Selection Methods for Optical Films, Polarizing Plates, Image Display Devices and Optical Films

Abstract

An optical film is provided that eliminates rainbow spots when viewed with the naked eye and exhibits good color uniformity when viewed obliquely. The optical film is formed by having a low-refractive-index layer on a plastic film. The plastic film has an axis with the highest in-plane refractive index (i.e., a slow axis) and an axis orthogonal to the slow axis (i.e., a fast axis) within the plane of the plastic film. The low-refractive-index layer is located on the surface of the optical film. Linearly polarized light is incident from the surface opposite the low-refractive-index layer of the optical film under specific conditions. The a* and b* values ​​of the transmitted light of the linearly polarized light in the L*a*b* color system are measured at 11 measurement points with different angles. Based on the measurements at the 11 measurement points, the sum of the squares of the differences in a* and b* between the adjacent measurement points is calculated at 10 adjacent points. The optical film's ΣT, representing the sum of the above sums, displays a specific range.

Description

[Technical Field]

[0001] This invention relates to an optical film, a polarizing plate, an image display device, and a method for screening optical films. [Previous Technology]

[0002] Various optical plastic films are mostly used in optical components such as image display devices. For example, in image display devices with polarizing plates on display elements, plastic films are used to protect the polarizers that constitute the polarizing plates. In this specification, "plastic film used to protect the polarizers" is sometimes referred to as "polarizer protective film".

[0003] Plastic films used in image display devices, such as polarizer protective films, are preferably those with excellent mechanical strength. Therefore, extended plastic films are preferably used as plastic films for image display devices.

[0004] When an extended plastic film is placed on a polarizer, the extended plastic film interferes with the polarization state of linearly polarized light passing through the polarizer, resulting in an observed rainbow pattern unevenness. To solve this rainbow pattern problem, technologies such as those in Patent Documents 1-3 have been proposed. Hereinafter, in this specification, "rainbow pattern unevenness" will sometimes be referred to as "rainbow unevenness."

[0005] Patent Document 1 discloses a liquid crystal display device that uses a specific white light source as the light source for an image display device, increases the in-plane retardation of the extended plastic film to between 3000 nm and 30000 nm, and arranges the absorption axis of the polarizer and the slow axis of the extended plastic film at approximately 45 degrees, thereby eliminating rainbow spots when viewing images through polarized sunglasses. However, the method in Patent Document 1 requires the use of an extended plastic film with a large in-plane retardation. Furthermore, extended plastic films with a large in-plane retardation are typically uniaxially extended, which can easily lead to breakage in the extension direction.

[0006] Patent Document 2 discloses a polarizing plate protective film with a reflectance within a specific range at the Brewster angle. Patent Document 3 discloses a polarizing plate protective film with a reflectance difference of less than 20% between the P-wave and S-wave at an incident angle of 50 degrees. The polarizing plate protective films of Patent Documents 2 and 3 eliminate rainbow spots when viewed with the naked eye by reducing the reflectance difference between the polarized components P-wave and S-wave of light from inside the image display device toward the viewer, without increasing the in-plane phase difference of the film as in Patent Document 1.

[0007] The polarizing plate protective films of Patent Documents 2 and 3 can improve the rainbow effect when viewed with the naked eye to a certain extent. However, the polarizing plate protective films of Patent Documents 2 and 3 have the problem that the hue changes with the viewing angle. That is, the polarizing plate protective films of Patent Documents 2 and 3 cannot meet the requirement of hue uniformity when viewed at an oblique angle. [Prior Art Documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 2011-107198 [Patent Document 2] Japanese Patent Application Publication No. 2009-14886 [Patent Document 3] Japanese Patent Application Publication No. 2010-204630 [Summary of the Invention]

[0009] [Problem to be Solved by the Invention] The problem of the present invention is to provide an optical film that eliminates rainbow spots when viewed with the naked eye and has good color uniformity when viewed at an angle, as well as a polarizing plate and an image display device using the above-mentioned optical film. The problem of the present invention is to provide a method for screening an optical film that eliminates rainbow spots when viewed with the naked eye and has good color uniformity when viewed at an angle. [Technical Means for Solving the Problem]

[0010] The present invention provides the following [1] to

[12] . [1] An optical film having a low refractive index layer on a plastic film, the plastic film having an axis with the largest in-plane refractive index, i.e., a slow axis, and an axis orthogonal to the slow axis in the plane of the plastic film, i.e., a fast axis, the low refractive index layer being located on the surface of the optical film, the optical film having a region where ΣT calculated from the following measurement condition 1 satisfies more than 0.04 but less than 0.20; <Measurement condition 1> Linearly polarized light is incident from the surface opposite to the low refractive index layer of the optical film; the incident light, i.e., the linearly polarized light, is defined as light L1; the transmitted light of light L1 passing through the optical film is defined as light L2; After fixing the angle between the slow axis and the vibration direction of light L1 to 45 degrees, the elevation angle of the vibration direction of light L1, with the plane of the optical film as a reference, is an angle of 50 degrees or more and 70 degrees or less, so that light L1 is incident on the optical film; within the range of 50 degrees or more and 70 degrees or less, the elevation angle is changed every 2 degrees, and light L2 is measured at 11 elevation angles; by means of this measurement, light L2 is measured at 11 measurement points; light L2 is converted to the conditions of C light source and 2-degree viewing angle; for light L2 at the nth measurement point among the 11 measurement points, the a* value and b* value of the L*a*b* color system are defined as a*n and b*n; and for light L2 at the (n+1)th measurement point among the 11 measurement points, the a* value and b* value of the L*a*b* color system are defined as a*n1 and b*n1; Based on the measurements of the 11 measurement points, the sum of the squares of the differences between adjacent measurement points a* and b* is calculated; the sum is calculated for each of the 10 adjacent points, and ΣT, representing the total sum, is calculated; ΣT can be expressed by the following Equation 1, ΣT=Σ[{a*n-a*n1}2+{b*n-b*n1}2] (Equation 1).

[0011] [2] As in the optical film of [1], when the maximum value of a* is defined as a*max, the minimum value of a* is defined as a*min, the maximum value of b* is defined as b*max, and the minimum value of b* is defined as b*min based on the measurement of the 11 measurement points, the following equations 2-1 and 2-2 are satisfied: a*max - a*min ≦ 0.250 (Equation 2-1), b*max - b*min ≦ 0.350 (Equation 2-2). [3] For the optical film of [1] or [2], the sum of the square of the difference between a* of adjacent measurement points and the square of the difference between b* of adjacent measurement points is calculated based on the measurements of the 11 measurement points; when the sum is defined as S, S can be expressed by the following Equation 3; when S is calculated at 10 adjacent points respectively, and the maximum value of S at the 10 points is defined as SMAX, SMAX is 0.010 or more and 0.050 or less, S = {a*n - a*n1}2 + {b*n - b*n1}2 (Equation 3). [4] For the optical film of any one of [1] to [3], when the visual reflectance Y value of the optical film is defined as R (%), the product of R and ΣT is 0.05 or more and 0.25 or less. [5] An optical film as described in any of [1] to [4], wherein when the average refractive index of the low refractive index layer is defined as n1 and the average refractive index of the layer adjacent to the low refractive index layer is defined as n2, n2 / n1 does not reach 1.23. [6] An optical film as described in any of [1] to [4], wherein when the average refractive index of the low refractive index layer is defined as n1 and the average refractive index of the layer adjacent to the low refractive index layer is defined as n2, n2 / n1 is 1.05 or more but does not reach 1.23. [7] An optical film as described in any of [1] to [6], wherein the in-plane phase difference of the plastic film is less than 2500 nm.

[0012] [8] An optical film of any one of [1] to [7], wherein the plastic film satisfies the following condition A: <Condition A> A sample of 50 mm in length × 50 mm in width is cut from the plastic film; a total of 5 points are taken as measurement points, including 1 point at the center of the sample and 4 points at each of the four corners of the sample that are 10 mm away from the center; the direction of the slow axis is measured at the 5 points of the sample; the angle between any side of the sample and the direction of the slow axis at each measurement point is defined as D1, D2, D3, D4, D5 respectively; the difference between the maximum value of D1 to D5 and the minimum value of D1 to D5 is 1.5 degrees or more. [9] An optical film of any one of [1] to [8] has one or more layers selected from hard coating and anti-glare layer between the plastic film and the low refractive index layer.

[0013]

[10] A polarizing plate having a polarizer, a first transparent protective plate located on one side of the polarizer and a second transparent protective plate located on the other side of the polarizer, wherein at least one of the first transparent protective plate and the second transparent protective plate is an optical film of any one of [1] to [9], and the low refractive index layer side of the optical film faces the opposite side of the polarizer.

[11] An image display device having a display element and a polarizer and an optical film disposed on the light emitting surface side of the display element, wherein the optical film is an optical film of any one of [1] to [7], and the low refractive index layer side of the optical film faces the opposite side of the display element.

[0014]

[12] A method for screening optical films in an image display device, the image display device having a polarizer and an optical film on the light emitting surface of a display element, the screening method selecting an optical film X that meets the following criteria (1) to (4) as the optical film: (1) the optical film X has a low refractive index layer on a plastic film; (2) the plastic film has an axis with the largest in-plane refractive index, i.e., a slow axis, and an axis orthogonal to the slow axis in the plane of the plastic film, i.e., a fast axis; (3) the low refractive index layer is located on the surface of the optical film X; and (4) the optical film X has a region where the ΣT calculated from the above-mentioned measurement condition 1 satisfies a value greater than 0.04 but less than 0.20. [Effects of the Invention]

[0015] The optical film of the present invention, as well as the polarizing plate and image display device using the above-mentioned optical film, can eliminate rainbow spots when viewed with the naked eye and can also provide good color uniformity when viewed at an angle. The screening method of the optical film of the present invention can efficiently screen optical films that can eliminate rainbow spots when viewed with the naked eye and provide good color uniformity when viewed at an angle.

Implementation Method

[0017] Hereinafter, embodiments of the optical film of the present invention will be described. [Optical Film] The optical film of the present invention is formed by having a low refractive index layer on a plastic film. The plastic film has an axis with the largest in-plane refractive index, namely the slow axis, and an axis orthogonal to the slow axis in the plane of the plastic film, namely the fast axis. The low refractive index layer is located on the surface of the optical film. The optical film has a region where ΣT calculated from the following measurement condition 1 satisfies a value greater than 0.04 but less than 0.20.

[0018] <Measurement Condition 1> Linearly polarized light is incident from the surface opposite to the low refractive index layer of the optical film. The linearly polarized light of the incident light is defined as light L1. The transmitted light of light L1 through the optical film is defined as light L2. After fixing the angle between the slow axis and the vibration direction of light L1 at 45 degrees, the elevation angle of the vibration direction of light L1, with the plane of the optical film as a reference, is an angle of 50 degrees or more and 70 degrees or less, so that light L1 is incident on the optical film. The elevation angle is changed every 2 degrees within the range of 50 degrees or more and 70 degrees or less, and light L2 is measured at 11 elevation angles. By the above measurement, light L2 is measured at 11 measurement points. The light L2 is converted to the conditions of C light source and 2-degree viewing angle. Regarding the light L2 of the nth measurement point among the 11 measurement points, the a* value and b* value of the L*a*b* color system are defined as a*n and b*n. Furthermore, regarding the light L2 at the (n+1)th of the 11 measurement points, the a* and b* values ​​of the L*a*b* color system are defined as a*n1 and b*n1, respectively. Based on the measurements at the aforementioned 11 measurement points, the sum of the squares of the differences in a* between adjacent measurement points and the squares of the differences in b* between adjacent measurement points is calculated. This sum is calculated at each of the 10 adjacent points, and ΣT, representing the total sum of the above sums, is calculated. ΣT can be expressed by the following Equation 1: ΣT=Σ[{a*n-a*n1}2+{b*n-b*n1}2] (Equation 1)

[0019] In this specification, the measurements under measurement condition 1 above, as well as the measurements described later (measurements of in-plane phase difference, phase difference in the thickness direction, direction of the slow axis, and visual reflectance Y value, etc.), unless otherwise specified, are performed in an environment with a temperature of 23℃±5℃ and a relative humidity of 40% or more and 65% or less. Furthermore, before each measurement, the sample is exposed to the above-mentioned environment for 30 minutes or more and 60 minutes or less.

[0020] In this specification, the a* and b* values ​​are based on the L*a*b* color system standardized by the International Commission on Illumination (CIE) in 1976. The L*a*b* color system is adopted in JIS Z8781-4:2013.

[0021] FIG1 is a cross-sectional view showing an embodiment of the optical film 100 of the present invention. As shown in FIG1, the optical film 100 of the present invention has a low refractive index layer 30 on the plastic film 10. The optical film 100 of the present invention may also have layers other than the plastic film 10 and the low refractive index layer 30. As layers other than the plastic film 10 and the low refractive index layer 30, examples include hard coating layers, anti-glare layers, and high refractive index layers. The optical film 100 of FIG1 has a hard coating layer 20 between the plastic film 10 and the low refractive index layer 30.

[0022] In this specification, the term "in-plane of the plastic film" means "in-plane in the direction orthogonal to the thickness direction of the plastic film" unless otherwise specified. In the case of Figure 1, the XY plane of the plastic film is equivalent to the in-plane of the plastic film.

[0023] Regarding Measurement Condition 1: Figure 2 is a schematic diagram showing an example of a measurement performed under Measurement Condition 1. Under Condition 1, linearly polarized light, i.e., light L1, is incident from the surface opposite to the low-refractive-index layer of the optical film. In Figure 2(a), the optical film 100 is disposed between the light source A1 and the detector A2. Also, in Figure 2(a), linearly polarized light, i.e., light L1, is emitted from the light source, and the aforementioned light L1 is incident on the surface opposite to the low-refractive-index layer 30 of the optical film 100. The detector A2 is a type that uses regularly transmitted light capable of detecting light L1.

[0024] Under measurement condition 1, light L1 is incident on optical film 100 after the angle between the slow axis of the plastic film and the vibration direction of light L1 is fixed at 45 degrees. Figure 2(b) is a diagram showing the angle between the slow axis "S" of the plastic film and the vibration direction "V" of light L1 when viewed from the XY plane direction of Figure 2(a). In Figure 2(b), θ1 represents the angle between the slow axis "S" of the plastic film and the vibration direction "V" of light L1. Under measurement condition 1, θ1 is fixed at 45 degrees. In addition, the vibration direction "V" of light L1 is actually tilted towards the Z-axis. Under measurement condition 1, after fixing θ1 at 45 degrees, the elevation angle of the vibration direction of light L1, with the plane of the optical film as a reference, is an angle of 50 degrees or more and 70 degrees or less, so that light L1 is incident on the optical film. In Figure 2(a), θ2 represents the elevation angle of the vibration direction of light L1, i.e., "V", with the plane of the optical film as the reference (0 degrees (reference)). In Figure 2(a), "F" represents the fast axis of the optical film. In Figure 2(a), the "F" of the fast axis extends along the Y-axis of Figure 2(a). In measurement condition 1, the elevation angle is changed every 2 degrees within a range of 50 degrees to 70 degrees, and light L2 is measured at 11 elevation angles. By the above measurement, light L2 is measured at 11 measurement points. As a means of changing the elevation angle every 2 degrees within a range of 50 degrees to 70 degrees, for example, the optical film 100 can be tilted by using the "F" of the fast axis as the center of rotation. In measurement condition 1, the elevation angle is set to 50 degrees to 70 degrees because the Brewster angle of plastic films such as polyester films is taken into account. The rainbow-colored spots on the plastic film are easily visible to the naked eye near Brewster Point.

[0025] In measurement condition 1, the light L2 is converted to a C light source and a viewing angle of 2 degrees. Through this conversion, the influence of the light source can be eliminated from the obtained a* and b* values. Regarding the light L2 of the nth measurement point out of the 11 measurement points, the a* and b* values ​​of the L*a*b* color system are defined as a*n and b*n. Furthermore, regarding the light L2 of the (n+1)th measurement point out of the 11 measurement points, the a* and b* values ​​of the L*a*b* color system are defined as a*n1 and b*n1. a*n, b*n, a*n1, and b*n1 are calculated from the converted light L2. The above measurements and calculations can be performed, for example, using a spectrophotometer from JASCO Corporation with product number "V-7100".

[0026] In measurement condition 1, based on the measurements of the above 11 measurement points, the sum of the squares of the differences a* between adjacent measurement points and the squares of the differences b* between adjacent measurement points is calculated. The above sums are calculated for each of the 10 adjacent points, and ΣT, representing the total sum of the above sums, is calculated. The above ΣT can be expressed by the following Equation 1: ΣT=Σ[{{a*n-a*n1}2+{b*n-b*n1}2] (Equation 1)

[0027] The optical film of the present invention requires "a region having a ΣT value greater than 0.04 but less than 0.20". The ΣT represents the change in transmitted hue when the optical film is viewed at an angle of 50 degrees or more and less than 70 degrees. The inventors have determined that the ΣT is an indicator of the visibility of rainbow spots on an optical film having a low-refractive-index layer on a plastic film. Furthermore, the inventors have determined that the lower the reflectivity of the optical film having a low-refractive-index layer on the plastic film, the smaller the ΣT, and the less likely rainbow spots are to be seen. Rainbow spots are caused by transmitted light. Based on the inventors' understanding, it is believed that reducing the ΣT improves visibility. However, the inventors have determined that reducing the ΣT increases the likelihood of hue changes when viewed from an oblique angle, leading to reduced hue uniformity. Furthermore, the inventors have determined that the main reason for reduced hue uniformity is reflected light, not transmitted light. Therefore, the inventors have discovered that by setting the aforementioned ΣT to a specific range, rainbow spots visible to the naked eye can be eliminated, and good color uniformity can be achieved when viewed from an oblique angle. Setting the aforementioned ΣT to a specific value or higher means increasing the reflectivity of the optical film having a low refractive index layer. That is, the inventors have discovered that by intentionally increasing the reflectivity of the optical film having a low refractive index layer, the problems of eliminating rainbow spots visible to the naked eye and achieving good color uniformity when viewed from an oblique angle can be solved. It is believed that by setting the aforementioned ΣT to a specific value or higher, interference of reflected light from the optical film having a low refractive index layer can be suppressed, and good color uniformity when viewed from an oblique angle can be easily achieved. (If ΣT increases, the refractive index of the low-refractive-index layer tends to increase. Furthermore, if the refractive index of the low-refractive-index layer increases, the refractive index difference between the low-refractive-index layer and the layer in contact with it tends to decrease, thus suppressing interference of reflected light from the optical film. As a result, it is believed that good tonal uniformity can be easily achieved when viewed at an oblique angle.)

[0028] When ΣT is 0.04 or less, the interference of reflected light becomes stronger, making it impossible to achieve good uniformity of color tone when viewed at an angle. When ΣT is 0.20 or more, it is impossible to eliminate rainbow spots when viewed with the naked eye. As mentioned above, rainbow spots are caused by transmitted light. Therefore, by making ΣT greater than 0.04 but less than 0.20, the influence of both transmitted and reflected light can be suppressed, resulting in extremely good visibility. ΣT is preferably 0.05 or more, more preferably 0.06 or more. ΣT is preferably 0.15 or less, more preferably 0.10 or less, and even more preferably 0.09 or less.

[0029] In the optical film of the present invention, the preferred range of ΣT may be greater than 0.04 but less than 0.15, greater than 0.04 but less than 0.10, greater than 0.04 but less than 0.09, greater than 0.05 but less than 0.20, greater than 0.05 and less than 0.15, greater than 0.05 and less than 0.10, greater than 0.05 and less than 0.09, greater than 0.06 but less than 0.20, greater than 0.06 and less than 0.15, greater than 0.06 and less than 0.10, greater than 0.06 and less than 0.09, etc. If the refractive index of the low refractive index layer is increased, ΣT tends to increase. If the refractive index of the low refractive index layer is decreased, ΣT tends to decrease. If the refractive index of the low refractive index layer is decreased, the mechanical strength of the low refractive index layer tends to decrease. If the refractive index of the low refractive index layer is increased, the reflectivity of the optical film tends to increase. In order to ensure that the reflectivity of the optical film is within an appropriate range and that the mechanical strength of the low refractive index layer is good, the above-mentioned ΣT is preferably above 0.05 and below 0.09.

