What are acid dyes? Describe properties and dyeing procedure of acid dyes.

Acid dyes

Acid dyes are anionic and water-soluble in nature, and it is applied to an acidic medium. 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 acid dye uptake in these fibers are very fast. The acid forms cations on the fibers and the temperature help to replace the negative part of the acid with the molecules of anionic dye.

Properties of acid dyes

i. Most of the acid dyes are sodium salts of sulphonic acids or carboxylic acids, and these are anionic in an aqueous solution.

ii. The acid dyes are dyes that are applied to textiles at low pH.

iii. These dyes have substantivity towards protein and polyamide fiber.

iv. They are suitable for wool, silk, polyamide, and modified acrylics.

v. They have no affinity for cotton cellulose, so these are not suitable for cellulosic fibers.

vi. There are molecular weights in the range of 300–1000 g mol^-1.

vii. Acid dyes create ionic bonds with fiber and hydrogen bond, and Vander Waals force of attraction also contribute.

viii. These are highly water-soluble in nature.

ix. The overall washing fastness of acid dyes is poor, although the lightfastness is quite good.

x. The active colored component is the dye anion in these dyes.

xi. The different bright shade is possible.

xii. Acid dyes are used as food colorants.

xiii. It can also be used to stain organelles in the medical field.

xiv. Some acid dyes are mutagenic and carcinogenic in nature for human health.

Dyeing procedure

At 40 – 50°C temperature, prepare the dye bath with the substrate. For maintaining 4.5 -5.5 pH, add leveling agent, acetic acid, salt, and other auxiliaries. Then run the bath for 10 – 20 minutes. Then check the dye solution pH, it is pH of (4.5 – 5.5) or is not. Raise the temperature to 90-100°C over 20 – 30 minutes gradually 1-2°/minute. Then run the dye bath for 30 – 50 minutes for dyeing. Cool down the bath temperature to 60 – 70°C. Then drain the dye bath and carry out the after-treatment process. Constant hot and cold washes are given to remove unrefined dyes from colored products. This is done to improve the color. It is then treated with a suitable fixing agent.

Approximate recipe for wool dyeing-

Wetting agent – 0.5 to 1.0 g/L

Leveling agent – 1.0 to 3.0 g/L

Wool protecting agent – 2.0 to 4.0 g/L

Dyes – X%

Glauber salt – 10 to 20 g/L

Acetic/ sulphuric acid – 1 to 4 g/L (to maintain pH 4.5 – 5.5)

Temperature – 90 to 100°C

Time – 30 to 50 minutes

M: L – 1:10