Polyester film and image display device using the same

Abstract

A polyester film with high in-plane retardation, excellent thickness uniformity, high productivity, good workability, and good planarity is provided. The polyester film disclosed has an in-plane retardation of 3000 nm or more and 30000 nm or less, an in-plane orientation degree of 0.128 or more and 0.155 or less, and a thickness non-uniformity in the film flow direction of less than 8% (it should be noted that the thickness non-uniformity is a value obtained by (maximum thickness - minimum thickness) / average thickness × 100 (%)).

Description

Technical Field

[0001] This invention relates to polyester films (e.g., optical polyester films). Specifically, this invention relates to polyester films suitable for use with various components of image display devices, such as polarizer protective films, substrate films (e.g., transparent electrode substrate films), anti-splash films, and screen surface protective films. Background Technology

[0002] Polyester films are used as optical films due to their excellent transparency, mechanical strength, and stability against chemicals. These optical polyester films are typically biaxially stretched films, exhibiting birefringence; therefore, when used in areas where polarized light passes through, they can produce rainbow-like color variations (iridescence).

[0003] On the other hand, the following technologies are known: technologies that eliminate blackouts and iris spots when viewing images with polarized sunglasses by applying a high-delay polyester film to the surface of a liquid crystal display device (e.g., Patent Document 1); technologies that eliminate iris spots by using it as a protective film for polarizing elements (e.g., Patent Document 2); and technologies that combine them as a touch panel substrate and an anti-splashing film (e.g., Patent Document 3).

[0004] In the case of stretching in only one direction to obtain a high-delay film, the tensile elongation in the direction orthogonal to the stretching direction is lower. Therefore, breakage may occur during film formation and processing of the resulting film, resulting in reduced productivity and workability. Alternatively, by first stretching at a low ratio in the direction orthogonal to the main stretching direction and then stretching in the main stretching direction, the tensile elongation in the direction orthogonal to the main stretching direction can be increased or the production speed can be increased, while ensuring the necessary delay. However, stretching in the direction orthogonal to the main stretching direction can sometimes result in uneven thickness and poor flatness.

[0005] The following technique is known: by stretching along the length direction (MD) by 2.0 times or less, preferably 1.3 times or less, and then stretching along the width direction (TD) by 4.15 times or more, the ratio of tensile strength in the length direction to tensile strength in the width direction is set to 0.25 to 0.6, thereby improving the tensile strength and elastic modulus in the MD direction (e.g., Patent Document 4). However, this technique still suffers from problems of poor thickness uniformity and planarity.

[0006] Furthermore, it has been proposed that a thin film obtained by stretching along the length direction (MD) by 1.0 to 3.4 times and then stretching along the width direction (TD) by 2.5 to 5.0 times be used as the folding direction in a foldable image display device (e.g., Patent Documents 5 and 6). However, in this technology, there is still room for improvement in thickness uniformity and planarity.

[0007] When the thickness is uneven in the MD direction, the following tendencies exist: it is easy to break not only during film formation, but also during cutting, especially when the cutting edge wears down and at high speed, resulting in poor productivity and workability.

[0008] Existing technical documents

[0009] Patent documents

[0010] Patent Document 1: International Publication No. 2011 / 058774

[0011] Patent Document 2: International Publication No. 2011 / 162198

[0012] Patent Document 3: International Publication No. 2014 / 123209

[0013] Patent Document 4: International Publication No. 2017 / 091031

[0014] Patent Document 5: International Publication No. 2018 / 159285

[0015] Patent Document 6: International Publication No. 2020 / 162119 Summary of the Invention

[0016] The problem the invention aims to solve

[0017] One object of the present invention is to provide a polyester film with high in-plane retardation, excellent thickness uniformity, good productivity, workability, and planarity. Another object of the present invention is to provide a polyester film that, when used in various applications as an image display device, exhibits good visibility and minimizes the appearance of iris spots, regardless of the type of image display device or the type of light source.

[0018] Solution for solving the problem

[0019] The inventors conducted in-depth research to achieve this objective, resulting in the completion of this invention.

[0020] The present invention includes the following methods.

[0021] Item 1:

[0022] A polyester film having an in-plane retardation of 3000 nm or more and 30000 nm or less.

[0023] The orientation degree is above 0.128 and below 0.155.

[0024] The thickness of the film in the flow direction is not all below 8%.

[0025] (Where, the thickness non-uniformity is calculated as (maximum thickness - minimum thickness) / average thickness × 100 (%)).

[0026] Item 2:

[0027] According to the polyester film of item 1, when performing a Fourier transform on the thickness measurement data in the film flow direction and replacing the frequency with the length period of the film, the ratio A / B of the following A to the following B is 5 or less.

[0028] A: For periods of 10cm or more, the average of the five largest amplitude values;

[0029] B: The average of the five largest amplitude values ​​for periods less than 10cm.

[0030] Item 3:

[0031] According to the polyester film of item 1 or 2, when performing a Fourier transform on the thickness measurement data in the film flow direction and replacing the frequency with the length period of the film, the ratio of Amax to B, Amax / B, is 7 or less.

[0032] Amax: The maximum amplitude when the period is 10cm or more.

[0033] Item 4:

[0034] The polyester film according to any one of items 1 to 3 has an NZ coefficient of 1.65 or higher and 3 or lower.

[0035] Item 5:

[0036] The polyester film according to any one of items 1 to 4 has a thickness of 25 μm or more and 150 μm or less.

[0037] Item 6:

[0038] The polyester film according to any one of items 1 to 5 has an elongation at break of 4% or more in the film-forming flow direction.

[0039] Item 7:

[0040] The polyester film according to any one of items 1 to 6 has a tensile strength of 50 MPa or more in the film-forming flow direction.

[0041] Item 8:

[0042] A polarizer protective film, which is formed from any one of the polyester films described in items 1 to 7.

[0043] Item 9:

[0044] A polarizing plate having a protective film for polarizing elements and a polarizing element as described in item 8 stacked thereon.

[0045] Item 10:

[0046] An image display device, wherein the polarizing plate described in item 9 is disposed on the visible side of the image display unit.

[0047] Item 11:

[0048] A transparent electrode substrate film, which is formed from any one of the polyester films described in items 1 to 7.

[0049] Item 12:

[0050] An anti-scattering film, which is formed from any one of the polyester films described in items 1 to 7.

[0051] Item 13:

[0052] A protective film for the surface of an image, which is formed from any one of the polyester films described in items 1 to 7.

[0053] Item 14:

[0054] An image display device comprising any one of the transparent electrode substrate film described in item 11, the anti-scattering film described in item 12, and the screen surface protective film described in item 13.

[0055] Item 15:

[0056] The image display device according to item 14 is a flexible image display device.

[0057] The effects of the invention

[0058] This invention provides a polyester film with high in-plane delay, uniform thickness, high productivity, excellent workability, and good planarity. Furthermore, the polyester film of this invention can suppress iris spots, and therefore is suitable for use in polarizer protective films, transparent electrode substrate films such as touch panels, anti-scattering films, and screen surface protective films for image display devices. When used in flexible image display devices, it exhibits excellent bending durability. Attached Figure Description

[0059] Figure 1 This is a graph showing the frequency analysis results of thickness unevenness in the flow direction of thin film A.

[0060] Figure 2This is a graph showing the frequency analysis results of the thickness unevenness in the film flow direction of thin film D. Detailed Implementation

[0061] Suitable examples of the polyester film of the present invention, from the viewpoints of increasing in-plane retardation and low moisture permeability or hygroscopicity, include polyethylene terephthalate (PET), polyethylene terephthalate (PTT), polyethylene terephthalate (PBT), polyethylene naphthalate (PEN), etc., with PET or PEN being preferred. These polyesters may copolymerize carboxylic acid components and / or glycol components other than the main constituent components. When the total amount of carboxylic acid components and glycol components is set to 100 mol%, the total amount of carboxylic acid components and / or glycol components other than the main constituent components is preferably 10 mol% or less, more preferably 5 mol% or less, and particularly preferably 2 mol% or less.

[0062] The intrinsic viscosity (IV) of the polyester resin constituting the polyester film of the present invention is preferably 0.45 dL / g or more and 1.5 dL / g or less.

[0063] In the case of PET, IV is preferably 0.5 dL / g or more and 1.5 dL / g or less. The lower limit of IV is more preferably 0.53 dL / g, and even more preferably 0.55 dL / g. The upper limit of IV is more preferably 1.2 dL / g, even more preferably 1 dL / g, and particularly preferably 0.8 dL / g.

[0064] In the case of PEN, the lower limit of IV is preferably 0.45 dL / g, more preferably 0.48 dL / g, even more preferably 0.5 dL / g, and particularly preferably 0.53 dL / g. The upper limit of IV is more preferably 1 dL / g, more preferably 0.8 dL / g, even more preferably 0.75 dL / g, and particularly preferably 0.7 dL / g.

[0065] By setting the above range, films with superior mechanical strength, such as impact resistance, can be formed, making film production more stable and efficient. Furthermore, films with less thickness variation can be manufactured without significantly burdening the machine.

[0066] In this invention, the lower limit of the in-plane retardation (Re) of the polyester film is preferably 3000 nm, more preferably 4000 nm, further preferably 4300 nm, particularly preferably 4500 nm, and most preferably 5000 nm. By setting it to the above lower limit or above, iris spots can be suppressed.

[0067] The upper limit of Re is preferably 30,000 nm, more preferably 15,000 nm, further preferably 12,000 nm, particularly preferably 10,000 nm, and most preferably 9,500 nm. By setting it below the above upper limit, it is not necessary to thicken the film to the required extent, thus easily addressing the thinning of image display devices and the like.

[0068] Furthermore, in order to broaden the angle at which the iris spot is suppressed when viewed from an oblique direction, Re is preferably 5500 nm or more, more preferably 6000 nm or more, further preferably 6000 nm or more, and particularly preferably 6500 nm or more. When the film is preferably a thin film, such as an anti-scattering film, a substrate film for a touch panel (e.g., a transparent electrode substrate film), a screen surface protective film, or a foldable film (e.g., a PET film), and even if the field of view is slightly narrow, Re is more preferably 7000 nm or less, further preferably 6500 nm or less, and particularly preferably 6000 nm or less.

