An electromagnetic shielding composite material for winding molding process and preparation method thereof

Abstract

The present invention discloses an electromagnetic shielding composite material for winding molding process and its preparation method, which belongs to the technical field of resin-based composite materials. The preparation method comprises the following steps: adding LiF and Ti3AlC2 precursor to hydrochloric acid solution in sequence and stirring the reaction to obtain Ti3C2T X ; Add the diluent and silver nanowires to the epoxy resin and mix well to obtain premix 1; Ti3C2T X The metal nanoparticles are added to the curing agent and ultrasonically treated to obtain premix 2; premix 1, premix 2, and the accelerator are uniformly mixed to obtain the product. The present invention also discloses an electromagnetic shielding composite material for a winding process prepared by the above method. The electromagnetic shielding composite material for a winding process of the present invention has excellent overall mechanical properties and electromagnetic shielding effectiveness, and has broad application prospects.

Description

Technical Field

[0001] The present invention belongs to the technical field of resin-based composite materials, and in particular relates to an electromagnetic shielding composite material used in a winding molding process and a preparation method thereof. Background Art

[0002] Wet winding refers to a composite material molding method in which the reinforcing material is directly wound around a core mold after being impregnated with resin and then formed after curing. As an efficient composite material manufacturing method, it is widely used in many fields such as aerospace, national defense and military, petroleum and chemical industry, marine engineering, automobile and energy due to its high molding precision, light product weight, high strength, overall molding and high production efficiency.

[0003] With technological advancements and the development of new materials, the application areas of winding molding are constantly expanding. Electronic equipment inside satellites and spacecraft, electronic control components of missile and weapon systems, the interior of missile launch tubes, medical imaging equipment, communication base stations, and antenna towers all require shielding from external electromagnetic waves to ensure normal operation. This places new demands on some winding molded products.

[0004] In order to give the wound product an electromagnetic shielding function, a conductive or magnetic material layer can be added during the winding process. These materials can be metal fibers, conductive polymers or composite materials containing conductive metal particles. For example, metal foil is added during the winding process or a conductive coating is sprayed on the surface of the finished product. This method requires additional steps, and the conductive layer is poorly bonded to the product, resulting in poor shielding effect. In addition, electromagnetic shielding function can also be achieved by adding conductive fillers to the resin system, but the amount of filler added is large, resulting in high resin viscosity, affecting the molding process, and not in line with the development trend of lightweighting. Summary of the Invention

[0005] The problem to be solved by the present invention is to provide an electromagnetic shielding composite material for a winding molding process and a preparation method thereof, so as to solve the problems of poor shielding effect and complicated molding process of existing electromagnetic shielding composite materials.

[0006] The technical solution adopted to solve the technical problem is to provide a method for preparing an electromagnetic shielding composite material for a winding molding process, comprising the following steps:

[0007] (1) LiF and Ti3AlC2 precursor were added to the hydrochloric acid solution in sequence and stirred to react to obtain Ti3C2T X ;

[0008] (2) adding the diluent and the silver nanowires to the epoxy resin and stirring evenly to obtain a premix 1;

[0009] (3) Ti3C2T Xand metal nanoparticles are added to the curing agent and ultrasonicated to obtain premix 2;

[0010] (4) The premix 1, the premix 2 and the accelerator are mixed to obtain an electromagnetic shielding composite material for a winding molding process.

[0011] The beneficial effects of the above technical solution of the present invention are as follows: X Anchoring metal nanoparticles on the nanosheets increases the roughness of the sheet and improves the electromagnetic shielding performance. At the same time, one-dimensional silver nanowires are introduced into the resin matrix, and their welding effect is used to construct an interconnected three-dimensional conductive network. While reducing the amount of metal conductive filler added to the system, the integrity of the conductive network is ensured, giving the wound product excellent electromagnetic shielding function. In addition, the formation of the three-dimensional network improves the overall mechanical properties of the material. This structural advantage enables the composite material to have good mechanical strength and toughness while maintaining excellent electromagnetic shielding performance. In the preparation method, LiF and Ti3AlC2 precursors are added to the hydrochloric acid solution for stirring reaction, which can efficiently and controllably prepare two-dimensional Ti3C2T X Materials, ensuring Ti3C2T X The high conductivity and good dispersion of Ti3C2T provide a basis for the subsequent electromagnetic shielding performance of the composite material; then the diluent and silver nanowires are added to the epoxy resin and stirred evenly to ensure the uniform dispersion of the silver nanowires in the resin matrix and avoid agglomeration, thereby improving the electrical conductivity and electromagnetic shielding effectiveness of the composite material; Ti3C2T X The metal nanoparticles are added into the curing agent process so that the metal nanoparticles are anchored on Ti3C2T X The continuity and integrity of the conductive network are further enhanced; finally, the accelerator is added to avoid premature curing between premixes and affecting the performance of the composite material.