[0030] The aforementioned ΣT can be easily set to the above range by "reducing the in-plane phase difference of the plastic film" and "reducing the parameters related to the refractive index, i.e., n2 / n1". When there is a hard coating between the plastic film and the low refractive index layer, the aforementioned ΣT can be easily set to the above range by using a multifunctional (meth)acrylate oligomer with a specific molecular weight range as the free radiation curing compound for forming the hard coating.

[0031] Within the optical film, the proportion of regions where ΣT satisfies ≥0.04 but ≥0.20 is preferably 50% or more, more preferably 70% or more, even more preferably 90% or more, and even more preferably 100%. Similarly, within the optical film, the proportion of regions satisfying various parameters other than ΣT (Equation 2-1, Equation 2-2, in-plane phase difference, phase difference in the thickness direction, etc.) is also preferably 50% or more, more preferably 70% or more, even more preferably 90% or more, and even more preferably 100%.

[0032] Preferably, the optical film of the present invention, based on the measurements at the above 11 measurement points, defines the maximum value of a* as a*max, the minimum value of a* as a*min, the maximum value of b* as b*max, and the minimum value of b* as b*min, satisfies the following equations 2-1 and 2-2: a*max - a*min ≦ 0.250 (Equation 2-1) b*max - b*min ≦ 0.350 (Equation 2-2)

[0033] By satisfying Equations 2-1 and 2-2, the variation in transmitted hue within the range of 50 degrees to 70 degrees becomes less perceptible. Therefore, by satisfying Equations 2-1 and 2-2, rainbow spots can be more easily eliminated when viewed with the naked eye. If ΣT is too small, the left side of Equations 2-1 and 2-2 tends to increase. This is believed to be because if ΣT decreases, the color change will be dominated by the change in the optical distance of the low-refractive-index layer. More specifically, it is believed that this is because the optical distance of the low-refractive-index layer changes linearly with the angle, causing the a* and b* values ​​to increase or decrease monotonically. Therefore, by satisfying Equations 2-1 and 2-2, it is also easy to achieve good hue uniformity when viewed at an angle. The a*max - a*min value in Equation 2-1 is preferably 0.230 or less, more preferably 0.210 or less, and even more preferably 0.200 or less. The lower limit of a*max - a*min in Equation 2-1 is not particularly limited, but is approximately 0.070. By making a*max - a*min above 0.070, excessive expansion of ΣT can be easily suppressed. The lower limit of b*max - b*min in Equation 2-2 is preferably below 0.300, more preferably below 0.250, more preferably below 0.230, more preferably below 0.210, and more preferably below 0.200. The lower limit of b*max - b*min in Equation 2-2 is not particularly limited, but is approximately 0.070. By making b*max - b*min above 0.070, excessive expansion of ΣT can be easily suppressed.

[0034] The preferred range of a*max - a*min in Equation 2-1 can be 0.070 or higher and 0.250 or lower, 0.070 or higher and 0.230 or lower, 0.070 or higher and 0.210 or lower, 0.070 or higher and 0.200 or lower, etc. The preferred range of b*max - b*min in Equation 2-2 can be 0.070 or higher and 0.350 or lower, 0.070 or higher and 0.300 or lower, 0.070 or higher and 0.250 or lower, 0.070 or higher and 0.230 or lower, 0.070 or higher and 0.210 or lower, 0.070 or higher and 0.200 or lower, etc.

[0035] Equations 2-1 and 2-2 can be easily satisfied by "reducing the in-plane phase difference of the plastic film" and "reducing the parameter related to the refractive index, i.e., n2 / n1". When there is a hard coating between the plastic film and the low refractive index layer, Equations 2-1 and 2-2 can be easily satisfied by using a multifunctional (meth)acrylate oligomer with a specific molecular weight range as the free radiation curing compound to form the hard coating.

[0036] a*max is preferably above -1.0 and below 0, more preferably above -0.8 and below -0.1. b*max is preferably above 0 and below 2.0, more preferably above 0.2 and below 1.8.

[0037] The optical film of the present invention preferably satisfies the following configuration. Based on the measurements at the above 11 measurement points, the sum of the squares of the differences a* between adjacent measurement points and the squares of the differences b* between adjacent measurement points is calculated. When the above sum is defined as S, S can be expressed by the following Equation 3. S is calculated at each of the 10 adjacent points. When the maximum value of S at the 10 points is defined as SMAX, SMAX is preferably 0.010 or more and 0.050 or less. S = {a*n - a*n1}² + {b*n - b*n1}² (Equation 3)

[0038] By setting SMAX to 0.050 or less, the variation in transmitted hue in the range of 50 degrees to 70 degrees becomes less perceptible. Therefore, by setting SMAX to 0.050 or less, rainbow spots can be more easily eliminated when viewed with the naked eye. If the reflectivity of the optical film with the low refractive index layer decreases, there is a tendency for SMAX to become smaller. That is, if SMAX is too small, there is a tendency for interference of reflected light from the optical film with the low refractive index layer to become difficult to suppress. Therefore, by setting SMAX to 0.010 or more, the hue uniformity when viewed at an angle can be easily improved. The lower limit of SMAX is preferably 0.011 or more, and even more preferably 0.012 or more. The upper limit of SMAX is preferably 0.040 or less, even more preferably 0.030 or less, and even more preferably 0.025 or less.

[0039] Preferred ranges for SMAX include 0.010 and 0.040, 0.010 and 0.030, 0.010 and 0.025, 0.011 and 0.050, 0.011 and 0.040, 0.011 and 0.030, 0.011 and 0.025, 0.012 and 0.050, 0.012 and 0.040, 0.012 and 0.030, and 0.012 and 0.025. By setting SMAX to 0.010 and 0.040, the increase of b* can be suppressed, thereby suppressing damage to the high-sensitivity of the image display device.

[0040] SMAX can easily be within the above range by "reducing the in-plane phase difference of the plastic film" and "reducing the parameters related to the refractive index, i.e., n2 / n1". When there is a hard coating between the plastic film and the low refractive index layer, SMAX can easily be within the above range by using a multifunctional (meth)acrylate oligomer with a specific molecular weight as the free radiation curing compound that forms the hard coating.

[0041] Preferably, when the visual reflectance Y value of the optical film is defined as R (%), the product of R and ΣT is 0.05 or more and 0.25 or less. By making the product 0.05 or more and 0.25 or less, the effect based on ΣT can be more easily achieved.

[0042] The product is preferably 0.06 or more, and even more preferably 0.07 or more. The product is preferably 0.19 or less, even more preferably 0.13 or less, and even more preferably 0.11 or less. In particular, if the product is 0.11 or less, the image display device can easily achieve a high-end feel.

[0043] The preferred range of the above-mentioned product may include 0.05 or more and 0.19 or less, 0.05 or more and 0.13 or less, 0.05 or more and 0.11 or less, 0.06 or more and 0.25 or less, 0.06 or more and 0.19 or less, 0.06 or more and 0.13 or less, 0.06 or more and 0.11 or less, 0.07 or more and 0.25 or less, 0.07 or more and 0.19 or less, 0.07 or more and 0.13 or less, and 0.07 or more and 0.11 or less.

[0044] <Plastic Film> Examples of resin components contained in plastic film include polyester, polyimide, polyether ether, polyurethane, polypropylene, polymethyl pentene, polyvinyl chloride, polyvinyl acetal, polyether ketone, polymethyl methacrylate, polycarbonate, polyurethane, triacetyl cellulose (TAC), and amorphous olefins (Cyclo-Olefin-Polymer: COP).

[0045] The plastic film preferably has a Brewster angle of 50 degrees or more and 70 degrees or less, more preferably 55 degrees or more and 65 degrees or less. The elevation angle of the optical film in the present invention, measured under condition 1, is 50 degrees or more and 70 degrees or less. Therefore, by making the Brewster angle of the plastic film 50 degrees or more and 70 degrees or less, the effects of the present invention can be easily achieved. Examples of resins with a Brewster angle of 50 degrees or more and 70 degrees or less include acrylic resins such as polymethyl methacrylate, polyesters, TAC, and COP. Among these, polyesters are preferred because they readily provide good mechanical strength.

[0046] Examples of polyesters include polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and polybutylene terephthalate (PBT). Among these, PET is preferred because it has inherently low birefringence and easily reduces in-plane phase difference.

[0047] Plastic film may also contain additives such as ultraviolet absorbers, light stabilizers, antioxidants, antistatic agents, flame retardants, anti-gelling agents, inorganic particles, organic particles, pigments, dyes, antifouling agents, crosslinking agents and surfactants.

[0048] To ensure good mechanical strength, the plastic film is preferably an extended film, and more preferably a biaxially extended film. Biaxially extended films also exhibit better tear resistance than uniaxially extended films. Therefore, the plastic film is preferably a biaxially extended plastic film. In this specification, various embodiments of the plastic film, such as "in-plane phase difference," "phase difference in the thickness direction," and "the difference between the maximum value of D1 to D5 and the minimum value of D1 to D5," are preferred. When the optical film of the present invention is a biaxially extended plastic film, it is even more preferable to satisfy the in-plane phase difference, etc.

[0049] To easily suppress rainbow spots, the in-plane phase difference of the plastic film is preferably 2500 nm or less. Furthermore, reducing the in-plane phase difference of the plastic film easily reduces the aforementioned ΣT. The in-plane phase difference of the plastic film is more preferably 2000 nm or less, more preferably 1500 nm or less, more preferably 1400 nm or less, more preferably 1250 nm or less, more preferably 1150 nm or less, more preferably 1100 nm or less, more preferably 1000 nm or less, more preferably 950 nm or less, more preferably 850 nm or less, and more preferably 600 nm or less. By making the in-plane phase difference of the plastic film 2000 nm or less, the aforementioned ΣT can be easily reduced. Furthermore, if the refractive index of the plastic film differs between the slow axis and fast axis directions, the reflectivity of the optical film will also differ between the slow axis and fast axis directions. To suppress the difference in reflectivity caused by orientation, it is preferable to reduce the refractive index difference between the slow axis and the fast axis of the plastic film. Therefore, the in-plane phase difference of the plastic film is preferably below 1250 nm.

[0050] In order to achieve good mechanical strength, the in-plane phase difference of the plastic film is preferably 20 nm or more. More preferably, the in-plane phase difference of the plastic film is 100 nm or more, more preferably 300 nm or more, more preferably 400 nm or more, and more preferably 520 nm or more.

[0051] The preferred range of in-plane phase difference of the plastic film may include: 20nm or more and 2000nm or less; 20nm or more and 1500nm or less; 20nm or more and 1400nm or less; 20nm or more and 1250nm or less; 20nm or more and 1150nm or less; 20nm or more and 1100nm or less; 20nm or more and 1000nm or less; 20nm or more and 950nm or less; 20nm or more and 850nm or less; 20nm or more and 600nm or less; 100nm or more and 2000nm or less; and 100nm or more and 500nm or less. Below, 100nm and above but below 1400nm, 100nm and above but below 1250nm, 100nm and above but below 1150nm, 20nm and above but below 1100nm, 100nm and above but below 1000nm, 100nm and above but below 950nm, 100nm and above but below 850nm, 100nm and above but below 600nm, 300nm and above but below 2000nm, 300nm and above but below 1500nm, 300nm and above but below 1400nm, 300nm and above but below 1250nm, 300nm and above but below 1 Below 150nm, above 300nm and below 1100nm, above 300nm and below 1000nm, above 300nm and below 950nm, above 300nm and below 850nm, above 300nm and below 600nm, above 400nm and below 2000nm, above 400nm and below 1500nm, above 400nm and below 400nm, above 400nm and below 1250nm, above 400nm and below 1150nm, above 400nm and below 1100nm, above 400nm and below 1000nm, 400 The following wavelength ranges are preferred for plastic films: ≥ 950nm, ≥ 400nm, ≥ 850nm, ≥ 400nm, ≥ 600nm, ≥ 520nm, ≥ 2000nm, ≥ 520nm, ≥ 1500nm, ≥ 520nm, ≥ 1400nm, ≥ 520nm, ≥ 1250nm, ≥ 520nm, ≥ 1150nm, ≥ 520nm, ≥ 1100nm, ≥ 520nm, ≥ 1000nm, ≥ 520nm, ≥ 950nm, ≥ 520nm, ≥ 850nm, and ≥ 520nm, ≤ 600nm. To ensure the in-plane phase difference of the plastic film falls within the aforementioned ranges, it is preferable that the longitudinal (travel direction) elongation ratio is close to the transverse (width direction) elongation ratio. Within the in-plane phase difference range of 520nm to 1400nm in the plastic film, rainbow spots can be easily suppressed and the aforementioned ΣT can be reduced, and the mechanical strength of the plastic film can be easily improved.Furthermore, when the in-plane phase difference of the plastic film is below 1250nm, differences in reflectivity caused by direction can be easily suppressed. By ensuring that the in-plane phase difference of the plastic film is above 50nm, blackout can be easily suppressed. This is because plastic films with an average in-plane phase difference of less than 50nm hardly interfere with linearly polarized light and directly transmit it. On the other hand, plastic films with an average in-plane phase difference of more than 50nm will interfere with linearly polarized light. Blackout refers to the phenomenon of overall darkening when viewing light that has passed through the polarizer and plastic film sequentially through polarized sunglasses.

[0052] The phase difference (Rth) in the thickness direction of the plastic film is preferably 2000 nm or more, more preferably 3000 nm or more, even more preferably 4000 nm or more, and even more preferably 5000 nm or more. The upper limit of Rth is about 10000 nm, preferably below 8000 nm, and even more preferably below 7000 nm. By making Rth within the above range, rainbow spots can be suppressed more easily. To suppress rainbow spots, it is especially preferable that Rth is above 5000 nm. Furthermore, in order to make the plastic film have good pencil hardness, Rth is preferably above 5000 nm. To suppress the breakage of the plastic film, it is preferable that Rth is below 10000 nm.

[0053] Preferred ranges for the Rth of the plastic film include 2000nm and above but less than 10000nm, 2000nm and above but less than 8000nm, 2000nm and above but less than 7000nm, 3000nm and above but less than 10000nm, 3000nm and above but less than 8000nm, 3000nm and above but less than 7000nm, 4000nm and above but less than 10000nm, 4000nm and above but less than 8000nm, 4000nm and above but less than 7000nm, 5000nm and above but less than 10000nm, 5000nm and above but less than 8000nm, and 5000nm and above but less than 7000nm. To achieve the Rth of the plastic film within the above ranges, it is preferable to increase the elongation ratio in both the longitudinal (travel direction) and transverse (width direction). By increasing the elongation ratio in both the travel and width directions, the refractive index in the thickness direction of the plastic film will decrease, thus easily increasing Rth.

[0054] By making the in-plane phase difference and the phase difference in the thickness direction within the above range, the degree of stretching of the plastic film can be made close to the biaxiality, which easily makes the mechanical strength of the plastic film good.

[0055] Regarding the in-plane phase difference (Re) and the thickness-direction phase difference (Rth) of the plastic film, when the refractive index in the slow axis direction is defined as nx, the refractive index in the fast axis direction is defined as ny, the refractive index in the thickness direction of the plastic film is defined as nz, and the thickness of the plastic film is defined as T [nm], it can be expressed by the following equations i and ii. Furthermore, in this specification, the refractive index, in-plane phase difference, and thickness-direction phase difference refer to values ​​at a wavelength of 590nm. Re = (nx - ny) × T [nm] (Equation i) Rth = ((nx + ny) / 2 - nz) × T [nm] (Equation ii)

[0056] The direction of the slow axis, the in-plane phase difference, and the phase difference in the thickness direction can be measured, for example, using the Otsuka Electronics Co., Ltd. trade name "RETS-100". When using the Otsuka Electronics Co., Ltd. trade name "RETS-100" to measure in-plane phase differences, it is preferable to prepare for the measurement in the following order (A1) to (A4).

[0057] (A1) First, to stabilize the RETS-100 light source, turn on the light source and leave it for at least 60 minutes. Then, select the rotating analyzer method and select the θ mode (mode for measuring angular direction phase difference and calculating Rth). By selecting this θ mode, the stage becomes a tilted rotating stage. (A2) Next, input the following measurement conditions into the RETS-100. (Measurement conditions) ・Retardation measurement range: rotating analyzer method ・Measurement point diameter: 5mm ・Tilting angle range: 0° ・Measurement wavelength range: 400nm and above and 800nm ​​and below ・Average refractive index of the plastic film. For example, in the case of PET film, set N = 1.617. In addition, the average refractive index N of the plastic film can be calculated based on nx, ny, and nz using the formula (N = (nx + ny + nz) / 3). • Thickness: The thickness is measured separately using SEM or optical microscope (A3). Next, background data is obtained without placing the sample on this device. This operation is performed each time the light source is turned on, with the device as a closed system. (A4) Then, the sample is placed on the stage inside the device for measurement.

[0058] Regarding the in-plane phase difference and the phase difference in the thickness direction, as well as the direction of the slow axis, it is preferable to cut a sample with a length of 50mm and a width of 50mm from the plastic film and set it as the average value of the five measurements of the above sample. The five measurement locations are one at the center of the sample and four at four points that move 10mm from the four corners of the sample toward the center of the sample (the five black dots in Figure 5).

[0059] The in-plane phase differences measured at the five locations of the above samples are defined as Re1, Re2, Re3, Re4, and Re5, respectively, and the phase differences in the thickness direction measured at the five locations of the above samples are defined as Rth1, Rth2, Rth3, Rth4, and Rth5, respectively. Preferably, the average of Re1 / Rth1, Re2 / Rth2, Re3 / Rth3, Re4 / Rth4, and Re5 / Rth5 for the plastic film is below 0.20.

[0060] A small ratio (Re / Rth) between the in-plane phase difference and the phase difference in the thickness direction indicates that the biaxial elongation of the plastic film is nearly uniform. Therefore, by making Re / Rth below 0.20, the mechanical strength of the plastic film is good. Re / Rth is more preferably below 0.18, and even more preferably below 0.16. The lower limit of Re / Rth is about 0.01. The Re / Rth of a fully uniaxially elongated plastic film is 2.0. General-purpose uniaxially elongated plastic films also slightly elongate in the direction of travel. Therefore, the Re / Rth of general-purpose uniaxially elongated plastic films is about 1.0.

[0061] The ratios Re1 / Rth1, Re2 / Rth2, Re3 / Rth3, Re4 / Rth4, and Re5 / Rth5 are preferably 0.20 or less, more preferably 0.18 or less, and even more preferably 0.16 or less. The lower limit of these ratios is about 0.01.

[0062] When a plastic film has layers and films that affect the values ​​of the in-plane phase difference and the phase difference in the thickness direction, the in-plane phase difference and the phase difference in the thickness direction of the plastic film can be measured after peeling off these layers and films. Alternatively, layers formed by coating generally do not affect the values ​​of the in-plane phase difference and the phase difference in the thickness direction. The following method can be used to peel off layers and films that affect the values ​​of the in-plane phase difference and the phase difference in the thickness direction: <Peeling Method> The following method can be used: Immerse a sample of 5 cm square or larger in warm water at 80°C or higher and 90°C or lower for 5 minutes. Then, remove the sample from the warm water and leave it at room temperature for 10 minutes or more. Then, immerse it in warm water again for 5 minutes. Remove the sample from the warm water. Use a cutting machine or the like to create cuts on the sample. Then, use the cuts to peel off the layers and films.

[0063] In the above-mentioned methods, it is preferable to immerse the sample in warm water while the edge of the sample is attached to a metal frame or the like.

[0064] The plastic film preferably satisfies the following condition A. <Condition A> The direction of the slow axis is measured at 5 locations of the above sample. When the angle between any side of the above sample and the direction of the slow axis at each measurement location is defined as D1, D2, D3, D4, and D5, respectively, the difference between the maximum value of D1 to D5 and the minimum value of D1 to D5 is 1.5 degrees or more.

[0065] If the slow axis of the plastic film is aligned neatly, the aforementioned ΣT tends to increase, making the rainbow spots easily visible. On the other hand, if the slow axis of the plastic film is altered, the rainbow spots become unclear and difficult to see. Therefore, by satisfying condition A, the phenomenon of rainbow spots being visible to the naked eye can be easily suppressed. In other words, by satisfying condition A, the aforementioned ΣT can easily meet the aforementioned range. Commonly used extended plastic films are designed so that the direction of the slow axis does not deviate. However, as mentioned above, by deliberately shifting the direction of the slow axis of the plastic film, rainbow spots can be easily suppressed. Furthermore, even if the slow axis is disordered and inconsistent in a large area, the suppression effect of rainbow spots is small, but in a relatively small area of ​​50mm x 50mm, the disordered and inconsistent slow axis can easily suppress rainbow spots.

