Method for producing an aqueous solution of a polyimide precursor, and polyamic acid salt

Abstract

The present invention aims to provide a method for producing an aqueous solution of a polyimide precursor that can be used to produce polyimides, using a polyamic acid in which pyromellitic dianhydride and 4,4'-diaminodiphenyl ether are essential monomers. [Solution] The method for producing an aqueous polyimide precursor according to the present invention comprises a salt formation step of mixing a solution containing the polyamic acid, a first organic base, and a poor solvent to precipitate a polyamic acid salt; a separation step of separating the precipitate containing the polyamic acid salt from the solvent in a solid-liquid manner; and a dissolution step of mixing the precipitate, a second organic base, and water to produce an aqueous polyimide precursor aqueous solution, wherein each of the first organic base and the second organic base is at least one selected from the group consisting of unsubstituted imidazole, monoalkylimidazole having only one C1-C3 alkyl group, and dimethylimidazole having only two methyl groups.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method for producing a polyimide precursor aqueous solution, and a polyamic acid salt.

BACKGROUND ART

[0002] Polyamic acid is a polymer of tetracarboxylic dianhydride and diamine, and is used as a polyimide precursor in polyimide synthesis. Examples of polyimide products include polyimide films produced using a polyimide precursor solution containing polyamic acid. In general, as the polyimide precursor solution, an organic solvent-based solution in which polyamic acid is dissolved in an organic solvent is known.

[0003] In addition, for the purpose of reducing the environmental load and the like, a polyimide precursor aqueous solution produced by dissolving polyamic acid in the form of a salt with an organic base (polyamic acid salt) in water has been proposed. For example, in the Examples of Patent Document 1, it is shown that for polyamic acid which is a polymer of a biphenyltetracarboxylic acid-type tetracarboxylic dianhydride and a bis(aminophenoxy)benzene-type diamine, a polyimide precursor aqueous solution capable of producing a polyimide film can be produced by allowing 1,2-dimethylimidazole and 1-methyl-2-ethylimidazole to act as organic bases.

[0004] In addition, in the Examples of Patent Document 2, it is shown that for polyamic acid which is a polymer of the same biphenyltetracarboxylic acid-type tetracarboxylic dianhydride as that of Patent Document 1 and a diamine such as p-phenylenediamine, a polyimide precursor aqueous solution capable of producing a polyimide film can be produced by allowing 1,2-dimethylimidazole and 2-ethyl-4-methylimidazole to act as organic bases. Further, Patent Document 2 shows that a polyimide precursor aqueous solution can be produced by allowing tetracarboxylic dianhydride, p-phenylenediamine, and the above-mentioned imidazole to act in water, thereby simultaneously performing synthesis of polyamic acid and formation of a polyamic acid salt. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2002-226582 [Patent Document 2] Japanese Patent Publication No. 2012-36382 [Overview of the project] [Problems that the invention aims to solve]

[0006] Incidentally, in addition to the polyamic acids described in Patent Documents 1 and 2, polyamic acids using pyromellitic dianhydride and 4,4'-diaminodiphenyl ether as essential monomers are known to impart high heat resistance to polyimides and are used in various polyimide products. Furthermore, there is a desire for the provision of aqueous polyimide precursor solutions that can produce polyimides from such polyamic acids. However, it is believed that such studies have not been sufficiently conducted to date.

[0007] In view of the above circumstances, the object of the present invention is to provide a method for producing an aqueous polyimide precursor solution that can produce polyimide, using a polyamic acid having pyromellitic dianhydride and 4,4'-diaminodiphenyl ether as essential monomers, and to provide a polyamic acid salt that can be used to produce an aqueous polyimide precursor solution. [Means for solving the problem]

[0008] The method for producing an aqueous solution of polyimide precursor according to the present invention is as follows. [1] A salt formation step in which a solution containing polyamic acid, a first organic base, and a poor solvent are mixed to precipitate a polyamic acid salt, A separation step for separating the precipitate containing the polyamic acid salt from the solvent in a solid-liquid manner, The system comprises a dissolution step of mixing the precipitate, a second organic base, and water to prepare an aqueous solution of a polyimide precursor, The polyamic acid is a polymer of pyromellitic dianhydride and 4,4'-diaminodiphenyl ether, or The polymer consists of pyromellitic dianhydride, 4,4'-diaminodiphenyl ether, 3,3',4,4'-biphenyltetracarboxylic acid dianhydride, and paraphenylenediamine. A method for producing an aqueous solution of a polyimide precursor, wherein each of the first organic base and the second organic base is selected from the group consisting of an unsubstituted imidazole, a monoalkylimidazole having only one C1-C3 alkyl group, and a dimethylimidazole having only two methyl groups.

