METHOD FOR MANUFACTURING A SHEET OF LEATHER

Abstract

The invention relates to a method for manufacturing a leather-based sheet, comprising preparing a pulp by extruding a material containing at least 70% leather by weight at a temperature between 10 and 40 °C and in the presence of water in a quantity such that the water-to-material weight ratio is between 1.7 and 6; then shaping this pulp into a sheet. The sheet thus obtained exhibits good cohesion, good mechanical properties, and an attractive aesthetic appearance.

Description

Title of the invention: METHOD FOR MANUFACTURING A LEATHER SHEET

[0001] The present invention falls within the field of valorization of materials of natural origin, more specifically of animal origin.

[0002] More particularly, the present invention relates to a process for preparing a sheet of leather from a leather-based material, in particular from leather waste.

[0003] The projected depletion of fossil resources has, for several years, prompted manufacturers to develop alternative solutions using renewable resources in all areas of industry. Recycling and the recovery of waste of animal or plant origin are therefore of increasing interest, from both an economic and environmental perspective. Prior art has thus proposed manufacturing articles from solid materials formed from waste of animal or plant origin, with the aim of ensuring that such articles take advantage of both the aesthetic and mechanical properties of the natural materials from which they are made.

[0004] Natural materials of the type with a high collagen content, such as leather, are of particular interest in this regard, due on the one hand to the large quantity of industrial waste from livestock farming or production scraps such as offcuts generated during leather processing, and on the other hand to the attractive aesthetic appearance of leather articles.

[0005] The reconstitution of a continuous sheet from leather waste nevertheless encounters a number of technical problems, and no solution has been proposed by the prior art to form, from such waste, a sheet of leather exhibiting good mechanical resistance properties and a satisfactory aesthetic appearance.

[0006] The present invention aims to propose a process for manufacturing, from leather waste, a sheet of leather having such characteristics.

[0007] The invention also aims to make this process simple and quick to implement, and as environmentally friendly as possible.

[0008] The present inventors have now discovered that these objectives can be achieved, and that it is possible, from leather waste, particularly in particulate form, to manufacture a sheet which has an attractive appearance, a flexibility equivalent to that of the original material and entirely satisfactory mechanical properties, by a process comprising more particularly a step of treating the leather waste by extrusion under conditions ensuring the defibration of the collagen fibers that make up the leather, without causing degradation The process involves shaping these fibers, followed by a sheet-forming step carried out directly on the resulting material, particularly using techniques conventionally employed in the paper industry for manufacturing cellulose-based sheets. In particular, the inventors have discovered that, surprisingly, it is then possible to obtain a sheet exhibiting good cohesion, in which the collagen fibers have reassembled into a cohesive network, even when the shaping technique used, as is typically the case for techniques implemented in the paper industry, is carried out under conditions of mild mechanical stress, at ambient temperature, in the presence of water only, and notably in the absence of any binder.

[0009] The present invention thus relates to a method of manufacturing a leather-based sheet from a material containing at least 70% by weight of leather, in particular leather waste(s), this leather being prepared from the skin of a non-human animal.

[0010] A sheet is defined in this description, classically in itself, as a thin plate, typically between 0.1 mm and a few millimeters thick, more or less flexible depending on the material which constitutes it.

[0011] In this description, the term "leather" is also understood in its conventional sense to mean animal skin that has undergone tanning and curing processes to render it rot-proof. Leather contains a high level of collagen fibers, a fibrous structural protein forming a triple helix, present in the connective tissues of animals and whose function is to give these tissues mechanical resistance to stretching. Collagen, more specifically type I collagen, is notably the major protein in the dermis of animals.

[0012] The leather waste used according to the invention can be of any type, including production waste from the leather industry, used leather articles or parts of articles, etc.

[0013] More specifically, the method according to the invention comprises: - the preparation of a paste by a processing step of the material containing at least 70% by weight of leather prepared from the skin of a non-human animal, hereinafter referred to as "initial material", in a twin-screw extruder at a temperature between 10 and 40 °C, in the presence of water in a quantity such that the ratio by weight of water to said material is between 1.7 and 6, preferably between 1.7 and 4; - and shaping this dough into sheets.

[0014] The initial material can be formed from a single material or a mixture of a plurality of materials, for example a mixture of several different types of leather waste.

