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Dyeing

Introduction to dyeing
The procedure of dyeing is meant at giving textile material (fibre, yarn, knitted or woven fabric and garment) its intended color, essential to its final use. Generally, however, dyes are used as solution with the substrate in the form of loose fiber, tow, slubbing, and sliver. It could be used for yarn or fabric dyeing. Woven and knitted fabrics are dyed with different dyeing method and dyeing machine, Now a days garments dyeing is very much popular and different types of garments dyeing machine is used commercially . Large quantities of fabric are also decorated with different types of printing method.

Theory of Dyeing
If we want to discuss about dyeing theory we have to understand the physical chemistry of dyeing, because the application method of a dye depends on its physical structure and chemical behavior. As for all theory, the goal is to provide a set of hypotheses that explain the characteristics of known dyeing systems, and which are suitable of predicting what will happen in a new condition. The theory of dyeing makes a vital role for qualitative analysis of practical dyeing. The physico-chemical dimensions on industrial coloration system that gives quantitative information are often far removed from Practical coloration practice.

The theory of dyeing included some subjects those are given bellow:
(1) Dyes must be mixed in dyeing solution before dyeing and in the fibre after dyeing. Liquefy or disperse the dyestuff in a dye bath (with conversional, semiautomatic and automatic color kitchens according to specific preset rules).

(2) The rates of dyeing processes and how these are influenced by mass-transfer of dye from the bath solution to the dye–fibre interface, and by diffusion of the dye from the interface into the fibre. Feed the dye solution in the machine after suitable filtering (automatic colour kitchen, supplementary vats, pumps and filters).

(3) The phenomena occurring at the dye–fibre interface such as dye molecule adsorption and the effects of surface potentials. Transfer the dye from the liquor to the fibre (process and machine).

(4) The nature of the interactions between dye and fibre molecules, which are the origin of substantively. Distribute the dye homogeneously on the fibre (process and machine).

(5) The treatment of dyeing as a thermodynamic equilibrium and its description in terms of thermodynamic variables. Let the dye penetrate in the fibre structure and fix it (time and temperature). - Wash or rinse the material to remove the dye on the surface or the unfixed dyeing liquor.

(6) The theory of fibre structure and how this influences dyeing rates and equilibrium.

Dyeing procedure may be carried out at any steps in the producing of textile material, including the yarn manufacturing of synthetic polymer or natural fiber. Considerable numbers of polymer is dyed by a specila dyeing method called mass pigmentation in which pigment becomes very heat-stable and isolated in the textile polymer molt. It could be very much appropriate system for olefin fibers. This process is widely used for nylon, polyester and regenareted viscose fibre. When the coloration progression is done during the first processing stages, for example on staple fibres, a superior colour fastness can be gained; bulk dyeing refers to the method used to dye a staple fiber before it is spun, this process is carried out in perforated baskets and although there may be areas where the dye does not penetrate completely, in subsequent yarn manufacturing process these areas are involved with the thoroughly dyed fiber, thus provide an excellent even color.

Dyeing or Coloration process of polymer is different. Suitable Dyestuffs are mixed evenly with fibre forming polymer solution according to the required shade then the fibre forming solution convert in to fibre filament. Dyeing of acrylic fibre could be the best example of synthetic polymeric fibre dyeing. Continuously produced acrylic filaments are passed within a solution of basic dyes. But there is a condition that the freshly extruded acrylic filament does not allow to dry out before Applying dyestuff. This process is widely used for acrylic fiber in tow form.

Yarn dyeing is also popular dyeing procedure. Dyeing of yarn is carried out for producing jacquard or strip fabric and sewing thread; this dyeing process provide a good colour fastness and wash fastness since the dyes particle are absorbed by fibres and penetrate in to the core of fibre. Skeins are dyed in hanks, spools are dyed in autoclaves and warp yarns are dyed in perforated beams loaded in autoclaves.

Garments dyeing or Piece dyeing is also popular dyeing system. Various types of machine and the material could be used for rope dyeing or piece dyeing. Several parameter and condition should immediately maintain and evaluate for level dyeing. Particular periodic decision is taken for achieving required color fastness that can be controlled only by means of subsequent laboratory tests.

The dyeing machines used are chosen according to the types of material to be dyed. The crucial requisites are the following:

- Protection of the substrate
- Perfection of the results
- Cost effectiveness of the process (based on dyeing time, automation quality of machine, liquor ratio, cost of the materials used and wastewater treatment plants).

To carry out a dyeing process it is necessary to:
There are two different methods to transfer the dye from the liquor to the fibre:
Exhaust dyeing (discontinuous systems).Dyeing liquor of dyeing bath will be produced dissolving or dispersing dyestuff. For proper dyeing the textile material will immersed in the dye solution. After transformation of dyes the textile material will removed from dye bath. Dyestuff will distribute homogeneously, well entered into the fibre core and fixed. At the end the unfixed dyes will be removed. The material will be washed or rinsed for removing dyes.

Pad dyeing (continuous or semi-continuous systems). Pad batch wetting machine is used This process is carried out using mechanical means (pad-batch wetting). The dyeing liquor is distributed homogeneously onto the fabric (i.e. also the dye is distributed homogeneously).

In a second stage the dye penetrates into the fabric and is then fixed. At the end of the process the material is washed.

Some operations must be carried out for both exhaust and pad dyeing:
- dissolve or disperse the dye in water and filter.
- achieve a homogeneous contact between the dyeing liquor and the fibre.
- make the dye penetrate into the fibre.
- fix the dye in the core of the fibre.
- Final washing.