Curable composition and cured product

Abstract

Provided are a curable composition and the like that include a polyimide precursor in which some of the imide bonds of a polyimide resin are hydrolyzed and an oxazoline compound having an oxazoline ring in the molecule thereof, and in which the quantity of oxazoline rings of the oxazoline compound relative to 1 mol of a repeating structural unit of the polyamide precursor is 0.20-1.60 mol.

Description

Curing composition and cured product CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority from Japanese Patent Application No. 2021-066611, the entire disclosure of which is incorporated herein by reference.

[0002] The present invention relates to, for example, a curable composition that becomes a cured product by a curing treatment, and a cured product obtained by curing the curable composition.

[0003] Conventionally, as a curable composition that cures by a curing treatment, for example, a curable composition containing at least a polyimide precursor in which a portion of the imide bonds of a polyimide resin have been hydrolyzed is known. In this type of curable composition, the polyimide precursor has, in its molecule, a plurality of cyclic imide structures and a plurality of amic acid structures (carboxy groups and amide groups) that can form the cyclic imide structures by an imidization reaction. When this type of curable composition is cured, the amic acid structures in the polyimide precursor are imidized (dehydration condensation reaction) to form imide bonds, thereby producing a cured product. Because the produced cured product has heat resistance and electrical insulation properties, this type of curable composition is used to form a cured product, such as an insulating coating material, by a curing treatment.

[0004] One known example of such a curable composition is one that contains an imide group-containing compound (polyimide precursor) obtained by partially hydrolyzing a polyimide molded article, an epoxy compound, and a blocked isocyanate compound, wherein the amount of epoxy compound is within the range of 10 to 100 parts by mass per 100 parts by mass of the polyimide precursor, and the amount of blocked isocyanate compound is within the range of 10 to 100 parts by mass, and when cured under heating conditions of 120°C x 60 minutes, the 10% mass loss temperature measured with a thermobalance is 200°C or higher (Patent Document 1). The curable composition described in Patent Document 1 can be cured by a curing treatment at a relatively low temperature, and can provide a cured product with good adhesion (tightness) to an adherend and good heat resistance.

[0005] Further, the above-mentioned curable composition may be, for example, a curable composition containing an imide group-containing compound (polyimide precursor) obtained by partially hydrolyzing a polyimide molded article, an aqueous solvent, and an amine compound having a boiling point of 85 to 145°C, wherein the imide group-containing compound has a wave number of 1500 cm originating from a benzene ring in an infrared spectroscopy chart. －１ and an absorption peak at a wavenumber of 1375 cm －１ and an absorption peak at a wave number of 1500 cm due to the benzene ring. －１ The height of the absorption peak at the wavenumber 1375 cm －１ A curable composition is known in which the ratio of S1 / S2 is a value within a range of 2 to 10, where S2 is the height of the absorption peak of the curable composition (Patent Document 2). The curable composition described in Patent Document 2 has good stability despite containing an aqueous solvent such as water, and can be cured by a curing treatment at a relatively low temperature.

[0006] Japanese Patent Publication No. 2017-014386 Japanese Patent No. 6402283

[0007] However, even if the curable composition described in Patent Document 1 is subjected to a curing treatment, the resulting cured product does not necessarily have good uniformity. Furthermore, even if the curable composition described in Patent Document 2 is subjected to a curing treatment, the resulting cured product does not necessarily have good flexibility. Thus, it is difficult for the cured product produced by the curing treatment of the above-described curable composition to have all the performance properties, such as uniformity, flexibility, and adhesion to an adherend. Therefore, there is a demand for a curable composition that can produce a cured product that has all the uniformity, flexibility, and adhesion to an adherend.

[0008] In view of the above problems and demands, an object of the present invention is to provide a curable composition that can produce a cured product that is uniform, flexible, and has adhesion to an adherend.

[0009] The curing composition according to the present invention is characterized in that it contains: a polyimide precursor in which imide bonds of a polyimide resin are partially hydrolyzed; and an oxazoline compound having an oxazoline ring in the molecule, wherein the amount of the oxazoline ring of the oxazoline compound relative to 1 mol of a repeating structural unit of the polyimide precursor is 0.20 mol or more and 1.60 mol or less.

[0010] In the curable composition according to the present invention, the oxazoline compound may be a polymer compound. The curable composition according to the present invention may further include an aqueous solvent. In the curable composition according to the present invention, the polyimide precursor has a benzene ring in its molecule, and an infrared spectrum of the polyimide precursor obtained by infrared spectroscopic analysis shows a peak at a wave number of 1500 cm due to the benzene ring. －１ The absorption peak P1 and the imide group have a wave number of 1375 cm －１ and an absorption peak P2 of

[0011] The cured product according to the present invention is obtained by curing the above-mentioned curable composition.

[0012] A diagram showing the schematic molecular structural formula of each example of polyimide resin before hydrolysis. A GPC chart of a polyimide precursor (hydrolysate). An IR chart of an example of polyimide resin before hydrolysis. An IR chart of another example of polyimide resin before hydrolysis. An IR chart of an example of polyimide precursor (hydrolysate). An IR chart of another example of polyimide precursor (hydrolysate). An IR chart of another example of polyimide precursor (hydrolysate).

[0013] Embodiments of the curable composition and the cured product according to the present invention will be described below.

[0014] The curable composition of the present embodiment comprises a polyimide precursor in which a portion of the imide bonds of a polyimide resin are hydrolyzed, and an oxazoline compound having an oxazoline ring in the molecule, wherein the amount of the oxazoline ring in the oxazoline compound per 1 mol of the repeating structural unit of the polyimide precursor is 0.20 mol to 1.60 mol. The curable composition of the present embodiment can produce a cured product that is uniform, flexible, and has good adhesion to an adherend.

[0015] In this embodiment, the polyimide precursor has a plurality of cyclic imide structures and a plurality of amic acid structures in its molecule. Each amic acid structure contains a carboxy group and an amide group. The carboxy group and the amide group in each amic acid structure can be imidized (dehydration condensation reaction) by a curing treatment such as heating to form an imide bond (imide group). This imidization newly creates a cyclic imide structure. The amic acid structure is a structure obtained by hydrolysis of the imide bond constituting the cyclic imide structure, and the cyclic imide structure is a structure obtained by imidization (dehydration condensation reaction) of the carboxy group and the amide group in the amic acid structure. Furthermore, since the polyimide precursor is a hydrolyzate of a polyimide resin, it may have a carboxy group at the molecular chain terminal.

[0016] Commercially available products can be used as the polyimide resin before hydrolysis. Examples of polyimide resin products that can be used include the "Kapton" series (manufactured by DuPont-Toray Co., Ltd.), the "Upilex" series (manufactured by Ube Industries, Ltd.), and the "Aurum" series (manufactured by Mitsui Chemicals, Inc.). Specific examples of the molecular structures of these polyimide resins will be described in detail later (see Figure 1). From the perspective of protecting the global environment, it is preferable to use a polyimide precursor obtained by hydrolyzing polyimide resin films that have traditionally been discarded as the polyimide resin before hydrolysis. This allows for active recycling of polyimide resins and also produces a cured product that combines uniformity, flexibility, and adhesion to adherends.

