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Dyeing

Introduction

Using dyeing and finishing techniques are simple ways to increase profit margins, product desirability, and performance. They are ubiquitous, used in areas as diverse as fabric-softened clothes, to fire-retardant upholstery, to water-shedding umbrellas or brightly coloured cloths.

A major concern to the textile industry regards the environmental impact of dyes on the environment, as dye works tend to produce vast quantities of waste water that often goes untreated. This has led to widespread research that hopes to solve this problem.

Terms and Definitions

Dyeing is the fixing of a dye to a substrate, and normally intends to give an even distribution. A dye is a colorant that is dispersed in its application medium before being absorbed, adsorbed, reacted with, or deposited on, a substrate. They are distinct from pigments because they are soluble in water and possess substantivity (attractiveness) for their substrate; pigments lack both of these features.

Dyes

Essential qualities of dyes are fastness, colour, low cost, and uniformity. They must also be water soluble, substantive (attractive) to the textile substrate, and reactive with the textile fibres. The colour of a dye depends on its chemical bonds:  its pi bonds, multiple bonds and unsaturated groups. Dyes are classified according to their method of application, or sometimes according to their chemical constitution. How they interact with their substrate depends on the nature of the substrate. Generally, cellulosic and proteinaceous fibres are Hydrophilic?. Fibres from a synthetic source tend to be hydrophobic. The types of dyes that can be used with each type of fibre are shown below. The fibre types are in blue, and the dye classes in red.

Dyeing 1

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There are three stages to the dyeing process. Stage one involves the migration and adsorption of the dye onto the fibre, stage 2 involves the diffusion of the dye from the fibre surface to the rest of the fibre, and stage 3 involves the anchoring of the dye. Anchoring takes places in 5 possible ways:

1) Ionic attraction between charged groups. Used by acid dyes on a polyamide or basic dyes on acrylic.
2) Formation of a large insoluble dye molecule inside the fibre. This is the method of attachment for vat, azo, and sulfur dyes on cotton fibres.
3) Formation of covalent bonds (these are chemical bonds with shared electrons). Reactive dyes on cotton or polyamide employ this method.
4) Formation of a solid solution inside the fibres. This is a hydrophobic interaction and is used by disperse dyes on polyester.
5) Formation of hydrogen bonds and van der Waals forces. This is a relatively weak method of binding and is used by disperse dyes on cotton.

There are five main types of dyes used on cotton: vat, sulfur, reactive, azoic and direct dyes. Vat dyes  are expensive and difficult to apply, but convey excellent wash and light fastness. Sulfur dyes  are economical and easy to apply but tend to be limited to dark colour shades. Sulfur black is the most used dye in the world. Reactive dyes  are the only type of dye to have an increasing worldwide use. They are relatively cheap, with excellent light and wash fastness and bright colours. However, 10-40% of reactive dyes are wasted and high salt concentrations pollute rivers. Direct dyes are cheap and applied at the boil, but bond only weakly to their substrate. Azoic dyes  form an insoluble dye inside the fibre and display very vivid colours.
Polyester is dyed with disperse dyes,  which are non-ionic. They are ground into a very fine powder before being dissolved in the dye bath. They form a dispersion in the dye bath and then diffuse into the fibre.

Reactive, disperse and acid dyes are used to dye nylon. Acid dyes  rely on ion-ion forces for their substantivity between fibre and dye. BarrĂ© patterns may result from acid dyeing. They are barred or striped patterns, usually undesirable, that occur due to physical and chemical differences between the fibres.

Acrylic is dyed using disperse or basic dyes. Basic dyes are positively charged (usually contain NH3+ groups) and give excellent light and wash fastness with brilliant hues.

Wool is dyed using acid, mordant or reactive dyes. Fittingly, mordant dyes are applied alongside a mordant, which is usually a metal-based compound. Together they form a complex that is retained by the substrate.

DP dyers 1
Lab dyeing. Small Samples to test recipes
(Picture Courtesy of W T Johnson & Sons, The Textile Finishers) 

DP Dyeing 27
Jet dyeing machine for high temperature dyeing.
(Picture Courtesy of W T Johnson & Sons, The Textile Finishers) 

DP Dyeing 44
Winch dyeing machine. Atmospheric dyeing for a wide variety of delicate fabrics.
(Picture Courtesy of W T Johnson & Sons, The Textile Finishers) 

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Dyeing (cont.)

Regulations

The HSE has produced a series of documents about the health and safety implications of using dyes,  of which some dyes are toxic, many are environmentally hazardous, and some are sensitizers, too.

They have produced a specialist document regarding reactive dyes

They have also produced a companion document regarding chemical finishes:

DEFRA also produce UK-wide regulations regarding emission controls:

Marks and Spencer provide a very informative article regarding banned and restricted dyes and finishes