High performance leather based on bio-based raw materials and method for its preparation

Abstract

This invention discloses a high-performance leather based on bio-based raw materials and its preparation method, belonging to the field of leather technology. The leather has a three-layer functional gradient structure, from bottom to top: a bottom layer, formed by cross-linking of thermoplastic starch matrix, plant tannins, and activated keratin via laccase catalysis; a middle foamed layer, formed by hot-pressing foaming of thermoplastic starch matrix, sodium bicarbonate, and citric acid; and a top layer, formed by cross-linking of thermoplastic starch matrix, sucrose, and nanocellulose via oxidase catalysis. The preparation method involves first preparing the bottom, middle, and top layer slurries separately, then performing semi-dry composite and integrated hot-pressing molding. The raw materials used in this invention are all renewable bio-based components, and the preparation process is green and environmentally friendly. The resulting leather possesses high tensile strength, excellent abrasion resistance and water resistance, tight interlayer bonding, good softness and cushioning, and excellent overall performance. It can be widely used in footwear, bags, and other fields, with promising industrialization prospects.

Description

Technical Field

[0001] This invention relates to the field of leather technology, and in particular to high-performance leather based on bio-based raw materials and its preparation method. Background Technology

[0002] Leather, as an important industrial material, is widely used in footwear, bags, and automotive interiors. However, the traditional tanning process for animal leather relies on chemical reagents such as chromium salts, generating large amounts of wastewater containing heavy metals, causing serious environmental pollution. Currently, global leather production mainly uses chrome tanning, which involves the discharge of high-concentration organic wastewater, resulting in high environmental remediation costs.

[0003] To address these issues, polyurethane or polyvinyl chloride-coated synthetic leather once became a major alternative. While these materials offer cost advantages, they are made from petrochemical products, resulting in volatile organic compound residues during production. Furthermore, they are difficult to degrade naturally after disposal, making them essentially unsustainable plastic products.

[0004] In recent years, bio-based leather alternatives made from plant waste or microorganisms have gradually emerged. Leather made from pineapple leaf fibers and apple leather made from apple pomace have entered the market. However, plant-derived materials lack the collagen fiber network structure unique to genuine leather, resulting in significantly insufficient mechanical properties. They often require the addition of synthetic fiber base fabric on the back or the addition of petroleum-based components to improve overall strength, leading to a low bio-based content in the final product.

[0005] Another type of research attempts to construct biomimetic structures using mycelium or gel materials. Mycelium-based leather has advantages in terms of biodegradability, but currently still suffers from problems such as insufficient tensile strength, poor water resistance, and low degree of process standardization. While gel-based materials offer structural controllability, they face challenges such as poor abrasion resistance, limited long-term stability, and difficulties in large-scale production. Existing technologies have not yet provided a bio-based leather solution that simultaneously satisfies high mechanical strength, green manufacturing throughout the entire process, and tunable structure and function. Summary of the Invention

[0006] The purpose of this invention is to address the shortcomings of existing technologies by proposing a high-performance leather based on bio-based raw materials and its preparation method.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A high-performance leather based on bio-based raw materials, wherein the leather has a three-layer functional gradient structure, comprising, from bottom to top:

[0009] The bottom layer, which is a reinforcing layer, is formed by cross-linking of a bottom slurry containing a thermoplastic starch matrix, plant tannins and activated keratin through laccase catalysis. The activated keratin is keratin powder that has been mechanically activated and is rich in free thiol groups.

[0010] An intermediate foaming layer, which is a buffer layer, is disposed on the bottom layer and is formed by hot-pressing foaming of an intermediate layer slurry containing thermoplastic starch matrix, sodium bicarbonate and citric acid.

[0011] The surface layer, which is a dense and wear-resistant layer, is disposed on the intermediate foamed layer and is formed by cross-linking of a surface slurry containing thermoplastic starch matrix, sucrose and nanocellulose through oxidase catalysis.

[0012] Preferably, the plant tannin is selected from one or more of tarara tannin, vitex bark tannin, and gallic tannin; the nanocellulose is bacterial cellulose or plant-derived nanocellulose.

[0013] Preferably, the thiol content of the activated keratin is >50 μmol / g.

[0014] A method for preparing high-performance leather based on bio-based raw materials includes the following steps:

[0015] Step 1: Preparation of basic thermoplastic starch mother liquor: Mix corn starch, glycerol and water in a mass ratio of 1:0.3-0.4:8-12, stir and gelatinize at 80-85℃ for 20-40 minutes, and cool to 40-45℃ to obtain basic thermoplastic starch mother liquor;

[0016] Step 2: Preparation of activated keratin powder: After cleaning and drying the waste keratin fibers, place them in a planetary ball mill and ball mill them for 2-4 hours at a ball-to-material ratio of 10:1 and a speed of 500-600 rpm. Then pass them through a 200-mesh sieve to obtain activated keratin powder.

