Plant extracts for tanning

Abstract

The invention relates to the use of a tanning agent-containing product for tanning and / or retanning a raw hide or fur or an intermediate product thereof produced during the production of leather, the tanning-containing material of plants is extracted by (i) the tanning-containing product in the form of an extract can be prepared or produced by means of an aqueous solution or dispersion comprising one or more polar organic solvents, or (ii) the tanning-containing product in the form of an extracted residue can be prepared or produced by means of a supercritical solvent, or (iii) the tanning agent-containing product in the form of an extracted residue can be prepared or produced by means of pure water, or (iv) the tanning agent-containing product in the form of an extracted residue can be prepared or produced by means of a mixture of a supercritical solvent and a polar solvent; the invention further relates to the subject of the invention.

Description

Technical Field

[0001] The present invention relates to the use of a product containing a tanning agent, which can be prepared or has been prepared by: (i) means of an aqueous solution or dispersion containing one or more polar organic solvents, or (ii) means of a supercritical solvent, or (iii) pure water, or (iv) a mixture of a supercritical solvent and a polar solvent, for extracting a plant tannin-containing product, as described hereinafter and in the claims, for tanning and / or retanning raw hides (Haut) or furs (Felle) or intermediate products produced during leather preparation, and / or also the subject matter of the present invention related thereto, as described hereinafter and especially in the examples. Background Art

[0002] During tanning, the collagen fibers from animal hides are crosslinked to resist bacterial or fungal degradation. Commonly used tanning agents are chromium sulfate, glutaraldehyde, vegetable tanning agents or synthetic tanning agents. Synthetic tanning agents, commonly referred to as Synthane / Syntane, are usually aromatic substances soluble by sulfonation or sulfomethylation, especially aldehyde condensation products of phenol, cresol, naphthalene and naphthol. Particularly preferred are condensates of naphthalenesulfonic acid, phenolsulfonic acid, diphenyl sulfone or sulfonated diaryl ethers with formaldehyde and / or urea. Such condensates are known, for example, from Herfeld (editor), "Bibliothek des Leders", Volume 3, pages 53 to 59, Umschau-Verlag 1984 or Reich, G. Theorie und Praxis der organischen Gerbstoffe, Teil 2: Synthetische organische Gerbstoffe, Das Leder, 1996, 157-171, and can be prepared according to the methods described in the references given therein.

[0003] With the widespread acceptance of chromium sulfate tanning or glutaraldehyde tanning, the use of vegetable tanning agents and synthetic tanning agents has now changed, and they are mainly used in retanning processes. In this process step, the collagen fibers have already been crosslinked, so the tanning effect is not as much concerned as the fine control of the desired leather properties. Here, since synthetic tanning agents are based on a more uniform chemical basis, the properties of the leather can also be controlled and adjusted more specifically. However, the effect of non-tanning agents in vegetable tanning agents filling the looser parts of raw hides results in a more uniform thickness across the entire leather surface, but this cannot be achieved with the same quality. For this purpose, a larger amount of synthetic tanning agents or other products usually based on different chemical bases, such as resin tanning agents or other polymers (such as acrylates), must be used in retanning. Another disadvantage lies in the fact that all these synthetic products are petroleum-based, thus running counter to the desire for a more sustainable leather value chain.

[0004] There have been attempts to overcome or limit the disadvantages of the technical and ecological properties of current synthetic tanning agents and to replace some of the components with renewable materials.

[0005] It is known that animal hides and furs are tanned with vegetable tanning (i.e., tanning with vegetable tanning agents or tannins, also known as vegetable tanning (VG)). See, for example, G. Moog, der Gerber, Professionelle Lederherstellung, Eugen Ulmer KG, 2016. Vegetable tanning agents are compounds with a wide variety and complex activities, also known as tannins or polyphenols. They are present in almost all plants. They are used there to prevent decay, damage by wild animals, and infestation by fungi or molds, and can be present in the outer bark (e.g., the outer bark of oak, spruce, mimosa, or willow), wood (e.g., oak, quebracho, or chestnut), roots (e.g., Bergenia (Badan), Rumex hymenosepalus (Canaigre)), leaves (e.g., sumac or Uncaria rhynchophylla), fruits (e.g., Valonea, Trillo, Terminalia chebula (Myobalanen), or tara), and weeds (e.g., galls or oak apples). These substances are usually leached with water into a tanning broth and they act as astringents.

