EP1297189B1 - Use of hydrolysates of vegetable tanning agents for stabilising chrome-iii against oxidation in chrome-tanned leather - Google Patents

Abstract

Hydrolysates of vegetable tanning agents are suitable for stabilising chrome-III against oxidation in chrome-tanned leather.

Description

The present invention relates to the use of vegetable hydrolysates Tannins for the stabilization of chromium III against unwanted oxidation in chrome-tanned leather.

State of the art

Known tanning is understood to be a process comprising several stages. which serves the unstable, perishable, rot and decomposition To permanently preserve the collagen of the animal skin freed from skin and hair.

The tanning agents used for tanning cause crosslinking of the collagen molecules the skin and thus the stabilization of the chain - like structures of the Skin protein.

Each tanning is divided into several phases, some of which are clearly separated from each other, but partly also merge into each other. In the preparation phase , the skin substance is brought to the required state for the process by soaking, pickling, liming, deliming, washing, pickling, tanning or conditioning. In the transport phase, the tanning agent migrates by diffusion to the binding sites. In this case, the transport medium is always water, with additions of diffusion-accelerating chemicals are possible. The bonding phase usually begins before the transport phase is completed. Transport phase and binding phase can be understood in some ways as "actual" tanning. Usually, after the introduction of the tanning substances, there also takes place a post- processing phase, which is characterized by processes such as mechanical processing, neutralization, retanning, possibly dyeing, fatliquoring, fixing and washing.

The tanning is carried out predominantly as so-called chrome tanning, in which Salts of trivalent chromium (chromium-III), for example, chromium sulfate used come. Chromium tanning is still the most important with a share of more than 80% Gerbart and because of their technical advantages and versatility Chrome-tanned leather is unanimous in the professional world the opinion that In the foreseeable future, the dominant role of chrome tanning will not change becomes.

• C.Hauber, H.P.Germann, "Untersuchungen zur Entstehung und Vermeidung von Chromat in Leder", Leder und Häutemarkt 1999, Seite 25-30
• Schill & Seilacher, "Trivalent chromium to hexavalent chromium?", Leather 1999 (Heft 12), Seite 33-34
• D.Graf, Dr. Boehme, "The influence of relative humidity of air during storage on the formation lowering of CR(VI) in chrome tanned leathers", World Leather 2000 (Mai), Seite 38-40)

There has recently been a discussion in the art about possible formation of hexavalent chromium (chromium VI) in leather. This discussion was prompted by publications in which the formation of chromium VI upon exposure to high-energy radiation or heat was pointed out:

• C.Hauber, HPGermann, "Investigations on the origin and prevention of chromate in leather", leather and skinning market 1999, page 25-30
• Schill & Seilacher, "Trivalent chromium to hexavalent chromium?", Leather 1999 (Issue 12), pages 33-34
• D.Graf, dr. Boehme, "Chrome leather tanned leathers" (VI), World Leather 2000 (May), pages 38-40)

Since chromium VI is toxic and therefore an undesirable pollutant, the experts dedicate Methods for stabilizing chromium III against unwanted oxidation are great Attention. There is therefore a great need to provide methods by which such stabilization is ensured or substances or compositions provided in the course of leather production in the leather and act as stabilizing additives for chromium III.

Description of the invention

The object of the present invention has been to provide a method for stabilization of chromium III in chrome-tanned leather. Another task has consisted of providing substances or compositions that are used in the In the course of leather production are introduced into the leather and as stabilizing Additives for chromium-III act.

It has been found that hydrolysates of vegetable tannins, said task excellent solution. They cause effective stabilization of chromium-III, without having a negative effect on the application properties of the leather.

It should be emphasized in this context that this latter point too for the expert is surprising, since the expert assumes that each introduction of substances in the leather structure of its properties - some very much sustainable - changes. According to the Applicant's investigations, vegetable tannins have as such, some effect with respect to stabilization of chromium III, however, the leathers treated with vegetable tannins have become significant inferior performance characteristics: the so treated leather show namely - in comparison to those with the hydrolysates of vegetable according to the invention Tannins - in addition to the lower effectiveness of chromium-III stabilization a lower fullness and a lower dyeability.

