Polyester can be dyed in different method. Polyester fabric dyeing with high temperature is the best method that is basically followed in our industries. This process requires high temperature in between 130 -135℃. Also this requires high pressure up to 170 kPa. For this reason it is called high pressure method also.
Polyester is highly crystalline fiber. To penetrate the disperse dye molecule inside the fiber, either liquor or material needs to circulate. At elevated temperature, the circulation or agitation helps to swell the fiber and allow the dye molecules to penetrate inside the fiber.
In this process no dye carriers are needed. Basically, carriers are used to swell the fiber. Carrier dyeing method has some drawbacks as well.
In high temperature high pressure method, high temperature does the action necessary for dyeing.
Recipe for dyeing:
Dispersing agent- 0.5 – 1.0 g/L
Sequestering agent- 1.0 – 2.0 g/L
Levelling agent- 1.0 – 2.0 g/L
Acetic acid (50%) – 0.5 – 2 g/L (to maintain pH)
Temperature- 125 – 135℃
Time- 30 – 60 min
pH- 4 – 5
High temperature dyeing method:
- Preparation of dyeing solution
Disperse dyes are water insoluble, thus need to make a dispersible (partially soluble) solution by stirring with warm water. If necessary add dispersing agent with it and stir rapidly.
- Set the dye bath with substrate at 50℃ temperature and add dispersing agent, acetic acid and other auxiliaries, then run the dye bath for 5 – 10 min.
- Add (dosing) dye solution and raise the temperature to 70 – 80℃ @ 2 – 3℃/min.
- Raise the temperature to 130 – 135℃ @ 1 – 1.5℃/min
- Run the bath for 30 – 60 min at the same temperature.
- Lower down the temperature to 70 – 80℃ over 15 – 30 min.
- Drop the dye bath and carry on the after-treatment process.
- Rinse twice with hot and cold water.
- Treat the fabric (for deep shade only) with-
- Caustic soda (36˚ Be) – 2 – 6 ml/L
- Sodium Hydrosulphit – 1 – 3 g/L
At 70 – 80℃ for 10 – 15 min. The amount of caustic and hydroze may vary with the shades. This process is called “Reduction Clearing.”
3. Rinse twice with hot and cold water.
4. Neutralize with acetic acid.
Time – Temperature profile or Process Curve
Fig: Process curve (dyeing of Polyester with Disperse dyes in HTHP method)
Advantages of High temperature method
- Shorter dyeing cycle under high temperature using rapid machineries.
- Faster diffusion of dye molecule in the fiber structure at elevated temperature
- No need of carriers
- Superior exhaustion of dye bath is achieved
- Deeper shades can be produced by this process without carriers. Thus a saving in expense is achieved and less time requires. This is so, as carrier rinsing out process is a time-consuming operation.
- A better combination of level dyeing and fastness properties are obtained.
Some Important Factors
>While it is the case of high temperature process, dye should be chosen that meets the requirements on fastness properties. Level dyeing behavior is an important factor.
With low molecular dye, a better coverage property is achieved such as B class > C class > D class. So, selecting a dye with high fastness properties is a sound practice.
>Optimum machine utilization in view of the high cost of pressurized machines compared to atmospheric machines, is another important factor while dyeing with high temperature method.
>Shading additions should be eliminated. For this, a sound recipe should be used altogether with small scale trial first. Then bulk order can be proceed.
>If possible, it is important to use a combination of dyes whose yield is relatively intensive to small variations in dyeing conditions.
In early dates, pressurized top dyeing machines were used for high temperature dyeing. After that pressurized jigger was developed but this is not very successful due to some mechanical difficulties and high cost.
Pressurized beam dyeing machine plays a successful role in case of fabric dyeing in this method. For flat polyester dyeing, this is a preferable high-temperature dyeing machine. Pressure exerted on the fabric by this machine don’t affect the appearance and handle of the fabric.
The need of dyeing circular knit fabric and texturiesd polyester fabric led the development of high temperature winch in 1960s. Various expected advantages are provided by this machine except a problem like elongation of the fabric with consequent reduction in width and rope marking or creasing.
High temperature jet dyeing machine is undoubtedly the most important machine for rope dyeing.
Polyester fabric dyeing with high temperature is the most used and convenient method.