- 1 Introduction
- 1.1 Properties Of Reactive Dyes
- 1.2 Classification Of Reactive Dyes
- 1.3 Classification Based On Reactive System
- 1.4 Chemistry Behind Reactive Dyeing
- 1.5 Cold Brand And Hot Brand Reactive Dyes
- 1.6 Types Of Reactive Dyes
- 1.7 Mechanism Of Reactive Dyes On Cotton
- 1.8 On-Going Research On Salt-free Dyeing Of Cotton Fabric Using Reactive Dyes
- 1.9 Related
Reactive dyes chemically react with cotton fiber and form the covalent bond and become a part of the fiber.The reactive dyes contain several groups that are shown in below.
- Water-solubilizing group.
- Chromophore group.
- Bridging group.
- Reactive group.
- Leaving group.
In 1956 Rattee + Stephen (ICI) introduced “first” reactive dyes– chlorotriazines.
Properties Of Reactive Dyes
- It has sulphonic acid groups in the molecules and readily soluble in water.
- Less substantive than direct dyes, hence more salt is required for exhaustion.
- Dyestuff reacts and combines chemically (covalently) with cellulose, so called reactive dyes.
- Easy penetration and good leveling property.
- Moderate to good light fastness and wash fastness properties.
- Formation of covalent bond occurs in alkaline medium.
- These dyes, unlike any other class of dye stuff, react and combine chemically (covalently) with cellulose and this leading to excellent wash fastness.
- These dyes give very bright shades such as orange, pink, magenta etc, which were not possible with other class of dyes.
- They do not react with water nearly as readily as with cellulosic hydroxyl in alkaline conditions so that they can be applied from an aqueous solution.
- Reactivity of the dyestuffs can be reduced when desirable by blocking one of the reactive chlorine atoms giving H-type Procions.
- Procions are dyes with small molecules; their molecules do not have to be very long as those of direct dyes to match the distance between absorption sites on the fiber. Short molecules bring two advantages (a) Clarity and brightness of hue and (b) easy penetration and therefore good leveling.
- Because there is some, even although not very much reaction between procions dyestuffs and water, it is very important to wash the dyed fiber thoroughly clean and free from the reaction product with water.
- The formation of the covalent bond between dye and fiber occurs under alkaline conditions. The presence of acids may reverse this process. Perspiration and atmospheric pollution which are both slightly acid may affect textile materials colored with reactive dyes and result in some fading
- Reactive dyes can be applied to cellulosic fibers by exhaust (batch), pad batch (semi-continuous), continuous dyeing method.
Classification Of Reactive Dyes
Reactive dyes can be classified basically into three groups
Group 1: Alkali Controllable
Group 2: Salt Controllable
Group 3: Temperature Controllable
On the basis of reactive system, reactive dyes can be classified as
Monofunctional Reactive Dyes
Dyes are characterized by the presence of reactive groups- one or more reactive species at individual locations in the dye molecule.
Examples of this type are mono chloro- triazine, dichloro triazine, and vinyl-sulphone dyes.
Bi-Functional Reactive Dyes
These dyes are characterized by the presence of two reactive groups of same type ( MCT or DCT) or different type(MCT&VS) at two different locations in the dye molecule.
Bifunctional again divided into
- Homo bifunctional- dyes having two reactive systems of same type (triazine or vinyl-sulphone).
- Hetero bifunctional – dyes having two reactive systems of mixed type (triazine – vinyl-sulphone).
- These dyes have High exhaustion and high fixation with good color yield.
- Less pollution.
- Very popular for exhaust dyeing applications.
- High exhaustion due to high molecular weight of dyes.
- High fixation due to the presence of two reactive systems.
Classification Based On Reactive System
Reactive dyes form a covalent bond between fiber and dye. They are classified depending on the reactive group present and the optimized conditions in which they are best used. Depending on the type of reaction, the reactive dyes are broadly divided into two categories:
- Dyes reacting through Nucleophilic substitution reactions.
- Dyes reacting through Nucleophilic addition reactions.
A. Dyes Reacting Through Nucleophilic Substitution Reactions
(1) Dichlorotriazynilamino Types Of Dyes
Structure Of Dichlorotriazynilamino Dyes
These are more reactive than a monochloro type of dyes and require lower temperature and milder alkali for dyeing and fixation. These are known as Cold brand reactive dyes.
(2) Monochlorotriazynylamino Type Of Dyes
Structure Of Monochlorotriazynylamino Dyes
These require higher temperature and stronger alkali for dyeing and fixation. These are called hot brand reactive dyes.
B. Dyes Reacting Through Nucleophilic Addition Reactions
Dyes Containing Vinyl Sulphone Group
As such this is not soluble in water, so it is marketed in its soluble form i.e., β – hydroxy ethylene sulphone sulphuric acid ester derivatives.
RSO2 –CH2 -CH2OSO3Na.
Chemistry Behind Reactive Dyeing
The dyeing principle is based on fiber reactivity and involves the reaction of a functional group of the dyestuff with a site on the fiber to form a covalent link between the dye molecule and the substance.
The Four structural feature of typical reactive dyes molecule are:
- The chromophoric grouping, contributing the color.
- The reactive system, enabling the dye to react with the hydroxy group in cellulose.
- A bridging group that links the reactive system to the chromophore.
- One or more solubilizing group, usually sulphuric acid substituent attached to the chromophoric group for their color, although the azo chromophore –N=N- is by itself the most important.
