Application of Acid Dyes | Application of Acid Dyes on Wool | Application of Acid Dyes on Nylon | Application of Acid Dyes on Silk

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 fibre polymer. The point of the fibre polymer at which the dye is attached is termed the dye site. In wool, the dye sites are of many amino group of the fibre. Under dyeing conditions, the amino group becomes positively charged and attracts the negatively charged dye anion.

There are a large number of amino groups are present in the wool fibre. 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 fibre, which results in relatively easy penetration of the fibre 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 thenylon, 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 fibre 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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