Carbon nanotube-containing TPU film for electromagnetic shielding and preparation method thereof

Abstract

The invention relates to a carbon nanotube-containing TPU film for electromagnetic shielding and a preparation method thereof, and belongs to the technical field of polymer synthesis. The TPU film has a three-layer structure, namely a high-conductivity outer layer, and graphene and auxiliaries are mixed, then TPU is added for mixing and melting, and the film is formed through coating; the intermediate layer for absorbing electromagnetic waves is prepared by mixing pretreated carbon nanotubes with an auxiliary agent, then adding grafted TPU for mixing and melting, and coating to form a film; the insulating inner layer is formed by mixing an insulating filler and an auxiliary agent, adding TPU for mixing and melting, and coating to form a film; and performing hot-pressing on each layer of material into a whole by using hot-pressing equipment. The TPU is modified and grafted on the middle layer, and meanwhile, the carbon nanotubes are pretreated, so that the uniform dispersion of the carbon nanotubes in the TPU is effectively improved, and the electromagnetic shielding effectiveness of the TPU film is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of polymer material research, and relates to a TPU film containing carbon nanotubes for electromagnetic shielding and a preparation method thereof. Background Art

[0002] With the rapid development of electronic devices and wireless communication technologies, the problem of electromagnetic interference (EMI) has become increasingly serious. EMI not only affects the normal operation of electronic devices but may also have an adverse impact on human health. Therefore, it is necessary to develop efficient electromagnetic shielding materials. Currently, the common electromagnetic shielding materials on the market mainly include metal shielding materials and carbon-based shielding materials. However, these materials have many defects in practical applications. Although metal shielding materials have excellent electrical conductivity and shielding effect, they are relatively heavy and not suitable for use in electronic devices with high lightweight requirements. In addition, metal materials have poor flexibility, complex processing and forming processes, and high costs. Carbon-based materials such as carbon black and carbon fiber, although having certain electrical conductivity, their shielding efficiency is often not as good as that of metal materials, and their dispersion in the polymer matrix is poor, easily forming aggregates, resulting in uneven shielding performance.

[0003] Thermoplastic polyurethane (TPU), as a new type of polymer material, has gradually attracted attention due to its excellent mechanical properties, flexibility and processability. TPU materials have significant advantages in electromagnetic shielding applications. First, TPU materials have good flexibility and ductility, suitable for various complex-shaped and bendable electronic devices. Second, TPU materials have a low density, which can meet the requirements of lightweight electronic devices. In addition, the processing process of TPU materials is simple and can be formed by various methods such as coating into films, injection molding, extrusion and compression molding, suitable for large-scale production. However, pure TPU materials themselves do not have electrical conductivity and need to add conductive fillers to improve their electromagnetic shielding efficiency. At the same time, the dispersion of conductive fillers in the TPU matrix directly affects the shielding performance, and how to achieve uniform dispersion is a key issue. The flame retardant performance of TPU materials is relatively poor and also needs to add flame retardants to improve their safety, especially in applications in high-temperature or fire environments. Summary of the Invention

[0004] The present invention relates to a carbon nanotube-containing TPU film for electromagnetic shielding and a preparation method thereof, belonging to the technical field of polymer synthesis. The TPU film has a three-layer structure, a highly conductive outer layer, which is formed by mixing graphene with additives, adding the mixture to TPU, melting them together, and coating to form a film; an intermediate layer for absorbing electromagnetic waves, which is formed by mixing pretreated carbon nanotubes with additives, adding the mixture to grafted TPU, melting them together, and coating to form a film; an insulating inner layer, which is formed by mixing insulating fillers with additives, adding the mixture to TPU, melting them together, and coating to form a film; and then using a hot pressing device to hot press each layer of materials into one body. The intermediate layer modifies and grafts TPU, and at the same time pretreats carbon nanotubes, effectively improving the uniform dispersion of carbon nanotubes in TPU and enhancing the electromagnetic shielding efficiency of the TPU film.

[0005] The object of the present invention can be achieved by the following technical solutions:

[0006] A carbon nanotube-containing TPU film for electromagnetic shielding, the TPU film is composed of a highly conductive outer layer, an intermediate layer for absorbing electromagnetic waves, and an insulating inner layer. The thickness ratio of the highly conductive outer layer, the intermediate layer for absorbing electromagnetic waves, and the insulating inner layer is 1:3:1. The components of the highly conductive outer layer include TPU, graphene, a dispersant, a coupling agent, a crosslinking agent, and a flame retardant. The components of the intermediate layer for absorbing electromagnetic waves include grafted TPU, pretreated carbon nanotubes, a dispersant, a coupling agent, a crosslinking agent, and a flame retardant. The components of the insulating inner layer include TPU, an insulating agent, a dispersant, a coupling agent, a crosslinking agent, and a flame retardant.

