Factors that affect the direct dyeing

Salt

Electrolytes in the dyebath promote the exhaustion of direct dyes. The negative charge of cellulose surface repels anionic dye molecules, and the salt provides sodium ions to counteract with the negative surface potential of wet cotton. The neutralization of charge allows the dye molecules to approach the fiber closely enough for the forces of attraction between the dye and the fiber to take effect.

The salt being very soluble in water may “salt out” or increase the degree of aggregation of dye molecules so that the dye is less held tenaciously by water. This less soluble dye could transfer more easily to the fiber surface from where it could diffuse into the fiber interior.

Temperature

Increasing temperature increase the rate of dyeing and the dye migration. Higher dyeing temperatures ensure good leveling and better penetration of the dye into the fibers within tightly packed yarns.

The diverse variations of direct dye exhaustion depend on two opposing influences of increasing dyeing temperature. These are the the usual effect of increasing temperature which decreases the dyebath exhaustion because the dyeing is exothermic, and the enhancement of the dyeing rate particularly at lower temperatures.Increasing temperature also promotes dye deaggregation in the dyeing solution liberating more individual dye molecules to enter the fiber.

Manufactures often cite a temperature of maximum exhaustion and provide optimum dyeing temperature profiles. The dyes C.I.Direct Red 81 and Direct Yellow28 have maximum exhaustion at 30℃，60℃ and 100℃。

Liquor ratio

Dyebath exhaustion should increase with decrease in dyeing liquor ratio. This means that the dyeing at low liquor ratios decreases the amount of waste dye in the effluent. It also consumes less water and steam, and allows a given salt concentration with less added salt.It should always be remembered , that the required amount of dye must be dissolved in a bath of reduced volume.It is important to ensure that the dye is ,in fact , in solution at the beginning of dyeing when the temperature is relatively low and after addition of salt.

After-treatment

After-treatment of dyeing of direct dyes on cellulosic materials aims to improve the washing fastness by increasing the dye’s molecular weight. This makes is less soluble and of slower diffusion. Some of these processes decrease the light fastness of the dyeing.The after-treatment of direct dyes can use resin finish, diazotization, cationic fixation agent and metal salts.After-treatment are difficult and costly to carry out ,and often give changes in hue that greatly impede shade correction and colour matching.