[0017] The polyimide precursor has, for example, a molecular structure represented by the following general formula (1): In general formula (1), Ar may contain at least one of, for example, an aromatic structure, a cyclic saturated hydrocarbon structure, a linear or branched saturated hydrocarbon structure, or an ether group. Meanwhile, Ar' may contain at least one of, for example, an aromatic structure, a cyclic saturated hydrocarbon structure, a branched saturated hydrocarbon structure, or an ether group. In general formula (1), m and n each represent a ratio in the molecule (m+n=10), and for example, m may be 5 or more and 9 or less, and n may be 1 or more and 5 or less.

[0018]

[0019] The repeating structural units in the polyimide precursor correspond to the repeating structural units in the polyimide resin before hydrolysis. Specifically, the polyimide resin before hydrolysis has a structure in which the structural units of the product of the reaction between a diamine monomer and a tetracarboxylic acid (dianhydride) monomer are repeated within the molecule. The polyimide precursor also has similar repeating structural units within the molecule. However, in the polyimide precursor produced by hydrolysis, a portion of the cyclic imide structure is converted to an amic acid structure by hydrolysis, so both the portion in parentheses on the left side of general formula (1) and the portion in parentheses on the right side of general formula (1) are repeating structural units. For example, the number of moles of the repeating structural units can be calculated as follows. The repeating structural unit of the polyimide resin before hydrolysis is, for example, the portion in parentheses on the right side of general formula (1). The number of moles of the repeating structural unit can be calculated by dividing the mass of the polyimide resin by the molecular weight of the portion in parentheses on the right side. The number of nitrogen atoms in the repeating structural unit is 2. Therefore, the amount of the oxazoline rings of the oxazoline compound relative to 1 mol of the repeating structural unit of the polyimide precursor can also be expressed as the amount per 2 mol of nitrogen atoms in the polyimide precursor.

[0020] As described above, the polyimide precursor is a hydrolyzate in which a portion of the imide bonds in a polyimide resin are hydrolyzed. Therefore, the polyimide precursor has, in its molecule, both the amic acid structure (the portion represented by β) represented on the left side of general formula (1) and the cyclic polyimide structure (the portion represented by α) represented on the right side.

[0021] The polyimide precursor preferably has an aromatic structure (particularly a benzene ring structure) in the molecule. The polyimide precursor having a benzene ring structure in the molecule has the advantage that the cured product can exhibit higher heat resistance.

[0022] In the molecule of the polyimide precursor, the molar ratio of the number of nitrogen atoms to one benzene ring may be 0.2 or more and 1.0 or less, or 0.4 or more and 0.8 or less, which can be analyzed by infrared spectroscopy or NMR measurement, which will be described later.

[0023] In general formula (1), for example, as shown below, Ar may be Ar-1 (a structure having one benzene ring) or Ar-2 (a structure having two benzene rings). Also, Ar' may be Ar'-1 (a structure having two benzene rings) or Ar'-2 (a structure having one benzene ring). In general formula (1), Ar may be Ar-1 (a structure having one benzene ring) and Ar' may be Ar'-1 (a structure having two benzene rings).

[0024] The polyimide precursor of the present embodiment is a hydrolysis reaction product of a polyimide resin. A method for producing the polyimide precursor will be described in detail later.

[0025] The average molecular weight (mass average molecular weight) of the polyimide precursor may be, for example, 1,000 or more and 100,000 or less. The average molecular weight (mass average molecular weight) of the polyimide precursor is preferably 3,000 or more, and more preferably 5,000 or more. The average molecular weight (mass average molecular weight) is preferably 60,000 or less, and more preferably 30,000 or less. When the mass average molecular weight of the polyimide precursor is within the above range, the solubility in the solvent described below is improved.

[0026] The mass average molecular weight of the polyimide precursor is determined by gel permeation chromatography (GPC). The details of the GPC measurement conditions are as follows: Apparatus name: Tosoh Corporation, product name "HLC-8320GPC" Detector: Differential refractive index (RI) detector Column: One styrene divinylbenzene (average particle size 3 μm, average pore size 2 nm) Column temperature: 40°C Standard substance for creating a calibration curve: Polyethylene glycol Data processing software: Software included with the above apparatus Eluent: N,N-dimethylacetamide (DMAc) [Number average molecular weight (Mn), mass average molecular weight (Mw), molecular weight distribution (Mw / Mn)]

[0027] The Mw / Mn of the polyimide precursor determined by the GPC measurement is preferably 1.0 or more, more preferably 1.5 or more. Such Mw / Mn is preferably 3.0 or less, more preferably 2.5 or less. Having the Mw / Mn of the polyimide precursor be 1.0 or more and 3.0 or less has the advantage of making the physical properties of the cured product more uniform and stable. The hydrolysis reaction product of the polyimide resin, i.e., the polyimide precursor produced by hydrolysis of the polyimide resin, can have an Mw / Mn in the above range.

[0028] In the infrared spectroscopy (IR) spectrum (chart) obtained by infrared spectroscopy of a polyimide precursor, for example, the following absorption peaks are observed. For example, the following absorption peaks are observed in the hydrolysis reaction product of a polyimide resin, i.e., a polyimide precursor produced by hydrolysis of a polyimide resin. Note that, as is commonly understood in the technical field of infrared spectroscopy, each wavenumber shown below is a value for defining an absorption peak near that wavenumber. In other words, each wavenumber shown below does not only indicate an absorption peak having a peak apex at that wavenumber.

[0029] Infrared spectroscopic analysis of the polyimide precursor is carried out under the following measurement conditions. Infrared spectroscopic analysis can be carried out under the following measurement conditions using a commercially available measuring instrument. Instrument name: Fourier transform infrared spectrophotometer ("FT / IR-4600" manufactured by JASCO Corporation) Analysis method: Attenuated total reflection (ATR) Measurement wavenumber range: 4000 cm －１ ~400cm －１

[0030] Infrared Spectroscopy (Absorption Peak P1 Due to Benzene Ring) When the polyimide precursor of the present embodiment is subjected to infrared spectroscopic analysis, it has an absorption peak P1 at a wave number of 1500 cm as shown in FIG. －１ The absorption peak P1 due to the benzene ring is shown (nearby).

[0031] Infrared Spectroscopy (Absorption Peak P2 Due to Imide Group) When the polyimide precursor of the present embodiment is subjected to infrared spectroscopic analysis, it has an absorption peak P2 at a wave number of 1375 cm as shown in FIG. －１ The absorption peak P2 due to the imide group is shown in (near). Since the polyimide precursor has an imide group in the molecule, the cured product produced by a curing treatment such as heating has good heat resistance.

