**5.2.2 Diffusion phase**

The dyeing of polyester is often described as a diffusion-controlled process. This is because of the diffusion phases shown in Fig. 16, including convective transfer through the liquor adsorption and molecular diffusion into the fibre, is the rate-determining step (Dawson & Todd, 1979). Where the time needed for the adsorption phase is largely influenced by the machine conditions, in the diffusion phase the time required at top temperature is directly related to the diffusion characteristics of individual dyes and dyeing depth, but generally, the standard time is 10-20 minutes for dyeing up to pale shades, 20-30 minutes for medium shades and 30-35 minutes for deep shades.

In the high temperature phase of the dyeing process, another important property of dyes is migration, or their tendency to level out. This phenomenon can be very important, particularly if dyes have been adsorbed in a non-uniform manner, perhaps due to inadequate liquor circulation or too rapid a heating rate. While the migration properties of disperse dyes may become a key factor if dyes are applied unevenly during the adsorption phase, the key parameter in the diffusion phase is the diffusion rate of the disperse dye.
