Acid desizing

Because they are unstable, hypobromites are usually prepared just before use for such jobs as textile bleaching and desizing. In alkaline solutions at 50—80°C bromine reacts to form bromide and bromate. This reaction is reversed in acidic solutions. 

A typical enzymatic stone-washing process (76) is as follows load garments into industrial laundry machine, add water, and heat to 50—60°C. Adjust pH to 6.0 with acetic acid or buffer. Desize garment with a-amylase for 10—15 min, and drain water. Add new water, heat to 55—60°C, adjust pH to 6.5—7.0, and add ceUulase. Tumble for 20—90 min, drain, rinse twice, and dry. 

Not all modified starches are suitable for removal by aqueous dissolution alone. Such modifications of natural starches are carried out to reduce solution viscosity, to improve adhesion and ostensibly to enhance aqueous solubility. Commercial brands vary [169], however, from readily soluble types to those of limited solubility. Indeed, some may be as difficult to dissolve as potato starch if they have been overdried. It is thus very important to be sure of the properties of any modified starch present. If there are any doubts about aqueous dissolution, desizing should be carried out by enzymatic or oxidative treatment. Even if the size polymer is sufficiently soluble, it is important to ensure that the washing-off range is adequate. Whilst the above comments relate to modified starches, other size polymers such as poly(vinyl acetate/alcohol) and acrylic acid copolymers vary from brand to brand with regard to ease of dissolution. 

The poor adhesion of carboxymethylcellulose to synthetic fibres means that where such fibres are present, it can only be effective in combination with a synthetic size polymer (Table 10.6). This needs to be taken into account when considering suitable desizing procedures. If this cellulose derivative is to be used in conjunction with an electrolyte-sensitive acrylic acid copolymer, it is advisable to choose a salt-free carboxymethylcellulose. 

Many acrylic acid copolymers are water-soluble but unlike poly(vinyl alcohol) they are not degraded by alkali. In fact they need alkali for effective desizing as they are more soluble at alkaline pH than in neutral solutions. They are sensitive to acidic media, which should not be used. Solubilisation occurs by the formation of sodium carboxylate groups from the anionic polyacid. The polyelectrolyte formed in this way is readily soluble and shows a rapid rate of dissolution. However, the presence of electrolytes such as magnesium or calcium salts from hard water can inhibit removal [191]. 

The desizing of water-soluble size polymers can be summarised as follows. Batchwise or continuous methods can be used in both cases an adequate supply of hot water is needed during the washing-off. Hot water and detergent are needed to remove polyfvinyl alcohol) or carboxymethylcellulose. The addition of alkali may be beneficial with carboxymethylcellulose. Alkali is essential with modified starches and acrylic acid copolymers. Polyfvinyl alcohol) can be degraded effectively by alkaline oxidation. 

Desizing of cotton fabric can be accomplished by physical, chemical or combination of physical and chemical mechanism, namely rot steeping, acid steeping, treatment with enzyme and oxidising agents. In desizing, the starches and polymers that are applied which are insoluble, are converted into water soluble com-... 

The pH of the desizing bath is maintained by adding acid or alkali. The effects of pH and temperature on enzyme activity are shown in Figs. 3-3 and 3-4 respectively. However storing the enzyme at a fixed temperature shows that the activity steadily decreases with time (Fig. 3-5). In general, changes in pH, tem-... 

Casein/cellulosic fibre blended fabric is first desized under mildly acidic conditions at 60-70 C for 30-60 min and then scouring is carried out in a fresh bath at pH 6 with 3-4 g/1 non-ionic detergent at 50-60°C for 30 min, and rinsed well, first in warm and then in cold water. 

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