Polyimide film preparation method and polyimide film

Abstract

The invention relates to a polyimide film preparation method and a polyimide film. The method comprises the following steps: by taking rigid diamine as a rigid monomer and flexible dianhydride as a flexible monomer, carrying out polycondensation for 6-10 hours at 35-45 DEG C according to a molar ratio of 1: 1 to form a polyamic acid solution with specific solid content, and constructing a'rigid framework-flexible joint 'structure. 2-5wt% of a cross-linking agent and 1-3wt% of a photoinitiator are added, dispersed and spin-coated on a flexible substrate, an initial cross-linked network is formed through pre-drying and ultraviolet irradiation, then imidization is completed through low-temperature thermocuring, and hardness and toughness are both considered. The method realizes low-temperature curing, avoids thermal damage of the substrate, and reduces the cost; product purity and film performance are improved by controlling conditions. According to the polyimide film prepared by the method, the pencil hardness reaches 4H, the friction-resistant cycle index is more than or equal to 40, the bright spot defect rate is less than or equal to 0.01%, and the problems that a traditional polyimide film is not suitable for a flexible substrate during high-temperature curing and the rigid-brittle contradiction is prominent are solved.

Description

Technical Field

[0001] This application relates to the field of display technology, and in particular to a method for preparing a polyimide film and the polyimide film itself. Background Technology

[0002] In display technology, emerging applications such as automotive displays and wearable devices are placing stringent demands on the performance of alignment films. As a key material in display technology, the performance of polyimide alignment films directly impacts the quality and reliability of display devices. However, traditional polyimide alignment films are increasingly facing performance bottlenecks when confronted with these new requirements, becoming a critical factor restricting the further development of display technology.

[0003] Currently, traditional polyimide alignment films have shortcomings in several key performance aspects. Regarding scratch resistance, their hardness is generally low, making them prone to scratches during device assembly when in contact with spacers and other components. This can lead to bright spot defects, which can damage the integrity of the display image and lower product yield. Repairing these defects is particularly costly and has limited effectiveness in high-end display device production. In terms of abrasion resistance, traditional films are easily damaged by friction during daily use or frequent handling, causing liquid crystal molecule alignment disorder, resulting in decreased display contrast, image distortion, and shortened product lifespan. This makes it difficult to meet the long-term reliability requirements of high-reliability devices. Furthermore, while adjusting the formula to increase film hardness and enhance scratch resistance, the film's toughness often decreases, becoming brittle and easily cracked. This makes it unable to withstand repeated bending of flexible substrates, resulting in poor performance in scenarios requiring frequent deformation, such as wearable devices.

[0004] To address these issues, the industry has made some attempts at improvement. Traditional solutions, which introduce rigid monomers to increase film hardness, suffer from excessively rigid molecular chains, leading to a significant increase in film brittleness and easy breakage under external forces. Simultaneously, the intermolecular bonding force is not enhanced, resulting in a decrease in abrasion resistance. Curing requires temperatures ≥250℃, while flexible substrates (such as PI and PET) have limited heat resistance, and high temperatures can cause substrate deformation, making the two incompatible. Insufficient film hardness (pencil hardness is typically below 2H) makes the film susceptible to scratches from spacers during device assembly, creating bright spot defects that directly reduce product yield and damage the integrity of the display image. Furthermore, there is an inherent contradiction between rigidity and flexibility: increasing hardness reduces toughness, while increasing toughness results in insufficient hardness, making it impossible to simultaneously meet the requirements for high hardness and bending resistance.

[0005] In summary, traditional polyimide alignment films have limitations in scratch resistance, abrasion resistance, rigidity-flexibility balance, and compatibility with flexible processes, making it difficult to meet the requirements for high reliability, flexibility, and long lifespan. This has become a bottleneck restricting the diversification and high-end upgrading of liquid crystal display technology. Therefore, developing a novel polyimide film preparation method and related film products that combine high scratch resistance, abrasion resistance, rigidity-flexibility balance, and compatibility with flexible processes has become one of the current industry needs. Summary of the Invention

[0006] The purpose of this application is to solve the problem of low hardness and susceptibility to bright spot defects caused by scratches from spacers in existing polyimide (PI) orientation films. By selecting specific materials and introducing a crosslinking agent, combined with a UV-initiated free radical polymerization mechanism, a high-density crosslinked network is constructed, improving the hardness, abrasion resistance, and stability of the orientation film, and reducing the bright spot defect rate. This objective is achieved through the following technical solution: the polyimide film preparation method of this application includes: Step S100: Add rigid diamine and flexible dianhydride to a solvent and stir to form a polyamic acid solution by polycondensation, wherein the solid content of the polyamic acid solution is in the range of 10wt%-25wt%. Step S200: Add a crosslinking agent to the polyamic acid solution, stir to fully dissolve and disperse the crosslinking agent, and then slowly add a photoinitiator, stirring under light-protected conditions to uniformly disperse the photoinitiator; Step S300: Spin-coat the solution obtained in step S200 onto the substrate and remove the solvent by heating at a temperature of less than 100°C. Step S400: Ultraviolet light irradiation is performed to initiate free radical polymerization and form an initial cross-linked network through the action of ultraviolet light; Step S500: Thermal curing is performed within a temperature range of 150°C to 200°C to form a polyimide film; The rigid diamine is selected from one or more of the following components: 4,4'-diaminodiphenyl ether, 4,4'-diamino-2,2'-dimethylbiphenyl and 2,6-diaminonaphthalene; The flexible dianhydride is selected from one or more of the following components: bisphenol A type diether dianhydride, 1,2,3,4-cyclobutanetetracarboxylic dianhydride, and diethylene glycol bis(triphenyl) anhydride.

