Dyeing Process | Process of Dyeing | The Chemistry of the Dyeing Process

The dyeing of a textile fiber is carried out in a solution, generally aqueous, known as the dye liquor or dye bath. For true dyeing to have taken place, coloration of fabric and absorption are important determinants. 

Coloration: 

The coloration must be relatively permanent: that is not readily removed by rinsing in water or by normal washing procedures. Moreover, the dyeing must not fade rapidly on exposure to light. 

Absorption: 

The process of attachment of the dye molecule to the fiber is one of absorption: that is the dye molecules concentrate on the fiber surface. There are four kinds of forces by which dye molecules are bound to the fiber: 

1) Ionic forces 2) Hydrogen bonding 3) Vander Wals’ forces and 4) Covalent chemical linkages

Dyeing of Wool: 

In the dyeing of wool which is a complex protein containing about 20 different amino acids, the sulfuric acid added to the dye bath forms ionic linkages with the amino groups of the protein. In the process of dyeing, the sulfate anion (negative ion) is replaced by a dye anion. In the dyeing of wool, silk and synthetic fibers, hydrogen bonds are probably set up between the azo, amino, alkyl amino and other groups and the amino Co-NH-groups. Covalent chemical links are brought about in the dye-bath by chemical reaction between a fiber-reactive dye molecule, one containing a chemically reactive center and a hydro-oxy group of a cotton fiber, in the presence of alkali. 

The Chemistry of the Dyeing Process : 

Exhaustion In any dyeing process, whatever the chemical class of dye being used, heat must be supplied to the dye bath; energy is used in transferring dye molecules from the solution to the fiber as well as in swelling the fiber to render it more receptive. The technical term for this process is exhaustion. Levelness: An Important Quality 

Evenness of dyeing, known as levelness is an important quality in the dyeing of all forms of natural and synthetic fibers. It may be attained by the control of dyeing conditions viz. 

• By agitation to ensure proper contact between dye liquor and substance being dyed and by use of restraining agents to control rate of dyeing or strike. Solvent Dyeing Serious consideration has recently been given to the methods of dyeing in which water as the medium is replaced by solvents such as the chlorinated hydrocarbons used in dry cleaning. The technological advantages in solvent dyeing are: 1. Rapid wetting of textiles 
• Less swelling 
• Increased speed of dyeing per given amount of material 
• Savings in energy, as less heat is required to heat or evaporate per-chloro-ethylene. Thus it eliminates the effluent (pollution) problems associated with the conventional methods of dyeing and finishing.

Dyeing Methods | Methods of Dyeing | Chain Dyeing | Cross Dyeing | Stock dyeing | Top dyeing | Yarn dyeing | Bale Dyeing | Batik Dyeing | Skein (Hank) Dyeing | Package Dyeing | Warp-beam Dyeing | Space Dyeing | Piece Dyeing | Beck dyeing | Jig dyeing | Pad dyeing | Jet dyeing | Solution pigmenting or dope dyeing | Garment dyeing | Random Dyeing

Dyeing Methods

Color is applied to fabric by different methods and at different stages of the textile manufacturing process. The process of applying color to fiber stock, yarn or fabric is called dyeing.” There may or may not be thorough penetration of the colorant into the fibers or yarns.

Dyes can be used on vegetable, animal or man made fibers only if they have affinity to them. Textile dyes include acid dyes, used mainly for dyeing wool, silk and nylon and direct or substantive dyes, which have a strong affinity for cellulose fibers. Mordant dyes require the addition of chemical substances, such as salts to give them an affinity for the material being dyed. They are applied to cellulose fibers, wool or silk after such materials have been treated with metal salts. Sulfur dyes, used to dye cellulose, are inexpensive, but produce colors lacking brilliance. Azoic dyes are insoluble pigments formed within the fiber by padding, first with a soluble coupling compound and then with a diazotized base. Vat dyes, insoluble in water, are converted into soluble colorless compounds by means of alkaline sodium hydrosulfite. These colorless compounds are absorbed by the cellulose, which are subsequently oxidized to an insoluble pigment. Such dyes are colorfast. Disperse dyes are suspensions of finely divided insoluble, organic pigments used to dye such hydrophobic fibers as polyesters, nylon and cellulose acetates.

Garment dyeing

Reactive dyes combine directly with the fiber, resulting in excellent colorfastness. The first ranges of reactive dyes for cellulose fibers were introduced in the mid-1950. Today, a wide variety is available.

Dyeing Methods | Methods of Dyeing

1. Chain Dyeing
2. Cross Dyeing
3. Stock dyeing
4. Top dyeing
5. Yarn dyeing
6. Bale Dyeing
7. Batik Dyeing
8. Skein (Hank) Dyeing
9. Package Dyeing
10. Warp-beam Dyeing
11. Space Dyeing
12. Piece Dyeing
13. Beck dyeing
14. Jig dyeing
15. Pad dyeing
16. Jet dyeing
17. Solution pigmenting or dope dyeing
18. Garment dyeing
19. Random Dyeing

Chain Dyeing
This is used when yarns and cloth are low in tensile strength. Several cuts or pieces of cloth are tacked end-to-end and run through in a continuous chain in the dye color. This method affords high production.

