Polyamide-imide copolymer and film using the same

Abstract

The problem of the present application is to provide a polyamide-imide copolymer having excellent toughness such as bending resistance, excellent transparency, and good wettability when formed into a film. The solution to this problem is to provide a polyamide-imide copolymer, which comprises an imide structural unit and an amide structural unit, wherein the imide structural unit comprises: an imide structural unit I-1 represented by the following formula (1) and at least one imide structural unit I-2 selected from the group consisting of those represented by the following formulas (2) to (5), and the amide structural unit comprises: an amide structural unit A-1 represented by the following formula (6).

Description

Technical Field

[0001] The present invention relates to a polyamide-imide copolymer that can be suitably used for a cover window of a bendable display element such as a foldable device. Background Art

[0002] Foldable devices have recently attracted significant attention to further enhance the portability of portable information devices such as smartphones and tablets. Components such as cover windows used in the flexible displays of these foldable devices require not only transparency but also flexibility. Specifically, they are being sought to exhibit extremely high flexibility, enabling 180° bending with a small curvature radius of approximately 2.5 mm.

[0003] In contrast, various studies have been conducted on materials made of flexible organic polymers as a substitute for rigid glass. For example, films containing polyimide resins have been studied and proposed as flexible organic polymers from the perspectives of transparency and heat resistance.

[0004] However, flexible displays using films containing such flexible organic polymers can sometimes produce pressure marks and bending marks on the display surface when touched with a finger or stylus, or when the display is held folded for extended periods. Therefore, films for such flexible displays are required to possess not only high flexibility but also a high elastic modulus. To address this issue, fluorine-substituted polyimide films have previously attracted significant attention as films for flexible displays that combine heat resistance, transparency, mechanical strength, surface hardness, and bending resistance.

[0005] Therefore, in the recent field of foldable devices, with the further improvement of functionality and productivity, and the diversification of design and application, the following have been proposed as cover window films for foldable devices: polyimide copolymers that further improve solubility in solvents and bending resistance during processing while maintaining transparency, and polyamide-imide copolymers that further improve mechanical strength, etc. (Patent Document 1, etc.).

[0006] Prior art literature

[0007] Patent Literature

[0008] Patent Document 1: Japanese Patent Application No. 2014-528490 Summary of the Invention

[0009] Problems to be solved by the invention

[0010] The polyamide-imide copolymer proposed in the above-mentioned patent document 1, etc. uses fluorine-substituted acid anhydrides such as 4,4'-(hexafluoroisopropylidene)diphthalic dianhydride (6FDA) as a tetracarboxylic dianhydride component that is beneficial to solubility and transparency, and has: an imide structural unit using an acid anhydride such as 3,3',4,4'-biphenyltetracarboxylic dianhydride (sBPDA) as a tetracarboxylic dianhydride component that is beneficial to bending resistance, and an amide structural unit using terephthaloyl chloride (TPC) as a dicarboxylic acid component that is beneficial to mechanical strength.

[0011] However, copolymers using fluorine-substituted acid anhydrides such as 6FDA have poor surface wettability and adhesion when formed into thin films, and require surface treatment when laminating the thin films with other substrates or coating materials.

[0012] Furthermore, while the use of an acid anhydride with intermolecular interactions, such as sBPDA, improves the copolymer's toughness, such as flex resistance, it tends to increase the copolymer's YI value (yellowish tint) due to π-electron conjugation. To reduce the YI value, a bluing agent must be added to the film. Consequently, light transmittance may be insufficient in applications requiring colorless transparency, such as window cover films.

[0013] Therefore, an object of the present invention is to provide a polyamide-imide copolymer having excellent toughness such as bending resistance and transparency, and also having good wettability and adhesion when formed into a film.

[0014] Solutions for solving problems

[0015] The present inventors focused on the imide structural unit of a polyamide-imide copolymer and examined various tetracarboxylic dianhydrides. They found that combining two specific tetracarboxylic dianhydrides could solve the aforementioned problems. The present invention was completed based on this finding. The gist of the present invention is as follows.

[0016] [1] A polyamide-imide copolymer comprising imide structural units and amide structural units,

[0017] The aforementioned imide structural unit comprises: an imide structural unit I-1 represented by the following formula (1);

[0018]

[0019] and,

[0020] At least one imide structural unit I-2 selected from the group consisting of those represented by the following formulae (2) to (5);

[0021]

[0022] (In formulae (1) to (5), X1 to X5 each independently represent a divalent organic group derived from a diamine.)

[0023] The aforementioned amide structural unit comprises: an amide structural unit A represented by the following formula (6);

[0024]

[0025] (In formula (6), X6 represents a divalent organic group derived from a diamine, and Y represents a divalent organic group derived from a dicarboxylic acid or a dicarboxylic acid derivative.)

[0026] [2] The polyamide-imide copolymer according to [1], wherein X1 to X6 are represented by the following formula (7):

[0027]

[0028] (In formula (7), * is a connecting group.)

[0029] [3] The polyamide-imide copolymer according to [1] or [2], wherein Y is at least one selected from the following formulas (8) to (10),

[0030]

[0031] (In formulas (8) to (10), * is a connecting group.)

[0032] [4] The polyamide-imide copolymer according to any one of [1] to [3], wherein the imide structural unit I-2 is represented by the formula (2).

[0033] [5] The polyamide-imide copolymer according to any one of [1] to [4], wherein the imide structural unit and the amide structural unit are contained in a molar ratio of 2:8 to 8:2.

[0034] [6] A film comprising the polyamide-imide copolymer according to any one of [1] to [5].

[0035] [7] The film according to [6], wherein the contact angle of water on the film surface having a thickness of 50 μm measured in accordance with JIS R3257:1999 is 55 degrees or less.

[0036] [8] The film according to [6] or [7], which is used as a cover window for a foldable device.

