Dyeing is a process in which a fiber, yarn or fabric is impregnated with color by using any of the available dyestuffs. The color applied on to the fabric must be appealing, uniform and fast to washing and wearing. The process should not be very expensive and should be environment friendly.
Although small yardages can be dyed at home or by the local dyer, dyeing is a commercial process carried out by specialized equipment with a variety of dyes or pigments. Hundreds of thousands of meters of fabric is dyed by professional dyers to avoid problems in the dyeing process and the final product.
The first thing that catches our attention in an item for apparel or furnishings is its color. Color is added on to the fabric by colorants such as dyes or pigments through the process of dyeing and printing. In earlier times, dyes and pigments used for dyeing were all natural colors obtained from plants, insects and minerals.
Synthetic dyes were invented in 1856 by William Henry Perkin and since then hundreds of coloring agents have been invented. The dye has to chemically react with the fabric for the color to be fast.
A wide variety of dyes are available, each one suitable for a specific group of fibers, that is, cellulosic, protein or man-made. During dyeing, the dye formulation needs to be kept consistent while dyeing each lot. Even then, slight variations occur in the final color due to variations in fiber structure, moisture in the air and temperature of the dye bath. It is therefore necessary to use materials from the same dye lot when making garments or furnishings.
At times, fabric colors much under one light source like daylight but not under another source like tube light. This phenomenon is known as metamerism. When a fabric is dyed with more than one color, at some places, two or more colors merge and produce another color effect. This effect is known as the bezold effect.
Although pigments have been invented later than dyes, they are used to color a whole lot of fabric. They are used for printing operations or at times to color some solid colored garments during the garment dyeing stage. Pigment prints used to be cheap and low quality, but their quality has improved over the years and they are now used for printing all types of products at reasonable cost. Pigments can also be added in the spinning solution to obtain colored man-made fibers.
Pigments are water insoluble color particles held on the surface of a fabric by a binder. In other words, the application process is mechanical. Pigments are also known as inks. The binder works like a glue and binds the pigment to the fiber as it has no fiber affinity. Binder selection depends on fiber content and performance expectation from the final product.
Pigment colors have become popular because of their ease of application, adaptability to all types of fibers and fabrics, large color range, excellent light fastness, low cost procedure and combination of the coloration operation with the finishing operation.
Another advantage of pigment colors is that they do not require washing after dyeing to remove excess color from the fabric surface. Pigment colors can be easily matched as compared to dyes, since they are held on the surface. As dyes undergo a chemical reaction, they at times shift color. Problems faced with pigments are stiffening of the fabric and color fading due to rubbing.
Pigments are used to print cotton of all weights, wool, rayon, acetate, nylons, polyesters and olefins. The light fastness is good to excellent and washing fastness is good. The fastness to ironing, dry-cleaning, perspiration, rubbing and sea water is also good.
Dye is an organic compound composed of the chromophore, the colored portion of the dye molecule, and an auxochrome, which slightly alters the color while making the dye soluble and able to bond with the fiber. The dye needs to be dissolved in water or any other agent for it to penetrate into the fiber. After entering the fiber, the dye chemically reacts with the fiber or gets locked inside the fiber molecules. The undissolved particles of the dye remain on the fabric surface, and therefore are removed on washing and rubbing. Dyes have much more color strength than pigments as a small amount of dye can color large quantities of fabric.
Dyes are applied in the required conditions of suitable water temperature, steam and dry heat to achieve perfect penetration. Certain chemicals, such as common salt or acids, are added to regulate this penetration. The dyeing procedure can be carried out at under any stage of the fabric manufacturing process but it is important that the dye reacts with the fabric under each stage. Hydrophilic fibers that have active sites for the dye molecules to react with can be dyed easily as compared to hydrophobic fibers.
First, the dye molecules react with the fiber molecules on the surface of the fiber. The fibers tend to smell because of the moisture and heat, pushing the polymer chains farther apart for the internal fiber sites. As a result, the fabric is available for reaction with the dye molecules. After the dyeing operation and during the cooling and drying of fibers, the polymer chains come back together, trapping the dye molecules inside the fiber. Thermoplastic fibers, due to their hydrophobic nature, are difficult to dye and need to be modified to accept the dyes.
Different dyes of different colors have different degrees of fastness under various conditions, such as washing, rubbing, light and perspiration. A complete range of shades is not available in any of the dye classes. The dyer chooses a single dye or a variety of dyes to attain the required color suitable as per the fiber content, end use of the product, and pricing of the product in relation to the process cost. Let’s not discuss the various dyes available in the market in alphabetical order.
