Method for tanning raw hide, method for preparing leather and detanning method

Abstract

The invention relates to a method for tanning a hide using a tanning solution comprising a silicon compound and free of metals. The invention also relates to a method for preparing leather comprising such a tanning method. The invention also relates to a tanned hide obtained by such a tanning process. The invention also relates to a method for detanning the raw hide obtained by the above tanning method.

Description

[0001] The present invention relates to a method for tanning raw hides using a silicon compound as a tanning agent, and a method for producing leather by implementing such a method. The present invention also relates to a method for detanning the tanned hides and leather thus obtained.

[0002] Making leather from hides requires many steps. All these steps can be divided into three main categories, which are defined as follows: i) steps for preparing the raw hides, ii) steps for the tanning operation, and iii) steps for grinding and finishing.

[0003] During the steps for preparing the raw hides, the native hides of animals are received. These hides can be classified according to their quality, size, weight, etc. During these preparation steps, the parts of the hides that are not desired to be retained (such as the legs) are cut off and removed. Then the hides are washed. For this purpose, the hides are immersed in a large amount of water, for example, in a fuller (rotating drum) or a trough (a fixed container with rotating blades). Preservatives can be added. After washing, the hides are, for example, depilated chemically. The hides also undergo a fleshing step, which consists of removing the subcutaneous tissue. These steps are called river steps.

[0004] At the end of the steps for preparing the raw hides, so-called "pickled" hides or so-called "delimed" hides are obtained. "Pickled" hides are hides with a high salinity and a low pH (usually less than 3.0). "Delimed" hides are hides that come out of the river steps and have a pH usually of 7.5 to 9.0.

[0005] During the tanning operation, the raw hides prepared as described above, such as "pickled" hides and "delimed" hides, are degreased. If necessary, these hides are rehydrated. At this stage, the hides are prone to decay and are ready for tanning. The tanning step itself consists of a chemical treatment that allows these perishable raw hides to be transformed into non-perishable and resistant leather. Then the resulting leather is drained, rinsed, and possibly spin-dried.

[0006] The grinding and finishing steps are the steps that enable the leather obtained at the end of the tanning operation to be used for manufacturing leather products. These steps can include spinning, thickening, stretching, retanning, dyeing, food steps. The finishing steps can include steps in which the leather is softened, smoothed, polished, printed, steps that impart to the leather properties such as stain resistance, abrasion resistance, water resistance, or light resistance. They can also impart aesthetic properties to the leather, such as gloss, color, velvet-like, satin, or textured appearance, etc.

[0007] Among the groups of the above tanning operations, the key step is the tanning step itself, during which the raw hide to be tanned is tanned so as to be transformed into non-perishable leather due to the action of the tanning agent. The raw hide is composed of collagen fibers. During tanning, the tanning agent forms bonds between the collagen fibers of the raw hide, and these bonds stabilize the raw hide and transform it into firm and non-perishable leather.

[0008] Since the release of the European REACH Regulation (Regulation (EC) No. 1907 / 2006 on Registration, Evaluation, Authorization and Restriction of Chemicals), the sale of consumer products has been increasingly controlled, especially in terms of chemical risks. Leather products are no exception and must comply with the new restrictions. In the leather industry, the most widely used tanning agent is chromium in the oxidation state of 3 (Cr 3 + ). In fact, Cr 3+ The chrome tanning method is easy to implement, efficient, and enables the production of leather with properties meeting market expectations. Under certain conditions, a small portion of Cr 3+ present in some leather can be transformed into allergenic Cr 6+ through skin contact. Therefore, the EU has imposed restrictions on the sale of products with a Cr 6+ content higher than 3 mg / kg in leather in contact with raw hides (Regulation EU No. 301 / 2014, which amended Annex XVII of Regulation 1907 / 2006). The EU is even considering reducing this threshold in the coming years. Therefore, the leather industry is looking for alternative solutions for chrome-tanned raw hides.

[0009] Glutaraldehyde can be used as a tanning agent: However, this compound has been listed as a substance of very high concern in the European REACH Regulation by ECHA (European Chemicals Agency). This substance is a respiratory and skin sensitizer.

[0010] Vegetable tannins can also be used to tan raw hides. However, the large amount of the required products and the hardness of the resulting leather limit the use of these tannins in specific applications.

[0011] Other tanning agents can be used, including metals such as aluminum, iron, titanium or zirconium, and synthetic organic compounds.

[0012] In addition, the production of leather or leather products generates leather waste (shavings, flakes, production waste). When raw hides are tanned in the presence of chromium, this waste is difficult to recycle.

[0013] Finally, the effluent from the chrome tanning method contains unreacted chromium. The sewage must be treated before being discharged into the natural environment.

[0014] The document US 3,116,965 describes a method for tanning raw hides using silatrioxanes.

[0015] Given the above limitations, the sustainable development of the leather industry requires alternative solutions for tanning raw hides. Therefore, there is still a need for a method of tanning raw hides that avoids the use of conventional metals (especially chromium) in order to obtain leather that ensures the safety of employees of leather industry companies, consumer protection (safety), recycling of leather waste (production waste or end-of-life items), and environmental protection. Therefore, there is still a need for a method of tanning raw hides that allows for the easy detanning of the tanned raw hides. There is also a need for a method of detanning such raw hides to allow for the recycling of tanned raw hides and leather.

[0016] The present invention relates to a method for tanning raw hides to be tanned, which comprises the following steps:

[0017] -A) Immerse the raw hides to be tanned in an aqueous solution in a tanning drum and adjust the pH of the raw hides and the aqueous solution to 1.5 to 10.5 by adding an acid or a base.

[0018] -B) Treat the raw hides from step A) with a tanning solution having the following composition, by mass relative to the mass of the raw hides:

[0019] o 1 to 1200% of water,

[0020] o Optionally, a sufficient amount of salt to prevent the raw hides from swelling, and

[0021] o 1 to 100% of a compound of formula (I):

[0022] [Formula 1]

[0023]

[0024] wherein Y1, Y2, Y3, and Y4 are each independently selected as a halogen atom, a -[OSiO m R 1 3-m n group or a -OR 2 group, two or more -[OSiO m R 1 3-m n and -OR 2 groups that can be covalently linked to each other, wherein:

[0025] -m is an integer from 0 to 3, n is an integer greater than or equal to 1, and R 1 is selected as a hydrogen atom, a C1-C 10 alkyl, an aryl, a C2-C​​10 alkenyl, C2-C 10 alkynyl, polyoxyalkylene group or a derivative thereof, R 1 optionally substituted by one or more amine, ammonium, amide, ketone, ester, aldehyde, thiol or thioether groups,

[0026] -R 2 selected as a hydrogen atom, C1-C 10 alkyl, aryl, C2-C 10 alkenyl, C2-C 10 alkynyl, polyoxyalkylene group or a derivative thereof, R 2 optionally substituted by one or more amine, ammonium, amide, ketone, ester, aldehyde, thiol or thioether groups,

[0027] Treat the raw hide with the tanning solution until the tanning solution penetrates the raw hide,

[0028] -C) Optionally, adjust the pH of the raw hide obtained at the end of step B) to a value of 2.1 to 10.5, preferably 4.0 to 8.0.