[0017] Step 3: Preparation of the base slurry: Take 100 parts by weight of the basic thermoplastic starch mother liquor from Step 1, add 1.5-3.0 parts of plant tannin, and stir to dissolve at 38-42℃; adjust the pH of the system to 5.0-6.5, then add laccase solution (laccase dosage is 500-1000 U / g tannin), and stir for 30-45 minutes; then add 2-5 parts of the activated keratin powder from Step 2, and stir for 1-2 hours at 600-800 rpm; heat to 75-80℃ for inactivation for 10-15 minutes, then degas under vacuum to obtain the base slurry;

[0018] Step 4: Prepare the intermediate foaming layer slurry: Take 100 parts of the basic thermoplastic starch mother liquor from Step 1 by weight, add 1.0-2.0 parts of glycerol, and stir evenly; add 1.5-3.0 parts of sodium bicarbonate and 1.0-2.0 parts of citric acid after premixing, stir for 1-2 minutes, and then proceed with the coating operation in Step 6, with an interval of no more than 10 minutes.

[0019] Step 5: Preparation of surface slurry: Take 100 parts by weight of the basic thermoplastic starch mother liquor from Step 1, add 2.0-5.0 parts of sucrose, and stir to dissolve at 30-40℃; add oxidase solution, with an oxidase dosage of 500-1000 U / g sucrose, and stir to react for 1-2 hours; add 0.5-2.0 parts of nanocellulose, and disperse at high speed at 600-800 rpm for 10-15 minutes; heat to 70-80℃ to inactivate for 10-15 minutes to obtain the surface slurry;

[0020] Step 6, Three-layer lamination and molding: Cast the bottom layer slurry from Step 3 and dry it at 50-60℃ for 20-30 minutes until it is semi-dry; apply the middle foaming layer slurry from Step 4 onto the semi-dry bottom layer, controlling the wet film thickness to 3.0-5.0 mm; immediately apply the surface layer slurry from Step 5 onto the middle layer, controlling the wet film thickness to 0.5-1.0 mm; place the three-layer composite preform in a hot press and hot press it at 150-170℃ and 0.2-0.5 MPa for 10-15 minutes. After cooling, remove it and humidify it at 20-25℃ and 45-55% relative humidity for 45-50 hours to obtain the high-performance leather based on bio-based raw materials.

[0021] Preferably, in step 5, the oxidase is pyranose oxidase or galactose oxidase.

[0022] Applications of high-performance leather based on bio-based raw materials in footwear, bags, automotive interiors, and furniture upholstery.

[0023] Based on the above technical solutions, the mechanisms and molding processes of each layer of the high-performance leather based on bio-based raw materials in this invention are explained as follows:

[0024] 1. Cross-linking mechanism of the bottom reinforcing layer: The bottom layer utilizes laccase to catalyze the oxidation of plant tannins, which then undergo a Michael addition reaction with mechanically activated keratin to construct a stable covalent cross-linked network. Mechanical ball milling breaks the disulfide bonds in the keratin molecules, generating free thiol groups in situ, thus reactivating the originally inert keratin. Laccase uses oxygen as an oxidant to oxidize the phenolic hydroxyl groups of plant tannins to quinone groups. Subsequently, the thiol groups on the keratin act as nucleophiles to attack the quinone groups, resulting in a Michael addition reaction to form stable thioether bonds. Through this "tannin bridge" effect, keratin is covalently linked to tannin molecules, while tannin itself is tightly bound to the starch matrix through hydrogen bonds, molecular chain intercalation, and etherification during hot pressing. This transforms keratin from a simple physical filler into a covalently reinforcing phase. This design gives the bottom layer the film-forming properties of starch, the hydrophobicity of tannin, and the rigidity of keratin. The three work synergistically through covalent and hydrogen bonds, significantly improving the mechanical strength and water resistance of the bottom layer and solving the technical problem of poor compatibility and weak binding force of waste keratin in the starch matrix.

[0025] 2. Crosslinking mechanism of dense wear-resistant surface layer: The surface layer uses oxidase to catalyze the oxidation of sucrose, introducing carbonyl groups into the sucrose molecules to form oxidized sucrose derivatives. The carbonyl groups of these oxidized sucrose derivatives react with the hydroxyl groups in the starch molecules to undergo hemiacetal / hemiketal and acetal / ketal reactions, constructing a dense covalent crosslinking network, thereby ensuring the wear resistance of the surface layer.

