A polyimide varnish, a modified polyimide varnish, its preparation method and application

Abstract

This invention provides a polyimide varnish, a modified polyimide varnish, its preparation method, and its applications, relating to the field of polymer material preparation technology. The polyimide varnish of this invention comprises polyimide and an organic solvent, wherein the polyimide has the structure shown in Formula I: where a is 0.55–0.58, b is 0.37–0.39, and c is 0.03–0.06. The polyimide varnish provided by this invention possesses a variety of rigid structural monomers with precise structures; it can also form interchain hydrogen bonds, exhibiting high molecular chain compactness, a high glass transition temperature, and excellent stability.

Description

Technical Field

[0001] This invention belongs to the field of polymer material preparation technology, specifically relating to a polyimide varnish, a modified polyimide varnish, its preparation method and application. Background Technology

[0002] Flexible display devices are lightweight, bendable, and even foldable flexible electronic products, including flexible thin-film transistor liquid crystal displays (TFT-LCDs) and flexible organic light-emitting diode (OLED) displays. They have gradually developed into one of the most promising high-tech industries. Flexible display technology requires materials with high glass transition temperatures, good flexibility, corrosion resistance, and low coefficients of thermal expansion. During the fabrication of flexible display devices, to improve the performance of thin-film transistors, annealing temperatures need to exceed 400°C. During annealing, it is crucial to ensure that the device and substrate do not warp, crack, or delaminate. Therefore, the substrate material needs to have a low coefficient of thermal expansion to prevent device delamination due to excessive deformation during annealing.

[0003] In existing technologies, a polyimide film is formed by coating the surface of a substrate material with a polyimide varnish to meet the fabrication requirements of flexible display devices. However, the polyimide varnish in these technologies suffers from poor stability. Summary of the Invention

[0004] The purpose of this invention is to provide a polyimide varnish, a modified polyimide varnish, a preparation method thereof, and its application. The polyimide varnish provided by this invention has excellent stability.

[0005] To achieve the objectives of this invention, the following technical solutions are provided:

[0006] A polyimide varnish comprising a polyimide and an organic solvent, wherein the polyimide has the structure shown in Formula I:

[0007]

[0008] Where a is 0.55–0.58, b is 0.37–0.39, and c is 0.03–0.06.

[0009] Preferably, the organic solvent includes one or more of N-methylpyrrolidone, N,N-dimethylacetamide, and N,N-dimethylformamide; the organic solvent has a mass percentage content of 81-84%.

[0010] This invention also provides a method for preparing the polyimide varnish described in the above technical solution, comprising the following steps:

[0011] p-phenylenediamine, 3,3',4,4'-biphenyltetracarboxylic acid dianhydride and organic solvent were mixed and subjected to a first polycondensation reaction to obtain a first polycondensation product system.

[0012] The first condensation product system was mixed with 2-(4-aminophenyl)-5-aminobenzimidazole and subjected to a second condensation reaction to obtain the second condensation product system.

[0013] The second polycondensation product system and pyromellitic dianhydride are mixed and subjected to a third polycondensation reaction to obtain the polyimide varnish.

[0014] Preferably, the molar ratio of p-phenylenediamine to 3,3',4,4'-biphenyltetracarboxylic acid dianhydride is 1:0.30 to 0.49.

[0015] Preferably, the molar ratio of p-phenylenediamine to 2-(4-aminophenyl)-5-aminobenzimidazole is 0.93-0.97:0.07-0.03.

[0016] Preferably, the molar ratio of 3,3',4,4'-biphenyltetracarboxylic acid dianhydride to pyromellitic dianhydride is 0.3-0.5:0.5-0.7.

[0017] Preferably, the temperatures of the first, second, and third polycondensation reactions are independently 15–35°C; the first, second, and third polycondensation reactions are carried out under stirring conditions; and the stirring rate is 100–200 rpm.

[0018] Preferably, the first polycondensation reaction takes 60–120 min; the second polycondensation reaction takes 20–40 min; and the third polycondensation reaction takes 8–12 h.

