High-temperature-resistant transparent polyimide film material capable of being used for flexible display and preparation method of high-temperature-resistant transparent polyimide film material

Abstract

The invention relates to the technical field of flexible display materials, in particular to a high-temperature-resistant transparent polyimide film material capable of being used for flexible display and a preparation method of the high-temperature-resistant transparent polyimide film material. The invention discloses a high-temperature-resistant transparent polyimide film material capable of being used for flexible display. The polyimide film material is prepared from aromatic diamine and dianhydride through a polymerization reaction. The aromatic diamine comprises a first diamine and a second diamine; the dianhydride comprises cyclobutane tetracarboxylic acid dianhydride; the molar ratio of the aromatic diamine to the dianhydride is 1: (1-1.1). The invention aims to effectively improve the problems of low transparency and poor heat resistance of the polyimide film, so that the polyimide film has favorable optical properties, keeps the heat resistance, dimensional stability and mechanical properties of the polyimide film, and meets the requirements of the market on using colorless and transparent polyimide as a cover film of a display screen.

Description

Technical Field

[0001] This application relates to the technical field of flexible display materials, and particularly relates to a high-temperature resistant transparent polyimide film material for flexible display and a preparation method thereof. Background Art

[0002] In the field of displays, the most commonly used material for traditional OLED and LCD display substrates and cover plates is glass. The disadvantages of glass, such as being thick, heavy, hard, and easily broken, cannot meet the requirements of the new generation of flexible displays. With the increasing market demand for thin, light, portable, and foldable display devices, the development of flexible display materials, flexible thin-film electrode materials, and flexible electronic devices with excellent performance has become an important topic in the field of material research and development.

[0003] Polyimide is a high-performance material with excellent thermal stability, chemical stability, and mechanical properties, and has broad application prospects in high-tech fields such as aerospace, electrical and electronics, and optoelectronics. However, traditional aromatic polyimides are prone to form intramolecular and intermolecular charge transfer complexes, resulting in low light transmittance of traditional polyimide films. Modified high-transparency polyimide films (CPI) have solved the transparency problem to a certain extent, but the transparency improvement of polyimide will reduce its own heat resistance, dimensional stability, and mechanical properties to a certain extent. Currently, transparent polyimides generally have defects such as insufficient high temperature resistance, insufficient dimensional stability, and poor mechanical strength, which limit the application of polyimide films in the optoelectronic field, especially in the flexible display field. There is an urgent need for colorless and transparent polyimides with excellent performance as the cover film of display screens. Summary of the Invention

[0004] In order to effectively improve the transparency of polyimide films and maintain their heat resistance, dimensional stability, and mechanical properties while having good optical properties, this application provides a high-temperature resistant transparent polyimide film material for flexible display and a preparation method thereof.

[0005] This application provides a high-temperature resistant transparent polyimide film material for flexible display and a preparation method thereof, adopting the following technical solutions: A high-temperature resistant transparent polyimide film material for flexible display, wherein the polyimide film material is prepared by a polymerization reaction of aromatic diamine and dianhydride; the aromatic diamine includes a first diamine and a second diamine; the dianhydride includes cyclobutane tetracarboxylic dianhydride; the molar ratio of the aromatic diamine to the dianhydride is 1:(1 - 1.1).

[0006] By adopting the above technical solution, polymerizing an aromatic diamine and a dianhydride within a specific molar ratio range can effectively regulate the structural characteristics of the polyimide molecular chain, thereby significantly improving its heat resistance while ensuring transparency. This helps to improve the dimensional stability and mechanical properties of the material, and the obtained polyimide film material has excellent transparency and high-temperature resistance.

[0007] Using cyclobutanetetracarboxylic dianhydride as the dianhydride, its unique alicyclic structure not only destroys the charge transfer complex effect in the polyimide molecular chain, thereby greatly improving the transparency of the film, but also the addition of aromatic diamine promotes the ordered arrangement between molecular chains, reduces the generation of defect structures, and improves the heat resistance and mechanical strength of the material. A specific combination of aromatic diamine and cyclobutanetetracarboxylic dianhydride within a certain proportion range effectively enhances the heat resistance of the material, enabling it to maintain good dimensional stability and mechanical strength in high-temperature environments, and can also optimize the molecular structure to reduce the coloring degree of the material, thereby greatly improving transparency and significantly enhancing the comprehensive performance of the polyimide film material. Among them, if the proportion of aromatic diamine is too low, the number of rigid units in the molecular chain structure will decrease, thereby reducing the heat resistance of the material; if the proportion of aromatic diamine is too high, it may increase the packing density between molecules, resulting in poor transparency. Therefore, a certain proportion range of aromatic diamine and cyclobutanetetracarboxylic dianhydride effectively breaks the charge transfer complex effect in the molecular structure, curbs the conjugation within the molecular chain, reduces the color characteristics of the material, and improves the visible light transmittance. At the same time, reasonable chemical structure design endows the material with excellent heat resistance, making it suitable for the harsh working environment in the flexible display field.