Cross Dyeing
This is a very popular method in which varied color effects are obtained in the one dye bath for a cloth which contains fibers with varying affinities for the dye used. For example, a blue dyestuff might give nylon 6 a dark blue shade, nylon 6, 6 a light blue shade, and have no affinity for polyester area unscathed or white.

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 in such a thorough way that it results 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
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 in such a thorough way that it results 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.

Bale Dyeing
This is a low cost method to dye cotton cloth. The material is sent without scouring or singeing, through a cold water bath where the sized warp has affinity for the dye. Imitation chambray and comparable fabrics are often dyed this way.

Batik Dyeing
This is one of the oldest forms known to man. It originated in Java. Portions of the fabric are coated with wax so that only un-waxed areas will take on the dye matter. The operation may be repeated several times and several colors may used for the bizarre effects. Motifs show a mlange, mottled or streaked effect, imitated in machine printing.

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 are 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 on to a perforated warp beam and then immersed in a tank for dyeing it 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 coloured 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 to set. After that it is passed through 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 are 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.

Random Dyeing
Coloring only certain designated portions of the yarn. There are three ways of doing this type of coloring:

Skeins may be tightly dyed in two or more places and dyed at one side of the dye with one color and at the other side with another one. Color may be printed onto the skeins which are spread out on the blanket fabric of the printing machine.

Cones or packages of yarn on hollow spindles may be arranged to form channels through which the yarn, by means of air-operated punch, and the dyestuff are drawn through these holes by suction. The yarn in the immediate area of the punch absorbs the dye and the random effects are thereby attained.

Dyeing Process | Different Types of Dye | Classification of Dyes | Various Classes and Types | Acid Dyes | Natural Dyes | Basic (Cationic) Dyes | Synthetic Dyes | Direct (substantive) Dyes | Disperse Dyes | Sulfur Dyes | Pigment Dyes | Mordant Dyes | Vat Dye | Reactive Dyes | Macromolecular Dyes | Metallized Dyes | Naphthol Dyes | Premetallized Dyes | Gel Dyeing | Developed Dyes | Azo Dyes | Aniline Dyes | Anthraquinone Dyes

A process of coloring fibers, yarns, or fabrics with either natural or synthetic dyes. Dyeing is an ancient art which predates written records. It was practised during the Bronze age in Europe. Primitive dyeing techniques included sticking plants to fabric or rubbing crushed pigments into cloth
. The methods became more sophisticated with time and techniques using natural dyes from crushed fruits, berries and other plants, which were boiled into the fabric and gave light and water fastness (resistance), were developed. Dyeing can be done at any stage of the manufacturing of textile- fiber, yarn, fabric or a finished textile product including garments and apparels. 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

Substances that add color to textiles. They are incorporated into the fiber by chemical reaction, absorption, or dispersion. Dyes differ in their resistance to sunlight, perspiration, washing, gas, alkalies, and other agents; their affinity for different fibers; their reaction to cleaning agents and methods; and their solubility and method of application. 

Various classes and types are listed below:

• Acid Dyes
• Natural Dyes 
• Basic (Cationic) Dyes
• Synthetic Dyes
• Direct (substantive) Dyes 
• Disperse Dyes
• Sulfur Dyes
• Pigment Dyes
• Mordant Dyes
• Vat Dyes
• Reactive Dyes
• Macromolecular Dyes
• Metallized Dyes
• Naphthol Dyes
• Premetallized Dyes
• Gel Dyeing
• Developed Dyes
• Azo Dyes
• Aniline Dyes
• Anthraquinone Dyes

Acid Dyes

A class of dyes used on wool, other animal fibers, and some manufactured fibers. Acid dyes are seldom used on cotton or linen since this process requires a mordant. Acid dyes are widely used on nylon when high washfastness is required. In some cases, even higher washfastness can be obtained by aftertreatment with fixatives.

Natural Dyes 

Direct Printing, it is the most common approach to apply a color pattern onto a 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.

Basic (Cationic) Dyes

 Basic 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 modacrylics. 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.

Synthetic Dyes

Synthetic dyes are classified based upon their chemical composition and the method of their application in the dyeing process.

Direct (substantive) Dyes 

Dyes Direct 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.

Disperse Dyes

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 Sulfur Dyes are insoluble and made soluble by the help of caustic soda and sodium sulfide. Dyeing is done at high temperature 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

Pigment Dyes Although pigments are not dyes in a true sense, they are extensively used for coloring fabrics like cotton,wool and other manmade 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.

Mordant Dyes

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 Vat dyes are insoluble in water and cannot dye fibers directly. However, They can be made soluble by reduction in alkaline solution which allows them to affix to the textile fibers. Subsequent oxidation or exposure to air restore 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 mordants to dye other fabrics such as wool, nylon, polyesters, acrylics and modacrylics.

