Reactive Dyes

Reactive dyes are so called because their molecules react chemically with the fire polymers  of  some  fibres  to  form  a  covalent  bond  between  the  dye  molecule  and  fibre polymer.The fibres most readily colored with reactive dyes are cellulose fibres, wool, silk and nylon. Reactive dyes are also called as Procion dyesWhen direct dyestuff is applied to cotton and to viscose rayon the process is one of Physical absorption there is no covalent union between dye and fibres.

Properties Of Reactive Dye

1. The Procion dyes, on account of the sulphuric acid groups in their molecule, are readily soluble in water.
2. These dyes, unlike any other class of dye stuffs, react and combine chemically (covalently) with cellulose and this leading to excellent wash-fastness.
3. These dyes give very bright shades such as orange, pink, magenta etc, which were not possible with other class of dyes.
4. They do not react with water nearly as readily as with cellulosic hydroxyl in alkaline conditions, so that they can be applied from an aqueous solution.
5. Reactivity of the dyestuffs can be reduced when desirable by blocking one of the reactive chlorine atoms giving H-type Procions.
6. Procion dyes are dyes with small molecules; their molecules do not have to be very long as those of direct dyes to match the distance between absorption sites on the fibre. Short molecules bring two advantages (a) Clarity and brightness of hue and (b) easy penetration and therefore good leveling.
7. Because there is some, even although not very much reaction between procion dyestuffs and water it is very important to wash the dyes fibre thoroughly clean and free from the reaction product with water.
8. Textile materials colored with reactive dyes have very good light fastness.

Preparation of Dye Solution

The dyestuff is dissolved by making them into paste with cold water, followed by dilution with more water, the temperature of which must not exceed 50ºC, A stock solution will not remain stable for more than about 4 hours, after which it will begin to lose strength due to hydrolysis.

Dyeing

The goods are loaded into the machine and 30 and 60 parts per 1000 of common salt are added, the quantity varying according to the depth of the shade. The liquor is then raised to the required temperature either 40ºC or 60ºC and the pre dissolved dye is added in the machine is then allowed to run for 20 to 30 minutes and the necessary amount of Soda ash is added over a period of 10 to 30 minutes.
Dyeing is continued for further 20 to 40 minutes after which the goods are rinsed to remove the salt and the alkali, and they are then washed off at the boil for 15 to 30 minutes to extract any of the products of hydrolyzed dye.
Considerable importance must be attached to the final washing of goods dyed with procion colors. The exhaustion is often not good and much dyestuff will be in retained in the liquor, but of greater significance is the fact that cellulose has some affinity for the hydrolyzed dye which is having lost its chlorine atoms, has not entered into chemical combination. The load is first rinsed thoroughly in as great a volume of cold water as the machine permits. The goods should then be rinsed in hot water nearly at the boil, after which they are soured at the boil, for 15 to 30 minutes with a synthetic detergent or soap and finally rinsed, first in hot and then in cold water.

Types of Reactive Dyes

1. Cold brand reactive dye M brand
2. Hot brand reactive dye H brand (or X brand)
3. High exhaust reactive dye HE brand
4. High exhaust reactive dye ME brand
5. Vinyl Sulphone reactive dye for dyeing and printing
6. Low salt reactive dye LS dyes

After treatment

• Treat the dyed material with 1 to 2 g/l of neutral soap at boil for 15 minutes and wash it.
• Treat the soaped material with 2 to 3 g/l of cationic dye fixing agent at 40ºC for 20 to 30 minutes and dry it.