The Chemistry of Paper

by J C Roberts

Overview

The manufacture of paper involves a large amount of chemistry, including carbohydrate chemistry, pigments and resins and colloid and surface chemistry, as well as elements of environmental and analytical chemistry. Providing an overview of the making of paper from a chemical perspective, this book deals with both the chemistry of paper as a material and the chemistry of its production. The book explores several chemical processes involved in the production of paper: the delignification of the wood fibres performed at elevated temperature and pressure, the bleaching of the cellulose-rich pulp using environmentally-friendly systems, the formation of the pulp into sheets of fibres strengthened by extensive inter-fibre hydrogen bonding, and finally the coating of the sheets in a manner appropriate to their end use. This book is an informative and entertaining overview for students and others who require an introduction to the chemistry of paper manufacture.

Product Details

ISBN-13:	9780854045181
Publisher:	RSC
Publication date:	09/27/1996
Series:	RSC Paperbacks , #10
Pages:	204
Product dimensions:	5.45(w) x 8.50(h) x (d)

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The Chemistry of Paper

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By J.C. Roberts

The Royal Society of Chemistry

Copyright © 1996 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85404-518-1

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CHAPTER 1

An Introduction to Paper

INTRODUCTION

Paper has been an essential part of our civilisation for at least two thousand years and, perhaps because of our familiarity with it, we do not tend to think of it as a particularly complex material. However nothing could be further from the truth. It is derived from plant sources and therefore has both morphological complexity and physical and chemical complexity. Even our understanding of its load-elongation behaviour, which might be expected to be relatively simple, is still far from complete. The production process itself is also highly sophisticated, involving what is in essence a high-speed filtration process yielding a weak wet fibrous network. This wet web, despite its weakness, must then be pulled continuously through the pressing and drying sections of the paper machine to the reel at speeds which these days approach 60 km h-1, during which the web undergoes some extension. To avoid frequent breaks, and to obtain good product uniformity therefore requires some of the most advanced control engineering technology available today.

This opening chapter is a brief introduction to the nature of paper, its history and to its modern day use.

DEFINITION OF PAPER

When we think of paper we think of it primarily as a writing and printing medium, and then perhaps as a wrapping and packaging material. However, because many other products — for example, tissue, board, filtration media, surgical wrap, etc. — are made by essentially the same process, a broader definition is more appropriate. For the purpose of this text therefore, paper will be defined in terms of its method of production, that is a sheet material made up of a network of natural cellulosic fibres which have been deposited from an aqueous suspension. The product which is obtained is a network of interlocking fibres with an approximately layered structure about 30-300 µm thick. The width of an individual fibre is in the range 10 to 50 µm, and a sheet of writing paper of 100 µm thickness would therefore be expected to be about 5 to 10 fibres thick (Figure 1.1).

The precise time and place at which paper was introduced into our civilisation is not known with any certainty. Before 700 BC, animals skins were certainly the medium of written communication, but these were displaced by papyrus by the Egyptians at some time around 600 BC. Papyrus, although derived from a plant source, is not strictly paper as defined above, as it is made by separating and spreading the pellicles of the aquatic papyrus plant on to a flat surface sprinkled with water rather than by depositing a network of fibres from an aqueous suspension. Various forms of parchment, which are close relatives of papyrus, were used by the Greeks and Chinese for the next 800 years, but it was not until around 200 AD that the Chinese introduced the art of making paper by reducing fibrous matter to a pulp in water and then forming it as a network. The Chinese are thus usually credited with the invention of modern paper manufacture.

The fibres from which paper is made are the structural cells of plants, and paper could therefore be made, in principle, from a wide variety of plant sources. In practice, the sources are limited by factors such as availability, crop yield per hectare, and quality of the fibre. In the late nineteenth and early twentieth centuries cotton in the form of rags was the main fibre source, and the pioneer paper-making factories grew up around the sites of the textile manufacturing industry. Since the early part of the twentieth century, as the demand for paper grew and the waste from the textile industry was no longer able to satisfy the demand, wood became increasingly used, so that now over 90% of virgin fibre (that is excluding any recycled fibre) is derived from wood.

PRODUCTION AND CONSUMPTION

The annual world production of paper and board is around 250 million metric tonnes, and well over half of this is produced in the US and EEC countries. A mere 1.2 million tonnes is produced in the whole of Africa. It is also consumed almost totally by the developed world and the per capita consumption of paper and board products varies hugely throughout the world (Table 1.1).

