Measuring Oil Stability, J. Am. Oil Chem. Soc. 53: 179–181 (1976). 282. Tan, C.P., Y.B. Che Man, J. Selamat, and M.S.A. Yusoff, Comparative Studies of Oxidative Stability of Edible Oils by Differential Scanning Calorimetry and Oxidative Stability Index Methods, Food Chem. 76: 385–389 (2002). 283. Raemy, A., I. Froelicher, and J. Loeliger, Oxidation of Lipids Studied by Isothermal Heat Flux Calorimetry, Thermochim. Acta 114: 159–164 (1987). 284. Stavinoha, L.L., and K.S. Kline, in Report, Oxidation Stability of Methyl Soyates– Modified ASTM D 5304 and D 6186 for Biodiesel B100, U.S. Army, TACOM, TARDEC, National Automotive Center, Warren, MI (2001). 285. Litwinienko, G., A. Daniluk, and T. Kasprzyska-Guttman, Study on Autoxidation Kinetics of Fats by Differential Scanning Calorimetry, 1. Saturated C12–C18 Fatty Acids and Their Esters, Ind. Eng. Chem. Res. 39: 7–12 (2000a). 286. Litwinienko, G., and T. Kasprzyska-Guttman, Study on the Autoxidation Kinetics of Fat Components by Differential Scanning Calorimetry, 2. Unsaturated Fatty Acids and Their Esters, Ind. Eng. Chem. Res. 39: 13–17 (2000b). 287. Litwinienko, G., A. Daniluk, and T. Kasprzyska-Guttman, A Differential Scanning Calorimetry Study on the Oxidation of C12–C18 Saturated Fatty Acids and Their Esters, J. Am. Oil Chem. Soc. 76: 655–657 (1999).
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Chapter 5
Biofuels for Home Heating Oils Bernard Y. Tao Purdue University, Department of Agricultural and Biological Engineering, West Lafayette, IN 47907
Introduction Petroleum-based liquid home heating oil is used to heat over eight million homes in the U.S., predominantly in the northeastern U.S. (1). This comprises ~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. This chapter presents a background on home heating systems and highlights recent research to develop renewable biofuels for home heating applications.
Home Heating Technologies History of Home Heating Historically, interior home heating has progressed from open wood fireplaces to modern enclosed high-efficiency furnaces. Centralized heating systems were initially developed during the early 19th century along with the demand for larger buildings and residences. While wood was the initial fuel used in fireplaces, coal was often preferred due to its higher energy density. As population density in urban settings increased, the environmental impacts of these heating systems became increasingly evident in the form of smoke, soot, and smog, with accompanying health issues. With the growth of the petroleum industry in the early 1900s, liquid fuels replaced solid fuels due to their ease of transportation and simplicity of use. In the mid-1900s, gaseous fuels, such as natural gas, became the fuel of choice, due to its ease of use and clean-burning properties. With the growth of electrical transmission systems, electrical heating/heat pumps have also become popular in the last four decades. Accompanying these changes in fuel technology were improvements in environmental emissions, such as reduced smoke/soot and sulfur oxides. Looking at the U.S., the geographical distribution of home heating systems follows this historical development of fuels technology. The northeast coast has 90
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Fig. 5.1. Residential heating fuel types (1).
the largest concentration of liquid fuel home heating systems, with natural gas and electrical systems becoming the majority in the central and western U.S. Figure 5.1 shows natural gas and electrical heat residences made up 55 and 30%, respectively, of the total heated households in the U.S., while fuel oil residences made up ~8%. The geographical distribution of annual sales of heating fuel are shown below in Figure 5.2. Since the 1970s, the heating oil industry has dropped from 20% of American households to less than 10%. Currently, most new housing is heated by natural gas or electrical energy. However, with recent record high prices in natural gas, the oil heat industry is eager to regain heating fuel markets. The National Oilheat Research Alliance (NORA) was created to address this concern by supporting new oil heat technologies and fuels as well as improving the public perception of oil heat (2).
Fig. 5.2. Sales of residential heating oil by location.
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With the long-term prospect of diminishing petroleum resources, sharply increasing prices, and environmental regulation, the oil heat industry is focusing on developing alternative fuels. Biofuels are particularly attractive due to their renewable nature, environmental benefits, and chemical similarity to petrochemical hydrocarbons. Additionally, there are potential political benefits based on domestic production and partnering with agriculture, which include reduction of national trade deficits, energy security, and domestic job creation. Fuel Oil Furnace Technology A typical home heating furnace is presented in Figure 5.3. Fuel is pumped into the combustion chamber where it is combined with external air and burned. A heat exchanger is used to transfer heat to a separate external air-flow to produce warm air for home heating. Due to low evaporative volatility, home heating oil furnaces require fuel to be atomized by pumping through nozzles prior to combustion. Atomization is the process of generating droplets from a liquid. The process is controlled by several factors including physical properties of the liquid, operating conditions and atomizer geometry. Nozzle configuration can have a significant effect on both combus-
Fig. 5.3. Typical residential heating furnace.
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tion and formation of nitrogen oxides. Liquid viscosity is the primary fuel factor in spray formation and hence combustion. Standard pump pressures are normally around 100 psi and produce nearly 55 billion drops/gal of fuel oil droplets which range in size from 0.002 in (5.1 µm) to 0.010 in (254 µm) (3). The efficiency of heat recovery is a critical performance factor. Therefore, controlling combustion air-flow is a key performance parameter. High life-cycle improves fuel combustion and may improve emissions, but results in loss of energy into the exhaust gases. Low air-flow may result in incomplete combustion and increased emissions. Ideally, hydrocarbon fuels composed of carbon, hydrogen, and oxygen would completely combust with oxygen to produce only water and carbon dioxide. In reality, some carbon monoxide forms, even under high-excess oxygen conditions due to incomplete combustion. Along with incomplete combustion, the other main emissions of concern are sulfur/nitrogen oxides. Sulfur oxides (SOx) are considered environmentally undesirable due to the formation of atmospheric sulfuric acid when combined with rainwater. Nitrogen oxides (NOx) function as a catalyst in the chain reaction formation of ozone and carbonyl groups in the upper atmosphere (4), creating smog. Natural sources of petroleum contain varying amounts of sulfur. This can be adjusted by refining; current laws/regulations exist to regulate SOx emissions. In general, SOx emissions are essentially quantitatively related to fuel sulfur content (5). Due to the presence of high amounts of nitrogen in the air used in combustion, NOx are also formed. Many major cities, such as Los Angeles and Texas, have restrictions on NOx emissions from sources including residential size boilers and furnaces (6). Eastern states have not yet restricted these emissions from residential boilers and furnaces, but that may change in the future. Since high temperatures are believed to be the primary catalyst in NOx formation (4), increased NOx formation occurs at higher combustion temperatures and in the presence of larger excesses of oxygen. To obtain complete combustion, excess air-flow is used (see Fig. 5.4). Ideally, the amount of air used minimizes carbon monoxide formation/maximizes carbon oxidation while trying to minimize the loss of energy in the outlet gases. However, maximizing the furnace temperature to provide high heat transfer in the presence of excess air produces increased levels of NOx. Therefore, furnaces must be carefully optimized to balance heating performance, while minimizing emissions by controlling air-flow, fuel combustion, and heating temperature. Fuel Oil Chemical Composition Petroleum hydrocarbons are composed of ~86% carbon and 14% hydrogen. Small amounts of sulfur, nitrogen, and oxygen may also be present depending on the source of extraction. Transportation fuel (gasoline, diesel, aviation, etc.) is the major use of petroleum in the U.S. This fuel requires significant refining to produce the highly refined hydrocarbon compositions needed by modern engine tech-
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Lack of air
Excess air i d
Flue gas loss
Flue gas components
e a Fuel/air → mixture
l
Carbon dioxide (CO2)
R a n g
Oxygen (O2)
e
Carbon monoxide (CO)
λ=1
Excess air
Fig. 5.4. Combustion exhaust gas properties (7).
nologies. Since the fuel oil combustion in furnaces is less stringent, fuel oil is less refined and contains higher molecular weight components (8). Typical carbon number range for #2 fuel oil is from C11 to C20 and may contain higher percentages aromatic hydrocarbons relative to kerosene or diesel fuel (9). Based on cold weather temperatures, different blends with lower freezing points may be created, for example heating fuels #1, #2, and #4 (10). Similar to automotive and industrial petroleum fuel use, environmental emissions are important for home heating fuels. The sulfur content is an important environmental and economic factor in petroleum heating oil. In 1991, the environmental protection agency (EPA) regulated the sulfur content of on-highway diesel vehicles to 0.05% or 500 ppm. Prior to that time, heating oil and on-highway diesel both shared the maximum sulfur content value of 0.5% or 5,000 ppm. In an attempt to simplify fuel distribution and improve emissions, the NORA board mandated that 80% of the heating oil consumed not contain more then 500 ppm by 2007 (5). This was supported by the recent ASTM adoption of specification for low sulfur (LS) #2 heating oil. Research into ultra-low sulfur (ULS) with less then 15 ppm sulfur heating oil has recently been conducted and approaches nitrogen free status (6). Additional environmental issues involved in fuel combustion technologies are the formation of nitrogen oxides, related to atmospheric smog formation, and particulate emissions (soot/smoke).
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Biofuels Biologically derived fuels from plant oils have four major benefits versus petroleum or natural gas. First, they are environmentally renewable. The main product of hydrocarbon combustion is carbon dioxide, which is believed to be related to global warming. Since growing plants actively recycle carbon dioxide to build oils and lipids, combusted biofuels are ecologically recycled, resulting in zero net carbon dioxide production. Second, plant oils from crops such as soybeans, corn, canola, etc., are domestically produced versus petroleum from foreign sources. This is a significant issue in today’s U.S. environment where the concern for homeland security is paramount. Third, the chemical structure of plant oils is chemically and physically similar to petroleum hydrocarbons. They are composed of triacylglycerols, whose long-chain fatty acid moieties are essentially hydrocarbons, hence the similarity in the common name “oil.” Finally, the current economics of plant oils are similar or slightly favorable versus petroleum and this is anticipated to become increasingly beneficial in the long-term future due to diminishing petroleum availability. With respect to liquid biofuels, there are currently two plant oil-based fuels being studied, degummed oil (triacylglycerides) and biodiesel fuel (monoalkyl esters). Degummed Vegetable Oil. Vegetable oil from common domestic sources, such as soybean oil, is a triacylglyceride composed of a glycerol molecule onto which three long-chain fatty acids are attached (see Fig. 5.5). The fatty acid chains are essentially linear hydrocarbons, some containing one or more double bonds, and may be different chain lengths at each glycerol position. Soybean oil is extracted from soybean seeds and purified through a series of processing steps, which includes degumming. Degumming is the process of removing phospholipids, such as lecithin, and is the initial processing step in the production of food-grade vegetable oils (Fig. 5.6). Degummed oil is an intermediate product in the production of food-grade vegetable oil and is approx. $0.05 to $0.10/gal cheaper than fully refined oil due to less processing. Approximately 2.8 billion gallons of soybean oil are produced domestically in the U.S. annually, with world production at ~10 billion gallons annually. Recent prices for food-grade soybean oil are in the range of $0.20–$0.25/lb or ~$1.50/gal. Mono-Alkyl Esters/Biodiesel. Alcohol transesterification of triacylglycerides chemically rearranges fats into glycerin and alkyl esters. The most common ester HO-CH2 | HO-CH | HO-CH2
CH3(CH2)7CH=CH(CH2)7C(O)O-CH2 | CH3(CH2)7CH=CH(CH2)7C(O)O-CH | CH3(CH2)14C(O)O-CH2
Fig. 5.5. Glycerol and triacylglyceride structures.
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Fig. 5.6. Soybean oil degumming process diagram (11).
currently formed uses methanol as the acceptor alcohol (ROH in Fig. 5.7) to make methyl esters (ROCOR′ in Fig. 5.7), which is commonly called biodiesel fuel. More information on this process can be found at www.biodiesel.org. Since biodiesel can be made from any triacylglyceride source, significant differences in physical properties have been observed. The use of animal fats or hydrogenated fats which contain higher saturated fat content have much higher freezing temperatures than standard soybean oil, which contains high levels of polyunsaturated fatty acids. In comparison to triacylglycerides, fatty acid methyl esters generally have lower viscosities and are much better solvents toward poly-
Fig. 5.7. Transesterification of fatty acid methyl esters.
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mers and elastomers. Their cold temperature properties are somewhat similar, but strongly depend on fatty acid structure and composition. In the U.S., current production capacity is ~80 million gallons annually with significant investments being made to increase production. Estimates of the current cost of biodiesel soy methyl esters are in the range of $2.50–$3.00/gal. Due to recent legislation for transportation applications of biodiesel fuel, subsidized support has been provided for this biofuel to reduce the cost by ~$1/gal, but it is not clear if this subsidy will apply to home heating fuels. Recent Research on Biofuels Usage in Home Heating Applications Low Temperature Behavior of Biofuel Blends. Solidification or gelling of home heating fuel is a concern due to cold weather climates. This phenomenon is quantified by the cloud point and the pour point temperatures, which are the temperatures at which the first crystals appear (cloud point) and at which the mixture no longer freely flows (pour point). These thermal properties increase with degummed soybean oil content in soybean oil heating fuels (SHO) (see Fig. 5.8). The cloud point is ~7°F (3.9°C) higher than the pour point for each blend. This behavior is similar to the results obtained for biodiesel blends published by Brookhaven National Laboratory (BNL) (12). Therefore, higher blends of biofuels can be an issue with respect to cold temperature fuel crystallization. Flash Point. The flash point of a fuel is a measurement of flammability. Higher flash points indicate fuels which are less flammable and hence safer to handle with respect to volatility. The reported flash points for the degummed soybean heating
Soybean Oil Content (vol%) Fig. 5.8. Cloud point of soybean oil/#2 home heating oil blends (13).
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Soybean Oil Content (vol%) Fig. 5.9. Flash point of soybean oil/#2 home heating oil blends (13).
oil blends (13) are similar to biodiesel results published by Krishna and Butcher (12). Analysis of both soybean oil and biodiesel heating oil blends showed a nearly constant flash point up to a 30% blend, then an increase with biofuel component composition (see Fig. 5.9). Therefore, higher blends of biofuels are safer from a volatility perspective. Energy Content. The energy content of the blended fuels is essentially linear with composition. Fuel oil contains ~10% more energy than soybean oil. Soybean oil has slightly higher energy content than methyl esters (see Table 5.1). Obviously, the energy content would be linearly proportional to the amount of biofuel component used. However, in practice, this difference in energy content is probably not significant, given that the operational parameters of furnaces have much greater impact on variations in energy output.
TABLE 5.1 Net Energy Content of Biofuels and Blends
#2 Fuel oil SHO20 (degummed) SHO50 (degummed) Degummed soybean oil Soy methyl esters
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Volume basis BTU/gal (MJ/L)
Mass basis BTU/lb (MJ/kg)
139,167 (38.8) 135,490 (37.8) 132,441 (36.9) 127,631 (35.6) 125,314 (34.9)
19,601 (45.6) 18,871 (43.9) 18,143 (42.2) 17,017 (39.5) 17,405 (40.4)
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Soybean Oil Content (vol%) Fig. 5.10. Soybean oil heating fuels kinematic viscosity at 100°F (13).
Viscosity. Figure 5.10 shows the kinematic viscosity at 100°F (37.8°C) for SHO heating oil blends. The kinematic viscosity of the SHO20 blend was 3.46 cSt which is within the ASTM recommendation for petroleum fuel oil. The main impact of the viscosity is in atomization of the fuel in the combustion chamber. Higher viscosities tend to increase particle size and reduce combustion efficiency. Research by VanLanningham (13) demonstrated that different atomization nozzle configurations could be used to effectively atomize viscosity biofuel blends higher than 20% if needed. Viscosity measurements by Krishna and McDonald (14) show that biodiesel fuel composition below ~40% falls within the current ASTM standard range for heating fuel. Environmental Impact of Heating Oil Replacement Both VanLanningham (13) and Krishna (14) observed linear decreases in SOx emissions with the addition of either biodiesel or vegetable oil fuels to #2 heating fuels. Therefore, SOx emissions reduction is basically due to decreased sulfur content of the biofuels. The nitrogen oxides (NOx) results from biofuel blends are less conclusive. Two studies show soy methyl ester blends reduce NOx emissions while a third study reports no statistical difference in emissions. The BNL biofuel study suggests higher concentrations of biofuels produce lower NOx emissions over most operating ranges (15). Lower flame temperatures and/or the presence of oxygenated compounds within the fuel were considered possible clues to the decrease in NOx emissions for biodiesel (12). This is supported by Bowman (16) who demonstrated that reducing either the burned gas temperature or the available oxygen will
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reduce the NO formation rate. The Brookhaven study also stated an observed peak in NOx emissions, and a general decrease in emissions as the excess combustion air increased. However, it was noted that combustion chamber design and the furnace design in general affected NOx emissions (17). This agrees with Vanlanningham’s observations that at 20% vegetable oil, different atomizing nozzle designs gave NOx levels from 60–120 ppm. Cold temperature combustion tests found NOx emissions not to be statistically different for the 20% SME blend and the petroleum heating oil (18). Krishna and Butcher (12) observed decreased emissions with the addition of biodiesel, but VanLanningham (13) observed no changes in NOx emissions with vegetable oil. In both cases, the excess oxygen content in the exhaust gases was ~3% range with NOx levels in the 110–120 ppm range. The reason for this disparity is unclear, but the difference may be more dependent on the air-flow operating conditions and fuel atomization than the type of biofuel. If higher excess air-flow is used, the NOx concentrations will be lower due to dilution in the outlet gases. Krishna and Butcher’s work shows slightly lower NOx emissions (115–117 ppm) at 3% oxygen in the exhaust, with decreasing NOx with increased air-flow down to ~90 ppm at 8% oxygen. VanLanningham only reports NOx levels at 3% oxygen in the exhaust gases which range from 60–120 ppm, depending on atomizing nozzle configuration. Alternatively, if higher combustion temperatures are obtained, NOx formation rates may be greater. In the VanLanningham study of vegetable oil (13), it was noted that increasing vegetable oil content above 50% did result in higher combustion temperatures. However, no significant temperature differeneces were noted at the 20% biofuel composition, so it is unclear why the observed NOx levels were higher. Analysis of emissions from combustion of a mixture of 20% soybean oil with #2 fuel oil showed a slight increase in NOx and slight decrease in SO2 (Table 5.2). Since environmental impact is a primary benefit of biofuels, a life-cycle analysis of the impacts of using soybean oil heating fuel blends was quantified (19) using the factors indicated in Fig. 5.11. Using a recently completed life-cycle inventory by the National Institute for Standards and Technology (NIST) for soybean production (20), the energy use and other environmental impacts from an acre of soybean production were calculated. Table 5.3 from the NIST LCI shows the total for inputs and outputs from soy agricultural production. Based on one kg of soybeans produced, the total primary energy requirement is 2 MJ for production. The crushing process requires 92,880 kcal/ton of soybeans or an additional 0.39 MJ/kg, bringing a total of 2.39 MJ/kg of oil or meal, if energy is allocated equally. Combustion data shows 17113.5 BTU/pound which equates to 39.7 MJ/kg of fuel burned for a net gain of 37.31 MJ/kg of oil. One kg of heating oil would produce 45.8 MJ/kg of energy. The NREL biodiesel life-cycle estimated a process energy requirement for diesel fuel of 9.16 MJ for extraction transportation and refining, leaving a net gain of 36.6 MJ/kg.
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TABLE 5.2 Emissions from #2 Fuel Oil/Soybean Oil Blends Fuel
T gas °C
CO2 %
O2 %
CO ppm
SO2 ppm
NOx ppm
Fuel Oil Fuel Oil Fuel Oil Fuel Oil Fuel Oil Fuel Oil Fuel Oil SHO20 SHO20 SHO20 SHO20 SHO20 SHO20 SHO20 SHO20 SHO20
252 252 247 240 238 238 239 246 245 239 253 258 262 239 235 236
10.7 11.4 11.3 10.4 10.1 9.9 9.7 11.9 11.9 10.3 10.3 11.3 11.3 11.9 11.3 11.3
6.5 5.6 5.8 6.9 7.3 7.5 7.9 4.8 4.9 7.1 7 5.7 5.7 4.8 5.6 5.7
11.2 11 10.9 10.3 9.7 9.8 9.6 11.3 11.5 10.9 10.6 11 10.5 9 8.6 8.5
99 78 74 104 83 79 78 92 90 94 93 82 81 77 69 69
72 84 81 75 76 77 78 91 91 92 93 98 99 93 85 84
Source: Reference 13.