[0008] In a specific feasible embodiment, the first diamine is selected from at least one of 2,2'-bis(trifluoromethyl)benzidine and 4,4'-diaminobiphenyl; the second diamine is selected from at least one of 4,4'-diaminobenzanilide and 4,4`-[isopropylidene bis(phenoxy)]diphenylamine.

[0009] The molar ratio of the first diamine to the second diamine is (2 - 4):1.

[0010] By adopting the above technical solution, through the diamine containing strongly electronegative groups, the charge transfer complex effect between polyimide chains is destroyed, thereby significantly improving the optical transmittance of the material, and the flexibility of the molecular chain segments can be adjusted, so that the material has both a high glass transition temperature and excellent tensile strength. Due to the strongly electronegative trifluoromethyl structure in 2,2'-bis(trifluoromethyl)benzidine diamine, the electron donor-acceptor interaction between molecules can be weakened, reducing the coloring degree of the material; the rigid alicyclic ring or special steric hindrance structure introduced in the second diamine can not only further inhibit the π-π conjugation effect to enhance transparency, but also improve the dimensional stability and mechanical properties of the material. Through the first diamine and the second diamine in the scope ratio of this application, the hydrogen bond network structure formed in the polyimide can be optimized, effectively enhancing the toughness and dimensional stability of the material and improving the heat resistance. If the molar amount of the first diamine is too low, the proportion of rigid units in the system will decrease, thereby affecting the overall heat resistance characteristics and dimensional stability; if the first diamine is too much, the molecular chain packing density will increase, which may cause problems such as the material becoming brittle and the transparency decreasing.

[0011] In a specific feasible embodiment, the dianhydride further includes 4,4'-(hexafluoroisopropylidene)diphthalic anhydride.

[0012] Preferably, the molar ratio of the cyclobutanetetracarboxylic dianhydride to 4,4'-(hexafluoroisopropylidene)diphthalic anhydride is (3 - 4):1.

[0013] Introducing 4,4'-(hexafluoroisopropylidene)diphthalic anhydride as one of the dianhydrides, the alicyclic ring structure introduced by the cyclobutanetetracarboxylic dianhydride can destroy the π-π conjugation in the polyimide molecular chain, effectively reducing the charge transfer complex effect, thereby improving the optical transparency of the film and enhancing the flexibility and dimensional stability; the fluorine atoms contained in 4,4'-(hexafluoroisopropylidene)diphthalic anhydride further weaken the intermolecular interaction, and can further optimize the comprehensive performance of the polyimide film material. The dianhydride containing strongly electronegative fluorine-containing groups can improve the transparency of the film, and its special molecular structure helps to improve the heat resistance and dimensional stability of the material, has better high-temperature environment adaptability and lower thermal expansion coefficient, and meets the deformation control requirements of high-performance materials in the flexible display field under high-temperature environments. However, when the proportion of 4,4'-(hexafluoroisopropylidene)diphthalic anhydride is too high, its structural characteristics will significantly affect the regularity and aggregation state structure of the polyimide molecular chain. The introduction of too many hexafluoroisopropylidene groups will cause the stacking between molecular chains to be closer, thereby increasing the light scattering phenomenon, and further reducing the transparency of the polyimide film.

[0014] In the second aspect, the present application provides a preparation method of a high-temperature resistant transparent polyimide film material for flexible display, adopting the following technical solution: Preparation method of high-temperature resistant transparent polyimide film material for flexible display, comprising the following steps: controlling the water bath temperature at 0-5°C, adding diamine monomer into a solvent and stirring until completely dissolved; then adding dianhydride and continuing to stir for 24-48 h to obtain a polyamic acid solution, adding imidization reagent and continuing to stir for 18-24 h to obtain a transparent polyimide solution, and finally performing filtration, coating, and heat treatment to obtain a transparent polyimide film.

[0015] The solvent is selected from at least one of N-methyl-2-pyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide, dimethyl sulfoxide, chloroform, and γ-butyrolactone.

[0016] In a specific feasible embodiment, the imidization reagent is composed of acetic anhydride and pyridine mixed in a molar ratio of (4-6):3.

[0017] The acetic anhydride and pyridine in the imidization reagent are mixed in a specific molar ratio, which can effectively promote the imidization reaction process of the polyamic acid solution and ensure that the molecular chain structure of the generated polyimide is uniform and stable. Acetic anhydride can effectively promote the ring-closing dehydration reaction of the polyamic acid solution to generate a stable polyimide structure; pyridine can not only accelerate the dehydration reaction process but also inhibit the occurrence of side reactions, ensuring the regularity of the molecular chain. The ratio optimizes the interaction between pyridine and acetic anhydride, avoiding side reactions while ensuring sufficient dehydration, thereby significantly improving the optical properties of the finally prepared transparent polyimide film, maintaining a low yellowness index, increasing the light transmittance, and improving the glass transition temperature, etc.

