EP0158077B1 - Process for retanning mineral-tanned leather - Google Patents

Description

The main tanning with mineral tanning agents alone is not sufficient to give finished leather the required variety of properties. To emphasize special properties or to achieve them in the first place, a retanning is connected to the main tanning.

In addition to the quality requirements of the processors, the leather producers still have to consider considerable environmental requirements. One of the main requirements relates to chromium in the wastewater, which is introduced into the wastewater through a main mineral tanning process with various chrome tanning agents. Many factories are therefore required to either have the main tanning section carried out by those companies that have the option of recovering chromium from the residual tanning liquor or to process the chrome-tanned wetblue merchandise on the market. However, problems of this type have not yet been completely solved, since astonishingly much, too much chromium is still transferred from the leather into the waste water during further processing of this product.

The chromium concentration in the rinsing or Nachgerbeflotten is largely dependent on the ability of the chromfixierenden Nachgerbemittels and is the present state of the art at about 0.1 to 2 g of Cr ₂ 0 _3/1 residual liquor. Since the permissible amount of chromium is handled criticism freely in the waste water only with 2 mg Cr / I (= 2.9 mg Cr ₂ 0 _3/1), are provided with the commonly occurring amounts of chromium considerable problems due to the concentration exceeded. There has been no shortage of attempts to generally reduce these problems by means of special processes and / or auxiliaries, but it was practically not possible - without impairing the quality of the leather - to significantly increase the chromium concentration of the remaining liquors below 0.1 g Cr ₂ O ₃ / l reduce.

In DE-OS 27 55 087 are described Vorgerbungen from pelt material, wherein by means of polymers of carboxyl-containing monomers at the end of tanning concentrations of 0.13 g of Cr ₂ 0 _3/1, 0.28 g of Cr ₂ 0 _3/1 or 0 , 3 g of Cr ₂ O ₃ / l of residual liquor can be reached. These polymers are also suitable for retanning. DE-OS 31 41 496 A 1 recommends copolymers of polyacrylamide and methacrylamide with cationic groupings in retanning, the focus being on coloring effects which, however, do not cause any abnormal chromium fixations.

The manufacturers of polymer tanning agents often recommend that the leather be rinsed before retanning, which, however, leads to pollution of the wastewater by chromium which has not been sufficiently fixed beforehand. Other types of polymer tanning agents are sensitive to acid and form precipitates, so that the chrome leather must also be neutralized before use, which, depending on the neutralizing agent used, causes particularly high concentrations of chromium in the waste water.

The external aspect plays a major role in achieving leather properties. For example, leather with the lightest possible color is sought in order to be able to dye very pastel or clear shades in the first place. The same requirements apply to leather that is undyed, dried and initially kept in stock before being dyed using a so-called head dye, depending on the fashion requirements. The versatility of such dry leathers in terms of color is very much dependent on a neutral, light base color.

The mode of action of polymer tanning agents can generally be described in terms of leather properties in such a way that the leather treated with them is soft and more or less full and full-grain. In the case of the polymer tanning agents designated as firm-graining, however, it must be ascertained that the term “firm-grained” is relatively stretchy and does not come up to the usual level of with specially designed auxiliaries, such as, for. B. leather treated with polyphosphates.

However, polyphosphates have a coloring disadvantage in the case of very clear colors, since the leather is given a greenish basic color and thus introduces parts of achromatic color in the complementary area. In addition, polyphosphates cannot achieve the very high levels of exhaustion required.

The present invention has therefore set itself the task of creating a retanning process for mineral-tanned, in particular chrome-tanned leather, which u. a. leads both to a very high brightness of the leather and to a very low pollution of the waste water. This object is achieved with the features set out in the claims.

The chromium-doped polymer-polymer complexes used here are new and can be prepared in an inventive manner by the manner also characterized in the claims.

The chromium polymer complexes according to the invention can be prepared in aqueous solution of the organic polymer component with the dissolved chromium salts at temperatures of 60-100 ° Celsius, preferably of 60-80 ° Celsius.