Another significant and favorable difference is the sequestration of CO2 from the atmosphere during the growth of the soybean crop. Combustion data shows that both soybean oil and #2 oil fuels release CO2 in equal amounts, but the NIST life-cycle shows that soybeans take up 1561 g of CO2/kg soybeans. Soybean oil is actually more dense in carbon than whole soybeans, containing ~75% elemental carbon by weight. Converting elemental carbon to CO2 shows that each kg of oil contains the equivalent of 2.77 kg of CO2. Allocating the CO2 release from agriculture based on the oil yield of 12.5% yields a net sequestration of 2.76 kg of CO2 with a release from combustion of ~1,200 g of CO2/kg of fuel burned for either soybean oil or #2 distillate. Therefore, the use of soybean oil for combustion shows a net sequestration of CO2 of between 1.5–1.6 kg/kg of fuel burned versus #2 fuel oil. Assuming a blend of 20% soybean oil with #2 distillate, the total CO2 contribution to the atmosphere is reduced from 1.2kg/kg of fuel to 0.66 kg/kg of fuel, a reduction in global warming potential of 45%. Testing of Biofuels in Home Heating Systems Current field testing of biofuels for home heating has focused on using blended fuels which do not require any modification of existing furnaces or fuel delivery infrastructures to minimize costs. Recent results reported by Batey (17) and Batey et al. (21) indicate that lower emissions of SOx, NOx, and CO2 based on single burner/boilers using 20% soy-based biodiesel blended with low sulfur (~0.05%) diesel fuel. In 2003, a blend of 20% biodiesel was used by the Warwick School
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Fig. 5.11. Life-cycle analysis parameters for soybean oil heating fuel blends.
Department in Rhode Island following a successful one year field testing where 10%, 15% and 20% blends of biodiesel were used experimentally (22). Field testing of 20% SHO heating fuel blends was conducted at two private residences over the past two years and demonstrated normal operations (14). Industrial furnace field tests have not been performed for soybean oil fuel blends to date. In all cases, no adverse performance or efficiency issues were noted.
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TABLE 5.3 NIST Life-Cycle Analysis Parameter Values Inventory: Soybeans Flow Input
Output
Reminders
(r) Coal (in ground) (r) Limestone (CaCO3, in ground) (r) Natural Gas (in ground) (r) Oil (in ground) (r) Phosphate Rock (in ground) (r) Potash (K2O, in ground) (r) Uranium (U, ore) Land Use: Cropland (Conservation Tillage) Land Use: Cropland (Conventional Tillage) Land Use: Cropland (Reduced Tillage) Water Used (total) Water: River Water: Well Carbon Dioxide (CO2, biomass)a Carbon Dioxide (CO2, fossil)a Carbon Monoxide (CO)a Methane (CH2)a Nitrogen Oxides (NOx as NO2)a Nitrous Oxide (N2O)a Particulates (unspecified)a Sulfur Oxides (SOx as SO2)a Nitrogenous Matter (unspecified, as N)b Phosphorous Matter (unspecified, as P)b Suspended Matter (unspecified)b Soybeans Soybean Residues Soybean Residues (collected) Waste (total) E Feedstock Energy E Fuel Energy E Nonrenewable Energy E Renewable Energy E Total Primary Energy
Soybean Cultivation Units
Total
kg kg kg kg kg kg kg m2/yr m2/yr m2/yr liter liter liter g g g g g g g g g g g kg kg kg kg MJ MJ MJ MJ MJ
0.0049 0.15 0.014 0.025 0.014 0.0078 1.3E-07 2.3 1.0 0.81 42 26 15 (1,561) 188 0.60 0.16 1.1 2.5 11 0.40 0.14 0.021 2,816 1.0 2.1 0.036 0.10 1.9 2.0 0.018 2.0
aAir
emission. emission. Abbreviation: NA, not available. bWater
Economics The partial replacement of #2 fuel oil through blending with crude degummed soybean oil demonstrated that up to 40% soybean oil mixtures produced a clean flame at manufacturer-recommended settings for air intake. On a heating energy basis, the energy content of soybean oil or methyl esters is roughly 10% less than #2 fuel oil on a weight basis (Table 5.4) and 8% less on a volume basis (Table 5.5).
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TABLE 5.4 Energy Content of Heating Fuels (Mass Basis) Material
BTU/lb
% Difference vs. #2 Fuel oil
#2 Fuel oil Methyl soyate Crude degummed soybean oil
19,601 17,405 17,114
0 –11 –13
Assuming 20% blends, the blended fuel would contain ~99% of the heating energy of #2 fuel oil, which would most likely not be significant for the average fuel user. Additionally, due to lower volatility, soybean oil and methyl esters would minimize the volatilization losses of the fuel during storage. Relative Prices of Soybean Oil, Methyl Esters, and #2 Heating Oil The prices of commodities such as heating oil and soybean oil fluctuate significantly. Since methyl esters are made from soybean oil, their prices are directly linked. Currently, methyl esters are ~$1/gal higher than the price of soybean oil. Historically, the price of soybean oil per pound is twice the price of raw soybeans per pound. Therefore, if soybeans are selling at $6.00/bu ($0.10/lb) the recovered crude oil should be selling at $0.18–$0.22/lb or $1.31–$1.60/gal. For comparison, recent soybean oil prices at the Chicago Board of Trade were at $0.2014/lb or $1.47/gal, whereas #2 heating oil was selling at $1.40–1.50/gal range (wholesale) and ~$2.00/gal (retail) in December of 2004. (Note: Degummed soybean oil is not traded but would carry a price of ~$0.03–$0.05/lb less than the CBT price.) Historical trends indicate that soybean oil prices have declined overall and appear to be trending downward despite the rise in 2003–2004 due to short crops. Crude oil prices and therefore on-season prices for #2 heating oil prices have increased more or less steadily with some fluctuation. The ten-year historical prices of soybean oil and #2 diesel fuel are shown below. During 2000–2002 soybean oil prices were actually lower than #2 heating fuel, despite being significantly higher previously. The recent spike in soybean oil prices in 2003–2004 to historical highs was due to decreased production due to weather and availability, but have
TABLE 5.5 Energy Content of Heating Fuels (Volume Basis) Material #2 Fuel oil Methyl soyate Crude degummed soybean oil
Copyright © 2005 AOCS Press
Weight lbs/gal
BTU/gal
% Difference vs. #2 Fuel oil
7.1 7.6 7.3
140067 127984 130217
0 –8.63 –7.03
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Cents / gallon
Per gallon Prices
June 1994–June 2002 Fig. 5.12. Historical prices for heating oil and soybean oil.
now returned to the $1.40/gal range in late 2004. Figure 5.12 shows the ten-year price history of soybean oil to #2 heating oil. The #2 heating oil prices are all national averages and do not account for local differences. An investigation of Energy Information Agency price history shows that retail heating oil spot prices on season have been by as much as $0.32/gal above the national average. Prices are generally highest in the east coast states with the highest prices occurring in the mid-Atlantic. In their March, 2004, U.S. Baseline Briefing Book, the Food and Agricultural Policy Research Institute (FAPRI) noted that “U.S. soybean yields in 2003 fell to the lowest level in ten years.” This, in turn, resulted in “sharply higher soybean prices in 2003–2004” (23). Looking forward, FAPRI projects a rebound of soybean production in 2004 to 2.87 billion bushels or an increase of 450 million bushels nationally. At this level, farm prices should fall from an average of $7.24/bu to $5.63/bu. FAPRI also projects soybean oil prices to fall steadily over the next decade from an average of $27.97/cwt in 2003–04 to only $19.45/cwt in 2013–2014. This
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Cents / gallon
Price Comparison with 20% Subsidy on Soy
June 1994–June 2002 Fig. 5.13. Comparative historical prices for heating oil and subsidized soybean oil.
happens due to a gradual increase in carryover stocks from 1 billion to 1.6 billion lbs. During this same period production increases by 500 million lbs while total utilization increases by 400 million lbs. If this is correct, the price of soybean oil as a biofuel for heating purposes would be ~$1.42/gal. One can only speculate what the price of petroleum may be in 2014 for comparison. Potential Economic Impact of Biodiesel Subsidies For biodiesel alkyl esters, the Commodity Credit Corporation (CCC) of the USDA has subsidized sales of biodiesel fuel replacements for petro-diesel fuel. While subject to a variety of constraints, the subsidies generally pay ~20% of the cost of the raw commodity soybeans. If this same subsidy were applied to soybean oil for use as a heating fuel, Figure 5.13 shows the relative prices versus #2 heating fuel. As this chart indicates, at subsidized levels similar to biodiesel fuel, historical soybean oil prices would have been consistently below wholesale prices for #2 heating oil for the last decade.
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Recently, the passage of the American JOBS Creation Act of 2004 created an economic incentive for the use of biodiesel fuels, including use as heating fuels. This incentive essentially reduces the heating fuel cost approximately one cent per percentage of biodiesel used in fuel blends. While this type of incentive does not yet exist for soybean oil used as heating fuel, a low level of subsidy would certainly be beneficial in opening new markets for soybean oil utilization and could help stabilize consumer cost for heating oil, as well as providing long-term domestic economic stimulus.
Conclusions Technical Advantages/Limitations of Biofuels The main technical advantages of vegetable-based biofuel blends are lower SOx emissions, potentially lower NOx emissions, zero net carbon dioxide emissions, and ease of use in existing fuel delivery and home furnaces without modification. Both biofuels have limitations with respect to cold temperature behavior and viscosity which restrict blends to ~20–30% to meet existing ASTM standards. These fuels also have significant benefits as domestically produced fuels to replace petrochemical fuels, with the commensurate benefits in energy security and support for domestic agricultural industries. As noted by VanLanningham (13) it may be possible to use higher levels of biofuels in home heating applications provided furnace designs can be modified to handle higher fuel viscosities. However, there are some significant differences between these fuels as summarized in the Table 5.6. It is unclear that the NOx emissions are significantly different for these fuels, given the differences in air-flow and operating conditions. The higher viscosity of vegetable oil is significant in that it affects the atomization process and hence the combustion process. This can be managed by changing fuel nozzles. The issue of methyl esters as excellent solvents may be an issue for existing furnaces. Methyl ester blends have been anecdotally observed to actually clean old residues and precipitated materials from fuel oil tanks and systems. It has been noted that methyl esters can swell and dissolve a variety of elastomers used in fuel applications, so care must be taken to replace seals in older pumps/furnaces if this biofuel is to be used. These issues do not occur with soybean oil biofuel blends. TABLE 5.6 Advantages and Limitations of Biofuels Type of biofuel
Advantages
Limitations
Vegetable oil
Low cost Ready availability
Higher viscosity Possibly higher NOx emissions
Biodiesel/methyl esters
Lower viscosity Higher blend compositions
Higher cost Pump elastomer compatibility
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The partial (20%) substitution of crude degummed heating oil or biodiesel for #2 heating oil is technically feasible with little or no change in existing equipment or infrastructure. The economic feasibility of using biofuels in heating fuel depends on the relative commodity prices of the fuels, which fluctuates significantly. Historically, the price of soybean oil has been above that of #2 fuel oil, other than for the 2000–2002 period, but with recent trends of increasing petroleum prices and future trends of soybean oil prices declining, soybean oil is predicted to be lower in cost than #2 heating oil. Therefore, use of soybean oil in home heating fuel applications appears to have a significant economic advantage. Biodiesel is currently significantly more expensive than #2 heating oil, almost twice as much. The use of biodiesel as a substitute for home heating oil is economically feasible only on a spot basis and only when subsidies are applied. If current subsidies available for biodiesel as a transportation fuel (roughly $1/gal) are available for home heating applications, this would bring the price of biodiesel to approximately the same level as current #2 heating oil. If such a subsidy were available for degummed soybean oil as a heating fuel, it would be a tremendous economic advantage, bringing the price down to ~$0.40/gal. Of course all this must be compared to the cost of petroleum as a heating fuel. Given the assumption that the price of petroleum will significantly rise over the next decade, the use of either biofuel in home heating applications promises to be both economically and environmentally beneficial. References 1. www.eia.doe.gov. Department of Energy, Residential Energy Consumption Survey 2001, Consumption and Expenditure Data Tables, http://www.eia.doe.gov/emeu/recs/ recs2001/detailcetbls.html#total, Total Energy Consumption in U.S. Households, 2001, http://www.eia.doe.gov/emeu/recs/recs2001/ce_pdf/enduse/cel-9c_ne_region2001.pdf, accessed 3/17/05. 2. DeRosa, L., BIOHEAT: Emerging Markets Emerging Markets, National Biodiesel Conference, Palm Springs, CA (2004). 3. Bamberg, S. C., A Total Look at Oil Burner Nozzles, Delavan, Inc., Bamberg, SC, p. 36 (2000). 4. Poulet, G., General Description of Atmospheric Chemistry, in Pollutants from Combustion: Formation and Impact on Atmospheric Chemistry, C. Vovelle, ed., Kluwer Academic Publishers, Boston, MA, p. 339 (2000). 5. McDonald, R. J., and J. Batey, Benefits and Advantages of Marketing Low Sulfur Heating Oil Including Results from a New York State Low Sulfur Market Demonstration, The 2003 National Oilheat Research Alliance Technology Symposium, Brookhaven National Laboratory, Boston, MA (2003). 6. Butcher, T., and C. R. Krishna, NOx—How Low is Achievable with Oilheating Combustion Systems, The 2003 National Oilheat Research Alliance Technology Symposium, Brookhaven National Laboratory, Boston, MA (2003). 7. Hedden, B., ed., NORA Oilheat Technician’s Manual, National Oilheat Research Alliance, Alexandra, VA (2003). 8. Schmidt, P. F., Fuel Oil Manual, Industrial Press Inc., New York (1969).
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9. Irwin, R. J., Environmental Contaminants Encyclopedia: Fuel Oil Number 2—Heating Oil Entry, Fort Collins, CO, National Park Service: 29 (1997). 10. Williams, A., Combustion of Liquid Fuel Sprays, Butterworths, London, England, (1989). 11. Snyder, H. E., and T. W. Kwon, Soybean Utilization, Van Nostrand Reinhold, New York (1987). 12. Krishna, C. R., and T. A. Butcher, Update on Use of Biodiesel Blends in Boilers, The 2002 National Oilheat Research Alliance Technology Symposium, Providence, RI, Brookhaven National Laboratory (2002). 13. VanLanningham, N.W., Soybean Oil Containing Triglycerides as a Renewable Component in Residential Heating Applications, M.S. Thesis, Purdue University (2003). 14. Krishna, C. R., Biodiesel Blends in Space Heating Equipment, National Renewable Energy Laboratory, Upton, NY, NREL/SR-510-33579 (2001). 15. Krishna, C. R. and R.J. McDonald, The Green Fuel Option for the Oilheat Industry— Biofuel Research, The 2003 National Oilheat Research Alliance Technology Symposium, Brookhaven National Laboratory, Boston, MA (2003). 16. Bowman, C. T., Gas-Phase Reaction Mechanisms for Nitrogen Oxide Formation and Removal in Combustion, in Pollutants from Combustion: Formation and Impact on Atmospheric Chemistry, C. Vovelle, ed., Kluwer Academic Publishers, Boston, MA, p. 339 (2000). 17. Batey, J., Combustion Testing of Biodiesel Fuel Oil Blends in Residential Oil Burning Equipment, The 2003 National Oilheat Research Alliance Technology Symposium, Brookhaven National Laboratory, Boston, MA (2003). 18. Lee, S. W., I. He, T. Herage, and B. Young, Laboratory Investigation on the Cold Temperature Combustion and Emission Performance of Biofuel Blends, The 2003 National Oilheat Research Alliance Technology Symposium, Brookhaven National Laboratory, Boston, MA (2003). 19. Martin, J., Value-Added Products from Soybeans: An Economic and Environmental Assessment of Potential Products Using Expelled Soybean Oil and Meal, OmniTech International (2004). 20. National Institute for Standards and Technology, NIST Life Cycle Analysis, 2004. 21. Batey, J., C.R. Krishna, T. Butcher, and R.J. McDonald, Combustion Testing of a Biodiesel Fuel Blend, National Biodiesel Conference, Palm Springs, CA January (2004). 22. Biodiesel, Next Step Home Heating Oil, www.biodiesel.org/markets, accessed 3/17/2005. 23. FAPRI, 2004 U.S. Baseline Briefing Book, FAPRI-UMC Technical Data Report 01-04 Published by the Food and Agricultural Policy Research Institute (FAPRI), University of Missouri-Columbia, 2004.
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Chapter 6
Vegetable Oils-Based Polyols Andrew Guo and Zoran Petrovic Kansas Polymer Research Center, Pittsburg, KS 66762
Introduction Natural fats and oils normally consist of triglycerides of a mixture of fatty acids (FA). The FA part represents around 90% by weight and the glycerine part around 10% of the fat/oil molecule. The individual FA are characterized by the number of carbon atoms in the hydrocarbon chain, ranging generally from C8 (caprylic acid) to C22 (erucic acid) and additionally by the number of the double bonds in the chain. Typical FA compositions of selected natural oils are shown in Table 6.1. One of the naturally occurring oils with an unusual chemical structure is castor oil, which typically comprises about 87% ricinoleic acid and has an average hydroxyl functionality of 2.7. Polyurethanes are probably 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. (1). 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 (2–8). 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 (2,9–47), but commercial production of oil-based polyols has been scarce. While the economics of the associated processes is certainly a large factor, properties of the product conforming to end-use play an important role. Many problems remain to be solved. For example, commercial polyols for rigid urethane foams require an OH number of 450–500 mg KOH/g. Rigid foam production also requires the viscosity of the polyol component to <2,000 centipoises under ambient conditions for the purposes of easy processing and optimal product performance. Most oil-based polyols do not satisfy both requirements at the same time. Some of them have the right viscosity but have a low OH content; others have a high OH content but have turned to greases. Still others have lost their triglyceride linkages—an additional crosslinking naturally built into each oil molecule that enhances crosslinking and thus rigidity. Secondly, studies on the effect of 110
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12:0
14:0
16:0
16:1
18:0
18:1
18:2
18:3
20:0
20:1
22:0
22:1
24:0
Iodine value range
Canola oil Castor oilb Coconut oil Corn Cottonseed oil Linseed oil Olive oil Palm oil Palm kernel oil Peanut oil Rapeseed oil Safflower oil Safflower oil (high oleic) Soybean oil Sunflower oil Sunflower oil (high oleic)
— — 7.1 — — — — — 3.3 — — — —
— — 6.0 — — — — — 3.4 — — — —
— — 47.1 — 0.1 — — 0.1 48.2 — — — —
0.1 — 18.5 0.1 0.7 — — 1.0 16.2 0.1 0.1 0.1 0.1
4.0 2.0 9.1 10.9 21.6 6.0 9.0 44.4 8.4 11.1 3.8 6.8 3.6
0.3 — — 0.2 0.6 — 0.6 0.2 — 0.2 0.3 0.1 0.1
1.8 1.0 2.8 2.0 2.6 4.0 2.7 4.1 2.5 2.4 1.2 2.3 5.2
60.9 7.0 6.8 25.4 18.6 22.0 80.3 39.3 15.3 46.7 18.5 12.0 81.5
21.0 3.0 1.9 59.6 54.4 16.0 6.3 10.0 2.3 32.0 14.5 77.7 7.3
8.8 — 0.1 1.2 0.7 52.0 0.7 0.4 — — 11.0 0.4 0.1
0.7 — 0.1 0.4 0.3 0.5 0.4 0.3 0.1 1.3 0.7 0.3 0.4
1.0 — — — — — — — 0.1 1.6 6.6 0.1 0.2
0.3 — — 0.1 0.2 — — 0.1 — 2.9 0.5 0.2 1.2
0.7 — — — — — — — — — 41.1 — —
0.2 — — — — — — — — 1.5 1.0 — 0.3
100–115 81–91 7–12 118–128 98–118 >177 76–88 50–55 14–19 84–100 100–115 140–150 82–92
— — —
— — —
— — —
0.1 0.1 —
10.6 7.0 3.7
0.1 0.1 0.1
4.0 4.5 5.4
23.3 18.7 81.3
53.7 67.5 9.0
7.6 0.8 —
0.3 0.4 0.4
— 0.1 —
0.3 0.7 0.1
— — —
— — —
123–139 125–140 81–91
aSome
oil compositions may not add to 100% due to the presence of minor fatty acids. Source: Reference 48. 87% OH-bearing ricinoleic acid (C18:1).
bContains
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TABLE 6.1 Typical Fatty Acid Compositions of Selected Plant Oilsa
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polyol structures derived from different vegetable oils on polyurethane properties have been largely lacking. However, such information is critical to the development of oil-based polyols, particularly regarding whether the methodologies employed for one oil can be adopted by another, or whether certain less available oils can be replaced by abundant oils in general polyurethane applications. On the other hand, oleochemical research peaked in the 1970s when a number of commercially feasible processes were identified (49). Since then the associated technologies have advanced a great deal, thus prompting new opportunities for research and development of better processes. For example, most current commercial processes for the epoxidation of vegetable oils still rely on methodologies developed in the late 1940s, which do not lead to complete epoxidation, while a recent procedure using phase-transfer reagents was reported to result in quantitative epoxidation (50,51). As a second example, hydroformylation of olefins traditionally uses the vaporization technique for product-catalyst separation, which is not feasible for low-volatile substrates such as vegetable oils. Such a situation changed after Union Carbide developed the decantation process in the 1980s (52–59). Meanwhile, bulky phosphite-modified rhodium catalysts have been developed that show high reaction rates toward less reactive internal or branched olefins including fatty esters (5,60–72). Therefore, the development of a commercial process for the hydroformylation of fatty substances may have become imminent.