[0026] The oxidation of sucrose is catalyzed by either pyranose oxidase or galactose oxidase. Pyranose oxidase can regioselectively oxidize sucrose under mild conditions to produce ketosucrose derivatives; galactose oxidase catalyzes the oxidation of primary alcohols on terminal galactose residues to aldehydes, making it suitable for substrate systems containing galactose. Both oxidases use oxygen from the air as an oxidant and react under mild conditions of 30-40°C, without the need for external chemical oxidants.

[0027] The carbonyl groups generated by oxidation do not require separation and purification; they can undergo in-situ condensation reactions with the hydroxyl groups on the surface of starch and nanocellulose to gradually form a stable cross-linked network. This in-situ generation of cross-linking agents eliminates the need for external chemical cross-linking agents, enhancing the greenness of the preparation process. Simultaneously, nanocellulose plays a synergistic reinforcing role in the surface layer. Its abundant surface hydroxyl groups can participate in the aforementioned condensation reaction and act as a rigid filler, tightly binding with the cross-linked network, further improving the surface layer's density and wear resistance, achieving a balance between greenness and functionality.

[0028] 3. Cushioning Mechanism of the Intermediate Foam Layer: The cushioning performance of the intermediate foam layer originates from the synergistic effect of sodium bicarbonate and citric acid during the hot-pressing process. Upon heating, the two undergo an acid-base reaction, releasing carbon dioxide gas and forming a uniform cell structure in the viscous system. Furthermore, as a polycarboxylic acid compound, under the high temperature and semi-dry environment of the hot-pressing process, the carboxyl groups of citric acid can undergo esterification with the hydroxyl groups on starch molecules, reinforcing the cell walls and thus constructing a closed-cell or open-cell foam structure with high resilience and cushioning performance, effectively improving the softness and cushioning properties of the leather material.

[0029] This invention employs a "semi-dry composite + integrated hot pressing" molding process to achieve the organic integration of a three-layer structure. The bottom layer slurry is pre-dried to a semi-dry state, effectively preventing interpenetration between the layers and ensuring the clarity of the functional gradient structure. Subsequently, it undergoes one-time hot pressing molding, with the foaming reaction of the middle layer and the deep cross-linking reaction of the surface and bottom layers proceeding simultaneously. During this process, the three-layer interfaces undergo melting, interpenetration, and chemical bonding under high temperature and pressure, ultimately forming a tightly bonded, complementary, integrated multi-layer functional gradient structure, achieving a synergistic effect of "wear-resistant surface, buffering middle layer, and tough bottom layer."

[0030] Compared with the prior art, the beneficial effects of the present invention are:

[0031] 1. This invention uses bio-based raw materials to construct a three-layer functional gradient structure for leather. The core raw materials are renewable resources such as corn starch, plant tannins, and waste keratin. It eliminates heavy metal tanning agents such as chromium salts and petroleum-based synthetic components. The preparation process leaves no volatile organic compound residues, and the product is biodegradable. It achieves green and environmentally friendly practices from raw material selection to production, solving the environmental pollution problems of traditional animal leather and petroleum-based synthetic leather.

[0032] 2. This invention achieves covalent bonding and network construction between raw materials through the enzymatic cross-linking of laccase and oxidase and the synergistic effect of thermo-pressing. The bottom layer forms a stable cross-linking system of keratin-tannin-starch, the surface layer forms a dense cross-linking network of oxidized sucrose-starch-nanocellulose, and the middle layer forms a pore structure enhanced by esterification. The functions of each layer are complementary and the interface is tightly bonded, so that the leather has high tensile strength, good elongation at break, excellent surface abrasion resistance and water resistance, and the mechanical properties and performance meet the requirements for practical application.

[0033] 3. The present invention adopts a molding process of "semi-dry composite + integrated hot pressing", which not only avoids the mutual penetration of the slurry between layers and ensures the clarity of the functional gradient structure, but also enables the foaming of the middle layer, the deep cross-linking of the top and bottom layers and the chemical bonding between layers to proceed simultaneously, thereby improving the interlayer peel strength and ensuring the stability of the overall leather structure. Moreover, the process steps are simple and the parameters are easy to control, making it suitable for large-scale industrial production.

[0034] 4. The bio-based leather prepared by this invention has comprehensive characteristics of surface wear resistance, middle cushioning, and bottom toughness. It also has good softness and resilience. Its performance can meet the usage requirements of multiple application scenarios such as shoe materials, bags, automotive interiors, and furniture coverings. It has a wide range of applications and provides a feasible solution for the industrial application of bio-based leather. Detailed Implementation

[0035] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with existing known technologies. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0036] Example 1: A method for producing high-performance leather based on bio-based raw materials, comprising the following steps:

[0037] (1) Preparation of basic thermoplastic starch mother liquor: Mix 100g corn starch, 30g glycerol and 800g water in a mass ratio of 1:0.3:8, stir and gelatinize at 80℃ for 20 minutes, and cool to 40℃ to obtain basic thermoplastic starch mother liquor;

[0038] (2) Preparation of activated keratin powder: After cleaning and drying the waste keratin fibers, place them in a planetary ball mill and ball mill them for 2 hours at a ball-to-material ratio of 10:1 and a speed of 500 rpm. Then pass them through a 200-mesh sieve to obtain activated keratin powder with a thiol content of 55 μmol / g.

