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Lignins: Structure and Distribution in Wood and Pulp

Lignins: Structure and Distribution in Wood and Pulp

In: Caulfield, D.F.; Passaretti, J.D.; Sobczynski, S.F., eds. Materials interactions relevant to the , , and industries:

Proceedings, Materials Research Society symposium; 11 1990 April 18-20;San Francisco, CA. Pittsburgh, PA: Materials Research Society; 1990: 11-20. Vol. 197. : STRUCTURE AND DlSTRlBUTlON IN WOOD AND PULP

John R. Obst USDA Service, Forest Products Laboratory,1 Madison, WI 53705-2398

ABSTRACT is the stuff that makes “woody.” Usually constituting from one-fifth to one-third of wood, lignin strongly influences its chemical and physical properties. A major use of wood is for the production of pulp for paper and products. The residual lignin in pulp greatly affects paper properties and, therefore, the uses of these pulps. Lignin is a rather unusual substance. Polymerized through coupling of propenylphenols, it forms several types of interunit bonds and a three-dimensional net­ work may result. The lignin of gymnosperms () is made up mostly of a single monomer type. As a result, the lignins of gymnosperms do not differ much from species to species. However, the lignins of angiosperms () do vary considerably among species because these lignins are derived from two monomer types which are often present in differing proportions. Furthermore, the ratio of monomer types may vary among different cell types and between cell regions within a species. This review, intended mainly for those unfamiliar with the details of lignin chemistry, will provide an overview of the formation, structure and distribution of lignins in wood and of the distribution of lignin in pulp fibers.

lNTRODUCTlON Lignin is the most important chemical constituent of wood. After all, while , and even some bacteria, can produce and , only lignified plants can be described as “woody.” Most of the unique chemical and physical properties of wood are determined by lignin. Indeed, the very word “lignin” is derived from the Latin “Lignum” meaning wood. Lignins occur in vascular plants and are especially plentiful in trees, ranging from about 18 percent to 35 percent of the wood. There are many mysteries concerning lignin, but none greater than the fact that relatively few people have ever heard of this remarkable substance. This lack of knowledge is particularly puzzling, because lignin is the second most abundant natural on after the . The reasons that wood utilization is a topic of increasing interest are obvious: wood is a very versatile material and has been used since prehistoric times; wood is renewable and abundant, with about one-third of the earth’s land mass covered in ; wood utiliza­ tion is extensive, being equivalent to twice the world’s production of steel, or 27 times the production of all plastics [1]. But perhaps one of the most important reasons that wood utilization is coming under increased scrutiny is that there are great economic and environ­ mental incentives to use wood, modified wood, wood pulps and wood composites in various engineered products. In the quest to best utilize wood, a thorough understanding of lignin is desirable. How­ ever, it is not possible to detail the intricacies of lignin here. To those new to wood and lignin science, I strongly advise you to review some of the excellent in these areas (e.g., 1-8). Also, if you have a particular interest or problem, do not hesitate to contact the author of a pertinent chapter in any of these texts-wood scientists are a friendly lot and always are eager to help. If the most important thing that I expect you to take from this paper is the bibliography, what can I expect to accomplish in the followingfew paragraphs? I hope to succeed in sharing my amazement and wonder in this marvelous material, lignin. I further hope to give you an

The Forest Products Laboratory is maintained in cooperation with the University of Wisconsin. This article was written and prepared by U.S. Government employees on official time, and it is therefore in the public domain and not subject to copyright.

Mat. Res. Soc. Symp. Proc. Vol. 197. ©1990 Materials Research Society 12 appreciation of the beauty and complexity of lignin and wood-and an appreciation of the basic knowledge already discerned and of some of the questions yet to be answered.

