Giving color decoration to fabric after it has been finished is known as dyeing. When the yarns are woven into the fabric the interesting and intricate design of weavers being to add beauty as well as service ability. The various finishing processes suggest additional means to enhance the appearance of newly from fabric. It remains for the dyeing and printing processes to provide beauty and delight to the eyes. Both these processes add color to the fabric. In the dyeing process fabric yarns or fabric is immersed in a solution of dyestuff and is then saturated with dye.
- SELECTION OF DYE
To select the proper dye for a fiber it is necessary to know which dye have an affinity for the vegetable, animal, manmade synthetic fiber. In general the dye used for cotton and Lenin may be used for viscose and cupramanium rayon but it is necessary to produced a special class of dye for dictate. TYPES
DIRECT DYE – When a dye, color the fabric directly with one fabric operation of immulsion without using the acid of fixing agent, the dye is said to be direct dye. Direct dyes are the easiest to produce, simplest to apply and cheapest in their initial cost as well in application. They are not fast to washing and to light.
MORDANT – A dye that has an affirmity for one type of fiber may not suit to the other side of fabric containing a mixture of fabric by using a mordant in the dye bath. Salts of metal and tannic acids are typical examples of such chemical agents.
NATURAL DYES – Primitive man obtained dyes from flower, nuts berries and other from of flowers and vegetable and animal sources. These sources have provided such natural dyes throughout civilization. They are no longer used in quantity by the dyeing industry. But they are still used in many parts of world for handicrafts.
ARTIFICIAL DYES – Although artificial dyes were first derived from coal tar in 1856, they were not developed in the US to any great extent until 1st world war. When the supply of imported synthetic dyes was cut off since then US have has built a dye industry in numerable dye compounds made from coal tar have now supplanted natural dyes. These artificial or synthetic dyes are constantly being improved as to beauty of color and color fastness. Lasting beauty of color is an important factor in consumers finished goods. Durability of colors depends upon.
Selection of important dye for the fabric to be dyed.
Selection of method of dyeing the fiber, yarn on fabric.
CLASSIFICATIONS OF SYNTHETIC DYES
The synthetic dyes are classified as-
- Neutral or substantive
The acid dyes are used for wool and basic dyes are used for silk. The neutral dyes are a new class specially adapted for cotton, linen and rayon. For this reason, they are called direct cotton dyes.
ACID DYES – In the textile industry, acid dyes are known as commercial colors. They are used as direct dyes for wool and they are also used when delicate tins are required for silk. They can be applied with some difficulty to nylon.
SULPHUR DYES – They are used for cotton, linen and rayon. These dyes are fast to washing and light but have one weakness. Excessive chlorine bleaching will strip the color. They are more expensive than direct dyes because unusual care is required in extras operation. Sulphur is insoluble in water but it is made soluble with aid of sodium sulphite and soda ash. The dyeing is done at high temperature with a large quantity of salt, which helps to drive the color into the fabric. After immersion in the dye bath followed by rinsing the fabric is oxidized to the desired shape by exposure to the air or chemically by the use of potassium bicarbonate and acetic acid. The oxidizing process must be carefully controlled because penetration of the dye is retarded by premature oxidation. Also oxidization changes sulphur to sulphuric acid, which may be harmful to the fabric. Excess chemicals and excess dye must be removed through washing. Sulphur dye penetrates more thoroughly than any other dye of high temperature. They are excellent for khaki and heavy piece goods used in work clothes. Sulphur dyes produce dull colors such as navy, brown and black. They are used for blacks more than any other dye. If stored for a great length of time fabrics becomes tender.
VAT DYES – These are the fastest dyed for cotton, linen and rayon. They can also be applied to nylon, Dacron, orlon, nicara with the use of modern dyes. Vat dyes are not only resistant to dyes and to acids but they are equally resistant to the strong oxidizing bleach used in commercial laundries. In this respect vat dyes excel sulphur dyes, which are not fast to chlorine washing. In reds and pinks however absolute colorfastness is sacrificed for brilliancy of dye. Consequently, a label stating that a garment that is vat dye is not a guarantee that the fabric is absolutely fast to washing if it is of brilliant color. The name “ imdanthrene” on labels indicate that a special type of dye has been used that is particularly fast to dye and washing. This type was among the finest of the synthetic vat dyes.
SELECTION OF METHOD OF DYEING
Textile may be dyed at any stage of their development from fiber into fabric.
* Stock dyeing in the fiber stage
* Yarn dyeing after the fiber has been put into yarn.
* Piece dyeing after the yarn has been constructed into fabric.