[0012] Preferably, in step (1), the molar ratio of Ti3AlC2 precursor to LiF is 1:(5-7.5); the stirring reaction temperature is 35-45°C, and the time is 24-48h; Ti3C2T X The size is 100nm~3μm.

[0013] Preferably, after the stirring reaction in step (1), the mixture is washed with water and centrifuged until the pH of the upper layer is greater than 6, and then the lower layer precipitate is cooled, added with water and ultrasonicated, and centrifuged again.

[0014] Preferably, the rotation speed of the two centrifugations is 3000-6000 rpm, the time is 10-15 min; the cooling temperature is 2-4° C., the time is 2-6 h; and the water addition and ultrasonication time is 30-60 min.

[0015] Preferably, the diluent is dibutyl alcohol diglycidyl ether; the epoxy resin is 4,4'-dihydroxydiphenylmethane; the silver nanowires have a diameter of 20-25 nm and a length of 15-30 μm; the metal nanoparticles are copper nanoparticles and / or silver nanoparticles; the curing agent is methyltetrahydrophthalic anhydride; and the accelerator is 2,4,6-tris(dimethylaminomethyl)phenol.

[0016] Preferably, the mass ratio of epoxy resin, diluent, curing agent and accelerator is (100-105):(85-95):(80-90):(1-2).

[0017] More preferably, Ti3C2T X The addition amount of silver nanowires is 1~3% of the total mass of the electromagnetic shielding composite material used for the winding molding process; the addition amount of metal nanoparticles is 0.3~1% of the total mass of the electromagnetic shielding composite material used for the winding molding process.

[0018] Preferably, the stirring temperature in step (2) is 40-65° C. and the stirring time is 3-6 h.

[0019] Preferably, the ultrasonication in step (3) is ice bath ultrasonication, and the time is 0.5 to 3 hours.

[0020] The present invention also provides an electromagnetic shielding composite material used in a winding molding process and obtained by the above preparation method.

[0021] The present invention has the following beneficial effects:

[0022] (1) In the preparation process of the electromagnetic shielding composite material for the winding molding process of the present invention, two-dimensional Ti3C2T X The material and highly conductive silver nanowires and metal nanoparticles are introduced into the composite material to construct an efficient conductive network, which significantly improves the electromagnetic shielding effectiveness of the composite material, ensures the stability and durability of the shielding effect, and meets the demand for high-performance electromagnetic shielding materials.

[0023] (2) The electromagnetic shielding composite material for winding molding process of the present invention is Ti3C2T X The combination of the two-dimensional structure of the composite and the one-dimensional characteristics of the silver nanowires not only enhances the conductivity of the composite material, but also improves the overall mechanical properties of the material through their synergistic effect. This structural advantage enables the composite material to maintain excellent electromagnetic shielding performance while also having good mechanical strength and toughness.

[0024] (3) In the preparation method of the present invention, each component is introduced and uniformly mixed at an appropriate stage without the need for complicated additional process steps. This not only simplifies the production process and reduces production costs, but also improves production efficiency, making large-scale industrial production possible. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 Shielding effectiveness diagrams of electromagnetic shielding composite materials used in winding molding processes according to Examples 1 to 5 and Comparative Examples 1 to 3;

[0026] Figure 2 The electromagnetic shielding performance diagram of wound products with different thicknesses. DETAILED DESCRIPTION

[0027] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. That is, the embodiments described are only part of the embodiments of the present invention, rather than all embodiments.

[0028] Therefore, the detailed description of the embodiments of the present invention provided below is not intended to limit the scope of the invention as claimed, but is merely intended to represent selected embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative work are within the scope of protection of the present invention.

