Assistants for Dyeing with Reactive Dyes | Dyeing Assistants ofReactive Dye

Assistants used for dyeing with reactive dyes: The following assistants are used in dye bath for dyeing with reactive dyes.

1) Salt:
As a salt NaCl is used widely. The salts do the following things-
- Salts are used to increase the affinity of dye to fibre.
- It decreases the hydrolysis rate of dyes.
- It neutralizes the electro negativity of fibre surface when immersed in solution.
- It puts extra energy to push the dye inside the fibre polymer i.e. increase absorption of dye.

The amount of salt and used depends upon the shade to be produced.

For light shade – 10gm/ L salt is used.
For medium shade – 20 gm/ L salt is used.
For dark shade – 30gm/L salt is used.

2) Alkali:
Alkali is used for the following purposes:
- Alkali is used to maintain proper pH in dye bath and thus to create alkaline condition.
- Alkali is used as a dye-fixing agent.
- Without alkali no dyeing take place.
- The strength of alkali used depends on the reactivity of dyes.
- As strong alkali caustic soda is used to create pH 12.5-12.
- As medium alkali soda ash (Na2CO3) is used to create pH 11-12 when dye is of medium reactivity.
- As weak alkali NaHCO3 is used to create pH 10-11 when dye is highly reactive.

3. Urea: 

Urea is used in continuous method of dying. It helps to get required shade of dye. To get dark shade more urea is used and to get light shade less amount of urea is used.

4. Soaping: 

By soaping, the extra color is removed from fibre surface. Thus washing fastness is improved. Soaping increases the brightness and stability of the dye.

Effect of Various Conditions on Disperse Dyeing | Effect of Temperature| Effect of pH | Heat Setting Procedure of Polyester Fabric

Effect of Temperature:
In case of dyeing with disperse dye, temperature plays an important role. For the swelling of fibre, temperature above 100°C is required if high temperature dyeing method is applied. Again in case of carrier dyeing method, this swelling occurs at 85-90°C. In case of thermasol dyeing method, if temperature is kept more, fabric is kept for less time in thermasol unit. Because in higher temperature, less time is enough for thermo fixation of dye. If it is kept for more time, then dye sublimation and loss of fabric strength may occur. Again though disperse dye is a water insoluble dye, its solubility increases with increasing temperature.

Effect of pH:
For disperse dyeing the dye bath should be acidic and pH should be in between 4.5-5.5. For maintaining this pH, generally acetic acid is used. We may also use any mineral acid like H3PO4. But those are strong and costly. So mild acid like acetic acid is used for controlling pH of the bath. At this pH dye exhaustion is satisfactory. During colour development, correct pH should be maintained otherwise fastness will be inferior and colour will be unstable.

Heat Setting Procedure of Polyester Fabric

The process of heat setting is used to stabilize yarn twist, remove residual shrinkage, increase wrinkle resistance and obtain durable pleat. Polyester may shrink 7% at boil and even more at higher temperature.

The importance of heat setting is as below:

· To modify crystalline structure.
· To improve dimensional stability.
· To resist wet creasing during washing.
· To increase safe ironing temperature.
· To avoid shade variation.
· Affect water inhibition.

The disadvantages of heat setting are:-

· Some dyes sublime at high temperature.
· Pre setting reduces dye uptake.

Heat setting can be done before or after dyeing. If the fabric is heat set before dyeing, the absorbency of fabric goes down and if it is done before scouring and bleaching then the dirt dust may permanently deposit on the fabric. If heat setting is done after dyeing then some dye may sublime at high temperature and thus cause shade variation.

The time of heat setting depends on the fabric state i.e. grey, scoured, bleached etc. It also depends on count of yarn in fabric. For heat setting hot air, hot roller or radiant machine can be used.