Lignins: Structure and Distribution in Wood and Pulp

In: Caulfield, D.F.; Passaretti, J.D.; Sobczynski, S.F., eds. Materials interactions relevant to the pulp, paper, and wood industries: Proceedings, Materials Research Society symposium; 11 1990 April 18-20;San Francisco, CA. Pittsburgh, PA: Materials Research Society; 1990: 11-20. Vol. 197. LIGNINS: STRUCTURE AND DlSTRlBUTlON IN WOOD AND PULP John R. Obst USDA Forest Service, Forest Products Laboratory,1 Madison, WI 53705-2398 ABSTRACT Lignin is the stuff that makes trees “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 paperboard products. The residual lignin in pulp fibers greatly affects paper properties and, therefore, the uses of these pulps. Lignin is a rather unusual substance. Polymerized through radical coupling of propenylphenols, it forms several types of interunit bonds and a three-dimensional net work may result. The lignin of gymnosperms (softwoods) 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 (hardwoods) 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 plants, and even some bacteria, can produce cellulose and hemicelluloses, 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 polymer on earth after the polysaccharides. 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 forests; 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 books 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 plant 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 water and food, and it is lignin that made these systems possible. Forests appeared and became dominant during the Carboniferous Period (360 million years ago). Although these first forests were unlike the gymnosperm and later angiosperm forests that followed, the tree-like lycophytes that prevailed were lignified. During this Period. huge amounts of atmospheric carbon dioxide were converted into woody biomass and enormous quantities of this biomass were buried, ultimately becoming coal deposits. But these early forests were hauntingly quiet, devoid of higher animals. The early atmospheres could not support much animal life, for oxygen 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 decomposition 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 biosynthesis 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), coniferyl alcohol (II) and sinapyl alcohol (III) (Fig. 1). Lignification is initiated when a phenolic hydroxyl hydrogen atom is abstracted by the enzyme peroxidase to form a phenoxy free radical. The phenoxy radical can be delocalized to aromatic and sidechain carbons. Such radicals then couple, leading to polymerization. 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 polymers. 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-polysaccharide 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 softwood lignin might look like, what about other kinds of lignin? First, the terms ((softwood” and “hardwood” 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 spruce, fir, cedar, and pine). Hardwoods are deciduous, broadleafed trees and are referred to more correctly as angiosperm woods. (Ginkgo is a broadleafed exception, being classified neither as a hardwood nor softwood; it is placed in its own division.) Oak, maple, birch, aspen, hickory, and walnut are angiosperms. An example of an angiosperm lignin, beech, is shown in Figure 3 [11].

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