A kind of polyimide film and preparation method thereof

Abstract

The invention relates to the technical field of polyimide films, and in particular to a polyimide film and a preparation method thereof. The polyimide film is prepared from a casting liquid composed of the following components through casting, biaxial stretching, imidization and winding processes, including: polyamide fat; polyamic acid; solvent; tertiary amine catalyst; the high intermolecular interaction force between the molecules of the polyamide fat and the solvent is utilized to facilitate the transportation and degassing of the casting liquid; the alkalinity strength of the tertiary amine catalyst is utilized to make the thermal cyclization reaction of the polyamide fat and the polyamic acid more uniform; the polyimide film of the invention has the characteristics of high imidization degree, uniform structure, excellent mechanical properties and the like.

Description

Technical Field

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

[0002] Artificial graphite film has a wide range of applications in thermal management of electronic appliances, aerospace and other products due to its high thermal conductivity, lightness and good electromagnetic shielding performance. Common artificial graphite film is mainly prepared by high-temperature treatment of polyimide film at 2800-3200 ℃. Compared with CVD method, graphite oxide reduction method and other methods, the graphite film prepared by this method has significant advantages such as complete graphite film structure, few crystal structure defects, and high order of carbon atoms. The thermal conductivity can reach 1400-1950 W m-1K-1.

[0003] At present, with the continuous improvement of the integration of microelectronic devices, higher requirements are put forward for thermally conductive graphite films, mainly developing in the direction of high thickness and high thermal conductivity. However, the current thickness of polyimide-based films is mainly 25-75 μm, which is difficult to meet the application requirements of high-thickness graphite films. In order to solve the above problems, the invention patent with announcement number CN201910055344.5 discloses a method for preparing a polyimide film by adding thermoplastic polyimide and blowout coating. The thickness of the polyimide film can reach 150-300 μm, but this method of introducing thermoplastic polyimide will damage the heat resistance stability of the polyimide film, and the process is complicated. In the invention patent with announcement number CN201910650702.7, the film-forming property of high-concentration and high-viscosity casting liquid is improved by adding a flow promoter. The thickness of the prepared polyimide film is 50-125μm. However, this method of adding small molecule flow promoters will inevitably form micro-defects in the film, affecting the uniformity and stability of the polyimide film. Compared with the traditional thickness of polyimide films, how to improve the fluidity and film-forming property of high-concentration and high-viscosity casting liquid, and how to achieve uniform mass transfer and heat transfer and uniform thermal cyclization of thick films during casting, longitudinal stretching, transverse stretching and thermal cyclization are the key points and difficulties in the development of polyimide films. In order to solve the above problems, a polyimide film and a preparation method thereof are proposed in the present invention. Summary of the invention

[0004] To achieve the above object, the present invention provides a polyimide film, which is prepared by a casting solution composed of the following components through casting, biaxial stretching, imidization and winding processes, including:

[0005] Polyamide fat, the weight percentage of which in the casting solution is 10%-12%;

[0006] Polyamic acid, the weight percentage of which in the casting solution is 83%-88%;

[0007] A solvent, the weight percentage of which in the casting solution is 83%-88%;

[0008] A tertiary amine catalyst, the weight percentage of which in the casting solution is 2%-5%;

[0009] Among them, the low interaction force between the molecules of the polyamide fat and the high interaction force between the molecules of the polyamide fat and the solvent are utilized to facilitate the transportation and degassing of the casting liquid, and the polyamide fat can be used as a flow promoter for high-concentration polyamic acid solution; the alkalinity strength of the tertiary amine catalyst is utilized to make the thermal cyclization reaction of the polyamide fat and the polyamic acid more uniform, thereby solving the problems of high polyimide film thickness, difficult heat and mass transfer, and uneven imidization degree in traditional processes.

[0010] Optionally, the concentration of the casting solution used is 25-35 wt %, and the apparent viscosity at room temperature is 1000-3000 poise.

[0011] Optionally, the polyimide film has a thickness of 75-200 μm.

[0012] Optionally, the polyimide film has a tensile strength of 140-290 MPa, a modulus of 2.2-7.5 GPa, and an elongation at break of 25%-65%.