[0032] Infrared Spectroscopy (Absorption Peak P3 Due to Amide Group) The polyimide precursor in this embodiment has an absorption peak at a wave number of 1600 cm as shown in FIG. －１The absorption peak P3 due to the amide group (-NHCO-) ​​is shown in (near). The polyimide precursor has an amide group in the molecule, so it can be cured at a relatively low temperature.

[0033] Infrared Spectroscopy (Absorption Peak P4 Due to Carboxy Group) The polyimide precursor in this embodiment has an absorption peak of 1413 cm as shown in FIG. －１ The absorption peak P4 due to the carboxy group (—COOH) is shown in (near). The polyimide precursor has a carboxy group in the molecule, and therefore has good solubility in solvents described below, and the cured product obtained by curing treatment can have good adhesion.

[0034] Infrared Spectroscopy (Absorption Peak P5 Due to Carbonyl Group) The polyimide precursor in this embodiment has an absorption peak of 1710 cm as shown in FIG. －１ The absorption peak P5 due to the carbonyl group (—CO—) is shown in (near). In this embodiment, the polyimide precursor has a carbonyl group in the molecule, and therefore has good solubility in the solvent described below.

[0035] When a polyimide precursor is obtained by partially hydrolyzing a polyimide resin, the amount of imide groups (height of the absorption peak), the amount of amide groups (height of the absorption peak), and the amount of carboxyl groups (height of the absorption peak) in the polyimide precursor can serve as indicators of the degree of hydrolysis of the polyimide resin. For example, the degree of hydrolysis can be confirmed by determining the height of the absorption peak of each of the above functional groups relative to the height of the absorption peak of a benzene ring, whose chemical structure does not change even upon hydrolysis. In other words, when a polyimide precursor is obtained by partially hydrolyzing a polyimide resin, the height of the absorption peak P2 due to the imide groups, the height of the absorption peak P3 due to the amide groups, and the height of the absorption peak P4 due to the carboxyl groups can each serve as indicators of the degree of hydrolysis of the polyimide resin. As a more accurate indicator of the degree of hydrolysis, the ratios of the above heights (P1 / P2, P1 / P3, P1 / P4) can be used.

[0036] The ratio of the height of the absorption peaks described above is calculated as follows: As a result of infrared spectroscopic analysis of the polyimide precursor, the horizontal axis represents the wave number [cm －１ ], and the vertical axis is the transmittance [%] (the upper side is the transmittance of 100%). －１ A horizontal line is drawn through the apex of the peak that protrudes upward and appears near the base line, and this horizontal line is used as the base line. Then, the absolute value of the difference between the height [%] of each absorption peak and the height [%] of the base line is calculated. For example, if the height [%] of the base line is 90 [%], the height of absorption peak P1 is 70 [%], and the height of absorption peak P2 is 80 [%], the ratio of the absorption peak heights (P1 / P2) is calculated using the following formula: (Calculation Example) (P1 / P2) = (|70-90| / |80-90|) = 2.0

[0037] Ratio of the height of the absorption peak P1 due to the benzene ring to the height of the absorption peak P2 due to the imide group: P1 / P2 In the infrared spectrum of the polyimide precursor of this embodiment, the wave number 1375 cm due to the imide group －１ The wave number of 1500 cm due to the benzene ring relative to the height of the absorption peak P2 －１ The height ratio (P1 / P2) of the absorption peak P1 in the polyimide precursor is preferably 2 or more, and may be 3 or more. Furthermore, the ratio (P1 / P2) is preferably 10 or less, more preferably 9 or less, and even more preferably 5 or less. When the height ratio (P1 / P2) is within the above range, the polyimide precursor can be cured at a lower temperature. Furthermore, the polyimide precursor can have better solubility in solvents described below. Furthermore, the cured product obtained by the curing treatment can have better adhesion, uniformity, and flexibility.

[0038] Ratio of the height of the absorption peak P1 due to the benzene ring to the height of the absorption peak P3 due to the amide group: P1 / P3 In the infrared spectrum of the polyimide precursor of this embodiment, the height of the absorption peak P1 due to the amide group at a wave number of 1600 cm －１ The wave number of 1500 cm due to the benzene ring relative to the height of the absorption peak P3 －１The height ratio (P1 / P3) of the absorption peak P1 in the polyimide precursor is preferably 2 or more, more preferably 3 or more. Furthermore, such a ratio (P1 / P3) is preferably 20 or less, more preferably 12 or less, and even more preferably 10 or less. When the height ratio (P1 / P3) is within the above range, the polyimide precursor can have better solubility in solvents described below. Furthermore, the cured product obtained by the curing treatment can have better adhesion, uniformity, and flexibility.

[0039] Ratio of the height of the absorption peak P1 due to the benzene ring to the height of the absorption peak P4 due to the carboxy group: P1 / P4 In the infrared spectrum of the polyimide precursor of this embodiment, the wave number 1413 cm due to the carboxy group －１ The wave number of 1500 cm due to the benzene ring relative to the height of the absorption peak P4 －１ The height ratio (P1 / P4) of the absorption peak P1 in the polyimide precursor is preferably 2 or more, more preferably 3 or more. Furthermore, the ratio (P1 / P4) is preferably 20 or less, more preferably 12 or less, and even more preferably 11 or less. When the height ratio (P1 / P4) is within the above range, the polyimide precursor can have better solubility in solvents described below. Furthermore, the cured product obtained by the curing treatment can have better adhesion, uniformity, and flexibility.

[0040] The benzene ring in the polyimide precursor may be an unsubstituted benzene ring, or a substituted benzene ring in which hydrogen atoms bonded to each of the carbon atoms constituting the benzene ring are replaced with substituents. The substituent on the substituted benzene ring may be, for example, an alkyl group having 1 to 3 carbon atoms, an alkoxy group having 1 to 3 carbon atoms, an alkoxysilyl group having 1 to 3 carbon atoms, or a trifluoromethyl group. Preferably, the benzene ring in the polyimide precursor is an unsubstituted benzene ring, a benzene ring substituted with an alkoxysilyl group having 1 to 3 carbon atoms, or a benzene ring substituted with a trifluoromethyl group. More preferably, the benzene ring in the polyimide precursor is an unsubstituted benzene ring. In other words, it is preferable that no monovalent substituent (such as a halogen atom) is bonded to the carbon atom constituting the benzene ring of the polyimide precursor. In other words, it is preferable that the monovalent group bonded to the carbon atom constituting the benzene ring is a hydrogen atom (-H).

[0041] As the polyimide precursor, for example, a polyimide precursor represented by the formula (3) described below can be used. Specifically, as the polyimide precursor, a partial hydrolyzate obtained by partially hydrolyzing a product named "Kapton H" (manufactured by DuPont-Toray Co., Ltd.) can be used. For example, the polyimide precursor of this embodiment can be obtained by subjecting a discarded polyimide resin film to a hydrolysis treatment.

