JP7569352B2 - Anti-reflection film, its manufacturing method, and image display device - Google Patents

Description

The present invention relates to an anti-reflection film having an anti-reflection layer on a hard coat layer of a hard coat film, and a manufacturing method thereof. Furthermore, the present invention relates to an image display device having the anti-reflection film.

Anti-reflection films are used on the viewing side surfaces of image display devices such as liquid crystal displays and organic EL displays to prevent degradation of image quality due to reflection of external light and to improve contrast. Anti-reflection films have an anti-reflection layer made of a laminate of multiple thin films with different refractive indices on a transparent film.

For example, Patent Document 1 discloses an antireflection film having a SiO primer layer on a hard coat film, and an antireflection layer thereon consisting of an alternating laminate of a niobium oxide (Nb ₂ O ₅ ) layer as a high refractive index layer and a silicon oxide (SiO ₂ ) layer as a low refractive index layer.

JP 2009-47876 A

In recent years, foldable image display devices (foldable displays) equipped with organic EL panels using foldable substrates (flexible substrates) such as resin films have been put to practical use. An anti-reflection film with an anti-reflection layer provided on a flexible film substrate is used as the cover window for foldable displays.

Foldable displays are generally stored in a folded state. In the folded state, compressive stress is applied to the inside of the fold (bent point) and tensile stress is applied to the outside. When the display is folded with the display surface on the inside, the anti-reflection film is folded with the surface on which the anti-reflection layer is formed on the inside. If the display is heated to a high temperature in this state, fine cracks may occur in the anti-reflection layer, causing a decrease in visibility of the display.

In view of the above, the present invention aims to provide an anti-reflection film that is unlikely to develop cracks in the anti-reflection layer and has excellent bending resistance, even when heated to high temperatures in a folded (bent) state.

The anti-reflection film comprises a hard coat film having a hard coat layer on one main surface of a transparent film substrate, and an anti-reflection layer provided on the hard coat layer. The hard coat layer may contain fine particles having an average primary particle size of 10 to 100 nm in addition to a binder resin. A primer layer made of an inorganic oxide may be provided between the hard coat layer and the anti-reflection layer. An anti-fouling layer may be provided on the anti-reflection layer.

The anti-reflection layer includes at least one high refractive index layer and at least one low refractive index layer. The anti-reflection layer may include two or more high refractive index layers, or may include two or more low refractive index layers. The anti-reflection layer is preferably an alternating laminate of multiple high refractive index layers and multiple low refractive index layers.

The high refractive index layers of the antireflection layer are thin films mainly composed of niobium oxide, and the high refractive index layer disposed farthest from the hard coat layer has a thickness of 40 nm or less and a film density of less than 4.47 g/cm ^3. When the antireflection layer includes a plurality of high refractive index layers (niobium oxide thin films), it is preferable that each high refractive index layer has a thickness of 40 nm or less and a film density of less than 4.47 g/cm ³ .

The arithmetic mean surface height of the anti-reflection layer is preferably 2.5 nm or more. If an anti-smudge layer is provided on the anti-reflection layer, the arithmetic mean surface height of the anti-smudge layer is preferably 2.5 nm or more.

The anti-reflection film of the present invention is less likely to develop cracks in the anti-reflection layer even when heated in a bent state with the anti-reflection layer-forming surface facing inward, and is also suitable for use in foldable displays.

FIG. 2 is a cross-sectional view showing a lamination form of an anti-reflection film. FIG. 2 is a cross-sectional view showing the configuration of a sample used in a bending resistance test.

Figure 1 is a cross-sectional view showing an example of a laminated structure of an anti-reflection film according to one embodiment of the present invention. The anti-reflection film 101 has an anti-reflection layer 5 on the hard coat layer 11 of the hard coat film 1. The hard coat film 1 has a hard coat layer 11 on one main surface of a transparent film substrate 10. The anti-reflection layer 5 is a laminate of two or more thin layers with different refractive indices, and includes at least one high refractive index layer and at least one low refractive index layer. A primer layer 3 may be provided between the hard coat layer 11 and the anti-reflection layer 5. An antifouling layer 7 may be provided on the anti-reflection layer 5.

[Hard coat film]
The hard coat film 1 includes a hard coat layer 11 on one main surface of a transparent film substrate 10. By providing the hard coat layer 11 on the surface on which the antireflection layer 5 is formed, the mechanical properties such as surface hardness and scratch resistance of the antireflection film can be improved.

<Transparent film substrate>
The visible light transmittance of the transparent film substrate 10 is preferably 80% or more, more preferably 90% or more. As the resin material constituting the transparent film substrate 10, for example, a resin material excellent in transparency, mechanical strength, and thermal stability is preferable. Specific examples of the resin material include cellulose-based resins such as triacetyl cellulose, polyester-based resins, polyethersulfone-based resins, polysulfone-based resins, polycarbonate-based resins, polyamide-based resins, polyimide-based resins, polyolefin-based resins, (meth)acrylic-based resins, cyclic polyolefin-based resins (norbornene-based resins), polyarylate-based resins, polystyrene-based resins, polyvinyl alcohol-based resins, and mixtures thereof.