Lignin and Life Many consider trees to be the highest form of life on earth. They are the oldest of living individuals; they are unsurpassed in height and mass on land. The beauty, grace, strength, and function of trees not only inspire poets, but trees touch the souls of all mankind. Yet, it is not appreciated that trees, or, more correctly, their lignified ancestors, also may have been responsible for life as we know it. While life began in the seas some 3,500 million years ago, it was not until about 400 million years ago that vascular plants began to colonize the land. It is held commonly that evolution of the ability to produce lignin was critical to the development of vascular land plants. Vascular plants contain efficient systems for support and for conducting and food, and it is lignin that made these systems possible. Forests appeared and became dominant during the Period (360 million years ago). Although these first forests were unlike the gymnosperm and later angiosperm forests that followed, the -like lycophytes that prevailed were lignified. During this Period. huge amounts of atmospheric dioxide were converted into woody and enormous quantities of this biomass were buried, ultimately becoming deposits. But these early forests were hauntingly quiet, devoid of higher animals. The early atmospheres could not support much animal life, for was present only as a trace gas. Over time, photosynthesis, combined with the burial of organic carbon, increased the oxygen content to levels that ultimately would support higher animal life. Although wet climates and swamps are suggested as having major roles in the process of carbon burial, it is curious that no subsequent time has equalled the Paleozoic Era in biomass burial [9]. Although lignin imparts decay resistance to trees, most of the of lignin is through biodegradation by wood-rotting fungi. However, it has been suggested recently that fungi did not develop the ability to degrade lignin effectively until hundreds of millions of years after its first production by plants [9]. The ability of plants to produce lignin and the inability of decay organisms to degrade it may have combined to make the atmosphere oxygen rich and, therefore, more hospitable for higher animal and, eventually, human life. While the specifics may be debated, the important role of lignin and trees in the creation and maintenance of the atmosphere, the climate, and the environment is without argument. It is ironic that some of man’s activities, for example, burning coal (which is burning the forests of the Carboniferous Period) and widespread deforestation, in turn threaten the entire biosphere.

NATURE OF LIGNIN An amusing, but perhaps apt, definition of lignin by James Lovelock is that “lignin is an enigmatic substance.” More precisely, lignin is a polymer of certain substituted cinnamyl alcohols. Now, that did not sound so enigmatic, did it? Although I will not review the of lignin monomers, it is important to look at lignin formation a little more closely. The lignin monomers, which differ only in their number of methoxyl substituents, are p-coumaryl alcohol (I), (II) and (III) (Fig. 1). Lignification is initiated when a phenolic hydroxyl hydrogen atom is abstracted by the peroxidase to form a phenoxy free radical. The phenoxy radical can be delocalized to aromatic and sidechain . Such radicals then couple, leading to . Extensive coupling occurs between phenoxy radicals and radicals localized at the beta (or second from the ring) sidechain carbon. The resultant ether linkage, a beta-0-4 bond, is the most frequent interunit linkage in lignin. If it were the only linkage, our story would be pretty well over: lignin would be a linear polymer similar to many other natural and synthetic . 13

Figure 1-Substituted cinnamyl alcohols are the lignin monomers biosynthesized by plants.

