Optical films and image display devices

Abstract

This invention relates to an optical film that achieves both high repeated bending properties and abrasion resistance, and an image display device using the same. [Solution] An optical film comprising a resin substrate with a thickness of 10 μm or more and 100 μm or less, on one side surface, a hard coat layer and a low refractive index layer in that order, wherein the thickness of the hard coat layer is 2.0 μm or more and 7.0 μm or less, the elongation ratio is 3.0% or more and 30.0% or less, the minimum diameter of a cylinder in which no cracks occur when the low refractive index layer is wound with the low refractive index layer on the inside is 1.0 mm or less, the minimum diameter of a cylinder in which no cracks occur when the low refractive index layer is wound with the low refractive index layer on the outside is 6.0 mm or less, and the water contact angle change rate after sliding an eraser 5000 times with a load of 1.0 kg / 6.0 mmΦ on the surface of the low refractive index layer is 20% or less.

Description

[Technical Field]

[0001] This invention relates to an optical film and an image display device using the same. [Background technology]

[0002] With the increasing size of smartphones and tablet devices, the development of foldable image display devices is progressing.

[0003] For example, Patent Document 1 discloses that an optical film is provided on a resin substrate made of one or more resins selected from polyimide resins, polyamideimide resins, polyamide resins, and polyester resins, with a first optical adjustment layer of 30 nm to 200 nm on a first surface, a third optical adjustment layer of 30 nm to 1 μm on a second surface opposite to the first surface, and a functional layer further provided on the first optical adjustment layer, and that no cracks or breaks occur when the optical film is folded 10,000 times with the functional layer facing inward. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Patent No. 7119424 [Overview of the project] [Problems that the invention aims to solve]

[0005] Generally, if the base material or hard coat layer is rigid, its flexibility decreases. Therefore, flexibility can be improved by using flexible materials or by thinning them. However, such configurations have the problem of reduced indentation strength, leading to poor abrasion resistance.

[0006] Optical films used in foldable image display devices require improved abrasion resistance in the outermost layer subjected to operation. Therefore, improvements in both repeated bending and abrasion resistance are required.

[0007] This invention relates to an optical film that achieves both high repeated bending properties and abrasion resistance, and an image display device using the same. [Means for solving the problem]

[0008] The present invention relates to the following [1] to [5]. [1] An optical film comprising a resin substrate with a thickness of 10 μm or more and 100 μm or less, on one side having a hard coat layer and a low refractive index layer in that order, wherein the thickness of the hard coat layer is 2.0 μm or more and 7.0 μm or less, The elongation rate is between 3.0% and 30.0%. The minimum diameter of a cylinder in which cracks do not occur when the low refractive index layer is wrapped around the material so that it faces inward is 1.0 mm or less. The minimum diameter of a cylinder in which cracks do not occur when the low refractive index layer is wrapped around the outside is 6.0 mm or less. The water contact angle change rate after 5000 cycles of rubbing an eraser against the low refractive index layer surface with a load of 1.0 kg / 6.0 mmΦ is 20% or less. Optical film. [2] An optical film comprising a resin substrate with a thickness of 10 μm or more and 100 μm or less, on one side having a hard coat layer and a low refractive index layer in that order, wherein the thickness of the hard coat layer is 2.0 μm or more and 7.0 μm or less, The integral value obtained by integrating the tensile stress until cracks occur in the hard coat layer with respect to the amount of elongation is 1,000 GPa·mm or more. The minimum diameter of a cylinder in which cracks do not occur when the low refractive index layer is wrapped around the material so that it faces inward is 1.0 mm or less. The minimum diameter of a cylinder in which cracks do not occur when the low refractive index layer is wrapped around the outside is 6.0 mm or less. The water contact angle change rate after 5000 cycles of rubbing an eraser against the low refractive index layer surface with a load of 1.0 kg / 6.0 mmΦ is 20% or less. Optical film. [3] An optical film comprising a resin substrate with a thickness of 10 μm or more and 100 μm or less, on one side having a hard coat layer and a low refractive index layer in that order, wherein the thickness of the hard coat layer is 2.0 μm or more and 7.0 μm or less, 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 in which cracks do not occur when the low refractive index layer is wrapped around the material so that it faces inward is 1.0 mm or less. The minimum diameter of a cylinder in which cracks do not occur when the low refractive index layer is wrapped around the outside is 6.0 mm or less. The water contact angle change rate after 5000 cycles of rubbing an eraser against the low refractive index layer surface with a load of 1.0 kg / 6.0 mmΦ is 20% or less. Optical film. [4] An optical film comprising a resin substrate with a thickness of 10 μm or more and 100 μm or less, on one side having a hard coat layer and a low refractive index layer in that order, wherein the thickness of the hard coat layer is 2.0 μm or more and 7.0 μm or less, The yield point is 100 GPa or higher. The minimum diameter of a cylinder in which cracks do not occur when the low refractive index layer is wrapped around the material so that it faces inward is 1.0 mm or less. The minimum diameter of a cylinder in which cracks do not occur when the low refractive index layer is wrapped around the outside is 6.0 mm or less. The water contact angle change rate after 5000 cycles of rubbing an eraser against the low refractive index layer surface with a load of 1.0 kg / 6.0 mmΦ is 20% or less. Optical film. [5] An image display device comprising an optical film as described in any of [1] to [4]. [Effects of the Invention]