[0009] The polyamic acid salt according to the present invention is as follows: [2] It contains polyamic acid and an organic base. The polyamic acid is a polymer of pyromellitic dianhydride and 4,4'-diaminodiphenyl ether, or The polymer consists of pyromellitic dianhydride, 4,4'-diaminodiphenyl ether, 3,3',4,4'-biphenyltetracarboxylic acid dianhydride, and paraphenylenediamine. The aforementioned organic base is a polyamic salt, wherein the organic base is at least one selected from the group consisting of unsubstituted imidazole, monoalkylimidazole having only one C1-C3 alkyl group, and dimethylimidazole having only two methyl groups. [Effects of the Invention]

[0010] According to the present invention, a method for producing an aqueous polyimide precursor solution capable of producing polyimides, using a polyamic acid having pyromellitic dianhydride and 4,4'-diaminodiphenyl ether as essential monomers, and a polyamic acid salt that can be used to produce the aqueous polyimide precursor solution can be provided. [Modes for carrying out the invention]

[0011] A method for producing an aqueous solution of a polyimide precursor according to the present invention will be described.

[0012] The aqueous polyimide precursor solution according to this embodiment can be used in the manufacture of polyimide products. Specifically, the aqueous polyimide precursor solution according to this embodiment can be used as an aqueous varnish for manufacturing polyimide films.

[0013] The method for producing the polyimide film typically comprises a casting step of forming a coating film of the aqueous varnish on a support surface such as a drum or belt; a first heating step of heating the coating film to remove the solvent and form a polyamic acid film; a peeling step of peeling the polyamic acid film from the support surface; and a second heating step of further heating the polyamic acid film to allow the imidation reaction to proceed and obtain the polyimide film. In addition, in the method for producing the polyimide film, after the first heating step, the second heating step may be performed on the polyamic acid film on the support surface to form the polyimide film, and a peeling step may be performed to peel the polyimide film from the support surface. In the production of polyimide films using an aqueous solution of a polyimide precursor, it is important whether a polyimide film having desired physical properties such as heat resistance, strength, and elastic modulus can be obtained by the polyamic acid salt. Furthermore, it is sometimes preferable that a polyamic acid film having self-supporting properties can be formed by the polyamic acid salt.

[0014] The polyamic acid is a polymer of tetracarboxylic dianhydride and diamine, more specifically, a polymer of aromatic tetracarboxylic dianhydride and aromatic diamine.

[0015] The mass average molecular weight of the polyamic acid used as a starting material is, for example, 10,000 or more, preferably 30,000 or more, and more preferably 40,000 or more. The mass average molecular weight of the polyamic acid may be, for example, 200,000 or less, 150,000 or less, or 130,000 or less. The mass average molecular weight is determined by gel permeation chromatography (GPC) under the following measurement conditions. (GPC Measurement Conditions) Apparatus: HLC-8320GPC manufactured by Tosoh Corporation Detector: Differential Refractometer (RI) Detector Column: 1 styrene-divinylbenzene copolymer column (TSKgel SuperH2000 manufactured by Tosoh Corporation, average particle diameter 3 μm, average pore size 2 nm) Column temperature: 40°C Standard substance for calibration curve preparation: Polyethylene oxide Eluent: N,N-Dimethylacetamide (DMAc), Lithium bromide (LiBr), Phosphoric acid (H3PO4)

[0016] The polyamic acid used in the present embodiment is a first polyamic acid that is a polymer of pyromellitic dianhydride and 4,4'-diaminodiphenyl ether, or a second polyamic acid that is a polymer of pyromellitic dianhydride, 4,4'-diaminodiphenyl ether, 3,3',4,4'-biphenyltetracarboxylic dianhydride, and p-phenylenediamine.

[0017] The first polyamic acid has a repeating structural unit composed of one molecule of pyromellitic dianhydride and one molecule of 4,4'-diaminodiphenyl ether.

[0018] The polymerization degree of the first polyamic acid is, for example, 20 or more, preferably 50 or more, more preferably 75 or more, and even more preferably 100 or more. Further, the polymerization degree of the first polyamic acid is, for example, 500 or less, preferably 400 or less, more preferably 300 or less, and even more preferably 200 or less.

