Why low affinity reactive dyes are preferred for dyeing?

Low affinity reactive dyes are preferred for dyeing:

             If the reactivity of the dye is increased considerably, the rate of reaction with the fibre increases. There fore, the dyeing can be carried out in a short time. However in this case the rate of 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 fibre also. The ultimate object of dyeing is to react as much of the dye ass possible with the fibre and minimize the hydrolysis of the dye. This is achieved in practice in two stages. The dyeing is first started from the aqueous medium under neutral conditions when the dye does not react either with the fibre or with water. Then gluber salt or common salt is added to exhaust the dye onto the fibre as much as possible. In this respect, this stage of dyeing (exhaustion) resembles the dyeing of direct dyes on cotton. Then the second step (that of fixation or reaction with the fibre) is carried out by adding the alkali (usually used soda ash).  Since the exhausted dye is already on the fibre, it is more likely that the exhausted dye reacts with the fibre in preference to water. However the dye present in the dye bath (which contains a substantial amount of the reactive dye) can now react with water since it is under alkaline condition. It is already   stated that the hydrolyzed dye cannot further react with the fibre but dye to the affinity forces; it is absorbed by the fibre and is retained in it. During the subsequent washing or soaping the substantivity held hydrolyzed dye gets stripped into the washing bath thereby reducing the washing fastness of the dyeing. If the affinity of the original dye is reduced to a very low value, this problem will not arise and a rigorous treatment of the dyeing with boiling soap or detergent solution removes almost all hydrolyzed dye. However if the affinity is very low, exhaustion of the dye bath prior to fixation cannot be achieved substantially. This results in a larger amount of the reactive dye remaining in the dye bath and getting hydrolyzed when alkali is added subsequently. If the dye has high affinity for cellulose like a direct dye, it becomes difficult to remove the hydrolyzed dye from the dyeing since it is also absorbed by and retained in the fibre by fairly strong affinity forces, through not as strong ass the covalent bond formed between the dye and the fibre. Hence in actual practice low affinity dyes are selected for converting in to reactive dyes.

Important factors for dyeing cellulose fibre

Important factors for dyeing cellulose fibre

 (with cold brand reactive dye in batching process):

1) pH of the dye bath:

          The optimum pH for fixing cold brand reactive dyes on cotton and viscose rayon depends on individual dyes, the temperature and time of dyeing. pH decreases with increasing temperature and time of dyeing. For most of the dyes the optimum pH is 10.8 to 11 at 20^o to 25^oC. Soda ash has been the best alkali for dyeing at 30^oC for cotton, mercerized cotton and linen. Increased fixation (due to higher temperature) and increased dye bath stability and better reproducibility are the advantages of soda ash as the fixing agent.

          For viscose rayon the optimum pH is 10.3 at 20^o to 25^oC.

2) Amount of alkali:

          The amount of alkali used for fixing depends on the depth of shade dyed and the liquor ratio employed. Some quantities of alkali required for fixing the reactive dyes are given table 28.

3) Dyeing temperature:

          Since increase in temperature affects the rate of physical and chemical processes involved in dyeing, it is important in dyeing reactive dyes also. The affinity of the dye for the fibre decreases with increases in temperature and at the same time the rate of hydrolysis of the dye increases and adversely affects the fixation of color yield. However the rate of diffusion of the dye in the fibre increases with increased temperature. At temperatures lower than 20^oc, the rate of fixation is very low. Hence for most of the dyes a temperature of 20^o to 25^oC is the recommended temperature while for some other dyeing at 50^o to 60^oC with sodium bicarbonate as the alkali gives maximum color value.

4) Electrolyte concentration:

          Since reactive dyes have low affinity for cellulose exhausting the dye bath by adding common salt or Glauber’s salt prior to fixation can increase the fixation. The amount of salt required producing adequate exhaustion decreases with decreasing liquor ratio. Thus for pale shade on cotton and viscose rayon 15 and 10 g/l of common salt used. The quantities may be increased to 30 and 20 to 30 g/l for medium and deep shades on these fibres.

5) Time of dyeing:

          Generally the dye may be added in two portions. The salt may also be added in two lots. The exhaustion takes place in 20 to 30 min. There is generally no advantage in extending the period beyond 30 min. The alkali is then added and the dyeing continued for 30 to 90 min. The depth of shade and reactivity of the dye decide the time of dyeing. For deeper shades larger times are required.

6) Liquor ratio:

          With decreased liquor ratio, both exhaustion and fixation take place to increased exert. However the rate of fixation of most of the dyes is not significantly affected. As the liquor ratio is decreased, the effectiveness of increasing salt addition also decreases. Hence lower amount of salt are sufficient to get optimum exhaustion.

