Polyester film for protecting polarizing plate, polarizing plate, and liquid crystal display device

Abstract

The polyester film for polarizer protection satisfies the following conditional expressions (1) and (2), where T1 (%) is the thermal shrinkage rate of the film before and after heat treatment at 80 DEG C in an a direction, which is a first direction, and T2 (%) is the thermal shrinkage rate of the film before and after heat treatment at 150 DEG C in the same direction as the a direction. T1 > = 0.2% (1) 1.0 lt; (2) T2 / T1 < = 5.0... (2).

Description

Technical Field

[0001] The present invention relates to a polyester film for protecting a polarizing plate, a polarizing plate and a liquid crystal display device. Background Art

[0002] Liquid crystal display devices are increasingly in demand for applications such as liquid crystal televisions and liquid crystal displays for personal computers. Generally, a liquid crystal display device is composed of a liquid crystal unit having transparent electrodes, a liquid crystal layer, a color filter, etc., sandwiched between glass plates, and two polarizing plates disposed on both sides thereof, each of which is composed of a polarizing plate (also called a polarizing film) sandwiched between two optical films (e.g., a polarizing plate protective film and a phase difference film).

[0003] In recent years, it has been confirmed that the LCD panel has warped and displayed unevenly due to the increase in the size and thickness of the LCD TV screen. In particular, the rigidity of the LCD panel changes due to the thickness of the glass substrate used in the LCD panel, and the slight shrinkage of the polarizer has an impact, causing the LCD panel to warp and display unevenly.

[0004] When the thickness of the glass substrate used in the liquid crystal panel is, for example, 0.7 mm or more, the high rigidity of the glass suppresses shrinkage of the polarizer, so that the liquid crystal panel does not warp and display unevenness tends not to be a problem.

[0005] In order to further reduce the thickness of the liquid crystal panel, when the thickness of the glass substrate is made thinner than 0.7 mm, the problem of uneven display occurs, and improvement is sought. The mechanism of uneven display is that it occurs mainly due to the shrinkage of the polarizer. When the polarizer is placed under high temperature and high humidity, the shrinkage force acts in the orientation direction in order to relax the orientation. As a result, it is believed that the liquid crystal panel warps and expands on the backlight unit side, thereby causing uneven display.

[0006] Patent Document 1 proposes a method for improving warpage and display unevenness of a liquid crystal panel by setting the shrinkage force of a polarizing plate protective polyester film laminated on one surface of a polarizing plate to a specific range.

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Document 1: WO2019 / 054406 Summary of the invention

[0010] Problem that the invention aims to solve

[0011] As liquid crystal panels become larger and glass substrates of liquid crystal cells become thinner, the present inventors have found that increasing the shrinkage stress of a polarizing plate protective polyester film is effective in suppressing warping of the liquid crystal panel.

[0012] When a large shrinkage stress is required, increasing the thermal shrinkage rate of the polarizer protective polyester film at 80°C is one of the effective countermeasures. However, when using such a polarizer protective polyester film with an increased thermal shrinkage rate, light leakage sometimes occurs slightly. In addition, if there is no light leakage initially, if the liquid crystal panel is placed in a high temperature environment for a long time, light leakage also occurs, and visibility sometimes deteriorates. These problems are particularly likely to occur in liquid crystal display devices of 50 inches or more (diagonal length is 50 inches or more).

[0013] The present invention is made in view of the above problems / situations, and the problem to be solved is to provide a polyester film for protecting a polarizer, a polarizing plate, and a liquid crystal display device that can suppress warping of a liquid crystal panel and light leakage. In particular, the problem is to provide a polyester film for protecting a polarizer, a polarizing plate, and a liquid crystal display device that can suppress warping of a liquid crystal panel even when the liquid crystal panel is placed in a high temperature environment for a long time.

[0014] Solutions for solving problems

[0015] The representative embodiments of the present invention are as follows.

[0016] Item 1:

[0017] A polyester film for protecting a polarizer, wherein the following conditional formulas (1) and (2) are satisfied when the heat shrinkage rate of the film in the a direction as the first direction before and after heat treatment at 80°C is set to T1(%) and the heat shrinkage rate of the film in the same direction as the a direction before and after heat treatment at 150°C is set to T2(%).

[0018] T1 ≥ 0.2% (1)

[0019] 1.0 <T2 / T1≤5.0···(2)

[0020] Item 2:

[0021] The polarizer protective polyester film according to item 1, wherein, when the heat shrinkage rate of the film before and after heat treatment at 80°C in the b direction which is a direction perpendicular to the a direction is defined as T3 (%), the following conditional expressions (3) and (4) are satisfied.

[0022] T3≥0.06%···(3)

[0023] 1.0 <T1 / T3≤6.0···(4)

[0024] Item 3:

[0025] The polarizing plate protective polyester film according to item 1 or 2, which has a retardation of 3000 to 30000 nm.

[0026] Item 4:

[0027] The polarizing plate-protecting polyester film according to any one of items 1 to 3, wherein the thickness is 25 to 200 μm.

[0028] Item 5:

[0029] The polarizer-protective polyester film according to any one of items 1 to 4, wherein the polarizer-protective polyester film has at least one layer selected from a hard coat layer, an antireflection layer, a low reflection layer, an antiglare layer and an antireflection antiglare layer on at least one surface.

[0030] Item 6:

[0031] A polarizing plate comprising the polarizer-protecting polyester film according to any one of items 1 to 5 on at least one surface of a polarizer.

[0032] Item 7:

[0033] An image display device comprising the polarizing plate described in item 6.

[0034] Item 8:

[0035] A liquid crystal display device comprising the polarizing plate described in item 6.

[0036] Effects of the Invention

[0037] According to the present invention, a polarizer protective film, a polarizing plate, and a liquid crystal display device that can suppress warping of a liquid crystal panel and light leakage can be provided. In particular, a polarizer protective polyester film, a polarizing plate, and a liquid crystal display device that can suppress warping of a liquid crystal panel and light leakage can be provided even when the liquid crystal panel is placed in a high temperature environment for a long time. Furthermore, an economically advantageous polarizer protective polyester film with improved productivity can be provided. DETAILED DESCRIPTION

[0038] The polarizer protective polyester film of the present invention is a polyester film formed of a polyester film and laminated on at least one surface of a polarizer to form a constituent member of a polarizing plate, and is generally referred to as a polarizer protective film. Hereinafter, the polarizer protective polyester film may be simply referred to as a polyester film.

[0039] First, the inventors speculated that the phenomenon of the above-mentioned subject caused the following situation. For example, when the polyester film is greatly stretched along the uniaxial stretching direction with the transverse direction as the main stretching direction, stress is also applied to the longitudinal direction due to Poisson contraction, but the ends of the film are clamped by the clamps and the central part is free, so the strain caused by the stress is easy to become uneven. In the state of uneven strain, the residual stress after the main stretching and heat setting is also easy to become uneven. In such a state, when a micro-stretching is performed after the main stretching, the influence of the residual stress is more significant and the planarity is deteriorated.

