Method for tanning leather, anionic surfactant composition, use of cationic surfactants for pre-tanning leather and tanned leather

Abstract

The invention discloses a method for tanning leather, an anionic active agent composition, the use of a cationic active agent for the pre-tanning of leather and tanned leather. The method comprises a pre-tanning step and a tanning step, characterized in that, in the pre-tanning step, the leather is treated with a cationic active agent comprising a quaternized ester of a fatty acid and of a triethanolamine, or a mixture thereof, and subsequently the leather is treated with an anionic active agent comprising a tannin plant extract and a natural or synthetic dispersing agent selected from the group consisting of lignosulfonates, stillage, sulfonated disulfonic acid-based polymers, sulfonated naphthalene polymers or mixtures thereof, to obtain a leather having a shrinkage temperature (ST) of at least 65°C, and wherein the pre-tanning step and the tanning step do not comprise a treatment with aldehydes or aldehyde donor compounds and / or a treatment with metal salts.

Description

TECHNICAL FIELD

[0001] The field of application of the invention described herein is in the tanning industry, in particular in the tanning of leather. BACKGROUND

[0002] Leather comes from the food industry and is a by-product of the processing of animal meat, in particular beef, pork, mutton and goat meat. The hides from the slaughterhouse, which at the slaughterhouse have been salted on the flesh side to be able to preserve them and prevent the proliferation of bacteria, are sent to the tanning industry process. In the tannery, they are partially desalted mechanically and sent to the first process called liming room. In the first step called soaking, the hides are washed to remove dirt and residual salt and at the same time to restore the water content. The second step is liming and unhairing, in which the hides are unhairing with calcium hydroxide, sodium bisulfate and sulfides at alkaline pH. In a mechanical operation called fleshing, the hides are separated from the fat and the horny part. At this point, the thickest bovine hides undergo a mechanical operation called splitting, in which the thickness is reduced approximately to the thickness required for the finished product. In this step, the thickness cannot be well controlled due to the consistency of the leather material, which is very swollen and full of moisture; therefore, a subsequent mechanical operation called shaving must be carried out on all types of hides to obtain the calibrated thickness required for application. At this point, the hide is called pelt.

[0003] The pelt skin is delimed and macerated with acids up to neutral pH and with enzymes, respectively, to eliminate the lime residues deriving from liming and unhairing and to remove the residues of keratin, fat, melanin and hair follicles, obtaining from the hide a material with a typical soft and elastic touch for the manufacture of all leather products, furniture and footwear.

[0004] After maceration, the hide has a neutral pH of about 6-7, and before the subsequent tanning step, the skin must be acidified, usually with formic acid and sulfuric acid in the presence of sodium chloride. These products are used as preservatives and therefore have the purpose of giving stability to the material, lowering the pH and increasing the osmotic pressure. The hide at this point is said to be in pickle. The pickle is a stable semi-finished product widely used for the storage, transport and sale of leather. The step to which the present invention relates is the step that follows this, i.e. tanning.

[0005] Tanning is the step that makes the leather stable against bacterial attack, thus becoming a reliable and durable material. Usually, about 5-10% of chromium salts (or other metals) or about 30-50% of vegetable tannins are used to tan the leather.

[0006] A pre-tanning step can precede the tanning step. In this case, the acid-treated leather is pre-tanned, which involves processing it with a product called pre-tanning for the first mechanical treatment. In this step, the anticipated mechanical treatment is pressing to remove excess moisture from the leather, followed by what is called shaving to obtain a material with reduced thickness, which involves removing a portion of the leather from the flesh side (the fibrous side opposite to the grain side of the leather). In this step, the leather is reduced to the thickness required for the finished product. Pre-tanned and pressed leather can be shaved to varying thicknesses, typically between 1.1-1.2 and 3.5 mm, depending on the intended final application of the product.

[0007] Sharpening is a machine-operated process that removes material from the flesh side of the leather to precisely achieve the desired final thickness. This has the distinct advantage of using less material in the tanning and subsequent processes, as this material is always proportional to the weight of the leather. Sharpening is an essential step for any tanned or pre-tanned leather; in fact, it is necessary for any final application: whether it's leather goods, footwear, furniture, or automobiles.