[0037] Effects of the Invention

[0038] According to the present invention, by forming an imide structural unit comprising a combination of two specific tetracarboxylic dianhydrides, a polyamide-imide copolymer having excellent toughness such as flex resistance and transparency, and good wettability and adhesion when formed into a film can be achieved. DETAILED DESCRIPTION

[0039] [Polyamide-imide copolymer]

[0040] The polyamide-imide copolymer of the present invention is a copolymer having an imide structure and an amide structure, and comprises, as imide structural units, an imide structural unit I-1 represented by the following formula (1) and at least one imide structural unit I-2 selected from the group consisting of those represented by the following formulas (2) to (5), and comprises, as an amide structural unit, an amide structural unit A represented by the following formula (6).

[0041]

[0042] In the present invention, as the imide structural unit constituting the polyamide-imide copolymer, by combining the structural unit I-1 represented by the above formula (1) with any one or more structural units I-2 of (2) to (5), the transparency of the polyamide-imide copolymer and its wettability when formed into a film and its adhesion to other components can be improved simultaneously. When a fluorine-substituted tetracarboxylic dianhydride such as 6FDA is used as the tetracarboxylic dianhydride component of the imide structural unit, as in conventional polyamide-imide copolymers, the transparency of the resulting polyamide-imide copolymer is improved, but the wettability and adhesion when formed into a film are insufficient. In the present invention, screening was conducted based on the electron affinity of various tetracarboxylic dianhydrides. As a result, by combining 3,4-oxydiphthalic dianhydride (aODPA) with at least one selected from the group consisting of 4,4'-oxydiphthalic dianhydride (sODPA), 2,2',3,3'-biphenyltetracarboxylic dianhydride (aBPDA), 9,9-bis(3,4-dicarboxyphenyl)fluorene dianhydride (BPAF), and 4,4'-(hexafluoroisopropylidene)diphthalic dianhydride (6FDA), the present invention achieves improvements in both resin transparency and wettability and adhesion when formed into a film, without compromising properties derived from the imide structure, such as heat resistance and mechanical strength, compared to conventional polyamide-imide copolymers comprising a combination of 6FDA and sBPDA.

[0043] Of the structural unit I-1 represented by the above formula (1) and the structural units I-2 of (2) to (5) in combination, the structural unit represented by the above formula (2) is preferred. That is, in the present invention, the tetracarboxylic dianhydride constituting the imide structural unit is preferably a combination of aODPA and sODPA.

[0044] The ratio of I-1 and I-2 constituting the imide structural unit of the polyamide-imide copolymer is preferably in the range of 2:1 to 1:2, more preferably in the range of 3:2 to 2:3, in terms of molar ratio.

[0045] In the polyamide-imide copolymer of the present invention, as an imide structural unit, components other than the above may be included within the range that does not impair the effects of the present invention. As the tetracarboxylic acid component constituting the imide structural unit, there are various tetracarboxylic acids or tetracarboxylic acid derivatives, and the tetracarboxylic acid derivatives can include anhydrides, preferably dianhydrides, and acid chlorides of tetracarboxylic acids. As tetracarboxylic acid compounds, for example, aromatic tetracarboxylic acids and their anhydrides, preferably aromatic tetracarboxylic acid compounds such as their dianhydrides; aliphatic tetracarboxylic acid compounds and their anhydrides, preferably aliphatic tetracarboxylic acid compounds such as their dianhydrides, etc. These tetracarboxylic acid compounds can be used alone or in combination of two or more.

[0046] Specific examples of aromatic tetracarboxylic dianhydrides include non-condensed polycyclic aromatic tetracarboxylic dianhydrides, monocyclic aromatic tetracarboxylic dianhydrides, and condensed polycyclic aromatic tetracarboxylic dianhydrides. Non-condensed polycyclic aromatic tetracarboxylic dianhydrides include 3,3',4,4'-biphenyltetracarboxylic dianhydride (sBPDA), 4,4'-(4,4'-isopropylidenediphenyloxy)diphthalic dianhydride (BPADA), 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 2,2',3,3'-benzophenonetetracarboxylic dianhydride, 3,3',4,4'-diphenylsulfonetetracarboxylic dianhydride, 2,2-bis(3,4-dicarboxyphenyl)propane dianhydride, 2,2-bis(2,3-dicarboxyphenyl)propane dianhydride, 2 Examples of the aromatic tetracarboxylic dianhydride include 1,2-bis(3,4-dicarboxyphenoxyphenyl)propane dianhydride, 1,2-bis(2,3-dicarboxyphenyl)ethane dianhydride, 1,1-bis(2,3-dicarboxyphenyl)ethane dianhydride, 1,2-bis(3,4-dicarboxyphenyl)ethane dianhydride, 1,1-bis(3,4-dicarboxyphenyl)ethane dianhydride, bis(3,4-dicarboxyphenyl)methane dianhydride, bis(2,3-dicarboxyphenyl)methane dianhydride, 4,4'-(p-phenylenedioxy)diphthalic dianhydride, and 4,4'-(m-phenylenedioxy)diphthalic dianhydride. Examples of the monocyclic aromatic tetracarboxylic dianhydride include 1,2,4,5-benzenetetracarboxylic dianhydride, and examples of the condensed polycyclic aromatic tetracarboxylic dianhydride include 2,3,6,7-naphthalenetetracarboxylic dianhydride.

[0047] As aliphatic tetracarboxylic dianhydride, cyclic or non-cyclic aliphatic tetracarboxylic dianhydride can be mentioned. Cyclic aliphatic tetracarboxylic dianhydride refers to a tetracarboxylic dianhydride having an alicyclic hydrocarbon structure. As specific examples thereof, cycloalkane tetracarboxylic dianhydrides such as 1,2,4,5-cyclohexane tetracarboxylic dianhydride (HPMDA), 1,2,3,4-cyclobutane tetracarboxylic dianhydride (CBDA), 1,2,3,4-cyclopentane tetracarboxylic dianhydride, bicyclo [2.2.2] oct-7-ene-2,3,5,6-tetracarboxylic dianhydride, dicyclohexyl -3,3 ',4,4 '-tetracarboxylic dianhydride (HBPDA) and positional isomers thereof can be mentioned. They can be used alone or in combination of two or more. As specific examples of non-cyclic aliphatic tetracarboxylic dianhydride, 1,2,3,4-butane tetracarboxylic dianhydride and 1,2,3,4-pentane tetracarboxylic dianhydride can be mentioned. They can be used alone or in combination of two or more. Moreover, you may use combining a cyclic aliphatic tetracarboxylic dianhydride and a non-cyclic aliphatic tetracarboxylic dianhydride.

