Optical film and image display device

Abstract

The present invention provides: an optical film which uses a film made from a polyester resin as a base material, is thin, and is excellent in strength and flatness; and an image display device using the same. In the optical film in which a surface treatment layer is formed on a base material film, the base material film is a polyester composite material having an average film thickness of 13-60 μm inclusive, the absolute value of the difference between heat shrinkage rates of the base material film in the MD direction and the TD direction is 1.3 or less, the average film thickness of the surface treatment layer is 2.0-15.0 μm inclusive, and a film thickness variation ratio Vr of the surface treatment layer defined by formula (1) is 15% or less.　Vr = (Vmax / Tavg) × 100 (1)

Description

Optical films and image display devices

[0001] This disclosure relates to an optical film and an image display device equipped therewith.

[0002] Polyester films are sometimes used as substrates for optical films placed on the surface of image display devices such as televisions and smartphones, from the viewpoint of low cost and moisture resistance (Patent Documents 1 and 2). Furthermore, such optical films are desired to be thin from the viewpoint of cost and weight reduction.

[0003] Japanese Patent Publication No. 2014-149320 Japanese Patent Publication No. 2020-192812

[0004] When the substrate is thinned, the phase difference (in-plane retardation) of the substrate decreases, suppressing rainbow-like unevenness originating from the substrate and improving visibility. Furthermore, the substrate becomes lighter, contributing to the weight reduction of the image display device. However, thinning reduces the strength of the substrate, and in particular, when using polyester-based films, insufficient strength may result, potentially causing the optical film to fail to withstand the operating environment.

[0005] Generally, the strength of a substrate is enhanced by laminating a surface treatment layer (hard coat layer) onto its surface. However, the formation of this surface treatment layer involves heating and film curing processes. Furthermore, polyester films used as substrates exhibit a biased in-plane orientation due to stretching, resulting in anisotropic thermal behavior. When a surface treatment layer is formed on such a substrate, the substrate can shrink unevenly due to the effects of the heat curing of the surface treatment layer, sometimes leading to poor surface properties and impaired flatness in the optical film. Optical films with impaired flatness are undesirable because they reduce the visibility of image display devices.

[0006] This disclosure has been made in view of the above-mentioned problems, and aims to provide an optical film that is thin, has excellent strength and flatness, and an image display device using the same, using a film made of polyester resin as a base material.

[0007] One aspect of this disclosure for solving the above problems is an optical film having a surface treatment layer formed on a base film, wherein the base film is a polyester composite material with an average film thickness of 13 μm or more and 60 μm or less, the absolute value of the difference between the thermal shrinkage rate in the MD direction and the thermal shrinkage rate in the TD direction of the base film is 1.3 or less, the average film thickness of the surface treatment layer is 2.0 μm or more and 15.0 μm or less, and the film thickness variation ratio Vr of the surface treatment layer, as defined by the following formula (1), is 15% or less. ｍａｘ / T ａｖｇ ) × 100 (1) Here, T ａｖｇ : The average value of the surface treatment layer thickness measured at any 15 points on the optical film, V ｍａｘ : Each measured value of the surface treatment layer thickness measured at any 15 points, and the average value T ａｖｇ This is the maximum absolute value of the difference between [the given value] and [the given value].

[0008] Another aspect of this disclosure is an image display device comprising an image display panel and the optical film provided on the front surface of the image display panel.

[0009] According to this disclosure, it is possible to provide an optical film that is thin, has excellent strength and flatness, and an image display device using the same, using a film made of polyester resin as a base material.

[0010] Figure 1 is a cross-sectional view showing the schematic configuration of an optical film according to an embodiment. Figure 2 is a diagram illustrating deformation of the base film that occurs when the difference in thermal shrinkage rate within the base film surface is 1.3 or more. Figure 3 is a diagram illustrating shrinkage of the surface treatment layer and the base film that occurs when the film thickness variation ratio Vr of the surface treatment layer exceeds 15%. Figure 4 is a diagram illustrating shrinkage of the surface treatment layer and the base film that occurs when there is a large bias in the amount of shrinkage within the base film surface and the film thickness variation ratio Vr of the surface treatment layer exceeds 15%.

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

[0012] (Optical Film) The optical film 1 has a surface treatment layer 3 on one side of a base film 2, and is provided on the front surface of the image display panel of an image display device as a component for protecting the polarizer. Examples of image display devices include smartphones, tablets, laptops, monitors, and televisions.

[0013] When the optical film 1 is used in large image display devices such as televisions, it is preferable that the optical film 1 be lightweight in order to improve handling during the manufacturing process. Furthermore, when it is used in mobile devices such as smartphones and tablets, it is desirable to make the optical film 1 lighter in order to lighten the device itself. From the viewpoint of such weight reduction, it is preferable that the optical film 1 be thin, and in this embodiment, the average film thickness of the optical film 1 (total of the base film 2 and the surface treatment layer 3) is set to 80 μm or less. The average film thickness is the average of the film thickness values ​​at different points on the optical film 1, and can be, for example, the average value of the film thickness measured at any 15 different points on the optical film 1.

[0014] Furthermore, it is undesirable for the visibility of the image display device to decrease due to the provision of the optical film 1. For example, if the flatness of the surface of the optical film 1 is impaired, distortion will occur in the displayed image, causing a decrease in visibility. However, as described above, when a surface treatment layer 3 is formed on a thin base film 2 for the purpose of reinforcing its strength, the flatness of the surface of the optical film 1 may be impaired due to shrinkage of the surface treatment layer 3 during curing and thermal shrinkage of the base film 2 caused by the heat treatment during curing. In particular, when a film having an anisotropic in-plane orientation, such as a polyester resin, is used as the base film 2, the base film 2 exhibits anisotropic behavior thermally as well, which can lead to surface defects based on the anisotropic thermal behavior of the base film 2, and impair the flatness of the surface of the optical film 1. Therefore, in this embodiment, the characteristics of the base film 2 and the surface treatment layer 3 constituting the optical film 1 are optimized to suppress surface defects of the optical film 1 and ensure flatness.

[0015] (Base Film)The base film 2 is the substrate of the optical film 1, and a material with excellent transparency and visible light transmittance is used. From the viewpoints of low cost and moisture permeability resistance, it is preferable to use a polyester-based film as the base film 2. In particular, it is preferably a polyester composite material obtained by laminating different first and second polyester-based resins and biaxially stretching them. The first and second polyester-based resins may be composed of a single resin or may be composed of a mixture of different resins. In particular, in order to reduce the retardation of the base film 2 described later, a configuration combining an amorphous polyester and a crystalline polyester is also preferable. In this case, the amorphous polyester causes a decrease in mechanical strength simultaneously with the decrease in retardation. Therefore, it is preferable to use the amorphous polyester in combination with the crystalline polyester and the surface treatment layer described later.

