CN117425738A

CN117425738A - Method for post-treating pelts, skins, leathers and furs with polyesters

Info

Publication number: CN117425738A
Application number: CN202280034949.6A
Authority: CN
Inventors: H·弗兰克; T·霍夫曼
Original assignee: Germany Sima Chemical Co ltd
Current assignee: Germany Sima Chemical Co ltd
Priority date: 2021-06-18
Filing date: 2022-06-14
Publication date: 2024-01-19
Also published as: EP4105342A1; WO2022263474A1; BR112023021563A2

Abstract

The invention relates to a method for the post-treatment of animal products selected from the group consisting of pelts, skins, leathers and furs, wherein the tanned animal products are subjected to retanning, wherein an aqueous solution of a polyester is used in retanning according to the invention, wherein the polyester is formed by polycondensation of monomers a and B, wherein monomer a is selected from the group consisting of linear or branched, saturated or unsaturated carboxylic acids having 2 to 10 carbon atoms, cyclic carboxylic acids having 5 to 10 carbon atoms and aromatic carboxylic acids having 8 to 10 carbon atoms or anhydrides thereof, wherein the carboxylic acids have at least two carboxyl groups and optionally one or more hydroxyl groups, and wherein monomer B is selected from the group consisting of polyols having 2 to 18 carbon atoms and at least 2 hydroxyl groups.

Description

Method for post-treating pelts, skins, leathers and furs with polyesters

Technical Field

The present invention relates to a method for the post-treatment of animal products selected from the group consisting of pelts, skins, leathers and furs, wherein polyesters are used as retanning agents. The invention also relates to the use of polyesters as retanning agents and in the post-treatment or retanning of pelts, skins, leather and pelts.

Background

In preserving hides, skins, leathers and furs, a distinction should be made between tanning and retanning. Each tanning is based on the permanent preservation of unstable, putrefying, decaying and decomposing collagen in the animal skin, which has been released from hair and subcutaneous connective tissue. Tanning agents used for tanning cause cross-linking of collagen molecules of the skin, thereby stabilizing the unique chain structure of skin proteins. The degree of crosslinking and the different stability of the different types of bonds achieved during tanning are manifested by an increase in the shrinkage temperature of the tanned pelts compared to the untanned pelts (Kurt Faber, bibliothek des Leders, volume 3, "Gerbmittel, gerbung und Nachgerbung" [ tanning agent, tanning and retanning ], publisher: umschau Verlag Frankfurt/M.1984, page 21). Thus, during tanning, cross-linking of collagen molecules occurs. The tanning agent must meet the following requirements: they must be at least bifunctional and steric such that they are small enough to penetrate the fine structure of collagen, but large enough to bridge the gap between two collagen molecules.

Examples of tanning agents are glutaraldehyde, which reacts with the free amino groups of collagen in the sense of a Mannich (Mannich) reaction, and chromium (III) salts, which form very stable complex bonds with the free carboxyl groups of collagen via polynuclear complexes.

Tanning can cause significant changes in both the chemical and physical properties of the pelt. This can be determined by measuring, for example, the shrinkage temperature. For example, depending on the type of tanning, the shrinkage temperature of the untanned pelt is about 40 ℃, and the shrinkage temperature of the tanned pelt is 65 ℃ to >100 ℃.

The tanning process is in contrast to the retanning process, which does not necessarily lead to cross-linking, resulting in a significant increase in shrinkage temperature. In leather production, the retanning process takes place as part of the so-called wet end process. The purpose of this operation is to tailor the desired properties of the finished leather according to its intended use. These properties include, inter alia, the fullness, softness or hardness of the leather, as well as the surface texture of the leather, such as hand or grain firmness. Other important properties that can be affected by retanning are, for example, heat and light fastness, dyeing behaviour, strength properties, shrinkage behaviour under the influence of heat and cold, water absorption and water vapour permeability, and many other properties. This is achieved by incorporating a retanning agent into the interstices of the fibrils and three-dimensional network of fibers of the leather. Depending on the type of retanning agent, there may be chemical interactions with the leather in the form of covalent or coordinate or ionic or hydrogen bonds, or purely physical interactions via cohesion or adhesion. The possibilities of interaction of the substances for retanning with the leather are as diverse. In addition to tanning multivalent metal salts such as chromium (III) or aluminum (III) compounds, kaolin, starch, especially tannin-containing plant extracts such as mimosa, tara and chestnut extracts are used. In the group of synthetic organic retanning agents, polycondensates are mainly used, such as formaldehyde condensation products with, for example, phenol, naphthalene, their sulfonates, dihydroxydiphenyl sulfone or nitrogen-containing compounds such as urea, melamine or dicyandiamide.