[0039] (3) Preparation of the bottom layer slurry: Take 100g of the basic thermoplastic starch mother liquor from step (1), add 1.5g of plant tannin, and stir to dissolve at 38℃; adjust the pH of the system to 5.0 with 0.1mol / L dilute sodium hydroxide solution, then add laccase solution (containing 1.5g of laccase), the amount of laccase is 500U / g tannin, and stir to react for 30 minutes; then add 2g of activated keratin powder from step (2), and stir to react for 1 hour at 600rpm; heat to 75℃ to inactivate for 10 minutes, then degas under vacuum to obtain the bottom layer slurry;

[0040] (4) Preparation of intermediate foaming layer slurry: Take 100g of the basic thermoplastic starch mother liquor from step (1), add 1g of glycerol, and stir evenly; add 1.5g of sodium bicarbonate and 1g of citric acid after premixing, stir for 1 minute, and then carry out the coating operation in step (6) with an interval of no more than 10 minutes.

[0041] (5) Preparation of surface slurry: Take 100g of the basic thermoplastic starch mother liquor from step (1), add 2g of sucrose, and stir to dissolve at 30℃; add pyranose oxidase (containing 370.37mg of pyranose oxidase) solution, the amount of oxidase is 500U / g sucrose, and stir to react for 1 hour; add 0.5g of bacterial cellulose, and disperse at 600rpm for 10 minutes; heat to 70℃ to inactivate for 10 minutes to obtain surface slurry;

[0042] (6) Three-layer composite and molding: The bottom layer slurry of step (3) is cast and molded, and dried at 50°C for 20 minutes to a semi-dry state; the middle foaming layer slurry of step (4) is coated on the semi-dry bottom layer, and the wet film thickness is controlled at 3.0 mm; the surface layer slurry of step (5) is immediately coated on the middle layer, and the wet film thickness is controlled at 0.5 mm; the three-layer composite preform is placed in a hot press and hot-pressed at 150°C and 0.2 MPa for 10 minutes, cooled and taken out, and humidified at 20°C and 45% relative humidity for 45 hours to obtain the high-performance leather based on bio-based raw materials.

[0043] The laccase was purchased from Shanghai Yuanye Biotechnology Co., Ltd., with catalog number S10188-100mg and specification ≥0.5u / mg solid; the pyranose oxidase was purchased from Shanghai Yuanye Biotechnology Co., Ltd., with catalog number V12446-50U and specification ≥2.7units / mg solid.

[0044] Example 2: A method for producing high-performance leather based on bio-based raw materials, comprising the following steps:

[0045] (1) Preparation of basic thermoplastic starch mother liquor: Mix 100g corn starch, 35g glycerol and 1000g water in a mass ratio of 1:0.35:10, stir and gelatinize at 82℃ for 30 minutes, and cool to 42℃ to obtain basic thermoplastic starch mother liquor;

[0046] (2) Preparation of activated keratin powder: After cleaning and drying the waste keratin fibers, place them in a planetary ball mill and ball mill them for 3 hours at a ball-to-material ratio of 10:1 and a speed of 550 rpm. Then pass them through a 200-mesh sieve to obtain activated keratin powder with a thiol content of 62 μmol / g.

[0047] (3) Preparation of the bottom layer slurry: Take 100g of the basic thermoplastic starch mother liquor from step (1), add 2.25g of plant tannin, and stir to dissolve at 40℃; adjust the pH of the system to 5.75 with 0.1mol / L dilute sodium hydroxide solution, then add laccase solution (containing 3.38g of laccase), the amount of laccase is 750U / g tannin, and stir to react for 38 minutes; then add 3.5g of activated keratin powder from step (2), and stir to react for 1.5 hours at 700rpm; heat to 78℃ to inactivate for 12 minutes, then degas under vacuum to obtain the bottom layer slurry;

[0048] (4) Preparation of intermediate foaming layer slurry: According to the weight, take 100g of the basic thermoplastic starch mother liquor from step (1), add 1.5g of glycerol, and stir evenly; add 2.25g of sodium bicarbonate and 1.5g of citric acid after premixing, stir for 1.5 minutes, and then carry out the coating operation of step (6) with an interval of no more than 10 minutes.

