Zeolite composition suitable for tanning leather

Abstract

The invention relates to leather tanning agents, and to a zeolite composition suitable as a single tanning agent, said zeolite composition comprising a zeolite, a first weak acid, a second weak acid and optionally a third weak acid. This zeolite composition enables effective and efficient chrome-free tanning.

Description

TECHNICAL FIELD

[0001] The present invention relates to the field of leather manufacturing and single tanning agents comprising zeolites. BACKGROUND

[0002] The production of leather involves the transformation of perishable hides or pelts into durable leather. For this purpose, the pre-treated hides are tanned with tanning agents. Several tanning agents are known in the art. Generally, three groups of tanning agents are identified: synthetic tanning agents, metal salt tanning agents and vegetable tanning agents.

[0003] Synthetic tanning agents (Synthetic tanning agent or syntan) are usually reactive aldehydes, however phenol-based condensation polymers are also used as synthetic tanning agents. In order to achieve high quality standards of leather, a combination of reactive aldehydes and phenol-based condensation polymers is usually used to tan leather. Their application is limited to a limited number of leather article types. Furthermore, aldehyde synthetic tanning agents are unhealthy for humans, thus special attention is required in industrial tanning processes, furthermore, tanning waste water from tanning processes can be contaminated with synthetic tanning agents.

[0004] Metal salt tanning agents are also known in the art as mineral tanning agents or tanning salts and comprise cations with a valence of three or higher, such as chromium, aluminium, zirconium, titanium and iron. These cations can interact with the collagen of the leather. Chromium tanning is by far the most predominant tanning method, as it provides excellent tanning effects, but also has recognized drawbacks in terms of environmental impact of chromium related to waste from tanning processes, such as chromium-containing leather. Chromium tanning also imparts a blue appearance to the leather. Tanning with alternative tanning salts, such as aluminium salts (mainly aluminium sulphate or alum), results in leather with poor water resistance; furthermore, the obtained leather is affected by aluminium dissolution, as the fixation of aluminium is less strong compared to chromium. Therefore, in the art, aluminium tanned leather is considered as incompletely tanned. Other metal salt alternatives, such as tanning with iron, are hindered by discoloration and limitations in leather finishing. Zirconium and titanium have a valence of 4 (IV), their application in tanning is limited. The penetration of cations with a valence of 4 means a much lower penetration rate into the hide and these cations cannot reach full saturation of the carboxyl groups, thus obtaining leather with greater stiffness than chromium tanned leather. Other problems of tanning with cation (IV) salts are the limited leather application types and the economic availability of the materials.

[0005] Vegetable tanning agents are derived from plants such as oak and spruce bark. The tanning agents in these plants are polyphenols. Vegetable tanning is time consuming, as the leather needs to absorb a considerable amount of vegetable tanning agent. Furthermore, the limited selection of colours hinders the economic use of a wide range of leather articles.

[0006] Since conventional tanning methods have drawbacks in terms of process efficiency, human health, environmental impact or leather properties, new methods and tanning materials are investigated, such as zeolite tanning based on aluminium silicate tanning agents.

[0007] WO 2013 / 114414 A1 describes a tanning method wherein zeolite is combined with a neutralizing agent and a tanning material for tanning leather. The examples illustrate the combination with synthetic tanning agents for complete tanning. The method has a high water consumption and requires undesired storage of the tanned hide after tanning.

[0008] WO 2013 / 045764 A1 describes a method wherein zeolite is combined with a mono-carboxylic acid and used as tanning agent. The method has a high water consumption. The zeolite mono-carboxylic acid combination can be further combined with co-tanning agents such as aluminium sulphate and poly-carboxylic acids. The tanning agent is preferably used in a two-step tanning method. SUMMARY

[0009] There is a continuous need for robust and efficient tanning agents that are environmentally friendly and harmless (non-toxic) to humans. Furthermore, there is a need for robust and efficient tanning agents that are easy to tailor to different leather properties and leather applications. There is also a need for tanning agents that are safe to apply and abundant in raw materials.

[0010] Even though the shrinkage temperature is increased, the zeolite compositions of the art as tanning agents are insufficient because their penetration at tanning is not acceptable, their absorption is poor and / or the leather quality of the produced leather is not suitable for commercial exploitation. Furthermore, the leather production at commercial scale is not robust because the leather quality fluctuates too much. The water consumption when using these tanning agents is also high. The conventional remedy of adding additional tanning agents such as tanning salts, synthetic tanning agents or vegetable tanning agents to the zeolite composition does not improve the tanning results and efficiency or brings the known drawbacks of such tanning agents.

[0011] The inventors now surprisingly found that the above needs can be met with a zeolite composition suitable as a single tanning agent, said zeolite composition comprising a zeolite, a first weak acid, a second weak acid and optionally a third weak acid, wherein the first weak acid, the second weak acid and the third weak acid are different acids. This zeolite composition was found to be efficient for tanning without the need for additional tanning agents, such as synthetic tanning agents, metal salt tanning agents and vegetable tanning agents. The zeolite composition of the invention is easy to implement in conventional tanning methods because it can be applied in one step and the tanning time is similar to conventional industrial standard tanning methods.

[0012] The inventors found that a zeolite composition comprising one weak acid, when used in a one-step tanning process, provided insufficient leather penetration and unacceptable leather sensory properties, regardless of whether the obtained leather had sufficient shrink temperature (Example 1). The insufficient penetration was even more clearly observed when scaled up to industrial scale leather tanning. The insufficient penetration resulted in leather having a non-uniform surface (appearance, physical and chemical properties varying over the surface) and was unsuitable for skinning. It was also found that the absorption was poor, which resulted in an inefficient use of the zeolite composition and waste water comprising a considerable amount of the zeolite composition.

[0013] Surprisingly, only when the zeolite composition comprises two specific weak acids (Example 2) or any combination of three weak acids (Example 3), sufficient penetration was achieved and the leather quality and absorption were good. In particular, sufficient absorption has been achieved in the case of two specific weak acids. Furthermore, the inventors unexpectedly found that excellent tanning results could be obtained in the absence of a metal tanning salt such as aluminum sulfate. Aluminum sulfate is a weak polyvalent inorganic acid salt comprising a cation and the cation is aluminum. The inventors unexpectedly realized that this tanning salt is counterproductive. As a result of the metal tanning salt such as aluminum sulfate generating a cationic surface charge to the collagen of the leather and the zeolite generating an anionic surface charge, the metal tanning salt such as aluminum sulfate interferes with the zeolite, which would result in a less efficient tanning. As a result, the penetration, shrink temperature, leather properties and / or absorption would be worse. This can result in a higher level of undesirable aluminum in the tanning waste stream and an inefficient use of the zeolite composition when a substantial part of the composition is discarded (Example 4).

[0014] The inventors unexpectedly found that for the zeolite composition to be suitable as a tanning agent, the presence of several weak acids is required to provide all the interactions needed for an efficient and effective tanning. The different interactions needed involve the dispersion of the zeolite in water, involve the interaction between the zeolite and the collagen and in some cases the need for pH stabilization. The different needed interactions require multiple acids; one acid helps the dispersion of the zeolite in water, another acid helps the interaction between the zeolite and the collagen of the hide and another acid can further support these interactions and / or support the pH stabilization. Each type of acid has a specific interaction with the zeolite and / or with the collagen of the hide, which is particularly observable in terms of leather properties.

[0015] The zeolite composition of the invention enables tailoring of the leather properties, such as leather feel and shrink temperature of the leather, while maintaining good permeability; tailoring can be achieved by selecting a specific combination of weak acids. The leather obtained by tanning with the zeolite composition of the invention has excellent dissolution (also referred to as wash-out) properties, and the tanning process with the zeolite composition of the invention has good tanning uptake (also referred to as uptake). Tanning uptake is generally the ratio of the amount of tanning agent taken up by the leather during tanning divided by the amount of tanning agent provided during the tanning step. For tanning with the zeolite composition of the invention as a single tanning agent, the tanning uptake is the amount of aluminum taken up by the leather during tanning divided by the amount of aluminum present in the zeolite composition provided during the tanning step. Tanning uptake is expressed in percentage.

[0016] Furthermore, the leather obtained by tanning with the zeolite composition of the invention has a white appearance and does not develop discoloration, such as the wet blue and wet white leather obtained by chrome tanning. As a result, the leather can be colored more freely, and bright colors can be applied.

[0017] The zeolite composition of the invention can be integrated in a conventional tanning process without the need for adaptations. The zeolite composition can be used as a single tanning agent without the use of other tanning agents. Furthermore, a two-step tanning process is not required; the zeolite composition is effective in a single tanning step. A single tanning step is more efficient, as the tanning agent only needs to be added once and the tanning time is shorter. In addition to this, in most cases the total amount of tanning agent in a two-step process is higher than the amount required in a one-step process.

[0018] The invention relates to a zeolite composition suitable as a single tanning agent, a method of preparing the zeolite composition, a zeolite composition obtained by the method of preparing, a method of producing leather wherein a hide is contacted with a tanning composition comprising the zeolite composition, and a leather obtained by the method of producing leather. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1A : Distribution of aluminum over the vertical cross-section of a leather sheet obtained from Example 5, determined by SEM-EDX. X-axis: length scale (micrometer) across the vertical cross-section; left side is flesh side, right side is grain side. Y-axis: intensity. Upper dotted line is concentration at the surface, lower dotted line is concentration at the center of the vertical cross-section of the leather sheet.

