Classification of Dyes according to their application to fibres
The person concerned with dyeing are interested in this type of classification of dyes because it gives various different methods of dyeing to different types of fabrics with different dyes.
The person concerned with dyeing are interested in this type of classification of dyes because it gives various different methods of dyeing to different types of fabrics with different dyes.
(ⅰ) Acid Dyes
These include those dyes which are the sodium salts of colour acids. The groups or phenolic group. The negative ion is responsible for the colour of the dye. Such dyes are used to dye annual fibres such as wool and silk directly. These are not very useful for unmordanted cotton and linen (vegetable fibres). Generally these dyes are used in acidic solutions and the fabric is dipped and strongly agitated in the hot solution of the dye on the presence of either an acid or its salt to get a uniform colour on the cloth to be dyed. Picric acid, martius yellow, orange Ⅱ, naphthol yellow are some of the important dyes that belong to this class.
(ⅱ) Basic Dyes
These are also called cationic dyes. This kind of dyes are usually the hydrochloride or the zinc chloride complexes of colour bases. The colour of these dyes depend on the positive ion present in them. These dyes can be used directly to dye silk and wool but not unmordanted cotton and linen. Generally those dyes are applied in basic medium. Methyl violet, crystal violet, methylene blue etc. are some dyes that belong to this class. They also include azo dyes and triphenylmethane dyes.
(ⅲ) Direct or Sustantive Dyes
These are salts of colour acids and are mostly azo dyes. They get strongly adsorbed on cellulose. These dyes are not considered as acid dyes through they contain sulphonic acid groups present in them are not used for attachment of dye to the fibre.
Since these dyes are held in cellulose fibres by adsorption so the dyling process with these dyes is reverible. These are quite cheap and easy to use. An important member belonging to this class is congo red.
(ⅳ) Mordant or Adjective Dyes
There days are always used after the prior treatment of the with some mordant such as salts of aluminium, chromium, iron etc. The mordant to be used depends upon the nature of the dye. If the dye is acidic then a basic mordant is used (salts of Cr, Al,Fe etc). If the dye is basic then an acidic mordant (tannin or tannic acid etc.) should be used. For dying the cloth is soaked in the mordant solution and then steamed to get mordant fibre. The mordanted fibre is then dyed. The mordant that gets attached to the fibre combines with the dye to form an insoluble complex (metal chelate). The formation of such chelated complexes can take place only when the resulting dye contains a five or six membered ring which is possible in the presence of -OH group in the ortho position to one of the following groups present in the dyestuff, -OH, CO, NO, NO₂, COOH, NH, NH₂, N＝N etc.
(ⅴ) Ingrain, Developed or Azoic Dyes
This refers to those dyes which are synthetised and so produced within the fibres. Since the azoic dyes are produced by azo coupling inside textile fibres so they are
fast soaked in a solution of phenol or amine (secondary component) and then it is soaked in a solution of dia-zonium salt. The significant example of Ingrain dye is para red.
The colour obtained by this type of dyeing can be of a variety of hues. The colours formed by this process of dyeing are sometimes called ice colours because diazotisation and coupling reactions are always carried out at low temperatures.
Aniline black may be considered as an oxidation dye but is generally regarded as a special case of ingrain dyes.It is due to the fact that the dye is produced within the fibre by oxidising the fibre padded with the aniline salt with some oxidising agent like KClO₄.
The colour obtained by this type of dyeing can be of a variety of hues. The colours formed by this process of dyeing are sometimes called ice colours because diazotisation and coupling reactions are always carried out at low temperatures.
Aniline black may be considered as an oxidation dye but is generally regarded as a special case of ingrain dyes.It is due to the fact that the dye is produced within the fibre by oxidising the fibre padded with the aniline salt with some oxidising agent like KClO₄.
(ⅵ) Vat Dyes
The majority of vat dyes are polycondensed aromatic carbonyl compounds, e.g., indanthrone and benzantbrone. As such, they bear an in-depth resemblance to anthraquinone dyes and many of the features of anthraquinone dyes apply to these dyes.
Other example of vat dyes are to be found in, for example, indigoid and sulphur dyes.
These are insoluble in water but their reduced forms are found to be soluble in water. The reduced form can be obtained by treating it with some reducing agent (e.g. sodium hyposulphite) in a large vat. The reduced form is then used for dyeing. The cloth to be dyed is dipped in a vat containing the reduced form of vat dye. After some time it is taken out and allowedd to aesial oxidation when the oxidised form of the dye is formed within the fibre. Since the oxidised form in water insoluble such dyes are found to be exceptionally fast. Some typical examples are:
Other example of vat dyes are to be found in, for example, indigoid and sulphur dyes.
These are insoluble in water but their reduced forms are found to be soluble in water. The reduced form can be obtained by treating it with some reducing agent (e.g. sodium hyposulphite) in a large vat. The reduced form is then used for dyeing. The cloth to be dyed is dipped in a vat containing the reduced form of vat dye. After some time it is taken out and allowedd to aesial oxidation when the oxidised form of the dye is formed within the fibre. Since the oxidised form in water insoluble such dyes are found to be exceptionally fast. Some typical examples are:
(ⅶ) Sulphur Dyes
These can be regarded as special class of vat dyes because these are also insoluble in water and ate used un their reduced form by a similar process as discussed in vat dyes. These generally process a dark colour. Sulphur black is an important member of this class. These are cheap and are exceptionally fast.
(ⅷ) Synthetic Fibre Dyes
The important amongst such dyes are:
⒜ Dyes for acetate rayon fibres
⒝ Dyes for nylon and terylene fibres
⒞ Dyes for orlon fibres and
⒟ Dyes for dacron fibres.
Since acetate rayon fibres are obtained by acetylation process in which hydroxyl group of cellulose has been acetylated so the substantive dyes are not suitable for them. In acidic or alkaline solutions there are good chances of acetate group being hydrolysed so the vat dyes and also the ingrain dyes are also not suitable for acetate rayon fibres. Thus the disperse dyes which are colloidal solutions of water insoluble dyes but which are soluble in cellulose acetate are most suitable for such fibres. Inoamines ( an azo dye ) is such an example of a dye.
During the process of dyeing with such dyes some compounds such as diphenyl or orthophenyl phenol are added which act as carriers for accelerating the penetration in the fabric. To further accelerate the penetration of dyes into fabric the process may be carried out at 100°C and under pressure.
Terylene is also dyed with disperse dyes and the same process as used for rayon is used for dyeing.
Nylon is basic and it can be dyed with acid dyes as also disperse dyes. Disperse dyes of used give a wet fastness in pale shades only but do not build heavy shades with good fastness.
Orlon having a free carboxyl groups is dyed with basic dyes. It can also be dyed with acetate dyes, vat dyes. To increase the penetration of dyes the process may be carried out at 120°C under pressure.
(ⅸ) Reactive Dyes
It is a new class of dyes and form covalent bonds with those fibres which possess hydroxyl or amino groups. In a particular class of such dyes chlorine atoms are present which are capable of reacting with hydroxyl groups in cellulose when applied to it in the presence of alkali. Orange azo dyes is an example of such a dye.
Another type of reactive dyes have activated vinyl groups which are capable of reacting with a cellulose hydroxyl group in the presence of a base as under.
(ⅹ) Solvent Dyes
This refers to dyes containing no sulpho or any other water solubilising group. They are soluble in organic solvents and are used for manufacture of stains, varnishes, ink lacquers, type writer ribbons, candles, soaps etc.
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