Reactive dyes hydrolysis

A feature of the chemistry of triazinyl reactive dyes, which is in fact common to all reactive dye systems, is that they undergo, to a certain extent, a hydrolysis reaction that involves reaction of the dye with OH anions present in the aqueous alkaline dyebath in competition with the dye-fibre reaction. The hydrolysis reaction is also illustrated in Scheme 8.1. Reactive dye hydrolysis is a highly undesirable feature of reactive dyeing for a variety of reasons. In the first instance, the hydrolysed dye 175b which is formed is no longer capable of reacting with the fibre and so must be washed out of the fibre after dyeing is complete, to ensure the... 

Fiber-reactive dye is also hydrolyzed by reaction with free OH ions in the aqueous phase. This is a nonreversible reaction and so active dye is lost from the system. Hydrolysis of active dye can take place both in the dyebath and on the fiber, although in the latter case there is a competition between the reactions with free hydroxyl ions and those with ionized ceUulose sites. The hydrolyzed dye estabHshes its own equUibrium between dyebath and fiber which could be different from the active dye because the hydrolyzed dye has different chemical potentials in the two phases. The various reactions taking place can be summarized as in Figure 2. 

The most important discovery in dyeing cellulose with reactive dyes was the appHcation of Schotten-Baumaun principles. Reaction of alcohols proceeds more readily and completely in the presence of dilute alkali, and the cellulose anion (cell- O ) is considerably more nucleophilic than is the hydroxide ion. Thus the fixation reaction (eq. 1) competes favorably with hydrolysis of the dye (eq. 2). 

A reactive dye—cellulose bond is subject to some slight hydrolysis during washing under alkaline conditions. 

Tria2inyl reactive dyes show less tendency toward hydrolysis during washing than do the vinyl sulfone type. 

There is no doubt that the major weakness of the reactive dyeing process is the hydrolysis reaction and the consequent need for a wash-off" process. The extent to which dye hydrolysis takes place in competition with dye-fibre reaction varies quite markedly within the range 10 40% depending upon the system in question. A considerable amount of research has therefore been devoted to the search for reactive dyes with improved fixation. The most successful approach to addressing this issue has involved the development of dyes with more than one fibre-reactive group in the molecule, which statistically improves the chances of dye fibre bond formation. Examples of products of this type are the Procion H-E... 

Copper phthalocyanine derivatives are well established as turquoise blue direct and reactive dyes for cellulosic fibres. Chlorosulphonation at the 3-position, followed by hydrolysis, yields sulphonated direct dyes such as Cl Direct Blue 86 (5.32 X = H) and Blue 87 (5.32 X = S03Na). Solubility and dyeing properties can be varied by introducing four chlorosulphonyl groups, some of which are hydrolysed and some converted to sulphonamide by reaction with ammonia or alkylamines. This approach is also the main route to reactive dyes of the copper phthalocyanine type. The reactive system Z is linked to a 3-sulphonyl site... 

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 ... 

Further work with the same dye (7.43) and carbodiimides (7.44 and 7.45) concentrated on this problem of limited efficiency. Cotton fabric padded with the dye phosphonate solution was aftertreated with the carbodiimide dissolved in various alcoholic solutions to avoid hydrolytic decomposition. Under these conditions cyanamide was much more effective than dicyandiamide. With conventional reactive dyes the efficiency of the dye-fibre reaction is limited by competing hydrolysis of the active dye. Although phosphonated or carboxylated reactive dyes do not hydrolyse, their level of fixation is limited by competing hydrolysis of the carbodiimide activator [46]. 

The most commercially successful reactive dye of all, Cl Reactive Black 5 (7.36) contains two sulpha toe thylsulphone precursor groups that contribute markedly to its initial solubility. When these are hydrolysed in alkali to release the reactive bis(vinylsulphone) form, the considerable increase in substantivity (Table 7.3) leads to highly efficient fixation. Further hydrolysis of the vinylsulphone groups to give the inactive bis(hydroxyethylsulphone) derivative, however, lowers the substantivity and hence contributes to favourable wash-off performance. 

