Effect of processing parameters on sodium hypochlorite bleach:

It is very important to maintain processing parameters like pH, time and temperature during hypochlorite bleaching process. The effect of various parameters on bleaching action given below:

i) Effect of pH: pH plays an important role for getting good bleaching effect without damaging cotton fibres.

If pH is less than 10, there will be almost no bleaching take place.

If pH is from 5.0 to 8.5, mainly HOCl is formed. Due to this the bleaching action will be very fast and this may lead to rapid degradation of fibre.

If pH is less than 5.0, chlorine gas is librated and there will be almost no bleaching action.

Therefore for hypochlrite bleaching process the optimum pH should be maintain in between 9 and 10. To maintain this pH, Sodium Carbonate is used.

ii) Effect of time and Temperature: Best results can be obtained if bleaching can be done at 35-40 degree C for 45 to 60 minutes.

Important note: Hypochlorite bleaching should be carried out in stainless steel. If the pot is made of copper or iron, it is possible that iron and copper catalyze the oxidation of cellulose by hypochlorite, which ultimately damage cellulose fibre. It is also advisable that the water used in the bleaching process should be free from iron and copper.

Bleaching process:

Following recipe may be used for bleaching of cotton using sodium hypochlorite:

NaOCl                    : 2.5% active bleach

Sodium Carbonate:  to maintain pH 9 to 10

Prepare the bath with the above ingredients and run the material from 45 to 60 minutes. The temperature should be maintained from 35 to 40 degree C. After the bleaching process, drain the bath and wash the material in cold water and then treat it with antichlor.

What is Antichlor? Fabrics bleached with hypochlorite will develop a distinctive chlorine odor. This odor is due to presence of residual chlorine in the cellulose material. This odor can easily be removed with an after treatment consisting of sodium bisulfite and acetic acid