[0029] The features and performance of the present invention are further described in detail below with reference to the embodiments.

[0030] Example 1

[0031] A method for preparing an electromagnetic shielding composite material for a winding molding process comprises the following steps:

[0032] (1) 8.5 g of LiF was added to the hydrochloric acid solution and stirred evenly. Then 10 g of Ti3AlC2 precursor was added and stirred evenly. Argon was introduced and sealed. The mixture was stirred and reacted at 35 °C for 24 h. After the reaction, the mixture was washed with water and centrifuged at 3500 rpm for 10 min until the pH of the upper liquid was greater than 6. The lower precipitate was then cooled at 3 °C for 5 h, and then water was added to fully shake it. After ice bath ultrasonication for 60 min, it was centrifuged again at 3500 rpm for 10 min to obtain Ti3C2T with a size of 100 nm~3 μm. X ;

[0033] (2) 900 g of dibutyl alcohol diglycidyl ether and 40.5 g of silver nanowires with a diameter of 20-25 nm and a length of 15-30 μm were added to 1000.5 g of 4,4′-dihydroxydiphenylmethane, and the mixture was stirred at 50° C. for 4 h until uniform, and cooled to room temperature to obtain premix 1;

[0034] (3) 90.3 g of Ti3C2T X and 16 g of copper nanoparticles were added to 850 g of methyltetrahydrophthalic anhydride and ultrasonicated in an ice bath for 2 h to obtain premix 2;

[0035] (4) Premix 1 and premix 2 were mixed, and 15.3 g of 2,4,6-tris(dimethylaminomethyl)phenol was added and mixed evenly to obtain an electromagnetic shielding composite material for winding molding process.

[0036] Example 2

[0037] A method for preparing an electromagnetic shielding composite material for a winding molding process comprises the following steps:

[0038] (1) 8.1 g of LiF was added to the hydrochloric acid solution and stirred evenly. Then, 10.5 g of Ti3AlC2 precursor was added and stirred evenly. Argon was introduced and sealed. The mixture was stirred and reacted at 35 °C for 48 h. After the reaction, the mixture was washed with water and centrifuged at 5000 rpm for 10 min until the pH of the upper liquid was greater than 6. The lower precipitate was then cooled at 3 °C for 5 h, and then water was added to fully shake the mixture. After that, the mixture was ultrasonicated in an ice bath for 30 min and centrifuged again at 5000 rpm for 10 min to obtain Ti3C2T with a size of 100 nm~3 μm. X ;

[0039] (2) 900.6 g of dibutyl alcohol diglycidyl ether and 60.3 g of silver nanowires with a diameter of 20-25 nm and a length of 15-30 μm were added to 1001 g of 4,4′-dihydroxydiphenylmethane, and the mixture was stirred at 50° C. for 4 h until uniform, and cooled to room temperature to obtain premix 1;

[0040] (3) 140.5g of Ti3C2T X and 20.2 g of copper nanoparticles were added to 850 g of methyltetrahydrophthalic anhydride and ultrasonicated in an ice bath for 2 h to obtain premix 2;

[0041] (4) Premix 1 and premix 2 were mixed, and 15.3 g of 2,4,6-tris(dimethylaminomethyl)phenol was added and mixed evenly to obtain an electromagnetic shielding composite material for winding molding process.

[0042] Example 3

[0043] A method for preparing an electromagnetic shielding composite material for a winding molding process comprises the following steps:

[0044] (1) 10.3 g of LiF was added to the hydrochloric acid solution and stirred evenly. Then 10 g of Ti3AlC2 precursor was added and stirred evenly. Argon was introduced and sealed. The mixture was stirred and reacted at 45 °C for 48 h. After the reaction, the mixture was washed with water and centrifuged at 5000 rpm for 10 min until the pH of the upper liquid was greater than 6. The lower precipitate was then cooled at 3 °C for 5 h, and then water was added to fully shake it. After ice bath ultrasonication for 30 min, it was centrifuged again at 5000 rpm for 10 min to obtain Ti3C2T with a size of 100 nm~3 μm. X ;

