Laminate and cover window of display using the same

Abstract

To provide a laminate which has high surface elastic modulus and surface hardness, is also excellent in bending resistance, and is suitable as a member such as a cover window.SOLUTION: A laminate includes a first resin layer, and a second resin layer provided on one main surface side of the first resin layer, wherein the first resin layer contains a resin component, and 15 to 65 mass% of a fibrous alumina nanofiller to the resin component, the second resin layer contains a resin component and 0 to 5 mass% of a fibrous alumina nanofiller to the resin component, the fibrous alumina nanofiller has an average fiber diameter of 4 to 30 nm and an average fiber length of 200 to 4,000 nm, and surface elastic modulus measured according to ISO14577 using a nanoindenter in the first resin layer is 6.5 GPa or more.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a laminate, and more particularly to a laminate that can be suitably used as a cover window for a display such as a foldable device, and a cover window for a display using the same.

Background Art

[0002] Foldable devices have recently attracted attention in order to further enhance the portability of portable information terminals such as smartphones and tablets. As a member such as a cover window used for a flexible display constituting such a foldable device, in addition to transparency, it is required to have flexibility. Specifically, a member having extremely high flexibility that can achieve a 180° bend with a small bending radius of about 2.5 mm is required.

[0003] As such a member, materials made of flexible organic polymers have been variously studied. For example, from the viewpoints of transparency and heat resistance, films containing polyimide resins have been studied and proposed as flexible organic polymers, and are expected as materials to replace conventional rigid glasses.

[0004] Although polyimide is a material having flexibility and excellent physical properties such as heat resistance, transparency, and mechanical strength, it has been insufficient as the performance required for a cover window (for example, heat resistance, transparency, mechanical strength, surface hardness, bend resistance). Therefore, composite materials in which inorganic oxides are added to resins have also been studied and are becoming commercially available. As such a material, for example, Patent Document 1 proposes a polyimide resin composition containing a polyimide resin having a specific molecular structure and silica fine particles as a film for a flexible display. Further, Patent Document 2 proposes blending a fibrous alumina nanofiller into a polyamideimide resin.

[0005] Furthermore, the display of a foldable device is required to be free from creases (bending marks) when opened and to be highly elastic so as not to cause pressure marks from touch or operation with a stylus. To address these requirements, for example, Patent Document 3 proposes a cover window member with a three-layer structure, comprising a polyimide resin film without inorganic fillers, a polyimide resin film containing alumina fillers, and a hard coat layer. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] International Publication No. 2016 / 060213 Brochure [Patent Document 2] International Publication No. 2022 / 221118 Brochure [Patent Document 3] Japanese Patent Publication No. 2021-169178 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] However, since high flexibility and high elasticity are in a trade-off relationship, prioritizing flexibility and creating a film with excellent flexibility leads to a decrease in surface modulus and surface hardness, resulting in problems such as insufficient resistance to touch and operation with a stylus. Therefore, the main objective of the present invention is to provide a laminate suitable for use as a component such as a cover window, which has high surface modulus and surface hardness, as well as excellent flexibility. Another objective of the present invention is to provide a cover window for a display using the laminate. [Means for solving the problem]

[0008] The inventors of the present invention have observed that incorporating fibrous alumina nanofillers into a resin component has the excellent effect of improving surface modulus and surface hardness. However, they also observed that when the amount of fibrous alumina nanofillers exceeds a certain level, the fibrous alumina nanofillers orient themselves in the film formation direction during film molding, resulting in anisotropy in the tensile modulus and insufficient bending resistance depending on the bending direction. Further investigations by the inventors revealed that by creating a two-layer laminated structure in which the fibrous alumina nanofillers are localized in one layer, a film can be obtained that has high surface modulus and surface hardness, as well as isotropic bending resistance. The present invention was completed based on these findings. The gist of the present invention is as follows.

[0009] [1] A laminate comprising a first resin layer and a second resin layer provided on one main surface side of the first resin layer, The first resin layer comprises a resin component and 15 to 65% by mass of fibrous alumina nanofiller relative to the resin component. The second resin layer comprises a resin component and 0 to 5% by mass of fibrous alumina nanofiller relative to the resin component. The fibrous alumina nanofiller has an average fiber diameter of 4 to 30 nm and an average fiber length of 200 to 4,000 nm. A laminate in which the surface modulus of the first resin layer, as measured in accordance with ISO 14577 using a nanoindenter, is 6.5 GPa or higher. [2] The laminate according to [1], wherein the thickness of the second resin layer is 10 to 80% of the sum of the thicknesses of the first resin layer and the second resin layer. [3] The laminate according to [1] or [2], wherein when the direction of film formation of the first resin layer is the MD direction and the direction perpendicular to the MD direction is the TD direction, the difference between the elastic modulus of the laminate in the MD direction and the elastic modulus of the laminate in the TD direction is 2 GPa or more. [4] A laminate according to any one of items [1] to [3], wherein the total thickness of the laminate is 10 to 250 μm. [5] The laminate according to any one of the claims [1] to [4], wherein at least one of the resin components of the first resin layer and the second resin layer comprises at least one resin selected from the group consisting of polyimide, polyamide, and polyamideimide. [6] A cover window for a display using a laminate described in any one of the items [1] to [5]. [7] The cover window according to [6], wherein the first resin layer of the laminate is positioned on the surface side. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a laminate that has high surface modulus and surface hardness, as well as excellent bending resistance, making it suitable as a component for cover windows and the like. [Brief explanation of the drawing]

