Polyimide film, preparation method therefor, and use thereof

Abstract

A polyimide film, a preparation method therefor, and a use thereof. The polyimide film has a thickness greater than or equal to 250 μm, and comprises an A film layer and at least one B film layer in contact with one another; the A film layer has a surface roughness Rz≤1.0 μm; in the B film layer, the number of micropores having a size greater than 0.1 mm is ≤1 / m², the B film layer has a density ≥1.45 g / cm3, and a surface roughness Rz≥1.2 μm. The polyimide film has the characteristics of large thickness, small linear thermal expansion coefficient, high pressure resistance, and low gas permeability.

Description

A polyimide film, its preparation method and application

[0001] This application claims priority to Chinese Patent Application No. 202411548016.6, filed on November 1, 2024, entitled "A polyimide film and its preparation method and application", the entire contents of which are incorporated herein by reference. Technical Field

[0002] This application relates to a polyimide film, and more particularly to a polyimide film, its preparation method and application, belonging to the field of polyimide film manufacturing technology. Background Technology

[0003] Ultra-thick polyimide films with a thickness greater than 100μm possess excellent heat resistance and insulation properties, outstanding mechanical and insulating properties, good chemical stability, and superior dielectric properties. They have been widely used as precursor films for high thermal conductivity artificial graphite films, substrate films, cover films, and reinforcing films for microelectronic circuits, high-temperature protective films in flexible copper-clad laminate processes, protective films for lithium-ion battery insulation systems, protective films for water-cooling systems in new energy vehicles, and insulating films for motors and special electrical appliances. In recent years, in particular, the market demand for ultra-thick polyimide films with a thickness greater than 100μm has been continuously increasing, driven by the transformation of 3C electronic products towards more compact structures and higher operational efficiency, and the development of new energy vehicles towards higher safety and reliability.

[0004] Polyimide is a stubborn polymer material that is difficult to dissolve and melt. Currently, industrialized polyimide film manufacturing technology mainly employs a two-step method. First, a polyamic acid solution is prepared by polycondensation of aromatic dianhydrides and aromatic diamines in a solvent. Then, a film containing a certain amount of solvent is prepared by casting the polyamic acid solution, or by casting a mixture of polyamic acid with dehydrating agents and catalysts. Finally, the film is stretched and subjected to high-temperature treatment to obtain the polyimide film. Using this manufacturing technology, as the thickness of the produced polyimide film increases, the solvent content of the film also increases exponentially. Controlling and effectively removing the solvent content during the process not only affects the performance of the final polyimide film but also directly relates to the stability and safety of the manufacturing process.

[0005] Currently, the preparation methods for polyimide films face the following technical challenges:

[0006] 1. The difficulty in controlling the uniformity of solvent evaporation in the thickness direction of the adhesive film increases. The solvent on the surface of the adhesive film is easy to evaporate and form a hard cured layer. The evaporation of the solvent in the inner layer is hindered, resulting in the phenomenon of "surface drying". The solvent cannot evaporate in time, resulting in poor mechanical properties and insufficient self-support of the adhesive film. It is easy to tear under tension, leading to production interruption or even production failure.

[0007] 2. When the adhesive film containing a large amount of solvent is dried on the smooth annular steel belt, the rapid and large-scale evaporation of the solvent can cause the adhesive film to shrink drastically, resulting in a narrowing width that cannot meet the processing width. Furthermore, when the adhesive film reaches the bottom of the annular steel belt, the increased weight of the adhesive film can cause insufficient adhesion between the adhesive film and the steel belt, leading to the adhesive film detaching from the steel belt and making production impossible.

[0008] 3. When the film enters the stretching process, the higher processing temperature will cause the film to shrink more violently, and the film is very easy to detach from the fixture; insufficient mechanical properties of the film will also cause the film to tear from the fixture and detach from the fixture; the film that has detached from the fixture will roll up and accumulate, and the large amount of solvent that cannot evaporate in time will cause fire safety problems.

[0009] 4. In addition to the above engineering problems, the rapid evaporation of the internal solvent during the film drying process can also lead to an increase in pore defects, insufficient mechanical properties of the prepared ultra-thick polyimide film, and quality problems. Ultimately, this will limit the application of the produced ultra-thick polyimide film, or even make it unusable.

[0010] Therefore, controlling the solvent content of intermediate adhesive films and balancing mechanical properties are key bottlenecks in the production of ultra-thick polyimide films.

[0011] US Patent US20220152912A1 discloses a method for manufacturing polyimide films with a thickness greater than 400 μm. This method avoids the aforementioned technical challenges by controlling the thickness and solvent content of the single-layer adhesive film and combining it with a lamination method. However, this method requires complex equipment, has high processing costs, and the produced ultra-thick polyimide films are expected to have significant interlayer interfaces, making them prone to delamination under high temperatures and stress. US Patent US20220152912A1 also describes polyimide films with a thickness greater than 200 μm. Production typically requires drying to remove solvent at speeds less than 1 m / min, resulting in extremely low production efficiency, high production costs, and an inability to produce polyimide films with a thickness greater than 400 μm. Chinese invention patent CN109647683 discloses a method for preparing polyimide films with a thickness of 50μm to 135μm through one or multiple spraying processes, which effectively avoids the "surface drying" problem. However, the process is relatively complex, and the mechanical and thermal properties of the produced polyimide films are difficult to meet the requirements of high-tech industry applications. Chinese invention patent CN108527745A discloses a specially designed drying device that uses a method of heating one side of the film while simultaneously cooling it to allow the solvent to diffuse and evaporate evenly from one side to the other. However, this method easily causes solvent to condense on the surface of the cooling chamber and drip back onto the film surface, forming blemishes. Chinese patent CN217654241U discloses an anti-condensation temperature control and fire extinguishing system for polyimide film production, which can effectively solve the fire problems caused by the film detaching from the fixture and film aggregation during high-temperature stretching. However, it does not address solutions for controlling the solvent content and balancing the mechanical properties of thick films. Chinese invention patents CN110423467B, CN101168598A, and CN113788478A all describe methods for manufacturing ultra-thick polyimide films, but do not explain specific solutions to the aforementioned technical challenges. The thickness of the ultra-thick polyimide films described in these patents is all less than 250 μm; manufacturing technologies for polyimide films with a thickness greater than 250 μm still require further development.

[0012] Therefore, developing a method for preparing a polyimide film with a thickness greater than 250 μm has become a key research direction in this field. Summary of the Invention

[0013] This application provides a polyimide film with the characteristics of large thickness, small linear thermal expansion coefficient, high pressure resistance and low gas permeability.

[0014] This application also provides a method for preparing a polyimide film, which has the advantages of being able to manufacture composite polyimide films with two or more layers and reducing the number of pores and defects inside the polyimide film.

[0015] This application also provides a circuit board with high dimensional stability.

[0016] This application also provides a gas barrier composite membrane, which has the characteristics of high gas barrier properties.

[0017] This application also provides an insulating composite film with high insulation properties.

[0018] This application provides a polyimide film, wherein the thickness of the polyimide film is greater than or equal to 250 μm, and includes an A film layer in contact with each other and at least one B film layer;

[0019] The surface roughness Rz of the A film layer is ≤1.0 μm;

[0020] The number of micropores larger than 0.1 mm in the B membrane layer is ≤1 per m. 2 The density of the B film layer is ≥1.45 g / cm³. 3 Surface roughness Rz≥1.2μm.

[0021] The polyimide film described above has a linear coefficient of thermal expansion of 12 ppm / ℃ to 35 ppm / ℃, a breakdown voltage ≥ 30 kV, and an H2 transmittance ≤ 1.0 × 10⁻⁶. - 15 mol·m / m 2 The glass transition temperature of the polyimide in the A film layer is 180℃~300℃.

[0022] In the polyimide film described above, the thickness of film layer A accounts for 20-50% of the total thickness of the polyimide film.

[0023] This application also provides a method for preparing a polyimide film, which can be used to obtain any of the above-mentioned polyimide films. The method includes the following steps:

[0024] 1) Mix the first aromatic dianhydride, the first aromatic diamine, the low-boiling-point solvent, the high-boiling-point solvent, and the filler to allow the first aromatic dianhydride and the first aromatic diamine to undergo a polycondensation reaction to obtain the first resin solution;

[0025] 2) Mix the second aromatic dianhydride, the second aromatic diamine, the high-boiling-point solvent, and the filler to allow the second aromatic dianhydride and the second aromatic diamine to undergo a polycondensation reaction to obtain the second resin solution;

[0026] 3) The solution film formed by casting the first resin solution is subjected to a first heating and drying treatment to obtain the initial A film layer;

[0027] 4) Mix the second resin solution with a chemical imidizing agent and a molecular weight reinforcing agent to obtain a second resin mixture solution; use the second resin mixture solution to perform at least one lamination-heating treatment on the initial A film layer to obtain an initial composite film;

[0028] 5) The initial composite film is subjected to at least one heating and drying treatment to obtain an intermediate composite film;

[0029] 6) The intermediate composite film is subjected to high-temperature heating and biaxial stretching treatment to obtain the polyimide film;

[0030] The first heating and drying treatment is performed at a temperature of 50℃ to 120℃ for a time of 2 min to 10 min.

