PI film and preparation method thereof

Abstract

The invention relates to the technical field of polyimide films, in particular to a PI film and a preparation method thereof. The PI film comprises the following raw materials in parts by mass: 100-150 parts of soluble polyimide resin, 5-15 parts of biphenyl epoxy resin, 5-10 parts of activated titanium dioxide, 1-5 parts of a curing agent and 1-3 parts of a curing accelerator, the soluble polyimide resin comprises the following raw materials in parts by mass: 5-10 parts of a first monomer, 25-30 parts of a second monomer, 50-60 parts of a third monomer, 20-30 parts of acetic anhydride and 5-15 parts of triethylamine. The size stability and heat resistance of the film can be remarkably improved, meanwhile, the outstanding transparency of the polyimide film can be guaranteed, and the application of the film in the field of flexible display is met.

Description

Technical Field

[0001] The invention relates to the technical field of polyimide films, and in particular to a PI film and a preparation method thereof. Background Art

[0002] With the rapid penetration of flexible electronic devices into wearable devices, foldable displays and other fields, display substrate materials are undergoing a major transformation from rigid glass to flexible polymer films. As the core substrate material of the flexible display industry, polyimide (PI) film has long dominated the market with its excellent thermal stability, excellent mechanical properties and good dielectric properties.

[0003] However, the rigid conjugated aromatic ring structure in the main chain of traditional polyimide films interacts strongly with the C=O group to form a charge transfer complex, which causes a significant decrease in the average transmittance of the film in the visible light region and exhibits a characteristic yellow light. At the same time, the high crystallinity and brittleness of the molecular chain segments lead to increased surface roughness after folding cycles, resulting in irreversible microcracks.

[0004] Existing technologies use fluorinated monomers to replace hydrogen atoms to improve light transmittance, but the fluorination process is complex and costly, and the introduction of rigid heteroatoms to destroy the conjugated system often leads to deterioration of thermal stability. Currently, nanocomposite modification has become the mainstream direction, among which nano-titanium dioxide is used to improve optical performance due to its high refractive index and UV blocking properties, but the introduction of nanoparticles easily agglomerates in the film to form stress defects, which damages the light transmittance and thermal stability of the film.

[0005] At present, how to improve the transparency, heat stability and bending resistance of polyimide films has become a technical problem that needs to be solved urgently. Summary of the invention

[0006] The purpose of the present invention is to solve the shortcomings of the prior art and to propose a PI film and a preparation method thereof.

[0007] A PI film, the raw materials of which include, by mass, 100-150 parts of soluble polyimide resin, 5-15 parts of biphenyl epoxy resin, 5-10 parts of activated titanium dioxide, 1-5 parts of curing agent, and 1-3 parts of curing accelerator. The raw materials of the soluble polyimide resin include, by mass, 5-10 parts of a first monomer, 25-30 parts of a second monomer, 50-60 parts of a third monomer, 20-30 parts of acetic anhydride, and 5-15 parts of triethylamine.

[0008] Preferably, the curing agent is methyltetrahydrophthalic anhydride.

[0009] Preferably, the curing accelerator is a tertiary amine accelerator, preferably 2-ethyl-4-methylimidazole.

[0010] Preferably, the first monomer is 3-(p-hydroxyphenoxy)-1,5-diaminobenzene or / and 4,4'-diamino-4''-hydroxytriphenylmethane.

[0011] Preferably, the second monomer is cis-1,4-cyclohexanediamine or / and 1-methyl-2,4-cyclohexanediamine.

[0012] Preferably, the third monomer is a dianhydride monomer, preferably 7-oxabicyclo[2.2.1]heptane tetracarboxylic dianhydride.

[0013] Among them, 7-oxabicyclo[2.2.1]heptane tetracarboxylic dianhydride can be prepared according to the detailed steps in the literature [1] (Zhang Anran. Synthesis of alicyclic dianhydrides using furan and norbornene monoanhydride as raw materials and properties of polyimides modified based on norbornene structure [D]. Nanchang University, 2012. DOI: 10.7666 / d.y2141352.).

