Wood and Cellulosic Chemistry, Second Edition Revised and Expanded

WOOD AND CELLULOSIC CHEMISTRY second edition, revised and expanded

edited by

David N.-S. Hon Clemson University Clemson, South Carolina

Nobuo Shiraishi Kyoto University Kyoto, Japan

M A R C E L

MARCEL DEKKER, INC. U E K K E R

NEWYORK BASEL

Library of Congress Cataloging-in-Publication Data

Woodandcellulosicchemistry / editedbyDavidN.-S.Hon, Nobuo Shiraishi.-2nded.,rev.and expanded. p. cm. Includes index. ISBN 0-8247-0024-4 (alk. paper) 1. Cellulose. 2.Wood-Chemistry. I. Hon,David N.-S. 11. Shiraishi,Nobuo. QD323.W662000 572'.56682"dc21 00-060

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Preface

Life and its surroundings are constantly changing within our dynamic world. As we stride into the new millennium, information technology and biotechnology continue to flourish. Rapid economic expansion, social development, and high demands for shelter, clothing, energy, and food for our overpopulated world have resulted in a desperate need for new and yet functional materials to support society’s infrastructure. Wood or lignocellulosic-based materials have made a significant contribution to the quality of living for human beings. With new developments in wood chemistry, scientists are confident that wood will continue to play an important role in fulfilling the needs of human beings. Over the past decade, the trend of emphasizingbio-basedtechnologieshasbeen observed worldwide. In February 1998, a long-term development project, PlanVCrop-based Renewable Resources 2020, was implemented among the U.S. Department of Agriculture, U.S. Department of Energy, and many U.S. companies, agricultural associations, and universities. The aim of the project was to obtain novel chemicals from plant- and crop-based renewable resources in order to widen the usage of crops, the yield of which has been significantly increased through bio-technological advancements. The recent movement of producing foods by means of genetically manipulated seeds should enhance the effectiveness of this project. Before the start of this project-which is considered the future of the petrochemicalindustry-majorchemicalcompanies in the UnitedStates,suchasDow Chemical,Dupont, and Monsanto,havebeenchanging their strategies in research and development.Theyhavestrengthened their bio-basedresearch field, trying to yield as many chemicals as possible from biomass. They are developing production technologies for ethanol, sorbitol, lysine, tryptophane, citric acid, lactic acid, poly(lactic acid), erythritol, 1,3-propanediol,etc.,frombiomass.Furthermore, in August of 1998 PresidentClinton issued an executive order, “Developing and Promoting Biobased Products and Bioenergy,” to further the development of a comprehensive national strategy that includes research. development, and private sector incentives to stimulate the creation and early adoption of technology needed to make bio-based products and bio-energy cost-competitive in national and international markets. Also. there has been research in so-called “green chemistry.” In this new methodology. biomass is the recommended MW material. Thc importance of wood and cellulose rescarch is thus rccognizcd. iii

iv

Preface

Since the publication of' the first edition of this book, considerablc advancement i n various fields ofwood chemistry has been made, as can be attested by many scientific publications in addition to well-attended international conferences. We contacted the contributors to the first edition, soliciting their opinions on revising and updating the book, and we received tremendous support from them as well as the publisher. Unfortunately, and inevitably, several authorswereunableto participate, buttheyrccomrnended their successors. Although most of the chapters in this new edition carry the same titles as those i n the previous edition, they have all been extensively revised and updated. In addition, this edition includes several new chapters representing important threads in the total fabric of wood chemistry. These new chapters cover the subjects of chemical synthesis of cellulose, preservation of wood, preservation of waterlogged wood, biodegradable polymers from lignocellulosics, recycling of wood and fiber products, and pulping chemistry. As editors, we feel fortunate to have been able to recruit some of the best talent in the field to this endeavor. We thank the contributors for their efforts. Any praise for the content should be addressed to them, and comments and criticisms to us will be welcome.

David N.-S. Hon NoDuo Shiruishi

Contents

1.

Ultrastructure and Formation of Wood Cell Wall

1

Minoru Fujittr trrlcl Hirnshi Hcrmdrr

2.

ChemicalComposition and Distribution

SI

Shirr) Strkrr

3.

Structure o f Cellulose: Recent Developments in Its Characterization

83

Frrrtlittrktr Hot-ii

4.

Chemistry of Lignin 109 Akirn Srrkrrkihrrr-rrcrrlrl Yoshillit-o Strrlo

S. Chemistry of Cell Wall Polysaccharides

175

Ttrtltrslli lsllii rrrltl Kuxrt1tr.w Shirr1i:rr

6.

Chemistry of Extractives Toshitrki U r ~ r t w ~ ~ r

7.

Chemistry of Bark

2 13

243

Kokki Sakoi

8. ChemicalCharacterization o f Wood and Its Components

275

Jrrirtw Htrexr cult1Jucrrlittr Frret-

9.

Color

IO.

3x5 NoDrryrr Mitlcwlrr.cr

~ t n dDiscoloration

D m i t l N . -S. Horl

trrld

Chemical Degradation

443

Krcrrl-ZotrgLrri

1 I. Weathering and Photochemistry o f Wood IltrlGcl N.-S. Hot1

S I3

V

vi

Contents

12. Microbial, Enzymatic, and Biomimetic Degradation of Lignin in Relation to Bioremediation 547 Rrkqfumi Huttnri und Mikio Shimadu 13. Chemical Modification of Wood

573

Misato Nothoto

14. Chemical Modification of Cellulose599 Akirn Isogai

15.

ChemicalSynthesis of Cellulose627 Furniaki Nukatsubo

16. Wood Plasticization

655

Nohuo Shiruishi

17. Wood-Polymer Composites

701

Hirnshi Mizunztrchi

18.

Adhesion and Adhesives

733

Hiroslli Mizurturc.hi

19. Pressure-SensitiveAdhesives and Forest Products765 Hiroshi Mizunlcrchi

20. 21.

Wood-InorganicCompositesas Shiro Suku Preservation of Wood

Prepared by the Sol-Gel Process

795

D u r r d D . NicAolcrs

22.

Preservation of Waterlogged Wood

807

David N.-S. Hot1

23.

Biodegradable Plastics from Lignocellulosics Muriko Yr)shioku m c l Nohuo Shirtrishi

74.

Recycling o f Wood and Fiber Products849 Tcrkcrrlori Arirrrn

25.

Pulping Chemistry 859 Giirn11 Gelle~rstcclt

827

781

Contributors

TakanoriArima Department of Biomaterial Sciences, Graduate School and Life Sciences, The University of Tokyo, Tokyo, Japan

of Agricultural

Jaime Baeza Departamento de Quimica, Facultad de Ciencias, Universidad de Concepcicin, Concepcicin, Chile Juanita Freer Departamento de Quimica, Facultad de Ciencias, Universidad deConcepcicin, Concepcicin, Chile Minoru Fujita Division of Forest and BiomaterialsScience,Graduate culture, Kyoto University, Kyoto, Japan Goran Gellerstedt Department of Pulp and PaperChemistry Institute of Technology, Stockholm, Sweden

School of Agri-

and Technology, Royal

Hiroshi Harada Division of Forest and Biomaterials Science, Graduate School riculture, Kyoto University, Kyoto, Japan

of Ag-

TakefumiHattori

Wood Research Institute,Kyoto University, Kyoto, Japan

David N.-S. Hon Carolina

School of Nature Resources, Clernson University, Clemson,South

FumitakaHorii

Institute for Chemical Research,

Kyoto University, Kyoto, Japan

TadashiIshii Division of Bio-Resources Technology, Forestry and Forest Products Research Institute, Ibaraki, Japan Akira Isogai Department of Biomaterial Science, The University of Tokyo, Tokyo, Japan Yuan-Zong Lai Faculty of Paper Science and Engineering, SUNY College of Environmental Science and Forestry, Syracuse, New York vii

viii

Contributors

NobuyaMinemura

Hokkaido Forest Products Research Institute,Hokkaido, Japan

Hiroshi Mizumachi

Professor Emeritus, The University of Tokyo. Tokyo, Japan

FumiakiNakatsubo Division of Forest and BionlaterialsScience,GraduateSchool Agriculture, Kyoto University. Kyoto, Japan Darrel D. Nicholas State, Mississippi MisatoNorimoto

Forest Products Laboratory, Mississippi State University, Mississippi

Wood Research Institute, Kyoto University, Kyoto. Japan

Shiro Saka Department of Socio-Environmental Energy Science,GraduateSchool Energy Science, Kyoto University, Kyoto, Japan KokkiSakai

of

of

Faculty of Agriculture, Kyushu University. Fukuoka, Japan

AkiraSakakibara Laboratory o f Wood Chemistry. Research Group of Bioorganic Chemistry, Division of Applied Bioscience, Hokkaido University. Sapporo. Japan Yoshihiro Sano Laboratory of Wood Chemistry. Research Group of Bioorganic Chemistry, Division of Applied Bioscience, Hokkaido University, Sapporo, Japan Mikio Shimada Wood Research Institute, Kyoto University, Kyoto, Japan Kazumasa Shimizu Division of Wood Chemistry, Forestry and Forest Products Research Institute. Ibaraki, Japan Nobuo Shiraishi Division of Forest and Biomaterials Science. Graduatc riculture. Kyoto University, Kyoto. Japan

School of Ag-

Toshiaki Umezawa Wood Research Institute. Kyoto University, Kyoto. Japan Mariko Yoshioka Division of Forest and Biolnaterials Science, Graduate School of Agriculture, Kyoto University, Kyoto, Japan

Ultrastructure and Formation of Wood Cell Wall Minoru Fujita and Hiroshi Harada Kyoto University, Kyoto, Japan

1.

A.

GENERAL STRUCTURE OFWOOD AND WOOD CELLS Wood

SoftwoodandHardwood In introduction it should be understood that the term “wood” refers to the secondary xylem formed by cell division in the vascular cambium of both gymnosperms (softwoods) and angiosperms(hardwoods). and especially i n Ginkgo. Similarsecondary xylem may be produced by plants of different form and structure, such as vines and shrubs, the xylem of which may be an important resource of pulping material. The structure and formation of the secondary xylem are discussed in this chapter. Both softwoods and hardwoods are widely distributed on earth, from tropical to arctic regions.The xylem of those species present in moderate-temperate to arcticregions is characterized by distinct growth rings, in which some anatomical differences can be noted. In the softwoods consisting mainly of tracheids (approximately 90% of wood volume), the latewood (summer wood) can be distinguished from the earlywood (spring wood) by its smaller radial dimensions and thickercells walls. Theseanatomicaldifferenccsare reflected in the higher density of the latewood compared with the earlywood. In softwoods growing i n tropical or warm areas, growth rings cannot be distinguished due to the indistinct boundary between earlywood and latewood. As with the softwoods, hardwoods are also present i n tropical to arctic regions. In colderregions,hardwoodspeciesaredeciduous, whereas i n tropical regions, they are predominantly evergreen and their growth rings are difficult to recognize. The macroscopic characteristics of hardwoods are reflected in the distribution and number of different ccll typessuch as vessels (pores),parenchyma, and fibers. Although fibers may account for only 25% of wood volume, in some cases, for hardwood, it may be as high as 50-70%. In contrast to the tracheid as the main cell in softwoods,hardwoods have a variety of cells. Some deciduous hardwoods such a s oak or elm have very large vessels concentrated at the beginning of annual rings. Suchwoodsare called “ring porous wood,”whereas otherdeciduousspecies and almost all evergreen hardwoods in which the vesselsare evenly dispersed over the annual ring are callcd “diffuse porous wood.” The above dis1.

1

Harada 2

and

Fujita

tinctions represent extremes and there are many intermediate arrangements of the vessels. Variations in arrangements of these vessels with other xylem tissues such as parenchyma are reflected in the “figure” and “grain” of the wood itself when it is cut from the tree. The physical properties of wood such as density also result from such arrangements of the cells.

2. Sapwood and Heartwood When a tree stem is cut transversely, a portion of “heartwood” can be seen frequently as a dark-colored zone near the center of the stem. This portion is surrounded by a lightcolored peripheral zone called “sapwood.” The sapwood or at least the outer part of the stem conducts water throughthe tissue where the water is transpired, and mineral nutrients are also carried with water from the roots into the wood. In addition, the sapwood has living parenchyma tissue, which often plays some physiological role such as the storage of starch or fat. From this point of view, the sapwood is considered an active xylem tissue. In contrast to sapwood, heartwood is dead xylem. As the tree matures, all parenchyma cells of the sapwood die, and other typesof cells such as tracheids or fibers become occluded with pigment composed of polyphenols and flavanoids supplied mainly from the ray parenchyma. The bordered pits of gymnosperms become aspirated, whereas the vessels are blocked by tyloses or gum in angiosperms. Thus, heartwood does not participate in water conduction. Although the conducting and physiological functions are lost in heartwood. the durability of wood against rot or insect decay is remarkably improved due to an addition of such pigments. Moreover, these pigments confer a variety of beautiful colors on wood. 3. Reaction Wood Reaction woods that appear on branches or a leaning stem by any force such as a landslide or snowfall have a peculiar nature. Once reaction wood is formed as a biological response, the living tree tries topreserve the original position of its stemorbranches.For the practical use of woods, the reaction woods have not been appreciated very much because of their different characteristics fromnormalwood in both a physical and a chemical sense. The occurrence and nature of reaction woods contrast quite a bit between softwood and hardwood. In softwood trees, the reaction wood forms at the lower side of a leaning stem or branches, where the compression stress reacts on the xylem. Therefore, this reaction wood is generally called “compression wood.” compression woodis heavy and appears dark brown on account of its highly lignified tracheid walls (see Section II), which seem to adapt to compression stress. Thus, compression wood is easily distinguished from normal wood by its dark color. The cambial activity at the lower position of a leaning stemorbranchacceleratesveryquicklyanddevelops a widercompressionareathan normal wood on the opposite side. Through the accumulation of compression wood tracheids over many years, a leaning stem will return gradually to the vertical position. The annual rings of such a stem, however, are conspicuously eccentric. On the contrary, reaction wood in many species of hardwoods is formed at the upper side of a leaning stem or branches where the xylem loads the tensile stress. Therefore, such reaction woods are called “tension wood.” Fibers of tension wood have a slightly lignified cell wall (see Section 11) that is adapted to the tensile stress just like a bowstring. It is not so easy to distinguish this area from a normal one on account of its slightly pale tone, in comparison to the case of compression wood.

Formation Ultrastructure and

Wall

of Cell

3

In fact, the occurrence of both reaction woods is averytroublesomeproblem in wood utilization. These reaction woods, however, are interesting material for the examination of wood structure and formation, as will be noted often in the following sections.

B. Wood Cells Wood cells are produced in the vascular cambium from two types of meristematic cells: the fusiform initial and the ray initial (Fig. l ) . Since cells derived fromthe fusiform initials that are upright in the stem occupy a major part of xylem, woods show remarkable anisotropism. The principal functions of xylem tissue are water conduction from roots to shoots, the mechanical support of a huge tree body, and a physiological role such as the storage of starch. Although these functions are common in both softwoods and hardwoods, the xylem of the latter is more evolved than that of the former, being adapted to each function.

softwood

hardwood

pits

%I

fusiform initials

a

&#

xial parenchyma cells

.B@'

ray tracheid FIGURE 1 hardwood.

axial parenchyma c e l l s

ray \ parenchyma ray cell initials p

Shapes of major wood cells fromthefusiformandrayinitialsinsoftwoodand

Harada 4

and

Fujita

I n softwoodsand Ginkgo, tracheids, beingmajorcells of xylem, are considered relatively underevolvedbecausetheyhavebothconductiveandmechanical properties. Bordered pits, the occurrence of which define a cell as a tracheid, are very important to the regulation of water flow. On the other hand, cell wall thickness is related directly to the strength of tracheids. The earlywood tracheids, therefore, seem to be well adapted to the conducting function whereas the latewood tracheids are loaded with the mechanical property, judging from their peculiar shapes. On the earlywood tracheids, well-developed pit pairs are distributed abundantly between the neighboring tracheids, and the cell walls of latewood tracheids are very thick. Only a small number of fusiform cells are subdivided into strand cells by horizontal partitions and compose an axial parenchyma. These parenchymatous cells survive in the sapwood for many years, being different from the tracheid, in which the protoplast is lost soon after differentiation (see Section III), and are part of some physiological functions. In somegenera of Pinaceae, axial resin canalssurrounded by epitherial cells are constructed. The occurrence and structure of resin canals are often used in the identification of softwoods, although the volume of such resin canals is very slight in wood. Ray cells are derived from the ray initials and elongated radially. A series of these ray cells make a ray parenchyma. Needless to say, these parenchyma cells are alive in the sapwood and are tied to the storage of nutrients such as starch or fat and also the transportation of some metabolites between the phloem and the heartwood. As a result, they must be related to the secretion of heartwood substance into the tracheids. Also, in some genera of Pinaceae, radial resin canals surrounded by epitherial cells are formed in many ray tissues, and more ray tracheids occur in the ray tissues. Hardwood xylem can be characterized by the development of vessel elements and wood fibers specialized for water conduction and the mechanical property, respectively. The vessel elements construct a very long and thick tube, namely, a vessel, being joined vertically with one another by a perforation that has a more developed style compared with the bordered pit pairs between tracheids. The occurrence of perforation distinguishes the vessel elements from the tracheids. Wood fibers elongate remarkably and possess very thick cell walls. The most developed type of cell, having simple pits (see Section II), is called libriform wood fiber. On the other hand, there are some intermediating cells from the tracheids to the vessel elements or wood fibers, i.e., vascular tracheids, vascentric tracheids, andfiber tracheids. The fiber tracheids are often included in the categoryof wood fibers. because there is no need to separate them from the libriform wood fibers in the practical use of wood. Vessel elements, wood fibers, and various types of tracheids in the hardwoods lose their protoplast just after the development of their secondary wall. However. in some hardwood species specialized wood fibers that remain alive for several years and often store starch grains are formed; they are called “living wood fibers.” Axial parenchyma cells, which are dispersed on the transverse section of softwoods, are clustered at the vessel periphery or form a group that is often linked tangentially. Resin canals that are surrounded by epitherial cells are formed in many genera of Dipterocarpaceae and a few Leguminosae. Ray parenchyma cells sometimes aggregate and develop a so-called broad ray. The broad rays make a peculiar figure on a board. especially on the radial surface, as observed in oak or beech. Cells contained i n the ray also vary in their anatomical features. Some of them are upright or square at the marginal position. These variations are used for the identitication of hardwoods [ l ] . Both axial and ray parenchyma cells are apparently concerned with physiological functions-for instance, the storage of nutrients or heartwood

Formation Ultrastructure and

Wall

of Cell

5

formation. Radial resin canals or latex tubes are formed in the ray tissue of some tropical hardwoods.

II.

ULTRASTRUCTURE OF WOOD CELL WALL

Wood is a natural composite material and a chemical complex of cellulose, lignin, hemicelluloses, and extractives [2]. Cellulose is the framework substance, comprising 40-50% of wood in the form of cellulose microfibrils, whereas hemicelluloses are the matrix substances present between cellulose microfibrils. Lignin, on the other hand, is the encrusting substance solidifying the cell wall associated with the matrix substances. The significance of lignin as the encrusting substance can be demonstrated by examination of the lignin skeleton created by the acid removal of carbohydrates (Fig. 2). The roles of these three chemical substances in the cell wall are compared to those of the constructing materials in the structures made from the reinforced concretein which cellulose, lignin, and hemicelluloses correspond, respectively, to the iron core, cement, and buffering material to improve their bonding.

A.

Cellulose Microfibrils

The crystalline nature of cellulose in wood has been demonstrated by studies with X-ray diffractometry and polarization microscopy. This crystalline nature was also confirmed by the electron diffraction patterns of the secondary walls of wood cells in selected areas [3]. Figure 3a isatransmissionelectronmicrograph of a longitudinalsection of latewood tracheids of Pinus densifloru, showing the intercellular layer (I), and the S, and S, layers. The electron diffraction diagram is of a selected area in S2 (Fig. 3b), which is represented by a small circle. The (101), (loi), and (002) of the equatorial reflections and (040) of

i

FIGURE 2 Electron micrograph of ultrathin transverse section of earlywood tracheids from Pinus densgora, showing thedistribution of lignin inthe cell wall, which was skeletonized using the hydrofluoric acid technique.

6

Fujita and Harada

FIGURE 3 (a) Electron micrograph of ultrathin longitudinal section of tension wood fibers from Pinus densiporu. (b) The corresponding diffraction diagram taken from the encircled area.

Formation Ultrastructure and

Wall

of Cell

7

the meridional reflection can be seen. It should be noted that crystallographic planes are based on the Meyer and Misch (1937) model of the unit cell of cellulose I, i n which the h axis (the fiber axis) is vertical. I t iswell known that in the wood cell wall, celluloseexists in the form ofthin threads with an indefinite length. Such threads are called cellulose microfibrils, and they play an important role in the chemical, physical, and mechanical properties of the wood. The greenalga, Kdorzia. which is oneform of Chlorophyceae,hasbeenstudied intensively by microscopists and crystallographers as an excellent material for the ultrastructural study of cellulose microfibril. Why then is W o t z i a used for the study of the cellulose microfibril of the wood cell wall? Because the cell walls of Valonia are unlignified, their microfibrils are readily isolated. Furthermore, as described later, Vrtlonicc microfibrils are approximately 20 nm in width, which is about five times larger than those of wood, and they are highly crystallized. However, the difference between algal microfibrils such as those of Vhlorli~zand ordinary ones produced by the higher plants also must be stressed. One of the differences is the selectively uniplaner orientation of algal microfibrils, that is, the ( 101) plane facing the cell surface, while cellulose microfibrils of higher plants are randomly oriented, although both microfibrils are laid along the cell surface i n their longitudinal direction [3,4]. The other is the crystallographic heterogeneity in algal microtibrils as detected by NMR [ S ] , and a triclinic system mixed with an ordinary monoclinic system was detected by electron diffraction [6]. The interface between these systems is not yet shown, although the former amounts to about 50%.

1. Dimensions of the Cellulose Microfibril As described above, it is clearly demonstrated through electron microscopy that the cellulose molecular chains are organized into strands as cellulose microfibrils. Figure 4 shows transmission electron micrographs of disintegrated cellulose microfibrils negatively stained withuranyl acetate. Figures .la and 4b, respectively, show the microfibrils of klonicr tnc~cmphyscrcell wall and the holocellulose of Pirlus drnsijur-a. A discrepancy in the size of the crystalline region of cellulose, obtained by X-ray diffractometry and electron microscopy, led to differing concepts as to the molecular organization of microfibrils. Frey-Wyssling 171 regarded the microfibril itself as being made up of a number of crystallites, each of which was separated by a paracrystalline region and later termed“elementary fibril” by Frey-WysslingandMuhlethaler 181. The term “elementary fibril” is therefore applied to the smallest cellulosic strand. Muhlethaler [ 10,l 11 applied this term to the cellulose fibril with a diameter of approximately 3.5 nm, using the negative-contrastpreparationtechnique for electron microscopy.Preston and Cronshaw [91,on the otherhand,considered the microfibril tohaveasinglecore of cellulose crystallite surrounded by a paracrystalline region. The width of cellulose microfibrils is reported to vary in different cellulose materials [ 121. For instance, as shown i n Fig. 4, Vrrlorzia cellulose microfibrils, being about 20 nm wide, are much larger than those of wood holocellulose. Shown in Table 1 are the crystallite size and microfibril width for several cellulose materials [ 131. The crystallite size was estimated with Scherrer’s equation at the reflection (002) or (101) of X-ray diffractometry, whereas the microfibril widthsweremeasured directly from the electron micrographs. The width range and mode width are also included in this table. It should be noted that the size of crystallites varies in different sources of cellulose materials, for results from both X-ray diffractometry and electron microscopy. According to Heyn [ 141, the negative stain can penetrate only the regions accessible to water. Thus, the translucent parts seen on the electron micrographs correspond to the

8

Fujita and Harada

FIGURE 4 Electron micrographs of the cellulose microfibrils of Vuloniu mucrophysa (a) and of Pinus densifloru holocellulose (b) (disintegration, negatively stained with uranyl acetate), showing the difference of cellulose microfibril width between wood and Vuloniu.

Formation Ultrastructure and

Wall

of Cell

9

TABLE 1 Crystallite Size and Microfibril Width

Crystallite Microfibril size"width Samples Pinus dens$ora

2.02 002)

-

Untreated 2.76Holocellulose

(2.5)b

Populus euramericana

4.1

layer Gelatinous Normal wood Valonia 15-30

(002)

2.2 (002) 14.3 11.9 (101)

(20.0)b

'Reflection examined. hModewldth. Source: Ref. 12.

crystalline regions of cellulose. Therefore, the difference in the microfibril width must be ascribed to that in the size of cellulose crystallites. In addition, the values obtained are not always equal to the 3.5 nm in elementary fibrils proposed by Muhlethaler [ l l ] .

2. Cross-Sectional View of Cellulose Microfibrils Figures 5a and 5b are similar electron micrographs of the ultrathin cross section of cellulose microfibrils from Valonia macrophysa and the gelatinous layer of Populus euramericana tension wood fiber. These were obtained by means of diffraction contrast in the bright-field mode foran epoxy resin-embedded section. This technique reveals a crystalline region as a dark zone dueto electron diffraction. Thus, cellulose microfibrils have a highly

FIGURE5 Electron micrograph of ultrathin transverse section of cellulose microfibrils (diffraction contrast in the bright field mode), showing their cross-sectional views: (a) from Valonia macrophysa; (b) from G layer of Populus eurarnericana.

Harada10

and

Fujlta

crystalline nature. It is interesting to note in Fig. 5a that a Vuloniu microfibril does not have any subunits corresponding to the elementary fibrils [13,17]. Additionally, cellulose microfibrils appear to be almost square in their cross section in both wood and Vuloniu [15-171.

3. Crystalline Structure of Cellulose Microfibrils Figure6shows Vuloniu macrophysu microfibrilsmechanicallydisintegratedwithacid, taken by diffraction contrast in the bright-field mode. Cellulose microfibrils can be seen as the dark areas, again indicating the highly crystalline structureof cellulose. However, the internal crystalline ultrastructure of cellulose microfibrils is not revealed by electron microscopic techniques such as negative staining and diffraction contrast, because lattice imagesof cellulose microfibrils are not obtained. The most important reason is that cellulose microfibrils are damaged by the electron beam and their crystalline nature is destroyed by irradiation under normal photographing conditions. Recently, the crystalline ultrastructure of cellulose microfibrils in Vuloniu macroby a specially developed technique for taking highphysu cell wall has been revealed resolution lattice images [15,16]. Figure 7 is an example of the lattice fringe substructures from disintegrated cellulose microfibrils. This micrograph shows the lattice image of 0.60 nm, corresponding to thatof the (101) plane. The lattice spacingof 0.60 nm is also shown in the electron and optical diffraction patterns. The lattice lines are observed regularly at about 20 nm width across the cellulose microfibril and are also visible along its length for more than 50 nm without any disruption. Figure 8 shows images of the cross section of cellulose microfibrils obtained using ultrathin sections. Lattice lines at0.60, 0.54, and 0.39 nm are visible in this figure. Therefore, a single microfibril is indicated as the individual crystal. Accordingly, it is suggested that the crystal line subunits as 3.5 nm elementary fibril and periodicity in its length does not exist inside the cellulosemicrofibril. Unfortunately, lattice images of cellulose microfibrils have not yet been taken in wood cellulose, since wood cellulose has low crystallinity and the size of the cellulose

FIGURE6 Electron micrograph of cellulose microfibrils fromVuloniu mucrophysu (disintegration, diffraction contrast in the bright-field mode), showing the crystalline nature of cellulose microfibrils.

Formation Ultrastructure and

Wall

of Cell

11

FIGURE 7 Lattice image of a disintegrated cellulose microfibril of Valonia mcrophysa, showing the lattice spacing of 0.60 nm.

FIGURE 8 Lattice images of the cross-sectional face of cellulose microfibrils from Valonia macrophysa, showing the lattice spacings of 0.60, 0.54, and 0.39 nm, respectively.

Fujita and Harada

12

microfibril is rather smaller compared with that of Valonia. In the near future, beam damage at room temperature against wood cellulose microfibrils would be reduced at least 10 times with cryo-electron microscopy. The cellulose microfibrils of the gelatinous layer of poplar (Populus eurntnictrna) tension wood in disintegrated samples are found to have many kinks, suggesting that the cellulose microfibril is highly crystalline [13]. However, the cellulose microfibrils of the gelatinous layer, about 100 nm in length prepared by ultramicrotome, become shorter than their original length upon hydrolysis [ 131. As a result, the crystalline regions in the cellulose microfibril of wood cell wall are thought to havesomecrystallinedislocations caused by chain ends [ 181. The cellulose microfibrils consist of a core crystalline region of cellulose surrounded by paracrystalline cellulose and short-chain hemicellulose. Lignin encases them and binds them into a rigid structure of wood cell wall.

B. Cell Wall Layers and Lamellae At the first step of differentiation of a woody cell, the living protoplasm produces a primary wall (P) that can be extensively increased in its surface as the cell develops. The substance between the primary walls of adjacent cells is called the intercellular layer (I) or the middle lamella. Since it is difficult to distinguish the region between the I layer and the P wall in the mature cell wall, the termcompoundmiddlelamella (CM) is generally used to designate the combined I layer and the two adjacent P walls. After the enlargement of the cell ceases. the cell wall layers are formed by the apposition of wall substances onto the inside of the primary wall. These wall layers are called the secondary wall (S) Although the primary wall andsecondarywallare classified by the ontogenetic process of plant cells, actual layered structures have been examined by the orientation of cellulose microfibrils. As a result the concept of lamellae, which are composed ofvery thin layers of only one or two cellulose microfibril width, is introduced. Cell walls were thickened by the appositional supply of these lamellae from the protoplast, so cellulosic interlamellae bridges are not accepted in the concept. The lamnella structure on the secondary wall is interesting in both physical and chemical properties of wood. Kerrer and Goring proposed a composite model with hemicelluloses and lignin [IS]. Although it is very intelligent, actual microfibril orientation on a lamella may fluctuate more [2O,2 I ] .

1.

Tracheids and Fibers

Figures 9 and I O are polarized photomicrographs at crosscd polars of transverse sections of tracheids and fibers, respectively. Both reveal the three-layered structure of the cell wall due to the differences in the orientation of cellulose microfibrils. According to the concept of Kerr and Bailey [22], normal wood cell wall consists of P and S walls, and the S wall is composed of a relatively narrowor thin outer layer (S,), an inner layer (S3), and a relatively thick middle layer (S?). However, the P wall cannot be distinguished in the figure due to the strong birefringence of the S , layer adjacent to the P wall. The S , and S, layers appear bright in the photographs, whereas the S, layer is at total extinction. That the birefringence of the S, layer occurs to a lesser degree than that of S , in the fibers of F q u s crewtcl indicates the poordevelopment of the S,. Despitesubsequentextensive studies with electron microscopy, the concept and terminology described above are still commonly accepted. Figure 1 1 is an electron micrograph ofan ultrathin transverse section from Cty7tomer-iajapotzicn, stained with silver protenate. It shows the intercellular layer (I), different

Ultrastructureand Formationof Cell Wall

13

I

FIGURE 9 Polarized-lightphotomicrograph of transverse section from earlywood tracheids of Pinus c/ensiforu, showing thethree-layeredstructure of the cell wall due to the birefringence of cellulose microfibrils.

layers of the secondary wall (S), and the warty layer (W) in an earlywood tracheid. The same layering structure from an earlywood tracheid of Pinus densiflot-a is shown more clearly in a longitudinal section that was skeletonized by the hydrofluoric acid technique (Fig. 12). Figure 16, (pg. 18). shows the texture of the P wall diagrammatically. The microfibril orientation in the primary wall was interpreted by the multinet growth hypothesis proposed by Roelofsen [23] and supported for the differentiating conifer tracheids by Wardrop[24].

FIGURE 10 Polarized-lightphotomicrograph of transverse section from wood fibers of Fugus crenatcr, showing the same structures as in Fig. 9.

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Fujita and Harada

FIGURE 11 Electronmicrograph of ultrathintransverse section of an earlywood tracheidfrom Cryptomeria japonica, showing I, S,, S*, S1,and W (warty layer) at the final differentiating stage of the cell wall.

FIGURE 12 Electronmicrograph of ultrathinlongitudinal section of earlywood tracheidsfrom Pinus densifom (skeletonized cell wall with the hydrofluoric acid method), showing the I, P, S,, Sl, and S3 of the cell wall.

Formation Ultrastructure and

Wall of Cell

15

In the multinet hypothesis, the microfibrils are first deposited transversely to the cell axis and passively shifted longitudinally during cell extension. From an opposite viewpoint, an orderedfibrilhypothesiswasproposed by Roland et al. [25] in order to interpret the crossed polylamellated structure in the primary wall of parenchyma cells. According to this hypothesis, whether the orientation of microfibrils becomes transverse, oblique, or longitudinal is determined at the time of deposition of cell wall and may not be changed thereafter. Recently, Fujii et al. [26] proposed a modified multinet hypothesis of microfibrils orientation in the primary wall. The difference between this conceptand Roelofsen’s theoryisthattheshift of microfibrilorientation during cell extension is made in the individual lamella and each lamella becomes thin on the outer surface of the P wall due to extension. The three layers of the secondary wall, designated S,,S2, and S3,are organized in a plywood type of construction. The S , or S3,with a large microfibril angle to the cell axis, is designated as a flat helix, and the S2,with a small angle, as a steep helix (see Fig. 16). It is also shown that the layers themselves are of lamellae of microfibrils with varying amounts of shift in orientation, visible in the transmission electron micrograph. The S , is composed of several lamellae with alternating S and Z helices of microfibril orientation [28,29], and this structure in the S , is termed “crossed fibrilar texture” [28]. Figure 13 is

FIGURE 13 Electronmicrograph of theradialinnersurfacein a differentiatingtracheidfrom Pirzus densgoru (direct carbon replica), showing the microfibrillar orientation of the newly deposited microlamella crossing that of the underlying microlamella in S,.

Harada16

and

Fujita

a transmission electron micrograph of a replica of the inner surface of the differentiating early wood tracheid of Pinus densgora forming the S,, showing the criss-crossed texture of the microfibril orientation in the two different lamellae. The middle layer of the secondary wall (S,) is the thickest within the layers of the secondary wall. Therefore, the S, contributes most to the bulk of the cell wall material and is a compact region in which a high degree of parallelism of microfibrils exists. The S, isathinlayer of flathelices of microfibrilorientationasseenin S,. As opposed to the highly oriented S,, the S, is loosely textured. The S,, birefringent to a somewhat lesser degree than theS, in wood fiber, shows that this layer is poorly developed. Althoughthe S2 exhibitsamicrofibrillarorientationwithsteephelices,there are transition lamellae on its inner and outer surfaces. Several lamellae in these regions show agradualshift of microfibrilanglesbetween S, and S, andbetween S, and SJ [30]. However, the gradual shift of microfibril angles is more abrupt between S, and S, than between S, and S,. The transition lamellae in the secondary wall are not detected in TEM micrographs of ultrathin sections, since this lamella is relatively thin compared with the S, and S,. The method for evaluating microfibril angles in the secondary wall of wood cells was proposed by Yamanaka [31]. Figure 14 is a "EM micrograph of a transversely oblique section of an earlywood tracheid from Pinus dens.ijZora (stained with KMnO,). The curve through black dots shows the changes of the microfibrillar angle with respect

F FIGURE 14 Electron micrograph of ultrathin oblique section of an earlywood tracheid in Pinus densijffloraand microfibril angles in the secondary wall: top, I. bottom, lumen side.

Formation Ultrastructure and

of Cell Wall

17

to the tracheid axis from the top S, to the bottom S,. The horizontal line in the upper part of the figure shows the angles of microfibrillar orientation, the symbols (-) and (+), respectively, referring to Z and S helices. The gradual changes of the microfibrillar angles from S, to S, and from S, to S, are shown there. The helical cellulose microfibril orientationin the S, is typically demonstrated in Xray diagrams of wood [32]. The arcs at 0.39, 0.54, and 0.60 nm in the X-ray diagram of wood show that the cellulose crystallites (microfibrils) lie in a helix around each wood fiber or tracheid. The microfibril orientations are Z helices in the S, and S helices in the S,, although S, is the crossed arrangement of S and Z helices. Preston [32] suggests that the structure with various microfibril angles in the secondary wall passes through only one cycle, but this may be the brief duration of wall thickening in higher-plant cell walls compared with that in algae. Roland and Mosiniak [33] presented a diagram regarding the changesof cellulose microfibril angles in the case of a secondary wall of tracheids and wood fibers (Fig. 15). Figure 15 illustrates the case between the S, and S, layer. The change of microfibril angle is regular and continuous between the S , and S, layers,butitstopsduringthe deposition of the S , layer. Afterwards the change of microfibril angle reopens toward S, layer deposition. The texture of cellulose microfibrils in the P and S walls of softwood tracheids and hardwood fibers is shown as a schematic diagram in Fig. 16. The thin primary wall (P) consists of a loose aggregation of microfibrils oriented more or less axially to the cell axis on the outer surface. The S, layer is a flat helix but with crossed structure, whereas the S, layer is a steep helix and the S, layer is a flat helix. There are intermediate layers: the S,,, present between the S , and S, layers; and the S,,, between the S, and S, layers. The spiral thickening is the ridge of microfibrils that exist on the inner surface of the S, layer. The spiral thickening is considered part of the S, layer becauseof its continuity with the S, layer and parallel arrangement to the S, layer microfibrils.

FIGURE 15 Schematic diagram of the change of microfibrilorientationfrom three-layered structure of the cell wall. (From Ref. 32.)

S, and Sz in the

Harada18

and

FuJlta

FIGURE 16 Schematic diagram of the microfibril orientation in the primary wall and different layers of the secondary wall from tracheids and fibers: Po,PI;outer and inner parts of the primary wall; SlzrS23, intermediate layers between S , and S , and between S2 and S,, respectively.

The warty layer is one of the major structural features of wood cells found by electron microscopy [34]. It was first foundin softwood tracheids and laterin the tracheids, vessels, and wood fibers of hardwoods (see Fig. 11). The major chemical constituents of warts arereported to be lignin and hemicelluloses according to examination by component removal treatment of ultrathin wood sections [35]. The warts are believed to arise from the extra wall materials and remains of cytoplasm that are deposited on the S, layer through the plasma membrane [36,37]. The warty layer is not found in all softwoods and hardwoods [30,38]. Parham and Baird [39] have pointed out that the appearance of warts in wood has a phylogenetic trend. Softwood tracheids and primitive hardwood cells nearly always have warts, but as the cell types become more advanced or specialized, they become wart-free.

2. Vessels The texture of cellulose microfibrils in the walls of specialized cells such as vessel elements and parenchyma cells cannot be readily described as in softwood tracheids and hardwood fibers. A concept of standardized cell wall organization in vessel elements was, however, represented by Kishi et al. [40,41]. The microfibrils in the primary wall extend straight and are arranged parallel to one another within one lamella, and the wall consists of three parts, P-outer, P-middle, and P-inner, each showing a different microfibril orientation. The microfibrils are oriented transversely with respect to the vessel axis in the Pouter and are oriented at random in the P-middle. The P-inner consists of a crossed polylamellatedstructure. It isalsoreportedfromthe examination of vessel elements from nearly 30 Japanese hardwoods with polarizing and electron microscopy that the layered structure of the secondary wall can be classified into three categories: the typical threelayered structure, an unlayered structure, and a multilayered structure. The typical threelayered structure consists of S,, S2,and S3 similar to those of softwood tracheids and hardwood fibers, although the S, and S3 layers are thicker than those of tracheids and

Formation Ultrastructure and

Wall of Cell

19

wood fibers. The unlayered structure has only microfibrils, with the orientation of a flat helix. The multilayered structure has more than four layers, in which microfibril angles to the vessel axis change. This type of structure contains in some cases the so-called bowshaped pattern. Figure 17 isa TEM micrograph of the transverse sectionof Cinnamomum camphora and shows the microfibril angle and helix in the part of the bow-shaped pattern appearing on the vessel wall of the multilayered type of structure. As shown in Fig. 17, the pattern results from the progressive changes of microfibrillar orientation in the wall from 90" to 0" and from 0" to 90".

3. Parenchyma Cells In spite of the fact that parenchyma cells had been generally considered to have only primary wall, thoseof wood are reported sometimes to develop secondary wall, in addition to complicated primary wall. It is evident from recent studies that ray and axial parenchyma cells in both softwoods and hardwoods have variations or complexities in their wall structure that are not observed in the cell walls of tracheids and wood fibers. In softwoods, the cell wall structure of the ray parenchyma cells was divided into five categories by Fujikawa and Ishida [42]. However, as shown in Fig. 18, it is fundamentally classified into two types; the firsttype consists of the primary walland protective

b

90 FIGURE 17 Electron micrograph of ultrathin oblique sectionof a vessel wall stained withKMnO., from Cinnamomum camphora, showing a bow-shaped pattern (a) and the microfibril angles and helices (b).

20

Fujita and Harada

L-

i

I l

I I

P

S1

i

S2

1

I

FIGURE 18 Schematicdiagram of themicrofibrilorientation in the cell wall of softwood ray parenchyma cell: (a) the first type; (b) the second type. (From Ref. 41.)

layer (Fig. 18a), and the second type consists of the primary wall, secondary wall, and protective layer (Fig. 18b) [42]. However, the protective layer and a random arrangement of microfibrils is omitted in this figure. The P, appears with microfibrils of almost parallel orientation to the ray cell axis, the P, with the network appearance of microfibrils, and the P3 with several crossed polylamellate at microfibrillar angles of 30-60". It is interesting to note that the ray parenchymacell wall in thediploxylem of Pinus develops in two stages: that is, the primary wall and inner protective layer are fornled in the sapwood, and just before the heartwood is developed, the secondary wall and protective layer are deposited. In the axial parenchyma cells of softwood, the cell wall texture is very similar to that of ray parenchyma cells, except that the microfibrils are arranged in a flat helix with respect to the cell axis in the P , . In hardwoods, the primary wall ofray parenchyma cells has the so-called polylamellated structure proposed by Chafe and Chauret 1431. It was pointed out by Chafe and Chauret [43] that an isotropic layer and protective layer characterize the layered structure of the secondary wall of xylem parenchyma cells in hardwoods. According to examinations of thechemicalcomponents of these two layers using aseries of treatments on serial ultrathin sections, both a protective layer and an isotropic layer are rich in hemicelluloses and contain some pectic substances and cellulose microfibrils, but they have little lignin at the first stage of their developing process and become lignin-rich after the deposition of the inner secondary walls on them [44]. Consequently, both layers are considered the

Ultrastructureand Formation Wall of Cell

21

same in their origin and are called “amorphous layer” by Fujii et al. [M].Figure 19 shows electron micrographs of transverse sections by ray parenchyma cell from Tiliu juponicu; Fig. 19a shows cell walls skeletonized with hydrofluoric acid, while Fig. 19b shows sodiumchloride-treatedcellwalls.Blackzonesshow an amorphous layer indicating the presence of much lignin (Fig. 19a), but these disappear through delignification as seen in Fig.19b. It has been reportedbyFujii et al. [45] fromtheexamination of ray and axial parenchyma cell walls from about 50 species of Japanese hardwoods that the secondary wall is composed of a lignified cellulosic layer (CL) and an amorphous layer (AL) and that the cell wall structure can be classified into three types according to the presenceand organization of these two kinds of layers. Figure 20 is a schematic diagram of the cell wall organization of hardwood ray parenchyma cells: (1) 3CL-type, (2) 3CL+AL-type, (3) 3CL+AL+IL-type. CL refers to the lignified cellulosic layer that is similar to the ordinary wood cell wall, whereas ICL refers to the lignified cellulosic layer inside the amorphous layer (AL). The 3CL-type wall structuremay be considered thestandard structure of parenchyma cells of hardwoods, whereas the 3CL+AL-type wall structure occurs in cells that have extensive pit contact with vessels.

FIGURE 19 Electron micrographs of ultrathin cross section of the ray parenchyma cell from Tilia japonica, showing the amorphous layer (AL) of the secondary wall: (a) delignified cell wall; (b) cell wall skeletonized using hydrofluoric acid treatment.

Harada22

and 3CL

1-q

AL

Fujita

ICL

.: .............,..,..........:..-

x:

(a)

(C)

FIGURE 20 Schematic diagram of the cell wall organization of hardwood ray parenchyma cell, showing three types of wall structure: (a) 3CL; (b) 3CL AL,(c) 3CL + AL ICL.

+

4.

+

Reaction Wood Cell Wall

As described above (see Section I), softwood reaction wood is called compression wood and hardwood reaction wood is called tension wood. Figure 21 is a polarizing micrograph of compression wood tracheids from Pinus densijlora, and it demonstratesthat the S, layer present in normal wood tracheids is lacking. This is clearly shown in an electron micrograph of a cross section of a compression wood tracheid from Pinus densijioru (Fig. 22), and the presence of deep spiral checks in the S, layer is also revealed. The microfibrillar orientation of the S, layer is nearly 45",

Harada24

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Fujita

cellulose of the G layer is highly crystalline. Its microfibrils are oriented parallel to the longitudinal axis of the fiber and the G layer is easily separated from the remainderof the fiber wall. Another structural feature of the tension wood fiber wall is that the G layer deposits on any one of the normal three secondary wall layers, S,, S2,and S,. The secondary wall of the tension wood fiber consists of three types, that is, S, G , S, S, G , and S, Sz + S, + G, depending on the wood species or part within a stem. Consequently, the G layer is called “the S, layer” when we refer to the S,, Sz,and S, layers.

+

C.

+

+

+

Sculpturing of the Wood Cell Wall

Cellulosic fibers such as cotton, ramie,and jute are relatively simple, smooth-walled composites of lamellae, but in wood the cell walls are almost invariably interrupted by gaps (pits) and sculpturing features.

1. Pit Structure Pits are gaps in the secondary wall of wood cells. There are two types of pits: bordered pits and simple pits. Generally, pits are present as pairs between two adjacent cells: bordered pit pairs, simple, and half-bordered pit pairs. In softwood, the pit border region of the cell wall is composed of border thickening (BT), S,, S2,and S, from the outer part of the cell wall as shown in Fig. 24. The presence of BT and thicker S, are features of the pit border wall. The microfibrils circle at theBT and sweep around thepit at the individual layers S,, Sz,and S,. In softwood bordered pit pairs, many species show a thickening at the center of the pit membrane. The torusis suspended from fine cellulosic strands to form a margin around the torus as shown in Fig. 25. The margin consists of an open net of

FIGURE 24 Schematic diagram of pit border organization in bordered pits of softwood tracheids. BT, initial pit border.

Formation Ultrastructure and

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25

FIGURE 25 Electronmicrograph of the surface of pitmembranefrom Cryptomeria japonica (direct carbon replica), showing the pit membrane structure. T, torus; M, margo.

radially oriented microfibrils superimposed on an unoriented primary wall network, and it extends from the torus to the pit border. The torus is generally convex lens-shaped in cross section. On the other hand, the torus is seldom thickened in other cases. The former is true in species of the Pinaceae and Sciadopityaceae families, and the latter case involves species of Ginkgoaceae, Taxaceae, Chephalotaxaceae, Cupressaceae, Podocarpaceae, and Araucariaceae. The pit membrane of a half-bordered pit pair between tracheids and ray or axial parenchyma cells is quite thick. There is no torus in the center of the pit membrane, and no openings can be seen even at high magnification with an electron microscope. The central feature of the membrane structure of simple pit pairs in the interparenchymatous pits is the presence of plasmodesmatal pores. In hardwoods, the cell wallof the pit border consists of BT, P, S,, S*, and S3in tracheids and fiber tracheids of hardwood, like softwood tracheids. However, the pit border of vessels lacks not only BT but also S, in some parts of the pit border region [47]. The pit membrane of the bordered,half-bordered, and simple pit pairs in hardwoods is equal in thickness, exhibiting the primarywall texture, and there is usually no evidence of a torus. However, the presence of a torus in the intervessel pit membrane is reported in several species of hardwoods [48]. The pit membrane of simple pit pairs has plasmodesmatal pores as seen in softwoods.

Fujita and Harada

26

2. Vesture Pits In hardwoods, the pit chamber and pit apertures that are decorated by outgrowths of wall material are known as vestured pits. The outer growths of vestured pits are constructed chemically of lignin, hemicelluloses, and a little pectin [35].The shape and size of the outgrowths of vestured pits are variable. The development of vestured pit outgrowths is regarded as similar to that of warts. 111.

GENERAL DEVELOPMENT OF WOOD AND WOOD CELLS

A.

Vascular Cambium and Cambial Activity

One of the characteristic features ofa tree is the formationof the vascularcambium cylindrically surroundingastem,branches,and roots. The vascularcambiumproduces xylem inward and phloem outward. This sequence allows a tree to make itself a huge body. The cylindrical vascular cambium occurs through a series of developing meristem, namely, the apical meristem, the occurrence of procambium in the ground meristem, the growth of the vascular bundle, and the connection of intrafasicular cambium by the development of interfasicular cambium. The vascular cambium is composed of two types of meristematic cells. One is the fusiform initial occupying the major part of meristematic cells, and the other is the ray initial. Through their active cell division, parts of xylem and phloem are produced. However, since their activity in cell division is to a great extent affected by the season and weather, the result is the formation of annual rings in temperate regions. These initials must also multiply themselves on the tangential plane according to the increment of stem diameter. These two types of cell divisions can be distinguished by the direction of the division. The former division is defined as “periclinal division,” and the latter is called “anticlinal division” (see Figs. 26 and 27). Periclinal division is the mostimportant in view of woodformationandthus is discussed in detail. Cell division of the initial is extremelyrapid in spring. Moreover, several derivative cells (xylem mother cells) just inside the initial also have the ability to multiply through periclinal division. It is practically impossible to determine the true initial cell among these dividing cells. Therefore, just for convenience, a group of these cells is consideredcambialcellsand their area is called the cambialzone. In softwoods, the fusiform cells derived from the cambial zone differentiate directly into the tracheids except in onlyafewcasesinvolving the formation of the parenchymastrand,whereasthey differentiate into vessel elements, wood fibers, and several types of tracheids and parenchyma cells in hardwoods. Carnbial activity and the derivative differentiation are very important sequences in the growth of trees, environmental preservation of forests, and production of wood as a biomaterial. That is, they are the major sink of organic substances which are synthesized on leaves by CO, fixation, and then the major source of other life activities such as insects and also human beings. A detailed review of the vascular cambium has been published by Larson [491.

B.

Differentiation of Wood Cells

The tern1 “differentiation” has several meanings in the fieldof biology. I n this chapter, the term will be applied to the restricted case of the process of cell devclopment from the just-forming state i n the meristematic tissue to the mature state at which it is accomplished.

P

..

R.

l!

I f

FIGURE 26 (a) Light micrograph around the cambial zone ( C ) ,phloem (Ph), and enlarging xylem (E) from a transverse section of Robinia pseudoacacia. Most fusiform cambial cells are undergoing periclinal division, except for a trace experiencing anticlinal division (arrow).(b) Electron micrograph of fusiform and ray cambial cells. (c) Cytoplasmic feature of enlarging cells. 27

28

Fujlta and Harada

FIGURE 26 Continued

For instance, the differentiation of tracheids implies their maturing process from birth at the cambial zone to death after the secondary wall formation, by which both water-conducting functions and mechanical properties are given to the tracheid. The method of differentiation of parenchyma cells is quite different from that of tracheids, because they have only the primary wall or an underdeveloped secondary wall on the primary wall. They may be already functioning at thecambial zoneand have the ability to redifferentiate. Therefore, their differentiation is not addressed here. First of all, the differentiationof softwood tracheids, which is the most basic process of wood cell formation, will be discussed in detail. When a specimen block around the cambial zone is taken from the stem of a living tree and then a transverse section is observed under a light microscope, it is noticed that cells are piling up on a radial row from the mature phloem to the mature xylem through the cambial zone (Fig. 27a). The differentiating zoneof tracheids is located between the cambial zone and the mature xylem area. If the whole life of a particular tracheid from birth to death could be traced in situ in a tree stem, the tracheid differentiation would be clearly elucidated. However, it is really impossible to do so because the cells must be fixed with some reagent to preserve their cytoplasmic structure. Regrettably, their dynamic cell actions evolve into static phase by fixation. Therefore, the differentiating process of a tracheid must be deduced from the static cell structure of a series of differentiating tracheids. From this point of view, the differentiating zone of earlywood tracheids is favored for the precise examinationof their differentiation. In the spring, the production of tracheids from the cambial zone is very

D

3

Q

n 0

7

3

s

0

3

FIGURE 27 (a) Light micrograph of the cambial zone (C) and the derivative tracheids in five differentiating stages (RE, S,, S,, S3, and F) between phloem (Ph) and mature xylem (MX)from Cryprorneriujuponicu. (b) Enlarged view of S, depositing cells.

h)

W

Harada30

and

Fujlta

FIGURE 27 Continued

constant and, as a result, a series of differentiating tracheids is lined up in an orderly fashion along a radialrow from the just-formed stage to the mature stage. This series can be considered a good substitute for the life story of a tracheid, and since it is possible to trace the series using many microscopic techniques, the differentiating processof a tracheid can be grasped dynamically by tracing these differentiating tracheids along radial rows (Fig. 27a). Thedifferentiation of tracheids will be separatedintoseveral developing stages. Tracheids are pushed out in an inward direction from the cambial zone so as to begin enlargement. In the case of tracheids, the enlargement proceeds mainly in the radial direction, whereas enlargement in the tangential and longitudinal directions is very slight. Therefore, it may be appropriate to call this stage the radial enlarging (RE) stage. This fact results in the thinner radial walls of tracheids and the reorientation of cellulose microfibrils that may occur during the extension of the wall. The tracheid in this stage is composed of primary wall similar to the wallof cambial stage (C) cells. The thinned wall is recovered by the supplement of new wall materials on the inner surface. The extended wall is so fragile that it is often damaged and tom off during sampling of a specimen block from a living stem. After the enlargement of cell size, tracheids thicken secondary wall layers with the formation of the S,, S?, and S, layers. These stages are performed by the active deposition of cellulose microfibrils. However, the outermost region of the cell wall, including the intercellular layer, the cell comers, and the primary wall, is lignified during the S, stage. This lignification, which will be called “intercellular layer W i g n i fication,” may play an important role in stabilizing the cell size and conjugating the differentiating cells with one another. This I-lignification is accomplished in the middle phase of the S2 stage. Hemicelluloses are also supplied just after the deposition of cellulose

Formation Ultrastructure and

of Cell Wall

31

microfibrils (see Section IV). The secondary wall, which is still porous and flexible after the deposition of hemicelluloses, is encrusted with lignin and becomes very rigid. The lignification of the secondary wall, which willbe called “S-lignification” in contrast to “I-lignification,” is the most active after the S, stage,namely, in thefinal (F) stage of differentiation, although its initiation can be detected already during the S, stage. In this F stage some decorative elements such as warts or helical thickenings are added on the inner surface of the wall. After the wall layers develop, tracheids lose their cytoplasm by autolysis. Amorphous substances that have embedded the pit membrane also dissolve enzymatically sometime in the F stage. Tracheid differentiation is completed as this point and water conduction is achieved in the mature xylem (MX). The differentiation of vessel elements is characterized by enormousexpansion in both the radial and tangential directions. Although the developing stages of tracheids cannotbe applied directly to those of vessel elementsdue to a different secondary wall structure, the relationship of enlargement to secondary wall thickening and lignification is consideredsimilar to the sequence of tracheid differentiation. Needless to say, the formation of perforation pores is completed by the disappearance of the membrane itself, apart from the removal of only an embedding substance in the bordered pit pairs. On the contrary, the differentiation of wood fibers is characterized by the remarkable elongation in cell length that occurs at cell tips [50], and the other properties of differentiation are quite similar to those of tracheids. In hardwood, although the differentiation of both vessel elements and wood fibers proceeds simultaneously, vessel elements differentiate faster than wood fibers. How long a wood cell needs for its differentiation is also an important question. The time requirement for differentiation has been deduced by several methods, but the results are conflicting. A detailed timerequirementwascalculated for young trees of several softwoods by means of periodic inclinations for the internal date marking on the xylem. By these markings and the cell numbers contained in each differentiating stage, a time requirementofabout three weeks for passingthrough the five developingstages of a tracheid (RE, S , , S?, S,, and F) was calculated [51].

C. Cytology of Wood Cells Cambial cells, the differentiating cells of the tracheid, vessel element, and wood fiber, and also living parenchyma cells possess protoplast in their cell lumens. The most peculiar cytoplasmic structure of the fusiformcambialcells is the existence of ahuge central vacuole (CV) (Fig. 26b). This vacuole is maintained during the differentiation of tracheids (Fig. 27b), vessel elements,and wood fibers, whereas the cytoplasmicregion(Cy) is restricted to the very narrow area between the plasma membrane (Pm) and the vacuole membrane tonoplast (T) (Fig. 26c). On the contrary, the ray cambial cells and their derivative parenchyma cells are full of cytoplasm in their cell lumen, although several smaller vacuoles sometimes occur (Fig. 26b). In the axial parenchyma cells formed by the redivision of a young fusiform derivative, the central vacuole becomes small, and the cytoplasmic area expands in the reverse way. In spite of the cambial zone and differentiating xylem existing under the circumstance of very high pressure between the bark and mature xylem, the cellscontained in this areahaveonlya thin wall. Althoughvacuolation is generally considered a symptom of cell decay, the conspicuous vacuolation of these cells is supposedtoplay a very important role in the sustainment of their cell shapeunder presure. The enlargement of cell volume also depends on the turgor pressure of the vac-

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uole. In fact, it can be pointed out by arealneasurements that the vacuole is the best developed of the cells at the RE stage, when cells are just expanding (Fig. 28a). A nucleous is located around the central position of a fusiform cell in the longitudinal direction [SO], but on the transverse plane it is still pushed to one side of the cell lumen by vacuolation (Fig. 26c). In the cytoplasm, ordinary cell organelles such as Golgi bodies (Go),rough and smooth endoplasmic reticula (r-ER and S-ER). mitochondria (M), plastids (P), small vesicles (v), ribosomes, microtubules, and so on, are contained in a very narrow cytoplasmic region, although the occurrence of these cell organelles except microtubules between plasma membrane (Pm) and tonoplast (T) is not so abundant during the difl’erentiation of tracheids, wood tibers, or vessel elements. On the contrary, the cytoplasm of differentiating ray and axial parenchyma cells is crowded with many cell organelles. Especially, starch grains in the plastids and lipid droplets are very abundant, and r-ER are also well developed, whereas microtubules are very scarce. Ray cambial cells and mature ray cells arc almost identical to the differentiating parenchyma cells in their cytoplasmic features(Fig.26b). However, the number and size of starchgrains and lipid droplets contained in mature parenchyma cells change during a year 152-541. Thecytoplasmicfeatures of tracheidschange 21 little both i n quality and quantity according to their differentiation. The increase and decrease of the cytoplasmic area and its constituents of cell organelles were revealed by the combined use of light microscopy (Fig. 28a) and electron microscopy (Fig. 2%) on the differentiating zones of normal and compression woods of Cqptmtwricr juponicu. Areas of cell outline (A,,,,,,,;,,),cytoplasmicsurface (A,,,,,,,,,),and central vacuole (A,,,,,,,,,) were measured on an enlarged light micrograph of the transversesections o f differentiating tracheids using a digitized system connected to a computer (Fig. 28a). The nxasuretnent was performedalongthedifferentiatingtracheids, which were numbered from the initiation of the S , stage, and about 30 radial rows were surveyed. These radial rows of tracheids were sectioned at random in their longitudinal direction, so that the average value of tracheids of the same cell number reflects the makeup of volume i n each region. Areas of cell wall (A,v,J and cytoplasm (A,,.,,,,,,,,,,,) can be calculated by finding the remainder between those of the cell outline, the cytoplasmic surface. and the central vacuole, respectively. These values are diagramed in Figs. 29 and 30. I t should be noted that the cytoplasmic volume of both the normal and compression woods has two peaks during tracheid differentiation. The earlier peak i n both cases is at the intermediating phase from the S , stage to the S, stage, whereas the later one is located just prior to the initiation of the S , or F stage. On the contrary, during the S2 thickening, the cytoplasm is rather poor. Following this, proportions i n RE, S , , early S?, middle S,, late S,, and S, stages (Fig. 29) show the relative constituents of major cell organelles surveyed by electron microscopy (Fig. 2%). The general change in these cell organelles can be grasped by inultiplying the relative value by the total area of cytoplasm diagramed in Fig. 29. In addition to the changes in these cell organelles, the plasma membrane, important to the transportation of materials in and out of the cytoplasm, is always observed during tracheid differentiation and disappears after the development of the cell wall. The cytoplasmic features of differentiating wood fibers and vessel elements are also similar to those of tracheid differentiation, although the vacuolation of vessel elements is Inore extreme. In some species, such as acer or black locust, the living wood tibers are formed during the later period of a growing season. Their protoplast remains after the development of a cell wall and stores many starch grains in the cytoplasm for several years. Therefore, the mature xylem i n the sapwood is composed of ray and axial parenchymacells and sometimes the living wood fibers as the cells have aprotoplast.The

Formation Ultrastructure and

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33

8 5

.. B

l I

FIGURE 28 Light (a) and electron (b) micrographs of transverse sections of S? depositing tracheids from Cryptomeria japonica. Areas of some cell organelles, for instance, Golgi bodies (Go), were measured with electron micrographs such as those shown in (b).

Fujita and Harada

9

B

r 6 S 4

3 2 1

0

0

c e l l number

C

RE

SI

s?r

Sam

Sar

S3

F

FIGURE 29 Changes of cytoplasmic volume during the differentiation of normal wood tracheids and proportions of some cell organelles at the stages of RE, S , , early S2, middle S,, late S?, S,, and F in Cryptonzeriu jqoniccr (see Figs. 28a and 28b).

FIGURE 30 A change of cytoplasmic volume during the differentiation of compression wood (see Fig. 28a).

Formation Ultrastructure and

Wall

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35

cytoplasmic features of these living cells are affected by the season, and also some of them seem to be specialized in their cell shape and cytoplasm. That is, the cells surrounding a vessel, particularly those directly contacted, become envelope-shaped and are very rich in Golgi bodies, r- and S-ERs, ribosomes, and mitochondria common to cells of active phase. On the other hand, their storage function seems to decay. These vessel-associated parenchyma cells are shown to concern the transportation of materials with vessel lumens [52,53] and also the formation of tyloses or gum that plugs the vessel lumen [55-571.

IV.

FORMATION OF WOOD CELL WALL

There is no doubt that cell walls are formed by the actions of cell organelles contained in each cell, even though some precursors of wall materials such as sugars may be supplied by the intercellular transport system.Therefore, cell wallformation is realized by the careful observation of cytoplasm that is undergoing cell wall development. It is also very important to select proper plant materials for precise examination of cell wall formation, becausegeneral plant cellsbearmanyphysiologicalfunctions in addition to cell wall formation. Moreover, the cell wall is composed of several types of chemical materials that are supposed to be metabolized by different cell actions, and their deposition on the wall may overlap. These complicated factorsare the major reason that the formation mechanism of plant cell walls has not yet been explained clearly, in spite of many investigations. Differentiating wood cells such as tracheids are very useful materials from this point of view. That is, they construct a very thick secondary wall, of which the ultrastructure and chemical components have been examined in detail, and the general sequence of cell wall formation can be traced through the series of differentiating cells along a radial row. Besides, the cell organellespossessed by thesecells are concernedonlywith cell wall formation, except vacuolation for the turgor pressure. In addition, if the depositing phase of individual wall materials such as cellulose, lignin, or hemicelluloses can be detected separately in differentiation, the relationship of cell organelleswith the metabolism of those materials would be grasped more clearly.

A.

CelluloseMicrofibrilDeposition

Cellulose is the mostbasic cell wall material in thewhole plant and it constructsthe framework structure of cell walls in the form of crystalline microfibrils as mentioned in Section 11. The formation has been studied using various plant cells from lower plants such as fungi or algae, to higher plants. Plasma membranes located just inside developing cell walls seem to be the most important cell organelles in relation to cellulose microfibril deposition. Although cross-sectional structures composed of unit membraneshadbeen observed by ordinary electron microscopic methods such as chemical fixation and ultrathin sectioning, faceviewsalongthemembranebecamepossiblewith the development of freeze-fracture or etching methods coupled with replication. Small particles on the outer surface of plasma membranes had been reported in various plant cells. The epoch-making discovery, however, was the characteristic assembly of granules located in the interior of the plasma membrane and revealed on the fractured surface in green algae such as Oocystis, Myclusteriu, or Vcloniu [58-601. Interesting structures have been reported using mainly single or naked cells such as algae [62], actobacteria [63,64], and cotton fiber [61], which can be frozen rapidly.

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Small granule assemblies at the tips of microfibrils are called the terminal complex. These granules are considered to be the enzyme for the polymerization of cellulose molecules and their alignment i n the conlplex in relation to crystallization [64]. In underevolvcd algae such as vrtlonicr, the assemblies are large and linear, corresponding to their thick microfibrils [62]. On the other hand, evolved plants have small groups called rosettes [6 11. Thus, the form of the complex is considered to be related to the shape of the cellulose microfibrils and also to the evolution of plants. The polymerization and crystallization of cellulose microfibrils have been surveyed in detail using AcetoDactor- . x - y l i t z ~ ( ~which ~/, produces a thin cellulosic thread [6S] and has various mutants (661. The sequence of cellulose synthesis described above has not been traced in differentiating wood cells yet, because the freeze-fracture method is difficult to apply to them. However, cellulosic frameworks of wood cell walls are supposed to be constructed by a similar way. perhaps by a rosette. Although the freeze-fracture method isvery effective for visualizing characteristic structures such as the terminal complex on the membrane, the overall structure of differentiating wood cells depositing cellulose microfibrils must be examined by ordinary sectioning methods. Especially in wood cells depositing secondary wall layers, a control lnechanism for microfibrillar orientation is a very interesting viewpoint. Also, as the cellulose deposition is accompanied by the synthesis and accumulation of hernicclluloses and lignin, actions of various cell organelles must be traced in detail. Hence, the deposition phase of cellulose microfibrils i n differentiating tracheids can be traced in both normal and reaction woods. The phase can be detected by the increment of cell wall thickness (671, by means of autoradiography [68-70] (Fig. 3 l ) , and by chemical analysis of selectively collected rnaterials in some developing stagesof tracheids [ 7 I 731 and wood fibers [74](Fig. 32). The results obtained by these methodsshow that cellulose microtibrils are supplied to the wall mainly in the early and middle phases of the S, and S2 deposition stages. In addition to these deposition stages of cellulose microfibrils, most noticeable were the deposition of the G layer in the tension wood fibers and the S, thickening stage in the compression wood tracheids. This stage is composed of the deposition of cellulose microfibrils and is followed by the depositing stages of hemicelluloses and lignin. Compared with other differentiating stages of tracheids and wood fibers, the cytoplasm of cells forming the G layer isvery poor in its activity due to the fact that the region between the plasma membrane and the tonoplast is very narrow and cell organclles are rare there (Fig. 3%) 1751. The exceptionally abundant cell organelle in the cytoplasm is microtubules (MT). They are regularly distributed just inside the plasmamembrane (Pm), keeping a constant space of approximately 8 nm to the inner membrane and also between themselves (Figs. 33a and 33b). They are exactly oriented parallel to the depositing cellulose microfibrils in the stages of the G layer as well as the S , and S2 layers (Fig. 33b). The diameter of the microtubules is approximately 23 nm, and their numbers increase up to 20 per I p m of the plasma membrane, as calculated by their transverse direction. This abundant distribution results in the covering of about 40% of the cytoplasmic surface (Fig. 33b). A feasible link between the microtubules and the inner layer of plasma membrane is also discernible (Fig. 3321).These characteristics strongly suggest that microtubules and plasma membrane comprise the outermost complex of cytoplasm. On the other hand, there are only traces of Golgi bodies, S- and r-ERs, and the vesicles derived from them in the cytoplasm, in spite of the very active synthesis of cellulose microfibrils in this phase of the cell. On the contrary, in the beginning of the S, thickening stage of compression wood tracheids, in which cellulose microfibrils are supplied to the wall at 45" to the cell axis,

Formation Ultrastructure and

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37

FIGURE 31 Serial light microscopic autoradiographs of “before section treatment” (a) and “after sectiontreatments”(b)withsodiumchloriteandhot 1.3% H,SO, from the differentiating compression wood tracheids in Cryptomeria japonica administered with 3H-glucose. Silver grains in (b) show the specific incorporation of radioactivity only on the inner surface of S , and S2 thickening tracheids, which reflects the deposition of cellulose by way of “apposition.” Removed activity can be detected in the intercellular layer of cells in the S , stage (arrows) and in the preexisting secondary wall of cells in the late S2stage (cells marked by an asterisk)by a comparison between (a) and (b) that implies lignin and hemicelluloses are supplied to the wall by wall of “intussusception.”

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FIGURE32 Electron microscopic autoradiographs showing the incorporation of ‘H-phenylalanine in the transitional cell from S , to Sz,namely, in the stage of I-lignification (a), and from S , to S, in the stage of S-lignification in Cryptomeria japonica. Radioactivity can be observed around the intercellular layer and also within the Golgi bodies and vesicles in (a). In (b), vesicular inclusion is supplied to the wall by exocytosis (arrows) and radioactivity is often detected in such vesicles and the secondary wall.

cytoplasm isvery dense andwide. A similar complex between the microtubules and plasma membrane, however, is still observed [76]; microtubules are very abundant beneath the plasma membrane, having a link to it (Fig. 34a). The direction of microtubules is also in this case parallel with depositing cellulose microfibrils. Various cell organelles in the cytoplasm, such as Golgi bodies or ER, were shown to be involved in the synthesisof lignin precursors for the succeeding lignin deposition into the S, layer (see the next section). The characteristic appearance of microtubules is always applied to the cells depositing other wall layers such as S , , S2,and S3 without exception. It is interesting to note that the reorientation of microtubules precedes that of depositing cellulose microfibrils in the transition from the completion of a wall layer to the initiation of the next layer (Fig. 35). Moreover, the treatment of colchicine in which microtubule construction is obstructed by the formation of conjugation with microtubule protein “tubulin” resulted in a remarkable disturbance of depositing cellulose microfibrils (Fig. 36) [77,78]. These results clearly show that although microtubules present inside the plasma membrane cannot readily synthesize long and rigid microfibrils, these are involved in a great extent in the control of depositing cellulose microfibril orientation.

Formation Ultrastructure and

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39

FIGURE 32 Continued

B. LigninDeposition Lignin is a very important cell wall component, particularly in wood cells, for the enhancement of the physical properties of cell walls and also for sealing the wall from prevention of waterleaks.Fortunately,lignincan be detected under an ordinary light microscope with the use of several stains such as a Wiesner reagent and Maule color reaction. Ultraviolet microscopy is especially useful for the quantitative analysis of lignin distribution in the cell wall [79] and more useful for studying the types of lignins present in the cell wall [80,81]. Needless to say, transmission electron microscopy coupled with potassium permanganate staining [82] or hydrofluoric acid treatment [83], electron probe microanalysis [84], and autoradiography [85-901 are also very useful for the observation of lignin from various points of view. The lignification of tracheid walls is generally known to last for a long period, from the S, stage to theF stage [79]. During this period, the lignification starts at the cell comer, spreads into the intercellular layer, and extends centripetally to the secondarywall. Lignin deposition, however, should be examined more closely in relation to the deposition of cellulose and hemicelluloses on each wall layer. This was attempted in the differentiating tracheids of compression wood, which were convenient for separating thelignification of the I region and S region because of their conspicuously highly lignified secondary wall, especiallyattheouterregion of the S, layer [67]. It has been shownthatthelignin deposition can be separated into two lignification stages, namely, I- and S-lignification. The former is active only during the early stage of secondary wall thickening, mainly at

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Fujita and Harada

FIGURE 33 Electron micrographs of the cytoplasm-cell wall region of a transverse section from a tension wood fiber of Populus euramericana depositmg G layer (a) and of an obliquely sliced section from a S,-depositing fiber (b).

the S , stage, and is soon finished. The shape of I-lignification seems to stop the enlargement of cell size and adheres firmly between neighboring cells. On the other hand, the latter proceeds mainly after the development of a secondary wall framework, even though it begins at the middle phase of S , thickening. At any rate, lignin precursors permeate deeply into the cellulose microfibril framework of both primary and secondary walls and accumulate by way of “intussusception.” These two types of lignification were also applied in the differentiationof normal wood tracheids[86,91,92]. Moreover, when the speed

Formation Ultrastructure and .

.

Wall

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41

..... .,v*

(b) FIGURE 33 Continued

of lignin accumulation and the distribution of peroxidase were compared between the two regions [86], I-lignin seemed to be richer in the “condensed-type lignin” caused by the bulk polymerization than the S-lignin. This would be so because lignification proceeds with the higher content of lignin monomers and peroxidasein a rather large space without microfibrils. This assumption was confirmed by selectively labeled precursors coupled with light microscopic autoradiography [89,90]. When the lignifying cells are observed from the viewpoint of cytology, the cytoplasm is wider and denser than that of cells depositing cellulose microfibrils. Especially in the compression wood tracheids, an enormous amountof lignin precursors must be synthesized

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Q t

p

FIGURE 34 Electron micrographs of 45"-inclined sections from compression wood tracheids in Cryptomeria japonica. (a) Shows the cytoplasmic feature in the cell just beginning S , deposition. At the cytoplasm-cell wall region (enlarged view), the distribution of microtubules (MT) is similar to that in Fig. 33a, although the cytoplasm is full of cell organelles, especially Golgi bodies. (b)

Shows the huge ridges and cavities in the of Golgi vesicles.

S , layer and poor cytoplasm after the active exmytocis

in their cytoplasm and thentransported from the cytoplasm to the wall. The area of cytoplasm becomes wider at the S , stage and also at the transition from Szto F (Fig. 30), where the cytoplasm becomes rich in Golgi bodies (Go) and ER (Fig. 34a). Although small vesicles (v) are produced mainly from Golgi bodies, they do not move to the cytoplasmic surfaceyet. These small vesicles increasein number and grow larger, occupying

43

Ultrastructure and Formation of Cell Wall

I

FIGURE 34 Continued

the largest part of the cytoplasm during the following late phase of the S, stage. S-lignification at the F stage is characterized by the active fusion of the well-developed vesicles to the plasma membrane and by the release of the vesicle inclusion to the wall area, namely, exocytocis. The cytoplasmic area resultsin the formationof an empty region after lignification (Fig. 34b). This sequence indicates thatlignin precursors are synthesized and stored in the vesicles that havebeen derived from Golgi bodies and on occasion from ER. In fact, the process was proven by autoradiography using tritiated lignin precursors [ S S ] . These sequences were also examined inboth lignifications at the I and S regions of normal woodtracheids(Figs. 32aand32b) [86]. InS-lignification, S-ER seems to be related to the lignification in addition to the Golgi bodies (Fig. 29), whereas I-lignification

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;S1

FIGURE 35 Electron micrograph of an obliquely sliced cytoplasm-cell wall region from a tension wood fiber of Populus euramericana, which is just traveling from S2to the G layer. Many microtubules (MT) are oriented parallel to the fiber axis, although several (large arrowheads) are still oblique. Fine striations in the cell wall (small arrowheads) show the deposition of the S layer, and cellulose microfibrils of the G layer cannot be detected yet.

is performed mainly by the action of Golgi vesicles, similar to the case of compression wood tracheids. The lignin of compression wood tracheids is generally known to be rich in the condensed-type lignin. The cytoplasmic features of these lignifying cells seem to be consistent with the types of lignin suggested by Takabe et al. [86]. That is, the I- and S-lignins of compression woods and also the I-lignin of normal wood are metabolized mainly by Golgi bodies and the derivative vesicles, being richin the condensed-type lignin,

Formation Ultrastructure and

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45

FIGURE 36 Scanningelectronmicrographs of theinnersurface of the developing S, layer of Crytomeria compression wood tracheids after incubatlon withoutcolchicine (a) and with colchicine (b). (a) Shows remarkably developed ridges and regularly depositing cellulose microfibrils parallel with the ridges, whereas (b) shows the disturbed deposition of them.

whereas the S-lignin of normal wood is synthesized through the cooperation of Golgi bodies, S-ER, and their vesicles, resulting in noncondensed-type lignin. It is interesting to note that the cytoplasmic regions becomewider, corresponding to both lignifications in the I- and S-regions in both normal and compression woods (Figs. 29 and 30). In addition, the peak of the S-lignification of compression wood is bigger than that of normal wood. The tendency of these peaks is to respond to the absolute amount of lignin that will be supplied to the separate wall regions. The precursors of lignin are most likely synthesized in the cytoplasm and stored temporarily, and then released from the cytoplasm to the wall, whereas the cytoplasm may possibly be rather narrow during the active depositing phase of cellulose microfibrils.

C.

HemicelluloseDeposition

Examination of hemicellulose deposition is divided into two groups. One covers the microscopic observations [68-70,931; the other is the chemical analyses of the tissues or cell walls collected selectively [71-741. Although the use of microscopy is a prerequisite for the observation of the microlevel localization of hemicelluloses, the specific staining method has not been improved enough to be applied on each hemicellulose and even mixed ones, being difficult to distinguish from cellulose or lignin. If any specific radio-

Harada46

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Fujita

active precursor of each hemicellulose can be applied to the differentiating xylem, autoradiography will provide invaluable information on the deposition of hemicelluloses. A similar effect, although applied only to the total hemicelluloses, was achieved by a combined technique involving autoradiography and the removal of hemicelluloses from the tissue [93] or sections [53] that had been administered with tritiated glucose as the general source of cell wall materials. It becomesclear by these methods that hemicelluloses, although not so deeply as in the cases of lignin, accumulate in the preexisting framework of cellulose microfibrils by way of “intussusception” (Fig. 31). The depositing phase also intermediates the deposition of cellulose microfibrils with lignin accumulation. The deposition of each hemicellulose must be traced by the sugar analyses of tracheids or wood fibers selectively collected from the differentiating zone according to their development. This technique was achieved qualitatively by Meier et al. [7 l ] and improved quantitatively by Takabe et al. [73]. Judging from the content of polysaccharides in wood cell walls and the sugar constituents of hemicelluloses, glucose, mannose, xylose, arabinose, and galactose are, respectively, reflected in cellulose, glucomannan, arabino-gluconoxylan, and galactoglucomannan. As shown in Fig. 37a, mannose is supplied to the wall just after the cellulose microfibril deposition, followed by the deposition of xylose in the tracheid differentiation. In the case of wood fibers (Fig. 37b), xylose deposition follows directly the deposition of cellulose microfibrils. On the contrary, galactose and arabinose seemto be supplied to the walltogether in both stages of I- and S-lignification. The disagreement between the depositing manners of xylose and arabinose, namely, arabinose showing more affinity for lignin than xylose, may suggest that a chain of arabino-glucurono-xylan is not polymerized at one time but separately. That is, in concert with lignification, arabinose may be added, possibly as a side chain of the backbone of xylan already deposited.

I 0’

1

.

.

.

.

.

.

.

.

a

’

.

L

1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 f rdctlonnumber

i r d c t l o nn u m b e r

Glucose

4 2 - 9

Hannole-A-,

rylor~-A-r

Ardblnose

- 0 - 1

GdIdctore”.--.

FIGURE 37 Depositionofpolysaccharidestocell wall duringdifferentiation ofnormalwood tracheids in Cryptomeria japonica (a) and of normal wood fibers in Juglans sieboldicrna (b).

Ultrastructure and Formation of Cell Wall

47

According to this speculation, galactose is also supplied to the glucomannose chain. On the other hand, chemical analyses of wood offer the evidence that groups of galactose, arabinose, and 4-0-methylglucuronic acid are combined directly between the polysaccharide and lignin in the so-called lignin-carbohydrate complex [941. These lines of evidence strongly suggest that the sugar groups forming branches of hemicelluloses are the ignition site of lignin accumulation. When the depositing periods and types of cell wall materials are coordinated with one another, the wood cell wall is concluded to develop by the following four processes: the appositional deposition of cellulose microfibrils on the preexisting wall, resulting in the construction of a framework of cell walls; the supply of hemicellulose main chains around the cellulose microfibrils and the reinforcement of the framework; the addition of hemicellulose branching chains such as galactose or arabinose;lignin accumulation starting on the branch and encrusting almost all spaces between the framework. The cytoplasmic relation to hemicellulose deposition has remained uncertain at many points. However, in contrast to the case of cellulose microfibrils that are synthesized at the surface of cytoplasm, the precursors of hemicelluloses are surely metabolized in the cytoplasm, judging from the combined observations from autoradiography and chemical treatments 1691.

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7. A. Frey-Wyssling. Scirrm,, //Y:80 (1954). 8. A. hey-Wyssling and K. Muhlethalcr. Mrrkrortlol. Clwrn., 62:25 (1963). 9. R. D. Preston and J. Cronshaw. Nature, 18/:248 (1958). I O . K. Miihlethaler. Bioh. Z. S c h ~ ~ i :For.stwreitf.. . 30:55 (1960). I I . K. Miihlethnler. i n Ccllltlrrr Ultrtrstrrrcturc~of Wooc!\ Plat1t.s (W.A. C W . Jr., ed.).Syracuse Univ.Press,NewYork. p. 191 ( 1965). 12. V. Bnlnshov and R . D. Preston, Ncrtrrw. 176:64 ( 1955). 13. H. Haradn and T. Goto, i n Ce//lrlo.sc, t r t d Other Ntrt/trrr/ PnlynrcJr .Sy.stcr/r.s (R. M. Brown. Jr.. ed.). Plenum Publishing, New York. p. 383 (1982). 14. A. N. J. Heyn. J . Cell Biol.. 2Y: I81 (1966). I S . J. Sugiyama. H. Haroda. Y. Fujiyoshi. and N. Uyeda. Pltrr~ttr.Ihh: I61 ( 1985). 16. J. Sugiyomn. H. Harada. Y. Fujiyoshi. and N. Uycdo. Moku:tri Gtrkktrishi. 31:6 I ( 1985). 17. J. F. Revol. J . Mrrtrt: Sci. Lett.. 4 : 1347 ( 1985). 18. K. Muhlethaler. J. P o / w w r Sci.. C2N:305 (1969). 19. A. J . Kcrr ancl D. A. I. Goring. Wood Sci.. Y: I36 ( 1977). 20. K.Rue1 and F. Barnoud, ISWPC Stockholm. I : I I (1981). 31. Y. Kataoka. S. Saiki. and M. Fujita. Mokrr:rri Gnkkctishi, 38:327 (1992). 32. T. Kerr and 1. W. Bailey. ./. A r d d Arborotrr/r~,/5:327 ( 1934). 33. P. A. Roelotken. i n A d 1 w r x ~ c . sirr Bo/trrlictr/ Rc,srtrrch. Vol. 2 (R. D. Preston. d . ) . Academic Prcss. L,ondon a n c l NcwYork. p. 69 ( 1959). 24. A. B. W d r o p . A r ~ s t r d ..I. Hot.. h:299 ( 1958). 25. J. C. Rolnnd, B. Vam. and D. Reis. J . Cc,// Sci.. / Y : 1 3 9 (1975). 36. T. Fujii.Pl1.D.thesis.KyotoUniversity.Kyoto. Japan. 1981.

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Fujita and Harada

27. 28. 29. 30. 31. 32.

H.Saiki. Mokuzui Gcrkkniski. /6:237 (1970). A.B.Wardrop. Hol;forschurlg, //:l02 (1964). Y. Imatnura,H.Harada,andH.Saiki, Bull. Kyoto Univ. For(>st,s,44: I83 ( 1972). H.Harada. Y. Miyazaki,andT.Wakashima, Bull. Go~vr.Forest Exp. Stu., 104:I ( 1958). K.Yamanaka,M.S.thesis,KyotoUniversity,Kyoto.Japan. 1969. R. D.Preston, The. Physictrl Biology of’ PlLmt Cell Walls. Chapman & Hall,London.p. 302

33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49.

SO. 5 1. 52.

53. 54. SS.

56. 57.

58. 59. 60. 61. 62.

63. 64.

65.

( 1 974). J . C.RolandandM.Mosiniak, IAWA BM//.,4:15 (1983). K. Kobayashiand N. Utsumi,unpublishedresults, 1951. N.Mori.M.Fjita,H.Saiki,andH.Harada, Bull. Kyoto U n i ~Forc..st.s,55:299 (1983). J. Cronshaw. Protoplasnl, 60:233 (1965). K.Takiya.H.Harada, and H. Saiki, Bull. Kyoto U I I ~ IForests, ! 48: 187 (1976). J. Ohtani, Bull. College Exp. Forests Hokkaido u/1j\J.. 33:4()7 (1979). R . A.Parhamand W. M.Baird, Wood Sei. Techrd., & l (1974). K.Kishi,H.Harada.andH.Saiki, Bull. Kyoto Unit,. Forc..sr,s.4Y:122 (1977). K.Kishi,H.Harada,andH.Saiki. Mok~cztriGtrkkaishi. 25521 (1979). S. Fujikawaand S. Ishida, Mokuzni Gakknishi, 2/:445 (1975). S. C. Chafeand G. Chauret, Protoplasm. ;YO:129 (1974). T. Fujii.H.Harada.andH.Saiki, Mok~rzniG~lkk.st.s, IJ. SO: 183 (1978). M.Fujita. Y. Shoji, and H. Harada, H u l l . Kyoto Urli1: Forrsts, 4Y: I16 (1977). N. Shibata,M.Fujita,H.Saiki.andH.Harada. &dl. Kyoto U I I ~ Forc..sts. IJ. SO: 174 (1978). R. M.Brown.Jr..and D. Montezinos, Proc. N d . Accrd Sci. USA. 73: l43 (1976). T.ltohand R. M.Brown,Jr.. Plarrtcr. /60:372 (1984). D.Montezinos,in The Cytoskeletou i r ~Plrrrlt Gron,th ctrld 1 l e w l o p r ) w ~ t(C. W. Lloid, ed.). Academic Press, London, p. 147 (1982). J. H. N. Willison, J. Appl. Po1yr11c.rSyrnp., 3791 (1983). A.M. C. Emons, i n Bio.syntlw.sis m r l BiotlrRrcr~lrtiorl (fCellulo.se (C. H.HaiglcrandP. J. Weimer, eds.). Marcel Dekker, New York. p. 7 I ( 1991). K. Zanr, .l. Cell Rio/.. A0773(1979). C. H.HaiglerandM.Benziman,in Cellrrlow C ~ I Other I ~ NLrturrrl Polyrr~crSy.sterr~.s(R. M. Brown, Jr.. ed.). Plenum Press, p. 273 (1982). C. H. Haigler, in Bio.syrrt/w.si.s c r r d Hioclrgrcrdrrtiorl o f Cdlulosc~(C. H. Haigler and P. J. Weimer,

eds.). Marcel Dekker, New York, p. 99 ( 1991). 66. S. Kt~ga,S. Takagi.and R. M.Brown, Jr., Polyrr~er.343291 (1993). 67. M.Fujita,H.Saiki,andH.Harada, Moklrzai Gakktrishi, 24:158 (1978). 68. M.Fujitaand H. Harada. Mokuzcri Gnkkaishi,24:435 (1978). 69. M.Fujita. K. Takabe.andH.Harada. Mokuzri Gdkoixhi. 27337 (1981). 70. K.Takabe.M.Fujita,H.Harada,andH.Saiki. Mokuzcri Gokknishi. -?(l:103 (1984). C. B.Wilkie. Hol~fi)rsckurlg,13: l77 (1959). 7 1. H.MeierandK. 72. H.Meier,in Bio.syr~rhe.si.sc m 1 BioLI~.grcr~Irriorlof’ Wood Corr~porz~r~ts (T. Higuchi. ed.). Academic Press. New York

(1985).

Ultrastructureand Formation of Cell Wall

73. 74. 7s. 76. 77. 78. 79. 80. 81. 82. 83.

84. 85. 86. 87. 88.

89. 90. 91. 92. 93. 94.

49

K. Takabe, M. Fujita, H. Harada. and H. Saiki. Mok~czoi GNkkaishi, 29:183 (1983). K. Takabe, M. Fujita, K. Tanaka, and H. Harada, Bull. Kyoto CJniv. Forests. .56:234 ( 1984). M. Fujita, H. Saiki, and H. Harada, Mokuzui Gakkaishi,20:147 (1974).

M. Fujita, H. Saiki, and H. Harada, Moktczai Gnkkaishi, 24:355 (1978). J. D. Pickett-Heaps, D e ~ dBiol.. /5:206 ( 1967). M.FijitaandH.Harada.“Colchicinetreatmentandmicrolibrilorientationindifferentiating compression wood tracheids,” Proc.. 27th A n r l d Meetirrg of Jup,crrwsc~Wood Resectr-chSociety. p. 311 (1977). A. B. Wardrop, TAPPI, 40:22S (l9S7). B. J . Fcrgus and D. A. I . Goring, H o l ~ f o r . s c l ~ ~ c24: r ~ 118 g , (1970). A. Yoshinaga, M. Fujita, and S. Saiki, Mokuzcli Gnkknishi, 38:629 (1992). M. Mauer and D. Fengel, Hol;for.schurlg. 44:4S3 (1990). I . H. Clark, TAPPI, 45:310 (1962). S. Sakaand D. A. I. Gorgin.in Biosy1lthesi.s crrltl Riorlesmrltrtiorl of’ Wood Cort~pot~c,r~t.s (T. Higuchi. ed.), Academic Press, New York, p. S 1 (1983. M. Fujita and H. Harada, Mokuzai Gakknishi, 25:89 (1979). K. Takabe, M. Fujita. and H. Harada. Mokuzcti Gtrkknishi, 31:613 (1985). N. Terashima. K. Fukushima, and K. Takabe, Holifi)r.sc~hwzg,40(Suppl.): 101 ( 1986). K. Fukushima and N. Terashima, J . Wood Cllern. T e c h o l . , /0:413 (1990). N. Terashima and K. Fukushima. Wood Sci. fi~cl~nol., 22:2S9 (1988). K. Fukushima and N. Terashima, Hol;fi~rsc.hur~g. 45:87 (1991). M. Fujita, K. Takabe, and H. Harada, ISWPC (Tsukuba), !:l4 (1983). K. Takabe, M. Fujita, H. Harada, and H. Saiki, Mokuzai Gczkkaishi, 27:813 (1981). P. M. Ray, J . Cell Biol.. 3.5:660 ( 1967). D. Fengel and G. Wegener, Wood, Walter de Gruyter. Berlin, 1984.

This Page Intentionally Left Blank

Chemical Composition and Distribution Shiro Saka Kyoto University, Kyoto, Japan

1.

INTRODUCTION

Wood is a complex of natural polymer substances: cellulose, hemicelluloses, and lignin. These polymer substances are not uniformly distributed within the wood cell walland their concentrations change from one morphological region to another. In order to understand the physical and chemical properties of wood, it is essential to study the topochemistry of these polymer substances. This chapter, therefore, deals with chemical composition andits distribution in normal and reaction woodsfrombothsoftwoodandhardwood species.

II. GENERAL FEATURES OF WOOD CELLS Woody plants have several different types of cells in bothsoftwoodsandhardwoods. However, the anatomy of softwoods is less complex than that of hardwoods. In softwoods, the principal types of cells are tracheid and parenchyma, whereas those in hardwoods are fiber, vessel, and parenchyma.Sincesoftwood tracheids andhardwood fibers constitute the majority of wood cells, they contribute in a major way to the physical and chemical properties of wood. The cell wall organization of typical softwood tracheids or hardwood fibers [ 1,2] is described in Fig. 1 . Basically, the cell wall consists of the primary (P) and secondary (S) wall layers. The P layer is formed during the surface growth of the cell wall, and the S layer is formed during the thickening of the cell wall. This layer is composed of three sublayers termed S , , Sz, and S3, based on differences in microfibril orientation. A layer called middle lamella (ML) is located between adjacent cells. Since it is difficult to differentiate the ML from the two P walls on either side, the term compound middle lamella (CML). which encompasses the ML and the two adjacent P wall layers, is frequently used (Fig. 2a). The pattern of the cell wall organization in reaction wood is somewhat different from that of normal wood [31. Compression wood, developed on the lower side of a leaning softwood stem or branch, lacks the S, layer but contains an extra layer of lignin [%(L)] located between the S , and S, layers (Fig. 2b). Tension wood, formed on the upper side of a leaning hardwood stem or branch, often lacks one or more of the three secondary 51

52

Saka

FIGURE 1 The gross structure of a typical softwood tracheid or hardwoodfiber.(Courtesy Prof. Emer. R. J. Thomas, North Carolina State University, Raleigh, NC.)

of

wall layers. Instead, the gelatinous layer (G layer) is usually deposited adjacent to the cell lumen (Fig. 2c). The G layer contains little or no lignin and consists mainly of cellulose microfibrils oriented parallel to the fiber axis.

111.

CHEMICAL COMPOSITION OF WOOD

A.

Normal Wood and Reaction Wood

The chemical constituents of wood are well known,and a numberof authors have provided an excellent review of this work [4-81. The major cell wall constituents are cellulose, hemicelluloses, and lignin. Other polymeric constituents, present in lesser and often varying quantities, are starch, pectin, and ash for the extractive-free wood. Tables 1 and 2 show comparisons of the chemical composition made by Time11 [9] for five hardwoodsand five softwoods, respectively. Although the cellulosecontent is more or less the same (43 -t 2%) for both groups, the hardwoods contain less lignin. The lignin content of hardwoods is usually in the range of 18-25%, whereas that of softwoods varies between 25% and 35%. However, tropical hardwoods can exceed the lignin content of many softwoods. The structure of lignin is different between these two groups: softwood lignins are composed mostly of guaiacyl units, whereas hardwood lignins consist of syringyl and guaiacyl moieties [ 101. The hemicelluloses found in these groupsvary both in structure and quality, as shown in Fig. 3. The predominant hardwood hemicellulose is a partly acetylated, glucuronoxylan (O-acetyl-4-O-methylglucuronoxylan),accounting for 20-35%, whereas softwoods containglucuronoarabinoxylan (arabino-4-0-methylglucuronoxylan)in therange of 10%. Hardwoods contain only a small quantityof glucomannan. In softwoods, however, a partly acetylated galactoglucomannan (0-acetylgalactoglucomannan) makes up as much as 18%. In additiontothesemajorcellwall components, pecticmaterialsandstarch are included in much smaller quantities inboth softwoods and hardwoods. Ash usually makes up between 0.1% and 0.5% of wood, but tropical species often exceed this range. Wood

FIGURE 2 (a) Cross section of brominated normal wood tracheids in Douglas fir [Pseudotsuga rnenziesii (Mirb.) Franco]. Transmission electron micrograph of ultrathin section. Dark zones indi-

cate the higher lignin concentration. (b) Cross section of compression wood tracheids in eastern white pine (Pinus strobus L.). Ultraviolet micrograph of thin section (upper portion) shows high concentration of lignin in the S,(L) layer indicated by an arrow. Polarized light micrograph (lower portion) shows a lack of the S3 layer.

53

Saka

54

4 FIGURE 2 Continued. (c) Cross section of tension wood fibers in Enoki [Celtis sinensis Pers vat. japonica (Planch) Nakai]. Transmission electron micrograph of KMn0,-stained ultrathin section.

Note a nonstained G layer deposited following the stained Prof. Emer. H. Harada, Kyoto University, Kyoto, Japan.)

S2 and S3 layers. (Courtesy of the late

in heartwood also contains varying quantitiesof extractives that are always more abundant than sapwood. The chemical composition of reaction wood differs from that of normal wood. Table 3 shows a comparison made by Timell [ 1l ] of the average chemical composition of normal and compression woods of many conifers. Pronounced compression wood contains, on average, 39% lignin and 30% cellulose, compared to 30% and 42% for normal wood,

TABLE 1 Chemical Composition of Wood from Five Hardwoods"

Cell wall constituent Cellulose Lignin Glucuronoxylan Glucomannan Pectin, starch, ash, etc. "All values in percent of Source: Ref. 9.

Acer rubrum

Betula papyrifera

Fagus grandifolia

Populus tremuloides

Ulmus americana

45

42

45

48

51

24

19 35 3

22 26 3 4

21

24

25 4 2

24

extractive-free wood.

1

19 3 4

4 2

"

omposition Chemical TABLE 2

55

Chemical Composition of Wood from Five Softwoods"

constituent wallCell Cellulose 27 Lignin Glucuronoarabinoxylan Galactoglucomannan Pectin, starch, ash, etc.

A hies hulsarnen

42 41 29 9 18 2

Picea glauca

Pinus strobus

41

41 29 9 18

13 18 1

3

Tsuga canudensis

33 7 16 3

TI1uja occidentalis 41 31 14 12

2

"All values in percent of extractive-free wood. Source: Ref. 9.

respectively. The content of galactoglucomannan is only 9%, half that in normal wood. The amount of xylan, on the other hand,is the same in thetwotissues.Compression woodcontains 2% of a 1,3-linked glucanand 10% of a galactan,bothpresentinonly trace amounts in normal wood. With regard to chemical composition of tension wood, the most characteristic feature is that it contains less lignin andxylan described as pentosan, but has much more cellulose

Saka

56

TABLE 3 Average Chemical Composition of Normaland Compression Woods of Softwoods"

Cell wall constituent Lignin Cellulose Galactoglucomannan 1.3-Glucan Galactan Glucuronoarabinoxylan Other polysaccharides

Normal wood Compression wood

30

39

42

30 9 2 IO

18

Trace Trace 8 2

8

2

"All values in percent of extractive-free wood. Source: Ref. I 1.

and galactose residues than normal woods (Table 4) [ 121. Furthermore, higher ash and uronicacidcontentsarereportedfortension woodthanin the side woodof Japanese beech (Fugus crenuru Blume) [ 131. The higher cellulose content in tension wood is due to the presence of a gelatinous layer, which is often quite thick and unlignified (Fig. 2c).

B. Tracheids and Ray Cells The tracheids in softwood or fibers in hardwood constitute more than 80% of the cells found in wood; thus, the global analyses reflect moreor less the composition of these types of cells. However, the chemical composition of ray cells may not be inferred from that of the whole wood. The data in Table 5 reveal a comparison made by Hoffmann and Timell [ 141 between the ray cells and tracheids from red pine (Pinus resinosu Ait.). The defibrated samples after delignification by acid chlorite were subjected to the separation of tracheids and ray cells through screening, followed by subsequent analysis of sugar residues. As reported in the literature [ 151, the ray cells contain more lignin. somewhat less cellulose, only half as much galactoglucomannan, and the same amounts of xylan and pectin as do the tracheids. A small amount of a 1,3-linked glucan also present in the ray cells appears to be absent

Chemical Composition of NormalandTension Woods from Euccclyptus gonioc.rr1y.r"

TABLE 4

Cell wall wood Tension wood Normal constituent 13.8 Lignin 57.3 Cellulose Pentosan Acetyl 7.4 Galactose residue

29.5 44.0

1s. I

11.0

3.0 2.5

I .9

"All values in percent of extroctlve-free wood. Solrrcr: Ref. 12.

omposition Chemical

l

57

Normal wood

wood" Colnpresslon

Tracheids cells Ray Tracheids Ray cells Cell

(%)

(%)

(c/o)

(c/o)

Lignin Cellulose Galactoglucomannon 1,3-Glucan Tracc Galactan Glucuronoarabinoxylan Pectin Other polysaccharides

40

28 42 20 -

40 35 II 2

40 30

Trace

IO

10 I

7 I

I

1

35 9 2

Trace 11

2 I

8 1 1

9 2

"From Ref. 14. hFrom Ref. 18.

in the tracheids. Overall, the results in Table 5 are in good accord with those by Perilii [ 161 and Perilii and Heitto [ 171. However, these authors indicated more xylan in ray cells than normal tracheids i n Scots pine (Pirzus sylvesfris L.). The lower content of xylan in Table 5 is due possibly to partial removal during chlorite delignification. Also included in Table 5 are the chemical compositions of tracheids and ray cells from red pine compression wood [ 181. Although compression wood tracheids have a different chemicalcompositionfromnormalwood tracheids, the ray cells in compression wood are chemicallyindistinguishablefromthose in normalwood.Comparedwith the compression wood tracheids, the ray cells have the same contents of galactoglucomannan, 1,3-glucan, and xylan. However, the galactan typical of grosscompressionwood is missing.

C. Earlywood and Latewood Meier [ 191 has studied the effect of the cell wall thickness on polysaccharide content by comparing earlywood and latewood from normal Scots pine(Pirzus sylvesfris L.). As shown in Table 6, the latewood contains more glucomannan and less glucuronoarabinoxylan than the earlywood. Since the proportion of the latewood tracheid S I layer to the whole wood is greater than that of earlywood, the observed differences are due mainly to the thicker S2 layer in latewood tracheids. I t may therefore be concluded that the tracheid S I layer hasmoreglucomannanand less glucuronoarabinoxylanthan doothermorphological regions, which agrees reasonably well with the results of Whiting and Goring 1201 for the secondary wall and middle lamella fractions of black spruce (Picecr rnnriarzcr Mill.). Also shown in Table 6 are the results of compression wood balsam fir [Abies ~ N I sanlecr (L.) Mill] 1211. Although earlywood and latewood have the same content of lignin [22],thepolysaccharidecomposition is different betweenthesetwo tissues. It canbe speculated in a similar way that the tracheid S, layer in compression wood contains more celluloseandgalactoglucomannanbut less galactan,arabinan,andxylan that doother morphological regions of compression wood.

58

Saka

TABLE 6 Polysaccharide Composition of Earlywood and Latewood from Normal Scots Pine and Balsam Fir Compression Wood

Balsam fir compression wood’ Polysaccharide

pine”

Scots

Normal

Latewood Earlywood Latewood Earlywood (%l

Cellulose 56.2 Galactan 3.1 Glucomannan Arabinan Glucuronoarabinoxylan

(%)

(%)

56.7

45.0

3.4 20.3

19.0 16.0

50.4 15.0 18.7’

1.o 18.6

(%)

24.8 1.8

14.1

0.9

0.6

19.1

15.3

“From Ref. 19. hFronl Ref. 2 I . ‘Values as galactoglucomannan.

IV.

DISTRIBUTION OF POLYSACCHARIDES

A.

Introduction

The distribution of cellulose is probably the easiest to study at the various morphological regions of wood.Onepossiblemethodinvolves the useofholocellulose after desired poststaining with heavy metals such as uranyl acetate [23-2.51 and lead citrate [26]. Although the orientation of the cellulose microfibrils is quite different in the various cell wall layers, cellulose is quite evenlydistributedthroughout the secondary wall. In the primary wall, however, microfibrils are rather loosely and randomly arranged (Fig. l), so the concentration of cellulose in the primary wall may be lower than that of the secondary wall. Unlikecellulose, a study of the distribution of hemicellulose is difficult. This is because histochemical techniques are generally nonspecific and frequently unreliable. In order to overcome these difficulties, BouteljeandHollmark [27] introduced the use of interference microscopycombinedwithenzymatic treatment. Sinner et al. 128,291 used electron microscopy to study the enzymatic degradation of the cell wall components by xylanases, mannanase, and avicellase for delignified spruce [Picea abies (L.) Karst.] and beech ( F c q p s sylvaticcr L.). It was found that xylan concentration is rather high in the S , and S, layers for both woods. Hoffmann and Parameswaran [30] made another attempt to study the polysaccharide distribution in spruce tracheids through oxidation of polysaccharides with heavy metal. Subsequent electron microscopic observations indicatedthe highest concentration of hemicelluloses in the S , layer. Awano et al. [31] have recently applied immunoelectron microscopy to studying the distribution of glucuronoxylan in buna (Fagus crencrtu Blume). An extensive study in the future will provide useful information on its distribution. The distribution of polysaccharides also has been studied by examination of holocelluloseskeletonsafterremoval of lignin with acid chlorite [21],atechniquefurther refined by Fujii et al. 1321by using ultrathin sections. An electron micrograph of the holocellulose skeleton from the compression wood of tamarack [Lcrrix laricim (Du Roi) K. Kochj is shown in Fig. 4. Forcomparison, Fig. 5 shows a micrograph ofa lignin skeleton of thc same wood. Although the presence of the residual lignin and some removal

omposition Chemical

59

FIGURE 4 Holocellulose skeleton of two tracheids in compression wood of tamarack [Lark lurkina (Du Roi) K. Koch]. Note the low concentration of polysaccharides inthe S,(L) layerand absence of substances in the middle lamella region. Transmission electron micrograph of a cross section. (Courtesy of Prof. Emer. W. A. C M , Jr., State University of New York, Syracuse, W.)

of polysaccharides may obscure the data, overall the results obtained by this method are in good agreement with the holocellulose distribution inferred from the lignin skeleton seen in Figs. 4 and 5 [33]. Some other methods also have been proposed; Parameswaran and Liese [34] have given an excellent review of these studies. For the localization of pectin, Albersheim et al. [35] have used the hydroxylamineiron method developed by McCready et al. [36,37] for the quantitative measurement of pectin. With this method, Parameswaran and Liese [34] have found a homogeneous distribution of pectin across the secondary wall. The middle lamella also was found to be highest in concentration. The use of ruthenium red and alcian blue also is proposed for staining pectin substances [38-401. For quantitative determinationof the polysaccharide distribution, the microdissection technique has often been used. One of the oldest is Bailey’s work in 1936 [41] for the pentosan content determinationof the middle lamella in Douglas fir. In 1959, Meier [42,43] adopted a similar technique for hardwood fibers (Betula verrucosa Ehrh.) and softwood tracheids (Pinus sylvestris L. and Picea abies Karst.) at different stages of development

60

Saka

FIGURE 5 Lignin skeleton of threetracheidsin compression wood of tamarack [ L a r i x luricina (Du Roi) K. Koch]. Note the high concentration of lignin in the S,(L) layer as well as in the middle lamella region. Transmission electron micrograph of a cross section. (Courtesy of Prof. Emer. W. A. CBtC, Jr., State University of New York, Syracuse, NY.)

that were microscopically distinguished, isolated, and subsequently subjected to microanalysis for sugar residues. From a knowledge of the chemical composition of different polysaccharides in wood, the contentsof polysaccharides at various morphological regions could be calculated. Although some doubt exists as to additional deposition of polysaccharides during the later stage of the secondary wall thickening, the technique developed by Meier remains applicable [44]. Later, Norberg and Meier [45] isolated the gelatinous layer(G layer) in tension wood fibers (Fig. 2c)by using ultrasonic treatment. Subsequent analysisindicated that it contains 98.5% glucose and 1.4% xylose, suggesting thepure cellulosic nature of the G layer. Luce [46] determined the radial variationin the content of hemicelluloses in softwood tracheids by a chemical peelingtechnique.Burkeetal. [47] measured the sugar content of the polysaccharides in the primary walls of a suspension-cultured Douglas fir. In 1981, Hardell and Westermark [48] have developed a method for peeling layers of the cell wall from a slightly delignified single tracheid of Norway spruce [Picea abies (L.) Karst.]. They reported only small differences in the relative amounts of polysaccha-

omposition Chemical

61

rides between the compound middle lamella and the secondary wall, a finding that is not in agreement with Meier's results [42]. It appears that a treatment of slight delignification may cause a partial dissolution as well as redistribution of hemicelluloses. The arabinose and galactose contents for the compound middle lamella were foundto be 7.3% and 7.6%, values that are considerably lower than those from nonlignified wood [6]. More recently, Whitinget al. [49]developedanothermethod of preparing wood tissue fractions from the compound middle lamella and secondary wall of black spruce (Piceu nzariana Mill.) by taking advantage of the difference in density ( p ) between lignin ( p = 1.4 g/mL)andpolysaccharide ( p zz 1.5 g/mL).Themost significant finding after analyses of carbohydrates for these wood tissue fractions [20] was that the concentrations of celluloseandglucomannan are smaller in the middlelamellathan in the secondary wall, whereas the concentrations of other polysaccharides are more or less the same in both the secondary wall and the middle lamella regions. Compared to the previous methods by Meier [42] or Hardell and Westermark [48], the method of Whiting et al. [49] is more reliable, due to only the physical treatment of specimens without introducing any chemical changes.

B.

Distribution of Polysaccharides in Normal Wood

Whiting and Goring [20] conducted carbohydrate analyses of fractions of tissue from the middle lamella and secondary wall of black spruce tracheids. Figure 6 shows the relationships between polysaccharide content and lignin content for the various tissue fractions. Each fraction is amixtureof the secondary wall, primary wall, andmiddlelamella in varying proportions. The fraction with a lignin content of 22% is from the secondary wall tissue, whereas the extrapolated results to a composition at which the cellulose content becomes zero would represent the polysaccharide composition of the true middle lamella with a lignin content of 70%. Figure 6, therefore, shows that the middle lamella contains less cellulose and glucomannan but more galactan and arabinan than the secondary wall.

t

U

0.2

0.3

0.4

0.5

0.6

0.7

FIGURE 6 Polysaccharide content versus the lignin content for thevarious black spruce (Picecc tnnriat~nMill.). (From Ref. 20.)

tissue fractions of

Saka

62 TABLE 7

l481

Relative Polysaccharide Percentages of the Secondary Wall Tissue

Polysaccharide Cellulose Glucomannan Glucuronoarabinoxylan 12.8 Galactan

WhitingGoring and [201

Meier Hardell Westermark and [421 63.0

1.1

0.0

Arabinan

58.1

60.0

20.8

23.7

14.3

10.7

4.8 2.0

4.1 1.5

However, the concentration of glucuronoarabinoxylan is essentially the same in both morphological regions. For the secondary wall tissue, Table 7 shows a comparison made by Whiting and Goring [20] of the relative polysaccharide percentages measured by Meier [42] and Hardell and Westermark [48] on Norway spruce (Picea abies Karst.) and by Whiting and Goring [20] on black spruce (Picea nzariarza Mill.). The values of Meier were calculated using a proportion of 90% secondary wall and 10% middle lamella for the whole wood [50].It is apparent that the agreement between the results obtained by three investigators is good, particularly between the data by Hardell and Westermark and those by Whiting and Goring. It istherefore likely that in the tracheid secondary wall thecontents of hemicelluloses decrease in thefollowingorder:glucomannan,glucuronoarabinoxylan,galactan, and arabinan. A comparison of the relative polysaccharide percentages of the middle lamella-rich fractions is shown in Table 8 for the same three investigators. The results of Whiting and Goring are the composition at the 70% lignin content in Fig. 6, which would be representative of the true middle lamella. For comparison, data from tissue fraction with 39% lignin (Fig. 6) are also included. This fraction includes part of the secondary and primary wall tissues, as well as the middle lamella fraction. In contrast to the excellent agreement for the secondary wall fractions (Table 7), the results on the middle lamella are at variance with each other. This is because the middle lamella fraction is most difficult to prepare in a pure state. It should be noted that the cellulose content of 50.3% for Hardell and Westermark (Table 8) is not much different from the value of 58.1 in Table 7 for the secondary wall fraction. Additionally, the results by Hardell and Westermark are i n good agreement with the data from the tissue fraction of 39%)lignin content by Whiting and Goring. Thus,

TABLE 8

Relativc Polysaccharide Percentages of Middle Lamella-Rich Fractions Hardell and Westermark Meier ~421

Polysaccharidc

(-)

50.3 Ccllulose Glucomannan

33.4 7.9 13.0 16.4 29.3

13.3 Glucuronoarnbinox~lat~ Galactan Arnbinan

Whiting and Goring [20]

148I (4I % lignin)

(70%. lignin) 0.0

50.8

22.6

12.5

21.6

15.4

(39% lignin)

37.5 7.6 6.2

29.2 20.8

7.2 5.0

Composition Chemical

63

a sample collected by Hardell and Westermark must be to some extent contaminated with the secondary wall fractions. It is of interest to note that both the results of Meier for the compound middle lamella and of Whiting and Goring for the true middle lamella show glucomannan to be the lowest among hemicelluloses in the middle lamella region. For overall trendsof the carbohydrate distribution across the cell wall, Meier [42,43] has indicated that, although the cellulose content is very low in the ML (middle lamella) and P (primary wall) regions, arabinan is almost completely confined to M P regions, and galactan is almost completely confined to M P S , regions. Glucomannan, however, increases from M + P to the S , layer in softwoods and remains at a rather constant low level in hardwoods. Glucuronoxylan in hardwoods has a higher concentration in the secondary wall than in the M P. Figure 7 shows the distribution of polysaccharides across the woodcellwall of Cryptomeria tracheids obtained by Takabe [U] through the technique of Meier [42]. Interestingly, cellulose is rich in the middle of the S , layer, whereas hemicelluloses of glucuronoarabinoxylan and galactoglucomannan are abundant in the S , and outer parts of the S, and S3 layers. The warty layer (W) is composed mainly of galactoglucomannan.

