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Overview
Product Details
| ISBN-13: | 9781853390289 |
|---|---|
| Publisher: | Practical Action Publishing |
| Publication date: | 12/01/1989 |
| Series: | Small-Scale Textiles Series |
| Edition description: | UK ed. |
| Pages: | 48 |
| Product dimensions: | 6.25(w) x 9.25(h) x 6.00(d) |
Read an Excerpt
CHAPTER 1
INTRODUCTION TO DYEING AND PRINTING
This handbook is limited to the consideration of dyeing and printing on a small scale — processes which can be carried out by hand or by the use of small power-driven equipment. It is also assumed that, as dyeing and printing are essentially service activities, they will form part of a larger activity of making textiles, probably as part of a small commercial operation. Considerations of cost, reproducibility, and the need to meet normal commercial demands for fastness properties will therefore be important considerations. The handbook will also be limited mainly to the dyeing and printing of wool, silk, and cotton, since a large range of other fibres, particularly synthetic fibres such as polyester, often require the use of expensive equipment and therefore fall outside its scope.
General considerations
Dyeing and printing as a way of enhancing the appearance of textiles has been used for a long time. Very little fundamental change took place in methods until the discovery of synthetic dyes in about 1856. Since that time there has been a proliferation of dyes designed to meet an ever-growing demand for textile products which are both attractive and practical. All dyes, whether from natural sources or manufactured, are coloured organic chemical substances which have the ability to be taken up and retained by fibres, normally from a solution of the dye in water. Because of the great number of different fibres and other materials which need to be dyed or coloured in some way, and the extremely wide variation in the kinds of wear and tear they must withstand in use, there are many thousands of different dyes. All must however possess certain basic properties if they are to be effective as dyes:
(a) Intense colour
(b) Solubility in water, either permanently or only during the dyeing operation.
(c) Ability to be absorbed and retained by fibres or to be chemically combined with them.
(d) Fastness — the ability to withstand removal or destruction by the processes which the fibre undergoes in manufacturing or in normal use.
(Source: Giles, C.H., Laboratory Course in Dyeing, p.29. Society of Dyers and Colourists.)
Modern dyes give the most satisfactory combination of these properties for any desired end use. Moreover, dyes are normally designed with specific fibres or groups of fibres in mind, and are usually grouped according to the type of fibre for which they are most suitable.
All fibres have a similar internal structure, in that they are composed of molecules which are extremely long and thin, like the fibres themselves. The way in which the molecules are arranged largely determines a fibre's physical properties, such as strength or elasticity, and some of the chemical properties, such as the ability to take up water or dyes. Sometimes the molecules are packed together lengthwise very closely and sometimes they are arranged in a more random manner. Where they are closely packed the fibre will be strong and rather rigid, and water and other substances cannot penetrate the fibre structure easily. Where they are more randomly arranged the fibre will tend to be weaker but elastic, and will readily take up water and other substances. All this, of course, is on the most minute scale, but the result is that fibres have a very large internal surface, composed of the walls and channels between molecules, and it is on these internal surfaces that dyes and other chemicals are taken up. (This list is not meant to be comprehensive. The same groups would be used for printing, with one or two additions. Coloured pigments have not been included. They are used mostly in printing, when they can be applied to most fibres by bonding to the surface.)
When a fibre is placed in water, the water rapidly penetrates and swells the fibre structure by entering through minute pores in the surface. Dyes and other chemicals are then able to diffuse into the spaces between the molecules and there combine with the fibre or form some other kind of link with the fibre or other dye molecules.
Although it is not normally possible to tell in a simple manner how the molecules are arranged in a fibre, a rough indication is given by the amount of water the fibre will normally hold in a moist atmosphere. This is called moisture regain (MR).
Fibres with a very low MR are in general rather difficult to dye, mostly because their internal molecular arrangement does not allow water or dyes to penetrate very easily under normal conditions.
Once a dye has penetrated the structure of a fibre, various chemical forces come into play so that the dye is gradually taken up and retained. The rate at which this takes place is determined by many factors. If the dye is taken up by the fibre very quickly from the dyebath the material being dyed may not be uniformly coloured. If the dye is taken up by the fibre too slowly, it will result in increased fuel and labour costs. In the first case an even dyeing, one in which every part of the fibre and the material being dyed is uniformly coloured, is difficult. In the second case the cost of dyeing becomes too high because of the increased fuel and labour costs, and the fibre may be damaged because of the prolonged dyeing time.