[0040] The polarizer protective film is subjected to surface processing such as an anti-reflection layer as required, and is bonded to a polarizer to form a polarizing plate, and the polarizing plate is further bonded to a liquid crystal unit to form a liquid crystal panel, but a certain degree of tension is applied during these processes. In a polarizer protective film with poor planarity, tension is applied unevenly, and in addition, in a polarizer protective film with uneven residual stress, strain is generated due to heat during the above-mentioned processing. For such a liquid crystal panel, the relationship between the crossed prisms of the two polarizers is offset at the end, and in particular, the displacement becomes larger due to uneven shrinkage over time, causing light leakage.

[0041] When manufacturing films in order to improve productivity, wide films are manufactured to match the required width of the polarizer protective film. Sometimes the films are divided into two or three parts or formed into slit rolls along the width direction. However, especially when divided into two or three parts, the flatness of the slit rolls at both ends is prone to deterioration, which can easily cause light leakage.

[0042] As a countermeasure based on the above speculation, the inventors considered that the stress generated in the main stretching can be improved by performing micro-stretching after the stress is sufficiently relaxed and eliminated, and thus completed the present invention.

[0043] Specifically, the present invention focuses on the relaxation process which has not been given much attention in the prior art. After stretching and heat setting, a relaxation process at an appropriate temperature and ratio is performed, and then micro-stretching is performed after the residual stress is fully removed. This can form a film with excellent planarity and excellent shrinkage characteristics at 80°C, and can form a polarizer protective film that does not cause light leakage and warping of the panel.

[0044] For the polarizer protective polyester film of the present invention, when the first direction is set to the a direction and the heat shrinkage rate of the polyester film in the a direction before and after the 80°C heat treatment is set to T1, the value of T1 is preferably 0.20% or more. By making T1 above 0.20%, the warping of the liquid crystal panel can be effectively suppressed. T1 is more preferably above 0.21%, and further preferably above 0.23%. In addition, by making T1 below 0.50%, the phenomenon that the liquid crystal panel will warp in the opposite direction due to excessive shrinkage stress can be suppressed. T1 is more preferably below 0.45%, and further preferably below 0.43%. The a direction can be the direction in which the 80°C heat shrinkage rate is greater in the TD direction or the MD direction. In addition, when the direction of TD is unclear, it can be the direction in which the 80°C heat shrinkage rate is greater in the slow axis direction or in the direction orthogonal to the slow axis direction. In addition, it can be the direction with the largest shrinkage rate.

[0045] As polarizers, polyvinyl alcohol films stretched in the MD direction are often used, so the a direction is preferably the TD direction. In addition, in order to stably produce a polyester film with a high retardation, stretching in the TD direction using a tenter is preferred, so the a direction is preferably the slow axis direction.

[0046] The slow axis direction can be determined using a molecular orientation meter (eg, MOA-6004 molecular orientation meter manufactured by Oji Instruments Co., Ltd.) etc. The direction with the largest shrinkage rate can be the direction with the highest shrinkage rate in the measurement method in the examples.

[0047] For the polarizer protective polyester film of the present invention, when the heat shrinkage rate of the polyester film in the a direction before and after the 150°C heat treatment is set to T2, the value of T2 is preferably 2.00% or less. By setting the value of T2 to 2.00% or less, when the polarizing plate is manufactured by a polarizing plate processing process, usually by a roll-to-roll process, bonding becomes easy, and light leakage can be suppressed. T2 is more preferably 1.95% or less, and further preferably 1.90% or less. In addition, T2 is preferably 0.50% or more. By setting it to 0.50% or more, the warping of the unit can be effectively suppressed. T2 is more preferably 0.55% or more, and further preferably 0.60% or more.

[0048] For the polarizer protective polyester film of the present invention, when the direction perpendicular to the above-mentioned a direction is set as the b direction and the heat shrinkage rate of the film in the b direction before and after the 80°C heat treatment is set as T3, the value of T3 is preferably 0.06% or more. By making T3 above 0.06%, the planarity of the film is excellent and leakage can be suppressed. T3 is more preferably above 0.07%, further preferably above 0.08%, and particularly preferably above 0.09%. In addition, T3 is preferably set to 0.20% or less, so that the warping of the liquid crystal panel can be effectively suppressed. T3 is more preferably below 0.18%, and further preferably below 0.16%.

[0049] The shrinkage rate in the MD direction is easy to produce differences according to the position of the film in the TD direction, but the shrinkage rate in the TD direction is not easy to produce differences based on the position in the TD direction. Thus, T1, T2 and T3 are measured at a total of 5 positions in the width direction of the polarizer protective polyester film, the central part, and the middle part of the middle place between the central part and the two ends, and the values ​​of T1, T2 and T3 at the position where the value of T3 is the smallest are adopted. It should be noted that the two ends are as follows: when the film ends are knurled, the distance between the film end side of the circumference of the circle for measuring the heat shrinkage rate described in the embodiment and the inner end of the knurling is made to be 1cm, and when the film end side of the aforementioned circumference is not knurled, the distance between the film end side and the film end is made to be 1cm. The 5 position measurements are preferably carried out in a manner that the measurement parts are arranged in a row parallel to the TD direction, but when the circles overlap, the measurement positions can be displaced in a manner that the circles alternately do not overlap along the MD direction.

[0050] In addition, when the polyester film is cut into single sheets and the TD direction is unclear, the heat shrinkage rates at 80°C and 150°C are measured at the above-mentioned five positions in both the long side direction and the short side direction of the cut rectangular film, and the direction in which the difference in heat shrinkage rates between the long side direction and the short side direction becomes larger can be set as the TD direction.

[0051] When the ratio of the above-mentioned heat shrinkage rate T2 to T1 is set to T2 / T1, it is considered that T2 / T1 becomes an indicator indicating the extent to which the residual stress in the main stretching is relaxed. The residual stress is manifested as strain in the MD direction and TD direction in the micro-stretching process after the main stretching, but it is difficult to directly evaluate it only by the physical properties of the film. After the main stretching, oriented crystallization is carried out in the heat setting process, and further appropriate relaxation treatment is carried out, thereby reducing the strain and residual stress caused by the main stretching. As a result, it is believed that the heat shrinkage rate at 150°C is reduced. Therefore, as an indicator for estimating the effect of the relaxation treatment, T2 / T1 is considered to be effective.

[0052] T2 / T1 is preferably 5.00 or less, more preferably 4.8 or less, further preferably 4.7 or less, particularly preferably 4.6 or less, and most preferably 4.5 or less. By setting it to the above or less, the relaxation treatment before micro-stretching is fully carried out, the strain and residual stress are relaxed / eliminated, and the planarity becomes good. In addition, T2 / T1 is preferably greater than 1.0, more preferably greater than 1.5, further preferably greater than 2.0, particularly preferably greater than 2.5, and most preferably greater than 3.0. By exceeding 1.0, T2 / T1 has excellent planarity of the film and can suppress light leakage.

