Physical Form And Handling Of Reactive Dyes

All reactive dyes are prone to hydrolysis in the presence of moisture, especially the highly reactive ranges, and they will deteriorate unless carefully handled and stored. Cool, dry conditions are essential and the lids of packages must be firmly replaced after use. Since reactive dyes in powder form may release dust when disturbed, it is always possible for respiratory allergies to arise with some workers who handle them [1]. For this reason suitable dust-excluding respirators should be used and weighing or dissolving procedures should be carried out in ventilated enclosures.

Conventional dye powders are usually dissolved by one of the following techniques:

1. Pasting with cold water followed by the steady addition, with stirring, of the required amount of water at the correct temperature.
2. Sprinkling a steady stream of dye powder into the vortex formed by the high-speed stirring of water at the correct temperature.

Few ranges of reactive dyes require boiling water, although Remazol (HOE) vinylsulphone dye powders are dissolved in boiling water followed by passing immediately through a fine sieve into the required volume of cold water. Highly reactive dyes, such as dichlorotriazine or chlorodifluoropyrimidine types, require water temperatures no higher than 50°C. Most dyes of lower reactivity, e.g. aminochlorotriazine or trichloropyrimidine systems, require a dissolving temperature of 80°C.

The dusting problem with some reactive dye powder brands can be avoided by working with granulated or liquid formulations. A recent improvement that ensures troublefree weighing and handling of small amounts for batchwise dyeing has been the development of cold-dissolving granular brands such as the Drimarene CDG (S) dyes [2–6]. These are non-dusting, free-flowing grains that dissolve readily in cold water and offer ease of handling in automatic dissolving and metering devices.

The marked tendency of reactive dyes to undergo hydrolysis in solution has delayed the development of liquid formulations until recent years. For continuous dyeing and printing, however, especially where automated metering equipment is installed, liquids are particularly convenient. Liquid brands of the relatively stable types, such as sulphatoethylsulphones, aminochlorotriazines and bifunctional dyes containing both of these systems [44], are well established in commercial use. They are essentially isotropic aqueous solutions of the dyes, often with auxiliaries such as a buffer, a hydrotropic agent such as urea or caprolactam, and often a polymeric stabiliser to inhibit settling out on storage [7].

References:

1. J M Wattie, J.S.D.C., 103 (1987) 304.
2. D Link and E J Moreau, Int. Text. Bull., 34 (1) (1988) 13.
3. D Link and E J Moreau, Tinctoria, 85 (1988) 55.
4. D Link and E J Moreau, Chemiefasern/Textilind., 39/91 (1989) 58.
5. D Link and E J Moreau, Textilveredlung, 24 (1989) 87.
6. C Oschatz, Text. Technol. Int., 194 (1991) 7.
7. A H M Renfrew and J A Taylor, Rev. Prog. Coloration, 20 (1990) 1.

Preparation And Chemicals Required To Color With Reactive Dye

Except in special instances, batchwise preparation before reactive dyeing is carried out in the dyeing machine itself or in equipment of similar design reserved for the preparation stage. The essential requirements are that the material must be made available for dyeing in a neutral, uniform and readily absorbent state. In contrast to the application of vat or sulphur dyes, typical reactive dyeing processes will not eliminate natural or added fats and waxes.

Acceptable results are often possible on knitgoods without lengthy pretreatment, as in the pad–batch or hot batchwise dyeing processes in the presence of a powerful wetting agent. Residual size must always be removed from woven goods because of the risk of dye wastage by reaction with hydroxyl groups in size components. Owing to the brilliant hues of many reactive dyes, sufficient brightness may be attainable on woven cotton without prebleaching. Thoroughly desized and scoured fabrics can be used in many cases. Where bleaching is necessary it is imperative to check that all traces of residual chlorine or peroxy compounds are removed prior to dyeing, otherwise loss of reactivity and even partial destruction of some dyes can occur. There is considerable variation in the ability of reactive dyes to cover dead or immature cotton. For this reason it may sometimes be necessary to causticise or mercerise woven fabrics in order to achieve a satisfactory appearance in certain hues. Such pretreatments give the further advantage of better colour yield.

