Introduction of Dyeing

The objective of dyeing is the uniform colouration of the fibers constituting the material, usually to match a pre-specified colour. Synthetic fibers can be manufactured ready coloured. Dye is added to the liquid even before it is forced through tiny holes to come out as filaments of thread. Other synthetic and natural fibers can be dyed in a solution in large drums. Mixing the colours of woollen fibers gives very pleasantly shaded yarns. Any significant difference in colour form that requested by the customer , and any unlevelness of the colour appearance. These include fiber characteristics such as luster, denier, staple length, texture and cross-section,as well as the cloth construction. Since a client,s coloured sample is rarely of the same material to that to be dyed, dyehouse laboratories devote a considerable time to dyeing trials aimed at developing recipes to reproduce the desired colour on the given goods.

Colouration of a textile material is achieved in a number of different ways:

1) Direct dyeing, in which the dye in an aqueous solution in contact with the material is gradually absorbed into the fibers because of its inherent substantivity.

2) Dyeing with a soluble precursor of the dye, which forms an insoluble pigment deep within the fibers on treatment after dyeing.

3) Direct dyeing followed by chemical reaction of the dye with appropriate groups in the fibers.

4) Adhesion of the dye or pigment to the surface of the fibers using an appropriate binder.

All of these methods but the last require that the fibers,at some stages, absorb the dye or an appropriate precursor form an aqueous solution, This process is essentially reversible.It is should be noted, however, that precipitation of a pigment and reaction with the fibers are irreversible chemical processes.

Dyeing is either a batch exhaustion process, or a continuous impregnation and fixation process.In the exhaust technique ,the textile is in repeated contact with all the dye liquor during dyeing and the fibers gradually absorb the dyes.Careful control of the dyeing temperature, PH value and auxiliary chemical concentration is often necessary to obtain level, well-penetrated dyeings. This is essential if the initially absorbed dye is unable to migrate form heavily dyed to poorly dyed areas during the process.

In continuous impregnation method, the fabric passes through a small bath containing the dye solution and then two rubber-covered rollers squeeze out the excess solution. This process is called padding. There is no migration of the dye from the point of impregnation expect into the fibers,which is assisted by the pressure rollers. Each small segment of fabric encounters the dye liquor only once so padding must be uniform across the fabric width and along its entire length. After padding, the dyes must diffuse into the fibers, This step is called fixation. It may be as simple as rolling up the fabric and batching it for several hours, or as complex as a rapid thermal treatment in a steamer or a hot air oven.

Other operations often precede or follow the actual process of dyeing.Pretreatments include scouring and bleaching, which remove natural and synthetic impurities from the goods and would result in uniform water absorbency. After dyeing , the material is rinsed to remove adhering solution but it may need additional treatment while still in the dyeing machine. These after-treatments may include:

1) Washing in detergent, at or near the boil, to remove any unfixed dye or loosely adhering pigment from the fiber surfaces -----this process is called soaping.

2) Treatment with chemicals to improve the fastness properties of the dyeing -----this often causes a change in shade and complicates colour matching.

3) Application of simple finishing chemicals such as softeners.