Sequential Oxidative/Reductive Bleaching and Dyeing of Wool in a Single Bath at Low Temperatures

Abstract

A new bleaching and dyeing process applicable to wool involves a sequential oxidative/reductive bleaching combined with a subsequent dyeing in a single bath. Apart from dyebath bleaching processes in which the bleaching agent is added towards the end of the dyeing process when almost all the dye is exhausted, bleaching and subsequent dyeing are normally done in separate baths due to the sensitivity of the dyes to bleaching agents. In the latter process, it is important that all bleaching agents be thoroughly washed off before dyeing, because any residual bleaching agent (hydrogen peroxide, reductive bleach) may adversely affect the results. The newly developed single-bath process begins with an oxidative hydrogen peroxide bleaching followed by addition of thiourea to the residual hydrogen peroxide in the same bath. Thiourea dioxide formed by the resulting chemical reaction hydrolyzes in solution to sulfonate anion and urea. Sulfinate anion is a strong reducing agent and effects reductive bleaching. At the end of the reductive bleaching stage, a small amount of hydrogen peroxide is added to oxidize all the reductive sulfur species in solution to the sulfate anion, which together with the urea serves subsequently as a dye assist. Once the temperature of the bath is lowered, dyes may be added to the same bath. The new sequential single-bath bleaching/dyeing process results in much brighter pastel shades upon dyeing, due to better whiteness obtained initially with sequential oxidative/reductive bleaching. Furthermore, there is a reduction in processing time, temperature, and effluent compared with conventional procedures and the utilization of byproducts from the initial bleaching assists during the subsequent dyeing step.