dyeing of leather

To give the substrate leather what is leather? any desired or fashionable shade and to improve its appearance, to make it adaptable for fashion styling and ultimately, to increase its value as a commodity means of soluble organic dyes
Dyeing with soluble, organic dyes gives the substrate leather may any desired or fashionable shade.
The following dyeing processes are possible on the requirments
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 dyes (according to application)

1. Anionic (they are further classified as follow)

a. Simple acid dyes

b. Substantive (direct ) dyes

c. Special leather dyes( they are divided into two groups)

1:1 metal complex dyes

1:2 metal complex 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, style="">

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.

2. Cationic (basic) dyes

3. Oxidative dyes

4. Reactive dyes

5. Dispersing dyes

6. Sulphur dyes

7. Natural mordant dyes

8. Fat and oil-soluble dyes

9. Developed Dyes

Method of applying dyes to leathers

Dyes are applied to leathers to in drums paddles or trays and by brush or spray. Drum and spray dye constitute then major means of applying to leather

Drum dyeing

Most leather is dyed in drums since this is efficient and economical. The drums are similar to and frequently identical to those used for tanning; only standard tanning equipment is needed. Neutralization, Retanning, dyeing and fat liquoring are done as successive operation in the drum; thus drum dyeing requires a minimum of leather dyeing handling.

The drums vary in size from 3 to 5 feet in width, and from 8 to 12 feet in diameter. They are fitted with a hollow axle or gudgeon through which dye solution and auxiliary agents may be added during rotation of the drum (12 to 18 rpm).

The inside face of the drum is fitted with pegs, shelves or combination of pages and these agitate the leather during the dying operation. The amount of solution in the drum varies depending on the type of leather being colored and the nature of the dyes employed. For leather dyed in the blue, crusted leather the usual ratio is 8 to 1. For pastel and light colors, the ratio of solution to the leather is greater than for dark or full shades. The solution should be enough to cover the leather when the drum is not moving, and the dyer should estimate the amount as full, medium or short, since quantities are seldom exact. Before addition of dye, the leather is washed thoroughly, to remove loose particles, adjust the surface solidity and raise the temperature. Emulsifying agents, usually of non ionic type, may be added during the washing to remove any superficial grease and ensure a clean surface for dyeing. After washing, water is added to obtain the desired float, the drum is closed and drumming is started.

Dye is added then through the gudgeon. The time of agitation depends upon the type of leather being treated, and the depth of penetration needed. For most grain leather 20 to 30 minutes is enough, though for suede a much longer time is needed because of the complete penetration is required. When the proper depth of penetration is attained, the dye is exhausted by the addition of a dilute acid, usually formic acid and adjusted pH 3.7 – 3.8