+

+ +

+

C.

Distribution of Polysaccharides in ReactionWood

C6t6 et al. [21] also used the technique of Meier [42] to study the polysaccharide distribution in compression wood tracheids of balsam fir [Abies balsamea (L.)Mill.]. Figure 8 shows the results obtained. It should be noted that the glactoglucomannan, arabinan, and xylan are homogeneously distributed across the secondary wall, whereas a higher concentration of galactan was found in the outer regionof the secondary wall. This fact was later confirmed by Larson [5 1,521. The content of cellulose is, on the other hand, higher at the inner portion of the cell wall. For the compound middle lamella (P M P), the high

+

+

:.:.:.:.:* ....:.:\.:\..:.., >>: ....... .:.:.:.:.:................. ....... ,...... :., ... ..... .A.

............... .....:..

...

... ...

m Glucuronoarabrnoxylan 0 blactoglucomannan 0 tcllulorc

FIGURE 7 The distribution of polysaccharides across the wood cell wall of tracheids in Cryptomeria japonica D. Don. (From Ref. 44.)

64

Saka

Percent

GALACTAN

ARABINAN

CELLULOSE

ARAEINOGLUCURONOXYLAN

GALACTOGLUCOMANNAN

FIGURE 8 Graphical representation of the distribution of polysaccharides in compression wood tracheids of balsam fir. (From Ref. 21.)

content of arabinan and galactan would be due to high pectin content. It is reported that chemical composition of the primary wall is the same in normal and compression woods [5 1,521.

V.

DISTRIBUTIONOF LIGNIN

A.

Introduction

Unlike polysaccharides, a number of reliable methods can be used to study the distribution of lignin in wood. One of the oldest procedures is selective staining, followed by study under the light microscope[53]. Although some doubt exists as to this specificity for lignin [54,55], potassiumpermanganatestaining [56] has been usedextensivelyforstudying lignin distribution by electron microscopy [57-601. Also reported were studies by electron microscopy of lignin skeletons (Fig.5 ) created by the carbohydrate removal by brown-rot fungi [61] or concentrated hydrofluoric acid [32,62-641. Although some alteration of the lignin through condensation may result and the possible presence of residual carbohydrates may obscure the data, overall the results obtained by this method are in reasonable agreement with those from potassium permanganate staining [59]. Although the above methods are useful in elucidating the presence of lignin in the various morphological regions of wood, they can provide only qualitative evaluation of thelignindistributionacrossthecellwall. For quantitativevisualization of thelignin distribution, ultraviolet (UV) microscopy with thin sections of wood has provided good results. This method was initiated by Lange [65], who estimated the weightconcentration of lignin to be, respectively, 16% and 73% for the secondary wall and compound middle lamella of Norway spruce tracheids. This result was in excellent agreement with Bailey’s value of 71% for the Douglas fir middle lamella fractions obtained by a direct analytical method [66]. Previous to Bailey’s work, Ritter [67] had concluded that approximately75% of the lignin in wood is located in the middle lamella, with the other 25% being located in the secondary wall. Apparently, a distinct difference exists between the results by LangeBailey [65,66] and Ritter [67]. However, considerable confusion has appeared in the lit-

Composition Chemical

65

erature. Some of this confusion could be due to the use of the symbol o/o to denote both the percentage fraction of the total wood lignin contained in a particular morphological region and the lignin content of that region. Therefore, g/g. i.e., g of lignin/g of cell wall substance. is used in this chapter to denote lignin concentration. The symbol o/o is then reserved for the proportion of total lignin in a particular morphological region. Later. Goring and co-workers [68-701 refined the UV microscopy method through a preparation of the thin section (0.5 p m ) to avoid errors caused by nonparallel illumination. Goring et al. then determinedquantitatively the distribution of lignin in wood [50,70-751 and proved that the result by Lange-Bailey[65,66] was correct.They also proved that the conclusiondrawn by Berlyn and Mark [76] is correct. that the middle lamella region can contain at most 40% of the total lignin in wood due to its small volume fraction of wood. In addition to these, they discovered that different lignins occur in different types of cells and different cell wall regions of wood [72-741. More recently, through the use of UV microscopy, Yang and Goring [77.78] have found that the secondary wall lignin of softwoodscontains twice as many phenolic groupsas the middlelamella.This finding was later confirmed by Whiting and Goring [79] from a study of the secondary wall and middle lamella fractions. Of other methods for the quantitative assay of the lignin distribution, Lange and Kjaer [SO] proposed the use of interference microscopy, and Boutelje [811 later refined this technique.More recently, Saka et al. [82-861 developeda new techniqueforthe quantitative determination of the lignin distribution in wood. The method involves a specific bromination for lignin in a nonaqueous system (CHCI,). Bromine concentrations in the various morphological regions of wood are then determined by electron microscopy (TEM or SEM) coupled with energy-dispersive X-ray analysis (EDXA). By knowing the lignin reactivity toward bromination, the distribution of lignin can be determined for various morphological regions of wood. Figure 9 shows the direct comparison made between two techniques of UV microscopy and EDXA measurement in bromination [S61 over the

1 .oo Earlywood

Latewood

I

0

.-C .-

l

SECONDARY WALL

0

I

I

I

15

10

5

I

I

1

5

10

Cell number FIGURE 9 Variation of lignin concentrationsacrosstheearlywood/latewood boundary of black spruce measured by U V microscopy ( 0 ) and the EDXA technique ( 0 ) .(From Ref. 86.)

Saka

66

earlywood/latewood boundary of black spruce (Piceu r~zuriur~a Mill.). It is quite apparent that the agreement between the results obtained by the two methods is good. With this EDXA technique, another method has also been developed by Westennark et al. [87] and Eriksson et al. [SS], based on a mercurization of lignin, followed by determination of mercury concentration in different morphological regions of wood.

B.

Distribution of Lignin in Softwoods

Table 9 shows the distribution of lignin in tracheids of black spruce (Picea marianu Mill.) as determined by UV microscopy [50]. The results show that the lignin concentration in the secondary wall (S) is considerably lower than that in the middle lamella (ML or ML,,). However, the secondary wall makes up a muchlarger proportion of the total tissue volume. Thus, the majority of the lignin is located in the secondary wall. Furthermore, the lignin is uniformly distributed across the secondary wall in black spruce tracheids, as seen in Fig. IO. For more detailed information, the distribution of lignin in the xylem of Douglas fir [Pseudotsuga tnenziesii (Mirb.) Franco] is given in Table 10 [74]. It should be noted that the distribution of lignin in the various morphological regions of the tracheids is basically the same as that shown for black spruce in Table 9. For the ray parenchyma secondary wall, the lignin concentration is higher than that for the tracheid secondary wall but lower than that for the middle lamella. However, the secondary wall of the tracheids does not differ much from that of ray tracheids in its lignin Concentration. Table I I shows the distribution of lignin in loblolly pine (Pinus ruedcl L.) tracheids as determined by bromination coupled with SEM-EDXA [85]. One of the advantages of this techniquecomparedwith UV microscopy is the ability to study the S,, S,, and S, layers in the secondary wall as a separate entity. Such resolution is often difficult with UV microscopy. It is interesting to note that the lignin concentration in the S? layer is lower than that in either the S , or S, layer. The line profile of the bromine X-rays in Fig. 1 1 showssuch differences clearly. FukazawaandImagawa[89]havealsoreporteda similar finding of high UV absorbance near the lumen/wall interface for juvenile wood tracheids of Japanese fir (Abies suchalinensis Fr. Schm.). A comparison of Tables 9-1 1 shows that, minor differences not withstanding, the trends in the distribution of lignin in the tracheids of the three softwoods are similar. For the ray parenchymacellsconstitutingabout 5% of the total xylem tissue in softwoods, Harada and Wardrop[ 151 have reported a lignin content of 0.44 g/gin Japanese

TABLE 9 The Distribution of Lignin in Black Spruce Tracheids UV Microscopy

as

Determined by

~~

Lignin

Tissue Wood

Morphological volume region

(%)

(g/g) conc. (% of total)

~~~

Earlywood Latewood

S

87

72

0.23

ML ML,,

9 4 94 4 2

16

0.50 0.85

S

ML ML,, Source:

Ref. SO.

12 82 IO 8

0.22 0.60 1 .oo

67

Composition Chemical

FIGURE 10 UVphotomicrographtakenat 240 nm of theearlywoodtracheidwallsinblack spruce. The densitometer tracing was conducted along the dotted line. (Courtesy of Prof. Emer. D. A. 1. Goring, University of Toronto, Toronto, Canada.)

[66]obtained cedar (CryptorneriujuponicuD. Don). By a microdisection technique, Bailey a value of 0.41 g/g for the segregated ray parenchyma cells of Douglas fir. Fergus et al. [50] also determined byUV microscopy a lignin concentration of 0.40 g/g forblack spruce. These results by a variety of methods are in good agreement with the data shownin Table 10 for Douglas fir earlywood parenchyma cells. Interestingly, the ray parenchyma cellsin softwoods possess significantly higher lignin contents than the whole wood.

TABLE 10 The Distribution of Lignin in Douglas Fir Xylem as Determined by W Microscopy

Lignin

Tracheid

racheid

Tissue Wood

Earlywood ray ray Latewood ray ray Source: Ref. 74.

Morphological volume region S Tracheid ML Tracheid ML, Paren. S Tracheid S S

Tracheid ML Tracheid ML,, Paren. S Tracheid S

(%) (g/g)

conc.(% total)

74 10 4 8 4

58

0.25

18

0.56 0.83

90

78 10

0.40 0.28 0.23 0.6

6

0.9

4

-

4 2 3 1

11

10 3

2

Saka

60

TABLE 11 The Distribution of Lignin In Loblolly Pine Tracheids as Determined by Bromination with SEM-EDXA

Lignin

Tissue

wood Earlywood

Morphological volume region SI S2

S, ML

ML, Latewood

SI S2 S,

ML ML,

. (glg)

(%) conc total)(70of

13 60

9 12 6 6 80

5 6 3

12

0.25

44

0.20

9

0.28

21

0.49 0.64 0.23 0.18

14 6 63 6 14

0.25

0.5 1

11

0.78

Source: Ref. 85.

Regarding the distribution of lignin in the compression wood of softwoods, Timell [ 1l] has given an excellent review. As observed in an electron micrograph shown in Fig. 5 of the lignin skeleton from the compression wood of tamarack [Lark luricina (Du Roi) K. Koch], the S, layer appears to have a slightly lower lignin concentration than the inner S,. However,aringpresent in the S , layer [&(L)] reveals a high lignin concentration about equal to that in the middle lamella. Table 12 shows acomparisonmadebyTimell[l13 of theligninconcentrations determined by Wood and Goring 1741 of Douglas fir and by Fukazawa [90] of Japanese fir (Abies sachalinensis Fr. Schm.). Although the lignin content in Japanese fir is lower in most of the morphological regions, the overall trends are basically the same.

C

FIGURE 11 Scanning electron micrograph (a) of brominated latewood trachids in loblolly pine (0.5-pm section). The distribution map (b) of Br-L X-rays was taken of the same area as the scanning electron micrograph. The distribution of bromine (c) was taken along the line across the double cell wall. (From Ref. 83.)

69

Chemical Composition and Distribution TABLE 12 TheDistribution of LignininCompression Wood Tracheids of Douglas Fir and Japanese Fir

Morphological Douglasregion

Lignin concentration (%)

49

29 42 26 49

75

65

40 54 36

"From Ref. 14. hFrom Ref. 90.

C.

Distribution of Lignin in Hardwoods

Hardwood lignins consist mainly of guaiacyl and syringyl residues, and its ratio seems to change from one morphological region to another. Fergus and Goring [72,73] attempted to determine the distribution of lignin in white birch (Betula papyriferem Marsh.) byUV spectral analysis. The syringyl and guaiacyl residues have, however, markedly different UV absorptivities. Thus, it is essential to know its exact ratio before the lignin concentration in a particular morphologicalregion is computedfrom the UV microscopy. In the 1980s Saka et al. 191,921 developed a new method to compute the ratio of guaiacyl and syringyl residues at the various morphological regions by combining UV microscopy with bromination-EDXA (UV-EDXA). This could be used to determine lignin distribution in hardwoods. Shown in Table 13 is the ratio of guaiacylkyringyl residues in various morphological regions of white birch wood as determined by the UV-EDXA technique 1911. For com-

TABLE 13 Distribution of GuaiacylandSyringylResidues White Birch

in Lignin i n

Guaiacy1:syringyl

DXA Morphological omination" with region

uv analysish spectral Syringyl Guaiacyl Syringyl 5

0

Guaiacyl 50:50 5050

"From Ref. 9 1. 'From Ref. 73. 'Fiber/fiber. dFiber/vesscl. 'Fibedray. 'Ray/ray.

70

Saka

parison, the results obtained by Fergus and Goring [73] through UV spectral analysis are also included. It is indicated by both methods that the fiber secondary wall (S2) contains predominantly syringyl residues, whereas the vessel secondary wall (S?) consists mostly of guaiacyl residues. The study by UV-EDXA [91] revealed that the ray parenchyma cell contains about equalproportions of guaiacylandsyringylresidues in lignin. However,apredominant amount of syringyl-type lignin was found by UV spectral analysis [73], as in the fiber secondary wall. For the cell corner middle lamella (ML,,), 80-100% of the lignin was found to be guaiacyl residue, with the remaining 0-20% being syringyl residue by the UV-EDXA technique [91]. This result is not in agreement with the data by UV spectral analysis [73]. However, it supports the later suggestion of Musha and Goring [75]that the middle lamella lignin consists entirely of guaiacyl residues. Itis therefore apparent in Table 13 that, in hardwoods, the ratio of guaiacyland syringyl residues in lignin varies in different morphological regions. These findings have been supported by several investigators; Wolter et al. [93] have shown that the vessels in aspen callus cultures contain a pure guaiacyl lignin. Kirk et al. [94] found that the fungal degradation of lignin in birch wood was consistent withthe presence of syringyl-rich lignin in the fiber walls. Furthermore,Yamasakiet al. [95] isolated syringyl-rich lignin from several hardwoods. Hardell et al. [96] fractionated birch wood to determine the syringyl and guaiacyl ratio, and indicated that lignins in both the middle lamella and vessel secondary wall are rich in guaiacyl units, whereas the ratio of syringyl/guaiacyl residues is high in the fiber and ray cell. Cho et al. [97] studied the filmlike substance isolated from the fines of birch in which a high proportion of the compound middle lamella was recognized.This material wasfound to possessalow ratio of syringyl to guaiacyl units. Terashima et al. [98] administered 'H-labeled guaiacyl and syringyl model compounds to magnolia shoots and determined their location in the growing cell wall by microautoradiography. They found that the vessel wall, cell comer, and compound middle lamella were lignified by the deposition of guaiacyl-type lignin, and the fiber wall was composed of syringyl-guaiacyl lignin. Recently, UV and visible-light microscopic spectrophotometry have been combined with the Maule color reaction for detecting syringyl lignin by Yoshinaga [99], and this method has been extended to taxonomic studies of the distribution of hardwood lignins [ 100- 1021. Table 14 shows the distribution of lignin in white birch wood as determined by UVthe results obtainedearlier by UV microscopy 1721 are EDXA [91].Forcomparison, included. For the fiber secondary wall, the lignin concentration in the S, layer is slightly lower than in either the S , or S2 layer. However, its difference is so small that the lignin may be considered to be distributed uniformly across the secondary wall. The vessel walls also reveal a uniform distribution of lignin, but the concentration is about 1.9 times higher than that of the fiber walls, which in turn is higher than that of ray parenchyma cells. The cell comer middle lamella (ML,,) associated with fibers and vessels has the highest lignin concentration. In spite of sufficient analytical resolution by the EDXA system, the middle lamella between cell corner areas (ML) was 10-30% lower in concentration than the cell corner middle lamella (ML,,). It is of interest to note that the lignin concentration in the middle lamella regions of hardwoods is lower than that of softwoods, as seen in Tables 9 and 14. A comparison of the data made between UV-EDXA and UV microscopy techniques indicates that lignin concentrations in fiber and vessel secondary walls are in agreement

Composition Chemical

71

TABLE 14 The Distribution of Lignin in WhiteBirch

Element

Lignin concentration (g/g)

TissueMorphological volume region

(%)

11.4 58.5 3.5 5.2 2.4 1.6 4.3 2.3 0.8 =O

8.0 2.0 =O =O

UV-EDXA

uv only”

0.14

-

0.14 0.12 0.36 0.45 0.26 0.26 0.27 0.40 0.58

0.16

0.12

0.22

0.38 0.47 0.41

-

0.34 0.72 -

0.22 -

0.35 -

“From Ref. 9 1. hCalculated using xylem lignin content of 0.199 g/g; from Ref. 72. “Fiberlfiber. ‘Fiber/vessel. ‘Fiberlray. ‘Raylray.

with each other. However, the lignin concentration in the ray parenchyma cells byUVEDXA[91] is nearly half aslow as the dataobtained by UV microscopyalone[72]. Although the middle lamella between two cell corners (ML) of fibers and vessels revealed similar values by these two techniques, the concentration in the cell corner middle lamella (ML,,) was lower by the UV-EDXA technique [91]. The observed discrepancies are due probably to the uncertainty in estimating the guaiacyl/syringyl ratio, as the analysis is made by UV microscopy alone.

VI.

DISTRIBUTION OF INORGANIC CONSTITUENTS

A fair amount of information is available on the inorganic constituents of wood [1031091 and bark [ 110,ll l]. In woods from temperate zones, elements other than carbon, hydrogen, oxygen, and nitrogen make up between 0. l % and 0.5% of the weight of wood [ 1 12,1131, whereas those from tropical regions make up to 5% [ 1 141. This proportion, although small, contains a wide variety of elements. For example, spectrographic analysis of grand fir [l031 revealed as many as 32 elements (Table 15). In many cases, alkali and alkali earth elements such as Ca, Mg, and K make up about 80% of the total inorganic constituents [l 151. These elements probably occur in wood as salts, e.g., oxalates, carbonates, and sulfates [ 1161, or inorganic moiety bound to the cell wall components such as carboxyl groups of pectic materials [ 115,117,1181. Some of the inorganicelementspresent in wood are essential forwoodgrowth, whereas others are not necessarily required. Metalic elements are often absorbed into the tree through the root system and are transported to all areas within the growing tree [ 1031.

Sa ka

72

TABLE 15 Classification, Function, and Approximate Level of Occurrence of Elements Found in Wood of Grand Fir (ppm of dry weight)

Essential Major

Constituent C

Ca

754

0 H N P S

K Mg Na

865

c1

Si

171 23 -

B Mn

Fe

0.9 19.3 2.6

Ag 0.23 AI 5.4 Ba 20.2

MO

0.005

CO

Cu Zn

2.5 0.9

Cr Ni Pb Rb Sr

0.0 1 0.05 0.1 1 0.12 2.0 10.2

Ti

0.11

Au

0.04

0.02 Ga In La

0.03

Li 0.003 Sn

0.13

0.04

v

0.001

Zr

0.002

Source: Ref. 103.

For seven species, Young and Guinn [ 1091 have determined the distribution of 12 inorganic elements in various tissue areas of a tree such as the roots, bark, wood, and leaves. The results indicated that both total ash content and concentration of each element vary significantly within and between the species. Therefore, unlike major cell wall components such as cellulose and lignin, the content of inorganic constituents varies to a great extent with the environmental conditions under which the tree has grown [ 105,1131. Little has been published regarding the morphological distribution of elements in the cell [ 1 19- 1221. By microincineration, Lange [ 1 191 found that mineral constituents of Swedish spruce are deposited predominantly in the compound middle lamella. Zicherman and Thomas [ 1201 also have pointed out that careful ashing of microtome sections of loblolly pine (Pinus r n e h L.), followed by electron microscopic observations, gives an ash residue distributed throughout the cell wall and concentrated in the compound middle lamella and S, layer. Wultsch [ 1211 stated that manganese is concentrated in ray cells, and Bergstrom [ 1221 reported that the phosphoruscontent is highest in the cambiumandadjacentxylem portions. Saka and Goring [ 1151 have studied the distribution of inorganic constituents from the pith to the outer ring of black spruce (Picea nznriann Mill.) by means of TEM-EDXA. The TEM-EDXA technique is a useful tool fordetectinganyelementaboveneonand recently above boron in the periodic table. Figure 12 shows seven morphological regions of the tracheids, ray tracheids, andrayparenchymacells investigated. Thedarkcircle indicates the location of the analysis and its diameter corresponds to the resolution of analysis (400 nm). Detected were 15 different elements, such as Na, Mg, AI, S, Cl, K, Ca, Cr, Fe, Ni, Cu, Zn, and Pb, above neon in the periodic table. The secondary walls of tracheids, ray tracheids, and ray parenchyma cells usually contain detectable concentrations of only four elements: sulfur, chlorine, potassium, and calcium. In contrast, almost all the elements were found to be localized and concentrated in the torus and half-bordered pit membrane regions (Fig. 13). The total content of inorganic constituents decreased in the order of torus (2%) > half-bordered pit membrane (1%) > middle lamella (0.4%) > ray parenchyma cell wall (0.3%) > tracheid secondary wall (0.1-0.15%). The total content of inorganic constituents was higherin earlywood than latewoodfor any of the morphological

omposition Chemical

and Distribution

73

FIGURE 12 Transmission electron micrographs of a cross section of black spruce showing the seven different morphological regions. All micrographs were takenat the same magnification. S , = secondary wall of the tracheid CC = cell comer middle lamella surrounded by tracheids TT = tours in an intertracheid pit pair SR = secondary wall of the ray parenchyma cell M = a half-bordered pit membrane between ray parenchyma cell and tracheid SRT = secondary wall of the ray tracheid TRT= torus in an intertracheid pit pair between ray tracheid and tracheid

74

Saka

FIGURE 13 EDXA spectra from the tracheid secondary wall and tracheid torus in black spruce. (From Ref. 115.)

regions studied. This is probably because the earlywood tracheids that have large lumens and abundant pits are the major water-conducting tissues, whereas thick-walled latewood tracheids with fewer pits may act as a physical or mechanical support for the wood. Bailey and Reeve [l231 have recently used imaging microprobe secondary ion mass spectrometry (SIMS) to determine the distribution of the trace elements in black spruce (Picea rnariana Mill.). This imaging microprobe S N S technique is a powerful tool for detecting inorganic elements with high spatial resolution and high sensitivity. Their overall findings correlatewell with results from the TEM-EDXA studyby Saka and Goring[ 1151. However, due to its higher sensitivity compared with the EDXA technique, the distribution of the elements within the cell wall could be more clearly demonstrated. Figure 14 is one example in which some elements are visualized and concentrated in the middle lamella region. Recently, Saka and Mimori [l241 have studied the distribution of inorganic constituents of Japanese birch wood (Betula platyphylla Sukatchev var. Japonika Hara) by the SEM-EDXA technique with thin sections. Figure 15 shows six morphological regions of the fibers, vessels, and ray parenchyma cells investigated. The dark circle corresponds to the resolution of analysis (800 nm). Detected were 11 different elements: Na, Mg, Al, Si, P, S, Cl, K, Ca, Fe, and Zn. The secondary walls of wood fibers, vessels, and ray parenchyma cells usually contained detectable concentrations of three elements, S, Cl, and Ca, while, in the amorphous layer of ray parenchyma cell and pit membrane between vessel and ray parenchyma cell, almost all of the detected elements were found to be localized and concentrated (Fig. 16). The total content of inorganic constituents decreased in the order amorphous layer (0.68%) > fiber middle lamella (0.54%) > vessel middle lamella

Composition Chemical

75

FIGURE 14 Ion image and its intensity for Ca. Fe, and Mn from a tangential section of a double cell wall of black spruce heartwood. (Courtesy of Prof. D. W. Reeve, Universityof Toronto, Toronto, Ontario, Canada.)

> fibersecondary wall (0.14%) > vessel (0.48%) > ray parenchema cell wall (0.15%) secondary wall (0.10%).This observed trend is basically the same as found in black spruce by Saka and Goring [115]. VII.

CELL WALL ORGANIZATION

In the previous sections, current knowledge of the distribution of cell wall constituents was described. In this section, therefore, how these constituents construct and organize the cell wall structure is discussed. In wood cell walls, cellulose acts as the structural framework in the formof cellulose microfibrils, while hemicellulose is the matrix substance present between these microfibrils. Lignin, on the other hand, is the encrusting substance binding the wood cells together and giving rigidity to the cell wall. Generally, the S2 layer increases with increasing wall thickness, whereas the S , and S , remain fairly constant. Because of its greater thickness, the S2 layer is largely responsible for the physical and mechanical properties of the cell walls. Figure 17 shows the relationship for softwoods between the lignin content and microfibrillar angle (e) in the tracheid S2 layer determined by the X-ray diffraction method. Since the majority of the lignin in softwoods is in the tracheid S , layer [50], the whole lignin content of wood must be closely correlated to the lignin concentration in the S , layer of the tracheid. Thus, from Fig. 17, the lignin concentration in theS , layer increases

76

Saka

c

FIGURE 15 Scanning electron micrographs of a transverse section of the Japanese birch wood showing the six different morphological regions considered in this study. Fs = secondarywall of thewoodfiber F,, = cell comer middle lamella surrounded by wood fibers Vs = secondarywall of thevessel VML= cell comer middle lamella surrounded by vessel and wood fibers Rs = secondarywall of therayparenchymacell RA, = amorphous layer in the ray parenchyma cell

with increasing microfibrillar angleof the tracheid S2 layer [ 1251. The biosynthetic origin

of this relationship is not known. However, it does suggest that, in order to construct the enforced plywood type of structure shown in Fig. 1, the three major chemical constituents of wood mutually interact and strengthen each other to make up a natural supercomposite material. Figure 18 shows such an ultrastructural arrangement of cellulose microfibrils, hemicellulose, and lignin in wood cell walls as proposed by Harada and CBtC [126]; around the core of cellulose microfibrils, paracrystalline regions of cellulose are thought to exist, which are associated with hemicellulose and lignin. Lignin encases them and binds them into the rigid structure of the wood cell wall. At the molecular level of arrangement of the chemical composition, the presence of a chemical bond between lignin and carbohydrate has been proved to be a lignin-carbohydrate complex (LCC) [l271 which is considered to be a compatibilizer-like substance localized at the interface between hydrophobic macromolecules of lignin and hydrophilic carbohydrates, by enhancing the physical and mechanical properties of wood [128].

77

Chemical Composition and Distribution

I

.

D-

4

I

I

FIGURE 16 Scanning electron micrograph (a) of a cross section of Japanese birch. The arrow shows the location of the EDXA analysis at the pit membrane between vessel and ray parenchyma cells from which the EDXA spectrum (b) was obtained.

45

2

40

Y

c

c

2 35 S

c *g 30 3

25 20 0

10 5 020 4 0 3 0 Microfibrillar angle Cel

FIGURE 17 Relationshipbetweenthemicrofibrillarangle lignin content of wood. (From Ref. 125.)

(e) inthetracheid

S2 layerandthe

78

Saka

FIGURE 18 Schematic diagram of the ultrastructural arrangement of a cellulose microfibril (Mf), hemicellulose (H), and lignin (L) in the wood cell wall. (From Ref. 126.)

VIII.

CONCLUDINGREMARKS

Knowledge of the chemical composition of woodis essential for studying the physical and chemical properties of wood. However, it can provide nothing but the average of the cell wall constituents. For a better understanding of wood properties, more detailed information is required about their distribution across the wood cell wall. However, in spite of a variety of methods proposed, all the methods have drawbacks and thus some discrepancy exists among investigators. A good method for resolving such discrepancies would be to separatevarioustypes of tissues physicallywithoutintroducinganychemicalchanges [49,96]. Analysis of the separated tissues could then provide definitive information on the distribution of the cell wall constituents at the various morphological regions of wood.

REFERENCES 1.

2. 3.

4. 5.

6.

7. 8. 9. 10. 11.

12. 13. 14.

A.B.Wardropand D. E. Bland, in Biochemistry of Wood (K. Kratzland G. Billek, eds.), Pergamon Press, London, p. 92 (1959). H. Harada, Mokuzai Gakkaishi, 30:513 (1984). F.F.P. Kollmann and W.A. C M , Jr., Principles of Wood Science and Technology, Vol. I, Solid Wood, Springer-Verlag, Berlin, p. 43 (1968). B. L. Browning, The Chemistty of Wood, Wiley-Interscience, New York, p. 57 (1963). T. E. Timell, in Cellular Ultrastructure of Woody Plants (W. A. CBtC, Jr., ed.), Syracuse Univ. Press, New York, p. 127 (1965). H. Meier, in Biosynthesis and Biodegradation of Wood Components (T. Higuchi, ed.), Academic Press, New York, p. 43 (1985). F.F.P. Kollmann and W. A. CBtC, Jr., Principles of Wood Science and Technology, Vol. I, Solid Wood, Springer-Verlag, Berlin, p. 55 (1968). W. A. C M , Jr., in Recent Advances in Phytochemistry, Vol. 11 (F. A.Loewusand V. C. Runeckles, eds.), Plenum Press, New York, p. 1 (1977). T. E. Timell, Wood Sci. Technol., 1:45 (1967). H. Higuchi, KASEAA, 13:206 (1975). T. E. Timell, Wood Sci. Technol., 16233 (1982). G. Schwerin, Holdorsch., 12:43 (1958). M. Fujii, J. Azuma, F. Tanaka, A. Kato, and T. Koshijima, Wood Res., 68:8 (1982). G. C. Hoffmann and T. E. Timell, Tappi, 55:733 (1972).

Chemical Composition and Distribution

79

15. H. Harada and A.B.Wardrop, MokuzaiGakknishi, 6:34 (1960). 16. 0. Perila, J. Pol.ymer: Sci., 51:19(1961). 17. 0. Perila and P. Heitto, Suomen Kemistilehti, B32:76 (1959). 18. G. C. Hoffmann and T. E. Timell, Tappi, 55:871 (1972). 19. H.Meier, Pure Appl. Chem., 5:37 (1962). 20. P. Whiting and D. A. I. Goring, Post-Grad. Res. Lab. Rep. 241, PPRICAN, Quebec, Canada (1981). 21. W. A. C8t6, Jr., N. P. Kutscha, B. W. Simson, and T. E. Timell, Tnppi, 51:33 (1968). 22. W. A. C M , Jr., A. C. Day, N. P. Kutscha, and T. E. Timell, Holz&rsch., 21: 180 (1967). 23. R. B. Hanna and W. A. C8t6, Jr., Cytobiologie, 1 0 1 0 2 (1974). 24. T. Goto, Ph.D. thesis, Dept. Wood Science Technology, Kyoto Univ., Kyoto, Japan (1976). 25. A. N. J. Heyn, Tippi, 60:l59 (1977). 26. G. Cox and B. Juniper, J. Microscopy, 97343 (1973). 27. J. B. Boutelje and B. H. Hollmark, Hol$orsch., 26:76 (1972). 28. V. M. Sinner, N. Parameswaran, H. H. Dietrichs, and W. Liese, Hol$orsch., 2 7 3 6 (1973). 29. M. N. Sinner, N. Parameswaran, N. Yamazaki, W. Liese, and H. H. Dietrichs, Appl. Polymer: Symp., 28:993 (1 976). 30. P. Hoffmannand N. Parameswarm, Holdorsch., 30:62 (1976). 3 1. T. Awano, K. Takabe, and M. Fujita, Abstr: 46th Annual Meeting of the Japun Wood Resenrch Society, p. 32 (1996). 32. T. Fujii, H. Harada, and H. Saiki, MokuzniGakknishi, 27:149 (1981). 33. W. A. C M , Jr., A. C. Day, and T. E. Timell, Wood Sci. Technol., 2 : I3 (1968). 34. N. Parameswaran and W. Liese, Holz.Roh.-Werkst.. 40:145 (1982). 35. P. Albersheim, K. Muhlethaler,and A. Frey-Wyssling. J. Biophys. Biochem. Cyrol., 8501 ( 1 960). 36. R. M. McCready and R. M. Reeve, Agric. Food Chem., 3:260 (1955). 37. M. Gee, R. M. Reeve, and R. M. McCready, Agric. Food Chem., 7 3 4 (1959). 38. E. M. Barmicheva and M. F. Danilova. Bo?.Zh., %':l278 (1973). 39. P. M. Colombo and N. Rascio, J . Ultrustruct. Res., 60:135 (1977). 40. Y. Czaninski, Biol. Cellulnire, 35:97(1979). . R389 (1936). 41.A. J. Bailey, lnd. Eng. Chem., A I Z U ~Ed., 42. H. Meier, J. Polymrc Sci.. 5 / :1 1 (1961 ). 43. H. Meierand K. C. B.Wilkie, HolO. 1%). The diaryl ether (g) and the biphenyl acids ( g )being , monocarboxylic acids, originated from substructures from which one of the side chains has been detached. The tricarboxylic acid (3) is one of the examples indicating a mixed radical coupling (coniferyl and p-coumaryl alcohols) and, analogously, the diaryl ether ( g ) is derived from a substructure formed from a sinapyl and a coniferylradical. The trimethoxylated ring in the trace constitutents were remarkable.

Chemistry of Lignin

139

The methoxyhydroquinone ring in these acids may be have been formed by reduction of a methoxy-p-quinone moiety which seems to be the hydrolysis product of a 2,4-cyclohexadienone diary1 ketal structure such as (3) being formed by dehydrogenative coupling [ 1131. It points to the presence in lignin of a methoxyhydroquinone.

B.

Hydrogenolysis

Previously, the hydrogenolysis of lignin wasstudiedtoproducechemicalsandalsoto obtain structural information. Recently, protolignins have been subjected again to catalytic

YHOH YHOH

R R=OMe R=H

OH Cez, R I = R 2 = b = H or OH R3=H or OMe

OH

0

Me0Q O M e OH

OH

CH2 CH20H

OMe MeO’

0

OMe (ep) R=H or OMe OCH2CH2OH

MeOQ

0

O M e OH

umz, CH2R

YH2OH

I

y

HC-

2

H&”O“CH I

MeO OH OH

OMe OMe

cuL1)R=H or OH

OMe

OH UM) R=H

or OH

OMe

MeO

OH

W

FIGURE 25 Products obtained by mild catalytic hydrogenolysis of protolignins with copper chromium oxide at about 240°C.

140 San0

and

Sakakibara

Me0 OH OH

R3

uQ6)R,=R2=H or OH R3=H or OMe

OMe

Q O M e OH

R=H or CHIOH

oc'o"-

y

m6

HOH27

2

7H2

H?-

CH2

OMe OH

OH

Me0

OC'O,

?H2

H?-

YH

CH2 /

OMe

/

OMe

OH

uu OH

FIGURE 25

W

Continued

hydrogenolysis under conditions at about 240°C with copper chromium oxide, leading to the isolation of various dimeric and trimeric compounds (94-111) besides a substantial amount of monomers [102,103,114-117,1211. The lignols isolated are summarized in Fig. 25. Hydrogenolysis cleaves most of aryl-alkyl ethers, but a few of these linkages remain intact as seen in compounds (95 - R, = OH, 96). Compound (96) - has an a-hydroxyl that

HC-0

141

Chemistry of Lignin R

I CH II CH

CH20H I

l CHOH

HOH27 Q O M e HOH2C HC-0 I CHOH

I

R

$$,

I

HC-

0

OMe

OH R=H or OMe

OH

0 Rl=CHCHCHzOH Rz=H

G OH O M e

R$

OMe

u1z) R,=CHCHCHO

R2=OMe

0 R,=CHzCH?CHzOH Rz=OMe

FIGURE 26

Products obtained by mild hydrolysis of protolignins with aqueous dioxane at 180°C.

142 San0

and

Sakakibara

(m)

(m)

has quite exceptionally remained unaffected. Compounds [ 102,1141 and [ 1031 should give metahemipinic acid (2) by permanganate oxidation of methylated lignins. Compound was isolated from the lignin of larch compression wood, but its occurrence in normalwood lignin isalsoprobable. Itcanbeconsidered that compounds (101,102) could be derived by reductive cleavage from pinoresinol and (E), respectively. However, a model experiment indicated that the alkyl-alkyl ether of the tetrahydrofuran ring is very stable toward hydrogenolysis, and the starting material (g) was almost completely recovered [ 1151. This fact suggests that compounds (E) may not be derived from compound after all, and these linkagepatterns may exist independently in lignin molecules. Compound (g) was isolated from the hydrogenolysis products of hardwood protolignin [ l 161, indicating that the alkyl-alkyl ether is fairly stable against the reductive cleavage. Compound (96)contains a 7-0-4 linkage [ 1171 so far not known. It is, however, very probable that a p-0-4 linkage can be enzymatically rearranged to a y0 - 4 linkage. The existence of 7-0-4 linked units waspostulatedatan early stage by Freudenbeng [ 1191 in 1933 without experimental support, but later he abandoned the idea. Subsequently, in 1960, Brauns et al. [l201 proposed the same substructure. However, this linkage pattern may be a minor one in the lignin macromolecule. Compound (F), which was isolated from hardwood protolignin [ 1211, has a heterocyclein the molecule involving a 4,5-dihydroxy-3-methoxyphenylmoiety. Compounds ( I 10,111) are trilignols with a y-lactone. They have no optical activity. The IR spectra of MWLs show a small shoulder at 1760 cm”, indicating the existence of y-lactones. The facts indicate that p-hydroxy cinnamic acids are also involved in the radical coupling scheme after enzymatic dehydrogenation. Nimz [95] cleaved protolignin from beech wood with thioacetic acid using boron trifluoride as a catalyst followed by reductionwithRaney nickel toproducemanydimericcompounds.Theproductswere somewhat different from those formed by hydrogenolysis.

(m)

(e)

(e)

C.

MildHydrolysis

1. Hydrolysis withWater

Nimz [125-1291 percolated extractive-free wood powder with water at 100°C for several weeks (“mild hydrolysis”), showing that beech wood loses about 40% of lignin, while only 20% of lignin in spruce wood goes into solution. From hydrolysis products of spruce were isolated eight dilignols, two diastereoisomeric trilignols, and one tetralignol: guaiacylglycerol-p-coniferyl ether [ 1261, a trilignol involving p-0-4 and p-1 linkages (123)[127], a tetralignol involving one 6-1 and two p-0-4 linkages (125)[ 1271, guaiacylglycerol-p-guaiacylglycerol ether [ 1261, and guaiacylglycerol-p-coniferylaldehyde (M)[ 1281, and from beech were isolated syringaresinol (g) [98] and three diarylpropanediols (115)[89,123].

(e) (e)

2. Hydrolysis with Dioxane and Water Sakakibara et al. [ 130-1391 found that 40-60% of lignin can be dissolved by treating wood powder with a dioxane and water (1: 1) mixture at 180°C. The many degradation products are almost the same as those obtained by Nimz (Fig. 26). Arylglycerol-p-aryl ether (114) [48], three diarylpropanediols [ 133,134,551,thephenylcoumarans (1 17,122) [76] and [ S ] , syringaresinol(45) [99], adilignol with an a-carbonyl group (g) [loo], the trilignols (122)[135], (123)[133], (124)[100,136], and the C6-C3-C3 lactone (Fig. 27) [ 1371 were isolated and identified, besides considerable amounts

(m)

(m)

(x)

143

Chemistry of Lignin

OH OH

M e o ~ o M e

+

H2?/"CH MeoQoMeI

Me0QOMe OH

OH

0

OH OH M e O A O M e

M e o ~ o M e

CH20H I

'CH

H2qA0"CH II

I

(b) R

6 0

HC$/C" I

CH20H

HCI

I

CH I

CH l

Me0

OMe+ Me0

OMe

OMe

0

OMe

0

/

0

J +H20 OMe H o H q G O H HOHC I

R Q OH O M

OMe e

R=H or OMe FIGURE 27

Proposedformation

(g). (From Ref.

+

Meo6

H2?C ,"H

I

HC-

CH

I

I

O+C-~-CH2 UZL)

mechanism of compound for biogenesis of substructure unit

137.)

of monolignols. A compound (M)with an w-propanol side chain supports that the existence of such reduced sidechains in the lignin structure is probable,considering the highly shielded signals in their NMR spectra, as discussed before 18,191. The formation of cornpound (E), which has eliminated an aromatic ring, may be explained i n two ways: ( a ) ring closure during hydrolysis or (b) displacement of the diarylpropanediol dur-

144

Sakakibara and Sano

+ OH

OH

OMe OMe

(m FIGURE 28 DegradationofcY,P-diarylethers (3 and aqueous dioxane at 180°C for 20 min. (From Ref. 139.)

g) by “mild

hydrolysis”using 50%

ing coupling ofradicals (Fig. 28) formed eitherin wood in situ or secondarly by homolysis of phenolic P-ethers in lignin, as shown in Fig. 29 [137,141]. As described before, the degradation products from hydrogenolysis and hydrolysis provide much important information about lignin structure and supportthe theory of lignin formation by enzymatic dehydrogenation of cinnamyl alcohols. Hydrolysis mainly cleaves the a-ethers of side chains in the lignin molecule in spite of poor model experiments, but nonphenolic P-ether cleavage occurs hardly at all [58,139]. The model experiments with p-hydroxyarylglycerol-a$-diary1 and -&aryl ethers [ 138- 1411 have indicated that homolysis occurs to a slight extent during hydrolysis, resulting in subsequent coupling of the radicals formed. But Sakakibara [l651 has stated that the formation of artificial products from protolignin by “mild hydrolysis” is negligible because of the following fact: Dehydrodiconiferyl alcohol (M)and pinoresinol and dehydrodiguaiacylpropane (106, R,=OCH,, R,=R,=H) that were found in the model experiment [138-1401 could not bedetectedeven in traces in the hydrolysatefromspruceprotolignins in spite of the proposed mechanism of compound shown in Fig. 27. The homolytic degradation of phenolic p-0-4 linkages will be summarized in the following section because of getting many informations on homolytic cleavage of phenolic P-ethers under the conditions similar to those for “mild hydrolysis” and “catalytic hydrogenolysis” of protolignins.

(e)

(m)

3. Homolysis With the aim of finding a method which would degrade lignin without involving simultaneous condensation reactions, Nimz [ 125- 1291 and Sakakibara [ 130- 1391 subjected extractive-freesoftwoodandhardwoodto“mildhydrolysis”under neutral or slightly acidic conditions followed by a percolation with hot water and a mild hydrolysis with

145

Chemistry of Lignin CH20H

CH20H

l

HCI

H A - 0 9

+

QOMe 0.

OMe

Rb

OH

0 Ra

uz&, (24.0%)

CH20H l CH II

m+RaorRb OMe 0-

OH

RC

(1) (2.6%)

OH .OMe

CHsCH-CH20H

CKOH

I

+

2xRa

CH20H H { - 0 9

FH20H j_\

OMe

Ra+Rc

-

OMe

+

H$-0 Q O M e Q O M e

Ra + Rb + RC

-

0 '

w(3.656)

polymerichydrolysisprodUCts of

FIGURE 29 Homolytic degradation of guaiacylglycerol-P-guaiacylether (g) by "mild hydrolysis" using 50% a q ~ ~ e o dioxane us (pH 3.54) at 180°C for 20 min (%: yield of the amount of starting material used). (From Ref. 140.)

146 San0

and

Sakakibara

aqueous dioxane at 1 80”C,respectively. Separately, they have isolated and identified many monolignol-to-trilignol hydrolysis products in small amounts, of which most compounds are identical to each other. The hydrolysis products due to cleavage of the benzyl aryl ether linkages have been thought to give valuable information on the end groups in lignin and the biosynthesis process of lignin, since it is probable that in the “mild hydrolysis” the hydrolytic cleavage of benzyl aryl ether linkages in lignin is the main reaction in spite of deficient experimental evidence. To obtain definitive information for the reaction mechanism of lignin under the conditions of “mild hydrolysis,” some lignin model compounds were subjected to mild hydrolysis. The hydrolysisofmodelcompounds(126,127)forphenolicandnonphenolic a$-diary1 ethers were carried out in 50% aqueous dioxane at 125-180°C for 20- 120 min [138-1411. The phenolic a-aryl ether bond in (126)was cleavaged by 41% and 83% at 140°C for 20 min and 120 min, and by 100% at 180°C for 20 min, and the nonphenolic a-aryl ether bond in by only 39% at 180°C for 20 min [139]. The latter was more resistant to mildhydrolysisthan the former. The nonphenolicP-ethercompoundwas recovered quantitatively from the reaction mixture of (g). New dimeric and trimeric compounds were obtained from the reaction mixture of (3) [138]. Guaiacylglycerol-P-guaiacyl ether was subjected to “mild hydrolysis” by two procedures, that is, 50% aqueous dioxane (pH 3.54) at 180°C for 20 min, and also water (pH 3.54) at 110°C for 48 h in place of the percolation procedures. Thin-layer chromatograms of reaction products obtained from the former were completelyidentical with those from the latter, indicating that the cleavage of the P-ether according to the two procedures proceeds by the same mechanism [140]. From the reaction mixture obtained by the former procedure of the starting material(24.0%),coniferylalcohol (1)(2.6%),pinoresinol (0.8%), 1,2-diguaiacyl1,3-propanediol (E) (1.4%), dehydrodiconiferyl alcohol (4.5%), and two trimeric compounds (1 29,130) were isolated and identified besides substantial amounts of unknown polymerizedmaterials [l401 as illustrated in Fig. 29. The compound (3.8%) was composed of phenylcoumaran and P-ether moieties, whereas (3.6%) had two P-aryl ether links. Gel filtration of the reaction mixtureshowed that their molecularweights increase with increasing reaction time, demonstrating that the degradation and polymerization of lignins take place simultaneously by “mild hydrolysis.” The polymerizationtookplaceduring the heat-up time up to 18O”C, and became predominent for 120 min to form more stabilized polylignols in large quantity. Both the relative absorbance of the “mild hydrolysis” products at A,,, about 280 nm for UV (neutral), and at A,,, about 300 nm for ionization differential spectrum ( A s i ) increase with increasing the reaction time, respectively, when measured immediately after the hydrolysis. In 348 h after the hydrolysis both of them decrease strikingly, and the absorbance at A,,,,,, about 255 nm for UV (neutral) increased reversely compared to every one when measured immediately. The UV spectrum for the reaction products, which was measured 348 h after “mild hydrolysis” of for 120 min, were very similar to that in spruce MWL except for slightly high absorbance at A 360 nm for A&,, due to phenolic conjugated carbonyl groups. The results obtained by UV analyses show that more reactive sites such as quinonemethides are present among the “mild hydrolysis” products of the model compounds, MWL, and wood-in-situ lignin [140]. When 1-guaiacyl-2-guaiacoxy- 1 -propene-3-01 (131) was subjected to mild hydrolysis with 50% aqueous dioxane at 180°C for 20 min, Hibbert’s monomers (3 1-34) were formed in addition to the starting material (15.6%) (Fig. 30). This implies that phenolic arylgly-

(m)

(m)

(e) (m)

(m),

(B)

(m)

(m)

II CH I

CHO OMe Ribkrt’s ketones

QOMe OH

(3.l-U (Total about 16%)

COOH

+ I

OH

(L31) (15.6%)

FIGURE 30 Degradation of I-guaiacyl-2-guaiacoxy- 1-propene (131)by “mild hydrolysis” using 50% aqueous dioxane (pH 3.54) at 180°C for 20 min [%: yield of the amount of (=)I. (From Ref. 140.)

148 Sano

and

Sakakibara

cerol-P-ethers in lignin are never degraded via corresponding enol-ethers under the “mild hydrolysis” conditions [ 1401. From the “mild hydrolysis” products of syringylglycerol-P-syringylether (133)by aqueous dioxane [ 14 l], syringol ( 1 4.5%), syringaldehyde (2)(1.1 YO),sinapyl aldehyde (134)( I . I %), 1-syringyl-2-syringoxy-3-hydroxypropanone-1 (136)(0.9%), D,L-syringaresinol (g) (5.6%), 2-formyl-3-hydroxymethyl- 1,4-bis-syringyl- 1,3-butadiene (3) (1.3%), and 1,2-syringyl- 1,3-propanediol (7.1 %) were isolated besides the starting material (33.3%) and polymeric compounds as shown in Fig. 3 1 . Sinapyl alcohol (2) and syringylglycerol-P-sinapylether which were not detected amongthe products, might be too thermolabile to exist in the reaction mixture, even if they were formed during the reaction process. The thermolabilities of (2,137) are apparent from the fact that they are synthesized only by reduction with LiAIH, at -30°C. The product patterns shown by “mild hydrolysis” of the two @-ether models and 134)by two procedures with aqueous dioxane at 180°C for 20 min and with water at 110°C for 48 h suggest that the reaction imitates the dehydrogenative formation of lignols and higher polymers from coniferyl alcohol and sinapyl alcohol (2). Accordingly, it is proposed that under the “mild hydrolysis” conditions phenolic arylglycerol-@aryl ether bonds may be subjected to homolytic degradation followed by polymerization as illustrated in Figs. 29 and 3 1. Phenolic @-aryl ethers as end groups in lignin lose a molecule of water to give the corresponding quinone-methides (QM), followed by homolytic cleavage of Paryl ether bonds with the formation of a p-hydroxycinnamyl alcohol and phenoxy radicals (Ra and Rb), whereas a radical transfer reaction between the former radical (Ra) and the phenolic hydroxyl groups yields a corresponding p-hydroxylcinnamyl alcohol (1, 2, or 3) and a new phenoxy radical (RC).“Endwise”addition of ap-hydroxycinnamylalcohol radical (Ra) to the latter phenoxy radical (RC)produces the trilignols in Fig. 29 and highermolecular lignols. Nimz et al. found that guaiacylglycerol-P-dihydroconiferylether gave dihydroconiferyl alcohol (41%) and dihydrodehydrodiconiferyl alcohol (phenylcoumaran, 32%), in addition to unknown products, when heated in water for 7 days [142]. Later, guaiacylglycerol-P-vanillyl ether (W)gave a phenylcoumaran coniferyl alcohol (l), vanillyl alcohol (g), and unknown condensed products as precipitates in addition to the starting material (35%) in yields of about 10, 18, 20, and 15%, respectively, when heated with water at 130°C for 4 h, and the yield of condensed products after 20 h at 130°C reachedabout30% in the absenceof starting material(Fig.32).Fromthese results Nimz concluded that a homolytic cleavage of the phenolic P-0-4 linkage occurs with water in neutral solution at 130°C [ 1431. In order to investigate the extent of homolytic reactions under the conditions of “mild hydrolysis,” Westtermark et al. [ 1441 subjected guaiacylglycerol-P-guaiacyl ether (128) and spruce wood to “mild hydrolysis” using dioxane:buffer ( I : 1) at different pH (3.69.6) and various temperatures ( I 30- 180°C). The main products obtained by the heating of spruce wood at 180°C were coniferyl alcohol (L), vanillin and coniferyl aldehyde ( 5 ) in order of peak area. The addition of a catalytic amountof FeZ+considerably increased the yield of (L) without influence on the rate of degradation. The amount of (l)formed was found to be strongly dependent on the temperatures; that is, the amount formed at 130°C was only one-fifth of that at 180°C. Its yield reached a maximum after about 100 millat 130”C, and a set of two maxima after 60 minand 120 min at 160°C as well as 180°C. then slowly decreased, showing that the coniferyl alcohol emerges from at least two different sources, and the formation and decomposition of (l)take place during the ‘‘mild hydrolysis.” The maximum amount of at 180°C exceeds the amount correspond-

(m)

(m),

”

(m

(L)

(m)

(x)

(E)

(m),

(z),

(L)

(L)

149

Chemistry of Lignin 7H20H OMe

HF-oa CHOH OMe

7H2OH OMe -H*O

0

H ? - 0 9 CH OMe

e Me0G O MOMe OH

OMe

7H20H

-6 '?H

+I[.

+M

Me0

0

0

OH

e O Q O M x e O Q O M e 0. OH

Meo60Me M e o ~ o M e

CH20H CH I

2XRa"+

HC-?H

l

I

CH2OH CH Me0O 0O M e

"t

(I4 S%)

Kb

7H2OH CH II

.CH

H+CH HC-o-kH,

Me0G O M e

FIGURE 31 Homolyticdegradation of syringylglycerol-P-syringyl ether (133)by "mild hydrolysis" using 50% aqueous dioxane (pH 3.54) at 180°C for 20 min (%: yield of the amount of (g)]. (From Ref. 141.)

150

Sakakibara and San0

...goMe CH

YH20H CH qH2OH

Ra+Rc

H-C

L 1

Q

CHo OMe

Me0

Ra+Rb

c-c

$H I

-

$oM;o Me0 OH

+ I I,O

Me0

OMe

0

Me0 OH U)(7 1%)

FIGURE 31

Continued

ing to the total content of coniferyl alcohol end groups (about 2%) in softwood lignin. This means that a considerable part of the coniferyl alcohol must originate from sources other than a-ether end group substructures in lignin. The stability of coniferyl alcohol (1) toward “mild hydrolysis” in 50% dioxane (pH 7) at 130°C demonstrated that coniferyl alcohol is degraded or polymerized fairly rapidly even at 130°C. And guaiacylglycerol-Pguaiacyl ether ( 128), which was degraded by 80% in the same medium after 40 min at 1 8O”C, a f f o r d e d 0 at a 20% mole yield. No Hibbert’s ketones (31-34) could be detected in the reaction mixtures even when about 50% of ( 128) was degraded in 50% dioxane at a pH as low as 2.7 and 160”C, indicating that a c i d o 5 i s does not occur by “mild hydrolysis.” Based on the above results, Lundquist et al. [ 1441 concluded that the use of “mild hydrolysis”asa tool in lignin analysis leads to erroneous and confusing results if the homolytic cleavage of the @-ethers is neglected. It is well-known that phenolic P-ethers in lignin are prone to undergo reactions via ionic intermediates under acidic and basic conditions. However, it has been reported recently that the P-etherbond i n syringylglycerol-P-syringylether (133)is subjected to homolysis under conditions similar to those in a soda cook [ 1451. Furthermore, Sipila et al.[l461 allowed syringylglycerol-P-syringylether to stand in aqueousdioxane solutions at pHof 4-7 at roomtemperatureandfoundsyringaresinol (G),P-l (E: R,=R2=OCH,) and 2,6-dimethoxy-p-quinones, giving further evidence for the existence of homolytic P-aryl ether cleavage in delignification of hardwoods. Sano [ 147,1481 subjected extractive-free oak wood meal, guaiacylglycerol-P-guaiacy1 ether and its 4-0-methyl ether to “solvolysis” in p-cresol-water ( 1 : 1 ) at 180°C for 30 min in order to explain the delignification mechanism of wood lignin by solvolysis pulping with aqueous phenol at elevated temperatures. Fromreaction products of the wood, six compounds (143-148) were isolated and identified, as the same cresolated compounds

(m)

(m),

151

Chemistry of Lignin

KMe OMe

+

Q O M e

OH

20 h

OH

OH

+

W (35%)

condensed products (30%)

FIGURE 32 Degradation of phenolic P-ethers (3 and 141)by “mild hydrolysis” with H 2 0 at 100°C or 130°C (%: yield of the amount of (139)or (E)]. (From Ref. 142.)

with guaiacyl and syringyl rings also obtained, respectively, from the reaction products of guaiacylglycerol-P-guaiacyl ether (128)and sinapyl alcohol (2) treated under the same conditions. The reaction mechanism by phenolysis has been illustrated asfollows(Fig. 33). The cleavage of phenolic @-ethers in lignin proceeds via homolysis to form two freeradicals, which are trapped by the phenols as a solvent and/or other active intermediates to give p-cinnamyl alcohols (1-3) from end groups and new phenol groups in lignin. The latter are further cleavaged in reaction sequences of the “peeling” type to lead the extensivedepolymerization of lignin.p-Hydroxycinnamylalcohols (1-3) aredehydrated to extended quinonemethides (E), which are condensed with the phenols by ionic reactions. The resulting resonance-stabilized phenolated compound are oxidized with the phenol radicals and/or lignin radicals to the corresponding radicals, followed by radical coupling. Interestingly, it may be noted that no syringaresinol (G)was detected among the solvolysis products of oak wood in spite of the inactive compound toward the solvolysis reaction [ 147,1481. Lin et al. [ 1501 recently reported that is subjected to homolysis under conditions for the liquefaction of wood with phenol at elevated temperatures. Thus, homolytic cleavages of phenolic P-arylethers in wood lignin can occur at elevated temperatures in many technical processes, for example, high-yield pulping and steam hydrolysis [ 1491, and in thedegradationproceduresfor structural studies under

(m)

(m)

152 San0

and

Sakakibara

coupling Radical coupling Radical Products

Products

W

FIGURE 33 Homolytic degradation of (3) by “solvolysis” using p-creso1:water:acetic acid (9: 1:O.l) at 180°C for 2 h. (From Ref. 147.)

neutral conditions at pH 2-9 and elevated temperatures, for example, mild hydrolysis and hydrogenolysis to yield p-hydroxycinnamyl alcohols and their radicals, and end phenoxy1 radicals in lignin, followed by formation of condensed polymers and chromophores, which influence the brightness and brightness stability of wood and high-yield pulps, and organosolv pulpings. The mild catalytic hydrogenolysis of protolignins has been studied extensively to obtain structural information. Sakakibara et al. [99-1171 subjected protolignins to catalytic hydrogenolysis in 60-90% dioxane containing a catalyst, copper chromium oxide,at 220240°C for 1 h. The hydrogenolysis of protolignins appears to proceed as follows: protolignins are depolymerized and solubilized in aqueous dioxane at 220-240°C higher than those for “mild hydrolysis,” then subjected to catalytic hydrogenation and hydrogenolysis to form “stabilized” hydrogenolysis products. The reaction mechanism for the solubilization and hydrogenolysis of protolignin needs clarification in details to apply the catalytic hydrogenolysis as a meaningful tool in the elucidation of lignin structures. On treatment of 1,l-diphenyl-2-picrylhydrazinewith aqueous dioxane at 180°C for 30 min, the hydrazyl radical is formed in large amounts, but not at below 140°C [ 1381, indicating that phenolic hydroxyl groups may be converted to phenoxy radicals at elevated temperatures.

153

Chemistry of Lignin

OH

-

CH II

OMe

’

O O C H 3 0

FIGURE 33 Continued

D. Acidolysis, Thioacetolysis, and Thioacidolysis

1. Acidolysis with 90% Aqueous Dioxane Containing HCl Since it had been found that refluxing of wood with 90% aqueous dioxane containing 0.2 M HCI, results in the formation of an ether-soluble oil in addition to a high-molecular lignin product, this treatment, “acidolysis,” was subsequently applied both to model compounds and to lignin preparations instead of ethanlysis. Upon 4 h acidolysis of guaiacylglycerol-P-guaiacyl ether (g), the P-ether linkage was cleaved, guaiacol being released, and furthermore, m-hydroxyl-guaiacylacetone (31) could be isolated in a yield of 53%. The latter was slowly further converted, yielding t h e isomeric ketols (3 1,32, I5 1 total yield: 15%), as well as small amounts of ketones in addition to only 3 S % of unchanged starting material as shown in Fig. 20 [IS l]. The acidolytic cleavage of the P-ether linkage in (128)is assumed to proceed via a benzylium ion and an enol ether (E), which is susceptible to acid hydrolysis, followed by formation of monolignols (Hibbert’s ketones 3 1 -34,15 1 ) with carbonyl groups. When spruce Bjokman lignin (MWL) was subjected to the acidolysis treatment under the same conditions, the low-molecular portion of the resulting mixture could be resolved by gel filtration into fractions containing monomeric. dimeric, and oligomeric compounds, respectively. As shown in Fig. 34, in the monolneric fraction the same ketones a s those

(e)

(2,s)

Sakakibara and Sano

154

HCO

l

y

2

H

+

+C,

um

Q O M e

0

I

OH

OH W

1

CH20H I

CH3 I

CH3

I

CH3

CH3

F="

c=o

6. 6 6 6 6 c=o

c=o l

-

1

I

I

I

H OH

OMe OH

OMe

' OMe

OH 0

U

m

0

CH20H

HCO

HS;0

c=o

HC

HC

I

I

66 I

1l

OH

OH

uiz,

0

W

II

OH 0

FIGURE 34 Monomeric products obtained by acidolysis of spruce MWL. (From Ref. 124.)

formed from model compounds (128) were detected, the predominating ketol (31)being obtained in yields of S-6% of the lignin. In addition, the presence of small amounts of homovanillin and formaldehyde (E) was demonstrated. The side reaction can be regarded as areverse Prins reaction of the benzyliumion intermediate. Theseresults constitute clear evidence of the substructures of the guaiacylglycerol-P-aryl ether type. The monomer fraction contained small amounts of ketol coniferyl aldehyde Q), and p-coumaraldehyde (E). In a similar monomeric fraction obtained from the acidolysis of birch MWL, a number of syringyl analogs were detected in addition to most of the compounds shown in Fig. 34 [ 1521. The yields of the syringyl monomers were higher than those of the guaiacylmonomers, although the ratio of syringyVguaiacyl is about 1 : 1 in birch. This is due to the fact that some of the guaiacyl units are linked to an adjacent unit by S - S , p-S, and S-0-4 bonds. which cannot occur in syringyl units. From the dimeric fraction obtainedfromspruce lignin, compounds (45,154- 160) were isolated and identified as summarized in Fig. 35. With the exception of trace constituents ( l S5,160), the dimeric fraction of birch lignin gave the same guaiacyl compounds,

(x)

(m),

"

CHpOH

CHpOH I c=0

I

c=o I

?H

?"

I

qH2

H3C,

+HC OM II.

Ff +OMe

6 GOMe

7-0

OH OMe

QOMe OH

OH

OH

ui8)

OH

(m)

HC-CH3

I

OMe

(rn)

0

'CH2

'

HpC' HCI

Me0

w

UIZ)

AH I

0 OH

Q O M e OH

(m

HC-YH I HpC-, /CH O I

(41,

Me0Q O M e OH

FIGURE 35 Dimeric products obtained by acidolysis of spruce MWL. (From Ref. 124.)

OH u.33

156

Sakakibara and San0

the corresponding syringyl analogs with one or two syringyl nuclei, and, furthermore, the stereoisomeric compounds D,L-syringaresinol ( S ) and D,L-epi-syringaresinol (E). The phenylcoumarone (154)and the stilbene (155)originate from a lignin substructure as shown in Fig. 36. The phenylcoumarone (%), which is formed in much higher yield than the latter, has a characteristic and very strong UV absorption 11531. This permitted its quantitative estimation, which indicated that about 10% of the C, units in the spruce lignin are connected to anadjacent unit byan a-0-4 aswellas a p-5 linkage, giving rise to a phenylcoumaran system. The acidolytic conversion of a hydroxylmethylsubstituted phenykoumdraninto a methyl-substitutedphenylcoumarone is readily explained by a sequence of ring opening, allylic rearrangement, and recyclization. The dimeric compounds (156-159)all exhibit only one side chain per two guaiacyl residues. It was postulated that these compounds could arise from a 1,2-diguaiacyl-1,3propanediol substructure (E)incorporated into lignin by acid-hydrolyzable linkages (Fig. 37). A plausible mechanism for their formation has been presented [ 1541. Compound (E) and related compoundscarryingone or twosyringyl nuclei were isolated from“mild hydrolysis” products of softwood and hardwood, respectively, [89,129,133,134], and (3) was also detected among the low-molecular productsof coniferyl alcohol dehydrogenation. The biogenesis of the lignin substructure (E)can be visualized as shown in Fig. 21. A p-l coupling between a coniferyl alcohol radical and the radical of a p-hydroxybenzyl alcohol end group forms (E)and a glyceraldehyde-2-aryl ether group Experimental evidence for the presence of glyceraldehyde end groups of type ( 4 0 ) was provided by the detection of pyruvaldehyde (150,methylglyoxal) in the acidolysis mixtures from spruce and birch MWL. The mechanism of the acidolytic formation of pyruvaldehyde is presented in Fig. 38. Colorimetric determination of the aldehyde formed on acidolysis of spruce and birch MWL, as well as of model compounds (g), indicated that only about 2% of the C, units of lignin were bound to glyceraldehyde as shown by formula (S). The results of ‘H-NMR studies on the aldehyde groups present in spruce MWL point to a similar value for the amount of glyceraldehyde groups. This figure, however, appears very low in view of the fair yields of degradation products of the p-1 type obtained by thioacetolysis of

(c).

s? r 6

2. a

a

L

Q O M e OH

w

OH (1121

acidolysis

Y O M e OH

w

(From Ref. 154.) 1,3-diolunits (2). FIGURE 37 Acidolysis of 1,2-diarylpropane-

m

I

Sakakibara andSan0

158

c-c-c

FIGURE 38 Acidolysis of glyceraldehyde-2-aryl ether units

(S).(From Ref. 90.)

beech wood. It has been proposed by Sarkanen that the unconjugated carbonyl groups in spruceMWL,whichamount to about 10 carbonylgroupsper 1 0 0 C, units, mightbe regarded as being present in glyceraldehyde groups. This proposal, however, does notfind support in the analytical results mentioned above. In Fig. 39, the substructures which have been disclosed by the acidolysis procedure are summarized. Of these structures, the arylglycerol-P-aryl ether structure undoubtedly is the most abundant one. As already mentioned, C, units which are linked to an adjacent unit by forming a phenylcoumaran system occur in spruce lignin in an amount of about 10%.D,L-syringaresinol ( g )and its stereoisomer D,L-episyringaresinol have also been found among the acidolysis products from birch. The corresponding guaiacyl compounds (S), however, could not be found in spruce or birch acidolysis mixtures. If pinoresinol (44) structures are present in lignin in

(m)

t; C,

C

c:

0’

C

I G O M e H2COH HC-0

MeoQoMe H,C*O-?H

Chemistry of Lignin

159

appreciable amounts, one must assume that they are linked to adjacent units by acid-stable bonds, i,e, 5-5 and 5-0-4 bonds, to an unexpectedly great extent. The "NMR spectrum of spruce lignin also indicates a very low content of pinoresinol structures. However, experimental evidence in favor of the occurrence of such structures in conifer lignin has been presented. A further acidolysis product exhibiting coupling, namely, D,L-divanillyltetrahydrofuran (E), was isolated in small amounts from spruce lignin acidolysis. The compound differs from the other dimeric acidolysis products in possessing a lower degree of oxidation. Degradation of beech wood by thioacidolysis afforded the syringyl analog. Its formation seems to involve an oxido-reduction process, but it remains open what substructures (@) in lignins originated from [154].

(e)

2. Thioacetolysis and Thioacidolysis Thioacidolysis causes cleavage of P-0-4 bonds, and brings a more deep-ground fragmentation of the lignin than acidolysis [95,155]. As much as 91% of the lignin of beech wood and 77% of the lignin of spruce wood were degraded to mixtures of monomeric to tetrameric products. The principle of the three-step degradation method has been formulated by Nimz as shown in Fig. 40. Treatment of wood with thioacetic acid and boron trifluoride converts the arylglycerol-P-aryl ether unit (40.1) via the benzylium ion ( 4 0 . 2 ) into the Sbenzyl thioacetate (=). Subsequent saponification with 2 N NaOH at 60°C gives a benzyl thiolate ion (40.4) which loses the P-aryloxy group by nucleophilic attack of the neighboring thiolate ion on the P-carbon atom to give an episulfide The latter dimerizes to dithianes or polymerizes to thioethers. In a final step, treatment with Raney nickel and alkali at 115°C removes the sulfur and yields the reduced phenolic reaction products. The 20 dimers obtained from beech wood are shown in Fig. 41. Most of the bond typesexhibited by these dimers are identical withthoserevealed by otherdegradation

(a).

cn,on l

I

I HCOAr 13 F,

R'

CH20COCHs

CHPOH

HCOAr Cl I$X)SI I

NaOl I

OMe

R'

OR (40.2)

OR (4.3)

CHZOH

CH2R

l

I

HqSo

R'

OMe OH

OH R=H or OMe K'=llorOlI (40.7)

FIGURE 40 Degradation of lignin by thioacetolysis with thioacetic acid.

(From Ref. 95.)

160

Sakakibara and San0

&Q'16 OH

R

OMe OH

H $ oOH O M e

R OMe / OMe OH

R=H or OMe

OH R=H or OMe

CH3

CH3

HC

I

Me0

OMeMeO OMe OH OH

OH

R

OMe Me0

OMe

OH

\

R=H or OMe , R'=H2or =O

/

u6i, (0.5%)

CH3

I

CH3

I

C

Me0

OMe

R

OHOH R=H R=OMe or OH

c166)(0.45%)

FIGURE41 95.)

R=OCH3.

R=OMe (142) (0.4%)

OH

OH R=H or OH

(0.1%)

0 (0.3%)

Dilignols obtained from beech protolignin by thioacetolysis (% of lignin). (From Ref.

Chemistry of Lignin

161

methods, especially acidolysis and oxidative degradation cited above. The a-p-junction (165) found in some compounds (0.5%) was assumed by Nimz and Das [95] to be present i n e e c h lignin, although it cannot be the result of a dehydrogenation. Its formation and that of some related structures is assumed to be due to a proton-catalyzed polymerization of coniferyl alcohol (1)or coniferyl alcohol end groups caused by the natural acidity of the cell sap. However, we must also point to the possibility mentioned by Nimz [ 1801 that (1)andsinapylalcohol (2) may arise during the degradation process, analogous to the formation of (L) as an intermediate in kraft cooking of spruce lignin. The heating with alkali in the last degradation step may cause Michael-type additions of the p-C atom of coniferyl alcohol to the a-position of quinonemethide structures whichalsocanbe assumed to be intermediates. The a-p-linked products may accordingly be artifacts. On the basis of the yields of crude and pure degradation products, Nimz has calculated the frequencies of the various bond types in beech lignin and has also proposed a structural scheme for this lignin (Fig. 46). Recently,a new acid degradationmethod, thioacidolysis (solvolysis in dioxaneethanethiol with boron trifluoride etherate) has been studied by means of a reproducible and mild routine procedure to obtain detailed structural information about lignin [ 1561591. The acid degradation of lignin was composed of two consecutive thioacidolysis and desulfuration of thioacidolysis products over Raney nickel as illustrated in Fig. 42, which is similar in principle to that proposed for thioacetolysis (Fig. 40). However, thioacidolysis is carried out using a few milligrams of sample in dioxane at 100°C for 4 h instead of thioacetolysis performed at 20°C for 1 week. Reaction conditions for the former seem to be more advantageous as a tool for structural studies of lignin than those for the latter. The thioethylated monolignols and the desulfurated dilignols of spruce MWL and wood are shown in Fig. 38 [1591. Among the thioethylated monomers from spruce MWL, the compounds (170,171), whichwereformedfromuncondensed p - 0 - 4 linked units, were obtained in a total yield of 9 3 % of the monolignols. They reflect the higher content of uncondensed p - 0 - 4 linkages in the MWL products and from coniferyl alcohol end groups were obtained in a total yield of 6.6% based on the monomers. The compounds and orginating from coniferyl aldehyde end groups and from dihydroconiferyl alcoholendgroups, respectively, weredetected in trace amounts.The yields of main dilignols obtained from spruce MWL and wood are shown in Fig. 43 and characterize the various types of condensed linkages in softwood lignin. From their yields, it can be concluded that p-S, 5-5, and p-1 linkages are present as major types of condensed interunit linkages in softwood lignin. In addition, the dimerswithdiphenylether (181)(4-0-S), phenylisocoumaran (p-5), and tetrahydrofuran (M)(p-p) structures, of which the structures were assigned only from their fragmentation patterns, were detected in trace amounts, and their linkages seem to be minor types among condensed linkages. However, the latter two dilignols have been assigned to be compounds with different structures in two papers reported by the same authors [ 159.1, indicating that they need to be isolated and identified. The total area of GC peaks due to the dilignols assigned accounts for more than 90% of all the peaks corresponding to dilignols in the chromatograms of spruce MWL and spruce protolignin. The total amount of the dilignols is about 30 mole% of the main thioacidolysis monomers obtained from both of the lignins. The relative importance of condensed interunit linkagesimplied by the mole ratio of the thioacidolysis dilignols is almostequal between spruce MWL and sprucewood lignin in situ. The yields of compounds which were characterized as monolignolsanddilignols among the thioacidolysis mixture were only 40-S0% of lignin [ 1593, which may reflect the limitation of thioacidolysis results to the degradable part of lignin.

(m)

(m) (m)

(m) (m),

162 San0

and

Sakakibara

H2COH HCSEt

I l

H2yH HCORp

HCOR2 HCOR2

I

@,R1

F 6

BF3 E(SH

_ I )

OMe

OR

H2COH

OMe9F3

H2COH

OR

I

I

COR2

H27OH

OH

HpCOH

-

I

I

6z; 6E":;61 OR

It

OH

!$p

I I

'OMe OR

H

OR

OR

OR

OH

OH

FIGURE 42 Reaction mechanism of p-0-4 substructure units by thioacidolysis and subsequently desulfuration with Raney nickel. (From Ref. 156.)

In order to use the interesting thioacidolysis method as a routine procedure for the characterization of the total structure of lignins, it is necessary that the thioacidolysis productscontaining trilignols to oligolignols be clearly characterizedandacid-derived condensation by thioacidolysis clarified.

V.

STRUCTURAL MODELS FOR LIGNINS

A.

Frequencies of Functional Groups and Typical Linkage Types in Lignins

Summarized in Fig. 44 are the main types of lignin structural units which were obtained from the various lignin degradation products described above. Frequencies of the functional groups and the typical linkage units in spruce and birchMWL, and beech protolignin are collected in Tables 10 and 11, respectively. The most important linkage types in the lignin molecule are p-0-4(B) and then p-5 (g),5-5 (E), p-1 (C), and a-0-4 (A). The pp linked units are represented by pinoresinol (g), syringaresinol (g), and dibenzyltetrahydrofurans F(c) (160 and Pinoresinol substructure (@-p)may be rather minor in softwood lignin, but syringaresinol (45) - is abundant in hardwood lignin. Though the com-

x).

Me

dsEt do"

Chemistry of Lignin

&Et

OMe

Et$

SEt Et$EtS

163

SEt

OMe

OH OH

OH

0

0

OH OH

OH

uze,

0

uz2,

W

R

Me0

OMe OH

5-5 series

p1 series

4-0-3 series

Meor i

H0

Me0

H O H z C e Me

R2

\

p5OH \ / series OMe OH

OMe HOH2CfMe p3OH \ senes

p5 series

CH3 R=H.

p5 senes

or CH20H

u8L,

OMe

OH

p p series

Rl=CH3 or CH20H R2=H 01 CH3

u&+M=33%)

FIGURE 43 ThioethylatedmonomersandRaneynickel-desulfurateddimersobtainedbythioacidolysis of spruce MWL (% of dimers). (From Ref. 158.)

(M)

pound was obtained as one of the important substructures next to A, B, C, D, and E typesfromacidolysisand thioacidolysis products of spruce MWL, it remainsopen whether is a substructure in lignin or acid-condensation products of (101:R=OH). P-Aryloxyglyceraldehyde units B(a) have been estimated by acidolysis and 'H-NMR. 40-5-Diphenyl ether and 0-6 units have been confirmed by not only oxidation but also catalytic hydrogenolysis. By the latter, frequencies of bond types has not been estimated. The yields of oxidation products were used in the estimation of the bond types in the MWLs, though certain assumptions, e.g., regarding the actual and theoretical yields of oxidation products, are involved [ 1611. Naturally, the frequency values in Tables 10 and

(m)

164

Sakakibara and San0

F

8-Q

E - 0 - 0 I

6" 6

6

C

A

B

"8 F'

go

F F

C-

FC

l

I

F

E

F

I

?

C I

D

F C?-

C

FC

F

cI

$

I

C

G

H

0 G(traces)

F F

FC

6-6 I

I

FIGURE 44 Typical linkageunitsin

lignin.

11 are to be regarded as approximate rather than accurate, although mostofthem are fairly reproducible. Some uncertainty is attached to the values (0.02-0.15) given for bond type C in spruce MWL in spite of differing estimation methods. Sarkanen [ 1721 has stated that p-1 units are main substructures in endwise lignin for the middlelamellaregion rather than in bulk lignin for the cell wall. However, Lapierre et al. [l581 have reported that p-

Chemistry of Lignin

165

TABLE 10 Functional Group and Structural Unit of Spruce

MWL

Functional Groups units

Functional groups and structural Aliphatic OH 168,701 Phenolic OH [82,701 Total carbonyl

0.93 0.33

c,,=o

Unconjugated C=O Ar-CH=CH=CHO Ar-CH=CH-CH20H Phenolic C,-OH Nonphenolic C,-OH

C&

References

1.09, 0.26, 0.20 0.06-0.07 0.10 0.03-0.04

1591 [S91 1591 1411

0.03

1441

0.05-0.06 0.15, 0.10

176.781 [59,781

~~

A: a-0-4 (open)

Phenolic Nonphenolic B: p - 0 - 4

c: p-5 Noncyclic D: p-l E: 5-5, 5-6 F: p-B

Pinoresinol units G: 4-0-5, 4-0-1

0.12, 0.07, 0.06-0.08 0.04, 0.02 0.05-0.09, 0.06 0.49-0.5 1, 0.50 (0.25-0.30, 0.3-0.5)'' 0.02b 0.14, 0.9-0.12 0.03 0.15, 0.02, 0.07 0.19-0.22, 0.10-0.1 1 0.13 0.05- 1 .O, 0.02-0.03 0.07-0.08, traces

"Except displaced side-chain unlts. "Arylglyceraldehyde-P-arylether. Source: Ref. 167.

1 units are almost as frequent as in spruce in-situ lignin and basis of the results obtained by thioacidolysis.

MWL preparations on the

B. Structural Models for Softwood Freudenberg [ 1631 attempted to constructa structural formula for softwood lignin, utilizing theknowledgeobtainedfromtheenzymaticdehydrogenation of coniferylalcohol. The formula, which was composed of 18 units, was later modified several times 1164- 1661. Adler [ 1671 has collected theprominent substructures of spruce ligninin a structural model comprising 16 C& units. Glasser [ 1681 has proposed a structuralmodelbasedon 81 phenylpropane units that was constructed by computer simulation. A structural model of softwood lignin consisting of 28 units was proposed by Sakakibara [ 1691 as shown in Fig. 45. Alternative units are indicated in brackets against letter (b). The linkage patterns between thephenylpropaneunitsarebased mainly on results obtained by hydrolysis and hydrogenolysis. The formula of the models for spruce lignin are calculated as C,H,,,~,O2,,,(OCH,),,,,, which agrees well with those of spruce MWL shown in Table 4. The formula is a tentative one and is constructed only from information obtained so far, omittingunits,theexistence of which isat present uncertain.Onlypart of theactual

and Sano

166 TABLE 11

StructuralUnits(per

1 0 0 C&, units) of Birch MWL and Beech Lignin

Beech [95]

Birch [ 1671 Units A: a-0-4 (open) B: p - 0 - 4 34-39

Total

G

S

6 60 2 7

22-28

p-0-4"

c: p-l D: p-5 E: 5-5 F: P-p

Total

6 4.5

2.3

4.5 3

P-B p-P and L Y - ~ ~ G: 4-0-5, 4-0-1 H: C(a)-2, C(a)-6 I: a-P

1 1-1.5

5.5 0.5- 1

6.5 1 .5-2.5

65b

15 6

5 2 0.5 1.5

2.5

"In glyceraldehyde-P-aryl ether. hA + B. 'In dibenzyltetrahydrofuran units. 'In tetralin units.

number of these units has been arbitrarily selected because of the lack of adequate quantitative data.

C. Structural Models for Hardwood Lignin Nimz [l701 proposed a constitutional scheme for beech lignin on the basis of the results from mild hydrolysis and thioacetic acid degradation of beech lignin (Fig. 46). This structural model consists of 25 phenylpropane units containing 14 guaiacyl, 10 syringyl, and one p-hydroxyphenyl moiety, of which six units can to some extent be replaced by the dilignol units enclosed in the brackets. The models give a representative section from a beech lignin molecule 10-20 times larger, in which the 10 different bond types are randomly distributed. Glyceraldehyde-2-aryl ether units B (a) are not detected on degradation of beech lignin, but their presence is evidenced as a counterpart by the occurrence of p1 dilignol units (C) as shown in Fig. 21. The formula of this structural model is calculated as C,H,,,,O,,,(OCH,),,,,, which is close to the formula shown in Fig. 4. Furthermore, the I3CNMR spectrum calculated for the proposed structure was compared with that observed for beech lignin.

D. Heterogeneity of Protolignin These structural modelsareonlyaverage pictures for lignin structures that may have different chemical configurations in the different morphological regions of the cell wall. Fergus and Goring [ 17 I ] indicated varying amounts ofsyringyl- and guaiacylpropaneunits in the various cell wall layers and middle lamella of birch wood by means of UV spectrophotometry. Matsukura et al. [ 1721 showed by oxidation and alcoholysis of spruce wood that the lignin polymer has a heterogeneous structure. Furthermore, it has been pointed out that lignin in the middle lamella region may possess more of the nature of an endwise

167

Chemistry of Lignin

H$?-0“

bH W O M e

FHOH FHOH CH20HI

FIGURE 45 A structure model for softwood lignin.

CH20H

168

Sakakibara and San0

7HpOH HV-

-0

OMe Me0 MeO MeO

MeO

OMe

OMe

Me0 OMe Me0 MeO

OMe -0

FIGURE 46

0-

-0

OH

A structure model for beechwood lignin. (From Ref. 170.)

polymer than that permeating the polysaccharide matrix in the S2 layer [ 1731. Compression wood contains not only more lignin but also more condensed-type units than normal wood lignin [21]. The reason may be explained by the fact that the outer S, layer of compression wood is highly lignified, whereas the middle lamella is not completely lignified [ 1741. The controversysurroundinglignin-carbohydratecomplexes(LCC)hasbeendebated for a long time. In spite of numerous studies, the question of whether the association between lignin andcarbohydrates is physicalorchemical in naturehas not yetbeen resolved. However, the evidence for chemicalbondsbetweenthemhasbeengrowing gradually. For instance, enzymatic hydrolysis of LCC preparations gives concentrated LCC

Chemistry of Lignin

169

fractions that indicate the existence of covalent bonds between lignin and carbohydrates, because enzymes do not cleave lignin-carbohydrate linkages [ 175- 1801. The structure of protolignin must take into consideration the presence of LCC linkages. Some typical forms of LCC linkages have been suggestedby Freudenberg et al., who isolated phenylpropanecane sugar compounds from the mixture arising from the simultaneous enzymatic dehydrogenation of coniferyl alcohol and cane sugar [ 181, l 821. Finally, a word should be mentioned about Brauns lignin (BL), which Brauns [ 1831 first isolated from black spruce by extraction with ethanol in 1939. Since then, BL has been consideredto be natural lignin and has been used for several basic studies of lignin. However, it is not clear whether BL is a true lignin or not, as Freudenberg [ 1841 pointed out that BL may be a fraction of resinous material. Recently, various new lignans have been isolated [ 1601. They consist of dimeric, trimeric, and tetrameric phenylpropanes that are very similar to the lignols from degradation product mixtures, as already mentioned, except for optical activity and some other details. The existence of monomeric, oligomeric, and polymeric phenylpropanes in the lignan fraction suggests that these constitute a continuous spectrum of lignans. In conclusion, BL can be considered a polymeric fraction of lignans and not a true lignin.

VI.

OUTLOOK

The concept of lignin as dehydrogenation polymer of p-hydroxycinnamylalcohols is now well established. The efforts to clarity the structures of the different types of lignin have resulted in a detailed picture of the various modes in which the C& units are linked together in the lignin polymer. Whereas there is good agreement regarding the frequency of the predominant type of linkage, that is, arylglycerol-p-aryl ether substructures, there is even now some uncertainty regarding the proportions of some linkage units, such as noncyclic cy-aryl ether (A), p-! (C), @-p(E), andothersoccurring in minoramounts. “Mild hydrolysis” and “catalytic hydrogenolysis,” which has been used as the conventional degradation methods to characterize the structures of protolignin, appear to lead to erroneous and confusing results by homolytic cleavage of phenolic @-aryl ether linkages and subsequent secondaryradical couplings, so the elucidation of their reaction mechanism of protolignin will be required. Acid-catalyzed degradation, such as acidolysis and thioacidolysis, which give rise to self-condensation of lignin, will deviate to a certain degree froma useful tool toanalyze total linkages of phenylpropane units in lignin. We will require much continued effort to analyze total linkage units in at least MWL and also unchanged lignin in wood. Lignin, which is the most abundant natural polymer next to cellulose, is produced toabout 80 milliontonsandburnedtorecoverkraft-pulpingchemicalsandtomake energies for pulping and papermaking. Regenerated wood biomass will have to be applied to the saving andor substitution of petroleum oil for both energy and chemicals in the future for the sustainable development of the world without environmental pollution. Although wood cellulose is the most predominant pulp material, it will be utilized together with other polysaccharides and old paper as raw materials for about 95% of petrochemicals. The polysaccharides may be converted easily to various chemicals, but the immense problem of finding industrial applications for lignin remains a great challenge to wood chemists. Continued efforts will be required to finish the conversion of wood biomass, that is, wood biomass is separated into pulp, hemicellulosic sugars, lignin, and extractives

170 San0

by a novel process with less amounts wood chemicals of high value.

and

of energy and environmental

Sakakibara

pollution to use as

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B. 0. Lindgren and H. Mikawa, Acta Chem.Scand., 11:826-835 (1957). T. Higuchi, Y. Ito, and I. Kawamura, Phytochemistry, 6:875-881 (1967). H.A. Stafford, PlantPhysiol., 37643-649 (1962). Y. San0 and A. Sakakibara, Mokuzai Gakkaishi, 16:81-86 (1970). S. Omori and A. Sakakibara, MokuzaiGakkaishi, 20:388-395 (1974). H.Nimz, Chern. Ber., 98:3153-3159 (1965). A.Yamaguchi, MokuzaiGakkaishi, 19:185-193 (1973). A. Sakakibara, H. Takeyama, and N. Morohoshi, Holdorsch., 20:45-47 (1966). T. Higuchi, H. Nakatsubo, and Y. Ikeda, Holdorsch., 28: 189-192 (1974). E. Adler and S. Yllner, Acta Chem. Scand., 7:570 (1953). K. Lundquist and R. Lungren, Acta Chem. Scand., 26:2005-2023(1972). M. Aoyama and A. Sakakibara, Mokuzai Gakkaishi, 24:422-423 (1978). T. Popoff and 0. Theander, Phytochemistry, 142065-2066 (1975). J. Gierer and S. S6derberg. Acta Chem. Scand., 13:127-137 (1959). J. Marton, E. Adler, and K. I. Person, Acta Chem. Scand., 15:384-392 (1961). E. Adler and J. Marton, Acta Chem. Scand., 13:75-96 (1959). Y. Hachihama, K. Nira, and T. Kyogoku, Kogyo Kagaku Zasshi, 47209-215 (1944). H. Mikawa, K. Sato,C.Takasaki,and K. Ebisawa, Bull.Chem. Soc. Japan, 28:653-660 ( I 955). K. Freudenberg and K. Dall, Naturwiss., 22:606-607 (1955). K.V. Sarkanen and C. Schuerch, Anal. Chem., 2 7 1245-1250 (1955). K. Freudenberg,, in Fortschritte d. Chem. org. Naturstoffe, XI, Zechrneister, Springer-Verlag, Berlin-Heidelberg, pp. 43-82 (1954). E. Adler and S. Hernestam, Acta Chem. Scand., 19:319-334 (1955). A. Bjorkmanand B. Person, Svensk Papperstidn., 60:285-292 (1957). A. Okay, Hul~ursch.,24:172-175 (1970). D. R. Robert and G. Brunow, Holdorsch., 3835-90 (1984). H.-M. Change, E. B. Cowling, and W. Brown, Holdorsch., 29:153-159 (1975). P. Miinsson, Hol$orsch., 37143-146 (1983). D. S. Argyropoulos, J. WoodChem.Technol., 14:65-82 (1994). 0. Faix, C. Grunwiind, and 0. Beinhoff, Hol$orsch., 46:425-432 (1992). 0. Faix, D. S. Argyropoulos, R. Danielle, and N. Vincent, Holdorsch., 48:387-394 (1994). E. Adler and K. Lundquist, Acta Chem. Scand., 15:223-224 (1961). K. Freudenberg, C.-L. Chen, J. M. Harkin, H. Nimz, and H. Renner, Chem. Commun., 224 ( 1965). M. Aoyama and A. Sakakibara, Mokuzai Gakkaishi, 22591-592 (1976). E. Adler, S. Delin, and K. Lundquist, Acta Chem. Scand., 13:2149-2150 (1959). E. Adler and J. Gierer, Acta Chem. Scand., 9534-93 (1955). J. Gierer, Acta Chern. Scand., 8:1319-1331 (1954). J.Gierer, Chem.Ber., 80257-262 (1956). E. Adler, H.-D. Becker, T. Ishihara, and A. Stamvik, Holdorsch., 20:3-11 (1966). H.-D. Becker and E. Adler, Acta Chem. Scand., 15:218-219 (1961). T. Higuchi, M. Tanahashi, and F. Nakatsubo, Wood Res. (Kyoto), 54:9-18 (1972). K. Freudenberg, J. M. Harkin, and H. K. Werner, Chem. Ber, 97:909-920 (1964). E. Adler, G. E. Miksche, and B. Johanson, Holtforsch., 22:171-174 (1968). E. Adler, J. M. Pepper, and E. Eriksoo, Ind. Eng. Chem, 49: 1391-1392 (1957). S. Larsson and G. E. Miksche, Acta Chem. Scand., 25:647-662 (1971). M. Erickson, S. Larsson, and G. E. Miksche, Acta Chem. Scand., 27:903-914 (1973). H.Nimz, Chem. Ber, 98:3160-3164 (1965). K.Lundquist, G. E. Miksche, and I. Berndtson, Tetrahedron. Lett., 46:4587-4591 (1967). G. Aulin-Erdtman and L. Haghom, Svensk Papperstidn., 61:187-210 (1958). C.-L. Chen,InauguralDissertation,UniversitatHeidelberg,Heidelberg,Germany, pp. 1-75 ( 1962).

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102. 103.

104. 105.

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K. FreudenbergandC.-LChen, Chem. Ber., 100:3683-3688(1967). S. Larsson and G. E. Miksche, Acru Chem Scund., 25:673-679 (1971). H. Nimzand K. Das, Chem. Bes, 1042359-2380(1971). S. Yasuda and A. Sakakibara, MokuzaiGukknishi, 23:383-387 (1977). K. Freudenbergand D. Rasenak, Clzem. Res, 86:755-758 (1953). H. Nimz and H. Gaber, Chem. Bes. 98:538-539(1965). S. Omori and A. Sakakibara, MokuzaiGakkuishi, 17464-467 (1971). S. Omori and A. Sakakibara, Mokuzni Gakkaishi, 21:170-176 (1975). K. Ogiyama and T. Kondo, Mokuzai Gnkknishi. f4:416-420 ( 1968). K. Sudoand A. Sakakibara, MokuzaiGakknishi, 20:396-401(1974). S. Yasuda and A. Sakakibara, MokuzniGukkaishi, 22:606-612(1976). K. Freudenberg, Z. Angew. Chem., 52362-366(1939). B. Leopold, Acrn Chetn. Scnnd., 6:38-48 (1952). B. Leopold and J. L. Malmstrom, Actcl Chem. Scund., S:936-940 (1951). J. C.Pew, J . Am. Chern. Soc.. 77:2831-2833(1955). D. L. Brink, Y. T. Wu, H. P. Naveau, J. G. Bicho, and M. M. Merriman, nippi, 55:719-721

( 1972). 109. H. P. Naveau, Y. T. Wu, D. L.Brink, M.M. Merriman,and J. G. Bicho, 72ppi. SS:13561361 (1972). 110. S. Larsson and G. E. Miksche,Acta Chern. Scand., 2/:1970-1971 (1967). I 1 1. M. Eriksson, S. Larsson, and G. E. Miksche, Actu Chem. Scancl., 2 7 127-140 (1973). 112. S. Larsson and G. E. Miksche, Actu Chern. Scand., 233337-3351(1969). 113. S. Larsson and G. E. Miksche, Actu Chem. Scatzd., 26:20-31 (1972). 114. S. Yasuda and A. Sakakibara, Mokuzui Gakkuishi, 23: 114-117 (1977). 1 15. M. Matsukura and A. Sakakibara, Mokuzni Gakknishi, 19:17 1-176 (1973). 116. K. Sudo. B. H. Hwang, and A. Sakakibara, MokuzaiGakkaishi. 24424-425 (1978). 117. B. H. Hwang, A. Sakakibara, and M. Miki, Hol7forsch., 35:229-232 (1981). 118. K. Miki, V. Renganathan, and M. H. Gold, Uiochenzistt-y, 25:4790-4796 (1986). 119. K. Freudenberg, in Rrnnin, Cellulose, Lignin, Springer-Verlag,Berlin, p. 133 (1933). 120. E. E. Brauns and D. A. Brauns, in The Chemistry of Lignin, Academic Press, New York, p. 626 ( 1960). 121. B. H. Hwang and A. Sakakibara, Hol CaClz > NaCI), acids, alkalis Chelating reagents, e.g., EGTA, CDTA, EDTA, oxalate, hexametaphosphate; low pH Organic solvents (Reagents that change molecular conformation?) Sugar hapten; denaturation

Abbr-evrattorls: Ara, arabinose; Fer, ferulate;Gal,galactose;Glc,glucose, Me, methyl;MeOH,methanol; NaOMe, sodium methoxlde; PS, polysaccharide; RG-I, rhamnogalacturonan-I; Rha, rhamnose; Tyr, tyrosine. Source: Modified from Ref. 28.

2. Diferuloyl Cross-Link The occurrence of ferulic and p-coumaric acids ester-linked to arabinoxylans in grasses [87], to pectic polysaccharides in spinach [39-411 and sugar beet [40], and to xyloglucan in bambooshoot [76] is well characterizedasdescribed in the previous section. The possibility of covalent linkages betweenesterified ferulic acid on wall polysaccharides was first proposed in 1971 by Geissman and Neukon [89]. The ferulic acid residues on feruloyl arabinoxylan from wheat flour have been cross-linked with peroxidase and hydrogen peroxidase to make a gel. This demonstrated that a dehydrogenative coupling between two esterified ferulic acid residues on arabinoxylan to form dehydrodiferulic acid had occurred (Fig. 27). Sugar beet pectin that contains ferulic acid esterified to arabinose and galactose residues [40] also make gel following peroxidase-catalyzed, oxidative cross-linking [90].

lshii and Shimizu

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The phenolic coupling has been invoked to explain termination of cell expansion. Small amounts of diferulate have been detected by alkaline hydrolysis of cell walls. The goal of detecting an oligosaccharide fragment, cross-linked by a diferuloyl bridge, was achieved in 1991 [91]. The linkage group was isolated and characterized from bamboo shoot arabinoxylan, providing definitive evidence for the existence of diferuloyl ester cross-link (Fig. 28). Ralph et al. [92] isolated a series of ferulic acid dehydrodimers in addition to 5-5 coupled dehydrodiferulate from saponified grass cell walls (Fig. 29). These dehydrodimers (8-5, 8-0-4, and 8-8) also are involved in cross-linking of polysaccharides in cell walls. Isolation ofwallfragmentscontainingthesedehydrodimers is animportant challenge. Other possibilities for dimerization of phenolic acid substituents of polysaccharides exist. A series of homo- and heterocyclodimers of the cyclobutane type, formed by headto-tail or head-to-head association of ester-linked p-coumaric acid and ferulic acids, were isolated [93] (Fig. 30). A similar peroxidase-catalyzed cross-link may occur between tyrosine residues of extensin (Fig. 31). The phenolic ether linkage of isotyrosine is known to form intramolecularly within extensin, and may also occur intermolecularly [28].

3. Ester Bonds Cold Na2C03,which hydrolyzes ester bonds but does not cause p elimination-degradation, solubilized pectin that is not solubilized by chelatingreagent or EPC [9]. Theseester bonds may be methyl esterified galacturonosyl residues, diferuloyl bridges, or ester bonds between uronic acids and neutral sugars. Although intermolecular ester bonds have been proposed [94], their identification has not yet been achieved.

4. Borate Diol-Diester Cross-Linkage Borate cross-links two RC-I1 molecules in pectic polysaccharide to form an RG-I1 dimer (Fig. 16). This cross-link is extremely acid-labile. Treatment of the borate-RG-I1 complex with 0.5 N HCl at room temperature for 30 min cleaved the borate ester linkage. The borate cross-linkage might play an important role in connecting pectin networks in the cell wall and may be involved in the acid-induced elongation of cells during growth.

"

O

H

c=C-C-OH 11

5 ) - a - ~ - A r a f -(1+3 ) -p-D-xylp- ( 1 4) -D-xylp

0

YCO

FIGURE 28 A diferuloyl arabinoxylan hexasaccharide isolated from bamboo shoot cell walls.

Chemistry of Cell Wall Polysaccharides

HOv

207

0

“ O v O OH

M S 0

FIGURE 29

B.

Structure of dehydrodimers of ferulates.

NoncovalentLinkages

As cell wall polysaccharides are polyhydroxylic, many hydrogen bonds form in the walls. Multiple hydrogen bondsare present within cellulose microfibrils. Hydrogen bondsare probably responsible for the incorporation of xylan, xyloglucan, and glucomannan into the cell wall. The hydrogen bonding between cellulose and xyloglucan, and between cellulose and xylan, has been demonstrated in vitro. Ionicbonds will be formed in the case of polymers that contain charged groups. Homogalacturonan and 4-O-methylglucuronoxylan have negative charges, while extensin has a positive charge. Ionic binding may occur between these charged polymers in the wall. Negatively charged galacturonic acid residues in homogalacturonan and RG-I can form cross-linking with Ca’+ to form an “egg-box” [95]. About 15-20 contiguousga-

lshii and Shimizu

208

OH

H0

FIGURE 30 Structure of cyclodimer of ferulic acid.

lacturonic acid residues are needed in each chain to make a stable complex. Methyl esterification in homogalacturonan and rhamnosyl residues in RG-I interrupt the concerted binding.Furthermore, acetylation occurs at 0 - 3 of galacturonicacidresidues in homogalacturonan [333 and at 0-2 and 0-3 of galacturonic acid residues of RG-I, respectively [35-371. Therefore,egg-boxcross-linkage is likely to be limited in the wall, although isolated pectin and pectin in jam and jellies are known to give rise to gel-like structures in vitro.

VIII.

CELL WALL MODEL OF GROWING PLANT CELL

The cell wall has a variety of components that assemble to form an extremely complicated structure. Several wall models have been proposed. An early cell wall model was proposed by Albersheim and co-workers [96]. This model contained covalent links between xyloglucan and RG-I and hydrogen bonds between cellulose and xyloglucan, leadingto indirect cross-linking of cellulose microfibrils through a series of hydrogen bonds and covalent bonds in the matrix. Some of the details of this model have been disproven by subsequent chemical analysis [ 181. Capita and Gibeaut [97] and McCann and Robert [98] have proposed structural models for primary cell walls basedon this new information. It is probably impossible to describe all cell wall components and their interactions with a single and simple model. However, the existence of two principal polysaccharide networks in the growing cell wall have been proposed: a tension-resistant load-bearing cellulose/xyloglucan network and a compression-resistant pectic polysaccharide network.

IX.

CONCLUDING REMARKS

Thischapter briefly summarizespresentknowledgeof the pectic polysaccharidesand hemicelluloses. Primary cell walls commonly contain cellulose, xyloglucan, arabinoxylan, homogalacturonan, RG-I, and RG-11. These six polysaccharides account for all or nearly all of the primary wall polysaccharides, and their primary structures have been well conserved among species. The six polysaccharides are to some extent cross-linkedby covalent and noncovalent bonds, making up a complicated macromolecular network in the primary

Chemistry of Cell Wall Polysaccharides

+

e e,

e, D

0

rc

209

lshii and Shimizu

210

walls. This chapter also discusses the functions of primary cell walls. The recent discovery that oligosaccharide fragments derived from cell wall polymers in growing tissues can act as potent and specific regulators of gene expression is of importance to the biochemistry of all living systems [see refs. 34,99,100].

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Chemistry of Cell Wall Polysaccharides

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122, 115(1983). 43. A. J . Whitcome, M. A. O’Neill, W. Steffan, P. Albersheim,and A. G. Darvill, Curbohyd,: Res., 271, 15 (1995). 44. V. Puvanesarajah, A. G. Darvill, and P. Albersheim, Curbohyd,: Res., 218, 21 1 (1991). 45. J. T. Thomas, A. G. Darvill, and P. Albersheim, Curbohydr. Res., 185, 261 (1989). 46. T. T. Stevenson, A. G. Darvill, and P. Albersheim, Curbohyd,: Res., 182, 207 (1988). J. Brasch, Curbohyd,: Res., 209, 191 (1991). 47. R. J . Regwell, L. D. Melton, and D. 48. T. Ishii, Mokuzui Gukkaishi, 41, 669 (1995). 49. M. Kobayashi, T. Matoh, and J. Azuma, Plant Physiol., 110, 1017 (1996). 50. T. Ishii and T. Matsunaga, Curbohydr. Res., 284, 1 (1 996). P. Pellerin, T. Doco, A. G. Darvill, and P. Alber51. M. A. O’Neill, D. Warrenfeltz, K. Kates, sheim, J. Biol. Chem., 271, 22923 (1996). Phytochemistry, 44, 243 (1997). 52. S. Kaneko, T. Ishii, and T. Matsunaga, 53. B. J. Shelp, in Boron and Its Role in Crop Production (U. C. Gupta, ed.), CRC Press, Boca Raton, FL, p. 58 ( 1992). 54. W. D. Loomos and R. W. Durst, Bio Factor, 3, 229 (1992). 55. H. 0. Bouveng, Acta Chem. Scand., 19, 953 (1965). G. J. Voragen, Carbohyd,: Res., 279, 265 56. H. A.Schols,E. J. Bakx,D.Schipper,andA. (1995). 57. I. Eriksson, R. Andersson, E. Westerlund, R. Andersson, and P. Aman, Curbohyd,: Res., 281, 161 (1996). G. Darvill,and P. Albersheim, Curbohydr: Res., 243, 373 58. P. Lerouge,M.A.O’Neill,A. ( 1 993). 59. M. McNeil, A. G. Aman, and P. Albersheim, Plant Physiol., 70, 1586 (1982). 60. J. M. Lau, M. McNeil, A. G. Darvill, and P. Albersheim, Curbohyd,: Res., 168, 245 (1988). 61. S. Eda, K. Miyabe, Y. Akiyama, A. Ohnishi, and K. Kato, Curbohyd,: Res., 158, 205 (1986). 62. A. M. Stephen, in The Polysaccharides, 2 (G. 0. Aspinal, ed.), Academic Press, New York, p. 97 (1980). 63. H. Du, A. E. Clarke, and A. Bacic, Trends Cell Biol., 6, 413 (1996). 64. T. Hayashi, AIUIU.Rev. Plant Physiol. Plant Mol. Biol., 40, 139 (1989). 65. S. C. Fry, J. Exp. Bot., 40,1 ( 1989). 66. S. C.Fry, W. S. York,P.Albersheim,A. G. Darvill, T. Hayashi, J.-P. Joseleau, Y. Kato, E. P. Lorences, G. A. Maclachan, M. McNeil, A. J. Mort, J . S. Reid, H. U. Seitz, R. R. Selvendran, A. G. J. Voragen, and A. R. White, Physiol. Plant, 89: 1 (1993). 67. W. S. York, J . E. Oates, H. van Halbeek, A. G. Darvill, and P. Albersheim, Carbohyd,: Res., 173, 113 (1988). 68. L. L. Kiefer. W. S. York, A. G. Darvill, and P. Albersheim, Phytochemistry, 28: 2105 (1989). 69. L. L. Kiefer. W. S. York, P. Albersheim, and A. G. Darvill, Carbohyd,: Res., 197, 137 (1990). 70. M. Hisamatsu, G. Impallomeni, W. S. York,P.Albersheim,andA. G. Darvill, Curbohyd,: Res., 211, 117 (1991). 71. M. Hisamatsu, W. S. York. A. G. Darvill, and P. Albersheim, Carbohyd,: Res., 227.45 (1992). 72. W. S. York,H.vanHalbeek,A. G. Darvill,and P. Albersheim. Carbohyd,:Res., 200. 9 (1990). 73. W. S. York, G. Impallomeni,M.Hisamatsu, P. Albersheim,andA. G. Darvill, Carbohyd,: Res., 267, 79 ( 1995). 74. W. S. York. V. S. K.Kolli, R. Orlando, P. Albersheim, and A. G. Darvill, Cur-bohydr. Res., 285. 99 (1996).

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75. 76. 77. 78. 79. 80. 81.

T. Ishii, J . R. Thomas, and T. Hiroi. Phytocher,~i.st~~, 29: 1999 (1990). T. Ishii and T. Hiroi, Curbohyrlr. Res., 206, 297 (1990). S. Aldington, G. J. McDougall, and S. C. Fry, Plant Cell, u r d Enr~iromnent,14: 625 (1991). W. S. York, A. G. Darvill, and P. Albersheim, Plant Physiol., 75: 295 (1984). G. J. McDougall and S. C . Fry, Plmta, 175: 412 (1988). G. J . McDougall and S. C . Fry, Plant Physiol., 93: 1042 (1990). J. Ralph and R. F. Helm, in Foruge Cell Wull Structure und Digestibility (H. G. Jung, D. R. Buxton, R. D. Hatfield, and J. Ralph, eds.), American Society of Agronomy, Madison, WI, p. 201 (1993). T. Ishii and T. Hiroi, Crrrbohydr: Res., 196: 175 (1990). T. Ishii, Phytochetnistry, 30: 23 17 (1991). T. Ishii, Mokrczai Cakkuishi, 42: S15 (1996). T. Ishii and H. Saka, Pltrnt Cell Physiol., 33: 321 (1992). T. Ishii and T. Nishijima, Plant Cell Physiol., 36: 1447 (1995). K. Iiyama, T. B.-T. Lam, and B. A. Stone, Plunt Physiol., 104: 3 IS (1994). Y. Kato and D. J. Nevins, Curbohydr: Res., 227: 315 (1992). T. Gerssman and H. Neukom, Helv. Chern. Acttr, 54: I108 (1971). F. M. Rombouts, J. F. Thibault, and C. Mercier, French Patent Appl. 83,07,208; Chern. Abstt;

82. 83. 84. 85.

86. 87. 88. 89. 90.

102: 60792u (1983). 91. T. Ishii, Curbohydr. Res., 21 9: IS ( 1 991). 92. J. Ralph. S. Quideau. J . H. Grabber, and R. D. Hatfield, J . Chern. Soc. Perkin Truns.. I : 3485 ( 1994). 93. W. H. Morrison, 111. R. D. Hartley, and D. S. Himmelsbach. J. Agric. Food Chem., 40: 766 ( 1992). 94. J. A. Brown and S. C. Fry, Plont Physiol., 103, 993 (1993). 95. D. A. Powell, E. R. Morris, M. J. Gidley, and D. A. Rees, J . Mol. Biol.. 155: 5 17 (1982). 96. P. Albersheim, M . Rev. Bioc.hern., 16: 127 (1978). 97. N. C. Carptia and D. M. Gibeaut, Plont J., 3: 1 (1993). 98. M. C . McCann and K. Roberts, J . Exp. Bot., 45. 1683 ( 1994). 99. S. A. Ldington and S. C. Fry, A h . Bot. Res., 19: I (1993). 100. F. CBte and M. G. Hahn, Plunt Mol. R i o / . , 26: 1379 ( 1994).

Chemistry of Extractives Toshiaki Umezawa Kyoto University, Kyoto, Japan

1.

INTRODUCTION

Extractives are the wood constituents which can be extracted with neutral solvents. They are obtained by extracting wood meal with organic solvents or water or by steam distillation, and some are obtained as exudates from wounded trees. The amountofextractives is small,generallyupto 5-10% in the woodin the temperate zone. However, in some tropical woods relatively high amounts of extractives are found [l]. Amongwood species, differences of chemical structures of three major cell wall components, cellulose, hemicellulose, and lignin, are few. However, a great diversity in extractive composition is found throughout wood species. Although the extractives are low in concentration compared with those of the cell wall polymers, this fraction characterizes eachwoodspecieschemically.Mostcomponents of woodextractives are classified as secondary metabolites, and the distribution of specific compounds is restricted in certain wood species. This feature provides the basis of chemotaxonomy of woody plants. Furthermore, individual compounds are often found in specific tissues of individual trees, and their amounts can vary from season to season even in the same tissue. Many phenolic compounds are accumulated in heartwood, whereas they are found only in trace amounts in the corresponding sapwood. Extractives are the predominant contributors to woodcolor, fragrance, and durability. Extractives also influencethe pulping, drying, adhesion, hygroscopicity, and acoustic properties of wood.Manyextractiveshave specific biological activities, andvariouswoods have been used as sources of crude drugs and medicines for centuries. Recent progress of the biosynthesis of secondary metabolites of woody plants including heartwood components [2] has suggested the possibility of biotechnological control of their biosynthesis. This may lead to biotechnological production of biologically active extractives, and to elucidating molecular mechanisms of heartwood formation. Progress in the chemistry and biochemistry of natural products including wood extractives has been reviewed in a number of articles, such as in Natural Product Reports. Thisjournalincludesregularreviews of the relevant literature publishedduringwelldefined periods with respect to individual topics in the fields.

213

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Umezawa

II. LIGNANS, NEOLIGNANS, AND RELATED COMPOUNDS Lignans and neolignans are phenylpropanoids that occur in many plants including softwoods, hardwoods, and medicinal plants [3-51. The term “lignan” was introduced by Haworth to describe a group of phenylpropanoid dimers, where the phenylpropane units were linked by the central carbon (C8) of their side chains [6]. Gottlieb coined “neolignan” for compounds containing two phenylpropane units that are linked otherwise than C8-C8’ [7]. Later, neolignans were redefined as the dimers of allyl- or propenylphenyls, while lignans were regarded as the dimers of cinnamyl alcohols [8]. However, in this review Haworth’s definition of lignans [6] and Gottlieb’s former definition of neolignans [7] will be used, because the original definitions are being applied by most researchers. Tri- and tetramers of phenylpropanoid units are referred to as sesqui- and dilignans, respectively. Lignans are classified into several subgroups:dibenzylbutanes,dibenzylbutyrolactones, furans, furofurans, aryltetralins, arylnaphthalenes, and dibenzocyclooctadienes [3,5] (Fig. I). Lignans often occur as glycosides. Examples of neolignans are shown in Fig. 2. Chemical structures of lignans and neolignans are similar to that of lignins. Thus (+)-pinoresin01 (5) and the corresponding (-)-enantiomer are typical lignans, while this structure is also involved in a lignin molecule as a substructure (i.e., pinoresinol or p-p’ substructure) [9]. Also the structures of neolignans are similar to lignin substructures of p-0-4 (arylglycerol p-aryl ether), 5-5’ (biphenyl), and p-5 (phenylcoumaran) types [IO]. In spite of the structural similarities, lignins differ sharply from lignans and neolignans in terms of optical activity. The former is optically inactive, while the latter is optically active, suggesting the difference in stereochemical mechanisms in their biosyntheses [9]. Following the first example of an in-vitro enantioselective formation of an optically pure lignan, (-)-secoisolariciresinol (l),with an enzyme preparation from Forsythia inin lignan biosynthesis termedia [ 1 l], much investigation has been done for enzyme systems of Forsythia spp. [9,12,13]. Figure 3a shows the conversion of coniferyl alcohol (14) into lignans with Forsythia enzymes. Recently, cDNA cloning of pinoresinol/lariciresinol reductase which catalyzes reduction of (+)-pinoresin01 (5) and (+)-lariciresinol (4) to (+)lariciresinol (4) and (-)-secoisolariciresinol (l),respectively, has been reported [ 141. In addition, enantioselective coupling of two coniferyl alcohol radicals giving rise to (+)pinoresinol (S) was reported [15]. The coupling was highly enantioselective only in the presence of dirigent protein isolated from Forsythia plant. Formation of (+)-secoisolariciresinol (lS), the oppositeenantiomerto the one in Forsythia spp.,with an enzyme preparation from Arcfiurn lappa petioles, has been reported (Fig. 3b) [16]. On the other hand, an enzyme preparation from seeds of A. lappa was recently found to catalyze enantioselective formation of (-)-secoisolariciresinol (1) from coniferyl alcohol (14), indicating that two enzyme preparations from different organs of a single plant species can catalyze the selective formation of different enantiomers of a lignan [ 171. The structural similarity with lignins suggests that neolignans maybe synthesized by enantioselective coupling of two phenolic phenylpropane units. Recently, two examples of enantioselectiveformation of neolignansfrom coniferyl alcohol (14) werereported [18,191. Lignanshavesuch biological activities asantitumor[podophyllotoxin (7) and steganacin (9)], antimitotic [podophyllotoxin (7)],antioxidant (nordihydroguaiaretic acid andsesaminol), and antiviral [podophyllotoxin (7) againstcytomegalovirusandherpes simplex l-virus, and (-)-arctigenin (3) against human immunodeficiency virus], etc. [3-

215

Chemistry of Extractives

Furan

Dibenzylbutane Dibenzylbutyrolactone

OH

H0 OCH3 -Secoisolariciresinol (-)

1

R=E (-) "atairesinol 2 Re83 (-) -Arctigenin 3

Aryltetralin

E'urofuran

OCH3

OCH3 R=E (+) -Pinoresin01 5

Podophyllotoxin 7

R=OC83 (+) -Syringaresinol 6

DibenzocycloGlycoside octadiene

Steganacin 9 FIGURE 1

Arctiin

10

Examples of lignans.

OCH3

Kadsurenone 11 FIGURE 2

(+) -Lariciresinol 4

Magnololl2 Eonokiol13

Examplcs o f neolignans.

Diphyllin

8

Umezawa

216

%" Coniferyl alcohol 14

(+) -Pinoresinol 5

(+) -Larici-

I

J

(-) -Secoisolariciresinol resinol 1

(-) "atai2

(-)

resinol 4

--ct

igenin 3

(b)

H3CO H O T : :

OCH3 Coniferyl OH

alcohol 14

\

0

OH

O C H ~ (+) -Secoisolariciresinoll5

FIGURE 3 Enzymatic lignan formation.

5,201. Antagonism toward the platelet-activating factor (veraguensin) and inhibitory activities toward certain enzymes have also been detected in many lignans [3-5,201. Among the biologically active lignans, antitumor podophyllotoxin (7)has attracted particular interest [see also Section XLC]. Mammalian lignans are known to be produced from plant lignans such as secoisolariciresinol diglucoside by the action of bacterial flora in the colon of human or animals. The mammalian lignans and their precursor secoisolariciresinol diglucoside have a protective effect on the promotion stage of mammary tumorigenesis [21], and are receiving widespread interest. A neolignan, kadsurenone (ll),has antagonistic activity to platelet-activating factor [22].Honokiol (13) is antibacterial and antifungal, and magnolol (12) has antimicrobial and muscle-relaxant activities [23,241.

111.

NORLIGNANS

Norlignans are diphenylpentane (C,-C,)-(Cz-C,) compounds, and typical examples are shown in Fig. 4. Although the biosynthetic sequences for norlignans have not been elucidated, the structures are seemingly composed of the phenylpropane (C,-C,) and phen-

217

Chemistry of Extractives

OH

Hinokiresinol 16

Sequirin (Sequirin-C) 18

Agatharesinol 17

Sugiresinol Hydroxysugiresinol Cryptoresinol (Sequirin-B) 20

21

( Sequirin-A) 19

HdHaWO H0 OH

Pueroside-A OH 22

H O f/l o H

cis-Hinokiresinol 23

\3

OH

Yateresinol Sequirin-D nyl)-2-cyclo-pentene24 25 FIGURE 4

2,3-Bis(p-hydroxy-phe l-one 26

Examples of norlignans.

ylethane (C,-C,) units connected via C8-C7' [e.g., hinokiresinol (16)] and in some cases C8-C8' [e.g., yateresinol (24)] and C9-C8' [e.g., sequirin-D (25)](Fig. 4). Most norlignans have been isolated from softwoods belonging to Cupressaceae, Taxodiaceae, and Araucariaceae [25-271, while some norlignans occur in herbaceous plants. cis-Hinokiresinol (23) was isolated from Anen7arrl1et~a crspI7ndt.loide.s (Liliaceae) 1281, and two norlignan glycosides, pueroside-A (22) and -B, were isolated from PuercrriLI lohtc1 (Leguminosae) [29] (Fig. 4). No concrete experimental evidence has been reported for the origin of the two aromatic nuclei of a norlignan molecule. Based on structural considerations, several possible schemes have been proposed for the biosynthesis of norlignans, involving coupling of two phenylpropane units followed by loss of one carbon atom 125,301. Norlignans are of particular interest with respect to heartwood coloration. The coloration of heartwood of Japanese cedar (Cty>tornc~ria japonica) is due to polymerization

Umezawa

218

of norlignans, e.g., hydroxysugiresinol (20) and sequirin-C (18) [31,32], andthat of hinoki cypress (Charnaecyparis obtusa) is related to hinokiresinol (16) [33]. Inhibitory activity on cyclic adenosine-3’,5’-monophosphatephosphodiesterase was reported for cis-hinoki resinol (23) [28]. Sugiresinol (19) and hydroxysugiresinol (20) have inhibitory effect on polymerization of methyl methacrylate [34].

IV.

FLAVONOIDS

Flavonoids are diphenylpropane (C,-C&,) compoundswhicharecomposedof the C,-C, (phenylpropane) fragment derived from the shikimate-cinnamate pathway and the C , fragmentderivedfrommalonyl-CoA (40). Flavonoids are classified into flavanones, flavones, chalcones, dihydroflavonols (flavanonols), flavonols, aurones, flavan-3-01s (catechins), flavan-3,4-diols (leucoanthocyanidins),anthocyanidins, isoflavonoids, andneoflavonoids (Fig. 5). The term “flavonoids” in the strict sense is sometimes applied to those except for isoflavonoidsandneoflavonoids.Proanthocyanidins(condensedtannins) are oligomers and polymers of polyhydroxyflavan-3-01 units [35] (see also Section IX). Typical examples of flavonoids are shown in Fig. 5. Flavonoids occur widely in the plant kingdom, and presentwidelyor specifically in barks, heartwoods, flowers, fruits, seeds, roots, etc. Flavonoids reported to occur in wood or bark are listed in Ref. [36], and the chemistry of flavonoids is reviewed in a number of works [36-411. Chalcone is biosynthesizedfromcinnamoyl-CoAs [especially, p-coumaroyl-CoA (39)], which is formed via the shikimate-cinnamatepathway,and three moleculesof malonyl-CoA (40). The transformation is catalyzed by chalcone synthase as shown in Fig. 6.6’-Deoxychalcone [e.g., isoliquiritigenin (30)] is likewisesynthesizedfrommalonylCoA (40) and 4-coumaroyl CoA (39) by chalcone synthase in coaction with a NADPHdependent polyketide reductase. The chalcones are the immediate precursors for all flavonoids (in the strict sense). Thechalcones are converted to flavanones, flavones, dihydroflavonols, flavonols, leucoanthocyanidins, anthocyanidins and their glycosides (anthocyanins), catechins, and aurones [42,43]. The enzymes which are responsible for these conversions and the genes encoding these enzymes have been well characterized [42,44481. Formation of isoflavonoidsinvolves the rearrangement of aphenylgroupof the flavanoneskeleton[42,46].On the otherhand, little is knownaboutbiosynthesisof neoflavonoids. Flavonoids have various biological activities 137,381. Thus, symbiotic nitrogen-fixing bacteria recognize flavonoids as signals for the activation of their nodulation genes [49]. Isoflavonoids are the major structural class of phytoalexins in legumes [49-511. Anthocyanins occur as flower pigments [52,53]. Green teas contain significant amounts of catechins which have an antioxidant activity [54]. The effects of flavonoids on mammalian biology are reviewed [55,56]. Flavonoids seem to protect plants from ultraviolet-induced injury 1571. Manyflavonoids are biosynthesized in response to externalstresses, e.g., ultraviolet light, microbial attack, and physical injury. Hence, the flavonoid biosynthesis is a metabolic event which is suitable to investigate stress-gene expression relation. Heartwood formation which does not occur in herbaceous plants is one of the metabolic events specific to woody plants, but little is known about its biochemical mechanisms. However, this metabolic event involves or is accompanied by deposition of significant amounts of secondary metabolites, so-called heartwood extractives suchas flavonoid, stilbene, lignan, norlignan, etc. Seasonal changes and site specificity of chalcone synthase activity have been examined in relation to heartwood flavonoid synthesis [58,59].

Chemistry of Extractives

219

Qfy

Flavanone

Cbalcone

Flavone

\

0

H OH 0 Sakuranetin 27

Dihydroflavonol (Flavanonol)

\

I OI

S

OH 0 Apigenin 28

H

O

\

R

S

0

R=on, Chalconaringenin 29 R=H, Isoliquiritigenin 30

Aurone

Flavonol

OH 0

H o e : : I

'

I

OH

OH 0

Dihydrokaempferol 31

Flavan-3-01 (Catechin)

Quercetin 32

OH

0

Sulfuretin 33

Flavan-3,I-diol (Leucoanthocyanidin) Anthocyanidin

OH

OH OH

H \o d : :

H o e OHo H

OH OH

(+)-Catechin 34

OH OH

Leucodelphinidin 35

Isoflavonoid

Neoflavonoid

% Daidzein 37

FIGURE 5 Examples of flavonoids.

OH Pelargonidin 36

Umezawa

220

p-Coumaroyl-CoA 39

Flavonoids

HOOC>SCoA

0 Malonyl-CoA 40 FIGURE 6 Biosynthesis of flavonoids and stilbenes.

Many genes of the enzymes involved in flavonoid synthesis have been cloned, and mechanisms of the gene expression have been investigated intensively [47,48]. Thus, flavonoids are one of the best-understood groups of plant secondary metabolites, especially in terms of biosynthesis. It should be noted that extensive genetic information available about flavonoids, especially anthocyanin pigments of flowers, has accelerated significantly the biosynthetic studies in this field.

STILBENES

V.

Historically, the termstilbene referred to compoundspossessing the 1,2-diphenylethene structure,butnowadays the newlydiscoveredbibenzylsandphenanthrenes,whichare composed of C,-C,-C, skeleton, are also involved in this group. Stilbenes occur in the Pinaceae, Moraceae, Betulaceae, Leguminosae, etc. [60]. Typical examples of this class are shown in Fig. 7. Stilbenes are elaborated from CoA esters of cinnamic acids, and there is a similarity in the biosynthesis of stilbenes with that of flavonoids (Fig. 6). Stilbene synthases catalyze

Resveratrol

Pinosylvin

Elydxangeic acid

42

I

OH

Lunularin

Batatasin I

45

FIGURE 7 Examples

or stilbenes.

Chemistry of Extractives

221

condensation of CoA estersof cinnamic acids [e.g., cinnamoyl-CoA and p-coumaroyl-CoA (39)J with three molecules of malonyl-CoA (40), as in chalcone synthesis catalyzed by chalcone synthase [2,61]. However, the cyclization of polyketide moiety of the C& polyketocarboxylic acid (41) occurs in a different wayfrom that by CHS to give rise to stilbenes (Fig. 6); resveratrol (42) and pinosylvin (43) are formed with elimination of one carbon atom (Fig. 6),while hydrangeic acid (44) is formed without the elimination [61,62]. Although pinosylvin (43) and pinosylvin monomethyl ether occur in sound heartwood of Pinus spp., they are formed as a response to stress such as fungal infections or UV light [601. Hence, the role of stilbene in decay resistance and induction of stilbene synthesis has attracted much attention [60]. A stilbene synthase from UV-stressed seedlings of Pinus sylvestris has been purified and characterized [63], while a stilbene synthase gene from grapevine (Vitis vintfera) was transferred to tobacco, and the regenerated plants were found to display increased resistance to Botrytis cinerea [64]. Pinosylvin (43) and pinosylvin monomethyl ether are also known as inhibitors of sulfite pulp cooking [65].

DIARYLHEPTANOIDS

VI.

Diarylheptanoids are composed of two phenyl rings connected with a C , carbon chain (Fig. 8). Many of this type of compound are isolated from plants belonging to the Betulaceae and Zingiberaceae. Besides these two families, the occurrence of diarylheptanoids in the following species was also reported: Centrolobium spp. (Leguminosae), Myrica spp. (Myricaceae), A c e r spp. (Aceraceae), and Garuga spp. (Burseraceae). Recently the chemistry and biological activity of diarylheptanoids have been reviewed [66,67].

OH OH

0 0

Ilannokinol 47

Curcumin 48

OH

H0

P l a t y p h y l l o s i d a 49 0

"'OH H 0 OH

Asadanin 50 FIGURE 8

Acerogenin

Examples of diarylheptanoids.

A 51

Urnezawa

222

Two different results of feeding experiments have been reported regarding biosynthetic precursors of diarylheptanoids. One suggested that one aromatic ring of curcumin (48) is derived from a cinnamate unit and the other from acetate (or malonate), based on administration of [ I4C]acetate, [I4C]malonate, and [14C]phenylalanine to Curcuma longa [68]. Other studies suggestedthat diarylheptanoids, acerogeninA (51), and platyphylloside (49) were derived from two phenylpropane units and one acetate (or malonate)unit, based on administration of [“C]cinnamate, [14C]phenylalanine, [ I4C]acetate, and [ 14C]malonate to Acer nikoense [acerogenin A (51)] [69], and of [14C]cinnamate and [I4C]malonate to Betula platyphylla [platyphylloside (49)] [70].

VII.

ISOPRENOIDS

lsoprenoids is the generic name of compounds composed of isoprene (C,H,) units connected linearly or cyclically. Isoprenoids consist of terpenoids, steroids, and tropolones. Since the chemistry of terpenoids, steroids, and tropolones has been developed independently in spite of their close relationship in biosynthesis, the three classes are usually treated separately. Terpenoids are divided into monoterpenes (C,,,), sesquiterpenes (C,,), diterpenes (C?”), sesterterpenes(C,,), triterpenes (C,,,), tetraterpenes (C,,), and polyterpenes (C,,,), depending on the number of the constituent isoprene (C,) units. Each subclass of terpenoids is further classified into many groups of different carbon skeletons. The terpenoid compounds are generally elaborated via the mevalonate pathway as outlined in Fig. 9. Although the mevalonate pathway has generally been accepted, a novel pathway concerning the early steps of isoprenoid biosynthesis toward isopentenyl pyrophosphate (56) has recently been demonstrated [71]. The novel pathway, which involves the condensation of a triose phosphate with activated acetaldehyde, has been characterized in several different bacteria [71,72]. In addition,Eisenreichetal.haveshown that the taxane carbon skeleton is not of mevalonate origin in Taxus chinensis [73] (see also Section X1.C). Isoprenoids are the largest group among plant secondary metabolites, and occur in a huge number of plants including woody plants. It is beyond the scope of this book to list all the plants producing this class of compounds and to describe their biosynthetic schemes. Comprehensive lists of the compounds, their biosynthesis, and biological activities are summarized elsewhere [2,37,74-771.

A.

Terpenoids

Specific fragrances of different woods are usually due to the composition of monoterpenes and volatile sesquiterpenes. They can be easily separated from wood by steam distillation, and the oily substance obtained is called “essential oil.” Turpentine, essential oil from Pinus spp., is obtained by steam distillation of exudates from pine trees (oleoresin); the residue is gumrosin,which is composedmainly of diterpene acids (rosin acids), e.g., abietic acid (91). Turpentines obtained from pine wood and those recovered from kraft pulp waste liquor are called wood turpentine and sulfate turpentine, respectively. Rosins are used for sizing of papers. Monoterpenes are derivedfromgeranylpyrophosphate (58). They are subdivided into acyclic and cyclic monoterpenes (Fig. IO) a-Pinene (64) and @-pinene (65) are major components of turpentine.

223

Chemistry of Extractives

y SCoA

y SCOA Acetyl-coA

Acetyl-coA 52

52

Ki,SCOA Acetoacetyl-coA 0 ,o

53

3-Eydro~y-3-methylglutaryl-CoA 54

Mevalonic

- JJPP

acid55

pyrophospxate 1 56

1

Isopenten

uopp

"-l l

Dimethylallyl pyrophosphate 57

I

OPP

Monoterpenoids

M Geran

1 pyropiosphate 58

Sesquiterpenoids Squalene \ TriFarnesyl pyrophosphate 59 terpenoids

'

.

Diterpenoids \ Tetraterpenoids c

G e r m lgeranyl 'Phytoene pyropxosphate 60

Sesterterpenoids Getan lfarnesyl pyropiosphate 61

Polyterpenoids FIGURE 9 Generalscheme of terpenoid biosynthesis. PP: pyrophosphategroup.

Sesquiterpenes are derived fromfarnesyl pyrophosphate (59), and constitute the largest class of terpenoids [74]. Some 120 distinct skeletal types of sesquiterpenes are known [74]. Figure 11 depicts the important sesquiterpene skeletal types from acyclic (e.g., farnesane) to tricyclic (e.g., thujopsane), which are often encountered as wood constituents. Qpical examples of each type of sesquiterpenes are also shown under the corresponding skeletal types in Fig. 11. Diterpenes are derived from geranylgeranyl pyrophosphate (60), and some 130 distinct skeletal types are reported [74]. Figure 12 shows the typical diterpene skeletal types

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224

P-Myrcene

(-) -Citronellol

62

63

(-) 64

-a-Pinene

(-) +-Pinene

65

(- ) -Limonene

1,B-Cineol

67

68

(+) -Camphor 66

FIGURE 10 Examples of monoterpenes.

with corresponding examples. A phytane diterpene, plaunotol (87), has anti-ulcer activities [78]. ent-Gibberellane diterpenes, gibberellins, are important plant hormones. Occurrence of sesterterpenes in higher plants is highly limited. Triterpenes are elaborated from squalene formed via tail-to-tail coupling (coupling between pyrophosphate ends) of two farnesyl pyrophosphate units 121. Two major classes of nonsteroidal triterpenes are tetracyclic and pentacyclic [74]. Examples of typical skeletal types of this class are shown in Fig. 13: lanostane, dammarane,euphane,limonoids [tetranor (C,,) compounds], quassinoids (mainly Czo compounds), lupane, and oleanane. Figure 13 also shows examples of compounds belonging to each skeletal type. Oleananes often occur as aglycons (sapogenins) of saponins [76].

B. Steroids Steroids, which are also derived from squalene, are compounds with cyclopentanoperhydrophenanthrene skeleton and their congeners elaborated from them. The basic structure of steroids is shown in Fig. 14. Positions on the same side as the angular methyl (18- and 19-CHJ are denoted as p, and those on the opposite side are denoted as a. Substituents on the 8, 9, 10, 13, 14, and 17 positions of steroids are projected to Sp, 9a, lop, 13P, 14a, and 17P positions, respectively. p-Sitosterol (or sitosterol) (108) (Fig. 14) is widely distributed in the plant kingdom. a- and P-Ecdysones isolated from silk worm (Bombyx rnori) have steroid skeletons, and are well known as molting hormones. Compounds with similar structure to the ecdysones were isolated from plants (e.g., Podocarpus and Taxus spp.), and are referred to as phytoecdysones [e.g., ponasterone A (109)] (Fig. 14) [79].

C. Tropolones Tropolones are nonbenzenoidaromaticcompoundshaving a seven-memberedenolone structure. They occur in Cupressaceae plants and exhibit antimicrobial activity. Examples

225

Chemistry of Extractives

B i s a b o lFaanren e s a n e

trans-Farnesol 69

(+) -P-Bisabolene 70

(+) -Juvabione

71

Cuparane

Caryophyllane Germacrane

Germacrone (+) -Costunolide 72 73

+-Caryophyllene 74 (-)

(+) -Cuparena

75

Eudesmane

OH

P-Eudesmol 76

(-) -CryptoT-Cadinol meridiol 78 77

FIGURE 11 Examples of sesquiterpenes.

of this class are shown in Fig. 15a. Hinokitiol(P-thujaplicin) (111) was isolated from Charnaecypuris ruiwunmsis by Nozoe [80,81]. a,P, and y-Thujaplicins, (110), (lll),and (112), were isolated from Thuju plicufu by Erdtman [82]. Tropolones are composed of 10 or 15 carbon atoms, and they have been regarded as mevalonate origin, i.e., a subclass of isoprenoids. Recently this has been supported by feedingexperiments.[“C]Mevalonic acid wasfound to beincorporatedinto hinokitiol (111) in suspensioncultures of Cupressus lusitunicu, suggesting that hinokitiol (111) is elaborated via the mevalonate pathway [83].

226

Umezawa

Guaiane

(-)

-Guaiol

79

Aramadendrane

Himachalane

(+) -Aromadendrene

(- 1 -a-Himachalene

80

81

Acorane

Chamigrane

Cedrane

Thuj o p s a n e

q y-Acora-

P-Chamigrene

diene

83

a-Cedrene 84

Thujopsene 85

82

FIGURE 11 Continued

On the other hand, the tropolone structure and aromatic ring of colchicine (114) (Fig. 15b),which is analkaloidfrom Colchicum autumnale, arederivedfromtyrosineand phenylalanine, respectively [84]. Thus colchicine (114) is a phenylpropanoid compound butnot an isoprenoid. In addition,tropolonesoccurring in microorganismsarebiosynthesized via the acetate-malonate pathway [85].

VIII.

QUINONES

Various types of quinones occur in many plant families, and most of them are benzoquinones, naphthoquinones, or anthraquinones. Most of the quinones found in nature are p quinones, but o-quinones also exist [86]. Spica1 examples of this class are shown in Fig. 16. Quinones are biosynthesized via various pathways, i.e., the shikimate, the mevalonate, and the acetate-malonate (polyketide) pathways [2]. Quinones are pigments and have various biological activities. Juglone (116), which occurs in black walnut (Juglans nigra), is skin-irritating [87-891. This compound is also well known as an allelochemical [90]. Tectoquinone (117) and related compounds have strongantitermite activity [91]. Mansonone A (118) and its congenercause allergies [89,92].

227

Chemistry of Extractives

Phytane

LJwwLL OH

m

HO-

OH

Plaunotol87

Phytol 86

Labdane

(+) -transCommunic acid 88

Pimarane,

Isopimarane

(-) -Sandaracopimaric acid

(+) -Pimaric acid 89

90

Nagilactone

Abietane

& p!?

COOH

(-) -Abietic acid

@ H 0

(+) -FerruginolInumaki-Nagilactone 92 lactone A 94

91

A

93

FIGURE 12 Examples of diterpenes.

IX. TANNINS Tannins are water-soluble phenolic compounds having molecular weights between500 and 3000. Besides giving the usual phenolic reactions, they have special properties such as the ability to precipitate alkaloids, gelatin, and other proteins [93,94]. This class of compounds also has high astringency, and gives blue or green coloration with femc chloride. Tanninsare distributed widely in wood, bark, andleaves of many plants. Tannins are classified into hydrolyzable and condensed tannins.

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ent -Kaurane

ent-Gibberellane

Ginkgolide

COOH (-)

-Kaurene

95

Gibberellin

A19

Ginkgolide B 97

96

Taxane

10-Deacetylbaccatin I11

Taxol 99

98

FIGURE 12 Continued

Hydrolyzable tannins (Fig. 17a) are esters of an aliphatic poly01 and phenolic acids (Fig. 17b), and can be hydrolyzed into the components. As shown in Fig. 17a, galloyl, hexahydroxydiphenoyl, and depside galloyl groups are esterified to the polyol, generally D-glucose. Hydrolyzable tannins that give gallic acid (120) by hydrolysis are referred to as gallotannins, while compounds that afford ellagic acid (122) are referred to as ellagitannins. Hexahydroxydiphenic acid (121) is lactonized to give rise to ellagic acid (122) in the hydrolysis. Some ellagitannins [e.g., casuarinin (119)] have C-glycosidic structures. Two different pathways have been proposed for the biosynthesis of the phenolic unit, gallic acid (120). One is P-oxidation of the side chain of cinnamates to give gallic acid (120), while the other is direct conversion of 3-dehydroshikimate to gallic acid (120) [95]. Recently, it has been shown that formation of gallic acid (120) via cinnamic acids can be ruled out as a major pathway in the fungus Pllycornyce.7 blakesleeanus and in young leaves of Rhus typhirza, and gallic acid (120) is probably formed from an early intermediate of the shikimate pathway, most probably 3-dehydroshikimate [96]. In the pathways to hydrolyzable tannins, the first specific intermediate is P-glucogallin ( l-O-galloyl-P-D-glucose), which is formed from UDP-glucose and gallic acid (120). Then, P-glucogallin is

229

Chemistry of Extractives

Lanostane

Dammarane

Euphane

H3CO"

Abieslactone 100

Limonoid

Dammarenediol I 101

Quassinoid

Euphol 102

Lupane

Oleanane

$$

,8

H0

0

H

OH

Cedrelone 103

Quassin 104 -

P-Amyrin

Betulin ( R S E , O E ) 105

Betulinic

107

acid

(R=COOH) 106

FIGURE 13 Examples of triterpenes.

converted to pentagalloylglucose and gallotannins. Enzymes which catalyze these conversions from gallic acid (120) to gallotannins have been studied intensively by Gross and co-workers, summarized in the review by Gross [97]. Condensedtannins(proanthocyanidins)(Fig.17c)areoligomers and polymersof polyhydroxyflavan-3-01 units [35]. The repeating unit is connected through C4-C6 or C4-C8 bonds. 3-Hydroxyl groups of condensed tannins are often galloylated (e.g., tannins of Diospyros kaki) [98]. Condensation of flavan-3,4-diols and flavan-3-01s may give rise to proanthocyanidins (condensed tannins), but possible mechanisms for the process and their enzymology are still unknown. In spite of their difference in the basic structures, hydrolyzable and condensed tannins have a similarity in that they have many phenolic units and therefore are often called plant polyphenols [94]. Besides the phenolic nature, tannins have the following general characteristics: antioxidant and radical-scavenging activities and the ability to complex with

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230

5P-Steroid 5a-Steroid

p-sitosterol

Ponasterone A

108

109

FIGURE 14 Examples of steroids. R: alkyl group.

a-Thujaplicin 110

Einokitiol (P-Thujaplicin)

y-Thu japlicin 112

111

Nootkatin 113

H3coq H&O

H3C0

CH3

.

0 OCH3

Colchicine 114 FIGURE 15 Examples of tropolones.

231

Chemistry of Extractives

OCH3

H3C0

0

0 118

0

J'uglone

2,C-Dimethoxyp-benzoquinone 117 16

TectoquinoneMansonone

A

1

115

FIGURE 16 Examples of quinones.

R=

yo

CO

/OR

OH

RO&&&%' OR Hydrolysable

tannin -

OH OH Hexahydroxydiphenoyl group

Depside galloyl group

OH

l

.n 119

(b)

Gallic acid

120

Haxahydroxydipbenic acid

OH Ellagic acid

122

121

FIGURE 17 (a): Hydrolyzableandcondensedtannins. of (c): hydrolyzable tannins.

(b): phenolic acids whicharecomponents

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232

metal ions and with other molecules such as proteins, polysaccharides, and alkaloids [54]. These properties underlie their biological activities as well as their industrial applications. Tannins exhibit various biological and pharmacological activities, e.g., bacteriocidal action, inhibition of HIV replication, as well as astringency [37,54]. Inhibitory effects of condensed tannins on the activities of Streptococcus sobrins glucosyltransferases involved in dental caries formation were also reported [99]. Recently, the role of plant phenolics in diets is attracting considerable interest, since epidemiological investigations suggested that the consumption of beverages containing plant phenolics (such as tannins and flavonoids), in particular, green tea and red wines, reduced the risk of certain degenerative diseases [54,100]. Recent study demonstratedthat the iron-chelating properties of polyphenols contribute to limit the growth of microorganisms [ l o l l . The ability of tannins to complex proteins has long been utilized for tanning agents in leather manufacturing,while that to complexmetal ions hasbeenappliedtodyeing [37,54].Condensed tannins, especiallywattle (ormimosa) tannins, havebeenused to produceadhesives for wood-basedmaterialssuchas particle board and plywood.The tannin adhesives have been successfully commercialized especially in South Africa, Australia, and New Zealand [ 102- 1041. Many reviews of chemical properties, biological significance, and commercial significance of tannins are available in a recent book [105]. Mechanisms of tannin-protein complexation will be described briefly in Section X1.B.

X.

OTHER COMPOUNDS

Besides the compoundsmentionedabove,sugars, triglycerides andwaxes,monomeric aromatic compounds and phenols, alkaloids, etc., occur as extractives in woody plants.

A.

Sugars

D-Glucose and D-fructose are found, along with sucrose, in the sapwood of woody plants, while L-arabinose is found in heartwood [ 1061. Larches (Larix spp.) contain significant amounts (IO-25%) of a water-extractable arabinogalactan [ 1071.

B. Glyceridesand Waxes Glycerides are esters of glycerol and long-chain fatty acids. Among the glycerides, triesters (triglycerides) are dominant. Triglycerides of pine woods cause pitch trouble in ground wood pulping. Waxes are complex mixtures of aliphatic compounds, and a majority of these compounds are wax esters composed of fatty acids and fatty alcohols, hydrocarbons and derivatives, long-chain fatty acids, and long-chain fatty alcohols [ 1081. C.

MonomericAromaticCompounds

Phenylpropanoid monomers are distributed widely in plants, including woody plants. Coniferin (124) and syringin (125) (Fig. 18), aglycons of which are precursors of lignins and lignans, were isolated from many woody plants [109,110]. A soil bacterium, Agrohacteriumtumefaciens, can initiate the neoplastic disease called crown gall on dicotyledonous plants. Tumor-inducing plasmid of the bacterium is used to construct a useful vector to produce transgenic plants. Virulence gene expression

233

Chemistry of Extractives

OH R=E

OH

Coni feryl alcohol 14

R=H Coniferin 124

R=OCH3

Sinapyl alcohol 123

Anethole 126

R=OCE3

Syringin 125

Eugenol 127

Umbelliferone 130

Safrole 128

Cinnamaldehyde 129

Aesculetin 131

R Urushiol 132 R=ClSH25-31 FIGURE 18 Examples of monomeric aromatic compounds.

of the bacterium is activated by coniferyl alcohol (14) and sinapyl alcohol (123) [ l I l ] (Fig. 18) as well as acetosyringone (3’,5’-dimethoxy-4’-hydroxyacetophenone) [ 1 121. Various phenylpropanoid monomers are components of essential oils, for example (Fig. 18), anethole (126) (star aniseed oil), eugenol (127) (clove oil), safrole (128) (sassafras oil), and cinnamaldehyde (129) (cassia oil), which have been used for spice and perfume. Coumarins constitute another class of phenylpropanoids which have 2H- 1 -benzopyran-2-one structure. They are distributed widely in plants, particularly Umbelliferae and Rutaceae [ 113- 1 151. Two examples, umbelliferone (130) and aesculetin (131), are shown in Fig. 18. Biological activities of coumarins are reviewed in the literature [37]. Urushiol (132) (Fig. 18) is a major component of urushi (Japanese lacquer), milky exudate from urushi (Rhus vernicijlua), which has been used to produce japan (Japanese lacquer ware) [ 116,1171.

234

Urnezawa

0 "+,/

OCH3

N

H

/

Berberine 133

H% H3COys

Quinine 134

9 H '

HH0

0 OH

-tine

,I

OCH3 OCH3

135

H 3 c 0H 3 ~ C: O H" w

0 OCH3

Yohimbine Strychnine Reserpine

OCH3 OCH3

136

Camptothecin FIGURE 19 Examples of alkaloids.

D. Alkaloids Although the occurrence of alkaloids is much less than other wood extractives, most of them are of considerable interest due to their biological activity. Examples of alkaloids from woody plants are as follows [37,118- 1201 (Fig. 19): berberine (133) from Phellodendron arnurense, which is antibacterial; quinine (134) from Cinchona pubescens, which is employed for the treatment of malaria; emetine (135) from Cephaelisipecacuanha, which is anamebicide;yohimbine (136) from Pausinystalia yohimbe, which is a selective inhibitor of the presynaptic a-2-adrenergic receptors; strychnine (137) from S t r y c h n o s n u - v o m i c a , a very toxic alkaloid; reserpine (138) from Rauwolfia serpentina, which is antihypertensive; and camptothecin (139) from Camprotheca acurninata. which is antitumor.

XI.

CONTRIBUTION OF EXTRACTIVES TO THE PROPERTIES OF WOOD AND UTILIZATIONOF WOOD EXTRACTIVES

Extractives of wood influence various properties of wood, e.g., color, fragrance, and durability. Some extractives have injurious effects on human health[89]. Troubles in pulping processes and adhesion in production of wood-based materials are sometimes due to extractives. They are described in detail in a book [ 1211 and outlined byKaiin the first edition of this book [122]. Therefore, several topics in these fields are described briefly in this section.

Chemistry of Extractives

A.

235

AcousticProperties

Recently, acoustic properties and internal friction (loss tangent, tan S) of several woods and a cane have been observed to be strongly influenced by extractives. The term tan S is an indication of decrement of vibration of solid materials, and materials with lower tan S exhibit higher sound radiation. Methanol extraction of heartwood specimens of western red cedar (Thuja plicata), which is usedfor the top plate of the guitar, increased tan S values by 15.3-36.9% [ 123,1241. The same effect of methanol extraction was observed for rosewood (Dalbergia spp.), black cherry(Prunus serorina), and padauk (Pterocarpus indicus),whereas no effects of heartwood contents or methanol extractives were detected in bubinga (Guibourtia demeusei) [ 123,1241. Pernambuco (Guilandinaechinata syn. Caesalpiniaechinata) andcane (Arundo donax) have been used for violin bows and reeds of woodwind instruments, respectively. Water extracts from pernambuco reduced tan S value [ 1251, while in the case of air-dried cane, water extracts increased tan S value [126]. It was suggested that the water extracts of the cane consisted mainly of oligoglucans [126].

B. Tannin-ProteinComplexation The ability to complex with proteins is one of the most important features of tannins, and the mechanisms for tannin-protein complexation have long been studied. These studies clarified effects of environmental factors (such as pH, temperature, and ionic strength) on the process, and the following three principal features of tannin structure and properties which are important in the complexation with protein have been established: molecular size, conformational flexibility, and water solubility of the tannin. [54,94,127-1291. The precipitation ability increases with an increase of the degree of polymerization of condensed tannin [130], while in the case of hydrolyzable tannin this ability is enhanced with the addition of galloyl ester group [ 13 l]. Selective interaction of the condensed tannin from Sorghum bicolor with various proteins was also investigated, and it was reported that proline-rich and flexible proteins have high affinity for the polyphenol [ 1321. As for the mode of interaction between tannins and proteins, involvement of hydrogen bonding and hydrophobic interaction was suggested [ 1271. Although the relative importance of these two types of interactions remains uncertain, the initial association of protein-polyphenol by hydrophobic interaction followed by reinforcement of the association by hydrogen bonding was proposed [ 1271. Since tannins have complicated structures, synthetic tannin models which have definite structures are useful to elucidate mechanisms of tannin-protein complexation. Based onexperimentswithaseries of synthesizedcondensedtanninmodels, the distribution pattern of the phenolic hydroxyl group in the tannin molecule, but not the existence of odihydroxyphenyl groups, was found to be important for higher protein-precipitating capacity [ 133,1341. Two mechanisms were proposed for tannin-protein co-precipitation [94,127]. One is a “cross-linking mechanism,” in which one tannin molecule binds more than two protein molecules simultaneously to form an aggregate to be precipitated. The other is a “twostage precipitation mechanism,” which consistsof initial complexation of tannin molecules to a protein molecule to form a complex, followed by aggregation of the complexes to give precipitates. Recently, a series of hydrolyzable tannin models has been synthesized, and studies withthe models suggested that the two-stage mechanism is involved in tanninprotein co-precipitation [129,135,136].

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236

C. Antitumor Taxol and Podophyllotoxin Recently, phytochemical studies of yew trees (Taxus spp.) have been developed exponentially [ 1371. This is due mainly to the plants’ producing taxane diterpene, taxol (99), which has strong antitumor activity. Taxol (99) is one of the most promising anticancer drugs and has been marketed under the name of TaxoP. Paclitaxel is the generic name for TaxoP. Because of great familiarity with the word taxol, however, it is used in this review in lieu of paclitaxel. Taxol (99) is present only in trace amounts in Taxus spp. (e.g., 0.0001-0.069% from 7: brevifolia bark) [ 1381. The sources of taxol (99) and related taxaneswerereviewed [ 1391. Interestingly, Taxomyces andreanae,an endophytic fungus of 7: brevifolia, produces taxol (99) andbaccatin 111 whenincubated in a semisynthetic liquid medium,although the quantities detected are very low (24-50 ng of taxolk) [140]. The limited production of taxol (99) in nature as well as the projected needs for the compound and its unique chemical structure have provided a very challenging target for syntheticorganicchemists.Recently,two total syntheses of this compoundhavebeen reported independently [ 141-1431. The production of taxol (99) through semisynthesis fromthemore readily available 10-deacetylbaccatin 111 (98) is nowwellestablished [ 144,1451. The biosynthesis of taxol (99) and its production by cultured cells have also received much interest. Recently,taxadienesynthasecatalyzing the cyclization of the universal diterpene precursor, geranylgeranylpyrophosphate (60), to the taxanesystem [taxa4(5), 1 l( 12)-diene] in a single enzymatic step was purified and characterized [ 1461. Very recently, Eisenreich et al. showed that the taxane ring system is not biosynthesized via mevalonate [73]. Production of taxol (99) by cultured cells is reviewed in the literature [120,145,147]. An aryltetralin lignan podophyllotoxin (7)has strong antitumor and antimitotic activity [4,5,20,148]. Since the lignan possesses severe gastrointestinal toxicity, semisynthetic derivatives of podophyllotoxin (7)have been developed to avoid the toxicity. Etoposide, which isone of the derivatives,hasbeenusedsuccessfully for cancerchemotherapy [5,148]. Podophyllotoxin (7)and its congeners occur in root and rhizome of Podophyllum spp. as well as other plants including woody plants, e.g., callus culture of Callitris drummondii (Cupressaceae) and leaves of Juniperus spp. (Cupressaceae) [20,148]. These lignans are elaborated from matairesinol [5,9,149,150].

D.

Pitch Trouble in Pulp and Paper Making

Wood extractiveshavecaused technical andeconomic pitch problems in the pulpand paper industry. Recently,twotypes of biochemicaltreatmentshavebeendeveloped to alleviate the problems. One employs enzymes, and the other uses a microorganism. Japanese red pine (Pinus densijora) isan important raw material for groundwood pulp. The wood contains significant amounts of resinousmaterialswhichcause pitch trouble in groundwood pulp process. The trouble can be partly avoided by seasoning the logs. Recently, lipase-catalyzed hydrolysis was foundto reduce triglycerides in the resinous materials to 70%, when the enzyme was added to groundwood pulp at 9000 U/kg. This treatment reduced the pitch deposits remarkably, and allowed the use of unseasoned logs up to 50% of total wood supply without pitch troubles [ 15 1,1521. Theother is a biological approach to decreasingextractivesfrom wood prior to pulping using the fungus Ophiostoma piliferum. A 2-week treatment of pine chips (50%

Chemistry of Extractives

237

Pinus taedcl and 50% Pinus virginiuna) with the fungus reduced ether-extractable pitch components by 22% compared with chips seasoned naturally for 2 weeks. GC-MS analysis of pitch components in fresh chips and in fungus-treated chips showed significant declines in the concentrations of triglycerides, fatty acids, and resin acids [ 1531.

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7

Chemistry of Bark Kokki Sakai Kyushu University, Fukuoka, Japan

1.

INTRODUCTION

Tree bark usually refers to all tissues external to and surrounding the vascular cambium. It occupies a much smaller volume than the wood of a mature tree stem because fewer bark cellsare produced than wood cells and also because the outermost bark cellsare continuously discarded in most tree species, while wood cells are retained and thus accumulate as the tree grows. In spite of its small volume, the bark plays important roles in a living tree. Tree barks often have developed complex anatomy and/or chemical compositions in order to manifest or maintain three main functions: (1) nutrient transport from the leaves to the rest of the tree, (2) protecting the sensitiveinnercambiumfromdesiccation,and (3) shielding from the environment as the primary defense of the tree against wildfire, mechanicalinjuriescaused by heavy wind, and attacks by phytopathogens,phytophagous insects, larger animals, and so on. The objective of this chapter is to provide the new information about the chemical composition and utilization of tree barks, mainly since 1986. The basic knowledge of bark chemistry and important findings in this field up until 1985 were well stated by Laks in the first edition of this book [ 11.

II. THE FORMATION AND ANATOMY OF TREE BARKS The formation and anatomy of bark are described very briefly, as quite detailed discussions of them were presented by Laks [ l ] . The epidermis. the cortex, and the primary phloem are produced during longitudinal growth from the apical meristem located at the apices of growing roots and branches. The secondary phloem and periderm are formed during radial growth of the tree. Accordingly, the primary phloem remains in only the outermost region of the bark of young trees. In most tree species the outer bark cracks and peels off as the tree grows, due to the successive formation of periderms within the bark and growth of the xylem. Thus the mature bark consists mainly of secondary phloem and periderm, a group of tissues including the phellogen or cork cambium, the phelloderm, and the phellem or cork tissue. As the sec243

Sakai

244

ondary phloem thickness, new periderm is formed within the phloem and any cells to the exterior of the periderm soon die. Most tree barks, therefore, have two zones, the inner bark that contains some living cells and the outer bark or rhytidome that does not contain any living cells. These regions are sometimes compared to the sapwood and heartwood in the xylem, respectively. Recently, Trockenbrodt [2] surveyed and discussed the terminology used in bark anatomy, andsuggestedterms for the tissue zones as illustrated in Fig. 1 . In the barkchemistry field, however, the terms “outer bark” and “inner bark” have been used for “rhytidome” and “secondary phloem up to the last-formed living secondary phloem,” respectively.

111.

CHEMICAL COMPOSITIONAND NONEXTRACTABLE COMPONENTS OF TREE BARKS

As described above, bark consists of the inner bark and outer bark zones. It is therefore preferable to determine the chemical composition accuratelyfor these two zones separately. However, chemical analyses have often been made with the whole bark because separation between the zones is sometimes cumbersome and time consuming, and because for most applications the whole bark is to be utilized. It has been known that chemical analyses by standard methods for wood often give erroneous results due to the presence and variability of suberin and high-molecular-weight tannins which are rarely or never found in wood. Surprisingly, some erroneous data were published evenin 1991 [ 3 ] .Preliminary extractions or corrections for these substances are very important for the accurate analysis of the bark of trees. Some of the general and comprehensive reviews onthe chemical composition of tree barks were referred to by Laks [ l ] . More recent examples of bark analysis are listed in Table 1 to show how different the chemical compositions of the tree bark are from those of wood. In general, bark contains much more extractives, slightly less lignin, and smaller amounts of holocellulose as compared to wood of the same tree. Nonextractable components in the bark consist of polysaccharides (cellulose, hemicellulose,and pectic substances),phenolicpolymers (lignin andhigh-molecular-weight tannins), and cross-linked polyesters (suberin and cutin). Significant parts of hemicellulose

/-

rhytidomc

secondary phlocm u p to thc last fonncd pc’idclnd living sccondaryphlocm calnbiunl xylcm FIGURE 1 Suggested tcrms for the tissue zones resulting from rhytidomc formation. Rcproduccd with permission of IAWA.

Chemistry of Bark

245

TABLE 1 Some BarkAnalyticalResults

(9%of DryMaterials)

Components Species

Extract. Lignin Ash

Holocellul. Cellulose Pentosan

1 % NaOH soh. Ref.

Eucalyptus glohulrts" 0.3

B kh

5 18.6 7.9 2.0

Wd' Populus Hybrid

1

NE388d B kh Wd' Pinus pirznster

28.7

B kh19.1 I? radiata Bkh Wd' I? sylvestris

Bkh Wd' Snlix roridrr

Bkh S. rkinuycrnngi Bkh

8.0

22.3

41.8 5.6

14.1 16.7

43.2 50. I 42.9 84.5

19.6 20.5

30.6 22.5

23.2 44.0 22.7

0.5 "2q.f -26 27.2 1.8

31.4

/.Pyro/ysis, 17: 305 ( 1990). 414.

415. 416. 417. 41 8.

419.

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Chemical Characterizationof Wood

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Color and Discoloration David N.-S. Hon Clernson University, Clemson, South Carolina

Nobuya Minemura Hokkaido Forest Products Research Institute, Hokkaido, Japan

1.

INTRODUCTION

Wood is an excellent material to absorb and reflect light. This physical interaction produces wood whose color may range from almost white, as in the sapwood of many species, to almost black, as in the heartwood of black ebony. The color characteristics depend on the chemical components of wood that interact with light. Hence, the reaction of wood components to light, heat, and chemicals will change the color of wood. Extensive studies and observations have shown that most, if not all, wood species of commercial importance, and in particular those used for furniture, paneling, and decks, are prone to discolor with age. Discoloration occurs both indoors and outdoors. Manyfactors and elements participate in the discoloration of wood. In this chapter, the major factors playing a role in discoloration, as well as the methods of removing and avoiding discoloration, are discussed. II. COLOR OF WOOD

The color of wood varies with wood species. In this section, a general concept of color, the coloration of wood, and characteristics of the color of wood species are discussed.

A.

Mechanisms of Coloration

Isaac Newton, the English physicist/mathematician, said, “Rays are not colored.” Color is recognized only when a rayof light enters the eye and is absorbed in the retina by light-sensitive receptor cells called cones and rods. Visible light, which produces the visual sense for human eyes, is part of an electromagnetic wave. Its wavelength ranges from 380 to 780 nm, as shown in Fig. 1. Ultraviolet (UV) light is at the lower end and infrared (IR) light at the upper end. Visible sensitivity varies with wavelength. In a dark place, a wavelength of 500 nm can be seen. In a bright place, the wavelength must be increased to 550 nm before the human eye can distinguish it. UV light does not reach the retina because it is absorbed into the cornea or crystalline lens; IR light reaches the retina but is not registered. 385

Minemura 386

and

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Cosmic ray

Waveforradio

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Wave U e l e v i s l o n

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Y' Yellow

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700 (nm) I

R G . Green

FIGURE 1 A portion of theelectromagneticspectrumshowingtherelationship region to other types of radiation.

of the visible

When visible light strikes an object, if all the light is reflected, we recognize the object's color as white. In contrast, when all the light is absorbed, we recognize the color as black. Most materials absorb certain wavelengths and reflect the rest. The reflected light is recognized as acolor, which is dependent onthe composition and amount of the reflected light. For example, the reflection of wavelengths longer than 590 nm produces an orange color. Absorption of light by a material excites its electrons. Generally, electrons are in the lowest energy state or ground state. If adequate energy is absorbed by the electron from outside, the electron will transit toahigherenergystate, or excited state. Light is an aggregate of photons that have energy, so depending on its wavelength, it can provide the energy necessary for electron excitation and transition. Betweentheenergyofaphoton ( E , kcal/mol) and thewavelength (A, nm), the following equation can be derived:

E=

2.86

X

10'

A

Figure 2 shows the relationship between energy and wavelength. The shorter the wavelength, the higher the energy will be The electron of an unsaturated bond (e.g., )C=C(, )C=O, )C=NH, -N=N-) can transfer easily to an excited state with a small amount of energy. In molecules containing many unsaturated groupings that are all conjugated, the molecular orbitals containing the electrons in the system will extend over these groups. The resulting high degree of delocalization of the electrons means that the energy required for a transition decreases. For example, one unit of )C=C( absorbs light at 190 nm, but @-carotene, in which I 1 units overlap, absorbs the light at 520 nm to give red. An atomic group having rr electron, such as an unsaturated bond, is called a chromophore. An atomic group having isolated electron pairs, such as " O H , " C O O H , and "OR, is called anauxochrome.Auxo-

387

Color and Discoloration (kcall

160' 150

~

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130. 120 110-

F

100-

E, 5

80-

6

60.

W

go-

c

70-

50. 40.

30

200

400

600

800

Wavelength

FIGURE 2

Relationship between light energy quanta and wavelength.

chromes assist the action of chromophores by intensifying the coloration or enabling the absorption of light having a longer wavelength.

B.

Representation and Determinationof Color

The reflection curve in the visible region most accurately represents the color of a material. However, a representation with a numbered value is often useful. The numerical representation of color can be derived by two methods. One way is based on a comparison with a color specimen in which various colors are classified and numbered. Another way is based on trichromatic quality, which means that any color can be made by mixing three other colors. Color specification systems such as XYZ, Lab, L*a*b*, and UVW have been used to determine trichromatic quality [ 1,2]. There are two methods of determining color mechanically: by determining the percent of spectral reflectance and by reading tristimulus values directly. A spectrophotometer and standard white plate of magnesium oxide or magnesium carbonate are used for the formermethod.Thewhitenessof the white plate is considered to be 100%. Relative spectral distribution to it is shown by the reflectance curve in the visible region. The proportion of up-and-down areas of the curves relates to lightness. The upper part of the curve means high lightness. A photoelectric colorimeter is used for photoelectric tristimulus colorimetry. A test specimen is irradiated with a xenon light, the reflected light is collected with an integrative globe that leads to the XYZ light receiver, and it is converted to electric current by a photocell to indicate the numerical value.

I

Minemura 388

and

C. Characteristics

Hon

of Wood Color

Wood absorbs visible light. Consequently, we see a wood’s color as red, brown, yellow, and so on. Thesurfaceofwood is not uniform like metal; it is composed of cells of various sizes. Various cell volumes and the difference of components cause delicate differences of color even on the same wood surface.

Coloring Substances of Wood The main structural materials in wood are cellulose, hemicellulose, and lignin. Cellulose and hemicellulose do not absorb visible light, Native lignins that are isolated with minimum chemical or physical changes are pale yellow. In coniferous wood, lignin color can be attributed to phenyl-substituted benzoquinone and dehydrogenative co-polymers of coniferyl aldehyde. In wood, it is assumed that lignin is incorporated into a cellulose matrix and absorbs wavelengths below 500 nm [3]. Moreover, many woods absorb light beyond 500 nm due to the presence of phenolic substances such as flavonoids, stilbene, lignan, tannin, and quinone. In Fig. 3, spectral reflectance curves of woods are shown [ S ] . The lightnesses of these woods are different from each other. Numerical values of the color by the Lab specification system are alsoshown in the figure. Allof the woodsshown absorb light beyond 500 nm, and darker-colored wood absorbs more light. Ordinary sapwood has a lighter color than heartwood. The transition of sapwood into heartwood is accompanied by the loss of its physiological activity and the formation of various organic substances with darker color. When darker-colored woods such as rosewood and ebony are extracted with organic solvents, the extracted solution colors strongly, as shown in Table 1. Mostcoloredmaterials are presumablyhigh-molecular-weightpigmentswhich are insoluble in solvent [56], while the existence of colored materials for low-molecular1.

L

Q @ @ @

White birch Japanese larch Mizunara Black walnut

78.1 63.0 48.8 36.2

a 2.0 11.3 8.9 5.7

0 400

500

600

700 (nm)

Wavelength

FIGURE 3 Spectral reflectance curves and numerical color values for some woods.

b 16.3 21.5

16. 2 8.6

389

Color and Discoloration TABLE 1 Color of Dark-ColoredVeneer

Determined Before and After Extraction with Acetone

Ebony Before After Rosewood Before After

L

a

b

2.6 19.2 19.9

2.2 2.0

2.6

7.7 37.0 44.0

7.9 9.4

8.9

weightpigmentsuch known.

as mansonon F[4]and4-methoxy-dalbergion

[57] arealsowell

2. Physical Factors Affecting Wood Color a. Irradiating Direction of Light. When light is irradiated on the surface of wood, one part is reflected directly and the other part enters cells having voids and pigmentsthat absorb some wavelengths of the entering light. The light that is not absorbed in the cell is emitted again through scattering, reflection, and transmission. We recognize the unabsorbed light as the wood color. Wood cells are slender in shape and arranged in layers in one direction. Therefore, the wood color will be slightly different according to the irradiating direction of the light. Figure 4 illustrates the change in color, as shown by a Hunter Lab system, when light is irradiated at various angles toward the fiber direction of wood at an incidence angle of 45" [5]. Lightness is lowest when the direction of the progress of an incidence light is in accordance with the direction of the wood fiber. Lightness is highest when the an incident light crosses the wood fiber at a right angle. The behaviors of a and b shown in Figs. 4a and 4b are completely contrary to the behavior of lightness. Saturation is, therefore, lowest when the light meets wood fiber at right angles. Values a and b show the same tendency of increase and decrease. This means that the hue does not change. When the direction of the incident light meets the fiber direction at right angles, the quantity of the light that reflects and scatters on the surface without penetration into a cell might become larger, causing lightness to rise and saturation to become lower. b. Moisture Content. Unseasoned wood contains a significant amount of free water in its cells. When the inside of a cell is filled with water, light is transmitted deep into the cell but is scattered slightly in the cell wall. This wood color is called wetting color. As shown in Table 2, lightness is lower in unseasoned wood than in seasoned wood. The wetting color of seasoned wood is similar to the color of wood coated with a clear paint. c. Roughness of theSurjiace. If the wood surface is not even, the reflectance and scattering of light on the surface become larger, causing the lightness to rise, as shown in Table 3.

3. Distribution Sphere of Wood Color The distribution sphere of wood color for about 100 wood species, which are frequently used in wood-working industries, is shown in Fig. 5 by use of the Lab specification system.

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180

(deg)

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Angle FIGURE 4 Relationship between chromaticity index a and angle ofirradiatedlight. when light is irradiated parallel to grain; a and b are chromaticity indices.

TABLE 2 Color of Todomatsu Sapwood Determined Before and After Drying

Green wood with moisture content of 175% Wood with moisture content of 13% dried at room temperature

L

a

b

68.9

5.3

20.8

80.9

1.4

16.0

TABLE 3 Color of Yew Determined After Planing and Grinding ~~

L

a

h

53.6 58.9

18.0 15.7

22.9 22.2

~

After planing Grinding with sandpaper #320

Angle is 0

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10

0

0

5

a

10

15

20

Distribution sphere of color of various woods.

Allof the woods distribution are in positive sphere of chromaticness indices a and b. Numerical values of lightness range from 20 to 85 [5]. In Figs. 6 and 7, the correlation of three color factors by the Munsell specification system is shown [55,58]. When reddish extent becomes large, Munsell value decreases. When yellowish extent becomes large, Munsell value increases. Saturation is in proportion to Munsell value for typical tropical woods.

Wood Color and Its Use Color affects human feelings in various ways. Wood is a natural material and its color seems compatible with human life. As compared with the color of plastic and concrete, that of wood conveys peace of mind and a feeling of natural gentleness. On the earth, forest resourceshavealwaysabounded,andwoodhasbeenusedwidelysinceancient timesasamaterial in construction, farming tools, furniture,carving,and so on. Most often, wood has been used because of its pleasant color as well as its warmth, hardness, and strength. a. White Wood. The image that white projects is purity, freshness, and sacredness. White woods include poplar, mangashinoro, white lauan, igem, yellow cypress, fir, white birch, shinanoki, and the sapwood of hinoki. These woods are used in building construction, obsequies, chopsticks, toothpicks, wood shavings, and so on. Because of hinoki's fragrance and excellent durability, its sapwood is considered the best construction material for shrines and palaces. White wood can be changed to any color by dyeing. White woods with high penetration are therefore used widely as basewood materials for dyeing. Because white wood shows dirt, it is normally used in areas that people do not touch. b. Red Wood. As it appears in autumn leaves or flowers, red is a passionate color. Because red harmonizes well with green, it works well as the exterior color of a house. The red heartwoods of sugi and hinoki have the highest value as building materials in

4.

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Hon and Minemura

393

Color and Discoloration A

AA

2

1

2

3

4

5

6

7

8

Saturation FIGURE 7

Relationship between Munsell value and saturation of typical tropical woods in Asia.

Japanese-style houses and are widely used as ceiling board, wall paneling, and posts. Red birch is popular for furniture and interior doors. Rosewood and Chinese quince are widely used as decorative materials in house construction, as well as for carving, instruments, high-quality art objects, and so on. c.Yellowish-BrownWood. Yellow and orange colors convey warm, homelike feelings. Teak and keyaki are widely used as building materials and for furniture. Tsuge is used for stamps and combs. Keyaki, which has a peculiar pattern, is the most precious wood. d.Light Brown Wood. Mizunara is a light-colored, expensive material. Mizunara and nire are used for furniture as well as window frames, building construction, highquality goods, and so on. Trend in color furniture has been considered an indication of the economic climate. For example, if light-colored woods such as mizunara are popular, it implies economic recovery. In contrast, if dark colors such as deep red are popular, it suggests an economic recession. e.BlackWood. Blackconveys the impressionoforderandcalm.Ebony is used for carvings, furniture, decorative materials in home construction, Buddhist family altars, and so on. 5 Contrast of Color in Sapwood and Heartwood. The color of heartwood and sapwood differs in manywoods.Thisdifference is often used decoratively. Forexample, decorative pillars of ebony display the brown color of heartwood and the white-yellow color of sapwood; decorative pillars of hinoki utilize the natural contrast of red and white. Such a contrast also is emphasized in fancy goods, souvenirs, cufflinks, and so on.

111.

DISCOLORATION OF WOOD

A.

Types of Discoloration

As wood is a biological material, it is decomposed by microorganisms and reacts chem-

ically when it comes into contact with substances such

as metal ions, acid, and alkali.

Hon and Minemura

394

Because wood is porous, water-soluble substances or salts are often deposited in its voids during the course of growth or after logging. Such deposits can change the wood’s color. Except in the caseofdecay by microorganisms,discoloration does not meanan accompanying loss of wood strength and is usually limited to the surface layer. Because wood color is an important factor that strongly affects value, discoloration is a serious problem from the viewpoint of commercial worth. Table 4 summarizes the characteristics of discoloration according to the factors that influence the change in color.

B. Characteristicsof Various Types of Wood Discoloration 1. Discoloration by Light A newspaper left in a window turns yellow in a few days. If a calendar is hung on a wall of natural wood, the part covered by the calendar retains its original color, whereas the uncovered part changes color. Photo-induced discoloration is consideredundesirable in manycases.Discoloration is alsoafactor in the manufactureofhigh-yieldpulps that contain a high amount of lignin. In this section, the characteristics of photo-induced discoloration of various wood species and methods for preventing it are described. a. Discoloration Behavior of Various Woods under Light Irradiation Classification of thePattern of Photo-Induced Discoloration. Every wood changes color with light irradiation, but the rate and course of change varies with wood species. Tables 5-8 show the quantity of photo-induced discoloration and the declining rate of whiteness of 100 commercially used wood species [ 5 ] . These results are obtained from the accelerating test by use of a carbon arc light as the light source. The measured values are classified according to the difference in discoloration with elapsed time. The color differences are calculated with the Lab system on the basis of the original color before light irradiation. A large color difference means a large amount of discoloration. Lowering rates of whiteness are calculated by dividing the difference in whiteness after light irradiation by the original whiteness before light irradiation. An increase in the lowering rate of whiteness indicates darkening; a decrease indicates lightening. As shown in Tables 5-8, discoloration is classified into five patterns in an elapsed time of 100 h: darkening only, darkening and then fading, darkening-fading-darkening, fading only, and fading and then darkening. The quantity of photo-induced discoloration after 100 h of exposure in most woods is beyond A E = 3, which is the limiting value that can be distinguished by the naked eye. The value of the most intensive discoloring wood is AE = 25. The relationship between the chromaticness index a of the original wood color and the quantity of photoinduced discoloration is shown in Fig. 8. It is well known that the magnitude of the a value has a positive correlation with the extent of redness. As illustrated in Fig. 8, woods with a white color show significant darkening. Many softwoods continue darkening with light irradiation, and many tropical woods discolor with a mixture of darkening and fading. In woods an intensive discoloration at the initial stage often is attributed to extractives. Change of Hue or Saturation. The discoloration of the above 100 wood species is classified by the change in a. The elapsed changes of hue andsaturation are also outlined as follows [ 5 ] . l.

Group showing an increase in a with light irradiation. Many woods in this group increase in saturation and discolor toward orange. As illustrated in Fig. 9, the value of b shows a small decrease at an initial state of irradiation, followed by a significant increase. It decreases again after 50 h of exposure. This tendency

TABLE 4 Characteristics of Wood Stain Classification After logging

-

Addition of source of stain from the outside

Cause of stain ~

Biological source

Propagation of microorganism

Blue stain

Chemical source

Bonding of metal ion Bonding of acid Bonding of alkali Heating Irradiation of light Metal ion Enzyme Resin Imperfect pruning Deposition of substance

Iron stain Reddish discoloring of zelkova Adhesion of cement Sticker mark Discoloration by sunlight Blackish discoloring of sugi Red discoloration of alder Exudation of resin Brown stripes Existence of specks

Physical source Immanence of source of stain In shade

Example

