Reductive bleaching of wool

For reductive bleaching of wool the two most popular chemicals are stabilized sodium dithionite (sodium hydrosulfite. Cl Reducing Agent 1) and thiourea dioxide (Cl Reducing agent 11). Most reductive bleaching of wool is carried out using stabilized dithionite (2—5 g/L) at pH 5.5—6 and 45—65°C for 1 h. Thiourea dioxide is more expensive than sodium dithionite, but is an effective bleach when appHed at the rate of 1—3 g/L at 80°C at pH 7 for an hour. 

Table 10.30 Recommended conditions for the reductive bleaching of wool [11,259]...

Reductive bleaching with non-stabilised sodium hydrosulphite [98], sodium hydrosulphite-based sulphoxylate formaldehyde [99] and sodium and zinc hydroxymethanesulphinate [100] have been shown to provide wool excellent whiteness, with a lower cystine content than in the case of bleaching wool with other reducing agents. The result obtained is permanent, in contrast to the unstable peroxide white. 

Hydrogen peroxide is not suitable for acrylic fibre at highly alkaline condition and moreover acrylic fibre turns yellowish on alkaline peroxide treatment. The discolouration can be improved by after-treatment with formic acid in presence of detergent. Acrylic/wool blends can also be bleached by a reduction bleach or by combination of peroxide and reduction bleaching process. 

Bleachit . [BASF] Reductive bleaching agent auxiliary for bleaching and optics whitening of wool and polyamide. 

It is possible to increase the brightness of wool by reduction bleaching. Bleaching agents for this process are sulfur dioxide (SO2), sulfurous acid (H2SO3), and hydrosulfite. 

The adverse effects of alkali on wool cause reduction in its dry strength only after fiber solubilization and considerable loss in its cystine content (105). Because of its sensitivity to alkali, wool is preferentially given a peroxide bleach with added formic acid silk does not degrade nearly so much under alkaline conditions and may be bleached at pH 10 with peroxide, using stabilizers such as sodium silicate (98). 

To obtain a high level of whiteness, comparable to white cotton and synthetics, wool must be treated with a FWA after bleaching. The normal procedure is to carry out a full bleaching process and include the FWA in the reductive... 

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