Biodegradable artificial leather and preparation method thereof

Abstract

The invention discloses biodegradable artificial leather and a preparation method thereof, and belongs to the technical field of artificial leather materials. The method aims at solving the problems that in traditional degradable artificial leather, the compatibility of starch and a matrix is poor, and the mechanical property and the degradability are difficult to consider at the same time. According to the technical scheme, the preparation method is characterized by comprising the following steps: firstly, carrying out ammoniation modification on starch, and then forming chitosan-polylactic acid nanoparticles with dual functions of a dispersant and a pore-forming template in a starch swelling solution by utilizing a solvent-induced self-assembly technology, so as to prepare a starch blended CS-PLA nanoparticle composite dispersion liquid; then mixing the dispersion liquid with waterborne polyurethane, coating on a base layer, heating, drying and curing, and removing the CS-PLA nanoparticle template in situ to finally obtain the biodegradable artificial leather with a uniform porous structure inside. Through the synergistic effect of ammoniation modification and a nanoparticle template method, the product has excellent mechanical properties, high air permeability and efficient biodegradability, and has wide application prospects in the fields of clothing, bags and the like.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of artificial leather materials, and particularly relates to a biodegradable artificial leather and a preparation method thereof. BACKGROUND

[0002] As a substitute for natural leather, artificial leather is widely used in clothing, luggage, furniture and other fields. However, traditional artificial leather (such as PVC and PU leather) is difficult to degrade in the natural environment, causing serious "white pollution" problems. To solve this problem, researchers have turned to the development of biodegradable materials.

[0003] Introducing natural degradable polymers such as starch into artificial leather is a common strategy. However, starch has strong polarity and poor compatibility with non-polar or weakly polar traditional artificial leather matrix (such as polyurethane), which easily aggregates, resulting in unsatisfactory mechanical properties and degradation performance of the material. At the same time, in order to ensure the texture, the traditional artificial leather structure is dense and has poor air permeability, affecting the use experience.

[0004] In the existing improvement scheme, some technologies improve the compatibility of starch and matrix by adding silane coupling agents, but only achieve surface physical modification, and the interfacial bonding force is weak, which is prone to peeling and performance degradation during long-term use. Another technology uses a foaming agent to construct a porous structure, but the foaming pore size is uneven and the distribution is disordered, which easily leads to a decrease in the mechanical strength of the material, and the residual foaming agent may affect the biodegradability. In addition, most degradable artificial leathers have the contradiction between degradation and mechanical properties, that is, either the low starch addition leads to insufficient degradation efficiency, or the high starch addition makes the material brittle and prone to cracking, which cannot meet the actual application requirements. Therefore, it is a technical problem to be solved in the industry to develop an artificial leather material with excellent compatibility, balanced mechanical properties, high air permeability and high-efficiency biodegradability.

[0005] Some existing degradable artificial leathers attempt to improve air permeability by water-containing pore formation of starch-based materials, for example, Chinese patents (CN120719541A and CN110791972A). Such technologies rely on rapid heating and rupture of starch particles and instantaneous vaporization of internal water to form bubbles to construct a porous structure. However, the bubbles formed by this method are mostly micron-sized large pores, and the bubbles are prone to fusion and collapse. The pore size distribution is uneven, which greatly destroys the continuity of the internal structure of the material, resulting in a significant decrease in key mechanical properties such as tensile strength and elongation at break of the artificial leather, which cannot meet the actual use strength requirements in clothing, luggage and other scenarios. At the same time, the large pore structure easily allows external stains and moisture to enter the internal material, further accelerating performance degradation, making it difficult to meet the dual requirements of air permeability and mechanical stability. SUMMARY

[0006] The present application aims to overcome the deficiencies of the prior art, and provides a biodegradable artificial leather and a preparation method thereof. The method adopts a polymer template method, utilizes chitosan-polylactic acid (CS-PLA) nanoparticles as a pore-forming template and a modifier, realizes the construction of a porous structure in the material and the modification of dispersibility, and significantly improves the compatibility of starch with waterborne polyurethane by ammonia modification of the starch, so that the artificial leather with excellent mechanical properties, high air permeability and high biodegradability is obtained.

[0007] To achieve the above object, the present application provides the following technical scheme.

