Polyamide-imide copolymer and film using the same

Abstract

[Problem] To provide a favorable polyamidimide copolymer which exhibits excellent toughness, such as elasticity, excellent transparency, and favorable wettability when made into a film. [Solution] A polyamidimide copolymer which contains an imide structural unit and an amide structural unit, wherein the imide structural unit includes an imide structural unit I-1 represented by formula (1) and one or more types of imide structural unit I-2 selected from the group consisting of those represented by formulas (2)-(5), and the amide structural unit includes an amide structural unit A-1 which is represented by 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 foldable display element such as a foldable device. [Background technology]

[0002] Foldable devices have recently attracted attention as they offer a way to further enhance the portability of mobile information terminals such as smartphones and tablets. Components such as cover windows used in flexible displays that make up such foldable devices must be flexible in addition to transparent. Specifically, there is a demand for components with extremely high flexibility that can be bent 180° with a small bending radius of approximately 2.5 mm.

[0003] In response to this, various materials made of flexible organic polymers have been studied as alternatives to rigid glass. For example, from the viewpoints of transparency and heat resistance, films containing polyimide resins have been studied and proposed as flexible organic polymers.

[0004] However, flexible displays using films containing such flexible organic polymers often suffer from pressure marks or flex marks on the display surface when touched with a finger or a touch pen, or when the display is kept folded for a long period of time. Therefore, films for such flexible displays are required to have not only high flexibility but also a high elastic modulus. To address this issue, fluorine-substituted polyimide films have attracted attention as films for flexible displays that combine heat resistance, transparency, mechanical strength, surface hardness, and flex resistance.

[0005] Meanwhile, in the recent field of foldable devices, with further improvements in functionality and productivity, and diversification of designs and applications, polyimide copolymers that have improved solubility in solvents and bending resistance during processing while maintaining transparency, and polyamide-imide copolymers that have improved mechanical strength, etc. have been proposed as cover window films for foldable devices (Patent Document 1, etc.). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Special Publication No. 2014-528490 Summary of the Invention [Problem to be solved by the invention]

[0007] The polyamide-imide copolymers proposed in Patent Document 1 and the like have imide structural units in which a fluorine-substituted acid anhydride such as 4,4'-(hexafluoroisopropylidene)diphthalic dianhydride (6FDA) is used as the tetracarboxylic dianhydride component that contributes to solubility and transparency, an acid anhydride such as 3,3',4,4'-biphenyltetracarboxylic dianhydride (sBPDA) is used as the tetracarboxylic dianhydride component that contributes to bending resistance, and an amide structural unit in which terephthalic acid chloride (TPC) is used as the dicarboxylic acid component that contributes to mechanical strength.

[0008] However, copolymers using fluorine-substituted acid anhydrides such as 6FDA have poor surface wettability and adhesion when formed into films, and require surface treatment when laminating the film to other substrates or coating materials.

[0009] Furthermore, although the use of acid anhydrides with intermolecular interactions such as sBPDA improves the toughness of the copolymer, such as its flex resistance, the YI value (yellowing) of the copolymer tends to be high due to π-electron conjugation, and in order to reduce the YI value, it was necessary to add a bluing agent to the film. As a result, in applications requiring colorless transparency, such as cover window films, the light transmittance was sometimes insufficient.

[0010] Therefore, an object of the present invention is to provide a polyamide-imide copolymer which has excellent toughness such as flex resistance, excellent transparency, and when formed into a film, has good wettability and adhesion. [Means for solving the problem]

[0011] The present inventors have focused on the imide structural unit of polyamide-imide copolymers and investigated various tetracarboxylic dianhydrides, and have found that the above-mentioned problems can be solved by combining two specific tetracarboxylic dianhydrides. The present invention has been completed based on this finding. The gist of the present invention is as follows.

[0012] [1] A polyamide-imide copolymer containing imide structural units and amide structural units, The imide structural unit is an imide structural unit I-1 represented by the following formula (1): [ka] and, At least one imide structural unit I-2 selected from the group consisting of those represented by the following formulas (2) to (5): [ka] (In the formulas (1) to (5), X1 to X5 each independently represent a divalent organic group derived from a diamine.) Including, The amide structural unit is an amide structural unit A represented by the following formula (6): [ka] (wherein 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.) A polyamide-imide copolymer comprising: [2] X1 to X6 are represented by the following formula (7): [ka] (In the formula, * represents a bonding group.) The polyamide-imide copolymer according to [1], represented by the formula: [3] Y is represented by the following formulas (8) to (10): [ka] (In the formula, * represents a bonding group.) The polyamide-imide copolymer according to [1] or [2], which is at least one selected from the following: [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). [5] The polyamide-imide copolymer according to any one of [1] to [4], wherein the imide structural units and the amide structural units are contained in a molar ratio of 2:8 to 8:2. [6] A film comprising the polyamide-imide copolymer according to any one of [1] to [5]. [7] The film according to [6], wherein the water contact angle of 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 [6] or [7], which is used as a cover window for a foldable device. [Effects of the Invention]

[0013] According to the present invention, by using an imide structural unit in which two specific tetracarboxylic dianhydrides are combined, it is possible to realize a polyamide-imide copolymer that is excellent in toughness such as flex resistance, excellent in transparency, and also has good wettability and adhesion when formed into a film.