[0020] Figure 1B : Distribution of silicon over the vertical cross-section of a leather sheet obtained from Example 5, determined by SEM-EDX. X-axis: length scale (micrometer) across the vertical cross-section; left side is flesh side, right side is grain side. Y-axis: intensity. Upper dotted line is concentration at the surface, lower dotted line is concentration at the center of the vertical cross-section of the leather sheet. DETAILED DESCRIPTION

[0021] In a first embodiment, the present invention relates to a zeolite composition suitable as a single tanning agent, comprising a zeolite, a first weak acid, a second weak acid and optionally a third weak acid, wherein the first weak acid, the second weak acid and the third weak acid are different acids, the amount of zeolite is at least 34 wt.%, preferably at least 50 wt.%, based on the total weight of the zeolite composition, and the amount of water is less than 25 wt.%, preferably less than 20 wt.%, based on the total weight of the zeolite composition.

[0022] Furthermore, the present invention relates to a method of preparing a zeolite composition, comprising

[0023] a) in case the first weak acid, the second weak acid and the third weak acid, when present, are solid at 20°C

[0024] i) mixing the first weak acid, the second weak acid, the zeolite and the third weak acid, when present, while keeping the temperature of the mixture below 100°C during mixing;

[0025] or

[0026] b) in case any of the first weak acid, the second weak acid or the third weak acid, when present, is liquid at 20°C

[0027] ii) mixing the weak acid or weak acids that are liquid at 20°C with the zeolite while keeping the temperature of the mixture below 100°C during mixing, and

[0028] iii) mixing any remaining weak acid or weak acids with the mixture obtained in step ii) while keeping the temperature of the mixture below 100°C during mixing.

[0029] The present invention also relates to a zeolite composition suitable as a single tanning agent, obtained by the method.

[0030] The present invention also relates to a method of producing leather, comprising a tanning step, wherein the hide is contacted with a tanning liquid, said tanning liquid comprising a zeolite composition and the concentration of the zeolite composition is between 1 wt.% and 15 wt.% based on the weight of the hide.

[0031] The present invention also relates to a leather, said leather having

[0032] - a shrinkage temperature higher than 60°C, preferably higher than 70°C, and

[0033] - an isoelectric point between 3 and 5, preferably between 3.5 and 4.5,

[0034] and said leather comprises more than 0.5 wt.% of aluminium based on the dry weight of the leather and more than 0.5 wt.% of silicon based on the dry weight of the leather.

[0035] The present invention also comprises the use of the zeolite composition of the present invention for improving tanning absorption, wherein the zeolite composition is a single tanning agent.

[0036] The present invention also comprises the use of the zeolite composition of the present invention for improving leather penetration, wherein the zeolite composition is a single tanning agent.

[0037] Zeolite

[0038] The zeolite composition comprises a zeolite. A zeolite is also known as an aluminosilicate. Zeolites known in the art are minerals having a crystal structure and can be characterized by the size of its pores, chemical composition and / or crystal structure. For the present invention, preferably, the zeolite is a zeolite having the general formula (Cat 2 / n O) X (Al2O3)(SiO2) Y wherein Cat is a cation, O is oxygen, Si is silicon and Al is aluminum. In preferred embodiments, when the cation is monovalent n = 1, X is 0.5 to 1.8, Y is 0.8 to 40 and H2O is present as crystal water, or when the cation is divalent n = 2, X is 0.5 to 1.8, Y is 0.8 to 40 and crystal water is present; more preferably, the cation is monovalent and n = 1, X is 0.5 to 1.8, Y is 0.8 to 14 and crystal water is present; most preferably the cation is sodium (Na), X is 0.5 to 1.8, Y is 0.8 to 14 and crystal water is present. Preferably, the Al2O3 weight percentage is 25 wt.% to 40 wt.% based on the total weight of the zeolite and the SiO2 weight percentage is 28 wt.% to 40 wt.% based on the total weight of the zeolite. Preferably, the zeolite has a pore size of 3 Angstroms or more, even more preferably 4 Angstroms or more, most preferably 5 Angstroms or more. Angstrom is a unit of length and is equal to 10 -10 meters. A larger pore size improves the interaction of the zeolite with the weak acid. Preferably, the zeolite is an A, Y, X or P type zeolite, more preferably the zeolite is an A type zeolite. Preferably, the zeolite has a loss on ignition of less than 40 wt.%, more preferably less than 25 wt.%, even more preferably less than 15 wt.%. Loss on ignition is known in the art and can be measured by measuring the initial weight, heating the zeolite to 800°C and measuring the weight reduction until the weight becomes stable. The loss on ignition is the difference between the initial weight and the stable weight after heating divided by the initial weight, expressed in weight percentage. A low loss on ignition equals a low water content, thus a more concentrated product is easier to use in production environments. Typically, the zeolite of the present invention has a water content before being contacted with the weak acid, thus the zeolite has a water content. This water content is typically between 1 wt.% and 25 wt.% of the total weight of the zeolite.

[0039] Acid

[0040] The zeolite composition of the present invention comprises a first weak acid, a second weak acid and optionally a third weak acid. Weak acids are known in the art and relate to acids that do not fully dissociate into a proton (H + ) and an anion when dissolved in water. Weak acids have an acid dissociation constant (pKa) value higher than -1.74.

[0041] Weak acids can be classified as organic acids, salts of organic acids, weak inorganic acids and weak inorganic acid salts. Weak acid salts comprise a cation and an anion. Preferably, the cation of the weak acid salts (organic and inorganic) of the zeolite composition of the present invention is a monovalent or divalent cation, more preferably the cation is selected from the group consisting of sodium, potassium, ammonium, calcium and magnesium, even more preferably the cation is sodium or potassium, most preferably the cation is sodium.

[0042] Other cations are undesirable because they are environmentally unfriendly, harmful to humans, expensive, cause poor solubility of the zeolite composition, cause discoloration of the leather and / or affect the effectiveness of the zeolite composition for tanning. For example, chromium is environmentally unfriendly, harmful to humans and causes the leather to be blue. Iron causes discoloration of the leather, while aluminum salts are known for their poor solubility effect, which means that the leather loses aluminum upon use, resulting in poor leather properties. Furthermore, aluminum salts have a lower binding efficiency to collagen compared to the zeolite composition, resulting in undesirably higher aluminum concentrations in the effluent and lower yields.

[0043] Furthermore, since the tanning effect of the zeolite composition is the result of the combination of the weak acid and the zeolite, there is no need for other tanning salts with tanning properties of the cation, such as chromium, aluminum, titanium, zirconium or iron salts. Even worse, the addition of such tanning salts to the zeolite composition would have a counteractive effect. Tanning salts interfere with the zeolite; the zeolite interacts with collagen through van der Waals association of the zeolite, resulting in a low isoelectric point of the zeolite-containing leather, while tanning salts interact with collagen through cation association, resulting in a high isoelectric point of the tanning salt-containing leather. Due to this interference, the tanning efficiency of the zeolite composition is reduced. The effect of the interaction is reflected in the isoelectric point measured with zeta potential, which is higher than 6.5 (pI value) for leather tanned with tanning salts, while it is between 3 and 5 for leather tanned according to the present invention.

[0044] Preferably, the first weak acid, the second weak acid and the third weak acid (when present) are selected from the group consisting of organic acids, salts of organic acids comprising a cation, weak inorganic acids and weak inorganic acid salts comprising a cation; wherein the cation is selected from the group consisting of sodium, potassium, ammonium, calcium and magnesium, preferably the cation is sodium or potassium, most preferably the cation is sodium.

[0045] Preferably, the third weak acid is present.

[0046] The preferred organic acid is selected from carboxylic acids and organic sulfonic acids, more preferably from carboxylic acids and aromatic sulfonic acids, even more preferably from carboxylic acids, phenol sulfonic acids, naphthalene sulfonic acids and p-aminobenzene sulfonic acids, and the most preferred organic acid is carboxylic acid.

[0047] Carboxylic acids can be monocarboxylic or polycarboxylic acids. Monocarboxylic acids have one carboxyl group per molecule, while polycarboxylic acids contain more than one carboxyl group per molecule. Dicarboxylic acids have two carboxyl groups per molecule, tricarboxylic acids have three carboxyl groups per molecule, and tetracarboxylic acids have four carboxyl groups per molecule. Preferably, when the organic acid is a carboxylic acid, it is a monocarboxylic acid; or conversely, preferably, when the organic acid is a carboxylic acid, it is a polycarboxylic acid; more preferably, when the organic acid is a carboxylic acid, it is a dicarboxylic acid or a tricarboxylic acid, and even more preferably, a dicarboxylic acid. Alternatively, when the organic acid is a carboxylic acid, it is preferred that the organic acid is a monocarboxylic acid or a dicarboxylic acid.

[0048] Preferred organic acids are selected from formic acid, acetic acid, propionic acid, butyric acid, benzoic acid, butenoic acid, valeric acid, adipic acid, citric acid, oxalic acid, galactobionic acid, gallic acid, gluconic acid, glucuronic acid, glycolic acid, lactic acid, nicotinic acid, ascorbic acid, malonic acid, maleic acid, succinic acid, glutaric acid, tartaric acid, phthalic acid, salicylic acid, 4-phenol sulfonic acid, naphthalene-1-sulfonic acid, naphthalene-2-sulfonic acid, p-aminobenzenesulfonic acid, and succinic acid. More preferred organic acids are selected from formic acid and acetic acid. Propionic acid, butyric acid, butenoic acid, valeric acid, adipic acid, citric acid, oxalic acid, galactobionic acid, gallic acid, gluconic acid, glucuronic acid, glycolic acid, lactic acid, nicotinic acid, ascorbic acid, malonic acid, maleic acid, succinic acid, glutaric acid, tartaric acid, phthalic acid, naphthalene-2-sulfonic acid, p-aminobenzenesulfonic acid, and even more preferably, organic acids selected from formic acid, gallic acid, oxalic acid, citric acid and tartaric acid, with the most preferred organic acids selected from formic acid, gallic acid, oxalic acid and tartaric acid.