Since the beginning of the 80 s of the 20 th century, more and more polymers have entered the leather manufacturing process as retanning agents. These are polymers or copolymers of acrylic acid, methacrylic acid, styrene, maleic acid, etc. dissolved or dispersed in water. An overview is given in Kurt Faber, bibliothek des Leders, volume 3, "Gerbmittel, gerbung und Nachgerbung" [ tanning agent, tanning and retanning ], publisher: umschau Verlag Frankfurt/M.1985, pages 234-235 or Eckhart Heidemann in Fundamentals of Leather Manufacturing, publisher: eduard Roether Verlag Darmstadt 1993, pages 421-423. Another good overview of polymeric tanning agents is given on pages 173-177 by E.Pfleiderer, das Leder 32 (1981). These polymeric tanning agents are characterized by a significantly higher filling effect than the other retanning agents mentioned. In particular, the formation of stable coordination bonds in chrome leather results in the anchoring of the nipple layer and the reticulation layer of the pelt, which results in very good grain compaction and smooth grain. However, the greatest advantage is the excellent thermal and photo stability of the polymeric tanning agent material, which is essential for leather types exposed to intense heat and light, such as automotive interior leather, and also for white and light-colored leather.

However, conventional polymeric tanning agents also have serious drawbacks. The monomer is entirely of petrochemical origin and therefore comes from non-renewable resources. However, in an effort to make leather production more sustainable, there is an increasing demand for products made from sustainable raw materials.

Furthermore, the described polymer tanning agents are not biodegradable, i.e. there is a risk that the polymer will partly enter the ocean of the world via waste water and stay there for a long time.

The monomers used are generally toxic. Despite efforts to keep the monomer content in the polymer as low as possible, the residual monomer content cannot be completely excluded.

Disclosure of Invention

It is therefore an object of the present invention to provide a process for post-treatment or retanning and an alternative retanning agent having comparable application properties such as filling effect, grain compaction but in particular heat and light fastness, since polymeric tanning agents and in particular polymeric tanning agents of non-petrochemical origin are less toxic or non-toxic and are more readily biodegradable and thus more likely to meet the criteria of sustainability, biodegradability and toxicity of the monomers.

This technical problem is solved by a method for the post-treatment of animal products selected from the group consisting of pelts, skins, leathers and furs, wherein the tanned animal products are subjected to retanning, wherein according to the invention an aqueous solution of a polyester formed by polycondensation of monomers a and B is used in retanning,

wherein monomer A is selected from the group consisting of linear or branched, saturated or unsaturated carboxylic acids having 2 to 10 carbon atoms, cyclic carboxylic acids having 5 to 10 carbon atoms and aromatic carboxylic acids having 8 to 10 carbon atoms or anhydrides thereof,

wherein the carboxylic acid has at least two carboxyl groups and optionally one or more hydroxyl groups, and

-wherein monomer B is selected from the group consisting of polyols having 2 to 18 carbon atoms and at least 2 hydroxyl groups.

Surprisingly, the polycondensates of carboxylic acids and polyols described above have been found to be suitable for retanning of pelts, skins, leathers and furs, with little concern regarding the toxic and eco-toxic properties, are completely harmless or completely non-toxic, are produced from renewable raw materials or can be produced from renewable raw materials, and are biodegradable in the forms described herein.

Polycondensates from the above-mentioned carboxylic acids and the above-mentioned polyols or polyols are readily available in terms of production or synthesis. The average molar mass can be adjusted via the reaction temperature and reaction time, as well as the molar ratio between monomer a and monomer B or the co-use of the esterification catalyst, so as to achieve various application properties. In retanning they produce an effect comparable to or better than the previous polymer retanning agents.

The method according to the invention is a retanning treatment. This means that the animal product that has been tanned is subjected to a post-treatment or retanning treatment.

Preferred methods include the step of tanning the animal product prior to post-treatment or retanning. The tanning agent used in the tanning step causes cross-linking of collagen molecules of the animal product (e.g., raw skin), thereby stabilizing the chain structure of the skin protein.