[0014] [Polyamide-imide copolymer] The polyamide-imide copolymer of the present invention is a copolymer having an imide structure and an amide structure, and includes, 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 includes, as amide structural units, an amide structural unit A represented by the following formula (6): [ka]

[0015] In the present invention, by combining the structural unit I-1 represented by the above formula (1) with one or more structural units I-2 represented by any one of (2) to (5) as the imide structural units constituting the polyamide-imide copolymer, it is possible to simultaneously improve the transparency of the polyamide-imide copolymer and the wettability and adhesion to other components when formed into a film. 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, various tetracarboxylic dianhydrides were screened from the viewpoint of electron affinity, and as a result, by combining at least one selected from the group consisting of 3,4-oxydiphthalic dianhydride (aODPA) and 4,4'-oxydiphthalic dianhydride (sODPA), 2,2',3,3'-biphenyltetracarboxylic dianhydride (aBPDA), 9,9-bis(3,4-dicarboxyphenyl)fluorene anhydride (BPAF), and 4,4'-(hexafluoroisopropylidene)diphthalic dianhydride (6FDA), the transparency of the resin and the wettability and adhesion when formed into a film are improved compared to conventional polyamide-imide copolymers that combine 6FDA and sBPDA, without impairing the various properties derived from the imide structure, such as heat resistance and mechanical strength.

[0016] Among the structural units I-2 (2) to (5) to be combined with the structural unit I-1 represented by the above formula (1), the structural unit represented by the above formula (2) is preferred. That is, in the present invention, a combination of aODPA and sODPA is preferred as the tetracarboxylic dianhydride constituting the imide structural unit.

[0017] The molar 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.

[0018] The polyamide-imide copolymer of the present invention may contain components other than those described above as imide structural units, provided that the effects of the present invention are not impaired. The tetracarboxylic acid component constituting the imide structural unit includes various tetracarboxylic acids or tetracarboxylic acid derivatives. Examples of tetracarboxylic acid derivatives include tetracarboxylic acid anhydrides, preferably dianhydrides, and acid chlorides. Examples of tetracarboxylic acid compounds include aromatic tetracarboxylic acids and their anhydrides, preferably dianhydrides; and aliphatic tetracarboxylic acid compounds, such as aliphatic tetracarboxylic acid compounds and their anhydrides, preferably dianhydrides. These tetracarboxylic acid compounds can be used alone or in combination.

[0019] Specific examples of aromatic tetracarboxylic acid dianhydrides include non-condensed polycyclic aromatic tetracarboxylic acid dianhydrides, monocyclic aromatic tetracarboxylic acid dianhydrides, and condensed polycyclic aromatic tetracarboxylic acid dianhydrides. Examples of non-condensed polycyclic aromatic tetracarboxylic acid dianhydrides include 3,3',4,4'-biphenyltetracarboxylic acid dianhydride (sBPDA), 4,4'-(4,4'-isopropylidenediphenoxy)diphthalic dianhydride (BPADA), 3,3',4,4'-benzophenonetetracarboxylic acid dianhydride, 2,2',3,3'-benzophenonetetracarboxylic acid dianhydride, 3,3',4,4'-diphenylsulfonetetracarboxylic acid dianhydride, 2,2-bis(3,4-dicarboxyphenyl)propane dianhydride, 2,2-bis(2,3-dicarboxyphenyl)propane dianhydride, 2,2-bis( Examples of the dianhydride include 1,2-bis(2,3-dicarboxyphenyl)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 acid dianhydride include 1,2,4,5-benzenetetracarboxylic acid dianhydride, and examples of the condensed polycyclic aromatic tetracarboxylic acid dianhydride include 2,3,6,7-naphthalenetetracarboxylic acid dianhydride.

