Dyeing of Polyester fabric with Disperse dyes.
What is Disperse Dye?
Dyes are colored, unsaturated organic chemical compounds capable of giving color to a substrate (a textile), i.e. coloring or dyeing it.
The term “disperse dye” have been applied to the organic coloring substances which are free from ionizing groups, are of low water solubility and are suitable for dyeing hydrophobic fibers. Disperse dyes have substantivity for one or more hydrophobic fibers e.g. cellulose acetate, nylon, polyester, acrylic and other synthetic fibers.
The negative charge on the surface of hydrophobic fibers like polyester can not be reduced by any means, so non-ionic dyes like disperse dyes are used which are not influenced by that surface charge.
In 1922, Green and Saunders made one type of colored azo compound, in which a solubilizing group (for example- methyl sulphate, -CH2-SO3H) is attached to amino group. In dye bath, they are slowly hydrolyzed and produce azo compound and formaldehyde bi sulphate. This free azo compound was capable of dyeing cellulose acetate fibers. This dye was named “ionamine”. But this ion amine did not give satisfactory result in dyeing.
Later in 1924, Baddiley and Ellis produced sulpho ricinoleic acid (SRA) for dyeing acetate fibers. This SRA was used as dispersing agent. Later it was seen that SRA was capable of dyeing Nylon, polyester, acrylic etc. In 1953 this dye was named as “Disperse Dye”.
Properties of Disperse Dyes:
- Disperse dyes are nonionic dyes. So they are free from ionizing group.
- They are ready made dyes and are insoluble in water or have very low water solubility.
- They are organic coloring substances which are suitable for dyeing hydrophobic fibers.
- Disperse dyes are used for dyeing man made cellulose ester and synthetic fibers specially acetate and polyester fibers and sometimes nylon and acrylic fibers.
- Carrier or dispersing agents are required for dyeing with disperse dyes.
- Disperse dyes have fair to good light fastness with rating about 4-5.
Classification of Disperse Dyes:
According to Chemical Structure:
- Nitro Dyes
- Amino Ketone dyes
- Anthraquinonoid dyes
- Mono azo dyes
- Di- azo dyes
Application Methods of Disperse Dyes:
- Method N: Normal dyeing method. Dyeing temperature is 80-100°C.
- Normal NC method: Method of dyeing at normal temperature with carriers. Dyeing temperature 80-100°C.
- Method HT: High temperature dyeing method. Dyeing temperature 105-140°C.
- Method T: Thermasol dyeing method. Dyeing temperature 180-220°C, continuous method of dyeing.
- Pad roll method: Semi continuous dyeing method.
- Pad steam method: Continuous dyeing method.
Dyeing Mechanism of Disperse Dye:
The dyeing of hydrophobic fibers like polyester fibers with disperse dyes may be considered as a process of dye transfer from liquid solvent (water) to a solid organic solvent (fiber). Disperse dyes are added to water with a surface active agent to form an aqueous dispersion. The insolubility of disperse dyes enables them to leave the dye liquor as they are more substantive to the organic fiber than to the inorganic dye liquor. The application of heat to the dye liquor increases the energy of dye molecules and accelerates the dyeing of textile fibers.
Heating of dye liquor swells the fiber to some extent and assists the dye to penetrate the fiber polymer system. Thus the dye molecule takes its place in the amorphous regions of the fiber. Once taking place within the fiber polymer system, the dye molecules are held by hydrogen bonds and Van Der Waals’ force.
The dyeing is considered to take place in the following simultaneous steps:
- Diffusion of dye in solid phase into water by breaking up into individual molecules. This diffusion depends on dispersibility and solubility of dyestuff and is aided by the presence of dispersing agents and increasing temperature.
- Adsorption of the dissolved dye from the solution onto the fiber surface. This dyestuff adsorption by fiber surface is influenced by the solubility of the dye in the dye bath and that in the fiber.
- Diffusion of the adsorbed dye from the fiber surface into the interior of the fiber substance towards the center. In normal condition, the adsorption rate is always higher than the diffusion rate. And this is the governing step of dyeing.
When equilibrium dyeing is reached, the following equilibria are also established:
- Dye dispersed in the bath ↔ Dye dissolved in the bath
- Dye dissolved in the bath ↔ Dye adsorbed on the fiber
- Dye adsorbed on the fiber ↔ Dye diffused in the fiber
Effect of Various Conditions on Disperse Dyeing:
Effect of Temperature:
In case of dyeing with disperse dye, temperature plays an important role. For the swelling of fiber, 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. If it is kept for more time, then dye sublimation and loss of fabric strength may occur.
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. At this pH dye exhaustion is satisfactory. During color development, correct pH should be maintained otherwise fastness will be inferior and color will be unstable.
CARRIER DYEING METHOD:
- At first, a paste of dye and dispersing agent is prepared and then water is added to it.
- Dye bath is kept at 60°C temperature and all the chemicals along with the material are added to it. Then the bath is kept for 15 min without raising the temperature.
- pH of bath is controlled by acetic acid at 4-5.5.
- Now temperature of dye bath is raised to 90°C and at that temperature the bath is kept for 60 min.
- Then temperature is lowered to 60°C and resist and reduction cleaning is done if required. Reduction cleaning is done only to improve the wash fastness.
- Material is again rinsed well after reduction cleaning and then dried.
HIGH TEMPERATURE DYEING METHOD:
- At first a paste of dye and dispersing agent is prepared and water is added to it.
- PH is controlled by adding acetic acid.
- This condition is kept for 15 minutes at temperature 60°C.
- Then the dye bath temperature is raised to 130°C and this temperature is maintained for 1 hour. Within this time, dye is diffused in dye bath, adsorbed by the fiber and thus required shade is obtained.
- The dye bath is cooled as early as possible after dyeing at 60°C.
- The fabric is hot rinsed and reduction cleaning is done if required.
- Then the fabric is finally rinsed and dried.
Thermosol Dyeing Method:
Dyeing of Polyester Fabric in Thermosol Dyeing Method
Thermosol method is continuous methods of dyeing with disperse dye. Here dyeing is performed at high temperature like 180-220°C in a close vessel. Here time of dyeing should be maintained very carefully to get required shade and to retain required fabric strength.
Pading—-→Drying—-→Thermo fixing—-→After treatment
- At first the fabric is padded with dye solution using above recipe in a three bowl padding mangle.
- Then the fabric is dried at 100°C temperature in dryer. For dyeing, infra red drying method is an ideal method by which water is evaporated from fabric in vapor form. This eliminates the migration of dye particles.
- Then the fabric is passed through thermosol unit where thermo fixing is done at about 205°C temp for 60-90 seconds depending on type of fiber, dye and depth of shade. In thermosol process about 75-90% dye is fixed on fabric.
- After thermo fixing the unfixed dyes are washed off along with thickener and other chemicals by warm water.
- Then soap wash or reduction cleaning is done if required. And finally the fabric is washed.
By this experiment we have acquired so much knowledge about dyeing of polyester with disperse dyes. We have known too much end uses of disperse dye.
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Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.