Hide tanning process, method for preparing leather, and detanning process

Abstract

The present invention relates to a process for tanning hides using a tanning solution that comprises a silicon compound and is free from metals. The invention also relates to a method for preparing leather that includes a tanning process of this type. The invention also relates to tanned hides obtained by such a tanning process. The invention further relates to a process for detanning the hides obtained by means of the above tanning process.

Description

[0001] DESCRIPTION

[0002] TITLE: Process for tanning hides, method for preparing leather and detanning process

[0003] The present invention relates to a process for tanning hides using a silicon compound as a tanning agent, a method for preparing leathers using such a process. The present invention also relates to a process for detanning tanned hides and the leathers thus obtained.

[0004] The preparation of leather from animal skins requires many steps. These steps can be classified into three main groups, which can be defined as follows: i) the skin preparation steps, ii) the tanning operation steps, and iii) the curing and finishing steps.

[0005] During the skin preparation stages, the raw hides of the animals are received. These hides can be sorted according to their quality, size, weight, etc. During these preparation stages, the parts of the hides that are not to be kept, such as the legs, are cut out and removed. The hides are then cleaned. To do this, the hides are soaked in a large volume of water, for example in a fuller (rotating barrel) or in a sewing machine (stationary tank with moving blades). An antiseptic may be added. Once cleaned, the hides are depilated to remove the hair, for example by chemical means. The hides also undergo a fleshing stage which consists of removing the subcutaneous tissue. These stages are called river stages.

[0006] At the end of the skin preparation stages, we obtain skins that can be so-called "pickled" skins or so-called "delimed" skins. "Pickled" skins are skins with high salinity and a low pH, generally less than 3.0. "Delimed" skins are skins coming out of the river stages with a pH generally between 7.5 and 9.0.

[0007] During the tanning process, the hides prepared as described above, for example "pickled" hides and "delimed" hides, are degreased. If necessary, these hides are rehydrated. At this stage, the hides are putrescible and ready to be tanned. The actual tanning stage consists of a chemical treatment to transform these putrescible hides into rot-proof and resistant leathers. The resulting leathers are then drained, rinsed and possibly wrung out.

[0008] The curing and finishing stages are the steps that make the leathers obtained at the end of the tanning operations usable for the manufacture of leather goods. These stages may include spinning, thicknessing, stretching, retanning, dyeing, and feeding. The finishing stages may include steps during which the leather is softened, smoothed, satin-finished, or printed, steps that give the leathers, for example, properties of resistance to stains, abrasion, water, or light. They can also give the leathers aesthetic properties, such as shine, color, a velvety, satiny, or grained appearance, etc.

[0009] In the group of tanning operations described above, the key step is the tanning stage itself, during which the hides ready for tanning are tanned in order to be transformed into rot-proof leathers thanks to the action of tanning agents. The hides are made up of collagen fibers. During tanning, the tanning agents form bonds between the collagen fibers of the hides, helping to stabilize these hides and transform them into strong and rot-proof leather.

[0010] Since the publication of the European REACH regulation (Regulation (EC) No. 1907 / 2006 on the Registration, Evaluation, Authorisation and Restriction of Chemicals), the marketing of consumer goods has been increasingly controlled, particularly with regard to chemical risks. Leather goods are no exception and must comply with new constraints. In the leather industry, the most widely used tanning agent is chromium in oxidation state 3 (Cr 3+ ). In fact, the chrome tanning process Cr 3+ is easy to implement, efficient and allows the production of leathers with properties that meet market expectations. Under certain conditions, a fraction of the Cr 3+ present in some leathers can transform into Cr 6+ allergen by skin contact. For this reason, the European Union imposes a restriction on the placing on the market of articles whose Cr content 6+in leathers in contact with the skin is higher than 3 mg / kg (EU Regulation No. 301 / 2014 amending Annex XVII of Regulation 1907 / 2006). The European Union is even considering lowering this threshold in the coming years. The leather industry is therefore looking for alternative solutions to chromium for tanning hides.

[0011] Glutaraldehyde can be used as a tanning agent; however, this compound has been classified as a Substance of Very High Concern by the European Chemicals Agency (ECHA) under the European REACH regulation. The substance is a respiratory and skin sensitizer.

[0012] Vegetable tannins can also be used for tanning hides. However, the large quantity of product required and the hardness of the resulting leathers limit the use of these tannins to specific applications.

[0013] Other tanning agents can be used, including metals such as aluminum, iron, titanium, or zirconium, as well as synthetic organic compounds. Furthermore, the manufacture of leather or leather goods generates leather waste (scratches, splits, manufacturing scraps). When the hides have been tanned in the presence of chrome, this waste is difficult to recycle.

[0014] Finally, the effluent from the chrome tanning process contains unreacted chromium. The effluent must be treated before being released into the natural environment.

[0015] Document US 3,116,965 describes a process for tanning hides, using silatrioxanes.

[0016] In view of the constraints described above, alternative solutions for tanning hides are necessary for the sustainability of the leather industry. There is therefore a need for a hide tanning process that does not require the use of metals in general, and chromium in particular, in order to obtain leathers that guarantee the safety of employees in leather industry companies, consumer protection (harmlessness), the recovery of leather waste (production scraps or end-of-life items) and environmental protection. There is therefore a need for a hide tanning process that can produce tanned hides that can be easily detanned. There is also a need for a detanning process for such hides, allowing the recovery of tanned hides and leathers.

[0017] The present invention relates to a process for tanning hides ready to be tanned comprising the following steps:

[0018] A) The hides ready to be tanned are soaked in a drum in an aqueous solution and the pH of the hides and the aqueous solution is brought to a value ranging from 1.5 to 10.5 by adding an acid or a base,

[0019] B) the skins from step A) are treated using a tanning solution consisting of, by mass relative to the mass of the skins: o 1 to 1200% of water, o optionally, a sufficient quantity of salt to prevent the skins from swelling, and o 1 to 100% of a compound of formula (I):

[0020] [Chem 1] in which Y1, Y2, Y3 and Y4 are independently chosen from each other as being a halogen atom, a group -[OSiO m R 1 3-m]n or a group -OR 2 , two or more of the groups -[OSiO m R 1 3-m]n and -OR 2 which can be connected to each other by a covalent bond, in which:

[0021] - m is an integer from 0 to 3, n is an integer greater than or equal to one, and R 1 is selected from hydrogen, C1-C10 alkyl, aryl, C2-C10 alkenyl, C2-C10 alkynyl, polyalkylene oxide or derivatives thereof, R 1 being optionally substituted by one or more amine, ammonium, amide, ketone, ester, aldehyde, thiol or thioether groups,

[0022] - R 2 is selected from hydrogen, C1-C10 alkyl, aryl, C2-C10 alkenyl, C2-C10 alkynyl, polyalkylene oxide or derivatives thereof, R 2 being optionally substituted by one or more amine, ammonium, amide, ketone, ester, aldehyde, thiol or thioether groups, said hides being treated using said tanning solution until said tanning solution penetrates the hides,

[0023] - C) optionally, the pH of the skins obtained at the end of step B) is brought to a value ranging from 2.1 to 10.5, preferably ranging from 4.0 to 8.0.