[0045] (2) 900.7 g of dibutyl alcohol diglycidyl ether and 40.6 g of silver nanowires with a diameter of 20-25 nm and a length of 15-30 μm were added to 1001.2 g of 4,4′-dihydroxydiphenylmethane, and the mixture was stirred at 50° C. for 4 h until uniform, and cooled to room temperature to obtain premix 1;

[0046] (3) 90.2g of Ti3C2T X and 15.9 g of copper nanoparticles were added to 850 g of methyltetrahydrophthalic anhydride and ultrasonicated in an ice bath for 2 h to obtain premix 2;

[0047] (4) Premix 1 and premix 2 were mixed, and 15.1 g of 2,4,6-tris(dimethylaminomethyl)phenol was added and mixed evenly to obtain an electromagnetic shielding composite material for winding molding process.

[0048] Example 4

[0049] A method for preparing an electromagnetic shielding composite material for a winding molding process comprises the following steps:

[0050] (1) 10.3 g of LiF was added to the hydrochloric acid solution and stirred evenly. Then 10 g of Ti3AlC2 precursor was added and stirred evenly. Argon was introduced and sealed. The mixture was stirred and reacted at 45 °C for 48 h. After the reaction, the mixture was washed with water and centrifuged at 5000 rpm for 10 min until the pH of the upper liquid was greater than 6. The lower precipitate was then cooled at 3 °C for 5 h, and then water was added to fully shake it. After ice bath ultrasonication for 30 min, it was centrifuged again at 5000 rpm for 10 min to obtain Ti3C2T with a size of 100 nm~3 μm. X ;

[0051] (2) 901 g of dibutyl alcohol diglycidyl ether and 60 g of silver nanowires with a diameter of 20-25 nm and a length of 15-30 μm were added to 1000.5 g of 4,4′-dihydroxydiphenylmethane, and the mixture was stirred at 50° C. for 4 h until uniform, and cooled to room temperature to obtain premix 1;

[0052] (3) 140.1 g of Ti3C2T X and 19.9 g of copper nanoparticles were added to 850 g of methyltetrahydrophthalic anhydride and ultrasonicated in an ice bath for 3 h to obtain premix 2;

[0053] (4) Premix 1 and premix 2 were mixed, and 15 g of 2,4,6-tris(dimethylaminomethyl)phenol was added and mixed evenly to obtain an electromagnetic shielding composite material for winding molding process.

[0054] Example 5

[0055] A method for preparing an electromagnetic shielding composite material for a winding molding process comprises the following steps:

[0056] (1) 10 g of LiF was added to the hydrochloric acid solution and stirred evenly. Then 10.1 g of Ti3AlC2 precursor was added and stirred evenly. Argon was introduced and sealed. The mixture was stirred and reacted at 45 °C for 30 h. After the reaction, the mixture was washed with water and centrifuged at 3500 rpm for 10 min until the pH of the upper liquid was greater than 6. The lower precipitate was then cooled at 3 °C for 5 h, and then water was added to fully shake it. After that, it was ultrasonicated in an ice bath for 30 min and centrifuged again at 3500 rpm for 10 min to obtain Ti3C2T with a size of 100 nm~3 μm. X ;

[0057] (2) 900.6 g of dibutyl alcohol diglycidyl ether and 50.6 g of silver nanowires with a diameter of 20-25 nm and a length of 15-30 μm were added to 1001.5 g of 4,4′-dihydroxydiphenylmethane, and the mixture was stirred at 50° C. for 4 h until uniform, and cooled to room temperature to obtain premix 1;

[0058] (3) 119.8 g of Ti3C2T X and 29.1 g of copper nanoparticles were added to 850 g of methyltetrahydrophthalic anhydride and ultrasonicated in an ice bath for 3 h to obtain premix 2;

[0059] (4) Premix 1 and premix 2 were mixed, and 15 g of 2,4,6-tris(dimethylaminomethyl)phenol was added and mixed evenly to obtain an electromagnetic shielding composite material for winding molding process.

[0060] Comparative Example 1

[0061] A method for preparing an electromagnetic shielding composite material for a winding molding process comprises the following steps:

[0062] Take 1000.9 g of 4,4'-dihydroxydiphenylmethane, add 901.2 g of dibutyl alcohol diglycidyl ether, 849.3 g of methyltetrahydrophthalic anhydride and 15 g of 2,4,6-tris(dimethylaminomethyl)phenol to 4,4'-dihydroxydiphenylmethane at room temperature, and stir evenly to obtain the product.