[0048] The imide structural unit may be any structural unit as long as it does not impair the effects of the present invention. In addition to the structural units described above, the imide structural unit may also include a water adduct of the tetracarboxylic dianhydride or a structural unit derived from a tricarboxylic acid compound. Examples of tricarboxylic acid compounds include aromatic tricarboxylic acids, aliphatic tricarboxylic acids, and similar acid chlorides and anhydrides thereof. A combination of two or more types may also be used. Specific examples include 1,2,4-benzenetricarboxylic acid anhydride; 2,3,6-naphthalenetricarboxylic acid-2,3-anhydride; and compounds in which phthalic anhydride and benzoic acid are linked by a single bond, -O-, -CH2-, -C(CH3)2-, -SO2-, or a phenylene group.

[0049] The polyamide-imide copolymer of the present invention comprises an amide structural unit A represented by the following formula (6). By forming a copolymer having an imide structure having rigid properties and an amide structure having flexible properties, it is possible to achieve a high level of mechanical properties, which are a compromise between excellent flexibility and high elasticity, without sacrificing transparency and wettability.

[0050]

[0051] (In formula (6), X6 represents a divalent organic group derived from a diamine, and Y represents a divalent organic group derived from a dicarboxylic acid or a dicarboxylic acid derivative.)

[0052] The polyamide-imide copolymer containing the amide structural unit A can be obtained by reacting a diamine compound, a tetracarboxylic acid compound, and a dicarboxylic acid compound as monomer components. Specifically, the diamine compound and the tetracarboxylic acid compound are reacted to synthesize a polymer having an imide precursor structure, and then the polymer is reacted with a dicarboxylic acid compound to synthesize a copolymer having an imide precursor structure and an amide structure. The imide precursor structure in the copolymer is subjected to a ring-closure reaction (imidization).

[0053] The polyamide-imide copolymer has a structure in which a residue obtained by reacting a diamine compound with a tetracarboxylic acid compound is bonded to a residue obtained by reacting a diamine compound with a tetracarboxylic acid compound via an amide structure in a structural unit in which a residue obtained by reacting a diamine compound with a tetracarboxylic acid compound is bonded via an imide structure.

[0054] In the amide structural unit A represented by the above formula (7), Y is a divalent organic group derived from a dicarboxylic acid or a dicarboxylic acid derivative. Examples of the dicarboxylic acid derivative include acid chlorides and esters of the dicarboxylic acid. The dicarboxylic acids can be used alone or in combination of two or more.

[0055] Specific examples of dicarboxylic acids include alicyclic dicarboxylic acids such as 1,3-cyclobutanedicarboxylic acid, 1,3-cyclopentanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, 4,4'-diphenyl ether dicarboxylic acid, terephthalic acid, isophthalic acid, 2,6-naphthalene dicarboxylic acid, 1,5-naphthalene dicarboxylic acid, 1,4-naphthalene dicarboxylic acid, 4,4'-biphenyl dicarboxylic acid, 3,3'-biphenyl dicarboxylic acid, and compounds in which two cyclohexane carboxylic acids or two benzoic acids are linked by a single bond, -CH2-, -C(CH3)2-, -C(CF3)2-, -SO2-, or a phenylene group, and their derivatives (e.g., acid chlorides and anhydrides); and aliphatic dicarboxylic acids such as dicarboxylic acid compounds of chain hydrocarbons having 8 or less carbon atoms and their derivatives (e.g., acid chlorides and esters). These dicarboxylic acid compounds can be used alone or in combination of two or more.

[0056] Among them, from the viewpoint of improving the elongation at break and elastic modulus as a film, it is preferred to use terephthalic acid, isophthalic acid, 4,4'-biphenyl dicarboxylic acid, or 4,4'-diphenyl ether dicarboxylic acid or a derivative thereof, and it is particularly preferred to use terephthaloyl chloride (TPC), isophthaloyl chloride (IPC), 4,4'-biphenyl dicarbonyl chloride (BPC), and 4,4'-oxybis(benzoyl chloride) (OBBC). Specifically, it is preferred to have an amide structural unit A-1 represented by the following formula (11) in which Y is a divalent organic group derived from TPC, an amide structural unit A-2 represented by the following formula (12) in which Y is a divalent organic group derived from IPC, or an amide structural unit A-3 represented by the following formula (13) in which Y is a divalent organic group derived from BPC, and it is particularly preferred to have the amide structural unit A-1.

[0057]

[0058] (In the formula, X6 represents a divalent organic group derived from diamine.)

[0059] As the amide structural unit, the amide structural unit A-1 may be used in combination with another amide structural unit. Examples of the other amide structural unit include those derived from the various dicarboxylic acids or dicarboxylic acid derivatives described above, but amide structural units A-2 or A-3 are preferred. When the amide structural units A-1 and A-2 or A-3 are used in combination as amide structural units, the ratio thereof is preferably in the range of 10:1 to 5:1 from the perspective of balancing optical properties such as transparency and mechanical properties such as mechanical strength.

[0060] The structural ratio (molar ratio) of the imide structure to the amide structure in the polyamide-imide copolymer of the present invention is preferably 0.5 to 4:3 to 6.5, more preferably 1.5 to 3.5:3.5 to 5.5, and particularly preferably 3:4. By achieving the above structural ratio of the imide structure to the amide structure, excellent softness and high elasticity can be achieved in a well-balanced manner.

