Dyeing of Reactive Dyes by Exhaust Method

 

REACTIVE DYES

Choice of Reactive class of Dyes has become indispensable for application of colours on the cellulosics to provide bright range of shades with reasonably good fastness features. No other class of colours can boast of the versatile range of shades with unmatched brilliance, yet economically viable and cost effective that this class of dyes can offer. Even as Reactive dyes are most popular for dyeing solid shades it is equally sought after for various resist and discharge printing styles, thanks to its suitability to be resisted or discharged readily and effectively 

The reaction mechanism is apparently simple in that on just altering the pH after exhaustion, formation of covalent bonds between the reactive group of the dye and the OH of cellulose proceeds. For the same reason of ready reactivity with Cell OH groups, it reacts with Water also to get hydrolyzed in which state the dye behaves no better than a direct cotton dye. The management of the various factors/variables that govern the transport of dye uniformly from an aqueous bath to the cellulose substrate and its preferential reactivity to the fibre than to water is far more complex and critical to perform to obtain a satisfactory dyeing. As the shades invariably are tertiary matchings, the behaviour of individual dyes with different exhaustion and reactivity characteristics, all the more compounds the complexity of the problems of differential shade build up, variations, uneven dyeings, reproducibility, fastness etc multifold. 

Though there are other methods of dyeing ‘Reactives’ like pad batch, pad –dry-cure or pad-dry-steam etc exhaust dyeing is practiced widely because of its flexibility to process fabrics in rope form and in the case of yarn and other packages, exhaust dyeing is the only alternative as on date. Tubular knit-ware, by its very physical form is more amenable to exhaust dyeing in ‘rope‘s form; however, advanced machineries obtainable in recent years claim satisfactory open width dyeing by Pad Batch technique.

The exhaust method of dyeing would include the following phases 

1. Primary exhaustion phase /Migration 

2. Secondary exhaustion phase, 

3. Fixation (Reaction) phase -Secondary exhaustion and Fixation can run concurrently/over lapping. 

4. Washing off phase. 

EXHAUSTION PHASE

Primary Exhaustion Phase

Exhaustion of dye from the dye bath to the cellulose during Primary Exhaustion phase is governed by the following three physical processes and the phenomenon of substantivity

• Adsorption

• Diffusion,

• Absorption/ Exhaustion/Migration

Adsorption

It would be relevant to briefly look at cellulose structure with respect to its Hydrogen bonding behaviour at the surface layers and in the interiors of the cellulose micro fibrils The interior layers contain both forms - 1Alpha and 1 Beta of Cellulose molecular chains that are packed compactly and there are intra molecular Hydrogen bonding parallel to the 1.4 Beta Glucoside link (OH of #2 to  #6 of  the succeeding glucose unit and  #3 OH with the ring O of the preceding Glucose Unit)  that stabilize the cellulose chain.

The other four hydroxyl groups are fully free for Hydrogen bonding. At the surface layers of cellulose even the O-3 (OH) and 2-6 Hydrogen bondings are reported to be absent and therefore all the six Hydroxyl groups in the Cellobiose repeat units at the surface are free to attract Hydrogen bonding with the water molecules.

Adsorption in an exhaust dyeing process is fundamentally the inter-phase phenomenon of a dye (solute) in its solution in water coming in to surface contact with the substrate and forming a surface layer/ coating. That is the starting phase for the rest of the diffusion and absorption phenomenon.  In the case of Cellulose exposed to a dye solution in water at slightly acidic pH there is no ionization of cellulose. However, with abundance of ‘free’ OH groups available at the surface (six numbers in each of the repeat Cellobiose unit), water molecules are drawn in clusters around the cellulose molecules to form hydrogen bonds causing an overall charge separation. Resultant surface thus carries a negative charge known as the zeta potential .

