A polyimide film, preparation and use thereof

Abstract

The application relates to a polyimide film and preparation and application thereof, and belongs to the field of advanced functional materials. The preparation comprises the following steps: synthesizing a polyamide acid glue solution from a dianhydride, a diamine monomer and 1,8-diazabicyclo[5.4.0]undec-7-ene phenolate, then performing casting extrusion, solvent evaporation, a chemical cyclization bath, longitudinal stretching, transverse stretching and heat setting to obtain the polyimide film. The polyimide film prepared by the application has a compact and uniform structure and excellent comprehensive performance, and has important application prospects in the fields of artificial graphite heat-conducting films, electricians and electronics.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of advanced functional materials, and particularly relates to a polyimide film and preparation and application thereof. BACKGROUND

[0002] Polyimide (PI) film has excellent heat stability, outstanding mechanical properties and electrical insulation properties, excellent dimensional stability and solvent corrosion resistance, and has been widely used in aerospace, microelectronics and integrated circuits, electrical insulation, advanced display and other fields. In addition, polyimide film is one of the most ideal substrates for preparing high-thermal-conductivity graphite film. By carbonizing the PI film under inert atmosphere and graphitizing at 2800-3200 ℃, a thermal-conductivity graphite can be prepared, which has a highly preferred orientation of graphite layers similar to that of highly oriented pyrolytic graphite, and has high crystallinity. This method has a simpler preparation process than the pyrolysis method for preparing graphite, greatly promoting the development of graphite as a heat dissipation material. With the development of microelectronic technology, the thermal-conductivity graphite film gradually develops in the direction of high thickness, thus putting forward very high requirements for the thickness of the PI base film.

[0003] PI films can be classified into ultra-thin films (d≤8 μm), conventional thin films (8 μm<d≤50 μm), thick films (50 μm<d≤125 μm) and super-thick films (d>125 μm). Relatively speaking, the preparation process of polyimide thick films and super-thick films is complex, and the main reason is that after the polyamic acid casting solution is extruded and cast onto the surface of a steel belt through a die, the cast film is heated by an upper and lower heating mode. Since the polyamic acid film is thick and contains a large amount of solvent, the outermost layer of the film (i.e. the surface that contacts the hot air) begins to solidify into a solid film after the solvent volatilizes rapidly upon heating. This solid film has high compactness, which is not conducive to the volatilization of the solvent inside the material. As the temperature inside the material rises, the vapor pressure also increases. Since the gas cannot be quickly volatilized, bubbles are generated when the temperature inside the material further rises, thereby affecting the structure and final performance of the film. Therefore, how to achieve uniform and rapid volatilization of the polyamic acid thick film during the casting process or how to effectively solve the problem of solvent diffusion in the polyamic acid film after casting is the key to the preparation of PI thick films. In order to solve the above problems, the disclosed invention patent (CN201811324383.2) proposes a method of adding a catalyst and a dehydrating agent to the polyamic acid solution and using a spraying method to prepare a polyimide thick film, in order to achieve rapid volatilization of the solvent. However, the addition of a catalyst and a dehydrating agent to the polyamic acid solution can easily cause gelation, making it difficult to process smoothly. The disclosed invention patent (CN201910650702.7) discloses a method of adding a flow promoter with a boiling point of 100-240 ℃ to achieve rapid volatilization of the solvent. However, the compatibility of the paraffin-based additive with the polyamic acid is not good, and the thickness of the prepared film is only 50 μm, making it difficult to prepare PI films with higher thickness. SUMMARY

[0004] The technical problem to be solved by the present application is to provide a polyimide film, its preparation and application, which solves the problems of uneven solvent volatilization and incomplete imidization in the traditional thermal imidization method, thereby preparing a polyimide film with dense and uniform structure and excellent comprehensive performance.

[0005] The present application provides a method for preparing a polyimide film, comprising:

[0006] Step (1) mixing a dianhydride monomer, a diamine monomer, 1, 8-diazabicyclo[5.4.0]undec-7-ene phenolate, and a solvent under the condition of a protective gas to obtain a polyamic acid solution by reaction;

[0007] Step (2) casting and extruding the polyamic acid solution, and evaporating the solvent to obtain a nascent film;

[0008] Step (3) immersing the nascent film in a chemical cyclization bath, and then drying to obtain a dried polyimide film;

[0009] Step (4) longitudinally stretching, transversely stretching, heat setting the dried polyimide film to obtain a polyimide film.