[0007] As a preferred technical solution of the present invention, the preparation method of the grafted TPU is as follows: Disperse TPU in absolute ethanol, add NaBH4 at 40 - 50 °C, stir for 3 - 8 hours, add dilute hydrochloric acid to neutralize the remaining NaBH4, centrifuge, filter, wash, and dry to obtain hydroxyl-containing TPU; dissolve γ-aminopropyltriethoxysilane in absolute ethanol, add 1 - 5 wt% deionized water, stir for 3 - 6 hours; add the hydroxyl-containing TPU, stir at 40 - 50 °C for 6 - 18 hours, and obtain grafted TPU through precipitation, filtration, washing, and drying.

[0008] As a preferred technical solution of the present invention, the preparation method of the pretreated carbon nanotubes is as follows: Add carbon nanotubes to a mixed solution of concentrated nitric acid and concentrated sulfuric acid, stir at 70 - 90 °C for 2 - 6 hours, then filter and wash with water to remove the acid solution until the washing liquid is neutral, and then perform a drying treatment to obtain pretreated carbon nanotubes with carboxyl groups on the surface.

[0009] As a preferred technical solution of the present invention, the dispersant is at least one of polyethylene wax, polypropylene wax, or EVA wax.

[0010] As a preferred technical solution of the present invention, the coupling agent is one of vinylmethyldimethoxysilane and vinyltriethoxysilane.

[0011] As a preferred technical solution of the present invention, the crosslinking agent is one of hydroquinone dihydroxyethyl ether and resorcinol bis(2-hydroxyethyl) ether.

[0012] As a preferred technical solution of the present invention, the flame retardant is at least one of aluminum diethyl phosphinate, melamine cyanurate or ammonium polyphosphate.

[0013] As a preferred technical solution of the present invention, the insulating agent is silicon dioxide.

[0014] Furthermore, a preparation method of the TPU film containing carbon nanotubes for electromagnetic shielding includes the following steps:

[0015] (1) Stir and mix TPU, graphene, dispersant, coupling agent, crosslinking agent, and flame retardant evenly at 180 - 200 °C and a pressure of 10 - 20 MPa to form a molten mixture, and use a casting coater to coat the molten mixture into a film, and dry it at a temperature of 80 - 100 °C for 2 - 4 hours to obtain a highly conductive outer layer;

[0016] (2) Stir and mix grafted TPU, pretreated carbon nanotubes, dispersant, coupling agent, crosslinking agent, and flame retardant evenly at 180 - 200 °C and a pressure of 10 - 20 MPa to form a molten mixture, and use a casting coater to coat the molten mixture into a film, and dry it at a temperature of 80 - 100 °C for 2 - 4 hours to obtain an electromagnetic wave absorbing intermediate layer;

[0017] (3) Stir and mix TPU, insulating agent, dispersant, coupling agent, crosslinking agent, and flame retardant evenly at 180 - 200 °C and a pressure of 10 - 20 MPa to form a molten mixture, and use a casting coater to coat the molten mixture into a film, and dry it at a temperature of 80 - 100 °C for 2 - 4 hours to obtain an insulating inner layer;

[0018] (4) Hot press the highly conductive outer layer, the electromagnetic wave absorbing intermediate layer, and the insulating inner layer through a hot pressing device at 150 - 180 °C and a pressure of 5 - 10 MPa to integrally hot press the materials of each layer to form a TPU film containing carbon nanotubes for electromagnetic shielding.

[0019] As a preferred technical solution of the present invention, the mass ratio of the TPU, graphene, dispersant, coupling agent, crosslinking agent, and flame retardant in step (1) is 16-23: 0.2-4: 0.3-1.6: 0.3-1: 0.2-0.6: 1.6-3.3; the mass ratio of the grafted TPU, pretreated carbon nanotubes, dispersant, coupling agent, crosslinking agent, and flame retardant in step (2) is 16-23: 5-15: 0.3-1.6: 0.3-1: 0.2-0.6: 1.6-3.3; the mass ratio of the TPU, insulating agent, dispersant, coupling agent, crosslinking agent, and flame retardant in step (3) is 16-23: 1-5: 0.3-1.6: 0.3-1: 0.2-0.6: 1.6-3.3.