[0024] The process according to the invention makes it possible to obtain tanned hides free of metals. In particular, the tanned hides obtained with the tanning process according to the invention are free of chromium. The hides obtained with the tanning process according to the invention have a satisfactory shrinkage temperature for the manufacture of leather articles (shoes, leather goods, gloves, automobiles, furniture). These hides can thus be used to shape leather objects without risk to the health of future users of these objects, whatever the conditions of use of these objects. In particular, due to the exclusive use of silicon in the tanned hides obtained according to the process of the invention, these hides can be detanned so as to recover their value.

[0025] The present invention also relates to tanned hides obtainable by the above tanning process.

[0026] The present invention also relates to a method for preparing leathers from raw hides comprising the following steps: i) a step of preparing the raw hides in order to obtain hides ready to be tanned, ii) a step of tanning the hides ready to be tanned, said tanning step being carried out in accordance with the tanning process as described above, iii) a step of curing and / or finishing the tanned hides obtained at the end of step ii) in order to obtain finished leathers.

[0027] The present invention also relates to a process for detanning hides obtained by the tanning process as described above and / or finished leathers obtained according to the method for preparing leathers described above, comprising at least one step during which said hides and / or leathers are soaked in a basifying solution in order to bring the pH of said hides and / or leathers to a value greater than or equal to 10.5, preferably to a value ranging from 11.5 to 13.0.

[0028] The skins concerned by the tanning and detanning processes according to the invention can be any skins containing collagen, whether these skins are synthetic or natural. These skins can, for example, come from animals whose skin contains collagen, such as cattle, sheep, goats, reptiles (snakes, lizards, crocodiles, alligatoridae), fish, birds, amphibians, pigs.

[0029] In the tanning process according to the invention, by "skins ready to be tanned" is meant skins resulting from the skin preparation operations as described above, for example pickled or delimed skins, which have also been degreased.

[0030] The rehydration and degreasing operations are well known to those skilled in the art. For example, to rehydrate and degrease pickled hides, the hides can be placed in a drum, water, salt and a degreaser added. The degreaser can, in a known manner, be chosen from surfactants, for example non-ionic and anionic surfactants. For example, the drum can be run without heat for a period ranging from 15 minutes to 10 hours, the drum can be drained and then the hides can be rinsed with salt water at room temperature. This produces hides ready for tanning.

[0031] In the method of preparing leathers according to the invention, the term "raw hides" means natural or synthetic hides containing collagen, but which have not yet undergone the steps of preparing the hides as described above, making these hides ready to be tanned.

[0032] According to a first step, step A), of the tanning process according to the invention, the skins ready to be tanned are soaked in a drum in an aqueous solution and the pH of the skins and of the aqueous solution is brought to a value ranging from 1.5 to 10.5 by adding an acid or a base.

[0033] The fulling mill is a rotating drum well known to those skilled in the art. In the present application, the term "fulling mill" means any container configured to receive the hides to be tanned and equipped with a system for mixing the hides and the aqueous solution. Thus, a sewing machine falls within the definition of a fulling mill according to the present application. Depending on the pH value chosen for soaking the hides, the aqueous solution may include a salt. The function of this salt is to prevent the hides from swelling in the aqueous solution. For the purposes of the present application, the term "salt" means an ionic compound of cations and anions forming a neutral product with no net electrical charge. The salt may, for example, be chosen 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 include a salt. The preparation of such a salt solution is within the scope of those skilled in the art.Determining the amount of salt, for example sodium chloride, necessary to prevent swelling of the skins is within the skill of the art. For example, the aqueous solution may comprise from 1 to 1200% water, by mass relative to the mass of the skins, and from 6 to 12% sodium chloride, by mass relative to the mass of water.

[0034] The pH of the skins and the aqueous solution is brought to a value ranging from 1.5 to 10.5 by adding an acid or a base.

[0035] In one embodiment, the pH of the hides and the aqueous solution is brought to a value ranging from 1.5 to 2.0, preferably ranging from 1.7 to 1.9, for example by adding one or more acids used alone or in a mixture, for example by adding an acid chosen from formic acid, sulfuric acid and their mixtures. As will appear from the remainder of the description, bringing the pH of the hides and the aqueous solution to such an acidic pH makes it possible to add directly into the drum the tanning solution in the form of a solution of the compound Si(OH)4, the latter having been previously prepared outside the drum from a compound of formula (I) above, for example from Si(OCH2CH3)4. As will also emerge from the rest of the description, bringing the pH of the hides and the aqueous solution to such an acid pH also makes it possible, alternatively, to prepare the tanning solution in situ, in other words in the fulling mill, by adding a solution of the compound Si(OCH2CH3)4.

[0036] In another embodiment, the pH of the hides and of the aqueous solution of step A) is brought to a value ranging from 2.1 to 10.5. As will appear from the remainder of the description, bringing the pH of the hides and of the aqueous solution of step A) to a value ranging from 2.1 to 10.5 makes it possible to prepare the tanning solution in situ, in other words in the drum, by adding a solution of the compound Si(OCH2CH3)4, while maintaining a relatively high pH during the step of treatment with the tanning solution, for example a pH of approximately 7.0, and to dispense with a subsequent basification step. Similarly, when the step of treatment with the tanning solution is carried out at a pH greater than or equal to 5.0, the addition of a salt to the tanning solution is not necessary. In a second step of the process according to the invention, step B), the hides from step A) are treated with a tanning solution. To do this, the aqueous solution from step A) may or may not be removed from the mill.In the case where the aqueous solution from step A) is drained from the drum, this draining is preferably done without rinsing the hides.

[0037] The tanning solution consists of, by mass relative to the mass of the hides: o 1 to 1200% of water, o optionally, a sufficient quantity of salt, preferably sodium chloride, to prevent the hides from swelling, and o 1 to 100% of a compound of formula (I): in which Y1, Y2, Y3 and Y4 are independently chosen from each other as being a halogen atom, a group -[OSiO m R 1 3-m]n or a group -OR 2 , two or more of the groups -[OSiO m R 1 3-m]n and -OR 2 which can be connected to each other by a covalent bond, in which:

[0038] - m is an integer from 0 to 3, n is an integer greater than or equal to one, and R 1is selected from hydrogen, C1-C10 alkyl, aryl, C2-C10 alkenyl, C2-C10 alkynyl, polyalkylene oxide or derivatives thereof, R 1 being optionally substituted by one or more amine, ammonium, amide, ketone, ester, aldehyde, thiol or thioether groups,

[0039] - R 2 is selected from hydrogen, C1-C10 alkyl, aryl, C2-C10 alkenyl, C2-C10 alkynyl, polyalkylene oxide or derivatives thereof, R 2 being optionally substituted by one or more amine, ammonium, amide, ketone, ester, aldehyde, thiol or thioether groups.