[0053] When the ratio of T1 to T3 of the above-mentioned heat shrinkage is set to T1 / T3, by making T1 / T3 appropriate, light leakage and unit warping can be further suppressed with high precision. For example, when the main stretching direction is the TD direction by tenter stretching, and the film is shrunk in the TD direction during the relaxation process, the residual stress in the TD direction is eliminated in the film after the relaxation process, but the residual stress in the MD direction is not completely eliminated. When heated, there is sometimes a shrinkage rate in the MD direction. On the other hand, there is a tendency for the shrinkage rate in the MD direction to further decrease during TD micro-stretching, which is speculated to imply MD shrinkage during micro-stretching. TD micro-stretching is performed to obtain the TD shrinkage rate required to suppress the warping of the unit, but if the micro-stretching is excessively increased, the MD shrinkage in the micro-stretching is excessive, and it is speculated that the planarity of the film is impaired. Therefore, it is believed that T1 / T3 is appropriate for the film before micro-stretching to indicate whether the micro-stretching is appropriate.

[0054] The polarizer protective polyester film of the present invention preferably has a T1 / T3 of 6.0 or less, more preferably 5.5 or less, further preferably 5.2 or less, particularly preferably 5.0 or less, and most preferably 4.8 or less. By setting it to be less than the above, the planarity becomes better.

[0055] Furthermore, T1 / T3 is preferably greater than 1, more preferably greater than 1.2, further preferably greater than 1.5, particularly preferably greater than 1.8, and most preferably greater than 2.0. By setting the above range, the warpage of the liquid crystal panel can be effectively reduced.

[0056] It should be noted that the above-mentioned relationship between the subject / effect of the present application and T2 / T1, T1 / T3 is described in the domestic context, including speculation, but the present invention is not limited to the scope of the above description.

[0057] Usually, in a liquid crystal display device, two polarizing plates are configured to form a cross prism relationship. If two polarizing plates are configured in a cross prism relationship, light usually does not pass through the two polarizing plates. However, due to the shrinkage or warping of the above-mentioned polarizer, the complete cross prism relationship collapses, and there is a concern about light leakage. From the viewpoint of suppressing light leakage, the angle between the direction in which the thermal shrinkage rate of the polarizer protective film is the largest and the transmission axis of the polarizer is preferably 25 degrees or less, more preferably 15 degrees or less, further preferably 10 degrees or less, particularly preferably 5 degrees or less, and most preferably 3 degrees or less.

[0058] The commonly used polarizer is a film made of polyvinyl alcohol containing dichroic pigments such as iodine and stretched in the MD direction, and such a polarizer shrinks in the MD direction. By laminating the polarizer and the polarizer protective film with a roll to a roll and then clamping the unit, it is believed that the shrinkage of the polarizer on one side is offset by the shrinkage of the polarizer protective film on the other side. Therefore, the angle between the direction in which the thermal shrinkage rate of the polarizer protective film is the largest and the TD direction of the polarizer protective film is preferably 10 degrees or less, more preferably 7 degrees or less, further preferably 5 degrees or less, particularly preferably 3 degrees or less, and most preferably 2 degrees or less.

[0059] The lower limit of the thickness of the polarizer protective polyester film of the present invention is preferably 25 μm, more preferably 30 μm, further preferably 35 μm, particularly preferably 40 μm, and the upper limit of the thickness is preferably 200 μm, more preferably 150 μm, further preferably 100 μm, particularly preferably 90 μm, and most preferably 85 μm.

[0060] By making the thickness of the polyester film within the above range, it is not easy to break, and in addition, the rigidity is maintained, thereby making it possible to make the planarity good and the fracture in the film making less likely to occur. In addition, the shrinkage stress of the film does not become excessively large, and the fluctuation of the shrinkage stress can be reduced, and its control becomes easy, and the increase in cost is also suppressed. The thickness of the polyester film can be measured by the method adopted in the embodiments described later.

[0061] For the polarizer protective polyester film of the present invention, from the viewpoint of suppressing the rainbow spots observed on the screen of the liquid crystal display device, it is preferred that the in-plane retardation is in a specific range. The in-plane retardation is preferably more than 3000nm, more preferably more than 5000nm, further preferably more than 6000nm, particularly preferably more than 7000nm, and most preferably more than 8000nm. The in-plane retardation is preferably less than 30000nm, more preferably less than 18000nm, further preferably less than 15000nm, particularly preferably less than 12000nm, and most preferably less than 10000nm. In particular, from the viewpoint of thin filmization, the in-plane retardation is preferably less than 10000nm and less than 9000nm. It should be noted that the in-plane retardation can be recorded as Re below.

[0062] The retardation of the polyester film can also be determined by measuring the refractive index and thickness in the biaxial directions, or by using a commercially available automatic birefringence measuring device such as KOBRA-21ADH (manufactured by Oji Scientific Instruments Co., Ltd.) The refractive index can be determined using an Abbe refractometer (measurement wavelength: 589 nm).

[0063] For the polarizer protective polyester film of the present invention, when the retardation in the thickness direction is set to Rth, the ratio of the in-plane retardation to the retardation in the thickness direction, i.e., Re / Rth, is preferably 0.2 or more, preferably 0.3 or more, preferably 0.4 or more, more preferably 0.5 or more, and further preferably 0.6 or more. The larger the Re / Rth, the more isotropic the effect of birefringence is, and the tendency that the occurrence of rainbow-like spots caused by the observation angle becomes less likely to occur. In a completely uniaxial (uniaxially symmetrical) film, Re / Rth becomes 2.0, and therefore, the upper limit of Re / Rth is preferably 2.0. The more preferred upper limit of Re / Rth is 1.2 or less, further preferably 1 or less, particularly preferably 0.98 or less, and most preferably 0.97 or less. It should be noted that the thickness direction phase difference refers to the average of the phase difference obtained by multiplying the two birefringence △Nxz and △Nyz when observing the film from the thickness direction cross section by the film thickness d.

[0064] For the polarizer protective polyester film of the present invention, from the viewpoint of further suppressing rainbow-shaped spots, the NZ coefficient of the polyester film is preferably less than 2.5, more preferably less than 2.0, further preferably less than 1.8, and further preferably less than 1.6. Moreover, in a completely uniaxial (uniaxially symmetrical) film, the NZ coefficient becomes 1.0, so the lower limit of the NZ coefficient is 1.0. However, as it approaches a completely uniaxial (uniaxially symmetrical) film, there is a tendency that the mechanical strength in the direction orthogonal to the orientation direction is significantly reduced, so it must be noted. The lower limit of the NZ coefficient is preferably 1.1, more preferably 1.2, further preferably 1.3, particularly preferably 1.4, and most preferably 1.45.

[0065] The NZ coefficient is represented by |Ny-Nz| / |Ny-Nx|, where Ny refers to the refractive index in the slow axis direction of the polyester film, Nx refers to the refractive index in the direction perpendicular to the slow axis (refractive index in the fast axis direction), and Nz refers to the refractive index in the thickness direction. The orientation axis of the film is obtained using a molecular orientation meter (MOA-6004 molecular orientation meter manufactured by Oji Instruments Co., Ltd.), and the biaxial refractive index (Ny, Nx, where Ny>Nx) in the orientation axis direction and the direction perpendicular to it and the refractive index in the thickness direction (Nz) are obtained using an Abbe refractometer (NAR-4T manufactured by ATAGO CO., LTD., measuring wavelength 589 nm). The NZ coefficient can be obtained by substituting the values ​​thus obtained into |Ny-Nz| / |Ny-Nx|.