The thickness of the transparent film substrate is not particularly limited, but from the viewpoints of workability such as strength and handling, thin layer property, etc., it is preferably about 5 to 300 μm, more preferably 10 to 250 μm, and even more preferably 20 to 200 μm.

<Hard Coat Layer>
A hard coat film 1 is formed by providing a hard coat layer 11 on the main surface of a transparent film substrate 10. The hard coat layer is a cured resin layer, and is formed by applying a composition containing a curable resin onto the transparent film substrate and curing the resin component. The hard coat layer may contain fine particles in addition to the cured resin.

(Hardening Resin)
As the curable resin (binder resin) of the hard coat layer 11, a curable resin such as a thermosetting resin, a photocurable resin, or an electron beam curable resin is preferably used. The types of curable resin include polyester-based, acrylic-based, urethane-based, acryl urethane-based, amide-based, silicone-based, silicate-based, epoxy-based, melamine-based, oxetane-based, and acryl urethane-based resins. Among these, acrylic-based resins, acryl urethane-based resins, and epoxy-based resins are preferred because of their high hardness and the ability to be photocured, and among these, acryl urethane-based resins are preferred.

The photocurable resin composition contains a polyfunctional compound having two or more photopolymerizable (preferably ultraviolet-polymerizable) functional groups. The polyfunctional compound may be a monomer or an oligomer. As the photopolymerizable polyfunctional compound, a compound containing two or more (meth)acryloyl groups in one molecule is preferably used.

Specific examples of polyfunctional compounds having two or more (meth)acryloyl groups in one molecule include tricyclodecane dimethanol diacrylate, pentaerythritol di(meth)acrylate, pentaerythritol tri(meth)acrylate, trimethylolpropane triacrylate, pentaerythritol tetra(meth)acrylate, dimethylolpropane tetraacrylate, dipentaerythritol hexa(meth)acrylate, 1,6-hexanediol (meth)acrylate, 1,9-nonanediol diacrylate, 1,10-decanediol (meth)acrylate, polyethylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, dipropylene glycol diacrylate, isocyanuric acid tri(meth)acrylate, ethoxylated glycerin triacrylate, ethoxylated pentaerythritol tetraacrylate, and oligomers or prepolymers thereof. In this specification, "(meth)acrylic" means acrylic and/or methacrylic.

A polyfunctional compound having two or more (meth)acryloyl groups in one molecule may have a hydroxyl group. By using a polyfunctional compound containing a hydroxyl group, the adhesion between the transparent film substrate and the hard coat layer tends to be improved. Examples of compounds having a hydroxyl group and two or more (meth)acryloyl groups in one molecule include pentaerythritol tri(meth)acrylate and dipentaerythritol penta(meth)acrylate.

The acrylic urethane resin contains a urethane (meth)acrylate monomer or oligomer as a polyfunctional compound. The number of (meth)acryloyl groups in the urethane (meth)acrylate is preferably 3 or more, more preferably 4 to 15, and even more preferably 6 to 12. The molecular weight of the urethane (meth)acrylate oligomer is, for example, 3,000 or less, preferably 500 to 2,500, and more preferably 800 to 2,000. The urethane (meth)acrylate is obtained, for example, by reacting a hydroxy (meth)acrylate obtained from (meth)acrylic acid or a (meth)acrylic acid ester and a polyol with a diisocyanate.

The content of the polyfunctional compound in the composition for forming the hard coat layer is preferably 50 parts by weight or more, more preferably 60 parts by weight or more, and even more preferably 70 parts by weight or more, per 100 parts by weight of the total of the resin components (monomers, oligomers, and prepolymers that form the binder resin upon curing). If the content of the polyfunctional monomer is within the above range, the hardness of the hard coat layer tends to be increased.

(Microparticles)
When the hard coat layer 11 contains fine particles, fine irregularities are formed on the surface, which tends to improve the adhesion and flex resistance of the antireflection layer.

As the fine particles, inorganic oxide fine particles such as silica, alumina, titania, zirconia, calcium oxide, tin oxide, indium oxide, cadmium oxide, and antimony oxide, glass fine particles, and crosslinked or uncrosslinked organic fine particles made of transparent polymers such as polymethyl methacrylate, polystyrene, polyurethane, acrylic-styrene copolymer, benzoguanamine, melamine, and polycarbonate can be used without any particular restrictions.

The average particle size (average primary particle size) of the microparticles is preferably about 10 nm to 10 μm. The average primary particle size is the weight average particle size measured by the Coulter count method. Depending on the particle size, the microparticles can be broadly classified into microparticles having a particle size of about 0.5 μm to 10 μm, or on the order of μm (hereinafter sometimes referred to as "microparticles"), microparticles having a particle size of about 10 nm to 100 nm (hereinafter sometimes referred to as "nanoparticles"), and microparticles having a particle size intermediate between microparticles and nanoparticles.

When the hard coat layer 11 contains nanoparticles, fine irregularities are formed on the surface, which tends to improve the adhesion between the hard coat layer 11 and the primer layer 3 and the anti-reflection layer 5. As the nanoparticles, inorganic fine particles are preferred, and inorganic oxide fine particles are particularly preferred. Among these, silica particles are preferred because they have a low refractive index and can reduce the difference in refractive index with the binder resin.

