Optical film and image display device

Abstract

Provided is an optical film comprising a hard coat layer and a low-refractive-index layer in the stated order on one surface of a resin base material having a thickness of 10-100 μm, the thickness of the hard coat layer being 2.0-7.0 μm, wherein: the optical film has a stretching ratio of 3.0-30.0%; the minimum diameter of a cylinder which is wound so that the low-refractive-index layer is on the inner side and in which cracks do not occur is 1.0 mm or less; the minimum diameter of a cylinder which is wound so that the low-refractive-index layer is on the outer side and in which cracks do not occur is 6.0 mm or less; and the surface of the low-refractive-index layer has a water contact angle change rate of 20% or less after an eraser is slid thereon 5000 times at a load of 1.0 kg / 6.0 mm Φ.

Description

Optical film and image display device

[0001] The present invention relates to an optical film and an image display device using the same.

[0002] 2. Description of the Related Art As the screens of smartphones, tablet terminals, and the like become larger, development of foldable image display devices is progressing.

[0003] For example, Patent Document 1 discloses that an optical film having a resin substrate made of one or more resins selected from polyimide-based resins, polyamideimide-based resins, polyamide-based resins, and polyester-based resins, a first optical adjustment layer of 30 nm or more and 200 nm or less on a first surface, and a third optical adjustment layer of 30 nm or more and 1 μm or less on a second surface opposite the first surface, and further having a functional layer on top of the first optical adjustment layer, does not crack or break when subjected to a test in which the functional layer is folded repeatedly 10,000 times with the functional layer facing inward.

[0004] Patent No. 7119424

[0005] Generally, if the substrate or hard coat layer is hard, flexibility is reduced, so flexibility can be improved by using a flexible material for these or by reducing the thickness of these layers. However, such a configuration has the problem of reducing the indentation strength and thereby worsening the abrasion resistance.

[0006] Optical films used in foldable image display devices are required to have improved abrasion resistance in the outermost layer that is subjected to handling, and therefore are required to have improved repeated folding properties and abrasion resistance.

[0007] The present invention relates to an optical film that combines high repeated bending properties with high abrasion resistance, and an image display device using the same.

[0008] The present invention relates to the following [1] to [5]: [1] An optical film comprising a hard coat layer and a low refractive index layer, in this order, on one surface of a resin substrate having a thickness of 10 μm to 100 μm, the hard coat layer having a thickness of 2.0 μm to 7.0 μm, wherein the stretching ratio is 3.0% to 30.0%, the minimum diameter of a cylinder that does not develop cracks when wrapped around the optical film with the low refractive index layer on the inside is 1.0 mm or less, the minimum diameter of a cylinder that does not develop cracks when wrapped around the optical film with the low refractive index layer on the outside is 6.0 mm or less, and the rate of change in water contact angle after sliding an eraser on the surface of the low refractive index layer 5,000 times with a load of 1.0 kg / 6.0 mmΦ is 20% or less. [2] An optical film comprising a hard coat layer and a low refractive index layer in this order on one side of a resin substrate having a thickness of 10 μm or more and 100 μm or less, the hard coat layer having a thickness of 2.0 μm or more and 7.0 μm or less, wherein an integrated value obtained by integrating the tensile stress until cracks occur in the hard coat layer over the amount of stretching is 1,000 GPa mm or more, the minimum diameter of a cylinder that does not cause cracks when wrapped around the optical film with the low refractive index layer on the inside is 1.0 mm or less, the minimum diameter of a cylinder that does not cause cracks when wrapped around the optical film with the low refractive index layer on the outside is 6.0 mm or less, and the rate of change in water contact angle after sliding an eraser on the surface of the low refractive index layer 5,000 times with a load of 1.0 kg / 6.0 mmΦ is 20% or less. [3] An optical film comprising a hard coat layer and a low refractive index layer in this order on one surface of a resin substrate having a thickness of 10 μm or more and 100 μm or less, the thickness of the hard coat layer being 2.0 μm or more and 7.0 μm or less, wherein the product of the tensile modulus and the thickness of the resin substrate is 700 GPa μm or less, the minimum diameter of a cylinder that can be wrapped around the optical film with the low refractive index layer on the inside and no cracks occurring is 1.0 mm or less, the minimum diameter of a cylinder that can be wrapped around the optical film with the low refractive index layer on the outside and no cracks occurring is 6.0 mm or less, and the rate of change in water contact angle after sliding an eraser on the surface of the low refractive index layer 5,000 times with a load of 1.0 kg / 6.0 mmΦ is 20% or less.[4] An optical film comprising a hard coat layer and a low refractive index layer in this order on one side of a resin substrate having a thickness of 10 μm to 100 μm, the hard coat layer having a thickness of 2.0 μm to 7.0 μm, wherein the optical film has a yield point of 100 GPa or more, a minimum diameter of a cylinder that can be wrapped around the optical film with the low refractive index layer on the inside and no cracks occurring is 1.0 mm or less, a minimum diameter of a cylinder that can be wrapped around the optical film with the low refractive index layer on the outside and no cracks occurring is 6.0 mm or less, and a water contact angle change rate of 20% or less after sliding an eraser over the surface of the low refractive index layer 5,000 times with a load of 1.0 kg / 6.0 mmΦ. [5] An image display device comprising the optical film according to any one of [1] to [4].

[0009] The optical film of the present invention exhibits the excellent effect of achieving both high repeated bending properties and high abrasion resistance.

[0010] Fig. 1 is a schematic cross-sectional view showing an example of the optical film of the present invention, and Fig. 2 is a schematic cross-sectional view showing another example of the optical film of the present invention.

[0011] FIG. 1 is a schematic cross-sectional view illustrating an example of an optical film according to an embodiment.

[0012] The optical film 10 includes a resin substrate 1, and a hard coat layer 2 and a low refractive index layer 3 in this order on one surface of the resin substrate 1.

[0013] FIG. 2 is a schematic cross-sectional view showing another example of the optical film according to the embodiment.

