DIRECT DYES

Direct dyes are one of the most versatile classes of dyestuff applicable to cellulose, wool, silk, nylon fibers. This dyes have inherent substantivity for cotton, and for other cellulosic fibres. Their aqueous solutions dye cotton usually in the presence of an electrolyte such as NaCl or Na₂SO₄ (sodium sulfate also known as Glauber's salt.

The direct dyes, also known as substantive colors, differ from other classes of dyes because they are strongly substantive towards cellulose fibers such as cotton and viscose. Many of them also dye wool and silk. In the year 1884, Bottigner discovered first direct dye popularly known as Congo Red, a derivative of benzidine, which was found to be carcinogenic and is now one of the banned amines. Prior to this discovery, cotton was dyed with vegetable dyes using mordants.

Direct dyes provide a simple and relatively inexpensive way of dyeing cotton. Complete range of colors is available in this class of dyes. However they are not as bright as reactive dyes. They can dye cotton without the use of chemicals unlike other classes of dyes; hence they are called direct dyes. They are anionic in nature (sodium salts of sulphonic acid) and are soluble in water. Majority of them are di azo or tri azo derivatives. They are represented by RSO3Na, where RSO3 is color bearing dye anion.

The light fastness of dyeings with direct dyes on cellulosic fibers varies from poor to fairly good, although some copper complex direct dyes have very good light fastness. As usual, the deeper the color of the dyeing, the lower the fastness to wet treatments, and the higher the fastness to light. Various after treatments of the dyeings improve the fastness to washing. In some cases, however, such after treatments decrease the light fastness. They also invariably cause a change in hue that makes shade correction and color matching more difficult. 

Mechanism of dyeing:

It is generally accepted that when direct dyes are dissolved in water the dye molecules disassociate and are present in water either as single molecule or in small aggregates.

During dyeing these small aggregates diffuse into fiber and form bigger aggregates which get deposited in fiber pores.

They do not get chemically bonded to the fiber as in the case of reactive dyes (it is generally believed that very weak bonding such as Van der Wall forces exists between the dye molecules and fiber). Therefore during washing by the end user, these bigger dye aggregates disassociate in water and come out of the garment, resulting in color bleeding.

The addition of electrolyte (salt) to dye bath tends to promote exhaustion of direct dyes. This is because when cotton is immersed in water, its surface acquires slightly a negative charge, known as zeta potential.

The dye being anionic in nature it also has negative charge. As both cotton and the dye possess identical charges, they repel. Therefore the dye will not have tendency to reach fiber surface and diffuse into fiber core.

The addition of salt neutralizes a part of negative charge developed on the fiber surface thus enabling the dye molecules to migrate towards the fiber surface and hence diffuse into fiber core. Addition of salt to the dye bath reduces the solubility of dye in water thus inducing the dye to migrate from water to the fiber core.

Therefore both suppression of fiber surface charge and reduction of dye solubility in water help in achieving high exhaustion of dye. Sodium chloride (common salt) and sodium sulphate (Glauber salt) are the most common agents used as exhausting agents. The quantity of salt to be added to the dye bath depends on depth of shade and material to liquor ratio. A large amount of salt is required for dark shades and high material to liquor ratios.

Classification of Direct Dyes:

    

The behavior of dyes in dye bath is influenced by salt concentration and dyeing temperature. Accordingly they are divided into three groups.

•      Group A or Self leveling dyes: In general this group of dyes does not pose problems while dyeing. They dye uniformly as they possess high leveling property. The unevenness produced during the initial phase of dyeing can be corrected by prolonging dyeing time.

•      Group B or Salt controllable dyes: These dyes have poor leveling property. Their leveling property is influenced by the addition of salt. If the required quantity of salt is added in one installment at the beginning of dyeing, the dye transfer to the fiber will be rapid and will result in uneven dyeing. It would be very difficult to level the shade if unlevelness occurs with this group of dyes. Therefore to dye this group of dyes, it is recommended to add salt in small portions over a span of time.

•   Group C or Temperature controllable dyes: These dyes are sensitive to temperature. Their leveling property can be controlled by controlling rate of heating. While dyeing with this group of dyes, the dye bath temperature must be slowly increased. Rapid increase in temperature will result in uneven dyeing. It is recommended that addition of salt should also be added in portions.

Dyeing Procedure

q  Set the dye bath with substrate at room temperature

q  Add dye solution with other auxiliaries ad raise the temperature at 90⁰c

q  Run the bath for 15-20 min and add salt step according to dye bath concentration

q  This is better to add salt after reaching the temperature to build point since at this period the maximum penetration is achieved all over the substrate.

q  Run the dye bath for 30 to 50 minutes at 90-95^oc for complete the dyeing cycle

q  Cold down the bath temperature to 60-70^oc

q  Drop the bath and rinse

q  Carry on the after treatment process to improve wet fastness