2 Colouration of Textiles

S. Ariharasudhan

1. 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 (λ) (the distance between the wave crests), or by the frequency (v) (the number of waves that pass a point in a given time).

The wavelength multiplied by the frequency gives the speed of wave propagation. This is always constant in a given medium (speed of light in a vacuum, c = 3.0 _ 108 m s–1). The human eye can detect electromagnetic waves with wavelengths in a narrow range between about 400 and 700 nm (1 nm = 1 X 10–9 m), comprising what we call visible light. We are also familiar with X-rays (λ = 0.3 nm), ultraviolet light (λ = 300 nm), infrared rays (λ= 3000 nm) and micro- and radio waves (λ > 3 x106 nm = 3 mm), whose wavelengths vary by many orders of magnitude. Spectral analysis of daylight, or white light – using a prism, for example – separates it into various coloured lights, as seen in the rainbow. The red, orange, yellow, green, blue and violet spectral colours of the rainbow correspond to lights with wavelengths of about 650, 600, 575, 525, 460 and 420 nm, respectively.

Table 1 Colours of typical spectral bands, and colours perceived after their absorption by a material viewed in white light

2. Basics of Colour

Unlike most organic compounds, dyes possess colour because they

a) Absorb light in the visible spectrum (400–700 nm),

b) Have at least one chromophore (colour-bearing group),

c) Have a conjugated system, i.e. a structure with alternating double and single bonds, and

d) Exhibit resonance of electrons, which is a stabilizing force in organic compounds.

When any one of these features is lacking from the molecular structure the colour is lost. In addition to chromophores, most dyes also contain groups known as auxochromes (colour helpers), examples of which are carboxylic acid, sulfonic acid, amino, and hydroxyl groups. While these are not responsible for colour, their presence can shift the colour of a colourant and they are most often used to influence dye solubility.

3. Classification of Dyes

There are basically two ways of classifying various dyestuffs.

1. According to application, and
2. According to chemical constitution.

The general term for a dyestuff is ‘colouring matter’. The application-wise classification of dyestuffs is more important from a practical point of view and is given below.

The term ‘synthetic dyes’ is used for all dyes that are available in coloured form as ‘readymade dyes’. Natural dyes are also included in the above classification as the extracts of a vast number of plants and some from animal origin are used for dyeing silk, wool and cotton.

Some vat and azoic pigments, and mineral and pthalocyanine pigments are available in insoluble form to the textile chemist. These are used without converting them into their soluble forms, generally for textile printing. Hence such colorants are classified under pigments. The differences between pigments and dyes have already been above. A few common dyes and pigments are described later.

The inclusion of optical brightening agents (optical whiteners) may be specially noted as these are regarded as colourless dyes having a fluorescent system.

4. Chemistry of Dyes and Pigments

Dyes containing one or more azo groups (i.e. azo dyes) comprise by far the largest family of organic dyes. Prominent types are

1. Acid dyes for polyamide and protein substrates such as nylon, wool, and silk;
2. Disperse dyes for hydrophobic substrates such as polyester and acetate, and Direct and reactive dyes for cellulosic substrates such as cotton, rayon, linen, and paper. Generally, the synthesis of azo dyes involves two steps.

  Step 1 is the conversion of an aromatic amine to a diazo compound (i.e. Ar-NH2 → ArN2+), a process known as diazotization, and

Step 2 is the reaction of the diazo compound with a phenol, naphthol, aromatic amine, or a compound that has an active methylene group, to produce the corresponding azo dye, a process known as diazo coupling (e.g. ArN2+ + Ar’-OH→ Ar-N=N-Ar’-OH). This process is suitable for forming both azo dyes and pigments.

Typical structures of colourants that fall into the two groups. Since the effectiveness of a dyeing or printing process often hinges on the affinity between the dye and substrate, dyes are designed with a specific substrate in mind.