This surface negative charge would repel the advances of the negatively charged ionized dyestuff anions. The zeta potential is partially overcome due to the presence of large amount of dye anions, some of which are forced across the electron cloud through increase in energy (raise in temperature) or through mechanical agitation to come within the effective distance for the inter molecular forces like Wander Vaal’s forces/secondary valence forces to facilitate the dye anion to get adsorbed on the surface of cellulose. Presence of electrolyte also helps in providing the positive charge that can effectively neutralize the zeta potential and improve the adsorption. (Discussed under ‘Role of Electrolyte’)
Diffusion phenomenon takes over followed by the absorption and migration of dyestuff across the cellulose membrane. Diffusion is influenced by the concentration gradient across the interface of cellulose surface and dye bath, the surface area of the cotton substrate in contact with the dye bath, temperature and time and the physical characteristics of the substrate. This is termed as the primary exhaustion phase. The term exhaustion would include the collective phenomenon of adsorption, absorption diffusion and migration in that order.

Diffusion

Diffusion process is explained by the relationship (Ficks Law of Diffusion in its simplest form.) 
F =      -D (C1-C2) / L                 And  D = Do e -E/RT        
Where
F = Mass flow of dye                gms/cm2 sec
D = Diffusion coefficient of the             dye m2/sec
D0 = Diffusion Coefficient at Infinite Temperature
C1 = Concentration of dye in the dye bath           g/cm3
C2 = Concentration of dye on surface of the fiber            g/cm3
L = Thickness of the layer          cm
e, E, R = Constants (E activation Energy; e  exponential;  R  Universal Gas Constant)
T = Temperature Kelvin

Applying the above relationship the following dynamics may be inferred during the diffusion / exhaustion stages of the dye to the cotton substrate.

F is the dyestuff sorbed across Unit area of the fiber surface in unit time (Rate) Greater the surface area of the fiber in contact with the dye bath greater is the dyestuff sorbed.  (C1-C2) concentration gradient during the process of diffusion.  The concentration gradient at the initial stages would be higher and therefore the rate of dyestuff transport to the fibre phase will be correspondingly higher tending towards zero at equilibrium.  D Diffusion coefficient Higher the Diffusion coefficient, lesser the time taken to reach the equilibrium. Time taken for dyeing 50% of the equilibrium depth of shade is an index of the speed  Temperature Increase in Temperature increases Diffusion coefficient.

Since surface area is a factor, the characteristics of the fiber and construction would influence the diffusion. Nature of cotton from different sources would have different shape, cross section, micronaire, fineness, impurities, etc and different packing densities of the cellulose molecular chains thus altering the surface area characteristics.  The corollary is that thinner the fibre/count and lower the density factor greater is the surface area available and better would be the diffusion.

Substantivity

The term substantivity is primarily a measure of the amount of the molecular dye chromophore that can penetrate/diffuse into the interstices of cellulose micro fibrils assisted by physical forces from an aqueous dye bath. This is influenced by the salt concentration in the dye bath, the liquor ratio, the temperature and the fibre surface area characteristics, besides the chemistry of the dye chromophore. Substantivity ratio is the unit concentration of dye on the fibre to the unit concentration of dye in the bath at the equilibrium state (both expressed in the same units)

The process of primary exhaustion proceeds to its limiting values dictated by the substantivity beyond which it ceases. In the absence of salt, the dye uptake by substantivity phenomenon as stated above is around 20 to 40% of the starting bath concentration or lower, a figure far too low to have any significant economically feasible colour yield. Therefore, as a general rule, without salt additions, substantvity by primary exhaustion of Reactive dye to cellulose cannot be improved or maximized, at the present status of Colouration technology.

[Efforts are on for reduced salt /salt-less systems based on changes in the chemistry of the dyes to exhibit reduced anionic behaviour, fibre substrate modification/sensitization to display cationic behavior to induce exhaustion with less/no salt, while retaining the reactive system for the ultimate fixation. Such developments are still in the R&D Labs and not presently available for bulk]