[0049] (5) Preparation of surface slurry: Take 100g of the basic thermoplastic starch mother liquor from step (1), add 3.5g of sucrose, and stir to dissolve at 35℃; add galactose oxidase solution (containing 87.5mg of galactose oxidase), with an oxidase dosage of 750U / g sucrose, and stir to react for 1.5 hours; add 1.25g of plant-derived nanocellulose, and disperse at 700rpm for 12 minutes; heat to 75℃ to inactivate for 12 minutes to obtain surface slurry;

[0050] (6) Three-layer composite and molding: The bottom layer slurry of step (3) is cast and molded, and dried at 55°C for 25 minutes to a semi-dry state; the middle foaming layer slurry of step (4) is coated on the semi-dry bottom layer, and the wet film thickness is controlled at 4.0 mm; the surface layer slurry of step (5) is immediately coated on the middle layer, and the wet film thickness is controlled at 0.8 mm; the three-layer composite preform is placed in a hot press and hot-pressed at 160°C and 0.4 MPa for 12 minutes, cooled and taken out, and humidified at 22°C and 50% relative humidity for 48 hours to obtain the high-performance leather based on bio-based raw materials.

[0051] The laccase was purchased from Shanghai Yuanye Biotechnology Co., Ltd., with catalog number S10188-100mg and specification ≥0.5u / mg solid; the galactose oxidase was purchased from Shanghai Yuanye Biotechnology Co., Ltd., with catalog number S10166-150u and specification high purity, 30u / mg.

[0052] Example 3: A method for producing high-performance leather based on bio-based raw materials, comprising the following steps:

[0053] (1) Preparation of basic thermoplastic starch mother liquor: Mix 100g corn starch, 40g glycerol and 1200g water in a mass ratio of 1:0.4:12, stir and gelatinize at 85℃ for 40 minutes, and cool to 45℃ to obtain basic thermoplastic starch mother liquor;

[0054] (2) Preparation of activated keratin powder: After cleaning and drying the waste keratin fibers, place them in a planetary ball mill and ball mill them for 4 hours at a ball-to-material ratio of 10:1 and a speed of 600 rpm. Then pass them through a 200-mesh sieve to obtain activated keratin powder with a thiol content of 74 μmol / g.

[0055] (3) Preparation of the bottom layer slurry: Take 100g of the basic thermoplastic starch mother liquor from step (1), add 3.0g of plant tannin, and stir to dissolve at 42℃; adjust the pH of the system to 6.5 with 0.1mol / L dilute sodium hydroxide solution, and then add laccase solution (containing 6.0g of laccase), with the amount of laccase being 1000U / g tannin, and stir to react for 45 minutes; then add 5g of activated keratin powder from step (2), and stir to react for 2 hours at 800rpm; heat to 80℃ to inactivate for 15 minutes, and then degas under vacuum to obtain the bottom layer slurry;

[0056] (4) Preparation of intermediate foaming layer slurry: Take 100g of the basic thermoplastic starch mother liquor from step (1), add 2.0g of glycerol, and stir evenly; add 3.0g of sodium bicarbonate and 2.0g of citric acid after premixing, stir for 2 minutes, and then carry out the coating operation in step (6) with an interval of no more than 10 minutes.

[0057] (5) Preparation of surface slurry: Take 100g of the basic thermoplastic starch mother liquor from step (1), add 5g of sucrose, and stir to dissolve at 40℃; add pyranose oxidase solution (containing 1.85g of pyranose oxidase), with an oxidase dosage of 1000U / g sucrose, and stir to react for 2 hours; add 2.0g of plant-derived nanocellulose, and disperse at 800rpm for 15 minutes; heat to 80℃ to inactivate for 15 minutes to obtain surface slurry;

[0058] (6) Three-layer composite and molding: The bottom layer slurry of step (3) is cast and molded, and dried at 60°C for 30 minutes to a semi-dry state; the middle foaming layer slurry of step (4) is coated on the semi-dry bottom layer, and the wet film thickness is controlled at 5.0 mm; the surface layer slurry of step (5) is immediately coated on the middle layer, and the wet film thickness is controlled at 1.0 mm; the three-layer composite preform is placed in a hot press and hot-pressed at 170°C and 0.5 MPa for 15 minutes, cooled and taken out, and humidified at 25°C and 55% relative humidity for 50 hours to obtain the high-performance leather based on bio-based raw materials.

[0059] The laccase was purchased from Shanghai Yuanye Biotechnology Co., Ltd., with catalog number S10188-100mg and specification ≥0.5u / mg solid; the pyranose oxidase was purchased from Shanghai Yuanye Biotechnology Co., Ltd., with catalog number V12446-50U and specification ≥2.7units / mg solid.

[0060] Comparative Example 1: Based on Example 2, the difference is that in step (3) when preparing the bottom slurry, only activated keratin powder and laccase solution are added, and no plant tannins are added. Only thermoplastic starch mother liquor, activated keratin and laccase are used as the bottom raw materials, and the other process parameters are exactly the same as in Example 2.