[0014] The optical film 20 comprises a resin substrate 1, a hard coat layer 2, and a low refractive index layer 3, in this order, on one surface of the resin substrate 1, and an easy-adhesion layer 4 laminated on the resin substrate 1. Specifically, the easy-adhesion layer 4, hard coat layer 2, and low refractive index layer 3, in this order, are provided on one surface of the resin substrate 1. The presence of the easy-adhesion layer improves adhesion even when the hard coat layer is thin, thereby further improving repeated bending properties and abrasion resistance. The other surface of the resin substrate 1 may have an easy-adhesion layer 4 laminated thereon to improve adhesion to other layers (for example, a display lamination adhesive layer) and thereby improve adhesion.

[0015] Each layer will be described in detail below.

[0016] The resin substrate is exemplified by one made of a polyester-based resin. Polyester-based resins are preferred from the viewpoint of imparting transparency and flexibility. Examples of polyester-based resins include polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate. The resin substrate may also be made of a polyimide-based resin or a polyamide-imide-based resin. In the present invention, from the viewpoint of bending resistance, one or more resins selected from the group consisting of polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polyimide, and polyamide-imide are preferred.

[0017] The thickness of the resin substrate is not particularly limited as long as it is 10 μm or more and 100 μm or less, but from the viewpoint of improving repeated bending properties, it is preferably 60 μm or less, more preferably 50 μm or less. The lower limit is not particularly limited, but from the viewpoint of strength, it is preferably 10 μm or more.

[0018] The surface of the resin substrate may be subjected to a surface modification treatment to improve adhesion with other layers to be laminated thereon, such as alkali treatment, corona treatment, plasma treatment, sputtering treatment, application of a surfactant or a silane coupling agent, or Si vapor deposition.

[0019] The hard coat layer is disposed on one surface of the resin substrate and provides flexibility to the optical film while improving impact resistance, and can improve repeated bending properties and abrasion resistance.

[0020] The hard coat layer can be formed by applying and curing a composition for forming a hard coat layer, which contains an active energy ray-curable compound, a photopolymerization initiator, and a solvent.

[0021] As the active energy ray-curable compound, for example, a monofunctional, difunctional, trifunctional or higher functional (meth)acrylate monomer can be used. In this specification, "(meth)acrylate" is a general term for both acrylate and methacrylate, and "(meth)acryloyl" is a general term for both acryloyl and methacryloyl.

[0022] Examples of monofunctional (meth)acrylate compounds include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, glycidyl (meth)acrylate, acryloylmorpholine, N-vinylpyrrolidone, tetrahydrofurfuryl acrylate, cyclohexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and isobornyl (meth)acrylate. acrylate, isodecyl (meth)acrylate, lauryl (meth)acrylate, tridecyl (meth)acrylate, cetyl (meth)acrylate, stearyl (meth)acrylate, benzyl (meth)acrylate, 2-ethoxyethyl (meth)acrylate, 3-methoxybutyl (meth)acrylate, ethyl carbitol (meth)acrylate, phosphate (meth)acrylate, ethylene oxide-modified phosphate (meth)acrylate, phenoxy (meth)acrylate, ethylene oxide-modified phenoxy (meth)acrylate, propylene oxide ethylene oxide-modified phenoxy (meth)acrylate, nonylphenol (meth)acrylate, ethylene oxide-modified nonylphenol (meth)acrylate, propylene oxide-modified nonylphenol (meth)acrylate, methoxydiethylene glycol (meth)acrylate, methoxypolyethylene glycol (meth)acrylate, methoxypropylene glycol (meth)acrylate, 2-(meth)acryloyloxyethyl-2-hydroxypropyl phthalate, 2-hydroxy-3-phenoxypropyl (meth)acrylate, 2-(meth)acryloyloxyethyl-2-hydroxypropyl phthalate, p) acryloyloxyethyl hydrogen phthalate, 2-(meth)acryloyloxypropyl hydrogen phthalate, 2-(meth)acryloyloxypropyl hexahydrohydrogen phthalate, 2-(meth)acryloyloxypropyl tetrahydrohydrogen phthalate, dimethylaminoethyl (meth)acrylate, trifluoroethyl (meth)acrylate, tetrafluoropropyl (meth)acrylate, hexafluoropropyl (meth)acrylate, octafluoropropyl (meth)acrylate, 2-adamantane,adamantane derivative mono(meth)acrylates such as adamantyl acrylate having a monovalent mono(meth)acrylate derived from adamantanediol, etc.

[0023] Examples of bifunctional (meth)acrylates include di(meth)acrylates such as ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, butanediol di(meth)acrylate, hexanediol di(meth)acrylate, nonanediol di(meth)acrylate, ethoxylated hexanediol di(meth)acrylate, propoxylated hexanediol di(meth)acrylate, diethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, neopentyl glycol di(meth)acrylate, ethoxylated neopentyl glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, and hydroxypivalic acid neopentyl glycol di(meth)acrylate.

[0024] Examples of tri- or higher functional (meth)acrylates include tri(meth)acrylates such as trimethylolpropane tri(meth)acrylate, ethoxylated trimethylolpropane tri(meth)acrylate, propoxylated trimethylolpropane tri(meth)acrylate, tris-2-hydroxyethyl isocyanurate tri(meth)acrylate, and glycerin tri(meth)acrylate; trifunctional (meth)acrylate compounds such as pentaerythritol tri(meth)acrylate, dipentaerythritol tri(meth)acrylate, and ditrimethylolpropane tri(meth)acrylate;

[0033] Examples of the polyfunctional (meth)acrylate include tri- or higher functional (meth)acrylate compounds such as pentaerythritol tetra(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, dipentaerythritol tetra(meth)acrylate, dipentaerythritol penta(meth)acrylate, ditrimethylolpropane penta(meth)acrylate, dipentaerythritol hexa(meth)acrylate, and ditrimethylolpropane hexa(meth)acrylate, as well as polyfunctional (meth)acrylate compounds in which a portion of these (meth)acrylates is substituted with an alkyl group or ε-caprolactone.