[0040] For the purposes of the present invention, the following is understood to mean: By “C1-C10 alkyl” group, a saturated, linear or branched hydrocarbon chain, comprising from 1 to 10 carbon atoms, such as for example a methyl, ethyl, isopropyl, tert-butyl, pentyl group, etc.

[0041] By aryl group, an aromatic group, preferably comprising from 5 to 10 carbon atoms, comprising one or more cycles and optionally comprising a heteroatom, in particular an oxygen, a nitrogen or a sulfur, such as for example a phenyl, furan, indole, pyridine, naphthalene group, etc.

[0042] By “C2-C10 alkenyl” group, a linear or branched hydrocarbon chain, comprising at least one unsaturation and comprising from 2 to 10 carbon atoms, such as for example an ethenyl (vinyl), propenyl, 2,4-hexafenyl group, etc.

[0043] By “C2-C10 alkynyl” group, a linear or branched hydrocarbon chain, comprising at least one double unsaturation and comprising from 2 to 10 carbon atoms, such as for example an acetylene, propyne, butyne group, etc.

[0044] By "polyalkylene oxide" group, polymers of ethylene oxide units, the number of ethylene oxide units is an integer greater than or equal to 1,

[0045] By polyalkylene oxide derivatives is meant one or more synthetic polymers chosen from copolymers of carboxylic acids and carboxylic esters of poly(alkylene glycol), copolymers of carboxylic acids and amide of poly(alkylene glycol), copolymers of carboxylic acids and imide of poly(alkylene glycol), copolymers of carboxylic acids and vinyl ethers of poly(alkylene glycol), neutralized or non-neutralized, and mixtures thereof.

[0046] The tanning solution is thus free of any metal other than silicon. In particular, the tanning solution is free of chromium.

[0047] In one embodiment, Y1, Y2, Y3 and Y4 are independently selected from one another as being a chlorine atom, an -OH group or an -OCH2CH3 group.

[0048] 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 hides and of the aqueous solution of step A) will have been previously brought to a value ranging from 1.5 to 2.0, preferably ranging from 1.7 to 1.9. The tanning solution may then consist of, by mass relative to the mass of the hides: o 1 to 1200% of water, o A sufficient quantity of salt, preferably sodium chloride, to prevent the hides from swelling, and o 5 to 60%, preferably 5 to 30%, more preferably 10 to 20%, of Si(OH)4.

[0049] In such an embodiment, the tanning solution is preferably prepared outside the drum by hydrolysis of the compound Si(OCH2CH3)4 to Si(OH)4, the tanning solution then being added to the hides in the drum. For example, the tanning solution may be prepared by mixing an aqueous solution of pH ranging from 1.5 to 2.0 and Si(OCH2CH3)4 in a volume ratio (v / v) ranging from 1 / 1 to 25 / 1, preferably ranging from 1 / 1 to 10 / 1.

[0050] The Si(OH)4 thus obtained is added to the drum, in a content, by mass relative to the mass of the hides, ranging from 5 to 60%, preferably from 5 to 30%, more preferably 10 to 20%. This content can be adjusted according to the type of hides to be tanned and according to the thickness of these hides. Furthermore, to prevent the hides from swelling, salt, such as sodium chloride, can also be added to the drum, for example in a content, by mass relative to the mass of Si(OH)4 added, ranging from 6 to 12%.

[0051] It is possible to add acidic salt water to the drum. Ultimately, within the drum, water can be present in a mass content relative to the mass of the hides, ranging from 1 to 1200%.

[0052] The drum is preferably left to rotate for a period of time allowing the silicic acid to penetrate the hides. This period of time may thus vary depending on the thickness of the hides and the nature of the hides: for example, the penetration of the hide to be tanned by the silicic acid may require a longer period of time for a bovine hide than for a sheep hide. The duration of this step can be easily determined by a person skilled in the art depending on the hide to be tanned. The rotation of the drum is preferably carried out at room temperature, in other words a temperature less than or equal to 30°C, preferably a temperature ranging from 20 to 25°C.

[0053] Once the silicic acid has penetrated the hides, the hides can be taken directly from the drum and left to dry. In this case, the hides are taken from the drum and left to dry after step B). The water they contain evaporates during this drying step. This evaporation leads to the polymerization of Si(OH)4 and the three-dimensional Si-O-Si network bound to the skin's collagen is created, thus stabilizing the hides. At the end of this drying step, dried leathers are obtained that can be directly stored.

[0054] In an alternative embodiment, once the silicic acid has penetrated the skins, the pH of the latter is brought to a value ranging from 2.1 to 10.5, preferably ranging from 4.0 to 8.0. This optional step C), will be described below. In another embodiment of the process according to the invention, the compound of formula (I) is Si(OCH2CH3)4.

[0055] The tanning solution may then consist of, by mass relative to the mass of the hides: o 1 to 1200% of water, o A sufficient quantity of salt, preferably sodium chloride, to prevent the hides from swelling, and o 5 to 100%, preferably 10 to 60%, more preferably 25 to 45%, of Si(OCH2CH3)4.

[0056] In such an embodiment, the tanning solution can be produced directly in situ by adding Si(OCH2CH3)4 to the drum in a content, by mass relative to the mass of the hides, ranging from 5 to 100%, preferably ranging from 10 to 60%, more preferably ranging from 25 to 45%.

[0057] If necessary, acidic salt water can be added. Ultimately, within the mill, water can be present in a mass content relative to the mass of the hides, ranging from 1 to 1200%.

[0058] In this embodiment, the hydrolysis of Si(OCH2CH3)4 into Si(OH)4 will thus take place within the drum. When Si(OCH2CH3)4 is added to the drum, two insoluble phases are present in the drum, water on the one hand, and Si(OCH2CH3)4 on the other hand.

[0059] The drum is preferably left to rotate for the time necessary for the complete hydrolysis of Si(OCH2CH3)4 into Si(OH)4, in order to obtain a single soluble phase of silicic acid solution and to allow the silicic acid to penetrate the hides. For example, this time can range from 1 hour to one week. As seen previously, the time necessary for the silicic acid to penetrate the hides may depend on the nature of the hides. This time is easily determined by a person skilled in the art. The rotation of the drum is preferably carried out at room temperature, in other words a temperature less than or equal to 30°C, preferably a temperature ranging from 20 to 25°C.

[0060] Furthermore, the duration of complete hydrolysis of Si(OCH2CH3)4 to Si(OH)4 may also depend on the pH of the tanning solution.

