Absorption type electromagnetic shielding solvent-free polyurethane microfiber leather and preparation method thereof

Abstract

The invention relates to the field of leather making, and discloses absorption type electromagnetic shielding solvent-free polyurethane microfiber leather and a preparation method thereof. The absorption type electromagnetic shielding solvent-free polyurethane microfiber leather comprises microfiber leather, a conductive metal layer and a solvent-free polyurethane low-conductive layer, wherein the conductive metal layer and the solvent-free polyurethane low-conductive layer are respectively arranged on two opposite surfaces of the microfiber leather; the conductive metal layer is used as a high-conductive shielding layer; the solvent-free polyurethane low-conductive layer is used as a low-conductive impedance matching layer; the solvent-free polyurethane low-conductive layer comprises a solvent-free polyurethane base material and a conductive nanofiller dispersed in the solvent-free polyurethane base material. According to the absorption type electromagnetic shielding solvent-free polyurethane microfiber leather, the high-conductivity shielding layer and the low-conductivity impedance matching layer are arranged on the two sides of the microfiber leather respectively, an impedance matching and multi-reflection system is constructed, and the prepared absorption type electromagnetic shielding solvent-free polyurethane microfiber leather not only has excellent electromagnetic shielding performance, but also has good mechanical performance and surface texture; in addition, the characteristics of light weight and environmental protection are realized.

Description

Technical Field

[0001] The present invention relates to the field of leather making, and particularly to an absorbent electromagnetic shielding solvent-free polyurethane superfine leather and a preparation method thereof. Background Art

[0002] Due to its excellent mechanical properties, abrasion resistance and environmental protection characteristics, superfine leather has been widely used in the fields of new energy vehicle interiors, electronic product casings, etc. However, with the popularization of intelligent manufacturing, a large number of electronic devices have been equipped in current new energy vehicles, such as sensing systems, navigation devices, electronic control systems, etc. These devices will generate or be affected by electromagnetic interference during operation, which may lead to signal transmission errors or even system failures, causing significant losses to health and property. Absorbent electromagnetic shielding superfine leather can effectively attenuate and absorb electromagnetic waves, reducing electromagnetic interference while ensuring the safe operation of the vehicle body system.

[0003] Polyurethane (PU) is widely used in the preparation of superfine leather by impregnating superfine fibers due to its excellent mechanical properties, abrasion resistance, weather resistance, etc. PU is mainly divided into the first-generation solvent-based PU and the second-generation water-based PU according to the dispersion medium. With the introduction of environmental protection policies for low VOC requirements, the third-generation solvent-free PU has emerged. While having good adhesion, abrasion resistance and processability, it can reduce VOC emissions. Compared with metal-based nano-fillers, carbon nano-materials are lighter while having good conductivity, and can significantly improve the electromagnetic shielding performance at low addition amounts. The team of Ren Wencai at the University of Science and Technology of China (Advanced Materials, 2020, 32(14), 1907411) used graphene as a raw material and prepared a highly conductive graphene film by a vacuum filtration method, achieving a high-efficiency electromagnetic shielding effect of 93 dB at a thickness of 100 μm. Combining SFPU with functional conductive fillers can prepare high-performance electromagnetic shielding materials. The team of Luo Xiaomin at Shaanxi University of Science & Technology used chrome-tanned waste leather and CNTs as conductive fillers and transition metal ions as coordination cross-linking agents to prepare flexible conductive collagen fibers, and constructed a flexible conductive foam with a double-layer and double-core cladding structure containing carbon nanotubes by a solvent-free polyurethane chemical foaming technology, with an electromagnetic shielding performance of up to 42.09 dB and a minimum reflection efficiency of 0.16% (Chemical Engineering Journal, 2024, 499, 156695). However, the solvent-free polyurethane system has the reaction characteristics of high viscosity and fast curing. How to achieve uniform dispersion of functional fillers and maintain the surface texture of superfine leather is still a technical difficulty.