[0025] Urethane (meth)acrylates can also be used as polyfunctional monomers. Examples of urethane (meth)acrylates include those obtained by reacting a polyester polyol with an isocyanate monomer or a prepolymer, and then reacting the resulting product with a (meth)acrylate monomer having a hydroxyl group.

[0026] Examples of urethane (meth)acrylates include pentaerythritol triacrylate hexamethylene diisocyanate urethane prepolymer, dipentaerythritol pentaacrylate hexamethylene diisocyanate urethane prepolymer, pentaerythritol triacrylate toluene diisocyanate urethane prepolymer, dipentaerythritol pentaacrylate toluene diisocyanate urethane prepolymer, pentaerythritol triacrylate isophorone diisocyanate urethane prepolymer, and dipentaerythritol pentaacrylate isophorone diisocyanate urethane prepolymer.

[0027] The active energy ray-curable compound may be used alone or in combination of two or more thereof. In addition, the active energy ray-curable compound may be a monomer in the coating liquid, or may be a partially polymerized oligomer.

[0028] The content of the active energy ray-curable compound is not particularly limited and is, for example, 10% by mass or more and 90% by mass or less of the total amount of solid components. In this specification, the total amount of solid components refers to the total content of all components of the composition other than the solvent.

[0029] The photopolymerization initiator may be any one that triggers a polymerization reaction when irradiated with ultraviolet light, an electron beam, or the like, and examples thereof include 2,2-ethoxyacetophenone, 1-hydroxycyclohexyl phenyl ketone, dibenzoyl, benzoin, benzoin methyl ether, benzoin ethyl ether, p-chlorobenzophenone, p-methoxybenzophenone, Michler's ketone, acetophenone, 2-chlorothioxanthone, etc. These may be used alone or in combination of two or more.

[0030] The content of the photopolymerization initiator is not particularly limited, and is, for example, 0.01% by mass or more and 20% by mass or less of the total amount of solid components.

[0031] Examples of the solvent include ketone solvents such as acetone, methyl ethyl ketone (MEK), and methyl isobutyl ketone (MIBK); alcohol solvents such as ethanol, methanol, isopropyl alcohol (IPA), and isobutanol; ether solvents such as ethylene glycol dimethyl ether and propylene glycol monomethyl ether (PGME); ester solvents such as ethyl acetate, propylene glycol monomethyl ether acetate (PGMEA), and 2-ethoxyethyl acetate; and aromatic hydrocarbon solvents such as toluene. These may be used alone or in combination of two or more.

[0032] The hard coat layer-forming composition preferably contains an antistatic agent. The antistatic agent not only imparts antistatic properties to the optical film itself but also improves the scratch resistance of the film surface, thereby maintaining the hard coat layer and thereby improving the repeated bending properties and abrasion resistance. The antistatic agent also improves the flexibility of the hard coat layer, thereby improving the tensile resistance of the optical film and improving the repeated bending properties and abrasion resistance. Any known antistatic agent can be used without particular limitation. For example, metal oxide particles such as antimony-doped tin oxide (ATO) and tin-doped indium oxide (ITO), as well as quaternary ammonium salts, can be used. A pre-mixture of an active energy ray-curable compound and an antistatic agent may also be used. The content of the antistatic agent is not particularly limited as long as antistatic properties are obtained. However, from the viewpoint of further improving the repeated bending properties and abrasion resistance, it is preferable that the content be, for example, 2% by mass or more and 10% by mass or less of the total solid components. In addition to the antistatic agent, various additives such as an antifoaming agent, an antioxidant, an ultraviolet absorber, an infrared absorber, a colorant, a light stabilizer, a polymerization inhibitor, a photosensitizer, a surface conditioner, etc. may be added as needed. The contents of these additives can be appropriately adjusted according to known techniques.

[0033] The thickness of the hard coat layer is 2.0 μm to 7.0 μm, preferably 2.0 μm to 6.0 μm. If it is less than 2.0 μm, the abrasion resistance is reduced, and if it exceeds 7.0 μm, the repeated bending property is reduced.

[0034] The low refractive index layer has a refractive index lower than that of the underlying hard coat layer, and can suppress reflection by optical interference.

[0035] The low refractive index layer can be formed by applying a composition containing an active energy ray-curable compound to the surface of the hard coat layer and curing the coating film. The low refractive index layer may contain a refractive index adjuster to adjust the refractive index.

[0036] Examples of the refractive index adjuster include LiF, MgF, 3NaF·AlF, and AlF (each having a refractive index of 1.4), and Na ３ AlF ６ Fine particles such as cryolite (refractive index 1.33) and silica fine particles having voids therein can be preferably used. Silica fine particles having voids therein can have the refractive index of the voids (approximately 1) of air, which is advantageous for lowering the refractive index of the low refractive index layer. Specifically, porous silica particles and silica particles with a shell structure can be used.

[0037] As the active energy ray-curable compound, the polymerizable compound described in the hard coat layer can be used. In addition, the above-mentioned polymerization initiator and solvent may be appropriately added to the composition for forming the low refractive index layer.

[0038] The composition for forming the low refractive index layer may contain components for improving antifouling properties, such as an antifouling agent, a leveling agent, an oil repellent, a water repellent, or an anti-fingerprint agent. Fluorine-containing compounds and silicone compounds can be suitably used as these additives. Other additives, such as antistatic agents, antifoaming agents, antioxidants, ultraviolet absorbers, infrared absorbers, colorants, light stabilizers, polymerization inhibitors, and photosensitizers, may also be added as needed. The content of these additives can be adjusted as appropriate using known techniques.

[0039] The thickness of the low refractive index layer is not particularly limited and may be, for example, 60 nm or more and 180 nm or less.

[0040] The easy-adhesion layer is formed, for example, by applying an anchor coating agent. However, the easy-adhesion layer is not essential, and the hard coat layer may be laminated directly on the resin substrate.

