Curing 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

Cross-reference of related applications

[0001] This application claims priority to Japanese Patent Application No. 2021-066611, which is incorporated into the description of this application by reference. [Technical Field]

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

[0003] Conventionally, as curing compositions that harden by a curing treatment, curing compositions are known that include, for example, a polyimide precursor in which some of the imide bonds of a polyimide resin have been hydrolyzed. In this type of curing composition, the polyimide precursor has multiple cyclic imide structures and multiple amic acid structures (carboxyl groups and amide groups) in its molecule that can form cyclic imide structures through an imidation reaction. When this type of curing composition is cured, the amic acid structures in the polyimide precursor undergo imidation (dehydration condensation reaction) to form imide bonds, thereby producing a cured product. Since the resulting cured product has heat resistance and electrical insulation properties, this type of curing composition is used to form cured products such as insulating coatings through a curing treatment.

[0004] As an example of a curing composition as described above, there is a known curing composition comprising 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 in the range of 10 to 100 parts by mass per 100 parts by mass of polyimide precursor, the amount of blocked isocyanate compound is in the range of 10 to 100 parts by mass, and when cured under heating conditions of 120°C for 60 minutes, the 10% by mass decrease temperature measured by a thermobalance is 200°C or higher (Patent Document 1). The curing composition described in Patent Document 1 can be cured by a curing treatment at a relatively low temperature, can provide good adhesion (bonding) of the cured product to the adherend, and can provide good heat resistance of the cured product.

[0005] Furthermore, the curing composition described above is, for example, a curing composition comprising an imide group-containing compound (polyimide precursor) obtained by partially hydrolyzing a polyimide molded product, an aqueous solvent, and an amine compound having a boiling point of 85 to 145°C, wherein the imide group-containing compound, in an infrared spectroscopy chart, has a wavenumber of 1500 cm² originating from the benzene ring. -1 The absorption peak at this location and the wavenumber 1375 cm² originating from the imide group. -1 It has an absorption peak and a wavenumber of 1500 cm, originating from the benzene ring. -1 The height of the absorption peak at this point is denoted as S1, and the wavenumber originating from the imide group is 1375 cm⁻¹. -1 A curing composition is known in which, when the height of the absorption peak is denoted as S2, the ratio of S1 / S2 is in the range of 2 to 10 (Patent Document 2). The curing composition described in Patent Document 2 contains an aqueous solvent such as water, has good stability, and can be cured by a curing treatment at a relatively low temperature. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 2017-014386 [Patent Document 2] Japanese Patent No. 6402283 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, even when the curing composition described in Patent Document 1 is subjected to a curing treatment, the resulting cured product does not necessarily have good uniformity. Furthermore, even when the curing 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 curing the above-described composition for curing to have properties such as uniformity, flexibility, and adhesion to the adherent body. Therefore, there is a demand for a composition for curing that can produce a cured product having uniformity, flexibility, and adhesion to the adherent body.

[0008] In view of the above problems, demands, etc., an object of the present invention is to provide a composition for curing that can produce a cured product having uniformity, flexibility, and adhesion to the adherent body.

Means for Solving the Problems

[0009] The composition for curing according to the present invention includes a polyimide precursor in which a part of the imide bonds of a polyimide resin is hydrolyzed, and an oxazoline compound having an oxazoline ring in the molecule, and the amount of the oxazoline ring of the oxazoline compound is 0.20 mol or more and 1.60 mol or less with respect to 1 mol of the repeating structural unit of the polyimide precursor.

[0010] In the composition for curing according to the present invention, the oxazoline compound may be a high molecular compound. Further, the composition for curing according to the present invention may further contain an aqueous solvent. In the composition for curing according to the present invention, the polyimide precursor has a benzene ring in the molecule, and the infrared absorption spectrum obtained by infrared spectroscopic analysis of the polyimide precursor shows an absorption peak P1 at a wave number of 1500 cm -1 due to the benzene ring and an absorption peak P2 at a wave number of 1375 cm -1 due to the imide group, and the ratio (P1 / P2) of the height of the absorption peak P1 to the height of the absorption peak P2 may be 2 or more and 10 or less.

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

Brief Description of the Drawings

[0012] [Figure 1] A diagram showing the schematic molecular structure of various polyimide resins before hydrolysis. [Figure 2] GPC chart of polyimide precursors (hydrolyzed products). [Figure 3] An IR chart of an example of polyimide resin before hydrolysis. [Figure 4] IR chart of another example of polyimide resin before hydrolysis. [Figure 5] An IR chart of an example of a polyimide precursor (hydrolyzate). [Figure 6] An IR chart of another example of a polyimide precursor (hydrolyzate). [Figure 7] IR chart of other polyimide precursors (hydrolyzed products). [Modes for carrying out the invention]

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

[0014] The curing composition of this embodiment comprises a polyimide precursor in which some of the imide bonds of the polyimide resin have been hydrolyzed, It includes an oxazoline compound having an oxazoline ring in its molecule, The amount of the oxazoline ring in the oxazoline compound relative to 1 mole of the repeating structural units of the polyimide precursor is 0.20 mol or more and 1.60 mol or less. The curing composition of this embodiment makes it possible to produce a cured product that possesses uniformity, flexibility, and adhesion to the 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 carboxyl group and an amide group. The carboxyl group and 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). A new cyclic imide structure is created by such imidization. The amic acid structure is a structure in which the imide bonds constituting the cyclic imide structure are hydrolyzed, and the cyclic imide structure is a structure in which the carboxyl group and amide group in the amic acid structure are imidized (dehydration condensation reaction). Furthermore, since the above-mentioned polyimide precursor is a hydrolysate of polyimide resin, it may have carboxyl groups at the end of its molecular chain.

