Two- bath chrome tanning

A two bath chrome tannage as outlined earlier is based on the penetration of the skin by dichromate which in turn is reduced by the addition of sodium thio sulfate during the tannage. This results in an in situ formation of trivalent chromium and fixation of this chromium by the skin two bath tannage also results in the formation of sulfur from the breakdown of sodium thio sulfate. The formation of sulfur inside the skin has a filling action and results in certain roundness, a fullness and tightness of grain that is desirable. Two – bath chrome tannage is costly in materials, labor and time the two- bath chrome tannage has disappeared from practical application in side upper leather production in the world but is still used in the production of some light leathers, such as glazed kid. The colloidal sulfur aspect of the use of sodium thio sulfate has led some tanners to incorporate thio sulfate into the neutralization process as a part of their normal one- bath chrome tannage.

Requirements on pretannage

1.   Reduced tanning time

2.   Increased shrinkage temperature

3.   Removal of water from the fiber texture

4.   Improvement of the penetration power and uniform binding of the subsequent tanning agents of the main tanning process.

5.   Adjustment of the pH value required for final tanning.

6.   Avoidance of case hardening.

7.   Prevention of loose grain, wrinkled grain or formation of a coarse grain.

8.   No change of the desired leather character as regard handles, softness or physical properties.

Possible types of pretannage

1.   Chrome tannage (often carried out in the pickling bath)

Pretannage with:	Improvement
Glutaraldehyde: Polymer tanning agents: Syntans: Alumin tanning agents: Resin tanning agents Alumin sodium silicates:	Cr absorption, fastness to perspiration Filling, fine grain, lightfastness Bleaching, filling Bleaching, firm grain Filling, firm grain, buffing properties Filling, firm grain, exhaustion

1.       Vegetable/synthetic  tannange

Pretannage with:	Improvement
Syntans with small-sized particles: Chrome syntans: Cr. Tanning agents: Alumin tanning agents: Glutaraldehyde: Polymer tanning agents: Polyphosphates: Resin tanning agents:	Penetration of tannin, grain elasticity Penetration of tannin, flexibility  Penetration of tannin, resistance to heat Bleaching, absorption of tannin Flexibility, fastness to perspiration Penetration of tannin, filling, lightfastness Bleaching, abrasion Filling, firm grain

2.       Aluminum tannage

Pretannage with:	Improvement
Glutaraldehyde: Polymer tanning agents:	Softness, fastness to perspiration Shrinking temperature handle