[0016] As the base film 2, it is preferable to use a low retardation film in which the in-plane retardation Re defined by the following formula (2) is 20 nm or more and 400 nm or less, and the thickness direction retardation Rth' defined by the following formula (3) is 0 nm or more and 1500 nm or less.Re = (n ｘ - n ｙ ) × d (2)Rth' = (n ｘ - n ｚ ) × d (3) Here, n ｘ : Refractive index in the slow axis direction (direction with a large refractive index) of the base film n ｙ : Refractive index in the fast axis direction of the base film n<000001Since the thickness of the base film 2 can be reduced, it is preferable that the in-plane phase difference Re and the thickness direction phase difference Rth' be small. Furthermore, if the upper limit of the in-plane phase difference Re of the base film 2 is 400 nm and the upper limit of the thickness direction phase difference Rth' is 1500 nm, the occurrence of rainbow unevenness that occurs when an image display device is constructed can be suppressed. On the other hand, if the lower limit of the in-plane phase difference Re is 20 nm, it is possible to suppress the base film 2 from being too thin, which would reduce handling performance or result in insufficient surface hardness when the optical film 1 is constructed. In addition, the condition of an in-plane phase difference Re of 20 nm or more is easy to achieve even with films using polyester resins, which reduces the difficulty of material design and suppresses high costs, making it preferable. Thus, it is preferable that the base film 2 be thin from the viewpoint of thinning the optical film 1 and suppressing rainbow unevenness, and the upper limit of the average film thickness of the base film 2 is preferably 60 μm. On the other hand, the lower limit of the average film thickness of the base film 2 is preferably 13 μm. By setting the lower limit to 13 μm, it is possible to suppress the decrease in handling performance due to excessive thinness, the decrease in surface hardness of the optical film 1, and the increase in cost. The average film thickness is the average of the film thickness values ​​at different locations on the base film 2, and can be, for example, the average value of the film thickness measured at 15 different arbitrary points.

[0018] The base film 2, composed of a polyester film, shrinks due to the heat generated during the curing process when forming the surface treatment layer 3. At this time, a smaller bias in the amount of shrinkage within the two surfaces of the base film is preferable to minimize the impairment of the flatness of the optical film 1. Therefore, it is preferable that the thermal shrinkage rate within the two surfaces of the base film be uniform. As an indicator of the uniformity of thermal shrinkage within the two surfaces of the base film, the absolute value of the difference between the thermal shrinkage rate in the MD direction and the thermal shrinkage rate in the TD direction of the base film 2 (hereinafter referred to as the thermal shrinkage rate difference) can be used. If the thermal shrinkage rate difference is 1.3 or less, the bias in the amount of shrinkage within the two surfaces of the base film caused by the heat curing process of the surface treatment layer 3 is small, and the impairment of the flatness of the optical film 1 can be suppressed. Note that the thermal shrinkage rate is the average of the thermal shrinkage rates at different locations on the base film 2, and can be, for example, the average value of the thermal shrinkage rates measured at 10 different arbitrary points.

[0019] (Surface Treatment Layer) The surface treatment layer 3 is a functional layer that coats the base film 2 which has been thinned and has reduced strength, and imparts hardness to the optical film 1. The surface treatment layer 3 can be formed by applying a coating liquid containing an active energy ray curable resin and curing it. Specifically, the surface treatment layer 3 is a hard coat layer (HC layer), a low reflection layer with a hard coat (HC-LR layer), an antiglare (AG) layer having antiglare properties, an antiglare low reflection (AGLR) layer, etc.

[0020] The upper limit of the film thickness variation ratio Vr defined by the following formula (1) for the surface treatment layer 3 is 15%. Vr = (V ｍａｘ / T ａｖｇ ) × 100 (1) Here, T ａｖｇ : The average value of the film thickness of the surface treatment layer 3 measured at any 15 points on the optical film 1, V ｍａｘ : The maximum value of the absolute value of the difference between each measured value of the film thickness of the surface treatment layer 3 measured at the above-mentioned arbitrary 15 points and the average value T ａｖｇ (the maximum value of the film thickness variation with respect to the average film thickness).

[0021] When the upper limit of the film thickness variation ratio Vr of the surface treatment layer 3 is set to 15%, the variation in the film thickness of the surface treatment layer 3 can be suppressed, and the flatness of the optical film 1 can be prevented from being impaired.

[0022] Also, the lower limit of the average film thickness of the surface treatment layer 3 is preferably 2.0 μm, and the upper limit is preferably 15.0 μm. The average film thickness is the average of the film thickness values at different locations of the surface treatment layer 3. For example, it can be the average value of the film thickness of the surface treatment layer 3 measured at any 15 different points (T ａｖｇ as described above). When the lower limit of the average film thickness of the surface treatment layer 3 is 2.0 μm, the surface treatment layer 3 can obtain sufficient surface hardness (pencil hardness) and impart sufficient strength to the optical film 1. Also, when the upper limit of the average film thickness is 15.0 μm, the shrinkage during curing of the surface treatment layer 3 can be suppressed, and the flatness of the surface of the optical film 1 can be prevented from being impaired. The thickness of the surface treatment layer 3 can be obtained, for example, by cutting out a cross-section of the optical film 1 with a microtome and measuring it with an optical microscope.

[0023] Furthermore, it is preferable that the surface treatment layer 3 contains organic or inorganic particles. The particles in the surface treatment layer 3 act as spacers during the curing of the surface treatment layer 3 and function to suppress the shrinkage of the surface treatment layer 3. Therefore, it is possible to suppress the formation of irregularities on the optical film surface of the surface treatment layer 3.

[0024] If the surface treatment layer 3 contains particles, the haze of the optical film 1 increases with the particle content. When the haze value is used as an indicator of particle content, the lower limit of the haze of the optical film 1 is preferably 0.8%. This haze can be the average value of the haze measured at any 15 different points on the surface treatment layer 3.

[0025] As organic particles, resin particles made from light-transmitting resin materials such as acrylic resin, polystyrene resin, styrene-(meth)acrylic acid ester copolymer, polyethylene resin, epoxy resin, silicone resin, polyvinylidene fluoride, and polyfluoroethylene resin can be used. The lower limit of the average particle size of the organic particles is preferably 0.5 μm, and the upper limit is preferably 10.0 μm.