The present invention initially relates to the use of hydrolysates of vegetable tannins to stabilize chromium III against unwanted Oxidation in chrome-tanned leather.

The expression "vegetable tannins" is known to the person skilled in the art and is to be understood as being synonymous with the term "vegetable tanning agents" (Hans Herfeld, "Library of the Leather; Volume 3: Tannin, Tanning and Retanning", Frankfurt 1985, page 31). Examples of vegetable tannins are Acacia negra, Algarobilla, Anjico, Avaram, Basbul, Badan, Barbatimao, Birch, Canaigre, Dahua, Divi-Divi Cascalote, Oak, Alder, Spruce, Chinese Galls, Japanese Galls, North African Galls, Turkish Galls, Gambir, Goran, Hemlock, Chestnut, Knoppern, Maletto, Mangrove, Mauto, Mimosa, Murici, Myrobalan, Myrtan, Quebracho, Sonali, Sumach, Tara, Tizerah, Trillo, Ulmo, Urunday, Valonea, Grasp Underground (compare: Hans Herfeld, "Library of the Leather, Volume 3: Tanner, Tanning and Retanning", Frankfurt 1985, pages 37-38).
Tannins were formerly classified according to their behavior when heated dry at 180 to 200 ° C according to the resulting main products in pyrogallol and catechol. However, this classic classification is now considered outdated and modern understanding distinguishes the hydrolyzable tannins, which are decomposed by hydrolyzing enzymes or acids into lower molecular weight substances, of condensed tannins, the action of mineral acids or oxidizing substances, the high molecular weight , form poorly soluble phlobaplene (see: Hans Herfeld, "Library of the Leather, Volume 3: Tanner, Tanning and Retanning", Frankfurt 1985, page 44).
Hydrolyzable vegetable tanning agents, such as are used in the context of the present invention as starting materials for the preparation of chromium III oxidation-stabilizing hydrolysates, are about chestnut, oak, Valonea, Myrobalans, Sumac, Algarobilla, Divi-Divi and the various bile tannins.
On the other hand, condensed tannins which are not used as starting materials for the preparation of the chromium III oxidation stabilizing hydrolyzates because of their non-hydrolysability are, for example, quebracho, mimosa, mangrove, myrtan, acacia negra, spruce, Hemlock, birch.

In order to obtain the compounds to be used according to the invention, hydrolysable vegetable tannins are subjected to hydrolysis. The type of hydrolysis is in principle subject to no restriction, it can be done, for example, acidic, alkaline or enzymatic. In the context of the present invention, however, acid hydrolysis is preferred.
In particular, the hydrolysis is carried out so that the degree of hydrolysis is in the range of 10 to 100%. The term "degree of hydrolysis" is understood to mean to what extent the vegetable tanning agent used for the hydrolysis has been "consumed" in the course of the hydrolysis. By measurement, this can be determined by determining in what amount the starting material is still present as such after hydrolysis.
If, for example, the vegetable tannin used after the hydrolysis is still present at 10% by weight, based on the total dry mass of hydrolyzate and the unhydrolysed starting material still present, then 90% by weight of the starting material have been hydrolyzed and the degree of hydrolysis is 90%. ,
Or: If the vegetable tannin used is still present after hydrolysis at 30% by weight, based on the total dry mass of hydrolyzate and unhydrolysed starting material still present, then 70% by weight of the starting material was hydrolyzed and the degree of hydrolysis is 70 %.
Or: If the vegetable tannin used is no longer detectable after the hydrolysis, then 100% by weight of the starting material were correspondingly hydrolyzed and the degree of hydrolysis is 100%.

