Leather tanning method, uses and articles thereof

Abstract

The present disclosure relates to the field of sustainable leather processing and environmentally friendly tanning technologies. The present disclosure relates to a leather tanning method providing glutaraldehyde-free leather suitable for high-performance applications such as the automotive, footwear, accessories, furniture, and sporting goods sectors. The method comprises sequential steps of washing / soaking in two consecutive baths green splitting to a thickness between 2.3 mm and 3.0 mm, liming under alkaline conditions, and organic pre-tanning with an organic polymer and a vegetable extract. The organic polymer is present in an amount of 5-13% (w / w green leather), and the vegetable extract in an amount of 4-15% (w / w green leather), under conditions of pH 4.5-6.5 and temperatures up to 60 °C. The process yields leather with enhanced softness, mechanical properties, and antifungal resistance, while ensuring ecological compliance. The resulting leather is particularly suitable for automotive upholstery and related articles.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to the field of sustainable leather processing and environmentally friendly tanning technologies.

[0002] In particular, the method disclosed in the present disclosure introduces green splitting of the raw hide prior to liming and then applies an organic pre-tanning step combining a polymer with a vegetable extract. This combination is directed at producing leather with high softness, elasticity, and stability, while explicitly avoiding chromium and glutaraldehyde. The technical focus is on obtaining an eco-friendly leather suitable for demanding applications, such as automotive interiors, with measurable physical property advantages.BACKGROUND

[0003] Leather tanning is an ancient practice that has evolved over millennia, adapting to the needs of different eras while incorporating technological advancements. The primary purpose of tanning is to convert raw hides and skins into durable, flexible, and rot-resistant leather, suitable for various applications, from fashion to industrial products. The most prevalent tanning methods to date include chromium tanning, vegetable tanning and alternative methods to chromium tanning, generically designated as wet-white technology, using alternative mineral tanning agents or organic chemicals, such as aldehyde, synthetic tannins and fish oil.

[0004] Vegetable tanning has been known for its use of natural tannins derived from plant sources. While environmentally friendly, this method is time-consuming and often results in leather that is less supple than desired for certain applications. It is also limited in the range of colors it can produce, predominantly offering shades of brown and black.

[0005] Chromium tanning revolutionized the leather industry with its ability to produce soft, flexible leather in a wide color spectrum. However, the environmental impact of chromium, particularly its disposal and the potential for heavy metal pollution, has raised significant environmental and health concerns.

[0006] Alternative tanning methods - wet-white - have been developed to address some of the shortcomings of traditional methods. These methods offer certain advantages, such as reduced environmental impact or specific aesthetic qualities of the leather. Nonetheless, they too have limitations, including cost, complexity, and in some cases, the durability of the finished product.

[0007] Despite advancements in tanning technology, the industry faces ongoing challenges related to environmental sustainability, processing time, and the ability to meet the evolving demands for high-quality leather with reduced health and ecological impacts.

[0008] As mentioned, despite being the most implemented tanning technology, the chrome tanning is the most pollutant system, mainly because of the difficulty on the valorisation of solid tanned wastes. The most common alternative tanning system is the "wet-white" process with glutaraldehyde and synthetic tannins. This technology also starts to be restricted, due to the high toxicity of the glutaraldehyde.

[0009] This kind of technology, usually designated as "wet-white", has two chemical basis: mineral and organic. The main tanning agents used in the production of "wet-white" comprise mineral tanning agents - establish coordinative links with the carboxylic groups of collagen matrix: ammonium titanyl sulfate; basic alumminium sulfate; basic zirconium sulfate; and also organic tanning agents: aldehydes - stablish covalent connections with collagen; vegetable extracts or synthetic tannins - stablish mainly hydrogen bridges with collagen.

[0010] In document TR201004878A2, it is disclosed a chrome or vegetable tanned leather production method and its feature, wherein the production method comprises the following steps: a) processing of the leather with natural tannins in the tanning stage; b) processing of the leather passing through the tanning stage with oils; c) taking the leather to a second oiling process after the oiling process.

[0011] In document GB189615044A, it is disclosed the extraction process of tannins from several vegetable raw materials, including the acorn cup. It also describes a simple tanning process by the immersion of the hides into the tanning liquor.

[0012] In document CN109735668B, it is disclosed a chromium-free and aldehyde-free tanning method for dirty hides, comprising the following steps: soaking with wetting agent at room temperature until the hides are completely hydrated and its fibrillar structure relaxed; fleshing; re-soaking at room temperature adding biocide; adjusting pH and degrease with emulsifying agent; washing; pickling, adding NaCl, acidic protease, and sulphuric acid to adjust the pH; tanning at 25°C, adding the tanning agent; fixing tanning agent by adding baking soda to bring the pH to 6.5, warm up, and discharge the bath; apply the fatliquoring agent to the flesh side of the goat tanned skin, and dry it in the shade; and staking / milling to turn the leather softer. The tanning agent described in such document does not contain formaldehyde. Tanned leather samples resulted with high contraction temperature, and the leather board and hair coat is white, no colouring phenomenon; eliminating the acid-impregnation step, saving time and cost, and reducing the pollution of industrial salt.

[0013] These facts are disclosed in order to illustrate the technical problem addressed by the present disclosure.GENERAL DESCRIPTION

[0014] The present disclosure relates to the field of sustainable leather processing, specifically to an innovative leather tanning method for tanning hides to produce leather. This method aims to improve upon existing tanning techniques by enhancing efficiency, environmental sustainability, and the quality of the finished leather product.

[0015] The present invention provides a leather tanning method that avoids the use of glutaraldehyde while ensuring leather with high softness, durability, and ecological compatibility. The method combines a sequence of washing / soaking, splitting, liming, and organic pre-tanning steps, wherein the organic pre-tanning employs a synergistic combination of an organic polymer and vegetable extracts under controlled pH and temperature conditions. The process results in leather with improved mechanical properties and resistance to fungal growth, making it especially suitable for demanding applications. The invention further relates to the leather obtained by said method and to articles manufactured therefrom, particularly in the automotive, footwear, furniture, and accessories sectors.

[0016] Unlike conventional approaches focusing only on efficiency and incremental variations of conventional tanning agents, the method now disclosed differentiated by its novel process steps order (new flowsheet) and chemical system that delivers both environmental benefits and specific leather performance characteristics.