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

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

BEST MODE FOR CARRYING OUT THE INVENTION

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

[0012] The optical film 10 includes a resin substrate 1, and a hard coat layer 2 and a low refractive index layer 3 are provided 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 includes a resin substrate 1, and a hard coat layer 2 and a low refractive index layer 3 are provided in this order on one surface of the resin substrate 1, but an easy adhesion layer 4 is laminated on the resin substrate 1. Specifically, an easy adhesion layer 4, a hard coat layer 2, and a low refractive index layer 3 are provided in this order on one surface of the resin substrate 1. The presence of the easy adhesion layer improves the adhesion even when the hard coat layer is thin, and can further improve the repeated bending property and the abrasion resistance. The easy adhesion layer 4 may be laminated on the other surface of the resin substrate 1 in order to improve the adhesion to other layers (for example, an adhesive layer for display attachment) and improve the adhesiveness.

[0015] Hereinafter, the details of each layer will be described.

[0016] Examples of resin substrates include those made of polyester resins. Polyester resins are preferred from the viewpoint of providing transparency and flexibility. Examples of polyester resins include polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate. The resin substrate may also be made of polyimide resin or polyamide-imide resin. In the present invention, from the viewpoint of bending resistance, it is preferable to use a material made of one or more selected from the group consisting of polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polyimide, and polyamide-imide.

[0017] The thickness of the resin substrate is not particularly limited as long as it is between 10 μm and 100 μm, but from the viewpoint of improving repeated bending properties, it is preferably 60 μm or less, and 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. Examples of surface modification treatments include alkali treatment, corona treatment, plasma treatment, sputtering, application of surfactants or silane coupling agents, and Si deposition.

[0019] The hard coat layer is placed on one side of the resin substrate. The hard coat layer provides flexibility to the optical film while also improving its impact resistance, thereby enhancing its ability to be repeatedly folded and its abrasion resistance.

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

[0021] As active energy ray curable compounds, for example, monofunctional, bifunctional, or trifunctional or more (meth)acrylate monomers 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, tetrahydrofurfluryl acrylate, cyclohexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and isobornyl (meth)acrylate. Phosphate, 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 2-(meth)acryloyloxyethyl-2-hydroxypropyl phthalate, 2-hydroxy-3-phenoxypropyl (meth)acrylate, 2-(meth)acryloyloxyethyl-2-hydroxypropyl phthalate, 2-(meth)acryloyloxyethyl-2-hydroxypropyl phthalate, 2-(meth)acryloyloxyethyl-2-hydroxypropyl phthalate, 2-(meth)acryloyloxyethyl-2-hydroxypropyl phthalate, 2-(meth)acryloyloxyethyl-2-hydroxypropyl phthalate, 2-(meth)acryloyloxyethyl-2-hydroxypropyl phthalate 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,Examples include adamantane derivative mono(meth)acrylates such as adamantyl acrylate, which has a monovalent mono(meth)acrylate derived from adamantanediol.

[0023] Examples of difunctional (meth)acrylates include 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, hydroxypivalate neopentyl glycol di(meth)acrylate, and other di(meth)acrylates.

[0024] Examples of trifunctional (meth)acrylates include trimethylolpropane tri(meth)acrylate, ethoxylated trimethylolpropane tri(meth)acrylate, propoxylated trimethylolpropane tri(meth)acrylate, tris-2-hydroxyethyl isocyanurate tri(meth)acrylate, glycerin tri(meth)acrylate, and other trifunctional (meth)acrylate compounds such as pentaerythritol tri(meth)acrylate, dipentaerythritol tri(meth)acrylate, and ditrimethylolpropane tri(meth)acrylate. Examples include polyfunctional (meth)acrylate compounds with three or more functions, 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 some of these (meth)acrylates are substituted with alkyl groups or ε-caprolactone.

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

[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 above-mentioned active energy ray curable compounds may be used individually or in combination of two or more. Furthermore, the active energy ray curable compounds may be monomers in the coating solution or oligomers in which a portion has been polymerized.