Preparation of Polyols from Vegetable Oils The hydroxylation of vegetable oils can be achieved via four main approaches (Scheme 6.1). The first approach is the epoxidation or oxidation of the unsaturation followed by the ring-opening of the epoxides with proton donors (Scheme 6.1, Route I) (9,11–14,16–18,21,38,43,44,73–87). Secondary OH groups normally result from these procedures. A second approach of introducing hydroxyl functionality is the catalytic hydroformylation of the oils followed by reduction of the aldehyde oils (Scheme 6.1, Route II) (10,37,39,68,74,88–107), with primary OH groups being formed. Hydroxyl functionality can also be obtained by transesterification of oils with various types of polyols (Scheme 6.1, Route III) (84,108–116). Microbial conversion of oils to obtain polyhydroxy substances is also an emerging field (Scheme 6.1, Route IV) (117–121). A large number of publications and patents have appeared in the literature over the years. The selection of vegetable oils depends mainly on the unsaturation level. While higher unsaturation of the oils leads to a higher OH content of the corresponding polyol, above a certain hydroxyl level the polyol suffers from a high viscosity. The theoretical OH numbers of vegetable polyols derived via the epoxidation route and the hydroformylation route are presented in Table 6.2. While coconut and palm kernel oils are of little value as polyol raw materials (i.e., OH number is too low), linseed oil has been reported to give a polyol behaving as a grease (10,96,97,122,123). Partial conversion of the double bonds in linseed oil
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Scheme 6.1. Hydroxylation of vegetable oils.
may be optionally pursued to avoid high viscosity. The epoxidation of castor oil will be complicated because of the coexistence of oxirane and hydroxy groups (124). The hydroformylation of castor oil will produce a mixture of primary and secondary OH groups (2). Some oils appear to have similar composition such as soybean, corn, and sunflower oils. Triolein, trilinolein, and trilinolenin are included for comparison purposes. Epoxidation and Alcoholysis Each double bond in the oil molecule can be monohydroxylated or dihydroxylated depending on the use of oxidizing agents. The most studied monohydroxylation reaction involves a first epoxidation step using hydrogen peroxide, followed by a ring-opening step (Scheme 6.1, Route I) (9,11–14,16–18,21,32,38,43,44,73–87). Swern and Greenspan were among the first to epoxidize vegetable oils using peracetic acid (78,80,125–138). Most commercial processes available today producing
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TABLE 6.2 Theoretical OH Number of Polyols from Plant Oils and Model Triglycerides Oil type Coconut oil Palm kernel oil Palm oil Olive oil Safflower oil (high oleic) Sunflower oil (high oleic) Triolein Peanut oil Rapeseed oil Canola oil Cottonseed oil Corn Soybean oil Sunflower oil Safflower oil Trilinolein Linseed oil Castor oild Trilinolenin
Oil molecular weight
Total unsaturationa
OH number Ab
OH number Bc
679 707 848 878 886 883 885 884 953 882 862 873 874 877 876 879 878 928 873
0.3 0.6 1.8 2.9 2.9 3.0 3.0 3.4 3.9 3.9 3.9 4.5 4.6 4.7 5.1 6.0 6.3 3.1 9.0
26 46 109 158 159 163 163 181 190 206 209 230 236 239 254 288 299 307 387
27 46 113 166 166 171 171 191 201 219 222 246 253 256 274 314 327 322 435
aNumber
of double bonds per triglyceride based on Table 6.1. followed by methanolysis (addition of HO– and CH3O– on each side of the double bond). cHydroformylation followed by hydrogenation (addition of HOCH – and H– on each side of the double bond). 2 dNatural OH groups have been counted into the total OH number. bEpoxidation
epoxidized oils are still based on the same principle. The process involves the in situ formation of peracid from glacial acetic or formic acid and H2O2 (Eq. 1), and oxidation of double bond by peracid to form epoxide regenerating acetic/formic acid at the same time (Eq. 2). H2O2 is consumed as a result: [1]
[2] Quantitative conversions of double bond to epoxide are expected if the reaction time is long enough. However, side reactions become competitive after a certain period of time. Such side reactions involve the ring-opening of oxiranes by acetic acid or water (by-product), and further reaction of formed hydroxyl groups with oxiranes leading to polyether formation, which results in loss of epoxide content and/or rise in product viscosity (78,80,126). Therefore most epoxidation processes are stopped prematurely, resulting in typically 75–90% yield of epoxide.
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While this level of epoxide content is sufficient for commercial products—whose uses are as plasticizers for polyvinyl chloride—the preparation of polyols desires a maximized epoxidation. A number of new procedures have developed in recent years toward expediting the epoxidation process by using catalysts, applying pressure, using phase-transfer reagents (PTR), or using other oxidizing agents (50,51,79,124, 139–158). Among others, catalysts used include ion-exchange resins, sulfuric acid, molybdenum or tungsten oxide, and methyltrioxorhenium/pyridine. Quaternary ammonium salts of tetrakis(diperoxotungsto)phosphates(3-) are PTR reportedly efficient for the epoxidation of vegetable oils (51). The use of these reagents in conjunction with medium-strength hydrogen peroxide (30%) as the primary oxidant in an aqueous/organic biphase system provides an efficient, versatile, and synthetically valuable catalytic method for olefin epoxidation. By this method, a variety of water-insoluble inactivated alkenes, internal or terminal, acyclic or cyclic, isolated or carrying diversified functionalities, were epoxidized in high yields under mild conditions and after relatively short reaction times. The process normally requires no solvents, and the use of medium-strength hydrogen peroxide also lessens safety concerns associated with the explosion hazard of various peroxides. Crivello has recently demonstrated that a series of vegetable oils were epoxidized in high yield by using either a PTR or a sulfonated ion-exchange resin (50,158–162). Methyltrioxorhenium/pyridine has also recently been reported to be a very efficient catalyst for the epoxidation of soybean oil (139), although the process suffers from the toxicity of pyridine. The dihydroxylation of fatty substances has been studied by a number of workers (124,126,130,153,163–167). Osmium, selenium, titanium, and other catalysts have been reported. Although they are efficient for discrete FA derivatives, some of the procedures involve strongly basic conditions that would hydrolyze the triglyceride and thus are generally not applicable to vegetable oils. For example, according to the authors’ observation, H2O2/OsO4 gives a grease for the oxidation of soybean oil with a low OH content, while SeO2 is believed to form a complicated mixture of products if applied to neat linoleic acid. The toxicity of osmium excludes its use in a commercial process, even in catalytic amounts. Therefore dihydroxylation procedures have been pursued scarcely. Although the very reactive oxirane groups in the epoxidized oil molecule enable a number of ring-opening reactions to convert the epoxidized oils to hydroxylated oils, such as carboxylation, alcoholysis, hydrolysis, or hydrogenation (Scheme 6.2) (9,11–14,16–18,21,33,34,37,44,47,73–87), not all are applicable to the preparation of polyols. According to the authors’ experience with the soybean oil system, carboxylation (Route A) leads to a number of complications (oligomerization, cyclization and esterification) resulting in a high viscosity and a low OH content of the polyol. Hydrohalogenation or hydrohyperhalogenation (Routes B and C) gives rise to very good conversion of the material, but the products are highly viscous greases under ambient conditions. Hydrogenation of ESBO (Route F) gives a semi-solid with a melting range of 40–60°C. Hydrolysis (Route E) produces a viscous product having half of the expected OH content owing to the presence of a number of side reactions.
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Scheme 6.2. Hydroxylation reactions of epoxidized oil, where Y = –OC(O)R for (A); X for (B); –OX for (C); –OR for (D); –OH for (E) and –H for (F), and X = Cl or Br.
Alcoholysis (Route D) is the only workable approach to prepare oil-based polyols. Epoxidized soybean oil can be alcoholyzed almost quantitatively in the presence of an acid catalyst, and the polyols produced as such possess low viscosities and nearly theoretical OH contents (18,33,34, 37,44,47). A series of vegetable oil-based polyols have been prepared via a two-step process starting from the vegetable oils, i.e., epoxidation by m-chloroperoxybenzoic acid, or by hydrogen peroxide catalyzed by a PTR or a sulfonated resin (50,158), followed by methanolysis (18,38). Some of the polyols prepared from m-chloroperoxybenzoic acid suffered from incomplete epoxidation as evidenced by Fourier transform infrared (FT-IR) and nuclear magnetic resonance (NMR) results of epoxidized samples, indicating that m-chloroperoxybenzoic acid is not an efficient peroxidant, while the PTR or resin-catalyzed epoxidation proceeded smoothly. The epoxidation products generally require purification before the hydroxylation step. FT-IR has proved to be an effective tool for the analysis of epoxidized oils. The double bond in the oil molecule shows a strong absorbance at 3011 cm–1 (C-H stretch), while the oxirane groups give a weak but distinctive twin band at 823 cm–1 (deformation) and 845 cm–1 (asymmetric stretch), and a shoulder band at 1263 cm–1 (symmetric stretch). The intensity change in these bands during epoxidation is sharp enough to monitor the progress, although 1H-NMR can be equally effective (5.4 ppm for vinylene protons, and 2.9/3.1 ppm for epoxy methine protons). However, FT-IR is only qualitative for the measurement of polyol OH content since the OH band at 3440 cm–1 is very broad and is highly sensitive to impurities containing OH groups. Catalytic Hydroformylation and Reductive Hydrogenation Hydroformylation of olefins has become a very mature and important industrial process since its discovery in 1938 (168). Olefins can be hydroformylated by treat-
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ment with syngas and a catalyst, yielding aldehydes that contain an additional carbon atom. This reaction is a mild and clean procedure for functionalizing unsaturated compounds and is compatible with many other functional groups, such as esters, carboxyls, amides, ketones and even hydroxyls. The atom economy of the reaction is 100%, i.e., all atoms of the reactants end up in the product (169), which results in a minimum of chemical waste discharge (Scheme 6.3). The aldehydes produced form the base for many subsequent transformations leading to alcohols (reduction), amines (reductive amination), carboxylic acids (oxidation), acetals, etc. Significant advancements in hydroformylation have been made over the last few decades. Aspects involve: (i) catalyst/ligand modification; (ii) product/catalyst separation; (iii) process modification including heterogeneous and biphasic catalysis, low pressure hydroformylation, and the addition of decanting process and catalyst recycling. Over the years, people all round the world have showed tremendous interest in the oxo processes. Hundreds of scientific publications and patents appear each year. The three main types of catalyst used commercially are (i) simple cobalt carbonyl complexes, (ii) cobalt carbonyl complexes modified by tertiary phosphine/ phosphite ligands, and (iii) tertiary phosphine/phosphite rhodium carbonyl species. Although most industrial processes still use cobalt, more than 90% of the oxo plants built since 1977 use rhodium-based catalysts and low-pressure oxo (LP OXO) technology developed by Union Carbide. Other metals such as iridium, platinum, ruthenium, manganese, iron, copper, silver, bimetallic species and metal clusters are also known catalysts. Polymer-supported and other heterogeneous catalysts have also been developed. These catalyst systems are all in the research stage, and still far from commercialization. Cobalt has been the conventional catalyst for hydroformylation. However, cobalt is not a very active catalyst. It requires high temperatures and high pressures [typically 140–180°C at 250–350 bars, see Table 6.3 (170)]. High temperatures
Scheme 6.3. Hydroformylation of olefins.
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TABLE 6.3 Comparison of Olefin Hydroformylation Catalystsa
Temperature (°C) Pressure (bar) Catalyst concentration (% metal/alkene) Linear/branched ratio Aldehydes (%) Alcohols (%) Alkanes (%) Other products (%) aUsing
Unmodified cobalt
Phosphine-modified cobalt
Phosphine-modified rhodium
140–180 250–350 0.1–1.0
160–200 50–100 0.5–1.0
80–120 15–25 10–2–10–3
3–4:1 80 10 1 9
6–8:1 none 80 15 5
10–14:1 96 none 2 2
α-olefin as feedstock (170).
degrade selectivities since side reactions have become a problem. Such side reactions include hydrogenation of aldehydes to alcohols and of olefin to alkanes, aldol additions of aldehydes to give dimer aldehydes and ester triols, hydroformylation of aldehydes to formate esters, and reaction of aldehydes with byproduct alcohols to give acetals. Meanwhile, high-pressure operations require expensive construction of process plants. Phosphine or phosphite modification of the cobalt catalyst leads to reduction of the process pressure (down to 50–100 bars). However, there is a significant amount of hydrogenation by-products being produced (15%) along with the rapid reduction of the aldehyde to alcohol (80%) during hydroformylation (although this anomaly may be advantageous to this project since the reduction step is eliminated). On the other hand, the rhodium catalyst requires only very low process pressures (15–25 bars) and also lower temperatures (80–120°C) resulting in nearly quantitative conversion (96%) of olefin to aldehyde. The discovery in the early 1980s by Bryant (Union Carbide, Charleston, WV) and van Leeuwen (Shell Lab, Amsterdam, The Netherlands) of bulky phosphite ligand modified catalysts showing high reaction rate for otherwise unreactive (internal and branched) olefins is a major breakthrough in hydroformylation catalysis (5,60–72). Owing to their high reactivity, these bulky phosphite-modified catalysts have offered new possibilities for the development of a commercial process for the hydroformylation of FA and derivatives including vegetable oils. Significant progress has also been made in the area of product/catalyst separation for the hydroformylation of higher olefins. Hydroformylation as a homogenous process has traditionally used the vaporization technique in commercial processes, but they are limited to those with volatile products, i.e., to the hydroformylation of lower olefins. The recent introduction by Union Carbide of a single-phase oxo process by using organic media-soluble ionic triarylphosphines or phosphites ligands and of product/catalyst separation by decantation has revolutionized the hydroformylation technology. This process permits the hydroformylation of higher molecular weight
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and non-volatile olefins such as fatty substances. The single-phase rhodium-ionic phosphine catalysts have the reactivity typical of conventional homogenous hydroformylation catalysts. However, they can be easily induced to separate into nonpolar (product) and polar (catalyst) phases, thereby providing an effective means for product separation and catalyst recovery (52,54–57,59,171–182). A variation of the Union Carbide single-phase oxo process has been under the extensive investigation of Fell et al. (98,183,184). This process allows the hydroformylation of polyunsaturated substances such as FA esters in a polar solvent (methanol) in the presence of a rhodium-ionic phosphine catalyst. After the reaction, the methanol is distilled off; the catalyst system becomes insoluble and can be separated from the reaction product by filtration or by extraction with water. The aqueous catalyst solution is evaporated to dryness and the catalyst system dissolved in methanol for a new reaction. However, the process is limited to polar media-soluble fatty substances applying a fairly high pressure (e.g., 70–200 bars). Heterogeneous systems using supported rhodium catalysts for the hydroformylation of fatty substances were also studied and it was concluded that these systems are limited to the hydroformylation of mono- and diunsaturated FA (e.g., oleates and linoleates) and are ineffective with triunsaturates (e.g., linolenates). USDA’s research activities on the hydroformylation of fatty substances were during the 1970s (2,10,88–90,92–96,100–103,106,107,122,185–190). A number of vegetable oils were investigated including soybean, linseed, castor, safflower oils, and their derivatives. While the hydroformylated fatty products were suggested for uses as plasticizers for polyvinyl chloride and as coatings, rigid urethane foams prepared from hydroformylated polyols were also reported. The best catalyst systems used at the time were triphenylphosphine- or triphenylphosphite-modified rhodium catalysts normally requiring at least 2,000 psig of syngas pressure, although a much lower pressure (200 psig) is needed for methyl oleate. Although monounsaturated fatty substances can be readily hydroformylated under mild conditions, di- and triunsaturated FA derivatives were found to require higher pressures owing to the presence of π-allylic intermediates, which slowly undergo hydroformylation. The isomerization of double bonds from cis to trans and that from nonconjugated to conjugated have been noted by several workers, and it has been understood that these phenomena are responsible for the slowdown in hydroformylation rate (61,62,64,69,71,72,95,103,191). Although much attention has been paid to the regioselectivity issue on the hydroformylation of various olefins, this information may have limited value as far as the preparation of oil-based polyol is concerned. The hydroxymethyl groups will be located internally in the FA chains, the positions of which differ by only one carbon atom. Since each triglyceride molecule contains many isomers randomly distributed in the three fatty chains, it would be impossible to identify the effect of isomerization on the properties of the crosslinked polyurethane materials, even if a model triglyceride is used. The same is true of the polyols prepared from the epoxidation approach, where the secondary OH groups may be placed in either side of the double bond.