[0015] Preferably, the starting material does not comprise any polymer substance or polymer precursor. During the extrusion processing step of the process According to the invention, the initial material is preferably not mixed with any polymer substance or polymer precursor. The extrusion treatment step of the process according to the invention is therefore not a compounding step, using a polymer binder, but a defibration step of the material under the effect of the forces exerted on it in the twin-screw extruder.

[0016] More specifically, and without presuming the physicochemical phenomena occurring within the material during the extrusion step of the process according to the invention, it can be assumed that during this step, carried out at a temperature that does not cause denaturation of the collagen, in the presence of a significant amount of water, a separation occurs of the collagen fibers that make up the initial material. The combination of the shear forces exerted on the material in the extruder and the excess water, in particular, causes the collagen fibers contained in the material to unwind and stretch, resulting in the formation of long, individual oblong fibers. These fibers reconstitute a network during the subsequent sheet-forming step and advantageously create interactions sufficiently strong to maintain the cohesion of the resulting sheet.

[0017] The process according to the invention is advantageously simple to implement, and it does not use any organic solvent or other substance harmful to the environment.

[0018] The method according to the invention may also meet one or more of the characteristics described below, implemented in isolation or in each of their technically operative combinations.

[0019] The starting material from which the process according to the invention is applied consists mainly of leather derived from the dermis of a non-human animal. This animal may, in particular, be a reptile or a mammal, for example, a bovine or a ovine.

[0020] This leather can be of any type, and in particular can be obtained by treating the dermis of animals with vegetable tanning agents, as well as by chrome tanning or by synthetic tannins.

[0021] According to the invention, the initial material can contain at least 80% by weight, or even at least 90% by weight or even at least 95% by weight, of leather, in particular of leather waste(s).

[0022] The initial material containing at least 70% by weight of leather is preferably in particulate form.

[0023] By particulate form, we mean that the initial material is in the form of particles of varying coarseness, ranging in size from a few tens or hundreds of micrometers or more, up to a few centimeters. Preferably, the particle size of the initial material, measured by sieving, is between 5 mm and 10 cm. When the initial material is in the form of larger pieces, the The process according to the invention preferably includes a preliminary step of cutting and / or grinding this material into smaller particles, for example by means of a knife mill.

[0024] The particles of matter can also have any shape and any moisture content.

[0025] The initial material can otherwise be in the form of a paste, based in particular on a mixture of leather and water.

[0026] The amount of water to be added to the initial material to obtain a water / initial material weight ratio (weight of dry material or weight of its dry extract when it already contains water) of between 1.7 and 6, preferably between 1.7 and 4, depends on the initial moisture content of the initial material. Thus, the process according to the invention may include a step of determining the moisture content of the initial material, then determining the amount of water to be added to it to obtain the desired water / dry material weight ratio, this ratio being between 1.7 and 6, preferably between 1.7 and 4.

[0027] In the present context, the term "moisture content" is understood, in a conventional sense, as the percentage by mass of water contained in the material, relative to the total mass of the material, under conditions of 60% relative humidity and approximately 20°C. This moisture content can, in particular, be determined by comparing the weight of a sample of the material with the weight of the same sample after it has been subjected to a drying step at over 100°C until a substantially constant sample weight is obtained.

[0028] The initial material can be brought into contact with water in different ways.

[0029] In preferred embodiments of the invention, the starting material is introduced into the extruder as is, and an aqueous solution is also introduced into the extruder, independently of the starting material. Thus, the initial material and water are brought into contact by introducing an aqueous solution into the twin-screw extruder.

[0030] The flow rate of aqueous solution fed into the extruder is then preferably constant and calculated to obtain the desired water / dry matter weight ratio. Such a calculation falls within the competence of a person skilled in the art, who will notably take into account the feed rate of the initial material into the extruder and the initial moisture content of this material.

[0031] In particular embodiments of the invention, the initial material and water are brought into contact by introducing an aqueous solution into the twin-screw extruder upstream of a first mixing zone comprising the twin-screw extruder, for example at the end of the conveying zone preceding the first stage. The initial material is then intimately mixed with the aqueous solution in this mixing zone.