[0029] The method according to the present invention enables the production of metal-free tanned raw hides. In particular, the tanned raw hides obtained by the tanning method according to the present invention are chromium-free. The raw hides obtained by the tanning method according to the present invention have a satisfactory shrinkage temperature for the manufacture of leather goods (shoes, leather goods, gloves, automobiles, furniture). Therefore, these raw hides can be used for shaping leather goods without endangering the health of future users of these goods, regardless of the conditions of use of these goods. In particular, due to the specific use of silicon in the tanned raw hides obtained by the method according to the present invention, these raw hides can be de-tanned for recycling them again.

[0030] The present invention also relates to tanned raw hides obtainable by the above tanning method.

[0031] The present invention also relates to a method for preparing leather from raw hides, which comprises the following steps:

[0032] -i) A step of preparing the raw hides to obtain the raw hides to be tanned,

[0033] -ii) A step of tanning the raw hides to be tanned, the tanning step being carried out according to the tanning method as described above,

[0034] -iii) A step of grinding and / or finishing the tanned raw hides obtained at the end of step ii) to obtain the finished leather.

[0035] The present invention also relates to a method for detanning raw hides and / or finished leather obtained by the tanning method as described above and / or according to the method for preparing leather described above. The method comprises at least one step during which the raw hides and / or leather are immersed in an alkalinizing solution to bring the pH of the raw hides and / or leather to a value greater than or equal to 10.5, preferably to a value between 11.5 and 13.0.

[0036] The raw hides involved in the tanning and detanning methods according to the present invention can be any raw hides containing collagen, whether these raw hides are synthetic or natural. These raw hides can be, for example, from animals whose raw hides contain collagen, such as bovine animals, sheep, goats, reptiles (snakes, lizards, crocodiles, alligators), fish, birds, amphibians, pigs.

[0037] In the tanning method according to the present invention, the term "raw hide to be tanned" refers to the raw hide produced by the operation of preparing the raw hide as described above, for example, a pickled or delimed raw hide that has also been degreased.

[0038] The rehydration and degreasing operations are well known to those skilled in the art. For example, to rehydrate and degrease a pickled raw hide, the raw hide can be placed in a tanning drum, and water, salt, and a degreasing agent can be added. The degreasing agent can be selected from surfactants in a known manner, such as nonionic and anionic surfactants. For example, the tanning drum can be rotated for a time between 15 minutes and 10 hours without heating, the tanning drum can be drained, and then the raw hide can be rinsed with brine at room temperature. The raw hide to be tanned is thus obtained.

[0039] In the method for preparing leather according to the present invention, the term "raw hide" refers to a natural or synthetic raw hide containing collagen, but which has not yet undergone the steps of preparing the raw hide as described above, so that these raw hides are to be tanned.

[0040] According to the first step, step A), of the tanning method according to the present invention, the raw hide to be tanned is immersed in an aqueous solution in a tanning drum, and the pH of the raw hide and the aqueous solution is brought to a value between 1.5 and 10.5 by adding an acid or a base.

[0041] The tanning drum is a rotating drum well-known to those skilled in the art. In the present application, the term "tanning drum" refers to any container configured to receive raw hides to be tanned and provided with a system allowing the mixing of raw hides and an aqueous solution. Thus, a water trough belongs to the definition of a tanning drum according to the present application. Depending on the pH value selected for soaking the raw hides, the aqueous solution may contain salts. The salts have the function of preventing the raw hides from swelling in the aqueous solution. For the purposes of the present application, the term "salt" refers to an ionic compound of cations and anions, forming a neutral product without a net charge. The salts may be selected, for example, from potassium chloride, sodium chloride, and mixtures thereof. For example, if the pH of the aqueous solution is less than 5.0, the aqueous solution will preferably contain salts. The preparation of such salt solutions is within the capabilities of those skilled in the art. Those skilled in the art are able to determine the amount of salt (such as sodium chloride) required to prevent the raw hides from swelling. For example, the aqueous solution may contain from 1 to 1200% by mass of water relative to the mass of the raw hides, and from 6 to 12% by mass of sodium chloride relative to the mass of the water.

[0042] By adding an acid or a base, the pH of the raw hides and the aqueous solution reaches a value of 1.5 to 10.5.

[0043] In one embodiment, the pH of the raw hides and the aqueous solution reaches a value of 1.5 - 2.0, preferably 1.7 - 1.9, for example by adding one or more acids used alone or in combination (such as by adding an acid selected from formic acid, sulfuric acid, and mixtures thereof). As can be seen from the remainder of the present specification, adjusting the pH of the raw hides and the aqueous solution to such an acidic pH enables the tanning solution to be added directly to the tanning drum in the form of a solution of the compound Si(OH)4, which has been previously prepared outside the tanning drum from the compound of formula (I) above (such as Si(OCH2CH3)4). As can also be seen from the remainder of the present specification, reaching such an acidic pH for the raw hides and the aqueous solution also enables the in-situ (in other words, in the tanning drum) preparation of the tanning solution by adding a solution of the compound Si(OCH2CH3)4.

[0044] In another embodiment, the pH of the raw hides and the aqueous solution in step A) reaches a value of 2.1 to 10.5. As can be seen from the remainder of the present specification, reaching a pH value of 2.1 to 10.5 for the raw hides and the aqueous solution in step A) enables the in-situ (in other words, in the tanning drum) preparation of the tanning solution by adding a solution of the compound Si(OCH2CH3)4, while maintaining a relatively high pH, such as a pH of approximately 7.0, during the treatment step with the tanning solution and without a subsequent alkalization step. Similarly, when the treatment step with the tanning solution is carried out at a pH greater than or equal to 5.0, it is not necessary to add salts to the tanning solution.

[0045] According to the second step of the method according to the invention, step B), the raw hide of step A) is treated with a tanning solution. For this purpose, the aqueous solution from step A) can be drained from the tanning drum or not. In the case where the aqueous solution from step A) is drained from the tanning drum, this draining is preferably carried out without rinsing the raw hide.

[0046] The tanning solution consists of the following, by mass relative to the mass of the raw hide:

[0047] o 1 to 1200% of water,

[0048] o Optionally, a sufficient amount of salt (preferably sodium chloride) to prevent the raw hide from swelling, and

[0049] o 1 to 100% of the compound of formula (I):

[0050] [Formula 1]

[0051]

[0052] wherein Y1, Y2, Y3 and Y4 are each independently selected as a halogen atom, -[OSiO m R 1 3-m n group or -OR 2 group, two or more -[OSiO m R 1 3-m n and -OR 2 groups, wherein:

[0053] -m is an integer from 0 to 3, n is an integer greater than or equal to 1, R 1 is selected as a hydrogen atom, C1-C 10 alkyl, aryl, C2-C 10 alkenyl, C2-C 10 alkynyl, polyoxyalkylene group or a derivative thereof, R 1 is optionally substituted by one or more amine, ammonium, amide, ketone, ester, aldehyde, thiol or thioether groups,

[0054] -R 2 is selected as a hydrogen atom, C1-C 10 alkyl, aryl, C2-C 10 alkenyl, C2-C 10 alkynyl, polyoxyalkylene group or a derivative thereof, R 2 is optionally substituted by one or more amine, ammonium, amide, ketone, ester, aldehyde, thiol or thioether groups.