With few exceptions, reactive dyes have good solubility in water. Although seldom sensitive to neutral hard water, precipitation of hardness constituents results at the alkaline fixation stage and thus soft water should be used for all dissolving and dyebath operations. Sources of water with variable bicarbonate content, as might arise in a water supply with temporary hardness, can adversely affect the reproducibility of the dye fixation conditions. The influence of the bicarbonate anions largely depends on the alkali used for fixation. Alkalis based on sodium hydroxide exhibit higher sensitivity than sodium carbonate systems [1].
The use of either common salt (sodium chloride) or Glauber’s salt (hydrated sodium sulphate), in large amounts, is essential to all batchwise dyeing processes for reactive dyes. The relative prices, purity and availability of these electrolytes vary considerably in different parts of the world and selection must take account of this. Common salt is widely used, but Glauber’s salt is preferred with certain bright royal blues based on anthraquinone chromogens and turquoise or green hues dyed with copper or nickel phthalocyanine derivatives. Common salt is more soluble and easier to dissolve than Glauber’s salt. Electrolyte, from whatever source, must be free from alkali, since the latter causes premature fixation or hydrolysis of the dye.

The impurities that can significantly influence the reproducibility of the dyeing process are the alkaline earth metals calcium and magnesium, as well as the transition elements copper and iron. Their adverse effect on the process can be seen as inferior reproducibility, unlevel dyeing and lower wet fastness. The quality and source of electrolyte have a major influence on the levels of trace metal impurities introduced into the dyebath. Common salt is obtained from underground deposits or from seawater. The levels of metal ion impurities vary according to source and degree of purification [47]. These impurities can be controlled by the appropriate use of sequestering agents, taking into account the pH and temperature of the dyeing process. Uncontrolled use of organic sequestrants, such as ethylenediaminetetra-acetic acid, can lead to problems of hue change and lower light fastness. All metal-complex reactive dyes, with the exception of the phthalocyanine derivatives, will give rise to this effect.

Whilst soda ash (98% anhydrous sodium carbonate) remains the most widely used alkali for reactive dyeing, sodium bicarbonate, sodium silicate, caustic soda and various phosphates are also important. Alkalis of high purity are recommended and additions of solid brands should be well-diluted beforehand. Hot, damp conditions of storage should be avoided and dry scoops should be used when weighing. Sodium bicarbonate should be dissolved at low temperatures and direct heating by steam injection must be avoided. Caustic soda and sodium silicate are normally marketed and used as concentrated liquors of known concentration. In recent years, liquid buffer systems and liquid alkali products such as Alkaflo (Tanatex) have been introduced mainly for automatic dosing and metering systems [48].

Provided goods have been prepared efficiently for exhaust dyeing, it is unnecessary to add wetting or levelling agents to the dyebath. In winch or overflow dyeing of tubular-knitted cotton, however, minimal addition of a wetting agent can provide a lubricating action for the avoidance of rope marks. After any pretreatment with wetting agent, thorough rinsing is advisable before a fresh bath is set for dyeing, in the interest of reproducibility. Unwanted foam in jet or overflow machines can be minimised by careful addition of selected antifoam agents. When dyeing in enclosed machines at temperatures higher than 70°C, certain azo reactive dyes may undergo reduction owing to the combined effects of heat, alkali and aldehydic groups in the cellulose. If this problem is expected to occur, it is advisable to add 1–2 gl-1(grams per liter) of sodium m-nitrobenzenesulphonate as a protecting agent.

References:

1. P S Collishaw, B Glover and M J Bradbury, J.S.D.C., 108 (1992) 13.
2. T D Fulmer, Am. Text. Int., 16 (1987) 69.

Batchwise Application of Reactive Dyes

In spite of the essential simplicity of reactive dyeing methods, there are few instances where dyes based on one type of reactive system show fully satisfactory compatibility with those of another type. Even if competing dyes have identical chromophoric groupings, there may well be marked differences in application behaviour and fastness properties of the resulting dyeings. In selecting a range of reactive dyes for batchwise dyeing, however, the dyer has considerable freedom of choice since almost all required hues are fully represented in every range. Selection is largely based on application technique and consideration of the end use of the material.

Reactive dyes can be applied by any conventional batchwise dyeing method for cellulosic materials, including circulating-liquor machines for loose stock, yarn or woven fabrics, as well as jets, winches or jigs for piece dyeing. The conventional dyeing process entails three stages:

1. Exhaustion from an aqueous bath containing electrolyte, normally under neutral conditions.
2. Addition of alkali to promote further uptake and chemical reaction of absorbed dye with the fibre at the optimal pH and temperature.
3. Washing of the dyed material to remove electrolyte, alkali and unfixed dye.

Numerous variants of this basic procedure in terms of chemical concentrations and fixation conditions have been devised to take account of the characteristic properties of the numerous ranges of reactive dyes now available.