All the reactive dyes contain sodium sulphonate group for solubility and dissolve in water to give colored sulphonate anions and sodium cations. Most reactive dyes have one to four of these sulphonate group, General form of the reactive dye is as follows:
S = Water solubility group.
R = Chromophore.
X = Reactive System.
B = Bond between reactive system and Chromophore.
Cold Brand And Hot Brand Reactive Dyes
The reactivity of these dyes is due to the chlorine atoms attached to the triazine ring. When two chlorine atoms are present in the dye molecule are called Dichloro Procions.
Dichloro procions are referred to as M-type procions and their characteristic is that they will combine with alkaline cellulose at room temperature (20oC to 30oC) and hence they are called cold brand reactive dyes when only one chlorine atom is present in the dyestuff molecule, the reactivity of the dye decreases considerably and the dyeing has to be carried out at a higher temperature (65o C to 80oC).
Hence these dyes are called hot brand reactive dyes. Mono chloro- procions are referred to as ‘H’ type procions. normally’ Brands are suitable for dyeing and H brands are suitable for printing.
The instability of the solutions of the cold-dyeing procion colors was a serious disadvantage in their application to textile printing. The stock solution for printing must be kept and used for several hours.
The reactive dyes having only one chlorine atom in their molecule are less reactive. Their aqueous solutions, therefore, are more stable and very suitable for printing.
Types Of Reactive Dyes
- Cold brand reactive dye -‘M’ brand
- Hot brand reactive dye -‘H’ brand (or ‘X’ brand)
- High exhaust reactive dye -‘HE’ brand
- High exhaust reactive dye -‘ME’ brand
- Vinyl Sulphone reactive dye – for dyeing and printing
- Low salt reactive dye – LS dyes
Mechanism Of Reactive Dyes On Cotton
Monochlorotriazinyl, Reactive Dyes (Or) Procion + H Brand
The mono chlorotriazinyl dyes or hot brand reactive dyes are less reactive. Their aqueous solutions are more stable but they do not react with cellulose so readily and the temperature of dyeing must be increased to 60oC to 70oC and in some cases, as high as 90oC to 95oC.
The dyestuffs are dissolved by making them into a paste with cold water, followed by dilution with more water.
Cibacron Brilliant Red B
The dye bath is made by dissolving sufficient common salt to give a concentration of 40 to 80 parts per 1000 according to the depth of the shade, the goods already having been entered. The temperature is brought up to 40oC and the machine is run for five to ten minutes in order that the fabric can be uniformly impregnated.
The pre-dissolved dye is then added in two portions at five-minute intervals followed by the alkali which should be 20 parts per 1000 of soda ash. The alkali should be added in two portions at five-minute intervals and the temperature then raised to between 60 to 90oC according to that which is recommended for the dye selected.
Dyeing is continued for a period of 20 to 40 minutes and then continued for 30 to 60 minutes when soda ash (Alkali) is used. After dyeing, the materials must be rinsed and soap boiled to remove hydrolyzed and unfixed dye.
- Treat the dyed material with 1 to 2 g/l of neutral soap at the boil for 15 minutes and wash it.
- Treat the soaped material with 2 to 3 g/l of cationic dye fixing agent at 40oC for 20 to 30 minutes and dry it.
On-Going Research On Salt-free Dyeing Of Cotton Fabric Using Reactive Dyes
The cellulosic fabric is dyed with reactive dyes require the large amount of salt, which pollutes fresh water.
Due to the hydrolysis of the dye, the dyeing effluent consists of a large amount of hydrolyzed dye, and it requires the high volume of water to remove the hydrolyzed dye in the washing process.
The cotton fabrics were dyed with reactive dyes using the conventional method and pretreating the fabric with Polyvinylamine Chloride.
Pretreated samples were dyed without using salt as an electrolyte.
It was found that pretreatment of cotton fabrics with Polyvinylamine Chloride increases the dye uptake and shows good wash fastness and rubbing fastness.
There was a slight increase in crease recovery angle and flexural rigidity in the pretreated sample. It is considered that Polyvinylamine Chloride is found to be effective for pretreatment in the salt-free dyeing of cotton fabrics.
It has been found that pretreatment of cotton before dyeing can offer a simple and effective method of improving dye-fiber affinity, avoiding the need for salt as an electrolyte in the dye bath.
It has been found that Poly (Vinylamine Chloride) [PVAmHCl] is a physical modifying agent.
Its wide range of properties has found use in Catalysis, Chelating, liquid chromatography, treatment of wastewater, recovery of oil and in polymeric dyes.it involving chemical modification of Cellulosic.
Nonreactive pre-treatments including some polymers with affinity for cellulose tend to be desorbed during dyeing and inhibit uptake of dye or cause it to precipitate.
This study has established the value of polymeric quaternary ammonium compounds, amines or amides, which may be attached to cotton by non-chemical mechanisms.
Despite the encouraging results obtained with non-reactive polymers in the salt-free dyeing of cotton, problems remain in dye selection and obtaining level results.
The aim of this work to determine the effectiveness of PVAmHCl as pretreatment agent of cotton fabrics in improving its dyeability with reactive dyes and in achieving evenness of dye uptake.
It was also to determine the effectiveness of pretreatment of dyed fabrics on K/S value and fastness properties like wash fastness and Rub fastness.
Various physical properties like tensile strength, flexural rigidity, cloth crease recovery angle and aerial density were also determined to see the effect of PVAmHCl.
Results obtained were analyzed to arrive at some advantages of the pretreatment.