[0055] For the purposes of the present invention: ​​

[0056] - “C1-C 10 The "-C1-C" alkyl group refers to a saturated, straight-chain or branched hydrocarbon chain containing 1 to 10 carbon atoms, such as methyl, ethyl, isopropyl, tert-butyl, pentyl and other groups.

[0057] - An aryl group refers to an aromatic group, preferably containing 5 to 10 carbon atoms, containing one or more rings, and optionally containing heteroatoms (especially oxygen, nitrogen or sulfur), such as phenyl, furan, indole, pyridine, naphthalene and other groups.

[0058] - “C2-C 10 The "-C2-C" alkenyl group refers to a straight-chain or branched hydrocarbon chain containing at least one unsaturated bond and containing 2 to 10 carbon atoms, such as ethenyl (vinyl), propenyl, 2,4-hexadienyl and other groups.

[0059] - “C2-C 10 The "-C2-C" alkynyl group refers to a straight-chain or branched hydrocarbon chain containing at least one double unsaturated bond and containing 2 to 10 carbon atoms, such as acetylene, propyne, butyne and other groups.

[0060] - The "-polyoxyalkylene" group refers to a polymer of ethylene oxide units, and the number of ethylene oxide units is an integer greater than or equal to 1.

[0061] - Polyoxyalkylene derivatives refer to one or more synthetic polymers selected from copolymers of neutralized or unneutralized carboxylic acids and carboxylic acid esters of poly(alkylene glycols), copolymers of carboxylic acids and poly(alkylene amide diols), copolymers of carboxylic acids and poly(alkylene glycol) imides, copolymers of carboxylic acids and poly(alkylene glycol) vinyl ethers, and mixtures thereof.

[0062] Therefore, the tanning solution does not contain any metal other than silicon. In particular, the tanning solution does not contain chromium.

[0063] In one embodiment, Y1, Y2, Y3 and Y4 are each independently selected from a chlorine atom, an -OH group or an -OCH2CH3 group.

[0064] In one embodiment of the method according to the invention, the compound of formula (I) is Si(OH)4. In such a case, the pH of the raw hide and the aqueous solution in step A) will be pre-adjusted to a value of 1.5 to 2.0, preferably 1.7 to 1.9. The tanning solution can thus consist of, by mass relative to the mass of the raw hide:

[0065] o 1 to 1200% water,

[0066] o A sufficient amount of salt (preferably sodium chloride) to prevent the raw hide from swelling, and

[0067] o 5 to 60%, preferably 5 to 30%, more preferably 10 to 20% of Si(OH)4.

[0068] In such an embodiment, the tanning solution is preferably prepared outside the tanning drum by hydrolyzing the compound Si(OCH2CH3)4 into Si(OH)4, and then the tanning solution is added to the raw hide in the tanning drum. For example, the tanning solution can be prepared by mixing an aqueous solution with a pH of 1.5 to 2.0 and Si(OCH2CH3)4 at a volume ratio (v / v) of 1 / 1 to 25 / 1, preferably 1 / 1 to 10 / 1.

[0069] The Si(OH)4 thus obtained is added to the tanning drum at a mass content of 5 to 60%, preferably 5 to 30%, more preferably 10 to 20% relative to the mass of the raw hide. This content can be adjusted according to the type of raw hide to be tanned and the thickness of these raw hides. In addition, in order to prevent the raw hide from swelling, salt (such as sodium chloride) can also be added to the tanning drum, for example, at a mass content of 6 to 12% relative to the mass of the added Si(OH)4.

[0070] More acidic brine can be added to the tanning drum. Finally, in the tanning drum, water can be present at a mass content of 1% to 1200% relative to the mass of the raw hide.

[0071] Preferably, the tanning drum is rotated for a period of time to allow the silicic acid to penetrate the raw hide. Therefore, this duration can vary according to the thickness and nature of the raw hide: for example, the penetration of the raw hide to be tanned with silicic acid may require a longer duration for cowhide than for sheepskin. Those skilled in the art can easily determine the duration of this step according to the raw hide to be tanned. The rotation of the tanning drum is preferably carried out at room temperature (in other words, the temperature is lower than or equal to 30 °C, preferably the temperature is 20 to 25 °C).

[0072] After the silicic acid penetrates the raw hide, the raw hide can be directly taken out of the tanning drum and dried. In such a case, at the end of step B), the raw hide is taken out of the tanning drum and dried. The moisture they contain evaporates during drying. This evaporation causes the polymerization of Si(OH)4, producing a three-dimensional Si-O-Si network connected to the raw hide collagen, thereby stabilizing the raw hide. At the end of this drying step, dried leather that can be directly stored is obtained.

[0073] In another embodiment, after the silicic acid penetrates the raw hide, the pH of the raw hide reaches a value of 2.1 to 10.5, preferably 4.0 to 8.0. This optional step C) will be described below.

[0074] In another embodiment of the method according to the invention, the compound of formula (I) is Si(OCH2CH3)4.

[0075] The tanning solution can thus consist of the following, by mass relative to the mass of the raw hide:

[0076] o 1 to 1200% of water,

[0077] o A sufficient amount of salt (preferably sodium chloride) to prevent the raw hide from swelling, and

[0078] o 5 to 100%, preferably 10 to 60%, more preferably 25 to 45% of Si(OCH2CH3)4.

[0079] In such an embodiment, the tanning solution can be generated directly in situ by adding Si(OCH2CH3)4 to the tanning drum in a mass content of 5 to 100%, preferably 10 to 60%, more preferably 25 to 45% relative to the mass of the raw hide.

[0080] If necessary, more acidic brine can be added. Finally, in the tanning drum, water can be present in a mass content of 1% to 1200% relative to the mass of the raw hide.

[0081] In this embodiment, the hydrolysis of Si(OCH2CH3)4 to Si(OH)4 will thus occur in the tanning drum. When Si(OCH2CH3)4 is added to the tanning drum, there are two immiscible phases in the tanning drum, one is water and the other is Si(OCH2CH3)4.

[0082] To obtain a single soluble phase of the silicic acid solution and allow the silicic acid to penetrate the raw hide, the tanning drum is preferably kept rotating for the duration required for the complete hydrolysis of Si(OCH2CH3)4 to Si(OH)4. For example, this duration can be from 1 hour to one week. As mentioned above, the duration required for the silicic acid to penetrate the raw hide depends on the nature of the raw hide. Those skilled in the art can easily determine this duration. The rotation of the tanning drum is preferably carried out at room temperature (in other words, the temperature is less than or equal to 30 °C, preferably the temperature is 20 to 25 °C).

[0083] In addition, the duration of the complete hydrolysis of Si(OCH2CH3)4 to Si(OH)4 can also depend on the pH of the tanning solution.

[0084] Thus, in one embodiment, the compound of formula (I) is Si(OCH2CH3)4, and the pH of the raw hide and the aqueous solution in step A) will be pre-adjusted to a value of 1.5 to 2.0, preferably 1.7 to 1.9. In this embodiment, after the silicic acid penetrates the raw hide, the raw hide can be directly taken out of the tanning drum and dried, as described above. Alternatively, they can be alkalized, as described below.

