Polyamide resin, polyamide film, and flexible display

Abstract

To provide a polyamide resin permitting formation of a film with good colorlessness and mechanical strength while employing a monomer that is not classified as a PFAS.SOLUTION: A polyamide resin has structural units derived from a diamine compound and structural units derived from a dicarboxylic acid compound, wherein the structural units derived from the diamine compound include a structural unit represented by a prescribed formula.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a polyamide resin, a polyamide film, and a flexible display. [Background technology]

[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. Conventionally, glass, such as ultra-thin glass (UTG), has been used as the front panel material for 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. Therefore, it is difficult to use such glass as the front panel material for flexible displays that can be bent or folded.

[0003] Therefore, films made from polymeric materials are being considered as an alternative to glass. Polymeric films are expected to be used in a variety of applications, such as flexible displays, because they are easily flexible.

[0004] Conventionally, when flexible displays are operated by touch or a touch pen, or when the display is kept folded for a long period of time, marks such as pressure marks and flex marks may appear on the display surface. Therefore, films for such flexible displays are required to have excellent mechanical strength (e.g., elastic modulus). In addition, films for such flexible displays are required to have excellent optical properties such as colorlessness and transparency. To meet these requirements, films containing polyamide-based resins have been studied and proposed.

[0005] For example, Patent Document 1 discloses a polyamide-imide resin having structural units derived from a specific tetracarboxylic dianhydride, structural units derived from a specific diamine, and structural units derived from a specific aromatic dicarboxylic acid dichloride, as a resin capable of forming a film that is excellent in mechanical properties, heat resistance, and transparency and further achieves reduced residual stress. The specific diamine is, in short, 2,2'-bis(trifluoromethyl)benzidine (TFMB).

[0006] Patent Document 2 discloses an aromatic polyamide film having a predetermined structural unit and a predetermined absorbance characteristic as a film having excellent transparency and resistance to solvents. The predetermined structural unit is, in short, a structural unit derived from 2,2'-bis(trifluoromethyl)benzidine (TFMB). [Prior art documents] [Patent documents]

[0007] [Patent Document 1] International Publication No. 2019 / 216151 [Patent Document 2] Japanese Patent Publication No. 2019-1853 Summary of the Invention [Problem to be solved by the invention]

[0008] Here, 2,2'-bis(trifluoromethyl)benzidine (TFMB), which is used as a diamine monomer in Patent Documents 1 and 2, has a trifluoromethyl group in its molecule and is a compound that falls under the category of so-called PFAS (Per- and polyfluoroalkyl substances). The use of such PFAS compounds is restricted or may be restricted by domestic and international laws and regulations due to concerns about their impact on the environment or human body.

[0009] Therefore, there is a need for polyamide-based resins that can exhibit film properties (colorlessness, mechanical strength) equivalent to or better than those obtained when TFMB is used as a diamine monomer, while avoiding the use of TFMB and, by extension, compounds that fall under the category of PFAS.

[0010] Therefore, an object of the present invention is to provide a polyamide resin that uses a monomer that does not fall under the category of PFAS and that can be used to produce a colorless film that has good mechanical strength. Another object of the present invention is to provide a film using such a polyamide resin and a flexible display using the film. [Means for solving the problem]

[0011] As a result of extensive research, the present inventors have found that the above problems can be solved by using a specific compound as a diamine monomer, and have thus completed the present invention. The gist and configuration of the present invention to solve the above problems is as follows.

[0012] [1] A copolymer having a structural unit derived from a diamine compound and a structural unit derived from a dicarboxylic acid compound, wherein the structural unit derived from the diamine compound is represented by the following formula (1): [ka] A polyamide resin characterized by containing a structural unit represented by the following formula:

[0013] [2] The polyamide resin according to [1], wherein the proportion of the structural units represented by the formula (1) in the structural units derived from the diamine compound is more than 50 mol %.

[0014] [3] The polyamide resin according to [1] or [2], which does not have a perfluoroalkyl skeleton.

[0015] [4] The polyamide resin according to any one of [1] to [3], which has a number average molecular weight of 5,000 or more and 200,000 or less.

[0016] [5] A polyamide film, characterized by comprising the polyamide resin according to any one of [1] to [4].

[0017] [6] A flexible display comprising the polyamide film according to [5]. [Effects of the Invention]

[0018] According to the present invention, it is possible to provide a polyamide resin that can be used to produce a colorless film with good mechanical strength by using a monomer that is not a PFAS. Furthermore, according to the present invention, it is possible to provide a polyamide film using such a polyamide resin, and a flexible display using the polyamide film. DETAILED DESCRIPTION OF THE INVENTION

[0019] The present invention will be described in detail below by way of example based on embodiments thereof.

[0020] <Polyamide resin> A polyamide resin according to one embodiment of the present invention (hereinafter sometimes referred to as "the polyamide resin according to the present embodiment") has a structural unit derived from a diamine compound and a structural unit derived from a dicarboxylic acid compound, and the structural unit derived from the diamine compound is represented by the following formula (1): [ka] The present invention is characterized in that it contains a structural unit represented by the following formula:

[0021] The polyamide resin of the present embodiment has both the structural unit represented by the above formula (1) and a structural unit derived from a dicarboxylic acid compound, and therefore can contribute to good colorlessness and mechanical strength when used in a film.