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A recent study has proved that the bulky phosphite-modified rhodium catalyst is several times more active than triphenylphosphine-modified catalyst (64). Vegetable oil-based polyols have been prepared using Rh/C (5% rhodium on carbon) or Rh(CO)2(acac) as catalyst precursors. Polyols of this type are expected to have an OH number and a viscosity in the same range of polyols prepared via the epoxidation route (see Table 6.2). When tris(2,4-di-t-butylphenyl) phosphite is used as ligand using an olefin/Rh molar ratio of 800–1,000, P/Rh ratio of 25, and a total pressure of 500–1,000 psig, the reaction is completed within a few hours at 100°C, as is evidenced from the soybean oil system. Bidentate phosphite ligands (192–196) and ionic phosphite ligands (54,57, 59,178) have also been examined. Whatever the case, the spent rhodium must be fully recovered in order to be commercially practical. The reductive hydrogenation of aldehyde oils has been conducted at 1,000 psig of hydrogen pressure using Raney nickel as the catalyst as demonstrated by many workers (2,186,197). Sodium borohydride can alternatively be used, but reaction conditions have to be controlled carefully in order not to hydrolyze the triglyceride linkages. Kinetic study has been carried out to characterize the catalyst reactivity. These aspects include varying the temperature, H2 and CO pressures, catalyst concentration, and ligand-to-metal ratio. Since the kinetics for the hydroformylation of vegetable oils is very complicated owing to the presence of a mixture of olefins (mono-, di-, and trienes), triolein, trilinolein, and trilinolenin have been used for the kinetic study (104,105). For the analysis of hydroformylated products, FT-IR is used. The aldehyde groups give a strong band at 1728 cm–1 (C=O stretch, shoulder to triglyceride carbonyl at 1744 cm–1) and a medium broad band at 2701 cm–1 (C-H stretch/bend). The intensity change in these bands can be used to monitor the progress of hydroformylation. NMR can also be optionally employed. Vinylene protons are at ca. 5.4 ppm and the aldehyde proton appears further downfield (ca. 10 ppm). The carbonyl content of the aldehyde oil can also be analyzed according to a titrimetric procedure (198). Transesterification The transesterification of vegetable oils with glycerine leads to the formation of monoand diglycerides. A polyol is formed in a single-step transformation and thus the process is of interest from the commercial point of view (84,108–116,199). Other polyhydroxy compounds can be used in the place of glycerine such as trimethylolpropane, pentaerythritol, among others. However, this process leads to no hydroxylation of the double bonds in the FA chain. The FA chains are simply dangling, and thus may not contribute to mechanical strength of the end products. These chains may actually lower the mechanical properties of the products due to their plasticizing effect. Microbial Conversion The microbial hydroxylation of oils is an emerging field (117–121,200). Polyols are produced in a single step under mild conditions using an enzyme as the catalyst
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and oxygen from the air and/or water as the reagents. In contrast to the chemical processes mentioned above (i.e., epoxidation, and hydroformylation) where the hydroxy groups are attached to the double bond positions in the FA chain, the hydroxyls generated enzymatically are normally alpha to the double bonds in the chain. Although these processes appear attractive environmentally, they are still far from the commercial stage mainly due to the overwhelming cost of the catalysts. References 1. Rupprecht, W., S.G. Wildes, and R.B. Turner, Plastics—A Market Opportunity Study, Omni Tech International, Midland, MI (1997). 2. Lyon, C.K., V.H. Garrett, and E.N. Frankel, Rigid Urethane Foams from Hydroxymethylated Castor Oil, Safflower Oil, Oleic Safflower Oil, and Polyol Esters of Castor Acids, J. Amer. Oil Chem. Soc. 51: 331–334 (1974). 3. Baser, S.A. and D.V. Khakhar, Castor Oil–Glycerol Blends as Polyols for Rigid Polyurethane Foams, Cell. Polym. 12: 390–401 (1993). 4. Somani, K.P., S.S. Kansara, N.K. Patel, and A.K. Rakshit, Castor Oil-Based Polyurethane Adhesives for Wood-to-Wood Bonding, Int. J. Adh. Adh. 23: 269–275 (2003). 5. Srivastava, A. and P. Singh, Gel Point Prediction of Metal-Filled Castor Oil-Based Polyurethanes System, Polym. Adv. Technol. 13: 1055–1066 (2002). 6. Yeganeh, H. and M.R. Mehdizadeh, Synthesis and Properties of Isocyanate Curable Millable Polyurethane Elastomers Based on Castor Oil as a Renewable Resource Polyol, Eur. Polym. J. 40: 1233–1238 (2004). 7. Linne, M.A., L.H. Sperling, and J.A. Manson, Simultaneous Interpenetrating Networks Prepared from Special Functional-Group Triglyceride Oils: Castor Oil, LesquerellaPalmeri, and Other Wild Plant Oils, J. Amer. Oil Chem. Soc. 62: 641 (1985). 8. Sperling, L.H., N. Devia, J.A. Manson, and A. Conde, Simultaneous Interpenetrating Networks Based on Castor-Oil Elastomers and Polystyrene—A Review of an International Program, ACS Symp. Series 121: 407–421 (1980). 9. Hoefer, R., G. Stoll, P. Daute, and R. Gruetzmacher, Ger. Offen. 3,943,080 (1991). 10. Khoe, T.H., F.H. Otey, and E.N. Frankel, Rigid Urethane Foams from Hydroxymethylated Linseed and Polyol Esters, J. Amer. Oil Chem. Soc. 49: 615–618 (1972). 11. Kluth, H., B. Gruber, A. Meffert, and W. Huebner, U.S. Patent 4,742,087 (1988). 12. Gruber, B., R. Hoefer, H. Kluth, and A. Meffert, Polyols on the Basis of Oleochemical Raw Materials, Fett Wiss. Technol. 89: 147–151 (1987). 13. Stoll, G., P. Daute, R. Hoefer, R. Gruetzmacher, and H. Kluth, U.S. Patent 5,266,714 (1993). 14. Kluth, H. and A. Meffert, U.S. Patent 4,508,853 (1985). 15. Hoefer, R., G. Stoll, P. Daute, and R. Gruetzmacher, U.S. Patent 5,302,626 (1994). 16. Peerman, D.E., E. DiDomenico, K.C. Frisch, and A. Meffert, U.S. Patent 4,546,120 (1985). 17. Daute, P., R. Gruetzmacher, J. Klein, H. Kluth, and R. Hoefer, U.S. Patent 5,688,989 (1997). 18. Petrovic, Z., A. Guo, and I. Javni, U.S. Patent 6,107,433 (2000). 19. Daute, P., R. Gruetzmacher, R. Hoefer, and A. Westfechtel, Saponification Resistant Polyols for Polyurethane Applications Based on Oleochemical Raw Materials, Fett Wiss. Technol. 95: 91–94 (1993).
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126. Khuddus, M.A., Y. Usui, and D. Swern, Improved Preparation of 9,10,12,13Tetrahydroxystearic Acids from Anti,Cis,Cis-9,10,12,13-Diepoxystearic Acid, J. Amer. Oil Chem. Soc. 50: 524–528 (1973). 127. Mancuso, A.J., S.L. Huang, and D. Swern, Oxidation of Long Chain and Related Alcohols to Carbonyls by Dimethyl Sulfoxide Activated by Oxalyl Chloride, J. Org. Chem. 43: 2480–2482 (1978). 128. Okimoto, T. and D. Swern, Phase Transfer Agents. II. Stereospecific Hydroxylation of Oleyl and Elaidyl Alcohol and Periodic Acid Cleavage of Epoxides, J. Amer. Oil Chem. Soc. 54: A867–A869 (1977). 129. Swern, D., T.W. Findley, and J.T. Scanlan, Epoxidation of Oleic Acid, Methyl Oleate and Oleyl Alcohol with Perbenzoic Acid, J. Amer. Chem. Soc. 66: 1925–1927 (1944). 130. Swern, D., Organic Peroxides, Vol. II, pp. 355–533 (1971). 131. Gall, R.J. and R.P. Greenspan, Epoxy Compounds from Unsaturated Fatty Acid Esters, I&EC 47: 147 (1955). 132. Greenspan, R.P. and R.J. Gall, Epoxy Fatty Acid Ester Plasticizers, Preparation and Properties, J. Amer. Oil Chem. Soc. 33: 391 (1956). 133. Greenspan, R.P. and R.J. Gall, U.S. Patent 2,810,732 (1957). 134. Sack, M. and H.C. Wohlers, Hydrogen Peroxide Variable in Increasing Epoxidation Efficiency, J. Amer. Oil Chem. Soc. 36: 623 (1959). 135. Hansen, L.I. and G.O. Sadgwick, U.S. Patent 3,051,729 (1962). 136. Sawaki, Y. and Y. Ogata, The Kinetics of the Acid-Catalyzed Formation of Peracetic Acid from Acetic Acid and Hydrogen Peroxide, Bull. Chem. Soc. Jpn. 38: 2103 (1965). 137. Ogata, Y. and Y. Sawaki, Kinetics of the Acid-Catalyzed Formation of Aliphatic Peracids from Hydrogen Peroxide and Aliphatic Acids in Dioxane, Tetrahedron 21: 3381 (1965). 138. Hansen, L.I. and A.G. Coutsicos, U.S. Patent 3,328,430 (1967). 139. Larock, R.C. and M. Hanson, U.S. Patent 6,211,315 (2001). 140. Eckwert, K., L. Jeromin, A. Meffert, E. Peukert, and B. Gutsche, U.S. Patent 4,647,678 (1987). 141. Ucciani, E., G. Rafaralahitsimba, and G. Cecchi, Catalytic Epoxidation of Rape, Soy and Sunflowerseed Oils by Means of Hydroperoxides and Molybdenum Compounds, Revue francaise des Corps gras 37: 97–101 (1990). 142. Kulkarni, A.S., R.R. Khoptal, S.P. Kulkarni, and H.A. Bhakare, Selective Methanolysis of Some Epoxidized Semi-Drying Oils, Paintindia 41: 49–50 (1991). 143. delaCuesta, P.J.M., E.R. Martinez, and V.R. Cortes, Epoxidation of Soybean Oil by Means of Isopropylbenzene Hydroperoxide and Molecular-Oxygen: Kinetic-Study, Afinidad 48: 123–127 (1991). 144. Gu, Q., X. Zhu, and S. Yu, Epoxidation Reaction of Soybean Oil, Huadong Huagong Xueyuan Xuebao 18: 825–829 (1992). 145. Xing, C., Y. Wu, and X. Xu, Preparation of Epoxidized Vegetable Oils, Yingyong Huaxue 11: 85–87 (1994). 146. Muturi-Mwangi, P., S. Dirlikov, and P.M. Gitu, Vernonia and Epoxidized Linseed and Soybean Oils as Low Yellowing Diluents in Alkyd Coatings, Pigment Resin Technol. 23: 3–7 (1994). 147. Kayama, R., H. Igarashi, and T. Suzuki, Jpn. Kokai Tokyo Koho JP 06,107,652 A2 (1994).
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148. Carlson, K.D., R. Kleiman, and M.O. Bagby, Epoxidation of Lesquerella and Limnanthes (Meadowfoam) Oils, J. Amer. Oil Chem. Soc. 71: 175–182 (1994). 149. Rangarajan, B., A. Havey, E.A. Grulke, and P.D. Culnan, Kinetic Parameters of a Two-Phase Model for In Situ Epoxidation of Soybean Oil, J. Amer. Oil Chem. Soc. 72: 1161–1169 (1995). 150. Motawie, A.M., E.A. Hassan, A.A. Manieh, M.E. Aboul-Fetouh, and A. Fakhr El-Din, Some Epoxidized Polyurethane and Polyester Resins Based on Linseed Oil, J. Appl. Polym. Sci. 55: 1725–1732 (1995). 151. Sonnet, P.E., M.E. Lankin, and G.P. McNeill, Reactions of Dioxiranes with Selected Oleochemicals, J. Amer. Oil Chem. Soc. 72: 199–204 (1995). 152. Gan, L.H., K.S. Ooi, S.H. Goh, L.M. Gan, and Y.C. Leong, Epoxidized Esters of Palm Olein as Plasticizers for Poly(vinyl chloride), Eur. Polym. J. 31: 719–724 (1995). 153. Sonnet, P.E. and T.A. Foglia, Epoxidation of Natural Triglycerides with Ethylmethyldioxirane, J. Amer. Oil Chem. Soc. 73: 461–464 (1996). 154. Klaas, M.R.G. and S. Warwel, Chemoenzymatic Epoxidation of Unsaturated Fatty Acid Esters and Plant Oils, J. Amer. Oil Chem. Soc. 73: 1453–1457 (1996). 155. Bombarda, I., E.M. Gaydou, J. Smadja, and R. Faure, Sesquiterpenic Epoxides and Alcohols Derived from Hydrocarbons of Vetiver Essential Oil, J. Agr. Food Chem. 44: 217–222 (1996). 156. Mel’nik, L.V., N.V. Kurchevskaya, and S.I. Kryukov, Synthesis of Epoxide Compounds and Products of Their Hydrogenation on the Basis of Fatty Acids of Tall Oil, Russian Chem. Ind. (English Trans.) 28: 9–13 (1996). 157. Warth, H., R. Muelhaupt, B. Hoffmann, and S. Lawson, Polyester Networks Based upon Epoxidized and Maleinated Natural Oils, Angew. Makromol. Chem. 249: 79–92 (1997). 158. Chakrapani, S. and J.V. Crivello, Synthesis and Photoinitiated Cationic Polymerization of Epoxidized Castor Oil and its Derivatives, J. Macromol. Sci., Pure Appl. Chem. A35: 691–710 (1998). 159. Crivello, J.V., S.S. Sternstein, and R. Narayan, Mechanical Characterization of Glass Fiber Reinforced/UV Cured Resins from Epoxidized Linseed Oil, Proc. of the 1995 ASME Int. Mech. Eng. Congress and Expo. Part 1 69-1: 175–180 (1995). 160. Crivello, J.V. and K.D. Carlson, Photoinitiated Cationic Polymerization of Naturally Occurring Epoxidized Triglycerides, Macromol. Reports 33: 251–262 (1996). 161. Crivello, J.V., R. Narayan, and S.S. Sternstein, Fabrication and Mechanical Characterization of Glass Fiber Reinforced UV-Cured Composites from Epoxidized Vegetable Oils, J. Appl. Polym. Sci. 64: 2073–2087 (1997). 162. Qureshi, S., Manson, J.A., J.C. Michel, R.W. Hertzberg, and L.H. Sperling, Characterization of Highly Crosslinked Polymers, ACS Symp. Series 243: 8 (1984). 163. Knothe, G., R.S. Glass, T.B. Schroeder, M.O. Bagby, and D. Weisleder, Reaction of Isolated Double Bonds with Selenium Dioxide-Hydrogen Peroxide: Formation of Novel Selenite Esters, Synthesis 57–60 (1997). 164. Knothe, G., M.O. Bagby, and D. Weisleder, Fatty Alcohols Through Hydroxylation of Symmetrical Alkenes with Selenium Dioxide/tert-butylhydroperoxide, J. Amer. Oil Chem. Soc. 72: 1021–1026 (1995). 165. Knothe, G., M.O. Bagby, D. Weisleder, and R.E. Peterson, Allylic Mono- and Di-hydroxylation of Isolated Double-Bonds with Selenium Dioxide-Ttert-Butyl Hydroperoxide-NMR Characterization of Long-Chain Enols, Allylic and Saturated 1,4-Diols, and Enones, J. Chem. Soc., Perkin Trans. 2: 1661–1669 (1994).
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166. Knothe, G., D. Weisleder, M.O. Bagby, and R.E. Peterson, Hydroxy Fatty Acids Through Hydroxylation of Oleic Acid with Selenium Dioxide/tert-butylhydroperoxide, J. Amer. Oil Chem. Soc. 70: 401–404 (1993). 167. Foglia, T.A., P.A. Barr, A.J. Malloy, and M.J. Costazo, Oxidation of Unsaturated Fatty Acids with Ruthenium and Osmium Tetroxide, J. Amer. Oil Chem. Soc. 54: 870A (1977). 168. Cornils, B. and W.A. Herrmann, Applied Homogeneous Catalysis with Organometallic Compounds, p. 2 v. (XXXVI, 1246 ) (1996). 169. Trost, B.M., Atom Economy—A Challenge for Organic Synthesis: Homogeneous Catalysis Leads the Way, Angew. Chem. Int. Ed. Engl. 34: 259–281 (1995). 170. Masters, C., Homogeneous Transition-Metal Catalysis, Chapman and Hall, New York, 1981. 171. Abatjoglou, A.G., D.R. Bryant, and J.M. Maher, U.S. Patent 5,756,855 (1998). 172. Packett, D.L., J.R. Briggs, D.R. Bryant, and A.G. Phillips, European Patent 853,609 A1 (1998). 173. Bryant, D.R., T.W. Leung, E. Billig, T.C. Eisenschmid, and J.C. Nicholson, U.S. Patent 5,741,944 (1998). 174. Becker, M.C., D.R. Bryant, D.L. Bunning, J.C. Nicholson, and E. Billig, U.S. Patent 5,728,893 (1998). 175. Billig, E. and D.R. Bryant, U.S. Patent 5,763,670 (1998). 176. Nicholson, J.C., D.R. Bryant, and J.R. Nelson, U.S. Patent 5,763,679 (1998). 177. Bryant, D.R. and J.C. Nicholson, U.S. Patent 5,763,671 (1998). 178. Abatjoglou, A.G. and D.R. Bryant, U.S. Patent 5,113,022 (1992). 179. Bryant, D.R., J.E. Babin, J.C. Nicholson, and J.D.J. Weintritt, U.S. Patent 5,183,943 (1993). 180. Miller, D.J., D.R. Bryant, E. Billig, and B.L. Shaw, U.S. Patent 4,929,767 (1990). 181. Maher, J.M., E. Billig, and D.R. Bryant, U.S. Patent 4,835,299 (1989). 182. Bryant, D.R. and R.A. Galley, U.S. Patent 4,297,239 (1981). 183. Fell, B., D. Leckel, and C. Schobben, Micellar 2-Phase Hydroformylation of Multiple Unsaturated Fatty Substances with Water-Soluble Rhodiumdicarbonyl/tert-Phosphine Catalyst Systems, Fett Wiss. Technol. 97: 219–228 (1995). 184. Fell, B. and W. Dolkemeyer, U.S. 4,188,363 (1980). 185. Dufek, E.J. and E.N. Frankel, Some Esters of Mono-, Di-, and Tricarboxystearic Acid as Plasticizers: Preparation and Evaluation, J. Amer. Oil Chem. Soc. 53: 198–203 (1976). 186. Frankel, E.N., Methyl 9(10)-Formylstearate by Selective Hydroformylation of Oleic Oil, J. Amer. Oil Chem. Soc. 48: 248–253 (1971). 187. Frankel, E.N., W.E. Neff, F.L. Thomas, T.H. Khoe, E.H. Pryde, and G.R. Riser, Acyl Esters from Oxo-Derived Hydroxymethylstearates as Plasticizers for PVC, J. Amer. Oil Chem. Soc. 52: 498–504 (1975). 188. Frankel, E.N. and E.H. Pryde, Catalytic Hydroformylation and Hydrocarboxylation of Unsaturated Fatty Compounds, J. Amer. Oil Chem. Soc. 54: A873–A881 (1977). 189. Awl, R.A., E.N. Frankel, and E.H. Pryde, Hydroformylation with Recycled Rhodium Catalyst and One-Step Esterification-Acetalation: A Process for Methyl 9(10)-methoxymethylenestearate from Oleic Acid, J. Amer. Oil Chem. Soc. 53: 190–195 (1976). 190. Khoe, T.H., L.E. Gast, E.N. Frankel, and J.C. Cowan, New Polyacetal, Poly(esteracetal) and Their Urethane Modified Coatings from Hydroformylated Linseed Oil, J. Amer. Oil Chem. Soc. 49: 134–136 (1972).
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Chapter 7
Development of Soy Composites by Direct Deposition Zengshe S. Liu and Sevim Z. Erhan Food and Industrial Oil Unit, National Center for Agricultural Utilization Research, Agricultural Research Service, United States Department of Agriculture, Peoria, IL 61604
Introduction Composites are materials comprised of two or more components that differ in their chemical and physical properties, which have been combined to promote specific characteristics for particular uses. In general, composites correspond to a class of heterogeneous multiphase materials. One of the components (discontinuous) supplies the strength (structural component or reinforcement), and the other (continuous) is the medium of stress transfer (matrix). These components are not mutually dissolved, but they act in a synchronized way, and the material as a whole offers a satisfactory performance (1). The polymeric matrices are more common in composites, although there are composites having other types of matrices, such as a metallic matrix. The most used thermosetting resins in high-performance advanced composites are the phenolics, epoxies, polyimides, bismaleimides, and cyanate ester resin systems. These resins exhibit both excellent solvent and high-temperature resistance. It has been estimated that >75% of all polymeric matrices in composites are thermoset polymers. Thermosetting polymers are resins that crosslink during the curing. Curing involves the application of heat and pressure or the addition of a catalyst. Polymeric materials prepared from renewable natural resources have been enjoying a continuous growing interest in the past decade from the academic and applied point of view. The advantages of these polymers are their low cost, easy availability, and possible biodegradability (2). Among products from agricultural resources, natural oils may constitute raw materials useful in polymer synthesis. Annually, the United States produces ~450 thousand tons of soybean oil in excess of current commercial need. Thus, developing new materials from soybean oil for industrial application has become highly desirable. These new materials can open needed new markets for this important crop. Epoxies have good adhesion, mechanical properties, low moisture absorption, chemical resistance, little shrinkage, and ease of processing. These excellent properties make this family of compounds one of the best matrix materials for many composites (3,4). However, epoxy resins are similar to other engineering resins in that they are either brittle, notch-sensitive, or both. For load-bearing purposes, this means that the product may be subject to catastrophic failure. A major effort over the years has focused on improving the toughness of epoxy structural systems. Qureshi et al. (5) 131
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reported that the use of 25% epoxidized crambe oil as a reactive diluent in bisphenol A and cycloaliphatic epoxy compounds improved resistance to fatigue crack propagation without significant sacrifice in tensile or impact strength and Young’s modulus. Massingill et al. (6) formulated neat epoxidized crambe oil with epoxy-amine systems to give two-phase thermosets. Fracture toughness values of the epoxy thermosets were increased ~100% by 5 and 10% epoxidized crambe oil. The glass transition temperature and mechanical properties were affected modestly. Soybean oil is a triglyceride that contains double bonds. These double bonds may also be converted into the more reactive oxirane moiety by reaction with peracids or peroxides. In the past, epoxidized soybean oil (ESO) has been used mainly as a plasticizer for polyvinyl chloride (PVC) compounds, chlorinated rubber, and polyvinyl acetate (PVA) emulsions. Epoxy-containing soybean oil used as raw material for the synthesis of new polymers suitable for liquid molding processes was reported by Wool and co-workers (7,8). The preparation of structurally strong soy-based composites is attractive from both the commercial and environmental perspectives. Reinforcement materials in soy-based composites can be glass or natural fiber. Composites Reinforced with Glass, Carbon, or Mineral Fibers Composites reinforced with carbon, aramid, and glass fiber dominate the aerospace, leisure, automotive, construction, and sporting industries. Glass fibers are most widely used to reinforce plastics due to their low cost (compared with aramid and carbon) and fairly good mechanical properties. These fibers have serious drawbacks as indicated in Table 7.1. The shortcomings have been highly exploited by proponents of natural fiber composites. Table 7.1 compares natural and glass fibers and clearly shows areas in which the former have distinct advantages over the latter. The carbon dioxide neutrality of natural fibers is particularly attractive. Attempts have been made to use natural fiber composites in place of glass mainly in nonstructural applications. A good number of automotive components previously
TABLE 7.1 Comparison Between Natural and Glass Fibers Natural fibers Density Cost Renewability Recyclability Energy consumption Distribution CO2 neutral Abrasion to machines Health risk when inhaled Disposal
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Low Low Yes Yes Low Wide Yes No No Biodegradable
Glass fibers Twice that of natural fibers Low, but higher than NF No No High Wide No Yes Yes Not biodegradable
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made with glass fiber composites are now being manufactured using environmentally friendly composites (10,11) . Composites Reinforced with Natural Fibers Currently, a great deal of research material is being generated on the potential of natural fiber as a reinforcement for plastics. Natural fiber may include hairs (cotton, kapok), fiber-sheaves of dicotylic plants, or vessel-sheaves of monocotylic plants, i.e., bast (flax, hemp, jute, ramie) and hard-fiber (sisal, henequen, coir). The availability of large qualities of such fiber with well-defined mechanical properties is a general prerequisite for the successful use of these materials and the lack of these large quantities is one of the drawbacks at present. Additionally, for several more technically oriented applications, the fibers have to be specially prepared or modified regarding the following: • • • • • •
homogenization of the fiber’s properties; degrees of elementarization and degumming; degrees of polymerization and crystallization; good adhesion between fiber and matrix; moisture repellence; and flame retardant properties.