[0014] Preferably, the soluble polyimide resin is prepared by the following steps: adding the first monomer, the second monomer, and the third monomer into N,N'-dimethylformamide for mixing, stirring at 1-4°C for 5-15 h under nitrogen protection, adding acetic anhydride and triethylamine and stirring for 10-15 h, adding the product into water, filtering, washing, and drying in vacuum to obtain the soluble polyimide resin.

[0015] Preferably, the activated titanium dioxide is prepared by the following steps: adding nano-titanium dioxide and a dispersant into an ethanol aqueous solution, adjusting the pH value of the system to 3-4, performing ultrasonic treatment for 1-2 h, adding terminal amino polyamidoamine, polyethylene glycol diacrylate, and azobisisobutyronitrile, stirring at 60-80°C for 1-2 h under nitrogen protection, raising the temperature to 85-90°C and continuing to stir for 10-30 min, cooling to room temperature, filtering, washing, and drying in vacuum.

[0016] More preferably, the mass ratio of nano-titanium dioxide, the dispersant, terminal amino polyamidoamine, polyethylene glycol diacrylate, and azobisisobutyronitrile is 5-10:1-2:1-3:1-2:0.1-1.

[0017] More preferably, the dispersant is polyvinylpyrrolidone

[0018] More preferably, the ultrasonic frequency is 50-80 kHz.

[0019] The preparation method of the above PI film includes the following steps: adding the soluble polyimide resin, biphenyl-type epoxy resin, activated titanium dioxide, curing agent, and curing accelerator into N,N'-dimethylformamide and stirring evenly, preparing a precursor film by the casting method, and performing heat treatment.

[0020] Preferably, the specific operation of the heat treatment is as follows: heat treatment at 120 - 130 °C for 25 - 35 min, then heat treatment at 180 - 200 °C for 50 - 70 min, and then heat treatment at 240 - 250 °C for 8 - 12 min.

[0021] Beneficial effects:

[0022] In the present invention, the first monomer and the second monomer are compounded, wherein the skeleton of the second monomer presents a V-shaped configuration, and in cooperation with the third monomer presenting a helical configuration, it can not only significantly improve the thermal stability of the polyimide, but also it is difficult to form creases on the surface when the film undergoes large-curvature bending, effectively solving the problem of poor bending resistance of the current film.

[0023] In the present invention, by introducing terminal amino polyamidoamine and polyethylene glycol diacrylate on the surface of nano-titanium dioxide particles, the flexibility of the polyethylene glycol diacrylate chain segment can effectively relieve the interfacial stress between titanium dioxide and the polyimide matrix and reduce light scattering; while the terminal amino polyamidoamine is combined with the soluble polyimide resin, which can not only reduce the resin viscosity during the casting process, but also avoid the settlement and secondary agglomeration problems of titanium dioxide particles.

[0024] The surface hyperbranched molecular structure of the activated titanium dioxide used in the present invention reduces chain entanglement, and the relatively large number of amino groups can enhance the compatibility between inorganic particles and the organic system. The formed cross-linked network structure can significantly improve the dimensional stability and heat resistance of the film, and at the same time can ensure the outstanding transparency of the polyimide film, meeting the application of this type of film in the flexible display field. Description of the drawings

[0025] Figure 1 It is a comparison chart of the fracture strength and total light transmittance of the PI films obtained in Example 5 and Comparative Examples 1 - 2.

[0026] Figure 2 It is a comparison chart of the glass transition temperature and coefficient of thermal expansion of the PI films obtained in Example 5 and Comparative Examples 1 - 2.

[0027] Figure 3 It is a comparison chart of the fracture strength retention rate of the PI films obtained in Example 5 and Comparative Examples 1 - 2 after 200,000 repeated foldings. Detailed implementation manners

[0028] The present invention will be further illustrated below with specific examples.