In the tanning step, a tanning agent, preferably selected from chromium (III) salts and glutaraldehyde, is used.

In chrome tanning, the appropriately prepared animal product is first treated (pickled) with an acid in a salt solution, such as sulfuric and/or formic acid, and adjusted to a pH of 1.5 to 4.0, preferably 2.5 to 3.5. The actual tanning process is started by adding a chromium (III) salt, preferably chromium (III) sulphate. 0.2 to 3%, preferably 1 to 2%, cr is used ₂ O ₃ Input (calculated as mass of animal product). After the chromium (III) salt penetration, the pH is raised to 3.5 to 4.5, preferably 3.8 to 4.2, by the addition of a base. Increasing the pH results in crosslinking of collagen molecules in the animal product by formation of chromium (III) polynuclear complexes via interaction with collagenThe very stable complexing bond of the free carboxyl group results in a stable animal product.

In tanning with glutaraldehyde, the preparation of animal products is carried out in a comparable manner. Tanning is initiated by adding glutaraldehyde (calculated as mass of animal product) at a concentration of 0.2% to 3%, preferably 0.5 to 2% at a pH of 1.5 to 4.0, preferably 2.5 to 3.5. After glutaraldehyde penetration, the pH is adjusted to 3.0 to 7.0, preferably 3.5 to 4.5 by adding a base, resulting in crosslinking of the collagen molecules via covalent binding to the amino groups of the collagen in the sense of a Mannich reaction, thus stabilizing the animal product.

Tanning results in significant changes in both the chemical and physical properties of the pelt. The degree of cross-linking achieved during tanning and the different stability of the different types of combinations can be represented by an increase in the shrinkage temperature of the tanned animal product compared to the non-tanned animal product. Thus, depending on the type of tanning, the shrinkage temperature of the untanned pelt is about 40 ℃ and the shrinkage temperature of the tanned pelt is 65 ℃ to >100 ℃.

Retanning does not necessarily lead to crosslinking, resulting in a significant increase in shrinkage temperature, but this is not completely excluded. In practice, retanning or post-treatment of tanned animal products preferably results in an increase in shrinkage temperature of 20 ℃ or less, particularly preferably 10 ℃ or less.

In a preferred process, monomer a is a linear or branched, saturated or unsaturated carboxylic acid having 4 to 10 carbon atoms, or an anhydride thereof, said carboxylic acid having at least two carboxyl groups and optionally one or more hydroxyl groups.

In a further preferred process, the monomers A represent linear or branched, saturated or unsaturated carboxylic acids having from 4 to 8 carbon atoms and having at least two carboxyl groups and optionally having at least one hydroxyl group, particularly preferably linear or branched, saturated or unsaturated carboxylic acids having from 4 to 8 carbon atoms and having at least two carboxyl groups and having at least one hydroxyl group, or anhydrides thereof.

In another preferred method, monomer a is a carboxylic acid selected from the group consisting of oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, cyclopentane-1, 2-dicarboxylic acid, 1, 2-cyclohexanedicarboxylic acid, 1, 3-cyclohexanedicarboxylic acid, 1, 4-cyclohexanedicarboxylic acid, maleic acid, fumaric acid, phthalic acid, isophthalic acid, terephthalic acid, tartaric acid (tatronic acid), malic acid (2-hydroxysuccinic acid), tartaric acid (2, 3-dihydroxysuccinic acid), hydroxyglutaric acid (2-hydroxyglutaric acid), 3-hydroxyglutaric acid, citric acid (2-hydroxypropane-1, 2, 3-tricarboxylic acid), isocitric acid (1-hydroxypropane-1, 2, 3-tricarboxylic acid), mucic acid, preferably citric acid (2-hydroxypropane-1, 2, 3-tricarboxylic acid), or anhydrides thereof.

