Water is an essential item in textile wet processing like scouring, bleaching, dyeing, printing and finishing. It is the medium in all processing operation and creates equally more pollution.
The properties of water are of high importance, since almost all the processes are carried out in aqueous solution. Therefore, the quality of water used in textile finishing is of great significance. The quality of water is estimated on the basis of the following characteristics: transparency, colour and hardness. Water hardness is determined by the total content of calcium and magnesium salts and is expressed in milligram equivalent per litre or parts per million (ppm). Water colour and its hardness have a great influence on the quality of textile material and the consumption of soap dyes and other reagents. The presence of iron, calcium, magnesium, and manganese salts in water causes roughening of fibres, unlevel dyeing, promotes stain formation and reduces colour fastness. The presence of iron and manganese salts in water causes formation of brownish stains and may provide local destruction of the fabric. The high sorption capacity of fibre towards iron and manganese salts prohibits the presence of these salts in water even in small amounts. The technological processes of treatment of fibrous materials require the use of water of a certain quality. The general requirements are : transparency of water, the presence in it of only traces of iron and manganese and a small amount of oxygen. The conventional classification of water according to its hardness are : Very soft 15 Soft 15 30 Medium Hard 30 60 Hard 60 100 Very Hard 100 + Different operations require water of different degree of hardness, characterized by a low oxygen content, transparency and pH from 7.0 to 8.5. The content of iron salts should not exceed 1 ppm. Solutions of soap, dyes, acids and alkalis are prepared with water of hardness of not over 2 4 ppm. It is allowed to use water containing iron in an equivalent upto 15 gpl without softening. It is desirable to use water with 18 to 35 ml, As an exception water of hardenss of 54 may temporarily be used.
Process used and Requirement of Water : The sequence of textile processing is mentioned below. Along with the process, the amount of water required to process 100 Kg of cotton material is mentioned below table : Table 1 Requirement of Water in Different Operation A For 100 Kg of Cotton Water in litre 700 1,500 1,100 4,500 3,000 1,000 5,000 15,000 5,000
1 2 3 4 5 6 7 8 9
Sizing Desizing Kierring Mercerising Scouring Bleaching Direct Vat Reactive
Naphthol B For 100 Kg of Silk
1 2 3 4 5 6 7 8 9 10 11 Water for Boiling
Plain Wash Degumming Hot Wash Cold Wash Bleaching Hot Wash Cold Wash Dyeing Hot Wash Cold Wash Finishing
Water in litre 2,000 2,000 1,000 1,000 2,000 1,000 1,000 2,000 2,000 1,000 1,000
Mostly pure water is converted into steam. The impurities in water causes (1) scale formation, (2) corrosion and (3) foaming. This means that water meant for steam must be soft and must not contain too much dissolved material otherwise a deposit of CaCO3 & CaSO4 mainly will be formed on the wall of the boiler. The formation of hard insulating crust, called boiler scale causes a much greater consumption of fuel and also a rapid depreciation of the boiler due to overheating. Under the stress of continued overheating, the boiler may give way and burst due to unequal expansion of the body of the boiler and scale. Further too much of dissolved matter in boiler water causes foaming and frothing. To avoid all these abnormalities, water should be softened by Bakelite process or by using sodium hexa meta phosphate. Water for Bleaching, Scouring & Washing In scouring, bleaching and washing, water is used again and again in each and every operation. The water may be hot or cold. Iron, Magnesium & Calcium salts should be removed for scouring and bleaching. Iron will form a dull shade after scouring and after bleaching; a yellow tinge will be obtained. For washing, water should be soft, otherwise much soap will be used for water and it results in unnecessary wastage of soap. Water containing suspended organic matter should not be used for scouring and bleaching purpose. Water for Dyeing Water is quite important to dye a fibre, yarn or fabric. The dyestuff is transferred to the fibre from the dye bath and it deposited on the fibre surface. Further, the dyestuff is held to the fibre by secondary forces. The dye bath is prepared by means of water, dyestuff and other chemicals. Iron salts should be removed from water before dyeing, as it would create a dull shade. In this case, sometimes, dyeing may be uneven. Water should be sufficiently rectified before dyeing for a better and uniform dyeing. Water for Printing Water is used as a solvent and also as medium with gum in printing. The amount of water used is relatively small than that of dyeing. The dyestuff, if soluble in water will be used by dissolving that in limited water. Insoluble dyestuffs in water are used in pasting. The water should be soft and must not contain any impurities like that of dyeing. After printing, water in form of
steam is applied on the printed cloth by means of pressure for a better penetration of the dye molecule in the fabric. Water for Finishing Water acts as excellent softening agent for many types of fabrics. Most of the fabrics feel softer in summer, after holding a larger amount of water than in winter. Water used for finishing should be soft and should not contain any impurities.