[0066] In this specification, "calculate the direction of the slow axis at the above ΣT" means "the average direction of the slow axis at 5 locations of the above sample".

[0067] Regarding any one side of the sample that is the reference for the angle between the sample and the slow axis direction in condition A, as long as all D1 to D5 are based on the same side, then either the longitudinal side or the transverse side of the sample is acceptable.

[0068] Furthermore, plastic films that satisfy condition A are preferable because they exhibit good bending resistance. On the other hand, general alignment films that do not satisfy condition A but have a consistent slow axis will break or develop strong bending habits after a bending test. Specifically, the uniaxially stretched film of Patent Document 1 breaks when bent along the slow axis, but develops strong bending habits when bent in a direction orthogonal to the slow axis. Similarly, general biaxially stretched films develop strong bending habits when bent in a direction orthogonal to the slow axis. Plastic films that satisfy condition A are preferable because they can suppress the development of bending habits or breakage after a bending test, regardless of the bending direction. Furthermore, plastic films that satisfy condition A are preferable because they can easily suppress micro-cracks in the plastic film after a bending test. Furthermore, plastic films that meet condition A are better because they easily maintain good bending resistance even with a hard pencil.

[0069] The difference between the maximum value of D1 to D5 and the minimum value of D1 to D5 is preferably 2.0 degrees or more, more preferably 3.0 degrees or more, and even more preferably 3.5 degrees or more. Furthermore, if the difference between the maximum value of D1 to D5 and the minimum value of D1 to D5 is too large, the orientation of the plastic film tends to decrease, and the mechanical strength decreases. Therefore, the above difference is preferably 20.0 degrees or less, more preferably 17.0 degrees or less, more preferably 15.0 degrees or less, more preferably 10.0 degrees or less, more preferably 9.0 degrees or less, and even more preferably 8.0 degrees or less.

[0070] In condition A, the preferred range of the difference between the maximum and minimum values ​​of D1 to D5 can be, for example, 1.5 degrees or more and 20.0 degrees or less, 2.0 degrees or more and 20.0 degrees or less, 3.0 degrees or more and 20.0 degrees or less, 3.5 degrees or more and 20.0 degrees or less, 1.5 degrees or more and 17.0 degrees or less, 2.0 degrees or more and 17.0 degrees or less, 3.0 degrees or more and 17.0 degrees or less, 3.5 degrees or more and 17.0 degrees or less, 1.5 degrees or more and 15.0 degrees or less, 2.0 degrees or more and 15.0 degrees or less, 3.0 degrees or more and 17 ... Below 15.0 degrees, above 3.5 degrees and below 15.0 degrees, above 1.5 degrees and below 10.0 degrees, above 2.0 degrees and below 10.0 degrees, above 3.0 degrees and below 10.0 degrees, above 3.5 degrees and below 10.0 degrees, above 1.5 degrees and below 9.0 degrees, above 2.0 degrees and below 9.0 degrees, above 3.0 degrees and below 9.0 degrees, above 3.5 degrees and below 9.0 degrees, above 1.5 degrees and below 8.0 degrees, above 2.0 degrees and below 8.0 degrees, above 3.0 degrees and below 8.0 degrees, above 3.5 degrees and below 8.0 degrees.

[0071] Preferably, the plastic film has D1 to D5 values ​​of 5 degrees or higher and 30 degrees or lower, or 60 degrees or higher and 85 degrees or lower, more preferably 7 degrees or higher and 25 degrees or higher, or 65 degrees or higher and 83 degrees or lower, and even more preferably 10 degrees or higher and 23 degrees or 67 degrees or higher and 80 degrees or lower. By ensuring that D1 to D5 are 5 degrees or higher or 85 degrees or lower, caking when viewed through polarized sunglasses can be easily suppressed. Furthermore, by ensuring that D1 to D5 are 30 degrees or lower or 60 degrees or higher, the reduction in mechanical strength caused by the decrease in the orientation of the plastic film can be easily suppressed.

[0072] Plastic film, for example, may be in sheet form or roll form. Regardless of whether it is sheet or roll, a sample measuring 50mm x 50mm can be cut from any part of the plastic film, provided the following criteria are followed. However, when the longitudinal and transverse orientations of the sheet or roll can be confirmed, the sample is cut along the confirmed longitudinal and transverse directions. For example, in the case of a roll, the direction of travel (MD direction) can be considered longitudinal, and the width direction (TD direction) can be considered transverse. When the direction of travel and width of the sheet can be confirmed, the direction of travel can be considered longitudinal, and the width direction can be considered transverse. When the direction of travel and width of the sheet are difficult to confirm, and the sheet is rectangular or square, the longitudinal and transverse orientations can be confirmed by the four sides forming the rectangle or square. When it is difficult to determine the direction of travel and width of the sheet, and the sheet is a shape other than a rectangle or square (circle, triangle, etc.), draw the largest rectangle or square whose area does not exceed the outer frame of the sheet. Use the sides of the drawn rectangle or square to determine the longitudinal and transverse directions. The sample is cut 10mm from the corner of the sheet or roll. In the case of a sheet shape, for a sample of 50mm x 50mm, prioritize cutting from the four corners of the sheet, followed by cutting from the center. Then, if there is still an area exceeding 50mm x 50mm after cutting from the four corners and the center, simply cut a sample of up to 50mm x 50mm from the remaining area. Furthermore, when multiple samples measuring 50mm x 50mm can be obtained from sheet plastic film, it is preferable that the proportion of samples satisfying condition A among the multiple samples is 50% or more, more preferably 70% or more, even more preferably 90% or more, and even more preferably 100%. The same applies to other parameters such as in-plane phase difference, thickness direction phase difference, and Re / Rth. Although the physical properties of rolled plastic film are prone to variation in the width direction, their physical properties are generally the same in the direction of travel. Therefore, when a sample obtained from a specific position in the width direction of the roll satisfies specific physical properties such as condition A, for the same position in the width direction, it can be designed to satisfy specific physical properties throughout the entire direction of travel of the roll.

[0073] Preferably, the plastic film will not crack or break after 100,000 folding tests as shown in the embodiment (more preferably after 300,000 tests). Furthermore, preferably, after 100,000 folding tests as shown in the embodiment (more preferably after 300,000 tests), when the test sample is placed on a horizontal stage, the angle at which the end of the sample tilts up from the stage is 20 degrees or less, more preferably 15 degrees or less. An angle at which the sample tilts up from the end of 15 degrees or less means that it is less likely to exhibit habits caused by folding. Also, the average of the slow axis direction and the average of the fast axis direction of the plastic film preferably exhibit the above results (no cracks, breaks, or habits caused by folding. The angle at which the end of the sample tilts up after the test is 20 degrees or less). In addition, if a uniaxially stretched plastic film is subjected to a folding test, it will break in the stretching direction and will strongly retain bending habits in the direction orthogonal to the stretching direction. Therefore, among stretched films, a biaxially stretched plastic film is preferred.

[0074] Regarding the thickness of the plastic film, the lower limit is preferably 10μm or more, more preferably 15μm or more, more preferably 21μm or more, more preferably 25μm or more, more preferably 30μm or more, and the upper limit is preferably 200μm or less, more preferably 180μm or less, more preferably 150μm or less, more preferably 100μm or less, more preferably 80μm or less, more preferably 60μm or less, more preferably 50μm or less. A thickness of 10μm or more easily ensures good mechanical strength. Furthermore, to reduce moisture permeability and extend the lifespan of the polarizing plate, a thickness of 21μm or more is preferred, more preferably 30μm or more. Also, if the panel size is large (50 inches or more), deformation can easily occur due to the weight of the plastic film itself when the panel is vertically erected. To suppress such deformation, a plastic film thickness of 30μm or more is preferred. By reducing the thickness to below 200μm, the in-plane phase difference of the plastic film can be easily reduced to below 2500nm. Furthermore, for the purpose of thinning panels and image display devices, the thickness of the plastic film is preferably below 60μm, and more preferably below 50μm.

[0075] A preferred range for the thickness of the plastic film, for example, is 10 μm or more and 200 μm or less, 15 μm or more and 200 μm or less, 21 μm or more and 200 μm or less, 25 μm or more and 200 μm or less, 30 μm or more and 200 μm or less, 10 μm or more and 180 μm or less, 15 μm or more and 180 μm or less, 21 μm or more and 180 μm or less, 25 μm or more and 180 μm or less, 30 μm or more and 180 μm or less, 10 μm or more and 150 μm or less, 15 μm or more and 150 μm or less, 21 μm or more and 150 μm or less, 25 μm or more and 150 μm or less, 30 μm or more and 150 μm or less, 10 μm or more and 100 μm or less, 15 μm or more. Above and below 100μm, above 21μm and below 100μm, above 25μm and below 100μm, above 30μm and below 100μm, above 10μm and below 80μm, above 15μm and below 80μm, above 21μm and below 80μm, above 25μm and below 80μm, above 30μm and below 80μm, above 10μm and below 60μm, above 15μm and below 60μm, above 21μm and below 60μm, above 25μm and below 60μm, above 30μm and below 60μm, above 10μm and below 50μm, above 15μm and below 50μm, above 21μm and below 50μm, above 25μm and below 50μm, above 30μm and below 50μm.

[0076] The plastic film preferably has a haze of 3.0% or less, more preferably 2.0% or less, even more preferably 1.5% or less, and even more preferably 1.0% or less, in accordance with JIS K7136:2000. The plastic film preferably has a total light transmittance of 80% or more, more preferably 85% or more, and even more preferably 90% or more, in accordance with JIS K7361-1:1997.

[0077] <Erosion Rate> When the average erosion rate of the plastic film from the surface to a depth of 20 μm is defined as E0-20, E0-20 is preferably 1.4 μm / g or higher.

[0078] In this specification, E0-20 is measured under the following test conditions. <Test Conditions> A test solution, prepared by mixing pure water, a dispersion, and spherical silica with an average particle size within ±8% of 4.2 μm at a mass ratio of 968:2:30, is placed in a container. The test solution in the container is then fed to a nozzle. Compressed air is supplied to the nozzle to accelerate the test solution within it. A specific amount of the test solution is vertically sprayed from the nozzle's nozzle tip orifice onto the first surface of the plastic film, causing the spherical silica particles in the test solution to impact the plastic film. The nozzle has a cross-sectional shape of a 1 mm × 1 mm square, and the distance between the nozzle orifice and the plastic film is 4 mm. Furthermore, the flow rates of the test solution and compressed air supplied to the nozzle, the pressure of the compressed air, and the pressure of the test solution within the nozzle are set to specific values ​​adjusted by the correction described later. After spraying a specific amount of the test solution, the spraying of the test solution is temporarily stopped. After temporarily stopping the spraying of the test solution, the cross-sectional profile of the plastic film at the point where the spherical silica particles in the test solution impact the film is measured. The operation is performed in a cycle consisting of three steps: "spraying a specific amount of the test solution from the nozzle," "temporarily stopping the spraying of the test solution after spraying a specific amount," and "measuring the cross-sectional profile after temporarily stopping the spraying of the test solution," until the depth of the cross-sectional profile exceeds 20 μm. Then, the erosion rate (μm / g) of the plastic film is calculated for each cycle up to a depth of 20 μm. The erosion rate of the plastic film for each cycle up to a depth of 20 μm is averaged to calculate E0-20.

[0079] <Correction> The test solution is stored in the container. The test solution in the container is fed to the nozzle. Compressed air is supplied to the nozzle to accelerate the test solution. An arbitrary amount of the test solution is vertically sprayed from the nozzle tip through the spray hole onto a 2mm thick acrylic plate, causing the spherical silica particles in the test solution to impact the acrylic plate. The nozzle has a cross-sectional shape of a 1mm × 1mm square, and the distance between the spray hole and the acrylic plate is 4mm. After spraying an arbitrary amount of the test solution, the spraying of the test solution is temporarily stopped. After temporarily stopping the spraying of the test solution, the cross-sectional profile of the point where the spherical silica particles in the test solution impact the acrylic plate is measured. The erosion rate (μm / g) of the acrylic plate is calculated by dividing the depth (μm) of the cross-sectional profile by the arbitrary amount (g). The erosion rate of the acrylic sheet is taken as 1.88 (μm / g) within ±5% as the acceptance condition. The flow rate of the test liquid, the pressure of the compressed air, and the pressure of the test liquid in the nozzle are adjusted and corrected in such a way that the erosion rate of the acrylic sheet is within the above range.

[0080] Hereinafter, the conditions for measuring the erosion rate and the technical significance of the erosion rate calculated by the above-mentioned conditions will be explained with reference to FIG7. As an apparatus for measuring the erosion rate as shown in FIG7, for example, the MSE test apparatus of Palmeso Co., Ltd. with product number "MSE-A203" can be cited.

[0081] Under the erosion rate determination conditions of the present invention, firstly, a test solution is prepared by mixing pure water, a dispersant, and spherical silica with an average particle size within ±8% of 4.2 μm at a mass ratio of 968:2:30 and storing it in a container (11). Preferably, the test solution is stirred in the container (11). There are no particular restrictions on the dispersant, which is a dispersible spherical silica. For example, the trade name "Demol N" of Koko Pure Chemical Industries Co., Ltd. can be cited. The phrase "with an average particle size within ±8% of 4.2 μm" means that the average particle size is 3.864 μm or more and 4.536 μm or less. In the erosion rate measurement conditions of this specification, the "average particle size of spherical silicon dioxide" is the morphological measurement of the volume average value d50 in the particle size distribution measurement using laser diffraction (the so-called "median particle size"). Regarding the aforementioned spherical silicon dioxide, in the results of the aforementioned particle size distribution measurement, when the occurrence rate of the particle size with the highest occurrence rate is normalized to 100, the width of the particle size with an occurrence rate of 50 is preferably within ±10% of 4.2 μm. The "width of the particle size with an occurrence rate of 50" is expressed as "X-Y (μm)" when "the particle size with an occurrence rate of 50 and located in the positive direction compared to the particle size with an occurrence rate of 100 is defined as X," and "the particle size with an occurrence rate of 50 and located in the negative direction compared to the particle size with an occurrence rate of 100 is defined as Y." Additionally, in this specification, the "width of the particle size at which the occurrence rate is displayed is sometimes referred to as the "full amplitude of the half-peak of the particle size distribution".

[0082] As for spherical silica with an average particle size within ±8% of 4.2 μm, the model number "MSE-BS-5-3" specified by Palmeso Co., Ltd. can be cited as an example. As for spherical silica conforming to the model number "MSE-BS-5-3" specified by Palmeso Co., Ltd., the product number "BS5-3" of Potters-Ballotini Co., Ltd. can be cited as an example.

[0083] The test solution in the container is fed into the nozzle (51). The test solution can be fed to the nozzle, for example, through a test solution piping (21). A flow meter (31) for measuring the flow rate of the test solution is preferably disposed between the container (11) and the nozzle (51). The flow rate of the test solution is set to a value adjusted by the above-mentioned calibration. In Figure 7, the nozzle (51) is disposed in the housing (52) constituting the spray section (50).

[0084] Compressed air is supplied to the nozzle (51). Compressed air can be supplied to the nozzle, for example, through a compressed air piping (22). The location where the compressed air is supplied to the nozzle is preferably upstream of the location where the test liquid is supplied. Upstream refers to the side furthest from the nozzle's injection orifice. Preferably, a flow meter (32) for measuring the flow rate of the compressed air and a pressure gauge (42) for measuring the pressure of the compressed air are installed before the compressed air reaches the nozzle (51). The compressed air can be supplied by an air compressor (not shown). The flow rate and pressure of the compressed air are set to values ​​adjusted by the above-described calibration.

[0085] If compressed air is supplied to the nozzle (51), the test liquid will be mixed with the compressed air and simultaneously accelerated. Then, the accelerated test liquid will be ejected from the spray hole at the front end of the nozzle (51) and impact the plastic film (70) vertically. The plastic film will be mainly abraded by the spherical silicon dioxide particles in the test liquid. Preferably, a pressure gauge (41) for measuring the pressure of the test liquid in the nozzle (51) is provided in the nozzle (51). The pressure gauge (41) is preferably located downstream of the position where the compressed air is supplied and the position where the test liquid is supplied. The pressure of the test liquid in the nozzle (51) is set to the value adjusted as described above.

[0086] The test liquid sprayed from the spray hole at the front end of the nozzle (51) is mixed with air and sprayed in a mist. Therefore, the impact pressure of spherical silica particles on the plastic film can be reduced. Therefore, the amount of wear on the plastic film caused by one spherical silica particle can be slightly suppressed. Figure 8 is a schematic diagram of the state of wear on the plastic film (70) caused by the test liquid containing pure water (A1) and spherical silica (A2) sprayed from the spray section (50). In Figure 8, symbol A3 represents air and symbol A4 represents the worn plastic film. In addition, since the test liquid contains water with excellent cooling effect, it can substantially eliminate the deformation and deterioration of the plastic film caused by the heat during impact. That is, it can substantially eliminate abnormal wear on the plastic film. In addition, water also has the function of cleaning the surface of the worn plastic film and achieving stable wear. In addition, water has the function of accelerating the spherical silica particles or controlling the flow of the test liquid. Furthermore, since the plastic film is subjected to impacts from a large number of spherical silica particles, the influence caused by subtle differences in the physical properties of individual spherical silica particles can be eliminated. Moreover, the measurement conditions of this invention are as follows: the flow rate of the test liquid supplied to the nozzle, the flow rate of the compressed air supplied to the nozzle, the pressure of the compressed air supplied to the nozzle, and the pressure of the test liquid inside the nozzle are set to the values ​​adjusted as described above; the cross-sectional shape of the nozzle is specified as a 1mm × 1mm square; and the distance between the spray hole and the plastic film is specified as 4mm. This is to specifically identify the factors affecting the amount of wear on the plastic film. The aforementioned distance is the distance shown as "d" in Figure 7, which refers to the perpendicular distance between the spray hole at the nozzle tip and the plastic film. Based on the above, the measurement conditions of this invention can be said to be capable of forming statistically stable wear marks on the plastic film.

[0087] The plastic film (70) can be installed on the sample mounting stage (81) of the measuring device (100). The plastic film (70) is preferably installed on the sample mounting stage (81) through a support (82) such as a stainless steel plate.

[0088] The test solution sprayed onto the plastic film (70) is preferably recovered by a receiver (12) and then returned to the container (11) through a return pipe (23). A return pump (24) is preferably provided between the receiver (12) and the return pipe (23).

[0089] In the measurement conditions of the present invention, the requirements are "after spraying a specific amount of test liquid, temporarily stopping the spraying of the test liquid" and "after temporarily stopping the spraying of the test liquid, measuring the cross-sectional profile of the point where the spherical silica particles in the test liquid impact the plastic film". The cross-sectional profile refers to the cross-sectional shape of the plastic film that is worn by the test liquid. The plastic film is mainly worn by the spherical silica particles in the test liquid. The cross-sectional profile can be measured, for example, by a cross-sectional profile acquisition part (60) such as a stylus-type surface shape measuring device and a laser interferometric surface shape measuring device. In addition, the cross-sectional profile acquisition part (60) is usually positioned separately from the plastic film (70) when the test liquid is sprayed. Therefore, it is preferable that at least one of the plastic film (70) and the cross-sectional profile acquisition part (60) is movable. The product number "MSE-A203" of the MSE testing apparatus of Palmeso Co., Ltd. is a stylus-type device for measuring cross-sectional profiles.

[0090] Furthermore, in the measurement conditions of the present invention, the operation is performed in a cycle consisting of three steps: "spraying a specific amount of test liquid from the nozzle," "temporarily stopping the spraying of the test liquid after spraying the specific amount of test liquid," and "measuring the cross-sectional profile after temporarily stopping the spraying of the test liquid," until the depth of the cross-sectional profile exceeds 20 μm. By performing the above operation, the erosion rate of the plastic film in each cycle can be measured, and the change in the erosion rate of the plastic film can be calculated. The above cycle can continue after the depth of the cross-sectional profile exceeds 20 μm, but it is preferable to end at the point when the depth of the cross-sectional profile exceeds 20 μm. The reason for setting the measurement to "20 μm from the surface of the plastic film" is that the physical properties of the plastic film are easily changed near the surface, and on the other hand, they are more stable towards the interior.