[0061] The diamine component (i.e., the divalent organic group represented by X1 to X6) constituting the above-mentioned imide structural unit and amide structural unit is not particularly limited, and diamine components used in conventionally known polyimides and polyamide-imides can be used, for example, aliphatic diamines, aromatic diamines, and mixtures thereof.

[0062] It should be noted that the term "aromatic diamine" herein refers to a diamine in which an amino group is directly bonded to an aromatic ring, and a portion of the structure may contain an aliphatic group or other substituents. The aromatic ring may be a single ring or a condensed ring, with examples including, but not limited to, a benzene ring, a naphthalene ring, an anthracene ring, and a fluorene ring. Among these, a benzene ring is preferred. Furthermore, the term "aliphatic diamine" refers to a diamine in which an amino group is directly bonded to an aliphatic group, and a portion of the structure may contain an aromatic ring or other substituents. Diamine compounds may be used alone or in combination of two or more.

[0063] Specific examples of aliphatic diamines include non-cyclic aliphatic diamines such as hexamethylenediamine, and cyclic aliphatic diamines such as 1,3-bis(aminomethyl)cyclohexane, 1,4-bis(aminomethyl)cyclohexane, norbornanediamine, and 4,4'-diaminodicyclohexylmethane. These can be used alone or in combination of two or more.

[0064] Specific examples of the aromatic diamine include aromatic diamines having one aromatic ring, such as p-phenylenediamine, m-phenylenediamine, 2,4-toluenediamine, m-phenylenediamine, p-phenylenediamine, 1,5-diaminonaphthalene, and 2,6-diaminonaphthalene; 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenylpropane, 4,4'-diaminodiphenylether, 3,4'-diaminodiphenylether, 3,3'-diaminodiphenylether, 4,4'-diaminodiphenylsulfone, 3,4'-diaminodiphenylsulfone, 3,3'-diaminodiphenylsulfone, 1,4-bis(4-aminophenoxy)benzene, 1,3-bis(4-aminophenoxy)benzene, bis[4-(4-aminophenoxy)phenyl]sulfone, bis[4-(3-aminophenoxy)phenyl]sulfone, Aromatic diamines having two or more aromatic rings, such as [4-(4-aminophenoxy)phenyl]sulfone, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, 2,2-bis[4-(3-aminophenoxy)phenyl]propane, 2,2'-dimethylbenzidine, 2,2'-bis(trifluoromethyl)benzidine, 4,4'-bis(4-aminophenoxy)biphenyl, 9,9-bis(4-aminophenyl)fluorene, 9,9-bis(4-amino-3-methylphenyl)fluorene, 9,9-bis(4-amino-3-chlorophenyl)fluorene, 9,9-bis(4-amino-3-fluorophenyl)fluorene, 4-aminophenyl-4'-aminobenzoate, (2-phenyl-4-aminophenyl)-4-aminobenzoate, and 4,4'-diaminobenzanilide. These can be used alone or in combination of two or more.

[0065] Among the above-mentioned diamine compounds, from the viewpoint of improving the colorless transparency and elasticity of the film, it is preferred to use one or more selected from the group consisting of aromatic diamines having a biphenyl structure, specifically one or more selected from the group consisting of 2,2'-dimethylbenzidine, 2,2'-bis(trifluoromethyl)benzidine, 4,4'-bis(4-aminophenoxy)biphenyl, and 4,4'-diaminodiphenyl ether. Furthermore, from the viewpoint of easily improving the colorless transparency, it is more preferred to use a diamine having a biphenyl structure in which a part or all of the hydrogen atoms on the aromatic ring are substituted with a substituent selected from a fluoro group, a trifluoromethyl group, or a trifluoromethoxy group, specifically 2,2'-bis(trifluoromethyl)benzidine (TFMB) represented by the following formula.

[0066]

[0067] The diamine compound may be used alone or in combination with TFMB or one or more of the above-mentioned diamine compounds. When TFMB and other diamine compounds are used in combination, the composition ratio is preferably within a range of 10:1 to 5:1 from the perspective of balancing optical properties such as transparency and mechanical properties such as mechanical strength.

[0068] In the synthesis of the polyamide-imide copolymer, the constituent ratio of the monomer components (diamine compound:tetracarboxylic acid compound:dicarboxylic acid compound) is preferably 7:0.5 to 4:3 to 6.5, more preferably 7:1.5 to 3.5:3.5 to 5.5, and particularly preferably 7:3:4 in terms of molar ratio.

[0069] The ring-closure reaction (imidation) of the imide precursor obtained by reacting the above-mentioned diamine compound with a tetracarboxylic acid compound, or the imide precursor obtained by reacting a diamine compound, a tetracarboxylic acid compound, and a dicarboxylic acid compound, can be either thermal imidization by adding an azeotropic solvent (e.g., toluene, xylene, etc.) that azeotropes with water and heating, or chemical imidization using a condensing agent and a reaction accelerator. However, chemical imidization is preferred from the perspective of maintaining colorless transparency.

[0070] Examples of reaction accelerators used in chemical imidization include triethylamine, diisopropylethylamine, N-methylpiperidine, pyridine, 2-methylpyridine, 3-methylpyridine, 4-methylpyridine, 3-ethylpyridine, 3,5-lutidine, 3,5-diethylpyridine, isoquinoline, imidazole, 1-methylimidazole, 2-methylimidazole, and 1,2-dimethylimidazole. These reaction accelerators may be used alone or in combination of two or more.

[0071] Examples of the condensing agent used in chemical imidization include acid anhydrides such as acetic anhydride, propionic anhydride, and trifluoroacetic anhydride, and phosphites such as triethyl phosphite, triethyl phosphite, tributyl phosphite, dimethyl phosphite, diethyl phosphite, and triphenyl phosphite. These condensing agents may be used alone or in combination of two or more.

