Knitted fabric properties

Wales per cm of knitted fabric

Wales per cm is a very much important properties knitted fabric. Wales are perpendicular lines of loop along the knitted fabric length. Wales is the column of loops along the length of the fabric. Wales determine the width of the knitted fabric and are measured as Wales per centimeter. The number of Wales per cm is depend the types of knitted fabric. For examples, 10 wales per cm is taken for single jersey knitted fabric and 7 wales per cm is taken for rib fabric knitting, it is measured along the course direction by using counting glass and needle.

  

Process sequence for measuring wales per cm of knitted fabric:

· When loop transfer occurs it is possible to transfer a wale A to another B and to recommence knitting with the second needle in which case more than one needle will have produced intermeshed loops in the same wale. 

· In warp knitting a wale can be produced from the same yarn if the same guide laps the same needle at successive knitting cycles . 

· Wales are connected together across the width of the fabric by sinker loop(weft Knitting) or under laps (Warp knitting) 

· Wales show most clearly on the technical face and technical back of single needle bed fabric. 

· For single jersey fabric wales show on face side as V shape and from back side as half circle.

Course per cm of knitted fabric:

A course is a predominantly horizontal raw of needle loops produced by adjacent needle during the same knitting cycles. Course is a row of loops across the width of fabric. A course determines the length of fabric and measured as course per centimeter. The number of course per centimeter is called course/cm. It Is measured along the wale direction by using counting glass and needle.

REACTIVE DYES FOR WOOL FIBRES

Fig: Types of reactive dyes for wool

 

Fig: Reactions of bromoacrylamido reactive dyes with wool

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BATCH DYEING OF WOOL WITH REACTIVE DYES

Wool fibre is a animal protein fibre. The chemical construction of the wool fibre is very much complex but contains a reactive functional group that used to react with the reactive group of reactive dyes. The somewhat late development of fibre-reactive dyes was partly caused by a lack of appreciation of the considerable reactivity of made of cellulose fibres (specially cotton fibre) or proteins fibres (specially wool fibre).

A number of dyes developed for wool with 2-sulphatoethylsulphone or chloroacetylamino groups were not immediately recognised as reactive dyes. In fact, the development of reactive dyes really started with the introduction of the Procion reactive dyes for cotton by ICI in 1955. Despite the many possible reactive groups in reactive dyes capable of covalent bond formation with nucleophilic groups in wool, only a limited number of types of reactive dye have been commercially successful. Figure-1 shows the major types of reactive groups. The dye chromophores are essentially those used for cotton fibre reactive dyeing. The most important reactive groups in wool fibre are all nucleophilic and are found mainly in the side-chains of amino acid residues.






































They are, in order of decreasing reactivity, thiol (the –SH of cysteine), amino (–NH– and –NH2 of say histidine and lysine) and hydroxyl groups (–OH of serine or tyrosine). Difluorochloropyrimidines undergo aromatic nucleophilic substitution of one or both fluorine atoms, the fluorine between the two nitrogen atoms being the most reactive. Bromoacrylamido groups are stable in boiling water at pH 7 and react by both nucleophilic addition to the double bond and nucleophilic substitution of the bromine atom. They can form a three-membered aziridine ring that can react further with the protein fibre resulting in a new crosslink. The actual reactive dyes are probably dibromopropionamides, which eliminate HBr on dissolving in hot water. Methyltaurine-ethylsulphones and 2 sulphatoethylsulphones form the vinyl sulphone reactive group of reactive dyes relatively slowly at pH 5–6 (1 h at the boil). This allows some textile levelling during dyeing before the vinyl sulphone dyes reacts with the wool fibre and becomes immobilized. 

This is useful in hank and winch dyeing where the liquor/goods interchange is less favourable. In fact, all commercial reactive dyes for wool have absorption rates that are greater than the rate of reaction with the wool fibre to allow some migration. Chloroacetylamino groups (–NHCOCH2Cl) react by an SN2 mechanism (Scheme 16.5). Wool reactive dyes are applied like acid dyes in weakly acidic solution. The degree of exhaustion and fixation are usually very high and clearing of unfixed dye from the goods may only be needed for deep shades. Reactive dyes for wool tend to be unlevel dyeing and are prone to give skittery dyeings. They are used more on loose fibre and slubbing than on piece goods, where they accentuate fibre nonuniformity and poor, uneven fabric preparation. A number of amphoteric or weakly cationic auxiliary products are available to assist level dyeing. Despite their good light fastness and very good washing fastness, they are still not widely used, partly because of their high cost. Red to maroon shades are very popular but there are no black reactive dyes available that can match the chrome blacks on wool fibre.

