Wool fibre is a natural protein fibre, which is used for the manufacturing of garments because of its excellent fire retardancy, stain-resistance, antistatic and odour control properties along with exceptional warmth and resilience. However, wool fibres extensively shrink during washing. To overcome this problem, wool fibres, especially those used in clothing, are frequently shrink-resist treated to make them machine-washable. A range of treatments have been developed over the years to make wool fabric felt and shrink resistant. Of all the treatments the chlorination treatment followed by coating with a polyamide-epichlorohydrin resin (known as chlorine-Hercosett treatment) is undoubtedly the most effective, and the cheapest shrink-resist treatment and it is believed that approximately 70% of all wool labelled as fully machine-washable is treated by the chlorine-Hercosett process.
Usually very bright colours cannot be obtained on wool because it does not start by having a white base, and it also yellows readily in sunlight, especially when wet. As a result, markets requiring vivid colours, bright whites and pastel shades such as women’s wear, baby wear, sports and leisure wear are dominated by polyester, nylon, acrylic and cotton, and are almost totally lost to wool.
Nonetheless, around 10% of the total world production of wool is bleached before sale.
At point of sale, some degree of brightness is desirable even if it is lost early during wear and a great deal of bleaching is carried out as a top-up whitening process in the “scour” which is the preparation process for wool. Normally Hydrogen peroxide is added to the last rinse in preparation and then some degree of bleaching takes place in the drier. This is done because the whiteness of scoured wool is taken into consideration in determining the value of the final fibre product before making up into garments.
Oxidative bleaching – In the fibre preparation oxidative bleaching usually gives the best whitening effect but oxidatively bleached wool yellows in use more readily than unbleached wool. Oxidative bleaching is usually carried out with Hydrogen peroxide or sodium percarbonate. Oxidative bleaching is also carried out in the presence of stabilisers, and activators. Oxidative bleaching always damages wool. – Bleaching with hydrogen peroxide can be carried out by batch or continuous methods, and at room or even higher temperatures. Why do they do this? Hydrogen peroxide is relatively inexpensive, does not release toxic chemicals or unpleasant odours, and does not cause corrosion of the preparation equipment making it a popular choice. The only by products released by this process are its de-composition into water and oxygen. When using hydrogen peroxide on fibres that are sensitive to oxidation, such as wool or cotton, damage can be, and in practice is usually, kept to a minimum provided that the bleaching is carried out carefully under the recommended conditions, in terms of pH, temperature etc.
Stabilisation of peroxide – Why do they need to add Stabilisers to wool bleaching solutions?
In the absence of stabilisers wool is yellowed by hydrogen peroxide. The reason being is that wool usually contains small amounts of transition metal ions that catalyse decomposition of hydrogen peroxide and prevent it from reacting with the wool. Stabilisers are metal complexing (sequestering) agents such as sodium silicate, EDTA and phosphates. Sodium silicate can be used as a stabiliser for peroxide bleaching because apart from its ability to sequester transition metal ions, it acts as a buffer at the proper pH of 10.5 -11.5, but insoluble deposits can be formed on the goods as well as on the dyeing equipment. These deposits can give the fabric a harsh handle, and may lead to unlevel dyeing, and many proprietary stabilisers used are formulated with complexing agents and buffers ‘built in’ and these consists of a combination of organic and inorganic salts in aqueous solution which enables acid hydrogen peroxide solutions to be used for bleaching wool at neutral or acid pH.
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