[0004] In summary, the development of an absorption-type electromagnetic shielding solvent-free polyurethane superfine leather and its preparation method not only conforms to the current development trend of materials science but also meets the urgent needs of high-performance electromagnetic shielding materials in fields such as new energy vehicles and intelligent devices, having important technical value and application prospects. Summary of the Invention

[0005] To solve the above technical problems, the present invention provides an absorption-type electromagnetic shielding solvent-free polyurethane superfine leather and its preparation method. By respectively arranging a high-conductivity shielding layer and a low-conductivity impedance matching layer on both sides of the superfine leather, an impedance matching and multiple reflection system is constructed. The prepared absorption-type electromagnetic shielding solvent-free polyurethane superfine leather not only has excellent electromagnetic shielding performance but also has good mechanical properties and surface texture; in addition, it also has the characteristics of light weight and environmental protection.

[0006] The specific technical solution of the present invention is as follows:

[0007] First of all, the present invention provides an absorption-type electromagnetic shielding solvent-free polyurethane superfine leather, which includes a superfine leather, and a conductive metal layer as a high-conductivity shielding layer and a solvent-free polyurethane low-conductivity layer as a low-conductivity impedance matching layer respectively arranged on two opposite surfaces of the superfine leather. Among them, the solvent-free polyurethane low-conductivity layer includes a solvent-free polyurethane substrate and conductive nano-fillers dispersed therein.

[0008] The core technical concept of the present invention is: by arranging a conductive metal layer on one side of the superfine leather as a high-conductivity shielding layer to achieve an efficient electromagnetic shielding effect; at the same time, using the reaction characteristics of an appropriate amount of conductive nano-fillers and solvent-free polyurethane with high viscosity and fast curing, a solvent-free polyurethane low-conductivity layer is prepared as a low-conductivity impedance matching layer. After respectively constructing a high-conductivity shielding layer and a low-conductivity impedance matching layer on both sides of the superfine leather, a multiple reflection and loss mechanism of electromagnetic waves inside the material is formed. Based on the synergistic action principle of "absorption-reflection-re-absorption", an absorption-type electromagnetic shielding superfine leather is prepared. This design not only significantly improves the electromagnetic shielding efficiency but also reduces the secondary reflection of electromagnetic waves, having the characteristics of high efficiency, light weight, and environmental protection.

[0009] Specifically, the present invention effectively combines the ultra-fine leather, the conductive metal layer, and the solvent-free polyurethane low-conductive layer to achieve efficient electromagnetic shielding and absorption behavior of electromagnetic waves. This is because the content of conductive nano-fillers in the surface solvent-free polyurethane low-conductive layer is relatively small, making the impedance of the low-conductive layer close to that of air, reducing the reflection of electromagnetic waves on the material surface, and making it easier for electromagnetic waves to enter the material interior. When the electromagnetic waves enter the interior and reach the conductive metal layer, due to the extremely high conductivity and impedance mismatch, a large amount of electromagnetic waves are reflected. The electromagnetic waves are reflected multiple times between the high-conductive layer and the low-conductive layer inside the material, and each reflection is accompanied by energy loss. There is an "absorption-reflection-reabsorption" mechanism, and the synergistic effect of the high-conductive layer and the low-conductive layer realizes the efficient shielding and absorption of electromagnetic waves. Moreover, with the increase of conductive nano-fillers in the solvent-free polyurethane low-conductive layer, the absorption performance is improved. This is because the gradual improvement of the solvent-free polyurethane low-conductive layer enhances the conductance loss of electromagnetic waves, and at the same time, the dispersion and interfacial effect of the conductive nano-fillers are improved, prolonging the propagation path of electromagnetic waves and constructing a more perfect "absorption-reflection-reabsorption" system.

[0010] Preferably, the conductive nano-filler is one or more of graphene (G), expanded graphite (EG), carbon nanotubes (CNTs), and titanium carbide (MXene).

[0011] Preferably, the addition amount of the conductive nano-filler is 1-6 wt% of the solvent-free polyurethane substrate.