[0011] [Figure 1] This is a cross-sectional view of a laminate according to one embodiment of the present invention. [Figure 2] This is a cross-sectional view of a laminate according to another embodiment of the present invention. [Figure 3] This is a cross-sectional view of a laminate according to another embodiment of the present invention. [Modes for carrying out the invention]

[0012] [Laminated structure] The laminate of the present invention will be described below with reference to the figures. The laminate has a longitudinal direction X, a transverse direction Y, and a thickness direction Z, all of which are orthogonal to each other. The longitudinal direction is the film formation direction (for example, the casting direction when film formation is done by casting, etc.). The cross-section of the laminate is an arbitrary cross-section, but for convenience, the drawings show the cross-section of the laminate parallel to the longitudinal direction X.

[0013] FIG. 1 is a cross-sectional view of a laminate according to an embodiment of the present invention. The laminate 1 of the present invention includes a first resin layer 10 and a second resin layer 20 provided on one main surface T1 side of the first resin layer 10. Further, as shown in FIG. 2, in the laminate according to another embodiment of the present invention, a protective layer 30 such as a hard coat layer is provided on the other main surface T2 side of the first resin layer 10. Further, as shown in FIG. 3, the protective layer 30 may be provided on one main surface U1 side of the second resin layer 20. Although not shown, in the laminate according to the embodiments shown in FIGS. 2 and 3, a functional layer such as a fingerprint adhesion prevention layer may be further provided on the surface on the protective layer 30 (hard coat layer) side. Hereinafter, each layer constituting the laminate of the present invention will be described.

[0014] <First resin layer> The first resin layer is a layer having a function of承担 surface elastic modulus and surface hardness in addition to bend resistance and transparency, and contains a resin component and fibrous alumina nanofiller as essential components.

[0015] As the resin component constituting the first resin layer, resins such as epoxy resin, polyimide resin, polyamideimide resin, and polybenzoxazole resin can be used, but resins having an imide structure and / or an amide structure can be preferably used. Examples of the resin having an imide structure and an amide structure include polyamideimide resin having a structure in which a structural unit containing an imide structure and a structural unit containing an amide structure are copolymerized, polyimide resin composed of a structural unit containing an imide structure, polyamide resin composed of a structural unit containing an amide structure, polyamideimide resin, polyimide resin, and a mixture of polyamide resin, a mixture of these resins, and the rigidity and flexibility can be appropriately adjusted from the selection and ratio of the amide structure, imide structure, and other skeletal structures. Then, in the resin composition containing these resin components, the fibrous alumina nanofiller described later is dispersed in a specific fiber dimension state, so that the first resin layer can have excellent bend resistance, transparency, high surface elastic modulus, and surface hardness.

[0016] It should be noted that the expression "承担 surface elastic modulus and surface hardness" in the translation of item may need to be adjusted according to the accurate technical meaning, perhaps it should be something like "responsible for surface elastic modulus and surface hardness". This is a literal translation based on the provided text.Polyimide resins are obtained by known synthesis methods such as the reaction of diamine compounds and tetracarboxylic acid compounds which are monomer components, polyamide resins are obtained by the reaction of diamine compounds and dicarboxylic acid compounds, and polyamide-imide resins are obtained by the reaction of diamine compounds, tetracarboxylic acid compounds and dicarboxylic acid compounds. For example, in the synthesis of polyamide-imide resins, a polymer having an imide precursor structure is synthesized by reacting a diamine compound and a tetracarboxylic acid compound, and then a copolymer having an imide precursor structure and an amide structure is synthesized by reacting the polymer and a dicarboxylic acid compound. After that, it can be obtained by subjecting the imide precursor structure in the copolymer to a ring-closing reaction (imidation). These resin components preferably contain at least one structure selected from the group consisting of structures in which aliphatic rings and aromatic rings are linked by an alkylene group which may be substituted with a sulfonyl group or a fluorine atom as the structure of the monomer components used.

[0017] Examples of the diamine compounds used in the synthesis of resins having an imide structure and / or an amide structure include aliphatic diamines, aromatic diamines, and mixtures thereof. In this specification, "aliphatic diamine" refers to a diamine in which an amino group is directly bonded to an aliphatic group, and a part of its structure may contain an aromatic ring or other substituents. Also, "aromatic diamine" refers to a diamine in which an amino group is directly bonded to an aromatic ring, and a part of its structure may contain an aliphatic group or other substituents. The aromatic ring may be a monocyclic ring or a condensed ring, and examples include, but are not limited to, benzene ring, naphthalene ring, anthracene ring, and fluorene ring. Among these, a benzene ring is preferred. These diamine compounds can be used alone or in combination of two or more.