Tanning liquors are used for the retanning according to the invention, which preferably contain between 0.1% and 20% chromium-doped polymer compound in complex-bound form.

To the great surprise, auxiliaries were found in the complexes of polymer compounds with trivalent chromium claimed according to the invention, with which chromium exhaustions that go far beyond the state of the art are achieved with a simultaneous retanning effect. although chrome is added to the retanning system. Since the leathers treated with the polymer complexes according to the invention are unusually pitted and at the same time retain their softness, an improvement over the prior art has also been achieved here. Another advantage that should not be underestimated is the characteristic, very bright basic color, which goes far beyond the usual size. The polymer complexes according to the invention are insensitive to acids and exchange unwanted acid residues in the leather, so that the neutralization step in the conventional sense can also be dispensed with. Alkaline additions are nevertheless possible, they serve to set a desired pH range.

In the case of leathers tanned with aluminum tanning agents, the polymer complexes according to the invention improve the strength of the scars, particularly in the loose skin regions. The yellow tint characteristic of leather tanned with aluminum can be compensated very well.

The polymer compounds can be prepared in a manner known per se (cf., for example, BE Müller, Houben-Weyl, Methods of Organic Chemistry, Volume XIV-I 1961, p.1010-1080 and Volume XIV-II, 1963, p. 631- 633 and 705-713). The K values according to Fikentscher are between 5 and 120, preferably between 10 and 50. In the further description, the polymer compound is in the form of an aqueous solution of 1 to 80% based on active ingredient (see above), preferably between 5 and 60% and particularly preferably between 10 and 40%.

By adjusting the doping ratio, i.e. H. the ratio of polymer component to chromium content, can surprisingly be chosen between the brightness of the leather and the chromium content of the waste water.

However, even with a low chromium content, there is a lower wastewater load than in the prior art.

The preparation of the polymer complexes themselves is extremely simple, as can be seen from the exemplary embodiments. It is sufficient to add carboxyl group-containing polymers, preferably acidic polymers and / or copolymers in aqueous solution with a soluble chromium III compound (e.g. chromium sulfate) and to heat the solution until a complex reaction occurs. Here, as well as in the further descriptions, chromium compounds are generally taken to mean the soluble, complex-capable chromium-III compounds, even if chromium-VI compounds were applied, which were reduced to chromium-III compounds for the purpose of complexation in the mixture with polymers.

For the polymerization or polycondensation preferred substances for the preparation of the polymers and / or copolymers claimed according to the invention are so-called monomer substances, such as. As acrylic acid and / or methacrylic acid and / or maleic acid and / or their hydroxy compounds, which can also be in the form of fully or partially neutralized salts.

The preferred doping ratio of chromium to carboxyl groups of the polymers is between 1: 500 and 1: 1, preferably between 1: 100 and 1: 3 and in particular between 1: 50 and 1: 5, the polymeric K values according to Fikentscher between 5 and 120, preferably between 10 and 50.

The retanning process itself is not critical in itself. You add the necessary amount of water to the piece of leather, which can be done without rinsing beforehand, and add the complex solution and roll in the tanning drum.

Particularly good results with chrome fixation are achieved if the folded wetblue leather without rinsing in a fleet of 100 to 300% water, based on the shaved weight of the wetblue leather, at a temperature of 35 to 60 ° C with 4 to 6 % of a complex solution, which contains 20 to 50% polymers, depending on the composition. The pH values necessary for the subsequent processes can be taken into account here, the alkalis required for a higher desired pH value of the liquor and / or leather from pH 4 to 7, such as. B. sodium bicarbonate and / or sodium formate and / or the like in solid or dissolved form in the process of chromium fixation can also be introduced. This compact way of working does not in any way impair the results of chromium depletion, whereby it can also be used to simplify the technology, because there is no need to separate or neutralize the leather as separate operations.

Licker and alkalis can, however, also be added separately in a manner known per se.