[0066] In addition, for the polyester film of the present invention, from the viewpoint of further suppressing iridescent color spots, the Ny-Nx value of the polyester film is preferably 0.05 or more, more preferably 0.07 or more, further preferably 0.08 or more, further preferably 0.09 or more, and most preferably 0.1 or more. The upper limit is not particularly limited, but in the case of polyethylene terephthalate films, the upper limit is preferably about 1.5, more preferably 1.2 or less, and further preferably 1.1.

[0067] It should be noted that the present invention can also be used for polyester films with an in-plane retardation of less than 3000 nm. If the in-plane retardation is less than 3000 nm, rainbow spots may sometimes occur, but by providing an anti-reflection layer or a low-reflection layer on at least one side of the polyester film, the rainbow spots can be made less conspicuous. In order to maintain the preferred film thickness as a polarizer protective film and form a film with stable optical properties, the in-plane retardation is preferably 100 nm or more, more preferably 200 nm or more, and further preferably 300 nm or more.

[0068] The polarizer protective polyester film of the present invention can be obtained from any polyester resin. The type of the polyester resin is not particularly limited, and any polyester resin obtained by condensing a dicarboxylic acid and a diol can be used.

[0069] Examples of the dicarboxylic acid component that can be used in the production of the polyester resin include terephthalic acid, isophthalic acid, phthalic acid, 2,5-naphthalene dicarboxylic acid, 2,6-naphthalene dicarboxylic acid, 1,4-naphthalene dicarboxylic acid, 1,5-naphthalene dicarboxylic acid, diphenyl carboxylic acid, diphenoxyethane dicarboxylic acid, diphenyl sulfone carboxylic acid, anthracene dicarboxylic acid, 1,3-cyclopentane dicarboxylic acid, 1,3-cyclohexane dicarboxylic acid, 1,4-cyclohexane dicarboxylic acid, hexahydroterephthalic acid, hexahydroisophthalic acid, malonic acid, dimethylmalonic acid, succinic acid, 3,3-diethylsuccinic acid, glutaric acid, 2,2-dimethylglutaric acid, adipic acid, 2-methyl adipic acid, trimethyl adipic acid, pimelic acid, azelaic acid, dimer acid, sebacic acid, suberic acid, and dodecanedicarboxylic acid.

[0070] Examples of the diol component that can be used in the production of the polyester resin include ethylene glycol, propylene glycol, hexylene glycol, neopentyl glycol, 1,2-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, decanediol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 2,2-bis(4-hydroxyphenyl)propane, and bis(4-hydroxyphenyl)sulfone.

[0071] The dicarboxylic acid component and the diol component constituting the polyester resin can be used in one or more than two kinds. As suitable polyester resins constituting the polyester film, for example, polyethylene terephthalate, polypropylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, etc. can be cited, and polyethylene terephthalate and polyethylene naphthalate can be more preferably cited, but they can further contain other copolymer components. These resins are excellent in transparency, and thermal properties and mechanical properties are also excellent. In particular, polyethylene terephthalate can achieve a high elastic modulus, and the control of heat shrinkage is relatively easy, so it is a suitable raw material.

[0072] In the case where it is necessary to highly increase the thermal shrinkage of the polyester film, it is desirable to add a copolymer component and appropriately reduce the degree of crystallinity. In addition, since the ratio of elastic strain to permanent strain is high for deformation below the vicinity of the glass transition temperature, it is usually difficult to highly increase the thermal shrinkage. Therefore, it is also a preferred embodiment to introduce a component with a low glass transition temperature as required. Specifically, propylene glycol, 1,3-propylene glycol, etc.

[0073] (Provision of an easy-adhesion layer)

[0074] In order to improve the adhesion to the polarizer, the polarizer protective polyester film may be subjected to a corona treatment, a coating treatment, a flame treatment, or the like.

[0075] In order to improve the adhesion between the functional layer such as the hard coat layer and the polarizer, it is preferred that the polyester film has an easy-adhesion layer on at least one side. The polyester film having such an easy-adhesion layer is also included in the polarizer-protecting polyester film of the present invention.

[0076] Preferably, at least one side of the polyester film has an easy-adhesive layer with at least one of polyester resin, polyurethane resin or polyacrylic resin as the main component. Here, "main component" refers to a component of more than 50% by mass in the solid component constituting the easy-adhesive layer. The coating liquid used in the formation of the easy-adhesive layer preferably contains at least one aqueous coating liquid in a water-soluble or water-dispersible copolyester resin, acrylic resin and polyurethane resin. As these coating liquids, for example, water-soluble or water-dispersible copolyester resin solutions, acrylic resin solutions, polyurethane resin solutions, etc. disclosed in Japanese Patent Gazette No. 3567927, Japanese Patent Gazette No. 3589232, Japanese Patent Gazette No. 3589233, Japanese Patent Gazette No. 3900191, Japanese Patent Gazette No. 4150982, etc. can be cited.

[0077] The easy-adhesive layer can be obtained by coating the coating solution on one or both sides of an unstretched film or a uniaxially stretched film in the longitudinal direction, drying at 100 to 150°C, and further stretching in the transverse direction. From the viewpoint of balancing adhesion and anti-blocking properties, the final coating amount of the easy-adhesive layer is preferably managed to be 0.05 to 0.20 g / m 2 When the adhesive layer is provided on both surfaces of the polyester film, the coating amounts of the adhesive layers on both surfaces may be the same or different, and may be independently set within the above range.

[0078] In order to give the easy-adhesive layer easy slip, preferably add particles. For the average particle size of microparticles, in order to prevent particles from falling off from the covering layer, preferably use particles below 2 μm. As the particles contained in the easy-adhesive layer, for example, inorganic particles such as titanium oxide, barium sulfate, calcium carbonate, calcium sulfate, silicon dioxide, aluminum oxide, talc, kaolin, clay, calcium phosphate, mica, hectorite, zirconium oxide, tungsten oxide, lithium fluoride, calcium fluoride, styrene, acrylic acid, melamine, benzoguanamine, silicone, etc. can be cited. Organic polymer particles of the system etc.. They can be added to the easy-adhesive layer alone, or two or more can be combined and added.

[0079] In addition, as a method for applying the coating liquid, a known method can be used, for example, a reverse roll coating method, a gravure coating method, a lip coating method, a roller brush method, a spray coating method, an air knife coating method, a wire rod coating method, a tubular doctor blade method, etc., and these methods can be used alone or in combination.

[0080] In addition, the average particle size of the above particles was measured according to the following method.

[0081] The particles were photographed using a scanning electron microscope (SEM), and the maximum diameters (the distance between the two most distant points) of 300 to 500 particles were measured at a magnification such that the size of the smallest particle was 2 to 5 mm. The average value was taken as the average particle size.

[0082] (Provision of functional layer)

[0083] In the polarizer protective polyester film of the present invention, at least one side may have a hard coat layer, an anti-glare layer, an anti-reflection layer, a low-reflection layer, a low anti-reflection layer, an anti-reflection anti-glare layer, a low-reflection anti-glare layer, an antistatic layer and other functional layers. Preferably, these functional layers are provided on the side of the polyester film opposite to the side of the laminated polarizer.

