Dyes can be divided either according to their chemical nature or according to their application and Chemists are generally like to former type of classification which has practically no value to dyers. Therefore, dyers are interested to following type of classification.
A. Chemical classification
The chemical constitutions of dyes are so varied that it is difficult too classify them into distinct group. The following classification is important and generally accepted.
- Nitro dyes
- Nitroso dyes
- Azo dyes
- Diphenyl methane dyes
- Triphenyl methane dyes
- xanthen dyes
- Diphenyl amine dyes
- Heterocyclic dyes
- Sulphur dyes
- Pthalocyanin dyes
- Anthraquinoid dyes etc
B. Dyers (according to application) classification
- Acid (anionic) dyes
- Basic (Cationic) dyes
- Direct dyes
- Vat dyes
- Developed dyes
- Sulphur dyes etc.
Tanners are generally interested with acid, basic, and direct dyes and sometimes with sulphur and developed dyes.
- Anionic dyes
The most commonly use dyes for dyeing chrome leathers or vegetably-synthetically tanned leathers which which have been submitted to a preliminary cationic treatment. By means of the pH value of the leather, tthe depth of penetration of the dyestuff can be controlled and of the dye-bath. If the adequate depth of penetration has been reached, binding of the dye to the fibre is achieved by reducing the pH to <4.0, mostly by means off formic acid. Appropriate dyeing auxiliaries can be widely influence the affinity of dyes in respect of absorption capacity or leveling.
a. Simple acid dyes
Mainly low molecular dyes, Their advantages consists in their low price and good dyeing and penetrating capacity, and in most cases they give clear, brilliant, shades.
Their disadvantages are inadequate lightness, and insufficient fastness to wetting perspiration and washing.
Acid dyes synthesis
The majority of acid dyes, chrome dyes and direct dyes are sodium salts of sulphuric acids.
A simple example is the azo dye orange-II.
The molecular weight of most of these dyes orange from 300 to 800 and there are usually one to four sulphonic groups present. The anion is obviously large compared with the cation and a dye may be expected to behave very differently in solution. This is the reason of behavior like non ideal solution.
Banned Azo dyes
Many aromatic amines have been used to make the so called azo dyes, but are known to bee toxic, particularly carcinogenic. Because there is a possibility of regenerating the amine from the azo derivative they are now excluded from this application, notability benzidine ( see figure) and its derivatives.
Other banned reagennts include 4-aminophenyl. toluidine, 2-napthylamine, dianisidine, cresidine etc.
b. Substantive (direct) dyes
Use for chrome leathers, mostly as surface dyes with good coating properties and corrective power for defects. On intermediately dried suede leathers they also enable a good dyeing effect with complete penetration and intensive colouring of fibres. Due to benzidine or benzidine derivatives contained in the dye complex they have been removed from the manufacturers range of products.
c. Special leathers dyes
Dyes developed for leathers with particular fastness properties, which are easily combined and have good levelling properties. These are mainly available in powder form. However a growing number off liquid dyes have been put on the market during the last few years. These evolve no dust , are easy to proportion and above all, they have a very low salt content. In general, they are divided into two groups..
1:1 Metal complex dye
In these dyes a dyestuff molecule is bound as ligand to a metallic central atom such as chromium, iron, copper or cobalt. The good lightfastness and washing properties and have good levelling capacity. However, rich deep shades are not obtained by means of these dyes. Their main field of application is dyeing of garment and glove leathers and pastel shades. For perfect dyeing results the applied temperatures should not be too high and the pH <5. Furthermore, combination with other classes of dyes or complexing substances should be avoided because demetallization inn the dye complex might occur, resulting in a shift off shade or loss of fastness.
1:2 Metal complex dyes
In these dyes two dyestuff molecule are bound as ligands to a metallic central atom. The dyes of this group are also very fast to light, wetting and perspiration. They are also good for dyeing vegetably-synthetically retanned chrome leathers. However, their levelling and penetration capacity is considerably reduced. This group is furtheer divided intto dyes which are free of sulpho groups and dyes which contain sulpho groups. If the solubilizing groups is not present in thee molecule, these products are only soluble in organic solvents and particularly suitable for spray-dyeing. However, they then have the disadvantage of being less fast to migration on flexible PVC or crepe material. With increasing content of these groups the dyes become increasingly soluble in water. When dyeing is performed in a dye-bath they are used as covering dyeing component in a separate bath, mostly after penetration dyeing with acid dyestuff.
2. Cationic (basic) dyes
Leather will only be dyed by means of these dyes if there is a sufficiently high content of negatively charged groups , i.e. vegetably-synthetically tanned or retanned leathers. They have the advantages of high covering power and provide brilliant shades. Their disadvantages are low lightfastness, inadequate dry and wet rub fastness, insufficient fastness to migration and accentuation of existing grain defects by more intense dyeing. Excessive amounts may result in undesired bronzing. Therefore, they are mostly used ass spate intermediate dyes in the so called sandwich method.
3. Oxidative dyes
Exclusively used for dyeing wool and hair fur skin, These dyes are mainly aromatic oxiamines and aminophenols which oxidize on the substrate or develop into dye.
4. Reactive dyes
Dyes with reactive groups such as vinyl sulphone, dichlorotriazine, trichloropyrinidine, dichloroquinoxaline, which link up with thee reactive substrate by a direct primary valency bond. A high degree off fastness to washing, dry-cleaning, migration and light is achieved on wool, cotton and poly-amide fibres. They have not yet gained importance for dyeing leather.
5. Dispersing dyes
Originally developed for dyeing artificial silk, nowadays they are also used for dyeing synthetic fibres. In leather dyeing these can sometimes be used to dye wool of fur skins in light and medium shades.
6. Sulphur dyes
These are used to dye chamois leather. They have to be dissolved by means of alkali sulfides. These give high fastness, but flat shades, some dyes available now have already been solubilized for special applications. These are used for chrome leather as low-price penetration dyes.
7. Natural (mordant)) dyes
These vegetable wood dye extracts have gradually lost importance and are used only special cases. Together with metal salts such as potassium alum, cuprous sulphate, ferric sulphate, titanium salts or ferrous lactates they produce different colourd compound the form of colour lakes. The dye grain side and flesh side evenly without accentuating the defects of leather.
8. Fat and oil-soluble dyes
They are free dye bases such as nigrosine bases or also some azo dyes which do not contain water-solubilizing groups dye neutral fats, oils and waxes. These are used for coloring wax finishes, shoe polishes, special fat mixtures or organic solvents.
Objectives of Dyeing
Dyeing with soluble, organic dyes gives the substrate leather any desired or fashionable shade. The following dyeing process are possible, depending on the requirements.
- Surface dyeing for leveling and correcting defects on the grain side.
- Deep dyeing to reduce the conspicuousness of patches damaged by impacts or scratches during use.
- Penetration dyeing of the entire cross-section of the leather in order to avoid bright edges on the goods.
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