Leather Tanning Agents

A) GUIDELINES FOR THE QUEBRACHO EXTRACTS USE

1. Pit and Drum Method

This is a mixed system with color and penetration pits and retannage in drums. When applying this method it is advisable to use a number of “color” pits with a liquor concentration of 2 to 6 ‹Bé and pH 5.0 / 5.2 and after that a number of tanning pits with liquors having 8 to 12 ‹Bé and pH 4.5 / 4.8. Tannin content, pH and salts concentration in the pits should be controlled in order to ensure uniform batch quality. In this process, delimed and pretanned leather is placed in color pits for 24 hours so as to provide an even, light color with an appropriate grain. In the next step the leather is transferred to the tanning pits until full penetration is achieved. It is very important to keep the tannin non tannin ratio of the liquors at a value of 1.5 to 2, with regular pit draining and addition of extracts. The recommended temperature to achieve faster penetration is between 22 and 26‹ C (5-10 days, depending on hide thickness). The purpose of drum retannage is to add more vegetable extracts and increase weight and resistance. In order to ensure this, it is necessary to monitor the concentration, pH, liquor circulation and temperature carefully. It is advisable to reach temperatures of 36/38‹ C and pH value of 3.8/3.9 at the end of tannage, so as to ensure good fixation and yield. In all cases it is advisable to begin the retanning process with soluble powder extracts (ATO, ATG, Lightan, CHD) and later, in the final stages, use up to 10/15% ATS, ATD, CHA extracts before acidification if necessary. Drum retannage may stop at 13 ‹Bé for light leather or the “load” level may be increased for sole leather having a final liquor value of 14 to 16 ‹Bé.

2. Only Drum Method (with or without fleet)

This method achieves tannage in shorter times than the previous method. It starts with pickled skins at  pH: 3.5 / 3.8 and liquor of above 6 ° Be. Tannage is forced by mechanical action, auxiliaries and the use of more concentrated liquors. It can be done with the same quebracho extracts and dosing order than the pool method drum. During the tanning it s necessary to work at low speed until total penetration is achieved, then acidify to pH: 3.8 / 3.9 and reach a final temperature of the process of 36/38 ° C. For sole leather an offer of approx. 45% of the different extracts is suggested and for pure Vachettas or Strap leather a suggested offer of 30% would be requires (weight percentages on pelt weight).

3. Chrome leather retan

Neutralization
It is advisable to use Buffer type neutralizers, which act as buffer salts, softening neutralization and reducing loose grain problems. In order to achieve better tensile strength, it is important to leave a small blue chrome vein. To a certain extent this has an effect on subsequent penetration of tannins and coloring matters (pH 4.2/4.4), which results in firmer leather. As pH increases, tensile strength is less and there is greater grain flexibility, which results in smoother leather (nappa).

Retan
the use of 2 / 3% soluble or semi soluble quebracho extract in natural leather in colored leather results in leather with clearly defined chrome characteristics but with a smoother grain, slight flank filling and better properties. Using 6 / 8% soluble or semi soluble quebracho extract results in leather that can easily be buffed, with full flanks having good thickness and which can be embossed. If Unitone is used, in addition to these characteristics we will have a base color enabling good savings in terms of dyestuff. The use of 10 / 20% extract will result in leather with distinct vegetable characteristics. It is important to use naphthalene sulphonic syntans during retannage which mitigates the astringent effect of extracts, reduces loose grain effect and eliminates the risk of grain overtanninig. If very light color leather is desired, it is advisable to use Lightan.

Fat liquoring
It should be remembered that leather stuffed before retannage has a smoother grain and more resistant fiber. On the other hand, if stuffing takes place after retannage vegetable features will-increase. As regards leather undergoing retannage with 10/20% extracts, it is advisable to pre-stuff with a high penetration oil, which enables higher fiber lubrication and increases tensile strength. This can be performed in two stages during neutralization and before retannage. This system can also be applied to upholstery and garment, which results in a firmer, more elastic grain.

4. Show upper leather

The use of 4/8% quebracho extract is recommended for pastel leather or 6/8% Unitone for color leather. It is advisable to neutralize at pH 4.2/4.5 and if allowed by penetration requirements, it is also good to leave a small chrome vein in order to increase tear strength. There can also be stuffing performed before retannage in order to achieve a smoother grain.

5. Nubuck

The use of 6/8% quebracho extract results in leather that can easily be buffed, with full flanks and good thickness. Due to the penetration requirements of this kind of leather, it is necessary to neutralize at an approximate pH value of 4.8 in order to improve resistance, pre-stuff in neutralization and before retannage, besides adding to the extracts a naphthalenic sulphonic syntan in order to reduce grain looseness.

6. Leather goods – Full Vegetable type

Retannage may be performed with 15/30% quebracho extracts for color leather. It is important to neutralize at a pH value of 4.8 in addition to pre-stuffing together with neutralization and before retannage so as to improve tensile strength. It is also important to use naphthalenic sulphonic syntans together with the extracts in order to help reduce grain looseness.

7. Upholstery / Garment

There are very good result in this field when retanning with 3/5% extract, which produces very warm touch. For pastel colors, we recommend using 3% ATO, ATG, Lightan, Unique CHD, CHA, Unitan ATD. In order to achieve the feel it is necessary to neutralize at pH 5+, with pre-stuffing in addition to the full stuffing before retannage.

B) TIPS FOR THE PROPER USE OF VEGETABLE EXTRACTS

1. To avoid contamination with iron, the use of sequestrants such as UNICLAR is recommended.

2. For an excellent effect in the filling of leathers, the extract added to the tanning and retanning drums should be in powder non dissolved and preferently in “short baths”.

3. The bath temperature should not exceed 40 grades C in pure vegetal tanning. When retanning wet blue higher temperature (normally used in retanning processes) can be used .

4. In retanning processes the neutralization pH should be above 4 in accordance to the type of article required. At a pH :4/4.2 a more sustained leather is obtained. As and when the pH is increased a more morbid leather will be obtained.

5. Quebracho extracts can be used in combination with any type of vegetable tanning products, synthetic and resin-based products.

6. Due to its characteristics (absence of sugars and presence of complexes derived from sodium bisulphate) Quebracho liquors are very resistant to bacteriological action and to hydrolisis.

7. In chrome re-tannage the use of 2-3% of extracts (ATO, ATG. ATS ,ATD, TL, CHA, CHD, UNITONES) will give leathers a defined characteristics of chrome, but with greater grain smoothness and better breaking point.

- With the use of 6-8% leathers with full flanks, good thickness and excellent buffing will be obtained .Good embossing effect and retention of same is achieved.

- With the use of UNITONE extracts, apart from aforesaid characteristics a base colour will be obtained that will enable good saving of dyes.

- With the use of quantities of extracts between 10 and 18% leathers with net vegetable characteristics will be obtained.

h. To improve strongly vegetable retanned leather resistance it is convenient to pre oil before the neutralizing and before or during the retanning process, completing same afterwards.

i. For a better use of the filling properties of the extract, it is advisable to add in two or three portions along with the auxiliaries.

j. For the evaluation of the different extracts to be used, it is important that same be compared in accordance with the same technical method of analysis. UNITAN uses the FILTER METHOD.