DYEING METHODS OF REACTIVE DYEING

Basic method of reactive dyeing:

Three basic steps are as follows for reactive dyeing of cellulose fibres with reactive dyes:
1. Exhaustion of the reactive dye from an aqueous bath containing common salt or Glauber’s salt normally used in neutral condition.
2. Addition of alkali to promote to further reactive dye uptake and chemical reaction of absorbed reactive dye with the fibers.
3. Dyed material with reactive dye is then rinsed and soaped to remove electrolyte, alkali and unfixed reactive dyes.

Parameters of dye absorption for reactive dyeing:
This is the first dyeing phase, that takes place in the same way as cellulose fibre dyeing with non reactive soluble dyes; in this phase the reactive dye does not decompose, but only diffuses towards the interior of the fibre, where it is absorbed by the cellulosic chains by secondary type forces.

The following dyeing parameters influence the absorption:

(a) Nature of the reactive dye
(b) Liquor ratio of reactive dye bath
(c) Electrolyte concentration in side the dye bath
(d) PH of the liquor
(e) Temperature of the reactive dye bath
(f) Type of the fiber to be dyed with reactive dye
(g) Reactive Dyeing time
(h) Alkali concentration in the liquor

Dyeing method for Reactive dyes: 

1.Discontinuoues method of reactive dyeing
2.Semi continuous method of reactive dyeing
   a) Pad jig reactive dye method
   b) Pad batch reactive dye method
3.Continuoues method of reactive dyeing
  a) Pad steam reactive dye method
  b) Pad thermofixation reactive dye method

CONTINUOUS METHOD OF REACTIVE DYEING:
Reactive Dyeing Process of Pad steam method:

This method is suitable for all types of reactive dyes. Recipe is given below:

Shade	Dyestuff g/l	Migration inhibitor	Soda ash g/l	Salt g/l	Urea g/l
Light	0-10	Small quantity of Na-alginate	5	40-60	150
Medium	10-20	Same	5-10	60-80	150
Heavy	>20	Same	10-20	80	150

Reactive Dyeing Process of pad steam method:
In pad steam method of reactive dyeing in the 1st bowl padding is done with reactive dye solution and urea at a temp of 60-800C. Here urea helps to dissolve the reactive dye completely in water and also to penetrate the reactive dye within the fiber. In some industries Na-alginate is used which act as a migration inhibitor may be add in the 1st bowl and other industries used Na-alginate in a separate bath just after padding with reactive dyes and urea. After padding with reactive dyes, urea and migration inhibitor, the fabric is then added with soda ash and salt at 600C which is known as chemical padding. Then the fabric is passed through the steam chamber at a temp of 100-1020C where it takes 30-60 sec for fixing the reactive dye with the fabric. Then the fabric is subsequently rinsed, soaped at boil and finally washed and dried to complete the dying operation.

Reactive Dying Process of Pad thermofixation method:
Recipe:
Dye – 2% on the weight of material
Soda ash – 1% on the weight of material
Salt – 8% on the weight of material
Urea – 15% on the weight of material
Na-alginate – Little amount

Process of reactive dyeing:
In this method reactive dyestuff, alkali(soda ash), salt, urea and Na-alginate are mixed in the dyebath. The fabric is padded in the padding bath at room temperature. Then the padded fabric is dried in the drying unit. After drying the fabric is treated for 5 minutes at 1600C in a heating chamber for thermofixation. After thermofixation the fabric is rinsed, soaped and rinsed. This method is suitable for moderate reactivity dye.

Semi continuous method of reactive Dyeing:
Pad batch reactive dyeing method:

Recipe:
Dye – 3% on the weight of material
Soda ash – 15 g/l
Salt - 60 g/l
Urea – 150 g/l
Na-alginate – Little amount

Process of reactive dyeing:

The fabric is padded with reactive dye, alkali, salt, urea and Na-alginate at a temp of 20-250 C. The padded fabric in roll form is transferred to a chamber where the temp is 200C and relative humidity is 100%. Four such rolls are prepared at the same time and they are kept at revolving condition so that dyes can not be settled due to centre of gravity at the lower part of the roll causing unevenness. On the basis of reactivity batching is done for 2-4 hours.Then the fabric is rinsed, soaped and rinsed. This method is suitable for high reactivity dye.

Pad-Jig method of reactive dyeing:
Process of dyeing:

In this method of reactive dyeing the fabric is padded with dye liquor in a padding mangle. No alkali and salt is added in this stage of padding. When the fabric is padded with reactive dye it is worked at room temperature. The padded fabric is then transferred to a jigger dyeing m/c. In this machine the dye is developed with the addition of salt and alkali. After the fixation of the dye in the jigger dyeing machine the fabric is rinsed, soaped and rinsed thoroughly. This method is suitable for closely woven fabric and also for fabrics made with hard twisted yarn.

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TYPES AND REACTIVITY OF REACTIVE DYES

Types of reactive groups of reactive dyes:
The reactive dyes fall essentially into two classes:
1. Those that react by nucleophilic substitution mechanism based on the presence of halogen
substituents in a heteroatomic systems e.g. the chlorotriazinyl dyes.

D – X-+ Cell-OH^-                  D – O – Cell + HX^-

Among the principle reactive systems of the type are the halogen substituted triazine, pyrimidine , pyrozine, quinoxaline, thiazole and pyridazone groups.

