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Processing, Dyeing & Finishing
  Reactive dyes: Past & future

Despite the many possible reactive groups in dyes capable of covalent bond formation with nucleophilic groups in wool, only a limited number of types of reactive dye have been commercially successful, say R H Deshpande and Y M Indi.

Reactive dyes for cellulosic materials were invented in 1954 and ICI in England introduced the first range of reactive dye popularly known as Procion dye in the year 1956. Invention of these dyes in textile industry has brought sea changes in the history of dyes. Reactive dyes are the most predominant class of dyes for cellulosics today and 50% of cellulosics are dyed with these dyes. They are also increasingly gaining importance for wool and polyamide fibres. Share of reactive dyes among all textile dyes is 29%, which is next to disperse dyes consumption (32.5%).

Easy application and choice of different kinds of application techniques like exhaust, semi-continuous & continuous, suitability to dye on any conventional or modern machines, presence of wide range of gamut of shades from dull to bright and pastel to dark, compatibility, possibility of getting acceptable allround fastness properties and cost effectiveness are the major key factors which are responsible for the growth of reactive dyes.

It is estimated that reactive dyes represent the only range of cellulosic dyes that is expected to increase in sales volume in the early part of the next century. Reactive dyes are expected to take share from other cellulosic dyes like sulphurs and azoics on environmental ground, vats on cost and application, directs on fastness. On the other hand, these dyes are also facing criticism on the ground of being highly polluting either during their manufacturing or their application on textile substrate.

Reactive dyes in general require high usage of salt (especially in exhaust method of application), alkali for fixation and high usage of urea (CDR and printing), which adds more load during treatment of effluent. Besides these, reactive dyes are facing issues like presence of AOX within the dye molecule, more water consumption, presence of heavy metal ion in dye molecules (Cu, Br, Cr, Co), relatively low fixation levels, difficulty in removing hydrolysed reactive dyes and removal of colour from effluent stream to meet legal limits.

Therefore, all research in dye chemistry is being directed towards the new generation reactive dyes that can meet all environmental norms as well as can satisfy the needs of the customer (ie, fastness requirements). Hence, it is a need to invent new generation reactive dyes with environmental & technical satisfaction.

Historical developments & different reactive systems

Reactive dyes are unique in that they contain specific chemical groups capable of forming covalent links with the textile substrate.

The original reactive system explored in the 1950s was the dichlorotriazinyl dyes. These were marketed by ICI under the trade name Procion MX. Due to incomplete utilisation of the dye bath (ie, 60 - 70% exhaustion and 30 - 40% hydrolysed dye still remain in the bath) and also due to low fixation property, compared to modern multi-functional dyes, these dyes show poor washing off and effluent related problems.

Application methods such as pad-dry-wash-off have been developed specially for this range of dyes. Due to high reactivity of DCT systems, these dyes were not suitable for printing application. This led to the development of Monochlorotriazinyl residue. These MCT dyes have remained the dominant chemical technology for direct printing as well as for the pad-dry-bake process. Due to less reactivity of MCT group, they demand higher temperature and more time for their fixation in comparison to DCT dyes.

These dyes require larger molecule in order to provide sufficient substantivity for the substrate at the dyeing temperature.

Then Remazol dyes from Hoechst were put into the market in the mid-late 1950s. These dyes contain Sulphato-ethyl sulphone residue, which generates the vinyl sulphone reactive group on addition of alkali. The special features of this low-medium substantivity and medium reactivity dyes are:

i) Exhaust application by warm dyeing at 50 - 60oC.
ii) Suitability for continuous dyeing application (pad–batch and pad–dry-pad–steam).
After the introduction of Chlorotriazine and Sulphato-ethyl Sulphone reactive systems in the market, search for new reactive groups has become comparatively less aggressive since both the reactive systems are firmly established for dyeing cellulose.
These dyes were not so fast to acid hydrolysis and peroxide bleaching.
The idea behind bi-functional (and tri/tetra functional) dyes was to produce a range of high temperature dyes for exhaust application with increased substantivity, exhaustion and fixation values compared with the dyes containing only one reactive group.
A bi–functional dye is one carrying two reactive groups. The groups could be similar (Homo bi-functional) or dissimilar (hetero bi–functional/mixed bi–functional).
The introduction of homo-bi-functional reactive dyes – Procion HE – from ICI was the first significant development in reactive systems, which contains reactive groups other than the exotic heterocyclic reactive system.
These bis-MCT dyes, originally introduced for dyeing cellulose component in P/C blends at long-liquor ratios, are characterised by their low reactivity (fixing at 80oC) and high substantivity.
In order to overcome the drawbacks like leveling, washing-off of the dye, and suitability to “Salt–at–start” process of HE dyes, improvement in the structure of the di–amine linking of the two chromophoric units was suggested. Due to presence of amino linkage there was improvement in fixation of the dye. This technology was successfully marketed by Zeneca under the Procion-HE-XL trade name.

