What is acid dyes? Dyeing mechanism of acid dyes.

Acid dyes

Acid dyes are anionic, water-soluble, and are mainly applied from acidic baths. These dyes contain acidic groups such as -COOH and -SO₃H that are applied to wool, silk, and nylon when an ionic bond is formed between the fibers of the protonated – NH₂ group and the acid group of the dye. Lightfastness is quite good though overall wash fastness is poor. Contrary to the electrical nature of such dyes and fibers, the strike rate and the absorption of acid dyes in these fibers are very fast; High-density electrolytes are added to the structure of dye uptake and leveled shades. The acid forms cations on the fiber and the temperature help to replace the negative part of the acid with anionic dye molecules.

Properties of Acid dyes

1. Acid dyes are large dyes consisting of one or more sulfonic or carboxylic acid salts.

2. Only fibers that develop a positive charge in the presence of acids like wool, silk and other protein fibers, nylon, and some modified synthetics are easily dyed by acid dyes.

3. The acid dyes in the fibers are lighter and reasonably colored for laundering, but modernizing improves the overall hardness properties of the dye.

4. Most of the acid dye is sodium salt of sulphuric acid and carboxylic acid where the anion group is the active colored component.

5. It is always applied to an acidic medium with an organic or inorganic acid.

6. It has a direct affinity toward protein fiber and polyamide fibers.

7. It cannot use in cellulose fiber.

8. Some acid dye has a good lightfastness and bright shades are found.

9. With these dyes the dyeing rate increases rapidly above 70 ° C, once the dye aggregates in the solution begin to break up with increasing temperature.

10. Anthraquinone acid dyes are offered in bright blue shades that are not found in azo dyes.

Types of acid dyes

1. Acidic dyes are classified according to their dyeing behavior, fastness requirements, migration ability, and dyeing pH. Such as below-

Leveling acid dyes: These dyes have relatively low molecular weight. As a result, they migrate more comfortably before settling and they show less wet fastness. They are generally not suitable for use as clothing fabric. They require an acidic dye bath, often using sulfuric acid and sodium sulfate mixtures (pH 2-4) with leveling agents like ethoxylated fatty amines.

Milling dyes: As these dyes are high molecular weight the result migrates slowly. As a result, they exhibit a wet fastness that is effective for dyeing wool materials. Sometimes it is called 'neutral acid dyes' because they do not require acidic dye baths. These are usually applied using acetic acid (pH 4-7).

Metallic complex acid dyes: These dyes are made up of metallic ionized complex acid dye molecules, usually chromium or cobalt. Metallic complex acid dyes have a high molecular weight which gives them low mobility and high wet fastness. For this reason, they are commonly used in nylon and synthetic polyamide fibers. Metallic complex acid dyes are economical, but they produce relatively dull shades. Metal complex acid dyes (pH 2-7) take a wider range of pH.

2. On the basis of strength, acid dyes are-

i. Neutral acid dye

ii. Weak acid dye

iii. Strong acid dye

Dyeing mechanism of acid dyes

Acid dyes are easily applied to wool, silk, and nylon fibers. During dyeing, H₂SO₄ or CH₃-COOH is present due to pigment conduction. These are water-soluble and produce color anions and sodium cations.

RSO₃ NA = RSO₃^- + Na ⁺

Wool fiber polymers can be represented as:

H₂N-W-COOH = Wool

Thus the polymer part has the current amino group and the other parts have a carboxyl group.

Under certain conditions, the hydrogen ion attached to the carboxyl group is transferred to the amino group at the other end of the macromolecule so that the two ends of the keratin molecule chain receive opposite electrical charges:

H₂N-W-COOH = H₃N ⁺ -W-COO^-

Thus, when wool fibers enter the acid solution, the negatively charged carboxylate group absorbs the hydrogen ions released from the hydrochloric acid group, and the chloride anions released from the HCl also absorb the positively charged amino edges of keratin macromolecules.

HCl = H⁺ Cl^-

NH₃ ⁺ -W-COO^- + H ⁺ Cl^- = ClNH₃-W-COOH

When wool is immersed in acid solution, the concentration of acid decreases with the formation of electrically positive sites whenever the amino group is present in the keratin molecule i.e. the acids act as a fatigue agent during acid dyeing on wool. This absorption continues until it is formed as a result of temporary absorption and sodium chloride forwarding reaction. In this way, if the reaction of salt in the dye bath is increased, the rate of backward reaction is increased. Actually, act as a resistor in dyeing acid dyes on wool. To get uniform color results, dyeing is started in the presence of salt with a little acid in winter, when the temperature is gradually increased. More acid is added to the dye bath and the dying continues.

Uses of acid dyes

1. Acid dyes in textiles are effective in protein fibers, such as animal hair fibers such as wool, alpaca, and mohair. These are also effective in silk. These synthetic fibers are effective in dyeing nylon, but there is a minimum interest in dyeing any other man-made fibers.

2. It can be used as food coloring, to enhance the appeal of certain foods and make them more attractive to consumers.

3. It is used in medicine like Lee's stain, Phosphotungstic Acid Hematoxylin stain, Eosin stain, etc.