[0069] The lower limit of the planar orientation degree (ΔP) of the polyester film of the present invention is preferably 0.128, more preferably 0.129, even more preferably 0.13, and particularly preferably more than 0.13. The upper limit of the planar orientation degree is preferably 0.155, more preferably 0.152, and even more preferably 0.15. By setting it within the above range, iris spots are suppressed and breakage is less likely to occur, thereby further improving the film formation stability.

[0070] When used as a protective film for polarizers, and where it is desirable to control the iris spot from the tilt direction more effectively, the upper limit of the planar orientation degree is more preferably 0.145, further preferably 0.14, particularly preferably 0.138, and most preferably 0.136.

[0071] In applications such as flexible image display devices where excellent bending resistance is desired, the lower limit of the face orientation degree (ΔP) is more preferably 0.135, further preferably 0.138, and particularly preferably 0.14.

[0072] The degree of orientation refers to the value obtained by (nx+ny) / 2-nz when the refractive index in the slow axis direction is set as ny, the refractive index in the fast axis (the direction orthogonal to the slow axis direction) is set as nx, and the refractive index in the thickness direction is set as nz.

[0073] The lower limit of the NZ coefficient of the polyester film of the present invention is preferably 1.65, more preferably 1.68, even more preferably 1.7, and particularly preferably more than 1.7.

[0074] The upper limit of the NZ coefficient is preferably 3, more preferably 2.7, even more preferably 2.5, and particularly preferably 2.3.

[0075] By setting the parameters within the aforementioned range, the stability of the film can be further improved by suppressing iris spots and reducing the likelihood of breakage.

[0076] When used as a protective film for polarizers, and where it is desirable to more effectively suppress iris spots from the tilt direction, the upper limit of the NZ coefficient is more preferably 1.9, further preferably 1.85, and particularly preferably 1.8.

[0077] In applications such as flexible image display devices where excellent bending resistance is desired, the lower limit of the NZ coefficient is more preferably 1.8, further preferably 1.85, and particularly preferably 1.9.

[0078] The NZ coefficient is obtained by calculating NZ = |ny-nz| / |ny-nx|.

[0079] By setting the surface orientation and NZ coefficient to the above range, it is also possible to ensure the tightness of adhesion to functional layers such as hard coatings, anti-reflective layers, and anti-glare layers, as well as the adhesion when bonding with polarizing components.

[0080] The lower limit of the thickness of the polyester film of the present invention is preferably 25 μm, and more preferably 30 μm, 40 μm, 45 μm, 50 μm, and 55 μm.

[0081] The upper limit of the thickness is preferably 150 μm, and further preferably 130 μm, 100 μm, 90 μm, and 85 μm. It should be noted that, in this specification, "preferred successively" means that the values ​​are preferred as the range gradually becomes narrower.

[0082] If the thickness is below the aforementioned upper limit, the thickness of the unstretched film also decreases when heated. Therefore, it is easy to uniformly increase the temperature along the thickness direction of the film in a short time, and it is easy to suppress thickness unevenness. In addition, it is easy to address the thinning of image display devices.

[0083] It should be noted that if it is a protective film for polarizing components, the thickness is preferably 40 to 85 μm; if it is a substrate film for anti-scattering film, touch panel, etc. (e.g., transparent electrode substrate film) or screen surface protective film for flexible image display device, the thickness is preferably 25 to 70 μm; and if it is a screen surface protective film for non-flexible image display device, the thickness is preferably 60 to 150 μm.

[0084] The thickness can be calculated as follows: For example, using a contact continuous thickness gauge, the thickness of a film of a specified size (e.g., about 50 mm wide and about 6 m long) is continuously measured at a specified speed (e.g., 1.5 m / min along the MD direction) and at a specified interval (e.g., 0.1 seconds). Based on the obtained data, a specified number of data points (e.g., 2048 consecutive points) are arbitrarily selected, and the thickness is calculated based on their average value.

[0085] The upper limit of the thickness unevenness of the polyester film of the present invention in the film-forming flow direction (hereinafter sometimes also referred to as the length direction, MD direction, and in the case of stretching, sometimes also referred to as the direction orthogonal to the main stretching direction) is preferably 8%, more preferably 7%, further preferably 6%, particularly preferably 5%, and most preferably 4%.

[0086] The thickness non-uniformity in the MD direction is preferably low. From a practical point of view, the lower limit is preferably 0.1%, and more preferably 0.5%.

[0087] It should be noted that the thickness non-uniformity is the value obtained in the following thickness measurement by (maximum thickness - minimum thickness) / average thickness × 100 (%).

[0088] In this invention, Fourier transform (e.g., high-speed Fourier transform) is performed on the data obtained by measuring the film thickness along the MD direction, and the results are analyzed using the length period of the film in the MD direction (specifically, the frequency is replaced by the length period). When A is set as the average of the top 5 values ​​with the largest amplitude and a period of 10 cm or more, and B is set as the average of the top 5 values ​​with the largest amplitude and a period of less than 10 cm, the lower limit of A / B is preferably 0.5, more preferably 1, further preferably 1.3, particularly preferably 1.5, and most preferably 1.8.

[0089] The upper limit of A / B is preferably 5, more preferably 4.5, even more preferably 4, and particularly preferably 3.5.

[0090] In the above description, when the period is 10 cm or more and the maximum amplitude is set as Amax, the lower limit of Amax / B is preferably 0.7, more preferably 1.4, further preferably 1.8, particularly preferably 2, and most preferably 2.2. The upper limit of Amax / B is 7, more preferably 6, further preferably 5, particularly preferably 4.5, and most preferably 4.

[0091] By setting A / B and / or Amax / B to the above range, higher productivity and stable production can be maintained, and breakage is less likely to occur during film making and post-processing. Even when using a liquid crystal display device with a light source having a steep peak, thin films with less noticeable color unevenness can be produced.

[0092] It should be noted that A, Amax, and B are preferably calculated using the following specific methods.

[0093] • The thickness of the central part of the film is continuously read at a speed of 1.5 m / min and at intervals of 0.1 seconds.

[0094] • Frequency analysis was performed using high-speed Fourier transform based on data obtained from 2048 consecutive points (a quantity with a length of 5.12m).

[0095] • Convert the frequency of the obtained analysis data into a length period and calculate the amplitude.

[0096] • In the data with a length period of more than 10cm, select 5 points in sequence starting from the maximum amplitude, set their average value as A, and set the maximum amplitude among these 5 points as Amax.

[0097] • In data with a period shorter than 10 cm, select five points sequentially starting from the maximum amplitude and set their average value as B. It should be noted that the latter half of the frequency analysis data, known as the "ghost" data, is ignored; only the first half of the analysis data is used.

[0098] According to the research of the inventors, without stretching in the MD direction, thickness unevenness in the MD direction is mostly caused by the movement of electrodes or vibration of the device used to electrostatically bond the molten resin to the cooling roller, or by the influence of the roundness or offset of the cooling roller during casting, or by the automatic adjustment of the die lip spacing. For example, thickness unevenness caused by electrode movement or device vibration is mostly less than a few centimeters in period. However, even with slight stretching along the MD direction, it is not possible to sufficiently suppress thickness unevenness, and thickness unevenness with periods of tens of centimeters to several meters becomes significant, causing problems caused by thickness unevenness.

[0099] Further research by the inventors revealed that when the film is slightly stretched along the MD direction, it is necessary to manage the preheating temperature and stretching temperature within an appropriate range. When the temperature deviates from the appropriate range, the following phenomena occur: uneven stretching, inconsistent stretching position, unstable stretching, film relaxation, failure of the film to peel off smoothly from the roller, significant thickness unevenness of tens of cm to several meters, and deterioration of flatness.

[0100] The preferred film-forming conditions for obtaining the polyester film of the present invention will be described below.

[0101] First, after drying the polyester resin (represented by PET), it is fed into an extruder and melted at 260–300°C. The melt is then extruded from the die as a sheet onto a cooling roller to obtain an unstretched film. At this stage, it is preferable to apply a charge to the resin by rapidly pressing it against the cooling roller, or to blow air or reduce pressure within the chamber. It is also preferable to perform the following operations: adjust the tension and fixing methods of the filament and strip electrodes to reduce electrode vibration; control the airflow and pressure reduction to ensure stability; and minimize the impact of vibrations from mechanical components such as electric motors on the casting equipment.

[0102] Next, the unstretched film is preheated and heated, and finally stretched by applying tension along the MD direction. In this case, the unstretched film is gradually heated in multiple stages. The highest temperature reached by the film surface in the heating process before the stretching process is called the preheating temperature, and the highest temperature reached by the film surface during stretching is called the stretching temperature. These will be explained separately.

[0103] To illustrate with a specific example, in MD stretching, the film is heated using multiple low-speed rollers, and stretched using the difference in circumferential speed between the low-speed rollers and the subsequent high-speed rollers. In this case, as a method for setting the film surface to the final stretching temperature, there are cases where heating is performed using an infrared heater (IR heater), etc., when heating is performed using the final roller of the low-speed rollers (hereinafter sometimes simply referred to as the "final roller").

[0104] When heating is performed using the final roll, the temperature at which the film leaves the final roll is called the stretching temperature, and the temperature at which the film leaves the preceding low-speed roll (heating roll) is called the preheating temperature. When heating is performed using an IR heater, the highest temperature in the area heated by the IR heater is called the stretching temperature, and the temperature at which the film leaves the final roll (heating roll) is called the preheating temperature.

[0105] The preheating temperature for MD stretching is preferably 60°C or higher, more preferably 65°C or higher, and even more preferably 70°C or higher. By setting the preheating temperature within the above range, the temperature difference in the thickness direction of the film can be reduced even at high film forming speeds, enabling stable stretching.