At present, the availability of plant fiber can be only partially ensured (as shown in Table 7.2). Their hydrophilic nature is a major problem for all natural fibers if they are used as reinforcements in plastics. The moisture content of the fibers, dependent on the content of noncrystalline parts and void content of the fiber, can be as much as 10 wt% under standard conditions (12). The hydrophilic nature of natural fiber influences the overall mechanical properties as well as other physical properties of the fiber itself (13). Physical (i.e., corona discharge) and chemical modification methods (coupling agents such as silanes) are used to change the surface structure of the fibers as well as to change their surface energy. TABLE 7.2 Production of Plant Fibers Compared with Production of Glass Fibers (1993)a Fiber Jute E-glass Flax Sisal Banana aSource:
Reference 9.
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Price in comparison to glass fibers (%)
Production (1000 t)
18 100 130 21 40
3600 1200 800 500 100
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Composite Molding Methods A wide range of different processes have been developed for the molding of composite parts ranging from very simple manual processes such as hand lay to very sophisticated highly industrialized processes such as sheet molding compounds. Each process has its own particular benefits and limitations making it suitable for particular applications. The choice of process is important to achieve the required technical performance economically. The most common molding techniques for the manufacturing of fiber reinforcement composites are resin transfer molding (RTM), vacuum-assisted resin injection (VARI), reinforced reaction injection molding (RRIM), structural reaction injection molding (S-RTM), vacuum infusion (VI), and solid free-form fabrication (SFF). These composite molding methods have been used successfully to manufacture products ranging from cosmetic parts with moderate demands for structural properties to highly load bearing parts of military and aerospace quality. Resin Transfer Molding (RTM) and Vacuum Assisted Resin Injection (VARI) Figure 7.1 shows a schematic of RTM. Fabrics are laid up as a dry stack of materials. These fabrics are sometimes prepressed to the mold shape, and held together by a binder. These “preforms” are then more easily laid into the mold tool. A second
Fig. 7.1. Schematic of resin transfer molding (RTM).
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mold tool is then clamped over the first, and resin is injected into the cavity. A vacuum can also be applied to the mold cavity to assist resin in being drawn into the fabrics. This is known as Vacuum Assisted Resin Injection (VARI). Once all of the fabric is wet, the resin inlets are closed, and the laminate is allowed to cure. Both injection and cure can take place at either ambient or elevated temperature. Generally epoxy, polyester, vinyl ester and phenolic resins are used at ambient temperature, although high-temperature resins such as bismaleimides can be used at elevated process temperatures. Any kind of fiber can be used as reinforcement. Stitched materials work well in this process because the gaps allow rapid resin transport. Some specially developed fabrics can assist with resin flow. The main advantages are that high fiber volume laminates can be obtained with very low void contents and with better safety and environmental control due to the enclosure of the resin. Both sides of the component have a molded surface. Medium large, complex, and highly integrated components can be produced with low capital costs and with a good working environment. The main disadvantages are that matched tooling is expensive and has to be heavy to be able to withstand pressures. Generally RTM is limited to smaller components. Reinforced Reaction Injection Molding (RRIM) and Structural Reaction Injection Molding (SRIM) This is a fabrication technique involving the extremely rapid impingement mixing of two chemically reactive liquid streams, injected into a mold that results in the simultaneous polymerization, cross-linking and formation of the part. This technique is a form of reactive injection molding (RIM). Figure 7.2 shows a schematic of RIM. When short fibers (1.6 mm), carbon, or glass or mineral fillers are incorporated into one of the two reactive constituents to increase the modulus and reduce the coefficient of expansion, the process is referred to as reinforced reaction injection molding (RRIM). The introduction of long-strand reinforcements, such as continuous filament mats, fabrics, complexes, or chopped strand preforms, into the mold before the injection takes place, allows parts with higher mechanical performance to be obtained. In this case, the process is known as structural reaction injection molding (SRIM). In each of these technologies, polymeric parts are produced directly from a mixture of the low-viscous reactants by its injection into a mold accompanied by fast polymerization and cross-linking reactions. The inherent advantages of RRIM and SRIM are that large parts can be produced with dimensional stability, chemical resistance, weatherability, and surface reproducibility. The main disadvantage is that it is difficult to control the volatile organic chemical (VOC) emissions from the open mold process. Vacuum Infusion (VI) The vacuum infusion (VI) process has become prominent in recent years due to increasing pressure on the control of VOC emissions from the open mold process.
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Fig. 7.2. Schematic of reaction injection molding (RIM).
Figure 7.3 shows a schematic of VI. The basic principal of VI is that reinforcing fibers are placed in a mold, which is sealed using a plastic film or vacuum bag, and resin is drawn into the mold under vacuum. It means lower injection pressure than in the RTM, with the pressure limited to 0.1 MPa. Molds for VI are fitted with a peripheral channel to enable a vacuum to be applied, and catalyzed resin is fed in at the center of the part and allowed to diffuse through the reinforcement to the edge of the mold. The design of the reinforcement and the setting up of the plastic film or vacuum bag, which normally incorporates tubes or channels to help even distribution of the resin, are absolutely critical. A major advantage of the VI process is that it can be reproduced exactly each time without the need to use skilled laminators. The mold is also fully enclosed during the molding process virtually eliminating VOC emissions. A further advantage of the use of the vacuum is
Fig. 7.3. Schematic of vacuum infusion (VI).
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that parts are extremely well consolidated, even with a high fiber content, with very low air content giving very good structural performance. The investment costs of VI molding are low, and it is possible to produce larger parts. One disadvantage of VI is that the excellent consolidation favors thin high-fiber content parts, which may not have sufficient stiffness. Other disadvantages are that the fiber content is usually lower than RTM and only one surface of the manufactured composites is well controlled. The thickness and consequently the fiber content are not as well defined and uniform as in laminates manufactured by RTM. Solid Free-Form Fabrication (SFF) Solid free-form fabrication (SFF) is a method of making shapes without molds. It is best known in its stereolithography forms as a method of rapid prototyping. In stereolithography a laser photopolymerizes successive thin layers of monomer to build up a solid object. Extrusion solid free-form fabrication was developed by the University of Arizona in collaboration with Advanced Ceramic Research (Tucson, AZ) (14). It functions essentially as a three-dimensional (3D) pen plotter. In this case, a slurry is extruded by a stepper motor pushing on a syringe and forcing the material through a needle. By moving the syringe over a computer-controlled path, nearly any geometry can be created (Fig. 7.4). Advantages are that this method has the potential to produce new materials and complex composites that could not be made in any other way, for example, objectives with loose enclosed parts (a ball in a box). A typical procedure for formation of soy composites using SFF is given below.
Fig. 7.4. Sketch of the extrusion free-form fabrication apparatus.
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Solid Free-Form Fabrication of Soy Composites ESO and Epon 828 resin are mixed thoroughly in the weight ratio of 1:0.3. The mixture is mixed with Aerosil R805 and fibrous fillers. A vacuum is applied to remove air bubbles. The fiber-filled slurries show a yield point, such that formed parts hold their shape until cured. A curing agent, diethylenetriamine (DETA), triethylenetetramine, 60% tech (TETA), or Jefamine EDR 148 is added and the paste is placed into a 20-mL plastic syringe. Bars 75 mm × 8 mm × 4 mm were formed by deposition of five layers, subsequently cured at higher temperature for some time depending on the curing agent used. Solid free-form fabrication is conducted using an Asymtek model 402 fluid dispensing system, equipped with small stepper motors (Oriel stepper mike) to drive the delivery syringe. The Asymtek and syringe are controlled by a program written in Microsoft Quick Basic. Solid bar samples are written as a series of lines.
Characterization of Soy Composites Developed by SFF Mechanical Testing The mechanical property testing is done using a 3-point bend test method with an Instron model 1100. The standard formula for the modulus, E and strength, σ in 3pt bending of a beam was used: E = PL3 / 4bd3 δ,σ = 3PL / 2bd2 where P is equal to the break load, L is the support span, δ is the deformation at the center under load P, d is the sample height, and b is the sample width. Composite Morphology Scanning electron microscopy (SEM) is performed to characterize the morphology of soy composites. Figure 7.5 shows the SEM of a freshly fractured surface for soy composites filled with (a) Franklin Fiber® H-45 fiber, and (b) carbon fiber. They clearly indicate that the interfacial adhesion between the fiber and matrix is fairly good. This can be readily seen from the physical contact between the two components. The fibers are broken up from the matrix. However, holes and spacing occur along the fiber, resulting in poor contact and inferior stress transfer between the phases. The use of a combination of glass or carbon fiber with mineral fiber in thermoset matrix was investigated recently by Peng (15). The experimental results show that the use of different fiber types combined in testing bars tends to yield a higher flexural modulus compared with the single type fiber-epoxy composites at same conditions. Liu et al. (16,17) reported the studies of single-fiber and fiber combination reinforced soy composites. An example of using a fiber combination is the use of short milled glass fibers (1/32 in) with Franklin Fiber‚ H-45, as well as short carbon fibers with Franklin Fiber‚ H-45. Table 7.3 shows that the moduli
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Fig. 7.5. Scanning electron microscope (SEM) photomicrograph of the freshly fractured surface of soybased composites filled with (a) Franklin fiber and (b) carbon fiber.
of the composites increase compared with single type fiber-epoxy composites under the same conditions. SEM of the fractured surfaces of the composites after the flexural test are shown in Figure 7.6 (a) and (b).The near absence of holes around the fibers resulted in good contact between the phases. A combination of two fibers can be used to achieve composites with higher strength and stiffness properties than can be obtained with a single fiber type. Influence of Fiber Orientation on Flexural Modulus The high degree of alignment has a great influence on the properties of a composite. Peng et al. (18) reported in glass fiber/epoxy composites, 90% of fibers are within 10° of the machine write direction, measured by microscopy of a polished section. By writing a series of test bars with write axes at different angles to the
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TABLE 7.3 Mechanical Properties of Free-Formed ESO/Epon/TETA or ESO/Epon/DETA with 32 vol% of E-Glass Fiber, 4.8 vol% Franklin Fiber® H-45a
Curing agent
Fiber 1
Fiber 2
Flexural strength (MPa)
TETA TETA TETA TETA DETA DETA
Glass Carbon Glass Carbon Glass Carbon
Franklin Franklin — — Franklin Franklin
110.0 96.3 69.0 63.0 104.3 86.0
aAbbreviations:
Flexural modulus (GPa)
Strain at break (%)
6.3 5.48 4.1 3.6 5.5 4.7
1.9 2.1 1.8 2.4 2.3 2.6
ESO, epoxidized soybean oil; TETA, triethylenetetramine; DETA, diethylenetriamine.
Fig. 7.6. Scanning electron microscope (SEM) photomicrograph of the freshly fractured surface of soy composites filled with the fiber combinations (a) E-glass fiber and Franklin fiber and (b) carbon fiber and Franklin fiber.
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TABLE 7.4 Effect of Orientation on Epoxidized Soybean Oil (ESO)/Epon/Jeffamine EDR-148 Reinforced with 23.4 wt% Franklin Fiber® H-45 Fiber orientation 0 30 45 60 90
Flexural modulus (GPa)
Flexural strength (MPa)
Strain at break (%)
0.97 0.42 0.34 0.32 0.29
21.0 14.0 16.0 22.0 8.1
2.2 5.0 5.0 4.3 2.2
long axis, they can vary the modulus by approximately a factor of 3. Therefore test bars from ESO/Epon/Jeffamine EDR 148 reinforced with 23.4 wt% of Franklin Fiber® H-45 are made by writing at varying angles relative to the axis of the test bars. The modulus can be varied by a factor of ~3.5 at writing angles parallel to the long axis than cross to the long axis (Table 7.4). This is significant because the composite modulus is at least as sensitive to orientation as it is to the fiber aspect ratio and volume fraction.
Summary Among the various composite molding methods, the solid free-forming method has the potential to produce new material and complex composites that could not be formed by other methods. Fiber-reinforced soy composite materials of high strength and stiffness can be formed by the free-form fabrication method. The higher strength and stiffness of soy composites can be achieved by a combination of two fibers. The fiber orientation follows the direction of motion of the write head that deposits the resins and has a large influence on the properties of the composite. These composite materials may be of great environmental interest because they comprise a high amount of agricultural resources. These can be used in agricultural equipment, the automotive industry, civil engineering, marine infrastructure, and the construction industry. References 1. Mano, E.B., Polímeros como Materiais de Engenharia, Edgard Blucher Ltda, São Paulo, 1991, pp. 115–129. 2. Kaplan, D.L., Biopolymers from Renewable Resources, Springer, New York, 1998. 3. Richardson, T., Composites Design Guide, Industrial Press Inc., New York, 1987. 4. Seymour, R.B., Polymeric Composites, New Concepts in Polymer Science, Jonge, CRHI, Utrecht, The Netherlands, USP, 1990. 5. Qureshi, S., J.A. Manson, R.W. Hertzberg, and L.H. Sperling, Mechanical Behavior of Some Epoxies with Epoxidized Natural Oils as Reactive Diluents, in ACS Division of ORPL Papers 48: 576 (1983).
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6. Raghavachar, R., R.J. Letasi, P.V. Kola, Z. Chen, and J.L. Massingill, J. Am. Oil Chem. Soc. 76: 511–516 (1999). 7. Wool, R.P., S.H. Kusefoglu, S.N. Khot, R. Zhao, G. Palmese, A. Boyd, C. Fisher, S. Bandypadhyay, A. Paesano, P. Dhurjati, J. LaScala, G. Williams, K. Gibbons, M. Bryner, J. Rhinehart, A. Robinson, C. Wang, and C. Soultoukis, Affordable Composites from Renewable Sources (ACRES), Polym. Preprints 39: 90 (1998). 8. Wool, R.P., S.H. Kusefoglu, R. Zhao, G. Palmese, A. Boyd, C. Fisher, S. Bandypadhyay, A. Paesano, S. Ranade, P. Dhurjati, S.N. Khot, J. LaScala, G. Williams, M. Ligon, K. Gibbons, C. Wang, C. Soultoukis, M. Bryner, J. Rhinehart, and A. Robinson, Affordable Composites from Renewable Sources (ACRES), 216th ACS National Meeting, Boston, August 23–27, 1998. 9. Baumgartl, H., and A. Schlarb, 2. Symposium Nachwachsend Rohstoffe-perspektiven für die Chemie, Frankfurt, 5–6 May, 1993. 10. Larbig, H., H. Scherzer, B. Dahlke, and R. Poltrock, J. Cell. Plastics 34: 361–379 (1998). 11. Leao, A., R. Rowell, and N. Tavares, 4th International Conference on Frontiers of Polymers and Advanced Materials Conference Proceedings, Cairo, Egypt, Plenum Press, New York, 1997, p. 755. 12. Gassan, J., and A.K. Bledzki, 6. Internationales Techtexil Symposium, Frankfurt, 15–17 July, 1994. 13. Wuppertal, E.W., Die textilen Rohstoffe, Dr. Spohr-Verlag, Frankfurt, 1981. 14. Stuffle, K., A. Mulligan, P. Calvert, and J. Lombardi, Solid Freeform Fabrication Symposium Proceedings, University of Texas, Austin, 1993, p. 60. 15. Peng, J., M.S. Thesis, The University of Arizona, Tucson, 1999. 16. Liu, Z.S., S.Z. Erhan, J. Xu, and P.D. Calvert, J. Appl. Polym. Sci. 85: 2100–2107 (2002). 17. Liu, Z.S., S.Z. Erhan, and P.D. Calvert, J. Appl. Polym. Sci. 93: 356–363 (2004). 18. Peng, J., T.L. Lin, and P. Calvert, Composites A 30: 133–138 (1999).
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Chapter 8
Vegetable Oils in Paint and Coatings Michael R. Van De Mark and Kathryn Sandefur University of Missouri-Rolla Coatings Institute, Rolla, MO 65409–1020
Introduction Triglycerides and their derivatives have been used as a binder or additive in coatings for possibly as long ago as 30,000 years going back to the days of cave paintings. The early oil-based products would simply be derived from the use of a drying oil selected from any naturally occurring plant or several fish oils and a naturally occurring pigment such as red iron oxide or carbon black. Because no catalyst was added, the curing or oxidation process was slow and yielded a soft coating. Technology over the centuries has improved the use of oils in coatings. In addition to the classic triglycerides, sulfonated oils and many other derivatives are used today, including lecithin, as additives to coatings. The primary use of oils in coatings is as a drying oil. Drying oils are highly unsaturated oils that will oligomerize or polymerize when exposed to the oxygen in air, usually in the presence of a catalyst. The result is an increase in the molecular weight including cross-linking. Oils have the distinct advantage of being able to penetrate into wood before polymerization (1) and thus are ideal wood primers. Some of the common fatty acids (2) found in drying oils are listed in Table 8.1. Only linoleic, linolenic, pinolenic, dehydrated ricinoleic, and α-eleostearic acids are truly drying oils. These all have two or more units of unsaturation separated by TABLE 8.1 Common Fatty Acids Found in Drying Oils Common name
Formula
Myristic acid Palmitic acid Stearic acid Oleic acid Linoleic acid Linolenic acid Pinolenic acid Ricinoleic acid α-Eleostearic acid
CH3(CH2)12CO2H CH3(CH2)14CO2H CH3(CH2)16CO2H CH3(CH2)7CH=CH(CH2)7CO2H CH3(CH2)4CH=CHCH2CH=CH(CH2)7CO2H CH3CH2CH=CHCH2CH=CHCH2CH=CH(CH2)7CO2H CH3(CH2)4CH=CHCH2C=CHCH2CH=CH(CH2)3CO2H CH3(CH2)5CH(OH)CH2CH=CH(CH2)7CO2H CH3(CH2)3CH=CHCH=CHCH=CH(CH2)7CO2H
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no more than one methylene. It should be noted that moving the unsaturation into conjugation, as is the case for α-eleostearic acid, increases the reactivity. Typical sources for drying oils include linseed, soybean, tung, sunflower, and dehydrated castor, as well as many fish oils. Their cost, coupled with the distribution of the various drying acids in the triglyceride, often governs their use. Trace contaminants in the oils can also influence their acceptability in a given market. For example, the presence of various phenolic contaminants can increase the color of the resin as it oxidizes in air. Formulations requiring a white or clear appearance may not allow their use. True Oil Coatings True drying oil coatings are those based upon an unsaturated oil as the only binder. Typically linseed or tung oil is used in these applications. Only oils with a high drying index are used (3). The drying index is equal to the sum of the percentage of linoleic acid plus two times the linolenic acid content of the oil. If this value is >70, it is considered to be a drying oil. Many use the iodine number which is the number of grams of iodine required per 100 g of oil as an indication of oxidative drying ability (4). Table 8.2 gives the iodine number, drying index, and a partial composition for some typical oils. It should be noted that genetic engineering has resulted in plants that are capable of producing oils with compositions substantially different from those shown. In the coming years, the available oils may be significantly different with very high unsaturation. Soybean oil currently can have the acid distribution shown in Table 8.3 or as much as 85% oleic acid with genetic engineering. Oil coatings are used to finish wood carvings, as wood deck stains and finishes, cedar shingle coatings, and other applications for which penetration is desired and a slow cure rate is not a significant problem. The deck finish market has grown from ~$30 million in 1980 to >$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 <115. TABLE 8.2 Iodine Number, Drying Index, and Partial Composition for Some Typical Oilsa Oils Drying oils Soybean Linseed Sunflower Tung Semidrying oils Corn Nondrying oils Coconut aNote:
Iodine number
Drying Index
% Saturated
130 185 139 165
66 123 66 >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 <40% oil, are soluble in aromatics but not mineral spirits, and are fast drying to touch and significantly harder than medium and long oil alkyds. Oils. The oils used to make alkyds vary based upon the cure speed desired, cost, odor, and many other factors. Table 8.3 shows several of the oils used in the manufacture of alkyds. The unsaturation number is used again to aid the resin formulator in choosing better drying oils. In general, the higher the unsaturation number, the better the oil is for drying. The desire is to have many di- or trienes in the oil, containing two and three double bonds, respectively. Excellent drying oils for alkyds include soybean oil, linseed oil, tung oil, and dehydrated castor oil. These oils are cooked with various polyols to produce a polyester resin called an alkyd. Figure 8.2 illustrates the formation of a typical alkyd. All alkyds, including modified alkyds, cure through air oxidation and require chemical driers, accelerators, and antiskinning agents as did the oils. Because the molecular weight of alkyds is larger and cross-linking is more probable, skinning or livering becomes a significantly larger problem than in simple oil coatings.