[0017] The aforementioned sustainable leather processing is performed through a leather tanning method, while effective in their respective domains, present a range of issues that the current invention seeks to address: Environmental Impact: The use of toxic chemicals in traditional tanning processes poses significant environmental and health risks; Efficiency and Cost: Many existing methods require lengthy processing times or involve expensive materials and processes, making them less economically viable. Leather Quality: There is a constant demand for higher quality leather with specific characteristics, such as enhanced durability, flexibility, and a broader palette of finishes that current tanning methods cannot always satisfactorily provide. The leather industry provides essential materials for various applications, such as footwear, clothing, accessories, and upholstery. Leather tanning is a crucial process that transforms raw animal hides and skins into durable and flexible materials suitable for these applications. However, traditional leather tanning methods often involve the use of harsh chemicals, such as chromium salts, which can have detrimental effects on the environment and human health and in the hide quality.

[0018] The environmental concerns associated with conventional leather tanning processes have led to an increasing demand for more sustainable and eco-friendly alternatives. The need for a tanning process that minimizes the environmental impact while maintaining the quality and durability of the leather has become vital in a conscious society.

[0019] In this disclosure, it is, for the first time, disclosed a leather tanning method that replace harmful chemicals with natural tanning agents, namely sustainable vegetable extracts, being it preceded by a "green splitting", during the beamhouse processing; and by a new organic pre-tanning.

[0020] In fact, the leather tanning method now disclosed based on vegetable extracts was quite difficult to achieve and at first sight, the production for the automotive sector was impossible due to two technical difficulties: vegetable extracts are composed mainly by polyphenols with high molecular weight, which led to difficult and slow diffusion across the collagen matrix, and; the reaction between these vegetable extracts and the collagen is also difficult and so, the kind of chemical bonds that are stablished are not strong.

[0021] The enounced difficulties generate a final product harder than what is usually required for the automotive sector.

[0022] Therefore, this specific application led to the development of a new leather tanning method using vegetable extracts to produce automotive leather comprising two innovative steps: green splitting; and organic pre-tanning.

[0023] In the step of green splitting, the hide is split in raw status, instead of doing it when the hide is already limed. Doing like this, the subsequent liming process is much more effective, and the same happens on the vegetable tanning step.

[0024] In the step of organic pre-tanning, new pre-tanning with an organic polymer that has reactivity not only with collagen, but also with the vegetable extracts, increasing the strength of the chemical bonds generated during the tanning.

[0025] Along this text, it is considered that the term "green leather" in the context of tanning refers to the initial stage of processed hides or skins before they undergo the complete tanning process to become leather. This term can be somewhat misleading, as it does not necessarily relate to the color green, nor does it imply the leather is finished in an environmentally friendly manner, although sustainable practices in leather production are increasingly common and may also be referred to as "green" in a different context. In traditional leather making, "green" refers to raw hides or skins, not yet tanned. The term "green" signifies that the hides are in their raw, untreated state beyond initial cleaning and preparation. At this stage, the hides are more susceptible to decay because the tanning process, which stabilizes the protein structure of the skin and makes it durable, rot-proof, and more resistant to water, has not yet been applied.

[0026] This new method can be applied in different technical fields, such as the automotive sector, resulting in articles that have the same properties like the ones obtained through the "wet-white" tanning process, but with a much lower environmental impact.

[0027] This new method introduces green splitting of the raw hide prior to liming and then applies an organic pre-tanning step combining a polymer with a vegetable extract. This combination is directed at producing leather with high softness, elasticity, and stability, while explicitly avoiding chromium and glutaraldehyde. The technical focus is on obtaining an eco-friendly leather suitable for demanding applications, such as automotive interiors, with measurable physical property advantages.

[0028] An aspect of the disclosure comprises a leather tanning method comprising the following steps: washing / soaking step; green splitting step; liming step; organic pre-tanning step; wherein the washing / soaking step comprises the immersion in two consecutive baths, wherein the first bath containing sodium carbonate, surfactants, and soaking enzymes, and the second bath consisting of water; wherein the green splitting step comprises the splitting of the leather in raw status to a thickness at a range from 2.3 mm to 3 mm is achieved; wherein the liming step comprises the controlled swelling of the fibrous structure of the hide and the dissolution of the undesired proteins, such as keratin, by the action of reducing agents on alkaline conditions, , preferably the pH is greater than 12; wherein the organic pre-tanning step comprises the addition of a pre-tanning organic polymer and a vegetable extract; wherein the amount of pre-tanning organic polymer ranges from 5% (w / w green leather ) to 13% (w / w green leather ); wherein the amount of vegetable extract ranges from 4% (w / w green leather ) to 15% (w / w green leather ); wherein the bath at the end of the organic pre-tanning step comprises a pH that ranges from 4.5 to 6.5 and an increase the temperature of up to 60°C.

[0029] The term "green splitting" as used herein refers to a mechanical splitting operation applied to hides or skins before the liming process. In contrast to conventional splitting, which is typically performed after liming, green splitting is carried out while the hide is still in its initial, non-limed state. This early splitting step reduces thickness at an earlier stage, improves the efficiency of subsequent liming and tanning, and facilitates more uniform penetration of chemicals.

[0030] The term "raw status" refers to the condition of a hide or skin prior to liming, after initial preparatory operations such as soaking and washing, but before chemical swelling or collagen matrix opening induced by lime or other alkaline agents. In this status, the fibrous structure of the hide is substantially intact, and the hide retains its natural, untreated characteristics.

[0031] The term "organic pre-tanning" refers to an intermediate treatment step performed before the final tanning of the hide, in which specific organic agents are introduced to partially stabilize the collagen structure and prepare it for subsequent tanning. In the present disclosure, the organic pre-tanning step involves the addition of an organic polymer that reacts both with collagen and with vegetable tannins, followed by the addition of vegetable extracts. This step enhances fixation of tannins, improves softness and elasticity, and avoids the use of chromium or glutaraldehyde.

[0032] The term "pre-fleshing step" refers to a mechanical operation performed on raw hides or skins prior to green splitting and liming, in which adhering subcutaneous tissue, fat, and residual flesh are removed from the flesh side of the hide. This operation reduces organic load, facilitates penetration of soaking and liming agents, prevents microbial degradation, and ensures a more uniform hide surface for subsequent processing.

[0033] As used in the present disclosure, the term pH refers to the negative logarithm of the hydrogen ion concentration measured in an aqueous solution of the solution at 25 °C, unless otherwise indicated. The pH values reported for baths, are determined measured in a sample of the bath, measured with a calibrated glass electrode pH meter according to ISO 10523:2008.

[0034] In an embodiment, the pre-tanning organic polymer used in the leather tanning method is selected from a list consisting of sodium-dichloro-triazin-amino-benzenesulphonate, naphthalene sulphonic acids, dihydroxy-diphenyl sulphones, or their combinations.

