Comparison Between Reactive Dye and Azoics Dye | Comparison Between VatDye and Azoic Dye

Vat Dye vs Azoics Dye 

1. Azoics are economical than Vat dyes.
2. Bright reds , oranges and deep maroon shades are possible in Azoics.
3. Limited Shade range in Azoics.
4. Bright pinks , bright Blue . olive green , green and blacks are not possible in Naphthols.
5. No fear of photodegradation in Naphthols.

6. Rubbing na dlight fastness is inferior in Naphthol dyeing.

7. More Suitable for Tie-dye and space dyeing.

Reactive Dye Vs Azoics Dye
1. Limited shade range in Azoics.

2. Dyeing method is very complex with too many variables in naphtholation , diazotisation and developing baths.
3. Color fastness to Chlorine is better in naphthols.
4. Rubbing and Light fastness are inferior in azoics.
5. Dark and bright reds , maroons and ornages can be produced in azoics which are difficult in reactives.
6. Azoics are suitable for Tie-dye and space dyeing.

Introduction of Reactive Dye | History of Rective Dye | Uses ofReactive Dye

Dye that reacts with the textile fiber to produce both a hydroxyl and an oxygen linkage, the chlorine combining with the hydroxyl to form a strong ether linkage; gives fast, brilliant colors.

History of Reactive Dyes
Reactive dyes first appeared commercially in 1956, after their invention in 1954 by Rattee and Stephens at the Imperial Chemical Industries Dyestuffs Division site in Blackley, Manchester, United Kingdom.

Reactive dyeing is now the most important method for the coloration of cellulosic fibres. Reactive dyes can also be applied on wool and nylon; in the latter case they are applied under weakly acidic conditions. Reactive dyes have a low utilization degree compared to other types of dyestuff, since the functional group also bonds to water, creating hydrolysis of Reactive Dye.

Reactive dyes have good fastness properties owing to the bonding that occurs during dyeing. Cotton is made of cellulose molecules which react with the dye .During reactive dyeing the H atom in the cellolose molecule combines with the cl atom in the dyeing process and results in a bond. Trifunctional dyestuffs also exist.

Uses of Reactive Dye

Reactive dyes are used to dye cellulosic fibres. The dyes contain a reactive group, either a haloheterocycle or an activated double bond, that, when applied to a fibre in an alkaline dye bath, forms a chemical bond with an hydroxyl group on the cellulosic fibre.

Hydrolysis of Reactive Dye | Removing Process of Hydrolysis of ReactiveDye | Why Low Affinity Reactive Dyes are Preferred for Dyeing?

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,
 D-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.

Why 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.

Stripping Process of Reactive Dye

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 

Important Factors of Cold Brand Reactive Dye | Factors of Reactive Dye

Important factors for dyeing cellulosic fibre with cold brand reactive dye in batching process:

The Important Factors are as Follows:

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 20o to 25oC. Soda ash has been the best alkali for dyeing at 30oC 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 20o to 25oC.

2) Amount of Alkali:

The amount of alkali used for fixing depends on the depth of shade dyed and the liquor ratio employed. 

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 20oc, the rate of fixation is very low. Hence for most of the dyes a temperature of 20o to 25oC is the recommended temperature while for some other dyeing at 50o to 60oC 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.

Different Methods of Reactive Dye Application | Pad-batch Method | PadDry Method | Pad Steam Method

Different methods of reactive dye application:

1) Pad-batch method.

Pad batch processes are of two types-
a) Pad (alkali)-batch (cold) process.
b) Pad (alkali)-batch (warm or hot) process. 

2) Pad dry method
3) Pad steam method.

1) Pad-batch method

a. Pad (alkali)-batch (cold) process.