[0016] Commercially available polyimide resins can be used before hydrolysis. Examples of such polyimide resins include the "Kapton" series (manufactured by Toray DuPont), the "Upirex" series (manufactured by Ube Industries), and "Aurum" (manufactured by Mitsui Chemicals). 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 conventionally discarded polyimide resin films, etc., as the polyimide resin before hydrolysis. This allows for the active recycling of polyimide resin and enables the production of a cured product that possesses uniformity, flexibility, and adhesion to the substrate.

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

[0018] [ka]

[0019] The repeating structural units in the polyimide precursor described above correspond to the repeating structural units in the polyimide resin before hydrolysis. More 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 described above also has similar repeating structural units in its molecule. However, in the polyimide precursor produced by hydrolysis, a part of the cyclic imide structure becomes an amic acid structure due to hydrolysis, so both the part in parentheses on the left side of general formula (1) and the part 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 units of polyimide resin before hydrolysis are, for example, the part in parentheses on the right side of the general formula (1) above. The number of moles of the repeating structural units can be calculated by dividing the mass of the polyimide resin by the molecular weight of the part in parentheses on the right side. Furthermore, the number of nitrogen atoms in the repeating structural unit described above is 2. Therefore, the amount of the oxazoline ring in the oxazoline compound per 1 mole of the repeating structural unit of the polyimide precursor can also be expressed as the amount per 2 moles of nitrogen atoms in the polyimide precursor.

[0020] As described above, the above polyimide precursor is a hydrolysate obtained by hydrolyzing some of the multiple imide bonds in the polyimide resin. Therefore, the above polyimide precursor has both an amical structure (the part indicated by β) represented on the left side of general formula (1) and a cyclic polyimide structure (the part indicated by α) represented on the right side in its molecule.

[0021] The polyimide precursor described above preferably has an aromatic structure (particularly a benzene ring structure) in its molecule. Having a benzene ring structure in the polyimide precursor has the advantage that the cured product exhibits higher heat resistance.

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

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

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

[0025] The average molecular weight (weight-average molecular weight) of the polyimide precursor may be, for example, 1,000 or more and 100,000 or less. Preferably, the average molecular weight (weight-average molecular weight) of the polyimide precursor is 3,000 or more, and more preferably 5,000 or more. Such an average molecular weight (weight-average molecular weight) is preferably 60,000 or less, and more preferably 30,000 or less. Having the weight-average molecular weight of the polyimide precursor within the above range results in better solubility in the solvent described later.

[0026] The mass-average molecular weight of polyimide precursors is determined by gel permeation chromatography (GPC) measurement. The details of the GPC measurement conditions are as follows: Device name: Tosoh Corporation product name "HLC-8320GPC" Detector: Differential refractive index (RI) detector Column: Styrene-divinylbenzene (average particle size 3 μm, average pore size 2 nm) 1 column Column temperature: 40℃ Standard material for creating calibration curves: polyethylene glycol Data processing software: Software included with the above-mentioned device. 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 above GPC measurement is preferably 1.0 or higher, and more preferably 1.5 or higher. Such an Mw / Mn is preferably 3.0 or lower, and more preferably 2.5 or lower. Having a polyimide precursor with a Mw / Mn ratio between 1.0 and 3.0 has the advantage of resulting in more uniform and stable physical properties in the cured product. In the hydrolysis reaction products of polyimide resin, i.e., the polyimide precursors produced by the hydrolysis of polyimide resin, the Mw / Mn ratio can be within the range described above.

[0028] In the infrared (IR) spectrum (chart) obtained by infrared spectroscopy analysis of a polyimide precursor, absorption peaks like the following can be observed. For example, in the hydrolysis reaction product of polyimide resin, i.e., the polyimide precursor produced by the hydrolysis of polyimide resin, absorption peaks like the following can be observed. As is commonly understood in the field of infrared spectroscopy, the wavenumbers shown below are values ​​that define the absorption peaks around that wavenumber. In other words, the wavenumbers shown below do not only indicate absorption peaks that have their peak peaks at that wavenumber.

[0029] Infrared spectroscopy of polyimide precursors is performed under the following measurement conditions. Infrared spectroscopy can be performed under these conditions using commercially available measuring instruments. Device Name: Fourier Transform Infrared Spectrophotometer (FT / IR-4600 manufactured by JASCO Corporation) Analysis Method: Attenuated Total Reflection (ATR) Measurement Wavenumber Range: 4000 cm -1 ~400 cm -1

[0030] ·Infrared Spectrum (Absorption Peak P1 by Benzene Ring) In the infrared spectrum when the polyimide precursor in this embodiment is subjected to infrared spectroscopic analysis, as shown in FIG. 5 for example, at a wavenumber of 1500 cm -1 (nearby), it shows an absorption peak P1 by the benzene ring.

[0031] ·Infrared Spectrum (Absorption Peak P2 by Imide Group) In the infrared spectrum when the polyimide precursor in this embodiment is subjected to infrared spectroscopic analysis, as shown in FIG. 5 for example, at a wavenumber of 1375 cm -1 (nearby), it shows an absorption peak P2 by the imide group. Since the above polyimide precursor has an imide group in the molecule, the cured product formed by a curing treatment such as heating has good heat resistance.

[0032] ·Infrared Spectrum (Absorption Peak P3 by Amide Group) In the polyimide precursor in this embodiment, as shown in FIG. 5 for example, at a wavenumber of 1600 cm -1 (nearby), it shows an absorption peak P3 by the amide group (-NHCO-). Since the above polyimide precursor has an amide group in the molecule, it can be cured at a relatively low temperature.