[0028] The content of the active energy ray curable compound is not particularly limited, but 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 can be any substance that triggers a polymerization reaction upon irradiation with ultraviolet light or electron beams, and examples include 2,2-ethoxyacetophenone, 1-hydroxycyclohexylphenyl ketone, dibenzoyl, benzoin, benzoin methyl ether, benzoin ethyl ether, p-chlorobenzophenone, p-methoxybenzophenone, Michler ketone, acetophenone, and 2-chlorothioxanthone. These may be used individually or in combination of two or more.

[0030] The content of the photopolymerization initiator is not particularly limited, but 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 solvents 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 individually or in combination of two or more.

[0032] The composition for forming the hard coat layer preferably contains an antistatic agent. The antistatic agent not only imparts antistatic properties to the optical film itself, but can also improve the scratch resistance of the film surface, thus contributing to the maintenance of the hard coat layer and, consequently, improving repeated bending and abrasion resistance. Furthermore, the antistatic agent also improves the flexibility of the hard coat layer, thereby improving the tensile strength of the optical film and improving repeated bending and abrasion resistance. The antistatic agent is not particularly limited as long as it is known and can be used. For example, metal oxide fine particles such as antimony-doped tin oxide (ATO) or tin-doped indium oxide (ITO), or quaternary ammonium salts can be used. A pre-mixed active energy ray curable compound and antistatic agent may also be used. The content of the antistatic agent is not particularly limited as long as antistatic properties are obtained, but from the viewpoint of further improving repeated bending and abrasion resistance, it is preferable that it be, for example, 2% by mass or more and 10% by mass or less of the total amount of solid components. In addition to antistatic agents, various additives such as defoamers, antioxidants, ultraviolet absorbers, infrared absorbers, colorants, light stabilizers, polymerization inhibitors, photosensitizers, and surface modifiers may be added as needed. The content of these additives can be adjusted appropriately according to known technologies.

[0033] The thickness of the hard coat layer is 2.0 μm or more and 7.0 μm or less, preferably 2.0 μm or more and 6.0 μm or less. If it is less than 2.0 μm, the abrasion resistance will decrease, and if it is greater than 7.0 μm, the repeated bending resistance will decrease.

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

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

[0036] As refractive index adjusting agents, fine particles such as LiF, MgF, 3NaF·AlF or AlF (all with a refractive index of 1.4), or Na3AlF6 (crylcite, refractive index 1.33), or silica fine particles with internal voids can be suitably used. Silica fine particles with internal voids are advantageous for lowering the refractive index of low refractive index layers because the refractive index of the void portion can be set to that of air (approximately 1). 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 section can be used. Furthermore, the polymerization initiators and solvents mentioned above may be added to the low refractive index layer forming composition as appropriate.

[0038] The composition for forming a low refractive index layer may contain antifouling agents, leveling agents, oil repellents, water repellents, and anti-fingerprint agents as components to improve antifouling properties. Fluorine-containing compounds and silicone compounds can be suitably used as these additives. Other additives such as antistatic agents, defoamers, antioxidants, ultraviolet absorbers, infrared absorbers, colorants, light stabilizers, polymerization inhibitors, and photosensitizers may be added as needed. The content of these additives can be appropriately adjusted according to known technologies.

[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 onto 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] One or more other functional layers, such as an adhesive layer, a coloring layer, a high refractive index layer, a medium refractive index layer, an antistatic layer, an electromagnetic wave shielding layer, an infrared absorbing layer, an ultraviolet absorbing layer, or a color correction layer, may be laminated between the resin substrate and the hard coat layer, or between the hard coat layer and the low refractive index layer. For example, the hard coat layer may have a high refractive index layer and a low refractive index layer in that order, or the hard coat layer may have a medium refractive index layer, a high refractive index layer, and a low refractive index layer in that order. When an easy-adhesion layer is present, examples include an easy-adhesion layer 4, a hard coat layer 2, a high refractive index layer, and a low refractive index layer 3 arranged in that order on one side of the resin substrate 1, or 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 arranged in that order on one side of the resin substrate 1.

[0043] The coating method for each of the above-mentioned layer compositions is not particularly limited, and can be used, for example, with a spin coater, roll coater, reverse roll coater, gravure coater, microgravure coater, knife coater, bar coater, wire bar coater, die coater, dip coater, spray coater, applicator, etc.