[0013] To achieve the above object, the present invention also provides a method for preparing the polyimide film, the method comprising the following steps:

[0014] S1: Under protective gas, dianhydride monomer and methanol or ethanol are heated to reflux at 70°C in tetrahydrofuran solvent for 2-4 hours, wherein the molar ratio of dianhydride monomer to methanol or ethanol is 1:2. After the reaction is completed, dicarboxylic acid ester is prepared by precipitation, washing and drying;

[0015] S2: Under nitrogen protection, the dicarboxylic acid ester is dissolved in a polar aprotic solvent. After the dissolution is complete, an equimolar amount of diamine monomer and catalyst N,N'-diisopropylcarbodiimide are added to the solution. The concentration of the reactants is controlled to be 20-35 wt%, and the reaction is stirred at room temperature for 12 h to synthesize a polyamide lipid solution.

[0016] S3: Under protective gas, an appropriate amount of polyamide lipid solution, an equimolar amount of diamine monomer and a dianhydride monomer are dissolved in a polar aprotic solvent at the same time, and reacted at room temperature for 12 h to obtain a polymer solution; after the reaction is completed, a tertiary amine catalyst is added to the above polymer solution, and the mixture is stirred and mixed evenly, and the casting solution is degassed to obtain a film casting solution for use;

[0017] S4: using the casting solution in S3 to form a self-supporting film by metering and casting onto a steel belt;

[0018] S5: The self-supporting film prepared in S4 is subjected to longitudinal and transverse stretching and imidization treatment, and finally rolled up to prepare a polyimide film.

[0019] Optionally, the structure of the dianhydride monomer in S1 is as follows:

[0020] .

[0021] Optionally, the structure of the diamine monomer in S2 is as follows:

[0022] .

[0023] Optionally, the polar aprotic solvent in S2 is one of N,N-dimethylformamide, N,N-dimethylacetamide or N-methyl-pyrrolidone.

[0024] Optionally, the mass fraction of the casting liquid in S3 is 25-35 wt%, wherein the mass fraction of the polyamide resin is 10 wt%-12 wt%.

[0025] Optionally, the tertiary amine catalyst in S3 is one of N-methylpyrrole, pyridine, and isoquinoline, and the added amount is 2wt%-5wt%.

[0026] Optionally, in S4, the temperature of the steel strip is controlled to be 120-160° C. and the film flow rate is controlled to be 2-5 m / min.

[0027] Optionally, in S5, the transverse stretching temperature is 150-180°C, the stretching ratio is 1:(1.1-1.3), the longitudinal stretching temperature is 350-400°C, the stretching ratio is 1:(1.01-1.2), and the high-temperature cyclization temperature is 400-450°C.

[0028] The beneficial effects of the present invention are as follows:

[0029] (1) The present invention introduces polyamide resin and polyamide acid solution into the composite, wherein the polyamide resin molecules have low interaction with each other, while the polyamide resin molecules have high interaction with solvent molecules, thereby exhibiting the characteristics of high concentration and low viscosity. As a flow promoter of high concentration polyamide acid solution, the film-forming property of high concentration and high viscosity casting solution is greatly improved.

[0030] (2) The process in which the polyamide resin added in the present invention can be converted into polyimide in a high temperature environment is continuous and controllable, and polyamide resin and polyimide have similarities in chemical structure. This conversion will not introduce new defects or discontinuities, and therefore will not damage the comprehensive properties of the polyimide film.

[0031] (3) In the present invention, a tertiary amine catalyst is added to the casting solution to promote the thermal cyclization reaction of the polyamide lipid and polyamide acid components during the biaxial stretching and imidization process of the polyimide film, thereby solving the problem of uniformity of the structure and performance of the polyimide film. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a physical picture of the polyimide film prepared in the present invention. DETAILED DESCRIPTION

[0033] In order to make the purpose, technical solutions and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. Unless otherwise defined, the technical terms or scientific terms used herein should be understood by people with general skills in the field to which the present invention belongs. "Including" and similar words used in this article mean that the elements or objects appearing before the word include the elements or objects listed after the word and their equivalents, without excluding other elements or objects.