[0008] A preparation method of a biodegradable artificial leather, comprising the following steps: S1, preparing a starch blended CS-PLA nanoparticle composite dispersion liquid: The starch is mixed with deionized water, then heated to swell, and then ammonia water is added for ammonia modification, so that part of the hydroxyl groups on the starch molecular chain are replaced by amino groups to obtain ammonia starch. Then chitosan (CS) and polylactic acid (PLA) are added, and dichloromethane is slowly added dropwise. Due to the change in solvent polarity, the CS and polylactic acid PLA molecular chains self-assemble to form nanoparticles (CS-PLA nanoparticles). During stirring, part of the nanoparticles are attached to the surface of the ammonia starch particles, and the other part is wrapped inside the starch, forming a stable starch blended CS-PLA nanoparticle composite dispersion liquid; After the starch is heated, swelled and ammonia modified, a viscous semi-gel dispersion system is formed, which can wrap dichloromethane droplets and avoid complete separation of oil and water, providing a stable environment for CS-PLA self-assembly.

[0009] S2, preparing a main body mixture: mixing the waterborne polyurethane with the starch blended CS-PLA nanoparticle composite dispersion liquid of step S1, adding a crosslinking agent and a dispersing aid, and stirring uniformly under high-speed shearing to form a main body mixture; S3, coating and forming: laying a base layer, and uniformly coating the main body mixture of step S2 on the surface of the base layer by using a coating machine; S4, curing and template removal: placing the coated composite layer into a drying tunnel for temperature rising and drying. During the temperature rising stage, dichloromethane is first volatilized, and at the same time, the balance of CS-PLA nanoparticles in water and dichloromethane is destroyed, the CS-PLA nanoparticles gradually dissolve and blend with the matrix or are degraded later, leaving pores of nanometer to micrometer size in situ; finally, cooling and curing to obtain a biodegradable artificial leather with a porous structure.

[0010] The core innovation mechanism of the present application is: Ammoniation modification improves compatibility: the starch is first treated by ammoniation, and amino groups (-NH2) are introduced into the molecular chain. The introduction of amino groups significantly improves the compatibility of the starch with the waterborne polyurethane matrix through acid-base interaction and by changing the surface energy of the starch, effectively preventing the agglomeration of starch in the matrix.

[0011] CS-PLA nanoparticles as a dual-function template: the application ingeniously uses solvent-induced self-assembly technology. After dissolving CS and PLA in the starch swelling liquid, the addition of dichloromethane induces the formation of nanoparticles of CS and PLA. These nanoparticles not only act as a dispersant for starch, making it more uniformly dispersed in the waterborne polyurethane through attachment and wrapping, but also act as a pore-forming template. During the subsequent heating process, as the dichloromethane evaporates, the nanoparticle balance is broken, leaving a porous structure inside the material. In addition, CS and PLA are also biodegradable polymers, further improving the overall degradation performance of the material.

[0012] Key pore-forming synergistic mechanism: the application forms interconnected nanoscale micropores through in-situ removal of the pores formed by CS-PLA nanoparticles. During the subsequent high-temperature drying stage, the water vapor generated by the vaporization of the residual water in the mixture can quickly and smoothly release through the interconnected micropore network, avoiding the formation of large pores caused by the retention of water vapor under high-temperature conditions. Ultimately, the material only retains uniform and controllable nanoscale micropores, balancing the air permeability and structural compactness, and ensuring that the mechanical properties do not deteriorate.

[0013] As a further preferred embodiment of the application: Further, in step S1, the starch, deionized water, and ammonia water are added in amounts of 10-12 g, 100-200 mL, and 5-8 mL, respectively; the mass fraction of the ammonia water is 28 wt%.

[0014] Further, in step S1, the CS and PLA are added in amounts of 1-2 g and 2-4 g, respectively; the volume ratio of dichloromethane to deionized water is 1-4:5.

[0015] Further, in step S1, the heating temperature is 60-75℃.

[0016] Further, in step S2, the waterborne polyurethane is 50-80 mL (solid content 20 wt%), the starch blended CS-PLA nanoparticle composite dispersion is 30-60 mL, the crosslinking agent is 0.5-1.0 g, and the dispersing aid is 0.5-2.0 g. The dispersing aid is sodium dodecyl benzene sulfonate, and the crosslinking agent is an aziridine crosslinking agent.