From the viewpoint of forming an uneven shape on the surface of the hard coat layer 11 that has excellent adhesion to the anti-reflection layer, the average primary particle diameter of the nanoparticles is preferably 20 to 80 nm, more preferably 25 to 70 nm, and even more preferably 30 to 60 nm.

The amount of nanoparticles in the hard coat layer 11 may be about 1 to 150 parts by weight per 100 parts by weight of the binder resin. From the viewpoint of forming a surface shape on the surface of the hard coat layer 11 that has excellent adhesion to the anti-reflection layer, the content of nanoparticles in the hard coat layer 11 is preferably 20 to 100 parts by weight, more preferably 25 to 90 parts by weight, and even more preferably 30 to 80 parts by weight per 100 parts by weight of the binder resin.

(Formation of hard coat layer)
The hard coat layer forming composition contains the above-mentioned binder resin component, and if necessary, contains a solvent capable of dissolving the binder resin component. As described above, the hard coat layer forming composition may contain fine particles. When the binder resin component is a photocurable resin, it is preferable that a photopolymerization initiator is contained in the composition. In addition to the above, the hard coat layer forming composition may contain additives such as a leveling agent, a thixotropic agent, an antistatic agent, an antiblocking agent, a dispersant, a dispersion stabilizer, an antioxidant, an ultraviolet absorber, an antifoaming agent, a thickener, a surfactant, and a lubricant.

A hard coat layer is formed by applying a composition for forming a hard coat layer on a transparent film substrate, and removing the solvent and curing the resin as necessary. Any suitable method such as a bar coating method, a roll coating method, a gravure coating method, a rod coating method, a slot orifice coating method, a curtain coating method, a fountain coating method, or a comma coating method can be adopted as a method for applying the composition for forming a hard coat layer. The heating temperature after application may be set to an appropriate temperature depending on the composition of the composition for forming a hard coat layer, and is, for example, about 50°C to 150°C. When the binder resin component is a photocurable resin, photocuring is performed by irradiating it with active energy rays such as ultraviolet rays. The integrated light amount of the irradiated light is preferably about 100 to 500 mJ/ ^cm2 .

The thickness of the hard coat layer 11 is not particularly limited, but from the viewpoint of achieving high hardness and appropriately controlling the surface shape, a thickness of about 1 to 10 μm is preferable, 2 to 9 μm is more preferable, and 3 to 8 μm is even more preferable.

(Surface treatment of hard coat layer)
Before forming the anti-reflection layer 5 on the hard coat layer 11, the hard coat layer 11 may be subjected to a surface treatment. Examples of the surface treatment include surface modification treatments such as corona treatment, plasma treatment, frame treatment, ozone treatment, glow treatment, alkali treatment, acid treatment, and treatment with a coupling agent. Vacuum plasma treatment may be performed as the surface treatment. The surface roughness of the hard coat layer can also be adjusted by the vacuum plasma treatment. For example, when the hard coat layer 11 contains inorganic fine particles in addition to the binder resin component (resin cured product), the resin component on the hard coat layer surface is easily selectively etched by the vacuum plasma treatment, and the inorganic particles are hardly etched and remain, so that the ratio of inorganic oxide particles present on the hard coat layer surface and its vicinity is increased, and the arithmetic average height Sa ₁ of the hard coat layer surface tends to be large.

The atmospheric gas in the vacuum plasma treatment is preferably an inert gas such as helium, neon, argon, krypton, xenon, radon, etc., and argon is particularly preferred. The effective power density in the vacuum plasma treatment is preferably 0.01 W·min/m·cm ² or more, and may be 0.03 W·min/m·cm ² or more, 0.05 W·min/m·cm ² or more, 0.07 W·min/m·cm ² or more, or 0.1 W·min/m·cm ² or more. The effective power density is the value obtained by dividing the power density (W/cm ² ) of the plasma output by the transport speed (m/min). The higher the effective power density, the larger the arithmetic mean height Sa ₁ of the hard coat layer surface becomes, and the adhesion and bending resistance of the antireflection layer formed on the hard coat layer tend to improve accordingly.

On the other hand, if the effective power density is too high, the etching of the binder resin proceeds excessively, which may lead to the roughness of the hard coat layer surface becoming larger or the adhesion being reduced due to the falling off of fine particles. Therefore, the effective power density is preferably 0.6 W·min/m·cm2 or ^less , and may be 0.43 W·min/m· ^cm2 or less or 0.22 W·min/m· ^cm2 or less.

(Surface shape of hard coat layer)
As described above, the hard coat layer 11 contains fine particles (nanoparticles), so that fine irregularities are formed on the surface. In addition, by carrying out a surface treatment such as a plasma treatment on the hard coat layer containing fine particles, the irregularities tend to become large, and the arithmetic mean height Sa ₁ tends to become large. The arithmetic mean height Sa is calculated according to ISO 25178 from an observation image of 1 μm square using an atomic force microscope (AFM).

The arithmetic mean height _Sa1 of the surface of the hard coat layer 11 is preferably 2.5 nm or more, more preferably 3.0 nm or more, and may be 3.5 nm or more, 4.0 nm or more, 4.5 nm or more, 5.0 nm or more, 5.3 nm or more, or 5.5 nm or more. The larger the arithmetic mean height _Sa1 of the hard coat layer 11, the more the adhesion of the antireflection layer tends to improve. In addition, when the arithmetic mean height _Sa1 of the hard coat layer that is the film formation base of the antireflection layer 5 is large, the antireflection layer 5 is easily grown in a columnar shape during sputtering film formation, and the bending resistance tends to improve.

