Reductive bleaching sodium hydrosulphite

Mechanical pulps are bleached by procedures that preserve lignin, but eliminate most coloured groups. Most mechanical pulps can be bleached to brightness levels of 75-80%, that are suitable for uncoated and coated papers. The principal bleaching procedures for mechanical pulps are reductive bleaching with sodium dithionite (hydrosulphite), and oxidative bleaching with peroxide under alkaline conditions. 

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. 

Reduction bleaching of silk is generally carried out with sodium hydrosulphite or appropriate stabilised commercial products on this basis. The silk fibrion is... 

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. 

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