[0017] Further, the base layer is a bamboo fiber fabric.

[0018] Further, in step S4, the temperature is raised at a rate of 3-4 DEG C / min to 120-150 DEG C, and after the temperature is raised, the drying time is 1-2 hours.

[0019] A biodegradable artificial leather prepared according to the above method.

[0020] The biodegradable artificial leather prepared according to the above method can be used as a degradable clothing fabric, a degradable luggage leather material, a degradable home sofa leather, a degradable soft packaging material, a degradable medical disposable protective cover leather, and a degradable shoe material leather surface, and meets the dual requirements of mechanical strength, air permeability and environmental degradation in various scenes.

[0021] Advantages of the present application (1) The present application applies the "polymer template method" to the preparation of artificial leather. By using CS-PLA nanoparticles as a pore-forming template, the nanoparticles are removed in situ during the curing process, forming a large number of nanoscale pores, which not only improves the air permeability and moisture permeability of the material, but also does not damage the continuity of the internal structure of the material.

[0022] (2) The connectivity of the nanoscale pores formed by the CS-PLA nanoparticles can quickly guide the water vapor generated during the high-temperature stage to avoid the retention and accumulation of water vapor to form large pores, and realize controllable pore size, which guarantees the stability of the mechanical properties of the material from the structural level, while ensuring the air permeability.

[0023] (3) Ammonia modification solves the compatibility problem and has excellent mechanical properties: by ammonia modification of starch, the problem of poor compatibility and easy agglomeration of starch and high molecular matrix is solved. The modified starch can be uniformly dispersed, so that the mechanical properties (such as tensile strength and elongation at break) of the composite material are guaranteed and improved.

[0024] (3) After dissolving CS and PLA in the starch swelling solution, dichloromethane is added to induce the formation of nanoparticles. These nanoparticles act as a modifier for starch, allowing it to be more uniformly dispersed in the water-based polyurethane, and also as a pore-forming template, distributed throughout the mixture. During the subsequent heating process, as the dichloromethane evaporates, the balance of the nanoparticles is broken, and the CS-PLA nanoparticles are gradually dissolved in deionized water, leaving a uniform and controllable porous structure inside the material, and achieving pore formation.

[0025] (4) The artificial leather of the present application is mainly composed of water-based polyurethane, degradable starch, CS, PLA and natural fiber base layer. Among them, starch and base layer can be degraded by microorganisms, and water-based polyurethane is also selected to be degradable or partially degradable. Therefore, the entire material can be efficiently degraded in a natural environment. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 : A scanning electron microscope (SEM) image of the artificial leather material prepared in Example 1 at 1.1k magnification; Figure 2 : A 50k scanning electron microscope (SEM) photograph of the artificial leather material prepared in Example 1; Figure 3 Nitrogen adsorption-desorption (BET) isotherms of the artificial leather materials prepared in Example 1 and Comparative Example 1; Figure 4 Fourier transform infrared spectra of the waterborne polyurethane and the main mixture in Example 1; Figure 5 (a)-(b) are images of a mixed dispersion of starch, deionized water, and ammonia, and a mixed dispersion of CS, PLA, and dichloromethane, respectively, illuminated with a laser pointer. Figure 6 (a)-(c) are images of CS dispersion, PLA dispersion and CS-PLA dispersion illuminated with a laser pointer, respectively. Figure 7 (a)-(b) are photographs of the main mixtures of Example 1 and Comparative Example 2, respectively; Figure 8 : Strain curves of Example 1, Comparative Example 1, and Comparative Example 3; Figure 9 : This is a scanning electron microscope image of Example 1 after 60 days of degradation. Detailed Implementation

[0027] To make the above-mentioned objectives, features, and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to examples. The following content is merely an example and illustration of the concept of the present invention. Those skilled in the art can make various modifications or additions to the described specific embodiments or use similar methods to replace them, as long as they do not deviate from the concept of the invention, they should all fall within the protection scope of the present invention.

[0028] The preparation method of the present invention will be described below through specific embodiments and comparative examples.

