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Textile Dyeing: Introduction

Dr. Tanveer Hussain

What is Dyeing?

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The Dyeing of Textile Materials, Texilia, Eurotex.

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Phases in Dyeing

1. Disaggregation of the dye 2. Exhaustion - Movement of the dye from dye-

bath towards the textile material 3. Adsorption of the dye on the surface of the

textile material 4. Diffusion/absorption/penetration of the dye

within the textile material 5. Fixation of the dye in the textile material

Effect of physical structure of the fiber on dyeing

Fiber molecules are grouped in the form of linear polymeric chains

Generally these linear chains of polymers are oriented along the axis of fibriller filaments

Dyeability of fibers depends on the orientation of these molecular chains

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In crystalline regions of the fiber, the chains are highly oriented posing difficulties in the dye penetration

In amorphous regions of the fiber, the chains are less oriented posing less difficulties in the dye penetration

Amorphous regions may be considered functioning as pores, thus allowing the dye penetration

The size of the fiber pores in relation with the size of the dye molecule determines the ease of penetration

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The size of pores in some fibers increases due to fiber swelling in the presence of water, thus further facilitating the dye penetration

Pore size of dry viscose rayon fibers is 5 A while 20-30 A when wet

Pore size of different cellulosic fibers range from 20-100 A

Porosity of viscose rayon > mercerized cotton > Un-mercerized cotton

In the same dyeing bath, colour intensity of viscose rayon > mercerized cotton > Un-mercerized cotton

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Pore size of wool fiber in dry state is 6A which increases to 41A when wet

Synthetic fibers have high orientation and crystallinity and poor swelling in wet state; consequently dye penetration is comparatively difficult in synthetic dyes

Effect of physical structure of the fiber }v]vPY

Effect of dye/fiber chemical structure on dyeing of cellulosic fibers

General factors for substantivity of dyes with cellulosic fibers 1. Dye structure linearity

a. Linear dyes have higher substantivity than the branched 2. Coplanarity

a. Dyes having coplanar aromatic nuclei have higher substantivity 3. Capability to form hydrogen bonds 4. Separation of hydrogen bond-capable entities by a

distance of 10.3 Angstrom or a multiple thereof 5. Existence of a system of conjugated double bonds 6. The presence of solublizing (sulphonic or carboxylic)

groups in the minimum possible quantity

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Effect of dye/fiber chemical structure on dyeing of cellulosic fibers

More Linear

Less Linear

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Hydrogen Bonding

Hydrogen atoms can accept electrons, especially when it is directly linked to nitrogen or oxygen

Nitrogen atom in the azo group can donate electrons Oxygen atom in hydroxyl or ether group can also donate

electrons Nitrogen and oxygen present in the dye can make hydrogen

bond with hydrogen in the hydroxyl group of cellulose Hydrogen present in the dye may form hydrogen bond with

oxygen in the cellulose

Hydrogen Bonding

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Cellulose acetate

Acetylation causes: Alteration of surface

characteristics

Decrease in hydrophilicity

Limited inter-miceller canal size due to less swelling because of water

Suitable dyes for dyeing cellulose acetate

Dyes with molecular size not exceeding 10 Angstrom

Molecules with least number of sulphonic groups

Molecules containing basic groups

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Protein Fibers

Contain tNH, -NH2 and COOH groups Are hydrophilic because of the above groups Have amphoteric nature In acidic pH, -NH3+ is formed In basic pH, -COO- is formed Normally dyeing is done in acidic pH

Nylon Fibers

typical nylon 6.6

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Nylon Fibers

Polyester Fibers

Normal polyester does not contain any ionic groups

The fiber crystallinity is very high Only suitable dye is disperse dye Either temperature > 100C required or special

auxiliary (carrier) required for dyeing Cationic dyeable modified polyester is also

available

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Acrylic Fibers

Normally contain acidic/anionic groups Sulphonic groups; and/or

Carboxylic groups

Because of the presence of acidic groups, acrylic fibers can be dyed with basic/cationic dyes

SO3- or COO- groups form bonds with cationic dye

Summary

Physical and chemical structure of fiber and dye greatly influence the dyeing process