[0029] The following biphenyl-type epoxy resin used is purchased from Shanghai Zhongmou Industry Co., Ltd., with the brand name YX4000HK and an epoxy equivalent of 192 g / eq. The following 7-oxo-bicyclo[2.2.1]heptane tetracarboxylic dianhydride is prepared by the method in reference [1].

[0030] Example 1

[0031] A PI film, the raw materials of which include: 100 g of soluble polyimide resin, 5 g of biphenyl-type epoxy resin, 5 g of activated titanium dioxide, 1 g of methyltetrahydrophthalic anhydride, and 1 g of 2-ethyl-4-methylimidazole.

[0032] The raw materials of the soluble polyimide resin include: 5 g of 3-(p-hydroxyphenoxy)-1,5-diaminobenzene, 25 g of 1-methyl-2,4-cyclohexanediamine, 50 g of 7-oxabicyclo[2.2.1]heptane tetracarboxylic dianhydride, 20 g of acetic anhydride, and 5 g of triethylamine. The soluble polyimide resin is prepared by the following steps: Add 3-(p-hydroxyphenoxy)-1,5-diaminobenzene, 1-methyl-2,4-cyclohexanediamine, and 7-oxabicyclo[2.2.1]heptane tetracarboxylic dianhydride to 200 g of N,N'-dimethylformamide and mix. Stir at a temperature of 1°C for 5 h under nitrogen protection. Add acetic anhydride and triethylamine and stir for 10 h. Add the product to 500 g of deionized water, filter, wash, and vacuum dry.

[0033] The activated titanium dioxide is prepared by the following steps: Add 5 g of nano-titanium dioxide and 1 g of polyvinylpyrrolidone to 50 g of an ethanol aqueous solution with a mass fraction of 40%. Adjust the pH value of the system to 3-4 with a nitric acid solution with a concentration of 1 mol / L. Perform ultrasonic treatment for 1 h with an ultrasonic frequency of 50 kHz. Add 1 g of terminal amino polyamidoamine, 1 g of polyethylene glycol diacrylate, and 0.1 g of azobisisobutyronitrile. Stir at a temperature of 60°C for 1 h under nitrogen protection. Raise the temperature to 85°C and continue to stir for 10 min. Cool to room temperature, filter, wash with ethanol and water, and vacuum dry.

[0034] The preparation method of the above PI film includes the following steps: Add the soluble polyimide resin, biphenyl-type epoxy resin, activated titanium dioxide, methyltetrahydrophthalic anhydride, and 2-ethyl-4-methylimidazole to 200 g of N,N'-dimethylformamide and stir evenly. Make a precursor film by the casting method; Heat-treat at a temperature of 120°C for 25 min, then heat-treat at a temperature of 180°C for 50 min, and then heat-treat at a temperature of 240°C for 8 min.

[0035] Example 2

[0036] A PI film, the raw materials of which include: 150 g of soluble polyimide resin, 15 g of biphenyl-type epoxy resin, 10 g of activated titanium dioxide, 5 g of methyltetrahydrophthalic anhydride, and 3 g of 2-ethyl-4-methylimidazole.

[0037] The raw materials of the soluble polyimide resin include: 10 g of 4,4'-diamino-4”-hydroxy triphenyl methane, 30 g of 1-methyl-2,4-cyclohexanediamine, 60 g of 7-oxa-bicyclo[2.2.1]heptane tetracarboxylic dianhydride, 30 g of acetic anhydride, and 15 g of triethylamine. The soluble polyimide resin is prepared by the following steps: adding 4,4'-diamino-4”-hydroxy triphenyl methane, 1-methyl-2,4-cyclohexanediamine, and 7-oxa-bicyclo[2.2.1]heptane tetracarboxylic dianhydride into 500 g of N,N'-dimethylformamide and mixing them. Stir at 4°C for 15 h under nitrogen protection, add acetic anhydride and triethylamine and stir for 15 h. Add the product into 1000 g of deionized water, filter, wash, and dry in vacuum.