[0018] In a specific feasible embodiment, the molar ratio of acetic anhydride to dianhydride is (4-6):1.

[0019] A certain range of molar ratios of acetic anhydride to dianhydride can effectively promote the imidization reaction process of the polyamic acid solution, ensure that the generated polyimide structure is more stable and uniform, and promote the formation of stable imide unit links at the side chain ends to construct a network three-dimensional framework structure. It endows the material with excellent mechanical properties and also makes it have excellent high-temperature resistance and mechanical properties. The amount of acetic anhydride within the ratio range of this application not only avoids the problem of increased side reactions due to excessive acetic anhydride affecting the material purity but also prevents the phenomenon of incomplete imidization due to insufficient acetic anhydride, thereby significantly improving the heat resistance and dimensional stability of the finally obtained transparent polyimide film.

[0020] The heat treatment includes vacuum low-temperature treatment and high-temperature treatment. Among them, the conditions for vacuum low-temperature treatment are a vacuum degree of 8-10 Pa, a temperature of 60-70 °C, and a time of 45-60 min; the high-temperature treatment is a heating process that sequentially passes through 90-100 °C / 30-45 min, 170-180 °C / 30-45 min, and 280-300 °C / 30-45 min under nitrogen protection, and the heating rate is 1-5 °C / min.

[0021] Vacuum low-temperature treatment ensures that the treatment process will neither overly affect the mechanical properties of the material nor fully guarantee the complete removal of the solvent, thus significantly improving the dimensional stability and surface flatness of the film. In the high-temperature treatment, it further promotes the ordered arrangement of polyimide molecular chains and the improvement of the cross-linking degree, making the finally obtained film have higher heat resistance and better optical properties. This precisely controlled heat treatment method is crucial for ensuring the overall heat-resistant quality of the transparent polyimide film.

[0022] In summary, this application includes at least one of the following beneficial technical effects: 1. By selecting aromatic diamines and dianhydrides within a specific molar ratio range for polymerization reaction, the structural characteristics of polyimide molecular chains can be effectively regulated, thereby significantly enhancing its heat resistance while ensuring transparency. It helps to improve the dimensional stability and mechanical properties of the material, and the obtained polyimide film material has excellent transparency and high-temperature resistance.

[0023] A specific combination of aromatic diamines and cyclobutane tetracarboxylic dianhydride within a certain proportion range effectively enhances the heat resistance of the material, enabling it to maintain good dimensional stability and mechanical strength in a high-temperature environment. It can also optimize the molecular structure and reduce the coloring degree of the material, thereby greatly improving transparency and significantly enhancing the comprehensive performance of the polyimide film material. Among them, if the proportion of aromatic diamines is too low, the number of rigid units in the molecular chain structure will decrease, thus reducing the heat resistance of the material; if the proportion of aromatic diamines is too high, it may increase the packing density between molecules, resulting in poor transparency. Therefore, a certain proportion range of aromatic diamines and cyclobutane tetracarboxylic dianhydride effectively breaks the charge transfer complex effect in the molecular structure, curbs the conjugation within the molecular chain, reduces the color characteristics of the material, and improves the visible light transmittance. At the same time, a reasonable chemical structure design endows the material with excellent heat resistance, making it suitable for the harsh working environment of the flexible display field.

[0024] 2. A certain range of the molar ratio of acetic anhydride to dianhydride can effectively promote the imidization reaction process of the polyamic acid solution and ensure that the resulting polyimide structure is more stable and uniform. The amount of acetic anhydride within the ratio range of this application not only avoids the problem of increased side reactions caused by excessive acetic anhydride, which affects the purity of the material, but also prevents the phenomenon of incomplete imidization due to insufficient acetic anhydride, thereby significantly improving the heat resistance and dimensional stability of the finally obtained transparent polyimide film.

[0025] 3. Vacuum low-temperature treatment ensures that the treatment process will neither overly affect the mechanical properties of the material nor fully guarantee the complete removal of the solvent, thereby significantly improving the dimensional stability and surface flatness of the film. And the high-temperature treatment further promotes the ordered arrangement of the polyimide molecular chains and the improvement of the crosslinking degree, making the finally obtained film have higher heat resistance and better optical properties. This precisely controlled heat treatment method is crucial for ensuring the overall heat-resistant quality of the transparent polyimide film. Detailed implementation mode

[0026] In this application, 4,4'-[isopropylidene bis(phenoxy)]dianiline (98%) was purchased from Macklin: all raw materials of this application can be obtained through commercial channels.

