Polyamide resin, method for producing polyamide resin, polyamide film, and flexible display

Abstract

The present invention addresses the problem of providing a polyamide resin that enables the production of a film having good mechanical properties while suppressing production costs. The problem is solved by a polyamide resin comprising structural units represented by formula (1) and structural units represented by formula (2).

Description

Polyamide resin, method for producing polyamide resin, polyamide film and flexible display

[0001] The present invention relates to a polyamide resin, a method for producing a polyamide resin, a polyamide film containing the polyamide resin, and a flexible display including the polyamide film.

[0002] Display device components, such as liquid crystal display devices and organic electroluminescence (EL) display devices, are widely used in a variety of applications, including mobile phones and tablets. Traditionally, glass has been used as the front panel of such display devices. However, while glass is highly transparent and, depending on the type, can exhibit high hardness, it is also very rigid and prone to breakage, making it difficult to use as a front panel material for flexible displays that are bent or folded. Therefore, films using polymeric materials are being considered as alternatives to glass. Because films containing polymeric materials are likely to exhibit flexible properties, they are expected to be used in a variety of applications, including flexible displays.

[0003] Conventionally, flexible displays have suffered from pressure marks and flex marks on their surfaces 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 (elongation at break) but also high mechanical strength (modulus of elasticity and maximum strength), i.e., excellent mechanical properties. Furthermore, films containing polyimide resins are also required to have excellent optical properties, such as transparency. To meet these requirements, films containing polyimide resins have been investigated and proposed.

[0004] For example, Patent Document 1 below discloses a polyamide-imide resin having a structural unit A derived from a tetracarboxylic dianhydride, a structural unit B derived from a diamine, and a structural unit C derived from an aromatic dicarboxylic acid dichloride, with the aim of applying a polyamide-imide resin that is excellent in mechanical properties (mechanical strength), heat resistance, and transparency and that can form a film that further achieves reduced residual stress.

[0005] In addition, in the following Patent Document 2, for the purpose of providing an aromatic polyamide having excellent transparency and resistance to solvents, a polyamide having a structural unit represented by the formula (1) described below, and having an absorbance ratio of 1630 cm by ATR measurement of FT-IR is disclosed. －１ / 1660cm －１ = 0.4 or more and 1.0 or less is disclosed.

[0006] International Publication No. 2019 / 216151 Japanese Patent Application Laid-Open No. 2019-1853

[0007] However, in the resin film having an imide structural unit as disclosed in Patent Document 1, an imidization step is required, which complicates the manufacturing process and increases the manufacturing cost. Therefore, there is a demand for a film that can be manufactured by a simple manufacturing process at reduced manufacturing cost and has good properties. Furthermore, Patent Document 2 proposes a polyamide resin, but while the polyamide resin has excellent transparency, there is room for improvement in its mechanical properties.

[0008] Therefore, an object of the present invention is to solve the above-mentioned problems of the conventional technology and to provide a polyamide resin from which a film having good mechanical properties can be produced while suppressing production costs. Another object of the present invention is to provide a method for producing such a polyamide resin, a polyamide film containing such a polyamide resin, and a flexible display including the polyamide film.

[0009] The present inventors have found that the above-mentioned problems can be solved by including in a polyamide resin a structural unit represented by the formula (1) and a structural unit represented by the formula (2) described below, and have completed the present invention.

[0010] The gist and configuration of the present invention to solve the above problems is as follows.

[0011] [1] A polyamide resin containing a structural unit represented by the following formula (1) and a structural unit represented by the following formula (2):

[0012] [2] The polyamide resin according to [1], further comprising a structural unit represented by the following formula (3):

[0013] [3] The polyamide resin according to [2], wherein the ratio of the number of moles of the structural unit represented by the formula (2) to the total number of moles of the structural unit represented by the formula (2) and the structural unit represented by the formula (3) is 10 to 90 mol%.

[0014] [4] The polyamide resin according to [2] or [3], wherein the ratio of the number of moles of the structural unit represented by the formula (2) to the total number of moles of the structural unit represented by the formula (2) and the structural unit represented by the formula (3) is 30 to 90 mol%.

[0015] [5] The polyamide resin according to any one of [1] to [4], having a number average molecular weight of 5,000 or more and 200,000 or less.

[0016] [6] A method for producing a polyamide resin according to any one of [1] to [5], comprising reacting a diamine with a dicarboxylic acid dichloride, wherein the diamine comprises an aromatic diamine represented by the following formula (1A), and the dicarboxylic acid dichloride comprises an aromatic dicarboxylic acid dichloride represented by the following formula (2A):

[0017] [7] The method for producing a polyamide resin according to [6], wherein the dicarboxylic acid dichloride further contains an aromatic dicarboxylic acid dichloride represented by the following formula (3A):

[0018] [8] A polyamide film comprising the polyamide resin according to any one of [1] to [5].