On the other hand, if the surface irregularities of the hard coat layer become too large, sufficient adhesion may not be achieved. Therefore, the arithmetic mean height _Sa1 of the hard coat layer surface is preferably 10 nm or less, more preferably 8.0 nm or less, and even more preferably 7.5 nm or less, and may be 7.0 nm or less or 6.5 nm or less.

[Anti-reflection film]
An antireflection layer 5 is formed on the hard coat layer 11 of the hard coat film 1, optionally with a primer layer 3 interposed therebetween, thereby forming an antireflection film.

<Primer layer>
A primer layer 3 is preferably provided between the hard coat layer 11 and the anti-reflection layer 5 of the hard coat film 1. Examples of materials for the primer layer 3 include metals such as silicon, nickel, chromium, tin, gold, silver, platinum, zinc, titanium, indium, tungsten, aluminum, zirconium, and palladium; alloys of these metals; oxides, fluorides, sulfides, or nitrides of these metals; and the like. Among these, the material for the primer layer is preferably an inorganic oxide, and silicon oxide or indium oxide is particularly preferred. The inorganic oxide constituting the primer layer 3 may be a composite oxide such as indium tin oxide (ITO).

The thickness of the primer layer 3 is, for example, about 1 to 20 nm, and preferably 3 to 15 nm. If the thickness of the primer layer is within the above range, it is possible to achieve both adhesion to the hard coat layer 11 and high light transmittance.

<Anti-reflection layer>
The antireflection layer 5 is a laminate of multiple thin films with different refractive indices, and includes at least one high refractive index layer and at least one low refractive index layer. In general, the optical thickness (product of refractive index and film thickness) of the thin films of the antireflection layer is adjusted so that the inverted phases of the incident light and the reflected light cancel each other out. The multi-layer laminate of multiple thin films with different refractive indices can reduce the reflectance in a wide wavelength range of visible light.

In the antireflection film 101 shown in Fig. 1, the antireflection layer 5 includes four layers, namely, a high refractive index layer 51, a low refractive index layer 52, a high refractive index layer 53, and a low refractive index layer 54, in this order from the hard coat film 1 side . The antireflection layer 5 is an alternating laminate of two high refractive index layers and two low refractive index layers. In order to reduce reflection at the air interface, the thin film 54 provided as the outermost layer (the layer farthest from the hard coat film 1) of the antireflection layer 5 is preferably a low refractive index layer.

The high refractive index layers 51 and 53 are thin films mainly composed of niobium oxide. Niobium oxide has a high refractive index, so by laminating it with a low refractive index layer, it is possible to efficiently reduce reflected light. The refractive index of the high refractive index layer is 2.0 or more, preferably 2.2 or more. The high refractive index layer may contain a metal oxide other than niobium oxide, but the niobium oxide content is 90% by weight or more, preferably 99% by weight or more.

The thickness of the niobium oxide thin film as the high refractive index layer is preferably 40 nm or less. When the anti-reflection layer includes multiple niobium oxide thin films 51, 53, it is preferable that at least the thickness of the niobium oxide thin film as the high refractive index layer 53 arranged farthest from the hard coat layer is 40 nm or less, and the thickness of all the high refractive index layers (niobium oxide thin films) is 40 nm or less. Since the thickness of the niobium oxide thin film is small, the anti-reflection layer has excellent bending resistance, and cracks are unlikely to occur in the anti-reflection layer even if the anti-reflection film is heated in a bent state. The thickness of the niobium oxide thin film is more preferably 35 nm or less, and may be 32 nm or less or 30 nm or less.

The density of the niobium oxide thin film is preferably 4.47 g/cm ³ or less, more preferably 4.40 g/cm ³ or less, and may be 4.35 g/cm ³ or less or 4.33 g/cm ³ or less. When the antireflection layer includes a plurality of niobium oxide thin films 51, 53, at least the niobium oxide thin film as the high refractive index layer 53 arranged farthest from the hard coat layer 11 preferably has a film density in the above range, and it is particularly preferable that all the film densities of the plurality of niobium oxide thin films 51, 53 are in the above range. The film density of the niobium oxide thin film is generally 4.0 g/cm ³ or more, and may be 4.1 g/cm ³ or more or 4.2 g/cm ³ or more. The film density is a value measured by the Rutherford backscattering (RBS) method, and the density is calculated using the film thickness obtained from cross-sectional observation.

The thicker the niobium oxide thin film, the greater the film density tends to be. As described above, by making the thickness of the niobium oxide thin film 40 nm or less, the film density of the niobium oxide thin film does not become excessively large, and the occurrence of cracks during bending tends to be suppressed. In addition, the greater the surface unevenness of the base of the niobium oxide thin film (the larger Sa), the smaller the film density of the niobium oxide thin film, and the more likely it is that cracks will be suppressed.