[0022] In this specification, the term "polyamide resin" refers to a resin having structural units derived from a diamine compound and structural units derived from a dicarboxylic acid compound, and in which an amide structure is formed. Such a polyamide resin can be obtained, for example, by reacting a diamine compound and a dicarboxylic acid compound, which are monomer components, to form an amide structure. In this specification, the term "polyamide resin" also encompasses resins (such as polyamideimide resins) containing structures other than amide structures, such as imide structures.

[0023] However, an imidization step is essential in the synthesis of polyamide-imide resins. Furthermore, in the synthesis of polyamide-imide resins, imidization agents may cause yellowing, and it is necessary to thoroughly remove such imidization agents. Therefore, from the viewpoint of suppressing increases in production costs associated with complicated steps and avoiding concerns about deterioration of colorlessness, it is preferable that the polyamide resin of this embodiment does not substantially contain an imide structure (i.e., it is not a polyamide-imide resin). Furthermore, it is more preferable that the structure that bonds the constituent monomer components of the polyamide resin of this embodiment consists essentially of an amide structure. Note that, in this embodiment, the structure that bonds the constituent monomer components does not include a structure in which a terminal blocking agent, described below, bonds to the polyamide resin (this is not subject to judgment).

[0024] (Structural unit derived from diamine compound) The polyamide resin of this embodiment has a structural unit represented by the above formula (1) as a structural unit derived from a diamine compound. The structural unit represented by the above formula (1) contains an aromatic ring (a benzene ring, a fluorene skeleton), and therefore has high rigidity and a large molecular volume. Furthermore, polymers having the structural unit represented by the above formula (1) have a bent structure (the aromatic rings are oriented in different directions) due to the inclusion of a fluorene skeleton, and therefore tend to have low birefringence (the phenomenon in which light is refracted in two different directions when incident on a substance). Furthermore, the polyamide resin of this embodiment has a high ionization potential of the structural unit represented by formula (1) due to the substituted fluorine. It is believed that these combined effects enable the polyamide resin of this embodiment to impart good colorlessness and mechanical strength to films.

[0025] The structural unit represented by formula (1) is a diamine monomer represented by the following formula (1A): [ka] It is preferable that the structural unit is derived from a diamine compound represented by the formula (1A): The diamine compound represented by the formula (1A) is 9,9-bis(4-amino-3-fluorophenyl)fluorene (FFDA).

[0026] FFDA, a diamine compound (monomer) that can be used to form the structural unit represented by formula (1), has the following characteristics: it is more reactive with various dicarboxylic acid compounds and the resulting resin (polymer) has better solubility than conventional diamine compounds (e.g., 2'-bis(trifluoromethyl)benzidine (TFMB)). In this regard, when a highly rigid dicarboxylic acid compound such as 4,4'-biphenyldicarbonyl chloride (BPC) or terephthaloyl chloride (TPC) is used in combination with a conventional diamine compound (e.g., 2'-bis(trifluoromethyl)benzidine (TFMB)), the resulting polyamide resin has low solubility, resulting in a solution that is not suitable for film production (coatable), a low molecular weight of the synthesized resin, and insufficient colorlessness. In contrast, when FFDA is used in combination with various dicarboxylic acid compounds in combination, a solution suitable for film production is obtained, resulting in a film with excellent properties (e.g., colorlessness, mechanical properties). That is, the polyamide resin of this embodiment has the advantage that FFDA, a diamine compound (monomer) that can be used in its synthesis, is versatile and easy to handle. The FFDA not only has a highly rigid structure but also a moderately curved structure, which allows solvent molecules to easily penetrate into gaps and weakens intermolecular forces. Therefore, polyamide resins using FFDA are presumed to have lower crystallinity and, as a result, superior solubility, etc., compared to polyamide resins using conventional diamine compounds such as TFMB.

[0027] The polyamide resin of this embodiment may contain structural units other than the structural unit represented by formula (1) above as structural units derived from a diamine compound, as long as the effects of the present invention are not impaired. In other words, in the synthesis of the polyamide resin of this embodiment, a diamine compound other than FFDA may be used as the diamine compound. Such a diamine compound other than FFDA may be an aromatic diamine compound or an aliphatic diamine compound. Furthermore, such a diamine compound other than FFDA may be used alone or in combination of two or more.

[0028] The polyamide resin of this embodiment preferably does not have a perfluoroalkyl skeleton. In this embodiment, the perfluoroalkyl skeleton refers to a —(CF) structure in which all hydrogen atoms on the alkyl chain (alkyl skeleton) have been substituted with fluorine atoms. n (n is an integer of 1 or more), and a specific example of such a perfluoroalkyl skeleton is a trifluoromethyl group (-CF3). When a polyamide resin has a perfluoroalkyl skeleton, the polyamide resin may be classified as a PFAS and may be subject to legal restrictions.