Reactive Dyes

Reactive Dyes Reactive dyes react with fiber molecules to form a chemical compound. These dyes, they are either applied from alkaline solution 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.

Macromolecular Dyes

 A group of inherently colored polymers. They are useful both as polymers and as dyes with high color yield. The chromophores fit the recognized CI classes, i.e., azo, anthraquinone, etc., although not all CI classes are represented. Used for mass dyeing, hair dyes, writing inks, etc.

Metallized Dyes

 A class of dyes that have metals in their molecular structure. They are applied from an acid bath.

Naphthol Dyes

A type of azo compound formed on the fiber by first treating the fiber with a phenolic compound. The fiber is then immersed in a second solution containing a diazonuim salt that reacts with the phenilic compound to produce a colored azo compound. Since the phenolic compound is dissolved in caustic solution, these dyes are mainly used for cellulose fiber, although other fibers can be dyed by modifying the process. (Also see DYES, Developed Dyes.)

Premetallized Dyes

Acid dyes that are treated with coordinating metals such as chromium. This type of dye has much better wetfastness than regular acid dye. Premetallized dyes are used on nylon, silk, and wool.

Gel Dyeing

Passing a wet-spun fiber that is in the gel state (not yet at full crystallinity or orientation) through a dyebath containing dye with affinity for the fiber. This process provides good accessibility of the dye sites.

Developed Dyes

Dyes that are formed by the use of a developer. The substrate is first dyed in a neutral solution with a dye base, usually colorless. The dye is then diazotized with sodium nitrate and an acid and afterwards treated with a solution of B-naphthol, or a similar substance, which is the developer. Direct dyes are developed to produce a different shade or to improve washfastness or lightfastness.

Azo Dyes

Dyes characterized by the presence of an azo group (-N=N-) as the chromophore. Azo dyes are found in many of the synthetic dye classes.

Aniline Dyes

Dyes derived chemically from aniline or other coal tar derivatives.

Anthraquinone Dyes

Dyes that have anthraquinone as their base and the carbonyl group (>C=O) as the chromophore. Anthraquinone-based dyes are found in most of the synthetic dye classes.

Natural Dyes | Classification of Natural Dyes | Plant Dyes | Plant Dye | Natural Plant Dyes | Insect Dyes | Insect Dye | Animal Dyes | Animal Dye

Dyes : Substances that add color to textiles. They are incorporated into the fiber by chemical reaction, absorption, or dispersion. Dyes differ in their resistance to sunlight, perspiration, washing, gas, alkalies, and other agents; their affinity for different fibers; their reaction to cleaning agents and methods; and their solubility and method of application. Various classes and types are listed below.

Natural Dyes

Natural dyes are dyes or colorants derived from plants, invertebrates,a or minerals. The majority of natural dyes are vegetable dyes from plant sources from roots, berries, bark, leaves, and wood and other organic sources such as fungi and lichens.

Plant Dyes | Plant Dye

There is a whole variety of plants which can be used to make plant dyes. Different parts of plants are used to make dyes – for example, the leaves, the skins of fruit, the bark, roots or wood. Lichens are very small plants which grow on rocks. There are many colours of lichens and they are very good for making dyes.

Natural Plant Dyes

• Alkanna tinctoria
• Indigofera suffruticosa
• Anthemis tinctoria
• Bixa orellana
• Caesalpinia echinata
• Catechu
• Common Tormentil
• Cordeauxia edulis
• Cryptocarya alba
• Dyewoods
• Eucalyptus fibrosa
• Eurasian smoketree
• Gardenia jasminoides
• Genista tinctoria
• Haematoxylum campechianum
• Haemodorum coccineum
• HennaIndigo dye
• Indigofera
• Indigofera tinctoria
• Isatis tinctoria
• Juglans regia
• Lithospermum caroliniense
• Lomatia hirsuta
• Maclura tinctoria
• Mallotus discolor
• Mallotus philippensis
• Morinda citrifolia
• Morinda tinctoria
• Oldenlandia umbellata
• Pentaglottis sempervirens
• Pokeweed
• Pomegranate
• Psorothamnus emoryi
• Quercus velutina
• Reseda (plant)
• Reseda luteola
• Rubia
• Rubia cordifolia
• Safflower
• Saw-wort
• Tansy
• Trihydroxyanthraquinone
• Turmeric
• Wild Angelica
• Wrightia tinctoria

Insect Dyes | Insect Dye

Example of an insect Dye is the Kermes dye obtained from the dried bodies of the female insects ( Kermes vermilio Planchon and Kermes ilicis ) which were found in southern Europe on the small evergreen kermes oak ( Quercus coccifera )The history of the Kermes dye dates back to the ancient Egyptians and Romans. Kermes dye was produced by a process of drying the bodies of the insects and then fermentation.

• Cochineal

Animal Dyes | Animal Dye

Animal-based dyes could be created from shellfish.

Carmine

Cochineal

Hexaplex trunculus

Polish cochineal

Tekhelet

Tyrian purple