Introduction
- Reactive dyes may be defined as colorants that contain reactive groups capable of forming covalent bonds with nucleophilic sites in fibrous substrates.
- They’re water-soluble anionic dyes and retain substitutive or additive reactive groups in the structure.
- Mostly contain azo, anthraquinone, triphenodioxazine, phthalocyanine etc. chromophores and are suitable for cellulosics, wool, nylon and silk
- Full shade range is available
- Affinity for wool is very high posing a problem on the production of level shar
- Light fastness seems to be very good in most of the cases
- Dyeings show excellent wash fastness except with cold brands
- Dyes underghydrolysis
Components of Reactive Dyes
The general formula of reactive dyes can be represented as: Where, S-C-B-R
S = Solubilizing groups (such as SO, Na, COONa etc.) which impart solubility. C = Chromophoric group like azo, anthraquinone etc. The chromophore is responsible for the color, affinity and diffusion of dye. B = Bridging group which attach the reactive system to the chromogen, this group is usually -NH, -O-, -NHCO-, -OCH,-, -SO-, etc. R = Reactive system or group, which reacts chemically with the functional group of the fiber with the formation of covalent bond between the dye and the fiber.
Classification:
On the basis of reactive group
- Halogen derivatives of heterocyclic nitrogen
- Triazine group (like Procion, Cibacron)
- Pyridimine group (like Reactone)
- Quinoxaline dyes (like Levafix)
- Activated vinyl compound:
- Vinyl sulphone (like Remazol)
- Vinyl acrylamide (like Primazine)
- Vinyl sulphonamide (like Levafix) On the basis of reactivity
Typical fibre-reactive groups in commercial reactive dyes
- High reactivity, e.g. Procion-E [Medium alkali (NaHCO3) used)
- Moderate reactivity, e.g. Levafix -E [Medium alkali (Na,CO₂) used)
- Low reactivity, e.g. Premazine [Strong alkali (NaOH) used)
On the basis of dyeing temperature:
- Cold brand-These types of dyes contain reactive group of high reactivity. So dyeing can be done in lower temperature i.e. 32-60°C. For example: Procion M, Levafix E.
- Medium brand-This type of dyes contains reactive groups of moderate reactivity.So dyeing is done in higher temperature than that of cold brand dyes i.e. in between 61°C and 71°C temperatures. For example, Remazol, Levafix are medium brand dyes.
- Hot brand-This type of dye contains reactive groups of least reactivity. So high temperature is required for dyeing i.e. 72-93°C temperature is required for dyeing. For example, Pricion H, Cibacron are hot brand dyes.
Basic Principle of Dyeing Cellulosics
Properly pre-treated substrate undergoes 3 stages of dyeing:
Exhaustion: Reactive dyes possess negligible affinity for cotton. Cotton and dye both acquire negative charge when dipped in bath and repel each other reducing the substantivity of dye. Salt is applied to reduce potential difference between the two resulting better exhaustion. It acts as exhausting agent and helps in migration of dye molecules from bath on to fibre.
Fixation: An appropriate alkali is added to the dyebath to increase its pH. This initiates the desired dye-fibre reaction. The hydroxyl groups in cellulose are 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.
Cell-OH + HO– ➤ Cell-O + H2O
After-treatment: After dyeing, any unreacted and hydrolysed dye present in the cotton must be removed by thorough washing. Otherwise, desorption of dye during washing by the consumer can cause staining of other materials. 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 cellulosate ion in the fibre. Being highly reactive due to presence of two chlorine atoms, cold brands are more susceptible to hydrolysis and fixation rate is also high. In contrast, due to presence of only one chlorine atom or reactive site, hot brand and remazol types are either hydrolysed or react with cotton but do not stain adjacent ground or garments during washing and are extensively used in padding and printing.
The washing processes involve three stages:-
Initially, the goods are rinsed in cold and warm water. This is a dilution stage aimed at removing as much salt and alkali from the goods as possible. 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 fibres to the point that the amount of unfixed dye carried over to the finaldrying is minimal.
This residual quantity of dye will be deposited on the fibre surface on evaporation of the water during drying and will be easily removed by later washing. Obviously, the amount must be as small as possible.
Application Methods
Discontinuous:
Conventional method
Exhaust or constant temperature method High-temperature method
Hot critical method.
Semi-continuous: The pad-dry process is only suitable for reactive dyes with fairly high reactivity. For dyes of lower reactivity, the dried fabric must be baked to promote further fixation. In pad-dry method, fabric is padded with dye, NaHCO3 (15-20 g/l), urea (50-100 g/l) and wetting agent (2-5 g/l), followed by batching for 2-12 h or drying at 100- 110°C when water evaporates and transports dye at the interior of fibre for fixation. NaHCO, raises dye bath pH to facilitate fixation. Urea acts as potential solvent, activates moisture to retain soluble form of dye and is essential for deeper shades. This helps the cotton fibres retain water during drying, possibly provides a fluid medium for dye diffusion in the fibres at low water contents, and increases dye solubility. Unfortunately, all this urea is removed on washing after fixation and poses a pollution problem. Urea is a source of nitrogen nutrients for algal growth.