[0019] This invention also provides a method for preparing modified polyimide varnish, comprising the following steps:

[0020] The polyimide varnish is heated and diluted sequentially to obtain diluted polyimide varnish; the polyimide varnish is the polyimide varnish described in the above technical solution or the polyimide varnish prepared by the preparation method described in the above technical solution;

[0021] The diluted polyimide varnish and the sealing agent are mixed and sealed to obtain the modified polyimide varnish.

[0022] The present invention also provides the application of the polyimide varnish described in the above technical solution, the polyimide varnish prepared by the preparation method described in the above technical solution, or the modified polyimide varnish described in the above technical solution in flexible display devices.

[0023] This invention provides a polyimide varnish comprising a polyimide and an organic solvent, wherein the polyimide has the structure shown in Formula I:

[0024]

[0025] Where a is 0.55–0.58, b is 0.37–0.39, and c is 0.03–0.06. The polyimide varnish provided by this invention possesses a variety of rigid structural monomers with precise structures; simultaneously, it can form interchain hydrogen bonds, resulting in a high degree of molecular chain compactness and a high glass transition temperature. As shown in Example 1 of this invention, the polyimide varnish provided by this invention has a glass transition temperature of 466℃, a coefficient of thermal expansion of 6.50 ppm / K, and excellent stability.

[0026] Furthermore, this invention also provides a modified polyimide varnish, which is modified in terms of viscosity, molecular weight distribution, solid content, and storage stability to further improve the stability of the polyimide varnish. As shown in Example 2 of this invention, the modified polyimide varnish prepared by this invention has a glass transition temperature of 485℃ and a coefficient of thermal expansion of 2.49 ppm / K, indicating further improved stability. Attached Figure Description

[0027] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0028] Figure 1 This is a flowchart illustrating the preparation process of the polyimide varnish described in Example 1;

[0029] Figure 2 This is a flowchart illustrating the preparation process of the modified polyimide varnish described in Example 2. Detailed Implementation

[0030] This invention provides a polyimide varnish comprising a polyimide and an organic solvent, wherein the polyimide has the structure shown in Formula I:

[0031]

[0032] Where a is 0.55–0.58, b is 0.37–0.39, and c is 0.03–0.06.

[0033] In this invention, a, b, and c represent the molar ratios of chain segment 1, chain segment 2, and chain segment 3, respectively. In this invention, a is 0.55–0.58, preferably 0.57; b is 0.37–0.39, preferably 0.38; and c is 0.03–0.06, preferably 0.05.

[0034] In this invention, the organic solvent preferably includes one or more of N-methylpyrrolidone, N,N-dimethylacetamide, and N,N-dimethylformamide, more preferably N-methylpyrrolidone. In this invention, the mass percentage of the organic solvent is preferably 81-84%, more preferably 83%.

[0035] This invention also provides a method for preparing the polyimide varnish described in the above technical solution, comprising the following steps:

[0036] p-phenylenediamine, 3,3',4,4'-biphenyltetracarboxylic acid dianhydride and organic solvent were mixed and subjected to a first polycondensation reaction to obtain a first polycondensation product system.

[0037] The first condensation product system was mixed with 2-(4-aminophenyl)-5-aminobenzimidazole and subjected to a second condensation reaction to obtain the second condensation product system.

[0038] The second polycondensation product system and pyromellitic dianhydride are mixed and subjected to a third polycondensation reaction to obtain the polyimide varnish.

[0039] In this invention, unless otherwise specified, all raw materials used in the preparation are preferably commercially available products.

[0040] In this invention, p-phenylenediamine, 3,3',4,4'-biphenyltetracarboxylic acid dianhydride, and an organic solvent are mixed and subjected to a first polycondensation reaction to obtain a first polycondensation product system. In this invention, the molar ratio of p-phenylenediamine to 3,3',4,4'-biphenyltetracarboxylic acid dianhydride is preferably 1:0.30 to 0.49, more preferably 1:0.412.