Necessity of alkali for dyeing with reactive dye

Necessity of alkali for dyeing with reactive dye

 (Described by vinyl sulphone reactive dye)

Reactive dyes belonging to vinyl sulphone system has become popular with the dyers as those belonging to the chlorotriazine system.

The structure of a typical dye of this group Remazol Red B is shown below:

These dyes may be represented by the general formula, 

D-SO₂-CH₂-CH₂-OSO₃K

where D is the color forming part and contains one or more solubilising groups.

          In this form of the dye, the sulphonyl induces polarization of aliphatic chain attached to it, by which the alpha carbon atom in the sulphonyl group has an electron restraining effect and the carbon atom in the beta position gains a positive charge –

                              D-SO₂-CH₂-CH₂-OSO₃Na

          The electrons belonging to the hydrogen atoms linked to the alpha carbon atom are nearer the carbon atom than the hydrogen atoms. However, the electrons in the neighborhood of the beta carbon atom are not as strongly linked to this atom. At the alpha carbon atom there is a tendency to liberate a proton and the –SO₃Na group linked to the beta proton is likely to separate as an anion. Both these tendencies favor the following reaction in an alkaline medium.

          D-SO₂-CH₂-CH₂-OSO₃Na + NaOH→ D-SO₂-CH=CH₂ + Na₂SO₄ + H₂O

          This reaction takes place almost immediately on addition of an alkali to a solution of the original dye in water.

          The vinyl sulphonyl group (-SO₂-CH=CH₂) shows strong polarization caused by the sulphonyl (-SO₂) group:

         

                                                  D-SO₂-CH=CH₂

          This activated double bond can then react with water, alcohols and cellulose, forming ethers by the addition reaction:

                              D-SO₂-CH=CH₂ + R-OH → D-SO₂-CH₂-CH₂-OR

          In the dyeing of cellulose with vinyl sulphone dyes the reaction with cellulose is always accompanied by reaction with water with the formation of hydroxyl ethyl sulphone dye, D-SO₂-CH₂-CH₂-OH, which then cannot react with cellulose.

          Both the reactions (with fibre and with water) are catalyzed by alkali. Thus alkali is required in the dyeing of cellulose with these reactive dyes for converting the suphato ethyl sulphone dye to vinyl sulphone dye with the cellulosic fibre.

(in the first case, alkali is consumed while the other alkali acts only as a catalyst)

The hydroxy ethyl sulphone dye (the reaction product of the vinyl sulphone dye and water) cannot react with cellulose and has very low affinity for cellulose.

In neutral solutions, the sulphatoethyl sulphone dyes (as marketed) do not undergo any chemical reaction; the aqueous dye solution may ever by boil without the risk of losing the reactivity by the formation of the hydroxy ethyl sulphone form of the dye.

Hydrolysis of reactive dye & prevention of hydrolysis

Hydrolysis of reactive dye:

    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 react with the hydroxyl group of water. This reaction of dye with water is known as hydrolysis of reactive dye. After hydrolysis dye cannot react with fibre. So hydrolysis increases the loss of dyes.

This hydrolysis 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.

1.    Hydrolysis of halogen containing reactive dye,

     R-Cl  +  H-OH     →     D-R-OH  +  H-Cl

2.    Hydrolysis of activated vinyl compound containing dye,

                D-F-CH₂-CH₂-OSO₃H  +  H-OH   →   D-F- CH₂-CH₂-OH + H₂SO₄

For preventing hydrolysis the following precautions are taken—

1.    As hydrolysis increases with increasing temperature during dissolving and application temperature should not be more than 40°C.

2.    Dye and alkali solution are prepared separately and mixed just before using.

3.    Dye and alkali should not be kept for long time after mixing.

Stripping of reactive dye

Stripping

        The reactive dye cannot be satisfactory stripped from fibre due to covalent bond between dye molecule and fibre. Stripping becomes necessary when uneven dyeing occurs.  By stripping azo group (--N=N--) from the dye is removed. Now the stripping processes are described:-

1.    Partial stripping:

   Partial stripping is obtained by treating the dyed fabric with dilute acetic acid or formic acid. Here temperature is raised to 70-100°C and treatment is continued until shade is removed by desired amount. After that a through washing is necessary to remove the product of hydrolysis. The amount of acid used is as below: -

Glacial acetic acid     : 5-10 parts

With water                        :1000 parts

Or

Formic acid                       :2.5 to 10 parts

With water                        :1000 parts

Temperature                      : 70 - 100°C

Time                                 : until desired shade is obtained.