[0029] Example 1 S1. Preparation of starch-blended CS-PLA nanoparticle composite dispersion: Mix 11g starch with 150mL deionized water, heat to 65℃ to allow it to swell, add 6mL of 28wt% ammonia water for ammoniation, stir for 30min, add 1.5g CS and 3g PLA, continue stirring for 1h, slowly add 60mL dichloromethane (dichloromethane to deionized water volume ratio 2:5), stir for 30min to form a stable starch-blended CS-PLA nanoparticle composite dispersion; S2. Preparation of the main mixture: Take 65 mL of waterborne polyurethane with a solid content of 20 wt%, add 45 mL of the composite dispersion prepared in step S1, then add 0.8 g of aziridine crosslinking agent and 1.2 g of sodium dodecylbenzenesulfonate, and stir for 20 min under high-speed shear at 1800 r / min to form a uniform main mixture. S3. Coating and molding: Lay the bamboo fiber fabric base layer flat, and use a coating machine to evenly coat the main mixture onto the base layer surface. The coating thickness is controlled to be 1.0mm. S4. Curing and template removal: Place the coated composite layer into the drying tunnel, heat it to 135℃ at 3.5℃ / min, keep it at that temperature for 1.5h, and cool it to room temperature to obtain biodegradable artificial leather.

[0030] Example 2 S1. Preparation of starch-blended CS-PLA nanoparticle composite dispersion: Mix 10g starch with 100mL deionized water, heat to 60℃ to allow it to swell, add 5mL of 28wt% ammonia water for ammoniation, stir for 30min, add 1g CS and 2g PLA, continue stirring for 1h, slowly add 20mL dichloromethane (dichloromethane to deionized water volume ratio 1:5), stir for 30min to form a stable starch-blended CS-PLA nanoparticle composite dispersion; S2. Preparation of the main mixture: Take 50 mL of waterborne polyurethane with a solid content of 20 wt%, add 30 mL of the composite dispersion prepared in step S1, then add 0.5 g of aziridine crosslinking agent and 0.5 g of sodium dodecylbenzene sulfonate, and stir for 20 min under high-speed shear at 1500 r / min to form a uniform main mixture. S3. Coating and molding: Lay the bamboo fiber fabric base layer flat, and use a coating machine to evenly coat the main mixture onto the base layer surface. The coating thickness is controlled to be 0.8mm. S4. Curing and template removal: Place the coated composite layer into the drying tunnel, heat it to 120°C at 3°C / min, keep it at that temperature for 1 hour, and then cool it to room temperature to obtain biodegradable artificial leather.

[0031] Example 3 S1. Preparation of starch-blended CS-PLA nanoparticle composite dispersion: Mix 12g starch with 200mL deionized water, heat to 75℃ to allow it to swell, add 8mL of 28wt% ammonia water for ammoniation, stir for 30min, add 2g CS and 4g PLA, continue stirring for 1h, slowly add 160mL dichloromethane (dichloromethane to deionized water volume ratio 4:5), stir for 30min to form a stable starch-blended CS-PLA nanoparticle composite dispersion; S2. Preparation of the main mixture: Take 80 mL of waterborne polyurethane with a solid content of 20 wt%, add 60 mL of the composite dispersion prepared in step S1, then add 1.0 g of aziridine crosslinking agent and 2.0 g of sodium dodecylbenzene sulfonate, and stir for 20 min under high-speed shear at 2000 r / min to form a uniform main mixture. S3. Coating and molding: Lay the bamboo fiber fabric base layer flat, and use a coating machine to evenly coat the main mixture onto the base layer surface. The coating thickness is controlled to be 1.2mm. S4. Curing and template removal: Place the coated composite layer into the drying tunnel, heat it to 150°C at 4°C / min, keep it at that temperature for 2 hours, and then cool it to room temperature to obtain biodegradable artificial leather.

[0032] Comparative Example 1 Starch was added without any modification.

[0033] S1. Mix 11g of starch with 150mL of deionized water, heat to 65℃ to make it swell, and directly obtain starch dispersion; S2. Take 65 mL of waterborne polyurethane with a solid content of 20 wt%, add 45 mL of starch dispersion, then add 0.8 g of aziridine crosslinking agent and 1.2 g of sodium dodecylbenzene sulfonate, and stir for 20 min under high-speed shear at 1800 r / min. S3. The coating, molding, and curing steps are the same as in Example 1, to obtain artificial leather.

[0034] Comparative Example 2 The starch was not modified by ammoniation.

