The most important class of dyestuff for cotton and other fibres is that of the Direct Dyes. This is chiefly due to their simple and cheap application, uniform affinity, good penetration and their property of retaining the original character of the material. That is luster, softness, suppleness etc.
Chemically Direct dyes are sodium salt of aromatic sulphonic acids and most of them contain an azo group as the main chromophore. Direct dyes are in general duller than the fibre reactive dyes, and exhibit poor wash fastness. Good dyed with Direct Dyes unless given a proper after treatment tent to bleed with every wash. The Direct Dyes in many cases exhibit a better light fastness as compared to the Reactive Dyes.
Our Direct Dyes form a special group within the Direct Dyestuff Category. Though otherwise equal to all other Direct Dyestuffs, they are favourably distinguished from them by a fastness to light which is considerably above the average. They are satisfactorily used where the production of fast to light dyeing is desired by the simplest method of application and fastness to washing is not particularly important.
The Direct Dyes are best dissolved in soft water free from lime, preferably in condensed water. The Direct Dyes is pasted up evenly with a little hot water and completely dissolved by pouring in boiling hot water while stirring. Usually 10 to 15 times its quantity of water in sufficient. The dissolved Direct Dyes should always be added to the bath through a cotton cloth or a fine meshed sieve.
Dyeing with Direct Dyes is done mainly in a weakly alkaline bath with addition of soaps, etc. if required. In some cases addition of sodium phosphate or acetic acid are used. Soda Serves to soften the water and also to increase the solubility of Direct Dyestuffs, some dyestuffs even show a greater affinity in the presence of soda.