[0020] Examples of aliphatic tetracarboxylic acid dianhydrides include cyclic and acyclic aliphatic tetracarboxylic acid dianhydrides. Cyclic aliphatic tetracarboxylic acid dianhydrides are tetracarboxylic acid dianhydrides having an alicyclic hydrocarbon structure, and specific examples include cycloalkane tetracarboxylic acid dianhydrides such as 1,2,4,5-cyclohexane tetracarboxylic acid dianhydride (HPMDA), 1,2,3,4-cyclobutane tetracarboxylic acid dianhydride (CBDA), and 1,2,3,4-cyclopentane tetracarboxylic acid dianhydride, bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic acid dianhydride, dicyclohexyl-3,3',4,4'-tetracarboxylic acid dianhydride (HBPDA), and positional isomers thereof. These can be used alone or in combination of two or more. Specific examples of the acyclic aliphatic tetracarboxylic dianhydride include 1,2,3,4-butanetetracarboxylic dianhydride and 1,2,3,4-pentanetetracarboxylic dianhydride, which can be used alone or in combination of two or more. Also, a cyclic aliphatic tetracarboxylic dianhydride and an acyclic aliphatic tetracarboxylic dianhydride can be used in combination.

[0021] The imide structural unit may contain, in addition to the above structural units, structural units derived from the above-mentioned tetracarboxylic dianhydride water adducts or tricarboxylic acid compounds, as long as the effects of the present invention are not impaired. Examples of tricarboxylic acid compounds include aromatic tricarboxylic acids, aliphatic tricarboxylic acids, and their related acid chloride compounds and acid anhydrides, and two or more of these may be used in combination. 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 via a single bond, -O-, -CH2-, -C(CH3)2-, -SO2-, or a phenylene group.

[0022] The polyamide-imide copolymer according to the present invention contains an amide structural unit A represented by the following formula (6): By forming a copolymer having an imide structure with rigid properties and an amide structure with flexible properties, it is possible to achieve a high level of mechanical properties, namely, excellent flexibility and high elasticity, which are in a trade-off relationship, without sacrificing transparency or wettability. [ka] (wherein 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.)

[0023] The polyamide-imide copolymer containing the above-mentioned amide structural unit A can be obtained by reacting the monomer components, that is, a diamine compound, a tetracarboxylic acid compound, and a dicarboxylic acid compound. Specifically, the polyamide-imide copolymer can be obtained by reacting a diamine compound with a tetracarboxylic acid compound to synthesize a polymer having an imide precursor structure, then reacting the polymer with a dicarboxylic acid compound to synthesize a copolymer having an imide precursor structure and an amide structure, and then subjecting the imide precursor structure in the copolymer to a ring-closing reaction (imidization).

[0024] The polyamide-imide copolymer has a structure in which a residue resulting from the reaction of a diamine compound with a tetracarboxylic acid compound is bonded via an imide structure to a structural unit in which a residue resulting from the reaction of a diamine compound with a tetracarboxylic acid compound is bonded via an amide structure.

[0025] 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 the acid chlorides and esters of the dicarboxylic acid. The dicarboxylic acids can be used alone or in combination.

[0026] Specific examples of dicarboxylic acids include alicyclic dicarboxylic acids or aromatic dicarboxylic acids and derivatives thereof (e.g., acid chlorides, acid anhydrides), such as 1,3-cyclobutanedicarboxylic acid, 1,3-cyclopentanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, 4,4'-oxybisbenzoic acid, terephthalic acid, isophthalic acid, 2,6-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 1,4-naphthalenedicarboxylic acid, 4,4'-biphenyldicarboxylic acid, 3,3'-biphenyldicarboxylic acid, and compounds in which two cyclohexanecarboxylic acids or two benzoic acids are linked by a single bond, -CH-, -C(CH)-, -C(CF)-, -SO-, or a phenylene group; and aliphatic dicarboxylic acids and derivatives thereof (e.g., acid chlorides, esters), such as dicarboxylic acid compounds of chain hydrocarbons having 8 or less carbon atoms. These dicarboxylic acid compounds can be used alone or in combination of two or more.

[0027] Among these, from the viewpoint of improving the elongation at break and elastic modulus of the film, it is preferable to use terephthalic acid, isophthalic acid, 4,4'-biphenyldicarboxylic acid, or 4,4'-oxybisbenzoic acid or a derivative thereof, in particular terephthalic acid chloride (TPC), isophthalic acid chloride (IPC), 4,4'-biphenyldicarbonyl chloride (BPC), or 4,4'-oxybis(benzoyl chloride) (OBBC). Specifically, it is preferable 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 preferable to have an amide structural unit A-1. [ka] (wherein X6 represents a divalent organic group derived from a diamine).

[0028] As the amide structural unit, the above-mentioned amide structural unit A-1 may be used in combination with other amide structural units, and examples of the other amide structural units include those derived from the various dicarboxylic acids or dicarboxylic acid derivatives described above, with the amide structural unit A-2 or A-3 being preferred. When the amide structural unit A-1 is used in combination with the amide structural unit A-2 or A-3 as the amide structural unit, the constituent ratio thereof is preferably in the range of 10:1 to 5:1, from the viewpoint of the balance between optical properties such as transparency and dynamic properties such as mechanical strength.