[0063] Comparative Example 2

[0064] A method for preparing an electromagnetic shielding composite material for a winding molding process comprises the following steps:

[0065] (1) 8.5 g of LiF was added to the hydrochloric acid solution and stirred evenly. Then 10 g of Ti3AlC2 precursor was added and stirred evenly. Argon was introduced and sealed. The mixture was stirred and reacted at 35 °C for 24 h. After the reaction, the mixture was washed with water and centrifuged at 3500 rpm for 10 min until the pH of the upper liquid was greater than 6. The lower precipitate was then cooled at 3 °C for 5 h, and then water was added to fully shake it. After ice bath ultrasonication for 60 min, it was centrifuged again at 3500 rpm for 10 min to obtain Ti3C2T with a size of 100 nm~3 μm. X ;

[0066] (2) 90g of Ti3C2T X and 16 g of copper nanoparticles were added to 850.5 g of methyltetrahydrophthalic anhydride and ultrasonicated in an ice bath for 2 h to obtain a premix;

[0067] (3) Add 1001 g of 4,4'-dihydroxydiphenylmethane, 900.2 g of dibutyl alcohol diglycidyl ether and 15.1 g of 2,4,6-tris(dimethylaminomethyl)phenol to the premix in sequence and stir evenly to obtain the product.

[0068] Comparative Example 3

[0069] A method for preparing an electromagnetic shielding composite material for a winding molding process comprises the following steps:

[0070] Take 1000g of 4,4'-dihydroxydiphenylmethane, add 899.6g of dibutyl alcohol diglycidyl ether and 40.5g of silver nanowires with a diameter of 20-25nm and a length of 15-30μm, and then stir at 50°C for 4h. After cooling to room temperature, add 850.5g of methyltetrahydrophthalic anhydride and 15.2g of 2,4,6-tris(dimethylaminomethyl)phenol, and stir evenly to obtain.

[0071] Experimental example

[0072] 1. Mechanical properties test

[0073] The electromagnetic shielding composite materials for the winding molding process prepared in Examples 1 to 5 and Comparative Examples 1 to 3 were placed in a vacuum oven for degassing for 1 hour, and then cast and cured according to the curing schedule of 60°C / 1 hour + 80°C / 1 hour + 150°C / 2 hours to obtain specimens. The mechanical properties of the specimens were tested according to GB / T 2567 standard. The results are shown in Table 1.

[0074] Table 1 Mechanical properties test results

[0075]

[0076] It can be seen from Table 1 that the tensile strength, flexural strength and impact toughness of the electromagnetic shielding composite material used in the winding molding process of the present invention are significantly better than those of the electromagnetic shielding composite materials prepared in Comparative Examples 1 to 3. This is because the present invention introduces two-dimensional Ti3C2T X and one-dimensional silver nanowires, which further improve the structural network integrity while reducing the amount of metal conductive filler added, and have better mechanical properties.

[0077] 2. Magnetic shielding performance test

[0078] A square cylindrical mold was selected, glass fiber yarn was selected as the reinforcement material, the number of yarn balls was 8, the tension control was set to 10N / ball, the temperature of the glue tank was controlled at 35±3℃, the winding was carried out according to the established procedure, the product thickness was controlled at 2mm, and the curing was carried out according to the procedure of 60℃ / 1h+80℃ / 1h+150℃ / 2h. The specimens were cut and the shielding effectiveness within the X-band frequency range was tested using the N5247A PNA vector network analyzer. The results are as follows Figure 1 As shown. Taking the electromagnetic shielding composite material for winding molding process prepared in Example 1 as the research object, a series of winding products with different thicknesses (1mm, 2mm, 3mm, 4mm, 5mm) were prepared under the same winding process, and the electromagnetic shielding effectiveness of the corresponding thickness was tested. The results are shown as follows Figure 2 shown.