[0072] The imidization rate is preferably 90% or more, more preferably 93% or more, and further preferably 96% or more. From the perspective of easily improving optical homogeneity such as transparency, a high imidization rate is preferably used. In addition, the upper limit of the imidization rate is 100% or less. The imidization rate represents the ratio of the molar amount of imide bonds in the imide structural unit relative to a value twice the molar amount of the structural unit derived from tetracarboxylic dianhydride in the imide structural unit. It should be noted that the imidization rate can be obtained by IR method, NMR method, etc.

[0073] The organic solvent used in the synthesis of the polyamide-imide copolymer is not particularly limited as long as it is an organic solvent inert to the reaction. Examples thereof include N,N-dimethylacetamide (DMAc), N-methyl-2-pyrrolidone, 1,3-dimethyl-2-imidazolidinone, dimethyl sulfoxide, m-cresol, γ-butyrolactone, cyclopentanone, cyclohexanone, and tetrahydrofuran. These organic solvents may be used alone or in combination of two or more.

[0074] The reaction conditions for the synthesis can be set to 10 to 50° C. for 1 to 27 hours or less. From the perspective of maintaining colorless transparency, the synthesis is preferably carried out under a nitrogen atmosphere.

[0075] The polyamide-imide copolymer can be isolated (isolated and purified) using commonly used methods, such as filtration, concentration, extraction, crystallization, recrystallization, column chromatography, and a combination of these separation methods. In a preferred embodiment, a large amount of alcohol such as methanol is added to a reaction solution containing a transparent polyamide-imide resin to precipitate the resin, and the resin is then concentrated, filtered, dried, and the like to achieve isolation.

[0076] The weight average molecular weight (Mw) of the resin having an imide structure obtained as described above is preferably in the range of 50,000 to 1,000,000, more preferably in the range of 80,000 to 800,000, and even more preferably in the range of 110,000 to 650,000, from the viewpoint of improving the elastic modulus and elongation at break. The weight average molecular weight (Mw) is a value calculated based on polystyrene conversion by GPC (gel permeation chromatography).

[0077] <Film>

[0078] The polyamide-imide copolymer of the present invention can be obtained as a film by dissolving it in a suitable solvent to form a resin composition (resin varnish), applying the resin composition to a support to form a coating film, drying the coating film, removing the solvent, and peeling it from the support to obtain a film. Any solvent that can dissolve the polyamide-imide can be used without particular limitation. However, from the perspectives of the coating properties of the resin varnish and the transparency of the resulting film, a solvent containing at least one selected from the group consisting of an ester group, an ether group, a ketone group, a hydroxyl group, a sulfo group, and a sulfinyl group is preferred.

[0079] Examples of the solvent having an ester group include ester-based solvents such as methyl acetate, ethyl acetate, butyl acetate, and dimethyl carbonate.

[0080] Examples of the solvent having a cyclic ester group include lactone-based solvents such as γ-butyrolactone (GBL), δ-valerolactone, ε-caprolactone, γ-butyrolactone, γ-caprolactone, α-methyl-γ-butyrolactone, γ-valerolactone, α-acetyl-γ-butyrolactone, and δ-caprolactone.

[0081] Examples of the solvent having an ether group include tetrahydrofuran, dioxane, and dibutyl ether.

[0082] Examples of the solvent having a ketone group include ketone-based solvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone.

[0083] Examples of the solvent having a hydroxyl group include phenolic solvents such as m-cresol.

[0084] Examples of the solvent having a sulfonic acid group include methyl sulfone, ethylphenyl sulfone, diethyl sulfone, diphenyl sulfone, sulfolane, bisphenol S, phenylpropyl sulfone, dapsone, bisphenol A polysulfone, and sulfolane.

[0085] Examples of the solvent having a sulfinyl group include sulfoxide-based solvents such as N,N-dimethylsulfoxide (DMSO).

[0086] In addition to the solvents listed above, amide solvents such as N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMAc) can also be used.

[0087] The resin composition for forming the film may also contain any components other than the polyamide-imide copolymer, for example, a leveling agent for improving the coating properties of the varnish when making the film, a dispersant, a surfactant, a delay regulator, an antioxidant, an ultraviolet light protectant, a light stabilizer, a plasticizer, waxes, a filler, a pigment, a dye, a foaming agent, a defoaming agent, a dehydrating agent, an antistatic agent, an antibacterial agent, a mildewproof agent, a bluing agent for reducing the yellowness of the film, etc.

[0088] The content of the polyamide-imide copolymer in the resin composition is preferably 65 to 100% by mass, more preferably 80 to 100% by mass, and even more preferably 90 to 100% by mass, based on the total solid content excluding the solvent.

[0089] As a means for coating the resin composition containing the polyamide-imide copolymer on the support, conventionally known means can be applied, for example, dip coating, flow coating, roll coating, bar coater method, blade coater method, screen printing method, curtain coating method, spray coating method, etc.

[0090] The drying conditions for the coating film are not particularly limited as long as the temperature is such that the solvent volatilizes. From the viewpoint of obtaining a thin film having excellent transparency, the drying conditions are preferably 60 to 250° C. for about 10 to 60 minutes.

[0091] The film of the present invention preferably has a thickness of 5 μm to 100 μm, more preferably 10 μm to 50 μm. By adjusting the film thickness within this range, a film with excellent bendability can be formed, making the film suitable for use as a cover window for foldable and flexible displays. The film thickness can be adjusted by adjusting the amount of resin composition applied.

[0092] As described above, the film of the present invention is formed from a polyamide-imide copolymer containing two specific imide structural units. Therefore, it has excellent toughness such as bending resistance, excellent transparency, and good surface wettability and adhesion to other materials. For example, in a single-layer film with a thickness of 50 μm formed from a polyamide-imide copolymer, the YI value can be set to 2.0 or less. It should be noted that, in this specification, the YI value (yellowness) refers to: according to ASTM E313-73, using a spectrophotometer to measure the transmittance of light at 360 to 780 nm, and to obtain the three stimulus values ​​(X, Y, Z), and the value calculated based on the formula YI = 100 × (1-0.847Z) / Y.