[0029] In the polyamide resin of this embodiment, the proportion of the structural units represented by the above formula (1) in the structural units derived from the diamine compound is preferably more than 50 mol %. In this case, the colorlessness and / or mechanical strength of the film can be improved. From the same viewpoint, the proportion of the structural units represented by the above formula (1) in the structural units derived from the diamine compound is more preferably 70 mol % or more, even more preferably 85 mol % or more, and particularly preferably 100 mol % (i.e., the structural units derived from the diamine compound consist solely of the structural units represented by the formula (1)).

[0030] (Structural units derived from dicarboxylic acid compounds) The polyamide resin of the present embodiment has a structural unit derived from a dicarboxylic acid compound. The structural unit derived from a dicarboxylic acid compound may be one type alone or a combination of two or more types.

[0031] In this specification, the term "dicarboxylic acid compound" includes dicarboxylic acids and derivatives of dicarboxylic acids, such as acid chlorides of dicarboxylic acids and esters of dicarboxylic acids.

[0032] Examples of the dicarboxylic acid compound include aromatic dicarboxylic acid compounds, aliphatic dicarboxylic acid compounds, etc. These dicarboxylic acid compounds may be used alone or in combination of two or more.

[0033] Examples of aromatic dicarboxylic acid compounds include terephthalic acid, isophthalic acid, 2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid, 1,4-naphthalenedicarboxylic acid, 1,4-phenylenedioxydiacetic acid, 1,3-phenylenedioxydiacetic acid, 4,4'-oxybisbenzoic acid, dibenzoic acid, diphenylmethane-4,4'-dicarboxylic acid, diphenylsulfone-4,4'-dicarboxylic acid, 4,4'-biphenyldicarboxylic acid, 3,3'-biphenyldicarboxylic acid, and derivatives thereof. These aromatic dicarboxylic acid compounds may be used alone or in combination of two or more.

[0034] Examples of the aliphatic dicarboxylic acid compound include succinic acid, 1,3-cyclobutanedicarboxylic acid, 1,3-cyclopentanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, 1,10-decanedicarboxylic acid, and derivatives thereof. These aliphatic dicarboxylic acid compounds may be used alone or in combination of two or more.

[0035] Among the above dicarboxylic acid compounds, from the viewpoint of further improving the elastic modulus of the film and enhancing the elongation at break and storage stability, it is preferable to use terephthalic acid or a derivative thereof, isophthalic acid or a derivative thereof, 4,4'-biphenyldicarboxylic acid or a derivative thereof, or 4,4'-oxybisbenzoic acid or a derivative thereof, and it is more preferable to use 4,4'-biphenyldicarbonyl chloride (BPC) or 4,4'-oxybis(benzoyl chloride) (DEDC). In particular, from the viewpoint of further enhancing colorlessness, it is even more preferable to use 4,4'-oxybis(benzoyl chloride) (DEDC).

[0036] (Structural units derived from other compounds) The polyamide resin of this embodiment may have structural units derived from compounds other than diamine compounds and dicarboxylic acid compounds. For example, the polyamide resin of this embodiment may have structural units derived from tetracarboxylic acid compounds (or their acid dianhydrides). When a tetracarboxylic acid compound is used as a monomer component in addition to a diamine compound and a dicarboxylic acid compound, an imide structure is formed, resulting in a polyamideimide resin. However, the polyamide resin of this embodiment preferably does not have structural units derived from tetracarboxylic acid compounds (or their acid dianhydrides). Furthermore, the polyamide resin of this embodiment preferably consists solely of structural units derived from diamine compounds and structural units derived from dicarboxylic acid compounds. In other words, in the synthesis of the polyamide resin of this embodiment, it is preferable to use only diamine compounds and dicarboxylic acid compounds as monomers. Note that in this embodiment, the above-mentioned monomers do not include the terminal blocking agent described below (not subject to judgment).

[0037] (Properties of polyamide resin) The polyamide resin of this embodiment preferably has a number average molecular weight (Mn) of 5,000 or more and 200,000 or less. When the number average molecular weight (Mn) of the polyamide resin is 5,000 or more, the colorlessness and / or mechanical properties of the film can be further improved, and when it is 200,000 or less, the handleability during synthesis of the polyamide resin and during production of the polyamide film can be further improved. From the same viewpoint, the number average molecular weight (Mn) of the polyamide resin of this embodiment is more preferably 10,000 or more and more preferably 180,000 or less. The number average molecular weight (Mn) is a value measured by GPC (gel permeation chromatography) and calculated in terms of polystyrene.

[0038] The polyamide resin of this embodiment preferably has a weight-average molecular weight (Mw) of 10,000 or more and 1,000,000 or less. When the weight-average molecular weight (Mw) of the polyamide resin is 10,000 or more, the colorlessness and / or mechanical properties of the film can be further improved, and when it is 1,000,000 or less, the handleability during synthesis of the polyamide resin and during production of the polyamide film can be further improved. From the same viewpoint, the weight-average molecular weight (Mw) of the polyamide resin of this embodiment is more preferably 50,000 or more, and even more preferably 100,000 or more, and more preferably 500,000 or less, and even more preferably 300,000 or less. The weight average molecular weight (Mw) is a value measured by GPC (gel permeation chromatography) and calculated in terms of polystyrene.