[0033] ·Infrared Spectrum (Absorption Peak P4 by Carboxy Group) In the polyimide precursor in this embodiment, as shown in FIG. 5 for example, at a wavenumber of 1413 cm -1 (nearby), it shows an absorption peak P4 by the carboxy group (-COOH). Because the above polyimide precursor has a carboxyl group in its molecule, it has good solubility in the solvents described later, and the cured product after curing treatment can have good adhesion.

[0034] • Infrared spectroscopy spectrum (absorption peak P5 due to carbonyl group) The polyimide precursor in this embodiment is, for example, as shown in Figure 5, at a wavenumber of 1710 cm⁻¹. -1 (Nearby) shows an absorption peak P5 due to the carbonyl group (-CO-). In this embodiment, the polyimide precursor has a carbonyl group in its molecule and therefore exhibits good solubility in the solvent described later.

[0035] When a polyimide precursor is obtained by partially hydrolyzing a polyimide resin, the amount of imide groups (height of absorption peak), amide groups (height of absorption peak), and carboxyl groups (height of 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 determined by comparing the height of the absorption peak of each of the above functional groups with the height of the absorption peak due to the benzene ring, which does not change its chemical structure even with 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 group, the height of the absorption peak P3 due to the amide group, and the height of the absorption peak P4 due to the carboxyl group can 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 ratios of the heights of the absorption peaks mentioned above are calculated as follows: The horizontal axis represents the wavenumber [cm] as a result of infrared spectroscopy analysis of the polyimide precursor. -1 Prepare an infrared spectral (IR chart) where the vertical axis represents transmittance [%] (the upper side is 100% transmittance). 1400~1430 cm⁻¹ -1Draw a horizontal line passing through the peaks of the upwardly protruding peaks that appear nearby, and use this horizontal line as the baseline. Then, find the absolute value of the difference between the height [%] of each absorption peak and the height [%] of the baseline. For example, if the baseline height [%] is 90[%], the height of absorption peak P1 is 70[%], and the height of absorption peak P2 is 80[%], the ratio of the heights of the absorption peaks (P1 / P2) is calculated by 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 spectral spectrum of the polyimide precursor in this embodiment, the wavenumber 1375 cm⁻¹ is due to the imide group. -1 The height of the absorption peak P2 is related to the benzene ring at wavenumber 1500 cm². -1 The ratio of the heights of the absorption peaks P1 (P1 / P2) in this mixture is preferably 2 or more, and may be 3 or more. Furthermore, this ratio (P1 / P2) is preferably 10 or less, more preferably 9 or less, and even more preferably 5 or less. The above polyimide precursor can be cured at a lower temperature because its height ratio (P1 / P2) is within the range described above. Furthermore, it can exhibit better solubility in the solvents described later. Additionally, the cured product after 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 spectral spectrum of the polyimide precursor in this embodiment, the amide group at wavenumber 1600 cm⁻¹ -1 The height of the absorption peak P3 is related to the benzene ring at wavenumber 1500 cm². -1The ratio of the heights of the absorption peaks P1 in the given solution (P1 / P3) is preferably 2 or greater, and more preferably 3 or greater. Furthermore, this ratio (P1 / P3) is preferably 20 or less, more preferably 12 or less, and even more preferably 10 or less. The above polyimide precursor can exhibit better solubility in the solvent described later because its height ratio (P1 / P3) is within the range described above. Furthermore, the cured product after curing treatment can exhibit 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 carboxyl group: P1 / P4 In the infrared spectral spectrum of the polyimide precursor in this embodiment, the wavenumber 1413 cm⁻¹ is due to the carboxyl group. -1 The height of the absorption peak P4 is related to the benzene ring at wavenumber 1500 cm². -1 The ratio of the heights of the absorption peaks P1 (P1 / P4) in this mixture is preferably 2 or more, and more preferably 3 or more. Furthermore, this ratio (P1 / P4) is preferably 20 or less, more preferably 12 or less, and even more preferably 11 or less. The above polyimide precursor can exhibit better solubility in the solvent described later because its height ratio (P1 / P4) is within the range described above. Furthermore, the cured product after curing treatment can exhibit better adhesion, uniformity, and flexibility.

[0040] The benzene ring in the above polyimide precursor may be an unsubstituted benzene ring, or it may be a substituted benzene ring in which hydrogen atoms bonded to each of the carbon atoms constituting the benzene ring are replaced by substituents. The substituents of 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 above polyimide precursor is an unsubstituted benzene ring. In other words, it is preferable that the carbon atoms constituting the benzene ring of the polyimide precursor do not have monovalent substituents (such as halogen atoms) bonded to them. In other words, it is preferable that the monovalent group bonded to the carbon atoms constituting the benzene ring is a hydrogen atom (-H).

[0041] As the polyimide precursor, for example, the polyimide precursor shown in formula (3) described later can be used. Specifically, as the polyimide precursor, a partially hydrolyzed product of the product name "Kapton H" (manufactured by Toray DuPont) can be used. For example, the polyimide precursor in this embodiment can be obtained by subjecting a discarded polyimide resin film to hydrolysis treatment.

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

[0043] The oxazoline compound may be a low molecular weight oxazoline compound with a molecular weight of less than 1000, or a high molecular weight oxazoline compound with a molecular weight of 1000 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, or compounds having three oxazoline rings.