In addition to fibre obtained directly from plant sources by chemical or mechanical treatment (virgin fibre), recycled fibre is also used and to an increasing extent for paper and board production. A breakdown of world fibre usage is given in Table 1.2 and the subject of paper recycling and its chemistry is discussed more fully in Chapter 9.

Recycled fibre now accounts for over a third of all fibrous raw material and, over the past few years, its use has steadily increased whilst that of virgin pulp has remained fairly constant. The extent to which recycled fibre is used varies greatly from country to country. In Europe, where there is a fibre deficiency, it accounts for over half of the total fibrous raw material whereas in North America and Canada, where wood is plentiful, recycling levels are much lower. There is still scope therefore to increase further the use of recycled fibre and, as the consumer is increasingly demanding it in paper and board products, the upward trend is expected to continue for some time yet. Recycled fibre is not distributed uniformly through all grades of products; some grades — for example many types of board — use 100% whereas others, such as speciality grades and some high quality writing grades use none at all. This subject is discussed more fully in Chapter 9.

FIBRE SOURCES

Although the amount of recycling could still be increased, there is almost certainly an ultimate limit to the extent to which recycled fibre can be used, and it is difficult to foresee a totally 'closed fibre' industry in which no new fibre is introduced. Most of the newly introduced fibre will also probably continue to be derived from wood, although annual crops can be expected to play an increasingly important role.

Approximately 30% of the earth's land surface is forested, and around half of this is harvested commercially. Over 80% of the wood for all industrial uses comes from the forests of North America, Europe and what was formerly the Soviet Union. Approximately two thirds of this is either sawn or peeled. Paper is generally made either from logs that are unsuitable for sawing or peeling or from residues arising from these processes.

Both hardwoods and softwoods are used for making paper and they have very different fibre morphologies and thus very different paper-making properties. The fibres of softwoods are longer and stronger than those of hardwoods and they make up the bulk of paper-making fibre worldwide (Table 1.3). However, because they easily form macroscopic floes of entangled fibres during the sheet forming process, they tend to produce a sheet with a relatively non-uniform mass distribution and hence a poorer quality of appearance (this is known by paper technologists as formation). It is common therefore to use blends of softwood and hardwood fibres to give an appropriate compromise between strength and formation. The characteristics of hardwood and softwood fibres are discussed at greater length in Chapter 2.

Non-woody fibre, although relatively small in volume is nevertheless important, particularly in the developing world where the use of indigenous raw materials can substantially reduce the amount of foreign exchange spent on importing costly wood pulp. The main sources of these fibres are bagasse, bamboo, jute, ramie, hemp, flax and cotton, and also various grasses and straws, such as esparto, wheat, barley or rice. Their main advantage over wood is that they can frequently be grown in areas which will not support trees, and in limited rainfall in low quality soil. In general, they produce an annual crop with a higher yield than wood. For example, straw can be produced at yields as high as 20 metric tons per hectare, which is considerably greater than the annual growth of most tree species. Non-woody plants can also be harvested relatively quickly — usually one or two years after planting — whereas trees require ten to twenty years to reach sufficient maturity.

The paper-making properties of all of these fibres are quite different from each other and also from wood. This is mostly due to the differing morphology and to some extent the differing chemistry of the fibre cells. The photomicrograph (Figure 1.2), shows a comparison between various non-woody fibre types.

PRODUCT TYPES

Just over 40% of all the paper which is produced throughout the world is used for communication purposes (newsprint and printing and writing), and over 50% is used for packaging and tissue (Figure 1.3). The remainder is used in rather specialised applications such as filtration media, tea bags and electrical insulation in transformers.

Paper is classified in terms of its weight per unit area (basis weight or grammage). Tissue grades are generally in the range 10-40 g m-2, newsprint around 40-50 g m-2, printing and writing grades around 60-90 g m-2, and boards are usually in excess of 100 g m-2. Because of the need to obtain specific characteristics in the final product, for example water absorbency or wet strength, there is a great difference in the chemistry and method of production of these grades.

