Dyeing Kinetics

The actual dyeing theory mathematically can be obtained from kinetics of dyeing or dyeing equilibria. The dyeing phenomena is found in principle of dyeing curve. The factors for uniform color and optimization of dye all are related to kinetic phenomena. Therefore kinetic dyeing is important in the dyeing process.

During dyeing process, two methods play a dominant role:

Dyeing kinetics: the rate of transfer of dye in solution (or dispersion) from the dye bath into substrate

Dyeing equilibiria: the position of sorption versus desorption after (theoretically) infinite time. Most of the equilibrium properties of dyeing system depend on three quantities:

• affinity

• heat of dyeing

• entropy change

The kinetic behavior of a dye is graphically depicted by rate of dyeing (or uptake) curve. The transfer consists of:

a. Convectional diffusion to the fiber surface occurring in dyebath.

b. Molecular diffusion through the hydrodynamic boundary layer

c. Adsorption at the outer surface

d. Molecular diffusion into the fiber (Absorption)

e. Anchoring of dye molecule

In the case of disperse dyeing (stage a) is preceded by the dissolution of disperse dye particles. Therefore the particle size distribution may influence the dyeing kinetics of disperse dye. In reactive dyeing, azoic dyeing, metallised dyes, vat and sulfuric acid esters of leucovat dyes, chemical reaction (stage e) takes place.

For the kinetics of over all dyeing processes stages a, c & d are important. Liquor circulation in the dye bath must be vigorous enough to ensure that stage a is short relative to stage d. The Sorption is faster than the preceding stages. Absorption leads to immbobilisation stage e. In equilibrium dyeing processes a complete immobilisation doesn’t take place.

The dyeing kinetic principle can be shown schematically

Dye in dyebath ^{Convective diffusion} Dye in boundary layer ^{Molecular diffusion} Dye on fiber surface ^{Adsorption + Diffusion} Dye in fiber surface ^Absorption dye

sorption ^Fixation Immobilized