[0085] In another embodiment, the compound of formula (I) is Si(OCH2CH3)4, and the pH of the hide and the aqueous solution in step A) will be pre-adjusted to a value of 2.1 to 10.5. In such a case, before adding Si(OCH2CH3)4 to the tanning drum, the pH of the aqueous solution and the hide is adjusted to a pH of 2.1 to 10.5, preferably 4.0 to 8.0, more preferably 4.0 to 5.0, as described above. Thus, the pH of the tanning solution is 2.1 to 10.5, preferably 4.0 to 8.0, more preferably 4.0 to 5.0. Such a pH of the tanning solution enables the reactions of hydrolysis of Si(OCH2CH3)4 to Si(OH)4 and the polymerization of Si(OH)4 to occur simultaneously in the tanning drum. The polymerization of Si(OH)4 will result in the formation of a three-dimensional Si-O-Si network and the establishment of a bond with the hide collagen fibers. This three-dimensional network will enable the conversion of the initially perishable hide into a non-perishable tanned hide, and thus into a stable and durable leather. Therefore, in an embodiment of the method according to the invention, where the pH of the tanning solution is 2.1 to 10.5, preferably 4.0 to 8.0, more preferably 4.0 to 5.0, it is not necessary to perform additional steps, such as a specific step of basifying the hide to tan it. In such an embodiment, the hide obtained at the end of step B) can be directly used for subsequent grinding steps or dried to complete the polymerization and storage. However, in another embodiment of the method according to the invention, where the pH of the tanning solution is 2.1 to 10.5, preferably 4.0 to 8.0, more preferably 4.0 to 5.0, a step of treating the hide with a basifying solution can be performed, as described below.

[0086] Therefore, in an embodiment, the tanning method according to the invention may include an additional step, step C). During step C), the pH of the hide obtained at the end of step B) is adjusted to a value of 2.1 to 10.5, preferably 4.0 to 8.0.

[0087] Therefore, when the tanning solution is too acidic to initiate the polymerization of Si(OH)4, for example when the pH of the tanning solution is less than or equal to 2.0, step C) is preferably implemented.

[0088] During step C), the hide can be treated, for example, with a basifying solution. The basification of the bath containing the hide will allow and / or accelerate the polymerization of Si(OH)4 within the hide. Bonds will be able to form between the silicon and the collagen of the hide fibers, and a three-dimensional Si-O-Si network connected to the hide collagen will be able to be produced, thus stabilizing the hide. A tanned hide will be obtained.

[0089] In one embodiment of the method according to the invention, the treatment with the alkalizing solution is carried out in a tanning drum by adding an alkalizing agent selected, for example, from sodium formate, sodium bicarbonate, sodium acetate, calcium formate, sodium carbonate and mixtures thereof, the mass content of the alkalizing agent being 0.5 - 20% relative to the mass of the raw hide.

[0090] In another embodiment, the treatment with the alkalizing solution can be carried out outside the tanning drum. For example, the treatment can include the following steps:

[0091] -C1) At the end of step B), removing the raw hide from the tanning drum,

[0092] -C2) Placing the raw hide flat and one on top of the other in a trough,

[0093] -C3) Filling the trough with the alkalizing solution to completely submerge all the raw hides, the alkalizing solution containing an alkalizing agent selected from sodium formate, sodium bicarbonate, sodium acetate, calcium formate, sodium carbonate and mixtures thereof,

[0094] -C4) Leaving the raw hides in the alkalizing solution in the trough and allowing to stand for, for example, at least 1 hour.

[0095] Calculate the content of the alkalizing agent according to the desired final pH. During step C1), the raw hide can be removed from the tanning drum with or without prior rinsing.

[0096] At any time during step C), the raw hides can be dried in order to polymerize Si(OH)4 by evaporation.

[0097] The raw hides obtained by the method according to the invention are tanned and not perishable. These raw hides are generally white. Their appearance makes them look rounder and fuller than their initial state before tanning. The shrinkage temperature of these raw hides is 65 to 90 °C. Such a shrinkage temperature allows these raw hides to withstand treatments during which they can be subjected to relatively high temperatures (e.g., 50 to 75 °C), such as thickening, hot dyeing for better fixation of dyes, etc., while maintaining their flexibility and appearance quality.

[0098] The invention also relates to a method for preparing leather from raw hides, which includes the following steps:

[0099] -i) A step of preparing the raw hides to obtain the raw hides to be tanned,

[0100] -ii) A step of tanning the raw hides to be tanned, the tanning step being carried out according to the above tanning method,

[0101] -iii) A step of grinding and / or finishing the tanned raw hides obtained at the end of step ii) to obtain the finished leather.

[0102] The present invention also relates to a method for detanning tanned hides and / or leather obtained by the above method, which comprises at least one step during which the hides and / or leather are immersed in an alkalizing solution so as to bring the pH of the hides and / or leather to a value greater than or equal to 10.5, preferably to a value between 11.5 and 13.0.

[0103] In the detanning method according to the invention, the tanned hides can be, for example, in the "dry white" state. For the purposes of the present invention, the term "dry white" means tanned, dry but unground hides. The tanned hides can optionally be in the wet white, stained, crust, finished or semi-finished leather, shavings, sheet form, these terms being well known to those skilled in the art. Leather waste and leather dust from wet white, "dry white", stained, crust or finished leather can also be detanned by the detanning method according to the invention.

[0104] For example, in order to carry out detanning, the tanned hides and / or finished leather can be immersed in an aqueous solution, for example in a tanning drum. When the tanned hides are in the form of leather waste and leather dust, they can optionally be placed in a container provided with stirring blades or other mixing systems. The aqueous solution can contain a degreasing agent. For example, the aqueous solution can contain the following substances in the following masses relative to the mass of the hides:

[0105] - 50 to 2000% water,

[0106] - 0.1 to 2% degreasing agent.

[0107] The degreasing agent can be selected from surfactants, for example selected from non-ionic and anionic surfactants.

[0108] The purpose of such a step is to degrease and rehydrate the hides and / or leather. Preferably, the tanning drum is rotated for a period of time (for example 15 min to overnight, for example 45 min to 1 hour 30 mins) at a temperature between 10 and 80 °C (for example between 30 and 45 °C) to allow this degreasing and this rehydration.

[0109] The tanning drum can then be emptied and the hides and / or leather can be rinsed in the tanning drum at room temperature.

[0110] The hides and / or leather are immersed in an aqueous solution containing an alkalizing agent so as to bring the pH of the hides and / or leather to a value greater than or equal to 10.5, preferably to a value between 11.5 and 13.0.

[0111] The alkalizing agent can be selected, for example, from sodium bicarbonate, sodium carbonate, ammonia, sodium hydroxide and mixtures thereof.

[0112] The basifying agent can be added to the aqueous solution, for example, in several steps, with or without different basifying agents being used in each step, and the working temperature can vary from one step to another.

[0113] For example, the first addition of the basifying agent can bring the pH of the hide and / or leather to a value of 7.0 to 10.0. For this purpose, for example, sodium bicarbonate and / or sodium carbonate and / or ammonia are used, and the temperature of the aqueous solution is maintained at a value of 10 to 80 °C, for example 30 to 45 °C.

[0114] Then a second addition of the basifying agent can be carried out to bring the pH of the hide and / or leather to a value of 11.5 to 13.0. For this purpose, for example, sodium hydroxide is used, and the temperature of the aqueous solution is maintained at a value of 10 to 30 °C, for example 20 to 25 °C.

