Bleaching is an operation to remove the colored impurities from textile fibers. Cotton in its natural form contains so many minerals, waxes, proteins and coloring matters, etc. In order to attain a bright substrate for dyeing, bleaching or printing and to make the fabric uniformly water absorbent, a pretreatment is essential.
So the first and foremost textile processing operation is called pretreatment, that remove remove the unwanted matters, such as color, minerals, waxes and oils and stains from the greige material. The pretreatment operation utilizes a lot of water and the quality of water plays a vital role in the cleansing of textile materials. Better the quality of water, better will be the processed goods.
As water and its quality play a very important role in wet processing, let us have a brief look into the quality of water required for wet processing, with an emphasis on reactive dyeing. We get water from various sources, like river, ponds, shallow wells and deep bore wells. According to the source of water, it contains many dissolved and suspended impurities. The water from a running river, contains many dissolved salts (solids) like Sulphates, Chlorides, silicates, Carbonates and Bicarbonates of heavy earth metals like Calcium, Magnesium, Iron, Aluminums, Sodium etc. The ratio of these salts varies according to the source of water. The general requirements of the water used in textile processing are given below:
The water should be colorless, clear and free from suspended impurities.
Should not be hard and have the tendency to deposit, scale on fabric or on water supply structures.
It should be non-corrosive.
It should be free from metals such as iron, manganese, aluminums and copper
It should neither be too alkaline or acidic.
Color is normally an indication of the presence of suspended and dissolved salts that may affect the fiber/yarn/fabric. So it has to be removed from water prior use in processing, by a suitable de-coloration technique.
Turbidity or Suspended solids are due to a fine suspension of inorganic salts like (clay. silica, calcium carbonate) or organic finely divided vegetable matter like algae, micro-organism etc. This should also be removed using a suitable filtration technique.
Dissolved solids - in water treatment and analysis this term is called Total Dissolved Solids (TDS). The TDS reflects the presence of unwanted elements in dissolved form, which has to be removed using a suitable method. Good quality water should not have a TDS more than 150 ppm.
pH value – Water with a pH value of more than 7 is alkaline and one below 7 is acidic. Most of the textile processing treatments are dependent on pH values.
Hardness (Calcium and Magnesium) – the presence of Calcium and Magnesium salts in water is called hardness of water.
Temporary Hardness: The presence of bicarbonates of Calcium and Magnesium in water is called temporary hardness. When the water containing these salts are heated to boil, the soluble bicarbonate salts will become insoluble carbonates and precipitate and the hardness disappears.
Permanent Hardness: The presence of carbonates, sulphates and chlorides of Calcium and Magnesium are called permanent Hardness, as this hardness cannot be removed by simple heating.
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