Sequential Oxidative and Reductive Bleaching of Stained and Pigmented Wool in a Single Bath

Abstract

A new process applicable to bleaching of both stained and pigmented wool combines a conventional bleaching process for pigmentation with a recently reported single- bath oxidative/reductive bleaching process. Conventionally, pigmented fibers in wool are selectively bleached by a three-step process: mordanting with ferrous ions, thorough and specific rinsing, and then bleaching with hydrogen peroxide. The process is de pendent on selective retention of ferrous species on melanin pigmentation, but some nonselective retention of ferrous ions on wool following rinsing leads, upon subsequent treatment with hydrogen peroxide, to an undesirable light brown discoloration from ferric species. The recently reported single-bath bleaching process, which was developed for stained wool, converts the unspent hydrogen peroxide of the initial oxidative bleaching step to a reductive substance in situ by reaction with thiourea followed by an appropriate pH adjustment to 7-8. When this process is combined with the ferrous mordanting procedure, pigmented and stained fibers are bleached, and any discoloring ferric species formed within the wool as a result of hydrogen peroxide are reduced to ferrous ions. Ferrous ions have a much lower affinity for wool than ferric ions and may be washed away easily. Overall, much superior bleaching with less physical damage is achievable by the new process than with conventional single- or multi-bath processes.