Polyester film, polarizing plate, and image display device

Abstract

Provided is a polyester film which exhibits good pencil hardness without becoming thick.　A surface of the polyester film has an average Martens hardness of 140.0 N / mm2 or more. The standard deviation 3σ of the Martens hardness of the surface of the polyester film is 9.0 N / mm2-36.0 N / mm2.

Description

[Technical Field]

[0001] This disclosure relates to polyester films, polarizing plates, and image display devices. [Background technology]

[0002] Various types of optical plastic films are often used in optical components such as image display devices. For example, in image display devices that have a polarizing plate on a display element, a plastic film is used to protect the polarizer that makes up the polarizing plate. In this specification, the "plastic film for protecting the polarizer" may be referred to as a "polarizer protection film."

[0003] Plastic films for image display devices, such as polarizer protective films, are preferably those with excellent mechanical strength. For this reason, stretched plastic films are preferably used for image display devices. Polyester films, which offer excellent mechanical strength and versatility, are frequently used as such plastic films. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2011-107198 [Overview of the project] [Problems that the invention aims to solve]

[0005] Polyester film is sometimes used as a surface material for image display devices, and therefore is expected to have a certain pencil hardness. In particular, since touch panel functionality is becoming standard in image display devices these days, it is extremely important to suppress scratches by increasing the pencil hardness. However, conventional polyester films, such as the polyester film described in Patent Document 1, lacked sufficient pencil hardness, and the polyester film alone was easily scratched. Therefore, when using the polyester film described in Patent Document 1 as a surface material, it was essential to form a hardened film, such as a hard coat layer, on the polyester film.

[0006] The pencil hardness of polyester film can be slightly improved by increasing the thickness of the polyester film. However, if the polyester film described in Patent Document 1 is made thick enough to achieve sufficient pencil hardness, it would be counterproductive to the trend towards thinner image display devices. Furthermore, since the polyester film described in Patent Document 1 is intended to be a uniaxially oriented film, it also has the problem of being prone to tearing in the stretching direction.

[0007] Furthermore, polyester films may have functional layers formed on them to improve optical properties, etc. Therefore, good adhesion is desirable for polyester films.

[0008] The object of this disclosure is to provide a polyester film, a polarizing plate, and an image display device that have good pencil hardness without increasing thickness and good adhesion to a functional layer. [Means for solving the problem]

[0009] This disclosure provides the following polyester film, polarizing plate, and image display device. [1] The average Martens hardness of the surface of the polyester film is 140.0 N / mm². 2 Furthermore, the standard deviation 3σ of the Martens hardness of the surface of the polyester film is 9.0 N / mm². 2 More than 36.0N / mm 2 The following is a polyester film. [2] The polyester film according to [1], wherein the standard deviation 3σ of the mean of the elastic deformation power of the surface of the polyester film is 7.0% or less. [3] The polyester film according to [1] or [2], wherein the average elastic deformation power of the surface of the polyester film is 40.0% or more. [4] The polyester film according to any one of [1] to [3], wherein the in-plane phase difference of the polyester film is 300 nm or more and 1450 nm or less. [5] A polyester film according to any one of [1] to [4], wherein the value obtained by dividing the in-plane phase difference of the polyester film by the phase difference in the thickness direction of the polyester film is 0.15 or less. [6] A polyester film as described in any of [1] to [5], having a thickness of 10 μm or more and 80 μm or less. [7] A polarizing plate having a polarizer, a first transparent protective plate disposed on one side of the polarizer, and a second transparent protective plate disposed on the other side of the polarizer, wherein at least one of the first transparent protective plate and the second transparent protective plate is a polyester film according to any one of [1] to [6]. [8] An image display device having a display element and a polyester film disposed on the light-emitting surface side of the display element, wherein the polyester film is the polyester film described in any of [1] to [6]. [9] The image display device according to [8], further comprising a polarizer between the display element and the polyester film. [Effects of the Invention]

[0010] The polyester film, polarizing plate, and image display device of this disclosure can achieve good pencil hardness without increasing the thickness of the polyester film, and can also achieve good adhesion with the functional layer. [Brief explanation of the drawing]

[0011] [Figure 1] This is a plan view illustrating the five measurement locations under Condition 1. [Figure 2] This is a cross-sectional view showing one embodiment of the image display device of the present disclosure. [Figure 3]It is a cross-sectional view showing another embodiment of the image display device of the present disclosure. [Figure 4] It is a figure which shows typically the state of a continuous folding test.

Mode for Carrying Out the Invention

[0012] Hereinafter, embodiments of the present disclosure will be described. [Polyester Film] The polyester film of the present disclosure has an average Martens hardness on the surface of the polyester film of 140.0 N / mm 2 or more, and a standard deviation 3σ of the Martens hardness on the surface of the polyester film of 9.0 N / mm 2 or more and 36.0 N / mm 2 or less.

[0013] When the average Martens hardness on the surface of the polyester film is less than 140.0 N / mm 2 the pencil hardness cannot be improved well. The average Martens hardness on the surface of the polyester film is preferably 145.0 N / mm 2 or more, more preferably 150.0 N / mm 2 or more, still more preferably 160.0 N / mm 2 or more.

[0014] If the average Martens hardness on the surface of the polyester film is too large, the processing suitability may decrease. Further, if the average Martens hardness is too large, the adhesion of the functional layer formed on the polyester film may decrease, and part or all of the functional layer may peel off, or cracks may occur in the functional layer. Further, if the average Martens hardness is too large, the bending resistance described later tends to decrease. Therefore, the average Martens hardness on the surface of the polyester film is preferably 280.0 N / mm 2 or less, more preferably 240.0 N / mm 2 or less, still more preferably 220.0 N / mm 2 or less, still more preferably 200.0 N / mm2 More preferably, 180.0 N / mm 2 More preferably, 170.0 N / mm 2 The following applies:

[0015] In the constituent elements shown herein, if multiple options are provided for both the upper and lower limits of a numerical value, one selected from the upper limit options and one selected from the lower limit options can be combined to form an embodiment of the numerical range. For example, the average Martens hardness of the surface of a polyester film is 140.0 N / mm². 2 More than 280.0N / mm 2 Below, 140.0N / mm 2 More than 240.0N / mm 2 Below, 140.0N / mm 2 More than 220.0N / mm 2 Below, 140.0N / mm 2 More than 200.0N / mm 2 Below, 140.0N / mm 2 More than 180.0N / mm 2 Below, 140.0N / mm 2 More than 170.0N / mm 2 Below, 145.0N / mm 2 More than 280.0N / mm 2 Below, 145.0N / mm 2 More than 240.0N / mm 2 Below, 145.0N / mm 2 More than 220.0N / mm 2 Below, 145.0N / mm 2 More than 200.0N / mm 2 Below, 145.0N / mm 2 More than 180.0N / mm 2 Below, 145.0N / mm 2 More than 170.0N / mm 2 Below, 150.0N / mm 2 More than 280.0N / mm 2 Below, 150.0N / mm 2 More than 240.0N / mm 2 Below, 150.0N / mm 2 More than 220.0N / mm2 Below, 150.0N / mm 2 More than 200.0N / mm 2 Below, 150.0N / mm 2 More than 180.0N / mm 2 Below, 150.0N / mm 2 More than 170.0N / mm 2 Below, 160.0N / mm 2 More than 280.0N / mm 2 Below, 160.0N / mm 2 More than 240.0N / mm 2 Below, 160.0N / mm 2 More than 220.0N / mm 2 Below, 160.0N / mm 2 More than 200.0N / mm 2 Below, 160.0N / mm 2 More than 180.0N / mm 2 Below, 160.0N / mm 2 More than 170.0N / mm 2 Examples of numerical range embodiments include the following.

[0016] To increase the average Martens hardness of a polyester film's surface, it is important to enhance the orientation of molecules within the film through stretching. Uniaxial stretching of a polyester film can seemingly align the molecules in one direction. However, uniaxial stretching alone does not allow the benzene rings of the molecules within the polyester film to face various directions. In other words, uniaxial stretching alone results in molecules within the polyester film being oriented two-dimensionally but not three-dimensionally. For this reason, uniaxially oriented polyester films do not easily achieve a high average surface Martens hardness. By biaxially stretching a polyester film, the orientation of the benzene rings of the molecules within the polyester can be more easily aligned in the plane. However, if the width stretching ratio in the second stretching stage is too small, the orientation of the benzene rings will not be sufficiently aligned in the plane, making it difficult to achieve a sufficiently high average Martens hardness on the surface. On the other hand, if the width stretching ratio is too large, the orientation balance within the plane may be disrupted. Therefore, in order to make it easier to bring the average Martens hardness of the surface of the polyester film within the above range, it is preferable to make the stretching ratio E2 in the width direction, which is the second stretching stage, about the same as the stretching ratio E1 in the flow direction, which is the first stretching stage, and it is more preferable to make E2 greater than E1. E2 / E1 is preferably 0.80 or more and 1.70 or less, more preferably 0.85 or more and 1.50 or less, even more preferably 0.90 or more and 1.40 or less, and even more preferably 1.00 or more and 1.30 or less. Furthermore, by setting E2 / E1 within the above range, the standard deviation 3σ of the Martens hardness is 36.0 N / mm 2 The following can be made easier.

[0017] In this specification, "the surface of the polyester film" in "the Martens hardness of the surface of the polyester film" and "the average elastic deformation power of the surface of the polyester film" refers to the planar surface of the polyester film. In other words, "the surface of the polyester film" does not refer to the cross-sectional surface of the polyester film. In this specification, the average Martens hardness and the average elastic deformation power refer to the average of 20 measurements. Furthermore, in this specification, the standard deviation of Martens hardness refers to the standard deviation of 20 measurements. The 20 measurement points are preferably any 20 locations inside a 0.5 cm margin from the outer edge of the measurement sample. In this case, it is preferable to select flat and clean areas without any unusual shapes or defects as measurement points.