Considerable research over several decades has led to the present selection of processes to apply each range of dyes in such a way that optimal fixation efficiency and level dyeing are achieved. In recent years these studies have been concerned particularly with dyes that contain bifunctional reactive systems. For example, a detailed investigation of levelling characteristics has compared a bis(vinylsulphone) and a bifunctional aminochlorotriazine–vinylsulphone with several conventional monofunctional vinylsulphone dyes [1]. Factors controlling the rates of exhaustion and level dyeing behaviour have been analysed to define optimised conditions recommended for batchwise dyeing with Procion H-EXL (Zeneca) bis(aminochlorotriazine) dyes [2] or with Sumifix Supra (NSK) aminochlorotriazine–vinylsulphone dyes [3].

References: 

1. N Harada et al., J.S.D.C., 107 (1991) 363.
2. M J Bradbury, P S Collishaw and D A S Phillips, J.S.D.C., 108 (1992) 430.
3. K Imada and N Harada, J.S.D.C., 108 (1992) 210.

Classification of Reactive Dyes

A valuable classification of reactive dye types has been formulated recently [1]. Three groups relating to the most important control parameter in each case may be distinguished.

Group 1: Alkali-controllable reactive dyes
These dyes have optimal temperatures of fixation between 40 and 60°C. They are characterised by relatively low exhaustion in neutral salt solution before alkali is added. They have high reactivity and care in addition of alkali is necessary to achieve level dyeing, preferably at a controlled dosage rate. Typical examples of dyes belonging to this group have dichlorotriazine, chlorodifluoropyrimidine, dichloroquinoxaline or vinylsulphone reactive systems.
Group 2: Salt-controllable reactive dyes
Dyes in this group show optimal fixation at a temperature between 80°C and the boil. Such dyes exhibit comparatively high exhaustion at neutral pH, so it is important to add salt carefully to ensure that dyeings are level. Electrolyte addition is often made portionwise or preferably at a controlled rate of dosage.

Dyes with these properties typically have low-reactivity systems such as trichloropyrimidine, aminochlorotriazine or bis(aminochlorotriazine). Aminofluorotriazine dyes in the Cibacron F (CGY) range have high substantivity and should thus be regarded as salt-controllable but they are sufficiently reactive for fixation at 60°C or even lower temperatures by batchwise application.

Group 3: Temperature-controllable reactive dyes
This group is represented by those dyes that react with cellulose at temperatures above the boil in the absence of alkali, although if desired they can be applied under the same conditions as the salt-controllable group with alkaline fixation at a temperature between 80°C and the boil. Dyes in this group have self-levelling characteristics so there is no need to use auxiliary products to facilitate level dyeing. Good results can be achieved by controlling the rate of temperature rise.

At present only the Kayacelon React (KYK) range of bis(aminonicotinotriazine) dyes belong to this group.

References:

1. T Sugimoto J.S.D.C., 108 (1992) 497.

Factors Governing Reactive Dye Uptake

All conventional reactive dyes for cellulose, irrespective of whether they react by nucleophilic addition, substitution, or both mechanisms, rely on the reactivity of the cellulosate anion as the nucleophilic reagent and hence hydrolysis of the dye by reaction with hydroxide ions from water will always compete with the desired fixation reaction. Reaction between the dye and cellulose can occur only when the dye has been absorbed into the cellulose phase. Thus the kinetics of the dye–cellulose reaction is strongly influenced by the rate of absorption of dye. The ratio of the rate constants for reaction of the dye with the fibre and with water is a constant for a given dye over a wide range of alkaline pH values.

The efficiency of fixation is a function of:

1. The reactivity ratio, the ratio of rate constants for the fixation reaction and hydrolysis;
2. The substantivity ratio, the relative concentrations of dye absorbed into the substrate and remaining in the dyebath;
3.  The diffusion coefficient of the dye in the substrate;
4. The liquor ratio; and
5. The surface area of the substrate available for absorption of dye [1].

The lower the linear density of the fibre, i.e. the greater the surface area per unit weight, the more efficient is the dyeing. The substantivity ratio is the most influential of the factors governing fixation efficiency. Dyes of higher substantivity diffuse more slowly than less substantive dyes. Changes in dyebath conditions that increase substantivity tend to decrease the diffusion coefficient. Lowering the liquor ratio favours increases in the rate and efficiency of fixation. The full effects of this are never completely realised, however, because the higher dyebath concentration necessary at the lower liquor ratio implies a decrease in substantivity ratio, offsetting some of the expected gain.