[0010] Preferably, the dianhydride monomer in the step (1) comprises one or more of:

[0011]

[0012] Preferably, the diamine monomer in the step (1) comprises one or more of:

[0013]

[0014] The structural formula of the 1, 8-diazabicyclo[5.4.0]undec-7-ene phenolate salt in the step (1) is:

[0015]

[0016] The solvent in the step (1) is an aprotic solvent, wherein the aprotic solvent comprises one or more of N, N-dimethylacetamide DMAc, N, N-dimethylformamide DMF, N-methyl-pyrrolidone NMP.

[0017] The protective gas in the step (1) is nitrogen.

[0018] The reaction in the step (1) is a room temperature stirring reaction for 12-24 h.

[0019] Preferably, the molar ratio of the dianhydride monomer to the diamine monomer in the step (1) is 1: (1-1.05).

[0020] Preferably, the 1, 8-diazabicyclo[5.4.0]undec-7-ene phenolate salt in the step (1) is 3-6% of the mass of solid substances, wherein the mass of solid substances refers to the total mass of the dianhydride monomer, the diamine monomer and the 1, 8-diazabicyclo[5.4.0]undec-7-ene phenolate salt.

[0021] ​​​​​​​​​​​​​​​​​Preferably, the mass fraction of the reactants after mixing in step (1) is 25-35 wt%; wherein the reactants refer to dianhydride monomers, diamine monomers and 1, 8-diazabicyclo [5.4.0] undec-7-ene phenolate.

[0022] The viscosity of the polyamic acid solution in step (1) is 2x10 4 ~ 4x10 4 mPa·s.

[0023] Preferably, the solvent evaporation of the polyamic acid solution in step (2) specifically includes: the polyamic acid solution is extruded through a die head and then cast onto the surface of a steel belt, the temperature of the steel belt and the box is controlled to be 160-220℃, and the residence time is 1-8 min; when the steel belt passes through the outlet of the oven, the nascent film is peeled off from the steel belt.

[0024] Preferably, the chemical cyclization bath in step (3) is composed of a dehydrating agent and a catalyst.

[0025] The dehydrating agent is one of acetic anhydride and propionic anhydride; the catalyst is one or more of pyridine, benzimidazole, isoquinoline and imidazole.

[0026] The molar ratio of the dehydrating agent to the catalyst is 1:1.

[0027] Preferably, the time for the nascent film to be immersed in the chemical cyclization bath in step (3) is 2-6 min; and the drying treatment is 160-180℃ for 2-5 min.

[0028] Preferably, the longitudinal stretching temperature in step (4) is 200-300℃, and the stretching multiple is 1.2-2.0; the transverse stretching temperature is 380-450℃, and the stretching multiple is 1.2-2.0.

[0029] Preferably, the heat setting temperature in step (4) is 450-480℃.

[0030] The present application provides a polyimide film prepared by the method, and the thickness of the polyimide film is 140μm or more.

[0031] Further, the thickness of the polyimide film is 140-160μm, and the internal structure is compact and uniform.

[0032] The tensile strength of the polyimide film is ≥145MPa, and the elongation at break is ≥35%.

[0033] The present application provides an application of the polyimide film in the field of artificial graphite heat-conducting film and electrical and electronic engineering.

[0034] Note: 1, 8-diazabicyclo [5.4.0] undec-7-ene phenolate is abbreviated as DBU phenolate.

[0035] The present application first synthesizes a high solid content and low viscosity polyamic acid glue solution by using dianhydride, diamine monomer and DBU phenolate, and no by-product is released in the reaction, and then the polyimide film is prepared by casting extrusion, solvent evaporation, chemical cyclization bath, high temperature drying, longitudinal stretching, transverse stretching and high temperature heat setting; wherein the DBU phenolate forms a cation-anion pair through the imidazole NH group and the polyamic acid COOH, breaks the interaction between the polyamic acid, can realize the solution high solid content and low viscosity characteristics, at the same time, the DBU phenolate can promote the cyclization reaction of polyamic acid in the casting section, and improve the mechanical properties of the nascent film; and the chemical cyclization bath can effectively promote the imidization reaction of the nascent film contacting the steel belt surface, and remove the residual solvent in the film, so as to ensure the structure uniformity and mechanical properties of the final polyimide film.