[0091] In this specification, the erosion rate of each cycle can be calculated by dividing the depth (μm) of the cross-sectional profile traversed in each cycle by the amount (g) of test liquid sprayed in each cycle. The depth (μm) of the cross-sectional profile of each cycle is set as the depth of the deepest position of the cross-sectional profile of each cycle.

[0092] The amount of test solution sprayed in each cycle is, in principle, "quantitative", but may vary slightly in each cycle. There is no particular limitation on the amount of test solution sprayed in each cycle, but the lower limit is preferably above 0.5g, more preferably above 1.0g, and the upper limit is preferably below 3.0g, more preferably below 2.0g.

[0093] In the measurement conditions of the present invention, the erosion rate (μm / g) is calculated in each cycle up to a depth of 20 μm in the cross-sectional profile. Then, the erosion rates of each cycle up to a depth of 20 μm in the cross-sectional profile are averaged to calculate E0-20. Although the above-mentioned cycles are performed up to a depth of more than 20 μm in the cross-sectional profile, the data of cycles with a depth of more than 20 μm in the cross-sectional profile will deviate from the data for calculating E0-20.

[0094] Generally speaking, softer plastic films are more easily damaged, while harder ones are less easily damaged. The inventors investigated using values ​​obtained from Picodenter evaluations including the depth direction (Marsh hardness, indentation hardness, elastic recovery work, etc.) as indicators of pencil hardness. However, the aforementioned parameters such as Marsh hardness, indentation hardness, and elastic recovery work are sometimes unsuitable as indicators of pencil hardness. Furthermore, plastic films tend to increase in strength when stretched. Specifically, compared to unstretched plastic films, uniaxially stretched plastic films tend to have better pencil hardness, while biaxially stretched plastic films tend to have better pencil hardness. However, even with biaxially stretched plastic films, pencil hardness is sometimes insufficient. The inventors studied the erosion rate as an indicator of the pencil hardness of plastic films. As mentioned above, since softer plastic films are more easily damaged, while harder ones are less easily damaged, a lower erosion rate is considered to result in better pencil hardness. However, on the contrary, the inventors have discovered that by increasing the erosion rate (E0-20) to 1.4 μm / g or higher, the pencil hardness of the plastic film can be good. Furthermore, regarding the erosion rate of the plastic film, the inventors have found that biaxially oriented plastic films more readily exhibit high values ​​compared to uniaxially oriented plastic films, and the pencil hardness of biaxially oriented plastic films can be determined by the erosion rate.

[0095] The reason why the erosion rate of the plastic film is related to the pencil hardness is believed to be as follows. As mentioned above, in the test conditions of the present invention, the test solution containing water and spherical silica is mixed with air and sprayed in a mist. Therefore, the impact pressure of the spherical silica particles on the plastic film is suppressed to a lower level. Therefore, it is believed that when the plastic film is soft, the stress when the spherical silica impacts the plastic film is easily dispersed, so the plastic film is not easily worn, and the erosion rate is lower. On the other hand, it is believed that when the plastic film is hard, the stress when the spherical silica impacts the plastic film is not easily dispersed, so the plastic film is easily worn, and the erosion rate is higher. Furthermore, the difference in the erosion rate in biaxially stretched plastic films is believed to be caused by differences in the extension of molecular chains and differences in molecular alignment. For example, in principle, molecules in biaxially stretched plastic films extend in-plane, but sometimes there are also molecules that are not fully extended locally in-plane. Therefore, it is believed that if the proportion of insufficiently stretched molecules in localized areas increases, the biaxially stretched plastic film will become locally softer, resulting in a lower erosion rate. Furthermore, it is believed that even biaxially stretched plastic films with equal in-plane phase differences will exhibit different erosion rates due to variations in the orientation of localized molecules. Conversely, even biaxially stretched plastic films with equal erosion rates may sometimes exhibit different in-plane phase differences due to differences in the ratio of the stretching ratio in the traveling direction to the stretching ratio in the width direction.

[0096] To ensure good pencil hardness of the plastic film, E0-20 is preferably 1.4 μm / g or higher, more preferably 1.6 μm / g or higher, more preferably 1.8 μm / g or higher, more preferably 1.9 μm / g or higher, and more preferably 2.0 μm / g or higher. As mentioned above, it is believed that if the proportion of molecules that are not fully extended locally within the plane increases, the erosion rate will decrease. In other words, it is believed that if the erosion rate is high, the proportion of molecules that are not fully extended locally within the plane will decrease. Therefore, by setting E0-20 to 1.4 μm / g or higher, wrinkling of the plastic film can be easily suppressed under high-temperature environments. To prevent the plastic film from cracking, E0-20 is preferably 3.0 μm / g or lower, more preferably 2.5 μm / g or lower, and even more preferably 2.2 μm / g or lower. Even if the value of E0-20 is the same, the characteristics of the plastic film may differ when there are different conditions such as in-plane phase difference. For example, even with the same E0-20 value, when the in-plane phase difference exceeds 1450 nm, the plastic film may still retain bending characteristics or break when folded. Furthermore, for plastic films with an E0-20 value less than 1.4 μm / g, even if a high-hardness curing film is formed on the plastic film, the insufficient hardness of the plastic film sometimes prevents the curing film from achieving good pencil hardness.

[0097] Examples of preferred numerical ranges for E0-20 include, for example, 1.4 μm / g or more and 3.0 μm / g or less, 1.4 μm / g or more and 2.5 μm / g or less, 1.4 μm / g or more and 2.2 μm / g or less, 1.5 μm / g or more and 3.0 μm / g or less, 1.5 μm / g or more and 2.5 μm / g or less, 1.5 μm / g or more and 2.2 μm / g or less, 1.6 μm / g or more and 3.0 μm / g or less, 1.6 μm / g or more and 2.5 μm / g or less, and 1.6 μm / g or more. ≥ 2.2 μm / g, ≥ 1.8 μm / g and ≤ 3.0 μm / g, ≥ 1.8 μm / g and ≤ 2.5 μm / g, ≥ 1.8 μm / g and ≤ 2.2 μm / g, ≥ 1.9 μm / g and ≤ 3.0 μm / g, ≥ 1.9 μm / g and ≤ 2.5 μm / g, ≥ 1.9 μm / g and ≤ 2.2 μm / g, ≥ 2.0 μm / g and ≤ 3.0 μm / g, ≥ 2.0 μm / g and ≤ 2.5 μm / g, ≥ 2.0 μm / g and ≤ 2.2 μm / g.

[0098] The plastic film has two planes, a front plane and a back plane. Preferably, the E0-20 measured from one plane side and the E0-20 measured from the other plane side are both the values ​​described above. Typically, the erosion rate of the plastic film measured from one plane side is approximately the same as the erosion rate measured from the other plane side.

[0099] Before measuring the above-mentioned erosion rate, the above-mentioned correction is performed. For example, the correction can be performed in the following manner.

[0100] <Correction> The test solution is stored in the container. The test solution in the container is fed to the nozzle. Compressed air is supplied to the nozzle to accelerate the test solution. An arbitrary amount of the test solution is vertically sprayed from the nozzle tip through the spray hole onto a 2mm thick acrylic plate, causing the spherical silicon dioxide in the test solution to impact the acrylic plate. The nozzle has a cross-sectional shape of a 1mm × 1mm square, and the distance between the spray hole and the acrylic plate is 4mm. After spraying an arbitrary amount of the test solution, the spraying of the test solution is temporarily stopped. After temporarily stopping the spraying of the test solution, the cross-sectional profile of the point where the spherical silicon dioxide in the test solution impacts the acrylic plate is measured. The erosion rate (μm / g) of the acrylic plate is calculated by dividing the depth (μm) of the cross-sectional profile by the arbitrary amount (g). The erosion rate of the acrylic sheet is taken as 1.88 (μm / g) within ±5% as the acceptance condition. The flow rate of the test liquid, the pressure of the compressed air, and the pressure of the test liquid in the nozzle are adjusted and corrected in such a way that the erosion rate of the acrylic sheet is within the above range.

[0101] The test solution used for calibration is set to be the same as the test solution used in the test conditions described later. Furthermore, the measuring apparatus used for calibration is set to be the same as the measuring apparatus used in the test conditions described later. The difference between calibration and the test conditions described later is that, for example, calibration uses a 2mm thick acrylic plate as a standard sample, while the test conditions use a plastic film as the sample.

[0102] The standard sample is a 2mm thick acrylic sheet, preferably a polymethyl methacrylate (PMMA) sheet. Furthermore, for the 2mm thick acrylic sheet used as the standard sample, when the average erosion rate of the acrylic sheet measured under the following test conditions A is defined as AcE, the AcE is 1.786 μm / g or more and 1.974 μm / g or less. Also, the spherical silica used in the following test conditions A can be the model "MSE-BS-5-3" specified by Palmeso Co., Ltd. As a spherical silica conforming to the model "MSE-BS-5-3" specified by Palmeso Co., Ltd., for example, is product number "BS5-3" from Potters-Ballotini Co., Ltd. <Test Condition A> A test solution prepared by mixing pure water, a dispersant, and spherical silica with an average particle size within ±8% of 4.2 μm at a mass ratio of 968:2:30 is stored in a container. The test solution in the container is then fed into a nozzle. Compressed air is supplied to the nozzle to accelerate the test solution. A specific amount of the test solution is vertically sprayed from the injection orifice at the tip of the nozzle onto the acrylic plate, causing the spherical silica particles in the test solution to impact the acrylic plate. The nozzle has a cross-sectional shape of a 1 mm × 1 mm square, and the distance between the injection orifice and the acrylic plate is 4 mm. Furthermore, regarding the flow rates of the test liquid and compressed air supplied to the nozzle, the pressure of the compressed air, and the pressure of the test liquid within the nozzle, it is assumed that the flow rate of the test liquid is 100 ml / min or more and 150 ml / min or less, the flow rate of the compressed air is 4.96 L / min or more and 7.44 L / min or less, the pressure of the compressed air is 0.184 MPa or more and 0.277 MPa or less, and the pressure of the test liquid within the nozzle is 0.169 MPa or more and 0.254 MPa or less. After spraying 4 g of the test liquid, the spraying of the test liquid is temporarily stopped. After temporarily stopping the spraying of the test liquid, the cross-sectional profile is measured at the point where the spherical silicon dioxide in the test liquid impacts the acrylic plate. Then, the erosion rate of the acrylic plate, AcE (unit: "μm / g"), is calculated by dividing the depth of the cross-sectional profile (μm) by the amount of test liquid sprayed (4 g).

[0103] The calibration system performs the following operation: taking the erosion rate of the acrylic plate as a reference of 1.88 (μm / g) within ±5% as the acceptance condition, and adjusting the flow rate of the test liquid and the compressed air, the pressure of the compressed air, and the pressure of the test liquid in the nozzle in such a way that the erosion rate of the acrylic plate falls within the aforementioned range. The phrase "erosion rate as a reference of 1.88 (μm / g) within ±5%" means, in other words, an erosion rate of 1.786 (μm / g) or higher and 1.974 (μm / g) or lower.

[0104] <σ0-20 / E0-20> Preferably, when the variation in erosion rate, calculated from the erosion rate from the surface of the aforementioned plastic film to a depth of 20 μm, is defined as σ0-20, σ0-20 / E0-20 is 0.100 or less. In this specification, σ0-20 can be calculated from the erosion rate of each cycle from the depth of the cross-sectional profile to 20 μm under the aforementioned measurement conditions.

[0105] σ0-20 / E0-20 represents the coefficient of variation of the erosion rate. A small σ0-20 / E0-20 means that the erosion rate is not easily affected by changes in the thickness direction of the plastic film. By keeping σ0-20 / E0-20 below 0.100, the erosion rate in the thickness direction can be stabilized, which can easily make the pencil harder.

[0106] The upper limit of σ0-20 / E0-20 is preferably 0.080 or less, further preferably 0.070 or less, further preferably 0.060 or less, and further preferably 0.055 or less. There is no particular limitation on the lower limit of σ0-20 / E0-20, but it is generally greater than 0, preferably 0.020 or more, and more preferably 0.035 or more. Furthermore, when the value of σ0-20 / E0-20 is low, the elongation of the plastic film is sometimes weak. Plastic films with weak elongation tend to have poor solvent resistance, are easily broken, and have low stability to heat and humidity. Therefore, σ0-20 / E0-20 is preferably 0.020 or more.

[0107] The preferred numerical range of σ0-20 / E0-20 may include, for example, values ​​greater than 0 but less than 0.100, greater than 0 but less than 0.080, greater than 0 but less than 0.070, greater than 0 but less than 0.060, greater than 0 but less than 0.055, greater than 0.020 and less than 0.100, greater than 0.020 and less than 0.080, greater than 0.020 and less than 0.070, greater than 0.020 and less than 0.060, greater than 0.020 and less than 0.055, greater than 0.035 and less than 0.100, greater than 0.035 and less than 0.080, greater than 0.035 and less than 0.070, greater than 0.035 and less than 0.060, and greater than 0.035 and less than 0.055.

[0108] The plastic film has two planes, a front plane and a back plane. Preferably, the σ0-20 / E0-20 of the plastic film, measured from one plane side and the σ0-20 / E0-20 measured from the other plane side, are both the values ​​described above.

[0109] The pencil hardness of the plastic film is preferably HB or higher, and more preferably F or higher. If the pencil hardness of the plastic film is too high, there is a tendency for the in-plane phase difference of the plastic film to increase. Therefore, the pencil hardness of the plastic film is preferably 2H or lower.

[0110] In this specification, the pencil hardness is measured and determined in the following order (1) to (6). (1) Prepare a sample by cutting the plastic film into 5cm×10cm pieces. (2) Heat the plastic film at 100°C for 10 minutes. After heating, place the plastic film in an environment at 24°C with a relative humidity of 40% to 60% for 30 to 60 minutes. (3) Determine the pencil hardness of the plastic film according to the scratch hardness (pencil method) of JIS K 5600-5-4:1999. Specifically, a pencil with a specific hardness is moved at a 45° angle and a load of 100g at a speed of 3.0mm / sec on the surface of the plastic film to apply a load. (4) After applying the load to the plastic film, heat the sample again at 100°C for 10 minutes. (5) Immediately after reheating, visually evaluate the damage to the plastic film. The environment for visual evaluation is set at 24°C and relative humidity between 40% and 60%. (6) Perform the above operations (1) to (5) 5 times. Then, take the hardest pencil among the pencils that were not damaged in 4 or more of the 5 times as the evaluation object to measure the pencil hardness of the plastic film.

[0111] In the above method for measuring and judging pencil hardness, when hardness B is not damaged in 4 out of 5 tests and hardness F is not damaged in 3 out of 5 tests, hardness B will be judged.

[0112] When the plastic film has a slow axis and a fast axis, it is preferable that the pencil hardness is B or higher in both the slow axis and fast axis directions. The slow axis of the plastic film refers to the direction with the highest refractive index within the surface of the plastic film. The fast axis of the plastic film refers to the direction orthogonal to the slow axis within the surface of the plastic film.

[0113] Regarding the lamination structure of the plastic film, single-layer and multi-layer structures can be cited. Among them, a single-layer structure is preferred. In order to achieve good mechanical strength and suppress rainbow spots, the plastic film is preferably a biaxially extended plastic film with a small in-plane phase difference. Furthermore, in order to minimize the in-plane phase difference of the extended plastic film, it is preferable to make the extension in the traveling direction and the width direction nearly equal. Also, in order to keep the erosion rate of the plastic film within the aforementioned range, it is preferable to make the molecules extend evenly within the plane of the plastic film. Therefore, in order to keep the average in-plane phase difference and erosion rate of the plastic film within the aforementioned range, the control of extension is important. Regarding extension control, in multi-layer structures, due to the differences in the physical properties of each layer, it is difficult to perform fine extension control. Single-layer structures are preferred because it is easier to perform fine extension control.

[0114] Example of Plastic Film Manufacturing The following describes an example of plastic film manufacturing, using biaxially stretched plastic film as a representative example. Biaxially stretched plastic film can be obtained by stretching a resin layer containing the components constituting the plastic film. Methods of stretching include stepwise biaxial stretching and simultaneous biaxial stretching.

[0115] -Step-by-Step Biaxial Extension- Step-by-step biaxial extension involves extending the cast film in the travel direction, followed by extension in the width direction. Extension in the travel direction is typically achieved by the difference in circumferential speed between a pair of extension rollers. Extension in the travel direction can be performed in one stage, or multiple extension roller pairs can be used to perform it in multiple stages. To suppress excessive changes in optical properties such as in-plane phase difference, it is preferable to bring multiple nip rolls close to the extension rollers. The extension ratio in the travel direction is typically 2 times or more and 15 times or less. To suppress excessive changes in optical properties such as in-plane phase difference, it is preferably 2 times or more and 7 times or less, more preferably 3 times or more and 5 times or less, and even more preferably 3 times or more and 4 times or less. To suppress excessive changes in optical properties such as in-plane phase difference, the extension temperature is preferably above the glass transfer temperature of the resin and below the glass transfer temperature +100°C. In the case of PET, the temperature is preferably 70°C or higher and 120°C or lower, more preferably 80°C or higher and 110°C or lower, and even more preferably 95°C or higher and 110°C or lower. The above-mentioned stretching temperature refers to the set temperature of the apparatus. Furthermore, even if the set temperature of the apparatus is set within the above range, time is required for the temperature to stabilize. Therefore, it is preferable to set the temperature within the above range and manufacture the plastic film after the temperature has stabilized. In this specification, the set temperature of the apparatus is described in multiple places. For other parts, it is preferable to manufacture the plastic film after the temperature has stabilized, similar to the above. Regarding the stretching temperature, by rapidly heating the film, the stretching range at low temperatures is shortened, tending to decrease the average value of the in-plane phase difference. On the other hand, by slowly heating the film, the stretching range at low temperatures is extended, tending to improve alignment, increase the average value of the in-plane phase difference, and decrease the variation of the slow axis. When heating during stretching, it is preferable to use a heater that generates turbulence. By heating with turbulent air, a temperature difference is generated in a tiny region within the film surface. This temperature difference causes a slight shift in the alignment axis, easily satisfying condition A. By satisfying condition A with the plastic film, the ΣT of the optical film can be easily made to fall within the aforementioned range.

[0116] Furthermore, regarding the extension in the direction of travel, shortening the extension time tends to decrease the erosion rate, while extending the extension time tends to increase the erosion rate. This is believed to be because if the extension time is short, the molecules cannot extend evenly within the surface of the plastic film; conversely, if the extension time is long, the molecules can easily extend evenly within the surface of the plastic film. That is, to achieve an E0-20 of 1.4 μm / g or higher, it is preferable to extend the extension time. Moreover, by appropriately increasing the extension ratio to a level where the physical properties do not change, and simultaneously extending the extension time, it is easier to achieve an E0-20 of 1.4 μm / g or higher.

[0117] Smoothness, adhesion, and antistatic properties can also be imparted to the film extending in the direction of travel by inline coating or offline coating. Surface treatments such as corona treatment, flame treatment, and plasma treatment may be performed as needed before inline or offline coating. In this specification, layers formed by inline or offline coating are not counted as layers constituting the plastic film.

[0118] The extension in the width direction is usually achieved using the tender method, where clamps hold both ends of the film while it is simultaneously transported, extending it in the width direction. The width extension ratio is typically 2 to 15 times, and more preferably 2 to 5 times to suppress excessive changes in optical properties such as in-plane phase difference, more preferably 3 to 5 times, and even more preferably 3 to 4.5 times. It is preferable that the width extension ratio is higher than the longitudinal extension ratio. The extension temperature is preferably above the glass transfer temperature of the resin and below the glass transfer temperature +110°C, and preferably the temperature gradually increases from upstream to downstream. The aforementioned extension temperature refers to the set temperature of the device. The upstream side refers to the side near the point where the width extension begins. The downstream side refers to the side near the point where the width extension ends. Specifically, when the transverse extension section is divided into two based on length, the temperature difference between the upstream and downstream sections is preferably 20°C or higher, more preferably 30°C or higher, even more preferably 35°C or higher, and still even more preferably 40°C or higher. In the case of PET, the extension temperature of the first stage is preferably 80°C or higher and 120°C or lower, more preferably 90°C or higher and 110°C or lower, and even more preferably 95°C or higher and 105°C or lower. By dividing the width extension section into two sections and differentiating the extension temperatures of the first and second stages, the surface temperatures of the film during the first and second stages of extension can be controlled at different temperatures. Therefore, in each extension stage, alignment and alignment crystallization will not proceed excessively, preventing the plastic film from becoming brittle, thus easily improving pencil hardness.