[0106] The preheating temperature for MD stretching is preferably 95°C or lower, more preferably 90°C or lower, further preferably 85°C or lower, particularly preferably 82°C or lower, and most preferably 80°C or lower. By setting the preheating temperature within the above range, adhesion between the film and the rollers can be suppressed, achieving stable film travel. Furthermore, since film relaxation between the preheating rollers can be suppressed and tension between them reduced, unnecessary film elongation during the preheating process can be reduced, suppressing thickness unevenness and reduced flatness. Additionally, when the molecular weight of the polyester is low (low IV), film relaxation is more likely to occur. For example, when IV is 0.7 dl / g or lower, 90°C or lower is preferred, and when IV is 0.65 dl / g or lower, 85°C or lower is preferred.

[0107] The stretching temperature for MD stretching is preferably 86°C or higher, more preferably 88°C or higher, even more preferably 89°C or higher, particularly preferably 90°C or higher, and most preferably 91°C or higher. At low stretching temperatures, in the SS (stress-strain) characteristics of the unstretched film, the stress may not increase slowly relative to the strain, resulting in stretching instability.

[0108] The stretching temperature for MD stretching is preferably below 110°C, and more preferably below 105°C, 102°C, 100°C, 98°C, and 96°C. By setting the stretching temperature within the above range, the film is not too soft, and relaxation during stretching can be suppressed. In particular, when tension is applied, the film tends to stretch from the latter half of 80°C, but if the stretching temperature is within the above range, stretching outside the intended position can be suppressed, and stretching can be stabilized. Furthermore, as mentioned above, the lower the molecular weight, the easier it is to relax; therefore, for example, when IV is below 0.7 dl / g, it is preferably below 100°C, and when IV is below 0.65 dl / g, it is preferably below 98°C.

[0109] In MD stretching, the difference between the stretching temperature and the preheating temperature is preferably 11°C or more, more preferably 12°C or more, and even more preferably 13°C or more. Furthermore, the difference between the stretching temperature and the preheating temperature is preferably 24°C or less, more preferably 23°C or less, and even more preferably 22°C or less.

[0110] By setting it to the above range, it is possible to suppress suspected stretching and film adhesion to the roller during preheating. Even at high film forming speeds, the temperature difference in the film thickness direction can be reduced during stretching, thus achieving stable stretching.

[0111] In MD stretching, as described above, it is necessary to rapidly raise the film temperature from the preheating temperature to the stretching temperature. However, even with excessively rapid heating, the temperature difference in the thickness direction of the film will increase, sometimes making stable stretching difficult. Therefore, when using a final roller for heating, it is preferable to increase the angle of the film relative to the final roller and prolong the contact time between the final heating roller and the film. The angle of contact is preferably 30 degrees or more, more preferably 45 degrees or more, further preferably 60 degrees or more, and particularly preferably 70 degrees or more.

[0112] Alternatively, if IR heaters are used, it is preferable to arrange multiple heaters along the MD direction, or to use heaters with a wide width in the MD direction.

[0113] The rollers used in the preheating process and the rollers used in the stretching process can be rollers with chrome plating, nickel plating, cobalt alloy plating, etc. on their surfaces. In cases where polyester resin is observed to adhere to the roller surface at high temperatures, rollers treated with fluororesin are preferred.

[0114] Improving the roundness and offset accuracy of the rollers used in the preheating process and the stretching process is also important. Roundness is preferably 30 μm or less, more preferably 20 μm or less, and even more preferably 10 μm or less, typically 0.1 μm or more. Offset is preferably 40 μm or less, more preferably 30 μm or less, and even more preferably 20 μm or less, typically 0.1 μm or more.

[0115] The diameter of the roller also depends on the size of the stretching machine. If the film to be produced is an unstretched film with a width of about 700 to 2500 mm, the diameter is preferably 100 to 500 mm, more preferably 150 to 400 mm, and even more preferably 170 to 350 mm.

[0116] The lower limit of the MD stretching ratio is preferably 1.05 times, more preferably 1.08 times, and even more preferably 1.1 times. The upper limit of the MD stretching ratio is preferably 2 times, more preferably 1.8 times, and even more preferably 1.7 times. By setting these ranges, it is possible to further produce a film with excellent workability and suppressed iris spots. In addition, when used as a protective film for polarizers, and where it is desirable to more effectively suppress iris spots from the tilt direction, the upper limit of the MD stretching ratio is more preferably 1.25 times, even more preferably 1.2 times, and particularly preferably 1.18 times. When used in flexible image display devices and where it is desirable to impart excellent bending resistance, the lower limit of the MD stretching ratio is more preferably 1.2 times, even more preferably 1.25 times, and particularly preferably 1.3 times.

[0117] In this invention, the upper limit of the thickness unevenness of the film in the direction orthogonal to the film-forming flow direction (TD direction) is preferably 5%, more preferably 4%, further preferably 3.5%, and particularly preferably 3%. The thickness unevenness in the TD direction is preferably low, and from a practical point of view, the lower limit is preferably 0.1%, and further preferably 0.5%.

[0118] The thickness in the TD direction can be achieved, for example, by controlling the die lip spacing during casting, reducing the unevenness of the film temperature in the TD direction during TD stretching, and setting the face orientation or NZ coefficient to an appropriate range.

[0119] In TD stretching, the film after MD stretching is preheated, preferably stretched at 80–130°C, more preferably at 90–120°C. The stretching ratio of TD stretching is preferably 3–6.5 times, more preferably 3.2–6.2 times, further preferably 3.5–6.0 times, and particularly preferably 3.7–5.8 times.

[0120] Preferably, heat curing is performed immediately after stretching. The heat curing temperature is preferably 150–250°C, more preferably 170–230°C. The heat curing time is preferably 3–60 seconds, more preferably 5–30 seconds.

[0121] During heat setting, relaxation treatment is preferably performed along the main stretching direction and / or in a direction orthogonal to it. The relaxation treatment is preferably 0.5% to 10%, more preferably 1% to 5%.

[0122] The lower limit of the elongation at break in the MD direction of the polyester film of the present invention is preferably 4%, and more preferably 5%, 6%, 7%, 8%, 9%, and 10% in sequence. The upper limit of the elongation at break in the MD direction is preferably 50%, more preferably 40%, more preferably 30%, particularly preferably 25%, and most preferably 20%.

[0123] The lower limit of the elongation at break in the TD direction of the polyester film of the present invention is preferably 50%, more preferably 60%. The upper limit of the elongation at break in the TD direction is preferably 200%, more preferably 150%, further preferably 120%, and particularly preferably 100%.

[0124] The lower limit of the tensile strength in the MD direction of the polyester film of the present invention is preferably 50 MPa, more preferably 55 MPa, further preferably 60 MPa, and particularly preferably 65 MPa. The upper limit of the tensile strength in the MD direction is preferably 150 MPa, more preferably 130 MPa, further preferably 120 MPa, particularly preferably 110 MPa, and most preferably 100 MPa.

[0125] The lower limit of the tensile strength in the TD direction of the polyester film of the present invention is preferably 300 MPa, more preferably 330 MPa, and even more preferably 350 MPa.

[0126] The upper limit of the fracture strength in the TD direction is preferably 500 MPa, more preferably 450 MPa, even more preferably 420 MPa, and particularly preferably 400 MPa.

[0127] By setting the elongation at break and tensile strength within the above-mentioned ranges, a film with superior workability is formed. The elongation at break and tensile strength are values ​​measured according to JIS K 7113.

[0128] Regarding the heat shrinkage rate of the polyester film of the present invention at 150°C, the lower limit is preferably -0.5% in both the MD and TD directions, more preferably -0.1%. Regarding the heat shrinkage rate at 150°C, the upper limit is preferably 3% in both the MD and TD directions, more preferably 2.7%, further preferably 2.5%, and particularly preferably 2%.

[0129] The transmittance of the polyester film of the present invention at a wavelength of 380 nm is preferably 20% or less. More preferably, the transmittance at a wavelength of 380 nm is 15% or less, even more preferably 10% or less, and particularly preferably 5% or less. If the aforementioned transmittance is 20% or less, the deterioration of iodine and dichroic pigments in the polarizing layer caused by ultraviolet light can be suppressed. It should be noted that the aforementioned transmittance is a value measured along a direction perpendicular to the plane of the film and can be measured using a spectrophotometer (e.g., Hitachi U-3500 type). In particular, when used as a protective film for polarizing elements, low ultraviolet transmittance is preferred.

[0130] Setting the transmittance of the polyester film of the present invention to 20% or less at a wavelength of 380 nm can be achieved, for example, by adding an ultraviolet absorber to the film, coating the film surface with a coating liquid containing an ultraviolet absorber, and appropriately adjusting the type, concentration, and film thickness of the ultraviolet absorber. Ultraviolet absorbers are known substances. Examples of ultraviolet absorbers include organic and inorganic types; from the viewpoint of transparency, organic ultraviolet absorbers are preferred.

[0131] Organic ultraviolet absorbers include benzotriazole, benzophenone, cyclic imino esters and their combinations, and there are no particular limitations as long as the desired absorbance range is met.

[0132] Furthermore, to improve slip properties, it is preferable to add particles with an average particle size of 0.05 to 2 μm to the polyester film of the present invention. Examples of particles include inorganic particles such as titanium dioxide, barium sulfate, calcium carbonate, calcium sulfate, silicon dioxide, alumina, talc, kaolin, clay, calcium phosphate, mica, lithium montmorillonite, zirconium oxide, tungsten oxide, lithium fluoride, and calcium fluoride; and organic polymer particles such as styrene-based, acrylic-based, melamine-based, benzoguanamine-based, and organosilicon-based particles.

[0133] These particles can be added to the entire film, or they can be formed into a skin-core co-extruded multilayer structure and added only to the skin layer. Alternatively, it is preferable that the film itself does not contain particles, but rather that the particles are added to the easily bondable layer described later.

