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

Abstract

This polyester film is for protecting a polarizer, and satisfies conditional expressions (1) and (2), where T1 represents the heat shrinkage rate (%) in direction 'a' that is a first direction, as observed between before and after the film has undergone a heat treatment at 80°C, and T2 represents the heat shrinkage rate (%) in the same direction as the direction 'a', as observed between before and after the film has undergone a heat treatment at 150°C.　(1): T1≥0.2% (2): 1.0<T2 / T1≤5.0

Description

Polyester film for protecting polarizers, polarizing plates and liquid crystal displays

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

[0002] Demand for liquid crystal display devices is expanding for applications such as liquid crystal televisions and liquid crystal displays for personal computers. Typically, a liquid crystal display device is composed of a liquid crystal cell in which transparent electrodes, a liquid crystal layer, a color filter, and the like are sandwiched between glass plates, and two polarizing plates provided on both sides of the cell, and each polarizing plate is configured such that a polarizer (also called a polarizing film) is sandwiched between two optical films (for example, a polarizer protective film and a retardation film).

[0003] In recent years, as LCD TV screens have become larger and thinner, LCD panels have become warped, resulting in uneven display. In particular, the rigidity of the glass substrates used in LCD panels varies depending on their thickness, and minute contractions in polarizers can cause LCD panel warping and result in uneven display.

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

[0005] When the thickness of the glass substrate is reduced to less than 0.7 mm in order to further reduce the thickness of liquid crystal panels, the problem of display unevenness occurs, and an improvement in this area is required. The mechanism by which display unevenness occurs is thought to be mainly due to the shrinkage of the polarizer, and when the polarizer is placed under high temperature and humidity, a shrinkage force acts in the orientation direction as it tries to relax the orientation, resulting in warping of the liquid crystal panel and swelling toward the backlight unit, which causes display unevenness.

[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 polyester film for protecting a polarizer, which is laminated on one surface of the polarizer, within a specific range.

[0007] WO2019 / 054406

[0008] As liquid crystal panels become larger and glass substrates of liquid crystal cells become thinner, the inventors have discovered that increasing the shrinkage stress of a polyester film for protecting a polarizer is effective in suppressing warpage of the liquid crystal panel.

[0009] When a large shrinkage stress is required, one effective measure is to increase the heat shrinkage rate of the polyester film for protecting a polarizer at 80°C. However, when a polyester film for protecting a polarizer with an increased heat shrinkage rate is used, slight light leakage may occur. Furthermore, even if there is no light leakage initially, light leakage may occur and visibility may deteriorate when the liquid crystal panel is placed in a high-temperature environment for a long period of time. These problems are particularly likely to occur in liquid crystal display devices of 50 types or larger (diagonal length of 50 inches or larger).

[0010] The present invention has been made in view of the above-mentioned problems and circumstances, and aims 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 while simultaneously suppressing light leakage, particularly, 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 period of time.

[0011] Representative aspects of the present invention are as follows. Item 1: A polyester film for protecting a polarizer, which satisfies the following conditional expressions (1) and (2), where T1 (%) is the heat shrinkage of the film in a direction a, which is a first direction, before and after a heat treatment at 80°C, and T2 (%) is the heat shrinkage of the film in the same direction as the a direction, before and after a heat treatment at 150°C. T1≧0.2% (1) 1.0<T2 / T1≦5.0 (2) Item 2: The polyester film for protecting a polarizer according to Item 1, which satisfies the following conditional expressions (3) and (4), where T3 (%) is the heat shrinkage of the film in a direction b, which is a direction perpendicular to the a direction, before and after a heat treatment at 80°C. T3≧0.06% (3) 1.0<T1 / T3≦6.0 (4) Item 3: The polyester film for protecting a polarizer according to Item 1 or 2, having a retardation of 3,000 to 30,000 nm. Item 4: The polyester film for protecting a polarizer according to any one of Items 1 to 3, having a thickness of 25 to 200 μm. Item 5: The polyester film for protecting a polarizer according to any one of Items 1 to 4, having at least one layer selected from a hard coat layer, an antireflection layer, a low reflection layer, an antiglare layer, and an antireflection and antiglare layer on at least one surface of the polyester film for protecting a polarizer. Item 6: A polarizing plate having the polyester film for protecting a polarizer according to any one of Items 1 to 5 on at least one surface of a polarizer. Item 7: An image display device having the polarizing plate according to Item 6. Item 8: A liquid crystal display device having the polarizing plate according to Item 6.

[0012] According to the present invention, it is possible to provide a polarizer protective film, a polarizing plate, and a liquid crystal display device that can suppress warping of a liquid crystal panel while simultaneously suppressing light leakage. In particular, it is possible 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 while simultaneously suppressing light leakage even when the liquid crystal panel is placed in a high-temperature environment for a long period of time. Furthermore, it is possible to provide an economically advantageous polyester film for protecting a polarizer with improved productivity.

[0013] The polyester film for protecting a polarizer of the present invention is made of a polyester film and is laminated on at least one surface of a polarizer to become a constituent member of a polarizing plate, and is generally called a polarizer protective film. Hereinafter, the polyester film for protecting a polarizer may be simply referred to as a polyester film.