[0038] Activated titanium dioxide is prepared by the following steps: adding 10 g of nano titanium dioxide and 2 g of polyvinylpyrrolidone into 100 g of an ethanol aqueous solution with a mass fraction of 60%. Adjust the pH value of the system to 3 - 4 with a nitric acid solution with a concentration of 3 mol / L, perform ultrasonic treatment for 2 h with an ultrasonic frequency of 80 kHz. Add 3 g of terminal amino polyamidoamine, 2 g of polyethylene glycol diacrylate, and 1 g of azobisisobutyronitrile. Stir at 80°C for 2 h under nitrogen protection, raise the temperature to 90°C and continue stirring for 30 min, cool to room temperature, filter, wash with ethanol and water, and dry in vacuum.

[0039] The preparation method of the above PI film includes the following steps: adding the soluble polyimide resin, biphenyl type epoxy resin, activated titanium dioxide, methyltetrahydrophthalic anhydride, and 2-ethyl-4-methylimidazole into 300 g of N,N'-dimethylformamide and stirring evenly to form a precursor film by the casting method; perform heat treatment at 130°C for 35 min, then at 200°C for 70 min, and then at 250°C for 12 min.

[0040] Example 3

[0041] A PI film, whose raw materials include: 110 g of soluble polyimide resin, 12 g of biphenyl type epoxy resin, 7 g of activated titanium dioxide, 4 g of methyltetrahydrophthalic anhydride, and 1.5 g of 2-ethyl-4-methylimidazole.

[0042] The raw materials of the soluble polyimide resin include: 9 g of 3-(p-hydroxyphenoxy)-1,5-diaminobenzene, 26 g of cis-1,4-cyclohexanediamine, 52 g of 7-oxabicyclo[2.2.1]heptane tetracarboxylic dianhydride, 28 g of acetic anhydride, and 7 g of triethylamine. The soluble polyimide resin is prepared by the following steps: Add 3-(p-hydroxyphenoxy)-1,5-diaminobenzene, cis-1,4-cyclohexanediamine, and 7-oxabicyclo[2.2.1]heptane tetracarboxylic dianhydride to 400 g of N,N'-dimethylformamide and mix. Stir at 2 °C for 12 h under nitrogen protection, add acetic anhydride and triethylamine and stir for 11 h. Add the product to 900 g of deionized water, filter, wash, and dry in vacuum.

[0043] The activated titanium dioxide is prepared by the following steps: Add 7 g of nano-titanium dioxide and 1.7 g of polyvinylpyrrolidone to 70 g of an ethanol aqueous solution with a mass fraction of 55%. Adjust the pH value of the system to 3-4 with a nitric acid solution with a concentration of 1.5 mol / L, perform ultrasonic treatment for 100 min at an ultrasonic frequency of 60 kHz. Add 2.5 g of terminal amino polyamidoamine, 1.2 g of polyethylene glycol diacrylate, and 0.7 g of azobisisobutyronitrile. Stir at 65 °C for 100 min under nitrogen protection, raise the temperature to 86 °C and continue stirring for 25 min. Cool to room temperature, filter, wash with ethanol and water, and dry in vacuum.

[0044] The preparation method of the above PI film includes the following steps: Add the soluble polyimide resin, biphenyl-type epoxy resin, activated titanium dioxide, methyltetrahydrophthalic anhydride, and 2-ethyl-4-methylimidazole to 220 g of N,N'-dimethylformamide and stir evenly to form a precursor film by the casting method; Heat-treat at 128 °C for 28 min, then heat-treat at 195 °C for 55 min, and then heat-treat at 248 °C for 9 min.

[0045] Example 4

[0046] A PI film, whose raw materials include: 130 g of soluble polyimide resin, 8 g of biphenyl-type epoxy resin, 9 g of activated titanium dioxide, 2 g of methyltetrahydrophthalic anhydride, and 2.5 g of 2-ethyl-4-methylimidazole.