[0026] As inorganic particles, silica particles, metal oxide particles, and various mineral particles can be used. As silica particles, for example, colloidal silica and silica particles surface-modified with reactive functional groups such as (meth)acryloyl groups can be used. As metal oxide particles, for example, alumina, zinc oxide, tin oxide, antimony oxide, indium oxide, titania, and zirconia can be used. As mineral particles, for example, mica, synthetic mica, vermiculite, montmorillonite, iron montmorillonite, bentonite, beidelite, saponite, hectorite, stevensite, nontronite, magadiite, islarite, kanemite, layered titanate, smectite, and synthetic smectite can be used. Mineral particles may be natural products or synthetic products (including substituted products and derivatives), or mixtures of both may be used. Among mineral particles, layered organic clay is more preferred. Layered organic clay refers to a material in which organic onium ions have been introduced between the layers of a swellable clay. The organic onium ions are not limited as long as they can be organicated by utilizing the cation exchange properties of the swelling clay. When layered organic clay minerals are used as mineral particles, the synthetic smectite described above can be suitably used. The inorganic particles are nanoparticles, and the lower limit of the average particle size of the inorganic particles is preferably 10 nm, and the upper limit is preferably 200 nm.

[0027] Since the surface treatment layer 3 is a layer that imparts strength to the base film 2, high hardness is required. Preferably, the pencil hardness of the surface treatment layer 3 is, for example, H or higher. By providing the surface treatment layer 3, an optical film 1 that can withstand the usage environment can be constructed even when a low-strength polyester-based film is used as the base film 2, or when the strength of the base film 2 is further reduced by thinning.

[0028] The surface treatment layer 3 preferably has a nanoindentation hardness higher than 0.30 GPa. A value higher than 0.30 GPa increases the strength of the base film 2 and improves its resistance to the usage environment. From the viewpoint of further improving resistance to the usage environment, the nanoindentation hardness of the surface treatment layer 3 is more preferably 0.31 GPa or higher, 0.32 GPa or higher, 0.33 GPa or higher, 0.35 GPa or higher, or 0.40 GPa or higher. Furthermore, if the nanoindentation hardness of the surface treatment layer 3 is too high, excessive film shrinkage may occur, affecting flatness, which may increase the difficulty of material design. For this reason, the nanoindentation hardness of the surface treatment layer 3 is preferably 0.80 GPa or lower, more preferably 0.60 GPa or lower, even more preferably 0.55 GPa or lower, and even more preferably 0.40 GPa. In this specification, nanoindentation hardness refers to the indentation hardness measured by the nanoindentation method in accordance with ISO 14577-1:2015.

[0029] To increase the nanoindentation hardness of the surface treatment layer 3 to more than 0.30 GPa, a resin composition containing an active energy ray curable compound may be used. The active energy ray curable compound is a resin that polymerizes and hardens upon irradiation with active energy rays such as ultraviolet light or electron beams, and 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.

[0030] Examples of monofunctional (meth)acrylate monomers 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. Lilate, 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 Phenoxy(meth)acrylate modified with ethylene oxide, nonylphenol(meth)acrylate, ethylene oxide modified nonylphenol(meth)acrylate, propylene oxide modified nonylphenol(meth)acrylate, methoxydiethylene glycol(meth)acrylate, methoxypolyethylene glycol(meth)acrylate, methoxypropylene glycol(meth)acrylate, 2-(meth)acryloyloxyethyl-2-hydroxypropyl phthalate, 2-hydroxy-3-phenoxypropyl(meth)acrylate, 2-(meth) Acryloyloxyethyl 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 methacrylate, which has a monovalent mono(meth)acrylate derived from adamantanediol.

[0031] Examples of difunctional (meth)acrylate monomers 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.

[0032] Examples of trifunctional (meth)acrylate monomers 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 monomers such as pentaerythritol tri(meth)acrylate, dipentaerythritol tri(meth)acrylate, and ditrimethylolpropane tri(meth)acrylate. Examples include polyfunctional (meth)acrylate monomers 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 monomers in which some of these (meth)acrylates are substituted with alkyl groups or ε-caprolactone.

[0033] 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.

[0034] 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.

[0035] These resin compositions exhibit shrinkage during curing. While various studies have been conducted on this property, it is mainly presumed to be due to a change in the bond species when monomers, existing as multiple molecules, are converted into a single molecule during polymerization. For example, see the "19th Polymer Materials Forum of the Society of Polymer Science, Japan" (November 17, 2010 https: / / www.ooc.co.jp / research / conference / pdf / 2010_02.pdf). The shrinkage characteristics of this surface treatment layer 3 during curing affect the flatness of the optical film 1, as will be explained below.

[0036] Figures 2 to 4 will be used to explain the cause of the loss of flatness on the surface of the optical film 1 during the curing of the surface treatment layer 3. Figure 2 illustrates the deformation of the base film that occurs when there is a large bias in the amount of shrinkage within the base film surface. Figure 3 illustrates the shrinkage of the surface treatment layer and the base film that occurs when the film thickness variation ratio Vr of the surface treatment layer exceeds 15%. Figure 4 illustrates the shrinkage of the surface treatment layer and the base film that occurs when there is a large bias in the amount of shrinkage within the base film surface and the film thickness variation ratio Vr of the surface treatment layer exceeds 15%.

[0037] Figure 2 shows a case where the film thickness of the surface treatment layer 3 is uniform (film thickness variation ratio Vr is 15% or less), but there is a large bias in the thermal shrinkage rate within the two surfaces of the base film (thermal shrinkage rate difference greater than 1.3). Within the two surfaces of the base film, there are areas with low thermal shrinkage rate (hatched areas in Figure 2) and areas with high thermal shrinkage rate (shaded areas in Figure 2). When the surface treatment layer 3 is cured in this state, the surface treatment layer 3 shrinks uniformly (left panel of Figure 2), but in the base film 2, a difference in shrinkage occurs between areas with high and low thermal shrinkage rate, impairing the flatness of the base film 2. This non-uniformity of shrinkage in the base film 2 manifests as irregularities on the surface of the surface treatment layer 3, impairing the flatness of the optical film 1 (right panel of Figure 2).

[0038] In the left diagram of Figure 3, the difference in thermal shrinkage rate between the two surfaces of the base film is small (the difference in thermal shrinkage rate is 1.3 or less), but the film thickness of the surface treatment layer 3 is non-uniform (film thickness variation ratio Vr is greater than 15%). In this case, there are areas where the film thickness of the surface treatment layer 3 is thin (hatched areas in the left diagram of Figure 3) and areas where it is thick (shaded areas in the left diagram of Figure 3). Thus, if there is a film thickness variation of a certain level or more in the surface treatment layer 3, this film thickness variation will be maintained, and the flatness of the optical film 1 will be impaired.

[0039] Furthermore, as shown in the left diagram of Figure 3, if the thickness of the surface treatment layer 3 is uneven, variations occur in the amount of heat received by the base film 2 during the heat curing process. That is, in areas where the thickness of the surface treatment layer 3 is thin, the amount of heat received by the base film 2 is large, and in areas where the thickness of the surface treatment layer 3 is thick, the amount of heat received by the base film 2 is small. As a result, an unevenness in the thermal shrinkage rate occurs within the surface of the base film 2, and the unevenness such as wrinkles and dimples is formed on the optical film 1, exacerbating the variations in the thickness of the surface treatment layer 3, and impairing its flatness (right diagram of Figure 3).