[0007] In one embodiment, the molar ratio of rigid diamine to flexible dianhydride is 1:1.

[0008] In one embodiment, the crosslinking agent content is in the range of 2 wt%-5 wt%, and the photoinitiator content is in the range of 1 wt%-3 wt%.

[0009] In one embodiment, the crosslinking agent is selected from one or more of the following components: trimethylolpropane triacrylate, pentaerythritol triacrylate, and ethoxylated trimethylolpropane triacrylate.

[0010] In one embodiment, in step S300, the thickness of the spin-coated film is controlled within the range of 80-100 nm.

[0011] In one embodiment, in step S400, the wavelength of the ultraviolet light is in the range of 350nm-400nm, and the energy is 180 mJ / cm². 2 -250mJ / cm 2 Within the range, the irradiation time is between 20s and 40s.

[0012] In one embodiment, in step S500, the thermosetting time is greater than 20 minutes.

[0013] In one embodiment, in step S100, the mixture is stirred continuously for 6-10 hours within a temperature range of 35°C to 45°C.

[0014] In one embodiment, each step is performed under an inert gas protective atmosphere.

[0015] In addition, this application also provides a polyimide film obtained by the aforementioned polyimide film preparation method.

[0016] Compared with the prior art, this application has the following beneficial effects: This application utilizes a polyamic acid with an alternating block structure formed by the polycondensation reaction of rigid diamine and flexible dianhydride. The two work synergistically to form a "rigid skeleton-flexible joint" structure, resolving the prominent contradiction between rigidity and brittleness in traditional polyimide films and achieving a balance between hardness and toughness. This allows the polyimide film to possess sufficient hardness while maintaining good flexibility, enabling it to withstand repeated bending of flexible substrates and perform excellently in scenarios requiring frequent deformation.

[0017] By introducing acrylate as a crosslinking agent, a free radical polymerization reaction is initiated by ultraviolet light. A high-density crosslinked network is constructed before thermosetting, effectively suppressing molecular chain slippage at high temperatures and enhancing intermolecular interactions. Compared to traditional polyimide oriented films, the film prepared in this application exhibits improved pencil hardness, meeting high hardness requirements and significantly reducing the risk of display devices being scratched by spacers during assembly and use. Simultaneously, thanks to the synergistic effect of the dense crosslinked network and the flexibility of the molecular chains, the film's abrasion resistance is greatly optimized.

[0018] This application employs a gradient thermosetting process, first removing the solvent through pre-baking, then raising the temperature for primary curing. It fully utilizes the electronic or steric effects of rigid diamines to help lower the activation energy, and leverages the synergistic self-cyclization activity of the alicyclic structure of flexible dianhydrides. Simultaneously, it incorporates ultraviolet pre-crosslinking technology, avoiding the drawbacks of traditional processes requiring ≥250℃ high-temperature curing. This low-temperature curing process is perfectly adaptable to flexible substrates, effectively avoiding high-temperature damage and expanding its application scenarios in the flexible display field. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of a method for preparing a polyimide film according to one embodiment of this application. Detailed Implementation

[0020] To make the above-mentioned objectives, features, and advantages of this application more apparent and understandable, the specific embodiments of this application will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the scope of this application. Furthermore, it should be noted that, for ease of description, only the parts relevant to this application are shown in the accompanying drawings, not the entire structure. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of this application.

[0021] The terms “comprising” and “having”, and any variations thereof, used in this application are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or apparatus that includes a series of steps or units is not limited to the steps or units listed, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to such process, method, product, or apparatus.

[0022] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0023] In the field of liquid crystal displays, polyimide alignment films, as the functional layer for arranging liquid crystal molecules, directly determine the image quality and reliability of display devices. However, traditional polyimide alignment films suffer from low hardness, making them easily scratched by spacers during display device assembly and use, leading to bright spot defects and affecting display performance. Furthermore, traditional curing processes require high temperatures, making them unsuitable for flexible substrates and limiting their application in flexible displays. This application proposes a novel method for preparing polyimide films, optimizing material selection and process design. By using specific rigid diamines and flexible dianhydride monomers, and introducing a crosslinking agent, combined with UV curing and gradient thermal curing processes, a high-density crosslinked network is first constructed. The specific steps and key points of this application's polyimide film preparation method are then described in detail. Please refer to [link to relevant documentation]. Figure 1 A preferred embodiment of the polyimide film preparation method of this application includes: adding a rigid diamine and a flexible dianhydride to a solvent, stirring and polycondensing to form a polyamic acid solution, wherein the solid content of the polyamic acid solution is in the range of 10wt%-25wt%; then adding a crosslinking agent to the polyamic acid solution, stirring to fully dissolve and disperse the crosslinking agent, then slowly adding a photoinitiator, stirring under light-protected conditions to uniformly disperse the photoinitiator; then spin-coating the solution obtained in the previous step onto a substrate, heating at a temperature below 100°C to remove the solvent; subjecting to ultraviolet light irradiation, initiating free radical polymerization through ultraviolet light to form an initial crosslinked network; and finally thermosetting at a temperature range of 150°C to 200°C to form a polyimide film.