[0119] For the plastic film that has undergone progressive biaxial stretching as described above, in order to impart planarity and dimensional stability, it is preferable to perform heat treatment at a temperature above the stretching temperature but below the melting point in a tenter frame. The aforementioned heat treatment temperature refers to the set temperature of the apparatus. Specifically, in the case of PET, it is preferable to perform heat curing in a range of 140°C to 240°C, and more preferably 200°C to 250°C. To suppress excessive changes in optical properties such as in-plane phase difference, it is preferable to perform additional stretching of 1% to 10% in the first half of the heat treatment. After heat treatment, the plastic film is slowly cooled to room temperature and then wound. If necessary, relaxation treatment may also be combined with heat treatment and slow cooling. To suppress excessive variations in optical properties such as in-plane phase difference, the relaxation rate during heat treatment is preferably 0.5% or more and 5% or less, more preferably 0.5% or more and 3% or less, even more preferably 0.8% or more and 2.5% or less, and even more preferably 1% or more and 2% or less. To suppress excessive variations in optical properties such as in-plane phase difference, the relaxation rate during slow cooling is preferably 0.5% or more and 3% or less, more preferably 0.5% or more and 2% or less, even more preferably 0.5% or more and 1.5% or less, and even more preferably 0.5% or more and 1.0% or less. The temperature during slow cooling, to ensure good planarity, is preferably 80°C or more and 140°C or less, more preferably 90°C or more and 130°C or less, even more preferably 100°C or more and 130°C or less, and even more preferably 100°C or more and 120°C or less. The aforementioned temperature during slow cooling refers to the set temperature of the device.

[0120] -Synchronous Biaxial Stretching- Synchronous biaxial stretching involves feeding the cast film into a synchronous biaxial tenter frame, gripping both ends of the film with clamps, and simultaneously transporting and / or stretching it in stages in the travel and width directions. Synchronous biaxial stretching machines include scaling type, spiral type, drive motor type, and linear motor type, but the stretching ratio can be arbitrarily changed. A drive motor type or linear motor type, which allows for relaxation processing at any location, is preferred.

[0121] The magnification of the synchronous biaxial extension is typically 6 times or more and 50 times or less as an area magnification. To suppress excessive variations in optical properties such as in-plane phase difference, the area magnification is preferably 8 times or more and 30 times or less, more preferably 9 times or more and 25 times or less, even more preferably 9 times or more and 20 times or less, and even more preferably 10 times or more and 15 times or less. The synchronous biaxial extension is preferably adjusted to achieve the aforementioned area magnification within the range of 2 times or more and 15 times or less in both the travel direction and the width direction. In the case of synchronous biaxial extension, to suppress in-plane alignment differences, it is preferable that the extension magnifications in the travel direction and the width direction are approximately the same, and that the extension speeds in the travel direction and the width direction are also approximately the same.

[0122] To suppress excessive changes in optical properties such as in-plane phase difference, the stretching temperature for synchronous biaxial stretching is preferably above the glass transition temperature of the resin and below the glass transition temperature +120°C. In the case of PET, it is preferably above 80°C and below 160°C, more preferably above 90°C and below 150°C, and even more preferably above 100°C and below 140°C. The above stretching temperature refers to the set temperature of the device.

[0123] To impart planarity and dimensional stability, the film after synchronous biaxial stretching is preferably subjected to heat treatment in the heat-setting chamber of a tenter frame at a temperature above the stretching temperature but below the melting point. The aforementioned heat treatment temperature refers to the set temperature of the apparatus. The aforementioned heat treatment conditions are the same as those for heat treatment after stepwise biaxial stretching.

[0124] <Low Refractive Index Layer> The low refractive index layer improves the anti-reflective properties of the optical film and makes it easier to suppress rainbow spots when viewed with the naked eye. The low refractive index layer is preferably located on the surface of the optical film on the side where the low refractive index layer is located, based on the plastic film. Functional layers such as anti-fouling layers and antistatic layers may also be provided on the low refractive index layer, without hindering the optical film effect of the present invention.

[0125] Light from inside the image display device toward the viewer is linearly polarized during its passage through the polarizer. However, after passing through the plastic film, the polarization state of the linearly polarized light is disturbed, resulting in light mixed with P-waves and S-waves. Since the reflectivity of P-waves differs from that of S-waves, and this reflectivity difference is wavelength-dependent, rainbow spots are visible to the naked eye. Here, when the plastic film has a low-refractive-index layer, it is believed that the aforementioned reflectivity difference can be reduced, thus easily suppressing rainbow spots.

[0126] However, as mentioned above, when the reflectivity of the optical film with the low refractive index layer is reduced, it becomes difficult to achieve good tone uniformity when viewed from the side. This is believed to be due to interference from reflected light from the optical film with the low refractive index layer. Therefore, when the average refractive index of the low refractive index layer is defined as n1, and the average refractive index of the layer adjacent to the low refractive index layer is defined as n2, it is preferable that n2 / n1 does not reach 1.23. By ensuring that n2 / n1 does not reach 1.23, interference from reflected light can be suppressed, and good tone uniformity when viewed from the side can be easily achieved.

[0127] n2 / n1 is preferably 1.20 or less, more preferably 1.15 or less, and even more preferably 1.13 or less. In particular, by making n2 / n1 1.05 or more and 1.15 or less, the wavelength dependence of reflectivity can be easily suppressed. Furthermore, by making n2 / n1 1.05 or more and 1.15 or less, the embrittlement of the low refractive index layer can be easily suppressed. If n2 / n1 is reduced excessively, the perceived reflectivity Y value of the optical film tends to increase. Therefore, n2 / n1 is preferably 1.05 or more, and more preferably 1.07 or more.

[0128] A preferred range for n2 / n1 could be 1.05 or higher but less than 1.23, 1.05 or higher but less than 1.20, 1.05 or higher but less than 1.15, 1.05 or higher but less than 1.13, 1.07 or higher but less than 1.23, 1.07 or higher but less than 1.20, 1.07 or higher but less than 1.15, 1.07 or higher but less than 1.13, etc.

[0129] In order to make it easier to make n2 / n1 fall within the above range, it is preferable to reduce the value of n2. Therefore, the layer adjacent to the low refractive index layer is preferably a plastic film or a hard coating, and more preferably a hard coating.

[0130] The average refractive index of each layer can be determined, for example, from a cross-sectional photograph of the laminate, whether the thickness of each layer is greater than 780 nm or less than 780 nm, and then measured or calculated by the following method.

[0131] - Average refractive index of layers with a thickness exceeding 780 nm- The average refractive index of a layer with a thickness exceeding 780 nm is determined by taking the refractive index of the adhesive component of the layer as the refractive index of the layer. The average refractive index of a layer with a thickness exceeding 780 nm can be calculated, for example, by Becke's method. The refractive index of plastic films, hard coatings, and anti-glare layers is preferably calculated by Becke's method. The Becke method involves cutting the layer to be measured for refractive index using a cutting machine or similar means to prepare a sample in which the adhesive component is in powder form, and then calculating the refractive index using Becke's method according to Method B (for transparent materials in powder or granular form) of JIS K7142:2008.

[0132] -Average refractive index of layers with a thickness of 780 nm or less- It is difficult to obtain adhesive components for layers with a thickness of 780 nm or less. Therefore, the average refractive index of layers with a thickness of 780 nm or less can be calculated, for example, by fabricating a laminate 1 having layers with a thickness of 780 nm or less, in the following order (Y1) and (Y2). The refractive index n1 of the low refractive index layer is preferably calculated in the following order (Y1) and (Y2). (Y1) The average refractive index of the layers constituting the laminate 1 with a thickness greater than 780 nm is calculated using the Beck method described above. Furthermore, the thickness of the layers with a thickness greater than 780 nm and the thickness of the layers with a thickness of 780 nm or less are calculated from a cross-sectional photograph of the laminate. (Y2) Using the average refractive index and thickness information of layers with a thickness exceeding 780 nm calculated in (Y1) above, and the thickness information of layers with a thickness of less than 780 nm, the average refractive index of layers with a thickness of less than 780 nm is calculated by the following fitting method. The average refractive index n1 of the low refractive index layer is preferably within the range described later. 《Fitting Method》 This method is used to calculate the average refractive index by fitting the reflectance spectrum measured by a reflectance spectrophotometer with the reflectance spectrum calculated from an optical model of a multilayer thin film using Fresnel coefficients.

[0133] The average refractive index n2 of the layer adjacent to the low refractive index layer is preferably greater than the average refractive index n1 of the low refractive index layer. n2 preferably makes n1 fall within the range described later, and is also set such that n2 / n1 satisfies the aforementioned range. n2 is preferably 1.42 or higher and 1.60 or lower, more preferably 1.45 or higher and 1.58 or lower.

[0134] From the viewpoint of suppressing rainbow spots, the refractive index of the low-refractive-index layer is preferably 1.45 or less, more preferably 1.43 or less, and even more preferably 1.40 or less. If the refractive index of the low-refractive-index layer is excessively reduced, the ΣT value of the optical film tends to be difficult to meet the above range. Therefore, the refractive index of the low-refractive-index layer is preferably 1.30 or more, more preferably 1.33 or more, and even more preferably 1.35 or more.

[0135] The thickness of the low-refractive-index layer is preferably 60 nm or more and 200 nm or less, more preferably 80 nm or more and 120 nm or less, even more preferably 85 nm or more and 110 nm or less, and even more preferably 90 nm or more and 105 nm or less. The thickness of the low-refractive-index layer is preferably greater than the average particle size of low-refractive-index particles such as hollow particles.

[0136] Methods for forming low-refractive-index layers can be broadly classified into wet methods and dry methods. Examples of wet methods include methods using metal alkoxides or similar materials via sol-gel processes, methods coating with low-refractive-index resins such as fluororesins, and methods using a coating solution containing low-refractive-index particles in the resin composition. Examples of dry methods include methods selecting particles with a desired refractive index from the low-refractive-index particles described later and forming the layer using physical vapor deposition or chemical vapor deposition. Wet methods are superior to dry methods in terms of production efficiency, suppression of oblique reflection tones, and chemical resistance. Among wet methods, for better adhesion, water resistance, scratch resistance, and low refractive index, it is preferable to form the layer using a coating solution containing low-refractive-index particles in the adhesive resin composition.

[0137] Low-refractive-index particles can be either hollow particles or non-hollow particles. As low-refractive-index particles, they may contain only either hollow or non-hollow particles, but it is preferable to contain both. By containing both hollow and non-hollow particles, the reduction in coating strength can be suppressed, and the refractive index of the low-refractive-index layer can be easily and appropriately reduced at the same time. On the other hand, when only hollow particles are contained, the refractive index of the low-refractive-index layer is excessively reduced, and the ΣT of the optical film is difficult to meet the aforementioned range. The materials of the hollow and non-hollow particles can be any inorganic or organic compound such as silicon dioxide and magnesium fluoride, but for the sake of low refractive index and strength, silicon dioxide is preferred. The following explanation focuses on hollow silicon dioxide particles and non-hollow silicon dioxide particles.

[0138] Hollow silica particles refer to particles with an outer shell made of silica, surrounded by a hollow interior containing air. Because they contain air, the refractive index of hollow silica particles is proportionally lower than that of silica itself due to the presence of air. Non-hollow silica particles, on the other hand, are particles that are not hollow like hollow silica particles. Non-hollow silica particles can be, for example, solid silica particles. The shape of hollow and non-hollow silica particles is not particularly limited; they can be true spheres, ellipsoids of revolution, or near-spherical shapes such as polyhedra that approximate spheres. However, considering scratch resistance, true spheres, ellipsoids of revolution, or near-spherical shapes are preferred.

[0139] Hollow silica particles, by containing air inside, achieve the effect of lowering the overall refractive index of the low-refractive-index layer. By using hollow silica particles with a larger particle size and an increased air ratio, the refractive index of the low-refractive-index layer can be further reduced. On the other hand, hollow silica particles tend to have poor mechanical strength. Especially when using hollow silica particles with a larger particle size and an increased air ratio, there is a tendency to easily reduce the scratch resistance of the low-refractive-index layer. Non-hollow silica particles, because they are dispersed in the adhesive resin, achieve the effect of improving the scratch resistance of the low-refractive-index layer.

[0140] In order to contain hollow silica particles and non-hollow silica particles at a high concentration in the adhesive resin, and simultaneously to uniformly disperse the particles in the film thickness direction within the resin, it is important to bring the hollow silica particles close together and to set the average particle size of the hollow silica particles and the average particle size of the non-hollow silica particles within a specific range, thereby allowing the non-hollow particles to enter between the hollow silica particles. The ratio of the average particle size of the non-hollow silica particles to the average particle size of the hollow silica particles (average particle size of non-hollow silica particles / average particle size of hollow silica particles) is preferably 0.29 or less, more preferably 0.27 or less. The above-mentioned average particle size ratio is preferably 0.05 or more, more preferably 0.10 or more. The average particle size of the hollow silicon dioxide particles is preferably smaller than the thickness of the low refractive index layer, for example, 1 nm or more and 150 nm or less. The average particle size of the hollow silicon dioxide particles is preferably 35 nm or more and 100 nm or less, more preferably 50 nm or more and 100 nm or less, and even more preferably 60 nm or more and 80 nm or less. The average particle size of the non-hollow silicon dioxide particles is preferably smaller than the thickness of the low refractive index layer, for example, 0.5 nm or more and 100 nm or less. The average particle size of the non-hollow silicon dioxide particles is preferably 1 nm or more and 30 nm or less, more preferably 5 nm or more and 20 nm or less, and even more preferably 10 nm or more and 15 nm or less.

[0141] The average particle size of low-refractive-index particles can be calculated by the following operations (y1) to (y3). (y1) Take a cross-section of the low-refractive-index layer using STEM. The accelerating voltage of the STEM is preferably set to 10 kV or more and 30 kV or less, and the magnification is preferably set to 50,000 times or more and 300,000 times or less. (y2) Sample 10 particles from the observed image and calculate the particle size of each particle. The particle size is measured as follows: when the cross-section of the particle is held between two parallel straight lines, the distance between the two lines is the distance between the two lines with the largest distance. When the particles are aggregated, the aggregated particles are considered as one particle for measurement. (y3) Perform the same operation 5 times in the observed images of other images of the same sample, and take the average value obtained from the total number of 50 particle sizes as the average particle size of the low-refractive-index particles.

[0142] Hollow silica particles and non-hollow silica particles are preferably coated with a silane coupling agent. Common silane coupling agents are acceptable, but those having (meth)acrylic or epoxy groups are preferred. By subjecting the silica particles to surface treatment with a silane coupling agent, the affinity between the silica particles and the adhesive resin is enhanced, making it less prone to agglomeration of the silica particles. Therefore, the dispersion of the silica particles becomes more uniform.

[0143] The higher the content of hollow silica particles, the higher the filling rate of hollow silica particles in the adhesive resin, and the lower the refractive index of the low-refractive-index layer. Therefore, the content of hollow silica particles relative to 100 parts by mass of adhesive resin is preferably 100 parts by mass or more, and more preferably 120 parts by mass or more. On the other hand, if the content of hollow silica particles is too high, the hollow silica particles tend to become easily damaged or detached, and the mechanical strength, such as scratch resistance, of the low-refractive-index layer decreases. If the content of hollow silica particles is too high, the refractive index of the low-refractive-index layer tends to decrease excessively, and the ΣT value of the optical film is difficult to meet the above range. Therefore, the content of hollow silica particles relative to 100 parts by mass of adhesive resin is preferably 200 parts by mass or less, more preferably 180 parts by mass or less, and even more preferably 160 parts by mass or less.

[0144] Regarding the content of non-hollow silica particles, in order to ensure good scratch resistance of the low refractive index layer, it is preferably 20 parts by mass or more, and more preferably 40 parts by mass or more, relative to 100 parts by mass of adhesive resin. On the other hand, if the content of non-hollow silica particles is too high, the non-hollow silica particles will easily aggregate. Therefore, the content of non-hollow silica particles is preferably 100 parts by mass or less, and more preferably 80 parts by mass or less, relative to 100 parts by mass of adhesive resin.

[0145] The adhesive resin of the low refractive index layer preferably contains a cured product of a free radiation curing resin composition. The free radiation curing resin composition is a composition containing a compound having a free radiation curing functional group (hereinafter also referred to as "free radiation curing compound"). Examples of free radiation curing functional groups include (meth)acrylic, vinyl, allyl, and other vinyl unsaturated groups, as well as epoxy, cyclobutane, etc. The free radiation curing compound is preferably a compound having a vinyl unsaturated group, more preferably a compound having two or more vinyl unsaturated groups, and even more preferably a polyfunctional (meth)acrylate compound having two or more vinyl unsaturated groups. Both monomers and oligomers can be used as polyfunctional (meth)acrylate compounds. Ionizing radiation refers to electromagnetic waves or charged particle rays that contain energy quanta that can cause molecules to aggregate or cross-link. Ultraviolet (UV) or electron beams (EB) are commonly used, but other types of radiation can also be used, such as X-rays, gamma rays, and charged particle rays like alpha rays and ion beams.

[0146] Among multifunctional (meth)acrylate compounds, examples of difunctional (meth)acrylate monomers include ethylene glycol dimethacrylate, bisphenol A tetraethoxydiacrylate, bisphenol A tetrapropoxydiacrylate, and 1,6-hexanediol diacrylate. Examples of trifunctional or higher-functional (meth)acrylate monomers include trimethylolpropane trimethacrylate, neopentyltetramethylol trimethacrylate, neopentyltetramethylol tetramethacrylate, dinepentyltetramethylol hexamethacrylate, dinepentyltetramethylol tetramethacrylate, and isocyanuric acid modified trimethacrylate. The above-mentioned (meth)acrylate monomers may be modified as part of the molecular backbone, or they may be modified with ethylene oxide, propylene oxide, caprolactone, isocyanuric acid, alkyl, cyclic alkyl, aromatic, or bisphenol.

[0147] Examples of multifunctional (meth)acrylate oligomers include amine (meth)acrylates, epoxy (meth)acrylates, polyester (meth)acrylates, polyether (meth)acrylates, and other acrylate polymers. Amino (meth)acrylates can be obtained, for example, by reacting polyols and organic diisocyanates with hydroxy (meth)acrylates. Preferred epoxy (meth)acrylates include (meth)acrylates obtained by reacting trifunctional or higher aromatic epoxy resins, alicyclic epoxy resins, and aliphatic epoxy resins with (meth)acrylate; (meth)acrylates obtained by reacting difunctional or higher aromatic epoxy resins, alicyclic epoxy resins, and aliphatic epoxy resins with polybasic acids and (meth)acrylate; and (meth)acrylates obtained by reacting difunctional or higher aromatic epoxy resins, alicyclic epoxy resins, and aliphatic epoxy resins with phenols and (meth)acrylate. Ionizing radiation hardening compounds can be used alone or in combination of two or more.

[0148] When the ionizing radiation curing compound is an ultraviolet curing compound, the ionizing radiation curing resin composition preferably contains additives such as a photopolymerization initiator or a photopolymerization accelerator. As a photopolymerization initiator, one or more may be selected from acetophenone, diphenyl ketone, α-hydroxyalkylphenone, milchnerone, benzoin, benzil dimethylketal, benzoylbenzoate, α-acyloxime ester, α-aminoalkylphenone, anthraquinone, halogenoketone, and 9-oxosulfur derivatives. The photopolymerization accelerator is one that can reduce polymerization hindrance caused by air during curing and accelerate the curing speed; for example, one or more may be selected from p-dimethylaminobenzoate isoamyl ester and p-dimethylaminobenzoate ethyl ester.

[0149] To improve stain resistance and surface smoothness, a leveling agent may also be included in the low-refractive-index layer. Examples of leveling agents include fluorine-based and polysiloxane-based agents, with polysiloxane being preferred. The inclusion of a polysiloxane-based leveling agent improves the slip properties and stain resistance of the low-reflectivity layer surface. Specific examples of "good stain resistance" include good fingerprint wiping properties and a large contact angle with pure water and hexadecane.

[0150] The content of the leveling agent relative to 100 parts by weight of the adhesive resin is preferably 0.01 parts by weight or more and 10 parts by weight or less, and more preferably 0.05 parts by weight or more and 1 part by weight or less.

[0151] A low refractive index layer may be formed, for example, by coating a low refractive index layer forming liquid and drying it, wherein the low refractive index layer forming liquid is formed by dissolving or dispersing the components constituting the low refractive index layer. In order to adjust the viscosity, the components may be dissolved or dispersed in the low refractive index layer forming liquid, and a solvent may also be contained.

