Sequestrant function of dyeing auxiliaries
by : Rahul Sharma
Sequestrant:
The most undesirable impurities in Fibre, Common salt, Glauber salt, Caustic Soda and Soda ash are the di- and tri-valent cations, e.g., Ca++, Mg ++ Cu ++, Fe+++ etc. These ions increase hardness of the process bath and generate iron oxides in the bath. Calcium and Magnesium reacts with alkali and precipitates as a sticky substance on the textile material, which creates patchy dyeing and discoloration of the fibre. The ferric oxide with cellulose and creates small pinhole on the fibres also damages the machinery by scale formation in the nozzles and base.
A sequestrant is a food additive which improves the quality and stability of foods. A sequestrant forms chelate complexes with polyvalent metal ions, especially copper, iron and nickel, which can prevent the oxidation of the fats in the food. Sequestrants are a type of preservative.
To overcome these deleterious effects in the scouring and bleaching bath adequate amount of sequestrant must be used. Sequestrants prevent di-and tri-valent metal ions, e.g., Cu++, Fe +++ , Mn ++, Ca++, Mg++ etc from interfering with the chemical processing of the textile material. It prevents catalytic damage of cellulosic fibres in bleaching hath during hydrogen peroxide bleaching.
In Dye-bath Ca++, Mg ++, Fe +++ attack the dye molecules and forms aggregates of molecules which deposits on the fabric as dye spots and also they prevent the reaction process. Dye bath sequestrant should be of different strength than that of the scouring and bleaching baths. Because some dyes have metal compounds and if powerful chelating agents are used than it will attack the metal compound of dye.