[0018] Specific examples of aliphatic diamines include acyclic aliphatic diamines such as hexamethylenediamine; and cyclic aliphatic diamines such as 1,3-bis(aminomethyl)cyclohexane, 1,4-bis(aminomethyl)cyclohexane, norbornanediamine, 4,4'-diaminodicyclohexylmethane, etc. These can be used alone or in combination of two or more.

[0019] Specific examples of aromatic diamines include aromatic diamines having one aromatic ring, such as p-phenylenediamine, m-phenylenediamine, 2,4-toluenediamine, m-xylylenediamine, p-xylylenediamine, 1,5-diaminonaphthalene, and 2,6-diaminonaphthalene; 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenylpropane, 4,4'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 3,3'-diaminodiphenyl ether, 4,4'-diaminodiphenyl sulfone, 3,4'-diaminodiphenyl sulfone, 3,3'-diaminodiphenyl sulfone, 1,4-bis(4-aminophenoxy)benzene, 1,3-bis(4-aminophenoxy)benzene, and bi Examples of aromatic diamines having two or more aromatic rings include s[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, 2,2'-bis(trifluoromethyl)benzidine (TFMB), 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 individually or in combination of two or more.

[0020] Among the diamine compounds described above, from the viewpoint of improving the colorless transparency and elasticity of the laminate, it is preferable to use one or more selected from the group consisting of aromatic diamines having a biphenyl structure, specifically one or more selected from the group consisting of 2,2'-dimethylbenzidine, 2,2'-bis(trifluoromethyl)benzidine, 4,4'-bis(4-aminophenoxy)biphenyl, and 4,4'-diaminodiphenyl ether. Furthermore, from the viewpoint of easily improving colorless transparency, it is even more preferable to use a diamine having a biphenyl structure in which some or all of the hydrogen atoms on the aromatic ring are substituted with substituents selected from a fluoro group, a trifluoromethyl group, or a trifluoromethoxy group, specifically 2,2'-bis(trifluoromethyl)benzidine (TFMB).

[0021] Tetracarboxylic acid compounds used in the synthesis of polyamide-imide resins include tetracarboxylic acids or tetracarboxylic acid derivatives. Tetracarboxylic acid derivatives include tetracarboxylic acid anhydrides, preferably dianhydrides, and acid chlorides. Examples of tetracarboxylic acid compounds include aromatic tetracarboxylic acids and their anhydrides, preferably dianhydrides; and aliphatic tetracarboxylic acid compounds and their anhydrides, preferably dianhydrides. These tetracarboxylic acid compounds can be used individually or in combination of two or more.

[0022] Specific examples of aromatic tetracarboxylic dianhydrides include non-condensed polycyclic aromatic tetracarboxylic dianhydrides, monocyclic aromatic tetracarboxylic dianhydrides, and condensed polycyclic aromatic tetracarboxylic dianhydrides. Examples of non-condensed polycyclic aromatic tetracarboxylic dianhydrides include 4,4'-oxydiphthalic acid dianhydride (ODPA), 3,4-oxydiphthalic acid dianhydride (aODPA), 4,4'-(4,4'-isopropylidene diphenoxy)diphthalic acid dianhydride (BPADA), 3,3',4,4'-benzophenone tetracarboxylic acid dianhydride, 2,2',3,3'-benzophenone tetracarboxylic acid dianhydride, 3,3',4,4'-biphenyltetracarboxylic acid dianhydride (sBPDA), 2,2',3,3'-biphenyltetracarboxylic acid dianhydride (aBPDA), 3,3',4,4'-diphenylsulfone tetracarboxylic acid dianhydride, 2,2-bis(3,4-dicarboxyphenyl)propane dianhydride, and 2,2-bis(2,3-dicarboxyphenyl) Examples include siphenyl)propane dianhydride, 2,2-bis(3,4-dicarboxyphenoxyphenyl)propane dianhydride, 4,4'-(hexafluoroisopropylidene)diphthalic acid dianhydride (6FDA), 1,2-bis(2,3-dicarboxyphenyl)ethane dianhydride, 1,1-bis(2,3-dicarboxyphenyl)ethane dianhydride, 1,2-bis(3,4-dicarboxyphenyl)ethane dianhydride, 1,1-bis(3,4-dicarboxyphenyl)ethane dianhydride, bis(3,4-dicarboxyphenyl)methane dianhydride, bis(2,3-dicarboxyphenyl)methane dianhydride, 4,4'-(p-phenylenedioxy)diphthalic acid dianhydride, and 4,4'-(m-phenylenedioxy)diphthalic acid dianhydride. Examples of monocyclic aromatic tetracarboxylic dianhydrides include 1,2,4,5-benzenetetracarboxylic dianhydride, while examples of condensed polycyclic aromatic tetracarboxylic dianhydrides include 2,3,6,7-naphthalenetetracarboxylic dianhydride.