[0032] In embodiments of the invention, the starting material is impregnated with an aqueous solution before being introduced into the extruder. This impregnation is preferably carried out over 12 to 30 hours, preferably at a temperature between 1 and 10 °C. The impregnation step preferably includes at least one initial agitation phase, so as to ensure homogeneous mixing of the water with the starting material. Agitation may be continued during all or part of the impregnation step. The respective quantities of aqueous solution and starting material mixed are determined to obtain the desired water / starting material weight ratio.

[0033] For either of the above embodiments, the aqueous solution may consist solely of water. Alternatively, it may contain at least one additional substance.

[0034] The aqueous solution may, in particular, contain an agent for decomplexing the chromium-collagen complexes that may have formed within the leather during the chrome tanning processes. The use of such a decomplexing agent in the aqueous solution advantageously facilitates the penetration of water into the initial material during the extrusion treatment step. This decomplexing agent may, in particular, consist of sodium hydroxide and / or magnesium hydroxide, for example at a concentration in water of between 10 and 40 g / L. Alternatively, it may, for example, be oxalate, and in particular ammonium oxalate and / or sodium oxalate, for example at a concentration in water of between 25 and 40 g / L. Sodium hydroxide, like oxalate, has the advantage of being readily available and having good solubility in water.

[0035] The aqueous solution may also or otherwise contain one or more surfactants.

[0036] The feed rate of the initial material into the twin-screw extruder is preferably constant. In particular embodiments of the invention, it is between 1 and 15 kg / h, expressed as a dry weight. As indicated above, the initial material introduced into the extruder can have any initial moisture content.

[0037] The temperature inside the twin-screw extruder is preferably between 15 and 35 °C, and more preferably between 20 and 30 °C. Preferably, this temperature is constant inside the extruder.

[0038] The twin-screw extruder implemented according to the invention can be of any conventional type. It is preferably of the co-rotating screw type, preferably of interpenetrating type. The screws used can have any profile and any thread pitch.

[0039] The rotational speed of the screws of the twin-screw extruder is preferably between 100 and 700 rpm, and preferably between 400 and 650 rpm. A speed within this range is advantageously, on the one hand, high enough to ensure good defibration of the collagen fibers within the material being processed in the extruder, and on the other hand, low enough to prevent the material from disintegrating in the extruder.

[0040] The twin-screw extruder implemented according to the invention may comprise one or more working zones. The material passes successively through these zones, from a first end, into which the initial material is introduced into the extruder, to a second end, from which the processed material exits the extruder.

[0041] The twin-screw extruder implemented according to the invention preferably comprises one or more conveying zones, with and / or without material compression, preferably of the direct-pitch type, and one or more mixing zones. Any combination of such zones in the extruder falls within the scope of the invention. In preferred embodiments of the invention, the twin-screw extruder comprises a conveying zone before and after each mixing zone.

[0042] Preferably, it does not include areas with reverse thread screws.

[0043] Preferably, the twin-screw extruder comprises at least three mixing zones, for example, 3 to 8 mixing zones. The mixing zones are preferably of the bilobe mixer type. Within each mixing zone, the mixers are preferably mounted on their respective screws with opposite inclinations, for example, between 5 and 90 degrees for one and between -5 and -90 degrees for the other. Their inclinations are, for example, equal to -30 and +30 degrees, respectively.

[0044] The residence time of the material in the twin-screw extruder depends on the rotation speed of the screws, and on the number and length of the mixing zones. For example, it ranges from 20 seconds to 6 minutes.

[0045] The material obtained at the extruder outlet is in the form of a moist paste. This paste can be ground and / or dried to form granules that can be used for the subsequent step of the process according to the invention. Alternatively, these granules can be incorporated into a polymer matrix, particularly a thermoplastic matrix, to manufacture a composite material.

[0046] Preferably, the paste obtained at the outlet of the extruder is used directly for the next step of the process according to the invention, without an intermediate drying step.

[0047] In particular embodiments of the invention, this paste is subjected, before being formed into a sheet, to at least one processing step in an ex- twin screw truder, at a temperature between 10 and 40 °C, in the presence of water in such a quantity that the ratio by weight of water to the weight of dry matter of said paste is between 1.7 and 6. This step of processing the paste by extrusion may meet one or more of the characteristics described above with reference to the step of processing the initial material by extrusion.

[0048] The paste obtained at the end of the extrusion treatment of the initial material can, for example, be successively subjected to three, four, or even more such extrusion treatment steps, before being subjected to the sheet forming step of the process according to the invention.