[0049] Preferred salts of cation-containing organic acids are selected from cationic carboxylates and cationic organic sulfonates. More preferably, salts of cation-containing organic acids are selected from cationic carboxylates, cationic phenol sulfonates, cationic naphthalene sulfonates, and cationic p-aminobenzoates, wherein the cation is selected from sodium, potassium, ammonium, calcium, and magnesium. Even more preferably, salts of cation-containing organic acids are selected from sodium formate, sodium acetate, sodium oxalate, and sodium naphthalene-2-sulfonate. More preferably, salts of cation-containing organic acids are selected from sodium formate, sodium oxalate, and sodium acetate.

[0050] The preferred weak inorganic acid salt containing a cation is selected from cationic sulfates, cationic bisulfates, and cationic acidic sulfates, wherein the cation is selected from sodium, potassium, ammonium, calcium, and magnesium, and preferably from sodium and potassium. Sodium acidic sulfate is a preferred weak inorganic acid salt containing a cation.

[0051] Preferably the weak inorganic acid is boric acid, phosphoric acid, silicic acid or sulfamic acid.

[0052] Preferably one or more of the first weak acid, the second weak acid and the third weak acid (when present) is an organic acid having a molecular diameter of less than 0.8 nm, more preferably less than 0.7 nm, even more preferably less than 0.6 nm, most preferably less than 0.5 nm. Preferably less than 0.8 nm More preferably less than 0.7 nm Most preferably less than 0.6 nm of an organic acid; more preferably the first weak acid is an organic acid having a molecular diameter of less than 0.8 nm, more preferably less than 0.7 nm, even more preferably less than 0.6 nm, most preferably less than 0.5 nm. Preferably less than 0.8 nm More preferably less than 0.7 nm Most preferably less than 0.6 nm of an organic acid.

[0053] The molecular diameter, also referred to as the kinetic diameter or critical diameter, is the smallest diameter of a molecule to enter a pore. Without being bound by theory, it is believed that small molecules are able to interact beneficially with the zeolite. The pore size of the zeolite structure allows smaller organic acids to penetrate into the structure. As such, the relationship of the molecular size of the acid to the pore size of the zeolite structure facilitates penetration. The small molecule acid that penetrates is able to interact beneficially with the zeolite and allows for better dispersion into the liquid phase during application.

[0054] In one embodiment, the first weak acid, the second weak acid and the third weak acid (when present) are selected from the group consisting of sodium diformate, sodium diacetate, p-aminobenzenesulfonic acid, 2-aminoglutaric acid, 2-oxopropanoic acid, 2-hydroxyacetic acid and 3-oxobutanoic acid, preferably from the group consisting of sodium diacetate, p-aminobenzenesulfonic acid, 2-aminoglutaric acid, 2-oxopropanoic acid, 2-hydroxyacetic acid and 3-oxobutanoic acid.

[0055] In another preferred embodiment, the first weak acid is an organic acid and the second weak acid and the third weak acid (when present) is an organic acid, a weak inorganic acid salt comprising a cation, a weak inorganic acid or a weak inorganic acid salt comprising a cation; and wherein the cation is selected from the group consisting of sodium, potassium, ammonium, calcium and magnesium. More preferably:

[0056] - the first weak acid is a carboxylic acid, most preferably a mono- or di-carboxylic acid,

[0057] - the second weak acid is a carboxylic acid, preferably a di- or tri-carboxylic acid, and

[0058] - the third weak acid is selected from the group consisting of a cationic sulfate, a cationic bisulfate, a cationic acid sulfate and a cationic naphthalene sulfonate, wherein the cation is selected from the group consisting of sodium, potassium, ammonium, calcium and magnesium, more preferably the third weak acid is a cationic naphthalene sulfonate or a cationic bisulfate, wherein the cation is selected from the group consisting of sodium, potassium, ammonium, calcium and magnesium, even more preferably the third weak acid is a cationic bisulfate, wherein the cation is selected from the group consisting of sodium, potassium, ammonium, calcium and magnesium.

[0059] Another embodiment of the present application relates to a zeolite composition, wherein

[0060] - the second weak acid is a carboxylic acid, a cationic organic sulfonic acid salt or a weak inorganic acid salt comprising a cation selected from the group consisting of sodium, potassium, ammonium, calcium and magnesium, and

[0061] - the third weak acid (when present) is a polycarboxylic acid, a cationic organic sulfonic acid salt or a weak inorganic acid salt comprising a cation selected from the group consisting of sodium, potassium, ammonium, calcium and magnesium.

[0062] In another preferred embodiment, the first weak acid is a carboxylic acid, most preferably a monocarboxylic acid or a dicarboxylic acid, and the second weak acid and the third weak acid (when present) are an organic acid, a weak inorganic salt comprising a cation or a weak inorganic acid salt comprising a cation; and wherein the cation is selected from the group consisting of sodium, potassium, ammonium, calcium and magnesium, more preferably the second weak acid and the third weak acid (when present) are an organic acid or a weak inorganic acid salt comprising a cation; and wherein the cation is selected from the group consisting of sodium, potassium, ammonium, calcium and magnesium. Even more preferably the second weak acid and the third weak acid (when present) are a carboxylic acid, a naphthalene sulfonic acid, a phenol sulfonic acid, a cationic carboxylic acid salt, a cationic organic sulfonic acid salt, a cationic naphthalene sulfonic acid salt, a cationic sulfate salt, a cationic hydrogen sulfate salt, a cationic acid sulfate salt, wherein the cation is selected from the group consisting of sodium, potassium, ammonium, calcium and magnesium. Even still more preferably the second weak acid and the third weak acid (when present) are a carboxylic acid, a cationic naphthalene sulfonic acid salt, a cationic hydrogen sulfate salt, wherein the cation is selected from the group consisting of sodium, potassium, ammonium, calcium and magnesium. In a particularly preferred embodiment, the second weak acid and the third weak acid (when present) are a carboxylic acid, a cationic naphthalene sulfonic acid salt, a cationic hydrogen sulfate salt, wherein the cation is selected from the group consisting of sodium, potassium, ammonium, calcium and magnesium, most preferably the second weak acid and the third weak acid (when present) are a carboxylic acid or a cationic hydrogen sulfate salt, wherein the cation is selected from the group consisting of sodium, potassium, ammonium, calcium and magnesium.

[0063] In another preferred embodiment, a third weak acid is present. For this embodiment, the first weak acid, the second weak acid and the third weak acid are selected from the group consisting of carboxylic acids and weak inorganic acid salts comprising a cation, wherein the cation is selected from the group consisting of sodium, potassium, ammonium, calcium and magnesium. Preferably in this embodiment, the first weak acid, the second weak acid and the third weak acid are selected from the group consisting of monocarboxylic acids, dicarboxylic acids, tricarboxylic acids and weak inorganic acid salts comprising a cation, wherein the cation is selected from the group consisting of sodium, potassium, ammonium, calcium and magnesium. More preferably the first weak acid is a carboxylic acid, the second weak acid is a carboxylic acid and the third weak acid is a carboxylic acid or a weak inorganic acid salt comprising a cation, wherein the cation is selected from the group consisting of sodium, potassium, ammonium, calcium and magnesium. Even more preferably the first weak acid is a monocarboxylic acid or a dicarboxylic acid, the second weak acid is a monocarboxylic acid, a dicarboxylic acid or a tricarboxylic acid, and the third weak acid is a monocarboxylic acid, a dicarboxylic acid or a weak inorganic acid salt comprising a cation, wherein the cation is selected from the group consisting of sodium, potassium, ammonium, calcium and magnesium. Preferably for this embodiment the cation is sodium or potassium.

[0064] In a preferred embodiment, the first weak acid is a carboxylic acid, preferably a mono- or di-carboxylic acid, the second weak acid is a carboxylic acid, preferably a mono-, di- or tri-carboxylic acid, and the third weak acid is a di-carboxylic acid, a tri-carboxylic acid or a weak inorganic acid salt comprising a cation selected from sodium, potassium, ammonium, calcium and magnesium. More preferably, the first weak acid is a mono- or di-carboxylic acid, the second weak acid is a mono-, di- or tri-carboxylic acid, and the third weak acid is a di-carboxylic acid or a weak inorganic acid salt comprising a cation selected from sodium, potassium, ammonium, calcium and magnesium.

[0065] Preferably, the first, second and third weak acids (when present) are selected from formic acid, adipic acid, citric acid, sodium bisulfate, oxalic acid, phthalic acid, salicylic acid, succinic acid, tartaric acid and maleic acid. More preferably, the first, second and third weak acids (when present) are selected from formic acid, oxalic acid, citric acid, galactaric acid, gallic acid, phthalic acid, succinic acid, tartaric acid and sodium bisulfate, most preferably from formic acid, gallic acid, oxalic acid, citric acid, tartaric acid and sodium bisulfate. Even more preferably, the first weak acid is selected from formic acid, citric acid, oxalic acid, phthalic acid and succinic acid, and the second and third weak acids (when present) are selected from citric acid, sodium bisulfate, galactaric acid, gallic acid, oxalic acid and tartaric acid; even more preferably, the first weak acid is formic acid or oxalic acid, and the second and third weak acids (when present) are selected from citric acid, sodium bisulfate, gallic acid, oxalic acid and tartaric acid.