[0023] Aliphatic tetracarboxylic dianhydrides include cyclic and acyclic aliphatic tetracarboxylic dianhydrides. Cyclic aliphatic tetracarboxylic dianhydrides are tetracarboxylic dianhydrides having an alicyclic hydrocarbon structure. Specific examples include cycloalkane tetracarboxylic dianhydrides such as 1,2,4,5-cyclohexanetetracarboxylic dianhydride (HPMDA), 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA), and 1,2,3,4-cyclopentanetetracarboxylic dianhydride, as well as bicyclo[2.2.2]octo-7-ene-2,3,5,6-tetracarboxylic dianhydride, dicyclohexyl-3,3',4,4'-tetracarboxylic dianhydride (HBPDA) and their positional isomers. These can be used individually or in combination of two or more. Specific examples of acyclic aliphatic tetracarboxylic dianhydrides include 1,2,3,4-butanetetracarboxylic dianhydride and 1,2,3,4-pentanetetracarboxylic dianhydride, which can be used individually or in combination of two or more. Furthermore, cyclic aliphatic tetracarboxylic dianhydrides and acyclic aliphatic tetracarboxylic dianhydrides may be used in combination.

[0024] Among the tetracarboxylic acid compounds described above, from the viewpoint of improving the flexibility and optical properties of the film, it is preferable to use aromatic tetracarboxylic acid dianhydrides in which some or all of the hydrogen atoms on the aromatic ring of the aromatic tetracarboxylic acid dianhydride are substituted with substituents selected from a fluoro group, a trifluoromethyl group, or a trifluoromethoxy group, specifically 4,4'-(hexafluoroisopropylidene)diphthalic acid dianhydride (6FDA), tetracarboxylic acid dianhydrides having a biphenyl structure or an alicyclic hydrocarbon structure, specifically 3,3',4,4'-biphenyltetracarboxylic acid dianhydride (sBPDA), 1,2,3,4-cyclobutanetetracarboxylic acid dianhydride (CBDA), dicyclohexyl-3,3',4,4'-tetracarboxylic acid dianhydride (HBDA), and aromatic tetracarboxylic acid dianhydrides having an ether bond, specifically 4,4'-oxydiphthalic acid dianhydride (ODPA) and 3,4-oxydiphthalic acid dianhydride (aODPA).

[0025] Dicarboxylic acid compounds used in the synthesis of resins having imide and / or amide structures include dicarboxylic acids or dicarboxylic acid derivatives. Examples of dicarboxylic acid derivatives include acid chlorides and esters of the dicarboxylic acid. Dicarboxylic acid compounds can be used alone or in combination of two or more.

[0026] Specific examples of dicarboxylic acid compounds include, for example, 1,3-cyclobutanedicarboxylic acid, 1,3-cyclopentanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, 4,4'-oxybisbenzoic acid (OBBC), terephthalic acid, isophthalic acid, 2,6-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 1,4-naphthalenedicarboxylic acid, 4,4'-biphenyldicarboxylic acid, 3,3'-biphenyldicarboxylic acid, alicyclic or aromatic dicarboxylic acids and their derivatives (e.g., acid chlorides, acid anhydrides) such as compounds in which two cyclohexanecarboxylic acids or two benzoic acids are linked by a single bond, -CH2-, -C(CH3)2-, -C(CF3)2-, -SO2-, or a phenylene group; and aliphatic dicarboxylic acids and their derivatives (e.g., acid chlorides, esters) such as dicarboxylic acid compounds of chain hydrocarbons with 8 or fewer carbon atoms. These dicarboxylic acid compounds can be used individually or in combination of two or more.

[0027] Among the dicarboxylic acid compounds mentioned above, it is preferable to use terephthalic acid or 4,4'-oxybisbenzoic acid or its derivatives, particularly terephthalic acid chloride (sometimes abbreviated as TPC) or 4,4'-oxybis(benzoyl chloride) (sometimes abbreviated as OBBC), from the viewpoint of improving the elongation at the breaking point and tensile modulus of the film.

[0028] In the synthesis of polyimide resins and polyamideimide resins, the ring-closing reaction (imidation) of the imide precursor can be carried out by either thermal imidation, which involves adding an azeotropic solvent that forms an azeotrope with water (e.g., toluene, xylene, etc.) and heating, or chemical imidation, which uses a condensing agent and a reaction accelerator. However, chemical imidation is preferred because it maintains colorless transparency.

[0029] Examples of reaction accelerators used in chemical imidation include triethylamine, diisopropylethylamine, N-methylpiperidine, pyridine, 2-methylpyridine, 3-methylpyridine, 4-methylpyridine, 3-ethylpyridine, 3,5-dimethylpyridine, 3,5-diethylpyridine, isoquinoline, imidazole, 1-methylimidazole, 2-methylimidazole, and 1,2-dimethylimidazole. These reaction accelerators may be used individually or in combination of two or more.

