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Textile Finishing ,Dyeing,Wet Process
Textile FinishingFinishing is the final series of operations that produces finished textile fabric from greige goods. Finishing operations are predominantly wet operations requiring large amounts of thermal energy for water heating and drying. Woven greige goods require some additional steps prior to dyeing, as compared to knit goods. As the first step in finishing woven goods, singeing burns protruding fibers by passing the fabric over an open flame or heated plates to produce a cleaner fabric and enhance future operations. Hot exhaust air is produced.
Sizes and other ingredients added during slashing in the weaving mill are removed in the desizing operation by washing them in a detergent solution at temperatures up to 200°F and then rinsing them with fresh water. The process produces a wastewater stream of 100 to 120°F. Scouring is another washing process using steam and detergents to remove oils and mineral material. The scouring can be done by batch in pressurized vessels known as kiers or on a continuous basis. In either case, high temperatures, to 250°F, and long retention times, up to 12 hours, are used to ensure thorough saturation and cleaning. Finally, the fabric is rinsed. A waste stream of warm, contaminated water is produced.
Next, the fabric is bleached, washed, and rinsed several times to achieve uniformity and improve its ability to absorb dyestuffs later in the finishing operation. Contaminated, warm wastewater is produced. Mercerizing is an optional step and consists of a caustic spray, tensioning, water rinse, water wash, acid dip, and final water wash, and produces a warm wastewater stream. For the drying step, conventional steam-heated drying cans arranged in sequence are most usually used. Finishing is the final wet process wherein size and/or other ingredients are applied to the fabric to provide particular characteristics, such as stiffness, water proofing, etc.
The fabric is dried and finally heat cured with a direct flame on a tender frame to bring it to its final dimensions. Large quantities of hot exhaust gas in the 200°F range are produced. The woven fabric is now ready for dyeing. Knit goods do not require desiring and bleaching. The finishing process requires only scouring to thoroughly clean
the goods prior to dyeing. As with woven fabric, the step involves hot water washes and rinsing, and produces warm, contaminated waste water.
Dyeing Process
The dyeing process varies considerably, depending on the type of dyestuff, type of fabric, and desired final characteristics (for example, solid color or prints.) Dyeing is a wet process and may be done in batches or continuously. Dye vats are known as dye becks and may be atmospheric dye becks (open) or pressure dye becks.
Frequently, a time-temperature ramp is employed. The dye beck may start at 120°F and may be slowly raised in temperature to 160°F or higher through steam-heated heat exchangers. After the dye is set by aging or heating, the fabric may be washed again and dried and/or heat cured. Large quantities of warm, contaminated wastewater are produced.
Finishing Wet Process
Finishing plants are thermal energy-intensive, requiring large quantities of hot water from 120 to 200°F for desiring, scouring, bleaching, mercerizing, dyeing, washing, and rinsing. The hot water is almost universally heated with steam using steam-to-water heat exchangers, direct sparing, or storage tank heating. Many plants recover heat from waste streams with heat exchangers to preheat the process water. Large quantities of steam are also used for heating drying cans and, to a lesser degree, for humidifying space conditioning air.
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TEXTILE DYEING PROCESS
Dyeing is the process of imparting colors to a textile material through a dye (colour). Dyes are obtained from flowers, nuts, berries and other forms of vegetables and plants as well as from animal and mineral sources. These are known as natural dyes.The other class of dyes is known as synthetic dyes. These are based on a particular type of chemical composition. Some of these dyes are- Acid ( Anionic) dyes, Basic ( Cationic) dyes, Neutral- Premetalized dyes, sulfur dyes, vat dyes, reactive dyes etc.
Dyeing Methods
Colour is applied to fabric by different methods of dyeing for different types of fiber and at different stages of the textile production process. These methods include Direct dyeing; Stock dyeing; Top dyeing; Yarn dyeing; Piece dyeing; Solution pigmenting or dope dyeing; Garment dyeing etc. Of these Direct dyeing and Yarn Dyeing methods are the most popular ones.
Direct Dyeing
When a dye is applied directly to the fabric without the aid of an affixing agent, it is called direct dyeing. In this method the dyestuff is either fermented (for natural dye) or chemically reduced ( for synthetic vat and sulfur dyes) before being applied. The direct dyes, which are largely used for dyeing cotton, are water soluble and can be applied directly to the fiber from an aqueous solution. Most other classes of synthetic dye, other than vat and sulfur dyes, are also applied in this way.
Yarn Dyeing
When dyeing is done after the fiber has been spun into yarn, it is called Yarn dyeing. There are many forms of yarn dyeing- Skein (Hank) Dyeing, Package Dyeing, Warp-beam Dyeing,and Space Dyeing.
Skein (Hank) Dyeing
The yarns are loosely arranged in skeins or hanks. These are then hung over a rung and immersed in a dyebath in a large container. In this method, the colour penetration is the best and the yarns retain a softer, loftier feel.
Package Dyeing
The yarns are wound on spools, cones or similar units and these packages of yarn are stacked on perforated rods in a rack and then immersed in a tank. In the tank, the dye is forced outward from the rods under pressure through the spools and then back to the packages towards the center to penetrate the entire yarn as thoroughly as possible.
Warp-beam Dyeing
It is similar to package dyeing but more economical. Here, yarn is wound on to a perforated warp beam, immersed in a tank and dyed under pressure.
Space Dyeing
In this method, the yarn is dyed at intervals along its length. For these two procedures- knit- deknit method and OPI Space-Dye Applicator- are adopted. In the first method, the yarn is knitted on either a circular or flat-bed knitting machine and the knitted cloth is then dyed and subsequently it is deknitted. Since the dye does not readily penetrate the areas of the yarn where it crosses itself, alternated dyed and undyed spaces appear. The OPI Space-Dye Applicator technique produces multi coloured space- dyed yarns. The yarns are dyed intermittently as they run at high speeds of upto 1000 yards (900 m) per minute through spaced dyebaths with continuous subjection to shock waves produced by compressed air assuming supersonic velocities.
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