[0008] Considering the heat generated by friction during the shaving process, the leather must be sufficiently heat-resistant to shave evenly without damaging it, i.e., it must have a shrinkage temperature of at least 65°C. The shrinkage temperature (ST) is the temperature at which a leather sample begins to shrink, losing its soft properties due to collagen denaturation. ST is measured using the standard method ISO 3380:2015 Leather—Physical and mechanical testing—Determining shrinkage temperatures up to 100°C. The maximum ST for raw hides and acid-treated leathers is approximately 54°C. To increase ST, the leather is treated with pre-tanning or tanning products.

[0009] The products used in pre-tanning also prepare the leather for tanning, meaning they make the products used in tanning work more effectively.

[0010] Aldehydes (or aldehyde donors) or synthetic tannins are typically used in the pre-tanning of leather. Glutaraldehyde is commonly used in the pre-tanning process to produce high-quality leather, such as that used in automobiles. However, glutaraldehyde is a highly reactive aldehyde, posing significant environmental problems due to its toxicity and high impact on tannery wastewater and the working environment during its use in tanneries. Alternatively, substances that are not true aldehydes but release aldehydes during use can be used, essentially functioning as aldehydes. So-called aldehyde donors (such as oxazolidine, which decomposes in water to release formaldehyde) fall into this category. For this reason, these substances can only be used with appropriate precautions to avoid excessive aldehyde release. Synthetic tannins (also used in pre-tanning, and are synthetic products obtained primarily from the polymerization of formaldehyde with phenol sulfonates in various ways) can cause formaldehyde or phenol contamination in treated leather.

[0011] Following the pretanning step, the leather thickness is reduced using a shaving process to achieve a calibrated thickness. For this reason, some of the product used to obtain the pretanned leather is lost, as it is part of the thickness that is eliminated. Therefore, it is convenient to pretan the leather using as little product as possible. However, there are technical limitations to using a small amount of pretanning agent; in fact, the leather must be relatively stable to some extent to the heat generated during the shaving process. Shrinkage temperature (ST) is used to measure thermal resistance.

[0012] To obtain leather with an acceptable stripping temperature (ST) of 65°C for even shaving, each pre-tanned product has its own minimum dosage. By weight of raw hide, if chromium salts are used, treatment with 7% basic chromium sulfate yields excellent results at STs exceeding 100°C. On the other hand, treatment with glutaraldehyde requires 2% to achieve an ST of approximately 75-78°C; approximately 25% of phenolic synthetic tannins achieves an ST of approximately 70°C, while the use of plant-based tannins requires 30-40% to achieve an ST of 80°C.

[0013] In the tanning process, pre-tanned leather can be treated with a variety of tanning products. Traditionally, leather is tanned with naturally derived tannins. However, in recent decades, most leather has been tanned with trivalent chromium salts, particularly basic chromium sulfate. Other alternative processes use aluminum or zirconium salts. After tanning, the tanned leather undergoes further processing steps, typically fatliquoring, dyeing, and finishing. The products used in tanning and pre-tanning are of particular importance because they are the primary source of much of the tactile and mechanical properties of the finished leather. To date, metals have been the substances that give leather its best mechanical, light, and heat resistance.

[0014] However, in recent years, there has been an increasing trend of trying to manufacture leather that is free of aldehydes or metals. This is an increasingly urgent demand from the leather tanning market, especially due to the environmental impact of the chemical industry that supplies tanning products, the overall environmental impact of the leather tanning industry, the risks to tanneries and their operators, and the risk of contact allergies to users.

[0015] Plant-based tannins appear to be the most interesting option for sustainable processes because they have a smaller environmental impact during production and pose less risk to industry operators and finished leather users.

[0016] However, there are known limitations to the use of plant tannins due to the fact that significant quantities must be used in the pretanning process to achieve an acceptable ST (sterile stress). This means that treatment with plant tannins is costly and industrially impractical, also considering the fact that, for pretanning, some pretanned leather is removed by leveling to obtain the final material. Plant tannins are highly reactive with the proteins contained in the leather, and at lower doses, only surface-tanned leather can be produced because the product cannot penetrate the interior of the leather but is depleted on its surface. The resulting leather is unsuitable for leveling operations and is unstable against bacterial invasion.