[0093] In addition, the film of the present invention also has excellent wettability and adhesion. For example, the water contact angle of a film formed on a glass substrate with a film thickness of 50 μm and an arithmetic average surface roughness Ra of 50 nm or less is 55 degrees or less. Thus, even if the film of the present invention has a smooth surface, its wettability is excellent. Therefore, when laminating the film to other substrates or laminating it with a coating formed from a decorative coloring material, unlike conventional polyimide amide films, no surface treatment of the film is required. It should be noted that the water contact angle refers to the water contact angle measured in accordance with JIS R3257:1999, and the arithmetic average surface roughness Ra refers to the value measured using a measuring device in accordance with JIS B0601-1994.

[0094] [use]

[0095] Examples of display components using the film of the present invention include thin, bendable, foldable organic EL displays, portable devices such as smartphones and watch-type terminals, display devices inside automobiles, flexible panels used in watches, and the like. Furthermore, the film can also be used in components for image display devices such as liquid crystal displays and organic EL displays, components for touch panels, components for solar cell panels such as flexible printed circuit boards, surface protection films, and substrate materials, components for optical waveguides, and semiconductor-related components. Among these, it is suitable for components such as cover windows and TFT substrates that constitute foldable organic EL displays.

[0096] [Overlay window of the display]

[0097] As a cover window for a display using the film of the present invention, the film can be arranged, for example, so that it is located on the surface of various displays. The method of arranging it on the surface is not particularly limited, and examples thereof include methods such as using an adhesive layer. As the material for the adhesive layer, conventional adhesive materials used for bonding display surface materials can be used. It should be noted that the cover window for a display using the film of the present invention can be provided with a protective layer such as a hard coat layer on the surface of the film, and further provided with an anti-fingerprint adhesion layer.

[0098] [TFT substrates for organic EL displays]

[0099] A TFT substrate for an organic EL display using the thin film of the present invention can be obtained, for example, by forming an amorphous silicon TFT (thin-film transistor) on the thin film of the present invention. The TFT comprises a gate metal layer, a silicon nitride gate dielectric layer, and an ITI pixel electrode. The structures required for the organic EL display can also be further formed thereon using known methods, and the method for forming the circuit, etc., is not particularly limited.

[0100] Example

[0101] Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to the Examples. It should be noted that "parts" and "%" in the following are all based on mass unless otherwise specified.

[0102] [Example 1]

[0103] <Preparation of polyamide-imide>

[0104] A 100 mL reactor was filled with 60.9 g of DMAc and 4.849 g (15.14 mmol) of TFMB. Subsequently, 1.007 g (3.245 mmol) of aODPA and 1.4875 g (3.245 mmol) of BPAF were added to the TFMB solution, and the mixture was stirred at 30°C for 2 hours to allow reaction to obtain a solution containing a polymer having an imide precursor structure. Subsequently, 1.757 g (8.653 mmol) of TPC was added to the solution, and the mixture was stirred for 1.5 hours while maintaining the solution temperature at 30°C to allow reaction to proceed, resulting in a solution containing a copolymer having an imide precursor structure and an amide structure.

[0105] Then, 2.09 g of pyridine, 2.45 g of acetic anhydride, and 8.53 g of DMAc were added, and the mixture was stirred at 20-30°C for 8 hours to obtain a polyamide-imide solution. Furthermore, 99 g of DMAc was added and stirred until uniform. The solution was then slowly added to a container containing 4 L of methanol to allow precipitation. The precipitated solid was filtered and pulverized, and then dried at 80°C under vacuum for 18 hours to obtain 8.2 g of a solid powder of a polyamide-imide copolymer (PAI-1). The polystyrene-equivalent weight average molecular weight determined by GPC was 597,000.

[0106] [Example 2]

[0107] A polyamide-imide solution was obtained in the same manner as in Example 1, except that 0.9547 g (3.245 mmol) of aBPDA was added instead of BPAF. The obtained polyamide-imide solution was purified in the same manner as in Example 1 to obtain 8.0 g of a polyamide-imide copolymer (PAI-2) as a solid powder. The polystyrene-equivalent weight average molecular weight determined by GPC was 201,000.

[0108] [Example 3]

[0109] A polyamide-imide solution was obtained in the same manner as in Example 1 except that 1.007 g (3.245 mmol) of sODPA was added instead of BPAF. The obtained polyamide-imide solution was purified in the same manner as in Example 1 to obtain 7.9 g of a polyamide-imide copolymer (PAI-3) as a solid powder. The weight average molecular weight based on polystyrene conversion according to GPC was 407,000.

[0110] [Example 4]

[0111] A polyamide-imide solution was obtained in the same manner as in Example 1, except that 1.442 g (3.245 mmol) of 6FDA was added instead of BPAF. The obtained polyamide-imide solution was purified in the same manner as in Example 1 to obtain 8.3 g of a polyamide-imide copolymer (PAI-4) as a solid powder. The polystyrene-equivalent weight average molecular weight determined by GPC was 311,000.

[0112] [Comparative Example 1]

[0113] A polyamide-imide solution was obtained in the same manner as in Example 1, except that 0.9547 g (3.245 mmol) of sBPDA was added instead of BPAF. The obtained polyamide-imide solution was purified in the same manner as in Example 1 to obtain 8.2 g of a polyamide-imide copolymer (PAI-5) as a solid powder. The polystyrene-equivalent weight average molecular weight determined by GPC was 625,000.

[0114] [Comparative Example 2]

[0115] A polyamide-imide solution was obtained in the same manner as in Example 1, except that 1.488 g (3.245 mmol) of BPAF and 0.9547 g (3.245 mmol) of aBPDA were added as tetracarboxylic dianhydride instead of aODPA and BPAF. The obtained polyamide-imide solution was purified in the same manner as in Example 1 to obtain 7.5 g of a polyamide-imide copolymer (PAI-6) as a solid powder. The polystyrene-equivalent weight average molecular weight determined by GPC was 114,000.