[0061] Thus, in one embodiment, the compound of formula (I) is Si(OCH2CH3)4 and the pH of the hides and of the aqueous solution of step A) will have been previously brought to a value ranging from 1.5 to 2.0, preferably ranging from 1.7 to 1.9. In this embodiment, once the silicic acid has penetrated the hides, the hides can be directly removed from the drum and left to dry, as described above. Alternatively, they can be subjected to a basifying treatment as will be described below. In another embodiment, the compound of formula (I) is Si(OCH2CH3)4 and the pH of the hides and of the aqueous solution of step A) will have been previously brought to a value ranging from 2.1 to 10.5. In such a case, the pH of the aqueous solution and the skins is adjusted to a pH ranging from 2.1 to 10.5, preferably from 4.0 to 8.0, more preferably from 4.0 to 5.0 prior to the addition of the Si(OCH2CH3)4 to the drum as described above.The tanning solution thus has a pH ranging from 2.1 to 10.5, preferably from 4.0 to 8.0, more preferably from 4.0 to 5.0. Such a pH of the tanning solution makes it possible to have simultaneously in the drum on the one hand a hydrolysis reaction of Si(OCH2CH3)4 into Si(OH)4, and on the other hand a polymerization reaction of Si(OH)4. The polymerization of Si(OH)4 will lead to the formation of a three-dimensional Si-O-Si network, as well as the establishment of bonds with the collagen fibers of the skin. This three-dimensional network will make it possible to transform the initially putrescible skin into a rot-proof tanned skin and thus into stable and resistant leather.Thus, in the embodiment of the process according to the invention in which the pH of the tanning solution ranges from 2.1 to 10.5, preferably from 4.0 to 8.0, more preferably from 4.0 to 5.0, it is not obligatory to carry out an additional step, such as for example a specific step of basifying the hides so that the latter are tanned. In such an embodiment, the hides obtained at the end of step B) can be directly used for subsequent curing steps or be dried to complete the polymerization and be stored. However, in an alternative embodiment of the process according to the invention in which the pH of the tanning solution ranges from 2.1 to 10.5, preferably from 4.0 to 8.0, more preferably from 4.0 to 5.0, it is possible to carry out a step of treating the hides with a basifying solution as described below.

[0062] Thus, in embodiments, the tanning process according to the invention may comprise an additional step, step C). During step C), the pH of the skins obtained at the end of step B) is brought to a value ranging from 2.1 to 10.5, preferably ranging from 4.0 to 8.0.

[0063] Thus, step C) is preferably carried out when the tanning solution is too acidic to trigger the polymerization of Si(OH)4, for example when the pH of the tanning solution is less than or equal to 2.0.

[0064] During step C), the hides can for example be treated using a basifying solution. The basification of the bath containing the hides will allow and / or accelerate the polymerization of Si(OH)4 within the hides. Bonds between the silicon and the collagen of the hide fibers will be able to form and a three-dimensional Si-O-Si network linked to the collagen of the hide will be able to be created, thus stabilizing the hides. Tanned hides will be obtained. In one embodiment of the method according to the invention, the treatment using a basifying solution is carried out inside the drum by adding a basifying agent for example chosen from sodium formate, sodium bicarbonate, sodium acetate, calcium formate, sodium carbonate and mixtures thereof, in a content, by mass relative to the mass of the hides, ranging from 0.5 to 20%.

[0065] In another embodiment, the treatment with a basifying solution may be carried out outside the drum. For example, the treatment may comprise the following steps:

[0066] C1) at the end of step B), the skins are removed from the fulling mill,

[0067] C2) the skins are placed in a tray, flat, one on top of the other,

[0068] C3) the tank is filled with said basifying solution so as to completely immerse all of the skins, said basifying solution comprising a basifying agent chosen from sodium formate, sodium bicarbonate, sodium acetate, calcium formate, sodium carbonate and mixtures thereof,

[0069] C4) the skins are left in the alkalizing solution in the tank, to rest, for example for at least 1 hour.

[0070] The alkalizing agent content is calculated according to the desired final pH. During step C1), the hides can be removed from the mill with or without prior rinsing.

[0071] At any time during step C), the skins can be dried in order to carry out the polymerization of Si(OH)4 by evaporation.

[0072] The hides obtained by the process according to the invention are tanned and rot-proof. These hides generally have a white color. They have an appearance which makes them appear round and fuller than in their initial state before tanning. These hides have a shrinkage temperature ranging from 65 to 90°C. Such a shrinkage temperature allows these hides to be suitable for treatments during which they can be subjected to relatively high temperatures, for example of the order of 50°C to 75°C, such as thicknessing, hot dyeing for better fixation of the dyes, etc. while retaining their flexibility and the quality of their appearance.

[0073] The present invention also relates to a method for preparing leathers from raw hides comprising the following steps: i) a step of preparing the raw hides in order to obtain hides ready to be tanned, ii) a step of tanning the hides ready to be tanned, said tanning step being carried out in accordance with the tanning process described above, iii) a step of curing and / or finishing the tanned hides obtained at the end of step ii) in order to obtain finished leathers.

[0074] The present invention also relates to a process for detanning tanned hides obtained by the process described above and / or leathers obtained from the preparation method described above, comprising at least one step during which said hides and / or leathers are soaked in a basifying solution in order to bring the pH of said hides and / or leathers to a value greater than or equal to 10.5, preferably to a value ranging from 11.5 to 13.0.

[0075] In the detanning process according to the invention, the tanned hides may, for example, be in the "dry white" state. For the purposes of the present invention, "dry white" means a tanned hide that is dried but not cured. The tanned hides may alternatively be in the wet white, stain, crust, finished or semi-finished leather, scraped, split, terms well known to those skilled in the art. Leather scraps and leather dust from wet white, "dry white", stain, crust or finished leather may also be detanned by the detanning process according to the invention.

[0076] For example, to carry out detanning, the tanned hides and / or finished leathers may be soaked in an aqueous solution, for example in a drum. When the tanned hides are in the form of leather scraps and leather dust, they may alternatively be placed in a tank having a stirring blade or other mixing system. The aqueous solution may comprise a degreaser. For example, the aqueous solution may comprise, by mass relative to the mass of the hides: from 50 to 2000% water, from 0.1 to 2% of a degreaser.

[0077] The degreaser may be chosen from surfactants, for example from non-ionic and anionic surfactants.

[0078] The purpose of such a step is to degrease and rehydrate the hides and / or leathers. Preferably, the drum is left to rotate for a period of time allowing this degreasing and rehydration, for example for a period ranging from 15 minutes to overnight, for example from 45 minutes to 1 hour 30 minutes, at a temperature ranging from 10 to 80°C, for example from 30 to 45°C.

[0079] The drum can then be drained and the hides and / or leathers can be rinsed in the drum at room temperature.

[0080] The skins and / or leathers are soaked in an aqueous solution comprising a basifying agent in order to bring the pH of the skins and / or leathers to a value greater than or equal to 10.5, preferably to a value ranging from 11.5 to 13.0.