[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 elongation 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 elongation ratio is less than 3.0%, the repeated bending properties will be poor, and a higher value indicates better repeated bending properties. Furthermore, from the viewpoint of preventing the optical film from becoming too hard and maintaining good handling properties, the upper limits are exemplified by, for example, 30.0% or less and 28.0% or less. In this specification, the elongation ratio of the film is determined by stretching a test piece cut to 100 × 10 mm using a tensile testing machine (chuck distance 50 mm, tensile speed 10 mm / min (usually within the range of 5 to 10 mm / min)), measuring the amount of film stretch when a crack appears in the hard coat layer visually, and calculating the rate of change from before stretching.

[0046] The optical film of the present invention exhibits good repeated bending properties, and therefore, it is preferable that the stress under tensile load (tensile stress) is at a predetermined level. In the present invention, the bending resistance of the optical film can be evaluated using this tensile stress, and for example, the integral value obtained by integrating the tensile stress with respect to the amount of stretching can be used as an indicator. 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 until a crack occurs in the hard coat layer is continuously measured. Then, the integral value of the obtained tensile stress and amount of stretching can be calculated and used. The higher the tensile stress, the stronger the resistance to external forces, and the larger the amount of stretching, the more flexible and flexible the bending resistance. However, for example, in the case of a hard material, although the tensile stress is high, cracks quickly form and the amount of stretching becomes small, so the integral value obtained by integrating the tensile stress with respect to the amount of stretching becomes small. Therefore, in the present invention, it is possible to demonstrate that when the integral value obtained by integrating the tensile stress with respect to the amount of stretching is specific, it possesses both flexibility that does not crack and high hardness. Cracks are visually inspected using a penlight to check for their presence or absence. Tensile stress can be adjusted by changing the ratio of antistatic agent and other factors. The optical film of the present invention preferably has an integral value of 1,000 GPa·mm or more, more preferably 1,500 GPa·mm or more, and even more preferably 1,700 GPa·mm or more. Furthermore, from the viewpoint of preventing the optical film from becoming too hard and maintaining excellent handling properties, upper limits include, for example, 5,000 GPa·mm or less and 4,500 GPa·mm or less.

[0047] The optical film of the present invention exhibits good repeated bending properties. Therefore, it is preferable that it has a tensile modulus that allows for easy deformation. On the other hand, since the repeated bending properties of an optical film depend on the film thickness, the superiority or inferiority of repeated bending properties cannot be accurately determined by the tensile modulus alone, and the resistance to deformation also depends on the layer thickness. Therefore, in the present invention, the bending resistance of the optical film is evaluated using the tensile modulus and the thickness of the resin substrate. Specifically, for example, 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 tester, and after measuring the tensile modulus, the value obtained by integrating the thickness of the resin substrate with the tensile modulus can be calculated and used. The tensile modulus can be adjusted by the ratio of the amount of antistatic agent, etc. In the optical film of the present invention, the integrated value is preferably 700 GPa·μm or less, more preferably 400 GPa·μm or less. There is no particular lower limit; for example, 10 GPa·μm or higher and 50 GPa·μm or higher are given as examples.

[0048] The optical film of the present invention is preferably composed of a material with a high yield point, from the viewpoint of having surface hardness optimal for foldable applications. The yield point is the stress at which the deformation transitions from elastic to plastic when a force is applied, and a higher yield point indicates superior 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 tester, the yield point is preferably 100 GPa or higher, more preferably 105 GPa or higher. There is no particular upper limit, and examples include 120 GPa or less and 115 GPa or less. The yield point can be adjusted by the ratio of antistatic agent and other factors.

[0049] The optical film of the present invention, when wrapped around a metal cylinder of a mandrel testing machine with the low refractive index layer facing inward (inward folding), has a minimum cylinder diameter of 1.0 mm or less that does not develop cracks. Furthermore, when wrapped with the low refractive index layer facing outward (outward folding), the minimum cylinder diameter that does not develop cracks is 6.0 mm or less, preferably 4.0 mm or less, and more preferably 1.0 mm or less. Cracks are confirmed by visual inspection. In this specification, if a fold mark remains on the optical film itself, it is considered that a crack has occurred. A minimum cylinder diameter of 1 mm or less indicates high flexibility and excellent bending resistance, but when the minimum cylinder diameter is 1 mm or less in both inward and outward folding cases, the flexibility of the film itself is further increased, resulting in excellent repeated bending performance. Moreover, the smaller the minimum cylinder diameter that does not develop cracks, the better; for example, lower limits include 0.01 mm, 0.05 mm, and 0.1 mm.