[0014] First, the inventors hypothesized that the following may be the cause of the phenomenon described above. For example, when a polyester film is largely stretched in a uniaxial stretching direction with the transverse direction as the main stretching direction, stress is also applied in the longitudinal direction due to Poisson contraction. However, because the film edges are held by clips while the center is free, the strain caused by the stress is likely to be uneven. When the strain is uneven, the residual stress after main stretching and heat setting is also likely to be uneven. If slight stretching is performed after main stretching under such conditions, the influence of the residual stress is significant, resulting in poor flatness.

[0015] The polarizer protective film is subjected to surface treatment such as an antireflection layer as necessary, and is then bonded to a polarizer to form a polarizing plate, which is then bonded to a liquid crystal cell to form a liquid crystal panel. During these processes, a certain amount of tension is applied. Polarizer protective films with poor flatness are subjected to uneven tension, and polarizer protective films with uneven residual stress may be distorted by heat or other factors during the above-mentioned processing. In such liquid crystal panels, the crossed-Nicol relationship between the two polarizing plates at the edges becomes misaligned, and the misalignment becomes particularly large due to uneven shrinkage over time, resulting in light leakage.

[0016] When producing a film to increase productivity, a wide film is produced and then divided into two or three widthwise to match the required width of the polarizer protective film to produce a slit roll film. However, when the film is divided into two or three, the flatness of the slit roll portions at both ends tends to deteriorate, making light leakage more likely.

[0017] As a countermeasure based on the above assumption, the inventors have completed the present invention, thinking that the improvement can be achieved by slightly stretching the film after sufficiently relaxing and eliminating the stress generated by the main stretching.

[0018] Specifically, by focusing on the relaxation treatment step, which was not considered important in the prior art, relaxation treatment is performed at an appropriate temperature and magnification after stretching and heat setting, and residual stress is sufficiently removed before slight stretching, a film with excellent flatness and excellent shrinkage characteristics at 80°C can be obtained, and a polarizer protective film that does not cause light leakage or panel warping can be obtained.

[0019] In the polyester film for protecting a polarizer of the present invention, when the first direction is the a-direction and the heat shrinkage rate of the polyester film in the a-direction before and after heat treatment at 80°C is T1, the value of T1 is preferably 0.20% or more. By setting T1 to 0.20% or more, warping of the liquid crystal panel can be effectively suppressed. T1 is more preferably 0.21% or more, and even more preferably 0.23% or more. Furthermore, by setting T1 to 0.50% or less, the phenomenon in which the liquid crystal panel warps in the opposite direction due to excessively strong shrinkage stress can be suppressed. T1 is more preferably 0.45% or less, and even more preferably 0.43% or less. The a-direction may be the direction having the larger 80°C heat shrinkage rate among the TD direction and the MD direction. Furthermore, when the TD direction is unknown, it may be the direction having the largest 80°C heat shrinkage rate among the slow axis direction or the direction perpendicular to the slow axis direction. Alternatively, it may be the direction having the largest shrinkage rate.

[0020] Since a film obtained by stretching polyvinyl alcohol in the MD direction is often used as a polarizer, the a-direction is preferably the TD direction. Furthermore, since stretching in the TD direction using a tenter is preferred for stably producing a polyester film with high retardation, the a-direction is preferably the slow axis direction.

[0021] The slow axis direction can be determined using a molecular orientation meter (for example, MOA-6004 molecular orientation meter manufactured by Oji Scientific Instruments Co., Ltd.) etc. The direction with the largest shrinkage rate can be determined by the measurement method in the examples.

[0022] In the polyester film for protecting a polarizer of the present invention, the heat shrinkage rate of the polyester film in the a direction before and after heat treatment at 150°C is defined as T2, and the T2 value is preferably 2.00% or less. By setting the T2 value to 2.00% or less, when a polarizing plate is manufactured through a polarizing plate processing process, typically a roll-to-roll process, lamination is facilitated and light leakage can be suppressed. T2 is more preferably 1.95% or less, and even more preferably 1.90% or less. Furthermore, T2 is preferably 0.50% or more. By setting it to 0.50% or more, cell warpage can be effectively suppressed. T2 is more preferably 0.55% or more, and even more preferably 0.60% or more.

[0023] In the polyester film for protecting a polarizer of the present invention, the direction perpendicular to the a direction is defined as the b direction, and the heat shrinkage rate of the film in the b direction before and after heat treatment at 80°C is defined as T3. By setting T3 to 0.06% or more, the film can have excellent flatness and suppress leakage. T3 is more preferably 0.07% or more, even more preferably 0.08% or more, and particularly preferably 0.09% or more. Furthermore, by setting T3 to 0.20% or less, warping of the liquid crystal panel can be effectively suppressed, which is preferable. T3 is more preferably 0.18% or less, and even more preferably 0.16% or less.

[0024] While the shrinkage rate in the MD direction is likely to vary depending on the position in the TD direction of the film, the shrinkage rate in the TD direction is unlikely to vary depending on the position in the TD direction. For this reason, T1, T2, and T3 are measured at a total of five locations in the width direction of the polyester film for protecting a polarizer: both ends, the center, and an intermediate portion midway between the center and both ends. The values ​​of T1, T2, and T3 at the location where the T3 value is smallest are adopted. Note that, for both ends, if the film ends are knurled, the distance between the film end side of the circumference of the circle used to measure the thermal shrinkage rate described in the Examples and the inner edge of the knurl is 1 cm. If the film ends are not knurled, the distance between the film end side of the circumference and the film end is 1 cm. It is preferable to measure the five locations so that the measurement locations are aligned parallel to the TD direction. However, if the circles overlap, the measurement positions may be shifted alternately in the MD direction so that the circles do not overlap.

