Process Control of Rope Dyeing for denim

1. Concentration of Hydrosulphite

It is measured by vatometer. It should be from 1.5 gpl to 2.5gpl , or by redox potential of dye bath which should be from -730 mV to -860 mV.

2. Caustic Soda or pH value

Should be from 11.5-12.5

3. Dye concentration in Dye bath

it is measured by spectrophotometer. It should be in g/l

Guidelines

High Indigo Concentration --> Shade is greener and lighter
Low Indigo Concentration --> Shade is dull and Red.

High pH or Caustic Concentration --> Redder and lighter
Low pH or caustic concentration --> greener and darker

Dipping Time

Longer the dipping time, better will be the penetration and lesser will be the ring dyeing effect. It varies from 15-22 seconds.

Squeeze Pressure
High pressure will lead to lower wet pick up and result in lesser color and better penetration. At rope dyeing, squeeze pressure is 5-10 tonnes, ie. wet pick up is as low as 60%. Hardness of squeeze roller is about 70-75 deg. shores. It sqeeze rolls are too hard then there are chances of slippage and uneven yarn tension.. If squeeze rollers are too soft then shading will occur. Surface of the squeeze rolls should be ground twice a year.

Airing Time

It should be 60-75 seconds. Longer airing time results in high tension on the yarn and subsequent processes will become difficult.

Drying

Insufficient or unevenly dried yarns will result in poor rebeaming

Calculation of Replenishing Dye feed/min

Conc. of stock vat is g/l= 90
range speed in yards/min=25
count = 7s
totoal ends = 4100

Wt of yarn dyed /min= (4100*25*1000)/(7*840*202)= 7924 gms
shade desired = 2%
Amount of dye to be replenished/min= 158.5 gms

Effect of pH

At pH of 10.5 to 11.5, there will be formation of more monophenolate ions, which lead to higher color yield, as strike rate of the dye to the yarn bundle is very high, and wash down activities will be very good.

At pH higher than this, dye penetration will be less and wash down characteristics are also poor.

Testing

1. Alkalanity in Dye Bath Liquor

Pipet 10.0 ml of vat liquor into 100ml of distilled water in a 150 ml beaker. place under continuous agitation and insert the electrodes of a pH meter caliberated at pH 7.0 with standard buffer solution.

Titrate with tenth normal HCl ( 0.1 HCl) to pH 7.0 (ml = A)

calculate
g/l of NaOH = A *0.40

2. Hydro in Dye bath Liquor

Add 2 ml of 37% HCHO to 150 ml beaker. Add 2 ml of dye range liquor . Add 6 ml of 25% glacial acetic acid solution prepared by diluting 1 part acid with 3 parts water. Add 2 ml of starch/KI indicator. Add ml of water. Titrate with 0.046 N ( prepared by diluting 460 ml of 0.1 N Iodine to one liter ) solution until the color changes from emarald green to bluish purple.

G/l of hydro= mo fo 0.046N of Iodine

Importance of High Concentration of Free Hydrosulphite

The clearest shades with minimum reddish streaks are observed at by relatively high conc. of hydrosulphite. On the other side, with lack of hydrosulphite, the leuco indigo is less dissolved and thereby adheres to a greater extent to the fibres. With lack of hydrosulphite furthermore, the amount of unreduced dyestuff by oxidation at the upper level of the liquor and through activiation of unfixed dyestuff, gets separated from the fibrous material would constantly rise as the reducing agent for creating leucoform would be missing. Under these circumstances a reddish bronze like shade results due to dispersion of not reduced dyestuff in the yarn. The min. proportion of hydrosulphite should be around 1.3 to 1.5 gpl in case of rope dyeing and 3-4 gpl in case of sheet dyeing. Also to avoid the lack of hydrosulphite or Indigo at certain places in the immersion, vat, the whole quantity of the liquor should be circulated 2-3 times every hour.

Reaction Time

At very short reaction time, an adequate liquor exchange ( i.e. the amount of chemicals consumed and replaced by fresh addition of reduced indigo) is not assured. This has a negative influence on dyeing and depth of dye penetration. In addition to this the time available for diffusion of dyestuff until oxidation commences is too short. To ensure an even and good depth of dye penetration by dyeing in several passages, the reaction time should be 20-30 sec. for each vat (eg. at a speed of 20m/min for a reaciton time of 10 seconds, the immersion path should be maximum 3.3 meters).

A reaction time exceeding 60 seconds should be avoided as the amount of dyestuff again get reduced and released may again supersede that of additionally take up dye stuff, resulting in higher shades.

Softening Agent: 8 g/lit

Drying: Rest humidity should be 30% and then sized.

Addition of chemicals

1. Red Tinge: reduce addition of NaOH, increase slightly Na2S2O3
2. Darkish Red: increase Hydro
3. Light Greenish: decrease Hydro
4. Dark Green: Increase Caustic

For further information you can read this article