Flexible PAN / TiO2 / rubber composite electromagnetic shielding film and its preparation method

Abstract

This invention discloses a method for preparing a flexible PAN / TiO2 / rubber composite electromagnetic shielding film, characterized by the following steps: (1) using PAN and 6% TiO2 as precursors, a casting solution is prepared, and electrospinning is used to spin a 10μm~20μm film; (2) after pre-oxidation at 180℃~200℃ and carbonization at 1200℃~1500℃, a C@TiO2 electromagnetic shielding film is obtained; (3) using chloroprene rubber as a matrix, the C@TiO2 electromagnetic shielding film / rubber sheet is stacked layer by layer through dry lamination hot vulcanization method for 10 alternating times to obtain a flexible PAN / TiO2 / rubber composite electromagnetic shielding film.

Description

Technical Field

[0001] This invention relates to a flexible PAN / TiO2 / rubber composite electromagnetic shielding film and its preparation method, belonging to the field of new material applications. Background Technology

[0002] The increasing severity of electromagnetic pollution poses a threat to the stability of electronic devices and human health, driving the research and development of high-performance electromagnetic shielding materials. Flexible polymer-based electromagnetic shielding materials have become a research hotspot due to their suitability for the trend towards thinner and more flexible electronic devices, offering advantages in both application flexibility and scenario adaptability. However, existing problems such as insufficient shielding effectiveness and a single loss mechanism limit their practical application. PAN-based carbon fiber films possess excellent flexibility and good conductivity potential, while the unique dielectric properties of TiO2 make it a high-quality functional filler for modifying and improving the shielding performance of materials. The combination of these two provides a feasible path for performance optimization. Heterogeneous interface construction is a core means to strengthen the synergistic shielding of electromagnetic waves through multiple mechanisms such as reflection and absorption, effectively overcoming the performance shortcomings of existing materials. This invention focuses on the above-mentioned material and interface regulation, possessing both significant scientific importance and practical application value. It provides new design ideas and technical support for improving the performance of flexible electromagnetic shielding materials, and can also promote their engineering applications in flexible electronics, portable devices, and other fields, helping to solve related practical electromagnetic protection problems. Summary of the Invention

[0003] The purpose of this application is to provide a method for preparing a flexible PAN / TiO2 / rubber composite electromagnetic shielding film, which has excellent electromagnetic shielding performance and multiple electromagnetic wave loss mechanisms, effectively solving the problems of insufficient shielding performance and single electromagnetic wave loss mechanism of existing flexible shielding materials.

[0004] The technical solution of the present invention is as follows: A method for preparing a flexible PAN / TiO2 / rubber composite electromagnetic shielding film includes the following steps: (1) using PAN and 6% TiO2 as precursors, a casting solution is prepared, and electrospinning is used to spin a 10μm~20μm film; (2) after pre-oxidation at 180℃~200℃ and carbonization at 1200℃~1500℃, a C@TiO2 electromagnetic shielding film is obtained; (3) using chloroprene rubber as the matrix, the C@TiO2 electromagnetic shielding film / rubber sheet is stacked layer by layer through dry lamination hot vulcanization method for 10 alternating times to obtain a flexible PAN / TiO2 / rubber composite electromagnetic shielding film.

[0005] The method for preparing the casting solution in step (1) is as follows: PAN and TiO2 are dissolved in DMAc, and the spinning solution is prepared by stirring in an 80℃ coagulation bath. After stirring at a constant temperature for 24 hours, the solution is allowed to stand for 2 hours to remove bubbles, resulting in a white spinning solution with a certain viscosity. The spinning solution is transferred to the syringe of an electrospinning machine, and the machine voltage is adjusted to 25kV, the feed speed to 3ml / h, the receiving distance to 20cm, and the spinning time to 6-12 hours to begin preparing the composite membrane.

[0006] The ratio of PAN to TiO2 is 1:1 to 2:1.5.

[0007] In step (2), the preferred method is to obtain a C@TiO2 electromagnetic shielding film after pre-oxidation at 200°C and carbonization at 1200°C.

[0008] The dry lamination hot vulcanization method described in step (3) is as follows: A multilayer electromagnetic shielding film / chloroprene rubber composite product is prepared using the dry lamination hot vulcanization method. First, the multilayer electromagnetic shielding film is cut to the specified size. The chloroprene rubber sheet is pretreated by ultrasonic cleaning with anhydrous ethanol for 10-15 minutes and vacuum drying at 60℃ for 2 hours to thoroughly remove surface impurities and moisture. Then, the two are tightly laid out according to the designed lamination sequence and placed in a lamination equipment. Dry pressing is performed at room temperature and 8-12MPa pressure for 3-5 minutes to form a pre-formed laminated blank with initially tightly bonded layers. The blank is then transferred to a hot vulcanization equipment and kept at 150-160℃ and 10-15MPa vulcanization pressure for 30-40 minutes to complete the hot vulcanization reaction, achieving interfacial chemical bonding and dense adhesion between the multilayer electromagnetic shielding film and the chloroprene rubber. Finally, after the vulcanized product naturally cools to room temperature, it is demolded, trimmed, and inspected for appearance to obtain the target composite product.

[0009] The flexible PAN / TiO2 / rubber composite electromagnetic shielding film has an EMI SE of 65dB, with absorption being the dominant factor: SEA / SET = 60–75%, followed by reflection: SER / SET = 20–30%, and multiple reflections accounting for >10%.

[0010] Shielding mechanism Absorption-dominant + Multi-interface, multi-reflection: 1. Reflection (SER): Continuous carbon film → High σ; 2. Absorption (SEA): Carbon defects + residual polar groups; 3. Multiple reflection (SEM): PAN / rubber multi-interface laminate.

[0011] The beneficial effects of this invention are: 1. Flexibility and stability: The areal density is significantly low (far lower than that of metal foil and high-filler rubber), and the total thickness of the composite film is about 1.5 mm to 3 mm, which has promising applications in flexible electronics, attachable EMI shielding, and lightweight packaging. 2. Replacing high-filler content with continuous carbon film achieves comparable performance with lower material complexity. It eliminates the need for multiple functional fillers working together and precise filler ratio control, resulting in a more stable process (carbon film performance can be independently adjusted). 3. Continuous carbon film, as a high-efficiency absorption-type EMI shielding unit, uses absorption as the main method and multi-interface synergy as the shielding principle. Attached Figure Description

[0012] Figure 1 Microscopic morphology images of PAN / TiO2 / rubber composite electromagnetic shielding films with different TiO2 contents; Figure 2 The conductivity of PAN / TiO2 / rubber composite electromagnetic shielding films with different TiO2 contents; Figure 3 The electromagnetic shielding effect of PAN / TiO2 / rubber composite electromagnetic shielding films with different TiO2 contents. Detailed Implementation

[0013] This application provides a method for preparing a flexible PAN / TiO2 / rubber composite electromagnetic shielding film, specifically including the following steps: (1) Using PAN and 6% TiO2 as precursors, a casting solution was prepared, and electrospinning was used to spin a 10μm~20μm film. Specifically, PAN and TiO2 can be dissolved in a suitable solvent, such as dimethylacetamide (DMAc), and mixed by stirring to prepare a uniform casting solution. The casting solution can be prepared in various ways, such as mechanical stirring at room temperature or stirring under heating conditions to accelerate dissolution. Subsequently, the prepared casting solution was transferred to an electrospinning device. The electrospinning process can be achieved by adjusting parameters such as voltage, feed speed, and receiving distance. For example, the voltage can be set to 20kV, the feed speed to 2ml / h, the receiving distance to 15cm, and the spinning time to 8h to obtain a film with a specific thickness. In addition to electrospinning, the film can also be prepared by solution casting, blade coating, or dip coating to form the desired film structure.

[0014] (2) After pre-oxidation at 180℃~200℃ and carbonization at 1200℃~1500℃, a C@TiO2 electromagnetic shielding film is obtained. Specifically, the film obtained in step (1) is placed in a pre-oxidation furnace and heat-treated in an oxidizing atmosphere (e.g., air). The pre-oxidation temperature can be set to 190℃, and the holding time is 1 hour. After pre-oxidation, the pre-oxidized film is transferred to a carbonization furnace and carbonized at high temperature in an inert atmosphere (e.g., nitrogen or argon). The carbonization temperature can be set to 1300℃, and the holding time is 2 hours. In addition to the above temperature range, pre-oxidation can also be carried out at a lower temperature such as 150℃, or at a higher temperature such as 220℃. The carbonization process can also use different temperatures, such as 1100℃ or 1600℃, or different heating rates and holding times.

[0015] (3) Using chloroprene rubber as the matrix, C@TiO2 electromagnetic shielding film / rubber sheet is stacked layer by layer in a dry lamination and hot vulcanization process, alternating 10 times, to obtain a flexible PAN / TiO2 / rubber composite electromagnetic shielding film. Specifically, the C@TiO2 electromagnetic shielding film obtained in step (2) is stacked alternately with chloroprene rubber sheet. For example, a layer of chloroprene rubber sheet can be placed first, followed by a layer of C@TiO2 electromagnetic shielding film, and this process can be repeated 10 times to form a multilayer stacked structure. After the stacking is completed, the stacked structure is placed in a lamination device and subjected to dry lamination and hot vulcanization treatment at a certain temperature and pressure. For example, a pressure of 5MPa can be applied at room temperature for pre-pressing, followed by vulcanization at 140℃ and 8MPa for 35 minutes. In addition to chloroprene rubber, other flexible polymer materials can also be selected as the matrix, such as silicone rubber or natural rubber. The lamination method can also be wet lamination, that is, adhesives are used in the lamination process. The number of times the effect is applied can also be adjusted according to actual needs, such as applying it 5 or 15 times.

[0016] Therefore, this application successfully prepared a flexible composite electromagnetic shielding film with multi-layered heterogeneous interfaces by integrating PAN, TiO2, and neoprene rubber materials and combining electrospinning, pre-oxidation carbonization, and lamination hot vulcanization processes. This film not only possesses excellent flexibility, adapting to the trend of thinner and more flexible electronic devices, but also effectively solves the problems of insufficient shielding effectiveness and a single loss mechanism in existing flexible electromagnetic shielding materials by utilizing the conductive loss of carbon materials, the dielectric loss of TiO2, and the multiple reflection losses caused by the multi-layered structure. Thus, it provides a new solution for high-performance flexible electromagnetic protection.

[0017] In some of the above-mentioned solutions of this application, a casting solution is prepared to form a spinning solution for electrospinning to prepare a thin film. However, in this process, the lack of specific operational details may lead to unstable quality of the spinning solution, such as bubbles, uneven viscosity, or uneven component distribution, which affects the uniformity and continuity of spinning, and thus reduces the electromagnetic shielding performance of the final composite film.

[0018] In response, this application further proposes a method for preparing a flexible PAN / TiO2 / rubber composite electromagnetic shielding film. The method for preparing the casting solution in step (1) includes: dissolving PAN and TiO2 in DMAc, stirring in an 80℃ coagulation bath to prepare a spinning solution, and then allowing the spinning solution to stand for 2 hours to remove bubbles, resulting in a white spinning solution with a certain viscosity. The spinning solution is then transferred to the syringe of an electrospinning machine, and the machine voltage is adjusted to 25kV, the feed speed to 3ml / h, the receiving distance to 20cm, and the spinning time to 6-12h to begin preparing the composite film.

[0019] Specifically, PAN (polyacrylonitrile) and TiO2 (titanium dioxide) are dissolved in DMAc (N,N-dimethylacetamide). DMAc acts as a solvent, providing a medium that allows for the complete dissolution of PAN and the uniform dispersion of TiO2. Besides DMAc, other polar aprotic solvents such as DMF (N,N-dimethylformamide) or DMSO (dimethyl sulfoxide) can also be used, as they also possess good PAN-dissolving ability and TiO2-dispersing properties. The dissolution process can be carried out at room temperature, with mechanical stirring or ultrasonic assistance used to accelerate dissolution and dispersion, ensuring that PAN and TiO2 form a homogeneous solution or suspension in DMAc.

[0020] The spinning solution is prepared by stirring in an 80°C coagulation bath. Here, the coagulation bath refers to an environment where stirring is carried out at a specific temperature to prepare the spinning solution. The 80°C temperature helps improve the dissolution rate of PAN and the dispersion efficiency of TiO2, while reducing the solution viscosity, making it easier to mix uniformly. Stirring ensures that PAN and TiO2 are fully mixed in DMAc to form a homogeneous spinning solution. The coagulation bath can be a water bath or oil bath, with a precise temperature control system maintaining a constant temperature of 80°C. Stirring can be done using magnetic or mechanical stirring to ensure thorough mixing. The stirring speed can be adjusted according to the viscosity and component characteristics of the solution; for example, a medium speed (e.g., 200-500 rpm) can be used for a longer stirring time to avoid introducing excessive air bubbles while ensuring uniform mixing.

[0021] After the spinning solution is kept at a constant temperature and stirred for 24 hours, the long-term (24-hour) constant-temperature stirring is a crucial step to ensure the full dissolution of the spinning solution components, uniform dispersion, and full stretching of molecular chains. This helps to eliminate local concentration gradients, improve the homogeneity of the solution, and thus ensure the stability of the subsequent electrospinning process and the quality of the obtained thin film. The constant-temperature stirring can be carried out in a sealed container to prevent solvent volatilization and entry of external impurities, ensuring the component stability of the spinning solution. After the stirring is completed, the uniformity and transparency of the spinning solution can be evaluated by visual inspection, viscosity measurement, or optical microscope observation, etc., to ensure that it meets the spinning requirements.

[0022] Let it stand for 2 hours to remove bubbles. Bubbles may be introduced into the spinning solution during the stirring process. These bubbles may cause fiber breakage, film defects or holes during the electrospinning process, thus affecting the performance of the final electromagnetic shielding film. Letting it stand for 2 hours is to allow the bubbles in the spinning solution to naturally float and escape, improving the purity of the spinning solution. The standing can be carried out at room temperature or slightly elevated temperature (such as 40 - 50 °C). Appropriate temperature increase can reduce the solution viscosity and accelerate the escape of bubbles. In addition to natural standing, auxiliary means such as vacuum degassing or centrifugal degassing can also be used to more efficiently remove the tiny bubbles in the spinning solution.

[0023] A white spinning solution with a certain viscosity is prepared. The color and viscosity of the spinning solution are important indicators of its quality. White usually indicates that the TiO2 particles are uniformly dispersed without obvious agglomeration; "having a certain viscosity" means that the rheological properties of the spinning solution are moderate, which can ensure the stable stretching of droplets during the electrospinning process and will not block the nozzle due to too high viscosity or fail to form continuous fibers due to too low viscosity. The viscosity of the spinning solution can be accurately measured by a rotary viscometer to ensure that it is within the range suitable for electrospinning. For example, it is usually between several hundred and several thousand mPa·s. The color and transparency of the spinning solution can be evaluated by visual observation or ultraviolet-visible spectrophotometer to confirm the dispersion state of TiO2.

[0024] Transfer the spinning solution into the syringe tube supporting the electrospinning machine. This step is to transfer the prepared spinning solution from the preparation container to the liquid supply system of the electrospinning equipment, ensuring that the spinning solution can be stably and continuously delivered to the spinneret. New bubbles or impurities should be avoided during the transfer process. Equipment such as syringes or peristaltic pumps can be used for slow and smooth transfer. The material and size of the syringe tube should match the properties of the spinning solution and the requirements of the electrospinning machine to ensure good compatibility and fluid transmission performance.

[0025] The machine voltage is adjusted to 25kV. The voltage during electrospinning is a key parameter driving the polymer solution to form fine filaments. 25kV provides sufficient electric field strength, allowing the spinning solution droplets to overcome surface tension at the spinneret, forming Taylor cones and being stretched into nanofibers. The voltage can be precisely controlled via the electrospinning machine's built-in high-voltage power supply, ensuring the stability and accuracy of the output voltage. In actual operation, the voltage selection also needs to consider factors such as the viscosity of the spinning solution, surface tension, and ambient humidity, optimizing the spinning effect through small-scale fine adjustments.

[0026] The feed rate of 3 ml / h controls the rate at which the spinning solution flows out of the syringe, directly affecting the formation of the Taylor cone and fiber yield. This 3 ml / h feed rate aims to provide a stable droplet supply, working in conjunction with the electric field strength and receiving distance to obtain uniform fibers. The feed rate is typically precisely controlled using an injection pump or peristaltic pump to ensure a continuous and constant supply of the spinning solution. Optimizing the feed rate requires matching it to the voltage and receiving distance; too fast a rate may result in droplet ejection rather than fiber formation, while too slow a rate may lead to spinning interruption.

[0027] The receiving distance is 20cm, which refers to the distance between the spinneret and the receiving plate. This 20cm distance provides sufficient space and time for the spinning solution to fully stretch, the solvent to evaporate, and the fibers to solidify under the influence of the electric field, thus forming long, thin, and uniform nanofibers. The receiving distance can be precisely set by adjusting the height of the receiving plate or the position of the spinneret, and calibrated using a ruler or laser rangefinder. The choice of receiving distance affects the fiber diameter and morphology; a greater distance generally benefits the full evaporation of the solvent and the refinement of the fibers, but too great a distance may lead to uneven fiber deposition.

[0028] The spinning time is 6-12 hours, which determines the final film thickness and yield. A spinning time of 6-12 hours aims to produce films of moderate thickness (10μm~20μm) to meet the requirements of subsequent pre-oxidation and carbonization processes, and to ensure sufficient electromagnetic shielding performance. The spinning time can be precisely controlled by a timer or automated control system to ensure the continuity and stability of the spinning process. The selection of the spinning time also needs to consider factors such as the target film thickness, the concentration of the spinning solution, and the feed rate, and should be optimized experimentally to achieve the best results.

[0029] Through the above technical solution, this application solves the problem of unstable spinning solution quality by refining the specific steps and parameters for preparing the casting solution. Specifically, by dissolving PAN and TiO2 in DMAc and conducting long-term (24h) constant-temperature stirring in an 80℃ coagulation bath, the full dissolution and uniform dispersion of PAN and TiO2 are ensured, eliminating local concentration differences. Subsequently, by allowing it to stand for 2h to defoam, any microbubbles that may exist in the spinning solution are effectively removed, avoiding spinning defects caused by bubbles. The final white spinning solution with a certain viscosity has significantly improved uniformity and stability, providing high-quality raw materials for the subsequent electrospinning process. In the electrospinning stage, by precisely controlling the machine voltage (25kV), feed speed (3ml / h), receiving distance (20cm), and spinning time (6-12h), the fiber formation and deposition process are synergistically optimized, ensuring the stability of spinning and the uniformity of film thickness. These meticulous operations and parameter controls work together in the preparation of the spinning solution and the spinning process, ensuring the quality and uniformity of the prepared 10μm~20μm films from the source. This lays a solid foundation for the subsequent preparation of high-performance C@TiO2 electromagnetic shielding films and effectively improves the electromagnetic shielding performance of the final flexible PAN / TiO2 / rubber composite electromagnetic shielding film.

[0030] In some of the above-mentioned schemes of this application, a method for preparing casting solution is proposed to dissolve PAN and TiO2 to form spinning solution. However, in this process, the ratio of PAN and TiO2 is not clearly defined, which may lead to unstable viscosity of spinning solution and uneven component distribution, affecting the smooth progress of electrospinning process and the uniformity of film, thereby reducing the performance reliability and consistency of the final composite electromagnetic shielding film.

[0031] To address this, this application further proposes a PAN to TiO2 ratio of 1:1 to 2:1.5. This ratio is designed to optimize the rheological properties and component dispersion uniformity of the spinning solution. Specifically, this ratio range ensures the spinning solution has a suitable viscosity, which is crucial for forming a stable and continuous fiber jet during electrospinning. If the PAN content is relatively low or the TiO2 content is relatively high, the spinning solution viscosity may be too high, resulting in poor flowability and difficulty in forming uniform nanofibers. Conversely, if the PAN content is relatively high or the TiO2 content is relatively low, the spinning solution viscosity may be insufficient, easily leading to droplets or short fibers, affecting the continuity and uniformity of the film. Furthermore, this ratio also helps to achieve uniform dispersion of TiO2 nanoparticles in the PAN matrix. Within the 1:1 to 2:1.5 ratio range, TiO2 can effectively avoid agglomeration, ensuring the formation of composite fibers with uniform composition during spinning, thereby providing a structurally consistent precursor for subsequent pre-oxidation and carbonization steps, ultimately improving the performance stability of the C@TiO2 electromagnetic shielding film.

[0032] By precisely controlling the ratio of PAN to TiO2 within the range of 1:1 to 2:1.5, the problems of unstable viscosity and uneven component distribution in the spinning solution are effectively solved. This optimized ratio allows for precise control of the rheological properties of the spinning solution, ensuring the smooth progress of the electrospinning process and avoiding spinning interruptions or film defects caused by unsuitable viscosity. Simultaneously, TiO2 nanoparticles achieve highly uniform dispersion in the PAN matrix, eliminating local agglomeration and ensuring the compositional consistency and microstructure uniformity of the prepared film. This provides a high-quality precursor for subsequent pre-oxidation and carbonization steps, significantly improving the performance reliability and consistency of the final flexible PAN / TiO2 / rubber composite electromagnetic shielding film, resulting in more stable performance in electromagnetic shielding applications.

[0033] In some of the above-mentioned schemes of this application, it is proposed to obtain C@TiO2 electromagnetic shielding film by pre-oxidation at 180℃~200℃ and carbonization at 1200℃~1500℃ in step (2). However, in this process, a wide temperature range may lead to insufficient structural transformation or performance fluctuation of the film, affecting the stability and optimization of electromagnetic shielding effectiveness.

[0034] In this regard, this application further proposes a preferred method in step (2) to prepare a C@TiO2 electromagnetic shielding film after pre-oxidation at 200°C and carbonization at 1200°C.

[0035] Specifically, the 200℃ pre-oxidation refers to heat-treating the PAN / TiO2 film obtained by electrospinning at a temperature of 200℃. This pre-oxidation process aims to promote the cyclization, dehydrogenation, and oxidative cross-linking reactions of the polyacrylonitrile (PAN) precursor, thereby stabilizing its molecular structure and laying the foundation for subsequent high-temperature carbonization. By precisely controlling the pre-oxidation temperature to 200℃, it is possible to effectively avoid insufficient oxidation due to excessively low temperatures, resulting in unstable film structure, or excessive degradation of PAN due to excessively high temperatures, affecting the performance of the final carbonized product. For example, the film can be placed in a muffle furnace or tube furnace in an air atmosphere or an inert atmosphere (such as nitrogen), heated to 200℃ at a heating rate of 5℃ / min, and held at that temperature for 1-2 hours. Another approach is to use segmented heating, first performing preliminary stabilization treatment at a lower temperature (such as 100-150℃), and then heating to 200℃ for deep pre-oxidation, to more precisely control the degree of oxidation and the structural evolution of the film.

[0036] Furthermore, the 1200℃ carbonization refers to the high-temperature pyrolysis of the pre-oxidized film at 1200℃. The carbonization process aims to transform the pre-oxidized polyacrylonitrile-based film into a conductive carbon material while maintaining the functional properties of the TiO2 component. Choosing 1200℃ as the carbonization temperature enables moderate graphitization of the carbon material, forming a regular carbon framework, thereby significantly improving the material's conductivity and electromagnetic wave absorption capacity. Simultaneously, this temperature effectively avoids potential TiO2 crystal transformation, functional loss, or damage to the overall material structure caused by excessively high temperatures. For example, the pre-oxidized film can be transferred to a high-temperature carbonization furnace (such as a graphite furnace) under an inert atmosphere (such as high-purity nitrogen or argon), heated to 1200℃ at a rate of 10℃ / min, held at that temperature for 1-3 hours, and then allowed to cool naturally. Alternatively, a vacuum carbonization furnace can be used to perform carbonization at 1200℃ in a vacuum environment to further reduce the introduction of impurities and promote the uniformity of the carbonization process, thereby obtaining a C@TiO2 electromagnetic shielding film with superior performance.

[0037] By precisely setting the pre-oxidation temperature to 200℃, the PAN precursor can be fully oxidized and its structure stabilized, avoiding incomplete conversion or excessive degradation caused by a wide temperature range. Simultaneously, setting the carbonization temperature to 1200℃ enables appropriate graphitization of the carbon material, significantly improving its conductivity and electromagnetic wave absorption capacity, and effectively protecting the functional integrity of TiO2, preventing damage from high temperatures. Therefore, this application, by optimizing the pre-oxidation and carbonization temperatures in step (2), solves the problem of insufficient membrane structure conversion or performance fluctuations caused by a wide temperature range in the prior art, thereby preparing a C@TiO2 electromagnetic shielding film with stable structure, excellent conductivity, strong electromagnetic wave absorption capacity, and highly stable performance. This ensures that the final flexible PAN / TiO2 / rubber composite electromagnetic shielding film has superior and stable electromagnetic shielding performance.

[0038] In some of the above-mentioned solutions in this application, a dry lamination hot vulcanization method is proposed to prepare multilayer composite electromagnetic shielding films. However, in the process of its implementation, there may be problems such as poor interfacial bonding, surface impurities affecting the bonding strength, and insufficient density of the composite film due to unoptimized vulcanization process parameters. In this regard, this application further proposes the following dry lamination hot vulcanization method in step (3): A multilayer electromagnetic shielding film / chloroprene rubber composite product is prepared using the dry lamination hot vulcanization method. First, the multilayer electromagnetic shielding film is cut to the specified size. The chloroprene rubber sheet is pretreated by ultrasonic cleaning with anhydrous ethanol for 10-15 minutes and vacuum drying at 60℃ for 2 hours to thoroughly remove surface impurities and moisture. Then, the two are tightly laid out according to the designed lamination sequence and placed in a lamination equipment. Dry pressing is performed at room temperature and 8-12 MPa pressure for 3-5 minutes to form a pre-formed laminated preform with initially tightly bonded layers. The preform is then transferred to a hot vulcanization equipment and kept at 150-160℃ and 10-15 MPa vulcanization pressure for 30-40 minutes to complete the hot vulcanization reaction, achieving interfacial chemical bonding and dense adhesion between the multilayer electromagnetic shielding film and the chloroprene rubber. Finally, after the vulcanized product naturally cools to room temperature, it is demolded, trimmed, and inspected for appearance to obtain the target composite product.

[0039] Specifically, cutting multi-layer electromagnetic shielding films to specified dimensions aims to ensure that the geometry and size of the electromagnetic shielding films meet the specifications of the final composite product, avoiding gaps or misalignments caused by dimensional mismatches during subsequent lamination, and laying the foundation for uniform pressing and tight bonding. For example, high-precision cutting equipment can be used to ensure neat edges and accurate dimensions; or die stamping or CNC cutting tools can be used to adapt to the production needs of different shapes and batches.

[0040] The pretreatment of chloroprene rubber sheets involves ultrasonic cleaning with anhydrous ethanol for 10-15 minutes followed by vacuum drying at 60°C for 2 hours. This pretreatment aims to clean and dry the surface of the chloroprene rubber sheets. This step thoroughly removes oil, dust, mold release agents, and other impurities, as well as adsorbed moisture, from the surface of the chloroprene rubber sheets. This provides a clean and dry substrate for subsequent interfacial bonding with the electromagnetic shielding film, thereby significantly improving the interfacial bonding strength and stability. Besides anhydrous ethanol, isopropanol, acetone, and other organic solvents can also be used for ultrasonic cleaning. These solvents have moderate solubility in rubber materials and good volatility. The temperature and time of vacuum drying can also be fine-tuned according to the specific type and thickness of the chloroprene rubber. For example, drying at 50°C to 70°C for 1.5 to 2.5 hours ensures complete removal of moisture without damaging the material.

[0041] The two materials are then laid out tightly according to the designed stacking sequence, aiming to precisely stack the pre-treated electromagnetic shielding film and neoprene sheet together according to the preset layered structure. This ensures that there are no gaps or misalignments between the layers, forming a uniform stacked structure that provides a good physical contact interface for subsequent pressing and vulcanization. For example, automated robotic arms can be used for precise alignment and laying, improving stacking efficiency and accuracy; or tooling fixtures with positioning pins or guide grooves can be designed to assist manual precise laying.

[0042] The tightly spread material is placed in a laminating device and dry-pressed at room temperature and 8-12 MPa for 3-5 minutes to form a pre-laminated preform with initially bonded layers. This step, without introducing heat, uses mechanical pressure to create physical contact between the layers, expelling air from the interfaces and forming a structurally stable preform. This provides a tight and uniform initial state for subsequent hot vulcanization, preventing delamination or bubbles during high-temperature vulcanization. The laminating device can be a flat vulcanizing machine or a hydraulic press, achieved through precise control of pressure and holding time. The pressure range can also be adjusted according to the material's hardness and thickness; for example, between 7 MPa and 13 MPa, the holding time can be between 2 and 6 minutes to achieve optimal initial bonding.

[0043] The preform is then transferred to a hot vulcanizing machine and held at 150-160℃ and 10-15MPa for 30-40 minutes to complete the hot vulcanization reaction, achieving interfacial chemical bonding and dense adhesion between the multilayer electromagnetic shielding film and the chloroprene rubber. This step, through precise control of temperature, pressure, and time, enables the formation of cross-linked structures between the chloroprene rubber molecular chains, while simultaneously promoting chemical bonding between the chloroprene rubber and the electromagnetic shielding film interface. This results in dense adhesion and curing of the overall structure, endowing the composite material with excellent mechanical properties and long-term stability. The hot vulcanizing machine is typically a flat-plate vulcanizing machine, whose heating and pressurizing systems can precisely control the vulcanization process. The vulcanization temperature and pressure can be optimized according to the chloroprene rubber formulation and the activity of the vulcanizing agent; for example, the temperature can be 145℃-165℃, the pressure 9MPa-16MPa, and the time 25-45 minutes to ensure a complete vulcanization reaction.

[0044] Finally, after the vulcanized product cools naturally to room temperature, it undergoes demolding, trimming, and visual inspection to obtain the target composite product. Natural cooling avoids thermal stress concentration caused by rapid cooling, thus preventing product deformation, cracking, or internal stress, and ensuring the dimensional stability and structural integrity of the composite product. Demolding, trimming, and visual inspection are crucial steps to ensure the final product meets design requirements and quality standards. Natural cooling can be carried out in air or in a controlled cooling environment to ensure a moderate cooling rate. Demolding can be done manually or mechanically, trimming can be done using tools or abrasives, and visual inspection is performed visually or with the aid of optical equipment.

[0045] Through the aforementioned refined dry lamination and hot vulcanization process, this application effectively solves the problems of weak interfacial adhesion and insufficient density in composite films. Cutting to the specified size ensures material dimensional consistency, creating conditions for close contact; the neoprene rubber sheet undergoes ultrasonic cleaning with anhydrous ethanol and vacuum drying pretreatment, thoroughly removing surface impurities and moisture, providing a clean and dry foundation for interfacial bonding and significantly enhancing adhesion strength; close spreading ensures precise alignment of each layer, laying a uniform contact foundation for pressing; dry pressure pressing at room temperature forms a pre-dense laminated preform, avoiding premature vulcanization interference and providing a stable intermediate for subsequent hot vulcanization; subsequently, precise control of temperature, pressure, and time in the hot vulcanization equipment promotes interfacial chemical bonding and dense adhesion between the multilayer electromagnetic shielding film and the neoprene rubber, ensuring the integrity of the composite film and its electromagnetic shielding effectiveness; finally, natural cooling, demolding, trimming, and appearance inspection further guarantee the structural integrity and product quality of the vulcanized product. This optimized interface bonding mechanism enables the formation of a highly dense and firmly bonded composite structure between the C@TiO2 electromagnetic shielding film prepared from PAN and TiO2 and the neoprene rubber matrix. This effectively improves the overall performance of the flexible PAN / TiO2 / rubber composite electromagnetic shielding film, especially in terms of electromagnetic shielding effectiveness, flexibility, and long-term stability. It can more effectively synergize the conductivity and dielectric loss characteristics of the C@TiO2 electromagnetic shielding film with the flexibility of neoprene rubber, thereby achieving a superior electromagnetic shielding effect.

[0046] In some of the solutions described above in this application, methods for preparing flexible PAN / TiO2 / rubber composite electromagnetic shielding films were proposed, aiming to improve electromagnetic shielding effectiveness and optimize loss mechanisms. However, in the actual preparation process, the obtained films may have insufficient shielding effectiveness or unbalanced electromagnetic wave loss mechanisms, resulting in an unsatisfactory ratio of absorption, reflection, and multiple reflections, making it difficult to meet the needs of high-performance applications.

[0047] In response, this application further proposes a flexible PAN / TiO2 / rubber composite electromagnetic shielding film, characterized by an EMI SE of 65dB, with absorption being dominant: SEA / SET = 60–75%, followed by reflection: SER / SET = 20–30%, and multiple reflections accounting for >10%.

[0048] Specifically, EMI SE (Electromagnetic Interference Shielding Effectiveness) is an indicator of a material's ability to block the propagation of electromagnetic waves, usually expressed in decibels (dB). A higher value indicates better shielding effectiveness. In this application, an EMI SE of 65 dB means that the power of the electromagnetic wave is attenuated by at least 10^6.5 times, or more than 3 million times. This indicates that the composite film has excellent electromagnetic shielding performance, effectively isolating and attenuating electromagnetic radiation, protecting internal electronic equipment from interference or preventing internal radiation leakage. Achieving a high EMI SE can be achieved by using a composite structure of highly conductive and high dielectric loss materials to enhance the absorption and reflection capabilities of electromagnetic waves. EMI SE is typically measured using the coaxial transmission line method or the free space method, calculated by comparing the difference in received electromagnetic wave power with and without shielding material.

[0049] Absorption dominance, i.e., SEA / SET = 60–75%, where SEA (Shielding Effectiveness due to Absorption) represents the attenuation effectiveness of electromagnetic waves due to absorption by the material, and SET (Total Shielding Effectiveness) represents the total shielding effectiveness. The absorption mechanism refers to the conversion of electromagnetic energy into heat or other forms of energy through dielectric loss, magnetic loss, or Joule heating after electromagnetic waves enter the material, thus achieving attenuation. Absorption dominance means that most of the electromagnetic wave energy is dissipated within the material, rather than simply reflected, which is crucial for preventing reflected waves from causing new interference to the surrounding environment or equipment. An absorption ratio of 60% to 75% indicates that the composite film performs excellently in electromagnetic wave absorption. Absorption capacity can be enhanced by introducing components with high dielectric or magnetic loss characteristics into the material. The absorption ratio can be calculated by measuring the complex permittivity and complex permeability of the material and combining them with transmission line theory, or by separating the absorption and reflection components through specialized testing methods.

[0050] Reflection is secondary, with SER / SET = 20–30%, where SER (Shielding Effectiveness due to Reflection) represents the attenuation effectiveness of electromagnetic waves reflected from the material surface. The reflection mechanism mainly occurs when electromagnetic waves encounter interfaces with impedance mismatches, where some energy is bounced back. The fact that reflection is secondary, and the proportion is controlled within a reasonable range of 20% to 30%, indicates that the composite film provides necessary reflective shielding while avoiding excessive reflection. An excessively high reflection ratio may cause electromagnetic waves to reflect multiple times within a confined space, forming new sources of interference. This ratio ensures that reflection acts as an auxiliary shielding mechanism, working in conjunction with the absorption mechanism. The reflection behavior of electromagnetic waves at the material interface can be controlled by adjusting the conductivity or dielectric constant of the material surface. The reflection ratio can also be quantitatively analyzed using electromagnetic testing techniques such as the S-parameter method.

[0051] Furthermore, a multiple reflection ratio greater than 10% refers to the process of electromagnetic waves being reflected and scattered multiple times within the material or between multilayer structures. These multiple-reflected electromagnetic waves travel longer paths within the material, thus increasing the chances of absorption and attenuation. A multiple reflection ratio greater than 10% indicates that the internal structure of the composite film can effectively induce multiple scattering and reflection of electromagnetic waves. This helps to further extend the transmission path of electromagnetic waves in the material, thereby enhancing the overall absorption effect, especially when the material thickness is limited, the multiple reflection mechanism can significantly improve shielding effectiveness. Multiple reflection can be enhanced by constructing multilayer heterostructures, introducing fillers with scattering centers, or designing microstructures with specific porosities. The contribution of multiple reflections can be evaluated through theoretical model calculations or experimental data analysis.

[0052] Through the above technical solutions, the flexible PAN / TiO2 / rubber composite electromagnetic shielding film prepared in this application can achieve a high level of electromagnetic shielding effectiveness, namely EMI SE reaching 65dB, thus effectively solving the problem of insufficient shielding effectiveness in existing technologies. This composite film optimizes the electromagnetic wave loss mechanism, with absorption as the dominant factor and an absorption ratio (SEA / SET) between 60% and 75%. This allows most of the electromagnetic energy to be efficiently dissipated within the material, significantly reducing secondary pollution caused by electromagnetic wave reflection and improving the cleanliness of electromagnetic protection. Simultaneously, the reflection mechanism, as a secondary contributor, has its ratio (SER / SET) controlled within a reasonable range of 20% to 30%, providing necessary surface reflection while avoiding excessive reflection that could lead to repeated bouncing of electromagnetic waves in space, creating new interference. Furthermore, the design with a multiple reflection ratio greater than 10% further enhances the scattering and attenuation of electromagnetic waves within the material, extending the effective path of electromagnetic waves and significantly improving the overall shielding effect even with limited material thickness. This synergistic mechanism, characterized by absorption as the primary factor, reflection as a secondary factor, and effective multiple reflections, ensures that the composite film not only possesses excellent overall shielding performance but also achieves balance and high efficiency in electromagnetic wave loss mechanisms. This comprehensively solves the problems of insufficient performance and unbalanced loss mechanisms that may occur during the preparation process, meeting the needs of high-performance electromagnetic shielding applications.

[0053] The following example will provide a more detailed explanation of the above technical solution: In one application scenario, to address the issues of insufficient shielding effectiveness and limited loss mechanisms of existing flexible polymer-based electromagnetic shielding materials, and to meet the electromagnetic protection requirements of flexible electronic devices for thinner and more flexible designs, this technical solution provides a method for preparing a flexible PAN / TiO2 / rubber composite electromagnetic shielding film.

[0054] First, the casting solution was prepared and the film was spun. Polyacrylonitrile (PAN) and 6% titanium dioxide (TiO2) were precisely weighed at a ratio of 1.5:1 and dissolved in dimethylacetamide (DMAc) solvent. The mixture was then placed in an 80°C coagulation bath and stirred to prepare a homogeneous spinning solution. To ensure the uniformity and stability of the spinning solution, it was stirred at a constant temperature for 24 hours and then allowed to stand for 2 hours to defoam, ultimately obtaining a white spinning solution with a specific viscosity. This step, through precise control of the component ratios and preparation conditions, lays the foundation for the subsequent formation of high-performance films. Compared with traditional single polymer solutions, the introduction of TiO2 aims to improve the dielectric loss capability of the material. Next, the prepared spinning solution was transferred to the syringe of an electrospinning machine. By adjusting the machine voltage to 25kV, setting the feed speed to 3ml / h, maintaining the receiving distance at 20cm, and continuously spinning for 8 hours, PAN / TiO2 composite films with thicknesses ranging from 10μm to 20μm were successfully prepared. Electrospinning technology plays a crucial role in the preparation of ultrafine fiber membranes, ensuring the flexibility and uniformity of the films.

[0055] Secondly, the PAN / TiO2 composite film is subjected to carbonization. The PAN / TiO2 composite film prepared above is first pre-oxidized at 200℃. This process helps stabilize the polymer structure and prepares for subsequent high-temperature carbonization. After pre-oxidation, the film is transferred to a high-temperature furnace for carbonization at 1200℃. Through a precisely controlled carbonization process, the PAN matrix is ​​transformed into a conductive carbon framework, while the TiO2 component is retained and uniformly dispersed within the carbon matrix, ultimately yielding a C@TiO2 electromagnetic shielding film. This carbonization process not only endows the material with excellent conductivity but also, through the composite of carbon and TiO2, constructs rich heterogeneous interfaces, significantly enhancing the absorption and reflection capabilities of electromagnetic waves and overcoming the limitations of a single conductive material's electromagnetic shielding mechanism.

[0056] Finally, a flexible composite film was prepared using a dry lamination and hot vulcanization method. Using neoprene rubber as the matrix, C@TiO2 electromagnetic shielding films pre-cut to a specified size were alternately stacked with neoprene rubber sheets. Before stacking, the neoprene rubber sheets underwent ultrasonic cleaning with anhydrous ethanol for 12 minutes and vacuum drying at 60°C for 2 hours to thoroughly remove surface impurities and moisture, ensuring strong adhesion. Subsequently, the C@TiO2 electromagnetic shielding film and the pre-treated neoprene rubber sheets were tightly laid out according to the designed stacking sequence and placed in a lamination device. Dry pressure bonding was performed for 4 minutes at room temperature and 10 MPa to form a pre-bonded laminated preform. This step achieved close physical contact between the layers, creating conditions for subsequent chemical bonding. Next, the preform was transferred to a hot vulcanization device and held at 155°C and 12 MPa for 35 minutes to complete the hot vulcanization reaction. The thermal vulcanization process induces cross-linking of the chloroprene rubber, forming interfacial chemical bonds and dense adhesion between it and the C@TiO2 electromagnetic shielding film, thereby achieving a stable multilayer structure. Compared with traditional adhesive bonding methods, the chemical bonds formed by thermal vulcanization significantly improve the interfacial bonding strength and long-term stability of the composite film, avoiding interfacial delamination problems. The entire stacking process is repeated 10 times to finally obtain a flexible PAN / TiO2 / rubber composite electromagnetic shielding film with a multilayer structure.

[0057] The flexible PAN / TiO2 / rubber composite electromagnetic shielding film obtained by the above preparation method achieves an electromagnetic shielding effectiveness (EMI SE) of 65 dB. Electromagnetic wave absorption loss is dominant, with an absorption-to-total shielding effectiveness ratio (SEA / SET) of 70%, while reflection loss is secondary, with a reflection-to-total shielding effectiveness ratio (SER / SET) of 25%. Multiple reflections account for over 10%. This multi-layered composite structure and heterogeneous interface construction allows electromagnetic waves to undergo multiple reflections, scattering, and absorptions within the material, effectively solving the problem of a single loss mechanism in existing materials. This significantly improves the overall shielding effectiveness, enabling it to effectively cope with complex electromagnetic environments and protect electronic equipment from electromagnetic interference.

[0058] The above description is merely an embodiment of this application and is not intended to limit the scope of protection of this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A method for preparing a flexible PAN / TiO2 / rubber composite electromagnetic shielding film, characterized in that: The process includes the following steps: (1) Using PAN and 6% TiO2 as precursors, a casting solution is prepared and electrospinned into a 10μm~20μm film using electrospinning technology; (2) After pre-oxidation at 180℃~200℃ and carbonization at 1200℃~1500℃, a C@TiO2 electromagnetic shielding film is obtained; (3) Using chloroprene rubber as the matrix, the C@TiO2 electromagnetic shielding film / rubber sheet is stacked layer by layer through dry lamination hot vulcanization method for 10 alternating times to obtain a flexible PAN / TiO2 / rubber composite electromagnetic shielding film.

2. The preparation method according to claim 1, characterized in that: The method for preparing the casting solution described in step (1) is as follows: PAN and TiO2 are dissolved in DMAc, and the spinning solution is prepared by stirring in an 80℃ coagulation bath. After stirring at a constant temperature for 24 hours, the solution is allowed to stand for 2 hours to remove bubbles, resulting in a white spinning solution with a certain viscosity. The spinning solution is then transferred to the syringe of an electrospinning machine. The machine voltage is adjusted to 25kV, the feed speed to 3ml / h, the receiving distance to 20cm, and the spinning time to 6-12 hours to begin preparing the composite membrane.

3. The preparation method according to claim 2, characterized in that: The ratio of PAN to TiO2 is 1:1 to 2:1.

5.

4. The preparation method according to claim 1, characterized in that: In step (2), the preferred method is to pre-oxidize at 200°C and carbonize at 1200°C to obtain a C@TiO2 electromagnetic shielding film.

5. The preparation method according to claim 1, characterized in that: The dry lamination hot vulcanization method described in step (3) is as follows: A multilayer electromagnetic shielding film / chloroprene rubber composite product is prepared using the dry lamination hot vulcanization method. First, the multilayer electromagnetic shielding film is cut to the specified size. The chloroprene rubber sheet is pretreated by ultrasonic cleaning with anhydrous ethanol for 10-15 minutes and vacuum drying at 60℃ for 2 hours to thoroughly remove surface impurities and moisture. Then, the two are tightly laid out according to the designed lamination sequence and placed in a lamination equipment. Dry pressure is maintained at room temperature and 8-12 MPa for 3-5 minutes to form a pre-formed laminated preform with initially tightly bonded layers. The preform is then transferred to a hot vulcanization equipment and kept at 150-160℃ and 10-15 MPa for 30-40 minutes to complete the hot vulcanization reaction, achieving interfacial chemical bonding and dense adhesion between the multilayer electromagnetic shielding film and the chloroprene rubber. Finally, after the vulcanized product naturally cools to room temperature, it is demolded, trimmed, and inspected for appearance to obtain the target composite product.

6. The preparation method according to claim 1, characterized in that: The flexible PAN / TiO2 / rubber composite electromagnetic shielding film has an EMI SE of 65dB, with absorption being dominant (SEA / SET = 60–75%), followed by reflection (SER / SET = 20–30%), and multiple reflections accounting for >10%.

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