Structural discussion of reactive dyes those suitable for cotton fibre

There are different types of reactive dyes that are usually compatible with cotton fibre. But molecular structure of cellolusic cotton fibre is complex. So the dyes which have more reactivity is suitable for that complex structure of cotton fibre. The molecular structures of reactive dyes resemble those of acid and simple direct cotton fibre dyes, but with an added reactive group. Usually the structure of  reactive dye assemble with azo anthraquinone  triphenodioxazine or copper phthalocyanine chromophores. The key structural features of a reactive dye are the chromophoric system, the sulphonate groups for water solubility, the reactive group, and the bridging group that attaches the reactive group either directly to the chromophore or to some other part of the reactive dye molecule. Each of these structural features can influence the dyeing and fastness properties of dyed textile material. Most of the commercial reactive dyes have a complete gamut of colours, many of which are particularly bright in color. Reactive dyes often have quite simple structures that can be synthesised with a minimum of coloured isomers and biproducts that tend to dull the shade of the more complex polyazo direct dyes. Some colours are difficult to obtain with simple chromophores. Dark blue and navy reactive dyes are often rather dull copper complexes of azo dyes and the production of bright green reactive dyes usually make. A wide range of possible fibre-reactive groups has been examined and evaluated by the dyestuff manufacturers. The final choices for commercial reactive dyes are limited by a number of constraints. The reactive group must exhibit adequate reactivity towards cotton fibre, but be of lower reactivity towards water that can deactivate it by hydrolysis. The hydrolysis of the dye’s reactive group is similar to its reaction with cellulose fibre but involves a hydroxyl ion in water rather than a cellulosate ion in the fibre. In addition, the dye–fibre bond, once formed, should have adequate stability to withstand repeated washing. Other factors involved are the ease of manufacture, the dye stability during storage and the cost of the final reactive dye.

Reactive groups are of two main types:

(1) Reactive dyes which are reacting with cellulose by nucleophilic substitution of a labile chlorine, fluorine, methyl sulphone or nicotinyl leaving group activated by an adjacent nitrogen atom in a heterocyclic ring.

(2) Reactive dyes those reacting with cellulose by nucleophilic addition to a carbon–carbon double bond, usually activated by an adjacent electron-attracting sulphone group. This type of vinyl sulphone group is usually generated in the dyebath by elimination of sulphate ion from a 2-sulphatoethylsulphone precursor group with alkali.

Although many of the early reactive dyes had only one reactive group in the dyestuff molecule, many of the newer reactive dyes are bifunctional with two or more identical or different reactive groups shows some typical fibre-reactive groups and the commonly used abbreviations for these groups. Dyes with nicotinyltriazine reactive groups (NT) will react with cotton on heating under neutral conditions. 

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After Treatment And Stripping of Reactive Dye

Washing-off of unfixed reactive dyes after dyeing:
Removal of hydrolysed and un-reacted dye from the goods is a vital step after dyeing. The amount of unfixed dye remaining in a cotton fabric dyed with reactive dyes may have to be less than 0.002% owf.

Both batch and continuous washing process involves three stages.
- Initially, the goods are rinsed in cold and warm water. This is a dilution stage in that removing as much salt and alkali from the goods as possible.
- Secondly, soaping stage much more efficient since at lower electrolyte concentrations the substantivity of the dyes less, making its desorption easier.
- Finally, again a warm rinsing to dilute the final dye solution adhering to the fibres to the point that the amount of unfixed dye carried over to the final drying 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.

After-treatment of Remazol dyes:
There are two factors are important during the aftertreatment of Remazol dyeing. The dyeing should not be soaped at the boil under alkaline conditions. This is ensured by neutralizing with acetic acid before soaping. The unfixed (hydrolysed) dye is best removed by boiling with a detergent solution at the boil. It is important to note that the substantivity of the unfixed dye is reduced at higher temperatures and at the boil, the dye has practically no affinity for the fibre and the loosely held dye rapidly diffuse out. If this dye is removed completely, it is not necessary to treat the dyeing with a cationic dye fixing agent to achieve the optimum perspiration and water fastness.

If the boiling after-treatment is carried out under alkaline conditions, some amount of the dye already fixed (reacted with the fibre) is separated by the rupture of the dye - fibre bond, since this bond is not stable to alkali especially at the boil.

