Reactive dyes exhaustion

In the name Procion Red H-E 7B, Procion is the Zeneca trade name for its range of reactive dyes for cotton. Red denotes the main color of the dye. H-E denotes the dye to be hot dyeing and an exhaust dye (high fixation), and 7B denotes it to be a very bluish red dye, ie, a magenta. 

A reactive dye for ceUulose contains a chemical group that reacts with ionized hydroxyl ions in the ceUulose to form a covalent bond. When alkaH is added to a dyebath containing ceUulose and a reactive dye, ionization of ceUulose and the reaction between dye and fiber is initiated. As this destroys the equihbrium more dye is then absorbed by the fiber in order to re-estabUsh the equUibrium between active dye in the dyebath and fiber phases. At the same time the addition of extra cations, eg, Na+ from using Na2C02 as alkaH, has the same effect as adding extra salt to a direct dye. Thus the addition of alkaH produces a secondary exhaustion. 

Electrolyte therefore plays three important roles increasing absorption in the neutral state, preventing desorption/promoting secondary exhaustion, and increasing the amount of ioni2ed ceHulose. Thus the amounts of salt used in the apphcation of fiber-reactive dyes are larger than for direct dyes. 

Eig. 3. Amounts and forms of fiber-reactive dye on the fiber as a function of time for a low affinity dye, where X represents the reactive group. Point A represents the amount of dye exhausted in neutral conditions B is the total amount of dye exhausted at the end of the dyeing process, ie, [dye—OH] +... 

Cold Exhaust Dyeings Fiber-Reactive Dyes. Start at 25—30°C optionally with a sequestrant and maintain. The dye is added over 5 min, then there is portionwise addition of salt every 10—15 min, increasing the size of the addition each time over 1 h. The amount of salt used (10—100 g/L) depends on the depth of shade. After the final addition of salt, wait 15 min, portionwise add soda ash (10—20 g/L) over 15 min, and continue dyeing for 30—45 min. Drop dyebath, cold water rinse, and use a sequence of hot washes to remove all loose "unfixed" dye. 

Electrolytes are used to promote the exhaustion of direct or reactive dyes on cellulosic fibres they may also be similarly used with vat or sulphur dyes in their leuco forms. In the case of anionic dyes on wool or nylon, however, their role is different as they are used to facilitate levelling rather than exhaustion. In these cases, addition of electrolyte decreases dye uptake due to the competitive absorption of inorganic anions by the fibre and a decrease in ionic attraction between dye and fibre. In most discussions of the effect of electrolyte on dye sorption, attention is given only to the ionic aspects of interaction. In most cases, this does not create a problem and so most adsorption isotherms of water-soluble dyes are interpreted on the basis of Langmuir or Donnan ionic interactions only. There are, however, some observed cases of apparently anomalous behaviour of dyes with respect to electrolytes that cannot be explained by ionic interactions alone. 

Polyepichlorohydrin and dimethylamine Polymerisation of epichlorohydrin in carbon tetrachloride with boron trifluoride/ether catalyst, then reaction with dimethylamine. Applied to cotton by exhaust method or pad-dry. Scheme 10.65 Good yields with direct dyes using only 2 g/l salt. Excellent build-up with most reactive dyes only 10% of normal salt usage needed for low-reactivity dyes and none for highly reactive types. Washing fastness very good but light fastness impaired. 

Dyes that have their optimal fixation temperature between 40 and 60 °C belong to this group. These dyes are characterised by relatively low exhaustion in neutral solution before the addition of alkali. This type of dye has high reactivity and careful addition of alkali must be made in order to obtain level dyeing. For these reasons, the name alkali-controllable reactive dyes has been chosen. 

This group includes dyes that have their optimal fixation temperature between 80 and 95 °C. Such dyes show comparatively high exhaustion before fixation so it is important to ensure that dyeings are level. Salt should be added portionwise at specified stages during the exhaustion process, hence they are termed salt-controllable reactive dyes . 

Figure 12.17 Dye exhaustion and fixation profiles in exhaust dyeing with atypical reactive dye [141]...

Table 12.21 Effect of a single washing treatment for various times at various temperatures on the removal of unfixed dye from exhaust or continuous dyeings with chlorodifluoropyrimidine reactive dyes [173]... 

Carliell CM, Barclay N, Buckley CA (1996) Treatment of exhausted reactive dye bath effluent using anaerobic digestion laboratory and full-scale trials. Wat SA 22 225-233... 

Ionisation of the hydroxy groups in cellulose is essential for the nucleophilic substitution reaction to take place. At neutral pH virtually no nucleophilic ionised groups are present and dye-fibre reaction does not occur. When satisfactory exhaustion of the reactive dye has taken place, alkali is added to raise the pH to 10-11, causing adequate ionisation of the cellulose hydroxy groups. The attacking nucleophile ( X ) can be either a cellulosate anion or a hydroxide ion (Scheme 7.8), the former resulting in fixation to the fibre and the latter in hydrolysis of the reactive dye. The fact that the cellulosic substrate competes effectively with water for the reactive dye can be attributed to three features of the reactive dye/ cellulosic fibre system ... 

Silk can be readily dyed with conventional high-reactivity dyes of the dichlorotriazine, dichloroquinoxaline or difluoropyrimidine classes. Exhaust dyeing at 60-70 °C and pH 5-6 gives satisfactory results, especially if a mildly alkaline aftertreatment is given to enhance fixation. Dichlorotriazine dyes can also be applied by pad-batch dyeing with bicarbonate and a batching time of 4-6 hours. The relatively low reactivity of aminochlorotriazine dyes, however, results in moderate to poor build-up on silk. Tertiary amine catalysts such as DABCO (7.66) can be used to accelerate the dye-fibre reaction and increase the fixation substantially [116], but it is difficult to achieve satisfactory compatibility in mixture dyeings by this method (section 7.4.2). 

Scheme 7.62 together with a variety of reactive intermediates or crosslinking agents intended to link together the dye and the substrate, raise some important questions. This research has encompassed variants of such systems on cotton, wool and nylon, mainly by exhaust application. Conventional reactive dyes have so far accounted for only about 20% of all dyes used worldwide on cellulosic fibres. Reactive dyes amount to only about 5% of all dyes used on wool and their current usage on nylon is negligible. 

Practically all the commercially important reactive dyes based on activated double bonds employ the vinyl sulfone reactive group (2.35), usually as the sulfatoethylsul-fone derivative (2.34), which converts into (2.35) in the dyebath under the influence of alkali at pH 11 to 12.5. They are usually applied onto the fibre by exhaustion from the dyebath at around 40-60 °C or by cold padding at 30 °C. The vinylsulfone group is most commonly attached to the dye chromophore via an aromatic ring, e.g. Cl Reactive Black 5 (2.2), but in a few cases a bridging aliphatic group is employed. ... 

Application Methods. There are many detailed application methods used for applying reactive dyes, and all have been described in detail. Examples of the main methods include cold exhaust dyeing fiber-reactive dyes, warm, hot exhaust dyeing dyes, migration exhaust technique for less than 0.5% depth of shade, all-in method, continuous dyeing, and cold pad-batch dyeing. 

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