Bleaching process by hypochlorite:
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Hypochlorite bleaching means bleaching of sodium hypochlorite (NaOCl) and calcium hypochlorite [Ca (OCI) 2].
Types of hypochlorite bleaching process:
Hypochlorite bleaching is done in two ways.
1. Machine Chemiching Process
2. Circulation Process
1. Machine Chemiching Process:
In this method, the wet cloth is dipped in the bleaching solution in a container like a rope washing machine with the help of a guide roller. The bleaching solution (sodium or calcium hypochlorite) also contains a few emulsion rollers through which the product is immersed in the liqueur of the container. Then by squeezing with the help of a pair of squeezing rollers the fabric is brought to the piling box and the piling is done.
The fabric is kept in the box for a certain time until the bleaching process takes place. Here the initial concentration of the solution is 3 to 4 towadels (TW) and the temperature is always kept below 40 degrees Celcius. Bleaching takes 8 to 12 hours, after which the product is first washed with water, then treated with 0.5% hydrochloric or sulfuric acid, and then washed with water.
2. Circulation Process:
Bleaching process by hypochlorite. Scouring cotton cloth in this method is kept in a cylindrical or rectangular container. This pot has a perforated false bottom. The required hypochlorite solution is placed in a tank at the bottom of the container. Which is sprinkled in the form of spray on the fabric processed with the help of a pump.
The concentration of this bleaching solution is 0.5 to 10 towadels and the ratio of liqueur in cloth is kept from 1: 4 to 1: 5. The solution is sprayed on the cloth for 2 to 4 hours. The pH of the solution is kept below 9.2 to 11 and the temperature below 40 degrees Celcius. The cloth is then washed with water and treated with 0.5% hydrochloric acid solution. Washed again with water and taken for further processing.
During hypochlorite bleaching, (OCI) ions react with proteins in the fiber to produce chloramine. Chloramine then turns yellowish on the cotton and when chloramine is broken down, hydrochloric acid is released which damages the fiber. Chloramine is soluble in alkali treatment. The bleached fabric can be treated for 20 to 30 minutes at room temperature in 0.5% to 1% sodium hydrosulfide solution to protect the fabric from the harmful effects of chloramine.
Precautions for hypochlorite bleaching:
1. During hypochlorite bleaching, the product must be protected from exposure to bright sunlight. Otherwise, the fiber will become weak.
2. The concentration of hypochlorite solution depends on the variation in summer and winter temperatures.
To be accomplished. This is because the rate of bleaching action doubles with every 8°Ctemperature increase.
4. Metal containers that are harmful to bleaching should not be used. This is because cotton is likely to be damaged during bleaching in contact with metal containers. For that, you have to use pots made of stone, concrete, or stainless steel.