2.    Full stripping:

  For complete stripping the goods are first treated with sodium hydrosulphite (hydrose) at boil then washed off and bleached with 1% sodium hypochlorite (NaOCl) at room temperature. This is carried out for 30 min.

                  The recipe is as below: -

                   Na-hypochlorite : 1% at room temperature

Na-hydrosulpite: at boil.

Time                  : 30 min

Application method of reactive dye

APPLICATION METHOD

Application method of reactive dyes varies significantly with various types of dyes, shade required, and available equipments in the mill. These are 3 application procedures available:

1.    Discontinuous method-

-Conventional method

-Exhaust or constant temperature method

-High temperature method

-Hot critical method.

      2. Continuous method-

                                 -Pad-steam method

                                 -Pad dry method

                                 - Pad thermofix method

     3. Semi continuous method-

                                        - Pad roll method

                                        - Pad jig method

                                        - Pad batch method.

Dyeing mechanism of reactive dye

Dyeing mechanism

The dyeing mechanism of material with reactive dye takes place in 3 stages:-

A.    Exhaustion of dye in presence of electrolyte or dye absorption.

B.    Fixation under the influence of alkali.

C.    wash-off the unfixed dye from material surface.

Now they are mentioned below:

A.    Dye absorption:

When fibre is immersed in dye liquor, an electrolyte is added to assist the exhaustion of dye. Here NaCl is used as the electrolyte. This electrolyte neutralize the negative charge formed in the fibre surface and puts extra energy to increase dye absorption. So when the textile material is introduces to dye liquor the dye is exhausted on to the fibre.

B.    Fixation:

  Fixation of dye means the reaction of reactive group of dye with terminal –OH or-NH₂ group of fibre and thus forming strong covalent bond with the fibre and thus forming strong covalent bond with the fibre. This is an important phase, which is controlled by maintaining proper pH by adding alkali. The alkali used for this purpose depends on brand of dye and dyeing temperature. Here generally caustic soda, soda ash or NaHCO₃ is used as alkali depending upon reactivity of dye. They create proper pH in dye bath and do as the dye-fixing agent. The reaction takes place in this stage is shown below: -

1. D-SO₂-CH₂-CH₂-OSO₃Na + OH-Cell  ⟶ D-SO₂-CH₂-CH₂-O-Cell + NaHSO₃ 

2. D-SO₂-CH₂-CH₂-OSO₃Na + OH-Wool ⟶ D-SO₂-CH₂-CH₂-O-Wool + NaHSO₃

3.

 

C. Wash-off: 

As the dyeing is completed, a good wash must be applied to the material to remove extra and unfixed dyes from material surface. This is necessary for level dyeing and good wash-fastness. It is done by a series of hot wash, cold wash and soap solution wash.

Criteria for cellulose for attracting reactive dye. Reactive rate of some compounds

Criteria for cellulose for attracting reactive dye

The chemical structure of cellulose macromolecule is given below:

In cellulose macromolecule glucose units are linked through oxygen bridges formed between C₁ position of one glucose and C₄ position of adjacent glucose unit. Each glucose unit contains one primary hydroxyl group (at C₆ position) and two secondary hydroxyl group =CHOH (at C₂ and C₃ positions). Again one end of this glucose

Unit has an additional secondary hydroxyl group at C4 position and the other end has an aldehyde or hemiacetal group at C1 position. Now the following things are considered.

1.    Primary hydroxyl group (-CH₂OH) at C₆ position is more reactive then the secondary hydroxyl groups (-CHOH) at C₂ and C₃ positions.

2.    C2 hydroxyl group is supported to be more acidic than c3 hydroxyl group under suitable alkaline condition and hence is more reactive.

3.    The hemiacetal group at C1 position is the most active while the additional hydroxyl group of C4 position is the least reactive.

4.    The reaction between reactive group and cellulose takes place predominantly with primary hydroxyl group to some extent.

5.    Longer carbon chain lowers the rate of reaction.

6.    Incase of monochloro triazinyl dyes, this reaction takes place 15 times more frequently with C6 hydroxyl group than with the hydroxyl group at C2 or C3 position.

7.    In case of dichlorotriazinyl dyes, this reaction takes place 3-7 times more frequently with hydroxyl group at C2 position than that with hydroxyl group at C1 or C3 position.

