Acid Dye Sources,application and uses

                      Acid Dye

Content

1.Define/Definition

2.Sources

3.Application of acid dye

4.Uses of acid dye

Acid dyes are sodium salts of sulphonic acid but there are a few containing carboxyl groups. The acid dyes have direct affinity towards protein fibers and polyamide fibers.

An acid dye is a dye that is typically applied to a textile at low pH. They are mainly used to dye wool, not cotton fabrics. Some acid dyes are used as food colorants

Sources:

The name “Acid dye” gives some insight into its chemical composition. As the name implies, acid dyes consist of acidic molecular groups that characterize the compounds. The process of using acid dyes predominantly takes place in water, when the acid dye is dissolved and therefore able to interact with the material being colored. Historically, acid dyes were most commonly used to dye materials consisting of protein fibers. The acidic pH of the dye-bath allows the dye molecules to bond to the protein fibers. Due to the low staining propensity of acid dyes, they are often used in soaps, detergents, and other cleaning products. The products composed of protein fibers that work well with acid dyes range from animal hair, wool, and feathers, to textiles and synthetic nylon.

Application

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 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 wool: The application of acid dyes to protein fibers results in an ionic or salt link between the dye molecule and the fibers 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.

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.

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.

                                                                   Uses

Fibers

In the laboratory, home, or art studio, the acid used in the dye-bath is often vinegar (acetic acid) or citric acid. The uptake rate of the dye is controlled with the use of sodium chloride. In textiles, acid dyes are effective on protein fibers, i.e. animal hair fibers like wool, alpaca and mohair. They are also effective on sillk.They are effective in dyeing the synthetic fiber nylon, but of minimum interest in dyeing any other synthetic fibers.

Medical

In staining during microscopic examination for diagnosis or research, acid dyes are used to color basic tissue proteins. In contrast, basic dyes are used to stain cell nuclei and some other acidic components of tissues.