In the examples described below, the brightness of the surface of the undyed leather was measured using an electric reflectance photometer (ELREPHO) when using a Y filter to determine the brightness (= E value). The resulting relative number indicates that the higher the number of the E value, the brighter the surface. White BaS0 ₄ powder e.g. B. has a value of 100. Leather pieces that reach or even exceed 60 can be described as exceptionally bright.

Furthermore, the improvements in leather and wastewater quality should be clarified in the following examples. All examples were based on the same wetblue material, with the best possible reproducibility of the leather up to the wetblue stage using the same raw material. Chemicals, process engineering and all mechanical work was guaranteed. The fold thickness of this starting material was 1.8 / 1.9 mm. Percentages refer to the fold weight of the wetblue pieces.

Example 1 (comparative example)

• Chromgehalt der Restflotte : 100 mg Cr₂0_:/1
• E-Wert des Fertigleders : 53,5.

A piece of wetblue was introduced into a float of 300% water at 45 ° C. together with 4% of a commercially available, 33 percent aqueous sodium polyacrylate solution without rinsing and drummed in a tanning drum for 1 hour. This neutralization liquor was drained off and the chromium content was determined. The leather piece was rinsed, treated with a standardized amount of 7% of a mixture of commercially available Licher for this series and finished. The result was a very soft, not very firm-grained, moderately filled, but little brightened leather.

• Chromium content of the remaining liquor: 100 mg Cr ₂ 0 _: / 1
• E-value of the finished leather: 53.5.

Example 2 (comparative example)

• Chromgehalt der Restflotte : 199,6 mg Cr₂0₃/1
• E-Wert des Fertigleders : 51,3.

Under the same conditions as in Example 1, a wet blue piece was treated with 5% of a 25 percent solution of a commercially available polymer product based on polyacrylamide instead of the sodium polyacrylate solution and finished. The result was a less soft, moderately filled, dark leather with moderate firm grain.

• Chromium content of the residual liquor: 199.6 mg Cr ₂ 0 _3/1
• E value of the finished leather: 51.3.

Example 3

500 g of a polyacrylic acid prepared by a process known per se with a content of 33% and a K value according to Fikentscher of approx. 23, which corresponds to the sodium polyacrylate mentioned in Example 1, were mixed with 16.7 g of a 33% basic chromium sulfate and stirred until completely dissolved. Instead of Cr (OH) (S0 ₄ ), other soluble chromium compounds can also be used, the preference arises, as in the following examples, for economic reasons. The solution is heated to 80 ° C, whereupon the complex reaction occurs. After cooling to room temperature, a water-soluble complex results with good flowability for practical handling. The doping ratio is approx. 40 carboxyl groups per chromoma.

• Chromgehalt der kompakten Restflotte : 10,1 mg Cr₂0₃/1
• E-Wert des Fertigleders : 60,3

A piece of wetblue leather was treated with 300% water at 45 ° C. and 5% above complex solution without rinsing and drummed in a tanning drum for 1 hour. After the addition of 1% solid sodium bicarbonate and a milling time of 15 minutes, 7% leaching mixture (commercial, water-emulsified fatliquor for leather) and another 1% sodium bicarbonate were added and the fabric was drummed for a further 45 minutes. The finished leather was soft, firm-grained than in example 1 and of a very light color.

• Chromium content of the compact residual liquor: 10.1 mg Cr ₂ 0 _3/1
• E value of the finished leather: 60.3

Example 4

500 g of a 40 percent polyacrylic acid with a K value according to Fikentscher 18 produced according to a known method were mixed with 20.3 g of a 33% basic chromium sulfate with stirring and dissolved. The mixture is now heated to 80 ° C. with stirring and then cooled again. The result is a pourable, water-soluble complex with a doping ratio of approximately 40 carboxyl groups per chromate.

• Chromgehalt der Resttlotte : 35,5 mg Cr₂O₃/l
• E-Wert des Fertigleders : 61,4.