[0029] The molar ratio of imide structures to amide structures in the polyamide-imide copolymer of the present invention is preferably 0.5-4:3-6.5, more preferably 1.5-3.5:3.5-5.5, and particularly preferably 3:4. When the molar ratio of imide structures to amide structures is within the above range, a good balance between excellent flexibility and high elasticity can be achieved.

[0030] The diamine components (i.e., divalent organic groups represented by X1 to X6) constituting the imide structural unit and amide structural unit described above are not particularly limited, and diamine components used in conventionally known polyimides and polyamideimides can be used, such as aliphatic diamines, aromatic diamines, and mixtures thereof. Here, "aromatic diamine" refers to a diamine in which an amino group is directly bonded to an aromatic ring, and may contain an aliphatic group or other substituent as part of its structure. The aromatic ring may be a single ring or a condensed ring, and examples thereof include a benzene ring, a naphthalene ring, an anthracene ring, and a fluorene ring, but are not limited to these. Among these, a benzene ring is preferred. Furthermore, "aliphatic diamine" refers to a diamine in which an amino group is directly bonded to an aliphatic group, and may contain an aromatic ring or other substituent as part of its structure. The diamine compounds can be used alone or in combination of two or more.

[0031] Specific examples of the aliphatic diamine include acyclic 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.

[0032] Specific examples of aromatic diamines include aromatic diamines having one aromatic ring, such as p-phenylenediamine, m-phenylenediamine, 2,4-toluenediamine, m-xylylenediamine, p-xylylenediamine, 1,5-diaminonaphthalene, and 2,6-diaminonaphthalene; 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenylpropane, 4,4'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 3,3'-diaminodiphenyl ether, 4,4'-diaminodiphenyl sulfone, 3,4'-diaminodiphenyl sulfone, 3,3'-diaminodiphenyl sulfone, 1,4-bis(4-aminophenoxy)benzene, 1,3-bis(4-aminophenoxy)benzene, bis[4-(4-aminophenoxy)phenyl]sulfone, and bis[4-(3 Examples of aromatic diamines include aromatic diamines having two or more aromatic rings, such as 2,2-bis[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.

[0033] Among the above diamine compounds, from the viewpoint of improving the colorless transparency and elasticity of the film, it is preferable 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 preferable to use a diamine having a biphenyl structure in which some or all of the hydrogen atoms on the aromatic ring are substituted with substituents selected from fluoro groups, trifluoromethyl groups, or trifluoromethoxy groups, specifically 2,2'-bis(trifluoromethyl)benzidine (TFMB) represented by the following formula. [ka]

[0034] As the diamine compound, the above-mentioned TFMB may be used alone, or TFMB may be used in combination with one or more of the above-mentioned various diamine compounds. When TFMB is used in combination with various other diamine compounds, the composition ratio is preferably in the range of 10:1 to 5:1, from the viewpoint of the balance between optical properties such as transparency and mechanical properties such as mechanical strength.

[0035] In the synthesis of the polyamide-imide copolymer, the molar ratio of the monomer components (diamine compound: tetracarboxylic acid compound: dicarboxylic acid compound) is preferably 7:0.5-4:3-6.5, more preferably 7:1.5-3.5:3.5-5.5, and particularly preferably 7:3:4.

[0036] The ring-closing reaction (imidization) 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 carried out by either thermal imidization, in which an azeotropic solvent (e.g., toluene, xylene, etc.) that forms an azeotrope with water is added and heated, or chemical imidization, in which a condensing agent and a reaction accelerator are used. However, chemical imidization is preferred because colorlessness and transparency are maintained.

[0037] Examples of reaction accelerators used in chemical imidization include triethylamine, diisopropylethylamine, N-methylpiperidine, pyridine, 2-methylpyridine, 3-methylpyridine, 4-methylpyridine, 3-ethylpyridine, 3,5-dimethylpyridine, 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.

[0038] Condensing agents used in chemical imidization include acid anhydrides such as acetic anhydride, propionic anhydride, and trifluoroacetic anhydride, and phosphites such as phosphites, 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.

[0039] The imidization rate is preferably 90% or more, more preferably 93% or more, and even more preferably 96% or more. From the viewpoint of easily improving optical homogeneity such as transparency, a higher imidization rate is preferable. The upper limit of the imidization rate is 100% or less. The imidization rate indicates the ratio of the molar amount of imide bonds in the imide structural unit to twice the molar amount of structural units derived from tetracarboxylic dianhydride in the imide structural unit. The imidization rate can be determined by IR method, NMR method, etc.