[0012] The reason for limiting the addition amount of the conductive nano-filler in the present invention within the above range is as follows: When the content of the conductive nano-filler is less than 1%, the distance between the conductive nano-fillers is too large to form a continuous conductive network, resulting in a large amount of electromagnetic waves passing through the material directly to the conductive metal layer, and a large amount of electromagnetic waves being reflected, unable to achieve electromagnetic absorption. On the other hand, too much conductive nano-filler will cause the conductive nano-fillers to agglomerate in the high-viscosity solvent-free polyurethane system, with poor compatibility. Failure to effectively establish chemical or physical connections between the conductive nano-fillers and the matrix will lead to difficult film formation. On the other hand, the viscosity of the solvent-free polyurethane system further increases, and the reaction between the isocyanate group and the amino group active groups is insufficient, and the active groups cannot form a cross-linked structure between the polymer chains, resulting in difficult film formation.

[0013] Preferably, the thickness of the solvent-free polyurethane low-conductive layer is 0.5-3.0 mm.

[0014] Preferably, the thickness of the conductive metal layer is 5-30 μm.

[0015] Preferably, the electromagnetic shielding performance of the conductive metal layer is 20-100 dB; more preferably 30-80 dB.

[0016] Preferably, the metal of the conductive metal layer is one or more of nickel, silver, copper, and gold.

[0017] Preferably, the conductive metal layer is deposited on one side surface of the microfiber leather by magnetron sputtering; the solvent-free polyurethane low-conductive layer is formed on the other side surface of the microfiber leather by coating and drying.

[0018] Secondly, the present invention provides a preparation method of an absorption-type electromagnetic shielding solvent-free polyurethane microfiber leather, which comprises the following steps:

[0019] S1. Deposit a conductive metal layer on one side of the microfiber leather by magnetron sputtering; the working gas is argon, the argon pressure is 1-5 Pa, the magnetic field strength is 30-150 gauss, the power of magnetron sputtering is 80-300 W, and the sputtering time is 5-60 min.

[0020] The present invention discovers that due to the relatively rough surface, softness and easy deformation of the microfiber leather, and its strong hydrophilicity, it will affect the diffusion and adsorption of metal ions during the magnetron sputtering process, and the binding force between the metal and the surface of the microfiber leather is weak, and the metal layer is prone to peeling or unevenness. Therefore, the present invention solves the above technical problems by optimizing the process parameters of magnetron sputtering. Among them, the power of magnetron sputtering determines the sputtering energy and rate, and thus affects the uniformity and fastness of the metal layer. The sputtering time directly affects the thickness and conductivity of the metal layer. As the working gas, the pressure of argon directly affects the energy, deposition rate and deposition uniformity of the sputtering particles. The magnetic field strength affects the movement path of the sputtering particles, and thus affects the uniformity and adhesion of the metal layer. Finally, the present invention discovers that a controllable conductive metal layer can be deposited on the surface of the microfiber leather through the above magnetron sputtering process.

[0021] S2. Use polyol, isocyanate, 1,4-butanediol, catalyst and conductive carbon nano filler as raw materials, set the free -NCO content to 20-50%, the R value to 0.8-1.5, and the hardness HS ratio to 20-50%. Mix and react, transfer the product to a plane, scrape and coat to form a film to obtain an undried solvent-free polyurethane low-conductive layer, and apply it to the other side surface of the microfiber leather and dry it to obtain the absorption-type electromagnetic shielding solvent-free polyurethane microfiber leather.

[0022] In S2, the reaction principle is as follows. In this reaction, the isocyanato group of the isocyanate combines with the hydroxyl group of the polyol to form a urethane intermediate, which further dehydrates to form a polyurethane chain. Among them, 1,4-butanediol has two -OH groups and a short molecule. During the synthesis of polyurethane, it reacts with the isocyanate to form new connection points and inserts into the polyurethane chain, extending the length of the polymer chain and playing the role of "chain extension". The increase in chain segments caused by the chain extension reaction simultaneously reduces the crosslinking degree of the polyurethane, making it more flexible and more suitable for the ultra-fine leather matrix, maintaining the original flexibility of the material.

