Denim bleach effect:
Tradition chemical process:
Denim Bleach is a process that can be used to decolorize indigo from denim In this process a strong oxidative bleaching agent such as sodium hypochlorite or KMnO4 is added during the washing with or without stone addition. Discoloration produced is usually more apparent depending on strength of the bleach liquor quantity, temperature and treatment time. It is preferable to have strong bleach with short treatment time. Care should be taken for the bleached goods so that they should be adequately antichlored or after washed with peroxide to minimize yellowing. Materials should be carefully sorted before processing for color uniformity.
Process is difficult to control i.e. difficult to reach the same level of bleaching in repeated runs. When desired level of bleaching reached the time span available to stop the bleaching is very narrow. Due to harshness of chemical, it may cause damage to cellulose resulting in severe strength losses and/or breaks or pinholes at the seam, pocket, etc.
- Harmful to human health and causes corrosion to stainless steel.
- Required antichlor treatment.
- Problem of yellowing is very frequent due to residual chlorine.
- Chlorinated organic substances occur as abundant products in bleaching, and
- Pass into the effluent where they cause severe environmental pollution
Enzymatic bleaching effect on denim:
Laccase is the newest enzyme class to be introduced into denim finishing. Laccases are multi-copper oxidizes that catalyze the oxidation of a wide range of phenols, including indigo, under simultaneous reduction of oxygen to water. Laccases alone are not effective in decolorizing indigo on denim and require a mediator, which mediates electron transfer from indigo to molecular oxygen. Since the laccase and mediator only degrade indigo, without affecting the weft yarns, the resulting finish exhibits unique wash-down of denim.
Conventional hypochlorite bleaching of denim is cheap, fast and efficient, but harsh to the environment and the denim. However, laccase and mediator bleach can be done under mild condition and is much simpler to control. Since the bleaching action is specific to phenol complexes, it is especially useful for stretch denim, without affecting its elasticity, unlike traditional hypochlorite bleach, which has a damaging effect on elastomeric yarn. The laccase and mediator can be alternatively used for abrasion enhancement by further brightening up the denim after cellulose treatment.
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