[0049] The paste obtained at the end of this or these additional extrusion step(s) is advantageously more flexible and homogeneous, particularly with regard to the size and diameter of the fibers it contains, the greater the number of successive passes in the extruder.

[0050] Preferably, during this or these additional extrusion step(s) of the paste, the specific mechanical energy (SME), expressing the work done by the screws of the twin-screw extruder to shear the paste, is between 100 and 800 Wh / kg, preferably between 100 and 500 Wh / kg.

[0051] The specific mechanical energy (SME) is defined by the following equation:

[0052] [Math.l] EMS =

[0053] in which U represents the voltage of the twin-screw extruder motor, expressed in volts; I represents the current of this motor (measured value), expressed in amperes; cosq> represents the motor correction coefficient (supplied by the manufacturer); N represents the rotation speed of the extruder screws, expressed in revolutions / min; Nmax represents the maximum rotation speed of the extruder screws, expressed in revolutions / min; and Q represents the input flow rate of solid material into the extruder, expressed in kg / h.

[0054] In particular embodiments of the invention, the step of shaping the dough into a sheet is carried out at room temperature, i.e. at a temperature between approximately 15 and 25 °C.

[0055] It is also preferably carried out without the addition of a binder, using only an aqueous solution. This aqueous solution may, where appropriate, contain one or more chemical additives, such as colorants, pigments, or plasticizing agents, in order to give the sheet more flexibility, such as glycerol, for example. Each of these additives is then introduced into the aqueous solution in a small quantity, less than or equal to 2% by weight relative to the total weight of the aqueous suspension. the total quantity of additive(s) not exceeding 10% by weight relative to the total weight of the aqueous suspension.

[0056] Preferably, the step of shaping the leather-based pulp into sheets is carried out according to a technique classically used in the paper industry for the formation of cellulose-based sheets.

[0057] Thus, in particular embodiments of the invention, the shaping of the sheet paste includes diluting this paste in an aqueous vehicle to form an aqueous suspension, projecting this aqueous suspension onto a sheet forming device, and then drying the sheet thus obtained.

[0058] The aqueous vehicle used is preferably free of binder, in particular polymer or polymer precursor.

[0059] Preferably, the aqueous suspension contains 5 to 15% by weight of the paste relative to the total weight of the aqueous suspension.

[0060] The sheet-forming device can be of any type used conventionally in the paper industry. It may, in particular, comprise a moving, especially rotating, fabric, wire, or grid against which the aqueous suspension is projected. The formation of a solid sheet is then accompanied by the simultaneous draining of this sheet.

[0061] By way of example, a device such as that marketed by Techpap can be used, in which a sheet is formed by projecting, in a vertical motion, an aqueous suspension of material onto a grid arranged in a rotating cylindrical container, this grid being completely immersed in a wall of water. The water contained in the forming sheet is then drained by centrifugal force. Once the sheet has drained, it can be removed from the device for possible pressing and drying.

[0062] The drying of the sheet obtained can be carried out in any conventional manner for a person skilled in the art, for example in the open air at room temperature, or in an oven, for example at a temperature between 20 and 50 °C, for a few minutes, for example between 2 and 60 minutes.

[0063] The leather-based sheet obtained by the process according to the invention can have any thickness, for example a thickness between 0.1 and 2.5 mm. Its density can vary between 180 and 300 g / m2.

[0064] It is also not very hygroscopic.

[0065] It advantageously presents an attractive aesthetic appearance, good uniformity of appearance, flexibility equivalent to that of the initial material, good cohesiveness, and good mechanical properties. In particular, it can exhibit, during a tensile test carried out in the longitudinal direction (the collagen fibers having naturally aligned themselves in this longitudinal direction during the manufacturing of the sheet), an al- 14% resistance to breakage.

[0066] The leather-based sheet obtained by the process according to the invention, consisting predominantly of leather, can advantageously be used for the manufacture of various articles such as leather goods, art and design objects, particularly in the field of fashion, protective articles, presentation articles, for example in the form of sheaths for presentation supports, etc.

