Continuing Professional Development

Transcription

1 Continuing Professional Development A-level Textiles Maximising student performance in the AS and A2 written papers (Units 1 and 3) Colour and Pattern in Fabrics Version 1.0

2 Permission to reproduce all copyright materials have been applied for. In some cases, efforts to contact copyright holders have been unsuccessful and AQA will be happy to rectify any omissions of acknowledgements in future documents if required. 2

3 Contents Page Dyeing of Fabrics 4 Preparing fabric for dyeing and printing 6 Dyeing Processes 7 Printing of Fabrics 9 Some recommended sources of information 13 3

4 Colour and Pattern in Fabrics Colour and pattern are major considerations when consumers select textile products for particular end uses. Colour is created when light is absorbed or reflected by different surfaces. It can be applied to textile products by dyeing and printing, and also through the use of embroidery and trimmings. Dyeing of fabrics There are many different dyes available for colouring fabrics; most of them have been developed for a particular application. Textile dyeing involves the permanent application of a colourant to a fibre to give a uniform colour. In order to be successful, the colourant must be able to be absorbed by, or react with, the textile fibre. It must also be soluble so that it can go into the spaces between the fibre molecules. Dyes are classified according to their ability to dye different fibres. Class of dye Direct dye Reactive dye Vat dye Disperse dye Acid dye Fibres it is used on Cellulosic fibres, including cotton Cellulosic and protein fibres, nylon Cellulosic fibres Acetate, polyamide, acrylic, polyester Protein fibres, polyamide Direct dyes are soluble in water and used for dyeing viscose, cotton and modal fibres. They are moderately fast to light but have poor wash fastness. Salt can be added to the dye bath to help the fibre to absorb the dye. Reactive dyes are water soluble dyes which form a strong chemical bond with cellulosic and protein fibres. They give bright colours which are fast to washing. Vat dyes are not soluble in water and give excellent wash and light fastness. Indigo, used to dye denim blue, is one of the best known vat dyes. In order to get the dye into the fibre, it has to be converted into a soluble form by removing oxygen. Once the dye is in the fibre, it is converted back to its insoluble form by oxidation, which makes the molecules too big to get out of the fibres. A lot of indigo dye sits on top of the fibre rather than being absorbed into it, and this is what causes it to rub off onto other fabrics and to lose some colour when washed. Disperse dyes are used to dye fibres which have hydrophobic (water-hating) properties. These dyes are almost insoluble in water and are applied to the fibre in the form of a fine aqueous dispersion. They are held inside the fibres by chemical bonds. 4

5 Acid dyes are soluble in water and are applied to the fabric in an acidic dye bath. They have good fastness to light but the wash fastness varies. In recent years, some solvent dyes have been developed. These are applied to fabric using organic solvents instead of water. This is because water usage and disposing of it afterwards in a form which will not cause pollution has become very expensive. Solvent dyes are also very expensive to use because there is a cost involved in recovering the organic solvent so that it can be used again. The equipment used is also very expensive, making these dyes not especially economical to use. There are a number of issues to consider when selecting dyes to use on a fabric for a particular end use: colour requirements, ie shade fastness requirements, ie how well the dye will stay on the fabric cost Generally dyes with better properties are more expensive but sometimes, in order to meet consumer demand, dyes with poor properties are used because they are the only ones available that will produce a given shade. The process of dyeing consists of three basic steps: immersing the textile into the dyebath the dye attaching itself to the textile fibre fixing of the dye within the fibre The strength with which the dye is held in the fibre depends on the physical forces resulting from the shape of the dye molecule and structure of the fibre, and also to the chemical forces that may be present. The strength with which the dye is retained within the fibre is related to the property of washing fastness. Colour fastness of textiles When selecting dyes for a particular end use, colour fastness must be considered. Fabrics may need to be have fastness to the following: Bleaching Dry cleaning Washing this will be important for clothing products worn regularly Light this will be important for furnishing products Perspiration this will be important for fashion products Gas fumes Rubbing (crocking) important for seating and some clothing products Miscellaneous 5