[0025] In addition, when the polyester film is cut into sheets and the TD direction is unknown, the thermal shrinkage rates at 80°C and 150°C are measured at the above-mentioned five locations in both the long and short side directions of the cut rectangular film, and the direction in which the difference in thermal shrinkage rate is greatest between the long and short side directions can be determined to be the TD direction.

[0026] If the ratio of the heat shrinkage rates T2 and T1 is defined as T2 / T1, T2 / T1 is considered to be an index representing the extent to which the residual stress caused by the main stretching has been relaxed. The residual stress appears as strain in the MD and TD directions during the micro-stretching process after the main stretching, but direct evaluation is difficult based on the film properties alone. After the main stretching, the film is oriented and crystallized in the heat setting process, and then subjected to an appropriate relaxation treatment, which can reduce the strain and residual stress caused by the main stretching, and as a result, the heat shrinkage rate at 150°C is considered to be reduced. Therefore, T2 / T1 is considered to be an effective index for estimating the effect of the relaxation treatment.

[0027] T2 / T1 is preferably 5.00 or less, more preferably 4.8 or less, even more preferably 4.7 or less, particularly preferably 4.6 or less, and most preferably 4.5 or less. By making it less than the above, the relaxation treatment before micro-stretching is sufficiently carried out, and distortion and residual stress are relaxed and eliminated, resulting in good flatness. Furthermore, T2 / T1 is preferably more than 1.0, more preferably 1.5 or more, even more preferably 2.0 or more, particularly preferably 2.5 or more, and most preferably 3.0 or more. By making T2 / T1 more than 1.0, the film has excellent flatness and light leakage can be suppressed.

[0028] If the ratio of the thermal shrinkage rates T1 and T3 is T1 / T3, optimizing T1 / T3 can suppress light leakage and cell warpage with even greater precision. For example, if 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 relaxation, but the residual stress in the MD direction is not completely eliminated, and when heated, the film may still have a shrinkage rate in the MD direction. On the other hand, the shrinkage rate in the MD direction tends to decrease during TD micro-stretching, which is presumed to indicate MD shrinkage during micro-stretching. TD micro-stretching is performed to obtain the TD shrinkage rate necessary to suppress cell warpage, but if the micro-stretching is too large, MD shrinkage during micro-stretching will proceed too much, presumably damaging the flatness of the film. From this, it is believed that T1 / T3 indicates whether the micro-stretching was appropriate for the film before micro-stretching.

[0029] In the polyester film for protecting a polarizer of the present invention, T1 / T3 is preferably 6.0 or less, more preferably 5.5 or less, even more preferably 5.2 or less, particularly preferably 5.0 or less, and most preferably 4.8 or less. By setting it to the above range or less, the flatness becomes better.

[0030] Furthermore, T1 / T3 is preferably greater than 1, more preferably 1.2 or greater, even more preferably 1.5 or greater, particularly preferably 1.8 or greater, and most preferably 2.0 or greater. By setting it within the above range, warpage of the liquid crystal panel can be effectively reduced.

[0031] Although the problems and effects of the present invention and the relationships between T2 / T1 and T1 / T3 have been described above, including estimates, the present invention is not limited to the scope of the above description.

[0032] Typically, in a liquid crystal display device, two polarizing plates are arranged in a crossed Nicol relationship. When two polarizing plates are arranged in a crossed Nicol relationship, light does not usually pass through the two polarizing plates. However, the shrinkage or warping of the polarizer described above can result in the complete crossed Nicol relationship being disrupted, which can lead to 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 maximum and the transmission axis of the polarizer is preferably about 25 degrees or less, more preferably 15 degrees or less, even more preferably 10 degrees or less, particularly preferably 5 degrees or less, and most preferably 3 degrees or less.

[0033] A commonly used polarizer is a film obtained by stretching polyvinyl alcohol containing a dichroic dye such as iodine in the MD direction, and such a polarizer shrinks in the MD direction. Based on the idea that a polarizer and a polarizer protective film are bonded together in a roll-to-roll manner and further sandwiched between a cell so that shrinkage of the polarizer on one side is offset by shrinkage of the polarizer protective film on the other side, the angle between the direction in which the heat shrinkage rate of the polarizer protective film is maximum and the TD direction of the polarizer protective film is preferably about 10 degrees or less, more preferably 7 degrees or less, even more preferably 5 degrees or less, particularly preferably 3 degrees or less, and most preferably 2 degrees or less.

[0034] The polyester film for protecting a polarizer of the present invention has a thickness of preferably 25 μm, more preferably 30 μm, even more preferably 35 μm, and particularly preferably 40 μm, and an upper limit of preferably 200 μm, more preferably 150 μm, even more preferably 100 μm, particularly preferably 90 μm, and most preferably 85 μm.

[0035] By setting the thickness of the polyester film within the above range, the film is less likely to crack, and by maintaining its rigidity, it can have good flatness and is less likely to break during film formation. Furthermore, the shrinkage stress of the film is not too large, so the variation in shrinkage stress can be reduced, making it easier to control, and costs can be reduced. The thickness of the polyester film can be measured by the method used in the examples described below.