[0045] Examples of compounds in which two oxazoline rings are directly bonded together include 2,2'-bis(2-oxazoline) and 2,2'-bis-4-benzyl-2-oxazoline. Examples of compounds in which two oxazoline rings are linked 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-oxazoline-2-yl)pyridine, 2,2'-methylenebis(4-tert-butyl-2-oxazoline), and 2,2'-methylenebis(4-phenyl-2-oxazoline). Examples of compounds containing three oxazoline rings include 1,2,4-tris-(2-oxazoline-2-yl)benzene.

[0046] Examples of high-molecular-weight oxazoline compounds include oxazoline ring-containing acrylic polymers that have an oxazoline ring at the end of their side chains and are polymerized with at least an acrylic acid ester. An oxazoline ring-containing acrylic polymer is a high-molecular-weight compound in which an acrylic acid ester monomer having an oxazoline ring is polymerized as at least one of the monomers. Such an oxazoline ring-containing acrylic polymer may be a homopolymer or a copolymer. Examples of oxazoline ring-containing acrylic polymers include the "Epocross WS" series (manufactured by Nippon Shokubai Co., Ltd.) and the "Epocross K-2000" series (manufactured by Nippon Shokubai Co., Ltd.).

[0047] In oxazoline compounds, it is preferable that the only monovalent group bonded to the carbon atoms constituting the oxazoline ring is a hydrogen atom (-H), as shown in formula (2) below. [ka]

[0048] As the oxazoline compound, high molecular weight oxazoline compounds are preferred because they have better reactivity with polyimide precursors and the cured product after curing treatment can have better storage stability. More preferably, oxazoline ring-containing acrylic polymers having an oxazoline ring at the end of the side chain are preferred.

[0049] The amount of oxazoline groups (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 a polymer oxazoline compound can be calculated based on the ratio of monomer units containing oxazoline groups to other monomer units that make up the polymer oxazoline compound. Such a ratio can be determined, for example, by nuclear magnetic resonance (NMR) analysis. 1 This can be determined based on the peak intensity derived from the oxazoline group and the peak intensity derived from other monomers in the 1H-NMR analysis results.

[0050] Furthermore, the oxazoline compound may be used alone or in combination of two or more types.

[0051] In the curing composition of this embodiment, as described above, the amount of the oxazoline ring of the oxazoline compound per 1 mol of repeating structural units of the polyimide precursor is 0.20 mol or more and 1.60 mol or less. The amount of the oxazoline ring may be 0.30 mol or more, 0.35 mol or more, or 0.39 mol or more. Furthermore, the amount of the oxazoline ring may be 1.50 mol or less, or 1.20 mol or less.

[0052] In the curing composition of this embodiment, the polyimide precursor is preferably a partially hydrolyzed polyimide resin, and the oxazoline compound is preferably a high-molecular-weight oxazoline compound (for example, an oxazoline ring-containing acrylic polymer having an oxazoline ring at the end of its side chain). Polyimide precursors derived from partial hydrolysis contain a higher amount of low-molecular-weight compounds than polyimide precursors composed of monomers, which can result in them becoming hard and brittle after curing. In contrast, the curing composition of this embodiment further contains a high-molecular-weight oxazoline compound, which can suppress the aforementioned hard and brittle properties. Therefore, the adhesion of the cured product to the adherend can be improved.

[0053] The curing composition of this embodiment preferably contains 5 to 95 parts by mass of an oxazoline compound per 100 parts by mass of a polyimide precursor. The curing composition of this embodiment may contain 8 parts by mass or more, or 10 parts by mass or more, of an oxazoline compound per 100 parts by mass of a polyimide precursor. The curing 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 an oxazoline compound per 100 parts by mass of a polyimide precursor.

[0054] The curing composition of this embodiment may contain a solvent. Examples of solvents include nonpolar solvents and polar solvents. Examples of nonpolar solvents include hydrocarbon solvents containing only carbon and hydrogen atoms in their molecules, and chlorine-based solvents such as carbon tetrachloride. The curing 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 that also contains atoms other than carbon and hydrogen atoms (e.g., oxygen atoms, nitrogen atoms) in its molecule. In the curing composition of this embodiment, both the polyimide precursor and the oxazoline compound are dissolved in a 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, halogen solvents, and water.

[0056] Examples of amine-based solvents include ammonia (water), diethylamine, ethylethanolamine, diethanolamine, triethanolamine, monoethanolamine, monopropanolamine, isopropanolamine, triethylamine, tolubutylamine, 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-based solvents include tetrahydrofuran, 1,3-dioxolane, 1,4-dioxane, and methylphenyl ether. Examples of pyrrolidone-based solvents include N-methyl-2-pyrrolidone. Examples of glycol ether solvents include methyl diglyme, ethyl diglyme, methyl triglime, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, propylene glycol monomethyl ether, 1-methoxy-2-propanol, ethylene glycol monoethyl ether, or acetates thereof. Examples of ester solvents include ethyl acetate, butyl acetate, and isopropyl acetate. Examples of alcohol-based 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-based solvents include glycerin. Examples of halogenated solvents include chloroform and dichloromethane.

[0057] As the polar solvent, an aqueous solvent is preferred. The aqueous solvent is water, or a hydrophilic organic solvent that dissolves in water in any amount ratio to water. The curing composition of this embodiment more preferably contains an aqueous solvent among the 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 to include an amine-based solvent in 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, used as amine solvents, have relatively high boiling points, and therefore gradually volatilize when the above-mentioned curable composition is subjected to a heating curing treatment. This allows the cured material to form a more uniform cured film as the curing progresses.

[0060] Multiple types of the above polar solvents may be used in combination. Similarly, multiple types of the above aqueous solvents may be used in combination.