[0019] The second polyamic acid is preferably a block copolymer of a polymer of 3,3',4,4'-biphenyltetracarboxylic acid dianhydride and 4,4'-diaminodiphenyl ether (hereinafter referred to as the first polymer) and a polymer of pyromellitic acid dianhydride and paraphenylenediamine (hereinafter referred to as the second polymer). The first polymer has a repeating structural unit consisting of one molecule of 3,3',4,4'-biphenyltetracarboxylic acid dianhydride and one molecule of 4,4'-diaminodiphenyl ether. The second polymer has a repeating structural unit consisting of one molecule of pyromellitic acid dianhydride and one molecule of paraphenylenediamine. In another embodiment, the second polyamic acid may be a block copolymer of a polymer of pyromellitic acid dianhydride and 4,4'-diaminodiphenyl ether and a polymer of 3,3',4,4'-biphenyltetracarboxylic acid dianhydride and paraphenylenediamine.

[0020] The degree of polymerization of the second polyamic acid is, for example, 20 or more, preferably 50 or more, more preferably 75 or more, even more preferably 100 or more, and even more preferably 120 or more. Furthermore, the degree of polymerization of the second polyamic acid is, for example, 500 or less, preferably 400 or less, more preferably 300 or less, and even more preferably 200 or less. When the second polyamic acid is the block copolymer, the degree of polymerization of the first polymer is, for example, 20 to 45. Furthermore, the degree of polymerization of the second polymer is, for example, 10 to 70. The molar ratio of the first polymer to the second polymer is preferably 0.6:1 to 4:1.

[0021] Furthermore, the method for producing an aqueous polyimide precursor according to this embodiment comprises a salt formation step of mixing a solution containing the first polyamic acid or the second polyamic acid, a first organic base, and a poor solvent to precipitate a polyamic acid salt; a separation step of separating the precipitate containing the polyamic acid salt from the solvent in a solid-liquid manner; and a dissolution step of mixing the separated precipitate, the second organic base, and water to produce an aqueous polyimide precursor solution.

[0022] The first polyamic acid and the second polyamic acid can be produced by conventionally known methods. These polyamic acids can be used in the form in which they remain dissolved in the organic solvent reaction solvent used in their synthesis. In this embodiment, such a polyamic acid solution (hereinafter referred to as the first solution) is used.

[0023] In the salt formation step of this embodiment, the first solution containing the first organic base is brought into contact with the poor solvent to form a precipitate containing the polyamic acid salt. In another embodiment, the first solution and the poor solvent containing the first organic base can be brought into contact to form the precipitate. In such a salt formation step, the polyamic acid salt can be formed without using water, thus eliminating the influence of water which may contribute to the decomposition of the polyamic acid.

[0024] Methods for bringing the first solution into contact with the poor solvent include adding the first solution (e.g., dropwise) to a tank filled with the poor solvent, adding the poor solvent (e.g., dropwise) to a tank containing the first solution, and a wet spinning method in which the first solution is gradually discharged from a nozzle inserted into the poor solvent in the tank to form fibrous polyamic acid salts. This makes it easier for the polyamic acid salts to precipitate.

[0025] The first organic base is at least one selected from the group consisting of unsubstituted imidazole, monoalkylimidazole having only one C1-C3 alkyl group as a substituent on the imidazole ring, and dimethylimidazole having only two methyl groups. That is, the first organic base may be one or more selected from the above group. These first organic bases can form precipitates with good properties and that are easy to handle.

[0026] Unsubstituted imidazole is the imidazole with the simplest structure, having no substituents. In other words, unsubstituted imidazole is an imidazole with a molecular weight of 68.1.

[0027] Examples of monoalkylimidazoles include monomethylimidazole with a molecular weight of 82.1, monoethylimidazole with a molecular weight of 96.1, and monopropylimidazole with a molecular weight of 110.2. Preferred monomethylimidazoles are readily available 1-methylimidazole, 2-methylimidazole, and 4-methylimidazole. Preferred monoethylimidazoles are 1-ethylimidazole and 2-ethylimidazole. Preferred monopropylimidazoles are 1-propylimidazole and 2-propylimidazole.

[0028] Dimethylimidazole is an imidazole with a molecular weight of 96.1. 1,2-dimethylimidazole is preferred as it is readily available.

