Reactive dyes are extensively used in textile in- dustry because of their wide variety of colour shades, high wet fastness, profiles, the simplicity of application, brilliant colour and minimum energy consumption. It is evident that this technology offers the dyers many options for producing full bright and tertiary shades on cotton, viscose , linen fibres. In light of above, there is still great scope for further research and development to maximize the benefits of reactive dye systems. Since their introduction in the 1950s re- active dyes have shown rapid growth and even today, developments are taking place. The combination of brightness, fastness, and ease of application associated with reactive dyes enables this class of dyes to dye cellulosic fibres.
The structure of a reactive dye is shown below:
Properties Of Reactive Dyes
- Reactive dye is anionic in
- Reactive dye is a water-soluble
- They have better wash and light fastness
- They have better
- They form a strong covalent bond with the cellulosic fibre
- The alkaline condition is a must required for
- The electrolyte is the must for exhaustion of dyes in the
- A wide range of colour can be
- Comparatively cheaper in
Classification Of Reactive Dyes
- Alkali-Controllable Dyes
Low substantivity reactive dyes, often described as Malakai-controllable dyes; addition exhibit rapid washing-off properties. Their level dyeing properties have to be regulated by slow addition of alkali. This is done in order to encourage migration of dye to the substrate during addition and also to prevent the phenomenon of simultaneous exhaustion and fixation.
- Salt-Controllable Dyes
Medium-high substantivity reactivity dyes often described as salt-controllable dyes, have to exhibit slow washing off characteristics. Their level of dyeing characters are to be controlled during the electrolyte addition stage.
- Temperature-Controllable Dyes
These dyes react with cellulose above the boil in the absence of alkali. They can be applied with group 2 dyes, with alkali fixation being done, between 80-100 degree C. These dyes are self-levelling.
Constitutional Characteristics Of Reactive Dyes
The four characteristic features of a typical reactive dye molecule are a reactive group, a chromophoric group, a bridging group and a solubilizing group.
- Reactive Groups
Reactive dyes owe their covalent bond forming ability to the presence of the reactive groups in their structure.
- Monofunctional Reactive
These systems can react only once with the nucleophilic groups in the fibre.
- Bifunctional Reactive
Bifunctional reactive dyes contain two separate reactive centres for reaction with suitable groups in the fibre. They also have the potential to combine with more than one group in the fibre chain molecule.
- Homobifunctional Reactive
These dyes consist of two similar reactive groups,e.g. of which are shown below:
- Heterobifunctional Reactive
These dyes consist of two different types of reactive groups.
- Chromophoric Groups
Chromophoric groups contribute colour to textile fibres. The proper selection of chromophores for com- mercial reactive dyes is essential to achieving a given shade area. In practice, mono-azo, diazo, metallized mono-azo, formazan, anthraquinone, triphenodioxazine & phthalocyanine chromophores have been used for the preparation of reactive dyes.
- Azo Reactive
- Anthraquinone Reactive
- Phthalocyanine Reactive
- Triphenodioxazine Reactive
- Formazan Reactive
- Bridging Groups
A bridging group is a group that links the reactive system to the chromophore. These groups are necessary for synthetic reasons, they also influence the reactivity, degree of fixation, stability of reactive dyeing & other dyeing characteristics such as substantivity & migration, significantly.
- Solubilising Groups
Solubilising groups provide characteristics such as water solubility, substantivity, migration & wash off. The dominant solubilising group in reactive dyes is the sulphonic substituent.
General Factors Affecting the Results of Reactive Dyeing
- Affinity Of The Dye
The affinity of a dye for a textile fibre is determined by its molecular structure and therefore is not under the control of the dyer except that he can select dyes from among the available dyes, those most suited to his particular process of dyeing.
- Material To Liquor Ratio
The liquor ratio has a powerful effect in determining exhaustion, and this is particularly significant with low-affinity reactive dyes.
- Concentration Of Electrolyte
In the first phase of dyeing, the reactive dyes behave
like direct dyes and therefore electrolytes are used for improving the exhaustion percentages.
- PH Of Dye Bath
The ph of fixation of reactive dyes on cotton and viscose actually depends on individual dyes and the temperature and time of dyeing.
- Time of Dyeing
The dye is generally added to the bath into the portions. The salt may also be added in two lots. Exhaustion normally takes place in 20-30 minutes. There is no specific advantage in extending the exhaustion phase beyond 30 minutes. The alkali is then added also in two lots, and the dyeing continued for 30-90 minutes.
- Nature Of Fibre
There are marked differences in the degree of exhaustion obtained with reacting to dyes on different cellulosic fibres. Viscose rayon gives the highest exhaustion and cotton the lowest with mercerized cotton occupying an intermediate position.
Versatility of Reactive Dyes
- Use Of Reactive Dye in Thermal Transfer Printing Process
A formulation and a method of printing anion or melt- able in a layer having a reactive dyes or mixtures of reactive dyes and disperse dyes as colorants. The melt layer also includes an alkaline substance, a binder and optionally, a heat activated printing additive. Permanently bonded colour images are provided by the reaction between the reactive dye and the final substrate, which may be any cellulosic, protein, or polyamide fibre material.
- New Reactive Dye Ink Set Added Dupont Artistry Ink
Dupont Digital Printing, a leading supplier of inkjet inks announced the addition of a new reactive dye in the set to their portfolio of digital textile inks at the Drupa 2008 trade show in Dusseldorf, Germany. The inks can be fixed by the various methods of fixation typically used in conventional printing with reactive dyes and provide high wash fastness and perspiration tolerance making them ideal for apparel application. Reactive dyes occupy an important position for dyeing cellulosic fibres, but this is not the case in the dyeing of natural and synthetic polyamide fibres; however, it is likely that environmental pressures will increase the usage of reactive dyes in the latter area. Cellulosic fibre reactive dye systems pose environmental question due to their current high salt requirements and coloured effluent discharge. In the case of polyamide fibres such as wool, reactive dyes give good uptake and fixation efficiencies and their usage is expected to grow since they offer the possibility to replace chrome dyes.
Many of the reactive dyes available today contain two reactive groups. Such dyes are known as bifunctional reactive dyes and are further classified as homobifunctional , containing two identical reactive groups (e.g. the ICI Procion HE dyes with two aminochloro-s-tri- azine groups), and hetero-bifunctional, containing two different reactive groups (e.g. the Sumitomo Sumifix Supra dyes with aminochloro-s-triazine and sulphatoethylsuphone groups) (Taylor, 2000). Technically, the bifunctional dyes have increased probability of reac- tion with the fibre.
However, these additional reactive groups can have an impact on important dyeing properties such as substantivity and migration because they change the molecular size and alter the extent and mode of reactivity. Therefore, the idea of additional reactive group (s) has been more of patenting interest (Taylor, 2000). A careful combination of reactive groups can make polyfunctional dyes superior to traditional dyes.
Commercial mixtures of selected reactive dyes have become popular because they provide an economic range of secondary and tertiary colours (Taylor, 2000). At the same time, the commercial mixtures can have improved dyeing properties and reduced sensitivity to dyeing variables such as temperature and pH and other factors.
Several tri-, tetra-, and penta-functional dyes have been developed. Some have been introduced to the market (Taylor, 2000; Lewis, 2009). Ongoing improvements from further R&D in this area are expected to come.