[0040] Furthermore, as shown in Figure 4 (left), if the area of ​​significant curing shrinkage of the surface treatment layer 3 overlaps with the area of ​​large difference in thermal shrinkage rate of the base film 2, the flatness will be more significantly impaired (Figure 4 (right)).

[0041] Therefore, by optimizing the difference in thermal shrinkage rates of the base film 2 constituting the optical film 1 and the film thickness variation ratio Vr of the surface treatment layer 3, it is possible to suppress surface defects of the optical film 1 and ensure flatness.

[0042] Furthermore, it is preferable that the optical film 1 has high scratch resistance (SW resistance). Scratch resistance can be evaluated by a scratch test using steel wool (SW), and in detail, the surface of the surface treatment layer 3 of the optical film 1 is subjected to a load of 1000 g / cm² on the steel wool. ２ For practical purposes, it is preferable that the number of scratches produced when the device is moved back and forth 10 times is 10 scratches / cm width or less, and even more preferable that it is 5 scratches / cm width or less.

[0043] As described above, in this embodiment, the optical film 1 has an average film thickness of the surface treatment layer 3 of 2.0 μm or more. As a result, the surface treatment layer 3 exhibits sufficient surface hardness (pencil hardness).

[0044] Furthermore, the base film 2 is a polyester composite material of an amorphous film and a crystalline film. This makes it possible to achieve both a balanced Young's modulus and a low phase difference through the stretching of the base film 2, thereby achieving both flatness and reduction of unevenness in the optical film 1.

[0045] Furthermore, the average thickness of the surface treatment layer 3 is 15.0 μm or less. This reduces shrinkage during the curing of the surface treatment layer 3, thereby suppressing damage to the flatness of the optical film surface.

[0046] Furthermore, the film thickness variation ratio Vr of the surface treatment layer 3 is 15% or less. This prevents uneven shrinkage of the surface treatment layer 3 during curing, which would impair the flatness of the optical film 1.

[0047] Furthermore, the difference in thermal shrinkage rates of the base film 2 is 1.3 or less. This reduces the unevenness in shrinkage within the base film 2 surface during curing of the surface treatment layer 3, thereby suppressing the loss of flatness of the optical film 1.

[0048] Furthermore, the in-plane phase difference Re of the base film 2 is 20 nm to 400 nm, and the phase difference Rth' in the thickness direction is 0 nm to 1500 nm. This allows the base film 2 to be given a suitable thickness, and reduces iridescence in the optical film 1.

[0049] Furthermore, the average film thickness of the base film 2 is 13 μm or more. This allows the optical film 1 to be constructed without compromising the handling properties or surface hardness of the optical film 1.

[0050] Furthermore, the average film thickness of the base film 2 is 60 μm or less. This allows the thickness of the optical film 1 to be reduced, contributing to the weight reduction of the optical film 1.

[0051] Furthermore, the surface treatment layer 3 contains particles, and the haze of the optical film 1 is 0.8% or more. This suppresses the formation of irregularities on the optical film surface during the curing of the surface treatment layer 3.

[0052] Furthermore, the pencil hardness of the surface of the surface treatment layer 3 is H or higher. This provides the optical film 1 with sufficient strength to withstand the usage environment.

[0053] Furthermore, the nanoindentation hardness of the surface treatment layer 3 is set to a value higher than 0.30 GPa. This increases the strength of the optical film 1.

[0054] The following describes specific examples of how this disclosure has been implemented.

[0055] (1) Preparation of base film First, polyester films to be used as base films were prepared in widths of 1.6 m each according to the following procedure. More specifically, five types were prepared: low phase difference polyester films a to b and e, high phase difference polyester film c, and medium phase difference polyester film d.

[0056] (Low phase difference polyester film a) First, the following resins Aa and Ba were prepared as materials for the polyester film. Resin Aa: Polyethylene terephthalate Resin Ba: Ethylene terephthalate 70 mol% - Cyclohexanedimethanol 30 mol% copolymer

[0057] Next, the following materials were prepared as surface coating agent α for the polyester film: Methyl methacrylate 64 parts by mass, Acrylic acid 33 parts by mass, Melanin-based crosslinking agent 1 part by mass, Colloidal silica (80 nm diameter) 1 part by mass, Fluorine-based surfactant 1 part by mass

[0058] Next, each material was extruded through the slit of a T-shaped die at 280°C so that the layer structure was resin Aa(2) / resin Ba(6) / resin Aa(2), and then cooled and solidified on a casting drum (electrostatically charged) at 23°C. Here, the numbers in parentheses indicate the thickness ratio of each material.

[0059] Next, the solidified cast film was heated with a heat roll at 80°C and stretched 3.3 times by the difference in peripheral speed, and then corona discharge treatment was performed on both sides.

[0060] Next, surface coating agent α was applied to both sides as an easy-adhesion coating using a reverse kissing method to a film thickness of 0.05 μm after drying.

[0061] Next, the material was stretched 4.3 times in the lateral direction under conditions of 140°C, and then subjected to heat treatment at 225°C.

[0062] Next, a 2% widthwise relaxation treatment was performed in a 225°C environment, followed by a 1% widthwise relaxation treatment in a 100°C environment.

[0063] By following the above procedure, a substrate film with a thickness of 60 μm ± 2.0 μm was obtained.

[0064] (Low Phase Difference Polyester Film b) First, the following resins Ab and Bb were prepared as materials for the polyester film. Resin Ab: Molten mixture of the following materials Polyethylene terephthalate 75 parts by mass Isosorbide (15 mol%) - cyclohexanedimethanol (20 mol%) - ethylene terephthalate (65 mol%) copolymer (PET / ISB / CHDM) 25 parts by mass Resin Bb: Molten mixture of the following materials Isosorbide (15 mol%) - cyclohexanedimethanol (20 mol%) - ethylene terephthalate (65 mol%) copolymer (PET / ISB / CHDM) 50 parts by mass Isophthalic acid (25 mol%) - ethylene terephthalate (75 mol%) copolymer (PET / I) 50 parts by mass

[0065] Next, surface coating agent α (the same as the low-phase-difference polyester film a) was prepared.

[0066] Next, each material was extruded through the slit of a T-shaped die at 280°C to form a 1090-layer laminated structure in which resins Ab and Bb were alternately stacked, and then cooled and solidified on a casting drum (with electrostatic discharge applied) at 23°C.

[0067] Next, the solidified cast film was heated with a heat roll at 110°C and stretched 3.3 times by the difference in peripheral speed, and then corona discharge treatment was performed on both sides.

