Composition for forming low-refractive-index layer, optical film, and image display device

Abstract

A composition for forming a low-refractive-index layer contains a polyfunctional (meth)acrylic monomer (A), a photopolymerization initiator (B), an antifouling additive (C), inorganic particles (D) having voids therein, a (meth)acrylic monomer (E) having a phosphate group, and a solvent (F), the polyfunctional (meth)acrylic monomer (A) having three or more (meth)acryloyl groups and one or more hydroxyl groups in each molecule, the content of the inorganic particles (D) having voids therein being 22-59 mass% of the total amount of solid components, and the content of the (meth)acrylic monomer (E) having a phosphate group being 0.5-10 mass% of the total amount of solid components.

Description

Composition for forming low refractive index layer, optical film and image display device

[0001] The present invention relates to a composition for forming a low refractive index layer used in the production of an optical film, an optical film, and an image display device.

[0002] Displays are required to have low reflectivity because the display quality deteriorates when external light is reflected on the display surface and mixed with the display light. Generally, an optical film having low reflectivity can be obtained by providing a low refractive index layer on the surface of a transparent plastic film substrate.

[0003] Meanwhile, in recent years, there has been a demand for low-reflectivity optical films for devices such as televisions, laptops, and tablets to have higher scratch resistance (steel wool resistance).

[0004] For example, Patent Document 1 discloses that by using a composition containing a (meth)acrylate compound having a first molecular weight, a (meth)acrylate compound having a second molecular weight greater than the first molecular weight, inorganic fine particles, and hollow particles, at least two layers can be formed by one coating, and an antireflection film having excellent interlayer interfacial adhesion and scratch resistance can be obtained.

[0005] Furthermore, as a film-forming technology, Patent Document 2 discloses that a film having excellent storage stability, a high water contact angle, and good dust-wipeability can be formed by using a film-forming coating liquid containing a polysiloxane having an organic group substituted with a fluorine atom on the side chain and a phosphate ester compound in which a hydroxyl group is bonded to a phosphorus atom.

[0006] Patent Publication No. 2014-529762 International Publication No. 2007 / 119805

[0007] However, optical films containing inorganic particles are prone to the inorganic particles falling off when subjected to impact, and the steel wool resistance is still insufficient, and further improvement is desired.

[0008] The present invention relates to a composition for forming a low refractive index layer that can provide a low refractive index layer that has excellent low reflectivity and also excellent steel wool resistance, and to an optical film and an image display device that include a low refractive index layer obtained from the composition.

[0009] The present invention relates to the following [1] to [3]. [1] A composition for forming a low refractive index layer, comprising a polyfunctional (meth)acrylic monomer (A), a photopolymerization initiator (B), an antifouling additive (C), inorganic particles (D) having internal voids, a (meth)acrylic monomer (E) having a phosphate group, and a solvent (F), wherein the polyfunctional (meth)acrylic monomer (A) has three or more (meth)acryloyl groups and one or more hydroxyl groups in one molecule, the inorganic particles (D) having internal voids account for 22 to 59 mass% of the total solid content, and the (meth)acrylic monomer (E) having a phosphate group accounts for 0.5 to 10 mass% of the total solid content. [2] An optical film comprising, on at least one surface of a transparent substrate, a hard coat layer and a low refractive index layer formed from the composition for forming a low refractive index layer according to [1], in this order. [3] An image display device comprising the optical film according to [2].

[0010] The composition for forming a low refractive index layer of the present invention has the excellent effect of being able to provide a low refractive index layer that has both low reflectivity and steel wool resistance.

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

[0012] The composition for forming a low refractive index layer of the present invention contains a polyfunctional (meth)acrylic monomer (A), a photopolymerization initiator (B), an antifouling additive (C), inorganic particles having internal voids (D), a (meth)acrylic monomer having a phosphate group (E), and a solvent (F). In this specification, the term "(meth)acrylic monomer" refers to an acrylic monomer, a methacrylic monomer, or both, and the term "(meth)acryloyl" refers to acryloyl, methacryloyl, or both.

[0013] The polyfunctional (meth)acrylic monomer (A) used in the present invention is characterized by having three or more (meth)acryloyl groups and one or more hydroxyl groups in one molecule, and does not have a phosphate group. The presence of multiple (meth)acryloyl groups forms a stronger crosslinked structure, which in turn improves the strength of the resulting coating. Furthermore, the presence of hydroxyl groups increases polarity, improving compatibility with inorganic particles. The number of (meth)acryloyl groups may be three or more from the viewpoint of improving layer strength, and the number of hydroxyl groups may be one or more from the viewpoint of improving steel wool resistance.