Figure: Pad-batch method

Steps:
1) The fabric is first padded in a padding mangle with reactive dye in presence of an alkali.
2) The padded fabric is rolled in a batch and the batches are wrapped by polyethylene sheets and stored in wet condition for 1-24 hours at 200-300C in a room.
3) During the storage period, the rolls may be kept slowly rotating to prevent seepage of the dye liquor.
4) After storing time is finished fabric is washed in a rope washing machine to remove the unfixed dye from fabric surface.

b. Pad batch (hot) process:

Figure: Pad batch (hot) process

Steps:
1) The fabric is first padded in a padding mangle with reactive dye in presence of an alkali.
2) The fabric is then passed in between infrared heater to preheat the padded fabric to 500C to 900C.
3) The fabric is then batched on a large diameter roller in a hot chamber. The batching is done under controlled conditions of temperature and humidity for a sufficient time to ensure diffusion and fixation of the dye in the fibre. During this period the batch is kept slowly rotating to avoid the seepage of dye liquor.
4) The cloth is then washed in a rope washing machine to remove the unfixed dyes.

2) Pad dry method:

Figure: Pad dry method

Steps:
1) Fabric is first padded in a padder with reactive dye in presence of an alkali.
2) Padded fabric is then passed through a squeezing roller into a dryer. As a dryer cylinder, stenter etc may be used. During drying due to higher temperature fixation of dye in fibre increases and at the same time water is removed by evaporation.
3) After drying fabric is washed in a washing machine to remove unfixed dye.

3) Pad steam method:

Figure: Pad steam method

Steps:
1) Fabric is first padded in a padder with the dye.
2) It is then passed through between two squeezing roller in a dryer. Drying should be done slowly; otherwise precipitation of dye due to quick removal of water may take place leading to lower color value.
3) After coming out from dryer fabric is padded in a padder containing salt and alkali. Due to salt exhaustion of dye takes place and due to alkali fixation occurs.
4) Fabric then passed through a steamer where it is kept for 15-19 second. Due to high temperature here fixation rate increases.
5) In this step fabric is washed in a washing machine to remove the unfixed dye. 

Knit Dyeing with Reactive Dyes(Hot Brand) | Knit Dyeing with Hot BrandReactive Dyes

Knit dyeing with reactive dyes :

Recipe:

1. Anti creasing agent = 0.3g/L
2. Sequestering agent = 0.5 g/L
3. Glauber salt or NaCl =80 g/L
4. Dye (reactive) = x%
5. Soda ash =5g/L
6. Or caustic soda =1 g/L
7. Acetic acid = 0.75 g/L
8. Soap = 0.25 g/L
9. M: L = 1:10 
10. Time =60 min
11. Temperature = 600-1000C

Function of these Ingredients:

•  Anti creasing agent is used to remove crease mark from fabric.
•  Sequestering agent is used to convert hard water into soft water.
•  Gluber salt is used for exhaustion of dye in the fibre.
•  Soda ash and caustic soda are used for fixation of dye in the fibre.
•  Acetic acid is used for neutralizing the dyed fabric.
•  Soap is used for washing the dyed fibre.

Dyeing Curve:

Dyeing Curve

Dyeing Procedure:

At first fabric, required water and required anti creasing agent is added in the dye bath. Then sequestering agent and gluber salt of required amount is added in the dye bath. Then the bath is kept rest for 5 minutes. After that reactive dye of required amount is added in the dye bath. After adding dye in the dye bath, the bath is kept for 30 minutes. During this period exhaustion of dye occurs in the fabric. Then required amount of alkali is added for fixation of dye into the fabric. After adding alkali we will wait for 50 minutes and then we will check the shade. If shade is all right then fabric will be taken for after treatment.

After Treatment of Reactive Dye:

1) At first dyed fabric will be treated with hot water at 800C for 10 minutes.
2) Then the fabric will be treated with stock solution of acetic acid for 10 minutes at 600Cfor neutralizing the fabric.
3) Then the fabric is washing with soap solution for 15 minutes at 950C.

Dyeing Mechanism of Reactive Dye

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

1. Exhaustion of dye in presence of electrolyte or dye absorption.
2. Fixation under the influence of alkali.
3. Wash-off the unfixed dye from material surface.

Now they are mentioned below:

1. 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.

2. 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: - 

3. 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.