[0031] The lamination-heat treatment includes attaching the second resin mixture solution to one side surface of the initial A film layer and performing a heat treatment at a temperature of 60°C to 150°C for a time of 5 min to 15 min.

[0032] The initial composite film is subjected to a heating and drying process at a temperature of 50°C to 180°C for 18 min to 60 min.

[0033] The first aromatic dianhydride is selected from at least one of the following: pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 2,3,3',4-biphenyltetracarboxylic dianhydride, 4,4'-biphenyl ether dianhydride, 3,4,3',4'-triphenyl diether dianhydride, 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]malonic dianhydride, 2,2-diphenylpropane-3,4,3',4'-tetracarboxylic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 3,3',4,4'-diphenylsulfone tetracarboxylic dianhydride, and p-phenylene-bisphenyltriester dianhydride;

[0034] The first aromatic diamine is selected from at least one of 4,4'-diaminodiphenyl ether, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, 4,4'-diaminodiphenylmethane, 2,2-bis(4-aminophenyl)propane, 1,3-bis(3-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)benzene, 2,4-diaminotoluene, and 1,3-bis(aminopropane)tetramethyldisilether;

[0035] The second aromatic dianhydride is selected from at least one of pyromellitic dianhydride and 3,3,4,4'-biphenyltetracarboxylic dianhydride;

[0036] The second aromatic diamine is selected from at least one of 4,4'-diaminodiphenyl ether and 1,4-phenylenediamine;

[0037] The solid content of the first resin solution is 10%-50%;

[0038] The solid content of the second resin solution is 10%-35%;

[0039] The low-boiling-point solvent is selected from at least one of acetone, butanone, toluene, ethyl acetate, tetrahydrofuran, and trichloromethane;

[0040] The high-boiling-point solvent is selected from at least one of N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone;

[0041] The filler is selected from at least one of calcium carbonate, calcium phosphate, calcium pyrophosphate, calcium hydrogen phosphate, silicon carbide, carbon nanotubes, graphene, carbon black, silicon dioxide, and titanium dioxide.

[0042] The chemical imidizing agent includes a dehydrating agent, a catalyst, a solvent, and an auxiliary agent;

[0043] The dehydrating agent is selected from at least one of acetic anhydride, trifluoroacetic anhydride, benzoic anhydride, acetyl chloride, and thionyl chloride;

[0044] The catalyst is selected from at least one of quinoline, isoquinoline, pyridine, 3-methylpyridine, triethylamine, and N-methylimidazole;

[0045] The solvent is selected from at least one of N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone;

[0046] The adjuvant is selected from at least one of tricresyl phosphate, triphenyl phosphate, toluene diphenyl phosphate, and trioctyl trimellitate;

[0047] The molecular weight reinforcing agent includes at least one of 1,2,4,5-pyromellitic acid, 3,3',4,4'-biphenyltetracarboxylic acid, tetrahydrofuran-2,3,4,5-tetracarboxylic acid, cyclohexane-1,2,4,5-tetracarboxylic acid, 1,2,3,4-cyclobutanetetracarboxylic acid, terephthalic acid, isophthalic acid, and terephthaloyl chloride.

[0048] The heating temperature for the high-temperature heating and biaxial stretching treatment described in step 6) is 150℃~600℃;

[0049] The molecular weight reinforcing agent also includes at least one of N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone.

[0050] In the preparation method described above, in step 1), the molar ratio of the first aromatic dianhydride to the first aromatic diamine is 0.99:1 to 1.05:1; and / or,

[0051] The molar ratio of the second aromatic dianhydride to the second aromatic diamine is 0.8:1 to 0.995:1.

[0052] In the preparation method described above, the solid content of the first resin solution is 30%-40%; and / or,

[0053] The solid content of the second resin solution is 25%-35%.

[0054] In the preparation method described above, the mass of the chemical imidizing agent is 1%-30% of the mass of the second resin solution; and / or,

[0055] The molar ratio of dehydrating agent to catalyst in the chemical imidizing reagent is 1:2 to 5:1; the mass fraction of the solvent is 10% to 40%; the mass fraction of the auxiliary agent is 1% to 10%; and / or,

[0056] The amount of filler added is 0.05% to 5% of the sum of the masses of the aromatic diamine and the aromatic dianhydride.

[0057] In the preparation method described above, the amount of the molecular weight reinforcing agent is equal to the difference between the amounts of the second aromatic diamine and the second aromatic dianhydride.

[0058] In the preparation method described above, step 5) includes heating and drying the upper and lower surfaces of the initial composite film, wherein the heating temperature of the upper surface is 50°C to 150°C and the heating temperature of the lower surface is 80°C to 180°C.

[0059] In the preparation method described above, step 5) involves heating and drying the initial composite film, which includes sequentially performing a second heating and drying process, a third heating and drying process, a fourth heating and drying process, a fifth heating and drying process, and a sixth heating and drying process.

[0060] The upper surface heating temperature of the second heating and drying treatment is 80℃~120℃, the lower surface heating temperature is 120℃~150℃, and the treatment time is 5min~15min;

[0061] The upper surface heating temperature of the third heating and drying process is 90℃~135℃, the lower surface heating temperature is 135℃~180℃, and the processing time is 1min~8min;

[0062] The upper surface heating temperature of the fourth heating and drying process is 80℃~120℃, the lower surface heating temperature is 120℃~150℃, and the processing time is 5min~15min.

[0063] The upper surface heating temperature of the fifth heating and drying process is 60℃~120℃, the lower surface heating temperature is 120℃~150℃, and the processing time is 5min~15min;

[0064] The upper surface heating temperature of the sixth heating and drying process is 50℃~100℃, the lower surface heating temperature is 100℃~120℃, and the processing time is 2min~10min.

[0065] In the preparation method described above, in the stacking-heat treatment, the heating temperature of the upper surface is 60℃~120℃, and the heating temperature of the lower surface is 120℃~180℃.

[0066] This application also provides a circuit board, wherein the circuit board includes any of the above-described polyimide films.

[0067] This application also provides a gas barrier composite membrane, wherein the gas barrier composite membrane comprises any of the above-mentioned polyimide membranes.

[0068] This application also provides an insulating composite film, wherein the insulating composite film comprises any of the above-mentioned polyimide films.

[0069] The polyimide film provided in this application has the characteristics of large thickness, small linear thermal expansion coefficient, high pressure resistance and low gas permeability. Attached Figure Description

[0070] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0071] Figure 1 is a structural diagram of a polyimide film production device.

[0072] Explanation of reference numerals in the attached drawings: 101-First extrusion die; 102-Second extrusion die; 103-Mixer; 104-First heating drive roller; 105-First heating driven roller; 106-Second heating driven roller; 107-Third heating driven roller; 108-Fourth heating driven roller; 109-Fifth heating driven roller; 110-Sixth heating driven roller; 111-Second heating drive roller; A-First drying zone; B-Second drying zone; C-Third drying zone; D-Fourth drying zone; E-Fifth drying zone; F-Sixth drying zone; G-Seventh drying zone. Detailed Implementation

[0073] To enable those skilled in the art to better understand the solutions of this application, a further detailed description of this application is provided below. The specific embodiments listed below are merely descriptions of the principles and features of this application; the examples are only for explaining this application and are not intended to limit its scope. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of this application.

[0074] The first aspect of this application provides a polyimide film with a thickness of 250 μm or more, comprising an A film layer in contact with each other and at least one B film layer.

[0075] The surface roughness Rz of film layer A is ≤1.0μm;

[0076] The number of micropores larger than 0.1 mm in the B membrane layer is ≤1 per square meter, and the density of the B membrane layer is ≥1.45 g / cm³. 3 Surface roughness Rz≥1.2μm.

[0077] The polyimide film provided in this application is a multilayer film with a total thickness of ≥250μm and ≥2 layers. This multilayer film comprises two components: a first component forming film layer A and a second component forming film layer B. Film layer A is a single layer located on one side of the polyimide film, while film layer B can be one or more layers. Film layers A and B are fused together in a stacked manner. Therefore, in one embodiment, when the polyimide film has a two-layer structure, the layers from bottom to top are: film layer A and film layer B; when the polyimide film has a three-layer structure, the layers from bottom to top are: film layer A, first film layer B, and second film layer B. Film layer B is the main component of the composite film, and film layer A is a functional layer that assists in the manufacture of film layer B. During molding, film layer A is located between the support (e.g., steel strip) and film layer B, resulting in a relatively low surface roughness (surface roughness Rz ≤ 1.0 μm). Film layer B is located above film layer A and in contact with air, resulting in a relatively high surface roughness (surface roughness Rz ≥ 1.2 μm). In the polyimide film provided in this application, the number of micropores larger than 0.1 mm in film layer B is ≤ 1 per m. 2 The density of the B-film layer is ≥1.45 g / cm³. 3 .

[0078] The polyimide film provided in this application has the characteristics of large thickness, low coefficient of thermal expansion, high breakdown voltage, and low gas permeability.

[0079] Furthermore, the polyimide film provided in this application has a linear coefficient of thermal expansion between 12 ppm / ℃ and 35 ppm / ℃, a breakdown voltage ≥30 kV, and an H2 transmittance ≤1.0 × 10⁻⁶. - 15mol·m / m 2 The glass transition temperature of the A film is 180℃~300℃.