[0014] Examples of the polyfunctional (meth)acrylic monomer (A) include pentaerythritol tri(meth)acrylate, dipentaerythritol tri(meth)acrylate, ditrimethylolpropane tri(meth)acrylate, dipentaerythritol tetra(meth)acrylate, and dipentaerythritol penta(meth)acrylate. Among these, it is preferable to include at least one of a pentaerythritol-based (meth)acrylic monomer and a dipentaerythritol-based (meth)acrylic monomer. As long as the number of (meth)acryloyl groups is three or more, some of the (meth)acryloyl groups may be substituted with alkyl groups or ε-caprolactone.

[0015] Furthermore, urethane (meth)acrylates can also be used as the polyfunctional (meth)acrylic monomer (A). Examples of urethane (meth)acrylates include those obtained by reacting a polyester polyol with an isocyanate monomer or a prepolymer, and then reacting the resulting product with a (meth)acrylate monomer having a hydroxyl group.

[0016] 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.

[0017] These polyfunctional (meth)acrylic monomers may be used alone or in combination of two or more. These may be monomers in the composition, or may be partially polymerized oligomers. As long as the effects of the present invention can be obtained, a polyfunctional (meth)acrylic monomer having two (meth)acryloyl groups may be contained in the composition.

[0018] From the viewpoint of the strength of the resulting coating film, the content of the polyfunctional (meth)acrylic monomer (A) is preferably 30% by mass or more, more preferably 32% by mass or more, of the total amount of solid components, and is preferably 80% by mass or less, more preferably 70% by mass or less. In this specification, the total amount of solid components refers to the total content of all components other than the solvent (F).

[0019] The photopolymerization initiator (B) may be any one that triggers a polymerization reaction when irradiated with ultraviolet light, an electron beam, or the like, and examples thereof include 2,2-ethoxyacetophenone, 1-hydroxycyclohexyl phenyl ketone, dibenzoyl, benzoin, benzoin methyl ether, benzoin ethyl ether, p-chlorobenzophenone, p-methoxybenzophenone, Michler's ketone, acetophenone, 2-chlorothioxanthone, etc. These may be used alone or in combination of two or more.

[0020] The content of the photopolymerization initiator (B) is preferably 0.01% by mass or more, more preferably 0.1% by mass or more, even more preferably 0.5% by mass or more, and is preferably 20% by mass or less, more preferably 10% by mass or less, even more preferably 5% by mass or less, based on the total amount of solid components.

[0021] The antifouling additive (C) reduces the surface tension of the resulting coating film to prevent adhesion of dirt, and examples thereof include acrylic additives, silicone additives, fluorine-based additives, vinyl-based additives, fluorine-silicone additives, etc. These may be used alone or in combination of two or more.

[0022] From the viewpoint of exhibiting antifouling properties in harmony with the monomers, the content of the antifouling additive (C) is preferably 1 part by mass or more, more preferably 3 parts by mass or more, even more preferably 5 parts by mass or more, and is preferably 20 parts by mass or less, more preferably 15 parts by mass or less, even more preferably 12 parts by mass or less, relative to 100 parts by mass of the polyfunctional (meth)acrylic monomer (A).

[0023] The content of the antifouling additive (C) is preferably 0.01 mass% or more, more preferably 0.1 mass% or more, even more preferably 0.5 mass% or more, of the total amount of solid components, from the viewpoint of reducing the surface tension of the resulting coating, and is preferably 20 mass% or less, more preferably 10 mass% or less, even more preferably 5 mass% or less.

[0024] The inorganic particles (D) having internal voids have minute voids therein, and examples thereof include silica, alumina, zeolite, talc, kaolin, and diatomaceous earth. These may be used alone or in combination of two or more. These particles may also be surface-treated with a known surface treatment agent.

[0025] The average particle size of the inorganic particles (D) having voids therein is preferably 30 nm or more, more preferably 40 nm or more, and even more preferably 50 nm or more, from the viewpoint of the refractive index lowering ability of the low refractive index layer and the film thickness, and is preferably 150 nm or less, more preferably 140 nm or less, even more preferably 130 nm or less, even more preferably 120 nm or less, and even more preferably 110 nm or less. In this specification, the average particle size of the inorganic particles is the volume median particle size (D) at which the cumulative volume frequency calculated by volume fraction is 50% calculated from the smallest particle size. ５０ ) means

[0026] The content of the inorganic particles (D) having internal voids is 22% by mass or more, preferably 23% by mass or more, more preferably 24% by mass or more, of the total amount of solid components, and is 59% by mass or less, preferably 58% by mass or less, more preferably 57% by mass or less.