[0040] The organic solvent used in the synthesis of polyamide-imide copolymers is not particularly limited as long as it is an organic solvent inert to the reaction. Examples 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.

[0041] The reaction conditions for the synthesis can be set to 10 to 50° C. for 1 to 27 hours or less, and the synthesis is preferably carried out in a nitrogen atmosphere in order to maintain colorlessness and transparency.

[0042] The polyamide-imide copolymer may be isolated (separated and purified) by a conventional method, for example, a separation means such as filtration, concentration, extraction, crystallization, recrystallization, column chromatography, or a combination of these. In a preferred embodiment, the resin can be isolated by adding a large amount of alcohol such as methanol to a reaction solution containing a transparent polyamide-imide resin to precipitate the resin, followed by concentration, filtration, drying, etc.

[0043] 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 modulus of elasticity and elongation at break. The weight-average molecular weight (Mw) is a value measured by GPC (gel permeation chromatography) and calculated in terms of polystyrene.

[0044] <Film> The polyamide-imide copolymer of the present invention can be dissolved in an appropriate solvent to form a resin composition (resin varnish), which can be applied to a support to form a coating film, dried to remove the solvent, and peeled off from the support to obtain a film. Any solvent can be used as long as it can dissolve polyamide-imide, but from the viewpoints of the coatability 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 sulfone group, and a sulfinyl group is preferred.

[0045] Examples of the solvent having an ester group include ester-based solvents such as methyl acetate, ethyl acetate, butyl acetate, and dimethyl carbonate. Examples of solvents having a cyclic ester group include lactone solvents such as γ-butyrolactone (GBL), δ-valerolactone, ε-caprolactone, γ-crotonolactone, γ-hexanolactone, α-methyl-γ-butyrolactone, γ-valerolactone, α-acetyl-γ-butyrolactone, and δ-hexanolactone. Examples of solvents having an ether group include tetrahydrofuran, dioxane, and dibutyl ether. Examples of the solvent having a ketone group include ketone-based solvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone. Examples of solvents having a hydroxyl group include phenolic solvents such as m-cresol. Examples of solvents having a sulfone group include methyl sulfone, ethyl phenyl sulfone, diethyl sulfone, diphenyl sulfone, sulfolane, bisphenol S, sorapsone, dapsone, bisphenol A polysulfone, and sulfolane. Examples of solvents having a sulfinyl group include sulfoxide solvents such as N,N-dimethyl sulfoxide (DMSO). 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.

[0046] The resin composition for forming a film may contain any component other than the polyamide-imide copolymer, and examples thereof include a leveling agent for improving the coatability of the varnish when producing the film, a dispersant, a surfactant, a retardation adjuster, an antioxidant, an ultraviolet inhibitor, a light stabilizer, a plasticizer, waxes, a filler, a pigment, a dye, a foaming agent, an antifoaming agent, a dehydrating agent, an antistatic agent, an antibacterial agent, an antifungal agent, and a bluing agent for reducing the yellowness of the film.

[0047] The content of the polyamide-imide copolymer in the resin composition is preferably in the range of 65 to 100 mass % relative to the total amount of solids excluding the solvent, more preferably in the range of 80 to 100 mass %, and even more preferably in the range of 90 to 100 mass %.

[0048] As a means for applying the resin composition containing a polyamide-imide copolymer onto a support, a conventionally known means can be applied, such as a dip coating method, a flow coating method, a roll coating method, a bar coater method, a blade coater method, a screen printing method, a curtain coating method, and a spray coating method.

[0049] There are no particular restrictions on the conditions for drying the coating film as long as the temperature is such that the solvent volatilizes, but from the viewpoint of obtaining a film with excellent transparency, drying at 60 to 250° C. for about 10 to 60 minutes is preferred.

[0050] The film of the present invention preferably has a thickness of 5 μm or more and 100 μm or less, more preferably 10 μm or more and 50 μm or less. By setting the thickness within the above range, a film with excellent flexibility can be obtained, and the film can be suitably used as a cover window for a foldable display or a flexible display. The film thickness can be adjusted by the amount of resin composition applied.

[0051] Because the film of the present invention is formed from a polyamide-imide copolymer containing two specific imide structural units as described above, it has excellent toughness, such as bending resistance, as well as excellent transparency, and also has good surface wettability and adhesion to other materials. For example, a 50 μm-thick monolayer film formed from a polyamide-imide copolymer can have a YI value of 2.0 or less. In this specification, the YI value (yellowness index) refers to a value calculated based on the formula YI = 100 × (1 − 0.847Z) / Y, obtained by measuring the transmittance of light from 360 to 780 nm using a spectrophotometer in accordance with ASTM E313-73 and determining the tristimulus values ​​(X, Y, Z).