[0030] Examples of condensing agents used in chemical imidation include acid anhydrides such as acetic anhydride, propionic anhydride, and trifluoroacetic anhydride, as well as phosphite esters such as triethyl phosphite, tributyl phosphite, dimethyl phosphite, diethyl phosphite, and triphenyl phosphite. These condensing agents may be used individually or in combination of two or more.

[0031] The organic solvent used in the synthesis of resins having imide and / or amide structures is not particularly limited, as long as it is an organic solvent that is inert to the reaction. Examples include N,N-dimethylacetamide (DMAc), N-methyl-2-pyrrolidone, 1,3-dimethyl-2-imidazolidinone, dimethyl sulfoxide, m-cresol, γ-butyrolactone, cyclopentanone, cyclohexanone, and tetrahydrofuran. These organic solvents may be used individually or in combination of two or more.

[0032] The reaction conditions for the synthesis of resins having imide and / or amide structures can be 10 to 50°C for 1 to 27 hours or less, and it is preferable to synthesize them under a nitrogen atmosphere in order to maintain colorlessness and transparency.

[0033] The weight-average molecular weight (Mw) of resins having an imide structure and / or amide structure is preferably in the range of 50,000 to 1,000,000, more preferably in the range of 80,000 to 800,000, and even more preferably in the range of 110,000 to 600,000, from the viewpoint of improving the tensile modulus and elongation at the breaking point. The weight-average molecular weight (Mw) is measured by GPC (gel permeation chromatography) and the value is calculated on a polystyrene basis.

[0034] The fibrous alumina nanofiller contained in the first resin layer described above is dispersed in the aforementioned resin components with an average fiber diameter of 4 to 30 nm and an average fiber length of 200 to 4,000 nm. In the present invention, the first resin layer contains 15 to 65% by mass of fibrous alumina nanofiller relative to the resin components constituting the first resin layer. By including the fibrous alumina nanofiller in the above-described proportions, the surface modulus and surface hardness of the first resin layer can be improved. Specifically, the surface modulus of the first resin layer, as measured using a nanoindenter in accordance with ISO 14577, can be set to 6 GPa or higher. In this specification, the surface modulus measured using a nanoindenter refers to the modulus obtained from the load-indenter-indenter depth curve, which is obtained by continuously measuring the load applied to the indenter and the indentation depth during loading and unloading when an indenter is pressed into the sample surface with a load of 30 mN. The measurement is performed with n=5 tests, and the average value is calculated. Furthermore, if other layers, such as the hard coat layer described above, are provided on the surface of the first resin layer, the other layers may be removed to expose the surface of the first resin layer and measure the surface modulus, or the other layers may be left in place and the tip of the indenter may be pressed into the surface of the first resin layer to measure the surface modulus. The surface modulus of the first resin layer measured using the nanoindenter is preferably 6.5 to 9 GPa.

[0035] Fibrous alumina nanofillers having the shape described above can impart superior mechanical strength (surface modulus and surface hardness) without impairing light transmittance compared to spherical or amorphous alumina fillers. The preferred amount of fibrous alumina nanofiller to be blended is 20 to 60% by mass relative to the resin component.

[0036] The fibrous alumina nanofillers are dispersed in a state where the average fiber diameter is 4 to 30 nm and the average fiber length is 200 to 4,000 nm, preferably with an average fiber diameter of 7 to 25 nm and an average fiber length of 500 to 3,000 nm, and more preferably with an average fiber diameter of 11 to 20 nm and an average fiber length of 700 to 2,000 nm. If the fibrous alumina nanofillers are dispersed in the resin composition within the above-mentioned ranges of average fiber diameter and average fiber length, the film made from this resin composition will have reduced discoloration and turbidity, and will achieve high elasticity while maintaining flexibility.

[0037] The "average fiber diameter" and "average fiber length" of fibrous alumina nanofillers in a dispersed state are measured by diluting the resin composition 10,000 times with methyl isobutyl ketone (MIBK), dropping one drop onto a cover glass (cover glass trophy, manufactured by Matsunami Glass Co., Ltd.), drying at 50°C, and then observing with an electron microscope image (for example, a 10,000x magnification image using a Hitachi High-Tech FE-SEM). The fibrous alumina nanofillers to be measured may be single fibers or bundles of multiple single fibers, as long as they are visible as a single fiber in the electron microscope image. The "average fiber diameter" is defined as the average measured length of the diameter in the short-side direction of 50 arbitrarily selected fibrous alumina nanofillers in the electron microscope image, and the "average fiber length" is defined as the average measured length in the long-side direction.

[0038] The fibrous alumina nanofiller is mixed with the resin components described above in powder or dispersion (sol) form, stirred, and kneaded as needed to adjust the dispersion state in the resin composition, i.e., the "average fiber diameter" and "average fiber length". For example, stirring or kneading can be performed using stirrers such as dissolvers or butterfly mixers, or kneaders such as roll mills or bead mills. Various conditions such as the rotation speed of the stirrer / kneader, the shape of the stirring blades / kneading device, stirring / kneading time, stirring / kneading temperature, bead filling rate, and roll spacing can be adjusted.