[0006] For example, a distinction is made between so-called (hydrolyzable) pyrogallol tanning agents and condensed catechol tanning agents. The former contain saccharide substances, which can lead to the formation of acids in the broth due to fermentation, and the latter contain insoluble large tanning agent particles (so-called phlobaphenes), which precipitate in a muddy form and then can no longer act as tanning agents. The tanner must know very precisely the nature of the tanning agent he wants to use - how each tanning agent behaves in terms of solubility, particle size, diffusion, binding, color, fullness, strength, odor, etc.

[0007] In the past few decades, attempts have been made to replace vegetable tanning agents with the synthetic tanning agents (synthetic tanning agents) already mentioned, which have a largely reproducible uniform particle size and consistent defined properties.

[0008] However, on the other hand, there is still a need for new vegetable tanning agents, and this has recently increased again, because they are natural substances and generally have good ecological and / or biocompatibility. When disposing of leather and its production, it is desirable to avoid synthetic materials and preferably use substances that can be naturally degraded. There have been attempts to overcome the disadvantages of the technical and ecological properties of current synthetic tanning agents or other tanning agents and to replace at least some of the components with renewable materials.

[0009] It has now been found that when the residual residues obtained with the aid of aqueous organic solvents, supercritical solvents such as supercritical carbon dioxide or mixtures of supercritical solvents and polar solvents, or in particular extracts of tannin-containing plant materials such as green tea are added as tannins to the bath, surprisingly good tanning and / or retanning effects are achieved, which makes it possible to produce leather / tanned furs having very good properties as described in more detail below, and in part, compared to known vegetable tannins, show favorable results and / or require or have lower concentrations of residues or extracts and / or other positive properties in tanning and retanning.

[0010] In the extract of green tea, the properties may be due not only to the content of polyphenols and / or proanthocyanidins.

[0011] Surprisingly, it has been found that especially in the case of green tea (with the aid of aqueous polar solvents, especially ethanol / water extract As a tannin-containing product, compared to using water extracts, compared to known vegetable tannins, it has better tanning effects and improved properties in the case of retanning, and at least comparable and in part better results in the case of leather production.

[0012] In contrast, when extracting with pure water (without solvent addition) or with supercritical solvents, in the residue residue (quasi "press cake") relatively tannin-rich products may be produced, which show stronger tanning effects than the plant starting materials.

[0013] Therefore, by using the residues or especially extracts to be used according to the present invention, compared to using known vegetable tannins such as leather or furs prepared from tara or mimosa, it is possible to obtain good or relatively improved shrinkage temperature ("TS") of the resulting leather or tanned fur, better or good penetration rate (and thus more uniform tanning), better or good levelness (uniformity of dyeing) and / or better or good colorability, better or good fullness, better or good softness, better or good compactness (tightness), better or good grain brittleness, better or good color strength or two or more of these properties. Generally, in the case of a comparable amount or concentration of tannins, a comparable or better grading is generally found.

[0014] The hydrothermal stability of leather, i.e., the so-called shrinkage temperature (TS), is considered an important measure of tanning. This can be measured according to the method of DIN 53336: 1977-01. The shrinkage temperature of untanned raw hides is advantageously below 60 °C. A further increase (e.g., to 65 °C or higher) indicates tanning. For example, in the case of tanning with vegetable tannins, the shrinkage temperature is greater than 60 °C according to the type, e.g., 65-85 °C.

[0015] The other properties mentioned are determined by expert appraisal and qualitatively graded according to the following values, where "+" indicates a positive impression of the property tested in each case and "-" indicates a negative impression:

[0016] +3 / -3 Extreme expression,

[0017] +2 / -2 Marked expression,

[0018] -1 / +1 Expression,

[0019] -0.5 / +0.5 Slight expression,

[0020] 0 Reference value.

[0021] Additional properties that are positively affected are the improved color intensity associated with good color uniformity and penetration; a pleasant round and smooth touch, and excellent grain brittleness of the leather before and after grinding.

[0022] A special feature is the excellent internal softness of the tanned or retanned material with good fullness and grain elasticity.

[0023] In particular, by extraction with a mixture of water and one or more polar solvents (always referring to organic polar solvents in this case), in the case of the same extract amount and especially polyphenol concentration, compared with known vegetable tanning agents (such as from mimosa or especially tara), even when using a lower absolute amount of extract and especially polyphenol in the tanning process or retanning process, it is possible to find substantially the same to better properties of the resulting tanned leather or fur, and even better properties in the case of a comparable absolute amount, especially when using an extract from green tea obtained by mixing water with one or more polar solvents.

