Wool bleaching

Bleaching of wool is necessary for the enhancement of whiteness and lustre. Using proteolytic enzymes alone [99] or in combination with peroxide [100], the degree of whiteness and hydrophilicity of the fibres are increased, comjjared with the oxidative treatment alone [99]. Serine protease stable to hydrogen peroxide is active in an alkaline medium and its activity increases with increasing peroxide level [94]. Higher whiteness index is caused by the decolourising action of the enzyme on natural colorants present in the wool fibre [100]. 

Protease, trypsin or papain are commonly used as enzymes for wool fibres. Protease is quite unreactive unless acted upon by a mild reducing agent, i.e. bisulphite or cysteine, so as to split disulphide to sulphydryl groups [110-113]. Other enzymes have been proposed [114,115], including bacterial alkaline proteases of the type used in washing powders. In some processes proteases are used to cut-off damaged fibres [ 116] or to achieve certain texturising effects [117,118]. 

Descaling is also achieved by the applicaiton of heat-resistant neutral protease, resulting in cashmere-like feel [119]. The combined use of the chlorinating agent, dichloroisocyanurate and proteolytic enzyme can improve handle properties [120, 121], 

Physical, as well as chemical, pretreatment processes have been combined with the enzyme treatment of wool. A low-temperature plasma is applied to the fibres prior to treatment with polymeric shrinkproofing agent [122]. Combined protease and heat treatment with a saturated steam [123] and the use of high frequency radiation on enzyme treated materials are reported. 

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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]...

Although various processes may have been subjected to optimisation in recent years as a result of economic pressures, a survey in the 1980s revealed disparities between different sectors of the industry as summarised in Table 10.29 for batchwise wool bleaching methods. 

Table 10.29 Worldwide practice of batchwise wool bleaching [259]...

Review of wool bleaching processes, IWS Process Development Bulletin No 35 (Oct 1996). 

Ltd, Huddersfield, England for a proprietary product of the hydrosulfite class for wool bleaching (Ref 2)... 

P. A. Duffield, IWS Review of Wool Bleaching Processes, International Wool Secretariat, Ilkley, U.K., 1996. 

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