PREPARATION OF THE GOODS FOR DYEING

In order to obtain satisfactory results the preparation of the cotton materials , prior to dyeing is most important . The goods should be de - sized and scoured well in order to impart adequate absorbency to the substrate and remove all impurities that may impede proper dye uptake in the subsequent dyeing process Level shades with optimum dye penetration and colour fastness are obtained only on well prepared material.

DISSOLVING OF DYESTUFF

The dyestuff is made to a smooth paste with cold water and dissolved by adding warm water, whilst stirring . Where solubility limits are exceeded satisfactory solution can be obtained by addition of urea.

DETAILS OF DYEING (EXHAUST METHOD)

This is recommended for cotton yarn in Open -vat (Beck) or Package -dyeing machines; cotton piece goods on Jigger, Winch. Jet -dyeing machines etc.

The bath is set at room temperature (about 25-35oC )with pre-dissolved dye and run for 10minutes . Pre - dissolved salt (Glauber's Salt or common Salt ) is added and run for 20-30 minutes . pre - dissolved alkali (Soda Ash) is then added and dyeing continued for 30-60 minutes . Finally , Washing -off as described at the end.

SALT & ALKALI REQUIREMENT :

TABLE-1

SEMI - CONTINUOUS METHODS OF APPLICATION FOR COTTON PIECE GOODS

In all above methods of application which involve padding , the following precautions are valuable to help avoid "tailing or " effects in the dyeings":

1. Employing a small pad trough "V" or "U" shaped ,having an optimum capacity of 5-10 liters.

2. Using high running speeds commensurate with the needs of the rest of the process and setting the padding mangle to give adequate squeeze .Normally, 60 or 70% nipexpression may be kept for cotton goods . The mangle squeeze should be even on the padded goods and the through ,fed with dye liquor to a constant level throughout .

3. In all dye- fixing operations , involving steaming in particular , addition of Resist Salt is recommended to the padding liquor to counteract any adverse reductive action of the dye.

Add 5-10 parts sodium bicarbonate (pre-dissolved) and 2-5 parts Calsolene Oil HS to dye padding solution at 20- 3oC . Migration during can be minimized by using suitable wetting agent and 10 parts of dissolved common salt or Glauber's salt (unhydrous) pad to give minimum pick up. Diffusion into viscose rayon is aided by batching and storing for I hour or more before drying. Dry at 100-120oC for 1-2 minutes in hot flue, or pin stenter or on cylinders set to dry slowly. Wash to clear agent and unfixed dye.

Pad-batch Method

Pad liquor composition:

Process

Dissolve dye urea and wetting agent .The dye solution and alkali are mixed just before padding . The goods are padded in above pad liquor solution and immediately batched on plastic sheet and kept for 2 hours and then washed

Washing -off procedure

In order to obtain maximum wet- fastness properties , brightness and purity of shades with consistent dyeing results , it is essential to give a through 'Soaping' to clear - off unreacted hydrolysed dye from the dyed fabric.

The dyed fabric is rinsed repeatedly in cold water to remove most of the alkali , salt and unfixed dye present and rinsed again in warm water not higher than 60oC. Then run in a bath containing.

Anionic detergent

1-2 gms /liter for 15 minutes at the boil. Then rinse in warm water (up to 60oC ) and finally in cold water . The most satisfactory results in washing - off particularly for piece goods ,are obtained by employing an open soaper or perforated Beam - washing machine. If such equipments are not available , conventional ones like jig or winch may be used .for yarn in the hank form open - vat is employed and for yarn in package form the package -dyeing machine itself is used.