[0052] The film of the present invention also has excellent wettability and adhesion. For example, a film formed on a glass substrate to a thickness of 50 μm and having an arithmetic mean surface roughness Ra of 50 nm or less has a water contact angle of 55° or less. Thus, because the film of the present invention has excellent wettability even when its surface is smooth, surface treatment of the film is not required, unlike conventional polyimide amide films, for example, when the film is bonded to another substrate or laminated with a coating film made of a decorative coloring material. The water contact angle refers to the water contact angle measured in accordance with JIS R3257:1999, and the arithmetic mean surface roughness Ra refers to a value measured using a measuring device in accordance with JIS B0601-1994.

[0053] [Application] Examples of display components using the film of the present invention include thin, bendable foldable organic EL displays, mobile terminals such as smartphones and wristwatch-type terminals, display devices inside automobiles, and flexible panels used in wristwatches. The film can also be used as components for image display devices such as liquid crystal displays and organic EL displays, touch panel components, flexible printed circuit boards, solar cell panel components such as surface protection films and substrate materials, optical waveguide components, and other semiconductor-related components. In particular, the film is preferably used as a component for cover windows and TFT substrates that constitute foldable organic EL displays.

[0054] [Display cover window] The cover window of a display using the film of the present invention can be used, for example, by placing the film on the surface of various displays. The method of placing the film on the surface is not particularly limited, and examples include a method using an adhesive layer. As the material for the adhesive layer, any conventionally known adhesive material that can be used to adhere display surface materials can be used. Note that the cover window of a display using the film of the present invention may be provided with a protective layer such as a hard coat layer on the film surface, and further with an anti-fingerprint layer.

[0055] [TFT substrate for organic EL displays] A TFT substrate for an organic EL display using the film of the present invention can be obtained, for example, by forming an amorphous silicon TFT (thin film transistor) on the film of the present invention. The TFT includes a gate metal layer, a silicon nitride gate dielectric layer, and an ITI pixel electrode. Furthermore, the structures required for the organic EL display can be formed on top of this by known methods, and the method for forming circuits, etc. is not particularly limited. [Example]

[0056] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. In the following, "parts" and "%" are all by mass unless otherwise specified.

[0057] [Example 1] <Preparation of Polyamide-imide> A 100 mL reactor was charged with 60.9 g of DMAc and 4.849 g (15.14 mmol) of TFMB was added. Next, 1.007 g (3.245 mmol) of aODPA and 1.4875 g (3.245 mmol) of BPAF were added to the TFMB solution and stirred at 30°C for 2 hours to produce a solution containing a polymer with an imide precursor structure. Then, 1.757 g (8.653 mmol) of TPC was added to the solution and stirred for 1.5 hours while maintaining the liquid temperature at 30°C to produce a solution containing a copolymer with an imide precursor structure and an amide structure. Next, 2.09 g of pyridine, 2.45 g of acetic anhydride, and 8.53 g of DMAc were added and stirred at 20-30°C for 8 hours to obtain a polyamideimide solution. 99 g of DMAc was then added and stirred until homogeneous. The solution was then gradually poured into a container containing 4 L of methanol to precipitate the solids. The precipitated solids were filtered and crushed, and then dried in a vacuum at 80°C for 18 hours to obtain 8.2 g of polyamideimide copolymer (PAI-1) as a solid powder. The weight-average molecular weight (Mw) of the polystyrene equivalent measured by GPC was 597,000.

[0058] [Example 2] A polyamideimide 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 polyamideimide solution was purified in the same manner as in Example 1, yielding 8.0 g of a polyamideimide copolymer (PAI-2) as a solid powder. The weight average molecular weight (as converted to polystyrene) measured by GPC was 201,000.

[0059] [Example 3] A polyamideimide 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 polyamideimide solution was purified in the same manner as in Example 1, yielding 7.9 g of a solid powder polyamideimide copolymer (PAI-3). The weight average molecular weight, calculated as polystyrene by GPC, was 407,000.

[0060] [Example 4] A polyamideimide 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 polyamideimide solution was purified in the same manner as in Example 1, yielding 8.3 g of a solid powder polyamideimide copolymer (PAI-4). The weight average molecular weight, calculated as polystyrene by GPC, was 311,000.

[0061] [Comparative Example 1] A polyamideimide 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 polyamideimide solution was purified in the same manner as in Example 1, yielding 8.2 g of a solid powder polyamideimide copolymer (PAI-5). The weight average molecular weight, calculated as polystyrene by GPC, was 625,000.

[0062] Comparative Example 2 A polyamideimide 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 instead of aODPA and BPAF as the tetracarboxylic dianhydride. The obtained polyamideimide solution was purified in the same manner as in Example 1, yielding 7.5 g of a polyamideimide copolymer (PAI-6) as a solid powder. The weight average molecular weight (Mw) of the polystyrene equivalent measured by GPC was 114,000.