~~~~

Hon and Minemura

396

TABLE 5 Quantity of Photo-Induced Discoloration and Declining Rate of Whiteness of Woods After Exposure to Carbon Arc Light After exposure for 100 h

Species Yellow cypress Sitka spruce Aspen Douglas fir Lawson cypress Noble fir Listwennitza, Larix dahurica Corean pine Red cedar Western hemlock Red oak White lauan Champaka New Guinea basswood Amberoi White cheesewood Tetrameles Manggasinoro Evodia Ipoh Celtis Ramin Canarium Sterculia Kedondong Antiaris Agathis Japanese poplar Todomatsu, Abies sachalinensis Shinanoki, Tilia japonica Shirakaba, Betula phatyphylla var. japonica Ezomatsu, Picea jezoensis Plane Hinoki, Charnaecyparis obrusa Buna fagus crenata Japanese yew Hiba, Thujopsis dolabrata Japanese larch Magnolia Painted maple Zelkova Camphor tree Sen, Kcdopanax pictum Japanese red birch Radiation on wood surface: 4032 cal/crn'.

Color difference (AE)

Declining rate of whiteness

23.4 21.7 21.6 19.1 17.2 16.1 15.2 14.8 14.4 11.5 7.7 21.1 20.8 19.8 19.7 16.9 16.1 13.5 13.3 12.3 12.0 10.9 10.1 7.2 6.7 6.3 5.4 24.7 23.8 21.8 21.3 21.2 20.3 18.5 18.3 17.5 15.2 14.7 13.7 11.2 9.8 9.0 6.2 5.5

32.9 19.3 27.5 30.3 25.9 26.2 23.2 24.5 27.4 18.2 13.8 28.4 32.6 26.8 26.1 22.6 26.2 22.4 17.8 15.4 15.9 17.6 17.4 7.4 10.6 9.3 9.8 31.4 33.7 27.8 26.7 30.2 23.5 29.5 25.1 20.1 20.8 25.7 22.8 19.4 14.3 13.3 9.0 10.5

(%)

397

Color and Discoloration TABLE 6 Quantity of Photo-Induced Discoloration and Declining Rate of Whiteness of Woods That Change from Dark to Faded and to Dark After Exposure to Carbon Arc Light After exposure for 100 h rateDeclining Color difference Species Redwood Jongkong Teak Santiria Yellow hardwood Terminalia Spondias Miwa mahogany Elaeocarpus Myristika Trichadenia Box wood Aogatsura, Cercidiphyllumjrcponicurn (dark) Formosan cypress Kiri, Paulowina tornentosa Kihada, Phellodendron arnurense Sugi, Cryptomeria japonica Swamp ash Higatsura, Cercidiphyllurnjnponicurn (pale) Japanese alder Values in parenthesesindicatehigherdiscolorationwithin determined after exposure of 1 0 0 h.

(

of whiteness

W

9.5 13.0 (13.6) 12.9 9.4 8.3 7.3 7.3 6.3 (6.7) 5.7 5.4 (7.1) 4.6 12.0 11.5 8.5 8.2 7.4 (7.9) 4.8 4.7 4.7 (5.1) 4.0

15.3 23.3 20.9 16.3 10.0

13.3 11.3 8.9 1.7 5.9 4.0 16.9 21.3 15.6 7.2 6.9 9.0 1.5

8.1 1.1

1 0 0 h of exposure,comparedtothevalue

often is observed in white wood, accompanying a high degree of discoloration. Spruce, Douglas fir, and todomatsu belong to this group. This group also contains woods that have only a decreasing value of b as well as no change in this value. At the final stage of light irradiation, woods in this group are deep red in color. 2. Group slightly showing changes in a . This group generally evidences significant fading. The value of b increases greatly and the color changes toward yellow. Rosewood and walnut belong to this group. 3. Group showing a decrease in a. Some woods in this group also show a decrease in the value of b and are nearly achromatic in color. Japanese yew and Chinese quince behave like this. In this case, lightness decreases and whiteness declines as the irradiation time increases. Otherwoods in this groupshow an initial decrease in h, followed by an increase. Many tropical woods with a dark color belong to this group. 4. Groupshowingrepetition of increaseanddecrease in thevalue of a. Many woods in this group show a slight photo-induced discoloration. The woods in this group also often show repetition of increase and decrease in the value of h.

Change of Lightness and Whiteness. Woods that alternately show darkening and fading have, in many cases, slight photo-induced discoloration. The quantity of photo-

Hon and Minemura

398 TABLE 7 Quantity of Photo-Induced Discoloration and Declining Rate

of Whiteness of Woods That Change from Dark to Faded After Exposure to Carbon Arc Light

After exposure for 100 h Color difference Species Melapi African mahogany Silkwood Nato Rosewood Pterocarpus Andes rose Artocarpus Kapur Calophyllum Malas Red lauan Rengas Kossipo Sapele Taun American walnut Eugenia Dao Zebra wood Sloanea Eurasian teak Kingiodendron Ebony Dracontomelon Maniltoa Yamaguwa, Morus bornb.ycia Shiurizakura, Prunus ssiori Locust tree Mountain cherry Mizunara, Quercus crispuln Japanese walnut Japanese hophornbeam

(

W

21.6 (23.6) 15.3 (15.9) 11.4 (12.6) 1 1.4 (15.5) 10.4 (9.5) 9.9 (8.2) 9.6 9.4 (10.3) 9.3 (10.4) 9.3 (10.6) 8.4 (8.8) 7.5 (8.0) 5.9 5.7 (4.5) 5.7 (6.9) 5.6 (6.4) 5.3 5.0 (7.7) 4.4 (5.0) 4.2 (4.5) 3.7 (7.1) 2.1(4.0) 4.0 (3.1) 3.5 (0.9) 2.1 (4.0) 2.0 (3.0) 19.9 (22.8) 12.9 (13.8) 11.7 (13.0) 8.7 (10.3) 5.0 (7.5)

Declining rate of whiteness (96)

30.0 28.8 18.8

17.3 12.2 4.7 -27.5 13.1 13.7 12.6 1.1

13.5 -5.2. 1.6 5.1 5.1 - 5.6

5.5 (1.8)

1.7 6.4 5.2 2.6 1.9 -6.3 - 15.6 -2.5 -5.1 26.7 21.4 16.3 13.3 8.6 -5.3

2.2 (4.9)

-0.5

Values in parentheses indicate greatest photo-Induced discoloration within exposure

of 1 0 0 h.

induced discoloration is calculated based on the total difference of the lightness and chromaticity indices before and after light irradiation. As lightness generally changes more than chromaticity indices, lightness and the quantity of photo-induced discoloration have a strong mutual relationship. Whiteness is also calculated from the sum of the lightness and chromaticity indices. As the lightness of wood is higher than chromaticity indices, whitenessandlightnesscorrelate in mostcases. In the case of asmallchange in the chromaticity indices, the declining rate of whiteness and the quantity of the photo-induced

399

Color and Discoloration TABLE 8 Quantity of Photo-Induced Discoloration and Declining Rate Whiteness of Woods That Fade After Exposure to Carbon Arc Light

of

After 100 h of exposure Declining rateColor difference Species

(

Fading only Rosewood Indian rosewood Fading-darkening Teijsmanniodendron Nire, Ulmus davidiann davidiana var. japonica

26

8.2 5.2

-9.7 -1.1

0

. A

8

18 -

A

0 0

0 0

0

1614 -

t L

12 -

5

10

V

- 30.3 -50.8

0

20 -

L

15.9 12.7

0

22 -

2

of whiteness (%Io)

-

24 -

%

W

0

0 0

0

00 A

B 0 0

A

o

-o

a

0

6 4 -

d o

0

0

02

AAA A 0

A

2 -

A

ta

0

0

8 -

A

A'

0 " " " " " ' 0 2 L 6 8 18 10 161412 a ChangeInduced by exposure 0 ; Darkening (0) , b ; D + Lightening (L) 0 ; D+L+D. '

FIGURE8 Relationship between chromaticity index LI and quantity of photo-induced discoloration for various woods during exposure t o a carbon arc light for 100 h.

400

Hon and Minemura

26 -

24

b

-

22-

/

20

-

l8 L 2

phenolic and wood substances (PI)

of iron ion

NaH,PO, ferrous phosphate substances phenolic hv oxalic acid

" +

NaH,PO,is a weak acidic substance that keeps the woodsurface in weak acidic condition after treatment with oxalic acid. It is possible to use phosphoric acid instead of oxalic acid. In such a case, however, the surface acidity is high. A weak acidic substance such as NaH,PO, must be used simultaneously to reduce the surface acidity. For some woodswith light stains, it is possibletoremove iron stains with NaH,PO, alone.The bonding strengths of iron ions and phenolic substances probably differ according to the kinds of phenolic substances involved.

422

Hon and Minernura

Color difference 0

Inthe

air

In N, gas

AE

10

20

'/////////////////////////=I

b

FIGURE 28 Quantity of photo-induced discoloration of iron-stained mizunara decolorized with oxalic acid when exposed to sunlight with an intensity of 400 mW min/cm2 in the UV region.

e. Prevention of IronStain. Because removing iron stains requires a great investment of labor, time, and chemicals, preventing the stain represents a better use of such resources. The main methods of preventing stains are as follows:

1. Preventingcontactwithiron-containingsubstances 2.Capturing iron ions 3. Controlling iron ionization 4. Usingasubstitute

These prevention methods are described here according to the wood processing involved. Prevention During Veneer Manufacturing. The mud that has adhered to the surface of a log or flitch must be removed carefully prior to boiling, and the vat for boiling must be made of stainless steel or concrete. Steam pipes madeof stainless steel or titanium must be used. If pipes made of iron are used, they must be coated sufficiently. If the water used contains iron ions, a chelating agent such as EDTA-2Na or a weak acidic phosphate

20 -

b

-

15-

e" -e

Unstained

Oxalicacld Oxalic acld lSodiumdihydrogenphosphate figure (Numerals In the mean exposure time.) e--.-.

10 L

5

4

loo

l

10

15

a FIGURE 29 Photo-induced discoloration of iron-stained zelkova decolorized with chemicals when exposed to carbon arc light for 100 h.

423

Color and Discoloration

0

Unstained

A

Oxalic acid

0 Oxalic

acid phosphate

0

+

Sodium dhydrogen-

50 75 Exposure time to carbon arc iight 25

100 (hr)

FIGURE 30 Quantity of photo-induceddiscoloration of iron-stainedmizunarathatwas ized with chemicals, painted with polyurethane, and exposed for 100 h.

TABLE 16 BondStrength of PolyurethaneFilms on IronStained Woods That Were Decorated with Chemicals used

rol)

Chemicals Water Oxalic acid Oxalic acid

(kgf/cm2)

+ dihydrogenphosphate

11.5 (36) 6.8 (48) 10.2 (64)

Numerical values in parentheses show the percentage of delamination area between wood and film.

decolor-

Minemura 424

and

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such as NaH,PO, should be added. If EDTA-2Na is used, it should be added in the amount of 0.5 g per gram of iron ion, as shown in Fig. 31 [32]. If water is dark brown in color due to the contamination of iron ions, the addition of aluminum potassium sulfate or a high-molecular-weight coagulant can remove the iron ions. If there is a possibility that the knife can break when a rotary lathe is used, the sliced veneer should be immersed in an EDTA-2Na solution. If condensed water drops from the surface of cold machinesin winter, the machines should be heated before use or the surface should be coated with paint. In the manufacture of laminated veneer lumber with a sidedriving system, gears made of stainless steel should be used. If veneer touches metal fittings fastened to the carrying belt when green veneer is carried to the dryer, the fittings should be coated or covered with vinyl tape. When a dryer is used, the wire netting on which the green veneer is placed should be made with stainless steel and the temperature of the hot wind should be kept above 140°C. High-temperature air brings on rapid vaporization of water from the veneer and does not produce a stain. High temperature also helps control deterioration of the exhaust pipe, because the moisture-containing acid component of the wood does not condense on the pipe. Prevention During Plywood Manufacturing. Because adhesives used in plywood production are water-soluble, it is desirable to useenameledironware or vessels made with stainless steel or plastic in preparing the glue. If a press is used in the gluing process, analuminum plate or duralumin plate withgoodheatconductivityshould be inserted between the plywood and hot plate to prevent staining. Prevention During Working of Lumber. To prevent stains where the surface of green lumber touches steel belts during packing, a piece of wood or cardboard should be inserted between the belt and the lumber. When packing a small amount of lumber, plastic tape should be used. If an iron vessel is used for mold-proofing wood with water-soluble chemicals, a vessel made from a thin plate of polyvinylchloride or stainless steel should be placed inside the iron vessel. A wooden vessel covered with thick polyethylene sheets can be used instead of an iron vessel. During the process of laminating wood, both the vessel used to prepare glue and the instrument for coating should be iron-free

VI Q,

20

x

EDTA.4Na No added

0

Q,

0 C

X

ul fd

5-10 ' ' ' -0.01 0.3 0.5 1.0 Concentration

8

L

1.5 0.01 0.3 0.5 1.0 ("'0) Concentration

1.5 ( %)

FIGURE 31 Relationship between concentration of EDTA solution and quantity of stain. Wood specimens were immersed in 1000 cm' of 0.01% ferric chloride solutionwith 10 cm7 of EDTA solution of various concentrations.

Color and

425

Stain Control in House Construction. When flooring is fixed on unset concrete, metal fittings containing less iron should be used. Nails made with stainless steel or brass should be used to nail wood on fences or outdoor wallboard.If iron nails are used, colored nails are preferred. The nail heads should also be coated. Metal fittings used for wooden entry doors or windows should be made with copper, aluminum, or stainless steel. Stain ControlDuring Furniture Manufacturing. If areddishsealer is required in the sealing process, a sealer that contains no ferric oxide should be used. If a sealer containing ferric oxide is used, EDTA-2Na should be added. 3. Discoloration by Acid Acid stain is caused by acid interacting with wood. Acidic substances are not used very often in woodworking processes. Therefore, acid stains do not occur very frequently. a.Occurrence of AcidStuin in WoodworkingProcesses. Aminoalkyd resin is widely used as an abrasive-resistant and inexpensive paint for wood coating. This paint is mixed with two liquors immediately before use. One of the liquors is a hardener such as paratoluenesulfonic acid. Wood paintedwithexcesshardenerand left in sunlight often turns red. Zelkova is usually used as a thin veneer because it is expensive but has a fine grain. It is stained easily with iron, and the surface of the flitch becomes black. Therefore, the flitch is often dipped into a solution of oxalic acid prior to slicing. When sliced veneer is glued to the base wood, the edge that contacts the solution of oxalic acid often turns red. Urea formaldehyde resin is moderately waterproof and is used widely as an inexpensive wood adhesive.Ammoniumchloride is added to it immediatelybefore use. This chemical reacts slowly with the formaldehyde in the resin and produces hydrochloric acid. On rare occasions, a plywood surface with this resin on it may turn red. b. Factors Affecting the Occurrence of Acid Stain pH. The acids involved in acid staining during woodworking are hydrochloric acid and oxalic acid. The extent of stains that occur when several woods are immersed into acid solutions with various pH values and then left indoors is shown in Fig. 32 [39]. In the pH range from 5 to 2, all wood species show weak stains that are not recognizable to the naked eye. However, at pH below 1.5, strong red or reddish-purple stains occur. Light. Figure 33 shows the change in color of four kinds of wood after immersion into a solution of oxalic acid with a pHof I , whichwere left to dry in a dark place, indoors or under the light of a mercury lamp [39]. Under the mercury lamp, a maximum color change was obtained after 5 min of exposure. With indoor exposure, 5 days were required to obtain the same extent of staining. When stored in a dark place, a slight change of color occurred. However, when the immersed specimens were exposed to indoor light, the colorchanged rapidly to reach the sameextent of staining as the indoor-exposed specimen. From this, it is evident that discoloration caused by acid stain is accelerated by UV light. Oxygen. When wood is immersed in a solution with a pH of 1 and then irradiated with a mercury lamp under nitrogen atmosphere, it shows the same extent of staining as wood irradiated in air [39]. This clearly indicates that oxygen does not participate in the stain’s development. Wood Extractives. Table 17 shows the relationship betweentannincontent and stain [39]. Softwood shows high staining, which might be due to the existence of condensed tannin. Catechol tannin causes acid stains. Because bubinga and koa contain leucoanthocianin, which easily turns red with acid, this substance might be the cause of the stains in both woods. When woods that show a significant stain are treated with acid after

426

Hon and Minemura

o\

15[

0 ;Sugi 0 ;Akamatsu

A ; Manggasinoro

-

0

1.0 2.0 3.0 4.0

5.0

PH FIGURE 32 RelationshipbetweenpH of oxalic acid solution usedforimmersion color difference after l-week exposure under indirect sunlight.

of woods and

hot-water extraction, they do not discolor. This indicates that the source of the acid stain is not lignin but a phenolic extractive. c. Removal of Acid Stain. If the stain is limited to the very top surface, it can be removed by planing or by sanding with sandpaper. For deeper stains, destruction with a bleaching agent or neutralization with alkali is effective to a certain degree. The use of sodium chlorite is desirable because it acts under acid conditions. Other bleaching agents such as hydrogen peroxide or sodium hypochlorite can also be used. Sodium bicarbonate or calcium carbonate can be used for neutralization. d. Prevention of Acid Stain. If paints or adhesives that harden with acid are used, the amount of hardener added should be kept to a minimum. To produce adhesion with

Indirect sunlight (thinline:darkplace)

Mercury lamp lot

.

0

10 20 Exposure time

30 (day)

0

----x-------

5 (hr)

10

15

time Exposure

FIGURE 33 Relationship between exposure time and color difference after soaking in oxalic acid solution at pH 1 . 0 : Sugi; 0 : Akamatsu; A: Manggasinoro; X: Buna.

427

Color and Discoloration TABLE 17 Sensitivity of Wood Species to AcidStain

Stain grade

Species

Akamatsu Strong Buna Bubinga Koa Painted maple Hinoki Sugi Douglas fir Medium Lawson

Weak

cypress Japanese red birch Manggasinoro Magnolia Sawagurumi Black walnut Japanese chestnut Kiri Swamp ash Mizunara Teak

Tannin content (%)

Color difference ( A E )

0.1 0.4 -

15.3 13.3

0.6

8.9 13.8 10.0

0.3

10.6 6.3

0.3

10.9

0.2 0.3 0.2 0.4

7.4

2.1 2.0

4.7 3.1

-

1.9 5.O

0.1

0.6

6.3 5.9

6.3

5.6

3.1 3.1

0.4

1.5

0.2

heat in plywood production, penetration of acid into the veneer should be prevented by raising the viscosity of the glue, diminishing the coating amount of the glue, lowering the moisture content of the veneer, and lowering the pressure and temperature. When oxalic acid is used to remove iron stains, sufficient washingwithwater or the addition of NaH,P04 is required.

4. Discoloration by Alkali Alkali stain is the discoloration caused by the reaction of alkali chemicals with wood. This stain is observedmoreoftenduring the useofwoodproductsthan in woodworking processes. a.Examples of Occurrence of theStain. Freshconcrete is strongly alkali. When wood contacts it in the presence of water, an alkali stain often occurs. Flooring is often bonded on concrete. If water overflows on the floor and reaches the concrete layer, the water in concrete becomes alkaline and penetrates into the flooring to form a brownish alkali stain. When an excess amount of alkaline water penetrates, even the surface of the flooring becomes discolored. A plate made with calcium silicate is used often as a flame-retardant board. On the plate, a decorative veneer with fine grain is often laminated and used in the interior field. The base plate is inorganic and alkali. When glue is coated on the base plate, the alkaline substances in the plate dissolve and react with the veneer to form a brownish stain. b. Factors Affecting the Occurrence of the Stain pH. Figure 34 shows the extentofalkalinestains in fourkinds of woodwhen immersed in solutions with various pH levels of calcium oxide or sodium hydroxide [40].

Hon and Minemura

428 Calciumhydroxid

30

$ 25

0 l

(L,

Sodium hydroxld

t

I

X

0 ,Sawagurumi X 0; Buna

/

"20

C

Q,

Z .U

15

-0b 10 V

5 9.0

1ao

11.0 12.0 13.0

FIGURE 34 Relationship between pH of an alkaline solution and quantity of stain when immersed in the solution for 5 min and then dried in air in a dark place.

Under pH 11.4, little staining occurs, but beyond this pH rapid discoloration is observed. The color differs according to pH. For example, the color of sugi is reddish-brown up to pH 12.5, but beyond this pH it becomes bluish. Light. The effect of light on stains is shown in Fig. 35 [41]. Woods immersed in a solution of pH 12 (in Fig. 34) were left in a dark place, indoors, and under a mercury lamp. The wood left in a dark place retained its original color at immersion, whereas the wood left indoors faded in color and the wood under a mercury lamp showed stronger fading. These results show that light is not required for staining to occur. The opposite is true with acid stains. Oxygen. Whenwoodimmersed in an alkali solution isleftin a nitrogenatmosphere, the extent of discoloration is less than in air. When wood in a nitrogen atmosphere is taken out and left in the air, it discolors to the same extent as wood left in the air from

2ot d" e"--,

Indirectsunlight (thin line : dark place)

Mercurylamp

-0

FIGURE 35 Relationship between exposuretimeand color differenceaftersoaking hydroxide solution at pH 12. 0 : Sawagurumi; 0 : B u m ; A: Sugi: X : Douglas fir.

in calcium

Color

429

the beginning. This means that the production of an alkali stain requires oxygen and the colored substance is formed by oxidative polymerization [40]. Wood Extractives. Table 18 shows the relationship betweentannincontentand amount of staining. In the 16 wood species tested, woods with high tannin content had more of a tendency to stain [41]. It has been found that if sufficient extraction with hot water is completedbefore the wood is immersed in analkalinesolution, the stain is scarcely recognized. Only a small yellowish-ocher stain occurs above pH 13. From these results, it can be surmised that the alkali stain is due mostly to the water-soluble phenolic components. Lignin also participates in discoloration under alkali conditions. c. Removal ofAlkali Stain. Most alkali stains occurring in a short period of time can be removed with bleaching agents. The stain on the surface of concrete blocks with alkalinecementdescribedearliercanbe easily removedwithhypochloritesolution, as shown in Table 19 [42]. Concreteblocks are highly alkali-resistant, but notvery acidresistant. Because the surface is rough and has a lot of voids, it is very difficult if not impossible to dissolve the colored substances of the stain with an alkaline solution. Hypochlorite is an alkaline bleaching agent;its solution can be used to decompose the colored substances without damaging the block. Stains on the top layers of wood surfaces can be removed by planing or sanding. Stains at a certain depth that cannotbecompletelyremoved by these methods can be bleached with a bleaching agent or coating with dilute acid. d. Prevention qf AlkaliStain. Plywood that does not discolorwith the alkali of cement or plywood coated with alkali-resistant paint can be used to frame concrete for hardening. When a decorative veneer is laminated on the alkaline inorganic plates, the use of an adhesive film is desirable. When a water-soluble adhesive is used, the following methods should be considered: increasing glue viscosity, diminishing water, lowering the moisture content of veneer, decreasing pressing time and temperature, coating the plate with an alkali-sealing paint, and so on.

5. Discoloration by Microorganisms Approximately half of wood components are carbohydrates, and in green wood they contain a moderate volume of water. When green wood is left under certain conditions, microorganisms propagate on the wood. This often is accompanied by the discoloration or lowering of wood strength. The microorganisms that cause discoloration are bacteria, mold, and basidiomycetes. Mold discolors the surface of wood but does not diminish its strength. Basidiomycetes cause a decline in strength. Among the basidiomycetes are brown-rot fungi and white-rot fungi. The former mainly decompose cellulose and hemicellulose, whereas the latter also decompose lignin. Bacteria may occur in stored wood when it is immersed in or sprayed with water. Discoloration is due to the pigments of the microorganism or coloring compounds produced by the reaction of the woodcomponentswith the secretions of the microorganism. a. ExcImples of Occurrence ?f Stains in Woodworking Processes or Wood Products. When fresh green lumber is stacked on a warm day under high humidity, many colonies of mold with various colors can grow on the surface of the wood overnight. When sliced veneer is transported without drying, discoloration caused by fungi and bacteria can occur on the surface. When logs are piled outdoors for a long period of time, brownish discol-

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TABLE 18 Sensitivity ofWoodSpeciestoAlkalineStain

Color Stain grade

gurumi

ash

and

Strong

Medium

Tannin content (%)

(

m

Mizunara Black walnut Buna Douglas fir Sugi Mizunara Painted maple

(heartwood) (heartwood) (heartwood) (heartwood) (heartwood) (heartwood) (sapwood) (heartwood)

2.1 5.6 2.0 0.4 0.3 0.3 1.2 0.6

25.6 11.9 9.5 20.9 15.2 15.3 18.2 16.0

Kiri Lawson cypress Lawson cypress Japanese red birch Manggasinoro Akamatsu Teak

(heartwood) (sapwood) (heartwood) (heartwood) (heartwood) (heartwood) (heartwood)

0.6 0.1 0.2 0.3 0.2 0.1 0.4

9.3 12.7 4.1 7.7 5.8 3.3 3.6

Magnolia Magnolia Hinoki Sugi

(heartwood) (sapwood) (heartwood) (heartwood) (sapwood)

0.2 0.2 0.4 0.1 0.1

2.9 1.5 1.6 8.2 3.4

Swamp Weak

oration may occur in the cross sections of both sapwood and heartwood. This stain is caused by basidiomycetes. A blue stain is observed only in sapwood and does not bring about a decline in strength. Ilomba wood often discolors after felling to give a reddish-brown color. This wood is normallyallsapwoodandcontains substrates which are suitablefor the growthof bacterial [68].Some bacterials propagated on the lumber produce ammonia as a metabolite and form colored substances by the reaction of the components with ammonia [43]. For wood components which are responsible for the discoloration, (+)-catechin and (-)-epi-

TABLE 19 Decoloring of Alkaline-Induced Color Substances of Wood on Concrete Block

Effect of

oring chemical Coated sol.(CIO)? 2% Ca 5% NaClO sol. 15% H 2 0 2sol. (pH 10) 10% NaOH sol. 0:

Excellent.

A: Common. X:

Poor.

0

0

A X

Color

431

catechin are confirmed [69]. In the brown-stained region of hemlock, dark-pigmented fungi are predominant.Thesefungiinducebrown discoloration in the sapwood.Browning is accompanied by an increase in pH from 5 to 7, a decrease in total soluble phenols, and oxidation of phenols such as catechin. Intensive discoloration occurs at pH 7, and oxygen is indispensable for the development of the discoloration [70-721. Brownish discoloration in beech wood is caused by bacterials which produce ammonia to give a pH 7.3 [73]. Yellow discoloration of oak heartwood is caused by mold fungus. It is assumed that metabolic compounds of the fungus react with hydrolyzable tannins and give yellow substances [74]. From the blue-stain fungi, the dark coloring pigments have been isolated. They are classed with the group of melanins and are associated with carbohydrates and proteinaceous components [75]. Concerning pink stains of angiosperm and gymnosperm woods caused by fungi, a red pigment has been isolated and identified as 5,8-dihydroxy-2,7-dimethoxy1,4-naphthalenedione [76]. Some injurious insects penetrate tropical woods to the inside. Such insects often carrymicroorganisms.Forexample, in aplacewhere Limnoria lives, there is awhite corpse of Ambrosia beetle. When the insect moves in the tangential direction in wood, the discoloration is dappled in the radial section and striped in the tangential section. Larvae of the sugi bark borer feed on the wood of living sugi trees and induce discoloration [77]. In the discoloredsapwoodand in the reaction zone of the soundsapwood-discolored sapwood boundary, potassium and magnesium begin to accumulate within one year, and calcium within two years. Discolored sapwood has a greater cation-exchange capacity. b. FactorsAffectingtheOccurrence cf Stains. Thefollowinggrowth factors are essential for the propagation of microorganisms: water, air (oxygen), moderate warmth, and nutrients. Wood itself is a nutrient. Generalgrowingconditionsare3-40°C, 90% relative humidity, and 20- 150% wood moisture content. c.Removal of Stains. Stainscaused by moldcan beremoved by planingor by coating with a bleaching agent. Because stains causedby basidiomycetes often occur deep in wood, they cannot be removed completely. In such cases, it is effective to immerse the wood in a bleaching agent. For example, the brownish stain on a shina log that is caused by invasion of basidiomycetes can be removed by immersing the log in a dilute solution of sodium hypochlorite for several hours [44]. For removing fungal stain of ponderosa pine sapwood, 2% hydrogen peroxide solution with sodium hydroxide and sodium silicate as a buffer give a good result [78]. Comparedwithchemical fungicides, biological control is generallybenign to the environment.Concerning the biological control of sapstain fungi,metabolitesobtained from two fungi were examined on stained pine veneer disks and it was found that they remove sapstain and kill existing fungal growth [79]. d. PreventingStains Addition of GrowthInhibitors. To preventstains, it is effective to adhere preservatives or antimold agents to wood. This can be done by coating, spraying, immersing, and pressure impregnation. The chemical should be selected on the basis of low toxicity and slight color. Organic compounds containing tin or iodine are soluble in organic solvents, and solutions of these materials have good permeability to wood. For prevention of mold growth during drying, pretreatment of green lumber with propionic acid is recommended [74]. As a preventing chemical for brown stain i n hemi fir, a quaternary ammonium compound, didecyldimethylammonium chloride, is effective [SO]. The preservative

Minemura 432

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treatmentmustbedoneassoonaspossible after sawing. Prior to outdoor storing, the treated wood should be kept away from sunlight and rain for at least one day. Controlling Growth. Stain preventioncanalsobeachieved by minimizing the growth factors described earlier. To reduce moisture, green fresh wood should be piled in a location with sufficient ventilation and transferred quickly to the seasoning process. To reduce the amount of air, logs should be stored in water or sprayed with water to cut off oxygen. Logs can be stored in snow and covered with sawdust or plastic foam to reduce the temperature. Sliced decorative veneer should be kept at a low temperature or dried with high frequency. The drying must be completed with bundling in order to prevent splitting. Lumber should be dried as soon as possible after sawing. Obstructing the Penetration of Microorganisms. Covering the cross section of the log with preservatives will prevent the penetration of microorganisms [45]. When wood is coated with preservatives, recoating with elastic paint such as polyurethane is even more effective. It is also important to keep the working place clean. Decayed wood and wood wastes must always be removed. It also is desirable to irradiate with UV light at night and to spray fungicides regularly.

6. Discoloration by Enzymes Variousenzymes in wood participate in manymetabolismsystems.Someenzymes are active even after logging. In sawing or veneering, when fresh green wood makes contact with oxygen, these enzymes often take part in discoloring the surface of the wood. U . Exurnples of Stains Occurring in Woodworking Processes. Alder generates reddish-orange discoloration immediately after felling. This discoloration is caused by interaction of catechol oxidase and hirsutoside, which is a xyloside of diarylheptanoid containing two catecholic nuclei [81]. Shina is widely used for plywood production. When veneer sliced with a rotary slicer is left without drying for several hours, the surface often becomes yellow. When sliced walnut veneer is allowed to stand in the same manner, it becomes black. The fresh green lumber of todomatsu become yellow. Kiri wood is widely used for furniture in Japan. This wood changes color to dark brown when sawn immediately after felling. This discoloration might be caused by catalytic oxidation with oxidase. Caffeic acid sugar esters have beenisolated as the compounds responsible for discoloration [82,83]. Peroxidase consists of heat-labile and heat-resistant enzymes. The activity of the latter enzyme occupies about 12% of the total [84]. Concerning brown stain in sapwood of Douglas fir, its enzymatic extract showed two pH optima for activity (pH 5.5 and 8.0) and highest activity at 35°C. It showed also highest activity for (-)-epicatechin and dihydroquercetin [85]. When beech wood chip is stored outdoors, it changes color to deep brown after a few days. As beechwoodcontains sufficient amounts of activeperoxidaseandmalate dehydrogenase,phenoxy radicals are formed in the lignin andsubsequently new chormophores are formed [86]. b. FuctorsAffectingDiscolorution by Enzymes. Moistureandhumidity significantly affect discoloration. For discoloration to occur, the surrounding humidity must be about 100%. Temperaturealsoinfluences discoloration, with discoloration occurring slowly below 20°C. Phenolic substances in wood might oxidate to colored substances by means of enzymes in the wood and oxygen in the air. c. Stuin Removul. Yellow stains can often be removed by bleaching with hydrogen peroxideor extraction with hot water. Because the sapwood of todomatsu is usedfor chopsticks, extraction with hot water is recommended.

hemical

Color

433

d. StainPrevetztiot?. In order to prevent the stain, it is necessary to create an environment in which the enzyme does not act. An enzyme is a protein and undergoes an irreversible change when it is heated or comes into contact with some chemical substances. To remove the yellow discoloration of shina and the sapwood of todomatsu, immersion of the wood into boiling water for half a minute is effective [46]. Radiation with microwaves in an oven also works. These treatments are recommended for woods used with foods. When treating with heat, a rapid rise of temperature is required. When a lotof wood is immersed in a small quantity of hot water, the temperature falls rapidly and can reach a temperature suitable for enzyme action. For prevention of brown stain of Douglas fir sapwood, steaming it to 212°F is recommended [87]. Coating with various chemicals is also effective. As shown in Table 20, coating with dilute acids, sulfites, and EDTA-2Na is valid [46]. Sulfites may act as reducing substances. EDTA-2Na may react with metal ions that are an essential part of a co-enzyme or react with the phenolic substances in wood. Optimum enzyme action occurs at weakly acidic pH. Therefore, the pH in the surface of the woodcan be loweredwithoutan acid stain occurring,or the pH canbe raised without an alkali stain occurring. Coating or immersing with a solution of dilute acid or carbonate is effective in changing the pH value. For white pine, immersion in a solution of sodium carbonate or sodium borate with a pH value of 10 is effective [47].

TABLE 20 Effect of Chemicals o n the Control of Orange Stain of Shim Coat weight (g/m') Coated Hydrochloric acid Sulfuric acid Phosphoric acid Hypophosphorous acid Nitric acid Boric acid Formic acid Acetic acid Oxalic acid Ascorbic acid Benzensulfonic acid Semicarbazide hydrochlorlde Sodium bisulfite Sodium sulfite Sodium hypophosphite Sodium nitrite Fornlaldehyde Urea Thiourea EDTA. disodium salt

0.1

I

5

A

e

e

0

0

A A A

0

X

X

X

X

e A A

X

X

X

A

0

e

X

A

0

X

0

X 0

A

A

0

0

A

A

0

X

X

X

X

X

X

X

X

0

X

X

X

X

X

0

A

0

0

a

e 0

Hon and Minemura

434

Forpreventionofdiscoloration, the removal of the causativecompounds is also effective. To prevent discoloration of kiri wood, sufficient natural seasoning of sawn timber has been conducted traditionally. During this seasoning, the compounds responsible for color change dissolve into rain water and are removed. As a more effective and rapid preventive method, impregnation of timbers in cold water and subsequent treatment with warm moisture are also recommended [88]. Immersion into urea solution is also suggested [511. Quick drying under good ventilation or storage at low temperatures is also valid. Discoloration by enzymes sometimes is encouraged in the wood industry. For example, in the manufacture of decorative walnut veneer, sliced green veneer is left until a blackbrown color develops, and the veneer is then heated with a roller press to stop the enzyme action [48].

7. Discoloration by Nonmicrobial Oxidation with or Without Heating It is well known that woods discolor when they are subjected to high temperatures. Even without being subjected to heat, however, some woods still change their color readily by oxidative reactions that arenotaccompanied by microbiologicalorenzymatic actions. These discolorations are described in this section. U . Churucteristics of Discolorution. When fresh sawedlumber is dried at ahigh temperature, the wood changes color. The color differs according to the wood species and drying temperature. It may be yellow, brown, red, gray, etc. Wood left at high temperatures for long periods of time usually becomes brown. Discolorationduringdryingincreasesas the temperatureandhumidityincrease. Hardwood generally discolors at a lower temperature than softwood. Heat discoloration of several woods is as follows [33,49]: Red color Maple (above 50°C and 65% RH) Beech (above 50°C and 65% RH) Brown color Oak (above 80°C and 65% RH) Sugar pine (above 65°C and 65% RH) Walnut alder (steaming) Spruce fir (above 90°C) The discoloration in artificial seasoning of todomatsu is shown i n Table 2 1 . Discoloration increases with the rise of temperature and the prolongation of time. The formation

TABLE 21

Color of Todomatsu Dried at High Temperature Drying condition

ying Wet-bulb Dry-bulb temperature temperature ("C) 100-1

IO

100-1 10 100

Color

("C) 100

100-86 100-80

ti me (h)

48.5 24.5 48.0

L

57. I

(1

63.7

11.9 9.0

62. I

6.9

h

26.6 33.4 27.8

Color

435

of colored substances from a phenolic compound oxidized with air and the formation of dark materials from hydrolysis of hemicellulose have been considered the causes of discoloration. When the material causing discoloration is water-soluble, these materials rise to the surface and accumulate there, discoloring the wood [49]. It is assumed that many substances that change color with heat also discolor with enzyme action. Brown discoloration oftenoccurs in Europeanoakwoodduring kiln drying.This discoloration occurs at a wood moisture content between 30% and 60%, kiln temperature above 25”C, and relative humidity of about 70% [89,90]. In brown stain of oak, colored polyphenolic polymers and complex esters, hexahydroxydiphenolesters, are found in larger amounts than in nonstained wood [91]. This discoloration is presumably due to oxidative coupling of compounds related to gallic acid. Discoloration during kiln drying may be the result of hydrolysis and oxidative transformation of ellagitannins 1921. Concerning sticker stain in sugar maple, chemical analysis of stained sapwood has been conducted [93]. The amount of acetone-water-soluble materials in the stained part was less than in the clear part. This suggests that phenolic extractives accumulated under the sticker stain during drying and then oxidized to insoluble polyphenolic compounds. Scopoletin was isolated from both the stained and unstained parts as the phenolic compound produced during drying. As anatomical characteristics of brown stain after kiln drying in hemlock, the stain exists in sapwood, particularly in the earlywood, and is recognized mainly in longitudinal tracheids [94]. From a study of the correlation between loss of brightness in mechanical pulp and storage time of western hemlock chip, it is suggested that d-catechin polymerizes oxidatively to give an insoluble polymer and to cause the brightness loss [95]. The heartwoodcolor of sugi is classified into three types:normalreddish-brown color type, black color type, and color-changeable type. The latter occurs when the wood is left at room temperature; the surface color changes from reddish brown to black in 30 min after sawing. This phenomenon is observed in wood grown on tree farms. This type of stain cannot be controlled by oxalic acid; therefore, it is not an iron stain. The characteristics of this phenomenon are as follows [96,97]: The stain occurs either in the dark or under light. Atmospheric oxygen is necessary. In nitrogen, discoloration does not occur. When the color-changeable sugi is extracted with water preliminarily, it gives no more black color. The water extract contains a potassium hydrogen carbonate(0.4% w/w), which keeps the wood weakly alkaline. When normal reddish-brown sugi wood is immersed into awater solution of KHC03, it changescolor to black. So, this inorganicchemical is recognized as one of the causative materials of the black discoloration. The characteristics of black sugi which is alreadyblack when standing in the forest, before logging, have been examined. The black sugi has high moisture content and alkalinity, and contains K’, Na’ , and HCO, [%l. The black substances are presumably a polymer of water-soluble norlignans such as segurin-C and plicatinaphthol [99]. Heartwood of murasakitagayasan changes rapidly i n color from brownish yellow to dark purple after sawing. For this discoloration, oxygen is indispensable. Light and water acceleratc the reaction [ 1001. As the substancewhich contributes to the discoloration, 7,3’,4’-triacctoxy-6’-n~cthoxyisoHav-3-enehas been isolated [ 1 0 1 1. This discoloration might be caused by the autooxidation of this compound to give the quinonoid structure. h. Rrr~~o\w/ of ~ i . s c ~ o / o r t r r i o rWhether ~. sawed lumber has been artitically seasoned or not. its surface is rough. When the lumber is used as an interior wood o r for furniture, its surfacc is usually planed. B ~ C ~ Lheat I S Cdiscoloration is mostly limited to the surface, a sound surface should reappcar after planing to ;l thickness of 2 mm.

436

Hon and Minemura

The yellow discoloration of todomatsu can be removed by immersing the stain in boiling water. Heat discoloration of a relatively light color can be removed by oxidative decomposition with a bleaching agent. c. Preventing Discoloration. A drying process generally consists of a natural seasoningandsubsequently an artificial seasoning. It is desirable to conductan artificial seasoning after enough natural seasoning in order to control discoloration. When sawed green lumber of todomatsu must be dried immediately after cutting, the drying must be done below 80% RH and at 50°C until the wood reaches the fiber saturation point. On drying, it is important to insert enough sticker to prevent close contact and dampness. Drying must be done as soon as possible afterlogging. Low-molecular-weight sugars or amino acids increase as time passes. These substances can cause discoloration [49]. As a method of preventing brown discoloration of oak, drying under a vacuum and superheated steam are effective [ 1021. Coating the wood with antioxidant, reducing agent, acid, and so on, is also an effective method to avoid discoloration. A solution witha concentration of 5-6% ammonia, ammonium carbamate, and zinc oxide is effective in preventing the brownish discoloration of white pine [50]. Sawn timber from water-stored oak logs develops gray stain in the sapwood sooner than does freshly cut logs. For prevention of this stain, a 5-min dip in a 5% sodium bisulfite solution is recommended for sawntimberfrom freshly cut logs, and 10% sodium bisulfite solution forsawntimber from water stored logs [ 1031. For logs stored in water for more than 3-4 weeks, however, this chemical does not give complete prevention. For prevention of nonmicrobial discoloration, methyl bromide fumigation has been tested [104]. This is effective for red alder, but not for western hemlock. This treatment causes rapid death and modification of living parenchyma cells. The effectiveness of coating shiurizakura with semicarbazide for heat discoloration is shown in Fig. 36 [5]. To prevent blackish discoloration of sugi, coating with acidic and chelating chemicals such as phosphoric acid, oxalic acid, nitric acid, formic acid, EDTA-disodium salt, etc., is effective [ 5 5 ] .

8. Discoloration with Exudation of Resin When resin in wood exudes to the surface, the color of the surface changes. This phenomenon is often observedondecorative thin veneer, on lumber that hasbeendried inadequately, and even on furniture or paneling.

W

Q

437

Color and Discoloration TABLE 22 Color of Hinoki Determined Before and After

Removal of Resin L

b

n

21.4 10.4 59.6 Before removal 8.5 62.2 After removal with methanol

19.4

a. Characteristics of Discoloration. Certain woods have resin canals in the direction of the stemand radiation. Such canals open onto the surface of the lumber after sawing.Lightpale resin exudes on the surface in softwood. This resin is a mixture of terpenoids with various boiling points. Exuded resin becomes hard with volatilization of substances with a lowboiling point and then exhibitsa wet color. There is a brown resinous material in the tracheid of mizunara and ash. b. Removal of Discoloration. Discolorationcanberemovedthroughphysical or chemical methods. The resin of hinoki dissolves well in alcoholic solvents such as methanol and ethanol. When only a small amount of resin is exuded, wiping the surface with a cloth impregnated with the solvent is recommended. Resin from thick lumber can be removed in the same way.If a lot of resin has exuded from thin veneer, immersing the veneer in a solvent is an effective method. The colors of hinoki before and after elution with methanol are shown in Table 22. The increase of lightness and loweringof saturation, as well as the disappearance of the wet color, are due to the removal of resin. Besides alcohol, methyl ethyl ketone and acetone also can be used. The resin of karamatsu dissolves well in hexane and trichloroethylene. Karamatsu is used for furniture or as a decorative material in interiors and often exudes resin during use. This phenomenon is mainly observed when the wood is used at high temperatures. To remove the resin, scrape off asmuch resin as possible, thenwipe it withacloth impregnated with a suitable solvent. After removal, coating with polyurethaneis desirable. This film is somewhat effective in controlling exudation of the resin. To remove the resinous products packed in the tracheid of mizunara or ash, it is effective to immerse the wood in a 1 % solution of polyethylene oxide or nonionic surfactant and then keep it at 80°C for half an hour [52].Table 23 presents data on the color before and after resin is removed. Lightness increases considerably after removal. c. Preventing Discoloration. Because the resin of softwood is light-colored, the use of an artificial seasoning condition that vaporizes substances with low boiling points is desirable. The following example is a typical procedure. At the beginning of seasoning, lumber is dried at 100% RH and 90°C. After that, the temperature is kept at 50°C and then raised by steps until 80°C is reached.

TABLE 23 Color of Mizunara Determined Before and After Removal of Resin L

8.6 Before removal After removal with 18.3 1% P E 0 solution 6.5

45.0 59.5

n

b

16.1

Hon and Minemura

438

9. Discoloration in a Standing Tree Stains in standing trees include spotted stains caused by deposit of inorganic or organic substances in tracheids, infestation of insects, or imperfect pruning. a. Troubles with Speck. Whenaninorganicmaterialabsorbed by the roots or an organicmaterialsynthesized in wood is depositedaswhite or yellow-brownish fillers, these materials appear on the surface of the sawn lumber or veneer as a speck. Silica. Silica is often contained in tropical wood and is recognized as white grain in tracheids, rays, and axial parenchyma, and so on. It hastens the abrasion of a saw blade in sawing. The solution of hydrogen fluoride dissolves silica, but cannot be used as a removal agent because of its poisonous properties and tendency to discolor wood. Unfortunately, there is no effective way to remove silica. Calcium Oxalate and Calcium Carbonate. Calcium compounds appear as white crystalline substances in tracheids. Calcium carbonate reacts with dilute hydrochloric acid to form carbon dioxide and water-soluble calcium chloride. Calcium oxalate dissolves in hydrochloricacid.Bothcompounds, therefore, canberemoved by coatingwithdilute hydrochloric acid. Washing or wiping with water must be done sufficiently after removal. When the surface is below pH 2 after treatment with water, it should be coated with a dilute solution of sodium carbonate until the surface becomes weakly acidic. Isoflavone. Eurasian teakwood is used widely in furniture. White spots or lines are often found in the tracheid of this wood. These are a mixture of isoflavones which consists of afrormosin and biochanin-A. As the melting points of these substancesare below 200"C, they can be removed when the wood is left in a hot press heated to 200°C [53]. Fisetin. There are yellow substances in the tracheid of melbau.Thesesubstances form spots or lines on the surface of lumber after sawing. This colored material contains fisetin, robinetin, and quercetin. As the melting point of these substances is above 300"C, removal during the heating process is impossible. These substances react with boric acid to form a water-soluble chelate compound, so when the wood is immersed in 2% boric acid solution for several hours, the stain can be removed [54]. The Rest. In planted teak, a lot of small bluish blackspots are frequently seen. These spots appear particularly in the heartwood like an annual ring at a frontier near to the sapwood, so they might be a dehydrotechtol [ 1051. For removal, the following methods are suggested [ S ] .The bluish-black color disappears immediately upon contact with organic solvents. However, it reappears again with larger area after evaporation of the solvent. Finishing with transparent paint with thinner is effective for disappearance of bluish black spots and maintenance of the characteristic color of teak. As shown in Table 24, sufficient immersion into organic solvents brings an increase of lightness and decrease of reddish component to lose the peculiar color of teak. When heated at high temperature, the bluish-black spots melt to give pale brown color and disappear.

TABLE 24 Color of Teak with Bluish-Black Spots Before and After Sufficient Immersion i n Solvent

Before extraction After extraction with ethanol 3.3 After extraction with acetone Normal unstained teak

33.3 63.6 64.2 55.2

-

0.4

2.8 9.3

7.0 18.9 19.8 22.2

Color

439

10. Stains Caused by Adhesives in Woodworking Processes There are many uses of adhesives in woodworking processes. Adhesives can be colored or can react with other substance and then discolor. These substances often stain wood. In this section, some examples are given and their control discussed. a. Stair1 Caused by Oozing of Adhesives. Adhesives containing phenolic substances (phenol-formaldehyde resin, resorcinol resin, tannin resin, etc.) have a dark red or reddishbrown color. When they are used asadhesives in plywood, they exude to the surface through the tracheid of athin-surface veneer. When the color of the surface veneer is white or light, the color of the adhesive layer reaches the surface and darkens the color of the surface veneer even if there is no exudation. To prevent such oozing, the following methods are effective: raising the glue viscosity, decreasing the moisture content of the veneer, reducing the pressing timeortemperature, and increasing the thickness of the veneer. In order to prevent reflection of the color of an adhesive, it is possible to mix a white pigment such as titanium oxideintotheadhesives without influencing bonding strength. b. Stains Caused b y Reactions with Adhesive Components. Vinyl urethane adhesive is composed of phenyl isocyanate and vinyl polymers. These two substances are mixed immediately before use. Thisisocyanate often makes a colored substance with tannin. When it is used for the adhesion of mizunara, the adhesive layer becomes gray. To prevent this, an aliphatic isocyanate should be used. c. Stciins Ccrusecl b y Whterproof Asphalt. The back side of flooring that contacts concrete directly should be coated with asphalt to prevent permeation of the water of the concrete. When much water penetratesinto the concrete (e.g., in a flood), the pressure caused by water evaporation acts on the asphalt coating layer of the flooring. Flooring of mizunara has a tracheid of large diameter, and this pressure can cause the asphalt to reach the surface through the tracheid. As a result, a stain composed of black specks may appear on the surface. To prevent this stain, an alkali- and waterproof sheet should be bonded on the back of the flooring rather than using asphalt coating, or the sheets may be placed on the concrete before setting the flooring. d. Stcrins During A.s.senzhIy with ( I Dowel. In the manufacture of furniture. assembly with a dowel is often practiced. The dowcl is coated with vinyl acetate adhesive and put into the opening. In this case, excess adhesive will squeeze out from the opening. When the excess adhesive is wiped off with a wet cloth, the wiped mark is often noticeable. This is caused by the tine fibers on the wood surface. This fiber is forced down at planing, but it stands up when water is absorbed.This stain can be removed by grinding with abrasive paper.