[0027] The (meth)acrylic monomer (E) having a phosphate group is characterized by having one or more phosphate groups in one molecule. In the present invention, in order to prevent the inorganic particles (D) having internal voids from falling off, attention is focused on adsorbing a resin component to the surface of the inorganic particles by a polar group, and in addition to the polyfunctional (meth)acrylic monomer (A) having one or more hydroxyl groups, a monomer having multiple hydroxyl groups is used from the viewpoint of obtaining stronger adsorption. It has been found that by using the (meth)acrylic monomer (E) having a phosphate group containing multiple hydroxyl groups, it is possible to strongly adsorb the inorganic particles (D) having internal voids while also forming a crosslinked structure with the polyfunctional (meth)acrylic monomer (A), thereby synergistically preventing the inorganic particles (D) having internal voids from falling off. The number of (meth)acryloyl groups may be one or more from the viewpoint of reactivity with the polyfunctional (meth)acrylic monomer (A), and the number of hydroxyl groups may be one or more from the viewpoint of compatibility with inorganic particles, and the hydroxyl groups derived from the phosphate groups may remain as they are or may be partially substituted.

[0028] Examples of the (meth)acrylic monomer (E) having a phosphate group include 2-(phosphonooxy)ethyl (meth)acrylate and 10-(meth)acryloyloxydecyl phosphate.

[0029] The content of the (meth)acrylic monomer (E) having a phosphate group is 0.5 mass% or more, preferably 0.8 mass% or more, more preferably 1.0 mass% or more, and 10 mass% or less, preferably 9.0 mass% or less, of the total amount of solid components, from the viewpoint of improving steel wool resistance.

[0030] Examples of the solvent (F) include ketone-based solvents such as acetone, methyl ethyl ketone (MEK), and methyl isobutyl ketone; alcohol-based solvents such as ethanol, methanol, isopropyl alcohol (IPA), and isobutanol; ether-based solvents such as ethylene glycol dimethyl ether and propylene glycol monomethyl ether; ester-based solvents such as ethyl acetate, propylene glycol monomethyl ether acetate, and 2-ethoxyethyl acetate; and aromatic hydrocarbon-based solvents such as toluene. These may be used alone or in combination of two or more.

[0031] In addition to the above-mentioned components, the composition for forming a low refractive index layer of the present invention may contain additives such as a dye, a leveling agent, an oil repellent, a water repellent, an antifingerprint agent, an antistatic agent, an ultraviolet absorber, an infrared absorber, an antifoaming agent, an antioxidant, a light stabilizer, a photosensitizer, a conductive material, a refractive index adjuster, an adhesion improver, a polymerization inhibitor, etc. The contents of these can be appropriately adjusted according to known techniques.

[0032] The low refractive index layer of the present invention can be produced by applying the composition for forming a low refractive index layer of the present invention onto a substrate (e.g., a transparent substrate) using a known coating method (e.g., a spin coater, a roll coater, a reverse roll coater, a gravure coater, a microgravure coater, a knife coater, a bar coater, a wire bar coater, a die coater, a dip coater, a spray coater, an applicator, etc.), drying, and photo-curing.

[0033] The refractive index of the low refractive index layer is preferably 1.25 to 1.45, more preferably 1.28 to 1.42.

[0034] The thickness of the low refractive index layer is not particularly limited and may be, for example, 50 nm to 200 nm.

[0035] The present invention also provides an optical film comprising a hard coat layer on at least one surface of a transparent substrate, and the low refractive index layer of the present invention on the hard coat layer in this order.

[0036] The transparent substrate may be a resin substrate or an inorganic compound substrate. Examples of materials for forming the resin substrate include triacetyl cellulose-based substrates, polyethylene naphthalate-based substrates, polyethylene terephthalate-based substrates, cycloolefin polymer-based substrates, polycarbonate-based substrates, acrylic-based substrates, polyimide-based substrates, and polyamide-based substrates. Examples of materials for forming the inorganic compound substrate include silicon dioxide, silicon oxynitride, and silicon nitride. These materials may be used alone or in combination of two or more. The transparent substrate may be prepared using the above-mentioned materials according to a known method, or a commercially available product may be used. Surface modification treatment may be performed to improve adhesion to other layers. Examples of surface modification treatment include alkali treatment, corona treatment, plasma treatment, sputtering treatment, application of surfactants or silane coupling agents, and Si vapor deposition.