In practice, therefore, the dyer must use various means to control the rate of dyeing, usually by varying the temperature of dyeing and by adding various chemicals to the dyebath. Raising the temperature apparently increases the rate at which the dye is taken up, and this is used in many dyeing methods to allow dyeing to take place in a reasonable time. It cannot be done too quickly or there will be a risk of uneven dyeing. The dyebath is therefore gradually heated (raised) to the final dyeing temperature, normally 80 to 100°C, and complete dyeing will take about one hour at that temperature. In the case of fibres with a very compact physical structure, such as polyester, the best way to obtain a reasonable dyeing time is to raise the temperature to about 130°C, which means that very expensive pressurized dyeing machines must be used.
It is important to note that because dyeing takes place on the internal surfaces of fibres, time must be allowed for the dye to penetrate the fibre structure, and so most dyeing methods are designed to allow this to take place in about one hour. If a fibre is cut across into very thin slices during dyeing and these are examined under a microscope, it will be seen that the dye gradually penetrates from the outside to the inside of the fibre. If dyeing is stopped with only the outer layers of the fibre dyed (ring dyeing), it would have poor colour value (weak colour) and fastness properties.
The addition of various chemicals to the dyebath is also used to control the way in which the dye is taken up by fibres and can thus also influence the evenness of the dyeing. The use of these will be described in detail when the dyeing of individual fibre groups is discussed in Chapter 2.
CHOICE OF DYE
Choosing the best dye for a particular dyeing method, which will give all the required fastness properties required in the final product, requires considerable knowledge and experience. Quite often, of course, the choice may be limited by considerations of cost, availability, customer requirements, or simply by colour. Where considerable choice is possible, the following factors should be considered when choosing a dye for a particular purpose:
(1) The form in which the material is dyed
If fibres are dyed in the loose state, the evenness of the dyeing is not so important since blending of the fibre mass takes place during yarn manufacture and so any unevenness of colour will tend to be lost. This may mean that faster dyes can be used which might have uneven dyeing properties: faster dyes may indeed be essential, since loose fibres must withstand much processing during manufacture into fabric. If fibres are dyed in yarn or fabric form, however, evenness of dyeing is important since the slightest unevenness will show in the finished fabric. If the dyeing method does not allow a very good circulation of the dye solution or movement of the material being dyed, which is often the case when dyeing by hand, it may be necessary to use dyes with very even dyeing properties or to modify dyeing methods to allow for this.
(2) The manufacturing process
Many parts of the manufacturing process after dyeing may influence the choice of dye. Woven cotton fabrics must generally be scoured in hot alkaline solutions and any dye used must withstand the process. Fabrics containing wool must often undergo severe wet treatments such as scouring and milling, and the dyes used must withstand these processes.
(3) The fastness properties required in use
More often than not these are dictated today by customer specifications, and dyes and methods must be chosen to meet these. However, where precise specifications are not available it is sensible to choose dyes with very good light fastness for articles which must withstand severe exposure to sunlight, or good wet fastness for those which must be washed frequently.
These are only a few of the considerations underlying the choice of a particular dye for a particular purpose. There are obviously many more and often a compromise must be reached between the colour which may be desired and that which is achievable to meet all the fastness requirements on a particular fibre.
Colour matching
This is the ability to reproduce any desired colour on the material being dyed. There are really two requirements when dyeing any colour: first, that the dyeing shall be carried out properly so that it is even and has good fastness; second, that it should be the desired colour. The first is relatively straightforward, the second requires experience and skill to carry out consistently.
The desired colour will usually be available, from a customer or from records, as a sample of dyed material. This should be of the same type as that being dyed: it is nearly impossible to match yarn to a coloured piece of pottery, for example. The most reliable way to reproduce the desired colour is of course to have a 'recipe' from a previous dyeing of that colour on that material. Failing this the dyer must rely upon his previous records of dyeing similar colours. It is most important that the dyer understands the basic principles involved in colour matching, so that any colour can be reproduced to order, rather than relying entirely upon recipes, and so that dyeings which are not the correct colour can be changed easily. Colour matching is obviously a large and important subject, since one of the most important requirements for dyeing in a commercial situation, even on a very small scale, is that the dyeing or print is the desired colour. These matters are discussed in more detail in Chapter 2.
Printing
Printing can be regarded in many ways as localized dyeing. Thus the same dyes would be used to print various fibres as to dye them: the fastness properties of the dye are the same dyed or printed, and colour matching is just as important in printing as in dyeing. Printing however can use many techniques not available to the dyer, and the printer must use different methods to 'fix' the dyes, which is the equivalent of carrying out the dyeing process successfully. Printing is much more concerned with technique and method than dyeing and these are described in detail in the appropriate Appendices.
The basic methods of printing fabrics are however straightforward and have been used for many years.