REFERENCES

Minemura 440

and

Hon

R. S. Williams, J. Appl. Polymer Sci., 28:2093(1983). D. N.-S. Hon. S.-T. Chang, and W. C. Feist, J. Appl. Polymer Sei., 30: 1429 (1985). 12. K. Kringstad, Tappi, 52:l070 ( 1969). 13. J. Gierer and S. Y. Lin, Svensk Pqerstidn., 75:233 (1972). 14. S. Y. Lin and K. P. Kringstad, Eppi, 53:l675 ( 1970). 15. K. Umehara and N.Minemura, J. Hokkaido Forest Res. I n s f . , 300:13 (1977). 16. W. C . Feist, USDA ForestServ. Res. Paper FPL 339, 1979. 17. N. Minemura, J. Hokkaido Forest Res. Inst., 311:18 (1977). 18. S. Imura and N.Minemura, J. Hokkaido Forest Res. Inst., 305:l (1977). 19. Y. Kai and M. Kawamura, Mokuzai Gakkaishi, 31:766 (1985). 20. V. Loras, Pulp Paper Mug. Can., T49 ( 1 968). 21. M. Takahashi, Abstracts of Papers Presented at the 26th Annu. Meeting Japan Wood Research Society, Shizuoka, pp. 3 18-3 19 ( 1 976). 22. N. Minemura and K. Umehara, J . Hokkaiclo Forest Res. Inst., 3/5:1 (1978). 23. N.Minemura, Japan Finishing, /7( 12):179 (1978). 24. N. Minemura, Mokuzoi Gakkuishi, 24587 (1978). 25. N. Minemuraand K. Umehara,Paper presentedatthe ACS/CSJChemicalCong.,No. 94, Honolulu.Hawaii(April 1979). 26. B. Ranby andJ. F. Rabek, Photode~rcldrltiotl.Photo-Oxidution und Photo.st~~bilizatio~l of’ Polynzers, Wiley, New York, p. 210 ( 1975). 27. N. Minemura, K. Umehara, and M. Sato, J. Hokkaido Forest Res. hsr., 380:11 (1983). 28. G. Gellerstedtand E.-L. Petterson, Svensk Puperstidn., 80:lS (1977). 29. E. A.McGinnes. Jr., Wood Sei., 7:270(1975). 30. T. Yoshimoto and M. Samejima, Mokuzui Gakkaishi, 23:601 (1977). 3 I . W. Sandermann and M. Luthgens, Holz Roh- Werkst., 11:435 (1953). 32. K. Takenami, Mokulai Kogyo (Wood I n d . ) , 24:263(1969). 33. F. Kollmann, R. Keylwerth, and H. Kubler. Holz Roh-Werkst., 9382 (1951). 34. T. Kondo, H. [to, and M. Suda, Nippon Nogeikagaku Kaishi, 30:28 1 ( 1956). 35. K. Takenami, Mokrczai Grrkkaishi, 10:22(1964). 36. T. Goto and H. Onishi, Bull. Shitnane Agric. Uni\j., 15(A-2):80(1967). 37. K. Takenami, Mokuzai K o g ~ o(Wood Ind.), 24:210 ( 1969). 38. N. Minemuraand K. Umehara,Abstracts of Papers Presentedatthe 12th Annu.Meeting Hokkaido Branch Japan Wood Research Society, Asahikawa, Hokkaido, pp. 59-62 (1980). 39. K . Takenami, Mokuxri Gukkaishi, 11:41 (1965). 40. K. Takenami, Mokuzai Kogvo (Wood Ind.), 2 4 3 14 ( l 969). 41. K. Takenami, Mokrczni Gukknishi, 11:47 (1965). 42. N. Minemura, Abstracts of PapersPresentedatthe 13th Annu. Meeting on Chemical Treatment of Wood by the Japan Wood Research Society, Tsukuba, Ibaragi, pp. 1-8 ( 1983). 43. J. Bauch, 0. Schmidt, Y. Yazaki, and M. Starck, Holqjorsch.. 39:249 (1985). 44. H . Kawakami, Annu. Rep. Hokkrrido Forest Prod. Res. I n s t . , 1980-198/, p. 1 I . 45. A. Nunomura. Abstracts of Papers Presented at the 16th Annu. Meeting on Studies of Forest Technology, Sapporo, Japan. pp. 325-326 (1976). 46. N. Minemura, Mokuzai Kogya (Wood Ind.), 38:363 (1983). 47. H. A. Hulme, Forest Prod. J., 25:38 (1975). 48. T. Yoshimoto. Ki no Hnnushi (Storcs 0 1 1 Wood). Outsuki. Tokyo, p. 60 (1983). 49. M. A.Millett, Forest Prod. J.. 2:232 (1952). 50. J. K. Shields, R. L. Desai, and M. R. Clarke, Forest Prod. J., 23:28 (1973). 5 1. K. Makino. Y. Kobayashi. T. Matsuura, and T. Ousako. Rep. Ind. Arts Inst. Hiroskirnu Pm-

IO. I 1.

,fecttcre, 9:24

(1980).

52. K.Umehara. Annu. Rep. Hokknido Forest Prod. RES. I n s t . , 1978-1979. p. 1 I . 53. H. Imamura, Y. Tanno, and T. Takahashi, M o k u x i Gukktrishi, 14:295 (1968). 54. H. Imamura, H. Fushiki, S. Ishihara,and H. Ohashi, Res. Bull. Fuc. Agric. G@ utli\?.3-7:

99 (1972).

Color

441

442

Hon and Minemura

100. R. Kondo, T. Mitsunaga, and H. Imamura, Mokuzai Gokkaishi, 32:462 (1986). 101. T. Mitsunaga, R. Kondo, and H. Imamura, MokuzaiGakknishi, 33:239 (1987).

102. B. Charrier and J. P. Haluk, Holz Roh- ur~d Wrrkst., 50:433 (1992). 103. P. G . Forsyth and T. L. Amburgey, Forvsr Prod. J., 42(4):59 (1992). 104. B. Kreber, E. L. Schmidt, and T. Byrne, ForestProd. J.. 44(10):63 (1994). 105. W. Sandermannand H. H. Dietrichs, Hol;forsch., 13:137 (1959).

10 Chemical Degradation Yuan-Zong Lai SUNY College of Environmental Science and Forestry, Syracuse, New York

1.

INTRODUCTION

Wood and other lignocellulosic materials are labile to a wide variety of chemical changes. These transformations, depending on the conditions of reaction environment, may vary from an undesirable discoloration (Chapter 9) to a selective breakdown of the major cell wallcomponents [ 1-31. In the chemical utilization of these lignocellulosic substrates, lignin usually plays a negative role, and must be modified, partially degraded, or totally removed, depending on the end uses of the final products. The commercial pulping and bleaching operations 141 generally are very nonselective, being accompanied by a significant degradation of the polysaccharide components. For example, the yield of lignin-free softwood pulp for the most widely used kraft process is only about 44%, as compared to a theoretical 67% for pine [ 5 ] .Thus, a great technical challenge for the paper industry is how to improve the delignification selectivity or carbohydrate stabilization. The fundamental chemistry of the degradation of isolated polysaccharide and lignin samples as well as related model compounds is now reasonably well understood [l-31. The detailed kinetics of reactions involving wood components in situ, however, are still not fully clarified, and are complicated by their heterogeneous nature across the cell wall [6,7], the possible role of lignin-carbohydrate complex (LCC) or linkages [ 1 -3,8.9], and the pore structure of the cell wall matrix. Theoretically, accessibility is a significant factor affecting the degradation behavior of wood polymers in situ, and its significance varies with the nature of chemical environments. This revised chapter largely retains the original format [21 for easy reference, and discusses the chemistry and controlling factors in the degradation of cellulose, hemicellulose, and lignin under acidic, alkaline, and oxidative conditions.

11.

REACTIVESITES

A.

Polysaccharides

The major functional units of wood polysaccharides are reducing end groups, glycosidic linkages, and hydroxyl groups. The reactivity of these units, however. varies considerably among the cellulosc and hemicellulosecomponents,contributing largely to their differences in supramolecular and chemical structures. 443

Lai

444

1.

ReducingEndgroup

Allnatut-al polysaccharide molecules contain a reducing end group which, being hemiacetal i n nature, is partially converted to an open-chain aldehyde function i n solution. This functional group can he reduced and oxidized to a n alditol and aldonic acid moiety, respectively. Also, the anomeric hydroxyl group (at the C1 position). being the nlost acidic [ I O ] . can be selectively etherified [ I I]. Reduction with sodium borohydride is often used for quantitative estimation of the reducing end-group contenl [13_]. The reported contents of g l ~ ~ c o smannose. c, and xylose end groups in wood 1 1 31 are gcnerally consistcnt with the molecular tnasscs established for cellulose. glucomannan, and xylan. Regarding the accessibility of reducing end groups. it was reported by Gentile et a l . [ 141 for a libroushydrocellulosesample based on the assumption that an amorphous cellulose was totally accessible. The latter sample was prepared by regenerating ;I cellulose solution i n a dimethyl sulfoxide (DMSO)-parafortnaldehyde (PF) solventsystem [ 121. Rcducing end groups were clctertnined by reduction with tritiated sodium borohydride i n dilute alkalis. Approximately 12%. of the reducing end groups i n the fibrous cellulose were shown to be inaccessible t o the borohydride treatment. Interestingly, a large difference i n hydroxyl accessibility between ;I native and ;I regenerated cellulosesample ( S 1 versus 99%) was previously indicated by the deuteration method [ IS]. Since the penetration of reagent into thc crystallites would be negligible under the mild conditions used (an 0.25 M borohydride solution a t ambient temperature). it appears that the concentration of reducing end gt-oups is significantly higher i n the amorphouscomponent than in thc crystallites. Reducing end groups i n alkalis undergo readily a series of the so-called Lobt-y de Bruyn-Alberda van Ekenstein transfonnations [ 1 S ) . and play a dominant rolc i n the a l kaline degradation of polysaccharides.

2. Glycosidic Linkages The glycosidic bonds. being acetal i n nature, are hydrolyzable under acidic, alkaline. and oxidative conditions. Acid hydrolysis proceeds very readily and forms the basis of :I saccharification proccss, whereas the alkaline cleavage reaction requires more drastic conditions. This hydrolytic reaction i n general weakens the mechmical properties of wood and fibers.

3. Hydroxyl Groups The intcrunits of ccllulosc and hemicelluloses contain one primary hydroxyl group for each :mydro-hcxose u n i t and two secondaryhydroxylsfor each anhydro-hcxosc and -pentose u n i t . Thew hydroxyl g r o ~ p saresusceptible to oxidation, and the resulting a l dehyde or keto g r o ~ ~may p initiate further degradation reactions. such ;IS dehydration and cleavage o f glycosidic linkages. Among the three hydroxyl groups, the ?-OH group is the most acidic 1 1 1,16- 191. and this has been gencrolly attributed t o an activating effect o f the nnomeric center. Hearne et a l . 1201 observed t h a t methyl P-D-ribopylanoside was more acidic t h m methyl P-D-xylopyranoside, and they differ only i n the C3 conformation. Thus. the acidity of the ?-OH group is also likely influenced by other factors such ;IS the hydrogen bonding system. The hydroxyl reactivities in a heterogeneous system are further affected by the Xcessibility factor. and thc reported data on cellulose have been shown t o vary considerably with the type and the conditions of reactions used. Inaccessibility may arise from either a

Chemical

region being inaccessible to a reagent or a other units.

B.

Degradation 445

functional group being hydrogen-bonded

to

Lignin

Lignin occurring in plants is well known for its variability or heterogeneity in terms of both morphological distribution and chemical characteristics. Significant variations have been observed between juvenile and mature woodlignins; among the normal, compression, and tension wood lignins; and for lignins in different morphological regions. Our present understanding of lignin structure has been obtained largely from analysis of milled wood lignin (MWL) preparations [9], which are usually obtained at less than 50% yield. The millingprocessused in MWLpreparation is knowntoinducesomechemicalchanges, notably an increase in the phenolic hydroxyl group content [21,22]. Additionally, MWL has been shown to originate mainly from the secondary wall lignin [8,23-251. Thus, the extent to which MWL may represent the lignin in situ requires further evaluation. Although the approximate contents of major lignin linkages are now reasonably well understood, the chemical structure of lignin, unlike that of cellulose or the hemicelluloses, cannot be defined precisely. Sincecarbon-carbonlinkages are generallyvery resistant tochemical attack, the degradation or fragmentation of lignin is limited largely to cleavages of ether units at the a - and P-positions. The nature of these hydrolyzable units and other functional groups having a significant impact on the reactivity of lignin is outlined below.

1. HydrolyzableEtherLinkages The hydrolyzable ether units in lignin are the P-aryl, a-aryl, and a-alkyl ether linkages (Fig. I ) . As summarized in Table I , the P-aryl ether based on phenyl propane (C,) units constitutes approximately 50% and 60% of spruce and birch MWL, respectively [21], and is present as two isomers. Proton and I3C NMR analysis indicates that spruce lignin contains about equal proportions of the erythro and threo forms, whereas the erythro form dominates in birch lignin [26,27]. Spruce MWL was reported to contain 6-9% of acid-labile (presumably noncyclic a-aryl) ether units by a mildly acidic hydrolysis reaction [28], but an appreciably lower value ( l 50°C) was associated witha rather high activation energy. A similar tendency was noted previously by Nelson [92]. Most of the valuesobtained below 100°C or using concentrated acids were close tothat of model glucoside, cellotriose, or methyl P-D-glucopyranoside. Solvents. The acid-catalyzed degradation of cellulose depends considerably on the nature of the solvent [ 102,1131. The addition of ethanol, propanol, or methyl ethyl ketone accelerates the degradation process, including the formation of nonglucoseproducts, whereas dimethyl sulfoxide (DMSO) has a negative effect. The solvent effect has been explained in terms of it affecting the hydronium ion reactivity [ 1131, or a r e h a t i o n of

TABLE 4

Variation of Hydrolytic Reactions in Activation Energy ~

Acid Sample Corn stover Douglas fir Cellulose I Mercerized Hollocellulose Cotton Mercerized cotton Decrystallized cotton Viscose rayon Cotton Cotton Regenerated cellulose Cellulose Cellotriose Methyl P-D-glucopyranoside

E,, Ref. (kcal/mol) 0.5- 1 .S% HZSO, 0.4- 1.6% HISO,

2% HZSO, 2%H,SO, 2% H2S0, 6 MHCI 6 M HCI 6 M HCI 6 M HCI 1 MHCI 0.1 M H2S0, 1 M HCI 51% HISO, 5 1% H2S0, 5 1 % H,SO,

155-240 170- 190 150- 170 150- 170 1 SO- 170 80808080-

100 100 100 100

30-50 50-70 30-50 18-30 18-30 18-30

45 43 41 43 38

111 1 05

33

97 97 I12 92 92 92 92

30

100

34 35 31

27 27 30 29 29

I00 100 1 07

96 96

456

Lai

structural stress in the cellulose [ 1021. Thus, nonswelling media may help preserve the structural stress of cellulose and enhance its hydrolytic degradation. In addition, cellulose can be effectively depolymerized in ethylene glycol at high temperatures(200-240°C) [ 1141 whileminimizing the oxidativedegradation reaction. Also, the depolymerized residues were shown to have a high accessibility toward cellulase in enzyme-catalyzed hydrolysis.

3. Hemicelluloses Hemicelluloses are amorphous materials and also contain a variety of nonglucose units [6,115- 1171. The nonglucose units, because of their different ring structures and hydroxyl configurations, generally have higher reactivity than the glucose residue, and often can be selectively removed from lignocellulosic substrates. Consistent with the behavior of simple glycosides (Table 2), the relative-hydrolysis rate of p-( 1 +4)-linked polysaccharides in a homogeneous system [ 118,1191 increased in the order cellulose (1) < mannan (2-2.5) < xylan (3.5-4) < galactan (4-5). A heterogeneous hydrolysis of these polysaccharides displayed even greater variations in reactivity, following an order cellulose (1) < mannan (60) < xylan (60-80) < galactan (300). This further demonstrates the important role of accessibility in acidic degradation reactions. Acetyl groups presents in hardwood xylans and softwood galactoglucomannans are also hydrolyzable by acid, especially at elevated temperatures. The acetic acid released contributes significantly to the acidity of reaction media. a. Xylan. The presence of uronic acid groups has a profound impact on the hydrolysis of xylans, as it reduces appreciably the hydrolysis rate of glycosidic linkages (Table 2). Thus, high yields of aldobiuronic acid dimers were generally obtained upon partial hydrolysis of xylans 16.1171. Also, the initial hydrolysis rate of various hardwood xylans (with water at 170°C) was closely related to their uronic acid contents [120]. A higher stability of softwood xylans compared to that of hardwood xylans in sulfite pulping may be partly attributed to their higher uronic acid group contents [6]. The acetyl groups in hardwoodxylanswerefoundto exhibit remarkable stability under the relatively drastic conditions of acid sulfite cooking [ 1211. They were reportedly, in steamtreatmentsof birch wood,more stable than the 4-0-methyl-glucuronosyl unit 11221. On the other hand, the arabinofuranosyl linkage in softwood xylans is very labile, and can be selectively hydrolyzed in dilute H2S0, (0.05 M at 97°C for 3 h) [ 1231 or oxalic acid (0.01 M at 100°C for 1.5 h) 11241. b. Glucornannan. The a-D-( 1+6)-galactosidiclinkage in galactoglucomannans is very labile, andcanbe selectively hydrolyzed in dilute oxalic acid (0.05 M) at100°C [125]. Its high reactivity 1125,1261, however, cannot be satisfactorily explained in terms of the behavior of simple glycosides (Table 2). Interestingly, the alkali-induced deacetylation of glucomannans increased its resistance toward acid hydrolysis, as evidenced in acid sulfite cook [ 127,1281. This was presumably caused by the deacetylated glucomannan being adsorbed onto the cellulose or partially crystallized.

B. Lignin 1. General Aspects Figure 8 illustrates the general types of acidic degradation reactions for a lignin model trimer (21b) containing both a- and @-etherunits [21,34,129,130]. The reaction is initiated by protonation of the benzyl oxygen, followedby a-ether elimination of the corresponding

457

Chemical Degradation GI-CH -C-CH, bH 8

CH2OH CHzOH

c 4

G,-C-CH .CH,

I

6

7 2

OCH,

OR

OR

21

OH

a: RI = H b: R,= a r y l

26ketones Hibberts

25

OR

30

@H2$

B

OCH,

H,CO OH

OH

31

t

T

+

OCH,

CH0

I

OR

28a

27

!ab

OCH)

FIGURE 8

Acidic degradations of a- and P-ether units.

phenol or alcohol to give the benzylic carbonium ion intermediate (23), which may undergo three major competing processes. Pathway A leads to the formation of the C&-enol ether (24), which readily undergoes acid-catalyzed hydrolysis to give the a-P-keto1 (25), and then Hibbert's ketones (26). Pathway B involves a carbon-carbon bond cleavage between the P- and y-positions via

Lai

458

reverse Prins reaction to give formaldehyde and the C,C,-enol ether (27), which is degraded slowly to homovanillin (28a). Pathway C consists of an intermolecular electrophilic addition of benzylic carbonium ion to another aromatic unit. giving mainly the a-6-diphenylmethane (DPM) unit (30) and some a-5 condensed structure. Additionally, formaldehyde generated from reaction B may condense with two aromatic units to form another DPM-type condensed structure (31). These condensation reactions have been well established in lignin model reactions [ 13I , 1321. Table 5 illustrates the relative hydrolysis rate of lignin model compound reactions (with 0.2 M HCI in 9: 1 dioxane-H20 solution at 50°C) reported by Johansson and Miksche [ 1331.It is evident that both the a - and P-aryl ether hydrolyses were enhanced by the presence of a phenolic hydroxyl group. Also, the a-aryl ether was much more reactive than the P-aryl ether, roughly by factors of 25 and 65 for the phenolic and etherified units, respectively. Reactivity of the a-ether units varies with their chemical nature [ 1361. Leary and Sawtell [l341 showed that a p-hydroxybenzyl a-aryl ether was about 400 times more reactive than a p-hydroxybenzyl alkyl ether.

2. a-ArylEther Model compound studies, especially by Meshgini and Sarkanen [ 1361, indicate that the overall a-aryl ether hydrolysis (pathway A, Fig. 8) was significantly affected by the nature of the benzyl and a-ether groups. As shown in Table 6, benzyl units (ring A) of the syringyl type, as compared to that of a guaiacyl or p-methoybenzyl moiety, reduced the hydrolysis rate, andhad a higher activation energy. The reaction was also retarded by the presence of a @-aryl ether unit. On the other hand, a syringyl moiety on the a-ether unit (R, group) had a positive effect. The activation energyvariedfrom 19 to 28 k c a h o l , and appears to be related to the stability of the ether linkage. Also, solvents play anoticeable role in the a-aryl ether hydrolysis 134,1361. The rate generally increases with increasing solvent polarity or a decrease in the proportion of organic solvent (dioxane or ethanol) in aqueous systems.

TABLE 5 Relative Rates for Hydrolysis of a- and P-Ether Lignin Model Compounds in 0.2 M HCI A q u e o u s Dioxane at 50°C Linkages Structure (Fig. ~~

~

I)

Relative rate

~

P - A r y l ether

Nonphenolic Phenolic a-Aryl ether Nonphenolic Phenolic

(la), R = CH,

(la), R = H (lb), R = CH, (lb), R = H

I 12

6.5 305

a-Alkyl

Phenolic” “Estimated from the data Source: Ref. 133.

G-O-(CH2)3“G in Ref. 134; G = guaiacyl.

0.8

Chemical Degradation

459

TABLE 6 RelativeRatesandActivationEnergies for the Hydrolysis of Nonphenolic a-Aryl Ether Linkages with 0.2 N HCI in 47.5% Aqueous Ethanol at 50°C Compound (39)rate

Relative

A E (kcal/mol)

Formula"

H,CO@-CH,-0-@CH, H,CO@-CH,-0-@CH, H,CO@-CH,-0-@CH,

H,co@-cH~-o-@ H,co@-cH~-o-@ H,CO@-CH,-0-@ H,CO@-CH-0-@CH,

1

30 30

20 33 3

12

24.5 21.7 23.7 21.3 18.9 21.9 22.8

I

h

O-@CH2 H,CO@-CH-O-@CH,

0.4

28.2

.'G = guaincyl; @ = syringyl: @ = p-hydroxybenzyl. Sortrce: Ref. 136.

3. /?-Aryl Ether The reactivity of P-aryl ether linkages, like that of a-aryl ethers, is substantially enhanced by a phenolic hydroxyl group [133,137-1411, and is influenced by their structures and the reaction conditions. a. Reaction Mechanism. The major degradation pathway of P-ether units shown in Fig. 8 is generally accepted as proceeding through an ionic mechanism under acid-catalyzed conditions [ 142- 1461. Under typical acidolysis conditions (0.2 M HCl in 9: 1 dioxane-water, 4 h at lOO"C), the ether cleavage reaction (pathway A) predominates, yielding Hibbert's ketones (26). These reactions have been used extensively in lignin analysis [21,144]. However, different mechanisms appear to be involved for hydrolytic reactions conducted in the absence of an acid catalyst. The phenolic P-aryl model compounds (32) and (33), when treated with water at 100- 130°C [ 138,1391 or with SO% aqueous dioxane at 180°C [ 147,1481 gave a variety of transformation products, including dimers of the P-S (35), P-l, and P-P types, and other condensation products (Fig. 9). Formation of these coupling products was explainedin terms of a radical mechanism by Sano et al. [147,148]. It should be noted that the etherified P-aryl ether dimer of (32) was unreactive in aqueous solution at temperatures below 130°C. Also, the phenolic Paryl ether dimer of the syringyl type, unlike the guaiacyl dimer, was reactive even under steam treatment conditions [140], and gave complicated products in aqueous dioxane at 180°C [ 1481. b. Solvent. Solventshave a significant influence on the overall degradationof Paryl ethers. The hydrolysis reaction conducted in an aqueous solution was enhanced by the addition of dioxane, and unlike the a-aryl ether hydrolysis [ 1361, was retarded in the presence of ethanol [ 1371. These organic solvents were shown to favor the ChC2enol ether formation (reaction B in Fig. S), especially at elevated temperature [l451 as illustrated in Fig. 10. Thus, the reduced ether hydrolysis in ethanol solution can be partially explained

Lai

460 OCH3

Q

RI

H20 130°C. 6 h

.

Condensation

c

products

OCH3

OCH3

OH

OH

(12.5%)

OH

32

34

(2 1.5%)

35

RI= CH20H (61.2%)

RI= CH =CH-C&OH

R1= CH20H (16%)

RI= CH=CH-C&OH (12.5%)

(6.9%)

CH3

Q OOi C H 3

OH

33

1

G- CH =CH -CH20H (2.6%)

38

G - p (p-5)G- CH=CH-CH20H H2O- Dioxane 1 80°C, 20 min

(4.5%) (1.4%)

+

v CHzOH

J 0

36

Condensation products

(65.9%)

QOCH3

0

37

39

W",

FIGURE 9 Hydrolysis products of phenolic guaiacylglycerol P-aryl ether in water at 130°C for 6 h (from Ref. 160), and in 50% aqueous dioxane at 180°C for 20 min (from Ref. 147).

in terms of increased formation of C&-enol ethers which, as noted earlier, are relatively resistant to acid hydrolysis. Hoo et al. [ 1451 studied the kinetics of acidic hydrolysis of P-aryl ether dimers in 50%aqueousethanolcontaining0.2 M HCl, and obtainedasimilaractivationenergy (-36 kcal/mol) for both the phenolic and etherified types. This value is substantially higher than that of the a-aryl ethers (Table 6) [ 1361.

461

Chemical Degradation

1.200r

l

I

8 I

i

m 17OoC 0

0.900

14OT} E ~ HHP:

f

1709c}Dioxane:H20

O 140°c 0 17OoC, Pure H 2 0

I

I

l

/

(0.002M HCI) /

l

/

Vol% Organic Solvent in HzO

FIGURE 10 The effect of solventcompositionand reaction temperatureontheethercleavage ( k , ) and enol ether formation (k,3)from acidic treatments of erythro-veratrylglycerol &(2-methoxyphenyl) ether. (From Ref. 145.)

Acidictreatments of @-aryletherdimers in concentratedorganicacids,however, resulted in only limited ether cleavages. These studies include using a 85% formic acid at reflux temperature [149,150] and a 75-90% acetic acid at 160- 180°C [ 1511.

4. Carbon-Carbon Linkages Acidic cleavages of the carbon-carbon linkages in lignin are limited mainly to the bond between the p- and y-carbon atoms, as indicated in reaction B (Fig. 8) for a p-0-4dimer with a simultaneous release of formaldehyde. Similar reactions may also occur with a p1 (40) or p-5 (42) type units (Fig. 11) [ 129,1521. These reactions do not contribute significantly to the formation of low-molecular-weight lignin products.Formaldehyde released, however, may participate in the lignin condensation reactions. Under acidolysis conditions [ 1.521, the formaldehyde yield from lignin model dimers decreased in the order p-1 (15%) > p - 5 (9%) > p-0-4 (3%). The main product from a p5 dimer (dihydrodehydrodiconiferyl alcohol) (42) was a phenyl courmarone (43b) (75%). The latter product has a characteristic UV absorption, and is often used in quantitative estimation of the p-S units [ 153,1541.

5. Lignin-Carbohydrate Complex Among the three possible types of lignin-carbohydrate (L-C) bonds (Fig. 2), the ester (5) and glycoside types (7) are probably more labile to acid hydrolysis than the ether type (6). Model compound reactions [ 1351 indicate that the benzyl ethers of vanillyl methyl

462

Lai

HCHO

OCH3

OCH3 OCH3 42

H 0 OCH3

OCH3

-

OCH3

43a

43b

FIGURE 11 Formation of formaldehyde from the acidic degradation of p-1 and p-5 lignin model dimers. (From Ref. 144.)

ether (44a) and methyl 4-0-veratryl a-D-glucopyranoside (44b) were substantially more stable than the glycosidic linkage of methyl a-D-glucopyranoside (44d) (Table 7). The reactivity of benzyl ethers was also significantly enhanced by the presence of a phenolic hydroxyl group [ 1331. Judging from the behavior of model compounds (Tables 5 and 7), LCC of the aether type, if present in lignin, can only be hydrolyzed slowly [135]. Also, the etherified units are more stable to acid hydrolysis than the glycosidic linkages.

6. Condensation Reactions Lignins are known to undergo condensation reactions even under mildly acidic treatments [9]. This is attributed mainly to the high reactivity of the benzyl hydroxyl groups. Three major types of lignin condensation processes are possible based on the lignin model compound reactions. a. Phenolation. This type of intermolecular condensations occurs between the benzyl carbon and another aromatic nucleus, mainly at the 6-position (30) (80%) (Fig. 8) plus some at the 5-position [131,155-1601. The condensation reaction varies with the nature of the phenyl units and reaction conditions. Yasuda and Ota [ 1601 reported that syringyl nuclei condensed more readily than guaiacyl nuclei upon reaction with vanillyl alcohol in 5% sulfuric acid at 100°C. The formation of benzyl chloride was observed upon treatment of p-0-4 dimers in hydrochloric acid, and this would reduce condensation at the benzyl position. b. Formaldehyde Addition. The condensation of aromatic units with formaldehyde results in the formationofmethylenecross-links (31) withpossiblysome1,3-dioxane derivatives [159]. Acidic hydroxymethylation occurs largely at the C, or C, position of aromatic nuclei, which may be phenolic or etherified. The initial hydroxymethylation for syringyl units was faster than for guaiacyl units, and was promoted by the presence of a phenolic hydroxyl group, whereas the subsequent cross-linking reaction was facilitated by an increase in acid concentration and reaction temperature [162]. c. Intramolecular Type. Yasuda et al. [ 1631 identified a phenylcoumaran-type condensation product (46) in the acidic treatment (5% H,SO, at 100°C) of a p-ether dimer

463

Chemical Degradation

Relative Hydrolysis Rates of BenzylEthersandMethyl Glucoside in 0.1 M HCI at 75°C

TABLE 7

Compounds rate

(44)

Relative

Structure

cH3of)--”cn,-ocH3 -

A

1

C H30

B

1 (3343

OH

CH20H

D

20 OCH3

OH

(45) (Fig. 12). Thisintramolecularcondensation was shown to be dominant in an 85% formic acid treatment [ 149,1501,whereas it was practically insignificant for acid-catalyzed (0.2 M HCI) reactions i n 50% aqueous ethanol at 135°C [ 1371.

7. Lignin In Situ The overall degradation of lignin, like model compound reactions, depends considerably on the acidic environment. In aqueous media, lignin condensation reactions dominate and lead to the formation of acid-insolubleresidues.This principle servesas the basis for quantitative determination of lignin content in plant materials [ 1641. Lignin condensation

45

R=HorCH3

FIGURE 12

46

An acid-catalyzed intl.nmolecular condcnsation reaction. (From Ref. 163.)

464

Lai

reactions, however, can be minimized by using mildly acidic conditions in the presence of organic solvents, or nucleophiles. a. Atyl-Ether Cleavages. Lai and Guo [ 130,1651 determined the acid-catalyzed hydrolysis of aryl ether linkages in wood lignin, which was evaluated in terms of the phenolic hydroxyl group generated. As indicated in Fig. 13, temperature had a significant influence on the aryl ether hydrolysis reaction. The low-temperature reaction (/]/.Po/yrrwr Sci., 42: 417 ( 1991 ). 98. A. Shorples. Trctr~s.Fcrrrrtkcy Soc.. 53: 1003 (1957). 99. A. Sharpies. Trctr~s.E'crrcrrkcy Soc... 5 4 3 I3 ( 1978). 1 0 0 . E. H. Daruwallu and M. G. Narsian, 7lrppi, JY(3): 106 ( 1996). 1 0 1 . R. H. Atalla. in Hydrolysis of' Ccllrrlosc~:Mec.hccrli.srrl of G~,-yr~lcrtic. trnd Acid C'tr/tr/y,si,s(R. D. Brown and L. Jurasek. cds.). Adv. Chem. Ser. No. 181, American Chemical Society. Washington. DC. p. SS (1979).

504

Lai

102. B. Philipp, V. Jacopian, F. Loth, W. Hirst, and G. Schulz, in Hydrolysis of Cellulose: Mecha n i s m of Enzyrnutic ccnd Acid Cuta1ysi.s (R. D. Brown and L. Jurasek, e&.), Adv. Chem. Ser. No. 18 I , American Chemical Society, Washington, DC, p. 127 (1979). 103. A. Sharpies, in Cellulose ancl Cellulose Derivutives (N. M. Bikales andL. Segal, e&.) WileyInterscience, New York, part V, p. 991 (1971). 104. M. A. Millett and M. J. Effland, in H.ydrolysis of Cellulose (R. D. Brown and L. Jurasek, eds.), Adv. Chem. Ser. No. 181, American Chemical Society, Washington, DC, p. 7 1 (1979). 105. J. F. Saeman, Ind. Eng. Chenl., 37( 1):43 (1945). 106. C. J. Biermann, Adv. Crrrbohydr. Clzem. Biochenz., 46:25 1 ( I 988). 107. K. Freudenberg and C. Blomqvist, Be,: Drsch. Chem., B68:2070 (1935). 108. I. S. Goldstein, H. Pereira, J. L. Pittman, B. A. Strouse, and F. P. Scaringelli, Biotech. Bioeng. Synzp., 13:17 (1983). 109. E Bayat-Makooi and I. S. Goldstein, in Cellulose and Its Derivatives (J. F. Kennedy, G. 0. Phillips, D. J. Wedlock, and P. A. Williams, eds.), Ellis Horwood, Chichester, U.K., p. 135 ( 1 985). 1 IO. K.Garves, Holzforsch., 4 7 149 (1993). 111. N. Bhandari, D. G. Macdonald, and N. N. Bakhshi, Biotechnol. Bioeng., 26:320 (1984). 112. D.H. Foster and A. B. Wardrop, Austrczl. J. Sci. Res., A4:412 (1951). 113. K. Garves, Cell. Chem. Technol., 18:3 (1984). 114. J. Bouchard, G. Garnier, P. Vidal, E. Chornet, and R. P. Overend, Wood Sci. Techno[., 24: 1S9 ( 1990). 1 15. K. C. B. Wilkie, Adv. Carbohydr. Chern. Biochem., 36:215 (1979). 116. K. Shimizu, in Wood and Cellulosic Chemist~y(D. N.-S. Hon and N. Shiraishi, eds.), Marcel Dekker, New York, p. 177 (1990). 117. T. E. Timell, Wood Sci. Technol., /:45 (1967). 118. J. N. BeMiller, Adv. Carbohyd. Chetn., 22:25 (1967). 119. J. N. BeMiller, in Starch Chernistr~and Technology (R. L. Whistler and E. F. Paschall, eds.), Academic Press. New York, vol. I , p. 495 ( 1 965). 120. E. Springer and L. L. Zoh, Tnppi, 51(5):214 (1968). 121. G. E. Annergren and I. Croon, Svensk Pnpperstidn., 64:618 (1961). 122. H. E. Korte, W. Offermann, and J. PUIS,Hol&r.sch., 45:419 (1991). 123. S. K. Banerjee and T. E. Timell, Tappi 43: 10 (1960). 124. K. Lundquist, R. Simonson, and K. Tingsvik, Svensk Papperstidn.. 86:R44 (1983). 125. J. K. Hamilton, E. V. Partlow, and N. S. Thompson, J. Am. Chern. Soc., 82:451 (1960). 126. T. E. Timell. Tuppi, 45:734 (1962). 127. G. E. Annergren, I. Croon, B. F. Enstrom, and S. A. Rydholm, Svensk Papperstidn., 64386 (1961). 128. I. Croon, Svensk Pupperstidn., 66: 1 (1963). 129. K. Lundquist, Appl. Poljvner. Sci., 28: 1393 ( 1976). Mcrtericr1.s (D. N.-S. Hon, ed.), Marcel 130. Y.-Z. Lai, in Chelniccrl Modifccrtion of Ligr~ocell~rlo.sic~ Dekker, New York, p. 35 (1996). 131. J. M. Harkin. Aclv. Chem. Set:. 59:65 (1966). 132. T. Ito, N. Terashima, and S. Yasuda, Mokuzai Gakktrishi, 27(6):484 (1981). 133. B. Johansson and G. E. Miksche. Actu Chern. Scarzd.. 26:289 (1972). 134. G. J. Leary and D. A. Sawtell, Hol 223 and h > 300 nm. More hydroperoxide is detectable from the specimens irra-

i=O CEO

-0

-0

QOCH,

-0 FIGURE 8

Norristype I photoreactionin lignin.

522

Hon Energytransferredhere

here

-0

"c.I

4

0

"0

FIGURE 9 Dissociation of the @-aryl etherlinkage in lignin by an energy-transferprocess via excited a-carbonyl group.

diated with the latter light source. Competitive reactions between hydroperoxide formation and decay revealed that at the initial 90 days of irradiation with the light of A > 300 nm, the rate of formation exceeded the rate of degradation. When the wood is irradiated with light of A > 223 nm, most of the hydroperoxide is generated and converted simultaneously into carbonyl group. This chemical conversion is also observed from the specimen irradiated above 65°C. ESR can also be used to monitor the formation and decay of hydroperoxide radical. A typical ESR spectrum of peroxy radicals in photoirradiated wood is shown in Fig. 10. The peroxy radicals seek to complete their unsatisfied valences, which may be done by abstracting a proton from a nearby molecule to form a hydroperoxide. The hydroperoxide is relatively unstable toward heat and light, and is usually transformed into a new chromophoricgroupsuchasacarbonyl or carboxylicgroup.Thehydroperoxideimpurities

FIGURE 10 A typical ESR spectrum of peroxy radicals in photo-irradiated southern yellow pine.

ochemistry andWeathering

of Wood

523

generated at woodsurfacescanbedetermined by spectrophotometrictechniquesusing iodometric and triphenylphosphine methods [56].

1. Mechanism of Hydroperoxidation Formation and Decomposition a. Kinetic of Initiated Oxidation. It has been known for a long time that chemical reaction between atmospheric oxygen and organic components at wood surfaces at ambient temperature is a very slow process. However, the rate can be enhanced by metal ions and light. In common with other radical chain reactions in polymers, photooxidation of wood surfaces can be divided into three separate processes: initiation, propagation, and termination, as indicated below:

hv

+ H*

(1)

+ 0, -% RO;

(2)

Initiation:

RH”+ R.

Propagation:

R.

RO; Termination:

RO;

+ R H & ROOH + R k + R05 a non-radicalproduct

R.

+ ROi

R.

+ R - ”% non-radical product

product non-radical

(3) (4) (5)

(6)

when RH represents chemical components, such as cellulose, hemicelluloses, and lignin, at the wood surface. From these mechanisms, the rate of oxidation can be illustrated as follows:

d[o’l - k,[R.][O2] ”

dt

Using the usual steady-state assumptions, the rate of chain initiation can be illustrated as follows:

+

+

R, = k,l[ROO*]2 2krl2[R*][ROO.] kf2[R.]’

(8)

If k,,’ is equal to (kflkf2)’”,Eq. (8) can be derivatized into Eq. (9): RI = (k,,[ROO*]+ k,JR-])’

(9)

At the initiation stage, if R * or ROO. is the only product or both are formed, the rate of oxidation can be illustrated in Eqs. (lo), ( 1 l), and (12), respectively.

524

Hon

If the system is rich in oxygen, then the rate of oxidation becomes

If the system is low in oxygen, then the rate of oxidation becomes l12 "

dt

b. Formation Mechanisms. When wood is irradiated with ultraviolet light, free radicals are generated at the surfaces due to the dehydrogenation, dehydroxylation, dehyroxymethylation, demethoxylation. and chain scission that occurred in cellulose, hemicellulose, and lignin distributed at the wood surface [Eq. ( l ) ] [52].The presence of oxygen in the system provides the opportunity for oxygen molecules to react with free radicals in wood to generate hydroperoxy radicals [Eq. ( 2 ) ] ,which in turn abstract protons to produce hydroperoxides [Eq. (3)]. This can be seen from the transformation of a multiplet signal of ESR, due to the various carbon radicals, to an asymmetric singlet signal, due to the hydroperoxy radicals. Singlet oxygen, resulting from the interaction of ultraviolet light and molecular oxygen, and its subsequent attack on wood surfaces, has been proved as another possible initiation route for hydroperoxide formation [571.

E.

Reactions of Gas Pollutants in PhotoirradiatedWood Surface and Its Components

Industrial pollutants are playing an ever-increasing role in our environment. Sulfur dioxide (SO,) and nitric oxide (NO) are important gases in air pollution. Due to their high reactivity, these gases are likely to have ill effects on the surface quality of wood when it is exposed to them [58,59]. Sulfur dioxide and nitric oxide accommodate unpaired electrons; hence they possess a paramagneticproperty that canbedetected by ESR. Since these gases are free radicals in nature, they will react with photo-induced free radicals in wood, which have been shown to be quite active toward oxygen to undergo hydroperoxidation. As discussed earlier, when wood or its chemical components, namely, cellulose, hemicelluloses, and lignin, is irradiated with light, various forms of free radicals are generated. In short,phenoxy and carbon radicals weregenerated in cellulose,hemicelluloses,and lignin. While most of the phenoxy radicals are stable at room temperature, carbon and alkoxy radicals are unstable at that temperature. Nitric oxide readily reacted with carbon and alkoxy radicals to produce nonradical nitroso and nitrite products, respectively. However, only a portion of carbon radicals react with sulfur dioxide to produce sulfonyl radicals. Alkoxy radicals are reactive with sulfur dioxide to produce sulfite radicals. Sulfonyl and sulfite radicals are unstable at room temperature and terminate rapidly to form sulfinic acid and sulfonate ester, respectively. Phenoxy radicals are inert toward sulfur dioxide and nitric oxide. BasedonsystematicESRstudies, the mechanisms of interaction betweensulfur dioxide and nitric oxide and photoinduced free radicals in wood have been elucidated [59]. When wood is irradiated with ultraviolet light, various free radicals are formed: Wood

-+ P-O(a).

+ P-O(s)* + R-0- + R(A)- + R(B).

(15)

where P-O(a)., P-O(s)., R-Os, R(A)., and R(B). are active phenoxy radicals, stable phenoxy radicals, alkoxy radical, group A carbon radicals, and group B carbon radicals, respectively. At 25"C, P-O(a) ., R(A) ., and R(B). decay rapidly and stabilize, whereas

hemistry andWeathering

P-O(s) dioxide,

of Wood

remainsstable.Whenphoto-inducedwood

525

free radicals are exposed to sulfur

+ SO, -+ no reaction + SO, -+ no reaction

P-O(s). P-O(a).

(16)

(17)

+ SO, -+ R-SO,. (formationofsulfonyl R(B). + SO, -+ no reaction

R(A).

R-0-

+ SO, -+ R-0-SO,.

(formation of

radical)

sulfite radical)

(20)

Sulfonyl and sulfite radicals are unstable at 25°C. They are likely to convert into sulfinic acid and sulfonate ester. When photo-induced wood free radicals are exposed to nitric oxide,

P-O(s). P-O(a)

1

+ NO -+ no reaction + NO "+ no reaction

+ NO "+ R-NO R(B)- + NO -+ R-NO + NO -+ R-0-NO R-0.

R(A)

(formation of nitroso group) (formation of nitroso group) (24) (formation of nitrite ester group) (25)

The R(B) group is more reactive with nitric oxide than sulfur dioxide. F. Effect of Water and Moisture on the Formation and Stability of Free Radicals

Wood used outdoors is exposed to the influence of water whether in the form of airborne humidity or as rain or dew. Water is considered to be a critical element in wood's photodegradability. Because water is a polar liquid, it readily penetrates and swells the wood cell walls. Water molecules may interact with free radicals generated by light. ESR studies [60] showed that when wood wasexposed to fluorescent light, the intensity, which is directly proportionalto the free-radical concentration(either in vacuoorair), initially increased as the moisture content increased from 0 to 3.2%, and reached a peak at 6.3%. At 15.9% moisture content, a significant decrease in intensity, i.e., decrease in free-radical concentration, was observed. At 31.4% moisturecontent,only a weakESR signal was detected (Fig. 1 l ) . From the stereotopochemistry point of view, it has been suggested that the principal role of water is to facilitate light penetration into the accessible regions and to open up the nonaccessible regions for light penetration. Thus, more free radicals are generated in these regions. The excess water molecules present probably trap free radicals to form a wood free-radical/water complex. The moisture content in wood cellulose itself also has exhibited a similar effect during photoirradiation. The presence of moisture in the range of 5-7% in photo-irradiated cellulose leads to a significant decrease in the ESR signal intensity, and when the moisture content increases beyond this critical range, the ESR signal intensities again increase (Fig. 12) [SO].

W . PARTICIPATION OF SINGLET OXYGEN IN THE PHOTOIRRADIATION PROCESS

In addition to sunlight and water, oxygenmolecules are among the mostubiquitous in nature. They play a unique role in many photophysical and photochemical processes. As

526

Hon

3.0

r

0% m.c.3.2%m.c.

6.3%m.c. i10.5%m.c. ;15.9%m.c.i 31.4%m.c.

L

c

Earlywood

FIGURE 11 Comparison of ESR relativesignalintensities (carriedout in a vacuum) of free radicals in earlywoodwithdifferentmoisturecontents. Key: SYP, southernyellowpine;WRC, western red cedar; DF, Douglas fir; RW, redwood.

discussed earlier, oxygen is an important element in promoting free-radical formation, and possibly that peroxide impurity is formed due to the interaction of free radicals and oxygen molecules.However, the rate of oxidation of mostpolymers is usuallyverysmall at ambient conditions without radiation. The acceleration of the reaction rate by electromagnetic energy may be due to the generation of excited oxygen species. Considerable evidenceexists that many photooxidation reactions involve the low-lyingsingletstateof oxygen ('A# and as intermediates [61]. The participation of singlet oxygen during the photoirradiation process has been reported [57]. Wood is a polymer blend containing cellulose, hemicellulose, lignin, and extractives. Thesewoodcomponentscontain internal chemicalentitiessuchascarbonyl,carboxyl, aldehyde, phenolic hydroxyl, and unsaturated double bonds, and external entities such as wax, fat, and metal ions. The absorption of light energy by these components may bring themtoanexcited triplet state that transfers the energy to triplet ground-stateoxygen

'x+)

527

Weathering and Photochemistryof Wood

.,

10

50

40

30

20

Moisture Content (“h) FIGURE 12 Relationship between ESR relative signal intensity and moisture content of cellulose irradiated with a high-pressure mercury lamp at 77 K for 60 min.

molecules to create singlet oxygen. The participation of singlet oxygen in the photooxidation ofwood wasevidenced by usingsinglet-oxygengeneratorsandsinglet-oxygen quenchersduring irradiation [57].Iodometrystudiesrevealed that hydroperoxidewas formed in wood photo-irradiated in the presence of oxygen. The formation rate of hydroperoxide at the wood surface increased when singlet-oxygen generators suchas rose bengal solutions were added to the wood prior to irradiation (Fig. 13). Peroxide radicals involved in the interimweredetected by an ESR spectrophotometer, i.e., anasymmetric single signal of peroxy radicals with an average g value of 2.021 (g,,= 2.034; g, = 2.007) was

Wood (control) Wood

wood

lrradlated In N,

1

lrrodlated In air

Wood lrradlated

In oxygen

. . . . . .

Wood nl th Rose Bengal Hood

wood with

nlth Rose Bengal Irradloted InN,

1

Rose Benpal Irradiated In air

Wood nlth Rose Bengal lrradlated In oxygen 1

0 0.4

0.2

.

1

.

0.6 0.8 O~tlcalDensltY at 360 M

1

1.0

.

1.2

FIGURE 13 Effect o f oxygen and rose bengal on the rate of peroxide formation in wood photoirrndiated for 24 h.

528

Hon

Wood (control1 Wood irradiated In air Wood wlth DABCO irrodlated in alr

I

Wood wl th Rose Bengal

IWoodwlthRoseBengalirradiated

in air

1

Wood wlth Rose Bengal andTEM Irradlated I n air

0

Wood wlth Rose Bengal and DABCO lrradlatedIn a i r 0.2 0.4 0.6 0.8 1.0 Optical Densltv at 360 nm

1

1.2

FIGURE 14 Inhibiting effect of DABCO and triethylamine o n peroxide formation in wood photoirradiated for 24 h.

detected. On the other hand, when singlet quenchers, such as triethylamine or 1,4-diazobicyclo[2,2,2]-octane (DABCO), wereusedunder identical experimentconditions, the hydroperoxide content was reduced in some cases, even in the presence of rose bengal (Fig. 14). This evidence supports the theory that singlet oxygen is formed during photoirradiation and that it interacts rapidly with free radicals of wood to produce hydroperoxides. Due to its instability against heat and light, the hydroperoxide decomposes rapidly under ambient conditions to create chromophoric groups, such as carbonyl and carboxyl groups. These groups contribute to the discoloration of wood surfaces.

V.

EFFECT OF ACID RAIN ON WOOD SURFACE QUALITY

The deleterious effects of acidrain on lakes, aquatic ecosystems, vegetation, forests, buildings, and artifacts have come to the fore in the past three decades and are the subjects of continuing investigation [62]. Wood materials offer an impressive range of attractive properties and in many of their applications they are exposed to the outdoorenvironment. Hence, they are subject to sunlight, weathering, and acid precipitation. Their increase use in outdoor applications has resulted in the needtounderstandweathering reactions involving acid rain. Although ultraviolet light is a major element in degrading wood polymers, it is to be expected that for lignocellulosic biopolymers, for which hydrolytic breakdown can be an important modeof deterioration 1631, acid rain will have a catalytic effect. In Section 1II.E we discussed that wood interacts readily with sulfur dioxide, one of the principal elements in acid rain, to trigger a wholeseries of free-radical reactions, especially in the presence of ultraviolet light. These reactions lead ultimately to discoloration and loss of surface integrity. Many factors are involved in wood deterioration in the polluted environment. The absorption of UV light and acid rain can lead to chemical changes on wood surfaces, and deterioration of tensile strength was proved experimentally. When wood surfaces are exposed to ultraviolet light, carbonyl-group content increased and lignin content decreased simultaneously. These changes are accelerated whenthey are also exposed to acid rain,

529

Weathering and Photochemistryof Wood TABLE 1 EffectofAcid Ram on Carbonyl-Group Content in Wood at 65°C and 6S% Relative Humidity in the Absence of UV Light Acid concentration (%)

Exposure time (h)

3.7 0 6 12 24 36 3.8 48

0.074 0 0.009 0.002 3.7.l 1

-

4.2

5.6I

4.2

-

-

3.9

-

I

"FTIR absorption peak r a t i o : 1735 c m '1895 c n - I.

i.e., a dilute sulfuric acid solution, especially at 65°C and 65% relative humidity. FTlR studies demonstrated that carbonyl groups are generated on photo-irradiated wood surfaces at ambient temperature, and the rate of increase in carbonyl groups is enhanced i n the presence of acid. When wood was exposed to an elevated temperature, i.e., 65°C and 65% relative humidity, its chemicalcomponents did not showa noticeable degradation. The carbonylcontent of untreated wood remained almostconstantas the exposuretime increased. The same is true for lignin content. In the presence of acid under such conditions, the carbonylgroupincreases slightly as a function of acid concentration (Table 1). No significant change in lignin is observed (Table 2). The effect of acid and U V light at 65°C and at 65% relative humidity on carbonyl-group and lignin content is significant (Table 3). When wood was exposed to UV light in the absence of acid at such conditions, it was noticed that carbonyl-groupcontent increased almost sixfold after 48 h of irradiation. About 80% loss of lignin content was observed (Table 4). Therate of degradation is further enhanced in the presence of acid. The higher the acid concentration, the more carbonyl group is found. Although lignin is resistant to acid attack, significant reduction of lignin content is observed under such conditions. The increase i n carbonylgroup and reduction of lignin content at wood surfaces being exposed to acid and U V light at ambicnt and at high temperature and high humidity

TABLE 2

Effect of Acid Rain on Lignin in Wood at 65°C and 6S% Relative Humidity i n thc Absence of U V Light Acid concentration (S)

Exposure time ( h )

4.4 0 3

6 3.8

5.5 12 24 5.8 36 48

0

0.003- 0.074 0.009

4.4" 4.3

.c>

3.8 4.2 4. I 3.9

3.8 3.8

-

-

-

5.2

-

5.0

Hon

530

TABLE 3 EffectofAcid Rain on Carbonyl-Group Content in Wood at 65°C and 65% Relative Humidity in the Presence of UV Light

Exposure time (h)

Acid concentration (%) 0.074 0 0.009 0.002

0

3.7"

6 12 24 36 48

8.2

3.7 9.8 16.0 19.0 19.3 23.0

3.7 11.7 16.5 20.4 21.3 23.7

"FTIR absorptionpeakratio: 1735 cm '1895 cm

3.7 11.9 19.2 22.8 24.4 25.4

I

signaled that chemical reactions take place in the polymeric components of wood, i.e., cellulose, hemicelluloses, and lignin. Since holocellulose is sensitive to acid hydrolysis, there is little doubt that the strength of wood will be reduced due to the chain scission reaction. The change in color of wood from pale yellow to dark brown and the embrittlement of wood specimens clearly indicate severe degradation of wood quality. Experimental results showed that oxidative degradation of wood surface is initiated by UV irradiation. However, photooxidation by itself appeared not to be a contributor to the loss of tensile strength. Acid rain appeared to be the major culprit contributing to the loss of strength [80]. Effects of acid rain on tensile strength at ambient temperature and at 65°C are shown in Figs. 15 and 16. As discussed earlier, the photo-induced free radicals in wood are capable of reacting with sulfur dioxide to produce various oxidized products. They, in turn. further influence the color and surface properties. The presence of UV light may also accelerate oxidation of sulfur dioxide to sulfur trioxide, which will subsequently react with water to produce sulfuricacid.Sulfuricacid may also form by othercatalyticprocessesinvolvingsulfur dioxide, water, and a catalyst present in the atmosphere. Hence, three plausible mechanisms that can lead to surface degradation of wood are worth considering. They are summarized below.

TABLE 4 Effect of Acid Rain on Lignin in Wood at 65°C and 65% Relative Humidity in the Presence of U V Light

Acid concentration (%I)

Exposure time (h)

0

6 12 24 36 48

0

4.44.4" I .8

0.002

0.009

4.4 2.1 I .3

1.2 I .o

0.8

0.9 0.X

"FTIR absorption peakratio: IS07 cn"/Xc)S c m

2.0 I .4 I .3 I .2 1.2 I.

0.074 4.5 2.5

1.7 I .S

I .3 I .3

531

Weathering and Photochemistryof Wood

"

t

d

IO

Exposure

40

30

20

Time

50

(H)

FIGURE 15 Effect of acid rain on tensile strength of wood at ambient temperature in the presence of UV light: (a) UV-irradiated without acid; (b) UV-irradiated with 0.002% acid; (c) UV-irradiated with 0.009% acid; (d) UV-irradiated with 0.074% acid.

100

-

0

y 80 v

C

.-0 60 W t

Q

a 40 c c

0 C

al

2 20

m

0

10

20 Exposure

30

40

50

Time (H)

FIGURE 16 Eftect o f acid rain 011 tensile strength of wood at 65°C: ( ; I ) control at 65°C: ( b ) UVirradintctl withoutacid: ( c ) UV-irradiated with 0.002% acid: (cl) UV-irradiatedwith 0.009%' acid: ( c ) UV-irrxliatcd with 0.074% acid.

Hon

532

1. 11.

+

Wood h u + woodfreeradicals Wood free radicals SO, + oxidized products

+ hu + so, + O? -3 so,

+ H,O H,SO, H,SO, + wood + degradation products 111. SO, + O2 + catalyst -+ [SO,. H 2 0 ] SO,

[S02-HrO]+ H2S0,

H,SO,

+ wood -+

degradation products

VI.

CHANGES IN PHYSICAL AND CHEMICAL PROPERTIES

A.

Discoloration

The effect of light on the color of wood is to cause it to fade or darken and to bring about changes in tone. Extensive studies and observations have shown that most, if not all, wood species of commercial importance are prone to discoloration with age. The rate of discoloration is usually related to the intensity of light and its wavelength, and also depends on the species of wood. Light of normal intensity tends to promote darkening generally, while that of high intensity usually causes fading, though often after an initial period of darkening. Pale-colored wood such as pine, oak,birch,beech, and sycamore generally respond to radiation below 400 nm, while the coloredspecies that absorblight in the visible spectrum also are sensitive to certain wavelengths i n the visible region. Discoloration is influenced by such factorsastemperature, water, and atmosphere.Thussome woods weathered by sunlight become red-brown; and in the presence of water, some woods become gray [64]. When wood is exposed to ultraviolet light fora relatively short time, changes in reflectance and color are readily observed. When birch and redwood are exposed to ultraviolet light, they darken during the first several hours in the atmospheres of air, oxygen, nitrogen, and argon. Upon continuing the UV exposure,the wood samples in air and oxygen stop darkening and become lighter, while those in nitrogen and argon continue to darken. The decreases in reflectance and color during 480 days of UV irradiation of several wood species in air are shown in Figs. 17 and 18, respectively. It is clear that, in addition to the change in reflectance, all wood species exposed to ultraviolet light changed in color from pale yellow to brown and to gray after 180 days of exposure. Changes in wood color reflect chemical changes in wood during ultraviolet irradiation. As discussed earlier, cellulose has a fair degree of resistance to photooxidation and is not known to discolor appreciably in ordinary light. Lignin, on the other hand, is more susceptible to photooxidation and readily undergoes structural changes i n ultraviolet light to generate chromophoric groups. The extent of lignin degradation has been analyzed in weathered wood from the brown underlayer to the outer gray layer of wood, as shown in Table S. The details of discoloration are discussed further in Chapter 9.

B.

MicroscopicChanges

Microscopicchangesaccompanythegross physical changes in wood during ultraviolet irradiatioll 13.65-671. The tirst sign of deterioration i n softwood surfaces is enlargemcnt

533

Weathering and Photochemistryof Wood

K

100

.-0 U

K

Southern Yellow Pine

Q) +

50

0

60

120

180

240 360 300

480

420

Weathering Time (Days)

FIGURE 17 Decrease in brightness of outdoorweathered wood.Key: 0, southern yellow pine; 0, Douglas fir.

M,

westernred

cedar;

0,

redwood;

loo 80

i

20

0

60

120 240

., .,

180

FIGURE 18 Change i n color of outdoor weathered wood. Key: wood;

Douglas fir;

420

360 300 Weathering Time (Days)

western red cedar.

0,

480

southern yellow pine;

D,

red-

Hon

534

TABLE 5 Change in Lignin Content of a Weathered Southern Pine-l0 Years Exposure

Centered portion Brown underlayer 24.2 Graylbrown underlayer Gray surface

A"

Bb

28.0 23.4 19.2 14.5

27.9 19.1 14.6

'Calculated from FlYR absorptionpeak at 1510 cm". hCalculatedfrom UV absorbance at 278 nm.

of apertures of bordered pits in radial walls of earlywood tracheids. Futo [68] observed that the degradation begins at a relatively low irradiation intensity with an attack on the compound middle lamellae. Higher intensities and longer exposure also degrade the secondary walls, as is made visible by the formation of cavernes. Elevated temperature intensifies the photolytic degradation process. The degradation of the wall substance during UV irradiation effects a contraction of the cell walls, resulting in microchecks along the compound middle lamellae and, particularly in latewood, along the border between S , and

FIGURE 19 Cross section of southern yellow pine (700X).

ochemistry andWeathering

of Wood

535

S , [69].Diagonal fissures that follow the fibril orientation of the Sz layer also have been observed. The apertures of the bordered pits in softwood were enlarged or ruptured by microchecks. The scanning electron microscope (SEM) is frequently used to study the breakdown of the structureof wood due toweathering. The surface texturesof woods from Norwegian stave churches and other wooden construction several hundred years old were studied by Borgin using an electron microscope [70,71]. Deterioration of wood surfaces after exposure to artificial W light was observed after wood was exposed for only 500 h [65]. Photodegradative effects on transverse, radial, and tangential surfaces of a typical southern yellow pine are described in the following sections.

1. Transverse Section The transverse sectionof southern yellow pine is normally quitesimple and homogeneous. Its axial system is essentially composed of wood tracheids, with only a relatively small number of parenchyma cells. An SEM micrograph of a transverse southern pine surface before exposure is shown in Fig. 19. A microtomed transverse wood surface was exposed to W light for 500 h. Surface deterioration of the exposed wood surface was observed readily from the SEM micrograph

FIGURE 20 Cross section of southern yellow pine exposed to UV light for 500 h (700X).

536

Hon

FIGURE 21 Cross section of southern yellow pine exposed to UV light for loo0 h (700X).

(Fig. 20). The cell walls were separated at the middle lamella zone. In the extreme case, the secondary wall almost collapsed. Roughening of the surfaces could be observed visually. Surface deterioration further developed when specimens were exposed for a total of lo00 h (Fig. 21). Bordered pits located at the tracheid walls were totally destroyed. The color of the exposed wood changed from pale yellow to light brown and then dark brown after 500 and 1000 h of W light exposure, respectively.

2. Radial Section Bordered pits in southern yellowpine could be observed at radial walls in both earlywood and latewood. Generally, bordered pits located in the earlywood were larger and more numerous than those in the latewood. Dpical SEM micrographs for half-bordered pitsand bordered pits at radial walls before W exposure are shown in Figs. 22 and 23. The first perceptible change in the anatomical structure of the radial sectionof southem yellow pine upon exposure appears to take place at the pits. After 500 h of UV exposure, half-bordered pits were damaged. Bordered pits also interacted with light, but to a lesser extent (Fig. 24). The bordered pits could still be recognized.In addition, checking and void formation in radial walls occasionally could be seen from the exposed specimen. After 1000 h of exposure, however, severe deterioration of the bordered pits was

chemistry andWeathering

of Wood

FIGURE 22 Half-bordered pit structures of southern yellow pine

537

on radial section (700X).

observed. The SEM micrograph (Fig. 25) shows that the apertures of bordered pits were enlarged to the limit of the pit chambers. The pit domes were destroyed completely. At the extreme, the deterioration also spread over the radial surface of the tracheid wall. Complete degradation of these cell walls would probably take place at a longer exposure time. Disappearance of bordered pits also has been observed in redwood exposed to UV light.

3. Tangential Section Bordered pits are rarely found in the tangential surfaces observed. SEM studies revealed that diagonal microchecks passing through bordered pits in tracheid cell walls were the most conspicuous anatomical change at the tangential section upon UV exposure. The narrow microchecks were oriented diagonally to the axis of the cell wall, thus indicating that microchecks occur at the fibril angles of the S2 cell wall (Figs. 26 and 27). Similar observations have been reported. The common appearance of the diagonal microchecks during UV exposure was suggested to be the resultof local concentrations of tensile stress at right angles to the fibril direction of the S , layer. Relatively wide diagonal checks were observed in the tangential section of tracheid walls of latewood.

Hon

538

FIGURE 23 Bordered pit structures of southern yellow pine on radial section (700X).

C.

ChemicalChanges

The consequences of photodegradation and photooxidation of wood are changes in chemical and physical properties. As discussed earlier, irradiated wood may exhibit a form of discoloration, loss of lightness, checking, cracking and rougheningof surfaces, damage of microstructure, and loss of weight. It is believed that these changes are caused by severe chemical modification of the structures of cellulose, hemicelluloses, and lignin. Over a century ago, Wiesner [72] reported that the intercellular substance of wood had been lostandtheremainingmembranes,consisting of chemicallypure or nearly chemically pure cellulose, were observed. As discussed in the earlier section on microscopic changes, it is clear that absorption of UV light by lignin in the middle lamella as well as in the secondary cell walls results in preferential lignin degradation. Most of the solubilized lignin degradation products are washed out by rain [54].Careful analyses of surface layers of a southern pine that had been weathered for about 10 years revealed that the top gray layer consistently exhibited a very low lignin content. The grayhrown layer had a higher lignin content than the outer gray layer but less than the brown underlayer and in the centered portion (see Table 5). Accordingly, it is obvious that ultraviolet light initiates significant modification to the wood polymeric system. From the ESCA study [73], the increase in signal intensities of carbon-oxygen bonds and oxygen-carbon-oxygen bonds (or unsaturated carbon-ox-

ochemistry andWeathering

of Wood

539

FIGURE 24 Deterioration of half-borderedpits and cell wall of southern yellow pine atradial section after exposure to UV light for 500 h (700X).

ygen bonds) and oxygen-to-carbon ratio, and the decrease in carbon-carbon and carbonhydrogen bonds of weathered and UV-irradiated wood surfaces, suggested that the wood surface was oxidized. The oxygen-to-carbon ratio data also revealed that weathered wood surface was rich in cellulose and poor in lignin. FTIR [55] studies showed the increase in carbonyl and hydroperoxide groups and the decrease in lignin content of UV-irradiated wood surfaces. On the whole, cellulose, hemicelluloses, and lignin are degraded as illustrated by increase in solubility and reducing power of cellulose and formation of carbonyl, carboxylic, hydroperoxide groups, quinone, and conjugated double bonds in lignin. The changes in carbonyl, carboxyl, and hydroperoxide groups in cellulose are shown in Table 6 [36]; and the changes in carbonyl and hydroperoxide groups in wood are shown in Table 7. During photoirradiation, in the initial stages up to 1 h, only CO, COz, Hz,and HzO were detected as gaseous products. At longer exposure times, however, methane, ethane and ethylene hydrocarbon gases were found. Figure28 shows the rateof formation of CO, COz, and H2 in a southern pine irradiated with ultraviolet light. Volatile products of formaldehyde, methanol,acetone,methylformate,acetaldehyde,propionaldehyde,vanillic acid, vanilin, and syringyl aldehyde also were found after longer exposures [57]. The photo-irradiated wood increased its solubility in water, benzene, alcohol, and alkaline aqueous solutions. Carbohydrates and phenolic compounds are detectable from

540

Hon

FIGURE 25 Deterioration of bordered pits and cell wall of southern yellow pine atradial section after exposure to UV light for lo00 h (700X).

the solutions. The contents of holocellulose, cellulose, and lignin in wood were decreased as a function of irradiation time (100 days) with different wavelengths. Table8 shows the results of such degradation. A study of the loss of weight by UV irradiation (A > 340-320 nm) was carried out by Futo [68,74]. He found that the loss of weight is highly influenced by the temperature and the irradiation energy. The loss of weight is much higher when wood is irradiated in the presence of water, which indicates that water-soluble products are formed in addition to gaseous and volatile products. The collection of water-soluble fragments was characterized using UVhisible spectroscopy by Hon and Chang [54]. They found that the lowmolecular-weight, water-soluble products are derived mostly from lignin. The degradation products contained carbonyl-conjugated phenolic hydroxyl groups and had a weight-average molecular weight of about 900 as determined by gel permeation chromatography. The degradation of cellulose under the influence of ultraviolet light is indicated by a decrease in strength and degree of polymerization, and an increase in alkali solubility and copper number. When a bleached softwood pulp was irradiated in vacuum and in oxygen for only 10 h, the DP of a-cellulose was reduced from 850 to about 380 and 260, respectively. The content of a-cellulosedecreasedfrom 88% toabout 50% and 40%, respectively [32]. Furthermore, ultraviolet light causes yellowing and browning and for-

Weathering and Photochemistryof Wood

541

M

l

"

FIGURE 26 Microchecks of cell wall of southern yellow pine at tangential section (earlywood) after exposure to UV light for 500 h (700X).

mation of carbonyl, carboxyl, and hydroperoxide groups along the cellulose chain, and a fragmentation of molecules to diversities of neutral and acidic nonvolatile, volatile, and gaseous products. Among the volatile degradation products of photo-irradiated cellulose are acetaldehyde, propionaldehyde, methyl formiate, acetone, methanol, ethanol, methane, and ethane [76]. Glucose, cellobiose, cellotriose, xylose, xylo-oligomers, arabinose, and 3-P-D-glucosido-D-arabinose wereidentified fromthesoluble degradation products [10,11,77]. The photodegradation of lignin also has been observed. The reduction of the methoxy1 content and the splitting of monomeric units were reported [15,75,78,79].

VII. CONCLUSIONS The deterioration of wood materials upon weathering involves a very complex reaction sequence. Penetration of W light into wood does not traverse deeper than 75pm. Nonetheless, wood surface reactions initiated or accelerated by light can be observed by discoloration, loss of brightness, and change in surface texture after artificial W light irradiation or long-term solar irradiation.

Hon

542

FIGURE 27 Microchecks of cell wall of southern yellow pine at tangential section (latewood) after exposure to UV light for 500 h (SOX).

TABLE 6 Formation of Carbonyl, Carboxyl, and Hydroperoxide Groups in Photo-Irradiated Cellulose" Hydroperoxide Carboxyl Carbonyl group Irradiation group time group (h)

(mmoV100 g)

0 3 5

10 15 20

0.1

(mmoV100 g)

(mmOV100g)

0.2

0.0

5.6 10.1

1.1

0.0 0.0

15.9

1.9 4.2

18.5

6.5

0.3 0.5

9.7

0.6

'Irradiated with a hlgh-pressure quartz mercury lamp (h > 253.7 nm) at amblent temperature.

543

Weathering and Photochemistryof Wood TABLE 7

Formation of Carbonyl and Hydroperoxide Groups Photo-Irradiated Wood

IrradiationCarbonyl time Hydroperoxide group group (mmoll100(h)

24

100 g)

~~

74

85

in

1.20

0

30 60 90 120

I .71 4.25

3.10

4.32 8.26

150

180 210 "Irradiated with a high-pressure quartz mercury-xenon compact arc lamp (A > 223 nm) at ambient temperature.

m 0) H

0

E

I

0

50

100 150 200 250

300

Irradiation Time (H) FIGURE 28 Rate of formation of (a) carbon dioxide, (b) carbon monoxide, and (c) hydrogen a southern yellow pine irradiated with ultraviolet light of A > 254 nm.

TABLE 8 Effect of Ultraviolet

Light on Holocellulose, Cellulose. Lignin, and Southern Pine Irradiated in Air for 100 Days

Chemical constituent wood Holocellulose Cellulose Lignin Extractives"

Extractives of

Content (%)

Unirradiated

68.1 45.3 27.8 10.5

in

A > 340 nm

A > 280 nm

A > 253 nm

58.4 43.7 22.1 16.4

56.8 41.5 14.6 27.8

34.5 32.7 8.6 40.2

"Obtained by a two-step extraction: first step, water extraction for 72 h; second step, benzynekthanol extraction for 72 h.

Hon

544

Various types of free-radical species, such as phenoxy, alkoxy, and carbon radicals, are readily generated in wood by light. Phenoxy radicals are quite stable at ambient ternperature, whereas alkoxy and carbon radicals decay rapidly at that temperature. Carbon radicals rapidly interact with oxygen to produce hydroperoxide impurities that are decomposed easily to produce chromophoric groups such as carbonyl and carboxyl groups. A competitive reaction between formation and destruction of hydroperoxide appears to be occurring during the photoirradiation, and the hydroperoxide is unstable above 65°C. The hydroperoxidation of wood surfaces can be readily detected by using the DRIFT technique without any sample preparation and destroying the surface. The ESR technique also providedvaluableinformation on the free-radical formationmechanism that explains the formation of hydroperoxide. The use of singlet-oxygen generators, such as rose bengal andmethylene blue, aswell as singlet-oxygenquenchers,such as 1,4-diazo-bicyclo[2.2.2]octane and triethylamine, suggests the participation of singlet oxygen as an effective intermediate in photooxidation reactions at the wood surface. Phenoxy radicals are inert toward sulfur dioxide and nitric oxide. All carbon radicals or alkoxy radicals are capable of reacting with nitric oxide to form nonradical products such as nitroso and nitrite groups. Somecarbon radicals are sensitivetoward sulfur dioxide to form sulfonyl and sulfite radicals andconvertedinto sulfinic acidandsulfonate ester. The presence of water in wood also influences the rate of free-radical formation. When moisture content in wood is increased from 0 to 6.3%. more free radicals are formed. Beyond this stage, the rate of radical decay increases. Infrared studies reveal that carbonyl groups are generated in cellulose and lignin. Water-soluble fractions of degraded wood exhibit characteristics of phenolic absorptions due to the loss of lignin. ESCA studies show that oxidized wood surfaces contain higher oxygen contents than unexposed wood surfaces. In addition to UV light, acid rain seems to be an important element contributing to deterioration of wood surface quality and tensile property of wood. Experimental results showed that while carbonyl group was generated on wood, lignin content simultaneously decreased when it was exposed to light; and this process was further enhanced when it was also exposed to acid rain. Either with or withoutultraviolet, at 65"C, wood deteriorated slightly faster than that at ambient temperature. Acid further accelerated the degradation process. Wood lost its tensile strength in the presence of acid both at ambient temperature and at 65°C.

REFERENCES 1. 2. 3. 4.

5. 6. 7. 8. 9. 10. 11.

D. N.-S. Hon and W. C. Feist, Wood t r n d Fibel; 12: 121 (1980). D. N.-S. Hon, J. Appl. Polymer Sci., Appl. Polymer Sywp., 3 7 845 (1983). H. Turkulin and J. Sell, Bericht 115/36 (1997). D. N.-S. Hon, Durability and Serviceability of Wood and Wood-based Products, presented at The New Tropical Timber Crops: Challenges in Processing and Utilization, International Tropical Wood Conference, June 17-20, 1997, Kuala Lumpur. Malaysia. D. Fengel andG. Wegener, Wood: Cl~en~istty, lJltrtrstrut~ture9 Recrctiorl, Walter de Gruyter, Berlin-New York ( 1984). T. Yoshinloto, Mokuztri Gtrkkniski, IS: 49 ( 1 972). W. C . Feist and D. N.-S. Hon, Ad\!. Cl~enz.SL'K,207: 401 (1984). H. Norrstrom, Sver~.Ptrpperstidn., 72: 25 (1969). N.-S. Hon, J . Polyrnt'r Sci., Polwwr Cllent. Ed., I S : 1347 (1975). A. Beelik and J. K. Hamilton, Pupel; 13: 77 (1959). A. Beelik and J. K. Hamilton, J . Org. Cllern., 26: 5074 ( 1961).

Weathering and Photochemistryof Wood

545

12. A. Box, J . Appl. Polyrner Sei.,16: 2567 ( 1972). D. N.-S.Hon and W. Glasser. Polymer-Ptrst. Teclznol. Eng.. 12: 159 (1979). 14. N.-S.Hon, J . Polymer Sei., Polyner Clzern. Ed., 13: 2641 (1975). 15. T. N.Kleinert, P q i e K 24: 207 (1970). 16. U.S.Forest Service Research Note, Wood Fini.shirt 0

m

1 5 0 200

The variations of E' and tan 6 with temperature at I 1 Hz lor the propylene oxidetreated wood areshown in Fig. 18 [57,58]. Thistreatment results in bonded cell-wall bulking as in the case of acetylation, except that the introduced group is hydrophilic. The E' was lower than those of the untreated samplesover the temperature range tested, especially above 100°C. Three relaxation processes, labeled a,,(,to 'y,.(,,were detected in

""-

kll """""

Chemical Wood Modification of

595

tan 6. As the introduced OCH,CH(OH)C,H5 group is bulkier and more flexible than the OCOCH, group. the cell wall polymers are much plasticized compared to those of acetylation and the a/.,,peak was observed within the temperature range tested. The treatment reduceshygroscopicity at low relative humidity levels 1291, so the &, processdue to adsorbed water became less distinct. The introduced side chain contains the OCH, group whose motion may be responsible for the y,.(, process as in the y/..process. The temperature dependence of E' and tan 6 at I 1 Hz for the wood-MMA composite is shown in Fig. 19. The E' was larger than that of the untreated wood within the temperature range tested. With respect to tan 6, five relaxation processes were observed, labeled aIV,PI,,, ylv, 6,,., and E,,. in order of decreasing temperature. A very low ASE shows that the cell wall remains untreated, while PMMA fills the lumen. Therefore, both relaxation processes observed in the untreated wood and PMMA are expected. Two relaxation processes assigned, respectively, to the micro-Brownian motion of the main chain and the motion of the OCOCH, side group were detected in PMMA 1701. These locations corresponded, respectively, exactly to the p,,. and yl,. processes. Thus. the remaining al,,, 6,,,, and cl,, processes can be ascribed to the same motions as those of the a(,to yI, process. Thetemperaturedependence of E' and tan 6 at 1 1 Hz for the PEG-impregnated wood is shown i n Fig. 20 [57,58]. The E' was larger than that of the untreated wood below O"C, but became lower above that temperature. Three peaks were observed in tan 6, labeled p,.(;,and y/.(;i n order of decreasingtemperature. I t is well known that PEG molecules with molecular weights of 1000 can penetrate into the cell wall [71,72] and act as a plasticizer, which leads to large reductions in the cohesive forces between the cell wall polymer molecules. The a/.(;was attributed to the micro-Brownian motions of the cell wall polymers plasticized with PEG molecules [60,72]. The tan 6 of the PEG-impregnated glass fibers had a peak between -50°C and 0°C. The apparent activation energy for this process estimated from the measurements at five different frequencies was about 40 kcal/mol, which suggested a mechanism involving chain backbonemotions of PEG

.

-

9

0.01

l

Norimoto

596

2 -51 (3 v

"""-

lo

W

5

FIGURE 20 Temperature variations of E' and tan 6 for untreated (dotted line) and polyethylene glycol-impregnated (solid line) woods.

molecules. On the other hand, the tan 6 increased remarkably above 20"C, which corresponds almost to the melting point of PEG. Therefore, this sharp increase in tan 6 may be related to the flow of PEG molecules. The p,.(; process was not recognized in PEG. PEG molecules in the cell wall plasticize the cell wall polymers, but at the same time their micro-Brownian motions must be restricted to some extent by the cell wall polymers. This may lead to a shift of the peak due to the micro-Brownian motions of PEG molecules toa higher temperature.The peak due to CHzOHgroup occurred about 20°C below the y,, peak temperature, and its value was rather large.

REFERENCES 1.

2. 3. 4.

S 6. 7. 8. 9. IO. II. 12.

R. M. Rowell, in Wood r r n d Ce//u/o.srChemistry (D. N.-S. Hon and N. Shiraishi, eds.), Marcel Dekker. New York and Basel, p. 703 (1991). N. Shiraishi, T. Aoki, M. Norimoto, and M. Okumura, C h e n ~ t c ~ h13: . 366 (1983). M. Norimoto, J . Gril, and R. M. Rowell, Wood Fiber Sci., 24: 25 (1992). M. Norimoto and J . Gril, in R w d Resenrrh U I I Wood m d W O O ~ - B ~ ~Mnrerids . S C > L ~ (N. Shiraish. H. Kajita, and M. Norimoto, eds.). Elsevier Applied Science, London and New York, p. 13.5 (1993). Modificrcriorl U/ o f Ligrzoce//[r/o.sicMrrfericcls (D. N.-S. Hon,ed.). M.Norimoto, in C / W I ? I ~ C Marcel Dekker. New York, Basel. and Hong Kong, p. 3 11 (1996). M. Inoue, N. Kadokawa, J. Nishio, and M. Norimoto. Wood Res. Techno/. Notes, No. 29: S4 ( 1993). R. M.Rowell and P. Konkol. FPL Gen. f i 4 . Rep. (USIIA). No. 55: 1 (1987). , 493 (1947). A. J. Stanm and H. Tarkow, J. Phys. Co//oir/C / ~ e m .31: M. Norimoto. Wood Res. 7 k ' / ? I l . Nures, No. 24: 13 ( 1988). R. M. Rowell. A.-M. Tillman. and R. Simonson. J . Wood C/7rw7. f i d t r r o l . , 15: 293 (1986). R. M. Rowell. Y. Imamura, S. Kawai. and M. Norimoto, Wood Fiber Sci.. 21: h7 (1989). R. M. Rowell and W. D. Ellis, FPL Res. Pup. (USDA), No. 451: 12 (1984).

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597

13. M. Inoue, S. Ogata, M. Nishikawa,Y. Otsuka, S. Kawai, and M. Norimoto,Mokuzai Gakkaishi, 39: 181 (1993). 14. M.Inoue, S. Ogata, S. Kawai,R.M.Rowell,and M. Norimoto, Wood FiberSci., 25: 404 (1993). 15. N. Hirai, N. Sobue, and I. Asano, MukuzuiGakknishi, 18: 535 (1972). 16. A. J . Stamm, Wood and Cellulose Science, Ronald Press, New York, p. 317 (1964). 17. M. Inoue and M, Norimoto, Wood Res. Technol. Notes, No. 27:31 (1991). 18. P. U. A. Grossman, Wood Sci. Tech., 10: 165 (1976). 19. M. Norimoto, J. Gril, K. Minato, K. Okarnura, J. Mukudai, and R. M. Rowell, Wood Ind., 42: 504 ( 1987). 20. R. Yasuda, K. Minato, and M. Norimoto, Wood Sci. Technol., 28: 209 (1994). 21. S. Takino, M. Norimoto, S. Kawai, and H. Sasai, Mokuzai Gakknishi, 35: 625 (1989). 22. A. H. Yano, J. Mukudai, and M. Norimoto, MokuzaiGnkknishi, 34: 94 (1988). 23. H. Yano, M. Norimoto, and R. M. Rowell, Wood and Fiber Sci., 25: 395 (1993). 24. C . Tanaka, T. Nakao, and T. Takahashi, Mokuzai Gakkaishi, 33: 8 I 1 (1987). 25. M. Norimoto, T. Ono, and Y. Watanabe, J. Soc. Rheol. Jpn.. 12: I 15 (1984). 26. M. Norimoto, MokuzaiGakknishi, 28: 407 (1982). 27. H. Akitsu, M. Norimoto, and T. Morooka, Mokuzai Gakknishi, 37: 590 (1991). 28. H. Akitsu, J. Gril, and M. Norimoto, Mokuzai Gakkaishi, 39: 258 (1993). 29. H. Akitsu, M. Norimoto, T. Morooka, and R. M. Rowell, Wood Fiber Sci., 25: 250 (1993). 30. T. Ono and M. Norimoto, Jpn. J . Appl. Phys., 22: 61 1 (1983). 3 1. T. Ono and M. Norimoto, Rheol. Acta, 23: 652 (1984). 32. M. Norimoto, J. Gril, and T. Sasaki, in Pruc. Eurupenn Scientific Colloquium on the Mechunicul Behnvior uf Wood, Bordeaux, France, p. 37 (1 988). 33. M. Norirnoto, F. Tanaka, T. Ohogama, and R. Ikimune, Wood Res. Technol. Notes, No. 22: 53 (1986). 34. T. Ono and M. Norimoto, Jyn. J. Appl. Phys., 24: 960 (1985). 35. T. Sasaki, M. Norimoto, T. Yamada, and R. M. Rowell, Mukuzui Gakkaishi, 34: 794 (1988). 36. M. Norimoto, T. Ohogama, T. Ono, and F. Tanaka, J. Soc. Rheol. Jpn., 9: I69 (198 I ) . 37. H. Yano and K. Minato, J . Acoust. Soc. Am., 92: 1222 (1992). 38. H. Yano, M. Norimoto, and T. Yamada, Mukuzui Gakkaishi, 32: 990 (1986). 39. T. Ono, Y. Katoh, and M. Norimoto, J . Acoust. Soc. Jpn., 9: 25 (1988). ~ . Am., 96: 3380 (1994). 40. H. Yano and K. Minato, J . A c ~ u s SOL.. 41. Y. Liu, M. Norimoto,and T. Morooka, MokuzniGakkni.shi, 39: I140 (1993). 42. M.Norimoto, MoklczuiGnkkcrisl~i, 39: 867 (1993). 43. M.Norimoto, Wood Res. Z~chnol.Nutes, No. 30: 1 (1994). 44. J. Gril and M. Norimoto, in Proc. COST-508 Wood Mechanics Wurkshop on Wood: Plasticity tend Danlccge, Ireland, p. I35 ( 1993). 45. Y. Liu, M. Norimoto. and T. Morooka, Wood Res. Technol. Nutes, No. 31: 44 (1995). 46. 1. Iida and M. Norimoto. and Y. Irnamura, Mokuzui Gnkknishi, 30: 354 (1984). 47. M.Inoue, T. Aoki, and G. Egawa, Wood Res. TeLhllol. Notes, No. 28: 59 (1992). 48. M. Norimoto and J. Gril, J. Micr-owntv Electromngn. Energy, 24: 203 (1989). 49. M. Inoue, K. Minato,and M. Norimoto, MokuzuiGcckkoishi, 40: 931 (1994). 50. K. Minato, N. Kubo, M. Norimoto, H. Sasaki. M. Sawada, and T. Yamamoto, Moku-ui Gnkkuishi. 38: 67 (1992). 51. W. Dwianto, M. Inoue, and M. Norimoto, Mukrrzcci Gukknishi, 43: 303 (1997). 52. M. Inoue. M. Norimoto. M. Tanahashi, and R. M. Rowell, Wood Fiber Sci., 25: 224 (1993). 53. W. Dwianto. F. Tanaka, M. Inouc, and M. Norimoto, Wood Res., No. 83:47 (1996). 54. H. E. Hsu, W. Schwald. J. Schwald, and J. A. Shields, Wood Sci. Technol.. 22: 281 (1988). 55. E. Obataya. M. Sugiyama.and M. Norimoto, Wood. Res, No. 83: 40 (1996). 56. M. Takayanagi. H. Harima, and Y. Iwata, Rep. Prog. Polymer- P l y . Jpn., 6: 1 13 ( 1963). 57. M. Sugiyama, E. Obataya, and M. Norimoto, Wood Res., No. 82, 31 (1995). Mokrrvti Grrkkoishi, 42: 1049 (1996). 58. M.SugiyamaandM.Norimoto.

598

Norimoto

H. Becker and D. Noack, Wood Sci. Techno/., 2: 213 ( 1968). T. Sadoh, Wood Sci. Techno/., 15: 57 (1981). E. Obataya, M. Yokoyama, and M. Norimoto, Mokuzui Gakkrcishi, 42: 243 (1996). M. Norimoto and T. Yamada. M o k u w i Grtkkaishi, 23: 99 (1977). G. Zhao, M. Norimoto, and T. Yamada, Mokuzrri Cukkrrishi. 36: 257 (1990). M. Norimoto and G. Zhao, Mokuzcti Gakknishi, 39: 249 (1993). G. P. Mikhailov, A. I. Arthyukhov, and V. A. Shevelev, Polwter Sci. USSR, 11: 628 (1969). M. Norimoto, Wood Res., No. 59/60: 106 (1976). M. Kimura and J. Nakdno, J . Polyrner Sci., Polymer Lett. Ed.. 14: 741 (1976). T. Morooka, M. Norimoto, T. Yamada, and N. Shiraishi. Wood Res., No. 72: 12 (1986). T. Morooka, M. Norimoto, T. Yamada. and N. Shiraishi, Wood Res., No. 69: 61 ( I 983). N. G. McCrum, B . E. Read, and G. Williams, Artelmtic crntl Dielecfric eflec'fs in Po/ytneric' Solids, Wiley, New York, p. 240 (1967). 71. H. Tarkow, W. C. Feist, and C. F. Southerland, Forest Prod. J.. 16: 61 ( 1966). 72. Y. Tominagd and T. Sadoh, Mok~rzniGukkrriski, 36:263 ( 1990).

59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70.

Chemical Modification of Cellulose Akira lsogai The University of Tokyo, Tokyo, Japan

1.

INTRODUCTION

Looking back on the history of the science and technologies of cellulosic materials, we realize that numerous technologies for modifying cellulose by chemical and mechanical methods have been developed. Some of these technologies have been practically applied to produce modified cellulosic materials for our daily and industrial necessities. Cellulosic materials are generally strong, hydrophilic, insoluble in water, stable to chemicals, safe to living bodies, reproducible, recyclable, and biodegradable. With these specific and advantageous characteristics of cellulose, “modification” techniques to reinforce these original properties or to add new functionalities to cellulose have been investigated, and have contributed to the development of cellulose science and technologies. Chemical treatments have been positioned at the center of the field of cellulose modification. The purposes of chemical modifications or derivatizations of cellulose are divided into three major groups: ( l ) to produce modified cellulosic materials having specific properties at commercial levels, ( 2 ) to characterize cellulosic materials at laboratory levels, and ( 3 ) to approach the nature of cellulose for scientific interests. Commercial cellulose derivatives having water solubility, organic-solvent solubility, ion-exchanging groups, or hydrophobic groups are used as aqueous thickeners, plastics, column-supporting materials for chromatography, and others. Cellulose carbamates and nitrates are used for determining molecular mass and molecular mass distribution of cellulose to evaluate cellulosic materials by size-exclusion chromatography. Furthermore, since there are still many unsolved and mysterious subjects in cellulose science, chemical modifications are sometimes used to study fundamental research subjects of cellulose, such as solid-state structures of cellulose (hydrogen bonding patterns, chain conformations, crystal and amorphous structures, etc.), interactions with other substances at molecular levels, and molecular dynamics in solution states. The typical modifications of cellulose are esterifications and etherifications at hydroxylgroups of cellulose. Most water-soluble and organicsolvent-solublecellulose derivatives are prepared by these substitution reactions, and drastic changes in the original properties of cellulose can usually be achieved by thesechemical modifications. Othersare ionic and radical grafting,acetalation,deoxyhalogenation,andoxidation. Since the usual cellulosic materials originating from wood and cotton pulps have aldehyde and carboxyl groups in quite small quantities, depending on the purity of the pulps, 599

lsogai

600

these minor groups are also target positions for chemical modifications. Figure l shows schematic representation of positions in the cellulose structure for chemical modifications. The methods for chemical modifications of cellulose are various, depending on the reactions. Cellulose esters such as cellulose acetate and nitrate are generally prepared from cellulose pulps with reagents in the presence of concentrated sulfuric acid as an acid catalyst. In the case of cellulose nitrate preparation, degree of substitution (DS) is controlled by water content in the reaction medium, the H,SO,-HNO, system, and the shape of the solid cellulose pulp is maintained during the nitration. Cellulose triacetate is prepared from solid cellulose pulp in a mixture of acetic anhydride, acetic acid, and sulfuric acid.The reaction mixture becomes transparent by dissolving celluloseacetate in the medium, as the DS values are increased by acetylation. Water is then added to the mixture to hydrolyze acetyl ester groups, in part for preparing cellulose diacetate, and this partial

Substitution reaction

Esterification Etherification Deoxyhalogenation

To carboxylic acid To aldehyde Acetalation

Oxidation

1

Acid hydrolysis Oxidative cleavage

H0 OH

OH

Etherification Deoxyhalogenation Acetalation

Reactions at

minor groups

Carboxyl group Aldehyde group

Esterification Amidation Reduction to alcohol Oxidation to carboxylic acid Reduction to alcohol

FIGURE 1 Positions in cellulosestructure for chemicalmodifications.

Chemical Modification of Cellulose

601

deacetylationproceeds under homogeneoussolution-stateconditions in the mixture. On the other hand, most cellulose ethers such as methyl-, ethyl-, carboxylmethyl-, hydrolyethyl-, and hydroxypropyl-celluloses are prepared from cellulose pulps through the alkalicellulosestage with or without i-propyl alcohol.Then, an etherifying reagent such as methyl chloride, ethyl chloride, monochloroacetic acid, ethylene oxide, or propylene oxide is added to the mixture for substitution reactions. Heterogeneous solid-liquid phase reactions are maintained during these etherifications of alkalicellulose. Degrees of substitution (DS) and distribution of substituents as well as molecular mass and molecular mass distribution strongly influence the properties of cellulose derivatives, and are significant factors for characterizing them. In the case of cellulose acetate, the controlling DS to a certain value i n the range 0.5-2.0 is difficult as long as conventional acetylation and the followingdeacetylationprocessesareadopted. DS values of cellulose ethers are controllable to some extent by selecting the etherification conditions. However, since the etherifications proceed heterogeneously to swollen alkali cellulose in solid-liquid phase,homogeneousdistribution of substituents is generally difficult to achieve. Here, the concept of distribution of substituents has the following different levels: ( I ) distribution of substituents among the three hydroxyl groups of one anhydroglucose residue, (2) that along one cellulose chain, (3) that among cellulose chains, and (4) that among fibrils or fibers (Fig. 2). Heterogeneous distribution of substituents leads to fluctuation of properties of the celluloseethers,eventhosehaving the same DS values. Furthermore, some new functionalities may be expected from cellulose ethers having homogeneous distribution of substituents. Since esterifications and etheritications are condensation reactions at hydroxyl groups of cellulose, the presence of water in the reaction medium generallycauseslower reaction efficiency by consumingreagentsto form byproducts. Therefore, preparation of cellulose ethers with high DS is difficult a s long as aqueous alkalicellulose systems are used. Based on this background, control of DS and distribution of substituents (or preparation of regioselectively substituted cellulose derivatives) have been extensively studied for cellulose derivatives from fundamental aspects. In some studies, homogeneous cellulose solutions were used as derivatization media, in place of heterogeneous solid-liquid phase reactions, in order to prepare regioselectively substitutedcellulosederivativesor those having high DS values. Various cellulosic materials have been investigated a s the starting cellulose samples for derivatizations (Table l ) . Softwood and hardwood bleached kraft pulps from low to high a-cellulose contents, linter cellulose, and bacterial cellulose have been used as native cellulose samples. Sometimes, as swollen or decrystallized cellulose samples, native celluloses pretreated with liquid ammonia, organic amines, or aqueous inorganic alkali (such as 15-20% ay. NaOH) were subjected to derivatizations. Since regenerated cellulose has generally lower crystallinity and higher accessibility to reagents than native celluloses, it was used in some reports as the starting cellulose material for derivatizations. Regenerated low-molecular-weight celluloses with degrees of polymerization (DP) of 7 and 15 can be prepared in good yields by dissolving native cellulose samples in about 85% phosphoric acid followed by pouring the solution into water or methanol [ l ] . These celluloses have highly crystalline cellulose I1 structure. Regenerated amorphous celluloses having stable amorphousstructuresevenunderaqueous media can be prepared by dissolving native cellulose samples with various DP values in SOz-ami~~e-dimethylsulfoxidesolvent systems followed by pouring the solution into water [2]. These low-DP crystalline celluloses and completely amorphous ones were used as cellulose samples for derivatization. Since commercial cellulose diacetate and some cellulose ethers are soluble in dimethylsulfoxide

602

lsogai

Among three OH groups

Along one cellulose chain

Among cellulose chains

Within one microfibril, and among microfibrils

Within one fiber, and among fibers FIGURE 2

Distribution of substituents at various levels.

and other organic solvents, these are also examined as the starting materials for derivatizations. From industrial and environmental points of view, the possibility of using lowera-cellulose-content pulps, especially for producing cellulose acetate, is a quite significant subject. Since detailed and systematic information about chemical modifications of cellulose have been already reviewed by Professor Ishizu in the first edition of this book [3], scientific and technical reports in this research field published after 1990 are supplemented in the following sections.

Chemical Modificationof Cellulose

603

TABLE 1 Cellulosic Materials and Cellulose Solutions Examined for Derivatizations in 1990- 1997 Native cellulose

Bleached softwood and hardwood chemical pulps Cotton linter with high a-cellulose content Bacterial cellulose Microcrystalline cellulose powder

Decrystallized cellulose

Native cellulose treated with liquid NH, Native cellulose treated with organic amine Native cellulose treated with aqueous alkali, including mercerized cellulose

Regenerated cellulose

Defibrated rayon (from cellulose xhantate solution) Cellulose regenerated from Cu(EDA),(OH), or Cu(NHA(OH), solution Porous regenerated cellulose beads for chromatography Amorphous cellulose regenerated from S02-amine-DMS0 solution Cellulose prepared by saponification of cellulose acetate Low-molecular-weight cellulose with DP of 7 or 15

Cellulose derivatives

Cellulose diacetate Cellulose ethers (MC, HPC, HEC, CMC, etc.) Methylolcellulose prepared from PF-DMSO system Triphenylmethylcellulose Cellulose p-toluenesulfonate Halodeoxycellulose Dialdehydecellulose