[0029] The amount of the first organic base added is preferably 0.2 equivalents or more, more preferably 0.3 equivalents or more, even more preferably 0.4 equivalents or more, and even more preferably 0.5 equivalents or more, relative to 1 mole of the first polyamic acid or 1 mole of the second polyamic acid. The amount of the first organic base added is, for example, 6 equivalents or less, and may also be 5 equivalents or less. In the equivalent calculation of the salt formation step, the molecular weight of the first polyamic acid can be 418, which is the sum of the molecular weight of the monomer pyromellitic dianhydride (218) and the molecular weight of 4,4'-diaminodiphenyl ether (200). Furthermore, the molecular weight of the second polyamic acid can be calculated by multiplying the molecular weights of the monomer pyromellitic dianhydride (218), 3,3',4,4'-biphenyltetracarboxylic acid dianhydride (294), 4,4'-diaminodiphenyl ether (200), and paraphenylenediamine (108) by their respective molar ratios and then adding them together.

[0030] Examples of the reaction solvent, i.e., organic solvents capable of dissolving the polyamic acid, include amide solvents such as N,N-dimethylformamide, N,N-diethylformamide, N,N-dimethylacetamide, N,N-diethylacetamide, N-methyl-2-pyrrolidone, and hexamethylphosphoramide; sulfoxide solvents such as dimethyl sulfoxide and diethyl sulfoxide; ester solvents such as γ-butyrolactone; phenolic solvents such as phenol, cresol, xylenol, halogenated phenol, and catechol; and aromatic hydrocarbons such as xylene and toluene. From the viewpoint of being able to dissolve both the polyamic acid and the polyamic salt, the amide solvents and sulfoxide solvents are preferred.

[0031] The water content in the reaction solvent is preferably 5% by volume or less, more preferably 1% by volume or less, and even more preferably 0.5% by volume or less. Furthermore, it is preferable that the reaction solvent is substantially water-free (for example, 0.1% by volume or less). This makes it possible to suppress the formation of gels and swollen substances when attempting to obtain precipitates containing polyamic acid salts. In other words, it is possible to obtain polyamic acid salts in an easy-to-handle solid or powder form. It also makes it possible to suppress the hydrolysis of polyamic acid.

[0032] The content of the polyamic acid in the first solution is, for example, 10% by mass or more and 30% by mass or less, relative to the mass of the first solution.

[0033] As the poor solvent for the polyamic acid salt, preferred solvents include alcohols with 4 or fewer carbon atoms, such as methanol, ethanol, and propanol, and ketones with 8 or fewer carbon atoms, such as acetone, methyl ethyl ketone, and isobutyl ketone. The alcohols are preferably monoalcohols. These poor solvents are preferred because they have boiling points lower than the temperature at which the imidation reaction of the polyamic acid begins.

[0034] The water content in the poor solvent is preferably 5% by volume or less, more preferably 1% by volume or less, and even more preferably 0.5% by volume or less. Furthermore, it is preferable that the poor solvent is substantially water-free (for example, 0.1% by volume or less). This makes it possible to suppress the formation of a gel-like precipitate and the inclusion of swollen material in the precipitate. In other words, it is possible to obtain a polyamic acid salt in a solid or powder form that is easy to handle. It is also possible to suppress the hydrolysis of the polyamic acid.

[0035] The amount of the poor solvent used is, for example, equal to or greater than the amount of the reaction solvent.

[0036] In the separation step, the precipitate suspended in a solvent consisting of the reaction solvent and the poor solvent is separated from the solvent into solid and liquid form. Examples of separation methods include vacuum filtration, pressure filtration, and centrifugation. In the separation step, the separated precipitate may be washed with the poor solvent. This can suppress the residue of the reaction solvent. From this viewpoint, the manufacturing method of this embodiment may include a washing step as the next step after the separation step, in which the separated precipitate is washed with the poor solvent.

[0037] The precipitate obtained through the separation step or the washing step may be used in the subsequent dissolution step while still wet. Alternatively, the precipitate may be used in the dissolution step after drying. Specifically, considering that polyamic acid is subject to hydrolysis by trace amounts of water that may be present in the solvent and trace amounts of water produced as a by-product during the slightly advanced imidization, it is preferable that the precipitate after separation and washing be stored in a dry state until the dissolution step is carried out. From this viewpoint, the manufacturing method of this embodiment may include a drying step as the next step after the separation step or the washing step to dry the precipitate in a wet state. In the drying step, heating and drying at a temperature higher than the boiling point of the poor solvent is preferable, and heating and drying at a temperature higher than the boiling point of the reaction solvent is more preferable. The temperature here is preferably lower than the temperature of the second heating step during the production of polyimide film.