[0036] Advantages

[0037] (1) The present application adds the thermal-induced base-producing agent DBU phenolate in the polyamic acid precursor glue solution, which mainly plays two roles: first, the positive and negative ion interaction between the benzimidazole in the DBU phenolate and the polyamic acid can weaken the hydrogen bond interaction between the polyamic acid molecular chains, realize the synthesis of high solid content and low viscosity polyamic acid, and thus weaken the solvent evaporation pressure in the casting section; second, the imidazole unit produced by the decomposition of DBU phenolate under high temperature can effectively catalyze the cyclization reaction of polyamic acid, weaken the interaction between polyamic acid and solvent molecules, also be beneficial to the volatilization of solvent, and improve the mechanical properties of the nascent film;

[0038] (2) The present application adds the chemical cyclization bath after casting, and the imidization reaction of the PAA film contacting the steel belt surface is caused by the action of the catalyst and the dehydrating agent, so as to promote the removal of solvent molecules, and thus prepare the film with dense and uniform structure;

[0039] (3) The present application prepares the polyimide film with thickness of 140 μm or more and strength of 145 MPa or more by the synergistic effect of the chemical cyclization bath and the high temperature bidirectional stretching treatment, which has outstanding comprehensive performance. BRIEF DESCRIPTION OF DRAWINGS

[0040] Figure 1 The SEM photo of the cross section of the PI super-thick film prepared in Example 1;

[0041] Figure 2 The stress-strain curves of the PI super-thick film prepared in Example 1 tested for five times. DETAILED DESCRIPTION

[0042] The application will be further described in connection with the following specific examples. It should be understood that the examples are only used to illustrate the application and not used to limit the scope of the application. Furthermore, it should be understood that after reading the content of the application, those skilled in the art can make various modifications or changes to the application, and these equivalent forms also fall within the scope defined by the appended claims.

[0043] The above-mentioned dianhydride and diamine monomers are provided by Tianjin Zhongtai Material Technology Co., Ltd., and the DBU phenolate is provided by Shanghai Huayuan Century Trading Co., Ltd.

[0044] The cross-section SEM of the PI super-thick film is tested by Hitachi High-Technologies SU3800;

[0045] The mechanical properties of the PI super-thick film are tested by Instron 3400, the tensile rate is 5 mm / min, the clamp height is 5 cm, and the test is performed five times to obtain the average value. -1

[0046] The polyimide in the examples and comparative examples is abbreviated as PI.

[0047] The DBU phenolate addition amount involved in the examples and comparative examples is the percentage content of the relative solid mass, wherein the solid mass refers to the total mass of the dianhydride monomer, the diamine monomer and the 1,8-diazabicyclo[5.4.0]undec-7-ene phenolate.

[0048] Example 1

[0049] Under the protection of nitrogen, equimolar PMDA and ODA monomers are dissolved in N, N-dimethylacetamide solvent, 3% of DBU phenolate is added, the mass fraction of the reactants is ensured to be 25 wt%, and the reaction is stirred at room temperature for 24 h to obtain a polyamic acid solution with an apparent viscosity of 2.5×10 4 mPa·s, which is used after degassing;

[0050] The synthesized polyamic acid solution is extruded through a die and cast onto the surface of a steel belt, the temperature of the steel belt and the box is controlled to be 180 ℃, and the residence time is 4 min; when the steel belt passes through the outlet of the oven, the nascent film is peeled off from the steel belt and then enters a chemical cyclization bath, which is composed of acetic anhydride and pyridine in a molar ratio of 1:1, and the residence time of the nascent film in the cyclization bath is ensured to be 3 min; after exiting the cyclization bath, the nascent film enters a drying box, the temperature is 180 ℃, and the drying time is 2 min; then the dried film is stretched longitudinally at 250 ℃, the stretching ratio is 1.5 times; then the film is stretched transversely at 400 ℃ by 1.5 times; finally, the PI film is subjected to high-temperature setting treatment at 450 ℃ to prepare a PI super-thick film.

[0051] For example, the PI film prepared in the above example 1 is used as a substrate, and a layer of silver is deposited on the surface of the PI film by magnetron sputtering to prepare a silver-coated PI film. Figure 1 ​The PI super-thick film is shown in the cross-sectional SEM photograph, the film is uniform and dense inside, and the thickness is 150 μm.

[0052] As shown in Figure 2 The stress-strain curve of the PI super-thick film is shown, the average tensile strength is 147 MPa, and the average breaking elongation is 43%.