CHEMICAL COMPOSITION OF PAPER

As paper is obtained from fibres which were, before chemical and mechanical treatment, the cells of land plants, it does not have a fixed chemical composition but one which is largely pre-determined by the fibre source. The cells of land plants are mostly composed of carbohydrate polymers (polysaccharides) which are impregnated to varying degrees, with lignin — a complex aromatic polymer the amount of which generally increases with the age of the plant and which is biosynthesised during the process of lignification. These components and their chemical structures and functions are discussed more fully in Chapters 2 and 4. The carbohydrate part of the cell is dominated by the structural polysaccharide cellulose, but there are also other polysaccharides of a non-structural nature and with a very much lower molecular weight which are known, somewhat misleadingly, as hemicelluloses and which play an important part in pulp and paper properties. The term hemicellulose seems to imply some relationship to cellulose and, at one time, they were thought to be biosynthetic precursors of cellulose. However, it is now well established that these polysaccharides are not involved in the biosynthesis of cellulose, but are a discrete group of polymers with their own specific function in the plant cell wall.

In addition to these main components there are also relatively small amounts of organic extractives and trace inorganic materials. The approximate distribution of the three main groups of components together with other trace materials is given in Table 1.4.

The overall composition of plant fibre cells in terms of carbon, hydrogen and oxygen is variable and dependent on the degree of lignification. For wood it is approximately 50% carbon, 6% hydrogen and 44% oxygen. Carbohydrates, because they all have more or less the same elemental composition of (CH2O)n, have a more or less uniform carbon content of around 40%. Lignin, on the other hand, is an aromatic polymer with the approximate composition C10H11O4 and therefore has a much higher average carbon content of about 60-65% (Table 1.5).

CONVERSION OF NATURAL FIBRES INTO PAPER

Paper can be made from fibre cells in their more or less unmodified form, by simple mechanical disintegration to disperse them in water, and then forming them into a web by the process described in Chapter 5. This process of mechanical pulping is suitable only for products with a short life span — because the lignin (which is not removed) discolours in sunlight as a result of photochemically catalysed oxidation processes, and the paper becomes yellow and brittle. The use of lignin-containing fibres is therefore restricted to products such as newsprint and disposable light-weight coated paper. For higher quality papers which are required to have a longer lifetime, it is necessary to remove the lignin by a chemical pulping process. This involves a high temperature and pressure reaction in which the lignin is solubilised under aqueous alkaline, neutral or acidic conditions. Non-aqueous solvent pulping procedures have also been developed but are not yet in full commercial use. The chemical removal of lignin produces a brown pulp, the colour of which is mostly due to chromophores associated with small amounts of residual lignin. It is therefore often followed by a bleaching operation which, in the past, has been almost exclusively chlorine-based but, as a result of environmental pressures, is being superseded by other methods. Chemical delignification and subsequent bleaching are discussed more fully in Chapter 3. Such fibres will be used in high quality printing and writing grades, and in high added-value speciality applications.

CHAPTER 2

The Material of Paper

INTRODUCTION

Unlike most chemical raw materials, the fibres which are used for paper making are produced not synthetically but biosynthetically as plant cells. The paper maker therefore, apart from using crop selection and strategies for growth and harvesting, has little control over fibre shape and chemical composition. As these have a profound influence upon the subsequent chemistry of the paper-making process, and also upon the physical and mechanical properties of the end product, it is important to understand something of the morphology, structure and chemical composition of paper-making fibres.

FIBRE MORPHOLOGY AND WOOD CELL STRUCTURE

Plant cell walls may have shapes varying from spherical to cylindrical, and sizes varying from under 1 mm to several centimetres. In higher plants, two types of functional cell walls can be distinguished. These are the primary cell wall, which surround the growing cell, and the secondary cell wall, which is laid down when growth has ceased. The cell wall is a complex composite material and contains both structural and non-structural components. These components are mainly polysaccharides, although lignin and proteins also play an important part. The structural component is usually partly crystalline, and exists in the form of microfibrils. The most common of these is cellulose, which is a linear β-1,4-linked polysaccharide of β-D-glucopyranose, the molecular and crystal structure of which is discussed more fully in Chapter 4. Some algae contain structural polysaccharides composed of mannose and xylose units, but these have no industrial importance in paper manufacture. The nonstructural polysaccharides are chemically more complex, and their function in the plant cell wall is still poorly understood.

(Continues...)

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Excerpted from The Chemistry of Paper by J.C. Roberts. Copyright © 1996 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

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Table of Contents

An Introduction to Paper;
The Material of Paper;
The Chemistry of Lignin and its Removal;
Cellulose Fibre Networks;
The Paper Formation Process;
The Surface Chemistry of Paper and the Paper-making System;
Chemical Additives in the Paper Formation Process;
The Surface Modification of Paper;
Recycling of Cellulose;
Paper Making and the Environment.