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

[0062] The curing composition described above may further contain, in addition to the components described above, an imidation 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 other resins mentioned above include acrylic resins, fluororesins, epoxy resins, phenolic resins, silicone resins, olefin resins, polyester resins, polyamide resins, and hydrocarbon resins. These components may be added for the purpose of improving the workability of the curing composition during application, or improving various properties of the cured product after the curing treatment.

[0063] The properties of the curing composition in this embodiment are not particularly limited, but for example, it is liquid. However, the curing composition in this embodiment may also be solid.

[0064] The curing composition of this embodiment can be produced, for example, by mixing the above-mentioned polyimide precursor, an oxazoline compound, and, if necessary, the above-mentioned polar solvent.

[0065] As described above, polyimide precursors can be obtained by partially hydrolyzing polyimide resin. A method for producing such a curing composition is, for example, A hydrolysis step to prepare a polyimide precursor by hydrolyzing a polyimide resin in the presence of water and an alkaline compound, The method comprises a mixing step of preparing a curing composition by mixing a polyimide precursor and an oxazoline compound such that the amount of oxazoline rings in the oxazoline compound is between 0.20 mol and 1.60 mol per mol of repeating structural units of the polyimide precursor.

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

[0067] ·Hydrolysis process As the polyimide resin that is partially hydrolyzed in the hydrolysis process, for example, discarded polyimide resin can be used. Specifically, molded articles of polyimide resin, or more specifically, waste polyimide resin film, can be used.

[0068] In the hydrolysis process, the polyimide resin can be hydrolyzed under temperature conditions of, for example, 50°C to 100°C. In the hydrolysis process, the duration of the hydrolysis treatment is, for example, between 1 hour and 24 hours. In the hydrolysis process, for example, sodium hydroxide or potassium hydroxide is used as the alkali compound.

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

[0070] In the hydrolysis process, for example, the hydrolysis treatment can be carried out until the mass-average molecular weight of the polyimide precursor is between 1,000 and 100,000. Furthermore, in the hydrolysis process, for example, the hydrolysis treatment can be carried out until the Mw / Mn ratio of the polyimide precursor, as determined by the GPC measurement described above, is between 1 and 4.

[0071] For example, in the hydrolysis process, the infrared spectral spectrum of the polyimide precursor shows a wavenumber of 1375 cm⁻¹ due to the imide group. -1 The height of the absorption peak P2 is related to the benzene ring at wavenumber 1500 cm². -1 Hydrolysis treatment can be carried out until the ratio of the heights of the absorption peaks P1 (P1 / P2) is between 2 and 10. Furthermore, in the hydrolysis process, the infrared spectral spectrum of the polyimide precursor showed that the amide group was present at wavenumber 1600 cm⁻¹. -1 The height of the absorption peak P3 is related to the benzene ring at wavenumber 1500 cm². -1 Hydrolysis treatment can be carried out until the ratio of the heights of the absorption peak P1 (P1 / P3) is between 2 and 20. Furthermore, in the hydrolysis process, the infrared spectral spectrum of the polyimide precursor showed a wavenumber of 1413 cm⁻¹ due to the carboxyl group. -1 The height of the absorption peak P4 is related to the benzene ring at wavenumber 1500 cm². -1 Hydrolysis treatment can be carried out until the ratio of the heights of the absorption peak P1 (P1 / P4) is between 2 and 20.

[0072] Alternatively, after the hydrolysis step, the next mixing step may be carried out with the polyimide precursor still dissolved. On the other hand, after the hydrolysis step, in order to powderize the polyimide precursor, the solvent used during the hydrolysis treatment may be volatilized by drying or other means.

[0073] The polyimide precursor obtained by the hydrolysis treatment described above can be represented, for example, by the following formula (3). The polyimide precursor represented by formula (3) has structural unit a and structural unit 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-hand portion) and structural unit b (right-hand portion) correspond to the repeating structural units described above, respectively. Furthermore, polyimide precursors may have two carboxyl groups at the ends of their molecular chains. These carboxyl groups can be generated by the complete hydrolysis of the cyclic imide structure during hydrolysis treatment. [ka]

[0074] In the hydrolysis process, increasing the temperature or increasing the pH of the reaction solution (making the reaction solution more alkaline) can reduce the mass-average molecular weight of the polyimide precursor and increase the proportion of amical structures in the polyimide precursor. Furthermore, in the hydrolysis process, the aforementioned Mw / Mn ratio can be reduced by increasing the hydrolysis reaction time.

[0075] ·Mixing process In the mixing step, for example, the powdered polyimide precursor prepared as described above, the oxazoline compound, and the solvent are mixed. A standard mixing apparatus can be used. If necessary, the mixture may be heated while mixing and stirring.

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

[0077] In the mixing step, for example, a solution is prepared containing a solvent containing at least water, the polyimide precursor described above, and an oxazoline compound. Alternatively, for example, the curing composition may be produced by mixing the powdered polyimide precursor described above with an aqueous solution in which the oxazoline compound is dissolved. It is preferable to prepare a solution-type curing composition by dissolving the polyimide precursor and the oxazoline compound in a solvent through a mixing step, and more preferably to prepare a curing composition in the form of an aqueous solution. Because the curing composition is in a solution state (especially an aqueous solution), it can be applied in various ways. Therefore, it has the advantage of broadening the range of applications for the curing composition. Furthermore, because the curing composition is an aqueous solution, it is not flammable, thus offering the advantage of increased safety.

[0078] The curing composition described above is used by being applied to a substrate such as an electric wire, film, flexible circuit board, or semiconductor. The applied curing composition becomes a cured product (specifically, a cured resin, etc.) when subjected to a curing treatment such as heating. The curing composition described above may be, for example, a curing composition for fiber coating, a curing composition for resin film coating, a curing composition for resin molded product coating, or a curing composition for metal coating. The curing composition described above, which becomes a cured product (for example, a cured resin molded product), may be used, for example, in applications such as films, paints, electrical insulating materials, heat-resistant components, heat-resistant containers, and fibers.