[0019] [9] A flexible display comprising the film according to [8].

[0020] According to the present invention, it is possible to provide a polyamide resin that can be used to produce a film having good mechanical properties while keeping production costs down, a method for producing the polyamide resin, a polyamide film containing the polyamide resin, and a flexible display that includes the polyamide film.

[0021] Hereinafter, the polyamide resin of the present invention, the method for producing the polyamide resin, the polyamide film containing the polyamide resin, and the flexible display including the polyamide film will be described in detail based on embodiments thereof.

[0022] In this specification, the expression "a to b" in the description of a range of values ​​means that the range is from a to b, unless otherwise specified.

[0023] <Polyamide Resin> The polyamide resin of the present invention is characterized by containing a structural unit represented by the following formula (1) and a structural unit represented by the following formula (2).

[0024] In this specification, "polyamide resin" refers to a resin in which structural units derived from a diamine compound and structural units derived from a dicarboxylic acid compound are repeated, and the repeating structural units are units having an amide structure. Furthermore, unlike polyamideimide resins, the polyamide resin of the present invention does not contain an imide component (imide structure), and therefore does not require complicated processes such as an imidization process in the production of the resin, thereby reducing production costs. Furthermore, when used in a film, the polyamide resin of the present invention can have good mechanical properties such as flexibility and mechanical strength by having both the structural unit represented by the above formula (1) and the structural unit represented by the above formula (2). The presence of the structural unit represented by the above formula (2) can also be advantageous in terms of the storage stability of a polyamide resin composition containing the polyamide resin of the present invention. It is preferable that the bonding mode between the repeating structural units of the polyamide resin of the present invention consists essentially of amide bonds.

[0025] (Structural Unit Represented by Formula (1)) The polyamide resin of the present invention contains the structural unit represented by the above formula (1) as a structural unit derived from a diamine compound. It is presumed that the inclusion of the structural unit represented by formula (1) (2,2'-bis(trifluoromethyl)biphenyl structural unit) in the polyamide resin provides a highly rigid structure containing an aromatic ring, a large molecular volume, and a high ionization potential of the structural unit represented by formula (1) due to the trifluoromethyl group, thereby imparting mechanical strength and colorless transparency to the polyamide resin.

[0026] The structural unit represented by formula (1) is preferably a structural unit derived from an aromatic diamine compound represented by the following formula (1A): The aromatic diamine represented by the following formula (1A) is 2,2'-bis(trifluoromethyl)benzidine (TFMB).

[0027] (Structural units based on other diamine compounds) In the polyamide resin of the present invention, the structural units based on the diamine compound may contain structural units other than the structural units represented by formula (1) within a range that does not impair the effects of the present invention. Moreover, the structural units other than the structural units represented by formula (1) may be one or more types. The ratio of the structural units represented by formula (1) in the structural units based on the diamine compound is preferably 70 mol% or more, more preferably 90 mol% or more, and particularly preferably 100 mol%, that is, the structural units based on the diamine compound do not substantially contain structural units other than formula (1), and are substantially only structural units represented by formula (1).

[0028] The structural unit based on a diamine compound that provides a structural unit other than the structural unit represented by formula (1) may be a structural unit based on an aromatic diamine compound or a structural unit based on an aliphatic diamine compound.

[0029] In this specification, the term "aromatic diamine compound" refers to a diamine in which an amino group is directly bonded to an aromatic ring, and the structure may contain an aliphatic group or other substituent. 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.

[0030] Specific examples of the aromatic diamine compound include, but are not limited to, 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'-diaminodiphenylether, 3,4'-diaminodiphenylether, 3,3'-diaminodiphenylether, 4,4'-diaminodiphenylsulfone, 3,4'-diaminodiphenylsulfone, 3,3'-diaminodiphenylsulfone, 1,4-bis(4-aminophenoxy)benzene ... Examples of aromatic diamines include aromatic diamines having two or more aromatic rings, such as 2,3-bis(4-aminophenoxy)benzene, bis[4-(4-aminophenoxy)phenyl]sulfone, bis[4-(3-aminophenoxy)phenyl]sulfone, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, 2,2-bis[4-(3-aminophenoxy)phenyl]propane, 2,2'-dimethylbenzidine, 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, and 9,9-bis(4-amino-3-fluorophenyl)fluorene. These can be used alone or in combination of two or more.