[0047] The raw materials of the soluble polyimide resin include: 7 g of 3-(p-hydroxyphenoxy)-1,5-diaminobenzene, 29 g of cis-1,4-cyclohexanediamine, 58 g of 7-oxabicyclo[2.2.1]heptane tetracarboxylic dianhydride, 22 g of acetic anhydride, and 13 g of triethylamine. The soluble polyimide resin is prepared by the following steps: Add 3-(p-hydroxyphenoxy)-1,5-diaminobenzene, cis-1,4-cyclohexanediamine, and 7-oxabicyclo[2.2.1]heptane tetracarboxylic dianhydride to 300 g of N,N'-dimethylformamide and mix. Stir at 3°C for 8 h under nitrogen protection. Add acetic anhydride and triethylamine and stir for 13 h. Add the product to 700 g of deionized water, filter, wash, and dry in vacuum.

[0048] The activated titanium dioxide is prepared by the following steps: Add 9 g of nano-titanium dioxide and 1.3 g of polyvinylpyrrolidone to 90 g of an ethanol aqueous solution with a mass fraction of 45%. Adjust the pH value of the system to 3 - 4 with a nitric acid solution with a concentration of 2.5 mol / L. Perform ultrasonic treatment for 80 min at an ultrasonic frequency of 70 kHz. Add 1.5 g of terminal amino polyamidoamine, 1.8 g of polyethylene glycol diacrylate, and 0.3 g of azobisisobutyronitrile. Stir at 75°C for 80 min under nitrogen protection. Raise the temperature to 88°C and continue stirring for 15 min. Cool to room temperature, filter, wash with ethanol and water, and dry in vacuum.

[0049] The preparation method of the above PI film includes the following steps: Add the soluble polyimide resin, biphenyl-type epoxy resin, activated titanium dioxide, methyltetrahydrophthalic anhydride, and 2-ethyl-4-methylimidazole to 280 g of N,N'-dimethylformamide and stir evenly. Make a precursor film by the casting method; Heat-treat at 122°C for 32 min, then heat-treat at 185°C for 65 min, and then heat-treat at 242°C for 11 min.

[0050] Example 5

[0051] A PI film, whose raw materials include: 120 g of soluble polyimide resin, 10 g of biphenyl-type epoxy resin, 8 g of activated titanium dioxide, 3 g of methyltetrahydrophthalic anhydride, and 2 g of 2-ethyl-4-methylimidazole.

[0052] The raw materials of the soluble polyimide resin include: 8 g of 4,4'-diamino-4''-hydroxytriphenylmethane, 27.5 g of cis-1,4-cyclohexanediamine, 55 g of 7-oxabicyclo[2.2.1]heptanetetracarboxylic dianhydride, 25 g of acetic anhydride, and 10 g of triethylamine. The soluble polyimide resin is prepared by the following steps: Add 4,4'-diamino-4''-hydroxytriphenylmethane, cis-1,4-cyclohexanediamine, and 7-oxabicyclo[2.2.1]heptanetetracarboxylic dianhydride to 350 g of N,N'-dimethylformamide and mix. Stir at 2 °C for 10 h under nitrogen protection, add acetic anhydride and triethylamine and stir for 12 h. Add the product to 800 g of deionized water, filter, wash, and dry in vacuum.

[0053] The activated titanium dioxide is prepared by the following steps: Add 8 g of nano-titanium dioxide and 1.5 g of polyvinylpyrrolidone to 80 g of an ethanol aqueous solution with a mass fraction of 50%. Adjust the pH value of the system to 3-4 with a 2 mol / L nitric acid solution, perform ultrasonic treatment for 90 min at an ultrasonic frequency of 65 kHz. Add 2 g of terminal amino polyamidoamine, 1.5 g of polyethylene glycol diacrylate, and 0.5 g of azobisisobutyronitrile. Stir at 70 °C for 90 min under nitrogen protection, raise the temperature to 87 °C and continue stirring for 20 min. Cool to room temperature, filter, wash with ethanol and water, and dry in vacuum.