[0061] Comparative Example 2: Based on Example 2, the difference is that: in step (3) when preparing the bottom slurry, no activated keratin powder is added, and only thermoplastic starch mother liquor and plant tannins are used as the bottom raw materials (i.e., laccase catalysis is retained, but no keratin is involved), and the other process parameters are exactly the same as in Example 2.

[0062] Comparative Example 3: Based on Example 2, the difference is that in step (2), ordinary keratin powder (thiol content of 22 μmol / g) that has not been ball-milled by a planetary ball mill is used directly to replace the activated keratin powder, and the remaining process parameters are exactly the same as in Example 2.

[0063] Comparative Example 4: Based on Example 2, the difference is that: in step (5) when preparing the surface slurry, no galactose oxidase solution is added, and sucrose is directly mixed and dispersed with nanocellulose and thermoplastic starch mother liquor. The remaining process parameters are exactly the same as in Example 2.

[0064] Comparative Example 5: Based on Example 2, the difference is that in step (4) when preparing the intermediate foaming layer slurry, citric acid is not added, only sodium bicarbonate is added as a foaming agent, and the other process parameters are exactly the same as in Example 2.

[0065] Comparative Example 6: Based on Example 2, the difference is that the preparation of the intermediate foaming layer is cancelled, and the surface slurry is directly coated on the semi-dry bottom slurry and integral hot pressing is performed. The remaining process parameters are exactly the same as those in Example 2.

[0066] Comparative Example 7: Based on Example 2, the difference is that in step (6) when the three layers are composited and molded, the bottom layer is first hot-pressed and molded separately, then the middle layer is coated on the bottom layer and hot-pressed and foamed, and finally the surface layer is coated and hot-pressed and cross-linked. The hot pressing is done in three stages, and the other process parameters are exactly the same as in Example 2.

[0067] Test example: Performance tests were conducted on the leathers prepared in Examples 1-3 and Comparative Examples 1-7. The test methods and results are as follows:

[0068] (1) Overall tensile strength and elongation at break: Referring to GB / T 1040.3-2006 "Determination of tensile properties of plastics - Part 3: Test conditions for films and sheets", the three-layer composite leather was cut into dumbbell-shaped specimens (total length 115 mm, gauge length 25 mm) and tensile tests were performed on a universal testing machine at a tensile speed of 50 mm / min. The maximum tensile strength and elongation at break were recorded. Five parallel samples were tested for each sample, and the average value was taken.

[0069] (2) Interlayer peel strength: Refer to QB / T 2714-2018 "Determination of adhesion strength of coatings in physical and mechanical tests of leather". Cut the three-layer composite leather into 25mm×150mm specimens. Pre-peel the top layer and the middle layer (or bottom layer) by about 50mm from one end. Clamp them in the upper and lower clamps of the universal testing machine and perform a T-type peel test at a speed of 100mm / min. Record the average peel force. Peel strength = average peel force / specimen width. Test 5 parallel samples for each sample and take the average value.

[0070] (3) Surface abrasion resistance: Refer to GB / T 1768-2006 "Determination of abrasion resistance of paints and varnishes - Rotary rubber grinding wheel method" and use a Taber abrasion tester. Cut the three-layer composite leather into circular samples with a diameter of 100 mm, select an H-18 grinding wheel, load 500 g, speed 60 r / min, and weigh the sample after 1000 revolutions. Test 3 parallel samples for each sample and take the average value.

[0071] (4) Overall softness: Referring to ISO 17235:2015 "Leather - Physical and mechanical tests - Determination of softness", a leather softness tester was used. The three-layer composite leather sample (50 mm in diameter) was placed on the test ring, and its deformation was measured. The larger the value, the better the softness. Five parallel samples were tested for each sample, and the average value was taken.

[0072] (5) Overall water resistance (water absorption rate): Referring to GB / T 1034-2008 "Determination of water absorption of plastics", the three-layer composite leather was cut into 50mm×50mm samples, dried in a vacuum drying oven at 50℃ to constant weight (m0), and then immersed in deionized water at 23℃ for 24h. After removing the samples and blotting the surface moisture with filter paper, the samples were weighed (m1). Water absorption rate = (m1-m0) / m0×100%. Three parallel samples were tested for each sample, and the average value was taken.

[0073] (6) Overall cushioning (resilience): Refer to GB / T 6670-2008 "Determination of rebound performance of flexible foam polymer materials by falling ball method". Lay three layers of composite leather flat on the test platform, and let a steel ball fall freely from a specified height to impact the surface of the sample. Measure the rebound height and calculate the rebound rate. Test 5 parallel samples for each sample and take the average value.

