Influence of pH in Reactive Dyeing:

Let us discuss about the general quality of all reactive classes now in the market.

All reactive dyestuffs are very much sensitive to the concentrations of alkali and pH in turn. The cleavage of one or two chloride ions from the dyestuff molecule and the reaction of the rest of the dyestuff molecule with the secondary hydroxyl group of cellulose, producing a strong covalent bonding is the essence of reactive dyeing in total.

But how much alkali and how much pH is required for each class of reactive dyestuff depend on the reactivity of the dyestuff itself.

M BRAND DYES are highly reactive requiring comparatively milder conditions in dye fixation. They are primarily of interest in dyeing at normal room temperature (about 25º - 30º Deg. C) using Soda Ash or Sodium Bicarbonate. M BRAND dyes are applicable to cellulosic textiles by Batch-wise, Semi-continuous and Continuous methods in conventional textile machinery like open-vat, Package, Jet, Jig, Winch and padding mangle. The versatility in various application procedures and the availability of wide range of bright and fast shades are the outstanding features of this class of dyestuffs.

Due to the very high reactivity of this class of dyestuff, they require lesser amount of alkali for fixation. A pH of 9.3 to 9.8 will be sufficient for proper fixation of the dye to cellulose.

Salt & Alkali Requirement for Cold Brand Reactive Dyes

Depth of Shade %	Salt g/l (Dye Liquor)	Soda Ash g/l (Dye Liquor)
Up to 0.5	30	3
0.5 to 2.0	40	4
2.0 to 4.0	50	7
Above 4.0	60	10

Since these dyestuffs are highly reactive, they are susceptible for easy hydrolysation even before getting fixed on to the fiber. There is a possibility of the dyestuff getting wasted due to prior hydrolysis in the water medium itself and a lot of unfixed dyestuff will be drained out in to the effluent. And the fastness level also will be poor.

The dyestuff should be stored in cool and dry conditions. The containers should be tightly closed when not in use. High temperature, Humidity and alkaline pH conditions reduce the stability of the dyes in solution

' VS' dyestuffs posses poor affinity for cellulosic fibers in absence of salt and alkalis. For this reasons, they are suitable for use on pad. Their substantivity can be increased by addition of glauber's salt or common salt and alkali, making the dyestuff suitable on all conventional dyeing machine for loose material, yarn in hanks and packages and piece goods. The dyeing methods may therefore, be classified under the padding and exhaust processes, standing baths are not recommended, as in alkaline medium inactivation of the dyestuffs by reaction with water takes place as a side reaction.

These dyes require higher dosage of alkali and salt for exhaustion and fixation. But these dyes have on continuous contact with the alkali and water medium for longer duration time dyeing dyeing process, about 30 to 40% will get hydrolyzed and become in active as dyestuff. But in order to make active and fix on the fiber we need to give higher dose of alkali and salt along with temperature of up to 60°C.

HE Dyes are Reactive Dyes for cellulosic material and are designed to give high fixation by exhaust dyeing methods when applied at the temperature 85° to 90º C. HE dyes are suitable for dying cotton and other cellulosic materials. These dyes process significantly higher exhaustion and Fixation efficiency which result in appreciable cost reduction, in comparison to Conventional reactive dyes. Some advantages of HE Dyes have:-
(i) Excellent compatibility (ii) Good built up. (iii) Excellent Reproducibility (iv) Wide Applicability
(v) Outstanding built up and consistency.

Set the dye bath at 50 and add dyestuff. Run for 15 Minutes. Add half of the glauber's salt and continue Dyeing Raise the temperature to 80º-85º c and run for 15 minutes. Add soda Ash solution and run 30-60 minutes. Wash properly the material.

These dyestuffs require higher alkali concentrations and higher dosage of salt and high amount of heat energy for making it to react and fix with the fiber. The pH requirement would around 10.8 to 11.2.

ME Dyes are low temperature High Exhaust dyes with higher grade of all round fastness properties. Because of low sensitivity to temperature and excellent alkali stability these dyes are considered to be more effective than other all classes dyes for exhaust dyeing. For fixation these dyes require a pH of 10.8 to 11.2 that can be achieved with some small addition of caustic soda also.

Influence of pH in reactive dyeing at every stage of dyeing:

In the beginning of dyeing, the water bath should be carefully adjusted to a neutral to slightly acidic pH, as otherwise premature hydrolysis of dyestuff will take place and cause (a) uneven dyeing and (b) lighter depths than the previous batches or in other words batch to batch variation will occur.
If the fabric or yarn has not been neutralized properly the core alkali presence will adversely affect the dyeing, forming patchy uneven dyeing. The places were alkali residue was high have the tendency to make deeper dyeing.
Lower alkali dosages and hence lower pH leads to partial reaction of reactive dyes; most of the dye may remain in water; the dyestuff that has got absorbed in to the fiber would also have less tendency to get fixed on to it, leading to poor washing and rubbing fastness.
Higher dosage of alkali may cause hydrolysis of dyestuff in the water itself. Thus lower depth of shade and poor washing and rubbing fastness.
After dyeing is over, when the alkali still fully remain on the fiber, if we do not neutralize the alkali properly with adequate quantity of acid, that also leads to higher amount of dyestuff bleeding during subsequent soaping and hot wash operations.
Finally after completing the dyeing, before unloading, if we do not keep the pH neutral - alkaline pH will slowly hydrolyze the dyestuff in the fiber and acid pH will tender the cotton fiber itself.
Every dyestuff appears in different tone under different pH conditions. Bright Lemon yellow, if allowed dry under alkaline pH, it will turn to a dull redder yellow and similarly Turquoise blues and royal blues will appear yellowier and duller in alkaline pH and brighter and redder in acidic pH. So make sure that the pH is exactly neutral or slightly acidic during final drying process.
Final cationic fixation and cationic softening treatment if not done in acidic pH, that will leave higher tonal changes and improper dye fixation and improper softening effect.