Continuous: Pad-dry-bake process is used mainly in the application of disperse dyes to cotton/polyester materials using the Thermosol. Baking temperatures of 200-220 °C for 1 min are typical but may be lower (150-170 °C) if only cotton is being dyed. In Pad-dry-pad-steam method, the fabric is padded with a neutral solution of the reactive dyes, dried and then padded with the alkali solution containing salt before steaming. Steaming may be done at 105°C for 5 min. To avoid the decomposition of dye during steaming, resist salt may be incorporated in padding liquor. Reactive dyes, being water soluble, migrate during drying throughout cotton from a point at lower temperature to a point at higher one as evaporation of water from latter point is fast causing movement of water molecules along with dye. Drying must be uniform at each point of fabric to avoid development of unlevelled shades.
Factors Affecting Dye Uptake
Affinity and reactivity of dye: Affinity of reactive dyes is directly proportional to their molecular structure-dyes of lower molecular weight have lower affinity, lower will be its hydrolysis and vice-versa. Selection of a dye with higher affinity results in higher dye uptake at shorter time indeed but with an inferior fastness-partially hydrolysed dyes are not completely removed during soaping and washing. So, fastness is to be ensured through repeated washing and
soaping. Otherwise, slow removal occurs during domestic washing cycles causing poor wash fastness.
Liquor Ratio: Hydrolysis of dye increases with increase in liquor in bath. A lower liquor ratio is the preferred one keeping in mind solubilisation of huge amount of dye, salt and alkali. But too lower a ratio may result uneven dyeing due to higher effective dye concentration in bath and higher strike ratio.
Alkalinity or pH of bath: Dye bath should be free of alkali till exhaustion is completed to resist premature fixation of dye and corresponding uneven dyeing. Rise in pH up to 11 increases exhaustion and reactivity, but beyond this, exhaustion decreases. Excess alkali in bath promotes hydrolysis of dye.
Nature of fibre: Dye uptake is also influenced by accessible free volume in fibre – higher the volume, higher will be dye uptake. Scoured cotton has the least free volume, bleached cotton has little higher and mercerized cotton has the highest free volume due to removal of more impurity or immature fibre. Viscose has more free volume than mercerized cotton and so dye uptake will be according to these free volume.
Time of dyeing: Reactive dye solution must not be stored for long time; otherwise hydrolysis will occur with loss of colour value.
Dyeing of Wool:- The most important reactive groups in wool are all nucleophilic and are found mainly in the side-chains of amino acid residues. They are, in order of decreasing reactivity, thiol (the -SH of cysteine), amino (-NH-and-NH, of say histidine and lysine) and hydroxyl groups (-OH of serine or tyrosine). Reaction with amino groups is most likely because their concentration in wool is relatively high. In wool at around pH 4.5 the number of dye sites is at a minimum and equally distributed between positive and negative. As the pH becomes more acidic more positive dye sites are produced. Going to pH above 4.5 increases the number of negative sites. With most reactive groups, the minimum hydrolysis does occur at about pH 5, which assists in improving yields and fastness. Lanasol reactive dyes contain one or two bromo-acrylamide reactive groups, which form a covalent bond with the nucleophilic groups of the wool’s amino acids during the dyeing process, resulting in outstanding wet fastness properties. Drimalan F dyes contain the 2,4-difluoro-5-chloro
-pyrimidine (FCP) reactive group. After dyeing, the material can be washed at 80°C for about 15 min using a dilute ammonia solution at pH 8.0-8.5, and then rinsed in water with a little acetic acid. To avoid any alkali damage to the wool, washing can be done with hexamine at pH 6.5, or with sodium bicarbonate.
Dyeing of Silk:- Silk is a natural polymer consisting of a large number of amino acids of the general formula NH,CHRCOOH. There are 17 amino acids in silk protein with glycine (45.9%), alanine (29.12%), serine (10.9%), and Tyrosine (5.2%) constituting 92%. Silk is damaged in an alkaline medium at high temperatures. Protonation of the amino and imino groups in silk results in a great reduction in their nucleophilicity. Dyes with higher reactivities are found to react with fibroin in the pH range 5.5-7.5. Salts may be required because the carboxylic acid groups of silk gain anionic charges in water which repulse the anionic reactive dyes.
Dyeing of Nylon:- At decreased pH, amount of protonated amino groups increases which can absorb more anionic reactive dyes. So, the exhaustion increases.But when it comes to fixation, nylon fiber can form covalent bond through nucleophilic substitution or addition
reaction with the free amine, not the protonated amine. Dyeing is reported at near boiling temperature for one hour.
Few of the references:
➤Basic Principles of Textile Coloration by Arthur D Broadbent
➤Fundamentals and practices in colouration of textiles by J N Chakraborty
➤https://www.woolwise.com/educational-resources/awttc-resources/course-4-contemporary- wool-dying-and-finishing/
➤ Advances in Silk Science and Technology edited by Arindam Basu
➤Handbook of Textile and Industrial Dyeing-Volume 1, edited by M. Clark
Author: Md. Rakibul Islam CA BUTEX