[0035] In an embodiment, the vegetable extract used in the leather tanning method is selected from a list consisting of cork extract, acorn cup extract, valonia extract, tara extract, or their combinations.

[0036] In an embodiment, the thickness of the leather after the green splitting step of the leather tanning method ranges from 2.3 mm to 3.0 mm, more preferably from 2.5 mm to 2.7 mm.

[0037] In an embodiment, the amount of pre-tanning organic polymer used in the leather tanning method ranges from 5% (w / w green leather) to 12% (w / w green leather), preferably from 6% (w / w green leather) to 11% (w / w green leather).

[0038] In an embodiment, the amount of vegetable extract used in the leather tanning method ranges from 4% (w / w green leather) to 13% (w / w green leather), preferably from 6% (w / w green leather) to 12% (w / w green leather).

[0039] In an embodiment, the pH in the organic pre-tanning step of the leather tanning method ranges from 5.0 to 6.5, preferably from 5.0 to 6.0.

[0040] In an embodiment, the increase of temperature in the organic pre-tanning step of the leather tanning method ranges from 35°C to 50°C, preferably from 40°C to 45°C.

[0041] In an embodiment, the leather tanning method further comprises a pre-fleshing step between washing / soaking step and green splitting step.

[0042] In an embodiment, the leather tanning method further comprises at least one of the following steps after the organic pre-tanning step: tanning step; splitting step; re-tanning step; drying step; finishing step.

[0043] In an embodiment, the leather tanning method further comprises the step of applying a post-tanning treatment that imbues the leather with antifungal properties.

[0044] In an embodiment, the addition of the vegetable extract in the leather tanning method is done after the addition of the pre-tanning organic polymer.

[0045] It is also described a leather obtained by the leather tanning method, wherein the leather is glutaraldehyde free and has the softness and elasticity suitable for use in the automotive industry.

[0046] In an embodiment, the softness of the leather is lower than -3.0 mm, preferably from -3.0 mm to -4.5 mm, more preferably from -3.0 mm to -4.0 mm.

[0047] In an embodiment, the elasticity of the leather ranges from 6% to 20%, preferably from 8% to 18%.

[0048] It is also described an article comprising the leather described.

[0049] In an embodiment, the article is used in the automotive sector, footwear, accessories sector, furniture sector, sporting goods or musical instruments.

[0050] In an embodiment, the article is a seat, an automobile upholstery, an automobile seat, a motorcycle seat, a rear window embellisher, a rear upper / lower trims, a trunk component, a boot door trim, a headliner, a roof component, a door panel, a door component, a trim of the seatback, a embellisher of seat cushion, a dashboard component, a glove box, a embellisher of column, a floor console, a gear lever, a door sash, a boot lid and a steering wheel.BRIEF DESCRIPTION OF THE DRAWINGS

[0051] The following figures provide preferred embodiments for illustrating the disclosure and should not be seen as limiting the scope of invention. Figure 1: Schematic representation of an embodiment of the microstructure of the leather using the prior art methods. Figure 2: Schematic representation of an embodiment of the microstructure of the leather along the leather tanning method. DETAILED DESCRIPTION

[0052] The present disclosure relates to the field of sustainable leather processing, specifically to an innovative leather tanning method for tanning hides and skins to produce leather. This method aims to improve upon existing tanning techniques by enhancing efficiency, environmental sustainability, and the quality of the finished leather product.

[0053] The present disclosure relates to a leather tanning method providing glutaraldehyde-free leather suitable for high-performance applications such as the automotive, footwear, accessories, furniture, and sporting goods sectors. The method comprises sequential steps of washing / soaking in two consecutive baths green splitting to a thickness between 2.3 mm and 3.0 mm, liming under alkaline conditions and organic pre-tanning with an organic polymer and a vegetable extract. The organic polymer is present in an amount of 5-13% (w / w green leather), and the vegetable extract in an amount of 4-15% (w / w green leather), under conditions of pH 4.5-6.5 and temperatures up to 60 °C. The process yields leather with enhanced softness, mechanical properties, and antifungal resistance, while ensuring ecological compliance. The resulting leather is particularly suitable for automotive upholstery and related articles.

[0054] Unlike conventional approaches focusing only on efficiency and incremental variations of conventional tanning agents, the method now disclosed differentiated by its novel process order and chemical system that delivers both environmental benefits and specific leather performance characteristics.

[0055] The tanning process now disclosed is based on vegetable extracts obtained from acorn cup - glutaraldehyde and metal free. It is less pollutant than the tanning with chromium sulfate because with this new tanning method is possible to valorize the tanned solid wastes. The toxicity of chromium and the restrictions that start to emerge relating the use of glutaraldehyde, created the necessity of develop the disclosed tanning method, that optimizes the distribution and fixation of a specific vegetable extract in the leather.

[0056] It is disclosed a leather tanning method comprising the following steps: washing / soaking step; green splitting step; liming step; organic pre-tanning step; wherein the washing / soaking step comprises the immersion in two consecutive baths containing sodium carbonate, surfactants and soaking enzymes; wherein the green splitting step comprises the splitting of the leather in raw status until a thickness within the range from 2.3 mm to 3.0 mm is achieved; wherein the liming step comprises the controlled swelling of the fibrous structure of the hide and the dissolution of the undesired proteins, such as keratin, by the action of reducing agents on alkaline conditions; wherein the organic pre-tanning step comprises the addition of a pre-tanning organic polymer and a vegetable extract; wherein the amount of pre-tanning organic polymer ranges from 5% (w / w green leather) to 13% (w / w green leather); wherein the amount of vegetable extract ranges from 4% (w / w green leather ) to 15% (w / w green leather ); wherein the organic pre-tanning step comprises a pH that ranges from 4.5 to 6.5 and an increase of temperature of up to 60°C.

[0057] In an embodiment, the pre-tanning organic polymer used in the leather tanning method is selected from a list consisting of sodium-dichloro-triazin-amino-benzenesulphonate, naphthalene sulphonic acids, dihydroxy-diphenyl sulphones, or their combinations, for better results.

[0058] In an embodiment, the vegetable extract used in the leather tanning method is selected from a list consisting of cork extract, acorn cup extract, valonia extract, tara extract, or their combinations, for better results.

[0059] In an embodiment, the thickness of the leather used in the leather tanning method after the green splitting step ranges from 2.3 mm to 3.0 mm, more preferably from 2.5 mm to 2.7 mm, for better results.

[0060] In an embodiment, the amount of pre-tanning organic polymer used in the leather tanning method ranges from 5% (w / w green leather) to 12% (w / w green leather), preferably from 6% (w / w green leather) to 11% (w / w green leather), for better results.

