Reactive Dyes Chemical Structure, Properties and Applications
Dyeing of Cotton Fabric with Reactive Dye
What is Reactive Dye in Textile Engineering:
Reactive Dyes have long been known to give a high degree of fixation because with two potential vinyl sulphone reactive groups there is an increased probability of reaction with the fiber. Many of the new reactive dyes are bi-functional with identical or different reactive groups in the dye molecule. Adding more reactive groups to a given chromophore increases the molecular weight but decreases the color per unit weight of dye since the reactive groups are not part of the chromophoric system. This problem can be partly overcome, for example by linking together two DCT dyes with a suitable diamine to give a dye with two MCT groups, as in the Procion H-E dyes. This increase in molecular size usually results, in an increase in substantivity. That is the value for exhaust dyeing with high liquor ratios but which can impede the washing-off of unfixed dye after dyeing
Bifunctional dyes with two reactive groups of different reactivity towards the cotton, which have different optimal fixation conditions, give a more uniform degree of fixation over a wide range of dyeing temperatures and fixation PH than dye containing two identical groups. This type of reactive dye gives quite high fixation yields and thus less color in the dyehouse effluent. Other important types of bifunctional reactive dyes include the MFT-VS type and the MCT-VS type used in the suffix supra dyes. The kayacelon react range for dyes are also bifunctional reactive dyes, having two NT reactive groups in each dye molecule
History of Reactive Dyes:
The dyeing of cotton with direct dyes has poor washing fastness. Because it has a weak bond with the dye molecules in the cellulose polymer chains. So direct dyes can easily come out from the fabric molecules. For best washing fastness insoluble pigment is attaching it with the cotton fibers. This type of dyeing process of vat and azoic dyes is much more difficult than direct dyeing.
The idea of forming a dye molecule covalent bond with the reactive group in a fiber originated in the early 1900s. Various chemicals were found that reacted with the hydroxyl group of cellulose and eventually, they converted into colored cellulosic derivatives. Cellulose was a relatively unreactive polymer. As a result, several dyes are known to be capable of forming bonds with wool. Initially, cotton was not considered a fiber-reactive dye, although it has good washing fastness.
In 1955, developed a procedure for dyeing cotton with fiber-reactive dyes. They established that dyeing cotton with these dyes under mild alkaline conditions resulted in reactive chlorine atoms. The role of alkali is to cause acidic dissociation of some of the hydroxy groups of cellulose and it is the cellulose ion that reacts with the dye
The dyeing has a very good washing fastness. The only possibility of bleeding dye from the cotton is after the hydrolysis of the covalent bond between the dye and the cellulose. Within five years of this development, all the major dyestuffs manufacturers were marketing reactive dyes of cotton and wool. Because of their good washing fastness, bright shades, versatile batch, and continuous dyeing method, reactive dyes have become one of the major classes of dye.
Reactive Dyes for Cotton:
Structure of reactive dyes for cotton:
The molecular structure of reactive dyes resembles those of acid and simple direct cotton dyes but with an added reactive group. Typical structures such as azo a) anthraquinone b) triphenodioxazine or copper phthalocyanine chromophores
The structural key features of reactive dyes are the chromophore system, the sulphonate group for water solubility, the reactive group, and the bridging group that attaches the reactive group either directly to the chromophore group or to some other part of the dye molecule. Each of these structural features can influence the dyeing and fastness properties in the dyeing textile manufacturing process.
Most commercial ranges of reactive dyes have a complete gamut of colors, many of which are particularly bright. Reactive dyes often have simple structures that can be synthesized with a minimum of colored isomers and byproducts that tend to dull the shade of the more complex palazzo-direct dyes. Some colors are difficult to obtain with simple chromophores. Dark blue and navy reactive dyes are often rather dull copper complexes of azo dyes and the production of bright green reactive dyes remains a problem.