[0042] The oxazoline compound contained in the curable composition of the present embodiment is a compound containing one or more oxazoline rings in the molecule. Examples of the oxazoline compound include low-molecular-weight compounds and high-molecular-weight compounds.

[0043] The oxazoline compound may be a low molecular weight oxazoline compound having a molecular weight of less than 1,000, or may be a high molecular weight oxazoline compound having a molecular weight of 1,000 or more.

[0044] Examples of low molecular weight oxazoline compounds include compounds in which two oxazoline rings are directly bonded to each other, compounds in which two oxazoline rings are bonded via an organic group, and compounds having three oxazoline rings.

[0045] Examples of compounds in which two oxazoline rings are directly bonded to each other include 2,2'-bis(2-oxazoline) and 2,2'-bis-4-benzyl-2-oxazoline. Examples of compounds in which two oxazoline rings are bonded via an organic group include 1,4-bis(4,5-dihydro-2-oxazolyl)benzene, 1,3-bis(4,5-dihydro-2-oxazolyl)benzene, 2,6-bis(isopropyl-2-oxazolin-2-yl)pyridine, 2,2'-methylenebis(4-tert-butyl-2-oxazoline), and 2,2'-methylenebis(4-phenyl-2-oxazoline). Examples of compounds having three oxazoline rings include 1,2,4-tris-(2-oxazolin-2-yl)benzene.

[0046] Examples of polymeric oxazoline compounds include oxazoline ring-containing acrylic polymers having an oxazoline ring at the end of a side chain and polymerized with at least an acrylic acid ester. Oxazoline ring-containing acrylic polymers are polymerized compounds in which an acrylic acid ester monomer having an oxazoline ring is polymerized as at least one type of monomer. Such oxazoline ring-containing acrylic polymers may be homopolymers or copolymers. Examples of oxazoline ring-containing acrylic polymers that can be used include those with the product names "Epocross WS" series (manufactured by Nippon Shokubai Co., Ltd.) and "Epocross K-2000" series (manufactured by Nippon Shokubai Co., Ltd.).

[0047] In the oxazoline compound, the monovalent group bonded to the carbon atom constituting the oxazoline ring is preferably only a hydrogen atom (—H), as shown in the following formula (2).

[0048] As the oxazoline compound, a polymeric oxazoline compound is preferred in that it has better reactivity with the polyimide precursor and that the cured product after curing treatment can have better storage stability, and the above-mentioned oxazoline ring-containing acrylic polymer having an oxazoline ring at the end of the side chain is more preferred.

[0049] The amount of oxazoline groups (amount of oxazoline rings) in the oxazoline compound is preferably 1 [mmol / g] or more and 10 [mmol / g] or less, and more preferably 3 [mmol / g] or more. The amount of oxazoline groups in the polymeric oxazoline compound can be calculated based on the ratio of the amount of the monomer unit having an oxazoline group to the amount of other monomer units constituting the polymeric oxazoline compound. Such an amount ratio can be determined, for example, by a nuclear magnetic resonance analyzer (NMR). １ It can be determined based on the peak intensity derived from the oxazoline group and the peak intensity derived from other monomers in the H-NMR analysis results.

[0050] As the oxazoline compound, one type may be used alone, or two or more types may be used in combination.

[0051] In the curable composition of this embodiment, as described above, the amount of the oxazoline rings in the oxazoline compound relative to 1 mol of the repeating structural unit of the polyimide precursor is 0.20 mol or more and 1.60 mol or less. The amount of the oxazoline rings may be 0.30 mol or more, 0.35 mol or more, or 0.39 mol or more. Furthermore, the amount of the oxazoline rings may be 1.50 mol or less, or 1.20 mol or less.

[0052] In the curable composition of this embodiment, the polyimide precursor is preferably a partial hydrolyzate of a polyimide resin, and the oxazoline compound is preferably a polymeric oxazoline compound (e.g., an oxazoline ring-containing acrylic polymer having an oxazoline ring at the end of a side chain). A polyimide precursor that is a partial hydrolyzate contains a larger amount of low-molecular-weight compound than a polyimide precursor composed of a monomer, and therefore may become hard and brittle when cured. In contrast, by further including a polymeric oxazoline compound in the curable composition of this embodiment, the hardness and brittleness can be suppressed. Therefore, the adhesion of the cured product to an adherend can be improved.

[0053] The curable composition of this embodiment preferably contains 5 parts by mass or more and 95 parts by mass or less of the oxazoline compound per 100 parts by mass of the polyimide precursor. The curable composition of this embodiment may contain 8 parts by mass or more, or 10 parts by mass or more, of the oxazoline compound per 100 parts by mass of the polyimide precursor. The curable composition of this embodiment may contain 90 parts by mass or less, 80 parts by mass or less, or 70 parts by mass or less of the oxazoline compound per 100 parts by mass of the polyimide precursor.

[0054] The curable composition of this embodiment may contain a solvent. Examples of the solvent include non-polar solvents and polar solvents. Examples of non-polar solvents include hydrocarbon solvents containing only carbon and hydrogen atoms in the molecule, and chlorine-based solvents such as carbon tetrachloride. The curable composition of this embodiment preferably contains a polar solvent for dissolving both the polyimide precursor and the oxazoline compound. A polar solvent is a compound containing atoms other than carbon and hydrogen atoms (e.g., oxygen atoms, nitrogen atoms) in the molecule. In the curable composition of this embodiment, both the polyimide precursor and the oxazoline compound are dissolved in the polar solvent.

[0055] Examples of polar solvents include amine solvents, amide solvents, ketone solvents, ether solvents, pyrrolidone solvents, glycol ether solvents, ester solvents, alcohol solvents, polyhydric alcohol solvents, halogenated solvents, and water.

[0056] Examples of amine solvents include ammonia (water), diethylamine, ethylethanolamine, diethanolamine, triethanolamine, monoethanolamine, monopropanolamine, isopropanolamine, triethylamine, tributylamine, dimethylaminoethanol, diethylaminoethanol, methylethanolamine, methyldiethanolamine, ethylaminoethanol, and diethanolamine. Examples of amide solvents include N,N-dimethylformamide and N,N-dimethylacetamide. Examples of ketone solvents include methyl ethyl ketone, methyl isobutyl ketone, γ-butyrolactone, cyclohexanone, and cyclopentanone. Examples of ether solvents include tetrahydrofuran, 1,3-dioxolane, 1,4-dioxane, and methyl phenyl ether. Examples of pyrrolidone solvents include N-methyl-2-pyrrolidone. Examples of glycol ether solvents include methyl diglyme, ethyl diglyme, methyl triglyme, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, propylene glycol monomethyl ether, 1-methoxy-2-propanol, ethylene glycol monoethyl ether, and the like, as well as acetates thereof. Examples of ester solvents include ethyl acetate, butyl acetate, and isopropyl acetate. Examples of alcohol solvents include methyl alcohol, ethyl alcohol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, 2-methyl-1-propanol, and 2-methyl-2-propanol. Examples of polyhydric alcohol solvents include glycerin. Examples of halogen-containing solvents include chloroform and dichloromethane.