[0039] The polyamide resin of the present embodiment 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.

[0040] (Production of polyamide resin) The polyamide resin of the present embodiment is not particularly limited and can be produced (synthesized) by a method known as a method for producing polyamide, such as a solution polymerization method, an interfacial polymerization method, a melt polymerization method, or a solid-state polymerization method.

[0041] Specifically, the polyamide resin of this embodiment can be produced by, for example, using a diamine compound and a dicarboxylic acid compound as monomers and reacting them. In this case, at least 9,9-bis(4-amino-3-fluorophenyl)fluorene (FFDA) represented by the above formula (1A) can be used as the diamine compound. In addition, the dicarboxylic acid compound used as the monomer is preferably a dicarboxylic acid dichloride (an acid chloride of a dicarboxylic acid).

[0042] The reaction can be carried out in an aprotic organic polar solvent. Examples of the aprotic organic polar solvent 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; hexamethylphosphoramide; and γ-butyrolactone. These aprotic organic polar solvents may be used alone or in combination. Aromatic hydrocarbons such as xylene and toluene can also be used as the solvent. Furthermore, to promote dissolution of the resulting polymer, 50% by mass or less of an alkali metal or alkaline earth metal salt can be added to the solvent.

[0043] The reaction of a diamine compound with a dicarboxylic acid dichloride produces hydrogen chloride as a by-product. Therefore, a neutralizing agent can be used to neutralize the reaction. Examples of the neutralizing agent include inorganic neutralizing agents such as calcium hydroxide, calcium carbonate, and lithium carbonate; and organic neutralizing agents such as 1,2-butylene oxide, ethylene oxide, propylene oxide, ethylene carbonate, glycidyl phenyl ether, ammonia, and pyridine.

[0044] When two or more diamine compounds are used as monomers, for example, the diamine compounds can be added one by one, and 10 to 99 mol % of a dicarboxylic acid dichloride relative to the diamine compounds can be added to react them, followed by adding another diamine compound and then a dicarboxylic acid dichloride, allowing the reaction to proceed stepwise. Alternatively, all the diamine compounds can be mixed and added, and then a dicarboxylic acid dichloride can be added to react them all at once. The same applies when two or more dicarboxylic acid dichlorides are used as monomers.

[0045] The ratio of the amounts of the diamine compound and the dicarboxylic acid dichloride charged (molar number of the diamine compound:molar number of the dicarboxylic acid dichloride) can be appropriately adjusted depending on the molecular weight of the target polyamide resin, and can be, for example, 49:51 to 51:49. In this case, a polyamide resin having a sufficiently large molecular weight and excellent mechanical properties can be obtained.

[0046] A yellowing inhibitor can be added to the reaction system to prevent discoloration of the resulting polyamide resin. Examples of such yellowing inhibitors include phosphorus compounds such as trimethyl phosphate, triethyl phosphate, triphenyl phosphate, and dimethyl phenylphosphonate. The yellowing inhibitor may be added during the step of preparing a solution containing a polyamide resin in the method for producing a polyamide film described below.

[0047] In the above reaction, depending on the ratio of the monomer raw materials used, the resulting polyamide resin may have an amine or carboxylic acid end. Therefore, in order to further improve the colorless transparency of the resulting film, it is preferable to end-capping the polyamide resin. Examples of compounds (end-capping agents) used to end-capping polyamide resins include amines other than the monomers used, carboxylic acid chlorides, and carboxylic acid anhydrides. More specifically, acetyl chloride, benzoyl chloride, substituted benzoyl chloride, acetic anhydride, 5-norbornene-2,3-dicarboxylic anhydride, 4-ethynylaniline, 4-phenylethynylphthalic anhydride, and maleic anhydride are examples.

[0048] The reaction conditions for synthesizing the polyamide resin may be a temperature of −30 to 50° C. and a time of 10 minutes to 27 hours. In order to maintain colorless transparency, the synthesis may be carried out in a nitrogen atmosphere.

[0049] When synthesizing a polyamideimide resin as the polyamide resin of this embodiment, FFDA may be used as the diamine compound, and a conventionally known method for synthesizing a polyamideimide resin may be used.

[0050] (Uses of polyamide resin) The applications of the polyamide resin of the present embodiment are not limited to films, which will be described later, but also include a wide range of applications such as materials for sheets, pipes, tubes, filaments, fibers, containers, etc.

[0051] <Polyamide film> A polyamide film according to one embodiment of the present invention (hereinafter sometimes referred to as the "polyamide film of the present embodiment") is characterized by containing the polyamide resin described above. Because the polyamide film of the present embodiment contains the polyamide resin described above, it exhibits good colorlessness and mechanical strength. More preferably, the polyamide film of the present embodiment can exhibit film properties equivalent to or superior to those obtained when TFMB is used as the diamine monomer.