[0018] For a polyester film, it is sufficient if the Martens hardness and standard deviation 3σ of one surface meet the predetermined range, but it is preferable that the Martens hardness and standard deviation 3σ of both surfaces meet the predetermined range.

[0019] Polyester film can come in various forms, such as sheets or rolls. In the case of a sheet-like polyester film, it is sufficient to identify the 20 locations mentioned above within the sheet-like structure. On the other hand, in the case of a roll of polyester film, a sheet of a predetermined size (for example, 100 mm vertically x 100 mm horizontally) can be cut out, and the 20 locations mentioned above can be identified in the form of the cut sheet. Furthermore, the physical properties of a roll of polyester film are generally consistent in the flow direction. Therefore, if a 100 mm vertically x 100 mm horizontally sheet cut out from any position α in the width direction satisfies predetermined conditions such as the average Martens hardness, then it can be assumed that the predetermined conditions are satisfied throughout the entire roll in the flow direction at any given position α.

[0020] In this specification, the atmosphere for measuring Martens hardness and elastic deformation power, as well as for various other measurements such as in-plane phase difference, thickness-direction phase difference, lagging axis direction, total light transmittance, and haze, described later, shall be a temperature of 23°C ± 5°C and a relative humidity of 40% to 65%, unless otherwise specified. Furthermore, the sample shall be exposed to the aforementioned atmosphere for 30 to 60 minutes before measurement.

[0021] The polyester film disclosed herein has a surface Martens hardness standard deviation 3σ of 9.0 N / mm². 2 More than 36.0N / mm 2 The following conditions must be met: The standard deviation 3σ of the Martens hardness on the surface of the polyester film is 36.0 N / mm². 2 If it exceeds this, the average Martens hardness is 140.0 N / mm². 2 Even if the above is true, the pencil will still penetrate the polyester film through areas with low Martens hardness. In other words, the average Martens hardness of the surface of the polyester film is 140.0 N / mm². 2 Furthermore, the standard deviation 3σ of the Martens hardness on the surface of the polyester film is set to 36.0 N / mm². 2 The pencil hardness of the polyester film can be improved by doing the following. Furthermore, if the standard deviation of the Martens hardness (3σ) is too large, the adhesion of the functional layer formed on the polyester film will decrease, potentially causing part or all of the functional layer to peel off or cracks to form in the functional layer. The standard deviation of the Martens hardness (3σ) on the surface of the polyester film should be 36.0 N / mm². 2 By doing the following, it becomes easier to improve the adhesion of the functional layer. The standard deviation 3σ of the Martens hardness on the surface of the polyester film is preferably 30.0 N / mm². 2 More preferably 28.5 N / mm 2 The following applies: In this specification, the standard deviation 3σ of the Martens hardness of the surface of the polyester film shall be calculated based on the measured values ​​in item 20 above.

[0022] The standard deviation 3σ of the Martens hardness on the surface of the polyester film is 9.0 N / mm². 2 If the standard deviation 3σ of the Martens hardness of the polyester film surface is less than 9.0 N / mm², the physical flatness of the surface increases, and the anchoring effect based on surface irregularities tends to decrease. In other words, if the standard deviation 3σ of the Martens hardness of the surface of the polyester film is less than 9.0 N / mm², 2 If the value is less than this, the adhesion of the functional layer is likely to decrease. Furthermore, if the standard deviation 3σ of the Martens hardness on the surface of the polyester film is too small, it tends to be difficult to satisfy condition 1, which will be described later. Moreover, if the standard deviation 3σ of the Martens hardness is too small, the processing suitability of the polyester film may decrease. One example of decreased processing suitability is the adhesion of polyester films to each other. Polyester films may be wound into rolls of about 100m to 8000m, or multiple sheets of polyester film may be layered together. In such cases, adhesion may occur at the contact surfaces of the polyester films. The standard deviation 3σ of the Martens hardness on the surface of the polyester film is preferably 12.0 N / mm². 2 More preferably, 14.0 N / mm 2 That's all.

[0023] The standard deviation 3σ of the Martens hardness on the surface of the polyester film is, for example, 9.0 N / mm 2 or more and 36.0 N / mm 2 or less and 9.0 N / mm 2 or more and 30.0 N / mm 2 or less and 9.0 N / mm 2 or more and 28.5 N / mm 2 or less and 12.0 N / mm 2 or more and 36.0 N / mm 2 or less and 12.0 N / mm 2 or more and 30.0 N / mm 2 or less and 12.0 N / mm 2 or more and 28.5 N / mm 2 or less and 14.0 N / mm 2 or more and 36.0 N / mm 2 or less and 14.0 N / mm 2 or more and 30.0 N / mm 2 or less and 14.0 N / mm 2 or more and 28.5 N / mm 2 Examples of embodiments include numerical ranges such as or less, etc.

[0024] The polyester film of the present disclosure preferably has a minimum value of the Martens hardness on the surface of the polyester film of 100.0 N / mm 2 or more, more preferably 120.0 N / mm 2 or more, and even more preferably 135.0 N / mm 2 or more. By setting the minimum value of the Martens hardness to 100.0 N / mm 2 or more, it is easier to improve the pencil hardness. In this specification, the "minimum value of the Martens hardness" means the minimum value of the 20 measured values described above.

[0025] The Martens hardness can be measured, for example, by the nanoindentation method. First, the indentation depth h (nm) corresponding to the indentation load F (N) is continuously measured to create a load-removal curve. The "load-removal curve" may also be referred to as the "load-displacement curve". Analyze the "maximum indentation depth hmax" from the load curve, and further, the projected area A at the time of hmaxC (mm 2 ) will be analyzed. Projection area A C This refers to the area where the indenter and the polyester film are in contact. Projected area A C The Martens hardness HM can be calculated by dividing by the maximum indentation load Pmax(N) (see formula (1) below). HM = Pmax / A C ...(1) Here, A C This is the contact projected area after correcting the indenter tip curvature using the standard method for the device.

[0026] Martens hardness can be measured using a device capable of performing nanoindentation. One such device is the Fischer Instruments PICODENTOR HM500. Martens hardness is preferably measured under the following conditions. <Measurement conditions> • Indenter used: Vickers indenter (square pyramidal shape, diamond material, model number: VV005, Fischer Instruments) • Push-in conditions: Maximum load • Maximum indentation load: 20mN • Load application time: 10 seconds • Holding time: Hold for 5 seconds under maximum indentation load. • Load release time: 10 seconds • Stage for placing samples: Suction stage (The suction stage used was SMC's model number "SP2130-AD".)

[0027] Before measuring the Martens hardness, it is preferable to perform a calibration to a standard. Standardization can be performed, for example, by conducting an indentation test using a standard sample with a known Martens hardness, and confirming that the Martens hardness obtained from the test results is within the reference range. It is preferable to perform standardization each time the sample is changed. However, if the sample is the same, it is preferable in terms of work efficiency to perform multiple Martens hardness measurements consecutively. Furthermore, if the measurement of Martens hardness continues for a long period of time, it is preferable to perform calibration at least 12 hours before it has elapsed. For example, even if calibration is not performed each time the sample is changed, it is preferable to perform calibration at least 12 hours before it has elapsed.

[0028] If the polyester film has layers and films, the Martens hardness of the polyester film can be measured after peeling off these layers and films. The following methods can be used to peel off the layers and films. <Methods of peeling> One method involves immersing a sample of 5 cm square or larger in warm water between 80°C and 90°C for 5 minutes. Then, removing the sample from the warm water and leaving it at room temperature for at least 10 minutes. After that, immersing it in warm water for another 5 minutes. Finally, removing the sample from the warm water. Another method involves making incisions in the sample with a cutter or similar tool, and then using these incisions as a starting point to peel off the layers and film.

[0029] In the above method, it is preferable to immerse the sample in warm water with the edge of the sample attached to a metal frame or the like.

[0030] The polyester film of this disclosure preferably has an average elastic deformation power of 65.0% or less on the surface of the polyester film. By setting the average elastic deformation power to 65.0% or less, elastic deformation can be reduced. More preferably, the average elastic deformation power should be 64.0% or less.

[0031] The elastic deformation power can be calculated using the following formula after determining the areas of the indentation work region, elastic work region, and plastic work region from the "load-removal curve" created when measuring Martens hardness. Elastic deformation power [%] = (Area of ​​elastic work region / Area of ​​compression work region) × 100

[0032] The average elastic deformation power is preferably 40.0% or higher, more preferably 50.0% or higher, and even more preferably 60.0% or higher, in order to suppress plastic deformation.

[0033] Examples of embodiments for the average elastic deformation power of polyester film include numerical ranges such as 40.0% to 65.0%, 40.0% to 64.0%, 50.0% to 65.0%, 50.0% to 64.0%, 60.0% to 65.0%, and 60.0% to 64.0%.

[0034] The average standard deviation 3σ of the elastic deformation power on the surface of the polyester film is preferably 7.0% or less, more preferably 5.0% or less, and even more preferably 4.0% or less. By setting the average standard deviation 3σ of the elastic deformation power to 7.0% or less, it becomes easier to suppress the pencil from sinking into the polyester film at points that are prone to plastic deformation, thereby making it easier to improve the pencil hardness of the polyester film. A smaller average standard deviation of 3σ of the elastic deformation power can result in better pencil hardness of the polyester film. However, setting the average standard deviation of 3σ of the elastic deformation power above a certain value can increase the productivity of the polyester film. In recent years, there has been a growing demand for high-quality, low-cost polyester films. For this reason, the average standard deviation of 3σ of the elastic deformation power on the surface of the polyester film is preferably 0.10% or higher, more preferably 0.20% or higher, and even more preferably 0.30% or higher.