The low refractive index layers 52 and 54 have a refractive index of 1.6 or less, preferably 1.5 or less. Examples of low refractive index materials include silicon oxide, titanium nitride, magnesium fluoride, barium fluoride, calcium fluoride, hafnium fluoride, and lanthanum fluoride. Among them, silicon oxide is preferred because it has a low refractive index, high hardness, and can efficiently reduce the reflectance by laminating with a niobium oxide high refractive index layer. The low refractive index layers 52 and 54 preferably have a silicon oxide content of 90% by weight or more, more preferably 99% by weight or more. The film density of the silicon oxide thin film is preferably 2.20 g/cm ³ or less, and may be 2.15 g/cm ³ or less, or 2.10 g/cm ³ or less.

When the low refractive index layer 54, which is the outermost layer of the antireflection layer, is a silicon oxide layer, its thickness is preferably greater than 85 nm. By making the low refractive index layer 54, which is the outermost layer of the antireflection layer 5, a silicon oxide layer having a thickness greater than 85 nm, the surface hardness of the antireflection layer 5 is increased, the scratch resistance of the antireflection layer is increased, and the abrasion resistance of the antifouling layer 7 formed thereon tends to be improved.

The thickness of the low refractive index layer 54 is preferably 87 nm or more, more preferably 90 nm or more, and may be 92 nm or more, 94 nm or more, or 95 nm or more. If the thickness of the low refractive index layer 54 is excessively large, it may cause cracks to occur or make it difficult to achieve an optical design with low reflectance and excellent anti-reflection properties. Therefore, the thickness of the low refractive index layer 54 is preferably 200 nm or less, more preferably 150 nm or less, and may be 130 nm or less, 120 nm or less, 115 nm or less, 110 nm or less, or 105 nm or less.

In one embodiment, the anti-reflection layer 5 is an alternating laminate of four layers, from the hard coat film 1 side, including a first layer: a niobium oxide thin film as a high refractive index layer 51, a second layer: a silicon oxide thin film as a low refractive index layer 52, a third layer: a niobium oxide thin film as a high refractive index layer 53, and a fourth layer: a silicon oxide thin film as a low refractive index layer 54. In another embodiment, the anti-reflection layer is an alternating laminate of six layers, including three layers each of niobium oxide thin films as high refractive index layers and silicon oxide thin films as low refractive index layers.

When the anti-reflection layer 5 is an alternating laminate of a total of four layers consisting of two high refractive index layers 51, 53 and two low refractive index layers 52, 54, the anti-reflection layer 5 may have a configuration in which, from the hard coat film 1 side, a niobium oxide thin film 51 having a thickness of 10 to 20 nm, a silicon oxide thin film 52 having a thickness of 35 to 45 nm, a niobium oxide thin film 53 having a thickness of 25 to 35 nm, and a silicon oxide thin film 54 having a thickness of 90 to 105 nm are arranged in this order.

(Deposition of primer layer and anti-reflection layer)
The method for forming the thin films constituting the primer layer 3 and the anti-reflection layer 5 is not particularly limited, and may be either a wet coating method or a dry coating method. Dry coating methods such as vacuum deposition, CVD, sputtering, and electron beam vapor deposition are preferred because they can form thin films with a uniform thickness. Among these, sputtering is preferred because it has excellent uniformity in thickness and is easy to form dense films.

In the sputtering method, a long hard coat film can be transported in one direction (longitudinal direction) using a roll-to-roll method while thin films are continuously formed, improving the productivity of anti-reflective films. In order to improve the productivity of anti-reflective films, it is preferable to form all of the thin films that make up the anti-reflective layer 5 by the sputtering method.

In the sputtering method, deposition is performed while introducing an inert gas such as argon, and if necessary, a reactive gas such as oxygen, into the chamber. Deposition of an oxide layer by sputtering can be performed using either an oxide target or reactive sputtering using a metal target. To deposit a metal oxide film at a high rate, reactive sputtering using a metal target is preferred.

The film density of the anti-reflective layer can be adjusted by adjusting the sputtering deposition conditions. For example, when the discharge voltage during sputtering deposition is low, the kinetic energy of the sputtered particles is low and diffusion on the substrate surface is suppressed, which promotes columnar growth and tends to reduce the film density.

In addition, if the pressure during film formation is high, the mean free path of the sputtered particles becomes smaller, and the directionality of the sputtered particles decreases, making them more likely to diffuse, which tends to result in a lower film density. In order to form a niobium oxide thin film with a low film density, the pressure during sputtering film formation is preferably 0.5 Pa or more, and may be 0.55 Pa or more or 0.6 Pa or more. On the other hand, if the film formation pressure is excessively high, the film formation rate is low and productivity is poor, so the film formation pressure is preferably 1.5 Pa or less, and may be 1 Pa or less or 0.9 Pa or less.

When a thin film is formed by a dry process such as a sputtering method, a thin film (initially formed) is strongly influenced by the base, and as the film thickness increases, it tends to have bulk-like characteristics. In particular, when the hard coat layer 11 has surface irregularities and _Sa1 is large, the film density tends to be small in the initial stage of film formation due to the influence of the base irregularities, but as the film thickness increases, it approaches the bulk characteristics and the film density tends to increase.