[0034] In view of the problems existing in the prior art, an embodiment of the present invention provides a polyimide film, wherein the polyimide film is prepared by a casting solution composed of the following components through casting, biaxial stretching, imidization and winding processes, including:

[0035] Polyamide fat, the weight percentage of which in the casting solution is 10%-12%;

[0036] Polyamic acid, the weight percentage of which in the casting solution is 83%-88%;

[0037] A solvent, the weight percentage of which in the casting solution is 83%-88%;

[0038] A tertiary amine catalyst, the weight percentage of which in the casting solution is 2%-5%;

[0039] Among them, the low interaction force between the molecules of the polyamide fat and the high interaction force between the molecules of the polyamide fat and the solvent are utilized to facilitate the transportation and degassing of the casting liquid; the alkalinity strength of the tertiary amine catalyst (the alkaline properties of the tertiary amine catalyst) is utilized to make the thermal cyclization reaction of the polyamide fat and the polyamic acid more uniform.

[0040] In this embodiment, the interaction between polyamide lipid molecules is low, while the interaction between polyamide lipid molecules and solvent molecules is high, thus showing the characteristics of high concentration and low viscosity, and the polyamide lipid can be used as a flow promoter for high-concentration polyamic acid solution, which is conducive to the transportation and deaeration of high-concentration casting liquid. Specifically, one, because the polyamide lipid molecules have lower polarity, the hydrogen bonds and other dipole-dipole interactions between the polyamide lipid molecules are lower, and the solvent molecules have higher polarity and can form stronger interactions with the polyamide lipid; second, the polyamide lipid molecules have a more linear or less branched structure, which reduces the van der Waals force and steric hindrance between the polyamide lipid molecules, making the interaction between the polyamide lipid molecules lower. The solvent molecules are smaller than the polyamide lipid molecules, and can more easily penetrate and surround the polyamide lipid molecules, enhancing the interaction between the polyamide lipid molecules and the solvent molecules; it can be seen that due to the strong interaction between the polyamide lipid molecules and the solvent molecules, the polyamide lipid can be used as a flow promoter to reduce the viscosity of the polyamic acid solution, making the polyamic acid solution easier to flow, which is more beneficial for the transportation and deaeration of the high-concentration casting liquid.

[0041] The addition of tertiary amine catalysts in this embodiment can effectively catalyze the thermal cyclization reaction of polyamide lipids and polyamic acid, solve the problems of high thickness of polyimide film, difficult heat and mass transfer, and uneven imidization degree in traditional processes, so that highly uniform polyimide films can be prepared. Specifically, firstly, tertiary amine catalysts are alkaline and can provide lone pairs of electrons, which enables tertiary amine catalysts to participate in the reaction as base catalysts. In the thermal cyclization reaction, tertiary amine catalysts can react with carboxyl groups (-COOH) to form carboxylates, thereby promoting dehydration reactions and accelerating the formation of imide rings (-NH-CO-NH-); secondly, in the traditional polyimide preparation process, when the polyimide film is thicker, the internal heat and mass transfer efficiency will be reduced, resulting in uneven imidization reactions. The addition of tertiary amine catalysts in the present invention can improve the uniformity of the reaction, because tertiary amine catalysts accelerate the reaction rate and reduce the limitations of heat and material transfer; thirdly, tertiary amine catalysts can improve the completion of the imidization reaction, so that more polyamic acid is converted into polyimide, thereby improving the thermal stability and mechanical properties of the polyimide film, and tertiary amine catalysts can improve the selectivity of the reaction and reduce the occurrence of side reactions, which helps to maintain the uniformity and performance of the polyimide film; it can be seen that tertiary amine catalysts effectively improve the uniformity of the cyclization reaction, so that a highly uniform polyimide film can be prepared, the defects and non-uniformity in the polyimide film are reduced, and the overall performance of the polyimide film is improved.

[0042] It should be noted that the solvent mentioned in this embodiment may be a tetrahydrofuran solvent and a polar aprotic solvent, but is certainly not limited to the tetrahydrofuran solvent and the polar aprotic solvent.

[0043] In one embodiment, the concentration of the casting liquid used is 25-35 wt%, and the apparent viscosity at room temperature is 1000-3000 poise. The configuration of this embodiment, setting the concentration of the casting liquid in the range of 25-35 wt%, can ensure that there is sufficient polymer content in the film forming process, thereby forming a thick and uniform polyimide film. Setting the viscosity of the casting liquid to 1000-3000 poise helps the casting liquid to be evenly coated during the casting process, reducing the unevenness and defects during the coating process. The low viscosity of the casting liquid is conducive to the escape of bubbles, reducing bubbles and pinholes during the film forming process, and improving the surface quality of the polyimide film and the uniformity of the internal structure.