[0035] It is preferable that the average and standard deviation 3σ of the elastic deformation power of one surface of the polyester film are within a predetermined range, but it is more preferable that the average and standard deviation 3σ of the elastic deformation power of both surfaces satisfy the predetermined range.

[0036] The polyester film disclosed herein preferably has an in-plane phase difference of 300 nm or more and 1450 nm or less. In this specification, "in-plane phase difference" may be denoted as "Re" and "phase difference in the thickness direction" may be denoted as "Rth". Polyester films with an Re of 300 nm or more can be said not to have an extremely insufficient molecular orientation or an extremely thin thickness. Therefore, by making the in-plane phase difference of the polyester film 300 nm or more, it is easier to improve the pencil hardness. By setting the Re of the polyester film to 1450 nm or less, the thickness of the polyester film can be suppressed, making it easier to create a thin image display device. Furthermore, by setting the Re of the polyester film to 1450 nm or less, the disruption of the polarization state when linearly polarized light passes through the polyester film can be suppressed, making it easier to suppress the unevenness of the rainbow pattern when viewed with the naked eye. In this specification, "unevenness of the rainbow pattern" may be referred to as "rainbow unevenness."

[0037] The Re of the polyester film has a lower limit of preferably 350 nm or more, more preferably 400 nm or more, more preferably 450 nm or more, more preferably 500 nm or more, and more preferably 550 nm or more. The Re of the polyester film is preferably 550 nm or more to improve its mechanical strength. The upper limit of the Re of the polyester film is preferably 1400 nm or less, more preferably 1200 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, more preferably 800 nm or less, more preferably 700 nm or less, and more preferably 650 nm or less. When the polyester film is thinned to a thickness of 10 μm or more and 50 μm or less, a Re of 1400 nm or less is preferred.

[0038] The Re of polyester film is, for example, 300nm to 1450nm, 350nm to 1450nm, 400nm to 1450nm, 450nm to 1450nm, 500nm to 1450nm, 550nm to 1450nm, 300nm to 1200nm, 350nm to 1200nm, 400nm to 1200nm, 450nm to 1200nm, 500nm to 1200nm, 550nm to 1200nm, and 300nm or less. Above 1100nm, 350nm to 1100nm, 400nm to 1100nm, 450nm to 1100nm, 500nm to 1100nm, 550nm to 1100nm, 300nm to 950nm, 350n m to 950nm, 400nm to 950nm, 450nm to 950nm, 500nm to 950nm, 550nm to 950nm, 300nm to 850nm, 350nm to 850nm, 400nm to 8 50nm or less, 450nm or more and 850nm or less, 500nm or more and 850nm or less, 550nm or more and 850nm or less, 300nm or more and 1000nm or less, 350nm or more and 1000nm or less, 400nm or more and 1000nm or less, 450nm or more1 000nm or less, 500nm or more and 1000nm or less, 550nm or more and 1000nm or less, 300nm or more and 800nm ​​or less, 350nm or more and 800nm ​​or less, 400nm or more and 800nm ​​or less, 450nm or more and 800nm ​​or less, 500nm or more and 80 Embodiments of numerical ranges such as 0 nm or less, 550 nm to 800 nm, 300 nm to 700 nm, 350 nm to 700 nm, 400 nm to 700 nm, 450 nm to 700 nm, 500 nm to 700 nm, 550 nm to 700 nm, 300 nm to 650 nm, 350 nm to 650 nm, 400 nm to 650 nm, 450 nm to 650 nm, 500 nm to 650 nm, and 550 nm to 650 nm are examples.

[0039] In this specification, the in-plane phase difference (Re) and the phase difference in the thickness direction (Rth) are expressed by the following equations (1) and (2), where nx is the refractive index in the slow axis direction, which is the direction in which the refractive index is greatest at each measurement point; ny is the refractive index in the fast axis direction, which is the direction perpendicular to the slow axis direction at each measurement point; nz is the refractive index in the thickness direction of the polyester film; and T [nm] is the thickness of the polyester film. In this specification, the in-plane phase difference and the phase difference in the thickness direction refer to the values ​​at a wavelength of 550 nm. Re = (nx - ny) × T [nm] (1) Rth = ((nx + ny) / 2 - nz) × T [nm] (2)

[0040] The direction of the slow axis, Re, and Rth can be measured, for example, using the "RETS-100" product manufactured by Otsuka Electronics Co., Ltd. When measuring Re etc. using the product name "RETS-100" manufactured by Otsuka Electronics Co., Ltd., it is preferable to prepare for measurement according to the following procedure (A1) to (A4).

[0041] (A1) First, to stabilize the light source of the RETS-100, leave it for at least 60 minutes after turning on the light source. Then, select the rotational analyzer method and the θ mode (mode for angular phase difference measurement and Rth calculation). By selecting this θ mode, the stage becomes a tilting rotation stage. (A2) Next, enter the following measurement conditions into the RETS-100. (Measurement conditions) • Measurement range of in-plane phase difference: Rotation analyzer method • Measurement spot diameter: φ5mm • Inclination angle range: 0° • Measurement wavelength range: 400nm to 800nm • The average refractive index of a polyester film. For example, in the case of polyethylene terephthalate film, enter N (average refractive index) = 1.617. Based on the input value of the average refractive index N of the polyester film, nz can be calculated using the formula "N = (nx + ny + nz) / 3". • Thickness: Thickness measured separately using SEM or optical microscope. (A3) Next, background data is obtained without placing a sample in the apparatus. The apparatus is a closed system, and this is done each time the light source is turned on. (A4) After that, the sample is placed on the stage inside the device and measured.

[0042] In this specification, in-plane phase difference, thickness-direction phase difference, total light transmittance, and haze refer to the average of 16 measurements unless otherwise specified. The 16 measurement points are preferably measured at the 16 intersection points when lines are drawn to divide the remaining area into five equal vertical and horizontal sections, after removing a 0.5 cm margin from the outer edge of the measurement sample. For example, if the measurement sample is rectangular, it is preferable to remove a 0.5 cm margin from the outer edge of the rectangle and measure at the 16 intersection points of the dotted lines that divide the remaining area into five equal vertical and horizontal sections. If the measurement sample is a shape other than a rectangle, such as a circle, ellipse, triangle, or pentagon, it is preferable to draw a rectangle inscribed in this shape and perform 16 measurements on the rectangle using the method described above.

[0043] The polyester film disclosed herein preferably has a value of 0.15 or less obtained by dividing the in-plane phase difference of the polyester film by the phase difference in the thickness direction of the polyester film. In this specification, the "value obtained by dividing the in-plane phase difference of the polyester film by the phase difference in the thickness direction of the polyester film" may be expressed as "Re / Rth".

[0044] A small Re / Rth ratio means that the degree of stretching of the polyester film approaches a uniform biaxial shape. Therefore, by setting the Re / Rth ratio to 0.15 or less, it is easier to improve the mechanical strength of the polyester film. Re / Rth is more preferably 0.13 or less, and even more preferably 0.10 or less. The lower limit of Re / Rth is preferably 0.01 or higher, more preferably 0.03 or higher, and even more preferably 0.05 or higher. When the stretching of the polyester film is weak, setting the ratio to 0.01 or higher makes it easier to suppress the brittleness of the polyester film. When the stretching of the polyester film is strong, setting the ratio to 0.01 or higher makes it easier to reduce Re. The Re / Rth of a perfectly uniaxially oriented polyester film is 2.0. General-purpose uniaxially oriented polyester films are slightly stretched in the flow direction as well. Therefore, the Re / Rth of general-purpose uniaxially oriented polyester films is approximately 1.0.

[0045] Examples of preferred numerical range embodiments for Re / Rth include 0.01 to 0.15, 0.01 to 0.13, 0.01 to 0.10, 0.03 to 0.15, 0.03 to 0.13, 0.03 to 0.10, 0.05 to 0.15, 0.05 to 0.13, and 0.05 to 0.10.

[0046] The polyester film of this disclosure preferably has an Rth of 2000 nm or more, more preferably 4000 nm or more, and even more preferably 5000 nm or more. By setting Rth to 2000nm or higher, it becomes easier to suppress blackout when viewing from an oblique angle through polarized sunglasses. Blackout is a phenomenon in which the entire screen appears black and the image cannot be seen. Furthermore, by setting Re to 300nm to 1450nm and Rth to 2000nm or higher, the stretching of the polyester film can be brought closer to a uniform biaxial shape, making it easier to improve the mechanical strength of the polyester film. In order to set the Rth of the polyester film within the above range, it is preferable to increase the stretching ratio in the flow direction and the width direction. By increasing the stretching ratio in the flow direction and the width direction, the refractive index nz in the thickness direction of the polyester film decreases, making it easier to increase the Rth. To improve the flexibility of the polyester film, the Rth is preferably 9500nm or less, more preferably 8000nm or less, and even more preferably 6000nm or less.

[0047] Examples of preferred numerical range embodiments for Rth include 2000nm to 9500nm, 2000nm to 8000nm, 2000nm to 6000nm, 4000nm to 9500nm, 4000nm to 8000nm, 4000nm to 6000nm, 5000nm to 9500nm, 5000nm to 8000nm, and 5000nm to 6000nm.