[0038] In the dissolution step of this embodiment, the dried precipitate is dissolved using the second organic base and water. Since the precipitate contains the first organic base, the second organic base is an organic base added in the dissolution step.

[0039] The second organic base, like the first organic base, is at least one selected from the group consisting of unsubstituted imidazole, monoalkylimidazole having only one C1-C3 alkyl group as a substituent on the imidazole ring, and dimethylimidazole having only two methyl groups. That is, the second organic base may be one or more selected from the group. It is preferable that the second organic base is the same as the first organic base. The precipitate can be dissolved in water by the addition of the second organic base. That is, an aqueous solution of the polyimide precursor can be prepared by the addition of the second organic base.

[0040] The amount of the second organic base added is preferably 1 equivalent or more, and more preferably 2 equivalents or more, per 1 mole of the first polyamic acid or 1 mole of the second polyamic acid. The amount of the second organic base added may be, for example, 5 equivalents or less, and may be 4 equivalents or less. The amount of the second organic base added is preferably such that the total equivalent amount of the first organic base and the second organic base per 1 mole of the first polyamic acid or 1 mole of the second polyamic acid is 1.5 equivalents or more, more preferably 2.0 equivalents or more, and even more preferably 3.0 equivalents or more. In the equivalent calculations here, it is assumed that the entire amount of the polyamic acid and the first organic base is contained in the precipitate without loss.

[0041] In the dissolution step, the second organic base may be added so that the pH of the aqueous solution of the polyimide precursor to be prepared becomes 7 or higher. The pH here may be, for example, 10 or less, 9 or less, or 8 or less.

[0042] The viscosity of the polyimide precursor aqueous solution can change depending on the type of the first or second organic base. For example, imidazoles in which a substituent is bonded to the nitrogen atom at position 1, such as 1-methylimidazole or 1-ethylimidazole, tend to suppress the increase in viscosity of the polyimide precursor aqueous solution. A low-viscosity polyimide precursor aqueous solution is advantageous because it is easy to increase the concentration of each component, such as polyamic acid, in the polyimide precursor aqueous solution, thus making it easier to adjust the thickness of the polyimide film. On the other hand, imidazoles in which the organic base is unsubstituted, imidazoles in which no substituent is bonded to the nitrogen atom at position 1, such as 2-methylimidazole, or polysubstituted imidazoles such as 1,2-dimethylimidazole, tend to increase the viscosity of the polyimide precursor aqueous solution. Such an increase in viscosity can make it difficult to increase the concentration of each component, such as polyamic acid, in the polyimide precursor aqueous solution, and thus can make it particularly difficult to form a thick polyimide film.

[0043] The aqueous solution of the polyimide precursor preferably has a viscosity of 1,000 to 50,000 mPa·s when measured at 40°C and 10 rpm (cone rotor 1°34' × R12) using an E-type viscometer (Toki Sangyo Co., Ltd., TVE-35H). Alternatively, the viscosity at 1 rpm may be 200,000 mPa·s to 500,000 mPa·s. When adjusting to such a viscosity, the content of the first polyamic acid or the second polyamic acid is preferably 5 to 40% by mass, and more preferably 10 to 30% by mass, relative to the mass of the aqueous solution of the polyimide precursor.

[0044] As described above, while exemplary embodiments have been presented, the method for producing an aqueous polyimide precursor solution and the polyamic acid salt according to the present invention are not limited to the configurations of the above embodiments. Furthermore, the method for producing an aqueous polyimide precursor solution and the polyamic acid salt according to the present invention are not limited by the effects described above. The method for producing an aqueous polyimide precursor solution and the polyamic acid salt according to the present invention can be modified in various ways without departing from the spirit of the present invention. [Examples]

[0045] The present invention will be further described below with reference to examples, but the present invention is not limited thereto.