[0152] <Reflectivity> The apparent reflectivity Y value of the optical film of the present invention, measured from the low refractive index layer side, is preferably 4.0% or less, more preferably 2.0% or less, even more preferably 1.7% or less, and even more preferably 1.5% or less. If the apparent reflectivity of the optical film is excessively reduced, the ΣT value tends to be difficult to meet the above range. Therefore, the apparent reflectivity Y value is preferably 0.5% or more, more preferably 0.7% or more, and even more preferably 1.0% or more.

[0153] In this specification, the perceived reflectance Y value refers to the perceived reflectance Y value of the CIE 1931 standard color system. The reflectance is preferably calculated as the average of the values ​​at 8 out of 10 points on a sample, excluding the maximum and minimum values. In this specification, the reflectance of the optical film is measured by preparing a sample with a black plate attached to the opposite side of the reflectance measurement surface of the optical film through a transparent adhesive layer, and then incident light at an angle of 5° from the low-refractive-index layer side of the sample. The light source used for measuring reflectance is preferably a C-type light source. The refractive index difference between the component (e.g., plastic film) in contact with the transparent adhesive layer of the sample and the transparent adhesive layer is preferably within 0.15, more preferably within 0.10, more preferably within 0.05, and more preferably within 0.01. The black sheet is preferably defined by JIS K7361-1:1997 as having a total light transmittance of less than 1%, more preferably less than 0%. The difference between the refractive index of the resin constituting the black sheet and the refractive index of the transparent adhesive layer is preferably less than 0.15, more preferably less than 0.10, more preferably less than 0.05, and more preferably less than 0.01.

[0154] <Haze, Total Light Transmittance> The optical film preferably has a haze of 5% or less, more preferably 4% or less, and even more preferably 3% or less, according to JIS K7136:2000. When anti-glare is required, the upper limit of the haze of the optical film can be 90% or less, or 65% or less, or 40% or less. The optical film preferably has a haze of 0.5% or more, more preferably 1.0% or more, and even more preferably 1.5% or more, according to JIS K7136:2000. The above haze refers to the overall haze of the optical film. The optical film preferably has a total light transmittance of 80% or more, more preferably 90% or more, even more preferably 91% or more, and even more preferably 92% or more, according to JIS K7361-1:1997.

[0155] <Other Layers> The optical film of the present invention may also have layers other than the plastic film and the low-refractive-index layer. The low-refractive-index layer and other layers are preferably optically isotropic. An optically isotropic layer refers to a layer with an in-plane phase difference of less than 20 nm, preferably less than 10 nm, and more preferably less than 5 nm. Examples of other layers besides the plastic film and the low-refractive-index layer include an anti-fouling layer, a hard coating layer, an anti-glare layer, and a high-refractive-index layer, with a hard coating layer and an anti-glare layer being preferred. That is, the optical film of the present invention preferably has one or more layers selected from a hard coating layer and an anti-glare layer between the plastic film and the low-refractive-index layer. Among these, a hard coating layer is preferred. An anti-fouling layer may also be provided on the opposite side of the plastic film side of the low-refractive-index layer, to a extent that does not impede the effect of the optical film of the present invention. For example, a plastic film, a low-refractive-index layer, and an anti-fouling layer may be provided sequentially to a extent that does not impede the effect of the optical film of the present invention.

[0156] 《Hard Coating Layer》 The hard coating layer may be formed as needed to improve the scratch resistance of the optical film. The hard coating layer is preferably formed between the plastic film and the low refractive index layer. When the optical film further has a high refractive index layer, the hard coating layer, the high refractive index layer and the low refractive index layer are preferably disposed on the plastic film in sequence.

[0157] In order to improve scratch resistance, the hard coating preferably contains a hardened material containing a thermosetting resin composition or a free radiation-curing resin composition, and more preferably contains a hardened material containing a free radiation-curing resin composition.

[0158] A thermosetting resin composition is a composition containing at least a thermosetting resin, which is a resin composition that is cured by heating. Examples of thermosetting resins include acrylic resins, amine resins, phenolic resins, urea melamine resins, epoxy resins, unsaturated polyester resins, and polysiloxane resins. A hardener may be added to the thermosetting resin composition as needed. Examples of ionizing radiation-curing resin compositions for hard coatings are the same as those exemplified in the low-refractive-index layer.

[0159] The ionizing radiation curing resin composition of the hard coating preferably contains a multifunctional (meth)acrylate oligomer as the ionizing radiation curing compound. The lower limit of the number average molecular weight of the multifunctional (meth)acrylate oligomer is preferably 2000 or more, more preferably 2500 or more, and the upper limit is preferably 6000 or less, more preferably 5000 or less. In a hard coating formed from a composition containing multifunctional (meth)acrylate oligomers with a number average molecular weight of 2000 or more, the solvent or ionizing radiation curing compound in the coating liquid for forming the low refractive index layer easily penetrates, thus suppressing reflection at the interface between the hard coating and the low refractive index layer. Therefore, interference of reflected light from the optical film can be easily suppressed, and ΣT can be easily made to the above range. A hard coating formed from a composition containing multifunctional (meth)acrylate oligomers with a number average molecular weight of 6000 or less can easily suppress the decrease in hardness of the hard coating.

[0160] Relative to the total amount of free radiation curing compounds in the free radiation curing resin composition of the hard coating, the content of multifunctional (meth)acrylate oligomers with a number average molecular weight of 2,000 or more and 6,000 or less is preferably 5% by mass or more, more preferably 10% by mass or more, and even more preferably 12% by mass or more.

[0161] To ensure good scratch resistance, the thickness of the hard coating is preferably 0.1 μm or more, more preferably 0.5 μm or more, even more preferably 1.0 μm or more, and even more preferably 2.0 μm or more. To suppress curling, the thickness of the hard coating is preferably 100 μm or less, more preferably 50 μm or less, more preferably 30 μm or less, more preferably 20 μm or less, more preferably 15 μm or less, and even more preferably 10 μm or less.

[0162] Anti-glare layer. The anti-glare layer may be formed, for example, by a coating liquid for forming an anti-glare layer containing an adhesive resin composition and particles. As the aforementioned adhesive resin composition, for example, a curable resin composition exemplified in the hard coating layer can be used.

[0163] Regarding particles, both organic and inorganic particles can be used. Examples of organic particles include those composed of polymethyl methacrylate, polyacrylic acid-styrene copolymer, melamine resin, polycarbonate, polystyrene, polyvinyl chloride, benzoguanidine-melamine-formaldehyde condensate, polysiloxane, fluorinated resins, and polyester resins. Examples of inorganic particles include those composed of silicon dioxide, aluminum oxide, antimony, zirconium oxide, and titanium oxide.

[0164] The average particle size of the organic particles in the anti-glare layer varies depending on the thickness of the anti-glare layer, and therefore cannot be generalized. However, it is preferably 0.5 μm or more and 10.0 μm or less, more preferably 1.0 μm or more and 8.0 μm or less, and even more preferably 1.5 μm or more and 6.0 μm or less. Inorganic particles tend to aggregate. Therefore, the average particle size of inorganic particles is not limited to the above, but is preferably 1 nm or more and 10 μm or less.

[0165] The average particle size of the anti-glare layer can be calculated by the following operations (z1) to (z3). (z1) Take an image of the anti-glare layer using an optical microscope or STEM. When the average particle size is in the micrometer range, it is preferable to use an optical microscope to take an image of the plane of the anti-glare layer. In this case, the magnification is preferably 500x or more and 2000x or less. When the average particle size is in the nanometer range, it is preferable to use STEM to take an image of the cross-section of the anti-glare layer. In this case, the magnification is preferably 20,000x or more and 100,000x or less. The accelerating voltage of the STEM is preferably 10kV or more and 30kV or less. (z2) Sample 10 random particles from the observed image and calculate the particle size of each particle. The particle size is determined as follows: when the cross-section of the particle is held between two parallel straight lines, the distance between the two lines is the distance between the two lines with the largest distance. (z3) Perform the same operation 5 times in the observation images of other images of the same sample, and take the average value obtained from the total number of 50 particle sizes as the average particle size of the particles in the anti-glare layer.

[0166] The particle content in the anti-glare layer varies depending on the degree of anti-glare required by the target material, and therefore cannot be generalized. However, relative to 100 parts by mass of resin content, it is preferably 1 part by mass and less than 100 parts by mass, more preferably 5 parts by mass and less than 50 parts by mass, and even more preferably 10 parts by mass and less than 30 parts by mass. In order to impart antistatic properties, control the refractive index, or adjust the shrinkage of the anti-glare layer caused by the curing of the curing resin composition, the anti-glare layer may also contain microparticles with an average particle size of less than 500 nm.

[0167] The thickness of the anti-glare layer is preferably 0.5 μm or more, more preferably 1.0 μm or more, and even more preferably 2.0 μm or more. The thickness of the anti-glare layer is preferably 50 μm or less, more preferably 30 μm or more, more preferably 20 μm or less, more preferably 15 μm or less, and more preferably 10 μm or less. To ensure good hardness of the anti-glare layer, the thickness of the anti-glare layer is preferably greater than the average particle size.

[0168] <Examples of Layer Structure> The following (1) to (5) are examples of the layer structure of the optical film of the present invention. Among the following structures, (2) and (4) are preferred. (1) A structure having a low refractive index layer on a plastic film. (2) A structure having a hard coating layer and a low refractive index layer sequentially on a plastic film. (3) A structure having a high refractive index layer and a low refractive index layer sequentially on a plastic film. (4) A structure having an anti-glare layer and a low refractive index layer sequentially on a plastic film. (5) A structure having a hard coating layer, a high refractive index layer and a low refractive index layer sequentially on a plastic film.

[0169] <Form, Size> The optical film can be in the form of sheets cut to a specific size, or in the form of a roll formed by winding a long sheet. The size of the sheet is not particularly limited, but the maximum diameter is approximately 2 inches or more and 500 inches or less. In this invention, the size of the sheet is preferably 30 inches or more and 100 inches or less, more preferably 40 inches or more and 100 inches or less. The term "maximum diameter" refers to the maximum length when connecting any two points of the optical film. For example, when the optical film is rectangular, the diagonal of the rectangular area is the maximum diameter. When the optical film is circular, the diameter is the maximum diameter. The width and length of the roll are not particularly limited, but generally, the width is approximately 500 mm or more and 5000 mm or less, and the length is approximately 100 m or more and 5000 m or less. The roll-shaped optical film can be cut into sheets to fit the size of image display devices, etc. When cutting, it is best to remove the roller ends with unstable physical properties. The shape of the film is not particularly limited; it can be polygonal (triangle, rectangle, pentagon, etc.), circular, or irregular. When the optical film is rectangular, the aspect ratio is not particularly limited if it is suitable for displaying the image. For example, aspect ratios such as 1:1, 4:3, 16:10, 16:9, 2:1, and 5:4 are acceptable.

[0170] <Applications> The optical film of the present invention is applicable as an optical film for image display devices. Furthermore, the optical film of the present invention is applicable as an optical film disposed on the light-emitting surface side of a display element in an image display device. In this case, it is preferable to have a polarizer between the display element and the optical film of the present invention. When the plastic film meets condition A, residual bending habit or breakage can be suppressed after a bending test, regardless of the bending direction. Therefore, when the plastic film meets condition A, it is more suitable as a plastic film for curved image display devices and foldable image display devices. Furthermore, the optical film of the present invention can also be used as a component in the manufacture of functional films. For example, the optical film of the present invention can be used as the substrate in a transfer sheet having a transfer layer on a substrate. In this case, the transfer layer can be formed on the side opposite to the side of the plastic film having a low refractive index layer. Furthermore, as the above-mentioned component, it can be used in the manufacturing process of functional films to protect or reinforce the substrate of the functional film.

[0171] [Polarizing plate] The polarizing plate of the present invention has a polarizer, a first transparent protective plate located on one side of the polarizer, and a second transparent protective plate located on the other side of the polarizer. At least one of the first transparent protective plate and the second transparent protective plate is the optical film of the present invention, and the low refractive index layer side of the optical film faces the opposite side of the polarizer.

[0172] FIG3 is a cross-sectional view showing an embodiment of the polarizing plate 700 of the present invention. The polarizing plate 700 of FIG3 includes a polarizer 300, a first transparent protective plate 500 disposed on one side of the polarizer, and a second transparent protective plate 600 disposed on the other side of the polarizer. The polarizing plate 700 of FIG3 uses an optical film 100 as the first transparent protective plate 500. In FIG3, the side of the optical film 100 with the low refractive index layer 30 faces the side opposite to the polarizer 300. In the polarizing plate 700 of FIG3, the polarizer 300, the first transparent protective plate 500, and the second transparent protective plate 600 are laminated together through an adhesive layer 400.

[0173] A polarizing plate, for example, is used to impart anti-reflective properties by combining it with a λ / 4 retardation plate. In this case, the λ / 4 retardation plate is placed on the display element of the image display device, and the polarizing plate is placed closer to the viewer than the λ / 4 retardation plate. When used in a liquid crystal display device, the polarizing plate is used to provide the liquid crystal shutter function. In this case, the liquid crystal display device has a lower polarizing plate, a liquid crystal display element, and an upper polarizing plate arranged sequentially from the backlight side, with the absorption axis of the polarizer of the lower polarizing plate and the absorption axis of the polarizer of the upper polarizing plate arranged orthogonally. In the configuration of the liquid crystal display device, the polarizing plate of the present invention can be used as both the upper and lower polarizing plates, and it is preferable to use the polarizing plate of the present invention as the upper polarizing plate. In the upper polarizing plate, it is preferable to use the optical film of the present invention as a transparent protective plate on the light emitting surface side of the polarizer. In the lower polarizing plate, it is preferable to use the optical film of the present invention as a transparent protective plate on the light-injection surface side of the polarizer.

[0174] <Transparent Protective Plate> The polarizing plate of the present invention uses the optical film of the present invention described above as at least one of the first transparent protective plate and the second transparent protective plate. Preferably, both the first transparent protective plate and the second transparent protective plate are optical films of the present invention described above.

[0175] When one of the first and second transparent protective plates is the optical film of the present invention described above, the other transparent protective plate is not particularly limited, but is preferably an optically isotropic transparent protective plate. In this specification, an optically isotropic transparent protective plate refers to one with an in-plane phase difference of less than 20 nm, preferably less than 10 nm, and more preferably less than 5 nm. Examples of optically isotropic transparent protective plates include acrylic films, triacetyl cellulose films, polycarbonate films, and amorphous olefin films.

[0176] <Polarizer> Examples of polarizers include sheet-type polarizers (polyvinyl alcohol film, polyethylene formaldehyde film, polyethylene acetal film, ethylene-vinyl acetate copolymer saponified film, etc.) formed by stretching a film dyed with iodine or the like, wire grid type polarizers composed of multiple parallel metal lines, coated type polarizers coated with lyotropic liquid crystals and dual-color host materials, and multilayer thin film type polarizers. These polarizers can be reflective polarizers that have the function of reflecting non-transmissive polarized light components.

[0177] Preferably, the angle between the polarizer's absorption axis and the slow axis of the plastic film is within 90 degrees ± 5 degrees. More preferably, the angle is within 90 degrees ± 3 degrees, and even more preferably, it is within 90 degrees ± 1 degree.

[0178] [Image Display Device] The image display device of the present invention includes a display element, a polarizer and an optical film disposed on the light emitting surface side of the display element, wherein the optical film is the optical film of the present invention, and the low refractive index layer side of the optical film faces the opposite side of the display element.

[0179] FIG4 is a cross-sectional view showing an embodiment of the image display device of the present invention. The image display device 1000 of FIG4 has an optical film 100 on the light emitting surface side (upper side of FIG4) of the display element 800. In FIG4, the low refractive index layer side of the optical film 100 faces the opposite side of the display element 800. The image display device 1000 of FIG4 has a polarizer 300 between the display element 800 and the optical film 100.

[0180] The image display device 1000 is not limited to the configuration shown in FIG. 4. For example, in FIG. 4, although the components constituting the image display device 1000 are arranged with specific spacing, the components are preferably integrally laminated together through an adhesive layer or the like. The image display device may also have other components not shown, such as optical films. For example, the image display device may also have surface plates such as glass plates and plastic plates. When the image display device has a surface plate, the optical film of the present invention may also be adhered to the surface plate.

[0181] In the image display device of the present invention, the angle between the absorption axis of the polarizer and the slow axis of the plastic film of the optical film is preferably within 90 degrees ± 5 degrees. More preferably, the angle is within 90 degrees ± 3 degrees, and even more preferably, it is within 90 degrees ± 1 degree.

[0182] <Display Element> Examples of display elements include liquid crystal display elements, EL display elements (organic EL display elements, inorganic EL display elements), plasma display elements, display elements using QD (quantum dot), and LED display elements such as mini LED and micro LED display elements. When the display element of the display device is a liquid crystal display element, the side opposite to the resin sheet of the liquid crystal display element must be a backlight.

[0183] The image display device may also be an image display device with a touch panel function. Examples of touch panels include resistive film, capacitive, electromagnetic induction, infrared, and ultrasonic types. The touch panel function may be an added function within the display element, such as an embedded touch panel liquid crystal display element, or it may be a touch panel mounted on the display element.

[0184] If the plastic film meets condition A, the optical film can suppress residual bending behavior or breakage after a bending test. Therefore, if the plastic film meets condition A, the image display device is preferably a curved image display device or a foldable image display device. When the image display device is a curved image display device or a foldable image display device, the display element is preferably an organic EL display element. When the image display device is a curved image display device or a foldable image display device, the glass included in the image display device is preferably thin glass. The thin glass is preferably 5 μm or more and 80 μm or less in thickness.

[0185] <Other Plastic Films> The image display device of the present invention may also have other plastic films without hindering the effects of the present invention. As other plastic films, those with optical isotropy are preferred.

[0186] [Method for screening optical films of image display devices] The method for screening optical films of image display devices of the present invention is a method for screening optical films of image display devices having a polarizer and an optical film on the light emitting surface of a display element. The screening method selects an optical film X that meets the following determination conditions (1) to (4) as the optical film: (1) is an optical film X having a low refractive index layer on a plastic film; (2) the plastic film has an axis with the largest in-plane refractive index, i.e., a slow axis, and an axis orthogonal to the slow axis in the plane of the plastic film, i.e., a fast axis; (3) the low refractive index layer is located on the surface of the optical film X; and (4) the optical film X has a region where ΣT calculated from the following measurement condition 1 satisfies more than 0.04 but less than 0.20.

[0187] <Measurement Condition 1> Linearly polarized light is incident from the surface opposite to the low-refractive-index layer of the optical film. This incident light, i.e., the linearly polarized light, is defined as light L1. The transmitted light L1 passing through the optical film is defined as light L2. The angle between the slow axis and the vibration direction of light L1 is fixed at 45 degrees. The elevation angle of the vibration direction of light L1, with the plane of the optical film as a reference, is set to 50 degrees or more and 70 degrees or less, so that light L1 is incident on the optical film. The elevation angle is changed every 2 degrees within the range of 50 degrees or more and 70 degrees or less, and light L2 is measured at 11 different elevation angles. Light L2 is measured at 11 measurement points using the above measurements. Light L2 is converted to a C-light source and a viewing angle of 2 degrees. Regarding the light L2 at the nth measurement point among the 11 measurement points, the a* and b* values ​​of the L*a*b* color system are defined as a*n and b*n. Furthermore, regarding the light L2 at the (n+1)th of the 11 measurement points, the a* and b* values ​​of the L*a*b* color system are defined as a*n1 and b*n1, respectively. Based on the measurements at the aforementioned 11 measurement points, the sum of the squares of the differences in a* between adjacent measurement points and the squares of the differences in b* between adjacent measurement points is calculated. This sum is calculated at each of the 10 adjacent points, and ΣT, representing the total sum of the above sums, is calculated. ΣT can be expressed by the following Equation 1: ΣT=Σ[{a*n-a*n1}2+{b*n-b*n1}2] (Equation 1)

[0188] In the method for screening optical films in the image display device of the present invention, the angle between the absorption axis of the polarizer and the slow axis of the plastic film of the optical film is preferably within 90 degrees ± 5 degrees. The above angle is more preferably within 90 degrees ± 3 degrees, and even more preferably within 90 degrees ± 1 degree.