[0031] In this specification, the term "aliphatic diamine compound" refers to a diamine in which an amino group is directly bonded to an aliphatic group, and the diamine compound may contain an aromatic ring or other substituents as part of its structure. The diamine compound may be used alone or in combination of two or more kinds.

[0032] Specific examples of the aliphatic diamine compound include, but are not limited to, 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.

[0033] (Structural Unit Represented by Formula (2)) The structural unit based on a dicarboxylic acid compound contains a structural unit represented by formula (2). It is presumed that the structural unit based on a dicarboxylic acid compound contains a structural unit represented by formula (2), which improves flexibility and storage stability while maintaining the colorless transparency of the polyamide resin.

[0034] In this specification, the term "dicarboxylic acid compound" refers to a dicarboxylic acid or a dicarboxylic acid derivative. Examples of the dicarboxylic acid derivative include acid chlorides and esters of aromatic dicarboxylic acids.

[0035] The structural unit based on a dicarboxylic acid compound is preferably a structural unit based on a dicarboxylic acid derivative, and more preferably a structural unit based on a dicarboxylic acid dichloride.

[0036] The structural unit based on the dicarboxylic acid compound preferably further contains a structural unit represented by the following formula (3).

[0037] When the polyamide resin of the present invention contains the structural unit represented by formula (3), the mechanical strength such as tensile modulus is improved.

[0038] The structural unit represented by formula (2) is preferably a structural unit derived from an aromatic dicarboxylic acid dichloride represented by the following formula (2A): The aromatic dicarboxylic acid dichloride represented by the following formula (2A) is oxybisbenzoyl chloride (DEDC).

[0039] The structural unit represented by formula (3) is preferably a structural unit derived from an aromatic dicarboxylic acid dichloride represented by the following formula (3A): The aromatic dicarboxylic acid dichloride represented by the following formula (3A) is terephthaloyl chloride (TPC).

[0040] In the polyamide resin of the present invention, the structural units based on the dicarboxylic acid compound may contain structural units other than the structural units represented by formula (2) and formula (3) within the range not impairing the effects of the present invention. The structural units other than the structural units represented by formula (2) and formula (3) may be of one type or two or more types.

[0041] When the structural unit represented by formula (3) is not contained, the ratio of the structural unit represented by formula (2) in the structural unit based on the dicarboxylic acid compound is preferably 70 mol% or more, more preferably 90 mol% or more, and particularly preferably 100 mol%, i.e., the structural unit based on the dicarboxylic acid compound does not substantially contain any structural unit other than formula (2), and the structural unit based on the dicarboxylic acid compound is substantially only the structural unit represented by formula (2). Furthermore, when the structural unit based on the dicarboxylic acid compound contains the structural unit represented by formula (2) and the structural unit represented by formula (3), the total ratio of the structural units represented by formula (2) and formula (3) in the structural unit based on the dicarboxylic acid compound is preferably 70 mol% or more, more preferably 90 mol% or more, and particularly preferably 100 mol%, i.e., the structural unit based on the dicarboxylic acid compound does not substantially contain any structural unit other than formula (2) and formula (3), and the structural unit based on the dicarboxylic acid compound is substantially only the structural unit represented by formula (2) and formula (3).

[0042] In the polyamide resin of the present invention, from the viewpoint of improving the flexibility of the film, it is preferable that the structural unit represented by formula (3) is not contained, and from the viewpoint of an excellent balance between the mechanical strength and flexibility of the film, it is preferable that the structural unit represented by formula (2) and formula (3) is contained. In the polyamide resin of the present invention, when the structural unit based on the dicarboxylic acid compound contains the structural unit represented by formula (2) and formula (3), from the viewpoint of the balance between the mechanical strength and flexibility of the film, the ratio of the number of moles of the structural unit represented by formula (2) to the total number of moles of the structural unit represented by formula (2) and the structural unit represented by formula (3) is preferably 10 to 90 mol%, and from the viewpoint of the balance between the optical properties and mechanical strength of the film, it is more preferably 30 to 90 mol%, and particularly preferably 30 to 60 mol%.

[0043] (Structural Units Based on Other Dicarboxylic Acid Compounds) The structural units based on dicarboxylic acid compounds that provide structural units other than those of formula (2) and formula (3) may be structural units based on aromatic dicarboxylic acid compounds or structural units based on aliphatic dicarboxylic acid compounds.

[0044] Specific examples of aromatic dicarboxylic acid compounds include, but are not limited to, isophthalic acid, 2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid, 1,4-naphthalenedicarboxylic acid, 1,4-phenylenedioxydiacetic acid, 1,3-phenylenedioxydiacetic acid, dibenzoic acid, diphenylmethane-4,4'-dicarboxylic acid, diphenylsulfone-4,4'-dicarboxylic acid, 4,4'-biphenyldicarboxylic acid, and derivatives thereof. These aromatic dicarboxylic acid compounds can be used alone or in combination of two or more.