[0041] The thickness of the easy-adhesion layer is not particularly limited and may be, for example, 50 nm or more and 150 nm or less.

[0042] Between the resin substrate and the hard coat layer or between the hard coat layer and the low refractive index layer, one or more other functional layers such as an adhesive layer, a colored layer, a high refractive index layer, a medium refractive index layer, an antistatic layer, an electromagnetic wave blocking layer, an infrared absorbing layer, an ultraviolet absorbing layer, a color correction layer, etc. may be laminated. For example, a high refractive index layer and a low refractive index layer may be provided in this order on the hard coat layer, or a medium refractive index layer, a high refractive index layer, and a low refractive index layer may be provided in this order on the hard coat layer. When an easy-adhesion layer is provided, an embodiment in which an easy-adhesion layer 4, a hard coat layer 2, a high refractive index layer, and a low refractive index layer 3 are provided in this order on one surface of the resin substrate 1, or an embodiment in which an easy-adhesion layer 4, a hard coat layer 2, a medium refractive index layer, a high refractive index layer, and a low refractive index layer 3 are provided in this order on one surface of the resin substrate 1 is exemplified.

[0043] The method for applying the composition for each layer described above is not particularly limited, and the composition can be applied using, for example, a spin coater, a roll coater, a reverse roll coater, a gravure coater, a microgravure coater, a knife coater, a bar coater, a wire bar coater, a die coater, a dip coater, a spray coater, an applicator, or the like.

[0044] The thickness of the optical film of the present invention is not particularly limited, and may be, for example, 50 μm or more and 120 μm or less.

[0045] The stretching ratio (tensile elongation, elongation at break) of the optical film of the present invention is preferably 3.0% or more, more preferably 5.0% or more, and even more preferably 10.0% or more. If the stretching ratio is less than 3.0%, the film will have poor repeated bending properties, and a higher stretching ratio indicates better repeated bending properties. Furthermore, from the viewpoint of preventing the optical film from becoming too hard and ensuring excellent handleability, the upper limit is, for example, 30.0% or less, or 28.0% or less. In this specification, the stretching ratio of the film is determined by stretching a test piece cut to 100 × 10 mm using a tensile tester (chuck distance 50 mm, tensile speed 10 mm / min (usually within the range of 5 to 10 mm / min)), visually measuring the amount of film stretching when cracks appear in the hard coat layer, and calculating the rate of change from before stretching.

[0046] Since the optical film of the present invention exhibits good repeated bending properties, it is preferable that the stress against a tensile load (tensile stress) be a predetermined value. In the present invention, this tensile stress can be used to evaluate the bending resistance of the optical film. For example, the integral value obtained by integrating the tensile stress with the stretch amount can be used as an index. Specifically, for example, using a Tensilon universal testing machine, a test piece (a dumbbell-shaped No. 5 test piece prepared in accordance with JIS K6251) is stretched at a test speed of 50 mm / min, and the tensile stress is continuously measured until cracks appear in the hard coat layer. The integral value obtained by calculating the tensile stress and the stretch amount can then be used. The higher the tensile stress, the stronger the resistance to external forces, and the greater the stretch amount, the more flexible and bending-resistant the material. However, for example, in the case of a hard material, although the tensile stress is high, cracks occur quickly and the stretch amount is small, so the integral value obtained by integrating the tensile stress with the stretch amount is small. Therefore, in the present invention, the integral value obtained by integrating the tensile stress with the stretch amount can indicate that the material has both crack-free flexibility and high hardness in a specific case. The occurrence of cracks is visually confirmed by shining a penlight on the film. The tensile stress can be adjusted by adjusting the ratio of the amount of antistatic agent, etc. In the optical film of the present invention, the integrated value is preferably 1,000 GPa mm or more, more preferably 1,500 GPa mm or more, and even more preferably 1,700 GPa mm or more. In addition, from the viewpoint of preventing the optical film from becoming too hard and achieving excellent handleability, the upper limit is, for example, 5,000 GPa mm or less, or 4,500 GPa mm or less.

[0047] The optical film of the present invention exhibits good repeated bending resistance. Therefore, it is preferable that the film has a tensile modulus sufficient to allow easy deformation. However, because the repeated bending resistance of an optical film depends on the film thickness, the superiority or inferiority of the repeated bending resistance cannot be accurately determined by the tensile modulus alone. Furthermore, the resistance to deformation also depends on the layer thickness. Therefore, in the present invention, the bending resistance of an optical film is evaluated using the tensile modulus and the thickness of the resin substrate. Specifically, for example, a test specimen (a dumbbell-shaped No. 5 test specimen prepared in accordance with JIS K6251) is stretched at a test speed of 50 mm / min using a Tensilon universal testing machine, and the tensile modulus is measured. The tensile modulus obtained is multiplied by the thickness of the resin substrate to calculate the tensile modulus. The tensile modulus can be adjusted by adjusting the ratio of the amount of antistatic agent, etc. The optical film of the present invention preferably has a multiplied value of 700 GPa·μm or less, more preferably 400 GPa·μm or less. The lower limit is not particularly limited, and examples thereof include 10 GPa·μm or more and 50 GPa·μm or more.

[0048] The optical film of the present invention is preferably made of a material with a high yield point in order to have an optimal surface hardness for foldable applications. The yield point is the stress at which elastic deformation transitions to plastic deformation when force is applied, and a higher yield point indicates better durability and strength. In the present invention, for example, when a test piece (a dumbbell-shaped No. 5 test piece prepared in accordance with JIS K6251) is stretched at a test speed of 50 mm / min using a Tensilon universal testing machine, the yield point is preferably 100 GPa or more, more preferably 105 GPa or more. The upper limit is not particularly limited, and examples include 120 GPa or less and 115 GPa or less. The yield point can be adjusted by adjusting the ratio of the amount of antistatic agent, etc.