[0057] The polar solvent is preferably an aqueous solvent. The aqueous solvent is water or a hydrophilic organic solvent that dissolves in water at any ratio relative to water. The curing composition of the present embodiment more preferably contains an aqueous solvent among polar solvents, and more preferably contains at least water as the aqueous solvent.

[0058] Examples of hydrophilic organic solvents include amine solvents such as dimethylaminoethanol and diethanolamine, amide solvents such as N,N-dimethylformamide and N,N-dimethylacetamide, pyrrolidone solvents such as N-methyl-2-pyrrolidone, alcohol solvents such as methyl alcohol, ethyl alcohol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, 2-methyl-1-propanol, and 2-methyl-2-propanol, alcohol ether solvents such as ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, 1-methoxy-2-propanol, and propylene glycol monomethyl ether, and polyhydric alcohol solvents such as glycerin.

[0059] Amine-based solvents have a high affinity for polyimide precursors and cured products of such polyimide precursors. Therefore, it is preferable that an amine-based solvent is blended into the curable composition for the purpose of improving the solubility of the polyimide precursor in the curable composition or for the purpose of forming a nearly uniform cured product. In particular, dimethylaminoethanol and diethanolamine as amine-based solvents have relatively high boiling points, so they gradually volatilize when the curable composition is subjected to a curing treatment by heating. This allows the cured product to form a more uniform cured film as the curing progresses.

[0060] The polar solvents may be used in combination of two or more kinds, and the aqueous solvents may be used in combination of two or more kinds.

[0061] The curable composition of this embodiment may contain 60% by mass or more of a polar solvent (particularly an aqueous solvent). Alternatively, the curable composition may contain 99% by mass or less of a polar solvent (particularly an aqueous solvent). In the curable composition of this embodiment, the total mass proportion of the polyimide precursor and the oxazoline compound may be 1% by mass or more. Alternatively, this proportion may be 40% by mass or less.

[0062] In addition to the components described above, the curable composition may further contain an imidization reaction catalyst, a surfactant, an antioxidant, a leveling agent, an antistatic agent, a dye or pigment, a viscosity modifier, an antifoaming agent, a stabilizer, a resin other than polyimide resin, or a coupling agent. Examples of the other resins include acrylic resins, fluororesins, epoxy resins, phenolic resins, silicone resins, olefin resins, polyester resins, polyamide resins, and hydrocarbon resins. These components may be blended for the purpose of improving the workability of the curable composition when it is applied or for the purpose of improving various performances of the cured product after curing treatment.

[0063] The state of the curable composition of the present embodiment is not particularly limited, but may be, for example, a liquid. The curable composition of the present embodiment may also be a solid.

[0064] The curable composition of the present embodiment can be produced, for example, by mixing the polyimide precursor, the oxazoline compound, and, if necessary, the polar solvent described above.

[0065] As described above, the polyimide precursor can be obtained by partially hydrolyzing a polyimide resin. A method for producing such a curable composition includes, for example, a hydrolysis step of hydrolyzing a polyimide resin in the presence of water and an alkali compound to prepare a polyimide precursor, and a mixing step of mixing the polyimide precursor with an oxazoline compound so that the amount of oxazoline rings of the oxazoline compound per 1 mol of the repeating structural unit of the polyimide precursor is 0.20 mol to 1.60 mol to prepare a curable composition.

[0066] The polyimide resin before hydrolysis is not particularly limited as long as it produces the polyimide precursor represented by the above general formula (1). Specific examples of the molecular structure of polyimide resins are shown in formulas (A) to (J) in Figure 1.

[0067] The polyimide resin to be partially hydrolyzed in the hydrolysis step may be, for example, a discarded polyimide resin. Specifically, a molded product of a polyimide resin, more specifically, a discarded polyimide resin film, may be used.

[0068] In the hydrolysis step, the polyimide resin can be hydrolyzed at a temperature of, for example, 50° C. to 100° C. The duration of the hydrolysis step is, for example, 1 hour to 24 hours. In the hydrolysis step, for example, sodium hydroxide or potassium hydroxide is used as the alkaline compound.

[0069] In the hydrolysis step, the hydrolysis treatment may be continued, for example, until the molecular weight of the polyimide precursor falls within the above-mentioned predetermined range, or until the ratio of the heights of the absorption peaks in the infrared spectrum obtained by infrared absorption analysis of the polyimide precursor falls within the above-mentioned predetermined range.

[0070] In the hydrolysis step, for example, the hydrolysis treatment can be carried out until the mass average molecular weight of the polyimide precursor reaches 1,000 or more and 100,000 or less. In addition, in the hydrolysis step, for example, the hydrolysis treatment can be carried out until the Mw / Mn ratio of the polyimide precursor determined by the above-mentioned GPC measurement reaches 1 or more and 4 or less.

[0071] For example, in the hydrolysis step, in the infrared spectrum of the polyimide precursor, a wave number of 1375 cm due to the imide group is observed. －１ The wave number of 1500 cm due to the benzene ring relative to the height of the absorption peak P2 －１The hydrolysis treatment can be carried out until the ratio of the height of the absorption peak P1 in the infrared spectrum of the polyimide precursor (P1 / P2) to the height of the absorption peak P1 in the infrared spectrum of the polyimide precursor (P1 / P2) becomes 2 or more and 10 or less. －１ The wave number of 1500 cm due to the benzene ring relative to the height of the absorption peak P3 －１ The hydrolysis treatment can be carried out until the ratio of the height of the absorption peak P1 in the infrared spectrum of the polyimide precursor (P1 / P3) to the height of the absorption peak P1 in the infrared spectrum of the polyimide precursor (P1 / P3) becomes 2 or more and 20 or less. －１ The wave number of 1500 cm due to the benzene ring relative to the height of the absorption peak P4 －１ The hydrolysis treatment can be carried out until the ratio of the height of the absorption peak P1 in the sample (P1 / P4) to the height of the absorption peak P1 in the sample (P1 / P4) becomes 2 or more and 20 or less.

[0072] After the hydrolysis step, the subsequent mixing step may be carried out with the polyimide precursor in a dissolved state. On the other hand, after the hydrolysis step, the solvent used in the hydrolysis treatment may be evaporated by a drying treatment or the like in order to powder the polyimide precursor.