However, the phenoxy radical and the beta radical are not the only ones that couple. Delocalization of the radical at other carbons, or formation of other radicals by hydrogen abstraction, can lead to other linkages, including carbon-carbon bonds. The formation of these other types of linkages, including bonds to more than one other phenylpropane unit, may result in a rather complicated polymer having a cross-linked and three-dimensional character. Additionally, lignin- bonds may be formed by free radical coupling or by addition reactions to quinone methides. Figure 2 depicts what a portion of a lignin molecule may look like [10]. When contemplating this figure, it should be remembered that this is an average representation of a lignin, and not a structural formula. Also, lignin is not necessarily two-dimensional, as drawn. While Figure 2 represents what a lignin might look like, what about other kinds of lignin? First, the terms ((softwood” and “” do not comment necessarily on the hardness of wood, but refer to botanical classifications. Softwoods, or conifers, sometimes are called evergreens, but are referred to more correctly as gymnosperms (examples are , , cedar, and ). Hardwoods are deciduous, broadleafed trees and are referred to more correctly as angiosperm . (Ginkgo is a broadleafed exception, being classified neither as a hardwood nor softwood; it is placed in its own division.) Oak, maple, , , hickory, and walnut are angiosperms. An example of an angiosperm lignin, beech, is shown in Figure 3 [11]. There are major differences between the gymnosperm and angiosperm lignins depicted in these figures. Almost all the aromatic units in Figure 2 contain one methoxyl group; about half the aromatic units in Figure 3 contain one methoxyl group, while most of the remainder have two methoxyl groups. As a result, major differences between these lignins are that angiosperm lignins generally have a greater number of beta-O-4ether bonds, fewer phenolic hydroxyls, and fewer cross links. Some of these chemical differences often are cited as the reasons for an easier chemical and biological degradation of angiosperm lignins compared to gymnosperm lignins. When the lignin units arising from the monomers p-coumaryl alcohol (I), coniferyl alcohol (II) and sinapyl alcohol (III) are discussed, they usually are referred to as p­ hydroxyphenyl, guaiacyl, and syringyl units, respectively. Gymnosperm lignins are all quite similar and they are classified as guaiacyl lignins that contain a small amount of p-hydroxyphenyl units. Angiosperm lignins are usually mixtures of syringyl and guaiacyl units, but their ratio varies widely among species. Most temperate zone angiosperm woods have a syringyl:guaiacyl ratio of about 1. However, there is no such thing as a typical an­ giosperm wood or lignin. The variety among angiosperm trees is almost overwhelming, and this variety extends to their lignin composition as well [12]. Among temperate angiosperms, the syringyl:guaiacyl ratio ranges from 0.1 for elder (Acer negundo) to over 2.5 for madrone (Arbutus menziesii). Most angiosperm tree species grow in the tropics, and among these are many that have very low syringyl contents. Another type of wood lignin is one that contains a relatively large amount of p­ hydroxyphenyl units (derived from p-coumaryl alcohol). Reaction wood is formed as trees react to external forces such as wind, mechanical stress, gravity, injury, and disease. In gym­ nosperms, the reaction wood is called compression wood. While there are many differences 14

Figure 2-Representationof a gymnosperm (softwood) lignin [10]. between normal wood and compression wood, the major chemical difference in compression wood lignin is that it contains a significant amount of p-hydroxyphenyl units. In addition, compression wood is more highly lignified than normal wood. Does it sound like “enigmatic” is beginning to fit lignin? Just wait, there’s more.

DISTRIBUTION OF LIGNIN IN WOOD Wood is not a homogeneous material. Not only are the chemical constituents of wood not uniformly distributed, there are various cell types to consider as well. These wood cells differ both chemically and physically. The two major cell types in gymnosperms are and ray parenchyma. Most of the wood (up to 95 percent) is composed of tracheids, which are classified further as earlywood or latewood. The latewood tracheids have thicker cell walls and this difference gives rise to the ease of distinguishing annual growth rings. Gymnosperm tracheids are about 3 to 5 mm in length. Parenchyma cells are short and occur predominantly in the radially oriented rays. Angiosperm woods are a little more complicated and contain fibers, sometimes tracheids, vessels, ray parenchyma, and longitudinal parenchyma cells. The ratio of cell 15

