How to Process Control of indigo rope dyeing ?

Process Control of indigo rope dyeing

Concentration of Hydrosulphite:

Hydrosulphite is measured by vatometer. It should be from 1.5 gpl to 2.5gpl. It can also be measured by Redox potential of dye bath which should be from -730 mV to -860 mV.

Relatively high conc. of hydrosulphite gives a clear shades with minimum reddish streaks. The indigo dye stuff in the dye bath should be kept in soluble stage. Indigo dye has a tendency to react with the oxygen in the air and get oxidized and transferred into their original insoluble stage. Hence an extra quantity of hydrosulphite is maintained in the dye bath to keep the indigo dyestuff in soluble stage.With lack of hydrosulphite, the leuco indigo is less dissolved and goes to insoluble stage and thereby adheres to a greater extent to the fibers. Further lack of hydrosulphite may cause higher unreduced dyestuff in the liquor. This results unfixed dyestuff onto the yarn. Under these circumstances a reddish bronze like shade is developed in the yarn.

Caustic Soda or pH value:

The pH of the dye bath should be around 10.5 - 11.5, as at this pH range, there will be formation of more monophenolate ions, which lead to higher color yield and good wash down effect. At higher pH, dye penetration is less and leads to poor wash down effect.

The pH influence the shades in the following way:

High pH or Caustic Concentration --> Redder and lighter

Low pH or caustic concentration --> greener and darker

Dye concentration in Dye bath:

Dye concentration is an important parameter in indigo dyeing. Generally the concentration of dye influence the shades as below:

High Indigo Concentration --> Shade is greener and lighter

Low Indigo Concentration --> Shade is dull and Red.

Dipping Time:

The dipping time of the ropes in the dye bath plays an important role in indigo dyeing. Dipping time varies from 15-22 seconds. Longer the dipping time, better will be the penetration. However, too long dipping time may dissolve the previously oxidized indigo. At very short dipping time, the liquor exchange, i.e. the amount of chemicals consumed and replaced by fresh addition of reduced indigo, will not be adequate. This may leads to poor dyeing and depth of dye penetration. However the time available for diffusion of dyestuff until oxidation commences is too short.

Squeeze Pressure:

The squeezing pressure should be optimum. High squeezing pressure may leads to low pick up of color and better penetration. At rope dyeing, squeeze pressure may be between 5-10 tones. Squeeze roller hardness should be about 70-75 deg. shores

Airing Time:

Airing time is an important consideration in rope dyeing for proper oxidation. The airing time should be 60- 75 seconds. Longer airing time leads to high tension on the yarn, whereas low airing time leads to poor oxidation.

Drying:

The dyed and washed rope should be properly dried with proper moister content. Insufficient or uneven, over or less dried yarns will result in poor opening and high end breakages in re-beaming.