[0039] Fibrous alumina nanofillers can be surface-treated or used as a dispersion (sol) in an organic solvent. Surface treatment or incorporation as a dispersion can stabilize the dispersion state. A dispersion (sol) should be used in which the dispersion state of the fibrous alumina nanofillers in the dispersion (sol) is adjusted to be the same as the dispersion state of the fibrous alumina nanofillers in the first resin layer, i.e., the "average fiber diameter" and "average fiber length".

[0040] The method for surface treatment of fibrous alumina nanofillers and for producing dispersions is not particularly limited. For example, surface treatment methods using coupling agents such as silane-based, titanate-based, aluminate-based, and zircoaluminate-based agents, or the method for producing an organic sulfonic acid-treated dispersion disclosed in Japanese Patent Application Publication No. 2008-31010 can be used.

[0041] The first resin layer may contain, in addition to the resin components and fibrous alumina nanofillers, further additives as needed. Examples of additives include surfactants that improve film-forming properties and defoaming properties.

[0042] Furthermore, the resin component may contain other resins other than those described above, as long as they do not impair the effects of the present invention. Examples of other resins include polyester resins such as polyethylene terephthalate and polyethylene naphthalate, polyphenylene sulfide resins, polyether ether ketone resins, polyether sulfone resins, polycarbonate resins, polyetherimide resins, phenolic resins, glass epoxy resins, polyphenylene ether resins, acrylic resins, polyolefin resins such as polyethylene and polypropylene, and polycycloolefins such as polynorbornene.

[0043] <Second resin layer> As described above, by incorporating a predetermined amount of fibrous alumina nanofiller into the resin component, transparency and mechanical strength can be imparted to the resin layer. However, if the fibrous alumina nanofiller is present in a proportion above a certain level, due to its shape characteristics, the fibrous alumina nanofiller will orient itself in the film-forming direction when forming the first resin layer (i.e., when forming a film using the resin composition). This inevitably results in anisotropy in properties such as flexural resistance between the MD direction (film-forming direction) and the TD direction (direction perpendicular to the film-forming direction). Therefore, when used in a cover window member, if the bending axis is perpendicular to the MD direction, flexural resistance may decrease, potentially leading to bending marks or fracture. In the laminate of the present invention, a second resin layer containing 0-5% by mass of fibrous alumina nanofiller relative to the resin component is provided on one main surface side of the first resin layer, thereby improving flexural resistance while maintaining high surface modulus and surface hardness.

[0044] The resin components constituting the second resin layer can be the same materials as those constituting the first resin layer described above. The resin components constituting the first and second resin layers may be the same or different.

[0045] The amount of fibrous alumina nanofiller in the second resin layer is preferably 0 to 3% by mass relative to the resin component, and more preferably 0% by mass.

[0046] In the laminate of the present invention, the thickness of the second resin layer is preferably 10 to 80% of the sum of the thicknesses of the first and second resin layers, and more preferably 40 to 60%. If the thickness of the second resin layer is too small, the bending resistance of the laminate may be insufficient, while if the thickness of the second resin layer is too large, the surface modulus (surface hardness) of the laminate may be impaired.

[0047] Depending on the intended use, when the laminate of the present invention is used as a cover window member for a display in a foldable device as described later, the overall thickness is preferably 10 to 250 μm, and more preferably 50 to 150 μm.

[0048] In that case, the thickness of the first resin layer is preferably 1 to 200 μm, and more preferably 20 to 30 μm. The thickness of the second resin layer is preferably 9 to 50 μm, and more preferably 20 to 30 μm.

[0049] In the laminate of the present invention, when the film formation direction of the first resin layer is the MD direction and the direction perpendicular to the MD direction is the TD direction, it is preferable that the difference between the elastic modulus of the laminate in the MD direction and the elastic modulus of the laminate in the TD direction is 5 GPa or less. Here, "elastic modulus of the laminate" refers to the elastic modulus obtained from the strength-elongation curve of the laminate measured using a tensile testing machine or the like.

[0050] In the laminate of the present invention, the second resin layer may be formed on one main surface side of the first resin layer after the first resin layer has been formed, or the first resin layer may be formed on one surface of the second resin layer after it has been formed. However, forming the second resin layer first and then the first resin layer results in a laminate with even higher elastic modulus and surface hardness.

[0051] Specifically, a second resin layer can be formed by applying a resin composition for forming a second resin layer (a varnish containing a solvent capable of dissolving the resin components) using a known application method, and then drying the applied film. Subsequently, a first resin layer can be formed by applying a resin composition for forming a first resin layer (a varnish containing a solvent capable of dissolving the resin components) to the surface of the second resin layer using a known application method, and then drying the applied film. The solvent can be used without particular limitations as long as it is capable of dissolving the resin components.

[0052] Conventional known methods can be applied as coating methods, such as dip coating, flow coating, roll coating, bar coating, blade coating, screen printing, curtain coating, spray coating, knife coating, and die coating.