[0045] Specific examples of the aliphatic dicarboxylic acid compound include, but are not limited to, aliphatic dicarboxylic acids such as 1,3-cyclobutanedicarboxylic acid, 1,3-cyclopentanedicarboxylic acid, and 1,4-cyclohexanedicarboxylic acid, or derivatives thereof. These can be used alone or in combination of two or more.

[0046] (Polyamide Resin) The polyamide resin contains the following structure (4) as a repeating unit. Preferably, 10 to 100 mol % of all repeating units of the polyamide resin are the following structure (4). Preferably, the polyamide resin contains the following structure (5) in addition to the following structural unit (4). When the polyamide resin contains the following structures (4) and (5) as repeating units, it is more preferable that 10 to 100 mol % of all repeating units are the following structures (4) and (5). It is particularly preferable that all repeating units consist essentially of the following structure (4), or essentially of the following structures (4) and (5).

[0047] From the viewpoint of improving mechanical properties, the polyamide resin of the present invention preferably has a number average molecular weight (Mn) of 5,000 to 200,000, more preferably 10,000 to 180,000. Here, the number average molecular weight (Mn) refers to a value measured by GPC (gel permeation chromatography) and calculated in terms of polystyrene.

[0048] From the viewpoint of improving mechanical properties, the polyamide resin of the present invention preferably has a weight average molecular weight (Mw) of 10,000 to 1,000,000, more preferably 50,000 to 500,000, and even more preferably 100,000 to 300,000. Here, the weight average molecular weight (Mw) refers to a value measured by GPC (gel permeation chromatography) and calculated in terms of polystyrene.

[0049] The polyamide resin of the present invention preferably has a polydispersity (Mw / Mn) of 1.0 or more and 20 or less, more preferably 1.0 or more and 15 or less, and even more preferably 1.0 or more and 4.0 or less.

[0050] The polyamide resin of the present invention can be used in a wide range of applications, including, but not limited to, materials for films, sheets, pipes, tubes, filaments, fibers, containers, etc. Among these, it is particularly suitable for use as a material for films.

[0051] <Method for producing polyamide resin> The polyamide resin of the present invention can be produced by reacting a diamine compound with a dicarboxylic acid compound (dicarboxylic acid or dicarboxylic acid derivative). The diamine compound includes a compound that provides a structural unit represented by formula (1), and the dicarboxylic acid compound includes a compound that provides a structural unit represented by formula (2). Furthermore, it is preferable that the dicarboxylic acid compound includes a compound that provides a structural unit represented by formula (3).

[0052] The method for producing a polyamide resin of the present invention includes reacting a diamine compound with a dicarboxylic acid dichloride, wherein the diamine compound includes an aromatic diamine represented by formula (1A) and the dicarboxylic acid dichloride includes an aromatic dicarboxylic acid dichloride represented by formula (2A). Preferably, the dicarboxylic acid dichloride further includes an aromatic dicarboxylic acid dichloride represented by formula (3A). More preferably, the diamine compound is an aromatic diamine represented by formula (1A) and the dicarboxylic acid dichloride is an aromatic dicarboxylic acid dichloride represented by formula (2A), or an aromatic dicarboxylic acid dichloride represented by formula (2A) and an aromatic dicarboxylic acid dichloride represented by formula (3A).

[0053] The polyamide resin can be produced by a known method for producing polyamides, such as solution polymerization, interfacial polymerization, melt polymerization, solid-state polymerization, etc. As the method for producing the polyamide resin of the present invention, the solution polymerization and interfacial polymerization are particularly preferably used.

[0054] Specifically, polyamide resins can be synthesized from dicarboxylic acid chlorides and diamine compounds by solution polymerization, in which the reaction can be carried out in an aprotic organic polar solvent.

[0055] In this reaction, hydrogen chloride is produced as a by-product, and to neutralize this, inorganic neutralizing agents such as calcium hydroxide, calcium carbonate, and lithium carbonate; and organic neutralizing agents such as ethylene oxide, propylene oxide, 1,2-butylene oxide, ammonia, and pyridine are used.