[0020] Advantages of the present invention:

[0021] (1) In the present invention, APTES is used to modify and graft TPU to obtain TPU containing amino groups; at the same time, carbon nanotubes are pretreated to obtain carbon nanotubes containing carboxyl groups; effectively improving the uniform dispersion and combination of carbon nanotubes in TPU and enhancing the electromagnetic shielding efficiency of the TPU film.

[0022] (2) The high-conductivity material in the outer layer of the present invention provides a primary shielding effect through reflection; the middle layer absorbs electromagnetic waves, reducing the multiple reflections of electromagnetic waves inside the material, thereby reducing the overall electromagnetic wave transmission intensity; the insulating property of the inner layer can prevent any residual electromagnetic waves from leaking into the internal or external environment of the shielding material; each layer of material plays its own role in shielding electromagnetic waves, forming a comprehensive and multiple shielding mechanism, thereby achieving a better electromagnetic shielding effect. Specific embodiments

[0023] To further elaborate on the technical means and effects adopted by the present invention to achieve the predetermined invention purpose, the following is a detailed description of the specific embodiments, structures, features, and effects of the present invention in combination with examples.

[0024] Example 1

[0025] A TPU film containing carbon nanotubes for electromagnetic shielding, the TPU film is composed of a high-conductivity outer layer, an electromagnetic wave-absorbing middle layer, and an insulating inner layer. The thickness ratio of the high-conductivity outer layer, the electromagnetic wave-absorbing middle layer, and the insulating inner layer is 1: 3: 1. The components of the high-conductivity outer layer include TPU, graphene, dispersant, coupling agent, crosslinking agent, and flame retardant. The components of the electromagnetic wave-absorbing middle layer include grafted TPU, pretreated carbon nanotubes, dispersant, coupling agent, crosslinking agent, and flame retardant. The components of the insulating inner layer include TPU, insulating agent, dispersant, coupling agent, crosslinking agent, and flame retardant.

[0026] The preparation method of the grafted TPU is as follows: Disperse 10 parts by weight of TPU in 80 parts by weight of absolute ethanol, add 0.5 part by weight of NaBH4 at 45°C, stir for 6 hours, add 0.1M dilute hydrochloric acid to neutralize the remaining NaBH4, centrifuge, filter, wash, and dry to obtain hydroxyl-containing TPU; Dissolve 1 part by weight of γ-aminopropyltriethoxysilane in 80 parts by weight of absolute ethanol, add 2 wt% deionized water, and stir for 5 hours; Add the hydroxyl-containing TPU and stir at 45°C for 12 hours, and obtain the grafted TPU through precipitation, filtration, washing, and drying.

[0027] The preparation method of the pretreated carbon nanotubes is as follows: Add 1 part by weight of carbon nanotubes to a mixed solution of 140 parts by weight of 63% concentrated nitric acid and 550 parts by weight of 98% concentrated sulfuric acid, stir at 80°C for 4 hours, then remove the acid solution through filtration and water washing until the washing solution is neutral, and then perform drying treatment to obtain pretreated carbon nanotubes with carboxyl groups on the surface.

[0028] The dispersant is polyethylene wax.

[0029] The coupling agent is vinylmethyldimethoxysilane.

[0030] The crosslinking agent is hydroquinone dihydroxyethyl ether.

[0031] The flame retardant is aluminum diethylphosphinate.

[0032] The insulating agent is silicon dioxide.

[0033] A preparation method of a TPU film containing carbon nanotubes for electromagnetic shielding includes the following steps:

[0034] (1) Stir and mix TPU, graphene, dispersant, coupling agent, crosslinking agent, and flame retardant evenly at 190°C under a pressure of 15 MPa to form a molten mixture, and use a casting coater to coat the molten mixture into a film with a thickness of 100 μm, and dry it at a temperature of 90°C for 2 hours as the highly conductive outer layer;

[0035] (2) Stir and mix the grafted TPU, pretreated carbon nanotubes, dispersant, coupling agent, crosslinking agent, and flame retardant evenly at 190°C under a pressure of 15 MPa to form a molten mixture, and use a casting coater to coat the molten mixture into a film with a thickness of 300 μm, and dry it at a temperature of 90°C for 3 hours as the electromagnetic wave absorbing intermediate layer;

[0036] (3) Stir and mix TPU, an insulating agent, a dispersant, a coupling agent, a crosslinking agent, and a flame retardant evenly at 190 °C under a pressure of 15 MPa to form a molten mixture. Use a casting coater to coat the molten mixture into a film with a thickness of 100 μm, and dry it at a temperature of 90 °C for 2 hours to obtain an insulating inner layer;

[0037] (4) Hot press the highly conductive outer layer, the electromagnetic wave absorbing intermediate layer, and the insulating inner layer through a hot pressing device at 160 °C and a pressure of 8 MPa to integrally hot press the materials of each layer to form a TPU film containing carbon nanotubes for electromagnetic shielding.