[0024] Rigid diamine monomers can be selected from 4,4'-diaminodiphenyl ether (ODA), 4,4'-diamino-2,2'-dimethylbiphenyl (DMDB), and 2,6-diaminonaphthalene (DAN). Among them, 4,4'-diaminodiphenyl ether (ODA) provides basic rigidity support due to the π-π stacking of benzene rings; 4,4'-diamino-2,2'-dimethylbiphenyl (DMDB), due to the presence of methyl side chains, can enhance the hydrophobicity of the film layer and adapt to humid environments (such as automotive display scenarios); 2,6-diaminonaphthalene (DAN) has even stronger naphthalene ring rigidity and can meet the requirements of ultra-high hardness (such as outdoor display devices).

[0025] Flexible dianhydrides can be selected from bisphenol A diether dianhydride (BPADA), 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA), and diethylene glycol dipreptyltriacyl anhydride (DEGDAn). The alicyclic backbone and isopropyl bridges of BPADA provide good chain flexibility; the more compact alicyclic structure of CBDA further enhances film flexibility, making it suitable for wearable devices and other applications requiring frequent deformation; and the ether-bonded flexible chain of DEGDAn is well-suited for high-frequency touch applications.

[0026] In the specific implementation process, the materials are first prepared. ODA, DMDB, and DAN are used as rigid monomers, BPADA, CBDA, and DEGDAn are used as flexible monomers, TMPTA, PETA, and EO-TMPTA are used as crosslinking agents, N-methylpyrrolidone (NMP, analytical grade, anhydrous grade) is used as a solvent, and Irgacure-2959 (CAS106797-53-9, BASF) is used as a photoinitiator.

[0027] Next, solution synthesis is performed to synthesize a mixed solution containing the above components. This mixed solution includes a solid substance formed by the condensation polymerization of a rigid diamine and a flexible dianhydride, with the solid content controlled within the range of 10wt%-25wt%. A specific example is as follows: Under nitrogen protection, ODA and BPADA are added to NMP at a 1:1 molar ratio, and the mixture is stirred at 40°C for 8 hours to form a polyamic acid solution with a solid content of 15wt%. A crosslinking agent and a photoinitiator are then added to the polyamic acid solution, and each is thoroughly dissolved and dispersed. Crosslinking agents such as trimethylolpropane triacrylate (TMPTA), pentaerythritol triacrylate (PETA), and ethoxylated trimethylolpropane triacrylate (EO-TMPTA) can be used. Trimethylolpropane triacrylate (TMPTA) can form a high-density cross-linked network through UV light initiation; pentaerythritol triacrylate (PETA), as a tetrafunctional cross-linking agent, can enhance network density; ethoxylated trimethylolpropane triacrylate (EO-TMPTA), containing ethoxy flexible chains, is more suitable for ultra-flexible substrate applications. A specific example is as follows: Place a polyamic acid solution on a magnetic stirrer, first add 3.5 wt% TMPTA (trimethylolpropane triacrylate), and stir continuously for 15 minutes to ensure the cross-linking agent is fully dissolved and dispersed. Then slowly add 3 wt% Irgacure-2959 (photoinitiator), and continue stirring for 30 minutes under light-protected conditions to ensure uniform dispersion of the photoinitiator and avoid premature decomposition.

[0028] After obtaining the solution, it can be directly spin-coated onto a flexible substrate. For example, the solution can be spin-coated onto a PI substrate with a film thickness controlled in the range of 80-100 nm.

[0029] Next is another important aspect of the method, namely the curing process. In this step, excess solvent needs to be removed first. Specifically, a pre-baking method can be used, such as baking at 80°C for 3 minutes to completely remove residual NMP solvent and form a bubble-free wet film. After forming the bubble-free wet film, UV pre-crosslinking is performed. Specifically, this can be done by irradiating with ultraviolet light at a wavelength of 365nm and an energy of 220mJ / cm² for 30 seconds to form an initial crosslinking network. Next, the substrate (with the UV-cured film on it) is placed in an oven and cured at 180°C for 30 minutes to complete imidization.

[0030] In this application, a balance between hardness and toughness is first achieved through component selection. At the molecular structure design level, a unique "rigid skeleton-flexible joint" structure is constructed by utilizing the rigid benzene ring structure of the rigid diamine and the flexible segment characteristics of the flexible dianhydride molecule. The rigid benzene rings are interconnected to form a stable skeleton, providing the necessary hardness support for the film, enabling it to withstand a certain amount of external pressure without significant deformation. The flexible segments act as buffers and connections between the rigid skeletons, giving the film good flexibility and impact resistance. This synergistic design of rigidity and flexibility allows the film to maintain good toughness while possessing high hardness, effectively avoiding the problem of brittle fracture caused by excessive hardness. In addition, a crosslinking agent is introduced. Under specific process conditions, multiple acrylate groups in TMPTA, PETA, and EO-TMPTA molecules can undergo free radical polymerization to form a high-density three-dimensional crosslinked network. The high-density crosslinked network tightly binds the polyimide molecular chains together, greatly enhancing the interaction force and cohesive energy between the molecular chains. On the one hand, the reinforcing effect of the cross-linking network effectively improves the hardness of the film, enabling it to resist scratches and wear from external objects and improve the friction resistance of the film. On the other hand, the presence of the cross-linking network also restricts the movement of molecular chains, reduces the plastic deformation of the film under stress, further enhances the mechanical strength and dimensional stability of the film, thereby comprehensively improving the overall performance of the film.