[0036] The polyester film for protecting a polarizer of the present invention preferably has an in-plane retardation within a specific range from the viewpoint of suppressing rainbow spots observed on the screen of a liquid crystal display device. The in-plane retardation is preferably 3,000 nm or more, more preferably 5,000 nm or more, even more preferably 6,000 nm or more, particularly preferably 7,000 nm or more, and most preferably 8,000 nm or more. The in-plane retardation is preferably 30,000 nm or less, more preferably 18,000 nm or less, even more preferably 15,000 nm or less, particularly preferably 12,000 nm or less, and most preferably 10,000 nm or less. In particular, from the viewpoint of thinning, the in-plane retardation is preferably less than 10,000 nm, and preferably 9,000 nm or less. Note that hereinafter, the in-plane retardation may be referred to as Re.

[0037] The retardation of the polyester film can be determined by measuring the refractive index in the biaxial direction and the thickness, 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 by an Abbe refractometer (measurement wavelength: 589 nm).

[0038] In the polyester film for protecting a polarizer of the present invention, when the retardation in the thickness direction is Rth, Re / Rth, which is the ratio of the in-plane retardation to the retardation in the thickness direction, is preferably 0.2 or more, preferably 0.3 or more, preferably 0.4 or more, more preferably 0.5 or more, and even more preferably 0.6 or more. The larger Re / Rth, the more isotropic the birefringence action becomes, and the less likely it is that rainbow-like color spots will occur depending on the observation angle. Since Re / Rth is 2.0 in a completely uniaxial (uniaxially symmetric) film, the upper limit of Re / Rth is preferably 2.0. The upper limit of Re / Rth is more preferably 1.2 or less, even more preferably 1 or less, particularly preferably 0.98 or less, and most preferably 0.97 or less. The thickness direction retardation refers to the average retardation obtained by multiplying the two birefringences ΔNxz and ΔNyz by the film thickness d when the film is viewed from a cross section in the thickness direction.

[0039] In order to further suppress rainbow color spots, the polyester film for protecting a polarizer of the present invention preferably has an NZ coefficient of 2.5 or less, more preferably 2.0 or less, even more preferably 1.8 or less, and even more preferably 1.6 or less. Since the NZ coefficient of a completely uniaxial (uniaxially symmetric) film is 1.0, the lower limit of the NZ coefficient is 1.0. However, it should be noted that as the film approaches a completely uniaxial (uniaxially symmetric) film, the mechanical strength in the direction perpendicular to the orientation direction tends to decrease significantly. The lower limit of the NZ coefficient is preferably 1.1, more preferably 1.2, even more preferably 1.3, particularly preferably 1.4, and most preferably 1.45.

[0040] The NZ coefficient is expressed as |Ny-Nz| / |Ny-Nx|, where Ny is the refractive index in the slow axis direction of the polyester film, Nx is the refractive index in the direction perpendicular to the slow axis (refractive index in the fast axis direction), and Nz is the refractive index in the thickness direction. The orientation axis of the film is determined using a molecular orientation meter (Oji Scientific Instruments, MOA-6004 molecular orientation meter), and the biaxial refractive indices (Ny, Nx, where Ny > Nx) in the orientation axis direction and the direction perpendicular thereto, and the refractive index in the thickness direction (Nz) are determined using an Abbe refractometer (Atago Co., Ltd., NAR-4T, measurement wavelength 589 nm). The NZ coefficient can be calculated by substituting the values ​​thus determined into |Ny-Nz| / |Ny-Nx|.

[0041] In order to further suppress iridescent color spots, the polyester film of the present invention preferably has an Ny-Nx value of 0.05 or more, more preferably 0.07 or more, even more preferably 0.08 or more, still more preferably 0.09 or more, and most preferably 0.1 or more. Although there is no particular upper limit, in the case of a polyethylene terephthalate film, the upper limit is preferably about 1.5, more preferably 1.2 or less, and even more preferably 1.1.

[0042] The present invention can also be applied to polyester films having an in-plane retardation of less than 3000 nm. When the in-plane retardation is less than 3000 nm, rainbow spots may occur. However, 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 noticeable. To obtain a film with stable optical properties while maintaining a preferred film thickness as a polarizer protective film, the in-plane retardation is preferably 100 nm or more, more preferably 200 nm or more, and even more preferably 300 nm or more.

[0043] The polyester film for protecting a polarizer of the present invention can be obtained from any polyester resin. The type of polyester resin is not particularly limited, and any polyester resin obtained by condensing a dicarboxylic acid and a diol can be used.

[0044] Examples of dicarboxylic acid components that can be used in the production of polyester resins include terephthalic acid, isophthalic acid, orthophthalic acid, 2,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, diphenylcarboxylic acid, diphenoxyethanedicarboxylic acid, diphenylsulfonecarboxylic acid, anthracenedicarboxylic acid, 1,3-cyclopentanedicarboxylic acid, 1,3-cyclohexanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, hexahydroterephthalic acid, hexahydroisophthalic acid, malonic acid, dimethylmalonic acid, succinic acid, 3,3-diethylsuccinic acid, glutaric acid, 2,2-dimethylglutaric acid, adipic acid, 2-methyladipic acid, trimethyladipic acid, pimelic acid, azelaic acid, dimer acid, sebacic acid, suberic acid, and dodecadicarboxylic acid.