[0037] The thickness of the transparent substrate is not particularly limited and may be, for example, 10 μm to 200 μm.

[0038] The hard coat layer is not particularly limited, and known materials can be used. It may be prepared according to a known method using a material containing a monofunctional, difunctional, trifunctional or higher functional (meth)acrylate, urethane (meth)acrylate, etc. For example, the hard coat layer can be formed by applying a hard coat layer composition containing the material to the surface of the transparent substrate and curing the coating film.

[0039] The thickness of the hard coat layer is not particularly limited and may be, for example, 2 μm to 15 μm.

[0040] The method for forming the low refractive index layer of the present invention on a hard coat layer is not particularly limited as long as the composition for forming a low refractive index layer of the present invention is used. For example, the low refractive index layer can be produced by forming a hard coat layer on a transparent substrate, and then forming a film of the composition for forming a low refractive index layer of the present invention by a known coating method.

[0041] In addition to the above-described layers, the optical film of the present invention may include other layers such as an adhesive layer, a colored layer, a high refractive index layer, a medium refractive index layer, an antistatic layer, an electromagnetic wave blocking layer, an infrared absorbing layer, an ultraviolet absorbing layer, a color correction layer, etc. For example, other layers may be included between the hard coat layer and the low refractive index layer of the present invention or between the transparent substrate and the hard coat layer.

[0042] The surface reflection characteristics (low reflectivity) of the optical film of the present invention are, for example, such that the spectral reflectance at an incident angle of 5° is preferably less than 2.0%, more preferably less than 1.5%. The lower limit is not particularly limited and may be about 0.1%. In this specification, the spectral reflectance is measured with a spectrophotometer after antireflection treatment is performed by applying a matte black dye to the surface of the transparent substrate on which the hard coat layer and the low refractive index layer are not formed.

[0043] The steel wool resistance of the optical film of the present invention can be evaluated by bringing steel wool into contact with the low refractive index layer on the surface of the film, and for example, an abrasion and friction tester can be used.

[0044] The phosphorus atom content of the optical film of the present invention is preferably 0.01 atm% or more, more preferably 0.05 atm% or more, even more preferably 0.1 atm% or more, and is preferably less than 1.5 atm%, more preferably 1.4 atm% or less, even more preferably 1.3 atm% or less. In this specification, the phosphorus atom content is measured by energy dispersive X-ray spectroscopy (EDS elemental analysis method).

[0045] The thickness of the optical film of the present invention is not particularly limited and may be, for example, 10 μm to 220 μm.

[0046] The present invention also provides an image display device comprising the optical film of the present invention, which may be used in televisions, monitors, mobile phones, portable game machines, personal digital assistants, personal computers, electronic books, video cameras, digital still cameras, head-mounted displays, navigation systems, audio playback devices (car audio, digital audio players, etc.), copiers, facsimiles, printers, multi-function printers, vending machines, automated teller machines (ATMs), personal authentication devices, optical communication devices, IC cards, etc.

[0047] The present invention will be specifically explained below with reference to examples, but the present invention is not limited to these examples in any way.

[0048] <Transparent substrate> A triacetyl cellulose film having a thickness of 60 μm was used.

[0049] <Hard Coat Layer> A composition for a hard coat layer was prepared by diluting 50 parts by mass of pentaerythritol triacrylate, 50 parts by mass of dipentaerythritol hexaacrylate, 1.5 parts by mass of diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, and 0.1 parts by mass of an acrylic polymer (manufactured by DIC, PC4300) in a solvent prepared by mixing acetone and propylene glycol monomethyl ether in a 70 / 30 (mass ratio) mixture, followed by stirring.

[0050] The obtained composition for hard coat layer was applied to a transparent substrate using a wire bar coater to form a coating film, which was then dried at 60°C for 60 seconds and then exposed to an exposure dose of 100 mJ / cm using a conveyor-type ultraviolet curing device in a nitrogen atmosphere. ２ A hard coat layer having a thickness of 5 μm was formed on the transparent substrate by irradiating the transparent substrate with ultraviolet light at 4000 kJ / cm.

