A Laboratory Study of Package Dyeing

Abstract

In package dyeing with vat dyes by the pigment method, the finely dispersed pig ment is circulated through the package and exhausts on the fiber in the oxidized state. The degree of pigmentation varies greatly from one part of the package to another, resulting in initial unlevelness of the package. The degree of levelness of the final dyeing will depend on the amount of migration that takes place after the pigment is reduced to the leuco vat dye. Dyes having leucos with high affinity for the fiber, as is shown by measurements of the free energy of dyeing, migrate less and produce less level package dyeings.

Reducing the particle size of the pigment sometimes gives more level pigmentation at the beginning, but this is not necessarily reflected in more level final dyeings. The vat acid process also leads to more uniform pigmentation, but still does not give level dyeings in every case; it is also quite inconvenient. Retarding agents promote levelness by tying up leuco and slowing the strike, but are uneconomical because much dye is left in the bath. Solvents slow the rate of dyeing and promote levelness, but also interfere with exhaustion and are very expensive. A higher temperature (212°-220° F.) during the pigmentation and reduction shifts the equilibrium in favor of the dye bath and promotes levelness; the temperature can then be lowered to put the remaining dye on the fiber. However, pressure machines are necessary; also. many dyes are unstable at the higher temperatures.

A migration test for vat dyes, whereby a dyed and undyed skein are placed in a caustic-hydro solution and allowed to reach equilibrium, has been found to give a good indication of the dye's leveling ability.