When sodium silicate is used as the alkali for fixing the dye, neutralization with acetic acid should not be done. If it is neutralized insoluble silicic acid may get deposited in the fibre producing a harsh feel. In this case, an overflow rinsing with warm water should be given before theboiling after-treatment. (Sodium silicate is more easily washed off than caustic acid). Sodium hexametaphosphate 2 g/l is to be added to the boiling after-treatment bath. 

Stripping of dyed materials:
Partial stripping:
The material is treated in a solution containing 5 to 10 ml glacial acetic acid per 1000 ml. water at 85-95°C until the shade is reduced to the desired extent. In the case of viscose rayon 2.5-10 ml of formic acid (85%) is used in place of acetic acid. The material is then rinsed and soaped at boil for 15 minutes.

Full stripping:
It is difficult to strip fully fixed dyeing completely to a white, but they can be reduced to a form suitable for re-dyeing to dark shades. The material is treated at boil in a solution containing 5 gms of sodium hydrosulphite and 2 gms of soda ash per litre of water for 20 minutes. It is then washed and treated at room temperature in l°Tw sodium hypochlorite, soured, rinsed and soaped at boil for 1 0 - 1 5 minutes.

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Vinyl sulphone dyes or Remazol dyes are good quality Reactive dyes

Vinyl sulphone dye:
The vinyl sulphone group has played an important part in the field of reactive dyes. In1944 Farbwerke Hoech AG prepared reactive dyes with the name of Remazol Which have the formulae

  D-SO₂ –CH₂  -CH₂ –OSO₃H    and    D- SO₂- CH₂ –CH₂Cl

These substance are precursor from which, in the presence of mild alkali, sulphone derivatives are formed in the dyebath.
Other precursor are:
Thio sulphato ethyl sulphone
Dialkayl aminoethyl sulphone
Phosphato ethyl sulphone

In case of both P^H  and temp promote the liberation of the vinyl sulphone group which confers reactivity with cellulosic and protein fibers.

D-SO₂- CH=CH₂  + HO-Cell   =   D-SO₂-CH₂-CH₂-O-Cell

D-SO₂-CH=CH₂  + H₂HR       =     D-SO₂-CH₂-CH₂-HNR  

The dyes according to their type, can be applied at ambient temperature 40⁰C and 60⁰C, the total time required for absorption and fixation being 2 hours, 1.5 hours and 1 hours respectively. 

Application of Remazol dye:

    The application in winch dyeing with Remazol dyes is very easy process. The dyebath is raised between 20⁰ C and  30⁰C and polyphosphate(e.g Calgon), the dissolved dye, electrolyte and appropriate quantity alkali are added. The temperature and the corresponding times of dyeing are as following:

20-30⁰C         Dye for 2 hours

40⁰C               Raise to 40⁰C in 10 minutes and continue to run for 90 minutes in 1.5 hours

60⁰C               Raise to 60⁰C in 20-30 minutes and continue to run for 60 minutes

Dyeing instruction regarding quantities of phosphate, electrolyte and alkali are tabulated below:

Liquor ratio is 1:15 to 1: 30

Dyeing temp                              60⁰C                  40⁰C                  20-30⁰C

Phosphate (g/l)                            1                           1                        1

NaCl (g/l)                                   50                         50                       50

Tri sodium phosphate (g/l)          5-10                    10-15                      -

Or Na₂CO₃(g/l)                          20                          -                          -

NaOH 32.5% ml/l                       1                          1-2                      2-4  

The main feature of vinyl sulphone dyes are:

           • Possess excellent brightness and good wet fastness.

           • The dyes are dischargeable.

           • The dyes are suitable for exhaust and different paddyeing methods and discharge printing.

           • Ease of washing off unfixed dyestuffs i.e.minimum staining of the white ground in printing.

           • They are applicable at 40°C and 60°C.

  

Precaution for dyeing with vinyl sulphone dves:

 For dyeing with vinyl sulphone dyes, it is advisable to ensure that the residual alkali has been removed from the fibre surface or neutralized prior to soaping since hydroxide ion can catalyse hydrolysis of the ether type dye-fibre bond and result in additional colour bleeding from the dye-fibre bond stability at around PH 4-5, whereas the corresponding value for dyes based on halogenated nitrogen heterocles is 6-7. the later type have dye-fibre bonds that are more sensitive to acid-catalysed hydrolysis.