[0017] Specifically, the method of the present invention aims to obtain pre-tanned leather with plant tannins, and the pre-tanned leather can be evenly shaved.

[0018] Besides their cost, the use of pre-tanned vegetable tannins is generally limited due to other reasons related to the characteristics and properties of finished leather, such as softness, color, and light and heat resistance. In fact, the large amounts of vegetable tannins typically required for pre-tanning leather result in stiff leather (called cardboard) with colors ranging from beige to dark reddish-brown. Because of this characteristic, it is impossible to obtain pre-tanned or tanned leather with vegetable tannins and light or soft colors. Leather made with vegetable tannins also has poor light and heat fastness. This is typically measured using the artificial light aging test ISO 105-B02:1994 – Leather – Light fastness: Xenon lamp. In this test, leather exposed to xenon lamp light and heat is compared to unexposed leather in terms of grayscale, and the color difference is expressed on a scale of 1 to 5, representing slight and high hues, respectively. Automotive leather is among the leathers with the most stringent acceptable requirements regarding light fastness, as it must meet the specifications set by the car manufacturer, who typically only accepts leather with a grayscale level below 1. Leather treated with plant tannins generally cannot meet this requirement because it requires a large amount of plant tannins.

[0019] For these needs, leather pre-tanned with aldehydes (especially glutaraldehyde) or tanned with chromium (especially basic chromium sulfate) is the most common. In particular, from a performance perspective, chromium is a treatment that yields better leather.

[0020] The main objective of this invention is to provide a method for obtaining multipurpose leather, such as leather obtained from glutaraldehyde or chromium, but without aldehydes or aldehyde donors or metals, using plant tannins without the generally known negative effects of these products.

[0021] Another object of the present invention is to provide a method for the sequential combination of non-toxic products with low environmental impact.

[0022] Another object of the present invention is to provide a method that allows the reuse of tanning baths, thereby reducing the load of organic matter present in tanneries wastewater. Summary of the Invention

[0023] The object of this invention is a tanning method, a pre-tanned product, and its use in pre-tanning.

[0024] The technical objective of this invention is based on the sequential use of two appropriately formulated substances in a pre-tanning step, or alternatively, the use of a first substance in pre-tanning and the distribution of a second substance between pre-tanning and tanning. Specifically, the invention provides the use of a cationic ester-based substance as a first treatment in the pre-tanning step. This makes the leather more cationic and therefore more reactive to a second anionic substance used for subsequent treatments. This final treatment may have already been performed in the pre-tanning step and may then be carried out during the tanning process. The anionic substance is based on a mixture of highly reactive natural tannins dispersed in a dispersant defined below, the dispersant being selected to ensure stability and moderate the reactivity of the natural tannins, which would otherwise be unusable for tanning leather.

[0025] Compared to leather obtained using natural tannins, this method imparts superior technical characteristics to the leather. Consequently, the resulting leather possesses sufficient properties to meet the demands of advanced applications in the automotive, footwear, and leather goods industries. The resulting leather is innovative because it is free of glutaraldehyde (or other aldehydes or aldehyde donors), chromium (or other metals), and possesses sensory and mechanical properties that have been unattainable until now without the use of the aforementioned pretanning agents and / or with the substantial use of plant tannins, which is not economically significant. Using the proposed technique, treatment with 1-3% cationic product followed by 10-15% (by weight percentage of the weight of the acid-soaked leather) anionic product is sufficient to obtain leather with a ST of 70-75°C and thus capable of withstanding the leveling step. Using the method according to the invention, even with relatively small amounts of plant tannins compared to the required standard amount, levelable pretanned leather can be obtained, thereby reducing product waste.

[0026] The proposed technology also offers another advantage: it allows for the reuse of tanning baths, which may be of interest as it reduces the load of polluting organic matter present in tannery wastewater. Indeed, some commonly used pre-tanning and tanning products do not allow for high recovery rates of bath solutions because the concentration of impurities increases rapidly and renders the bath unusable if not present in small quantities in the tanning bath. On the other hand, bath solutions treated using the proposed technology can be completely reused, achieving effective and problem-free depletion on the resulting leather.