[0038] The mass ratio of the TPU, graphene, dispersant, coupling agent, crosslinking agent, and flame retardant in step (1) is 12:3:0.5:0.5:0.2:2; the mass ratio of the grafted TPU, pretreated carbon nanotubes, dispersant, coupling agent, crosslinking agent, and flame retardant in step (2) is 36:9:1.5:1.5:0.6:6; the mass ratio of the TPU, insulating agent, dispersant, coupling agent, crosslinking agent, and flame retardant in step (3) is 12:3:0.5:0.5:0.2:2.

[0039] Example 2

[0040] A TPU film containing carbon nanotubes for electromagnetic shielding, the TPU film is composed of a highly conductive outer layer, an electromagnetic wave absorbing intermediate layer, and an insulating inner layer. The thickness ratio of the highly conductive outer layer, the electromagnetic wave absorbing intermediate layer, and the insulating inner layer is 1:3:1. The components of the highly conductive outer layer include TPU, graphene, a dispersant, a coupling agent, a crosslinking agent, and a flame retardant. The components of the electromagnetic wave absorbing intermediate layer include grafted TPU, pretreated carbon nanotubes, a dispersant, a coupling agent, a crosslinking agent, and a flame retardant. The components of the insulating inner layer include TPU, an insulating agent, a dispersant, a coupling agent, a crosslinking agent, and a flame retardant.

[0041] The preparation method of the grafted TPU is as follows: Disperse 10 parts by weight of TPU in 80 parts by weight of absolute ethanol, add 0.5 part by weight of NaBH4 at 40 °C, stir for 6 hours, add 0.1 M dilute hydrochloric acid to neutralize the remaining NaBH4, centrifuge, filter, wash, and dry to obtain hydroxyl-containing TPU; dissolve 1 part by weight of γ-aminopropyltriethoxysilane in 80 parts by weight of absolute ethanol, add 3 wt% deionized water, and stir for 5 hours; add the hydroxyl-containing TPU and stir at 40 °C for 8 hours, and obtain grafted TPU through precipitation, filtration, washing, and drying.

[0042] The preparation method of the pretreated carbon nanotubes is as follows: Add 1 part by weight of carbon nanotubes to a mixed solution of 140 parts by weight of 63% concentrated nitric acid and 550 parts by weight of 98% concentrated sulfuric acid, stir at 80 °C for 4 hours, then remove the acid solution by filtration and water washing until the washing solution is neutral, and then perform a drying treatment to obtain the pretreated carbon nanotubes with carboxyl groups on the surface.

[0043] The dispersant is EVA wax.

[0044] The coupling agent is vinylmethyldimethoxysilane.

[0045] The crosslinking agent is resorcinol bis(2-hydroxyethyl) ether.

[0046] The flame retardant is ammonium polyphosphate.

[0047] The insulating agent is silicon dioxide.

[0048] A preparation method of a TPU film containing carbon nanotubes for electromagnetic shielding includes the following steps:

[0049] (1) Mix TPU, graphene, dispersant, coupling agent, crosslinking agent, and flame retardant evenly at 180 °C under a pressure of 10 MPa to form a molten mixture, and use a casting coater to coat the molten mixture into a film with a thickness of 100 μm, and dry it at a temperature of 90 °C for 2 hours as the highly conductive outer layer;

[0050] (2) Mix grafted TPU, pretreated carbon nanotubes, dispersant, coupling agent, crosslinking agent, and flame retardant evenly at 180 °C under a pressure of 10 MPa to form a molten mixture, and use a casting coater to coat the molten mixture into a film with a thickness of 300 μm, and dry it at a temperature of 90 °C for 3 hours as the electromagnetic wave absorbing intermediate layer;

[0051] (3) Mix TPU, insulating agent, dispersant, coupling agent, crosslinking agent, and flame retardant evenly at 180 °C under a pressure of 10 MPa to form a molten mixture, and use a casting coater to coat the molten mixture into a film with a thickness of 100 μm, and dry it at a temperature of 90 °C for 2 hours as the insulating inner layer;

[0052] (4) Hot press the highly conductive outer layer, the electromagnetic wave absorbing intermediate layer, and the insulating inner layer into one body at 160 °C under a pressure of 8 MPa through a hot pressing device to form a TPU film containing carbon nanotubes for electromagnetic shielding.

