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.

Polyester dyeing
Fig: Polyester dyeing
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:-
  1. Some dyes sublime at high temperature.
  2. 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. 
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Mazharul Islam Kiron is a textile consultant and researcher on online business promotion. He is working with one European textile machinery company as a country agent. He is also a contributor of Wikipedia.

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