[0066] Another embodiment of the present application relates to a zeolite composition, wherein a third weak acid is present and the first, second and third weak acids are selected from formic acid, citric acid, oxalic acid, tartaric acid, gallic acid and sodium bisulfate; preferably wherein a third weak acid is present, the first weak acid is selected from formic acid and oxalic acid, the second weak acid is selected from citric acid, tartaric acid, gallic acid and oxalic acid, and the third weak acid is selected from sodium bisulfate, tartaric acid, gallic acid and oxalic acid.

[0067] In another preferred embodiment, the first weak acid and the second weak acid are selected from the group consisting of monocarboxylic acids, dicarboxylic acids, and weak inorganic acid salts comprising a cation selected from the group consisting of sodium, potassium, ammonium, calcium, and magnesium. More preferably, the first weak acid is a monocarboxylic acid or a dicarboxylic acid, and the second weak acid is selected from the group consisting of monocarboxylic acids, dicarboxylic acids, and weak inorganic acid salts comprising a cation selected from the group consisting of sodium, potassium, ammonium, calcium, and magnesium. Even more preferably, the first weak acid is a dicarboxylic acid, and the second weak acid is selected from the group consisting of monocarboxylic acids, dicarboxylic acids, and weak inorganic acid salts comprising a cation selected from the group consisting of sodium, potassium, ammonium, calcium, and magnesium. Most preferably, the first weak acid is a dicarboxylic acid, and the second weak acid is a weak inorganic acid salt comprising a cation selected from the group consisting of sodium, potassium, ammonium, calcium, and magnesium. Preferably, for this embodiment, the cation is selected from the group consisting of sodium and potassium, more preferably, for this embodiment, the weak inorganic acid salt comprising a cation is sodium bisulfate. The combination of acids of this embodiment provides a zeolite composition that is suitable as a single tanning agent without the need for other tanning agents and has improved absorption compared to other acid combinations.

[0068] In one embodiment, preferably the first weak acid and the second weak acid are selected from the group consisting of phthalic acid, succinic acid, formic acid, oxalic acid, tartaric acid, gallic acid, and sodium bisulfate; more preferably wherein the first weak acid is selected from the group consisting of phthalic acid, succinic acid, formic acid, oxalic acid, and sodium bisulfate, and the second weak acid is selected from the group consisting of sodium bisulfate, tartaric acid, oxalic acid, and gallic acid; even more preferably the first weak acid is selected from the group consisting of phthalic acid, succinic acid, and oxalic acid, and the second weak acid is selected from the group consisting of sodium bisulfate, tartaric acid, oxalic acid, and gallic acid; most preferably the first weak acid is selected from the group consisting of phthalic acid, succinic acid, and oxalic acid, and the second weak acid is sodium bisulfate. The combination of acids of this embodiment provides a zeolite composition that is suitable as a single tanning agent without the need for other tanning agents and has improved absorption compared to other acid combinations.

[0069] In a preferred embodiment, the first weak acid and the second weak acid are selected from the group consisting of formic acid, oxalic acid, tartaric acid, gallic acid, and sodium bisulfate; preferably wherein the first weak acid is selected from the group consisting of formic acid, sodium bisulfate, and oxalic acid, and the second weak acid is selected from the group consisting of sodium bisulfate, tartaric acid, gallic acid, and oxalic acid; more preferably the first weak acid is selected from the group consisting of formic acid and oxalic acid, and the second weak acid is selected from the group consisting of sodium bisulfate, tartaric acid, gallic acid, and oxalic acid. The combination of acids of this embodiment provides a zeolite composition that is suitable as a single tanning agent without the need for other tanning agents and has improved penetration and absorption compared to other acid combinations.

[0070] In a preferred embodiment, the combination of the first weak acid with the second weak acid and the third weak acid (when present) is selected from the following combinations:

[0071] - the first weak acid is formic acid, and the second weak acid is tartaric acid,

[0072] - the first weak acid is formic acid, and the second weak acid is oxalic acid,

[0073] - the first weak acid is oxalic acid and the second weak acid is sodium bisulfate,

[0074] - the first weak acid is sodium bisulfate and the second weak acid is oxalic acid,

[0075] - the first weak acid is formic acid and the second weak acid is sodium bisulfate,

[0076] - the first weak acid is formic acid and the second weak acid is gallic acid,

[0077] - the first weak acid is formic acid, the second weak acid is citric acid, and the third weak acid is sodium bisulfate,

[0078] - the first weak acid is formic acid, the second weak acid is tartaric acid, and the third weak acid is sodium bisulfate,

[0079] - the first weak acid is formic acid, the second weak acid is oxalic acid, and the third weak acid is sodium bisulfate,

[0080] - the first weak acid is formic acid, the second weak acid is citric acid, and the third weak acid is oxalic acid,

[0081] - the first weak acid is oxalic acid, the second weak acid is gallic acid, and the third weak acid is tartaric acid,

[0082] - the first weak acid is oxalic acid, the second weak acid is tartaric acid, and the third weak acid is sodium bisulfate, and

[0083] - the first weak acid is formic acid, the second weak acid is oxalic acid, and the third weak acid is gallic acid.

[0084] More preferably, the combination of the first weak acid with the second weak acid and the third weak acid (when present) is selected from the following combinations:

[0085] - the first weak acid is formic acid, the second weak acid is citric acid, and the third weak acid is sodium bisulfate,

[0086] - the first weak acid is formic acid, the second weak acid is tartaric acid, and the third weak acid is sodium bisulfate,

[0087] - the first weak acid is formic acid, the second weak acid is oxalic acid, and the third weak acid is sodium bisulfate,

[0088] - the first weak acid is formic acid, the second weak acid is citric acid, and the third weak acid is oxalic acid,

[0089] - the first weak acid is oxalic acid, the second weak acid is gallic acid, and the third weak acid is tartaric acid,

[0090] - the first weak acid is oxalic acid, the second weak acid is tartaric acid, and the third weak acid is sodium bisulfate, and

[0091] - the first weak acid is formic acid, the second weak acid is oxalic acid, and the third weak acid is gallic acid.

[0092] Most preferably, the combination of the first weak acid with the second weak acid and the third weak acid, when present, is selected from the following combinations:

[0093] - the first weak acid is formic acid, the second weak acid is oxalic acid, and the third weak acid is sodium bisulfate. In a preferred embodiment, the combination of the first weak acid with the second weak acid is selected from the following combinations:

[0094] - the first weak acid is phthalic acid, and the second weak acid is sodium bisulfate,

[0095] - the first weak acid is succinic acid, and the second weak acid is sodium bisulfate,

[0096] - the first weak acid is formic acid, and the second weak acid is tartaric acid,

[0097] - the first weak acid is formic acid, and the second weak acid is oxalic acid,

[0098] - the first weak acid is oxalic acid, and the second weak acid is sodium bisulfate,

[0099] - the first weak acid is sodium bisulfate, and the second weak acid is oxalic acid,

[0100] - the first weak acid is formic acid, and the second weak acid is sodium bisulfate,

[0101] - the first weak acid is formic acid, and the second weak acid is gallic acid,

[0102] In a more preferred embodiment, the combination of the first weak acid with the second weak acid is selected from the following combinations:

[0103] - the first weak acid is phthalic acid, and the second weak acid is sodium bisulfate,

[0104] - the first weak acid is succinic acid, and the second weak acid is sodium bisulfate,

[0105] - the first weak acid is oxalic acid, and the second weak acid is sodium bisulfate.

[0106] The weak acid can comprise water prior to mixing with the zeolite. In order to keep the water content of the zeolite composition low, and in order to improve the effectiveness of the zeolite composition, it is preferred that the weak acid is a concentrated weak acid. It is preferred that the concentration of the weak acid is at least 80 wt.%, more preferably at least 90 wt.%, even more preferably at least 95 wt.%, based on the total weight of the weak acid.

[0107] Preferably the first, second and / or third weak acid is solid at 20 °C, more preferably the second and third weak acid (when present) is solid at 20 °C. In another embodiment, preferably the first, second and / or third weak acid is liquid at 20 °C, more preferably the first weak acid is liquid at 20 °C. Preferably, in case one or more weak acids are liquid at 20 °C, the ratio of total liquid weak acid to zeolite is such that the combination of liquid weak acid and zeolite forms a solid material, preferably a powder, after mixing. Preferably the ratio of total liquid weak acid to zeolite is lower than 70 wt.%, more preferably lower than 55 wt.%, most preferably lower than 35 wt.%. The wt.% of these ratios is the weight of total liquid weak acid divided by the weight of zeolite, expressed in percentage.

[0108] Zeolite composition

[0109] In the context of the present invention, tanning agent is leather tanning agent and tanning is leather tanning. The zeolite composition of the present invention comprises water. Preferably the zeolite composition comprises less than 25 wt.% water based on the total weight of the zeolite composition, more preferably less than 20 wt.% water based on the total weight of the zeolite composition, most preferably less than 15 wt.% water based on the total weight of the zeolite composition. A high water content negatively influences processability. Furthermore, a higher water content is not desirable as water does not increase the functionality of the tanning; the presence of water increases the transportation costs. Furthermore, the present inventors believe that a low water content is necessary for positive tanning results as it improves the zeolite-acid interaction.

[0110] The zeolite composition comprises zeolite. Preferably the zeolite composition comprises at least 34 wt.% zeolite based on the total weight of the zeolite composition, more preferably at least 50 wt.% zeolite based on the total weight of the zeolite composition, most preferably at least 60 wt.% zeolite based on the total weight of the zeolite composition. A higher concentration of zeolite results in a more economic use of the zeolite composition as a single tanning agent, as the zeolite interacts with the collagen of the leather and provides tanning. Preferably the structure of the zeolite is intact in the zeolite composition, i.e. analysis shows that there is no breakage or disintegration of the zeolite structure.