[0046] [Raw materials used] (Polyamic acid solution) • A DMAc solution containing 20% ​​by mass of a polymer of pyromellitic dianhydride (PMDA) and 4,4'-diaminodiphenyl ether (ODA) (PMDA / ODA) (mass-average molecular weight of polyamic acid: 118,000, degree of polymerization of polyamic acid: 115, molecular weight for determining the amount of organic base to be charged: 418) A DMAc solution containing 20% ​​by mass of a block copolymer ((BPDA / ODA) / (PMDA / PPD)) of a first polymer (BPDA / ODA) of 3,3',4,4'-biphenyltetracarboxylic acid dianhydride (BPDA) and 4,4'-diaminodiphenyl ether (ODA), and a second polymer (PMDA / PPD) of pyromellitic acid dianhydride (PMDA) and paraphenylenediamine (PPD) (mass-average molecular weight of polyamic acid: 122,000, degree of polymerization of polyamic acid: 130, molecular weight for determining the amount of organic base to be charged: 425) • A DMAc solution containing 9.1% by mass of a polymer of 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) and paraphenylenediamine (PPD) (Molecular weight for determining the amount of organic base to be added: 402) • A DMAc solution containing 10% by mass of a polymer of 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) and 1,3-bis(4-aminophenoxy)benzene (TPE-R) (Molecular weight for determining the amount of organic base to be added: 586) (Organic bases) • Imidazole (unsubstituted imidazole, molecular weight: 68) • 1-Methylimidazole (Molecular weight: 82) • 2-Methylimidazole (Molecular weight: 82) • 1-Ethylimidazole (Molecular weight: 96) • 1,2-dimethylimidazole (molecular weight: 96) • 2-Ethyl-4-methylimidazole (Molecular weight: 110) • Diethanolamine (molecular weight: 105)

[0047] [Salt formation test] Salt formation was evaluated by reacting PMDA / ODA with organic bases to form precipitates containing polyamic salts and observing the properties of the precipitates. Specifically, at room temperature, each organic base shown in Table 1 was added to a DMAc solution of PMDA / ODA, and then added dropwise to methanol as a poor solvent to precipitate the product. Solid-liquid separation was performed by filtration to obtain the precipitate. Salt formation was evaluated based on the evaluation criteria below. The results are shown in Table 1. (Evaluation Criteria) ○: No fusion was observed in the precipitate, and the separation performance in solid-liquid separation was good. △: Some of the precipitate shows fusion, resulting in poor separation performance in solid-liquid separation. ×: Swelling material precipitates, resulting in poor separation during solid-liquid separation.

[0048] [Table 1]

[0049] The evaluation results shown in Table 1 suggest that unsubstituted imidazole and monomethylimidazole are favorable for salt formation of PMDA / ODA. Based on these results, we decided to first investigate whether polyamic acid salts can be synthesized and formed in an aqueous solvent.

[0050] [Comparative Example 1] 100 g of deionized water was mixed with 6.35 g (0.031 mol) of ODA and 9.98 g (0.121 mol, 3.9 equivalents) of 1-methylimidazole, and stirred at 25°C for 1 hour (ODA did not dissolve). Next, 6.8 g (0.031 mol) of PMDA was added, and the mixture was stirred at 70°C for 3.5 hours. However, none of the raw materials dissolved, and no increase in viscosity was observed. In other words, an aqueous solution of polyimide precursor could not be obtained.

[0051] The results from Comparative Example 1 suggested that, depending on the monomer constituting the polyamic acid, simultaneous synthesis of polyamic acid or formation of its salt in an aqueous solvent may be difficult. Therefore, we decided to investigate whether it is possible to form a polyamic salt or dissolve a polyamic salt in water using polyamic acid synthesized in an organic solvent. Furthermore, if an aqueous solution of a polyimide precursor, which is an aqueous solution of a polyamic salt, is obtained, we decided to prepare a polyimide film and evaluate its physical properties.