[0049] When the optical film of the present invention is wound around a metal cylinder of a mandrel tester with the low refractive index layer facing inward (inward folding), the minimum diameter of the cylinder at which cracks do not occur is 1.0 mm or less. Furthermore, when the optical film is wound around a metal cylinder with the low refractive index layer facing outward (outward folding), the minimum diameter of the cylinder at which cracks do not occur is 6.0 mm or less, preferably 4.0 mm or less, and more preferably 1.0 mm or less. Cracks are visually confirmed. In this specification, if fold marks remain on the optical film itself, it is considered that cracks have occurred. A minimum diameter of 1 mm or less indicates high flexibility and excellent bending resistance, and when the minimum diameter of the cylinder is 1 mm or less in both the inward folding and outward folding cases, the flexibility of the film itself is higher and it exhibits excellent repeated folding resistance. Furthermore, the smaller the minimum diameter of the cylinder at which cracks do not occur, the better. Examples of lower limits include 0.01 mm, 0.05 mm, and 0.1 mm.

[0050] When the optical film of the present invention is repeatedly subjected to a folding test in which a test piece (30 × 80 mm) is placed in a bending durability tester so that the low refractive index layer faces inward when folded, it is preferable that, although not particularly limited, no cracks occur even after 200,000 folds when the spacing between opposing sides of the test piece is 3 mm. Furthermore, when a similar test is performed with the low refractive index layer facing outward, it is preferable that no cracks occur even after 200,000 folds when the spacing between opposing sides of the test piece is 3 mm. In this specification, evaluation can be performed using a bending durability tester (DMX-FS, manufactured by Yuasa Systems). The smaller the spacing between the sides of the test piece where cracks do not occur, the higher the flexibility, and the more times the test piece can be folded without cracks occurring, the higher the repetitive foldability. Therefore, by performing a durability test so that the spacing between the sides when folded is 3 mm, it is possible to confirm that no cracks occur even with a narrower spacing, demonstrating the film's adaptability to thinner designs.

[0051] The pencil hardness (according to JIS K5600-5-4) of the optical film of the present invention is not particularly limited, but is preferably 2H or more. If the pencil hardness is less than 2H, the surface hardness (scratch hardness) will be poor, and a higher value indicates better surface hardness. In this specification, the pencil hardness refers to the maximum hardness that does not result in NG, with a scratch test being performed five times at each hardness while changing the pencil hardness, and the case where two or more scratches are found on the film surface (low refractive index layer) being NG.

[0052] The optical film of the present invention was measured using steel wool (Bonstar #0000, manufactured by Nippon Steel Wool Co., Ltd.) with a load of 1.5 kg (4 × 4 cm ２ When a test is conducted in which the film surface (low refractive index layer) is reciprocated 50 times using an abrasion tester, the number of scratches formed is not particularly limited, but is preferably 5 or less. If the number of scratches is 5 or more, the film has poor scratch resistance, and a smaller number indicates better scratch resistance. In this specification, for example, an abrasion friction tester can be used.

[0053] The optical film of the present invention has a water contact angle change rate of 20% or less after a test in which the film surface (low refractive index layer) is slid 5,000 times with an eraser under a load of 1.0 kg (1.0 kg / 6.0 mmΦ). The smaller the water contact angle change rate, the better the abrasion resistance, and the lower limit may be 1% or more, 2% or more, or 3% or more. A water contact angle change rate of more than 20% indicates poor abrasion resistance. Furthermore, the optical film of the present invention has a water contact angle of approximately 110° or more before the test and a water contact angle of approximately 90 to 105° after the test. In this specification, the water contact angle of the film surface can be measured using a contact angle measuring instrument.

[0054] Furthermore, the optical film of the present invention preferably has a water contact angle change rate of 20% or less after the eraser test (2000 times of sliding) while dropping ethanol onto the film surface (low refractive index layer). When the water contact angle change rate is 20% or more, the film has poor ethanol resistance, and a smaller value indicates better ethanol resistance.

[0055] The optical film of the present invention preferably has a surface resistance of 1.0×10 ８ Ω / □ or more, more preferably 1.0×10 ９ The upper limit is not particularly limited, and is, for example, 1.0 × 10 １４ Ω / □ or less, 5.0×10 １３ Ω / □ or less. ８ A surface resistance of Ω / □ or more indicates excellent antistatic properties. In this specification, the surface resistance value can be measured using a surface resistivity meter.

[0056] When fingerprints are left on the surface (low refractive index layer) of the optical film of the present invention and then wiped off with tissue paper, the number of times that the fingerprints are wiped off until they are no longer visible to the naked eye is not particularly limited, but is preferably 30 or less. A number of wipes of 30 or less indicates good fingerprint wiping properties, and a smaller value indicates better fingerprint wiping properties.

[0057] The total light transmittance (according to JIS K7361-1) of the optical film of the present invention is not particularly limited, but is preferably 92.0% or more, more preferably 93.0% or more, and even more preferably 94.0% or more. A total light transmittance of 92% or more indicates excellent optical properties, and a higher value indicates excellent optical properties.

[0058] The luminous reflectance of the optical film of the present invention is not particularly limited, but is preferably 1.0% or less, more preferably 0.7% or less, and even more preferably 0.5% or less. In this specification, the luminous reflectance of the film can be evaluated using a spectrophotometer (U-4100, manufactured by Hitachi, Ltd.). A luminous reflectance of 1.0% or less indicates excellent optical properties, and a lower value indicates excellent optical properties.

[0059] The present invention also provides an image display device comprising the optical film of the present invention. The image display device is not particularly limited as long as it is a foldable display device, and examples of the image display device include the following foldable devices, such as smartphones, tablets, and portable information terminals (PDAs) such as portable game consoles. The image display device may also be a rollable display device, such as a rollable television. On the other hand, the image display device may also be applied to non-foldable devices, such as televisions, monitors, mobile phones, portable game consoles, PDAs, personal computers, e-books, video cameras, digital still cameras, head-mounted displays, and navigation systems.

[0060] The present invention will be specifically explained below with reference to examples, but the present invention is not limited to these examples in any way.