[0052] The thickness of the polyamide film of the present embodiment is not particularly limited and can be appropriately set depending on the application. The thickness of the polyamide film can be, for example, 10 μm or more, or 20 μm or more, and can be 150 μm or less, or 100 μm or less. The thickness of the polyamide film can be measured by the method described in the Examples.

[0053] The polyamide film of the present embodiment preferably has a YI value (yellow index) of less than 3.5, more preferably less than 3.0, and particularly preferably less than 2.0. The YI value (yellow index) can be measured by the method described in the examples.

[0054] From the viewpoint of mechanical strength, the polyamide film of this embodiment preferably has a tensile modulus of 2.7 GPa or more, more preferably greater than 3.2 GPa, even more preferably greater than 4.5 GPa, and even more preferably greater than 6.0 GPa. Furthermore, 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. In the case where the polyamide film has anisotropy between the tensile modulus in the MD direction and the tensile modulus in the TD direction, the above-mentioned tensile modulus refers to the average value of the tensile modulus in the MD direction and the tensile modulus in the TD direction, unless otherwise specified. The tensile modulus can be measured by the method described in the Examples.

[0055] The polyamide film of this embodiment may contain, in addition to the polyamide resin of this embodiment described above, any other components within the scope of the present invention, such as resins other than the polyamide resin of this embodiment described above, leveling agents for improving coatability during film production, dispersants, surfactants, retardation adjusters, antioxidants, UV inhibitors, light stabilizers, plasticizers, waxes, fillers, pigments, dyes, foaming agents, antifoaming agents, dehydrating agents, antistatic agents, antibacterial agents, antifungal agents, bluing agents for reducing the yellowness of the film, pH adjusters, crosslinking agents, and lubricants.

[0056] The polyamide film of the present embodiment preferably does not substantially contain a compound having a perfluoroalkyl skeleton, because such a compound falls under the category of PFAS and may be subject to legal restrictions.

[0057] When the polyamide film of the present embodiment contains a filler, the material of the filler is not particularly limited, but examples thereof include silica, titanium oxide, alumina (including alumina hydrate), silicon nitride, boron nitride, calcium hydrogen phosphate, calcium phosphate, and mica. Examples of the alumina hydrate include boehmite and pseudo-boehmite. These fillers may be used alone or in combination of two or more. Among these, alumina (including alumina hydrate) is preferred as the filler material, with boehmite or pseudo-boehmite being more preferred, and pseudo-boehmite being even more preferred. That is, the filler is preferably an alumina filler, more preferably a boehmite or pseudo-boehmite alumina filler, and even more preferably a pseudo-boehmite alumina filler. The filler is preferably fibrous. The term "fibrous" refers to a shape having an aspect ratio (filler length / filler diameter) of 5 or more. When the polyamide film of the present embodiment contains a filler, it is particularly preferable that the filler is a fibrous alumina filler.

[0058] The fibrous alumina filler is preferably dispersed with an average fiber diameter of 1 to 30 nm and an average fiber length of 100 to 4,000 nm, preferably with an average fiber diameter of 2 to 25 nm and an average fiber length of 200 to 3,000 nm, and more preferably with an average fiber diameter of 3 to 20 nm and an average fiber length of 500 to 2,000 nm. If the fibrous alumina filler is dispersed in the polyamide film with the average fiber diameter and average fiber length in the above-mentioned ranges, the film can achieve high elasticity while maintaining transparency. The "average fiber diameter" and "average fiber length" of the dispersed fibrous alumina filler are measured by dissolving and diluting the polyamide film at a concentration of 10,000 times with the solvent used in its preparation (e.g., methyl isobutyl ketone, N,N-dimethylacetamide, etc.), placing one drop on a cover glass (Cover Glass Trophy, manufactured by Matsunami Glass Co., Ltd.), drying at 50°C, and then observing the image with an electron microscope (e.g., 10,000x magnification observation image using a Hitachi High-Tech FE-SEM). Furthermore, the fibrous alumina filler to be measured may be in the form of either a single fiber or a fiber bundle consisting of multiple aggregated single fibers, as long as it can be seen as a single fiber in the electron microscope image. The average measured length of the diameter in the short side direction of 50 fibrous alumina fillers arbitrarily selected in the electron microscope image is taken as the "average fiber diameter," and the average measured length in the long side direction is taken as the "average fiber length."

[0059] Furthermore, when the filler is used, it is preferable to further use a dispersant. That is, when the polyamide film of this embodiment contains a filler, it is preferable to further contain a dispersant. Examples of the dispersant, from the viewpoint of modifying the filler and stabilizing the viscosity of the resin-containing solution, include organic carboxylic acid compounds such as acetic acid, benzoic acid, terephthalic acid, citric acid, succinic acid, and lactic acid; organic phosphoric acid compounds and organic phosphonic acid compounds; organic sulfonic acid compounds such as benzenesulfonic acid, p-toluenesulfonic acid, and dodecylbenzenesulfonic acid; and organic bases such as pyridine, tetraethylamine, diisopropylethylamine, 2,6-dimethylpyridine, isoquinoline, and triethylenediamine.