[0035] Except for step S1, where ammonia was not added for ammoniation, the amount of other raw materials and process parameters were completely consistent with those in Example 1.

[0036] Comparative Example 3 The preparation steps for simulating starch water absorption and vaporization to create pores are as follows: S1. Preparation of the main mixture: Mix 70 parts by weight of waterborne polyurethane with 40 parts by weight of deionized water and stir at 800 r / min for 12 min to form a premixed sol; add 30 parts by weight of modified starch composition, 0.6 parts by weight of benzoyl peroxide and 1.5 parts by weight of sodium lauryl sulfate, heat to 45℃ and shear and stir at 2200 r / min for 35 min to form the main mixture; In this process, corn starch and polyvinyl alcohol are mixed at a mass ratio of 10:1, an appropriate amount of deionized water is added, and the mixture is stirred at 50°C for 1 hour until the starch granules fully absorb water and swell. The mixture is then vacuum dried at 30°C for 2 hours.

[0037] S2. Preparation of foamed composite layer: 100 parts by weight of bamboo fiber knitted fabric base layer are laid flat, and the main mixture of step S1 is coated on the base layer surface using a coating machine. The coating environment temperature is 25℃.

[0038] S3. Rapid heating and pore-forming treatment: Place the coated composite layer into a hot air drying tunnel and preheat it at 65°C for 4 minutes; rapidly heat it to 110°C at a rate of 17°C / min and hold it at that temperature for 25 minutes; allow it to cool naturally to room temperature and cure to obtain a biodegradable artificial leather product.

[0039] Figure 1 SEM image of artificial leather in Example 1 (1.1k magnification).

[0040] This image presents the overall microstructure of the material at low magnification. It shows that the surface of the artificial leather is dense, without cracks or pores, and the base layer and coating are tightly bonded without peeling. This invention utilizes CS-PLA nanoparticles to create micropores only within the material, without compromising surface density. This solves the problems of surface roughness and structural discontinuity caused by traditional foaming methods, ensuring both the material's appearance and durability, while balancing porosity and structural integrity.

[0041] Figure 2 This is a SEM image (50kx magnification) of artificial leather from Example 1.

[0042] Under high magnification, numerous uniformly distributed, interconnected nanoscale pores were clearly observed inside the material. The pore size matched the size of the CS-PLA nanoparticles, with no pore aggregation, collapse, or fusion. CS-PLA nanoparticles, acting as pore-forming templates, were removed in situ to form controllable nanopores, unlike the non-uniform large pores of traditional pore-forming methods. This provides a structural basis for achieving a balance between air permeability and mechanical properties.

[0043] Figure 3 The nitrogen adsorption-desorption (BET) isotherms are for the artificial leather materials prepared in Example 1 and Comparative Example 1. The isotherm of Example 1 shows that it has a larger specific surface area and pore volume. The isotherm of Comparative Example 1 exhibits the characteristics of typical non-porous or microporous materials, with very small specific surface area and pore volume.

[0044] Figure 4 The Fourier transform infrared spectrum of the waterborne polyurethane and the main mixture in Example 1; in the infrared spectrum of Example 1, it should be possible to detect the components at a specific wavenumber (e.g., ~3417 cm⁻¹). -1 A new absorption peak attributable to the NH stretching vibration was observed in the vicinity, or a significant change in the peak shape was observed in this region. This directly proves that ammonia treatment successfully introduced amino groups (-NH2) into the starch molecular chain. The introduction of amino groups reduces the surface energy of starch and enables strong interactions with groups on the waterborne polyurethane molecular chain, thereby improving the compatibility between the two.

[0045] Figure 5 (a) Starch-deionized water-ammonia water dispersion; (b) Composite dispersion with added CS, PLA and dichloromethane (laser irradiation).

[0046] Figure 5 (a) Laser irradiation does not exhibit the Tyndall effect. Figure 5 (b) A significant Tyndall effect was observed, proving the formation of nanoscale particles in the system. This indicates that dichloromethane induced solvent self-assembly between CS and PLA, successfully forming CS-PLA nanoparticles, which were stably dispersed in the starch-swollen system, verifying the feasibility of nanoparticle preparation.

