Chemistry in the Utilization of Wood

206 Pages·1967·3.66 MB·English

by  R. H. Farmer and Jack Kape (Auth.)

PERGAMON SERIES OF MONOGRAPHS ON
FURNITURE AND TIMBER
VOLUME 1 Brown — An Introduction to the Seasoning of Timber
VOLUME 2 Parker — Costing in the Furniture Industry
VOLUME 3 Clark — Veneering and Wood Bending in the Furniture
Industry
VOLUME 4 Akers — Particle Board and Hardboard
VOLUME 5 Findlay — Timber Pests and Diseases
VOLUME 6 Collier — Woodfinishing
VOLUME 7 Oliver — The Development and Structure of the Furniture
Industry
VOLUME 8 Silvester — Timber: its Mechanical Properties and
Factors Affecting its Structural Use
Chemistry in the
Utilization of Wood
R. H. FARMER
B.A., D.Sc. (Tech.), F.R.I.C, F.I.W.Sc.
Officer in Charge, Chemistry Section
Forest Products Research Laboratory
Princes Risborough, England
PERGAMON PRESS
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Copyright © 1967
Pergamon Press Ltd.
First edition 1967
Library of Congress Catalog Card No. 66-29789
3077/67
Foreword
WOOD is a ubiquitous raw material employed for many widely
varying purposes. It is mainly used as a constructional ma-
terial and for fuel but it also serves as the basic raw material
for chemical processing, the most important being for the
manufacture of paper.
These highly specialized processes have their own literature
but the chemical aspects of wood affect all its uses from ease
of sawing to changes in colour.
This book is a much shorter and less detailed treatment of
the chemistry of wood than the existing textbooks and is
largely concerned with its technical and practical application
over a wide field. It is primarily intended for technical men in
the wood-using industries who have an interest in wood and
some knowledge of chemistry. It will also serve as a valuable
textbook for students entering any field of wood technology.
Princes Risborough, J. BRYAN
Aylesbury, Bucks. Director, Forest Products Research
vi
Preface
THE spread of scientific education and the introduction of new
science-based techniques into the wood-using industries make
it both possible and necessary for the wood technologist of
the future to have a better understanding of the fundamental
properties of his material. It might be thought that there are
already sufficient books on the chemistry of wood to fill this
need. Indeed, the excellent textbooks of Wise and Jahn,
Hagglund, Stamm and Harris, Sandermann, Hillis, Browning
and Stamm provide full and detailed accounts of this complex
subject which are indispensable to the specialist and the
research worker. Nevertheless, it is felt that there is a need for
a shorter and simpler treatment that will be better suited to the
requirements of students of wood technology and of technical
men in the wood-using industries, who have some basic know-
ledge of chemistry and wish to apply this knowledge to the
practical problems arising in the utilization of wood.
The emphasis in the present book is on the practical applica-
tions of the chemistry of wood. The discussion of the chemical
composition of wood has intentionally been kept short and
concise, and it is assumed that the reader has sufficient know-
ledge of chemistry to grasp the essential principles without
lengthy explanations. The principal aim of the book is to
show how the chemical nature of wood influences its proper-
ties and utilization, both as a constructional material and as
the raw material for the manufacture of secondary products,
such as pulp and paper, cellulose derivatives, board materials,
etc.
Full literature references have not been provided, but at the
end of each chapter will be found a short list of books and
vn
PREFACE
articles which is intended to assist the reader who wishes to
study the various subjects in greater detail.
In the preparation of this book the author has drawn freely
on existing publications on wood chemistry, and particularly
on the works mentioned above. He also gratefully acknowledges
the help and advice of his colleagues at the Forest Products
Research Laboratory.
Princes Risborough, R. H. FARMER
England
vm
CHAPTER 1
Chemical Composition of Wood
THROUGHOUT the history of civilization, wood has always been
one of man's most widely used and versatile raw materials,
both as a building and constructional material and as fuel.
While its use as timber is still of primary importance, the past
hundred years have been characterized by the tremendous
growth of other industries based on wood. The manufacture
of pulp and paper, packaging materials, board products,
cellulosic fibre and plastics, and chemical products from wood
constitute vast industries and consume very large quantities of
wood. All of these secondary industries are dependent for
their successful operation upon a knowledge of the chemistry
of wood, and even in the utilization of wood as such, chemical
factors are often of major importance, as will be seen in later
chapters. It is perhaps significant that the study of wood
chemistry also commenced rather more than 100 years ago.
In order to understand the chemical nature of wood it is
necessary to consider briefly its anatomical structure. The
discussion will be confined to those aspects of wood structure
that are essential for the present purpose, and for a more
detailed treatment of this subject, reference should be made to
books listed in the bibliography at the end of the chapter.