[0024] Vice versa, the same is true for the residue in which polyphenols are enriched relative to the total mass.

[0025] Generally, the products containing tanning agents to be used according to the present invention advantageously have a more enriched share of tanning agents such as polyphenols relative to the material of the original plant tanning agent.

[0026] Polyphenols can be determined, for example, by the Folin / Ciocalteu assay, sugar content by the phenol-sulfuric acid assay, proanthocyanidin content by the acid-butanol assay, and measured by UV / visible spectroscopy according to the SOP of the Biel University of Applied Sciences (Biel, Switzerland).

[0027] In a first embodiment, the present invention relates to the use of a tannin-containing product for tanning and / or retanning raw hides or pelts or intermediate products produced during leather preparation, by extracting the tannin-containing material of a plant by:

[0028] (i) The tannin-containing product in the form of an extract can be prepared or obtained by means of an aqueous solution or dispersion containing one or more polar organic solvents, or

[0029] (ii) The tannin-containing product in the form of an extraction residue can be prepared or obtained by means of a supercritical solvent, or

[0030] (iii) The tannin-containing product in the form of an extraction residue can be prepared or obtained by means of pure water, or

[0031] (iv) The tannin-containing product in the form of an extraction residue can be prepared or obtained by means of a mixture of a supercritical solvent and a polar solvent.

[0032] Advantageously, or in a second embodiment of the present invention, the tannin-containing product to be used is an extract obtainable or obtained according to variant (i), in the form of a liquid or a dried form of an aqueous dispersion or in particular a solution containing one or more polar organic solvents.

[0033] Alternatively, or in a third embodiment of the present invention, the tannin-containing product to be used is a solid or a dispersed residue obtainable or obtained according to one of variants (ii) or (iii) by extraction with pure water or in particular with a supercritical solvent, in particular "press cake".

[0034] In an advantageous embodiment of the present invention, in particular the embodiments already mentioned, phenolic compounds, in particular polyphenols and / or proanthocyanidins, are present as tannins in the tannin-containing product to be used.

[0035] Preferably, in another particular embodiment of the present invention, the tannin-containing product to be used is an extract of green tea in liquid or dried form, which contains or is in particular a product extracted by means of an aqueous dispersion or in particular a solution containing one or more polar organic solvents. Preferably, the extract of green tea is used for retanning intermediate products produced during the preparation of leather from raw hides or pelts, characterized in that the (one or more) intermediate products are materials obtained after pickling, in particular bare hides, and each of the intermediate products is tanned or tanned and then retanned using the extract of green tea. One or more intermediate products are, for example, materials obtained after a first tanning step, in particular wet white leather, wet brown leather or wet blue leather, where each of the materials is retanned using the extract of green tea.

[0036] In another embodiment of the present invention, an extract of green tea can in particular be added to the respective baths, in solid form such as granules or especially powder, to the baths for tanning or retanning.

[0037] Another embodiment of the present invention provides that the extract of green tea is added to the respective baths in liquid form, especially in the form of a water-ethanol solution.

[0038] Another particular embodiment of the present invention each relates to one of the embodiments mentioned in the first three paragraphs, in which the extract of green tea is obtained ( = can be obtained or is obtained) in water with a polar solvent or a mixture of polar solvents with a weight ratio of polar solvent (mixture) to water of 5:95 to 90:10, preferably 10:90 to 80:20, especially 30:70 to 70:30, based on the total weight of dry green tea, water and polar solvent (mixture), preferably by extraction in the range up to the boiling point of the respective mixture, for example at 80 to 105 °C, where the extract is used in liquid form or in solid form after drying, in which case especially as granules or preferably as powder.

[0039] In all embodiments of the present invention, in each case, the polar solvent is preferably understood to be a polar solvent miscible with water and is selected from alcohols, especially methanol, ethanol, propanol or isopropanol, diols, especially ethylene glycol, diethylene glycol or triethylene glycol, ketones, especially acetone or methyl ethyl ketone, di-lower-alkyl sulfoxides, especially dimethyl sulfoxide or diethyl sulfoxide, nitriles, such as acetonitrile, water-soluble ethers, such as dioxane or tetrahydrofuran, amides, such as formamide, N-methylformamide, dimethylformamide or diethylformamide, and heterocyclic solvents, such as piperidine or N-methyl-2-pyrrolidone, or a mixture of two or more thereof.