Figure 3-Representationof an angiosperm (hardwood) lignin [11]. types varies. For example, birch is composed of 64 percent fibers, 21 percent vessels and 15 percent parenchyma cells, whereas basswood has 36 percent fibers, 56 percent vessels and 8 percent parenchyma. The organization, or arrangement, of vessels may differ among species (for example, ring porous versus diffuse porous) and some angiosperm woods do not even contain vessels. Angiosperm fibers, often between 0.5 and 2.3 mm long, are considerably shorter than gymnosperm tracheids. Short fibers are generally unsuitable for . This is not, of course, an adequate lesson in wood anatomy, but it serves as an introduction to the heterogeneity and variability of woods. Before we discuss the distribution of lignin in wood cells, we need to be aware of the ultrastructure of the (see Fig. 2 in Paper U1.l of these Proceedings). The different cell wall layers have varying chemical compositions and different orientations of the cellulose fibrils. The thin layer between adjoining cells is the middle lamella, and the substances present in it act as a glue to hold the cells together. The thicker portion of the middle lamella where more than two cells come together is referred to as a cell corner. The first layer inward from the middle lamella is the primary wall. The next layer is the S1 region of the secondary wall, followed by the S2 region, which is the thickest layer. The final region is the S3, which is adjacent to the lumen, the hollow interior of the cell. This is far from a comprehensive discussion of cell wall ultrastructure, but this minimal introduction is essential to begin to appreciate the organization of wood and its structural components. The distribution of cellulose, , and lignin is not uniform throughout the wood. The concentration of lignin is generally greatest in the middle lamella and lowest in the S2 region of the secondary wall. But because the middle lamella is very thin, and the S2 is the thickest layer, most of the lignin in wood is contained in the S2. Up to this point, 16

Table I -Lignin concentration of middle lamella cell corners (CC) and S2 secondary wall of earlywood (EW) and latewood (LW) gymnosperm tracheids. Lignin concentration (%) S2 CC

Sample EW LW EW LW Method Reference Norway spruce 23 23 55 58 Hg tag EDXA 15 Loblolly pine 20 18 64 78 Br tag EDXA 17 Douglas-fir 25 23 83 90 UV 18 Black spruce 23 22 85 100 UV 19 Norway spruce TMPa 25 48 UVb 16

a Thermomechanical pulp. b UV microscopy was not on embedded cross sections, but on middle lamella “flakes.” Earlywood/latewood separations were not made. there is little disagreement among researchers on these descriptions. However, the actual quantitative distribution of lignin in wood cells is still somewhat unresolved. Table I presents a selected summary of the results of several lignin distribution studies on gymnosperms. The table shows that although there is fair agreement on the values for lignin concentration in the S2 layer, there is poor agreement on lignin concentrations in the middle lamella cell corners. A critical evaluation of the experimental procedures leads to the strong suggestion that the method of analysis may significantly influence the results. For example, while UV microscopy long has been used to indicate the distribution of lignin, the application of this method to achieve stringent quantitative measurements is uncertain [13]. A novel method for quantitatively determining lignin distributions was first to tag the lignin by reaction with bromine, then to measure the bromine distribution by scanning electron microscopy with energy dispersive X-ray analysis (SEM-EDXA). While attempts were made to correlate bromination results with the UV data, there was concern about the reactions that occurred and the quantitative results obtained [14, 15]. This technique was improved by replacing the bromination step with a derivatization of the lignin with mercuric acetate [13, 15]. This latter method has been reported to be insensitive to structural variation in lignin, and it labels each of the lignin aromatic rings with one mercuric derivative. As shown in Table I, the lignin content of the middle lamella cell corners in both earlywood (EW) and latewood (LW) is much lower by this method than as determined by UV microscopy. However, it is noted that a different application of UV microscopy, using unembedded flake samples [16], gave results more similar to the mercurization values. While the method of mercurization/SEM-EDXA has not been investigated rigorously or verified, it appears at this time to offer the most promise for determining lignin distributions. It is accepted generally that lignin distributions in gymnosperms are all rather similar. However, there may be some species effects. It would be best if the various methods could be compared using not only the same species but also wood from the same specimen. Application of some experimental methods to the distribution of lignin in angiosperm woods is complicated in that guaiacyl and syringyl composition may not be uniform. For example, in birch, vessel cell walls and the middle lamellae between all cell types are reported to be guaiacyl rich [17]. Analysis of isolated cell types and of cell fragments supports this nonuniform distribution for some woods [20-22]. Syringyl and guaiacyl units do not absorb the same amount of UV light nor do they react to the same extent on bromination. Critical assumptions must be made to use these methods for quantitative distributions, including an unambiguous method for determining the syringyl:guaiacyl ratio (which has not yet been 17