[0081] The basifying agent may, for example, be chosen from sodium bicarbonate, sodium carbonate, ammonia, sodium hydroxide and mixtures thereof. The basifying agent may, for example, be added to the aqueous solution in several stages, with the use of different or different basifying agents for each stage, and at a working temperature that may vary from one stage to another.

[0082] For example, an initial addition of a basifying agent can raise the pH of the skins and / or leathers to a value ranging from 7.0 to 10.0. For example, sodium bicarbonate, and / or sodium carbonate and / or ammonia can be used for this purpose, and the temperature of the aqueous solution can be maintained at a value ranging from 10 to 80°C, for example from 30 to 45°C.

[0083] Then a second addition of basifying agent can be made in order to bring the pH of the skins and / or leathers to a value ranging from 11.5 to 13.0. For example, sodium hydroxide can be used to do this, and the temperature of the aqueous solution can be maintained at a value ranging from 10 to 30°C, for example from 20 to 25°C.

[0084] Bringing the hides and / or leathers to a pH greater than or equal to 10.5, preferably ranging from 11.5 to 13.0, results in the three-dimensional Si-O-Si network bound to the skin's collagen breaking down. At such a pH, the hides and / or leathers, for example wet whites, "dry whites", stains, finished crust leathers, dust and leather scraps, lose their resistance to the shrinkage temperature: this loss indicates detanning of the hides and / or leathers.

[0085] Wet whites, dry whites, stains, crusts, finished leathers, scratches, splits, dust and leather scraps thus detanned can either remain in the form of non-crosslinked protein or be revalued, for example by being tanned again or by being used for the manufacture of gelatin, the manufacture of protein for animal feed or the manufacture of fertilizer for fields, etc.

[0086] EXPERIMENTAL PART

[0087] Example 1

[0088] Preparation of SifOH from

[0089] Mix 1 L of Si(OCH2CH3)4 and 1 L of water.

[0090] The pH of this mixture is adjusted to a value ranging from 1.5 to 2.0 using a strong acid, such as hydrochloric acid or sulfuric acid, this list is not exhaustive.

[0091] The hydrolysis reaction of Si(OCH2CH3)4 to Si(OH)4 is exothermic. Thus, preferably, the mixture is maintained at the desired temperature, for example around 30°C by means of a cooling bath. Preferably, the mixture is placed under vigorous stirring for 15 min to 120 min. The duration of the hydrolysis reaction can range from 20 min to 2 h. At the end of this reaction, a clear solution of silicic acid is obtained, with the chemical formula Si(OH)4. 10% by mass of sodium chloride is added, relative to the total mass of the previously prepared silicic acid solution.

[0092] Example 2

[0093] Sheep skins are tanned using the tanning process according to the invention as follows:

[0094] Step A)

[0095] The hides ready for tanning are pickled hides from a storage pickle or delimed hides previously degreased and rehydrated according to the know-how of the person skilled in the art. The hides are weighed.

[0096] The hides are placed in a drum in a salt solution. The pH of the hides and the salt solution is brought to a value ranging from 1.5 to 2.0, for example by adding sulfuric acid alone or in a mixture with formic acid.

[0097] Step B)

[0098] In the fulling mill, we add in mass relative to the mass of the hides:

[0099] 50% cold water, at 25°C,

[0100] 5% sodium chloride,

[0101] 13% Si(OH)4 hydrolyzed from Si(OCH2CH3)4 as described in Example 1.

[0102] The drum is rotated for 4 hours at room temperature, i.e. 25°C.

[0103] Step C)

[0104] First, 5% sodium formate by mass relative to the mass of the hides is added to the drum at a temperature of 25°C. The drum is rotated for 1 hour. The pH measured on the hides ranges from 3.8 to 4.0.

[0105] In a second step, 5% sodium formate by mass relative to the mass of the hides is added to the drum at a temperature of 25°C. The drum is rotated for 1 night. The final pH measured on the hides ranges from 4.0 to 4.2.

[0106] The fulling mill is drained and the hides are rinsed. The result is tanned hides free of metals, particularly chrome.

[0107] The skins are white, round and have a beautiful appearance.

[0108] Example 3

[0109] Bovine hides are tanned using the tanning process according to the invention as follows:

[0110] Step A)

[0111] The hides ready for tanning are pickled hides from a storage pickle or delimed hides previously degreased and rehydrated according to the know-how of the person skilled in the art. The hides are weighed.

[0112] The hides are placed in a drum in a salt solution. The pH of the hides and the salt solution is brought to a value ranging from 1.5 to 2.0, for example, by adding sulfuric acid alone or mixed with formic acid.

[0113] Step B)

[0114] In the fulling mill, we add in mass relative to the mass of the hides:

[0115] 50% cold water, at 25°C,

[0116] 5% sodium chloride,

[0117] 20% Si(OH)4 hydrolyzed from Si(OCH2CH3)4 as described in Example 1.

[0118] The drum is rotated overnight at room temperature, i.e. 25°C.

[0119] Step C)

[0120] First, 5% sodium formate by mass relative to the mass of the hides is added to the drum at a temperature of 25°C. The drum is rotated for 1 hour. The pH measured on the hides ranges from 3.8 to 4.0.

[0121] In a second step, 5% sodium formate by mass relative to the mass of the hides is added to the drum at a temperature of 25°C. The drum is rotated for 1 night. The final pH measured on the hides ranges from 4.0 to 4.2.

[0122] The fulling mill is emptied and the hides are rinsed.

[0123] The result is tanned hides free of metals, especially chrome. The hides are white, round and have a beautiful appearance.

[0124] Example 4

[0125] Sheep skins are tanned using the tanning process according to the invention as follows:

[0126] Step A)

[0127] The hides ready for tanning are pickled hides from a storage pickle or delimed hides previously degreased and rehydrated according to the know-how of the person skilled in the art. The hides are weighed.

[0128] The hides are placed in a drum in a salt solution. The pH of the hides and the salt solution is brought to a value ranging from 1.5 to 2.0, for example, by adding sulfuric acid alone or mixed with formic acid.

[0129] Step B)

[0130] In the fulling mill, we add in mass relative to the mass of the hides:

[0131] 50% cold water, at 25°C,

[0132] 5% sodium chloride,

[0133] 13% Si(OH)4 hydrolyzed from Si(OCH2CH3)4 as described in Example 1.

[0134] The drum is rotated for 4 hours at room temperature, i.e. 25°C.

[0135] Step C)

[0136] First, 5% sodium formate by mass relative to the mass of the hides is added to the drum at a temperature of 25°C. The drum is rotated for one minute and then stopped for one hour. The pH measured on the hides ranges from 3.8 to 4.0.

[0137] In a second step, 5% sodium formate by mass relative to the mass of the hides is added to the drum at a temperature of 25°C. The drum is rotated for one minute and then stopped overnight. The final pH measured on the hides ranges from 4.0 to 4.2.

[0138] The result is tanned hides free of metals, especially chrome. The hides are white, round and have a beautiful appearance.