[0111] The zeolite composition comprises a first weak acid, a second weak acid and optionally a third weak acid. Preferably, the zeolite composition comprises a first weak acid, a second weak acid and a third weak acid. Preferably, the amount of the first weak acid is 2 to 35 wt.%, more preferably 5 to 25 wt.%, even more preferably 7.5 to 20 wt.% based on the total weight of the zeolite composition. Preferably, the amount of the second weak acid is 2 to 25 wt.%, more preferably 5 to 20 wt.%, most preferably 10 to 15 wt.% based on the total weight of the zeolite composition. Preferably, the amount of the third weak acid (when present) is 2 to 25 wt.%, preferably 5 to 20 wt.%, most preferably 10 to 15 wt.% based on the total weight of the zeolite composition.

[0112] Preferably, the amount of the first weak acid is 2 to 35 wt.%, more preferably 5 to 25 wt.%, even more preferably 7.5 to 20 wt.% based on the total weight of the zeolite composition, and preferably, the amount of the second weak acid is 2 to 25 wt.%, more preferably 5 to 20 wt.%, most preferably 10 to 15 wt.% based on the total weight of the zeolite composition, and preferably, the amount of the third weak acid (when present) is 2 to 25 wt.%, preferably 5 to 20 wt.%, most preferably 10 to 15 wt.% based on the total weight of the zeolite composition.

[0113] Preferably, the zeolite composition does not comprise tanning salts, more preferably the zeolite composition does not comprise tanning salts, synthetic tannins or vegetable tannins. Preferably, the zeolite composition comprises less than 20 wt.%, more preferably less than 10 wt.%, even more preferably less than 5 wt.% of tanning salts based on the total weight of the zeolite composition; more preferably, the zeolite composition comprises less than 20 wt.%, more preferably less than 10 wt.%, even more preferably less than 5 wt.% of the total amount of tanning salts, synthetic tannins and vegetable tannins based on the total weight of the zeolite composition. Preferably, the zeolite composition comprises less than 5 wt.% of aluminium sulfate based on the total weight of the zeolite composition, most preferably, the zeolite composition does not comprise aluminium sulfate.

[0114] Preferably, the zeolite composition is a powder.

[0115] Method for manufacturing a zeolite composition

[0116] The zeolite is a powder. In the method for manufacturing the zeolite composition, the zeolite is mixed with two or more weak acids. Depending on the aggregation state of the weak acids, the manufacturing method needs to be chosen.

[0117] In case the first weak acid, the second weak acid and the third weak acid, when present, are solids at 20 °C, mixing powders is involved. The technique of mixing powders is commonly used in the art. In some cases, commonly known measures can be required to remove heat, as the zeolite powders can interact to some extent with the weak acid powders. Therefore, in case the first weak acid, the second weak acid and the third weak acid, when present, are solids at 20 °C, in the process for preparing the zeolite composition, the temperature of the mixture during mixing is kept below 100 °C, preferably below 90 °C, most preferably below 60 °C. In case all weak acids are solids, the order of mixing the different components does not have a particular meaning.

[0118] In case any of the first weak acid, the second weak acid or the third weak acid, when present, is a liquid at 20 °C, combining the zeolite and the liquid weak acid(s) can generate considerable heat, which can affect the product structure of the zeolite composition. Furthermore, combining a liquid weak acid with a solid weak acid, thus liquid at 20 °C and solid at 20 °C, can lead to caking, which will result in poor handling properties and undesired inhomogeneity of the further mixture with the zeolite. Therefore, in case any of the first weak acid, the second weak acid or the third weak acid, when present, is a liquid at 20 °C, in the process for preparing the zeolite composition, the liquid weak acid(s) is / are first mixed with the zeolite, before the other solid weak acid(s) is / are mixed with the obtained mixture. Mixing of powders and liquids is common in the art. The mixing of a zeolite and a liquid weak acid is known from WO 2013 / 045764 A1.

[0119] In case any of the first weak acid, the second weak acid or the third weak acid, when present, is a liquid at 20 °C, in the process for preparing the zeolite composition, the temperature of the mixture during mixing is kept below 100 °C, preferably below 90 °C, most preferably below 60 °C.

[0120] In the process for preparing the zeolite, mixing is preferably carried out in a closed vessel, and the mixture is cooled, e.g. using a cooling jacket. In one embodiment of the process for preparing the zeolite, in case any of the first weak acid, the second weak acid or the third weak acid, when present, is a liquid at 20 °C, the liquid weak acid(s) at 20 °C is / are continuously provided over a period of at least 10 minutes, while step ii) is carried out.

[0121] Process for producing leather

[0122] The production of leather is well known in the art. Typically, for producing leather, first a hide is pretreated, followed by tanning. Typically, the last pretreatment step before tanning is pickling the hide in a pickling bath. In most cases, the tanning step is followed by one or more post-tanning steps to further tailor the leather properties.

[0123] In one embodiment, the present application relates to a method of producing leather, comprising a tanning step, wherein the hide is contacted with a tanning liquid, the tanning liquid comprising the zeolite composition of the present application, and the concentration of the zeolite composition is 1 wt.% to 15 wt.% based on the weight of the hide, preferably the concentration of the zeolite composition is 3 wt.% to 10 wt.% based on the weight of the hide, most preferably the concentration of the zeolite composition is 4 wt.% to 8 wt.% based on the weight of the hide.

[0124] In the context of the present application, the weight of the hide is the weight of the limed hide.

[0125] Preferably, the hide is contacted with the tanning liquid comprising the zeolite composition of the present application for a period of 10 to 1500 minutes, more preferably 50 to 500 minutes, even more preferably 100 to 300 minutes. Preferably, the contacting of the hide with the tanning liquid comprising the zeolite composition of the present application is carried out at a temperature of 10 °C to 95 °C, preferably 15 °C to 75 °C, more preferably 20 °C to 55 °C. Preferably, the concentration of the tanning liquid is 100 wt.% or less based on the weight of the hide, more preferably 50 wt.% or less based on the weight of the hide, even more preferably 35 wt.% or less based on the weight of the hide. A low concentration of the tanning liquid improves the tanning efficiency and keeps the environmental impact low.

[0126] The hide is pickled before tanning as is common in the art. Pickling makes the fibers of the hide more susceptible to tanning. Preferably, in the method of producing leather, the hide is pickled before tanning, wherein pickling is contacting the hide with a pickling liquid comprising one or more acids and salts. Preferably, the concentration of the pickling liquid is 100 wt.% or less based on the weight of the hide, more preferably 50 wt.% or less based on the weight of the hide, even more preferably 35 wt.% or less based on the weight of the hide.

[0127] Preferably, the pickling liquid comprises an organic acid. An organic acid is necessary for good quality of the leather. Preferably, the concentration of the organic acid in the pickling liquid is 1 wt.% to 5 wt.% based on the weight of the hide, more preferably 2 wt.% to 3 wt.% based on the weight of the hide, most preferably 2.25 wt.% to 2.75 wt.% based on the weight of the hide. Preferably, the organic acid in the pickling liquid is one or more selected from the group consisting of formic acid, acetic acid, and oxalic acid. The presence of salts in the pickling liquid is for balancing the ionic strength. The pickling liquid preferably comprises 1 wt.% to 10 wt.% of salts based on the weight of the hide, more preferably 4 wt.% to 8 wt.% of salts based on the weight of the hide. Preferably, the initial pH of the pickling liquid is below 3.5, more preferably below 3, most preferably below 2.8. The initial pH of the pickling liquid is the pH of the liquid just before the tanning agent is added to the pickling liquid comprising the hide.

[0128] Preferably in the method of producing leather, the tanning liquid comprising the hide is brought to a higher pH by adding the zeolite composition of the present invention to the pickling liquid comprising the hide.

[0129] As is common in the art, the tanning liquid comprising the hide is brought to a higher pH at the end of the tanning step. In this step, the zeolite is activated to interact with the collagen of the hide. Preferably in the method of producing leather, the pH of the tanning liquid comprising the hide is increased to a pH of preferably 4.0 to 6.0, more preferably 4.5 to 5.5, most preferably 4.8 to 5.3 at the end of the tanning step. The pH is preferably increased by adding one or more alkaline reagents selected from the group consisting of sodium hydroxide, sodium bicarbonate, sodium carbonate, soda ash, magnesium oxide, magnesium carbonate, dolomite and potassium hydroxide. Preferably the alkaline reagents are added at different moments in time, preferably wherein said moments in time are at least 5 minutes apart. Preferably the alkaline reagents are added in a total amount of more than 1 wt.%, more preferably 2 wt.% based on the weight of the hide added.

[0130] Leather

[0131] In one embodiment, the present invention relates to leather obtained by a method of producing leather, said method comprising a tanning step wherein a hide is contacted with a tanning liquid, said tanning liquid comprising the zeolite composition of the present invention and the concentration of the zeolite composition is 1 wt.% to 15 wt.% based on the weight of the hide. The present invention also relates to leather.

[0132] The leather of the present invention can be characterized in various ways. The leather shrinkage temperature or shrinkage temperature (Ts) can be determined according to ASTM D D6076-08 and represents the temperature at which a fully wetted leather sample undergoes shrinkage. The shrinkage temperature is representative of the hydrothermal stability of the leather and has developed into an industry standard for quality control of tanned leather. Preferably the shrinkage temperature of the leather of the present invention is higher than 60°C, more preferably higher than 70°C. Preferably the shrinkage temperature of the leather of the present invention is lower than 100°C, more preferably lower than 90°C. A high shrinkage temperature is typical for chrome tanned leather.

