Dyeing Process by Jigger Dyeing Machine with limitation

Dyeing Process 

The dyeing process on jigger is regarded as a series of intermittent padding operation followed by dwelling periods on the main roller , during which the dyeing action and diffusion takes place.

In the dyeing on jigger machines the cloth revolves on two main rollers , The open-width fabric passes from one roller through the dye bath at the bottom of the machine and then onto a driven take-up roller on the other side. When all the fabric has passed through the bath, the direction is reversed. Each passage is called an end. Dyeing always involves an even number of ends. The dye bath has one or more guide rollers , around which the cloth travels , and during this immersion achieves the desired contact with the dye liquor. During this passage the fabric picks up adequate quantity of dye liquor , excess of which is drained out but still a good quantity is held in the fabric . During rotation of rollers this dye penetrates and diffuse into the fabric. The real dyeing takes place not in the dye liquor but when the cloth is on the rollers, since only a very small length of fabric is in the dyebath and major part is on the rollers . Therefore the speed of cloth during immersion in dye liquor has a very little effect on percentage of shade produced.

Some critical problems related to the conventional jigger dyeing machines ( which are minimized in the modern day machines) The major problems are side-to-centre color variations, called listing, and lengthways color variations, called ending.

The factors controlling the rate of dye absorption are

1. The amount of interstitial dye liquor retained in the interstices of the fabric weaves.
2. The exhaustion of the interstitial liquor in the dwell period between successive immersions.
3. The degree of interchange of liquor during one immersion (interchange factor).

Problems of Jigger dyeing

• Temperature control from side-to-side and end-to-end of the roll
• Tension control from end-to-end
• Constant speed control from end-to-end
• Prevention of creases
• Prevention of air

Limitations of Jigger Dyeing

1.     Jigs exert considerable lengthwise tension on the fabric and are more suitable for the dyeing of woven than knitted fabrics.

2.     In textile preparation due to the swelling and dissolution of size, which makes the fabric slippery and unstable in roll form.

3.     The low liquor ratio makes washing-off difficult.

4.     There is little mechanical action in a jig machine and it is less suitable where vigorous scouring is required before dyeing.

5.     Moiré effects or water marks may arise on some acetate and nylon fabrics because of pressure flattening the structure of the rolled fabric.

Important factors for dyeing cellulose fibre

Important factors for dyeing cellulose fibre

 (with cold brand reactive dye in batching process):

1) pH of the dye bath:

          The optimum pH for fixing cold brand reactive dyes on cotton and viscose rayon depends on individual dyes, the temperature and time of dyeing. pH decreases with increasing temperature and time of dyeing. For most of the dyes the optimum pH is 10.8 to 11 at 20^o to 25^oC. Soda ash has been the best alkali for dyeing at 30^oC for cotton, mercerized cotton and linen. Increased fixation (due to higher temperature) and increased dye bath stability and better reproducibility are the advantages of soda ash as the fixing agent.

          For viscose rayon the optimum pH is 10.3 at 20^o to 25^oC.

2) Amount of alkali:

          The amount of alkali used for fixing depends on the depth of shade dyed and the liquor ratio employed. Some quantities of alkali required for fixing the reactive dyes are given table 28.

3) Dyeing temperature:

          Since increase in temperature affects the rate of physical and chemical processes involved in dyeing, it is important in dyeing reactive dyes also. The affinity of the dye for the fibre decreases with increases in temperature and at the same time the rate of hydrolysis of the dye increases and adversely affects the fixation of color yield. However the rate of diffusion of the dye in the fibre increases with increased temperature. At temperatures lower than 20^oc, the rate of fixation is very low. Hence for most of the dyes a temperature of 20^o to 25^oC is the recommended temperature while for some other dyeing at 50^o to 60^oC with sodium bicarbonate as the alkali gives maximum color value.

4) Electrolyte concentration:

          Since reactive dyes have low affinity for cellulose exhausting the dye bath by adding common salt or Glauber’s salt prior to fixation can increase the fixation. The amount of salt required producing adequate exhaustion decreases with decreasing liquor ratio. Thus for pale shade on cotton and viscose rayon 15 and 10 g/l of common salt used. The quantities may be increased to 30 and 20 to 30 g/l for medium and deep shades on these fibres.

5) Time of dyeing:

          Generally the dye may be added in two portions. The salt may also be added in two lots. The exhaustion takes place in 20 to 30 min. There is generally no advantage in extending the period beyond 30 min. The alkali is then added and the dyeing continued for 30 to 90 min. The depth of shade and reactivity of the dye decide the time of dyeing. For deeper shades larger times are required.

6) Liquor ratio:

          With decreased liquor ratio, both exhaustion and fixation take place to increased exert. However the rate of fixation of most of the dyes is not significantly affected. As the liquor ratio is decreased, the effectiveness of increasing salt addition also decreases. Hence lower amount of salt are sufficient to get optimum exhaustion.