6 Preparing fabric for dyeing and printing When the fabric comes from the loom or knitting machine it is not ready to be dyed or used straight away. At this stage it is referred to as Grey (greige) or loomstate cloth and it often has a natural creamy colour. Fibres, especially the natural fibres, have natural impurities in them, eg fats, waxes and salts. There will also be other impurities picked up during the processing, eg oil and dirt from machines, and size (starch) added to strengthen yarns before weaving. These have to be removed before the fabric can be dyed or finished, otherwise the colour or finish will not attach itself evenly to the fabric. The following are some of the important preparation processes. Desizing Size is a starch, gum or gelatine type of substance which is applied to warp yarns before they are woven into fabrics. This helps to strengthen them so that they will be more able to stand up to the constant movements of the loom. Most of these substances are soluble in water so they can easily be removed by washing. Scouring Scouring removes fatty and waxy impurities which would prevent the fabric from being wetted. Cotton fabrics are scoured by boiling them in caustic soda solution. This method would not be appropriate for wool fabrics as they would be damaged by the heat and the alkali; instead they are moved through warm detergent solutions to remove the fatty deposits in the fibres. More modern methods involve washing the fabrics in solvents to remove the oils and greases. Bleaching Fabrics are sometimes bleached to obtain a fabric which is evenly white before colour is added. Cotton fabrics are bleached using carefully controlled amounts of hypochlorite bleach or hydrogen peroxide. Both of these are oxidizing bleaches, ie they remove unwanted colour by adding oxygen to the stain to make it colourless. Wool is often left in its naturally creamy colour. It is never treated with chlorine bleaches as these will damage the scales on the wool fibres. It can be bleached using hydrogen peroxide (as used to bleach human hair), or by using sulphur dioxide gas, a reducing agent which works by removing oxygen from the stain to leave it colourless. Fluorescent whitening agents are commonly used to whiten all fibre types, especially where a very white fabric is required. These are colourless dyes which cause the fabric to reflect an ultraviolet light which makes it appear to be whiter and brighter. These fluorescent dyes will be broken down by the bleaches commonly used in home laundering, and this is the reason why consumers are sometimes advised not to bleach white cotton fabrics. 6

7 Dyeing textiles Textiles may be dyed at various stages in processing during which they are transformed from the fibre to the finished article: Dope dyeing dope or spun dyeing is a process whereby the dye is added to the spinning solution of man-made fibres prior to spinning the fibre. This method results in the textiles having good light and wash fastness. Stock dyeing when textile fibres are dyed in a loose form the term stock dyeing is used. This method is relatively cheap and has the advantage that if the dyeing is uneven it will be removed by the blending processes that follow when converting the fibre into yarn. Yarn dyeing Piece dyeing Yarns are dyed before being made into fabric. This is when woven or knitted fabrics are dyed in the piece. Garment dyeing made-up garments are dyed as required. This allows manufacturers of fashion products to apply colour at the last minute as and when there is consumer demand for the latest shades. Dyeing processes When dyeing fibres, yarns and fabrics it is important to apply the colour evenly and to avoid damaging the textile. Yarns should not become tangled and fabrics and garments should not be creased, or their shape/size be distorted. Synthetic fibres can only be dyed at temperatures above 100 C and therefore machines which are able to operate under pressure must be used. The dyeing process can be continuous, discontinuous (batch), or semi-continuous. In continuous dyeing, the fabric is passed through a small pad bath which contains the dye. It is then squeezed between rubbercovered rollers which ensure that the dye penetrates the fabric evenly. The fabric then moves on to the fixing machinery which fixes the dye within the fibre. Batch dyeing processes In these processes a specific weight of fabric is treated in a self-contained machine, in a dye bath which contains a specific weight of dye. Different methods are used according to the type of fabric being dyed. 7

8 When fibres and yarns are dyed, they are put into perforated containers and the dye liquor is pumped through. Hanks or skeins of yarn can be hung on rotating arms which move through the dye bath, although modern methods are more likely to use stationary arms with the dye liquor circulated through the yarns. Fabrics (piece goods) can be dyed using the winch machine. The fabric is sewn so that it forms a continuous band of fabric which is then passed round and round, over the winch and through the dye liquor. This method is used for lightweight woven fabrics and knitted fabrics. Jig dyeing is used for fabrics which must not be creased. The jig dyeing machine has two rollers which sit above the dye bath. The fabric is wound from one roller to the other so that the fabric passes through the dye bath. The direction is then reversed and the fabric is wound back on to the first roller, again passing through the dye liquor. This is repeated until the required depth of colour is achieved. Jet dyeing is similar to winch dyeing except that the fabric is moved round the machine by a high pressure jet of the dye liquor. 8