[0063] Comparative Example 3 A polyamideimide 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 instead of aODPA and BPAF as the tetracarboxylic dianhydride. The obtained polyamideimide solution was purified in the same manner as in Example 1, yielding 7.3 g of a polyamideimide copolymer (PAI-7) as a solid powder. The weight average molecular weight (Mw) of the polystyrene equivalent measured by GPC was 100,000.

[0064] Comparative Example 4 A polyamideimide 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 instead of aODPA and BPAF as the tetracarboxylic dianhydride. The obtained polyamideimide solution was purified in the same manner as in Example 1, yielding 7.0 g of a polyamideimide copolymer (PAI-8) as a solid powder. The weight average molecular weight (Mw) of the polystyrene equivalent measured by GPC was 97,000.

[0065] Comparative Example 5 A polyamideimide 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 instead of aODPA and BPAF as the tetracarboxylic dianhydride. The obtained polyamideimide solution was purified in the same manner as in Example 1, yielding 8.4 g of a polyamideimide copolymer (PAI-9) as a solid powder. The weight average molecular weight (Mw) of the polystyrene equivalent measured by GPC was 478,000.

[0066] Comparative Example 6 A polyamideimide 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 instead of aODPA and BPAF as the tetracarboxylic dianhydride. The obtained polyamideimide solution was purified in the same manner as in Example 1, yielding 8.0 g of a polyamideimide copolymer (PAI-10) as a solid powder. The weight average molecular weight (Mw) of the polystyrene equivalent measured by GPC was 179,000.

[0067] Comparative Example 7 A polyamideimide 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 instead of aODPA and BPAF as the tetracarboxylic dianhydrides. The obtained polyamideimide solution was purified in the same manner as in Example 1, yielding 8.3 g of a polyamideimide copolymer (PAI-11) in the form of a solid powder. The weight-average molecular weight (Mw) of the polystyrene equivalent measured by GPC was 593,000.

[0068] [Comparative Example 8] A polyamideimide 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 instead of aODPA and BPAF as the tetracarboxylic dianhydrides. The obtained polyamideimide solution was purified in the same manner as in Example 1, yielding 7.9 g of a polyamideimide copolymer (PAI-12) as a solid powder. The weight average molecular weight (Mw) of the polystyrene equivalent measured by GPC was 124,000.

[0069] Comparative Example 9 A polyamideimide 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 instead of aODPA and BPAF as the tetracarboxylic dianhydride. The obtained polyamideimide solution was purified in the same manner as in Example 1, yielding 8.1 g of a polyamideimide copolymer (PAI-13) as a solid powder. The weight average molecular weight (Mw) of the polystyrene equivalent measured by GPC was 77,000.

[0070] [Comparative Example 10] A polyamideimide 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 instead of aODPA and BPAF as the tetracarboxylic dianhydride. The obtained polyamideimide solution was purified in the same manner as in Example 1, yielding 8.4 g of a polyamideimide copolymer (PAI-14) as a solid powder. The weight average molecular weight (Mw) of the polystyrene equivalent measured by GPC was 391,000.

[0071] [Comparative Example 11] A polyamideimide 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 instead of aODPA and BPAF as the tetracarboxylic dianhydride. The obtained polyamideimide solution was purified in the same manner as in Example 1, yielding 7.8 g of a polyamideimide copolymer (PAI-15) as a solid powder. The weight average molecular weight (Mw) of the polystyrene equivalent measured by GPC was 429,000.

[0072] Table 1 shows the molar ratios of the components constituting each polyamide-imide copolymer obtained as described above.

[0073] <Film Preparation> 5.0 g of each polyamideimide obtained as described above was dissolved in 45 g of DMAc to prepare a resin varnish. Subsequently, it was applied onto a glass plate using a table coater (AFA-standard manufactured by Coatec), and dried in an inert gas oven (INL-45N1 manufactured by Yamato Scientific Co., Ltd.) at 70 °C for 1 hour and then at 250 °C for 1 hour, and a film was formed by peeling it from the glass plate. The film thickness of the obtained film was as shown in Table 1 below. Using each of the obtained films as evaluation samples, the following evaluations were performed.

[0074] <Measurement of total light transmittance and haze> Each film was cut into a size of 30 mm × 30 mm, and in accordance with ASTM D 1003, the total light transmittance and haze were measured using a haze meter (NDH 7000 II manufactured by Nippon Denshoku Industries Co., Ltd.). Each measured value was normalized by the film thickness (50 μm thickness). The measurement results were as shown in Table 1 below.