When an anti-reflection film containing a thick niobium oxide thin film as a high refractive index layer is heated while bent with the anti-reflection layer side facing inward, cracks may occur in the anti-reflection layer. Even when the thickness of a low refractive index layer such as silicon oxide is thick, the occurrence of cracks is suppressed if the thickness of the niobium oxide thin film is small, so it is thought that the occurrence of cracks is caused by the thick niobium oxide thin film.

When an anti-reflective film is bent so that the surface on which the anti-reflective layer is formed is on the inside, a compressive strain is generated in the anti-reflective layer. When heated in this state, the film substrate thermally expands, generating a tensile strain (a force that tries to stretch the film) on the surface of the thin film facing the hard coat layer. When the niobium oxide thin film is thick, the difference in strain between the front and back of the thin film becomes large, and the high film density makes it difficult to alleviate the strain within the film, which is thought to be one of the causes of cracks in the anti-reflective layer.

In particular, the niobium oxide thin film 53 on the surface side of the anti-reflection layer 5 is more susceptible to cracking due to its greater distortion when bent than the niobium oxide thin film 51 located closer to the hard coat layer 11. In an anti-reflection film in which a niobium oxide thin film as a high refractive index layer and a silicon oxide thin film as a low refractive index layer are alternately laminated, a configuration in which the niobium oxide thin film on the surface side of the anti-reflection layer is made thick is adopted as an optical design for reducing reflectance.

In contrast, in the present invention, the thickness of the niobium oxide thin film 53 is 40 nm or less, so even if the anti-reflection film is heated in a bent state, the distortion of the niobium oxide thin film is small, and it is believed that the occurrence of cracks is suppressed. In addition, since the hard coat layer contains fine particles and fine irregularities are formed on the surface of the hard coat layer, columnar growth is promoted when a thin film is formed thereon, and the film density of the niobium oxide thin film is reduced, which is also believed to contribute to improved bending resistance (suppression of cracks).

The arithmetic mean height Sa of the surface of the anti-reflection layer 5 is preferably 2.5 nm or more, more preferably 2.8 nm or more, even more preferably 3.0 nm or more, and may be 3.5 nm or more or 4.0 nm or more. The arithmetic mean height Sa of the anti-reflection layer 5 is preferably 8 nm or less, more preferably 7.5 nm or less, even more preferably 7 nm or less, and may be 6 nm or less or 5.5 nm or less.

<Anti-stain layer>
The antireflection film preferably has an antifouling layer 7 as an outermost layer (topcoat layer) on the antireflection layer 5. By providing the antifouling layer on the outermost surface, the influence of contamination (fingerprints, hand dirt, dust, etc.) from the external environment can be reduced and contaminants attached to the surface can be easily removed.

In order to maintain the anti-reflection properties of the anti-reflection layer 5, it is preferable that the anti-smudge layer 7 has a small difference in refractive index from the low refractive index layer 54, which is the outermost layer of the anti-reflection layer 5. The refractive index of the anti-smudge layer 7 is preferably 1.6 or less, and more preferably 1.55 or less.

As the material of the antifouling layer 7, a fluorine-containing compound is preferred. The fluorine-containing compound can impart antifouling properties and also contribute to lowering the refractive index. Among them, a fluorine-based polymer containing a perfluoropolyether skeleton is preferred because it has excellent water repellency and can exhibit high antifouling properties. From the viewpoint of enhancing antifouling properties, perfluoropolyethers having a main chain structure that can be rigidly arranged in parallel are particularly preferred. As the structural unit of the main chain skeleton of perfluoropolyether, a perfluoroalkylene oxide having 1 to 4 carbon atoms that may have a branch is preferred, and examples thereof include perfluoromethylene oxide (-CF ₂ O-), perfluoroethylene oxide (-CF ₂ CF ₂ O-), perfluoropropylene oxide (-CF ₂ CF ₂ CF ₂ O-), and perfluoroisopropylene oxide (-CF(CF ₃ )CF ₂ O-).

The antifouling layer 7 can be formed by a wet method such as reverse coating, die coating, or gravure coating, or a dry method such as CVD. The thickness of the antifouling layer is usually about 2 to 50 nm. The larger the thickness of the antifouling layer 7, the more the antifouling properties tend to improve. In addition, the larger the thickness of the antifouling layer 7, the more the deterioration of the antifouling properties due to wear tends to be suppressed. The thickness of the antifouling layer is preferably 5 nm or more, more preferably 7 nm or more, and even more preferably 8 nm or more. On the other hand, from the viewpoint of forming a surface shape on the surface of the antifouling layer that reflects the uneven shape of the hard coat layer surface and imparting slipperiness, the thickness of the antifouling layer is preferably 30 nm or less, and more preferably 20 nm or less.

The arithmetic mean height _Sa2 of the surface of the antifouling layer 7 is preferably 2.5 nm or more, more preferably 2.8 nm or more, even more preferably 3.0 nm or more, and may be 3.5 nm or more, 3.7 nm or more, 3.9 nm or more, or 4.0 nm or more. The arithmetic mean height _Sa2 of the antifouling layer 7 is preferably 8 nm or less, more preferably 7.5 nm or less, even more preferably 7 nm or less, and may be 6.5 nm or less, 6 nm or less, or 5.5 nm or less.