[0067] The features and advantages of the invention will become more apparent in the light of the following implementation examples, provided by way of illustration only and in no way limiting the invention, with the support of [Fig. 1], in which:

[0068] [Fig.1] Fig.1 shows photographs of leather-based sheets obtained according to the invention, in a / , b / and c / from synthetically tanned leather scraps, the sheet forming step having been carried out in a / after a single extrusion treatment step of the material, in b / after two successive extrusion treatment steps of the material, in c / after three successive extrusion treatment steps of the material, and in d / from pre-ground chrome-tanned full-grain leather, the sheet forming step having been carried out after one extrusion treatment step of the material.

[0069] Example 1 - Obtaining paste by extruding leather in particulate form

[0070] A CLEXTRAL BC-45 twin-screw extruder is used, equipped with a screw-driving motor with a power of 66 kW and a DKM water supply pump with a flow rate of 201 / h.

[0071] The rotation speed of the screws is fixed at 600 rpm.

[0072] The material to be processed is introduced into the extruder via a hopper combined with a calibrated scale.

[0073] A series of modules are assembled according to an arrangement ensuring an alternation of conveying zones and mixing zones.

[0074] A first conveying module, of the trapezoidal groove double-threaded direct-pitch screw type, is positioned at the beginning of the screw, at the raw material feed inlet. The other 10 conveying modules are of the U-shaped groove double-threaded direct-pitch screw type, allowing both the conveying of the material and its compression by reducing the thread pitch.

[0075] The seven mixing modules are of the bilobe mixer type, which allow water to penetrate the material and defibrate it. These mixers are mounted inclined on the two respective screws at angles of 30 and -30 degrees respectively, which increases the shear stress experienced by the material, ensures good mixing of the material, and provides resistance to its advancement, thus promoting good defibration.

[0076] In each of the conveyor modules, the screw pitches are between 25 and 70 mm. The length of each module is between 50 and 100 mm.

[0077] The temperature applied in the extruder is 30°C.

[0078] For all experiments, the water / dry matter ratio to be treated is equal to 4.

[0079] The input flow rate of the initial material into the extruder is set at 7 kg / h, expressed in dry matter.

[0080] When the material is subjected to several successive passes in the extruder, the input flow rate of the paste into the extruder, for the second and subsequent passes, is set at 28.8 kg / h, expressed in wet matter.

[0081] For each paste obtained, the characteristics of the starting material, the number of passes through the extruder, and the characteristics of the resulting material are indicated in Table 1 below. In this table, Synth. refers to synthetically tanned leather scraps in the form of particles between 1 cm and 10 cm in length, and Nat. refers to pre-ground chrome-tanned full-grain leather in the form of chips approximately 1 cm long.

[0082] [Tables 1] Paste PSI PS2 PS3 PN1 Initial material Synth. Synth. Synth. Nat. Number of passes through the extruder 1 2 3 1 Dry matter content in the final paste 19.2% 20% 20.4% 41.1%

[0083] Each of the resulting pastes contains long collagen fibers separated from each other, this separation increasing with the number of passes in the extruder, as does the homogeneity of fiber length.

[0084] For PSI and PN1 pastes, it is noted that there are some unprocessed areas with lumps in the paste. These lumps decrease in size and number with each subsequent pass through the extruder.

[0085] Example 2 - Shaping the dough

[0086] The pastes obtained in example 1 are shaped into sheets.

[0087] For this purpose, each of the pastes is suspended in water, with a water / paste mass ratio of 250, equivalent to 2% by weight of paste in the aqueous suspension.

[0088] Each aqueous suspension thus obtained is processed in an FDA-compliant automatic sheet forming machine from Techpap. The aqueous suspension is sprayed from top to bottom onto a grid arranged in the rapidly rotating circular drum of the machine. Once the solution has been sprayed, the resulting sheet is collected and left to dry either in open air or in an oven at 30°C for 30 minutes.

[0089] Sheets with thicknesses between 0.2 and 0.75 mm are thus prepared from the pastes obtained in example 1.

[0090] It is observed that once dry, each of these leaves exhibits good cohesion. The fibers have naturally arranged themselves along the longitudinal direction of the leaf.

[0091] The density of the sheets obtained varies between 180 and 300 g / m2.

[0092] Their hydroscopy rate varies between 2 and 5%.

[0093] Photographs of sheets obtained from the PSI, PS2, PS3 and PN1 pastes are shown in [Fig. 1], respectively in a, b, c and d. They all exhibit a pleasing aesthetic appearance and good continuity. It can be observed that the greater the number of passes through the extruder, the more homogeneous the appearance of the resulting sheet.