* Cross-dyeing a combination of either stock dyeing or yarn dyeing with subsequent piece dyeing.
* Solution pigmenting or dyeing before the man made of synthetic fiber.
STOCK DYEING – In stock Dyeing, the fibers are immersed in a dye bath and the dyed stuff is allowed to penetrate them. This method is considered most desirable because maximum may be expected when material is in this form. A dye that penetrates to the center of the fiber will resist rubbing. Therefore, stock dyed fabrics are likely to be colorfast. Yarns spun from dyed fabrics are uniformly colored. If there is any irregularity of color it is evened up in the subsequent carding, drawing out and spinning operations. Dye fibers are more difficult to spin than un dyed ones. However, because of degree of flexibility is lost in the dyeing process, woolens are often stock dyed. The completeness of this method is reflected in the expression “dyed in the wool”, which is used to attribute the quality of thoroughness. Stock Dyeing produces mixture effects and color effects and color blends of which Oxford Suiting and Tweed Home Spuns are examples.
YARN DYEING – When dyeing is done after the fibre has been spun into yarn, the fabric is described as yarn dyed. This method is also sometimes referred to as ‘skein dyeing’. The yarn is in on spools or in the form of skein when immersed in the dye bath. The dyed stuff has a chance to penetrate to the core of yarn. Similar to the penetration of fibre in stock dyeing, yarn dyed fabrics are usually deeper and richer in colour and tend to be more colour fast than when the fabric is dyed after it is woven. When yarns have been dyed separately, interesting colour effects can be introduced into the woven process by combing yarns of different colours. In such fabrics the warp yarn is usually one colour and the filling yarn in another colour.
PIECE DYEING – When cloth is dyed after it is woven the process is called piece dyeing or sometimes dip dyeing. In piece dyed fabrics the dye doesn’t penetrate and envelope the fibres or yarns as in stock or tarn dyeing. But this methods is advantageous to the manufacturer, therefore it permits him to keep his productions close to changing fashion demands. When fabrics is to be piece dyed it is woven in the grey and can be dyed to supply and current seasonal colour. This method is also economical therefore less dyestuff is required. It is commonly used for the less expensive fabrics. Piece dyeing is limited to the production of solid colour but 2 colour effects may be obtained by constructing the fabric of fibres or yarns that have been specially treated so as to take a deeper colour or to absorb no colour at all. From the consumers point or view piece dyeing is the least desirable process therefore complete penetration doesn’t take place and the fabric may fade. When light spots show at frequent intervals this is an indication that the fabric is piece dyed. In this case only the surface of the yarn took the dye. The consumer can generally detect piece dyeing by revolving and untwisting a single yarn and examining the inner most part to see whether the dye has penetrated. Piece dyeing is used for a wide variety or fabrics like linen, silk, crepe and cottons such as organdi and voil.
CROSS DYEING – Cross dye is a combination of stock dyeing or of yarn dyeing with subsequent piece dyeing. Cross dyeing produce various effects for e.g. either warp or the filling yarn may be stock or yarn dyed and one set of yarn being left un dyed. The fabric is piece dyed after weaving thus colour is given to the un dyed yarns in a second dye bath and the yarns that were originally stock dyed or dyed acquire some additional colouring which blends with piece dyed portion of the fabric.
If yarns of vegetable fibres have been combined with yarns of animal fibres in a fibres that is to be piece dyed. The 2 separate dye baths must be used. The fabric is dipped into both the solutions, each of which affects the fibre for which it has an affinity. This provides colourful affects. A mordant can be included in a singles dye bath to cause the dye to adhere to the fibre for which it does not have an affinity. Thus, the more expensive method of cross dyeing requiring 2 dye baths need not be used.
Still another method of cross dyeing is to immerse a fabric composed of 2 different types of fibres into 1 dye both containing 2 different dyes. One specific for each of the fibres. For ex. A fibre composed of viscose rayon and acetate yarns may be cross-dyed in this manner. When the fabric is removed from the dye bath the viscose rayon yarns will be one colour and the acetate yarns will be another colour.
SOLUTION PIGMENTIONG OR DYEING – In producing man made or synthetic fibres a great deal of time and money can be saved if the dye is added to the solution before it is extrudent through spinnrets into filaments. This method also gives a greater degree of colourfastness. A process called solution pigmenting as dyeing has been used with varying degree of success for man-made and synthetic fibre ranging from rayon and fiberglass. Some effective results have been obtained. 2 outstanding examples of the solution pigmenting process acetate. Fibres are chromo spun and celaperm.