Later in 1980s with the introduction of Sumifix supra dyes by Sumitomo, textile processors have experienced a trend of incorporating different reactive groupings into the same molecule. These Hetero–Bi-functional Sumifix supra dyes contain both MCT and a Vinyl Sulphone reactive grouping. Due to presence of these two different reactive systems, these colours offered good fixation, over a wide range of dyeing temperature and being smaller in size, also offered better wash-off characteristics.
Year 1990s witnessed the development of Fluoro-triazine and low salt reactive systems, which resulted in excellent compatibility for achieving batch–to–batch reproducibility and use of low salt in dyeing. Fluoro-triazine system offers many advantages over chlorotriazines.

The Kayacelon React range of dyes (Nippon Kayaku) is also bi-functional reactive dyes having two Nicotenyltriazine reactive groups in each dye molecule.

Traditional reactive dyes require the addition of a large amount of salt to achieve exhaustion. Salt not only facilitates the binding process of reactive dyes to cellulosic fibre, but also prevents the large scale bonding of water molecules to the -vely charged dyes, which produces inert ‘Dead dye’. This large amount of salt when discharged into bodies of water causes an increase in ecological salinity.

With the use of sophisticated molecular engineering techniques, it has been possible to design reactive dyes (eg, low-salt reactive dyes) with considerably higher performance than traditional reactive dyes. With the introduction of these revolutionary dyes, it is possible to reduce salt requirement by 50-60% based on the weight of fabric dyed. This was a very important development in the history of reactive dyes on the ecological point of view.

Due to introduction of highly electrophilic ‘F’ group in the reactive system, more stable bond formed between cellulose and dye resulted into excellent overall fastness.

Some typical examples of reactive systems for cellulose fibres are reported below.
The late development of fibre-reactive dyes was partly caused by a lack of considerable reactivity of fibres made of cellulose or proteins. Despite the many possible reactive groups in dyes capable of covalent bond formation with nucleophilic groups in wool, only a limited number of types of reactive dye have been commercially successful. Some typical examples of reactive systems for wool or polyamide fibres are reported below.

The most impotent reactive groups in wool are all nucleophlic and found mainly in the side-chains of amino acid residues. They are, in order of decreasing reactivity, thiol, amino and hydroxyl groups. The actual dyes are probably dibromopropionamides, which eliminate HBr on dissolving in hot water. Mythyltaurine-ethylsulphones and 2-sulphatoethylsuphones form the vinyl sulphone reactive group relatively slowly at pH 5-6. This allows some levelling during dyeing before the vinylsulphone dye reacts with the wool and becomes immobilised. Bromoacrylamido groups are stable in boiling water at pH 7 and react by both nucleophilic addition and nucleophilic substitution reaction.

Current trends in reactive dyes & their application
* Warm dyeing application offers many advantages on the ground of utilisation of energy. So, there is a perceptible shift from hot dyeing application to warm dyeing.
* The chlorotriazine group together with vinyl sulphone has dominated on both technical & commercial grounds.
* Vat dyes are being replaced more & more with reactive dyes due to availability of new generation reactive dyes.
* The demand for Right-First Dyeing is increasing.
* The pressure on environmental issues & cost economics is increasing due to stiff competition in the global market.

Future trends in reactive dyes
Research is being carried out for:
* Increasing the robustness of individual dyes and dye combinations in trichromatic systems.
* Enhancing reproducibility of trichromatic combinations used in most commonly applied dyeing processes.
* Reducing salt consumption and/or unused dye in the effluent. (Dyes with no salt, low alkali addition & 100% fixation).
* Improving fastness properties (eg, light fastness, fastness to repeated laundering).
* Polyfunctional dye chemistry to improve reactivity, fixation levels & reproducibility.
* Reactive dyes exclusively for printings, which are different from low- molecular weight MCT dyes.

References
1. Proceedings of ‘Golden Jubilee of Reactive Dyes- Future Trends’- SDC conference organised by Mumbai region.
2. “Cellulosics Dyeing”, Edited by John Shore; SDC, 1995.
3. “Basic Principles of Textile Coloration” by Arthur D Broadbent; SDC, 2001.
4. “Wool Dyeing” by D M Lewis; SDC, 1992.
5. “Textile Processing & Properties” by T L Vigo; ELSEVIER, 1997.

Acknowledgement
The authors are thankful to Prof (Dr) A I Wasif, I/C Principal, DKTES’s Textile and Engineering Institute, Ichalkaranji and management of DKTES’s Textile and Engineering Institute, Ichalkaranji for their encouragement and permission to publish this article.

Note: For detailed version of this article please refer the print version of The Indian Textile Journal February 2010 issue.

R H Deshpande
D.K.T.E.’S. Textile and Engineering Institute,‘RAJWADA’
Ichalkaranji, Dist Kolhapur, Maharashtra 416 115.

Y M Indi
D.K.T.E.’S. Textile and Engineering Institute,‘RAJWADA’
Ichalkaranji, Dist Kolhapur, Maharashtra 416 115.
Mobile: 099234 21408. Email: ymindi@gmail.com.

published February , 2010
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