[0050] The optical film of the present invention, when a test specimen (30 x 80 mm) is set in a bending durability testing machine so that the low refractive index layer faces inward when bent, is subjected to repeated folding tests. While not particularly limited, it is preferable that no cracks occur even after 200,000 folds with a gap of 3 mm between opposing edges of the specimen. Similarly, when the low refractive index layer faces outward, it is also preferable that no cracks occur even after 200,000 folds with a gap of 3 mm between opposing edges of the specimen. In this specification, evaluation can be performed using a bending durability testing machine (Yuasa Systems, DMX-FS). A smaller gap between the edges of a specimen that does not crack indicates higher flexibility, and a higher number of folds without cracking indicates higher repeated bending resistance. Therefore, by conducting durability tests with a gap of 3 mm between the edges when folded, it is possible to confirm that no cracks occur even with narrow gaps, demonstrating that the film can be made thinner.

[0051] The pencil hardness (according to JIS K5600-5-4) of the optical film of the present invention is not particularly limited, but 2H or higher is preferred. If the pencil hardness is less than 2H, the surface hardness (scratch hardness) will be inferior, and a higher value indicates superior surface hardness. In this specification, pencil hardness is defined as the maximum hardness at which no scratches occur, after performing five scratch tests at each pencil hardness, with two or more scratches on the film surface (low refractive index layer) being considered NG.

[0052] The optical film of the present invention can withstand a load of 1.5 kg (4 x 4 cm) using steel wool (Bonstar #0000, manufactured by Nippon Steel Wool). 2 When a test is performed in which the film surface (low refractive index layer) is moved back and forth 50 times with a device, the number of scratches formed is not particularly limited, but it is preferable that it be 5 or less. If the number of scratches is 5 or more, the scratch resistance will be poor, 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 exhibits a water contact angle change rate of 20% or less after a test in which the film surface (low refractive index layer) is slid 5000 times with an eraser under a load of 1.0 kg (1.0 kg / 6.0 mmΦ). A smaller water contact angle change rate indicates superior abrasion resistance, and the lower limit may be 1% or more, 2% or more, or 3% or more. A water contact angle change rate exceeding 20% ​​indicates poor abrasion resistance. Furthermore, the water contact angle of the optical film of the present invention is approximately 110° or more before the test, and approximately 90-105° 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, it is preferable that the optical film of the present invention also exhibits a change rate of 20% or less in the water contact angle after the eraser test (2000 swiping cycles) while dropping ethanol onto the film surface (low refractive index layer). A change rate of 20% or more indicates poor ethanol resistance, while a smaller value indicates superior ethanol resistance.

[0055] The optical film of the present invention preferably has a surface resistance value of 1.0 × 10⁻⁶. 8 Ω / □ or greater, more preferably 1.0 × 10 9 It is desirable that the ratio be greater than or equal to Ω / □. There is no particular upper limit; for example, 1.0 × 10 14 Ω / □ or less, 5.0×10 13 Examples include values ​​less than Ω / □. A surface resistance of 1.0 × 10⁻⁶ is given. 8 A value of Ω / □ or greater indicates excellent antistatic properties. In this specification, the surface resistance value can be measured using a surface resistivity meter.

[0056] The optical film of the present invention is characterized by the fact that, when fingerprints are applied to the film surface (low refractive index layer) and wiped with tissue paper, the number of times it takes to wipe away the fingerprints until they are no longer visible is not particularly limited, but preferably 30 times or less. A number of wipes of 30 times or less indicates good fingerprint wiping performance, and a smaller number indicates superior fingerprint wiping performance.

[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 higher, more preferably 93.0% or higher, and even more preferably 94.0% or higher. A total light transmittance of 92% or higher indicates excellent optical properties, and a higher value indicates better 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 (Hitachi, Ltd., U-4100). A luminous reflectance of 1.0% or less indicates excellent optical properties, and a lower value indicates better 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 foldable devices include smartphones, tablets, portable game consoles and other personal information terminals. It may also be a rollable display device, for example, it may be applied to a rollable television. On the other hand, it may also be applied to non-foldable devices, such as televisions, monitors, mobile phones, portable game consoles, personal information terminals, personal computers, e-books, video cameras, digital still cameras, head-mounted displays and navigation systems. [Examples]

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

[0061] Examples 1-10 and Comparative Examples 1-8 <Resin substrate> The resin substrate shown below was used. Polyethylene terephthalate film (O700E50, with easy-adhesion layers on both sides, 50 μm thick, manufactured by Mitsubishi Chemical) Polyethylene terephthalate film (with easy-adhesion layers on both sides, 8 μm thick) Polyethylene terephthalate film (with easy-adhesion layers on both sides, 10 μm thick) Polyethylene terephthalate film (with easy-adhesion layers on both sides, 100 μm thick) Polyethylene terephthalate film (with easy-adhesion layers on both sides, 110 μm thick) Polybutylene terephthalate film (with easy-adhesion layers on both sides, 50 μm thick) Polyethylene naphthalate film (with easy-adhesion layers on both sides, 50 μm thick) Polyimide film (with easy-adhesion layers on both sides, 50 μm thick) Triacetylcellulose film (TJ25UL, 25 μm thick, manufactured by Fujifilm)