[0040] Regarding the use of supercritical solvents, in another embodiment, the present invention relates to supercritical solvents selected from nitrous oxide, sulfur dioxide, fluorinated hydrocarbons or especially carbon dioxide; mixtures of polar organic solvents and supercritical solvents are also possible.

[0041] In a specific embodiment, the present invention relates to a use, in which the extract of green tea is such a green tea extract: in which 100 to 300 parts of green tea, 300 to 900 parts of water and 200 to 600 parts (each based on weight) of a polar solvent, especially ethanol, are used for preparing the extract.

[0042] In the use according to the present invention, it is advantageous that, during extraction, the weight ratio of green tea to water to polar solvent, especially ethanol, is (10 - 30):(40 - 60):(25 - 40), preferably such that the sum of the ratio numbers is 100.

[0043] Particular embodiments of the present invention also relate to tanning agents and / or substances for retanning, which comprise tannin-containing products that can be prepared or have been prepared as defined in the foregoing embodiments, in particular for tanning and / or retanning raw hides or furs or intermediate products produced in the preparation of their leather. In a particular variant of this embodiment, the tanning agent and / or the substance for retanning (i.e., the corresponding material) comprises an extract of green tea as defined in one of the foregoing embodiments. Combinations with synthetic tanning agents and / or lignin are also contemplated embodiments of the present invention.

[0044] Another embodiment of the present invention relates to a method for vegetable tanning and / or retanning, characterized in that a tannin-containing product prepared according to one of the foregoing embodiments or a tanning agent and / or a substance for retanning according to the previous paragraph is added, in an aqueous solution or dispersion, with or without additional additives, to a fur, a raw hide or, in the case of retanning, to a material that has already been tanned with said material, and the batch is incubated for a relatively long time, for example 0.1 to 5 days, for example 0.5 to 3 days, each preferably with agitation.

[0045] The present invention also relates to the embodiments defined in the claims, which are incorporated herein by reference.

[0046] The most important preferred embodiments of the various methods and processes are described below.

[0047] Extraction with water and one or more polar (organic) solvents is preferably carried out by extracting tannin-containing plant materials, such as outer bark (e.g., oak, spruce, mimosa or willow bark), wood (e.g., oak, quebracho or chestnut wood), roots (e.g., Bergenia (Badan), Rumex hymenosepalus (Canaigre)), stems, leaves or flower petals (e.g., sumac or Uncaria rhynchophylla, or especially green tea from Camellia sinensis or also assamica), fruits (e.g., acorns, Trillo, Terminalia chebula or tara) or weeds (e.g., galls or Chinese gallnuts) or seeds, in an unchanged or dried form, each either uncrushed or crushed (e.g., chopped or ground).

[0048] The extraction is preferably carried out with an extractant (a mixture of water and one or more polar solvents) at a warm temperature, e.g., between 50 °C and the boiling point of each batch (= extractant plus plant material to be extracted), for a sufficient time, e.g., 0.5 to 10 hours, such as in particular a period of 2 to 5 hours, for example (based on the mass of the plant material to be extracted or the weight under normal conditions of the earth's gravity), with a mass ratio of plant material:extractant of 10:1 to 1:100, preferably 5:1 to 1:25, especially 3:1 to 1:10.

[0049] The extract thus obtained or obtainable can optionally be obtained and stored (e.g., under cooling) in the form of a liquid filtered from solid components, diluted further, or as such, or in a concentrated form by partially removing the extraction agent (e.g., by distillation or evaporation, with or without reduced pressure), or in a dried form after complete drying (e.g., by evaporating the extraction agent, spray drying, fluidized bed method, freeze drying, etc.), such as a solid substance, granules, or (especially after grinding) as a powder, and subsequently added to the respective bath solutions during tanning or retanning.

[0050] The residue after extraction with water or a supercritical solvent as the extraction agent is obtained similarly. The extractable components of the tannin-containing (e.g., tannin) plant material are transferred to the solution with the extraction agent and discarded, and the remaining residue is now used as the matrix for the tanning agent.