[0079] from Figure 1 It can be seen that the introduction of Ti3C2T X The electromagnetic shielding effectiveness of the winding product with metal nanoparticles is 30.2dB, and the shielding effectiveness of the winding product with silver nanowires is 21.6dB. X The electromagnetic shielding effectiveness of the products obtained by adding and removing metal nanoparticles is greater than 53.9dB, and the maximum can reach 60.6dB, which shows excellent shielding effectiveness. This is because the introduction of silver nanowires alone, due to the limited addition amount and weak interaction between nanowires, makes the conductive network formed loose; the introduction of Ti3C2TX With metal nanoparticles, the mutual adhesion between the two-dimensional sheets makes the system less reflective of electromagnetic waves; at the same time, silver nanowires, Ti3C2T X And metal nanoparticles are introduced into the wound products, and the welding effect of silver nanowires is used to obtain a tighter and more complete three-dimensional conductive network. At the same time, metal nanoparticles increase the roughness of the two-dimensional sheet, improve the reflection and absorption of electromagnetic waves, thereby greatly improving the electromagnetic shielding effectiveness.

[0080] from Figure 2 It can be seen that as the thickness of the product increases, the shielding effectiveness further increases. This is because the winding process makes the conductive filler well dispersed in the product. As the thickness increases, the reflection and absorption of electromagnetic waves by the product are further enhanced, and the shielding effectiveness increases accordingly.

[0081] The present invention has been described in accordance with the above embodiments. It should be understood that the above embodiments do not limit the present invention in any form, and any technical solutions obtained by equivalent replacement or equivalent transformation fall within the scope of the present invention.

Claims

1. A method for preparing an electromagnetic shielding composite material for a winding molding process, characterized in that: The following steps are involved: (1) LiF and Ti3AlC2 precursor were added to the hydrochloric acid solution in sequence and stirred to react to obtain Ti3C2T X ; (2) adding the diluent and the silver nanowires to the epoxy resin and stirring evenly to obtain a premix 1; (3) Ti3C2T X and metal nanoparticles are added to the curing agent and ultrasonicated to obtain premix 2; (4) mixing the premix 1, the premix 2 and the accelerator to obtain an electromagnetic shielding composite material for a winding molding process; In the step (1), after the stirring reaction, the mixture is washed with water and centrifuged until the pH of the upper layer is greater than 6, and then the lower layer precipitate is cooled, ultrasonicated with water, and centrifuged again; the speed of the two centrifuges is 3000-6000 rpm, and the time is 10-15 minutes; the cooling temperature is 2-4°C, the time is 2-6 hours; and the ultrasonication time is 30-60 minutes; The Ti3C2T X The addition amount of the silver nanowire is 3-7% of the total mass of the electromagnetic shielding composite material used for the winding molding process; the addition amount of the silver nanowire is 1-3% of the total mass of the electromagnetic shielding composite material used for the winding molding process; the addition amount of the metal nanoparticles is 0.3-1% of the total mass of the electromagnetic shielding composite material used for the winding molding process.

2. The method for preparing an electromagnetic shielding composite material for a winding molding process according to claim 1, wherein: The molar ratio of the Ti3AlC2 precursor to LiF in step (1) is 1:(5-7.5); the stirring reaction temperature is 35-45°C and the time is 24-48h; the Ti3C2T X The size is 100nm~3μm.

3. The method for preparing an electromagnetic shielding composite material for a winding molding process according to claim 1, wherein: The diluent is dibutyl alcohol diglycidyl ether; the silver nanowires have a diameter of 20 to 25 nm and a length of 15 to 30 μm; the metal nanoparticles are copper nanoparticles and / or silver nanoparticles; the curing agent is methyltetrahydrophthalic anhydride; and the accelerator is 2,4,6-tris(dimethylaminomethyl)phenol.

4. The method for preparing an electromagnetic shielding composite material for a winding molding process according to claim 1 or 3, wherein: The mass ratio of the epoxy resin, diluent, curing agent and accelerator is (100-105):(85-95):(80-90):(1-2).

5. The method for preparing an electromagnetic shielding composite material for a winding molding process according to claim 1, wherein: In the step (2), the stirring temperature is 40-65° C. and the stirring time is 3-6 h.

6. The method for preparing an electromagnetic shielding composite material for a winding molding process according to claim 1, wherein: The ultrasonication in step (3) is performed in an ice bath for 0.5 to 3 hours.

7. An electromagnetic shielding composite material for a winding molding process, prepared by the preparation method according to any one of claims 1 to 6.

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