[0139] Example 5

[0140] Bovine hides are tanned using the tanning process according to the invention as follows:

[0141] Step A)

[0142] The hides ready for tanning are pickled hides from a storage pickle or delimed hides previously degreased and rehydrated according to the know-how of the person skilled in the art. The hides are weighed.

[0143] The hides are placed in a drum in a salt solution. The pH of the hides and the salt solution is brought to a value ranging from 1.5 to 2.0, for example, by adding sulfuric acid alone or mixed with formic acid.

[0144] Step B)

[0145] In the fulling mill, we add in mass relative to the mass of the hides:

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

[0150] Step C)

[0151] First, 5% sodium formate by mass relative to the mass of the hides is added to the drum at a temperature of 25°C. The drum is rotated for one minute and then stopped for one hour. The pH measured on the hides ranges from 3.8 to 4.0.

[0152] In a second step, 5% sodium formate by mass relative to the mass of the hides is added to the drum at a temperature of 25°C. The drum is rotated for one minute and then stopped overnight. The final pH measured on the hides ranges from 4.0 to 4.2.

[0153] The result is tanned hides free of metals, especially chrome. The hides are white, round and have a beautiful appearance.

[0154] Example 6

[0155] Sheep skins are tanned using the tanning process according to the invention as follows:

[0156] Step A)

[0157] The hides ready for tanning are pickled hides from a storage pickle or delimed hides previously degreased and rehydrated according to the know-how of the person skilled in the art. The hides are weighed.

[0158] The hides are placed in a drum in a salt solution. The pH of the hides and the salt solution is brought to a value ranging from 1.5 to 2.0, for example by adding sulfuric acid alone or in a mixture with formic acid.

[0159] Step B)

[0160] In the fulling mill, we add in mass relative to the mass of the hides:

[0161] - 28% Si(OCH2CH3)4.

[0162] This quantity of Si(OCH2CH3)4 represents the equivalent of 13% of Si(OH)4 by mass relative to the mass of the skins.

[0163] The drum is rotated for 4 hours at room temperature, i.e. 25°C.

[0164] Then we add to the fulling mill, in mass relative to the mass of the skins:

[0165] - 150% water at 25°C

[0166] 15% sodium chloride

[0167] The drum is rotated for 20 minutes at 25°C.

[0168] Step C)

[0169] First, 5% sodium formate by mass relative to the weight of the hides is added to the drum at a temperature of 25°C. The drum is rotated for 1 hour. The pH measured on the hides ranges from 3.8 to 4.0. Second, 5% sodium formate by mass relative to the weight of the hides is added to the drum at a temperature of 25°C. The drum is rotated for 1 night. The final pH measured on the hides ranges from 4.0 to 4.2.

[0170] The fulling mill is emptied and the hides are rinsed.

[0171] Tanned skins are obtained free from metals, particularly chrome.

[0172] The skins are white, round and have a beautiful appearance.

[0173] Example 7

[0174] Sheep skins are tanned using the tanning process according to the invention as follows:

[0175] Step A)

[0176] The hides ready for tanning are pickled hides from a storage pickle or delimed hides previously degreased and rehydrated according to the know-how of the person skilled in the art. The hides are weighed.

[0177] The hides are placed in a drum in a salt solution. The pH of the hides and the salt solution is brought to a value ranging from 1.5 to 2.0, for example by adding sulfuric acid alone or in a mixture with formic acid.

[0178] Step B)

[0179] In the fulling mill, we add in mass relative to the mass of the hides:

[0180] - 28% Si(OCH2CH3)4.

[0181] This quantity of Si(OCH2CH3)4 represents the equivalent of 13% of Si(OH)4 by mass relative to the mass of the skins.

[0182] The drum is rotated for 4 hours at room temperature, i.e. 25°C.

[0183] Then we add to the fulling mill, in mass relative to the mass of the skins:

[0184] - 150% water at 25°C

[0185] 15% sodium chloride

[0186] The drum is rotated for 20 minutes at 25°C.

[0187] Step C) First, 5% sodium formate by mass relative to the weight of the hides is added to the drum at a temperature of 25°C. The drum is rotated for one minute and then stopped for one hour. The pH measured on the hides ranges from 3.8 to 4.0.

[0188] In a second step, 5% sodium formate by mass relative to the weight of the hides is added to the drum at a temperature of 25°C. The drum is rotated for one minute and then stopped overnight. The final pH measured on the hides ranges from 4.0 to 4.2.

[0189] The fulling mill is emptied and the hides are rinsed.

[0190] The result is tanned hides free of metals, especially chrome. The hides are white, round and have a beautiful appearance.

[0191] Example 8

[0192] Bovine hides are tanned using the tanning process according to the invention as follows:

[0193] Step A)

[0194] The hides ready for tanning are pickled hides from a storage pickle or delimed hides previously degreased and rehydrated according to the know-how of the person skilled in the art. The hides are weighed.

[0195] The hides are placed in a drum in a salt solution. The pH of the hides and the salt solution is brought to a value ranging from 1.5 to 2.0, for example, by adding sulfuric acid alone or mixed with formic acid.

[0196] Step B)

[0197] Si(OH)4 hydrolyzed from Si(OCH2CH3)4 as described in Example 1 is added in several stages according to the following procedure. The percentages are given by mass relative to the mass of the skins:

[0198] Addition of 0.6% Si(OH)4 then rotation of the drum for 4 hours at 25°C,

[0199] Addition of 0.6% Si(OH)4 then rotation of the drum for 4 hours at 25°C,

[0200] Addition of 3.8% Si(OH)4 then rotation of the drum overnight at 25°C, Addition of 5% Si(OH)4 then rotation of the drum for 10 hours at 25°C, Addition of 5% Si(OH)4 then rotation of the drum overnight at 25°C, Addition of 7.5% Si(OH)4 then rotation of the drum for 24 hours at 25°C, Addition of 7.5% Si(OH)4 then rotation of the drum for 24 hours at 25°C.

[0201] Then we add to the drum, in mass relative to the mass of the skins: 100% water at 25°C, 10% sodium chloride at 25°C.

[0202] The drum is rotated for 20 minutes.

[0203] Step C)

[0204] 1.5% sodium formate by mass relative to the mass of the hides is added to the drum at a temperature of 25°C. The drum is rotated for one minute every 30 minutes, for a period of 12 hours. The pH measured on the hides is 3.88.

[0205] Then 1.5% of sodium formate by mass relative to the mass of the hides is added to the drum at a temperature of 25°C. The drum is rotated for one minute every 30 minutes, for a period of 12 hours. The pH measured on the hides is 4.40.

[0206] Then 4% sodium formate by mass relative to the mass of the hides is added to the drum at a temperature of 25°C. The drum is rotated for one minute every 30 minutes, for a period of 6 hours. The pH measured on the hides is 4.50.

[0207] The fulling mill is emptied and the hides are rinsed.