[0044] Specifically, in the casting solution, the interaction between the polymer (polyamide and polyamide acid) and the solvent determines the rheological properties of the solution. The appropriate concentration (25-35 wt%) and viscosity (1000-3000 poise) can achieve a balance between the interaction between the polymer and the solvent, maintaining the solubility of the polymer and ensuring the fluidity of the solution. Moreover, the interaction between polymer molecules affects the viscosity of the solution. At a concentration of 25-35wt%, the attraction and repulsion between polymer molecules reach a balance, so that the casting solution has sufficient strength while maintaining a low viscosity. The viscosity of the casting solution directly affects its flow behavior during the casting process. The viscosity range of 1000-3000 poise ensures that the casting solution has good fluidity during coating and maintains the stability of the coating before curing.

[0045] In one embodiment, the thickness of the polyimide film is 75-200 μm. Figure 1 shown.

[0046] In one embodiment, the polyimide film has a tensile strength of 140-290 MPa, a modulus of 2.2-7.5 GPa, and an elongation at break of 25%-65%.

[0047] In view of the problems existing in the prior art, an embodiment of the present invention further provides a method for preparing the polyimide film, the method comprising the following steps:

[0048] S1: Under protective gas, dianhydride monomer and methanol or ethanol are heated to reflux at 70°C in tetrahydrofuran solvent for 2-4 h, wherein the molar ratio of dianhydride monomer to methanol or ethanol is 1:2. After the reaction is completed, dicarboxylic acid ester is prepared by precipitation, washing and drying; the protective gas includes inert gas and active gas, wherein the inert gas includes helium (He) and argon (Ar); the active gas includes carbon dioxide (CO2), oxygen (O2), nitrogen (N2) and hydrogen (H2).

[0049] S2: Under nitrogen protection, the dicarboxylic acid ester is dissolved in a polar aprotic solvent. After the dissolution is complete, an equimolar amount of diamine monomer and catalyst N,N'-diisopropylcarbodiimide are added to the solution. The concentration of the reactants is controlled to be 20-35 wt%, and the reaction is stirred at room temperature for 12 h to synthesize a polyamide lipid solution.

[0050] S3: Under protective gas, an appropriate amount of polyamide lipid solution, an equimolar amount of diamine monomer and a dianhydride monomer are dissolved in a polar aprotic solvent at the same time, and reacted at room temperature for 12 h to obtain a polymer solution; after the reaction is completed, a tertiary amine catalyst is added to the above polymer solution, and the mixture is stirred and mixed evenly, and the casting solution is degassed to obtain a film casting solution for use;

[0051] S4: using the casting solution in S3 to form a self-supporting film by metering and casting onto a steel belt;

[0052] S5: The self-supporting film prepared in S4 is longitudinally and transversely stretched and imidized (the imidization treatment needs to be performed at a high temperature), and finally rolled up to prepare a polyimide film.

[0053] This embodiment can prepare a polyimide film with a film thickness of 75-200 μm through the above-mentioned polyimide film preparation method. At the same time, the prepared polyimide film has the characteristics of high degree of imidization, uniform structure, excellent mechanical properties, etc., and has important application prospects in the fields of high-thickness artificial graphite film and so on.

[0054] In one embodiment, the structure of the dianhydride monomer in S1 is as follows:

[0055] .

[0056] In one embodiment, the structure of the diamine monomer in S2 is as follows:

[0057] .

[0058] In one embodiment, the polar aprotic solvent in S2 is one of N,N-dimethylformamide, N,N-dimethylacetamide or N-methyl-pyrrolidone. The selection of polar aprotic solvents in this embodiment has the following main functions: 1. Solubility: These solvents have good solubility for polyamic acid and polyimide precursors; 2. These solvents are relatively stable chemically and are not easy to react with polyamic acid or polyimide, which helps to maintain the purity and performance of the polymer; 3. These solvents will not provide protons (H+) to other molecules in the solution, which helps to avoid affecting the charge balance and reaction kinetics of the polymer. Of course, in other embodiments, polar aprotic solvents are not limited to one of N,N-dimethylformamide, N,N-dimethylacetamide or N-methyl-pyrrolidone.