[0023]

[0024] Further preferably, in S2, the free -NCO content is 25 - 40%, the R value is 0.9 - 1.3, and the hardness HS ratio is 25 - 40%.

[0025] The reason for the present invention to define the above parameters is that: the free -NCO group is a key reaction site during the synthesis of polyurethane. Controlling the free -NCO content can adjust the progress of the reaction. An excessively high free -NCO content usually means more unreacted isocyanate groups, which affects the hardness, toughness, and surface smoothness of the material. An excessively low free -NCO content results in too low crosslinking degree of the material, thus affecting the surface texture and causing the surface to be rough or uneven; the R value controls the reaction ratio of the isocyanate to the polyol and affects the structure and crosslinking degree of the polyurethane chain; the hardness HS ratio affects the crosslinking degree, chain length, and molecular weight of the polyurethane. A higher hardness ratio makes the material relatively hard, while a relatively lower hardness ratio makes the material more flexible and the surface smoother.

[0026] Preferably, in S2, the polyol is selected from one or more of castor oil, polytetrahydrofuran ether glycol (PTMEG), polycarbonate diol (PCDL), and polycaprolactone diol (PCL).

[0027] Preferably, in S2, the isocyanate is selected from one or more of isophorone diisocyanate (IPDI), hexamethylene diisocyanate (HDI), 4,4'-diisocyanatodicyclohexylmethane (HMDI), and diphenylmethane diisocyanate (MDI).

[0028] Preferably, in S2, the temperature of the mixed reaction is 30 - 100 °C, and the reaction time is 0.5 - 3 hours; further preferably, the reaction temperature is 40 - 80 °C, and the reaction time is 1 - 2 hours.

[0029] Compared with the prior art, the beneficial effects of the present invention are:

[0030] (1) By respectively arranging a high-conductivity shielding layer and a low-conductance impedance matching layer on both sides of the microfiber leather, an impedance matching and multiple reflection system is constructed. The prepared absorption-type electromagnetic shielding solvent-free polyurethane microfiber leather not only has excellent electromagnetic shielding performance, but also has good mechanical properties and surface texture. It can be widely applied to new energy vehicles, electromagnetic protection, aerospace and other fields, and has excellent application value.

[0031] (2) Compared with the preparation of traditional microfiber leather electromagnetic shielding composite materials, the microfiber leather electromagnetic shielding composite material prepared in the present invention uses solvent-free polyurethane as the coating, without VOC emissions; in addition, it also has the characteristics of light weight.

[0032] (3) Compared with the preparation of microfiber leather electromagnetic shielding composite materials, the microfiber leather electromagnetic shielding composite material prepared in the present invention has a low content of conductive fillers, realizing impedance matching. Description of the Drawings

[0033] Figure 1 It is a diagram of the electromagnetic shielding effect and absorption coefficient A of different comparative examples;

[0034] Figure 2 It is a diagram of the electromagnetic shielding effect and absorption coefficient A of SFPU-CNTs-x-y;

[0035] Figure 3 It is a diagram of the electromagnetic shielding effect and absorption coefficient A of SFPU-G-x-y;

[0036] Figure 4 It is a diagram of the electromagnetic shielding effect and absorption coefficient A of SFPU-MXene-x-y;

[0037] Figure 5 It is a diagram of the electromagnetic shielding effect and absorption coefficient A of SFPU-EG-x-y. Detailed Embodiments

[0038] The above content of the present invention will be further described in detail below through examples, but it should not be understood that the scope of the above subject matter of the present invention is limited to the following examples. All technologies implemented based on the above content of the present invention belong to the scope of the present invention.

[0039] Example 1

[0040] S1. Through the physical sputtering process, control the sputtering of the silver metal layer, with an argon gas pressure of 2 Pa, a magnetic field strength of 60 Gauss, the sputtering power controlled at 120 W, the sputtering time of 5 min, and a thickness of 5 μm, to obtain a high-conductivity microfiber with 30 dB.