[0115] Bringing the hide and / or leather to a pH greater than or equal to 10.5, preferably 11.5 to 13.0, causes the decomposition of the three-dimensional Si-O-Si network linked to the hide collagen. At such a pH, the hide and / or leather (such as wet white, "dry white", dyed, finished green leather, leather dust and waste) lose their resistance to the shrinkage temperature: this loss indicates the de-tanning of the hide and / or leather.

[0116] The thus de-tanned wet white, "dry white", dyed, green leather, finished leather, shavings, flakes, leather dust and waste can be retained or recycled again in the form of non-crosslinked proteins, for example, by re-tanning or by being used for manufacturing gelatin, manufacturing animal feed proteins or manufacturing agricultural fertilizers, etc.

[0117] Experimental section

[0118] Example 1

[0119] Preparation of Si(OH)4 from Si(OCH2CH3)4

[0120] 1 L of Si(OCH2CH3)4 was mixed with 1 L of water.

[0121] Using a strong acid (such as hydrochloric acid or sulfuric acid, this list is not exhaustive), the pH of the mixture was adjusted to a value of 1.5 to 2.0.

[0122] The reaction of Si(OCH2CH3)4 hydrolyzing into Si(OH)4 is exothermic. Therefore, preferably, the mixture was maintained at the required temperature, for example, around 30 °C, by a cooling bath. Preferably, the mixture was stirred vigorously for 15 min to 120 min. The duration of the hydrolysis reaction can be 20 min to 2 hours. At the end of this reaction, a transparent silicic acid solution with the chemical formula Si(OH)4 was obtained. 10% by mass of sodium chloride was added relative to the total mass of the pre-prepared silicic acid solution.

[0123] Example 2

[0124] Tanning raw sheepskins according to the tanning method of the present invention is as follows:

[0125] Step A)

[0126] The raw skins to be tanned are pickled raw skins from stored brine or unhaired skins that have been pre-degreased and rehydrated according to the expertise of those skilled in the art. Weigh the raw skins.

[0127] Place the raw skins in a tanning drum in a salt solution. Adjust the pH of the raw skins and the salt solution to a value of 1.5 to 2.0, for example, by adding sulfuric acid alone or sulfuric acid mixed with formic acid.

[0128] Step B)

[0129] - 50% cold water, at 25 °C,

[0130] - 5% sodium chloride,

[0131] - 13% Si(OH)4 hydrolyzed from Si(OCH2CH3)4 as described in Example 1,

[0132] are added to the tanning drum, by mass relative to the mass of the raw skins.

[0133] The tanning drum is rotated at room temperature (i.e., 25 °C) for 4 hours.

[0134] Step C)

[0135] First, at a temperature of 25 °C, 5% by mass of sodium formate relative to the mass of the raw skins is added to the tanning drum. The tanning drum is rotated for 1 hour. The pH measured on the raw skins is 3.8 to 4.0.

[0136] Second, at a temperature of 25 °C, 5% by mass of sodium formate relative to the mass of the raw skins is added to the tanning drum. The tanning drum is rotated overnight. The final pH measured on the raw skins is 4.0 to 4.2.

[0137] Empty the tanning drum and rinse the raw skins.

[0138] Tanned raw skins without metals (especially chromium) are obtained.

[0139] The raw skins are white, round and have a nice appearance.

[0140] Example 3

[0141] Tanning raw cowhides according to the tanning method of the present invention is as follows:

[0142] Step A)

[0143] The raw hide to be tanned is a pickled raw hide from stored brine or unhaired raw hide, which has been pre - degreased and re - hydrated according to the expertise of those skilled in the art. Weigh the raw hide.

[0144] Place the raw hide in a tanning drum in a salt solution. Adjust the pH of the raw hide and the salt solution to a value of 1.5 to 2.0, for example, by adding sulfuric acid alone or sulfuric acid mixed with formic acid.

[0145] Step B)

[0146] - 50% cold water, at 25 °C,

[0147] - 5% sodium chloride,

[0148] - 20% Si(OH)4 hydrolyzed from Si(OCH2CH3)4 as described in Example 1,

[0149] is added to the tanning drum, by mass relative to the mass of the raw hide.

[0150] The tanning drum is rotated overnight at room temperature (i.e., 25 °C).

[0151] Step C)

[0152] First, at a temperature of 25 °C, 5% by mass of sodium formate relative to the mass of the raw hide is added to the tanning drum. The tanning drum is rotated for 1 hour. The pH measured on the raw hide is 3.8 to 4.0.

[0153] Second, at a temperature of 25 °C, 5% by mass of sodium formate relative to the mass of the raw hide is added to the tanning drum. The tanning drum is rotated for 1 night. The final pH measured on the raw hide is 4.0 to 4.2.

[0154] Empty the tanning drum and rinse the raw hide.

[0155] A tanned raw hide without metals (especially chromium) is obtained.

[0156] The raw hide is white, round and has a nice appearance.

[0157] Example 4

[0158] Tan sheep raw hide according to the tanning method of the present invention as follows:

[0159] Step A)

[0160] The raw hide to be tanned is a pickled raw hide from stored brine or unhaired raw hide that has been pre - degreased and re - hydrated according to the expertise of a person skilled in the art. Weigh the raw hide.

[0161] Place the raw hide in a tanning drum in a salt solution. Adjust the pH of the raw hide and the salt solution to a value of 1.5 to 2.0, for example, by adding sulfuric acid alone or sulfuric acid mixed with formic acid.

[0162] Step B)

[0163] - 50% cold water, at 25 °C,

[0164] - 5% sodium chloride,

[0165] - 13% Si(OH)4 hydrolyzed from Si(OCH2CH3)4 as described in Example 1,

[0166] is added to the tanning drum, by mass relative to the mass of the raw hide.

[0167] The tanning drum is rotated at room temperature (i.e., 25 °C) for 4 hours.

[0168] Step C)

[0169] First, at a temperature of 25 °C, 5% by mass of sodium formate relative to the mass of the raw hide is added to the tanning drum. The tanning drum is rotated for one minute and then stopped for 1 hour. The pH measured on the raw hide is 3.8 to 4.0.

[0170] Second, at a temperature of 25 °C, 5% by mass of sodium formate relative to the mass of the raw hide is added to the tanning drum. The tanning drum is rotated for one minute and then stopped for 1 night. The final pH measured on the raw hide is 4.0 to 4.2.

[0171] A tanned raw hide free of metals (especially chromium) is obtained.

[0172] The raw hide is white, rounded and has a nice appearance.

[0173] Example 5

[0174] Tan bovine raw hides according to the tanning method according to the present invention, as follows:

[0175] Step A)

[0176] The raw hide to be tanned is a pickled raw hide from stored brine or unhaired raw hide that has been pre - degreased and re - hydrated according to the expertise of a person skilled in the art. Weigh the raw hide.

[0177] Place the raw hide in a tanning drum in a salt solution. Adjust the pH of the raw hide and the salt solution to a value of 1.5 to 2.0, for example, by adding sulfuric acid alone or sulfuric acid mixed with formic acid.

[0178] Step B)

[0179] - 50% cold water, at 25 °C,

[0180] - 5% sodium chloride,

[0181] - 20% Si(OH)4 hydrolyzed from Si(OCH2CH3)4 as described in Example 1,

[0182] is added to the tanning drum, by mass relative to the mass of the raw hide.