[0048] The polyester film preferably satisfies the following condition 1. <Condition 1> A sample measuring 50mm x 50mm is cut from a polyester film. The direction of the slow axis is measured at a total of five locations: one point in the center of the sample and four points 10mm from each of the four corners toward the center. When the angles formed by any one side of the sample and the direction of the slow axis at each measurement point are 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.

[0049] If the direction of the slow axis of the polyester film is misaligned, the rainbow pattern becomes blurred and difficult to see. Therefore, by satisfying condition 1, it is possible to suppress the visibility of the rainbow pattern to the naked eye. General-purpose stretched polyester films are designed so that the direction of the slow axis does not shift. However, as described above, intentionally shifting the direction of the slow axis of the polyester film can make it easier to suppress iridescence. Furthermore, while the effect of suppressing iridescence is small when the slow axis is varied over a large area, it is easier to suppress iridescence when the slow axis is varied over a relatively small area of ​​50mm x 50mm. The five black circles in Figure 1 correspond to the five locations where the direction of the slow axis is measured under condition 1.

[0050] In condition 1, any one side of the sample that serves as the reference for the angle with respect to the direction of the slow axis can be either the vertical or horizontal side of the sample, as long as all sides D1 to D5 refer to the same side.

[0051] Furthermore, satisfying condition 1 is preferable because it allows for good bending resistance of the polyester film. On the other hand, general-purpose polyester films that do not satisfy condition 1 and have aligned slow phase axes may break or retain a strong bending curvature after bending tests. Specifically, a uniaxially oriented film like the one in Patent Document 1 will break when bent along the slow phase axis, and will retain a strong bending curvature when bent in a direction perpendicular to the slow phase axis. Similarly, general-purpose biaxially oriented films will retain a strong bending curvature when bent in a direction perpendicular to the slow phase axis. A polyester film that satisfies condition 1 is preferable because it can suppress the retention of kinks or breakage after a bending test, regardless of the direction of bending.

[0052] Furthermore, polyester films that satisfy condition 1 have a Martens hardness of 9.0 N / mm² with a standard deviation of 3σ of the surface hardness of the polyester film. 2 This makes it easier to do the above.

[0053] 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, even more preferably 3.5 degrees or more, and even less preferably 4.5 degrees or more. Furthermore, if the difference between the maximum value and the minimum value of D1 to D5 is too large, the orientation of the polyester film will decrease, making it difficult to keep the average Martens hardness within the above range and tending to reduce mechanical strength. For this reason, the difference is preferably 20.0 degrees or less, more preferably 15.0 degrees or less, even more preferably 12.0 degrees or less, and even more preferably 8.0 degrees or less.

[0054] In condition 1, the preferred range for the difference between the maximum and minimum values ​​of D1 to D5 is, 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, 4.5 degrees or more and 20.0 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 15.0 degrees or less, 3.5 degrees or more and 15.0 Examples include temperatures below 1 degree, 4.5 degrees to 15.0 degrees, 1.5 degrees to 12.0 degrees, 2.0 degrees to 12.0 degrees, 3.0 degrees to 12.0 degrees, 3.5 degrees to 12.0 degrees, 4.5 degrees to 12.0 degrees, 1.5 degrees to 8.0 degrees, 2.0 degrees to 8.0 degrees, 3.0 degrees to 8.0 degrees, 3.5 degrees to 8.0 degrees, and 4.5 degrees to 8.0 degrees.

[0055] If a polyester film has layers and films that affect the measurement of in-plane phase difference, thickness-direction phase difference, and the direction of the slow axis, these layers and films should be removed before measuring the in-plane phase difference, thickness-direction phase difference, and the direction of the slow axis of the polyester film. Note that layers formed by coatings usually do not affect the measurement of in-plane phase difference, thickness-direction phase difference, and the direction of the slow axis. The means for peeling off layers and films that affect in-plane phase differences, etc., are as described above.

[0056] The polyester film is preferably free from cracking or breaking after 100,000 folding tests as shown in the examples, and more preferably free from cracking or breaking after 300,000 such tests. Furthermore, after performing the folding test shown in the examples 100,000 times, when the measurement sample is placed on a horizontal table, it is preferable that the angle at which the edge of the sample lifts off the table is 20 degrees or less, and more preferably 15 degrees or less. After performing the above test 300,000 times, it is even more preferable that the angle at which the edge of the sample lifts off satisfies the above condition. An angle of 20 degrees or less from the edge of the sample means that it is less prone to creasing due to folding. Furthermore, it is preferable that the polyester film satisfies the above performance requirements whether it is folded in the direction of the slow phase axis or the direction of the fast phase axis. Furthermore, when uniaxially oriented polyester film is subjected to folding tests, it breaks in the stretching direction and retains a strong bending tendency in the direction perpendicular to the stretching direction. For this reason, biaxially oriented polyester film is preferred among stretched polyester films.

[0057] Regarding condition 1, if multiple samples measuring 50 mm in length and 50 mm in width can be taken from a sheet of polyester film, it is preferable that the proportion of samples satisfying condition 1 among the multiple samples be 50% or more, more preferably 70% or more, even more preferably 90% or more, and even more preferably 100%. Furthermore, the physical properties of a roll of polyester film are generally consistent in the flow direction. Therefore, if a 50mm x 500mm sheet cut from any position β in the width direction satisfies condition 1, then it can be assumed that condition 1 is satisfied throughout the entire roll in the flow direction at any given position β.

[0058] Polyester film lamination structures include single-layer and multi-layer structures. The polyester film disclosed herein has an average surface Martens hardness of 140.0 N / mm². 2 Furthermore, the standard deviation 3σ of the surface Martens hardness is 9.0 N / mm². 2 More than 36.0N / mm 2 The following conditions must be met. In order to bring the average Martens hardness and the standard deviation of Martens hardness 3σ of the surface of the polyester film within the aforementioned range, it is preferable to make the stretching ratios in the flow direction and width direction as close to uniform as possible. Therefore, in order to bring the average Martens hardness and the standard deviation of Martens hardness 3σ of the surface of the polyester film within the above-mentioned range, stretching control is essential. Single-layer polyester films are preferred over multi-layer polyester films in that stretching is easier to control.

[0059] Polyesters that make up polyester films include polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and polybutylene terephthalate (PBT). Among these, PET is preferred because it has a low intrinsic birefringence and makes it easy to reduce the in-plane phase difference.

[0060] The polyester film may contain additives such as UV absorbers and other absorbers that absorb specific wavelengths, light stabilizers, antioxidants, antistatic agents, flame retardants, gelling inhibitors, dyes, pigments, organic particles, inorganic particles, antifouling agents, crosslinking agents, and surfactants.

[0061] The thickness of the polyester film is preferably 10 μm or more at the lower limit, more preferably 15 μm or more, more preferably 20 μm or more, more preferably 25 μm or more, and more preferably 30 μm or more, and preferably 80 μm or less at the upper limit, more preferably 60 μm or less, more preferably 55 μm or less, and more preferably 50 μm or less. For thinning and flexibility, the thickness of the polyester film is preferably 50 μm or less. By increasing the thickness to 10 μm or more, it becomes easier to achieve good mechanical strength. Conversely, by reducing the thickness to 80 μm or less, it becomes easier to minimize in-plane phase difference.

[0062] Examples of preferred numerical ranges for the thickness of the polyester film include, for example, 10 μm to 80 μm, 10 μm to 60 μm, 10 μm to 55 μm, 10 μm to 50 μm, 15 μm to 80 μm, 15 μm to 60 μm, 15 μm to 55 μm, 15 μm to 50 μm, 20 μm to 80 μm, 20 μm to 60 μm, 20 μm to 55 μm, 20 μm to 50 μm, 25 μm to 80 μm, 25 μm to 60 μm, 25 μm to 55 μm, 25 μm to 50 μm, 30 μm to 80 μm, 30 μm to 60 μm, 30 μm to 55 μm, and 30 μm to 50 μm.

[0063] The polyester 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, according to JIS K7136:2000. Furthermore, the polyester film preferably has a total light transmittance of 80% or more, more preferably 85% or more, and even more preferably 90% or more, according to JIS K7361-1:1997.

[0064] The surface of the polyester film preferably has a pencil hardness of HB or higher in both the slow-phase axis direction and the fast-phase axis direction, and more preferably F or higher. If the pencil hardness of a polyester film is too high, its flexibility and the adhesion of the functional layer tend to decrease. For this reason, the surface of the polyester film preferably has a pencil hardness of 2H or less in both the slow phase axis direction and the fast phase axis direction, and more preferably H or less. Examples of preferred numerical ranges for the pencil hardness of the surface of the polyester film include, for example, HB to 2H, HB to H, F to 2H, and F to H. It is preferable that the pencil hardness of one surface of the polyester film satisfies the above range, but it is more preferable that the pencil hardness of both surfaces satisfies the above range.

[0065] In this specification, the pencil hardness of polyester films was determined by pressing a test pencil specified in JIS S6006 onto the surface of the polyester film, and the following conditions were modified, except as described below, in accordance with JIS K5600-5-4:1999. <Condition> Test load: 0.98N Test speed: 3mm / s Passing criteria: Each polyester film was subjected to a pencil hardness test five times in the slow phase axis direction. If the polyester film was not scratched in three or more tests with a pencil of a specified hardness, the specified hardness in the slow phase axis direction was considered to have passed. Furthermore, each polyester film was subjected to a pencil hardness test five times in the fast phase axis direction. If the polyester film was not scratched in three or more tests with a pencil of a specified hardness, the specified hardness in the fast phase axis direction was considered to have passed.