[0056] When two or more diamine compounds are used for polymerization, various methods are available, including adding diamine compounds one by one, adding 10 to 99 mol% of dicarboxylic acid dichloride relative to the diamine compounds, and then reacting the diamine compounds. Then, adding other diamine compounds and then dicarboxylic acid dichloride, and then reacting the diamine compounds. Alternatively, all diamine compounds are mixed and added, and then dicarboxylic acid dichloride is added and reacted. Similarly, when two or more dicarboxylic acid dichlorides are used, various methods are available, including adding the diamine compounds one by one and simultaneously. The molar ratio of all diamine compounds to all dicarboxylic acid dichlorides (moles of all diamine compounds:moles of all dicarboxylic acid dichlorides) can be adjusted appropriately depending on the molecular weight of the target polyamide. For example, a ratio of 49:51 to 51:49 can be used to obtain a polyamide resin with a sufficiently high molecular weight and excellent mechanical properties.

[0057] When a diamine compound and a dicarboxylic acid dichloride are used as raw materials, the end terminals are either amine or carboxylic acid depending on the composition ratio of the raw materials. From the viewpoint of improving the colorless transparency of the film, it is preferable to perform end-capping with other amines, carboxylic acid chlorides, or carboxylic acid anhydrides.

[0058] Examples of compounds used for terminal blocking include benzoyl chloride, acetyl chloride, substituted benzoyl chloride, acetic anhydride, 5-norbornene-2,3-dicarboxylic anhydride, 4-ethynylaniline, 4-phenylethynylphthalic anhydride, maleic anhydride, etc. Terminal blocking may not be performed, in which case the terminal group can be used as a crosslinking point, etc.

[0059] Examples of aprotic polar solvents used in the production of polyamide resins include sulfoxide solvents such as dimethyl sulfoxide and diethyl sulfoxide; formamide solvents such as N,N-dimethylformamide and N,N-diethylformamide; acetamide solvents such as N,N-dimethylacetamide and N,N-diethylacetamide; pyrrolidone solvents such as N-methyl-2-pyrrolidone and N-vinyl-2-pyrrolidone; and hexamethylphosphoramide and γ-butyrolactone. These aprotic polar solvents are preferably used alone or as a mixture, but aromatic hydrocarbons such as xylene and toluene can also be used. Furthermore, up to 50% by mass of an alkali metal or alkaline earth metal salt can be added to the solvent to promote dissolution of the polymer.

[0060] The reaction conditions for synthesizing the polyamide resin can be 10 to 50° C. for 10 minutes to 27 hours, and the synthesis may be carried out in a nitrogen atmosphere in order to maintain colorless transparency.

[0061] <Polyamide Film> The present invention also relates to a polyamide film containing the polyamide resin of the present invention. The polyamide film of the present invention has good mechanical properties.

[0062] The thickness of the polyamide film is not particularly limited and can be appropriately set depending on the application. The thickness of the polyamide film is preferably 10 μm or more and 150 μm or less, and more preferably 20 μm or more and 100 μm or less. The thickness can be measured using a micrometer (manufactured by Mitutoyo Corporation) and is the average value of measurements taken at five points along the width direction of the film.

[0063] The polyamide film of the present invention preferably has a total light transmittance of 80% or more, more preferably 85% or more, and even more preferably 88% or more. The total light transmittance can be measured in accordance with ASTM D1003 using a haze meter (NDH 7000 II, manufactured by Nippon Denshoku Industries Co., Ltd.).

[0064] The polyamide film of the present invention preferably has a haze of 10% or less. When the haze of the polyamide film is 10% or less, the film has excellent transparency and is therefore suitable as a film for displays, etc. Furthermore, the haze of the polyamide film is more preferably 5% or less, even more preferably 2% or less, and particularly preferably 1% or less. The haze can be measured using a haze meter (NDH 7000 II, manufactured by Nippon Denshoku Industries Co., Ltd.) in accordance with ASTM D1003.

[0065] The polyamide film of the present invention preferably has a YI value (yellow index) of 0 or more and 20 or less, more preferably 0 or more and 10 or less, and even more preferably 0 or more and 5 or less. The YI value (yellow index) can be measured in accordance with ASTM E313-73 using a spectrophotometer (CM-5, manufactured by Konica Minolta, Inc.).