[0133] The leather of the present invention also has a surface charge and an isoelectric point (pi). The isoelectric point can be determined as known in the art. Wang et al. describe a method to determine the isoelectric point (JALCA, Vol. 112, 2017, p224) in which the zeta potential of the leather is measured at different pH values and the pi is considered to be the pH at which the zeta potential is zero. Preferably the isoelectric point of the leather is determined by measuring the zeta potential of the leather at different pH values, wherein the isoelectric point is the pH value at which the zeta potential is zero. The pi of the leather depends on the type of tanning agent. Metal salt tanned leather has an isoelectric point higher than 6.5 because the metal interacts with the leather through cationic association (positively charged metal ions interact with negatively charged collagen). Synthetic or vegetable tanned leather has an isoelectric point lower than 5 because the active groups of the synthetic or vegetable tanning agent interact through anionic association with the leather (negatively charged active groups interact with positively charged collagen). The present inventors now found that by tanning with the zeolite composition of the present invention, a leather can be obtained with an isoelectric point lower than 5. Preferably the isoelectric point of the leather of the present invention is between 3 and 5, more preferably between 3.5 and 4.5.

[0134] Tanning agent of the leather

[0135] The leather of the present invention comprises the elements aluminium (Al) and silicon (Si). Both aluminium and silicon are homogeneously distributed in the leather of the present invention. Preferably the leather comprises more than 0.3 wt.% of aluminium based on the dry weight of the leather, more preferably more than 0.5 wt.% of aluminium based on the dry weight of the leather, most preferably more than 1 wt.% of aluminium based on the dry weight of the leather. Preferably the leather comprises more than 0.3 wt.% of silicon based on the dry weight of the leather, more preferably more than 0.5 wt.% of silicon based on the dry weight of the leather, most preferably more than 1 wt.% of silicon based on the dry weight of the leather. The amount of aluminium can be determined by the method ISO 17072-2:2019 for the content of mineral tanning agent in leather after digestion. For silicon, the amount can be based on a modified method of ISO 17072-2:2019 or based on the relative intensities of aluminium and silicon in SEM-EDX analysis. Preferably the amount of aluminium (Al) based on the dry weight of the leather is determined according to 17072-2:2019. For this method, the leather is dried before analysis. The dry weight of the leather mentioned with respect to the aluminium and silicon concentration is the weight of the leather dried according to ISO 17072-2:2019.

[0136] In the production of leather, a uniform distribution of the tanning agent is sought. For tanning with a zeolite composition as tanning agent, it is believed that after tanning, the zeolite absorbed by the leather is not intact, but also not degraded into its elements; it is believed that the zeolite is present in the leather in a network. For tanning with a zeolite, a uniform distribution can be established by measuring the distribution of aluminium and silicon in the leather. For a leather of the invention with a uniform distribution of the zeolite composition, or in other words, for a preferred leather with a uniform distribution of aluminium and silicon in the leather. A uniform distribution is important, because the appearance as well as the physical and chemical stability and properties are dependent on the uniform distribution. A uniform distribution is known in the art as full penetration and can be determined by tests known in the art. Penetration refers to the distribution of the tanning agent over the surface of the leather as well as the distribution of the tanning agent perpendicular to the surface of the leather. The distribution over the surface is determined by sensory inspection: by inspecting whether there are no significant differences in colour and / or feel of the leather over the surface. A good penetration or full penetration is achieved when the tanning agent is uniformly distributed, so when the tanning agent has a uniform distribution over the surface of the leather as well as perpendicular to the surface of the leather. For tanning with a zeolite composition as tanning agent, a uniform distribution is present when the lowest local concentration of aluminium and / or silicon in the leather is at least 20% of the highest local concentration of aluminium and / or silicon in the leather. A leather that is not fully penetrated can show colour differences between different areas of the surface, and show differences in feel and physical-chemical properties between different areas of the surface. Furthermore, because usually the leather is skived after tanning, a new surface will be formed underneath the surface before and during tanning. Usually, if the penetration in the plane perpendicular to the surface is not sufficient, so if the leather is not fully penetrated, after skiving, a burnt area will appear on the newly formed surface.

[0137] It is preferred that the leather has a uniform distribution of the zeolite composition, wherein uniform distribution indeed means that the leather is fully penetrated.

[0138] A method to determine whether a leather has a uniform distribution of the zeolite composition is to measure the distribution of aluminium and silicon in the leather. The spatial distribution of silicon and aluminium can be determined by SEM-EDX. By SEM-EDX the (relative) concentration distribution of aluminium and silicon can be measured. A leather sheet can be cut perpendicular to the surface direction, exposing a vertical cross section. The surface of the vertical cross section can be scanned with a finely focused electron beam (SEM). The X-rays emitted due to excitation with the electron beam can be measured by energy dispersive X-ray spectroscopy (EDX). In this way, the elemental composition of each position on the vertical cross section can be determined. Intensity profiles of aluminium and silicon over the vertical cross section of the leather sheet can be constructed based on this measurement. The intensity is linearly related to the (local) concentration.

[0139] Leather can be defined by an upper side surface (grain side) and a lower side surface (flesh side) extending in parallel and a perpendicular cross section perpendicular to these surfaces. Preferably, for the leather of the present invention, the concentration of aluminum at the center of the perpendicular cross section is at least 20%, preferably at least 30%, most preferably at least 50% of the concentration at each surface side of the perpendicular cross section, and / or wherein for the leather of the present invention, the concentration of silicon at the center of the perpendicular cross section is at least 20%, preferably at least 30%, most preferably at least 50% of the concentration at each surface side of the perpendicular cross section. Preferably, the length of the perpendicular cross section is at least 0.5 millimeter, more preferably at least 1 millimeter, most preferably at least 2 millimeters.

[0140] The ratio of silicon to aluminum of the leather of the present invention is constant compared to leather obtained by other tanning methods, for example tanning with a combination of zeolites and aluminum salts as tanning agents. The constant ratio guarantees a similar surface coverage and related surface charge throughout the leather, which is beneficial for further processing of the leather and uniform leather quality. Preferably, the ratio of aluminum to silicon at the center of the perpendicular cross section of the leather is within 40%, more preferably within 60%, most preferably within 75% of the ratio of aluminum to silicon at each surface side of the perpendicular cross section.

[0141] Dissolution

[0142] When the dissolution value is low relative to the total content, the tanning interaction is considered strong and the tanning agent is well fixed. The dissolution of a specific mineral element from the leather can be determined by the industrial standard method “Determination of soluble mineral tanning agents in leather” ISO 17072-1 :2019. The better the uptake of the tanning agent, the higher the content in the leather and the better the tanning interaction, the lower the soluble mineral tanning agent. For a specific metal element of the tanned leather, the soluble mineral tanning agent is expressed in percentage of the total content of the mineral tanning agent in the leather. Preferably, for Si, the soluble mineral tanning agent is below 5 wt.%, preferably below 2 wt.%, more preferably below 1 wt.% relative to the total tanning agent in the leather. Preferably, for Al, the soluble mineral tanning agent is below 5 wt.%, preferably below 2 wt.%, more preferably below 1 wt.% relative to the total tanning agent in the leather.

[0143] Considering that the zeolite composition provides excellent tanning results, there is no need to use other tanning agents and the leather of the present invention does not comprise other tanning agents. Preferably, the leather of the present invention does not comprise chromium, more preferably the leather of the present invention does not comprise chromium, titanium and zirconium, even more preferably the leather of the present invention does not comprise chromium, titanium, zirconium and synthetic tanning agents, most preferably the leather of the present invention does not comprise chromium, titanium, zirconium, synthetic tanning agents and vegetable tanning agents.

[0144] Examples

[0145] Example 1 - Tanning of leather with a single tanning agent comprising a zeolite and a weak acid

[0146] Preparation

[0147] Single tannins were prepared by mixing zeolite powder (water content less than 20 wt.% of the total weight of the zeolite powder) and weak acid. For all single tannins, the zeolite used was A-type zeolite.

[0148] In case the weak acid is a liquid at room temperature, the single tannins were prepared according to the method of WO 2013 / 045764 Al (Example 2). In short, the concentrated liquid weak acid was slowly and continuously added to the zeolite while mixing with a dynamic mixer. The temperature was kept below 85°C by mixing and by external cooling. A powder was obtained.

[0149] In case the weak acid is a solid, the zeolite and weak acid were dry mixed. Some heat was generated during dry mixing, but the temperature did not exceed 50°C and no cooling was required. The single tannins prepared are listed in Table 2. Also a reference (variant 1A) was tested, which did not contain a weak acid. Each weak acid was added to the zeolite in a concentrated form, which had a water content of less than 20 wt.%. For each single tannin, the zeolite content was 65 wt.% of the total weight and the weak acid content was 35 wt.% of the total weight. The water content of the final single tannin was in all cases less than 20 wt.% of the total weight of the single tannin. Each example tanned about 12 kg of raw hide.

[0150] Tanning

[0151] The dehaired bull hides were limed, delimed and bated, washed, then pickled and tanned according to industrial standard methods. The recipes of Table 1 are as follows. In this table, the weight percentages of the components used for pickling and tanning are relative to the weight of the limed hide. Between brackets is the dilution factor of the added components. The components were diluted with water before addition. The concentration of the sulphuric acid before dilution was 98% (total weight percentage) and the concentration of the formic acid before dilution was 85% (total weight percentage). The single tannins were added at the concentrations listed in Table 1 and Table 2 (“tannin concentration”). The industrial standard is to let the hide stand in the pickling liquor overnight. This is also possible for the zeolite tanning. For the variants of Examples 1 to 3, overnight pickling was tested and did not affect the results. The hide was let stand overnight at 5 minutes rotation / hour after the last addition of sulphuric acid. If necessary, a final adjustment of the pH was made in the morning and the single tannin was added.