[0053] Example 2

[0054] Under the protection of nitrogen, equimolar monomers of dianhydride PMDA and mixed diamine (molar ratio of ODA to p-PDA is 1:1) were dissolved in N, N-dimethylacetamide solvent, 3% DBU phenolate was added, the mass fraction of the reactants was ensured to be 25 wt%, and the reaction was stirred at room temperature for 24 h to obtain a polyamic acid solution with an apparent viscosity of 3.2×10 4 mPa·s, which was used after degassing;

[0055] The synthesized polyamic acid solution was extruded through a die and cast onto the surface of a steel belt, the temperature of the steel belt and the box was controlled to be 180 ℃, and the residence time was 4 min; when the steel belt passed through the oven outlet, the nascent film was peeled off from the steel belt and then entered the chemical cyclization bath, which was composed of acetic anhydride and pyridine with a molar ratio of 1:1, and the residence time of the nascent film in the cyclization bath was 3 min; after leaving the cyclization bath, the nascent film entered the drying box, the temperature was 180 ℃, and the drying time was 2 min; then the dried film was stretched longitudinally at 250 ℃, the stretching ratio was 1.5 times; then the film was stretched transversely at 400 ℃ by 1.5 times; finally, the PI film was subjected to high-temperature setting treatment at 450 ℃ to prepare a PI super-thick film.

[0056] Example 3

[0057] Under the protection of nitrogen, equimolar monomers of dianhydride BPDA and diamine p-PDA were dissolved in N, N-dimethylacetamide solvent, 4% DBU phenolate was added, the mass fraction of the reactants was ensured to be 28 wt%, and the reaction was stirred at room temperature for 24 h to obtain a polyamic acid solution with an apparent viscosity of 4.8×10 4 mPa·s, which was used after degassing;

[0058] The synthesized polyamic acid solution was extruded through a die and cast onto the surface of a steel belt, the temperature of the steel belt and the tank was controlled at 180 °C, the residence time was 4 min; when the steel belt passed through the oven outlet, the nascent film was peeled off from the steel belt and then entered a chemical cyclization bath, which was composed of acetic anhydride and pyridine in a molar ratio of 1:1, and the residence time of the nascent film in the cyclization bath was ensured to be 3 min; after leaving the cyclization bath, the nascent film entered a drying oven, the temperature was 180 °C, and the drying time was 2 min; then the dried film was stretched longitudinally at 250 °C, the stretching ratio was 1.5 times; then the film was stretched transversely at 400 °C by 1.5 times; finally, the PI film was subjected to high-temperature setting treatment at 480 °C to prepare a PI ultra-thick film.

[0059] Example 4

[0060] Under the protection of nitrogen, equimolar ratio of PMDA and ODA monomers were dissolved in N, N-dimethylacetamide solvent, 3% DBU phenolate was added at the same time, the mass fraction of the reactants was ensured to be 30 wt%, the reaction was stirred at room temperature for 24 h, and a polyamic acid solution with an apparent viscosity of 3.1 x 10 4 mPa·s was obtained, which was ready for use after degassing;

[0061] The synthesized polyamic acid solution was extruded through a die and cast onto the surface of a steel belt, the temperature of the steel belt and the tank was controlled at 180 °C, the residence time was 4 min; when the steel belt passed through the oven outlet, the nascent film was peeled off from the steel belt and then entered a chemical cyclization bath, which was composed of acetic anhydride and pyridine in a molar ratio of 1:1, and the residence time of the nascent film in the cyclization bath was ensured to be 3 min; after leaving the cyclization bath, the nascent film entered a drying oven, the temperature was 180 °C, and the drying time was 2 min; then the dried film was stretched longitudinally at 250 °C, the stretching ratio was 1.5 times; then the film was stretched transversely at 400 °C by 1.5 times; finally, the PI film was subjected to high-temperature setting treatment at 480 °C to prepare a PI ultra-thick film.