[0059] In one embodiment, the mass fraction of the casting solution in S3 is 25-35 wt%, wherein the mass fraction of polyamide fat is 10 wt%-12 wt%. As an important component of the casting solution, the mass fraction of polyamide fat has a significant effect on the formation and performance of the film. Polyamide fat can provide enough polymer chains to form a tough film structure within the range of 10 wt%-12 wt%. And within the above mass fraction, the interaction between polyamide fat molecules and the interaction with the solvent can reach a balance, thereby optimizing the fluidity of the casting solution and the uniformity of the coating. It can be seen that in this embodiment, by accurately controlling the mass fractions of the total reactants and polyamide fat in S3, the processing properties and physical properties of the polyimide film can be optimized to ensure the consistency and reliability of the product. The control of these parameters is based on a deep understanding of the chemical and physical properties of polyimide, and precise regulation of processing conditions.

[0060] In one embodiment, the tertiary amine catalyst in S3 is one of N-methylpyrrole, pyridine, and isoquinoline, and the addition amount is 2 wt%-5 wt%. The selection of tertiary amine catalysts in this embodiment has the following beneficial effects: it is possible to increase the reaction rate, specifically, the tertiary amine catalyst can significantly increase the reaction rate of polyamic acid cyclization to polyimide, shortening the production cycle; it is possible to improve the uniformity of the film, specifically, the uniform distribution of the catalyst contributes to the uniform cyclization reaction, thereby improving the uniformity of the polyimide film; it is possible to improve product quality, specifically, the addition of the catalyst can increase the degree of imidization of the polyimide film, reduce unreacted polyamic acid, and improve product quality. Of course, in other embodiments, the tertiary amine catalyst is not limited to one of N-methylpyrrole, pyridine, and isoquinoline.

[0061] In one embodiment, the temperature of the steel strip is controlled to be 120-160°C and the film flow rate is 2-5 m / min in S4. The configuration of this embodiment can optimize the forming process of the polyimide film, improve product quality, and ensure the continuity and stability of production by accurately controlling the temperature of the steel strip and the film flow rate. Specifically:

[0062] Uniform film formation: Appropriate steel belt temperature helps the casting liquid to spread evenly on the steel belt to form a film with uniform thickness; improving film quality: maintaining an appropriate steel belt temperature can avoid local overheating or incomplete solidification of the film, thereby ensuring the quality and performance of the film; improving production efficiency: Control of the film flow rate helps to improve the efficiency of the production line and ensure the continuous and stable production of high-quality polyimide films.

[0063] In one embodiment, the transverse stretching temperature in S5 is 150-180°C, the stretching ratio is 1: (1.1-1.3), the longitudinal stretching temperature is 350-400°C, the stretching ratio is 1: (1.01-1.2), and the high-temperature cyclization temperature is 400-450°C. In this embodiment, by precisely controlling the temperature and ratio of stretching and cyclization, the structure and performance of the polyimide film can be optimized to meet specific application requirements. It has the following beneficial effects:

[0064] Molecular chain orientation: During the stretching process, the polyimide molecular chains are arranged along the stretching direction, which increases the interaction between molecules and thus improves the mechanical properties of the film; Heat treatment effect: High-temperature cyclization helps to complete the transformation of polyamic acid to polyimide, form a stable imide ring, and improve the thermal stability of the film; Surface and internal quality: High-temperature cyclization can improve the surface and internal quality of the polyimide film and reduce holes and unevenness.

[0065] The polyimide film and its preparation method of the present invention are described in detail below through specific embodiments:

[0066] Example 1

[0067] Under nitrogen protection, dianhydride monomer (PMDA) and ethanol were heated to reflux at 70 °C in tetrahydrofuran solvent for 4 h, wherein the molar ratio of dianhydride monomer to ethanol was 1:2. After the reaction was completed, dicarboxylic acid ester was prepared by precipitation, washing and drying. Under nitrogen protection, the above dicarboxylic acid ester was dissolved in a polar aprotic solvent (DMAc). After complete dissolution, an equimolar amount of diamine monomer (ODA) monomer and catalyst N,N'-diisopropylcarbodiimide were added to the solution, and the concentration of the reactants was controlled to be 20 wt%. The reaction was stirred at room temperature for 12 h to synthesize a polyamide ester solution. An appropriate amount of polyamide ester solution and equimolar amounts of ODA and PMDA monomers were dissolved in DMAc solvent at the same time, and the reaction was carried out at room temperature for 12 h. The concentration of the reactants was controlled to be 25 wt%, wherein the mass fraction of the polyamide ester was 10 wt%. After the reaction was completed, a tertiary amine catalyst was added to the above polymer solution in an amount of 2 The obtained casting solution was metered and extruded onto a steel belt at a temperature of 150 °C and a film flow rate of 2 m / min to form a self-supporting film. The self-supporting film was then stretched transversely by 1.1 times at 180 °C and 1.1 times at 380 °C in sequence, and finally imidized at 450 °C for 2 min to form a polyimide film.