[0047] Figure 6 (a)-(c) are photographs of CS dispersion, PLA dispersion, and CS-PLA dispersion illuminated with a laser pointer, respectively. It can be seen that only... Figure 6 (c) The Tyndall effect was generated, combined with Figure 5 This indicates that CS-PLA formed nanoparticles in the dispersion.

[0048] Figure 7 (a)-(b) are photographs of the main mixtures of Example 1 and Comparative Example 2, respectively; Comparative Example 2 has poor dispersibility. Figure 7 (a) (Example 1) shows the main mixture after amination modification. The liquid exhibits a uniform and stable emulsion state, without obvious particles or sediment. Figure 7 (b) (Comparative Example 2) shows the bulk mixture without ammoniation modification of the starch. Layering can be seen in the liquid, indicating that the starch particles agglomerated and separated due to the significant difference in polarity between the starch and the waterborne polyurethane matrix.

[0049] Figure 8 Strain curves of Example 1, Comparative Example 1, and Comparative Example 3.

[0050] The tensile strength and elongation at break of Example 1 were significantly higher than those of Comparative Examples 1 and 3. Comparative Example 1 exhibited a steep drop in strain curve and extremely poor mechanical properties due to starch agglomeration. Comparative Example 3, with its large pore size created by water vaporization, showed a significant decrease in tensile strength and insufficient elongation at break. This demonstrates that the present invention balances degradability and mechanical properties; ammoniation modification solves the embrittlement problem caused by starch agglomeration; and CS-PLA nanoparticle pore creation does not disrupt structural continuity, compared to traditional pore-forming and unmodified systems.

[0051] Figure 9 This is a scanning electron microscope image of the material after 60 days of degradation in Example 1. It can be seen that the material surface has become very rough and porous, with numerous holes and cracks formed by microbial erosion, and the original structure has been severely damaged.

Claims

1. A method for preparing biodegradable artificial leather, characterized in that, Includes the following steps: S1. Preparation of starch-blended CS-PLA nanoparticle composite dispersion: Starch is mixed with deionized water and heated to swell. Ammonia is added to ammonify the starch to obtain ammonified starch. Then CS and PLA are added, and dichloromethane is added dropwise to combine with the ammonified starch to form a stable starch-blended CS-PLA nanoparticle composite dispersion. S2. Preparation of the main mixture: The aqueous polyurethane and the starch-blended CS-PLA nanoparticle composite dispersion from step S1 are mixed, a crosslinking agent and a dispersing agent are added, and the mixture is stirred evenly under high-speed shearing to form the main mixture. S3. Coating and molding: Lay the base layer flat and use a coating machine to evenly coat the main mixture from step S2 onto the surface of the base layer; S4. Curing and Template Removal: The coated composite layer is placed in an oven and heated to dry. The volatilization of dichloromethane and the balanced destruction of CS-PLA nanoparticles form nano- to micron-sized pores. After cooling and curing, biodegradable artificial leather is obtained.

2. The preparation method according to claim 1, characterized in that, In step S1, the amount of starch, deionized water, and ammonia added is 10-12g: 100-200mL: 5-8mL; the mass fraction of the ammonia is 28wt%.

3. The preparation method according to claim 1, characterized in that, In step S1, the amounts of CS and PLA added are 1~2g and 2~4g, respectively; the volume ratio of dichloromethane to deionized water is 1~4:

5.

4. The preparation method according to claim 1, characterized in that, In step S1, the temperature for heating and swelling is 60~75℃.

5. The preparation method according to claim 1, characterized in that, In step S2, the aqueous polyurethane is 50-80 mL with a solid content of 20 wt%, the starch-blended CS-PLA nanoparticle composite dispersion is 30-60 mL, the crosslinking agent is 0.5-1.0 g, and the dispersing aid is 0.5-2.0 g; the dispersing aid is sodium dodecylbenzenesulfonate, and the crosslinking agent is an aziridine crosslinking agent.

6. The preparation method according to claim 1, characterized in that, The base layer is made of bamboo fiber fabric.

7. The preparation method according to claim 1, characterized in that, In step S4, the heating rate of the heating and drying process is 3~4℃ / min, the final temperature of the heating process is 120~150℃, and the drying and holding time after the heating process is 1~2h.

8. A biodegradable artificial leather obtained by the preparation method according to any one of claims 1 to 7.

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