[0084] The polarizing plate using the polarizer protective polyester film is ideally integrated with the glass plate of the liquid crystal unit under the condition that the aforementioned heat shrinkage T1, T2, T3 and the ratio T2 / T1, T1 / T3 of the polyester film maintain the aforementioned conditions. Therefore, in the case of imparting these functional layers, it is an ideal embodiment to set the drying temperature relatively low and to perform the process by a method with a small thermal history such as UV irradiation and electron beam irradiation. In addition, imparting these functional layers in the film-making process of the polyester film is also an ideal embodiment. That is, in the present invention, in the laminated body having the functional layer laminated on the polarizer protective polyester film, it is also preferred that the aforementioned heat shrinkage T1, T2, T3 and the ratio T2 / T1, T1 / T3 are the aforementioned conditions.

[0085] (Method for producing polyester film)

[0086] The polarizer protective polyester film of the present invention can be manufactured according to the usual method for manufacturing polyester films. For example, the following method can be cited: the non-oriented polyester obtained by melt-extruding the polyester resin into a sheet and molding it is stretched in the longitudinal direction as the film flow direction during film making by using the speed difference of the roller at a temperature above the glass transition temperature, and then stretched in the transverse direction as a direction orthogonal to the longitudinal direction by using a tenter, and heat-treated. It can be a uniaxially stretched film or a biaxially stretched film. In the case of forming a polyester film with an in-plane retardation of 3000 to 30000 nm, it is preferably a uniaxially stretched film obtained by strongly stretching mainly in the transverse direction, and it can also be slightly stretched in a direction perpendicular to the main stretching direction. It should be noted that the longitudinal direction is also referred to as the film flow direction, the length direction, the longitudinal direction, and the MD direction, and MD is the abbreviation of Machine Direction. In addition, the transverse direction is also referred to as the width direction and the TD direction, and TD is the abbreviation of Transverse Direction.

[0087] When the film-making conditions of the polyester film are specifically described, the longitudinal stretching temperature and the transverse stretching temperature are preferably 80 to 130°C, and particularly preferably 90 to 120°C. In order to control the delay amount to the above range, it is preferred to control the ratio of the longitudinal stretching ratio to the transverse stretching ratio and increase the stretching ratio of one. For example, if the slow axis is in the TD direction, the longitudinal stretching ratio is preferably 1.0 to 3.5 times, more preferably 3.0 times or less, further preferably 2.5 times or less, particularly preferably 2.0 times or less, and most preferably 1.5 times or less. In order to obtain a film with a low deviation between the slow axis and the TD direction and a film with uniform heat shrinkage characteristics in the TD direction, the longitudinal stretching ratio is preferably 1.2 times or less, and further preferably 1.1 times or less. By setting it as above, the waste portion at the end of the film in the width direction can also be reduced.

[0088] In addition, the transverse stretching ratio is preferably 2.5 to 6.0 times, more preferably 3.0 to 5.7 times. Particularly preferably 3.5 to 5.5 times. In the subsequent heat treatment (heat setting), the treatment temperature is preferably 150 to 250°C, particularly preferably 180 to 240°C. The heat setting time is preferably 1 to 60 seconds, more preferably 3 to 40 seconds, and further preferably 5 to 30 seconds.

[0089] In addition, when forming a polyester film having an in-plane retardation of less than 3000 nm, the preferred range of the longitudinal stretching ratio is the same as the range of the transverse stretching ratio described above.

[0090] Preferably, after stretching, heat setting and relaxation, micro-stretching is performed in the direction in which the shrinkage is desired to be imparted. The direction of micro-stretching is preferably the a direction.

[0091] In the relaxation treatment, the relaxation rate in the direction in which the shrinkage rate is to be imparted is preferably 0.25% or more, more preferably 0.3% or more, further preferably 0.35% or more, particularly preferably 0.38% or more, and most preferably 0.4% or more. In addition, it is preferably 2% or less, further preferably 1.7% or less, more preferably 1.5% or less, particularly preferably 1.3% or less, and most preferably 1.2% or less. By setting the above range, the heat shrinkage rate at 80°C can be within an appropriate range without excessively increasing the micro-stretching ratio, and good flatness can be obtained. It should be noted that the relaxation rate can be expressed as (film width at the start of heat setting - film width at the end of the relaxation process) / film width at the start of heat setting × 100 (%) in the case of stretching and relaxation treatment in a tenter.

[0092] The temperature of the relaxation treatment is preferably below the heat setting temperature and above the micro-stretching temperature. The upper limit of the temperature of the relaxation treatment may be -5°C of the heat setting temperature, -10°C of the heat setting temperature, -15°C of the heat setting temperature, or -20°C of the heat setting temperature. In addition, the lower limit may be +5°C of the micro-stretching temperature, +10°C of the micro-stretching temperature, +15°C of the micro-stretching temperature, or +20°C of the micro-stretching temperature. The preferred temperature range may be appropriately determined according to the heat setting temperature and the micro-stretching temperature. It should be noted that the temperature of the relaxation treatment does not need to be a constant temperature, and may be carried out in the process of gradually cooling from the heat setting temperature to the micro-stretching temperature. In this case, the temperature at the beginning and end of the relaxation treatment is preferably in the above range.

[0093] The relaxation treatment time is preferably 1 to 60 seconds, more preferably 1.5 to 40 seconds, and still more preferably 2.0 to 30 seconds. By setting the temperature and time within the above ranges, the residual stress of the film can be uniformly eliminated, and the productivity can be improved.

[0094] It should be noted that, when the relaxation treatment is a relaxation treatment in a tentering machine, the time when the width of the film begins to be reduced is taken as the start time, and the time when the film exits the zone with the same set temperature as the zone set temperature of the oven where the film reduction is finished is taken as the end time, and the end time - start time is set as the relaxation treatment time.

[0095] The lower limit of the micro-stretching ratio is preferably 1.5%, more preferably 1.7%, further preferably 1.8%, particularly preferably 1.9%, and most preferably 2%. The upper limit of the micro-stretching ratio is preferably 5%, more preferably 4.5%, further preferably 4%, particularly preferably 3.5%, and most preferably 3%. By setting it in the above range, the shrinkage force required to suppress warping and good flatness can be obtained. In the case of micro-stretching in a tenter, the micro-stretching ratio can be expressed by (film width after micro-stretching - film width before micro-stretching) / film width before micro-stretching × 100 (%).

[0096] The micro-stretching temperature is preferably 80° C. or higher, more preferably 85° C. or higher, further preferably 90° C. or higher, and particularly preferably 95° C. or higher. In addition, the micro-stretching temperature is preferably 150° C. or lower, more preferably 140° C. or lower, further preferably 135° C. or lower, and particularly preferably 130° C. or lower. By setting the temperature within the above range, the shrinkage force required to suppress warping and good planarity can be obtained.

[0097] The micro-stretch ratio / relaxation rate, which is the ratio of the micro-stretch ratio to the relaxation rate, is preferably 1.5 or more, more preferably 1.8 or more, further preferably 2.0 or more, and particularly preferably 2.2 or more. The micro-stretch ratio / relaxation rate is preferably 10 or less, more preferably 9 or less, further preferably 8 or less, and particularly preferably 7 or less. The difference between the micro-stretch ratio and the relaxation rate, that is, the value of the micro-stretch ratio-relaxation rate is preferably 0.8% or more, more preferably 0.9% or more, further preferably 1.0% or more, and particularly preferably 1.1% or more. By setting the above range, the shrinkage force required to suppress warping and good flatness can be obtained.