2. Those that reacting with cellulose by nucleophilic addition to a carbon – carbon double bond, usually activated by an adjacent electron – attracting

sulphone group.

Dye – SO₂ – CH = CH₂ + Cell – OH ^-                    Dye – S0₂ – CH₂ – CH₂ – O Cell  

Classification of Reactive dye:
1.On the basis of Reactive group of reactive dyes there are two types
 a) Halogenated heterocycles (Example: Diclorotriazine, Tetrachloropyrimidine, Monochlorotriazine etc)
 b) Activated vinyl compound (Example: Vinyl sulphone, Vinyl acrylamide, Vinyl sulphonamide etc)

2.On the basis of Reactivity of reactive dyes there are three types
 a) High reactivity (Example: Dichlorotriazine)
 b) Moderate reactivity (Example: Vinyl sulphone)
 c) Low reactivity (Example: Trichloropyrimidine, Dichloroquinoxaline etc) 

Modern classification of reactive dyes:  Reactive dyes have recently been classified as

1. Alkali-controllable reactive dyes, which have relatively high reactivity and only moderate substantivity. The reactive dyes are applied at relatively low temperatures and level dyeing requires careful control of the addition of the alkali to initiate the fixation stage. Examples include DCT, DFCP and VS reactive dyes. 

2. Salt-controllable dyes. These are dyes of relatively low reactivity towards cotton under alkaline conditions and therefore the'dyeing temperature will be as high as 80°C. They have appreciable substantivity and level dyeing requires careful addition of salt to promote exhaustion. Examples in this class include TCP, MCT as well as MFT reactive dyes. 

3. Temperature-controllable dyes, which undergo fixation at high temperatures even under neutral conditions. The NT dyes are in this class.A

Reactivity and affinity of the dyes:
If the reactivity of the dye is increased considerably, the rate of reaction with the fiber increases. Therefore, the dyeing can be carried out in a short time. However, in this case the rate of hydrolysis of the dye also increases, leading to deactivation of a part of the dye. This results in wastage of the dye. If, on the other hand, the reactivity of the dye is decreased, the extent of hydrolysis can be reduced considerably. However, this results in the slower rate of reaction with the fiber also.

Role of electrolyte in the dyebath:
When a fiber is immersed in water, a negative electrostatic charge develops on its surface. This charge repels any dye anions present in the solution, so that, the fiber cannot be dyed satisfactorily. If, however, the dyebath also contains an electrolyte such as sodium chloride or sodium sulphate, a diffuse layer of positive sodium ions forms at the fibre surface, neutralizing its charge. The dye ions are then able to approach sufficiently closely to the fiber for the inherent attractive forces between the dye and the fiber to operate.

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Characteristic, Properties and Nature of Reactive Dyes

Nature of reactive dyes:  

The molecular structure of reactive dyes are smaller than those of direct dyes, and their smaller size is accompanied by a correspondingly lower substantivity. The molecules of  direct dyes are made deliberately large so as to build up the physical attraction between fibre and dye, thus making them more substantive. Smaller molecules may be suitable for use as reactive dyes because one covalent bond between textile fibres and reactive group of reactive dye is about thirty times as strong as one Vander Waals bond between fibre and dye. As the size of a dye molecule becomes larger the colour it imparts becomes duller. Reactive dye molecules therefore confer very bright colours to dyeings because their molecules need be no larger than those of simple acid dyes.

Properties of Reactive dye:

1. During the dyeing process the reactive group of the dye forms covalent bond with the fibre. 
2. Solubility in water of those dyes are very effective.
3. Reactive dyes are normally used for dyeing cellulose(usually cotton fibre) and protein(usually wool) fibre. 
4. All types of shades are available from reactive dyes. 
5. The dyeing is done in alkaline condition. 
6. Wet fastness of finished textile materials are well. 
7. The brightness and rubbing fastness are comparatively well. 
8. The costs of  reactive dyes are cheap. The dyeing process is very easy. 

Comparison among  properties of major types of Reactive dye:

Reactive Group	Commercial name	Reactivity	Exhaust Dyeing Temp(°C)
Dichlorotriazine (DCT)	Procion MX (Zeneca)	High	2 5 - 40
Monochlorotriazine (MCT)	Procion H (Zeneca)  Procion EXL(Zeneca) Basilen (BASF) Cibacron E (Ciba)	Low	8 0 - 85
Monofluorotriazine (MFT)	Cibacron F (Ciba)	Moderate	40 - 60
Difluorochloropyrimidine (DFCP)	Drimarene K (Clariant) LevafixE-A(Dystar) Levafix P-A(Dystar)	Moderate to low	3 0 - 50
Dichloroquinoxaline (DCQ)	LevafixE (Bayer/ Dystar)	Low	5 0 - 70
Vinyl sulphone (VS)	Remazol (Dystar)	Moderate	4 0 - 60
Trichloropyrimidine	DrimareneX (Clariant) Cibacron T (Ciba)	Low	8 0 - 95
Nicotinyltriazine (NT)	Kayacelon React (NipponKayaku)	Moderate to high	100-130

Characteristic structural feature of reactive dyes:

The simplest way of representing the reactive dyes follows:
S       C       B         X
Where,

S is the solubilising group
C is the chromophoric group
B is the bridging group
X is the halogen containing reactive group

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