[0068] The synthesis steps of polyamic acid are as follows:

[0069] preparing PMDA monomer and ODA monomer;

[0070] Prepare a solvent such as N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), or N-methyl-pyrrolidone (NMP);

[0071] dissolving PMDA monomer in a solvent at room temperature to obtain a PMDA solution;

[0072] The ODA monomer is added to the PMDA solution under stirring, during which the molar ratio of the ODA monomer to the PMDA monomer is maintained at 1:1, and the mixing process is performed at 60° C. for 5 hours to form a polyamic acid solution;

[0073] The polyamic acid solution obtained above is partially treated by evaporation to obtain polyamic acid.

[0074] Example 2

[0075] Under nitrogen protection, dianhydride monomer (BPDA) and ethanol were heated to reflux at 70 °C in tetrahydrofuran solvent for 4 h, wherein the molar ratio of dianhydride monomer to ethanol was 1:2. After the reaction was completed, dicarboxylic acid ester was prepared by precipitation, washing and drying. Under nitrogen protection, the above dicarboxylic acid ester was dissolved in a polar aprotic solvent (DMAc). After complete dissolution, an equimolar amount of diamine monomer (PDA) and catalyst N,N'-diisopropylcarbodiimide were added to the solution, and the concentration of the reactants was controlled to be 20wt%. The reaction was stirred at room temperature for 12 h to synthesize a polyamide lipid solution. An appropriate amount of polyamide lipid solution and equimolar PDA and BPDA monomers were dissolved in DMAc solvent at the same time, and the reaction was carried out at room temperature for 12 h. The concentration of the reactants was controlled to be 25 wt%, wherein the mass fraction of the polyamide lipid was 10 wt%. After the reaction was completed, a tertiary amine catalyst was added to the above polymer solution in an amount of 2 The obtained casting solution was metered and extruded onto a steel belt at a temperature of 150 °C and a film flow rate of 2 m / min to form a self-supporting film. The self-supporting film was then stretched transversely by 1.1 times at 180 °C and 1.1 times at 380 °C in sequence, and finally imidized at 450 °C for 2 min to form a polyimide film.

[0076] Example 3

[0077] Under nitrogen protection, dianhydride monomer (PMDA) and ethanol were heated to reflux at 70 °C in tetrahydrofuran solvent for 4 h, wherein the molar ratio of dianhydride monomer to ethanol was 1:2. After the reaction was completed, dicarboxylic acid ester was prepared by precipitation, washing and drying. Under nitrogen protection, the above dicarboxylic acid ester was dissolved in a polar aprotic solvent (DMAc). After complete dissolution, an equimolar amount of diamine monomer (ODA) monomer and catalyst N,N'-diisopropylcarbodiimide were added to the solution, and the concentration of the reactants was controlled to be 30 wt%. The reaction was stirred at room temperature for 12 h to synthesize a polyamide lipid solution. An appropriate amount of polyamide lipid solution and equimolar amounts of ODA and PMDA monomers were dissolved in DMAc solvent at the same time, and the reaction was carried out at room temperature for 12 h. The concentration of the reactants was controlled to be 30 wt%, wherein the mass fraction of the polyamide lipid was 12 wt%. After the reaction was completed, a tertiary amine catalyst was added to the above polymer solution in an amount of 5 The obtained casting solution was metered and extruded onto a steel belt at a temperature of 150 °C and a film flow rate of 2 m / min to form a self-supporting film. The self-supporting film was then stretched transversely by 1.1 times at 180 °C and 1.1 times at 400 °C in sequence, and finally imidized at 450 °C for 2 min to form a polyimide film.