[0066] The polyester film is preferably a stretched polyester film in order to obtain good mechanical strength. Furthermore, the stretched polyester film is more preferably a single-layer structure of polyester resin layers.

[0067] Stretched polyester films can be obtained by stretching a resin layer containing the components that make up a polyester film. Stretching methods include biaxial stretching such as sequential biaxial stretching and simultaneous biaxial stretching, and uniaxial stretching such as longitudinal uniaxial stretching. Among these, biaxial stretching is preferred because it easily reduces the in-plane phase difference and increases mechanical strength. In other words, the stretched polyester film is preferably a biaxially oriented polyester film. Furthermore, among biaxially oriented polyester films, biaxially oriented polyethylene terephthalate film is preferred.

[0068] -Sequential biaxial stretching- In sequential biaxial stretching, the casting film is stretched in the flow direction first, and then stretched in the width direction of the film. Flow-direction stretching is usually performed by the difference in peripheral speed of a pair of stretching rolls. Flow-direction stretching may be performed in one stage, or in multiple stages using multiple pairs of stretching rolls. To suppress excessive variations in physical properties such as Martens hardness and in-plane phase difference, it is preferable to place multiple nip rolls close to the stretching rolls. The flow-direction stretching ratio is usually between 2.0 and 15.0 times. To suppress excessive variations in physical properties such as Martens hardness and in-plane phase difference, it is preferably between 2.0 and 7.0 times, more preferably between 3.0 and 5.0 times, and even more preferably between 3.0 and 4.0 times. The stretching temperature is preferably between the glass transition temperature of the resin and the glass transition temperature + 100°C in order to impart appropriate variation to the Martens hardness and suppress excessive variation in physical properties such as in-plane phase difference. In the case of PET, 70°C to 120°C is preferred, 80°C to 110°C is more preferred, and 95°C to 110°C is even more preferred. The stretching temperature refers to the set temperature of the apparatus. Even if the set temperature of the apparatus is set within the above range, it takes time for the temperature to stabilize. For this reason, it is preferable to set the temperature within the above range and then manufacture the polyester film after the temperature has stabilized. In this specification, the set temperature of the apparatus is described in multiple places. As with the set temperatures in other places, it is preferable to manufacture the polyester film after the temperature has stabilized, as described above. Regarding the stretching temperature, shortening the stretching period at low temperatures by rapidly heating the film tends to reduce the in-plane phase difference. On the other hand, lengthening the stretching period at low temperatures by slowly heating the film increases the orientation, leading to a larger in-plane phase difference and a tendency for less variation in the slow axis. Furthermore, lengthening the stretching period at low temperatures by slowly heating the film makes it easier to bring the average Martens hardness and the standard deviation of Martens hardness (3σ) within the above range. When heating during stretching, it is preferable to use a heater that generates turbulence. Heating with turbulent air creates temperature differences in minute regions within the film surface, and these temperature differences cause minute misalignments in the orientation axis, making it easier to satisfy condition 1, and also lowering the standard deviation of the Martens hardness (3σ) to 9.0 N / mm². 2 This makes it easier to achieve the above. However, it is preferable to adjust the intensity of the turbulence, taking into consideration that if the turbulence is made too strong, the average Martens hardness may become too small or the standard deviation of Martens hardness (3σ) may become too large.

[0069] A film stretched in the direction of flow may be given functions such as slipperiness, adhesion, and antistatic properties by in-line coating or off-line coating. Furthermore, surface treatments such as corona treatment, flame treatment, or plasma treatment may be applied before in-line or off-line coating, as needed. In this specification, layers formed by in-line coating or offline coating shall not be counted as part of the number of layers constituting the polyester film.

[0070] Stretching in the width direction is usually performed using the tenter method, where the film is transported while being held at both ends with clips to stretch it in the width direction. The stretching ratio in the width direction is usually between 2.0 and 15.0 times, preferably between 2.0 and 7.0 times, more preferably between 3.0 and 6.0 times, and even more preferably between 4.0 and 6.0 times, in order to suppress excessive variations in physical properties such as in-plane phase difference. In order to make it easier to bring the average Martens hardness and the standard deviation of Martens hardness 3σ within the above range, it is preferable to make the stretching ratio E2 in the width direction, which is the second stage of stretching, about the same as the stretching ratio E1 in the flow direction, which is the first stage of stretching, and it is more preferable to make E2 larger than E1. E2 / E1 is preferably 0.80 or more and 1.70 or less, more preferably 0.85 or more and 1.50 or less, even more preferably 0.90 or more and 1.40 or less, and even more preferably 1.00 or more and 1.30 or less. The stretching temperature is preferably above the glass transition temperature of the resin and below the glass transition temperature + 110°C, and it is preferable that the temperature increases from upstream to downstream. The stretching temperature refers to the set temperature of the apparatus. The upstream side is the side closer to the point where stretching in the width direction begins. The downstream side is the side closer to the point where stretching in the width direction ends. Specifically, when the stretching section in the width direction is divided into two based on length, the difference between the upstream temperature and the downstream temperature is preferably 20°C or more, more preferably 30°C or more, even more preferably 35°C or more, and even more preferably 40°C or more. In the case of PET, the stretching temperature for the first stage is preferably 80°C to 120°C, more preferably 90°C to 110°C, and even more preferably 95°C to 105°C. By dividing the stretching section in the width direction into two and creating a difference in stretching temperature between the first and second stages, the surface temperature of the film during the first stage of stretching and the surface temperature of the film during the second stage of stretching can be controlled to be different temperatures. Therefore, at each stretching stage, orientation and orientation crystallization do not progress too much, preventing the plastic film from becoming brittle, and thus making it easier to improve pencil hardness.

[0071] As described above, the successively biaxially stretched polyester film is preferably subjected to heat treatment in a tenter, above the stretching temperature but below the melting point, in order to impart flatness and dimensional stability. The heat treatment temperature refers to the set temperature of the apparatus. Specifically, in the case of PET, it is preferable to perform heat setting in the range of 140°C to 240°C, and more preferably 200°C to 250°C. Furthermore, in order to suppress excessive variations in physical properties such as Martens hardness and in-plane phase difference, it is preferable to perform a further stretch of 1% to 10% in the first half of the heat treatment. After heat treatment, the polyester film is slowly cooled to room temperature before being wound up. Additionally, relaxation treatments may be used in conjunction with heat treatment and slow cooling as needed. The relaxation rate during heat treatment is preferably 0.5% to 5%, more preferably 0.5% to 3%, even more preferably 0.8% to 2.5%, and even more preferably 1% to 2%. Similarly, the relaxation rate during slow cooling is preferably 0.5% to 3%, more preferably 0.5% to 2%, even more preferably 0.5% to 1.5%, and even more preferably 0.5% to 1.0%. The temperature during slow cooling is preferably 80°C to 140°C, more preferably 90°C to 130°C, even more preferably 100°C to 130°C, and even more preferably 100°C to 120°C, in order to ensure good flatness. The temperature during slow cooling refers to the set temperature of the apparatus.

[0072] -Simultaneous biaxial stretching- Simultaneous biaxial stretching involves guiding the casting film to a simultaneous biaxial tenter, where both ends of the film are held with clips during transport, and stretching simultaneously and / or in stages in the flow direction and width direction. Simultaneous biaxial stretchers can be of the pantograph type, screw type, drive motor type, or linear motor type, but the drive motor type or linear motor type is preferred because it allows for arbitrary changes in the stretching ratio and enables slackening at any point.

[0073] The magnification ratio for simultaneous biaxial stretching is typically 6.0 to 50.0 times as an area magnification ratio. The area magnification ratio is preferably 8.0 to 30.0 times, more preferably 9.0 to 25.0 times, even more preferably 9.0 to 20.0 times, and even more preferably 10.0 to 15.0 times, in order to suppress excessive variations in physical properties such as Martens hardness and in-plane phase difference. In simultaneous biaxial stretching, it is preferable to adjust the area magnification ratio so that the stretching ratio in the flow direction and the stretching ratio in the width direction are within the range of 2.0 to 15.0 times. Furthermore, in the case of simultaneous biaxial stretching, it is preferable to make the stretching ratio in the flow direction and the width direction the same, and to make the stretching speed approximately equal, in order to suppress the orientation difference within the plane. In addition, by making the stretching ratio in the flow direction and the width direction the same, and making the stretching speed approximately equal, it is easier to bring the average Martens hardness and the standard deviation of Martens hardness 3σ within the above range.

[0074] For simultaneous biaxial stretching, the stretching temperature is preferably above the glass transition temperature of the resin and below the glass transition temperature + 120°C in order to suppress excessive variations in optical properties such as in-plane phase difference. In the case of PET, 80°C to 160°C is preferred, 90°C to 150°C is more preferred, and 100°C to 140°C is even more preferred. The stretching temperature refers to the set temperature of the apparatus.

[0075] The simultaneously biaxially stretched film is preferably subsequently subjected to a heat treatment in a heat-setting chamber within a tenter, where the heat treatment is performed above the stretching temperature but below the melting point, in order to impart flatness and dimensional stability. The temperature of the heat treatment refers to the set temperature of the apparatus. The conditions for the heat treatment are the same as those for the heat treatment after sequential biaxial stretching.