[0066] From the viewpoint of the mechanical strength of the film, the polyamide film of the present invention preferably has a tensile modulus of 2.5 GPa or more, more preferably 3.0 GPa or more, and even more preferably 3.5 GPa or more. From the viewpoint of the balance between the mechanical strength and flexibility of the film, the tensile modulus may be 10.0 GPa or less, or may be 8.0 GPa or less. From the viewpoint of flexibility, the polyamide film preferably has a breaking elongation of 5% or more, more preferably 7% or more, and even more preferably 10% or more. From the viewpoint of the balance between flexibility and mechanical strength, the breaking elongation may be 30% or less. From the viewpoint of the mechanical strength of the film, the polyamide film of the present invention preferably has a maximum strength of 100 MPa or more, more preferably 120 MPa or more. From the viewpoint of the balance between the mechanical strength and optical properties of the polyamide film, the maximum strength may be 250 MPa or less. The tensile modulus, breaking elongation, and maximum strength can be measured using an EX-SX (manufactured by Shimadzu Corporation) under the following conditions. The tensile modulus can be determined from the slope of the obtained stress-strain curve at strains of 0.2% to 0.5%. [Test conditions] Sample size: 80 mm x 10 mm Distance between grippers: 50 mm Speed: 5 mm / min Number of measurements: 3

[0067] The polyamide film of the present invention may contain other components in addition to the polyamide resin, provided that the effects of the present invention are not impaired. Examples of such other components include fillers, filler dispersants, crosslinking agents, bluing agents, UV absorbers, leveling agents, pH adjusters, lubricants, phosphorus compounds, surfactants, antioxidants, light stabilizers, plasticizers, waxes, pigments, dyes, foaming agents, antifoaming agents, dehydrating agents, antistatic agents, antibacterial agents, and antifungal agents. When the polyamide film of the present invention contains a filler, the filler is preferably silica or alumina from the viewpoints of transparency and mechanical strength, and a fibrous alumina filler is particularly preferred. Here, "alumina" encompasses alumina and alumina hydrate, and examples of alumina hydrate include boehmite and pseudo-boehmite. Furthermore, "fibrous" refers to a shape having an aspect ratio (filler length / filler diameter) of 5 or more.

[0068] A polyamide film can be produced by casting a solution containing a polyamide resin onto a support, drying the resulting coating, and forming it into a film. The solution containing a polyamide resin can be obtained by dissolving a polyamide resin in a solvent. The solvent is not particularly limited as long as it can dissolve the polyamide resin. However, from the viewpoint of film transparency, etc., a solvent having a functional group selected from the group consisting of an ester group, an ether group, a ketone group, a hydroxyl group, a sulfonic acid group, and a sulfinyl group is preferred. The organic solvent may be one type, or a combination of two or more types in any ratio. Examples of solvents having an ester group include γ-butyrolactone (boiling point 204°C), ε-caprolactone (boiling point 230°C), γ-hexanolactone (boiling point 219°C), γ-valerolactone (boiling point 207°C), benzyl benzoate (boiling point 323°C), ethyl benzoate (boiling point 212°C), ethylene glycol monobutyl ether acetate (boiling point 191.5°C), ethylene glycol monoethyl ether acetate (boiling point 156.3°C), butyl lactate (boiling point 188°C), ethyl lactate (boiling point 154°C), and ethyl 3-ethoxypropionate (boiling point 169°C).

[0069] Examples of solvents having an ether group include 2-(2-butoxyethoxy)ethyl acetate (boiling point 245°C), 2-(2-ethoxyethoxy)ethyl acetate (boiling point 217°C), propyl cellosolve (boiling point 150°C), and triethylene glycol dimethyl ether (boiling point 216°C).

[0070] Examples of solvents having a ketone group include cyclohexanone (boiling point 156° C.), 1-phenylethanone (boiling point 202° C.), and benzaldehyde (boiling point 179° C.).

[0071] Examples of solvents having a hydroxyl group include 2-methylphenol (boiling point 190° C.), 3-methylphenol (boiling point 202° C.), and octyl alcohol (boiling point 195° C.).

[0072] Examples of solvents having a sulfonic group include methanesulfonic acid (boiling point 167°C), dimethyl sulfone (boiling point 238°C), diethyl sulfone (boiling point 238°C), sulfolane (boiling point 285°C), and dapsone (boiling point 177°C).

[0073] Examples of the solvent having a sulfinyl group include dimethyl sulfoxide (boiling point: 189° C.).

[0074] Examples of amide solvents that can be used include N-methyl-2-pyrrolidone (boiling point: 202°C), N,N-dimethylformamide (boiling point: 153°C), N,N-dimethylacetamide (boiling point: 165°C), and 3-butoxy-N,N-dimethylpropanamide (boiling point: 252°C).

[0075] Among these solvents, from the viewpoint of obtaining a film with a low YI value, amide solvents are more preferable, and N,N-dimethylacetamide is particularly preferable.

[0076] The solution containing the polyamide resin may contain any component other than the polyamide resin, as long as the object of the present invention is not impaired. Examples of the optional component include the other components that can be contained in the polyamide film described above.