Rate constants and the products formed in the hydrolysis of Cl Reactive Red 194 (7.76) at 50 °C and pH values in the 10-12 region were determined by high-pressure liquid chromatography. In addition to the normal hydrolysis of the two reactive systems, the imino link between the triazine and benzene nuclei was also hydrolysed [67]. The heterobifunctional copper formazan dye Cl Reactive Blue 221 and two blue anthraquinone monofunctional reactive dyes of the bromamine acid type, namely the aminochlorotriazine Blue 5 and the sulphatoethylsulphone Blue 19, were compared in terms of their sensitivity to... 

The kinetics of alkaline hydrolysis of a series of eleven vinylsulphone reactive dyes fixed on cellulose have been investigated at 50 °C and pH 11. Bimodal hydrolytic behaviour was observed under these conditions, the reaction rates being rapid at first but becoming slower as the concentration of fixed dye remaining gradually decreased. These results were attributed to differences in the degree of accessibility of the sites of reaction of the dyes within the fibre structure [87]. 

In contrast to cellulosic dyeing with reactive dyes, the fibroin-dye bonds are remarkably stable in aqueous media of pH 4 to 10 [117]. Since there exists only a negligible amount of bond hydrolysis even at high temperature and in a medium of pH 2, the cleavage of the fibroin-dye bond is not a problem in reactive-dyed silk. The stability of these bonds when dyeing with difluoropyrimidine dyes is the highest in both acidic and basic media [118]. 

RP-HPLC was also employed for the investigation of the hydrolysis behaviour of two fluorotriazine reactive dyes Cibacron blue F-R and Cibacron yellow F-4G. Chromatographic measurements were performed in an ODS column (50 X 4.6 mm i.d. particle size 5 /jm) at room temperature. The mobile phase was ACN-0.05 M ammonium acetate buffer containing 1 mM acetyhrimethylammonium bromide (47 53, v/v). Flow rates were 0.8 and 0.6 ml/min depending on the dyes to be separated. Dyes were detected at 275 nm. The hydrolysis of dyes was investigated both in the absence and presence of... 

Figure 2.18 Dye fibre bond formation and hydrolysis in reactive dyeing of cellulose.

Sulfonated azo dyes (reactive dyes) are widely used in the textile industry. Due to the simultaneous hydrolysis in the dyeing process, 15-60 % of the reactive dyes reach the waste water system. In addition, their use strongly increases with the colouring of natural fibres, and so a total of 60 % of all dyes emitted to the waste water are reactive materials. 

Most mono-anchor dyes are derivatives of cyanuric chloride (2,4,6-trichloro-1,3,5-triazine [108-77-0]), a molecule of wide synthetic potential because the three chlorine atoms on the triazine ring differ in their reactivities [12], The first chlorine atom exchanges with nucleophiles in water at 0 - 5 °C, the second at 35 -40 °C, and the third at 80 - 85 °C. A wide variety of triazinyl dyes can thus be prepared by careful selection of the reaction conditions. Condensation of cyanuric chloride with a chromophore ( Chrom. ) containing an amino group yields the highly reactive dichlorotriazinyl dyes 1 [11]. These very reactive dyes are sensitive to hydrolysis, and a suitable buffer is usually added to the powdered dye to increase its stability [13],... 

On leather, reactive dyes attach to the amino group of lysine and hydroxyly-sine moieties of collagen. A tanning effect may occur if one reactive group reacts with leather. However, the reaction of the electrophilic group of reactive dyes with water (hydrolysis) competes with the fixation reaction of forming a covalent bond between the dye and the substrate. The hydrolyzed dye cannot react with the fiber. Leather absorbs the noncovalently bound dye like a conventional anionic dye. Unlike on textiles, these hydrolyzed dyes cannot be easily washed off. That is the reason why sometimes no decisive wetfastness improvement can be achieved. 

Long-term dye stability—Low-temperature reactive dyes will slowly hydrolyze with water over long periods of time, even at room temperature. To prevent slow hydrolysis, the dye impregnation bath was kept at a slightly acid pH (- 6.0). This technique proved successful, with dye solutions maintained for several weeks without detectable degradation. 

The requirement for alkali in the application of reactive dyes to cotton leads to an undesirable side reaction, namely hydrolysis of the reactive groups before dye-fiber fixa-... 

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