A whole range of possible fiber reactive groups has been examined and evaluated by the dyestuff manufacturers. The final choices for commercial dyes are limited by several constraints. The reactive group must exhibit adequate reactivity towards water that can deactivate it by hydrolysis. The hydrolysis of the dye's reactive group is similar to its reaction with cellulose but involves a hydroxyl ion in water rather than a cellulose ion in the fiber. In addition, the dye fiber bond, once formed should have adequate stability to withstand repeated washing. Other factors involved are the ease of manufacture, the dye stability during storage, and the cost of the final reactive dye
HO-+ Dye -Cl = Dye-OH + Cl-
Types of Reactive Dyes:
Although many of the early reactive dyes had only one reactive group in the dyestuff molecule, many of the newer reactive dyes function with two or more identical or different reactive groups. Some typical fiber reactive groups and the commonly used abbreviations for these groups. Dyes with nicotinyltriazine reactive groups react with cotton on heating under neutral conditions.
Basic Principle of Reactive Dyeing Process of Cotton Fabric:
The relatively simple procedure for batch dyeing of cotton materials with reactive dyes is still used for all types of reactive dyes irrespective of their particular reactive group. Dyeing is commenced in a neutral solution, often in the presence of salt to promote exhaustion of the dye onto the cotton. During this period, the dye does not react with the fiber, and migration from fiber to fiber is possible. Then, an appropriate alkali is added to the dye bath to increase its PH. This initiates the desired dye fiber reaction. The hydroxyl group in cellulose4 is weakly acidic and absorption of hydroxide ions causes some dissociation, forming cellulosate ions. It is these that react with the dye by
nucleophilic addition or substitution. In general, the lower the reactivity of the reactive group towards the alkaline cellulose, the higher the final dyeing temperature and the higher the PH of the dye bath.
Unfortunately, under alkaline conditions, hydroxide ions also react with the reactive group of the dye. This produces the hydrolyzed dye, which is incapable of reacting with the fiber. Hydrolysis of the dye is slower than the reaction with the alkaline cotton but it is significant and reduces the efficiency of the fixation process. After dyeing, an unreacted and hydrolyzed dye present in the cotton must be removed through washing. This ensures that no color will bleed from the cotton on subsequent washing during use. The higher the substantivity of the reactive dye for the cotton, the more difficult to wash out the unfixed dye from the material. Many of the first reactive dyes had quite simple molecular structures and low substantivity for cotton so the removal of hydrolyzed dye from the material by washing was relatively easy. This is not necessarily true for reactive dyes of more complicated molecular structures.
Dye Reactivity, Storage, and Application of Reactive Dyes:
The reactive groups of various types of dye have different chemical structures and show a wide range of reactivities. They were originally divided into cold and hot dyeing types but many current ranges would be better called warm dyeing. The most reactive dyes such as DCT reactive dyes are applied at lower temperatures (20-40)C and only require a weak alkali such as NaHCO3 or Na2CO3 for fixation. The less reactive dyes such as MCT dyes, need higher temperatures(80-90) C and stronger alkali such as Na2CO3 Plus NaOH. Many dyestuff manufacturers now market several ranges of reactive dyes for cotton, each with its own particular recommended dyeing procedure. Below are some examples based on the type of reactive grouping:
The above reactive groups react under neutral conditions. Because most reactive dyes are prone to hydrolysis, their handling and use require care. Most are readily water soluble and the dye solution is prepared in the usual way by pasting it with water and then adding more water. The temperature of the water used depends upon the ease of solution and the reactivity of the dye. Hot water is not recommended for dissolving dyes of high reactivity, because of the risk of the hydrolysis of the reactive group, but it is suitable for the less reactive dyes. Once the dye solution has been prepared, it cannot be stored for later without some risk of hydrolysis of the reactive group. This decreases its fixation ability and it is a particular problem with most of the reactive dye. Dyes containing 2- sulphatoethylsulphone group, however, can be dissolved in neutral water in the boil without risk of hydrolysis. Formation of the reactive vinyl sulphone group requires the addition of an alkali.