[0077] The support is not particularly limited, and examples thereof include substrates and films such as glass, ceramic, metal (aluminum foil, etc.), and resin films (polyester films such as PET and PEN, polyimide films, polyamideimide films, polypropylene films, polystyrene films, etc.). These substrates and films may have circuits formed thereon using copper or the like.

[0078] The method for applying the solution containing the polyamide resin to the support is not particularly limited, and examples thereof include dip coating, flow coating, roll coating, bar coating, blade coating, screen printing, curtain coating, and spray coating.

[0079] After application, the solution containing the polyamide resin is dried to remove the solvent. Examples of drying methods include vacuum drying, heat drying, and a combination of these. When drying at normal pressure, the temperature can be 30 to 350°C. From the viewpoint of obtaining a colorless, highly transparent film, drying at 60 to 250°C for approximately 30 seconds to 180 minutes is preferred. In such a drying method, stepwise drying can be performed by gradually increasing the temperature from a low temperature within the above-mentioned temperature and time ranges. Drying may also be performed under a nitrogen atmosphere.

[0080] The polyamide film of the present invention can be used as a glass substitute material for cover films, base films, etc. of various components. Specifically, the polyamide film of the present invention can be used as a component for displays (particularly flexible displays) such as liquid crystal displays and organic EL displays. The polyamide film of the present invention can also be used as a component for touch panels, flexible printed circuit boards, solar cell panel components, optical waveguide components, other semiconductor-related components, etc.

[0081] <Flexible Display> The present invention also relates to a flexible display comprising the polyamide film of the present invention. For example, the polyamide film of the present invention can be used as part of a layer of a flexible display, such as a cover window, and bonded to other layers (such as an impact-absorbing layer or a shatterproof layer), thereby producing a flexible display. Such flexible displays can be used in flexible devices such as foldable mobile phones and tablets, and deformable solar panels.

[0082] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples in any way.

[0083] <Measurement Methods> The physical properties of the polyamide resins and polyamide films prepared in the examples were measured as follows: (1) Film Thickness The film thickness of the prepared films was measured using a micrometer (manufactured by Mitutoyo Corporation).

[0084] (2) Total Light Transmittance and Haze The total light transmittance and haze of the film were measured by cutting the film of each example into a size of 30 mm x 30 mm to prepare an evaluation sample, and each evaluation sample was measured using a haze meter (manufactured by Nippon Denshoku Industries Co., Ltd., NDH 7000 II) in accordance with ASTM D1003. Note that a larger total light transmittance value indicates better transparency. Also, a smaller haze value indicates better transparency.

[0085] (3) YI Value (Yellow Index) The film of each example was cut into a size of 30 mm x 30 mm, and the YI value of each film was measured using a spectrophotometer (CM-5, manufactured by Konica Minolta, Inc.) in accordance with ASTM E313-73. The closer the YI value (yellow index) is to 0, the more excellent the colorlessness is.

[0086] (4) Tensile Modulus, Elongation at Break, and Maximum Strength The tensile modulus, elongation at break, and maximum strength were measured using an EZ-SX (Shimadzu Corporation) under the following conditions. The tensile modulus was determined from the slope of the obtained stress-strain curve at strains of 0.2% to 0.5%. [Test Conditions] Sample size: 80 mm x 10 mm Grip distance: 50 mm Speed: 5 mm / min Number of measurements: 3 Note that larger values ​​for the tensile modulus and maximum strength indicate better mechanical strength. Also, larger values ​​for the elongation at break indicate better flexibility.

[0087] (5) Number-average molecular weight (Mn) and weight-average molecular weight (Mw) The number-average molecular weight (Mn) and weight-average molecular weight (Mw) of the polyamide resin were determined by gel permeation chromatography (GPC) under the following conditions. The number-average molecular weight (Mn) and weight-average molecular weight (Mw) are polystyrene equivalents. Apparatus: GL7700 manufactured by GL Science Co., Ltd. Column: TSKgel αM (manufactured by Tosoh Corporation) Column temperature: 40°C Eluent composition: 100 mmol / L H ３ P.O. ４ (H ３ P.O. ４ NMP solution containing 85% aqueous solution as raw material) and 10 mmol / L LiBr Eluent flow rate: 0.7 mL / min Calibration standard reagent: polystyrene Detector wavelength: 260 nm and 300 nm Detector temperature: room temperature Baseline range during analysis: 15 to 40 minutes Molecular weight calculation range during analysis: 20 to 35 minutes

[0088] <Raw Materials> The chemical structures of the compounds used in the examples are as follows: [Aromatic Diamine Compound] 2,2'-bis(trifluoromethyl)benzidine (TFMB) represented by the following formula (1A) was used as a compound that provides a structural unit based on an aromatic diamine compound.