[0053] The mass ratio of the TPU, graphene, dispersant, coupling agent, crosslinking agent, and flame retardant in step (1) is 10:2:0.2:0.2:0.1:1; the mass ratio of the grafted TPU, pretreated carbon nanotubes, dispersant, coupling agent, crosslinking agent, and flame retardant in step (2) is 30:6:0.6:0.6:0.3:3; the mass ratio of the TPU, insulating agent, dispersant, coupling agent, crosslinking agent, and flame retardant in step (3) is 10:2:0.2:0.2:0.1:1.

[0054] Comparative Example 1

[0055] On the basis of Example 1, the highly conductive layer is not prepared: graphene is not added, and the mass ratio of the grafted TPU, pretreated carbon nanotubes, dispersant, coupling agent, crosslinking agent, and flame retardant in step (2) is 48:12:2:2:0.8:8, and a film with a thickness of 400 μm is coated; the mass ratio of the TPU, insulating agent, dispersant, coupling agent, crosslinking agent, and flame retardant in step (3) is 12:3:0.5:0.5:0.2:2, and a film with a thickness of 100 μm is coated, and the rest is the same as in Example 1.

[0056] Comparative Example 2

[0057] On the basis of Example 1, the electromagnetic wave absorption layer is not prepared: carbon nanotubes are not added, and the mass ratio of the TPU, graphene, dispersant, coupling agent, crosslinking agent, and flame retardant in (1) is 48:12:2:2:0.8:8, and a film with a thickness of 400 μm is coated; the mass ratio of the TPU, insulating agent, dispersant, coupling agent, crosslinking agent, and flame retardant in step (3) is 12:3:0.5:0.5:0.2:2, and a film with a thickness of 100 μm is coated, and the rest is the same as in Example 1.

[0058] Comparative Example 3

[0059] On the basis of Example 1, the grafting treatment of the intermediate layer TPU and the pretreatment of carbon nanotubes are not carried out, and the rest is the same as in Example 1.

[0060] Performance Test

[0061] According to the GB / T30142-2013 standard, the average value of the electromagnetic shielding effectiveness of the material from 0.5 MHz to 20 GHz is measured. The shielding effectiveness of the material is expressed by shielding attenuation. The larger the shielding attenuation value, the better the shielding effectiveness. The unit is decibel (dB), and the calculation formula is SE = 20lg(E1 / E2) (dB), where E1 is the field strength without shielding and E2 is the field strength with shielding. The test results are shown in the table.

[0062] Example 1 Example 2 Comparative Example 1 Comparative Example 2 Comparative Example 3 Shielding effectiveness 66.9 66.4 53.5 29.4 37.6

[0063] It can be seen from the test results that the present invention combines a highly conductive layer, an electromagnetic wave absorbing layer and an insulating layer to form a comprehensive and multiple shielding mechanism, thereby achieving a better electromagnetic shielding effect; and by modifying and grafting TPU and pre-treating carbon nanotubes, the uniform dispersion and combination of carbon nanotubes in TPU are effectively improved, and the electromagnetic shielding efficiency of the TPU film is improved.

[0064] The above are only the preferred embodiments of the present invention, and do not impose any form of limitation on the present invention. Although the present invention has been disclosed above with the preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some changes or modifications to equivalent embodiments by using the disclosed technical content within the scope of the technical solution of the present invention. However, as long as it does not depart from the content of the technical solution of the present invention, any brief modifications, equivalent changes and modifications made to the above embodiments according to the technical essence of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A TPU film containing carbon nanotubes for electromagnetic shielding, characterized in that: The TPU film consists of a highly conductive outer layer, an electromagnetic wave absorbing middle layer and an insulating inner layer. The thickness ratio of the highly conductive outer layer, the electromagnetic wave absorbing middle layer and the insulating inner layer is 1:3:

1. The components of the highly conductive outer layer include TPU, graphene, a dispersant, a coupling agent, a cross-linking agent, and a flame retardant. The components of the electromagnetic wave absorbing middle layer include grafted TPU, pretreated carbon nanotubes, a dispersant, a coupling agent, a cross-linking agent, and a flame retardant. The components of the insulating inner layer include TPU, an insulating agent, a dispersant, a coupling agent, a cross-linking agent, and a flame retardant.

