Advancements in Reactive Textile Dyes

Application Method of Reactive Dyes

Reactive dyes are known for their versatility in application technique on to the cotton substrates. Depending upon the reactive system, the application method varies.

Broadly, the application of the reactive dyes is classified into:

• Batch wise method
• Semi-continuous method
• Continuous method

This gives flexibility to a dyer in the production and programming and choice of an alternate method, which the situation demands.

Exhaust Method

The object of exhaust dyeing is to achieve level dyeing and reproducibility in practical dyeing times with maximum colour yield. The most common method in exhaust dyeing is to apply the first under the condition of no reaction by simplex exhaust procedure similar to that used for direct dyes and then to add alkali to allow the dye to react after most of the dyes are exhausted. At the completion of dyeing the fibres is removed by soaping achieve dyeing that is fast to wet treatments.¹⁷

This method may be done on jigger or winch. It comprises of two stages:

Exhaustion: During this stage, the dye is exhausted from the dye-bath and comes into the fibre phase. The material is run in a bath of water, dye and salt, both dye and Glauber’s salt are added into two parts. The condition for dying and fixation for the cold and hot brand that reacts through substitution mechanism is given in the table.

Fixation: The fixation takes place in presence of alkali. After the addition of alkali, migration takes place either with the fibre or with the water molecules. The dyes present on the fibre preferentially react with the fibre.

The dyeing procedure for exhaust dyeing in Jigger is, Load the Jigger > Add dye in 2 ends > Run for in 2 ends > Add salt for 2 ends > Run for 2 ends > Add alkali in 2 ends > Run for 2 to 6 ends> Drain out the Jigger > Cold wash in 2 ends > Soaping in 2 ends > Hot washing in 2 ends > Cold washing in 2 ends.

Exhaust dyeing in Jigger using Remazol dyes.

In this method, the dye bath is heated to the recommended temperature. Then the dye solution and other additives are added in the bath in the following fashion.

• 1^st End: Half the amount of dyestuff and half amount of salt.
• 2^nd End: Remaining quantity of dyes stuff and salt.
• 3^rd End: Half the amount of alkali.
• 4^th End: Remaining quantity of alkali

Dyeing is then continued for 60 to 90 minutes depending upon the dyeing temperature. The processing data for jigger dyeing with Remazol of

Several temperatures are given in the table:

After the required time of batching the cloth is washed properly removed all unfixed dye.

Padding Methods

There are various padding methods.

Pad-batch method:

All pad-batch process is following a general sequence.

• Impregnation of the well prepared dry fabric as it of the dye and alkali at ambient temperature.
• Uniform squeezing of surplus liquor from the fabric as it passes through the magnet hip.
• Wrapping of the batch roll of wet fabric in Polythene film and storage at ambient temperature for specific well times.

It is a semi-continuous process and is an economical alternative to conventional exhaust method. The process is found to be very useful for medium batch size low water consumption and minimizing pollution. The process also ensures high reproducibility and the low cost of dying. ³

For dyeing by pad-batch method use of Na-silicate as alkali is giving popularity because of increased stability of dye bath and higher colour yield. Normally 100gpl (130tw) Na silicate with little caustic soda, 2gpl are added to the padding bath. The wet pickup of the fabric should be kept around 65.7 if it is more seepage of dye liquor take place. Addition of little Na alginate (40%paste) 5-10gpl help in minimizing the problem of seeping bath help easy removal of Na-silicate, the levelness of pad dyeing depends entirely on uniform dye liquor application.

In particular, it is not always easy to ensure this in the centre and the edge of food with padding of modern design. The pressure and the applied amount of dye liquor can be established across the width of the machine to compensate for a difference in the material itself by varying the pressure. Such a system also enables fabric of different with to be processed without problem. Expression and liquor add on can be continually monitored and corrected by means of modern humidity measuring in equipment.

Pad dry steam method:

In this dying system, a drying step is involved. The replace the exhaustion step in batch dyeing and consequently substantively is not needed to transport the dye to the fibre. The process is essentially same as that of the pad-batch method, but for this the addition of migration inhibitors Na-bicarbonate and wetting agent are added prior to padding the material is padded in cold, maintaining minimum pick up to reduce the tendency of migration and finally dried, in hot air stenter for 1-2 minute for fixation.

In the absence, if migration inhibitor in padding liquor the dye may migrate during dye result in level dyeing when the fabric is padded with the liquor the inter febrile arises, are swollen with liquor the dye dispersion can circulate freely through this capillary network. Install the average dyestuff particles size. During the dying, all the capillaries decrease in size as water is evaporated from fabric eventually the largest capillary diameter shrink to the size of the smallest dyestuff particles.³

Pad-dry pad steam method:

This method can be used for continuous production and for longer yardage suitable equipment such as padding mangle and a dryer are synchronized with a pad steam unit and a soaper. The cellulosic material to be dyed by this method must be highly absorbent free from finishing and sizing agent. The padding method is the same as in former use. The padded goods after drying are passed through a bath containing 250gpl caustic soda and the dyes are fixed by steaming for 20-30 second at 103-105⁰C.

Pad-dry cure method:

The machinery required for these methods is a good padding mangle with a small through. And a hot flue or flood dryer, which dries and fixes the dyestuff simultaneously. The dyestuff is dissolved with required quantity of water and the dissolved dyestuff solution is further diluted with cold water. Fixation does not take place under dry condition, so humectants are required. Urea about 5-100gpl is added to the cold solution for this purpose and other additives such as resist salt-o about 10gpl and soda ash about 10-30gpl are added to padding liquor 50-100pgl Na-alginate thickener may be used where fear of dyestuff migration is there. For fixation, the fabric is dried at 120-140⁰C for 4-2 minute.³

This process after several advantages over pad dry pad steam method such as:

• Lower machinery cost (pad dry range only)
• Less chemical
• Lesser energy (no steam)
• Easier wash off (no salt to remove)
• Better reproducibility (less strength to control)

After Treatment

Two factors are important during the after treatment of reactive dyeing. The dying should not be soaped of the boil under alkaline condition. This is ensured by neutralization with acetic acid before soaping. The hydrolysed dyes are the best removal buy boiling with a detergent solution at the boil. It is reduced at a higher temperature and at the boil the dye rapidly diffuse out. If this dye is removed completely it is not necessary to treat the dying with a cationic dye-fixing agent to achieve the optimum perspiration and water fastness.

When sodium silicate is used as the alkali for fixing the dye then neutralization with acetic acid should not be done. If in neutralized insoluble silica acid may get deposited in the fibres producing a harsh feel in this case, on overflow rinsing with warm water should be given before the boiling after treatment.