[0080] In one embodiment, the thickness of film layer A accounts for 20%-50% of the total thickness of the polyimide film. Polyimide films that meet the above thickness percentages have more prominent characteristics such as a small linear coefficient of thermal expansion, a high breakdown voltage, and a low gas permeability.

[0081] The second aspect of this application provides a method for preparing a polyimide film, which can be used to obtain the polyimide film provided in the first aspect of this application. The method includes the following steps:

[0082] 1) Mix the first aromatic dianhydride, the first aromatic diamine, the low-boiling-point solvent, the high-boiling-point solvent, and the filler to allow the first aromatic dianhydride and the first aromatic diamine to undergo a polycondensation reaction to obtain the first resin solution;

[0083] 2) Mix the second aromatic dianhydride, the second aromatic diamine, the high-boiling-point solvent, and the filler to allow the second aromatic dianhydride and the second aromatic diamine to undergo a polycondensation reaction to obtain the second resin solution;

[0084] 3) The solution film formed by casting the first resin solution is subjected to a first heating and drying treatment to obtain the initial A film layer;

[0085] 4) Mix the second resin solution with a chemical imidizing agent and a molecular weight reinforcing agent to obtain a second resin mixture solution; use the second resin mixture solution to perform at least one lamination-heat treatment on the initial A film layer to obtain the initial composite film;

[0086] 5) The initial composite film is subjected to at least one heating and drying treatment to obtain the intermediate composite film;

[0087] 6) The intermediate composite film is subjected to high-temperature heating and biaxial stretching to obtain a polyimide film;

[0088] The first heating and drying treatment is carried out at a temperature of 50–120°C for 2–10 minutes.

[0089] The lamination-heat treatment includes attaching a second resin mixture solution to one side surface of the initial A film layer and performing a heat treatment at a temperature of 60°C to 150°C for 5 min to 15 min.

[0090] The initial composite film is heated and dried at a temperature of 50℃ to 180℃ for a time of 18 min to 60 min.

[0091] The first aromatic dianhydride is selected from at least one of the following: pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 2,3,3',4-biphenyltetracarboxylic dianhydride, 4,4'-biphenyl ether dianhydride, 3,4,3',4'-triphenyl diether dianhydride, 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]malonic dianhydride, 2,2-diphenylpropane-3,4,3',4'-tetracarboxylic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 3,3',4,4'-diphenylsulfone tetracarboxylic dianhydride, and p-phenylene-bisphenyltriester dianhydride;

[0092] The first aromatic diamine is selected from at least one of 4,4'-diaminodiphenyl ether, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, 4,4'-diaminodiphenylmethane, 2,2-bis(4-aminophenyl)propane, 1,3-bis(3-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)benzene, 2,4-diaminotoluene, and 1,3-bis(aminopropane)tetramethyldisilether;

[0093] The second aromatic dianhydride is selected from at least one of pyromellitic dianhydride and 3,3,4,4'-biphenyltetracarboxylic dianhydride;

[0094] The second aromatic diamine is selected from at least one of 4,4'-diaminodiphenyl ether and 1,4-phenylenediamine;

[0095] The solid content of the first resin solution is 10%-50%;

[0096] The solid content of the second resin solution is 10%-35%;

[0097] The low-boiling-point solvent is selected from at least one of acetone, butanone, toluene, ethyl acetate, tetrahydrofuran, and chloroform;

[0098] The high-boiling-point solvent is selected from at least one of N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone;

[0099] The filler is selected from at least one of calcium carbonate, calcium phosphate, calcium pyrophosphate, calcium hydrogen phosphate, silicon carbide, carbon nanotubes, graphene, carbon black, silicon dioxide, and titanium dioxide;

[0100] Chemical imidizing agents include dehydrating agents, catalysts, imidizing solvents, and auxiliaries;

[0101] The dehydrating agent is selected from at least one of acetic anhydride, trifluoroacetic anhydride, benzoic anhydride, acetyl chloride, and thionyl chloride;

[0102] The catalyst is selected from at least one of quinoline, isoquinoline, pyridine, 3-methylpyridine, triethylamine, and N-methylimidazole;

[0103] The imidizing solvent is selected from at least one of N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone;

[0104] The adjuvant is selected from at least one of tricresyl phosphate, triphenyl phosphate, toluene diphenyl phosphate, and trioctyl trimellitate;

[0105] The molecular weight reinforcing agent includes at least one of 1,2,4,5-pyromellitic acid, 3,3',4,4'-biphenyltetracarboxylic acid, tetrahydrofuran-2,3,4,5-tetracarboxylic acid, cyclohexane-1,2,4,5-tetracarboxylic acid, 1,2,3,4-cyclobutanetetracarboxylic acid, terephthalic acid, isophthalic acid, and terephthaloyl chloride; the heating temperature for the high-temperature heating and biaxial stretching treatment in step 6) is 150℃~600℃.

[0106] Specifically, in step 1), the first aromatic dianhydride and the first aromatic diamine are used to undergo a polycondensation reaction. This application does not limit the types and number of the first aromatic dianhydride and the first aromatic diamine; one or more of the first aromatic dianhydride and the first aromatic diamine may be used.

[0107] Low-boiling-point solvents, defined as solvents with a boiling point range of 55℃ to 120℃, are selected from at least one of acetone, butanone, toluene, ethyl acetate, tetrahydrofuran, and chloroform. High-boiling-point solvents, defined as solvents with a boiling point range of 150℃ to 230℃, are selected from at least one of N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone. The combination of low-boiling-point and high-boiling-point solvents results in a solvent that is easily removed during the heating and drying process. This allows for the design objective of controlling the total amount of solvent evaporation from the coating stage, significantly reducing film shrinkage caused by solvent evaporation and mitigating the "surface drying" phenomenon (i.e., during the heating and drying process, the surface layer of the substrate dries before the inner layers, making it difficult for the solvent in the inner layers to evaporate).

[0108] This application does not limit the ratio of low-boiling-point solvent to high-boiling-point solvent. In one embodiment, the mass ratio of low-boiling-point solvent to high-boiling-point solvent is 20:100 to 40:100.

[0109] Chemical imidizing agents can convert polyamic acid into polyimide at low temperatures. These agents include dehydrating agents, catalysts, imidizing solvents, and auxiliaries.

[0110] The dehydrating agent is selected from at least one of acetic anhydride, trifluoroacetic anhydride, benzoic anhydride, acetyl chloride, and thionyl chloride; the catalyst is selected from at least one of quinoline, isoquinoline, pyridine, 3-methylpyridine, triethylamine, and N-methylimidazole; the imidizing solvent is selected from at least one of N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone; the auxiliary agent is selected from at least one of tricresyl phosphate, triphenyl phosphate, diphenyl phosphate, and trioctyl trimellitate; and the molecular weight reinforcing agent includes at least one of 1,2,4,5-pyromellitic acid, 3,3',4,4'-biphenyltetracarboxylic acid, tetrahydrofuran-2,3,4,5-tetracarboxylic acid, cyclohexane-1,2,4,5-tetracarboxylic acid, 1,2,3,4-cyclobutanetetracarboxylic acid, terephthalic acid, isophthalic acid, and terephthaloyl chloride. The molecular weight reinforcing agent is soluble in a solvent, which may be selected from at least one of N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone.

[0111] This application does not limit the proportions of dehydrating agent, catalyst, solvent, and additives. In one embodiment, the mass ratio of dehydrating agent, catalyst, solvent, and additives is 1099:501:400:100.

[0112] Furthermore, the filler's role is to improve the opening performance of the polyimide film, providing antistatic and anti-blocking effects. The function of the molecular weight reinforcing agent is to extend the chain of the polyimide molecules during the polyimide film production process, thereby increasing the molecular weight of the polyimide.

[0113] This application does not limit the specific method of performing step 1), as long as it satisfies the requirement that the first aromatic dianhydride and the first aromatic diamine can undergo a condensation reaction to obtain the first resin solution. Specifically, in step 1), any of the following methods can be used to prepare the first resin solution:

[0114] a) Under inert gas protection, a low-boiling-point solvent and a high-boiling-point solvent are added sequentially to the reactor; stirring is started; packing material is added to the reactor and stirred; the first aromatic diamine is slowly added to the reactor and stirred until dissolved; the first aromatic dianhydride is slowly added to the reactor; the first aromatic dianhydride and the first aromatic diamine polymerize for a period of time to obtain a first resin solution; the solution is then frozen and stored at low temperature. In this solution, the resin molecule structure is a polymer of one first aromatic diamine and one first aromatic dianhydride.

[0115] b) Under inert gas protection, low-boiling-point solvent and high-boiling-point solvent are added sequentially to the reactor; stirring is started; packing material is added to the reactor and stirred; the first type of aromatic diamine is slowly added to the reactor and stirred; the first type of aromatic dianhydride is slowly added to the reactor; the first polymerization is carried out, and after reacting for a period of time, the second type of aromatic dianhydride is slowly added to the reactor for the second polymerization. The addition of the third and more types of aromatic dianhydrides is carried out in accordance with the feeding method of the second type of aromatic dianhydride and reacted, finally obtaining the first resin solution; it is frozen and stored at low temperature; in this solution, the resin molecular structure is a random copolymer of one type of aromatic diamine and two or more types of aromatic dianhydrides.