[0060] The method for producing the polyamide film of this embodiment is not particularly limited. For example, the polyamide film of this embodiment can be produced by the steps of: (1) dissolving or dispersing the polyamide resin of this embodiment and any other components in a solvent to prepare a solution containing the polyamide resin, (2) applying the solution to a substrate to obtain a coating film, (3) drying the coating film, and (4) peeling the dried coating film from the substrate to obtain a film.

[0061] The solvent is not particularly limited as long as it can dissolve the polyamide resin, but from the viewpoint of film transparency, etc., solvents having a functional group 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, and amide solvents are preferred. Such solvents may be used alone or in combination of two or more in any ratio.

[0062] 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).

[0063] 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).

[0064] Examples of solvents having a ketone group include cyclohexanone (boiling point 156°C), 1-phenylethanone (boiling point 202°C), benzaldehyde (boiling point 179°C), and methyl isobutyl ketone.

[0065] 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).

[0066] 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).

[0067] Examples of solvents having a sulfinyl group include dimethyl sulfoxide (boiling point 189° C.).

[0068] 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).

[0069] Among these solvents, from the viewpoint of obtaining a film with excellent colorlessness, amide solvents are more preferred, and N,N-dimethylacetamide is more preferred.

[0070] When a solvent is used in the synthesis of the polyamide resin, the solvent may be used as it is in the production of the polyamide film.

[0071] The substrate is not particularly limited and may be glass, ceramic, metal (aluminum foil, etc.), resin film (polyester film such as PET or PEN, polyimide film, polyamideimide film, polypropylene film, polystyrene film, etc.), or other thermoplastic film. These substrates may have a circuit formed thereon using copper or the like.

[0072] As a means for applying the solution onto the substrate, 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.

[0073] Methods for drying the coating film include, for example, reduced-pressure drying, heat drying, and a combination of these. When drying at normal pressure, the temperature can be 30 to 350°C, and 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.

[0074] The polyamide film of this embodiment can be used as a glass substitute material for cover films, base films, etc. of various components. Specifically, the polyamide film of this embodiment can be suitably used as a component for displays such as liquid crystal display devices and organic EL display devices (particularly flexible displays, which will be described later). In addition to the above, the polyamide film of this embodiment 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.

[0075] <Flexible display> A flexible display according to one embodiment of the present invention is characterized by including the polyamide film described above. Since the flexible display uses the polyamide film described above, it has good colorlessness and mechanical strength.

[0076] For example, a flexible display can be produced by using the polyamide film of this embodiment as part of a layer of the flexible display, such as a cover window, and adhering it to other layers (such as an impact absorbing layer or a shatterproof layer). Such flexible displays can be used in flexible devices such as foldable mobile phones or tablets, and deformable solar panels. [Example]

[0077] The present invention will be explained in more detail below by way of examples, but the present invention is not limited to the following examples and can be modified as appropriate within the scope of the present invention.

[0078] <Measurement method> Various measurements of the polyamide resins and polyamide films produced in the examples were carried out according to the following procedures.

[0079] (1) Number average molecular weight (Mn), weight average molecular weight (Mw) The number average molecular weight (Mn) and weight average molecular weight (Mw) of the polyamide resin were measured using a GL7700 manufactured by GL Science Co., Ltd. The number average molecular weight (Mn) and weight average molecular weight (Mw) are polystyrene equivalent values.

[0080] (2) Film thickness The thickness of the film was measured using a micrometer (manufactured by Mitutoyo Corporation).

[0081] (3) YI value (yellow index) Each film was cut to 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 D1925. For comparison between films, the measured YI value was converted to a value for a film thickness of 55 μm, and the YI value of each example (55 μm_YI) was used. The closer the YI value (55 μm_YI) is to 0, the better the colorlessness is.

[0082] (4) Tensile modulus The tensile modulus (unit: GPa) of the prepared film was measured using Shimadzu Corporation's "EZ-SX" under the following conditions. [Test conditions] Sample size: 80mm x 10mm Grip distance: 50 mm Speed: 5mm / min Number of measurements: 3 For the filler-added samples, the tensile modulus was measured in both the direction parallel to the coating direction (MD) and the direction perpendicular to the coating direction (TD) when the film was produced. The larger the tensile modulus value (the tensile modulus value in the MD direction for a filler-added sample), the more excellent the mechanical strength.

[0083] <Monomer raw material: diamine compound> The diamine compounds used as raw monomer materials in each example are as follows: FFDA: 9,9-bis(4-amino-3-fluorophenyl)fluorene FDA: 9,9-bis(4-aminophenyl)fluorene 33DDS: Bis(3-aminophenyl) sulfone 44DDS: Bis(4-aminophenyl) sulfone DHB: 3,3'-dihydroxybenzidine TPE-M: 1,3-bis(3-aminophenoxy)benzene 3-APBP: 4,4'-bis(3-aminophenoxy)biphenyl PHBAAB: (2-phenyl-4-aminophenyl)-4-aminobenzoate TFMB: 2,2'-bis(trifluoromethyl)benzidine

[0084] The chemical structures of the above diamine compounds are as follows: [ka] [ka]