In this regard, dyes must be designed that have

1. Greater affinity for the substrate than the medium (usually water) from which it is applied and
2. A high degree of permanence under end-use conditions (e.g. Stability to fading upon exposures to water (wet fast) and/or sunlight (light fast)).

4.1. Forces in dyeing systems

1. Electrostatic force
2. Hydrogen Bond
3. Covalent bond
4. Van der Waals force
5. Physical force
6. Hydrophobic interactions/Entropy factors

4.1.1. Electrostatic force:

The forces have a range about 100A°.Electrostatic force formed when the dye particle and fiber surfaces are oppositely charged. Such force exists in the dyeing of wool, silk, polyamides with anionic dyes (or fibers containing anion with cationic dyes). The polymers of these fibers contain amino and carboxyl groups depending on pH value in water, these groups are either neutral (-COOH, -NH2), cationic (-NH3+) or anion (-COO-).

4.1.2. Hydrogen Bond:

When hydrogen atoms are united with strongly electronegative group element, the latter by attracting the electron of the hydrogen atom, gives to it a positive bias. This positively charged hydrogen atom may form bond with groups containing unshared pale of electrons. They are of short range 1A° to 5A° (0.1 to 0.5nm)

R-δ——H+δ … …. … …ö= C O—1.5-1.9Aº—–H …1Aº…… ö

Hydrogen bonds are formed because of extra attraction between such atoms. It is a weak type of bond. This bond may be intermolecular or intramolecular.

4.1.3. Covalent bond

The covalent bond between carbon atom in most organic compound is very stable. They are of short range 1A° to 5A° (0.1nm to 0.5nm). Covalent bonds are formed when dyes react chemically with fibers. All reactive dyes form covalent bonds so fastness properties of such dyes are generally good.

4.1.4. Van der Waals force

Van der waals forces are only effective for sorption of dyes to fiber molecules if the distance between the dye and fiber is very small. These are weak forces and depend on atoms being at certain relative position.

4.1.5. Physical force

It is found that although –OH, -NH2, -N=N- and –CO groups might be responsible for attachment by hydrogen bonds to the fiber but this explanation is to a great extent discounted because the coordinating power of these groups is satisfied by chelation within dye molecules which is due to nonpolar or physical force.

4.1.6. Hydrophobic interactions/Entropy factors:

It is found that increasing the no of aromatic rings or unbranched aliphatic chain makes a much greater increase in affinity than does the introduction of potential bond forming groups. This is assumed that the hydrophobic part of unbranched aliphatic chain dissolved in water because of ice-like structure of the water molecules in the immediate vicinity of hydrophobic molecules, which is of completely entropy factors

4.2. Dye Fiber Interaction/ Anchoring system

Dye fiber interaction system can be divided into

1. Nonionic system
2. Ionic system
3. Reactive system
4. Hydrogen bond system
5. Other interactions

4.2.1. Nonionic system: PET, acrylic, polyamide etc.

4.2.2. Ionic system

• Fiber which possess charged group:

   Anionic and cationic: Acrylic fiber (contain negatively charged sulfonic or –COOH group) and basic dyes Wool, Silk, Nylons (contain charged –NH4+ groups) & acid dye Fiber which contain no charged groups

Anionic and Anionic: Cellulose is dyed with direct & vat dyes both of which carry negative charges. The dye is absorbed by virtue of its attraction to the fiber & in doing so it is accompanied by other ions of electrolytes e.g. Na+ & Cl-.

4.2.3. Reactive system: cellulose, wool and reactive dye

4.3. Role of fiber functional groups in dye fiber interaction systems Cotton: ionic system and covalent bond forces and H-bond Cotton fiber has –OH groups, which is highly electronegative and is capable of hydrogen bonding. It is also capable of reacting with reactive groups of reactive dyes and form covalent bonds.

Protein: ionic system

Wool fiber has –COOH and –NH2 groups which are capable of ionizing and at certain pH are positively or negatively charged. So it can be dyed with basic and acid dyes.

