dyeing of leather

To give the substrate leather any desired or fashionable shade by means of soluble organic dyes the following dyeing process are possible depending upon the requirements.
1. Surface dyeing for leveling and correcting defects on the grain side.
2. Deep dyeing to reduce the conspicuousness of patches damaged by impacts or scratches during use.
3. Penetration dyeing of the entire cross-section of the leather in order to avoid bright edges on the goods.
Classification of the dyes
1. Anionic dyes
The most commonly used dyes for dyeing chrome leathers or vegetably –synthetically tanned leather which has been submitted to a preliminary cationic treatment the depth of penetration of the dyestuff can be controlled by means of the pH value of the leather and dye bath. The penetration of anionic dye is optimum at the pH 6-7 when the desired depth of penetration has been reached, binding of the dye to the fiber is achieved by reducing the pH to <4.0, mostly by means of formic acid. The affinity of dyes in respect of absorption capacity or leveling can be widely influence by appropriate dyeing auxiliaries.
Simple acid dyes
Mainly low-molecular dyes. Their advantage consists in their low price and good dyeing and penetrating capacity, and in most cases they give clear, brilliant shade. They are very suitable for no or less finish articles. Their disadvantages are inadequate, light fastness and significant fastness to wetting perspiration and washing.
Substantive (direct) dyes
Used for chrome tanned leather, mostly as surface dyes with good coating properties and corrective power for defects. On intermediately dried suede leather they also enable a good dyeing effect with complete penetration and intensive coloring of fibers. Due to benzidine or benzidine derivatives contained in the dye complex they have been removed from the manufactures’ range of products.