Reactive dyes for printing are usually dyes of low reactivity so that the print paste can be stored for some time at room temperature without deterioration from hydrolysis of the reactive group. Reactive dyes of low reactivity and relatively high substantivity are valuable for dyeing using long liquor ratios, using a winch machine. Exhaust dyeing with low reactivity dyes at the higher temperature required for fixation allows better penetration of the dyes into the cotton fibers. For continuous dyeing of reactive dyes stabilized liquid forms are available. Although these contain special PH buffers and stabilizers to minimize the hydrolysis reaction, they only have a limited shelf life
Many commercial reactive dyes are dusty powders but all physical forms must be handled with care. These dyes react with the amino groups in proteins in the skin and on mucous surfaces. Inhalation of the dust is dangerous and a dust mask is obligatory during handling. Reactive dye powders and grains are sometimes hygroscopic and drums must be carefully resealed. Most reactive dyes have a limited storage period, after which some deterioration can be expected. Standardization and comparison of reactive dye powders or liquids cannot be done by the usual spectrophotometric procedure involving absorbance measurements of standard solutions. Both the reactive dyes and their hydrolyzed forms are equally colored. but only the former is capable of reacting with the cellulose during dyeing. Chromatographic techniques usually allow separation and quantitative measurements of the relative amount of a reactive dye and its hydrolysis product in a given dye
Fastness Properties of Reactive Dyes on Cellulosic Fibers
In general, reactive dyes on cellulosic fibers give good dyeing with very good washing fastness and other wet processes. Apparent inferior fastness to washing is usually because of incomplete removal of unreacted and hydrolyzed dye from the material by washing after dyeing. The presence of unfixed dye can easily be tested for by hot pressing a wet sample of the dyeing sandwiched between two pieces of dry white cotton. Color transfer to the white material indicates the presence of unfixed dye in the dyeing. It is more likely to be a problem for deep dyeing.
The use of reactive dyes is growing rapidly, faster than for any other dye application class. This is because these dyes also give dyeing of moderate to good washing fastness, allow relatively simple and diverse dyeing methods, and are available in a range of bright colors. They have contributed significantly to the decline of direct cotton dyes. Their fastness properties, though generally good, do not match those of pigmented cotton dyed with vat dyes. In particular, the fastness to bleaching by chlorine and to a lesser extent by peroxide present in modern household detergents is often only moderate.
Evidence for Covalent Bond Formation of Reactive Group with Cellulose
The good fastness to washing of dyeings with reactive dyes on cellulosic fibers is a consequence of the stable covalent bond formed between the dye's reactive group and the cellulosic polymer. There is considerable evidence to support the formation of this dye-fiber bond. Dyeing is resistant to color stripping with hot aqueous pyridine, a solvent that effectively removes direct dyes from cotton. The dyeing of cotton obtained with bifunctional dyes often exhibits reduced swelling and decreased solubility in cuprammonium solution. Adye molecule with two reactive groups crosslinking two different cellulose chains would explain this. If the color of dyeing obtained with an azo-reactive dye is destroyed by chemical reduction with alkaline sodium hydrosulfite, which cleaves the azo dye into two primary aromatic amines, the amine remaining attached to the celllulose can be diazotized and coupled with an appropriate phenol to reform a colored fiber. Finally, bacterial degradation of reactive dyes cotton that depolymerizes the cellulose, but avoids breaking the dye-fiber bond, gives colored products containing the original dye still bonded to glucose
Batch Dyeing of Cotton with Reactive Dyes in Textile Engineering
Preparation for Dyeing of Reactive Dyes
Level well-penetrated dyeings require careful preparation of the material. All sizing chemicals capable of reacting with the dye, such as starch or polyvinyl alcohol, must be removed from the material and any traces of residual alkali must be uniformly neutralized. Good alkali boiling to remove wax is essential for goods to be dyed with cold dyeing reactive dyes because the penetration of the dyes into the fibers is more difficult at lower dyeing temperatures. Reactive dyes often give such bright colors that bleaching may not be necessary. Once size has been removed, grey cotton goods can sometimes be simultaneously scoured and dyed using hot dyeing reactive dyes and an effective detergent. Because of the sensibility to bleaching by chlorine of some reactive dyes, over-chlorinated water must be avoided or treated with a reducing agent such as sodium bisulfite or thiosulphate.
