Bleach Washing

Bleach washing is normally carried out with a strong oxidative bleaching agent such as sodium hypochlorite (NaOCl) or potassium permanganate (KMnO4). Bleach washing may be carried out with or without the addition of stone. The bleach washing effect and discoloration usually depend on strength of the bleach liquor, liquor quantity, temperature and treatment time. The bleached fabric materials should be properly antichlored or after washed with peroxide to reduce the subsequent yellowing or tendering of the bleached denim fabric.

At any process stage, if sodium hypochlorite is used, characteristics of the chemical must be considered to assure minimum fabric degradation while achieving the desired color fading effect. In storage, the active chlorine content of sodium hypochlorite solution loses daily especially in hot weather. Therefore, concentration should be checked before dosing the washer. The pH range used in sodium hypochlorite bleach washing is critical because of the formation of hypochlorous acid. It is often not possible to use pH paper for measuring sodium hypochlorite solutions because many indicators get bleached during the measurement. However, when a slight excess of hydrogen peroxide is added to the test solution, the hypochlorite is destroyed without altering the pH. There are many indicators such as universal indicators, phenolphthalein and thymol blue which are not sensitive to traces of hydrogen peroxide.

There is greater danger in pH when used in bleaching than is the case with sodium hypochlorite because of carbonate formations (from atmospheric carbon dioxide). Sodium carbonate is formed in the solution which can help buffer the pH. However, an alkaline buffer is recommended for sodium hypochlorite bath. In the sodium hypochlorite process, as an after treatment, all chlorine must be cleared from the fabric by use of an antichlor, such as hydrogen peroxide, sodium bisulphite or sodium thiosulphate. Hydrogen peroxide is often chosen as it not only neutralizes chlorine but also offers the advantage of additional whitening during the process. On the other hand, bisulphite or thiosulphate are used for the antichlor process.

The normal pH use range for sodium hypochlorite bleaching is 9–11.5, where bleaching species of the chemical are most stable. The stability decreases rapidly above 55–60 °C. Therefore, these variables must be controlled to ensure that the cellulose substrate is not damaged. If damage is occurring, lower temperature or concentration or a shorter cycle should be considered. Buffers should also be added to sodium hypochlorite bath to aid controlling of the pH. Some surfactants perform this role, acting as dual agents to provide alkalinity and aiding in wetting and penetration of the fabric by the chemical bath. Another negative property of sodium hypochlorite is the ability to form chloramines with protein impurities in cotton, which tend toward yellowing and ageing. This source of yellowing is a function of impurities in cotton itself.

In case of potassium permanganate, manganese oxide (MnO²), a brown and water insoluble compound is produced as a by-product. Some of the preventive methods involve high temperature water rinsing, use of chelating agents, good detergent and mechanical agitation, and especially a enough reduction stages, scouring and rinses to completely remove the residual materials. However, the most effective way to remove MnO² is treating the fabric with oxalic acid. Limitations of bleach washing:

• The same level of bleaching is very difficult to achieve in repeated runs.
• Bleaching treatment sometimes damages cellulose, resulting in strength loss and or pinholes at the seam, pocket, etc.
• Bleaching liquor is harmful to human health. This may also cause corrosion to machine parts.
• Bleaching treatment needs antichlor treatment to eliminate the subsequent yellowness in the fabric.
• Chlorinated substances occur as abundant products in bleaching and pass into the effluent where they cause severe environmental pollution.