[0052] [Example 1] (Salt formation process) A DMAc solution containing 22.6 g (0.0541 mol) of PMDA / ODA (Solution 1) and 2.5 g (0.0305 mol, 0.56 equivalents) of 1-methylimidazole as the first organic base were added to a container to prepare Solution 2. Next, using a separate container and a stirring device, 1000 mL of methanol as a poor solvent was added dropwise while stirring, and stirring was continued for 10 minutes. The precipitate was separated from the solvent by solid-liquid separation by filtration. The separated precipitate was dried at 50°C for 14 hours to obtain a dried product. In this salt formation process, it was assumed that the total amounts of PMDA / ODA and 1-methylimidazole used were contained in the dried product without loss. (melting process) First, 20 g of the dried material was collected. This 20 g of dried material was assumed to have been produced in the salt formation process using 18.0 g (0.0431 mol) of PMDA / ODA and 1.99 g (0.0243 mol) of 1-methylimidazole. The collected 20 g of dried material, 88.5 mL of water, and 11.5 g (3.26 mol) of 1-methylimidazole as the second organic base were added to a container, and the dried material was dissolved by stirring with a stirring device to obtain an aqueous solution of the polyimide precursor. (Film manufacturing process) An aqueous solution of the polyimide precursor obtained in the dissolution process was used as an aqueous varnish, and a coating film with a thickness of 250 μm (wet film thickness) and a rectangular shape in plan view was formed on a glass plate, which served as the substrate, by bar coating. The coating film was heated at 60°C for 30 minutes (corresponding to the first heating step) to form a polyamic acid film on the glass plate. The polyamic acid film was peeled off the glass plate (corresponding to the peeling step), and the polyamic acid film was fixed to a rectangular metal frame with multiple needles protruding from the frame portion by piercing the multiple needles. It was then heated at 250°C for 10 minutes, and then at 400°C for 5 minutes (corresponding to the second heating step) to obtain a polyimide film.

[0053] [Example 2] As shown in Table 2, a polyimide film was obtained in the same manner as in Example 1, except that the amount of 1-methylimidazole charged in the salt formation step was changed to 4.4 g (0.0537 mol, 0.99 equivalents).

[0054] [Example 3] As shown in Table 2, a polyimide film was obtained in the same manner as in Example 1, except that 2.2 g (0.0324 mol) of unsubstituted imidazole was used in the salt formation step and 9.5 g (0.140 mol) of unsubstituted imidazole was used in the dissolution step.

[0055] [Example 4] As shown in Table 2, a polyimide film was obtained in the same manner as in Example 1, except that 2.7 g (0.0329 mol) of 2-methylimidazole was used in the salt formation step and 11.5 g (0.140 mol) of 2-methylimidazole was used in the dissolution step.

[0056] [Example 5] As shown in Table 2, a polyimide film was obtained in the same manner as in Example 1, except that 3.0 g (0.0313 mol) of 1-ethylimidazole was used in the salt formation step and 13.5 g (0.141 mol) of 1-ethylimidazole was used in the dissolution step.

[0057] [Example 6] As shown in Table 2, a polyimide film was obtained in the same manner as in Example 1, except that 5.2 g (0.0542 mol) of 1,2-dimethylimidazole was used in the salt formation step and 13.5 g (0.141 mol) of 1,2-dimethylimidazole was used in the dissolution step.

[0058] [Example 7] As shown in Table 2, a polyimide film was obtained in the same manner as in Example 6, except that 2.8 g (0.0292 mol) of 1,2-dimethylimidazole was used in the salt formation step and 11.5 g (0.140 mol) of 1-methylimidazole was used in the dissolution step.

[0059] [Comparative Example 2] As shown in Table 2, a polyimide film was obtained in the same manner as in Example 1, except that 6.0 g (0.0545 mol) of 2-ethyl-4-methylimidazole was used in the salt formation step and 15.4 g (0.140 mol) of 2-ethyl-4-methylimidazole was used in the dissolution step.

[0060] [Example 8] As shown in Table 3, a polyimide film was obtained in the same manner as in Example 2, except that the amount of 1-methylimidazole added in the dissolution step was changed to 11.5 g (0.140 mol).

[0061] [Example 9] As shown in Table 3, a polyimide film was obtained in the same manner as in Example 2, except that the amount of 1-methylimidazole added in the dissolution step was changed to 8.0 g (0.0976 mol).

[0062] [Comparative Example 3] As shown in Table 3, the procedure up to the dissolution step was carried out in the same manner as in Example 2, except that 1-methylimidazole was not added during the dissolution step.