[0061] Examples 1 to 10 and Comparative Examples 1 to 8 <Resin Substrate> The following resin substrates were used: Polyethylene terephthalate film (O700E50, with adhesive layers on both sides, thickness 50 μm, manufactured by Mitsubishi Chemical Corporation) Polyethylene terephthalate film (with adhesive layers on both sides, thickness 8 μm) Polyethylene terephthalate film (with adhesive layers on both sides, thickness 10 μm) Polyethylene terephthalate film (with adhesive layers on both sides, thickness 100 μm) Polyethylene terephthalate film (with adhesive layers on both sides, thickness 110 μm) Polybutylene terephthalate film (with adhesive layers on both sides, thickness 50 μm) Polyethylene naphthalate film (with adhesive layers on both sides, thickness 50 μm) Polyimide film (with adhesive layers on both sides, thickness 50 μm) Triacetyl cellulose film (TJ25UL, thickness 25 μm, manufactured by Fujifilm Corporation)

[0062] <Hard Coat Layer> 99.9 parts by mass of a photocurable resin (hard coating agent containing quaternary ammonium salt, manufactured by Arakawa Chemical Industries, Ltd.) and 0.1 parts by mass of an acrylic polymer (TEGO (registered trademark) Flow 300, manufactured by Evonik) were diluted in a mixed solvent of propylene glycol monomethyl ether / methyl ethyl ketone / isopropyl alcohol (mass ratio 25 / 50 / 20) and stirred to prepare a composition for forming a hard coat layer. The amount of the quaternary ammonium salt added was adjusted according to a known technique and was separately described as the content in the hard coat layer.

[0063] The obtained composition for forming a hard coat layer was applied to one side of the substrate using a wire bar coater to form a coating film, which was then dried at 60°C for 60 seconds and then cured in a nitrogen atmosphere using a conveyor-type ultraviolet curing device at an exposure dose of 200 mJ / cm. ２ By irradiating the coated film with ultraviolet light at a temperature of 100° C., a hard coat layer having a thickness shown in Tables 1 to 4 was formed.

[0064] <Low Refractive Index Layer> A composition for forming a low refractive index layer was prepared by diluting and stirring 30.0 parts by mass of hollow silica fine particles (Sururia 5320, manufactured by JGC Catalysts and Chemicals), 62.0 parts by mass of an acrylic monomer (pentaerythritol triacrylate, Viscoat #300, manufactured by Osaka Organic Chemical Industry), 5.0 parts by mass of a fluorine-based antifouling agent (KY-1203, manufactured by Shin-Etsu Chemical Co., Ltd.), and 3.0 parts by mass of a photopolymerization initiator (Omnirad (registered trademark) 184, manufactured by IGM Resins) in a methyl isobutyl ketone / propylene glycol monomethyl ether acetate (mass ratio 50 / 50) mixed solvent.

[0065] The obtained composition for forming a low refractive index layer was applied to the upper surface of the hard coat layer obtained above using a wire bar coater to form a coating film, which was then dried at 60°C for 60 seconds and then exposed to an exposure dose of 200 mJ / cm using a conveyor-type ultraviolet curing device in a nitrogen atmosphere. ２ A low refractive index layer having a thickness of 100 nm was formed by irradiating the film with ultraviolet light at 400 nm, thereby obtaining an optical film.

[0066] Test Example 1 [Repeated Bending Property 1] The repeated bending property 1 was evaluated by a mandrel test in accordance with JIS K 5600-1. A metal cylinder with a diameter of 1.0 mm, 2.0 mm, 4.0 mm, 6.0 mm, 8.0 mm, or 10.0 mm was set in a mandrel tester (manufactured by COTEC). The optical film was fixed so that the low refractive index layer was on the side that contacted the cylinder (inward folding) or the side that did not contact the cylinder (outward folding). The film was then bent 180° and wrapped around the cylinder. The film surface was visually observed, and the smallest cylinder diameter at which cracks did not occur (minimum mandrel diameter) was used as the evaluation value. A minimum mandrel diameter of 1.0 mm or less when folded inward and 6.0 mm or less when folded outward was considered to be acceptable, and a minimum mandrel diameter of 1.0 mm or less in both the inward and outward folding cases was considered to be even better, and the film itself was evaluated for its excellent repeated bending property. The results are shown in Tables 1 to 4.

[0067] Test Example 2 [Repeated Bending Property 2] A sheet-shaped unloaded U-shaped stretch test jig (DMX-FS) was attached to a tabletop durability tester (manufactured by Yuasa Systems). A test piece cut to 30 x 80 mm was fixed so that the low refractive index layer was on the side that would be folded inward or outward when folded, and then the film was folded continuously so that the distance between the opposing films was 3 mm. The folding operation was performed once per second for a total of 200,000 times, and cracks on the film surface were visually observed. The results were evaluated as "Good" if no cracks were observed, and "Poor" if cracks were observed. The results are shown in Tables 1 to 4.

[0068] Test Example 3 [Tensile Properties 1: Stretchability] A test piece cut to 100 x 10 mm was set (chuck distance 50 mm) in a tensile tester (STB-1225L, manufactured by A&D Manufacturing) and stretched. The film length was measured visually when a crack appeared in the film, and the stretch rate (%) was calculated from the amount of change from before stretching. A stretch rate of 3.0% or more and 30.0% or less was considered to be acceptable. The results are shown in Tables 1 to 4. Stretch rate (%) = 100 x (film length at break - film length before test) / film length before test

[0069] Test Example 4 [Tensile Properties 2: Tensile Stress] A dumbbell-shaped No. 5 test piece punched in accordance with JIS K6251 was set in a Tensilon universal testing machine (STB-1225S, manufactured by A&D Co., Ltd.) (chuck distance: 80 mm), stretched at a rate of 50 mm / min, and the tensile stress was continuously measured visually until cracks appeared in the hard coat layer. The integral of the stretch amount until cracks appeared and the tensile stress obtained during stretching was calculated, and the integral value was calculated by integrating the tensile stress with the stretch amount. Specifically, a tensile stress-stretch amount curve was obtained with tensile stress on the vertical axis and stretch amount on the horizontal axis, and the area in the range of stretch amount up to the point where cracks appeared on this curve was calculated to obtain the integral value. An integral value of 1,000 GPa mm or more was considered acceptable. The results are shown in Tables 1 to 4.