[0116] c) Under inert gas protection, a low-boiling-point solvent and a high-boiling-point solvent are added sequentially to the reactor; stirring is started; packing material is added to the reactor and stirred; the first aromatic diamine is slowly added to the reactor and stirred; the first aromatic dianhydride is slowly added to the reactor; the first polymerization is carried out, and after a period of reaction, the second aromatic diamine is slowly added to the reactor for the second polymerization; after a period of reaction, the second aromatic dianhydride is slowly added to the reactor for the third polymerization; after a period of reaction, the first resin solution is obtained; it is then frozen and stored at low temperature. In this solution, the resin molecule structure is a block copolymer of two aromatic diamines and two aromatic dianhydrides; block copolymers of two or more aromatic diamines and two or more aromatic dianhydrides can be produced using this method.

[0117] d) Under inert gas protection, add low-boiling-point solvent and high-boiling-point solvent to the reactor; start stirring; add filler to the reactor and stir; slowly add two or more first aromatic diamines to the reactor and stir; slowly add one first aromatic dianhydride to the reactor; polymerize for a period of time to obtain a first resin solution; freeze and store at low temperature; in this solution, the resin molecular structure is a random copolymer of one first aromatic dianhydride and two or more first aromatic diamines.

[0118] e) Under inert gas protection, add low-boiling-point solvent and high-boiling-point solvent to the reactor; start stirring; add packing material to the reactor and stir; slowly add the first type of aromatic diamine to the reactor and stir; slowly add the first type of aromatic dianhydride to the reactor; carry out the first polymerization; after reacting for a period of time, slowly add the second type of aromatic diamine to the reactor and carry out the second polymerization; the addition of the third and more types of aromatic diamines is carried out according to the feeding method of the second type of aromatic diamine and reacted, finally obtaining the first resin solution; freeze and store at low temperature; in this solution, the resin molecular structure is a block copolymer of one type of aromatic dianhydride and two or more types of aromatic diamines.

[0119] That is, the first resin solution includes, but is not limited to: a block copolymer or random copolymer of one first aromatic dianhydride and one first aromatic diamine or two or more first aromatic diamines; a block copolymer or random copolymer of one first aromatic diamine and one first aromatic dianhydride or two or more first aromatic dianhydrides; or a block copolymer or random copolymer of two or more first aromatic dianhydrides and two or more first aromatic diamines.

[0120] In step 2), the second aromatic dianhydride and the second aromatic diamine are used to undergo a polycondensation reaction. This application does not limit the types and quantities of the second aromatic dianhydride and the second aromatic diamine selected; one or more of the second aromatic dianhydride and the second aromatic diamine may be selected.

[0121] This application does not limit the specific method of performing step 2), as long as it meets the requirement that the second aromatic dianhydride and the second aromatic diamine can undergo a condensation reaction to obtain the second resin solution. Specifically, the second resin solution in step 2) can be prepared using the same method as in step 1).

[0122] The second resin solution includes, but is not limited to, a block copolymer or random copolymer of one second aromatic dianhydride and one or two second aromatic diamines, a block copolymer or random copolymer of one second aromatic diamine and one or two second aromatic dianhydrides, or a block copolymer or random copolymer of two second aromatic dianhydrides and two second aromatic diamines.

[0123] Further, in step 3), the solution film formed by casting the first resin solution undergoes a first heating and drying treatment to obtain an initial A film layer. During the first heating and drying treatment, the solvent in the first resin solution evaporates at a high temperature, and the other components in the first resin solution form a solid initial A film layer. The treatment temperature of the first heating and drying treatment is 50℃~120℃, and the treatment time is 2min~10min. The above treatment conditions allow the solvent in the solution film formed by casting the first resin solution to evaporate at a suitable rate, forming the initial A film layer.

[0124] Understandably, to facilitate the drying of the solution film formed by the casting of the first resin solution, in one embodiment, the first resin solution can be extruded and cast onto a support in a suitable form, and then the extruded first resin solution on the support can be subjected to a first heating and drying treatment to obtain the initial A film layer. This application does not limit the specific structure of the support; it only needs to be able to support the extruded resin solution. In one embodiment, the support is a steel strip.

[0125] Furthermore, in one embodiment, the first heating and drying process heats the upper and lower surfaces of the extruded first resin solution separately, with the upper surface heated to 80°C and the lower surface heated to 100°C. The upper surface refers to the side of the extruded first resin solution that is not in contact with the support, and the lower surface refers to the side of the extruded first resin solution that is in contact with the support. Heating the upper and lower surfaces of the extruded first resin solution separately using different methods guides the volatile organic compounds in the film to evaporate uniformly and vertically outward from the support side, effectively controlling the solvent content in the film and avoiding the "surface drying" problem.

[0126] Further, in step 4), the initial A film layer of the second resin mixture solution, obtained by mixing the second resin solution with a chemical imidizing agent and a molecular weight reinforcing agent, is subjected to at least one lamination-heat treatment to obtain an initial composite film. Specifically, the lamination-heat treatment includes attaching the second resin mixture solution to one side surface of the initial A film layer and performing a heat treatment at a temperature of 60℃~150℃ for 5min~15min, so that the organic volatiles in the second resin mixture solution in contact with the upper surface of the initial A film layer slowly evaporate and form a film. At the same time, the chemical imidizing agent and molecular weight reinforcing agent included in the second resin mixture solution react with the second resin solution, so that the film layer formed by the second resin mixture solution has higher mechanical strength.

[0127] Thus, the above process ultimately yields an initial composite film. This initial composite film comprises an initial A film layer and a film layer formed by drying a second resin mixture solution.

[0128] Furthermore, the above-mentioned lamination-heating treatment can be performed multiple times. That is, in the first lamination-heating treatment, the second resin mixture solution is extruded onto the upper surface of the initial A film layer and then heated to form a film (called the newly formed film). Subsequently, the second resin mixture solution is extruded again onto the surface of the newly formed film facing away from the initial A film layer and then heated to form an initial composite film with a three-layer structure.

[0129] This application does not limit the specific number of times the above-mentioned lamination-heating treatment is performed. The specific number of performances can be determined by the number of layers of the initial composite film to be obtained, and the specific number of performances is one less than the number of layers of the initial composite film to be obtained. For example, if the number of layers of the initial composite film to be obtained is 2, then the lamination-heating treatment is performed once; if the number of layers of the initial composite film to be obtained is 3, then the lamination-heating treatment is performed twice. And so on.

[0130] Further, the initial composite film is subjected to at least one heating and drying treatment to obtain an intermediate composite film. During the heating and drying process, the solvent contained in the initial composite film continues to evaporate, and the chemical imidizing agent, molecular weight reinforcing agent and other components contained in the second resin solution continue to react to obtain an intermediate composite film. The intermediate composite film is then subjected to biaxial stretching treatment (including transverse stretching treatment and longitudinal stretching treatment) to obtain the polyimide film provided in this application.

[0131] The temperature for each heating and drying process is 50℃-180℃, and the processing time is 18min-60min. This application does not limit the specific number of heating and drying processes, which can be determined according to requirements.

[0132] Furthermore, the solid content of the first resin solution is 10%-50%, and the solid content of the second resin solution is 10%-35%. The solid content in the first resin solution refers to the ratio of the total mass of the first aromatic dianhydride, the first aromatic diamine, and the filler to the mass of the first resin solution, and the solid content in the second resin solution refers to the ratio of the total mass of the second aromatic dianhydride, the second aromatic diamine, and the filler to the mass of the second resin solution.

[0133] The first resin solution has high adhesion. The initial A film layer prepared by casting and drying is located at the bottom layer and is in direct contact with the support. This not only greatly improves the adhesion to the support, but also stably supports the weight of the subsequent laminated films, effectively solving the problem of detachment caused by insufficient adhesion between the ultra-thick film and the support.

[0134] The second type of resin solution is used to prepare the adhesive film, which is located above the initial A film layer and does not directly contact the support. This avoids the shrinkage and delamination problems caused by insufficient adhesion to the support. The second type of resin solution reacts with chemical imidizing agents and, combined with the effect of molecular weight reinforcing agents, can produce an adhesive film with high mechanical properties, thus solving the tearing problem caused by insufficient mechanical properties during the production of the adhesive film.

[0135] Furthermore, both the first and second resin solutions have high solid content, which can control the initial organic volatiles of the cast film to a low mass percentage, significantly reducing the evaporation of organic volatiles per unit time. This not only helps control the shrinkage size of the film and avoid the problem of stripping, but also helps reduce the number of internal pore defects in the film caused by the evaporation of organic volatiles. This allows for the preparation of intermediate films with controllable solvent content and high mechanical properties, thereby increasing the density of the polyimide film, reducing the linear thermal expansion coefficient of the polyimide film, and increasing the gas permeability.

[0136] The polyimide film preparation method provided in this application uses a first resin solution and a second resin solution, both of which have high solid content. The first resin solution contains both high-boiling-point and low-boiling-point solvents, making it easy to remove solvents during heat drying. This reduces the solvent content in the inner layer of the film, mitigating surface drying and reducing shrinkage of the support layer during drying, thus preventing detachment.