[0076] <Shape, size> The polyester film may be in the form of a single sheet cut to a predetermined size, or in the form of a roll formed by winding a long sheet into a roll. The size of the sheet is not particularly limited, but the maximum diameter is approximately 2 inches to 500 inches. In this disclosure, the size of the sheet is preferably 30 inches to 100 inches, and more preferably 40 inches to 100 inches. "Maximum diameter" refers to the maximum length when any two points on the polyester film are connected. For example, if the polyester film is rectangular, the diagonal of the rectangular area is the maximum diameter. If the polyester 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 between 200 mm and 8000 mm, and the length is between 100 m and 8000 m. The polyester film in roll form can be cut into sheets to match the size of an image display device or the like. When cutting, it is preferable to remove the roll ends, which have unstable physical properties. Furthermore, the shape of the sheets is not particularly limited; for example, they may be polygonal (triangle, quadrilateral, pentagon, etc.), circular, or randomly irregular in shape. More specifically, if the polyester film is rectangular, the aspect ratio is not particularly limited as long as it does not pose a problem as a display screen. Examples include width:height = 1:1, 4:3, 16:10, 16:9, 2:1, 5:4, etc.

[0077] <Application> The polyester film of this disclosure can achieve good pencil hardness without increasing its thickness. For this reason, the polyester film of this disclosure can be suitably used as a polyester film for image display devices, and in particular, can be suitably used as a polyester film for image display devices equipped with a touch panel. Furthermore, the polyester film of one embodiment of the present disclosure that satisfies condition 1 can suppress the retention of kinks or breakage after a bending test, regardless of the bending direction, and is therefore suitable for use as a polyester film for curved image display devices and foldable image display devices. Examples of polyester films used in image display devices include polyester films that protect polarizers, and polyester films used as base materials for various functional films such as surface protection films, anti-reflective films, and conductive films that constitute touch panels.

[0078] [Optical laminate] The polyester film of this disclosure may be further formed with functional layers such as a protective layer, an anti-reflective layer, a hard coat layer, an anti-glare layer, a phase difference layer, an adhesive layer, a transparent conductive layer, an anti-static layer, and an anti-fouling layer, and may be used as an optical laminate. The functional layer of the optical laminate preferably includes an anti-reflective layer. The anti-reflective layer is preferably placed on the outermost surface of the polyester film on the side with the functional layer. Having an anti-reflective layer as the functional layer of the optical laminate makes it easier to suppress rainbow-like unevenness.

[0079] Furthermore, it is more preferable that the functional layer includes a hard coat layer and an anti-reflective layer. When the functional layer includes a hard coat layer and an anti-reflective layer, it is preferable that the hard coat layer and the anti-reflective layer are arranged on the polyester film in this order. General-purpose hard coat and anti-reflective coatings can be used.

[0080] [Polarizing plate] The polarizing plate of the present disclosure is a polarizing plate having a polarizer, a first transparent protective plate disposed on one side of the polarizer, and a second transparent protective plate disposed on the other side of the polarizer, wherein at least one of the first transparent protective plate and the second transparent protective plate is the polyester film of the present disclosure described above.

[0081] Polarizing plates are used, for example, in combination with λ / 4 phase difference plates to provide anti-reflective properties. In this case, the λ / 4 phase difference plate is placed on the display element of an image display device, and the polarizing plate is placed on the viewer side of the λ / 4 phase difference plate. In liquid crystal display devices, polarizers are used to provide the function of a liquid crystal shutter. In this case, the liquid crystal display device is arranged in the order of lower polarizer, liquid crystal display element, and upper polarizer from the backlight side, and the absorption axis of the polarizer of the lower polarizer and the absorption axis of the polarizer of the upper polarizer are arranged orthogonally. In the configuration of the liquid crystal display device, the polarizers of the present disclosure can be used as the upper polarizer and the lower polarizer, and it is preferable to use the polarizer of the present disclosure as the upper polarizer. In the upper polarizer, it is preferable to use the optical film of the present disclosure as a transparent protective plate on the light emission surface side of the polarizer. In the lower polarizer, it is preferable to use the optical film of the present disclosure as a transparent protective plate on the light incident surface side of the polarizer.

[0082] <Transparent protection plate> The polarizing plate of this disclosure uses the optical film of this disclosure 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 the optical film of this disclosure described above.

[0083] When one of the first and second transparent protective plates is the optical film of this disclosure described above, the other transparent protective plate is not particularly limited, but an optically isotropic transparent protective plate or a transparent protective plate containing a biomass material is preferred. In this specification, an optically isotropic transparent protective plate refers to one with an in-plane phase difference of less than 20 nm, preferably 10 nm or less, and more preferably 5 nm or less. Examples of optically isotropic transparent protective plates include acrylic films, triacetylcellulose films, polycarbonate films, amorphous olefin films, and the like. Furthermore, if only one of the first transparent protective plate and the second transparent protective plate is the optical film of the present disclosure described above, it is preferable to use the optical film of the present disclosure described above as the transparent protective plate on the light-emitting side. The first transparent protective plate, the second transparent protective plate, and the polarizer may be in direct contact with each other, or they may be in contact with each other via an adhesive layer.

[0084] <Polarizer> Examples of polarizers include sheet-type polarizers made by stretching a film dyed with iodine or the like (polyvinyl alcohol film, polyvinyl formal film, polyvinyl acetal film, ethylene-vinyl acetate copolymer saponified film, etc.), wire grid-type polarizers made of numerous parallel metal wires, coated polarizers coated with lyotropic liquid crystal and dichroic guest-host materials, and multilayer thin-film polarizers. These polarizers may also be reflective polarizers that have the function of reflecting polarization components that do not transmit through them.

[0085] It is preferable that the polarizer is positioned such that the angle between its absorption axis and the slow axis of the polyester film is within 90 degrees ± 5 degrees. More preferably, this angle is within 90 degrees ± 3 degrees, and even more preferably within 90 degrees ± 1 degree.

[0086] [Image display device] The image display device of the present disclosure is an image display device having a display element and a polyester film disposed on the light-emitting surface side of the display element, wherein the polyester film is the polyester film of the present disclosure described above.

[0087] Figures 2 and 3 are cross-sectional views showing embodiments of the image display device 100 of the present disclosure. The image display devices 100 in Figures 2 and 3 have a polyester film 10 on the light-emitting side (upper side in Figures 2 and 3) of the display element 20. Both the image display devices 100 in Figures 2 and 3 also have a polarizer 31 between the display element 20 and the polyester film 10. Furthermore, in Figures 2 and 3, a first transparent protective plate (32) and a second transparent protective plate (33) are laminated on both sides of the polarizer 31. In the image display device in Figure 3, the polyester film 10 is used as the first transparent protective plate (32).

[0088] The image display device preferably has a polarizer between the display element and the polyester film. When an image display device has a polarizer, it is preferable to arrange the polarizer so that the angle between the absorption axis of the polarizer and the slow axis of the polyester film is within 90 degrees ± 5 degrees. More preferably, this angle is within 90 degrees ± 3 degrees, and even more preferably within 90 degrees ± 1 degree.

[0089] The image display device 100 is not limited to the forms shown in Figures 2 and 3. For example, in Figures 2 and 3, the components constituting the image display device 100 are arranged at predetermined intervals, but the components may be integrated by means of 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 have surface plates such as glass plates and plastic plates. If the image display device has surface plates, the optical film of this disclosure may be laminated to the surface plates.

[0090] <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, and LED display elements such as mini-LEDs and micro-LEDs, as well as liquid crystal display elements and LED display elements using QDs. If the display element of a display device is a liquid crystal display element, a backlight is required on the side of the liquid crystal display element opposite the resin sheet.

[0091] Furthermore, the image display device may also be an image display device equipped with a touch panel function. Examples of touch panel technologies include resistive, capacitive, electromagnetic induction, infrared, and ultrasonic types. The touch panel function may be implemented by adding the function to the display element, such as an in-cell touch panel liquid crystal display element, or by mounting a touch panel on top of the display element.

[0092] If the polyester film satisfies condition 1, the optical film can be prevented from retaining its bending shape or breaking after the bending test. For this reason, if the polyester film satisfies condition 1, the image display device is preferably a curved image display device or a foldable image display device. If 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.

[0093] Examples of polyester films placed on the light-emitting surface side of a display element include polyester films that protect polarizers, and polyester films used as base materials for various functional films such as surface protection films, anti-reflective films, and conductive films that constitute touch panels.

[0094] <Other plastic films> The image display device disclosed herein may have other plastic films to the extent that they do not impair the effects of the disclosure. There may be only one or more of these other plastic films. When there are multiple plastic films on the display element, it is preferable that the plastic film furthest from the display element be the polyester film of this disclosure. [Examples]

[0095] Next, the present disclosure will be described in more detail by examples, but the present disclosure is not limited in any way by these examples.

[0096] 1. Measurement and evaluation The atmosphere for measurement and evaluation in sections 1-1 to 1-8 below shall be a temperature of 23°C ± 5°C and a relative humidity of 40% to 65%. Furthermore, before measurement and evaluation, the sample for measurement shall be exposed to the aforementioned atmosphere for 30 to 60 minutes. The sample for measurement shall be taken from a clean and undamaged area. Measurement and evaluation shall be carried out with the sample in a state of good flatness. The results are shown in Table 1.