[0085] <Monomer raw material: dicarboxylic acid compound> The dicarboxylic acid compounds used as raw monomer materials in each example are as follows: DEDC: 4,4'-oxybisbenzoyl chloride (also known as 4,4'-oxybis(benzoyl chloride)) TPC: Terephthalic acid chloride BPC: 4,4'-biphenyldicarbonyl chloride

[0086] The chemical structures of the above dicarboxylic acid compounds are as follows: [ka]

[0087] <Examples 1 to 3 and Comparative Examples 1 to 11: Filler-free polyamide films> (Synthesis of polyamide resin) In each example, N,N-dimethylacetamide (DMAc) was added as a solvent to a vial containing a stir bar, and a diamine compound shown in Tables 1 and 2 was dissolved therein to obtain a solution. In each example, the amount of N,N-dimethylacetamide added was adjusted to achieve the desired solids concentration after the polymerization reaction (as shown in Tables 1 and 2). Next, a neutralizing agent (1,2-butylene oxide) and, if necessary, a yellowing inhibitor (triethyl phosphate) were added to this solution (Tables 1 and 2 indicate whether triethyl phosphate was added in each example). Next, a dicarboxylic acid compound shown in Tables 1 and 2 was added, and the mixture was stirred for 1 hour under ice-cooling (5°C) or at room temperature (approximately 25°C) (the selected polymerization temperature is shown in Tables 1 and 2). Subsequently, an end-capping agent (acetyl chloride) was added to this solution and stirred for 1 hour to obtain a solution containing the polyamide resin synthesized by the above reaction. The polyamide resin obtained by using FFDA as the diamine compound had a structural unit represented by formula (1). In each example, the molar ratio of diamine compound:dicarboxylic acid compound:1,2-butylene oxide:acetyl chloride was 1:1:2.5:0.01. In each example, the amount of triethyl phosphate added was adjusted to 20% by mass of the yield of the resulting polyamide resin.

[0088] The number average molecular weight (Mn) and weight average molecular weight (Mw) of the obtained polyamide resin are shown in Tables 1 and 2.

[0089] (Preparation of polyamide film) The resulting polyamide resin solution (coating solution) was then applied to a glass substrate using an applicator to a target thickness of 55 μm after drying. The coating was then subjected to primary drying at 120°C for 40 minutes in a precision incubator (Fine Oven DH612, manufactured by Yamato Scientific Co., Ltd.), followed by secondary drying at 220°C for 30 minutes. After drying, the coating was peeled off from the glass substrate to obtain a polyamide film.

[0090] The thickness, YI value (55 μm_YI) and tensile modulus of the obtained polyamide film are shown in Tables 1 and 2.

[0091] For filler-free polyamide films using DEDC as the dicarboxylic acid compound, a comprehensive evaluation was performed as follows. The YI value (55 μm_YI) was evaluated (colorlessness) as follows: 2 points if less than 2.0, 1 point if 2.0 or more and 10.0 or less, and 0 point if more than 10.0. Furthermore, the tensile modulus (mechanical strength) was evaluated as follows: 2 points if more than 3.2 GPa, 1 point if 2.7 GPa or more and 3.2 GPa or less, and 0 point if less than 2.7 GPa. These scores were then added together to perform a comprehensive evaluation. The results are shown in Table 1.

[0092] [Table 1]

[0093] [Table 2]

[0094] Table 1 shows that Example 1, which uses a polyamide resin obtained by reacting FFDA as a diamine compound with a dicarboxylic acid compound, has a higher overall rating than Comparative Examples 1 to 7, i.e., better colorlessness and mechanical strength. In particular, Example 1 is comparable to or better than Comparative Examples 8 and 9, which use TFMB, a PFAS, in both colorlessness and mechanical strength of the film. Furthermore, Example 1 is the only example in which the YI value (55 μm_YI) is better than Comparative Examples 8 and 9, which use TFMB, a PFAS, which is also noteworthy.

[0095] Table 2 also shows that in Examples 2 and 3, which used polyamide resins obtained by reacting FFDA as the diamine compound with TPC or BPC as the dicarboxylic acid compound, films with good colorlessness and mechanical strength were obtained. In contrast, in Comparative Example 10, in which TFMB was used as the diamine compound and reacted with TPC as the dicarboxylic acid compound, the resulting polyamide resin was very brittle and could not be peeled off from the substrate to form a free-standing film. In Comparative Example 11, in which TFMB was used as the diamine compound and reacted with BPC as the dicarboxylic acid compound, the resulting polyamide resin had low solubility in solvents, making it impossible to obtain a polyamide resin with a sufficiently high molecular weight. Furthermore, it was not possible to prepare a solution containing the polyamide resin, making it impossible to form a film.