[0137] Furthermore, the first resin solution also has the characteristic of high adhesion. The initial A film layer prepared by the first heating and drying treatment is located at the bottom and is in direct contact with the support. This not only greatly improves the adhesion to the support, but also stably supports the weight of the subsequent laminated films, effectively solving the problem of delamination caused by insufficient adhesion between the ultra-thick film and the steel strip. Furthermore, the second and higher layers of film prepared by the second resin solution are located above the initial A film layer and are not in direct contact with the support. This avoids the shrinkage and delamination problems caused by insufficient adhesion to the support. The reaction of the second resin solution with the chemical imidizing agent and the effect of the molecular weight reinforcing agent can produce a film with high mechanical properties, and can also solve the tearing problem caused by insufficient mechanical properties during the film production process.

[0138] In one embodiment, the heating temperature for the high-temperature heating and biaxial stretching treatment in step 6) can be controlled to be between 150°C and 600°C. These treatment conditions allow for more uniform stretching of the intermediate composite film, resulting in a polyimide film.

[0139] In one embodiment, in step 1), the molar ratio of the first aromatic dianhydride to the first aromatic diamine is 0.99:1 to 1.05:1. In a preferred embodiment, the molar ratio is 0.995:1 to 1.02:1. Further, the molar ratio of the second aromatic dianhydride to the second aromatic diamine is 0.8:1 to 0.995:1. In a preferred embodiment, the molar ratio is 0.88:1 to 0.98:1. By controlling the aromatic dianhydride and aromatic diamine to meet the above molar ratios during the polycondensation reaction, a polyamic acid resin solution of a certain viscosity can be precisely synthesized.

[0140] Furthermore, in one embodiment, the solid content of the first resin solution is 30%-40%; further, the solid content of the second resin solution is 25%-35%. The high solid content of the first and second resin solutions allows for control of the total amount of volatile organic compounds at the coating front end, significantly reducing the shrinkage size of the film caused by solvent evaporation and alleviating the "surface drying" phenomenon. This not only enables the stable preparation of intermediate composite films but also helps to obtain high-quality polyimide films with fewer internal "pore" defects.

[0141] In one embodiment, the mass of the chemical imidizing agent is 1%-30% of the mass of the second resin solution; in a preferred embodiment, the mass of the chemical imidizing agent is 5%-15% of the mass of the second resin solution. Further, the molar ratio of the dehydrating agent to the catalyst in the chemical imidizing agent is 1:2 to 5:1, preferably 2:1 in a preferred embodiment; the mass fraction of the solvent is 10%-40%, preferably 20% in a preferred embodiment; the mass fraction of the additive is 1%-10%, preferably 5% in a preferred embodiment; further, the mass of the filler is 0.05%-5% of the sum of the masses of the aromatic diamine and aromatic dianhydride, preferably 0.1%. The above-mentioned amount of chemical imidizing agent allows polyamic acid to be converted into polyimide at low temperatures via chemical imidization, and the prepared polyimide film has the advantages of high tensile strength and a small linear coefficient of thermal expansion.

[0142] Furthermore, in one embodiment, the amount of the molecular weight reinforcing agent is equal to the difference in amount between the second aromatic diamine and the second aromatic dianhydride. When the amount of the molecular weight reinforcing agent satisfies the above condition, the mechanical properties of the prepared polyimide film can be improved by increasing the molecular weight of the polyimide.

[0143] In one embodiment, step 5), the heating and drying process includes heating the upper and lower surfaces of the initial composite film separately, wherein the heating temperature of the upper surface is 50℃~150℃, and the heating temperature of the lower surface is 80℃~180℃. The upper surface refers to the surface of the initial composite film that has undergone the lamination-heating treatment. The lower surface refers to the surface of the initial composite film that has not undergone the lamination-heating treatment, i.e., the surface in contact with the support. Heating the upper and lower surfaces of the initial composite film separately guides the volatile organic compounds in the film to evaporate uniformly from the support side vertically outward, effectively controlling the solvent content in the film and avoiding the "surface drying" problem.

[0144] Furthermore, in one embodiment, step 5) involves heating and drying the initial composite film, which includes sequentially performing a second heating and drying process, a third heating and drying process, a fourth heating and drying process, a fifth heating and drying process, and a sixth heating and drying process.

[0145] The upper surface of the second heating and drying process is heated at 80℃~120℃, the lower surface is heated at 120℃~150℃, and the processing time is 5min~15min.

[0146] The upper surface of the third heating and drying process is heated at 90℃~135℃, the lower surface is heated at 135℃~180℃, and the processing time is 1min~8min.

[0147] The upper surface of the fourth heating and drying treatment is heated at 80℃~120℃, the lower surface is heated at 120℃~150℃, and the treatment time is 5min~15min;

[0148] The fifth heating and drying process involves heating the upper surface at 60℃~120℃ and the lower surface at 120℃~150℃ for 5min~15min.

[0149] The upper surface of the sixth heating and drying process is heated at 50℃~100℃, the lower surface is heated at 100℃~120℃, and the processing time is 2min~10min.

[0150] Heating the upper and lower surfaces of the initial composite film under the conditions described above can guide the organic volatiles in the film to evaporate evenly from the support side vertically outward, effectively controlling the solvent content in the film and avoiding the "surface drying" problem.

[0151] Furthermore, in one embodiment, during the lamination-heat treatment, the upper surface heating temperature is 60°C to 120°C, and the lower surface heating temperature is 120°C to 180°C. The lower surface is the side of the initial A film layer that contacts the support, and the upper surface is the side loaded with the second resin mixture solution. These treatment conditions guide the volatile organic compounds in the film to evaporate uniformly and vertically outward from the support side, effectively controlling the solvent content in the film and avoiding the "surface drying" problem.

[0152] A third aspect of this application provides a circuit board comprising the polyimide film provided in the first aspect of this application. Further, in one embodiment, the circuit board is a flexible circuit board, and the polyimide film provided in the first aspect of this application serves as the substrate of the flexible circuit board. Because the polyimide film provided in the first aspect of this application has a linear coefficient of thermal expansion between 12 ppm / ℃ and 35 ppm / ℃, the circuit board exhibits high dimensional stability.

[0153] A fourth aspect of this application provides a gas barrier membrane composite film, which is the polyimide film provided in the first aspect of this application or includes the polyimide film provided in the first aspect of this application. Further, in one embodiment, the gas barrier membrane is a lining material for a hydrogen transport pipeline, and the polyimide film provided in the first aspect of this application serves as or is part of that lining material. Because the polyimide film provided in the first aspect has an H2 transmittance ≤1.0 × 10⁻⁶... -15 mol·m / m 2 Due to the characteristics of ·s·Pa, the gas barrier membrane composite membrane has the characteristic of high gas barrier properties.

[0154] The fifth aspect of this application provides an insulating composite film, which includes the polyimide film provided in the first aspect of this application. Further, in one embodiment, the insulating composite film is the insulation layer of a high-voltage resistant cable, and the polyimide film provided in the first aspect of this application is part of that insulation layer. Because the polyimide film provided in the first aspect has a breakdown voltage ≥30kV, the insulating composite film has high insulation properties.

[0155] Figure 1 shows an apparatus for performing the method for preparing a polyimide film provided in the second aspect of this application. As shown in Figure 1, the apparatus includes 101-first extrusion die, 102-second extrusion die, 103-mixer, 104-first heating drive roller, 105-first heating driven roller, 106-second heating driven roller, 107-third heating driven roller, 108-fourth heating driven roller, 109-fifth heating driven roller, 110-sixth heating driven roller, 111-second heating drive roller, A-first drying zone (hereinafter referred to as zone A), B-second drying zone (hereinafter referred to as zone B), C-third drying zone (hereinafter referred to as zone C), D-fourth drying zone (hereinafter referred to as zone D), E-fifth drying zone (hereinafter referred to as zone E), F-sixth drying zone (hereinafter referred to as zone F), and G-seventh drying zone (hereinafter referred to as zone G).

[0156] Wherein, 101-first extrusion die is used to cast the first resin solution to form a solution film; 103-mixer is used to mix the chemical imidizing agent, the second resin solution and the molecular weight reinforcing agent to obtain the second resin mixture solution, which then enters 102-second extrusion die, and 102-second extrusion die extrudes the second resin mixture solution to perform at least one lamination-heating treatment on the initial A film layer.

[0157] 104-First heating drive roller, 105-First heating driven roller, 106-Second heating driven roller, 107-Third heating driven roller, 108-Fourth heating driven roller, 109-Fifth heating driven roller, 110-Sixth heating driven roller, and 111-Second heating drive roller are moving structures with steel strips attached to their outer surfaces, allowing the steel strips to rotate counterclockwise around the aforementioned moving structures.

[0158] A - First drying zone (hereinafter referred to as Zone A), B - Second drying zone (hereinafter referred to as Zone B), C - Third drying zone (hereinafter referred to as Zone C), D - Fourth drying zone (hereinafter referred to as Zone D), E - Fifth drying zone (hereinafter referred to as Zone E), F - Sixth drying zone (hereinafter referred to as Zone F), G - Seventh drying zone (hereinafter referred to as Zone G). These are heating plates with internal heat sources, arranged around the steel belt, used to perform the heating and drying process.

