COLOUR, DYES AND DYEING

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👉 Light and colour :

Colour sensation is a characteristic of human experience. Nature provides a

particularly vivid display of colour. We use colours in many varied ways; for

example, for clothes, paints, foods, lighting, cosmetics, paper, furnishings, and for

identification and security. Despite our familiarity with it, there is no simple

answer to the question ‘What is colour, and how do we see it?’: we understand so

very little of the complex processes involved in colour vision. There are three main

stages in the perception of colour, but each one consists of numerous complicated

processes:

(1) absorption of coloured light entering the eye by the sensitive cells in the

retina lining the back of the eyeball;

(2) transmission of nerve impulses from the retina to the brain via the optic nerve;

(3) interpretation of these signals when they reach the visual cortex in the brain.

To understand colour, some knowledge of the nature of light is essential. Light is a

form of energy usually considered as being propagated at high speed in the form of

electromagnetic waves. All types of electromagnetic radiation are characterised by
their wavelength (႓) (the distance between the wave crests), or by the frequency
(v) (the number of waves that pass a point in a given time). Figure 1.8 illustrates
the variations of the electric and magnetic fields associated with an
electromagnetic wave.

Figure 1.8 Variations of the electric and magnetic fields associated with an
electromagnetic wave

Colours of typical spectral bands, and colours perceived after their absorption by a material viewed in white light Light absorbed by the material Wavelength Colour of Perceived colour of

band (nm) the light absorbed the reflected light

• 400–440 Violet Greenish-yellow
• 440–480 Blue Yellow
• 480–510 Blue-green Orange
• 510–540 Green Red
• 540–570 Yellowish-green Magenta
• 570–580 Yellow Blue
• 580–610 Orange Greenish-blue (cyan)
• 610–700 Red Blue-green

👉 Colorants, dyes and pigments :

A colorant is a substance capable of imparting its colour to a given substrate, such

as paint, paper or cotton, in which it is present. Not all colorants are dyes. A dye

must be soluble in the application medium, usually water, at some point during the

coloration process. It will also usually exhibit some substantivity for the material

being dyed and be absorbed from the aqueous solution. On the other hand,

pigments are colorants composed of particles that are insoluble in the application

medium. They have no substantivity for the material. Since the particles are too

large to penetrate into the substrate, they are usually present on the substrate

surface. The pigment is therefore easily removed unless fixed with an adhesive.

Most textile dyeing processes initially involve transfer of the coloured chemical,

or its precursor, from the aqueous solution onto the fibre surface; a process called

adsorption. From there, the dye may slowly diffuse into the fibre. This occurs down

pores, or between fibre polymer molecules, depending on the internal structure of
the fibre. The overall process of adsorption and penetration of the dye into the
fibre is called absorption. Absorption is a reversible process. The dye can therefore
return to the aqueous medium from the dyed material during washing, a process
called desorption. Besides direct absorption, coloration of a fibre may also involve
precipitation of a dye inside the fibre, or its chemical reaction with the fibre. We
have already seen that these two types of process result in better fastness to
washing, because they are essentially irreversible processes.
For diffusion into a fibre, dyes must be present in the water in the form of
individual molecules. These are often coloured anions; for example, sodium salts
of sulphonic acids such as Congo Red (2, Figure 1.3). They may also be cations
such as Mauveine (1, Figure 1.1), or neutral molecules with slight solubility in
water, such as disperse dyes (3, Figure 1.6). The dye must have some attraction for
the fibre under the dyeing conditions so that the solution gradually becomes
depleted. In dyeing terminology, we say that the dye has substantivity for the fibre
and the dyebath becomes exhausted.
The four major characteristics of dyes are:
(1) intense colour;
(2) solubility in water at some point during the dyeing cycle;
(3) some substantivity for the fibre being dyed;
(4) reasonable fastness properties of the dyeing produced.

👉 Dye classification and nomenclature :

Classification of dyes according      Classification according to chemical 

to chemical constitution                   constitution to textile usage

 

Azo dyes Anthraquinone dyes Heterocyclic dyes* Indigoid dyes Nitro dyes Phthalocyanine dyes Polymethine dyes Stilbene dyes Sulphur dyes Triphenylmethane dyes

Acid dyes Azoic dyes Basic dyes Direct dyes Disperse dyes Mordant dyes Pigments Reactive dyes Sulphur dyes Vat dyes