Since the surface shape of the antifouling layer 7 reflects the surface shapes of the hard coat layer 11 and the antireflection layer 5 provided thereon, the larger the arithmetic mean height _Sa1 of the hard coat layer 11, the larger the arithmetic mean height _Sa2 of the antifouling layer 7 tends to be. In addition, if the antireflection layer 5 grows columnarly during deposition, the unevenness increases, and therefore the arithmetic mean height _Sa2 of the antifouling layer 7 tends to be larger.

[Usage of anti-reflection film]
Antireflection films are used by being disposed on the surface of image display devices such as liquid crystal displays, organic EL displays, etc. For example, by disposing an antireflection film on the viewing side surface of a panel including an image display medium such as a liquid crystal cell or an organic EL cell, reflection of external light can be reduced and visibility of the image display device can be improved.

The anti-reflection film of the present invention has excellent bending resistance, and even if it is held in a bent state with the anti-reflection layer-forming surface on the inside, cracks in the anti-reflection layer are unlikely to occur at the bent portion, so it can also be suitably used for foldable displays.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following specific examples.

[Preparation of hard coat film]
An organosilica sol ("MEK-ST-L" manufactured by Nissan Chemical Industries, Ltd., average primary particle size of silica particles: 50 nm, particle size distribution of silica particles: 30 nm to 130 nm, solid content 30 wt%) was added to an ultraviolet-curable acrylic resin composition (manufactured by DIC, product name "GRANDIC PC-1070") so that the amount of silica particles was 40 parts by weight relative to 100 parts by weight of the resin component, and mixed to prepare a composition for forming a hard coat layer.

The above composition was applied to one side of a triacetyl cellulose film (FUJITAC, manufactured by FUJIFILM Corporation) having a thickness of 80 μm so that the thickness after drying would be 4 μm, and dried for 3 minutes at 80° C. Thereafter, the coating layer was cured by irradiating it with ultraviolet light at an integrated light quantity of 200 mJ/cm ² using a high-pressure mercury lamp to form a hard coat layer.

[Example 1]
(Surface treatment of hard coat layer)
While the hard coat film was transported under a vacuum atmosphere of 0.5 Pa, an argon plasma treatment was performed on the surface of the hard coat layer at an effective power density of 0.014 W·min/m·cm ^2. The arithmetic mean height Sa ₁ of the hard coat layer after the argon plasma treatment was 4.9 nm.

(Formation of primer layer)
The hard coat film after the plasma treatment was introduced into a roll-to-toll sputtering deposition apparatus, and the pressure inside the chamber was reduced to 1×10 ⁻⁴ Pa. Then, while the film was running, argon gas and oxygen gas were introduced in a volume ratio of 98:2 so that the pressure became 0.5 Pa, and an ITO primer layer with a thickness of 2 nm was formed by sputtering under the conditions of a power source: MFAC and input power: 6 kW. For the formation of the ITO primer layer, a sintered target containing indium oxide and tin oxide in a weight ratio of 90:10 was used as the target material.

(Formation of Antireflection Layer)
Following the formation of the ITO primer layer, a first layer: a 14 nm Nb ₂ O ₅ layer (refractive index: 2.32), a second layer: a 40 nm SiO ₂ layer (refractive index: 1.46), a third layer: a 29 nm Nb ₂ O ₅ layer, and a fourth layer: a 94 nm SiO ₂ layer were sequentially formed by reactive sputtering to form an anti-reflection layer. For the formation of the Nb ₂ O ₅ layer (first and third layers), a Nb target was used, argon gas and oxygen gas were introduced in a volume ratio of 90:10 so that the pressure was 0.6 Pa, and sputtering was performed with an input power of 30 kW. For the formation of the SiO ₂ layer (second and fourth layers), a Si target was used, argon gas and oxygen gas were introduced in a volume ratio of 70:30 so that the pressure was 0.5 Pa, and sputtering was performed with an input power of 20 kW.

(Formation of antifouling layer)
A fluorine-based antifouling coating agent ("SHIN-ETSU SUBELYN KY1903-1" manufactured by Shin-Etsu Chemical Co., Ltd.) that had been dried and solidified was used as a deposition source to form an antifouling layer with a thickness of 7 nm on the antireflection layer by vacuum deposition at a heating temperature of 260°C.

[Example 2]
The discharge power during the plasma treatment was changed to an effective power density of 0.14 W·min/m·cm ^2. Otherwise, the plasma treatment of the hard coat layer, the formation of the antireflection layer, and the formation of the antifouling layer were performed in the same manner as in Example 1 to produce an antireflection film. The arithmetic mean height Sa ₁ of the hard coat layer after the argon plasma treatment was 5.7 nm.

[Comparative Example 1]
In forming the antireflection layer, the film thickness of each layer was changed as shown in Table 1. Otherwise, the plasma treatment of the hard coat layer, the formation of the antireflection layer, and the formation of the antifouling layer were carried out in the same manner as in Example 1 to prepare an antireflection film.

[Comparative Example 2]
In forming the antireflection layer, the pressure during deposition of the first and third Nb ₂ O ₅ layers was changed to 0.4 Pa. Otherwise, the plasma treatment of the hard coat layer, the formation of the antireflection layer, and the formation of the antifouling layer were performed in the same manner as in Example 1 to prepare an antireflection film.

