Wool bleaching with

Figure 10.38 Effect of treatment time on the alkali solubility and Jaquemart whiteness index of wool bleached with 2-vol. hydrogen peroxide at 40 °C and 50 °C [259,264]...

The most common chemical bleaching procedures are hypochlorite bleach for cotton hydrogen peroxide bleach for wool and cotton sodium chlorite bleach for cotton, polyamide, polyester, and polyacrylonitrile and reductive bleaching with dithionite for wool and polyamide. 

Wool and Silk. Wool must be caretiiUy bleached to avoid fiber damage. It is usually bleached with 1—5% hydrogen peroxide solutions at pH 8—9 for several hours at 40—55°C or at pH 5.5—8 for 20—60 min at 70—80°C. Silk is bleached similarly, but at slightly higher temperatures. 

Wool with dark pigmented fibers is treated with ferrous sulfate, sodium dithionite, and formaldehyde before it is bleached with hydrogen peroxide. The ferrous ions are absorbed by the dark pigments where they increase the bleaching done by the peroxide. 

Wool may also be bleached with reducing agents, usually after bleaching with hydrogen peroxide. This is the normal practice with wool blends. In the reducing step, 0.2—0.5% sodium dithionite solutions are often used at pH 5.5—7 for 1—2 h at 45—65°C. Faster bleaching is obtained with 2inc hydroxymethane-sulfinate [24887-06-7] below pH 3 and above 80°C. 

For satisfactory whiteness on wool, it is essential for the fibre to be well scoured and bleached, either oxidatively with hydrogen peroxide or by reduction using stabilised sodium dithionite. Brightener is usually applied together with the dithionite bleach. To achieve the highest possible whiteness, the wool should first be scoured to remove natural waxes and other contaminants, then bleached with peroxide and finally treated with FBA during a second bleach with dithionite. 

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). 

Wool has been successfully bleached with a stable emulsion of hydrogen peroxide [49] or permonosulphuric acid [50] in perchloroethylene. By this means it appears to be possible to obtain the same degree of whiteness as that attainable in a standard aqueous bleach, but more rapidly and with the use of less peroxide. No stabilising or activating agents are needed, but the bleaching of wool in perchloroethylene seems to be more deleterious to mechanical properties of bleached wool than equivalent aqueous procedure. 

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. 

The wool portion contained in the blend show reversion to a creamy colour and yellowing of the fabric. In general, blends containing wool and polyester fibres can be bleached with hydrogen peorxide either in acid or alkaline medium without risk of damage. 

For acrylic/wool blends, the acrylic portion is brightened in the normal way and then the wool is bleached with hydrogen peroxide and subsequent reduction bleaching is done in a bath containing suitable brightener (1-2%) and stabilised sodium hydrosulphite (3-5 g/1) at 85 C for 30 min. The fabric is then rinsed and dried. 

Wool can also be bleached with sodium peroxide, using sulphuric acid to neutralize the excess of sodium hydroxide. The liquor is made up of the following ... 

Wool can be bleached with sodium hydrosulphite or its derivatives, but in practice they do not find much application, partly because they tend to impart a harsh handle. They are used as stripping agents to remove dyes when this is necessary. 

---