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The Three-Step Exhaust Dyeing Process of Reactive Dyes
A typical exhaust dyeing process for cellulosic materials using reactive dyes has three distinct processes;
Reactive Dyes Classification in Textile Engineering:
Reactive is very versatile and allows a variety of different approaches for controlling the rate of absorption, the rate of dye migration, and dye fixation. Salt addition, temperature variations, and alkali additions are used alone or in combination to control the dyeing process. Reactive dyes have recently been classified as:
Difference Between Direct Dye And Reactive Dyes:
Direct dyes and reactive dyes have the potential to provide the full gamut of colors. The main difference and the advantage of reactive dyes is that they give much higher fastness than direct dyes. The reactive dye bonds covalently, double bond, with the end groups on the cotton. Providing a strong bond, which in washing running tests etc. would easily show grade 4 or 5. The direct dye would not be able to achieve these levels of performance, the addition of binders, etc. would improve the fastness but not to the reactive level and would have a detrimental effect on the handle of the fiber. Now coming back to the navy blue shade the main options would be direct dye combination, reactive dye combination, and vat dye combination. For direct dye, the total dye percent could be 2%. Considering the average cost of direct dye as Rs800/kg the cost of shade is Rs16. To get it with reactive dye it would require, say a 6 % shade. With an average dye cost of Rs300/kg, this would mean a dye cost of Rs18. The reactive dyes come in many variants MCT(Monochloro triazine) DCT( Dichloro triazine) and Vinyl sulphone(Few are suitable for discharge prints) Now the reactive dyes are available in Bifunctional groups which make the application easy at 60 degrees centigrade.
Reactive Dyes are most common for cotton dyeing in the industry for their ease of application, covalent bonding, satisfactory fastness, brilliance in colors, range in colors, and economical.
Direct dyes are easy to apply but have poor wash fastness due to a weaker bond. some cheap/ economical clothes are dyed with direct dyes only. Tie-dye is one of the known applications.
Advantages of Reactive Dyes:
The objective of dyeing with reactive dyes is to obtain the maximum degree of reaction between the dye and fiber, with a minimum of dye lost through hydrolysis of the reactive group, and under conditions where the color of the dyed material is uniform. Dyes with different reactive groups and molecular structures have different amounts of added salt to obtain economic exhaustion. Some reactive dyes, including most of the oldest, have relatively simple molecular structures and have low substantivity towards cellulosic fibers. They need a high concentration of salt, up to 100gm/L, particularly when dyeing deep shades or using low liquor ratios. At such high salt concentrations, there is always a risk of precipitation, particularly when dyeing at lower temperatures. Although Glubar salt is more expensive than sodium chloride, it is preferred for dyes prone to aggregation to low dyeing temperatures such as the turquoise copper phthalocyanine reactive dyes. The advantages of dyes with low substantivity are that they diffuse easily in the fibers and are easy to wash out of the material after dyeing. The higher the substantivity of the dyes for the cellulosic fibers, the higher the exhaustion and the greater the chance of reaction with the fiber, but the greater the difficulty of removing the unfixed dye after the final washing.
For dyeing with vinyl sulphone dyes, it is advisable to ensure that the residual alkali has been removed or neutralized to soaping. Since hydroxide ions can catalyze the hydrolysis of the ether-type dye-fiber bond and result in additional color bleeding from the goods. Dyeing with vinyl sulphone dyes has maximum fiber bond stability around pH 4.5, whereas the corresponding value for dyes halogenated nitrogen heterocycles is 6-7. The latter have dye fiber bonds that are more sensible to acid-catalyzed. hydrolysis.
b Dye-O-Cell +H2O = Dye-OH + Cell- OH
The dyeing temperature and the nature and concentration of the alkali required are determined by the reactivity of the dye, its degree of sulphonation, and its substantivity. For tightly twisted yarns and compact woven fabrics, the migration phase of dyeing with low reactivity dye can be carried out at higher temperatures up to 120C to promote initial migration and penetration into the textile. This is well above the later fixation temperatures in the presence of alkali 80 C. Azo copper complex dye will not withstand these conditions and tends to lose the copper ion. For a higher fixation temperature, a reaction with the fiber occurs at lower PH, using a lower alkali or a lower concentration of the usual alkali. As in the case of salt addition, a deeper shade, or a higher liquor ratio, will require more alkali.
It is not usual to exceed a dyebath Ph of 11, even with the less reactive dyes since this invariably leads to lower color yields because of high hydrolysis
The effect of increasing the dyeing Ph during the fixation phase is complex but usually involves an increase in the rate of reaction of the dye with the fiber and with hydroxide ion. For polysulphone dyes, one effect of dyeing at Ph above 11 is the decrease in substantivity of the dye for the increasingly anionic dissociated cellulose. The greater negative charge of more cellulose ions repels the dye anions. In fact, some dyes actually desorb from the fiber into the dyebath when the alkali is added at the start of the fixation stage giving a sudden decrease in the degree of exhaustion at that point. In addition, with polysulphonated dyes, the substantivity decreases as dye fixation proceeds because the cotton contains more and more bound anionic dye molecules, which also repels the unfixed dye. These effects can be counteracted by a concentration of salt in the dyebath
Dye hydrolysis is more pronounced and exhaustion is less at a high liquor ratio. Therefore, In winch dyeing, reactive dyes of high substantivity are preferred. In recent years, there has been a considerable shift to dyeing with reactive dyed-on machines with low liquor ratios. This gives more efficient dyeing and reduces the consumption of dyes, salt, and alkali.