ACID OR ANIONIC DYES:
Acid or Anionic dyes have a complete color range and can be used on protein fibers and man-made fibers, such as polyester, modified acrylic, and modified rayon. The dyes produce bright colors but have poor fastness to washing, and therefore fabrics need to be dry-cleaned. The dye tends to bleed and stain adjacent fibers. The fastness to light is usually good but can vary from poor to excellent. It has fair resistance to perspiration and sea water, but excellent rubbing fastness.
Azoic dyes are applied in cold bath with ice, as this helps lower the dye-bath temperature for efficient dyeing. They are also known as ice colors. The dyes have a complete color of bright shades in yellows, reds and blacks. The dyes are cheap and are mainly used for cottons along with limited use on acetate and nylon.
The dyes are mainly used for resist printing, such as batik and applied in cold bath and discharge printing. This is because colors are easily dischargeable and work well with other classes of dyes. To apply a dye, the fabric is first immersed in napthol solution, which impregnates the fibers. It is then dipped into the diazotixed color bath to develop the color. These dyes usually have good to excellent light, wash, ironing and dry-cleaning fastness. The resistance to perspiration and sea water is also good. The dye sometimes bleeds during washing or fades due to rubbing.
BASIC OR CATIONIC DYES:
The first synthetic dyes produced belonged to the class of Basic or Cationic dyes. These dyes also have a complete color range with bright colors, but have limited use these days. They can be used to dye cottons with the help of mordants, while other fibers, such as wool, silk, nylon, acrylic and polyester can be dyed without mordanting. Basic dyes are mainly used on acrylics and for direct printing on acetate a discharge printing on cottons.
The dyes have poor light and dry-cleaning fastness, except when used on acrylics. The wash fastness is poor on cottons while being good for other fibers. It tends to bleed on protein fibers, but not on man-made fibers. The resistance to perspiration, sea water, and rubbing is also good, but only in the case of acrylics.
DIRECT DYES (Developed):
Direct (developed) dyes are developed in the fiber with the help of copper salts. These dyes also have a complete color range though the colors are not as bright as in the case of acid or basic dyes. They are used to dye cellulosic fibers, but some can dye wool, silk and nylon fibers. They are also used for union dying of cotton wool blends. In printing, these find much usage for printing on dischargeable dyed backgrounds.
These dyes have good to excellent light fastness and good resistance to ironing, dry cleaning solvents, color bleeding, perspiration and rubbing. However, their major drawback is that they have poor to fair washing fastness and therefore the dyed fabric needs to be treated with a color fastener.
DIRECT DYES (Substantive)
Direct (substantive) is the largest and most commercially significant dye class. These dyes, like developed dyes, also have a complete color range, though the colors are not as bright as in the case of acid or basic dyes. Their dyeing and printing capabilities are similar to developed dyes. The fastness qualities of the dye are also similar to the developed dyes.
Disperse class of dyes was developed for acetates but is presently used for all man-made fibers. The dye is insoluble in water; the dye particles disperse in water so as to dissolve in fibers. The shade range is good.
The dye fastness to light is fair to excellent while having fair to good wash fastness. Ironing at high temperatures tend to change the dye color and it does not have very good resistance to fading due to atmospheric gases, especially in the case of blue and violet colors. Fastness to perspiration, rubbing and sea water is good, but it can stain wool badly.
Mordants have a fair color range but the colors produced are lighter than acid dyes shades. Mordants require addition of chrome and are used for dyeing mainly protein fibers, which have good fastness qualities. Man-made fibers can also be dyed with these.
Mordants give good wash and light fastness but they can easily stain other fibers, such as silk and nylon. The resistance to perspiration, rubbing and sea water is also good.
Natural dyes are obtained from the natural sources such as flowers, nuts, plants, insects and minerals and are available as madder, saffron, indigo, etc., They are usually difficult to obtain and have a limited color range. The color fastness varies on the basis of dye type, application procedure, and developing procedure.
OPTICAL BRIGHTENERS (Colorless Dyes)
Optical Brighteners are also known as fluorescent whiteners. Optical brighteners brighten the fabric by absorbing ultraviolet light rays and reflecting visible blue light. Brighteners are mainly used on cottons but can also be used on wool, acetate, nylon and acrylic.
These have fair light fastness and washing fastness tends to vary and generally improves with the building up of whiteners in household detergents.