[0183] The tanning drum is rotated overnight at room temperature (i.e., 25 °C).

[0184] Step C)

[0185] First, at a temperature of 25 °C, 5% by mass of sodium formate relative to the mass of the raw hide is added to the tanning drum. The tanning drum is rotated for one minute and then stopped for 1 hour. The pH measured on the raw hide is 3.8 to 4.0.

[0186] Second, at a temperature of 25 °C, 5% by mass of sodium formate relative to the mass of the raw hide is added to the tanning drum. The tanning drum is rotated for one minute and then stopped for 1 night. The final pH measured on the raw hide is 4.0 to 4.2.

[0187] Tanned raw hides free of metals (especially chromium) are obtained.

[0188] The raw hides are white, round and have a nice appearance.

[0189] Example 6

[0190] Tan sheep raw hides according to the tanning method according to the present invention as follows:

[0191] Step A)

[0192] The raw hides to be tanned are pickled hides from stored brine or unhaired hides that have been pre - degreased and re - hydrated according to the expertise of those skilled in the art. Weigh the raw hides.

[0193] Place the raw hide in a tanning drum in a salt solution. Adjust the pH of the raw hide and the salt solution to a value of 1.5 to 2.0, for example, by adding sulfuric acid alone or sulfuric acid mixed with formic acid.

[0194] Step B)

[0195] -28% of Si(OCH2CH3)4

[0196] is added to the tanning drum, by mass relative to the mass of the raw hide.

[0197] The amount of Si(OCH2CH3)4 is equivalent to 13% by mass of Si(OH)4 relative to the mass of the raw hide.

[0198] The tanning drum is rotated for 4 hours at room temperature (i.e., 25 °C).

[0199] Then:

[0200] -150% of water, at 25 °C

[0201] -15% of sodium chloride

[0202] is added to the tanning drum, by mass relative to the mass of the raw hide.

[0203] The tanning drum is rotated for 20 minutes at 25 °C.

[0204] Step C)

[0205] First, at a temperature of 25 °C, 5% by mass of sodium formate relative to the weight of the raw hide is added to the tanning drum. The tanning drum is rotated for 1 hour. The pH measured on the raw hide is 3.8 to 4.0.

[0206] Second, at a temperature of 25 °C, 5% by mass of sodium formate relative to the weight of the raw hide is added to the tanning drum. The tanning drum is rotated overnight. The final pH measured on the raw hide is 4.0 to 4.2.

[0207] The tanning drum is emptied and the raw hide is rinsed.

[0208] Tanned raw hides free of metals (especially chromium) are obtained.

[0209] The raw hides are white, rounded and have a nice appearance.

[0210] Example 7

[0211] Tanning sheep raw hides according to the tanning method according to the invention is as follows:

[0212] Step A)

[0213] The raw hides to be tanned are pickled hides from stored brine or dehaired raw hides that have been pre-degreased and rehydrated according to the expertise of those skilled in the art. Weigh the raw hides.

[0214] The raw hide is placed in a salt solution in a tanning drum. The pH of the raw hide and the salt solution is brought to a value of 1.5 to 2.0, for example, by adding sulfuric acid alone or sulfuric acid mixed with formic acid.

[0215] Step B)

[0216] - 28% of Si(OCH2CH3)4

[0217] is added to the tanning drum, by mass relative to the mass of the raw hide.

[0218] The amount of Si(OCH2CH3)4 corresponds to 13% by mass of Si(OH)4 relative to the mass of the raw hide.

[0219] The tanning drum is rotated at room temperature (i.e., 25 °C) for 4 hours.

[0220] Then:

[0221] - 150% of water, at 25 °C

[0222] - 15% of sodium chloride

[0223] is added to the tanning drum, by mass relative to the mass of the raw hide.

[0224] The tanning drum is rotated at 25 °C for a time of 20 minutes.

[0225] Step C)

[0226] First, at a temperature of 25 °C, 5% by volume of sodium formate relative to the weight of the raw hide is added to the tanning drum. The tanning drum is rotated for a time of one minute and then stopped for 1 hour. The pH measured on the raw hide is 3.8 to 4.0.

[0227] Second, at a temperature of 25 °C, 5% by mass of sodium formate relative to the weight of the raw hide is added to the tanning drum. The tanning drum is rotated for a time of one minute and then stopped overnight. The final pH measured on the raw hide is 4.0 to 4.2.

[0228] The tanning drum is emptied and the raw hide is rinsed.

[0229] Tanned raw hides free of metals (especially chromium) are obtained.

[0230] The raw hides are white, round and have a nice appearance.

[0231] Example 8

[0232] According to the tanning method according to the invention, bovine raw hides are tanned as follows:

[0233] Step A)

[0234] The raw hide to be tanned is a pickled hide from stored brine or unhaired hide that has been pre - degreased and re - hydrated according to the expertise of those skilled in the art. Weigh the raw hide.

[0235] Place the raw hide in a tanning drum in a salt solution. Adjust the pH of the raw hide and the salt solution to a value of 1.5 to 2.0, for example, by adding sulfuric acid alone or sulfuric acid mixed with formic acid.

[0236] Step B)

[0237] Add Si(OH)4 hydrolyzed from Si(OCH2CH3)4 as described in Example 1 in multiple steps according to the following procedure. Given as a mass percentage relative to the mass of the raw hide:

[0238] - Add 0.6% of Si(OH)4, then rotate the tanning drum at 25 °C for 4 hours,

[0239] - Add 0.6% of Si(OH)4, then rotate the tanning drum at 25 °C for 4 hours,

[0240] - Add 3.8% of Si(OH)4, then rotate the tanning drum at 25 °C overnight,

[0241] - Add 5% of Si(OH)4, then rotate the tanning drum at 25 °C for 10 hours,

[0242] - Add 5% of Si(OH)4, then rotate the tanning drum at 25 °C overnight,

[0243] - Add 7.5% of Si(OH)4, then rotate the tanning drum at 25 °C for 24 hours,

[0244] - Add 7.5% of Si(OH)4, then rotate the tanning drum at 25 °C for 24 hours.

[0245] Then:

[0246] - 100% water, at 25 °C,

[0247] - 10% sodium chloride, at 25 °C

[0248] Are added to the tanning drum, by mass relative to the mass of the raw hide.

[0249] The tanning drum is rotated for 20 minutes.

[0250] Step C)

[0251] At a temperature of 25 °C, 1.5% by weight of sodium formate relative to the weight of the raw hide is added to the tanning drum. The tanning drum rotates for one minute every 30 minutes for a period of 12 hours. The pH measured on the raw hide is 3.88.

[0252] Then, at a temperature of 25 °C, 1.5% by mass of sodium formate relative to the mass of the raw hide is added to the tanning drum. The tanning drum rotates for one minute every 30 minutes for a period of 12 hours. The pH measured on the raw hide is 4.40.

[0253] Then, at a temperature of 25 °C, 4% by mass of sodium formate relative to the mass of the raw hide is added to the tanning drum. The tanning drum rotates for one minute every 30 minutes for a period of 6 hours. The pH measured on the raw hide is 4.50.

[0254] Empty the tanning drum and rinse the raw hide.

[0255] A tanned raw hide free of metals (especially chromium) is obtained.