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TABLE 8.3 (left) Typical Composition of Vegetable Oils Average fatty acid concentration (% of total fatty acid) Vegetable oil Carbon atoms (n) Double bonds (n) Castor Coconut Corn Cottonseed Crambe Linseed Menhadenb Mustard Oiticicac Olive 80 Palm kernel Peanut Rapeseed Rice Bran Safflower oil Sardine, Pilchard Sesame Soybean Sunflower Tallow Tung (Regular)d Tung (African)e Walnut (English)
Iodine numbera
Caprylic 8 0
85 8 124 107 94 185 170 120 150 20 90 101 102 141 190 110 130 139 40 165 160 150
8.0
Capric
Lauric
10 0
12 0
14 0
48.0
18.0
7.0
Myristic
1.0
7.0
3.0
6.0
50.0
15.0
5.0
3.0
1.0
Palmitic
Stearic
16 0 1.0 9.0 13.0 22.0 3.0 6.0 16.0 2.0 7.0 8.0 8.0 7.0 2.0 17.0 6.0 14.0 9.0 8.0 6.0 31.0 4.0 4.0 9.0
18 0 2.0 2.0 4.0 2.0 2.0 4.0 1.0 6.0 2.0 1.0 6.0 2.0 1.0 2.0 3.0 4.0 6.0 2.0 22.0 1.0 1.0 1.0
aIndicates
the degree of unsaturation. contains 17.0% palmitoleic fatty acids 16 C-1 double bonds. cContains 82% licanic acid. dContains 82% oleostearic acid. eContains 71% oleostearic acid. bAlso
Alkyds, in general, do not function well over galvanized steel or on concrete. The alkalinity of the substrate coupled with traces of water causes the oils to hydrolyze to produce long-chained carboxylic acids. The calcium or zinc salts of these “fatty acids” are basically a mold-release agent. Because this mold release agent is formed under the paint film, it can cause the paint to easily peal off in large sections. This phenomenon can be observed when alkyds are used to paint galvanized duct work on the roof of many buildings. If moisture can be avoided, alkyds may work on these substrates. Soybean oil is often used as the base oil but will often yellow. Various types of linseed oil are also very popular because are very readily cured. Several other
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TABLE 8.3 (right) (Continued) Average fatty acid concentration (% of total fatty acid) Oleic 18 1 2.0 6.0 29.0 21.0 18.0 20.0 27.0 24.0 5.0 82.0 16.0 60.0 16.0 47.0 13.0 10.0 46.0 28.0 26.0 42.0 5.0 9.0 16.0
Linoleic 18 2 5.0 2.0 54.0 54.0 10.0 17.0
Linolenic 18 3
PalmiticOleic Ricinoleic 16 1
5.0 53.0
18 1 90.0
Erucic 22 1
56.0 17.0
20.0 8.0 1.0 22.0 16.0 35.0 79.0 15.0 41.0 50.0 66.0 2.0 8.0 15.0 60.0
Saturated fatty acids
20–22 3+
20–24
3.0
2.0
32.0
6.0
43.0
8.0
45.0
12.0
Unsaturated fatty acids
5.0
6.0
5.0 4.0
41.0
8.0
13.0
highly unsaturated oils such as sunflower oil are gaining popularity as the supply grows. Sunflower oil is a very good drying oil and produces less color. General Issues of Alkyds. Air dry curing may require several weeks to reach full performance. In the laboratory, curing can be accelerated by aging at 50°C for 7 d. This is a well-accepted test criterion to evaluate corrosion, flexibility, hardness, and many other variables. To cure, the oxygen in the air must penetrate through the coating all the way to the substrate. The coating thickness should be limited to <3 mil/coat. Recoating should be done only when sufficient time has been allotted to allow the antiskinning agent to escape and the oxygen to react with the alkyd. The common belief is that solvent evaporation and oxygen permeation are the only issues; however, the loss of the antiskinning agent may be more important to the overall performance. Unless sufficient time has been allowed, the cohesive strength, adhesion and pro-
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Fig. 8.1. Radical-induced polymerization.
tection may all be poor. Our tests indicated a direct proportional drop in adhesive and cohesive performance of an alkyd with film thickness. All alkyd waste, overspray, spray booth filters, rags, and applicators containing alkyd or its washes are prone to spontaneous combustion under certain circumstances. All of these materials should be stored properly to prevent storage fires. Filters should be changed regularly and disposed of properly and safely. All of these waste materials are capable of starting a fire. Most alkyds are solvent-borne and require significant volatile organic chemicals (VOC). Aromatic solvents are to be avoided in air quality control areas that are not in compliance. Utilization of oxygenated solvents in conjunction with mineral spirits can substitute for the aromatics but do raise the cost slightly. Modified Alkyds Modified alkyds utilize the general alkyd backbone or chemistry and typically modify it with a more costly component that improves the performance of the resin. Many of the aspects of alkyds are very desirable. These properties include penetration into wood and air oxidative cross-linking. However, alkyds are not
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Fig. 8.2. The formation of a typical alkyd.
very abrasion resistant, solvent resistant, or hard. Chemistries that enhance these properties have been well received including urethane, epoxy, silicone, and many other technologies (6). Epoxy. Typically 5–45% of the resin is based upon epoxy chemistry. This imparts better solvent resistance and the alkyd is often used as a primer. Ultraviolet (UV)
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stability is poor. Both bisphenol A and other phenolic-based epoxy chemistries have been used. These coatings are not as solvent resistant as the typical two-component epoxy primers but offer better performance than a simple alkyd. The most common modification of this type is based upon the fatty acid reacting with the epoxy group to form an ester (Fig. 8.3). As for most alkyds, the final curing is oxidative. Acrylic. From 15 to 50% modification is typical and can produce a quicker hard film. These are useful for OEM applications but require significant VOC. In this chemistry, the normal low-molecular-weight alkyd is prepared and cooled to ~120–140°C and the monomers added with a radical initiator, usually benzoyl per-
Fig. 8.3. The formation of an ester from the reaction of the fatty acid with the epoxy
group.
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oxide. The monomer polymerizes and is grafted onto the alkyd. This chemistry works equally well for styrene and vinyl toluene (VT). Although this modification improves the properties of the alkyd, it increases the molecular weight and thus the viscosity also increases. Therefore, the VOC for these coatings is generally high. Styrene. Typically a graft of styrene of 10 to 50% onto the alkyd is used. The properties it improves are gloss, cost, and hardness with rapid drying. The disadvantages include poor UV stability with rapid yellowing, significantly higher VOC required for application, and decreased adhesion to many substrates. Both styrene- and VT-modified alkyds will photooxidize. If three or more styrene units are in a row in a polymer chain, it will appear as a yellowing with exterior exposure. Vinyl Toluene. Commonly called VT-modified alkyds, these have properties and problems very similar to those of the styrene-modified alkyds. Melamine. The use of melamine derivatives allows alkyds and modified alkyds to be cured thermally. These are baking systems. The melamine derivative acts as a cross-linking agent to produce a cross-linked film when baked. These are typically baked at 120°C for 30 min, with shorter times for elevated temperatures. The structure of a typical melamine used in mineral spirits-based alkyds is shown in Figure 8.4. Toluene sulfonic acid or the amine salt of a sulfonic acid is usually used as a catalyst at a concentration of 0.5% based upon resin solids. These resins are soluble in aromatics, ketones, esters and, when hot, alcohols such an n-butyl alcohol. Use of n-butyl alcohol as 10% of the solvent helps to keep the surface open longer to allow solvent to escape during baking, thus preventing solvent popping.
Fig. 8.4. Melamine derivatives for alkyd formulations.
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Urethane Modified Alkyds. This system is a one-can coating based upon the use of an alkyd polymer that was modified with 15–30% urethane chemistry (Fig. 8.5). These systems have better solvent resistance, abrasion resistance, and overall properties than typical alkyds. They are usually 3.5–4.5 lb/gal VOC and dry relatively quickly to handle rates of 30 min to 24 h, depending upon the oil content. The higher the oil content, the longer the drying time. These alkyds have been applied to OEM, maintenance, and architectural coatings. Their performance is very good and their cost is relatively low (3). As with all urethane systems, these come in aliphatic or aromatic urethanes. Usually toluene diisocyanate, an aromatic isocyanate, or hexamethylene diisocyanate, an aliphatic isocyanate, are reacted with a monoglyceride made by reacting a di-, tri- or tetraol with the oil. The resultant urethane oils are then used as clear or pigmented coats for wood. The aliphatic systems are excellent for exterior use or where UV exposure is possible. The aromatic-based urethane systems usually have better abrasion resistance but will yellow and degrade if exposed to UV, even through a normal window. The higher the urethane content, the better the coating will generally perform, but the more VOC will generally be required and the higher the cost. A novel approach to the synthesis of urethane oils utilized a chemoenzymatic synthesis. The use of enzymes produces better control of stereochemistry and thus can impart unique properties. This methodology may have future potential (7). Silicone. Silicone coatings are based mainly on dimethyl or diphenyl siloxane with significant cross-linking (Fig. 8.6). These coatings are very expensive because the resin can cost >$14/lb. The advantage of silicone coatings is a high degree of thermal stability, typically stable to 250°C for >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 <8 µm in diameter. In recent years, researchers have been developing an oil-modified latex technology (12–14). The goal is to reduce the need for a coalescent aid by incorporation of the oil into the latex resin. After application, the oil portion of the resin,
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which lowered the glass transition temperature of the resin to allow coalescence into a film, cross-links the resin to produce a hard, durable finish. Acrylated oils are an excellent co-monomer to use in the synthesis of the latex. Alkyd emulsions are readily prepared and can be employed for OEM coatings as well as for architectural applications. These resins have been utilized as an additive resin for years to improve the performance of latex paint on chalking surfaces. Because the resin is a liquid, it can penetrate the chalk and improve adhesion. Typical use levels are between 0.1 and 1 lb/gal. To emulsify the resin, typically an acid-rich resin neutralized with an amine is sheared into the paint. The submicron size droplets are stabilized by the thickeners (15–17). The use of ultrasonic technology has also been utilized in the preparation of mini-emulsions (18). Water-reducible technology relies upon the presence of water-loving groups, usually the salt of carboxylic acids, on the polymer chain to act as an internal emulsifying agent. The polymer is dissolved in a high-boiling, water-loving solvent such as ethylene glycol monobutyl ether. An amine such as ammonia or triethyl amine is added to form the salt of the acid groups. Then water is added very slowly. The result is the formation of very small particles of polymer, typically from 5 to 100 nm. If formulated correctly, the polymer solution is clear and the solution is thermodynamically stable due to Brownian motion keeping the particles suspended. The polymer is not water soluble but is actually dispersed, and many of the particles are single polymer chains. To obtain the requisite acid functionality on the chain, maleated alkyds are often used. In Figure 8.7, maleic anhydride is reacted thermally with a conjugated oil component in the alkyd through a Diels Alder reaction. Water-reducible resin systems have been available for over 50 years but did not gain popularity until the mid-1980s when environmental regulations caused the industry to move more to water-borne coatings. The formulation and manufacture of these coatings are much more difficult and less forgiving than those of either latex or alkyd coatings. The amount of added base, the molecular weight of the resin, the acid value of the resin, and the rate and amount of water added all play an important role in their properties. As water is added to the resin, the viscosity drops with the first small amount of water, then it rises to a very high level. When the solvent composition becomes more like water, the resin's polymer/polymer interactions overcome the polymer/solvent interaction and the resin collapses into a sphere with the carboxylate ions on the exterior. A spherical geometry is typically observed because this places the ions at the furthest points from each other. After application, the water evaporates and the solvent redissolves the particles and coalesces into a film. The use of water-borne driers promotes air oxidation and the subsequent decomposition of the hydroperoxide to cross-link the resin (19,20). Water-reducible coatings can be used for many OEM and maintenance applications. For direct application of the coatings to metal, flash rusting is a frequently encountered problem. Flash rust inhibitors must be used when water-borne coatings are applied directly to metal, and nitrites are commonly employed.
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Fig. 8.7. Diels Alder Reaction.
Inorganic/Oil Hybrid Coating A new approach to producing coatings with better adhesion, corrosion resistance, and hardness is to combine inorganic and oil technologies into a hybrid system. Several different approaches have been used to produce high-quality durable films. Tuman and Soucek (21) investigated the use of a sol-gel precursor based upon titanium(di-ipropoxide) bis (acetyl-acetonate) and titanium (IV) isopropoxide in conjunction with either linseed or sunflower oil with a zirconium drier. In this system, the titanium solgel formed a cluster when moisture entered the film. To improve performance, the coating was baked at 210°C after ambient drying. The sol-gel inclusion at the 5–50% level produced coatings that were stable at 284°C to as high as 306°C with only 5% weight loss by thermogravimetric analysis. In the scheme depicted in Figure 8.8, the –OR, which originally was the isopropoxide, is presumably now the polymerized oil. Thus the resin and inorganic segments are linked together after the baking process, adding strength and cross-linking density. A similar approach was used with zirconium n-propoxide, which performed analogously (22). Both the zirconate alone and titanium (IV) isopropoxide sol-gel alone improved performance. However, the results indicated a synergistic effect of a 5% titanium (IV) isopropoxide:5% zirconium n-
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Fig. 8.8. Sol-gel.
propoxide sol-gel in linseed oil, which showed better hardness, up to 6H, and better adhesion to metal substrates. The fracture toughness was also found to improve through the use of titanium and zirconium sol-gels (23–26). High-solid alkyds have also been improved by the addition of aluminum complexes. The addition of typically l% of an aluminum complex enhances the hardness and general performance of alkyds. However, accelerated weathering or exterior exposure results in crack formation due to embrittlement from oxidative degradation (27,28). UV and Radiation Cure Technology One of the fastest growing areas of coatings is that of radiation cure. This area is divided into three areas; radical UV cure systems, ionic cure systems, and e-beam. Of the three, the radical cure systems are the most popular in part due to their cost
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and their cure speed. Radical cure processes require only seconds to reach full cure, whereas ionic cure systems can require 20 min. The e-beam and a few other high-energy cure systems have small markets, but the ionic and radical systems are the largest markets. The use of oils in this market is growing as new, more compatible and reactive monomers are created from low-cost oil feedstocks. Castor acrylated monomers as well as various acrylate adducts of epoxidized oils such as lesquerella oil are a few of the monomers used in the radical curing systems. In general, acrylates cure readily through UV radical polymerization, whereas styrenic, methacrylates, and vinyl ethers react too readily with oxygen in the air to be as useful. These latter monomers require a blanket of inert gas over the coating during curing to reduce the oxygen radical scavenging. The scheme in Figure 8.9 illustrates the typical reaction of an acrylate. Castor oil is simply esteri-
Castor Oil Triacrylate The Acrylate group is free radically polymerizable Fig. 8.9. Typical reaction of an acrylate.
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fied as the triacrylate. When cured, all of the acrylates are incorporated into the backbones of various chains creating a highly cross-linked film. The oil side chains act as internal plasticizers to keep the coating flexible. It is important to note that over time, these coatings will gain hardness due to the oxidative cure of the residual unsaturation (29). Many oil-based products are not compatible with the very polar acrylate monomers, and urethane derivatives are also frequently used in the radical cure systems to improve adhesion and impart toughness. In these cases, the amount of oil-based monomer must be kept low to avoid phase separation during polymerization. Cationic cure systems have seen significant interest in recent years even though they are only ~8% of the UV cure market. The cost of highly epoxidized soybean, linseed, and other oils is relatively low compared with the typical aliphatic epoxy monomers used in cationic cure systems. The epoxidized oils are significantly more polar and thus more compatible with the other components of the cation cure system. Unlike the radical cure systems, these have generally less shrinkage, can cure more deeply on pigmented systems, and are much slower to cure. Vernonia oil and epoxidized soybean oil were studied by Thames and Yu (30). They found excellent adhesion, impact, UV stability, gloss retention, and corrosion resistance with the oils. However, the pencil hardness and tensile strength of the coating decreased with increasing oil content. Gu et al. (31) found that as the percentage of epoxidized soybean oil increases in a cationic UV cure system with a cycloaliphatic epoxy, the adhesion and solvent resistance increase. The hardness, however, generally decreases unless a postbake is performed after UV exposure. The postbake is often employed to cause a subsequent increase in the rate of cationic reaction, including acid-catalyzed reactions. Usually the cross-link density and subsequent hardness increase due to baking. Epoxidized palm oil was also used and may prove useful for coatings in the immobilization of herbicides (32). One study utilized oil-modified polyols derived from Chinese melon oil (Momordica charantia) and tung oil. These oils are highly unsaturated and excellent drying oils. The system was formulated with the oil-modified polyol, cycloaliphatic epoxides, and a cationic initiator. These systems had excellent gloss and adhesion with improved impact resistance and flexibility (33). Zou and Soucek (34) successfully modified linseed oil by reaction with 1,3-butadiene and subsequent epoxidation. This new monomer can also be used for cationic UV cure systems and should be cost effective. In another study, Soucek et al. (35) utilized tetraethoxysilane oligomers in conjunction with epoxidized norobornene-derivitized linseed oil to form an organic/inorganic hybrid cationic UV cure coating. The performance enhancement was attributed to the nanoscale phase separation of the inorganic and larger organic phases. The inorganic phase was discontinuous, whereas the organic formed a continuous phase around the islands of inorganic polymer. The use of inorganic ceramic-type building blocks can enhance both the abrasion resistance and the adhesion of the coating to metals.
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Oil-Based Additives in Coatings The environmental movement of the past 25 years has driven a deep interest in natural products and their potential use in coatings. The general goal of VOC elimination has been the primary driver. Oils have a high potential of reducing VOC because they are a liquid at room temperature and polymerize, or at least oligomerize, with air oxidation. Therefore, with time, they become a solid. Today, several of the additives, especially in water-borne coatings, are being replaced with unsaturated oils and their derivatives. The goal is to replace additives that may degrade the performance of the coating or are VOC with an additive that adds to the performance but not the VOC. The major VOC in a latex paint is the coalescent aid; typically an ester or ether such as 2-butoxyethanol is used. Van De Mark and Jirautumnukul (36,37) utilized simple monoesters of unsaturated oils with ethylene, diethylene, triethylene, propylene, and dipropylene glycols and other simple alcohols such as the methyl and ethyl ester of unsaturated fatty acids as potential coalescent aids. Vegetable oils as potential coalescent aids are not compatible with latex paint; they are too nonpolar and phase separate from the resin. Oils such as soybean oil were transesterified with a glycol. The simple monoesters were highly compatible for most commercial latex resins, particularly ethylene glycol and propylene glycol monoesters. Virtually a 1:1 replacement amount of the new coalescent aid was effective for many conventional coalescent aids. The new oil-based esters were nonvolatile. As they age, the properties of the latex paint improved very much like the conventional aids as they evaporated over several days or weeks. The amount of resin needed decreased slightly and the efficacy of the associative thickener improved; thus less is required. This additive is now a commercial product. Unsaturated oils are being used as surfactants as the fatty acid or are derivatized. These additives are targeting the air oxidative cure mechanism of alkyds to make the additive lose its mobility and become a binder in the coating. Thus the additive does its job, and then becomes part of the binder (38–40). References 1. Nussbaum, R.M., E.J. Sutcliffe, and A.C Hellgren, Microautoradiographic Studies of the Penetration of Alkyd, Alkyd Emulsion and Linseed Oil Coatings into Wood, J. Coatings Technol. Res. 70: 49–57 (1998). 2. Hofer, R., Daute, P., Grutzmacher, R., and Westfechtel, A., Oleochemical Polyols—A New Raw Material Source for Polyurethane Coatings and Floorings, J. Coatings Technol. Res. 69: 65–72 (1997). 3. Wicks, Z.W. Jr., F.N. Jones, and S.P. Pappas, Organic Coatings Science and Technology, 2nd ed., Wiley-Interscience, New York, 1998, pp. 258–285. 4. Rheineck, A.E., and R.O. Austin, Drying Oils, in Treatise on Coatings, Vol. 1, No. 2, edited by R.R. Meyers and J.S. Long, Marcel Dekker, New York, 1968, pp.181–248. 5. Swaraj, P., Surface Coatings Science & Technology, 2nd ed., John Wiley, New York, 1996, pp. 82–161.