Oxidation bases are one of the earlies synthetic dyes. Aniline black belongs to this class and is one of the most intense and fast black colors available till date. This class also has excellent browns. The bases are mainly used to dye cottons, but can also be used for wool, silk and acetate dyeing. Printing carried out with these bases is direct, resist and discharge.
The dye has very good light, washing and dry cleaning fastness. It has very good resistance to bleeding and rubbing while having good resistance to perspiration.
Reactive dyes are available in several varieties, such as hot and cold. These dyes form chemical combinations with the fiber and produce the brightest shades on cottons. The dye provides good all-around fastness and is used for dyeing at the fiber stage, yarn stage, fabric stage, and is also used in printing. Some reactive dyes can be used for wool, silk, nylons, acrylics and blends.
These dyes have good light, wash and dry cleaning fastness. They also have good resistance to bleeding, perspiration, rubbing and sea water.
Sulfur dyes are generally insoluble in water. The colors are not bright and shade range is also incomplete. The dye can be applied at the fiber, yarn or fabric stage. It is used for both heavy woven and cotton knits, and jute and linen. It is not used much for printing.
These dyes have poor to fair light fastness for yellows and browns, but it is good for darker shades. Washing fastness is also good but the color is sensitive to chlorine bleaches. It has good fastness to perspiration and sea water, but rubbing fastness can vary from good to poor and stored goods tend to become tender.
Vat dyes are insoluble in water and require reduction for their application. Their shade range is incomplete. They are mainly used for cottons and sometimes woolens, mostly for printing.
These dyes have good light, wash and dry cleaning fastness along with having good resistance to perspiration, rubbing, sea water and bleeding.
STAGES OF DYEING:
The dyeing procedure can be carried out at any stage of manufacturing of fabric, starting from the fiber till the garment stage. The various stages at which the dyeing process can be carried out are:
- Stock dyeing at the fiber stage.
- Top dyeing at the sliver stage
- Dope dyeing before a man-made fiber is extruded from the spinneret
- Yarn dyeing after the fiber has been spun into yarn
- Piece dyeing after the yarn has been constructed into fabric
- Garment dyeing after the apparel has been constructed
The dyeing of a staple fiber before it is spun into a yarn is known as stock dyeing. There are two methods for stock dyeing. In the older and widely practiced procedure, the packed fibers are removed from the bales and then packed in large vats and dye liquor is circulated through the mass of a fiber at elevated temperatures.
The newer method known as bale dyeing, is used for wool and all types of man-made fibers. In this, the bale covering is split on all 6 sides and placed in a specially designed machine. The dye liquor is forced through the bale of fiber. This method saves time and labor cost, but sometimes the dye may not penetrate the entire fiber.
Stock dyeing is the most effective and expensive method of dyeing; the color penetrates into the fiber and does not crock readily. Stock dyed fibers do not undergo spinning as readily as undyed fibers because they lose some of their flexibility. However, lubricants added in the final rinsing overcome most of this difficulty.
Top dyeing is a term used in the worsted industry. The top is wool that has been combed to take out the short fibers. The top is then delivered from the combs in a rope like form about 1 ¼ inches thick, wound on perforated spools, and the dye liquor circulated through it. Even dyeing is possible with this method.
Dope dyeing is the process of adding colored pigments or dyes to the spinning solution before it is extruded through the spinneret to form the fiber. This colors each fiber as it is spun into a yarn. This process is called by various names, such as mass pigmentation, solution pigmenting, spun dyeing, and mass coloration. Dope dyeing saves time, money and gives a greater degree of color fastness. This is used for dyeing any type of man-made fiber from rayon to glass fiber. Pigment colors are used for this dyeing.
Color can also be introduced in the fiber, such as acrylic, while it is in the soft gel state, that is the time between fiber extrusion and fiber coagulation. The process is known as gel dyeing.
Chip dyeing refers to the addition of special dyes to the fiber polymer for the production of fibers, such as nylon, prior to melt spinning of the chips. Such dyes will not fade or run.
Yarn Dyeing has several methods. The purpose is to have the dyestuff the fibers in the cord of the yarn. Yarn-dyed fibers are usually deep and rich the color. The yarn dyeing is helpful in creating interesting checks, stripes and plaids with different colored yarns in the weaving process.
- Skein Dyeing
- Package Dyeing
- Warp Beam Dyeing
- Space Dyeing
Yarn can be prepared in skein or hand form and then dyed. The loose arrangements of yarn in this form helps in deep dye penetration. The skeins are hung on a rung and then immersed in a dye bath in a large container. It is the costliest method of yarn dyeing, but the color penetration is the best and the yarn retains a softer and loftier hand.
