Optical films and image display devices

Abstract

The present invention provides a thin optical film with excellent strength and flatness, and an image display device using the same, which uses a film made of polyester resin as a base material. [Solution] 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)

Description

[Technical Field]

[0001] The present invention relates to an optical film and an image display device equipped therewith. [Background technology]

[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. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2014-149320 [Patent Document 2] Japanese Patent Publication No. 2020-192812 [Overview of the Initiative] [Problems that the invention aims to solve]

[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 supplemented by laminating a surface treatment layer (hard coat layer) on the substrate surface. However, the formation of the surface treatment layer involves processes such as heating and film curing. Further, the polyester-based film used as the substrate exhibits a biased in-plane orientation due to stretching, and thus shows anisotropic characteristics thermally as well. When forming a surface treatment layer on such a substrate, the substrate is affected by the heat curing of the surface treatment layer and shrinks unevenly, and there were cases where surface unevenness occurred in the optical film and flatness was impaired. An optical film with impaired flatness is not preferable because it causes a decrease in the visibility of an image display device.

[0006] The present invention has been made in view of the above problems, and an object thereof is to provide an optical film that is thin, excellent in strength and flatness, using a film made of a polyester-based resin as a substrate, and an image display device using the same.

Means for Solving the Problems

[0007] One aspect of the present invention for solving the above problems is an optical film having a surface treatment layer formed on a substrate film, wherein the substrate film is a polyester composite material having 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 substrate 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 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 : The maximum value of the absolute value of the difference between each measured value of the film thickness of the surface treatment layer measured at any 15 points and the average value T avg is.

[0008] Another aspect of the present invention is an image display device including an image display panel and the above optical film provided on the front surface of the image display panel. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide an optical film that is thin, has excellent strength and flatness, and uses a polyester resin film as a base material, as well as an image display device using the same. [Brief explanation of the drawing]

[0010] [Figure 1] Cross-sectional view showing the schematic configuration of the optical film according to the embodiment. [Figure 2] This diagram illustrates the deformation of the base film that occurs when the difference in thermal shrinkage coefficient within the base film surface is 1.3 or greater. [Figure 3] This diagram illustrates the shrinkage of the surface treatment layer and the substrate film that occurs when the thickness variation ratio Vr of the surface treatment layer exceeds 15%. [Figure 4] This diagram illustrates the shrinkage of the surface treatment layer and the base film that occurs when there is a large unevenness in the amount of shrinkage within the base film surface and the thickness variation ratio Vr of the surface treatment layer exceeds 15%. [Modes for carrying out the invention]

[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 of the image display panel of an image display device to protect 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. Similarly, when used in mobile devices such as smartphones and tablets, it is desirable to lighten the optical film 1 in order to lighten the device itself. From this viewpoint of 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 substrate film 2 is the base of the optical film 1, and a material excellent in transparency and visible light transmittance is used. As the substrate film 2, it is preferable to use a polyester-based film from the viewpoints of low cost and moisture permeability resistance. 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 substrate 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 substrate 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 substrate film n y : Refractive index in the fast axis direction of the substrate film n z : Refractive index in the thickness direction d: Thickness of the substrate film That is.

[0017] Since 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 prevent 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 easily achievable 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 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 any 15 different 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 is 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; for example, it can be the average value of the thermal shrinkage rates measured at 10 different points.

[0019] (Surface treatment layer) The surface treatment layer 3 is a functional layer that coats the thinned and weakened base film 2, thereby imparting 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 includes a hard coat layer (HC layer), a hard coat-equipped low-reflection layer (HC-LR layer), an anti-glare (AG) layer with anti-glare properties, and an anti-glare low-reflection (AGLR) layer.

[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 : Each measured value of the film thickness of the surface treatment layer 3 measured at any 15 points, and the average value T avg The maximum absolute value of the difference (the maximum variation in film thickness relative to the average film thickness) That is the case.

