Stages/Steps Involved in Reactive Dyeing

Steps Involved in Reactive Dyeing:
Three basic steps involves in reactive dyeing are as follows:
  1. Exhaustion of the dye from an aqueous bath containing common salt or Gluber’s salt normally under neutral condition (PH – 7).
  2. Addition of an alkali to promote to further dye uptake and chemical reaction of absorbed dye with the fiber.
  3. Dyed material is then rinsed and soaped to remove electrolyte, alkali and unfixed dye. 
Dissolving the Dyes:
The dyes are in most cases readily water-soluble. They are dissolved either by passing with cold water to which is then added hot water or by strewing the dye powder into hot water which is stirred at high speed. Usually a temperature not greater than 80ºC is used for dissolution. In the case of highly reactive dyes such as Procion MX (ICI) warm (50-60ºC) water is used. Since the dyes are prone to hydrolysis stock solutions should not be stored for long periods.

Reaction with Cellulosic Fiber:
Reactive dyes formed covalent bond with cellulosic fiber by Nucleophilic substitution and addition reaction. The reactions are as follows ---

Substitution reaction:

D – R – Cl + HO - Cell ® D – R – O – Cell + HCl

Addition reaction:

D – F – CH2 = CH2 + HO – Cell ® D – F – CH2 – CH2 – O – Cell

Reaction with water:
The cold brand reactive dyes are highly reactive in nature. They react also with water and get hydrolyzed. The reaction is as follows ----

D – R – Cl + HOH ® D – R – OH + HCl

The hydrolyzed dye is also colored substances and has very low substantivity for the cellulosic fiber. So they are attached with fiber surface after dyeing. If they do not washed away from the fiber surface the fastness property of the fiber must be very low.

Salts Used for Reactive Dyeing:
Depends on the substantivity to increase the exhaustion, salt like common salt (NaCl) or Glouber’s salt (Na2SO4) is used.

Alkalis for Reactive Dyeing:
The common alkalis used for reactive dyeing are sodium bi-carbonate (NaHCO3), sodium carbonate (NaCO3), sodium hydroxide (NaOH), sodium silicate (Na2SiO3) and sodium phosphate (Na3PO4). Sodium silicate is very strong alkali and only used for deep dyeing. Sodium carbonate is mostly used alkali in reactive dyeing.

Eliminating the Hydrolyzed Dye:
The hydrolyzed dye is also colored substances and has very low substantivity for the cellulosic fiber. They tend to stay on the fiber surface after dyeing for sometimes although there is no bond between the dye and the fiber. These unfixed dyes come of the fibers when washed or rubbed subsequently.

If they do not washed away from the fiber surface the rubbing and wash fastness property of the fiber must be very low.

The presence of electrolytes in the washing water causes an increase in the hydrolyzed dye affinity making it difficult to extract, therefore very saline water must be avoided even if this water conditioned.

Discharging Defective Dyeing:
It cannot be said that dyeing with reactive dyes excessive leveling problems, since the dyes have a low affinity.

However, it can happen that it is necessary to discharge a dyeing, which can be more or less difficult depending on the situation of the dye in relation to the fiber.

Thus, in the case of cellulosic fibers dye discharging before the alkalinization of the dye – bath does not present an difficulty, since hot washing with a detergent with extract the main part of the dye deposited in the fiber, as it has a low affinity.

When instead the dye has already been covalently fixed to the fiber, it is more difficult to eliminate, as it is necessary to destroy the molecule chemically by means of oxidizer and or reduction processes.

In the reduction process, the process is started at 50 – 600C the temperature is raised to 980C and held for 20 minutes, and then the goods are rinsed in hydrogen peroxide before re – dyeing.

Absorption of the Dye:
This is the first dyeing phase, that takes place in the same way as cellulose dyeing with non-reactive soluble dyes; in this phase the reactive dye does not decompose, but only diffuses towards the interior of the fiber, when it is absorbed by the cellulosic chains by secondary type forces.

The following parameters influence the absorption:
  1. Nature of the dye.
  2. Liquor ratio.
  3. Electrolyte concentration.
  4. pH.
  5. Temperature.
  6. Type of fiber. 
 

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