[0070] Test Example 5 [Tensile Properties 3: Tensile Modulus] A dumbbell-shaped No. 5 test piece punched in accordance with JIS K6251 was set (chuck distance 80 mm) in a Tensilon universal testing machine (STB-1225S, manufactured by A&D Co., Ltd.) and stretched at a speed of 50 mm / min to measure the tensile modulus. The obtained tensile modulus was multiplied by the thickness of the resin substrate to calculate the product of the tensile modulus and the thickness of the resin substrate. When the resin substrate was a commercially available product, the thickness of the resin substrate was determined to be the numerical value listed in the catalog. A value of this product of 700 GPa μm or less was considered acceptable. The results are shown in Tables 1 to 4.

[0071] Test Example 6 [Tensile Properties 4: Yield Point] A dumbbell-shaped No. 5 test piece punched in accordance with JIS K6251 was set in a Tensilon universal testing machine (STB-1225S, manufactured by A&D Co., Ltd.) (chuck distance: 80 mm), stretched at a speed of 50 mm / min, and the yield point was measured. A yield point of 100 GPa or more was considered to be acceptable. The results are shown in Tables 1 to 4.

[0072] Test Example 7 [Abrasion Resistance] Using a reciprocating abrasion tester (TYPE: 30S, manufactured by Shinto Scientific), an eraser (cylindrical, 6.0 mm diameter, manufactured by Hwarang) was set on the surface (low refractive index layer) of the optical film and slid 5,000 times under a load of 1.0 kg (1.0 kg / 6.0 mmΦ). A contact angle meter (Drop Master 300, manufactured by Kyowa Interface Science) was used to measure the water contact angle of the film surface before and after the test, 5 seconds after pure water was applied to the surface, and the water contact change rate (%) from before the test was calculated. A change rate of 20% or less was considered acceptable. The results are shown in Tables 1 to 4. Change rate (%) = 100 x (water contact angle before test - water contact angle after test) / water contact angle before test

[0073] Test Example 8 [Ethanol Resistance] The water contact change rate (%) from before the test was calculated in the same manner as in Test Example 7, except that the film surface was slid 2,000 times while ethanol was dropped on it during the test. The water contact angle before the test was the value from Test Example 7. A change rate of 20% or less was considered to be acceptable. The results are shown in Tables 1 to 4.

[0074] Test Example 9 [Scratch Hardness] Pencil hardness was evaluated in accordance with JIS K5600-5-4. The pencil hardness of the surface (low refractive index layer) of the optical film was measured using a pencil (uni, manufactured by Mitsubishi Pencil Co., Ltd.) and a Clemens-type scratch tester (HA-301, manufactured by Tester Sangyo Co., Ltd.). The test was repeated while changing the pencil hardness, and changes in appearance due to scratches were visually observed. The maximum hardness at which no scratches were observed four or more times out of five times was taken as the evaluation value. A pencil hardness of 2H or more was considered to be acceptable. The results are shown in Tables 1 to 4.

[0075] Test Example 10 [Abrasion Resistance] Using an abrasion and friction tester (AB-301, manufactured by Tester Sangyo), steel wool (Bonstar #0000) was brought into contact with the surface of the low refractive index layer of the optical film, and a test load of 1.5 kg / 4 cm was applied. ２ After 50 reciprocal strokes under pressure, the surface of the low refractive index layer was visually observed and the number of scratches formed was counted. A PET film (Kukkiri Miel, manufactured by TOMOEGAWA) was attached to the back of the film before visual observation. A film with 5 or fewer scratches was considered to pass. The results are shown in Tables 1 to 4.

[0076] Test Example 11 [Antistatic Property] Using a surface resistance meter (HIRESTA-UP MCP-HT, manufactured by Nitto Seiko Analytech Co., Ltd.), the element was pressed against the film surface (low refractive index layer), and the surface resistance was measured 10 seconds after applying 250 V. When the surface resistance was 1.0 × 10 ８ A value of Ω / □ or higher was considered to be acceptable. The results are shown in Tables 1 to 4.

[0077] Test Example 12 [Total Light Transmittance] Total light transmittance was evaluated in accordance with JIS K7105. It was measured using a haze meter (NDH7000SP, manufactured by Nippon Denshoku Industries Co., Ltd.). A total light transmittance of 92.0% or more was considered to be acceptable. The results are shown in Tables 1 to 4.

[0078] Test Example 13 [Luminous Reflectance] The spectral reflectance of the surface of the low refractive index layer of the optical film was measured using an automatic spectrophotometer (U-4100, manufactured by Hitachi, Ltd.). In the spectral reflectance measurement, a matte black tape (#302 black, Teraoka vinyl tape) was attached to the back surface of the optical film (the surface opposite to the surface on which the low refractive index layer was provided) to provide an anti-reflection treatment, and specular reflection was measured at an incident angle of 5° of incident light. The luminous reflectance (Y value) was calculated from the obtained spectral reflectance curve in accordance with JIS R 3106. A luminous reflectance of 1.0% or less was considered acceptable. The results are shown in Tables 1 to 4.

[0079]

[0080]

[0081]

[0082]

[0083] From Table 1, Comparative Example 1 has a thin hard coat layer thickness of 1.0 μm, so it has excellent repeated bending resistance, but is easily elongated with an elongation rate of over 30.0%, has a yield point of 95 GPa, and is brittle, so has poor tensile properties, and also has a large change in surface properties due to eraser wear. Comparative Examples 2 and 3 have a thick hard coat layer thickness, so it has excellent abrasion resistance, but not only is it poor in repeated bending resistance, but it also has a small elongation rate, is difficult to elongate, and has poor tensile properties, such as an integrated tensile stress of 499 and 387 GPa mm. On the other hand, Examples 1 to 3 have a hard coat layer thickness of 2.0 μm or more and 7.0 μm or less, so it can be seen that it has excellent tensile properties and combines repeated bending resistance and abrasion resistance.