[0256] The raw hide is white, round and has a nice appearance.

[0257] Example 9

[0258] Tan sheep raw hides according to the tanning method according to the present invention as follows:

[0259] Step A)

[0260] The raw hide to be tanned is a pickled raw hide from stored brine or unhaired raw hide that has been pre-degreased and rehydrated according to the expertise of those skilled in the art. Weigh the raw hide.

[0261] Place the raw hide in a salt solution in the tanning drum. Adjust the pH of the raw hide and the salt solution to a value of 1.5 to 2.0, for example, by adding sulfuric acid alone or sulfuric acid mixed with formic acid.

[0262] Step B)

[0263] - 50% cold water, at 25 °C

[0264] - 5% sodium chloride

[0265] - 13% Si(OH)4, hydrolyzed from Si(OCH2CH3)4 as described in Example 1

[0266] is added to the tanning drum, by mass relative to the mass of the raw hide.

[0267] The tanning drum rotates at room temperature (i.e., 25 °C) for 4 hours.

[0268] Empty the tanning drum and rinse the raw hide.

[0269] Step C)

[0270] Take the raw hide out of the tanning drum and place it on a rack for the time required to drain the surface moisture. Then place the raw hide in a damp frame and leave it outdoors to dry, usually overnight or longer, depending on the thickness of the raw hide and the daily weather.

[0271] The next day, after the moisture contained in the raw hide has evaporated, the raw hide becomes dry, white and tanned.

[0272] Example 10

[0273] Tan bovine raw hides according to the tanning method according to the present invention as follows:

[0274] Step A)

[0275] The raw hides to be tanned are pickled raw hides from stored brine or unhaired hides that have been pre-degreased and rehydrated according to the expertise of those skilled in the art. Weigh the raw hides.

[0276] Place the raw hides in a salt solution in the tanning drum. Adjust the pH of the raw hides and the salt solution to a value of 1.5 to 2.0, for example, by adding sulfuric acid alone or sulfuric acid mixed with formic acid.

[0277] Step B)

[0278] - 50% cold water, at 25 °C,

[0279] - 5% sodium chloride,

[0280] - 20% Si(OH)4 hydrolyzed from Si(OCH2CH3)4 as described in Example 1,

[0281] is added to the tanning drum, by mass relative to the mass of the raw hides.

[0282] The tanning drum is rotated overnight at room temperature (i.e., 25 °C).

[0283] Empty the tanning drum and rinse the raw hides.

[0284] Step C)

[0285] Take the raw hides out of the tanning drum and place them on a rack for the time required to drain the surface moisture. Then place the raw hides in a damp frame and leave it outdoors to dry, usually overnight or longer, depending on the thickness of the raw hides and the daily weather.

[0286] On the second day, after the moisture contained in the raw hide evaporates, the raw hide becomes dry, white and tanned.

[0287] Example 11

[0288] Tan sheep raw hides according to the tanning method of the present invention as follows:

[0289] Step A)

[0290] The raw hides to be tanned are pickled raw hides from stored brine or unhaired raw hides that have been pre-degreased and rehydrated according to the expertise of those skilled in the art. Weigh the raw hides.

[0291] Place the raw hides in a tanning drum in a salt solution. Adjust the pH of the raw hides and the salt solution to a value of 2.1 to 10.5, for example, by adding sulfuric acid alone or sulfuric acid mixed with formic acid, or by adding an alkalizing agent.

[0292] Step B)

[0293] Adjust the pH of the raw hides and the salt solution to a value of 4.0 to 5.0, for example, 4.2.

[0294] - 28% of Si(OCH2CH3)4

[0295] Is added to the tanning drum, by mass relative to the mass of the raw hides.

[0296] The amount of Si(OCH2CH3)4 is equivalent to 13% by mass of Si(OH)4 relative to the mass of the raw hides.

[0297] The tanning drum is rotated at room temperature (i.e., 25 °C) for 7 days.

[0298] Then empty the tanning drum and rinse the raw hides.

[0299] Obtain tanned raw hides.

[0300] Example 12

[0301] Measurement of shrinkage temperature

[0302] Measure the shrinkage temperature of the tanned raw hides obtained in Examples 2 to 9 and 11 according to the following protocol:

[0303] - Cut a piece of the tanned raw hide approximately 2 cm by 1 cm at the collar,

[0304] - Place one side of the prepared sample on the fixed area of a retractometer,

[0305] - Place the other side of the sample on the movable hook of the retractometer and balance it with a weight,

[0306] - Place the retractor containing the sample in a beaker filled with cold water (temperature strictly below 25 °C): the raw hide sample must be completely immersed.

[0307] - Place the thermometer in the same beaker and fix it to the retractor. The bottom of the thermometer is placed in front of the tanned raw hide sample.

[0308] - Set the contraction indicating arrow to zero.

[0309] - Place the assembly (beaker filled with water + retractor containing the sample + thermometer) on the hot plate.

[0310] - Then turn on the hot plate.

[0311] - When the arrow initially set to zero starts to move, record the temperature value. This temperature indicates the moment of raw hide contraction.

[0312] The results obtained by tanning raw hides according to Examples 2 to 9 and 11 above are summarized below Table I as follows:

[0313] [Table 1]

[0314] Example Shrinkage temperature (in °C) Example 2 74 Example 3 75 Example 4 73 Example 5 75 Example 6 73 Example 7 73 Example 8 74 Example 9 72 Example 11 73

[0315] Table I : Shrinkage temperature

[0316] The shrinkage temperature of the raw hides tanned with the tanning method according to the invention is particularly satisfactory. Such a shrinkage temperature allows these raw hides to be suitable for undergoing treatments during which they can withstand relatively high temperatures (for example in the range of 50 to 75 °C), such treatments as thickening, hot dyeing for better fixing of dyes, etc., while maintaining their flexibility and appearance quality.

[0317] Example 13

[0318] According to the detanning method according to the present invention below, the raw hides obtained in Examples 2 to 11 above can be easily detanned.

[0319] Introduce the tanned raw hide into the tanning drum. Weigh the raw hide. The mass will be used as a reference.

[0320] - 50 to 2000% water,

[0321] - 0.1 to 2% degreasing agent,

[0322] are added to the tanning drum, by mass relative to the mass of the raw hide.

[0323] The tanning drum rotates for a time of 15 minutes to overnight, desirably 45 minutes to 1 hour 30 minutes, at a temperature of 10 to 80 °C, desirably 30 to 45 °C.

[0324] Empty the tanning drum and rinse the raw hide at room temperature (i.e., 25 °C).

[0325] Add an alkalizing agent selected from sodium bicarbonate, sodium carbonate, ammonia, and mixtures thereof to the tanning drum to bring the pH of the raw hide to a value of 7.0 to 10.0, preferably to a value of 8.0 to 9.0. The alkalizing agent is added in a mass content of 0.1 to 15%, preferably 1 to 5%, relative to the mass of the raw hide.

[0326] The tanning drum rotates for a time of 15 to 120 minutes (e.g., 45 minutes to 90 minutes). The temperature is 10 to 80 °C (e.g., 30 to 45 °C).

[0327] Then add sodium hydroxide to the tanning drum to bring the pH of the raw hide to a value greater than or equal to 10.5, preferably to a value of 11.5 to 13.0.