[0045] Examples of diol components that can be used in producing polyester resins include ethylene glycol, propylene glycol, hexamethylene glycol, neopentyl glycol, 1,2-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, decamethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexadiol, 2,2-bis(4-hydroxyphenyl)propane, and bis(4-hydroxyphenyl)sulfone.

[0046] The dicarboxylic acid component and diol component constituting the polyester resin can each be one or more types. Suitable polyester resins constituting the polyester film include, for example, polyethylene terephthalate, polypropylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate. More preferred examples include polyethylene terephthalate and polyethylene naphthalate, but these may further contain other copolymer components. These resins have excellent transparency and also excellent thermal and mechanical properties. In particular, polyethylene terephthalate is a suitable material because it can achieve a high elastic modulus and its heat shrinkage rate can be relatively easily controlled.

[0047] When it is necessary to highly increase the heat shrinkage of a polyester film, it is desirable to add a copolymer component to moderately reduce the crystallinity. Furthermore, since the rate of elastic strain and permanent set is high with respect to deformation below the glass transition temperature, it is generally difficult to highly increase the heat shrinkage. Therefore, it is a preferred embodiment to incorporate a component with a low glass transition temperature as needed. Specific examples include propylene glycol and 1,3-propanediol.

[0048] (Providing an Adhesion-Enhancing Layer) The polyester film for protecting a polarizer may be subjected to corona treatment, coating treatment, flame treatment, or the like in order to improve adhesion to the polarizer.

[0049] In order to improve adhesion to a functional layer such as a hard coat layer or a polarizer, the polyester film preferably has an easy-adhesion layer on at least one surface thereof. The polyester film having such an easy-adhesion layer is also included in the polyester film for protecting a polarizer of the present invention.

[0050] It is preferable that at least one side of the polyester film has an easy-adhesion layer mainly composed of at least one of polyester resin, polyurethane resin, and polyacrylic resin. Here, "main component" refers to a component that accounts for 50% by mass or more of the solid components constituting the easy-adhesion layer. The coating liquid used to form the easy-adhesion layer is preferably an aqueous coating liquid containing at least one of water-soluble or water-dispersible copolymer polyester resin, acrylic resin, and polyurethane resin. Examples of such coating liquids include water-soluble or water-dispersible copolymer polyester resin solutions, acrylic resin solutions, and polyurethane resin solutions disclosed in Japanese Patent No. 3,567,927, Japanese Patent No. 3,589,232, Japanese Patent No. 3,589,233, Japanese Patent No. 3,900,191, and Japanese Patent No. 4,150,982.

[0051] The easy-adhesion layer can be obtained by applying the coating liquid to one or both sides of an unstretched film or a uniaxially stretched film in the machine direction, drying the film at 100 to 150°C, and then stretching the film in the transverse direction. The final coating amount of the easy-adhesion layer is 0.05 to 0.20 g / m from the viewpoint of achieving both adhesiveness and blocking resistance.２ When the easy-adhesion layer is provided on both sides of the polyester film, the coating amounts of the easy-adhesion layer on both sides may be the same or different, and can be set independently within the above ranges.

[0052] It is preferable to add particles to the adhesion layer to impart slipperiness. The average particle size of the fine particles is preferably 2 μm or less to prevent the particles from falling off the coating layer. Examples of particles to be contained in the adhesion layer include inorganic particles such as titanium oxide, barium sulfate, calcium carbonate, calcium sulfate, silica, alumina, talc, kaolin, clay, calcium phosphate, mica, hectorite, zirconia, tungsten oxide, lithium fluoride, and calcium fluoride, and organic polymer particles such as styrene-based, acrylic-based, melamine-based, benzoguanamine-based, and silicone-based particles. These may be added to the adhesion layer alone or in combination of two or more.

[0053] The coating solution can be applied by any known method, such as reverse roll coating, gravure coating, kiss coating, roll brushing, spray coating, air knife coating, wire bar coating, or pipe doctor coating, which can be used alone or in combination.

[0054] The average particle size of the particles is measured by the following method.

[0055] The particles are photographed using a scanning electron microscope (SEM), and the maximum diameters (the distance between the two most distant points) of 300 to 500 particles are measured at a magnification such that the size of the smallest particle is 2 to 5 mm, and the average value is taken as the average particle size.

[0056] (Providing a Functional Layer) The polyester film for protecting a polarizer of the present invention may have, on at least one surface thereof, one or more functional layers such as a hard coat layer, an antiglare layer, an antireflection layer, a low reflection layer, a low antireflection layer, an antireflection antiglare layer, a low reflection antiglare layer, and an antistatic layer. These functional layers are preferably provided on the surface of the polyester film opposite to the surface on which the polarizer is laminated.

[0057] A polarizing plate using a polyester film for protecting a polarizer is preferably integrated with a glass plate of a liquid crystal cell while maintaining the heat shrinkage rates T1, T2, and T3 and the ratios T2 / T1 and T1 / T3 of the polyester film within the above-described conditions. Therefore, when these functional layers are applied, it is desirable to set the drying temperature low or to use a method that minimizes thermal history, such as UV irradiation or electron beam irradiation. It is also desirable to apply these functional layers during the film-forming process of the polyester film. That is, in the present invention, it is also desirable that the heat shrinkage rates T1, T2, and T3 and the ratios T2 / T1 and T1 / T3 of the polyester film be within the above-described conditions in a laminate in which a functional layer is laminated on a polyester film for protecting a polarizer.