[0031] Regarding the curing process, the preparation method of this application employs a curing temperature below 200℃. Compared to the high-temperature curing conditions of ≥250℃ in traditional processes, the lower curing temperature effectively avoids problems such as thermal shrinkage, deformation, or even decomposition of flexible substrates (e.g., polyimide (PI), polyethylene terephthalate (PET)) at high temperatures, ensuring the dimensional stability and structural integrity of the flexible substrate. Simultaneously, the low-temperature curing process also reduces energy consumption and production costs. A gradient thermosetting process of "80℃ pre-baking for 3 minutes + 180℃ curing for 30 minutes" is adopted, suitable for flexible substrates such as PI and PET, avoiding high-temperature damage.

[0032] In the preparation of polyimide films, 4,4'-diaminodiphenyl ether (ODA) and bisphenol A type diether dianhydride (BPADA) can be selected, and the feeding ratio of 4,4'-diaminodiphenyl ether (ODA) and bisphenol A type diether dianhydride (BPADA) is precisely controlled, with the molar ratio strictly set at 1:1. In the reaction system, ODA, as a diamine monomer, contains two amino groups (-NH2) in its molecular structure, which have high reactivity and can undergo a stepwise polycondensation reaction with the two anhydride groups (-CO-O-CO-) in the BPADA molecule. When ODA and BPADA are reacted in a 1:1 molar ratio, they react fully, ensuring that each anhydride group binds to an amino group, and each amino group participates in the reaction. This maximizes the formation of a well-ordered polyamic acid prepolymer structure, resulting in a more regular and ordered arrangement of repeating units in the polyamic acid molecular chain. This well-ordered molecular structure facilitates intramolecular and intermolecular dehydration cyclization reactions during subsequent imidization, forming uniformly structured polyimide molecular chains. The more compact and ordered arrangement of these well-ordered polyimide molecular chains in the film reduces defects and gaps between molecular chains, thereby improving the film's density and uniformity. This contributes to enhancing the film's mechanical properties, such as tensile strength and elongation at break. Simultaneously, it avoids side reactions caused by monomer imbalances.

[0033] In summary, the low-temperature curing mechanism of this application is mainly achieved through the synergistic effect of molecular structure design and process, with the core being the reduction of the activation energy of the imidization reaction and the stabilization of the molecular structure through pre-crosslinking. Traditional polyimides require curing at temperatures ≥250℃ because the activation energy of the imidization reaction (-COOH and -NH2 cyclization dehydration) is relatively high, requiring high temperatures to provide sufficient energy to drive the reaction to completion. This application reduces this activation energy through the structural synergy of rigid diamines and flexible dianhydrides: flexible dianhydrides (such as BPADA, CBDA, DEGDAn, etc.) generally contain alicyclic structures (such as the cyclohexane skeleton of BPADA and the cyclobutane structure of CBDA). These structures have low steric hindrance, and when reacting with the amino groups of rigid diamines, they can reduce the steric repulsion between molecules, making the cyclization dehydration reaction easier to proceed, thereby reducing the activation energy required for the reaction.

[0034] Meanwhile, the functional groups contained in rigid diamines (such as ODA, DMDB, DAN, etc.) further promote low-temperature cyclization: taking ODA as an example, the ether bond (-O-) in its molecule has a weak electron-donating effect, which can enhance the formation of intramolecular hydrogen bonds in the amic acid chain segment, accelerate the cyclization and dehydration process, and ensure that the degree of imidization can still reach 98% at 180℃; other rigid diamines (such as the methyl side chain of DMDB and the naphthalene ring of DAN) mainly regulate the hydrophobicity or hardness of the film layer, but the reaction mechanism of the amine group and the flexible dianhydride cycloaliphatic structure is consistent, and they can all help reduce the activation energy through similar electronic or steric effects.

[0035] In addition, the initial cross-linking network formed by the UV pre-cross-linking process (365nm ultraviolet light, 220mJ / cm²) plays a key auxiliary role: this network can fix the molecular chain morphology and avoid the structural loosening caused by excessive slippage of molecular chains due to thermal motion at low temperatures. When curing at 180℃, the molecular chains can complete imidization in an ordered state, and finally form a dense and complete structure. It can achieve full curing without high temperature driving, which is suitable for the heat resistance requirements of flexible substrates such as PI and PET.

[0036] Furthermore, the content of the crosslinking agent is in the range of 2wt%-5wt%, and the content of the photoinitiator is in the range of 1wt%-3wt%. If the content of the crosslinking agent is less than 2wt%, the crosslinking network is too sparse, the connection between the molecular chains is not tight enough, the mechanical strength of the film is insufficient, and it is easy to deform or be damaged under external force; while when the content of the crosslinking agent exceeds 5wt%, the crosslinking network is too dense, the movement of the molecular chains is excessively restricted, the film becomes brittle and hard, the tensile strength and elongation at break decrease, and it loses a certain degree of flexibility and toughness.