A piece of wetblue leather was mixed without prior rinsing with 300% water at 45 ° C. and 5% of the above complex solution and drummed for one hour in a tanning drum. Thereafter, 0.9% sodium bicarbonate was added, the process was continued for 15 minutes and another 0.6% sodium bicarbonate was added together with 7% of a licker mixture and the process was continued for 45 minutes. The result was a soft, light, filled and well-grained leather.

• Chromium content of the residual liquor: 35.5 mg Cr ₂ O ₃ / l
• E value of the finished leather: 61.4.

Example 5

3,000 g of a 40 percent polyacrylic acid with a K value according to Fikentscher of approx. 18 produced according to a process known per se were mixed with 121.8 g of a 33% basic chromium sulfate and stirred until completely dissolved. After heating to 80 ° C and cooling, a viscous but water-soluble paste with a doping ratio of approx. 40 carboxyl groups per chromate results.

• Chromgehalt der Resttlotte : 8,9 mg Cr₂0₃/1
• E-Wert des Fertigleders : 59,6.

A wetblue piece was mixed with 5% of the above paste and 300% of water at 45 ° C. without rinsing and drummed for 1 hour in a tanning drum. Then 0.8% sodium bicarbonate was added, drummed for 15 minutes, 7% of a licker mixture and 1% sodium bicarbonate was added. After 45 minutes of flexing, the leather piece was finished as before. The result was a firmer, but still soft, light leather with very good firm grain.

• Chromium content of the Resttlotte: 8.9 mg Cr ₂ 0 _3/1
• E-value of the finished leather: 59.6.

Example 6

250 g of a 40% polyacrylic acid with a K value according to Fikentscher of approx. 18, which was produced according to a known method, was diluted with 12.5 ml of distilled water and mixed with 10.2 g of 33% basic chromium sulfate. The mixture was heated to 80 ° C. with stirring and cooled again. The result was a well-liquid, water-soluble complex with a doping ratio of approx. 40 carboxyl groups per chromate.

• Chromgehalt der Restflotte : 11,4 mg Cr₂O₃/l
• E-Wert des Fertigleders : 60,4.

A wet blue piece was treated exactly as in Example 5 with a 5 percent complex solution. The result was a slightly softer, light leather with still good firm grain.

• Chromium content of the remaining liquor: 11.4 mg Cr ₂ O ₃ / l
• E value of the finished leather: 60.4.

Example 7

250 g of a 40 percent polyacrylic acid with a K value according to Fikentscher of approx. 18 produced according to a known method were diluted with 250 ml of distilled water, mixed with 40.6 g of 33% basic chromium sulfate, heated to 75 ° C. and cooled again . The result was a very tough, but water-soluble paste with a doping ratio of 10 carboxyl groups per chromate.

• Chromgehalt der Restflotte : 11,4 mg Cr₂0₃/1
• E-Wert des Fertigleders : 61,1.

A wet blue piece was treated exactly as in Examples 5 and 6 with 5% paste above. The result was a soft, full, not very firm grain, very light leather.

• Chromium content of the residual liquor: 11.4 mg Cr ₂ 0 _3/1
• E value of the finished leather: 61.1.

Example 8

Using a process known per se, a monomer content of 90% was obtained. Acrylic acid and 10% methacrylic acid a 32 percent polymethacrylic acid with a Fikentscher K value of approx. 20 and neutralized to pH 7.5 with sodium hydroxide solution. 2,000 g of this neutralized polymethacrylic acid were mixed with a solution of 63.7 g of 33% basic chromium sulfate and heated to 80 ° C. A complex solution with good flowability resulted.

A wetblue piece was mixed without prior rinsing with 300% water at 45 ° C. and 5% solution above and drummed for 1 hour in a tanning drum. The fleet was then drained and checked for chromium. The leather piece was washed in 200% water at 60 ° C for 10 minutes of flexing. In a new fleet of 200% water at 60 ° C., 7% licker mixture was added, drumming was carried out for 20 minutes and the fat substances of the licker were fixed with 0.5% formic acid for 45 minutes.