[0089] [Aromatic dicarboxylic acid compound] Oxybisbenzoyl chloride (DEDC) represented by the following formula (2A) and terephthaloyl chloride (TPC) represented by the following formula (3A) were used as compounds that provide structural units based on an aromatic dicarboxylic acid compound.

[0090] Example 1 - Synthesis of Polyamide Resin - A 100 mL reactor was charged with 60.0 g of dimethylacetamide (DMAc), and 5.33 g (16.63 mmol) of TFMB and 2.64 g (36.59 mmol) of 1,2-butylene oxide were added. Next, 0.97 g (3.29 mmol) of DEDC and 2.67 g (13.17 mmol) of TPC were added to the TFMB solution, and the mixture was stirred at 30°C for 2 hours to react. Thereafter, 0.026 g (0.33 mmol) of acetyl chloride was added to the solution, and the mixture was stirred at 30°C for 30 minutes to react, yielding a solution containing a polymer having an amide structure. The weight average molecular weight measured by GPC in terms of polystyrene was 123,522. In Example 1, the ratio of the number of moles of DEDC to the total number of moles of DEDC and TPC was 20 mol%.

[0091] - Preparation of polyamide film - A resin composition for film production was prepared by adjusting the solids concentration to 12.5 wt% using polyamide powder and a solvent (DMAc) in the amounts shown in Table 1, and then dispersing and homogenizing. Next, this resin composition was coated on a glass plate using a table coater (AFA-standard, manufactured by Coatec Co., Ltd.), dried in a precision incubator (Fine Oven DH612, manufactured by Yamato Scientific Co., Ltd.) at 120°C for 40 minutes, then at 220°C for 30 minutes, and peeled off from the glass plate to form a film.

[0092] Examples 2 to 4: Polyamide resins, solutions containing polyamide resins, and polyamide films were prepared in the same manner as in Example 1, except that the amounts (molar ratios) of TFMB, DEDC, and TPC added were set to the values ​​listed in Table 1. The ratio of DEDC moles to the total moles of DEDC and TPC was 40 mol% in Example 2, 80 mol% in Example 3, and 100 mol% in Example 4. Comparative Example 1: Polyamide resins, solutions containing polyamide resins, and polyamide films were prepared in the same manner as in Example 1, except that only TFMB and TPC were used in a 1:1 mole ratio. The polyamide film of Comparative Example 1 exhibited whitening throughout the film, and was extremely brittle, making it impossible to peel from the glass plate. Therefore, it is believed that the transparency and mechanical properties were significantly inferior to those of the films of the Examples. Since a free-standing film could not be obtained from the film of Comparative Example 1, its physical properties were not evaluated.

[0093]

[0094] From Table 1, it can be seen that polyamide resins containing the structural unit represented by formula (1) and the structural unit represented by formula (2), or the structural unit represented by formula (2) and the structural unit represented by formula (3), have good mechanical properties when formed into a film. It can also be seen that by setting the molar ratio of DEDC to TPC within a specific range, optical properties such as YI value and haze become more excellent.

[0095] According to the present invention, it is possible to provide a polyamide resin from which a film having good mechanical properties can be produced while suppressing production costs.

Claims

1. A polyamide resin comprising a structural unit represented by the following formula (1) and a structural unit represented by the following formula (2):

2. The polyamide resin according to claim 1, further comprising a structural unit represented by formula (3).

3. The polyamide resin according to claim 2, wherein the ratio of the number of moles of the structural unit represented by formula (2) to the total number of moles of the structural unit represented by formula (2) and the structural unit represented by formula (3) is 10 to 90 mol %.

4. The polyamide resin according to claim 3, wherein the ratio of the number of moles of the structural unit represented by formula (2) to the total number of moles of the structural unit represented by formula (2) and the structural unit represented by formula (3) is 30 to 90 mol %.

5. The polyamide resin according to claim 1, having a number average molecular weight of 5,000 or more and 200,000 or less.

6. A method for producing a polyamide resin according to any one of claims 1 to 5, comprising reacting a diamine with a dicarboxylic acid dichloride, wherein the diamine comprises an aromatic diamine represented by the following formula (1A), and the dicarboxylic acid dichloride comprises an aromatic dicarboxylic acid dichloride represented by the following formula (2A).

7. The method for producing a polyamide resin according to claim 6, wherein the dicarboxylic acid dichloride further comprises an aromatic dicarboxylic acid dichloride represented by the following formula (3A):

8. A polyamide film comprising the polyamide resin according to any one of claims 1 to 5.

9. A flexible display comprising the film according to claim 8.

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