[0061] In an embodiment, the amount of vegetable extract usd in the leather tanning method ranges from 4% (w / w green leather) to 13% (w / w green leather), preferably from 6% (w / w green leather) to 12% (w / w green leather), for better results.

[0062] In an embodiment, the pH in the organic pre-tanning step of the leather tanning method ranges from 5.0 to 6.5, preferably from 5.0 to 6.0, for better results.

[0063] In an embodiment, the increase of temperature in the organic pre-tanning step of the leather tanning method ranges from 35°C to 50°C, preferably from 40°C to 45°C, for better results.

[0064] In an embodiment, the leather tanning method further comprises a pre-fleshing step between washing / soaking step and green splitting step.

[0065] In an embodiment, the leather tanning method further comprises at least one of the following steps after the organic pre-tanning step: tanning step; splitting step; re-tanning step; drying step; finishing step, for better results.

[0066] In an embodiment, the leather tanning method further comprises a liming step between the green splitting step and the organic pre-tanning step, for better results.

[0067] In an embodiment, the leather tanning method further comprises the step of applying a post-tanning treatment that imbues the leather with antifungal properties, for better results.

[0068] In an embodiment, the addition of the vegetable extract in the leather tanning method is done after the addition of the pre-tanning organic polymer.

[0069] It is also described the leather obtained by the disclosed leather tanning method, which is glutaraldehyde free and has the softness and elasticity suitable for use in the automotive industry, for better results.

[0070] In an embodiment, the softness of the leather is lower than -3.0 mm, preferably from -3,0 mm to -4,5 mm, more preferably from -3,0 mm to -4,0 mm, for better results. The softness has been measured according to the test established by the standard DIN EN ISO 17235:2015 Leather - Physical and mechanical tests - Determination of softness.

[0071] In an embodiment, the elasticity of the leather ranges from 6% to 20%, more preferably from 8% to 18%, for better results. The elasticity has been measured according to the test established by the standard PV 3909 / RV -2019 - Measurement of Static and Permanent Elongation.

[0072] It is also disclosed an article comprising the leather described.

[0073] In an embodiment, the article is used in the automotive sector, footwear, accessories sector, furniture sector, sporting goods or musical instruments, for better results.

[0074] In an embodiment, the article is at least one of the following elements: a seat, an automobile upholstery, an automobile seat, a motorcycle seat, a rear window embellisher, a rear upper / lower trims, a trunk component, a boot door trim, a headliner, a roof component, a door panel, a door component, a trim of the seatback, a embellisher of seat cushion, a dashboard component, a glove box, a embellisher of column, a floor console, a gear lever, a door sash, a boot lid or a steering wheel, for better results.

[0075] The search for alternatives to the tanning processes with chromium salts are day by day more explored and demanded by the market, especially for the automotive industry.

[0076] The indirect measure of the tanning degree is the shrinkage temperature, when the collagen denaturation occurs. The shrinkage temperature of an hide tanned with chromium is around 100°C. The wet-white hardly achieve shrinkage temperatures above 80°C and in the case of the tanning process now disclosed is around 72°C.

[0077] Figure 1 shows a schematic representation of an embodiment of the microstructure of the leather along the leather tanning method where: 1) represents detail of the microstructure of leather already limed, previously split in green; 2) diffusion of the organic polymer, preparing the skin to receive the tannic extracts; 3) fixing the organic polymer to the collagen; 4) diffusion of the plant extracts through the fibre structure; 5) fixation of tannins - tanned leather.

[0078] Figure 2 shows a schematic representation of an embodiment of the microstructure of the leather along the leather tanning method where: 1) detail of the hide with the liming process already done and after splitting; 2) Diffusion of the vegetable extracts across the collagen matrix; 3) Fixation of the tannins - tanned leather.

[0079] The term "comprising" whenever used in this document is intended to indicate the presence of stated features, integers, steps, components, but not to preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

[0080] The disclosure should not be seen in any way restricted to the embodiments described and a person with ordinary skill in the art will foresee many possibilities to modifications thereof. The above-described embodiments are combinable.

[0081] The following dependent claims further set out particular embodiments of the disclosure.

Examples

Embodiment Construction

[0052]The present disclosure relates to the field of sustainable leather processing, specifically to an innovative leather tanning method for tanning hides and skins to produce leather. This method aims to improve upon existing tanning techniques by enhancing efficiency, environmental sustainability, and the quality of the finished leather product.

[0053]The present disclosure relates to a leather tanning method providing glutaraldehyde-free leather suitable for high-performance applications such as the automotive, footwear, accessories, furniture, and sporting goods sectors. The method comprises sequential steps of washing / soaking in two consecutive baths green splitting to a thickness between 2.3 mm and 3.0 mm, liming under alkaline conditions and organic pre-tanning with an organic polymer and a vegetable extract. The organic polymer is present in an amount of 5-13% (w / w green leather), and the vegetable extract in an amount of 4-15% (w / w green leather), under conditions of pH 4.5...

TECHNICAL FIELD

[0001] The present disclosure relates to the field of sustainable leather processing and environmentally friendly tanning technologies.

[0002] In particular, the method disclosed in the present disclosure introduces green splitting of the raw hide prior to liming and then applies an organic pre-tanning step combining a polymer with a vegetable extract. This combination is directed at producing leather with high softness, elasticity, and stability, while explicitly avoiding chromium and glutaraldehyde. The technical focus is on obtaining an eco-friendly leather suitable for demanding applications, such as automotive interiors, with measurable physical property advantages.BACKGROUND

[0003] Leather tanning is an ancient practice that has evolved over millennia, adapting to the needs of different eras while incorporating technological advancements. The primary purpose of tanning is to convert raw hides and skins into durable, flexible, and rot-resistant leather, suitable for various applications, from fashion to industrial products. The most prevalent tanning methods to date include chromium tanning, vegetable tanning and alternative methods to chromium tanning, generically designated as wet-white technology, using alternative mineral tanning agents or organic chemicals, such as aldehyde, synthetic tannins and fish oil.

[0004] Vegetable tanning has been known for its use of natural tannins derived from plant sources. While environmentally friendly, this method is time-consuming and often results in leather that is less supple than desired for certain applications. It is also limited in the range of colors it can produce, predominantly offering shades of brown and black.

[0005] Chromium tanning revolutionized the leather industry with its ability to produce soft, flexible leather in a wide color spectrum. However, the environmental impact of chromium, particularly its disposal and the potential for heavy metal pollution, has raised significant environmental and health concerns.

