Dyeing
Dyeing is a method that imparts beauty to the textile by applying various colors and their shades onto fabric. Dyeing can be done at any stage of the manufacturing of textile- fiber, yarn, fabric, or a finished textile product including garments and apparel. The property of color fastness depends upon two factors- selection of proper dye according to the textile material to be dyed and selection of the method for dyeing the fiber, yarn, or fabric.
Dyes
Dyes are used for coloring the fabrics. Dyes are molecules that absorb and reflect light at specific wavelengths to give human eyes the sense of color. There are two major types of dyes – natural and synthetic dyes. Natural dyes are extracted from natural substances such as plants, animals, or minerals. Synthetic dyes are made in a laboratory. Chemicals are synthesized for making synthetic dyes. Some of the synthetic dyes contain metals too.
Natural Dyes
It is the most common approach to apply a color pattern to fabric. If done on colored fabric, it is known as overprinting. The desired pattern is produced by pressing dye on the fabric in a paste form. To prepare the print paste, a thickening agent is added to a limited amount of water and dye is dissolved in it. Earlier starch was preferred as a thickening agent for printing. Nowadays gums or alginates derived from seaweed are preferred as they allow better penetration of color and are easier to wash out. Most pigment printing is done without thickeners because the mixing up of resins, solvents and water produces thickening anyway.
Synthetic Dyes
Synthetic dyes are classified based upon their chemical composition and the method of their application in the dyeing process.
Basic (Cationic) Dyes
Basic (Cationic) Dyes are water-soluble and are mainly used to dye acrylic fibers. They are mostly used with a mordant. A mordant is a chemical agent which is used to set dyes on fabrics by forming an insoluble compound with the dye. With mordant, basic dyes are used for cotton, linen, acetate, nylon, polyesters, acrylics and mod acrylics. Other than acrylic, basic dyes are not very suitable for any other fiber as they are not fast to light, washing or perspiration. Thus, they are generally used for giving an after-treatment to the fabrics that have already been dyed with acid dyes.
Direct (substantive) Dyes
Direct (substantive) Dyes color cellulose fibers directly without the use of mordants. They are used for dyeing wool, silk, nylon, cotton, rayon, etc. These dyes are not very bright and have poor fastness to washing although they are fairly fast to light.
Mordant Dyes
The mordant or chrome dyes are acidic in character. Sodium or potassium bichromate is used with them in the dyebath or after the process of dyeing is completed. This is done for getting the binding action of the chrome. They are mostly used for wool which gets a good color fastness after treatment with mordant dyes. They are also used for cotton, linen, silk, rayon and nylon but are less effective for them.
Vat Dyes
Vat dyes are insoluble in water and cannot dye fibers directly. However, they can be made soluble by a reduction in alkaline solution which allows them to affix to the textile fibers. Subsequent oxidation or exposure to air restores the dye to its insoluble form. Indigo is the original vat dye. These dyes are the fastest dyes for cotton, linen and rayon. They are used with mordents to dye other fabrics such as wool, nylon, polyesters, acrylics and mod acrylics.
Reactive Dyes
Reactive dyes react with fiber molecules to form a chemical compound. These dyes are either applied from alkaline solutions or from neutral solutions which are then alkalized in a separate process. Sometimes heat treatment is also used for developing different shades. After dyeing, the fabric is washed well with soap so as to remove any unfixed dye. Reactive dyes were originally used for cellulose fibers only but now their various types are used for wool, silk, nylon, acrylics and their blends as well.
Disperse Dyes
Disperse dyes are water-insoluble. These dyes are finely ground and are available as a paste or a powder that gets dispersed in water. These particles dissolve in the fibers and impart color to them. These dyes were originally developed for the dyeing of cellulose acetate but now they are used to dye nylon, cellulose triacetate, and acrylic fibers too.
Sulfur Dyes
Sulfur Dyes are insoluble and made soluble with the help of caustic soda and sodium sulfide. Dyeing is done at high temperatures with large quantities of salt so that the color penetrates into the fiber. After dyeing the fabric is oxidized for getting desired shades by exposure to air or by using chemicals. Excess dyes and chemicals are removed by thorough washing. These dyes are fast to light, washing and perspiration and are mostly used for cotton and linen.