[0074] (7) Intermediate layer density: Referring to the principle of GB / T 6343-2009 "Determination of Apparent Density of Foamed Plastics and Rubber", the cross-sectional analysis method was adopted. The three-layer composite leather was cut along the cross-section, and the thickness of the intermediate foam layer was measured using a stereomicroscope. The volume of the intermediate layer was calculated. A unit area sample was cut and weighed. After deducting the mass of the top and bottom layers, the mass of the intermediate layer was obtained, and the apparent density of the intermediate layer was calculated. Three parallel samples were tested for each sample, and the average value was taken.

[0075] The test results are as follows:

[0076] Table 1. Performance test results of the examples and comparative examples.

[0077] Data Analysis: According to the test results in Table 1, the three-layer composite leather prepared in Examples 1-3 exhibits excellent comprehensive performance. Its tensile strength remains at 15.8-16.3 MPa, and its elongation at break is 89-94%, while also possessing low water absorption (23-26%) and good softness (4.1-4.4 mm). This indicates that the present invention, through reasonable material blending and structural design, successfully constructs a bio-based leather material that combines high strength, high toughness, water resistance, and a comfortable feel, with significant synergistic effects among the layers.

[0078] Comparative Example 1, where plant tannins were removed during the preparation of the bottom layer, showed a significant decrease in tensile strength to 8.1 MPa, a reduction in elongation at break to 54%, and a sharp increase in water absorption to 72%. This result indicates that in the absence of tannins as a cross-linking medium, activated keratin cannot form effective chemical bonds with the thermoplastic starch matrix and can only play a physical filling role, leading to the loss of the internal cross-linking network, the collapse of mechanical properties, and a severe deterioration in water resistance.

[0079] Comparative Example 2, which eliminated activated keratin, exhibited a tensile strength of only 5.9 MPa, the lowest among all experimental groups, and an elongation at break of only 43%. This indicates that keratin in the system is not merely a filler but also plays a role in providing a rigid reinforcing framework. Although the tannin cross-linking system was retained, the lack of keratin support resulted in a lack of rigid nodes in the internal network, leading to a significant decrease in strength. Combined with the data analysis of Comparative Example 1, this fully validates the bidirectional cross-linking mechanism of tannin as a "bridge" connecting keratin and the starch matrix, with the three working synergistically to form a stable network structure.

[0080] Comparative Example 3 used ordinary keratin powder without mechanical activation, with a tensile strength of 9.4 MPa and a water absorption rate of 52%, both significantly lower than those of Example 2. This is because ordinary keratin molecules have a compact structure, with thiol groups encapsulated within, resulting in low reactivity and difficulty in achieving sufficient covalent cross-linking with tannins. This comparison verifies the crucial role of mechanical activation treatment in improving keratin surface activity, exposing thiol sites, and promoting deep cross-linking reactions.

[0081] Comparative Example 4, which did not include galactose oxidase in its surface preparation, achieved a surface abrasion resistance test result of 39 mg / 1000 r, significantly higher than the value in the example. This indicates that in the absence of enzyme catalysis, sucrose cannot be oxidized in situ to generate cross-linking active products, and a dense cross-linked network cannot be constructed on the surface. This data clearly demonstrates the decisive influence of the enzyme-catalyzed oxidative cross-linking system on improving surface density and abrasion resistance.

[0082] Comparative Example 5 used only sodium bicarbonate in the intermediate foaming layer without adding citric acid, resulting in an overall resilience rate of 49% and an intermediate layer density increase to 0.53 g / cm³. This indicates that the gas production of a single blowing agent is unstable and lacks the reinforcing effect of esterification and cross-linking, leading to uneven cell structure and insufficient cell wall strength, making it prone to collapse. This result verifies the synergistic effect of citric acid as a compound blowing agent and esterification and cross-linking agent in stabilizing cell structure and improving the buffering and resilience performance of the intermediate layer.

[0083] Comparative Example 6 simplifies the structure by removing the intermediate foam layer. While its tensile strength increases to 17.2 MPa, its elongation at break drops to 52%, its softness is only 1.8 mm, and its resilience is a mere 22%. This characteristic of "high strength, low toughness, and extreme hardness and brittleness" indicates that removing the intermediate foam layer transforms the material into a dense sheet structure, completely losing the soft feel and cushioning properties that leather should possess. This comparison powerfully demonstrates the necessity of a three-layer functional gradient structure design; the intermediate layer, while connecting the top and bottom layers, provides the material with indispensable cushioning and toughening effects.