[0053] [Display cover window] The cover window of the display of the present invention consists of the laminate of the present invention and is used by being positioned on the surface of various displays. In this case, it is preferable that the first resin layer of the laminate is positioned on the surface side. Compared to the case where the second resin layer of the laminate is positioned on the surface side, the flexibility, surface modulus, and surface hardness are further improved.

[0054] The display cover window can be used in various known displays, and its applications are not particularly limited. It can be used as a replacement for display cover windows that conventionally used thin glass plates or other glass products. In addition to heat resistance and transparency, the laminate of the present invention possesses high elasticity, surface hardness, and flexibility, which could not be achieved with conventional technology. Therefore, it can be suitably used as a substrate or cover window component for displays that can accommodate curved surfaces. In particular, it can be especially suitably used as a cover window for displays mounted on foldable devices.

[0055] The laminate of the present invention can be suitably used, for example, in thin, flexible, foldable organic EL displays, mobile devices such as smartphones and smartwatches, automotive in-car display devices, and flexible panels used in watches. It can also be applied to components for image display devices such as liquid crystal displays and organic EL displays, components for touch panels, flexible printed circuit boards, components for solar cell panels such as surface protective films and substrate materials, components for optical waveguides, and other semiconductor-related components.

[0056] The method for positioning the display cover window on the display surface is not particularly limited, but examples include using an adhesive layer. Conventional adhesive layers that can be used for bonding display surface materials can be used as the adhesive layer. [Examples]

[0057] The present invention will be described more specifically below with reference to examples, but the present invention is not limited to these examples. In the following, "parts" and "%" all refer to mass unless otherwise specified.

[0058] [Preparation of resin components] A 500 mL reactor equipped with a stirrer, nitrogen injection device, dropping funnel, temperature controller, and condenser was filled with 130 g of DMAc while passing nitrogen through it, and 32.27 g (100.8 mmol) of TFMB was added and dissolved. Next, 2.558 g (5.758 mmol) of 6FDA and 2.541 g (8.637 mmol) of sBPDA were added to this TFMB solution, and the mixture was stirred at 20°C for 2 hours to obtain a solution containing a polymer having an imide precursor structure. Subsequently, 17.54 g (86.37 mol) of TPC was added to this solution, and the mixture was stirred at 20°C for 1 hour to obtain a solution containing a copolymer having an imide precursor structure and an amide structure. Subsequently, 1.14 g of pyridine, 3.00 g of acetic anhydride, and 234 g of DMAc were added and the mixture was stirred at 20-30°C for 1 hour, and then stirred again at 15-25°C for 18 hours to obtain a polyamide-imide solution. Furthermore, 660 g of DMAc was added and stirred until homogeneous. This solution was then gradually added to a container with 4 L of methanol to precipitate, and the precipitated solid was filtered and pulverized. The mixture was then dried under vacuum at 80°C for 18 hours to obtain 48.9 g of solid powder polyamide-imide (PAI). The weight-average molecular weight in polystyrene equivalent, calculated by GPC, was 167,000.

[0059] A 500 mL reactor equipped with a stirrer, nitrogen injection device, dropping funnel, temperature controller, and condenser was filled with 200 g of DMAc while passing nitrogen through it, and 15.00 g (46.8 mmol) of TFMB was added and dissolved. Next, 13.55 g (45.9 mmol) of OBBC was added to this TFMB solution, and the reaction was carried out with stirring at 20°C for 2 hours to obtain a polyamide-containing solution. The weight-average molecular weight of the obtained polyamide (PA) in terms of polystyrene, calculated by GPC, was 280,000.

[0060] A 500 mL reactor equipped with a stirrer, nitrogen injection device, dropping funnel, temperature controller, and condenser was filled with 206 g of DMAc while passing nitrogen through it, and 15.00 g (46.8 mmol) of TFMB was added and dissolved. Next, 14.24 g (45.9 mmol) of ODPA was added to this TFMB solution, and the mixture was stirred at 20°C for 2 hours to obtain a solution containing a polyimide precursor. Subsequently, 13.49 g of pyridine, 19.11 g of acetic anhydride, and 300 g of DMAc were added and the mixture was stirred at 20-30°C for 1 hour, and then stirred again at 15-25°C for 18 hours to obtain a polyimide solution. This solution was gradually added to a container with 4 L of methanol to precipitate, and the precipitated solid was filtered and pulverized. The mixture was then dried under vacuum at 80°C for 18 hours to obtain 26.1 g of polyimide (PI) solid powder. The weight-average molecular weight in polystyrene terms, calculated by GPC, was 126,000.

[0061] In addition, a 25 μm thick polyethylene naphthalate film (manufactured by Teijin Limited, Teonex) was prepared.

[0062] [Preparation of fibrous alumina nanofillers] A slurry (manufactured by Kawaken Fine Chemicals Co., Ltd.) was prepared by dispersing fibrous alumina nanofillers with an average fiber diameter of 18.9 nm and an average fiber length of 1600 nm in MIBK at a concentration of 5%.