[0068] Next, surface coating agent α was applied to both sides as an easy-adhesion coating using a reverse kissing method to a film thickness of 0.05 μm after drying.

[0069] Next, the material was stretched four times in the lateral direction under conditions of 140°C, and then subjected to heat treatment at 215°C.

[0070] Next, a 1% widthwise relaxation treatment was performed in a 190°C environment, followed by a 1% widthwise relaxation treatment in a 150°C environment, and then a 3.5% longitudinal relaxation treatment was performed in a 150°C environment.

[0071] By following the above procedure, a substrate film with a thickness of 50 μm ± 2.0 μm was obtained.

[0072] (High-phase-difference polyester film c) First, the following resin Ac was prepared as the material for the polyester film. Resin Ac: Polyethylene terephthalate

[0073] Next, surface coating agent α (the same as the low-phase-difference polyester film a) was prepared.

[0074] Next, the resin Ac was extruded through the slit of the T-shaped die at 280°C and cooled and solidified on a casting drum (electrostatically charged) at 23°C.

[0075] Next, corona discharge treatment was performed on both sides of the solidified cast film.

[0076] Next, surface coating agent α was applied to both sides as an easy-adhesion coating using a reverse kissing method to a film thickness of 0.05 μm after drying.

[0077] Next, the substrate was stretched 4.0 times using tenter clips in a 125°C environment, and then heat-treated at 225°C.

[0078] Next, a 3.0% widthwise relaxation treatment was performed under a 225°C environment.

[0079] By following the above procedure, a substrate film with a thickness of 80 μm ± 3.0 μm was obtained.

[0080] (Intermediate phase difference polyester film d) First, the following resin Ad was prepared as the material for the polyester film. Resin Ad: Polyethylene terephthalate

[0081] Next, surface coating agent α (the same as the low-phase-difference polyester film a) was prepared.

[0082] Next, the resin Ad was extruded through the slit of the T-shaped die at 280°C and cooled and solidified on a casting drum (electrostatically charged) at 23°C.

[0083] Next, the solidified cast film was heated with a heat roll at 110°C and stretched 1.5 times by the difference in peripheral speed, and then corona discharge treatment was performed on both sides.

[0084] Next, surface coating agent α was applied to both sides as an easy-adhesion coating using a reverse kissing method to a film thickness of 0.05 μm after drying.

[0085] Next, the substrate was stretched 4.0 times in the width direction at 110°C, and then subjected to offline annealing at 90°C for 5 minutes.

[0086] By following the above procedure, a substrate film with a thickness of 50 μm ± 2.0 μm was obtained.

[0087] (Low Phase Difference Polyester Film e) First, the following resins Ae and Be were prepared as materials for the polyester film. Resin Ae: Polyethylene terephthalate Resin Be: Spiroglycol 15 mol% - Cyclohexanedicarboxylic acid 25 mol% - Ethylene terephthalate 60 mol% copolymer

[0088] Next, surface coating agent α (the same as the low-phase-difference polyester film a) was prepared.

[0089] Next, each material was extruded through the slit of a T-shaped die at 280°C to form an 800-layer laminated structure in which resin Ae and resin Be were alternately stacked, and then cooled and solidified on a casting drum (electrostatically charged) at 23°C.

[0090] Next, the solidified cast film was heated with a 100°C heat roll to stretch it 4.5 times due to the difference in peripheral speed, and then corona discharge treatment was performed on both sides.

[0091] Next, surface coating agent α was applied to both sides as an easy-adhesion coating using a reverse kissing method to a film thickness of 0.05 μm after drying.

[0092] Next, the substrate was stretched 1.1 times in the transverse direction under conditions of 140°C, and then subjected to heat treatment at 225°C.

[0093] Next, a 1% widthwise relaxation treatment was performed in a 225°C environment, followed by another 1% widthwise relaxation treatment in a 100°C environment.

[0094] By following the above procedure, a substrate film with a thickness of 40 μm ± 1.0 μm was obtained.

[0095] (2) Evaluation of the base film (polyester film) For the prepared polyester films a to e, the in-plane phase difference Re, the thickness direction phase difference Rth', the thermal shrinkage rate in MD-TD, and the film thickness were measured, and the difference in thermal shrinkage rate and the average film thickness T' were measured. ａｖｇ , average film thickness T' ａｖｇ Maximum value V' of film thickness variation ｍａｘ The film thickness variation ratio V'r was also calculated.

[0096] (Phase Difference) The in-plane phase difference Re and the phase difference Rth' in the thickness direction were measured using a phase difference film / optical material inspection device (RETS-100, manufactured by Otsuka Electronics Co., Ltd.) under the following measurement conditions. [Measurement Conditions] ・Phase difference measurement method: Rotational analyzer method ・Measurement spot diameter: φ5 mm ・Tilt angle range: 0° ・Measurement wavelength range: 400 nm to 800 nm ・Average refractive index N of light-transmitting substrate: n ｘ , n ｙ and n ｚ Based on this, N = (n ｘ +n ｙ +n ｚ The value was calculated using the formula ) / 3. Note that the in-plane phase difference Re and the phase difference Rth' in the thickness direction are values ​​at a wavelength of 589 nm.

[0097] (Heat Shrinkage Rate) Samples were prepared for measurement by cutting out sections of polyester film 150 mm in the longitudinal direction (MD) and 150 mm in the width direction (TD) from areas other than the 50 mm at both ends in the width direction. Ten such samples were prepared for each of polyester films a to e (representing 10 different locations on the polyester film). A pair of marks were placed in the center of each sample, spaced 100 mm apart in the longitudinal direction, and another pair of marks were placed in the width direction, spaced 100 mm apart. The samples were treated in an oven at 150°C for 30 minutes, then cooled to room temperature, and the distance between each pair of marks was measured. The distance between the marks before heat treatment was defined as L0, and the distance between the marks after heat treatment as L1. The heat shrinkage rates in the longitudinal and width directions were calculated based on {(L0 - L1) / L0} × 100. The average of the heat shrinkage rates of each of the 10 samples was calculated for both the longitudinal and width directions, and this was defined as the heat shrinkage rate of the polyester film.

[0098] (Difference in thermal shrinkage rate) The absolute value of the difference between the thermal shrinkage rate in the longitudinal direction and the thermal shrinkage rate in the width direction of the base film was determined as the difference in thermal shrinkage rate.

[0099] (Film Thickness) In the width direction (TD) of the polyester film, in areas other than the 50 mm at both ends, the thickness was measured at 15 different arbitrary points using a contact-type film thickness gauge Lightmatic VL-50A (manufactured by Mitutoyo Corporation, 10.5 mmφ carbide spherical measuring tip, measuring load 0.06 N), and the average value of the thicknesses T' was calculated. ａｖｇ This was determined as the thickness of the polyester film.