Cellulose solutions

LiCI-DMAc LiBr-DMAc N@-DMF Chloral-pyridine-DMF PF-DMSO SO,-amine-DMSO

~~~

~

EDA,ethylenediamine;MC,methylcellulose;HPC,hydroxypropylcellulose;HEC,hydroxyethylcellulose; CMC,carboxylmethylcelluloseNasalt; PF, paraformaldehyde; DMSO,dimethylsulfoxide;DMAc,N,N-dimethylacetamide; DMF, N,N-dimethylformamide.

II.

ESTERIFICATION

On the subject of esterification, new processes or methods for preparing conventional cellulose esters in laboratory and industrial levels, preparation of new cellulose derivatives, applications to new analytical methods, and others have been reported. From the industrial viewpoint, developments of new processes for producing cellulose diacetate (DS = 2.5) from bleached sulfite pulps with lower a-cellulose content are quite innovative. On the other hand, for fundamental aspects, the LiCl-DMAc cellulose solvent system has been widely used as a homogeneous esterification medium to prepare new cellulose derivatives

lsogai

604

or to control DS and distribution of substituents. Carbanilation or preparation of ccllulose carbamates is included in this section for convenience.

A.

New Acetylation Process

Cellulose diacetate is being manufactured in thc largest quantity among cellulose derivatives, and is used as fibers for cigarette filters, cloths, plastics, and others. In order to reduce the process energy in acetylation and to utilize the lowest-a-cellulose-content wood pulps possible, a new acetylation process has been developed [4,5]. This includes ( I ) an acetylation stage at high temperature under reduced pressure together with accurate computer control of the temperature, (2) a deacetylation stage at high temperature, and (3) successive flash evaporation stage for separation of concentrated acetic acid from the reaction mixture. Eventually, this process leads to huge reductions in process energy and amounts of reagents, and to great improvement in productivity. Furthermore, sulfite pulps with lower a-cellulose content for not only softwood but also hardwood have come to be applicable to cellulose diacetate production by the new process. When conventional acetylation is applied to softwood sulfite pulps with low a-cellulose content, a relatively large quantity of the acetone-insoluble fraction, which originates from hemicellulose acetate and causes serious problems in the spinning process. is formed into cellulose diacetate. On the other hand, thenew process can extremely reduce this fraction, because hemicellulose acetate becomes soluble in acetone by partial depolymerization under the acetylation process at higher temperature. The difference between the conventional and new acetylation processes is illustrated in Fig. 3 [4]. Ueda et al. studied detailed structures of the acetone-insoluble fraction of cellulose diacetate prepared from softwood sulfite pulp with low a-cellulose content, by size-exclusion chromatography and some analytical techniques [ S ] . The mechanisnl proposed for the formation of the acetone-insoluble fraction is connected with the interaction between cellulose and hemicellulose partly present in the native wood state. Namely, a part of cellulose and hemicellulose molecules are chemically and/or physically bound to each other in the original pulp and native wood as well, and acetylation of these cellulose molecules proceeds with hen~icellulosethereby entangled. Under these restraining conditions, cellulose triacetate having the crystal structure of cellulose triacetate I instead of 11 is formed, and is hard to deacetylate i n the following hydrolysis stage. Generally, the structure of cellulose triacetate I, which can be defined from X-ray diffraction patterns, is formed by solid-phase acetylation of cellulose using, for example, toluene as a poor solvent of cellulose triacetate. Hence, cellulose triacetate having low solubility in acetone forms a gel-like and acetoneinsoluble fraction coagulated with hemicellulose acetate at molecular levels. The new acetylation and deacetylation processes at high temperatures must release the cellulosehemicellulose interactions present in the pulp by partial degradation of hemicellulose, and thus the acetone-soluble fraction increases even lor sulfite pulps with lower a-cellulose content. Saka et al. 17-91 further studied some analyses ofthe insoluble fractions of cellulose triacetate prepared from softwood and hardwood bleached kraft pulps, and proposed some techniques to reduce the insoluble fraction by pretreatment of the pulps with an acetic acid-sulfuric acid mixture. At the present timc, bleached softwood and hardwood sulfite pulps with a-cellulose content from 98% down to about 93% have come to be utilized for cellulose acetate manufacturing at industrial levels by the above new acetylation process. Further innovative techniques, by which normal bleached kraft pulps can be used in acetylation, will hopefully be developed someday in future.

605

Chemical Modificationof Cellulose

Conventional process ~. ...............

,

.................. :ii .............. H,O i ~

/ ................

Dissolvlng pulp Lmter or pulp

:H

t

1

I

cellulose De-acetylation Crude diacetate

T

I

trlacetate

I

-

I ,

HP1

I oniatA;; process

purification process Separation and

i

H,S;

I

I

.......... ........., .....................

/

' ! A70 A;OH Ac,O production orocess

I

.............. .................. i DilutedAcOH j

~.................... . ........................................ . . . .....................

4

1 4

~

~

I

G,-

............ ........:

{ ACOHprocess recovery I

A

;

AcOH

L.....

...............

I

New process .......................

:

L

>

I .................... Steam i

IChernc la1 l1

v Acetylation at

4 ...................

.......................................... . . . ................... :......H,SO, Ac,O AcOH .................c ....., ..................... L

~

t

i

Ac,O production

,

~

l

I

-.......... ........

iL.. .................. AcOH

process

I

FIGURE 3

..................

New and conventionalacetylation

processes 141.

B. Other New Esterification Methods In the case of cellulose esters other than cellulose acetate, some were prepared by new methods at laboratory levels, often using cellulose solvents to prepare cellulose derivatives having various DS values or homogeneous distribution of substituents. Shimizu et al. found that cellulose esters having a wide range of DS values can be prepared with organic acid (or organic acid salt), pyridine, and p-toluenesulfonyl chloride (tosyl chloride: TsCI) with or without DMF [ 10,l l ] . In the case of acetylation of cellulose

606

lsogai

with sodium acetate, pyridine, and TsCl in DMF, cellulose acetates with DS 1.4-2.8 were obtained, depending on the reaction conditions. The effect of molar ratios of TsCl/AcONa on the DS values was investigated in detail. Tri-0-substituted cellulose benzoates having various substituents at the benzene ring were also prepared by a similar method without using DMF. The substituents at the benzene ring had little influence on the DS values obtained. In these systems, the organic acid anhydrides or organic acid chlorides seem to be formed in situ in the organic acid-TsCI-pyridine mixture, and thus these esterifying reagents do react with cellulose hydroxyl groups in the presence of pyridine [ 121. The LiCl-DMAc cellulose solvent system was often used for homogeneous esterifications of cellulose in solution states. When N,N-dicyclohexylcarbodiimide,organic acids (or organic acid anhydrides) and 4-pyrolidinopyridine were added to acellulose/LiClDMAc solution, cellulose esters having DS values lower than 2.5 were prepared [ 13,141. In the case of cinnamoylation of cellulose in the LiCl-DMAc system, cinnamoyl groups were introduced almost selectively into C6 hydroxyl groups at DS values lower than about 1 [ 151. Cellulose propionates with DS values lower than 1.76 were prepared using the LiCI-DMAc system and propionyl chlorideasacellulose solvent and an etherifying reagent. Solution-state properties including liquid crystalline behavior of the products were studied, and rigidity of cellulose propionate molecules in solution states decreased with an increase in the DS values [16]. When cellulose is dissolved in N,O,-DMF or N,O,DMSOsystems, all hydroxyl groups finally form unstable nitrite esters in the solution [ 171, where the ease of nitrosation among the hydroxyl groups is in the order of C6 > C2 > C3. On the other hand, when the cellulose solutions are poured into water, all nitrite ester groups are finally removed from cellulose by hydrolysis to produce regenerated cellulose, where the ease of denitrosation is of the order of C6 >> C3 > C2 [ 181. Some water-soluble polysaccharides having sulfate esters have blood anticoagulant activity and also sometimes an anti-HIV one in the medical field. Preparation of watersoluble cellulose sulfate esters has been tried from these points of view. as well as ion exchanging and other specific properties suchasCMC in aqueoussolutions.Since the conventional sulfonation of cellulose with S03-DMF complex leads to severedepolymerization of cellulose, even though the DS values reach up to about 2, a homogeneous solution-state sulfonation with various reagents was examined [191. The results obtained showed that SOzClz gavethe highest efficiency of sulfonation when N,O,-DMF was used as the cellulosesolvent.Thesulfate groups were distributed asC6-OH > C2-OH on sulfonation at 20°C and C2-OH = C6-OH at -20°C. Permethylation analysis of cellulose sulfate with DS 0.52- 1.37 was carried out for elucidating distribution of sulfate groups among the three hydroxyl groups of one anhydroglucose residue by gas chromatography and I3C-NMR 1201. Since p-toluenesulfonate ester groups (tosyl esters) can be introduced selectively into primary hydroxyl groups of carbohydrates under certain conditions and also can be subjected to the following deoxy-substitution reactions, preparations of cellulose tosyl esters have been examined for a long time. When microcrystalline cellulose was reacted with TsCl and pyridine under heterogeneous solid-liquid phase conditions, the corresponding esters with DS 2.3 were obtained [21]. Arai and Aoki reported preparation of cellulose tosylates with DS 0.79-0.92 under heterogeneous (i.e.,solid-liquidphase)conditions, and the cellulose tosylates obtained were further reacted with sodium bisulfite to prepare cellulose deoxysulfonic acid with DS 0.065-0.1 [22]. However, the products obtained still contained tosyl groups tosome extent. The LiCI-DMAc cellulose solvent system was introduced to the solution-state homogeneous tosylation of cellulose by Heinze et al. [23.24]. Their DS values were in the range 0.4-2.3, and were controllable by the reaction

dificationChemical

of Cellulose

607

conditions. Thermal properties of these cellulose tosylates and further esterifications were examined using the cellulose tosylates with pyridine, sodium acetate, and esterifying agents (organic acid anhydrides). Regioselective tosylation at C6 hydroxyl groups of cellulose does not seem to be achieved even by the homogeneous tosylation, and severe depolymerization may occur on cellulose chains during the reactions.

C. New Cellulose Esters New cellulose esters were prepared using new reagents and/or organic cellulose solvent systems in terms of both fundamental and practical aspects of cellulose modifications. Cellulose derivatives containing relatively long aliphatic chains such as ester groups were prepared with fatty acid chloride and pyridine under heterogeneous solid-liquid phase conditions, and their liquid crystalline properties were studied [2S]. When the LiCl-DMAc cellulose solvent system was used with p-toluenesulfonic acid and organic acid anhydrides, waxy cellulose esters having C12-C20 fatty acid ester groups in the DS range 2.8-2.9 were prepared. Liquid crystalline properties of the products and crystallization behavior among their long aliphatic side chains were studied [26]. lwata et al. prepared regioselectively substituted cellulose esters, 6-0-acetyl-2,3-di-0-propanoylcellulose and 6-0-propanoyl-2,3-di-O-acetylcellulose,from 6-O-tritylcellulose, which was prepared beforehand from cellulose with DP S7 using a cellulose/LiCl-DMAc solution. Their crystal structures were made clear by X-ray and electron-diffraction analyses [27-301. Although liquid crystalline properties of cellulose derivatives have not yet been applied at practical levels, the accumulation of such fundamental information contributes to our understanding of the nature of cellulose molecules in solid, mesophase, and liquid states. Fluorine-containing polymers have unique properties such as thermal resistance, water and oil repellency, small dielectricity, and clear piezoelectricity. From these aspects, introduction of fluorine-containing groups through ester linkages into cellulose has been investigated (Fig. 4). When cellulose was dissolved in trifluoroacetic acid at room temperature, trifluoroacetate ester groups were introduced almost regioselectively at C6 hydroxyl groups of cellulose [31]. On the other hand, cellulose trifluoroacetate esters with DS 1.5-2.1 and DP 170-800 were prepared by reactions of cellulose with a trifluoroacetic acid-trifluoroacetic anhydride mixture at 20-150°C for 4 h under high pressures [32]. The ester bonds are, however, unstable to moisture or water and hydrolyzed to some extent, when the cellulose trifluoroacetate esters are exposedto air atmosphere. Furthermore, depolymerization of cellulose is inevitable under such strong acid conditions, being especially influenced by water content in the reaction media. Cellulose was reacted with 4-perfluorononeyloxy phthalic anhydride and either pyridine or triethylamine as a base in a LiC1-DMAc cellulose solution, and the corresponding half-esters with DS < 2.1 were obtained [33]. Introduction of fluorine-containing groups into cyanoethylcellulose by esterification was also studied using fluorine-containing alkanoyl fluoride, and thermal fluidity. Physical properties of the obtained products (DS of fluorine-containing ester groups < O S ) were studied [34]. Perfluorooctanoate ester groups were introduced into hydroxypropylcellulose, and its unique liquid-crystalline properties were studied [3S]. Some cellulose triesters and tricarbamates have been practically applied as columnpacking materials for chromatographic enantiomer separation using the homochiral property of the cellulose backbone of the derivatives. This is one of the most exciting topics in these two decades for the subject of “functionalizations of cellulose.” Figure S shows representative cellulose derivatives used for this purpose [36].

608

lsogai

CF3COOH

Cellulose

Dissolution

U

(CF3CO)zO t CF3COOH

Cellulose

W

Cellulose-0-CCF, II

Dissolution

DS = 1.5-2.1

F

+

pyridine or triethylarnme

Cellulose

*

-q F&

perfluorononenyloxyphtharic anhydride

Cellulose-o

LiCI-DMAc DS < 2.1

cF3

COOH

1 .l ,2,2,3-pentafluoropropyloxy-

22-difluoroporpionly fluorlde t

pyridine or triethylamine

Cellulose

LiCI-DMAc

*

Cellulose-o-c,

0 I1 Fz Hz

/c, /c,

C

DS CH3 Cellulose-0

m

e

Cellulose-0

>Q

Cellulose-0

c

l

dification Chemical CellUlOSe

of

609

Other new celluloseesters were prepared fromcellulose or commercial cellulose derivatives with new reagents or methods. Reactions between cellulose andA'-oxazolinones in aPF-DMSOcellulose solution or in the presence of hydrazine gave unique cellulose esters 137,381. When CMC, ethylcellulose, and other commercial cellulose ethers were reacted with chloroformiate 2-hydroxymethyl methacrylate, new cellulose derivatives containing photosensitive groups with DS 0.07-0.165 were obtained. UV-light irradiation to the products led to disappearance of the double bonds, indicating that intra- and intermolecular cross-linking occurred in the product [39]. The behavior of photo-cross-linking of cellulose cinnamate and allylcellulose cinnamate was also studied [40]. Preparation of highly water-absorbable materials was attempted from cellulose sulfate by cross-linking with glutaraldehyde [41]. Sulfonation of CMC with CIS0,H or SO, in pyridine gave CMC sulfate with DS 0.4- 1.48 [42].

D. Analyses of Distribution of Substituents I n the case of cellulose esters with DS values lower than 3 and those containing substituents of more than two different groups (cellulose heteroesters) such as cellulose acetate butylate, distribution of substituents is one of the significant factors which influence the properties of cellulose esters for end use. 'H- and ',C-NMR have been used for obtaining the information about distribution of substituents among the three hydroxyl groups of one anhydroglucose residue, i.e.,C2-OH,C3-OH, and C6-OH. Kowsaka et al. determined distribution of sulfate groups of cellulose sulfate sodium salts by 'H- and '.'C-NMR, together with their correlation spectra which were prepared from cellulose with SO,-DMF complex 1431. In the case of cellulose acetate, the residual free hydroxyl groups were first esterified with propionic anhydride and pyridine, and the chloroform-solublecellulose acetate propionate obtained was subjected to I3C-NMR analysis for determining distribution of acetyl groups [44]. Cellulose heteroesters such as cellulose acetate butylate and 02-hydroxypropyl-0-methylcellulose acetate succinate were also analyzed by asimilar method L45.461.

E. Application of Cellulose Esterification to Analytical Methods Phenylcarbanilation, or preparation of tri-0-phenylcarbamated cellulose, has been used for conversion of cellulosic materials into tetrahydrofuran-soluble derivatives with phenylisocyanate and pyridine at about 70°C for 1-2 days, and their molecular mass and molecular mass distribution can be evaluated as phenylcarbamate derivatives by size-exclusion chromatography (SEC)[47]. For this purpose, depolymerization of cellulose must be avoided as much as possible. Evans et al. examined the optimum conditions for preparing tri-0-phenylcarbamated cellulose with less depolymerization, in terms of the addition of DMSO or DMF as a co-solvent or that of amines to the reaction media [48,49]. This phenylcarbanilation seems to be better than nitration of cellulose for SEC analysis, because H' ions are not formed during the former reaction. Evans proposed that depolymerization of cellulose occurring to some extent during the phenylcarbanilation is due to oxidative cleavage of the celluloseglycosidebond.This carbanilation requires relatively large amounts of cellulosic samples and the following isolation stage. Therefore, heterogeneous solid-liquid phase nitration with a mixture of nitric acid-phosphoric acid-phosphorous pentoxide has been sometimes applied to cellulosic samples in small quantity, such as bacterial cellulose obtained by cultivation under specific conditions, for determining molecular mass and molecular mass distribution of cellulose by SEC [50].

lsogai

610

F. Mechanisms of AKD and ASA Sizing of Cellulose The esterifications described so far in the above sections bring about drastic changes in cellulose properties by introducing ester groups, whose DS values are generally more than 0.5. On the otherhand, so-called reactive sizes have been practically used as wet-end chemicalsforaddingsuitable water repellency to hydrophilic paper sheets in alkaline papermaking systems, where CaC03 is present as a filler. Alkylketene dimers (AKD) and alkenylsuccinic anhydrides (ASA) are included in this category. These reactive sizes are added as cationic emulsions 0.1- 1.5 p m in diameter to paper stock. Paper sizing with the reactive sizes has the following characteristics. 1.

2. 3.

4. 5. 6.

7.

Addition of small amounts of these sizes to paper stock gives sufficient sizing to paper (generally 0.05-0.2% on dry weight of pulp). It takes generally within 1 min between the size addition stageto paper stock and the final rolling stage of dried paper. Even though a large amount of water is present in paper stock (l-2% pulp consistency), good sizing can be given to paper by the reactive sizes. Basic properties of cellulosic pulp fibers are unchanged by the size treatments; only surface modifications of cellulosic fibers occur. Neither isolation nor purification of the sized paper to remove by-products and others is required. Thus, the sizing of paper with the reactive sizes is a quite efficient modification method for surfaces of cellulosic materials. So far, the covalent bond formation between these reactive size molecules and hydroxyl groups of cellulose on pulp fiber surfaces have been believed to be essential for giving sizing features to paper so efficiently.

Figure 6 shows possible reactions occurring in paper sheets during the papermaking process. It is true that AKD and ASA form the corresponding esters with cellulose under nonaqueous conditions in the presence of a base catalyst. However, the most controversial question is whether the covalent bonds are really formed under such aqueous conditions as papermaking within a short time without reactions with water to form hydrolyzed byproducts. Thefollowing are the major reasons why covalent-bond formation has been supported by many researchers.

1. The reactive structures of AKD and ASA are necessary for efficient paper sizing; hydrolyzed products give no sizing effect at the same addition levels. 2. Sizingdegrees are unchanged evenafterSoxhletextraction of the AKD- and ASA-sized papers; sizingfeatures must be lost by the extraction, if the size components are present in the paper sheets without forming the covalent bonds with cellulose hydroxyl groups [51,521. 3. Some analytical studies such as FT-IR, "C-labeling of the size molecules, and solid-state "C-NMR analysis of the sized papers prepared thereby, indicated the presence of covalent bonds in the paper sheets [53,54]. In the case of usual esterifications of cellulose, additions of large amounts of reagents as well as careful control of water content in the reaction media are required. In contrast, since highly efficient esterifications can be achieved when AKD and ASA emulsions are used as reagents even in the presence of water, these reactions must be applicable also to other cellulosic materials as more efficient modification techniques, if the covalent bond formation is the true mechanism f o r the paper sizing. Detailed reexaminations of sizing

61 1

Chemical Modificationof Cellulose R-CHXH-CH-R*

*

I

Cellulose-OH

I

0

OH

0-Cellulose

ASA-cellulose half ester

CH, -CH o=c,

l F=O

o=y

R-CH=CH-CH-R*

I

l

R-CH=CH-CH-R* HO , (OH- )

ASA

y=o o=y OH

OH

ASAcid Cellulose-OH

R-CH,-C=O

I

i R- CH

I

-P

R-CH=F:

-c=o

R*

F=O 0- Cellulose AKD-cellulose P-keto ester

HO , (OH-)

AKD

R-CH,-fi-CH,-R*

0

CO,' CO,

Ketone

FIGURE 6 Possible reactions of AKD and ASA inpaper sheet.

mechanisms of paper by AKD and ASA were carried out using some analytical techniques, and the following conclusions were obtained 155-611. Synthesis of "C-labeled AKD and the following solid-state "C-NMR analysis of the handsheets prepared thereby revealed that nearly no covalent bonds are present in the AKD-sized handsheets, when resonance peaks were assigned correctly. Thus, the hydrolyzed products of the reactive sizes, i.e., ketones for AKD and alkenylsuccinic acid for ASA, must consequently contribute to sizing performance of paper. 2. Most size components, i.e., hydrolyzed products, are physically anchored to pulp fiber surfaces, and these components are extractable from fibers when the paper sheetsare defibrated at 60°Cin1% Tween 80, which is one of the nonionic surfactants. 3. The structures of AKD and ASA that are reactive with water are necessary for achieving homogeneously distributed hydrophobic sizingcomponents with smaller coagulants as possible on hydrophilic cellulosic pulp fiber surfaces by in-situ hydrolysis of the sizing components in paper. 4. In the pulpsuspensions,cationicsize emulsion particles are adsorbed on pulp fiber surfaces by forming ionic bonds between the cationic groups on the sizing emulsion surfaces and carboxyl groups on pulp fiber surfaces, which are present as minor groups of 0.02-0.08 mmol/g for bleached kraft pulps.

1.

612

lsogai

From the aspect of esterifications of cellulose, therefore, the sizing of paper with AKD or ASA is not included in the story of this chapter, “chemical modification of cellulose.” However, such efficient surface modification techniques of cellulose as paper sizing with reactive sizes may give some clues to developingnew techniques for modifying cellulosic materials efficiently with reagents that are reactive with “water” under usual conditions, where water is always present and has strong interactions with cellulose.

111.

ETHERIFICATION

Since ether linkages are more stable than ester linkages to acid and alkaline conditions, various cellulose ethers have been prepared for both practical and fundamental purposes. Especially in the case of commercial cellulose etherifications, control of DS values and distribution of substituents at various levels (Fig. 2) are quite important subjects for manufacturing, because generally etherifications proceed heterogeneously to solid cellulose pulps through swollen alkalicellulose.Thus,some modified processes to prepare commercial cellulose ethers having more homogeneous distribution of substituents have been developed at industrial levels. Also, regioselectively substituted cellulose ethers were prepared, sometimes using cellulose solvent systems or multistage reactions, to obtain fundamental information about cellulose ethers and cellulose itself. As to new cellulose ethers, hydrophobically modified water-soluble cellulose ethers have been found to have unique viscosity behavior in aqueous solutions.

A.New

CarboxymethylationProcess

Carboxymethylcellulose sodium salt(CMC) is produced in the largest quantity among water-soluble cellulose derivatives, and is utilized as a thickener or dispersant in various fields. Taguchi et al. studied the effect of distribution of substituents of CMC samples on stability of their solution properties, and a new carboxymethylation process of cellulose was developed on the basis of the results obtained thereby [62]. Distribution of substituents of CMC samples along one cellulosechain and that among cellulose chains were evaluated by cellulase degradation and electrophoresis, respectively. The results showed that CMC samples having more homogeneous distribution of substituents had higher stability of their solution properties to enzymes and salt concentrations, even at the same DS values and the same distribution of substituents among the three hydroxyl groups of one anhydroglucose residue. The new process had two key points for producing CMC having more homogeneous distribution of substituents: ( I ) the use of i-propyl monochloroacetate ester as the reagent instead of monochloroacetic acid or (2) separate additions of NaOH solutions to the reaction mixture. This process has been already utilized at practical levels. Other new carboxymethylation methods were reported for fundamental aspects. 60-triphenylmethylcellulose (tritylcellulose) was first prepared under homogeneous conditions using the LiCI-DMAc cellulose solvent system, and residual hydroxyl groups at C2 and C3 of tritylcellulose were partially or completely carboxymethylated.Carboxymethylcellulose partially substituted at C2 and C3 hydroxyl groups were obtained by detritylation of the carboxymethylated tritylcellulose with HCI.Liu et al. reported that these CMC samples became water-soluble at DS values more than 0.3. thus suggesting that distribution of carboxymethyl groups strongly influences the solubility behavior in water [63]. 2,3-Di-O-carboxymethylcellulosewas prepared from tritylcellulose by ( l ) repeated carboxymethylation with monochloroacetic acid sodium salt and powdered NaOH

Modification Chemical

of Cellulose

613

using a tritylcellulose/DMSO solution and (2) the following detritylation [64]. 6-0-Carboxymethylcellulose has not been prepared yet.

B. Other New Etherification Methods Some new etherification methods have been reported in order to prepare regioselectively substituted cellulose ethers and to clarify their solution properties or interactions with other materials. 2,3-Di-O-alkylcelluloseswere prepared from tritylcellulose dissolved in DMSO with the Corresponding alkyl iodide and powdered NaOH followed by detritylation of the 2,3-di-0-alkyl-6-0-tritylcellulosewith HCl [65]. Preparation of6-0-alkylcellulose required multistage reactions: ( l ) tritylation at C6-OH in LiCI-DMAc, (2) complete allylation of the tritylcellulose at C2-OH and C3-OH in DMSO, (3) HC1 gas treatment in CH,CI, for detritylation of the 2,2-di-O-allyl-6-O-tritylcellulose, (4) isomerization of the allyl groups in the 2,3-di-O-allylcellulose to l-propenyl ones with potassium t-butoxide in DMSO, ( 5 ) alkylation at C6-OH of the 2,3-di-O-propenylcellulosewith the corresponding alkyl iodide in DMSO, and (6) treatment with 0.1 N HCl in 90% methanol to remove the 1 -propenyl groups of the 2,3-0-propenyl-6-0-alkylcellulose[66]. Methylcellulose with DS 0.9-2.2 was prepared under homogeneous conditions with methylsulfinyl carbanion, which was formed from NaH and DMSO, using a LiCI-DMAc cellulose solution [67]. Residual hydroxyl groups at C2-OH and C3-OH in tritylcellulose were partially or completely methylated with powdered NaOH and methyl iodide in DMSO, and methylcellulose samples with DS 0.53-2 at C2 and C3 were obtained by detritylation of the methylated tritylcellulose. The minimum DS value required for complete dissolution of methylcellulose in water was investigated using these samples [68]. Cellulose was reacted with propylene oxide to prepare hydroxypropylcellulose in a homogeneous cellulose/LiCl-DMAc solution or in a heterogeneous alkalicelluloseli-propanol system. Their solubility in water, as well as liquid-crystalline behavior, were compared in terms of distribution of substituents. Hydroxypropylcellulose prepared under heterogeneous conditions had lower solubility in water, high viscosity and some surfaceactive properties, compared with that prepared under homogeneous conditions [69]. The following three methods for alkylation of all hydroxyl groups of cellulose to prepare tri0-alkylcellulose ethers by one step have been reported so far, and were used for permethylation analysis of cellulosic materials and regioselective etherifications:

I.

2. 3.

C.New

Alkylation of cellulose with the corresponding alkyl iodide or alkyl bromide and powdered NaOH in homogeneous cellulose/SO,-diethylamine-DMSO solutions [70,7 11 In-situ deacetylation and simultaneous alkylation of cellulose acetate in homogeneous DMSO solutions with the same reagents as the above 1721 Alkylation of regenerated cellulose dissolved in LiCI-DMAc with alkyl iodide and methylsulfinyl carbanion [73]

CelluloseEthers

Preparation of hydrophobically modified water-soluble cellulose ethers and characterization of their solution properties are significant current topics for cellulose ethers [74]. For example, commercially available hydrophobically modified hydroxyethylcellulose (HMHEC) is prepared from HEC by reactions with epoxides having long alkyl chains of C12C24. This HM-HEC contains l-2% hydrophobic groups, i.e., DS 0.01-0.03, and thus is

614

lsogai

Hydrophobic

FIGURE 7 1751.

Interaction between molecules of hydrophobically modified cellulose ethers in water

water-soluble and nonionic. The molecules of hydrophobically modified water-soluble cellulose ethers form network structures in water by hydrophobic association at more than about 0.2% concentration (Fig. 7) [75],and thus their solutions sometimes have extremely high viscosity compared with unmodified cellulose ethers (Fig.8) [74]. Since HM-cellulose ethers, including methylated and ethylated hydroxypropyl- and hydroxyethyl-celluloses, have both hydrophilic and hydrophobic structures in the molecules, they behave as sur-

15

I

l / / Hydrophoblcally modified 1 / hydroxyethylcellulose 1 / / l / / / / / / / / /

-

U! 1 0 Kl

a

v

.a m 0 0 v)

5

I

5

/

Normal

/ " 0 0

"

"

0

1

2

3

Concentration of polymer in water ("h)

FIGURE 8 Viscosity behavior of aqueoussolutions of hydrophobically modified hydroxyethylcellulose and normal hydroxyethylcellulose [74].

Modification Chemical

of Cellulose

615

factants in water and have some interactions with other compounds in water. Therefore, the solution properties of these HM-cellulose ethers have been studied in terms of temperatures, phase-separation behavior, and interactions with water, surfactants, or latex particles, which have hydrophobic sites on their surfaces [76-791. The unique solution properties due to HM-cellulose ethers may be further utilized in the future in various fields. Pentyl ether of hydroxypropylcellulose and its cross-linked gels were prepared in nonaqueous solutions, and their thermotropic liquid-crystalline behavior was studied on the basis of changes in the optical pitch by heating [80]. Also, liquid-crystalline properties of cellulose ethers having long alkyl chains (cellulose-0-C,,,H,,,,-OH:m = 12-24) were studied by Liu et al. [81]. Comb-shaped amphiphilic cellulose derivatives, 0-(2-hydroxy3-butoxypropyl)cellulose, with MS 0.4- 1.4 were prepared with butylglycidilether in celluloseLiC1-DMAc solutions. The substitution occurred at the three hydroxyl groups in the order C6 > C2 >> C3, and the products were soluble in water or DMSO, depending on their DS values [82]. When cellulose was reacted with p-methoxytriphenylmethyl chloride in a cellulose/LiCl-DMAc solution at 25-70°C for 4-96 h, the corresponding cellulose ether-like tritylcellulose with DS of about l was obtained. This cellulose ether is soluble in DMF, DMAc, and DMSO [83]. Amine group-containing cellulose ethers were prepared fromepoxide group-containing cellulosederivatives by reaction with hexamethylene diamine or polyethylene imine, and the products obtained had some antimicrobial activity [84]. Cellulose ethers containing Michler's ketone groups as substituents of DS 0.56 were prepared, and their photoregulable properties were investigated [ U ] . SUIfoethyl groups were introduced into cellulose by reacting cellulose with BrCH,CH,SO,Na and powdered NaOH in the cellulose/SO,-diethylamine-DMSO solution or by reacting celluloseacetate with the same reagents in the celluloseacetate/DMSO solution. The water-soluble products prepared by the latter method had some anti-HIV activity [86]. Further derivatizations of CMC have been studied in some reports. Etherifications of CMC with diethylaminoethyl chloride HCl salt or 3-chloro-2-hydroxypropyltrimethylammonium chloride gaveamphoteric cellulose derivativescontaining both carboxyl groups and either tertiary amine or quaternary amine groups. Size-exclusion chromatography of these amphoteric cellulose derivatives indicated that some hydrophobic interactions were present among the molecules in the diluted aqueous solutions [87,88]. Carboxyl groups of CMC were partly amidated by hexadecylamine in DMSO containing dicyclohexylcarbodiimide, and the products obtained were analyzed by I3C-NMR after acid hydrolysis [89]. CMC xanthate was prepared from CMC by the addition of NaOH and CS, in the aqueous CMC solutions, and it had some metal-adsorptive properties [90]. CMC is soluble in formic acid, forming CMC formyl ester with DS0.4-2.0[91].Some new cellulose ethers reported recently are illustrated in Fig. 9.

D. Analyses of Distribution of Substituents As described previously, distribution of substituents of cellulose ethers at various levels has great influence on their properties, and various analytical methods have been proposed. Although average distribution of substituents of CMC among the three hydroxyl groups of one anhydroglucose residue have been established by the 'H-NMR method using 50% D,SO,/D,O [92], more detailed information about distribution of substituents was obtained by the sequence permethylation-reductive degradation-acetylation-GC analysis [93] and "C-NMR analysis [94]. Distribution of substituentsalongonecellulose chain and that among cellulose chains for CMC were measured by the previously described methods in Section 1II.A [62]. Distribution of substituents along one cellulosechain in methylcellulose

616

lsogai NMe2

l

Cellulose - 0 OH

Cellulose- 0 - CH , 2,

-OH

(m = 12-24)

NMe,

-(

Cellulose- 0 CH2CH2 f OCH, Cellulose- 0 -(CH2CH2 fOCH2CH3 n

Cellulose- 0- CH2CHCH2-0

l

- CH2CH2CH2CH3

OH

Cellulose - 0 -CH2CHCH2-NH - CH2CH2CH2CH2CH2CH2 - NH,

I

OH Cellulose-0-CH2CH2-N

I

\CH2CH3

OCH2COONa

Cellulose-O-CH2CHCH2-N+-CH3

I

OCH2COONa OH

I

I

Hydroxyethylcellulose - 0 - CH2CH

I

OH Hydroxyethylcellulose- 0 -CH2CH2CH2CH2CH3

FIGURE 9 Chemical structures of new cellulose ethers.

was studied by partial acid hydrolysis of methylcellulose followed by GC and FAB-MS analyses of the hydrolyzates [95]. Distribution of substituents of hydroxyethylcellulose with high molarsubstitutionvalueswereanalyzed by permethylation, acid hydrolysis, acetylation, and then GC-MS method. Relative reactivity among C2-OH, C3-OH, C6-OH, and substituted ethoxyl-OH was evaluated for hydroxyethylcellulose with various DS values [96].

Chemical Modification of Cellulose

IV.

617

GRAFTING

Grafting is also one of the chemical modification methods of cellulose. In order to introduce fluorine-containing groups into cellulose by grafting, cellulose was reacted with perfluorooctylethylacrylate as a reagent and either ammonium persulfate or AIBN as a catalyst using acellulose/PF-DMSOorcellulose/LiCl-DMAc solution [97]. The grafting efficiency was evaluated for the products, and the PF-DMSO system with ammonium persulfate gave good results. On the other hand, aqueous and heterogeneous solid-liquid phase reactions gave poor efficiency in cellulose grafting. Thermal and chemical properties of the grafted products were studied from various aspects [98]. Grafting of cellulose acetate with N-vinylcarbazol or epoxide-containing cellulose derivatives with various vinyl monomers was also investigated [99,100]. When cellulose was reacted with diethylaminosulfur trifluoride (DAST) in a cellulose/LiCl-DMAc solution, a polysaccharide having C6-0-C1 branch structures of the cellulose backbone was obtained [ I O I 1.

V.

DEOXYHALOGENATION

Preparation of cellulose derivatives having C-X (X: halogen) groups have been studied often using nonaqueous cellulose solvent systems(Fig. lo), and in some reports these deoxyhalogenated cellulose derivatives were further subjected to substitution reactions to add functionalities to cellulose. Deoxychlorination of cellulose was carried out in a cellulose/LiCl-DMAc solution with N-chlorosuccinimide-triphenylphosphine (TPP). At the early stage the reaction occurred only at C6-OH, and then it also occurred at C3-OH with Walden inversion as the deoxychlorination proceeded. The maximum DS value was 1.86 by this chlorination [ 1021. When cellulose was reacted with sulfuryl chloride in a cellulose/LiCl-DMAc solution, deoxychlorination occurred at C6-OH and C3-OH, with Walden inversion, up to DS 1.8. In this case, however, sulfur-containing groups were also introduced into cellulose [103]. Since cellulose is soluble in the LiBr-DMAc system, deoxybromination was performed with N-bromosuccinimide-TPP in acellulose/LiBr-DMAc solution. The products obtained had deoxybromide groups only at C6 with DS 0.9 [104]. Although cellulose was reacted with tribomoimidazole-TPP under heterogeneous solid-liquid phase conditions, the products had deoxybromide groups with DS less than 0.6 [ 1051. On the other hand, when the homogeneous deoxybromination was adopted with the same reagents using a cellulose/LiBr-DMAc solution, DS values of deoxybromide groups reached up to 1.6 by reacting at C6-OH and C3-OH with Walden inversion [106]. Halodeoxycelluloses thus prepared were further subjected to nucleophilic substitution reactions with tiols. inorganic compounds, or long aliphatic amines [ 107- 1091. 6-Deoxyfluorocellulose acetate with DS less than 0.6 was prepared without depolymerization of cellulose by reacting cellulose acetate with diethylaminosulfur trifluoride (DAST) in dioxane [ 1 IO]. Regioselectively substituted 6-deoxyfluorocellulose with DS 0.9 was prepared through the following steps: ( I ) preparation of 6-0-tritylcellulose using the LiCI-DMAc cellulose solvent system, ( 2 ) acylation (benzoylation or acetylation) of the tritylcellulose to prepare 2,3-di-0-acyl-6-0-tritylcellulose, ( 3 ) HBr treatment for detritylation, (4) reaction with DAST to prepare 6-deoxyfluoro-2,3-di-O-acylcellulose, and (5) saponification of the acyl groups with methanolic sodium methoxide [ 11 11.

618

lsogai

Q +O

N-Cl

LiCI-DMAc

OH

P

CH2CI

%?

Cl DS < 1.86 OH

di-2 CH20H

CH2CI SOpCI, LiCI-DMAc

-Q 1 1 1 1 1 l l l l l

c &-p 6*o c2 OH

Cl

OH

DS < 1.8

N-Br

CH20H

+ e

p

CH@

O

LiBr-DMAc

OH

OH DS = 0.9

Br

CH20H

CH2Br

Br

H

*

LiBr-DMAc

OH

Br

OH

DS < 1.6

1) Tritylation 5) De-acylation in LiCI-DMAc in MeONdMeOH 2) Acylation with AcPOor BzpO

TH20H

HBr

OH

3) De-tritylation with F 4) Fluorinationwith F-S-NEt, I DAST F

CH2F

l

OH DS= 1

FIGURE 10 Deoxyhalogenation of cellulose.

VI.

OXIDATION

Many studies about cellulose oxidation have been done, concerning both chemical modifications of cellulose and oxidative bleaching of residual lignin in chemical pulps. Oxidation reactions applied to cellulose for chemical modifications in this decade are summarized in Fig. 11. Some oxidation reactions occur on cellulose selectively at particular

619

Chemical Modificationof Cellulose CH20H

C CH O

YHC

OH

I

I

I

NaBH,

I

NaCIO,

c2

CH20H

+ side reactions

fQ

N204in CHCI,

*a OH

OH CH20H pH 10-11

TEMPO-NaBr-NaCIO

OH

Pulp-CH0

OH

HCIO,, pH 4-5

*

Pulp-COOH

FIGURE 11 Oxidation of cellulose.

positions. The periodate oxidation is typically involved in this category, and has been used to prepare dialdehyde cellulose at laboratory levels. Generally, the oxidation requires severaldays at room temperature in the darktopreparedialdehydecellulose from solid cellulose samples, whose C2-C3 bonds are mostly cleaved, and thus depolymerization is inevitable during the periodate oxidation. When acelluloseRF-DMSO solution was poured into methanol, partly methyloylated cellulose was obtained as a precipitate. Since

I

lsogai

620

this methyloylcellulose was soluble in water, the periodate oxidation proceeded homogeneously in the aqueous solution, and almost completely oxidized dialdehyde cellulose was obtained within 20 h [ 1121. Since the aldehyde groups of dialdehyde cellulose form intraand intermolecular hemiacetal linkages, the isolated and dried dialdehyde cellulose, even having 100% oxidized structures at the C2-C3 bond, is insoluble in water. The dialdehyde cellulose was oxidized to the corresponding dicarboxyl cellulose with sodium chlorite, or reduced to the corresponding dialcohol cellulose with sodium borohydride. These oxidized and reduced products were soluble in water, and characterized from various aspects [ 1 121 1 81. Dialdehyde cellulose was further modified to the corresponding hydroxamic acid derivatives by three-step reactions, and their behavior to form metal complexes in water was investigated [ l 19,120). Periodate oxidation was also applied to cellulose beads for chromatography [ 12 1 1. It is well known that primary alcohol groups of cellulose are partly converted to carboxyl ones by oxidation with N20, in chloroform. However, side reactions are inevitable during this N20, oxidation. On the other hand, recently a new water-soluble reagent, 2,2,6,6-tetramethylpipelidine- I -oxy1 radical (TEMPO), has become commercially available, and TEMPO can oxidize primary alcohol groups of water-soluble polysaccharides such as starch to carboxyl ones in good yields and selectivity in the presence of a cooxidizing agent at pH 9- l l [ 1221. The TEMPO-NaBr-NaC10 system was first applied to native cellulose by Chang and Robyt [ 1231, although their method could not give watersoluble cello-uronic acid [ 1241. Then the TEMPO-mediated oxidation of cellulose samples under various conditions was examined to prepare water-soluble cello-uronic acid having high DP. Although only small amounts of carboxyl groups were introduced into native cellulose samples by this oxidation, water-soluble cello-urionic acid sodium salts were obtained quantitatively by using regenerated and mercerized celluloses as starting materials (Fig. 12) [ 1241. DP values of the cello-uronic acids thus prepared were greatly influenced by the oxidation conditions, and partial depolymerization occurred on cellulose chains by p-elimination under alkaline conditions. Since cello-uronic acids prepared by the TEMPO system regularly consist of the glucuronic acid repeating unit, differing from the conven-

-C,OONa

l

220 200

I

I

180

I

160

I

140 120

I

I

loo

I

80

I

60

ppm

FIGURE 12 "C-NMR spectrum of cello-uronic acid Na salt dissolved in D,O 11241.

odification Chemical

of Cellulose

621

tional water-soluble cellulose derivatives, they may have some unique solution properties or bioactivity.

VII.

CHEMICAL REACTIONS AT MINOR GROUPS

Cellulosic fibers usually contain carboxyl and aldehyde groups in quite small quantities, depending on the purity of the fibers. Usual bleached kraft pulps have carboxyl contents of 0.02-0.08 mmol/g and aldehyde contents of 0.01-0.03 mmol/g. Cotton linter pulps have much lower values. The carboxyl groups originate from those present in the original hemicellulose and/or those formed during pulping (especially anthraquinone-added kraft pulping) and bleaching. The C 1 groups at the reducing ends of cellulose and hemicellulose chains are accounted forasaldehydegroups.Carboxylgroups in cellulosic fibers play significant roles in fiber processing treatments: dyeing and surface treatments in fiber processing, and efficient adsorption of wet-end additives on pulp fibers at the wet end in papermaking. Aldehyde groups in cellulosic materials can be oxidized with ClO; to carboxylic acids, and thus their carboxyl contents are increased by this oxidation [60]. Since watersoluble carbodiimide, WSC: 1-ethyl-3-(3-dimethylaminopropyl)carbodiimideHCI salt, has become commercially available, carboxyl groups can be selectively converted to amides by WSC and compounds having primary amine groups under aqueous conditions at pH 4.75. More than 95% of the carboxyl groups in a bleached kraft pulp were converted into nonionic or cationicamides with WSC and the corresponding amine in aqueous pulp suspensions (Fig. 13) [57-60,1251. Here, the cationic pulps prepared by this method can be regarded as true cationic pulps, because the cationic groups are introduced into pulps by blocking the anionic carboxylgroups. On the other hand, the usual cationic pulps prepared from normal pulps by etherification with, for example, diethylaminoethyl chloride HCI salt at hydroxyl groups, should be called “amphoteric” pulps. Fundamental properties such as pulp freeness were unchanged by the amidation, and thus only slight chemical modifications of pulp fibers can be achieved by this amidation. However, when handsheets were prepared from the nonionic pulp, which was prepared from normal bleached kraft pulp by amidation with methylamine and WSC, effects of wet-end additives, such as sizes, wet-strength resins, and retention aids,decreased drastically. This is because retention values of these additives clearly drop by blocking the anionic carboxyl groups with nonionic methylamide groups in the pulp fibers. Thus, carboxyl groups in pulp fibers, even though their quantity is quite low, behave as essential retention sites for wet-end additives in paper stock by ionic interactions [57-60,1251.

VIII.

CONCLUSION

As described above, chemical modifications of cellulose have been studied extensively from both fundamental and practical points of view by many researchers, and so much information about fundamental properties of cellulose and cellulose derivatives has been accumulated. Some results have been applied practically at industrial levels. Since cellulose always has some strong interactions with water, large amounts of energy are required for complete removal of water from cellulosic material. Thus, substitution reactions occurring selectively at hydroxyl groups of cellulose instead of water (or OH” ions) must be significant to achieve efficient substitution reactions in cellulose.

622

lsogai

Pulp-COOM (M = Na, Ca, H)

-----

t

PuI~-COOH"

PuIP-COOCH~

Solvent exchange from water to ether CH2N2 in ether through methanol and n-hexane

Pulp-COOM (M = Na, Ca, H)

1

EtN=C=N(CH2)3NMe2*HCI (WSC), pH 4.75

/ N+H-Et

o=c

&N+H-Et

Pulp-coo-c,

Pulp-CONH-R NH(CH,),NMe,

RNHz, pH 4.75 R : -CH3 (nonionic) R : -CH2CH2NHMe2 (tertiary amine) R : -NHNHCOCH2N+Me3 Cl- (quaternaryamine)

R : -CH2(CH2)&H3 (hydrophobic)

(hydrophobic)

FIGURE 13 Methylation and amidations of carboxyl groups in bleached kraft pulp [125].

Preparations of regioselectively substituted or oxidized cellulose derivatives and their characterizations may bring about development of cellulose derivatives having new functionalities. The nonaqueous cellulose solvent systems have often been used to prepare cellulosederivatives in laboratory levels. For practical purposes, these multicomponent solvent systems, usually having high boiling points, are hard to utilize unless the cellulose derivatives obtained have quite unusual functionalities. If highly regioselective reactions to the three hydroxyl groups of one anhydroglucose residue are applicable to solid cellulosic materials in one step under aqueous conditions, they must be valuable for practical use. Not only drastic changes in the properties of cellulose by esterifications or etherifications but also some efficient surface modification techniques of cellulose, such as paper sizing, may also be utilized for other purposes in functionalizations of cellulosic materials.

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15 Chemical Synthesis of Cellulose Fumiaki Nakatsubo Kyoto University, Kyoto, japan

I. INTRODUCTION Cellulose is the most abundant natural organic polymer existing as a main plant cell wall component and is important as a biodegradable and renewable organic resource [l]. The study of cellulose, therefore, has continued for more than 150 years. However, there are still several problems which should be solved: biosynthesis, crystal structure, chemical synthesis, regiospecific substitution reactions, structure-function relationships of their derivatives, and so on [ 1,2]. The synthesis of cellulose has been a very important but extremely difficult problem to solve, sinceSchlubach first tried the synthesis in 1941 [3]. Recently, Kobayashi and co-workers reported enzymic synthesis of cellulose [4]. Their synthetic method with cellulase is important and interesting as the first in-vitro synthesis using enzyme. The method, however, does not satisfy the recent molecular design of cellulose derivatives having special functions,because it may not enable us to regiospecifically introduce the special functional group into only the desired hydroxyl groups in the repeating pyranose units of cellulose. There are many functional cellulose derivatives: cellulose esters and ethers having liquid crystalline properties [5] and chiral recognition ability [6], sulfated cellulose with anticoagulant activity like a heparin [7], branched cellulosederivatives with antitumor activity [8], and so on. Much is still unknown about the relationship between the structure and properties, which derivatives are more functional or active among those substituted at 2,3- or 6-positions. For these studies and,furthermore,for molecular design of the advanced materials from cellulose, methods which make it possible to prepare cellulose derivatives with functional groups at special positions among 2,3,6-hydroxyl groups in the repeating pyranose unit of cellulose are indispensable. Polycondensation and ring-opening polymerization methods using glucose derivative as a starting monomer satisfy the above requirements [9], but all trials synthesizing cellulose by these methods have been unsuccessful. Husemann and Muller [lo] and Hirano [ I l l reported the condensation of 2,3,6glucose tricarbanilate with phosphorus pentoxide in a mixture of chlorofoddimethylsulfoxideto give a cellulose-like polymer containing branched polymer and about 1% phosphorus. Micheel et al. [ 121 and Uryu et al. [ 131 tried cationic polymerization of 1,4anhydro-2,3,6-tri-O-benzyl-a-~-glucose initiated with various Lewis acids, but stereoregular (1 +4)-P-D-glUCOpyranan was not obtained. Furthermore, Uryu et al. [ 141 reported 627

Nakatsubo

628

the first synthesis of cellulose-type glycopyranan, (1-4)-/3-~-ribopyrananby cationic ringopening polymerization of 1,4-anhydro-a-~-ribopyranose derivatives. However, their strategy is not applicable to the synthesis of cellulose although it is useful for the preparation of glucan with the same hydroxylation pattern as ribose. Very recently, Kochetkov described in his review [9cl that Malysheva synthesized completely stereoregular (l+4)-pglucan from 1,2-O-cyanoethylidene derivative only at high pressure, but the paper describing the method in detail has not appeared. Recently, the author and his co-workers succeeded in the first chemical synthesis of cellulose [ 151. In this chapter, the first chemical synthesis of cello-oligosaccharides and cellulosederivatives by both astepwisesynthetic method and a cationic ring-opening polymerization, and conversion to cellulose by removing their protective groups, are described. These synthetic methods are now drawing attention in the world [ 16).

II. SYNTHESIS OF CELLO-OLIGOSACCHARIDES Cello-oligosaccharides are generally defined asa series of saccharidescontaining from dimer, cellobiose, up to cellodecaose [ 171, and have an important role as a bridge between commercially available glucose and cellobiose with low molecular weight, and cellulose with high molecular weight. For the elucidation of chemical structure, and chemical and biological reactivity of a complex macromolecule, model compounds have played a significant role in the history of lignin chemistry 1181. A few cases use such model compounds in the field of cellulose research, because the chemical structure of cellulose is simple. We may, however, discover new merits again in using cellulose model compounds to clarify several problems which still exist after about 150 years of cellulose research as described above. Thus,cellooligosaccharides are quite useful as the model compounds. These cello-oligosaccharides may be obtained by the partial hydrolysis of cellulose and appear as single peaks on the chromatogram [191, but it is extremely difficult actually to isolate a suitable amount of these oligosaccharides for model experiments, especially in the case of higher-molecular-weight compounds. Consequently, chemical synthesis is the most promising method for obtaining suitable amounts of these pure oligosaccharides, when synthetic problems suchas selection of the protective groups of glucose and pglycosylation methods are solved. There appear to be a few papers on the synthesis of cello-oligosaccharides. Around 1930, Helferich et al. [20] and Freudenberg et al. [ 2 11 reported the synthesis of cellobiose. In I97 I , Hall et al. [22] reported erroneous synthesis of cellotriose, but Takeo et al. [231 succeeded in the first true synthesis of cellotriose. In 1980, Schmidt et al. 1241 developed a new glycosylation method called the imidate method, using trichloroimidoyl groups as a leaving group from the C,-anomeric position, and applied it to the first synthesisof cellotetraose 1251. Later, Takeo et al. [26] also reported cellotetraose synthesis, but their methods do not seem to apply for the synthesis of the higher-molecular-weight oligosaccharides. Thus, a new synthetic strategy completely different from their methods must be considered for the higher oligosaccharides (271.

A.

BasicSyntheticStrategy

In principle, the synthesis of cellulose seems tobe simple because only three synthetic problems need to be solved: regiospecific control and stereospecific control, and increasing the molecular weight (Fig. l ) .

629

Chemical Synthesis of Cellulose

Stereospecific control ( P -glucosidic bond)

n OH

U Regiospecific control (1,4-bond)

Regiospecific control means to control 1,4-bond formation, which could be done by the selection of 2,3,6-tri-O-substituted glucopyranose derivative as a starting material. stereospecific control means to control P-glucosidic bond formation, which could be accomplished by stereospecific P-glycosylation. Stereospecific control would be extremely difficult and a key problem. Thus, we may obtain useful information about stereospecific P-glycosylation from the synthesis of cello-oligosaccharides without having to deal with the molecular-weight problem. Two basic synthetic methods, i.e., linear and convergent synthetic methods, are conceivable for the cello-oligosaccharides and also for cellulose, as shown in Fig. 2 [28]. For these synthetic designs, the starting material, D-glucose, should have three kinds of protective groups, X, Y, and R, for the regulation of regiospecificity. Here, X and Y groups are “temporary” protective groups and the R group is a “persistent” protective group [29]. It is a prerequisite for the selection of these three protective groups that the Y and R groups or the X and R groups must not change upon the removal of the temporary X and Y groups, respectively. That is, it is most important that each of these temporary groups be removed independently without any influence on the other functional groups. In a linear synthetic route as shown in Fig. 2, after repeating two reactions, deprotection of the Y group and P-glycosylation, a series of cello-oligosaccharides whose degrees of polymerization (DPs) are 2, 3, 4, . . . could be theoretically obtained. On the other hand, in a convergent synthesis as shown in Fig. 2, after repeating a set of two reactions, deprotection and P-glycosylation, n times, we will theoretically obtain a cellooligosaccharide whose DP is 2 . The convergent synthesis is preferred for obtaining higherDP cello-oligosaccharides by minimum reaction steps [30]. Thus, the selection of the three kinds of protective groups and the P-glycosylation method are extremely important. The possible combinations of the protective groups are shown in Fig. 3 [31].

B.

Synthesis of Cello-Octaose by a Linear SyntheticMethod

Kawada et al. established the first synthetic route for cello-octaose acetate starting from allyl 2,3,6-tri-0-benzyl-4-(p-methoxylbenzyl)-~-~-glucopyranoside ( l )as shown in Fig. 4 [32].Here allyl, p-methoxybenzy (PMB), and benzyl (Bn) groups were selected as the X, Y, and R groups, respectively. ThePMBgroup of the starting compound (1) can be removed selectively by oxidative reaction conditions with cerium(1V) ammonium nitrate (CAN) in acetonitrile-H,O to afford glycosyl acceptor (2).The allyl group can be removed by the two reactions, migration of the double bond with Ko-‘Bu in DMSO and subsequent acid hydrolysis of the enol ether obtained with HCl in acetone to give a glycosyl donor

630

X

0

Nak:atsubo

631

Chemical Synthesis of Cellulose

OR CICHpCO-

J

1 -CHpCH=CHp (All)

-COCH,( Ac) R = -CO-C(CH& (Pi") -CH2C& (Bn)

[

FIGURE 3 Possible combination of X. Y, and R protective groups.

which is further converted to the activated compound, a-imidate (3). The glycosylation of the acceptor (2) with the donor (3) catalyzed by BF, etherate in CH,CIZ at -70°C gave the expectedcellobiosederivative (4) in 85% yield without any production of the a anomer, via a completely S,2 reaction mechanism as proposed by Schmidt et al. [33]. Repeating the set of two reactions, deprotection of the PMB group and subsequent P-glycosylation with donor (3), six times, finally affords a cello-octaose derivative (10) which is converted to an acetyl derivative after deprotection and subsequent acetylation. The cello-octaose acetate obtained was interestingly compared with the acetate from hydrolysis product of cellulose reported by Buchanan et al. 1341. Thus, the selection of both combinations of the protective groups, allyl, PMB, and Bn groups, and the P-glycosylation method (imidatemethod)werefound to be quite suitable for the linear synthesis of cello-oligosaccharides. However, it was revealed that selection of the protective groups is not suitable for the convergent synthesis, because of instability of the dimeric and tetrameric imidates: imidoylation of the donor derived from cellobiose (4) and cellotetraose (6) derivatives always gave a mixture consisting of a - and P-anomers which cannot be obtained as a pure compound, i.e., these imidates are very unstable, resulting in decomposition upon separation by silica gel TLC. Generally, an electron-donatingprotectivegroup, such as a Bn group,accelerates reactivity of the glycosyl donor, but an electron-withdrawing group such as the acyl group reduces it [ 3 5 ] .Consequently, it is expected that we must design suitable starting materials with both optimum reactivity and stability by a partial exchange of several Bn groups with acyl groups in compound (1). C.

Substituent Effects on P-Glycosylation and Selection of the Starting Material for the Convergent Synthesis

Takano et al. 1361 obtained surprising but very interesting experimental results, leading to success in the first chemical synthesis of cellulose as a consequence, on glycosylation with a-imidate (3), which was used as the most suitable glycosyl donor, giving only @-glycoside for the linear synthesis of cello-octaose derivative as described in Section 1I.B. As shown in Fig. 5, upon glycosylation with a-imidate (3), glycosyl acceptor (2) having benzyl groups gave the only expected P-glucoside in a high yield, but contrary to their expectation, glycosyl acceptor (11). having acetyl groups, gave a mixture consisting of a - (29%) (12) and P-glucoside ( 16%) in low yield. Then, Takano et al. systematically examined the effect of the protective groups of the glycosyl acceptor and donor on the glycosylation reaction in order to select the best protective group system. The results are summarized in Table 1 . It is clear from the results

632 Nakatsubo

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Chemical Synthesis of Cellulose

”.

O , Bn

. s m 0 0 -4 OBn

OBn

OBn

OBn

no a-Glucoside

-0 HO,

--

-/

-

12 [a-Glucoside (29%)

-U

ethcrate gave the expected tetramer (22) with a 71% yield. Glycosylation between tetrameric donor (23) and acceptor (24) at 0°C using 0. 15 eq. of BF,-ethrate gave cello-octaose derivative (25) with a 95% yield. The cello-octaose derivative (25) was converted to cello-tetraose (27) by removing the protective groups. Thus, the first convergent synthesis of cello-octaose was established by reexamining the protective groups and by considering the reaction conditions.

E. Synthesis of Cello-Oligosaccharides up to Eicosaose Derivatives by a Stepwise Synthetic Method For the clongation of the carbohydrate chain with a minimum of reaction steps based on the same convergent synthetic method as that shown i n Fig. 7, cello-octaose derivatives (25) must beconvertedinto both glycosyl donor and acceptor. However, there wasa problem in the preparation of a-imidate from compound (25). That is. imidoylation reaction always gave P-inlidate as an oil in quantitative yield, which is a kinetically controlled product, but the product was an unstable compound. This compound decomposed within a few days at room temperature to afford the hydrolysis product. Thus. elongation of the carbohydrate chain from cello-octaose derivative (25) was carried out via a stcpwise synthetic routc as shown in Fig. 8 [41]. Here, tetrameric a-imidate (23) was used a s the sole glycosyl donor. Cello-octaosederivative (25) wasconvertedintothcglycosylacceptor (28). then glycosylated with tetrameric imidate (23), to afford the expected cello-dodecaose derivative (29) with a high yield. After repeating thc set of two reactions, removal of acetyl group and subsequentglycosylation with imidate (23) twice. cello-eicosaosederivative (33) was obtained.Compound (33) was convertedinto the acetyl derivativeafter the

Chemical Synthesis of Cellulose

637

638 Nakatsubo

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639

removal of protective groups and subsequent acetylation. The ‘H-NMR spectrum of the synthesized acetyl cello-eicosaose derivative was found to be almost identical to that of authentic cellulose triacetate (CTA). Generally, in carbohydrate chemistry the term oligosaccharide refers to a substance having two to ten monosaccharide units 1171. According to this terminology, derivatives with degree of polymerization (DP) above 12 are not oligosaccharides, although they may not be macromolecules. Concerning this point, Kobayashi et al. regarded their synthesized product having a DP value of 22 as a polysaccharide, that is, “cellulose” [4]. In fact, at around this DP, several characteristics of cellulose such as crystal structure seem to appear [4]. Isogai et al. also regarded samples having DP values of 15 as low-molecular-weight cellulose [42]. Thus, the synthesized compounds (29)-(33) may be regarded as “cellulose,” more strictly as low-molecular-weight or medium-size cellulose. Here, the stepwise synthesis of cellulose was achieved for the first time. Interesting changes in several of the properties accompanying an increase in DP of a series of the synthesized medium-size molecular weight compounds were found. Peak patterns of both ‘H- and “C-NMR spectra were gradually simplified with an increase of DP, leading to peaks attributable only to the internal repeating units. Thespectrum of acetyl cello-eicosaose derivative was almost identical to that of CTA. Upon gel permeation chromatography (GPC) analysis, the plots of logarithm of molecular weight (log M> versus elution volume are clearly linear, as shown in Fig. 9. Segal reported measurement of the molecular weight of cellulose trinitrates by GPC, but the values obtained were fairly large in comparison with those obtained viscometrically [43]. In this investigation it was found that, at least with high-DPcello-oligosaccharidesor medium-size cellulose up to eicosaose, the molecular weight can be determined almost exactly using this calibration curve. According to Freudenberg [44], if the linkages in a polymer-homologous series are uniform, the plot of[M] n/n against ( n - I)/n yields a straight line. Here, [m n = molecular rotation and 11 = DP. Figure 10 shows a plot of molecular rotations versus DP. The Freudenberg theory was found to be applicable to a series of a-D-acetates of cello-oligosaccharides up to DP 7 [45]. For the synthesized cellulose derivatives,the theory appears to fit up to DP 16. The point for DP 20 deviates from this straight line in spite of uniformity of the glycosidic linkages. It may be assumed that a conformational change due to a polymer effect starts to appear at around cello-eicosaose. Kajiwara et al. proposed an extended zigzag structure with an inflection point at DP 20, by the Monte Carlo simulation method based on the molecular mechanics [46]. For the proof of this possibility, synthesis of cellulose derivatives above eicosaose and their properties have to be studied.

111.

SYNTHESIS OF CELLULOSE BY A RING-OPENING POLYMERIZATION

A.

Synthetic Characteristicsand Problems of Ring-Opening Polymerization

Let us consider synthesizing cello-eicosaose derivative by the two typical synthetic methods, stepwise (linear and convergent syntheses) and ring-opening polymerization. Linear and convergent synthetic methods need 40 and 18 reaction steps for the synthesis of celloeicosaose derivative from XYR-glucose derivative as shown in Figs. 4, 7 , and 8, respectively, although the product is single-dispersed.

a a C

0:

1 ooooc

~o~ys~yrene

10000

2 0,

0 -

1000

1 0 0 " " " " " "

13

14

15

16

17

Elution time (rnin)

18

19

100 13

14

15

16

17

18

19

20

Elution time (rnin)

FIGURE 9 GPC analyses of synthesized 2,6-di-O-pivaloyI-3-O-benzyl cellulose and cellulose acetate series. Column, Shodex GPC KF-802 + KF-803: solvent. THF ( 1 mL/rnin).

641

Chemical Synthesis of Cellulose

c c

\

-5000

-10000

-1

/4

I

I

I

I

I

0.5

0.6

0.7

0.8

0.9

1

(n-1 )/n

FIGURE 10 Relation between degree of polymerization and molecular rotation: n, degree of polymerization; IM] ! l . molecular rotation; 0.2,h-di-O-pivaloyl-3-O-benzyl cellulose series; (Y-Dacctatc of cello-oligosaccharides series rcported by Wolforrn and Dacons 1451.

*,

A*,, 36: (1-4)-a-P

37: (1+4)-p-P

On the other hand, a ring-opening polymerization requires only one reaction for the syntheses of cello-eicosaose and also polysaccharides from the starting monomer, although usually it gives a polydispersed polymer. Thus, ring-opening polymerization is very useful for the preparation of high-molecular-weight polysaccharides. This method, however, has to overcome two extremely difficult problems, regiospecificity and stereospecificity, at the same time during the polymcrization. That is, ring-opening polymerizations of 1,4-anhydro glucose (34) and glucose 1,2,4-orthoester (35) monomers usually yield four possible structural units, that is, 1,4-anhydro glucose monomer (34) usually gives the (1+4)-a (36) and (1+4)-p (37) -D-glucopyranosidic units and the ( I + S ) - a (38) and ( I "+S)-p (39) -Dglucofuranosidic units. And glucopyranose l ,2,4-orthoestermonomer (35) gives the

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(1-+4)-a (36) and (1+4)-P (37)-~-glucopyranosidicunits, and (1 -+2)-a (40) and (1+2)P (41)-~-glucopyranosidicunits. That is, for synthesis of cellulose, glucan consisting of only (1-4)-P-~-glucopyranosidic unit in four possible units has to be made. Furthermore, the stereo- and regioselectivity must be extremely high during the polymerization. For example, 99% stereoselectivity yielding 99% P- and l% a-glucosides upon glycosylation is a satisfactory result in the stepwise synthesis, because the undesired a-anomer can be separated by chromatography after glycosylation. However, 99% stereoselectivity is not acceptable for the ring-opening polymerization. This is because the eicosaose derivative obtained by polymerization with 99% selectivity is theoretically a mixture consisting of 524,288 (= 2'2')"') molecular species containing 82.6% (= 0.99(20"))of a stereoregular cello-eicosaose derivative. This is why no one so far has succeeded in the synthesis of cellulose by ring-opening polymerization.

B.

Cationic Ring-Opening Polymerization of 1,4-AnhydroglucopyranoseDerivative

The synthetic approach to cellulose via ring-opening polymerization of 1,4-anhydro-2,3,6tri-0-benzyl-a-D-glucopyranosewas reported for the first time by Micheel et al. [l21 to yield a cellulose-like polymer. Uryu etal. [ 131 also tried polymerization of the same monomer, but obtained an unexpected stereoregular ( l +5)-a-D-glUCOfUranan, with stereochemistry expected on the basis of the antiperiplanar theory of Deslongchamps [47]. Since I ,4-anhydro-a-~-glucopyranose, which may also be regarded as l ,5-anhydro-P-D-glucofuranose,has two ring-opening modes, that is, 1,4- or 1,5-ring scission, there are four possible structural units in the polymer obtained, which are caused by the ring-opening modes and anomeric a- and @-configurationsas described in Section 1II.A. Ring-opening polymerization is affected by reaction conditions, and there is a possibility of achieving the chemical synthesis of cellulose by finding optimum reaction conditions. In Chapter 2, the substituent effects on the highly stereoselective glycosylation in the syntheses of cello-oligosaccharides are described. The benzyl group at 3-0 was indispensable to obtain P-linked glucosides in high yield, and the pivaloyl group introduced into the 2 - 0 led to P-glycosidic linkage by the P-side attack of glycosyl acceptor because of the neighboring-group participation. Thus, there is a good possibility of synthesizing the expected P-(1-+4)-glucan with both stereo- and regioselectivity by ring-opening polymerization utilizing such substituent effects. In fact, Ichikawa et al. [48] and Kobayashi et al. [49] reported the syntheses of (1+6)-~-~-galacto-oligosaccharidesby applying the neighboring-group participation of the 2-0-acyl group. However, there are no papers describing such substituent effects in ring-opening polymerization of 1,4-anhydro-a-~-glucopyranose derivatives. In this section, substituent effects on the ring-opening polymerization of 1,4-anhydroglucose derivatives toward the chemical synthesis of cellulose are described. In orderto study substituent effects on ring-opening polymerization of 1,Canhydroglucopyranose derivatives, Kamitakahara et al. [50] selected four starting monomers, 1,4-anhydro-2,3-di-O-benzyl-6-O-pivaloyl-a-~-glucopyranose (42), 1,4-anhydro-3-O-benzyl-2,6-di-O-pivaloyl-a-~-glucopyranose (43). 1,4-anhydro-3-O-benzyI-2,6-di-O-pivaloyla!-D-glUCOpyranOSe (44), and 1,4-anhydro-6-O-benzyl-2,3-di-O-pivaloyl-a-~-glucopyranose ( 4 9 , and polymerized them under several reaction conditions. The results are summarized in Table 3.

Chemical Synthesis of Cellulose

643

TABLE 3 Polymerization of 1,4-Anhydro-a-~-Glucopyranose Derivatives“ Yield Time Temp (“C)

Exp. Monomer Initiator no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

19 20 21 22 23 24 25

42

30 30 - 30 0 20 20 20 -30 - 30 - 30 20 20 20 - 30 - 30 - 30 20 20 20 - 30 - 30 - 30 20 20 20 -

43

44

(%)

25 21 20 20 17 20 16 34 60 20

65h 66h 80h 82h 1Ih 5~6~ 62h 87h 78’ 100’ 74h 86h ca. 100 Trace 42’ 100 Trace 100 100 Trace 7d 7* 35* ca. 100’ 7*

15

20 1

239 136 20 240 18 1.5

26 17 17 24 16 16 ~

[a], (deg)

(h)

~

10-’MGKe

DP,,

+79.7 +72.9 +43.4 +88.9 +68. I +58.1 +27.2 -66.9 -69.3 -65.6 -5.8 - 19.0 -23.4

10.4 8.0 I .9 9.3 2.7 5.1 3.1 9.4 7.5 3.4 1.4 4.7 2.2

24.3 18.9 4.3 21.8 6.4 12.1 7.3 22.0‘ 17.6‘ 8.1 ‘ 3.3 11.0 5.2‘

-59.3 -57.9

10.0 6.3

23.4 ‘ 14.9’

15.7 -48.0

3.2 4.6

1 1 .of

I .4 2.0 1.9 2.5 1.6

3.3 4.8 4.4 6.2 3.7

-

-26.0

7.6

~-

“Initiator concentration, 5 mol%; solvent, CH,Cl,; monomer/solvent, 50 g/lOO mL. *Polymer was insoluble fractlon in n-hexane. ‘No unreacted monomer was detected. ‘Polymer was separated from unreacted monomer by TLC. ‘Number-averaged molecular weight was determined by GPC using polystyrene standards. ‘Stereoregular (1-1S)-P-o-glucofuranan derivative was given.

1. Substituent Effect on Molecular Weight of Polysaccharides Molecular weights ofpoly(42)s obtained by the polymerization of monomer (42), poly(43)s from (43), and poly(44)s from (44) decreased with an increase of reaction temperature, but those of poly(45)s from (45) were low under all reaction conditions tried. The polymerizability of four monomers is in the order (42) G (43) = (44) >> (45), as judged from the highest molecular weight obtained from these monomers (Table 3, experiments 1, 8, 15, and 24) and from the yield of polymer at -30°C.

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Generally, the electron-donating benzyl group accelerates reactivity. but the electronwithdrawing acetyl group retards both the glycosylations and the polymerizations of anhydro-sugars. For example, Zachoval et al. [51 ] reported that I ,6-anhydro-P-D-glucopyranose triacetate was less reactive than 1,6-anhydro-P-1~-glucopyranose tribenzylether. Monomers (42) and (43), having two benzyl groups. expectedly afforded polymer with high molecular weight. Monomer(45), having one benzyl group, did not afford polymer under various reaction conditions (Table 3, experiments 20-25), but unexpectedly, monomer (44), having only one benzyl group, afforded polymer with high molecular weight. Consequently, the present results indicate that the important factors for accelerating polymerizability are not only the number of the substituent benzyl groups but also the positions where benzyl groups are attached. The comnlon substituent among monomers (42), (43), and (44) is a benzyl group at the 3 - 0 position: monomer (45) does not have a benzyl group at the 3 - 0 position. Thus, the benzyl group at the 3 - 0 position is indispensable for yielding glucan with high molecular weight. Comparing monomers (43) having a benzyl group at the 6-0 position and (44) having a pivaloyl group at the 6-0 position, there is little difference in molecular weight under optimum reaction conditions (Table 3, experiments 8 and 15). Consequently, the benzyl group at the 6 - 0 position did not have much effect on polymerizability. 2.

Substituent Effecton Stereoregularity of Polysaccharides

Substituent groups at 0 - 2 greatly affect specific rotation. as shown in Table 3. All poly(42)s are dextrorotatory. The polymerization of (42), having a benzyl group at the 2 - 0 position, gave a nonstereoregular polymer consisting mainly of ( 1 jS)-cY-glucofuranosidic units. On the other hand. all poly(43)s, poly(44)s, and poly(45)s are levorotatory. All these monomers have a pivaloyl group at the 2 - 0 position. The polymerization of (43) gave a stereoregular ( I ~ 5 ) - P - ~ - g l u c o f u r a n derivative an (Table 3, experiments 8- 10 and 13). Polymerization of (44) also gave a stereoregular ( 1 +5)-P-~-glucofurananderivative (Table 3, experiments IS, 16, and 19). Polymerization of (45) tended to give (l+S)-P-furanosidic units, but none of the conditions afforded a stereoregular poIy(45), although monomer (45) has the same 2-0-pivaloyl group as monomers(43) and (44) (Table 3, experiments20-25).Consequently, it turned out that the benzyl group at the 3-0 position is indispensable for obtaining a stereoregular polysaccharide. It is predicted that the favorable complexation of Lewis acids with both a C , and a C , oxygen tends to result in enhancement of polymerizability and stereoregularity, because the electron-donating 3-0-benzyl group raises the electron density of the C, oxygen and consequently elevates the coordination power with Lewis acids. The electron-withdrawing pivaloyl group at the 3-0 position, on the contrary, weakens the coordination power of the Lewis acids so that polymerizability and stereoregularity are lowered. The fact that both polymerization of (43), having a benzyl group at the 6 - 0 position, and that of (44), having pivaloyl group at the 6 - 0 position, gave stereoregular (I-+5)-PD-glucofuranan derivatives with almost the same under these optimum conditions (Table 3, experiments 8 and IS) indicates that the substituent group at the 6 - 0 position hardly affects either stereoregularity or polymerizability.

3. Importance of 3-0-Benzyl Group of 1,4-Anhydro-cw-~-Glucopyranose Derivative on Ring-Opening Polymerization Substituent groups at the 6-0 position did not remarkably affect stereoregularity or polymerizability. comparing results from monomers (43) and (44). It was confirmed that the

Chemical Synthesis of Cellulose

645

benzyl group at the 3 - 0 position has a special function for yielding ;I stereoregular polysaccharide with high molecular weight in a ring-opening polymerization [results from monomers (43) and (45)). It was reconfirmed that the presence of the pivaloyl group at the 2 - 0 position makes the polysaccharide take the P-configuration [results from monomers (42) and (44)]. Polysaccharides with high molecular weight tend to have high stereoregularity as shown in Table 3 (experiments 8, 9, 15, and 16). Consequently. both the pivaloyl group at the 2 - 0 position and the benzyl group at the 3-0 position are indispensable for yielding stereoregular ( 1 +5)-/3-~-glucofuranan derivatives with high molecular weight. Furthermore, polymerization of 1,4-anhydro-a-~-glucopyranose was found to always preferentially afford ( 1 +5)-~-glucofuranose units, not ( 1 ~4)-P-D-giucopyranoseunits. These results agreed with the cases of the ring-opening polymerization of 2,7-dioxabicyclo-[2.2.1]heptane [52] and that of 1,4-anhydro-2,3,6-tri-O-benzyl-a-~-glucopyranose [ 131. Thus, it is also concluded that the 1,4-anhydro-a-~-glucopyranoseskeleton is not suitable for yielding a ( 1 +4)-P-~-gIucopyranan, that is, a cellulose molecule. It was revealed from the results of the polyrnerizations of 1,4-anhydroglucose derivatives that at least one of the most important problems i n the synthesis of cellulose, i.e., stereospecificity giving a stereoregular P-glucosidic polymer, could be successfully solved by applying substituent effects of 2-O-pivaloyl and 3-O-benzyl groups obtained from synthetic studies of cellooligosaccharides.

C.

Cationic Ring-Opening Polymerization of Glucopyranose 1,2,4-Orthoester Derivatives

1.

First Chemical Synthesis of Cellulose by a Cationic Ring-Opening Polymerization of Glucose 1,2,4-Orthopivalate Derivative

As described in Section III.B, polymerization of l ,4-anhydro-a-~-glucopyranose (43) was found always to preferentially afford ( 1+5)-~-glucofuranose units [poIy(43), not ( l +4)P-D-glucopyranose units poIy(47)]. One strategy for realizing the highly regioselective 1,4-scission is to substitute a l ,4-ether bond of 1,4-anhydro-a-~-glucopyranose derivatives (43) for another, more reactive linkage such as that of orthoester derivative (47), as shown in Fig. 11. Several cationic ring-opening polymerizations of such tricyclic intramolecular orthoesters prepared from arabinose and xylose have been studied extensively by Bochkov, Kochetkov, and their co-workers 1531. but they neither considered the substituent effect on polymerization nor achieved a stereoregular polymer. N,N'-carbonyldiimidazoleis usually used for preparing cyclic carbonate [54). In our initial synthetic plan, if such a 1,4-cyclic carbonate derivative was obtained from compound (46) by treatment with N,N'-carbonyldiimidazole, the carbonate might also be used as a starting monomer for the polymerization, as expected in the polymerization of Ncarboxy-a-amino acid anhydride resulting in poly-a-amino acid [ S ] .Thus, compound (47) was treated with N,N'-carbonyldiimidazole and then the product was polymerized with triphenylcabenium tetrafluoroborate. Surprisingly, the polymerization product obtained as crystals was found to be a stereoregular (1+4)-P-~-glucopyranan [poly (47)] which was identified by 'H- and I3C-NMR spectra (Fig. 12). I t was an exciting and memorable day for us on the Saturday evening of July 25 in 1994. We had thought at that time that the starting monomer was a I,4-cyclic carbonate, but latcr the starting monomer was found to be an orthopivalate (47). Stereoregular poly(47) was converted into a cellulose triacetate (49) by deprotections and subsequent aectylation, and identified by 'H- and "C-NMR spectra as shown in Fig.

Nakatsubo

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43 OPiv 46

t \

OBn BnO /

OR1 poly(43): R1 = Piv, R2 = R3 = Bn poly(43)': R1 = R2 = R3 = H

\

C

X"",, 47

poly(47): R, = Piv, R2 = R3 = Bn

48:R1 = Ac, R2 = R3 = Bn 49: R1 = R2 = R3 =Ac 50: R1 = R2 = R3

H

FIGURE 11 Synthetic route for cellulose by a cationic ring-opening polymerization. "p-TsOH/ benzene/reflux/55%,"PF,/toluene/-3O0C, 'N,N'-carbonyldiimidazole/benzene/reflux, 62.8%. "Ph,CBFJ CH2Cl,/r.t.

100

95

90

85

80

75

70

6 (PPW FIGURE 12 2D-NMR spectra of poIy(47) from C-H COSY experiment (CDCI? as solvent).

647

Chemical Synthesisof Cellulose C2-OAc

.

t

.-,OAc

I

.oO *- AcO OAC

Authentic CTA

SyntheticCTA

7.0

8.09.0

6.0

t

1

5.0

t

~ - t

4.0

3.0

2.0

1.0

80

60

2040

0.0

6 (ppm)

120 140 l60 180

100

6 (PPW FIGURE 13 solvent).

'H- and I3C-NMR spectra of authentic and synthetic cellulose triacetate (CDCI, as

13. Finally, the cellulose triacetate (49) thus obtained was converted into cellulose (50) by deacetylation with NaOCH, in THF/methanol. The IR spectrum and X-ray diagram (Fig. 14) of cellulose prepared in this way were completely identical with those of regenerated cellulose with the cellulose-I1 crystal structure. Thus, the first chemical synthesis of cellulose by a cationic ring-opening polymerization had been achieved [ 151.

Nakatsubo

10

20

30

40

28 (deg.) FIGURE 14 X-ray diffractograms of (A) Whatman cellulose CFI 1. (B) regenerated celluhe. and (C) synthetic cellulose.

Substituent Effects at the 3-0- and 6-0-Positions on Stereo- and Regioregularity of Polysaccharides For examining the substituent effects at the 3-0- and 6-0-positions on the cationic ringopening polymerization of glucose 1,2,4-orthopivalate, Kamitakahara et al. [56] selected three additional orthopivalates, (Sl), (52), (53), and polymerized them under several reaction conditions. The results are summarized in Table 4.

2.

The molecular weights of poly(47). poly(51), and poly(52) were almost equal, but those of poly(53) were low under all reaction conditions tried. The polymerizability of four Inonomers is in the order (47) (51) >> (53), as judged from the highest molecular weight and yield obtained from these monomers (Table 4, experiments 3, S , 8, and 12). It can be said that polymerizability of (53) could notbe compared exactly with that of (47), (Sl), and (52) because polymerization of (53) was conducted at lower monomer concentration than that of (47), (Sl), and (52), but the above-mentioned trend must be true and substantial. In cases of ring-opening polymerizations of I ,4-anhydro-c~-~-glucopyranose derivatives, the benzyl group at the 3-0 position is indispensable for yielding glucan with high molecular weight. Similarly, in the present cases of cY-D-glUcOpyranOSe 1,2,4-orthopivalate derivatives, the benzyl group at the 3-0 position is indispensable for realizing stereoregularity, butis dispensable for yielding glucan with high molecular weight: that is, monomer (52), having a pivaloyl group at the 3-0 position, polymerized well but without yielding stereoregularity. Monomers having one benzyl group and two pivaloyl groups (including an orthopivaloyl group), that is, monomers (51) and (52), had the same polymerizability as monomer (47), having two benzyl groups and one pivaloyl group. Monomer (53), however, had

Chemical Synthesis of Cellulose

649

TABLE 4 Polymerizations of a-D-Glucopyranose 1,2,4-Orthopivalate Derivatives" Exp. no. 1

2 3

[a]::, (deg)

Temp. ("C)

Time (h)

Yield

Ph,CBF, Ph,CBF, Ph,CBF,

-30 20

16 18 14

59 96 93

-30 0 20

96 22 2

21

- 1.4

51 60

-3.7 + 1.9 7.5

4. I 4.9 3.2

9.7 11.6

33 30 0 20

96 49 15

-26.8 - 12.6 -18.8

2.9 3.7 3.0

6.9

47 58 Trace Trace 1.4

-24.8

Monomer

Initiator

47 47 47 S1

0

4 5 6

51 S1

Ph7CBF4 Ph,CBF, Ph7CBF,

7 8 9

52 52 52

Ph,CBF, Ph,CBF, Ph,CBF,

-

10" I l" 12''

53 S3 S3

Ph,CBF., Ph,CBF, Ph,CBF,

-30 0 1720

97 96 17 3.5

(76)

6.9 -20.1 -32.9 -35.2

DP,,

10

2.9 3.8 4.5

8.9 10.5

x.x 7. I

"Initiator concentration, S mol%; solvent. CH2CI,: monomer/solv.. 50 g/IOO mL "Monomer/solv.. 25 gll00 mL.

remarkably low polymerizability among the four monomers. Consequently, at least one benzyl group is indispensable for yielding a polymer with high molecular weight. Polymerizations of (47) and (51), having 3-0-benzylgroups, gave stereoregular ( I +4)-p-D-glUCOpyranan derivatives (Table 4, experiments 1-6). However. polymerizations of (52) did afford a stereo-irregular polysaccharide consisting of ( 1 +2)-a-P, ( 1 +2)p-P, and ( 1 -94)-p-P units. The (l+2)-bond formation increased with an decrease in temperature. That is, while the probability of coordination of Ph,CBF., with a C., oxygen was equal to that with a C., oxygen at 0°C and at 20°C, the coordination with the C? oxygen took place in preference to that with the C, oxygen at -30°C: the product ratios were ( I +2)-a-P/( 1 +2)-p-P/( 1+4)-p-P = 1 : I :2 [Table 4, experiment 8 (0°C) and 9 (20"C)I and (l+2)-a-P/( 1+2)-p-P/( 1 +4)-p-P = 3:3:4 [Table 4, experiment 7 (-30"C)l. The products ratio was calculated from the peak areas of the anomeric carbons of the corresponding constitutional units. The results indicate that in the case of coordination of a catalyst with the C, oxygen, the probability of an a-side attack resulting in the formation of (l+2)-aP was equal to that of a @-side attack resulting in formation of (1+2)-p-P. After all, the benzyl group at the 3 - 0 position is indispensable for yielding a stereoregular cellulose derivative, i.e., a ( 1 -4)-P-~-glucopyranan derivative. This fact agreed with the case for yielding stereoregular ( 1 +5)-/3-~-glucofuranan derivatives from 1,4anhydro-a-glucopyranose derivatives.

3.

Substituent Effects of Orthoester Groups on the Cationic Ring-Opening Polymerization of Glucopyranose 1,2,4-Orthoesters Generally, an electron-donating substituent increases the reactivities of both glycosyl donors and acceptors, resulting in highly stereoselective glycosylation with high yield, but an electron-withdrawing substituent exhibits the oppositeeffect.This means that ether groups are superior to acyl groups. Thus, the pivaloyl groups with relatively small electronwithdrawing effects in acyl groups are highly effective as protective groups of sugar hy-

Nakatsubo

650

droxyl groups on the glycosylation. The electron-withdrawing abilities of acyl groups are associated with the pK,, values of the corresponding carboxylic acids. OBn

47: R=-C(CH& 54: R=-CHPCH~ 55: R=-CH3 56: R=-CBHS R

Hori et al. [57] selected three additional orthoesters, 1,2,4-orthopropionate (54), orthoacetate ( S ) , and orthobenzoate (56), as starting monomers for cationic ring-opening polymerization and prepared from propionic (pK, 4.88), acetic (pK, 4.76), and benzoic (pK,, 4.20) acids to investigate the electronic effects of the orthoester groups on the cationic ring-opening polymerization. The results were compared with that of orthopivalate (47) (pK,, of pivalic acid, 5.05), whose polymerization gave a completely stereoregular cellulose derivative as described previously. Polymerizations of 3,6-di-O-benzyl-a-~-glucopyranose 1,2,4-orthoester derivatives (54)-(56), were camed out under the same reaction conditions as those giving cellulose derivative with BE 19.3 in the polymerization of (47). That is, all polymerizations were carried out at 20°C in the presence of triphenylcarbenium tetrafluoroborate as an initiator, in 5 mol% of initiator concentration and in 100 g/100 mL of monomer concentration. The results are summarized in Table 5. Specific rotations of polys(54)-(56) were positive values, 1.56", 12.9", and 11.8", respectively, as compared with poly(47), a cellulose derivative which has a large negative specific rotation, -37.2". These specific rotation values suggest that not all polymers newly obtained from monomers (54)-(56) are stereoregular. In fact, all these polymers were found to be nonregioregular, consisting of ( 1 -2)-and ( 1 +4)-P units, although with P-glycosidic linkage, that is, with stereospecificity but without regiospecificity. These data indicate that there is no distinct relationship between the production of (1-+4)-p-Punit and the pK,, values of the carboxylic acid groups introduced at the 2-0positions of the monomers. Only the 2-0-pivaloyl group has a characteristic effect on the fate of ring-opening polymerization, resulting in the formation of stereoregular (1+4)-p-

+

+

+

TABLE 5 Polymerization of 3,6-O-dibenzyl-ar-~-Glucopyranose 1,2,4-OrthoesterDerivatives"

~~

47 54 55 56

4.88 4.20

2 6 I .5 1.5

62 56 72 94

-37.2 +67 IS6 + 77 12.9 81 +11.8

19.3 15.8 10.4 9.4

0 33 23 19

100

"Initiator, Ph,CBF,; initiator concentration, S mol%; solvent, CH,CI,; monomer/solvent, 100 g / l O mL; reaction

temperature, 20°C. hpK,,values are those of the carboxylic acid groups introduced into 2-0-pos~tionsof monomers. 'Polymer is insoluble fraction in chloroformh-hexane (ca. 1/S. v/v). 'Calculated from polystyrene standard. "Calculated from the anomeric peak ratios in "C-NMR spectra of polymers.

thesis

Chemical

of Cellulose

651

glucopyranan. 2-0-acyl groups affect highly stereoselective P-glucosidic bond formation, and, in addition, the pivaloyl group in the acyl groups also affects further highly regioselective, (1+4)-glycosidic bond formation, probably because of steric effects, not electronic effects. Orthoester derivatives (52) having a 3-0-pivaloyl group gave polymerized products consisting of almost the same amount of (1+2)-a- and (1+2)-P-P units in the case of (1+2)-glycosidic bond formation (Section III.C.2). However, the present polymerizations of orthoesters (54)-(56) having a 3-0-benzyl group gave only P-glucosidic linkages upon (1+2)-bond formation. Consequently, the benzyl group at the 3-0-position has a great electronic effect upon the highly stereoselective P-glucosidic bond formation in the polymerization of a-D-glucopyranose l ,2,4-orthoester derivatives. Thus, both the 3-0-benzyl group and the orthopivaloyl group are indispensable substituents for the synthesis of stereoregular ( 1+4)-P-~-glucopyrananderivatives, cellulose derivatives, in the ring-opening polymerization of a-D-glucopyranose 1,2,4-orthoester derivatives.

IV.

FUTUREPROSPECTS

In spite of all efforts at chemical synthesis of cellulose for about55 years since Schulubach first started synthetic study, no one reached the difficult goal. Recently, the author and his co-workers succeeded in the first chemical synthesis of cellulose. In this chapter, their story of search for synthesis for a period of 15 years was described briefly. In this story, extremely important findings are on the substituent effects on P-glycosylation. That is, both 2-0-pivaloyl and 3-0-benzyl groups were found to be indispensable for obtaining Pglycoside with high stereoselectivity, from the results obtained on both synthetic methods, i.e., stepwise synthesisgiving cello-eicosaose derivative (DP20) and cationic ring-opening polymerization giving cellulose derivative with DP about 20 (DP 45 in the recent experimental results). Furthermore, interesting results were obtained that the 2-0-pivaloyl group also participates in the highly regioselective 1,4-bond formation in ring-opening polymerization. However, the way that both stereo- and regioselectivity can be overcome by the effects of both 2-0-pivaloyl and 3-0-benzyl groups are still unknown, especially highly regioselective control by the 2-0-pivaloyl group on ring-opening polymerization of the orthoester derivatives, although the neighboring participation of the 2-0-pivaloyl group giving P-glycoside is well known. Furthermore, additional problems which cannot be understood are that the present ring-opening polymerization method developed for the synthesis of cellulose cannot be applied for the synthesis of (1 +4)-P-xylopyranan. The ring-opening polymerization of 3-0-benzyl-xylopyranose 1,2,4-orthoester always gives a polymer consisting of both (1+2) and (1+4)-P-xylopyranose units [58]. We expect that research and the solution of the aboveproblems in synthesis may lead to the logical molecular design for the synthesis of other natural polysaccharides. We have succeeded in the syntheses of stereoregular galactofuranan and arabinofuranan [59]. On the other hand, one of the future aims in cellulose application is development of functional derivatives with special functions. For this, preparation of highly regioselectively substituted cellulose derivatives is a key. At present, 6-0-substituted cellulose derivatives can be synthesized, but the cellulose derivatives substituted at only the 2-0- and 3-0-positions cannot be obtained. Cellulose derivatives (44) in Fig. 8 and poly(47) and poly(51) may be theoretically used as starting materials for the preparation of regioselec-

Nakatsubo

652

tively 2-0- and 3-0-substituted cellulose, because the 2-0- and 3-0-positions of these derivatives are protected with ester and ether groups, respectively, which can be deprotected independently. At present, we have succeeded in the synthesis of all kinds of pos1591. sibleregioselectivelymethylatedcellulosederivativesfrompoly(47)andpoly(51) The studies on the relationship between structure and function of these regioselectively substituted cellulose derivatives may enable us to design new functional cellulose derivatives in the future. Thus, the present chemical synthetic methods for cellulose are expected to apply to the solution of basic problems of cellulose chemistry and technology.

REFERENCES I.

2.

3. 4. S.

6.

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13. 14.

IS.

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16 wood Plasticization Nobuo Shiraishi Kyoto University, Kyoto,

1.

lapan

INTRODUCTION

Methods for processing wood are very limited. While metals, plastics, and glass can be processed in the liquid phase at high temperatures, natural wood cannot. This difference is due to the lack of plasticity of wood, so that it cannot be melted, dissolved, or softened sufficiently for molding. Consequently, the scope for utilizing wood is restricted, which sometimes makes wood less valuable as a material. If plastic properties could be imparted to wood, it would become a more useful material. This need, and the desirability of using wood waste and renewable forest product resources better, have resulted in extensive new studies on wood chemicals, modified natural polymers, new pulping methods, and reconstituted wood products. Inherent in this work is the plasticization of wood by simple chemical processing [ 1-71.

II. THERMOPLASTICITYOF WOOD The name “plastics” is given to the numerous macromolecular organic materials that can be softened, melted, and molded by heat and pressure or by mechanical means, such as mixing and calendering. Wood, which is also composed of macromolecular organic materials, differs from the plastics in such basic properties as plasticity, thermoplasticity, and dissolubility in organic or aqueous solvents. In other words, wood lacks plasticity, which means that it cannot be softened sufficiently for molding, melted, or dissolved. A summary of the previous literature on thermoplasticities of wood reveals that only the phenomena up to the second-order transition and/or thermal softening have been investigated, but not the phenomena of thermal flow [&lo]. Wood is composed of 50-55% cellulose, 15-25% hemicellulose, and 20-30% lignin, with small quantities of ash and extractives. The main components make up an interwoven network in the cell walls and middle lamella. The minor components are mostly in cell lumens or special tissues such as resin canals, and are directly or indirectly related to the physiology of trees. Thus, the components that are directly related to the fundamental properties of wood, such as thermoplasticity, are considered to be the main ones, and such properties are 655

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formed not as the simple summation of properties of individual components but as the integration of these properties as a result of mutual interactions by these components. This has been observed in previous studies on thermoplasticity of wood. Lignin and hemicellulose are amorphous polymers and are much more thermoplastic than cellulose, which is a highly crystalline polymer. Goring's data [S1 concerning the thermoplasticity of wood components are typical examples of measurements of this kind. They show that, in a dry state, while lignin and hemicellulose have thermal softening temperatures around 127-235°C and 167-2 17"C, respectively, for cellulose the thermal softening temperature is around 231 -253°C. The variation in the value of the softening temperature found in each wood component is caused chiefly by the difference in the method of isolating the components from wood. On the other hand, the thermoplasticity of lignin, hen~icellulose, and cellulose can be increased by wetting the samples. When the water content is increased to 20%, lignin and hemicellulose show softening points around 72- 128°C and 54- 142"C, respectively, much lower than for the dry state. By contrast, cellulose shows a decrease of only about 6-9°C in a wet state. These differences indicate how water can act a s a plasticizer and how the softening temperature depends on the two distinct states, crystalline and amorphous, of the components. On the other hand, an examination of thermal softening of wood as a whole reveals that no thermal softening can be identified similar to that of individual components. Wood does not show thermal softening until it is heated to a much higher temperature.This suggests that interaction among the main wood components plays an important role. Goring [8] found an insignificant softening at temperatures over 200"C, and Chow [9] reported that thermal softening of wood begins at 180°C and reaches the maximum rate of softening at 380°C. Back et al. proved that the second-order transition point of hardboard made from wood fiber is around 230-350°C. Baldwin and Goring [ 101 found that steamed wood has a lower softening point, around 200°C. This may be because steaming reduces the interactions among the main components of wood, causing amorphous portions of the main components to soften independently of the cellulose. This implies that the thermoplasticity of wood is governed by cellulose. The thermal behavior of lignin and hemicellulose is restricted by interactions due to secondary intermolecular bonding with cellulose. The difference in the effect of sorbed water on thermal softening of each of the main components implies that the crystallinity of cellulose contributes greatly to the degree of thermoplasticity of wood.

111.

REASONS FOR LOW THERMOPLASTICITYOF WOOD

As described above, although wood shows thermal softening, it occurs only at temperature over 200°C. Thermal fluidity is not observed for wood. Thus, wood is known as a nonplastic material. The fact that wood is insensitive to heat and shows no thermal fluidity and plasticity can be attributed to the following reasons:

1. 2. 3.

Cellulose is a crystalline polymer with about 50-70% crystallinity. Lignin has a three-dimensional molecular structure with very high molecular weight. Chemical bonds are formed even between main components of wood, such as in lignin-carbohydrate complexes (LCCs).

Cellulose is a linear high polymer having glucose residue as a repeating unit. Glucose has three hydroxyl groups, which means that cellulose has the ability to form significant

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hydrogen bonding. The resulting high intermolecularforces, in addition to the regular structure of the polymer, result in its high degree of crystallinity. The crystalline melting point of cellulose is considerably above its decomposition temperature. Thus, no melting of cellulose can occur at temperatures that do not cause pyrolysis. Therefore, cellulose is a material with low thermoplasticity. However, the above properties of cellulose can be altered by converting cellulose into derivatives, which enables it to become more plastic. For example, cellulose nitrate, cellulose acetate, benzylcellulose, and so forth, are known as cellulose plastics. I n this sense, it may be easily postulated that if cellulose in wood could be derivatized in situ, it would show thermoplasticity up to melting. The idea of converting wood as a whole into plastic material did not appear until recently. This is natural when one considers that lignin has a three-dimensional gel structure. Furthermore, lignin has been proved to exist as a stereoscopic spongelike aggregate within the cell walls of wood. Cellulose is interwoven as fibril bundles through the network of this aggregatestructure of lignin, and the spacesare filled with hemicellulose. The crystalline portions of cellulose fibrils can be regarded as rigid junctions among the linear polymer chains, and in this sense it is considered that cellulose aggregates also form a three-dimensional network. The cell wall can be considered as an interpenetrating polymer network (IPN) with partial chemical interactions between the main components of wood, such as those in LCCs. All these factors make wood a thermally insensitive material.

IV. THERMOPLASTICIZATIONOF WOOD The preceding section suggests that it maybe difficult to change wood intoplastics; however, the author has recently found that wood can be converted into a thermally flowable material by chemical modifications such asesterification,etherification, and some other dcrivatizations 11 -7,l I , 121. Chemical modification does not necessarily require special techniques, and therefore conventional and simplemethods can be used for this purpose. This phenomenon can be explained most simply in terms of the internal plasticization of wood. That is, the internal plasticization of wood through chemical modification can producea meaningful change in fundamentalpropertiesincluding thermoplasticity. The degree of change in the plastic properties of wood is dependent on the molecular size of the substituentgroupsintroduced, the degree of substitution, as well as the reaction method.

A.

Large-Substituent Modification

Accordingly, the introduction of large substituent groups into wood can result in a chemically modified wood with high thermoplasticity. Since the change in thermoplasticity is significant in this case, early studies dealt with thermoplasticization of wood reacted with large groups. Plasticization of wood was first observed after esterification with a series of higher fatty acids in a nonaqueous cellulose solvent medium (N,O,-dimethylformamidc) [ I , 121. In this case, acylation of more than one-third of the available hydroxyl groups in the wood is sufficient for the products to show thcrmofluidity. The thermofluidity was proved with a thermomechanical analyzer, a flow tester, and scanning electron microscope (SEM) observations. Figure 1 shows the SEM of heat-treated birchwood meal and that of lauroylatcd wood meal [ 121. While the untreated wood meal which was heat-treated at 270°C (Fig.

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FIGURE 1 (a) Untreated birch sawdust and (b) lauroylated sawdust with 93% ester content, both heat-treated at 270°C. Observed under scanning electron microscope (SEM).

la) shows normal wood tissues such as parenchyma and wood fibers, the corresponding lauroylated wood meal (Fig. lb) reveals clear melting with the disappearance ofwood tissue. In the latter case, about one-third of the hydroxyl groups in the wood have been lauroylated. It has also been proved that this thermal flow of the modified wood is not caused by chemical degradation during chemical modification or thermal deterioration of the sample during molding under heat and pressure [ 1l]. Films molded by heat and pressure were crushed and reexamined by a thermomechanical analyzer to show that their thermodiagrams were not different from those of the samples prior to hot-press molding. The thermal softening behavior of samples first acylated and then completely saponified were also found to be essentially the same as that of untreated wood meal. It was thus shown that the acylated wood, obtained underspecial reaction conditions by using a nonaqueous cellulose solvent as a reaction medium, showed thermofluidity. Subsequently, studies were continued to determine whether wood can be plasticized in the same manner by more general methods of higher-aliphatic-acid acylation. Preparations of cellulose esters of a series of higher aliphatic acids are generally carried out by reaction with a trifluoroacetic anhydride-fatty acid mixture (TFAAmethod) or a fatty acid chloride-pyridine system (acid chloride method). We performed the former reaction under conditions of temperature 30-50°C and reaction time 0.5-24 h; and the latter under conditions of temperature lOO"C, reaction time 2-8 h with DMF as a solvent. Both methods yield acylated woods with thermofluidity. Typicalthermograms of the acylated woodmeasuredwith the thermomechanical analyzer are shown in Fig. 2 [13,14]. The figure compares the thermoplasticity of the

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0 lauroylaled wood

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FIGURE 2 Thermomechanicalcurvesfor (a) untreated, (b) acetylated. (c) propionylated, and (d) lauroylated sawdust.

lauroylated and, as examples of lower fatty acid esters, the acetylated and propionylated wood meal, with that of untreated wood meal. Acylation in all the above cases was carried out by the TFAA method. Measurements were carried out by following the collapse of a column of powder sample under a constant load of 3 kgfkm' in a heated glass capillary tube in the temperature range 20-350°C at a programmed heating rate of l"C/min. The untreated wood meal shows no thermal softening until it is heated to a relatively high temperature (around 200°C) and seems to have a softening point around 270°C. Measurements covering carbonization temperature ranges prove that the untreated wood sample as a whole does not show any thermal flow. On the other hand, the lauroylated wood meal shows a sharp drop dueto the complete flowof the sample at 195°C. Itis alsorecognizable in the figure that acetylated and propionylated woods, both prepared by the TFAA method, can have complete flow state. At this stage, it should be clarified whether the thermal flow is due to the complete melting of all the components of the acylated wood or of only some of them, with others remaining in a solid phase and flowing together with the former. To examine this point as well as to make other observations, films were molded from the acylated wood meals under heat and pressure. An example of the results is shown in Fig. 3. Conditions for the hot-pressing in this case were 140"C, 150 kgfkm', and 2 min. Transparent films, as shown in the figure, were obtained, which did not show any solid residue when observed under an optical microscope. This suggests that the flow behavior observed in the thermomechanical analyzer is caused by the thermal flow of all the main components of the chemically modified wood. The complete melting of all the components of lauroylated wood was also confirmed by measurement with a flow tester. It was found that lauroylated wood could be easily extruded from the flow tester under a rather low pressure (10 kgfkm')

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FIGURE 3 Lauroylatedsawdust (left) and film(right)madeup perature, 140°C; time, 2 min; pressure, 150 kg/cm2.

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of it. Molding conditions: tem-

from a nozzle with a diameter of 0.5 mm, when heated to about 150-200°C. If any one of the major acylated woodcomponents could not reacha flow state, then it was impossible to extrude the whole material through the nozzle. The conclusion that the thermal flow of the chemically modified wood accompanies the flow of all of its main components should be reasonable when the IPN structure of wood components described above is considered. In addition, the temperature actually appliedin the molding mentionedabove is much lower than the flow temperature obtained with a thermomechanical analyzer for the same acylated wood shown in Fig. 2. In connection with this, it was proved experimentally that this flow temperature decreased with the increasing load applied. In general, amorphous polymers, such as higher-fatty-acid esterified wood, incontrast to the crystalline polymers, do not have a definable melting point but do have an apparent melting point. This point varies depending on the conditions of measurement, such as the load pressure or the rate of heating. If different substituent groups are introduced into wood, the apparent melting point (flow temperature) will be different. Even if the same groups are substituted, the apparent melting points of the products may be different, depending on the method and conditions of the reactions. In this regard, and also as another example of plasticization of wood by introducing large substituent groups into wood, results of benzylation are shown in Fig. 4 W]. This figure shows how the extent of benzylation affects the thermoplasticity of the etherified wood. Only sample 4, which has an ether content of 43%, gives a transparent film. Other filmsfromtheetherifiedwoodof lower substitution comparedwiththat of sample 4 contain solid granules. These results agree wellwiththe other experimental data [l61 obtained with the thermomechanical analyzer. Benzylated wood with ether content less than 40% does not show complete flow, but that with ether content within the 40-50% rangerevealsanapparentmeltingpointofabout 300°C. Further increasein the ether content results in a steady decrease in the melting temperature, which finally reaches about 200°C with an ether content of more than 60%.

FIGURE 4 Benzylated wood films. Conditions for benzylation were as follows:

Sample 1 2 3 4

40%Sodium hydroxide aq. (ml/g wood)

Benzylchloride (ml/g wood)

3.5 3.5 3.5 7.0

3.6 3.6 3.6 7.2

Reaction time" (h)

aeactlon temperature 100°C.

B. Small-SubstituentModification Larger substituents are not easily introduced using practical chemical modification methods. Therefore, as the next stage of this study, the author has been examining the plasticization of wood by introducing smaller groups within wood [ 13,14,17]. One of the most practical chemical modifications of wood is acetylation. The thermoplasticity of acetylated wood was found to vary with the acetylation method adopted. The plasticity of acetylated wood preparedby the TFAA method is high and shows a clear melting phenomenon at about 300-320°C even under a low pressure of 3 kgf/cm2, as shown in Fig. 2. However, thisis an exceptional case, as acetylated wood samples prepared

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by other conventional procedures do not show thermal flow. For example, acetylated wood samples prepared by an acetic anhydride-acetic acid in the presence of catalyst (sulfuric acid or perchloric acid) did not show thermal flow under the above pressure. Techniques for converting thermally nonmeltable acetylated wood prepared by the conventional method into plastic material have been investigated.The first method involves partial saponification subsequent to acetylation, which enhances the plasticity of cellulose acetate within wood. Figure 5 shows the thermodiagram of peracetylated wood and that of partially saponified wood. Although the peracetylated wood does not show thermofluidity, complete thermal flow can be seen for the sample after partial saponification. Considering that cellulose triacetate is a polymer that can be partially crystallized and has an apparent melting temperature as high as 300°C, and that cellulose acetate with high thermoplasticity is a diacetate with a degree of substitution of around 2.4, we can understand the effect of saponification as found above. The second method is mixed esterification with other acyl groups such as butyryl or propionyl groups. As shown in Fig. 6 , thermofluidity is conferred by this kind of mixed acetyl-propionylation. Mixed esterification is known to reduce the crystallinity and enhance the plasticity of cellulose as compared to cellulose triacetate. The third method for obtaining thermofluidity in acetylated wood isby replacing acetic acid by trifluoroacetic acid (TFA) in the pretreatment step of acetylation [ 1S]. This

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FIGURE 5 Effect of saponification on the thermoplasticity of acetylatedwood. N o aging: peracetylated wood: Aging: partially saponified acetylated wood (acetylated hy acetic anhydride-acetic acid-sulfuric acid system at 50°C for 3 h, followed hy aging at 70°C for 3 h after partial neutration and dilution).

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FIGURE6 Thermoplasticity (differential thermomechanical diagrams) of untreated wood and ;teetyl-propionylated woods prepared with different molar ratios of acetic anhydride ( A . A . ) and propionic anhydride (P.A.). Ratio of numerical values i n the figure represents the molar ratio of A.A. and P.A. used i n the esterification reaction.

method is considered as a simulation of the TFAA method. which can be used to prepare thermaily flowable acetylated wood asdescribed previously. Figure 7 shows that this method of acetylation can also give thermofluidity to acetylatedwood.Whatis more interesting is that the acetylated wood obtained shows an apparent melting temperature of about 210°C. This temperature is almost 90°C lower than the apparent melting temperature of ordinal acctylated wood or cellulose triacetate. Actually, the apparent melting temperature of cellulose triacetate is about 300°C (Fig. 8). The figure shows the differential thermonlechanical diagrams of a series of fractionated

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Ace.arhyd(O.26).TFA(L.6). 50°C

0.. 2 -

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FIGURE7 Effect of pretreatnlent time on thermal softening and flow behavior of acetylated wood. Pretreatment: TFA (4.6 lnllg wood)-acetic anhydride (0.28 mllg wood). S O T , 2-6 h; acetylation: 50°C. 3 h.

cellulose acetates. The apparent melting temperature is shown as the peak at the higher temperature. Even the cellulose acetate with very low molecular weight (0.61 X 10') flows around 300°C. I n this connection, the findings of the previous figure that the acetylated wood showed apparent melting temperature of about 210°C are quite characteristic. The compatibility and/or the mutual plasticization among the acetylated wood components can be considered to account for the shift of the flow temperature. Then, plasticization of celluloseacetate with an interaction with acetylated lignin could bc postulated. In order to confirm this. variations i n the thermofluidity of acetylated wood were investigated in relation to a stepwise extraction of acetylated lignin from the sample. The results are shown in Fig. 9. The curve for the acetylated wood before methanol extractionshowsa flow at almost 200°C. Thecurvefor the sampledelignitied by the peracetic acid method behaves like cellulose triacetate. The thern~omechanical diagrams for the acetylated wood partially delignitied by methanol extraction appear between the above-mentioned two curves. The methanol extracts were characterized by IR and NMR, and were found to be acetylated lignin contaminated with a small amount of acetylated hemicellulose.

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Cellulose triacetate

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o : Not ext. withMeOH 0

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: 24 h MeOH ext. : 48 h MeOH ext.

0 . 8 t o : 7 2 h MeOHext.

t

m : 120 h MeCti ext.

x : Delignification

From these results, it can be concluded that acetylated wood that was prepared aftcr TFA pretreatment flows at a rather low temperature. and this is caused by the plasticization of cellulose acetate by acetylated lignin within the acetylated wood cell wall. The splitting of the benzylaryl ether and, less frequently, the existing ester bond in the lignin macromolecule by the action of trifluoroacetic acid could play a large role in making the acetylated wood prepared after TFA pretreatment flowable at a n unexpectedly low temperaturc. I t can also be said that the same effect makes acetylated wood prepared by the TFAA method thermally flowable. I n the latter case, however, acetylated and partially cleaved lignin was removed during the purification stage, i.e., during washing with methanol. Thus, acetylated wood prepared by the TFAA method cannot flowat around 200°C but reveals an apparent melting temperature of about 300°C as mentioned above. These findings, in addition to the above-mentioned finding that acetylated wood prepared by the traditional method does not show thertnofluidity, suggest that the lignin inherently suppresses thermal f o w i n acetylated wood if i t is not partly split.

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As a fourth method to confer thermofluidity on acetylated wood, the author examined the explosion of wood as a pretreatment for acetylation. In this case, the explosion pretreatment activates lignin so that self-condensation of the lignin occurs during acetylation in an acidic medium. This could be prevented by the addition of a nucleophilic reagent, such as P-naphthyl methyl ether, during the acetylation. Figure 10 shows that this method is also successful in offering thermally flowable acetylated wood. The fifth method of offering thermofluidity to acetylated wood is the additional use of external plasticization. By mixing appropriate plasticizers, synthetic polymers, or both, acetylated wood can be converted to thermally flowable materials. Figure 1 1 shows how effective the use of some external plasticizers is. That is, even though the acetylated wood pulp prepared by the perchloric acid catalyst method cannot be molded into a film, it can be brought to moldability by blending with an equal weight of PMMA or, more effectively, with PMMA and dimethylphthalate in a 5 : 3 : 2 weight ratio. As otherexamples in this fifth investigation, the casesfor allylation as well as carboxymethylation of wood [ 171 are described here.

0

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FIGURE 10 Thermofluidity (thermoplasticity) of the acetylated woodpreparedafter explosion pretreatment at 179°C for I O min. Acetylation was conducted at 50°C for 3 or 6 h in the presence of ;I small amount of P-naphthyl ethyl ether (0.1 m o l per mole of phenyl propane unit estimated to exist in exploded wood n x A ) .

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FIGURE 11 Appearance of moldings from acetylated wood (wood pulp), acetylated wood-polymethyl methacrylate (PMMA) [ 1:1 (wt)], and acetylated wood-PMMA-dimethyl phthalate (DMP) [5:3:2 (wt)] blends.

In the upper part of Fig. 12, untreated wood meals are shown on the left, allylated wood meals in the middle, and carboxymethylated wood meals on the right. In the lower part thecorresponding molded materials made by hot-pressing are shown in the same order. It can be seen that none of these could be molded into a transparent film. Examinations were carried out to see if these allylated or carboxymethylated wood meals could be converted into thermally flowable material by mixing with plasticizers, synthetic polymers, or both. Figure 13 shows the results for allylated wood. The molded allylated wood (left), a film molded after mixing 25% with dimethylphthalate (DMP) (middle), and a film molded after mixing 100% with polymethylmethacrylate (PMMA) and 25% with DMP (right) are shown. The spots found in the film mixed only with DMP are bubbles formed during hotpressing. The film on the right is transparent, showing the complete thermofluidity of the sample. These results also indicate that conventional allylation does not cause sufficient internal plasticization and hence cannot confer thermofluidity on wood. The addition of adequate external plasticizer can confer thermofluidity on the allylated wood. Similar results were obtained for carboxymethylated wood (CM wood) (Fig. 14). CM wood powder is shown on the left, and after molding in the middle. A transparent film could not be obtained in this case. CM wood was converted to a thermally flowable material, however, after mixing with resorcinol (right). In other experiments it was found that hydroxyethylated wood meal can be molded into a transparent film after mixing and kneading with a small quantity of water. In all of these examples, it should be pointed out that the appearance of chemically modified wood meal is not very altered from that of untreated wood meal, as can be seen in Figs. 12-14.

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FIGURE 12 Appearance of etherified wood meal and moldings. Upper: untreated wood (left), allylated wood (center), and carboxymethylated wood (right). Lower: moldings from untreated wood (left), allylated wood (center), and carboxymethylated wood (right).

I

i

FIGURE 13 Plasticization of allylated wood by blending with DMP and PMMA-DMP. Appearance of moldings of allylated wood (left), allylated wood-DMP blend (center), and allylated woodPMMA-DMP blend.

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FIGURE 14 Plasticization of carboxymethylated wood by blending with resorcinol. Appearance of carboxymethylated wood meal (left), and moldings of carboxymethylated wood (center) and its blend with resorcinol (right).

From the above observations andfindings, it can be said that, in principle, regardless of the molecular sizeof the substituent group introduced, wood can be given thermoplastic properties by internal plasticization, that is, chemical modification, supplemented, if necessary, by external plasticization [17]. The sixth method for conferring thermofluidity on acetylated wood is thatof grafting vinyl monomerssubsequenttothe acetylation [13,14]. Inthiscasetheresultant vinyl polymer acts as a plasticizer within wood. Figure 15 shows that acetylated wood prepared by the perchloric acid catalyst method does not reveal thermal flow, whereas subsequent grafting with styrene changes the acetylated wood to a thermally flowable material. This grafting method is similar in principal to the method supplemented by external plasticization with mixing with synthetic polymer. In that case it was not easy to find appropriatesyntheticpolymers as suitableplasticizersforacetylated wood. A suitable polymer-plasticizer should be one that is compatible with the polymer component of the acetylated wood at the molecular level. Compatibility between polymers, however, rarely occurs. To overcomethis difficulty, graftingtechniquescanbe used. Compatibility of acetylated wood components and synthetic polymers is attainable by the grafting technique. Even with a combination of acetylated wood and synthetic polymers, with which a homogeneous transparent film cannot be effected merely by blending, the use of grafting techniques can result in a transparent molded film. The method of grafting is also important. It is desirable to select a method by which grafting or polymerization of monomer compounds occurs homogeneously within acetylated wood to produce products with high compatibility. A representative experimental result (Fig. 16) shows how the grafting technique is superior to blending in yielding high compatibility among the components of

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O~

a

0.5

1 .o

FIGURE 15 Thermomechanical behavior of acetylated wood ( 0 ) and acetylated wood-polystyrene composite prepared by the y-ray-induced graft copolymerization in a pyridine medium (e). Conditions: total dose 2 Mrad; resultant weight increase 76.7%.

acetylated wood and synthetic polymers. The figure illustrates the temperature dependence of loss tangent obtained by a viscoelastic measurement for composites prepared by blending acetylated wood [acetylated radiata pine refiner ground pulp (RGP)] with PMMA with a kneading technique or by grafting MMA onto the acetylated wood. This figure shows that grafting results in forced compatibility of acetylated wood with PMMA. Although the blended material of acetylated wood and PMMA shows two T, peaks ascribable to the two constituting components, the corresponding grafted material reveals only one intermediate T, peak. This shows that graft products of PMMA onto acetylated wood com-

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ponents can be partially produced which can act as colnpatibilizers and which enable the total mixing and interaction of the acetylated wood and PMMA to be enhanced. On the other hand, some investigations have been advancing from the point of view of not only conferring but also enhancing the thermofluidity of chemically modified wood. Among them, the method of using TFA as a reagent for the pretreatment, to split selectively the parts of bonds combining lignin units [IS], a s well as methods chlorinating lignin's aromatic ring parts of chemically modified wood [ 19,201, are very effective and specific. The former method has already been explained i n detail as the third method of offering thermoplasticity to acetylated wood. It should be noted here that since a significant shift of the apparent melting temperature toward the lower temperature can be effected as mentioned previously, the former method is also useful as a method for enhancing the thermofluidity of modified wood. The latter method is concerned with a postchlorination of the chemically modified wood. Morita, Sakata, and co-workers have been investigating the effect of postchlorination of cyanoethylated wood on the thermofluidity of modified wood [ 19,201. The chlorination was conducted by soaking the modified wood in a delute aqueous chlorinesolution (0.1-0.2%), at a low temperature, for a short time (O'C, 5 min) under stirring, followed by washing with water and drying. Under these mild conditions of chlorination, chlorine is known to react selectively with the aromatic ring of lignin. The chlorination of lignin can be considered to cause ( I ) chlorine substitution reactions at the S - and 6-positions of lignin aromatic rings; (2) chlorination substitutions at the I-position of the aromatic ring,

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eliminating the propane side chain; (3) hydrolysis of ether bonds (demethylation of methoxy groups and hydrolysis of phenyl ether bonds); and (4) oxidation of the aromatic rings. Among these, the former two reactions are primary reactions, while the latter two are side reactions. As representative results of these studies, it has been shown that the flow temperature ofthe cyanothylated wood can be shifted downward by 100- 120°C (Fig. 17). At this point, decrystallization of the samplcs is known to proceed without degradation of the cyanoethyl cellulose fraction [19,20]. The lowering oftheflow temperature of cyanoethylated woodis thought to be attributable to a loosening of the lignin structure by chlorine substitution, action ofthe chlorinated lignin as an external plasticizer for cyanoethyl cellulosc within the modified wood (mutual interaction among modifiedwood components), a s well as decrystallization produced by the chlorination [X)].Lowering the flow temperature is considered not to be caused by the degradation or depolymerization of the components of the chemically modified wood. This is concluded from the observations that moderate chlorination, as described above, results in a tremendous drop in the flow temperature of the chemically modified wood, as well as the fact that the viscosity change of the aqueous solution of cyanoethyl cellulose before and after the chlorine treatment is very low (7%) 1201. Similar phenomena have been found with other chemically modified woods. For example, chlorination causes the lowering of the flow temperature of benzylated wood, and results in thermally flowable hydroxyethylated wood that does not show thermofluidity before the chlorination.

