Why Is Salt 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 for 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).
Functions Of Salt In Dyeing Process
The salt in the reactive dyeing increases the affinity of the dye towards the Cellulosic substrate. Salt increases the exhaustion rate of reactive dyestuffs.
As reactive dyestuffs have a lower affinity, more inorganic salt is required when using reactive dyestuffs in order to accelerate absorption.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.
Role Of Salt In Reactive Dyeing
Inorganic salts have two main functions in exhaustion dyeing with reactive dyestuffs:
- Improving the affinity of the dyestuff.
- Acceleration of the dyestuff’s association and lowering of its solubility.
Generally, reactive dyes contain 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 the sulphonic acid (-SO3Na) which is soluble in water.
Reactive dye – SO3H + Na⁺ → Reactive dye SO3Na
Figure Salt reaction
Generally, when the reactive dye goes in the water, it is solubilised giving dye anions and sodium cations
Reactive dye – SO3Na + Water — → Reactive dye – SO3⁻ + Na ⁺
(Dye anion) (Sodium cation)
Why Salt-Free Dyeing?
In recent years there has been an increasing awareness about environmental friendliness in all human activities. The textile industry is a water intensive industry with water being used in every stage of wet processing from sizing, desizing, scouring and bleaching of fibres to the dyeing, finishing, and printing of fabrics. Every textile plant requires large volumes of water and produces high volumes of effluent wastewater. The typical textile dye wastewater composition is quite complex. The demand for environmental friendly dyes and application processes is therefore very strong.
Reactive dyes have become very popular for cotton due to its brilliance, variety of hue, high wet fastness, convenient usage and high applicability
Reactive dyes are anionic in character and cotton fibres also adopt anionic surface charge in water causing limited exhaustion of dye due to charge repulsion. Large quantities of electrolyte (30-100 g/l) are thus added to overcome this problem. One of the major problem of reactive dyeing is the large amount of electrolyte required for exhaust and pad application which leads to the environmental problem. In addition, inadequate dye exhaustion and fixation result in colored effluents. As environmental problems arising from dyeing with reactive dyes have become critical, many studies have been devoted to improving the substantivity of cotton fibre for reactive dyes, thus reducing or eliminating the amount of electrolyte used.