[0041] S2. Using PTMEG, HMDI, 1,4-butanediol, the catalyst dibutyltin dilaurate, and carbon nanotubes as raw materials, setting the free -NCO content at 20%, the R value at 1.1, the HS at 30% ratio, and three gradient carbon nanotube contents of 2%, 3%, and 4%, reacting at 60 °C for 1 h, transferring the product to a centrifuge paper, scraping to form a film, and then laminating the non-conductive layer superfiber and polyurethane (1 mm) through a laminating process, and drying in an oven to prepare a solvent-free polyurethane-based absorptive electromagnetic shielding superfiber named SFPU-CNTs-x-y, where x represents the shielding effect of the high-conductive layer and y represents the carbon nanotube filler amount. Table 1 is the preparation parameter table of SFPU-CNTs-x-y.

[0042] To characterize its conductivity and electromagnetic shielding performance, a four-point probe is used for conductivity testing; a vector network analyzer is used to obtain the S parameters through the waveguide method, and its electromagnetic shielding performance and electromagnetic wave behavior are calculated and analyzed.

[0043] Table 1: Preparation parameter table of SFPU-CNTs-x-y

[0044]

[0045]

[0046] Example 2

[0047] S1. Through a physical sputtering process, controlling the sputtered copper metal layer, with an argon gas pressure of 3 Pa, a magnetic field strength of 80 Gauss, a sputtering power controlled at 150 W, a sputtering time of 10 min, and a thickness of 10 μm, a high-conductive superfiber with 50 dB is achieved.

[0048] S2. Using PCDL and PTMEG (1:1), HDI, 1,4-butanediol, the catalyst dibutyltin dilaurate, and graphene as raw materials, setting the free -NCO content at 25%, the R value at 1.0, the HS at 35% ratio, and three gradient graphene contents of 1%, 3%, and 5%, reacting at 70 °C for 1.5 h, transferring the product to a centrifuge paper, scraping to form a film, and then laminating the non-conductive layer superfiber and polyurethane (1.5 mm) through a laminating process, and drying in an oven to prepare a solvent-free polyurethane-based absorptive electromagnetic shielding superfiber named SFPU-G-x-y, where x represents the shielding effect of the high-conductive layer and y represents the graphene filler amount. Table 2 is the preparation parameter table of SFPU-G-x-y.

[0049] To characterize its conductivity and electromagnetic shielding performance, a four-point probe is used for conductivity testing; a vector network analyzer is used to obtain the S parameters through the waveguide method, and its electromagnetic shielding performance and electromagnetic wave behavior are calculated and analyzed.

[0050] Table 2: Preparation Parameter Table of SFPU-G-x-y

[0051]

[0052] Example 3

[0053] S1. Through the physical sputtering process, control the sputtering of the nickel metal layer. The argon gas pressure is 3.3 Pa, the magnetic field strength is 90 Gauss, the sputtering power is controlled at 300 w, the sputtering time is 30 min, and the thickness is 15 μm to achieve a highly conductive superfiber with 70 dB.

[0054] S2. Using castor oil, TDI, 1,4-butanediol, the catalyst dibutyltin dilaurate, and MXene as raw materials, set the free -NCO content to 30%, the R value to 1.2, the HS to 33%, and three gradient MXene contents of 2%, 4%, and 6%. React at 80 °C for 0.5 h, transfer the product to a centrifugal paper, scrape and coat to form a film, then laminate the non-conductive layer superfiber with polyurethane (2 mm) through the laminating process, and place it in an oven to dry to prepare a solvent-free polyurethane-based absorption-type electromagnetic shielding superfiber named SFPU-MXene-x-y, where x represents the shielding effect of the highly conductive layer and y represents the amount of MXene filler. Table 3 is the preparation parameter table of SFPU-MXene-x-y.

[0055] To characterize its conductivity and electromagnetic shielding performance, use a four-point probe to test the conductivity; use a vector network analyzer to obtain the S parameters through the waveguide method and calculate and analyze its electromagnetic shielding performance and electromagnetic wave behavior.

