Bleach is a chemical that removes colour or whitens a fabric via oxidation or reduction method.
Many types of bleach have strong bactericidal properties, and are used for disinfecting and sterilizing.
Bleaching is another important pretreatment next to scouring, performed on cotton fibres.
This treatment is given to decolourize the natural colouring matter present in the cotton fabrics and impart a pure white colour.
It increases the ability of the textile materials for dyeing and printing by removing any traces of colour present in it.
Normally oxidative bleaching action is performed in the industries on cotton fibre substrates.
Though a number of bleaching agents are available in the chemical market, few bleaching agents are used extensively.
Calcium hypochlorite (CaOCl), Sodium hypochlorite (NaOCl) and Hydrogen peroxide (H2O2) are the most frequently used bleaching agents in the conventional cotton processing units.
Hydrogen peroxide is considered a universal bleaching agent, as it is suitable for all sorts of textiles.
It is stable at neutral and value is near neutral pH. The pH of hydrogen peroxide solution can be modified based on the suitability.
Processing pH of cellulosic is between 9 and 11.5; proteins 2.5 to 6.0 and for synthetics it is near neutral acidic pH.
In order to bleach cotton, the pH of hydrogen peroxide solution is maintained at around 11.
When alkali is added the stability of hydrogen peroxide is reduced and decomposed at a fast speed.
To control the rapid decomposition a stabilizer is added to the solution.
The ingredients added during peroxide bleaching are normally; hydrogen peroxide (bleaching agent, 1-3% owm), sodium hydroxide or carbonate (bleaching promoters, 0.25 to 1.0% owm), sodium silicate (buffer or stabilizers, 0.5 to 1.0% owm)
In the alkaline condition the instability of peroxide is continued by the concentration.
The liberation of nascent oxygen is utilized for the oxidation reaction in a controlled manner by selecting the stabilizers to get uniform application.
Preparation of bleaching powder:
Bleaching powder is produced by passing chlorine gas through dry slaked lime (calcium hydroxide).