[0134] The polyester film of the present invention can also be subjected to corona treatment, flame treatment, plasma treatment, or other treatments to improve its adhesion.

[0135] (Easy-to-bond layer)

[0136] To improve the adhesion of adhesives and coatings, the polyester film of the present invention may be provided with an easy-to-adhere layer.

[0137] The resin used in the easy-bonding layer can be polyester resin, polyurethane resin, polycarbonate resin, acrylic resin, etc., preferably polyester resin, polyester polyurethane resin, polycarbonate polyurethane resin, or acrylic resin. The easy-bonding layer is preferably cross-linked. Examples of cross-linking agents include isocyanate compounds, melamine compounds, epoxy resins, and oxazoline compounds. Additionally, adding water-soluble resins such as polyvinyl alcohol to the easy-bonding layer is also a useful method to improve its adhesion to the polarizing element.

[0138] The easy-to-adhere layer can be formed by applying a water-based coating containing these resins and, as needed, crosslinking agents, particles, etc., to a film and then drying it. Examples of particles include those used in the aforementioned substrate.

[0139] The easy-to-adhesive layer can be applied to the film (e.g., a stretched film) offline, but is preferably applied online during the film-forming process. When applied online, it can be applied either before longitudinal stretching (MD stretching) or transverse stretching (TD stretching), preferably just before transverse stretching, and then dried and crosslinked during preheating, heating, and heat treatment processes based on a tenter frame. It should be noted that when online coating is performed just before longitudinal stretching using rollers, it is preferable to dry the coating using a horizontal dryer before guiding it to the stretching rollers.

[0140] The coating amount of the easy-to-adhere layer (the coating amount after drying) is preferably 0.01–1.0 g / m². 2More preferably, it is 0.03–0.5 g / m 2 .

[0141] (Functional layer)

[0142] The polyester film of the present invention is preferably provided with functional layers such as a hard coating layer, an anti-reflection layer, a low-reflection layer, an anti-glare layer, and an antistatic layer. The anti-reflection layer, low-reflection layer, and anti-glare layer are collectively referred to as reflection reduction layers. The reflection reduction layer also has the following functions: it not only prevents external light from reflecting onto the display screen and making it difficult to observe, but also suppresses interface reflection, thereby reducing or making the iris less noticeable. Furthermore, in the case of a substrate film (e.g., a transparent electrode substrate film) used as a touch panel, a refractive index adjustment layer is preferably provided to make the transparent electrode layer less noticeable. It should be noted that in polyester films with functional layers, the film in its state before the functional layers are provided is referred to as the substrate film. It should also be noted that the substrate film sometimes includes the aforementioned easy-to-adhere layer.

[0143] The upper limit of the reflectivity of the polyester film measured from the reflectance reduction layer side is preferably 5%, more preferably 4%, further preferably 3%, particularly preferably 2%, and most preferably 1.5%. If it is below the above upper limit, the reflection of external light can be reduced, and the visibility of the image can be improved. The lower limit of reflectivity is not particularly limited, but from a practical point of view, it is preferably 0.01%, and more preferably 0.1%.

[0144] As a reflection reduction layer, there are various types such as low-reflection layer, anti-reflection layer, and anti-glare layer.

[0145] (Low-reflective layer)

[0146] A low-reflection layer is a layer that functions as follows: by setting a low-refractive-index layer on the surface of a substrate film, the difference between its refractive index and that of air is reduced, thereby lowering the reflectivity.

[0147] (Anti-reflective layer)

[0148] The anti-reflective layer controls reflection by interfering with the reflected light at the interface through controlling the thickness of the low-refractive-index layer. The thickness of the low-refractive-index layer is preferably approximately the wavelength of visible light (400–700 nm) / (refractive index of the low-refractive-index layer × 4).

[0149] Placing a high-refractive-index layer between the anti-reflective layer and the substrate film is also a preferred approach. Two or more layers of low-refractive-index and / or high-refractive-index layers can be applied, using multiple interferences to further enhance the anti-reflective effect. Sometimes, the high-refractive-index layer and the low-refractive-index layer are collectively referred to as the anti-reflective layer.

[0150] In the case of an anti-reflective layer, the upper limit of reflectivity is preferably 2%, more preferably 1.5%, further preferably 1.2%, and particularly preferably 1%.

[0151] (Low-refractive-index layer)

[0152] The refractive index of the low-refractive-index layer is preferably 1.45 or less, more preferably 1.42 or less. Furthermore, the refractive index of the low-refractive-index layer is preferably 1.2 or more, more preferably 1.25 or more.

[0153] It should be noted that the refractive index of the low-refractive-index layer was measured using a wavelength of 589 nm.

[0154] The thickness of the low-refractive-index layer is not limited and can usually be appropriately set within the range of about 30 nm to 1 μm. However, when used as an anti-reflective layer, the thickness of the low-refractive-index layer is preferably 70 to 120 nm, and more preferably 75 to 110 nm.

[0155] Preferably, the following are examples of low refractive index layers: (1) a layer formed from a resin composition containing a binder resin and low refractive index particles; (2) a layer formed from a fluorinated resin that is a low refractive index resin; (3) a layer formed from a fluorinated resin composition containing silicon dioxide or magnesium fluoride; and (4) a thin film of a low refractive index material such as silicon dioxide or magnesium fluoride.

[0156] The binder resin contained in the resin composition of (1) can be polyester, polyurethane, polyamide, polycarbonate, acrylic, etc., without particular limitation. Among them, acrylic is preferred, and resin obtained by polymerizing (crosslinking) a photopolymerizable compound by light irradiation is even more preferred.

[0157] Examples of photopolymerizable compounds include photopolymerizable monomers, photopolymerizable oligomers, and photopolymerizable polymers, which can be appropriately modified and used. Preferably, a combination of a photopolymerizable monomer and a photopolymerizable oligomer or photopolymerizable polymer is used. These photopolymerizable monomers, photopolymerizable oligomers, and photopolymerizable polymers are preferably multifunctional substances.

[0158] Examples of multifunctional monomers include pentaerythritol triacrylate (PETA), dipentaerythritol hexaacrylate (DPHA), pentaerythritol tetraacrylate (PETTA), and dipentaerythritol pentaacrylate (DPPA). It should be noted that monofunctional monomers can be used in combination to adjust coating viscosity and hardness.

[0159] Examples of multifunctional oligomers include polyester (meth)acrylates, (meth)acrylate urethanes, polyester-(meth)acrylate urethanes, polyether (meth)acrylates, polyol (meth)acrylates, melamine (meth)acrylates, isocyanurate (meth)acrylates, and epoxy (meth)acrylates.

[0160] Examples of multifunctional polymers include (meth)acrylate urethane, isocyanurate (meth)acrylate, polyester-(meth)acrylate urethane, and epoxy (meth)acrylate.

[0161] In addition to the above-mentioned components, the resin composition of (1) may also contain polymerization initiators, crosslinking catalysts, polymerization inhibitors, antioxidants, ultraviolet absorbers, leveling agents, surfactants, etc.

[0162] Examples of low-refractive-index particles included in the resin composition of (1) include silica particles (e.g., hollow silica particles) and magnesium fluoride particles, among which hollow silica particles are preferred. Such hollow silica particles can be manufactured by, for example, the manufacturing method described in the examples of Japanese Patent Application Publication No. 2005-099778.

[0163] The average particle size of the primary particles of the low refractive index particles is preferably 5-200 nm, more preferably 5-100 nm, and even more preferably 10-80 nm.

[0164] The low refractive index particles are more preferably surface-treated with a silane coupling agent, wherein the surface treatment is preferably performed with a silane coupling agent having a (meth)acryloyl group.

[0165] The content of low-refractive-index particles in the low-refractive-index layer is preferably 10 to 400 parts by weight relative to 100 parts by weight of the binder resin, more preferably 10 to 250 parts by weight, further preferably 50 to 200 parts by weight, particularly preferably 80 to 180 parts by weight, and most preferably 100 to 180 parts by weight.

[0166] As the fluorinated resin in (2), a polymeric compound or its polymer containing at least fluorine atoms in its molecule can be used. There are no particular limitations on the polymeric compound, but curing reactive groups such as photopolymerizable functional groups and thermosetting polar groups are preferred. Alternatively, a compound having multiple curing reactive groups can be used. The polymer, in contrast to the polymeric compound, does not possess the aforementioned curing reactive groups.

[0167] As compounds with photopolymerizable functional groups, fluorinated monomers, such as those with olefinic unsaturated bonds, can be widely used.

[0168] To improve fingerprint resistance, it is also preferable to add appropriate amounts of known polysiloxane-based or fluorine-based antifouling agents to the low refractive index layer.

[0169] To achieve anti-glare properties, the surface of the low-refractive-index layer can be uneven, but is preferably smooth.

[0170] When the surface of the low-refractive-index layer is smooth, the arithmetic mean roughness SRa (JIS B0601:1994) of the surface of the low-refractive-index layer is preferably 20 nm or less, more preferably 15 nm or less, even more preferably 10 nm or less, and particularly preferably 1 to 8 nm. Furthermore, the ten-point mean roughness Rz (JIS B0601:1994) of the surface of the low-refractive-index layer is preferably 160 nm or less, more preferably 50 to 155 nm.

[0171] (High refractive index layer)

[0172] The refractive index of the high refractive index layer is preferably set to 1.55 to 1.85, and more preferably to 1.56 to 1.7.

[0173] It should be noted that the refractive index of the high refractive index layer was measured using a wavelength of 589 nm.

[0174] The thickness of the high refractive index layer is preferably 30–200 nm, more preferably 50–180 nm. The high refractive index layer can consist of multiple layers, preferably two or fewer, and more preferably a single layer. In the case of multiple layers, the total thickness of the multiple layers is preferably within the above-mentioned range.

[0175] When there are two high refractive index layers, it is preferable to further increase the refractive index of the high refractive index layer on the low refractive index layer side. Specifically, the refractive index of the high refractive index layer on the low refractive index layer side is preferably 1.6 to 1.85, and the refractive index of the other high refractive index layer is preferably 1.55 to 1.7.