[0027] Example 1 At room temperature, 0.042 mol of 2,2'-bis(trifluoromethyl)benzidine diamine and 0.018 mol of 4,4'-diaminobenzanilide were added to 114 ml of N-methylpyrrolidone solvent and stirred until completely dissolved. Then, the water bath temperature was controlled at 5°C; 0.060 mol of cyclobutane tetracarboxylic dianhydride was added, and stirring was continued for 48 h to obtain a transparent polyamic acid solution. A mixed solution of 0.301 mol of acetic anhydride and 0.149 mol of pyridine was added to the transparent polyamic acid solution, and stirring was continued for 24 h to obtain a transparent polyimide solution; after filtering the transparent polyimide solution, it was coated. The wet film was dried in a vacuum of 10 Pa and 70°C for 60 min, and then treated in a nitrogen oven at a heating rate of 5°C / min through 100°C / 30 min, 180°C / 30 min, and 300°C / 30 min to obtain a 20-μm-thick high-temperature-resistant transparent polyimide film for flexible display.

[0028] Example 2 At room temperature, 0.042 mol of 2,2'-bis(trifluoromethyl)benzidine and 0.018 mol of 4,4'-diaminobenzanilide were added to 114 ml of N-methylpyrrolidone solvent and stirred until completely dissolved. Then, the water bath temperature was controlled at 5 °C. 0.048 mol of cyclobutanetetracarboxylic dianhydride and 0.012 mol of 4,4'-(hexafluoroisopropylidene)dibenzoic anhydride were added, and stirring was continued for 48 h to obtain a transparent polyamic acid solution. A mixed solution of 0.301 mol of acetic anhydride and 0.149 mol of pyridine was added to the transparent polyamic acid solution, and stirring was continued for 24 h to obtain a transparent polyimide solution. After filtering the transparent polyimide solution, it was coated. The wet film was placed in a vacuum of 10 Pa and 70 °C and dried for 60 min. Then, in a nitrogen oven, it was treated at 100 °C / 30 min, 180 °C / 30 min, and 300 °C / 30 min with a heating rate of 5 °C / min to obtain a 20-μm-thick high-temperature-resistant transparent polyimide film for flexible display.

[0029] Example 3 At room temperature, 0.042 mol of 2,2'-bis(trifluoromethyl)benzidine and 0.018 mol of 4,4'-diaminobenzanilide were added to 114 ml of N-methylpyrrolidone solvent and stirred until completely dissolved. Then, the water bath temperature was controlled at 5 °C. 0.03 mol of cyclobutanetetracarboxylic dianhydride and 0.03 mol of 4,4'-(hexafluoroisopropylidene)dibenzoic anhydride were added, and stirring was continued for 48 h to obtain a transparent polyamic acid solution. A mixed solution of 0.301 mol of acetic anhydride and 0.149 mol of pyridine was added to the transparent polyamic acid solution, and stirring was continued for 24 h to obtain a transparent polyimide solution. After filtering the transparent polyimide solution, it was coated. The wet film was placed in a vacuum of 10 Pa and 70 °C and dried for 60 min. Then, in a nitrogen oven, it was treated at 100 °C / 30 min, 180 °C / 30 min, and 300 °C / 30 min with a heating rate of 5 °C / min to obtain a 20-μm-thick high-temperature-resistant transparent polyimide film for flexible display.

[0030] Example 4 At room temperature, 0.03 mol of 2,2'-bis(trifluoromethyl)benzidine diamine and 0.03 mol of 4,4'-diaminobenzanilide were added to 114 ml of N-methylpyrrolidone solvent and stirred until completely dissolved. Then, the water bath temperature was controlled at 5 °C. 0.048 mol of cyclobutane tetracarboxylic dianhydride and 0.012 mol of 4,4'-(hexafluoroisopropylidene)dibenzoic anhydride were added, and stirring was continued for 48 h to obtain a transparent polyamic acid solution. A mixed solution of 0.301 mol of acetic anhydride and 0.149 mol of pyridine was added to the transparent polyamic acid solution, and stirring was continued for 24 h to obtain a transparent polyimide solution. After filtering the transparent polyimide solution, coating was carried out. The wet film was placed in a vacuum of 10 Pa and 70 °C and dried for 60 min. Then, in a nitrogen oven, it was treated at 100 °C / 30 min, 180 °C / 30 min, 300 °C / 30 min with a heating rate of 5 °C / min to obtain a 20-μm-thick high-temperature resistant transparent polyimide film for flexible display.