[0159] The above-mentioned apparatus is used for the production of polyimide films as follows: 101 - The first extrusion die extrudes a first resin solution onto a steel strip, causing it to flow and form a solution film. A - The first drying zone and 104 - The first heating drive roller, 105 - The first heating driven roller, and 106 - The second heating driven roller are located on the upper and lower surfaces of the solution film, respectively, and subject it to a first heating and drying treatment to obtain an initial A film layer. When the initial A film layer moves counterclockwise to directly below 102 - The second extrusion die, 102 - The second extrusion die extrudes a second resin mixture solution. The solution film formed by this flowing solution covers the upper surface of the initial A film layer and moves to a position below B - The second drying zone and above 107 - The third heating driven roller and 108 - The fourth heating driven roller for heating. This process essentially involves using the second resin mixture solution to perform a lamination-heating treatment on the initial A film layer to obtain an initial composite film.

[0160] The initial composite film moves counterclockwise, undergoing a second heating and drying process below the third drying zone (C), above the fifth heating driven roller (109), and above the sixth heating driven roller (110), and so on. After a sixth heating and drying process above the seventh drying zone (G), below the first heating drive roller (104), the first heating driven roller (105), and the second heating driven roller (106), an intermediate composite film is obtained. The intermediate composite film is then longitudinally stretched at room temperature and transversely stretched within the range of 150℃ to 600℃, followed by cooling to obtain a polyimide film.

[0161] This application also provides a standard for the solvent content and tensile strength of an intermediate composite film used in the production of polyimide films. The solvent content and tensile strength of the intermediate composite film meet the standards shown in Table 1, and the intermediate composite film is subjected to biaxial stretching and high-temperature treatment to produce a polyimide film of the corresponding thickness. The solvent content of the composite film is calculated as: [(total weight of the composite film - weight of the composite film after high-temperature drying) / weight of the composite film after high-temperature drying] × 100%; the weight of the composite film after high-temperature drying is the weight after drying at 450℃ for 2 hours; the tensile strength of the intermediate composite film is determined using an Instron 68SC-05 universal tensile testing machine.

[0162] Table 1. Solvent content and tensile strength standards for intermediate composite films used in the production of ultra-thick polyimide films.

[0163] The following examples further illustrate the polyimide film and its preparation method provided in this application.

[0164] Example 1

[0165] This embodiment uses the following method and the equipment shown in Figure 1 to prepare a polyimide film:

[0166] 1) Preparation of the first resin solution

[0167] Under nitrogen protection, 3139g of DMF and 1046g of THF were added to a 15L reactor; stirring was started; 1243g of BAPP and 606g of 3,4-ODA were weighed and added to the reactor sequentially, and stirred at room temperature for 30 minutes; 3g of calcium pyrophosphate was weighed and added to the reactor; stirring was carried out for 20 minutes; 1880g of ODPA was weighed and slowly added to the reactor; stirring was carried out for 300 minutes; a first resin solution with a solid content of 47% and a viscosity of 1815P was obtained; the solution was frozen and stored at -10℃ for later use.

[0168] 2) Preparation of the second resin solution

[0169] Under nitrogen protection, 10484g of DMF was added to a 20L reactor; stirring was started; 1170g of ODA and 632g of PDA were weighed and added to the reactor and stirred at room temperature for 30 minutes; 3g of calcium pyrophosphate was weighed and added to the reactor; stirring was carried out for 20 minutes; 1174g of PMDA and 1514g of s-BPDA were weighed and slowly added to the reactor in sequence; stirring was carried out for 300 minutes; a second polyamic acid resin solution with a solid content of 30% and a viscosity of 2005P was obtained; the solution was frozen and stored at -10℃ for later use.

[0170] 3) Preparation of bilayer polyimide composite film

[0171] ① Preparation of chemical imidizing reagent: Under nitrogen protection, add 800g DMF, 2198g acetic anhydride, 1002g 3-methylpyridine, and 200g triphenyl phosphate to a 5L reactor in sequence and stir for 180min.

[0172] ② Preparation of molecular weight reinforcing agent: Under nitrogen protection, add 1200g DMF and 600g 1,2,4,5-pyromellitic acid to a 2L reactor and stir at 60℃ for 120min for later use.

[0173] ③ The first polyamic acid solution is pumped into the first extrusion die and cast onto the surface of the steel belt. The first film layer moves with the steel belt in area A and undergoes the first heating and drying treatment to obtain the initial A film layer.

[0174] The chemical imidizing agent and molecular weight reinforcing agent in ① and ②, along with the second resin solution, are quantitatively pumped into a mixer and thoroughly mixed to obtain a second resin mixture solution. The second resin mixture solution is then pumped into a second extrusion die and cast onto the upper surface of the initial A film layer. Heat treatment is then performed in region B to obtain an initial composite film with a two-layer structure (i.e., the initial A film layer is subjected to a lamination-heat treatment using the second resin mixture solution).

[0175] The initial composite film undergoes a second drying process in zone C, a third drying process in zone D, a fourth drying process in zone E, a fifth drying process in zone F, and a sixth drying process in zone G, resulting in an intermediate composite film with a certain solvent content and mechanical properties. The intermediate composite film is then subjected to longitudinal stretching at room temperature and transverse stretching within the range of 150℃ to 600℃, followed by cooling to obtain a polyimide film with a thickness of 252 μm.

[0176] The quantitative relationships between the first resin solution, the second resin solution, the chemical imidizing reagent, and the molecular weight reinforcing agent are shown in Table 2.

[0177] Table 2

[0178] Specifically, in Example 1, the processing time, air surface temperature (upper surface heating temperature), and steel strip surface temperature (lower surface heating temperature) of zones A (first heating and drying treatment), B (first lamination-heating treatment), C (second heating and drying treatment), D (third heating and drying treatment), E (fourth heating and drying treatment), F (fifth heating and drying treatment), and G (sixth heating and drying treatment) are shown in Table 3:

[0179] Table 3

[0180] Examples 2-8

[0181] Examples 2-8 are basically the same as Example 1, except that the feed flow rates of the first and second extrusion dies are different. Specific feed flow rate data are shown in Table 4.

[0182] Table 4

[0183] In Examples 2-8, the processing time, air surface temperature (upper surface heating temperature), and steel strip surface temperature (lower surface heating temperature) for areas A (first heating and drying treatment), B (first lamination-heating treatment), C (second heating and drying treatment), D (third heating and drying treatment), E (fourth heating and drying treatment), F (fifth heating and drying treatment), and G (sixth heating and drying treatment) are shown in Table 5.

[0184] Table 5

[0185] Examples 9-10

[0186] Examples 9-10 use another apparatus to prepare polyimide films with a three-layer structure. This apparatus is essentially the same as the apparatus in Figure 1, except that it further includes a third extrusion die located at the front end of region C and the rear end of region B (with the steel strip running in the front direction). Examples 9-10 are essentially the same as Example 1, except that:

[0187] The chemical imidizing agent and molecular weight reinforcing agent from ① and ②, along with the second resin solution, are quantitatively pumped into a mixer and thoroughly mixed to obtain a second resin mixture solution. The second resin mixture solution is then pumped into the second extrusion die and the third extrusion die, respectively. The second resin mixture solution extruded from the second extrusion die flows onto the upper surface of the initial A film layer and is heated in region B. When the composite system reaches the junction of region B and region C, the second resin mixture solution extruded from the third extrusion die flows onto the upper surface of the composite system and is heated in region C, resulting in an initial composite film with a three-layer structure (i.e., the initial A film layer is subjected to two lamination-heat treatments using the second resin mixture solution).

[0188] The initial composite film is carried by a steel belt through zone D for a second drying process, zone E for a third drying process, zone F for a fourth drying process, and zone G for a fifth drying process, resulting in an intermediate composite film. The intermediate composite film is then subjected to longitudinal stretching at room temperature and transverse stretching within the range of 150℃ to 600℃, followed by cooling to obtain a three-layer polyimide film.

[0189] The specific feed flow rate data for each extrusion die is shown in Table 6:

[0190] Table 6

[0191] In Examples 9-10, the processing time, air surface temperature (upper surface heating temperature), and steel strip surface temperature (lower surface heating temperature) for zones A (first heating and drying treatment), B (first lamination-heating treatment), C (second lamination-heating treatment), D (second heating and drying treatment), E (third heating and drying treatment), F (fourth heating and drying treatment), and G (fifth heating and drying treatment) are shown in Table 7.

[0192] Table 7

[0193] Example 11

[0194] This embodiment is basically the same as Embodiment 2, except that the preparation process of the first resin solution is as follows:

[0195] Under nitrogen protection, 4000g DMAc and 2000g butanone were added to a 15L reactor; stirring was started; 1006g GAPDS and 495g DAT were weighed and added to the reactor and stirred at room temperature for 30min; 4g calcium pyrophosphate was weighed and added to the reactor; stirring was carried out for 20min; 1306g BTDA and 1192g α-BPDA were weighed and slowly added to the reactor in sequence; stirring was carried out for 300min; a first polyamic acid resin solution with a solid content of 40% and a viscosity of 1903P was obtained; it was frozen and stored at -10℃ for later use.

[0196] Example 12

[0197] This embodiment is basically the same as Embodiment 2, except that the preparation process of the second resin solution is as follows:

[0198] Under nitrogen protection, 10080g of DMF was added to a 20L reactor; stirring was started; 1638g of ODA and 380g of PDA were weighed and added to the reactor and stirred at room temperature for 30 minutes; 4.5g of calcium pyrophosphate was weighed and added to the reactor; stirring was carried out for 20 minutes; 2296g of PMDA was weighed and slowly added to the reactor sequentially; stirring was carried out for 300 minutes; a second polyamic acid resin solution with a solid content of 30% and a viscosity of 2536p was obtained; the solution was frozen and stored at -10℃ for later use.

