Direct dyeing process in textile process

The first subjective or 'direct' dyes discovered in 1884 were diazo dyes obtained from terazotised benzidine.The direct dyes are called substantive dyes they have strong affinity for Cellulosic fibers.  It was called a direct dye because it was the first dye to become available for coloring cellulose 'directly' without the use of a any other chemical such as mordant. The direct dyes are called substantive dyes as they have strong affinity for cotton and other Cellulosic fibers.

Properties:

They are available in a full range of hues but are not noted for their color brilliance. They have poor moderate fastness to washing but can be improved by after - treatment with copper salts & cationic fixing agents. The light fastness of with direct dyes on cellulose fibers varies from poor to fairly good, although some copper complex direct dyes have good light fastness.  SDC committee also classified Direct Dye on three categories based on their leveling ability and their response to increase in the dyeing temperature and to added salt during exhaust dyeing.

Class A (Self Leveling): Dyes with good migration or leveling properties.
Class B (Salt Controllable): Dyes with poor leveling but which can be controlled by addition of salt.
 Class C (Temperature Controllable): Dyes which are not               self-leveling and are highly sensitive to salt; the exhaustion of  these dyes needs addition of salt and temperature.

FACTORS FOR DIRECT DYEING:

 The Society of Dyers and Colorists (SDC) Committee in 1946, on the basis of the dyeing Properties of direct dyes on cotton determined four factors for direct dyeing of cotton:

                migration (or leveling power)

                salt controllability

                 the influence of temperature

                liquor ration on exhaustion

Photochromism of direct dyeing:

On certain direct dyes, hue changes from bright turquoise to violet or reddish blue on prolonged exposure to sunlight or UV radiation. This is known as Phototropism. The hue reverts gradually to the original turquoise when the illuminant withdrawn. An electrode is a substance that, because of its chemical nature, conducts electricity in solution. When simple electrolytes like NaCl dissolve in water they spilt up (dissociate) into separate ions. It is for this reason that they will allow an electric current to pass through the water.

Mordanting & Fixing of dyeing

Tannic acid is used here as a mordant. Generally cotton is Mordant with tannic acid which is thus times the dye used. Tannic acid solution is boiled, where the cellulose is immersed and the temperature is lowered to 60⁰C within 1–2 hour. For this treatment the affinity of cellulose for tannic acid is increased. Then the material is withdraw from the bath and squeezed. Then the Mordanted cotton is dipped into tartar emetic solution (Dye: Tannic acid: Tartar emetic = 1:2:1) where it is worked for 30 min at room temperature where tannic acid is fixed on the cotton fibre.

The Mordanted cotton is then dyed with above recipe. The dye stuff is pasted with equal amount of acetic acid or methylated spirit. Then the dye solution is made with required amount of hot water. The Mordanted cotton is then dyed in the dye bath at 70⁰C for 1-1.5 hrs. the dyed material is then rinsed, squeezed and dried.