[0037] In polyimide systems, acrylates, through their carbon-carbon double bonds, can react with unsaturated groups that may exist in the polyimide prepolymer, or generate free radicals through specific initiation methods, to polymerize on the active sites of the polyimide molecular chains. This forms cross-links between the polyimide molecular chains, constructing a three-dimensional cross-linked network structure. After the acrylate cross-linking agent forms a cross-linked network in the polyimide film, the formation of this network restricts the thermal motion of the polyimide molecular chains. Without cross-linking, the molecular chains will move freely when heated to a certain temperature, causing the film to soften and lose its shape stability. The network formed by the acrylate cross-linking agent fixes the molecular chains within a certain range. Furthermore, when the amount and degree of cross-linking of the acrylate cross-linking agent are properly controlled, the resulting cross-linked network structure is uniform and dense, reducing light scattering and absorption within the film.

[0038] In step S300, the spin-coated film thickness is controlled within the range of 80-100 nm. A suitable film thickness helps to form a uniform and dense molecular structure. At a thickness of 80-100 nm, the polyimide molecular chains can fully align and cross-link, forming strong intermolecular forces and chemical bonds. When subjected to tensile force, these forces and chemical bonds can effectively transfer stress, allowing the molecular chains to collectively bear the external force, thereby improving the tensile strength of the film. Compared to films that are too thin or too thick, films within this thickness range are less prone to breakage during stretching and can withstand greater tensile forces.

[0039] In step S400, ultraviolet light with a wavelength in the range of 350nm-400nm is selected, and the energy is set to 180mJ / cm². 2 -250mJ / cm 2 Within the specified range, the irradiation time is controlled within 20-40 seconds. Ultraviolet light with wavelengths in the 350-400 nm range can be effectively absorbed by the photoinitiator in the polyimide prepolymer. After absorbing ultraviolet light, the photoinitiator transitions from the ground state to the excited state, then decomposes to generate free radicals. These free radicals have high reactivity, initiating polymerization reactions of the unsaturated groups in the prepolymer to form polyimide molecular chains. A suitable wavelength range ensures that the photoinitiator can efficiently absorb light energy and generate a sufficient number of free radicals, thereby promoting the full progress of the polymerization reaction. A suitable energy range provides appropriate energy conditions for the polymerization reaction. The aforementioned irradiation time of 20-40 seconds ensures sufficient time for the reaction to proceed, but avoids excessive cross-linking of the molecular chains or other side reactions due to excessive time.

[0040] To ensure that the polyimide prepolymer can fully and with high quality complete the transformation to a stable polyimide structure, the thermosetting time in step S500 is greater than 20 minutes. During the thermosetting process, the molecular chains gradually align from a disordered state to an ordered state. A thermosetting time of more than 20 minutes provides sufficient time for the rearrangement of molecular chains, allowing them to better align along specific directions and form a more regular crystal structure or orientation structure.

[0041] In step S100, continuous stirring within a temperature range of 35℃ to 45℃ for 6-10 hours minimizes the occurrence of side reactions. Within this temperature range, the reaction mainly proceeds along the target reaction pathway, generating the desired product, and effectively avoiding the formation of impurities due to excessively high temperatures. Continuous stirring ensures uniform distribution of substances in the reaction system, avoiding uneven reaction caused by excessively high or low local concentrations. If there are areas of excessively high local concentrations in the reaction system, side reactions or impure products may occur. Stirring ensures uniform mixing of reactants and catalysts in the solution, ensuring synchronous reaction throughout the entire system, reducing the formation of impure products, and thus improving the overall purity of the product.

[0042] Each step is carried out under an inert gas protective atmosphere. Specifically, before the reaction begins, the reaction vessel is thoroughly evacuated to remove as much residual air and other reactive gaseous components as possible. Then, high-purity inert gas, such as nitrogen or argon, is slowly introduced into the vessel to achieve a positive pressure, preventing the re-entry of outside air. In subsequent steps, including material addition, reaction proceeding, product separation, and purification, inert gas is continuously introduced to ensure the reaction system remains completely enveloped in inert gas, forming a closed environment effectively isolated from external oxygen, moisture, and other reactive substances.

[0043] In addition, this application also provides a polyimide film obtained by the aforementioned polyimide film preparation method. Specific Implementation The following will further introduce some specific implementation methods to provide a more detailed explanation of the technical solution of this application.

[0045] Example 1 Under nitrogen protection, ODA and BPADA were added to NMP at a 1:1 molar ratio, and the mixture was stirred at 35°C for 10 hours to form a polyamic acid solution with a solid content of 10 wt%. The polyamic acid solution was placed on a magnetic stirrer, and 2 wt% TMPTA was added first, stirring continuously for 15 minutes to ensure the crosslinking agent was fully dissolved and dispersed. Then, 3 wt% Irgacure-2959 (photoinitiator) was slowly added, and stirring continued for 30 minutes under light-protected conditions.

[0046] The solution was spin-coated onto a PI substrate with a film thickness controlled at 80 nm. The substrate was then baked at 80 °C for 3 minutes to completely remove the residual solvent NMP, forming a bubble-free wet film.

[0047] Irradiation with ultraviolet light (365nm, 220mJ / cm²) for 20 seconds forms an initial cross-linked network.

[0048] The substrate was placed in an oven and cured at 150°C for 45 minutes to complete imidization.