Polyester:

contains –COOH, -OH as functional groups but don’t undergo ionisation, so it is not possible to dye them with ionic dyes. So nonionic system and hydrophobic interaction and Van der waals force exist.

PAN: ionic and nonionic system

Contains –OSO3H, can be dyed with cationic/basic dyes Rayon: Ionic system

Contains –OH groups, -COOCH3 groups

5. Dyeing Mechanism

• Affinity

It is the difference between the chemical potential of dye in its standard state in the fiber & the corresponding chemical potential in the dye bath i.e. tendency of a dye to move from dye bath into a substance. It is expressed in Joule or cal (per mole) and quantitative expression of substantivity.

2. Substantivity

The attraction between a substrate and a dye or other substance under the precise condition of test whereby the test is selectively extracted from the application medium of substrate. It is the qualitative expression of affinity. Substantivity depends on temperature, type of fiber, electrolyte concentration. Substantive dyes have affinity and are soluble.

5.1. Affinity of Dyes for Fibres

As fibres vary considerably in chemical structure and as dyes too have different chemical groups that are characteristic of them, it can be easily understood that all dyes will not have the same affinity for all fibres. The disperse dyes, for example, have excellent affinity for the commonly dyed synthetic fibres like nylon, polyester and acrylic but none for the cellulose or protein fibres. Similarly, azoic and vat dyes are applicable only to the cellulose fibres but not to the other common fibres. The following table presents the affinities of different dyes for different fibres, an asterisk representing affinity and a blank space no affinity.

Direct dyes

•  Direct dyes are mainly applied to cellulose fibres such as cotton, viscose, cuprammonium rayon, etc. The cellulose fibres have a good substantivity (affinity) for direct dyes so they can be dyed ‘directly’ from an aqueous solution; hence these dyes are called ‘direct cotton dyes’. As these dyes have excellent substantivity for cellulose, they are also called ‘substantive dyes’.
• The direct dyes are attached to the fibre mainly by hydrogen bondingand van der Waals’ forces.
• The direct dyes have poor light and washing fastness but they are used extensively due to their low cost and simplicity of the dyeing process.

Reactive dyes

a.Since very strong covalent chemical linkages ‘fix’ the dyes in the fibre these dyes are called ‘reactive’ dyes. The reactive dyes are soluble in water and are predominantly applied to the cellulose fibres. The dye-fibre reaction takes place only when alkali is added to the dye bath.

ii.A schematic representation of the covalent bond between the chlorine atom in a reactive dye and the hydroxyl group in a cellulose fibre is shown below.

1. The application of these dyes to cotton materials involves two distinct steps:
2. Dyeing with the dye in the presence of common salt to effect as much exhaustion as possible.
3. Chemically reacting the dye with the fibre in the presence of an alkali like soda ash, caustic soda, sodium silicate, etc.
4. Reactive dyes are mostly applied to cellulose fibres; they also dye silk and nylon.
5. The reactive dyes in general have good washing and light fastness, but suffer from poor hypochlorite bleaching fastness.

• Vat dyes

1. Vat dyes are the most important dyes for the coloration of cotton and other cellulose fibres.
2. The washing, light, perspiration, rubbing, etc fastness properties of vat dyes are excellent.
3. The vat dyes are insoluble in water and have to be dissolved by using sodium hydroxide and sodium hydrosulphite (a reducing agent), usually at 50°C, the treatment lasting for 15 to 20 minutes. The solubilisation process is referred to as ‘vatting’.

In the vatting process, the dye is reduced by the sodium hydrosulphite to the ‘leuco-vat dye’, which in turn is converted into the water-soluble sodium salt in the presence of the sodium hydroxide. The solubilised dye is taken up by the fibre and then brought back to the insoluble state in the fibre by oxidation.

1. The steps in the dissolution of a vat dye, its take up by the fibre and the reversion of the dye back to its insoluble form in the fibre can be listed as follows.