[0031] Example 5 At room temperature, 0.03 mol of 2,2'-bis(trifluoromethyl)benzidine diamine and 0.03 mol of 4,4'-diaminobenzanilide were added to 114 ml of N-methylpyrrolidone solvent and stirred until completely dissolved. Then, the water bath temperature was controlled at 5 °C. 0.03 mol of cyclobutane tetracarboxylic dianhydride and 0.03 mol of 4,4'-(hexafluoroisopropylidene)dibenzoic anhydride were added, and stirring was continued for 48 h to obtain a transparent polyamic acid solution. A mixed solution of 0.301 mol of acetic anhydride and 0.149 mol of pyridine was added to the transparent polyamic acid solution, and stirring was continued for 24 h to obtain a transparent polyimide solution. After filtering the transparent polyimide solution, coating was carried out. The wet film was placed in a vacuum of 10 Pa and 70 °C and dried for 60 min. Then, in a nitrogen oven, it was treated at 100 °C / 30 min, 180 °C / 30 min, 300 °C / 30 min with a heating rate of 5 °C / min to obtain a 20-μm-thick high-temperature resistant transparent polyimide film for flexible display.

[0032] Example 6 At room temperature, 0.042 mol of 2,2'-bis(trifluoromethyl)benzidine and 0.018 mol of 4,4'-[isopropylidene bis(phenoxy)]diphenylamine were added to 114 ml of N-methylpyrrolidone solvent and stirred until completely dissolved. Then, the water bath temperature was controlled at 5 °C. 0.048 mol of cyclobutanetetracarboxylic dianhydride and 0.012 mol of 4,4'-(hexafluoroisopropylidene)dibenzoic anhydride were added, and stirring was continued for 48 h to obtain a transparent polyamic acid solution. A mixed solution of 0.301 mol of acetic anhydride and 0.149 mol of pyridine was added to the transparent polyamic acid solution, and stirring was continued for 24 h to obtain a transparent polyimide solution. After filtering the transparent polyimide solution, coating was carried out. The wet film was placed in a vacuum of 10 Pa and 70 °C and dried for 60 min. Then, in a nitrogen oven, it was treated at 100 °C / 30 min, 180 °C / 30 min, and 300 °C / 30 min with a heating rate of 5 °C / min to obtain a 20-μm-thick high-temperature-resistant transparent polyimide film for flexible display.

[0033] Example 7 At room temperature, 0.042 mol of 2,2'-bis(trifluoromethyl)benzidine and 0.018 mol of 4,4'-diaminobenzanilide were added to 114 ml of N-methylpyrrolidone solvent and stirred until completely dissolved. Then, the water bath temperature was controlled at 5 °C. 0.060 mol of cyclobutanetetracarboxylic dianhydride was added, and stirring was continued for 48 h to obtain a transparent polyamic acid solution. A mixed solution of 0.248 mol of acetic anhydride and 0.247 mol of pyridine was added to the transparent polyamic acid solution, and stirring was continued for 24 h to obtain a transparent polyimide solution. After filtering the transparent polyimide solution, coating was carried out. The wet film was placed in a vacuum of 10 Pa and 70 °C and dried for 60 min. Then, in a nitrogen oven, it was treated at 100 °C / 30 min, 180 °C / 30 min, and 300 °C / 30 min with a heating rate of 5 °C / min to obtain a 20-μm-thick high-temperature-resistant transparent polyimide film for flexible display.

[0034] Example 8 At room temperature, 0.042 mol of 2,2'-bis(trifluoromethyl)benzidine and 0.018 mol of 4,4'-diaminobenzanilide were added to 114 ml of N-methylpyrrolidone solvent and stirred until completely dissolved. Then, the water bath temperature was controlled at 5 °C. 0.060 mol of cyclobutane tetracarboxylic dianhydride was added and stirring was continued for 48 h to obtain a transparent polyamic acid solution. A mixed solution of 0.36 mol of acetic anhydride and 0.23 mol of pyridine was added to the transparent polyamic acid solution, and stirring was continued for 24 h to obtain a transparent polyimide solution. After filtering the transparent polyimide solution, it was coated. The wet film was placed in a vacuum of 10 Pa and 70 °C and dried for 60 min. Then, under a nitrogen oven, it was treated at 100 °C / 30 min, 180 °C / 30 min, 300 °C / 30 min with a heating rate of 5 °C / min to obtain a 20-μm-thick high-temperature resistant transparent polyimide film for flexible display.

[0035] Example 9 At room temperature, 0.042 mol of 2,2'-bis(trifluoromethyl)benzidine and 0.018 mol of 4,4'-diaminobenzanilide were added to 114 ml of N-methylpyrrolidone solvent and stirred until completely dissolved. Then, the water bath temperature was controlled at 5 °C. 0.060 mol of cyclobutane tetracarboxylic dianhydride was added and stirring was continued for 48 h to obtain a transparent polyamic acid solution. A mixed solution of 0.18 mol of acetic anhydride and 0.11 mol of pyridine was added to the transparent polyamic acid solution, and stirring was continued for 24 h to obtain a transparent polyimide solution. After filtering the transparent polyimide solution, it was coated. The wet film was placed in a vacuum of 10 Pa and 70 °C and dried for 60 min. Then, under a nitrogen oven, it was treated at 100 °C / 30 min, 180 °C / 30 min, 300 °C / 30 min with a heating rate of 5 °C / min to obtain a 20-μm-thick high-temperature resistant transparent polyimide film for flexible display.