In package dyeing yarn is wound on spools, cones, or similar units and then dyed. The package of yarn is stacked on perforated rods in a rack and then immersed in a tank. The dye is forced outward from the rods under pressure through the spools and then back through passages towards the center so as to penetrate the yarn as thoroughly as possible.
WARP BEAM DYEING:
Warp beam dyeing method is similar to package dyeing but is more economical. Yarn is wound on a perforated warp beam, immersed in a tank and dyed under pressure. This method is used when fabrics are to be woven with dyed warp yarns.
Space dyed yarns are dyed at intervals along their length. Dyeing is carried out by the knit-deknit method or a space dye applicator.
The dyeing of fabric after it has been woven or knitted is known as piece dyeing. Piece-dyed fabrics demonstrate good amount of levelness, penetration, and overall fastness. A fabric may be piece dyed, whether it is composed of one type of yarn or is a blend of different fibers or combination of different yarns.
When the fabric is made of one kind of a fiber or yarn, then dyeing is carried out easily because one dye is used. However, when the fabric contains a blend of fibers or a combination of yarns, then special procedures are required, which employ different dyes that are each specific for that particular fiber.
Dyeing can be carried out by one dye method or two dye bath processes. In one dye bath, fabrics composed of two different types of fibers are dyed in a single dye bath containing two different dyes, one specific for each type of fiber and in two dye bath, blended fabrics are dyed in both solutions and each fiber takes up the color for which it has an affinity.
In union dyeing, the blended fabric is dyed in single uniform color and in cross dyeing, the blended fabric is dyed in two or more colors, one for each fiber. Both union dyeing and cross dyeing can be carried out in one dye bath or two dye bath.
Let’s now describe the different methods of piece dyeing.
– Beck or Winch or Box Dyeing
– Jig Dyeing
– Pad Dyeing
– Beam Dyeing
– Jet Dyeing
– Vaccum Impregnation
– Foam Dyeing
– Solvent Dyeing
BECK or WINCH or BOX DYEING:
In beck dyeing method, long lengths of the fabrics are passed in a tension free rope form through the dye-bath. The rope of cloth moves over a rail on to a reel, which immerses into the dye-bath and then draws the fabric up and forward to the front of the machine. The process is repeated as long as it is necessary to dye the material uniformly to the desired intensity of the color. The original softness and fullness of the fabric is retained and this method is therefore widely used for woolens and worsteds, woven and knitted fabrics.
In this process of dyeing, a pressurized liquor ratio of 5:1 or 4:1 is maintained. It is generally used for fabric ranging from 50 to 100 meters in length. This uses large amounts of water, chemicals and energy and causes abrasion, creasing and distortion of some fabrics when dyed in rope form.
The method of jig dyeing utilizes the basic procedure of beck dyeing. However, the fabric is held on rollers at full width rather than in the rope form as it is passed through a stationary dye-bath. The fabric is carried along the rolls in open width, which roll back and forth through the dye-bath every 20 minutes or so.
Large runs of fabric, of several thousand meters are dyed with this method. The procedure places some tension on goods causing flatness and loss of fullness. If there is an uneven tension of the material, it causes a slight variation in shade from the center to the selvedge or from one end to another. This dyeing is less costly than back dyeing, but cannot be used for woolens and stretch fabrics, because these fabrics should not be subjected to tension.
The pad dyeing method also dyes the fabric in full width. The fabric is passed through a trough (tray like) containing dye and then between two heavy rollers, which forces the dye into the cloth and squeezes out the rest. This method uses very little dye liquor, making it an economical method of piece dyeing. The cloth runs through the machines at rapid rate of 30 (27.5 m) to 300 (275m) yards per minute. It is generally does on a continuous basis to accommodate a large amount of fabric.
The material is run in a single operation through a pad into a heat or steam chamber, where the dye is set and then passed into a washer, a rinser and a dryer.
Beam dyeing method is similar to the method of warp beam dyeing of yarn, as the fabric is wound on a beam. Only lightweight fabrics of open construction can be beam-dyed, because the dye liquor cannot circulate through the compactly constructed material. It does not subject the material to stress or tension. It is a rapid and economical method.
The jet dyeing method is similar to beck dyeing. Here, the fabric is processed as a continuous loop. The technique is especially useful for delicate fabrics of polyester, but depending on the machine, almost any weight structure or fiber type can be used. It involves vigorous agitation of the dye bath and the textile. Because of the rapid speed of the process, wrinkling of the fabric is minimum. Low warp tension helps develop bulk and fullness. High temperature results in rapid dyeing, increasing the efficiency of dyes and lower use of energy.
