More description about Reactive dye is given below:
Classification of Reactive Dye
1) On the Basis of Reactive Group: Two types
A) Halogenated heterocycles:
B) Activated Vinyl compounds:
a) Vinyl Sulphone (D-SO2-CH2-CH2-) Example: Ramazol
b) Vinyl Sulphonamide (D-SO2-NH-CH2-CH2-) Example: Levafix
c) Vinyl acrylamide (D-NH-CO-CH2-CH2-) Example: Primazine
2) Classification on the basis of reactivity
On the basis of reactivity reactive dyes are of three types. These are:
i)Low reactive dye
Reactivity of these dyes is low. So, highly alkaline environment is required for the fixation of these dyes with substrate. Here pH is maintained 12-12.5 by using NaOH in bath.Example: Cibacron T, Drimarene
ii) Medium reactive dyes:
These are medium reactive dyes. Here pH is maintained 11-12 by using Na2CO3 (Soda ash) in dye bath. Example: Levafix E, Remazol
iii) Higher reactive dye:
These dyes are highly reactive .So, fixation of these dyes is easy and lower alkaline medium is kept. Here pH is maintained 10-11 by using NaHCO3 in dye bath. Example: Procion MX, Drimarene KLR
3) On the basis of Temperature and method
i) Cold Brand Dyes
These types of dyes are highly reactive as they posses highly reactive group in their constituent. So dyeing of these dyes can be done in lower temperature i.e 32-60o C. Example : Procion Mx, Levafix E etc.
ii) Medium Brand
These types of dye contain reactive group of moderate reactivity.So dyeing is done in higher temperature than that of cold brand. Here temperature is kept between 60o-71oC
iii) Hot brand
These types of dyes contain reactive groups of least reactivity. So high temperature is required for dyeing i.e. 72-90o C temperature is kept for dyeing. Example: Procion H, Cibacron T etc.
Chemical Classification of Reactive dyes:
Reactive dyes can be classified chemically into three different groups
1) Chlorotriazinyl Reactive dyes
a) Monochloro dyes
2) Vinyl Sulphone dyes
3) Heterocyclic helogen containing Reactive dyes-
Modern classification of reactive dyes:
Reactive dyes have recently been classified as
Alkali-controllable reactive dyes
- These dye 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
- These are dyes of relatively low reactivity towards cotton under alkaline
conditions and therefore the dyeing temperature will be as high as 80oC.
- They have appreciable substantivity and level dyeing requires careful addition of salt to promote exhaustion
- Example in this class include TCP, MCT as well as MFT reactive dyes.
- Temperature-controllable dyes, which undergo fixation at high temperatures even under neutral conditions. The NT dyes are in this class.
Hydrolysis is a chemical process in which a molecule is converted into two parts by the addition of molecule of water. One fragment of parent molecule gains a hydrogen ion (H+) from the additional water molecule. The other group collects the remaining hydroxyl group (OH-)
Hydrolysis of Reactive Dye/Technical defficiency of R.D:
Under alkaline condition, Reactive dyes react with the terminal hydroxyl group of cellulose. But if the solution of the dye is kept for long time, its concentration drops. Then the dye reacts with the hydroxyl group of water. The reaction of drops. Then the dye reacts with the hydroxyl group of water. The reaction of dye with water is called Hydrolysis of reactive dye.
After hydrolysis dye cannot react with fibre. So hydrolysis increases the loss of dyes.
This hydrolysis dye occurs in two stages. At first the concentration of dye initially increases and then begins to decrease. where as the concentration of hydroxyl compound increases continuously. Then the hydroxyl compound cannot react with dye.
i) Incase of Triazinyl dyes:
ii) In case of Vinyl Sulphone Dyes:
- b) Dye-SO2CH=CH2+ H-OH==Dye-SO2 CH2CH2OH
For preventing hydrolysis the following precautions are taken-
- As hydrolysis increases with increasing temperature during dissolving and application temperature should not be more than 40oC.
- Dye and alkali solution are prepared separately and mixed just before using.
- Dye and alkali should not be kept for long time after mixing.
Factors affecting Dye Hydrolysis:
- Liquor Ratio: Longer liquor ration increases hydrolysis and vice versa
- Salt Concentration: Higher salt concentration increases hydrolysis and vice versa
- pH: Higher pH increases hydrolysis and vice versa
- Temperature: Higher temperature increases hydrolysis and vice versa
- Dye reactivity:Increase both hydrolysis and fixation rate
- Time: Higher dyeing time increases hydrolysis and vice versa
- Type and number of reactive group: Vinyl Sulphone is more prone to hydrolysis than Triazinyl group
- Types of bridging group: The oxide (O-) and sulphide bridges are less stable in alkaline hydrolysis. But more stable bridges decreases reaction rate along with the hydrolysis
Potential problems due to dye hydrolysisl:
1)Hydrolysis accompanies fixation, resulting in incomplete utilization of dye. Hence dye wastage occurs. Up to 40-60 % dyes (avg. 50%) are wasted in this case
2) Relatively large amount of electrolyte are required for exhaust, otherwise dye hydrolysis wil occur greatlydye bath
3) Laborious removal of unreacted and hydrolyzed dye is required- often a longer operation than the dyeing step itself and not always entirely satisfactory.
4) Longer washing operation for removal of unreacted and hydrolyzed dye often costs 50% of total dyeing cost.
5) Hydrolyzed dye is discharged as colored effluent and effluent cost risen up.Moreover, color is not easily removed by effluent treatment processes and in many cases the dyes are not readily biodegradable.
6) Unhydrolyzed, unfixed heterocyclic reactive dyes may pose in environment problems associated can occur due to hydrolysis.
7) Dispite reactive dyes being covalently bond with the substrate, fastness
8) Less storage stability
9) Training problems for continuous dyeing.
10) Running shade in batch process