Wool sodium hydrosulphite

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. 

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 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. 

An interesting aspect is that moths only thrive on keratin which contains disulphide bonds. If these are ruptured by reduction with sodium hydrosulphite and thioglycoUic acid and new links are built by methods such as joining primary amino groups with glyoxal, the wool is immune from attack by moths. 

In the case of the pale shades the two fibres can be dyed in the same liquor with a disperse and an acid milling or a premetallized dye, and subsequently cleared with permanganate in the manner described in the previous paragraph. With dark colours it is better to dye the polyester first then clear the wool with sodium hydrosulphite, ammonia, and a non-ionu dispersing agent, and finally match the protein fibre with acid or premetallized colours. It must be borne in mind that when after-chrome dyes art used, some disperse dyestuffs are modified by the dichromate, with change of colour. C.I. disperse red 11 (Cibacet Brilliant Pink 4BN) anc C.I. DISPERSE RED 15 (Duranol Red 2B) are particularly susceptible, anc C.I. DISPERSE RED 13 (Cibacet Rubine R) should be substituted. 

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