Dyeing
Dyeing operations are used at various stages of production to add color and intricacy to textiles and increase product value. Most dyeing is performed either by the finishing division of vertically integrated textile companies, or by specialty dyehouses. Specialty dyehouses operate either on a commission basis or purchase greige goods and finish them before selling them to apparel and other product manufacturers. Textiles are dyed using a wide range of dyestuffs,
techniques and equipment.
Dyes used by the textile industry are largely synthetic, typically derived from coal tar and petroleum-based intermediates. Dyes are sold as powders, granules,
pastes and liquid dispersions, with concentrations of active ingredients ranging typically from 20 to 80 percent.
Methods of Dyeing
Dyeing can be performed using continuous or batch processes. In batch dyeing, a certain amount of textile substrate, usually 100 to 1,000 kilograms, is loaded into a dyeing machine and brought to equilibrium, or near equilibrium, with a solution containing the dye. Because the dyes have an affinity for the fibers, the dye molecules leave the dye solution and enter the fibers over a period of minutes to hours, depending on the type of dye and fabric used. Auxiliary chemicals and controlled dyebath conditions (mainly
temperature) accelerate and optimize the action. The dye is fixed in the fiber using heat and/or chemicals, and the tinted textile substrate is washed to remove unfixed dyes and chemicals. Common methods of batch, or exhaust, dyeing include beam, beck,
jet and jig
processing (see picture to the left). Pad dyeing can be performed by either batch or continuous processes.
In continuous dyeing processes, textiles are fed continuously into a dye range at speeds usually between 50 and 250 meters per minute. Continuous dyeing accounts for about 60 percent of total yardage of product dyed in the industry (Snowden-Swan, 1995). To be economical, this may require the dyer to process 10,000 meters of textiles or more per color, although specialty ranges are now being designed to run as little as 2,000 meters economically. Continuous dyeing processes typically consist of dye application, dye fixation with chemicals or heat, and washing.
Dye fixation is a measure of the amount of the percentage of dye in a bath that will fix to the fibers of the textile material. Dye fixation on the fiber occurs much more rapidly in continuous dying than in batch dyeing. Each dyeing process requires different amounts of dye per unit of fabric to be dyed. This is significant since color and salts in wastewater from spent dyes are often a pollution concern for textile facilities. In addition, less dye used results in energy conservation and chemical savings. The amounts of dye used depends on the dye
exhausted from the dyebaths which determines the required dyebath ratio. The dyebath ratio is the ratio of the units of dye required per unit of fabric and typically ranges from 5 to 50 depending on the type of dye, dyeing
system and affinity of the dyes for the fibers.
Dyeing processes may take place at any of several stages of the manufacturing process (fibers, yarn, piece-dyeing). Stock dyeing is used to dye fibers. Top dyeing is used to dye combed wool sliver. Yarn dyeing and piece
dyeing is done after the yarn has been constructed into fabric.
Yarn Dyeing. Yarn dyeing is used to create interesting checks,
stripes and plaids with different-colored yarns in the weaving process. In yarn dyeing, dyestuff penetrates the fibers in the core of the yarn. Some methods of yarn dyeing are stock,
package and skein dyeing. Stock dyeing dyes fiber using perforated tubes. In package dyeing, spools of yarn are stacked on perforated rods in a rack and immersed in a tank where dye is then forced outward from the rods under pressure. The dye is then pressured back through the packages toward the center to fully penetrate the entire yarn. Most carded and combed cotton used for knitted outerwear is package-dyed. In skein dyeing, yarn is loosely coiled on a reel and then dyed. The coils, or skeins, are hung over a rung and immersed in a dyebath (Corbman, 1975). Skein-dyed yarn is used for bulky acrylic and wool yarns. Typical capacity for package dyeing equipment is 1,210
lbs (550 kg) and for skein dyeing equipment is 220 lbs (100 kg).
Piece Dyeing. Most dyed fabric is piece-dyed since this method gives the manufacturer maximum inventory flexibility to meet color demands as fashion changes. In terms of overall volume, the largest amount of dyeing is performed using beck and jig equipment (Figure 11). Beck dyeing is a versatile, continuous process used to dye long yards of fabric. About 1,980
lbs (900 kg) of fabric can be dyed on beck equipment at a time. The fabric is passed in rope form through the dyebath. The rope moves over a rail onto a reel which immerses it into the dye and then draws the fabric up and forward to the front of the machine. This process is repeated as long as necessary to dye the material uniformly to the desired color intensity. Jig dyeing uses the same procedure of beck
dyeing; however, the fabric is held on rollers at full width rather than in rope form as it is passed through the dyebath (Corbman, 1975). This reduces fabric tendency to crack or crease. Jig dyeing equipment can handle 550
lbs (250 kg) of fabric. Other piece dyeing methods include jet dyeing and pad dyeing. Fabric can be jet-dyed
[at up to 1,100 lbs (500 kg)] by placing it in a heated tube or column where jets of dye solution are forced through it at high pressures. The dye is continually recirculated as the fabric is moved along the tube. Pad dyeing, like jig dyeing, dyes the fabric at full width. The fabric is passed through a trough containing dye and then between two heavy rollers which force the dye into the cloth and squeeze out the excess (Corbman, 1975).
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