As explained above, according to the invention, monomer B is selected from the group consisting of polyols having 2 to 18 carbon atoms and at least 2 hydroxyl groups. Polyols having 2 to 18 carbon atoms and at least 2 hydroxyl groups include the corresponding alkanols, diols, glycol ethers, pentaerythritol and sugar alcohols. Thus, it is further preferred that the monomer B is selected from the group consisting of polyhydric alkanols having 2 to 12 carbon atoms and at least 2 hydroxyl groups, polyhydric alkanols having 2 to 8 carbon atoms and at least 2 hydroxyl groups, polyhydric alkanols having 2 to 6 carbon atoms and at least 2 hydroxyl groups, such as ethylene glycol, 1, 3-propanediol, 1, 4-butanediol, polyhydric alkanols having 3 to 12 carbon atoms and at least 3 hydroxyl groups, polyhydric alkanols having 3 to 8 carbon atoms and at least 3 hydroxyl groups, polyhydric alkanols having 3 to 6 carbon atoms and at least 3 hydroxyl groups, glycol ethers, such as diethylene glycol, pentaerythritol and sugar alcohols, in particular glycerol, butane-1, 2,3, 4-tetraols, such as threitol, erythritol; pentane 1,2,3,4, 5-pentanol, such as xylitol, arabitol, ribitol; hexane-1, 2,3,4,5, 6-hexanol, such as sorbitol, mannitol; and glycerol is particularly preferred. Among glycol ethers, diethylene glycol is particularly preferred.

The polyesters used in the process according to the invention are water-soluble. Preferably, the polyester has a water solubility of at least 50g/l, more preferably at least 100 g/l.

In a further preferred process, the molar ratio of monomers B to monomers A for polycondensation is from 1:0.1 to 1:10, preferably from 1:0.5 to 1:5, more preferably from 1:1 to 1:3.

In a very particularly preferred method, monomer a is citric acid and monomer B is glycerol, i.e. the polyester is formed from citric acid and glycerol. In the case of polyesters of citric acid and glycerol, the molar ratio of glycerol to citric acid used for the polycondensation is from 1:0.5 to 1:5, particularly preferably from 1:1 to 1:3.

In a further preferred process, the polyester has a weight average molecular weight of from 200 to 2000g/mol.

Furthermore, a method in which an aqueous solution having a dry matter content of 5.0 to 95.0 wt%, particularly preferably 20.0 to 80 wt% of polyester is used is preferable. The solution preferably has a viscosity of 1000 to 5000 mPa-s, more preferably 2000 to 4000 mPa-s, measured with a Brookfield viscometer using a spindle No. 2 at a temperature of 20 ℃ and a stirring speed of 12 rpm.

In another preferred method, the polyester according to the invention is used in an amount of 1.0 to 20.0 wt.%, preferably 3.0 to 10.0 wt.%, based on the weight of the split leather (split weight). "skiving weight" is defined as the weight of a skived (i.e., caliper adjusted) pre-tanned animal product (leather, pelt, skin or pelt).

In another preferred method, an aqueous solution of the polyester used has a pH of 2.0 to 8.0.

The aqueous solutions of polyesters may be used directly, but are preferably present partly or completely as alkali metal, alkaline earth metal or ammonium salts, or as salts of other bases, preferably organic bases.

In a further preferred method, a float length (float length) of at most 300% is used. For this purpose, it corresponds to 100% based on the weight of the animal product. For example, a float length of 300% means that 300kg of treatment liquor is used for a weight of 100kg of pre-tanned leather.

In another preferred embodiment, the process is carried out at a temperature in the range of 20 ℃ to 70 ℃.

In a further preferred process, in the retanning process, a synthetic or vegetable tanning agent is used in addition to the polyester.

The invention further relates to an animal product selected from the group consisting of pelts, skins, leathers and furs obtainable by the method according to the invention.

The invention further provides an animal product selected from the group consisting of pelts, skins, leathers and furs, comprising a polyester as defined above, wherein the polyester is inserted into a collagen network of the animal product, wherein further

i) At least a portion of the collagen molecules of the animal product are crosslinked by the formation of covalent bonds, and/or

ii) at least a portion of the carboxyl groups of the collagen molecules of the animal product have formed a complex with ions selected from the group consisting of chromium, aluminum, zirconium and iron ions.

The ability of the polyester to be inserted into the collagen net of an animal product means that the polyester is present in the animal product, in particular in the space between the upper and lower side of the skin portion or leather portion of the animal product.