In one embodiment, the hydrolysis is carried out so that the to be hydrolyzed vegetable tannins dissolved or suspended in a liquid system or be emulsified. In this case, aqueous systems - especially water itself - as liquid systems are preferred

• Die Temperatur im Bereich von 20 bis 200 °C, vorzugsweise im Bereich von 60 bis 100 °C
• Die Reaktionszeit im Bereich von 30 bis 720 Minuten
• Der Druck im Bereich von 0,5 bis 10 bar, vorzugsweise bei etwa 1 bar (Atmosphärendruck)

As already stated, acid hydrolysis is preferred in the context of the present invention. In the case of acid hydrolysis, the parameters are set in particular as follows:

• The temperature in the range of 20 to 200 ° C, preferably in the range of 60 to 100 ° C.
• The reaction time in the range of 30 to 720 minutes
• The pressure in the range of 0.5 to 10 bar, preferably at about 1 bar (atmospheric pressure)

Preferably, the procedure for the preparation of a hydrolyzate suitable according to the invention as follows: Add to a mixture of 100 g hydrolyzable vegetable Tannin and 100-150 g of water about 5 g of concentrated sulfuric acid and stir the entire mixture at a temperature of about 80-90 ° C for about 30 150 minutes. Subsequently, it is adjusted with sodium hydroxide solution or aqueous ammonia a pH in the range of 3 - 5 a. The whole process is at atmospheric pressure, i.e. carried out about 1 bar.

The hydrolysates of vegetable tannins become according to the invention in particular in the course of the retanning into chrome-tanned leather introduced.

The amount of hydrolysates of vegetable tannins depends essentially on the amount of chromium present in the leather. The hydrolyzates of vegetable tannins are preferably used in an amount which is 40 to 50% by weight of the total amount of the hydrolyzate, based on the trivalent chromium introduced into the leather during tanning, this chromium being calculated as Cr ₂ O ₃ .

In the context of the present invention, in principle all hydrolysable vegetable tanning agents can be used for the preparation of the hydrolysates to be used.
In a preferred embodiment, such hydrolysates of vegetable tannins for the stabilization of chromium-III, which are obtainable by hydrolysis of vegetable tannins selected from the group chestnut, oak, Valonea, Myrobalans, Sumach, Algarobilla, Divi-Divi, Tara and the various bile tannins. Chestnut, Tara and Valonea are very particularly preferred as starting materials for the preparation of the hydrolysates. Tara hydrolyzate, in whose production per 100 g of tare 150 g of water are used, and wherein for the hydrolysis 5 g of sulfuric acid for 150 minutes at 80 - 90 ° C act and then with aqueous sodium hydroxide solution, the pH to 3-5 sets , is very particularly preferred.

In a further embodiment, the hydrolysates of vegetable tannins are used in combination with reducing agents. Reducing agents are substances which are capable of reducing hexavalent chromium to trivalent chromium. Examples of suitable reducing agents are Na ₂ S ₂ O ₅ , Na ₂ S ₂ O ₃ , phyllosilicates, Na ₂ SO ₃ and iron (II) salts.

In a further embodiment, the hydrolysates of vegetable tannins are used in combination with radical scavengers. Among radical scavengers, substances are too who are able to bind free radicals. Examples of suitable radical scavengers are vitamin C, vitamin E, sterically hindered phenols as well as the Expert known HALS compounds (hindered amine light stabilizer).

Another object of the invention is a method for the treatment of leather, wherein man wet blue at any point in the course of further processing to the finished Leather - preferably in the course of retanning - with one or more hydrolysates treated by vegetable tannins. The expression familiar to the person skilled in the art "wet blue" is used in the context of the invention for chrome-tanned hides, independently of whether these skins are ready for use after their production Further processed leather or whether commercially available chrome-tanned hides, so wet blue in the strict sense, at a later date ready for use Leather further processed.

With regard to further parameters with regard to the type of hydrolysis, the production the hydrolysates, the amount of hydrolysates used, the possible co-use of reducing agents or radical scavengers, etc., the above applies.