Pigment Dyes
Although pigments are not dyes in a true sense, they are extensively used for coloring fabrics like cotton, wool and other artificial fibers due to their excellent light fastness. They do not have any affinity to the fibers and are affixed to the fabric with the help of resins. After dyeing, the fabrics are subjected to high temperatures.
Dyeing Methods
Color is applied to the fabric by different methods and at different stages of the textile manufacturing process.
Stock dyeing: The fiber is dyed even before it is spun.
Top dyeing: Top is the combed wool. In this method, the fiber is dyed on the stage just before the appearance of the finished yarn.
Piece dyeing: In this method, small batches of constructed natural-colored fabric are dyed according to the demands for a given color.
Solution pigmenting or dope dyeing: Dye is added to the solution before it is extruded through the spinnerets for making synthetic filaments.
Garment dyeing: Garment dyeing Dye is applied to finished products such as apparel and garments.
Stock dyeing: Stock dyeing is used to dye fibers. In this process, the staple fibers are packed into a vessel and then dye liquid is forced through them. Although the dye solution is pumped in large quantities, the dye may not penetrate completely into the fibers and some areas may be left without dyeing. However, the following blending and spinning processes mix up the fibers thoroughly, resulting in an overall even color. Woolens are usually stock dyed.
Top dyeing: Top is the combed wool sliver. It is wound on perforated spools and the dye solution is circulated through it. This method results in very even dyeing.
Yarn dyeing: In this process, the staple fibers are packed into a vessel, and then dye liquid is forced through them. Although the dye solution is pumped in large quantities, the dye may not penetrate completely into the fibers and some areas may be left without dyeing. However, the following blending and spinning processes mix up the fibers thoroughly, resulting in an overall even color. Woolens are usually stock dyed.
When dyeing is done after the fiber has been spun into yarn, it is called yarn dyeing. In this method, the dyestuff penetrates the fibers to the core of the yarn. 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 coils. 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. It is mostly used for bulky acrylic and wool yarns.
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. Mostly, the carded and combed cotton which is used for knitted outerwear is dyed through this method.
Warp-beam Dyeing: It is similar to package dyeing but more economical. Here, the yarn is wound onto a perforated warp beam and then immersed in a tank for dyeing it by applying 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-colored space- dyed yarns. The yarns are dyed intermittently as they run at very high speeds through spaced dyebaths. They are continuously subjected to shock waves produced by compressed air having supersonic velocities.
Piece Dyeing: The constructed fabrics are piece dyed for the flexibility they provide. The textile manufacturer can dye the whole fabric in batches according to the fashion demands of the time thus avoiding wastage and resultantly loss. There are several methods prevalent or piece dyeing.
Beck dyeing: It is used for dyeing long yards of fabric. The fabric is passed in rope form through the dyebath. This rope of the fabric moves over a rail onto a reel which immerses it into the dye and then draws the fabric up and forward and brings it to the front of the machine. This process is repeated many times until the desired color intensity is obtained.
Jig dyeing: It is similar to the process of beck dyeing with a slight variation. The fabric in jig dyeing is held on rollers at full width rather than in rope form as it is passed through the dyebath.
Pad dyeing: Padding is also done while holding the fabric at full width. The fabric is passed through a trough having dye in it. Then it is passed between two heavy rollers which force the dye into the cloth and squeeze out the excess dye. Then it is passed through a heat chamber for letting the dye set. After that, it is passed through a washer, rinser and dryer for completing the process.
Jet dyeing: Fabric is placed in a heated tube where jets of dye solution are forced through it at high pressures. The fabric too moves along the tube. The solution moves faster than the cloth while coloring it thoroughly.
Solution pigmenting or dope dyeing: This is a method applied for dyeing the synthetic fibers. Dye is added to the solution before it is extruded through the spinnerets for making synthetic filaments. This gives a colorfast fiber as the pigments are used which are the fastest known colors.
Garment dyeing: When the finished textile product such as hosiery or sweaters is dyed, it is called garment dyeing. A number of garments are packed loosely in a nylon net and put into a dyestuff-filled tub with a motor-driven paddle. The dye is thrown upon the garments by the moving paddles’ effect.