Furthermore, the invention relates to the use of an aqueous solution containing a polyester for the post-treatment of tanned animal products selected from the group consisting of pelts, skins, leathers and furs, wherein the polyester is formed according to the invention by polycondensation of monomers A and B,

wherein monomer A is selected from the group consisting of straight-chain or branched, saturated or unsaturated carboxylic acids having 2 to 10 carbon atoms, cyclic carboxylic acids having 5 to 10 carbon atoms and aromatic carboxylic acids having 8 to 10 carbon atoms or anhydrides thereof,

wherein monomer B is selected from the group consisting of polyols having 2 to 18 carbon atoms and at least 2 hydroxyl groups.

The present invention will be described in more detail with reference to the following examples.

Examples

Example 1: synthesis of citric acid glyceride

111g of glycerin were weighed in a 1L four-necked flask equipped with a KPG stirrer, a thermometer, a gas introduction tube and a distillation bridge (including a distillation receiver), and heated to 90 ℃. 504g of citric acid monohydrate are added in portions so that the temperature does not drop below 80 ℃. The molar ratio of glycerin to citric acid was 1:2. After complete addition of citric acid, the mixture was heated to 125 ℃ with the introduction of nitrogen. Condensation starts immediately. The progress of the reaction was followed by the increase in the amount of water collected and the viscosity of the reaction product. After 4 hours, the condensation was stopped by cooling to below 100 ℃ and adding 266g of water. A pale yellow, almost clear viscous liquid was obtained showing the following analytical data:

dry solids: 69.7%

Viscosity: 3280 mPa.s (20 ℃ C.)

pH value: 1.9

Molar mass (Mw): 298g/mol

Example 2: synthesis of citric acid glyceride

The synthesis was carried out analogously to example 1, but with a molar ratio of glycerol to citric acid of 1:1.5. Again a pale yellow, almost clear viscous liquid was obtained showing the following analytical data:

dry matter: 70.2%

Viscosity: 2670 mPa.s (20 ℃ C.)

pH value: 2.1

Molar mass (Mw): 463g/mol

Example 3: synthesis of citric acid glyceride

The synthesis was carried out analogously to example 1, but with a molar ratio of glycerol to citric acid of 1:1. Again a pale yellow, almost clear viscous liquid was obtained showing the following analytical data:

dry matter: 69.5%

Viscosity: 2500 mPa.s (20 ℃ C.)

pH value: 2.2

Molar mass (Mw): 470g/mol

Example 4: synthesis of citric acid glyceride

207g of glycerol was weighed in a 1L four-necked flask equipped with a KPG stirrer, thermometer, gas introduction tube and distillation bridge (including distillation receiver), and heated to 90 ℃. 378g of citric acid monohydrate are added in portions so that the temperature does not drop below 80 ℃. The molar ratio of glycerin to citric acid was 1:0.8. After complete addition of citric acid, the mixture was heated to 120 ℃ with the introduction of nitrogen. Condensation starts immediately. The progress of the reaction was followed by the increase in the amount of water collected and the viscosity of the reaction product. After 4 hours, the condensation was stopped by cooling to below 100 ℃ and adding 75g of water. 183g of 50% NaOH solution was added to the resulting reaction product via a dropping funnel over 2 hours. A pale yellow, clear viscous liquid was obtained showing the following analytical data:

dry matter: 70.4%

Viscosity: 2750 mPa.s

pH value: 4.5

Molar mass (Mw): 404g/mol

Example 5: biodegradability of the material

The biodegradability of the citric acid glycerides synthesized in examples 1 to 4 is determined according to EN ISO 9408:1999 "determination of the completely aerobic biodegradability of organic substances in aqueous medium by determination of oxygen demand in closed respirators". In this test, the test substance was stirred in a closed test vessel at a concentration of 100mg/l in a system of water and mixed inoculum taken from a municipal sewage plant for 28 days. Biodegradability is determined from oxygen consumption measured continuously via pressure measurement. The results are summarized in table 1 below:

TABLE 1

Example 6: synthesis of adipic acid glyceride

212g of glycerin was weighed in a 1L four-necked flask equipped with a KPG stirrer, a thermometer, a gas introduction tube and a distillation bridge (including a distillation receiver), and heated to 90 ℃. 336g of adipic acid were added in portions so that the temperature did not drop below 80 ℃. The molar ratio of glycerin to adipic acid was 1:1. After complete addition of adipic acid, the mixture was heated to 135 ℃ with the introduction of nitrogen. Condensation starts immediately. The progress of the reaction was followed by the amount of water collected and the increase in viscosity of the reaction product. After 5 hours, the condensation was stopped by cooling to below 100 ℃ and adding 225g of water. 65g of 50% NaOH solution was added to the resulting reaction product via a dropping funnel over 2 hours. A nearly colorless, clear viscous liquid was obtained showing the following analytical data:

dry matter: 67.2%

Viscosity: 2450 mPas

pH value: 7.0

Molar mass (Mw): 404g/mol

Example 7: synthesis of diethylene glycol malate

244g of diethylene glycol was weighed into a 1L four-necked flask equipped with a KPG stirrer, a thermometer, a gas introduction tube and a distillation bridge (including a distillation receiver), and heated to 90 ℃. 308g of DL-malic acid were added in portions so that the temperature did not drop below 80 ℃. The molar ratio of diethylene glycol to DL-malic acid is 1:1. After complete addition of DL-malic acid, the mixture was heated to 125 ℃ with the introduction of nitrogen. Condensation starts immediately. The progress of the reaction was followed by the increase in the amount of water collected and the viscosity of the reaction product. After 7 hours, the condensation was stopped by cooling to below 100 ℃ and adding 75g of water. 65g of 50% NaOH solution was added to the resulting reaction product via a dropping funnel over 2 hours. A pale yellow, clear viscous liquid was obtained showing the following analytical data:

dry matter: 80.4%

Viscosity: 2750 mPa.s

pH value: 4.0

Molar mass (Mw): 513g/mol

Example 8: application test: post-treatment of leather

The esters synthesized in examples 1-4, 6 and 7 were tested for their properties as retanning agents using pre-tanned leather (wet blue leather). Wet blue refers to wet leather tanned with chromium (III) salts. For comparison a commercial polyacrylate (NOVALTAN 2999 from Zschimmer & Schwarz GmbH & Co.KG) with an average molecular weight of 30000g/mol was used.

Wet blue leather having 1.8-2.0mm shaving material was treated as follows:

TABLE 2

^* The amount refers to the weight of the animal product used, which corresponds to 100%;

^** retanning was performed using the esters synthesized in examples 1-4, 6 and 7; as comparative examples, commercial polyacrylates (from Zschimmer) having an average molecular weight of 30000g/mol were used&Schwarz GmbH&NOVALTAN 2999 of co.kg).

The post-treated leather was flattened, dried in vacuo at 60 ℃ for 2 minutes, air dried, conditioned and softened.

Subsequently, the wet blue leather after the post-treatment was first checked for thermal yellowing and light fastness, whereby the light fastness was measured by irradiating the leather sample with a xenon lamp having a spectrum similar to sunlight for 72 hours. SUNTEST CPS+ from Atlas Material Testing Solutions was used as the exposure device. The color change in terms of thermal yellowing and light fastness was evaluated according to DIN EN ISO 105A 05. In addition, the fullness and grain compactibility of the leather were also examined. The assessment of fullness and grain compactibility was a tactile test and visual test, respectively, performed subjectively by leather specialists on a graded scale, with a 5 scale indicating very good and a 1 scale indicating very poor.

The results are summarized in table 3 below:

TABLE 3 Table 3

^* The evaluation was subjectively done on a hierarchical scale, with 5 scale representing very good and 1 scale representing very poor.

After evaluation of the products used according to the invention, the products according to examples 1 to 4, 6 and 7 as leather retanning agents showed an improvement over the previously used retanning agents made of polyacrylates. The described polycondensates have the advantage that they can be produced entirely from renewable biological raw materials and that they exhibit good biodegradability.

Claims

1. Method for the post-treatment of an animal product selected from the group consisting of pelts, skins, leathers and furs, wherein the tanned animal product is subjected to retanning, characterized in that an aqueous solution of a polyester is used in the retanning, wherein the polyester is formed by polycondensation of monomers A and B,

2. The process according to claim 1, wherein monomer a is a linear or branched, saturated or unsaturated carboxylic acid having 4 to 8 carbon atoms and at least two carboxyl groups and optionally having at least one hydroxyl group, preferably a linear or branched, saturated or unsaturated carboxylic acid having 4 to 8 carbon atoms and at least two carboxyl groups and having at least one hydroxyl group, or an anhydride thereof.

