Dyeing Process of Textile for Dyes and Pigments

Dyeing Process of Textile for Dyes and Pigments

1. Unit-III Dyeing Processes of Textiles Prepared by Dr. Nandan C. Pomal Assistant Professor Faculty of Science Sigma University, Vadodara, Gujarat, India. 3MSO05 Synthetic Dyes and Pigments

2. Need of Pretreatment of Textiles The basic objective of pretreatment processing is removal of added or natural impurities present in textile fibers to improve the absorbency. The textile pretreatment process can help to eliminate sizing agents, oils, waxes, seed hulls, soils, pectin and other impurities, while improving whiteness and feel of the fabric. A typical pretreatment process include three steps; desizing, scouring and bleaching. In general, synthetic fibres do not have any natural impurity. Added impurities are the additives or chemicals which are added to ease certain processes like weaving (sizing) or prevention of static (lubrication/anti-static treatment during melt spinning). Additionally, textile processors want to go beyond the simple coating, waterproofing, and softening to provide good fabrics which possess combination of comfort, performance and style.

3. Improves the wetting and fabric penetration properties. Emulsify and disperse impurities. Prevent foam in the harsh manufacturing and process conditions. Imparts the socks, gloves and sportwear clothes the high-grip properties. Cationize the cotton to enable unique, brighter colours and reduces the use of resources. Produces the UV-resistant and water resistant fabrics. Improve tear strength, abrasion resistance and wash durability. Add unique moisture-management, wrinkle-free, stretch and recovery capability. Importance of Pretreatment of Textiles

4. Singeing Process: Removal of hairs from fibres or fabrics by means of burning having three types-gas flame, rollers, and hot plates. Desizing Process: Desizing is a process that removes size material from woven fabric's warp yarn before further processing. Scouring Process: All natural and additive impurities are removed to produce hydrophilic fabric. Bleaching Process: It is the process of removal of natural colour from the fabric. Process in Pretreatment

5. Dyeing process of Textiles Azoic dyes are insoluble pigments formed within the fibre by padding, first with a soluble coupling compound and then with a diazotized base. Vat dyes, insoluble in water, are converted into soluble colourless compounds by means of alkaline sodium hydrosulphite. These colourless compounds are absorbed by the cellulose, which is subsequently oxidised to an insoluble pigment. Such dyes are colourfast. Disperse dyes are suspensions of finely divided insoluble organic pigments used to dye hydrophobic fibres such as polyesters, nylon, and cellulose acetates. The process of applying colour to fibre stock, yarn, or fabric is called dyeing. Textile dyes include acid dyes, used mainly for dyeing wool, silk, and nylon, and direct or substantive dyes, which have a strong affinity for cellulose fibres. Mordant dyes require the addition of chemical substances, such as salts, to give them an affinity for the material being dyed. They are applied to cellulose fibres, wool, or silk after such materials have been treated with metal salts. Sulphur dyes, used to dye cellulose, are inexpensive but produce colours lacking brilliance.

6. YARN TEXTILE FABRIC

7. Bale Dyeing:  A low-cost method to dye cotton cloth. The material is sent without scouring or singeing through a cold-water bath where the sized warp has an affinity for the dye. Batik Dyeing:  One of the oldest forms of dyeing, originated in Java. Portions of the fabric are coated with wax so that only unwaxed areas are dyed. The operation may be repeated several times, using multiple colours for a mottled or streaked effect. Beam Dyeing:  The warp is dyed before weaving by winding it onto a perforated beam and forcing the dye through the perforations, saturating the yarn with colour. Methods of Dyeing Burl or Speck Dyeing:  Mostly done on woolens. Coloured spots are covered by using special-coloured inks in many colours and shades. It is a hand operation. Chain Dyeing:  Used when yarns and clothes have low tensile strength. Several cuts or pieces of clothes are tacked end-to-end and run through a continuous chain in the dye colour, allowing for high production. Cross Dyeing:  A popular method where different colour effects are obtained in one dye bath for cloth containing fibres with different affinities for the dye used. For example, a blue dyestuff might give nylon 6 a dark blue shade, nylon 6,6 a light blue shade, and have no affinity for polyester, leaving it white.

8. Jig Dyeing: Done in a jig, kier, vat, beck, or vessel in an open formation of the goods. The fabric goes from one roller to another through a deep dye bath until the desired shade is achieved. Piece Dyeing: The dyeing of fabrics in cut, bolt, or piece form. It follows the weaving of the goods and provides a single colour for the material, such as blue serge or green organdy. Raw Stock Dyeing: Dyeing of fibre stock before spinning of the yarn, following the degreasing of wool fibres and drying of the stock. …Methods of Dyeing Random Dyeing: Colouring only certain designated portions of the yarn. There are three ways of doing this type of colouring: skeins may be tightly dyed in two or more places and dyed on one side of the dye with one colour and the other side with another; colour may be printed onto skeins spread out on the blanket fabric of the printing machine; or cones or packages of yarn on hollow spindles may be arranged to form channels through which yarn and dyestuff are drawn by suction, achieving random effects.