[0051] Examples 1 to 7 and Comparative Examples 1 to 6 The composition raw materials shown in Table 1 or 2 were added to and mixed with a mixed solvent of propylene glycol monomethyl ether acetate / propylene glycol monomethyl ether / methyl isobutyl ketone / isopropyl alcohol (mass ratio 30 / 10 / 50 / 10) to prepare compositions for forming low refractive index layers (diluted so that the layer thickness after curing would be 100 nm). The composition was applied to the hard coat layer formed on the transparent substrate obtained above using a wire bar coater to form a coating film, which was then dried under conditions of 60°C and 60 seconds, and then cured in a nitrogen atmosphere using a conveyor-type UV curing device at an exposure dose of 300 mJ / cm. ２ A low refractive index layer was formed by irradiating the film with ultraviolet light at 65.1 μm, thereby obtaining an optical film (film thickness: 65.1 μm).

[0052] Test Example 1 [Low Reflectivity] The spectral reflectivity of the surface of the low refractive index layer of the optical film was measured at an incident angle of 5° using a spectrophotometer (U-4100, manufactured by Hitachi High-Tech Science), and the low reflectivity was evaluated according to the following evaluation criteria. The results are shown in Table 1. For the measurement, the surface of the triacetyl cellulose film serving as the transparent substrate, on which the hard coat layer and low refractive index layer were not formed, was treated with a matte black finish. The results are shown in Tables 1 and 2.

[0053] [Evaluation criteria for low reflectivity] ◎: Reflectance less than 1.5% ○: Reflectance 1.5% or more but less than 2.0% △: Reflectance 2.0% or more but less than 2.5% ×: Reflectance 2.5% or more

[0054] Test Example 2 [Steel Wool Resistance] A rubbing test was carried out on the surface of the low refractive index layer of the optical film under the following test conditions using an abrasion and friction tester (manufactured by Tester Sangyo, AB-301), and the steel wool resistance was evaluated according to the following evaluation criteria. The results are shown in Tables 1 and 2. (Test Conditions) Rubbing material: Bonstar #0000 (manufactured by Nippon Steel Wool) Stroke: 12 cm Rubbing speed: 30 cpm Load: 300 g / cm ２ Number of rubs: 30 times

[0055] [Evaluation criteria for steel wool resistance] After the test, the surface of the low refractive index layer was visually observed and the number of scratches formed was counted. ◎: 0 to 5 scratches ○: 6 to 10 scratches △: 11 to 20 scratches ×: 21 or more scratches

[0056] Test Example 3 [Phosphorus Atom Content] The surface of the low refractive index layer of the optical film was subjected to platinum sputtering treatment, and the phosphorus atom content (atm %) was quantified by energy dispersive X-ray spectroscopy using a scanning electron microscope SU-8020 (manufactured by Hitachi High-Technologies Corporation) at 10 kV output for 5 minutes. The results are shown in Tables 1 and 2.

[0057]

[0058]

[0059] The above results show that, compared with Comparative Examples 1 to 6, Examples 1 to 7 exhibit low reflectivity while also exhibiting excellent steel wool resistance. When the polyfunctional monomer has one acryloyl group (Comparative Example 1) or no hydroxyl group (Comparative Example 2), steel wool resistance is poor. A low content of inorganic particles with internal voids results in insufficient low reflectivity (Comparative Example 3), while a high content results in poor steel wool resistance (Comparative Example 4). Furthermore, Comparative Example 5 indicates that a low content of phosphate group-containing monomers fails to retain inorganic particles with internal voids, resulting in insufficient steel wool resistance. Comparative Example 6 also suggests that a high content of phosphate group-containing monomers fails to adequately form a crosslinked structure, reducing the hardness of the layer itself and worsening steel wool resistance.

[0060] On the other hand, when a polyfunctional (meth)acrylic monomer (A) having three or more (meth)acryloyl groups and one or more hydroxyl groups in one molecule is used, even if the contents of the (A) component and the inorganic particles (D) having voids therein vary, as long as the content of the phosphate group-containing (meth)acrylic monomer (E) is within the range of 0.5 to 10 mass% of the total solid content, it is found that both low reflectivity and steel wool resistance can be achieved (Examples 1 to 4).Furthermore, when dipentaerythritol pentaacrylate having five (meth)acryloyl groups and one hydroxyl group in one molecule is used as the polyfunctional (meth)acrylic monomer (A), it is found that both low reflectivity and steel wool resistance can be achieved similarly (Example 5).