[0036] Example 10 At room temperature, 0.042 mol of 2,2'-bis(trifluoromethyl)benzidine diamine and 0.018 mol of 4,4'-diaminobenzanilide were added to 114 ml of N-methylpyrrolidone solvent and stirred until completely dissolved. Then, the water bath temperature was controlled at 5 °C. 0.060 mol of cyclobutane tetracarboxylic dianhydride was added, and stirring was continued for 48 h to obtain a transparent polyamic acid solution. A mixed solution of 0.301 mol of acetic anhydride and 0.149 mol of pyridine was added to the transparent polyamic acid solution, and stirring was continued for 24 h to obtain a transparent polyimide solution. After filtering the transparent polyimide solution, it was coated. The wet film was placed in a nitrogen oven and treated at 100 °C / 30 min, 180 °C / 30 min, 300 °C / 30 min with a heating rate of 5 °C / min to obtain a 20-μm-thick high-temperature resistant transparent polyimide film for flexible display.

[0037] Comparative Example 1 At room temperature, 0.06 mol of 2,2'-bis(trifluoromethyl)benzidine diamine was added to 114 ml of N-methylpyrrolidone solvent and stirred until completely dissolved. Then, the water bath temperature was controlled at 5 °C. 0.060 mol of cyclobutane tetracarboxylic dianhydride was added, and stirring was continued for 48 h to obtain a transparent polyamic acid solution. A mixed solution of 0.301 mol of acetic anhydride and 0.149 mol of pyridine was added to the transparent polyamic acid solution, and stirring was continued for 24 h to obtain a transparent polyimide solution. After filtering the transparent polyimide solution, it was coated. The wet film was placed in a vacuum of 10 Pa at 70 °C and dried for 60 min, and then in a nitrogen oven, and treated at 100 °C / 30 min, 180 °C / 30 min, 300 °C / 30 min with a heating rate of 5 °C / min to obtain a 20-μm-thick high-temperature resistant transparent polyimide film for flexible display.

[0038] Comparative Example 2 At room temperature, 0.06 mol of 4,4'-diaminobenzanilide was added to 114 ml of N-methylpyrrolidone solvent and stirred until completely dissolved. Then, the water bath temperature was controlled at 5 °C. 0.060 mol of cyclobutane tetracarboxylic dianhydride was added, and stirring was continued for 48 h to obtain a transparent polyamic acid solution. A mixed solution of 0.301 mol of acetic anhydride and 0.149 mol of pyridine was added to the transparent polyamic acid solution, and stirring was continued for 24 h to obtain a transparent polyimide solution. After filtering the transparent polyimide solution, it was coated. The wet film was placed in a vacuum of 10 Pa at 70 °C and dried for 60 min, and then in a nitrogen oven, and treated at 100 °C / 30 min, 180 °C / 30 min, 300 °C / 30 min with a heating rate of 5 °C / min to obtain a 20-μm-thick high-temperature resistant transparent polyimide film for flexible display.

[0039] Comparative Example 3 At room temperature, 0.042 mol of 2,2'-bis(trifluoromethyl)benzidine and 0.018 mol of 4,4'-diaminobenzanilide were added to 114 ml of N-methylpyrrolidone solvent and stirred until completely dissolved. Then the water bath temperature was controlled at 5 °C. 0.060 mol of 4,4'-(hexafluoroisopropylidene)diphthalic anhydride was added and stirring was continued for 48 h to obtain a transparent polyamic acid solution. A mixed solution of 0.301 mol of acetic anhydride and 0.149 mol of pyridine was added to the transparent polyamic acid solution, and stirring was continued for 24 h to obtain a transparent polyimide solution. After filtering the transparent polyimide solution, it was coated. The wet film was placed in a vacuum of 10 Pa and 70 °C and dried for 60 min. Then, in a nitrogen oven, it was treated at 100 °C / 30 min, 180 °C / 30 min, 300 °C / 30 min with a heating rate of 5 °C / min to obtain a 20-μm-thick high-temperature-resistant transparent polyimide film for flexible display.

[0040] Comparative Example 4 At room temperature, 0.06 mol of 2,2'-bis(trifluoromethyl)benzidine was added to 114 ml of N-methylpyrrolidone solvent and stirred until completely dissolved. Then the water bath temperature was controlled at 5 °C. 0.060 mol of 4,4'-(hexafluoroisopropylidene)diphthalic anhydride was added and stirring was continued for 48 h to obtain a transparent polyamic acid solution. A mixed solution of 0.301 mol of acetic anhydride and 0.149 mol of pyridine was added to the transparent polyamic acid solution, and stirring was continued for 24 h to obtain a transparent polyimide solution. After filtering the transparent polyimide solution, it was coated. The wet film was placed in a vacuum of 10 Pa and 70 °C and dried for 60 min. Then, in a nitrogen oven, it was treated at 100 °C / 30 min, 180 °C / 30 min, 300 °C / 30 min with a heating rate of 5 °C / min to obtain a 20-μm-thick high-temperature-resistant transparent polyimide film for flexible display.