Substantivity ratio remains approximately constant within the pH range 7–11 at a given electrolyte concentration, but above pH 11 there is a marked fall in substantivity, especially with highly sulphonated dyes. As the applied con-centration of dye is increased at constant electrolyte concentration, the substantivity ratio and hence the efficiency of fixation are lowered. Thus fulldepth dyeings require longer for completion of the reaction and the percentage fixation is usually inferior. In order to attain the maximum rate and efficiency of fixation, more electrolytes are needed, but this increases the risk of aggregation and possible precipitation with dyes of limited solubility.

An increase of dyeing temperature lowers the substantivity ratio and accelerates the rate of hydrolysis of the dye; both of these effects reduce the fixation efficiency. The rates of diffusion into and reaction with the fibre are also accelerated, however, and these factors both favour fixation of the dye. An increase in electrolyte concentration always enhances substantivity without impairing reactivity providing the dye remains completely dissolved. The beneficial effects of electrolyte addition are most evident with the more highly sulphonated dyes at relatively high pH and applied depth.

Informative studies of the relationships between dye structure and substantivity [2] and between dye structure and levelling properties [3] are available. The interaction between these dyeing properties and the controlling parameters in exhaust dyeing with reactive dyes, such as applied concentration, pH, temperature and electrolyte addition, is the key to achieving successful and reproducible dyeing.

References:

1. H H Sumner and C D Weston, Am. Dyestuff Rep., 52 (1963) 442.
2. M Haelters, Melliand Textilber., 61 (1980) 1016.
3. N Harada et al., J.S.D.C., 107 (1991) 363.

Aminofluorotriazine-sulphatoethylsulphone Dyes

Early in 1988 Ciba-Geigy launched the Cibacron C range of mixed bifunctional dyes. They contain a new aliphatic vinylsulphone system and either a monofluorotriazine bridging group (Figure 1) or an arylvinylsulphone function [1]. They are designed mainly for pad applications and appear to be characterised by medium to low affinity, good build-up, easy wash-off and high fixation. Their outstanding bath stability and high fixation make them especially suitable for pad–batch dyeing [2]. The manufacturing cost of these structures is believed to be relatively high but the purchase cost to the dyer may be offset by enhanced cost-effectiveness in use attributable to efficient fixation and easy wash-off, possibly the best approach so far towards environmentally acceptable reactive dyes [3].

An important feature of the Sumifix Supra (NSK) type of bifunctional system (Figure 2) is the major difference in reactivities between the amino- chlorotriazine moiety and the much more reactive vinylsulphone group.

There are some practical conditions, notably in pad–batch application, that do not allow full advantage to be taken of both types of reactive group present. The combination of aminofluorotriazine and vinylsulphone in the Cibacron C (CGY) system, both groups offering effective fixation under  virtually the same conditions, exploits the concept of bifunctionality more effectively.

These factors have been demonstrated elegantly in a detailed evaluation by pad–batch dyeing of cotton with three commercially important copper formazan reactive dyes:

1. Cibacron Blue F-R (CI Reactive Blue 182) with a monofunctional 2-fluoro- 4-sulphoanilinotriazine system.
2. Sumifix Supra Blue BRF (CI Reactive Blue 221) with a mixed bifunctional 2-chloro-4-vinylsulphonylanilinotriazine system.
3. Cibacron Blue C-R with a synchronised bifunctional 2-fluoro-4-vinylsulphonylalkylaminotriazine system.

Under the relatively mild conditions of pad–batch fixation within 6 h batching time, the mixed bifunctional system behaved more like a monofunctional vinylsulphone dye and only the synchronised Cibacron C (CGY) system showed truly bifunctional performance [4].

Reference:

1. J P Luttringer and A Tzikas, Textilveredlung, 25 (1990) 311.
2. J M Sire and P Browne, Melliand Textilber., 72 (1991) 465.
3. A H M Renfrew and J A Taylor, Rev. Prog. Coloration, 20 (1990) 1.
4. P Scheibli and S Koller, Proceedings of the Conference on Reactive Dyes, Leeds University, Leeds (Sep 1989).