[0063] [Example 10] (Salt formation process) A DMAc solution (Solution 1) containing 18.7 g (0.0440 mol) of (BPDA / ODA) / (PMDA / PPD) and 3.6 g (0.0439 mol, 1.00 equivalent) of 1-methylimidazole as the first organic base were added to a container to prepare Solution 2. Next, using a separate container and a stirring device, 1000 mL of methanol as a poor solvent was added dropwise while stirring, and stirring was continued for 10 minutes. The precipitate was separated from the solvent by solid-liquid separation by filtration. The separated precipitate was dried at 50°C for 14 hours to obtain a dried product. In this salt formation process, it was assumed that the total amounts of (BPDA / ODA) / (PMDA / PPD) and 1-methylimidazole used were contained in the dried product without loss. (melting process) First, 20 g of the dried material was collected. This 20 g of dried material was assumed to have been produced in the salt formation process using 16.8 g (0.0395 mol) of (BPDA / ODA) / (PMDA / PPD) and 3.23 g (0.0394 mol) of 1-methylimidazole. The collected 20 g of dried material, 88.5 mL of water, and 11.5 g (3.26 mol) of 1-methylimidazole as the second organic base were added to a container, and the dried material was dissolved by stirring with a stirring device to obtain an aqueous solution of the polyimide precursor. (Film manufacturing process) A film was prepared in the same manner as in Example 1.

[0064] [Example 11] As shown in Table 4, a polyimide film was obtained in the same manner as in Example 10, except that 3.0 g (0.0441 mol) of (unsubstituted) imidazole was used in the salt formation step and 9.5 g (0.140 mol) of (unsubstituted) imidazole was used in the dissolution step.

[0065] [Comparative Example 4] As shown in Table 4, the procedure was carried out in the same manner as in Example 10, up to the dissolution step, except that 9.1 g (0.0226 mol) of BPDA / PPD was used as the polyamic acid and 1.85 g (0.0226 mol, 1.0 equivalent) of 1-methylimidazole was used in the salt formation step.

[0066] [Comparative Example 5] As shown in Table 4, BPDA / TPE-R 10.0 g (0.0171 mol) was used as the polyamic acid, and the procedure up to the dissolution step was carried out in the same manner as in Example 10, except that 1.40 g (0.0171 mol, 1.0 equivalent) of 1-methylimidazole was used in the salt formation step.

[0067] [Viscosity of polyimide precursor solution] The viscosity was measured at 40°C and 10 rpm using the E-type viscometer described above. If torque overload occurred at 10 rpm, the viscosity was measured at 1 rpm.

[0068] [Evaluation methods for polyimide films] When polyimide films were obtained, their film thickness (JIS K7130), tensile strength (JIS K7161), tensile elongation (JIS K7161), and tensile modulus (JIS K7161) were measured according to the measurement methods specified in JIS. The results are shown in Tables 2 and 4. Measurement values ​​for the same parameters were also obtained for a polyimide film produced as Reference Example 1 using the DMAc solution (first solution) of PMDA / ODA used as a raw material. The results are shown in Table 5. The performance of the polyimide films was then evaluated by comparing the measurement values ​​of the examples and comparative examples with those of Reference Example 1.

[0069] [Table 2]

[0070] [Table 3]

[0071] [Table 4]

[0072] [Table 5]

[0073] The polyimide films in each example are considered to have performance equivalent to that of Reference Example 1.

Claims

1. A salt formation step in which a solution containing polyamic acid, a first organic base, and a poor solvent are mixed to precipitate a polyamic acid salt, A separation step for separating the precipitate containing the polyamic acid salt from the solvent in a solid-liquid manner, The system comprises a dissolution step of mixing the precipitate, a second organic base, and water to prepare an aqueous solution of a polyimide precursor, The polyamic acid is a polymer of pyromellitic dianhydride and 4,4'-diaminodiphenyl ether, or It is a polymer of pyromellitic dianhydride, 4,4'-diaminodiphenyl ether, 3,3',4,4'-biphenyltetracarboxylic acid dianhydride, and paraphenylenediamine. A method for producing an aqueous solution of a polyimide precursor, wherein each of the first organic base and the second organic base is selected from the group consisting of an unsubstituted imidazole, a monoalkylimidazole having only one C1-C3 alkyl group, and a dimethylimidazole having only two methyl groups.

2. It contains polyamic acid and an organic base. The polyamic acid is a polymer of pyromellitic dianhydride and 4,4'-diaminodiphenyl ether, or It is a polymer of pyromellitic dianhydride, 4,4'-diaminodiphenyl ether, 3,3',4,4'-biphenyltetracarboxylic acid dianhydride, and paraphenylenediamine. The aforementioned organic base is a polyamic salt, selected from the group consisting of unsubstituted imidazole, monoalkylimidazole having only one C1-C3 alkyl group, and dimethylimidazole having only two methyl groups.

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