Application of Acid Dyes:
Acid dyes are so-called because they are applied in presence of organic and inorganic acid in dyebath solutions. Acid dye is water soluble dye. Acid dyes are sodium salts of sullphonic acid but there are a few containing carboxyl group. It’s have a direct affinity towards protein fibers and polyamide fibers. Acid dyes are commercially applied mainly on natural protein (wool and silk), synthetic polyamide (nylon) and to a small extent to acrylic and blends of these materials.
The acid dyes are usually applied under acidic conditions. The application of acid dyes on wool or nylon results in ionic bond or salt links between the anionic dye and the positively charged groups in the fiber under acidic conditions. In addition to ionic bonds, hydrogen bonds as well as Van der Waal’s forces may also be formed between the fiber polymer system and the acid dye molecules.
Because of the high dye-fiber affinity due to opposite charges, there is risk of rushing of dye molecules towards the fibers at high rate with possibility of unlevel dyeing. To avoid unlevel dyeing, some retardation in the dyeing rate is obtained by making use of sodium sulphate.
The acid dyes are further classified into three main groups:
- Levelling dyes
- Milling dyes
- Super-milling dyes
The main differences in the above three types of dyes include: their molecular weight, affinity for fiber, levelling properties, amount of levelling agent required, dyeing pH and fastness properties. Levelling dyes have the lowest affinity and the best levelling properties but poor wash fastness. Super-milling dyes have the highest affinity, the worst levelling properties but good wash fastness. The levelling properties of dyes can be improved with careful control of dyeing parameters.
Application of acid dyes on wool:
The application of acid dyes to protein fibers results in an ionic or salt link between the dye molecule and the fiber polymer. The point of the fiber polymer at which the dye is attached is termed the dye site. In wool, the dye sites are of many amino group of the fiber. Under dyeing conditions, the amino group becomes positively charged and attracts the negatively charged dye anion.
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There are a large number of amino groups are present in the wool fiber. As a guide, there are approximately twenty times as many amino groups on wool as on nylon and five times as many amino groups on wool as on silk. Dark shades can be readily be obtained on wool because of the highly amorphous nature of the fiber, which results in relatively easy penetration of the fiber polymer by the dye molecule and because of the presence of minor groups.
Method of dyeing with acid dyes on nylon:
The dyeing properties of acid dyes with regard to nylon and wool are similar. The shades are very similar to the corresponding colors on wool, but the saturation point is lower with nylon. When the pH of the dye solution is 2 or lower, nylon has greatly increased affinity for acid dyes. In practice, dyeing cannot be carried out in the pH region of 2 to 2.5 because the degradation of the nylon would be excessive.
Acid dyes requiring strong acid are applied from a dye liquor containing 3 to 5% of formic acid. Sulphuric acid should not be used because it can cause degradation of the nylon, and the addition of Glauber‟s salt is omitted because it has no beneficial effect. Non-ionic leveling agent, either alone or mixed with cationic products, are used. The goods are entered cold and the dye bath is brought to the boil and dyeing continued at this temperature for ¾ to 1 hour. With these acid dyes exhaustion takes place well with weaker acid, 1 to 3% acetic acid (80%), may be substituted for the formic acid or, alternatively, 1 to 3% of ammonium acetate may be used.
The application of acid dyes to nylon also results in ionic bonds or salt links between the dye molecules and the polymer. The point at which the ionic is formed is the terminal amino groups of nylon. The greater crystalline fiber structure of nylon compared with wool as well as the relatively lower number of amino groups means that dark shades on nylon cannot be obtained with acid dyes.
Application of acid dyes on silk:
Although silk has an affinity for acid dyes the colors tend to be less fast than on wool. Silk will exert its affinity for acid dyes at lower temperature than is the case with wool, and dyeing is usually commenced at 40ºC and the temperature is not allowed to rise above 85ºC. Glauber‟s salt is not suitable for use with silk as it diminishes its luster. Sulfuric acid damages the silk. Acid used should be acetic acid. While using boiled off liquor the bath must be neutral or only faintly acidic.
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Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.