1.1. HISTORY OF DYE
A review of history of dyeing shows that the art of dyeing is staged far back to the primitive period. Over 30,000 years ago, finely ground coloured minerals dispersed in water were used in paints but they are easily washed off with material coloured with them. Also, use of dye stuff on textile has being practised for thousands of years with the written record of it dated at 2600BC in China. All dyes were natural substances obtained from plant, animal or mineral sources. In Greco-Romans time insect. One precious animal dye with a glorious history which becomes available for home consumption was "Tyrian purple" which was obtained from Mediterranean shellfish.
Among the ancient dyes were fustic madder, indigo extracted from the leave of the plant indigofera also kernes, which produces scarlet colour. Kernes use is recorded as early as 1727B.C and textile fibres dyed with it was first found in France. In 1507, France, Holland and German begin the cultivation of dye plant as an industry with few exception dyers use limited number of natural dye available for their work until the advent of aniline dyes in the middle of nineteen century.
In 1856, credit was given to a talented British chemist, William H. Perkin, working in a suburb of London; synthesize what he called Mauveine by oxidation of mixture of aniline bases. This dye on application to silk gave a bright violet hue, since then a great number of the intense research activity of the chemist for other textile fibres such as wool, nylon, polyester, cotton and acrylics and the use of natural dye has almost ceased.
In 1859, E. Verguin discovered fuchsine; p. Griess in England laid the foundation of azo dye by discovering of diazo compound which is the largest class of synthetic dyes, namely the azo compounds. Bismarck Brown is the first azo dye developed by Marius in 1863.
The establishment of quadrivalent of carbon (1858) and the constitution of benzene (1865) by August Von Kekule opened way for the preparation of purely synthetic dyes, as well as for the artificial production of natural dyes. The first success to be mentioned is the elucidation of the constitution (1868) and the synthesis immediately afterwards, of alizarin (1, 2-dihydroxy anthraquinone) by Graebe and Lieberman. The elucidation of the constitution and the synthesis, of indigo by Adolf Von Baeyer in (1870) and K Heuman in 1890. In the beginning of the 20th century, the development of indigoid dyes has reached a climax as a result of the work of P. Fried Lander and G. Engine who synthesis Ciba blue and thioindigo respectively.
In 1826, a brewery chemist named Peter Griess discovered the azo dyes. Griess did some fundamental research into the diazotization of aromatic amines and went on to discover the coupling reaction by which virtually all azo dyes are now synthesized.
1.2 DEFINITION OF DYE
A dye is a coloured organic compound that have ability to impart colour on a substrate e.g textile material, paper, plastic, leather and wood. Dyes are generally applied in an aqueous solution and require a mordant to improve the fastness of the dyes on the fibre.
Dyes are coloured compounds that have affinity for substrate to which it is applied (fibre) because of the presence of both Chromophores and Auxochromes in the molecules. Chromophores are unsaturated group(s) with multuple bonds present in organic compounds which absorbed white light and impart colour to substance. While Auxochrome make it a dye for dyeing a particular fibre by acting as a fixing agents between the dye and the fibre through salt formation or association. (Allen. 1971)
1.2.1 QUALITIES OF A GOOD DYES
A good dye must have the following qualities
- The colour must be attractive
- Dye must have a suitable colour
- A dye must be fast to washing, dry cleaning, perspiration, light, heat, and other agencies.
- Dye must be able to able to form stable and good dispersion in the medium i.e water or solvent.
- Dye must be able to attach itself to material or solution.
- The substrate to the dye must have a natural affinity of an appropriate dye and must be able to absorb it from solution in the presence of auxiliary substances under suitable conditions of concentration, temperature and pH.
1.3 BASIS FOR COLOUR OF DYES
Unlike most organic compounds, dye posses colour because they:
- Absorb light in the visible spectrum (400-500nm)
- Have at least one chromophore (colour-bearing group)
- Have a conjugated system, i.e. structure with alternating double bond and single bonds
- Exhibit resonance of electrons, which is stabilizing force in organic compounds when any of this features is lacking from molecular structure, the colour is lost.
In addition to chromophore most dye also contains group know as auxochromes (colour helper), example of which is carboxylic acid, sulphonic acid, amino and hydroxyl groups. While these are not responsible for the colour, their presence can shift the colorant and they are often used to influence dye solubility.
As ealier defined, chromophore (literally colour-bearing) are unsaturated groups present in organic compound which absorb white light. The chromophore is a group of atoms which control the colour of dye. A chromophore group is a functional group, not conjugated with another group, which exhibits a characteristic absorption spectrum in the ultra-violet or visible region. Example of chromophore include:
– NO, – NO2, – N=N – NH and – C=O.
If any of the simple chromophore is conjugated with another (of the same type or different type) a multiple chromophore is formed having a new absorption band which is more intensed at a longer wavelength than the strong bands of the simple chromophores. (F.T Duarte et 1990).
The displacement of absorption maxima towards a longer wavelength (i.e. from blue to red) is termed a Bathochromic shift. The displacement of absorption maxima from red to ultra-violet is termed Hypsochromic shift.
The colour of a dye molecule may be intensified by groups called auxochrome which generally do not absorb significantly in the 200-800nm region, but will affect the spectrum of the chromophore to which it is attached. The most important auxochrome groups are –OH, -NH2, -CH3and –CL and their properties are acidic (phenolic) or basic.
The actual effect of an auxochrome on a chromophore depends on the polalarity of the auxochrome, e.g. groups like CH3 – CH3CH2 and –Cl have a very little effect, usually a small shift of 5-10nm. Other group such as –NH2 and –NO2are very popular and completely alter the spectra of chromophores. In general it should be possible to predict the effect of non-polar or weakly polar auxochromes, but the effect of strongly polar auxochromes is difficult to predict. In addition, the availability of non-bonding electrons which may enter into transition also contributes greatly to the effect of an auxochromes. There are two tyes of auxochromes.
- Bathromic auxuchromes: These groups increase the length of the colour by shifting the adsorption maxima from the violet towards red leading to deepen of the colour. It is called Red-shift.
- Hypsochromic auxochromes: These groups decrease the length of the colour by shifting the absorption maxima from red to the violet thereby resulting in colour fading by shifting. It is called Blue-shift.
1.4 DYES AND PIGMENTS
There are some basic differences between dyes and pigments. These are:
- Dyes are the materials which can be applied on a fabric or yarn and show colour visible to the human eye in the presence of light While Pigments both inorganic and organic types, are almost always applied in an aggregated or crystalline insoluble form that requires a binder to form a coating on the surface of a substrate.
- Dyes can be exist either solid or liquid forms while pigments are solids.
- Considerable amounts of dyes are insoluble in water while pigment are 100% insoluble
- The fastness properties of dyes are good while the fastness properties of pigments are average.
- Dyes are comprises of organic while pigment are both organic and inorganic materials.
- Dyes are selectively applied to some fibres, while pigment are applied to all fibres.
1.5 NOMENCLATURE OF DYES
There is no systematic nomenclature for naming dyes. Many names have been given to them by the manufactures, and it is not unusual to find a given dye having several names. Generally, each dye has a trade name (or names), and the shade is indicated by a letter. For example, Y for yellow, R for Red, or B for Blue. Sometimes the letter is repeated, the number of letters indicating roughly the intensity of the colour, e.g methyl violet 6B is a very deep purple (close to blue) sometimes the letter have other meanings. For example, alizarin blue D, Here the D means that, this dye is direct cotton colour, fuchsine S, the S indicating that the dye is an acid (sauer). The letter F is often used to indicate that the dye is fast to light.