[0051] The tanning of the extract (or residue) to be used according to the present invention is carried out according to conventional methods, e.g., in a bath solution in a method for tanning and / or retanning raw hides and / or furs, especially naked hides, or in the case of retanning, pre-tanned hides and / or furs, including the following steps:

[0052] - In the bath solution, provide the raw hide and / or fur or naked hide or, in the case of retanning, the pre-tanned hide or pre-tanned fur.

[0053] - Add the extract (or residue) to the raw hide, fur, naked hide, or pre-tanned fur or leather or two or more of said materials in the bath solution.

[0054] - Allow the extract added to the bath solution to act on the said material during the penetration step, with or without coloring.

[0055] Advantageously, a fatliquoring step is subsequently carried out with a fatliquor, and if necessary, the pH is adjusted to between 2.5 and 5, especially between 3 and 4, with an organic acid, especially formic acid, and after washing with water, the resulting vegetable-tanned or vegetable-retanned raw hide and / or fur product is obtained.

[0056] Subsequently, conventional post-treatment steps can be carried out, such as loosening (Auflockern), propping up, aligning, drying (e.g., in a vacuum), pulling soft, or drumming.

[0057] The methods, methods for measuring parameters, and embodiments of other steps are shown in the examples.

[0058] The following examples are used to illustrate the present invention without limiting its scope.

[0059] Definition of the brand products mentioned:

[0060] SAF - Anionic stabilizer for fatliquors with softening effect

[0061] SLG - Anionic fatliquor based on a combination of natural and synthetic fats, for all soft and hard grain leather goods

[0062] BZE - N - Anionic fatliquor for upper leather based on a combination of sulfated natural fats

[0063] TANIGAN BN pwd. - Anionic exchange tanning agent based on methylene condensation products of arylsulfonic acids and hydroxyaryl acids, for tanning and retanning of soft and plump leather

[0064] SELLATAN MBS gran. - Tanning agent with good lightfastness based on anionic condensation products of phenolsulfonic acids

[0065] CV conditioner

[0066] Pure A bio - filler

[0067] FAST Brown HF - R metal complex dye

[0068] FIX E formaldehyde - free fixing agent for anionic dyes

[0069] They are always trademarks of TFL Ledertechnik GmbH, Rheinfelden, Germany.

[0070] Example 1: Obtaining green tea extract with water or a water / ethanol mixture

[0071] a) Extraction with water:

[0072] 400 g of green tea and 2000 g of softened water are placed in a 3 - liter reactor with a three - blade stirrer (2 - stage). The stirrer is started and the temperature is raised to the boiling point (temperature set at 99 °C). After 3 hours and 30 minutes, it is cooled with stirring (temperature adjusted to 20 °C). After another 2 hours and 30 minutes, the reactor is placed in a water bath at 47 °C, the stirring is stopped, the reactor is emptied, and the mixture is filtered through a stocking filter (Strumpffilter). Based on the initial filtrate, after drying at 105 °C, the filtrate has a dry content of 6.75 wt%.

[0073] b) Extraction with water and ethanol:

[0074] In a 3-liter reactor equipped with a three-blade stirrer (2-stage), 400 g of green tea, 1200 g of softened water (50%, based on 60% of the two solvents), and 800 g of ethanol (33.33%, based on 40% of the two solvents) were pre-placed. The stirrer was adjusted to 150 revolutions per minute and warmed in a water bath adjusted to 50 °C for 20 min, then warmed in a water bath adjusted to 60 °C for 25 min, then warmed in a water bath adjusted to 70 °C for 15 min, then warmed in a water bath adjusted to 80 °C for 15 min, and then warmed in a water bath adjusted to 90 °C for 15 min until the water / ethanol mixture boiled after an additional 15 min. The mixture was boiled with stirring for 3 hours and 30 minutes. Then it was cooled with stirring (temperature adjusted to 20 °C). After an additional 2 hours, the batch was placed in a water bath at 53.6 °C, the stirring was stopped, the reactor was emptied, and the mixture was filtered through a sock filter. Based on the initial filtrate, after drying at 105 °C, the filtrate had a dry content of 6.5 wt%.

[0075] The two extracts were stored as liquids in a refrigerator at 10 °C until use before use.

[0076] Example 2: Importance of ethanol for the effectiveness as a tanning agent when extracting green tea, taking the liquid extract of green tea as an example importance

[0077] product shoe leather

[0078] bare skin, sheet

[0079] material deacidified bare skin

[0080] color natural

[0081]

[0082]

[0083] Set up overnight, hang dry, and pull soft

[0084] TS = shrinkage temperature (a measure of the strength of crosslinking compounds in the fine structure of raw hides) was determined as described in the general part of the specification.