[0073] The polyimide precursor obtained by the above-described hydrolysis treatment is represented, for example, by the following formula (3). The polyimide precursor represented by formula (3) has structural units a and b in its molecule. Structural unit a has a portion where the imide group has been hydrolyzed (a portion where the cyclic imide structure has become an amic acid structure). Structural unit b has a portion where the imide group has not been hydrolyzed and remains. In formula (3), structural unit a (left portion) and structural unit b (right portion) correspond to the repeating structural units described above, respectively. The polyimide precursor may have two carboxy groups at the ends of the molecular chain. These carboxy groups may be generated by complete hydrolysis of the cyclic imide structure by the hydrolysis treatment.

[0074] In the hydrolysis step, by increasing the temperature or increasing the pH of the reaction solution (making the reaction solution more alkaline), the weight-average molecular weight of the polyimide precursor can be reduced, and the proportion of amic acid structures in the polyimide precursor can be increased. Furthermore, by extending the hydrolysis reaction time in the hydrolysis step, the above-mentioned Mw / Mn can be reduced.

[0075] In the mixing step, for example, the powdered polyimide precursor prepared as described above, an oxazoline compound, and a solvent are mixed. A general mixing device can be used. If necessary, the mixture may be heated while being stirred.

[0076] In the mixing step, the polyimide precursor and the oxazoline compound are mixed so that the oxazoline compound is mixed in the specific ratio as described above relative to 100 parts by mass of the polyimide precursor.

[0077] In the mixing step, for example, a solution containing at least water, the polyimide precursor, and an oxazoline compound is prepared. Alternatively, for example, the curable composition may be produced by mixing the powdery polyimide precursor with an aqueous solution in which the oxazoline compound is dissolved. It is preferable to prepare a solution-type curable composition by dissolving the polyimide precursor and the oxazoline compound in a solvent in the mixing step, and it is more preferable to prepare a curable composition in the form of an aqueous solution. The curable composition in the form of a solution (particularly an aqueous solution) can be applied by various methods. This has the advantage of broadening the range of uses for the curable composition. Furthermore, the curable composition in the form of an aqueous solution has the advantage of being non-flammable, thereby increasing safety.

[0078] The curable composition is used by being applied to an adherend such as an electric wire, a film, a flexible circuit board, or a semiconductor. The applied curable composition is subjected to a curing treatment such as heating to become a cured product (specifically, a cured resin, etc.). The curable composition may be, for example, a curable composition for application to fibers, a curable composition for application to resin films, a curable composition for application to resin molded products, or a curable composition for application to metals. The curable composition that becomes a cured product (e.g., a cured resin molded product) may be used, for example, for films, paints, electrical insulating materials, heat-resistant parts, heat-resistant containers, fibers, etc.

[0079] The method for using the curable composition (in other words, the method for producing a cured product by curing the curable composition) is not particularly limited. In such a method, for example, the curable composition can be applied to an object to be coated (an adherend such as a substrate) or impregnated into the object to be coated, followed by curing to form a cured product (such as a cured coating film). Typical application methods for the curable composition include, for example, spray coating, dip coating, spin coating, die coating, and gravure coating. The solvent contained in the curable composition can be volatilized by subjecting the curable composition applied to the substrate and the substrate to a heat treatment. The heat treatment is not particularly limited, and typical methods such as hot air heating and infrared heating can be used. Heating conditions for the heat treatment are, for example, 60°C to 100°C for 30 minutes. Subsequently, a heat treatment at a higher temperature can be performed to further promote thermal curing. Such a heat treatment at a higher temperature is also not particularly limited, and can be performed by a typical method. The temperature condition for the high-temperature heat treatment may be 200°C or higher, preferably 300°C to 400°C. The heating time may be 10 minutes to 5 hours, preferably 10 to 60 minutes. During this high-temperature heat treatment, the volatilization of the solvent further progresses, and the imidization of the polyimide resin precursor further progresses, resulting in the formation of a cured film. The heat treatment can also be carried out in an inert gas atmosphere or under reduced pressure conditions.

[0080] The curable composition and cured product of the present invention are as exemplified above, but the present invention is not limited to the above-exemplified embodiments. In addition, in the present invention, various forms adopted in general curable compositions and the like can be adopted within a range that does not impair the effects of the present invention.

[0081] The present specification discloses the following: (1) A curable composition comprising: a polyimide precursor in which a portion of the imide bonds of a polyimide resin are hydrolyzed; and an oxazoline compound having an oxazoline ring in the molecule, wherein the amount of the oxazoline ring in the oxazoline compound relative to 1 mol of the repeating structural unit of the polyimide precursor is 0.20 mol to 1.60 mol. (2) The curable composition according to (1) above, wherein the oxazoline compound is either a low-molecular-weight oxazoline compound having a molecular weight of less than 1,000 or a high-molecular-weight oxazoline compound having a molecular weight of 1,000 or more. (3) The curable composition according to (1) or (2) above, further comprising an aqueous solvent. (4) The curable composition according to (3) above, wherein the aqueous solvent comprises water and a hydrophilic organic solvent soluble in water at any ratio relative to the water. (5) The curable composition according to (4) above, wherein the hydrophilic organic solvent comprises at least one of dimethylaminoethanol and diethanolamine. (6) The curable composition according to any one of (1) to (5), wherein the molar ratio of the number of nitrogen atoms per benzene ring in the molecule of the polyimide precursor is 0.2 or more and 1.0 or less. (7) The polyimide precursor has a benzene ring in its molecule, and the infrared spectrum obtained by infrared spectroscopic analysis of the polyimide precursor has a wavelength of 1500 cm due to the benzene ring. －１ The absorption peak P1 and the imide group have a wave number of 1375 cm －１ and an absorption peak P2 at a wave number of 1500 cm due to the benzene ring, and a ratio (P1 / P2) of the height of the absorption peak P1 to the height of the absorption peak P2 is 2 or more and 10 or less. (8) The curable composition according to any one of the above (1) to (6), wherein the polyimide precursor has a benzene ring in its molecule, and the infrared spectrum obtained by infrared spectroscopic analysis of the polyimide precursor has a wave number of 1500 cm due to the benzene ring. －１The absorption peak P1 is due to the amide group at a wave number of 1600 cm －１ and an absorption peak P3 at a wavenumber of 1500 cm due to the benzene ring, and a ratio (P1 / P3) of the height of the absorption peak P1 to the height of the absorption peak P3 is 2 or more and 20 or less. (9) The curable composition according to any one of the above (1) to (7), wherein the polyimide precursor has a benzene ring in its molecule, and the infrared spectrum obtained by infrared spectroscopic analysis of the polyimide precursor has a benzene ring at a wavenumber of 1500 cm due to the benzene ring. －１ The absorption peak P1 and the carboxyl group have a wave number of 1413 cm －１ and an absorption peak P4 of (1) above, wherein the ratio of the height of the absorption peak P1 to the height of the absorption peak P4 (P1 / P4) is 2 or more and 20 or less. (10) The curable composition according to any of (1) to (9) above, which is a curable composition for coating fibers, a curable composition for coating resin films, a curable composition for coating resin molded products, or a curable composition for coating metals. (11) A cured product obtained by curing the curable composition according to any of (1) to (10) above. (12) The cured product according to (11) above, which is used for a film, a paint, an electrical insulating material, a heat-resistant part, a heat-resistant container, or a fiber. (13) A method for using the curable composition, comprising applying the curable composition according to any of (1) to (10) above to an object to be coated or impregnating the object with the curable composition, and then curing the composition to form a cured product.