[0199] Example 13

[0200] This embodiment is basically the same as Embodiment 2, except that the preparation process of the first resin solution is as follows:

[0201] Under nitrogen protection, 3669g DMAc and 1223g butanone were added to a 15L reactor; stirring was started; 1006g GAPDS and 495g DAT were weighed and added to the reactor and stirred at room temperature for 30min; 4g calcium pyrophosphate was weighed and added to the reactor; stirring was carried out for 20min; 1306g BTDA and 1192g α-BPDA were weighed and slowly added to the reactor in sequence; stirring was carried out for 300min; a first polyamic acid resin solution with a solid content of 45% and a viscosity of 1822P was obtained; it was frozen and stored at -10℃ for later use.

[0202] Example 14

[0203] This embodiment is basically the same as Embodiment 2, except that the preparation process of the second resin solution is as follows:

[0204] Under nitrogen protection, 8070g of DMF was added to a 20L reactor; stirring was started; 1638g of ODA and 380g of PDA were weighed and added to the reactor and stirred at room temperature for 30 minutes; 4.5g of calcium pyrophosphate was weighed and added to the reactor; stirring was carried out for 20 minutes; 2246g of PMDA was weighed and slowly added to the reactor sequentially; stirring was carried out for 300 minutes; a second polyamic acid resin solution with a solid content of 34.5% and a viscosity of 2337P was obtained; the solution was frozen and stored at -10℃ for later use.

[0205] Example 15

[0206] This embodiment is basically the same as Example 2, except that the chemical imidizing reagent is prepared as follows:

[0207] Under nitrogen protection, 800g DMF, 2198g acetic anhydride, 1390g isoquinoline, and 200g triphenyl phosphate were added sequentially to a 5L reactor and stirred for 180 minutes.

[0208] Example 16

[0209] This embodiment is basically the same as Embodiment 2, except that the molecular weight reinforcing agent is prepared as follows:

[0210] Under nitrogen protection, 1200g DMF and 600g 1,2,3,4-cyclobutanetetracarboxylic acid were added sequentially to a 2L reactor and stirred at room temperature for 120min.

[0211] Comparative Example 1

[0212] This comparative example uses the second resin mixing solution (including the second resin solution, chemical imidizing agent, and molecular weight reinforcing agent solution) used in Example 2 to prepare a single-layer ultra-thick polyimide film. That is, the second resin solution, chemical imidizing agent, and molecular weight reinforcing agent solution are quantitatively pumped into the mixer, mixed, and then pumped into the second extrusion die. The rest is the same as in Example 2.

[0213] Comparative Example 2

[0214] The single-layer ultra-thick polyimide film was prepared using the first resin solution in Example 2, that is, the first polyamic acid resin solution was pumped into the first extrusion die, and the rest was the same as in Example 2.

[0215] Comparative Example 3

[0216] Film layer A and film layer B were prepared using the second resin solution, chemical imidizing agent, molecular weight reinforcing agent and first resin solution of Example 2, respectively, and the rest were the same as in Example 2.

[0217] Comparative Example 4

[0218] 1) Preparation of the first polyamic acid resin solution

[0219] Under nitrogen protection, 4767g DMF and 1559g THF were added to a 15L reactor; stirring was started; 622g BAPP was weighed and added to the reactor and stirred at room temperature for 30min; 1.5g calcium pyrophosphate was weighed and added to the reactor; stirring was carried out for 20min; 469g ODPA and 303g 3,4-ODA were weighed and slowly added to the reactor in sequence; stirring was carried out for 300min; a first polyamic acid resin solution with a solid content of 18% and a viscosity of 2239p was obtained; it was frozen and stored at -10℃ for later use.

[0220] 2) Preparation of the second type of polyamic acid resin solution

[0221] Under nitrogen protection, 10972g of DMF was added to a 20L reactor; stirring was started; 585g of ODA and 316g of PDA were weighed and added to the reactor and stirred at room temperature for 30min; 1.5g of calcium pyrophosphate was weighed and added to the reactor; stirring was carried out for 20min; 637g of PMDA and 869g of s-BPDA were weighed and slowly added to the reactor in sequence; stirring was carried out for 300min; a second polyamic acid resin solution with a solid content of 18% and a viscosity of 1706P was obtained; the solution was frozen and stored at -10℃ for later use.

[0222] The rest is the same as in Example 2. The solvent content of the prepared two-layer composite film is 561%, the tensile strength is 21 MPa, and the film detaches from the fixture during high-temperature treatment, making it impossible to stably produce a 300 μm thick polyimide film.

[0223] Comparative Example 5

[0224] This comparative example is basically the same as Example 2, except that the processing time, air surface temperature (upper surface heating temperature), and steel strip surface temperature (lower surface heating temperature) for areas A (first heating and drying treatment), B (first lamination-heating treatment), C (second heating and drying treatment), D (third heating and drying treatment), E (fourth heating and drying treatment), F (fifth heating and drying treatment), and G (sixth heating and drying treatment) are shown in Table 8.

[0225] Table 8

[0226] Comparative Example 6

[0227] This comparative example is basically the same as Example 2, except that the processing time, air surface temperature (upper surface heating temperature), and steel strip surface temperature (lower surface heating temperature) for areas A (first heating and drying treatment), B (first lamination-heating treatment), C (second heating and drying treatment), D (third heating and drying treatment), E (fourth heating and drying treatment), F (fifth heating and drying treatment), and G (sixth heating and drying treatment) are shown in Table 9.

[0228] Table 9

[0229] Test case

[0230] The solvent content, tensile strength, and thickness of the produced ultra-thick polyimide film in the above examples and comparative examples are statistically analyzed and are shown in Table 10.

[0231] Table 10

[0232] Note: × - Unable to produce a film for testing; * - The film surface has blemishes.

[0233] in:

[0234] 1) During the preparation of Comparative Example 1, the film shrank and tore during the drying process, making production impossible. This indicates that a single-layer polyimide film cannot be prepared using only the second resin mixture solution;

[0235] 2) During the preparation process of Comparative Example 2, the adhesive film adhered to the steel strip after drying and was difficult to peel off. Furthermore, the adhesive film tore under tension, making production impossible. This indicates that a single-layer polyimide film cannot be prepared using only the first resin solution.

[0236] 3) During the preparation of Comparative Example 3, the film shrank and tore during the drying process, making production impossible. This indicates that when the correspondence between the first and second resin solutions and the A and B film layers is reversed, a polyimide film cannot be obtained.

[0237] 4) The solvent content in the two-layer intermediate composite film prepared in Comparative Example 4 was 561%, and the tensile strength was 21 MPa. During high-temperature treatment, the film detached from the fixture, making it impossible to stably produce a 300 μm thick polyimide film. This indicates that it is difficult to obtain polyimide film products when the solvent content of the intermediate composite film is too high.

[0238] 5) The intermediate composite film prepared in Comparative Example 5 had a solvent content of 372% and a tensile strength of 36 MPa. During high-temperature treatment, the film detached from the fixture, making it impossible to stably produce a 300 μm thick polyimide film. This indicates that it is difficult to obtain polyimide film products when the solvent content of the intermediate composite film is too high.

[0239] 6) During the drying process of the film in Comparative Example 6, a large amount of solvent dripped back onto the surface, forming blemishes. The prepared film had a solvent content of 167% and a tensile strength of 72 MPa. After biaxial stretching and high-temperature treatment, a polyimide film with a thickness of 295 μm could be produced. This indicates that when the air surface temperature is too low, a large amount of solvent drips back onto the surface, forming blemishes.

[0240] 7) In addition, the low temperature of the air surface heating and the large temperature difference between the air surface heating and the steel strip heating not only cause solvent to drip back onto the film surface and form defects, but also cause the film to dry insufficiently, resulting in a high solvent content in the product and a low overall performance of the final composite film.

[0241] The statistical analysis of the structure and properties of the ultra-thick polyimide films in the above embodiments is shown in Table 11. The measurement methods are as follows:

[0242] Thickness measurement method: The CHY-CA mechanical contact thickness gauge from China Saicheng Instruments was used for measurement.

[0243] CTE measurement method: The CTE was measured using a TMA450 static thermomechanical analyzer from TA Instruments, USA, with a heating rate of 3℃ / min.

[0244] Breakdown voltage measurement method: The SDJ-150KV thin film voltage breakdown tester from China Guanheng Precision Instruments Equipment Co., Ltd. was used for measurement, with a voltage boost rate of 500V / s;

[0245] H2 transmittance measurement method: SMT-275 membrane separation test and analyzer of China Sike Testing Technology, hydrogen inlet rate 50ml / min;

[0246] Surface roughness RZ measurement method: The surface roughness was measured using a Marsurf VD140 280 profile roughness measuring instrument from Germany.

[0247] Glass transition temperature (Tg) measurement method: DMA303 from Netzsch, Germany, was used for measurement, with a heating rate of 3℃ / min;

[0248] Micropore count measurement method: The cross-section of the polyimide film sample was examined using a Zeiss GeminiSEM 360 scanning electron microscope (Germany), and the number of micropores with a size greater than 0.1 mm was counted.