[0041] In this invention, p-phenylenediamine and a portion of an organic solvent are mixed to dissolve the p-phenylenediamine, yielding a p-phenylenediamine solution. In this invention, the mixing is preferably carried out under stirring conditions, with the stirring rate preferably being 200–300 rpm, more preferably 250 rpm. In this invention, the mass ratio of the organic solvent to p-phenylenediamine is preferably 1:0.08–0.09, more preferably 1:0.085.

[0042] After obtaining the p-phenylenediamine solution, the present invention mixes the p-phenylenediamine solution, 3,3',4,4'-biphenyltetracarboxylic acid dianhydride, and the remaining organic solvent to carry out a first polycondensation reaction, thereby obtaining a first polycondensation product system. In the present invention, the mixing is preferably carried out under stirring conditions, and the stirring rate is preferably 100-200 rpm, more preferably 150 rpm. In the present invention, the mass ratio of the remaining organic solvent to the 3,3',4,4'-biphenyltetracarboxylic acid dianhydride is preferably 1:0.8-0.85, more preferably 1:0.83.

[0043] In this invention, the first polycondensation reaction is preferably carried out under stirring conditions; the stirring rate is preferably 150-250 rpm, more preferably 200 rpm. In this invention, the temperature of the first polycondensation reaction is preferably 15-35°C, more preferably 25-30°C; the time of the first polycondensation reaction is preferably 60-120 min, more preferably 90 min.

[0044] After obtaining the first condensation product system, the present invention mixes the first condensation product system with 2-(4-aminophenyl)-5-aminobenzimidazole to carry out a second condensation reaction to obtain the second condensation product system. In the present invention, the molar ratio of p-phenylenediamine to 2-(4-aminophenyl)-5-aminobenzimidazole is preferably 0.93-0.97:0.03-0.07, more preferably 0.95:0.05.

[0045] In this invention, an organic solvent is preferably added during the second polycondensation reaction. Specifically, the first polycondensation product system, 2-(4-aminophenyl)-5-aminobenzimidazole, and the added organic solvent are mixed. In this invention, the mixing is preferably carried out under stirring conditions, and the stirring speed is preferably 100-200 rpm, more preferably 150 rpm. In this invention, the mass ratio of the added organic solvent to 2-(4-aminophenyl)-5-aminobenzimidazole is preferably 1:0.1-0.15, more preferably 1:0.13.

[0046] In this invention, the temperature and stirring rate of the second polycondensation reaction and the first polycondensation reaction are preferably the same. In this invention, the time for the second polycondensation reaction is preferably 20–40 min, more preferably 30 min.

[0047] After obtaining the second polycondensation product system, the present invention mixes the second polycondensation product system with pyromellitic dianhydride and carries out a third polycondensation reaction to obtain the polyimide varnish. In the present invention, the molar ratio of 3,3',4,4'-biphenyltetracarboxylic acid dianhydride to pyromellitic dianhydride is preferably 0.3-0.5:0.5-0.7, more preferably 0.4:0.6.

[0048] In this invention, it is preferable to add an organic solvent during the third polycondensation reaction. Specifically, the second polycondensation product system, pyromellitic dianhydride, and the added organic solvent are mixed. In this invention, the mixing is preferably carried out under stirring conditions, and the stirring speed is preferably 100–200 rpm, more preferably 150 rpm. In this invention, the mass ratio of the added organic solvent to pyromellitic dianhydride is preferably 1:1.60–1.70, more preferably 1:1.68.

[0049] In this invention, the temperature and stirring rate of the third polycondensation reaction are preferably the same as those of the first polycondensation reaction. In this invention, the duration of the third polycondensation reaction is preferably 8–12 hours, more preferably 10 hours.

[0050] In this invention, the first, second, and third polycondensation reactions are preferably carried out under a nitrogen atmosphere; the nitrogen flow rate is preferably 4 to 7 mL / min, more preferably 5 mL / min.