Dye – O – Cell + H2 0 = Dye – OH + Cell – OH

The dyeing temperature and the nature and concentration of the alkali required are determined by the reactivity of the dye. Its degree of sulphonation and its substantivity.

Bi-functional reactive dyes:

The most obvious deficiency of reactive dyes lies in the fact that their dyeing efficiency is significantly less than 100% and may be nearer 70%. More recently, improvements have been made by introducing more than one reactive group into the reactive dye molecule so that even though one group may hydrolyse, there is another left for reaction with cellulose. Dyes with suitable diffsubstantivity properties, but carrying two reactive groups, have been carefully selected.

Bi - functional reactive dyes with two reactive groups of different reactivity towards the cotton, which have different optimal fixation conditions, sjive a more uniform decree of fixation over a wide range of dyeing temperature and fixation PH than dyes containing two identical groups.

Therefore, process control does not need to be so stringent. These types of reactive dyes give quite high fixation yields and thus less colour in the dye house effluent. Other important types of bi-functional reactive dyes include the MFT-VS type (Cibacron C, Ciba) and the MCT-Vs type used in the Sumifix Supra dyes (Sumito). The Kayacelon react range of dyes (Nippon Kayaku) are also bi-functional reactive dyes, having two NT reactive groups in each dye molecule.

There are two types of bi-functional reactive dyes, where one type is homo-functional which have two reactive groups are similar type in nature and another is haterofunctional which have two reactive groups are dissimilar type in nature reactive dyes.      

A major advantage of MCT/VS, dyes over the dyes containing either MCT or VS reactive group is the higher degree of fixation of the former and is 1.3 to 2.3 times  more than the latter. It can easily be assessed that dyeswith two identical reactive groups and dyes with two different reactive group exhibit a higher fixation yield than dyes with one group.

Studies have demonstrated that their excellent solubility, higher degree of fixation, good leveling and good –to-excellent fastness properties etc.

Dyeing method of bi-functional reactive dyes

Exhaust dyeing method:

Bi-functional reactive dyes are applicable by exhaust dyeing method in following different ways.

              1.Increase temperature method

              2.Constant temperature method

              3.All-in method 

Increase temperature method of bi-functional Reactive dye:
Procedure:

 Set the dye bath at 25⁰ C with dye solution and carry out dyeing for 10 minutes. Then add half amount of salt and continue dyeing by raising temperature to 45°C in 10 minutes. Add remaining half the amount of salt at 45°C and. Raise the temperature to 60°C in 10 minutes with continued dyeing. Continue dyeing at 60°C for further 5 minutes and then add half amount of soda ash and continue dyeing for 10 minutes. Add the remaining half amount of soda ash and dyeing is continued for further 50minutes at 60°C. The goods are then washed off. 

Constant temperature method of bi-functional Reactive dye:
Procedure:

Set theusion anSet the dye bath at 60°C with dye solution and carry out dyeing at 60°C for 10 minutes. Then add salt at 60°C and continue dyeing 25 minutes. Add half amount of soda ash

and continue dyeing for 10 minutes. Then add the remaining half amount of soda ash and dye ing is continued for further 50 minutes at 60°C. The goods are

then washed off.

All-in method

This method is particularly suitable for unmercerised cotton to produce deep shades. Moreover this method is applicable to vinylsulphone dyes and hence combination of vinylsulphone dyes is possible.

Procedure

Set the dye bath at 20-30°C. Add salt and continue dyeing for 5-10 minutes. Then add dye solution and continue for 15-20 minutes. Now add half amount of alkali and dye for 5-10 minutes. Add remaining half the amount of alkali and dye for further 5-10 minutes. Then raise the temperature to 60 C in 20 minutes (1-5⁰ C rise/minutes) and continue dyeing at 60°C for 45-60 minutes.

Salt and alkali requirements:

% shade	Glaubers salt (g/l)	Soda ash (g/l)
1.0 to 2.0	35-50	15
2.0 to above	50	15-20

For light shades of less than 1%, quantity of Glauber’s salt and soda ash to be reduced appropriately.

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