By "wood" is meant the main tissue of the stem, roots and
branches of so-called woody plants. These plants include
conifers, dicotyledonous trees and shrubs, and certain tree-like
monocotyledons such as the palms. For practical purposes we
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CHEMISTRY IN THE UTILIZATION OF WOOD
may restrict ourselves to the wood of the coniferous and
broad-leaved trees, commonly referred to as softwoods and
hardwoods respectively.
GROWTH OF A TREE
A tree grows as a result of two separate processes. First,
elongation of the stem and branches occurs by growth at the
ends of these. This is called "primary growth" and controls
the ultimate form of the mature tree, because the increase in
length and the extent to which branching takes place at the
growing points are characteristic of the species.
In addition to primary or apical growth, increase in thickness
of the stem and branches of the tree occurs as the result of the
activity of a growing layer (cambium) between the wood and
the bark. This secondary growth, or secondary thickening, is
of greater importance than the primary growth from the point
of view of the features of the wood that is formed. The
cambium consists of a thin growing layer, one cell in thick-
ness, surrounding the wood of the stem. The cambial cells are
capable of dividing with formation of a pair of cells, one of
which remains as a cambial cell, while the other forms part of
the xylem or wood or, less frequently, the phloem or bark.
In this way the stem and branches continually increase in
thickness during the whole life of the tree. The amount of bark
formed is comparatively small in relation to the production
of wood and, furthermore, the phloem cells are compressed
by bark pressure and portions of the outer bark may be lost
from time to time. For these reasons, the increase in thickness
of the bark is relatively small and the bark only makes up a
small fraction of the total volume of the stem.
GROSS ANATOMICAL FEATURES OF WOOD
Figure 1 (see art section) shows a wedge-shaped section from
the stem of a tree. This illustrates an important feature of the
wood in many kinds of trees, namely the existence of a series
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CHEMICAL COMPOSITION OF WOOD
of concentric rings around the pith as centre. These are the
annual rings, or growth rings, and consist of alternate layers
ofmoreorless dense tissue, each complete layer corresponding
to one year's growth. The cells that are laid down in the early
part of the growing season are thin walled and have a wide
lumen, so the "spring wood" or "early wood" has a relatively
low density. Later in the season the increase in thickness of
the stem is slower, and the cell walls are thicker. The "summer
wood" or "late wood" that is formed is therefore higher in den-
sity than the spring wood, and is also often darker in colour.
Formation of summer wood ceases late in the summer, and
the growth of spring wood commences again the next spring.
The cycle is then repeated year after year.
The annual rings, although always present, are not equally
distinct in the wood of all trees. The conifers generally show
clearly marked annual rings, but these are much less sharply
defined in many of the deciduous woods (hardwoods). In
addition, growth increments differ greatly in width and
density according to the conditions of growth of the tree. In
trees grown in tropical climates, where the seasonal differences
are small, the growth rings may be absent, and are in any case
less prominent than in trees grown in temperate regions.
The wood of the outermost growth rings of the stem of a
tree (sapwood) is light in colour and performs certain physio-
logical functions, of which the most important is sap conduc-
tion. After a few years' growth, however, there is a more or
less abrupt change in colour of the wood in many species of
tree, the wood in the centre of the stem (heartwood) being
darker in colour. The heartwood no longer participates in sap
conduction or the storage of reserve food, and may be regarded
as dead tissue. Its dark colour in many species is due to the
deposition in the cell walls or cell cavities of extraneous
materials, such as resins, tannins and colouring matters. These
components will be discussed in greater detail in Chapter 2.
Woody tissue consists essentially of a large number of cells
(fibres or tracheids) which are cemented together by inter-
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CHEMISTRY IN THE UTILIZATION OF WOOD
cellular substance, together with a proportion of other special-
ized cells (ray cells, parenchyma cells, vessels, resin canals,
etc.). The percentages of the various types of tissue vary in
different woods and in different parts of the stem, but in
general the greater part of the wood substance consists of
fibres. A study of the chemical composition of these structural
units will therefore provide a good indication of the composi-
tion of the wood as a whole.
CHEMISTRY OF THE CELL WALLS OF WOOD
The accompanying diagram (Fig. 2) of the enlarged trans-
verse section of a small group of typical wood cells illustrates
FIG. 2. Enlarged cross-section of a group of wood cells
the various layers which may be distinguished in the cell wall.
Commencing at the outside of the cell and proceeding inwards,
there is first the zone that is shown black on the diagram. This
4