[0208] Tanned skins are obtained free from metals, particularly chrome.

[0209] The skins are white, round and have a beautiful appearance.

[0210] Example 9

[0211] Sheep skins are tanned using the tanning process according to the invention as follows:

[0212] Step A)

[0213] The hides ready for tanning are pickled hides from a storage pickle or from delimed hides previously degreased and rehydrated according to the know-how of the person skilled in the art. The hides are weighed. The hides are placed in a drum in a salt solution. The pH of the hides and the salt solution is brought to a value ranging from 1.5 to 2.0, for example by adding sulfuric acid alone or in a mixture with formic acid.

[0214] Step B)

[0215] In the fulling mill, we add in mass relative to the mass of the hides:

[0216] 50% cold water, at 25°C,

[0217] 5% sodium chloride,

[0218] 13% Si(OH)4 hydrolyzed from Si(OCH2CH3)4 as described in Example 1.

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

[0220] The fulling mill is emptied and the hides are rinsed.

[0221] Step C)

[0222] The hides are removed from the mill and placed on a rack to drain, the time needed for the surface water to drain. The hides are then framed while damp and left to rest in the open air while they dry, generally overnight or a little longer depending on the thickness of the hide and the daily weather.

[0223] The next day, after the water contained in the skins has evaporated, they are dry, white and tanned.

[0224] Example 10

[0225] Bovine hides are tanned using the tanning process according to the invention as follows:

[0226] Step A)

[0227] The hides ready for tanning are pickled hides from a storage pickle or from delimed hides previously degreased and rehydrated according to the know-how of the person skilled in the art. The hides are weighed. The hides are placed in a drum in a salt solution. The pH of the hides and the salt solution is brought to a value ranging from 1.5 to 2.0, for example by adding sulfuric acid alone or in a mixture with formic acid.

[0228] Step B)

[0229] In the fulling mill, we add in mass relative to the mass of the hides:

[0230] 50% cold water, at 25°C,

[0231] 5% sodium chloride,

[0232] 20% Si(OH)4 hydrolyzed from Si(OCH2CH3)4 as described in Example 1.

[0233] The drum is rotated overnight at room temperature, i.e. 25°C.

[0234] The fulling mill is emptied and the hides are rinsed.

[0235] Step C)

[0236] The hides are removed from the mill and placed on a rack to drain, the time needed for the surface water to drain. The hides are then framed while damp and left to rest in the open air while they dry, generally overnight or a little longer depending on the thickness of the hide and the daily weather.

[0237] The next day, after the water contained in the skins has evaporated, they are dry, white and tanned.

[0238] Example 11

[0239] Sheep skins are tanned using the tanning process according to the invention as follows:

[0240] Step A)

[0241] The hides ready to be tanned are pickled hides from a storage pickle or from delimed hides previously degreased and rehydrated according to the know-how of the person skilled in the art. The hides are weighed. The hides are placed in a drum in a salt solution. The pH of the hides and the salt solution is brought to a value ranging from 2.1 to 10.5, for example, by adding sulfuric acid alone or in a mixture with formic acid or by adding alkalizing agents.

[0242] Step B)

[0243] The pH of the skins and the saline solution is adjusted to a value ranging from 4.0 to 5.0, for example to 4.2.

[0244] Then we add to the fulling mill, in mass relative to the mass of the skins:

[0245] - 28% Si(OCH2CH3)4.

[0246] This quantity of Si(OCH2CH3)4 represents the equivalent of 13% of Si(OH)4 by mass relative to the mass of the skins.

[0247] The drum is rotated for a period of 7 days at room temperature, i.e. 25°C

[0248] The fulling mill is then emptied and the hides are rinsed.

[0249] Tanned skins are obtained.

[0250] Example 12

[0251] Shrink temperature measurement

[0252] The shrinkage temperature for the tanned hides obtained in Examples 2 to 9 and 11 was measured according to the following protocol:

[0253] - cutting a piece of tanned skin, approximately 2cm by 1cm, at the collar,

[0254] - one side of the prepared sample is placed at the fixed area of ​​a retractometer,

[0255] - the opposite side of the sample is placed at the level of the movable hook of the retractometer counterbalanced by a weight,

[0256] - the retractometer containing the sample is placed in a beaker filled with cold water (temperature strictly below 25°C): the skin sample must be completely immersed, - a thermometer is placed in the same beaker, fixed on the retractometer. The base of the thermometer is placed opposite the tanned skin sample,

[0257] - the retraction indicator arrow is set to zero,

[0258] - The assembly (beaker filled with water + shrinkometer containing the sample + thermometer) is placed on a heating plate,

[0259] - the hotplate is then turned on,

[0260] - when the arrow, initially placed at zero, begins to move, the temperature value is recorded. This temperature indicates the moment when the skin retracts.

[0261] The results obtained for the skins tanned according to Examples 2 to 9 and 11 above are collected in the following Table I:

[0262] [Table 1]

[0263] Table I: Shrinkage temperature

[0264] The shrinkage temperatures of the hides tanned with the tanning process according to the invention are particularly satisfactory. Such shrinkage temperatures make these hides suitable for treatments during which they can be subjected to relatively high temperatures, for example of the order of 50 to 75°C, such as thicknessing, hot dyeing for better fixation of the dyes, etc., while retaining their flexibility and the quality of their appearance.

[0265] Example 13 The skins obtained in Examples 2 to 11 above can be easily detanned, according to the detanning process according to the invention described below.

[0266] The tanned hides are introduced into a drum. The hides are weighed. The mass will serve as a reference.

[0267] In the drum, the following are added in mass relative to the mass of the hides: from 50 to 2000% water, from 0.1 to 2% degreaser.

[0268] The drum is rotated for a period ranging from 15 minutes to overnight, ideally between 45 minutes and 1 hour 30 minutes, at a temperature ranging from 10 to 80°C, ideally 30 to 45°C.

[0269] The mill is emptied and the hides are rinsed at room temperature, i.e. 25°C.

[0270] A basifying agent selected from sodium bicarbonate, sodium carbonate, ammonia and mixtures thereof is added to the drum in order to bring the pH of the hides to a value ranging from 7.0 to 10.0, preferably to a value ranging from 8.0 to 9.0. The basifying agent is added at a content, by mass relative to the mass of the hides, ranging from 0.1 to 15%, preferably ranging from 1 to 5%.

[0271] The drum is rotated for a period of time ranging from 15 to 120 minutes, for example from 45 to 90 minutes. The temperature ranges from 10 to 80°C, for example from 30 to 45°C.

[0272] Sodium hydroxide is then added to the drum to bring the pH of the hides to a value greater than or equal to 10.5, preferably to a value ranging from 11.5 to 13.0.

[0273] The drum is rotated for a period of time ranging from 30 minutes to 24 hours, for example overnight. The temperature ranges from 10 to 30°C, for example from 20 to 25°C.