[0051] This invention also provides a method for preparing modified polyimide varnish, comprising the following steps:

[0052] The polyimide varnish is heated and diluted sequentially to obtain diluted polyimide varnish; the polyimide varnish is the polyimide varnish described in the above technical solution or the polyimide varnish prepared by the preparation method described in the above technical solution;

[0053] The diluted polyimide varnish and the sealing agent are mixed and sealed to obtain the modified polyimide varnish.

[0054] This invention involves sequentially heating and diluting a polyimide varnish to obtain a diluted polyimide varnish. In this invention, the heating temperature is preferably 60–75°C, more preferably 65°C; the heating holding time is preferably 1–2 hours, more preferably 1.5 hours. In this invention, the heating process reduces the molecular weight distribution of the polyimide varnish.

[0055] In this invention, the organic solvent used for dilution is preferably the same as the organic solvent described above, and the mass ratio of the organic solvent used for dilution to the polyimide varnish is preferably 0.16–0.17:0.84–0.83, more preferably 0.165:0.835. In this invention, the dilution is preferably carried out under stirring conditions, the stirring time is preferably 2–4 hours, more preferably 3 hours; the stirring speed is preferably 50–110 rpm, more preferably 80 rpm. In this invention, the dilution is preferably carried out under a nitrogen atmosphere. In this invention, the dilution has the effect of reducing the viscosity of the polyimide varnish.

[0056] In this invention, the viscosity of the diluted polyimide varnish is preferably 4000-6000 cp, more preferably 5000 cp.

[0057] After obtaining the diluted polyimide varnish, the present invention mixes the diluted polyimide varnish with a sealing agent and performs end-capping to obtain the modified polyimide varnish.

[0058] In this invention, the end-capping agent is preferably benzoynyl phthalic anhydride or trimellitic anhydride, more preferably trimellitic anhydride. In this invention, the molar ratio of the end-capping agent to the polyimide in the diluted polyimide varnish is preferably 0.01–0.03:1, more preferably 0.02:1.

[0059] In this invention, the end-capping is preferably performed at room temperature under stirring conditions, and the end-capping time is preferably 30–75 min, more preferably 60 min. In this invention, the stirring rate is preferably 100–200 rpm, more preferably 150 rpm. In this invention, the end-capping is preferably performed under a nitrogen atmosphere. In this invention, the use of an end-capping agent improves the storage stability of the polyimide varnish.

[0060] The present invention also preferably involves solid-liquid separation of the end-capped polyimide varnish to obtain the modified polyimide varnish. In the present invention, the solid-liquid separation preferably employs a positive pressure filtration device. In the present invention, the filter element of the positive pressure filtration device is preferably 0.1–0.3 μm, more preferably 0.2 μm; the pressure is preferably 0.3–0.6 MPa, more preferably 0.4–0.5 MPa. In the present invention, positive pressure filtration can remove heterogeneous insoluble impurities from the polyimide varnish.

[0061] The present invention also provides the application of the polyimide varnish described in the above technical solution, the polyimide varnish prepared by the preparation method described in the above technical solution, or the modified polyimide varnish described in the above technical solution in flexible display devices.

[0062] In this invention, the polyimide varnish or modified polyimide varnish is preferably coated onto a glass substrate, and after drying, a polyimide film or modified polyimide film is obtained.

[0063] In this invention, the thickness of the polyimide film and the modified polyimide film is preferably 8-13 μm, more preferably 10-12 μm.

[0064] After coating, the present invention preferably heats the polyimide varnish or modified polyimide varnish to remove some of the organic solvent. In the present invention, the heating temperature is preferably 60-100°C, more preferably 80°C.

[0065] The present invention preferably uses an oxygen-free oven for heating and curing. The preferred parameters of the oxygen-free oven are: oxygen content below 10,000 rpm; and the preferred heating program is 120℃ for 30 min, 150℃ for 30 min, 250℃ for 30 min, 350℃ for 30 min, and 450℃ for 30 min.

[0066] The present invention preferably involves cleaning the cured polyimide film or modified polyimide film. The present invention preferably uses deionized water for cleaning; the temperature of the deionized water is preferably 80°C.

