The major classes of dyestuffs used in the textile industry are as follows

Dyeing-Dyes and Environmental Consequence Kazy Mohammad Iqbal Hossain

I do not have much knowledge about textile dyes and pigments but with the blessing Google, became able to get some information on this which I would like to share here to create a general awareness among the community. Also believe, this may help the professionals who are engaged in environmental management of dyeing factory to negotiate with their respective dye house professionals regarding the use of dyes with high fixation ratio.

Dyeing and Dyes Dyeing is the process of adding color to textile products like fibers, yarns, and fabrics. Dyeing is normally done in a special solution containing dyes and particular chemical material. After dyeing, dye molecules have uncut chemical bond with fiber molecules. The temperature and time controlling are two key factors in dyeing.

A dye is a colored substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and may require a mordant to improve the fastness of the dye on the fiber. Both dyes and pigments are colored because they absorb some wavelengths of light more than others. In contrast to dyes, pigments are insoluble and have no affinity for the substrate.

Classification of Dyes According to Applications The major classes of dyestuffs used in the textile industry are as follows: • Acid Dyes: Mainly used on wool, silk and polyamide fibers. They give very bright colors, whose fastness ranges from very poor (allowing colors to run) to very good.

• Basic Dyes: Usually applied to acrylics and polyesters to produce very bright colors.

• Direct Dyes: Commonly applied to rayon and cotton.

• Disperse Dyes: Applied to cellulose acetate, polyamide and polyester fibers.

• Reactive Dyes: This group produces a range of bright shades, and commonly used for cellulose textiles.

• Sulphur Dyes: Most commonly used for dyeing cotton, rayon and cotton-synthetic blends and produce strong, deep colors in the final fabric.

• Vat Dyes: These cover an almost full range of shades and are particularly important in the dyeing of cellulose fibers (such as cotton).

• Azoic Dyes: Produce deep shades of blue, violet, yellow, orange and scarlet.

Table-1: Unfixed fraction of selected dyes and pigments (collected from different sources) Dyestuff

Percent of unfixed portion which directly goes to wastewater USEPA

Disperse dyes

OECD

ECHA

Danish EPA

ETAD

5-25

1-12

1-12

0

1-12

Direct dyes

30

4-36

4-36

12

4-36

Reactive Dyes

40

3-40

3-45

32

4-45

Acid Dyes

20

2-15

2-15

10

2-15

Basic dyes

10

0-4

0-4

0-4

0-4

-

-

0-2

2

-

Pigment

Environmental Impact of Dyes and Pigments

Dyes cause a lot of problems in the environment. The problems which are caused in environment by dyes are given below: 1. Depending on exposure time and dye concentration, dyes can have acute and/or chronic effects on exposed organisms.

2. The presence of very small quantities of dyes in water (less then 1 ppm) is highly visible due to their brilliance.

3. The greatest environmental concern with dyes is their absorption and reflection of sunlight entering the water. Light absorption diminishes photosynthetic activity of algae and seriously influence on the food chain.

4. Dyes can remain in the environment for an extended period of time, because of high thermal and photo stability. For instance, the half-life of hydrolysed Reactive Blue 19 is about 46 years at pH 7 and 25 째C. 5. Many dyes and their breakdown products are carcinogenic, mutagenic and/or toxic to life.

Dyes are mostly introduced into the environment through industrial effluents.