[0079] The method of using the above curing composition (in other words, the method of producing a cured product obtained by curing the above curing composition) is not particularly limited. In such a method of use, for example, a cured product (such as a cured coating film) can be formed by applying the above curing composition onto an object to be coated (such as a substrate or other adherend) or by impregnating the object to be coated with the composition and then curing it. Common application methods for the curing composition include, for example, spray coating, dip coating, spin coating, die coating, and gravure coating. By applying a heat treatment to the curing composition coated on the substrate and to the substrate itself, the solvent contained in the curing composition can be volatilized. The heat treatment is not particularly limited, and general methods such as hot air heating and infrared heating can be used. The heating conditions in the heat treatment are, for example, 60°C to 100°C for 30 minutes. Subsequently, further heat curing can be advanced by performing a heat treatment at a higher temperature. Such higher temperature heat treatments are not particularly limited and can be carried out using general methods. The temperature conditions 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 progresses further, imidation in the polyimide resin precursor progresses further, and a cured film is formed. The heat treatment can also be carried out under an inert gas atmosphere or under reduced pressure conditions.

[0080] The curing composition and cured product of the present invention are as illustrated above, but the present invention is not limited to the embodiments described above. Furthermore, in the present invention, various forms used in general curing compositions and the like can be adopted as long as they do not impair the effects of the present invention.

[0081] The matters disclosed herein include the following: (1) A polyimide precursor in which some of the imide bonds of the polyimide resin have been hydrolyzed, It includes an oxazoline compound having an oxazoline ring in its molecule, A curing composition in which the amount of the oxazoline ring of the oxazoline compound is 0.20 mol or more and 1.60 mol or less per mol of repeating structural units of the polyimide precursor. (2) The curing composition according to (1) above, wherein the oxazoline compound is either a low molecular weight oxazoline compound with a molecular weight of less than 1000, or a high molecular weight oxazoline compound with a molecular weight of 1000 or more. (3) The curing composition according to (1) or (2) above, further comprising an aqueous solvent. (4) The curing composition according to (3) above, wherein the aqueous solvent comprises water and a hydrophilic organic solvent that dissolves in water in any amount ratio with respect to water. (5) The curing composition according to (4) above, wherein the hydrophilic organic solvent comprises at least one of dimethylaminoethanol and diethanolamine. (6) The curing composition according to any one of (1) to (5) above, wherein 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 in molar ratio. (7) The polyimide precursor has a benzene ring in its molecule, Infrared spectroscopy analysis of the aforementioned polyimide precursor revealed an infrared spectral spectrum with a wavenumber of 1500 cm⁻¹ due to the benzene ring. -1 The absorption peak P1 and the wavenumber 1375 cm⁻¹ due to the imide group -1 The absorption peak P2 is shown, The curing composition according to any one of (1) to (6) above, wherein the ratio of the height of the absorption peak P1 to the height of the absorption peak P2 (P1 / P2) is 2 or more and 10 or less. (8) The polyimide precursor has a benzene ring in its molecule, Infrared spectroscopy analysis of the aforementioned polyimide precursor revealed an infrared spectral spectrum with a wavenumber of 1500 cm⁻¹ due to the benzene ring. -1 The absorption peak P1 and the wavenumber 1600 cm due to the amide group -1 The absorption peak P3 is shown, The curing composition according to any one of (1) to (7) above, wherein the ratio of the height of the absorption peak P1 to the height of the absorption peak P3 (P1 / P3) is 2 or more and 20 or less. (9) The polyimide precursor has a benzene ring in its molecule, Infrared spectroscopy analysis of the aforementioned polyimide precursor revealed an infrared spectral spectrum with a wavenumber of 1500 cm⁻¹ due to the benzene ring. -1 Absorption peak P1 and wavenumber 1413 cm⁻¹ due to the carboxyl group. -1 The absorption peak P4 is shown, The curing composition according to any one of (1) to (8) 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) A curing composition according to any one of (1) to (9) above, which is a curing composition for fiber coating, a curing composition for resin film coating, a curing composition for resin molded product coating, or a curing composition for metal coating. (11) A cured product obtained by curing a curing composition described in any of (1) to (10) above. (12) The cured product described in (11) above, used in applications of film, paint, electrical insulating material, heat-resistant component, heat-resistant container, or fiber. (13) A method of using a curing composition, wherein a curing composition described in any of (1) to (10) above is applied to or impregnated into a workpiece, and then cured to form a cured product. [Examples]