[0058] (Method of Manufacturing Polyester Film) The polyester film for protecting a polarizer of the present invention can be manufactured according to a general method of manufacturing a polyester film. For example, a polyester resin is melted, and the non-oriented polyester is extruded into a sheet. The non-oriented polyester is stretched in the machine direction, which is the film flow direction during film formation, at a temperature equal to or higher than the glass transition temperature by utilizing the speed difference between rolls, and then stretched in the transverse direction, which is the direction perpendicular to the machine direction, using a tenter, and then heat-treated. The film may be uniaxially or biaxially stretched. When a polyester film having an in-plane retardation of 3,000 to 30,000 nm is to be obtained, the film is preferably a uniaxially stretched film that is strongly stretched mainly in the transverse direction, and may also be slightly stretched in a direction perpendicular to the main stretching direction. The machine direction is also called the film flow direction, longitudinal direction, machine direction, or MD direction, where MD stands for Machine Direction. The transverse direction is also called the width direction or TD direction, where TD stands for Transverse Direction.

[0059] Specifically, the film-forming conditions for the polyester film are preferably 80 to 130°C, and particularly preferably 90 to 120°C, for the longitudinal and transverse stretching temperatures. To control the retardation within the above range, it is preferable to control the ratio between the longitudinal and transverse stretching magnifications and increase one of the stretching magnifications. For example, when the slow axis is oriented in the TD direction, the longitudinal stretching magnification is preferably 1.0 to 3.5 times, more preferably 3.0 times or less, even more preferably 2.5 times or less, particularly preferably 2.0 times or less, and most preferably 1.5 times or less. To obtain a film with a low deviation of the slow axis from the TD direction or a film with uniform heat shrinkage properties in the TD direction, the longitudinal stretching ratio is preferably 1.2 times or less, and even more preferably 1.1 times or less. By adopting the above-described method, it is also possible to reduce the waste portion at the widthwise end of the film.

[0060] The transverse draw ratio is preferably 2.5 to 6.0 times, more preferably 3.0 to 5.7 times, and 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 even more preferably 5 to 30 seconds.

[0061] When a polyester film having an in-plane retardation of less than 3000 nm is to be produced, the preferable range of the longitudinal stretching ratio is the same as the above-mentioned range of the transverse stretching ratio.

[0062] After stretching and heat setting, it is preferable to carry out a relaxation treatment and then slightly stretch in the direction in which it is desired to impart a shrinkage ratio. The direction of the slightly stretching is preferably the a-direction.

[0063] 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, even more preferably 0.35% or more, particularly preferably 0.38% or more, and most preferably 0.4% or more. It is also preferably 2% or less, more 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 it within the above range, the heat shrinkage rate at 80°C can be kept within an appropriate range without excessively increasing the micro-stretching ratio, thereby achieving good flatness. In the case of stretching and relaxation treatment using a tenter, the relaxation rate can be expressed as (film width at the start of heat setting - film width at the end of the relaxation step) / film width at the start of heat setting x 100 (%).

[0064] The temperature of the relaxation treatment is preferably below the heat setting temperature and above the slight stretching temperature. The upper limit of the temperature of the relaxation treatment may be the heat setting temperature -5°C, the heat setting temperature -10°C, the heat setting temperature -15°C, or the heat setting temperature -20°C. The lower limit may be the slight stretching temperature +5°C, the slight stretching temperature +10°C, the slight stretching temperature +15°C, or the slight stretching temperature +20°C. A suitable temperature range can be determined according to the heat setting temperature and the slight stretching temperature. The temperature of the relaxation treatment does not need to be a constant temperature, and may be performed during the cooling process from the heat setting temperature to the slight stretching temperature. In this case, it is preferable that the temperature at the start and end of the relaxation treatment be within the above range.

[0065] The relaxation treatment time is preferably 1 to 60 seconds, more preferably 1.5 to 40 seconds, and even more preferably 2.0 to 30 seconds. By setting the temperature and time within the above ranges, the residual stress in the film can be uniformly eliminated, improving productivity.

[0066] In the case of relaxation treatment using a tenter, the start time is the time when the film starts to narrow its width, and the end time is the time when the film exits the zone with the same set temperature as the oven zone set temperature at the point where narrowing is completed, and the relaxation treatment time is the end time minus the start time.

[0067] The lower limit of the micro-stretching ratio is preferably 1.5%, more preferably 1.7%, even more 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%, even more preferably 4%, particularly preferably 3.5%, and most preferably 3%. By setting it within the above range, the shrinkage force necessary to suppress warping and good flatness can be obtained. In the case of micro-stretching using a tenter, the micro-stretching ratio can be expressed as (film width after micro-stretching - film width before micro-stretching) / film width before micro-stretching × 100 (%).

[0068] The temperature for the slight stretching is preferably 80° C. or higher, more preferably 85° C. or higher, even more preferably 90° C. or higher, and particularly preferably 95° C. or higher. The temperature for the slight stretching is preferably 150° C. or lower, more preferably 140° C. or lower, even more preferably 135° C. or lower, and particularly preferably 130° C. or lower. By setting the temperature within the above range, a shrinkage force necessary to suppress warping and good flatness can be obtained.