[0085] Evaluation of the resulting dried leather:

[0086]

[0087]

[0088] Instructions for table use: +3 / -3 extreme expression, +2 / -2 significant expression, -1 / +1 expression, -0.5 / +0.5 slight expression, 0 reference value.

[0089] The extract prepared with ethanol / water according to the present invention shows surprisingly obvious advantages in terms of tanning effect (high TS) and high levelness. The tanning performance is improved by extraction with ethanol / water. Adding ethanol to the aqueous green tea extract does not show improvement. Therefore, the higher tanning effect depends on extraction with ethanol water, rather than just the ethanol content in the bath.

[0090] Example 3: Tanning compared with mimosa extract in powder form

[0091] Article: Upper

[0092] Raw hide, sheet, deacidified

[0093] Material: +20% by weight of deacidified

[0094] Color: Natural

[0095]

[0096]

[0097]

[0098] Instructions for table use: +3 / -3 extreme expression, +2 / -2 significant expression, -1 / +1 expression, -0.5 / +0.5 slight expression, 0 reference value

[0099] Evaluation results:

[0100]

[0101] In the above experiment, particularly and truly unusually, an amount of 40% powdered mimosa extract can more or less be replaced with 20% green tea extract. This makes it possible to achieve a significant saving in tanning agents.

[0102] Example 4: Retanning leather tanned with glutaraldehyde with green tea extract powder (extracted with ethanol / water)

[0103] Article: Shoe

[0104] Material: Glutaraldehyde-tanned leather, 1.1 mm

[0105] Color: Beige

[0106]

[0107] Prop up overnight, align, vacuum dry at 45 °C for 3 minutes, soften, drum roll.

[0108] Evaluation results:

[0109]

[0110] Instructions for using the table: +3 / -3 extreme expression, +2 / -2 significant expression, -1 / +1 expression, -0.5 / +0.5 slight expression, 0 reference value

[0111] Surprisingly, it has been found that for a similar tanning effect using green tea extract, the amount of powder required is only half that of tara, which is known as a vegetable tanning agent. However, comparable leather properties have been found.

[0112] Example 5: Retanning of chrome-tanned leather (better results at the same dosage)

[0113] Material US wet blue

[0114] 1.1mm 1 raw hide, 4 pieces

[0115] % based on unhaired weight Color: brown

[0116]

[0117]

[0118] Instructions for using the table: +3 / -3 extreme expression, +2 / -2 significant expression, -1 / +1 expression, -0.5 / +0.5 slight expression, 0 reference value

[0119] Surprisingly, it has been found that for the same amount of extract, the leather has comparable or even better properties.

[0120] Example 6: Tanning for the preparation of vegetable leather

[0121]

[0122]

[0123] Tara is a vegetable tannin obtained from the small tara tree (Cesalpina spinosa).

[0124] Leather evaluation:

[0125]

[0126] 1 / +1 expression, -0.5 / +0.5 slight expression, 0 reference value

[0127] It shows that green tea extract provides results roughly as good as those of mimosa extract and is better than tara.

Claims

1. Use of a tanning agent-containing product for tanning and / or retanning raw hides or pelts or intermediate products produced therefrom in leather preparation, by extracting the tanning agent-containing material of a plant in the following manner: (i) The tanning agent-containing product in the form of an extract can be prepared or obtained by means of an aqueous solution or dispersion containing one or more polar organic solvents, or (ii) The tanning agent-containing product in the form of an extraction residue can be prepared or obtained by means of a supercritical solvent, or (iii) The tanning agent-containing product in the form of an extraction residue can be prepared or obtained by means of pure water, or (iv) The tanning agent-containing product in the form of an extraction residue can be prepared or obtained by means of a mixture of a supercritical solvent and a polar solvent.

2. The use according to claim 1, wherein the tanning agent-containing product is an extract obtainable or obtained according to variant (i), which is a liquid or a dried, aqueous dispersion or in particular a solution containing one or more polar organic solvents.

3. The use according to claim 1, wherein the tanning agent-containing product is a solid or a dispersed residue obtainable or obtained according to one of variants (ii) or (iii) by extraction with pure water or in particular with a supercritical solvent, in particular "press cake".