[0152] Analysis

[0153] The leather samples were subjected to sensory evaluation by a panel of experts. The leather was evaluated for leather feel, where the hardness (smooth, firm, hard, very hard) and fullness (empty, average, full) were determined. In addition, the penetration was evaluated; the incision was visually observed and the even distribution and penetration of the surface area of the hide was observed. The penetration was also observed by SEM-EDX analysis of cross sections of different locations. The penetration was summarized as “bad”, “partially”, “completely” and “empty spots” when some parts of the leather were not penetrated. The shrinkage temperature was determined by differential scanning calorimetry (DSC) according to ASTM D6076-08 known in the art. In the art, DSC is the accepted method used in D6076-08. The absorption was determined based on aluminum. High absorption is preferred, as in that case the single tanning agent is effective. For the examples, the amount of aluminum per dry weight of tanned leather was measured by ICP-MS and multiplied by the total amount of dry tanned leather, resulting in the amount of aluminum retained. The amount of aluminum supplied was calculated based on the amount of single tanning agent added. The ICP-MS was performed according to ISO 17072-2:2019.

[0154]

[0155]

[0156] Table 1 : Tanning formulation

[0157] For some examples, the dissolution was determined according to the industry standard method “Determination of soluble mineral tanning agents in leather” ISO 17072-1 :2019. Dissolution refers to the amount of mineral that can be extracted from the leather after tanning with the tanning agent, and wherein the mineral is the mineral of the tanning agent. For the examples, the mineral that was determined was aluminum. The dissolution is expressed as a percentage of the total content of the mineral in the leather.

[0158] Results

[0159] The resulting leather properties are listed in Table 2. The zeolite as single tanning agent without the addition of an acid resulted in poor tanning results at all concentrations applied. Although the zeolite increased the shrinkage temperature, the penetration and leather feel were poor. The introduction of a weak acid in the single tanning agent slightly improved the feel and penetration, but the feel and penetration were still not acceptable. The leather feel could be controlled by the choice of weak acid. The leather retained its natural color without fading. Higher amounts of the single tanning agent increased the shrinkage temperature, but did not increase any other parameter.

[0160]

[0161]

[0162] Table 2: Results of single tanning agents comprising zeolite and weak acid. The tanning agent concentration is based on the weight of the limed hide.

[0163] Example 2 - Tanning of leather with a single tanning agent comprising a zeolite, a first weak acid and a second weak acid

[0164] The zeolite and the first weak acid were mixed according to Example 1. The obtained mixture was mixed together with a second weak acid to obtain a single tanning agent. The combinations tested are listed in Table 3. Each weak acid was added in concentrated form having a water content of less than 20 wt.%. For each single tanning agent, the zeolite content was 70 wt.% of the total weight of the single tanning agent, the first weak acid content was 15 wt.% of the total weight of the single tanning agent, and the second weak acid content was 15 wt.% of the total weight of the single tanning agent. The water concentration of the final single tanning agent was below 20 wt.% of the total weight of the single tanning agent in all cases. The leather was tanned according to Example 1 (Table 1). The amount of single tanning agent added during tanning is in Table 3.

[0165] The tanning efficiency and the obtained leather were evaluated according to Example 1, the results are listed in Table 3. In general, the use of a single tanning agent comprising two weak acids improved the tanning performance compared to a single tanning agent comprising one weak acid. For variants 2F to 2K, improved penetration and a wider range of structural attributes could be obtained. For combinations (2D to 2K), the absorption was improved. The shrinkage temperature of Example 2 was in the same range as Example 1. The leather kept its natural color without discoloration.

[0166]

[0167] Table 3: Results of single tanning agents comprising a zeolite, a first weak acid and a second weak acid. The tanning agent concentration is based on the weight of the limed hide.

[0168] Example 3 - Tanning of leather with a single tanning agent comprising a zeolite, a first weak acid, a second weak acid and a third weak acid

[0169] The zeolite and the first weak acid were mixed according to Example 1. The obtained mixture was mixed together with a second weak acid and a third weak acid to obtain a single tanning agent. The combinations tested are listed in Table 4. Each weak acid was added in concentrated form to the zeolite, the concentrated form having a water content of less than 20 wt.%. For each single tanning agent, the zeolite content was 60 wt.% of the total weight of the single tanning agent, the first weak acid content was 10 wt.% of the total weight of the single tanning agent, the second weak acid content was 15 wt.% of the total weight of the single tanning agent, and the third weak acid content was 15 wt.% of the total weight of the single tanning agent. The water concentration of the final single tanning agent was below 20 wt.% of the total weight of the single tanning agent in all cases. The leather was tanned according to Example 1 (Table 1). The amount of single tanning agent added during tanning is in Table 4. Unlike Example 1 and Example 2, the average total weight of the hide in each trial was about 65 kg.

[0170]

[0171] Table 4: Results of single tannage comprising zeolite, first weak acid, second weak acid and third weak acid. Tannage concentration based on pickled hide weight.

[0172] The leather obtained according to example 1 was evaluated, the results are listed in table 4. In general, the use of a single tannage comprising three weak acids improves the tanning performance compared to single tannages comprising one or two acids. Improved penetration and absorption and a broader range of structural properties can be obtained. In addition, the shrink temperature is increased. The leather retains its natural color without discoloration.

[0173] Surprisingly, a single tannage comprising zeolite and three acids provides excellent tanning results without the addition of any other tannage. The single tannage is easy to use, as it can be added as a powder. In addition, the known drawbacks of conventional tannages can be prevented, such as toxicity (e.g. chromium and aluminum salts, aldehydes), long penetration times (natural tannins and tannin extracts), solubility (aluminum, zirconium titanium salts), limited leather article applications (aluminum, zirconium titanium salts), consistent leather quality (aluminum, zirconium titanium salts), limited color (natural tannins and tannin extracts) and discoloration of the leather (e.g. chromium salts, iron salts).

[0174] Example 4 - Tanning of leather with a single tanning agent comprising a weak inorganic acid salt comprising aluminum

[0175] As a supplement to example 3, leather was tanned in analogy to the method of example 3 with a single tannage comprising three weak acids, wherein one acid is a weak inorganic acid salt comprising aluminum. Compared to example 3, MgO was used instead of sodium bicarbonate to increase the pH at the end of tanning, MgO is known to improve absorption compared to sodium bicarbonate. The single tannage was prepared according to example 3. The results are listed in table 5. The leather with this single tannage felt and penetrated less well. The replacement of part of the weak acids of the single tannage of example 2a (single tannage comprising zeolite, formic acid and citric acid) by aluminum sulfate decreased the shrink temperature and the leather felt worse, in addition, the absorption was hardly improved even when MgO was used to increase the pH.

[0176]

[0177] Table 5: Results of single tannage comprising zeolite, first weak acid, second weak acid and third weak acid, wherein one of the acids is a weak inorganic acid salt with aluminum as cation. Tannage concentration based on pickled hide weight.

[0178] Example 5 - Large scale application of a single tanning agent

[0179] In a large scale test, the zeolite composition according to combination 3C in table 4 was applied as single tannage. The zeolite composition was prepared according to example 3.

[0180] Tanning

[0181] Depilatory bullhides were limed, delimed and bated according to industrial standard methods, washed and then the hides were pickled and tanned. The recipes of Table 6 are as follows. In this table, the weight percentages of the components used for pickling and tanning are relative to the weight of the limed hide. Between brackets is the dilution factor of the added components. The components were diluted with water before addition. The concentration of sulphuric acid before dilution was 98% (total weight percentage), the concentration of formic acid before dilution was 85% (total weight percentage). The single tanning agent was added at the concentration listed in Table 6 and Table 7 (column “Tanning agent concentration”). Large scale tests were performed with hides of a total weight of 550 kg, which is a typical amount for large scale industrial leather tanning applications. The leather was tanned according to the table below.

[0182]

[0183] Table 6: Large scale tanning recipes

[0184] The results of the large scale tests are listed in Table 7. The large scale experiments show excellent tanning results with full penetration, very high uptake and good shrinkage temperature.

[0185] The aluminium content of Table 7 was measured by ICP-MS according to ISO 17072-2:2019.

[0186] Furthermore, the tanned leather pieces obtained by the large scale tests were analysed by SEM-EDX. The (relative) concentration profiles of aluminium and silicon were measured by SEM-EDX. The leather pieces were cut perpendicular to the surface direction, exposing a vertical cross section. The surface of the vertical cross section was scanned with a fine focused electron beam (SEM). The X-rays emitted due to excitation with the electron beam were measured by energy dispersive X-ray spectroscopy (EDX). The analysis was performed with a Tescan Vega 3 SEM equipped with an Oxford Instruments X-Max 80 X-ray source, the results were analysed by the corresponding software provided by the instrument supplier.

[0187] In this way, the elemental composition of each position on the vertical cross section can be determined. The intensity profiles of aluminium and silicon on the vertical cross section of the leather pieces were constructed based on this measurement Figure 1A and Figure 1B ). The left side (around 200 pm) is the flesh side, the right side (around 1800 pm) is the grain side. The intensity is linearly related to the concentration. The upper dotted line is the concentration / intensity at the surface, the lower dotted line is the concentration / intensity at the centre of the vertical cross section of the leather piece. The increase in intensity from around 1900 pm or higher is due to an anomaly related to the tilt of the image, resulting in higher intensities due to the increased measurement area.