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

[0022] Furthermore, the lower limit of the average thickness of the surface treatment layer 3 is preferably 2.0 μm, and the upper limit is preferably 15.0 μm. The average thickness is the average of the thickness values ​​of different locations on the surface treatment layer 3, for example, the average value of the thickness of the surface treatment layer 3 measured at 15 different arbitrary points (T as described above). avg) can be set as follows. If the lower limit of the average film thickness of the surface treatment layer 3 is set to 2.0 μm, the surface treatment layer 3 can obtain sufficient surface hardness (pencil hardness), and sufficient strength can be imparted to the optical film 1. Furthermore, if the upper limit of the average film thickness is set to 15.0 μm, shrinkage during curing of the surface treatment layer 3 can be suppressed, and the loss of flatness of the surface of the optical film 1 can be prevented. The thickness of the surface treatment layer 3 can be determined, for example, by cutting 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 increasing 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 type of swellable clay in which organic onium ions have been introduced between the layers. The organic onium ions are not limited as long as they can be organicated by utilizing the cation exchange properties of the swollen 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 2-(meth)acryloyloxyethyl-2-hydroxypropyl phthalate, 2-hydroxy-3-phenoxypropyl (meth)acrylate, 2-(meth)acryloyloxyethyl-2-hydroxypropyl phthalate, 2-(meth)acryloyloxyethyl-2-hydroxypropyl phthalate, 2-(meth)acryloyloxyethyl-2-hydroxypropyl phthalate, 2-(meth)acryloyloxyethyl-2-hydroxypropyl phthalate, 2-(meth)acryloyloxyethyl-2-hydroxypropyl phthalate, 2-(meth)acryloyloxyethyl-2-hydroxypropyl phthalate Acryloyloxyethyl hydrogen phthalate, 2-(meth)acryloyloxypropyl hydrogen phthalate, 2-(meth)acryloyloxypropyl hexahydrohydrogen phthalate, 2-(meth)acryloyloxypropyl tetrahydrohydrogen phthalate, dimethylaminoethyl (meth)acrylate, trifluoroethyl (meth)acrylate, tetrafluoropropyl (meth)acrylate, hexafluoropropyl (meth)acrylate, octafluoropropyl (meth)acrylate, 2-adamantane,Examples include adamantane derivative mono(meth)acrylates such as adamantyl methacrylate, which has a monovalent mono(meth)acrylate derived from adamantanediol.

[0031] Examples of difunctional (meth)acrylate monomers include di(meth)acrylates such as ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, butanediol di(meth)acrylate, hexanediol di(meth)acrylate, nonanediol di(meth)acrylate, ethoxylated hexanediol di(meth)acrylate, propoxylated hexanediol di(meth)acrylate, diethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, neopentyl glycol di(meth)acrylate, ethoxylated neopentyl glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, and hydroxypivalic acid neopentyl glycol di(meth)acrylate.

[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 replaced 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 investigated this property, it is primarily suspected to be due to a change in bonding species when monomers, existing as multiple molecules, are converted to 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 curing shrinkage characteristics of this surface treatment layer 3 affect the flatness of the optical film 1, as explained below.

[0036] Figures 2 to 4 illustrate 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 panel 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 panel of Figure 3) and areas where it is thick (shaded areas in the left panel of Figure 3). Thus, if there is a certain level of film thickness variation in the surface treatment layer 3, this 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 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, surface defects of the optical film 1 can be suppressed and flatness can be ensured.

[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 surfaces during the 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 between 20 nm and 400 nm, and the phase difference Rth' in the thickness direction is between 0 nm and 1500 nm. This allows for a suitable thickness to be given to the base film 2, 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 its handling properties or surface hardness.