[0328] The tanning drum rotates for a time of 30 minutes to 24 hours (e.g., overnight). The temperature is 10 to 30 °C (e.g., 20 to 25 °C).

[0329] During this step, the three-dimensional Si-O-Si network linked to the raw hide collagen is unraveled and the raw hide is detanned. The detanned raw hide can be kept in the form of non-crosslinked proteins or recycled again. The detanned raw hide is free of metals (especially chromium).

Claims

1. A method for tanning raw hides to be tanned, which comprises the following steps: - A) Immerse the raw hides to be tanned in an aqueous solution in a tanning drum, and adjust the pH of the raw hides and the aqueous solution to a value of 1.5 to 10.5 by adding an acid or a base. - B) Treat the raw hides from step A) with a tanning solution, which consists of the following, by mass relative to the mass of the raw hides: o 1 to 1200% of water, o Optionally, a sufficient amount of salt to prevent the raw hides from swelling, and o 1 to 100% of the compound of formula (I): [Formula 1] wherein Y1, Y2, Y3 and Y4 are each independently selected from a halogen atom, -[OSiO m R 1 3-m n group or -OR 2 group, two or more -[OSiO m R 1 3-m n and -OR 2 groups that can be covalently linked to each other, wherein:​​ -m is an integer from 0 to 3, n is an integer greater than or equal to 1, R 1 is selected from a hydrogen atom, C1-C 10 alkyl, aryl, C2-C 10 alkenyl, C2-C 10 alkynyl, polyoxyalkylene group or a derivative thereof, R 1 optionally substituted by one or more amine, ammonium, amide, ketone, ester, aldehyde, thiol or thioether groups, -R 2 Selected as a hydrogen atom, C1-C 10 alkyl group, aryl group, C2-C 10 alkenyl group, C2-C 10 alkynyl group, polyoxyalkylene group or a derivative thereof, R 2 Optionally substituted by one or more amine, ammonium, amide, ketone, ester, aldehyde, thiol or thioether groups, Treat the raw hides with the tanning solution until the tanning solution penetrates the raw hides. - C) Optionally, adjust the pH of the raw hides obtained at the end of step B) to a value of 2.1 to 10.5, preferably 4.0 to 8.

0.

2. The method according to claim 1, characterized in that Y1, Y2, Y3 and Y4 are each independently selected as a chlorine atom, an -OH group or an -OCH2CH3 group.

3. The method according to claim 1 or 2, characterized in that, Adjust the pH of the raw hides and the aqueous solution in step A) to a value of 1.5 to 2.0, preferably 1.7 to 1.9, for example by adding an acid selected from formic acid, sulfuric acid and mixtures thereof.

4. The method according to claim 3, wherein The compound of formula (I) is Si(OH)4.

5. The method according to claim 4, wherein The tanning solution consists of the following, by mass relative to the mass of the raw hides: o 1 to 1200% of water, o A sufficient amount of salt, preferably sodium chloride, to prevent the raw hides from swelling, and o 5 to 60%, preferably 5 to 30%, more preferably 10 to 20% of Si(OH)4.

6. The method according to any one of claims 4 to 5, characterized in that During the implementation of step B), prepare the tanning solution outside the tanning drum by hydrolyzing the compound Si(OCH2CH3)4 into Si(OH)4, and then add the tanning solution to the raw hides in the tanning drum.

7. The method according to claim 6, characterized in that, Prepare the tanning solution by mixing an aqueous solution with a pH of 1.5 to 2.0 and Si(OCH2CH3)4 at a volume ratio (v / v) of 1 / 1 to 25 / 1, preferably 1 / 1 to 10 / 1.

8. The method according to claim 1 or 2, characterized in that, The compound of formula (I) is Si(OCH2CH3)4.

9. The method according to claim 8, wherein The tanning solution consists of the following, by mass relative to the mass of the raw hides: o 1 to 1200% of water, o A sufficient amount of salt, preferably sodium chloride, to prevent the raw hides from swelling, and o 5 to 100%, preferably 10 to 60%, more preferably 25 to 45% of Si(OCH2CH3)4.

10. The method according to claim 8 or 9, characterized in that During the implementation of step B), directly prepare the tanning solution in situ by adding 5 to 100%, preferably 10 to 60%, more preferably 25 to 45% by mass of Si(OCH2CH3)4 relative to the mass of the raw hides in the tanning drum.

11. The method according to any one of claims 8 to 10, characterized in that Adjust the pH of the raw hides and the aqueous solution in step A) to a value of 1.5 to 2.0, preferably 1.7 to 1.9, for example by adding an acid selected from formic acid, sulfuric acid and mixtures thereof.

12. The method according to any one of claims 8 to 10, characterized in that Adjust the pH of the raw hides and the aqueous solution in step A) to a value of 2.1 to 10.

5.

13. The method according to claim 12, characterized in that, Before adding Si(OCH2CH3)4 to the tanning drum, adjust the pH of the aqueous solution and the raw hides to a pH of 2.1 to 10.5, preferably 4.0 to 8.0, more preferably 4.0 to 5.

0.

14. The method according to any one of claims 1 to 13, characterized in that, At the end of step B), the green hide is removed from the tanning drum and dried.

15. The method according to any one of claims 1 to 13, characterized in that, It includes step C).

16. The method according to claim 15, wherein During step C), the green hide is treated with an alkalizing solution.

17. The method according to claim 16, wherein Step C) is carried out in the tanning drum by adding an alkalizing agent in a mass content of 0.5 - 20% relative to the mass of the green hide, and the alkalizing agent is selected from the following: sodium formate, sodium bicarbonate, sodium acetate, calcium formate, sodium carbonate and mixtures thereof.

18. The method according to claim 16, characterized in that, Step C) is carried out outside the tanning drum, and includes the following steps: - C1) At the end of step B), the green hide is removed from the tanning drum, - C2) The green hide is placed flat and one layer on top of the other in a trough, - C3) The trough is filled with the alkalizing solution so as to completely immerse all the green hides, and the alkalizing solution contains an alkalizing agent selected from sodium formate, sodium bicarbonate, sodium acetate, calcium formate, sodium carbonate and mixtures thereof, - C4) The green hides are left in the alkalizing solution in the trough and left standing for at least 1 hour, for example.

19. The method according to claim 18, characterized in that, The alkalizing agent is selected from sodium bicarbonate, sodium carbonate and mixtures thereof, and is present in the alkalizing solution in a mass content of 0.1 to 10% relative to the mass of the green hide.

20. A tanned green hide obtainable by the tanning method according to any one of claims 1 to 19.

21. A method for preparing leather from raw hides, which includes the following steps: - i) A step of preparing the raw hides to obtain the green hides to be tanned, - ii) A step of tanning the green hides to be tanned, and the tanning step is carried out according to the tanning method according to any one of claims 1 to 19, - iii) A step of grinding and / or finishing the tanned green hides obtained at the end of step ii) to obtain the finished leather.

22. A method for detanning the green hide according to claim 20 and / or the green hide obtained by the method according to any one of claims 1 to 19 and / or the finished leather obtained according to claim 21, which includes at least one step during which the green hide and / or the leather is soaked in an alkalizing solution to bring the pH of the green hide and / or the leather to a value greater than or equal to 10.5, preferably to a value of 11.5 to 13.0.

Images