[0189] The embodiment of measurement condition 1 in the method for screening optical films of the image display device of the present invention is the same as the embodiment of measurement condition 1 of optical films of the present invention described above.

[0190] The method for screening optical films in an image display device according to the present invention preferably further includes additional determination conditions. Examples of these additional determination conditions include preferred embodiments of the optical films of the present invention (e.g., Formula 2-1, Formula 2-2, n2 / n1, in-plane phase difference of the plastic film, etc.). The method for screening optical films in an image display device according to the present invention is useful for screening optical films in an image display device having a polarizer on the light-emitting surface of the display element. [Example]

[0191] Next, the present invention will be described in further detail by way of examples, but the present invention is not limited to these examples in any way.

[0192] 1. The environment for the following measurements and evaluations is set at a temperature of 23℃±5℃ and a relative humidity of 40% to 65%. Furthermore, before the measurements and evaluations, the samples used for the measurements are exposed to the above environment for 30 minutes to 60 minutes. The samples used for the measurements are obtained from clean and undamaged sources. The measurements and evaluations are performed with the samples in good flatness.

[0193] 1-1. Measurement under Measurement Condition 1: A 5cm × 5cm sample was cut from the optical film of the experimental example. Measurement under Measurement Condition 1 was performed on the above sample. The measuring apparatus was a spectrophotometer with product number "V-7100" from JASCO Corporation. Based on the above measurement results, "ΣT of Equation 1", "(a*max - a*min) of Equation 2-1", "(b*max - b*min) of Equation 2-2", and "maximum sum (SMAX)" were calculated. In Measurement Condition 1, the sum of the squares of the differences of a* between adjacent measurement points and the squares of the differences of b* between adjacent measurement points was calculated at 10 adjacent points. "Maximum sum (SMAX)" refers to the maximum value of the sum at 10 points. Furthermore, the visual reflectance Y value of the optical film in the experimental example was defined as "R (%)", and "R×ΣT" was calculated (the visual reflectance Y value was determined by the methods described in 1-6 below). The results are shown in Table 1.

[0194] 1-2. n1 and n2 For the optical film of the experimental example, the average refractive index n1 of the low refractive index layer was determined by combining the Beck method and the fitting method described herein. Furthermore, for the optical film of the experimental example, the average refractive index n2 of the layer adjacent to the low refractive index layer was measured. Regardless of whether the layer adjacent to the low refractive index layer was a plastic film or a hard coating, n2 was determined by the Beck method described herein. The results are shown in Table 1.

[0195] 1-3. Rainbow Spot Preparation: A liquid crystal display device (EIZO's trade name "EV2450", width: 527.0 mm, height: 596.4 mm, the absorption axis of the polarizer is parallel to the vertical axis of the screen, backlight: using a white light-emitting diode backlight) was prepared. A laminate was formed by depositing the optical film of the experimental example onto the above-mentioned liquid crystal display device through an adhesive layer. At this time, the absorption axis of the polarizer and the slow axis of the plastic film of the optical film were aligned at 90 degrees. Then, the above-mentioned laminate was displayed in white in a dark room environment, and viewed from all positions and directions at a distance of 30 cm to 100 cm away from the laminate. The evaluators were healthy individuals aged 20 to 40 with visual acuity of 0.7 or above, and the presence or absence of rainbow spots was evaluated by the naked eye according to the following criteria. The above visual acuity also includes corrected visual acuity. The results are shown in Table 1. AA: The rainbow pattern is not visible from all positions and directions. A: There are locations where the rainbow pattern can be seen in a very small area, or directions where the rainbow pattern can be seen in a very small area. B: There are numerous locations where the rainbow pattern can be seen in a very small area, or directions where the rainbow pattern can be seen in a very small area. B-: There are numerous locations where the rainbow pattern can be seen in a part of the area, or directions where the rainbow pattern can be seen in a part of the area. C: There are numerous locations where the rainbow pattern can be seen in most of the area, or directions where the rainbow pattern can be seen in most of the area.

[0196] 1-4. Uniformity of Tone: With the power off, the laminates prepared in 1-3 were visually observed in a well-lit room. The well-lit room conditions were set so that the brightness of the laminate surface was between 1000 lux and 1500 lux. Observation was performed from three directions: the front of the laminate, approximately 50 degrees relative to the laminate, and approximately 70 degrees relative to the laminate. The distance between the laminate and the evaluator's eyes was set between 30 cm and 100 cm. Twenty healthy individuals aged 20 to 40 with visual acuity of 0.7 or better were used as evaluators. The uniformity of tone was evaluated when viewed obliquely, according to the following criteria. The results are shown in Table 1. A: When comparing the tone in the three directions, more than 18 people answered that they did not perceive any tone change. B: When comparing the tone in the three directions, more than 15 people answered that they did not perceive any tone change. C: When comparing hues from three directions, 10 to 14 people reported not perceiving any hue change. D: When comparing hues from three directions, 5 to 9 people reported not perceiving any hue change. E: When comparing hues from three directions, 4 or fewer people reported not perceiving any hue change.

[0197] 1-5. Based on the chroma of reflected light, a sample (5cm × 5cm) was prepared by bonding a black plate (Kuraray Co., Ltd., Comoglas DFA2CG 502K (Black) type), with a total light transmittance of 0%, thickness of 2mm, and refractive index of 1.49, to the plastic film side of the optical film of the experimental example. The sample was prepared by setting the direction perpendicular to the surface of the low refractive index layer of the sample to 0 degrees, and incident light on the sample from directions of 5 degrees, 50 degrees, and 70 degrees. The chroma was measured based on the regular reflection of the incident light. Chroma was measured at 10 locations for each sample, and the average value was taken as the chroma for each sample at each angle. Chroma (C*) can be calculated based on the a* and b* values ​​of the L*a*b* color system using the following formula: C* = {(a*)² + (b*)²}¹ / ². The measuring apparatus used was a spectrophotometer with product number "V-7100" from JASCO Corporation. The measurements were performed in the wavelength range of 380 nm to 780 nm, and then converted to brightness perceptible to the human eye using software [software built into the measuring apparatus <JASCO Corporation product number "JASCO spectrum manager">. Conditions for calculating reflectance: C light source and viewing angle of 2 degrees]. The results are shown in Table 1. Chroma was not measured for optical films without a low refractive index layer.

[0198] 1-6. Visual Reflectance Y-value (Reflectance) When the direction perpendicular to the low-refractive-index layer side of the sample prepared in 1-5 is 0 degrees, incident light is shone onto the sample from a direction of 5 degrees, and the reflectance (visual reflectance Y-value) is measured based on the regular reflection of the incident light. The measuring device is a spectrophotometer with product number "V-7100" from JASCO Corporation. The above measuring device is used to measure wavelengths from 380 nm to 780 nm, and then the conversion is performed using software that converts the brightness to what the human eye perceives [software built into the above measuring device <JASCO Corporation product number "JASCO spectrum manager">. Conditions for calculating reflectance: C light source and viewing angle 2 degrees]. The reflectance was measured at 10 locations for each sample, and the average value was taken as the visual reflectance Y-value for each sample. The results are shown in Table 1. The optical film without a low refractive index layer was not subjected to measurement of its visual reflectance Y value.

[0199] 1-7. In-plane phase difference (Re), thickness direction phase difference (Rth), and slow axis direction: A sample measuring 50mm in length and 50mm in width was cut from the plastic film used in the experimental and reference examples prepared or made in "2" below. The direction of travel of the plastic film (MD direction) was considered longitudinal, and the width direction (TD direction) was considered transverse. The in-plane phase difference, thickness direction phase difference, and slow axis direction were measured at four points 10mm from the four corners of the sample towards the center, and a total of five points at the center of the sample. The average values ​​of Re1 to Re5 were calculated from the measurement results. The results are shown in Table 2. The measuring apparatus used was the Otsuka Electronics Co., Ltd. trade name "RETS-100 (measuring point: diameter 5mm)". The direction of the slow axis is measured within a range of 0 degrees above 0 degrees and below 90 degrees, with the direction of travel of the plastic film (MD direction) as the reference.

[0200] 1-8. Bending Resistance From the plastic film used in the experimental and reference examples prepared or made in "2" below, cut a long, narrow sample with a width direction (TD direction) of 30mm and a travel direction (MD direction) of 100mm. Fix both ends of the short side (30mm side) of the above sample to a durability testing machine (product name "DLDMLH-FS", YUASA SYSTEM CO., LTD.) and perform 100,000 consecutive folding tests with 180-degree folds. The two ends of the short side of the sample are fixed in an area 10mm away from the front end of the sample. The folding speed is set to 120 times per minute. A more detailed method for the folding test is disclosed below. After the folding test, place the long, narrow sample on a horizontal platform and measure the angle at which the end of the sample rises from the platform. If the angle is less than 15 degrees, it is considered acceptable. If the sample breaks during the test, it is recorded as "breakage". The results are shown in Table 2. This evaluation allows for assessment of the bending resistance in the TD direction (≒ slow axis direction). <MD direction> From the biaxially stretched plastic film used in the embodiments and comparative examples described in "2" below, an elongated sample measuring 30mm in the MD direction and 100mm in the TD direction is cut, and the same evaluation is performed as above. This evaluation allows for assessment of the bending resistance in the MD direction (≒ fast axis direction).

[0201] <Details of the Folding Test> As shown in Figure 6(A), in the continuous folding test, firstly, the edge 10C of the plastic film 10 and the edge 10D opposite to the edge 10C are fixed with parallel fixed parts 60. The fixed parts 60 can slide in the horizontal direction. Next, as shown in Figure 6(B), the fixed parts 60 are moved closer to each other, thereby deforming the plastic film 10 into a fold. Furthermore, as shown in Figure 6(C), after moving the fixed parts 60 to a position where the distance between the two opposite edges of the plastic film 10 fixed by the fixed parts 60 is 10mm, the fixed parts 60 are moved in the opposite direction, thereby eliminating the deformation of the plastic film 10. As shown in Figures 6(A) to (C), by moving the fixed parts 60, the plastic film 10 can be folded 180 degrees. Furthermore, a continuous folding test is conducted in such a way that the curved portion 10E of the plastic film 10 does not extend beyond the lower end of the fixing portion 60, and the interval when the fixing portions 60 are closest is controlled to be 10mm, thereby making the interval between the two opposing sides of the optical film 10 10mm.

[0202] 1-9. Pencil Hardness The pencil hardness of polyester films 1-5 described in "2" below was measured. The method for measuring pencil hardness was in accordance with the order of (1) to (6) in this instruction manual. For commercially available polyester films with an easy-adhesion layer pre-formed on one side, the pencil hardness of the side without the easy-adhesion layer was measured. The pencil hardness was measured on both the slow axis and the fast axis. The results are shown in Table 2.

[0203] 1-10. The erosion rate was measured using an erosion rate measuring device (MSE test apparatus of Palmeso Co., Ltd., product number "MSE-A203", nozzle cross-sectional shape is a square of 1mm × 1mm, measuring method of cross-sectional profile: stylus type) to measure the erosion rate of polyester films 1 to 5 in "2" below, and E0-20 was calculated. The measurement area of ​​the erosion rate was 1mm × 1mm. The erosion rate of each sample was measured after calibration using the standard acrylic plate as described below. In addition, the test solution was prepared before calibration and dispersed before calibration. In addition, the standard acrylic plate mentioned above is one in which the AcE (average erosion rate of the acrylic plate measured under measurement condition A) is in the range of 1.786μm / g or more and 1.974μm / g or less.

[0204] (0-1) Preparation of the test solution: The test solution is prepared in a beaker and then mixed with a glass rod. The test solution is prepared by mixing pure water, dispersant (Demol N, trade name of Wako Pure Chemical Industries Co., Ltd.), and spherical silicon dioxide with an average particle size (median particle size) of 3.94 μm (Palmeso Co., Ltd. model "MSE-BS-5-3", particle size distribution half-peak amplitude: 4.2 μm) at a mass ratio of 968:2:30. After placing the prepared test solution and stirrer into the container (jar), the jar is covered and the clamp is installed. Then, the jar is placed in the measuring device. In this embodiment, the product number "BS5-3" of Potters-Ballotini Co., Ltd. is used as the model "MSE-BS-5-3" designated by Palmeso Co., Ltd. (0-2) Dispersed Operation: After placing the container containing the test solution into the testing device, place the virtual sample on the sample mounting stage. Next, press the "Erosion Force Setting" and "Proceed" buttons on the operation panel of the main body of the testing device in sequence. Then, input specific values ​​for the flow rate of the test solution and compressed air, the pressure of the compressed air, and the pressure of the test solution in the nozzle, and project the test solution onto the virtual sample. After stopping the projection, press the "Return," "Complete," and "Confirm" buttons on the operation panel in sequence.

[0205] (1) Calibration: A 4mm thick acrylic plate for calibration is fixed to the sample mounting stage of the measuring device using a double-sided tape (Kapton double-stick tape from Nitto Denko America, product number: P-223 1-6299-01). The acrylic plate is a PMMA plate. Next, the sample mounting stage with the acrylic plate fixed is placed on the measuring device. Next, the micro gauge is unlocked, and the height of the sample mounting stage is adjusted using the height gauge. The distance between the spray nozzle of the measuring device and the acrylic plate is adjusted to 4mm. Next, the "Enter Processing Condition Input Screen" button on the operation panel of the measuring device is pressed, and the setting is "Step Number: 1, Specified Spray Amount g × 1 time". The spray amount is set to 4g. Next, the "Setting Complete", "Start Operation", and "Yes" buttons on the operation panel are pressed in sequence. The flow rates of the test solution and compressed air, the pressure of the compressed air, and the pressure of the test solution in the nozzle are maintained at the values ​​input in "(0-2) Dispersion Operation". Next, click "Connect" on the operation screen of the data processing PC to disconnect the connection and change it to offline. Next, click "Descend" on the operation screen to lower the stylus of the stylus-type step gauge in the section profile acquisition section. Next, confirm that the micro gauge lock is released and raise the micro gauge. At this time, adjust it so that the red arrow on the screen is centered. By the above adjustment, the stylus of the stylus-type step gauge can be made to contact the surface of the calibration sample, and the height direction, i.e., the 0 point of the z-axis, can be adjusted. Next, switch the micro gauge lock from OFF to ON. Next, click "Rise" to raise the stylus of the stylus-type step gauge in the section profile acquisition section. Next, click "Offline" on the operation screen of the data processing PC to disconnect the offline connection and change it to connected. Next, close the cover of the measuring device body, press the "Confirm" button on the operation panel of the measuring device body, and spray 4g of test solution. After stopping the spraying of the test solution, click "Proceed" to calculate the erosion rate. If the erosion rate is within ±5% of 1.88 (μm / g), the calibration is complete. If the erosion rate exceeds the above range, adjust the flow rate of the test solution, the flow rate of the compressed air, the pressure of the compressed air, and the pressure of the test solution in the nozzle, and repeat the calibration until the erosion rate is within the above range.

[0206] (2) Determination of the erosion rate of each sample (2-1) Sample installation and preparation The sample (polyester films 1 to 5 in "2" below) is attached to the stainless steel plate laminate. The laminate is then fixed to the sample mounting stage using double-sided tape (Kapton double-stick tape from Nitto Denko America, product number: P-223 1-6299-01). The sample is set to a size of 1cm × 1cm. Next, the sample mounting stage is placed on the measuring device. Then, the micro gauge is released and the height of the sample mounting stage is adjusted using the height gauge. The distance between the spray hole of the measuring device and the plastic film is adjusted to 4mm. Next, the "Enter Processing Condition Input Screen" button on the operation panel of the measuring device is pressed, and the number of steps is entered. The spray volume of the test liquid (g / time) for each step is entered. The spray volume for each step is set to a range of 0.5g to 3.0g. The flow rate of the test liquid and compressed air, the pressure of the compressed air, and the pressure of the test liquid in the nozzle are maintained at the conditions that have passed the "(1) calibration". Next, press the "Setting Complete", "Start Operation", and "Yes" buttons on the operation panel in sequence. Next, click "Connection" on the operation screen of the data processing PC to disconnect the connection and change it to offline. Next, click "Descend" on the operation screen to lower the stylus of the stylus step gauge of the cross-sectional profile acquisition part. Next, confirm that the micro gauge lock is released and raise the micro gauge. At this time, adjust it so that the red arrow on the screen is centered. By the above adjustment, the stylus of the stylus step gauge can be made to contact the surface of the calibration sample, and the height direction, i.e., the 0 point of the z-axis, can be adjusted. Next, switch the micro gauge lock from OFF to ON. Next, click "Up" to raise the stylus of the stylus step gauge of the cross-sectional profile acquisition part. Next, click "Offline" on the operation screen of the data processing PC to disconnect the offline connection and change it to connection.

[0207] (2-2) To begin the measurement, close the cover of the main body of the measuring device, press the "Confirm" button on the operation panel of the main body of the measuring device, and perform a measurement cycle of spraying the test liquid and measuring the cross-sectional profile until the depth of the cross-sectional profile exceeds 20 μm. Specifically, the depth of the cross-sectional profile is between 25 μm and 30 μm. After the measurement, start the auxiliary software "MseCalc" and click "Analysis Method". Next, click "Average Analysis". Next, click "Add" twice on the average analysis screen to display "A-1" and "A-2" in the analysis name field. Double-click the "Baseline" field of "A-1" to display "0" in the baseline field. Next, click "A-1" on the average analysis screen to enable it and operate the position of the X-axis position bar. The position of the position bar is set to the part of the plastic film in the cross-sectional profile screen that is not worn. Next, click "A-2" on the average analysis screen to enable it and operate the position of the X-axis position bar. The position of the aforementioned position bar is defined as the deepest point of wear on the plastic film within the cross-sectional profile image. Next, the cross-sectional profile and erosion rate data from each step are output as a CSV file, and the erosion rate E0-20 is calculated. Specifically, the erosion rate (corrected) for depths between 0 μm and 20 μm in the CSV output data is averaged to calculate the erosion rate E0-20. The results are shown in Table 2.

[0208] 2. Production and preparation of plastic film [Polyester film 1] 1 kg of PET (melting point 258℃, absorption center wavelength: 320nm) and 0.1 kg of UV absorber (2,2'-(1,4-epenylphenyl)bis(4H-3,1-benzophenone-4-one)) are melt-mixed in a kneader at 280℃ to produce granules containing UV absorber. The granules and PET with a melting point of 258℃ are fed into a single-shaft extruder and melt-kneaded at 280℃. The mixture is then extruded from a T-die and cast onto a casting drum with a surface temperature controlled at 25℃ to obtain a cast film. The amount of UV absorber in the cast film is 1 part by mass relative to 100 parts by mass of PET. The resulting cast film is heated by a roller array set to 95°C. Then, with the film temperature at 103°C at 250mm of a 400mm extension section, both sides of the film are heated by a radiant heater. Simultaneously, the film is extended 3.3 times its travel length in the direction of travel and then temporarily cooled to obtain a uniaxially extended film. The extension section begins at extension roller A and ends at extension roller B, with each roller having two clamping rollers. During radiant heating, a 92°C, 4m / s airflow is blown towards the film from the opposite side of the radiant heater, thereby creating turbulence on both sides of the film and disrupting the temperature uniformity. Next, corona discharge treatment was applied to both sides of the uniaxially stretched film in air to reduce the wetting tension of the substrate film to 55 mN / m. Then, a smoothing coating liquid containing polyester resin with a glass transition temperature of 18°C, polyester resin with a glass transition temperature of 82°C, and silicon dioxide particles with an average particle size of 100 nm was applied online to the corona discharge treated surfaces of the film to form a smoothing layer. Next, the uniaxially stretched film was introduced into a tenter frame and preheated with hot air at 95°C. It was then stretched 4.5 times its original width in the first stage at 105°C and in the second stage at 140°C. Here, when the width-direction stretching section is divided into two parts, the film stretch at the midpoint of the width-direction stretching section is stretched in two stages to achieve 80% of the stretch at the end of the width-direction stretching section. The aforementioned "stretch" refers to the difference between the film width at the measurement point and the film width before stretching. The film stretched in the width direction was then directly heat-treated with hot air in the tenter frame. The hot air temperature was progressively increased from 180°C to 245°C. Next, a 1% relaxation treatment was performed in the width direction at this temperature, followed by rapid cooling to 100°C and another 1% relaxation treatment in the width direction. Finally, it was wound to obtain a biaxially stretched polyester film 1 with a thickness of 40 μm. Polyester film 1 was used as the plastic film in Experimental Example 3.