3. The process according to claim 1, wherein the monomer a is a carboxylic acid selected from the group consisting of oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, cyclopentane-1, 2-dicarboxylic acid, 1, 2-cyclohexanedicarboxylic acid, 1, 3-cyclohexanedicarboxylic acid, 1, 4-cyclohexanedicarboxylic acid, maleic acid, fumaric acid, phthalic acid, isophthalic acid, terephthalic acid, tartaric acid, malic acid (2-hydroxysuccinic acid), tartaric acid (2, 3-dihydroxysuccinic acid), hydroxyglutaric acid (2-hydroxyglutaric acid), 3-hydroxyglutaric acid (3-hydroxyglutaric acid), citric acid (2-hydroxypropane-1, 2, 3-tricarboxylic acid), isocitric acid (1-hydroxypropane-1, 2, 3-tricarboxylic acid), mucic acid, preferably citric acid (2-hydroxypropane-1, 2, 3-tricarboxylic acid), and anhydrides thereof.

4. A process according to any one of claims 1 to 3, wherein monomer B is selected from the group consisting of a polyhydric alkanol having 2 to 12 carbon atoms and at least 2 hydroxyl groups, a polyhydric alkanol having 2 to 8 carbon atoms and at least 2 hydroxyl groups, a polyhydric alkanol having 2 to 6 carbon atoms and at least 2 hydroxyl groups, a polyhydric alkanol having 3 to 12 carbon atoms and at least 3 hydroxyl groups, a polyhydric alkanol having 3 to 8 carbon atoms and at least 3 hydroxyl groups, a polyhydric alkanol having 3 to 6 carbon atoms and at least 3 hydroxyl groups, a glycol ether, pentaerythritol and a sugar alcohol, preferably glycerol.

5. The process according to any one of claims 1 to 4, wherein the molar ratio of monomer B to monomer a for the polycondensation is from 1:0.1 to 1:10, preferably from 1:0.5 to 1:5, more preferably from 1:1 to 1:3.

6. The method of any one of claims 1 to 4, wherein the polyester is formed from citric acid and glycerol.

7. The process according to claim 6, wherein the molar ratio of glycerol to citric acid used for the polycondensation is from 1:0.5 to 1:5, preferably from 1:1 to 1:3.

8. The process of any one of claims 1 to 7, wherein the weight average molecular weight of the polyester is 200 to 2000g/mol.

9. The process according to any one of claims 1 to 8, wherein the aqueous solution of the polyester has a viscosity of 1000 to 5000 mPa-s, preferably 2000 to 4000 mPa-s, measured with a Brookfield viscometer at a temperature of 20 ℃ using a number 2 spindle and a stirring speed of 12rpm at a dry matter content of 5.0 to 95.0% by weight of the polyester.

10. The method according to any one of claims 1 to 9, wherein the aqueous solution comprises 1.0 to 20.0 wt.%, preferably 3.0 to 10.0 wt.% dry matter based on the skiving weight of the polyester.

11. The method of any one of claims 1 to 10, wherein the aqueous solution of polyester has a pH of 2.0 to 8.0.

12. The process according to any one of claims 1 to 11, wherein the polyester is present partly or completely as an alkali metal, alkaline earth metal or ammonium salt, or as a salt of another base, preferably an organic base.

13. The method according to any one of claims 1 to 12, wherein a float length of at most 300% is used.

14. The method according to any one of claims 1 to 13, wherein the method is performed at a temperature in the range of 20 ℃ to 70 ℃.

15. The method according to any one of claims 1 to 14, wherein in the retanning a synthetic or vegetable tanning agent is used in addition to the polyester.

16. Animal products selected from the group consisting of pelts, skins, leathers and furs, obtainable by the method according to any one of claims 1 to 15.

17. Animal product selected from the group consisting of pelts, skins, leathers and furs, comprising said polyester as defined in any one of claims 1 to 8, wherein said polyester is inserted into a collagen network of said animal product, wherein further

18. Use of an aqueous solution containing a polyester for the post-treatment of tanned animal products selected from the group consisting of pelts, skins, leathers and furs, characterized in that the polyester is formed by polycondensation of monomers A and B,

wherein monomer A is selected from the group consisting of straight-chain or branched, saturated or unsaturated carboxylic acids having 2 to 10 carbon atoms, cyclic carboxylic acids having 5 to 10 carbon atoms and aromatic carboxylic acids having 8 to 10 carbon atoms, or anhydrides thereof,