[0054] The preparation method of the above PI film includes the following steps: Add the soluble polyimide resin, biphenyl-type epoxy resin, activated titanium dioxide, methyltetrahydrophthalic anhydride, and 2-ethyl-4-methylimidazole to 250 g of N,N'-dimethylformamide and stir evenly to form a precursor film by the casting method; Heat-treat at 125 °C for 30 min, then heat-treat at 190 °C for 60 min, and then heat-treat at 245 °C for 10 min.

[0055] Comparative Example 1

[0056] A PI film, whose raw materials include: 120 g of soluble polyimide resin, 10 g of biphenyl-type epoxy resin, 8 g of activated titanium dioxide, 3 g of methyltetrahydrophthalic anhydride, and 2 g of 2-ethyl-4-methylimidazole.

[0057] The raw materials of the soluble polyimide resin include: 8 g of 4,4'-diamino-4"-hydroxytriphenylmethane, 27.5 g of cis-1,4-cyclohexanediamine, 55 g of 7-oxabicyclo[2.2.1]heptanetetracarboxylic dianhydride, 25 g of acetic anhydride, and 10 g of triethylamine. The soluble polyimide resin is prepared by the following steps: Add 4,4'-diamino-4"-hydroxytriphenylmethane, cis-1,4-cyclohexanediamine, and 7-oxabicyclo[2.2.1]heptanetetracarboxylic dianhydride to 350 g of N,N'-dimethylformamide and mix. Stir at 2 °C for 10 h under nitrogen protection. Add acetic anhydride and triethylamine and stir for 12 h. Add the product to 800 g of deionized water, filter, wash, and dry in vacuum.

[0058] Activated titanium dioxide is prepared by the following steps: Add 8 g of nano-titanium dioxide and 1.5 g of polyvinylpyrrolidone to 80 g of an ethanol aqueous solution with a mass fraction of 50%. Adjust the pH value of the system to 3 - 4 with a 2 mol / L nitric acid solution. Perform ultrasonic treatment for 90 min at an ultrasonic frequency of 65 kHz. Add 3.5 g of amino-terminated polyamidoamine and 0.5 g of azobisisobutyronitrile. Stir at 70 °C for 90 min under nitrogen protection. Raise the temperature to 87 °C and continue stirring for 20 min. Cool to room temperature, filter, wash with ethanol and water, and dry in vacuum.

[0059] The preparation method of the above PI film includes the following steps: Add the soluble polyimide resin, biphenyl-type epoxy resin, activated titanium dioxide, methyltetrahydrophthalic anhydride, and 2-ethyl-4-methylimidazole to 250 g of N,N'-dimethylformamide and stir evenly. Make a precursor film by the casting method; Heat-treat at 125 °C for 30 min, then heat-treat at 190 °C for 60 min, and then heat-treat at 245 °C for 10 min.

[0060] Comparative Example 2

[0061] A PI film, whose raw materials include: 120 g of soluble polyimide resin, 10 g of biphenyl-type epoxy resin, 8 g of activated titanium dioxide, 3 g of methyltetrahydrophthalic anhydride, and 2 g of 2-ethyl-4-methylimidazole.

[0062] The raw materials of the soluble polyimide resin include: 8 g of 4,4'-diamino-4"-hydroxytriphenylmethane, 27.5 g of 4,4'-diaminodiphenyl ether, 55 g of 7-oxabicyclo[2.2.1]heptanetetracarboxylic dianhydride, 25 g of acetic anhydride, and 10 g of triethylamine. The soluble polyimide resin is prepared by the following steps: Add 4,4'-diamino-4"-hydroxytriphenylmethane, 4,4'-diaminodiphenyl ether, and 7-oxabicyclo[2.2.1]heptanetetracarboxylic dianhydride to 350 g of N,N'-dimethylformamide and mix. Stir at a temperature of 2°C for 10 h under nitrogen protection. Add acetic anhydride and triethylamine and stir for 12 h. Add the product to 800 g of deionized water, filter, wash, and dry in vacuo.