[0027] Description of cationic products:

[0028] Cationic products contain active substances belonging to the quaternary ammonium salt family, particularly quaternized esters of fatty acids and triethanolamine. The quaternizing agent is typically dimethyl sulfate, but other quaternizing agents can be used. Preferably, the cationic product contains a dialkyl ester of triethanolamine methyl sulfate, wherein the alkyl group is straight-chain or branched and has 12 to 24 carbon atoms, also known as dialkyl dihydroxyethylammonium methyl sulfate or ester quaternary ammonium salt. Within this family, the following substances are of particular interest:

[0029] - Distearoylethyl hydroxyethylammonium methyl sulfate (also known as distearoylethyl hydroxyethylammonium methyl sulfate) or (2-hydroxyethyl)methylbis[2-[(1-oxooctadecyl)oxy]ethyl]ammonium methyl sulfate (commercially also known as Esterquat 75);

[0030] - Distearyl ethyl hydroxyethyl ammonium methyl sulfate mixed with cetearyl alcohol (commercially also known as Dehyquart F75);

[0031] - Docosanyltrimethylammonium methylsulfate (also known as docosanyltrimethylammonium methylsulfate).

[0032] The described active ingredients are a class of non-toxic, plant-derived substances composed of fatty acids derived from vegetable oils (such as rapeseed oil or coconut oil). These ingredients are commonly used as cationic surfactants and conditioning agents in cosmetics (primarily shampoos).

[0033] Quaternary ammonium ions with long-chain substituents possess surfactant properties; for this reason, depending on the substituents present, they are used as detergents and as biocides. Products of particular interest according to the invention are known and commonly used as fabric softeners or as hair conditioners.

[0034] The aforementioned cationic products can be prepared, for example, by the method described in U.S. Patent 5,705,663, which relates to a method and use for producing quaternized esters of fatty acids and triethanolamine prepared from triethanolamine, fatty acids, fatty acid esters, and a quaternizing agent. According to the disclosure, the quaternized esters of fatty acids and triethanolamine thus prepared can be used to produce fabric softeners with high activity content, resistance to rinse cycles, and particularly stable viscosity during storage.

[0035] For example, a production process is also reported in US document 2002 / 0002298, which describes a method for producing esterquat salts, wherein triglycerides are transesterified with alkanolamines, and then the resulting fatty acid is quaternized with an alkyl halide or a dialkyl sulfate in the presence of a solvent, characterized by the continuous removal of glycerol released during the transesterification process from the reaction equilibrium.

[0036] As previously mentioned, these substances are known and used as surfactants, and their softening properties are known. They are primarily used on fabrics and hair. In some cases, they are also known to be used as softeners for finished leather, but only as touch modifiers for chrome leather, as products used in post-tanning leather and imparting softness only in the fatliquoring step. They have never been described in any instance as being used in the pre-tanning step to prepare tanned leather, and therefore are more reactive to the products used.

[0037] To facilitate its use in the method according to the invention, the cationic active substance product is preferably formulated in a preparation comprising a solvent (e.g., water, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, tert-butanol, glycerol, ethoxylated alcohols, propoxylated alcohols, hydroformylation products of olefins, or mixtures thereof) and a water-compatible dispersant (e.g., sodium dodecyl sulfate, 40 moles of castor oil ethoxylate, sulphosuccinate, monoesters of C12-C14 oxoalcohols, or mixtures thereof). The solvent and dispersant also have the functions and advantages of improving stability over time, ease of dispersibility in water, and effectiveness.

[0038] Description of anionic products:

[0039] The anionic product can be in liquid or powder form. The powder form is obtained by removing water from the liquid form (conveniently by spray drying). The liquid form of the anionic product preferably comprises the following components (parts by weight):

[0040] - 1 part of tannin plant extract in an aqueous solution (preferably 45-50% dry).

[0041] -3 to 20 parts dispersant

[0042] -0 to 1 part solvent or stabilizer.

[0043] Tannin plant extracts that can be used in this technology may include hydrolyzable or concentrated tannins, and optionally other plant tannins, but conveniently they are hydrolyzable tannins, especially gallic type tannins.