[0176] The high refractive index layer is preferably formed from a resin composition comprising high refractive index particles and resin.

[0177] Preferred high-refractive-index particles include antimony pentoxide particles, zinc oxide particles, titanium oxide particles, cerium oxide particles, tin-doped indium oxide particles, antimony-doped tin oxide particles, yttrium oxide particles, and zirconium oxide particles. Among these, titanium oxide particles and zirconium oxide particles are particularly suitable.

[0178] Two or more high-refractive-index particles can be used in combination. In particular, to prevent aggregation, it is preferable to add a first high-refractive-index particle and a second high-refractive-index particle with a lower surface charge.

[0179] In addition to fluorinated resins, other resins used in high-refractive-index layers can also be listed as resins that are the same as those listed in low-refractive-index layers.

[0180] To ensure the flatness of the low-refractive-index layer disposed on the high-refractive-index layer, it is preferable that the surface of the high-refractive-index layer is also flat. As a method for flattening the surface of the high-refractive-index layer, the same method used to flatten the aforementioned low-refractive-index layer can be employed.

[0181] The average particle size of the high-refractive-index particles and the primary particles of the high-refractive-index particles is preferably 5-200 nm, more preferably 5-100 nm, and even more preferably 10-80 nm.

[0182] These particles are more preferably surface-treated, more preferably surface-treated with a silane coupling agent, wherein, preferably, surface-treated with a silane coupling agent having a (meth)acryloyl group.

[0183] The content of high refractive index particles in the high refractive index layer is preferably 10 to 400 parts by weight relative to 100 parts by weight of the binder resin, more preferably 10 to 250 parts by weight, further preferably 50 to 200 parts by weight, particularly preferably 80 to 180 parts by weight, and most preferably 100 to 180 parts by weight.

[0184] High-refractive-index layers and low-refractive-index layers can be formed, for example, by coating a resin composition containing photopolymerizable compounds onto a substrate film and drying it, and then irradiating the coated resin composition with light such as ultraviolet light to polymerize (crosslink) the photopolymerizable compounds.

[0185] In the resin composition containing the high-refractive-index layer and the low-refractive-index layer, thermoplastic resins, thermosetting resins, solvents, polymerization initiators, and combinations thereof may be added as needed. Furthermore, dispersants, surfactants, antistatic agents, silane coupling agents, thickeners, anti-coloring agents, colorants (pigments, dyes), defoamers, leveling agents, flame retardants, UV absorbers, adhesives, polymerization inhibitors, antioxidants, surface modifiers, lubricants, and combinations thereof may be added.

[0186] (Anti-glare layer)

[0187] An anti-glare layer is a layer that reduces glare or reflects light by creating uneven surfaces to diffuse the light.

[0188] The arithmetic mean roughness (SRa) of the surface unevenness of the anti-glare layer is preferably 0.02 to 0.25 μm, more preferably 0.02 to 0.15 μm, and even more preferably 0.02 to 0.12 μm.

[0189] The surface roughness (Rzjis) of the anti-glare layer at ten points is preferably 0.15 to 2 μm, more preferably 0.2 to 1.2 μm, and even more preferably 0.3 to 0.8 μm.

[0190] SRa and Rzjis are calculated according to JIS B0601-1994 or JIS B0601-2001, based on the roughness curves measured using a contact roughness tester.

[0191] As a method for providing an anti-glare layer on a substrate film, the following methods can be cited as examples.

[0192] • Coatings for applying anti-glare layers containing particles (fillers), etc.

[0193] • The resin used for the anti-glare layer cures when in contact with a mold having an uneven structure.

[0194] • Apply the anti-glare layer with resin to a mold with an uneven structure, and then transfer it to a substrate film.

[0195] • Coatings that undergo spinosporosis during drying and film formation.

[0196] The minimum thickness of the anti-glare layer is preferably 0.1 μm, more preferably 0.5 μm. The maximum thickness of the anti-glare layer is preferably 100 μm, more preferably 50 μm, and even more preferably 20 μm.

[0197] The refractive index of the anti-glare layer is preferably 1.2 to 1.8, and more preferably 1.4 to 1.7.

[0198] It should be noted that the refractive index of the anti-glare layer is a value measured using a wavelength of 589nm.

[0199] A low-refractive-index layer can be made into an anti-glare, low-reflection layer by creating an uneven surface, or a low-refractive-index layer can be placed on the surface of a hard coating or a high-refractive-index layer to give it anti-reflection function, thereby creating an anti-glare, anti-reflection layer.

[0200] (Hard coating)

[0201] As the lower layer of the aforementioned reflection reduction layer, providing a hard coating is also a preferred configuration.

[0202] The hard coating is preferably H or higher on a pencil hardness tester, more preferably 2H or higher. The hard coating can be applied, for example, by coating a composition (solution) containing a thermosetting resin or a radiation-curing resin and allowing it to cure.

[0203] Examples of thermosetting resins include acrylic resins, urethane resins, phenolic resins, urea-melamine resins, epoxy resins, unsaturated polyester resins, silicone resins, and combinations thereof. In thermosetting resin compositions, a curing agent is added to these curable resins as needed.

[0204] The radiation-curable resin is preferably a compound having radiation-curable functional groups (radiation-curable compounds). Examples of radiation-curable functional groups include (meth)acryloyl, vinyl, allyl, and other olefinically unsaturated groups, epoxy groups, and oxetyl groups. Among these, compounds having olefinically unsaturated groups are preferred as ionizing radiation-curable compounds; more preferably, compounds having two or more olefinically unsaturated groups are preferred; and even more preferably, polyfunctional (meth)acrylate compounds having two or more olefinically unsaturated groups are preferred. Polyfunctional (meth)acrylate compounds can be monomers, oligomers, or polymers.

[0205] As specific examples, substances listed above as adhesive resins can be used.

[0206] To achieve the desired hardness for a hard coating, in compounds having radiation-curable functional groups, monomers with two or more functionalities are preferably 50% by mass or more, more preferably 70% by mass or more. Furthermore, in compounds having radiation-curable functional groups, monomers with three or more functionalities are preferably 50% by mass or more, more preferably 70% by mass or more.

[0207] The compounds with radiation-curable functional groups mentioned above can be used alone or in combination of two or more.

[0208] The thickness of the hard coating is preferably in the range of 0.1 to 100 μm, more preferably in the range of 0.8 to 20 μm.

[0209] The refractive index of the hard coating is more preferably 1.45 to 1.7, and even more preferably 1.5 to 1.6.

[0210] It should be noted that the refractive index of the hard coating was measured using a wavelength of 589 nm.

[0211] To adjust the refractive index of the hard coating, methods such as adjusting it to the resin refractive index and adjusting the particle refractive index when adding particles can be listed.

[0212] As examples of particles, particles that serve as anti-glare layers can be cited.

[0213] It should be noted that this invention also includes a hard coating layer, sometimes referred to as a reflection reduction layer.

[0214] When a functional layer is provided, an easy-adhesion layer can be provided between the functional layer and the substrate film. Suitable materials for the easy-adhesion layer include resins and crosslinking agents listed above. Furthermore, the easy-adhesion layer can be provided on both sides of the substrate film; in this case, the easy-adhesion layers on both sides can have the same composition or different compositions.

[0215] (Polarizing plate)

[0216] The polyester film of the present invention can be suitably used as a protective film for polarizing elements. The polyester film of the present invention can be laminated with polarizing elements to form a polarizing plate.

[0217] (Polarized parts)

[0218] As a polarizing element, there are no particular limitations on the following types of polarizing elements: polarizing elements obtained by adsorbing iodine or organic dichroic pigments onto uniaxially stretched polyvinyl alcohol (PVA); liquid crystal polarizing elements containing liquid crystal compounds and substances formed by orienting organic dichroic pigments or liquid crystal dichroic pigments; and polarizing elements of the wire grid type.

[0219] A thin-film polarizing element can be bonded to a rolled-up protective film for polarizing elements using PVA-based or UV-curable adhesives or binders, and then rolled into a roll. The thin-film polarizing element is obtained by adsorbing iodine or an organic dichroic pigment onto uniaxially stretched polyvinyl alcohol (PVA). The thickness of this type of polarizing element is preferably 5–30 μm, more preferably 8–25 μm, and particularly preferably 10–20 μm. The thickness of the adhesive or binder is preferably 1–10 μm, more preferably 2–5 μm.

[0220] Alternatively, polarizing elements that have PVA coated on unstretched substrates such as PET and polypropylene, and which have been uniaxially stretched together with the substrate to adsorb iodine or organic dichroic pigments, are also preferred. When using this polarizing element, the polarizing element surface (the surface without the substrate) of the polarizing element laminated to the substrate is bonded to the polarizing element protective film using an adhesive or binder. Then, the substrate used in manufacturing the polarizing element is peeled off, thereby bonding the polarizing element protective film to the polarizing element. In this case, it is also preferable to bond and wind the polarizing element in roll form. The thickness of this type of polarizing element is preferably 1–10 μm, more preferably 2–8 μm, and particularly preferably 3–6 μm. The thickness of the adhesive or binder is preferably 1–10 μm, more preferably 2–5 μm.

[0221] In the case of liquid crystal polarizers, a polarizing plate can be manufactured by laminating a polarizer containing a liquid crystal compound and an organic dichroic pigment onto a protective film of the polarizer, thereby aligning the polarizer, or by coating a protective film of the polarizer with a coating liquid containing a liquid crystal dichroic pigment, drying it, and then photocuring or heat-curing it, and then laminating the polarizer. Methods for aligning the liquid crystal polarizer include: brushing the surface of the object to be coated, and irradiating it with polarized ultraviolet light to simultaneously align and cure the liquid crystal polarizer. The surface of the protective film of the polarizer can be directly brushed and coated with a coating liquid, or the protective film of the polarizer can be directly coated with a coating liquid and then irradiated with polarized ultraviolet light. Furthermore, it is also preferable to provide an alignment layer on the protective film of the polarizer before setting the liquid crystal polarizer (i.e., to laminate the liquid crystal polarizer on the protective film with an alignment layer in between). Methods for providing an alignment layer include:

[0222] • A method for forming an orientation layer (brushing orientation layer) by coating polyvinyl alcohol and its derivatives, polyimide and its derivatives, acrylic resins, polysiloxane derivatives, etc., with their surfaces brushed.