[0100] (Maximum variation in film thickness) Each measured value of the film thickness of the base film measured at the aforementioned 15 points, and the average value T' ａｖｇ The maximum absolute value of the difference between the two is the maximum value of the variation in the thickness of the polyester film, V'. ｍａｘ It was calculated as follows.

[0101] (Film thickness variation ratio) The film thickness variation ratio Vr' of the polyester film was calculated based on the following formula: Vr' = (V' ｍａｘ / T' ａｖｇ ) × 100

[0102] Polyester films a to e: material, in-plane phase difference Re, thickness direction phase difference Rth', thermal shrinkage rate in MD-TD, film thickness, thermal shrinkage rate difference, average film thickness T'. ａｖｇ , Maximum film thickness variation V' ｍａｘ The film thickness variation ratio V'r is shown in Table 1.

[0103]

[0104] (3) Surface treatment Next, hard coat treatment solutions i to iii were prepared. Each of the hard coat treatment solutions i to iii was obtained by stirring and mixing the components shown below. Note that hard coat treatment solutions ii and iii both contain particles.

[0105] (Hard coat treatment solution i) Adamantate HM (manufactured by Idemitsu Kosan Co., Ltd.) 20.82 parts by mass Light acrylate PE-3A photocurable resin (manufactured by Kyoeisha Chemical Co., Ltd.) 10.50 parts by mass Acrylic resin D 20.88 parts by mass Photopolymerization initiator Omnirad® 184 (1-hydroxycyclohexyl phenyl ketone), manufactured by IGM Resins B.V. 1.40 parts by mass LUCIRIN TPO (manufactured by IGM Resins B.V.) 1.40 parts by mass Dimethyl carbonate 6.70 parts by mass Methyl isobutyl ketone 38.30 parts by mass

[0106] The above acrylic resin D was obtained by the following method. [Synthesis of Acrylic Resin D] 800 parts of cyclohexanone were placed in a reaction vessel, and while injecting nitrogen gas into the vessel, it was heated to 100°C. At the same temperature, the following mixture of monomers and thermal polymerization initiators was added dropwise over 1 hour to carry out the polymerization reaction. Styrene 60.0 parts Methacrylic acid 60.0 parts Methyl methacrylate 65.0 parts Butyl methacrylate 65.0 parts Azobisisobutyronitrile 10.0 parts

[0107] After dropwise addition, the mixture was reacted at 100°C for 2 hours. Then, a solution of 2.0 parts azobisisobutyronitrile dissolved in 50 parts cyclohexanone was added, and the reaction was continued at 100°C for 1 hour to synthesize acrylic resin D with a weight-average molecular weight of 8000. (Hard coat treatment solution ii) Light curing resin Light Acrylate PE-3A (manufactured by Kyoeisha Chemical Co., Ltd.) 92.93 parts by mass Photopolymerization initiator Omnirad® 184 (1-hydroxycyclohexyl-phenyl ketone), IGM Resins B. V. 4.57 parts by mass of particle synthesis smectite manufactured by the company; 2 parts by mass of leveling material F565; 0.5 parts by mass of solvent toluene; 100 parts by mass of (hard coat treatment solution iii); 88.45 parts by mass of photocurable resin light acrylate PE-3A; photopolymerization initiator Omnirad® 184 (1-hydroxycyclohexyl-phenyl ketone), IGM Resins B. V. 4.66 parts by mass of acrylic / polystyrene copolymer particles (n=1.515 particle system, 3.5 μm) manufactured by the company 3.15 parts by mass of acrylic / polystyrene copolymer particles (n=1.565 particle system, 3.5 μm) 0.35 parts by mass of particle synthesis smectite 2 parts by mass of leveling material F565 0.4 parts by mass of nanoparticle MEK-ST-49 1 part by mass of solvent toluene 100 parts by mass (hard coat treatment liquid iv) Light curing resin Light acrylate PE-3A (manufactured by Kyoeisha Chemical Co., Ltd.) 94.93 parts by mass of photopolymerization initiator Omnirad® 184 (1-hydroxycyclohexyl-phenyl ketone), manufactured by IGM Resins B.V.4.57 parts by mass Leveling material F565 0.5 parts by mass Solvent Toluene 100 parts by mass (Hard coat treatment solution v) Pentaerythritol triacrylate (PE3A) 15.0 parts by mass Pentaerythritol tetraacrylate (PE4A) 15.0 parts by mass Urethane acrylate (UA) 25.0 parts by mass Omnirad 184 3.0 parts by mass BYK-350 0.1 parts by mass Methyl ethyl ketone (MEK) 41.9 parts by mass (Hard coat treatment solution vi) NK ester A9300-1CL 47 parts by mass Omnirad 184 3.0 parts by mass Methyl ethyl ketone 50 parts by mass

[0108] Next, hard coat treatment solutions i to vi are applied to polyester films a to e using a die coater or bar coater, heated at 70°C for 1 minute, and then heated with a metal halide lamp at 400 mJ / cm². ２ The films were cured by irradiation with ultraviolet light to obtain optical films according to Examples 1 to 15 and Comparative Examples 1 to 10. Hereinafter, the combinations of polyester films a to e and hard coat treatment solutions i to vi, and the coating method used (whether coating was performed with a die coater or a bar coater) in each example and comparative example are shown in Table 2.

[0109] (4) Evaluation of the optical film For the optical films according to Examples 1 to 15 and Comparative Examples 1 to 10, the film thickness of the optical film was measured and the average film thickness T'' was measured. ａｖｇ , average film thickness T'' ａｖｇ Maximum value V'' of film thickness variation ｍａｘ The film thickness variation ratio V''r was calculated. Furthermore, thin-film properties, pencil hardness, haze, flatness, iridescence, and resistance to steel wool were evaluated.

[0110] (Film Thickness) The thickness of 15 different points in the optical film, excluding the 50 mm at both ends in the width direction (TD), was measured using a contact-type film thickness gauge, Lightmatic VL-50A (Mitutoyo Corporation, 10.5 mmφ carbide spherical probe, measuring load 0.06 N). The average value of the 15 thicknesses was T''. ａｖｇ This was determined as the thickness of the optical film.

[0111] (Maximum film thickness variation) The measured values ​​of the optical film thickness measured at any 15 points, and the average value T'' ａｖｇ The maximum absolute value of the difference between the two is the maximum value of the variation in the thickness of the optical film, V''. ｍａｘ It was calculated as follows.

[0112] (Film Thickness Variation Ratio) The film thickness variation ratio Vr'' of the optical film was calculated based on the following formula: Vr'' = (V'' ｍａｘ / T'' ａｖｇ ) × 100

[0113] (Thin film properties) The average value T'' of the optical film thickness obtained above. ａｖｇ Based on this, the thin-film properties were evaluated. A score of ○ was used to indicate an optical film thickness of 80 μm or less, and a score of × was used to indicate an optical film thickness exceeding 80 μm.