[0044] More preferably, the following extracts and mixtures thereof are used:

[0045] - A liquid extract of Peruvian Tara, preferably obtained by the following procedure: extraction of the pods of Peruvian Tara (Caesalpinia spinosa) with ethanol (e.g., 80% by volume). The obtained extract is removed from the solvent using a steam extraction process. The obtained product is concentrated under vacuum to remove water to preferably up to 44-46% dry matter. The minimum content of tannins, according to ISO 14088 method, is preferably 96-98% on dry weight.

[0046] - A liquid extract of gallnut, preferably obtained as described above, but can also be obtained by extracting and grinding gallnut (a natural growth produced from the leaves of the Chinese sumac plant). The minimum content of tannins, on a dry weight basis, is preferably higher than 98% according to ISO 14088 method.

[0047] - A liquid extract of Turkish gall, preferably obtained as described above, but can also be obtained by extracting and grinding gall (a natural growth produced from the bark of Aleppo oak (Quercus infectoria)). The minimum content of tannins, on a dry weight basis, is preferably higher than 98% according to ISO 14088 method.

[0048] - Liquid chestnut extract, preferably obtained by hot water extraction of chestnut wood (European chestnut) followed by concentration under vacuum to remove water to a dry matter content preferably up to 46-48%. The minimum content of tanning agents, on a dry weight basis, is preferably 78-80% according to ISO 14088 method.

[0049] The preferred dispersants are natural and synthetic polyphenols. Among natural dispersants, lignosulfonate, distillation stillage, or mixtures thereof may be mentioned as non-limiting examples.

[0050] Lignosulfonates originate from the cellulose production process in the paper industry.

[0051] Distillate is the residue from alcoholic musts distilled from various agricultural products, such as grapes, potatoes, cereals, beet molasses, or other sugar-containing raw materials, such as those used in bioethanol production processes.

[0052] Particularly convenient is the use of liquid quebracho sulfitized extract and liquid ammonium lignosulfonate, both preferably in a 45-50% dry aqueous solution. In the synthesis of the dispersant, sulfonated disulfonic polymers [also defined as "reaction products with formaldehyde and sulfonylbis[phenol], sodium salts" (CAS No. 90218-44-3)], sulfonated naphthalene polymers [also defined as "reaction products with formaldehyde" (CAS No. 91078-68-1)], or mixtures thereof are preferred. Particularly convenient is the use of liquid sulfonated disulfonic polymers in a 45-50% dry aqueous solution.

[0053] The solvent is preferably a polar solvent with stabilizing properties; non-limiting examples include: glycerol, monopropylene glycol, ethylene glycol, diethylene glycol, triethylene glycol, the above-mentioned methoxylated derivatives, ethoxylated derivatives, and propoxylated derivatives, water, or mixtures thereof. A 50 / 50 mixture of water and monopropylene glycol is particularly suitable. Detailed Implementation

[0054] Example 1:

[0055] Preparation of cationic products

[0056] In the mixer, add the following ingredients in order of weight percentage:

[0057] Distearyl ethyl hydroxyethyl ammonium methyl sulfate 44%

[0058] Isopropanol 4%

[0059] Oxygenated alcohols C12-14 2.5%

[0060] Heat slowly to 45°C for 30 minutes, stirring constantly, then add the following water four times:

[0061] Water 49.5%

[0062] Continue stirring for 60 minutes and check for lumps. At 20°C and 35°C, the product appears as an ivory-white paste; in 10% water, it forms a translucent to semi-emulsive emulsion. The cationic product thus formulated can be used at 0.5% to 7% by weight relative to the weight of the acid-treated leather, preferably 1% to 4%, conveniently 2% to 3%.

[0063] Example 2:

[0064] Preparation of anionic products in liquid form. The following components must be added in the order shown to a suitable homogenizer for room temperature. Percentages are by weight:

[0065] Dispersant, 45-50% dry aqueous solution:

[0066] - Sulfonated disulfonic acid polymers 63.3%

[0067] - Sulfonated naphthalene polymer 18%

[0068] Tannin plant extract, 45-50% dry aqueous solution:

[0069] - 2.6% liquid extract of gallnut

[0070] - Liquid extract of gallnut 4.4%

[0071] Solvent:

[0072] -Water and monopropylene glycol 50 / 50 11.7%

[0073] Example 3:

[0074] Preparation of anionic products in powder form.