[0098] The polarizing plate has a polarizing plate protective polyester film of the present invention on at least one surface of the polarizer. Preferably, a film without birefringence such as a TAC film, an acrylic film, or a norbornene film is provided on the other surface of the polarizer. In addition, from the viewpoint of thinness, a polarizing plate without laminating any film on the other surface of the polarizer is also a preferred mode. In this case, there is no film on the other surface of the polarizer, but a coating layer may also be provided on the polarizer. As the coating layer, it may be a functional layer such as a hard coat layer, or it may be a phase difference film formed by transfer coating or coating film.

[0099] It should be noted that, when a film or coating layer other than the polarizer protective polyester film of the present invention is provided on the polarizer, the shrinkage stress of the film or coating layer other than the polarizer protective polyester film in the direction parallel to the transmission axis of the polarizer, and the shrinkage stress of the film or coating layer other than the polarizer protective polyester film in the direction parallel to the light absorption axis of the polarizer are preferably both lower than the value of the heat shrinkage rate of the polarizer protective polyester film in TD, and more preferably both lower than the value of the heat shrinkage rate of the polarizer protective polyester film in MD. In addition, in the process of manufacturing the polarizing plate, in order to maintain the heat shrinkage characteristics of the polyester resin used as the polarizer protective film, it is also preferred to minimize the thermal history.

[0100] Industrially, polarizing plates are obtained by laminating a long strip of polarizer and a long strip of polarizer protective polyester film in a roll-to-roll form with the aid of an adhesive. Moreover, polarizers are usually manufactured by stretching in the longitudinal direction, and therefore have an absorption axis in the MD direction and a transmission axis in the TD direction. The long strip of polarizer protective polyester film is preferably in the form of a roll wound on a core. In this case, the length of the long film is preferably more than 200m, more preferably more than 500m. The length of the long film is preferably less than 20000m, more preferably less than 10000m. The width of the long film is preferably more than 500mm, more preferably more than 700mm. The width of the long film is preferably less than 4000mm, more preferably less than 3000mm.

[0101] The polarizer and the polarizer protective polyester film are preferably laminated in a manner that the transmission axis of the polarizer and the TD direction of the polarizer protective polyester film are roughly parallel. Thus, in the polarizing plate, the transmission axis direction of the polarizer is preferably the a direction. That is, the direction in which the shrinkage rate of the polarizer is large is preferably roughly perpendicular to the direction in which the shrinkage rate of the polarizer protective film is large.

[0102] It should be noted that, roughly parallel here means that the angle formed by the transmission axis of the polarizer and the TD direction of the polarizer protective polyester film is preferably set to 15° or less, more preferably set to 10° or less, further preferably set to 8° or less, further more preferably set to 5° or less, particularly preferably 3° or less, and most preferably 0°, with respect to an error of 0°. In addition, roughly perpendicular is also set to the same range as above with respect to an error of 90°.

[0103] In addition, the polarizer and the polarizer protective polyester film are preferably laminated in a manner that the transmission axis of the polarizer and the slow axis of the polarizer protective polyester film are roughly parallel. Here, roughly parallel means that the angle formed by the transmission axis of the polarizer and the slow axis of the polarizer protective polyester film is preferably 0°±15° or less, more preferably 0°±10° or less, further preferably 0°±8° or less, further more preferably 0°±5° or less, particularly preferably 0°±3° or less, and most preferably 0°. The angle formed by the transmission axis of the polarizer and the slow axis of the polarizer protective polyester film is preferably as above in the entire area of ​​the polarizing plate.

[0104] The liquid crystal display device has at least a backlight source and a liquid crystal unit disposed between two polarizing plates. Preferably, at least one of the two polarizing plates is a polarizing plate in which the polarizer protective polyester film of the present invention is used as a polarizer protective film. The polarizing plates of the present invention can be used for both of the two polarizing plates of the liquid crystal display device.

[0105] The thickness of the glass substrate as a constituent member of the liquid crystal cell is preferably 0.7 mm or less, more preferably 0.6 mm or less, further preferably 0.5 mm or less, and most preferably 0.4 mm or less. The thickness of the glass substrate is preferably 0.05 mm or more, more preferably 0.1 mm or more to ensure the rigidity of the cell.

[0106] The polarizer-protecting polyester film of the present invention can be used in liquid crystal display devices of any size, but can be used in liquid crystal display devices having a diagonal length of preferably 42 inches or more, more preferably 46 inches or more, further preferably 50 inches or more, further more preferably 55 inches or more, and particularly preferably 60 inches or more.

[0107] The polarizer protective polyester film of the present invention is preferably used for the polarizer protective film on the visible side starting from the polarizer of the visible side polarizing plate and / or the polarizer protective film on the light source side starting from the polarizer of the light source side polarizing plate.

[0108] Generally, a liquid crystal display device is generally rectangular in shape, and the two polarizing plates used in the liquid crystal display device are also rectangular, and are arranged in such a way that the long side of one polarizing plate is parallel to the light absorption axis, the long side of the other polarizing plate is parallel to the transmission axis, and the light absorption axes are perpendicular to each other. In addition, generally, the polarizing plate whose long side is parallel to the light absorption axis is used as the polarizing plate on the viewing side of the liquid crystal display device, and the polarizing plate whose long side is parallel to the transmission axis is used as the polarizing plate on the light source side of the liquid crystal display device.

[0109] Generally, curling tends to occur in the long side direction. The polarizer with large shrinkage stress has a light absorption axis direction that becomes the long side, causing the polarizer to shrink and become prone to curling due to shape factors, and the asymmetric structure of the upper and lower polarizers in the liquid crystal panel. The liquid crystal panel tends to bulge toward the side of the polarizer whose transmission axis is the long side of the upper and lower polarizers arranged in the cross prism.

[0110] From the viewpoint of suppressing the warping of the liquid crystal panel, it is preferred to use the polarizing plate of the present invention at least as a polarizing plate in which the long side of the polarizing plate is parallel to the transmission axis. In addition, it is also preferred to use the polarizing plate of the present invention for both a polarizing plate in which the long side of the polarizing plate is parallel to the transmission axis and a polarizing plate in which the long side of the polarizing plate is parallel to the light absorption axis.

[0111] Example

[0112] Hereinafter, the present invention will be described in more detail with reference to the examples, but the present invention is not limited to the following examples and can be implemented with appropriate modifications within the scope consistent with the gist of the present invention, and all of them are included in the protection scope of the present invention.

[0113] (1) Thermal shrinkage of polyester film

[0114] After the polyester film was left to stand in an environment of 25°C and 50 RH% for 168 hours, a circle with a diameter of 80 mm and a straight line D with a length of about 50 mm and passing through the center of the circle and parallel to the TD direction of the film were drawn. The circle and the straight line D were drawn with a black oily ink using an ultra-fine pen with a thickness of 0.5 mm or less, and the pen pressure was kept constant so that the line was as uniform as possible. When drawing the circle, a template or a compass can be used.