[0078] Example 4

[0079] Under nitrogen protection, dianhydride monomer (PMDA) and ethanol were heated to reflux at 70 °C in tetrahydrofuran solvent for 4 h, wherein the molar ratio of dianhydride monomer to ethanol was 1:2. After the reaction was completed, dicarboxylic acid ester was prepared by precipitation, washing and drying. Under nitrogen protection, the above dicarboxylic acid ester was dissolved in a polar aprotic solvent (DMAc). After complete dissolution, an equimolar amount of diamine monomer (ODA) monomer and catalyst N,N'-diisopropylcarbodiimide were added to the solution, and the concentration of the reactants was controlled to be 30 wt%. The reaction was stirred at room temperature for 12 h to synthesize a polyamide ester solution. An appropriate amount of polyamide ester solution and equimolar amounts of ODA and PMDA monomers were dissolved in DMAc solvent at the same time, and the reaction was carried out at room temperature for 12 h. The concentration of the reactants was controlled to be 35 wt%, wherein the mass fraction of the polyamide ester was 12 wt%. After the reaction was completed, a tertiary amine catalyst was added to the above polymer solution in an amount of 5 The obtained casting solution was metered and extruded onto a steel belt at a temperature of 150 °C and a film flow rate of 2 m / min to form a self-supporting film. The self-supporting film was then stretched transversely by 1.3 times at 180 °C and 1.2 times at 400 °C in sequence, and finally imidized at 450 °C for 5 min to form a polyimide film.

[0080] Comparative Example 1

[0081] Under nitrogen protection, equimolar ODA and PMDA monomers were dissolved in DMAc solvent at the same time, and reacted at room temperature for 12 hours, and the concentration of the reactants was controlled to be 25 wt%; after the reaction was completed, a tertiary amine catalyst was added to the above polymer solution in an amount of 2 wt%, and the mixture was fully stirred and mixed, and the casting liquid was degassed for standby use; the synthesized casting liquid was metered and extruded onto a steel belt by casting, the steel belt temperature was 150 ℃, and the film flow rate was 2 m / min to form a self-supporting film; the self-supporting film was then stretched transversely by 1.1 times at 180 ℃ and 1.1 times at 380 ℃, and finally imidized at 450 ℃ for 2 min to form a polyimide film.

[0082] Comparative Example 2

[0083] Under nitrogen protection, equimolar ODA and PMDA monomers were dissolved in DMAc solvent at the same time, and reacted at room temperature for 12 hours, and the concentration of the reactants was controlled to be 30 wt%; after the reaction was completed, a tertiary amine catalyst was added to the above polymer solution in an amount of 2 wt%, and the mixture was fully stirred and mixed, and the casting liquid was degassed for standby use; the synthesized casting liquid was metered and extruded onto a steel belt by casting, the steel belt temperature was 150 ℃, and the film flow rate was 2 m / min to form a self-supporting film; the self-supporting film was then stretched transversely by 1.1 times at 180 ℃ and 1.1 times at 380 ℃, and finally imidized at 450 ℃ for 2 min to form a polyimide film.

[0084] Comparative Example 3

[0085] Under nitrogen protection, dianhydride monomer (PMDA) and ethanol were heated to reflux at 70 ℃ in tetrahydrofuran solvent for 4 h, wherein the molar ratio of dianhydride monomer to ethanol was 1:2. After the reaction was completed, dicarboxylic acid ester was prepared by precipitation, washing and drying. Under nitrogen protection, the above dicarboxylic acid ester was dissolved in polar aprotic solvent (DMAc). After complete dissolution, equimolar amounts of diamine monomer (ODA) monomer and catalyst N,N'-diisopropylcarbodiimide were added to the solution, and the concentration of the reactants was controlled to be 20 wt%. The reaction was stirred at room temperature for 12 h to synthesize polyamide ester solution. An appropriate amount of polyamide ester solution and equimolar amounts of ODA and PMDA monomers were dissolved in DMAc solvent at the same time, and the reaction was carried out at room temperature for 12 h. The concentration of the reactants was controlled to be 25 wt%, wherein the mass fraction of polyamide ester was 10 wt%, and the casting solution was degassed for standby use. The synthesized casting solution was metered and extruded onto a steel belt by casting. The temperature of the steel belt was 150 ℃ and the film flow rate was 2 m / min to form a self-supporting film; then the self-supporting film was sequentially stretched transversely by 1.1 times at 180 °C, stretched transversely by 1.1 times at 380 °C, and finally imidized at 450 °C for 2 min to form a polyimide film.