Viscose fibers give higher fixation and exhaustion of reactive dye than cotton. In fact, for identical conditions, exhaustion and fixation increase in the order, cotton, mercerized cotton, viscose. The washing fastness of reactive dyes on viscose is also somewhat better than cotton. Because of the ease of swelling of viscose, the dyeing Ph and temperature for a given dye may be different than for cotton, particularly if dye penetration may be problematic.
The so-called 'all-in' dyeing method has the advantage that no additions are required during dyeing and is therefore the most rapid dyeing method. The goods are run in the filled machine with all the added salt, alkali, and the reactive dye solution added over 10-15 min. The rate of dye fixation is controlled by controlled for good reproducibility. The NT reactive dyes, mixed with disperse dyes are useful for dyeing cotton/polyester at 130 C under neutral conditions.
Washing-Off of Unfixed Reactive Dyes
Removal of hydrolyzed and unreactive dye from the goods is a vital step after dyeing. The amount of unfixed dye remaining in a cotton fabric dyed with reactive dyes may have to be less than 0.002%. Although bleeding out of such a small amount during subsequent washing by the consumer will not significantly alter the shade of the depth of the material, it can visibly stain adjacent white goods. This is usually unacceptable
Both batch and continuous washing processes involve three stages. Initially, the goods are rinsed in cold and warm water. This is a dilute stage aimed at removing as much salt and alkali as possible from the goods. This makes the next soaping stage much more efficient since at lower electrolyte concentrations the substantivity of the dye is less, making its desorption easier. The final stage is again a warm rinsing stage to dilute the final dye solution adhering to the fibers to the point that the amount of unfixed dye carried out to the final drying is minimal. This residual quantity of dye will deposited on the fiber surface on the evaporation of the water during drying and will be easily removed by water washing. Obviously, the amount must be as small as possible/
The entire washing operation involves achieving a compromise between the effectiveness of the removal of unfixed dye and the cost of the large volumes of water used, including the heading cost. Low liquor ratio washing saves water but gives less dilution of the washing liquors. Some dyeing machines, allow overflow rinsing but this consumes much more water. For winch and jig dyeing machines operating at atmospheric pressures, it is not possible to carry out soaping at temperatures above 90C, even when they are closed. For any given dyeing machine, each protocol is established. One point requires particular attention. After each stage in the washing cycle, dye transfers one bath to the next, in the solution retained by the fabric. This transfer must be as low as possible. This means that machines should be completely drained between stages and wash boxes in continuous washing must have effective mangles to squeeze out as much solution as possible from the fabric leaving the box
Fluorescent Reactive Dyes for Cotton Fabric:
Nowadays, Cotton fabric dyeing with reactive fluorescent dyes has limited color shades and brilliance. Thus, they are a group of colorants in the textile industry for dyeing cotton fabric with high demand. This research designed and synthesized two novel fluorescent reactive dyes based on fluorescein. In this respect, 4,4' -diamino stilbene-2,2- sulfonic acid and 4-aminophenyl-4-B-hydroxyl ethyl sulfone sulfate ester react with the cyanuric chloride, separately. Those that reacted with fluorescein and two reactive dyes D1 with dual-emission wavelength and D2 with an emission spectrum. The characterizations of two dyes were carried out using the TLC, HNMR, CNMR, FTIR, elemental analysis, ultraviolet spectra, and fluorimeter techniques. The absorption and emission parameters of dyes in solutions were obtained. The result indicated that the parameters of D1 were more than D2. The results show that two dyes have a positive solvatochromism effect. The synthesized dyes were applied to the cotton fabric, and their characteristics, fixation, and exhaustion percentages were studied
Dyeing Compatibility of Reactive Dyes:
Ideally, reactive dyes in a mixture should all exhaust and react with the fiber at about the same rate so that the shade builds up on tone. Dyes from different ranges,, with different reactive groups, can rarely be used together because of their different dyeing characteristics and reactivities. It is therefore usual to mix dyes with the same type of reactive group having about the same substantivity. Since there is often a great deal of uncertainty about the particular type of reactive group in a given reactive dye, dye selection must often be from one particular manufacturer's dye range and based on his recommendations
Compatible dyeing behavior is a function of all the process variables and requires careful control of the dyeing temperature, salt and alkali concentrations, dyeing time, and liquor ratios. Once the dy reacts with the cellulose, it is completely immobilized and cannot migrate. Control of the process variables determines whether a given shade will be reproducible from batch to batch. When dyeing with mixtures of reactive dyes, shading is usually possible by the addition of low substantivity dyes to the alkaline bath. The dye bath may be partially drained and refilled with cold water, the solution of shading dyes added and the bath then reheated if necessary. Further additions of salt or alkali are often not required.