[0006] Alternative tanning methods - wet-white - have been developed to address some of the shortcomings of traditional methods. These methods offer certain advantages, such as reduced environmental impact or specific aesthetic qualities of the leather. Nonetheless, they too have limitations, including cost, complexity, and in some cases, the durability of the finished product.

[0007] Despite advancements in tanning technology, the industry faces ongoing challenges related to environmental sustainability, processing time, and the ability to meet the evolving demands for high-quality leather with reduced health and ecological impacts.

[0008] As mentioned, despite being the most implemented tanning technology, the chrome tanning is the most pollutant system, mainly because of the difficulty on the valorisation of solid tanned wastes. The most common alternative tanning system is the "wet-white" process with glutaraldehyde and synthetic tannins. This technology also starts to be restricted, due to the high toxicity of the glutaraldehyde.

[0009] This kind of technology, usually designated as "wet-white", has two chemical basis: mineral and organic. The main tanning agents used in the production of "wet-white" comprise mineral tanning agents - establish coordinative links with the carboxylic groups of collagen matrix: ammonium titanyl sulfate; basic alumminium sulfate; basic zirconium sulfate; and also organic tanning agents: aldehydes - stablish covalent connections with collagen; vegetable extracts or synthetic tannins - stablish mainly hydrogen bridges with collagen.

[0010] In document TR201004878A2, it is disclosed a chrome or vegetable tanned leather production method and its feature, wherein the production method comprises the following steps: a) processing of the leather with natural tannins in the tanning stage; b) processing of the leather passing through the tanning stage with oils; c) taking the leather to a second oiling process after the oiling process.

[0011] In document GB189615044A, it is disclosed the extraction process of tannins from several vegetable raw materials, including the acorn cup. It also describes a simple tanning process by the immersion of the hides into the tanning liquor.

[0012] In document CN109735668B, it is disclosed a chromium-free and aldehyde-free tanning method for dirty hides, comprising the following steps: soaking with wetting agent at room temperature until the hides are completely hydrated and its fibrillar structure relaxed; fleshing; re-soaking at room temperature adding biocide; adjusting pH and degrease with emulsifying agent; washing; pickling, adding NaCl, acidic protease, and sulphuric acid to adjust the pH; tanning at 25°C, adding the tanning agent; fixing tanning agent by adding baking soda to bring the pH to 6.5, warm up, and discharge the bath; apply the fatliquoring agent to the flesh side of the goat tanned skin, and dry it in the shade; and staking / milling to turn the leather softer. The tanning agent described in such document does not contain formaldehyde. Tanned leather samples resulted with high contraction temperature, and the leather board and hair coat is white, no colouring phenomenon; eliminating the acid-impregnation step, saving time and cost, and reducing the pollution of industrial salt.

[0013] These facts are disclosed in order to illustrate the technical problem addressed by the present disclosure.GENERAL DESCRIPTION

[0014] The present disclosure relates to the field of sustainable leather processing, specifically to an innovative leather tanning method for tanning hides to produce leather. This method aims to improve upon existing tanning techniques by enhancing efficiency, environmental sustainability, and the quality of the finished leather product.

[0015] The present invention provides a leather tanning method that avoids the use of glutaraldehyde while ensuring leather with high softness, durability, and ecological compatibility. The method combines a sequence of washing / soaking, splitting, liming, and organic pre-tanning steps, wherein the organic pre-tanning employs a synergistic combination of an organic polymer and vegetable extracts under controlled pH and temperature conditions. The process results in leather with improved mechanical properties and resistance to fungal growth, making it especially suitable for demanding applications. The invention further relates to the leather obtained by said method and to articles manufactured therefrom, particularly in the automotive, footwear, furniture, and accessories sectors.

[0016] Unlike conventional approaches focusing only on efficiency and incremental variations of conventional tanning agents, the method now disclosed differentiated by its novel process steps order (new flowsheet) and chemical system that delivers both environmental benefits and specific leather performance characteristics.

[0017] The aforementioned sustainable leather processing is performed through a leather tanning method, while effective in their respective domains, present a range of issues that the current invention seeks to address: Environmental Impact: The use of toxic chemicals in traditional tanning processes poses significant environmental and health risks; Efficiency and Cost: Many existing methods require lengthy processing times or involve expensive materials and processes, making them less economically viable. Leather Quality: There is a constant demand for higher quality leather with specific characteristics, such as enhanced durability, flexibility, and a broader palette of finishes that current tanning methods cannot always satisfactorily provide. The leather industry provides essential materials for various applications, such as footwear, clothing, accessories, and upholstery. Leather tanning is a crucial process that transforms raw animal hides and skins into durable and flexible materials suitable for these applications. However, traditional leather tanning methods often involve the use of harsh chemicals, such as chromium salts, which can have detrimental effects on the environment and human health and in the hide quality.

[0018] The environmental concerns associated with conventional leather tanning processes have led to an increasing demand for more sustainable and eco-friendly alternatives. The need for a tanning process that minimizes the environmental impact while maintaining the quality and durability of the leather has become vital in a conscious society.

[0019] In this disclosure, it is, for the first time, disclosed a leather tanning method that replace harmful chemicals with natural tanning agents, namely sustainable vegetable extracts, being it preceded by a "green splitting", during the beamhouse processing; and by a new organic pre-tanning.

[0020] In fact, the leather tanning method now disclosed based on vegetable extracts was quite difficult to achieve and at first sight, the production for the automotive sector was impossible due to two technical difficulties: vegetable extracts are composed mainly by polyphenols with high molecular weight, which led to difficult and slow diffusion across the collagen matrix, and; the reaction between these vegetable extracts and the collagen is also difficult and so, the kind of chemical bonds that are stablished are not strong.

[0021] The enounced difficulties generate a final product harder than what is usually required for the automotive sector.

[0022] Therefore, this specific application led to the development of a new leather tanning method using vegetable extracts to produce automotive leather comprising two innovative steps: green splitting; and organic pre-tanning.

[0023] In the step of green splitting, the hide is split in raw status, instead of doing it when the hide is already limed. Doing like this, the subsequent liming process is much more effective, and the same happens on the vegetable tanning step.

[0024] In the step of organic pre-tanning, new pre-tanning with an organic polymer that has reactivity not only with collagen, but also with the vegetable extracts, increasing the strength of the chemical bonds generated during the tanning.