[0041] Comparative Example 5 At room temperature, 0.06 mol of 2,2'-bis(trifluoromethyl)benzidine was added to 114 ml of N-methylpyrrolidone solvent and stirred until completely dissolved. Then, the water bath temperature was controlled at 5 °C. 0.060 mol of cyclobutane tetracarboxylic dianhydride was added and stirring continued for 48 h to obtain a transparent polyamic acid solution. A mixed solution of 0.301 mol of acetic anhydride and 0.149 mol of pyridine was added to the transparent polyamic acid solution and stirring continued for 24 h to obtain a transparent polyimide solution. After filtering the transparent polyimide solution, it was coated. The wet film was placed in a vacuum of 10 Pa and 70 °C and dried for 60 min. Then, in a nitrogen oven, it was treated at 100 °C / 30 min, 180 °C / 30 min, 300 °C / 30 min with a heating rate of 5 °C / min to obtain a 20-μm-thick high-temperature resistant transparent polyimide film for flexible display.

[0042] Comparative Example 6 At room temperature, 0.06 mol of 4,4'-diaminodicyclohexylmethane was added to 114 ml of N-methylpyrrolidone solvent and stirred until completely dissolved. Then, the water bath temperature was controlled at 5 °C. 0.060 mol of cyclobutane tetracarboxylic dianhydride was added and stirring continued for 48 h to obtain a transparent polyamic acid solution. A mixed solution of 0.301 mol of acetic anhydride and 0.149 mol of pyridine was added to the transparent polyamic acid solution and stirring continued for 24 h to obtain a transparent polyimide solution. After filtering the transparent polyimide solution, it was coated. The wet film was placed in a vacuum of 10 Pa and 70 °C and dried for 60 min. Then, in a nitrogen oven, it was treated at 100 °C / 30 min, 180 °C / 30 min, 300 °C / 30 min with a heating rate of 5 °C / min to obtain a 20-μm-thick high-temperature resistant transparent polyimide film for flexible display.

[0043] Performance testing The performance of the high-temperature resistant transparent polyimide films prepared in the examples and comparative examples for flexible display was tested by the following methods: (1) Transmittance (T 450 %) and yellowness index YI were tested using an X-rite Ci7800 spectrophotometer; (2) Tensile strength was tested using a Shimadzu AG-X plus, 1 KN, with a test speed of 5 mm / min, sample size 10 mm wide * 15 mm long, test gauge length: 50 mm, extensometer gauge length 20 mm.

[0044] The glass transition temperature (Tg) was measured using a dynamic mechanical analyzer (DMA850) under the following test conditions: a load of 0.05 N, a heating rate of 3 °C / min, and measurement was carried out in a nitrogen atmosphere within the temperature range of 200 - 400 °C. The inflection point of the curve with the maximum value was recorded as the glass transition temperature.

[0045] (4) The coefficient of thermal expansion (CTE) was tested using a thermomechanical analyzer (TMA 7100C) under the following conditions: a load of 20 mN, a heating rate of 5 °C / min, and a temperature range of 50 - 200 °C. The comparison results are shown in Table 1 below: Table 1 Performance test results Comparing Examples 1 - 6 and Comparative Examples 1 - 4, it can be seen that using cyclobutane tetracarboxylic dianhydride as the dianhydride, its unique alicyclic structure not only destroys the charge transfer complex effect in the polyimide molecular chain, thus greatly improving the transparency of the film, but also the addition of aromatic diamine promotes the ordered arrangement between molecular chains, reduces the generation of defect structures, and improves the heat resistance and mechanical strength of the material. A specific combination of aromatic diamine and cyclobutane tetracarboxylic dianhydride within a certain proportion range effectively enhances the heat resistance of the material, enabling it to maintain good dimensional stability and mechanical strength in a high-temperature environment, and can also optimize the molecular structure to reduce the coloring degree of the material, thereby greatly improving the transparency and significantly enhancing the comprehensive performance of the polyimide film material. If the proportion of aromatic diamine is too low, the number of rigid units in the molecular chain structure will decrease, thus reducing the heat resistance of the material; if the proportion of aromatic diamine is too high, it may increase the packing density between molecules, resulting in poor transparency.

[0046] The fluorine atoms contained in 4,4'-(hexafluoroisopropylidene)dibenzoic anhydride further weaken the intermolecular interaction, and can further optimize the comprehensive performance of the polyimide film material. However, when the proportion of 4,4'-(hexafluoroisopropylidene)dibenzoic anhydride is too high, its structural characteristics will significantly affect the regularity and aggregation state structure of the polyimide molecular chain. The introduction of too many hexafluoroisopropylidene groups will lead to closer stacking between molecular chains, thus increasing the light scattering phenomenon, and further reducing the transparency of the polyimide film.