[0223] • A method for preparing an orientation layer (photo-orientation layer) by coating a coating liquid containing a solvent and a polymer or monomer having photoreactive groups such as cinnamyl and chalcone groups, and then irradiating it with polarized ultraviolet light to make it oriented and cured.

[0224] A liquid crystal polarizing element is provided on a release film according to the above method. The liquid crystal polarizing element surface is then bonded to the polarizing film using an adhesive or bonding agent. Afterward, the release film is peeled off. In this way, the polarizing film and the polarizing element can also be bonded together.

[0225] The thickness of the liquid crystal polarizing element is preferably 0.1–7 μm, more preferably 0.3–5 μm, and particularly preferably 0.5–3 μm. The thickness of the adhesive or binder is preferably 1–10 μm, more preferably 2–5 μm.

[0226] (Layering of polarizing element and protective film for polarizing element)

[0227] The polyester film of the present invention is preferably laminated on the surface of the polarizer opposite to the image display unit side. When the polarizer and the film are laminated to form a polarizing plate, the angle between the absorption axis of the polarizer and the slow axis of the film is preferably about 90 degrees or about 0 degrees. In this specification, "about" means an error of 7 degrees or less. The error is preferably 5 degrees or less, more preferably 3 degrees or less, particularly preferably 2 degrees or less, and most preferably 1.5 degrees or less. Preferably, the above angle is achieved throughout the entire range of the polarizing plate.

[0228] It should be noted that if the polarizing element is a liquid crystal polarizing element or a wire grid type, the absorption axis of the polarizing element is also prone to tilting relative to the slow axis of the polyester film. The angle between the slow axis and the absorption axis can be 30 to 60 degrees, preferably about 45 degrees.

[0229] (The side of the image display unit of the polarizing element)

[0230] When a polarizing plate is used in a liquid crystal display device, the liquid crystal cell side of the polarizing element can be either unlaminated or made of adhesive. A curing layer can be provided on the polarizing element, or a polarizing element protective film different from the aforementioned polarizing element protective film can be provided. As a preferred curing layer, the aforementioned hard coating layer can be cited as an example. In the case of an adhesive, a release film can be further laminated. Furthermore, in the cases of unlaminated, cured, or polarizing element protective films, a peelable protective film can be separately laminated.

[0231] The polarizer protective film on the liquid crystal cell side of the polarizer can be made of cellulose-based (TAC) film, acrylic film, polycyclic olefin (COP) film, etc. The retardation of the polarizer protective film on the liquid crystal cell side can be almost zero, or it can be a phase retardation film called an optical compensation film, used to control the color tone change when viewing the displayed image from an oblique direction.

[0232] To achieve the desired phase difference using an optical compensation film, methods include: stretching the film; coating a phase difference layer such as a liquid crystal compound onto the film; and separately depositing a phase difference layer such as a liquid crystal compound onto a release film and then transferring it. The liquid crystal compound used to form the phase difference layer can be rod-shaped, disk-shaped, or the like, depending on the desired phase difference characteristics. To maintain a fixed orientation, the liquid crystal compound preferably has photocurable reactive groups such as double bonds. To align the liquid crystal compound and thus impart a phase difference, for example, by setting an alignment layer as the lower layer of the phase difference layer and brushing the alignment layer, or by irradiating it with polarized ultraviolet light, the alignment of the liquid crystal compound coated thereon can be controlled, allowing it to be aligned in a specific direction.

[0233] The phase difference of the optical compensation film can be appropriately set by the type of liquid crystal cell used and the desired field of view.

[0234] A phase retardation layer can be formed by coating a phase retardation layer composition. This composition can contain solvents, polymerization initiators, sensitizers, polymerization inhibitors, leveling agents, polymerizable non-liquid crystal compounds, crosslinking agents, or combinations thereof. These can be used in the orientation control layer and liquid crystal polarizing elements as described above.

[0235] A phase retardation layer is formed by applying a composite coating to the release surface of a release film or an orientation control layer, followed by drying, heating, and curing.

[0236] Regarding their conditions, the conditions described in the sections on the orientation control layer and the polarizing element of liquid crystal will also be used as preferred conditions.

[0237] When bonding polarizing elements to polarizing element protective films and phase retardation films, adhesives or bonding agents are used. Water-based adhesives such as polyvinyl alcohol-based adhesives or photocurable adhesives are preferred. Examples of photocurable adhesives include acrylic and epoxy-based adhesives. Acrylic adhesives are preferred.

[0238] (Liquid Crystal Unit)

[0239] The liquid crystal cell is obtained by sealing a liquid crystal compound between thin substrates such as glass where circuits are formed. When the substrate is glass, the thickness is preferably 1 mm or less, and from the viewpoint of thinning, the thickness is more preferably 0.7 mm or less, even more preferably 0.5 mm or less, and particularly preferably 0.4 mm or less.

[0240] There is no particular limitation on the type of liquid crystal cell. The VA and IPS types are those with less color shift when viewed from an oblique direction. In these types, the absorption axis of the polarizing plate is parallel or orthogonal to the long side of the liquid crystal cell. Therefore, they are the preferred types for applying the present invention.

[0241] As a color filter incorporated into the liquid crystal cell, the maximum and minimum transmittance of the blue pixel in the wavelength range of 420nm to 460nm are preferably both 80% or more, and more preferably 85% or more. The difference between the maximum and minimum transmittance in the wavelength range of 420nm to 460nm is preferably 4% or less, and more preferably 3% or less.

[0242] (LCD panel)

[0243] A liquid crystal display panel is preferably manufactured by bonding polarizing plates to both the viewable side and the light source side of the liquid crystal cell. Bonding is preferably performed using an adhesive. An acrylic adhesive is preferably used.

[0244] In a liquid crystal panel, the polarizing plate obtained using the aforementioned polyester film can be either a polarizing plate on the light source side or a polarizing plate on the viewing side, or even both types of polarizing plates.

[0245] When the image display device is an organic or inorganic electroluminescent unit, micro LED, etc., the polarizing plate is preferably a circular polarizing plate. Typically, a circular polarizing plate has a quarter-wavelength layer stacked on the visible side of the polarizing element. The quarter-wavelength layer includes not only objects with only one quarter-wavelength layer, but also objects combining a quarter-wavelength layer and a half-wavelength layer, and objects obtained by applying a phase retardation layer such as a C-plate to them. The quarter-wavelength layer, half-wavelength layer, C-plate, and other phase retardation layers can be thin films or coatings. If the phase difference and orientation of these phase retardation layers are appropriate, they can be objects described in the phase retardation layers of the polarizing plate.

[0246] (Transparent electrode substrate film)

[0247] The polyester film of the present invention can be suitably used as a transparent electrode substrate film for touch panels and the like. The transparent conductive layer can be disposed on at least one side of the polyester film, or on both sides.

[0248] Examples of transparent conductive layers include conductive paste mesh printing, coatings containing carbon nanotubes, self-assembled nano-silver coatings, coatings containing needle-like conductive fillers, and metal oxide films. Among these, metal oxide films are preferred, and as preferred examples, films made of indium oxide, zinc oxide, tin oxide, indium tin oxide (ITO), antimony tin oxide, aluminum zinc oxide, and zinc indium oxide are also included.

[0249] The transparent conductive layer is preferably formed into a patterned shape such as lines or grids by etching.

[0250] The thickness of the transparent conductive layer is preferably 5–500 nm, more preferably 15–250 nm, and even more preferably 20–100 nm. This thickness ensures conductivity and suppresses coloration caused by the conductive layer.

[0251] Transparent conductive layers can be formed using known methods such as vacuum evaporation, sputtering, CVD, ion plating, spraying, and sol-gel methods.

[0252] The transparent conductive layer can be formed by etching after the film is formed, by photolithography to form a resist mask with a specified pattern.

[0253] The transparent conductive layer can be amorphous. Preferably, the amorphous transparent conductive layer is heated at 130-180°C for 0.5-2 hours to grow crystals, thus forming a crystalline transparent conductive layer and improving conductivity.

[0254] As the lower layer of the transparent conductive layer, it is also a preferred configuration to provide a hard coating layer and a refractive index adjustment layer. The refractive index adjustment layer can be a layer with a refractive index close to that of the transparent conductive layer (a high refractive index layer), or a high refractive index layer and a low refractive index layer can be provided sequentially. It is particularly preferred to provide a high refractive index layer and a low refractive index layer sequentially.

[0255] The polyester film of the present invention is preferably used as an anti-scattering film. When glass is used as a substrate for touch panels, display overlays, etc., the anti-scattering film is adhered to the glass to prevent fragments from damaging the internal structure or scattering or protruding outwards in the event of glass breakage. The anti-scattering film can be laminated on either the visible side or the reverse visible side of the glass. When laminated on the glass, it is preferable to use an optical substrate-free adhesive called OCA for adhesion.

[0256] The polyester film of the present invention is preferably used as a screen surface protective film. The screen surface protective film is laminated on the visible side of the screen of the image display device, protecting the internal image display unit from external impacts or preventing surface damage. The screen surface protective film is preferably adhered to the image display section using an adhesive. The screen surface protective film is also preferably of the type located on the outermost surface of the image display section and can be peeled off and replaced in case of damage. In this case, the adhesive is preferably of a strength sufficient to allow for manual peeling.

[0257] The polyester film of the present invention is also preferably used in flexible image display devices, and can be used in the form of a polarizer protective film, a back cover film, a transparent electrode substrate film, an image surface protective film, etc. of flexible image display devices. Among these, it is preferred to use it as a back cover film or an image surface protective film.