[0025] Along this text, it is considered that the term "green leather" in the context of tanning refers to the initial stage of processed hides or skins before they undergo the complete tanning process to become leather. This term can be somewhat misleading, as it does not necessarily relate to the color green, nor does it imply the leather is finished in an environmentally friendly manner, although sustainable practices in leather production are increasingly common and may also be referred to as "green" in a different context. In traditional leather making, "green" refers to raw hides or skins, not yet tanned. The term "green" signifies that the hides are in their raw, untreated state beyond initial cleaning and preparation. At this stage, the hides are more susceptible to decay because the tanning process, which stabilizes the protein structure of the skin and makes it durable, rot-proof, and more resistant to water, has not yet been applied.

[0026] This new method can be applied in different technical fields, such as the automotive sector, resulting in articles that have the same properties like the ones obtained through the "wet-white" tanning process, but with a much lower environmental impact.

[0027] This new method introduces green splitting of the raw hide prior to liming and then applies an organic pre-tanning step combining a polymer with a vegetable extract. This combination is directed at producing leather with high softness, elasticity, and stability, while explicitly avoiding chromium and glutaraldehyde. The technical focus is on obtaining an eco-friendly leather suitable for demanding applications, such as automotive interiors, with measurable physical property advantages.

[0028] An aspect of the disclosure comprises a leather tanning method comprising the following steps: washing / soaking step; green splitting step; liming step; organic pre-tanning step; wherein the washing / soaking step comprises the immersion in two consecutive baths, wherein the first bath containing sodium carbonate, surfactants, and soaking enzymes, and the second bath consisting of water; wherein the green splitting step comprises the splitting of the leather in raw status to a thickness at a range from 2.3 mm to 3 mm is achieved; wherein the liming step comprises the controlled swelling of the fibrous structure of the hide and the dissolution of the undesired proteins, such as keratin, by the action of reducing agents on alkaline conditions, , preferably the pH is greater than 12; wherein the organic pre-tanning step comprises the addition of a pre-tanning organic polymer and a vegetable extract; wherein the amount of pre-tanning organic polymer ranges from 5% (w / w green leather ) to 13% (w / w green leather ); wherein the amount of vegetable extract ranges from 4% (w / w green leather ) to 15% (w / w green leather ); wherein the bath at the end of the organic pre-tanning step comprises a pH that ranges from 4.5 to 6.5 and an increase the temperature of up to 60°C.

[0029] The term "green splitting" as used herein refers to a mechanical splitting operation applied to hides or skins before the liming process. In contrast to conventional splitting, which is typically performed after liming, green splitting is carried out while the hide is still in its initial, non-limed state. This early splitting step reduces thickness at an earlier stage, improves the efficiency of subsequent liming and tanning, and facilitates more uniform penetration of chemicals.

[0030] The term "raw status" refers to the condition of a hide or skin prior to liming, after initial preparatory operations such as soaking and washing, but before chemical swelling or collagen matrix opening induced by lime or other alkaline agents. In this status, the fibrous structure of the hide is substantially intact, and the hide retains its natural, untreated characteristics.

[0031] The term "organic pre-tanning" refers to an intermediate treatment step performed before the final tanning of the hide, in which specific organic agents are introduced to partially stabilize the collagen structure and prepare it for subsequent tanning. In the present disclosure, the organic pre-tanning step involves the addition of an organic polymer that reacts both with collagen and with vegetable tannins, followed by the addition of vegetable extracts. This step enhances fixation of tannins, improves softness and elasticity, and avoids the use of chromium or glutaraldehyde.

[0032] The term "pre-fleshing step" refers to a mechanical operation performed on raw hides or skins prior to green splitting and liming, in which adhering subcutaneous tissue, fat, and residual flesh are removed from the flesh side of the hide. This operation reduces organic load, facilitates penetration of soaking and liming agents, prevents microbial degradation, and ensures a more uniform hide surface for subsequent processing.

[0033] As used in the present disclosure, the term pH refers to the negative logarithm of the hydrogen ion concentration measured in an aqueous solution of the solution at 25 °C, unless otherwise indicated. The pH values reported for baths, are determined measured in a sample of the bath, measured with a calibrated glass electrode pH meter according to ISO 10523:2008.

[0034] In an embodiment, the pre-tanning organic polymer used in the leather tanning method is selected from a list consisting of sodium-dichloro-triazin-amino-benzenesulphonate, naphthalene sulphonic acids, dihydroxy-diphenyl sulphones, or their combinations.

[0035] In an embodiment, the vegetable extract used in the leather tanning method is selected from a list consisting of cork extract, acorn cup extract, valonia extract, tara extract, or their combinations.

[0036] In an embodiment, the thickness of the leather after the green splitting step of the leather tanning method ranges from 2.3 mm to 3.0 mm, more preferably from 2.5 mm to 2.7 mm.

[0037] In an embodiment, the amount of pre-tanning organic polymer used in the leather tanning method ranges from 5% (w / w green leather) to 12% (w / w green leather), preferably from 6% (w / w green leather) to 11% (w / w green leather).

[0038] In an embodiment, the amount of vegetable extract used in the leather tanning method ranges from 4% (w / w green leather) to 13% (w / w green leather), preferably from 6% (w / w green leather) to 12% (w / w green leather).

[0039] In an embodiment, the pH in the organic pre-tanning step of the leather tanning method ranges from 5.0 to 6.5, preferably from 5.0 to 6.0.

[0040] In an embodiment, the increase of temperature in the organic pre-tanning step of the leather tanning method ranges from 35°C to 50°C, preferably from 40°C to 45°C.

[0041] In an embodiment, the leather tanning method further comprises a pre-fleshing step between washing / soaking step and green splitting step.

[0042] In an embodiment, the leather tanning method further comprises at least one of the following steps after the organic pre-tanning step: tanning step; splitting step; re-tanning step; drying step; finishing step.

[0043] In an embodiment, the leather tanning method further comprises the step of applying a post-tanning treatment that imbues the leather with antifungal properties.

[0044] In an embodiment, the addition of the vegetable extract in the leather tanning method is done after the addition of the pre-tanning organic polymer.

[0045] It is also described a leather obtained by the leather tanning method, wherein the leather is glutaraldehyde free and has the softness and elasticity suitable for use in the automotive industry.

[0046] In an embodiment, the softness of the leather is lower than -3.0 mm, preferably from -3.0 mm to -4.5 mm, more preferably from -3.0 mm to -4.0 mm.

[0047] In an embodiment, the elasticity of the leather ranges from 6% to 20%, preferably from 8% to 18%.

[0048] It is also described an article comprising the leather described.

[0049] In an embodiment, the article is used in the automotive sector, footwear, accessories sector, furniture sector, sporting goods or musical instruments.