4. The use according to any one of claims 1 to 3, wherein phenolic compounds, in particular polyphenols and / or proanthocyanidins, are present as tanning agents in the respective products.

5. The use according to claim 1, wherein the tanning agent-containing product is an extract of green tea in liquid or dried form, which is a product obtained by extraction with an aqueous dispersion or in particular a solution containing one or more polar organic solvents.

6. Use of the extract of green tea according to claim 5 for retanning intermediate products produced from raw hides or pelts in leather preparation, characterized in that the one or more intermediate products are materials obtained after pickling, in particular naked hides, and the intermediate products are in each case tanned with the extract of green tea or tanned together with subsequent retanning.

7. The use according to claim 6, characterized in that the one or more intermediate products are materials obtainable after a first tanning step, in particular wet white leather, wet brown leather or wet blue leather, and each material is retanned while using the extract of green tea.

8. The use according to any one of claims 5 to 7, wherein the extract of green tea of each bath solution is added to the bath solution for tanning or retanning in solid form, such as granules or in particular powder form.

9. The use according to any one of claims 5 to 8, wherein the extract of green tea is added to one or each bath solution in liquid form, in particular as a water-ethanol solution.

10. Use according to any one of claims 5 to 9, wherein the extract of green tea is obtained by means of a polar solvent or a mixture of polar solvents in water with a weight ratio of the polar solvent (mixture) to water of 5:95 to 90:10, preferably 10:90 to 80:20, in particular 30:70 to 70:30, based on the total weight of the dry green tea, water and polar solvent (mixture), preferably by extraction in the range up to the boiling point of the respective mixture, for example at 80 to 105 °C, wherein the extract is used in liquid form or in solid form after drying, in which case it is used in particular as granules or preferably as a powder.

11. Use according to any one of claims 1 to 10, characterized in that the polar solvent is understood to be a polar solvent miscible with water and is selected from alcohols, in particular methanol, ethanol, propanol or isopropanol, diols, in particular ethylene glycol, diethylene glycol or triethylene glycol, ketones, in particular acetone or methyl ethyl ketone, di-lower-alkyl sulfoxides, in particular dimethyl sulfoxide or diethyl sulfoxide, nitriles, such as acetonitrile, water-soluble ethers, such as dioxane or tetrahydrofuran, amides, such as formamide, N-methylformamide, dimethylformamide or diethylformamide, and heterocyclic solvents, such as piperidine or N-methyl-2-pyrrolidone, or a mixture of two or more thereof.

12. Use according to claim 1, 3 or 4, wherein the supercritical solvent is selected from dinitrogen monoxide, sulfur dioxide, fluorinated hydrocarbons or in particular carbon dioxide; mixtures of polar organic solvents with supercritical solvents are also possible.

13. Use according to any one of claims 5 to 12, wherein the extract of green tea is such a green tea extract: wherein 100 to 300 parts of green tea, 300 to 900 parts of water and 200 to 600 parts (each based on weight) of a polar solvent, in particular ethanol, are used for the preparation of the extract.

14. Use according to any one of claims 5 to 12, characterized in that at the time of extraction, the weight ratio of green tea to water to polar solvent, in particular ethanol, is (10 - 30):(40 - 60):(25 - 40).

15. A tanning agent and / or a substance for retanning, which comprises a tanning agent-containing product that can be prepared or obtained according to any one of claims 1 to 14 as a tanning component, in particular for tanning and / or retanning raw hides or furs or intermediate products produced in the preparation of their leathers.

16. The tanning agent and / or the substance for retanning according to claim 15, which comprises an extract of green tea as defined in any one of claims 1 to 14.

17. A method for vegetable tanning and / or retanning, characterized in that the tanning agent-containing product prepared according to any one of claims 1 to 14 or the tanning agent and / or the substance for retanning according to claim 15 or 16 is added, in an aqueous solution or dispersion, with or without additional additives, to raw hides, furs, crusts, or, in the case of retanning, to materials produced from said materials that have already been tanned, and the batch is incubated for a longer period of time, for example 0.1 to 5 days, preferably with agitation.

18. A method for retanning leather semi-finished products, especially after tanning with glutaraldehyde or chromium, comprising treating the leather semi-finished products in an aqueous solution with an extract of green tea as a tanning agent as defined in any one of claims 5 to 15, optionally in combination with one or more other conventional products or auxiliaries for leather preparation.

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