[0069] The ratio of the micro-stretching ratio to the relaxation rate, that is, the micro-stretching ratio / relaxation rate, is preferably 1.5 or more, more preferably 1.8 or more, even more preferably 2.0 or more, and particularly preferably 2.2 or more. The micro-stretching ratio / relaxation rate is preferably 10 or less, more preferably 9 or less, even more preferably 8 or less, and particularly preferably 7 or less. The difference between the micro-stretching ratio and the relaxation rate, that is, the value of micro-stretching ratio - relaxation rate, is preferably 0.8% or more, more preferably 0.9% or more, even more preferably 1.0% or more, and particularly preferably 1.1% or more. By setting it within the above range, a shrinkage force necessary to suppress warping and good flatness can be obtained.

[0070] The polarizing plate has the polyester film for protecting a polarizer of the present invention on at least one surface of the polarizer. It is preferable that the other surface of the polarizer has a film that does not have birefringence, such as a TAC film, an acrylic film, or a norbornene film. A polarizing plate having no film laminated on the other surface of the polarizer is also a preferred embodiment from the viewpoint of thinness. In this case, no film is present on the other surface of the polarizer, but a coating layer may be provided on the polarizer. The coating layer may be a functional layer such as a hard coat layer, or a retardation film formed by coating or by transferring a coating film.

[0071] When a film or coating layer other than the polyester film for protecting a polarizer of the present invention is provided on a polarizer, the shrinkage stress of the film or coating layer other than the polyester film for protecting a polarizer in the direction parallel to the transmission axis of the polarizer and the shrinkage stress of the film or coating layer other than the polyester film for protecting a polarizer in the direction parallel to the absorption axis of the polarizer are both preferably equal to or less than the value of the heat shrinkage percentage in the TD of the polyester film for protecting a polarizer, more preferably equal to or less than the value of the heat shrinkage percentage in the MD of the polyester film for protecting a polarizer. Also, in the process of producing a polarizing plate, it is preferable to minimize the thermal history as much as possible in order to maintain the heat shrinkage properties of the polyester resin used as the polarizer protective film.

[0072] Industrially, polarizing plates are produced by laminating a long polarizer and a long polarizer-protecting polyester film via an adhesive in a roll-to-roll manner. Since polarizers are usually produced by stretching them in the machine direction, they have an absorption axis in the machine direction and a transmission axis in the transverse direction. The long polarizer-protecting polyester film is preferably in the form of a roll wound around a core. In this case, the length of the long film is preferably 200 m or more, more preferably 500 m or more. The length of the long film is preferably 20,000 m or less, more preferably 10,000 m or more. The width of the long film is preferably 500 mm or more, more preferably 700 mm or more. The width of the long film is preferably 4,000 mm or less, more preferably 3,000 mm or less.

[0073] The polarizer and the polyester film for protecting a polarizer are preferably laminated so that the transmission axis of the polarizer is approximately parallel to the TD direction of the polyester film for protecting a polarizer. For this reason, the transmission axis direction of the polarizer in the polarizing plate is preferably the a-direction. That is, it is preferable that the direction in which the polarizer has a large shrinkage rate and the direction in which the polarizer protective film has a large shrinkage rate are approximately perpendicular to each other.

[0074] Here, "substantially parallel" means that the angle formed by the transmission axis of the polarizer and the TD direction of the polyester film for protecting a polarizer has an error from 0° of preferably 15° or less, more preferably 10° or less, even more preferably 8° or less, still more preferably 5° or less, particularly preferably 3° or less, and most preferably 0°. Similarly, "substantially perpendicular" means that the error from 90° is in the same range as above.

[0075] The polarizer and the polyester film for protecting a polarizer are preferably laminated so that the transmission axis of the polarizer and the slow axis of the polyester film for protecting a polarizer are substantially parallel. Here, "substantially parallel" means that the angle formed by the transmission axis of the polarizer and the slow axis of the polyester film for protecting a polarizer is preferably 0°±15° or less, more preferably 0°±10° or less, even more preferably 0°±8° or less, still 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 polyester film for protecting a polarizer is preferably as described above throughout the entire polarizing plate.

[0076] The liquid crystal display device has at least a backlight source and a liquid crystal cell disposed between two polarizing plates. At least one of the two polarizing plates is preferably a polarizing plate that uses the polyester film for protecting a polarizer of the present invention as a polarizer protective film. The liquid crystal display device may use the polarizing plate of the present invention as both of the two polarizing plates.

[0077] The thickness of the glass substrate, which is a component of the liquid crystal cell, is preferably 0.7 mm or less, more preferably 0.6 mm or less, even more preferably 0.5 mm or less, and most preferably 0.4 mm or less. In order to ensure the rigidity of the cell, the thickness of the glass substrate is preferably 0.05 mm or more, more preferably 0.1 mm or more.

[0078] The polyester film for protecting a polarizer of the present invention can be used in liquid crystal display devices of any size, but can be used preferably in liquid crystal display devices having a screen diagonal length of 42 inches or more, more preferably 46 inches or more, even more preferably 50 inches or more, still more preferably 55 inches or more, and particularly preferably 60 inches or more.

[0079] The polyester film for protecting a polarizer of the present invention is preferably used as a polarizer protective film on the viewer side relative to the polarizer of the viewer-side polarizing plate and / or as a polarizer protective film on the light source side relative to the polarizer of the light source-side polarizing plate.