[0061] Furthermore, Example 6, which used 2-methacryloyloxyethyl acid phosphate (P-2) having one phosphate group (one hydroxyl group in the phosphate group) and two (meth)acryloyl groups in one molecule, was able to achieve both low reflectivity and steel wool resistance, similar to Example 3, which used 2-methacryloyloxyethyl acid phosphate (P-1) having one phosphate group (two hydroxyl groups in the phosphate group) and one (meth)acryloyl group in one molecule. Furthermore, Example 7 shows that even if the chain structure between the phosphate group and the (meth)acryloyl group is large, as long as there is one phosphate group (two hydroxyl groups in the phosphate group) and one (meth)acryloyl group, both low reflectivity and steel wool resistance can be achieved. These results show that even if the (meth)acrylic monomer (E) having a phosphate group has one (meth)acryloyl group per molecule, as long as it has one or more phosphate groups, it forms a crosslinked structure with the polyfunctional (meth)acrylic monomer (A) and is effective in suppressing the shedding of the inorganic particles (D). The (meth)acrylic monomer (E) having a phosphate group preferably has one or more (meth)acryloyl groups and one or more phosphate groups per molecule, and more preferably has one or two (meth)acryloyl groups and one or more phosphate groups.

[0062] The composition for forming a low refractive index layer of the present invention can be used to form a low refractive index layer in an optical film used in an image display device, and is particularly suitable for use as a protective film for a folding display device.

[0063] REFERENCE SIGNS LIST 1 transparent substrate 2 hard coat layer 3 low refractive index layer 10 optical film

Claims

1. A composition for forming a low refractive index layer, comprising a polyfunctional (meth)acrylic monomer (A), a photopolymerization initiator (B), an antifouling additive (C), inorganic particles having internal voids (D), a (meth)acrylic monomer having a phosphate group (E), and a solvent (F), wherein the polyfunctional (meth)acrylic monomer (A) has three or more (meth)acryloyl groups and one or more hydroxyl groups in one molecule, the content of the inorganic particles having internal voids (D) is 22 to 59% by mass of the total solid components, and the content of the (meth)acrylic monomer having a phosphate group (E) is 0.5 to 10% by mass of the total solid components.

2. The composition for forming a low refractive index layer according to claim 1, wherein the polyfunctional (meth)acrylic monomer (A) comprises at least one of a pentaerythritol-based (meth)acrylic monomer and a dipentaerythritol-based (meth)acrylic monomer.

3. The composition for forming a low refractive index layer according to claim 1, wherein the content of the polyfunctional (meth)acrylic monomer (A) is 30 to 70% by mass of the total amount of solid components.

4. The composition for forming a low refractive index layer according to claim 1, wherein the inorganic particles (D) having voids therein are silica particles.

5. The composition for forming a low refractive index layer according to claim 1, wherein the inorganic particles (D) having voids therein have an average particle size within the range of 30 to 150 nm.

6. A composition for forming a low refractive index layer according to claim 1, wherein the antifouling additive (C) is one or more compounds selected from the group consisting of acrylic compounds, vinyl compounds, silicone compounds, fluorine compounds, and fluorine silicone compounds.

7. The composition for forming a low refractive index layer according to claim 1, wherein the content of the antifouling additive (C) is 0.01 to 20% by mass of the total amount of solid components.

8. A composition for forming a low refractive index layer according to claim 1, wherein the (meth)acrylic monomer (E) having a phosphate group has one or more (meth)acryloyl groups and one or more phosphate groups in one molecule.

9. An optical film comprising a transparent substrate and, on at least one surface thereof, a hard coat layer and a low refractive index layer formed from the composition for forming a low refractive index layer according to any one of claims 2 to 8, in this order.

10. An optical film comprising a transparent substrate and a hard coat layer and a low refractive index layer formed from the composition for forming a low refractive index layer according to claim 1, in this order, on at least one surface of the substrate.

11. The optical film according to claim 10, wherein the phosphorus atom content calculated by energy dispersive X-ray spectroscopy (EDS elemental analysis) is less than 1.5 atomic %.

12. The optical film according to claim 10, which has a spectral reflectance of less than 2.0% at an incident angle of 5°.

13. The optical film according to claim 10, wherein the transparent substrate is selected from the group consisting of triacetyl cellulose-based substrates, polyethylene naphthalate-based substrates, polyethylene terephthalate-based substrates, cycloolefin polymer-based substrates, polycarbonate-based substrates, acrylic-based substrates, polyimide-based substrates, and polyamide-based substrates.

14. An image display device comprising the optical film according to claim 9.

15. An image display device comprising the optical film according to any one of claims 10 to 13.

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