Aminochlorotriazine-sulphatoethylsulphone Dyes

Reaction of a dichlorotriazine dye with an arylamine containing a 2- sulphatoethylsulphone grouping is the preferred route to mixed bifunctional reactive dyes capable of reacting with cellulose either via a monochlorotriazine moiety or a vinylsulphone group. A typical example is Sumifix Supra Brilliant Red 2BF (NSK) shown in Figure 1 [1]. Both reactive systems can contribute to the fixation process but the relatively greater reactivity of the vinylsulphone group ensures that most of the fixation arises via this function [2]. Further benefits of this type of structure, however, include the higher substantivity contributed by the triazine bridging residue and the capability this gives to link a sulphatoethylsulphone grouping to a wide range of chromogens.

The presence of two reactive groups that differ in reactivity gives dyes that are less sensitive to exhaust dyeing temperature than typical monofunctional reactive dyes. They can be applied over a wider range of temperatures (50–80°C) and reproducibility of hue in mixture recipes is improved. Moreover, they show minimal sensitivity to electrolyte concentration and are less affected by changes in liquor ratio [3]. Low dyeing temperatures favour reaction via the vinylsulphone group and at higher temperatures the contribution of the chlorotriazine system to fixation becomes more important [4].

The presence of two types of dye–fibre bond has certain consequences for fastness properties. Mixed bifunctional dyes show superior fastness to acid storage than dichlorotriazine or dichloroquinoxaline systems and better fastness to peroxide washing than difluoropyrimidine or dichloroquinoxaline dyes [5]. Conversely, mixed bifunctional dyes do show some of the weakness to alkaline treatments characteristic of vinylsulphone dyes, although less so than monofunctional dyes relying solely on these groups for fixation [6].

An admirable investigation of 18 isomeric dyes of the 7-phenylazo-N- (anilinochlorotriazinyl) H acid series, including CI Reactive Red 194 (Figure 1) and two other commercial products, was reported recently [1]. Allpossible combinations of o-, m-, p-orientation of the sulpho and sulphatoethylsulphonyl groups in the phenylazo and anilino rings at opposite ends of the molecule were evaluated. Interestingly, the three commercially utilised structures showed the highest fixation yields amongst the series of 18 dyes studied.

Reference: 

1. S Abeta et al., J.S.D.C., 107 (1991) 12.
2. U Meyer and S Muller, Text. Chem. Colorist, 22 (1990) 26.
3. S Abeta, T Yoshida and K Imada, Am. Dyestuff Rep., 73 (July 1984) 26.
4. K Imada, M Sasakura and T Yoshida, Text. Chem. Colorist, 22 (1990) 18.
5. S Fujioka and S Abeta, Dyes and Pigments, 3 (1982) 281.
6. M Matsui, U Meyer and H Zollinger, J.S.D.C., 104 (1988) 425.

Bifunctional Systems of Reactive Dyes

In the early years of the commercial exploitation of reactive dyes, it was soon noted that anomalously high values of cuprammonium fluidity were observed for dyeings of many reactive dyes in full depths, although tests of tensile strength demonstrated that the cellulose remained undamaged. Investigation showed that these anomalous results were associated with those dyes capable of forming crosslinks between neighbouring cellulose chains, such as the bis(sulphatoethylsulphone) dye CI Reactive Black 5 (Figure 1) or the dichlorotriazine dyes with two chloro substituents capable of attack by cellulosate segments of the polymer chains, such as CI Reactive Red 1 (Scheme 1).

Figure-1

Scheme-1

These phenomena attracted further interest when ICI introduced the ProcionH-E dyes, a full range of high-fixation dyes containing two aminochlorotriazine groups per molecule. A detailed study of representative members of this range, as well as other potentially crosslinking reactive systems (dichlorotriazine, chloromethoxytriazine, dichloroquinoxaline, trichloropyrimidine and chlorodifluoropyrimidine) provided convincing evidence of the extent of crosslinking that could take place. The degree of crosslinking was non-existent or relatively insignificant for typical pad–batch dyeings at ambient temperature, but thermal fixation by the pad–dry–steam method resulted in a much higher proportion of crosslinked dye molecules.

Recently attention has turned to the more difficult problem of analysing the mode of fixation to cellulose of a bifunctional reactive dye of the Sumifix Supra (NSK) range that contains two dissimilar reactive systems (vinylsulphone and chlorotriazine). Controlled enzymatic degradation of a mechanically milled cotton fabric that had been batchwise dyed at 60°C yielded interesting results.

1. About 80% of the vinylsulphone groups had reacted with the cellulose.
2. About 50% of the chlorotriazine groups had not reacted with cellulose and only half of these had hydrolysed to OH groups.
3. A considerable proportion of the dye molecules had formed crosslinks by reacting via both mechanisms.