[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] The raw materials for manufacturing the curing composition are described below. <Main ingredients> (A-1) Polyimide precursor (powder form) • Polyimide resin molded products before hydrolysis: Polyimide resin, product name "Kapton H" (manufactured by Toray DuPont) Polyimide resin with the molecular structure shown in equation (A) in Figure 1 • Mass-average molecular weight (Mw) after hydrolysis: 11,000 • Mw / Mn after hydrolysis: 1.6 The above (A-1) was prepared as follows. First, the above polyimide resin molded product was cut into pieces approximately 5 mm square with scissors to obtain pulverized polyimide resin. Meanwhile, an alkaline aqueous solution was prepared by dissolving 40 g of potassium hydroxide in 600 g of deionized water in a 1000 mL container equipped with a stirrer. 100 g of the pulverized polyimide resin was added to this alkaline aqueous solution and hydrolysis treatment was carried out at 80-90°C for 3 hours to obtain a crude polyimide precursor solution. Next, the crude polyimide precursor solution was neutralized to obtain a precipitate. This precipitate was filtered and then washed with deionized water. After removing the neutralized salt compound and excess acidic compounds, it was dried. It was also ground into a powder. In this way, a powdered polyimide precursor was prepared. reference: (A-2) Polyimide precursor (powder form) • Polyimide resin molded products before hydrolysis: Polyimide resin, product name "Kapton H" (manufactured by Toray DuPont) • Hydrolysis conditions: Different from the hydrolysis conditions described above. reference: (A-3) Polyimide precursor (powder form) • Polyimide resin molded products before hydrolysis: Polyimide resin, product name "Kapton EN" (manufactured by Toray DuPont) Polyimide resin with the molecular structure shown in equation (J) in Figure 1 • Hydrolysis conditions: Different from the hydrolysis conditions described above. (B) Oxazoline compounds (B-1) High molecular weight oxazoline compound (oxazoline structure-containing acrylic polymer) 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) Oxazoline group content: 4.5 [mmol / g, per solid content] Molecular weight: Mn=20,000, Mw=70,000, Mw / Mn=3.5 (B-2) High molecular weight oxazoline compounds (acrylic polymers containing oxazoline structure) Product name: "Epocross WS-700" (manufactured by Nippon Shokubai Co., Ltd.) Dissolves in water (solid content 25% by mass) Oxazoline group content: 4.5 [mmol / g, per solid content] Molecular weight: Mn=20,000, Mw=40,000, Mw / Mn=2.0 (B-3) High molecular weight oxazoline compounds (acrylic polymers containing oxazoline structure) Product name: "Epocross WS-300" (manufactured by Nippon Shokubai Co., Ltd.) Dissolves in water (solid content 10% by mass) Oxazoline group content: 7.7 [mmol / g, per solid content] Molecular weight: Mn=40,000, Mw=120,000, Mw / Mn=3.0 (C) Alternative compounds (comparative compounds) for the above (B) (C-1) Polycarbodiimide (Polycarbodiimide with added hydrophilic segment) Product name: "Carbodilite V-02-L2" (manufactured by Nisshinbo Chemical Co., Ltd.) Dissolve in an aqueous solvent (40% solids by mass) (C-2) Polycarbodiimide (Polycarbodiimide with added hydrophilic segment) Product name: "Carbodilite V-02" (manufactured by Nisshinbo Chemical Co., Ltd.) Dissolve in an aqueous solvent (40% solids by mass) (C-3) Polycarbodiimide (Polycarbodiimide with added hydrophilic segment) Product name: "Carbodilite V-04" (manufactured by Nisshinbo Chemical Co., Ltd.) Dissolve in an aqueous solvent (40% solids 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" (manufactured by Mitsubishi Chemical Corporation)

[0084] [GPC chart of polyimide precursors produced by hydrolysis] Figure 2 shows the GPC chart obtained when polyimide precursor (A-1), a hydrolysis product of polyimide resin, was measured under the GPC measurement conditions described above.

[0085] [IR chart of polyimide resin before hydrolysis] Figure 3 shows the infrared spectral spectrum (IR chart) obtained when the polyimide resin before hydrolysis of the polyimide precursor (A-1) was subjected to infrared spectroscopy analysis using the analytical conditions and methods described above. Furthermore, Figure 4 shows the infrared spectroscopic spectrum (IR chart) obtained when the polyimide resin (A-3) was analyzed in the same manner.

[0086] [IR chart of polyimide precursor produced by hydrolysis (for reference)] Figure 5 shows the infrared spectral spectrum (IR chart) obtained by infrared spectroscopy analysis of polyimide precursor (A-1), which is a hydrolysis product of polyimide resin, using the analytical conditions and methods described above. Figure 6 shows the infrared spectroscopic spectrum (IR chart) obtained when polyimide precursor (A-2), a hydrolysis product of polyimide resin, was analyzed in the same manner. Figure 7 shows the infrared spectroscopic spectrum (IR chart) obtained when polyimide precursor (A-3), a hydrolysis product of polyimide resin, was analyzed in the same manner.

[0087] Based on Figure 5, the ratio of each peak height mentioned above was calculated as follows: (P1 / P2): 4.04 (P1 / P3): 4.95 (P1 / P4): 5.35 Based on Figure 6, the ratio of each peak height mentioned above was calculated as follows: (P1 / P2): 2.89 (P1 / P3): 8.00 (P1 / P4): 10.46 Based on Figure 7, the ratio of each peak height mentioned above was calculated as follows: (P1 / P2): 2.27 (P1 / P3): 9.08 (P1 / P4): 5.89

[0088] (Examples 1-9, Comparative Examples 1-13) The curing compositions for each example and comparative example were prepared by mixing the above-mentioned raw materials in the amounts shown in the following tables and dissolving (A-1). The composition of each curing composition is shown in Table 1. Each curing composition was prepared so that the total amount (solid content) of (A) and (B) was approximately 20% by mass. Specifically, 68.1 g of deionized 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 (A) polyimide precursor was added and stirred for 60 minutes. Subsequently, a specified amount of (B) oxazoline ring-containing compound or (C) comparative compound, and additives (leveling agent, stabilizer) were added, and deionized water was added as needed to prepare the curing composition. Note that in each table, the amounts of (B) and (C) are shown on a solid content basis.

[0089] <Solution properties of the composition> The solution, mixed according to the curing composition (Table 1), was stored at room temperature for two days. The properties of the solution were then examined.