[0063] [Fabrication of laminates] After dissolving PAI, PA, or PI powder in DMAc, fibrous alumina nanofillers were added in the amounts shown in Table 1, dispersed, and homogenized to prepare a composition for forming the first resin layer. The amounts in the table are based on the mass of non-volatile solids.

[0064] First, a second resin layer-forming composition, prepared by dissolving PAI, PA, or PI powder in DMAc, was applied to a glass plate using a table coater (AFA-standard, manufactured by Cortec Co., Ltd.) so that the film thickness after drying would be the thickness shown in Table 1. The second resin layer was then formed by drying in an inert gas oven (INL-45N1, manufactured by Yamato Scientific Co., Ltd.) at 250°C for 1 hour. In Example 14, a polyethylene naphthalate film with a thickness of 25 μm was used as the second resin layer.

[0065] Next, the above-mentioned composition for forming the first resin layer was applied to one surface of the second resin layer using a table coater (Co-Tec Co., Ltd., AFA-standard) so that the film thickness after drying would be the thickness shown in Table 1. The first resin layer was then formed by drying in an inert gas oven (Yamato Scientific Co., Ltd., INL-45N1) at 250°C for 1 hour.

[0066] Subsequently, the second resin layer was peeled off from the glass plate to obtain a laminate consisting of the first resin layer and the second resin layer.

[0067] [Measurement of Tensile Modulus] For each laminate obtained as described above, the difference in tensile modulus (laminate modulus) between the film formation direction (MD direction) and its orthogonal direction (TD direction) during the formation of the first resin layer was measured using a tensile testing machine (Shimadzu Corporation, EZ-SX) under the following conditions. The laminate modulus was determined from the slope of the obtained stress-strain diagram at stresses from 5 MPa to 10 MPa. <Test Conditions> Sample size: 50mm x 5mm Distance between gripping devices: 30mm Speed: 1mm / min Number of measurements: 5

[0068] [Measurement of surface modulus using nanoindenter] For each laminate, the surface modulus of the first resin layer was measured using a nanoindenter in accordance with ISO 14577. Specifically, the displacement-load hysteresis curve obtained by pressing a probe (indenter) against the surface was numerically processed using the software (triboscan) attached to the measuring device to measure the surface modulus, and the average of five measurements was used as the measured value. <Measurement device and measurement conditions> Equipment: Nanoindenter (manufactured by Elionix Co., Ltd., ENT-2000) Measurement method: Single indentation method Test load: 30mN Load time: 100000msec Holding time: 1000msec Unloading time: 100000msec Probe: Made of diamond, Berkovich type (triangular pyramidal shape)

[0069] [Flexural resistance test] For each laminate, repeated bending tests were performed using a benchtop durability testing machine to evaluate its bending resistance. Specifically, under the measurement conditions described below, each laminate was repeatedly bent 180 degrees in a direction perpendicular to the direction of high elastic modulus of the laminate, and the appearance of defects such as cracks or whitening in the bent area was evaluated according to the following criteria. <Measurement device and measurement conditions> Equipment: Benchtop durability testing machine (Yuasa Systems Corporation, Tension-Free® Folding Clamshell-type) Sample size: 10mm x 100mm (longer side parallel to the direction of high-laminated material elastic modulus) Bending radius: 1.5 mm Bending speed: 30 cyc / min Number of bends: 200,000 Observation method: Optical microscope <Evaluation Criteria> ○: No cracks or whitening were observed after 200,000 folds. ×: After 200,000 folds, cracks or whitening were observed.

[0070] [Table 1] [Explanation of Symbols]

[0071] 10 1st resin layer 20 Second resin layer 30 protective layer

Claims

1. A laminate comprising a first resin layer and a second resin layer provided on one main surface side of the first resin layer, The first resin layer comprises a resin component and 15 to 65% by mass of fibrous alumina nanofiller relative to the resin component. The second resin layer comprises a resin component and 0 to 5% by mass of fibrous alumina nanofiller relative to the resin component. The fibrous alumina nanofiller has an average fiber diameter of 4 to 30 nm and an average fiber length of 200 to 4,000 nm. In the first resin layer, the surface modulus measured using a nanoindenter in accordance with ISO 14577 is 6.5 GPa or higher. A laminate in which, when the film formation direction of the first resin layer is defined as the MD direction and the direction perpendicular to the MD direction is defined as the TD direction, the difference between the elastic modulus of the laminate in the MD direction and the elastic modulus of the laminate in the TD direction is 2 GPa or more and 5 GPa or less.

2. The laminate according to claim 1, wherein the thickness of the second resin layer is 10 to 80% of the sum of the thicknesses of the first resin layer and the second resin layer.

3. The laminate according to claim 1, wherein the total thickness of the laminate is 10 to 250 μm.

4. The laminate according to claim 1, wherein at least one of the resin components of the first resin layer and the second resin layer comprises at least one resin selected from the group consisting of polyimide, polyamide, and polyamideimide.

5. A cover window for a display using the laminate described in any one of claims 1 to 4.

6. The cover window according to claim 5, wherein the first resin layer of the laminate is arranged to be on the surface side.

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