[0050] In an embodiment, the article is a seat, an automobile upholstery, an automobile seat, a motorcycle seat, a rear window embellisher, a rear upper / lower trims, a trunk component, a boot door trim, a headliner, a roof component, a door panel, a door component, a trim of the seatback, a embellisher of seat cushion, a dashboard component, a glove box, a embellisher of column, a floor console, a gear lever, a door sash, a boot lid and a steering wheel.BRIEF DESCRIPTION OF THE DRAWINGS

[0051] The following figures provide preferred embodiments for illustrating the disclosure and should not be seen as limiting the scope of invention. Figure 1: Schematic representation of an embodiment of the microstructure of the leather using the prior art methods. Figure 2: Schematic representation of an embodiment of the microstructure of the leather along the leather tanning method. DETAILED DESCRIPTION

[0052] The present disclosure relates to the field of sustainable leather processing, specifically to an innovative leather tanning method for tanning hides and skins to produce leather. This method aims to improve upon existing tanning techniques by enhancing efficiency, environmental sustainability, and the quality of the finished leather product.

[0053] The present disclosure relates to a leather tanning method providing glutaraldehyde-free leather suitable for high-performance applications such as the automotive, footwear, accessories, furniture, and sporting goods sectors. The method comprises sequential steps of washing / soaking in two consecutive baths green splitting to a thickness between 2.3 mm and 3.0 mm, liming under alkaline conditions and organic pre-tanning with an organic polymer and a vegetable extract. The organic polymer is present in an amount of 5-13% (w / w green leather), and the vegetable extract in an amount of 4-15% (w / w green leather), under conditions of pH 4.5-6.5 and temperatures up to 60 °C. The process yields leather with enhanced softness, mechanical properties, and antifungal resistance, while ensuring ecological compliance. The resulting leather is particularly suitable for automotive upholstery and related articles.

[0054] Unlike conventional approaches focusing only on efficiency and incremental variations of conventional tanning agents, the method now disclosed differentiated by its novel process order and chemical system that delivers both environmental benefits and specific leather performance characteristics.

[0055] The tanning process now disclosed is based on vegetable extracts obtained from acorn cup - glutaraldehyde and metal free. It is less pollutant than the tanning with chromium sulfate because with this new tanning method is possible to valorize the tanned solid wastes. The toxicity of chromium and the restrictions that start to emerge relating the use of glutaraldehyde, created the necessity of develop the disclosed tanning method, that optimizes the distribution and fixation of a specific vegetable extract in the leather.

[0056] It is disclosed a leather tanning method comprising the following steps: washing / soaking step; green splitting step; liming step; organic pre-tanning step; wherein the washing / soaking step comprises the immersion in two consecutive baths containing sodium carbonate, surfactants and soaking enzymes; wherein the green splitting step comprises the splitting of the leather in raw status until a thickness within the range from 2.3 mm to 3.0 mm is achieved; wherein the liming step comprises the controlled swelling of the fibrous structure of the hide and the dissolution of the undesired proteins, such as keratin, by the action of reducing agents on alkaline conditions; wherein the organic pre-tanning step comprises the addition of a pre-tanning organic polymer and a vegetable extract; wherein the amount of pre-tanning organic polymer ranges from 5% (w / w green leather) to 13% (w / w green leather); wherein the amount of vegetable extract ranges from 4% (w / w green leather ) to 15% (w / w green leather ); wherein the organic pre-tanning step comprises a pH that ranges from 4.5 to 6.5 and an increase of temperature of up to 60°C.

[0057] In an embodiment, the pre-tanning organic polymer used in the leather tanning method is selected from a list consisting of sodium-dichloro-triazin-amino-benzenesulphonate, naphthalene sulphonic acids, dihydroxy-diphenyl sulphones, or their combinations, for better results.

[0058] In an embodiment, the vegetable extract used in the leather tanning method is selected from a list consisting of cork extract, acorn cup extract, valonia extract, tara extract, or their combinations, for better results.

[0059] In an embodiment, the thickness of the leather used in the leather tanning method after the green splitting step ranges from 2.3 mm to 3.0 mm, more preferably from 2.5 mm to 2.7 mm, for better results.

[0060] In an embodiment, the amount of pre-tanning organic polymer used in the leather tanning method ranges from 5% (w / w green leather) to 12% (w / w green leather), preferably from 6% (w / w green leather) to 11% (w / w green leather), for better results.

[0061] In an embodiment, the amount of vegetable extract usd in the leather tanning method ranges from 4% (w / w green leather) to 13% (w / w green leather), preferably from 6% (w / w green leather) to 12% (w / w green leather), for better results.

[0062] In an embodiment, the pH in the organic pre-tanning step of the leather tanning method ranges from 5.0 to 6.5, preferably from 5.0 to 6.0, for better results.

[0063] In an embodiment, the increase of temperature in the organic pre-tanning step of the leather tanning method ranges from 35°C to 50°C, preferably from 40°C to 45°C, for better results.

[0064] In an embodiment, the leather tanning method further comprises a pre-fleshing step between washing / soaking step and green splitting step.

[0065] In an embodiment, the leather tanning method further comprises at least one of the following steps after the organic pre-tanning step: tanning step; splitting step; re-tanning step; drying step; finishing step, for better results.

[0066] In an embodiment, the leather tanning method further comprises a liming step between the green splitting step and the organic pre-tanning step, for better results.

[0067] In an embodiment, the leather tanning method further comprises the step of applying a post-tanning treatment that imbues the leather with antifungal properties, for better results.

[0068] In an embodiment, the addition of the vegetable extract in the leather tanning method is done after the addition of the pre-tanning organic polymer.

[0069] It is also described the leather obtained by the disclosed leather tanning method, which is glutaraldehyde free and has the softness and elasticity suitable for use in the automotive industry, for better results.

[0070] In an embodiment, the softness of the leather is lower than -3.0 mm, preferably from -3,0 mm to -4,5 mm, more preferably from -3,0 mm to -4,0 mm, for better results. The softness has been measured according to the test established by the standard DIN EN ISO 17235:2015 Leather - Physical and mechanical tests - Determination of softness.

[0071] In an embodiment, the elasticity of the leather ranges from 6% to 20%, more preferably from 8% to 18%, for better results. The elasticity has been measured according to the test established by the standard PV 3909 / RV -2019 - Measurement of Static and Permanent Elongation.

[0072] It is also disclosed an article comprising the leather described.

[0073] In an embodiment, the article is used in the automotive sector, footwear, accessories sector, furniture sector, sporting goods or musical instruments, for better results.