[0090] [Table 1]

[0091] <Hardening treatment> The curing compositions for each example and comparative example were applied to polyimide film (Kapton H, manufactured by Toray DuPont) and stainless steel plate (SUS304, manufactured by TP Giken Co., Ltd.) using a bar coater. After application, a pre-drying treatment was performed at 60°C for 30 minutes, and then a curing treatment was performed by heating at 300°C for 30 minutes to produce a cured coating film (cured product) with a thickness of 20 μm.

[0092] <Uniformity of the cured coating film> The curing compositions for each example and comparative example were applied to aluminum foil (manufactured by UACJ Foil) using a bar coater. After application, the materials were 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 thickness of 20 μm. The uniformity of the cured coating film was determined by visual observation. Good (〇): It is a brown, transparent membrane. Slightly good (△): Slightly cloudy Poor (×): Cloudy

[0093] <Flexibility of the cured coating film> The cured coating surface of each example and comparative example was placed facing upwards, and the sample was folded 180 degrees so that the cured coating surface was facing outwards. The cured coating surface was then returned to its original flat state. Adhesive tape (Nichiban's "Cellotape (registered trademark) CT405AP-24") was applied to the folded portion of the coating and peeled off. The flexibility was measured by measuring the width of the peeled coating, and this was used as an indicator of the flexibility of the cured coating. Polyimide film was used as the substrate. Good (〇): 1mm or less Slightly good (△): 1-2mm Defective (×): 2mm or more

[0094] <Adhesion of hardened coating film> The adhesion of the cured coating film in each example and comparative example was measured by making a grid pattern of cuts with a utility knife and then peeling it off using the adhesive tape described above. The measurement was carried out in accordance with JIS K5600-5-6:1999 General Test Methods for Paints Part 5: Mechanical Properties of Coating Films Section 6: Adhesion (Cross-cut method, also known as grid test). Polyimide film and SUS were used as the adherends, respectively. The degree of adhesion was evaluated according to the following criteria. Good (〇): Number of remaining items is 90-100 / 100 Fair (△): Number of remaining items is 50-90 / 100 Defective (×): Number of remaining items is 0-50 / 100

[0095] Table 2 shows the results of evaluating each of the above performance aspects for each curing composition. Note that for Comparative Examples 4-7, other evaluations (flexibility, adhesion) were not performed because their uniformity was not deemed satisfactory.

[0096] [Table 2]

[0097] The reason why the cured product (cured coating) of the curing composition in the examples exhibits good performance is thought to be as follows: When the curing composition of the examples is subjected to a curing treatment such as heating, at least some of the multiple amic acid structures in the polyimide precursor are imidized. This forms a new cyclic polyimide structure. In addition, the carboxyl groups in some of the amic acid structures of the polyimide precursor, and the carboxyl groups at the molecular chain ends of the polyimide precursor, can be amide-ester bonded with the oxazoline ring of the oxazoline compound. This allows the polyimide precursor to be bonded with a relatively large amount of the oxazoline compound. Since the oxazoline rings that remain without being bonded to the carboxyl groups have an affinity for the polar groups present on the surface of the adherend, the cured product (cured resin, cured coating, etc.) obtained by curing the curing composition can exhibit good adhesion to the adherend. Furthermore, cracking of the cured product can be suppressed, and the cured product can be flexible. Moreover, the cured product also exhibits good uniformity.

[0098] As can be seen from the results in Tables 1 and 2, the cured coating film obtained by curing the curing composition of the examples was good in terms of uniformity, good adhesion to the substrate, and good in terms of flexibility (i.e., flexibility). In other words, the cured coating film obtained by curing the curing composition of the examples simultaneously possessed uniformity, flexibility, and good adhesion to the substrate. In curing compositions that do not contain oxazoline compounds, or curing compositions that contain less than a specified amount of oxazoline compounds, the cured coating film hardened by the curing treatment is unable to relieve its internal stress, resulting in reduced adhesion and flexibility of the cured coating film. Furthermore, when the oxazoline compound content exceeds a predetermined amount, the compatibility between the polyimide precursor and the oxazoline compound decreases, resulting in reduced uniformity of the cured coating film. Furthermore, the curing compositions containing the polyimide precursors (A-2) and (A-3) shown above for reference can also be said to be capable of producing good cured coating films. [Industrial applicability]

[0099] The curing composition of the present invention is used by being applied to, for example, metals, resin molded products, 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 some of the imide bonds of the polyimide resin have been hydrolyzed, It includes an oxazoline compound having an oxazoline ring in its molecule, The oxazoline compound is an oxazoline ring-containing water-soluble polymer having an oxazoline ring at the end of its side chain. A curing composition in which the amount of the oxazoline ring of the oxazoline compound is 0.20 mol or more and 1.60 mol or less per mol of repeating structural units of the polyimide precursor.

2. The curing composition according to claim 1, wherein the oxazoline compound is an oxazoline ring-containing acrylic polymer having an oxazoline ring at the end of a side chain.

3. Further comprising an aqueous solvent containing at least water, the aqueous solvent containing 60% by mass or more, The curing composition according to claim 1 or 2, wherein the oxazoline compound is dissolved in water.

4. The polyimide precursor has a benzene ring in its molecule, Infrared spectroscopy analysis of the aforementioned polyimide precursor revealed that the infrared spectral spectrum was obtained from the benzene ring at a wavenumber of 1500 cm⁻¹. －１ Absorption peak P1 and wavenumber 1375 cm⁻¹ due to the imide group. －１ The absorption peak P2 is shown, The curing composition according to claim 1 or 2, wherein the ratio of the height of the absorption peak P1 to the height of the absorption peak P2 (P1 / P2) is 2 or more and 10 or less.

5. A cured product obtained by curing the curing composition described in claim 1 or 2.

Images