Table 11-Relative lignin distributions in white birch by several methods [13]. The lignin concentration (g/g) of the S2 layer by the UV method was 0.16, and by the bromination method, 0.14 [17]. UV Cell type Morphological region Mercurization microscopy Bromination Fibers Secondary wall (S2) 1 1 1 Fibers Middle lamella cell 3.0 4.5 3.1 corners Vessels Secondary wall (S2) 1.6 1.4 1.8 Ray cells Secondary wall (S2) 1.5 1.4 0.8

achieved). To illustrate this, the reported syringyl:guaiacyl ratios in various morphological parts of birch wood have varied significantly [17, 23, 24]. Because syringyl and guaiacyl groups react to the same extent with mercuric acetate, it appears that the method of mercurization/SEM-EDXA provides the best tool for lignin distribution studies in angiosperm woods. The results from the various methods for birch are summarized in Table 11. While there is some agreement among the methods, significant differences in relative lignin concentrations are seen. It has already been mentioned that there is no typical angiosperm wood, and this may be extended to state that it is not likely that there is a standard syringyl-guaiacyl distribution in angiosperm lignins [12]. It is more likely that the actual lignin distribution and the specific syringyl:guaiacyl distribution may be unique for many species. The definition of lignin as an “enigmatic substance’’ probably seems more understandable now. Although quite a bit is known, much about lignin composition and distribution remains to be determined.

DISTRIBUTION OF LIGNIN IN PULP FIBERS The amount and distribution of lignin in papermaking fibers are important because they will influence the properties of the resultant paper. The distribution of lignin also may be important in the development of new products such as composites. Wood can be treated mechanically and/or chemically to separate the cells into pulps for paper and production. Just as the properties of wood are influenced by lignin, so are the properties of products made from wood fibers. (As an aside, when papermakers refer to “fibers” they include all of the long cells useful for papermaking. In this context, “fibers” refer not only to the fibers of angiosperm woods but also to gymnosperm tracheids. In contrast, the “fiber” that is common to materials science is generally thought of as a thin, thread-like substance of rather uniform chemical composition.) To illustrate the extremes of the effect of lignin on paper products, the comparison between a bleached chemical pulp and a mechanical pulp often is made. Mechanical pulps often are prepared by grinding the wood to produce the papermaking fibers. Sometimes a relatively mild chemical treatment is used to reduce the grinding energy and improve pulp properties (referred to as chemomechanical pulps). These pulps have high lignin contents, and made from them reflect this contribution by lignin. An example of a high lignin paper is . Newsprint is not very strong, will rapidly discolor on exposure to light, and becomes brittle on aging. These properties do not impair seriously the utility of newsprint because of the short time that the product is used. The most common chemical pulp, kraft, is produced by treating wood at high temper­ ature (150°C to 170°C) with strong sodium in the presence of ions. Most of the lignin is depolymerized and dissolved. The resulting brown-colored fibers are very strong (hence the German word “kraft”); paper grocery are one example of a kraft 18