[0063] The activated titanium dioxide is prepared by the following steps: Add 8 g of nano-titanium dioxide and 1.5 g of polyvinylpyrrolidone to 80 g of an ethanol aqueous solution with a mass fraction of 50%. Adjust the pH value of the system to 3 - 4 using a nitric acid solution with a concentration of 2 mol / L. Perform ultrasonic treatment for 90 min at an ultrasonic frequency of 65 kHz. Add 2 g of terminal amino polyamidoamine, 1.5 g of polyethylene glycol diacrylate, and 0.5 g of azobisisobutyronitrile. Stir at a temperature of 70°C for 90 min under nitrogen protection. Raise the temperature to 87°C and continue stirring for 20 min. Cool to room temperature, filter, wash with ethanol and water, and dry in vacuo.

[0064] The preparation method of the above PI film includes the following steps: Add the soluble polyimide resin, biphenyl-type epoxy resin, activated titanium dioxide, methyltetrahydrophthalic anhydride, and 2-ethyl-4-methylimidazole to 250 g of N,N'-dimethylformamide and stir evenly. Make a precursor film by the casting method; Heat-treat at a temperature of 125°C for 30 min, then heat-treat at a temperature of 190°C for 60 min, and then heat-treat at a temperature of 245°C for 10 min.

[0065] Measure the thicknesses of the PI films obtained in Example 5 and Comparative Examples 1 - 2. The thicknesses of all three are within the range of 20 ± 1 μm. Subsequently, refer to T / CIEP 0106-2024 "Transparent Polyimide Films for Flexible Touch Substrates" to measure the fracture strength, total light transmittance, glass transition temperature, and coefficient of thermal expansion of the PI films obtained in Example 5 and Comparative Examples 1 - 2.

[0066] As Figure 1 and Figure 2 shown, the PI film obtained in Example 5 has the highest fracture strength, total light transmittance, and glass transition temperature, and the smallest coefficient of thermal expansion, which is superior to Comparative Examples 1 - 2 (P < 0.05). It is confirmed that the PI film obtained by the present invention has excellent transparency, heat resistance stability, and mechanical properties.

[0067] The PI films obtained in Example 5 and Comparative Examples 1-2 were repeatedly folded 200,000 times (with a curvature radius of 5 mm) using a MIT folding endurance tester. Creases were found on the PI films obtained in Comparative Examples 1-2, while the PI film obtained in Example 5 showed no change. Then, the breaking strength of the three groups of PI films after repeated folding was measured, and the breaking strength retention rate was calculated to characterize their bending resistance performance.

[0068] As Figure 3 shown, the PI film obtained in Example 5 had the highest breaking strength retention rate, which was better than that of Comparative Examples 1-2 (P < 0.05), that is, the PI film obtained in Example 5 had the best bending resistance performance.

[0069] The applicant believes that: This is because the present invention uses a combination of a first monomer and a second monomer, wherein the backbone of the second monomer presents a V-shaped configuration, and in cooperation with the third monomer having a helical configuration, it can not only significantly improve the thermal stability of the polyimide, but also make it difficult to form creases on the surface when the film undergoes bending with a large curvature, effectively solving the problem of poor bending resistance performance of current films. The present invention introduces terminal amino polyamidoamine and polyethylene glycol diacrylate on the surface of nano-titanium dioxide particles. The flexibility of the polyethylene glycol diacrylate chain segment can effectively relieve the interfacial stress between the titanium dioxide and the polyimide matrix and reduce light scattering; while the terminal amino polyamidoamine cooperates with the soluble polyimide resin, which can not only reduce the resin viscosity during the casting process, but also avoid the settlement and secondary agglomeration problems of titanium dioxide particles. The surface hyperbranched molecular structure of the activated titanium dioxide used in the present invention reduces chain entanglement, and the relatively large number of amino groups can enhance the compatibility between the inorganic particles and the organic system. The formed cross-linked network structure can significantly improve the dimensional stability and heat resistance of the film, and at the same time ensure the excellent transparency of the polyimide film, meeting the application requirements of such films in the flexible display field.