[0097] 1-1. Mean Martens hardness (HM), standard deviation of Martens hardness (HM3σ), and elastic deformation power Samples measuring 50 mm in the flow direction and 50 mm in the width direction were cut from the polyester films of the examples and comparative examples prepared in "2" below. Using a Fischer Instruments "PICODENTOR HM500" measuring device, measurements were performed at 20 locations on the samples under the following measurement conditions, and the average Martens hardness, the standard deviation of Martens hardness (3σ), the elastic deformation power, and the standard deviation of the elastic deformation power (3σ) were calculated. The 20 measurements within a single sample were performed consecutively. The measurement and analysis software used was the software included with the measuring device (product name "WIN-HCU", version 3.5). Part of the operation procedure of the software during measurement is shown below. Similarly, the standard deviation of Martens hardness (3σ) was calculated for Reference Examples 1 to 3. The average indentation depth of the indenter was 2.2 μm for Examples 1-3, 2.0 μm for Example 4, 2.5 μm for Comparative Example 1, 2.4 μm for Comparative Example 2, 2.2 μm for Comparative Example 3, 2.1 μm for Comparative Example 4, 2.0 μm for Reference Examples 1-2, and 1.8 μm for Reference Example 3. In Tables 1 and 2, "Average Martens hardness" is denoted as "HM" and "Standard deviation of Martens hardness" is denoted as "HM3σ". The initial measurement for each sample was not used as data because it may produce outliers. Therefore, in practice, 21 measurements were taken for each sample, and HM, HM3σ, and elastic deformation power were calculated using data from the 2nd to the 21st measurements. Furthermore, before measuring each sample, a standard indentation test was performed using a standard sample with a known Martens hardness (acrylic plate manufactured by Fischer Instruments) to confirm that the Martens hardness obtained from the test results was within the standard range. Standard calibration was performed each time the sample was changed. <Measurement conditions> • Indenter used: Vickers indenter (square pyramidal shape, diamond material, model number: VV005, Fischer Instruments) • Push-in conditions: Maximum load • Maximum indentation load: 20mN • Load application time: 10 seconds • Holding time: Hold for 5 seconds under maximum indentation load. • Load release time: 10 seconds • Stage for placing samples: Suction stage (The suction stage used was SMC's model number "SP2130-AD".) <Partial operating procedure of the aforementioned software> Click the "Create New" button. Select "Application Properties" from the "Edit" tab to open the Application Properties. In the "Test Parameter Settings" section of the Application Properties, click the ">>>" icon for load increase settings to open "Load Increase Parameters". In the Load Increase Parameters, confirm that "dSQRT(F) / dt=Constant" is selected and that "Maximum Load" is selected. Furthermore, in the Load Increase Parameters, enter 20[mN] as the maximum test load, select "Load Application Time Setting", and enter 10[s] for the time.

[0098] 1-2. In-plane phase difference (Re) and phase difference in the thickness direction (Rth) Using the same sample as in 1-1, the in-plane phase difference and the phase difference in the thickness direction were measured at 16 locations according to the description in the specification, and the average of the 16 locations was calculated. The Re, Rth, and Re / Rth calculated from the measurement results are shown in Table 1. The measuring device used was the "RETS-100" (measurement spot: 5 mm diameter) manufactured by Otsuka Electronics Co., Ltd.

[0099] 1-3. Direction of the slow axis The direction of the slow phase axis was measured at four locations 10 mm from each corner of the same sample as in 1-1, and at a total of five locations in the center of the sample. The measuring device used was the "RETS-100" (measurement spot: 5 mm in diameter) manufactured by Otsuka Electronics Co., Ltd. The direction of the slow phase axis was measured within a range of 0 to ±90 degrees, with the flow direction of the polyester film as the reference 0 degrees.

[0100] 1-4.Pencil hardness A pencil hardness test was performed on the surface of the polyester films of the examples and comparative examples by pressing a test pencil specified in JIS S6006 against it. The pencil hardness test was conducted in accordance with JIS K5600-5-4:1999, except for the following changes to the conditions. Table 1 shows the highest hardness among the pencils that met the following acceptance conditions in the slow phase axis direction and the fast phase axis direction of each polyester film. <Condition> Test load: 0.98N Test speed: 3mm / s Passing criteria: Each polyester film was subjected to a pencil hardness test five times in the slow phase axis direction. If the polyester film was not scratched in three or more tests with a pencil of a specified hardness, the specified hardness in the slow phase axis direction was considered to have passed. Furthermore, each polyester film was subjected to a pencil hardness test five times in the fast phase axis direction. If the polyester film was not scratched in three or more tests with a pencil of a specified hardness, the specified hardness in the fast phase axis direction was considered to have passed.

[0101] 1-5. Flexibility <Width direction> Strip-shaped samples measuring 30 mm in the short side (width direction) and 100 mm in the long side (flow direction) were cut from the polyester films of the examples and comparative examples. The short side (30 mm) of the samples was fixed to a durability testing machine (product name "DLDMLH-FS", manufactured by Yuasa System Equipment Co., Ltd.) (fixing a 10 mm area from the tip), and a continuous folding test of 180 degrees was performed 100,000 times. The folding speed was set to 120 times per minute. A more detailed method of the folding test is shown below. After the folding test, the strip-shaped samples were placed on a horizontal table, and the angle at which the ends of the samples lifted off the table was measured. A smaller angle indicates that the sample is closer to its original flat state before folding. The results are shown in Table 1. Samples that broke midway were marked as "broken". <Flow direction> Strip-shaped samples measuring 30 mm (short side, flow direction) x 100 mm (long side, width direction) were cut from the polyester films of the examples and comparative examples, and evaluated in the same manner as described above.

[0102] <Details of the folding test> As shown in Figure 4(A), in the continuous folding test, first, the edge 10C of the polyester film 10 and the edge 10D opposite to edge 10C are fixed by parallel fixing parts 60. The fixing parts 60 are slidable in the horizontal direction. Next, as shown in Figure 4(B), the fixing parts 60 are moved closer to each other to deform the polyester film 10 into a folded state. Furthermore, as shown in Figure 4(C), the fixing parts 60 are moved to a position where the distance between the two opposing sides of the polyester film 10 fixed by the fixing parts 60 is 7 mm, and then the fixing parts 60 are moved in the opposite direction to release the deformation of the plastic film 10. As shown in Figures 4(A) to (C), the polyester film 10 can be folded 180 degrees by moving the fixing part 60. Furthermore, by performing a continuous folding test so that the bent portion 10E of the polyester film 10 does not protrude from the lower end of the fixing part 60, and by controlling the distance between the fixing parts 60 when they are closest to each other to 7 mm, the distance between the two opposing sides of the polyester film can be set to 7 mm.

[0103] 1-6. Rainbow pattern The sample prepared in 1-1 was placed on the viewing-side polarizing plate of the image display device with the configuration described below, so that the width of the sample was parallel to the horizontal direction of the screen. Next, the image display device was turned on in a darkroom environment, and observed with the naked eye from various angles to evaluate the presence or absence of rainbow unevenness according to the following criteria. A: The rainbow pattern is not visible. B: Rainbow-colored blemishes are visible in a very limited area. C: Rainbow patterns are visible in most areas. <Configuration of the image display device> (1) Backlight source: White LED or cold cathode tube (2) Light source side polarizer: TAC is used as a protective film on both sides of the polarizer, which is made of PVA and iodine. It has a film. The polarizer is positioned so that the direction of its absorption axis is perpendicular to the horizontal direction of the screen. (3) Image display cell: Liquid crystal cell (4) Viewing side polarizer: A polarizer protective film made of PVA and iodine, such as TAC A polarizing plate using film. The polarizer is positioned so that the direction of its absorption axis is perpendicular to the parallel direction of the screen. (5) Size: 10 inches diagonally

[0104] 1-7. Anti-sticking properties Prepare five 100mm x 100mm pieces each of polyester film for the examples, comparative examples, and reference examples. Align the cut direction of the five cut pieces of film, stack them, and test them using a permanent strain tester (constant load type), model number: CO-201, pressure surface: φ80mm, manufactured by Tester Sangyo Co., Ltd., at 500g / cm². 2 A load was applied. The film was left at 40°C for 3 days with the load still applied. The adhesion resistance was evaluated at the four interfaces where the films were in contact, according to the following criteria. A: All four interfaces can be easily peeled off. B: 4 The world Materials that can be easily peeled off at three or more interfaces within a surface. C: 4 The world A surface with two or fewer interfaces that can be easily peeled off.

[0105] 1-8. Adhesion A hard coat layer coating solution with the following composition was applied to the polyester films of the Examples, Comparative Examples, and Reference Examples, and then dried at 70°C for 1 minute to evaporate the solvent. Subsequently, ultraviolet irradiation (100 mJ / cm²) was performed. 2 A hard coat layer (dry thickness 5 μm) was formed, and a laminate having a hard coat layer on a polyester film was obtained. A strip-shaped sample measuring 30 mm on the short side (flow direction) and 100 mm on the long side (width direction) was cut from the laminate, and a continuous folding test similar to that in 1-5 was performed. Two types of tests were conducted: one with a spacing of 7 mm when the fixing part 60 was closest, and another with a spacing of 5 mm. After the folding test, the adhesion between the polyester film and the hard coat layer was visually evaluated according to the following criteria. A: The hard coat layer does not peel off from the polyester film even after 100,000 folding tests. B: When the number of folding tests reached 100,000, a small portion of the hard coat layer peeled off from the polyester film, but when the number of folding tests reached 50,000, the hard coat layer did not peel off from the polyester film. C: When the number of folding tests reached 100,000, a large portion of the hard coat layer peeled off from the polyester film. Or, when the number of folding tests reached 50,000, at least a portion of the hard coat layer peeled off from the polyester film.

[0106] <Hard coat coating liquid> ·Ionizing radiation curable compound 1: 0.6 parts by mass (Compound α synthesized in the following process) ·Ionizing radiation curable compound 2: 0.2 parts by mass (Daicel Corporation, product name "EBECRYL230", solid content 100%) ·Ionizing radiation curable compound 3: 0.2 parts by mass (Kyoeisha Chemical Co., Ltd., product name "Light Acrylate IAA", solids content 100%) • Leveling agent: 0.01 parts by mass (Dainichiseika Kagyo Co., Ltd., product name "10-28(TL)", solid content 10% by mass) • Photopolymerization initiator: 0.1 parts by mass (IGM Resins BV, product name "Omnirad 184") ·solvent (A 5:5 mixed solvent of methyl isobutyl ketone and cyclohexanone. The solvent is used in an amount that results in a solid content of 35% by mass in the coating solution.)