[0074] In an embodiment, the article is at least one of the following elements: a seat, an automobile upholstery, an automobile seat, a motorcycle seat, a rear window embellisher, a rear upper / lower trims, a trunk component, a boot door trim, a headliner, a roof component, a door panel, a door component, a trim of the seatback, a embellisher of seat cushion, a dashboard component, a glove box, a embellisher of column, a floor console, a gear lever, a door sash, a boot lid or a steering wheel, for better results.

[0075] The search for alternatives to the tanning processes with chromium salts are day by day more explored and demanded by the market, especially for the automotive industry.

[0076] The indirect measure of the tanning degree is the shrinkage temperature, when the collagen denaturation occurs. The shrinkage temperature of an hide tanned with chromium is around 100°C. The wet-white hardly achieve shrinkage temperatures above 80°C and in the case of the tanning process now disclosed is around 72°C.

[0077] Figure 1 shows a schematic representation of an embodiment of the microstructure of the leather along the leather tanning method where: 1) represents detail of the microstructure of leather already limed, previously split in green; 2) diffusion of the organic polymer, preparing the skin to receive the tannic extracts; 3) fixing the organic polymer to the collagen; 4) diffusion of the plant extracts through the fibre structure; 5) fixation of tannins - tanned leather.

[0078] Figure 2 shows a schematic representation of an embodiment of the microstructure of the leather along the leather tanning method where: 1) detail of the hide with the liming process already done and after splitting; 2) Diffusion of the vegetable extracts across the collagen matrix; 3) Fixation of the tannins - tanned leather.

[0079] The term "comprising" whenever used in this document is intended to indicate the presence of stated features, integers, steps, components, but not to preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

[0080] The disclosure should not be seen in any way restricted to the embodiments described and a person with ordinary skill in the art will foresee many possibilities to modifications thereof. The above-described embodiments are combinable.

[0081] The following dependent claims further set out particular embodiments of the disclosure.

Examples

Embodiment Construction

[0052]The present disclosure relates to the field of sustainable leather processing, specifically to an innovative leather tanning method for tanning hides and skins to produce leather. This method aims to improve upon existing tanning techniques by enhancing efficiency, environmental sustainability, and the quality of the finished leather product.

[0053]The present disclosure relates to a leather tanning method providing glutaraldehyde-free leather suitable for high-performance applications such as the automotive, footwear, accessories, furniture, and sporting goods sectors. The method comprises sequential steps of washing / soaking in two consecutive baths green splitting to a thickness between 2.3 mm and 3.0 mm, liming under alkaline conditions and organic pre-tanning with an organic polymer and a vegetable extract. The organic polymer is present in an amount of 5-13% (w / w green leather), and the vegetable extract in an amount of 4-15% (w / w green leather), under conditions of pH 4.5...

Claims

1. A leather tanning method comprising the following steps: washing / soaking step; green splitting step; liming step; organic pre-tanning step; wherein the washing / soaking step comprises the immersion in two consecutive baths, wherein the first bath containing sodium carbonate, surfactants, and soaking enzymes, and the second bath of water rinse; wherein the green splitting step comprises the splitting of the leather in raw status to a thickness at a range from 2.3 mm to 3 mm is achieved; wherein the liming step comprises the controlled swelling of the fibrous structure of the hide and the dissolution of the undesired proteins by the action of reducing agents on alkaline conditions; wherein the organic pre-tanning step comprises the addition of a pre-tanning organic polymer and a vegetable extract; wherein the amount of pre-tanning organic polymer ranges from 5% (w / wgreen leather) to 13% (w / wgreen leather); wherein the amount of vegetable extract ranges from 4% (w / wgreen leather) to 15% (w / wgreen leather); wherein the bath at the end of the organic pre-tanning step comprises a pH that ranges from 4.5 to 6.5 and an increasing the temperature of up to 60°C.

2. The leather tanning method according to claim 1, wherein the pre-tanning organic polymer is selected from a list consisting of sodium-dichloro-triazin-amino-benzenesulphonate, naphthalene sulphonic acids, dihydroxy-diphenyl sulphones, or mixtures thereof.

3. The leather tanning method according to any of the previous claims, wherein the vegetable extract is selected from a list consisting of cork extract, acorn cup extract, valonia extract, tara extract, or their combinations; preferably acorn cup extract, valonia extract, or mixtures thereof.

4. The leather tanning method according to any of the previous claims, wherein the thickness of the leather after the green splitting step ranges from 2.3 mm to 3.0 mm, more preferably from 2.5 mm to 2.7 mm.

5. The leather tanning method according to any of the previous claims, wherein the amount of pre-tanning organic polymer ranges from 5% (w / wgreen leather) to 12% (w / wgreen leather), preferably from 6% (w / wgreen leather) to 11% (w / wgreen leather).

6. The leather tanning method according to any of the previous claims, wherein the amount of vegetable extract ranges from 4% (w / wgreen leather) to 13% (w / wgreen leather), preferably from 6% (w / wgreen leather) to 12% (w / wgreen leather).

7. The leather tanning method according to any of the previous claims, wherein the pH in the bath of the organic pre-tanning step ranges from 5.0 to 6.5, preferably from 5.0 to 6.0.

8. The leather tanning method according to any of the previous claims, wherein the increase of temperature in the organic pre-tanning step ranges from 35°C to 50°C, preferably from 40°C to 45°C.

9. The leather tanning method according to any of the previous claims, further comprising a pre-fleshing step between washing / soaking step and green splitting step.

10. The leather tanning method according to any of the previous claims, further comprising at least one of the following steps after the organic pre-tanning step: tanning step; splitting step; re-tanning step; drying step; finishing step.

11. The leather tanning method according to any of the previous claims, further comprising the step of applying a post-tanning treatment that imbues the leather with antifungal properties.

12. The leather tanning method according to any of the previous claims, wherein the addition of the vegetable extract is done after the addition of the pre-tanning organic polymer.

13. A leather obtained by the leather tanning method described in any of the previous claims wherein the leather is glutaraldehyde-free and is suitable for use in the automotive industry.

14. The leather according to the previous claim, wherein the softness is lower than - 3.0 mm, preferably from -3.0 mm to -4.5 mm, more preferably from -3.0 mm to - 4,0 mm.

15. An article comprising the leather described in the previous claim, in particular wherein the article is used in the automotive sector, footwear, accessories sector, furniture sector, sporting goods or musical instruments, more particularly wherein said article is a seat, an automobile upholstery, an automobile seat, a motorcycle seat, a rear window embellisher, a rear upper / lower trims, a trunk component, a boot door trim, a headliner, a roof component, a door panel, a door component, a trim of the seatback, a embellisher of seat cushion, a dashboard component, a glove box, a embellisher of column, a floor console, a gear lever, a door sash, a boot lid and a steering wheel.

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