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6. Lin, K.F., Alkyd Resins, in Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed., Vol. 2, Wiley, New York, 1992, pp. 53–85. 7. Athawale, V.D., and M.D. Bhabhe, Chemoenzymatic Synthesis and Characterization of Urethane Oils for Surface Coatings, J. Coatings Technol. Res. 70: 43–48 (1998). 8. Thames, S.F., H. Yu, and M.D. Wang, Air-Dry Primer Coatings from Dehydrated Lesquerella Oil, Ind. Crops Prod. 6: 169–175 (1997). 9. Dirlikov, S., I. Frischinger, M.S. Islam, and T.J. Lepkowski, Vernonia Oil: A New Reactive Monomer, Biotechnol. Polym. 79–93 (1990). 10. Muturi, P., D. Wang, and S. Dirlikov, Epoxidized Vegetable Oils as Reactive Diluents. I. Comparison of Vernonia, Epoxidized Soybean and Epoxidized Linseed Oils, Prog. Org. Coatings 25: 85–94 (1994). 11. Derksen, J.T.P., F.P. Cuperus, and P. Kolster, Paints and Coatings from Renewable Resources, Ind. Crops Prod. 3: 225–236 (1995). 12. Thames, S., K.G. Panjnani, and O.S. Fruchey, Latex Compositions Containing Ethylenically Unsaturated Esters of Long-Chain Alkenols, U.S. Patent 6,001,913 (1999). 13. Thames, S., C.L. King, T. Williams, and O.W. Smith, Oxidative Curing Profiles for CAM Based Acrylic Latexes, Proceedings of the 27th Annual International Waterborne, High-Solids, and Powder Coatings Symposium, New Orleans, LA, 2000, pp. 436–447. 14. Thames, S., E.H. Brister, T. Johnston, and C.L. King, New Monomers from Vegetable Oils, Specialty Monomers and Their Polymers., ACS Symposium Series 755, 2000, pp. 159–169. 15. Weissenborn, P.K., and A. Moteijauskaite, Drying of Alkyd Emulsion Paints, J. Coatings Technol. Res. 72: 65–74 (2000). 16. El-Aasser, M.E., and E.D. Sudol, Miniemulsions: Overview of Research and Applications, J. Coatings Technol. Res. 76: 21–31 (2004). 17. Tsavals, J.G., and F.J. Shork, Particle Morphology Development in Hybrid Miniemulsion Polymerization, J. Coatings Technol. Res. 76: 53–63 (2004). 18. Landfester, K., J. Eisenblatter, and R. Rothe, Preparation of Polymerizable Miniemulsions by Ultrasonication, J. Coatings Technol. Res. 76: 65–68 (2004). 19. Van De Mark, M.R., and J.R. Schnelten, Water Reducible Acrylic Resins, Am. Paint Coatings J. 14, 1997. 20. Van De Mark, M.R., and K. Loftin, Advances in Waterborne Resin Technology for OEM Application, Ind. Paint Powder Mag. 75: 54–55 (1999). 21. Tuman, S.J., and M.D. Soucek, Novel Inorganic/Organic Coatings Based on Linseed Oil and Sunflower Oil with Sol-Gel Precursors, J. Coatings Technol. Res. 68: 73–81, (1996). 22. Wold, C.R., and M.D. Soucek, Mixed Metal Oxide Inorganic/Organic Coatings, J. Coatings Technol. Res. 70: 43–51 (1998). 23. Ballard, R.L., R.A. Sailer, B. Larson, and M.D. Soucek, Fracture Toughness of Inorganic-Organic Hybrid Coatings, J. Coatings Technol. Res. 73: 107–114 (2001). 24. Tuman, S.J., M.S. Thesis, North Dakota State University, Fargo, 1995. 25. Wold, C.R., H. Ni, and M.D. Soucek, Model Compound Study of Sol-Gel Precursor Interaction with Free Fatty Acids, Polym. Preprints 40: 793–794 (1999). 26. Johnson, A.H., L.E. Meemken, and M.D. Soucek, UV Curable Linseed Oil Based Ceramers, Polym. Preprints 42: 747–748, (2001). 27. Mulzebelt, W.J., J.C. Hubert, R.A.M. Venderbosch, A.J.H. Lansbergen, R.M. Klaasen, and K.H. Zabel, Aluminum Compounds as Additional Crosslinkers for Air-Drying High-Solids Alkyd Paints, J. Coatings Technol. Res. 70: 53–59 (1998).
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28. Love, D.J., Aluminum Organic Compounds in High-Solids Alkyd Coatings, J. Coatings Technol. Res. 53: 55 (1981). 29. Thames, S., M.D. Wang, H. Yu, and T.P. Schuman, Acrylated Lesquerella Oil in Ultraviolet Cured Coatings, Prog. Org. Coatings 28: 299–305 (1996). 30. Thames, S.F., and H. Yu, Cationic UV-Cured Coatings of Epoxide Containing Vegetable Oils, Surface Coatings Technol. 115: 208–214 (1999). 31. Gu, H., K. Ren, D. Martin, T. Marino, and D.C. Neckers, Cationic UV-Cured Coatings Containing Epoxidized Soybean Oil Initiated by New Onium Salts Containing Tetrakis (pentafluorophenyl)gallate Anion, J. Coatings Technol. Res. 74: 49–52 (2002). 32. Guthrie, J.T., J.G. Tait, and A.G. Sager, Surface Coatings Int. 83: 278–284 (2000). 33. Thames, S.F., and R. Subramanian, Cationic Ultraviolet Curable Coatings from Chinese Melon Oil and Tung Oil, Macromolecules—New Frontiers, Proceedings of the IUPAC International Symposium on Advances in Polymer Science and Technology 2: 1011–1013 (1998). 34. Zou, K., and M.D. Soucek, Novel UV-Curable Cycloaliphatic Epoxy Based on Linseed Oil Synthesis and Characterization, Polym. Mater. Sci. Eng. 89: 823–824 (2003). 35. Zong, Z., M.D. Soucek, and C. Xue, Polym. Mater. Sci. Eng. 89: 761–762 (2003). 36. Jiratumnukul, N., and M.R. Van De Mark, Preparation of Glycol Esters of Soybean Oil Fatty Acids and Their Potential as Coalescent Aids in Paint Formulations, J. Am. Oil Chem. Soc. 77: 691–697 (2000). 37. Jiratumnukul, N., and M.R. Van De Mark, Australian Patent 767116 (2004). 38. Thames, S.F., M.D. Blanton, S. Mendon, R. Subramanian, and H. Yu, Surfactants and Fatty Acids: Plant Oils, Biopolym. Renewable Resources, 249–280 (1998). 39. Holmberg, K., Prog. Colloid Polym. Sci. 101: 69–74 (1996). 40. Bloom, P.D., G.B. Poppe, and A.F. Rich,, Polyunsaturated Fatty Acids as Part of Reactive Structures for Latex Paints: Thickeners, Surfactants and Dispersants, U.S. Patent 20030187103 (2003).
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Chapter 9
Printing Inks Sevim Z. Erhan USDA, ARS, NCAUR, Peoria, Illinois 61604
Introduction Printing inks are applied to substrates by printing presses of various designs and are divided into five categories according to the type of printing press. Major printing processes used by the industry are lithography (offset), letterpress, flexography, gravure, and screen printing (1). Letterpress and lithographic inks are viscous and paste-like. Flexographic and gravure inks are extremely fluid and they are generally called liquid inks. On the other hand, screen inks are intermediate in viscosity between the liquid flexographic and gravure inks, and the paste-like lithographic inks. Vegetable oils are mainly used in paste inks; therefore, the role of vegetable oils in paste ink formulations will be described.
Lithographic Inks Lithographic inks, the leading type of ink both in volume and dollar shipments, accounted for an estimated 1,230 lbs, valued at $1,770 million in 2001 (2). They represent about 41% of all ink shipments as measured in dollars. Publication and commercial printing represents an estimated 80% of consumption. Of this, newspapers account for 40% of the total. In approximate order, the balance is: advertising printing; magazines and periodicals; catalogs and directories; financial and legal printing; labels and wrappers; and other commercial printing, including packaging. The petroleum shortage in the 1970s stimulated research into vegetable-oil based inks as a substitute for petroleum-based products. Inks containing vegetable oils have been formulated for various specialized applications (3–6). In the early 1980s, the American Newspaper Publishers Association (ANPA) (later changed to the Newspaper Association of America/ NAA) developed a series of ink formulations comprising a blend of “gilsonite” and tall-oil fatty acids with carbon black pigment (7–9). The cost and availability of tall oil and the difficulty of equipment cleanup created by the gilsonite limited the acceptance of these inks by the industry. A later approach by ANPA to produce an ink vehicle from renewable materials resulted in a lithographic news ink formulated with a commercial ANPA vehicle consisting of alkali-refined soybean oil, a
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hydrocarbon resin, and carbon black pigment (10). This black ink prints as well as the mineral oil-based commercial ink, but costs 30 to 50% more. The color inks are formulated similarly, with a good print quality, but cost about 5 to 10% more than the petroleum-based commercial inks. Both the black and color inks contain 20 to 25% hydrocarbon resin. Thus, industry has continued to search for a 100% vegetable oil-based ink to replace the petroleum-based inks. Ink Vehicles Printing ink consists of two components: the colorant (pigment) and the vehicle. The ink vehicle is the liquid or fluid portion of the ink which (i) functions as the carrier and transport system for the pigment, and (ii) upon reaching the substrate, must be converted to a solid and anchor the pigment to the substrate. Important properties of the vehicle as a pigment carrier and transport system include (i) pigment wetting and dispersion; (ii) ink rheology (ink transfer, ink misting, and press stability); and (iii) ink/water balance for lithography. Ink rheology relates to both viscosity and body or structure of the vehicle (or ink). For example, misting indicates that the vehicle has to have more rheology, or more body. The lithographic (offset) printer plate consists of two distinct areas. One area has been rendered hydrophobic (image area) while the non-image area is hydrophilic. Thus, the offset printing process involves a two-phase system consisting of an oil phase (the ink) and aqueous phase (the fountain solution). During the printing process, these phases must not form stable emulsions, or they will not separate properly on the printer plates. Poorly separated phases lead to smudged or ill-defined print (11). Important properties of the vehicle as the pigment binder or anchor to the substrate area (12) include (i) ink setting; (ii) ink drying; and (iii) gloss and rub-resistance of dried ink. Setting and Drying. Setting has to occur very quickly. Slow setting results in undesirable smearing. Drying occurs after the ink was set and ink is considered dry when its viscosity reaches one million centipoises (10,000 ps). Substrate plays an important role during the setting and drying process. Setting and drying mechanisms include one or more of the following (13): (i) solvent/oil penetration; (ii) solvent evaporation; (iii) oxidation polymerization; (iv) catalytic polymerization; (v) resin precipitation. Drying Oils Drying oils are mixtures of fatty acid triglycerides. Fatty acids can exist alone as free fatty acids or in a combined form of esters. A majority of the combined forms are esters with glycerol (propane 1,2,3-triol) (14). They are called triacylglycerols or triglycerides. Upon hydrolysis, each triglyceride molecule can release three fatty acids and one glycerol (Fig. 9.1). Types of fatty acids in common drying oils are tabulated in Table 9.1.
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Fig. 9.1. Triglyceride and hydrolyzed components.
In order to dry by oxidation polymerization, the predominant fatty acid component of the vegetable oil must have at least two unsaturated groups. Oxidation occurs across the double bonds and it involves absorption of oxygen from air. It causes those double bonds to open up and form peroxides, which are unstable, break down, and are crosslinked. One fatty acid chain gets attached to the next one and they form a more rigid structure which essentially gives dry ink. For example, in oleic acid, there is one double bond, which is not enough to oxidize properly. On the other hand, linoleic acid has two double bonds but they are not conjugated. A conjugated structure is very important to the oxidation process. Therefore the linoleic acid is a slower oxidizer than linolenic or eleostearic acid. The double bonds in eleostearic acid are conjugated. In the case of linolenic acid, there are three double bonds and more opportunity for oxidation to occur, but anything that changes the non-conjugated double bonds to conjugated double bonds will enhance the drying process. The drying index indicates how well or how poorly certain drying oils will oxidize. The drying index of drying oil, semi-drying oil, and non-drying oil is >70, 50–70, and <50, respectively. If a particular drying oil has a blend of two and three double bond fatty acids, the percentages of the fatty acids should be included in the determination of the drying index. For example, linseed oil has 52% linolenic and TABLE 9.1 Fatty Acid Composition of Common Drying Oils Common name
Chemical name
Myristic Palmitic Stearic Arachidic Oleic Linoleic Linolenic Eleostearic
Tetradecanoic Hexadecanoic Octadecanoic Eicosanoic 9-Octadecaenoic 9,12-Octadecadienoic 9,12,15-Octadecatrienoic 9,11,13-Octadecatrienoic
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Soybean (%)
Safflower (%)
Linseed (%)
Tung (%)
Tr–0.5 7–11 2–6 0.3–3 15–33 43–56 5–11 —
Tr 3–6 1–4 Tr–0.2 13–21 73–79 Tr —
— 4–7 2–5 0.3–1 12–34 17–24 35–60 —
— 3–5 Tr–1 — 4–9 8–10 2–3 77–86
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15% linoleic acid, therefore the drying index of linseed oil is: (52 × 2) + 16 = 120. Similarly, the drying index of tung oil (Chinawood oil), soybean oil, and safflower oil is calculated as 170, 69, and 77, respectively. Alkali refined vegetable oils are used in ink formulation. Alkali refining removes the gums, waxes, free fatty acids and gives the oil greater clarity and lighter color. The presence of any one of these materials will interfere with the desirable hydrophobic characteristics of the vehicle and the ultimate ink formulation (15). The most frequent use of drying oils in ink vehicles is as the oil component in liquid resins (alkyds). Alkyds are oil modified polyesters with well defined chemistry (16). Modified oil can be used as an ink vehicle without the need of resin in the formulation. The vehicles can be prepared from vegetable oils by two methods (17–18). In one method, vegetable oils are heat-polymerized at a constant temperature in a nitrogen atmosphere to the desired viscosity. In another method, the heat polymerization reaction is permitted to proceed to a gel point, and then the gel is mixed with vegetable oils to obtain the desired viscosity. To further characterize the gels and ink vehicles prepared from vegetable oils, the viscosities and apparent molecular weights of these vehicles are determined by gel permeation chromatography. The prepared vehicles typically had viscosities in the range of G–Y on the Gardner-Holdt viscometer scale, or about 1.6–18 P (19–20). These viscosities corresponded to apparent average molecular weights of 2600–8900. For all oils studied, the viscosity (from crosslinking and polymerization) increased with time at the heat-bodying temperature of 330 ± 3°C. The reaction time necessary to reach a desired viscosity depends on the mass and the structure of the reactants and the rate of heat transfer and agitation. As expected, oils with higher unsaturation polymerized more rapidly than those with lower unsaturation. Gelling times for safflower (I.V. = 143.1), soybean (I.V. = 127.7), sunflower (I.V. = 133.4), cottonseed (I.V. = 112.9), and canola (I.V. = 110.2) oil were 110, 255, 390, and 540 min, respectively. Although the iodine values of cottonseed and canola oil are similar, canola oil with its greater oleic and low linoleic acid content required a longer reaction time. During heat-bodying, conjugated dienes are formed by bond migration in polyunsaturated fatty acids. These can form six-membered rings by intra- or intermolecular reaction with the double bonds of other fatty acids. If these reactants come from different triglycerides, the molecular weight increases for the system. As heating continues, another conjugated group can add to the previously formed unsaturated ring structure. Triglycerides consisting of three polyunsaturated fatty acids where addition may occur increase the probability of forming very complex highly branched structures. Viscosity increases are directly proportional to increases in apparent molecular weight and the degree of polymerization. Apparent molecular weight at the same viscosity of different oils may be due to differences in lin-
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Fig. 9.2. (A) Gel permeation chromatograms of alkali refined, 60 and 105 minute heat-bodied soybean oil. (B) Gel permeation chromatograms of 125, 150, and 270 minute heat-bodied soybean oil.
earity of the bodied oils. As heat-bodying time was increased, the ratio of polymerized oil to unpolymerized oil increased. These behaviors are readily seen by the plots in Figure 9.2. At the heat-bodying time of zero, the peak shows the unmodified soybean oil. When heat-bodying time of the oil increases to 60 minutes, a shoulder appears, resulting from formation of the polymer, and this shoulder becomes dominant as the heating time increases from 60 to 270 minutes. Such heat-bodied oils of different viscosities can be blended to produce viscosities of any desirable value. Also, blending different proportions of the gel and unmodified oil gave different vehicle viscosities. Additives which may be formulated into the inks include driers, lubricants and antioxidants. The thickening effect of the pigment on the base vehicle should be considered in preselecting a vehicle viscosity (21–23). Biodegradation Biodegradation plays an important role in the transformation of many organic compounds in the environment. Several articles speculate that vegetable oil (soybean oil) should biodegrade more readily than mineral oil (24–25). Cavagnaro and Kaszubowski (26) have reviewed the biodegradation of food oils and greases. Erhan et al. studied and reported the biodegradation of soybean oil and the ink vehicles by using the microorganisms that are commonly found in soil (27–28). Also, biodegradation of printing inks was tested by using the “Modified Sturm Test” by Erhan et al. (29). Rosinski (30) reported the results of a de-inking study conducted at Western Michigan University, Kalamazoo, Michigan.
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Conclusion Technology is available for manufacturing vegetable oil-based printing ink vehicles with the desired commercial characteristics. This technology allows manufacturers to increase the vegetable oil content of the ink formulations and in turn improve environmental properties by increasing the biodegradability, lowering the volatile organic compound content, and improving the de-inking properties. References 1. Leah, R.H., The Printing Ink Manual, 4th ed., Society of British Printing Ink Manufacturers Ltd., Van Nostrand Reinhold International, 1988, pp. 3–5. 2. The Rauch Guide to the U.S. Ink Industry, 2002–2004, Impact Marketing Consultants, pp. 95–96. 3. Kobayashi, S. and Nozaki, K., Japanese Patent 17716 (1978). 4. Gupta, P. K., Rai, J., Singh, H., Indian Patent 154760 (1984). 5. Ono, T., Otake, K., Japanese Patent 61,123681 (86, 123681) (1986). 6. Kuzuwata, M., Japanese Patent 63,227287 (88, 227287) (1988). 7. Moynihan, J.T., U.S. Patent 4419132 (1983) 8. Moynihan, J.T., U.S. Patent 4519841 (1985). 9. Manufacturing Directions for Soybean Oil-Based ANPA-Ink, American Soybean Association, St. Louis, Missouri, 1988, White paper. ANPA-INK, Reston, VA. 10. Moynihan, J.T., U.S. Patent 4554019 (1985). 11. Erhan, S.Z., and M.O. Bagby, Vegetable Oil-Based Printing Ink Formulation and Degradation, Ind. Crops and Prod. 238 (1994). 12. Advanced Physical Chemistry of Printing Inks, Summer Course, Lehigh University, National Printing Ink Research Institute, Bethlehem, PA, 1989. 13. Printing Ink Handbook, 5th ed., National Association of Printing Ink Manufacturers, Woodbridge, NJ, 1999, p. 15. 14. Wan, P.J., Properties of Fats and Oils, in Introduction to Fats and Oils Technology, 2nd edn., (O’Brien, R.D., Farr, W.E., Wan, P.J., eds.), AOCS Press, Champaign, IL, 2000, p. 23. 15. Erhan, S.Z., Vegetable Oil-Based Printing Inks and Environmental Advantages, in Recent Research Developments in Oil Chemistry 1, Transworld Research Network, Trivandrum, India. 159–164 (1997). 16. Hui, Y.H., Bailey’s Handbook on Fats and Oils, 5th ed., Wiley Interscience, New York, 1996, p. 240. 17. Erhan, S.Z., and M.O. Bagby, Lithographic and Letterpress Ink Vehicles from Vegetable Oils, J. Am. Oil Chem. Soc. 68: 635–638 (1991). 18. Erhan, S.Z., and M.O. Bagby, U.S. Patent 5,122,188 (1992). 19. Erhan, S.Z., and M.O. Bagby, Gel Permeation Chromatography of Vegetable Oil-Based Printing Ink Vehicles, J. Appl. Polym. Sci. 46: 1859–1862 (1992). 20. Erhan, S.Z., and M.O. Bagby, Polymerization of Vegetable Oils and Their Uses in Printing Inks, J. Am. Oil Chem. Soc. 71: 1223–1226 (1994). 21. Erhan, S.Z., M.O. Bagby, and H.W. Cunningham, Vegetable Oil-Based Printing Inks, J. Am. Oil Chem. Soc. 69: 251–256 (1992).