[0115] The image size measuring device (Image measure IM6500 manufactured by KEYENCE) was used for continuous measurement, and the direction of the straight line D was set to 0°. The diameter of the circle was set and measured every 1°, and the length of the diameter of the circle was obtained at each angle before the heat treatment. Next, the heat treatment was performed for 30 minutes using a Gill aging oven set at 80°C, and then cooled for 10 minutes in an environment set at room temperature of 25°C, and the length of the diameter of the circle was obtained at each angle after the heat treatment in the same manner as before the treatment. It should be noted that the above treatment was performed with a polyester film for polarizer protection alone.

[0116] The heat shrinkage rate was evaluated at each angle using the following calculation formula.

[0117] Thermal shrinkage = (length before heat treatment - length after heat treatment) / length before heat treatment × 100

[0118] Thus, for the polyester film, the heat shrinkage of the polyester film in the direction where the shrinkage is the largest and in the direction orthogonal thereto, and the angle of the acute angle formed by the direction where the shrinkage is the largest and the straight line D are obtained. It should be noted that in the measurement, the measurement is performed at a total of five positions in the width direction of the slit film, namely, the two ends, the center, and the middle part between the two ends and the center, and the data at the position where T3 shows the lowest value is used as the value of T3. The data at the same position as T3 is also used for T1 and T2.

[0119] (2) Film thickness

[0120] The thickness (mm) of the polyester film was measured by using an electric micrometer (Militron 1245D manufactured by Feinpruf) after leaving the film to stand for 168 hours in an environment of 25°C and 50 RH%.

[0121] (3) Warping of LCD panels

[0122] In a Gill aging oven set at 80°C and 5%RH, the liquid crystal panel produced in each embodiment / comparative example was placed on the prisms at the four corners in a state where the panel was floating, and subjected to heat treatment for 2 hours. After that, it was cooled for 30 minutes in an environment set at room temperature of 25°C and 50%RH, and then placed on a horizontal surface with the convex side facing downward. The heights of the four corners were measured with a measuring instrument, and the maximum value was taken as the warpage amount. The warpage amount was evaluated as follows.

[0123] ○: 0 mm or more and less than 1.5 mm

[0124] ×: 1.5mm or more

[0125] (4) Light leakage evaluation method

[0126] The panel measured in the warping of the above-mentioned liquid crystal panel was installed in a liquid crystal display device, and the brightness was set to the maximum in a dark room as a white display, and the four corners of the panel were visually evaluated for light leakage. It should be noted that when the warping amount of the panel becomes more than 1.5 mm, there is a concern that it will be damaged, so the light leakage evaluation was not performed.

[0127] ○: No light leakage was detected

[0128] ×: Light leakage confirmed

[0129] (5) Refractive index and in-plane retardation (Re) of polyester film

[0130] The retardation is a parameter defined by the product (△Nxy×d) of the orthogonal biaxial refractive index anisotropy (△Nxy=|Nx-Ny|) on the film and the film thickness d (nm), and is a measure of optical isotropy and anisotropy. The biaxial refractive index anisotropy (△Nxy) is determined by the following method. The slow axis direction of the film is determined using a molecular orientation meter (MOA-6004 molecular orientation meter manufactured by Oji Instruments Co., Ltd.), and the sample is cut into a 4 cm × 2 cm rectangle in such a way that the slow axis direction is parallel to the long side of the sample for measurement. For this sample, the orthogonal biaxial refractive index (refractive index in the slow axis direction: Ny, refractive index in the direction orthogonal to the slow axis direction: Nx) and the refractive index in the thickness direction (Nz) were obtained using an Abbe refractometer (manufactured by ATAGO CO., LTD., NAR-4T, measuring wavelength 589 nm), and the absolute value of the biaxial refractive index difference (|Nx-Ny|) was taken as the anisotropy of the refractive index (ΔNxy). The retardation (Re) was obtained from the product (ΔNxy×d) of the anisotropy of the refractive index (ΔNxy) and the thickness d (nm) of the film.

[0131] (6) Thickness direction retardation (Rth)

[0132] The thickness direction retardation is a parameter that represents the average of the retardation obtained by multiplying the two birefringences ΔNxz (=|Nx-Nz|) and ΔNyz (=|Ny-Nz|) by the film thickness d when observed from a cross section in the film thickness direction. Nx, Ny, Nz and the film thickness d (nm) are obtained by the same method as the measurement of the in-plane retardation, and the average value of (ΔNxz×d) and (ΔNyz×d) is calculated to obtain the thickness direction retardation (Rth).

[0133] (Production Example 1-Polyester A)

[0134] The esterification reaction tank was heated, and when it reached 200°C, 86.4 parts by mass of terephthalic acid and 64.6 parts by mass of ethylene glycol were added, and 0.017 parts by mass of antimony trioxide, 0.064 parts by mass of magnesium acetate tetrahydrate, and 0.16 parts by mass of triethylamine as catalysts were added while stirring. Next, the pressure was increased and the temperature was increased, and the pressure esterification reaction was carried out under the conditions of a gauge pressure of 0.34 MPa and 240°C. After that, the esterification reaction tank was returned to normal pressure, and 0.014 parts by mass of phosphoric acid was added. Furthermore, the temperature was increased to 260°C in 15 minutes, and 0.012 parts by mass of trimethyl phosphate was added. Then, after 15 minutes, a high-pressure disperser was used for dispersion treatment, and after 15 minutes, the obtained esterification reaction product was transferred to a polycondensation reaction tank, and a polycondensation reaction was carried out at 280°C and under reduced pressure.

[0135] After the polycondensation reaction is completed, the mixture is filtered with a NASLON filter having a 95% cutoff diameter of 5 μm, extruded from a nozzle into strands, cooled and solidified using cooling water that has been filtered in advance (pore size: 1 μm or less), and cut into pellets. The obtained polyethylene terephthalate resin (A) has an intrinsic viscosity of 0.62 dl / g and contains substantially no inactive particles or internal precipitated particles. (Hereinafter, abbreviated as PET (A).)

[0136] (Production Example 2-Polyester B)

[0137] 10 parts by mass of the dried ultraviolet absorber (2,2'-(1,4-phenylene)bis(4H-3,1-benzoxazin-4-one) and 90 parts by mass of PET (A) (intrinsic viscosity: 0.62 dl / g) containing no particles were mixed and a mixing extruder was used to obtain a polyethylene terephthalate resin (B) containing an ultraviolet absorber. (hereinafter referred to as PET (B))

[0138] (Manufacturing Example 3-Adjustment of Adhesion-Modifying Coating Liquid)

[0139] A water-dispersible sulfonic acid metal salt-containing copolyester resin was prepared by carrying out an ester exchange reaction and a polycondensation reaction by a conventional method, wherein as dicarboxylic acid components (relative to the total dicarboxylic acid components), 46 mol% of terephthalic acid, 46 mol% of isophthalic acid and 8 mol% of sodium 5-sulfoisophthalate were used, and as diol components (relative to the total diol components), 50 mol% of ethylene glycol and 50 mol% of neopentyl glycol were used. Subsequently, 51.4 parts by mass of water, 38 parts by mass of isopropyl alcohol, 5 parts by mass of n-butyl cellosolve and 0.06 parts by mass of a nonionic surfactant were mixed, and the mixture was heated and stirred. After the mixture reached 77° C., 5 parts by mass of the water-dispersible sulfonic acid metal salt-containing copolyester resin was added, and the mixture was stirred until the lumps of the resin disappeared. The resin aqueous dispersion was then cooled to room temperature to obtain a uniform water-dispersible copolyester resin liquid having a solid content concentration of 5.0% by mass. Furthermore, after dispersing 3 parts by mass of aggregated silica particles (SYLYSIA 310 manufactured by FUJI SYLYSIA CHEMICAL LTD.) 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 water-dispersible copolyester resin solution, and 20 parts by mass of water was added while stirring to obtain an adhesion-modified coating solution.

