Direct Dyes: Direct dyes are also called substantive dyes because of their excellent substantivity for cellulosic textile materials like cotton and viscose rayon. This class of dyes derives its name from its property of having direct affinity for cellulosic fibres, when applied from an aqueous solution.
Properties of Direct Dyes: Chemically, direct dyes are sodium salts of aromatic sulphonic acids and most of them contain an azo group as the main chromophore. They are in general duller than the fibre reactive dyes, and exhibit poor wash fastness. Goods dyed with direct dyes unless; given a proper after treatment tend to bleed with every wash. The direct dyes in many cases exhibit a better light fastness as compared to the reactive dyes.
Classification of Direct Dyes: The direct dye in the colour index system refers to various planar, highly conjugated molecular structures containing one or more sulfonate groups which render solubility to the dye molecule. Their planar structure and length enable them to lie along side cellulose fibres and maximize the Van-der-waals, dipole and hydrogen bonds. As in other classes of dyes, the direct dyes can also be classified in many ways like the chemical nature of the chromophore, or by their dyeing characteristics and performance properties, but in general the most popular way of classification by the dyes is by means of their dyeing characteristics.
The classification of direct dyes by SDC is based essentially on the compatibility of different groups of direct dyes with one another under certain conditions of batch dyeing. There are three groups: A, B, and C.
Class A: Self Leveling Direct Dyes: These dyes have good leveling characteristics and are capable of dyeing uniformly even when the electrolyte is added at the beginning of the dyeing operation. Dyes in this group have good leveling properties and are capable of dyeing uniformly even when the electrolyte is added at the beginning of the dyeing operation.
Class B: Salt Controllable Direct Dyes: these dyes have poor leveling or migration properties. They can be controlled by addition of electrolyte, usually after the dye bath has reached the dyeing temperature. They require relatively large amount of salt to exhaust well.
Class C: Salt and Temperature Controllable Dyes. These dyes show relatively poor leveling or migration and their substantivity increases very rapidly with increasing temperature. Controlling rate of rise of the dye bath temperature, as well as controlling the salt addition, controls their rate of dyeing.
Besides, the dyeing behavior of different group of dyes, the dyeing process also operates at a wide range of liquor to good ratio (MLR), ranging from approximately 30-40:1, for loose raw stock in open vessels and for garment dyeing, approximately 20-30:1for becks; about 5-10:1 for jet dyeing machines; about 3-5:1 for package dyeing yarns and anywhere from about 0.6-2:1 for padding applications. As the Liquor ratios get lower, the solubility of the dye and the effect of the salt on the solubility get more critical. The solubilities of commercial direct dyes can range from about 5-300gms/litre at the boil. Dyes of high salt sensitivity, particularly at low liquor-to-goods ratios, can easily cause unlevel dyeings, because the strike rate is too high to allow for the dyeing uniformity, which comes from good circulation between the dye liquor and the fibre. In padding applications, differential strike rates in mixtures can lead to the initial preferential sorption of one or more of the components of the dye mix by the fabric. This leads to the phenomenon known as tailing or end-to end shading. The dyer especially in case of exhaust dyeing, consistently tries to optimize the liquor ratio, dyeing time and dyeing levelness, as a lower liquor ratio increases the chances of unlevel dyeing and requires a higher holding time to allow for the dye to migrate so that satisfactory dyeing in terms of shade build up and levelness is obtained.
Fibro has recently launched its latest series of process auxiliaries especially for the direct dyes, which consists of a leveling agent Fibrolev-DLV, and Dirofix-ECO an ecofriendly dye fixing agent. Both the products have been especially formulated for direct dyes, which are used for fabrics whose end-use do not involve regular washing.