[0082] The present invention will now be described in more detail with reference to examples, but the present invention is not limited to these examples.

[0083] Each raw material used to produce the curable composition is described below. <Major Raw Materials> (A-1) Polyimide Precursor (Powder) - Polyimide Resin Molded Product Before Hydrolysis: Polyimide Resin Product Name "Kapton H" (manufactured by DuPont Toray Co., Ltd.) Polyimide Resin with the Molecular Structure Shown in Formula (A) in Figure 1 - Mass Average Molecular Weight (Mw) After Hydrolysis: 11,000 - Mw / Mn After Hydrolysis: 1.6 (A-1) was prepared as follows. First, the polyimide resin molded product was finely cut with scissors into pieces approximately 5 mm square to obtain a pulverized polyimide resin. Meanwhile, in a 1000 mL container equipped with a stirrer, 40 g of potassium hydroxide was dissolved in 600 g of ion-exchanged water to prepare an alkaline aqueous solution. 100 g of pulverized polyimide resin was added to this alkaline aqueous solution, and hydrolysis was carried out at 80 to 90°C for 3 hours to obtain a crude polyimide precursor solution. Subsequently, the crude polyimide precursor solution was neutralized to obtain a precipitate. The precipitate was filtered and washed with ion-exchanged water. After removing the neutralized salt compound and excess acidic compound, the precipitate was dried and pulverized to obtain a powder. Thus, a powdery polyimide precursor was prepared. Reference: (A-2) Polyimide precursor (powder form) Polyimide resin molded product before hydrolysis: Polyimide resin Product name "Kapton H" (manufactured by DuPont Toray Co., Ltd.) Hydrolysis conditions: Different from the hydrolysis conditions above Reference: (A-3) Polyimide precursor (powder form) Polyimide resin molded product before hydrolysis: Polyimide resin Product name "Kapton EN" (manufactured by DuPont Toray Co., Ltd.) Polyimide resin with a molecular structure shown in formula (J) in Figure 1 Hydrolysis conditions: Different from the hydrolysis conditions above (B) Oxazoline compound (B-1) Polymeric oxazoline compound (acrylic polymer containing oxazoline structure) Product name "Epocross WS-500" (manufactured by Nippon Shokubai Co., Ltd.) Dissolved in an aqueous solution containing 1-methoxy-2-propanol (solid content 39% by mass) Amount of oxazoline group: 4.5 [mmol / g,(B-1) Polymeric oxazoline compound (acrylic polymer containing oxazoline structure) Product name: "EPOCROS WS-700" (manufactured by Nippon Shokubai Co., Ltd.) Dissolved in water (solid content 25% by mass) Amount of oxazoline group: 4.5 [mmol / g, per solid content] Molecular weight: Mn = 20,000, Mw = 40,000, Mw / Mn = 2.0 (B-2) Polymeric oxazoline compound (acrylic polymer containing oxazoline structure) Product name: "EPOCROS WS-700" (manufactured by Nippon Shokubai Co., Ltd.) Dissolved in water (solid content 25% by mass) Amount of oxazoline group: 4.5 [mmol / g, per solid content] Molecular weight: Mn = 20,000, Mw = 40,000, Mw / Mn = 2.0 (B-3) Polymeric oxazoline compound (acrylic polymer containing oxazoline structure) Product name: "EPOCROS WS-300" (manufactured by Nippon Shokubai Co., Ltd.) Dissolved in water (solid content 10% by mass) Amount of oxazoline group: 7.7 [mmol / g, per solid content] Molecular weight: Mn = 40,000, Mw = 120,000, Mw / Mn = 3.0 (C) Substitute compound for (B) above (comparative compound) (C-1) Polycarbodiimide (polycarbodiimide with hydrophilic segments added) Product name: "Carbodilite V-02-L2" (manufactured by Nisshinbo Chemical Inc.) Dissolved in aqueous solvents (solid content: 40% by mass) (C-2) Polycarbodiimide (polycarbodiimide with hydrophilic segments added) Product name: "Carbodilite V-02" (manufactured by Nisshinbo Chemical Inc.) Dissolved in aqueous solvents (solid content: 40% by mass) (C-3) Polycarbodiimide (polycarbodiimide with hydrophilic segments added) Product name: "Carbodilite V-04" (manufactured by Nisshinbo Chemical Inc.) Dissolved in aqueous solvents (solid content: 40% by mass) (C-4) Epoxy resin (emulsified state, solid content: 70% by mass) Product name: "JER W2821R70" (manufactured by Mitsubishi Chemical Corporation) (C-5) Epoxy resin (emulsified state, solid content: 67% by mass) Product name: "JER W3435R67" (Mitsubishi Chemical Corporation),

[0084] [GPC Chart of Polyimide Precursor Produced by Hydrolysis] FIG. 2 shows a GPC chart of the polyimide precursor (A-1), which is a hydrolyzate of a polyimide resin, measured under the gel permeation chromatography (GPC) measurement conditions described above.

[0085] [IR Chart of Polyimide Resin Before Hydrolysis] The infrared spectrum (IR chart) of the polyimide resin before hydrolysis of the polyimide precursor (A-1) was analyzed by infrared spectroscopy under the above-mentioned analytical conditions and by the above-mentioned analytical method is shown in Fig. 3. Furthermore, the infrared spectrum (IR chart) of the polyimide resin before hydrolysis of the polyimide precursor (A-3) was analyzed in the same manner is shown in Fig. 4.

[0086] [IR Chart (Reference) of Polyimide Precursor Produced by Hydrolysis] The infrared spectrum (IR chart) of polyimide precursor (A-1), a hydrolyzate of polyimide resin, analyzed by infrared spectroscopy under the above-mentioned analytical conditions and method is shown in Figure 5. The infrared spectrum (IR chart) of polyimide precursor (A-2), a hydrolyzate of polyimide resin, analyzed in the same manner is shown in Figure 6. The infrared spectrum (IR chart) of polyimide precursor (A-3), a hydrolyzate of polyimide resin, analyzed in the same manner is shown in Figure 7.