In this method, the fabric is dyed by placing it in a heated tube or column where jets of dye solution are forced through it at pressure of up to 300 pounds / jet. The dye is continuously recirculated as the cloth is moved along the tube as the speed of 300 yard (275m) / min. The fluid moves faster than the cloth so that the cloth floats through the tube without touching the wall. In this method, equipment and maintenance costs are high, foaming can be a problem and certain fabrics may be spoiled in the process.
The Vaccum impregnation method of dyeing fabrics differs from the above methods as it depends on initial of air followed by normal air pressure to impregnate the fibers with the dye. It is based on the principle that air entrapped in the substrates of the textile fibers in the fabrics and in the folds of the fabrics itself hinders the passing of dyeing through to the fiber sites. Also, normal atmospheric pressure can be substituted for the higher pressure, eliminating the cost of developing and using higher pressure.
In this process, air is evacuated from the dye tank to near zero pressure. The fabric is covered with dye liquid in vaccum state and the system is opened to return to normal atmospheric pressure. This results in fast wetting, complete saturation and impregnation of the dye into the fibers.
In the foam dyeing technique, the dyestuff is suspended in liquid that is foamed with a special mixture and is applied to the fabric. The reason for using this method of dyeing is to conserve energy used in steaming and dyeing and reduce water consumption and pollution. The method is based on the fact that foam has high volume and viscosity, making it possible to apply relatively small quantity of concentrated dye liquor as compared to pad dyeing.
It is carried out by feeding the fabric into the trough with an aid and a doctor blade so as to ensure that a uniform layer of foam of defined thickness is there on the applicator roll. The foam is pressed into the fabric as it comes in contact with the roll. The front and back of the fabric can be simultaneously dyed in one step Heavy pile fabric and carpets can also be foam dyed.
All dyeing techniques discussed so far dissolve dyestuff in water. These aqueous solutions have their limitations with regards to factors such as:
- Dye solubility in water
- Dye liquor viscosity
- Water permeability of the fiber
- Local water pollution.
The solvent dyeing uses other liquids, such as ammonia, perchloroethylene and trichlorobenzene, as they have found to be superior dye vehicles. These liquids are better dye solvents and penetrate better in a wide variety of man-made fibers. There is also saving in terms of energy and cost of production, because high temperature for dyeing are not needed and the exhausted solvents are recovered and therefore do not cause pollution.
Some apparel goods, such as hosiery and sweater, can be dyed using garment dyeing as they are each made of a single fiber type. Careful selection of components, such as buttons, threads and trims, is required so that they do not attain a different color because of differences in dye absorption between the various product parts. Allowance should be made for anticipated shrinkage while dyeing.
PADDLE MACHINE OR ROTARY DRUM OR TUMBLER DYEING:
Paddle Machine, Rotary Drum or Tumbler Dyeing are used primarily for product or garment dyeing. Both the dye bath and the product are circulated by a paddle or rotation of the drum, tumblers are similar to rotary drums, except that they tilt forward for easier loading and unloading.
METHODS OF DYEING:
The process of dyeing can also be classified on the basis of the dyeing methods. The following are the three methods of combining the dye with the textile:
- Batch Dyeing: Textile is circulated in a dye bath
- Package Dyeing: Dye-bath is circulated around the textile
- Combination Dyeing: Both textile and dye bath are circulated together
Batch dyeing is also known as exhaust dyeing. In this process, the textile is circulated through the dye-bath. It can be used in any stage of the production of textiles, from fiber to product, but is more appropriate for smaller or shorter yardages. The process has good flexibility in terms of color selection and low cost. Temperature can be controlled depending on the dye fiber combination. Back dyeing, Jig dyeing and pad dyeing are some examples of this type of dyeing.
In package dyeing, the dye both is forced through the textile. Normally, the textile is in yarn stage but the technique is also used for some fiber or fabric dyeing. Examples include beam dyeing, skein dyeing, package dyeing, stock dyeing and top dyeing.
In combination dyeing, both textile and dye both are circulated, for example: jet dyeing, paddle machine dyeing.
Continuous dyeing is carried out on continuous machines called ranges and is used for large fabric lots. Ranges include compartment for wetting out, dyeing, after treatment, washing, and rinsing. In continuous dyeing, fabric or yarns are used. The fabrics are often cotton polyester blends, warp yarns for denim and carpets. This method is efficient for long runs and it is most commonly used in union dyeing for blends, but it can be used in cross-dyeing too.