2. The TPU film containing carbon nanotubes for electromagnetic shielding according to claim 1, characterized in that: The preparation method of the grafted TPU is as follows: dispersing TPU in anhydrous ethanol, adding NaBH4 at 40-50°C, stirring for 3-8 hours, adding dilute hydrochloric acid to neutralize the remaining NaBH4, centrifuging, filtering, washing, and drying to obtain hydroxyl-containing TPU; dissolving γ-aminopropyltriethoxysilane in anhydrous ethanol, adding 1-5wt% deionized water, and stirring for 3-6 hours; adding hydroxyl-containing TPU, stirring at 40-50°C for 6-18 hours, and obtaining grafted TPU by precipitation, filtering, washing, and drying.

3. The TPU film containing carbon nanotubes for electromagnetic shielding according to claim 1, characterized in that: The preparation method of the pretreated carbon nanotubes is as follows: adding carbon nanotubes to a mixed solution of concentrated nitric acid and concentrated sulfuric acid, stirring at 70-90° C. for 2-6 hours, filtering and washing with water to remove the acid solution until the washing solution is neutral, and then drying to obtain pretreated carbon nanotubes with carboxyl groups on the surface.

4. The TPU film containing carbon nanotubes for electromagnetic shielding according to claim 1, characterized in that: The dispersant is at least one of polyethylene wax, polypropylene wax or EVA wax.

5. The TPU film containing carbon nanotubes for electromagnetic shielding according to claim 1, characterized in that: The coupling agent is one of vinylmethyldimethoxysilane and vinyltriethoxysilane.

6. The TPU film containing carbon nanotubes for electromagnetic shielding according to claim 1, characterized in that: The cross-linking agent is one of hydroquinone dihydroxyethyl ether and resorcinol di(2-hydroxyethyl) ether.

7. The TPU film containing carbon nanotubes for electromagnetic shielding according to claim 1, characterized in that: The flame retardant is at least one of diethyl aluminum hypophosphite, melamine cyanurate or ammonium polyphosphate.

8. The TPU film containing carbon nanotubes for electromagnetic shielding according to claim 1, characterized in that: The insulating agent is silicon dioxide.

9. A method for preparing a TPU film containing carbon nanotubes for electromagnetic shielding according to any one of claims 1 to 8, characterized in that: The following steps are involved: (1) stirring and mixing TPU with graphene, dispersant, coupling agent, cross-linking agent and flame retardant at 180-200° C. and pressure of 10-20 MPa to form a molten mixture, coating the molten mixture into a film using a cast coater, and drying at a temperature of 80-100° C. for 2-4 hours to form a highly conductive outer layer; (2) The grafted TPU, pretreated carbon nanotubes, dispersant, coupling agent, crosslinking agent, and flame retardant are stirred and mixed at 180-200° C. and a pressure of 10-20 MPa to form a molten mixture, and the molten mixture is coated into a film using a cast coater, and dried at a temperature of 80-100° C. for 2-4 hours to serve as an intermediate layer for absorbing electromagnetic waves; (3) stirring and mixing TPU with an insulating agent, a dispersant, a coupling agent, a cross-linking agent, and a flame retardant at 180-200° C. and a pressure of 10-20 MPa to form a molten mixture, coating the molten mixture into a film using a cast coater, and drying at a temperature of 80-100° C. for 2-4 hours to serve as an insulating inner layer; (4) The highly conductive outer layer, the electromagnetic wave absorbing middle layer and the insulating inner layer are hot pressed together at 150-180° C. and 5-10 MPa pressure by hot pressing equipment to form a TPU film containing carbon nanotubes for electromagnetic shielding.

10. The method for preparing a TPU film containing carbon nanotubes for electromagnetic shielding according to claim 9, characterized in that: The mass ratio of the TPU, graphene, dispersant, coupling agent, cross-linking agent and flame retardant in step (1) is 16-23: 0.2-4: 0.3-1.6: 0.3-1: 0.2-0.6: 1.6-3.3; the mass ratio of the grafted TPU, pretreated carbon nanotubes, dispersant, coupling agent, cross-linking agent and flame retardant in step (2) is 16-23: 5-15: 0.3-1.6: 0.3-1: 0.2-0.6: 1.6-3.3; the mass ratio of the TPU, insulating agent, dispersant, coupling agent, cross-linking agent and flame retardant in step (3) is 16-23: 1-5: 0.3-1.6: 0.3-1: 0.2-0.6: 1.6-3.3.