Reactive Dye (Fundamental)

ð  Reactive dyes are developed in the 1950s.

ð  Reactive dyes are mainly applied on cellulosic fibres such as cotton, viscose rayon, cuprammonium rayon, Wool, Nylon etc.

ð  Generally, these dyes are easily applied on cellulosic fibres and can be directly dyed from simple solution in water with alkali.

ð  These dyes chemically react with fibre forming covalent bond; hence they are sometimes called “Fibre Reactive Dye”.

Properties

ð  Reactive dyes are highly soluble in water. Solubility increases with the addition of urea.

ð  Reactive dyes are anionic in nature.

ð  Cold brand has higher affinity for cotton, so suitable for exhaust dyeing.

ð  Hot brand and Remazol are suitable for padding and printing due to their poor affinity for cotton.

ð  Reactive dyes show Excellent washing fastness (Except cold brands) & light fastness.

ð  Reactive dyes show poor bleaching fastness. (Bleaching Powder)

Structure of Reactive Dye

ð  Characteristic structural features of a reactive dye is shown below:- 

S = Solubilising Group like SO₃Na, or COONa, or combination of both

C = Chromophoric Group, responsible for affinity and diffusion of dye

B = Bridging Group, attach the reactive system to chromogen C

X = Reactive system, reacts with the functional group of fibre forming homo-polar bond 

Classification

Reaction Mechanism

ð  Cold brand reactive dye, being highly reactive due to presence of two chlorine atoms, it is more susceptible to hydrolysis and also possessing high fixation rate,

ð  One chlorine atom of dye attach to the functional group of fibre, while another one reacts with water.

ð  Reaction with water results into hydrolysis of dye, and so the wash fastness is poor as partially hydrolysed dye can not be removed from the dyed cotton.

ð  In hot brand reactive dye, only single chlorine atom is present, so it can either react with cellulose or with water. But do not stain adjacent ground.

Reaction with Cotton and Water

ð  Hydroxyl group present in cotton reacts with reactive dye with the liberation of acid, even at neutral pH.

ð  To increase the fixation, liberated acid must be neutralize with the addition of alkali.

ð  Hydrolysis process also produces acid, while reacting with dye.

ð  Rate of hydrolysis is very less to the rate of dyeing, with strictly controlling parameters of efficient dyeing.

 ð  Remazol dyes also react with cotton and water in same manner.

ð  Hot brand and Remazol react with cotton at primary hydroxyl group at C₆ position, while cold brand reacts with two different hydroxyl groups at a time.

  References

1) Fundamentals and practices in colouration of textiles by J N Chakraborty, Woodhead Publishing India, 2010, pp. 57-60

2)  Chemical technology in the coloration of textiles by S R Karmarkar, Colour publication Pvt ltd, 2007, pp. 181-187