[0084] Table 2 also shows that the repeated bending properties and abrasion resistance change depending on the thickness of the resin substrate. For example, Comparative Example 4, in which the resin substrate is 8 μm thick, has an elongation rate of 34.0% and a yield point of 98 GPa, making it easy to stretch but brittle, and therefore has excellent repeated bending properties but poor abrasion resistance and pencil hardness. Comparative Example 5, in which the resin substrate is 110 μm thick, has a large minimum mandrel diameter for both inward and outward folding, and is therefore poor in repeated bending properties. On the other hand, Examples 4 and 5, in which the resin substrate is 10 μm or more and 100 μm or less in thickness, are found to have excellent tensile properties while achieving both repeated bending properties and abrasion resistance.

[0085] Table 3 shows that, when the thickness of the resin substrate is the same, the repeated bending property and abrasion resistance change depending on the hard coat layer configuration. For example, Comparative Example 6, in which the antistatic agent content in the hard coat layer is 1% by mass, has poor stretchability and poor tensile properties, such as a small integrated value of tensile stress, and is also poor in repeated bending property. Comparative Example 7, in which the antistatic agent content is 11% by mass, is easily stretched but becomes brittle, so it has excellent repeated bending property but poor abrasion resistance and pencil hardness. On the other hand, Examples 6 and 7 have small changes in the minimum mandrel diameter between inward and outward folding and in the water contact angle after load abrasion, and thus achieve both repeated bending property and abrasion resistance.

[0086] Table 4 shows that even when the type of resin substrate is different, repeated bending properties and abrasion resistance can be achieved by using substrates such as polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, and polyimide, suggesting that various substrates can be used as protective films for foldable display devices.

[0087] The optical film of the present invention is suitably used as a protective film for a folding display device.

[0088] REFERENCE SIGNS LIST 1 resin substrate 2 hard coat layer 3 low refractive index layer 4 easy-adhesion layer 10 optical film 20 optical film

Claims

1. An optical film comprising a hard coat layer and a low refractive index layer, in this order, on one side of a resin substrate having a thickness of 10 μm or more and 100 μm or less, wherein the thickness of the hard coat layer is 2.0 μm or more and 7.0 μm or less, wherein the stretching ratio is 3.0% or more and 30.0% or less, wherein the minimum diameter of a cylinder that can be wrapped around the optical film with the low refractive index layer on the inside and no cracks occurring is 1.0 mm or less, and the minimum diameter of a cylinder that can be wrapped around the optical film with the low refractive index layer on the outside and no cracks occurring is 6.0 mm or less, and wherein the change in water contact angle after sliding an eraser over the surface of the low refractive index layer 5,000 times with a load of 1.0 kg / 6.0 mmΦ is 20% or less.

2. An optical film comprising a hard coat layer and a low refractive index layer, in this order, on one side of a resin substrate having a thickness of 10 μm or more and 100 μm or less, the hard coat layer having a thickness of 2.0 μm or more and 7.0 μm or less, wherein the integral value of the tensile stress until cracks occur in the hard coat layer, integrated over the amount of stretching, is 1,000 GPa·mm or more, the minimum diameter of a cylinder that does not generate cracks when wrapped around the optical film with the low refractive index layer on the inside is 1.0 mm or less, the minimum diameter of a cylinder that does not generate cracks when wrapped around the optical film with the low refractive index layer on the outside is 6.0 mm or less, and the rate of change in water contact angle after sliding an eraser over the surface of the low refractive index layer 5,000 times with a load of 1.0 kg / 6.0 mmΦ is 20% or less.

3. An optical film comprising a hard coat layer and a low refractive index layer, in this order, on one side of a resin substrate having a thickness of 10 μm or more and 100 μm or less, the hard coat layer having a thickness of 2 μm or more and 7 μm or less, wherein the product of the tensile modulus and the thickness of the resin substrate is 700 GPa μm or less, the minimum diameter of a cylinder that does not develop cracks when wrapped around the substrate with the low refractive index layer on the inside is 1.0 mm or less, the minimum diameter of a cylinder that does not develop cracks when wrapped around the substrate with the low refractive index layer on the outside is 6.0 mm or less, and the rate of change in water contact angle after sliding an eraser over the surface of the low refractive index layer 5,000 times with a load of 1.0 kg / 6.0 mmΦ is 20% or less.

4. An optical film comprising a hard coat layer and a low refractive index layer, in this order, on one side of a resin substrate having a thickness of 10 μm or more and 100 μm or less, wherein the thickness of the hard coat layer is 2.0 μm or more and 7.0 μm or less, wherein the optical film has a yield point of 100 GPa or more, a minimum diameter of a cylinder that can be wrapped around the optical film with the low refractive index layer on the inside and no cracks occurring is 1.0 mm or less, a minimum diameter of a cylinder that can be wrapped around the optical film with the low refractive index layer on the outside and no cracks occurring is 6.0 mm or less, and a water contact angle change rate of 20% or less after sliding an eraser over the surface of the low refractive index layer 5,000 times with a load of 1.0 kg / 6.0 mmΦ.

5. The optical film according to any one of claims 1 to 4, which has a pencil hardness of 2H or more in a scratch hardness test in accordance with JIS K5600-5-4.

6. The optical film according to any one of claims 1 to 4, wherein the hard coat layer has antistatic properties.

7. The optical film according to any one of claims 1 to 4, which has a high refractive index layer between the hard coat layer and the low refractive index layer, or has a medium refractive index layer, a high refractive index layer and the low refractive index layer on the hard coat layer in this order.

8. The optical film according to any one of claims 1 to 4, wherein the resin substrate is made of one or more materials selected from the group consisting of polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polyimide, and polyamideimide.

9. An image display device comprising the optical film according to any one of claims 1 to 8.

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