[0086] Table 1 is a comparison of the properties of polyimide films

[0087]

[0088] Polyimide membranes were prepared in Examples 1-4. Compared with Example 1, no polyamide lipid component was introduced in Comparative Example 1, so the apparent viscosity of the 25 wt% casting solution was high, the casting solution had poor fluidity during film formation, the coefficient of deviation of the film thickness was high, and the mechanical properties of the polyimide membrane were significantly lower than those of Example 1; in Comparative Example 2, when a casting solution with a concentration of 30 wt% was used, there was no fluidity and no film could be formed; in Comparative Example 3, since no tertiary amine catalyst was added, when preparing a thick film, the heat and mass transfer was uneven, and the imidization was uneven, so the thickness of the polyimide membrane was uneven, and the mechanical properties were significantly lower than those of Example 1.

[0089] It should be noted that the above-mentioned dianhydride monomers and diamine monomers are all provided by Tianjin Zotye New Material Technology Co., Ltd., and various solvents, methanol, ethanol and tertiary amine catalysts are all provided by Sinopharm Group.

[0090] Although the embodiments of the present invention are described in detail above, it is obvious to those skilled in the art that various modifications and variations can be made to these embodiments. However, it should be understood that such modifications and variations are within the scope and spirit of the present invention as described in the claims. Moreover, the present invention described herein may have other embodiments and may be implemented or realized in a variety of ways.

Claims

1. A method for preparing a polyimide film, characterized in that: The preparation method comprises the following steps: S1: Under protective gas, dianhydride monomer and methanol or ethanol are heated to reflux at 70°C in tetrahydrofuran solvent for 2-4 hours, wherein the molar ratio of dianhydride monomer to methanol or ethanol is 1:

2. After the reaction is completed, dicarboxylic acid ester is prepared by precipitation, washing and drying; S2: Under nitrogen protection, the dicarboxylic acid ester is dissolved in a polar aprotic solvent. After the dissolution is complete, an equimolar amount of diamine monomer and catalyst N,N'-diisopropylcarbodiimide are added to the solution, the concentration of the reactants is controlled to be 20-35wt%, and the reaction is stirred at room temperature for 12 h to synthesize a polyamide ester solution; S3: Under protective gas, an appropriate amount of polyamide ester solution, an equimolar amount of diamine monomer and a dianhydride monomer are simultaneously dissolved in a polar aprotic solvent, and reacted at room temperature for 12 h to obtain a polymer solution; after the reaction is completed, a tertiary amine catalyst is added to the above polymer solution, and the mixture is fully stirred and mixed, and the casting solution is degassed to obtain a casting solution for use; S4: using the casting solution in S3 to form a self-supporting film by metering and casting onto a steel belt; S5: longitudinally and transversely stretching and imidization treatment of the self-supporting film prepared in S4, and finally rolling up to prepare a polyimide film; The concentration of the casting solution used is 25-35 wt %, and the apparent viscosity at room temperature is 1000-3000 poise; The mass fraction of the casting solution in S3 is 25-35 wt%, wherein the mass fraction of the polyamide ester is 10 wt%-12wt%; The tertiary amine catalyst in S3 is one of N-methylpyrrole, pyridine and isoquinoline, and the added amount is 2 wt%-5 wt%.

2. The preparation method according to claim 1, characterized in that: The thickness of the polyimide film is 75-200 μm.

3. The preparation method according to claim 1, characterized in that: The polyimide film has a tensile strength of 140-290 MPa, a modulus of 2.2-7.5 GPa, and an elongation at break of 25%-65%.

4. The preparation method according to claim 1, characterized in that: The structure of the dianhydride monomer in S1 is as follows: 。 5. The preparation method according to claim 1, characterized in that: The structure of the diamine monomer in S2 is as follows: 。 6. The preparation method according to claim 1, characterized in that: The polar aprotic solvent in S2 is one of N,N-dimethylformamide, N,N-dimethylacetamide or N-methyl-pyrrolidone.

7. The preparation method according to claim 1, characterized in that: In the S4, the temperature of the steel strip is controlled to be 120-160° C., and the film flow rate is 2-5 m / min.

8. The preparation method according to claim 2, characterized in that: In the S5, the transverse stretching temperature is 150-180°C, the stretching ratio is 1:(1.1-1.3), the longitudinal stretching temperature is 350-400°C, the stretching ratio is 1:(1.01-1.2), and the high-temperature cyclization temperature is 400-450°C.

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