[0062] <Hard coat layer> A hard coat layer-forming composition was prepared by diluting 99.9 parts by mass of a photocurable resin (hard coat agent containing a quaternary ammonium salt, manufactured by Arakawa Chemical Industries) and 0.1 parts by mass of an acrylic polymer (TEGO® Flow 300, manufactured by Evonik) in a mixed solvent of propylene glycol monomethyl ether / methyl ethyl ketone / isopropyl alcohol (mass ratio 25 / 50 / 20) and stirring. The amount of quaternary ammonium salt added was adjusted according to known techniques and is described separately as its content in the hard coat layer.

[0063] The obtained hard coat layer forming composition was applied to one side of the substrate using a wire bar coater to form a coating film, dried at 60°C for 60 seconds, and then cured in a nitrogen atmosphere using a conveyor-type ultraviolet curing device with an exposure of 200 mJ / cm². 2 By irradiating with ultraviolet light, hard coat layers of the thicknesses shown in Tables 1-4 were fabricated.

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

[0065] The obtained low refractive index layer-forming composition was applied to the upper surface of the hard coat layer obtained above using a wire bar coater to form a coating film. After drying at 60°C for 60 seconds, it was cured in a nitrogen atmosphere using a conveyor-type ultraviolet curing device with an exposure of 200 mJ / cm². 2 By irradiating with ultraviolet light, a low refractive index layer with a thickness of 100 nm was fabricated, and an optical film was obtained.

[0066] Test Example 1 [Repeated Folding Resistance 1] The repeatable folding resistance 1 was evaluated by a mandrel test in accordance with JIS K 5600-1. Metal cylinders with diameters of 1.0 mm, 2.0 mm, 4.0 mm, 6.0 mm, 8.0 mm, or 10.0 mm were set in a mandrel testing machine (manufactured by COTEC), and the optical film was fixed so that the low refractive index layer was on the side in contact with the cylinder (inward fold) or the side not in contact with the cylinder (outward fold). The film surface was then visually observed after folding it 180° and wrapping it around the cylinder, and the smallest cylinder diameter at which no cracks occurred (minimum mandrel diameter) was used as the evaluation value. A minimum mandrel diameter of 1.0 mm or less when inward fold and 6.0 mm or less when outward fold was considered acceptable, and a minimum mandrel diameter of 1.0 mm in both inward and outward folds was considered better, indicating excellent repeatable folding performance of the film itself. The results are shown in Tables 1 to 4.

[0067] Test Example 2 [Repeated Folding Resistance 2] A tabletop durability testing machine (manufactured by Yuasa Systems) was fitted with a planar no-load U-shaped stretch test fixture (DMX-FS). A 30 x 80 mm test piece was fixed so that the low refractive index layer was on either the inward or outward folding side when bent. The piece was then continuously folded so that the distance between 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. No cracks were observed ("○") and cracks were observed ("×"). The results are shown in Tables 1-4.

[0068] Test Example 3 [Tensile Properties 1: Stretchability] A 100 x 10 mm test piece was set in a tensile testing machine (STB-1225L, manufactured by A&D Manufacturing) with a chuck distance of 50 mm, stretched, and the length of the film at which a crack appeared was measured visually. The elongation rate (%) was calculated from the change in length from before stretching. An elongation rate of 3.0% to 30.0% was considered acceptable. The results are shown in Tables 1-4. Elongation (%) = 100 × (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 specimen, punched according to JIS K6251, was set in a Tensilon universal testing machine (STB-1225S, manufactured by A&D Co., Ltd.) (chuck distance 80 mm). The specimen was stretched at a speed of 50 mm / min, and the tensile stress until a crack occurred in the hard coat layer was continuously measured visually. The integral value of the tensile stress obtained during stretching and the amount of stretching until a crack occurred was calculated, and the integral value of the tensile stress integrated with respect to the amount of stretching was determined. Specifically, a tensile stress-stretch curve was obtained with tensile stress on the vertical axis and stretching amount on the horizontal axis, and the area of ​​this curve up to the stretching amount at which a crack occurred was calculated to obtain the integral value. An integral value of 1,000 GPa·mm or higher was considered acceptable. The results are shown in Tables 1 to 4.

