Process sequence in Indigo rope dyeing in denim :

In rope dyeing, 12-36 ball warping ropes of yarn are fed side-by-side simultaneously into the rope dyeing range for application of the indigo dyeing. The process sequence is as below :

Fig. : The passage of yarn in rope dyeing

Pre-scouring :

In the rope dyeing range the ropes are first passed through one or more scouring baths. It consist of wetting agents, detergents and caustic. The objectives of pre-scouring is to remove the wax content from cotton, naturally occurring impurities in cotton fiber yarn such as dirt, minerals, ash, pectin, and naturally occurring waxes, removal of trapped air from cotton yarns through wetting the yarn at temperature of 90 to 95ºC. Removal of these materials is very important in order to achieve uniform wetting and uniform dyeing. The trapped air is essential to be removed as it may cause uneven dyeing. There is approx. 2 liters of air in 1 kg of yarn. One liter of air decomposes 2 liters of Sodium hydroxide which also increase consumption of sodium hydroxide.


The following agents normally used in pre-scouring.

CausticSoda:

In pre-scouring,3–5% sodium hydroxide (depends upon the quantity of cotton fibers) is used, which removes the wax by the action of soapanification.

Wetting agent: 

Anionic wetting agent are normally used.

Sequestering agent: 

It is used to make the water soft, as it is very difficult to find the desired softness in water (about 1-3 ppm) even with the use of soft water.


The ropes are subsequently fed into one or more water rinsing baths.

Hot wash

The ropes is hot washed 80 – 90ºC to remove the sodium hydroxide left in the yarn after pre-scouring, otherwise it will change the pH values of dye – bath.

Cold wash

After hot wash, the yarn ropes is fed through a cold wash bath to brought back to its room temperature.

Dyeing

The ropes of yarn are then fed into the indigo dye baths and skyed after each dip. As the constant of substantivty for indigo it is only 2.7, where as for other color, it is 3.0. Hence there is a need of repeated dipping of 5 to 6 dye baths with multi – dip and multi –nip facility to increase the penetration.

As indigo belongs to IK class of vat dyes, the dyeing is done on room temperature, and oxidation is done by air. Sodium hydrosulphide is used to reduce the indigo, as Indigo is not soluble in water. Then caustic soda is also used to make sodium salt of vat color to make it soluble.

The pH of the indigo dye bath should be maintained between 10.5 to 11.5, as at this level, the dye affinity is very high. This is because at this pH, sodium salt of indigo is mono phenolic form. pH is maintained by the addition of caustic soda.

Washing

After the dyeing process, the ropes of yarn are rinsed in several water baths to remove any unfixed dye. Rubbing fastness of indigo, which is a very important parameter, can be improved through washing. In a typical indigo dyeing range, there are four wash bath after the dye bath. The typical washing process are in Fig.

Fig. : Typical washing Process

The softener treatment at the last was box is very important in rope dyeing. The dyed rope is to be opened at the next operation, Long Chain Beamer. The softener helps easy opening of the rope, hence less end breakages.Cationic softener is generally used with 1.2% of the weight of the yarn and pH in the range of 4 to 5.5. Softening treatment is done at room temperature, as high temperature may cause tendering of yarn.

Drying and Coiling

After the washing, the ropes are dried by hot cylinder and coiled in can.

Fig. : Rope cans