[0258] As a flexible image display device, the image display section can be folded in a V-shape, a left-right split shape, a W-shape, or rolled up.

[0259] In the case of a flexible image display device, it is preferable to configure the polyester film so that the slow axis is orthogonal to the folding direction; in other words, the crease is configured as the slow axis.

[0260] When used as a transparent electrode substrate film, anti-splash film, or screen surface protective film, which is located on the visible side of the polarizer of an image display device, the polyester film is configured such that its slow axis is at an angle of 30 to 60 degrees relative to the absorption axis of the polarizer, preferably about 45 degrees. This configuration is preferred because it does not cause shadows or iris spots when viewed with sunglasses.

[0261] Example

[0262] The present invention will now be described in more detail with reference to embodiments. However, the present invention is not limited to the embodiments described below, and may be implemented by appropriate modifications within the scope that conforms to the spirit of the present invention, all of which fall within the technical scope of the present invention. It should be noted that the evaluation methods for physical properties, etc., in the following embodiments are as follows.

[0263] (1) Refractive index of polyester film

[0264] Using a molecular orientation meter (Oji Instruments, MOA-6004), the slow axis direction of the thin film was determined. A 4cm × 2cm rectangle was cut with the slow axis direction parallel to the long side as the sample for measurement. For this sample, the refractive indices of the orthogonal biaxial axes (refractive index along the slow axis: ny, refractive index along the fast axis (orthogonal to the slow axis): nx) and the refractive index along the thickness direction (nz) were determined using an Abbe refractometer (ATAGO, NAR-4T, measurement wavelength 589nm).

[0265] (2) In-plane delay (Re)

[0266] In-plane retardation is a parameter defined by the product of the anisotropy of the refractive index of the orthogonal biaxial refractive index on the thin film (ΔNxy = |nx - ny|) and the film thickness d (nm) (ΔNxy × d). It is a scale representing optical isotropy and anisotropy. Using the method described in (1) above, the absolute value of the aforementioned biaxial refractive index difference (|nx - ny|) is calculated as the biaxial refractive index anisotropy (ΔNxy). The in-plane retardation (Re) is obtained by multiplying the refractive index anisotropy (ΔNxy) and the film thickness d (nm) (ΔNxy × d).

[0267] (3) Delay in the thickness direction (Rth)

[0268] Thickness retardation is a parameter representing the average retardation obtained by multiplying the two birefringences ΔNxz (=|nx-nz|) and ΔNyz (=|ny-nz|) by the film thickness d when viewed from the cross section along the film thickness direction. nx, ny, and nz are obtained using the method described in (1) above, and the average of (ΔNxz×d) and (ΔNyz×d) is calculated to determine the thickness retardation (Rth).

[0269] (4) Planar orientation

[0270] Substitute nx, ny, and nz into the equation shown in |nx+ny| / 2-nz to calculate the degree of orientation of the surface.

[0271] (5) NZ coefficient

[0272] Substitute nx, ny, and nz into the equation shown in |ny-nz| / |ny-nx| to obtain the NZ coefficient.

[0273] (6) Absorption axis of polarizer

[0274] A polarizing filter with a known absorption axis is aligned with a polarizing element and placed on a surface light source. The direction that forms a 90-degree angle with the absorption axis of the polarizing filter, which causes the polarizing filter to rotate and become darkest, is taken as the absorption axis direction of the polarizing element. It should be noted that in the case of a long strip-shaped polarizing element made of PVA stretched along its length, the length direction becomes the absorption axis direction; therefore, the length direction can be used as the absorption axis direction.

[0275] (7) Slow axis direction of the thin film

[0276] The determination was performed using a molecular orientation meter (manufactured by Oji Instruments Co., Ltd., MOA-6004 type molecular orientation meter).

[0277] (8) Transmittance at a wavelength of 380nm

[0278] Using a spectrophotometer (Hitachi, U-3500 model), the transmittance in the wavelength range of 300–500 nm was measured with the air layer as a standard, and the transmittance at wavelength 380 nm was calculated.

[0279] (9) Intrinsic viscosity

[0280] Dissolve 0.2 g of the sample in 50 ml of a mixed solvent of phenol / 1,1,2,2-tetrachloroethane (60 / 40 (weight ratio)) and measure the viscosity using an Ostwald viscometer at 30 °C.

[0281] (10) Thin film thickness and frequency characteristics after Fourier transform

[0282] Using a contact-type continuous thickness gauge based on MIKURON (the thickness gauge part was manufactured by Anritsu Electric Co., Ltd.), a sample with a width of about 50 mm and a length of about 6 m in the MD direction was cut from the center of the width direction of the obtained film. The thickness was measured along the MD direction at a speed of 1.5 m / min, and the data was read continuously at an interval of 0.1 seconds.

[0283] From the obtained data, 2048 points (each with a length of 5.12 m) were randomly selected consecutively, and the average thickness of these points was taken as the film thickness. In the measured data, the value calculated as (maximum thickness - minimum thickness) / average thickness × 100 was taken as the thickness non-uniformity (%) in the MD direction.

[0284] Furthermore, based on the selected 2048 data points, frequency analysis was performed using Microsoft Excel (registered trademark) with high-speed Fourier transform. The frequencies of the analyzed data were then converted into length periods, and their respective amplitudes were calculated.

[0285] Five points with large amplitudes are selected from the values ​​with a period greater than 10 cm, and their average is taken as A. The maximum amplitude among these five points is taken as Amax. Then, five points with large amplitudes are selected from the values ​​with a period less than 10 cm, and their average is taken as B. The second half of the frequency analysis data for the phantom is ignored, and only the first half of the analysis data is used.

[0286] Calculate A / B and Amax / B based on the obtained values ​​of A, Amax, and B. The uneven thickness in the TD direction is calculated as follows: Cut a 1000mm wide section from the central part of the film in the TD direction after film preparation, and cut out a sample with a TD direction of 1000mm × width of 50mm. Similarly, measure the thickness using a continuous thickness gauge. Calculate the thickness using the obtained data in the form of (maximum thickness - minimum thickness) / average thickness × 100.

[0287] It should be noted that a length period of 10cm or more corresponds to a frequency of 0.25Hz or less.

[0288] (11) Fracture strength and elongation at break

[0289] Based on JIS K 7113. Samples with a width of 10 mm and a length of 100 mm in both the longitudinal and width directions were cut using a razor. After being placed in an atmosphere of 23°C and 65% RH for 12 hours, measurements were performed under the following conditions: 23°C, 65% RH, chuck spacing of 100 mm, and tensile speed of 200 mm / min. The average of five measurements was used. The measuring apparatus was an Autograph AG5000A manufactured by Shimadzu Corporation.

[0290] (12) Thin film temperature

[0291] The measurement was performed using a radiation thermometer (IR-BZPHGN1, manufactured by CHINO), inserted into the detection unit next to the film forming machine. The measurement data was smoothed for 10 seconds.

[0292] (13) Productivity

[0293] The evaluation was based on the number of breaks when the central portion of the obtained film was cut into 1000 mm wide sections. It should be noted that the cutting blade used for cutting was used to cut existing films equivalent to the comparative examples. Samples that were removed due to exceeding the specified amount were picked up again, and the cutting was performed at a speed of 90% of the maximum design speed of the slitting machine.

[0294] ○: The number of breaks per day is 0.

[0295] △: The number of fractures per day is 1.

[0296] ×: The number of fractures in one day is more than 2.

[0297] (14) Planarity

[0298] Ion-exchanged water was dropped onto a flat glass plate, and a sample obtained by cutting a 1000 mm wide film into a 2000 mm long piece was placed on it. The sample was then adhered using a roller to ensure a uniform water layer. The planarity was evaluated by observing the fluorescent light shining into the top of the film from an inclined direction.

[0299] ○: The reflected fluorescent light has minimal bending and good flatness.

[0300] △: The incoming fluorescent light is distorted, but acceptable.

[0301] ×: The reflected fluorescent light has a large curvature and poor flatness.

[0302] • Polyester A (PET(A))

[0303] Polyethylene terephthalate with an intrinsic viscosity of 0.62 dl / g

[0304] • Polyester B (PET(B))

[0305] A melt mixture of 10 parts by weight of ultraviolet absorber (2,2'-(1,4-phenylene)bis(4H-3,1-benzoxazin-4-one) and 90 parts by weight of PET(A).

[0306] (Preparation of adhesive-modified coating solution)

[0307] A water-dispersible copolyester resin containing sulfonic acid metal salts was prepared by transesterification and polycondensation using conventional methods. The composition consisted of 46 mol% terephthalic acid, 46 mol% isophthalic acid, and 8 mol% sodium isophthalate 5-sulfonate as dicarboxylic acid components (relative to the overall dicarboxylic acid composition), and 50 mol% ethylene glycol and 50 mol% neopentyl glycol as diol components (relative to the overall diol composition). Next, 51.4 parts by weight of water, 38 parts by weight of isopropanol, 5 parts by weight of n-butyl cellosolve, and 0.06 parts by weight of a nonionic surfactant were mixed and heated with stirring. After reaching 77°C, 5 parts by weight of the aforementioned water-dispersible copolyester resin containing sulfonic acid metal salts were added. Stirring continued until the resin solidified without disappearing. The resin dispersion was then cooled to room temperature to obtain a uniformly dispersed water-dispersible copolyester resin solution with a solid content concentration of 5.0% by weight. Subsequently, after dispersing 3 parts by mass of aggregated silica particles (manufactured by Fuji-Silysia, SYLYSIA 310) in 50 parts by mass of water, 0.54 parts by mass of the aqueous dispersion of SYLYSIA 310 was added to 99.46 parts by mass of the above-mentioned water-dispersible copolyester resin liquid, and 20 parts by mass of water was added while stirring to obtain an adhesive-modified coating liquid.

[0308] (Polarized parts)

[0309] A roll of polyvinyl alcohol film with a thickness of 80 μm, which is continuously dyed in an iodine aqueous solution, is stretched 5 times along the transport direction, treated in a boric acid solution, and then washed and dried to obtain a long strip of polarizing element.