9. Jig Dyeing: Done in a jig, kier, vat, or vessel in an open formation of the goods. The fabric goes from one roller to another through a deep dye bath until the desired shade is achieved. Piece Dyeing: The dyeing of fabrics in cut, bolt, or piece form. It follows the weaving of the goods and provides a single colour for the material, such as blue serge or green organdy. Raw Stock Dyeing: Dyeing of fibre stock before spinning of the yarn, following the degreasing of wool fibres and drying of the stock. …Methods of Dyeing

10. A finish is anything that is done to a fabric after weaving or knitting, to changes its appearance, hand and performance. When a finish is applied, say on cotton, it might become more shiny, stronger or resist shrinking on washing. Similarly, other finishes may make the fabric softer or stiffer; water or stain resistant; coloured or designed. Textile Finishes

11. Finishes can be classified as: Renewable and Durable Routine (Basic) and Special Routine finishes are applied to almost all fabrics with an aim to improve their appearance. Special finishes are applied with a specific purpose or end use in mind. Classification Of Finishes

12. Basic Finishes (i) Scouring/cleaning Fabrics received as gray cloth have a lot of impurities naturally present in them. These may be oils, waxes and dirty stains acquired during construction of the fabric. Complete removal or cleaning of these impurities is important before applying any other finish. This cleaning is called scouring and is done to all fabrics with the help of soap solutions and chemicals. After cleaning, the fabric becomes smooth, neat and more absorbent. Activity: Do and See: Take a new and an old washed fabric and put them in water. What do you observe? The old one will sink faster because it is more absorbent. As soon as it absorbs water, it becomes heavier and sinks. …Textile Finishes

13. (ii) Bleaching: When fabrics are made, they are not white in colour, due to impurities and colouring material present in them. To make them white or to dye them in light colours they are bleached. Suitable bleaching agents are used to remove the colour from the fabric. Bleaching is done for cottons, woolens and silks. Man-made fabrics do not need bleaching as they are naturally white. Bleaching has to be done very carefully as the chemical which can destroy the colour may also damage the fabric to some extent. Hydrogen peroxide is a universal bleach which can be applied to all kinds of fabrics. …Textile Finishes

14. (iii) Stiffening: Stiffening means the fabric which is generally limp becomes stiff when a stiffening agent is applied. For stiffening your cotton clothes at home, you use maida, starch or rice water. For stiffening silk, gums are used. Stiffening gives body, smoothness and lustre to the fabric. This practice is sometimes used to cheat the customer. You must have observed that sometimes, if you rub a fabric between your hands, some white powder comes out. It is because the fabric has been over-starched. Inferior fabrics are over-starched to look dense and better. Avoid buying such fabrics. Activity: Take a starched fabric and rub it against your fingers. Then hold it against the light. The weave will look little more open in that area, as the starch has been removed. This will clearly indicate if the fabric is of good quality or inferior quality. This activity will enable you to select a good quality fabric. …Textile Finishes

15. Special Finishes (i) Mercerization: Before finishing, cotton is a dull and rough fabric which wrinkles easily. When it is mercerized by using chemicals for example sodium hydroxide it becomes strong, lustrous and dyes well as it is now more absorbent. This is a durable finish. Now-a-days this finish has become almost a routine finish for all cottons. Threads used for stitching are also mercerized. …Textile Finishes

16. (ii) Shrinkage control or anti-shrink: When your new shirt becomes small after washing, reduction in size of a fabric after it is washed is known as shrinkage. If the label on the fabric reads ‘sanforised’ or ‘anti-shrink’ or ‘shrink-proof’ then it means the fabric has received a finish for shrinkage control, and such a fabric will not shrink on washing. If this marking is not there on the fabric, you may shrink the fabric yourself at home. You probably do it many a times. You soak the fall in water before putting it on the saree or soak the cloth before getting a suit made out of it. Purchase the fabric, a little more than what you need, soak it overnight, squeeze and dry it. The garment made from this fabric will not shrink on further washing. Activity: Find a new (unwashed) khadi cloth. Cut a 15 cm square piece. Wash the cloth and dry it. Measure the dry cloth. Is there any difference in the size? If the size remains unchanged it means that the fabric has not shrunk. The difference in size clearly indicates shrinkage of fabric. …Textile Finishes

17. (iii) Water proofing: Fabrics to be used as raincoats, umbrellas, and tarpaulins have to be treated with chemicals to give them a property which makes them water resistant. The finish is called waterproofing and it is a durable finish. (iv) Parchmentisation : Have you observed something different about the organdy fabric? Yes, it is stiff though thin. At the same time it is quite transparent and when washed the stiffness still remains. This is because of a finish called parchmentisation resulting in a permanent stiffness. You don’t need to apply starch to organdy sarees. (v) Wash 'n' Wear: You know that cotton fabrics crush badly when in use. By applying the finish of wash-n-wear, cotton fabrics become easy to maintain as now they do not require repeated ironing and do not wrinkle also. This finish is a durable one and is produced by use of chemicals like resins. …Textile Finishes

18. (vi) Dyeing and Printing: In the market, you see fabrics in plain colours or colorful designs apart from white ones. The processes of producing colours and designs are called dyeing and printing. Dyeing gives a solid colour to the fabric whereas printing is an application of dye at specified areas to create a design. It is very important for the dyed and printed fabric to be ‘colourfast’, otherwise, if the colour runs on washing, rubbing or ironing the design is destroyed. A simple test to check colorfastness is to rub a wet white hanky against the coloured fabric. If colour comes on it, the colour is not fast and the fabric should not be bought. If you have bought a cotton fabric whose colour runs, wash it only in cold water with salt added to it. This will help in fixing the colour to some extent.

19. Finishing With Colour: When ‘colour’ is applied to a fabric it is termed as dyeing. Dyeing and printing of fabrics is usually done after routine or basic finishes but prior to the application of other finishes. It is mainly done to give colour to the fabric and thus improve the appearance of the fabric. …Textile Finishes

20. …Textile Auxiliaries

21. To be continue…