[0087] Based on Figure 5, the ratios of the heights of the above-mentioned peaks were calculated, and the results were as follows: (P1 / P2): 4.04 (P1 / P3): 4.95 (P1 / P4): 5.35 Based on Figure 6, the ratios of the heights of the above-mentioned peaks were calculated, and the results were as follows: (P1 / P2): 2.89 (P1 / P3): 8.00 (P1 / P4): 10.46 Based on Figure 7, the ratios of the heights of the above-mentioned peaks were calculated, and the results were as follows: (P1 / P2): 2.27 (P1 / P3): 9.08 (P1 / P4): 5.89

[0088] (Examples 1 to 9, Comparative Examples 1 to 13) The above-mentioned raw materials were mixed in the amounts shown in the tables below, and (A-1) was dissolved to prepare curable compositions for each Example and Comparative Example. The formulation of each curable composition is shown in Table 1. Each curable composition was prepared so that the total amount (solid content) of (A) and (B) was approximately 20% by mass. Specifically, 68.1 g of ion-exchanged water and 11.6 g of diethanolamine were placed in a container equipped with a stirrer and heated to 60°C. Next, 20 g of the above-mentioned (A) polyimide precursor was added and stirred for 60 minutes. Subsequently, the specified amounts of (B) oxazoline ring-containing compound or (C) comparative compound, and additives (leveling agent, stabilizer) were added, and ion-exchanged water was also added as necessary to prepare the curable composition. In each table, the amounts of (B) and (C) are shown in terms of solid content.

[0089] <Solution Properties of Composition> A solution prepared according to the formulation of the curing composition (Table 1) was stored at room temperature for 2 days, after which the solution properties of the composition were confirmed.

[0090]

[0091] <Curing Treatment> The curing composition of each Example and Comparative Example was applied using a bar coater to a polyimide film (Kapton H, manufactured by DuPont-Toray Co., Ltd.) and a stainless steel plate (SUS304, manufactured by TP Giken Co., Ltd.) After application, the coating was pre-dried at 60°C for 30 minutes, and then cured by heating at 300°C for 30 minutes to produce a cured coating film (cured product) with a film thickness of 20 μm.

[0092] <Uniformity of cured coating film> The curable composition of each example and comparative example was applied onto aluminum foil (manufactured by UACJ Foil Co., Ltd.) using a bar coater. After application, the composition was pre-dried at 60°C for 30 minutes, and then cured by heating at 300°C for 30 minutes to produce a cured coating film (cured product) with a film thickness of 20 μm. The uniformity of the cured coating film was judged by visual observation. Good (◯): Brown, transparent film Fairly good (Δ): Slightly cloudy Poor (×): Cloudy

[0093] <Flexibility (Flexibility) of Cured Coating Film> Each example and comparative example was placed with the cured coating film side facing up and folded 180 degrees so that the cured coating film side was on the outside, and then returned to its original flat state. Adhesive tape (Nichiban Co., Ltd.'s "Cellotape (registered trademark) CT405AP-24") was applied to the coating film at the folded portion and then peeled off. Flexibility was measured by measuring the width of the peeled coating film, and this was used as an index of the flexibility of the cured coating film. Note that a polyimide film was used as the adherend. Good (◯): 1 mm or less Fairly good (△): 1 to 2 mm Poor (×): 2 mm or more

[0094] <Adhesion of cured coating film> The cured coating film of each example and comparative example was measured for adhesion by making grid-like cuts with a cutter knife and then peeling using the adhesive tape. The measurement was carried out in accordance with JIS K5600-5-6:1999 General Test Methods for Paints, Part 5: Mechanical Properties of Coatings, Section 6: Adhesion (Cross-cut Method, so-called grid-cut test). Polyimide film and SUS were used as adherends. Adhesion was evaluated according to the following criteria: Good (◯): 90-100 / 100 remaining Fair (△): 50-90 / 100 remaining Poor (×): 0-50 / 100 remaining

[0095] The results of evaluating the above-mentioned performance of each curing composition are shown in Table 2. Note that, for the cured coating films of Comparative Examples 4 to 7, it was confirmed that the uniformity was not good, so other evaluations (flexibility, adhesion) were not performed.

[0096]

[0097] The reason why the cured products (cured coating films) of the curing compositions of the examples exhibit good performance is believed to be as follows. By subjecting the curing compositions of the examples to a curing treatment such as heating, at least a portion of the multiple amic acid structures in the polyimide precursor are imidized. This results in the formation of a new cyclic polyimide structure. Furthermore, carboxy groups in some of the amic acid structures of the polyimide precursor and carboxy groups at the molecular chain terminals of the polyimide precursor can form amide ester bonds with the oxazoline rings of the oxazoline compound. This allows a relatively large amount of the oxazoline compound to be bonded to the polyimide precursor. The remaining oxazoline rings that are not bonded to carboxy groups have affinity for polar groups present on the surface of the adherend, allowing the cured products (cured resins, cured coating films, etc.) obtained by curing the curing compositions to exhibit good adhesion to the adherend. Furthermore, cracking of the cured product can be suppressed, and the cured product can be flexible. The cured product also exhibits good uniformity.

[0098] As can be seen from the results in Tables 1 and 2, the cured coating films obtained by curing the curable compositions of the Examples were excellent in terms of uniformity, had good adhesion to the adherend, and were also excellent in terms of flexibility (i.e., flexibility). That is, the cured coating films obtained by curing the curable compositions of the Examples simultaneously possessed uniformity, flexibility, and adhesion to the adherend. It is believed that with curable compositions that did not contain an oxazoline compound or curable compositions containing less than a specific amount of oxazoline compound, the cured coating film cured by the curing treatment was unable to relieve its internal stress, resulting in reduced adhesion and flexibility of the cured coating film. Furthermore, it is believed that when the content of the oxazoline compound was greater than the specified amount, the compatibility between the polyimide precursor and the oxazoline compound decreased, resulting in reduced uniformity of the cured coating film. It can also be said that the curable compositions containing the polyimide precursors (A-2) and (A-3) shown above for reference can produce excellent cured coating films.

[0099] The curable composition of the present invention is used by being applied to, for example, metals, resin moldings, fibers, and films in order to produce cured products having properties such as heat resistance, electrical insulation, and chemical resistance.

Claims

1. a polyimide precursor in which a portion of the imide bond of a polyimide resin is hydrolyzed; and an oxazoline compound having an oxazoline ring in the molecule, the amount of the oxazoline ring of the oxazoline compound relative to 1 mol of the repeating structural unit of the polyimide precursor is 0.20 mol or more and 1.60 mol or less.

2. The curable composition according to claim 1 , wherein the oxazoline compound is a polymer compound.

3. The curing composition according to claim 1 or 2, further comprising an aqueous solvent.

4. the polyimide precursor has a benzene ring in its molecule, The infrared spectrum of the polyimide precursor obtained by infrared spectroscopy showed a wave number of 1500 cm due to the benzene ring. －１ The absorption peak P1 due to the imide group and the wave number 1375 cm －１ and an absorption peak P2 of The curing composition according to claim 1 or 2, wherein a ratio (P1 / P2) of a height of the absorption peak P1 to a height of the absorption peak P2 is 2 or more and 10 or less.

5. A cured product obtained by curing the curing composition according to claim 1 or 2.