[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. [Examples]

[0054] The following describes specific examples of how the present invention is 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-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, after which 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 under a 225°C environment, followed by a 1% widthwise relaxation treatment under 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: A 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 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 Ac resin was extruded through the slit of the T-shaped die at 280°C and cooled and solidified on a casting drum (with electrostatic discharge applied) 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 (with electrostatic discharge applied) 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 (with electrostatic discharge applied) 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 under a 225°C environment, followed by another 1% widthwise relaxation treatment under 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 base film (polyester film) For the fabricated polyester films a-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 V' is the maximum value of the 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: Rotation analyzer method • Measurement spot diameter: φ5mm • Inclination angle range: 0° • Measurement wavelength range: 400nm to 800nm • Average refractive index N:n of light-transmitting substrate x , n y and n z Based on this, N=(n x +ny +n z The value 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] (Thermal shrinkage rate) From the polyester film, excluding the 50 mm at both ends in the width direction, sections of 150 mm in the longitudinal direction (MD) and 150 mm in the width direction (TD) were cut out to serve as measurement samples. Ten of these samples were prepared for each of the 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 spaced 100 mm apart in the width direction. After treating the samples in an oven at 150°C for 30 minutes and then cooling to room temperature, 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 thermal shrinkage rates in the longitudinal and width directions were calculated based on {(L0-L1) / L0} × 100. The average thermal shrinkage rate of each of the 10 samples was calculated for both the longitudinal and width directions and was used as the thermal 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 thermal shrinkage rate difference.

[0099] (film thickness) In the width direction (TD) of the polyester film, excluding 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 thickness 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 substrate 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'. maxIt 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] Material of polyester films a-e, 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] [Table 1]

[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 following components. 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-curing resin, Light Acrylate PE-3A (manufactured by Kyoeisha Chemical Co., Ltd.) 10.50 parts by mass Acrylic resin D: 20.88 parts by mass Photopolymerization initiator Omnirad(registered trademark)184(1-hydroxycyclohexyl -Phenyl ketone), manufactured by IGM Resins BV, 1.40 parts by mass LUCIRIN TPO (manufactured by IGM Resins BV) 1.40 parts by mass Dimethyl carbonate 6.70 parts by mass Methyl isobutyl ketone 38.30 parts by mass

[0106] The acrylic resin D mentioned above was obtained by the following method. [Synthesis of acrylic resin D] 800 parts of cyclohexanone were placed in a reaction vessel, and the vessel was heated to 100°C while nitrogen gas was injected into it. At the same temperature, the following mixture of monomer and thermal polymerization initiator was added dropwise over 1 hour to carry out the polymerization reaction. 60.0 parts styrene 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 another 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(registered trademark)184(1-hydroxycyclohexyl - Phenyl ketone), manufactured by IGM Resins BV, 4.57 parts by mass Particle synthetic smectite 2 parts by mass Leveling material F565 0.5 parts by mass Solvent: Toluene 100 parts by mass (Hard coat treatment solution iii) Light-curing resin, Light Acrylate PE-3A, 88.45 parts by mass Photopolymerization initiator Omnirad® 184 (1-hydroxycyclohexyl phenyl ketone), manufactured by IGM Resins BV, 4.66 parts by mass Acrylic / polystyrene copolymer particles (n=1.515 particle system, 3.5μm) 3.15 parts by mass Acrylic / polystyrene copolymer particles (n=1.565 particle system, 3.5 μm) 0.35 parts by mass Particle synthetic smectite 2 parts by mass Leveling material F565 0.4 parts by mass Nanoparticles MEK-ST-49 1 part by mass Solvent: Toluene 100 parts by mass (Hard coat treatment solution iv) Light-curing resin, Light Acrylate PE-3A (manufactured by Kyoeisha Chemical Co., Ltd.) 94.93 parts by mass Photopolymerization initiator Omnirad® 184 (1-hydroxycyclohexyl phenyl ketone), manufactured by IGM Resins BV, 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 Omnirad184 3.0 parts by mass BYK-350 0.1 part by mass Methyl ethyl ketone (MEK) 41.9 parts by mass (Hard coat treatment solution vi) NK Ester A9300-1CL 47 parts by mass Omnirad184 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 treated 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 optical film For the optical films in Examples 1-15 and Comparative Examples 1-10, the film thickness was measured, and the average film thickness T'' was determined. avg , average film thickness T'' avg V'' is the maximum value of the 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's width direction (TD), excluding the 50 mm at both ends, 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 variation in film thickness) Each measured value of the optical film thickness 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 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) 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. The change in appearance due to scratches was 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) In accordance with JIS K7136, the haze of the optical film was measured using a haze meter NDH4000 (manufactured by Nippon Denshoku Industries Co., Ltd.). The average of the measurements taken at 15 different points was defined as the haze of the optical film.

