Factors which influence the leather dyeing

Choice of dyestuff

The choice of dyestuff depends primarily on the demanded fastness for the types of leather to the produced, on the desired shade and on the respective main tanning method or the retanning agents used. Perfect combinability of the dyes is a further criterion. This depends on the build-up properties, on the absorption rate and chemical structure of the dyes. In principle, the dyer should adjust the desired shades by means of the smallest possible number of shading dyestuffs in order to restrict the possibility of introducing additional defects. Price is a further factor which influences the choice of dyestuff. However, when using low price dyestuff their fastness and intensity should be examined thoroughly. Often they have a high content of extenders or extremely different mixed suppliers’ sample cards should be considered when choosing dyes according to fastness
Solubility of dyes
Anionic readily soluble dyes are dissolved in water at 70-80 ⁰ C in a ratio of 1:10 to 1:20 briefly and if they are intended for high-quality aniline leathers filtered through a fine-meshed cloth to be on the safe side.
Cationic dyes are made into a paste by means of acetic acid with about half the weight of the dye, especially if hard water is used and dissolved in water at 80-90 ⁰ C. Boiling should be avoided anionic and cationic dyes should not be dissolved together as otherwise precipitation or Color Lake may occur. Liquid dyes have the advantage of being dust-free and better to handle because they are easily diluted.


Quantity of dyes
In the case of wet leathers the quantity is calculated in relation to the shaved weight of the leathers, in the case of intermediately dried leathers according to the dry weight. However, it would be more favorable to relate the calculation to the leather surface because the thinner a leather, the greater is its surface per kilogram. In this case better constancy of shade without significant variations is achieved for the subsequent lots.
Addition of dye
For high-quantity aniline leathers the addition of dyes in dissolved form and in several portions is always advantageous for an even and leveled absorption. Liquid dye should therefore also be added in diluted form. Powder dyes in short float processes mostly result in reduced depth and brilliance of color. With this form of application it is necessary to ensure that all dye components are evenly and are readily soluble.

Float ratio
A large of dye-liquor promotes the distribution of the dyes and auxiliary agents used. Furthermore, it prevents an excessive rate of absorption on the surface or reverses side. This should be considered especially when dyeing pastel shades which should therefore be dyed in larger quantities of dye-liquor. The quantities of dye-liquor commonly used are 100-250 % and up to 400 % for pastel shades.
The “dyeing without float” or “short-float dyeing methods” with floats of 0-30 % have to be performed at low temperatures in order to avoid unleveled dyeing because in short penetration rapidly increases but the leveling become uneven. Short floats should not be used for very thin leathers to avoid tearing or tangling of the leather material. These processes should be performed more gently in the automatic chamber or sector machine.