[0114] (Pencil Hardness) In accordance with JIS K5400-1900, the pencil hardness of the optical film surface (surface treatment layer) was evaluated using a uni pencil (manufactured by Mitsubishi Pencil Co., Ltd.) and a Clemens-type scratch tester HA-30 (manufactured by Tester Sangyo Co., Ltd.). The load used was 250g. Changes in appearance due to scratches were observed visually, and the maximum pencil hardness at which no scratches were observed was determined. A pencil hardness of H or higher was considered good, and a pencil hardness below H was considered poor.

[0115] (Haze Value) The haze of the optical film was measured using a haze meter NDH4000 (manufactured by Nippon Denshoku Industries Co., Ltd.) in accordance with JIS K7136. The average of the measurements taken at 15 different points was taken as the haze value of the optical film.

[0116] (Flatness) A rectangular sample with sides of 1 m or more was cut from the optical film, and light was shone onto one side of the sample using a three-wavelength light source (fluorescent lamp-like linear light source) with an illuminance of 1000 lux or more. The sample was observed visually from the opposite side of the illuminated light, and the quality of the flatness was evaluated in four ranks from S to C based on the visible surface condition of the sample (whether the optical film was flat or not) and the degree of distortion of the linear light source reflected on the optical film. The evaluation criteria for each rank are as follows, with rank A or higher being good and rank B or lower being poor. S: The optical film surface is flat and no distortion of the linear light source is visible. A: The optical film surface is flat, but some distortion of the linear light source is visible. B: Irregularities are present on the optical film surface, and distortion of the linear light source is visible across the entire surface. C: Irregularities are present on the optical film surface, and reflected images other than those of the linear light source are distorted.

[0117] (Rainbow pattern) Backlight (5000 cd / m²) ２ A first polarizer, an optical film, and a second polarizer were arranged in order on a display with a certain degree of polarity. In this case, the second polarizer was intended to be a polarized sunglasses. Furthermore, the absorption axis of the first polarizer was positioned perpendicular to the vertical direction of the display, and the absorption axis of the second polarizer was positioned perpendicular to the absorption axis of the first polarizer.

[0118] The orientation reference (0°) was set when the vertical direction of the display and the slow axis of the transparent film were parallel. From there, the transparent film was rotated 45° clockwise, and then another 45° (90° from the reference), and the rainbow pattern was observed at each orientation. Observations were made at a distance of 50-60 cm from the display, both directly in front of the display and at a 45° angle to the display. The degree of rainbow pattern was evaluated based on the following criteria: —: Blackout occurs, making it impossible to determine the rainbow pattern (unusable) ×: Rainbow pattern occurs and unusable △: Rainbow pattern occurs, but no practical problems ○: Slight rainbow pattern occurs, but no practical problems ◎: No rainbow pattern occurs (outstandingly good)

[0119] (5) Evaluation of the surface treatment layer Using the optical films according to Examples 1 to 15 and Comparative Examples 1 to 10, measure the thickness of the surface treatment layer and measure the average thickness T ａｖｇ , average film thickness T ａｖｇ Maximum value V of film thickness variation ｍａｘ The film thickness variation ratio Vr was also calculated.

[0120] (Film Thickness) The thickness of the surface treatment layer was measured using an optical microscope after cutting a cross-section of the optical film with a microtome. Specifically, first, an optical film cut to 2 mm x 5 mm was placed in a silicone embedding plate, and epoxy resin was poured in to embed the entire optical film in resin. It was then left at 65°C for 12 hours or more to cure. Next, using an ultramicrotome EM UC7 (Leica Microsystems), the feed thickness was set to 100 nm, and ultrathin sections were prepared as measurement samples. The cross-section of the optical film of the measurement sample was observed with an optical microscope BX51 (Olympus Corporation), and the thickness of the surface treatment layer was determined. The above procedure was repeated, and the thickness of 15 arbitrary points with different surface treatment layers was measured, and the average thickness T was calculated. ａｖｇ This was determined as the thickness of the surface treatment layer.

[0121] (Maximum film thickness variation) The measured values ​​of the film thickness of the surface treatment layer measured at any 15 points, and the average value T ａｖｇ The maximum absolute value of the difference between the two is the maximum value of the thickness variation of the surface treatment layer V. ｍａｘ It was calculated as follows.

[0122] (Film thickness variation ratio) The film thickness variation ratio Vr of the surface treatment layer was calculated based on the following formula: Vr = (V ｍａｘ / T ａｖｇ ) × 100

[0123] (Nanoindentation (NI) Hardness Measurement) The nanoindentation hardness of the substrate film and surface treatment layer was measured using the optical films according to Examples 1 to 16 and Comparative Examples 1 to 10. Specifically, an optical film cut to 2 mm x 5 mm was placed in a silicone embedding plate, and epoxy resin was poured in to embed the entire optical film in resin. The film was then left at 65°C for 12 hours or more to cure. Next, sections were prepared using an ultramicrotome EM UC7 (Leica Microsystems) to be used as measurement samples. Then, the nanoindentation hardness was measured by pressing a Berkovich indenter perpendicular to the cross-section obtained by cutting the section of the measurement sample to a depth of 50 nm. The measurement was performed by indentation hardness according to the nanoindentation method in accordance with ISO 14577-1:2015, and the test machine used was the microhardness tester nanoindenter "TI Premier" (Bruker Japan). The surface treatment layer was measured by pressing a Berkovich indenter 0.2 μm away from the surface layer, and the base film was measured by pressing a Berkovich indenter 3.0 μm away from the interface with the surface treatment layer. The indentation hardness was calculated from the maximum indentation load and contact projected area.

[0124] (Steel wool resistance) 1000g / cm² on top of steel wool (SW) of product number #0000 manufactured by Japan Steel Wool Co., Ltd. ２ The surface of the optical film's surface treatment layer was moved back and forth 10 times under a load. The number of scratches per 1 cm width after 10 back and forth movements was measured, and the scratch resistance was evaluated according to the following criteria. The apparatus used to move the steel wool back and forth was a friction and abrasion tester (Tribote Station TYPE: 32, travel speed 1000 mm / min.) from Shinto Kagaku Co., Ltd. ◎: Number of scratches 5 or less per cm width ○: Number of scratches 6 to 10 per cm width △: Number of scratches 11 to 20 per cm width ×: Number of scratches 21 or more per cm width

[0125] Tables 2 and 3 show the average film thickness, maximum film thickness variation, film thickness variation ratio, nanoindation hardness, and evaluation results for the thinness, pencil hardness, haze, flatness, iridescence, and steel wool resistance (SW resistance) of the optical film.

