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DYEING AND IT'S TYPE :
"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.
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The 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.
METHOD OF DYEING A FABRIC :
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 :
1.
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.
2.
Color may be printed onto the skeins which are spread out on the blanket fabric of the printing machine.
3.
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.
Cellulose and non-Cellulose 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 Walls 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
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.
5.
Thus it eliminates the effluent (pollution) problems associated with the conventional methods of dyeing and
finishing. |
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