[0070] Test Example 5 [Tensile Properties 3: Tensile Modulus of Elasticity] A dumbbell-shaped No. 5 test specimen, punched according to JIS K6251, was set in a Tensilon universal testing machine (STB-1225S, manufactured by A&D Co., Ltd.) (chuck distance 80 mm), and the tensile modulus was measured by stretching at a speed of 50 mm / min. The obtained tensile modulus was multiplied by the thickness of the resin substrate, and the product of the tensile modulus and the thickness of the resin substrate was calculated. In the case of commercially available resin substrates, the thickness of the resin substrate was the value stated in the catalog. A product value 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 specimen, punched according to JIS K6251, was set in a Tensilon universal testing machine (STB-1225S, manufactured by A&D Corporation) (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 higher was considered acceptable. The results are shown in Tables 1-4.

[0072] Test Example 7 [Abrasion Resistance] Using a reciprocating abrasion tester (TYPE: 30S, manufactured by Shinto Kagaku), an eraser (cylindrical, 6.0 mm in diameter, manufactured by Hwarang) was placed on the surface (low refractive index layer) of an optical film and slid 5000 times under a load of 1.0 kg (1.0 kg / 6.0 mmΦ). For the film surface before and after the test, a contact angle meter (Drop Master 300, manufactured by Kyowa Interface Science) was used to measure the water contact angle 5 seconds after pure water was applied to the surface, and the percentage change in water contact from before the test was calculated. A change of 20% or less was considered acceptable. The results are shown in Tables 1-4. Rate of change (%) = 100 × (Water contact angle before test - Water contact angle after test) / Water contact angle before test

[0073] Test Example 8 [Ethanol Tolerance] The percentage change in water contact from before the test was calculated in the same manner as in Test Example 7, except that ethanol was dropped onto the film surface and rubbed 2000 times during the test. The water contact angle before the test was the value from Test Example 7. A change of 20% or less was considered a pass. 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 optical film surface (low refractive index layer) was measured using a pencil (uni, manufactured by Mitsubishi Pencil) and a Clemens scratch tester (HA-301, manufactured by Tester Sangyo). The test was repeated while varying the pencil hardness, and the change in appearance due to scratches was visually observed. The highest hardness at which no scratches were observed in 4 or more out of 5 tests was used as the evaluation value. A pencil hardness of 2H or higher was considered acceptable. The results are shown in Tables 1-4.

[0075] Test example 10 [Scratch resistance] Using a wear and friction tester (AB-301, manufactured by Tester Industry Co., Ltd.), steel wool (Bonseter #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 2 was applied and reciprocated 50 times. After that, the surface of the low refractive index layer was visually observed, and the number of formed scratches was measured. When visually observing, it was carried out after laminating a PET film (Kukkiri Mire, manufactured by Tomoegawa) on the back surface of the film. Five or less scratches were regarded as passing. 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 Analytic Co., Ltd.), an element was pressed against the film surface (low refractive index layer), 250 V was applied, and the surface resistance value after 10 seconds was measured. A surface resistance value of 1.0×10 8 Ω / sq or more was regarded as passing. The results are shown in Tables 1 to 4.

[0077] Test Example 12 [Total Light Transmittance] The 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 regarded as passing. The results are shown in Tables 1 to 4.

[0078] Test Example 13 [Visual Reflectance] The spectral reflectance on 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, on the back surface of the optical film (the surface opposite to the surface provided with the low refractive index layer), an anti-reflection treatment was performed by laminating a black tape for dulling (#302 black, Teraoka Vinyl Tape), and a normal reflection measurement was performed at an incident light angle of 5°. The visual reflectance (Y value) was calculated in accordance with JIS R 3106 from the obtained spectral reflectance curve. A visual reflectance of 1.0% or less was regarded as passing. The results are shown in Tables 1 to 4.

[0079]

Table 1

[0080] [Table 2]

[0081] [Table 3]

[0082] [Table 4]

[0083] Table 1 shows that Comparative Example 1, with its thin hard coat layer of 1.0 μm, exhibited excellent repeated bending properties, but its elongation rate was over 30.0%, making it easily stretched, yet its yield point was a brittle 95 GPa, resulting in poor tensile properties. Furthermore, its surface properties changed significantly due to eraser abrasion. Comparative Examples 2 and 3, with their thicker hard coat layers, exhibited excellent wear resistance, but not only were they poorly flexible, but they also had a low elongation rate, making them difficult to stretch, and their tensile properties were poor, with integrated tensile stress values ​​of 499 and 387 GPa·mm. On the other hand, Examples 1 to 3, with hard coat layers between 2.0 μm and 7.0 μm, demonstrated excellent tensile properties and achieved a balance between repeated bending and wear resistance.