[0107] <Synthesis of compound α> Air gas was introduced into a reaction vessel equipped with a stirrer, thermometer, condenser, and nitrogen gas inlet. Next, 57 parts by mass of pentaerythritol triacrylate, 43 parts by mass of pentaerythritol 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 charged into the reaction vessel, and the temperature was raised to 60°C while stirring under a nitrogen stream. Then, 30 parts by mass of hexamethylene diisocyanate was charged into a dropping container and uniformly added to the reaction vessel over 1 hour. After the addition, the temperature of the reaction vessel was raised to 75°C and maintained at 75±3°C for 6 hours. Subsequently, 150 parts by mass of methyl ethyl ketone was added to obtain a clear resin solution. Finally, the solvent was removed using an evaporator to obtain compound α. Compound α is an ionizing radiation-curable compound.

[0108] 2. Preparation and manufacturing of polyester film [Example 1] A pellet containing the ultraviolet absorber was prepared by melting and mixing 1 kg of PET (melting point 258°C, absorption center wavelength: 320 nm) and 0.1 kg of ultraviolet absorber (2,2'-(1,4-phenylene)bis(4H-3,1-benzoxazinon-4-one) in a kneader at 280°C. This pellet and PET with a melting point of 258°C were fed into a single-screw extruder and melt-kneaded at 280°C. The mixture was extruded through a T-die and cast onto a casting drum with a surface temperature controlled to 25°C to obtain a casting film. The amount of ultraviolet absorber in the casting film was 1 part by mass per 100 parts by mass of PET. The obtained casting film was heated in a group of rolls set to 95°C, and then stretched 5.1 times in the flow direction while heating both sides of the film with a radiation heater until the film temperature reached 103°C at the 250mm point of a 400mm stretching section (starting point is stretching roll A, ending point is stretching roll B; stretching rolls A and B each have two nip rolls). After that, the film was cooled. During heating with the radiation heater, a 92°C, 4m / s airflow was blown towards the film from the opposite side of the radiation heater to create turbulence on both sides of the film, thereby disrupting the film's temperature uniformity. Next, corona discharge treatment was performed on both sides of the uniaxially oriented film in air to set the wetting tension of the base film to 55 mN / m. Then, a smooth layer coating solution containing a polyester resin with a glass transition temperature of 18°C, a polyester resin with a glass transition temperature of 82°C, and silica particles with an average particle size of 100 nm was inline coated onto the corona discharge treated surfaces on both sides of the film to form a smooth layer. Next, the uniaxially oriented film was guided into a tenter and preheated with hot air at 95°C. After that, it was stretched 4.5 times in the width direction at temperatures of 105°C for the first stage and 140°C for the second stage. Here, when the stretching section in the width direction was divided into two parts, the film was stretched in two stages so that the amount of stretching of the film at the midpoint of the stretching section in the width direction (film width at the measurement point - film width before stretching) was 80% of the amount of stretching at the end of the stretching section in the width direction. The film stretched in the width direction was then heat-treated with hot air in the tenter for 7 seconds. The temperature of the hot air was gradually increased from 180°C to 245°C. Subsequently, a 1% relaxation treatment was performed in the width direction at 245°C under the same temperature conditions, and after rapid cooling to 100°C, a 1% relaxation treatment was performed in the width direction. After that, it was wound up to obtain the biaxially oriented polyester film of Example 1 with a thickness of 40 μm.

[0109] [Example 2] Except for changing the stretching ratio in the flow direction from 5.1 times to 3.8 times, a biaxially oriented polyester film of Example 2 with a thickness of 40 μm was obtained in the same manner as in Example 1.

[0110] [Example 3] The biaxially oriented polyester film of Example 3 was obtained in the same manner as in Example 1, except that the thickness of the casting film was increased to a final thickness of 80 μm, and the stretching ratio in the flow direction was changed from 5.1 times to 3.8 times.

[0111] [Example 4] The biaxially oriented polyester film of Example 4 was obtained in the same manner as in Example 1, except that the stretching ratio in the flow direction was changed from 5.1 times to 4.7 times, and the stretching ratio in the width direction was changed from 4.5 times to 5.2 times.

[0112] [Comparative Example 1] For Comparative Example 1, a commercially available biaxially oriented polyester film (Toyobo Co., Ltd., product name: Cosmoshine A4300, thickness: 23 μm) was prepared as the polyester film.

[0113] [Comparative Example 2] For Comparative Example 2, a commercially available uniaxially oriented polyester film (Toyobo Co., Ltd., product name "Cosmoshine TA044", thickness: 80 μm) was prepared as the polyester film.

[0114] [Comparative Example 3] A biaxially oriented polyester film of Comparative Example 3 was obtained in the same manner as in Example 1, except that the stretching ratio in the flow direction was changed from 5.1 times to 3.6 times, and the stretching ratio in the width direction was changed from 4.5 times to 5.0 times.

[0115] [Comparative Example 4] The stretching ratio in the flow direction was changed from 5.1 times to 5.5 times, and the stretching ratio in the width direction was changed from 4.5 times to 4.7 times. Furthermore, during stretching in the flow direction, the airflow of 92°C, 4 m / s was not blown towards the film from the opposite side of the film of the radiation heater, in order to prevent turbulence from being generated on both sides of the film. Except for these changes, the biaxially oriented polyester film of Comparative Example 4 was obtained in the same manner as in Example 1.

[0116] [Reference example 1] A biaxially oriented polyester film of Reference Example 1 with a thickness of 40 μm was obtained in the same manner as in Example 1, except that the stretching ratio in the flow direction was changed from 5.1 times to 4.1 times.

[0117] [Reference example 2] A biaxially oriented polyester film of Reference Example 2 with a thickness of 40 μm was obtained in the same manner as in Example 1, except that the stretching ratio in the flow direction was changed from 5.1 times to 4.3 times.

[0118] [Reference example 3] Under a nitrogen atmosphere, 4.85 g of dimethyl terephthalate, 4.4 g of 1,2-propylene glycol, 6.8 g of p-toluic acid, and 10 mg of tetraisopropyl titanate were mixed and stirred at 140°C for 2 hours, followed by stirring at 210°C for 16 hours. Next, the temperature was lowered to 170°C, and unreacted 1,2-propylene glycol was removed by distillation under reduced pressure to obtain polyester pellets. As in Example 1, the amount of UV absorber in the pellets was 1 part by mass per 100 parts by mass of PET. Since the polyester pellets use twice the molar ratio of monocarboxylic acid to dicarboxylic acid, it is thought that the terminals are toluic acid esters. A biaxially oriented polyester film of Reference Example 3 with a thickness of 40 μm was manufactured in the same manner as in Example 1, except that the PET material system was changed, the stretching temperature was changed to 175°C, the stretching ratio in the flow direction was changed to 1.5 times, and the stretching ratio in the width direction was changed to 1.5 times. The film movement speed during heating and stretching was set to 20 m / min. Furthermore, stretching was performed with the temperature of the preheating zone set to 175°C and the temperature of the cooling zone set to 160°C.

[0119] [Table 1]

[0120] From the results in Table 1, the average Martens hardness was 140.0 N / mm². 2 Furthermore, the standard deviation of the Martens hardness (3σ) is 9.0 N / mm². 2 More than 36.0N / mm 2 The polyester film of the following example exhibits good pencil hardness regardless of thickness, and it can also be confirmed to have good adhesion to the functional layer. Furthermore, by satisfying condition 1, the polyester film of the example can be confirmed to suppress retaining a bend or breaking after a bending test, regardless of the bending direction. [Explanation of symbols]

[0121] 10: Polyester film 20: Display elements 30: Polarizing plate 31: Polarizer 32: First transparent protective plate 33: Second transparent protective plate 50: Cabinet 100: Image display device

Claims

1. The average Martens hardness of the surface of the polyester film is 140.0 N / mm². ２ Furthermore, the standard deviation 3σ of the Martens hardness of the surface of the polyester film is 9.0 N / mm². ２ 36.0N / mm or more ２ The following is a polyester film.

2. The polyester film according to claim 1, wherein the standard deviation 3σ of the mean of the elastic deformation power of the surface of the polyester film is 7.0% or less.

3. The polyester film according to claim 1 or 2, wherein the average elastic deformation power of the surface of the polyester film is 40.0% or more.

4. The polyester film according to any one of claims 1 to 3, wherein the in-plane phase difference of the polyester film is 300 nm or more and 1450 nm or less.

5. The polyester film according to any one of claims 1 to 4, wherein the value obtained by dividing the in-plane phase difference of the polyester film by the phase difference in the thickness direction of the polyester film is 0.15 or less.

6. The polyester film according to any one of claims 1 to 5, wherein the thickness of the polyester film is 10 μm or more and 80 μm or less.

7. A polarizing plate having a polarizer, a first transparent protective plate disposed on one side of the polarizer, and a second transparent protective plate disposed on the other side of the polarizer, wherein at least one of the first transparent protective plate and the second transparent protective plate is a polyester film according to any one of claims 1 to 6.

8. An image display device having a display element and a polyester film disposed on the light-emitting surface side of the display element, wherein the polyester film is the polyester film described in any one of claims 1 to 6.

9. The image display device according to claim 8, further comprising a polarizer between the display element and the polyester film.

Images