V.

APPLICATION OF THERMOPLASTICIZATIONOF WOOD

It has become possible to confer thermofluidity on wood, as described in the previous section, and based on these findings, several attempts have been made to apply the result to useful products. Examples are the preparation of films, sheets, and other moldings from chemically modified wood; preparation of three-dimensionally cured plasticlike woodboard; preparation of deep-drawable hardboard; application to hot-melt adhesives; surfacelayer plasticization of wood intended to develop surface densitification and emboss processings; and preparation of modified plywood, LVL (laminated veneer lumber), particle board with densified surface, flame resistance, decay resistance, and insect resistance. The preparation of films, sheets, and other moldings from chemically modified woods or their blends with other materials, mentioned above as the first example, has been attempted, and the mechanical as well as the viscoelastic properties of the parts of the moldings have been studied. The mechanical and other properties of the modified wood film depend on the type of substituent groups, the extent of substitution, and the method of modification. Figure 18 [21] shows a stress-strain curve for a film from caprylated wood. Curve a is a stressstrain curve for Hinoki cypress stretched in the fiber direction, and curve b is for the same wood stretched perpendicular to the fiber axis, both of which are shown as reference. Curve c is caprylated wood. In the case of the untreated wood specimens (a and b), the elongation at break is small ( 1 -2%). On the other hand, the elongation at break found for the caprylated wood is very large (as high as 100%). In this case, a yield point is recognizable, and the breaking strength is almost equal to that of Hinoki cypress perpendicular to the fiber direction.Thus, caprylated wood film can be said to exhibit elastomerlike mechanical properties.

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.-

E

d

l00 200 Temperature ( "C l FIGURE 1 7 Thermal softening and flow curves for CE-wood treated with chlorine.

The temperature dependences of dynamic modulus E' and loss tangent tan S for the caprylated wood film are shown in Fig. 19 [22]. For comparison, the same relation was obtained for an untreated Beihi cypress specimen (fiber direction) and is shown as a dashed line in the figure. The dynamic modulus E' for the untreated wood shows a small decrease within the sameorder of magnitude when the temperature increases from - 180°C to

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60

80

E: (%) FIGURE 1 8 Stress (n)-strain ( E ) curve for caprylated wood film (c) and wood specimens (Hinoki) with two different edge cuttings (a and b).

Wood Plasticization

675 E'

-----------------------------""~~""""""""""""~""" """

-1

1010-

rt I

N

S

2 C z

-0.1

109-

E

"-"

iJl

108

"_"

- 0.01

107, -200

-100

0

100

200

T (C)

FIGURE 19 Temperaturedependenceof

E' and tan 6 forcaprylated

wood film and wood

specimen.

250°C. Although the dynamic modulus E' for caprylated wood film at - 170°C is very close to that for the untreated wood, it decreases with temperature and, especially, is dramatically lowered between -90 and -40°C and between 30 to 100°C. As a result, the E' value becomes three orders of magnitude lower than that of the untreated wood at 140°C. With respect to tan S , three relaxation processes are detectable within the experimental conditions. They are labeled as a, p, and y process in order of decreasing temperature. These processes were identified on the basis of the information obtained from a series of cellulose acetates. The a process is assigned to be due to a micro-Brownian motion of the main chain of the caprylated wood (the process corresponding to the glassrubber transition); the p process to the motion of the capryl side chain introduced, and the y process to the motion of a part of the side chain (the motion initiated by a minimum of three methylene groups in addition to the oxycarbonyl group of the side chain). Figure 19 shows that a large decrease in E' is caused by the motion initiated by the whole acyl side chain ( p process) and that the glass-rubber transition temperature of the caprylated wood is higher than room temperature. These results are interesting in relation to the results shown in Fig. 18. That is, the observation in Fig. 18 that the caprylated wood film behaves as though it is an elastomer at the measuring temperature (room temperature) can be directly explained by the effect of the micro-Brownian motion of the acyl side chain, and not by micro-Brownian motion of the main chain. As another example of the mechanical properties of a modified wood with large substituent groups, stress-strain curves for a benzylated wood film measured at various temperatures are shown in Fig. 20 [15]. The stress-strain curve varies widely with the temperature. It clearly shows a yield point when measured at 55 and 81°C while the film reveals brittleness at room temperature. The breaking strength of the film decreases with increasing temperature, and the breaking elongation increases rapidly with temperature up to around 1 17°C and then decreases beyond it.

676

Shiraishi

n

E ob)

FIGURE 20

Stress (u)-strain (c)curve for benzylated wood f i l m .

I n Fig. 2 I , the temperature dependence of dynamic modulus E' and loss tangent tan 6 for the benzylated wood film is shown [ 151. Compared with the corresponding results with a Beihi cypress specimen (fiber direction), results with the benzylated wood specimen are similar to those obtained with general amorphous synthetic polymers. Within its glassystate region, where the E' value is more than 10"' dynes/cm2, a secondary dispersion ( p process), accompanied by a very slight decrease in E', is recognized around -9O"C, and the tan 6 curve reveals a corresponding peak. This process is assigned to be due to the motion of the benzyl side chain introduced. The effect of the motion of the benzyl side chain on the decrease i n the E' value is considerably smaller compared to that caused by the motion of the capryl side chain mentioncd above. On the other hand, a primary dispersion ( a process) appears around 118°C in this benzylated wood. The findings in Fig. 21 explain the observations shown in Fig. 20;i.e., benzylated woodfilm behaves asa brittle and glassy polymer film at room temperature, and the breaking elongation increases with the measuring temperature up to around 118°C (the glass transition temperature) and then decreases beyond it. In Table I , mechanical properties of the films from certain benzylated woods are compared with those of several common synthetic polymers [ 161. It can be seen that there are no fundamental differences between the two. The physical properties of benzylated wood can be altered or, more desirably, enhanced by blending with synthetic polymers and/or low-molecular-weight plasticizers. Based on similarity in chemical structure between the benzylated wood and polystyrene, good compatibility can be expected between them. Thus, the related blending has been studied [23]. In Fig. 22, apparent melting points (T,) for the benzylated wood, the polystyrene, their blends in several ratios. as well as a typical chlorinated benzylated wood (Cl-BzW) are shown. From this figure, it is apparent that the thermoplasticity of benzylated

677

Wood Plasticization llT 1

lO$O

71-3 -

FIGURE 21 specimen.

l

I

l

100

0

100

Temperaturedependence

200

of E' and tan S for benzylatcd wood film and wood

TABLE 1 Comparison of Mechanical Properties of Benzylated Wood Films (C-l to C-7) with Those of Common Synthetic Polymers Sample plastics

c-1 C-3

c-4 c-7 Vinyl chloride resin (hard) Polystyrene A S resin Methacrylic resin Polypropylene Polypropylene (glass reinforcement) Cellulose acetate Ethylcellulose Polyurethane

Tensile strength (kg/cm')

Elongation (%)

Young's modulus (kg/cm')

300 24 1 320 337 420-530 350-530 630-840 490-770 300-390 420- 1020 130-630 140-560 320-590

5.5 2.5 3.2 4.7 40-80 1 .o-2.5 1 S-3.7 2.0- 10 200-700 2.0-3.6 6.0-70 5.0-40 100-650

10,057 15.051 14,477 14,140 25,000-42,000 28.000-42.000 28,000-39,000 27.000-32,000 1 I ,000- 16,000 32.000-63.000 5,000-28,000 7,000-2 1,000 700-25.000

Shiraishi

678

,

1010 614812

,

I

.

,

.

,

416 218 BzWPS (wt/wt)

.

0110

FIGURE 22 Apparent melting temperature (TJ for benzylated wood (BzW), polystyrene (PS), and their blends with various ratios. Cl-BzW should be referred to the text.

wood can be markedly enhanced. Especially, the samples obtained by blending more than 50% of polystyrene are known to have T , values almost equal to that of the polystyrene. In this connection, Morita and Sakata [l91 reported that the apparent melting temperature of the chemically modified wood can be considerably lowered by a weak chlorination aftertreatment, which gives the Cl-BzW mentioned above. Dynamic viscoelastic properties of the benzylated wood, the polystyrene, and a blend of the two in equal weight ratio were measured, and the obtained temperature dependencies of logarithmic decrement (aT)are shown in Fig. 23. In general, the comparison of this kind of viscoelastic relaxation curves is suitable for judging qualitatively the compatibility of a polymer blend. That is, in the case of the compatible polymer blend, the glass transition temperature (T,) appears as a single peak in a position proportional to the existing amounts of component polymers, whereas in the case of an incompatible polymer blend, the individual phase domains retain the glass transitions of their respective parent homopolymers. These viscoelastic relaxation data were correlated with electron microscopic observation results and the following have been pointed out. In the case of the compatible blend system, the component polymers were found to exist as domain mixtures, whose grain diameter was less than 15 nm (150 whereas in the case of the incompatible blends, they were often existing as individual phase domains whose diameters were larger than 100 nm. I n the case of the semicompatible blend, in which even though a number

W),

FIGURE23 Temperature dependencies of logarithmic decrement (a,)for benzylated wood (BzW) (H),polystyrene (PS) ( 0 ) . and their blend with BzWPS = 5/5 by weight (A).

Wood Plasticization

679

of T, peaks can be observed, the transitions are broadened and their temperatures become closer together, and the individual phase domains have grain diameters in the range 20100 nm. Consequently, the experimental results shown in Fig. 23 indicate that relatively significant molecular mixing takes place in the benzylated woodpolystyrene blend. When the transparency of the molded sheets from this benzylated woodpolystyrene blend is also taken into consideration, the two- or multicomponent polymer system is concluded to be a compatible polymer blend, and the individual phase domains have grain diameters of less than15 nm. Figure 24 shows the results of tensile tests obtained for films prepared from benzylated wood, polystyrene, and their blended mixtures. From this figure, it can be seen that the tensile properties obtained for these blending composites are not very good. That is, even though the benzylated woodpolystyrene blends are miscible, the interfacial bonding of the related individual phase domains is not enough for the corresponding molded sheets to have desirable mechanical properties. This result is not necessarily unusual, and in order to overcome this situation, compatibilizers can be used in various ways. In this case, a styrene-maleic anhydride copolymer (SMA: Arc0 Chemical, Inc., Dylark 232; maleic anhydride content 8 wt%) was tested in order to make clear whether it can work as a compatibilizer ornot when certain amounts of it are kneaded with the benzylated wood and polystyrene at a temperature as high as 160°C. The result is shown in Figs. 25 and 26, in which the amount of benzylated wood was fixed to 50 wt% and the mixing ratio of polystyrene to SMA, the total amount of which was 50 wt%, was changed. From the figures, it is seen that both the tensile strength and the breaking elongation take the maximum values when SMA is added at 5 wt% to the total weight of the blended composites. This result shows clearly that benzylated w o o d SMA graft copolymers, which have been produced by kneading [blending at high temperature (16O"C)], can act as a compatibilizer. In other words, due to the localized existence of thus-produced benzylated wood components/SMA graft copolymers at the interfaces between the benzylated wood and the polystyrene phase domains, as shown in Fig. 27, their interfacial bonding is considered to be improved, giving materials with excellent physical or mechanical properties. In Fig. 25, when the amounts of SMA added to polystyrene exceed 1096, the tensile strength of the composite films obtained shows a monotonic decrease. This is considered 15 14 13 12 11

515

1010

BzWPS (wtlwt)

FIGURE 24 Tensile properties for benzylated wood, polystyrene, andtheir blending mixtures. 0 , tensile strength (um1.,J; A, breaking elongation m, Young's modulus ( E ) .

Shiraishi

680

FIGURE 25 Polystyrene-to-SMAratiodependency of thetensilestrength of themoldedfilms prepared from their blends with benzylated wood (the benzylated wood content is fixed at SO wt%).

L

1010

, . , . , . , . 812

614 416 218 PSISMA (wtlwt)

0110

FIGURE 26 Polystyrene-to-SMA ratio dependency of the breaking elongation of the molded films prepared from their blends with benzylated wood (the benzylated wood content is fixed to 50 wt%).

SMA Chain

P

FIGURE 27 Schematic diagram showing the localized existence of benzylated wood components/ SMA graft copolymer at the interface between the benzylated wood and the polystyrene phase.

Wood Plasticization

681

to be caused not by the decline of the mechanical properties of products with increasing SMA content, but from the formation of crosslinking within the benzylated wood components phase, the amounts of which increase with the amount of SMA added. The formation of crosslinking during kneading or blending results in products with less moldable thermomechanical properties. Films can sometimes be molded directly even from modified wood with small-molecular-sized substituents. For example, acetylated wood prepared by the TFAA method and the TFA pretreatment method, as well as acetyl-butyrylated wood, can be molded into films. Table 2 is a n example of the experimental data. Even though the films can be molded from wood substituted by small groups, the films show brittleness. In order to improve the physical properties of these moldings and to enhance the thermofluidity and moldability of modified woods having small-molecular-sized substituent groups, these modified woods have also been blended and plasticized with plasticizers, synthetic polymers, or both. Graftcopolymerization with synthetic polymers has also been tried, and the resulting physical properties have been measured. Mechanical properties of films prepared after the external plasticization of allylated as well as carboxymethylated wood meals with synthetic polymers andor low-molecularweight plasticizers [ 171 are shown in Table 3. Corresponding data for each of the synthetic polymers used as the external plasticizer and for a benzylated wood film are also included for comparison. It is known that the films from the externally plasticized allylated as well as carboxymethylated wood reveal mechanical properties comparable to those of the corresponding synthetic polymers. Reinforcement because of the presence of modified wood can be recognized. It can be concluded that the physical properties of chemically modified wood alloyed with synthetic polymers with or without other plasticizers are dependent on the species of synthetic polymers and the composition of the alloys. Generally, external plasticization by graft-copolymerization results in greater enhancement of thermofluidity (moldability) as well as better physical properties of the moldings than those provided by mere mixing (blending). In the case of graft-copolymerization, the method of grafting also plays an important role. Wood-synthetic polymer composites with higher thermofluidity and better physical properties can be obtained when grafting techniques resulting in a uniform distribution of polymers within the wood cell wall are adopted (seeFigs. 16 and 28). As shown in Fig. 28, however, even though the graft-polymerization technique is adopted, two peaks assignable to the respective primary dispersion of acetylated wood and that of polymethyl methacrylate (PMMA) can be found in the tan S-versus-temperature curve of a graft product prepared by use ofan azobisisobutyronitrile initiator (AIBN catalyst

TABLE 2 Mechanical Properties of Films from Acetyl-Butyrylated Woods Prepared by TFAA Method Sample (acety1:butyryl)

0: 10 1 :9 3:7 1:l

Tensilc strength (kghn’)

Elongation

263 294 313 414

4.9 6.0 12.8 12.5

(%l

Young’s modulus (kgkm’) 1 8,600

8.840 7,720 10,700

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682

TABLE 3 Mechanical Properties of Films from Blends of Allylated Wood (AW) or Carboxymethylated Wood (CMW) with Synthetic Polymers and/or Plasticizers. Data on Synthetic Polymers and Benzylated Wood Are Also Shown as References

Sample AW:PE = 1:2, blend PE AW:PP = 1.2, blend PP AW:PMMA:DMP = 4:4: 1, blend PMMA

CMWResorsinol = 1: I , blend Benzylated wood

X

Tensile strength IO-" (dynedcm')

Elongation

9.22

14.6

(%)

0.41-3.82

20- 1000

15.9

3.8

2.94-3.82

200-700

13.1

4.1

4.80-6.23

3-10

8.73 2.77-4.06

28.2 1.6 1-9.49

Young's modulus (dynes/cm')

X 10""

2.43 0.096-1.3 4.35 1.10-1.55

3.28 -3.142.56 1.62-4.17

method). This result is quite different from that of Fig. 16, where a curve for a grafted product prepared by a vinyl polymerization without initiator in the presence of water is included. In this case, only one primary dispersion peak appears, suggesting a high compatibility at the molecular level among the components of the acetylated wood and PMMA. As already mentioned, when individual phase domains composing the blended composite have grain diameters less than 15 nm, the composite can exhibit one principal glass transition. Itis also known that the tensile strength and the elongation at the break of the molded films become larger when higher interfacial bondings among the individual phase domains are developed. Further addition of adequate plasticizers, such as dimethyl phthalate (DMP), di-2-ethylhexyl phthalate (DOP), tricresyl phosphate (TCP), etc., in suitable quantities, to chemically modified wood-synthetic polymer composites can substantially enhance the thermofluidity of the polymer alloys. More homogeneous films are thus obtainable by hot-pressing. From the standpoint of preparing molding materials with excellent properties from chemically modified wood, it is possible to make use of chemicals that can act as plasticizers for the modified wood as well as that can react with the components of the modified wood during kneading and hot-pressing. One example of this is the preparation of films from blends of acetylated wood-resorcinol paste with various synthetic polymers (polyvinyl acetate, ethyl acrylate, acrylonitril-butadiene rubber, etc.), to which a definite amount of formalin was added. The mixture was reacted under heat and kneading, and then molded into films by hot-pressing. Three-dimensionalcuringoccursduring molding. Physical properties are shown in Fig. 29. In this case, formalin was added in a quantity of 0.5 mol of formaldehyde for each mole of resorcinol. Although the mechanical properties of the films are largely dependent on the kind of synthetic polymers added, tensile strengths up to 770 kgf/cm2 could be obtained even within the small number of experimental trials. Another example of the use of chemicals as plasticizers as well as reacting agents for chemically modified wood is that of preparing three-dimensionally cured plasticlike

683

Wood Plasticization

PMMA+AcP

X.method N.method

F.method

c t!

A.method

Blend ( DMP)

Blend

-50

0

50

100 T ("C1

150

200

FIGURE 28 Temperature dependence of ay(= tan 6) for various PMMA-acetylated wood graft products prepared by different procedures and that for PMMA-acetylated wood blends. X.method, xanthate method; N.method, noncatalyzed grafting method; Emethod, redox method with Fenton's reagent; A.method, catalyzed method with AIBN; Blend (DMP), blending method using kneader in the presence of DMP; Blend, blending method using kneader.

wooden board, developed by Matsuda and Ueda [24]. The special feature of this method is the combined use of carboxyl group-bearing esterified wood, such as maleoylate wood (maleic acid half-esterified wood), phthaloylated wood (half-ester), and bisphenol A diglycidyl ether. In this case, a crosslinking reaction accompanied by plasticization occurs during the hot-press molding stage, resulting in new types of cured wood. Various molded boards having plasticlike appearance as shown in Fig. 30 were obtained. The color of the board depends on the species of the esterified wood. Most common are red-brown, yellowbrown, or black-brown colored boards. When the wood content of the boards falls to 6070%, materials with high water resistance can be obtained. The physical properties, such as strength, elongation, etc., of the boards are superior to those of conventional boards (fiberboard, particle board, etc.). Cured board from meleoylated wood is superior in blending strength, while boards from phthaloylated wood have excellent compressive strength

684

Shiraishi

l

aoo

Ac -Res - PVAc

7: 3 1 3 (0.5rnol HCHO)

56oo

U

\

0

x

m

m

E

I

v1

k-Res-NBRI

7:3:3

(0.5mol HCHO 1 L

0

20

40 Strain(%1

60

FIGURE 29 Stress-strain curves for the acetylated wood-resorcinol-formaldehyde-synthetic polymer blended and cured films.

(2026 kgfkm'), hardness, thermomoldability, and water resistance. Esterification of wood with melaic anhydride, or phthalic anhydride, is very simple and can be accomplished by heating without the use of solvents 1251. Maleic anhydride and phthalic anhydride are cheaper than acetic anhydride, and the use of a catalyst is not usually necessary. When a catalyst is required, sodium carbonate can be used with satisfactory results. An example of a three-dimensionally moldable fiberboard-in other words-deepdrawable fiberboard, is shown in Fig. 3 1 [26,27]. In this case, wood fibers were chemically modified to levels that made the product thermoplastic but not thernlofluid. As shown in the figure, when wood fibers are chemically modified to an appropriate extent by acetylation o r other lower-aliphatic-acid esterification, fiberboards that are highly moldable under hot-pressing can be obtained. Application of chemically modified wood, especially blends with synthetic polymers, to the film-type hot-melt adhesives has been studied. This application makes use of the thennofluidity of chemically modified wood, which can be enhanced by blending with appropriate synthetic polymers. An example of the results of this study is shown in Fig. 32. Butyrylated wood was blended with polyvinyl acetate (PVAc) in various blending ratios, and adhesiveness as hot-melt adhesive was estimated based on Japanese Industrial Standard (JIS) K 6852. The figure shows that the adhesive strength of the hot-melt adhesive can be enhanced by blending with the synthetic polymer (PVAc). These blended adhesives give stronger adhesion than PVAc film. The maximum compressive shear ad-

Wood Plasticization

685

i

FIGURE 30 Three-dimensionally cured wooden boards prepared by curing phthaloylated woodis 20 X bisphenol A diglycidyl ether blends. The maximum dimension of the board in the figure 20 X 0.7 cm.

FIGURE 31 Hot-pressdrawability of hardboard withadensity of approximately 1.0: ordinary hardboard (left) and hardboard prepared from acetylpropionylated wood pulp (right).

686

Shiraishi e e

e

e m

e

m

150

e

m

e

e

~~

0

0.3

0.5

0.7

1.o PVAc

Butylated Wood Composition

FIGURE 32 Relationbetweenblendingratio

for butylatedwood-PVAcsystemsandadhesion

strength.

hesive strength found in Fig. 32 is more than 150 kgf/cm2, which is 1.5 times higher than the JIS specification demands.

VI.

LIQUEFACTION AND DISSOLUTION OF CHEMICALLY MODIFIED WOOD

Chemically modified wood has been found to liquefy and/or dissolve in various neutral aqueous solvents, organic solvents, or organic solutions, depending on the characteristics of the modified wood [2-7,28-341. So far, three methods have been found for this purpose. The first trial of the liquefaction and/or dissolution of chemically modified wood was accomplished by treating chemically modified wood in the presence of organic or aqueous solvents, or solutions at various temperatures in the range 80-290°C [34]. One example used wood samples esterified with aseries of aliphatic acids which could be liquefied in benzyl ether, styrene oxide, phenol, resorcinol, benzaldehyde, aqueous phenol,

Wood Plasticization

687

chloroform-dioxane mixture, benzene-acetone mixture, etc., after treating at 200-270°C for 20-150 min. Examples of the results of the liquefaction are shown in Fig. 33. Benzyl ether solutions of a series of acylated woodsare shown. A series of aliphatic acid-acylated wood, with almost complete substitution, prepared by the TFAA method, were liquefied in benzyl ether by heating at 250°C for 20 min. All the samples except the acetylated wood were shown to be homogeneously liquefied. Even the acetylated wood was found to be completely liquefied in the solvent if it was treated at 270°C for 20 min. Carboxylated wood, allylated wood, and hydroxyethylated woodhave been found to dissolve or liquefy in phenol, resorcinol, or their aqueous solutions, formalin, etc., after allowing them to stand or with stimng at 170°C for 30-60 min [17]. Benzylated wood could be dissolved in DMSO when heated at 80°C. Another method for liquefaction makes use of solvolysis during the process [33,35]. By using conditions which allow phenolysis of part of the lignin, especially in the presence of an appropriate catalyst, the liquefaction ofchemically modified wood into phenols could be accomplished under milder conditions (80°C for 30-150 min). Allylated wood, methylated wood, ethylated wood, hydroxyethylated wood, acetylated wood, and others have been found to liquefy in polyhydric alcohols, such as 1,6-hexanediol, l,Cbutanediol, 1,2ethanediol, 1,2,3-propane triol (glycerol), and bisphenol A, using liquefaction conditions as just described above. Each of these materials caused partial alcoholysis of lignin macromolecules [22]. The liquefaction process can produce a pastelike solution with a high concentration of wood solute (70%). The third method of liquefaction or dissolution involves postchlorination [%l. When chemically modifiedwoods are chlorinated, their solubility in solvents is tremendously enhanced. For example, at room temperature, cyanoethylated wood can dissolve in o-cresol

FIGURE 33 Dissolution of fatty acid-acylated wood prepared by TFFA process in benzyl ether (C = number of carbon atoms in the acyl group). Dissolution conditions: 250°C and 20-40 min.

Shiraishi

688

by only 9.25%. However, once chlorinated, it can dissolve almost completely in the same solvent at room temperature.

VII.

APPLICATION OF THE LIQUEFACTION AND/OR DISSOLUTION OF CHEMICALLY MODIFIED WOOD

There are many potential applications for the liquefaction and/or dissolution of chemically modified wood.Examples include the fractionation of modified wood components [34,36,37], the preparation of solvent-sensitive and/or reaction-sensitive wood-based adhesives 123 1,35331, the preparation of resinified wood-based moldings such as the foam type 131 1, and the preparation of wood-based fibers and their conversion to carbon fibers (391. To fractionate modified wood components, the dissolution-precipitation technique has been successfully used [34,36.37]. Attempts to prepare wood-based adhesive as well a s their curing based on the reaction of modified wood and the reactive solvents are reported in the literature. In these cases, phenols, bisphenols, and polyhydric alcohols have been used as reactive solvents [31,35,38,40,41 I. Combined use of these reactive solvents with reactive agents, crosslinking agents, and/or hardeners has given rise to phenol-formaldehyde resins (such as resol resin), polyurethane resins, epoxy resins, etc., all of which contain meaningful amounts of chemically modified wood. The chemically modified woods are not designed merely to dissolve and disperse in the final resins, but to react chemically and bond to thc resins. This can be achieved by liquefaction of the chemically modified wood into the reactive solvent using solvolysis techniques. In the case of epoxy resins. it can also be achieved by reacting various alcoholic hydroxyl groups remaining in the modified woods with epichlorohydrin, resulting i n introduction of glycidyl groups. Crosslinking within and between degraded wood components, especially between degraded polysaccharide components during the last stage of resinification, by reaction with crosslinking agcnts, can also be used. In order to prepare wood-based resins with meaningful amounts of the wood component, it is very important to liquefy or dissolve the chemically modified wood into reactive solvents in high concentrations (more than 50% is preferable). When hydrophilic chemically modified woods, such as carboxymethylated woods, hydroxyethylated woods, or ethylated woods are used in wood-based adhesives, aqueous resol resin adhesives that maintain their solution state during the preparation-that is, from the period of the completion of the liquefaction or dissolution in phenol to the final stage of prepolymer formation-are obtainable[35,38,40,41 I, Whcn a phenol solution with a concentration more than 50% is obtained, the chemically modified wood powder cannot bc completely immersed in phenol but can be only partly penetrated by the phenol during the first stage of dissolution (see Fig. 34). However, when the heterogeneous mixture is allowed to stand for about 30 min at 80°C (without stirring in the presence of appropriate amounts of hydrochloric acid as catalyst), a homogeneous paste can be obtained. Subsequent stirring of the paste for about I - 1 .S h enhances the liquefaction. In this liquefaction process, a certain degree of phenolysis ofwood components. especially that of lignin, takes place, which makes it easy to liquefy and dissolve them in phenol. After neutralizing the paste with aqueous sodium hydroxide, il definite amount of formalin and sodium hydroxide are added and resinified in accordance with the conventional procedure for preparing the resol resin adhesives. The appearance of thc resin obtained (Fig. 3 5 ) is similar to that of the corresponding commercial phenol resin adhesive.

Wood Plasticization

689

FIGURE 34 Early stage of liquefaction of hydroxyethylated wood meal into phenol. Photograph was taken after 10 min of liquefaction at 80°C.

The wood-based resol resin adhesives have superior gluability and workability. The adhesives can be used with fillers, thickeners, and fortifiers such as wheat flour, coconut shell, walnut flour, and polymeric MD1 (4,4’-diphenyl methane diisocyanate). It has been found that by using mildconditions for the first liquefaction and dissolution process, which does not completely dissolve all of the wood meal, the addition of fillers and thickeners into the adhesives become unnecessary. On the other hand, the addition of appropriate fortifiers, especially crosslinking agents such as polymeric MDI, into the wood-based adhesives enhances their dry-bond and waterproof gluabilities remarkably. In the cases of utilizing hydrophobic chemically modifiedwoodssuch as acetylated wood, butyrylated wood, etc., the liquefaction and dissolutionof these chemically modified woods into phenol is possible, but the products tend to become solids when they are subsequently resinified. In these cases, resinification should bedone within the kneader so that the products become solid powders (Fig. 36, center). The products can be dissolved in suitable solvents such as ethyl acetate, resulting in solutions as shown on the right side of Fig. 36, and can be used as reaction-sensitive liquid adhesives. For the preparation of wood-based polyurethane as well as epoxy resin adhesives, the above-mentioned hydrophilic chemically modified woods, prepared by conventional methods, are liquefied and dissolved in polyhydric alcohols or bisphenol A in a manner similar to the dissolution in phenol [31]. Concentration of the modified wood is usually more than 50%. Diluents such as ethanol or methanol are also often added to the dissolution system according to the requirements. After the liquefaction, the pastes are neutralized and the diluents are distilled off. When the pastes are used in combination with suitable polyisocyanatecompounds,theybecomewood-based polyurethane adhesives. When the pastes are further reacted with epichlorohydrin, glycidyl etherified resins are formed. These can be used with hardeners such as amines and acid anhydrides, and they

690

Shiraishi

FIGURE 35 Appearance of hydroxyethylated wood-phenol resin adhesive prepared.

become wood-based epoxy resin adhesives. Figure 37 shows epoxy resin adhesives prepared under various conditions after liquefying allylated wood into the same weight of bisphenolA. These are comparedwith a commercial epoxyresinadhesive. Figure 38 shows the fluidity of one of the epoxy resin adhesives prepared from allylated wood. Generally, the wood-based epoxy resins tend to become very viscose or solid, depending on the conditions of preparation, and require dilution or dissolution with solvents such as ethyl acetate, acetone, etc. These resins make satisfactory adhesives, which can be used in waterproof glues. Molding materials such as foams or shaped moldings can be obtained from chemically modified wood solutions of polyhydricalcohols, phenols, and bisphenolsas described above [31]. An example is shown in Fig.39. These can be prepared by adding anadequate amount of water as a foaming agent and a polyisocyanate compound (polymeric MDI) as a hardener to the 1,6-hexanediol solution of allylated wood, mixing well, and heating. When heated at 100°C, foaming and resinification of the resins are initiated within 2 min and are completed within several minutes. If promotors such as triethylamine are added, rapid reactions occur even at room temperature and foams can be obtained within several minutes. The foam shown in Fig. 39 has a very low apparent density (0.04 g/cm’). It also

Wood Plasticlzatlon

691

FIGURE 36 Preparation of acetylatedwood-phenolformaldehyde adhesive. Acetylated wood meal (left); phenol-resinified acetylated wood (center); ethyl acetate solution of phenol-resinified acetylated wood (right).

FIGURE 37 Appearance of epoxy resins prepared by liquefaction and dissolution of allylated wood into bisphenol A followed by glycidyl etherification, as well as a commercial epoxy resin (Araldite, CIBA-GAYGET, upper left).

692

Shlraishi

FIGURE 38 An example of the solution property shown by theappearance of allylated woodepoxy resin.

FIGURE 39 Appearance of polyurethane foams from allylated wood, prepared by liquefying and dissolving in 1,6-hexanediol, adding a foaming agent (water) and reacting agent (polymeric MDI), and resinifying.

Wood Plasticization

693

has substantial strength and a restoring force for compression deformation, as can be presumed from Fig. 40. The apparent density of the foams is dependent on the amount of foaming agent as well as the kind of reactive solvent (such as species of polyhydric alcohols, phenols, bisphenols, anddiluents) used. Foams preparedfrom the modified wood solution of bisphenol A using a similar procedure tend to have apparent densities around 0.1 g/cm3 and considerable strength. In order to elucidate the role of the liquefied chemically modified wood within the foams, comparative experiments of preparing the foams without the chemically modified wood have also been conducted. It was found that foaming actually occurs during the resinifying processbut, immediately after that, a contraction in volume of the foam occurs, resulting in resin moldings with apparent densities around 0.2 g/cm3 with little foamingcell structure remaining. This result is understandable because the open-mold, one-shot process used here is said to require rather high-molecular-weight polyhydric alcohols as one raw material of the foam. This result also reveals that the liquefied chemically modified wood plays a positive role in maintaining the shape of the foam during formation. One other application of the phenomenon of liquefying modified wood is in the formation of filaments or fibers produced by spinning methods. Tsujimoto [39] prepared wood-based fibers from acetylated wood. Acetylated wood is first liquefied and dissolved in phenol, and hexamethylenetetramine is added to the solution in various proportions. This is followed by heating to 150°C to promote addition-condensation, resulting in a resinified solution with high spinnability. From the spinning solution, filaments are spun and hardened in a heating oven at a definite heating rate (maximum temperature 250°C). An example of the filament is shown in Fig. 41. These filaments can be carbonized to give carbon filaments. Carbonization is carried out in an electrically heated furnace at a temperature of 900°C with a heating rate of 5.5"Umin. The strength of carbon filaments is measured according to JISR7601, and tensile strengths up to 120 kgf/mm' has been

FIGURE 40 Deformation and restoring property of foam prepared from allylated wood.

Shiraishi

694

FIGURE 41 Appearance of acetylated wood-phenol resinified filament. obtained, which is comparable to that of pitch carbon fibers of general-purposegrade. This strength value is expected to be increased by improvements in spinning and carbonization methods.

VIII.

LIQUEFACTION OF UNTREATED WOOD

The liquefaction and dissolution of chemically modified woods have been reviewed so far.Morerecently,untreatedwood has also beenfound to liquefyin several organic solvents [3,5-7,33,42-551. For example, after treating at around 250°C for 15-180 min, wood chips and wood meals were liquefied in phenols, bisphenols, alcohols (benzyl alcohol), polyhydric alcohols (1,6-hexanediol, l ,4-butanediol, and glycerin), and hydroxyethers (methyl cellosolve, ethyl cellosolve, diethylene glycol, triethylene glycol, and polyethylene glycol). The liquefaction of untreated wood can also be achieved at a lower temperature of 150°C and at atmospheric pressure inthe presence of an acid catalyst, phenolsulfonic acid, sulfuric acid, phosphoric acid, oxalic acid, and hydrochloric acid having been used [3,57,33,42-551. It is possible to obtain pastelike solutions with a high concentration of wood solute of up to 70%. After liquefaction, the wood components were found to have degraded and became reactive, which will be shown more detail in the section on the liquefaction mechanism for wood. The obtained wooden solute can be used to prepare adhesives and other moldings, opening a practical new field for utilization of wood materials, the details of which will also be explained later. During the liquefaction of wood, especially in the presence of acid catalyst, recondensation of degraded wood components occurs, as has been also observed inthe explosion and autohydrolysis process for wood. Because of the recondensation, it becomes very

Wood Plasticization

695

difficultto obtain a liquid with a large wood concentration, which is often considered undesirable from the viewpoint of biomass utilization. On the other hand, starch is very easy to liquefy even at a very small liquid ratio and catalyst concentration, when a catalyst is even necessary. Based on this background, a combined liquefaction process for wood and starch was proposed as a practical method for preparing large-biomass-content liquids [45]. That is, a stepwise liquefaction procedure in which the wood could be preliquefied alone at a relatively large liquid ratio, followed by the addition and liquefaction of the starch, was proposed. By this procedure, a large-biomass-content liquid was prepared with relatively small unliquefied residue [45]. In order to find an appropriate method for accurately determining the amounts of unliquefied residues, the soluble properties of liquefied wood and starch were investigated using a series of diluent solvents [47]. It was found that the solubility behavior of a liquefied biomass in a certain solvent was a kind of fractionation of the degraded and liquefied biomass components. In most cases, any single solvent could not dissolve all of the liquefied components completely. Several binary solvent mixtures composed of solvents considerably different in polarity were found to be good diluent solvents for liquefied biomasses. These phenomena can be illustrated by consulting previous works on physicochemical properties of binary solvent mixtures. Among several satisfactory binaries, the binary of dioxane and water has been studied in detail and found to be widely suitable for liquefied biomasses prepared in different liquefaction solvents. The range of dioxane/ water mixing ratio usable for the complete dilution of liquefied biomasses was wide enough for practical usage. Especially, a binary with a dioxanelwater composition of 812 was recommended as a universal diluent for liquefied biomass [47]. Phosphoric acid and even oxalic acid were found to be usable as catalysts for the liquefaction of wood [46,48-501. In the latter case, a small amount of hydrochloric acid was used simultaneously. These uses of catalyst were evaluated in connection with the flow properties and reactivities or curing properties of the liquefied wood. In this extension, phenolated woodlphenollformaldehyde co-condensed resins were proposed [53]. Wood was first liquefied in the presence of phenol by using an acid catalyst to produce a phenolated wood, and after the liquefaction, formalin was added to conduct a condensation reaction forconverting the remaining nonreacted phenol into resin components. It was found that this procedure can convert almost all the phenol remaining after liquefaction into resins, and therefore significantly upgrades the practical value of the liquefaction technique. Another advantage of this co-condensation isthat it can greatly improve the thermofluidity of the phenolated wood resins and the mechanical properties of their molded products. The flow temperatures and melt viscosities of the co-condensed resins were much lower than those of the phenolated wood resins. That is, these two properties were more or less similar to those of the conventional Novolac resin, resulting in excellent processability. The flexural properties of the molded products made from the co-condensed resins, although this point should be discussed in the next section, were much higher than those of the phenolated wood and also somewhat superior to those of the conventional Novolac resin [53].

A.

Comments on the Liquefaction of Wood

The term “liquefaction of lignocellulose” has hitherto referred chiefly to those procedures for producing oil from biomass under very severe conditions for conversion 156-581. For example, Appel et al. have converted cellulosics to oil by using homogeneous Na,CO, catalyst in water and a high-boiling-point mixture (anthracene oil, cresol, etc.)at a pressure

696

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of 140-240 atm, with synthesis gas CO/H2 1581. Treatment for 1 h at 300-350°C resulted in a 40-60% yield of benzene solubles(oil) and 95-99% conversion of the starting materials. This type of liquefaction can be more precisely called the oilification of lignocellulosics. The review presents recent progress on lignocellulosic liquefaction under milder treatment conditions, that is, at temperatures of 80-150°C with an acidic catalyst. One special group of chemically modified woods can be dissolved in cresols even at room temperature, as shown previously.

B. Application of the Liquefactionof Untreated Wood Almost the same products have been prepared from liquefaction solutions of untreated wood as from chemically modified wood. For example, resol-type phenolic resin adhesives prepared from five parts wood chips and two parts phenol did not require severe adhesion conditions and were comparable to the corresponding commercial adhesives in gluability. Acceptable waterproof adhesion was attained from the adhesives after gluing wood veneers at 120- 130°C with a hot-pressing time of 0.5 min to l-mm-thick plywood. This adhesion temperature of 120°C is at least 15°C lower than that ordinarily usedwith resol resin adhesives [43]. As a second example, foams can be prepared from untreated wood-polyethylene glycol solutions [59). Both soft and hard types of foams can be produced according to the preparation conditions. The prepared foams had a density of around 0.04 &m3, substantial strength, and strong restoring force against deformation. These results imply that the wood components were not merely blended within the foam bubbles, but also played an important role in maintaining the dimensional stability of the foams. Rigid polyurethane foams from combined liquefaction mixtures of wood and starch were proposed [ 5 I]. In this case. the large-biomass-content polyols were first prepared from a combined liquefaction of wood and starch, and the application of these polyols to the preparation of polyurethane foam was studied. The viscosity of a biomass polyol was influenced greatly by the composition of the biomass. At a constant total biomass content of 50%, an increase i n the wood content (i.e., decrease in starch content) drastically increased the viscosity of the polyol. Rigid polyurethane foams have been prepared successfully from the large-biomass-contcnt polyols. The foams had densities of about 0.03 g/cm', compressive strength of 80- 150 kPa, and elastic moduli of 3- I O MPa, being comparable to those of the conventional rigid polyurethane foams. The biomass composition in a biomass polyol had a significant influence on the properties of the resulting foams. The foams prepared from a biomass polyol containing only liquefied starch showed the greatest compressive strength and elastic modulus, but they were brittle and revealed poor restorability after deformation.Thefoams made from biomass polyols containing both wood and starch had somewhat smaller compressive strength and elastic moduli, but were much more resilient, revealing good balance in overall properties. Water-absorbing polyurethane foams were also prepared [54]. These were prepared from liquefied starch polyols and diphenylmethane diisocyanate (MDI) by using a cellopening foaming surfactant. The liquefied starch polyols were obtained by the liquefaction of starch in the presence of polyethylene glycol-dominant reaction reagents by using sulfuric acid :IS :l catalyst under either a refluxing condition or a reduced-pressure condition. The influences of the liquefaction conditions onthe properties of the liquefied starch polyols were investigated, taking into account the requirements for preparing appropriate polyurethane foam. Feasible formulation for the preparation of the water-absorbing foams

Wood Plasticization

were proposed and theproperties

697

of the foamsobtained

were systematically reported

[541. The third application example is Novolacresin-typemoldings prepared from untreated wood-phenol solutions [60]. After one part wood meal had been liquefied in two parts phenol, the untreated phenols were distilled under reduced pressure. The resulting liquefied and reacted wood-phenol powder could be used directly after wood meal filler and hexamethylene tetramine had been added and hot-pressed at 170-200°C. The flexural strength of the molding was comparable to those made from the commercialNovolac, when the curing temperatures for the former were settled 20°C or somewhat higher than that for the commercial Novolac. In connection with these moldings, it was described in the last part of the previous section that when the free phenol existing within the liquefied phenol solution was subsequently reacted with appropriate amounts of formaldehyde to give co-condensed resin, the thermal fluidity, the curing property of the liquefied wood, as well as the mechanical properties of the molding can be enhanced considerably [ 5 3 ] .Additionally, it was found that the flexural properties of the liquefied wood moldings were enhanced with an increase in the amount of combined phenol within the liquefied wood and became comparable to those of the commercial Novolac when the amounts of combined phenol were greater than 75%. Furthermore, with an increase in the content of wood fillers, the flexural properties of the liquefied wood moldings were enhanced more effectively than was the case for the commercial Novolac molding, showing that liquefied wood resins can gain a greater reinforcedeffect from compounding with wood fillers than did thecommercialNovolac resins. And the greater the amount of combined phenol, the higher was the reinforcing performance of the wood fillers. In addition, water-sorption measurements and SEM observations of the moldings indicated that the liquefied wood resins had much greater hydrophilicity than the Novolac and revealed greater compatibility with wood fillers 1521. Carbon fiber, already described in the section on the application of liquefied solutions from chemically modified wood, could also be prepared from an untreated wood solution, and atensilestrength ofup to 1.2 GPa has been obtained so far. Even better physical properties can be expected with more development 1391.

C.

Liquefaction Mechanism for Wood and Related Compounds

As described above, liquefaction of wood and its application have been developed during the last ten and more years. More recently, considerable studies have been carried out to elucidate the liquefaction mechanism for wood and its model compounds. First, cellobiose was used as the model compound for cellulose, and its liquefaction mechanism i n the presence of polyhydricalcoholor phenol and acatalyticamount of sulfuric acid was studied. As the conclusion, the following were shown: ( 1 ) Liquefaction of polysaccharides in the presence of alcohols or phenol with catalytic amount of sulfuric acid is accomplished via alcoholysis or phenolysis in the glucosidic linkage. (2) During this liquefaction reaction i n the presence of alcohols, the anomeric hydroxyl groups of the reducing end group or that of the free glucose are protonated and alcoholated, resulting in the same glucoside as is yielded by the above alcoholysis. (3) The rate of liquefaction depends on the accessibility of the liquefaction solvent to the polysaccharide. The liquefaction of an amorphouspolysaccharide, such asstarch, is very rapid, whereas that of crystalline cellulose proceeds at a much slower rate. which obeys pseudo-first-order kinetics. (4) The initial product of the liquefaction in the presence of an alcohol or phenol is the corresponding alcohol or phenol glucosides. (5)The reaction between polysaccharide

Shiraishi

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and phenol is more complicated compared with that between polysaccharide and alcohols, because of the multifunctionality of phenol. As a result, liquefaction products prepared in the presence of phenol tends to convert to higher-molecular-weight substances with increase in the reaction time. On the other hand, the liquefaction mechanism for lignin in the presence of phenol was studied in relatively wider ranges, that is, without and with acidic catalysts [61-631. As the model compound for lignin, guaiacylglycerol-P-guaiacylether (GC) was used and the range of the liquefaction studied was as follows: ( I ) under elevated temperature (200250°C) without catalyst; (2) under an elevated temperature of 200-250°C in the presence of acetic acid (catalyst); (3) under a moderate temperature of 150°C in the presence of acetic acid (catalyst); (4) under a moderate temperature of 150°C in the presence of sulfuric acid (catalyst), which corresponds to the study on the liquefaction of cellobiose described above. Conclusions obtained from this study are as follows: ( 1 ) The liquefaction of GC in the presence of phenol under elevated temperature without catalyst proceeds very rapidly through homolysis, producing coniferyl alcohol radical and guaiacol radical through quinone methide as the initial main intermediate. However, various homolytic cleavages occur, which give various kinds of radical compounds. As the result, considerable compounds are produced through reaction among these radical species, with reactions among coniferyl alcohol radical, guaiacol radical, and phenoxy or phenyl radicals resulting in dominating reaction pathways. (2) Acetic acid can greatly promote the homolysis reaction of GG, but does not alter the reaction mechanism; that is, in the presence of acetic acid, homolytic cleavage and coupling can occur even at a mild temperature of 150"C, and the resulting reaction products resemble those obtained under elevated temperature without catalysts. (3) Under catalysis with sulfuric acid, GG is first transferred into mainly benzyl cation. Benzyl cation rapidly condenses with phenol to give four condensed products as the initial reaction intermediates, which are no sooner produced thanthey are further subjected to extensivecleavage in their P-0-4 linkages and C&, bondings. The resulting cleaved fragments further react with phenol to form various phenolated products. The characteristic of this liquefaction reaction is heterolytic, that is, ionic, giving relatively small numbers of products when compared with the homolytic reactions [61-63].

IX.

CONCLUSION

The present state of studies on wood plasticization has been briefly reviewed. The scope of the description has been somewhat limited, that is, somewhat focused on works of the author and his colleagues. This is because the study of wood plasticization isnewand immature and sometimes tends to confusion. However, it can be said that this is a new field for chemical processing of wood with high future potential, although more studies need to be undertaken.

REFERENCES

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699

N. Shiraishi, in N. Shiraishi, H. Kajita and M. Norimoto (eds.), ReceI?t Research on b%od C I l d Wood-Based Materials, Elsevier Applied Science, London, p. 155 (1993). 6. N. Shiraishi, in Y. Doi (ed.), Handbook on Biodegracluble Plastics, STS,Tokyo, p. 139 (1995). 7. N. Shiraishi and M. Yoshioka, in T.Haraguchi, I . Sakata, K. Shimizu, N. Shiraishi, M.Norimoto and G. Meshimha (eds.), Handbook on Wooden New Muterials, Gihoudou, Tokyo, pp. 45, 62, 72, 79, 81, 118, 152 (1996). 8. D. A. 1. Goring, Pulp Paper M a g . C m . , 64: T-517(1963). 9. S.-Z. Chow, Wood and Fiber; 3 : 166(1972). IO. S. H. Baldwinand D.A.I. Goring, Svensk Paperstid., 71: 641 (1968). 1 I . N. Shiraishi, T. Matsunaga, and T. Yokota, J . Appl. Polynwr Sci., 24: 2361 (1979). 12. H. Fnakoshi, N. Shiraishi, M. Norimoto, T. Aoki, S. Hayashi. and T. Yokota. Holiforsch.. 33: 159 ( 1979). 13. N. Shiraishi, T.Aoki, M. Norimoto,and M. Okumura, in D.N.-S.Hon(ed.), Grq? rnerizatiorl of Lignocellulosic Fibers, ACS Syrnp. Series, 187, AmericanChemical Society, Washington, DC, p. 32 I (1982). 14. N. Shiraishi, T. Aoki, M. Norimoto, and M. Okumura, Cltemteclz, p. 366 (June 1983). 15. M. Norimoto, T. Morooka, T. Aoki, N. Shiraishi, T. Yamada, and F. Tanaka. Wood Res. Tech. Notes. 1 7 181 (1983). 16. N. Shiraishi, H. Matsui, K. Tsubouchi, T. Yokota, and T.Aoki, A b s t ~Pcrpers . Presented u t 31st Nutl. Meeting, J q m n Wood Research Society. Tokyo, p. 262 ( I98 1 ). 17. N. Shiraishi, and Coda, Mokuzoi Kogyo, 39: 329(1984). 18. N. Shiraishi and M.Yoshioka, Sen-i Gakkoishi, 42(6): T-346 (1986). 19. M.Moritaand I . Sakata, J. Apj~l. Polyn~e,: Sci., 3 1 : 831 (1986). 20. M. Morita, K. Shigematsu, and 1. Sakata, Abstr. Papers Prc..sented ut 35th Nntl. Meeting, J q m r l Wood Re.search Society, Tokyo, pp. 215. 2 16 (1985). 21. M. Norimoto. KGK J.. 16(9): 18 (1982). 22. T. Morroka, M. Norimoto, T. Yamada, and N. Shiraishi, Wood Res. Tech. Notes, 17: 75 (1983). 23. N. Shiraishi and K. Shiratsuchi,JapanPatent PublicationUnexamined. 1989-40560. 24. H. Matsudaand M. Ueda, Mokuzcli Gnkktrislti, 31: 903 (1985). 25. H. Matsuda, M. Ueda, K. Murakami, and M. Hara, Mokuzni Gakknishi, 30: 735, 100.3 ( 1984); Mokuzcri Gnkkrrishi, 31: 103, 215, 267, 468 (1985). 26.N. Shiraishi, Y. Murakami, T. Yokota,M. Noritnoto, T. Aoki, 0. Fujioka,and K. Ito, Abstl: Pcrper.s Presrrtted (11 32rld Nrrtl. Meetinsq. J q x r r z Wood Rrserrrdl SocYety. Fukuoka, p. 326 ( 1982). 27. N. Shiraishi, H. Karakane, T. Yokota. M. Norimoto, 0. FLtjioka, and R. Kitamura, A h t , : P C I I J C ~ ~ S ~, p. 156 ( 1983). Preserlted (11 33rd N u t / . Meetir~g.J a p m Wood Resetrrch S o c i c ~ r Kyoto, 28. N. Shiraishi, in C1wrnisrr;y of' Wood Utili-trtiorl. Kyoritsu,Tokyo. p. 294 (1983). 29. N. Shiraishi. in Advmcwl Techticpe and FLrturc. A p p ~ ~ );)ld lWool/ C/lc>l?lic.tr/,y, CMC, Tokyo, p. 271 ( 1983). 30. N. Shiraishi. Sert-i t o K o g y o . 39: 95 (1983). 31. N. Shiraishi, S. Onodcra, M. Ohotani. and T. Masunloto, M o k u z ; Gctkkai,y/t;,3 1 : 418 (1985). 32. N. Shiraishi,Japan PatentPublication Examined. 1988-1992 (appl. June 6. 1986). 33. N. Shiraishi. N. Tsujimoto,and S. Pu. Japan PatentPublication UnexiUl1ined 1986-261358 (appl. May 14, 1985). 34. N. Shiraishi,Japan PatentPublication Unexamincd, l982-2301: lc)82-236(). 35. N. Shiraishi.and H. Kishi. J. App/. Po1ym.r: Sri., 32: 3189 (1986). I. Sakata, Cellulose Chcw~.7 i d t l l o l . . 21: 255 (1987). 36. M. Morita and 37. R. A. Yong, S. Achmadi. and D. Barkalow, Preprints of CELLUCON'84, Cartl.eIlc-Wrexhaln. Walcs, p. 64 ( 1984). 38. H. Kishi, and N. Shiraishi, Mnku:cri Gddkrishi, 32: 520 ( 1986). 3'9. N. Tsujimoto, i n N. Shilaishi. H. Kajita. and M. Noril1loto (e&.), R e c w l t Kc>,sc.trrc/ro l l woo^/ t r d Wr)od-Bcr.sd M ~ t ~ r i ~ r 1 Elsevier .s. Applied Science. London, p. I69 ( 1993). 40. N. Shiraishi, H. Ito, and S. V. Lonikar. J. Wood Chml. pdutol., 7 405 ( I 987). 5.

700 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54.

55. 56. 57. 58. 59.

60. 61. 62. 63.

Shiraishi M. Morita, Y. Yatnawaki, M. Shigematsu. and I . Sakata, Mokrrxri Gokkrrishi, 36: 659 (1990). S. Pu. and N. Shiraishi, M o k u x i Gcrkktrishi, 39: 446, 453 (1993). S. Pu. M. Yoshioka, Y. Tanihara, and N. Shiraishi. in C.-Y. Hse and B. Tomita (eds.),Adho.si~~e.s m d Ror~dedWood Proc/wts, Forest Products Society (USA). Madison, WI, p. 344 (1994). S. Pu, and N . Shiraishi, Mokuzcri G~rkktrishi.40: 824 (1994). Y. Yao. M. Yoshioka. and N. Shiraish, Mokrrz(ri Gtlkkniski. 39: 930 ( 1 993). L. Lin, M. Yoshioka, Y. Yao, and N. Shiraish, J. A/?/)/.P d y r ~ r t ~Sci., r 52: 1629 (1094). Y. Yao. M. Yoshioka. and N. Shiraish, Mokrc=.ni Gtrkkaiski, 40: 176 (1994). M. H. Alma. M. Yoshioka, Y. Yao, and N. Shiraishi, M o k ~ t z Gtrkktrishi, ~i 4 / : 1122 (1995). M. H. Alma. M. Yoshioka. Y. Yao, and N. Shiraishi. J. A p p / . Po/yrn~r:Sci.. 61: 675 (1996). M. H. Alma, M. Yoshioka, Y. Yao, and N. Shiraishi. Ho/$~r.sc~I~.. 50: 85 (1996). Y. Yao, M. Yoshioka, and N. Shiraish. Mokrzrri Gnkktrishi, 41: h59 (1995). L. Lin. M. Yoshioka, Y. Yao. and N. Shiraish. J . A/)/?/. Po/yr~rer:Sci.. 55: 1563 ( 1995). L. Lin, M. Yoshioka, Y. Yao, and N. Shiraish, J . A/>/>/.Po/yrrrtv: Sci.. 58: 1297 ( I 995). Y. Yao, M. Yoshioka. and N. Shiraish. J . AI?/>/.Po/yrrwc Sci., 60: 1939 (1996). M. H. Alma, M. Yoshioka, Y. Yao. and N. Shiraishi, Mokrrztri Gttkkoishi, 4 / : 741 (1995). C. Vanasse. E. Chornet, and R. P. Overend, C m . J . Cl~cwt.Ens., 66: I12 (1988). H. R. Appel. I. Wender. and R. D. Miller, U.S. Bur: Mirles. Ech. Prog. Kcp., 25: S (1969). H . R . Appel. Y. C. Fu, E. G. Illig, F. W. Stetfgen, and R. D. Miller, U.S. H r o : M i r ~ c xHI 801.3, p. 27 ( I 975). N. Shiraishi. in H. Inagaki and G. 0. Phillips (eds.). Ct4lctln.sic.s Ufiliztrfiorl; Htwrrrch t r r r d Rcwnrcls in Ce//rr/o.sic.s.Elsevier Applied Science, London, p. 97 ( 1989). N.Shtraishi, Japan Patent Publication Unexamined. 1989-179483. L. Lin. Y. Yao. M. Yoshioka. and N. Shiraishi, Ho/$)r.sch.. 51: 316 (1997). L. Lin, M. Yoshioka. Y. Yao. and N. Shiraishi. Ho/$orsch.. 51: 325 (1997). L. Lin. M. Yoshioka, Y. Yao, and N.Shimishi, Ho/$orsc./~..51: 333 (1997).

17 Wood-Polymer Composites Hiroshi Mizumachi The University of Tokyo, Tokyo, lapan

1.

INTRODUCTION

In composite systems consisting of polymers and other solid materials, polymer molecules are in close contact with surfaces of solids, and it is quite reasonable to think that there are some interactions between the two materials near the surface. Polymer segments in the vicinity of the surface will be less mobile than those of the bulk polymers beyond the range of influence of the surface. Physical properties of polymers filled with various solid particles have been extensively studied in the past, and aresummarized by Kraus [ l ] , Flory [ 2 ] , and Nielsen [3]. There is much literature showing that carbon black particles in vulcanized natural rubber modify the mobility of the polymer chains as a result of chemical linkages formed between the two materials. This modified polymer layer is estimated to be about SO W i n thickness. Therefore, the modulus and the ultimate strength of the filled rubber are greater than those of the unfilled one. Kraus and Gugone [4] studied the adsorption of elastomers onreinforcing fillers from solutions,analyzedtheadsorptionisothermsaccording to a statistical mechanical theory of polymer adsorption developed by Simha, Frisch, and Eirich [S-71, and obtained valuable information on the polymer-filler interaction responsible for reinforcement. Kwei [S] studied the sorption of water vapor by Ti02-filled and unfilled epoxy polymer, and performed a thermodynamic analysis that showed that polymer segments at a distance less than IS00 A from the surface of the filler are under the influence of the filler. Kambe and Kat0 191 studieddynamicmechanicalproperties of a series of amorphousepoxy resin films filled with homogeneous polyethyl methacrylate (PEMA) beads with different diameters, and showed that larger increases of T,of PEMA are found for smaller particles, and that the loss peak per unit volume of PEMA is lowered with increase in the particle size, thus indicating some interactions between beads and the matrix phase. I t is evident from these several examples that we need detailed knowledge of the interfacial interactions between the two phases if we want to study the physical properties of composite materials. We need to measure the physical or physicochemical properties of the composite matcrials a s a whole, as well as those of the individual constituents, in order to know the properties of the bound polymer near the surface, which reflect the interactions between the component phases. Physical properties such as dimensional stability, bending strength, abrasion, etc., of wood-plastic composites have been widely studied from a practical point of view, as 701

Mizumachi

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reviewed by Kent et al. [IO], Siau et al. [ l l ] , and Burmester [12]. However. the intermolecular interactions between the two materials through the boundary surface has not been studied in detail from the physicochemical standpoint. In this chapter, some typical studies of the interactions between polymers and woods or wood components are reviewed.

II. DYNAMIC MECHANICAL PROPERTIES OF COMPOSITE SYSTEMS If a polymeric material is subjected to a sinusoidal strain E

= ell exp(iwr)

where is the amplitude of strain, W is the angular frequency of vibration, and r is time, then the stress U also will vary sinusoidally with the same angular frequency W, with the amplitude of v,,, and with a certain phase angle 6, namely, v=

v,, exp i(wr +

6)

The dynamic modulus of the material is expressed as a complex number,

where E' and E" are called storage modulus and loss modulus, respectively. Mechanical energy applied to a material is split into two parts; one is stored in the material as the mechanical energy, which is responsible for the elastic response of the material, and the other is converted to heat as a result of internal friction. The former is proportional to E' and the latter to E". Both E' and E" are functions of frequency and temperature. Typical curves of E' and E" at a constant frequency for an amorphous uncrosslinked polymer are shown schematically in Fig. 1 as a function of temperature. Generally, we find a few dispersions (stepwise drops of E ' ) accompanied by the same number of mechanical absorptions (peaks in E"). Each dispersion and absorption, which must occur simultaneously, correspond to a mode of molecular motion. For example, the primary absorption of a-absorption (or a-dispersion) that appears at the highest temperature is assigned to the initiation of micro-Brownian motion of polymer backbone segments. Around the temperature of this primary absorption, E' of the material changes drastically from IO"' to lo7 dyneskm'. Below the temperature of this absorption, backbone chains of the polymer are frozen. and the material is glassy. However, some kinds of local motions, such as rotation of side groups or local twisting of the chains, can happen at low temperature, and this iswhy some secondary absorptions-P-absorption, y-absorption, and so on. appear. We can get information on molecular motions of polymers from their dynamic mechanical properties measured over a wide temperature range. Therefore, if we measure the dynamic mechanical properties of polymers in the composite systems and compare them with those of the bulk polymers, useful information on the interactions between the component phases can be obtained.

703

Wood-Polymer Composites

log E '

l o g E"

TEMPERATURE FIGURE 1 Schematic representation of E' and E" at constant frequency as a function of temperature for a linear amorphous polymer.

A.

Dynamic Mechanical Propertiesof Wood Impregnated with Polymers

Figure 2 shows E" at 100 H z of Sugi (Cryptomeria Japonica D. Don, early wood, lon[ 131. There are two broad absorption gitudinal direction) as a function of temperature peaks: one around 0°C and the other below -50°C. Mechanisms of these absorptions are not known exactly. However, the change of dynamic mechanical properties of woods is generallyverysmallcomparedwiththoseofsyntheticpolymers.Wearenowmostly

10'

I

G

3.5 .:**tVC"..r-*"

lo8 '

.

-100

-50 0 TEMPERATURE

100

50 "C

FIGURE 2 Dynamic loss moduli of Sugi (earlywood, longitudinal direction) as a function of temperature at various frequencies. Different symbols refer to different specimens in each frequency of measurement.

Mizumachi

704

interested in the interactions between woods and polymers-in other words, in the extent to which molecular motions of polymers are influenced by solid materials. When polymers are impregnated into woods or filter papers by immersing the specimens in polymer solutions, a very thin polymer layer is formed on the very complicated surfaces of the wood or filter paper. These surfaces are porous and have such heterogeneous structures that the effective surface areas for polymer adsorption differ from specimen to specimen. Therefore, it is difficult to control, or even to measure, the thickness of the polymer layer or the surface of the material. Because an interaction between the solid surface and polymer molecules, if any, must be the consequence of a very shortranged molecular force (van der Waals force), it may be predicted that the thinner the polymer layer, the greater the influence of the solid surface. On the contrary, as the polymer layerbecomes thicker, the properties of the layer approach those of the bulk polymer which lies beyond the region of influence of the surface. Figure 3b shows variation of temperature of a-absorption, 7&!&x), with the amount of polymer. Obviously, T(E,',:c,x) of a-absorption of polymer in the composite systems is higher than that of the polymer itself, suggesting that polymer chains on the surfaces of cellulosic materials are somewhat im-

l

-40 0 40 TEMPERATURE

0

5 w t .'lo

80

"C

10 100 of polyrncr

(b)

FIGURE 3 Variation of dynamic loss moduli at 110 Hz with the amount of polymer impregnated: (a) log E" as a function of temperature; (h) T(E:i,,,x)as a function of the amount of polymer. Here a commercial NBR adhesive polymer is used.

Wood-Polymer Composites

705

mobilized, and are falling toward that of the bulk polymer as the amount of polymer increases. But when the polymer content is very small, i.e., below 10 wt% in the case of filter paper, T(E::,:,Jvaries very slightly. In other words, it is substantially constant within the experilnental error a s shown by the upper dashed line in Fig. 3b. When wood specimens are immersed into 5 % polymer solutions and evacuated to remove solvent, the amount of polymers impregnated in the wood generally falls within the region where T (E:;,~!,) is substantially constant. AT(E::,.d,or the difference between of polymer under the influence of the solid surface and that of the bulk polymer, which corresponds to the difference between the two dashed lines in Fig. 3b, will be one of the appropriate parameters to express the degree of interfacial interactions between the two components. AT(E::,,,)is approximately equal to ATv or the difference between T V of polymers in the colnposite and in the bulk phase. Typical data on the dynamic mechanical properties of wood-polymer composites are shown in Figs. 4-7. Data for the respective bulk polymer and for wood are shown for comparison. Generally, in composite systems the drop of E' is smaller and the E" peak shifts to higher temperature than in the respective bulk polymer. Dynamic mechanical properties of the composite materials can be calculated by means of the theory of polymer blends. Takayanagi et al. [ 141 proposed mechanical models like the ones shown in Fig. 8 to describe the viscoelastic behavior of polymer blends in terms of the known properties of the two component polymers. If phase cl is dispersed in phase M', there are two possible equivalent models for the system, model I and model 11. Complex moduli of these models are expressed as follows:

E2' (model I) = (+/[AE$ E* (model 11) = A[@E$

+ (1

+ (1

+ ( 1 - AYE:) ' A)/E,T]" + ( 1 - A)E:

-

A)E:l

where E;: and E: are complex moduli of phase N and phase W , respectively; the values of A and 4 correspond to the thickness fraction and the length fraction, respectively, of phase N as shown in Fig. 8, and the product A4 is equal to the volume fraction ( U , , ) of phase N. Practically, A and 4 are called the parameters of the mixing state, because the values of these parameters naturally vary with the change of mixing state for a system with the same value of U 10"' dynes/cm') is

743

Adhesion and Adhesives

I

2

121

.

Emulsion B

ul

0 . 0

0 . - . .1 -150

D

' 0 0, . . - . 1 . . . . ~ . " 1 . ~ - - - ~ l - @ - ~ l

-100

-50 0 50 Temperature ('C)

l00

IS0

FIGURE 10 Temperature dependence of adhesive shear strength of woods bonded with two emulsion adhesives.

called A-region (A refers to Anchors), and failure modes in this region are illustrated schematically in Fig. 14. When the test temperature is raised a little more or the rate of strain is lowered, or when glassy polymers are plasticized so as to make E' of the adhesive between 10" and 10"' (mostly when 5 X 10') dyneskm', adhesion through the boundary surface between adherend and adhesive becomes mostly effective, and in this region both adhesive tensile strength and shear strength for woods as well as for metal adherends become maximum. This region is called the B-region (B refers to Boundary surface), meaning that adhesion at the boundary surface predominates there. When the adhesive becomes extremely soft (E' < 10' dyneskm?), failureoccurs always in the adhesive phase, which means that adhesion at the boundary surface exceeds the cohesive strength of the adhesives. This region is called the C-region (C refers to Cohesion), because the experimentally measured adhesive strength is determined solely by the cohesive strength of the adhesives.

-

C.

Peel Strength

In peel tests, an adherend is largely bent and the external force is applied so as to pull it. Therefore, it is not possible to perform peel tests on wood adherends.

744

Mizurnachi

I

I

1

10 Epikote828 Epikate87I Weight fraction FIGURE 11 Plot of adhesive shear strength of woods bonded with epoxy resin adhesives against weight fraction of Epikote 871 in Epikote 828/Epikote 871 blends. Wood failure is also shown.

Birch/ PSt /Birch

FIGURE 12 Adhesive shear strength of polystyrene as a function of temperature, where Kaba and aluminum ( 0 ) are used as adherends.

(0)

745

Adhesion and Adhesives

5x109

dyne/crn2

B region

-#

E of adhesives

Temperature of adhesion tests

(

Degree of plasticization etc.

FIGURE 13 Dependence of adhesive strengths on storage modulus

*-

m

of adhesives.

n I

S hear

Tensile

FIGURE 14 Comparison of failure modes for shear and tensile tests in the case when the adhesive is in glassy state (A-region).

Mizumachi

746

Hofrichter and McLaren [ 1 l ] measured peel strength of cellophane, using terpolymers of vinyl acetatehinyl chloride/maleic acid as adhesives, and obtained the following empirical relation:

P = k[-COOH]" where k and n are constants ( n = 0.5-0.75). They believed that as the concentration of the "COOH group increases in the adhesive, the probability of interaction between the functional group and " O H group on the surface of the cellophane increases, and consequently peel strength also increases. However, if cohesive failure must be partly involved in adhesion tests, their discussion does not seem reasonable. Mizumachi et al. [ 121 studied the influence of chemical structures and dynamic mechanical properties of a series of methacrylic co-polymers on peel strength of cellophane. Monomers used in the study are listed in Fig. 15. One can easily prepare adhesive polymers with various viscoelastic properties and with various concentrations of functional groups such as hydroxyl groups and epoxy groups by combining these monomers. Monomer mixtures are polymerized in contact with adherend in the glass cell shown in Fig. 16, and after polymerization is completed, a peel test is performed. Some of the typical data are shown. Figure 17 shows the cases of co-polymers of 2-ethylhexyl methacrylate (EH) and 2-hydroxyethyl methacrylate (HO) or glycidyl methacrylate (G). It is true that

Methyl methacrylate (M)

CHt'

Ethyl methacrylate

(€1

F C.0

Butyl methacrvlate (B)

cnZ=

?43

c

$90

0 FH2

42M25 Lauryl methacrylate

'4'9'2'5

(L)

2 Ethyl hexyl methacrylate (EH) ~

P F"2 P 2 OH

2 - Hydroxy ethyl methacrylate WO)

FIGURE 15 Monomers.

Glycidyl methacrylate (G)

747

Adhesion and Adhesives

UV light

111 monomers cellophane

FIGURE 1 6 Glass cell used for polymerization and peel test.

1.0

EH

H0

G FIGURE 1 7 Peel strength of cellophane as a function of co-polymer composition for EH/HO and EH/G ( 0 ) systems.

(0)

748

Mizumachi

peel strength increases as concentration of hydroxyl group or epoxy group increases in some range. Hofrichter et al. made their experiments within a very narrow range of concentration (less than 5.7%). However, if the functional group in the adhesive is increased more and more, peel strength goes down. It does not keep on increasing, nor does it level off, but it decreases to almost zero. The left-hand side of the peak corresponds to cohesive failure, and the right-hand side corresponds to interfacial failure. These data show that chemical structure (or functional group) is not the only factor in adhesion. The dynamic mechanical properties of the copolymers were also measured. The temperature of the viscoelastic absorption peak at 100 Hz, which is close to the glass transition temperature, was plotted as a function of co-polymer composition as shown in Fig. 18. These curves will go down if frequency of measurement is lower. At temperatures below these curves, the backbone chains of the co-polymers are frozen in and the polymers are in the glassy state. At temperatures near the curves, micro-Brownian motion is initiated; and at temperatures above the curves, molecular motion will be great and the polymers will be in a rubbery state and then in a fluid state. It is evident that when the adhesive is rubbery or fluid at room temperature, the cohesive failure occurs at room temperature, regardless of the concentration of functional groups, and peel strength is low; and that when the adhesive is glassy, interfacial failure occurs and the peel strength is still low. Peel strength becomes maximum when the viscoelastic absorption peak of the polymer is near room temperature, where the peel test is performed.

120

I

FIGURE 18 Temperature of viscoelastic absorption peak at 100 Hz as a function of co-polymer composition for EH/HO ( 0 ) and EH/G ( 0 ) systems.

749

Adhesion and Adhesives

0

8 0.5 E

0 0

0

.

0

0

1.0

H0 C

FIGURE 19 Peel strength of cellophane as a function of co-polymer composition for E/HO and E/G ( 0 ) systems.

(0)

Figure 19 shows the cases of ethyl methacrylate and the two functional monomers ( H 0 and G ) . These co-polymers are glassy at room temperature over the whole range of co-polymer composition as shown in Fig. 20. Peel strength is almost zero, in spite of the fact that the concentration of functional groups increases to a great extent. Figure 2 1 shows the case of lauryl methacrylate (L) and methyl methacrylate (M), where no functional group is involved and only viscoelastic properties of the copolymers are varied, as shown in Fig. 22. Here again, maximum peel strength is obtained when the temperature of the viscoelastic absorption peak is near room temperature. Master curves of the rheological functions of some copolymers were obtained and compared with the data on peel strength. It was concluded that the viscoelastic properties of adhesives are the dominant factor in adhesion. Peel strength becomes maximum when the viscoelastic absorption peak of the adhesive at room temperature in the test appears at the frequency that corresponds to peel velocity. These characteristics are summarized as follows. The absolute value of the peel strength can be somewhat different if the chemical structure of the copolymerization system is different, but maximum peel strength is always obtained when the storage modulus E‘ of the adhesive is about 10’ dyneskm’ as shown in Fig. 13. If Hofrichter et al. had performed their experiments over a much wider range of concentration, they might have come to somewhat different conclusions.

D.

Tack of Pressure-Sensitive Adhesives

Tackis one of the most important properties of a pressure-sensitive adhesive, and it is measured mainly by two kinds of methods, which are summarized by Johnston [13]; one is the probe tack test and the other is the rolling-ball tack test. Because the former must

750

Mizumachi

120 100

-

0. 0

**.**-

n

u

0

-80

.m

*

-

0. 0"

be-" \o-o

60 h 8

:: W

c "10

20

.

O0oA E

a

A

0.5 '

a

'

a

1.0 H0 G

FIGURE 20 Temperature of viscoelastic absorption peak at 100 Hz as a function of co-polymer composition for E/HO ( 0 ) and E/G ( 0 ) systems.

L FIGURE 21

M Peel strength of cellophane as a function of co-polymer composition for L h 4 system.

751

Adhesion and Adhesives

120 100

’

n

u

-

0

-eo 60 n I 0

: E W

g40

-

/

0

OakL

0.5 ’ ’ ’

*

m

1.o M

FIGURE 22 Temperature of viscoelastic absorption peak at 100 Hz as a function of co-polymer composition for L/M system.

be classified as one of the adhesive tensile tests, only the latter is discussed here. It is believed that rolling motion of a ball on a pressure-sensitive adhesive reflects tackiness of the adhesive, and therefore rolling-ball tests have been employed in many countries for a very long time. In J. Dow’s method of measuring rolling-ball tack, balls are rolled on an inclined surface. A pressure-sensitive adhesive 10 cm in length is placed on a part of the surface. Angle of inclination is 30°, and leading distance is 10 cm. If a ball is too large, it may roll out across the pressure-sensitive adhesive zone and go down farther. Then a little smaller ball is rolled, and so on. If a ball of a certain diameter, namely, 11/32 in., stops within the pressure-sensitive adhesive zone, tack of the pressure-sensitive adhesive is expressed as “ball number 17.” In the PSTC-6 method, a ball of 14/32 in. diameter rolls down on an inclined path and onto a horizontal surface of a pressure-sensitive adhesive. Here, tack of the pressure-sensitive adhesive is expressed in terms of the rollout distance, because the reciprocal of it is considered to be proportional to the tack of the pressuresensitive adhesive. These ways of expressing tack are useful in some practical cases, but the physical meaning of the value is not necessarily clear. If the angle of inclination is different, or if the length of path on a rigid substrate is different, the ball number or the rollout distance might be different for the same pressure-sensitive adhesive. It is hoped that a method can be developed by which tack of pressure-sensitive adhesive can be expressed in terms of significant physical meaning. Mizumachi [ 14,151 pointed out that tack must reasonably be described in terms of the rolling friction coeffi-

Mizumachi

752

cient ( f ) of the pressure-sensitive adhesive, because rolling motion of a ball can be expressed by a set of equations of motion, where f is involved and ,f does not depend on such parameters as the angle of inclination of the surface or the leading distance, but depends only on the physical properties of the materials on which the ball rolls. He solved the equations of motion of a ball, assuming that

f = 40 + 4 l V and derived the following equations: cos a)/(5g4?cos’a)]

(x - x(,),, = [(7R(R sin a X

log[(R sin a

-

(+(,

+ ~ I v , ) c o sa)l(R sin a - 4(,cos a ) ]

+ 7Rv,,/(5g4f COS a ) where (x - x(,),, is the rollout distance of a ball on a pressure-sensitive adhesive, R is the radius of a ball, a is the angle of inclination of the surface, and v,, is the initial velocity of the ball in the pressure-sensitive adhesive zone, which is given as follows: v,, = ( X

-

X(,)”’( 1Og/7R)”’(R sin a - A) cos a)”’

where ( X - X(,) is the leading distance, and.f;, is the rolling friction coefficient of the rigid substrate, which is nearly equal to zero. Urushizaki et al. [ 161 made extensive experiments on rolling motion of a ball on pressure-sensitive adhesives, and showed that the data can be analyzed successfully according to the above equations. A typical example is given in Fig. 23. Values of the two parameters, 4” and 41,are determined so as to minimize the sum of the deviations of experimental data from the theoretical curves. Agreement between the theoretical curves and the experimental data is satisfactory. In this case, +(, = 0.67 cm and = -0.0043 S , regardless of the leading distance or the angle of inclination of the surface.

x-x0 FIGURE 23

(CM 1

Plot of rollout distance against leading distancc

o f a ball

753

Adhesion and Adhesives

“

FIGURE 24 Rollingcylindermethod

Mizumachi and Saito [ 171 also clarified that rolling processes of a ball can be analyzed by the theory over the whole range of velocity, and at the same time rollout distance can also be analyzed by the same theory. In any case, one must perform a rather complicated analysis in order to determine the values of ,f (or 4,)and because velocity of a ball changes at every moment and at the same time ,f varies as a function of velocity. However, Mizumachi [ 14,15,18] point out that it we adopt the pulling cylinder method as illustrated in Fig. 24, we can easily determine the value offwithout any elaborate analysis. Suppose that a cylinder of radius R, length 0,and weight M g is pulled by a force P on a horizontal plane of a pressure-sensitive adhesive at a constant velocity v. Then f of the material is given as

This type of experiment and also the analysis can be easily made, and we do not need any postulate concerning the dependence offon U . If we want to know f as a function of 71, we only have to pull a cylinder at several constant velocities. Naturally, .f‘depends on the physical properties of a pressure-sensitive adhesive, and if the value off is plotted against log 7~ for a viscoelastic material over a very wide range of velocity, it is expected that some curve will be obtained that increases from a relatively low value to a certain maximum and then decreases as the velocity becomes greater. A typical example is given in Fig. 25.

W. RHEOLOGICAL THEORYOF ADHESIVE STRENGTH If we measure stress and strain at break (c/,, c,,)of a viscoelastic material, and plot c, against E/,, a curve of common shape such as given in Fig. 26 is obtained. This is the “failure envelope” of Smith [ 191. Several attempts 120-241 have been made to interpret the failure envelope theoretically, among which Hata’s theory [23] is the simplest. He showed that the failure envelope of viscoelastic materials (adhesives) can be reproduced if we choose a simple mechanical model, a parallel connection of two Maxwell elements, and assume some failure criteria. Parameters concerning the elements in the model are as shown in Fig. 27. Failure starts in the weak point (the tirst Maxwell element), and then the residual part (the second Maxwell element) breaks down, carrying the whole load. The failure at the first Maxwell element may occur if either of the following two conditions is fulfilled: I.

Strain of the spring ( E ~ , reaches ) a certain critical value (cIIC). This corresponds to the region where strain rate ( r ) is high or temperature ( T ) is low.

754

Mizumachi

lr

FIGURE 25 adhesive.

Plot off measured by pulling-cylinder method against log v for a pressure-sensitive

C

-

Strain

FIGURE 26

Schematic representation of a failure envelope of a viscoelastic material.

FIGURE 27

Parallel connection of two Maxwellelements.

Adhesion

755

11. Strain of the dashpot (cl2) reaches a certain critical value to the region where r is low or T is high.

Thiscorresponds

Then the following equations are derived:

Results of the numerical calculations, substituting appropriate values for the parameters,are shown in Fig. 28, where characteristic features of the failureenvelopeare reproduced. It is possible to show according to this theory that the failure point (U!,, E!,) moves counterclockwise along the failure envelope as the temperature is lowered or the strain rate is increased. In the case of failure of an adhesively bonded joint, the same treatment must be possible in the region where cohesive failure occurs. However, it is evident that an additional failure criterion is needed in order to interpret the interfacial failure. Hata [24] postulated the third criterion: 111. Interfacial failure occurs when energy stored in the springs of the model (W) reaches a certain critical value (Wc-):

Y. Hatano and H. Mizumachi [25,26] calculated adhesive strength of various kinds according to the theory, and some of the results are shown. A.

Adhesive Tensile Strength and Adhesive Shear Strength

Theoretical expressions for both adhesive tensile strength and shear strength have already been given above. However, the parameters must be properly chosen, as illustrated in Fig. 29. An example of the numerical calculation of adhesive shear strength is given in Fig. 30. The curves vary naturally if we choose different values for the parameters. For example, if and W,. is larger, curves I and 111 will go up in almost parallel, and if elZC becomes larger, the slope of curve I1 will be greater. And if only the relaxation time 7,= q / E , varies, all the curves will shift along the log U axis.

d

“Q c-

-

””“”

& FIGURE 28

Explanatory diagrams of the model theory for the failure envelope.

Mizumachi

756

l Tens i l e

Shear FIGURE 29

Comparison of shear and tensile deformation: Shear

Strain Strain rate Modulus Viscosity

E

=d h = 7dh G 77

d&/ldt

Tcnsile E = .r/h d&/lrlt = lllh E (= 3G)

77,

Because we have to select the failure mechanism with the lowest m,, at every velocity, the solid line in the figure expresses the overall adhesive shear strength as a function of velocity. According to the rheological principle, larger velocity is equivalent to lower temperature, or vice versa, and therefore we can imagine how adhesive shear strength depends on temperature at a constant velocity. Curves I, 11, and 111 in Fig. 30 correspond to the C-region, B-region, and A-region in Fig. 13, respectively. If we want to take into

Mizumachi

758 10-

il \‘

0-

\’4

g

6-

. U? P

m

Y

4-

h

(L

2-

104

10.1

10.2

100

101

Velocity (crn/s)

FIGURE 32 An example of the results ofthe numerical calculations of peel strength, which is plotted against log 7). Values of the parameters are as follows: b = 1.5 cm, h = 0.025 cm, W,, = 5 X 10’ dyneskm’, E , = 10“’ dyneslcm’, 77, = 10’ poise, E’ = 10“’ dyneslcm’, 77’ = 10’ poise, q I C = 0.02, = 0.005, W , = 10’ ergslcm’. It is assumed that a suddenjumpfromcurve I tocurve 111 occurs at the same strain rate as in the case of shear test.

interesting to notice that the absolute value of P is very low (order of magnitude of 10” kg/1.5 cm), in spite of the fact that the adhesive shear strength for the same model is about 10’ kg/cm?. A second interesting point is that the velocity corresponding to maximum peel strength is much lower than that corresponding to maximum shear strength, which is in good agreement with the previously described fact that adhesive shear strength becomes maximum when E‘ is about 5 X 10’ dyneslcm’, while peel strength becomes maximum when the adhesive is much softer (E’ = lo8 dyneslcm’).

C.Tack

(Rolling Friction Coefficient)

Mizumachi [ 181 developed a rheological theory on rolling friction of pressure-sensitive adhesives in the case of a cylinder of radius R , length b, and weight M g pulled by a force P at a constant velocity 71, using the same model. Parameters of a cylinder are shown in Fig. 33. The corresponding equations are as follows:

FIGURE 33 Cylinder tacktest.

759

Adhesion and Adhesives

v (cmlsl

FIGURE 34 An example of the results of the numerical calculations of,f. which is plotted agianst log V. Values of the parameters are as follows: 17 = 2.0 cm, R = 1 . 0 cm, IT = 0.001 cm, M g = 6.0 X 10' dynes. E , = IO7 dyneskm', r], = IO7 poise. E, = IO7 dyneskm', = 10' poise. E,,' = 3.0, cl'(.= 7.0, W , = 7.0 X IO7 ergs/cm'.

r].

V.

FRACTURE MECHANICS OF ADHESIVE JOINTS

I t has been clarified in the previous section that the values of adhesiveshearstrength, adhesive tensile strength, and ped strength can be calculated theoretically as a function of rate of deformation, according to the mechanical model proposed by Hata 123,241. If we postulate an additional criterion concerning the abrupt transition from cohesive failure to interfacial failure, we can construct a curve having a peak at some rate for each adhesive strength. The rate at which peel strength becomes maximum is lower than that at which both adhesive shear strength and adhesive tensile strength do. The peak in each adhesive

Mizumachi

760

strength shifts toward the higher-rate side as the relaxation time of the viscoelastic material (adhesive) decreases. or vice versa. In other words, for a particular adhesive, the peak shifts toward the higher-rate side when the temperature of measurements is raised, and when we compare the curves for various adhesives at some fixed temperature, an adhesive of lower T, will have its maximum adhesive strength in a higher-rate region. These features are in agreement with most of our experimental observations so far. However, there is a large discrepancy, that is, adhesive tensile strength is calculated to be much greater than adhesive shear strength, whereas the experimentally measured values are generally in the reverse order. In spite of this discrepancy, this simplified theory may be useful in describing qualitatively the complicated phenomena of adhesion, but we have to think seriously of the case of the discrepancy. Of course, the biggest problem is stress concentration. It is postulated in the abovementioned theory that stress and strain are uniform within the specimen, but this is not true especially in case where the modulus of the adhesive is high. When an external force is applied to the adhesive joint, stress is extremely concentrated at the edge or corner of the adhesive layer, and failure of the joint initiates there and propagates along the bond line as the force reaches a critical value. Stress concentration within a material and its fracture are the major objects of the fracture mechanics which has been developed in the fields of solid materials such as glass, ceramics, hard plastics, and others, and has also been applied to the fracture of adhesive joints [27-391. In fracture mechanics, the critical strain energy release rate G,. is an important parameter, a measure of the toughness of the material, and is defined as follows:

G,. =

(2)(g)

where. P,. and h refer to load at failure and width of the specimen, respectively, and ( K / ilA) is a partial differentiation of compliance (C) with respect to crack length ( A ) . G , is the energy required to increase the unit area of the crack surface in the specimen, and is different if the deformation mode is different. There are three typical deformation modes: mode I or tensile-opening mode, mode I1 or in-plane shear mode, and mode I11 or antiplane shear mode, which are illustrated schematically in Fig. 35. Lim et al. [38,393 measured G, for three deformation modes: G,,. for mode I, GIICfor mode 11, and GI,,,. for mode 111, using double-cantilever beam specimens of wood joints by the compliance method. Characteristics of both adhesives and adherends are listed i n Tables 3 and 4, respectively.

Adhesion and Adhesives

AVUT EPOO 1

EP007 EC34569 EsetR PM200 KU224 KU66 1/2 CH18 Y 400 SGA

A-ff 3000DXH

761

Water-based vinyl polymer isocyanates H-3" Epoxy polyamines" Epoxy polyamines" Epoxy polyamines" Epoxy polyamines" Epoxy silicon" Polyurethanes Polyester (polyol) polyisocyanates" Polyacetates Polyacrylates polyacrylates" Polyacrylates polyamines" PoIy(cY-cyano acrylates)" PoIy(a-cyano acrylates)"

7.5 x I O " -54.7 64.6 84 55.5 -51.3 21 46.3 27.3 72.2 -11.3

1.54

X IO" 1.0 x IO"' 1.12 x 10"' 1.16 x IO"'

4.65 X IOx 1.04 x IO"' 1.01 x 10"' 1.08 x IO"' I . 18 x IO"' 6.06

X

10"

4.34

X

10'

1.00 X 1 0 ' 3.42 X IO"

3.03 x I O " 5.81 x IO' 6.23 X IO" 2.99 x 10" 2.64 X I O " 1.13 x IO'' 7.58 x I O "

0.075 0.26 0.0382 0.0305 0.026 0.125 0.06

0.027 0.245 0.096 0. I25

The experimentally obtained GC values for the joints. where the commercially available adhesives are used, are summarized in Table S. It is shown that G,,. < G,,,,. < G,,,., and this tendency is in agreement with that found in the literature 134,361. An interesting G, value and the corresponding conquestion is what kind of correlation exists between in Fig. 36. whereadhesivctensile ventionaladhesivestrength.Ancxampleisshown strength of a series of the joints is plotted against the square root of G,,, because they are obtained by the tests of similar deformation mode, and G,, is proportional to the square of the load at failure. The correlation coefficientin this case is not necessarily high enough, but if well-characterized adhesives are employed, a moresignificantcorrelation will be o n the state of molecular found. It is quite natural to think that the correlation depends motion of the adhesive (i.e., glassy state, transient state, rubbery state, or fluid state), and systematic data are being accumulated 1401. A rheological approach as well as a fracture mechanical approach will be necessary i n order to clarify the mechanism of adhcsion.

TABLE 4

Characteristics o f Adherends Specilic gravity

Moisture content

Young's modulus ( 1 Or kgl/cm')

Adherends

Air

Dry

(%)

E , . E,,

Ell,

Kaba 1 Kahn2

0.6X

0.64 0.78

14.8-16.5 14.9

I .38 1.16

1.34 1.12

0.88

Mizumachi

762 TABLE 5

Critical Strain Energy Release Rates for Modes

I, II, and I11

GllC

Adhesives

G,,

(kgf cm/cm')

G,,,,.

AV UT EP007 EPOO I PM200

0.34 0.28 1.18 0.34 0.19 0.39 0.10 0.20 0.24 0.07 0.24 0.10 0.1 1

2.05 2.77 2.15 2.80 2.57 5.55 I .72 3.7 I 2.93 2.1 1 2.54 9.47 4.06

0.93 0.72 0.52 0.5 I 0.69 2.53 0.47 0.80 0.82 0.76 0.50 0.83 1.36

EsetR EC3569 KU224 KU66 1/2 CH18 Y400 SGA A- a

3000DHX

VI.

GIICGC

6.0 9.9 11.9 8.1 13.3 14.2 16.6 19.0 12.3 30.4 10.8 54.8 38.3

GlllC/Gl,

2.7 2.6 2.9 1.S 3.6 6.5 4.5 4.1 3.4 10.9 2. I 8.6 12.9

CONCLUDING REMARKS

Adhesion involves a variety of factors, among which structures and propertiesof adhesives and/or adherends are most important, because failure of the materials is always involved in adhesion tests. Takayanagi [41] has pointed out that physical properties of a material ( Z ) are generally expressed in terms of two variables, namely, the state of aggregation ( X ) of molecules and the state of molecular motion (Y). The variable X represents such aspects of material ascrystalline/amorphous,orientedhonoriented, and homogeneous/heterogeneous, as well as superstructures. Information related to X can be obtained through studies utilizing electron microscopy, light-scattering observation, and X-ray diffraction measurements. The variable X also includes thermodynamic measurements with respect to compatibility among the components of the material, and phase separation. 1o+"----

.

FIGURE 36 Relationbetweenadhesivetensilestrength various adhesives.

and strainenergyreleaserate.

GI,., for

Adhesion

763

On the other hand, the variable Y represents such aspects as the micro-Brownian motion of molecular segments and some local modes of molecular motion. Information concerning these aspectscan be obtained through such observationsas viscoelasticity, dielectric characterization, and NMR. Let us assume, for example, that we have two materials, both of which are completely amorphous and located at the same point on the X axis. If the micro-Brownian motion of the backbone chains is restricted in one material and nonrestricted in the other-in other words, if they are located at different points on the Y axis-then the two materials will show completely different physical properties. The former will be in a glassy state with modulus E' of the order of IO"' dynes/cm', while the latter will be a rubbery material with E' of about lo7 dynedcm'. If we take two materials, each having a two-phase structure, the overall material characteristics of a system consisting of a continuous phase in a glassy state and a dispersed phase in a rubbery state would differ extremely from those of the other in which the state of each phase is reversed. This indicates that a conclusion with generality cannot always be achieved if relations of the adhesion performance with either of the variables X and Y are investigated separately. And if some data on the practical performance of adhesion can be connected to Z as a function of X and Y , it will enhance the understanding of adhesion phenomena from the viewpoint of polymer science. The so-called adhesion theories and adhesion principles presented in the past may be said to have clarified some theoretical or empirical laws to connect practical adhesion performance to physical properties of materials. Even in the case where a law is only an empirical one, it will not only be helpful for the efficient development of new materials, it will also serve to stimulate theoretical advancement in the future if the law is expressed in terms of Z =,fix,Y ) . In this chapter, adhesive strengths of various kinds are connected to dynamic mechanical properties of an adhesive, and it became evident that there exists some common tendency, which was interpreted by a simplified rheological theory. We have come to know that although adhesion is a very complicated phenomenon, lots of common elementary processes are involved in various aspects of adhesion. If we continue to accumulate data on adhesion systematically, using well-characterized adhesives (molecular-characterized as well as material-characterized adhesives), the mechanism of adhesion will be clarified scientifically in more detail.

REFERENCES 1

2. 3. 4. 5. 6. 7. 8. 9. 10. II. 12. 13. 14.

IS.

E. A. Davies, Adhesion t r n d Arlhesives, Furldnrrlentnls rrnd Prcrctice, Society of the Chemical Industry (1954). Seccyczk~c-Riron to Oqo, Kobunshi Gakkai ( 1 959). K. Motohashi, B. Tomita, and H. Mizumachi, Holdi~rsch..36: 183 (1982). K. Motohashi, B. Tomita, H. Mizumachi, and H. Sakaguchi, Wood Fiber Sci., 16: 72 (1984). Y. Hatano, B. Tomita, and H. Mizumachi, Moklczcri Gakkaishi, 29: 578 (1983). H. Ishii and Y. Yamaguchi, J . Adlwsiorl Soc. Japan, l / : 59 (1975). S. Koizumi and T. Matsunaga, J. Adkesior~Soc. Jtrparl. 6 : 437 (1970). Seccynkrc Hcrrldbook, Nikkan Kogyo ( 1980). H. Mizumachi, Mokuzcri K o g y , 36: 3 (1981). H. Mizumachi, Mokuzai Kogyo, 36: 57 (1981 ). C. F. Hifrichter and A. D. McLaren, / m f . Eng. Chern., 40: 329 (1948). H. Mizurnachi, M. Tsukiji, Y. Konishi, and A. Tsujita, J. Adhesior? Sor. J t p t r r l , 12: 378 (1976). J. Johnston, A d h e s i ~Age, ~ ~ 26: 34 (1983). H . Mizumachi, Zcriry Gijutsu, 2: 72 (1984). H. Mizumachi, J . Adhesiorl Soc. h p m t , 20: 522 (1984).

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16. F. Urushizaki, H. Yamaguchi. and H. Mizumachi, J . Atlhe.rior~Soc. J q x r n , 20: 295 (1984). 17. H. Mizurnachi and T. Saito, J. Arlhrsiorl. 20: 83 (1986). 18. H. Mizumachi, J. App/. P o / w w r Sci., 30: 2675 (1985). 19. T. L. Smith, J. P o / y l r r Sci., 32: 99 (1958). 20. R.Sato. K o h r m s / z i . 15: 665 (1966). 21. R. Sato. K o h l r n s h i . 15: 768 (1966). 22. T. Saito. K o b u r ~ s / R ~ iO I I ~ L ~ I I41: S / Z19 L I( ,1984). 23. T. Hata, Z ~ r i t y ,1 3 : 322 ( 1964). 24. T. Hnta, ./. Adlwsiorl Soc. J q x r t l . h': 64 ( 1972). 25. Y. Hatano and H. Mizumachi, Mokuxri Gnkkrtishi. 3.5: 243 ( 1989). 26. H. Mizumachi, Semi t o K o g x o . 42: 33 (1986). 27. S. Mostovoy and E. J. Ripling, J . Appl. P o / y r w r Sci., 15: 641 (1971). 28. S. Mostovoy and E. 3 . Ripling. J. AI?/?/. P o / w v r Sci., 1 5 : 661 ( I 97 I ) . 29. W. D. Bascom, R. L. Cottington. R. L. Jones, and P. Peyser. J . Appl. Polyr~lrrSci.. / Y : 2545 (1975). 30. R. Ebewele, B. River, and J. Koutsky. Wood Fiher; l / : 197 (1979). 31. J. L. Binter. J. L. Rushford, W. S. Rose. D. L. Hunston, and C. K. Riew, J. Ad/w.sior~.13: 3 32. 33. 34. 35. 36. 37. 38. 39. 40. 41.

(1981). M. Takatani, R. Hamada, and H. Sasaki. M o k l r x i Gtrkkaishi, 3 0 : 130 ( 1984). H. Chai, ASTM STP 893. 209 ( 1986). H. Chai. / t i t . ./. Frrrcmrc., 3 7 137 ( 1988). M. B. Ouezdou and A. Chudnovsky. J . A d 1 ~ 4 0 r l25: . I69 ( 1988). K. M. Liechti and T. Freda. J . At/hr.siotl. 28: I45 ( I 989). T. Kobayashi, Y. Hatano, and H. Mizumachi, Mok~r:cri Gtrkkrri.shi, 37: 331 (1991). W. W. Lirn, Y. Hatano. and H. Mizumachi. J. App/. P o / w w r Sci., 52: 967 (1994). W. W. Lim and H. Mizumachi, J. AppL Po!\rtrc>r Sci.. 57: 55 (1995). W. W. Lim. Ph.D. thesis. The University of Tokyo. 1995. M. Takayanagi. Kohtcrd1i, 3 1 : 142 (1982).

Pressure-Sensitive Adhesives and Forest Products Hiroshi Mizumachi The University of Tokyo, Tokyo, lapan

1.

INTRODUCTION

Because pressure-sensitive adhesives (PSA) are used to bond one material to another, they can be regarded as a kind of adhesive, but in industry they are treated substantially as if they were materials which are different from the so-called adhesives other than PSA. The industrial societies are organized separately, and various statistical data are collected separately. If we look at the situation in detail, we come to know that there surely are some differences between PSA and other adhesives. First, other adhesives are converted from liquid state to solid state after they are applied on the surface of the adherends, but PSA is never hardened. Second, other adhesives are usually supplied to the consumer as liquids or solids (hot-melt adhesives), but PSA is supplied as a PSA product in which PSA is coated on some film material. Usually, PSA itself, eitherasa solid or a liquid, is not available commercially. The most abundant PSA products are PSA tapes, which are used in the fields of packaging, office uses, electronics, vehicles, buildings, medicine, etc. The next abundant PSA products are PSA labels or decals, for which no explanation will be needed about the labels. Decals are sometimes called “sticking paints,” meaning that the necessary information is printed on a film and PSA is coated on the back side of the print so that one can stick them anywhere one likes. Decals can be conveniently used instead of coatings. (For example, in many cases decals are applied on the outsides of cars and trains or on the walls and windows of buildings.) Many kinds of PSA products for medical uses have been developed recently. The industrial scale of PSA-related fields in Japan in 1990 is shown in Fig. 1 [ 1,2]. The total scale of the PSA industry was 452,000 million Y (about $3,480 million U S . ) . On the other hand, the total industrial scale of adhesives other than PSA in the same year was 246,000 million Y (about $1,900 million U.S.), according to the statistics of the Industrial Society of Adhesives in Japan. Scale ofthe former approximately doubles that of the latter. The amount of polymers used as PSA is about 10%ofthatused as adhesives other than PSA, but nevertheless PSA has such a large industrial scale not simply because it includes the cost of various substrates such as papers, fabrics, plastic films, metal foils, etc., but because PSA has a tremendous function of combining chemical components of PSA and substrates. Values of polymers in PSA are added greatly. 765

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FIGURE 1 Market share of products in Japan in 1990. (a) Area of PSA products (%). Total area is about 2,268 million m’. (b) Amount of money (%). Total money is $3,480 million U.S.

There are several kinds of PSA, such as rubber-based PSA, hot-melt PSA, acrylic PSA, silicone-based PSA, and others. Rubber-based PSA includes natural rubber and synthetic rubber, but the former is much more used than the latter. Because rubber alone is not sticky enough as PSA, we have to blend some tackifier resins with the rubber. It must be pointed out that solvents must be used in the manufacturing process of the rubberbased PSA. Polymers mostly used in hot-melt PSA are SIS, SBS (block co-polymers of styrene-isoprene-styrene or styrene-butadiene-styrene) and SEBS (hydrogenated SBS). Tackifier resins must be blended with these block co-polymers, too. No solvent is needed for these co-polymers either in the blending process or in the coating process. This is why these systems are called hot-melt PSA, and this is a great advantage which this type of PSA has. There are many kinds of acrylic PSA because they are produced by combining various acrylic co-monomers (acrylates and/or methacrylates), some of which are made by solution polymerization and others by emulsion polymerization. Everybody thinks that any industrial products must be environmentally friendly, and therefore in the field of PSA, people have tried to reduce solvent-based materials as much as possible, which is the reason why the production of water-based (emulsion) PSA is gradually increasing. Tackifier resins used not to be blended with acrylic co-polymers because it is quite easy to make polymers with any T, and modulus by controlling the kind and composition of co-monomers. Recently, however,there have been many cases where tackifier resins have been introduced in this type of PSA for the purpose of modifying the adhesion properties against polyolefins. Silicone-based PSA is a blend of silicone rubbers and silicone resins. Because surface tension of this series of polymers is low,they easily stick to Teflon, polyimides, silicone rubbers, etc., which are known to be inert to most adhesives. At the same time, this type of PSA is resistant to heat, chemicals, weathering, etc. All these blends are coated on papers, cellophane, fabrics, plastic film, or metal foil to produce PSAtapes, labels, or decals, and theyare abundantly used in industry, offices, in and in homes.

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II. PSAAND FOREST PRODUCTS Figure 2 [1,2] shows the market shares of both PSA products (substrate plus PSA) and PSA itself in 1990 in Japan. Natural rubber, which is a forest product, has the biggest share: 43.3% by area of PSA products (tapes and labels) and 53.3% by weight of PSA itself. SBR (random co-polymers of styrene and butadiene), which is the most popular synthetic rubber, is used at about 10% of natural rubber as PSA. Block co-polymers such as SIS, SBS, and SEBS, which are used as hot-melt PSA, are sometimes called thermoplastic rubbers, and there are cases where they are classified as one of the rubber-based PSAs. Anyway, the amount of these block co-polymers used as PSA is much lower than that of natural rubber. Recently, the legal regulations on environmental problems have become more andmore severe, and manyresearchers have been seriously trying to develop some production systems where solvents are not needed. However, according to the anticipation by PSA specialists [3], natural rubber will continue to be used for some period of time, in spite of the fact that we cannot avoid the solvent problem in this system. This may be due not only to the low cost, but to the tremendous accumulation oftechnological data, and also to the fact that this type of PSA has delicate performance which the other PSA do not have. It must be pointed out that there is a great difference between the solvent problem in PSA and that in other adhesives or coatings. In the case of adhesives (other than PSA) or coatings, solvent will vaporize into the air anywhere they are used. On the other hand, in the case of PSA, solvent is used within the plant, andwhen the PSA products are transferred to consumers, the solvent problem does not occur at all. If the solvent is

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recovered effectively within the plant as is the case in most of the PSA industry, there is almost no problem. Nevertheless, much effort has been concentrated to developsome manufacturing processes without any solvents, such as emulsion techniques, hot-melt techniques, radiation-cure techniques, and others. As mentioned earlier, the modulus and viscosity of rubber itself are usually too high for PSA, and we have to add some tackifier resins to the system. The most popular tackifier resins which have been used in PSA are rosins and terpene resins. Rosin is harvested from pine trees, and its main component is abietic acid. Terpene resins are polymers of CYpinene, P-pinene, dipentene, etc. Molecular weight of the resins is around 1000. There are many kinds of tackifier resins because both rosin and polyterpenes are chemically modified in various ways. They are hydrogenated, dehydrogenated, dimerized, or polymerized further, esterified with glycol, glycerol, or pentaerythritol. In some cases, they are co-polymerized with phenolic compounds. Most of the resins are solid, although there are some liquid tackifiers. The solids are very brittle, and if mechanical shock is given to them by a hammer, they are tinely divided into very small fragments or powders, but if we blend the resins with natural rubber, the viscosity of the system becomes extremely low, as shown in Fig. 3 [4]. Thisseems to be an anomalousphenomenon. If molecules of natural rubber and tackifier resins share the free volumes within the material, then the viscosity of the blend must vary monotonically with the blend ratio [ 5 ] .How to analyze this phenomenon on a scientific basis is a great problem which has not been

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TABLE 1 Tackifier Resins I. Resin made from forest products A. Polar compounds 1. Rosin or rosin derivatives (a) Rosin: gum rosin, tall oil resin, wood rosin (b) Modified rosin: hydrogenated rosin, disproportionated rosin, polymerized rosin (c) Rosin ester: rosins or modified rosins esterified with glycol, glycerol, or pentaerythritol 2. Terpene-phenol resin B. Nonpolar compounds 1. Terpene resin: a-pinene resin, ppinene resin, dipentene resin 2. Terpene resin modified by hydrocarbon II. Resin made from petroleum, etc. A. Polymerization resin 1. Petroleum resin: aliphatic resin, cycloaliphatic resin, aromatic resin 2. Cumarone-indene resin 3. Styrenic resin: styrene resin, substituted styrene resin B. Condensation resin 1. Phenolic resin: alkyl phenolic resin, rosin-modified resin 2. Xylene resin

solved yet. Practically,PSA specialists are interested in the blend ratio where the viscosity and modulus are extremely low. There are tackifier resins -which are synthesized from petroleum, besides those which come from forest resources, and all of themare competing in the industrial markets. Table 1 [6] shows the classification of the tackifier resins. Thus, the fact that not only natural rubber, but also such tackifier resins as rosin derivatives and polyterpene derivatives, which originate from forest products, are used abundantly in industry will be attracting the interest of many forest products researchers. In addition, it must be pointed out that a large amount of kraft paper is used as backing material for PSA, as shown in Fig. 4. For example, OPP (oriented polypropylene) film is mostly used inthe United States as the backing material for packaging tapes, but inJapan, kraft paper is used in the same area as the majority of all the film materials. ,Others

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Of course, such plastic films as polyethylene terephthalate (PET), polyvinyl chloride, etc., are used to a great extent in areas other than packaging.

111.

PRACTICAL PERFORMANCE OF PSA-THREE FUNDAMENTAL PSA PERFORMANCE CHARACTERISTICS

PSA tapes stick easily to any material upon light touch, which is the most important property of PSA. They are not usually expected to show strong adhesive strength, contrary to the case of structural adhesives. In some applications low adhesive strength is a great merit of the products. On the other hand, there are some PSA specialists who are trying to develop PSA products which stick to various adherends like ordinary PSA does, and at the same time can be used in some structural applications. So, there are tremendous numbers of PSA products which are commercially available now. The practically important properties of PSA are sometimes called “the three fundamental PSA performances.” Testing methods are standardized by the JIS (Japan Industrial Standards), ASTM (American Society for Testing Materials), PSTC (Pressure Sensitive Tape Council), and others. Some of the typical testing methods are shown in Figs. 5-7 [ 6 ] .

A.

Adhesion

“Adhesion” expresses a degree of adhesion of PSA under a normal condition, and it is measured by 180” peel test after PSA tape is thoroughly adhered on an adherend. Theoretical approaches have been tried by many PSA researchers [ 7 ] .

B. Tack “Tack” means the instantaneous adhesion of PSA. There are three types of tack test. The first one is called the rolling-ball method, where steel balls are rolled on PSA, and a rollout distance or some related quantities are regarded as a measure of tack of PSA. Mizumachi et al. [8-191 analyzed the rolling motion of a ball on PSA according to the equation of motion of a solid ball, and proposed that the rolling friction coefficient f of PSA must be

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taken as a measure of tack. The second method is called the “quick stick” or “loop tack” method, where a looped tape with the sticky surface facing outside is hung in a tube, and upon light touch on an adherend it is pulled away. The measured resistance is a measure of tack. The last test is the “probe tack” test. This is a simulation of the finger test. An end surface of a cylinder touches the PSA surface and moves away. The resistance is regarded as the probe tack of the PSA tape.

C . Cohesion or Holding Power The upper part of a strip of PSA tape is adhered on an upright adherend surface, and a specified load is applied on the lower unbonded part of the tape. Then the PSA tape bears a constant shear stress U(,, and the tape finally falls down in time r),. This is called the holding power. Of course, r , is dependent on such factors as the dimensions of the bonded part and load, which is why they are specified in the standards mentioned above. For example, when we close a corrugated paper box using PSA tape for packaging, the tendency of the box to open is suppressed by the tape, which means that the PSA layer of the tape is continuously bearing the shear stress. It is a very serious problem to know to what extent of stress and how long the PSA tape can resist the shear. This is why this type of testing has been standardized. The three fundamental PSA performances thus obtained depend on the temperature and time scale of the measurements (i.e., contact time, rate of separation, etc.), and therefore the conditions of measurements are specified in detail by some standards, as mentioned above. Because PSAs are viscoelastic materials, the PSA performances are a function of the physical properties of PSA, such as phase structure, viscoelastic properties, T,, viscosity, molecular weight, and so on. Figure 8 [20] shows the dependence of the three fundamental PSA performances on T, of the PSA. Similar empirical relations have been found for other factors as well. PSA specialists are trying to develop a variety of PSA products by taking

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FIGURE 7 Polykenprobetack test: B. backing: A, pressure sensitiveadhesive: W. weight; P, probe: C. carrier; I, insulation: E, electric contacts; CC, collect chuck (probe holder);G, force gauge; D, dashpot; L. lead screw; CL, clutches; T, transmission (multispeed); H, motor; TC. timer and controls.

into account the balance of the three performances which willbe appropriate for the specific applications. For example, in the case of memorandum notes which stick on anything, the necessary conditions are that some degree of tack is needed and at the same time the adhesive strength (peel strength) must not be high so as not to withdraw any fibers from the paper to which it sticks. Similarly, in the case of bandages where human skins are expected to be the adherends, the peel strength must be controlled at a relatively low level in order to avoid damaging the skin. On the other hand, in the case of packaging tape or industrial tapes for semistructural uses (e.g., VHB), both peel strength and holding power must be especially great. If the practical requirements for a variety of applications are clarified quantitatively, we can control the appropriate levels of the three PSA performances by

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choosing chemical structures, composition, molecular weight, and tackifier resins and their concentrations. There are many empirical guidelines which have been proposed by some well-known specialists [2 l]. However, if we want to understand the phenomena related to PSA on a scientific basis, we are usually confronted with some difficulties because the empirical rules of PSA are expressed in terms of the PSA performances measured within a very narrow range of experimental conditions (temperature, time scale, stress level, etc.), and we cannot analyze them according to the principles of physics, physicochemistry, surface chemistry, and/or rheology. Experimentally evaluated quantities of PSA performances vary systematically in accordance with the change of the experimental condition, and it is recommended that the PSA performances be measured over a wide range of experimental conditions, and the results compared with the physical properties of PSA such as viscoelastic properties, or other structure/property of the materials. Accumulation of such data will result in clarification of the mechanisms of PSA phenomena.

IV.

COMPARISON BETWEEN ACRYLIC PSA AND NATURAL RUBBER-BASED PSA

The PSA performances depend greatly on the viscoelastic properties of the PSA, which may suggest in turn that PSA of the same viscoelastic properties will have the same PSA performances, but actually the PSA performances are delicately different from one PSA to another. Here, the rheological aspects of the PSA performances of acrylic PSA are compared with those of natural rubber-based PSA.

A.

Viscoelastic Properties [22]

Figure 9 shows the temperature dependence of the Young’s modulus of a series of acrylic polymers. The shape of the curve is almost the same for any acrylic polymer, and if the

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FIGURE 9 Young’s modulus of acrylic polymers: 4, polymethylacrylate; 0, polyethylacrylate; H, polyisobutylacrylate; 0 , poly-n-butylacrylate; 0 , polyethylhexylacrylate.

curves are shifted along the temperature axis by the difference of T,, all the curves can be superimposed. Similar behavior is seen in a series of co-polymers. This means that the modulus at the rubbery plateau region is higher for a polymer of higher T,q. On the other hand, Figure I O shows similar plots for a natural rubber-based PSA which is a blend of natural rubber and glycerol ester of hydrogenated rosin. Crossing over

FIGURE 10 Young’s modulus of thenatural rubber-based PSA (blends of natural rubberand hydrogenated rosin esterified with glycerol). Content of tackifier resin: f , 0%; 0 , 20%; 0, 40%; 0 , 60%; A, 80%.

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of the curves is seen in the figure, i.e., the modulus at the plateau region is lower for higher T,. These trends are sometimes seen in natural rubber-resin systems.

B. Adhesion or Peel Strength [23,24] Figure I 1 shows peel strength P of an acrylic PSA (a co-polymer of butyl acrylate and acrylic acid, 90/10) as a function of rate of separation v. In the region of very low rate, P increases with increase of U , and cohesive failure is observed. At around a certain v, failure mode abruptly changes from cohesive to interfacial, and P decreases drastically. In the region of very high v, P becomes nearly constant (or increases very slightly), and the value of P there depends on the critical surface tension yc of the adherends. Curves in the figure shift along the log v axis toward the higher-v side as the relaxation time of the PSA becomes shorter (or as the temperature is elevated, or as T, of the PSA is lowered). Figure 12 shows similar plots for natural rubber-based PSA. It is evident that the shape of the curves is quite different from that in Fig. 11. However, there are some similarities: P at very high v depends on yc.of the adherends, and the curve shifts along the rate axis according to the change of relaxation time of the PSA.

C. Tack

or Rolling Friction Coefficient [23,24]

Mizumachi et al. proposed that the rolling friction coefficient f of PSA must be regarded as a measure of tack of the PSA and can be evaluated by a simple method of pulling a cylinder on PSA. Figure 13 shows the plots off of an acrylic PSA (a co-polymer of butyl acrylate and acrylic acid, 90/10) against rate of pulling v. Curves of the plots of P against log U are similar in shape to those o f f against log v. In the region of very low rate, f is an increasing function of U , and cohesive failure is observed there. After v reaches a certain level, failure mode changes from cohesive to interfacial, and f decreases drastically. The curves shift toward the rate axis according to the change of the relaxation time of the PSA.

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There is a remarkable difference between f and P in the region of extremely high U . It is evident that j ' decreases to almost zero at high U , whereas P becomes constant or slightly increases in the same region. Mizumachi et al. tried to analyze these phenomena by assuming that f is related to both the bonding process and the debonding process at the same time, and that P is related to the debonding process only. Figure 14 shows a plot off against log 71 for natural rubber-based PSA. Here some differences are found in the curves of the two PSA series, and specific dependence on yc is seen for the two systems. It is true that practical performances of any PSA system are dependent upon structure and

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D.

Holding Power or Shear Creep Resistance [23,24]

A strip of PSA tape is adhered on a vertical adherend surface, and a constant load is applied on the end of the tape. Then a bonded part of the tape resists a constant shear stress CT[, due to the load, and at last the tape falls off after a time tl, has passed. Of course, the larger the stress U[), the smaller the time to break l!,,or vice versa. A plot of LT,, against log t,, gives a descending curve. If these curves are obtained at different temperatures, we can construct a master curve according to the time-temperature superposition principle. Figure 15 shows some examples of the master curve for acrylic PSA. It is evident that the higher the T q of acrylic co-polymer, the more the curve shifts toward longer time scale. Figure 16 shows similar curves for natural rubber-based PSA, where the opposite trend is seen: the higher the T, FIGURE 3 Relationship between antiswelling efficiency (ASE) and weightpercent gain (WPG) in various wood-inorganic composites. (From Ref. 6.)

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dimensional stabilization can be achieved if the SiO, gels are bound directly with cell wall components through isocyanate- or epoxy-type silane coupling agents [7]. On the other hand, for wood-inorganic composites with TiO,, AI2O3,and ZrO, gels (type 11 or 111), in which inorganic substances are present in the cell cavities, dimensional stabilization cannot be achieved. Interestingly, however, TiO, wood-inorganic composites from titanium chelates (type I) as mentioned above revealed an improvement of dimensional stabilization [lo].

2. Fire-RetardantProperties Figure 4 shows the combustibility test after 45-S ignition of model houses made from 2mm-thick veneers of hinoki (Charnaecyparis obfusa Endl.). It is apparent that the SiO, wood-inorganic composites (right) show much higher resistance to burning, compared with the untreated wood (left). Figures 5a-5c show micrographs obtained by scanning electron microscope (SEM) of the transverse surfaces after the combustibility test [4].It should be noted that the untreated wood (a) has thin cell walls, whereas thecomposites prepared fromthe moistureconditioned specimens with 9.5 WPG (b) retain rather thick cell walls by the deposition of SiO, gels and carbonizationof the cell walls. This apparently shows that a small amount of SiO, gel formed within the cell walls can enhance fire resistance. On the other hand, the composites with 40 WPG prepared with water-saturated specimens (c) cannot protect the cell walls and the cell walls are as thin as in the untreated wood (a).

3. TermiteDeterioration Figure 6 shows the results of termite tests with Reticulitermes sperafus Kolbe. On the relationship between the number of dead termites and the test periods [4,6]. Compared with untreated specimens, the composites from water-saturated specimens with 70 WPG (type IV) show significant resistance to termite attack, due perhaps to the blocking effects of the gels. However, the composites prepared from moisture-conditioned specimens (type I) also reveal termite resistance with only 10 WPG. It is therefore noteworthy that a small amount of SiO, gel formed within the cell walls is as effective against termite attack as gels formed in the cell cavities.

FIGURE 4 Comparison of combustibility tests of model houses made from untreated (left) and SiO, composite wood veneer (right) after 45-S of ignition.

Saka

FIGURE 5 SEM micrographs of wood-inorganic composites after combustibility tests: (a) untreated wood; (b) composites with Si02 gels in the cell walls (9.5 WPG); (c) composites with Si02 gels in the cell cavities (40WPG); (d) composites with Si0,-P,O, gels (16.9 WC);(e) composites with Si02-B20, gels (38.7WPG); (f) composites with SiO2-P2O5-B2O3 gels (34.3WPG). (From Refs. 4.8.)

100 W

a,

c, .l4

E 8

rd

8

50

a %l

0

35

= o 0

10

20

30

40

Test Period (days) FIGURE 6 Relationship between the number of dead termites and the test periods for various wood-inorganic composites: 0 SiOz composites (type W ) ;0 SiO, composites (type I); 0 TiO, composites (type 11); ZrO, composites (type 111); A A1,0, composites (type 111); A untreated wood. (From Refs. 4,6.)