[0126]

[0127]

[0128] (Regarding thin film properties) The optical films in Examples 1 to 15 and Comparative Examples 4 to 10 all have an average film thickness T'. ａｖｇ Since the base film used was 60 μm or less (i.e., polyester films a, b, d, and e), the overall thickness of the optical film could be kept down, and an optical film with excellent thin-film properties could be obtained. On the other hand, the optical films according to Comparative Examples 1 to 3 all used base films with a film thickness exceeding 60 μm (i.e., polyester film c), so the overall thickness of the optical film increased, and the thin-film properties were impaired.

[0129] (Regarding pencil hardness) The optical films in Examples 1 to 15 and Comparative Examples 2 to 5 and 9 all have an average film thickness T of the surface treatment layer. ａｖｇ Since the surface treatment layer thickness was 2.0 μm or more, it showed sufficient pencil hardness. On the other hand, the optical films in Comparative Examples 1, 6-8, and 10 all had a surface treatment layer thickness of less than 2.0 μm, so their pencil hardness was HB or lower, and sufficient surface hardness could not be obtained. Furthermore, among the optical films in Comparative Examples 1, 6-8, and 10, the optical films in Comparative Examples 6-8 and 10 had particularly poor pencil hardness. This is because the polyester film a used in Comparative Examples 6-8 and 10 was a composite polyester, and therefore softer than the polyester film c composed of PET alone used in Comparative Example 1.

[0130] (Regarding flatness) The optical films in Examples 1 to 15 and Comparative Examples 4, 8, and 10 all used base films (i.e., polyester films a, b, and d) with a thermal shrinkage difference of 1.3 or less, and the variation ratio Vr of the surface treatment layer thickness was 15% or less. Therefore, the flatness of the optical films was excellent.

[0131] Examples 6 and 7 had a higher variation ratio Vr of the surface treatment layer than Examples 1-5 and 13, resulting in inferior flatness compared to Examples 1-5 and 13. Examples 8-12 (especially Example 12) also had a higher variation ratio Vr of the surface treatment layer than Examples 1-6, but their flatness was superior to Examples 6 and 7. This is thought to be because the inclusion of particles in the surface treatment layer suppressed the shrinkage of the surface treatment layer, preventing the formation of irregularities on the optical film surface. Furthermore, while Example 14 showed flatness comparable to Examples 6 and 7, this is presumed to be because the use of the surface treatment liquid v with the highest NI hardness resulted in significant curing shrinkage, which affected the flatness.

[0132] On the other hand, the optical films in Comparative Examples 2, 3, and 5 used base films (i.e., polyester films c and e) with a thermal shrinkage difference of more than 1.3, and the optical films in Comparative Examples 3 and 9 had a surface treatment layer thickness variation ratio Vr exceeding 15%. As a result, the flatness of the optical film surface was poor. In particular, Comparative Example 3 had especially poor flatness because neither the thermal shrinkage difference of the base film nor the surface treatment layer thickness variation ratio Vr met the favorable conditions.

[0133] However, even when a base film with a thermal shrinkage rate difference exceeding 1.3 (i.e., polyester films c and e) is used, or when the variation ratio Vr of the surface treatment layer thickness exceeds 15%, good flatness (rating A or higher) may still be achieved. Specifically, the optical film according to Comparative Example 1 exhibited good flatness despite the base film (polyester film c) having a thermal shrinkage rate difference exceeding 1.3 and a large bias in in-plane shrinkage. This is thought to be because the thickness of the surface treatment layer on the base film was very thin at 1.5 μm, so the deformation of the base film did not manifest as an abnormality in flatness. Furthermore, the optical films according to Comparative Examples 6 and 7 exhibited good flatness despite the variation ratio Vr of the surface treatment layer thickness exceeding 15%. This is thought to be because the thickness of the surface treatment layer was very thin at 1.5 μm, so the difference in thermal shrinkage of the surface treatment layer did not manifest as an abnormality in flatness.

[0134] (Regarding Rainbow Irregularities) The optical films in Examples 1 to 15 and Comparative Examples 5 to 10 all used low-phase-difference base films (i.e., polyester films a, b, and e) in which the in-plane phase difference Re was in the range of 20 nm to 400 nm and the thickness-direction phase difference Rth' was in the range of 0 nm to 1500 nm, so no rainbow irregularities occurred in the optical films. In addition, the optical films in Comparative Examples 1 to 3 all used base films with very high phase differences (i.e., polyester film c), so the rainbow irregularities were suppressed to a level that did not cause practical problems. On the other hand, the optical film in Comparative Example 4 used a base film with medium phase difference (i.e., polyester film d), so rainbow irregularities occurred in the optical film, making it difficult to use in practice.

[0135] In the examples and comparative examples, a die coater or a bar coater was used to coat the surface treatment layer. Using a die coater resulted in a smaller variation ratio Vr of the surface treatment layer thickness. Furthermore, when particles were contained in the surface treatment layer, the haze was 0.8% or higher.

[0136] This disclosure can be used as an optical film and an image display device using the same.

[0137] 1: Optical film 2: Substrate film 3: Surface treatment layer

Claims

1. An optical film having a surface treatment layer formed on a base film, wherein the base film is a polyester composite material with an average film thickness of 13 μm or more and 60 μm or less, the absolute value of the difference between the thermal shrinkage rate in the MD direction and the thermal shrinkage rate in the TD direction of the base film is 1.3 or less, the average film thickness of the surface treatment layer is 2.0 μm or more and 15.0 μm or less, and the film thickness variation ratio Vr of the surface treatment layer, as defined by the following formula (1), is 15% or less. Vr = (V ｍａｘ / T ａｖｇ ) × 100 (1) Here, T ａｖｇ : The average value of the film thickness of the surface treatment layer measured at any 15 points on the optical film, V ｍａｘ : Each measured value of the film thickness of the surface treatment layer measured at any 15 points, and the average value T ａｖｇ This is the maximum absolute value of the difference between [the given value] and [the given value].

2. The optical film according to claim 1, wherein the in-plane phase difference Re of the substrate film is 20 nm or more and 400 nm or less, and the phase difference Rth' in the thickness direction is 0 nm or more and 1500 nm or less.

3. The optical film according to claim 1, wherein the surface treatment layer contains particles.

4. The optical film according to claim 1, wherein the haze is 0.8% or more.

5. The optical film according to claim 1, wherein the pencil hardness of the surface of the surface treatment layer is H or higher.

6. The optical film according to claim 1, wherein the nanoindentation hardness of the surface treatment layer is higher than 0.30 GPa.

7. An image display device comprising an image display panel and an optical film according to any one of claims 1 to 6 provided on the front surface of the image display panel.

Images