[0116] (flatness) A rectangular sample with sides of 1m or more was cut from the optical film, and one side of the sample was illuminated with light from 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 flatness was evaluated on a four-level scale 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 considered good and rank B or lower being considered poor. S: The optical film surface is flat and distortion of the line light source is not visible. A: The optical film surface is flat, but some distortion of the linear light source is visible. B: Irregularities appear on the surface of the optical film, and distortion of the linear light source is visible across the entire surface. C: Irregularities occur on the surface of the optical film, causing distortion of reflected images even from sources other than line light sources.

[0117] (Rainbow pattern) Backlight (5000 cd / m²) 2A 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 unevenness 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 unevenness was evaluated based on the following criteria. ―: The screen blacks out, making it impossible to determine the presence of rainbow-colored blemishes (unusable). ×: Rainbow pattern occurs and it is not practical. △: Rainbow-colored unevenness occurs, but there are no practical problems. ○: A slight rainbow pattern may occur, but it does not pose a practical problem. ◎: No rainbow pattern (outstandingly good)

[0119] (5) Evaluation of surface treatment layer Using the optical films according to Examples 1-15 and Comparative Examples 1-10, the film thickness of the surface treatment layer was measured, and the average film thickness T was determined. 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 more than 12 hours 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 variation in film thickness) Each measured value of the surface treatment layer thickness measured at the aforementioned 15 arbitrary 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 described in Examples 1-16 and Comparative Examples 1-10. 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 serve 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 using 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 resistant) On top of steel wool (SW) of product number #0000 manufactured by Nippon Steel Wool Co., Ltd., 1000g / cm² 2 A load was applied, and the surface of the optical film's surface treatment layer was moved back and forth 10 times. 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 testing machine (Tribostation TYPE:32, travel speed 1000 mm / min.) manufactured by Shinto Kagaku Co., Ltd. ◎: When the number of wounds is 5 or less per cm width ○: When the number of scars is 6-10 per cm width △: When the number of scars is 11-20 per cm width ×: If the number of scars is 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] [Table 2]

[0127] [Table 3]

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

[0129] (Regarding pencil hardness) The optical films in Examples 1-15 and Comparative Examples 2-5 and 9 all have an average surface treatment layer thickness T. 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 1-15 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 the flatness of Example 14 was similar to that of 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 with a thermal shrinkage difference of more than 1.3 (i.e., polyester films c and e), 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 in 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. Similarly, the optical films in 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-colored spots) The optical films in Examples 1-15 and Comparative Examples 5-10 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 unevenness occurred in the optical films. In addition, the optical films in Comparative Examples 1-3 all used base films with very high phase differences (i.e., polyester film c), so the rainbow unevenness was suppressed to a level that was not a problem for practical use. 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 unevenness 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 included in the surface treatment layer, the haze was 0.8% or higher. [Industrial applicability]

[0136] This invention can be used as an optical film and an image display device using the same. [Explanation of Symbols]

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

Claims

1. An optical film having a surface treatment layer formed on a base film, The aforementioned 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 of the base film in the MD direction and the thermal shrinkage rate in the TD direction is 1.3 or less. The average thickness of the surface treatment layer is 2.0 μm or more and 15.0 μm or less. An optical film in which 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 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 The maximum absolute value of the difference between the two. That is the case.

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

Images