9 Semi-continuous dyeing The dye is put onto the fabric in the same way as for continuous dyeing. The fabric is then wound up onto a batching roller this is capable of holding very long lengths of fabric. The dye can then be fixed onto the fabric whilst it is on the batching roller, or it can be fixed in a separate process. Printing of fabrics Printing is a technique of applying colour to fabric via some medium which carries a design. It offers certain advantages over dyeing: greater scope for designing more colours can be produced complicated designs can be produced by printing methods of printing are quite varied usually less costly Printed fabrics can be distinguished from woven fabrics because only the printed side shows the printed design quite clearly as the dye applied during printing does not penetrate right through the fabric. When designing for a print particular attention is paid to the repeat of a design as it must fit evenly into the width of the cloth; lengthways it is only limited by the size of the equipment used for printing. The design may be full drop (A), a half drop which is a vertical stripe (B) or a brick repeat which is a horizontal stripe (C). The repeats are most important as a fabric may be spoiled by the design forming unintentional lines, spaces or gaps. When the design repeat has been established there are various methods of printing the colours on to the fabrics. 9

10 Dyes and pigments Dyes are soluble in water, pigments are not. Dyes used for printing are made up into a viscous paste by using a thickener which may be a starch, gum or alginate. Pigments are another technique used to colour fabrics. The pigments, which in themselves have no affinity for the fibre, are fixed by means of a synthetic resin binder which holds them onto the fabric. Techniques of printing The techniques of printing are many and varied but only roller printing, screen printing and rotary screen printing are of commercial importance. In recent years roller printing has declined significantly through the introduction of rotary screen printing. Screen printing this technique is derived from the Japanese method of stencilling delicate patterns on fine fabrics. A mesh screen is prepared for each colour in the design. There are two different methods of screen printing flat screen printing and rotary screen printing. Flat screen printing: The fabric to be printed is on a roll at one end of the printing table. The fabric is fed on to the table by means of a conveyor belt which carries the cloth under the printing screens. When the fabric stops, the screens are lowered onto the printing table. The screens are placed side by side a few centimetres above the table, and are lowered on to the cloth when printing commences. Each screen prints a different colour across the width of the fabric. Two fine blades called squeegees move across the screens pressing the printing paste through the mesh of the screen. After the blades have pressed the dye through to the cloth the screens rise and the conveyor moves the fabric along into position under the next screen. Principle of flat screen printing From the printing table the fabric goes to a drying chamber at the end of the printing table. The printed fabric is finished by passing through a steaming chamber, acid baths and a scouring unit to fix the dye and remove the gum used in printing. The cloth may be given any further finishing processes that are required. 10

11 Rotary screen printing: Rotary screen printing is the most popular method of screen printing because it is quick and efficient and allows a greater quantity of fabric to be printed in a short time. The screen is a seamless metal mesh roller; a screen is prepared for each colour in the design. All the rollers are fitted on to the printing machine. The stationary squeegee is fitted through the centre of the roller and the dye is automatically fed in through a pipe attachment. Each screen roller prints one colour across the width of the fabric. While the rollers are revolving in one direction the conveyor belt carrying the fabric moves in the opposite. The cloth travels under the rollers and is printed by each in turn. The fabric is dried and finished as before. Principle of rotary screen printing Roller printing print the design. this is a similar technique to rotary screen printing but uses engraved rollers to Principle of roller printing 11

12 Transfer printing This is a similar technique to ironing transfers on to fabric. The design is printed on to a special type of paper which is wound on to a roller and placed in position in the machine. The design is the full width of the cloth to be printed. The fabric, on another roller, is placed opposite the design paper roller and both fabric and paper are brought together in a dry heat press which transfers the design from the paper to the cloth. The temperature of the heat press is high enough to cause the dye to turn into a vapour which then transfers, under pressure, to the fabric. This is also known as sublimation printing. Principle of sublimation printing Colour application Printing may involve putting colour onto a fabric, or preventing colour from going onto the fabric. This may be achieved by the following techniques: Resist technique the fabric is printed with chemicals which prevent dye being taken up in the chemically treated areas. Dyed technique the fabric is printed with a mordant and the true colour is achieved by applying the second part of the colouring compound during dyeing. Direct technique the design is printed directly on to the fabric. Discharge technique the dyed fabric is printed with a paste containing chemicals which destroy the dye in areas according to the printed design. Other resist methods of applying colour include tie and dye, which uses tightly tied string to prevent dye from reaching some parts of the fabric and batik, where wax is used to prevent the dye from colouring the fabric. Many commercially produced fabrics which have a tie-dye or batik design have had the pattern printed on to them as it is very costly to produce these effects by hand. 12

13 Some recommended sources of information All images taken from Clothing Technology Title Author(s) Publisher ISBN Technology of Marjorie A Taylor Forbes Publications Textile Properties Textiles at the Lesley Cresswell Forbes Publications Cutting Edge Textile Science E.P.G. Gohl and Longman Cheshire L.D. Vilensky Clothing Europa Lehrmittel Technology A Handbook of Textiles Ann M Collier Arnold Wheaton