• Chromgehalt der Restflotte : 60,8 mg Cr₂0₃/1
• E-Wert des Fertigleders : 63,6.

The finished leather was soft, full, astonishingly grained and exceptionally bright.

• Chromium content of the residual liquor: 60.8 mg Cr ₂ 0 _3/1
• E value of the finished leather: 63.6.

(See table on page 6 f.)

Summary of data on chrome depletion in the liquors and brightness of the leather surfaces:

Claims (14)

1. A process for the retanning of mineral tanned leather, particularly chrome tanned leather in an aqueous bath preferably at elevated temperatures comprising the step of : treating the leather in an aqueous solution of an organic polymer complex comprising carboxyl groups and chromium, wherein the doping with chromium is effected in a 1-80 weight % aqueous solution of said polymer at temperatures between 60 and 100°C.

2. A process according to claim 1, wherein said organic component is selected from the group consisting of acid polymers or copolymers of acrylic acid and/or methacrylic acid and/or maleic acid and/or a hydroxy compound of maleic acid, wherein said polymers or copolymers may be in the form of totally or partly neutralised salts.

3. A process according to claim 2, wherein said organic component has a K-value according to Fikentscher in the range from 5 to about 120, preferably from 10 to about 50.

4. A process according to claim 1 or 2, wherein the polymer compound is applied to the doping with chromium, as an aqueous solution with a content between 5 and 60 %, preferably between 10 and 40 % of the active compound.

5. A process according to claim 1 or 2, further comprising the step of doping said organic component with a water soluble chromium salt or an alkalized chromium salt, preferably an alkalized chromium sulfate.

6. A process according to claim 1 to 5, wherein said chromium is present in a molar ratio to said carboxyl groups of from 1 : 500 to 1 1, preferably 1 100 to 1 : 3, especially from 1 : 50 to 1 : 3.

7. A process according to claim 1 to 6, wherein the reaction between said aqueous organic polymer complex with the optionally dissolved chromium salt is conducted at a temperature of from 60 °C to 80 °C.

8. A process according to claim 1 to 7, wherein said tanning bath comprises from 0,1 % to 20 % by weight polymer complex, doped with chromium.

9. A retanning agent for retanning mineral tanned leather on the basis of polymeric compounds containing carboxy groups, wherein the carboxyl groups containing polymeric compounds have K-values according to Fikentscher in the range from 5 to 120, preferably 10 to 50, and comprise chromium bound in complex form, wherein the doping with chromium is effected in an aqueous solution of the polymer with a concentration of 1 to 80 % at temperatures from 60-100 °C.

10: A retanning agent according to claim 9, wherein said carboxyl group containing polymer is selected from the group consisting of acid polymers and/or copolymers of a acrylic acid and/or methacrylic acid, maleic acid and/or a hydroxy compound of maleic acid, wherein said polymers are produced according to known methods and may be in the form of totally or partly neutralised salts.

11. A retanning agent according to claim 9, wherein the polymeric compound contains chromium in a ratio of 1 : 500, preferably 1 : 100 to 1 : 3, most preferably 1 : 50 to 1 : 3.

' 12. A process to produce carboxylgroups containing polymeric compounds containing chromium in complex form, wherein a water soluble polymeric compound with a K-value according to Fikentscher of 5 to 120, preferably 10-50 is introduced as an aqueous solution with a content of 1-80 % into an aqueous solution of a chromium salt and heated at a temperature from 60-100 °C, preferably about 80 °C.

13. A process according to claim 12, wherein the ratio of chromium to polymeric compound is 1 : 50 to 1 : 3, preferably 1 : 50 to 1 : 3.

14. A water soluble homo- or copolymerisate selected from the group consisting of acid polymers or copolymers of acrylic acid and/or methacrylic acid and/or maleic acid and/or a hydroxy compound of maleic acid, wherein said polymers or copolymers may be in the form of totally or partly neutralised salts, producable according to the process of claim 12 or 13.