[0056] Table 3: Preparation Parameter Table of SFPU-MXene-x-y

[0057]

[0058] Example 4

[0059] S1. Through the physical sputtering process, control the sputtering of the gold metal layer. The argon gas pressure is 4 Pa, the magnetic field strength is 90 Gauss, the sputtering power is controlled at 250 w, the sputtering time is 40 min, and the thickness is 25 μm to achieve a highly conductive superfiber with 80 dB.

[0060] S2. Using PCL and PTMEG (1:1), MDI and TDI (1:1), 1,4-butanediol, the catalyst dibutyltin dilaurate, and EG as raw materials, setting the free -NCO content to 35%, the R value to 1.3, HS to 38%, and three gradient EG contents of 3%, 4%, and 5%. React at 75°C for 1.5 h, transfer the product to filter paper, scrape and coat to form a film, then laminate the non-conductive layer superfiber and polyurethane (2.5 mm) through the laminating process, and place it in an oven to dry. The solvent-free polyurethane-based absorbent electromagnetic shielding superfiber is prepared and named SFPU-EG-x-y, where x represents the shielding effect of the high-conductive layer and y represents the EG filler content. Table 4 is the preparation parameter table of SFPU-EG-x-y.

[0061] Table 4: Preparation Parameter Table of SFPU-EG-x-y

[0062]

[0063] Comparative Example 1

[0064] Using the high-conductive superfiber of Example 1 and 3% CNTs content, without adding polyol, and controlling other conditions to be exactly the same, 1-CNTs-30-3 is prepared.

[0065] Comparative Example 2

[0066] Using the high-conductive superfiber of Example 1 and 3% CNTs content, without adding isocyanate, and controlling other conditions to be exactly the same, 2-CNTs-30-3 is prepared.

[0067] Comparative Example 3

[0068] Using the high-conductive superfiber of Example 2 and the solvent-free polyurethane PU coating, without adding conductive nano-fillers, and controlling other conditions to be exactly the same, SFPU-G-50-0 is prepared.

[0069] Comparative Example 4

[0070] Using the solvent-free polyurethane PU coating of Example 3 with an MXene addition amount of 3%, and the other layer of the superfiber not depositing a conductive metal layer of 0 dB, and controlling other conditions to be exactly the same, SFPU-MXene-0-3 is prepared.

[0071] Comparative Example 5

[0072] Using the solvent-free polyurethane PU coating of Example 4 with an EG addition amount of 0%, and the other layer of the superfiber not depositing a conductive metal layer of 0 dB, and controlling other conditions to be exactly the same, SFPU-EG-0-0 is prepared.

[0073] Comparative Example 6

[0074] Using the highly conductive superfine fiber of Example 2 and a solvent-free polyurethane (PU) coating, with a graphene addition amount of 10%, and controlling other conditions to be exactly the same, SFPU-G-50-10 was prepared.

[0075] Comparative Example 7

[0076] Using the highly conductive superfine fiber of Example 1 and a 3% CNTs content, with a free - NCO content of 70%, and controlling other conditions to be exactly the same, SFPU-CNTs-30-3-1 was prepared.

[0077] Comparative Example 8

[0078] Using the highly conductive superfine fiber of Example 2 and a solvent-free polyurethane (PU) coating, with a graphene addition amount of 3% and an R value set to 0.5, and controlling other conditions to be exactly the same, SFPU-G-50-3-1 was prepared.

[0079] Performance Test

[0080] Table 5: Test Results of Conductive Performance and Electromagnetic Shielding Performance

[0081]

[0082]

[0083] It can be seen from the data in the above table that:

[0084] Compared with Example 1, in Comparative Example 1, the polyol is missing, and in Comparative Example 2, the isocyanate is missing. The hydroxyl group and the isocyanate group cannot react to form a urethane bond, resulting in the inability to form a molecular chain and construct a crosslinked network, leading to the inability to form a film and no shielding effect. Therefore, ensuring the proper ratio and sufficient reaction of the polyol and the isocyanate are the keys to synthesizing a solvent-free polyurethane film.