[0067] To further illustrate the present invention, the polyimide varnish provided by the present invention will be described in detail below with reference to the accompanying drawings and embodiments, but these should not be construed as limiting the scope of protection of the present invention.

[0068] Example 1

[0069] Under a nitrogen atmosphere (nitrogen flow rate of 5 mL / min), 155 g of N-methylpyrrolidone was placed in a three-necked flask and stirred at 30 °C and 250 rpm for 30 min. Then, it was mixed with 0.194 mol of p-phenylenediamine and 93 g of N-methylpyrrolidone for another 30 min to obtain a p-phenylenediamine solution. After cooling to 25 °C, 0.08 mol of 3,3',4,4'-biphenyltetracarboxylic acid dianhydride, 31 g of N-methylpyrrolidone and the above p-phenylenediamine solution were mixed at 150 rpm. Then, the mixture was stirred at 25 °C and 200 rpm for 90 min to obtain the first polycondensation product system.

[0070] The temperature was raised to 30°C, and 0.01 mol of 2-(4-aminophenyl)-5-aminobenzimidazole, 15.5 g of N-methylpyrrolidone and the above first condensation product system were mixed at 150 rpm. Then the mixture was stirred at 30°C and 200 rpm for 30 min to obtain the second condensation product system.

[0071] The mixture was cooled to 25°C and mixed with 0.12 mol pyromellitic dianhydride, 15.5 g N-methylpyrrolidone and the above second condensation product system at 150 rpm. The mixture was then stirred at 25°C and 100 rpm for 10 h to obtain polyimide varnish.

[0072] Example 2

[0073] Under a nitrogen atmosphere (nitrogen flow rate of 5 mL / min), the polyimide varnish prepared in Example 1 was heated to 65°C and kept at that temperature for 2 h. Then, 58 g of N-methylpyrrolidone was added and stirred at 80 rpm for 2 h to obtain diluted polyimide varnish.

[0074] 0.004 mol trimellitic anhydride was added to the above diluted polyimide varnish, and the mixture was stirred at 150 rpm for 60 min. Then, solid-liquid separation was performed using a positive pressure filtration device with a 0.6 MPa and a 0.2 μm filter element to obtain the modified polyimide varnish.

[0075] Comparative Example 1

[0076] Under a nitrogen atmosphere (nitrogen flow rate of 5 mL / min), 129 g of N-methylpyrrolidone was placed in a three-necked flask and stirred at 30 °C and 250 rpm for 30 min. Then, it was mixed with 0.153 mol of p-phenylenediamine and 103 g of N-methylpyrrolidone for another 30 min to obtain a p-phenylenediamine solution. After cooling to 25 °C, 0.15 mol of 3,3',4,4'-biphenyltetracarboxylic acid dianhydride and 26 g of N-methylpyrrolidone were mixed with the above p-phenylenediamine solution at 150 rpm. Then, the mixture was stirred at 25 °C and 200 rpm for 90 min to obtain a polyimide varnish.

[0077] Comparative Example 2

[0078] Under a nitrogen atmosphere (nitrogen flow rate of 5 mL / min), the polyimide varnish prepared in Comparative Example 1 was heated to 65 °C and kept at that temperature for 2 h. Then, 49 g of N-methylpyrrolidone was added and stirred at 80 rpm for 2 h to obtain diluted polyimide varnish.

[0079] 0.003 mol trimellitic anhydride was added to the above diluted polyimide varnish, and the mixture was stirred at 150 rpm for 60 min. Then, solid-liquid separation was performed using a positive pressure filtration device with a 0.6 MPa and a 0.2 μm filter element to obtain the modified polyimide varnish.

[0080] Test case

[0081] The polyimide varnishes described in Examples 1-2 and Comparative Example 1 were coated onto a clean glass substrate using a spin coater. The substrate was heated at 80°C for 10 minutes to remove organic solvents, and then placed in an oxygen-free oven for heating treatment. The oxygen content was below 10,000 rpm. The heating program was: 120°C for 30 minutes, 150°C for 30 minutes, 250°C for 30 minutes, 350°C for 30 minutes, and 450°C for 30 minutes. After cooling to room temperature, the film was peeled off in deionized water at 80°C to obtain a polyimide film.