[0274] During this step, the three-dimensional Si-O-Si network bound to the skin's collagen is broken down and the skins are detanned. The detanned skins can either remain in the form of uncrosslinked protein or be revalued. The detanned skins are free of metals, particularly chromium.

Claims

CLAIMS 1. Process for tanning hides ready to be tanned comprising the following steps: A) The hides ready to be tanned are soaked in a drum in an aqueous solution and the pH of the hides and the aqueous solution is brought to a value ranging from 1.5 to 10.5 by adding an acid or a base, B) the skins from step A) are treated using a tanning solution consisting of, by mass relative to the mass of the skins: o 1 to 1200% of water, o optionally, a sufficient quantity of salt to prevent the skins from swelling, and o 1 to 100% of a compound of formula (I): [Chem 1] in which Y1, Y2, Y3 and Y4 are independently chosen from each other as being a halogen atom, a group -[OSiO m R 1 3-m]n or a group -OR 2 , two or more of the groups -[OSiO m R 1 3-m]n and -OR 2which can be connected to each other by a covalent bond, in which: - m is an integer from 0 to 3, n is an integer greater than or equal to one, and R 1 is selected from hydrogen, C1-C10 alkyl, aryl, C2-C10 alkenyl, C2-C10 alkynyl, polyalkylene oxide or derivatives thereof, R 1 being optionally substituted by one or more amine, ammonium, amide, ketone, ester, aldehyde, thiol or thioether groups, - R 2 is selected from hydrogen, C1-C10 alkyl, aryl, C2-C10 alkenyl, C2-C10 alkynyl, polyalkylene oxide or derivatives thereof, R 2 being possibly substituted by one or several amine, ammonium, amide, ketone, ester, aldehyde, thiol or thioether groups, said hides being treated using said tanning solution until said tanning solution penetrates the hides, - C) optionally, the pH of the skins obtained at the end of step B) is brought to a value ranging from 2.1 to 10.5, preferably ranging from 4.0 to 8.

0.

2. Method according to claim 1, characterized in that Y1, Y2, Y3 and Y4 are chosen independently of each other as being a chlorine atom, an -OH group or an -OCH2CH3 group.

3. Method according to claim 1 or 2, characterized in that the pH of the skins and of the aqueous solution of step A) is brought to a value ranging from 1.5 to 2.0, preferably ranging from 1.7 to 1.9, for example by adding an acid chosen from formic acid, sulfuric acid and their mixtures.

4. Method according to claim 3, characterized in that the compound of formula (I) is Si(OH)4.

5. Method according to claim 4, characterized in that the tanning solution consists of, by mass relative to the mass of the skins: o 1 to 1200% of water, o A sufficient quantity of salt, preferably sodium chloride, to prevent swelling of the skins, and o 5 to 60%, preferably 5 to 30%, more preferably 10 to 20%, of Si(OH)4.

6. Method according to any one of claims 4 to 5, characterized in that during the implementation of step B), the tanning solution is prepared outside the drum by hydrolysis of the compound Si(OCH2CH3)4 into Si(OH)4, the tanning solution then being added to the hides in the drum.

7. Method according to claim 6, characterized in that the tanning solution is prepared by mixing an aqueous solution with a pH ranging from 1.5 to 2.0 and Si(OCH2CH3)4 in a volume ratio (v / v) ranging from 1 / 1 to 25 / 1, preferably ranging from 1 / 1 to 10 / 1.

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

9. Method according to claim 8, characterized in that the tanning solution consists of, by mass relative to the mass of the skins: o 1 to 1200% of water, o A sufficient quantity of salt, preferably sodium chloride, to prevent swelling of the skins, and o 5 to 100%, preferably 10 to 60%, more preferably 25 to 45%, of Si(OCH2CH3)4.

10. Method according to claim 8 or 9, characterized in that during the implementation of step B), the preparation of the tanning solution is carried out directly in situ by adding Si(OCH2CH3)4 to the drum in a content, by mass relative to the mass of the hides, ranging from 5 to 100%, preferably ranging from 10 to 60%, more preferably from 25 to 45%.

11. Method according to any one of claims 8 to 10, characterized in that the pH of the skins and of the aqueous solution of step A) is brought to a value ranging from 1.5 to 2.0, preferably ranging from 1.7 to 1.9, for example by adding an acid chosen from formic acid, sulfuric acid and their mixtures.

12. Method according to any one of claims 8 to 10, characterized in that the pH of the skins and of the aqueous solution of step A) is brought to a value ranging from 2.1 to 10.

5.

13. Method according to claim 12, characterized in that the pH of the aqueous solution and of the skins is adjusted to a pH ranging from 2.1 to 10.5, preferably from 4.0 to 8.0, more preferably from 4.0 to 5.0 prior to the addition of the Si(OCH2CH3)4 in the drum.

14. Method according to any one of claims 1 to 13, characterized in that the skins are removed from the mill and left to dry at the end of step B).

15. Method according to any one of claims 1 to 13, characterized in that it comprises step C).

16. Method according to claim 15, characterized in that during step C), the skins are treated using a basifying solution.

17. Method according to claim 16, characterized in that step C) is carried out inside the drum by adding a basifying agent chosen from sodium formate, sodium bicarbonate, sodium acetate, calcium formate, sodium carbonate and mixtures thereof, in a content, by mass relative to the mass of the skins, ranging from 0.5 to 20%.

18. Method according to claim 16, characterized in that step C) is carried out outside the drum and comprises the following steps: C1) at the end of step B), the skins are removed from the fulling mill, C2) the skins are placed in a tray, flat, one on top of the other, C3) the tank is filled with said basifying solution so as to completely immerse all of the skins, said basifying solution comprising a basifying agent chosen from sodium formate, sodium bicarbonate, sodium acetate, calcium formate, sodium carbonate and mixtures thereof, C4) the skins are left in the alkalizing solution in the tank, to rest, for example for at least 1 hour.

19. Method according to claim 18, characterized in that the basifying agent is chosen from sodium bicarbonate, sodium carbonate and their mixtures, and is present in the basifying solution in a content, by mass relative to the mass of the skins, ranging from 0.1 to 10%.

20. Tanned hides obtainable by the tanning process according to any one of claims 1 to 19.

21. Method for preparing leathers from raw hides comprising the following steps: i) a step of preparing the raw hides in order to obtain hides ready to be tanned, ii) a step of tanning the hides ready to be tanned, said tanning step being carried out in accordance with the tanning process according to any one of claims 1 to 19, iii) a step of curing and / or finishing the tanned hides obtained at the end of step ii) in order to obtain finished leathers.

22. Process for detanning hides according to claim 20 and / or obtained by the process according to any one of claims 1 to 19 and / or finished leathers obtained according to claim 21, comprising at least one step during which said hides and / or leathers are soaked in a basifying solution in order to bring the pH of said hides and / or leathers to a value greater than or equal to 10.5, preferably to a value ranging from 11.5 to 13.0.

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