[0096] <Examples 4 to 6 and Comparative Examples 12 to 13: Polyamide Films with Filler> (Synthesis of polyamide resin) In each example, N,N-dimethylacetamide (DMAc) was added as a solvent to a vial containing a stir bar, and the diamine compound shown in Table 3 was dissolved therein to obtain a solution. In each example, the amount of N,N-dimethylacetamide added was adjusted to achieve the desired solids concentration after the polymerization reaction (as shown in Table 3). Next, a neutralizing agent (1,2-butylene oxide) and a yellowing inhibitor (triethyl phosphate) were added to the solution. Next, a dicarboxylic acid compound shown in Table 3 was added, and the mixture was stirred for 1 hour under ice cooling (5°C) to allow the reaction to proceed. Then, an end-capping agent (acetyl chloride) was added to the solution and stirred for 1 hour to obtain a solution containing the polyamide resin synthesized by the above reaction. The polyamide resin obtained using FFDA as the diamine compound had a structural unit represented by formula (1). In each example, the molar ratio of diamine compound:dicarboxylic acid compound:1,2-butylene oxide:acetyl chloride was 1:1:2.5:0.01. In each example, the amount of triethyl phosphate added was adjusted to 20% by mass of the yield of the resulting polyamide resin.

[0097] Table 3 shows the number average molecular weight (Mn) and weight average molecular weight (Mw) of the obtained polyamide resin.

[0098] (purification) In the examples indicated as "Implemented" in the "Purification" column in Table 3, the following operations were then carried out. Methanol and water were added as poor solvents to the obtained solution containing the polyamide resin, and reprecipitation was carried out. Subsequently, suction filtration and drying under reduced pressure were carried out to obtain a polyamide resin powder. In Examples in which only purification was carried out, the purified powder was dissolved in N,N-dimethylacetamide (DMAc) to obtain a solution containing the purified polyamide resin.

[0099] (filtration) In the examples indicated as "Implemented" in the "Filtration" column in Table 3, the following operations were then carried out. The polyamide resin powder obtained by the above purification was dissolved in N,N-dimethylacetamide (DMAc). The obtained solution containing the polyamide resin was filtered using a nonwoven fabric filter (mesh opening: 5 μm) at an air pressure of 0.1 to 0.3 MPa to obtain a solution containing the filtered polyamide resin.

[0100] (Preparation of coating liquid) Next, a filler (pseudoboehmite-type fibrous alumina filler, manufactured by Kawaken Fine Chemicals, average fiber diameter: 5 nm, average fiber length: 800 nm, dimethylacetamide solution with a solid content of 10% by mass), a dispersant (phenylphosphonic acid), and a leveling agent (manufactured by BYK-Chemie, "BYK-361N") were added to the resulting solution containing the polyamide resin and stirred to prepare a coating liquid. In preparing the coating liquid, the filler was added in an amount of 20 parts by mass, the dispersant in an amount of 5 parts by mass, and the leveling agent in an amount of 0.012 parts by mass relative to 100 parts by mass of the polyamide resin.

[0101] (Preparation of polyamide film) The resulting polyamide resin solution (coating solution) was then applied to a glass substrate in a fixed direction using an applicator to a target thickness of 55 μm after drying. This was followed by primary drying at 120°C for 40 minutes in a precision incubator (Fine Oven DH612, manufactured by Yamato Scientific Co., Ltd.), followed by secondary drying at 220°C for 30 minutes. After drying, the film was peeled off from the glass substrate to obtain a polyamide film.

[0102] The thickness, YI value (55 μm_YI) and tensile modulus of the obtained polyamide film are shown in Table 3.

[0103] The polyamide films containing filler were comprehensively evaluated as follows. The YI value (55 μm_YI) was evaluated (colorlessness) by assigning a score of 1 if it was less than 3.5 and a score of 0 if it was 3.5 or greater. Furthermore, the tensile modulus (mechanical strength) was evaluated by assigning a score of 1 if the tensile modulus in the MD direction was greater than 6.5 GPa and a score of 0 if it was 6.5 GPa or less. These scores were then added together to form a comprehensive evaluation. The results are shown in Table 3.

[0104] [Table 3]

[0105] Table 3 shows that Examples 4 to 6, which used polyamide resins obtained by reacting FFDA as a diamine compound with a dicarboxylic acid compound, had higher overall evaluations, i.e., better colorlessness and mechanical strength, than Comparative Example 12. Furthermore, it can be seen that the colorlessness and mechanical strength of the films of Examples 4 to 6 are equivalent to those of Comparative Example 13, which used TFMB, which corresponds to PFAS. [Industrial Applicability]

[0106] According to the present invention, it is possible to provide a polyamide resin that can be used to produce a colorless film with good mechanical strength, using only monomers that are not PFAS. Furthermore, according to the present invention, it is possible to provide a polyamide film using such a polyamide resin, and a flexible display using the polyamide film.

Claims

1. The copolymer has a structural unit derived from a diamine compound and a structural unit derived from a dicarboxylic acid compound, and the structural unit derived from the diamine compound is represented by the following formula (1): 【Chemistry 1】 A polyamide resin characterized by containing a structural unit represented by the following formula:

2. 2. The polyamide resin according to claim 1, wherein the structural units represented by formula (1) account for more than 50 mol% of the structural units derived from the diamine compound.

3. The polyamide resin according to claim 1, which does not have a perfluoroalkyl skeleton.

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

5. A polyamide film comprising the polyamide resin according to any one of claims 1 to 4.

6. A flexible display comprising the polyamide film according to claim 5.

Images