[0082] The glass transition temperature, coefficient of thermal expansion, tensile strength, elongation at break and tensile modulus of the above polyimide film were measured, and the results are shown in Table 1.

[0083] Table 1. Performance tests of polyimide films prepared from the polyimide varnishes described in Examples 1-2 and Comparative Example 1.

[0084]

[0085] As shown in Table 1, the glass transition temperature, coefficient of thermal expansion, tensile strength, elongation at break and tensile modulus of the polyimide varnish prepared by the present invention are significantly improved.

[0086] The stability of the modified polyimide varnishes prepared in Examples 1-2 and Comparative Examples 1-2 was tested, and the test results are shown in Tables 2 and 3.

[0087] Table 2 shows the storage stability test results of the modified polyimide varnishes prepared in Examples 1-2 and Comparative Examples 1-2 at room temperature (25°C).

[0088]

[0089]

[0090] Table 3 shows the storage stability test results of the modified polyimide varnishes prepared in Examples 1-2 and Comparative Examples 1-2 at -15°C in a cold medium.

[0091]

[0092] As shown in Tables 2 and 3, the polyimide varnish prepared in this invention exhibits good stability, with no significant change in apparent viscosity after 60 days of storage in a refrigerant medium.

[0093] Although the above embodiments have provided a detailed description of the present invention, they are only some embodiments of the present invention, and not all embodiments. People can obtain other embodiments based on these embodiments without creative effort, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A method for preparing a modified polyimide varnish, comprising the following steps: The polyimide varnish was heated and diluted sequentially to obtain diluted polyimide varnish; The diluted polyimide varnish and the end-capping agent are mixed and then end-capped to obtain the modified polyimide varnish; the end-capping agent is benzoynyl phthalic anhydride or trimellitic anhydride. The polyimide varnish comprises polyimide and an organic solvent, wherein the polyimide has the structure shown in Formula I: Where a is 0.55~0.58, b is 0.37~0.39, and c is 0.03~0.06; The preparation method of the polyimide varnish includes the following steps: p-phenylenediamine, 3,3',4,4'-biphenyltetracarboxylic acid dianhydride and organic solvent were mixed and subjected to a first polycondensation reaction to obtain a first polycondensation product system. The first condensation product system was mixed with 2-(4-aminophenyl)-5-aminobenzimidazole and subjected to a second condensation reaction to obtain the second condensation product system. The second polycondensation product system and pyromellitic dianhydride were mixed and subjected to a third polycondensation reaction to obtain the polyimide varnish. The molar ratio of p-phenylenediamine to 3,3',4,4'-biphenyltetracarboxylic acid dianhydride is 1:0.30~0.49; The molar ratio of p-phenylenediamine to 2-(4-aminophenyl)-5-aminobenzimidazole is 0.93~0.97:0.07~0.03; The molar ratio of 3,3',4,4'-biphenyltetracarboxylic acid dianhydride to pyromellitic dianhydride is 0.3~0.5:0.5~0.

7.

2. The method for preparing the modified polyimide varnish according to claim 1, characterized in that, The organic solvent includes one or more of N-methylpyrrolidone, N,N-dimethylacetamide, and N,N-dimethylformamide; the organic solvent has a mass percentage of 81-84%.

3. The method for preparing the modified polyimide varnish according to claim 1, characterized in that, The temperatures of the first, second, and third polycondensation reactions are independently 15-35°C; the first, second, and third polycondensation reactions are carried out under stirring conditions, and the stirring rate is independently 100-250 rpm.

4. The method for preparing the modified polyimide varnish according to claim 1 or 3, characterized in that, The first polycondensation reaction takes 60-120 minutes; the second polycondensation reaction takes 20-40 minutes; and the third polycondensation reaction takes 8-12 hours.

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