787

Wood-Inorganic Composites

WaterRepellency Figure 7 shows the change of water absorption ratio (WAR) in Si02 wood-inorganic composites prepared with and without a water-repellent agent as a property enhancer. As SiO, wood-inorganic composites were prepared in the reaction medium of tetraethoxysilane (TEOS)/ethanol(EtOH)/acetic acid (catalyst) with a molar ratio of 1 :1:0.01, the water-repellent agent silicon alkoxide with a long-chain alkyl residue or perfluoroalkyl residue was added to the reaction medium in a molar ratio of 0.0 I . These water-repellent agents are, for example, DTMOS [decyltrimethoxysilane, CH,(CH,),)Si(OCH,),] and HFOETMOS 12-heptadecafluorooctylethyltrimethoxysilane;CF,(CF,),CH2CH2Si(OCH3),],and have a large molecular weight so that they are mainly distributed over the surfaces of the cell cavities, as observed by SEM-EDXA study. However, due to the trimethoxysilyl residue present in the water-repellent agent, it is bound with SiO, gels formed within the cell walls as described below and thus fixed in the wood cells with low-surface-energy residues (R') exposed over the surface of the cell cavities.

4.

l SiO,

\

+

OH

\

H-0-Si-R'

\

Y

- ( CH,) ,CH,

R ;

or

-CH,CH, (CF, ) ,CF,

Compared with SiO, composites and untreated wood, composites with a water-repellent agent revealed lower WARin Fig. 7, indicating a water-repellent property added to the composites [ S ] .

5. Antibacterial Properties Creosote and chromated copper arsenate (CCA) are widely used as preservatives for antibacterial treatment of wood. In spite of their excellence in this property, they have some

Untrealed

TFPTMOS-SiO,

HFOETMOS-SiO,

1

0

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

5 10 15 Test period ( days )

FIGURE 7 Change of waterabsorptionratio in Si02 wood-inorganic composites prepared with and without a water-rcpellentagent:TFPTMOS, 3,3,3-trifluoropropyItrimethoxysilane;DTMOS, decyltrimethoxysilane: HFOETMOS. 2-heptadecaHuorooctylcthyltrin~ethoxysilane. (From Rcf. 9.)

Saka

788

drawbacks in terms of toxicity. Therefore, less toxic and environmentally acceptable chemicals are expected to be used. Quarternary alkylammonium salts are among the candidates for the antimicrobial treatment ofwood.Inthisstudyofwood-inorganic composites, trimethoxysilylpropyldimethyloctadecylammonium chloride (TMSAC), shown below, was used as a property enhancer to add an antibacterial property to wood [ll].

L As in water-repellent agents, TMSAC can be expected to be bound with SiO, gels and fixed in the wood cells, with quarternary alkylammonium salt residue exposed over the surface of the cell cavities. Therefore, TMSAC with a molar ratio of 0.005 was added to the reaction medium of TEOSEtOWacetic acid (molar ratio 1: 1:O.Ol).It is quite apparent in Fig. 8 that TMSAC-added SiO, composites (TMSAC-Si02) reveal a significant resistance against attack by white-rot fungi [Coriolus versicolor (L.ex Fr.) Quell. It should be noted further that TMSAC-Si0, composites are more resistant than TMSAC wood. This result canbeexplained by Fig. 9, inwhichTMSAC-SiO, composites are more water repellent compared with TMSAC wood, due perhaps to more uniformly distributed TMSAC with its long alkyl residue, chemically bound with Si02 gels in wood [l l]. In summary, if the composites have inorganic gels distributed selectively within the cell walls, as in type I, effectivelyenhanced properties can be achieved.However,in composites of type 11 or 111, improvement of the properties cannot be expected. In types IV and V, property improvement can be expected to some extent, but the porous structure characteristic of wood would be diminished. The results obtained therefore indicate that it is more effective for enhancement of the wood properties to incorporate inorganic substances into wood in neighbor wood cell wall components rather than to deposite them in the cell cavities, far from the cell wall components. For the property enhancers of silicon alkoxide with hydrophobic residue, such as a long-chain alkyl or perfluoroalkyl residue, a water-repellent property can be added to woodby covering the surfaces of the cell cavities. Similarly, property enhancers with a quarternary alkylammonium salt residue can add an antibacterial property to wood. Therefore, it may be concluded that the topochem-

FIGURE8 Comparisons of fungal attack by whlte-rot fungi:SiOz composites (6.7 WPG); TMSAC wood (0.7 WPG); TMSAC-SiO, composites (4.1 WPG). (From Ref. 11.)

789

Wood-Inorganic Composites

1.o

l

~

I

~~

Untreated *

x

v

,a

E 0

Y

.m

e 0.5

c)

2

0

I

I

5

10

15

Test periods ( days ) FIGURE 9 Changes of water absorption ratio for various composites: SiO, composites (6.7 WPG); TMSAC wood (0.7 WPG); TMSAC-SiO, composites (4.1 WPG). (From Ref. 1 I .) ical effects of inorganic substances and property-enhancers exist for wood property enhancement in wood-inorganic composites.

IV.

MULTICOMPONENTWOOD-INORGANICCOMPOSITES

Through a study of monocomponent wood-inorganic composites, S i 0 2 composites have been found to be harmless and environmentally friendly. Additionally, composites prepared from moisture-conditioned wood specimens have porous structures characteristic of untreated wood. To improve the properties of these SiO, composites further, a study of multicomponent wood-inorganic composites has been made with two or more kinds of metal alkoxides.

A.

Si0,-P,O,-B,O,

Wood-InorganicComposites

The preparation of Si02-P,0s, Si02-B203, and SiOZ-Pz0,-B2O3 wood-inorganic composites is based on the reaction medium of TEOS/EtOH/acetic acid (molar ratio 1: 1:0.01) with trimethylphosphite (TMP) and/or trimethylborate (TMB) with a molar ratio of 0.05 181. Figure I O shows the results of thermogravirnetric (TG) analyses of the composites obtained. In the TG curve of the untreated wood (a), an abrupt decrease in its weight can be observed in a temperature range between 300 and 350°C due to the flaming. However, in the SiO, composites formed within the cell walls (8.4 WPG) (b),the flaming temperature was shifted higher with the higher residual weight, compared with untreated wood. The difference between (a) and (b) in the TG curves would have reflected upon the differences observed in Figs. 4 and 5. On the other hand, binary and ternary composites (c, d, and e)

790

Sa ka

-

Flaming.

100-

.

Glowing

A

x

v

c,

-c

M

.F-

a

M

2

50-

2

-...---

v1

a,

_._-__ e

d "." C _______.__ -. b

CT

. " 4 2 " " -

-0-1

0

I

l

I

100

200

300

I

1

500 Temperature("C) 400

""""

a

I

l

I

600

700

800

FIGURE 10 Thermogravimetricanalyses of variouswood-inorganiccomposites: (a) untreated wood: (b) SiOzcomposites (8.4 WPG); (c) Si02-P20, composites (16.9 WPG); (d) SiO,-BzO2 composites (38.7 WPG); (e) Si02-Pz0,-Bz0, composites (34.3 WPG). (From Ref. 8.)

revealed fairly high residual weight for flaming, showing very high tire resistance. Furthermore, for glowing at a higher temperature over 350°C in (d), or over 300°C in (c) and (e), both binary and ternary composites show fairly high tire resistance. These composites have the highest WPG at 38.7, keeping the cell cavities nearly empty. Figure 11 shows comparisons of the combustibility tests among five plywood veneers made from untreated wood (left) and inorganic composites (right) [S]. After 30-S of ignition with a gas burner, the burner was removed and combustibility with the remaining flame was observed. Compared with SiOLcomposites, binary and ternary composites reveal stronger fire resistance. Furthermore, as shown in Figs. Sd-Sf, SEM micrographs of these carbonized composites after this combustibility test apparently show that the thinning of the cell walls is much less in these binary and ternary composites, compared with untreated wood (Fig. Sa). Figure 12 shows the results of differential thermal analyses (DTA) of those binary and ternary composites [S]. In the untreated wood (a), significant endothermic peaks corresponding to flaming and glowing can be observed. However, these peaks weaken i n the SiO, composites (b). and in binary and ternary composites they disappear and broaden to the higher temperature in SiO,-B,O, composites (d). In Si0,-P,O, (c) and Si02-P,0,B,O, composites (e), the endothermic peaks broaden to both higher and lower temperatures, revealing high resistance to combustion. For the tire retardance observed above, the mechanism of B,O, gels is different from that of P,O, gels. In Fig. 10, the flaming temperature is shifted lower for composites with P,O, gels, whereas for the composites with B,O, gels or SiO? composites, it is shifted higher. Thegels in the latter composites are believed to be melted during the flaming process and form a glassy layer to cover the cell wall components. As a result, fireretardant properties are added through the physical barrier against heat and oxygen. On the other hand, for composites with P,O, gels, chemical effect by dehydration with phosphorus has promoted the carbonization of the composites which have revealed fire-retardant properties [ IS]. Therefore, as seen in Fig. IO, ternary composites with Si0,-P,O,-

791

Wood-Inorganic Composites -

~-

~.

~

~.~

FIGURE 11 Comparisons of various wood-inorganic composites after combustibility tests: (left) untreated wood veneer; (right) wood-inorganiccomposites corresponding to those in Fig. 10. (From Ref. 8.)

I

0

I

100

I

I

I

I

1

200

300

400

500

600

l

700

l

800

Temperature ("C1 FIGURE 12 Differential thermal analyses of variouswood-inorganic composites: (a) untreated wood; (b) SiO, composites (8.4 WPG); (c) Si0,-P,O, composites (16.9 W C ) ; (d) Si0,-B,O, composites (38.7 WPG);(e) Si0,-PZ05-B,0, composites (34.3 WPG). (From Ref. 8.)

792

Saka

B20, gels would have high fire-retardant properties through effects of both a physical barrier and chemical reaction as mentioned above.

B. Leachability of Inorganic Substances For those wood-inorganic composites with fire-retardant properties, a leaching test for inorganic substances was made under the severe conditions of water stirred 160 times per minute in a 250-mL beaker [9]. The results obtained clearly indicated that the P20, and B,O, gels are leached out readily, but SiO, gels are stable in composites. To overcome this property, we tried to use the water-repellent agents studied in Fig. 7 with a molar ratio of 0.01 on the reaction system of TEOS/EtOH/acetic acid with TMP or/and TMB(molar ratio 1: 1:0.01 :O.OS:O.OS). The water-repellent agents used were DTMOS (decyltrimethoxysilane) and HFOETMOS (2-heptadecafluorooctylethyltrimethoxy silane), as already mentioned. Figure 13 shows one example of the results obtained, which clearly indicate that the leaching of P,O, gels is fairly prevented with an addition of such property enhancers, particularly HFOETMOS, and of course, fire-retardant property was maintained. Similar results were obtained with B 2 0 , gels 191. However, leachability was slightly higher than for P,O, gels, so some improvement would be necessary for permanent achievement of antileachability.

V.

WOOD-INORGANIC COMPOSITES WITH MULTICOMPONENT OLIGOMERS

In a practical sense, TMP needs to be handled cautiously because of its odor and toxicity, while BzO, gels from TMB are not stable and are readily leachable. Additionally, consid-

Treatment time (h) FIGURE 13 Leachability of P,O, gels in Si0,-P,O, compositcs and effects of the water-repellent agent added to composites on the prevention of leaching. (From Ref. 9.)

Wood-Inorganic Composites

793

ering operational and processing environments, methyltrimcthoxysilane (MTMOS) is more appropriate than TEOS because it is a safer agent, and its potential for displacement of TEOS has already been demonstrated to prepare SiO, wood-inorganic composites [ 121. Therefore, to pursue the development of “superwood” with topochemical effects, some silicon alkoxide oligomers with ethylphosphite and boric hydroxide residues were prepared from MTMOS, TMP and boric acid, and applied for the preparation of SiO2-P2O,-Bz0, wood-inorganic composites [ 131. The oligomers prepared are described below.

Me

I

OMe

I

Me

I

MeO-Si-0-Si-0-Si-0-Si-OMe I l I 0 Me OMe

0 I

I

Me

R = CH2CH3 or H

Through their evaluation, the composites had a fire resistance as high as the composites prepared from the reaction system TEOS/EtOH/acetic acid with TMP and TMB. Furthermore, leaching of the gels was prevented to ;I greater extent, due perhaps to chemical bonding of multicomponents in oligomer levels which could have stabilized inorganic substances in wood cell walls. Additionally, the oligomers prepared are nontoxic, so environmental safety in their preparation was achieved. By adding HFOETMOS in a small quantity as a property enhancer to the oligomer reaction system, the composites obtained could improve further the antileachability of PzO, and B,O, gels.

VI.

CONCLUDING REMARKS

To develop wood with high function and remarkable properties, we have tried to prepare inorganic composites of wood without losing characteristic properties of the wood as seen in its porous structure. As mentioned already, in spite of the same inorganic substances used, the observed properties are different if inorganic substances are distributed differently in the wood cells.This is because topochemical effects exist in wood for property enhancement.Our goal fordeveloping ideal “superwood” is to achievewood-inorganic composites from environmentally friendly materials with their minimal use and maximal effect on property enhancement. More extensive study of topochemistry in wood property enhancement will provide a clue to a development of “superwood.”

794 7. K. Ogisoand S. Saka. Mokuzcci Gakkaishi,40:1100 (1994). 8. H. Miyafuji and S. Saka, M o k l c x i Gakkaishi, 42:74 (1996). 9. S. Saka and F. Tanno. Mokuzni Gdknishi, 42:81 (1996). IO. H. Miyafuji and S. Saka, Wood Sci. 7 2 ~ . h r 1 o l .3/:449 , (1997). 1 1. F. Tanno, S. Saka, and K. Takabe, Mater: Sci. Res. h f . , 3: 137 (1997). 12. S. Saka and T. Ueno. Wood Sci. Techr~ol.. 31:4S7 (1997). 13. H. Miyafuji, S. Saka. and A. YaInamoto, Hol;for.sch. 52:410 (1998). 14. S. Sakka, Scicwcc. by Sol-Gd Process, Agune-shofusha, Tokyo, p. 8 (1988). IS. F. L. Browne, U.S. FPL Rep. 2136, U.S. Forest Service, Forest Products Lab. (1958).

Sa ka

21 Preservation of Wood Darrel D. Nicholas Mississippi State University, Mississippi State, Mississippi

1.

INTRODUCTION

A number of books and summaries that deal with wood preservation are currently available [ 1-51. Consequently, there is little need for another general review of this subject. However, in the past few years there have been significant changes in the wood preserving industry, so a review of these trends seems appropriate. Accordingly, in this chapter emphasis will be placed on new developments and trends in the wood preserving industry. New developments in wood preservation have been mainly in the area of new preservatives, so this chapter will focus heavily on these advances.

II. TREATMENT PROCESSES AND TECHNOLOGY The selection of wood preservatives, formulations, and treatment methods is dependent on the product and type of protection required. For example, control of sapstain and mold in green lumber is accomplished by dip- or spray-treating the wood with aqueous formulations. Since only short-term protection is required, this type of treatment is adequate. Millwork is also treated by the dip method, but somewhat higher preservative loadings are attained in dry, highly permeable wood species. Pressure processes are used for products which are used in adverse environments. The use of pressure/vacuum systems makes it possible to achieve good penetration and retention of the biocidcs, which are required for these products.

A.

PressureProcesses

The majority of wood products are treated by conventional pressure methods using either the full-cell or empty-cell process. The full-cell process uses an initial vacuum to evacuate air from the wood, followed by filling the cylinder with preservative solution under vacuum prior to the application of pressure. This process is generally used for water-borne preservatives, where maximum treating solution retention is desired. In recent years the lnoditied full-cell process has become increasingly popular. The same basic cycle is used with the exception that the initial vacuum is reduced by about 40-50% and a final vacuum is applied after the pressure period. Use of the modified full-cell treatmentprovidesa 795

Nicholas

796

means of reducing the final solution retention. which minimizes the dripping of preservative solution and subsequent holding time after treatment. Empty-cell processes (Lowry and Rueping) do not employ an initial vacuum and as a consequence result in much lower net preservative solution retentions. By applying some degree of initial air pressure and filling the cylinder at this pressure (Rueping process), the net solution retention can be reduced even further. These treating processes are generally used with oil-borne preservatives where it is desirable to minimize the amount of carrier oil used.

B. Vapor-PhaseProcess A major concern i n the treatment of many wood species is achieving adequate penetration of the preservative. One possible approach to this problem is to use vapor-phase treatments. The validity of this concept has been demonstrated in New Zealand, where they successfully treated various wood products with borates Barnes and Murphy [6]. In this process, trimethyl borate is vaporized by heating and then introduced into an evacuated cylinder containing wood at a low moisture content. The trimethyl borate rapidly diffuses into the wood and reacts with residual water to form boric acid in situ.

C. Supercritical-Fluid Process Another approach to the problem of poor preservative penetration is to use supercritical fluids as carrier solvents in the treating process. Supercritical fluids are capable of penetrating the small openings i n wood because they effectively eliminate the interfacial problems associated with conventional liquids. The feasibility of this process has been demonstrated in laboratory studies 171. However, a substantial amount of research willbe required before it can be determined whether this process has any commercial applications.

111.

PRESERVATIVES

Biodeterioration of wood products by microorganisms and insects is a major problem and results in millions of dollars lost annually. A substantial portion of this money could be saved if appropriate control methods were used. Some wood species are naturally resistant to biodegradation because they contain toxic heartwood extractive. However, as a result of considerable variation in durability among trees and a shortage of material, nondurable species are generally used for these applications. In order to attain a reasonable service life from nondurable woods used in exterior applications, they must be treated with wood preservatives. Currently, all commercial wood preservative formulations contain chemicals that are toxic to microorganisms and insects. These chemicals protect wood by preventing the attack of wood-decay fungi and in some cases insects. In order t o be commercially viable wood preservatives, biocide formulations must have the following characteristics: Cost effective Good permanence in the wood under use conditions No significant cff'cct on the strength properties of wood Low corrosivity to metal F'Istcners Good penetration properties Safe to handle and use

Preservation of Wood

797

Low mammalian toxicity N o detrimental effects on the environment

A.

Types of Preservatives

Wood preservatives are generally classified into two basic types-oil-borne and waterborne-which are distinguished by the type of carrier used to solubilize the biocides.

1. Oil-borne Preservatives Creosote and pentachlorophenol (penta) are the major wood preservatives currently being used. Another oil-borne biocide that has been used on a limited basis is tributylin oxide (TBTO). Abrief discussion of the general characteristics of these preservatives is presented below. a. Creosote. The use of creosote as a wood preservative was patented in 1838 by Bethell, and since that time creosote has remained an effective, widely used chemical for treating wood [S]. Creosote is a complex mixture containing at least 200 identifiable compounds. However, it is generally agreed that several thousand different compounds are present in very small amounts [91. The greater part of the composition of creosote consists of neutral fractions (Table l ) . Tar acids, such as phenol and the creosols,as well as such tar bases as pyridenes, quinolines, and acridines, constitute a rather small percentage of the total weight of creosote. Unlike the neutral fractions, the tar acids and bases are usually soluble in water and hence contribute very little to the efficacy of creosote as a wood preservative. It follows

TABLE l ChemicalComposition of a Typical Creosote Produced in the United States Compound or Component Naphthalene Methyl naphthalene Diphenyl dimethylnaphthalene Biphenyl Acenaphthene Dimethylnnphthalcne Diphcnyloxide Dibenzofuran Fluorene-related compounds Methyl Huorenes Phenanthrene Anthracene Carbazole Methylphcnanthrene Methyl anthracenes Fluoranthene Pyrene Benzofluorene Chrysene Other components not identified

U S . Creosote [34] 3.0 2.1

0.8 9.0 2.0 -

S.o 10.0 3.o 21.0 2.0 2.0 3.0 4.0 10.0 8.5 2.0 3.0

Nicholas

798

from the foregoing statements that the chemistry of creosote and that of the coal-tar neutral fractions are quite similar. So, for that matter, is the chemistry of the parent materialcoal tar. Compositional data for coke-oven coal tar produced in the United States is given in Table 2. The majority of the compounds in creosote are aromatic hydrocarbons with condensed ring systems. In addition, tar acids, which are heterocyclic compounds containing nitrogen plus some neutral oxygenated compounds, are present [ 101. The tar acids and bases contain a wide range of fungicidal constituents. The fungicidal activity of creosote fractions obtained by distillation varies widely. In this regard, Schulze and Becker [ 1 l ] investigated the fungicidal activity of I5 distillate fractions boiling between 120 and 360°C against three wood-decay fungi. They found that the toxicity of the different fractions varied widely, with those boiling between 180 and

TABLE 2 Chemical Composition of U.S. Coke-Oven Tars Component Water, '70 Carbon, o/o (on dry tar) Hydrogen, 96 (on dry tar) Sulfur, 95 (on dry tar) Nitrogen, C7c. (on dry tar) Ash, '3- (on dry tar) Toluene insolubles, c/c (on dry tar) Components wt, Oh (on dry tar) Benzene Toluene o-Xylene rtl-Xylene />-Xylene Ethylbenzene Styrene Phenol 0-Cresol m-Cresol p-Cresol Xylenols Higher-boiling tar acids Naphtha fraction (bp 150-200°C) l-Methyl naphthalene 2-Methyl naphthalene Acenaphthene Fluorenc Diphenylenr oxide Anthracene Phenanthrene Carbazole Tar bases Medium-soft pitch (70°C. R, and B softening pt.)

Wt (95) 2.2 91.3 5. I I .2 0.67 0.03 9.1 0.12 0.25 0.04 0.07 0.03 0.02 0.02 0.6 1 0.25 0.45 0.27 0.36 0.83 0.97 8.80 0.65 1.23 0.84 0.75 1.66 0.60 2.08 63.5

Preservation of Wood

799

240°C being the most active. Within these fractions, thionaphthene, 2-naphtho1, 2-methylnaphthalene, and isoquinoline were the most active compounds. However, it is likely that some of the components in creosote exhibit synergism, so the toxicity of individual components is probably not of major significance. In recent years attempts have been made to improve the surface characteristics of creosote-treated wood. Thisconcept was termed “clean”creosote andis achieved by reducing the xylene insolubles (XI) present in creosote solutions [ 121. By reducing the XI content to 0.1% or lower, a much cleaner product is produced. b. Pentachlor-ophenol (Petzta). Penta was first used asa wood preservative in the 1930s and rapidly became established as the most widely used single oil-soluble biocide in wood preservation. The performance of penta-treated wood for ground-contact applications is highly dependent on the carrier system used [ 13- 171. The effect of the carrier oil on performance has been attributed to the following factors: ( 1 ) its effect on penta depletion; (2) its effect on distribution of penta in the wood structure; and (3) its intrinsic biological activity [ 171. The fungicidal activity of the carriers varies considerably among the petroleum oils used for this purpose, with some oils demonstrating reasonably good performance in the soil block test [ 171. With regard to penta depletion from treated wood, there is considerable variation among the oils. Furthermore, there appears to be an inverse relationship between the penta depletion rate and performance of field stakes [ 171. Over the years, a number of different carriers other than petroleum oils have been used for penta treatments. These include transient light solvent systems-liquefied petroleum gas(LPG), methylene chloride, and mineral spirits-and water-based emulsions. The LPG (Cellon) and methylene chloride (Dow process) systems were used extensively for a number of years to treat utility poles. However, poor performance of the treated wood due to erratic penta distribution and environmental concerns resulted in abandonment of these processes. The water-borne emulsion penta system also failed due to poor performance of the treated wood products. Inadequate performance was probably due to excessive leaching of penta from treated products subjected to exterior exposure.Another water-borne system, the water-soluble sodium salt of penta, was used extensively for dip or spray treatment of green wood tocontrol stain and mold fungi. Because of environmental concerns this compound is no longer used for this application. c. Tributylin Oxide. Organotin compounds were shown to have fungicidal properties in the 1950s. Subsequently, laboratory and field tests demonstrated that tributylin oxide (TBTO) was the best wood preservative on the basis of cost, permanence, and mammalian toxicity [ 181. TBTO has been used extensively in Europe as a wood preservative for aboveground applications. It has also been used to a limited extent in the United States as a replacement for penta in millwork applications. However, recent studies have shown that this compound is not stable in contact with wood and gradually decomposes over a period of time [19]. As a consequence, TBTO is no longer used in the United States. In Europe, tributylin naphthenate has replaced TBTO and apparently is equally as effective and more stable.

2. Water-Borne Preservatives Chromated copper arsenate (CCA) and ammoniacal copper zinc arsenate (ACZA) are the major water-borne wood preservatives currently being used commercially in the United States. A brief description of the general characteristics of these preservatives is presented below.

Nicholas

800

(1. Chromlted Copper Arsenate. CCA is unquestionably the most important wood preservative in the United States, representing 78% of all preservatives used in 1993 [6]. Although CCA is water-soluble, it undergoes a series of complex fixation reactions in wood. These reactions involve both lignin and carbohydratecomplexesas well as inorganic precipitates (Fig. l ) . As a consequence of these reactions, CCA is highly fixed in wood and resists leaching even under severe exposure conditions. This interaction with wood results in a decrease in strength properties, with toughness being particularly sensitive to this treatment. However, these strength losses can be minimized to 10% or less by drying the treated wood at temperatures of 71°C or less 120-221. CCA is a very effective wood preservative andis used for numerous applications suchas lumber fordecks, utility poles, marine piling, etc. When the wood is properly treated. an extremely long service life can be obtained with these products. h. Anmoniacal Copper Zinc Arsenrrre. The useof ACZA is limited to the West Coast area, where it is used to treat Douglas fir and other local wood species. The alkaline solution provides better penetration of these relatively refractory wood species. ACZA is not as highly fixed as CCA, and the chemical reactions responsible for this insolubilization are not clearly understood. The main mechanism of fixation of copper and zinc is postulated to be the formation of insolublecopper arsenate and zinc arsenate. However, the overall mechanism is undoubtedly more complex because cuprammonium ions react by ion exchange with functional groups in wood 1231. In addition, both copper and zinc complexes can be formed with the wood substrate, but copper and zinc complex formation do not appear to be related.

3. New Wood PreservativeSystems In recent years considerable research has been directed at the development of new wood preservatives. This activity was stimulated by the actions of the U.S. Environmental Protection Agency, which questioned the environmental impact of the major wood preservatives used in the United States. As a result of this work, a number of biocides have been identified as potential new wood preservatives (Table 3 ) . The majority of these chemicals have been used for other applications in agriculture, paints, etc. Others, such as copper

CCA (Cu”, Crs+, As5’)

Cu2+,Cr3+,As5+

W + , CuCrO,,

l

l

Carbohydrate

Inorganic Precipitates

cu2+

CrAsO,

FIGURE 1 CCA reactions with wood.

Cr(OH1,. CrAsO,, Cr,(OH),CrO,, Cu(OH)CuAsO,

TABLE 3 Biocides withPotential a s Wood Preservatives Trade name

Chemical name 3-Iodo-2-propynyl butyl carbamate

Ti mbor2'In

Disodium octaborate tetrahydratc

Copper naphthcnate

Copper( 11) naphthenatc

Oxine copper/ copper-8

Copper-8-quinolinolate

structureChemical

H 9 l I -c=c-~-o--~-N - C * H ~

A

Na2B,0,,.4H20

4,S-Dichloro-2-,1-octyl-4isothiazolin-3-one

Busan 3 ( P / TCMTB

2-(Thiocyanomcthylthio) benzothiaxole

WocosenW propiconazole

(2RS, 4RS)-2-(2,4dichlorophenyl)-2-[ 1- 1H ( I .2,4-triazoIe)methyI1-4propyl- 1.3-dioxolanc

Tebuconazole

(3RS)-5-(4-~hlorophenyl)2. 2-dimethylethyl-3-1H [ I .2.4-triazole)methyl)-3pentanol

Amical 4 8 O

Diiodomethy-/,-tolysulfone

Chlorothalonill tuffgard@>B

2,4,5.6-Tetrachloroisophthalonitrile

Cl

I

OH C I O C H ~ - C H Z - C - C (I C H ~ ) ~ ?Hz

C=N

C=N

802

Nicholas

naphthenate and Cu-8, have been used to a limited extent as wood preservatives in the past but are now gaining more popularity because of their relatively low mammalian toxicity. A brief discussion of each of these biocides is presented below. N. Polyphusea (ZPBC). IPBC is an organic biocide that exhibits low mammalian toxicity and has broad-range activity against common wood decay, mold, and stain fungi, but is not effective against wood-destroying insects. It is currently being used to treat wood for above-ground applications for millwork and similar products. A combination of IPBC and DDAC is effective against mold and sapstain fungi and is used extensively for controlling these microorganisms in freshly sawn lumber. b . TinzborQ ( B o r d B o r i c Acid). T i m b o e is an inorganic biocide with boron being the active component. It hasa very low mammalian toxicity and exhibits broad-range activity against both wood-decay fungi and insects. It is highly soluble in water and readily diffuses in and out of wet wood. Consequently, its use as a wood preservative is limited to above-ground applications which are protected from the weather. It is currently being used to a limited extent in the United States for commercial products. c. Copper- Nuphthenate. Copper naphthenate is an organometallic compound that is normally prepared by the direct reaction of copper hydroxide with naphthenic acid at elevated temperatures in a hydrocarbon solvent. It exhibits low mammalian toxicity and has broad-range activity against wood-decay fungi and insects. Copper naphthenate is not a new wood preservative and has been used to a limited extent as a preservative for a variety of wood and textile products over the years. As a result of recent environmental concerns with the major wood preservatives, interest in copper naphthenate has increased. This led to its use by several companies as a preservative for utility poles. However, some utilities have experienced early failure of some of these poles, and this has resulted in curtailment in the use of copper naphthenate. The exact cause of these early failures is not known, but it appears that it may be due at least in part to inactivation of copper naphthenate when it is used to treat green poles that are steam-conditioned. In any event, this experience will undoubtedly result in limited use of this preservative in the future. d. Oxine Copper (Cu-S). Cu-8 is an organometallic compound formed by the reaction of copper with 8-quinolinol. It exhibits low mammalian toxicity and has broadrange activity against wood-decay fungi and insects. Cu-8 is very insoluble in water and most organic solvents, butan oil-soluble form can be made by reaction with 2-ethyl hexoate. A water-soluble form can be made with dodecylbenzene sulfonic acid, but this formulation is highly corrosive to metals. Cu-8 is not a new preservative and has been used to a limited extent as apreservative for a variety of wood and textile products over the years. It is currently the only preservative that is approved for treating wood that is in contact with foodstuffs. It isused primarily for treating specialty items such as food pallets and picnic tables. e. Burduc 2 2 a (DDAC). DDAC is an organic biocide that exhibits low mammalian toxicity and has broad-range activity against wood-decay fungi and insects. It is a watersoluble compound, but undergoes an ion-exchange reaction with wood which greatly reduces its leachability from wood exposed to water or wet soil. In order to improve the efficacy of DDAC as a wood preservative, it is generally combined with other biocides. In this regard, DDAC is currently used in two commercial wood preservative formulations.As mentioned previously, a formulation composed of DDAC and IPBC is widely used to control mold/sapstain in green wood. More recently, a combination of DDAC and ammoniacal copper (ACQ) has emerged as a commercially viable alternative to CCA for many applications.

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J: Kathon 9 3 0 a ( R H 287). RH 287 is an organic biocide which exhibits low mammalian toxicity and has broad-range activity against wood-decay fungi and insects. It is readily soluble in hydrocarbon solvents and is practically insoluble in water. This compound is not currently being used commercially as a wood preservative, but it has considerable future potential. g. Busan 30Q (TCMTB). TCMTB is an organic biocide which exhibits low mammalian toxicity and has broad-range activity against wood-decay fungi and insects. It is readily soluble in hydrocarbon solvents and is practically insoluble in water. A TCMTB formulation which contains methylene bis thiocyanate is currently being used commercially for sapstain/mold control in freshly sawn lumber. 11. Propiconcczole ( WocnsenQ). Propiconazole is an organic triazole biocide which has low mammalian toxicity and exhibits broad-range activity against wood-decay fungi, sapstain/mold fungi, and insects. It is readily soluble in organic solvents and exhibits very low solubility in water. Propiconazole is currently being used commercially for above-ground treatments and sapstain/mold control applications in Europe and Canada. i. Tebuconazole. Tebconazole is similar in structure to propiconizole and exhibits many of the same properties. It has a somewhat lower mammalian toxicity and exhibits good activity against wood-decay fungi. It has no commercial applications as a wood preservative at the present time. j . Arnica1 4 8 a . Amical 4 8 0 is an organic biocide that has extremely low mammalian toxicity and exhibits broad-range activity against wood-decay fungi and insects. It is readily soluble in a number of organic solvents and exhibits low solubility in water. Currently, it does not have any commercial applications as a wood preservative. k. Chlorothalonil (Nopcocidea, Tuffgarda). Chlorothalonil is an organic biocide which exhibits extremely low mammalian toxicity and has broad-range activity against wood-decay fungi and insects. It has limited solubility in organic solvents and very low solubility in water. Chlorothalonil is currently being used to a limited degree as an additive for mold control in CCA-treated wood. Because of its relatively low cost and good efficacy, it has considerable potential for both above-ground and ground-contact applications. Itis also used as a sapstain chemical as NexGenQ.

4. TrendsinWoodPreservativeDevelopment The current trend in the development of wood preservatives is to use biocide combinations. These combinations include both inorganic-organic and organic-organic binary mixtures. Examples of these are ammoniacal copper quat (ACQ), copper dimethyldithiocarbamate (CDDC), ammoniacal copper citrate (ACC), ammoniacal copper azole, DDAC-IPBC (NPl), chlorothalonil-chlorpyrfos,and DDAC-Na omadine. ACQ has two different formulations-ACQ B and ACQ D. The only difference in these formulations is the copper-complexing agent. Type B contains ammonia and type D contains ethanolamine. In both formulations, the active ingredients are copper and DDAC in a ratio of 2: 1 (Cu0:DDAC). This combination provides broad-range efficacy against wood-decay fungi and insects in both above-ground and ground-contact applications. CDDC is formulated with copper ethanolamine and sodium dimethyldithiocarbamate (SDDC). Since copper reacts rapidly with SDDC to form an insoluble complex, a twostep treating process is required with this preservative system. Accordingly, the wood is first treated with a copper ethanolamine solution in one treating cylinder and then moved

Nicholas

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to a second treating cylinder for treatment with SDDC.The reaction product is a 1.2 c0pper:dimethyldithiocarbamate chelate which is highly insoluble in water. This chelate appears to have reasonable broad-range efficacy against wood-decay fungi and insects in both above-ground and ground-contact applications. ACC is formulated with ammoniacal copper carbonate and citric acid, using a ratio of 1.6S:l (Cu0:citric acid). Although the AWPA has developed a Standard, the value of this wood preservative system is questionable because both soil block and field stake test data indicate that the treated wood is not resistant to copper-tolerant fungi. This weakness is not surprising, since it does not contain a co-biocide. NP- I Q is formulated with DDAC and IPBC at an 8.5: 1 ratio of DDAC:IPBC. NP-I is used mainly for sapstain and mold control in freshly sawn wood but may have potential for millwork and similar applications. Copper azole is formulated with a combination of copper (49%), boric acid (49%) and tebuconazole (2%), using ethanolamine as the complexing agent for copper. This system is still undergoing evaluation, but shows promise as a viable wood preservative for some applications. The trend toward the use of binary and tertiary biocide combinations in wood preservative formulations is expected to continue.This formulation strategy is particularly significant when the biocides exhibit synergism. Indeed, the development ofwood preservatives on the basis of synergistic mixtures is currently being explored and appears to have considerable potential [24].

5. Biocontrol Another approach to wood preservation is to use antagonistic microorganisms rather than toxic chemicals. The first report concerning the potential to this approach was by Richard and Bollen [2S], using Scytalidiun~sp. to inhibit Antrodicr cclrhonica in Douglas fir poles. The results of this study stimulated additional research on the validity of this method for wood preservation, and this led to the development of Binab AB@. This bioprotectant has been marketed in Europe a s a preservative for Scots pine [26,27]. Although some trials were very positive, there is concern about the long-term effectiveness of this formulation 127-291. Binab AB was also evaluated by Morrell and Sexton [30] as a possible preservative for Douglas fir and southern pine. Poor results were obtained in this latter study, and failure of the system was attributed to the fact that it did not control all the decay fungi that colonize these wood species. Consequently, it was concluded that biocontrol is not a currently viable method for preserving wood products against decay fungi. Although the possibility of using microorganisms for controlling wood-decay fungi is discouraging, this approach may have potential for controlling sapstain/mold fungi in freshly sawn wood products [2]. This particular application requires only short-term protection, so concern about long-term survival of the microorganisms is eliminated. Both bacteria [31] and fungi 1321 show promise for this application.

IV.

FUTUREDEVELOPMENTS

Environmental issues have been the major driving forces behind the development of new wood preservatives in recent years. This trend will undoubtedly continue into the foreseeable future because there currently is considerable concern about disposal of treated wood after it has ended its useful life. This is particularly true for CCA-treated wood, which is

Preservation of Wood

805

not amenable to disposal by incineration. This dilemma will encourage research to develop practical methods for removing CCA from treated wood so that it can be recycled. At the same time, efforts will continue toward the development of alternative wood preservatives that do not pose disposal problems for treated wood. The development of ACQ, which eliminated chromium and arsenic, was a step in the right direction, but the presence of copper complicates disposal problems. The major thrust in wood preservative development in the future will probably be based on total organic biocide systems. If the mammalian toxicity of these systems is low, then the environmental concerns will be effectively eliminated. Other, more specific, and possibly nonbiocidal,approaches to wood preservation may develop as we learn more about the complex reactions involved in the overall decay mechanisms utilized by fungi. The problem of poor weathering characteristics of treated wood, especially CCAtreated wood, will probably alsoreceiveconsiderable attention in the future. If a high degree of water repellency canbeimpartedtowood, it will minimize the weathering problem and also minimize the biocide levels required to inhibit biodeterioration. Significant advancements in this area will help pave the way for development of cost-effective organic wood preservative systems. With regard to treating processes, significant improvements have been minimal in the past, and this trend probably will continue. One of the main problems that needs to be addressed is providing adequate treatment of refractory heartwood. More research is needed in this area, and progress in solving this problem could be very rewarding. Continued progress with the supercritical-fluid treatment process could make a major contribution in this area [ 3 3 ] .Other developments, such as the shock-wave treating process and a compression/vibration pretreatment of the wood, may also prove to be effective methods for improving the treatability of refractory woods.

REFERENCES I.

2. 3. 4. 5. 6. 7.

X. 9.

IO.

11.

12. 13.

R. A. Eaton and M. D. C. Hale, Wood D e c q Pests trr~dProtectiorl. Chapman & Hall, London ( 199.3). R. A. Zabel and J. J. Morrell, Wood Microbiology: 1)ccay m l d I t s Prclvrltiorl, Academic Press, New York ( 1992). J. G. Wilkinson, Irldrtstrid Tirnher Preservntiorz, Associated Business Press, London ( 1979). D. D. Nicholas(ed.), Wood Deteriorntion clnd I t s P revention by Presenwtivo Trcyttnlerlts, Syracuse University Press, Syracuse, NY (1973). G.M. Hunt and G. A. Garrett, Wood Preservntiorz. 3rd ed., McGraw-Hill, New York (1967). H. M. Barnes and R. J. Murphy, Forest Prod. J . , 4.5(9):16 (1995). J. J. Morrell, K. L. Levien, E. S. Demessie, S. Kutnar, S. Smith, and H. M.Barnes, Proc. Curl. Wood Preservntiorl Assrl., 14:6 ( I 993). M. P. Levi, in Wood Drteriortrtiorl crntl I t s Prevelltion by Presenrrtive Trmtnlents, Vol. l , D e g r d r t i o n u ~ l dProtection of Wood (D. D. Nicholas, ed.), Syracuse University Press. Syrocuse, NY. p. I83 ( 1973). Anonymous, The Biologic and Economic Assessment of Pcntachlorophenol, Inorganic Arsenicals and Creosote, Vol. I. Wood Preservatives. USDA Tech. Bull. 16.58-1:28 (1981 ). W. H. Hartford, in Wood Deteriorrrtion and I t s Preverltior? by Pre.ser\*nti\v Tremmv1t.s. Vol. 11, Presenutivcv trrltl Prescwcrtive Systerus, Syracuse University Press,Syracuse, NY, p. I O ( 1973). B. Schulze and G. Becker, Holz$~r.scl~.,2 3 7 (1948). H. M. Barnes and L. L. Ingram, Jr.. Pmc. Am. Wood Pre.srl?.c.r.s’A.s.srl.,Y1:108 (1995). R. H. Baechler and H. G. Roth, Forest Prod. J., 12: 187 (1962).

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14. I. Hatfield and S. S. Sakornbut. Forest Prod J., 5:361 (1955). 15. F. J. Meyer and R. M. Gooch. Forest Prod. J., 6:117 (1956). 16. W. C. Kelso. E. A. Behr, and R. E. Hill, Forest Prod. J., 5369 (1955). 17. D. D. Nicholas, L. Sites, H. M. Barnes, and H. Ng, Proc. Am. Wood Pre.sen~ers'As.srr.,Y0:44 (1994). 18. T. Hof. J . Inst. Woocl Sci., 4(5)23:19 ( 1 969). 19. R. Meder and K. J. Archer, Holgorsch.,45(2):103 (1991). 20. J. E. Winandy, B. A. Bendsten. and R. S. Boone, Forest Prod. J.. 33(6):53 (1983). 2 1. H. M. Barnesand J. E. Winandy, The InterrrcttionnlResecrrch Group on Wood Preserwtion IRG/WP/3543 ( 1 989). 22. H. M. Barnes, J. E. Winandy, and P. H. Mitchell, Inst. Wood Sci., 11(6):222 (1990). 23. M. A. Hulme, Record Anrrucrl Convention British Wood Preservers' Assn., pp. 38 (1979). 24. T. P. Schultzand D. D. Nicholas, Proc., Wood Presenntiorr i n the '90s nnd Beyond, Forest Products Society, Madison, WI, p. I87 ( 1995). 25. J. L. Richard and W. B.Bollen. Cm. J. Botany, 46:643 (1967). 26. J. L. Richard, J . I n s t . Wood Sci., 7(4):6 (1976). 27. P. 1. Morris, D. J. Dickinson,and J. F. Levy, R e c ~ r dAnrr~ralConvention British Wood Preservers' Assn.. p. 42 (1984). 28. P. I . Morris and D. J. Dickinson, Tlw Intenrntiorrcrl Re.seorch Group o n Wood Pmservotion IKG/WP/IISO, Stockholm, Sweden (1981). 29. A. Bruce and B. King, Materictl tcncl Orgctnisrrren. 18(3):171 (1983). 30. J. J. Morrell and C. M. Sexton, Wood Fiber Sci.. 22:10 (1990). 31. R. K. Velicheti and J . J. Morrell, in Wood Presrrvcrtion i n the '90s crnd Beyond, Forest Products Society Proc. 7308, Madison, WI, p. 245 (1995). 32. S. C.Croan, Tlw Internntiorrcrl Resecrrch Group o n Wood Prrservution IRG/wP/96-10158 (1996). 33. J. J. Morrell and K. L. Levien, in Wood Preservcctiorr i n the '90s crnd Beyond, Forest Products Society Proc. 7308, Madison, W1 (1994). 34. L. F. Lorenzand L. R. Gjovik, Proc. A m Wood Presc.rvers'As.srr.. 6832 (1972).

Preservation of Waterlogged Wood David N.-S. Hon Clernson University, Clenlson, South Carolina

Hay surlk, a shattered visage lies . . . Nothing beside remnirls. Round the clecccy Of that colossal wreck, hourldless c m 1 hare, The lone c z r d level .sc~r~d.s stretch fiw clwcly.

“Ozymandias” ( 1817) Percy Bysshe Shelley

1.

INTRODUCTION

Wood is a versatile material which has been used since the dawn of civilization. At the basic level wood satisfied human beings’ needs or wants in shelter, defense, transport, and leisure. Archeologists have discovered, from time to time, weapons, domestic utensils, tools, building materials, and boats made of wood. It is a durable material under a benign environment.Since woodis acomposite biological material, it inevitably continues to deteriorate asa result of physical, chemical, mechanical, and biological processes. To prolong the service life of wood, many chemical treatments have been developed to protect it from attack by microorganisms such as bacteria and fungi, and insects such as termites (see Chapters 12 and 21). Surface treatments are used to protect wood against moisture and weathering (see Chapters 9 and 11). Since wood is one of the oldest materials used, many artifacts, from cradlesto coffins, which were used in the past have been discovered by archeologists. Thus, specific kinds of treatments have been developed to preserve waterlogged wood which has been stored under wet conditions such as burial in soil below the permanent water table, at the bottom of rivers or lakes, or i n a marine environment. More than 40% of all shipwreck losses in the Western Hemisphere have occurred due to ships wrecking in shallow water. Wood is a biological material; when it is submerged in the marine environment, it comes under immediate attack from the teredos, fungi, and different bacteria. The more wood items are exposed to salt water, the more they suffer and the more quickly some of them vanish. Oftentimes, wood-based materials, when buried deep under sediment, will suffer less, or not at all, and on occasion can be discovered in an excellent state of preservation. Sometimes, wood is completely preserved by being saturated by iron oxide while lying 807

808

Hon

close to iron objects under water, as they tend to become mineralized and hard in texture. Wood will also be preserved if it is saturated in fresh water. In 1997, Titanic was a popular movie which broke the box-office sale record for the century. The story was based on the S.S. Titanic, which went down in the Atlantic Ocean in April 19 12, eighty-eight years ago. On July 14, 1986, the sunken Tirctrlic was discovered, and more than 3000 artifacts have been lifted from the debris field. Contrary to many expectations, the deep ocean did not preserve the Titanic from decay. Organic materials such as wood, paper, and cloth all perished. The wood decking and furniture were nearly all gone. Media attention, in print and on television, was usually intense and at times frenzied, as anyone who remembers the discovery of the wreck of the Titanic in 1985 can attest. When the Titunic- broke the surface of the water after 87 years of marine burial, we can only imagine the depth of emotions experienced by those who were there. Hearts must have been bursting and tears flowing. The sight of the ship must have conjured up many different thoughts in the people watching: thoughts of its preciousness as an object of antiquity and the incredible tie it forms between the people of today and those 87 years ago; thoughts about the ship and its repository of information about past ship-building technology. To study shipwrecks is to study human history. To study wood in the past is also to study human activity. Such studies have brought to light cultures that existed long before written records, and have transformed featureless prehistory into a fascinating landscape of evolution, cultural change, and technological advance. They have delineated sequences of events in the past and have discovered and illustrated the course of human civilization. Because wood-based artifacts provide a rich and varied record of our early activities and technology, there is a great need to preserve them in the many forms that remain from ancient to modern times. It is not only because it will be interesting to future generations, but to use it to study the cultures, human behavior, and stratagems of intelligent human minds throughout history and to study the wood aging process itself. Cultural materials used by society exist in systemic context. Scientists and technologists study waterlogged woods, including shipwrecks, in archeological context in order to understand the systemic contexts of past societies. Preservation can be viewed as actions by a contemporary society to slow the rate of deterioration and destruction of cultural materials. The majority of waterlogged objects need to be preserved with special chemicals. If this is not done quickly and properly, the artifacts will rapidly deteriorate. The conservation and preservation of waterlogged wood tend to be expensive, require specialized knowledge and facilities, and be complex and time consuming. Preservation of waterlogged wood depends greatly on the condition of the wood and the conditions of the environment where the wood will be stored after preservation. Present techniques range from art to science. The history of preservation or conservation treatments is a story of success and failures [ l ] . Until recently, many techniques have been based on empirical approaches rather than hard scientific data. Because of the understanding of degradation mechanisms of wood and availability of new instrumentations [2], scientists are able to tackle preservation problems with more scientific approaches and thus acquire better results. For most waterlogged woods, preservation consists of both careful removal of water from the wood to minimize shrinkage, and introduction of a substance into the wood to improve its strength. Generally, the processes developed to preserve waterlogged woods are classified into two groups. One consists of dehydrating the wood first, before treating it with a consolidant. The process is commonly employed only on small objects, because careful removal of water from wood involves solvent exchange, a process which takes longer as the size of the object increases. The other group, which is employed with

Preservation of Waterlogged Wood

809

larger artifacts such as entire ships, uses injection of a consolidant first and dehydrating the artifact after treatment. These processes use water-soluble consolidants and introduce the consolidants into the wood through the time-consuming process of diffusion. In this chapter. many methods that have been developed to preserve waterlogged wood are reviewed. Additional information can be obtained from several excellent monographs [3S]. A case study of preservation of the historic gunboat U.S.S. Cairo is included.

II.

PROPERTIES OF WATERLOGGEDWOOD

As mentioned earlier, wood is a biological material that deteriorates in almost any environmental condition. If wood is kept in a very moist or wet environment, it will absorb water and eventually become waterlogged. Wood normally decays under combined biological and chemical attack when buried in the ground or submerged in water. Buried wood will generally not survive unless it becomes waterlogged to create the anaerobic conditions necessary for protection from decay fungi and inserts. Under this condition, wood is still vulnerable to chemical degradation and attack by anaerobic bacteria. In time, changes will occur in the wood structure which will affect its physical and chemical properties. These changes adversely affect the integrity of the wood. At the outset, the readily water-soluble extractives and mineral salts in the wood will diffuse into the surrounding medium, followed by readily hydrolyzed compounds such as the pectins and pentosans. Then a microbiological degradation of the more stable hemicelluloses and cellulose will follow. Finally, mainly lignin remains, which can also be decomposed slowly by microorganisms in the anaerobicenvironment. Both fungi and bacteria cause the microbiological breakdown of wood and both use extracellular enzymes to hydrolyze cellulose, hemicelluloses, and lignin. In the case of afungus breakdown, degradation spreads throughout the capillary system of the wood, at least as far as the extracellular enzymes can penetrate into this system. Because of the loss of polymeric components in the cell structure, voids increase throughout the cell and the wood becomes more porous and permeable to water. Hence, for highly waterlogged wood, it is not unusual to have a moisture content of over 800% (based on oven-dried weight). Thehygroscopicity of waterlogged wood tends to increase, and equilibrium moisture content values as much as twice those of recent wood have been found [6]. Because of increased hygroscopicity, deteriorated waterlogged wood also exhibits significant increase in shrinkage, reaching values of the order of 70% for volumetric shrinkage. The shrinkage in volume is found to be related linearly to maximum moisture content of wood 171. As long as the waterlogged wood is kept wet, it will retain its shape. If the wood is exposed to the air, it cannot redistribute internal moisture properly during drying due to various factors. The most important one is due to capillary tension collapse above the fiber saturation point in the deteriorated wood. Cell wall shrinkage occurs below the fiber saturation point because of desorption and resultant dimensional change [ 7 ] . Typically, moisture attacks the secondary cell wall, reducing the resistant strength and flexibility of the wood [g]. Hence, when the excess water evaporates, the resulting surface tension forces of the evaporating water cause the weakened cell walls to collapse. Such an action subsequently leads to significant warping, shrinkage, and cracking. Drying of highly deteriorated waterlogged wood therefore results in severe damage and distortion of artifacts. Shrinkage values of more than 30% in the tangential and more than 10% in the longitudinal directions have been recorded [6]. Moreover, because of the loss of major polymeric structural components in the cell, deteriorated waterlogged wood also shows

Hon

81 0

major losses in strength. To prevent shrinkage, stabilization treatments should be performed to control dimensional change as well as the strength and stiffness of the wood. The degradation of the wood also results in a decreased swelling capacity of the old waterlogged wood when compared to new wood. The amount of shrinkage upon drying occurring in old waterlogged wood appears to be correlated with the amount of degradation of the wood as reflected in its chemical composition. It should be borne in mind that wood can be preserved for long periods of time under anoxic, waterlogged conditions. Thus, ideally, no submerged sites should be excavated unless archeologists can guarantee a proper preservation method for the recovered artifacts. Or, after excavating and surveying, the waterlogged wood should be left or reburied and the site preserved in situ, rather than “collecting” items that often deteriorate in air. These simple approaches would allow us to preserve cultural heritage, without all of the expense of conservation risk to the archeological or waterlogged materials. To do this effectively we must be able to measure how stable the archeological material is within a site.

Ill. PRESERVATIONTREATMENTS Conservation treatments for waterlogged wood have been designed to prevent dramatic dimensional changes caused by cell collapse and cell wall shrinkage during drying. Wood artifacts to be conserved are usually physically weak and chemically very complex. The most active phase of a conservation treatment is in controlling the processes as the waterlogged wood is transferred from its deposition to its new “home.” Nonnally, one of the first steps that must be taken is to remove contaminants such as sea salt and iron sulfide which will be unstable in the new environment and damage the wood’s structure. Hence, the specimens must be first cleaned and kept wet after removal from the discovery site. If they are allowed to dry, they will shrink and split. Normally it is recommended to let wood soak for 1-12 months in fresh water, depending on the size of the artifact. Hardwoods can handle a mild solution ( 5 % ) of muriatic acid. In this case, no steel or iron should be involved, as muriatic acid will destroy these items. After muriatic acid is used, the wood must be soaked again in fresh water. This will help remove the smell of acid. Preservation of waterlogged archeological wood involves stabilization a s tosize, shape, and durability. The state of the wood surface as seen from an esthetic viewpoint is also taken into consideration in preservation, whereas questions of restoration, completion, and related aspects are considered outside of preservation. Most if not all processes developed to preserve waterlogged wood have as their objectives to achieve dimensional stabilization and improvement of wood strength. Wood dimensions can be stabilized by three different means or combination of these means. These are (1) reducing the hygroscopicity of the wood so that less water can be taken up, ( 2 ) forming crosslinks between cellulose chains so as to minimize separation of these units, and (3) depositing a bulking agent within the swollen wood so as to reduce shrinkage 191.

A.

On-Site Preservation

On-site preservation consists of preservation measures that are employed at the excavation site. Immediate treatments are usually aimed at preventing shrinkage and further deterioration of the wood before it can be treated at a conservation facility. Techniques include spraying with a 5% solution of sodium borate, boric acid, or any other wood-preservative

Preservation of Waterlogged Wood

811

to arrest and prevent further attack of deteriorating organisms, immersing in fresh water, or covering with plastic foam sheets saturated with water to prevent drying and shrinkage.

B. Classic Impregnation Treatments One of the earliest methods for treating waterlogged wood was to impregnate it with a mixture of petroleum and a drying oil, such as linseed oil, while the wood was allowed to dry slowly. Most of the drying oils do not penetrate well, and they tend to deteriorate in time. The treated object becomes sticky and dark. Glycerin had been used in the early 1990s. It was used to replace water in the wood. It evaporates extremely slowly at ambient conditions. Since air does not enter the wood, the shrinkage which is caused by the surface tension at the interface between air and water does not occur. Glycerin is a highly hygroscopic material; it absorbs and releases moisture as the atmospheric humidity changes. It gives the treated object a wet and sticky appearance. It also does not strengthen the wood. Aluminum potassium sulfate or alum has also been used. When waterlogged wood is treated with alum in water solution, the potassium, aluminum, and sulfate ions will diffuse into wood. When the wood is cooled, alum recrystallizes to function as a bulking agent to prevent wood from shrinking. The treated object may be brushed with warm linseed oil and a thin coat of shellac to prevent reabsorption of moisture from the air. Unfortunately, alum treatment only reinforces a thin surface layer of wood.

C.

Consolidation Using Water-Borne Chemicals

Consolidation using water-borne chemicals relies on strengthening the wood structure through the introduction of a consolidant into the cells. The method utilizes chemicals that are soluble in water and are introduced directly into the wood without drying the wood. Consequently, preserving an object using water-borne chemicals does not take as long as using resins or other consolidants that are not water-soluble. Nevertheless, the treatment time is governed by the rate of diffusion of consolidant through the wood to where it is required. These water-borne chemicals include sugars,salts,alum, polyethylene glycol (PEG), tetraethyl ortho silicate (TEOS), and phenol. Of these, PEG is the only one which has gained wide acceptance among conservators. PEG has been considered since the 1950s to be the most reliable method for treating waterlogged wood. PEG is a synthetic polymer of ethylene oxide with the general formula HOCH2-(CH20CH2),,-CH20H, where n represents the average degree of polymerization. PEGs are available in molecular weights (MW) ranging from 200 to 6000. The fractions with MWs ranging from 200 to 600 are liquid at 20°C and are soluble in water inall proportions. Those with MWs above 600 are white and waxy, with the higher MWs being more viscous than the lower MWs. PEGs with higher MWs are soluble freely in alcohols, such as ethanol and methanol, as well as water. For treating waterlogged wood, the low-MW PEGs are dissolved in water or alcohol and diffuse into the wood slowly at 60°C for a period of several days to weeks to replace water in the wood. Then the next range of higher-MWs PEGs is successively and gradually introduced into the solution and allowed to diffuse into the wood to replace the low-MW PEGs. Apparently the low-MW PEGs have higher rates of diffusion and penetration into the cell wall. Eventually, higher-MW PEGs (>3000) are used. The size of the PEG increments is dependent on the condition, size, and species of the wood. While low-MW PEGs are most effective for less degraded wood, higher-MW PEGs tend to be more suitable for highly degraded wood. In any cases, it is a very time-consuming process. Fungicides may

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be used simultaneously with PEG. After PEG treatment, the excess wax is wiped off and the wool is allowed to cool. When cooled, any excess that was on the surface is removed with a hot-air gun or with hot water. Alcohols may finally be used to clean the surface to remove dark color and to regenerate woodlike color. When the Swedish warship Wasrl was resurrected in 1961 after 333 years at the bottom of Stockholm Harbor, PEG was selected to treat the entire vessel [IO]. Even though PEG treatment is a popular method, it has several significant drawbacks. It is a relatively expensive and time-consuming process. Perhaps most significantly, PEGtreated wood is sensitive to heat and high humidity. These unfavorable characteristics require that treated wood be curated in climate-controlled facilities.

D. SucroseTreatment To overcome the PEG-related limitations, sucrose has been developed as an alternative method [ 11. In practice, the procedure is identical to that described for PEG. where sucrose is applied by aqueous diffusion as a bulking agent. Itis less expensive and penetrates wood quite well. For better results, refined white sugar (white sucrose) is recommended. Unbleached, brown-colored sugar should be avoided because of its high hygroscopic property. For less degraded waterlogged wood, a sucrose solution with a sufficiently low concentration ( 1 - 5 % ) may be used to avoid dehydration of sound wood. For highly degraded wood, a higher concentration of sucrose may be used. It is recommended to start with low percentage increases, for example, 1-596, until a concentration of 50% is reached. Then the solution can be increased in 10% increments. The treatment continues until sucrose concentration reaches 70% and the wood has equalized at this concentration. During the treatment, an antimicrobial agent may be added to the first batch of solution. When the wood has reached equilibrium with the highest solution concentration, the treated wood is air-dried slowly under conditions of controlled high humidity. The properly dried wood should be, if possible, stored under conditions of less than 70% humidity. Sucrose-treated wood has the appearance, density, and much of the strength of nondegraded wood.

E. Acetone-RosinTreatment Acetone-rosin treatment involves the exchange of free water in the waterlogged wood with a natural rosin, such as pine rosin. In this method, the wood is dehydrated completely with acetone or a mixture of acetone and ether [ I 1 , l 21. It is important to remove all the water because it is not compatible with rosin. The well-dehydrated wood is placed in a saturated solution (67%) of rosin dissolved in acetone at about 50-55°C. After treatment, treated wood is removed from the solution and the excess rosin is wiped off, This method can give excellent results, but it involves heating volatile flammable solvents and is difficult to control. Acetone is also an expensive solvent. It is recommended only for treating small artifacts.

F. Alcohol-EtherTreatment In alcohol-ether treatment, waterlogged wood isfirst immersed in successive baths of alcohol until all the free water is replaced by either isopropyl alcohol or ethanol. This is followed by successive baths of acetone. The treated wood is then dried under vacuum to remove ether. Since ether has a very low surface tension (0.7 dynekm), when it evaporates,

Preservation of Waterlogged Wood

813

the surface tension forces are so low that there is no appreciable collapse of the weakened cell wall. Like acetone, alcohols and ether are highly flammable solvents, and they must be handled very carefully. After the water is replaced by alcohols, camphor can be used instead of ether. After the camphor fills the cavities and cell walls of the waterlogged wood, the camphor then slowly sublimates without exerting any surface tension on the cell walls. Thus the wood does not collapse, shrink, or distort.

G.

In-SituPolymerization

PEG is a polymer. It takes time for the polymer to diffuse or penetrate into wood cells. To overcome this problem, considerable work has been done in an attempt to develop monomers which can be polymerized in situ after infusion [ 13,141. The polymerization process maybe initiated using either heat or high-energy radiation to convert monomer into polymer. The polymer functions as a consolidant, providing strength and dimensional stability for the degraded wood cells. Many monomers can be used for this purpose. Styrene, vinyl acetate, acrylonitrile, acrylates, and methacrylates are among the most commonly used monomers. Melamine formaldehyde resin has been used quite successfully for this purpose. After infusion, copolymerization of unsaturated polyester oligomeric resins with styrene by high-energy radiation has been employed successfully. The treated products are mechanically strong. durable, and stable to a wide range of environmental conditions [ 131.

H.Freeze-DryingTreatment Freeze-drying is not actually a conservation process but rather only a dehydration process. It is a physical process which sublimes ice. Water in the wood is frozen and leaves in the form of gas without actually passing through the liquid state. There are several variations of this process, including directly freeze-drying the wood, freeze-drying after impregnation with a consolidant, freeze-drying after exchanging water in the wood with another solvent, and consolidating the wood before freeze-drying and then freeze-drying in a natural environment [ 15,161. The technique is not as straightforward as it might seem to be, as there is a considerable reduction in density when water freezes, and hence a physical expansion of the material within the specimen. Accordingly, for freeze-drying to be effective in treating waterlogged wood, a cryoprotectant must be added. The purpose of a cryoprotectant is to reduce the volume change by continuing to increase in density as the mixture cools through the water’s freezing point. The most commonly used cryoprotectant is PEG with a MWof 400. I t is usual for waterlogged wood to be i n equilibrium with a 20% solution of PEG prior to commencing the freeze-drying process. Vacuum freeze-drying to remove water from waterlogged wood has been done at several conservation laboratories. Of the many methods tested, pretreatment with PEG 200 and 400 worked well with undeteriorated wood. Highly deteriorated woods responded best to pretreatment with a combination of higher-molecular-weight PEG or PEG dissolved in solvents such as f-butyl alcohol [ 171. Directly freeze-drying wood was the process chosen in the conservation of some parts of the Mary Rose [ 5 ] ,which was a ship built as part of King Henry VIII’s naval expansion program.

1.

SupercriticalDrying

The supercritical drying method was invented in the 1950s by Kistler 1181. This method uses a high-density or supercritical fluid to replace the water in the wood. The supercritical

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fluid is then removed from the wood by decompression, without forming a liquid phase. As shrinkage is due to surface tension forces at a liquid surface, supercritical drying does not damage the artifact. Carbon dioxide is a suitable supercritical fluid that can be used. For treatment, water in the wood is first replaced with an organic solvent or alcohol. At high pressure the carbon dioxide’s density is similar to thatof common liquids, and it readily dissolves the alcohols.

IV. PRESERVATION OF HISTORIC GUNBOAT A CASE STUDY1221 A.

U.S.S. CAIRO.

Background and History of Chemical Treatments [ 19-21]

The U.S.S. Cairo was constructed at Mound City, Illinois, in the fall and early winter of 1861. It was 175 ft in length, 5 1 ft in breadth, and 15 ft in height. The vessel was a flatbottomed, three-keeled ship, designed for use solely on Western rivers. Theship was constructed of heavy timber, mostly white oak, with additional protection provided by 2.5in.-thick armor plates to protect the front casemate, sides abreast machinery, and the pilothouse. Approximate tonnage of the vessel was 512 tons, and it was manned by a crew of 174 officers and seamen. The U.S.S. Cairo served the Union forces for approximately 1 1 months, from January 1862 to December 1862, when she was sunk by a submerged Confederate“torpedo” (mine) in the Yazoo River near Vicksburg, Mississippi, on December 12, 1862. The U.S.S. Cairo had retained virtually all its structural integrity beneath the muddy left bank of the Yazoo riverbed when it was discovered in 1956. Following a three-year hiatus of exploration, the exact site of the Cairo was determined conclusively and the gunboat was found to be in an apparently excellent state of preservation. In 1960, a gun port, winched from its mounting on an inclined casemate, the pilothouse, and a naval cannon were the first items to be raised. However, the entire vessel was not raised until 1964 by the State of Mississippi. During this process, unfortunately, the vessel was heavily damaged and ultimately was cut into three pieces to get it ashore, i.e., to the barge. The immediate need for preservation of wooden objects was recognized. A huge polyethylene tank (45 ft X 7 ft X 4 ft) was made, in which smaller wooden artifacts were preserved with 25% polyethylene glycol (PEG-1000) solution. Sodium salt of pentachlorophenate was also added to the solution. No preservation or protection of large wooden objects was done. In 1965, the Cairo was barged to Pascagoula, Mississippi, for mock-up and restoration. The bulk of the wooden remains of the Cairo was comprised of approximately 17 large separate wood sections after the gunboat was moved to the Ingalls’ shipyard at Pascagoula. After the armor was separated, the wooden parts (ranging in size from whole sections of the casemate to single planks) were stored and stacked. Due to the high cost of polyethylene glycol, the wooden fabric was placed under sprinklers of tap water for occasional “sprinkling treatment.” This practice was terminated in the early- to mid- 1970s, but the Cairo stayed outdoors without any protection at Pascagoula until 1977. In that year, the Cairo was removed to the National Military Park at Vicksburg, Mississippi. In the course of preparing the Cairo for transport, workers discarded unidentifiable, highly deteriorated lumber that had separated from the gunboat sections reassembled in 1965. In order to facilitate truck transit to the park, many of the 17wood sections were further divided by chain saws into smaller sections. By the time the Cairo arrived at Vicksburg National Military Park, there were approximately 27 separate significant wood sections. All of the wood sections that were transported to Vicksburg were sprayed with a hydrozol

Preservation Wood of Waterlogged

815

5% pentachlorophenol solution before shipment.No further chemical stabilization of wood fabric was done until 1979, when the National Park Service decided on spray treatment of the Cairo remains with polyethylene glycol (PEG 4000) and copper-o-quinolinolate (PQ-57) solution six times between 1979 and 1982. Since then, the Cairo has been occasionally treated with insecticide. Meanwhile, a shelter with a covering structure for the gunboat was completed in 1980. B. Specimens from the Gunboat Specimens from various location of the gunboat, as shown in Fig. 1, are listedbelow. They were collected for chemical analysis. 1. Port (Right, Upper,Forward) 2. Port (Right, Lower,Forward) 3. Hurricane Deck (Left, Under,Forward) 4. Port (Left,Upper, Forward) 5. Port (Right, Upper,Middle) 6. Gundeck (Right, Upper,Middle)

FIGURE 1 Specific locations of wood specimens collected from the U.S.S. Cairo, Vicksburg Military Park, Vicksburg, MS.

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7. Gundeck (Right, Upper, Middle) 8. Hurricane Deck (Left, Under, Middle) 9. Gundeck (Center, Upper, Middle) 10. Hurricane Deck (Left, Under, Middle) 11. Hurricane Deck (Left, Under, Middle) 12. Hurricane Frame-Cross Section (Left, Under, Middle) 13. Gundeck (Right, Upper, Forward, near Torpedo-damaged bow) 14. Gundeck (Right, Upper, Forward, near Torpedo-damaged bow) 1 S . Gundeck. Side Plank (Right, Upper, Forward. near Torpedo-hit site) 16. Gundeck (Right, Upper, Middle) 17. Gundeck (Center, Upper, Middle, in front of Paddlerwheel) 18. Hurricane Deck (Right, Under, Middle) 19. Hurricane Deck (Center, Under Paddlewheel, Middle) 20. Gundeck, Side Plank (Left, Under, Aft) Most of the wood elements of the Cairo are deteriorated-some to a critical state. For example, the wood fabric located in the port areas is light in weight, very brittle, and fragile. Planks located at the gundeck and hurricane deck are significantly discolored and weathered, but have retained their strength. Portions of deteriorated wood regularly fall from the structure and canbefound on the floor under the boat. In general, the wood located on the upper part of the gunboat is suffering more degradation than that at the bottom. This implies that the degradation process continues to proceed due to weathering and other environmental factors. The bulk of the remaining Cairo fabric is white oak that is essentially sound. However, there is considerable surface decay caused largely by biological agents. There is also some chemical decay and significant weathering. The deep cracks in most members are usually the result of repeated wet/dry cycles. These checks can present serious preservation problemswhere they have allowedmoisture to penetrateto the timbercenter, thereby causing internal pockets of decay. The chemicalcomposition of the Cairo fabric was analyzed for the specimens collected from the Cairo. Scanning electron microscopy was also used to examine the ultrastructures of the wood specimens. Details of these studies will be discussed in a subsequent section. Chemicalcomposition of wood specimens from the Cairo wasanalyzed using a variety of laboratory techniques.Holocellulose,alpha-cellulose, Klasson lignin,extractives, and ash were determined based on wood chemistry standard methods described in the literature. A scanning electron microscope (SEM; JEOL JSM-IC848) was used to study the ultrastructure of the wood specimens.

C. Wood Identification The wood used in the construction of the U.S.S. Cairo was identified as a species of the white oak group (QuCrcu.7 sp.). Some of the gundecks of the ship were made of southern yellow pine (Pirzus sp.). It is not possible to identify the specific species of these group of woods on the basis of their wood anatomyalone; fruit and flowers are needed for positive species identification.

1. Chemical Analysis Chemical analysis of specimens from the gunboat revealed that the wood fabric is severely degraded, particularly at the surface layers. Chemical composition of the wood is tabulated

Preservation of Waterlogged Wood TABLE 1 Sample no.''

817

Chemical Composition of Deteriorated Wood Fabric of U.S.S. Ctriro Alpha-

Holocellulose cellulose

82.27 55.23 56.84 61.99 47.96 66. I0 47.07 62.04 45.48 6 1.92 49.65 67.67 48.43 66.9 I 5 I .68

47.68 23.09 18.29 27.90 -

40.46 -

36.17 -

36.97 -

35.93 -

33.10

Klason lignin

Ethohenzene extractives extractives

1% NaOH

20.76 23.39 29.20 22.55 25.98 25.97 29.68 27.06 33.2 I 24.69 25.28 22.47 28. I 1 23. I 1 27.19

3.67 10.06 5.25 5.03 14.95 5.04 12.77 4.59 11.51 8.53 15.25 5.41 12.36 6.88 10.06

18.76 52.12 55.44 47.26

Ash

-

28.15 66.18 34.78 59.30 37.9 1 54.16 33.26 56.94 34.47 56.13

0 . I3

2.0 I 1.25 1.42 1.85 0.60 0.93 1.10

2.67 0.9 I 1.30 1.43 1.77 1.95 2.69

in Table I . It is apparent that cellulose and hemicelluloses content have decreased significantly. Lignin appeared to suffer less degradation than cellulose. (The increase in lignin content was due to the loss of cellulose and hemicelluloses.) Reduction of degree of polymerization of cellulose is shown in Table 2. It also revealed that low-molecular-weight fragments of detcriorated products can be extracted with co-solvents of ether and benzene and with 1 % sodium hydroxide solution.

2. Scanning ElectronMicroscopy Examination The wood of white oak is made up of six types of cells: springwood vessels. summerwood vessels, vasicentric tracheids, longitudinal parenchyma, ray parenchyma, and libriform fibers. SEM micrographs of the cross section of a normal piece of white oak show extremely thick-walled fibers which make up the highest proportion of the cell types in white oak. The vessel elements, vasicentric tracheids, and parenchyma arc generally thin-walled cell. SEM micrographs of the wood from the U.S.S. Cairo show considerable deterioration and reduction in the secondary walls of the thick-walled fibers as well a s complete deterioration of some of the thin-walled ccllular elements (i.e., latewood vessels and vascicentric tracheids). The deterioration which has occurred to the anatomical structure is due in part to fungi, evidenced by the presence ofan abundance of hyphae (Figs. 2 and 3) and bore holes in the cell walls, as well as possibly some bacteria (Fig. 2).

D. Consolidation of Historic U.S.S. Cairo Gunboat

In order to restore or improve the strength of the critically deteriorated, once-waterlogged gunboat, conventional polyethylenc glycol trcatments are undesirable because the wood structure has already shrunk and collapsed. I t is very difficult for polyethylene glycol to penetrate into such wood. Likewise, the use of borate has the same problem for oncewaterlogged wood. In addition, borate is too casily dissolved in water. It will leach out

3

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TABLE 2 Changes in Degree of Polymerization of Cellulose from the U.S.S. Cairo Sample no.'

Degree of polymerization ~~

Control 1

4 5(c) 6(c) 9(c) 9(s) 17(c) 17(s) 19(c) 19(s) 20(c) 20(s)

1471 467 934 1634 770

957 1447. 1027

'See Fig. 1 for location of speclmens; c, core; S, surface.

FIGURE 2 Springwoodzoneshowingspringwoodvesselelements in lowerrightcornerwith tyloses and smallroundocclusionswhicharepossiblybacteria.Secondarywall of allcellsis practically nonexistent. Magnification 15OX.

Preservation of Waterlogged Wood

819

FIGURE 3 Cross section of libriform fiber tissue showing extreme deterioration of the secondary walls of the fiber. Normally these cells would have extremely thick walls with a very small lumen. Magnification 300X.

readily from wood in a high-humidity climate. The addition of polyethylene glycol and borate would only increase the risk of further damage of the ruined fabric. One hundred and two years of submersion in the Yazoo River as well as a decade-long weathering in Pascagoula and Vicksburg have brought the gunboat to a state of extreme fragility. As discussed earlier, the gunboat has lost most of its polysaccharides(i.e., cellulose and hemicellulose) and some parts of its lignin. Loss of these chemical components and physical stress have resulted in the formation of cracks, checks, crevices, and cavities in the wood structure at the macro- and microscopic levels. Given these conditions, the buildup of polymer consolidants within the cell walls appears to be the most acceptable approach to restoring fabric solidity. Six different types of polymer resins were selected to consolidate the deteriorated wood samples. The resins were phenol formaldehyde, resorcinol-phenol-formaldehyde, epoxy, polyacrylic, polyurethane, and wood rosin. With the exception of wood rosin, all the polymer consolidants favorably restored the strength of the wood fabric. Of these polymers, only resorcinol-phenol-formaldehyde resin exhibited acceptable appearance, i.e., color and texture of wood after impregnation; wood treated withother resins displayed a glossy surface which is not acceptable for restoration and conservation purposes. The resorcinol-phenol-formaldehyde-treated wood specimens exhibited significant improvement of hardness, reduced water pick-up, and good dimensional stability. 1. Resorcinol-Phenol-Formaldehyde Treatments Various concentrations of resorcinol-phenol-formaldehyde resins can be prepared by using water and alcohol as the diluents, as shown in Tables 3 and 4. Several solutions were

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TABLE 3 Alternative Formulations of Resorcinol-Phenol-Formaldehyde Resin (R301)" for Application to Wood R30

Sample

I

Hardener Viscosity Alcohol Water

100 100

I 00 100 I00

20

16 30 30 30 20

"K301 is a rcsorclnol-phenol-formaldehyde "cp = centipoise.

0 0 0 0 I20

80 80 120 I60

120

236 76 36 52

rcsin manufactured hy Koppcrs Company. Inc.

used in tests in which wood fabric was treated-each with a given solution and cured at room temperature. Allof the formulations worked well with the wood fabric to achieve good consolidation. Strength was improved. Original color and texture were retained. It was found that the more alcohol used, the darker the color of wood surf'ace resulted. The use of filler (peanut flour) could fill the large gaps and cavities properly. Scanning electron micrographs showed that the resorcinol-phenol-formaldehyde resin deposited on the surface of cell lumens properly to achieve good consolidation. N. Chamcteristics qf Resorcinolic Resins. Resorcinolic resins are condensation products of resorcinol with formaldehyde, or with various phenol-formaldehyde resoles. The latter were used for this study due to their much lower cost. Although the resorcinolic adhesives were initially found to be capable of being efficiently cured under neutral conditions, it has generally been found to be advantageous for commercial purposes to cure them under mildly alkaline conditions. Basic catalysts may provide bonds of strength at low curing temperature, such as that used in this work. Resins are prepared from resorcinol by reaction with efficient amounts of formaldehyde. The theoretical amounts necessary to produce a cured resin are slightly in excess of 1 molof formaldehyde per mole of resorcinol. However, even 80% of this amount would be sufficient to give an unstable or gelatinous product, since the molecules react disproportionately in bulk situations, so that

TABLE 4 Altcrnative Formulations of Resorcinol-Phenol-Formaldehyde Resin (G 1260-A)" for Application to Wood Samplc

1 2 3 4 S

6 7 8 9 10

G 1260-A

Hardcner

Water

Alcohol

100 100 100 100 I 00 1 00

20 20 20 20 20 20 20 20 20 20

0 0 S0

30 60 0

IO0 100 100 100

IS

18

25 35

2s 35 40 20

40 h0

80 80

IS 0

(cP)" Viscosity 280 70 200 280 120 72 64 S2 64 -

Preservationof Waterlogged Wood

821

OH

OH

7

OH I

CH2OH

FIGURE 4

Substitutionreaction in a resorcinol structure.

some of the resorcinol molecules are left unreacted, while some of the first-formed oligomers acquire a superior share of the formaldehyde to form polymers of higher molecular weights. Resorcinolic resins of between 0.5 and 0.7 mole of formaldehyde per mole of resorcinol are made having infinite stability. At the point of eventual use, some additional formaldehyde is provided and the resin is converted, within a short period of time, to a very highly cross-linked resin. This product is characteristically insoluble, infusible, and physically strong when properly aged. b. Chemistry of' Resorcinol- Fornlaldehyde Resin Formation. Resorcinol readily combines with formaldehyde to form methylol derivatives, with the methylol groups occupying either the positions ortho to both hydroxyl groups, or ortho to one and para to the other. The meta position is not ordinarily reacted (see Fig. 4). The reactivity of these methylol derivativesis so high that they cannot easily be isolated in pure stable form. They continue to react under ambient, uncatalyzed conditions with formaldehyde,resorcinol,phenol, or othermethylol-containingmolecules to form polymer chains of higher molecular weight, with branched, as well as linear, configurations of great complexity. These reactions continue until spatial considerations prevent further interaction. In these polymers the resorcinol nuclei are joined together through methylene linkages to give complex molecules as shown schematically in Fig. 5 . c. Resorcinol-Pherzol- Fr~rnluldehyne Resin. In orderto produce resorcinol-phenol-co-polymer resin, the phenol is combined with the formaldehyde before the resorcinol is introduced. If the resorcinol were added to the initial charge, it would preempt most of

&

CH,---

CH,2H:+

OH

FIGURE 5

&CH2

OH

9 CH,---

OH

CH,---

CH2---

A typical polymcric structurc of resorcinol-formaldehyde rcsin

822

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the formaldehyde and form a gel, because it is many times as reactive as phenol. As a result, most of the phenol would remain unreacted. In order to obtain a block co-polymer, phenol would be combined withformaldehyde in one reactor to form a resole, and resorcinol would be combined with formaldehyde in a second reactor. The two resins would then be mixed, and the methylol groups on the resole would combine with available nuclear positions of the resorcinol to form mixed polymers. The other way to do it is to let phenol react with an excess of formaldehyde to form a low-condensed resol. Resorcinol would then be added in sufficient proportion to combine with all of the methylol groups, thus forming a resinous co-polymer.

2. ScanningElectronMicroscopyExamination Since the resorcinol-phenol-formaldehyde resin consolidates wood fabric successfully, it is assumed that the penetration and solidification of the polymer in the cell walls is efficient. SEM was used to studythe characteristics of resin treated wood fabric. Typical SEM pictures are shown in Figs. 6-9. Figures 6 and 7 showed that the cross section of the red oak was coated with resorcinol resin. Some of the small cells still can be seen. Figure 6 shows that most of the cells in the cross section were filled with resins. A large vessel on the left was also filled. Vessels on the right were still left empty. Figure 8 shows the tangential surface aspect. Some of the procumbent cells in the body of the ray still can be seen, but are heavily coated with the resin. The surface of the radial section is also heavily coated with resorcinol resin, in which some of the pit cavity was filled with resin too (Fig. 9).

FIGURE 6 SEM micrograph of the cross section of red oak fabric of the gunboat, treated with resorcinol-phenol-formaldehyde resin. The vessel at the left was filled with resin. The surface of the lumen was also coated with the resin. Magnification 250X.

Preservation of Waterlogged Wood

823

FIGURE 7 SEM micrograph of the cross section of red oak fabric of the gunboat, treated with resorcinol-formaldehyde resin. The surface was heavily coated with the resin. Many small cells still can be seen. Magnlfication 250X.

FIGURE 8 SEM picture of the tangential section of red oak fabric of the gunboat, treated with resorcinol-formaldehyde resin. One of the ray cells at the left was filled with resin. Magnification 250X.

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FIGURE 9 SEM micrograph of radial section of red oak of the gunboat, treated with resorcinolformaldehyde resin. The surface was heavily coated with the resin. Some of the pits were entirely coated. Magnification 250X.

It is obvious that resorcinol resin can penetrate, wet, and consolidate fabric favorably to increase the strength of the deteriorated fabric. It may serve as a useful resin to preserve the U.S.S. Cairo gunboat.

3. Large-scale Treatment of U.S.S. Cairo Gunboat Fabrics In order to evaluate alternative treatments ofwood fabric, initial tests were conducted using small samples. Since resorcinol-phenol-formaldehyde resin exhibited positive effect on consolidation, a large-scale consolidation with resorcinol-phenol-formaldehyde wasattempted. Wood specimens with dimensions of 12 cm X 12 cm X 30 cmwere treated. Similar proportions of resin were used as for the small-scale study. Results are very impressive. The penetration of resins into the wood fabric was good, and the appearance, hardness, dimensional stability, and water pick-up properties were excellent. It is obvious that by proper treatments, mechanical and physical propertiesof Cairo gunboat fabric can be successfully consolidated. The improvement of 62.6%and 78.7% of hardness for severely decayed wood specimens and moderately decayed wood specimens, respectively, was a good demonstration.

REFERENCES 1.

2.

D. W. Grattan and R. W. Clarke, in Conservation of Marine Archaeological Objects (C. Pearson, ed.). Butterworths, London, p. 164 (1987). J. I. Hedges, in Archaeological Wood (R. M. Rowel1 and R. J. Barbour, eds.), American Chemical Society, Washington, DC, p. 111 (1990).

Preservation of Waterlogged Wood 3. 4.

5. 6. 7. 8. 9.

IO. 1I.

12.

13.

14. 15. 16.

17. 18. 19.

20. 21.

22.

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R.M.Rowell and R. J. Barbour (eds.), Arc~htreologic~rrl Wood, American Chemical Society. Washington. DC ( 1990). D. L. Hamilton. Rnsic Metl~odsof’ Cor~.servir~~g Urlder-rvnterArr./~rreolo~~icrr/ Matcricrl Crrlt~rre. U.S. Department of Defense, Legacy Resource Management Program,Washington, DC ( 1 996). B. B. Christensen, The Conser~~crtior~ of Wcrterlogg~rl Woodi r l the Ntrtiorltrl M I I S P I I I~fI I DCIIm~rrk,The National Museum of Denmark. Copenhagen ( 1970). R. J. Barbour and L. Leney, in Proc. ICOM Wtrterloggecl Wood Workiug Grorcp Conf: (D. W. Grattan and J. C. McCawley, eds.), International Council of Museums. Ottawa (1982). P. Hoffmann, Strrd. Corlser-l~., 3 1 : 103 (1986). L. F. Hawley, Wood Liquid R&tiom, USDA Tech. Bull. 248. Washington, DC ( 1937). A. J. Stamm, Dimensional Stabilization of Wood with Carbowaxes, Forest Prod. J . , 6(5):201 (1956). B. Hafors. in Archaeologiccrl Wood (R. M. Rowell and R. J . Barbour, eds.), American Chemical Society, Washington, DC. p. 195 ( 1990). H. McKerrel. E. Roger, and A. Varsanyi. Strrtl. Cor~ser-~~.. 17:lIl (1972). T. Bryce, H. McKerrel,and A. Varsanyi, in Prob1ml.s i n the Cor~.s~rl.ntior~ of’ Wrrterloggc~d Wood, Marine Monographs and Reports No. 16 (W. A. Oddy). National Maritime Museum, London ( 1975). Q.-K. Tran, R. Ranliere, and A. Ginier-Gillet, in Archtreologicd Wood (R. M. Rowell and R. J. Barbour. eds.). American Chemical Society, Washington, DC, p. 217 (1990). R. W. Clarke and J. P. Squirrel, in Proc. ICOM Wtrterlo~ggerl Wood Workir~~g Gro~q’Con$ (D. W. Grattan and J. C. McCawley, eds.),International Council of Museums, Ottawa, p. 9 ( 1982). W. R. Ambrose, in IIC Ne,$. York Corlfc~rel1c.e0 1 1 ComcJr-wrtiorlof‘ Stone a r l d Woodcw Ol?jrc.t.s, 2nd ed.. 25.3 (1970). J. Watson, in Proc. ICOM Wrrterlogged Wood Working Gmtrp Cor!/: (D. W. Grattan and J. C. McCawley, cds.), International Council of Museums, Ottawa, p. 19 (1982). R. J. Barbour, in Arr~hrrrolo,gir~rr/ Wood (R.M.RowellandR. J. Barbour, eds.), American Chemical Society. Washington, DC, p. 176 (1990). S. Kistler. Nlrtrrre. 127:741 (1931). E. C. BCXSS, Htrrd1rrc.k 11-orlclncl: Tllc. Sir~kirzgt r r d Srrll1age of’ the Ctriro, LouisianaState University Press, Baton Rouge and London ( 1980). V. C. Jones and H. L.Peterson. The Sro~;v of’ ( I C i ~ ~War i l Grrnhoot: U.S.S. Ct1;r.o. U.S. Department of Defense, Washington. DC ( l97 l ). T. McGrath and D. Ashley, Historic Structure Report: U.S.S. Cairo, Vicksburg National Military Park, Vicksburg. Mississippi. U.S. Department ofInterior, Denver Service Centcr.Denver, C O (1981). D. N.-S. Hon and M. A. Taras. A.ssc~.s.srnc~r~t of‘ USS Clriro t r r r d R c c , o ~ l l ~ ~ ~ e r l t l f rft bj [r’ ~Preser~.s lwfintl ~ ~ ~ ~ I I I I ~ ~Vicksburg I I / , s , National Military Park, Southeast Region, National Park Service ( 1988).

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Biodegradable Plastics from Lignocellulosics Mariko Yoshioka and Nobuo Shiraishi Kyoto University, Kyoto, Japan

1.

INTRODUCTION

During the past 50 years, synthetic polymers utilizing petroleum as their raw material have been advanced. A wide variety of plastic materials is now being used commercially that supportour daily life with considerablecomfort. Even plastics which have properties similar to those of metals have appeared as engineering plastics. Before the start of the synthetic polymer industry, there were a number of attempts to obtain moldable materials from natural polymers, mainly from cellulose. Trials of developing cellulose derivatives into industrially acceptable materials [ I ] , as well as efforts to identify excellent plasticizers for cellulose acetate (CA) [2,3], provide good examples. However, these effortsbecame unpopular with thestart-up of theindustrialization of petrochemistry. Actually, many synthetic polymers exhibit properties that polymers of natural origin do not possess, especially in relation to melt processability. Many opinions expressed i n textbooks claim that cellulose has such a rigid backbone that it cannot be converted to plastic materials. Because of these circumstances, it becomes understandable that attempts to develop plastics of natural origin have not been emphasized during the past half-century. Recently, however, several changes have occurred and provided motivation for the authors to start and continue studies on the conversion of biomass into plastics. The first change concerns gradually growing demands for circulating materials which are desirably originated from biomass. One of the works of the authors’ group must be includable, in which wood could be converted into thermally flowable material by chemical modification, such as esterification and etherification [4-71. Thatis, plastics couldbeobtainedfrom suchlow-cost materials as wood wastes. At the present, there are actually almost no sophisticated methods or technologies generally available that can make use of biomass wastes for the purpose of adding satisfactory value.Thus, wood plasticization canbe considered as one attempt to pursue recycling technology. The second motivation resulted from a recently occurring requirement for developing biodegradable plastics. To meet this need,development of biodegradable plastics from natural polymers becomes attractive, together with that of bacterial polyesters and synthetic 827

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polymers (aliphatic polyesters and water-soluble polymers). Many reviews concerning the biodegradable plastics have appeared recently [S- I O ] . Among these biodegradable plastics, investigations in the field of bacterially produced polymers and synthetic polymers are more actively and extensively pursued than polymers from natural origin, especially those from lignocellulosic and cellulosic materials. Actually, even in recently published books on biodegradable polymers, it is difficult to find descriptions of lignocellulosic biomass wastes being converted to meaningful biodegradable plastics [8-10]. These biodegradable polymers must not only be cost-effective; they must also have performance characteristics that are comparable to common synthetic polymers and they must be degradable in the cnvironment. These requirements, however, are often mutually exclusive, and practical biodegradable polymers have not yet been realized. It has been pointed out that it will take S - 10 more years before the development of biodegradable polymers reaches a practical level [ l l]. It is known that the biodegradable chemical intermonomer bonds include glycosides, peptides, and aliphatic esters [ 121. Thus, some of the most attractive materials with greatest potential i n terms of cost, material applications, and environmental compatibility include cellulose derivatives, especially cellulose esters. Among the cellulose esters, cellulose acetate (CA) has been produced industrially in the largest amount. Thus, the big interest has recently focused on the potential biodegradability of CAS. Until the end of the 1960s, it was accepted as axiomatic that cellulose acetates having DS 2 1.0 are resistant to hydrolysis by enzymes [ 131. In 1969, however, Cantor and Mechalas [ 141 found that even cellulose diacetates (CDAs) having a degree of substitution up to 2.5 could be degraded by microbial attack. Their investigation was carried out with the objective of relating losses i n semipermeability of cellulose acetate (DS 2.5) reverseosmosis membranes to microbiological degradation. They were discussing the durability of cellulose diacetate used for the reverse-osmosis membranes; that is, they did not have any interest in biodegradable plastics. In that sense it can be said that the first people to find the microbiological degradation of CDA in relation to biodegradable polymers were the research groups of Eastman Chemical Company and the University of Massachusetts. Buchanan et al. [IS] and Komarek et al. (161 demonstrated that CA with a degree of substitution (DS) up to 2.5 can be degraded microbially. In the former study [IS], they used two separate assay systems to evaluate the biodegradability of CA: an in-vitro cnrichment cultivation technique (closed batch system), and a system in which CDA films were suspended in a water treatment system (open continuous-feed system). The in-vitro assay employed a stable enrichment culture. which was initiated by activated sludge into a basal salts medium containing CA with 5 % (v/v). Extensive degradation of CDA (DS = 2.5) fibers was found after 2-3 weeks of incubation. In-vitro enrichments with CMA (DS = 1.7) films were able to degrade 80% of the films in 4-5 days. Films prepared from cellulose triacetate remained essentially unchanged after 28 days in the in-vitro assay. The wastewater treatment assay was less active than the in-vitro enrichment system. For example, approximately 27 days were required for 70% degradation of CMA (DS = 1.7) fill11s to occur, while CDA (DS = 2.5) films required approximately 10 weeks before significant degradation was obtained.Evidence for the biodegradation ofCA was also obtained through the conversion of cellulose~l-“C]-acetateto “CO, in the in-VitI-0 assay [ 151. The last viewpoint was conclusively established in their successive study by use of naturally derived mixed microbial culture derived from activated sludge and “C-labeled CA and “C-labeled cellulose propionate [ 161. Biodegradation was measured in an in-VitrO aerobic culture system that was designed to capture “CO, produced by the aerobic mi-

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crobial metabolism of cellulose esters. More than 80% and 60% of the original lJc-polymeric carbon was biodegraded to ]'CO; for CA substrates with a DS of 1.85 and those of 2.07 and 2.57, respectively, over periods of 14-31 days. The amount of biodegradation that was observed with cellulose[ I-'"C] propionate with DS of 2.1 I , 2.44, and 2.64 was lower than that of the corresponding acetyl ester and ranged from 0.09% to 1.1 %. However, cellulose[I-"C] propionate with a DS of 1.77 and 1.84 underwent very rapid degradation in the mixed culture system, with from 70% to over 80% conversion of labeled polymeric carbon metabolized to "CO2 in 29 days. The high level of microbial utilization of carbon from both cellulose esters and its conversion to CO, confirms the biodegradability of these polymers and the potential they have for total mineralization in natural microbiologically active environments [ 16). Gu et al. [ 171 studied the cellulose acetate biodegradation upon exposure to simulated aerobic composting and anaerobic bioreactor environments. CA films with DS of 1.7 and 2.5 were exposed to biologically active in-laboratory aerobic composting test vessels maintained at approximately 53°C. The CA 1.7- and 2.5-DS films (thickness values of 1.3 X 10"-2.5 X 10 ' and 5.1 X 10" mm,respectively) had completelydisappeared by the end of 7- and 18-day exposure time periods in the biologically active bioreactors, respectively. On the other hand, almost no change in CA film weight losses was observed when the samples were exposed in the poisoned control vessels(aqueous KCN was added), showing the conclusion that CA film erosion during the composting exposures resulted from, at least in part, biologically mediated processes.Treatments of CA 1.7-DS film samples (1.3 X 10 '"2.5 X IO-' and 5. l X 10 mm thickness) in anaerobic serum bottles with or without KCN poison gave the same natures of results mentioned above. Therefore, it was concluded that degradation of the CA 1.7-DS films upon exposure to the anaerobic bioreactors was due, also at least in part, to biologically mediated processes. Gu et al. [ 181 also reported the degradation and mineralization of CA in simulated thermophilic compost environments. They studied the aerobic degradation of CA (DS 1.7 and 2.5) films exposed for up to 7 and 18 days, respectively. The number- and weight-average molecular weight (Mn and MW) values for both 1.7- and 2.5-DS CAS decreased significantly for extended composting exposure times. For example, Mn of residual polymers (CA 1.7 and 2.5 DS) decreased by 30.4% by day 5 and 20.3% by day 16, respectively. Furthermore, a decrease i n the DS from 1.69 to 1.27 (4-day exposure) and from 2.5 1 to 2.18 (12-day exposure) was observed for the respective CA samples. In contrast, CA films (DS 1.7 and 2.5) which were exposed to poisoned control vessels for identical time periods showed no significant changes in Mn and DS. Scanning electron microscopy (SEM) photographs of CA (DS 1.7 and 2.5) film surfaces after compost exposures revealed severe erosion and corresponding microbial colonization. Similar exposure times for CA films in poisoned control vessels resulted in only minor changes in surface characterization by SEM observations. Theconversion of CAS (DS 1.7 and 2.5) to CO, was monitored by respirometry. A lag phase of IO- and 25-day duration for CA DS 1.7 and DS 2.5, respectively, was observed, after which the rate of degradation increased rapidly. Mineralization of 1.7- and 2.5-DS CA powders, reported as the percentage of theoretical CO, recovered, reached 72.4% and 77.6% in 24 and 60 days, respectively. The results of this study demonstrated that microbial degradation of CA films exposed to aerobic thermophilic compost reactors not only results in film weight loss but also causes severe film pitting and a corresponding decrease in chain Mn and DS for the residual material. Furthermore, a high degree of CA mineralization was observed by the significant attainments of CO, conversion. Buchanan et al. [ 191 studied the influence of DS on blend miscibility and biodegradation of cellulose acetate blends. They reported their findings on blends of CA having a

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DS of 2.49 with those having a DS of 2.06. This blend system was examined over the composition range of 0- 100% 2.06-DS CA employing both solvent casting of films (no plasticizer) and thern-ral processing (melt-compressed films and injection molding) using poly(ethy1ene glycol) as a common plasticizer. Thermal analysis and measurement of physical properties indicate that blends in the middle composition range are partially miscible. while those at the end of the composition range are fully miscible. The miscibility of these cellulose acetate blends is suggested as being influenced primarily by the monomer composition of the co-polymers. Bench-scale simulated municipal composting confirmed the biodestructability of these blends and indicated that incorporation of a plasticizer accelerated the composting rate of the blends. In-vitro aerobic biodegradation testing involving radiochemical labeling demonstrated conclusively that both the lower DS CA (DS 2.06) and plasticizer significantly enhanced the biodegradation of the more highly substituted CA (DS 2.49). This last point, that the presence of the lower DSCA significantly enriches the biodegradability of the more highly substituted CA, was already reported by Itoh et al. [20,21]. They reported that, when the former, more degradable CA, exists in an amount of more than 10%. the more highly substituted, thus less degradable CA can be significantly enhanced in its biodegradability. It was estimated that because of the presence of the lower-DS CA, microorganisms. usually not workable for destroying the higher-DS CA, can attain the degradation ability through so-called acclimatization. On the other hand, Sakai et al. [22] have searched for fungi that decompose CAS. They demonstrated that Neisseritr SI'CCN can degrade CA with a DS at least up to 2.3. The isolated strains, identified as Neisseria s i c m , degraded CA membrane filter (DS mixture of 2.8 and 2.0) and textiles (DS 2.34) in a cultivating medium. Biodegradation of 1.81and 2.34-DS CAS on the basis of biochemical oxygen demand reached S 1-60 and 4045%, respectively, in the culture of N . sicccr within 20 days. It also has been suggested that CA would undergo an enzymatic splitting by acetyl esterase in a first stage, down to a DS of 1 .O, before the degradation would continue by the action of cellulase enzymes [ l6.17,22]. Since cellulose diacetate (CDA) became thus recognized as a biodegradable polymer, various trials have been undertaken to impart sufficient thermoplasticity to CAS in order to render them melt-processable. This is because CDA, which has the greatest thermoplasticity among all kinds of CAS, fails to show adequate melting behavior without decomposition or discoloring. Thus, lowering the flow temperature of CAS is necessary, and it requires the addition of plasticizers and/or flow promoters. Traditional plasticization of CAS has been accomplished by using conventional p k ticizers with low molecular weights, such a s phthalates. glycerol derivatives, phosphates. etc. At present. phthalates and phosphates are used industrially i n procedures that are often very time-c(~nsuming (i.e.,4-5 h per batch). These plasticizers are usLlally not suitable for the prep:1ration of biodegradable polymers because of the harmful natures of their decomposition products. In this connection, there have been several attempts to utilize aliphatic polyesters of bacterial origins a s well as synthetic ones as plasticizers for CAS 123,341. Of these experimental studies, the following are includable as typical examples. Scandola et a l . reported miscibility of bacterial poly(3-hydroxybutyrate) (P(3HB)) with cellL1lose esters i n 1992 1231. They prepared blends of P(3HB) with cellulose acetate butyrate (CAB) and cellulose acetate propionate (CAP) by melt compounding. It is known that P(3HB)/CAB blendscontaining S-SOQI P(3HB) and P(.?HB)/CAP blends with 5h()(%p(3HB) arc transparent. stable homogeneous amorphous glasses, while blends with highel- p(3HB) content are partially crystalline. When i n the amorphous state. both P(3HB)/

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83 1

CAB and P(3HB)/CAP blends show aglass transition which decreases regularly with increasing P(3HB) content, in excellent agreement with the behavior predicted for totally miscible blends. Both dynamic mechanical thermal analysis (DMTA) and differential scanning calorimetric (DSc) show that P(3HB) and CAB can crystallize from the blends only at temperatures higher than the composition-dependent T,. When crystallization is induced by thermal treatments. the melting temperature of the crystalline phase obtained depends on composition, a s expected for miscible blends of crystallizable polymers. Besides the strongly composition-dependent glass transition, another relaxation is observed, located in proximity to the T , of P(3HB) and slightly shifting to higher temperature with increasing CAB or CAP content. That is, another relaxation associated with mobilization of the IOWT, component is observed at alowertemperature. It was suggested that the two glass transitions are the manifestation of two mobilization processes coexisting in blends which appear in all respects to be single-phase, homogenous mixtures. After getting this information, Scandola’s group studied the effect of a low-molecular-weight plasticizer on the thermal and viscoelastic properties of the miscible blends of P(3HB) with CAB [24]. The low-molecular-weight plasticizer selected was di-n-butyl phthalate (DBP). The plasicizer DBP is miscible i n all proportions with both CAB and PHB. It is known that, analogous to the polymeric CAB/P(3HB) blends, the two polymer/diluent systems [CAB/DBP and P(3HB)/DBP] show a dual dependence on T, in composition. Thus, it can be said that i n binary mixtures such behavior appears to be independent on the macromolecular or lowmolecular-weight nature of the Iow-T, component. On the other hand, addition of a fixed amount of DBP plasticizer to CAB/P(3HB)blends with varyingcomposition [P(3HB) content from 0 to 100%] causes a significant decrease of T, of the binary polymer blends; the higher the amount of DBP in the ternary blend, the greater the T, depression. Concomitant with the expected plasticizing effect on T,, the presence of DBP also induces a decrease in the characteristic temperature of the additional low-temperature transition observed in CAB/P(3HB) blends. In the ternary blends, the temperature of such a transition is a function of DBP content only, being independent of the relative amount of the two polymers [CAB and P(3HB)I. Buchanan et al. (251 reported on their work on CAB and bacterial poly(hydroxybutyrateco-valerate)(PHBV)co-polymerblends.They prepared blends in the composition range 20-80 wt% of CAB and aco-polymer of PHBV by thermal compounding. Measurement by I3C-NMR and gel permeation chromatography (GPC) showed that no transesterification occurred during thermal mixing and that little change in molecular weight occurred.Blendscontaining20-50% PHBV were found to be amorphous, optically clear, miscible blends, while the blends containing 60-80% PHBV were semicrystalline, partially miscible blends. Both thermal and DMTA revealed the presence of a high-temperature transition that was sensitive to blend composition and a low-temperature transition whose position was uninfluenced by the blend composition. The high-temperature transitions of the 20-50% PHBV blendsclosely match calculated T V ’ s fora fully miscible blend. It was proposed that the dual transitions in the blends containing 20-50% PHBV arise from dynamic heterogeneity and not from a classical miscibility gap. Blend morphology was found to strongly influence physical properties such as tensile strength and tangent modulus. Blends containing 70% and 80% PHBV were found to exhibit tear strengths that were superior to either of the blend components. Buchanan et al. [26] also studied CAP and synthetic poly(tetramethy1ene glutarate) (PTC) blends. They prepared blends of synthetic polyester, PTC, and CAP, in the range of 50-90 wt% of the latter, by thermal compounding. During the compounding, no transesterification and little loss in molecular weight occurred. PTC was found to be a low-

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melting (39"C), low-T, (-%"C), semicrystalline polymer. CAP is known as an amorphous, high-T, (136°C) polymer. It is known that the obtained CAP/PTG blends are optically clear and, when quenched from the melt, amorphous. Some blend compositions did exhibit small crystallization exotherms and melting endotherms in DSC experiments. The temperature of these melting endotherms lowered linearly from ca.168 to 148°C with decreasing CAP content over the range 85-60% CAP, while the AH, reached a maximum at 75% CAP in the blend. The T, of the blends containing more than 50% PTG lie near or below room temperature, and they are tacky and difficult to handle. No attempt was made to prepare and analyze these blends containing more than 50% PTG. Like the parent CAP,the blends containing 50-90% CAP exhibit a single relaxation process in their DMTA data, with no evidence of a low-temperature transition that could be associated with the polyester. The T, of these same blends agreed well with predicted values from Wood's equation. This fact offers good evidence for the miscibility of these blends. Buchanan et al. 1271 studied the influence of diol length of aliphatic polyesters on blend miscibility with CAP. A series of aliphatic polyesters containing a C5 dicarboxylic acid (glutaric acid) and C2-C8 straight-chain diols were blended with CAP at different composition levels. Characterization by DMTA revealed that, when blended with CAP, the polyesters prepared from C2-C6diols formed transparent, stable, amorphous glasses which exhibited a single composition-dependent T q . Upon reaching a C8 diol, the blend became partially miscible. It is known that within the miscible blends, analysis of their DMTA spectra indicates that the polyester prepared from the C4 diol had the highest level of miscibility with CAP, while the polyesters prepared from C5 and -CH,-CH,-0 -CH2-CH2diols gave the lowest degree of miscibility. Sub-T, mobilization processes, centered in the range -60 to -5O"C, were observed for the blends prepared from polyesters which contained C2, -CH2CH,0CH2CH2and C6 diols. In this connection, the activation energy for the sub-T, relaxation process for 40% poly(diethy1ene glutarakCAP blend was measured (210 kJ/mol). This result suggests cooperative, localized motion of a CAP-polyester complex. However, no relation was found between low-temperature relaxation processes and blend miscibility. Buchanan's group also reported a study on mechanical properties of CAP/synthetic aliphatic polyester blends 1281. This study was done because useful blends of cellulose ester with other high-molecular-weight polymers are generally unknown. Two aliphatic polyesters, PTC and poly(tetramethy1ene succinate)(PTS), have been thermally compounded with CAP in the range of 10-40% polyester. These blends have been injectionmolded, and the rnechanical properties of the molded bars were compared to bars molded from CAP plasticized with a low-molecular-weight diester, dioctyl adipate (DOA). The CAP/aliphatic polyester blends have significantly higher tensile strengths, flexural moduli, heat deflection temperatures, and greater hardness values than the corresponding CAP/ DOA blends. Buchanan et al. [29] studied the biodegradation of cellulose esters and cellulose ester/ diluent mixtures by composting.They evaluated a number of biodegradable polymers including CA with different DS and celluloseesteddiluent mixtures in a static, benchscale simulated municipal compost environment. Of the polymers evaluated,cellulose acetate (DS < 2.2), PHBV, and polycaprolactone (PCL) exhibited the fastest composting rate, disappearing completely after 14 days.Compression-molded films and injectionlnolded bars of CA (DS 2.06)hriethyl citrate (TEC) and of a series of miscible blends consisting of CAP andpoly(ethy1ene glutarate) (PEG) or PTC were evaluated in c m posting. Samples were removed from the compost at different intervals and evaluated by gravimetric analysis, GPC, and 'H-NMR. As expected, the CA/TEC film disappeared rap-

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idly upon composting, while the injection-molded bars exhibited weight loss of 10- 12%. For the CAP/polyester blends. the type of polyester (PEG versus PTC) in the blends made no difference in cornposting rate. In general, a s the DS of the CAP decreased and the amount of polyester in the blend increased, the rate of composting and the weight loss due to colnposting increased. When the CAP was highly substituted, almost all the weight loss was ascribed to loss to polyester. When the DS of CAP was below approximately 2.0, both components degraded. Buchanan et al. also reported their work on composting of miscible CAPhliphatic polyester blends in another journal 1301. In the article, they evaluated a series of miscible blends consisting of CAP and PEG or PTC in a static bench-scale simulated nlunicipal compost environment. Samples were removed from the compost at different intervals, and the weight loss was determined before evaluation by GPC, SEM, and 'H-NMR. The type of polyester (PEG versus PTC) in the blend made no difference in composting rate. At fixed CAP DS. when thecontent of polyester in the blend was increased, the rate of composting and the weight loss due to composting increased. When the CAP was highly substituted, little degradation was observed within 30 days and almost all of the weight loss wasascribedto loss of polyesters. Although the polyesters were still observed to degrade faster, when the CAP DS was below approximately 2.0, both components were observed to degrade. The data obtainedin this experiment suggested that initial degradation of the polyester is by chemical hydrolysis and the rate of this hydrolysis is very dependent on the temperature profile of the compost and on the DS of CAP. Buchanan's group also developed and reported a bench-scale compost methodology that emulates a high-efficiency municipal windrow composting operation [ 31 1. A series of CA films, differing in DS,wasevaluated in this bench-scalesystem. In addition,commercially availablebiodegradablepolymers such as PHBV and PCLwere included as points of reference. Based on film disintegration and on film weight loss, CA having DS less than approximately 2.2 composts at rates comparable to that ofPHBV. NMR and GPC analyses of composted films indicate that low-molecular-weightfractionsare removed preferentially from the more highly substituted and more slowly degrading CA. Buchanan et al. submitted a patent application consisting of various data included in their above-mentioned publications 1321. Vazquez-Torres and Cruz-Ramos [33] studied binary blends of PCL with cellulose esters(CDA,CAB. and CTA) by using D S c , DMTA, and wide-angle X-ray scattering (WAXS)techniques.Aqualitativecomparisonwasmade with the results obtained by polarizing optical microscopy. PCL having MW of 35,000-45,000 was used. The PCL/ CAB system was proved to be partially miscible, whereas PCL/CDA and PCL/CTA appeared to be immiscible. A double-melting behavior was showed for PCL/CAB and PCL/ CTA blends. As these peaks did not shift by varying the heating rate of D S c run, this behavior can be due to melting of two populations of crystals of PCL, which maybe different in size. On the other hand, blends of PCL containing a small amount of CAB or CDA seem to develop more crystallinity for the PCL than this polymer alone. The solvent seems to have a certain influence on the thermal and morphological behaviors of the ascast blends of these three systems, affecting the extent of crystallinity of PCL, as well as its T,,, and AH,. This finding is discussed in the light ofWAXS and polarizing optical microscopy results. Zhang et al. [34] studied the melting and crystallization behavior and phase morphology of bacterial P(3HB) and hydroxyethyl cellulose acetate (HECA) blends prepared by casting films by D S c , Fourier-transform infrared spectroscopy (FT-R), SEM, and POlarizing optical microscopy. The melting temperatures of P(3HB) in the blends were in-

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dependent of the blend composition with P(3HB) contents above 20%. The melting enthalpy of the blends decreased with increase in the HECA component and was close to the additive value of the enthalpy of the two components.The glass transition temperatures of P(3HB) in the blend were constant at about 8°C. No specific interaction between the two components was found by FT-IR. The crystallization of P(3HB) in the blend was affected by the HECA component, especially in the P(3HB)/HECA (20/80) blend. During the DSC cooling run at a lower cooling rate, two separate transitions werefoundfor P(3HB)IHECA (80/20), (60/40), and (40/60) blends, which corresponded to the crystallization of P(3HB) and the phase transformation of HECA from an isotropic phase to mesophase in the blends, respectively. The phase transformation of HECA from an isotropic phase to a mesophase was almost independent of the P(3HB) component. As has been shown in the above literatures [23-341, thermoplastic miscibilities could be found between bacterial or synthetic polyesters and cellulose esters, and amorphous optically clear miscible blends could be formed in certain ranges of combinations. It can be pointed out that novel thermoplastic materials moldable to give transparent homogeneous sheets could be prepared by these blendings. The problems with these are the main uses of cellulose esterssuch as CAB, the biodegradabilities of which have not been studied to a sufficient level, and higher cost performance of CAB compared with CA. At any rate, these studies have been continued with the intent to enhance the biodegradabilities of the materials obtained and furthermore to increase their thermoplasticities by blending the polymeric materials with high biodegradabilities. On the other hand, the addition of low-molecular-weight plasticizers has been taken as the second type of plasticization of cellulose acetates. Buchanan et al. [29] reported that CA can be effectively plasticized by thermal compounding with TEC.Thecompounded resins were converted to compression-molded film and injection-molded bars. Their biodegradabilities were evaluated and confirmed by composting. Concerning this plasticization of CA with low-molecular-weight plasticizers, there appeared in newspapers several announcements of commercialization of biodegradable plasticized CA. One was an announcement from Planet Polymer Technologies. Inc. (California, USA) that CA plasticized with triacetine was entering the Japanese market with the trade name of Lunare. Theother was from Daicel Chemical Industries Ltd., using polycaprolactone oligomer having a molecular weight of 500 as plasticizer for CA. As for methods for plasticization of CA, in addition to the two kinds of external plasticizations mentioned above, which are usually adopted, it is possible to make use of internal plasticization-that is, chemical modification or grafting methods. In view of this situation, we have been attempting in the past 5 years to find novel plasticizers and plasticizing procedures by which biodegradable thermoplastic polymers can be obtained from CAS. In the early stage, attempts have been made to introduce oligoester side chains into CA molecules by reaction of CA with dicarboxylic acid anhydrides such as maleic anhydride (MA) and succinic anhydride (SA) together with monoepoxides such as phenyl glycidyl ether (PGE), styrene oxide (SO), and allyl glycidyl ether (AGE). Theoligoesterifications of CAS have been carried out by the use of a compounding machine at high temperatures with constant kneading speed. The results of these attempts are shown later, With the advancement of this study, it became clear that CA must be sufficiently graft co-polymerized to prevent the bleeding of co-produced homo-oligomers or homopolymers. From these findings came the understanding that the more effective the grafting attained, the more ideal plasticization of CA can be effected. With this idea in mind, the

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authors have come to grafting work, in which CAS are plasticized by cyclic ester grafting using tin(II)2-ethylhexanoate (SnEht,) as catalyst.

II. PLASTICS FROM CELLULOSE ACETATE A.

Cellulose Acetate Plasticized by Reaction with Dibasic Acid Anhydrides and Monoepoxides During Melt Processing, and Their Biodegradabilities

The authors tried to develop a methodology for the plasticization of CAS by reaction with dibasic acid anhydrides and monoepoxide during melt processing [35].In this case, oligoesterified CA, prepared by the reaction of CA with SA and PGE, for example, at a temperature between 70 and 1 80"C, has the hypothetical structure shown in Fig. 1. This kind of introduction of the oligoester side chain into the CA molecule would enhance the thermoplasticity of CA (internal plasticization). At the same time, homooligomers of the oligoester would be produced and these would be able to act as external plasticizers. The effect of the plasticization treatment can be seen in Fig. 2. From the figure, it is known that CDA, though it has the greatest thermoplasticity among cellulose acetates, still lacks melt processability, whereas when it was reacted with SA and PGE at 120°C for 20 min under kneading conditions, it was converted easily into a thermoplastic material. Thus, the above supposition, that the formation of oligoester chains as grafted branches of CAS together with homo-oligomers enhances the thermoplasticity of the products, was confirmed. Since these results are very promising, the plasticization of CAS was examined under different conditions, including amounts of reactive plasticizers added, kneading reaction temperature, molding (hot pressing) time, and so forth. Thus, moldable plasticized products having various mechanical and thermal properties could be obtained. Representative stress-strain curves of CDA and cellulose monoacetate(CMA) oligoesterified are shown comparatively in Fig. 3, along with those for polystyrene (PS) and polyethyrene (PE). From this figure, it appears that both SP-10 and SP-40 have attractive mechanical properties. They are tougher than PS and stronger than PE. Thermal softening curves of L-40 and its plasticized materials are shown in Fig. 4. Among the latter, SS-40 (oligoesterified L-40 reacted with SA and SO) is included as well. From the figure, it can be seen that when CDA was oligoesterified by the reaction with SA and PGE, for example, i n an amount of 30.9 wt% at 120°C for 20 min in the kneader, the flow temperature dropped to 170°C from 250°C. Considerabledecrease in the flow temperature of CDA can be attained by this oligoesterification grafting.