[0084] Furthermore, Table 2 shows that the repeated bending properties and abrasion resistance change depending on the thickness of the resin substrate. For example, Comparative Example 4, with a resin substrate thickness of 8 μm, has an elongation rate of 34.0% and a yield point of 98 GPa, making it easy to stretch but brittle. Therefore, while it has excellent repeated bending properties, it is inferior in abrasion resistance and pencil hardness. Comparative Example 5, with a resin substrate thickness of 110 μm, has a large minimum mandrel diameter for both inward and outward folding, indicating poor repeated bending properties. On the other hand, Examples 4 and 5 have resin substrate thicknesses of 10 μm to 100 μm, demonstrating that they achieve both excellent repeated bending properties and abrasion resistance while also exhibiting superior tensile properties.

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

[0086] Table 4 shows that even when using different types of resin substrates, it is possible to achieve both repeated bending and abrasion resistance by using substrates such as polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, and polyimide. This suggests that various substrates can be applied as protective films for foldable display devices. [Industrial applicability]

[0087] The optical film of the present invention is suitably used as a protective film for foldable display devices. [Explanation of symbols]

[0088] 1. Resin substrate 2. Hard court layer 3. Low refractive index layer 4 Easy adhesive layer 10 Optical film 20 Optical Films

Claims

1. An optical film comprising a resin substrate with a thickness of 10 μm or more and 100 μm or less, on one side having a hard coat layer and a low refractive index layer in that order, wherein the thickness of the hard coat layer is 2.0 μm or more and 7.0 μm or less, The elongation rate is between 3.0% and 30.0%. The minimum diameter of a cylinder in which cracks do not occur when the low refractive index layer is wrapped around the cylinder so that it is on the inside is 1.0 mm or less. The minimum diameter of a cylinder in which cracks do not occur when the low refractive index layer is wrapped around the cylinder so that it faces outwards is 6.0 mm or less. The water contact angle change rate after sliding an eraser 5000 times with a load of 1.0 kg / 6.0 mmΦ across the surface of the low refractive index layer is 20% or less. Optical film.

2. An optical film comprising a resin substrate with a thickness of 10 μm or more and 100 μm or less, on one side having a hard coat layer and a low refractive index layer in that order, wherein the thickness of the hard coat layer is 2.0 μm or more and 7.0 μm or less, The integral value obtained by integrating the tensile stress until a crack occurs in the hard coat layer with respect to the amount of elongation is 1,000 GPa·mm or more. The minimum diameter of a cylinder in which cracks do not occur when the low refractive index layer is wrapped around the cylinder so that it is on the inside is 1.0 mm or less. The minimum diameter of a cylinder in which cracks do not occur when the low refractive index layer is wrapped around the cylinder so that it faces outwards is 6.0 mm or less. The water contact angle change rate after sliding an eraser 5000 times with a load of 1.0 kg / 6.0 mmΦ across the surface of the low refractive index layer is 20% or less. Optical film.

3. An optical film comprising a resin substrate with a thickness of 10 μm or more and 100 μm or less, on one side having a hard coat layer and a low refractive index layer in that order, wherein the thickness of the hard coat layer is 2 μm or more and 7 μm or less, 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 in which cracks do not occur when the low refractive index layer is wrapped around the cylinder so that it is on the inside is 1.0 mm or less. The minimum diameter of a cylinder in which cracks do not occur when the low refractive index layer is wrapped around the cylinder so that it faces outwards is 6.0 mm or less. The water contact angle change rate after sliding an eraser 5000 times with a load of 1.0 kg / 6.0 mmΦ across the surface of the low refractive index layer is 20% or less. Optical film.

4. An optical film comprising a resin substrate with a thickness of 10 μm or more and 100 μm or less, on one side having a hard coat layer and a low refractive index layer in that order, wherein the thickness of the hard coat layer is 2.0 μm or more and 7.0 μm or less, The yield point is 100 GPa or higher. The minimum diameter of a cylinder in which cracks do not occur when the low refractive index layer is wrapped around the cylinder so that it is on the inside is 1.0 mm or less. The minimum diameter of a cylinder in which cracks do not occur when the low refractive index layer is wrapped around the cylinder so that it faces outwards is 6.0 mm or less. The water contact angle change rate after sliding an eraser 5000 times with a load of 1.0 kg / 6.0 mmΦ across the surface of the low refractive index layer is 20% or less. Optical film.

5. An optical film according to any one of claims 1 to 4, wherein the pencil hardness in a scratch hardness test in accordance with JIS K5600-5-4 is 2H or higher.

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, wherein a high refractive index layer is provided between the hard coat layer and the low refractive index layer, or a medium refractive index layer, a high refractive index layer, and the low refractive index layer are provided 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 consists of one or more selected from the group consisting of polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polyimide, and polyamideimide.

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

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