[0140] (Example 1)

[0141] <Manufacture of Polarizer Protective Polyester Film 1>

[0142] 90 parts by weight of PET (A) resin pellets without particles and 10 parts by weight of PET (B) resin pellets containing an ultraviolet absorber, which are raw materials for the intermediate layer of the substrate film, were dried under reduced pressure (1 Torr) at 135°C for 6 hours and then supplied to extruder 2 (for the intermediate layer II). In addition, PET (A) was dried by a conventional method and supplied to extruder 1 (for the outer layer I and the outer layer III) respectively, and dissolved at 285°C. The two polymers were filtered using a filter material (nominal filtration accuracy 10 μm particles 95% cutoff) of a stainless steel sintered body, respectively, and stacked using two types of three-layer confluence blocks, extruded from a nozzle into a sheet, and then wound on a casting drum with a surface temperature of 30°C by an electrostatic casting method, cooled and solidified to produce an unstretched film. At this time, the discharge amount of each extruder was adjusted so that the thickness ratio of the I layer, the II layer, and the III layer became 10:80:10.

[0143] Next, the coating was dried by a reverse roll method to a coating weight of 0.08 g / m 2 The adhesiveness-modified coating liquid was applied to both sides of the unstretched PET film in a slurry-free manner and then dried at 80° C. for 20 seconds.

[0144] The unstretched film with the coating layer is introduced into a tenter stretching machine, and while clamping the ends of the film with a clamp, it is introduced into a hot air zone with a temperature of 105°C, stretched to 4.0 times along the TD, and then heat treated at a temperature of 180°C for 15 seconds. After that, a relaxation treatment is performed, and the film cooled to 100°C is slightly stretched in the width direction. Then, the clamps clamping the two ends of the film cooled to 60°C are opened, the edges are cut off, and the film is collected at a tension of 350N / m to collect a giant roll formed by a uniaxially oriented PET film. The obtained giant roll is divided into 2 equal parts to obtain 2 slit rolls (L (left side), R (right side)). A polarizer protective polyester film 1 with a thickness of 80μm, a width of 1500mm, and a length of 1500m is obtained from the slit roll located at L. Re is 8160nm, Rth is 9520nm, and the NZ coefficient is 1.67. The relaxation treatment conditions, micro-stretching ratio and other conditions are described in Table 1.

[0145] <Production of Liquid Crystal Panel>

[0146] On one side of the polarizer containing PVA, iodine and boron, a polarizer protective polyester film 1 is adhered in such a manner that the transmission axis of the polarizer is parallel to the TD of the polarizer protective polyester film 1. In addition, a TAC film (manufactured by Fujifilm Co., Ltd., thickness 60 μm) is adhered to the opposite side of the polarizer to prepare a polarizing plate. It should be noted that the angle difference between the maximum heat shrinkage direction of the polarizer protective polyester film 1 and the TD direction is within 25 degrees, and the a direction is set as the TD direction.

[0147] <Production of Liquid Crystal Panel>

[0148] The liquid crystal panel was taken out from a 65-inch IPS-type liquid crystal television using a glass substrate with a thickness of 0.4 mm in the liquid crystal cell. The light source side and the visible side polarizing plates were peeled off from the liquid crystal panel, and the polarizing plates prepared above were replaced by being bonded to the liquid crystal cell with a commercially available optical adhesive sheet in such a way that the transmission axis of the polarizer was aligned with the transmission axis direction of the polarizing plate before peeling, thereby preparing a liquid crystal panel.

[0149] In addition, the polarizing plate was bonded to the liquid crystal cell so that the polarizing plate protective polyester film 1 and the liquid crystal cell were on the far side (opposite side).

[0150] (Examples 2 to 11, Comparative Examples 1 to 6)

[0151] In the production of the polarizer protective polyester film, the relaxation rate, micro-stretching ratio, etc. were set as shown in Table 1. Polyester films 2 to 17 were produced in the same manner as in Example 1 to produce polarizing plates and liquid crystal panels. In film 10, Re was 6630 nm, 7735 nm, and the NZ coefficient was 1.67. Except for this, the Re, Rth, and NZ coefficient of each film were substantially the same as those of film 1.

[0152] [Table 1]

[0153]

[0154] From the results shown in Table 1, it was confirmed that the polarizing plate using the polarizer-protective polyester film of the present invention can suppress warping of the panel and light leakage as compared with the polarizing plate of the comparative example.

[0155] Industrial Applicability

[0156] According to the present invention, a polarizer protective film, a polarizing plate, and a liquid crystal display device that can suppress warping of a liquid crystal panel and light leakage can be provided. In particular, a polarizer protective polyester film, a polarizing plate, and a liquid crystal display device that can suppress warping of a liquid crystal panel and light leakage can be provided even when the liquid crystal panel is placed in a high temperature environment for a long time. Furthermore, due to the good planarity, the manufactured film can be widely used as a polarizer protective film, and an economically advantageous polarizer protective polyester film with improved productivity can be provided.

Claims

1. A polyester film for protecting a polarizer, wherein: When the heat shrinkage rate of the film in the direction a as the first direction before and after the heat treatment at 80°C is defined as T1(%) and the heat shrinkage rate of the film in the same direction as the direction a before and after the heat treatment at 150°C is defined as T2(%), the following conditional expressions (1) and (2) are satisfied: T1≥0.2％···(1) 1.0 <T2 / T1≤5.0···(2)。 2. The polarizer protective polyester film according to claim 1, wherein When the heat shrinkage rate of the film before and after the 80°C heat treatment in the b direction which is a direction perpendicular to the a direction is defined as T3 (%), the following conditional expressions (3) and (4) are satisfied: T3≥0.06％···(3) 1.0 <T1 / T3≤6.0···(4)。 3. The polarizing plate protective polyester film according to claim 1 or 2, which has a retardation of 3000 to 30000 nm. 4 . The polarizing plate protective polyester film according to claim 1 , which has a thickness of 25 to 200 μm.

5. The polarizer-protecting polyester film according to any one of claims 1 to 4, wherein The polarizing plate protective polyester film has at least one layer selected from the group consisting of a hard coat layer, an antireflection layer, a low reflection layer, an antiglare layer and an antireflection antiglare layer on at least one surface. 6 . A polarizing plate comprising the polarizer-protective polyester film according to claim 1 on at least one surface of a polarizer. 7 . An image display device comprising the polarizing plate according to claim 6 . A liquid crystal display device comprising the polarizing plate according to claim 6.

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