[0075] <YI value (Yellow Index) evaluation> Each film was cut into a size of 30 mm × 30 mm, and in accordance with ASTM E313, the YI value was determined using a spectrophotometer (CM-5 manufactured by Konica Minolta Co., Ltd.), and the YI value was evaluated according to the following evaluation criteria. ○: YI value is less than 2 △: YI value is 2 or more and less than 3 ×: YI value is 3 or more The evaluation results were as shown in Table 1 below.

[0076] <Toughness evaluation> Each film was cut into a predetermined size, and the elastic modulus, breaking point stress, and breaking point elongation were measured using a small bench-top testing machine (EZ-SX manufactured by Shimadzu Corporation). The elastic modulus was determined from the slope of the stress-strain diagram where the stress was from 5 MPa to 10 MPa. Also, the breaking energy was calculated by obtaining the product of stress and strain (i.e., the area formed by the stress-strain curve) from the stress-strain curve. Based on the breaking energy, the toughness was evaluated according to the following evaluation criteria. ○: Breaking energy is 0.6 J or more △: Breaking energy is 0.4J or more and less than 0.6J ×: Breaking energy is less than 0.4 J The evaluation results are shown in Table 1 below.

[0077] <Film wettability> The contact angle with water on the surface of each film that had been 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. ○: Water contact angle is 55 degrees or less △: Water contact angle is over 55 degrees and less than 65 degrees ×: Water contact angle is over 65 degrees The evaluation results are shown in Table 1 below.

[0078] <Film adhesion> A decorative coloring material was applied by screen printing to the surface of each film that had been in contact with the glass plate so that the film thickness after drying would be 2 to 3 μm, and the film was dried by heating at 80°C for 30 minutes to form a colored coating film, thereby obtaining a sample for evaluating adhesion. The decorative coloring material was prepared as follows. (decorative coloring material) A 500 ml separable flask equipped with a nitrogen inlet tube and a stirrer was charged with 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)propanoic dianhydride] (BPADA), 94.64 g of ethyl benzoate, 0.47 g (6 mmol) of pyridine, and 10 g of toluene. The mixture was reacted under a nitrogen atmosphere at 180°C for 4 hours while removing the toluene from the system during the reaction, thereby preparing a polyimide compound solution with a solids content of 20 mass %. Next, 12.5 g of carbon black and 0.5 g of DBP (a phosphoric ester-based adhesion aid manufactured by Johoku Chemical Industry Co., Ltd.) were mixed with 50 g of this solution and dispersed until uniform, thereby preparing a decorative coloring material.

[0079] For each sample obtained as described above, the adhesion between the film and the colored coating was evaluated in accordance with JIS K 5600-5-6 (ISO 2409). Specifically, 100 grid-like cuts were made in the colored coating with a single-edged blade at 1 mm intervals, and then "Cellotape" (registered trademark) was applied. The Cellotape (registered trademark) was then peeled off, and the condition of the colored coating was visually inspected and evaluated based on the evaluation criteria of JIS K 5600-5-6 (ISO 2409) (a 6-point scale from 0 to 5, with adhesion being highest). The evaluation results are shown in Table 1 below.

[0080] [Table 1]

[0081] As is clear from the evaluation results in Table 1, the polyamide-imide copolymers (Examples 1 to 4) that use a combination of an imide structural unit (unit I-1) derived from aODPA and a specific imide structural unit (unit I-2) as the tetracarboxylic dianhydride component are superior in all of the optical properties, mechanical properties, wettability, and adhesion to the polyamide-imide copolymer (Comparative Example 1) that uses a combination of an imide structural unit (unit I-1) and another imide structural unit (sBPDA) and the polyamide-imide copolymers that do not have a structural unit derived from aODPA (Comparative Examples 2 to 11).

Claims

1. A polyamide-imide copolymer containing imide structural units and amide structural units, The imide structural unit is an imide structural unit I-1 represented by the following formula (1): 【Chemistry 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): 【Chemistry 2】 (In the formulas (1) to (5), X １ ~X ５ each independently represents a divalent organic group derived from 2,2'-bis(trifluoromethyl)benzidine as a diamine. Including, The amide structural unit is an amide structural unit A represented by the following formula (6): 【Transformation 3】 (In the formula, X ６ represents a divalent organic group derived from 2,2'-bis(trifluoromethyl)benzidine as a diamine, and Y represents a divalent organic group derived from terephthalic acid chloride as a dicarboxylic acid. A polyamide-imide copolymer comprising:

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

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

2.

4. A film comprising the polyamide-imide copolymer according to any one of claims 1 to 3.

5. 5. The film according to claim 4, wherein the water contact angle of the film surface having a thickness of 50 μm measured in accordance with JIS R3257:1999 is 55 degrees or less.

6. The film according to claim 4 or 5, which is used as a cover window for a foldable device.

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