Function of Salt in the Textile Wet Processing

Why Salt is Used in Dyeing?
The textile substrate and dye molecule, not necessarily should have of homogeneous characteristics to combine with each other. In such case, we require some catalyst to facilitate dyeing action on fabric. Salt plays this crucial role of catalyst. Salt has an extremely high affinity for water. Broadly speaking, Salt is necessary in three ways, firstly, to drive dye into textile during the dyeing process in textile. Secondly, use of salt leads to maximum exhaustion of dye molecules during dyeing process in textiles. Thirdly it is used as an electrolyte for migration, adsorption and fixation of the dyestuff to the cellulose material.

Salts plays important role in reactive dyeing by improving the affinity of the dyestuff towards the fibre and acceleration of the dyestuff's association and lowering its solubility. Normally, Glauber's salt or common salt/ vacuum salt is used for this purpose. The presence of chlorine ion in the common salt may cause corrosion of the equipment. Hence, Glauber's salt is always preferred over common salt. Glauber's salt is a common name for sodium sulfate decahydrate, Na2SO4.10H2O; it occurs as white or colorless monoclinic crystals. Upon exposure to fairly dry air it effloresces, forming powdery anhydrous sodium sulfate. Johann Glauber’s was the first to produce the salt (from Hungarian spring waters). Glauber's salt is water soluble, has a salty, bitter taste, and is sometimes used in medicine as a mild laxative; it is also widely used in dyeing. Vacuum salt is the common name of sodium chloride (NaCl).

Role of Inorganic Salt in Reactive Dyeing:
Inorganic salts have two main functions in exhaustion dyeing with reactive dyestuffs:
  1. Improving the affinity of the dyestuff
  2. Acceleration of the dyestuff's association and lowering of its solubility.
Generally reactive dyes contains sulphonic acid (-SO3H) group which is insoluble in water. During the manufacturing of the reactive dyes these sulphonic acid groups are converted into the sodium salt of sulphonic acid (-SO3Na) which is soluble in water.

Reactive dye – SO3H + Na⁺ → Reactive dye SO3Na

Generally when the reactive dye goes in the water, it is solublised giving dye anions and sodium cations

Reactive dye – SO3Na + Water -- → Reactive dye – SO3⁻ + Na ⁺

                                                             (Dye anion)           (Sodium cation)

Dyeing Mechanism:
In actual dyeing mechanism vegetable fibres contains cellulose which ionizes in the water

Cell – OH -- → Cell – O⁻ + H⁺

While reactive dye goes in the water, it is soubise giving dye anions and sodium cations

Reactive dye – SO3Na + Water -- → Reactive dye – SO3⁻ + Na ⁺

                                                            (Dye anion)               (Sodium cation)

During dyeing both the negative ions of dye and cellulose repels each other in the absence of salt and thus no exhaustion or very little exhaustion is done but in the presence of salt , it will ionize as follows,

NaCl -- → Na ⁺ + Cl ⁻ (Common Salt) or

Na2SO4 - → 2 Na ⁺ + SO4 ⁻ (Glauber’s Salt)

Thus the salt neutralize the negative ion of the cellulose and facilating the exhaustion,

(Cell – O⁻ + H⁺ )+ (Na ⁺ + Cl ⁻ )-- → Cell – ONa

Cell – ONa + SO3⁻ – Reactive dye - → Cell – O – Reactive dye

                                                        (Exhausted dye on the substrate)

Thus the presence of salt in the reactive dyeing increases the affinity of the dye towards the Cellulosic substrate. Since reactive dyes have low affinity for cellulose, the fixation can be increased by exhausting the dye bath by adding Glauber’s salt prior to fixation. The amount of the salts required to produce adequate exhaustion decreases with decreasing liquor ratio.

Function of Salt in the Dyeing Process:
  1. The salt in the reactive dyeing increases the affinity of the dye towards the Cellulosic substrate.
  2. Salt increases the exhaustion rate of reactive dyestuffs.
  3. As reactive dyestuffs have a lower affinity, more inorganic salt is required when using reactive dyestuffs in order to accelerate absorption.
  4. While the amount of inorganic salt used varies according to the type of dyestuff used, recently developed high-fixation dyestuffs with improved affinity allow the amount of inorganic salt to be reduced.
Due to considerations of effectiveness and cost, both Glauber's salt and common salt (sodium chloride) are used in dyeing. In terms of their role as an inorganic salt, these two are effectively the same because of the sodium cation active in both.

Author of This Article:
Md. Jasimuddin Mandal
Govt. College of Engineering and Textile Technology,
Serampore (Under West Bengal University of Technology)
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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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