Cotton Dyeing

 

 

Cotton Dyeing: Cotton can be dyed with the following dyestuffs., viz., Direct, Reactive, Vat, Suplur, Pigment and Solubilized vat dyes. Cotton is anionic in nature and can be dyed easily with cationic chromophores.

Batch Dyeing: When small quantities of up to 300 to 500 meters or 100 to 500 kgs of yarn or fiber to be dyed, then batch dyeing machines would be used for dyeing. Cotton batch dyeing machines are viz., open beck dyeing machine, spray dyeing machine (for hank yarn dyeing), cabinet dyeing machine (for hank dyeing), jigger and winch dyeing machines for small fabric batches, enclosed yarn and fiber dyeing machines and jet dyeing machines.

Semi Continuous and Continuous dyeing: When medium size batches of fabric length 1000 to 2000 meters are to be dyed, then Cold Pad Batch method is most suitable with reactive dyes.With vat dyes pad, dry, jig develop is suitable. When long batch lengths of more than 3000 meters of a same shade to be dyed then continuous dyeing - pad - dry- chemical-pad - dry - steam or thermosol dyeing is the most suitable methods.

Direct dyes: The name direct dye explains that this dye application is simple and direct. The dyestuff should be dissolved in water and the well prepared RFD (Ready For Dyeing) fabric or yarn is entered in to the dye solution and worked for 30 to 45 minutes at boil. Afterwards,the dyed material would be given a short cold wash and fixed with some cationic dye fixing agent.

Reactive dyes: The first synthetic organic dyes were called substantive dyes or direct dyes. These dyestuffs do not have attraction towards cotton fiber. However due to high temperature (90 to 100°C) it has been forced to enter the cellulose molecules. Using a strong cationic fixing agent these dyes used to get fixed on the fiber. But the final fastness towards washing and rubbing of these dyestuffs were very poor. In order to solve this poor wash fastness qualities, a dyestuff that would react with cellulose and become part and parcel of it was developed; since these dyes react with cellulose and form a strong covalent bond, these are called reactive dyes.
Classes of reactive dyes: Reactive dyes are classified according to their chemical names and their reactivity or presence of number of reactive groups. The general classifications are as below:

Vat Dyes: Vat dyes do not dissolve in water, while when reduced to be leuco salt by reducing agent under alkaline conditions, they can dissolve in water and get feature of immediacy with cellulose fibers, which is the way to achieve the purpose of dyeing. Then stable shade and good color fastness would come out via oxidation and soaping.

Reduction of dyes: Insoluble reducing dye will change into soluble leuco.

.Different levels of the best staining methods are brought out according to the reduction dyes' reduction potential and level of its immediacy to fibers.

Dye-uptake of leuco: The leuco is adsorbed by fiber and then it is diffusing into the fiber.

It is necessary to use soft water or softened water in dyeing process. Hereby it is suggested to add the water softener for 1g/L to some poor quality cotton fabric. The water softener would not only absorb calcium, magnesium ions, but also the iron, copper ions which are absorbed by the pipe into the dye bath, and finally keep the solubility of leuco.

The sodium sulfate could increase the dyeing absorption rate, and surely it is also a good promoter. So it can be added into the dye bathes with medium or low immediacy according to the demand.

Batch dyeing characteristics:  No single classification of vat dyes by dyeing characteristics has been as useful  or as generally accepted as has the classification of direct dyes in to groups A, B and C.

One method of classification of vat dyes in to four principle sub-groups: IK, IW, IN and IN Special, which still leaves a few dyes out, such as C.I. Vat Black 9. This classification is based on the different substantivities of the leuco vat anions and the corresponding differences in dyeing temperatures and the salt, caustic soda and hydro concentrations necessary to give the best over all dyeing results. The importance of these traditional groupings is restricted to batch dyeing with a leuco vat anions. substantivities of the leuco vat anions and the corresponding differences in dyeing temperatures and the salt, caustic soda and hydro concentrations necessary to give the best over all dyeing results. The importance of these traditional groupings is restricted to batch dyeing with a leuco vat anions.

The 'I' stands for 'Indanthrene' . "K" stand for the German word, Kalt, meaning cold. Dyes (reduced leuco vat anions) in this group are dyed at room temperature with a relative high salt concentration ( common salt or anhydrous sodium sulphate) and relatively low sodium hydroxide (caustic soda) concentration, all of which tend to promote higher substantivity and exhaustion of equilibrium.Indanthrene' . "K" stand for the German word, Kalt, meaning cold. Dyes (reduced leuco vat anions) in this group are dyed at room temperature with a relative high salt concentration ( common salt or anhydrous sodium sulphate) and relatively low sodium hydroxide (caustic soda) concentration, all of which tend to promote higher substantivity and exhaustion of equilibrium.

"W" stands  for German/English word warm. Dyes in this group are more substantive and can be dyed at 40 to 50 C (100 to 120 F) with less salt and slightly more alkali.

"N" stands for the German/English word normal. Such dyes are even more substantive, require more alkali but no salt, and can be dyed at 60 C (140 F). The IN special dyes require even more alkali.