1. Rate of dyeing and levelness

The following three factors should be considered when setting reasonable dyeing conditions;

1) The rate of dyeing should be controlled not to exceed the leveling capacity of the dyeing machine, and the dye should be absorbed evenly into the fiber.

2) The dyeing conditions necessary for the migration of the absorbed dye should be secured.

3) The temperature and the time necessary for complete diffusion of dye into the fiber should be secured.

Especially, concrete information on the rate of dyeing is very important for appropriate control of the first step of dyeing 1).

The step of dyeing can be divided into three parts;

1) Dye in the dyebath diffuses to the surface of the fiber

2) Dye is absorbed on the surface of the fiber

3) Absorbed dye diffuses into the fiber.

Research on the rate of dyeing is traditionally conducted mainly on the rate of diffusion, because the rate-determining step of dyeing is the rate of diffusion of dye in the fiber, and there is a correlation between the rate of dyeing and the rate of diffusion.

However, the practical rate of dyeing should be considered on the basis of the rate of dyeing curve (rate of absorption curve) measured in the actual dyeing system, because in bulk dyeing, factors affecting the rate of dyeing are not only the rate of diffusion but also the circulation of dyebath and the relation between the amount of absorbable dye and the amount of dye in the dyebath.

The rate of dyeing curve is generally measured on a basis of fixed dye, however, the absorbed dye on the fiber surface should be included when considering the levelness of dyeing.

In most classes of dyes, the difference between the absorbed dye and fixed dye is relatively small, and the rate of absorption curve is very close to the rate of dyeing (fixed dye) curve, therefore, an analysis of the rate of dyeing curve can give useful information on level dyeing. However, in the case of reactive dye, absorption (exhaustion) is completely different from dyeing (fixation), so the absorption curve must be carefully studied.