[0310] (Polyester film A~H)

[0311] As a raw material for the intermediate layer of the substrate film, 90 parts by weight of PET(A) resin granules (without particles) and 10 parts by weight of PET(B) resin granules (containing UV absorber) are dried under reduced pressure (1 Torr) at 135°C for 6 hours and then fed into extruder 2 (for intermediate layer II). Meanwhile, PET(A) is dried using conventional methods and fed into extruder 1 (for outer layer I and outer layer III) for dissolution at 285°C. Both polymers are filtered separately using stainless steel sintered filter media (removing 95% of particles with a nominal filtration accuracy of 10 μm). The mixture is then laminated using two types of three-layer confluence blocks, extruded into sheets from the nozzle, and then wound onto a casting drum with a surface temperature of 30°C using an electrostatic casting method. After cooling and curing, an unstretched film is produced. At this time, the ejection rate of each extruder is adjusted to a thickness ratio of 10:80:10 for layers I, II, and III.

[0312] The unstretched PET film is fed into an MD stretching machine consisting of low-speed and high-speed rollers. The film is heated to a preheating temperature using multiple low-speed rollers, and then heated to a stretching temperature using an infrared heater located between the low-speed and high-speed rollers. Stretching is performed using the difference in circumferential speed between the infrared heater and the high-speed roller.

[0313] The preheating temperature, stretching temperature, and stretching ratio are shown in Table 1. It should be noted that the number of illuminated lamps in the infrared heaters refers to the number of columns of infrared heaters arranged along the MD direction that are lit for heating purposes.

[0314] It should be noted that the rollers of the MD stretching machine are chrome-plated with a diameter of 180-250mm, and the roundness and offset are all less than 10μm and less than 20μm.

[0315] Next, a coating of 0.08 g / m is applied to both sides of the MD-stretched film after drying. 2 The adhesive-modified coating liquid was applied in the above manner and allowed to dry. The resulting film with the coated layer was then fed into a tenter frame, and while the ends of the film were held by clamps, it was fed into the tenter frame at 100°C and stretched along the width direction. Next, while maintaining the width of the stretch along the width direction, it was treated at 200°C for 10 seconds in a heat-fixing zone, and then a 2% relaxation treatment was performed along the width direction to obtain a stretched PET film.

[0316] (Thin film I and J)

[0317] The unstretched PET film is fed into an MD stretching machine. While being heated using multiple rollers, it is simultaneously heated to the preheating temperature using the second roller starting from the final roller of the low-speed roller, and then heated to the stretching temperature using the final low-speed roller. The film is stretched using the difference in circumferential speed between the film and the high-speed rollers. It should be noted that the final low-speed roller and the rolls mounted on it are rollers with a fluoropolymer-coated surface. Subsequently, the same operation as described above is performed for TD stretching and heat setting to obtain the stretched PET film.

[0318] The film-forming conditions and thickness non-uniformity of these films are shown in Table 1. It should be noted that, regarding the transmittance at a wavelength of 380 nm, except for film H which is 8.5%, all are in the range of 2.3% to 2.5%.

[0319] Furthermore, as an example of the frequency analysis results for thickness non-uniformity, the results for thin film A are shown in [the figure]. Figure 1 The results of thin film D are shown in Figure 2 .

[0320] [Table 1]

[0321]

[0322] Table 1 clearly shows that film A may have large thickness non-uniformity due to its low MD stretching temperature. However, by increasing the MD stretching temperature, as with films B and C, the thickness non-uniformity in the MD direction decreases, and the A / B ratio also decreases. This is evident from... Figure 1 (Thin film A) and Figure 2 It can also be known that the amplitude of thin film A with a frequency below 0.25 Hz (period above 10 cm) is greater than that with a frequency above 0.25 Hz (period less than 10 cm) compared to that of thin film D.

[0323] On the other hand, as with film E, if the MD stretching temperature is too high, it may result in slackness or irregular stretching position, leading to increased thickness unevenness in the MD direction. Additionally, film F has a high preheating temperature, which sometimes causes slackness and film adhesion to the rollers during the preheating process, resulting in increased thickness unevenness in the MD direction.

[0324] Similar to films G and H, even with an increased MD stretching ratio, thickness unevenness in the MD direction can be suppressed as long as the preheating and stretching temperatures are appropriate. In stretching via roller heating, as with film I, excessively high preheating temperatures and excessively low stretching temperatures result in significant thickness unevenness, but this unevenness can be reduced by optimizing the temperature.

[0325] Films B to D, E to G, and J are suitable for use, for example, as protective films for polarizing elements. Additionally, film H is suitable for use, for example, as a transparent electrode substrate film, an anti-spray film, and a screen surface protective film for flexible image display devices.

[0326] (Making of polarizing plates)

[0327] In the polyester films produced in the embodiments, films C, D, and J with particularly good thickness uniformity were used to fabricate polarizing plates as follows.

[0328] A polarizing plate is fabricated by roller-to-roll bonding of the aforementioned polyester film to one side of the polarizer and a cellulose triacetate film (40 μm thick) to the opposite side. A UV-curable adhesive is used for bonding. The angle between the slow axis of the polyester film and the absorption axis of the polarizer is 90 degrees, with an offset of less than 0.5 degrees.

[0329] (Evaluation of image display devices)

[0330] The obtained polarizing plate is cut and replaced with the viewing side polarizing plate of a commercially available 42-inch LCD TV. No iris is visible even when viewed from an angle, demonstrating excellent visibility.

[0331] Industrial availability

[0332] This invention provides a polyester film with high in-plane delay, excellent thickness uniformity, high productivity, good workability, and good planarity. Regardless of the type of image display device or light source, the iris of this polyester film is not easily noticeable, exhibiting good visibility and making it suitable for various applications in image display devices.

Claims

1. A polyester film having an in-plane retardation of 4000 nm or more and 30000 nm or less. The orientation degree is above 0.133 and below 0.

155. The NZ coefficient is greater than 1.7 and less than 3. The thickness of the film in the flow direction is not uniformly below 8%. in, Thickness non-uniformity is calculated as (maximum thickness - minimum thickness) / average thickness × 100 (%). When performing a Fourier transform on the thickness measurement data in the film flow direction and replacing the frequency with the film length period, the ratio A / B of the following A to the following B is 5 or less. A: For periods of 10cm or more, the average of the five largest amplitude values; B: The average of the five largest amplitude values ​​for periods less than 10cm.

2. The polyester film according to claim 1, wherein the in-plane retardation is above 4000 nm and below 6500 nm.

3. The polyester film according to claim 1, wherein the in-plane retardation is above 6500 nm and below 15000 nm.

4. The polyester film according to claim 1, wherein the in-plane retardation is above 6500 nm and below 9500 nm.

5. The polyester film according to claim 1, wherein the degree of planar orientation is 0.138 or higher.

6. The polyester film according to claim 1, wherein the degree of planar orientation is 0.14 or higher.

7. The polyester film according to claim 1, wherein the thickness in the film-forming flow direction is not all below 4%.

8. The polyester film according to claim 1, wherein the NZ coefficient is 1.8 or higher and 3 or lower.

9. The polyester film according to claim 1, wherein the NZ coefficient is 1.9 or higher and 2.5 or lower.

10. The polyester film according to claim 1, wherein, The A / B ratio is above 1.8 and below 3.

5.

11. The polyester film according to claim 1, wherein, When performing a Fourier transform on the thickness measurement data in the film flow direction and replacing the frequency with the film length period, the following ratio of Amax to B, Amax / B, is less than 7. Amax: The maximum amplitude when the period is 10cm or more.

12. The polyester film according to claim 11, wherein, The Amax / B ratio is greater than 2.2 and less than 4.

13. The polyester film according to claim 1, wherein the thickness is 25 μm or more and 150 μm or less.

14. The polyester film according to claim 1, wherein the thickness is 30 μm or more and 70 μm or less.

15. The polyester film according to claim 1, wherein the thickness is 50 μm or more and 85 μm or less.

16. The polyester film according to claim 1, wherein the thickness is 60 μm or more and 130 μm or less.

17. The polyester film according to claim 1, wherein the elongation at break in the film-forming flow direction is 4% or more.

18. The polyester film according to claim 1, wherein the elongation at break in the film-forming flow direction is 10% or more.

19. The polyester film according to claim 1, wherein the elongation at break in the film-forming flow direction is less than 50%.

20. The polyester film according to claim 1, wherein the elongation at break in a direction orthogonal to the film-forming flow direction is 50% or more and 120% or less.

21. The polyester film according to claim 1, wherein the tensile strength in the film-forming flow direction is 50 MPa or higher.

22. The polyester film according to claim 1, wherein the tensile strength in the film-forming flow direction is 65 MPa or higher.

23. The polyester film according to claim 1, wherein the tensile strength in the film-forming flow direction is less than 100 MPa.

24. The polyester film according to claim 1, wherein the tensile strength in a direction orthogonal to the film-forming flow direction is 330 MPa or more and 420 MPa or less.

25. The polyester film according to claim 1, wherein the thickness in the direction orthogonal to the film-forming flow direction is not all greater than 0.5% and less than 3%.

26. The polyester film according to claim 1, wherein it is a polyethylene terephthalate film.

27. A polarizer protective film formed from any one of claims 1 to 26.

28. A polarizing plate having a polarizing element protective film and a polarizing element as described in claim 27 laminated thereon.

29. An image display device, wherein, The polarizing plate of claim 28 is disposed on the visible side of the image display unit.

30. A transparent electrode substrate film formed from any one of claims 1 to 26.

31. An anti-scattering film, formed from any one of claims 1 to 26.

32. A protective film for a screen surface, formed from a polyester film according to any one of claims 1 to 26.

33. An image display device comprising any one of the transparent electrode substrate film of claim 30, the anti-scattering film of claim 31, and the screen surface protective film of claim 32.

34. The image display device according to claim 33, wherein it is a flexible image display device.

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