The reactive rate of some compounds are mentioned below:

COMPOUND                       STRUCTURE                            REACTIVITY 

Water                                    H-OH                                        1.0

Iso-propanol                         CH₃-CHOH-CH₃                       0.7

Ethanol                                 CH₃-CH₂-OH                            7.4

Methanol                               H-CH₂-OH                               12.3

Glucose                                C₆H₁₂O₆                                   5.5

   So from the above table it is obvious that secondary hydroxyl group is the beast reactive while primary one is the most reactive.

Reaction mechanism of reactive dye and cellulose

Reaction mechanism of dye and cellulose

The bonding behavior of dye and cellulose are mentioned below:

1.  Hydroxyl group of cotton polymer takes part in reaction with reactive group of   dye

2. Reactive group of dye react preferably with the hydroxyl group of cellulose than that of water.

3. The activation energy of dye – water reaction is 16.4 – 26.2 kcal and that of dye cellulose is 9.2 – 15.8 kcal. As the latter is less, so that occurs predominately.  

4.Higher activation energy causes slower reaction.

5.The reaction with water and dye takes place in a smaller extent.

6.The strength of covalent bond formed between cellulose polymer and reactive group is more than hydrogen bonding, vander wall’s force of attraction and metal co-ordination bonds.

7.Extreme acidic and alkaline condition should be avoided, otherwise hydrolysis will take place resulting bond breakage and poor wash fastness.

Factors considered for selection of dyes

Dye selection depends upon the following factors:

1. Selection of dying method: 

dye selection depends on dyeing method, which may be -

          1. Batch wise/ discontinuous method

2. Semi-continuous method i.e.

                              a) Pad – batch method

                              b) Pad - jig method

                              c) Pad – roll method

3. Continuous method i.e.

                              a) Pad - steam method

.                             b) Pad – dry method

                              c) Pad – thermo fix method

     

         This dyeing method selection depends on:

1.    Speed of dye diffusion on the fibre.

2.    Affinity of dye to fibre

3.    Reactivity to dye stuff.

2. Selection of brand: Brand selection is important. It may be

1.    Hot brand – Less reactive dye (temp 72-93)

2.    Medium brand – Medium reactive dye.

3.    Cold brand – Most reactive dye.

3. Economy of production

4. Availability of dyes

5. Storage of dyes.

6. Bond stability i.e. kind of bonding           

7. Fastness of dye i.e. washes, light, rubbing fastness       

8. Re – producibility

Assistants used for dyeing with reactive dyes

The following assistants are used in dye bath for dyeing with reactive dyes.

1)    Salt:

  As a salt NaCl is used widely. The salts do the following things-

-        Salts are used to increase the affinity of dye to fibre.

-        It decreases the hydrolysis rate of dyes.

-        It neutralizes the electro negativity of fibre surface when immersed in solution.

-        It puts extra energy to push the dye inside the fibre polymer i.e. increase absorption of dye.

           The amount of salt and used depends upon the shade to be produced.

                   For light shade – 10gm/ L salt is used.

                   For medium shade – 20 gm/ L salt is used.

                   For dark shade – 30gm/L salt is used.

2)    Alkali:

 Alkali is used for the following purposes:

-        Alkali is used to maintain proper pH in dye bath and thus to create alkaline condition.

-        Alkali is used as a dye-fixing agent.

-        Without alkali no dyeing take place.

-        The strength of alkali used depends on the reactivity of dyes.

-        As strong alkali caustic soda is used to create pH 12.5-12.

-        As medium alkali soda ash (Na2CO3) is used to create pH 11-12 when dye is of medium reactivity.

-        As weak alkali NaHCO₃ is used to create pH 10-11 when dye is highly reactive.

 3. Urea:    Urea is used in continuous method of dying. It helps to get required shade of dye.   To get dark shade more urea is used and to get light shade less amount of urea is used.

     

     4. Soaping:   By soaping, the extra color is removed from fibre surface. Thus  washing fastness is improved. Soaping increases the brightness and stability of the dye.

Reason of popularity of reactive dye. Why it is superior to vat dyes?

Popularity of reactive dye

Reactive are mostly used for dyeing cellulosic fibres. At past cellulosic fibres were dyed with direct and vat dyes, but after the introduction of reactive dyes there utility has become limited. Reactive dyes are superior to direct dye in the following aspects:

i)                Ability to procedure bright shades of wide range.

ii)              High leveling quality.

iii)            Good washing fastness.

iv)            Good light fastness.

And it is superior to vat dyes in the following aspects:

i)                Simple dyeing method therefore one stage dyeing.

ii)              Low temperature dyeing (below 100⁰C)

iii)            Lower cost, i.e. cheaper.

Again its dyeing process is fast and gives brighter shades than metallized azo dyes. For the above reasons reactive dyes are more popular.