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Informative articles on Dye reactivity, Application and Storage of Reactive dyes 

Structural discussion of reactive dyes those suitable for cotton fibre 

After Treatment And Stripping of Reactive Dye 

Vinyl sulphone dyes or Remazol dyes are good quality Reactive dyes

  

REFERENCES

1. J Shore, in Cellulosics Dyeing, J Shore, Ed (Bradford: SDC, 1995).

2. D M Lewis, in Wool Dyeing, D M Lewis, Ed (Bradford: SDC, 1992).

3. A H M Renfrew, Adv. Colour Sci. Technol., 1 (1998) 12.

4. A H M Renfrew and J A Taylor, Rev. Prog. Coloration, 20 (1990) 1.

5. D A S Phillips, Adv. Colour Sci. Technol., 1 (1998) 1.

6. J A Taylor, Rev. Prog. Coloration, 30 (2000) 93

Informative articles on Dye reactivity, Application and Storage of Reactive dyes

Dye reactivity is very important terms for dyeing cellulose fibre. Shed of dyed textile fabric directly depend on the reactivity of reactive group. Highly reactive group of reactive dyes makes strong covalent bond with cotton fibre structure. The reactive groups of different types of reactive dyes have different chemical structures and show a wide range of reactivities. They were originally divided into cold- and hot-dyeing types but many current ranges would be better called warm dyeing. The most reactive types, such as DCT reactive dyes, are applied at lower temperatures (20–40 °C) and for dyeing process only require a weak alkali such as NaHCO3 or Na2CO3 for fixation. The less reactive types, such as MCT dyes required higher temperatures (80–90 °C) and stronger alkalis such as Na2CO3 plus NaOH. Many reactive dyes manufacturers now market several ranges of reactive dyes for cotton fibre , each with its own particular recommended dyeing procedure.

Gives some typical examples of reactive dyes based on the type of reactive grouping.
 

Reactive group	Commercial name of reactive dyes	Reactivity	Exhaust dyeing temperature (°C)
DCT	Procion MX (BASF)	high	25–40
MCT	Procion H (BASF) Basilen (BASF) Cibacron (Ciba)	low	80–85
MFT	Cibacron F (Ciba)	moderate	40–60
DCQ	Levafix E (DyStar)	low	50–70
DFCP	Drimarene K (Clariant) Levafix E-A (DyStar)	moderate to high	30–50
VS	Remazol (DyStar)	moderate	40–60
TCP	Drimarene X (Clariant)	low	80–95
NT	Kayacelon React (Nippon Kayaku)	moderate to high	100–130   *

Because most reactive dyes are prone to hydrolysis, their handling and use requires care. Most of the reactive dyes are readily water-soluble dye and the dye solution in dye bath is prepared in the usual way by pasting with water and then adding more water. The temperature of the water used depends upon the ease of solution and the reactivity of the dye. Hot water is not recommended for dissolving dyes of high reactivity, because of the risk of hydrolysis of the reactive group, but is suitable for the less reactive types.
 Special care must be taken for storing reactive dyes. Highly reactive dyes could be react with air. Once the dye solution of reactive dye has been prepared, it cannot be stored for later use without some risk of hydrolysis of the reactive group of reactive dyes. This decreases its fixation ability after dyeing and is a particular problem with the most reactive types of dye. Dyes containing a 2 sulphatoethylsulphone group, however, can be dissolved in neutral water at the boil without risk of hydrolysis. Formation of the reactive vinyl sulphone group requires the addition of alkali. Reactive dyes for printing are usually dyes of low reactivity so that the print paste can be stored for some time at room temperature without deterioration from hydrolysis of the reactive group. Reactive dyes of low reactivity and relatively high substantivity are valuable for dyeing using long (high) liquor ratios, using a winch machine for knitted fabric and twill tape dyeing. Exhaust dyeing method with low reactivity of reactive dyes at the higher temperatures required for fixation allows better penetration of the dyes into the cotton fibres. For continuous dyeing with reactive dyes stabilised liquid forms are available. Although these contain special pH buffers and stabilisers to minimise the hydrolysis reaction, they only have a limited shelf life. Many commercial reactive dyes are dusty powders but all physical forms must be handled with care. These dyes react with the amino groups in proteins in the skin and on mucous surfaces. Inhalation of the dust is dangerous and a dust mask is obligatory during handling. Reactive dye powders and grains are sometimes hygroscopic and drums must be carefully re-sealed. Most reactive dyes have a limited storage period, after which some deterioration can be expected. Standardisation and comparison of reactive dye powders or liquids cannot be done by the usual spectrophotometric procedure involving absorbance measurements of standard solutions. Both the reactive dye and its hydrolysed form are evenly coloured, but only the former is capable of reaction with the cellulose fibre during dyeing. Therefore, dyeing must be prepared and their colors compared with standard dyeings. Chromatographic techniques usually allow separation and quantitative measurement of the relative amounts of a reactive dye and its hydrolysis product in a given dye.
 
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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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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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