[0188] Figure 1A and Figure 1BThe single tannage has penetrated throughout the leather. The leather does not typically contain silicon and aluminum, and the figures show that both silicon and aluminum from the single tannage are present throughout the vertical cross-section. The aluminum concentration at the center of the vertical cross-section is about 50% of the concentration at the surface (7000 / 14000 is 50%). The silicon concentration at the center of the vertical cross-section is about 40% of the concentration at the surface (8000 / 20000 is 40%). The strength ratio of aluminum to silicon is constant throughout the cross-section.

[0189] The isoelectric point of the leather was 3.8. The isoelectric point was determined on a SurPass® electrokinetic analyzer (SurPass® Anton Paar) for solid surface analysis. The leather sample was dried in air, split and cut to a diameter of 12.7 mm and mounted on a gap cell. The starting solution contained 0.01 M KCI adjusted to pH 9.8 with KOH, and was titrated with 0.01 M HC1 during the experiment. During the experiment, the pressure gradient and current were measured to allow calculation of the zeta potential. TM

[0190]

[0191] Table 7: Results of large scale tannage using a single tannage containing a zeolite, a first weak acid, a second weak acid, and a third weak acid. The tannage concentration is based on the weight of the limed hide. The aluminum and silicon concentrations refer to the amount in the tanned leather (aluminum / silicon weight based on the total weight of the leather).​

Claims

1. A zeolite composition suitable as a single tanning agent, comprising a zeolite, a first weak acid, a second weak acid and optionally a third weak acid, wherein the first weak acid, the second weak acid and the third weak acid are different acids, the amount of the zeolite is at least 50 wt.% based on the total weight of the zeolite composition, and the amount of water is less than 25 wt.% based on the total weight of the zeolite composition, and wherein the first weak acid is a monocarboxylic acid or a dicarboxylic acid, and the second weak acid is selected from the group consisting of a monocarboxylic acid, a dicarboxylic acid and a weak inorganic acid salt comprising a cation, wherein the cation is selected from the group consisting of sodium, potassium, ammonium, calcium and magnesium, and wherein the amount of the first weak acid is from 2 wt.% to 35 wt.% based on the total weight of the zeolite composition, and the amount of the second weak acid is from 2 wt.% to 25 wt.% based on the total weight of the zeolite composition, and wherein the amount of the third weak acid, when present, is from 2 wt.% to 25 wt.% based on the total weight of the zeolite composition, wherein the zeolite is a zeolite having the general formula (Cat 2 / n O) X (Al203)(Si02) Y wherein Cat is a cation, O is oxygen, Si is silicon, and Al is aluminum, and the Al203weight percent is from 25 wt.% to 40 wt.% and the Si02weight percent is from 28 wt.% to 40 wt.% based on the total weight of the zeolite, and wherein the weak acid is an acid having an acid dissociation constant value higher than -1.74, and wherein the zeolite composition is a powder, wherein the zeolite composition comprises less than 5 wt.% aluminium sulfate based on the total weight of the zeolite composition.

2. The zeolite composition according to claim 1, wherein the amount of water is less than 20 wt.% based on the total weight of the zeolite composition, the amount of the first weak acid is from 5 wt.% to 25 wt.% based on the total weight of the zeolite composition, the amount of the second weak acid is from 5 wt.% to 20 wt.% based on the total weight of the zeolite composition, and wherein the amount of the third weak acid, when present, is from 5 wt.% to 20 wt.% based on the total weight of the zeolite composition.

3. The zeolite composition according to claim 1 or 2, wherein the first weak acid is a dicarboxylic acid, and the second weak acid is a monocarboxylic acid, a dicarboxylic acid or a weak inorganic acid salt comprising a cation, wherein the cation is selected from the group consisting of sodium, potassium, ammonium, calcium and magnesium.

4. The zeolite composition according to claim 1 or 2, wherein the first weak acid is selected from the group consisting of phthalic acid, succinic acid, formic acid, oxalic acid, tartaric acid and gallic acid, and the second weak acid is selected from the group consisting of phthalic acid, succinic acid, formic acid, oxalic acid, tartaric acid, gallic acid and sodium bisulfate.

5. The zeolite composition according to claim 4, wherein the first weak acid is selected from the group consisting of phthalic acid, succinic acid and oxalic acid, and the second weak acid is selected from the group consisting of sodium bisulfate, tartaric acid, gallic acid and oxalic acid.

6. The zeolite composition according to claim 1 or 2, wherein the first weak acid is selected from the group consisting of formic acid, oxalic acid, tartaric acid and gallic acid, and the second weak acid is selected from the group consisting of formic acid, oxalic acid, tartaric acid, gallic acid and sodium bisulfate.

7. The zeolite composition according to claim 6, wherein the first weak acid is selected from the group consisting of formic acid and oxalic acid, and the second weak acid is selected from the group consisting of sodium bisulfate, tartaric acid, gallic acid and oxalic acid.

8. The zeolite composition according to claim 1 or 2, wherein the third weak acid is present, the first weak acid is selected from the group consisting of formic acid, oxalic acid, tartaric acid and gallic acid, the second weak acid is selected from the group consisting of formic acid, oxalic acid, tartaric acid, gallic acid and sodium bisulfate, and the third weak acid is selected from the group consisting of formic acid, citric acid, oxalic acid, tartaric acid, gallic acid and sodium bisulfate.

9. The zeolite composition according to claim 8, wherein the third weak acid is present, and the first weak acid is selected from the group consisting of formic acid and oxalic acid, and the second weak acid is selected from the group consisting of tartaric acid, gallic acid and oxalic acid, and the third weak acid is selected from the group consisting of sodium bisulfate, tartaric acid, gallic acid and oxalic acid.

10. The zeolite composition according to claim 1 or 2, wherein the zeolite composition does not comprise aluminum sulfate.

11. The zeolite composition according to claim 1, wherein the composition does not comprise a synthetic tanning agent or a vegetable tanning agent.

12. A method of making the zeolite composition according to claim 1, comprising: a) in case the first weak acid, the second weak acid and, when present, the third weak acid are solid at 20 °C i) providing a mixture by mixing the first weak acid, the second weak acid, the zeolite and, when present, the third weak acid, while keeping the temperature of the mixture below 100 °C during mixing; or b) in case any of the first weak acid, the second weak acid or, when present, the third weak acid is liquid at 20 °C ii) mixing the weak acid or weak acids that are liquid at 20 °C with the zeolite, while keeping the temperature of the mixture below 100 °C during mixing, and iii) mixing any remaining weak acid or weak acids with the mixture obtained in step ii), while keeping the temperature of the mixture below 100 °C during mixing.

13. A method of producing leather, the method comprising a tanning step, wherein a hide is contacted with a tanning liquid, the tanning liquid comprising the zeolite composition of claim 1, and the concentration of the zeolite composition is from 1 wt.% to 15 wt.% based on the weight of the hide.

14. The method according to claim 13, wherein no tanning agent based on chromium, aluminum, titanium, zirconium and iron salts is contacted with the hide before, during or after the tanning step.

15. The method according to claim 13 or 14, further comprising one or more of a retanning step, a fat liquoring step, a finishing step or any combination thereof.

16. Leather, the leather having Shrinkage temperature above 60°C, and 3 to 5 isoelectric point, and the leather comprises more than 0.5 wt.% of aluminum based on the dry weight of the leather and more than 0.5 wt.% of silicon based on the dry weight of the leather, wherein the shrinkage temperature is determined according to ASTM D6076-08 and the isoelectric point of the leather is determined by measuring the zeta potential of the leather at different pH values, wherein the isoelectric point is the pH value at which the zeta potential is zero, wherein the amount of aluminum is measured according to ISO method ISO 17072-2:2019 and the amount of silicon is determined based on the relative intensities of aluminum and silicon determined by SEM-EDX analysis and based on the amount of aluminum measured according to ISO method ISO 17072-2:2019, wherein the leather is obtained by a method of producing leather, the method comprising a tanning step, wherein a hide is contacted with a tanning liquid, the tanning liquid comprising the zeolite composition of claim 1 and the concentration of the zeolite composition is 1 wt.% to 15 wt.% based on the weight of the hide.

17. The leather according to claim 16, having a shrinkage temperature higher than 70 °C and having an isoelectric point of 3.5 to 4.

5.

18. The leather according to claim 16, wherein the leather is obtained by a method of producing leather, wherein the method of producing leather is free of contact of the hide with tanning agents based on chromium, aluminum, titanium, zirconium and iron salts before, during and after the tanning step.

19. The leather according to claim 16, obtained by a method of producing leather, wherein the method of producing leather comprises one or more of a retanning step, a fat-liquoring step, a finishing step or any combination thereof.

20. The leather according to claim 16, comprising an upper side surface and a lower side surface extending in parallel and a perpendicular cross-section perpendicular to these surfaces, wherein the concentration of aluminum at the center of the perpendicular cross-section is at least 30% of the concentration at each surface side of the perpendicular cross-section and / or wherein the concentration of silicon at the center of the perpendicular cross-section is at least 30% of the concentration at each surface side of the perpendicular cross-section, the spatial distribution of silicon and aluminum is determined by SEM-EDX, wherein a leather sheet is cut perpendicular to the surface direction, exposing a perpendicular cross-section; the surface of the perpendicular cross-section is scanned with a finely focused electron beam and the X-rays emitted due to excitation with the electron beam are measured by energy dispersive X-ray spectroscopy and intensity profiles of aluminum and silicon on the perpendicular cross-section of the leather sheet are constructed, wherein intensity is linearly related to concentration.

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