Examples example 1

100 g of chestnut (hydrolyzable vegetable tannin) and 100 g of water were with 5 g of concentrated sulfuric acid stirred at 85 ° C for 60 minutes. It was allowed to cool and added 8.9 g of a 33% sodium hydroxide solution to thereby have a pH of 4.2.

Example 2 (B2)

1000 g wet blue (origin: Turkish beef, tanned with 8% chrome tanning agent) was retanned with 20 g of the hydrolyzate according to Example 1 and then with a Greased fish oil sulphate.

The leather thus produced was tested for softness and fullness. The result can be seen in Table 1.
Furthermore, the leather was aged for 24 hours at 80 ° C and then examined for its content of chromium-VI. The result can also be seen in Table 1.

Comparative Example 1 (V1)

Example 2 was repeated except that the addition of the hydrolyzate of Example 1 was omitted (blind sample). The result can be seen in Table 1.

Comparative Examples 2 to 6 (V2 to V6)

V2) Quebracho

V3) Tara

V4) Kastanie

V5) Valonea

V6) Mimosa

Example 2 was repeated, using instead of the hydrolyzate according to Example 1 the following non- hydrolyzed vegetable tanning agents:

V2) Quebracho

V3) Tara

V4) Chestnut

V5) Valonea

V6) Mimosa

The results can be seen in Table 1. attempt softness wealth ppm Cr-VI Example B2 9 6.5 3.0 Comparative Example C1 (Blank) 10 5.5 16.0 Comparative Example V2 8th 7 9.2 Comparative Example C3 7 9 4.0 Comparative Example V4 8th 7.5 5.5 Comparative Example V5 7 9 4.5 Comparative Example V6 8th 7 8.0

From Table 1 shows that the inventive example 2 in terms of Content of chromium VI to the comparative examples - sometimes very clearly - is superior. Another positive effect is that in terms of fullness and softness of the leather Example 2 according to the invention only insignificantly from comparison 1, the blank, differs. The use of non-hydrolysed vegetable tannins (cf. 2 to 6) on the other hand shows qualitatively worse results in fullness and softness of the leather.

Individual stabilization test

Investigations by the applicant have shown that the effect of additives for the stabilization of chromium-III in leather can be simulated by an in vitro study, in which each 0.5 g of the test substance to be tested for 30 minutes at 20 ° C. an aqueous solution containing 25 ppm chromium VI ions (prepared by dissolving potassium dichromate in water), and then determines the content of the system of chromium-VI. The results of this individual stabilization test can be seen in Table 2. Added test substance ppm chromium VI none (blank value) 25 acrylic retanning agent 25 Chestnut, not hydrolyzed 5.5 Chestnut, acid hydrolysed 3 Valonea, acid hydrolysed 1.5 Tara, acid hydrolysed 1

Claims (8)

1. The use of hydrolyzates of vegetable tannins for stabilizing chromium(III) against oxidation in chrome-tanned leather.
2. The use claimed in claim 1, characterized in that the hydrolyzates of vegetable tannins are introduced into chrome-tanned leather in the course of retanning.
3. The use claimed in claim 1 or 2, characterized in that hydrolyzates of vegetable tannins obtainable by acidic hydrolysis of vegetable tannins are used.
4. The use claimed in any of claims 1 to 3, characterized in that hydrolyzates of vegetable tannins obtainable by hydrolysis of vegetable tannins selected from chestnut, tara and valonea are used.
5. The use claimed in any of claims 1 to 4, characterized in that the hydrolyzates of vegetable tannins are used in combination with reducing agents.
6. The use claimed in any of claims 1 to 4, characterized in that the hydrolyzates of vegetable tannins are used in combination with radical traps.
7. A process for the treatment of leather, characterized in that wet blue is treated with one or more hydrolyzates of vegetable tannins at any stage of its further processing to the final leather.
8. A process for the treatment of leather, characterized in that wet blue is treated with one or more hydrolyzates of vegetable tannins in the course of retanning.