[0085] In Comparative Examples 3 and 4, the separate presence of a conductive metal layer or a solvent-free polyurethane low-conductive layer on the surface of the superfine leather cannot achieve high-efficiency electromagnetic shielding and absorption performance, mainly reflecting electromagnetic waves. The ratio of the hard and soft segments is appropriate, and the Shore hardness is 73 and 70 respectively, with certain flexibility and wear resistance. The data of Comparative Example 5 also prove that the superfine leather has no shielding performance, and the solvent-free polyurethane layer without conductive fillers has no shielding performance. The Shore hardness is 75, which is lower than that of Example 4. This is because the loading of conductive fillers usually has higher rigidity and hardness. When these rigid materials are added to the polyurethane matrix, the hardness of the overall material will be increased, and the filler acts as a reinforcing agent in the polyurethane.

[0086] In Comparative Example 6, on the one hand, excessive conductive fillers can cause aggregation of conductive nanofillers in the high-viscosity solvent-free polyurethane system, with poor compatibility. The failure to effectively establish chemical or physical connections between the fillers and the matrix can lead to difficulty in film formation. On the other hand, the viscosity of the solvent-free polyurethane system further increases, and the reaction between the isocyanate groups and the amino groups (active groups) is insufficient. The active groups cannot form a cross-linked structure between the polymer chains, resulting in difficulty in film formation.

[0087] In Comparative Example 7, a higher free - NCO content was set compared to Example 1. It was found that its Shore hardness was as high as 93, being overall hard, but the toughness of the material was poor, and it was prone to brittle fracture or cracking, which was not conducive to practical use.

[0088] In Comparative Example 8, a lower R value was set compared to Example 2. It was found that its Shore hardness was only 53, being overall soft and more susceptible to wear. Especially in a high-friction environment, its durability was poor, and it was prone to cracks or damage, which was not conducive to practical use.

[0089] It was Figures 1-5 found that the effective combination of the ultra-fine leather, the conductive metal layer, and the solvent-free polyurethane low-conductive layer can achieve efficient electromagnetic shielding and electromagnetic wave absorption behavior for electromagnetic waves. This is because the content of conductive fillers in the surface solvent-free polyurethane low-conductive layer is less, making the impedance of the low-conductive layer close to that of air, reducing the reflection of electromagnetic waves on the material surface and making it easier for electromagnetic waves to enter the material interior. When the electromagnetic waves enter the interior and reach the conductive metal layer, due to the extremely high conductivity and impedance mismatch, a large amount of electromagnetic waves are reflected. The electromagnetic waves are reflected multiple times between the high-conductive layer and the low-conductive layer inside the material, and each reflection is accompanied by energy loss, presenting an "absorption - reflection - reabsorption" mechanism. The synergistic effect of the high-conductive layer and the low-conductive layer realizes the efficient shielding and absorption of electromagnetic waves. Moreover, with the increase in the conductive fillers in the solvent-free polyurethane low-conductive layer, the absorption performance is improved. This is because the gradual improvement of the solvent-free polyurethane low-conductive layer enhances the conductance loss of electromagnetic waves, and at the same time, the dispersibility of the fillers and the interfacial effect are improved, extending the propagation path of electromagnetic waves and constructing a more perfect "absorption - reflection - reabsorption" system.

[0090] In the embodiments, appropriate free - NCO content, R value and hard segment Hs are set. The Shore hardness of the overall material is between 60 and 85, with moderate hardness. It not only maintains sufficient toughness and tenacity but also does not appear overly rigid, being relatively soft to use. At the same time, the microfiber leather within this hardness range has moderate abrasion resistance, can withstand a certain degree of friction and pressure, and can adapt to a certain degree of bending and stretching, reducing breakage or deformation caused by excessive hardness and extending the service life. In each embodiment, the Shore hardness shows an increasing trend with the content of the conductive filler. This is because conductive nanomaterials (such as CNTs, MXene, graphene, etc.) generally have high rigidity and hardness. When these rigid materials are added to the polyurethane matrix, the hardness of the overall material will increase because these fillers act as reinforcing agents in polyurethane, simultaneously causing cross - linking between polyurethane chain segments or increasing the cross - link density of the polyurethane matrix. The network structure of polyurethane becomes more robust, resulting in an increase in material hardness, which helps to improve the mechanical strength and hardness of the material.

[0091] Unless otherwise specified, the raw materials and equipment used in the present invention are common raw materials and equipment in the art; unless otherwise specified, the methods used in the present invention are conventional methods in the art.

[0092] The above - mentioned are only the preferred embodiments of the present invention and do not impose any limitations on the present invention. Any simple modifications, changes, and equivalent transformations made to the above - mentioned embodiments based on the technical essence of the present invention still fall within the protection scope of the technical solution of the present invention.

Claims

1. An absorbent electromagnetic shielding solvent-free polyurethane microfiber leather, characterized in that: It includes a microfiber leather, and a conductive metal layer as a high-conductive shielding layer and a solvent-free polyurethane low-conductive layer as a low-conductive impedance matching layer respectively provided on two opposite surfaces of the microfiber leather; The solvent-free polyurethane low-conductive layer includes a solvent-free polyurethane substrate and conductive nano-fillers dispersed therein; the addition amount of the conductive nano-fillers is 1-6 wt% of the solvent-free polyurethane substrate.

2. The preparation method according to claim 1, characterized in that, The conductive nano-fillers are one or more of graphene, expanded graphite, carbon nanotubes and titanium carbide.

3. The preparation method according to claim 1 or 2, characterized in that, The thickness of the solvent-free polyurethane low-conductive layer is 0.5-3.0 mm.

4. The preparation method according to claim 1, characterized in that, The electromagnetic shielding performance of the conductive metal layer is 20-100 dB; The thickness of the conductive metal layer is 5-30 μm.

5. The preparation method according to claim 1 or 4, characterized in that, The metal of the conductive metal layer is one or more of nickel, silver, copper and gold.

6. The preparation method according to claim 1 or 5, characterized in that The conductive metal layer is deposited on one side surface of the microfiber leather by magnetron sputtering; The solvent-free polyurethane low-conductive layer is formed on the other side surface of the microfiber leather by the method of coating and drying.

7. A preparation method of the absorbent electromagnetic shielding solvent-free polyurethane superfine leather as described in any one of claims 1-6, characterized in that, It includes the following steps: S1. Deposit a conductive metal layer on one side of the microfiber leather by magnetron sputtering; the working gas is argon, the argon pressure is 1-5 Pa, the magnetic field strength is 30-150 gauss, the power of magnetron sputtering is 80-300 W, and the sputtering time is 5-60 min; S2. Use polyol, isocyanate, 1,4-butanediol, catalyst and conductive carbon nanotubes as raw materials, set the free -NCO content to 20-50%, the R value to 0.8-1.5, and the hardness HS ratio to 20-50%, mix and react, transfer the product to a plane, scrape and coat to form a film to obtain an undried solvent-free polyurethane low-conductive layer, and coat it on the other side surface of the microfiber leather and dry it to obtain an absorption-type electromagnetic shielding solvent-free polyurethane microfiber leather.

8. The preparation method according to claim 7, wherein In S2, the polyol is selected from one or more of castor oil, polytetrahydrofuran ether diol, polycarbonate diol and polycaprolactone diol.

9. The preparation method according to claim 7, characterized in that, In S2, the isocyanate is selected from one or more of isophorone diisocyanate, hexamethylene diisocyanate, 4,4'-diisocyanate dicyclohexylmethane and diphenylmethane diisocyanate.

10. The preparation method according to claim 7, characterized in that, In S2, the temperature of the mixing reaction is 30-100 °C, and the reaction time is 0.5-3 hours.

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