[0062] Example 5

[0063] Under the protection of nitrogen, equimolar ratio of PMDA and ODA monomers were dissolved in N, N-dimethylacetamide solvent, 3% DBU phenolate was added at the same time, the mass fraction of the reactants was ensured to be 30 wt%, the reaction was stirred at room temperature for 24 h, and a polyamic acid solution with an apparent viscosity of 3.1 x 10 4 mPa·s was obtained, which was ready for use after degassing;

[0064] The synthesized polyamide acid solution is extruded through a die and cast onto the surface of a steel belt, the temperature of the steel belt and the tank is controlled at 180 °C, the residence time is 4 min; when the steel belt passes through the outlet of the oven, the nascent film is peeled off from the steel belt and then enters a chemical cyclization bath, which consists of acetic anhydride and pyridine in a molar ratio of 1:1, the residence time of the nascent film in the cyclization bath is ensured to be 3 min; after exiting the cyclization bath, the nascent film enters a drying tank, the temperature is 180 °C, the drying time is 2 min; then the dried film is stretched longitudinally at 250 °C, the stretching ratio is 1.5 times; then the film is stretched transversely at 400 °C by 1.5 times; finally, the PI film is subjected to high-temperature setting treatment at 450 °C to prepare a PI ultra-thick film.

[0065] Comparative Example 1

[0066] Under the protection of nitrogen, equimolar PMDA and ODA monomers are dissolved in N, N-dimethylacetamide solvent, the mass fraction of the reactants is ensured to be 25 wt%, the reaction is stirred at room temperature for 24 h, a polyamide acid solution with an apparent viscosity of 2×10 5 mPa·s is obtained, and is ready for use after degassing;

[0067] The synthesized polyamide acid solution is extruded through a die and cast onto the surface of a steel belt, the temperature of the steel belt and the tank is controlled at 180 °C, the residence time is 4 min; when the steel belt passes through the outlet of the oven, the nascent film is peeled off from the steel belt and then enters a chemical cyclization bath, which consists of acetic anhydride and pyridine in a molar ratio of 1:1, the residence time of the nascent film in the cyclization bath is ensured to be 3 min; after exiting the cyclization bath, the nascent film enters a drying tank, the temperature is 180 °C, the drying time is 2 min; then the dried film is stretched longitudinally at 250 °C, the stretching ratio is 1.5 times; then the film is stretched transversely at 400 °C by 1.5 times; finally, the PI film is subjected to high-temperature setting treatment at 450 °C to prepare a PI ultra-thick film.

[0068] Comparative Example 2

[0069] Under the protection of nitrogen, equimolar PMDA and ODA monomers are dissolved in N, N-dimethylacetamide solvent, 3% DBU phenoxide is added at the same time, the mass fraction of the reactants is ensured to be 25 wt%, the reaction is stirred at room temperature for 24 h, a polyamide acid solution with an apparent viscosity of 2.5×10 4 mPa·s is obtained, and is ready for use after degassing;

[0070] The synthesized polyamide acid solution was extruded through a die and cast onto the surface of a steel belt, the temperature of the steel belt and the box was controlled at 180 ℃, the residence time was 4 min; when the steel belt passed through the oven outlet, the nascent film was peeled off from the steel belt and then entered into a chemical cyclization bath, the composition of which was acetic anhydride and pyridine in a molar ratio of 1:1, to ensure that the nascent film stayed in the cyclization bath for 3 min; after leaving the cyclization bath, the nascent film entered a drying oven, the temperature of which was 180 ℃, and the drying time was 2 min; finally, the PI film was subjected to high-temperature setting treatment at 450 ℃ to prepare a PI ultra-thick film.

[0071] Comparative Example 3

[0072] Under the protection of nitrogen, equimolar PMDA and ODA monomers were dissolved in N, N-dimethylacetamide solvent, 3% DBU phenolate was added at the same time, the mass fraction of the reactants was ensured to be 25 wt%, and the reaction was stirred at room temperature for 24 h to obtain a polyamide acid solution with an apparent viscosity of 2.5×10 4 mPa·s, which was ready for use after degassing;

[0073] The synthesized polyamide acid solution was extruded through a die and cast onto the surface of a steel belt, the temperature of the steel belt and the box was controlled at 180 ℃, the residence time was 4 min; when the steel belt passed through the oven outlet, the nascent film was peeled off from the steel belt and then entered into a chemical cyclization bath, the composition of which was acetic anhydride and pyridine in a molar ratio of 1:1, to ensure that the nascent film stayed in the cyclization bath for 3 min; after leaving the cyclization bath, the nascent film entered a drying oven, the temperature of which was 180 ℃, and the drying time was 2 min; finally, the PI film was subjected to high-temperature setting treatment at 450 ℃ to prepare a PI ultra-thick film.

[0074] The performance comparison of the PI ultra-thick films of the examples and comparative examples is shown in Table 1.

