Dyeing

"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 the 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.

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.

Methods of Dyeing

1) 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.

2) 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 mélange, mottled or streaked effect, imitated in machine printing.

3) Beam Dyeing:
In this method the warp is dyed prior to weaving. It is wound onto a perforated beam and the dye is forced through the perforations thereby saturating the yarn with color.

4) Burl or speck Dyeing:
This is done mostly on woolens or worsteds, colored specks and blemishes are covered by the use of special colored links which come in many colors and shades. It is a hand operation.

5) 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.

6) 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.

7) Jig Dyeing:
This is done in a jig, kier, vat, beck or vessel in an open formation of the goods. The fabric goes from one roller to another through a deep dye bath until the desired shade is achieved.

8) Piece Dyeing:
The dyeing of fabrics in the cut, bolt or piece form is called piece dyeing. It follows the weaving of the goods and provides a single color for the material, such as blue serge, a green organdy.

9) Random Dyeing : Coloring
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

10) Raw Stock Dyeing:
Dyeing of fiber stock precedes spinning of the yarn. Dyeing follows the degreasing of wool fibers and drying of the stock.

11) Solution Dyeing:
This is also called dope dyeing or spun dyeing, the pigment color is bonded-in in the solution and is picked up as the filaments are being formed in the liquor. Cellulosic and non-Cellulosic fibers are dyed to perfection by this method. The colors are bright, clear, clean and fast.

12) Yarn dyed:
Yarn which has been dyed prior to the weaving of the goods; follows spinning of the yarn. It may be done in either partial immersion or total immersion of the yarn.

Process of Dyeing
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.,

1. 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
2. Less swelling
3. Increased speed of dyeing per given amount of material
4. 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.

Machinery and Equipment
Modern dyeing machines are made from stainless steels. Steels containing up to 4% molybdenum are favored to withstand the acid conditions that are common. 

A dyeing machine consists essentially of a vessel to contain the dye liquor, provided with equipment for heating, cooling and circulating the liquor into and around the goods to be dyed or moving the goods through the dye liquor. The kind of machine employed depends on the nature of the goods to be dyed. Labor and energy costs are high in relation to total dyeing costs: the dyers aim is to shorten dyeing times to save steam and electrical power and to avoid spoilage of goods.

The conical-pan loose-stock machine is a widely used machine. Fibers are held in an inner truncated conical vessel while the hot dye liquor is mechanically pumped through. The fiber mass tends to become compressed in the upper narrow half of the cone, assisting efficient circulation. Leveling problems are less important as uniformity may be achieved by blending the dyed fibers prior to spinning.

The Hussong machine is the traditional apparatus. It has a long, square-ended tank as a dye bath into which a framework of poles carrying hanks can be lowered. The dye liquor is circulated by an impeller and moves through a perforated false bottom that also houses the open steam pipe for heating. In modern machines, circulation is improved at the points of contact between hank and pole. This leads to better leveling and elimination of irregularities caused by uneven cooling.
 
In package-dyeing machines dye color may be pumped in rather two directions:

1)Through the perforated central spindle and outward through the package                                                                                  or
2) By the reverse path into the outer layers of the package and out of the spindle. In either cases levelness is important.

Some package-dyeing machines are capable of working under pressure at temperatures up to 130C.

The winch is the oldest piece of dyeing machine and takes its name from the slated roller that moves an endless rope of cloth or endless belt of cloth at full width through the dye liquor. Pressurized-winch machines have been developed in the U.S. 

In an entirely new concept, the Gaston County jet machine circulates fabric in rope form through a pipe by means of a high-pressure jet of dye color. The jet machine is increasingly important in high-temperature dyeing of synthetic fibers, especially polyester fabrics. Another machine is the jig. It has a V-shaped trough holding the dye color and guide rollers to carry the cloth at full width between two external, powered rollers, the cloth is wound onto each roller alternately, that is, the cloth is first moved forward, then backward through the dye color until dyeing is complete. Modern machines, automatically controlled and programmed, can be built to work under pressure.