[0249] Density measurement method: The DH-300 thin film rapid density meter of China Hongtuo Instruments was used for detection.

[0250] Table 11 Statistical Table of Structural Properties of Polyimide Films

[0251] Where: CTE: coefficient of linear thermal expansion;

[0252] BV: Breakdown voltage;

[0253] Rz: Surface roughness;

[0254] Tg: Glass transition temperature;

[0255] ρ: density.

[0256] As shown in Table 11, the polyimide films prepared in each embodiment of this application have the characteristics of large thickness, low thermal expansion, high pressure resistance and low gas permeability.

[0257] In the description of the embodiments of this application, it should be noted that, unless otherwise expressly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, an indirect connection through an intermediate medium, or the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this application according to the specific circumstances.

[0258] The terms “first,” “second,” “third,” “fourth,” etc. (if present) in the specification, claims, and accompanying drawings of this application are used to distinguish similar objects and are not necessarily used to describe a particular order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of this application described herein can be implemented, for example, in orders other than those illustrated or described herein. Furthermore, the terms “comprising” and “having,” and any variations thereof, are intended to cover a non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0259] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the technical solutions of the embodiments of this application.

Claims

1. A polyimide film, wherein, The polyimide film has a thickness of 250 μm or more and includes an A film layer that is in contact with each other and at least one B film layer. The surface roughness Rz of the A film layer is ≤1.0 μm; The number of micropores larger than 0.1 mm in the B membrane layer is ≤1 per square meter, and the density of the B membrane layer is ≥1.45 g / cm³. 3 Surface roughness Rz≥1.2μm.

2. The polyimide film as described in claim 1, wherein, The polyimide film has a linear coefficient of thermal expansion of 12ppm / ℃ to 35ppm / ℃, a breakdown voltage ≥30kV, and an H2 transmittance ≤1.0×10⁻⁶. - 15 mol·m / m 2 The glass transition temperature of the polyimide in the A film layer is 180℃~300℃.

3. The polyimide film as described in claim 1, wherein, The thickness of the A film layer accounts for 20%-50% of the total thickness of the polyimide film.

4. A method for preparing the polyimide film according to any one of claims 1-3, wherein, Includes the following steps: 1) Mix the first aromatic dianhydride, the first aromatic diamine, the low-boiling-point solvent, the high-boiling-point solvent, and the filler to allow the first aromatic dianhydride and the first aromatic diamine to undergo a polycondensation reaction to obtain the first resin solution; 2) Mix the second aromatic dianhydride, the second aromatic diamine, the high-boiling-point solvent, and the filler to allow the second aromatic dianhydride and the second aromatic diamine to undergo a polycondensation reaction to obtain the second resin solution; 3) The solution film formed by casting the first resin solution is subjected to a first heating and drying treatment to obtain the initial A film layer; 4) The second resin solution is mixed with a chemical imidizing agent and a molecular weight reinforcing agent to obtain a second resin mixed solution; The initial A film layer is subjected to at least one lamination-heating treatment using the second resin mixture solution to obtain the initial composite film. 5) The initial composite film is subjected to at least one heating and drying treatment to obtain an intermediate composite film; 6) The intermediate composite film is subjected to high-temperature heating and biaxial stretching treatment to obtain the polyimide film; The first heating and drying treatment is performed at a temperature of 50℃ to 120℃ for a time of 2 min to 10 min. The lamination-heat treatment includes attaching the second resin mixture solution to one side surface of the initial A film layer and performing a heat treatment at a temperature of 60°C to 150°C for a time of 5 min to 15 min. The initial composite film is subjected to a heating and drying process at a temperature of 50°C to 180°C for 18 min to 60 min. The first aromatic dianhydride is selected from at least one of the following: pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 2,3,3',4-biphenyltetracarboxylic dianhydride, 4,4'-biphenyl ether dianhydride, 3,4,3',4'-triphenyl diether dianhydride, 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]malonic dianhydride, 2,2-diphenylpropane-3,4,3',4'-tetracarboxylic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 3,3',4,4'-diphenylsulfone tetracarboxylic dianhydride, and p-phenylene-bisphenyltriester dianhydride; The first aromatic diamine is selected from at least one of 4,4'-diaminodiphenyl ether, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, 4,4'-diaminodiphenylmethane, 2,2-bis(4-aminophenyl)propane, 1,3-bis(3-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)benzene, 2,4-diaminotoluene, and 1,3-bis(aminopropane)tetramethyldisilether; The second aromatic dianhydride is selected from at least one of pyromellitic dianhydride and 3,3,4,4'-biphenyltetracarboxylic dianhydride; The second aromatic diamine is selected from at least one of 4,4'-diaminodiphenyl ether and 1,4-phenylenediamine; The solid content of the first resin solution is 10%-50%; The solid content of the second resin solution is 10%-35%; The low-boiling-point solvent is selected from at least one of acetone, butanone, toluene, ethyl acetate, tetrahydrofuran, and trichloromethane; The high-boiling-point solvent is selected from at least one of N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone; The filler is selected from at least one of calcium carbonate, calcium phosphate, calcium pyrophosphate, calcium hydrogen phosphate, silicon carbide, carbon nanotubes, graphene, carbon black, silicon dioxide, and titanium dioxide. The chemical imidizing agent includes a dehydrating agent, a catalyst, an imidizing solvent, and an auxiliary agent; The dehydrating agent is selected from at least one of acetic anhydride, trifluoroacetic anhydride, benzoic anhydride, acetyl chloride, and thionyl chloride; The catalyst is selected from at least one of quinoline, isoquinoline, pyridine, 3-methylpyridine, triethylamine, and N-methylimidazole; The imidizing solvent is selected from at least one of N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone; The adjuvant is selected from at least one of tricresyl phosphate, triphenyl phosphate, toluene diphenyl phosphate, and trioctyl trimellitate; The molecular weight reinforcing agent includes at least one of 1,2,4,5-pyromellitic acid, 3,3',4,4'-biphenyltetracarboxylic acid, tetrahydrofuran-2,3,4,5-tetracarboxylic acid, cyclohexane-1,2,4,5-tetracarboxylic acid, 1,2,3,4-cyclobutanetetracarboxylic acid, terephthalic acid, isophthalic acid, and terephthaloyl chloride. The heating temperature for the high-temperature heating and biaxial stretching treatment in step 6) is 150℃~600℃.

5. The method for preparing the polyimide film according to claim 4, wherein, In step 1), the molar ratio of the first aromatic dianhydride to the first aromatic diamine is 0.99:1 to 1.05:1; and / or, The molar ratio of the second aromatic dianhydride to the second aromatic diamine is 0.8:1 to 0.995:

1.

6. The method for preparing the polyimide film according to claim 4, wherein, The solid content of the first resin solution is 30%-40%; and / or, The solid content of the second resin solution is 25%-35%.

7. The method for preparing the polyimide film according to claim 4, wherein, The mass of the chemical imidizing agent is 1%-30% of the mass of the second resin solution; and / or, The molar ratio of dehydrating agent to catalyst in the chemical imidizing reagent is 1:2 to 5:1; the mass fraction of the solvent is 10% to 40%; the mass fraction of the auxiliary agent is 1% to 10%; and / or, The mass of the filler is 0.05% to 5% of the sum of the masses of the aromatic diamine and the aromatic dianhydride.

8. The polyimide film as claimed in claim 4, wherein, The amount of the molecular weight reinforcing agent is equal to the difference between the amounts of the second aromatic diamine and the second aromatic dianhydride.

9. The method for preparing the polyimide film according to claim 4, wherein, In step 5), the heating and drying process includes heating the upper and lower surfaces of the initial composite film respectively, wherein the heating temperature of the upper surface is 50℃~150℃ and the heating temperature of the lower surface is 80℃~180℃.

10. The method for preparing the polyimide film according to claim 4, wherein, In step 5), the heating and drying process performed on the initial composite film includes a second heating and drying process, a third heating and drying process, a fourth heating and drying process, a fifth heating and drying process, and a sixth heating and drying process performed sequentially. The upper surface heating temperature of the second heating and drying treatment is 80℃~120℃, the lower surface heating temperature is 120℃~150℃, and the treatment time is 5min~15min; The upper surface heating temperature of the third heating and drying process is 90℃~135℃, the lower surface heating temperature is 135℃~180℃, and the processing time is 1min~8min; The upper surface heating temperature of the fourth heating and drying process is 80℃~120℃, the lower surface heating temperature is 120℃~150℃, and the processing time is 5min~15min. The upper surface heating temperature of the fifth heating and drying process is 60℃~120℃, the lower surface heating temperature is 120℃~150℃, and the processing time is 5min~15min; The upper surface heating temperature of the sixth heating and drying process is 50℃~100℃, the lower surface heating temperature is 100℃~120℃, and the processing time is 2min~10min.

11. The method for preparing the polyimide film according to claim 4, wherein, In the stacked-heat treatment, the heating temperature of the upper surface is 60℃~120℃, and the heating temperature of the lower surface is 120℃~180℃.

12. A circuit board, wherein, The polyimide film included in any one of claims 1-3.

13. A gas barrier composite membrane, wherein, The polyimide film included in any one of claims 1-3.

14. An insulating composite film, wherein, The polyimide film included in any one of claims 1-3.

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