[0116] [Comparative Example 3]

[0117] A polyamide-imide solution was obtained in the same manner as in Example 1, except that 1.488 g (3.245 mmol) of BPAF and 1.007 g (3.245 mmol) of sOPDA were added as tetracarboxylic dianhydride instead of aODPA and BPAF. The obtained polyamide-imide solution was purified in the same manner as in Example 1 to obtain 7.3 g of a polyamide-imide copolymer (PAI-7) as a solid powder. The polystyrene-equivalent weight average molecular weight determined by GPC was 100,000.

[0118] [Comparative Example 4]

[0119] A polyamide-imide solution was obtained in the same manner as in Example 1, except that 0.9547 g (3.245 mmol) of aBPDA and 1.007 g (3.245 mmol) of sODPA were added as tetracarboxylic dianhydride instead of aODPA and BPAF. The obtained polyamide-imide solution was purified in the same manner as in Example 1 to obtain 7.0 g of a polyamide-imide copolymer (PAI-8) as a solid powder. The polystyrene-equivalent weight average molecular weight determined by GPC was 97,000.

[0120] [Comparative Example 5]

[0121] A polyamide-imide solution was obtained in the same manner as in Example 1, except that 1.007 g (3.245 mmol) of sODPA and 1.441 g (3.245 mmol) of 6FDA were added as tetracarboxylic dianhydride instead of aODPA and BPAF. The obtained polyamide-imide solution was purified in the same manner as in Example 1 to obtain 8.4 g of a polyamide-imide copolymer (PAI-9) as a solid powder. The polystyrene-equivalent weight average molecular weight determined by GPC was 478,000.

[0122] [Comparative Example 6]

[0123] A polyamide-imide solution was obtained in the same manner as in Example 1, except that 1.007 g (3.245 mmol) of 6FDA and 0.9547 g (3.245 mmol) of sBPDA were added as tetracarboxylic dianhydride instead of aODPA and BPAF. The obtained polyamide-imide solution was purified in the same manner as in Example 1 to obtain 8.0 g of a polyamide-imide copolymer (PAI-10) as a solid powder. The polystyrene-equivalent weight average molecular weight determined by GPC was 179,000.

[0124] [Comparative Example 7]

[0125] A polyamide-imide solution was obtained in the same manner as in Example 1, except that 1.007 g (3.245 mmol) of sODPA and 0.9547 g (3.245 mmol) of sBPDA were added as tetracarboxylic dianhydride instead of aODPA and BPAF. The obtained polyamide-imide solution was purified in the same manner as in Example 1 to obtain 8.3 g of a polyamide-imide copolymer (PAI-11) as a solid powder. The weight average molecular weight based on polystyrene conversion according to GPC was 593,000.

[0126] [Comparative Example 8]

[0127] A polyamide-imide solution was obtained in the same manner as in Example 1, except that 0.9547 g (3.245 mmol) of aBPDA and 0.9547 g (3.245 mmol) of sBPDA were added as tetracarboxylic dianhydride instead of aODPA and BPAF. The obtained polyamide-imide solution was purified in the same manner as in Example 1 to obtain 7.9 g of a polyamide-imide copolymer (PAI-12) as a solid powder. The polystyrene-equivalent weight average molecular weight determined by GPC was 124,000.

[0128] [Comparative Example 9]

[0129] A polyamide-imide solution was obtained in the same manner as in Example 1, except that 0.9547 g (3.245 mmol) of aBPDA and 1.441 g (3.245 mmol) of 6FDA were added as tetracarboxylic dianhydride instead of aODPA and BPAF. The obtained polyamide-imide solution was purified in the same manner as in Example 1 to obtain 8.1 g of a polyamide-imide copolymer (PAI-13) as a solid powder. The polystyrene-equivalent weight average molecular weight determined by GPC was 77,000.

[0130] [Comparative Example 10]

[0131] A polyamide-imide solution was obtained in the same manner as in Example 1, except that 1.488 g (3.245 mmol) of BPAF and 0.9547 g (3.245 mmol) of sBPDA were added as tetracarboxylic dianhydride instead of aODPA and BPAF. The obtained polyamide-imide solution was purified in the same manner as in Example 1 to obtain 8.4 g of a polyamide-imide copolymer (PAI-14) as a solid powder. The polystyrene-equivalent weight average molecular weight determined by GPC was 391,000.

[0132] [Comparative Example 11]

[0133] A polyamide-imide solution was obtained in the same manner as in Example 1, except that 1.488 g (3.245 mmol) of BPAF and 1.441 g (3.245 mmol) of 6FDA were added as tetracarboxylic dianhydride instead of aODPA and BPAF. The obtained polyamide-imide solution was purified in the same manner as in Example 1 to obtain 7.8 g of a polyamide-imide copolymer (PAI-15) as a solid powder. The polystyrene-equivalent weight average molecular weight determined by GPC was 429,000.

[0134] Table 1 shows the molar ratios of the components constituting the polyamide-imide copolymers obtained as described above.

[0135] <Film Production>

[0136] 5.0 g of each polyamideimide obtained above was dissolved in 45 g of DMAc to prepare a resin varnish. This was then applied to a glass plate using a tabletop coater (AFA-standard, manufactured by COTEC), dried in an inert gas oven (INL-45N1, manufactured by YAMATOSCIENTIFIC CO., LTD.) at 70°C for 1 hour and then at 250°C for 1 hour, and peeled from the glass plate to form a thin film. The thickness of the resulting film is shown in Table 1 below.

[0137] The obtained films were used as evaluation samples and the following evaluations were performed.

[0138] <Measurement of total light transmittance and haze>

[0139] Each film was cut into 30 mm x 30 mm pieces and the total light transmittance and haze were measured using a haze meter (NDH 7000II, manufactured by Nippon Denshoku Industries, Ltd.) according to ASTM D 1003. The measured values ​​were normalized by the film thickness (50 μm). The measurement results are shown in Table 1 below.