[0084] Comparative Example 7, which employed a layered hot-pressing process, exhibited an interlayer peel strength of only 1.4 N / cm, significantly lower than the 3.4 N / cm of Example 2. This indicates that layered hot pressing results in interlayer interfaces relying solely on physical bonding, lacking deep chemical bonding and fusion interpenetration, leading to extremely weak interfacial adhesion. This data validates the significant advantages of integrated hot pressing in promoting interlayer interface fusion and enhancing the overall structural bonding strength.

[0085] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A high-performance leather based on bio-based raw materials, characterized in that, The leather has a three-layer functional gradient structure, comprising, from bottom to top: The bottom layer, which is a reinforcing layer, is formed by cross-linking of a bottom slurry containing a thermoplastic starch matrix, plant tannins and activated keratin through laccase catalysis. The activated keratin is keratin powder that has been mechanically activated and is rich in free thiol groups. An intermediate foaming layer, which is a buffer layer, is disposed on the bottom layer and is formed by hot-pressing foaming of an intermediate layer slurry containing thermoplastic starch matrix, sodium bicarbonate and citric acid. The surface layer, which is a dense and wear-resistant layer, is disposed on the intermediate foamed layer and is formed by cross-linking of a surface slurry containing thermoplastic starch matrix, sucrose and nanocellulose through oxidase catalysis.

2. The high-performance leather based on bio-based raw materials according to claim 1, characterized in that, The plant tannins are selected from one or more of tarara tannins and vitex bark tannins; the nanocellulose is bacterial cellulose or plant-derived nanocellulose.

3. The high-performance leather based on bio-based raw materials according to claim 1, characterized in that, The activated keratin has a thiol content >50 μmol / g.

4. A method for preparing high-performance leather based on bio-based raw materials as described in any one of claims 1-3, characterized in that, Includes the following steps: Step 1: Preparation of basic thermoplastic starch mother liquor: Mix corn starch, glycerol and water in a mass ratio of 1:0.3-0.4:8-12, stir and gelatinize at 80-85℃ for 20-40 minutes, and cool to 40-45℃ to obtain basic thermoplastic starch mother liquor; Step 2: Preparation of activated keratin powder: After cleaning and drying the waste keratin fibers, place them in a planetary ball mill and ball mill them for 2-4 hours at a ball-to-material ratio of 10:1 and a speed of 500-600 rpm. Then pass them through a 200-mesh sieve to obtain activated keratin powder. Step 3: Prepare the base slurry: Take 100 parts by weight of the basic thermoplastic starch mother liquor from Step 1, add 1.5-3.0 parts of plant tannin, and stir to dissolve at 38-42℃; adjust the pH of the system to 5.0-6.5, then add laccase solution, with the amount of laccase being 500-1000 U / g tannin, and stir to react for 30-45 minutes; then add 2-5 parts of the activated keratin powder from Step 2, and stir to react for 1-2 hours at 600-800 rpm. Heat to 75-80℃ for 10-15 minutes to inactivate, then degas under vacuum to obtain the bottom layer slurry; Step 4: Prepare the intermediate foaming layer slurry: Take 100 parts of the basic thermoplastic starch mother liquor from Step 1 by weight, add 1.0-2.0 parts of glycerol, and stir evenly; add 1.5-3.0 parts of sodium bicarbonate and 1.0-2.0 parts of citric acid after premixing, stir for 1-2 minutes, and then proceed with the coating operation in Step 6, with an interval of no more than 10 minutes. Step 5: Preparation of surface slurry: Take 100 parts by weight of the basic thermoplastic starch mother liquor from Step 1, add 2.0-5.0 parts of sucrose, and stir to dissolve at 30-40℃; add oxidase solution, with an oxidase dosage of 500-1000 U / g sucrose, and stir to react for 1-2 hours; add 0.5-2.0 parts of nanocellulose, and disperse at high speed at 600-800 rpm for 10-15 minutes. Inactivate the surface slurry by heating to 70-80℃ for 10-15 minutes; Step 6, Three-layer lamination and molding: Cast the bottom layer slurry from Step 3 and dry it at 50-60℃ for 20-30 minutes until it is semi-dry; apply the middle foaming layer slurry from Step 4 onto the semi-dry bottom layer, controlling the wet film thickness to 3.0-5.0 mm; immediately apply the surface layer slurry from Step 5 onto the middle layer, controlling the wet film thickness to 0.5-1.0 mm; place the three-layer composite preform in a hot press and hot press it at 150-170℃ and 0.2-0.5 MPa for 10-15 minutes. After cooling, remove it and humidify it at 20-25℃ and 45-55% relative humidity for 45-50 hours to obtain the high-performance leather based on bio-based raw materials.

5. The method according to claim 4, characterized in that, In step 5, the oxidase is either pyranose oxidase or galactose oxidase.

6. The application of the high-performance leather based on bio-based raw materials as described in any one of claims 1-3 in footwear materials, bags, automotive interiors, and furniture upholstery.

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