Ariharasudhan

1.  Vat dye dispersion in water + Sodium hydrosulphite dye
2.  2. Leuco-vat dye + Sodium hydroxide Sodium salt of the leuco-vat dye
3. Sodium salt of the leuco-vat dye dissolves in the aqueous bath.
4. The fibre takes up the dissolved leuco-vat dye molecules during the dyeing process.
5. The absorbed leuco-vat dye is oxidised back to the original insoluble vat dyeinside the fibre.

• Disperse dyes

1. Disperse dyes are non–ionic in nature, insoluble in water and relatively much smaller in size compared with other dye classes.
2. These dyes are mechanically ground to a very small particle size and with the help of dispersing agents, they can be dispersed in the dye bath.
3. The dispersed dyes were originally developed for cellulose acetate fibre. The disperse dyes of today are used to colour the synthetic fibres like nylon, polyester and acrylic fibres. The disperse dyeing of polyester fibres is carried out at high temperature (135° C in the regular dyeing process and even at 200 C using a special process called the ‘thermosol’ process).
4. Disperse dyes in general have fairly good all-round colour fastness.

Pigments

• Pigments are insoluble in water.
• They have no substantivity (affinity) for any fibre and do not have any fibre reactive groups that can form chemical bonds with fibres.
• Pigment particles are much larger in molecular size compared with dyes. Therefore they are not capable of penetrating fibres; they are held mechanically on the fibre surface by means of special chemicals. Dyes, on theother hand, are much smaller in size and therefore they penetrate the fibre structure and get fixed inside the fibre, as stated earlier.
• On account of point No. (iii), pigments are applicable to all fibres and are used mostly in textile printing rather than dyeing.
• Pigments have moderate to good fastness to wet treatments if they have been properly fixed to the substrate surface by binders.

6. Reproducibility of Shades

The shade of the dyes should be reproducible when required. Certain dyes have ability to overcome the factors like liquor ratio, pH, temperature etc. which affect the reproducibility.

Characteristics of highly reproducible dyes are:

•  Highly soluble
•  Medium substantivity
•  Medium reactivity
•  Good wash off properties
•  Highly diffusible

7. Optimization of Dye

The principle is to carry out dyeing in a manner in which the dyestuffs absorbed by substrate almost uniformly with less dye wastage.

7.1.   Substrate

1. Affinity
2. Circulation speed
3. Action of chemicals before

  7.2. Dyestuff

1. Depth of shade
2. Optimum quantity/yield
3. Diffusion ability and regularity
4. Color fastness
5. Combination & mixability
6. Chromphore percentage

7.3. Auxiliary Products

• Optimum quantity
• Compatibility with dyestuff and with each other
• Levelness
• Control of PH in final exhaustion
• Reproducibility
• No adverse effect

7.4. Temperature and time

Low initial temperature to avoid rapid absorption of dye

Control of critical temperature zone for maximum exhaustion

Sufficient time for penetration and fixing

7. 5. Machine

• Control of batch
• Volume of flow
• Temperature regulation

The actual dyeing theory can be obtained mathematically from kinetics of dyeing or dyeing equilibria. The dyeing phenomena found in principle of dyeing curve. The factors for uniform color & optimization of dye all are related to kinetic phenomena. Therefore, kinetic dyeing is important in the dyeing process.

REFERENCES and URLs

1. A K Roy Choudhary, “Textile Preparation & Dyeing”, Science Publishers, USA, 2006.
2. Broadbent D.A., “Basic Principles of Colouration”, Society of Dyers & Colourists, 2001.
3. Trotman, E.R., “Dyeing and Chemical Technology of Textile Fibres”, Charles Griffin and Co. Ltd., London. 1991.
4. Bhagwat R.S “Handbook of Textile Processing”, Colour Publication, Mumbai, 1999.
5. Shenai, V.A., “Principle and Practice of Dyeing”, Sevak Publisher, Bombay,1991.