Disadvantages of Reactive Dyes:
One of the major problems in exhaust dyeing with many reactive dyes is the rather low-level fixation, particularly when dyeing using a high liquor ratio. Often less than 70% of the original dye reacts with the fiber. This results in appreciable dye concentrations in the dyehouse effluent. This environmental problem is compounded if high salt concentrations are also present. Newer range of reactive dyes, particularly with those more than one reactive group that give higher fixation, have attempted to address the problem of color in the effluent with some success. Several duye manufacturers now offer reactive dyes requiring smaller amount of salt for exhaust dyeing. The use of less salts demand a higher degree of substantivity but this impedes efficient washing-off after dyeing. The is counteracted by using dyes that give good fixation
It is possible to eliminate color bleeding and staining of adjacent material when an article is first washed during use by after treating the dyeing with a cationic fixative. This type of product reacts with any residual unfixed anionic dye, forming an organic salt of greatly increased molecular size and fo lower water solubility and diffusion rate. Such cationic fixative lack permanence on repeated washing but this is not a problem since the unfixed dye will have gradually been removed by that point. They may however, reduce the light fastness of the dyeing and are therefore more suitable for treatment of deep shades. Such an after treatment is not a remedy for inefficient washing-off of unfixed dye. If the amount of unfixed dye remaining in the goods is significant there is a risk of the preciptate of dye auxiliary complex rubbing off particularly on wet abrasion.
Dyeing with a few red DCT reactive dyes on cotton are prone to increased color bleeding because of hydrolysis of the dye fiber bond. The color bleeding can be counteracted by adding a polyamine to the final rinse water that reacts with any residual reactive chlorine atoms in the bound dye.
As for direct dyes, some reactive dyes may be reduced by the cellulose when dyeing at high temperatures in the presence of alkali. This can lead a significant decrease in color strength when dyein g viscose with reactive dyes, for example. Addition of the mild oxidant m-nitrobenzene sulphonate usually prevents this.
Thers is always a risk of anionic reactive dyes being precipitated by calcium, magnesium or heavy metal ions in the water supply, or of the formation of insoluble hydroxide of these metals under the alkaline dyeing conditions. To avoid these problems a limited addition of a polyphosphate sequestering agent may be required. EDTA should be avoided with azo copper complex dyes since demetallisation can occur with a dramatic change in hue. The copper in the stable copper phthalocyanine dyes is, however, unaffected by EDTA.
Finally, stripping reactive dyes is by no means easy. Hydrolysis of the dye fiber bond using hot acetic acid solution, followed by good washing, may give partial stripping. Complete color discharge si often possible with alkaline sodium hydrosulphite solutions followed by hypochlorite. Before such treatments complexed metals should be removed using EDTA.
Application of Reactive Dyes and Finishing Chemicals
Reactive dyes may be applied to undyed cellulosic materials at the same time as the usual crease -resist and durable press resin finishes, thus combine dyeing and finishing into one step. During curing, when the methyloamino groups of the polyfunctional finishing chemicals react with hydroxyl group in the cellulose. The dye reacts with the free amino groups in these agent rather than with the fiber. Since the dye is fixed to the finishing agent, the fastness properties of the final dyeing depend on the permanence of the finish.
List of Reactive Dyes Manufacturers in India: