Dopo modified pmia nanofiber insulation film and preparation method thereof

Abstract

The application discloses a DOPO modified PMIA nanofiber insulation film and a preparation method thereof, and belongs to the technical field of polymer insulation materials and energy storage battery safety protection. The preparation method comprises the following steps: dissolving PMIA in a DMAc / LiCl mixed solvent system, adding a DOPO flame retardant, stirring and standing to remove bubbles to obtain a uniform spinning solution; adopting an electrostatic spinning process to prepare a DOPO / PMIA nanofiber film; removing residual solvent and ions through deionized water immersion and drying treatment; and then performing heat pressing densification treatment to make the fiber network plastically flow and rearrange the structure, so that a structure-dense insulation film is obtained. Through the molecular-level uniform dispersion of DOPO, efficient flame retardation is realized, the hydrogen bond reconstruction and heat pressing densification are cooperated to reduce the porosity and eliminate the local electric field concentration, the synergistic improvement of the flame retardation performance, the electrical insulation performance and the mechanical performance is realized under a low DOPO addition amount. The obtained insulation film is suitable for the fields of energy storage battery cell outer insulation and power equipment insulation protection.

Description

Technical Field

[0001] This invention belongs to the field of polymer insulating materials and energy storage battery safety protection technology, and particularly relates to a DOPO modified PMIA nanofiber insulating film and its preparation method. Background Technology

[0002] As energy storage battery systems rapidly develop towards higher energy density and greater integration, the energy density of individual cells continues to increase, posing increasingly severe challenges to thermal management and safety protection within battery modules. Against this backdrop, insulating materials used in cell insulation systems not only need excellent electrical insulation and heat resistance but also good flame retardant properties to effectively suppress flame spread and heat diffusion after thermal runaway of a single cell, reducing the risk of thermal runaway propagation throughout the entire energy storage system. Meta-aramid (PMIA), due to the strong hydrogen bonding between its molecular chains, exhibits excellent heat resistance, mechanical strength, and electrical insulation properties, and has been widely used in the electrical insulation field, becoming an important matrix material for producing high-performance insulating paper and films. However, traditional PMIA materials still have significant room for improvement in flame retardant performance, and they struggle to provide sufficient fire protection when facing continuous open flames or high heat flux environments. Furthermore, PMIA nanofiber membranes prepared using processes such as electrospinning often exhibit a loosely packed structure with high porosity. While this structure is advantageous for air permeability requirements in certain specific applications, for electrical insulation applications, a large number of pores can form low-density defect areas, which are prone to local electric field concentration and distortion under the influence of an electric field. This leads to a decrease in the breakdown strength of the material and affects its mechanical properties, making it difficult to reliably integrate flame-retardant functions while maintaining high insulation reliability.

[0003] 9,10-Dihydro-9-oxa-10-phosphonaphenanthrene-10-oxide (DOPO) is a highly efficient halogen-free phosphorus-containing flame retardant. Its biphenyl ring and phosphorus-containing heterocyclic structure endow it with excellent thermal stability and char-promoting ability. During heating or combustion, DOPO can release phosphorus-containing free radicals to capture highly reactive free radicals in the gas phase, terminating the combustion chain reaction and exerting a gas-phase flame-retardant effect. Simultaneously, its decomposition products can promote the rapid dehydration and char formation of the polymer matrix in the condensed phase, forming a dense char layer barrier that isolates heat and oxygen transfer, thus exhibiting a synergistic flame-retardant effect in both the gas and condensed phases. Currently, research on the application of DOPO mainly focuses on its introduction into thermosetting or thermoplastic resin systems such as epoxy resins and polyesters, preparing flame-retardant resin materials through chemical reactions or physical blending. However, research on the preparation technology of DOPO-modified PMIA nanofiber insulating films, especially how to achieve synergistic improvement of flame-retardant performance, electrical insulation performance, and mechanical properties through process design, remains limited, lacking systematic technical solutions and in-depth exploration of its mechanisms of action. Therefore, it is necessary to develop a new PMIA insulating film material that combines excellent flame retardant properties, electrical insulation properties, and mechanical properties to meet the urgent needs of high-end application fields such as external insulation of energy storage cells.

[0004] While traditional meta-aramid insulating films possess good heat resistance and insulation properties, their intrinsic flame retardant performance is insufficient to meet the high flame retardant requirements of external insulation systems for energy storage cells. Especially in the event of thermal runaway, ordinary PMIA films may lose their insulation and mechanical protection functions due to continuous combustion. To improve the flame retardant properties of PMIA materials, researchers have attempted to introduce phosphorus-containing flame retardants such as DOPO. However, existing modification schemes mostly employ direct blending, mechanically dispersing DOPO particles in the PMIA solution or matrix. Due to the significant differences in chemical structure between DOPO and the PMIA matrix, their compatibility is limited. Simple blending processes struggle to achieve uniform dispersion of the flame retardant, leading to DOPO agglomeration within the system, forming micron-sized flame retardant enrichment regions. This uneven dispersion not only reduces flame retardant efficiency, requiring higher addition amounts to achieve the target flame retardant level, but also causes agglomerated particles to become defects in the fiber structure, leading to instability in the spinning jet during electrospinning and affecting fiber continuity and diameter uniformity. Meanwhile, the fiber membranes formed by electrospinning themselves have high porosity and multi-scale pore structures. These pores can cause local electric field distortion under a high-voltage electric field, leading to charge concentration at the pores and significantly reducing the overall breakdown strength and insulation reliability of the material. Therefore, simply introducing flame retardants through physical blending often fails to achieve a balance between flame retardant properties and electrical insulation properties.

[0005] Furthermore, to achieve high flame retardant properties, existing technologies typically require maintaining a high level of DOPO addition. However, high addition levels significantly alter the viscoelasticity and conductivity of the spinning solution, easily leading to problems such as jet breakage, droplet ejection, and widening of fiber diameter distribution during electrospinning, severely impacting fiber quality and the structural stability of the fiber network. More importantly, high levels of flame retardant doping often degrade the dielectric properties of the PMIA matrix, resulting in increased dielectric constant and dielectric loss, further reducing the insulation reliability of the insulating film under high-frequency electric fields. Therefore, under traditional technical approaches, improving flame retardant performance often comes at the expense of insulation performance and fiber quality, making it difficult to achieve a true balance between flame retardant and insulation properties.

[0006] On the other hand, most existing flame-retardant modification methods focus on improving flame retardancy by adjusting the type and amount of flame retardants, with insufficient attention to the control of the microstructure of nanofibers. In electrospinning and subsequent processing, factors such as fiber orientation, packing state, interfacial bonding strength between fibers, and the overall network density have a decisive impact on the material's mechanical properties, thermal stability, and insulation performance. Neglecting microstructure control can easily lead to decreased fiber continuity and insufficient load transfer capacity between fibers, resulting in loss of mechanical properties and reduced thermal stability. Even if a higher limiting oxygen index is achieved by increasing the amount of flame retardant, if the fiber structure is loose and lacks strength, the material may still suffer structural damage and insulation failure under assembly stress and operational vibration. Therefore, focusing solely on flame retardant performance while neglecting the integrity and density of the fiber structure makes it difficult to achieve synergistic optimization of flame retardant, insulation, and mechanical properties.

[0007] In summary, there is currently a lack of a high-performance flame-retardant insulating film suitable for the external insulation system of energy storage cells, capable of achieving efficient flame retardancy and long-term service stability while maintaining excellent insulation and mechanical properties. Existing material systems and preparation methods are insufficient to simultaneously meet the application requirements of energy storage equipment for lightweight, high safety, and high reliability. There is an urgent need to develop new material formulations, structural designs, and preparation processes to achieve an integrated improvement in flame retardancy, insulation, and mechanical properties. Summary of the Invention

[0008] This invention addresses the problems of insufficient flame retardancy of existing meta-aramid insulating films, the high porosity of nanofiber films leading to limited breakdown strength and mechanical properties, and the difficulty in achieving both flame retardancy and electrical insulation properties. It provides a DOPO-modified PMIA nanofiber insulating film and its preparation method. By introducing DOPO flame retardant into the PMIA spinning system and combining it with electrospinning and hot-pressing densification processes, the flame retardancy, electrical insulation, and mechanical properties are synergistically improved.

[0009] To achieve the above objectives, the present invention provides the following technical solution: A method for preparing a DOPO-modified PMIA nanofiber insulating film includes the following steps: (1) dissolving PMIA in a DMAc / LiCl mixed solvent system, adding DOPO, and then stirring and allowing it to stand for degassing to obtain a uniform DOPO / PMIA spinning solution; (2) using an electrospinning process, spraying the spinning solution under a high voltage electric field to form nanofibers and depositing them on the surface of a collector to obtain a DOPO / PMIA nanofiber membrane; (3) soaking the nanofiber membrane in deionized water and then drying it to remove residual solvent and ions; (4) hot-pressing densification treatment on the dried nanofiber membrane to cause plastic flow and structural rearrangement of the fiber network to obtain a dense DOPO-modified PMIA nanofiber insulating film.

[0010] In the above preparation method, the amount of DOPO added in step (1) is 0.5wt% to 12wt% based on the mass of PMIA. Further, the amount of DOPO added based on the mass of PMIA is preferably 3wt% to 10wt%. By controlling the DOPO content within this range, the synergistic flame retardant effect of DOPO in the gas phase and condensed phase can be fully utilized at a lower addition amount, while avoiding the adverse effects of high addition amount on the spinning fiber quality and dielectric properties.

[0011] The electrospinning process parameters in step (2) are as follows: a positive DC voltage of 10kV to 18kV is applied to the needle tip, a negative DC voltage of 4kV to 8kV is applied to the collector end, the distance between the needle tip and the collector is 100mm to 200mm, and the propulsion speed of the spinning solution is 0.02mm / min to 0.06mm / min. By reasonably controlling the spinning parameters within the above process window, a nanofiber membrane with uniform fiber diameter distribution and continuous and stable structure can be obtained.

[0012] In step (3), the deionized water soaking time is 12 to 36 hours; the drying treatment involves first drying at 60℃ to 100℃ for 2 to 6 hours, and then drying at 160℃ to 200℃ for 2 to 6 hours. The water soaking treatment can effectively remove residual Li from the fiber membrane. + and Cl - Plasma simultaneously promotes the reconstruction and restoration of the hydrogen bond structure between PMIA molecules; staged drying effectively removes moisture and residual DMAc solvent, ensuring the stability of the fiber structure before subsequent hot pressing.

[0013] In step (4), the hot-pressing densification process is carried out at a temperature of 260℃ to 300℃, a pressure of 10MPa to 20MPa, and a time of 5min to 15min. During the hot-pressing process, the nanofibers undergo plastic flow and structural rearrangement under the synergistic effect of temperature and pressure. The originally loose and porous fiber network structure collapses and transforms into a continuous and dense film structure. The interfacial bonding between fibers is significantly enhanced, and the porosity is greatly reduced, thereby significantly improving the mechanical properties and electrical insulation properties of the material.

[0014] In step (1), DOPO can be added directly to the DMAc / LiCl solution of PMIA, or DOPO can be dissolved in DMAc first, and then LiCl and PMIA can be added for dissolution. Both methods can achieve uniform molecular-level dispersion of DOPO in the spinning solution system, thus inhibiting phase separation and agglomeration from the source.

[0015] This invention also provides a DOPO-modified PMIA nanofiber insulating film prepared by the above-described method. The insulating film is formed by hot-pressing and densification of composite nanofibers composed of DOPO and PMIA, exhibiting a continuous and dense film structure, with DOPO uniformly dispersed within the PMIA matrix. This insulating film can be applied to fields such as external insulation of energy storage battery cells, insulation protection of power equipment, and insulation protection of electrical equipment.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: By introducing DOPO flame retardant into the PMIA nanofiber system, DOPO promotes dehydration and char formation during combustion and forms a dense char layer in the condensed phase. At the same time, it releases phosphorus-containing free radicals to capture gaseous active free radicals, achieving synergistic flame retardancy between the gas and condensed phases. This significantly improves the limiting oxygen index and flame retardancy rating of the material, reducing the fire risk during the operation of energy storage batteries and electrical equipment.

[0017] A homogeneous, highly polar solution environment was constructed using a DMAc / LiCl mixed solvent system, which allowed PMIA to fully dissolve and form a molecular-level dispersion system. At the same time, DOPO achieved uniform dissolution and stable distribution in this system, suppressing phase separation from the source and solving the problems of poor compatibility and easy aggregation between DOPO and PMIA in traditional blending methods.

[0018] Based on the formation of a continuous and uniform nanofiber network by electrospinning, residual conductive ions are removed by water immersion treatment and the reconstruction of intermolecular hydrogen bonds of PMIA is promoted. Then, hot pressing densification treatment is combined to cause plastic flow and structural rearrangement of the fiber network, transforming the loose and porous nanofiber structure into a continuous and dense film structure. This significantly reduces the internal porosity and local electric field concentration effect, effectively improves the breakdown strength and reduces the conductivity and dielectric loss.

[0019] By optimizing the DOPO addition ratio and adopting a homogeneous solution system, a high flame retardant effect can be achieved with a low addition amount of DOPO, avoiding problems such as sudden changes in system viscosity, unstable spinning jets, and decreased dielectric properties caused by traditional high filler content, thus achieving a good balance between flame retardant and insulation properties.

[0020] Through a triple structural control mechanism of "electrospinning orientation regulation + water treatment hydrogen bond reconstruction + hot pressing densification", the loose stacking of fibers is transformed into a continuous and dense multi-scale network structure, which significantly enhances the load transfer capacity and interfacial bonding strength. While improving flame retardant performance, the mechanical properties and thermal stability of the material are maintained or even improved, achieving synergistic optimization of flame retardancy, insulation and mechanical properties.

[0021] The preparation process is simple and the raw materials are readily available. High-performance insulating films can be prepared by electrospinning combined with hot pressing. It has the advantages of low cost, good repeatability and easy mass production. The prepared insulating films can be widely used in the external insulation of energy storage battery cells, insulation protection of power equipment and other high-performance insulation fields, and have good application prospects and promotion value. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 This is a schematic diagram illustrating the process of preparing and testing the DOPO / PMIA nanofiber insulating film of this invention. Detailed Implementation

[0024] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0025] All raw materials used in this invention are not particularly restricted in their source; they can be purchased from the market or prepared using conventional methods known to those skilled in the art.

[0026] There are no particular restrictions on the purity of any of the raw materials used in this invention. However, this invention preferably uses raw materials of analytical grade or purity commonly used in the field of chemical synthesis.

[0027] The preparation method of the DOPO-modified PMIA nanofiber insulating film of the present invention has the following overall process flow: Figure 1 As shown. Figure 1This is a schematic diagram of the process for preparing and testing the DOPO / PMIA nanofiber insulating film of the present invention. First, the electrospinning solution is prepared: a certain mass of meta-aramid (PMIA) is weighed and dissolved in a mixed solvent system composed of N,N-dimethylacetamide (DMAc) and lithium chloride (LiCl); DOPO flame retardant is weighed according to a set mass fraction and added to the above PMIA solution. The mixture is continuously stirred magnetically at room temperature to ensure that the PMIA is fully dissolved and uniformly mixed with the DOPO, forming a homogeneous and stable spinning solution; after stirring, the solution is allowed to stand for degassing until the system is completely stable and clear to eliminate any undissolved particles and microbubbles that may exist in the solution, thereby obtaining a homogeneous spinning solution suitable for electrospinning. When preparing the spinning solution, the amount of DOPO added can be adjusted from 0.5wt% to 12wt% based on the mass of PMIA, preferably from 3wt% to 10wt%, and more preferably from 5wt% to 8wt%, to obtain a good balance between flame retardant properties, insulation properties, and mechanical properties. The stirring time is generally 3 to 8 hours, preferably around 5 hours, to ensure that all components are fully dissolved and mixed evenly. The settling and degassing time can be appropriately determined according to the solution volume and viscosity, usually 1 to 4 hours, until the system is completely clear and there are no visible bubbles.

[0028] After the spinning solution is prepared, the electrospinning stage begins. The spinning solution is extracted using a syringe and thoroughly defoamed before being installed in the electrospinning propulsion device. A release paper layer is pre-coated onto the surface of the roller collector to facilitate the complete peeling of the nanofiber membrane. During electrospinning, a positive DC high voltage is applied to the needle tip, while a negative DC high voltage is applied to the roller collector end, maintaining an appropriate distance between the needle tip and the roller collector surface. The spinning solution is extruded from the needle tip at a constant propulsion speed. Under the action of the high-voltage electric field, the solution forms a Taylor cone structure at the needle tip. When the electric field force overcomes the surface tension of the solution, a jet is generated. Under the combined action of the electric field force and solvent evaporation, the jet continuously stretches and refines, eventually depositing on the surface of the roller collector to form a continuous and uniform nanofiber network structure, resulting in a PMIA nanofiber membrane. The key process parameters for electrospinning can be selected within the following ranges: the positive high voltage applied to the needle tip is 10kV to 18kV, preferably 13kV to 15kV; the negative high voltage applied to the collector end is 4kV to 8kV, preferably 5kV to 7kV; the distance between the needle tip and the collector is 100mm to 200mm, preferably 140mm to 160mm; and the propulsion speed of the spinning solution is 0.02mm / min to 0.06mm / min, preferably 0.03mm / min to 0.05mm / min. By reasonably controlling the spinning parameters within the above process window, nanofiber membranes with uniform fiber diameter distribution and stable structure can be obtained.

[0029] After electrospinning, the resulting aramid nanofiber membrane is completely peeled off from the release paper and proceeds to the post-treatment stage. First, the fiber membrane is subjected to a deionized water immersion treatment, where it is soaked in deionized water at room temperature for 12 to 36 hours, preferably 24 hours. The main purpose of this water immersion treatment is to thoroughly remove residual Li from the fiber membrane. + With Cl - Plasma; simultaneously, water molecules can penetrate into the spaces between PMIA molecular chains, promoting the reconstruction and restoration of the partially damaged intermolecular hydrogen bonds during dissolution, thereby achieving initial stabilization of the fiber structure. After water immersion treatment, the wet film is placed in a vacuum drying oven for drying. The drying process can be carried out using a staged heating method: first, drying at 60℃ to 100℃ for 2 to 6 hours to remove most of the free water in the film; then further drying at 160℃ to 200℃ for 2 to 6 hours to completely remove residual DMAc solvent.

[0030] After drying, the fibrous membrane undergoes hot-pressing densification, a key step in achieving structural densification and improved overall performance. The dried nanofiber membrane is placed in a hot-pressing device and subjected to hot-pressing under specific temperature and pressure conditions. The temperature, pressure, and time of the hot-pressing process can be selected within the following ranges: hot-pressing temperature: 260℃ to 300℃, preferably 270℃ to 290℃; hot-pressing pressure: 10MPa to 20MPa, preferably 13MPa to 17MPa; hot-pressing time: 5min to 15min, preferably 8min to 12min. During the hot-pressing process, the nanofibers undergo plastic flow, rearrangement, and fusion under the synergistic effect of temperature and pressure. The originally loose and porous nanofiber network structure collapses and transforms into a continuous and dense film structure, significantly enhancing the interfacial bonding between fibers and drastically reducing porosity. After hot-pressing densification, a dense and high-performance DOPO-modified PMIA nanofiber insulating membrane is finally obtained.

[0031] Through the complete process route described above, namely "homogeneous preparation of electrospinning solution → electrospinning web formation → water treatment hydrogen bond reconstruction → hot pressing densification", this invention achieves molecular-level uniform dispersion of flame-retardant components in PMIA matrix and controllable transformation of nanofiber membrane from loose porous structure to continuous dense film structure. Thus, it synergistically improves flame-retardant performance, electrical insulation performance, thermal stability and mechanical properties at both the material composition and microstructure levels.

[0032] Example 1 The DOPO-modified PMIA nanofiber insulating film was prepared according to the following steps: (1) Preparation of spinning solution: Weigh a certain mass of PMIA and dissolve it in a DMAc / LiCl mixed solvent system to form a homogeneous solution. In this embodiment, no DOPO flame retardant is added (i.e., the DOPO mass fraction is 0 wt%). Stir magnetically for 5 hours at room temperature, and then let stand to degas until the solution is completely stable and clear to obtain a homogeneous pure PMIA spinning solution.

[0033] (2) Electrospinning: The above spinning solution is drawn into the electrospinning propulsion device by a syringe and fully defoamed. A layer of release paper is pre-covered on the surface of the roller collector. The spinning process parameters are set as follows: a positive DC voltage of 14kV is applied to the needle tip, a negative DC voltage of 6kV is applied to the roller collector end, the distance between the needle tip and the roller collector is 150mm, and the propulsion speed of the spinning solution is 0.04mm / min. Under the action of the high voltage electric field, the solution is ejected from the needle tip to form a jet, which is stretched, refined, and evaporated by the solvent before being deposited on the surface of the collector to form a PMIA nanofiber membrane.

[0034] (3) Post-treatment: The obtained nanofiber membrane was completely peeled off from the release paper and soaked in deionized water at room temperature for 24 hours. Then the wet membrane was placed in a vacuum drying oven and dried at 80°C for 4 hours, and then dried at 180°C for 4 hours.

[0035] (4) Hot pressing densification: The dried fiber membrane was hot pressed at 280°C and 15MPa for 10 minutes to obtain a pure PMIA nanofiber insulating membrane.

[0036] The insulating film obtained in this embodiment is designated as Sample 1.

[0037] Example 2 The difference between this embodiment and Embodiment 1 is that in step (1) of preparing the spinning solution, DOPO flame retardant is added to the PMIA solution at a ratio of 3wt% DOPO mass fraction. The remaining steps and process parameters are exactly the same as in Embodiment 1. The resulting insulating film is designated as Sample 2.

[0038] Example 3 The difference between this embodiment and Embodiment 1 is that in step (1) of preparing the spinning solution, DOPO flame retardant is added to the PMIA solution at a DOPO mass fraction of 5wt%, while the remaining steps and process parameters are exactly the same as in Embodiment 1. The resulting insulating film is designated as Sample 3.

[0039] Example 4 The difference between this embodiment and Embodiment 1 is that in step (1) of preparing the spinning solution, DOPO flame retardant is added to the PMIA solution at a DOPO mass fraction of 7wt%. The remaining steps and process parameters are exactly the same as in Embodiment 1. The resulting insulating film is designated as Sample 4.

[0040] Example 5 The difference between this embodiment and Embodiment 1 is that in step (1) of preparing the spinning solution, DOPO flame retardant is added to the PMIA solution at a ratio of 10wt% DOPO mass fraction. The remaining steps and process parameters are exactly the same as in Embodiment 1. The resulting insulating film is designated as Sample 5.

[0041] Example 6 The difference between this embodiment and embodiment 4 is that in step (4) hot pressing densification, the hot pressing temperature is adjusted to 260℃, the hot pressing pressure is adjusted to 10MPa, and the hot pressing time is adjusted to 5 minutes. The remaining steps and process parameters are exactly the same as in embodiment 4.

[0042] Example 7 The difference between this embodiment and embodiment 4 is that in step (4) hot pressing densification, the hot pressing temperature is adjusted to 300℃, the hot pressing pressure is adjusted to 20MPa, and the hot pressing time is adjusted to 15 minutes. The remaining steps and process parameters are exactly the same as in embodiment 4.

[0043] Example 8 The difference between this embodiment and embodiment 4 is that in step (2) electrospinning, the DC positive high voltage applied to the needle tip is adjusted to 18kV, and the distance between the needle tip and the roller collector is adjusted to 100mm. The remaining steps and process parameters are exactly the same as in embodiment 4.

[0044] Example 9 The difference between this embodiment and embodiment 4 is that in step (1) of preparing the spinning solution, the order of adding materials is adjusted so that the DOPO flame retardant is first dissolved in DMAc solvent, and after thorough stirring and dissolution, LiCl and PMIA are added. The other operating conditions are the same. After magnetic stirring for 5 hours and standing to remove bubbles, a uniform spinning solution is obtained. The subsequent steps and process parameters are exactly the same as in embodiment 4.

[0045] Comparative Example 1 The difference between this comparative example and Example 4 is that step (4) hot-pressing densification is omitted. That is, after the post-treatment (water soaking and drying) of the nanofiber membrane with a DOPO mass fraction of 7wt%, the hot-pressing treatment is not performed, and it is directly used as the final sample for performance testing. The remaining steps and process parameters are exactly the same as those in Example 4.

[0046] Comparative Example 2 The difference between this comparative example and Example 4 is that in step (1) of preparing the spinning solution, the DMAc / LiCl mixed solvent system is not used. Instead, pure DMAc solvent (without adding LiCl) is used to dissolve PMIA. Then, DOPO powder is directly added to the above solution by physical blending. After magnetic stirring for 5 hours, the dispersion state of DOPO is significantly different from the homogeneous molecular-level dispersion of Example 4. The remaining steps and process parameters are exactly the same as those in Example 4.

[0047] Comparative Example 3 The difference between this comparative example and Example 4 is that: in step (1), the spinning solution is prepared using the same DMAc / LiCl homogeneous solvent system as in Example 4 (i.e., DOPO is uniformly dispersed at the molecular level), but step (4) hot pressing densification treatment is omitted. That is, the DOPO mass fraction is 7wt%, and after homogeneous solution spinning, water soaking and drying, hot pressing is not performed, and the test is performed directly.

[0048] Comparative Example 4 The difference between this comparative example and Example 1 is that in step (1) of preparing the spinning solution, the amount of DOPO added is adjusted to 15 wt% based on the mass of PMIA, that is, the DOPO mass fraction exceeds the upper limit of the preferred range of this invention. The remaining steps and process parameters are exactly the same as in Example 1.

[0049] Comparative Example 5 This comparative example uses a commercially available meta-aramid film prepared by the conventional casting method as a control sample. This sample was not electrospun into a web, does not contain DOPO flame retardant components, and has not undergone the water immersion and hot-pressing densification treatments described in this invention.

[0050] Comparative Example 6 The difference between this comparative example and Example 4 is that the deionized water soaking treatment in step (3) is omitted. That is, the nanofiber membrane with a DOPO mass fraction of 7wt% obtained by electrospinning is directly put into the vacuum drying step after being peeled off from the release paper, and then subjected to hot pressing densification treatment under the same conditions. The remaining steps and process parameters are exactly the same as those in Example 4.

[0051] Test case The insulating film samples prepared in Examples 1 to 9 and Comparative Examples 1 to 6 were subjected to the following performance tests: (1) Surface roughness: The surface roughness (arithmetic mean roughness Ra value) of the film is measured by atomic force microscope or optical profilometer, which reflects the smoothness and density of the film surface.

[0052] (2) Conductivity: The volume conductivity of the sample is measured under DC conditions using a high resistance meter or dielectric spectrometer to characterize the insulation level of the material.

[0053] (3) DC breakdown strength: In accordance with relevant electrical insulation test standards, DC withstand voltage test is performed in transformer oil or air using ball-plate electrodes or column-column electrodes, and the electric field strength at the breakdown point is recorded.

[0054] (4) Dielectric constant: The capacitance of the sample is measured at room temperature and a specific frequency (1kHz) using a precision impedance analyzer or dielectric spectrometer, and the relative dielectric constant is calculated.

[0055] (5) Young's modulus: The tensile stress-strain curve of the sample is measured by a universal testing machine or a nanoindenter. The slope of the initial linear segment is taken as Young's modulus, which reflects the rigidity of the material.

[0056] (6) Limiting oxygen index: The minimum oxygen concentration required for a sample to maintain stable combustion in a nitrogen-oxygen mixed gas flow according to standard methods, expressed as a volume fraction, reflecting the flame retardant performance of the material.

[0057] Table 1 summarizes the main performance test results of samples 1 to 5 of Examples 1-5.

[0058] Table 1. Comparison of the properties of PMIA nanofiber insulating films with different DOPO contents

[0059] As shown in Table 1, the limiting oxygen index of the insulating film continuously increases with the increase of DOPO content, rising from 28% in pure PMIA (Example 1) to 41.5% in Example 5 (10wt% DOPO), indicating that the introduction of DOPO significantly enhances the flame retardant properties of the material. Regarding insulation and mechanical properties, Example 4 (7wt% DOPO) exhibits the best overall performance: its DC breakdown strength reaches 484 kV / mm, an increase of approximately 22.5% compared to the pure PMIA film (395 kV / mm); its Young's modulus reaches 8.64 GPa, an increase of approximately 22.4% compared to the pure PMIA film (7.06 GPa). It is noteworthy that the breakdown strength and Young's modulus of Example 5 (10wt% DOPO) decrease compared to Example 4, reaching 452 kV / mm and 6.86 GPa respectively, indicating that there is an optimal range for DOPO addition; exceeding this range leads to a decrease in fiber structure uniformity and a loss of mechanical properties. Example 4 achieved the best balance of flame retardant properties (limiting oxygen index 38%), insulation properties (breakdown strength 484kV / mm), and mechanical properties (Young's modulus 8.64GPa) with an addition of 7wt% DOPO.

[0060] Table 2 summarizes the performance test results of Examples 4, 6-9, and Comparative Examples 1-6.

[0061] Table 2 Performance comparison of Example 4 with other examples and comparative examples

[0062] The following conclusions can be drawn from Table 2: (1) Effect of hot-pressing densification step: Comparing Example 4 with Comparative Example 1 (without hot pressing) and Comparative Example 3 (homogeneous dispersion but without hot pressing), it can be seen that hot-pressing densification treatment has a decisive effect on improving the breakdown strength and mechanical properties of the insulating film. The breakdown strength of Comparative Example 1 is only 180 kV / mm, and the Young's modulus is only 4.52 GPa, which is about 62.8% and 47.7% lower than that of Example 4 (484 kV / mm and 8.64 GPa, respectively). Although Comparative Example 3 achieved homogeneous dispersion of DOPO (breakdown strength 375 kV / mm, Young's modulus 4.86 GPa), due to the lack of hot pressing, a large number of pores and defects still exist inside the nanofiber membrane, resulting in electrical insulation performance and mechanical properties that are also much lower than those of Example 4. This fully demonstrates that hot-pressing densification is an indispensable step to achieve structural density and performance improvement.

[0063] (2) Effect of homogeneous solvent system: Comparing Example 4 and Comparative Example 2 (conventional physical blending, without LiCl system), Comparative Example 2, due to the uneven dispersion and agglomeration of DOPO in the PMIA matrix, had significantly lower breakdown strength (396 kV / mm) and limiting oxygen index (34.1%) than Example 4, and its conductivity (5.20 × 10⁻⁶) was also lower. -15 S / m was higher than that of Example 4 (2.77 × 10⁻⁶). -15 The S / m ratio reflects the negative impact of DOPO aggregates as defect centers on insulation performance. This indicates that the use of a DMAc / LiCl mixed solvent system to achieve uniform dispersion of DOPO at the molecular level is an important prerequisite for achieving high-efficiency flame retardancy and excellent insulation performance with low addition amount.

[0064] (3) Synergistic effect of homogeneous dispersion and hot-press densification: A comprehensive analysis of Comparative Example 3 (homogeneous dispersion but no hot pressing) with Example 4 and Comparative Example 1 reveals that, with only homogeneous dispersion and no hot-press densification (Comparative Example 3), the breakdown strength and Young's modulus remain at a low level; with only hot-press densification but uneven DOPO dispersion (Comparative Example 2's performance is also lower than Example 4). Only by organically combining the two core technologies of "homogeneous dispersion" and "hot-press densification" (Example 4) can the synergistic maximization of flame retardant performance, insulation performance, and mechanical performance be achieved simultaneously.

[0065] (4) Optimization range of DOPO addition amount: Although the limiting oxygen index of Comparative Example 4 (DOPO 15wt%) was further increased to 43.0%, its breakdown strength decreased significantly to 398kV / mm, Young's modulus decreased to 5.21GPa, and electrical conductivity increased to 1.25×10 -14S / m indicates that excessive addition of DOPO will severely degrade insulation and mechanical properties. Based on the patterns observed in Examples 1-5, a good balance between flame retardancy and insulation properties can be achieved with an addition of DOPO in the range of 3wt% to 10wt%, with approximately 7wt% being the optimal addition amount.

[0066] (5) Effect of water immersion treatment: Comparing Example 4 and Comparative Example 6 (without water immersion treatment), the conductivity of Comparative Example 6 (8.90 × 10⁻⁶) was significantly lower. -15 The S / m ratio was significantly higher than that of Example 4 (2.77 × 10⁻⁶). -15 The breakdown strength (327 kV / mm) and Young's modulus (7.45 GPa) were also lower than those of Example 4. This indicates that water immersion treatment is effective in removing residual conductive ions (Li). + Cl - It plays an important role in promoting the reconstruction of intermolecular hydrogen bonds in PMIA and makes a substantial contribution to improving the electrical insulation and mechanical properties of insulating films.

[0067] (6) Comprehensive comparison with traditional products: The performance indicators of Comparative Example 5 (commercially available cast PMIA film) are significantly lower than those of the embodiments of the present invention: the breakdown strength is only 285kV / mm, the Young's modulus is only 4.12GPa, and the limiting oxygen index is only 27.5%. This fully demonstrates that the present invention, through the comprehensive technical solution of "DOPO flame retardant modification + electrospun nanofiber network + hot pressing densification", has achieved a significant improvement over traditional PMIA insulating films in terms of flame retardant performance, insulation performance, and mechanical properties.

[0068] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for preparing a DOPO-modified PMIA nanofiber insulating film, characterized in that, Includes the following steps: (1) Dissolve PMIA in a DMAc / LiCl mixed solvent system, add DOPO, and after stirring and standing to remove bubbles, obtain a uniform DOPO / PMIA spinning solution. (2) Using electrospinning process, the spinning solution is sprayed under the action of high voltage electric field to form nanofibers and deposited on the surface of the collector to obtain DOPO / PMIA nanofiber membrane; (3) The nanofiber membrane is soaked in deionized water and then dried to remove residual solvent and ions; (4) The dried nanofiber membrane is subjected to hot pressing densification treatment to cause plastic flow and structural rearrangement of the fiber network, thereby obtaining a dense DOPO modified PMIA nanofiber insulating membrane.

2. The preparation method according to claim 1, characterized in that, In step (1), the amount of DOPO added is 0.5wt% to 12wt% based on the mass of PMIA.

3. The preparation method according to claim 2, characterized in that, The amount of DOPO added is 3 wt% to 10 wt% based on the mass of PMIA.

4. The preparation method according to claim 1, characterized in that, In step (2), the electrospinning process parameters are as follows: a positive DC voltage of 10kV to 18kV is applied to the needle tip, a negative DC voltage of 4kV to 8kV is applied to the collector end, the distance between the needle tip and the collector is 100mm to 200mm, and the propulsion speed of the spinning solution is 0.02mm / min to 0.06mm / min.

5. The preparation method according to claim 1, characterized in that, In step (3), the soaking time in deionized water is 12 to 36 hours; the drying process is to first dry at 60°C to 100°C for 2 to 6 hours, and then dry at 160°C to 200°C for 2 to 6 hours.

6. The preparation method according to claim 1, characterized in that, In step (4), the temperature of the hot pressing densification treatment is 260℃~300℃, the pressure is 10MPa~20MPa, and the time is 5min~15min.

7. The preparation method according to any one of claims 1 to 6, characterized in that, In step (1), DOPO is added directly to the DMAc / LiCl solution of PMIA, or DOPO is first dissolved in DMAc, and then LiCl and PMIA are added for dissolution.

8. A DOPO-modified PMIA nanofiber insulating film, characterized in that, The insulating film is prepared by any one of claims 1 to 7; the insulating film is formed by hot pressing and densification of composite nanofibers composed of DOPO and PMIA, and has a continuous and dense film structure.

9. The application of the DOPO-modified PMIA nanofiber insulating film according to claim 8 in the external insulation of energy storage battery cells.

10. The application of the DOPO-modified PMIA nanofiber insulating film according to claim 8 in the insulation protection of electrical equipment.

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