[0075] Table 1

[0076]

[0077] Examples 1-5 prepared PI ultra-thick films with excellent mechanical properties. Compared with Example 1, no DBU phenolate was added in Comparative Example 1, so the viscosity of the synthesized glue solution reached 2×10 5 mPa·s, a large amount of gel appeared in the glue solution, the solvent was wrapped in the gel block and was difficult to volatilize, so a large amount of solvent volatilized instantaneously during high-temperature heat treatment, resulting in a large number of hole defects in the PI ultra-thick film, and the mechanical properties of the PI ultra-thick film were poor, but the film thickness increased; compared with Example 1, the nascent film in Comparative Example 2 did not pass through the chemical cyclization bath, so, similar to Comparative Example 1, a large amount of residual solvent volatilized instantaneously during high-temperature heat treatment, resulting in a large number of hole defects in the PI ultra-thick film, and the mechanical properties of the PI ultra-thick film were poor, but the film thickness increased; and in Comparative Example 3, no bidirectional heat stretching was applied, therefore, the comprehensive mechanical properties of the PI ultra-thick film were significantly lower than those of Example 1.

Claims

1. A method for producing a polyimide film, characterized by, Comprising: Step (1) mixing dianhydride monomer, diamine monomer, 1, 8-diazabicyclo [5.4.0] undec-7-ene phenolate, solvent under the condition of protective gas, and obtaining polyamic acid solution by reaction; Step (2) casting extrusion and solvent evaporation of the polyamic acid solution to obtain nascent film; Step (3) immersing the nascent film into chemical cyclization bath and then drying to obtain dried polyimide film; Step (4) longitudinally stretching, transversely stretching and heat setting the dried polyimide film to obtain polyimide film.

2. The method of claim 1, wherein, The dianhydride monomer in step (1) comprises: , , , , , one or more of the following: The diamine monomer in step (1) comprises: , , , , , , , , , one or more of the following:

3. The preparation method according to claim 1, characterized in that, The molar ratio of the dianhydride monomer to the diamine monomer in step (1) is 1: (1-1.05) ; The 1, 8-diazabicyclo [5.4.0] undec-7-ene phenolate in step (1) is 3-6% of the solid substance mass.

4. The preparation method according to claim 1, characterized in that, The mass fraction of the mixed reactants in step (1) is 25-35 wt%; the viscosity of the polyamic acid solution is 2 x 10 4 mPa·s. 4 mPa·s.

5. The method of claim 1, wherein the step of forming the first and second layers is performed by a process selected from the group consisting of: sputtering, evaporation, and chemical vapor deposition. The casting extrusion and solvent evaporation of the polyamic acid solution in step (2) specifically comprises: the polyamic acid solution is extruded through a die head and then cast onto the surface of a steel belt, the temperature of the steel belt and the box is controlled to be 160-220 ℃, and the residence time is 1-8 min, and the nascent film is peeled off from the steel belt when the steel belt passes through the outlet of the oven.

6. The method of claim 1, wherein the step of forming the first and second layers is performed by a process selected from the group consisting of: sputtering, evaporation, and chemical vapor deposition. The chemical cyclization bath in step (3) is composed of a dehydrating agent and a catalyst; The dehydrating agent is one of acetic anhydride and propionic anhydride; the catalyst is one or more of pyridine, benzimidazole, isoquinoline and imidazole; The molar ratio of the dehydrating agent to the catalyst is 1:

1.

7. The method of claim 1, wherein the step of forming the first and second layers is performed by a process selected from the group consisting of: sputtering, evaporation, and chemical vapor deposition. The immersion time of the nascent film in the chemical cyclization bath in step (3) is 2-6 min; The drying treatment in step (3) is 160-180 ℃ for 2-5 min.

8. The method of claim 1, wherein the step of forming the first and second layers is performed by a process selected from the group consisting of: sputtering, evaporation, and chemical vapor deposition. The longitudinal stretching temperature in step (4) is 200-300 ℃, and the stretching multiple is 1.2-2.0; the transverse stretching temperature is 380-450 ℃, and the stretching multiple is 1.2-2.0; The heat setting temperature in step (4) is 450-480 ℃.

9. A polyimide film prepared by the method of any one of claims 1 to 8, characterized in that, The thickness of the polyimide film is more than 140 μm.

10. Application of the polyimide film of claim 9 in artificial graphite heat-conducting film and electrical and electronic fields.

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