[evaluation]
<Surface shape>
Using an atomic force microscope (AFM), the surface shapes of the hard coat layer and the antireflection film (antifouling layer) were measured under the following conditions, and the arithmetic mean surface height Sa was measured in accordance with ISO 25178.
Apparatus: Bruker Dimension 3100, Controller: Nanoscope V
Measurement mode: Tapping mode Cantilever: Si single crystal Measurement field of view: 1 μm x 1 μm

<Thickness and density of thin film>
The antireflection film was processed using a focused ion beam processing device ("FB2200" manufactured by Hitachi High-Technologies) to prepare a sample for cross-sectional observation, and the cross-section was observed using a field emission transmission electron microscope ("JEM-2800" manufactured by JEOL Ltd.) to measure the film thickness of the thin film constituting the antireflection layer. The film density of the thin film constituting the antireflection layer was measured by Rutherford backscattering (RBS) method. The film density was calculated using the film thickness obtained from the cross-sectional observation.

<Bending resistance test>
The anti-reflection film was cut into a size of 10 mm wide x 100 mm long, curved 180° so that the surface on the anti-reflection layer side was on the inside, and both ends in the length direction were attached to a spacer of thickness D so that the bending radius was constant at D/2, as shown in Figure 2. The sample was heated in an oven at 100 ° C for 30 minutes, then removed and visually confirmed for the presence or absence of cracks (opacity of the anti-reflection layer) in the curved portion. Evaluation was performed when the spacer thickness D was 9.2 mm, 7.8 mm, and 5.2 mm (bending radius D/2 was 4.6 mm, 3.9 mm, and 2.6 mm), and the minimum bending radius at which no cracks occurred was taken as the bending resistance radius.

Table 1 shows the effective power density of the plasma treatment of the hard coat layer of the antireflective films of the examples and comparative examples, the arithmetic mean height Sa ₁ after plasma treatment, the film thickness of each layer constituting the antireflective layer , the film density of the third Nb ₂ O ₅ layer, the arithmetic mean height Sa ₂ of the antifouling layer surface, and the bending resistance radius of the antireflective films.

In Comparative Example 1, in which the film thickness of the third layer was 95 nm, the bending radius was 4.6 mm, whereas Examples 1 and 2, in which the film thicknesses of the Nb ₂ O ₅ layers (first and third layers) were all 40 nm or less, had a smaller bending radius and better bending resistance than Comparative Example 1. In Examples 1 and 2, the film density of the third layer was smaller than that of Comparative Example 1, and it is believed that the decrease in film density contributed to the improvement of bending resistance.

In Comparative Example 2, the bending resistance was inferior even though the film thickness of the third layer was equivalent to that of Examples 1 and 2. It is considered that the bending resistance of Comparative Example 2 was lower than that of Examples 1 and 2 because the pressure during deposition of the _five Nb ₂ O layers was low and the film density was high.

In comparison between Example 1 and Example 2, Example 2, in which the power during the plasma treatment of the hard coat layer was high, had a smaller bending radius and better bending resistance. In Example 2, in which the discharge power was high, the surface unevenness (arithmetic mean height Sa ₁ ) of the hard coat layer was large, which promoted columnar growth and made it difficult for the film density to increase, which is thought to contribute to the improvement of bending resistance.

REFERENCE SIGNS LIST 1 Hard coat film 10 Transparent film substrate 11 Hard coat layer 3 Primer layer 5 Anti-reflection layer 51, 53 High refractive index layer (niobium oxide layer)
52, 54 Low refractive index layer (silicon oxide layer)
7 Antifouling layer 101 Antireflection film

Claims (10)

A hard coat film having a hard coat layer on one main surface of a foldable, flexible transparent resin film substrate; and an antireflection layer provided on the hard coat layer of the hard coat film,
The antireflection layer is composed of a total of four layers including two high refractive index layers and two low refractive index layers alternately laminated together,
the high refractive index layer is a thin film containing niobium oxide as a main component,
the high refractive index layer disposed farthest from the hard coat layer has a thickness of 40 nm or less and a film density of less than 4.47 g/ ^cm3 ;
Anti-reflective film. The anti-reflection film according to claim 1, wherein the two high refractive index layers are both thin films having a thickness of 40 nm or less and made mainly of niobium oxide. The anti-reflection film according to claim 1 or 2, wherein the arithmetic mean surface height of the anti-reflection layer is 2.5 nm or more. The anti-reflection film according to claim 1 or 2, comprising a primer layer made of an inorganic oxide between the hard coat layer and the anti-reflection layer. The anti-reflection film according to claim 1 or 2, wherein the hard coat layer contains a binder resin and fine particles having an average primary particle size of 10 to 100 nm. The anti-reflection film according to claim 1 or 2, comprising an anti-fouling layer on the anti-reflection layer. The anti-reflection film according to claim 6, wherein the arithmetic mean surface height of the antifouling layer is 2.5 nm or more. A method for producing the anti-reflection film according to claim 1 or 2, comprising the steps of:
The method for producing an antireflection film comprises forming the antireflection layer by a sputtering method. The method for producing an anti-reflection film according to claim 8, wherein the pressure when forming the high refractive index layer of the anti-reflection layer by sputtering is 0.5 Pa or more. An image display device in which the anti-reflection film according to claim 1 or 2 is disposed on the viewing side surface of an image display medium.

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