[0094] Example 3 - Characterization of the mechanical properties of the sheets

[0095] Test specimens are cut using a punch from each of the sheets formed in Example 2, some along the length of the sheet (longitudinal specimens), and others perpendicular to this length (transverse specimens). These specimens are then stabilized in a climate chamber at 23 °C, 50% relative humidity for 7 days.

[0096] The mechanical performance of the sheets is determined by tensile tests carried out on each of these specimens, on a tensile testing bench, using the method described in ISO 527-1:2012. The force is applied at a constant speed of 2.5 mm / min at 23 °C. A 500 N force sensor is used to measure the force.

[0097] The results obtained are shown in Table 2 below. In this table, the results are given for sheets that have been oven-dried at 30 °C for 30 min, unless otherwise indicated as natural drying (“SN”). “Length.” further indicates a tensile test carried out along the longitudinal direction of the sheet, i.e. on the longitudinal test pieces taken from the sheet, and “Width” indicates a tensile test carried out in the direction perpendicular to the longitudinal direction of the sheet, i.e. on the transverse test pieces taken from the sheet.

[0098] [Tables2] Length. Paste PSI PSI PS2 PS2 (SN) PS3 PN1 (SN) Average thickness (mm) 0.73 0.40 0.36 0.31 0.25 0.20 Max. stress (MPa) 38 36 39.6 43 42 4 Tensile modulus (GPa) 0.74 1.09 0.77 0.76 0.82 0.14 Elongation (%) 7 12 11 13 14 4 Width Average thickness (mm) 0.60 0.41 0.36 0.31 0.22 0.20 Max. stress (MPa) 19 21 16 22 21 2 Tensile modulus (GPa) 0.47 0.57 0.39 0.43 0.49 0.18 Elongation (%) 5 8 9 9 8 5

[0099] It is observed that good mechanical properties are obtained for all the sheets prepared according to the invention. The thickness of the sheet plays a minor role in the tensile performance, which demonstrates the homogeneity of the material within the sheets.

[0100] Overall mechanical performance increases with the number of successive passes through the extruder. The sheet obtained from the PS3 paste, corresponding to three successive treatments in the extruder, not only has a smoother and more homogeneous appearance than the others, but also the best overall mechanical properties.

Claims

Demands

1. A method for manufacturing a leather-based sheet, characterized in that it comprises: - preparing a pulp by a processing step of a material containing at least 70% by weight of leather prepared from the skin of a non-human animal in a twin-screw extruder at a temperature between 10 and 40 °C, in the presence of water in an amount such that the weight ratio of water to said material is between 1.7 and 6; - and shaping said pulp into a sheet.

2. A method according to claim 1, wherein said material is in particulate form, preferably with a particle size between 5 mm and 10 cm.

3. A method according to any one of claims 1 or 2, wherein said twin-screw extruder is a co-rotating screw extruder.

4. A method according to any one of claims 1 to 3, wherein the rotation speed of the screws of the twin-screw extruder is between 100 and 700 rpm.

5. A method according to any one of claims 1 to 4, wherein said twin-screw extruder comprises at least three mixing zones.

6. A method according to any one of claims 1 to 5, wherein the input flow rate of said material into the twin-screw extruder is between 1 and 15 kg of dry material per hour.

7. A method according to any one of claims 1 to 6, wherein the bringing together of said material and water is carried out by introducing an aqueous solution into the twin-screw extruder, preferably before a first mixing zone of said twin-screw extruder.

8. A method according to any one of claims 1 to 7, wherein said paste is subjected, before being formed into a sheet, to at least one processing step in a twin-screw extruder, at a temperature between 10 and 40 °C, in the presence of water in an amount such that the weight ratio of water to dry weight of said paste is between 1.7 and 6.

9. A method according to any one of claims 1 to 8, wherein the shaping of said sheet paste comprises diluting said paste in an aqueous vehicle to form an aqueous suspension, projection of said aqueous suspension onto a sheet-forming device, then drying of the sheet thus obtained.

10. A method according to any one of claims 1 to 9, wherein said aqueous suspension contains 5 to 15% by weight of said paste relative to the total weight of said aqueous suspension.

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