0

0

II

II

OH

FIGURE 1 Schematic chemical structure of CA oligoesterified with SA and PGE.

036

Yoshioka and Shiralshi

FIGURE 2 Effects of oligoesterification on L-40: upper left, untreated L-40; upper right, L-40 hot-pressed at 190°C under 15 MPa for 30 S; lower left, L-40/SA/PGE (100;11.0/33.8) just after being kneaded at 120°C for 20 min without pretreatment; lower right, a sheet from the kneaded sample (lower left) prepared by hot-pressing at 190°C under 15 MPa for 30 S.

Although these results concerning thermoplasticization of CDA and the mechanical properties of the products were extremely fascinating, there often occurred a plasticizer migration (bleeding) problem. This was caused by fugitive plasticizers which are mostly homo-oligomers. The occurrence of plasticizer bleeding was found to be much more pronounced for plasticized CDA than for plasticized CMA (Fig. 5). In the latter, almost no bleeding was found. This difference is thought to be caused by a lack of miscibility, that is, a lack of affinity between the CAS and the oligoesters. To make immiscible polymers (or polymer A and oligomer B) miscible or compatible, a compatibilizer is often added. As one type of compatibilizer, A-B blockor graft co-polymers have been found to beeffective. There-

FIGURE 3 Stress-strain curves of representative CAS plasticized with SA and PGE. SP-l0 (40): LL-l0 (L-40)/SA/PGE = 100/11.0/33.8; prepared without pretreatment; kneading condition 120"C, 30 rpm (5 min)-90 rpm (20 min); hot-pressing condition 190"C,15 MPa, 30 S ; tensile test, span 40 mm, crosshead speed 5 mmlmin.

rom

Plastics Biodegradable

Llgnoce~~u~osics

837

I

50

100 150 200 250 300 Temperature ('C)

FIGURE 4 Thermal softening curves for L-40 and its plasticized materials. L-40: untreated CDA, SP-40: L-40ISNPGE = 100/11.0/33.8, MP-40: L40IMAPGE = 100/10.8/33.8, SS-40: U O I S A I S O = 100/11.0/27.0. Prepared without pretreatment; kneading condition 120°C, 30 rpm (5 min)-90 rpm (20 min); flow test condition, load 10 kgf; heating rate lO"C/min; T,, flow temperature.

FIGURE 5 Bleeding of plasticizer from sheets of plasticized L-40 and LL-10. SP-10: LL-IOISAI PGE = 100/11.0/33.8; prepared on April 21, 1993; SP-40: L40/SA/PGE = 100/11.0/33.8; prepared onApril19,1993: kneading condition 120°C, 30 rpm (5 min)-90 rpm (20 min); hot-pressing condition 190°C, 15 MPa, 30 S; photographed on June 26, 1995.

838

Yoshioka and Shiraishi

A

B

C

D

E

FIGURE 6 Effect of catalyst (Na2COS) on weight gain of cellulose acetate. Kneading, 12OoC, 90 rpm, 15 min; flow test, die diameter 1 mm, length 2 mm; plunger, 1 cm2;load 5 MPa; heating rate 1O0C/min. A, L-4O/MA/PGE = 100/16.9/25.9 (30%); B, L-40/SAPGE = 100/17.3/25.9 (30%); C, L-40OISAPGE = 100/21.7/32.6 (35%); D, LL-lO/SA/PGE = 100/21.7/32.6 (35%); E, L-40/SA/GMA = 100/17.7/25.1 (30%). Each value in parentheses shows reactive plasticizers content (wt%) in the startingmaterial. 0,withoutcatalyst(Na,CO,);withcatalyst(Na,CO,).

fore, a large amount of oligoester side chain attached to the CA molecule can beexpected to enhance the affinity betweenthe modified CAS andthe homo-oligomer. Thatis, grafting can effectively suppress or prevent the bleeding of nongrafting homo-oligomers. CMA can be proceeded by grafting to a higher level compared with the case for CDA. Based on these considerations, methods for enhancing the amount of grafting were pursued by varying the combination of dibasic acid anhydride and monoepoxide, by extending the kneading reaction period, by using grafting catalyst (Fig. 6), and so forth [36]. By these trials, the grafting could be enhanced, which actually resulted in the suppression of the plasticizer bleeding. The biodegradabilities of the representative samples obtained were examined by a soil burial test in a controlled environment (30°C, 80% RH) and by the measurement of oxygen consumption in a closed system where test samples were exposed to standard activated sludge [36]. Concerning the former, it was found in a few examples, as in Fig. 7, that the plasticized CA samples were degraded and disappeared within relatively short periods (i.e., within 3-12 months). Results of the biodegradation test measured by means of the oxygen consumption within a closed activated sludge suspension are shown in Fig. 8. It is apparent that all samples are subjected to significant biodegradation. The CMA control sample, of which biodegradation had been literarily confirmed, was degraded more slowly than any of the oligoesterified samples.

B. Cellulose Acetate Plasticized by Grafting with Cyclic Esters [36-391 In the previous section, it was shown that CA must be sufficiently graft polymerized to achieve effective plasticization and to prevent the bleeding of homo-oligomers. Thus, the amounts of grafting and the graft efficiency were intended to increase. As an extensionof the efforts, the authors have come to grafting work in which cyclic esters are reacted with CA using SnEhtz as a catalyst. This is based on the following information found in relatively recent publications. It is often said that little is known about the polymerization mechanism of cyclic esters in the presence of SnEht, [40,41]. Ikada described the same in his review article [40]on “polylactic acid,” butintroduced a mechanismbywhichhomo-polymers are produced predominantly, even though the ring-opening polymerization of cyclic esters is

Samples disapped after 3 months

Samples disappeared

l

*er l2 months

FIGURE 7 Changes SA-l0 and SP-IO specimens during the soil burial test in the incubator.

839

Shiraishi

840

Yoshioka and

Exposure time (week) FIGURE 8 Results of' the exposure test on the closed activated sludge system. Degree of degradation was calculated using oxygen consumption and theoretical initial oxygen demand. 0.LL-IO: 0, SP-40: L40/SA/PGE = 100/11.0/25.9; kneading, 120°C. 20 min. A, MP-IO: LL- IO/MA/F'GE = 100111.0/33.1; kneading,120°C.20 min, V,SA-IO:LL-IO/SA/AGE = 100/11.0/2S.9; kneading. 80°C, 15 min.

conducted in the presence of CDA. On the contrary, Kricheldorf and his co-workers studied the polymerization of L-lactide catalyzed with SnEht, i n the presence or absence of benzyl alcohol [42]. When SnEhtz and benzyl alcohol are used as a catalyst and a co-initiator, respectively, NMR spectroscopicexamination of all polylactidesobtained by the ringopening polymerization revealed the presence of benzyl ester end groups but the absence of 2-ethylhexanoate end groups [30]. This result means that the hydroxyl group of alcohols plays an essential and direct role in initiating the ring-opening polymerization of cyclic esters. In this sense, graft polymerization can be expected to occur selectively when the cyclic esters are polymerized in the presence of CA and SnEht?. Thus, the authors started a co-grafting study of s-caprolactone (CL) and I,-lactide (LACD) onto CDA using SnEhtz as a catalystin order to realize grafting with considerably

2011

O

O

,

, 20

,

, 40

,

, 60

Reaction time (min)

FIGURE 9 Effects of thereactiontime on theyield.Reactiontemperature140°C;L-40/(LACD + CL)/catalyst, 100/600/15 (by weight); LACD/CL, I.O/I.O (by mole).

Biodegradable Plastics from Lignocellulosics

0

20 40 Reaction time (min)

841

60

FIGURE 10 Effects o f thereactiontimeonthe flow temperature. Reaction temperature 140°C; L-40/(LACD CL)/catalyst. 100/600/15 (by weight): LACDICL, 1.0/1.0 (by mole).

+

high graft efficiency. In these cases, the reaction temperature was kept constant at 140°C and the reaction time was changed from 0 to 60 min. The other reaction conditions are shown in the footnotes of the relevant figures (Figs. 9-12). One of the characteristics of this grafting is that the grafting reaction can proceed rapidly and can be completed within 10-30 min (Fig. 9), and in correspondence with this, the flow temperature decreases to about halfof that of CDA(Fig. 10). The products obtained are moldable to films or sheets without using any plasticizer. Furthermore, it is known from Fig. 1 1 that LACD is grafted more rapidly than CL, producing relatively rigid and brittle products in the earlier stages and elastomer-like ones in the latter stages of the grafting. From Fig. I I , it can also be said that the total molar substitution reached its theoretical maximum value after 10 min of grafting. This result confirms that the graft reaction proceeds at a high rate and shows that selective grafting is achieved completely without significant production of homo-polymers or homo-oligomers. This supports the reaction mechanism proposed by Kricheldorf et al. (421. Transparent sheets were obtained depending on the reaction conditions, showing their amorphous nature. In this regard, the triad structure of the grafted side chain was analyzed by means of high-resolution NMR spectroscopy. An example of the NMR data is shown in Fig. 12, in which the E-oxycaproyl unit is denoted by C and the lactidyl unit by LL. Each of the splitting spectral lines for a, /3, y, &methylene carbons of c-oxycaproyl units and the methyl carbon of lactidyl unit

LL -1""

20 40 Rcaction timc (min)

GO

FIGURE 11 Effects of thereaction time on the molar substitution. Reaction temperature 140°C; L40/(LACD + CL)/catalyst, 100/600/15 (by weight); LACD/CL, I.O/I.O(by mole). 0 , LACD; A, CL: 0 , LACD + CL; , theoretical maximum value of (LACD + CL). ~

842

Yoshioka and Shiraishi

Y

38

3‘

31

0

1I

26

PP*

14

B

10

I,

l8

11

I1

FIGURE 12 ”C-NMR spectrum of (CL-CO-LACD) grafted CDA. H- 1. Region of a-,p-, y -, Smethylenecarbonatoms of coxycaproyl units, methylcarbonatom of lactidylunit, and acetyl methyl carbon atoms of CDA.

in a grafted product was assigned by reference to Kasperczyk and Bero’s work 1431. The spectrum of Fig. 12 means that even for a grafted CDA (H-l) prepared by using a liquid ratio of 2, LACD/CL = 2/5 (by mole), reaction time of 30 min, and reaction temperature of 140°C, the introduced graft side chains are long enough to reveal triad sensitivity. Appearance of LLCC, CCLL, and LLCLL sequences, total signal strength of which are comparable or larger than that due to the CCC sequence, reveals meaningful occurrences of random polymerization of CL and LACD within the graft side chains. This fact confers irregularity in graft chains and is considered to be related to the appearance of the high thermoplasticity and amorphous nature found and discussed above. In other words, it can be said that the analysis of the structure of the grafting side chain by means of NMR spectroscopy showed that, although the side chains are composed of large amounts of c-oxycaproyl or lactidyl block polymer portions, considerable amounts of randomly polymerized parts coexist in the grafting chains, which confers high thermoplasticity and amorphous nature on the grafted product obtained.

111.

PLASTICS FROM LIGNOCELLULOSE AND APPLICATION TO BIODEGRADABLE POLYMERS

A.

Plasticization of Wood by Benzylation and Blending with Polycaprolactone

Since lignocellulosics, including wood, are not thermally flowable materials, the methods for processing them are limited. If appropriate thermoplastic properties could be imparted to the lignocellulosics, they would become more useful materials. More than 15 years ago, it was found that wood could be converted into a plastic material by chemical modification, such as by esterification and etherification [4,5]. Chem-

Biodegradable Lignocellulosics Plasticsfrom

843

ical modification does not necessarily require special techniques; conventional and simple methods work satisfactorily for this purpose [6,7]. The phenomenon of thermal flow can be explained in terms of internal plasticization of wood. The introduction of large nonpolar substituent groups into wood can result in a chemically modified material with high thermoplasticity. When small substituent groups and/or polar groups are introduced, such thermal fluidity cannot be achieved; therefore, the latter modification alone cannot produce plastic properties. However, this lack of plasticity can be solved by external plasticization. All of the above reveals that chemically modified wood derivatives can be considered as novel biobased plastic materials, and experimental studies have targeted the development of composites with enhanced physical properties. There are various types of thermoplasticized wood derivatives. Among them, BzW is known as a moldable material giving excellent mechanical properties. Indeed, a molded sample of BzW showed a tensile strength of 42.7 MPa, which is fairly high when compared, for example, with that of polystyrene (PS) (Styron 666, a product of Asahi Chemical Industry Co. Ltd.). The tensile strength of Styron is 29.4 MPa under the same conditions. Differential scanning calorimetric measurements have revealed that both polymers are amorphous. Thus, the thermal softening behavior is similar, qualitatively, when measured with a thermomechanical analyzer. Quantitatively, there aresomedifferences. Experimental observations with a flow tester revealed that PS undergoes flow at 153"C, while BzW starts to flow at 175°C. Almost identical results are found when BzW is compared with polypropylene (PP) (J700G; MI = 7, a product of Idemitsu Petrochemical Co. Ltd.), which is also widely used as a thermoplastic polymer. However, even though the flow temperature of BzW is higher than that of PS or PP, the temperature difference can be reduced to almost zero by blending polycaprolactone (PCL) (PLACCEL H-4; Mn = 40,000, a product of Daicel Chemical Industries, Ltd.) in an amount of 20% with BzW. Figure 13 shows the dependence of melt viscosity (p)and shear stress (TU) on the shear rate ( 7 ) of BzW and PP, measured with a Capirograph. Itis seen that both the

7 n

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.-0

v)

c

W

5t

FIGURE 13 Melt viscosity and shearstress versus shear rate for BzW and PP.

Yoshioka and Shiraishi

844

viscosity and the shear stress of BzW reveal similar shear rate dependences as those for PP. However, the values for BzW are one order larger than those for PP. This means that there exists a greater resistance to thermal processing for BzW. In order to reduce this resistance, blends of BzW with PCL were studied. since the latter has considerably greater thermoplasticity. Since PCL itself has a low melting point (ca. 60°C) and low tensile strength (19 MPa) at room temperature, its compounding with BzW would be of great significance. Results obtained are shown in Fig. 14. As BzW content increases. both the viscosity and shear stress are seen to decrease. At a PCL content of 30%, the characteristic melt flow values converge with those for PP, which are shown as dotted lines. Thus, the blended composite consisting of 70% BzW and 30% PCL has practically the same fluidity as PP. However, the mechanical properties of the blends were reduced by this blending as shown in Fig. 15. This is a widely observed phenomenon in polymer blends. The mechanical properties of the latter. can, however, be improved by the useof appropriate compatibilizers, which enhance the adhesion of interfaces between or anlong the domains or phases of blended polymers. As one of such attempts, the styrene-maleic acid anhydride co-polymer (SMA) was used and its effect as a compatibilizer was investigated. The results obtained are shown in Fig. 16. With an increase in the amount of added SMA, the tensile strength of the BzW/ PCL sheet recovered; and when the SMA content became 596, the tensile strength rose to 200 kgflcm' ( 19.6 MPa). The composite also showed practical moldability as shown in Fig. 17. The photograph displays trays that were vacuum-formed from sheets of the blends. This kind of tray is commonly used in grocery stores. These trays possess not only reasonable strength but they also have functional properties as biodegradability and photodegradability, as shown later.

B.

Bio- and Photodegradabilities of BzW and BzW/PCL Composites

Novel thermoplastic materials, which have been developed by the chemical modification of wood, have been described in Section 1II.A. Although they are of biological origin, an

n

W

.-m0

c

U

a M 0 -

M

0 -

845

Biodegradable Plastics from Lignocellulosics

GOO

-

-600 - 6

-

0 : Tensile strength

0:Break elongation A

'

Tensile modulus

0

o

400 -A

- 4 0 0 .c2 . 4

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FIGURE 15

-U .U

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c

0

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? I

I

A c

I .

0 -0

0/10

(BzW/PCL)

Relationship between mechanical properties of the BzW sheets and PCL content

evaluation of the biodegradability and photodegradability characteristics of these thermoplasticized wood derivatives is indispensable. Thus, the degradability of BzW was compared with that of synthetic polymers including PP, PS, polycarbonate, and others.The results reveal that while the sheets of synthetic polymers did not lose any strength at all even after 80 days of immersion in aerobic and anaerobic activated sludge, sheets of BzW exhibited continuous deterioration and strength reduction. The results for BzW are shown in Table 1 . The results were most impressive since, although PP is said to be photodegradable to a certain extent, it did not reveal any change in its external appearance or its strength; whereas BzW became very brittle and was destroyed after 80 days of exposure to sunlight.

SMA (part)

FIGURE 16 Relationship betweentensilestrength content.

of BzW/PCL 1713 (w/w)lsheetsand

SMA

Shlraishi 846

and

Yoshloka

FIGURE 17 Trays from BzWPCL blended composites.

Furthermore, the BzWrPCL sheets were found to undergo faster biodegradation than those of each individual sheet component, as revealed by tests carried out under the same conditions (not shown). This fact is remarkable inasmuch as PCL is well known for its high biodegradability.

W.

CONCLUSION

In this review, recent studies on biodegradable plastics from cellulose and lignocellulose were elucidated.It is known that thesestudies have just been developing. Even elastomeric biodegradable polymers could be introduced from cellulose, which had been considered a rigid polymer. This new development anticipates a wide and prospective future for that kind of development.

TABLE 1

Biodegradability andPhotodegradability of BzW

Control (BzW) Dipping in aerobic activated sludge for 80 days Dipping in anaerobic activated4.8 sludge for 80 days Dipping in sea water for 80 days Exposuring to sunlight

Tensile strength (MW

Tensile breaking elongation (%)

42.7 37.7 37.9 39.8

9.8 4.1

-

6.1

-

Biodegradable Plastics from Lignocellulosics

047

REFERENCES 1.

2.

3. 4. S.

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IS. 16. 17. 18. 19.

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R. M. Gardner, C. M. Buchanan, R. Komarek. D. Dorschel. C. Boggs, and A. W. White, J . Appl. Po/yr?~er Sci.. 52: 1477 ( 1 994). C. M. Buchanan, R. M. Gardner, M. D. Wood, A. W. White, S. C. Gedon. and F. D. Barlow. U.S. Patent 5292783 (1994). H. Viizquez-Torres and C. A. Cruz-Ramos, J. Appl. Polynrer Sci.. 54: 1141 (1994). L. Zhang, X. Deng. S. Zhao, and Z. Huang. Polyrr~er;38(24):6001 (1997). M. Yoshioka, T. Miyazaki, and N. Shiraishi, Mokuzcti Gtrkknishi, 42(4):406 (1996). M. Yoshioka, K. Okajima, T. Miyazaki, and N. Shiraishi. J. Wood Sci., 46( 1):22 (2000). M. Yoshioka, Preprints of YN-2 R e g d m Meeting o f t h e Society j b r rhe Stcrcly c f Eco-rrlutc,ricrl. Society o f Polymer Scientists, Japan. p. I (1998). M. Yoshioka, H. Mizumoto, N. Hagiwara, and N. Shiraishi, Pwprirlr oj' '98 Cellrrlo.se K & D , 5th Annual Meeting of the Cellulose Society. Japan, p. 26 (1998). M. Yoshioka, N. Hagiwara, and N. Shiraishi. Cellulose, 6 ( 3 ) :193 (1999). Y. Ikada, in Hurdhook of Hiodcgrurlcrble Plastics (Y. Doi et al.. eds.), N. T. S. Ltd.. Tokyo. p. 279 (1995). A. Kowalski, A. Duda, and S. Penczek, Mncrorrrol. Kcrpid Cornmrr~..19567 (1998). H. R. Kricheldorf, 1. Kreiser-Saunders, and C. Boetlcher, Po/ym>r:36(6):1253 (1995). J. Kasperczyk and M. Bero, Mrlcrortzol. C / I ~ I I192: I . , 1777 (199 I ).

Recycling of Wood and Fiber Products Takanori Arima The University of Tokyo, Tokyo, Japan

Wood has been the most important construction and decorative material since ancient times. The importance of wood will certainly increase in future years, since among all the construction materials, it is the only major renewable resource. Further, in view of growing energy consciousness and the resulting emissions of carbon dioxide to the earth's atmosphere, greater wood utilization is to be encouraged, since wood processing requires relatively smallamounts of energy. Since wood is a typical organic compound which is constituted by absorbing carbon dioxide from the atmosphere, biodegradation and burning leads to the release of carbon dioxide to the atmosphere. The effective use of wood waste produced from industries that are processing or consuming wood and of timber recovered from demolished houses has become a serious issue. Solving this problem could be an effective means for saving wood resources and reducing the emission of carbon dioxide, even though the processing requires a reasonable amount of energy. For wood-based resources, the breakdown ofraw material basically follows a sequence of steps from logs to lumber, to chips, to fibers, to charcoal, and tinally to fuel. After wood-based products such as building products, furniture. paper, etc., leave the factory, they are utilized and eventually disposed of. With disposal, they again become available as raw materials to various wood-utilizing industries if they are collected, shipped, and reprocessed. In such a case, it can be said that there is relatively little negative impact associated with this type of recycling. For example, there are many cases in Japan where, after removing foreign matter, wood recovered from demolished timber structures is reused in chip form for particleboard production or as a fuel source. If the material is prepared in this way, it presents no technical difficulties and can therefore be recycled. However, when the recycling conditions are not met, the wood material will simply be thrown away or incinerated along with other garbage. The essential point is to address adequately the problems of (1) usable life spans of wooden materials such as furniture and building materials; ( 2 ) their subsequent quality considerations asa recyclable raw material, including the removal of foreign debris; and ( 3 ) the collection ofthewood material. It is vital that this should be coordinated in order to address effectively the issues of environmental protection and energy concerns. In this chapter, an overview is given of the current conditions and issues related to the recycling of wood materials both from the wood industries and from the demolition of wooden structures in Japan. 849

Arirna

850

1. A.

EXISTING DISPOSAL AND UTILIZATIONIN WOOD INDUSTRY General

An outline of the use of wood wastes from wood industries is shown in Fig. 1. The breakdown of raw materials basically follows a sequence of steps in change of form from the unprocessed or semiprocessed state (such as logs or large timbers to be further processed) to lumber to chips and finally to fibers. One can imagine this process as a kind of “cascade” as the material is sequentially broken down into smaller units. Accordingly, provided that foreign objects or otherimpurities do not enter into thisprocess, it is possible to capture and utilize this “cascade”for the production of wood-based materials. For example, particleboard producers utilize the waste of lumber and plywood production which are farther upstream in this “cascade,” usually in chipform,as do many pulp producers as well. The waste trimmings, sawdust, bark, etc., of the wood-utilizing industries such as plywood, particleboard, fiberboard, and so on, are often burned to supplement the energy needs of the factories where the wood waste is generated. Although there has been a considerable amount of technical research into the uses of wood waste, whether it is economical for industry depends very much on its location and on the method of transport and the cost. Most of the residues from lumber mills are sent to the paper, particleboard, fiberboard, and wood-particle cement board industries as chips or flakes. Existing disposal and utilization of sawdust produced from lumber mills consists of fertilizer, briquettes, charcoal, and fuel. Approximately 70% of sawdust seems to be used effectively. Other wood waste is burned as fuel in the wood waste-generating industries’ own boilers or is incinerated or dumped. On the other hand, most of the wood waste from the building, furniture, and pallet industries seems to be disposed of by dumping or burning. Effective utilization of the wood waste generated on construction sites, such as used concrete-form plywood and the timber recovered from demolished houses, remains as one of the most important problems to be solved technically and economically, as described in Section 11.

B. Wood Industry Integration Plan The wood industry integration plan calls for the integration of the various industries that produce or use wood into segregated areas removed from residential districts. Such integration aims to improve the circulation of raw materials and products and to solve problems of pollution. In 1997, about 100 such areas were completed or under construction, and this seems to be a reasonable achievement. However, they still present some problems, especially cooperation between various manufacturers and the difficulty of incorporating the building and furniture industries. These areas have helped to solve the disposal problem and have enabled better utilization of wood waste for various purposes, since the production of waste from many industries is concentrated in a particular area.

C.Wood

Chip Industry

Residues produced from forest, lumber, and plywood mills, consisting of branches, timber, and plywood off-cuts, are generally used to produce wood chips for pulp, particleboard, and fiberboard. The volume and type of chips produced from the various sources are shown in Table 1. The softwood chips are produced from wastes from lumber sawmills more than from logs of small diameter. Use of the waste from demolition of wooden buildings and pallets, which are composed mainly of softwood, tends lo be gradually increasing

851

Recycling of Wood and Fiber Products

Wood waste from forest l l

l-lCharcoal Ir

Wood particle cement board Fiberboard Pulp cement board

FIGURE 1

Brief outline of distribution of woodwaste.

around the urban areas. Most of the hardwood chips are produced from logs of miscellaneous species. As most of the lumber mills in Japan are minor enterprises and are widely dispersed, transportation costs and the low prices of imported chips often restrict the effective use of wood waste. The wood integration plan as described could make a definite contribution to better utilization of wood waste.

D.

ParticleboardandFiberboard

The particleboard and fiberboard industries have made a considerable contribution to the utilization ofwood waste produced from lumber and plywood mills, so-called ordinary chips. Figure 2 shows the annual particleboard production in 11 Japanese industries, which produce about 80% of the total amount of Japanese production. The shaded portion indicates the ratio of recycled wood chips. The sum of the shaded portion reached about 20% of the total production. According to a survey 10 years ago, only a few mills used

TABLE 1 Origin of Wood Chips, 1997 ( X 1000 m') [3] Wastes from Grouping

Total

~ _ _ __

~~

Total Softwood Species Type of mill

Log

Own mill

Wastes . . from from Other forest

Wastes demolition

~~

11,165 5,812 4,708

Hardwood Lumber and chips Plywood, Ilooring. and chips Chips only

Total

mill

7,009 1,234 4,156 3,474 7,083 4.533 4.952 2,063 76

4,006

4,623 5,290 4,244 522143 379

141.189 1,046 419

76 -

76

2,645

784

-

14

770

7 7 7

63 I 47 8 IS3 61

-

-

7

570

852

Arlma

0

4.’

0 0 0 0

Recycledwoodchip

10

A

B

C

D

E

F

G

H

I

J

K

PB Maker

FIGURE 2 Particleboard production by 1 1 makers in 1992, and the proportion of recycled wood used.

recycled chips. Changes in conditions for procuring raw material could have elevated the importance of recycled wood in particleboard production over the last decade, as described in Section 111.

E.Wood-ParticleCementBoard Wood-particlecementboard of Japanese Industrial Standard is used as sheathing and fireproof material for buildings. The wood chips are produced from lumber, plywood, and from the demolition lumber of wooden structures. InJapan, large quantities of plywood are used as formworkpanels for concrete construction in buildings and civil engineering works. The waste panels from this work are contaminated withcementand sand, and are usuallydumped or burned.Ways of utilizing this material by crushing the waste panels and producinga type of wood-particle cementboardhavebeen investigated, butwhetherthis is technically or economically feasible depends on collecting wood waste constantly and expanding the market for this type of board. II. WOOD RESOURCES FROM DEMOLISHED BUILDINGS According to the Ministry of Health and Welfare’s “Change in Waste by Industry” report, in 1994 approximately 8 million tons of wood debris were transported from demolition sites. When this is converted to a volume basis, the figure exceeds 11 million cubic meters of wood waste. The ratio of reuse is approximately 36%. Additional waste such as furniture, pallets, leftover waste materials from new construction sites, etc., are thrown together, making up the difference. The actual volume of wood which can be reused varies considerably according to the demolition method employed. Below, the desired situation and problems of recycling or cascade use of wood demolition material are stated. (1) The nature of wood waste has changed, the wood particles have become smaller, more mixed with foreign matter, and are of many varieties. The use of a magnet can easily remove the steel objects such as nails, but nonferrous objects such as aluminum, plastic, cement, gypsum board, etc., present difficulttechnical problems inthe sorting and selection of chips, which need to be solved.

Recycling of Wood and Fiber Products

853

( 2 ) The condition and amount of the chips are determined by the manner in which the building is demolished-primarily, whether it was done by machine or by hand. AS shown in Table 2, this determines whether the chips are of suitable quality for pulping, board production, fuel chips, or just plain garbage, and has a tremendous effect on the price and volume of chipsobtained.Therefore, it is necessary to establish a means of sorting the chips according to quality so that they can be properly utilized to their maximum potential. Already, there is a slowly growing trend at the chip collection sites for a minimum acceptable level of chip quality. On a regional basis, there are instances where one can see thought has been put into the recycling of demolition material as well as cases where there appears to be no consideration at all as to the reuse of the recovered wood waste. The important deciding factor seems to be whether there is a facility to handle the wood waste in the locality, or sufficient volume to justify such a facility. (3) Ideally, the demolition and site preparation would be done carefully, but in many instances these steps are rushed, exacerbated by a shortage of labor that does not allow for hand demolition andwood waste segregation. As a result, the reliance on machine demolition damages muchof the material and introduces foreign matter into thewood waste. This also adversely affects the efficiency of the waste transportation, as it becomes impossible to load the machine-demolished and -loaded material in a way which maximizes the use of the truck. This adds further cost to a low-value waste. By wayof comparison, let us lookat an example of a machine-demolished but material-segregated case and a totally machine-demolished case where no effort is made to recover any of the material. In the sorted case, where the segregated demolition material can be more neatly loaded onto a truck, the load will weigh about 0.72 ton per cubic meter. compared to 0.38 ton per cubic meter for the unsorted case. Thus, a truck in the sorted case will carry almost twice the load of one in the unsorted case. Also, among the mixed waste to dump, the volume of wood in the unsorted case is about five times that of the sorted case. If the demolition material were to be separated, it would be possible to recycle a greater portion of it; however, when this is not done, the material will be disposed of as mixed waste and will not be recycled. The reliance on mechanical demolition arises from a limited amount of time available to devote to the job, the ease with which demolition may be conducted by machine compared to hand demolition,and a general shortage of labor. As a result, a larger volume of garbage is generated which is difficult to reuse or recycle.

TABLE 2

Wood Waste from Timber Structures Demolished by Machine or by Hand 171 VoIume/floor area (m’/rn2)

Demolished by machine Demolished by machine and hand Dcmolished by hand Raw material for

Mixed wood waste with debris

Wood waste without debris

0.038 0.033 0.07 1

0.070 0 . I00 0.120

Fuel Particleboard

Paper Fiberboard Particleboard

Total IO8 0. I33

0 .

0.19 1

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854

(4) The generation and collection of waste material varies widely across regions and also by seasons, making it very difficult to obtain a stable supply and quality. Therefore, individual companies that must consider profit implications of using a new material will be reluctant to do so unless the supply is stable and the costs are low. Presently, the price of chips from demolition material is determined by their quality and suitability for fuel, board, or pulp production. At the same time, chips of pulping quality must compete with chips of a virgin nature, while fuel chips must compete with fossil fuels. Foreign currencies have a huge impact on the competition of these materials. (S) With greater urbanization, it has become very difficult to build recycling facilities, due to site restrictions. operating hours, and noise-prevention problems. For more distantly located sites, working hours and transportation considerations create a large burden, making it unprofitable and unfeasible to collect and process waste wood material. Because efforts on behalf of the government and individual companies are being conducted piecemeal and according to individual agendas, there is little active effort at coordinating the policies of recycling and garbage disposal. (6) The wood products industry has accepted the inevitability of using demolition from only a cost-and-supply viewpoint, but the usage will be determined by a balance of technological risk associated with manufacturing and the cost of raw materials. Accordingly. if a new raw material were to have stable price and supply, the wood products industry would naturally tend to use the new material. The recovered wood waste would tend to become a reduction i n trash. In this scenario, competition between new raw materials and the demolition wood waste on these simple economic terms would potcntially negate environmental conservation efforts in regard to the utilization of wood waste from demolition sites.

111.

THE PRESENT SITUATION FOR DEMOLITION WOOD CHIPS

From a technological standpoint it has become possible to utilize wood resources, including recovered demolition material. in a systematic cascade-type approach described earlier, where finally the wood is converted to carbon dioxide (CO,) by incineration. However, the great volumc of material generated creates difficulties in recycling and incineration with respect to social and economic efficiencies. According to the different methods of demolition, primarily mechanical and hand. the quality of the wood waste is changed where the material may be suitable raw material for manufactured wood products, chips, or unusable trash. I n order to prevent the recovered wood from becoming trash to be thrown away, it has become clear that an active and aggressive effort is needed. In spite of the fact that wood waste recycling efforts have shown positive results compared to the past, the recycling of demolition wood waste (demolition chips) still finds itself i n a very difficult situation. Wood chips arc generally segregated for utilization i n pulp, wood-based composite boards, or fuel production. In the regions around Tokyo, Chubu, and Osaka which are close to the typical urban areas, the composition of the chip material in the region’s 35 chip facilities is shown i n Table 3 . Approximately half of the chip materials are collected from the waste of house demolition. The waste from concrete-form. pallet, and packaging demolition also is considerably used because it is easy to collect and also to remove foreign debris. I n the casc of the board industrics, the large proportion of chips originating from the demolition of houses can easily be seen, a s shown i n Table 4.

Recycling of Wood and Fiber Products

855

TABLE 3 Composition Ratio of Chip Materials in Chip Recycling Facilities

[lo]

Waste from m i l l

Grouping

X 1000

area of Total Kanto

Chubu Kansai

(tons) 654 21 I 272 3.9171

Waste from demolition (%)

(%o)

Total

Wood product Housing House 5.3 1.9 9.2 3.2

Other Pallet

Concrete

Packaging

5 .4 12.0 I .2

54.4 48.4 54.1 62.3

5.9 6.7 4.8 6.5

8.5 9.5 7.0 9.7

forms

(c/)

13.2 19.2 IO. I 10.8

7.3 2.3 13.6 3.5

Table 5 shows the breakdown of industries which utilize wood waste chips, where the users are forest products-related industries.Particleboard and wood-particlecement board industries use recycled chip materials mostly as raw material. In fiberboard and pulp industries, the ratio of fuel use tends to increase because the quality level of raw material is required to be high. In the plywood, gypsum board, and dye industries, wood chips are used as fuel for drying. Table 6 shows, in order of relative importance, the primary considerations in chip quality as a raw material for production of pulp and boards, and for use asfuel.Pulp producers presently accept very little demolition wood chips for pulp production and are not actively pursuing furtherexpansion i n demolition wood chipacquisition.Thepulp sectors which do not accept demolition chips are involved primarily in the production of paperboard products such as cardboard. Demolition chips have great difficulty competing with virgin fiber sources due to the presence of foreign particles in the chips, cost, and other factors which make demolition chips unattractive to pulp producers. At present the supply and price of imported chips and waste from other wood industry sectors is both stable and economical, which is probably the main reason for the lack of interest and effort regarding greater demolition wood chip utilization in the pulp industry. In Japan, the recycling of old paper such as newspaper is very high; nevertheless, the industry is in a difficult situation and the incineration of large volumes of paper presents a considerable burden when considering society's economic and social efficiencies and priorities. If the chips from demolition wood waste are sorted, the negative aspects of these chips become negligible or nonexistent and the wood-based boards produced from such material can receive recognition by means of the Ecomark seal. Using demolition chips alsosaves on dryingexpenses,as they are usually drier than material from alternative sources. However, in recent years the percentage of nonferrous waste material mixed with

TABLE 4

Composition Ratio of Recycling Chip Materials in theBoard Industries [ I O ] Waste from demolition ( 7 6 )

Grouping of

Total xIO00

products

(tons)

House

Packaging

Pallet

form

I37 95 35

67.7 67.7 98.0

13.8 8.6

18.7 18.7

12.9 I .4 0.9

Particleboard Fiberboard Wood-particle ccment board

Concretc

1.1

~

Other (c/)

3.6 3.6 -

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856

TABLE 5 UsageRatio of Recycling Chip Materials in Industries [ I O ]

Grouping of

material Fuel, Raw

industry

tons

tons (%)

Particleboard Fiberboard Pulp Wood-particle cement board Plywood Gypsum board Dye Other Total

(%)

Other, tons

Total, tons

(%l

(%Io)

29,956 (2 1.6) 3,8 18 (2.7) 85,264 (6 I .4) 9,920 (7.1) 960 (0.7) 4,000 (2.9) 0 (0) 4,968 (3.6)

38,316 (7.5) 170,993 (33.5) 125,300 (24.5) 63,125 ( 12.4)

4,120 ( 100)

34,168 (5.2) 20,575 (3.1) 177,449 (27.1) 9,920 ( 1 .S) 39,276 (6.0) 174,993 (26.8) 125,300 ( 19.2) 8,093 ( I 1 .O)

138,886 (100) (2 I .2)

510,888 (100) (78.1)

4,120 ( 100) (0.6)

653.894 ( 100) (100)

4,2 I2 (0.8) 16,757 (3.3) 92,185 (18.0) 0 (0)

the wood chips has created difficulties, as the simple use of a magnetic sorting system is no longer sufficient. Accordingly, it becomesquiteimportant to what degreesortingis carried out at the demolition site. Moreover, the nonselection of materials, especially ones which will create waste disposal problems when demolition ultimately occurs, becomes vital from theconception of the building process. Along theselines, material selection based simply on functionality should be discarded in favor of greater consideration of the complete life cycle of the materials up to the ultimate conversion to CO, with incineration. Because the scale of wood-based material factories, such a particleboard, is limited due to site restrictions among other factors, geographic factors become important when considering the use of demolition chips for board production. In the wood industries, the waste raw material which does not find its way into pulp or board production is often burned to supplement energy requirements of the mills or sent for incineration elsewhere. Demolition wood chips used for fuel have comeinto greater use as an economic alternative to fossil fuels, and are often used as fuel for public baths and burned in boilers of various industries. However, in recent years the use of fuel chips has come into difficult times due to sagging prices and volumes. The economic costs of conveyor maintenance, ash disposal, and personnel for wood waste boilers have become increasingly burdensome, and many facilities are increasingly switching to oil and, especially, gas.

TABLE 6 Problems in Utilizing Demolition Chips as a RawMaterial Input (ranked in order of importance) [ 101 Rank

1 2 3 4 5 6

Pulp

Foreign matter Collection Chip quality Cost Finished goods quality Demand

Fuel Board production Collection Foreign matter Chip quality Demand Finished goods quality Lack of' awareness

Collection Demand Pricing Chip quality

Foreign nlatter Cost

Recycling Products Fiber of Wood and

857

The oil shocks of the past sounded the alarm of too much reliance on limited oil resources across all industries and sectors, including those related to building materials, where too much energy was consumed in the production, assembly, transportation, and demolition relatcd to those materials, at the same time came increased awareness of wasteful resource consumption. The use of wood and wood materials as a supplemental and alternative energy source was examined; however, burning wood has a lower heat efficiency, and in ordertoobtain the sameamount of energyas by burningoil, a greater volume of CO2 would be generated and released to the atmosphere, contributing a greater amount to the global warmingphenomenon. In otherwords, the move toward burning wood as an alternative to oil, though possible, provides an inferior choice concerning the environmental problems associated with excessive COz generation. When viewed from the point of efficiency and economics, the only likely trend is a move to oil and gas. However, at the end of its life cycle, wood will eventually be burned, and wood waste destined for incineration (versus fuel generation) combined with the burning of oil or gas used to fuel the incinerators themselves results in essentially a double release of carbon to the atmosphere. As a consequence, the burning of wood waste should be the last step i n a series of steps which utilizes the wood through a cascade-type recycling process. In this vein, it is necessary to give due consideration to effective use of waste materials, including the safety of the waste and any by-products from utilizing that waste. In other words, simply incinerating wood waste materials should be avoided if possible, and consideration should be given to systems which can capture and utilize the energy contained in the wood. The city of Sapporo,Japan, has wastedisposalresourcefacilities which produce solid-waste fuel, fuel chips, and chips for particleboard production. The uses of this waste include heat generation for greenhouses, electricity generation, and supplemental energy for industrial facilities. In order to carry out the proper sorting and material selection and to adjust to the seasonal variations in the waste types and quality, the use of silos becomes a vital factor in enabling operations to proceed smoothly.

IV. CONVERTING WOOD WASTE TO CHARCOAL AS A MEANS OF UTILIZATION AND DISPOSAL For Japan’s construction industry, and the housing industry in particular, the era of emphasis on volume of construction has yielded to a new era where emphasis is placed on quality. From this has arisen the “scrap and build” phenomenon, where large numbers of structures built i n previous years when volume was emphasized are demolished and new, high-quality buildings take their place. Because of the large numbers of buildings being scrapped after a relatively short life span, the problem of waste disposal and recycling has become an unavoidable issue. It is possible to reuse and recycle the waste from residential demolition sites as well asother public works and construction/demolitionsitesforvarious uses such as pulp, board, and fuel chips in the cascade-type manner describedearlicr. However, the huge volumes of waste being generated and the economic efficiency problems associated with utilizing waste chips have created a very difficult situation regarding the full utilization of the demolition chips. To meet the demand for reducing the volume of waste, i n the worst case, it could simply be incinerated or decomposed in a manner with comparatively few pollution problems, but disposing of the huge volumes of waste i n such a manner would emit large quantities of CO, into the atmosphere, which should be avoided to the degree possible.

Arima

858

From this background, the goal is to develop further the reuse and recycling alternatives while simultaneously reducing the volume of wood waste and keeping the CO, in a stored form (i.e., not released into the atmosphere until as late a time as possible) by converting waste wood resources into charcoal products with sequential carbonizing heat treatment steps. In the past the production of the charcoals, from ordinary general-purpose charcoal to special types such as for filters, has relied on relatively clean wood resources such as virgin material, and not material such as demolition waste. Charcoal is a very stable material compared to other biodegradable materials. Because of this, it is possible to change the method of disposal of charcoal, since it may be added to just about any soil anywhere without any adverse effects, thus alleviating some pressure on landfills. Because the physical properties differ according to the different levels of heat treatment, proper heat treatment is aimed at producing a charcoal suitable for prescribed uses. Charcoal is used for soil improvement, absorption of odors, moisture, and for preservatives. Charcoal was mixed with soil in ordinary house planters and plant growth and soil conditions were shown to be improved.

REFERENCES I. 2. 3. 4. S.

6. 7. 8. 9.

IO. 11.

12.

13. 14. 15.

16. 17. 1 8.

T. Arima, Rev. Forest Culture, 13:IO9 ( 1992). Research Group on Forest Products Administration, Wood Supply and Wood Industry 1997, Wood Industry Integration Plan Areas 27 1-283 ( 1997). Research Group on Forest Products Administration, Wood Supply and Wood Industry 1997, Wood Chip Industry, 189-196 (1997). S. Suzuki, Proc. Int. RlLEM Workxllop, Tokyo, p. 223 (1995). T. Arima, Proc. Preserlt arrd Future of‘ Wood Wctstc~,Japan Wood Research Society Hokkaido Branch. p. I14 (1993). T. Arima, J. J p 7 . Agric. Sys. Soc.. 8:69 ( 1992). Osaka Industrial Waste Corporation. Report on Generating and Recycling of Wood Waste i n the Construction Site ( 1988). Japan Housing Wood Technology Center, Report on Recycling of Wood Waste, p. 36 ( 1992). Association Housing Demolition, Composition Analysis on Waste from Wooden House Demolition ( 1993). Japan Housing Wood Technology Center, Report on Recycling of Wood Waste (I). p. 6 (1994). Japan Wood Research Society, Wood Scirncc~m t d T ~ ~ . k r ~ o /IV o g l~~t/rt.sfrirrl y r r r d 1Ir~il.vW ~ t c . . Wood Chip, p. 7 ( 1996). Japan Wood Research Society. Wood Scierrcc. ( I d Techrrology IV Irltlustrirrl rrrrd Drti1.v W~ISIL’, Roctrd, p. 104 (1996). T. Arirna. Rcsectrck Ptcp”:s o r 1 H o u s i n g trntl L ~ I I I ~211255 , ( 1997). K. Fujike, Pro(.. Prt~serrrt r r r d Future qf’ Wood Wrl.stC.Japan Wood Research Society Hokkaido Branch, p.22 (1993). T. Arima, Reports on Grant-in-Aid for Developmental Scientific Research Development of Fundamental Technics for Recycling Wood Resources, p. 42 ( 1994). T. Arima. S. Kobayashi, F. Socda, and M. Tokuda, RC): Forest Culfttrr. IS: I78 ( 1 997). Japan Wood Research Society. Wood Scicwcc t u r d T d ~ ~ r o l o II! g y Irtthrstritrl rrrlrl L)tti!\. Wtr.stt>. Chcrrt.otr/. p. 78 (1996). Japan Housing Wood Technology Center. Report on Recycling 01’ Wood Waste, Charcoal. p. 66 ( 1997).

25 Pulping Chemistry Goran Gellerstedt Royal Institute of Technology, Stockholm, Sweden

1.

INTRODUCTION

The pulping of wood for the production of paper, board, and other cellulosic products has a large economic impact in several countries in the world. North America, Scandinavia and, more recently, SouthAmerica are importantareasforpulpproduction,sincethe availability of suitable wood species is high. Japan has a large pulp industry which, to a large extent, is based on imported wood, whereas the pulp industry in Western Europe is of amoderatesize. In otherareas of theworld,suchasChina, annual plants play an important role as fiber supply, but here, as well as in other developing areas, new pulp mills utilizing wood are rapidly increasing in number. Secondary fibers constitute an important fiber resource that has been used for paper production in countries with no or little domestic wood supply. In the future, the importance of secondary fibers will increase considerably, and legislation in several countries has been used to force the paper industry to further increase the content of secondary fibers in a variety of paper products. The chemistry of pulping has developed in parallel with the development in pulping technologies. In several pulp-producingcountries, central researchinstitutes have been created to support the industry with fundamental research on selected problem areas of technical significance. Today, the chemicalknowledgeaboutpulping and bleaching of wood is comprehensive, and numerous articles, reviews, and book chapters are available on the subject. Nevertheless, due to the complexity of wood as a composite of polymers, our knowledge about the chemistry and chemical reactions involved when wood is converted to pulp of different types is still far from being complete. In this chapter, the chemistry involved in the production of unbleached and bleached mechanical and chemical pulps is discussed. with an emphasis on reactions which are thought to play a significant role in technical pulping systems.

II.

(CHEM1)MECHANICALPULPING

A.

TechnicalOverview

In mechanical pulping, wood is disintegrated into fibers by grinding or refining, using a rotating grindstonc or a disk refiner, respcctively. In the former case, wood logs are pressed against the rough stone surface with simultaneous water spraying. In the latter case, refin859

Gellerstedt

860

ing is done by forcing wood chipsto pass from the center and outward between two circular disks, with at least one rotating at a high speed. The inner surfaces of the disks are covered with bars and grooves of successively smaller dimensions. The disintegration of wood to mechanical pulp results in a broad distribution of fiber sizes, including course fibers and fiber bundles, whole fibers, fiber fragments, and small particles. To some extent the relative distribution of the various fractions is determined by the process and the conditions needed to meet various end uses. The resulting pulps are termed stone groundwood (SGW) and thermomechanical pulp (TMP), respectively. Refining is also used to make refiner mechanical pulp (RMP), which differs from TMP by the way the chips are pretreated in the process. Fresh spruce is the preferred wood species for making mechanical pulp, although other species can be used as well. The majority of the mechanical pulp produced is used for making newsprint and other “wood-containing” printing papers in large integrated mills. The pulp yield in mechanical pulping is usually of the order of 95-989’0; i.e., a l l the wood constituents are retained i n the finished product. Brightness isan important quality parameter for spruce mechanical pulps. Values around 60-63% IS0 are generally encountered. For several paper products this is too low, and bleaching is frequently used to adjust the brightness to different levels in the range of around 80% ISO, which, at present, constitutes the upper technical limit. The bleaching of mechanical pulps is done in such a way that a l l wood constituents are retained, using either dithionite under neutral conditions or alkaline hydrogen peroxide. This“ligninretaining bleaching” results in bright pulps which, however, suffer from brightness reversion caused by either heat or light. For this reason, bleached mechanical pulps cannot replace bleached chemical pulps in paper products where brightness permanence is important. Mechanical pulping can also be performed on chips which have been pretreated with small amounts of chemicals, usually sodium sulfite, either alone (softwoods) or in combination with sodium hydroxide (hardwoods). A somewhat lower pulp yield is obtained, but strength and brightness properties are improved. These pulps are referred to as chemithermomechanical (CTMP) or chemimechanical pulps (CMP), and they are often further bleached with alkaline hydrogen peroxide.

B.

Reactive Structures in Wood

The native color of wood can vary largely between species, but in bright softwoods, such as spruce, the predominant contribution comes from the lignin. Although the majority of lignin structures are colorless, the yellowish hue of wood can be attributed to the presence of small amounts of colored lignin structures. The quantitatively most important of these are end groups of the cinnamaldehyde type, which are present in softwoods with a frequency of approximately 3-4 units per 100 phenylpropane units [ 11. Among other chromophoric structures in native lignin, trace amounts of ortho- and pum-quinones are thought to be major contributors [2,3], although their presence is difficult to demonstrate directly. Phenolic lignin structures containing a-carbonyl groups may also contribute to the color of wood, but also in this case, no unequivocal proof of their existence is available. The structures and wavelengths of maximum absorbance for some wood chromophores are given in Fig. I . Native lignin also contains a variety of colorlcss structures which under rather mild reaction conditions may be converted into chromophores. The presence of small amounts of hydroquinoncs and catechols has been demonstrated 1431 and are probably the result

861

Pulping Chemistry

0

OCH,

0

+o 0

0

a,

-

amax

-

-

a,,

390 410 nm (420 440 nm)

a,,

-

-

-

335 350 nm (340 350 nm)

-

360 nm (410 - 420 nm)

-

310 nm

FIGURE 1 Suggested chromophores in wood and their approximate light absorption maxima (values in parentheses are for the solid state).

of redox reactions and incomplete methylation, respectively, in the growing tree. Under oxidative conditions, as in the presence of air, such structures are easily oxidized, with formation of the corresponding quinone structure [6]. End groups of the cinnamyl alcohol type do not contribute to the wood color directly. They constitute, however, structures of high reactivity in, e.g., oxidative processes. Since the frequency of these structures is rather high in the native lignin (approximately 3-4 per 100 phenylpropaneunits) 171, evena reaction proceeding to aminuteextent may contribute to color changes. Among the major structural units in lignin, the p-1 structures are of particular interest. In mild acidic or alkaline conditions such structures are easily converted into the corresponding stilbene structure according to the reaction shown in Fig. 2 [ & S ] . The stilbene, in turn, is easily oxidized to form deeply colored quinones of various types.

C.

Reactions with Sodium Sulfite

The pretreatment of chips at temperatures around 120- 130°C with 1-4% sodium sulfite solution, followed by refining, results in the production of CTMP. The weakly alkaline sulfitesolution reacts with the lignin to someextent,asshown in Fig. 3 [ 101. These reactions are not, however, uniform throughout the lignin matrix but concentrated to the outer part of the fiber [ I I ] . The preferential hydrophilization of the primary wall gives a

Gellerstedt

862

H'or

HO-

P

OH

OH

FIGURE 2 Formation of a stilbene from p- 1 structures in wood.

swelling effect which directs the fiber-fiber fracture to this cell wall layer. This results in a pulp having more intact and homogeneous fibers as compared to a mechanical pulp. The reactions between sulfite and wood are thought to involve sulfonation of reactive lignin structures as the predominant reaction mode. Thus, coniferyl alcohol structures, which are abundant in the primary wall 1121, react readily with the strongly nucleophilic sulfite ions to form a series of sulfonated coniferyl alcohol structures [ 13,141. The related structure, coniferaldehyde, is even more reactive and easily converted into sulfonatesunder CTMP conditions [ 151. Other aldehydes, such as vanillin structures, can be assumed to form a-hydroxy-sulfonates in the well-known aldehyde-bisulfite equilibrium reaction. Conjugated carbonyl structures, such as orrho- and para-quinones, will also react with the sulfite ion to form the corresponding aromatic sulfonates [ 161. The various reactions taking place between sulfite and wood chromophores result in a certain bleaching effect, leading to CTMP being somewhat brighter as compared to TMP.

D. Chemical Changes in Refining

1. Stone Groundwood and TMP The defibration of wood to mechanical pulp involves exposure of the wood constituents to high temperatures (150- 170°C) in the presence of water. These conditions result in a

Sulfur content, mmoVkg wood

5

10

15 Time, min

FIGURE 3 Sulfur uptake in wood under CTMP conditions [ I O ] .

Pulping Chemistry

863

color formation which, together with the native wood chromophores. determinesthe brightness of the final pulp. Several factors can influence the degree of color formation in the fibers, including wood quality, content ofbark in the wood, content of transition metal ions in the wood, pH during fiber liberation, presence of sulfite, and presence of oxygen. The importance of these parameters indicates that autoxidation reactions play a role in the discoloration reactions. This is further supported by experiments both with wood [ 17,l S] and with various model compounds [6]. The major contribution is likely to come from the oxidation of reactive phenols in ligninlike catechols and hydroquinones. Such reactions occur rapidly even at room temperature and result in the formation of the corresponding quinone structure. In the presence of metal ions, such as copper or manganese, the oxidation rates are further enhanced. Catechol structures in lignin are also able to form strong chelates with ferrous/ferric ions; these are colored and, once formed,difficult or impossible to remove [19]. Thus, the presence of high amounts of iron in the wood results in very dark-colored mechanical pulp [20]. The presence of bark in defibration is detrimental, since reactive phenolic compounds are easily leached out in the surrounding aqueous solution. Subsequently, these may participate in lignin autoxidation reactions, and new, and more intensely colored addition products linked to the fiber, may be formed [6]. The mechanical action on wood in grinding or refining can alsoalter the lignin structure in such a way that more reactive structures are created. In a series of investigations, several different lignin model compounds have been subjected to either ball milling at room temperature or added to coarse mechanical pulp which subsequently was subjected to further refining [21]. The products obtained from the model compounds reveal that mechanical action on lignin structures may result in both homolytic and heterolytic reactions. Important products include structures with an a-carbonyl group in the side chain, as well as stilbenes; the later originate from p-5 and p-l structures. One example is shown in Fig. 4.

2.

CTMP

The addition of sulfite to wood prior to defibration results in a certain brightening effect, since some wood chromophores are eliminated due to sulfonation. The effect is limited,

CH0

Hi:

II

CH

I

I

HC

It

CH

uo FIGURE 4

Chemicalchange in a p-5 structure as the result of mechanical treatment of wood.

Gellerstedt

864

however, since new chromophores are created in the defibration process. Therefore, the net result on brightness of the simultaneous creation and elimination of chromophores can vary widely due to the exact reaction conditions before and during refining. In particular, the presence of transition metal ions in the wood and/or the process water will affect the extent of autoxidation and other homolytic processes. The simultaneous presence of sulfite and a strong chelating agent such as EDTA or DTPAin the defibration process has been shown to give a synergistic bleaching effect, since the color-forming reactions are minimized while the bleaching effect of sulfite is still operating. Thus, a brightness gain of the order of 8-10% I S 0 is attainable using this mixture [22].

E.

Bleaching Chemistry

1.

Dithionite

Sodium dithionite (sodium hydrosulfite) is a mild reductive bleaching agent which is used to obtain small or moderate brightness gains in mechanical pulps with a maximum increase of around 8 I S 0 units. Although the reducing power in alkaline solution is much higher (eo = 1.12 V versus e,, = 0.66 V), such conditions result in discoloration of the pulp due to rapid alkali-induced discoloration reactions. Consequently, dithionite isusedat a pH around 5-6 and at temperatures around 60°C. Under aerobic conditions, dithionite easily decomposes into sulfite and sulfate according to reaction ( l ) , resulting in a loss of bleaching power, even though the sulfite formed may participate in reactions of the type discussed above. A slower decomposition is encountered under anaerobic conditions, resulting in the formation of sulfite and thiosulfate [reaction (2)]. Thus, the bleaching solution mustbe freshly prepared, which is done either by simple dissolution of the sodium salt in water or by reduction of sulfur dioxide by sodium borohydride. S,Oi-

2S,O:-

+ H,O + 0, = HSO, + HSO, + H,O = 2HSO; + S @ -

The bleaching chemistry of dithionite is a simple reduction of easily reduced chromophores, such as quinones present in lignin [23]. Accordingly. the maximum color reduction is found in the wavelength region of 440-480 nm.

Hydrogen Peroxide The bleaching of mechanical pulp with hydrogen peroxide in alkaline media requires a prior elimination of transition metal ions from the pulp since, otherwise, a rapid decomposition of hydrogen peroxide to oxygen and water takes place according to reaction (3). The metal ion concentration is reduced by treatment of the pulp with DTPA or EDTA at a neutral or slightly alkaline pH, followed by dewatering. The bleaching itself is carried out in the presence of silicate, which acts as both a buffer agent and a stabilizer for the peroxide. The brightness gain in peroxide bleaching is dependent on the charge of peroxide and alkali; however, the bleaching liquor cannot be allowed to contain a large excess of alkali, sincesuchconditions will result in darkening reactions in competition with the bleaching reactions [24]. 2.

H,O,

+ H 0 2 = O2 + H,O + HO-

(3)

The chemistry of peroxide bleaching has been thoroughly studied both with lignin model compounds and with pulps. Analysis of pulp reflectance spectra before and after a

865

Pulping Chemistry

1.6 1.4 1.2

t

A

400

1 I

l

0.8

600

500

wavelength, nm

I

457

FIGURE 5

Relative reflectance spectra of mechanical pulp beforehfter a peroxide stage [25].

peroxide bleaching reveals that two major areas of the spectrum are subject to changes, viz., 320-400 nm and 400-600 nm, as shown in Fig. 5 [25]. The former of these is attributed mainly to coniferaldehyde structures, which are known to react rapidly with the peroxy anions to form the corresponding aromatic aldehyde(Fig. 6) [26]. In phenolic units, the aldehyde may react further in a Dakin reaction, with formation of a hydroquinone structure [27,28].

CH0

I

CH

I

-

CH0

H202 I HO-

OCH,

/

2 HCOOH

+

HCOOH

OCH,

/

OR

+

OR

PH OCH,

0

OH

FIGURE 6

Alkaline peroxide oxidation of a coniferaldehyde structure in wood.

Gellerstedt

866

4

H*~dHO”

OCHj

+

COOH

others

COOH

0

FIGURE 7

Alkaline peroxide oxidation of a pnm-quinone structure in wood.

The elimination of coniferaldehyde structures with peroxide bleaching has been supported both by analysis of wood tissue using UV microscopy [29] and by chemical analysis of pulp samples [30].However, this elimination does not seem to be quantitativeand, even in highly bleached mechanical pulp, coniferaldehyde structures can still be found, thus indicating that the bleaching process is not optimal. Ortho- and yaru-quinones are also attacked by the strongly nucleophilic peroxide anion and degraded to a mixture of carboxylic acids (Fig. 7) [31]. In a second reaction pathway, quinones may also react with peroxide to form the corresponding hydroxy-quinone. Although occurring to only a minute extent, this reaction creates a new chromophore which, despite being a quinone, has a low reactivity toward alkaline peroxide, due to the acidic hydroxyl group. The stoichiometry in peroxide bleaching has been found to vary depending on the charge of peroxide. Thus, at high charges of peroxide, more peroxide is consumed to obtain a certain brightening effect (Fig. 8) [32]. A high charge of peroxide is, however, necessary to reach the highest brightness levels and, consequently, such bleaching systems are quite expensive. The reason for the increase in peroxide consumption seems to be a formation of radicals through a homolytical decomposition reaction of hydrogen peroxide according to reaction (4) [33-351. This reaction is catalyzed by certain transition metal ions, such as manganese, and results in the formation of hydroxyl radicals and superoxide ions. In the absence of other substrates, these radicals will combine to give oxygen and water [reaction (5)]; but the presence of pulp may alter the reaction pathways and a large

Relative bleaching effect

Relative bleaching effect

5-

5-

L-

10.7 11.1 11.5 pH;

L-

P

3-

3-

2-

2-

‘1

1-

I

0.2

0.6

1.0

Consumption of

0.2

b o 2, mol/kg

0.6

1.0

pulp

FIGURE 8 Stoichiometry in the alkaline peroxide bleaching of mechanical pulp [ 3 2 ] .PH, denotes the initial pH in the pure bleaching liquor.

Pulping Chemistry

867

number of both lignin and carbohydrate reactions are possible. The effect of these radicals on wood chromophores is unknown; however, it has been shown that bleaching mechanical pulp in the presence of a radical scavenger results i n asomewhat reduced bleaching response under otherwise identical conditions [36].

+ HO, = 0,- + HO' + H,O + HO' = 0, + HO-

H20,

(4)

0;

(5)

A complete conversion of coniferaldehyde and quinone structures to the reaction products outlined in Figs. 6 and 7 would require a consumption of hydrogen peroxide of the order of 0.20-0.24 mol/kg of pulp. Such values are found when pulp is bleached with a low charge of peroxide, as shown in Fig. 8. Therefore, the presence of unknown types of chromophores in wood, which have a more unfavorable stoichiometry or, alternatively, require the presence of oxygen radicals to be eliminated, cannot be ruled out. An alternative explanation, involving the formation of new chromophores, e.g., due to alkali- and/ or peroxide-induced reactions, is also possible.

F. Chemistry of Yellowing Both unbleached and bleached mechanical pulps undergo brightness reversion when exposed to daylight or heat treatment. For a given set of aging conditions, the extent of color formation is highest when the brightness of the original pulp is high [37]. For bleached mechanical pulps, these yellowing reactions prohibit a wide use of the pulp in high-quality paper products and thus constitute a severe technical problem. The reflectance spectra of bleached mechanical pulp before and after accelerated light-induced yellowing reveal that two wavelength areas change as the result of irradiation, viz., 310-350 nrn and 380-470 nm (Fig. 9) [38]. The former of these can be attributed to the formation of carbonyl groups in conjugation with aromatic rings; the latter is from quinone structures. The possibility of reducing or eliminating the yellowing of mechanical pulps is small, due to the fact that several functional groups present in the lignin absorb daylight and therefore act as sensitizers for photochemical oxidation reactions. The addition of ultra-

AAbs

0.3

t

~ 0 5

0.2

0.1

3

0

250

350

L50

550 Xnm

FIGURE 9 Absorption spectra beforehfter light-induced yellowing of mechanical pulp [ X ] .

868

Gellerstedt

0

2

1

3

Accelerated (h)

Irradiation time & 1

2

3

Daylight (months)

FIGURE 10 Light-induced yellowing of mechanical pulp as a function of irradiation time [40].

violet absorbers, such as hydroxylated benzophenones [ 3 7 ] , or radical scavengers, such as the combination of sodium sulfite and ascorbic acid [391, to paper containing mechanical pulp has been suggested, but the cost is high. The chemistry of’ light-induced yellowing involves at least two different types of reactions. One initial fast reaction takes place within the first hour of accelerated laboratory irradiation and is followed by a second reaction proceeding at a lower rate (Fig. IO) [40]. A suggested mechanism for the initial yellowing reaction is based on the photo-induced oxidation of reactive phenols in lignin, such as catechols and hydroquinones (Fig. 11) [41]. Such structures are present in the native lignin, and additional amounts can be formed on

OH

0

? FIGURE 11 Suggested mechanism for the light-induced yellowing of mechanical pulps.

Pulping Chemistry

869

dithionite or peroxide bleaching, as discussed above. The first step in the reaction, the absorption of light and formation of a phenoxy radical, can be accomplished by the phenol itself [42,43] or by a reaction involving an excited conjugated carbonyl structure [44-461. The latter, in turn, can be either native or formed in the peroxide bleaching stage. At a later stage of yellowing, unspecific monohydric phenols in lignin can be converted to phenoxy radicals and further oxidized to quinones by the action of light and oxygen. Again, conjugated carbonyl structures can act as photosensitizers. In addition to available carbonyl structures, lignin end groups containing conjugated double bonds are themselves photo-active. Forexample, coniferyl alcohol structures can be oxidatively cleaved to yield further aromatic aldehydes [47]. Photo-induced oxidation of benzyl alcohol groups to the corresponding ketyl radical may also constitute an important pathway for discoloration, since such structures, when present in P-aryl ether structures, induce a cleavage of the P-aryl ether linkage and direct formation of a phenoxy radical (Fig. 12) [46, cf. 451. The heat-induced (thermal) aging of mechanical pulps can take place i n the manufacturing process itself, during pulp storage, and in the finished paper product. The color formation seems to be due to autoxidation processes of catechols and hydroquinones, resulting in the formation of the correspondingquinones in reactions similar to those discussed above [6]. In the absence of light, these reactions proceed through phenolate anion intermediates, making them dependent on pH with a minimum around pH 5 . The presence of certain transition metal ions, such as copper or manganese, accelerates the reactions. A simple way of reducing the autooxidation of mechanical pulps is by addition of sulfite at a pH of around 5-6. The simultaneous presence of a chelating agent such as DTPA or EDTA prevents autooxidation of the sulfite and leads to long-term stabilization [37].

-

CHzOH I

Hi:

I: a

+

hv

OCH,

CH30 OCH,

/O

further reactions

FIGURE 12 Direct photo-induced cleavage of an oxidized p-0-4 structure in lignin with formation of a phenoxy radical.

Gellerstedt

870

111.

CHEMICAL PULPING

A.

TechnicalOverview

In chemical pulping, the cellulosic fibers are separated from each other by dissolution of lignin (and part of the carbohydrates) under acidic or alkaline hydrolytic conditions. All chemical pulping processes give fibers which still contain some residual lignin. This is usually measured in an indirect way by determining the consumption of a strong oxidant, potassium permanganate, in a given amount of pulp. The resulting value is termed the kappa number and, although not measuring only lignin 1481, is frequently used to classify the process and the quality of the pulp. The predominant process, kraft pulping, is done by reacting wood chips with a mixture of sodium hydroxide and sodium sulfide for 1-2 h at a temperature in the range of 140-170°C depending on wood species. The process gives brownish pulps with high fiber strength which can be used in a broad range of paper products. Unbleached kraft pulps in a yield range of SO-60% still contain some 8- 15% lignin and are used as such, whereas pulps in lower yield (4S-S0%) with 3-5% residual lignin usually are bleached to full brightness with oxidative bleaching agents. Major advantages of the kraft process are the low sensitivity to wood species and the possibility of producing pulp from wood of inferior quality. However, the malodorous compounds produced in the process and the high capital costs involved in new installations constitute important drawbacks. Nevertheless, the kraft process is the totally dominating chemical pulping process in the world. An alternative to kraft pulping, sulfite pulping, is somewhat olderasa technical process and gives brighter and more easily bleached pulps. Pulping is carried out in an acidic solution containing sulfur dioxide and either calcium, sodium, or magnesium ions at temperatures in the range of 120- 150°C for 6-9 h. The sulfite process is more sensitive to wood species and the resulting fibers are somewhat weaker than those from the kraft process. For special purposes, sulfite pulping of hardwood at a neutral or slightly alkaline pH is carried out. That process results in a high-yield sulfite pulp (NSSC) used for making corrugated medium in board. Several alternative pulping processes have been suggested but, with a few exceptions, these have not reached commercial scale. Various modifications of the kraft process. which are the most important of these, involve addition of either polysulfide, anthraquinone (AQ), or both to the cooking liquor. The resulting pulps are obtained at a somewhat higher yield for a given amount of residual lignin. Sulfur-free pulping with only sodium hydroxide and AQ is a further interesting alternative, resulting in “kraftlike” pulps. Certain hardwood species such as aspen are easily delignified, and solvent pulping can be used to obtain chemical pulp. In this process ethanol is used and the wood chips are pulped at 180- 190°C for 4-6 h.

B.

Chemistry of Pulping in Acidic Media

1. Sulfite Pulping The pulping ofspruce wood using an aqueous solution of acid bisulfite started as an industrial process in Sweden in 1874. The early industry was based on calcium as a counter ion, resulting in severe pollution problems since the used liquor could not be burned to recover the pulping chemicals. Calcium based mills are still in operation in the world, but the more modern mills utilize either sodium or magnesium, thus permitting chemical recovery and energy production through burning of the partially evaporated spent liquor.

Pulping Chemistry

871

The sulfite system contains two equilibrium reactions [reactions (6) and (7)], and the pulping liquor is usually prepared by dissolution of sulfur dioxide in an aqueous solution or a slurry of the appropriate hydroxide. The proportions are chosen such that a certain excess of sulfur dioxide is obtained, resulting in a pH in the range of 1.5-4.0, depending on process and product requirements. In NSSC (neutral sulfite semi-chemical) pulping, a solution of sodium sulfite is used at a slightly alkaline pH.

+ H,O = HSO; + H30' + H,O = SO:- + H30'

SOz H,O

pK,, = 1.9

(6)

HSO,

pK,, = 7.0

(7)

The presence of a base is essential in sulfite pulping, and pulping with sulfur dioxide alone cannot be done since such conditions would result in a dark wood residue with very little lignin dissolution.This is due to the fact that the strongly acidic aqueoussulfur dioxide promotes condensation reactions in the lignin, e.g., between aromatic and benzyl alcoholic carbon atoms, and does not lead to any appreciable amount of sulfonation reaction. The relationship between the total amount of sulfurdioxide and the required amount of base in the cooking liquor has been determined experimentally. Thus, if a certain relationship between the combined and total sulfur dioxide is maintained in the system, the proportions between sulfonation and condensation are such that sulfonation is favored

[W. The desired chemical reaction types in sulfite pulping are sulfonation and acid hydrolysis. Acid catalysed condensation is a nondesirable side reaction. In these reactions, the lignin is solubilized through the introduction of a large number of sulfonate groups in the lignin side chains. This solubilization is further facilitated by the acid-catalyzed hydrolysis of alkyl aryl ether linkages in lignin and of benzyl alkyl ether linkages between lignin and carbohydrates. Further acid hydrolysis takes place in the polysaccharides, resulting in a certain loss of pulp yield and in the formation of low-molecular-weight neutral sugars suitable for fermentation to ethanol. ( I . Lignin Chemistry. The rates for sulfonation and dissolution of lignin in sulfite pulping have been shown to be dependent on the pH of the liquor. Thus, sulfonation is always the fastest reaction type, and at a low pH a complete sulfonation of all lignin units takes place within a few hours of pulping (Fig. 13) [50].The hydrolytic reactions resulting in cleavage of ether linkages between lignin and polysaccharides, as well as between lignin units, seem to be somewhat slower. Together with possible restrictions in the mobility of dissolved lignin fragments through the fiber wall, the resulting delignification thus shows an apparent retardation as compared to sulfonation. Around neutral pH values, the sulfonation of lignin is much more selective and only around 20% of the phenylpropane units react, although at a comparably fast rate. Therefore, the dissolution of lignin is limited. The initial sulfonation reactions in the pH region are the same as those described for CTMP. Under more strongly alkaline conditions, sulfite pulping gives chemical pulps with characteristics similar to kraft pulps [51,52]. The reactions of lignin in acid sulfite pulping have been studied using a variety of model compounds representing the different structural units [53,54].The major reaction mode is a sulfonation of benzylic carbon atoms through the intermediate formationof carbonium ions, as outlined in Fig. 14. In principle, both phenolic and etherified lignin units can react, although the rates for phenolic structures seem to be somewhat higher [ S ] . A stronginfluence on the rate of sulfonation is also exerted by the pH under otherwise identical conditions. At higher pH values, i.e., around neutral and above, the sulfonation of lignin becomes more selective and only phenolic structures react. The reaction involves addition of sulfite

872

Gellerstedt

FIGURE 13 Rate of sulfonation (-pulping [ 5 0 ] .

-)

and dissolution (-

) of lignin

in acidic sulfite

CHZOH

I

CHzOH

I

CHzOH

I

HC-

HC-

I

HC-

HC-0-R

I

HSO,0

SOz/ H @

o +

-e

' @

OCH,

' -e

/O

OCH,

'

R-OH

+ H

OCH,

/O

FIGURE 14 Mechanism of sulfonation of lignin in acidic sulfitepulping.

CHzOH

CHzOH

I HCI

I

HC-

I

HC-OR

CHzOH

I

HC-

I

H0 ~

OCH3 OH

OCH,

0

OCH3 OH

FIGURE 15 Sulfonation of a phenolic lignin structure in neutral sulfite pulping.

further reactions

0

Pulping Chemistry

873

ions to intermediate quinone methide structures, as demonstrated in Fig. 15 1561. Under these conditions,further sulfonation may occurand, in P-aryl ether structures, the P-substituent can be eliminated, resulting in a partial degradation of the lignin macromolecule. Alkaline sulfite pulping conditions will result in more fragmentation of the lignin, since the cleavage of @-aryl ether structures is enhanced due to the presence of hydroxyl ions in addition to the sulfite 1541. The degree of sulfonation of the lignin will decrease, however, due to secondary elimination reactions of sulfonate groups 1571. The facile formation of new carbon-carbon bonds in lignin during acid conditions is due to the presence of free aromatic carbon atoms in the para position to a methoxyl group (C-6 position). This carbon atom will easily combine with an adjacent carbonium ion formed through elimination of the oxygen function from a benzyl alcohol or ether. In model experiments, this type of condensation has been shown to be favored over sulfonation in structures where both types of carbon atoms are present in a sterically suitable arrangement [58,591. It is also well known that acid sulfite pulping of wood species such as pine is less favorable than of spruce, since the heartwood in pine contains extractives of the pinosylvin type [60]. Under acidicconditions, the C-6 carbon atom in the 3 5 dihydroxy (or 3-hydroxy-5-methoxy)-substitutedaromatic ring in pinosylvin will compete with the bisulfite ion for the intermediate carbonium ions in lignin. As a result, several positions in the lignin will never become sulfonated and, consequently, such lignin fragments will be less soluble in the pulping liquor. b. Carbohydrates.The major reaction of carbohydrates in acid sulfite pulping is the hydrolysis of glucosidic linkages, resulting in a loss of polysaccharides and thus of yield. In particular, the low-molecular-weight hemicelluloses are lost in the process; a spruce sulfite pulp may contain less than 30% of the original glucomannan and around 50% of the xylan 1611. Most of the degraded polysaccharides are present in the pulping liquor as low-molecular-weight neutral sugars. To some extent these may be further converted in acid-catalyzed reactions to furfural (from pentoses) and hydroxymethylfurfural by repeated water elimination, followed by cyclization.

2. Organosolv Pulping Pulping with ethanol at a high temperature can be done with wood species such as aspen and poplar, as well as with annual plants [62,63]. Due to the facile liberation of lowmolecular-weight organic acids, such as acetic acid, the pulping process is carried out in a weakly acidic environment. Hydrolytic reactions, resulting in a liberation of lignin from the lignin-carbohydrate matrix, is assumed to play an important role [64]. In poplar species this is particularly facilitated since the lignin contains para-hydroxybenzoic acid end groups that are linked to carbohydrates through ester linkages 165,661. In grass species, ferulic acid plays a similar role as “spacer” 1671. Homolytic cleavage of linkages in lignin may, however, also occur and contribute to a lignin fragmentation and subsequent dissolution in the solvent 168,691. The carbohydrates in organosolv pulping seem to undergo reactions similar to those occurring in sulfite pulping; neutral sugars, as well as furfural, can be found in the spent liquor 1621.

C. Chemistry of Alkaline Pulping 1. Introduction Approximately 10 years after the first sulfite mill was started in the world, kraft pulping was introduced on a commercial scale. The process gained considerable value with the

Gellerstedt

874

finding that the addition of sulfide to a soda process gave a pulp of superior strength as compared to soda alone. In addition, the pulping time could be considerably shortened. Although the pulp was dark in color, the possibility of obtaining strong pulps from a variety ofraw materials made the process attractive. The poor bleachability prevented major expansion, however, until chlorine dioxide was developed as a bleaching agent in the 1940s, thus permitting the production of fully bleached kraft pulps. The kraft pulping liquor is a mixture of sodium hydroxide and sodium sulfide; the charge of each chemical is usually expressed on a wood basis as effective alkali and sulfidity, respectively. The sulfide system contains two equilibria, as shown in reactions (8) and (9). The equilibrium constants are such that, under all conditions prevailing in the digester, the sulfur is present as hydrosulfide ions [70]. These ions accelerate the rate of delignification, the rate increase being proportional to the charge of sodium sulfide. No similar influence on the rate of carbohydrate dissolution is found according to Fig. 16 [71], but from the figure it is obvious that the carbohydrate losses in kraft pulping are substantial.

+ H,O = HS- + H,O+ HS- + HO- = S” + H 2 0 H,S

pK,, = 7.1

(8)

pK,, = -0.7

(9)

gOl 80

J

20

\mc. 0

135OC. 165OC.

40

9

80

-

I I I 160 200 240 I20 COOKING TIME (TOTAL) MIN

FIGURE 16 Dissolved amounts of lignin and carbohydrates in kraft and soda pulping [71].

Pulping Chemistry

a75

If the dissolutions of lignin and carbohydrates in kraft pulping are plotted against each other, the resulting curve can be divided into three distinct phases. These are termed initial, bulk, and residual delignification, respectively; each has a different selectivity with respect to carbohydrate losses. The kinetics and activation energies for the initial and bulk phases have been determined from laboratory kraft cooks and are given in reactions ( 1 0) and ( 1 1) [72]. Obviously, there is no apparent influence of either alkali or sulfide on the delignification in the initial phase. The activation energy here is low. The bulk phase, on the other hand, shows a strong dependency on alkali and has an activation energy typical of chemical reactions. A certain influence of sulfide is also noticed.

dL = k . L - [HO-I0[HS-]” dr

--

E ,= 40 kJ/mol

The apparent slight influence of sulfide on the delignification kinetics has, however, a great impact on the resulting pulp characteristics. Both the transition point from bulk to residual delignification and the viscosity of the pulp at a given kappa number will be greatly changed if the sulfide concentration is so low that a sulfide deficiency occurs toward the end of the initial phase [73,74]. Thus, a low availability of sulfide in this part of the cook will result in a high amount of lignin when the cook reaches the residual (slow) delignification phase. Continued cooking results in a pulp of inferior quality with a low viscosity. In the traditional kraft cook, the whole charge of chemicals present in the “white liquor” is mixed directly with the wood, resulting in a very high concentration of alkali at the beginning of pulping. Simultaneously, the charge of sulfide is such that after a short period of pulping, the sulfide concentration may decrease to very low levels before it again increases [70]. These concentration profiles are highly detrimental for pulping selectivity and, in modern pulping, split charge of white liquor and recirculation of the used pulping liquor (black liquor) have been introduced. These changes result in a more even alkali profile during the cook and in a high initial concentration of sulfide, thus permitting a more selective delignification [75].

2. Lignin Reactions a. @Aryl EtherStructures. The dissolution of lignin in kraft pulping is mainly a consequence of the cleavage of p-aryl ether linkagesin p-0-4 structures, resulting in lignin fragmentation, together with the liberation of free phenolic hydroxyl groups. The latter, together with the originally present phenolic lignin end groups, are ionized in the alkaline solution. Phenolate salts are more soluble in water than un-ionized phenols, and this results in dissolution of lignin fragments.The reactions of p-aryletherstructures have been studied extensively, both in experiments with low-molecular-weight model compounds and through analyses of black liquor and pulp lignin samples [76-811. In phenolic structures of the guaiacyl type, the rate of cleavage has been found to follow the expression given in reaction (12), which, in accordance with the relationship found for wood, shows no influence of alkali or sulfide. In nonphenolic units [reaction (13)1, the corresponding relationship shows a first-order dependency on alkali [82,83].

876

Gellerstedt

The published pseudo-first-order rate constants for these reactions [84] suggest that the kinetic half-life for a phenolic P-aryl ether structure under realistic pulping conditions is around 1.3 min at 170°C, whereas the corresponding value for a nonphenolic structure is 45 min. These values are based on model compound studies and can be regarded as ideal, since complete accessibility between model and pulping liquor exists. The reaction mechanism for cleavage of a phenolic P-0-4 structure is shown in Fig. 17. The first reaction step, the formation of a quinone methide, is reversible, but once it is formed this intermediate may react further with nucleophiles present in the pulping liquor. The most reactive nucleophiles are hydrosulfide and/or polysulfide ions. Under conditions where sulfide is absent or present in very low concentration, carbanions from

Reduction

Condensation

Further reactions

FIGURE 17 Reactions of phenolic p-0-4structuresinkraft pulping.

Pulping Chemistry

077

lignin or carbohydrates may compete successfully [85-881. The latter reactions would result in condensation products, the presence of which is discussed further below. By addition of hydrosulfide or polysulfide to the quinone methide, a thiol (polythio) structure is formed (Fig.17) [76,89-911. In the alkaline solution this may undergo an intramolecular attack to form a cyclic intermediate with simultaneous release of the psubstituent. In further reaction steps, the cyclic intermediate loses elemental sulfur, resulting in a temporary loss of sulfide ions in the pulping liquor. In the alkaline solution, elemental sulfur reacts with sulfide to form polysulfide, which finally disproportionates into hydrosulfide and thiosulfate (Fig. 18). Although this reaction sequence is not fully supported by experimental data under conditions prevailing in a digester [92], it offers an explanation of the formation of thiosulfate and the re-formation of hydrosulfide in kraft pulping. In competition with the addition of a hydrosulfide ion or a carbanion to the intermediate quinone methide, two further reaction routes are possible on this intermediate. The first of these is elimination of either the P-hydrogen or the y-hydroxymethyl group, resulting in the formation of an enol ether structure 1761. Once formed, such a structure is relatively stable under alkaline conditions and may thus survive the kraft cook [79]. As a consequence, the lignin fragmentation will not proceed to an optimum extent. Thesecond reaction type is less established but involves a reduction of quinone methides to the corresponding aromatic a-methylene structure, a reaction which has been indicated by analysis of lignin samples with "C-NMR [93]. Further support for this reaction type has been obtained by alkaline treatment of quinone methide model compounds with alkali in the presence of either glucose, anthrahydroquinone, or both [94]. Again, such reactions would constitute a limitation of the lignin fragmentation, since p-0-4 structures are withdrawn from the desirable cleavage reaction. The cleavage of nonphenolic p-0-4 structures is dependent on the concentration of hydroxy ions and, as shown above, the reaction is comparably slow. The mechanism involves a formation ofan epoxide between the a - and the p-carbon atoms as shown in Fig. 19 1951. Subsequent hydrolysis would give a glycerol structure, together with the released @-substituent. Again, a competing reaction iwolving anions from lignin or carbohydrates may lead to condensation products [96]. The chemistry of p-0-4 structures in kraft pulping has been supported by analyses of the remaining fiber lignin, as well as the corresponding dissolved lignin, as a function of delignification. Analysis by acidolysis or thioacidolysis provides semiquantitative data of the amount of remaining p-0-4 structures in lignin. Figure 20shows the expected decrease in p-0-4 structures as pulping progresses but also that there is an amount remaining at the end of the cook (in softwood lignins). Furthermore, the dissolved lignin contains an appreciable amount of this structure throughout the cook [78,81]. Obviously, the rapid cleavage reaction indicated by the kinetic data above is not fully supported by the analytical data. The reason for this is not known, but factors such as accessibility of hydrosulfide, both in the fiber wall and in the lignin after dissolution, must be assumed

4nSo + 4HS- + 4HO- =

&,S2-

43,s'-

+ 4H20

+ 4(n-l)HO = nS2O3'- + 2(n+2)HS + (n4)H20 4S0 + 4HO- = S203'- + 2HS'

+ H20

FIGURE 18 Inorganic reactions of elemental sulfur in kraft pulping.

878

Gellerstedt CH2OH

I

HC-OR

I

HC-OH

CHIOH H

I

L

CH2OH -

O

-

I

e

HC-OH

+

"Q/ \ CH30

OCH,

/o

o /

CHZOH

I

HC-OH

I

HC-OH

I

FIGURE 19 The cleavage reaction of a nonphenolic p-0-4 structure in alkaline pulping.

879

Pulping Chemistry P-aryl ether structures l m o l l g of lignin

500

-

300 -

100

I

1

0

1

I

I

1

20 40 60 80 100 Degree of delignificotion. D/'' on wood

FIGURE 20 The presence of noncondensed phenolic p-0-4structures in pulp and dissolved lignin as a function of delignification (analysis by acidolysis).

to be important. The consumption of hydrosulfide and formation of sulfur in the fiber wall may, for example, result in a certain deficit of sulfide at some of the reactive sites during pulping. Even if the charge of sulfide is high and added to the wood through an efficient impregnation stage, this type of deficit seems to be present during pulping. The presence of p - 0 - 4 structures in the dissolved lignin can, for similar reasons, be assumed to be due to the formation of molecules or molecular aggregates having negatively charged shells with an interior that is inaccessible to the hydrosulfide ions. b. Other Lignin Substructure.s. Phenolicphenylcoumaran (p-S) and 1,2-diarylpropane-l ,3-diol (p-1) structures in lignin are reactive under alkaline conditions, since they contain an a-hydroxy or a-ether function that can be lost in the reversible formation of a quinone methide. In analogy with the reactions of p-aryl ether structures discussed above, addition of a nucleophile or elimination of a hydrogen or formaldehyde may take place. Thepresence of acarbon-carbonlinkage between the phenylpropane units, however, prevents any fragmentation of the lignin macromolecule around this linkage. Model studies indicate that phenolic p-5 or p-l structures preferentially undergo elimination reactions, resulting in the formation of stilbene structures (cf. Fig. 2) [97]. The nonphenolic counterparts, on the other hand, are essentially stable under alkaline pulping conditions. Other major lignin linkages, such as those in biphenyl (S-S) and biphenyl ether (40 - S ) structures, are completely stable in alkaline pulping, whether they are phenolic or not. In phenolic units, the side chains in such structures can, however, react via formation of a quinone methide according to the reactions outlined above. Recently, the behavior of a phenolic dibenzodioxocin (S-S-0-4) structure has been studied by the use of a model compound 1981. Under kraft cookingconditions, the aryl etherlinkageiscleaved to a large extent, leaving the S-S structure as a major reaction product. c. Aronzrrtic Metl1o.ry Groups. In softwood species, the lignin contains one aromatic methoxyl group per phenylpropane unit, with the exception of compression wood lignin, where some pcrrrr-hydroxy-phenylpropane units are present. In hardwoods, a mixture of

880

Gellerstedt

guaiacyl and syringyl structures are found, resulting in a methoxyl value per C-9 unit of between 1 and 2, depending on species. In kraft pulping, these methoxyl groups are attacked by the nucleophilic hydrosulfide ions, resulting in a partial demethylation and formation of methyl mercaptan. The formation of a new phenolic hydroxyl group in lignin adds to the hydrophilicity, whereas the other product, methyl mercaptan, is able to react further as a nucleophile to form dimethyl sulfide by further attack on aromatic methoxyl groups (Fig. 21) [99]. The two low-molecular-weight sulfur compounds survive the kraft cook and constitute major components in the mixture of malodorous gases leaving the digester. From both softwoods and hardwoods, the amount of these products corresponds to a degree of demethylation in the lignin of around 1-2%. In modern kraft mills, methyl mercaptan, dimethyl sulfide, and small amounts of dimethyl disulfide, formed through air (oxygen) oxidation of methyl mercaptan, and some hydrogen sulfide, are usually collected and burned to eliminate almost all of the odor. d. Condensation Reuctions. The presence of condensation reactions in lignin has been used frequently to explain the slow rate of delignification toward the end of kraft pulping, as well as the high demand of chemicals in a subsequent bleaching operation. This assumption is further supported by a variety of lignin model studies demonstrating that condensation reactions may easily occur, e.g., between quinone methides and different types of carbanions or between carbanions and liberated formaldehyde (Fig. 22) [85].The possibility of establishingequilibria between quinone methides and (ionized) hydroxyl groups, as well as reactions between quinone methides and enol structures, have also been suggested as modes of formation of new linkages between lignin and carbohydrates [87,88]. However, conclusive evidence for the formation of condensed structures in the wood residue during pulping, as well as their presence in the unbleached pulp, is not available. Although the existence of comprehensive condensation reactions giving rise to diarylmethane structures during pulping has been suggested [ 100,101], the analyses used in that work cannot be used unequivocally for making such an interpretation [ 1021. The presence of lignin-carbohydrate linkages in unbleached pulps. however, is strongly supported by indirect means; but, as discussed further below, it is not known whether these are native or not. The presence of condensed structures and products in dissolved lignin have been identified, although the extent of such reactions seems to be small [1031. Recently, it has also been suggested that cellulose and glucomannan. when subjected to a kraft cook in the presence of coniferyl alcohol, will result in a “grafting” reaction [ 1041. In summary,

(R = H or CH,) CHS-S-S-CH,

FIGURE 21 Formation of the malodorouscompoundsmethylmercaptan,dimethylsulfide.and dimethyldisulfide in kraft pulping.

Pulping Chemistry

W

(v (v

881

882

Gellerstedt

however, the presence of condensation reactions in draft pulping is still a matter of uncertainty which requires further attention.

3. Carbohydrate Reactions In alkaline pulping, the initial consumption of alkali is caused mainly by the rapid hydrolysis of acetyl groups from hemicelluloses: galactoglucomannans in softwood and glucuronoxylan in hardwoods. Essentially all acetyl groupsare eliminated as acetic acid, requiring an equivalent molar amount of sodium hydroxide. Thus, in softwood pulping, this reaction consumes approximately 0.35 mol of alkali per kilogram of wood, whereas the corresponding figure for hardwoods is in the range of 0.9 mol. The yield loss in kraft pulping due to degradation and dissolution of polysaccharides is substantial and constitutes a serious drawback of the process. Typical values for the amount of wood (pulp) components after kraft pulping of pine and birch are shown in Fig. 23. Thus, in addition to the desirable dissolution of lignin, all types of polysaccharides are degraded to some extent. In softwoods, substantial amounts of the xylan and around 75% of the dominating hemicellulose, glucomannan, are lost. In hardwoods, the xylan loss is almost 50%. The predominant reaction responsible for the degradation and dissolution of polysaccharides is an alkali-induced stepwise elimination of monomeric sugar units, starting from the reducing end; this is the so-called peeling reaction. Even at low temperatures, around 100°C, the rate of reaction is substantial and a decrease in the degree of polymerization (DP) of the polysaccharide is encountered [ 1051. For hemicelluloses having a short chain length, such as in glucomannans, the change in DP facilitates nearly acomplete dissolution already in the early part of the kraft cook. Xylans, being somewhat larger polymers and having a less efficient peeling reaction, survive the cook to a larger extent, but the remaining xylan chains can be assumed to have suffered a substantial loss i n DP. The mechanism for the peeling reaction is shown in Fig. 24. The major polysaccharides in wood are linked through 1 + 4 glucosidic linkages; the overall reaction results in a loss of the terminal sugar moiety as an isosaccarinic acid and the formation of a new reducing end group capable of undergoing the same reaction. Intermediate reaction steps involve a reversible opening of the hemiacetal ring, reversible isomerization to a fructose unit, a p-elimination of the 4-substituent, a keto-enol equilibrium, and finally a benzilic acid rearrangement. As a side reaction, the p-elimination reaction may occur already in the first-formed intermediate, the open aldehyde structure, with formation of a mdcr-~ac-

Pine Cellulose Glycomannan Xylan Other carbohydrates and various components Sum of carbohydrates Lignin Pitch Sum of components (yield) a

35

-

Birch

34

4 5

1 16

44 3 0.5 47

51

2

0.5 53

Figures in parentheses refer to original wood composition,

FIGURE 23

Yield loss in kraft pulping for pine and birch. (All figures based on wood.)

883

Pulping Chemistry

(OH

cell-0 H QH 0

H

OH

&

H0

-

(OH

/y-"oz,

cell-oH 0

OH

Metasaccharinic acid ("stopping" reaction)

-

p-elirn

cell-0

CHZOH 0

(Fructose unit)

I

OH

H

HoYCooH CHIOH

lsosaccharinic acid

FIGURE 24

The mechanism for thepeelingreaction

in alkaline pulping.

carinic acid. For cellulose, the latter reaction, termed the stopping reaction, occurs when an average of 65-70 sugar units have been peeled off the chain. In softwood xylans, the presence of arabinose substituents in the 3-position favors a direct @-elimination from the corresponding open aldehyde form of the end group. This results in a somewhat higher stability of this xylan as compared to a hardwood xylan, since the stopping reaction is facilitated. Both softwood and hardwood xylans also contain 4-0-methyl-glucuronic acid as substituent in the 2-positions, thereby making the isomerization to a terminal keto sugar more difficult and decreasing the extent of the peeling reaction. In alkaline pulping, the 4-0-methyl-glucuronic acid substituents in xylans are to a large extent converted to the corresponding unsaturated structure by loss of methanol (Fig. 25) [ 1061. The formed "hexenuronic acid" groups, which are still attached to the xylan backbone, are rclativcly stable under alkaline conditions and contributc a substantial proportion of the acidic groups in kraft pulps (Fig. 26) [ 1071.

Gellerstedt

884

HO'

cH30*1

~

- CHaOH

H0

H0

H0

"Xyl-xyl-xyl-

"Xyl-xyl-xyl-

FIGURE 25 Alkali-inducedelimination ofmethanolfrom4-OMe-glucuronicacid wood and hardwood xylans.

H0

units in soft-

The extent of peeling reactions is high in the beginning of a kraft cook, resulting in a rapid loss of hemicelluloses from the wood. In later parts of the cook, a further loss of carbohydrates can be initiated through alkaline hydrolysis of polysaccharide chains. This reaction occurs randomly along the chain and results in the formation of shorter chains or alkali-soluble chain fragments(Fig. 27). Since each cleavage reaction alsoleads to the formation of a new reducing end group, this in turn may rapidly undergo further degradation by the peeling reaction ("secondary peeling").

4. Additives in Alkaline Pulping The large yield loss in kraft pulping due to the degradation of polysaccharidescanbe reduced if the carbohydrate end groups are either reduced or oxidized, thus reducing the extent of the peeling reaction [ 108- 1 IO]. Both reaction modes have been tried successfully on a laboratory scale with polysulfide oranthraquinone (AQ) asoxidizingagents and sodiumborohydrideor hydrogen sulfide as reducingagents. Today, however, only the oxidants are used commercially.

20

I

18

-

16 W

4 3;

1412-

J

0 10-

S

0

o

i0 E

a6'

"

175 l70

HexA

-8-

165

"

MeGlcA

160

"

Ara

155

-El-

Y

g 3 5a W

150 t l

remperature

4

145

5F

4 '

140

2' n

"0

50

150

100

200

- 135 250

TIME, min FIGURE 26 The presence of hexenuronic acid groups i n pulp xylan as a function of cooking time in a kraft cook [1071.

Pulping Chemistry

+

'0 Xt

9+

+

0

X

C .*

885

Gellerstedt

886

The reaction mechanism in the oxidation of a carbohydrate end group can be envisaged as involving the enediol intermediate which is oxidized to the corresponding diketo structure (Fig. 28). A benzilic acid rearrangement then results in the formation of aldonic acid structures [cf. 11 I]. The reductive reactions give rise to the corresponding sugar alcohol or thioalcohol end groups with borohydride or hydrogen sulfide, respectively. The oxidation of carbohydrates with either polysulfide or AQ results in the formation of hydrosulfide ions or anthrahydroquinone (AHQ). In the alkaline pulping liquor, the latter is ionized and may add to quinone methides in lignin analogously to the hydrosulfide ion. The adduct formed, when present in a P-0-4 structure, has been shown to decompose with cleavage of the P-aryl ether linkage and re-formation of AQ, thus resulting in an efficient AQ redox cycle (Fig. 29) [ 112,113]. An alternative mechanism, which has been supported by several experimental studies, indicates that the reaction of AHQ with lignin involves single-electron-transfer reactions, leading to efficient P-aryl ether cleavage [ 1 141. With either mechanism, the cleavage reactions result in the formation of a cinnamyl alcoholstructure. In analogy to kraft pulping, a mixture of sodium hydroxide and AQ therefore acts as a powerful pulping system able to give “kraftlike” chemical pulps. In practice, however, AQ is used mostly as an additive in kraft pulping and, like polysulfide addition, this will result in a yield increase. The oxidizing power of polysulfide is not restricted to the carbohydrates; certain lignin structures can be oxidized as well. Some examples are shown in Fig. 30 [89,115]. Since oxidized lignin structures generally are more easily degraded in alkaline media, it can be assumed that the presence of polysulfide ions in kraft pulping will facilitate the delignification. Further support for such reactions was recently obtained by impregnating wood with polysulfide prior to kraft pulping and observing a better selectivity in the cook [ 1161.

5. The Chemical Structure of Unbleached Pulp Lignin. The comprehensive degradation and dissolution of lignin in kraft pulping is the result of cleavage of P-0-4 structures. However, the cleavage reaction is not complete, and several P-aryl ether linkages survive the kraft cook. Analytical data, although a.

GOH

CHIOH

= HOQ

-0

OH

C C H O -0

7

-0

OH

CHIOH

I

OCH3

0

0

0

OH

FIGURE 29

Mechanism for cleavage of a phenolic p-0-4structure in alkaline anthraquinone pulping. 03 03 -l

888

Gellerstedt

HC-0

PS

0. CH,

I

c=o

+

&H3

OCH,

OCH,

OH

OH

CH0

CHZOH

I

I

CH

CH

II

CH3

I

It

PS OCH3 OH

FIGURE 30 Alkaline polysulfide (PS) oxidation of an enol ether and a coniferyl alcohol structure.

at best semiquantitative, indicate a remaining amount, around 15% in spruce kraft pulp lignin [78]; this figure probably constitutes a lower limit. In birch pulp, on the other hand, the figure seems to be much lower [SO]. In addition to intact p-0-4 structures in the remaining kraft pulp lignin, some enol ether structures can also be found [79]. Obviously, the availability of hydrogen sulfide ions at the reacting sites in the fiber wall is not sufficient to suppress the elimination reactions completely throughout the cook, as discussed above. The presence of phenolic hydroxyl groups in lignin determines the solubility in alkaline solution. To some extent, such groups are present in the native lignin, but the liberation ofnew such groupsduring pulping is essential for the lignin dissolution to proceed efficiently. In dissolved lignins from a spruce kraft cook, it has been demonstrated that the content of phenols should be in the range of 3-4 mmol/g of lignin for attaining solubility in the pulping liquor; higher values are needed for the higher-molecular-weight molecules. In addition,a kraft lignin contains around 0.7 mmol/g of carboxyl groups 1I17,l IS]. In the residual fiber lignin, the values are much lower and the content of phenolic hydroxyl groups changes from approximately 0.7 in wood to 1.5 mmol/g of lignin in the unbleached pulp with a kappa number around 30 [ 1 17,1191. The desirable fragmentation of lignin in kraft pulping, brought about by the cleavage of p-0-4 structures, can be restricted due to a low availability of hydrosulfide and by nondesirable side reactions. The reduction of quinone methides mentioned above is one such reaction, but condensation reactions between lignin and polysaccharides would also

Pulping Chemistry

889

act in a restrictive way, since the resulting product would be of much higher molecular weight and have the possibility of being firmly attached to the polysaccharides. The formation of such linkages during pulping has been suggested [ 1201, but lignin-carbohydrate linkages may also be native. It has been found that the residual delignification phase is accompanied by a slow dissolution of lignin, which successively contains higher amounts of polysaccharides, most notably xylan [ 1181. Extraction of unbleached kraft pulp with strong alkali at an elevated temperature will also result in the dissolution of a material which contains lignin linked to xylan as the predominant polysaccharide [ 12 1 1. It is reasonable to believe that the lignin reactions in kraft pulping proceed toa different extent in different parts of the fiber wall and in different types of fibers. Therefore, all analytical data on residual lignin must be regarded as averages, and the true values for phenolic hydroxyl groups, etc., must, in fact, include a range of values. The lignin macromolecules also include a range of structural features from rather intact “native” molecules to those which have been severely degraded and chemically changed as a result of the pulping conditions. In line with this hypothesis, it has been shown that a substantial portion of the residual lignin can be leached out of the fibers in a process which is governed by a high temperature and the presence of alkali [ 122,1231. b. C~rr1~okyclmte.s.The peeling reactions occurring in the polysaccharides during pulping result in the formation of acidic end groups. In addition, some side groups in the hemicelluloses are either eliminated, such as galactose from galactoglucomannan, or chemically modified, like 4-0-methyl-glucuronic acid from xylan. The latter, together with the formed unsaturated hexenuronic acid groups, add a substantial portion of the acid groups i n unbleached kraft pulps, the total of which can be around 60-80 mmolkg of (softwood) pulp [ 1241. The presence of hexenuronic acid groups also add to the kappa number of the pulp (Fig. 3 1 ) [ l 25,1261. In recent work, it has been found that this structure alone corresponds to around 2 kappa units inan unbleached softwood kraft pulp (kappa no. 18) whereas the corresponding figure for a hardwood pulp (kappa no. 1.5) is around 5 units 11271.

Pulp

Kappa Hexenuronic number

acid, pMollg pulp

Kappa number equivalence

Birch, unbleached 11.3 14.0 14.5 15.9 18.8

64 85

3.6 5.5 4.9 5.6 5.8

Birch, bleached 6.0 4.5

53

39

4.6 3.4

22

1.2 1.9

Pine, unbleached 18.2 18.6 18.0

FIGURE 31 kraft pulps.

Contribution to kappa nulnbcr of hcxcnuronic acid in solnc unbleached and bleached

890

Gellerstedt

W. PULP BLEACHING A.

TechnicalOverview

The poor selectivity and the slow delignification encountered in the final delignification phase in kraft (as well as in sulfite) pulping cannot be tolerated from a pulp quality point of view. Consequently, the cook is interrupted and, when required, the pulp is further delignified and bleached using oxidation as the principal reaction mode. The residual lignin in chemical pulps, notably kraft pulps, is difficult to remove, and several oxidative steps are required with intermittent extraction of the lignin which was oxidized in the previous acidic stage. Contrary to mechanical pulp bleaching, the complete bleaching of chemical pulps to full brightness must be accompanied by the almost total removal of all residual lignin from the fibers. Obviously, the chromophoric structures introduced in the pulping process are such that no bleaching agent can be used for the selective oxidation of the colored groups without a simultaneous dissolution of the lignin. The most efficient bleaching agent for chemical pulps is elemental chlorine, which has been used for several decades (together with hypochlorite) as the predominant oxidant in chemical pulp bleaching. With the development of chlorine dioxide bleaching, the possibility of making fully bleached kraft pulp in sequences such as C E D E D* was realized and introduced in the 1940s. The growing awareness that elemental chlorine might produce chlorinated products with a negative impact on the environment and the large leakage of organic matter occurring from bleaching plants paved the way for further changes in the bleaching technology. Today, these changes are aimed at strongly decreased levels of chlorinated organic products. As a consequence, new possibilities arise for reducing the use of fresh water and closing up the pulp mills. The discovery that magnesium compounds excerted a positive influence on the selectivity in oxygen (0)bleaching opened up the possibility of starting the bleaching sequence with an 0 stage. Here, around 50-6570 of the residual lignin in a softwood kraft pulp can be eliminated without detrimentally affecting the pulp strength characteristics. The dissolved material can be introduced into the chemical recovery system, thus considerably reducing the content of organic matter in the bleaching effluent. In modern bleaching technology, further changes have been introduced together with new bleaching agents. The use of elemental chlorinehas been reduced or completely eliminated, while the use of chlorinedioxide has increased. In elemental chlorine-free (ECF) bleaching, only chlorinedioxide is used. Alternatives, not involving the useof chlorine-containing bleaching agents, have been developed (totally chlorine-free, TCF, bleaching) and include oxygen, hydrogen peroxide (P), and ozone ( Z ) as major oxidants. This development has resulted in large improvements in the effluent quality and in increasing possibilities for reducing the discharge of both water and organic material from pulp mills. Bleaching reactions with oxygen, hydrogen peroxide, or ozone involves, in addition to ionic reactions, hydroxyl radical reactions. Oxygen is itself a radical, and therefore the initial reaction with lignin is a one-electron reaction giving rise to the formation of SUperoxide ions, hydrogen peroxide, and organic peroxides, which in turn can give hydroxyl radicals in secondary reactions. Hydroxyl radicals can also be formed in hydrogen peroxide and in ozone bleaching stages. The formation of the highly reactive and unselective hy-

*:C = chlorine, E = alkaline extraction, D = chlorine dioxide.

Pulping Chemistry

891

droxyl radical in these bleaching systems will result in a certain oxidation and degradation of the cellulose unless special precautions are taken. Even so, certain cellulose degradation cannot be completely avoided.

B. Oxygen 1. Lignin Reactions In oxygen &lignification, carried out in an alkaline medium, a partial oxidation of phenolic structures in lignin takes place. In structures with conjugated double bonds, such as in enol ethers and stilbenes, the oxidation may also take place in the side chain, resulting in a fragmentation of the lignin. The reaction starts with the formation of a phenoxy radical by electron transfer to the oxygen. In subsequent reaction steps, the phenoxy radical is converted into a hydroperoxide, which in turn reacts further, with formation of oxidized products. The reaction sequence is outlined in Fig. 32 [ 128,1291. The kinetics for the oxidation of a series of ligninlike phenols has been studied and found to be rather slow [ 1301. Thus, the kinetic half-life for phenolic structures having a guaiacyl structure is of the order of 14 min under technically relevant conditions. For structures like stilbenes and diphenols, such as catechols, much higher reaction rates are encountered. The importance of these structures for the lignin degradation is, however, limited due to their low abundance. Since the solubility of oxygen in water is low, it must be assumed that the extent of oxygen oxidation of lignin is highly dependent on the efficiency of mixing the pulp and the oxygen gas. Once the concentration of oxygen in the aqueous phase has ceased, only a very limited further oxidation of phenols can take place and secondary reactions, such as alkaline hydrolysis and lignin extraction from the fibers, become important. These reactions are accompanied by a certain brightening action caused by the liberated hydrogen peroxide in the system. The latter, together with organic peroxides, may also give rise to hydroxyl radicals by metal ion-induced decomposition and, thus, degradation of the cellulose. The consumption of phenolic hydroxyl groups in oxygen delignification has been found to be rather limited [ 131,l 321. In accordance with the kinetics discussed above, the residual lignin after an oxygen stage contains a considerable amount of remaining phenolic groups, whereas the solubilized lignin has a higher amount of phenolic groups as compared to the average for the residual lignin in the unbleached pulp. Thus, it seems likely that the oxygen oxidation occurs predominantly with those parts of the residual lignin which are the most phenolic and the most accessible, i.e., a lignin which already is rather soluble but trapped in the fiber wall for reasons of molecular size or a few remaining linkages to other pulp constituents. In addition to phenolic groups,thedissolved lignin from the oxygen stage contains an increased number of carboxyl groups. The total number of acidic groups is of the order of 5 mmol/g of lignin. After an oxygen stage, the remaining lignin contains a higher amount of carboxyl and a lower amount of phenolic groups as compared to the unbleached fiber lignin. Other differences are small, but the residual lignin has a somewhat higher amount of condensed structures [ 1321 and the content of p-0-4 structures has decreased in comparison with the residual lignin after the kraft cook. A second oxygen stage results in further &lignification of the pulp, but again the structural differences in the residual lignin are minor [ 1331. 2. Carbohydrate Reactions The carbohydrate reactions in oxygen delignification result in a loss of viscosity due to attack by hydroxyl radicals along the cellulose chains. The reaction, which is shown in

892

9

\ \

0

c

Gellerstedt

Pulping Chemistry

a93

Fig. 33, involves the formation of a carbonyl group in the sugar moiety, followed by a pelimination reaction that results in a chain scission [134]. Unless magnesium ions are present in the bleaching liquor, the viscosity loss isof such a magnitude that technical utilization of oxygen delignification is prohibited 11351. Addition of magnesium ions, however, will stabilize the formed hydrogen peroxide to some extent [136], thereby reducing the formation of hydroxyl radicals. It has also been shown that the presence of magnesium ions gives a certain reduction in the rate of reaction between hydroxyl radicals and carbohydrate structures [ 1371. The yield loss in oxygen delignification is limited, since the carbohydrate peeling reactions (see above) are suppressed in favor of an oxidative stabilization reaction. Thus, the reducing end groups in the polysaccharide chains are converted to aldonic acid groups through oxygen oxidation of the open enediol structure as shown in Fig. 28 [ 1381. After a kraft cook, the pulp contains a xylan in which a major fraction of the 4-OMe-glucuronic acid groups has been converted into the corresponding unsaturated structure, "hexenuronic acid," by loss of methanol, as described in Fig. 25. A subsequent oxygen delignification step is not able to degrade this structure, and consequently the relative contribution to the resulting pulp kappa number from the hexenuronic acid groups will increase after the 0 stage.

CHpOH H0

o

H0

'

o

m0 H

o

'0

aOQow HH 00

CH20H

0

- Hop H0

CHpOH

t "

0

0

H0

1

'0

- @-OH

CHZOH

-

H CHpOH

rearrangement

H0

0

HeLow coo0

FIGURE 33 Mechanism for the oxidative degradation of cellulose by hydroxyl radicals.

894

C.

Gellerstedt

HydrogenPeroxide

Alkaline hydrogen peroxide in the presence of sodium silicate is the traditional bleaching system for mechanical pulps, as discussed earlier. An efficient bleaching action requires that all transition metal ions be removed as completely as possible prior to bleaching, an operation usually done by pretreatment of the pulp with a strong chelant such as EDTA or DTPA. The same criteria must be fulfilled in the peroxide bleaching of chemical pulps. Unlike mechanical pulps, however, chemical pulps cannot be bleached to high brightness values with hydrogen peroxide unless the residual lignin is removed, since the chromophoric systems are much less reactive [ 1391. Efficient delignification and bleaching of chemical (kraft) pulps can be obtained if the bleaching stage is preceded by a chelating stage with EDTA, carried out under controlled conditions [140,141] in order to eliminate manganese, iron, etc., as completely as possible while keeping the concentration of ions such as magnesium and calcium high. The bleaching stage itself can be done with alkaline hydrogen peroxide (without silicate) at temperatures around or slightly above 100°C for 2-4 h.In the latter case, an oxygen pressure can be applied (POstage). Addition of magnesium ions to the bleaching liquor is beneficial. The ability of alkaline hydrogen peroxide to attack and degrade chromophoric structures, such as conjugated carbonyl structures, is well known. In the bleaching of chemical pulp, however, this mode of reaction must be accompanied by reactions resulting in a more comprehensive lignin oxidation and dissolution. One possibility for achieving such a reaction would be a radical-induced oxidation of phenolic structures in the residual lignin. Thus, the (controlled) decomposition of hydrogen peroxide leading to hydroxyl radicals and superoxide ions should result in the degradation of aromatic rings through reactions similar to those occurring in oxygen delignification. Analyses of the structure of the dissolved and residual lignin from a peroxide bleaching stage do not, however, unequivocally support such a mechanism since, i n contrast to the corresponding lignins from an oxygen stage, the content of aromatic rings in these lignins does not change to any large extent [ 1321. On the other hand, the content of carboxyl groups i n lignins from a peroxide stage is high, demonstrating the capability of hydrogen peroxide as an oxidant for lignin. Based on I3C-NMR data, the majority of the formed carboxyl groups seem to be aliphatic rather than aromatic, thus indicating that side-chain oxidation in lignin may be a predominant reaction mode in peroxide bleaching [142]. Recently, this view was further supported by a model compound study in which a phenolic lignin structure was shown to be easily degraded by the action of alkaline hydrogen peroxide at an elevated temperature (Fig. 34) [ 1431. Analysis by "C-NMR of isolated residual lignin after a peroxide stage (in the sequence OQP) demonstrates that this lignin, although coming from a pulp with less than 1 % lignin, still has most of the features of a lignin structure; the major differences are a low amount of p-0-4 structures, the presence of carboxyl groups, and the presence of aliphatic methylene and methine groups [142]. Therefore, the successive delignification that takes place in the kraft cook and in the subsequent oxygen delignification and hydrogen peroxide bleaching stage does not seem to alter (degrade) the remaining lignin structure in a profound way. The reactions of carbohydrates in a peroxide bleaching stage seem to be very similar to an oxygen stage. Thus, any decomposition of hydrogen peroxide, resulting in the formation of hydroxyl radicals, is probably inducing an oxidation of hydroxyl groups to carbonyl groups along the polysaccharide chains followed by a chain scission. In analogy with oxygen delignification, the hexenuronic acid groups present in the xylan will survive an alkaline peroxide stage, thus resulting in a comprehensive contribution to the kappa nlltnher :d'ter the stape

(Fig. 31).

Pulping Chemistry

895

I

I

HC-0-OH

-

OCH,

/

0

0

OH

CHZOH

I

HC-O*

I

HC-O* coo@

+

I

HC

-

OCH3

OCH,

+

ox.

f"

-

I

HO'

OH

I

OCH, 0

FIGURE 34

OCH,

OH

Oxidation of a phenolic p-0-4 structure with alkaline hydrogen peroxide.

D. Chlorine and Chlorine Dioxide In the bleaching of pulp with chlorine, both chlorination and oxidation reactions take place in the lignin. The dissolution of lignin is limited, however, unless a subsequent alkaline extraction is carried out in which the formed carboxyl groups are ionized. Other acidic bleaching stages behave similarly. Experiments with pulp have demonstrated that despite the great efficiency of chlorine bleaching (in combination with alkaline extraction), one DE sequence is not sufficient to remove the residual lignin completely; in fact, several DE sequences applied on the same pulp do not result in a complete lignin removal, as measured by the kappa number (Fig. 35) [ 1441. The reason for this behavior is thought to depend on the formation of specific groups in lignin which prevent any further penetration of aqueous chlorine into the lignin matrix. Alkaline extraction removes that portion of the lignin containing these groups, and new unoxidized lignin structures become exposed for an oxidationlchlorination sequence. Indirect support for this view is found in chlorine analysis of kraft fibers before and after an E stage in a CE sequence. That part of the lignin which had a high content of chlorine (and presumably of oxidized groups) could be removed by alkali, leaving a residual lignin with a uniform and low residual amount of chlorine [145]. Estimation of the chlorine mass balance from the bleaching sequence (Css + D,J E D E D (Fig. 36) [l461 reveals that the majority of added chlorine ends up in the effluent

Gellerstedt

30

6

-i

0 CE 0 CRECE A C~ECRECE cR: 2 min,

l§

5.74% cl2 on 0.d. pulp C : t mln, 5.74% Cl2 on 0.d. pulp

10

51k:n*

,

om*

,

0

0

10

20

30

40

o%

, 50

Time (t), min

FIGURE 35 Successive dissolution of lignin in a kraft pulp by repeated treatment with followed by an alkaline extraction stage [144].

chlorine

as chloride ions, most of which are formed in the E stage. In addition, chlorate is formed in the D stages.The remaining chlorine is found as organically bound chlorine in the effluent and, to a small extent, in the bleached pulp. The reactions between lignin and aqueous chlorine have been thoroughly studied, and four reaction principles have been identified I 1341. Chlorination of aromatic rings may

Cl (