[0075] The liquid product prepared as described in Example 2 is fed into the atomizer. The feed flow rate of the liquid product and the air temperature entering the atomization chamber are maintained at 90 ± 2°C. Alternatively, other types of evaporators can be used, such as freeze dryers, vacuum atomizing evaporators, or falling film evaporators with a solid scraping system. Approximately 2.4 kg of liquid product is required to obtain 1 kg of powder product. For the weight of acid-treated leather, the anionic powder product thus produced can be conveniently used at 8 to 25% by weight, preferably 12 to 20%, and reasonably 15 to 18%.

[0076] Example 4:

[0077] The following is a recipe for pre-tanning leather in drums at a pilot tannery using the described technique. The percentages of products used refer to the weight of the acid-treated leather loaded into the drums, which also corresponds to the weight of the raw hide.

[0078] Load into the roller:

[0079] 1) Two halves of a cowhide (half-hide) that have been pickled. The raw material originates from France, with a thickness of 1.8 to 2.0 mm. The total weight of the two halves is 17.3 kg, and the pH of the raw hide is 3.0-3.1.

[0080] 2) If necessary, add wastewater from the previous pre-tanning process to reduce COD, run the drum for 30 minutes, and then drain.

[0081] 3) Add (50%) 8.65 kg of 8% NaCl aqueous solution (density 7 Baume).

[0082] 4) Spin the roller for 20 minutes.

[0083] 5) Add (2%) 346g of the cationic product described in Example 1, and rotate for 2 hours.

[0084] 6) Add 2.595 kg (15%) of the anionic product in powder form as described in Example 3, continuously rotate for 2 hours, and then alternately rotate for 10 minutes and wait for 30 minutes at room temperature (20-25°C) for the next 10 hours, and then continuously rotate for another 2 hours at 38°C.

[0085] 7) Add (50%) 8.65 kg of water at 38℃.

[0086] 8) Rotate the drum for 20 minutes.

[0087] 9) Drain the bath liquid.

[0088] 10) Wash at room temperature by adding (200%) 34.6 kg of water.

[0089] 11) Drain the bath liquid and leather.

[0090] 12) Place the leather on a rack and let it sit for 24 hours.

[0091] 13) ST is 72℃.

[0092] 14) Squeeze the pre-tanned leather to remove excess water.

[0093] 15) Shave the leather to a thickness of 1.1 to 1.2 mm.

[0094] The leveled leather is then re-tanned using a standard formula (which is also commonly used for leather pre-tanned with glutaraldehyde). Standard re-tanning involves various mixtures of natural tannins, such as powdered tara pods (subulcanum), synthetic tannins, such as disulfonic acid polymers, and possibly acrylic polymers. In the same re-tanning step, the leather is also dyed with acidic aniline dyes. The leather is then greased using a typically natural sulfitized grease. After this step, the leather is dried and moved to subsequent finishing and cutting steps.

[0095] Compared to the recycling that is typically possible after classic retanning, the bath liquid discharged after pretanning (step 9) can be recycled more effectively (possibly step 2), a step that offers no particular advantage in classic retanning. In fact, if wastewater (step 9), which typically has a COD (chemical oxygen demand) value of around 80,000 mg / L, is used in the subsequent second step of pretanning, its COD content is effectively reduced due to the presence of particularly reactive organic matter on the tanned leather, and thus it can be discharged at a COD of 12,000–15,000 mg / L. From an environmental impact perspective, this is an advantageous aspect of leather tanning methods using this technology.

[0096] Example 5:

[0097] - The embodiment was carried out according to Example 4, but using 2% of the cationic product described in Example 1 and 18% of the anionic product in powder form described in Example 3, ST = 74°C.

[0098] Example 6:

[0099] - The example was carried out according to Example 4, but using 1% of the cationic product described in Example 1 and 18% of the anionic product in powder form described in Example 3, ST = 73°C.

[0100] Example 7:

[0101] - The example was carried out according to Example 4, but using 3% of the cationic product described in Example 1 and 15% of the anionic product in powder form described in Example 3, ST = 72°C.

[0102] Example 8:

[0103] - The embodiment was carried out according to Example 4, but using 3% of the cationic product described in Example 1 and 20% of the anionic product in powder form described in Example 3, ST = 75°C.