Making designs directly on the fabric
This is probably the oldest method of patterning a fabric using dyes or pigments, but of course the person making the pattern must be skilled in drawing. The simplest method is to paint directly on the fabric and there are many techniques which allow dyes and pigments to be used in this way. Once the dye or pigment has been applied the fabric must be given a suitable treatment to allow the dye to be properly taken up or the pigment to be fixed.
The first methods of working directly on the fabric probably relied upon painting the pattern on the fabric with a substance which formed a physical barrier to the dye: when the fabric was then dyed these areas resisted the dye to form the pattern ('resist printing'). This method was used before dyes existed which gave good fixation simply by direct hand-painting. A wide variety of substances can be used as a resist for dyes, from those which form a simple barrier such as starches, gums, clay, wax, or methods of sewing or tying the fabric tightly in patterns, to the use of resist pastes which contain mordants or chemical resists.
The essential feature of all these methods is that the 'designer', by working directly on the fabric, produces a unique article which cannot be repeated exactly. The methods are therefore most suited to the production of relatively small articles, but the capital outlay needed can be very low.
Making designs directly and transferring them to the fabric
This involves a wide variety of techniques, all of which take the original' design', such as a painting or a photograph, which can be converted into a form which can be transferred to a fabric. If the original design contains many colours each of these must be separated out, since almost all printing processes apply colours to fabric separately, and all the colours must of course 'fit' together when they are applied to the fabric and so reproduce the original design. There are three ways the design can be converted into a form suitable for printing on fabric:
(a) by making each colour in the design into a raised surface, on a block of wood or metal. The raised surface is used to pick up dye or pigment to transfer to the fabric, and each block must 'fit' so that the design is reproduced exactly;
(b) by making each colour in the design into a pattern cut below a flat metal surface. The dye is applied to the whole surface, then the excess is scraped off the flat area to leave dye only in the part cut below the surface, which can then be transferred to the fabric. Again, each colour must fit;
(c) by making each colour in the design into a pattern cut through a metal or paper sheet or formed on a fine mesh or screen. The dye is pushed through the holes in the stencil or screen on to the fabric. Each stencil or screen must again fit.
The essential features of all these methods is the separation of the production of the original design from the method used to transfer it to fabric. This means that the design can be repeated exactly as many times as needed, and the methods are therefore suited to the production of many identical articles or to printing long lengths of fabric. The capital cost can vary from moderate to very high depending on the degree of mechanization which is introduced.
Transferring designs directly to fabrics using a high degree of mechanization
The latest developments allow the production of multicolour designs on computer screen which can then be transferred to fabrics directly by means of electronically controlled printing systems. The essential features of these methods are extreme flexibility and speed of operation which makes them suitable for a wide variety of printed effects, but the capital cost is extremely high.
All the methods described, with the exception of painting by hand, have been developed in a number of ways for different purposes. Block printing has been mechanized by mounting the blocks on rollers: one version uses blocks made from plastic foam to print carpets. Screen-printing has been developed in similar ways using rotary screens. In every case, as the degree of mechanization and speed of operation increases the capital costs rise and the latest printing systems are extremely complex and expensive. For these reasons the only printing methods which will be considered in this handbook are those which can be carried out simply by hand, that is, direct hand-painting, hand block-printing, and hand screen-printing.
No matter which method of printing is used it is essential that there is good collaboration between those who originate the designs to be printed and those who carry out the printing.
(Continues…)
Excerpted from "Dyeing and Printing"
by .
Copyright © 1990 Intermediate Technology Publications.
Excerpted by permission of Practical Action Publishing Ltd.
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.
Table of Contents
Acknowledgements, vi,
Foreword, vii,
Preface, viii,
Chapter 1 Introduction to dyeing and printing, 1,
Chapter 2 Basic principles and processes, 8,
Chapter 3 Simple methods of testing, 23,
Chapter 4 Specifications of small-scale dyeing machines, 28,
Chapter 5 Planning for production, 33,
Chapter 6 Equipment and materials suppliers, 39,
Chapter 7 Sources of further information, 42,
APPENDICES,
1 Some technical terms used in dyeing, 45,
2 Preparation and after treatment of yam and fabric, 47,
3 Dyeing wool with acid and metal complex dyes, 50,
4 Dyeing cotton, viscose rayon, or linen with direct dyes, 51,
5 Dyeing cotton, viscose rayon, or linen with reactive dyes, 52,
6 Dyeing silk with acid, metal complex, and reactive dyes, 53,
7 Printing wool and silk with reactive dyes without steaming, 54,
8 Printing cotton fabrics with reactive dyes, 56,
9 Printing cotton fabrics with pigments, 58,
10 Effluent disposal, 60,