[0047] Comparing Comparative Example 1 with Example 7, it can be seen that when acetic anhydride and pyridine in the imidization reagent are mixed in a specific molar ratio, acetic anhydride can effectively promote the ring-closure dehydration reaction of the polyamic acid solution to form a stable polyimide structure; pyridine can not only accelerate the dehydration reaction process, but also inhibit the occurrence of side reactions, ensuring the regularity of the molecular chain. The ratio optimizes the interaction between pyridine and acetic anhydride, avoiding side reactions while ensuring sufficient dehydration, thereby significantly improving the optical properties of the finally obtained transparent polyimide film, maintaining a low yellowness index, increasing the light transmittance, and improving the glass transition temperature, etc.

[0048] Comparing Comparative Example 1 with Examples 8 - 9, it can be seen that acetic anhydride and dianhydride in a certain proportion range can effectively promote the imidization reaction process of the polyamic acid solution, ensuring that the generated polyimide structure is more stable and uniform. The amount of acetic anhydride within the proportion range of this application avoids the problem of increased side reactions due to excessive acetic anhydride, which affects the material purity, and prevents the phenomenon of incomplete imidization due to insufficient acetic anhydride, thereby significantly improving the heat resistance and dimensional stability of the finally obtained transparent polyimide film.

[0049] Comparing Comparative Example 1 with Example 10, it can be seen that vacuum low-temperature treatment ensures that the treatment process will neither overly affect the mechanical properties of the material nor fully guarantee the complete removal of the solvent, thereby significantly improving the dimensional stability and surface flatness of the film, and to a certain extent improving the stability and light transmittance.

[0050] The above are all preferred embodiments of this application. The protection scope of this application is not limited thereby. Therefore, all equivalent changes made according to the structure, shape, and principle of this application should be covered within the protection scope of this application.

Claims

1. A high-temperature resistant transparent polyimide film material for flexible display, characterized in that: The polyimide film material is prepared by the polymerization reaction of aromatic diamines and dianhydrides; the aromatic diamines include a first diamine and a second diamine; the dianhydrides include cyclobutane tetracarboxylic dianhydride; the molar ratio of the aromatic diamines to the dianhydrides is 1:(1 - 1.1).

2. The high-temperature resistant transparent polyimide film material for flexible display according to claim 1, wherein: The first diamine is selected from at least one of 2,2'-bis(trifluoromethyl)benzidine and 4,4'-diaminobiphenyl; the second diamine is selected from at least one of 4,4'-diaminobenzanilide and 4,4`-[isopropylidene bis(phenoxy)]diphenylamine.

3. The high-temperature resistant transparent polyimide film material for flexible display according to claim 1, wherein: The molar ratio of the first diamine to the second diamine is (2 - 4):

1.

4. The high-temperature resistant transparent polyimide film material for flexible display according to claim 1, characterized in that: The dianhydrides also include 4,4'-(hexafluoroisopropylidene)diphthalic anhydride.

5. The preparation method of the high-temperature resistant transparent polyimide film material for flexible display according to any one of claims 1-4, characterized in that It includes the following steps: controlling the water bath temperature at 0 - 5°C, adding the diamine monomer to the solvent and stirring until completely dissolved; then adding the dianhydride and continuing to stir for 24 - 48 h to obtain a polyamic acid solution, adding an imidization reagent and continuing to stir for 18 - 24 h to obtain a transparent polyimide solution, and finally performing filtration, coating, and heat treatment to obtain a transparent polyimide film.

6. The preparation method of the high-temperature resistant transparent polyimide film material for flexible display according to claim 5, characterized in that: The solvent is selected from at least one of N-methyl-2-pyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide, dimethyl sulfoxide, chloroform, and γ-butyrolactone.

7. The preparation method of the high-temperature resistant transparent polyimide film material for flexible display according to claim 5, characterized in that: The imidization reagent is formed by mixing acetic anhydride and pyridine in a molar ratio of (4 - 6):

3.

8. The preparation method of the high-temperature resistant transparent polyimide film material for flexible display according to claim 7, wherein: The molar ratio of the acetic anhydride to the dianhydride is (4 - 6):

1.

9. The high-temperature resistant transparent polyimide film material for flexible display according to claim 5, characterized in that: The heat treatment includes vacuum low-temperature treatment and high-temperature treatment. Among them, the vacuum low-temperature treatment conditions are a vacuum degree of 8 - 10 Pa, a temperature of 60 - 70°C, and a time of 45 - 60 min; the high-temperature treatment is a heating process that sequentially passes through 90 - 100°C / 30 - 45 min, 170 - 180°C / 30 - 45 min, and 280 - 300°C / 30 - 45 min under nitrogen protection, and the heating rate is 1 - 5°C / min.

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