[0209] [Polyester film 2] Except that the elongation ratio in the width direction was changed from 4.5 times to 5.1 times, a biaxially stretched polyester film 2 with a thickness of 40 μm was obtained in the same manner as the biaxially stretched polyester film 1. The polyester film 2 was used as the plastic film in Experimental Example 2.

[0210] [Polyester film 3] A commercially available biaxially stretched polyester film (TOYOBO CO., LTD., trade name: Cosmoshine A4300, thickness: 38μm) was prepared as polyester film 3. Polyester film 3 was used as the plastic film in Experiment 1.

[0211] [Polyester film 4] A commercially available biaxially stretched polyester film (TOYOBO CO., LTD., trade name: Cosmoshine A4100, thickness: 50μm) is prepared as polyester film 4. Polyester film 4 is used as the plastic film of Reference Example 1.

[0212] [Polyester film 5] A commercially available uniaxially stretched polyester film (TOYOBO CO., LTD., trade name "Cosmoshine TA044", thickness: 80μm) is prepared as polyester film 5. Polyester film 5 is used as the plastic film of Reference Example 2.

[0213] 3. Synthesis of the Compound The compound α used in "4. Preparation of the Coating Solution" was synthesized by the following method. Air gas was introduced into a reaction vessel equipped with a stirrer, thermometer, cooling pipe, and nitrogen inlet pipe. The pressure of the reaction vessel during air introduction was controlled at 1.0 atm ± 0.1 atm. Next, 57 parts by mass of neopentyl terephthalate triacrylate, 43 parts by mass of neopentyl terephthalate tetraacrylate, 0.02 parts by mass of dibutyltin dilaurate, 0.02 parts by mass of p-methoxyphenol, and 30 parts by mass of butyl acetate were added to the reaction vessel, and the mixture was heated to 60°C under a nitrogen flow while stirring. The pressure of the reaction vessel under a nitrogen flow was controlled at 1.2 atm ± 0.1 atm. (By making the pressure under a nitrogen flow higher than atmospheric pressure, the oxygen concentration in the reaction vessel can be reduced more efficiently.) Next, 30 parts by mass of hexamethylene diisocyanate were added to a dropping container and uniformly dropped into the reaction vessel over a period of 1 hour. After the addition, the reaction vessel temperature was raised to 75°C and maintained at 75±3°C for 6 hours. Then, 150 parts by mass of methyl ethyl ketone were added to obtain a transparent resin solution. Finally, the solvent was removed using an evaporator to obtain compound α. Compound α is a free radiation-curing compound. The number-average molecular weight of compound α is approximately 4500.

[0214] 4. Preparation of coating solution The coating solution used in "5. Fabrication of optical film" is prepared. <Coating Solution A for Hard Coating Formation> • Ionizing radiation curing compound 1: 0.6 parts by mass (compound α synthesized in "3") • Ionizing radiation curing compound 2: 0.2 parts by mass (Daicel Inc., trade name "EBECRYL230", 100% solids content) • Ionizing radiation curing compound 3: 0.2 parts by mass (Kyoei Chemical Co., Ltd., trade name "LIGHT ACRYLATE IAA", 100% solids content) • Leveling agent: 0.01 parts by mass (Dai Nippon Seika Kogyo Co., Ltd., trade name "10-28 (TL)", 10% by mass solids content) • Photopolymerization initiator: 0.1 parts by mass (IGM Resins BV, trade name "Omnirad 184") • Solvent (a 5:5 mixture of methyl isobutyl ketone and cyclohexanone. The solvent is used in an amount where the solids content of the coating solution is 35% by mass.)

[0215] <Coating Solution B for Hard Coating Formation> ・Ionizing radiation curing compound 1:1 parts by mass (compound α synthesized in "3") ・Acrylic resin particles: 0.1 parts by mass (average particle size: 2 μm, refractive index: 1.535) ・Leveling agent: 0.01 parts by mass (Dainichi Seika Co., Ltd., trade name "10-28 (TL)", solid content 10% by mass) ・Photopolymerization initiator: 0.1 parts by mass (IGM Resins BV, trade name "Omnirad 184") ・Solvent (a 5:5 mixture of methyl isobutyl ketone and cyclohexanone. The solvent is used in an amount where the solid content of the coating solution is 35% by mass.)

[0216] <Coating Solution C for Hard Coating Formation> ・Ionizing radiation curing compound 1: 0.625 parts by mass (compound α synthesized in "3") ・Ionizing radiation curing compound 4: 0.375 parts by mass (Arakawa Chemical Industry Co., Ltd., trade name "OPSTAR Z7415", 100% solids content) ・Leveling agent: 0.01 parts by mass (Dai Nippon Seika Industry Co., Ltd., trade name "10-28 (TL)", 10% by mass solids content) ・Photopolymerization initiator: 0.1 parts by mass (IGM Resins BV, trade name "Omnirad 184") ・Solvent (a 5:5 mixture of methyl isobutyl ketone and cyclohexanone. The solvent is used in an amount where the solids content of the coating solution is 35% by mass.)

[0217] <Coating Solution for Low Refractive Index Layer Formation i> ・Composition containing UV-curable acrylate: 1 part by weight (Nippon Kayaku Co., Ltd., trade name "KAYARAD PET-30", 100% solids content) ・Photopolymerization initiator: 0.1 part by weight (IGM Resins BV, trade name "Omnirad 127") ・Hollow silica particles: 1.3 parts by weight (average primary particle size 60 nm) ・Solid silica particles: 0.7 parts by weight (average primary particle size 15 nm) ・Leveling agent: 0.1 part by weight (Shin-Etsu Chemical Co., Ltd., trade name "X-22-164E") ・Solvent (a 5:5 mixture of methyl isobutyl ketone and cyclohexanone. The solvent is used in an amount where the solids content of the coating solution is 2% by weight.)

[0218] <Coating Solution for Low Refractive Index Layer Formation ii> ・Composition containing UV-curable acrylate: 1 part by weight (Nippon Kayaku Co., Ltd., trade name "KAYARAD PET-30", 100% solids content) ・Photopolymerization initiator: 0.1 part by weight (IGM Resins BV, trade name "Omnirad 127") ・Hollow silica particles: 1.55 parts by weight (average primary particle size 60 nm) ・Solid silica particles: 0.45 parts by weight (average primary particle size 15 nm) ・Leveling agent: 0.1 part by weight (Shin-Etsu Chemical Co., Ltd., trade name "X-22-164E") ・Solvent (a 5:5 mixture of methyl isobutyl ketone and cyclohexanone. The solvent is used in an amount equal to 2% by weight of the solids content of the coating solution.)

[0219] <Coating Solution for Low Refractive Index Layer Formation iii> ・Composition containing UV-curable acrylate: 1 part by weight (Nippon Kayaku Co., Ltd., trade name "KAYARAD PET-30", 100% solids content) ・Photopolymerization initiator: 0.1 part by weight (IGM Resins BV, trade name "Omnirad 127") ・Hollow silica particles: 2 parts by weight (average primary particle size 60 nm) ・Leveling agent: 0.1 part by weight (Shin-Etsu Chemical Co., Ltd., trade name "X-22-164E") ・Solvent (a 5:5 mixture of methyl isobutyl ketone and cyclohexanone. The solvent is used in an amount where the solids content of the coating solution is 2% by weight.)

[0220] 5. Fabrication of optical film [Experimental Example 1-1] The polyester film 3 prepared in "2" is used as the optical film of Experimental Example 1-1. The optical film of Experimental Example 1-1 does not have a hard coating and a low refractive index layer on the polyester film 3.

[0221] [Experimental Examples 1-2] A hard coating forming solution A was applied to the polyester film 3 prepared in "2", and then dried at 70°C for 1 minute to allow the solvent to evaporate. Next, it was irradiated with ultraviolet light (100 mJ / cm²) to form a hard coating (dry thickness 10 μm). A low refractive index layer forming solution i was applied to the hard coating, and then dried at 60°C for 1 minute to allow the solvent to evaporate. Next, it was irradiated with ultraviolet light (200 mJ / cm²) to form a low refractive index layer (dry thickness 100 nm), thus obtaining the optical film of Experimental Example 1-2.

[0222] [Experimental Examples 1-3, 1-4] Except for using the coating liquids listed in Table 1 as coating liquids for forming hard coatings and low refractive index layers, the optical films of Experimental Examples 1-3 and 1-4 were obtained in the same manner as Experimental Examples 1-2.

[0223] [Experimental Examples 1-5] Except that a low refractive index layer was formed directly on the polyester film without forming a hard coating, and the coating liquid listed in Table 1 was used as the coating liquid for forming the low refractive index layer, the optical films of Experimental Examples 1-5 were obtained in the same manner as Experimental Examples 1-2.

[0224] [Experimental Example 2-1] The polyester film 2 prepared in "2" is used as the optical film of Experimental Example 2-1. The optical film of Experimental Example 2-1 does not have a hard coating and a low refractive index layer on the polyester film 2.

[0225] [Experimental Example 2-2] Except that the polyester film 3 was changed to the polyester film 2, the optical film of Experimental Example 2-2 was obtained in the same manner as Experimental Example 1-2.

[0226] [Experimental Examples 2-3, 2-4] Except for using the coating liquids listed in Table 1 as coating liquids for forming hard coatings and low refractive index layers, the optical films of Experimental Examples 2-3 and 2-4 were obtained in the same manner as Experimental Example 2-2.

[0227] [Experimental Examples 2-5, 2-6] Except that a low refractive index layer was formed directly on the polyester film without forming a hard coating, and the coating liquid described in Table 1 was used as the coating liquid for forming the low refractive index layer, the optical films of Experimental Examples 2-5 and 2-6 were obtained in the same manner as Experimental Example 2-2.

[0228] [Experimental Examples 3-1, 3-2] Except that polyester film 3 was replaced with polyester film 1, and the coating liquid for forming hard coating layer and low refractive index layer as described in Table 1 were used, the optical films of Experimental Examples 3-1 and 3-2 were obtained in the same manner as Experimental Examples 1-2.

[0229] [Experimental Example 3-3] Except that the polyester film 3 was changed to the polyester film 1, and the low refractive index layer was formed directly on the polyester film without forming a hard coating, and the coating liquid listed in Table 1 was used as the coating liquid for forming the low refractive index layer, the optical film of Experimental Example 3-3 was obtained in the same manner as Experimental Example 1-2.

[0230] [Table 1]

[0231] [Table 2]

[0232] From the results in Table 1, it can be confirmed that optical films with ΣT exceeding 0.04 but not reaching 0.20 can eliminate rainbow spots when viewed with the naked eye and can also provide good color uniformity when viewed at an angle. In the experimental examples in Table 1, the equivalent examples are experimental examples 1-2, 1-3, 1-4, 2-2, 2-3, 2-4, 2-6, 3-1, and 3-2. Furthermore, from the results in Tables 1 and 2, it can be confirmed that "plastic films with small in-plane phase differences" and "plastic films with large differences between the maximum and minimum values ​​in the slow axis direction" easily achieve appropriate ΣT values. Furthermore, from the results in Table 2, it can be confirmed that polyester films 1 and 2, regardless of the bending direction, can suppress residual bending habits or breakage after the bending test. Polyester films 1 and 2 are "plastic films with large differences between the maximum and minimum values ​​in the slow axis direction". Furthermore, no microcracks were detected in polyester films 1 and 2 after the bending test. Microcracks can be observed in the following ways.

[0233] Microcracks can be observed using a digital microscope. For example, the "VHX-5000" manufactured by Keynes Corporation can be used as a digital microscope. Microcracks are observed using ring illumination and dark-field and reflected light. Specifically, first, the sample after the bending test is slowly unfolded and then fixed to the microscope stage with tape. When the sample has a strong folding tendency, the observation area should be made as flat as possible. During this operation, care should be taken to avoid touching the bent portion of the sample in the area being evaluated, and care should be taken to ensure that the bent portion is not subjected to stress. Then, the portion that became the inner side and the portion that became the outer side during the bending test are evaluated. Microcrack observation is conducted in a brightly lit room with white illumination (illuminance 1000 lux to 2000 lux). [Simplified Explanation of the Diagram]

[0016] [Fig. 1] is a cross-sectional view showing one embodiment of the optical film of the present invention. [Fig. 2] is a schematic diagram showing an example of a measurement performed under measurement condition 1. [Fig. 3] is a cross-sectional view showing one embodiment of the polarizing plate of the present invention. [Fig. 4] is a cross-sectional view showing one embodiment of the image display device of the present invention. [Fig. 5] is a plan view illustrating five measurement positions within the sample when the in-plane phase difference is calculated from the sample. [Fig. 6] is a schematic diagram showing the continuous folding test. [Fig. 7] is a schematic cross-sectional view of the erosion rate measuring device. [Fig. 8] is a schematic diagram showing the state of wear of the plastic film due to the test liquid containing pure water and spherical silicon dioxide sprayed from the spraying part.

Claims

1. An optical film having a low-refractive-index layer on a plastic film, the plastic film having an axis with the highest in-plane refractive index, i.e., a slow axis, and an axis orthogonal to the slow axis in-plane of the plastic film, i.e., a fast axis, the low-refractive-index layer being located on the surface of the optical film, the optical film having a region where ΣT calculated from the following measurement condition 1 satisfies a value greater than 0.04 but less than 0.20, and the following values ​​of the plastic film, D1 to D5, are respectively greater than 5 degrees and less than 30 degrees or greater than 60 degrees and less than 85 degrees; <Measurement condition 1> Linearly polarized light is incident from the surface opposite to the low-refractive-index layer of the optical film; the incident light, i.e., the linearly polarized light, is defined as light L1; the transmitted light L1 passing through the optical film is defined as light L2; After fixing the angle between the slow axis and the vibration direction of light L1 to 45 degrees, the elevation angle of the vibration direction of light L1, with the plane of the optical film as a reference, is an angle of 50 degrees or more and 70 degrees or less, so that light L1 is incident on the optical film; within the range of 50 degrees or more and 70 degrees or less, the elevation angle is changed every 2 degrees, and light L2 is measured at 11 elevation angles; by means of this measurement, light L2 is measured at 11 measurement points; light L2 is converted to the conditions of C light source and 2-degree viewing angle; for light L2 at the nth measurement point among the 11 measurement points, the a* value and b* value of the L*a*b* color system are defined as a*n and b*n; and for light L2 at the (n+1)th measurement point among the 11 measurement points, the a* value and b* value of the L*a*b* color system are defined as a*n1 and b*n1; Based on the measurements at the 11 measurement points, the sum of the squares of the differences between adjacent measurement points a* and b* is calculated; the sum is calculated at each of the 10 adjacent points, and ΣT, representing the total sum, is calculated; ΣT can be expressed by the following formula 1, ΣT=Σ[{a*n-a*n1}2+{b*n-b*n1}2 ] (Formula 1; <D1~D5> A sample of 50mm in length × 50mm in width is cut from the plastic film; the 1 point in the center of the sample and the 4 points 10mm from each of the four corners of the sample toward the center are taken as the 5 measurement points; the direction of the slow axis is measured at these 5 points of the sample; the angle between any side of the sample and the direction of the slow axis of each measurement point is defined as D1, D2, D3, D4, D5 respectively.

2. The optical film as claimed in claim 1, wherein, When the maximum value of a* is defined as a*max, the minimum value of a* is defined as a*min, the maximum value of b* is defined as b*max, and the minimum value of b* is defined as b*min, based on the measurements of these 11 measurement points, the following equations 2-1 and 2-2 are satisfied: a*max - a*min ≦ 0.250 (Equation 2-1), b*max - b*min ≦ 0.350 (Equation 2-2).

3. The optical film as claimed in claim 1 or 2, wherein, Based on the measurements of these 11 measurement points, the sum of the squares of the differences between adjacent measurement points a* and b* is calculated. When this sum is defined as S, S can be expressed by the following Equation 3. When S is calculated at 10 adjacent points respectively, and the maximum value of S at the 10 points is defined as SMAX, SMAX is greater than 0.010 and less than 0.050, S = {a*n - a*n1}2 + {b*n - b*n1}2 (Equation 3).

4. The optical film as claimed in claim 1 or 2, wherein, When the visual reflectance Y value of the optical film is defined as R (%), the product of R and ΣT is greater than 0.05 and less than 0.

25.

5. The optical film as claimed in claim 1 or 2, wherein, When the average refractive index of the low-refractive-index layer is defined as n1, and the average refractive index of the layer adjacent to the low-refractive-index layer is defined as n2, n2 / n1 does not reach 1.

23.

6. The optical film as claimed in claim 1 or 2, wherein, When the average refractive index of the low refractive index layer is defined as n1, and the average refractive index of the layer adjacent to the low refractive index layer is defined as n2, n2 / n1 is greater than 1.05 but less than 1.

23.

7. The optical film as claimed in claim 1 or 2, wherein, The in-plane phase difference of the plastic film is less than 2500nm.

8. The optical film as claimed in claim 1 or 2, wherein, The plastic film meets the following condition A: <Condition A> The difference between the maximum value of D1 to D5 and the minimum value of D1 to D5 is greater than 1.5 degrees.

9. The optical film of claim 1 or 2, wherein between the plastic film and the low refractive index layer, there is one or more layers selected from the hard coating layer and the anti-glare layer.

10. The optical film of claim 1 or 2 has a visual reflectance Y value of 4.0% or less when measured from the low refractive index layer side.

11. A polarizing plate having a polarizer, a first transparent protective plate located on one side of the polarizer and a second transparent protective plate located on the other side of the polarizer, wherein at least one of the first transparent protective plate and the second transparent protective plate is an optical film of any one of claims 1 to 10, and the low refractive index layer side of the optical film faces the opposite side of the polarizer.

12. An image display device having a display element, a polarizer and an optical film disposed on the light emitting surface side of the display element, wherein the optical film is any one of claims 1 to 10, and the low refractive index layer side of the optical film faces the opposite side of the display element.

13. A method for selecting an optical film for an image display device, the image display device having a polarizer and an optical film on the light emitting surface of a display element, the method selecting an optical film X that meets the following criteria (1) to (5) as the optical film: (1) the optical film X has a low refractive index layer on a plastic film; (2) the plastic film has an axis with the largest in-plane refractive index, i.e., a slow axis, and an axis orthogonal to the slow axis in the plane of the plastic film, i.e., a fast axis; (3) the low refractive index layer is located on the surface of the optical film X; (4) the optical film X has a region where ΣT calculated from the following measurement condition 1 satisfies a value greater than 0.04 but less than 0.20; and (5) the following values ​​D1 to D5 of the plastic film are respectively greater than 5 degrees and less than 30 degrees or greater than 60 degrees and less than 85 degrees; <Measurement Condition 1> Linearly polarized light is incident from the surface opposite to the low-refractive-index layer of the optical film; this incident light, i.e., the linearly polarized light, is defined as light L1; the transmitted light L1 that passes through the optical film is defined as light L2; after fixing the angle between the slow axis and the vibration direction of light L1 at 45 degrees, the elevation angle of the vibration direction of light L1, with the plane of the optical film as a reference, is an angle of 50 degrees or more and 70 degrees or less, so that light L1 is incident on the optical film; the elevation angle is changed every 2 degrees within the range of 50 degrees or more and 70 degrees or less, and light L2 is measured at 11 elevation angles; by means of this measurement, light L2 is measured at 11 measurement points; The light L2 is converted to a C light source with a viewing angle of 2 degrees. For the light L2 at the nth measurement point out of 11 measurement points, the a* and b* values ​​of the L*a*b* color system are defined as a*n and b*n, respectively. Also, for the light L2 at the (n+1)th measurement point out of 11 measurement points, the a* and b* values ​​of the L*a*b* color system are defined as a*n1 and b*n1, respectively. Based on the measurements at these 11 measurement points, the sum of the squares of the differences in a* between adjacent measurement points and the squares of the differences in b* between adjacent measurement points is calculated. This sum is calculated at each of the 10 adjacent points, and ΣT, representing the total sum, is calculated. ΣT can be expressed by the following Equation 1: ΣT=Σ[{a*n-a*n1}2 +{b*n-b*n1}2 ] ( Equation 1; <D1~D5> Cut a sample measuring 50mm x 50mm from the plastic film; take five points as measurement points: one at the center of the sample and four points 10mm from each of the four corners of the sample toward the center; measure the direction of the slow axis at these five points; define the angle between any side of the sample and the direction of the slow axis at each measurement point as D1, D2, D3, D4, and D5, respectively.

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