[0049] Example 2 Under nitrogen protection, ODA and CBDA were added to NMP at a 1:1 molar ratio, and the mixture was stirred at 40°C for 8 hours to form a polyamic acid solution with a solid content of 20 wt%. The polyamic acid solution was placed on a magnetic stirrer, and 3 wt% TMPTA was added first, stirring continuously for 20 minutes to ensure the crosslinking agent was fully dissolved and dispersed. Then, 1 wt% Irgacure-2959 (photoinitiator) was slowly added, and stirring continued for 20 minutes under light-protected conditions.

[0050] The solution was spin-coated onto a PI substrate with a film thickness controlled at 100 nm. The substrate was then baked at 90 °C for 2 minutes to completely remove the residual solvent NMP, forming a bubble-free wet film.

[0051] Irradiation with ultraviolet light (365nm, 220mJ / cm²) for 30 seconds forms an initial cross-linked network.

[0052] The substrate was placed in an oven and cured at 180°C for 40 minutes to complete imidization.

[0053] Example 3 Under nitrogen protection, ODA and DEGDAn were added to NMP at a 1:1 molar ratio, and the mixture was stirred at 35°C for 6 hours to form a polyamic acid solution with a solid content of 10 wt%. The polyamic acid solution was placed on a magnetic stirrer, and 2 wt% TMPTA was added first, stirring continuously for 15 minutes to ensure the crosslinking agent was fully dissolved and dispersed. Then, 3 wt% Irgacure-2959 (photoinitiator) was slowly added, and stirring continued for 30 minutes under light-protected conditions.

[0054] The solution was spin-coated onto a PI substrate with a film thickness controlled at 80-100 nm. The substrate was then baked at 80°C for 3 minutes to completely remove the residual solvent NMP, forming a bubble-free wet film.

[0055] Irradiation with ultraviolet light (365nm, 220mJ / cm²) for 40 seconds forms an initial cross-linked network.

[0056] The substrate was placed in an oven and cured at 200°C for 30 minutes to complete imidization.

[0057] Example 4 Under nitrogen protection, DMDB and CBDA were added to NMP at a 1:1 molar ratio, and the mixture was stirred at 35°C for 6 hours to form a polyamic acid solution with a solid content of 10 wt%. The polyamic acid solution was placed on a magnetic stirrer, and 2 wt% PETA was added first, stirring continuously for 15 minutes to ensure the crosslinking agent was fully dissolved and dispersed. Then, 3 wt% Irgacure-2959 (photoinitiator) was slowly added, and stirring continued for 30 minutes under light-protected conditions.

[0058] The solution was spin-coated onto a PI substrate with a film thickness controlled at 80-100 nm. The substrate was then baked at 80°C for 3 minutes to completely remove the residual solvent NMP, forming a bubble-free wet film.

[0059] Irradiation with ultraviolet light (365nm, 220mJ / cm²) for 40 seconds forms an initial cross-linked network.

[0060] The substrate was placed in an oven and cured at 200°C for 30 minutes to complete imidization.

[0061] Example 5 Under nitrogen protection, DAN and DEGDAn were added to NMP at a 1:1 molar ratio, and the mixture was stirred at 35°C for 6 hours to form a polyamic acid solution with a solid content of 10 wt%. The polyamic acid solution was placed on a magnetic stirrer, and 2 wt% EO-TMPTA was added first, and stirring was continued for 15 minutes to ensure that the crosslinking agent was fully dissolved and dispersed. Then, 3 wt% Irgacure-2959 (photoinitiator) was slowly added, and stirring was continued for 30 minutes under light-protected conditions.

[0062] The solution was spin-coated onto a PI substrate with a film thickness controlled at 80-100 nm. The substrate was then baked at 80°C for 3 minutes to completely remove the residual solvent NMP, forming a bubble-free wet film.

[0063] Irradiation with ultraviolet light (365nm, 220mJ / cm²) for 30 seconds forms an initial cross-linked network.

[0064] The substrate was placed in an oven and cured at 200°C for 30 minutes to complete imidization.

[0065] Example 6 Under nitrogen protection, DAN and TMPTA were added to NMP at a 1:1 molar ratio, and the mixture was stirred at 35°C for 6 hours to form a polyamic acid solution with a solid content of 10 wt%. The polyamic acid solution was placed on a magnetic stirrer, and 2 wt% PETA was added first, stirring continuously for 15 minutes to ensure the crosslinking agent was fully dissolved and dispersed. Then, 3 wt% Irgacure-2959 (photoinitiator) was slowly added, and stirring continued for 30 minutes under light-protected conditions.

[0066] The solution was spin-coated onto a PI substrate with a film thickness controlled at 80-100 nm. The substrate was then baked at 80°C for 3 minutes to completely remove the residual solvent NMP, forming a bubble-free wet film.

[0067] Irradiation with ultraviolet light (365nm, 220mJ / cm²) for 20 seconds forms an initial cross-linked network.

[0068] The substrate was placed in an oven and cured at 200°C for 30 minutes to complete imidization.

[0069] Comparative Example 1 Comparative Example 1 uses a traditional PI-oriented film.

[0070] The examples and comparative examples were tested, including pencil hardness, number of abrasion cycles, and bright spot defect rate. The test methods are as follows: Pencil hardness test: According to ASTM D3363-05, a 2H / 4H pencil was used to scratch the pencil at a 45° angle and a 1000g load.