[0070] The above is only the preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, making equivalent substitutions or changes, should be covered by the protection scope of the present invention.

Claims

1. A PI film, characterized in that, Its raw materials by mass parts include: 100 - 150 parts of soluble polyimide resin, 5 - 15 parts of biphenyl type epoxy resin, 5 - 10 parts of activated titanium dioxide, 1 - 5 parts of curing agent, and 1 - 3 parts of curing accelerator; The raw materials of the soluble polyimide resin by mass parts include: 5 - 10 parts of the first monomer, 25 - 30 parts of the second monomer, 50 - 60 parts of the third monomer, 20 - 30 parts of acetic anhydride, and 5 - 15 parts of triethylamine.

2. The PI film according to claim 1, wherein The curing agent is methyltetrahydrophthalic anhydride, and the curing accelerator is a tertiary amine accelerator.

3. The PI film according to claim 1, wherein The first monomer is 3-(p - hydroxyphenoxy)-1,5 - diaminobenzene or / and 4,4’ - diamino - 4” - hydroxytriphenylmethane.

4. The PI film according to claim 1, characterized in that, The second monomer is cis - 1,4 - cyclohexanediamine or / and 1 - methyl - 2,4 - cyclohexanediamine.

5. The PI film according to claim 1, wherein The third monomer is a dianhydride monomer, preferably 7 - oxa - bicyclo[2.2.1]heptane tetracarboxylic dianhydride.

6. The PI film according to claim 1, wherein The soluble polyimide resin is prepared by the following steps: adding the first monomer, the second monomer, and the third monomer into N,N’ - dimethylformamide for mixing, stirring at 1 - 4°C for 5 - 15 h under nitrogen protection, adding acetic anhydride and triethylamine and stirring for 10 - 15 h, adding the product into water, filtering, washing, and drying under vacuum to obtain the soluble polyimide resin.

7. The PI film according to claim 1, wherein The activated titanium dioxide is prepared by the following steps: adding nano - titanium dioxide and a dispersant into an ethanol - aqueous solution, adjusting the pH value of the system to 3 - 4, performing ultrasonic treatment for 1 - 2 h, adding amino - terminated polyamide - amine, polyethylene glycol diacrylate, and azobisisobutyronitrile, stirring at 60 - 80°C for 1 - 2 h under nitrogen protection, raising the temperature to 85 - 90°C and continuing to stir for 10 - 30 min, cooling to room temperature, filtering, washing, and drying under vacuum.

8. The PI film according to claim 7, wherein The mass ratio of nano - titanium dioxide, dispersant, amino - terminated polyamide - amine, polyethylene glycol diacrylate, and azobisisobutyronitrile is 5 - 10:1 - 2:1 - 3:1 - 2:0.1 - 1.

9. A method for preparing a PI film according to any one of claims 1-8, characterized in that, It includes the following steps: adding the soluble polyimide resin, biphenyl type epoxy resin, activated titanium dioxide, curing agent, and curing accelerator into N,N’ - dimethylformamide and stirring evenly, preparing a precursor film by the casting method, and performing heat treatment.

10. The preparation method of the PI film according to claim 9, characterized in that, The specific operation of the heat treatment is as follows: heat treatment at 120 - 130°C for 25 - 35 min, then heat treatment at 180 - 200°C for 50 - 70 min, and then heat treatment at 240 - 250°C for 8 - 12 min.

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