Controlling factors of chrome tanning:
(ⅰ) pH → The suitable for chrome tanning is the range between 3.5 - 3.8.
(ⅱ) Temperature → The optimum temperature for the chrome tanning is 25 - 30°C.
(ⅲ) Time→ To get good pelts 1.5 - 2 hours chrome tanning would be enough.
Chrome tanned won its rightful place in leather manufacture because of some advantages like
Therefore, there is no wonder that thus properties should induces the leather manufacturers to increasing use of the chromium compounds in the manufacture of various types of leather.
Chemical controls in chrome tanning :
(ⅰ) Effect of pH value in chrome tanning: With the amount of acid or alkali present in the liquor or added from outside, the tanning behaviors of a chrome liquor and therefore, the quality of the leather produced are well related. The addition of alkali to a chrome liquor causes the molecules of the chromium salt to become larger and larger. This increase in size reduces the rate of penetration of the grain. Free acid present in the chrome liquor penetrates the skin more readily than the basic chrome salt, thereby causing the chrome liquor to become more basic and more colloidal due to hydrolysis. If the pelt is pickled, the acid from the pelt diffuse out into the chrome liquor first and thereby the basicity of the external liquor decreases so much that the risk of over tannage of the grain is removed. Since there is no fixation of chromium when there is sufficient acid in the pelt and liquor, the chrome liquor penetrates easily into the pelt and is uniformly distributed everywhere. The addition of some alkali like Na₂CO₃, borax, etc., after uniform distribution of chromium, is necessary to fix up the chromium to collagen.
(ⅱ) Effect of concentration: When chromium liquor is diluted, the chromium compound is hydrolyzed and thereby the basicity and molecular size of the chrome complex increase even though it does not affect the basicity of the liquor as a whole. The pH of the liquor does not appreciably decrease due to dilution even though free acid is released due to hydrolysis of chromium salts. This increase in astringency of chrome compound due to dilution may harden the grain due to over tannage, if diluted liquor is used at the beginning. Thus the quality of finished leather may change due to the use of dilute liquor at the start, instead of fairly concentrated liquor.
(ⅱ) Effect of concentration: When chromium liquor is diluted, the chromium compound is hydrolyzed and thereby the basicity and molecular size of the chrome complex increase even though it does not affect the basicity of the liquor as a whole. The pH of the liquor does not appreciably decrease due to dilution even though free acid is released due to hydrolysis of chromium salts. This increase in astringency of chrome compound due to dilution may harden the grain due to over tannage, if diluted liquor is used at the beginning. Thus the quality of finished leather may change due to the use of dilute liquor at the start, instead of fairly concentrated liquor.
(ⅲ) Effect of salt in one bath chrome tanning : The effects of salt-
❶ on the chrome liquor are -
⒜ To lower the pH value.
⒝ To increase the real concentration of the chrome liquor.
⒞ To affect the nature of chrome complex.
⒟ To change the precipitation figure of the chrome liquor.
❷ on the leather are-
⒜ To change the chrome fixation by the collagen.
⒝ To change the shrinkage temperature and other qualities like feel, fullness etc. of the leather.
(ⅳ) Effect of basicity on pelt:
An increase in basicity brings about an increase in the number of OH groups in the chromium complex, increase olation and causes an increase in the size of the chromium complexes. The rate of penetration of the chromium into the pelt is slowed down because of the increased size of the chromium complexes, the precipitation figure is decreased, the pH value of liquor increases and there is a corresponding decrease in the swelling power of the liquor.
The most suitable basicity for practical purposes depends upon very many factors, such as the kind of pelt, the type of leather to be produced, previous treatment of the pelt, the nature of processes after tanning, the kind and concentration of the chrome liquor, its content of neutral salts, and so on.
Too low a basicity gives an empty, flat hard, tinny leather with a tender grain.Too high a basicity overloads the fibres with tan and particularly the grain, resulting sometimes in a crack grain and an under tannage of the middle layer.
(ⅴ) Pilling up after tanning: Since the fixation of chromium compound to pelt does not become complete in the tanning drum, the chrome blue must be aged in the piled condition on the floor at least for overnight.
During piling up of chrome blues on the floor care must be taken to avoid pleats or folds, otherwise permanent fold - marks will be developed on the tanned leather.
❶ on the chrome liquor are -
⒜ To lower the pH value.
⒝ To increase the real concentration of the chrome liquor.
⒞ To affect the nature of chrome complex.
⒟ To change the precipitation figure of the chrome liquor.
❷ on the leather are-
⒜ To change the chrome fixation by the collagen.
⒝ To change the shrinkage temperature and other qualities like feel, fullness etc. of the leather.
(ⅳ) Effect of basicity on pelt:
An increase in basicity brings about an increase in the number of OH groups in the chromium complex, increase olation and causes an increase in the size of the chromium complexes. The rate of penetration of the chromium into the pelt is slowed down because of the increased size of the chromium complexes, the precipitation figure is decreased, the pH value of liquor increases and there is a corresponding decrease in the swelling power of the liquor.
The most suitable basicity for practical purposes depends upon very many factors, such as the kind of pelt, the type of leather to be produced, previous treatment of the pelt, the nature of processes after tanning, the kind and concentration of the chrome liquor, its content of neutral salts, and so on.
Too low a basicity gives an empty, flat hard, tinny leather with a tender grain.Too high a basicity overloads the fibres with tan and particularly the grain, resulting sometimes in a crack grain and an under tannage of the middle layer.
(ⅴ) Pilling up after tanning: Since the fixation of chromium compound to pelt does not become complete in the tanning drum, the chrome blue must be aged in the piled condition on the floor at least for overnight.
During piling up of chrome blues on the floor care must be taken to avoid pleats or folds, otherwise permanent fold - marks will be developed on the tanned leather.
Why chrome tanned leather is so popular?
- It can be made by simple and short process.
- It is light in weight.
- It has long durability.
- Resistance to heat and water.
- It is easy to dye and finish.
Masking agent and the advantages of masking agent in Chrome tanning
Masking agent: There are some chemicals mostly organic acids and their salts which when added to basic chrome sulphate liquors, will raise precipitation point, buffer the solution and reduce tanning affinity of the chrome. The organic acids or their salt which bring about such change are called masking agent.
Effects of masking agent: Masking agent changes any on all of the following properties -
Effective masking agent: Na adipate, Na ptalate.
Advantages of masking agent: Masking agent has the following advantages -
Effects of masking agent: Masking agent changes any on all of the following properties -
- The composition of Cr compound.
- The pH of Cr liquor.
- The electrical migration of Cr compound.
- The ability of the liquor to resist precipitation.
Effective masking agent: Na adipate, Na ptalate.
Advantages of masking agent: Masking agent has the following advantages -
- The Pickling of the pelt can be very light or sometimes may be omitted.
- Tannage is quicker.
- Tannage is straight forward and no adjustment of basicity is required.
- Considerable saving in the amount of Cr2O3 needed for satisfactory tannage, better exhaustion of tan - bath.
- Production of leather of higher Cr2O3 content.
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