paper product. can be bleached to remove the color of the residual lignin to give white papers for writing and uses. Very high-yield pulps have not been delignified significantly and the lignin distribution has not changed appreciably from the untreated wood. However, depending on the pulping method, the surfaces of the papermaking fibers can be quite different. Mechanical pulps generally contain not only individual fibers, but also a large number of fiber bundles and broken fibers. Because the wood cells will be split mechanically at various locations, the exposed surfaces of mechanical pulp fibers are mixes of the various layers (middle lamella, primary wall, and secondary wall). Thus, the papermaking fibers will have both lignin-rich and cellulose-rich surfaces. Thermomechanical pulp (TMP) is prepared by pretreating wood chips with steam, fol­ lowed by fiberization in a disk refiner. At high temperatures (165°C to 185°C), the middle lamella is softened, and fiberization occurs due to separation at this layer (Asplund process). Although the lignin distribution has not changed, the exposed surface of the fiber is now mainly the middle lamella. This type of fiber therefore has a lignin-rich surface and is gen­ erally used in fiberboard production. If a TMP is produced at lower temperature (115°C to 130°C), the secondary wall is ruptured. Many of these fibers have surfaces composed mainly of cellulose. Also, because these surfaces become fibrillated, this kind of TMP fiber bonds better than some mechanical pulps. There are a number of other high- and moderate-yield pulp types. In general, if the pulping process does not remove much lignin, the lignin distribution in the pulp fibers will be determined by the mechanical degradation that occurs on fiberization. For pulps that have undergone significant delignification, the distribution of lignin in the pulp fibers will be determined by the topochemistry of delignification and any mechanical degradation that occurs in processing. In chemical pulping, delignification of the wood cells may not be uniform. For example, upon kraft or acid sulfite pulping, the secondary wall delignifies faster than the middle lamella in the initial part of the cook [25, 26]. Oxygen delignification of soda pulp fibers gives the opposite result: the middle lamella is delignified before the secondary wall [27]. Little topochemical effect was seen on neutral sulfite pulping and there was no effect reported for acid chlorite delignification [26]. Thus, the lignin distribution of the pulp fiber in chemical pulps will be determined to a large degree by the topochemistry of the pulping procedure used and by the extent of delignification. The topochemistry of alkaline delignification and the distribution of lignin type in birch have led to an interesting quandary. If the secondary wall delignifies preferentially, and if the secondary wall contains a syringyl-rich lignin, then the prediction is made that the lignin removed in the initial part of a kraft cook of birch should be syringyl rich. Not only is this prediction wrong, the opposite occurs: guaiacyl-rich lignin is dissolved in the early stages of delignification. One explanation offered for this failed prediction is that vessels, which may contain guaiacyl-type lignin, delignify first due to better accessibility. However, recent results in our laboratory (which included using vessel-enriched and fiber-enriched fractions of black oak in one series of experiments; madrone, which has syringyl-rich vessel lignin, in another series; and vessel-less angiosperm woods in a third series), indicated that vessels were not the sole source of the guaiacyl-rich dissolved lignin. These results were consistent, however, with another suggestion that a low molecular weight, reactive, guaiacyl-rich lignin may be distributed throughout the wood.

LIGNIN REACTIONS Lignin contains a number of functional groups and reactive sites. Reactive types include primary and benzyl alcohols, phenols, aldehydes, ketones, olefins, and ethers. Reactive aro­ matic sites include carbons ortho- and para- to phenoxy and methoxy substituents. There­ fore, it is not surprising that lignin can undergo a myriad of reactions, such as addition, derivatization, oxidation, reduction, condensation, and depolymerization. It is this latter 19 category, depolymerization, that has been studied most extensively, because the delignifica­ tion of wood is of immense economic importance in the production of papermaking fibers. Additionally, degradative depolymerization reactions have been used extensively in schemes. However, the other reaction types are often of more interest in developing new wood-based products. Considerable efforts have been made to utilize the waste lignins produced by chemical pulping. However, it must be recognized that processed lignins generally have significantly different chemical and physical properties from that of the lignin in wood. Waste lignin from kraft pulping can be used in formulations, but even after activation by methylola­ tion, kraft lignin must be combined with phenol-formaldehyde to achieve satisfactory performance [28]. Most of the research effort to utilize lignin in thermosetting resins has concentrated on phenolics. There is interest in incorporating lignin into other thermosetting resins, including polyurethanes, polyesters, polyamines and epoxies. However, reactions such as sulfonation, phenolation, alkoxylation, acrylation, carboxylation, and carboxymethylation must be used to modify the lignin to achieve compatibility in these systems [29]. While progress has been made, and the incentive to use renewable resources is great, the actual utilization of lignin as a replacement for synthetic polymers remains low.

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