[0071] Abrasion resistance: Refer to JISK5600-5-9, use chamois cloth to rub back and forth under 500g pressure, and observe the scratch area through an optical microscope (500x magnification).

[0072] Bright spot defect rate: The number of bright spots with a diameter ≥5μm on a 100×100mm² substrate was counted using an automated optical inspection (AOI) system.

[0073] The specific test results are shown in Table 1.

[0074] Table 1. Test results for each embodiment and comparative example. The tests above show that the pencil hardness of Examples 1 to 6 is 4H. The alternating block structure of "rigid skeleton-flexible joint" constructed from rigid diamine and flexible dianhydride achieves a synergistic balance between hardness and toughness. It leverages the reinforcing effect of the aromatic ring structure and cross-linking network of the rigid monomer to increase the pencil hardness from 2H of traditional PI films to 4H, significantly reducing the risk of scratches from spacers during device assembly. Furthermore, the moderate deformation capability of the flexible segments avoids the embrittlement problem caused by simply increasing hardness, adapting to the repeated bending requirements of flexible substrates. This demonstrates that the technical solution of this application effectively improves the problem of insufficient hardness in traditional polyimide alignment films. Through the rigid-flexible balance design of the rigid monomer ODA and the flexible monomer BPADA, combined with the reinforcing effect of the TMPTA cross-linking network, the resulting alignment film pencil hardness is significantly higher than that of traditional polyimide alignment films (typically below 2H), meeting high hardness requirements and reducing the risk of scratches from spacers during display device assembly and use.

[0075] Examples 1 and 6 showed no significant damage after 40 friction cycles, Examples 2 and 3 showed no significant damage after 50 friction cycles, and Examples 4 and 5 showed no significant damage after 35 friction cycles. The bright spot defect rate of Example 1 was 0.01%, and the bright spot defect rates of Examples 2 to 6 were 0.008%, 0.007%, 0.011%, 0.013%, and 0.016%, respectively. All performance characteristics were superior to traditional PI alignment films. The high-density "dot crosslinking" network formed by the crosslinking agent in this application enhances the wear resistance of the film layer, increasing the number of friction cycles from the traditional ≤10 cycles to ≥35 cycles. This significantly reduces liquid crystal alignment disorder caused by friction in daily use, extending the device's lifespan, and is particularly suitable for high-frequency applications such as automotive displays and outdoor equipment. Simultaneously, the wear resistance is optimized. Compared to traditional polyimide alignment films (generally resistant to ≤10 friction cycles), this application, through the synergy of a dense crosslinking network and molecular chain flexibility, can withstand more friction cycles without significant damage, improving the durability of liquid crystal display devices. The bright spot defect rate is significantly reduced, showing a marked improvement compared to traditional alignment films (>0.1%), which helps to improve display performance and product yield.

[0076] Furthermore, the reduced curing temperature, achieved through synergistic UV pre-crosslinking and molecular self-cyclization, avoids the drawbacks of traditional processes requiring ≥250℃ high-temperature curing, making it compatible with flexible substrates (such as PI and PET) and expanding its application scenarios in the flexible display field. The monomers (ODA, BPADA) and crosslinking agent (TMPTA) used are commercially available products, eliminating the need for in-house synthesis. The process is compatible with existing LCD device manufacturing processes, providing a solid foundation for industrial application. The significant reduction in bright spot defect rate is attributed to the scratch resistance and material density resulting from high hardness, reducing display damage caused by scratches and lowering repair costs for high-end devices. Through the synergistic effect of the low steric hindrance of the flexible dianhydride cycloaliphatic structure and the rigid diamine functional groups, the curing temperature can be reduced from the traditional ≥250℃ to 180℃, avoiding damage to flexible substrates such as PI and PET from high temperatures, providing process feasibility for emerging applications such as wearable devices and flexible screens. Furthermore, tests were conducted on the orderliness of liquid crystal molecule arrangement, pretilt angle stability, and orientation retention after friction. The orderliness test involved observing the consistency of molecule arrangement within the liquid crystal cell using a polarizing microscope and statistically analyzing the percentage of non-orientationally disordered areas. The pretilt angle stability test used an ellipsometer to measure the pretilt angle of the alignment film and measured the rate of change of the pretilt angle after 50 bends (5mm radius). The orientation retention test after friction followed the JIS K5600-5-9 friction test standard. After 50 rubs, the liquid crystal molecules were observed using a polarizing microscope to determine if they remained orderly arranged along the friction direction, and the percentage of disordered area was recorded (≤5% was considered acceptable).

[0077] Test results show that in Examples 1 to 6, the proportion of non-orientation disordered areas observed under polarized light microscopy is ≥98%, indicating good molecular alignment consistency. Regarding the pretilt angle, with an initial pretilt angle of 3.2°, the change rates in Examples 1 to 6 are 6.15%, 6.25%, 6.00%, 6.25%, 6.00%, and 6.00%, respectively, demonstrating excellent stability. Regarding orientation retention after rubbing, after 50 rubbing cycles, the proportions of orientation disordered areas are 2.3%, 2.6%, 2.4%, 2.3%, 2.3%, and 2.6%, respectively, indicating that the liquid crystal molecules remain orderly arranged along the rubbing direction without significant disorder. The orientation properties of this application did not decrease due to the improvement in mechanical properties. The core reason is that the aromatic ring structure of the rigid diamine retains stable π-π stacking anchor points, and the alternating block structure of the flexible dianhydride and the rigid diamine maintains the orderliness of the molecular chain. The "point crosslinking" spacing of the crosslinking system is larger than the size of the liquid crystal molecule, without spatial obstruction. The synergistic effect of UV pre-crosslinking and 180℃ thermosetting ensures the strength of the crosslinking network and reserves space for the movement of molecular chains in orientation alignment, ultimately achieving synergistic optimization of mechanical properties and orientation properties.