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22. Erhan, S.Z., and M.O. Bagby, Vegetable Oil-Based Sheetfed and Heatset Ink Formulations and Comparison of VOC Test Results with Commercial Inks, TAGA Proceedings, Technical Association of the Graphic Arts, TAGA Office, Book Crafters, Inc., Chelsea, MI, 1997, pp. 807–814. 23. Erhan, S.Z., and M.O. Bagby, U.S. Patent 5,713,990 (1998). 24. Ticer, J.M., Environmental Factors in Disposal, Recycling, and Work Place, Soy Oil Symposium Proceedings, Section VII, 1988. 25. Ellis, S., Soy Based Inks, American Ink Maker 69: 36–38 (1991). 26. Cavagnaro, P.V., K.E. Kaszubowski, and H. Needles, Pretreatment Limits for Fats, Oil and Grease in Domestic Wastewater, Proceedings of the 43rd Industrial Waste Conference, Purdue University, West Lafayette, IN, 1988, pp. 777–789. 27. Erhan, S.Z., and M.O. Bagby, Biodegradation of News Ink Vehicles, TAGA Proceedings, Technical Association of the Graphic Arts, TAGA Office, Book Crafters, Inc., Chelsea, MI, 1993, pp. 314–326. 28. Erhan, S.Z., M.O. Bagby, and T.C. Nelsen, Statistical Evaluation of Biodegradation of News Ink Vehicles and Ink Formulations, J. Am. Oil Chem. Soc. 74: 707–712 (1997). 29. Erhan, S.Z., M.O. Bagby, and T.C. Nelsen, Biodegradation of News Inks with Modified Sturm Test, TAGA Proceedings, Technical Association of the Graphic Arts, TAGA Office, Book Crafters, Inc., Chelsea, MI, 1995, pp. 184–203. 30. Erhan, S.Z., and M.O. Bagby, Environmental Aspects of Vegetable Oil-based Lithographic News Inks, TAGA Proceedings, Technical Association of the Graphic Arts, TAGA Office, Vol. 1, Book Crafters, Inc., Chelsea, MI, 1995, pp. 952–962.
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Chapter 10
Synthesis of Surfactants from Vegetable Oil Feedstocks Ronald A. Holser USDA, ARS, NCAUR, Food and Industrial Oils Research Group, Peoria, Illinois 61604
Introduction Triglycerides obtained from domestic oilseed crops such as soybean represent a renewable source of medium chain length fatty acids (FA) that are suitable for the preparation of surfactants and related products (1). These compounds find numerous industrial applications as detergents, dispersants, emulsifiers, softeners, and wetting agents (2). The hydrolysis of vegetable oil yields FA and glycerol. The FA provide the chemical structures that are the building blocks of surfactants. A FA combines an alkyl structure that imparts hydrophobic properties with a carbonyl or other solubilizing structure that imparts hydrophilic properties. Both attributes are essential to surfactant performance (3). The length of the alkyl chain determines the hydrophobic character while the solubilizing group determines the hydrophilic character, e.g., anionic, cationic, or non-ionic (4). The presence and location of substituent groups further influence surfactant properties and performance in a particular application (5). For example, the tropical oilseeds—coconut and palm kernel—are rich sources of lauric and myristic acids which are used in the production of detergents, whereas the derivatives of longer chain FA perform better as dispersants or emulsifiers (6). FA may be neutralized with base to produce the corresponding salts. Such salts or soaps combine the hydrophilic and hydrophobic functionalities that are characteristic of surfactant compounds. However, the performance and production of soaps is surpassed by the ether, ester, and amide derivatives that can be prepared from FA (7). Oilseed crops are an attractive source of FA since they represent a renewable and sustainable raw material that can replace petrochemicals for industrial synthesis. A surfactant will exhibit the same physical and chemical properties whether prepared from oleochemical or petrochemical raw materials. In fact, certain structures may be more advantageously prepared from an oleochemical raw material. For example, the fatty alcohols that are commonly used in the preparation of surfactants may be produced by the reduction of FA obtained from oilseed triglycerides. Other factors that affect the choice of oleochemical or petrochemical feedstocks include availability, consumer preference, cost, process economics, and purity.
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Additionally, initiatives to promote the development of biobased products that can replace products currently derived from petroleum sources have been implemented by the U.S. government and the European Union over the past several years. In the U.S., the Biomass Research and Development Act of 2000 and executive order 13134, “Developing and promoting bio-based products and bio-energy,” have established the goal to significantly increase the use of agricultural materials as a source for the production of consumer goods, industrial chemicals, lubricants, and alternative fuels. Historically, agricultural materials were used in this capacity prior to the development of petroleum as a raw material for the chemical industry. The shift from agriculture to petroleum as the source of industrial raw materials occurred due to the availability of inexpensive petrochemicals. A return to agricultural sources may be expected as petroleum prices increase. Agricultural Raw Materials in Industrial Applications The current interest in agricultural raw materials for industrial applications is a response to both economic and environmental concerns associated with petroleum. The reserves of petroleum and other fossil fuels are finite, in contrast to renewable sources of oleochemicals, such as those obtained from oilseed crops. The cost of petroleum is expected to increase as the supply is depleted, while the corresponding cost of a sustainable agricultural raw material should remain relatively stable. In addition, the use of domestically produced oleochemical materials instead of petrochemical feedstocks helps to conserve the global supply of fossil resources while reducing the dependence of the U.S. on foreign petroleum supplies. The U.S. currently imports the majority of petroleum it uses. Much of this supply is located in politically sensitive regions such as the Middle East, where a change in the social order can interrupt the production of petroleum and lead to price fluctuations or shortages. Reducing the levels of imported petroleum offers a strategic benefit and promotes domestic economic security. The U.S. Department of Energy has recognized this and established goals to increase the use of biomass as a source of raw materials for the chemical industry (8). The domestic petroleum resources in the U.S. are located in environmentally fragile areas where development remains extremely controversial, such as the Coastal Plain of the Arctic National Wildlife Refuge, and offshore regions of the Gulf and the Pacific coasts. Leasing and drilling activities in these areas would provide only a temporary solution to a long-term problem. In contrast, agricultural raw materials offer a renewable alternative with distinct economic advantages to replace petrochemical feedstocks for industrial applications. The domestic production of industrial agricultural crops promotes a stable and secure supply of the raw materials obtained from these sources. This presents both economic and strategic benefits to the U.S. There is an economic benefit to the farmers and local processors involved in the production and post-harvest handling of the crop. Additional economic stability exists for the farmer who is supply-
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ing a crop with both edible and industrial applications. For example, a decline in the price of a crop or derivative agricultural material in the edible market may be offset by the price in the industrial market. This stability improves the rural economy and stimulates diversity in the agricultural marketplace. The national economy also benefits from this stability. The increased agricultural production of oilseed crops for industrial oleochemicals will provide a secure source of raw materials for the chemical process industry and reduce dependence on imported petroleum. The economic benefit associated with substituting an agricultural raw material for a petrochemical raw material may not be measured simply by the raw material cost or through evaluation of conventional direct production costs. A more accurate life-cycle analysis must be evaluated for each particular case. Comprehensive life-cycle assessments are not trivial to perform and the results obtained for one crop do not necessarily transfer to a similar crop (9). Such analyses must consider the agronomic expenses of fertilizer, pesticides, cultivation, harvest, transport, and storage of the crop plus the additional post-harvest processing that may be required to recover or prepare the agricultural material for use as an industrial raw material. For example, crushing, expelling, or refining operations are necessary to recover the natural oils from an oilseed crop followed by hydrolysis or transesterification of the component FA. The strategic plan prepared by the Biomass Research and Development Board in response to the Biomass Research and Development Act of 2000 and executive order 13134 (which set the national goal of “tripling the use of biobased products and bioenergy by 2010”) proposed the coordination of federal, state, and industrial activities to accelerate the commercialization of biobased products and technologies without adversely affecting environmental and public health (10). The environmental aspects of developing biobased products and implementing the corresponding production technologies need to be evaluated critically. Life-cycle analyses have also been performed to evaluate the environmental impact of manufacturing a particular product from agricultural raw materials versus one manufactured from petrochemicals. A review performed in Germany found that the use of vegetable oils, specifically rapeseed oil, to produce surfactants presented definite advantages both in the reduction of greenhouse gases such as carbon dioxide, and energy requirements (11).
Surfactants Surfactants are applicable to both edible and industrial products. Edible applications include use as emulsifiers or thickening agents. The monostearate and oleate glycerides are used in the highest volume in the food industry where these compounds act as viscosity modifiers and stabilizers that provide texture to processed foods (12). Industrial applications of surfactants are numerous. Surfactants are important components in formulations of agrochemicals, corrosion inhibitors, cosmetics, detergents, lubricants, metal working and oil drilling fluids, polymers, and
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textile finishes (3). They can act as emulsifiers, dispersants, stabilizers for oil-inwater (o/w) or water-in-oil (w/o) emulsions, viscosity modifiers or flocculents. Surfactants can impart foaming, leveling, release, or wetting properties to formulations. The attributes of the surfactant that make it suitable for a specific application are determined by the chemical structure which can be designed for optimal performance in a particular formulation. Surfactants are classified as anionic, cationic, amphoteric, or non-ionic based on the ionization of the structure in an aqueous system. Surfactants from all of these classes may be produced from vegetable oils. The typical surfactant structure consists of a hydrophilic group attached to a hydrophobic moiety. Common hydrophiles include the carboxylate, sulfate, sulfonate, or phosphate groups for anionic surfactants and amine or ammonium groups for cationic surfactants. Common hydrophobes or lipophiles include the linear hydrocarbons of medium chain length (C12–C18). FA obtained from natural fats and oils provide this structure, and their salts were some of the earliest surface active compounds prepared. However, FA may be easily converted into a variety of derivative compounds with a wider range of surfactant properties through reaction of the carboxylic acid group (13). The FA may undergo amidation, esterification, or ethoxylation to produce derivative structures with specific surfactant properties. Alternatively, the FA may be reduced to the fatty alcohol which is another important material for the production of surfactants. Fatty alcohols produced from vegetable oils in this way compete directly with those obtained from petrochemical sources. In some instances the alkyl portion of the FA may contain hydroxyl or epoxide functionalities. The presence of such functional groups permits chemical modifications to be made along the hydrocarbon chain that can further alter the surfactant characteristics. If these functionalities do not occur naturally in the FA they may be introduced by addition reactions at unsaturated sites. Several important surfactant compounds that may be synthesized from vegetable oils are described in the following sections. Esters FA esters are formed by the condensation of an alcohol with the free FA or by transesterification of the triglyceride in the presence of an acid or base catalyst. Esters of primary and secondary alcohols such as methyl, ethyl, isopropyl, butyl, octyl, decyl, myristyl, and cetyl are commonly used in cosmetic and personal care products (14). These compounds act as excipients, lubricants, plasticizers, softeners, solubilizers, and wetting agents in cleansing creams, lipsticks, insect repellants, nail varnishes, perfumes, bath oils, creams, lotions, antiperspirants, and deodorant formulations. The methyl esters are also intermediates for the synthesis of other surfactants such as the α-sulfonates. Esters produced by reaction of the FA with polyhydroxy alcohols such as glycol, glycerol, pentaerythritol, or sorbitol produce compounds with greater hydrophilic characteristics (15). These materials act as emulsifying, opacifying, thick-
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ening, dispersing, pearling, and anti-foaming agents that find applications in the preparation of o/w and w/o emulsions, absorption bases, pomades, emollient creams, and shampoos. Sucrose esters display high biodegradability and physiological inertness (16). The best detersive performance is reportedly obtained with saturated C18 FA. These compounds find applications in cosmetic and detergent formulations. Esters prepared with polyethylene glycol (PEG) provide a non-ionic o/w emulsifier for cosmetic and pharmaceutical formulations. The emulsifier is used as a thickening agent in creams, a suspension agent for solid substances, and a stabilizer in formulations with a high proportion of electrolytes. It has also been used as an anti-static agent for plastics. Ester Sulfonates. A sulfonated FA methyl ester is prepared by the transesterification of a vegetable oil to obtain the corresponding methyl esters followed by sulfonation with sulfur trioxide. The hydrophilic sulfonate group increases the water solubility while the linear hydrocarbon moiety provides the necessary lipophilic character. Ester sulfonates exhibit favorable wetting, emulsifying, and dispersant properties (17,18). This is an efficient method for the production of these anionic surfactants (19). These compounds have been prepared from coconut oil and compare favorably to C12–C14 fatty alcohol ether sulfates and C14–C16 olefin sulfonates (20,21). Yields greater than 95% are obtained when the vegetable oil is hydrogenated prior to sulfonation. If the oil is unsaturated then numerous reaction products are generated. Further reaction of the sulfonate with diethanolamine (DEA) or triethanolamine (TEA) produces improved foaming characteristics (22). The sodium DEA and TEA salts of α-sulfomyristate exhibit better detergency than sodium lauryl sulfate with less irritation in physiological tests (23). Ester Ethoxylates. Fatty ester ethoxylates are prepared by catalytic ethoxylation of the FA methyl esters. These ethoxylates are comparable to the alcohol ethoxylates but with increased water solubility (24). Glycerol esters prepared with 7 moles of ethoxylate are used as dispersing and solubilizing agents. These non-ionic ethoxylates promote the solution of other lipid ingredients in alcoholic formulations. Applications include emulsifying, opacifying, thickening and dispersing agents for in oil-in-water emulsions. Sorbitan esters prepared with 4–20 moles of ethoxylate also act as emulsifying, dispersing, and solubilizing agents for vitamins, essential oils, fragrances, tannins, and other lipid components in cosmetic formulations and topical medicinal preparations. Amides FA or their methyl esters can react with primary and secondary amines such as monoethanolamine (MEA) and diethanolamine (DEA) to produce the corresponding amides (25). However, the reaction of FA with MEA or DEA can produce
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unwanted ester by-products, whereas reaction of the corresponding methyl ester with MEA or DEA produces the desired alkanolamide with over 90% yield. Amido alkoxylated ammonium compounds are also prepared from either the FA or the triglyceride by reaction with diethylamine triamine followed by ethoxylation and conversion to the methyl sulfate derivative with dimethyl sulfate (26). The amido imidizolines are produced by a similar reaction scheme but at elevated temperatures cyclization occurs (27). These compounds are used industrially as fabric softeners, flotation agents, corrosion inhibitors (13,28). The amides also serve as intermediates for structures such as the betaines which exhibit good foaming power and favorable skin tolerance. The reaction of a FA with an amino acid such as N-methyl glycine produces the important class of N-acyl-sarcosine surfactants. Additional condensation products may be obtained by reaction of FA with protein hydrolysates. These compounds have diverse properties, and applications including use as rust inhibitors, fuel oil additives, and emulsifiers. Alcohols Fatty alcohols are not produced in significant quantities by commodity oilseed crops although some specialty crops such as jojoba produce wax esters rather than triglycerides. However, it would not be practical to obtain fatty alcohols from jojoba oil because of the value of the oil. Alternatively, fatty alcohols may be produced by the hydrogenation of the triglycerides, FA, or FA esters obtained from vegetable oil (29,30). The resulting fatty alcohols provide an alternative to the petrochemical source. The fatty alcohol is reacted to form ethoxylated, sulfated, or aminated surfactant compounds. The alcohol ethoxylates are prepared by reacting the fatty alcohol with ethylene oxide. The ethylene oxide chain provides the hydrophilic character for these non-ionic surfactants. These compounds are biodegradable and are used in formulating low foaming degreasers and liquid cleansers (31–33). Subsequent sulfation of these alcohol ethoxylates produces biodegradable surfactants that exhibit exceptional foaming and detergency, and are used in shampoos. The fatty alcohol may also be sulfated directly with sulfur trioxide to provide the alcohol sulfates which are used extensively as foaming agents in personal care products (34,35). Amines Amines can be produced by the reaction of fatty alcohols with alkylamines or by reduction of the fatty nitrile (36). Primary, secondary, tertiary, and quaternary amines are prepared via the fatty nitrile which is formed by the reaction of the FA with ammonia, followed by hydrogenation to produce the corresponding fatty amine. Amine oxides can be prepared from the fatty amine by heating with hydro-
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gen peroxide. These compounds are mild with good foaming properties, and are used in shampoos and cosmetic products.
Summary Clearly, a variety of vegetable oil-based surfactants can be prepared from the reaction of FA or the corresponding methyl esters with various alcohols or amines. A surfactant of every type is represented, such as anionic, cationic, non-ionic, and amphoteric, with applications in personal care products, household detergents, and industrial cleansers. Surfactants are essential components in formulations in such diverse areas as emulsion polymerization, textile treatments, agrochemicals, paper and textile finishes, coatings, metal working fluids, and corrosion inhibitors. Currently, the consumer has expressed interest in bio-based products. In response, initiatives for using renewable raw materials have been established by the federal government. Similarly, applicable industries will evaluate the economic benefits of using natural raw materials to replace petrochemicals. References 1. Gervasio, G.C., FA and Derivatives from Coconut Oil, in Bailey’s Industrial Oil & Fat Products, 5th ed., edited by Y.H. Hui, Wiley Interscience, NY, 1996, Vol. 5, pp. 33–91. 2. Heidrich, J.F., Oleochemicals: Feedstock or Auxiliary, J. Am. Oil Chem. Soc. 61: 271–275 (1984). 3. Lynn, Jr., J.L., and B.H. Bory, Surfactants, in Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed., edited by Kirk-Othmer, Wiley Interscience, NY, 1997, Vol. 23, pp. 478–541. 4. Klevens, H.B., Structure and Aggregation in Dilute Solution of Surface Active Agents, J. Am. Oil Chem. Soc. 30: 74–80 (1953). 5. Mankowich, A.M., Micellar Molecular Weights of Selected Surface Active Agents, J. Phys. Chem. 58: 1027–1030 (1954). 6. Ogoshi, T., and Y. Miyawaki, Soap and Related Products: Palm and Lauric Oil, J. Am. Oil Chem. Soc. 62: 331–335 (1985). 7. Borghetty, H.C., and C.A. Bergman, Synthetic Detergents in the Soap Industry, J. Am. Oil Chem. Soc. 27: 88–90 (1950). 8. US DOE: Plant/Crop-based Renewable Resources 2020—A Vision to Enhance U.S. Economic Security Through Renewable Plant/Crop-Based Resource Use, DOE/G10098–3385, 1998. 9. Zemanek, G., and G.A. Reinhardt, Notes on Life-Cycle Assessments of Vegetable Oils, Fett/Lipid 101: 321–327 (1999). 10. Fostering the Bio-Economic Revolution in Biobased Products and Bioenergy: An Environmental Approach, Biomass Research and Development Board, 2001, pp. 1–20. 11. Patel, M., G.A. Reinhardt, and G. Zemanek, Vegetable Oils for Biofuels Versus Surfactants: An Ecological Comparison for Energy and Greenhouse Gases, Fett/Lipid 101: 314–320 (1999). 12. Van Haften, J.L., Fat-Based Food Emulsifiers, J. Am. Oil Chem. Soc. 56: 831A–835A (1979).
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