[0140] <YI value (yellow index) evaluation>

[0141] Each film was cut into a size of 30 mm×30 mm, and the YI value was determined using a spectrophotometer (CM-5 manufactured by Konica Minolta, Inc.) in accordance with ASTM E313. The YI value was evaluated according to the following evaluation criteria.

[0142] ○: YI value is less than 2

[0143] △: YI value is 2 or more and less than 3

[0144] ×: YI value is 3 or more

[0145] The evaluation results are shown in Table 1 below.

[0146] <Toughness Evaluation>

[0147] Each film is cut into a specified size and measured using a small tabletop testing machine (Shimadzu Corporation, EZ-SX). It should be noted that the elastic modulus is obtained from the slope of the stress of the stress-strain diagram obtained from 5 MPa to 10 MPa. In addition, the product of stress and strain (i.e., the area presented by the stress-strain curve) is obtained from the stress-strain curve to calculate the fracture energy. Based on the fracture energy, toughness is evaluated according to the following evaluation criteria.

[0148] ○: Fracture energy is 0.6 J or more

[0149] △: Fracture energy is 0.4J or more and less than 0.6J

[0150] ×: Fracture energy is less than 0.4J

[0151] The evaluation results are shown in Table 1 below.

[0152] <Film Wettability>

[0153] The contact angle of each film with water on the surface in contact with the glass plate was measured using a contact angle meter (DM300) manufactured by KYOWA INTERFACE SCIENCE Co., Ltd. Based on the measured contact angle, the wettability of the film was evaluated according to the following evaluation criteria.

[0154] ○: Water contact angle is 55 degrees or less

[0155] △: Water contact angle is more than 55 degrees and less than 65 degrees

[0156] ×: Water contact angle exceeds 65 degrees

[0157] The evaluation results are shown in Table 1 below.

[0158] <Film Adhesion>

[0159] The decorative coloring material was applied to the surface of each film in contact with the glass plate by screen printing to a film thickness of 2 to 3 μm after drying, and then heated and dried at 80° C. for 30 minutes to form a colored coating film, thereby obtaining a sample for adhesion evaluation. The decorative coloring material was prepared as follows.

[0160] (Decorative coloring material)

[0161] In a 500 ml separable flask equipped with a nitrogen inlet tube and a stirrer, 4.57 g of (2-phenyl-4-aminophenyl)-4-aminobenzoate (PHBAAB), 4.29 g of 4,4'-diamino-3,3'-dicarboxydiphenylmethane (MBAA), 15.61 g of 2,2'-bis[4-(3,4-dicarboxyphenoxy)propionic dianhydride] (BPADA), 94.64 g of ethyl benzoate, 0.47 g (6 mmol) of pyridine, and 10 g of toluene were placed. The mixture was reacted for 4 hours at 180° C. under a nitrogen atmosphere while removing toluene from the system during the reaction, thereby preparing a polyimide compound solution having a solid content of 20% by mass.

[0162] Next, 12.5 g of carbon black and 0.5 g of DBP (manufactured by Johoku Chemical Industry Co., Ltd., a phosphate-based adhesion aid) were added to 50 g of the solution and dispersed until uniform, thereby producing a decorative coloring material.

[0163] The adhesion between the film and the colored coating was evaluated for each sample obtained as described above in accordance with JIS K 5600-5-6 (ISO 2409). Specifically, a single-edged blade was used to make 100 grid-like cuts at 1 mm intervals in the colored coating. "Cellotape" (registered trademark) was then applied to the film. The Cellotape (registered trademark) was then peeled off, and the state of the colored coating was visually inspected. Evaluation was then performed based on the JIS K 5600-5-6 (ISO 2409) evaluation criteria (adhesion was evaluated on a six-step scale of 0 to 5, with the highest adhesion rating being the highest). The evaluation results are shown in Table 1 below.

[0164] [Table 1]

[0165]

[0166] As is clear from the evaluation results in Table 1, the polyamide-imide copolymers (Examples 1 to 4) using an imide structural unit (unit I-1) derived from aODPA as a tetracarboxylic dianhydride component and a specific imide structural unit (unit I-2) are superior to the polyamide-imide copolymers (Comparative Example 1) using an imide structural unit (unit I-1) and another imide structural unit (sBPDA) and the polyamide-imide copolymers (Comparative Examples 2 to 11) without a structural unit derived from aODPA. All of them have excellent optical properties, mechanical properties, wettability, and adhesion.

Claims

1. A polyamide-imide copolymer, characterized in that: comprising an imide structural unit and an amide structural unit, The imide structural unit comprises: an imide structural unit I-1 represented by the following formula (1); and, At least one imide structural unit I-2 selected from the group consisting of those represented by the following formulae (2) to (5); In the formulae (1) to (5), X1 to X5 each independently represent a divalent organic group derived from a diamine, The amide structural unit comprises: an amide structural unit A represented by the following formula (6); In formula (6), X6 represents a divalent organic group derived from a diamine, and Y represents a divalent organic group derived from a dicarboxylic acid or a dicarboxylic acid derivative.

2. The polyamide-imide copolymer according to claim 1, wherein X1 to X6 are represented by the following formula (7): In formula (7), * is a linking group.

3. The polyamide-imide copolymer according to claim 1 or 2, wherein Y is at least one selected from the following formulas (8) to (10), In formulae (8) to (10), * represents a linking group.

4. The polyamide-imide copolymer according to claim 1 or 2, wherein The imide structural unit I-2 is represented by the formula (2).

5. The polyamide-imide copolymer according to claim 1 or 2, wherein The imide structural unit and the amide structural unit are contained in a molar ratio of 2:8 to 8:

2. A film comprising the polyamide-imide copolymer according to any one of claims 1 to 5. 7 . The film according to claim 6 , wherein the contact angle of water on the film surface having a thickness of 50 μm measured in accordance with JIS R3257:1999 is 55 degrees or less.

8. The film according to claim 6 or 7, which is used as a cover window of a foldable device.

Images