Claims

1. A method for tanning leather, comprising a pre-tanning step and a tanning step, characterized in that, In the pretanning step, the leather is treated with a cationic surfactant comprising quaternized esters of fatty acids and triethanolamine, or mixtures thereof, and subsequently treated with an anionic surfactant comprising tannin plant extracts and natural or synthetic dispersants to obtain leather with a shrinkage temperature of at least 65°C as measured using the standard method ISO 3380:2015. The natural or synthetic dispersants are selected from the group consisting of lignin sulfonates, distillate, sulfonated disulfonic acid polymers, sulfonated naphthalene polymers, or mixtures thereof. The pretanning and tanning steps do not involve treatment with aldehydes or aldehyde donor compounds and / or with metal salts. The distillate is a residue from the distillation of alcoholic fermentation mash derived from the fermentation processes of various agricultural products.

2. The method according to claim 1, characterized in that, The tanning step includes treating the pre-tanned leather with an anionic surfactant having chemical properties consistent with those of the anionic surfactant used in the pre-tanning step.

3. The method according to claim 1 or 2, characterized in that, The cationic surfactant is a solution of a dialkyl ester comprising triethanolamine methyl sulfate, wherein the alkyl group is a straight-chain or branched group having 12 to 24 carbon atoms.

4. The method according to claim 1 or 2, characterized in that, The cationic surfactant is a compound selected from the group consisting of: distearyl ethyl hydroxyethyl ammonium methyl sulfate, a mixture of distearyl ethyl hydroxyethyl ammonium methyl sulfate and cetearyl alcohol, and docosyl trimethyl ammonium methyl sulfate and mixtures thereof.

5. The method according to claim 1 or 2, characterized in that, The cationic surfactant is a solution comprising a solvent and optional dispersing additives. The solvent is selected from the group consisting of water, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, tert-butanol, glycerol, ethoxylated alcohols, propoxylated alcohols, hydroformylation products of olefins, or mixtures thereof. The dispersing additives are selected from sodium dodecyl sulfate, ethoxylated castor oil, sulfosuccinate / esters, C... 12 -C 14 The group consisting of monoesters of oxoalcohols or mixtures thereof.

6. The method according to claim 1 or 2, characterized in that, The anionic surfactant includes gallnut-type hydrolyzable tannins.

7. The method according to claim 1, characterized in that, In the anionic surfactant, the dispersant is selected from liquid sulfite extract of white privet and liquid ammonium lignin sulfonate.

8. The method according to claim 1, characterized in that, In the anionic surfactant, the dispersant is selected from "the reaction product of sodium hydroxybenzenesulfonic acid with formaldehyde and sodium sulfonyl bis(phenol) salt" and "the reaction product of naphthalenesulfonic acid with formaldehyde" and mixtures thereof.

9. The method according to claim 1 or 2, characterized in that, The anionic surfactant comprises a tannin plant extract in a polar solvent at a ratio of 1:3 to 1:20 and the dispersant, wherein the ratio is expressed as the weight ratio between a solution of 45-50% plant extract and the dispersant.

10. The method according to claim 9, characterized in that, The polar solvent is selected from the group consisting of: glycerol, monopropylene glycol, ethylene glycol, diethylene glycol, triethylene glycol, their methoxylated derivatives, ethoxylated derivatives or propoxylated derivatives, water and mixtures thereof.

11. The method according to claim 1, characterized in that, The tanning step includes using natural tannins, optionally in combination with synthetic tannins and optionally acrylic polymers, as tanning agents.

12. The method according to claim 1 or 2, characterized in that, The anionic surfactant comprises extracts selected from the group consisting of: Tara liquid extract, Gallnut liquid extract, Gallnut liquid extract, and Chestnut liquid extract, or mixtures thereof.

13. The method according to claim 11, characterized in that, The natural tannins are from tara beans.

14. The method according to claim 11, characterized in that, The synthesized tannin is a disulfonic acid polymer.

15. Tanned leather obtainable by the method according to any one of claims 1 to 14, wherein the tanned leather is substantially free of aldehydes, aldehyde donor compounds and / or metal salts derived from tanning or pretanning.