[0078] As described above, this application proposes a method for preparing polyimide films and the corresponding films. This method achieves a comprehensive improvement in film performance through precise component selection and process control. First, rigid diamine is used as a rigid monomer and flexible dianhydride as a flexible monomer, and polycondensation is carried out at a 1:1 molar ratio at 35℃-45℃ for 6-10 hours to form a polyamic acid solution with a solid content of 10wt%-25wt%, constructing a "rigid skeleton-flexible joint" structure to balance hardness and toughness. Next, 2wt%-5wt% of crosslinking agent and 1wt%-3wt% of photoinitiator are added and thoroughly stirred and dispersed. The solution is spin-coated onto a flexible substrate, with the film thickness controlled at 80-100nm. The film is then pre-baked at 80℃-90℃ to remove the solvent, forming a bubble-free wet film. Irradiation with ultraviolet light at a wavelength of 350nm-400nm and an energy of 180mJ / cm²-250mJ / cm² for 20s-40s initiates free radical polymerization to form an initial crosslinked network. Finally, the polyimide film is thermally cured at 150℃-200℃ for at least 20 minutes to complete imidization and form a dense and uniform polyimide film. This method employs a low-temperature curing process to avoid thermal damage to the flexible substrate, reducing energy consumption and cost. By controlling the content of crosslinking agent and photoinitiator, the crosslinking network density is optimized, improving mechanical strength and dimensional stability. Adjusting the monomer ratio and reaction conditions reduces side reactions and improves product purity. The entire preparation process is carried out under inert gas protection to ensure reaction stability. The polyimide film prepared by this method has a pencil hardness of 4H, a friction cycle resistance of ≥40 cycles, and a bright spot defect rate of ≤0.01%, significantly outperforming traditional PI oriented films and meeting the requirements for high hardness and high durability.

[0079] The above is only one specific implementation of this application, and any other improvements made based on the concept of this application shall be considered within the scope of protection of this application.

Claims

1. A method for producing a polyimide film, characterized by, The application relates to a preparation method of a polyimide film. Step S100: rigid diamine and flexible dianhydride are added into a solvent, and polyamide acid solution is formed by stirring and condensation polymerization, wherein the solid content of the polyamide acid solution is in the range of 10wt%-25wt%; Step S200: a crosslinking agent is added into the polyamide acid solution, and the crosslinking agent is fully dissolved and dispersed by stirring, then a photoinitiator is slowly added, and the photoinitiator is uniformly dispersed by stirring under light-proof conditions; Step S300: the solution obtained in step S200 is spin-coated on a substrate, and the solvent is removed by heating at a temperature lower than 100 DEG C; Step S400: ultraviolet light irradiation is performed to form an initial crosslinking network by initiating free radical polymerization through the action of ultraviolet light; Step S500: heat curing is performed at a temperature in the range of 150 DEG C-200 DEG C to form a polyimide film; The rigid diamine is selected from one or more of the following components: 4,4'-diamino diphenyl ether, 4,4'-diamino-2,2'-dimethyl diphenyl and 2,6-diamino naphthalene; The flexible dianhydride is selected from one or more of the following components: bisphenol A type diether dianhydride, 1,2,3,4-cyclobutane tetracarboxylic dianhydride and diethylene glycol bis-trimellitic anhydride.

2. The polyimide film production method according to claim 1, wherein The molar ratio of the rigid diamine to the flexible dianhydride is 1:

1.

3. The method for preparing polyimide film according to claim 2, characterized in that, The content of the crosslinking agent is in the range of 2wt%-5wt%, and the content of the photoinitiator is in the range of 1wt%-3wt%.

4. The method for preparing polyimide film according to claim 3, characterized in that, The crosslinking agent is selected from one or more of the following components: trimethylolpropane triacrylate, pentaerythritol triacrylate and ethoxylated trimethylolpropane triacrylate.

5. The method for preparing polyimide film according to claim 1, characterized in that, In step S300, the film thickness of the spin coating is controlled in the range of 80-100nm.

6. The method for preparing polyimide film according to claim 1, characterized in that, In step S400, the wavelength of the ultraviolet light is in the range of 350 nm to 400 nm, the energy is in the range of 180 mJ / cm 2 -250 mJ / cm 2 2, and the irradiation time is in the range of 20 s to 40 s.

7. The method for preparing polyimide film according to claim 6, characterized in that, In step S500, the heat curing time is greater than 20min.

8. The method for preparing polyimide film according to claim 1, characterized in that, In step S100, the stirring is continuously performed at a temperature in the range of 35 DEG C-45 DEG C for 6-10 hours.

9. The method for preparing polyimide film according to claim 1, characterized in that, Each step is performed under an inert gas protection atmosphere.

10. A polyimide film characterized by comprising: The polyimide film is prepared by the polyimide film preparation method according to any one of claims 1-9.

Images