[0080] Typically, a liquid crystal display device is generally rectangular, and the two polarizing plates used therein are also rectangular, with the long side of one polarizing plate parallel to its absorption axis and the long side of the other polarizing plate parallel to its transmission axis, and the absorption axes of the two polarizing plates are arranged perpendicular to each other. Typically, the polarizing plate whose long side and absorption axis are parallel is used as the viewer-side polarizing plate of a liquid crystal display device, and the polarizing plate whose long side and transmission axis are parallel is used as the light source-side polarizing plate of a liquid crystal display device.

[0081] Generally, curling is likely to occur in the long side direction. Due to the shape factor that makes curling more likely to occur when a polarizing plate whose long side is in the absorption axis direction of a polarizer with large shrinkage stress shrinks, and the influence of the asymmetric configuration of the upper and lower polarizing plates in a liquid crystal panel, liquid crystal panels tend to be convex toward the polarizing plate whose long side is the polarizer transmission axis of the upper and lower polarizing plates arranged in crossed Nicols.

[0082] From the viewpoint of suppressing warpage of a liquid crystal panel, it is preferable to use the polarizing plate of the present invention at least as a polarizing plate having a long side parallel to its transmission axis, and it is also preferable to use the polarizing plate of the present invention as both a polarizing plate having a long side parallel to its transmission axis and a polarizing plate having a long side parallel to its absorption axis.

[0083] The present invention will be described in more detail below with reference to examples. However, the present invention is not limited to the following examples, and can be practiced by making appropriate modifications within the scope that is compatible with the spirit of the present invention, and all of these modifications are included in the technical scope of the present invention.

[0084] (1) Heat shrinkage rate of polyester film After leaving the polyester film at rest for 168 hours in an environment of 25°C and 50% RH, a circle with a diameter of 80 mm and a straight line D with a length of approximately 50 mm, passing through the center of the circle and parallel to the TD direction of the film, were drawn. The circle and straight line D were drawn with a fine-tip pen capable of drawing lines with a thickness of 0.5 mm or less using black oil-based ink, with constant writing pressure to ensure that the line was as uniform in thickness as possible. A template or compass could be used to draw the circle.

[0085] The diameter of the circle was measured continuously using an image dimension measuring instrument (Image Measure IM6500 manufactured by KEYENCE Corporation) at 1° intervals, with the direction of line D being set to 0°, and the length of the diameter of the circle at each angle before the heat treatment was determined. Next, heat treatment was performed for 30 minutes using a gear oven set to 80°C, and then cooled for 10 minutes in an environment set to room temperature of 25°C. After that, the length of the diameter of the circle at each angle after the heat treatment was determined in the same manner as before the treatment. Note that the above treatment was performed on the polyester film for protecting a polarizer alone.

[0086] The heat shrinkage rate was evaluated at each angle using the following formula: Heat shrinkage rate = (length before heat treatment - length after heat treatment) / length before heat treatment x 100

[0087] In this way, the heat shrinkage of the polyester film in the direction in which the shrinkage was greatest and in the direction perpendicular thereto, as well as the acute angle formed by the direction in which the shrinkage was greatest and the line D, were determined. Measurements were taken at five locations in total, namely, both ends in the width direction of the slit film, the center, and a middle portion between both ends and the center, and the data at the position where T3 showed the lowest value was used as the value of T3. Data at the same positions as T3 were also used for T1 and T2.

[0088] (2) Film Thickness The thickness (mm) of the polyester film was measured using an electric micrometer (Militron 1245D, manufactured by Fine Leaf Co., Ltd.) after leaving it to stand for 168 hours in an environment of 25°C and 50% RH.

[0089] (3) Warpage of Liquid Crystal Panel The liquid crystal panels produced in each Example and Comparative Example were placed in a gear oven set at 80°C and 5% RH, with the panel floating on the square pillars at the four corners, and subjected to heat treatment for 2 hours. After that, the panels were 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 down, and the heights of the four corners were measured with a tape measure. The maximum value was taken as the amount of warpage. The amount of warpage was evaluated as follows: ○: 0 mm or more, less than 1.5 mm ×: 1.5 mm or more

[0090] (4) Light Leakage Evaluation Method The panel measured for warpage of the liquid crystal panel above was attached to a liquid crystal display device, and white was displayed at maximum brightness in a dark room. The four corners of the panel were visually inspected for light leakage. Panels with a warpage of 1.5 mm or more were not evaluated for light leakage due to the risk of breakage. ○: No light leakage was observed ×: Light leakage was observed

[0091] (5) Refractive index and in-plane retardation (Re) of polyester film Retardation is a parameter defined by the product (ΔNxy × d) of the anisotropy of the refractive index of two orthogonal axes on the film (ΔNxy = |Nx - Ny|) and the film thickness d (nm), and is a measure of optical isotropy and anisotropy. The biaxial refractive index anisotropy (ΔNxy) was determined by the following method. Using a molecular orientation meter (MOA-6004 molecular orientation meter manufactured by Oji Measurement Instruments Co., Ltd.), the slow axis direction of the film was determined, and a 4 cm × 2 cm rectangle was cut out so that the slow axis direction was parallel to the long side of the measurement sample, and used as a measurement sample. For this sample, the refractive indexes of two orthogonal axes (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 measured using an Abbe refractometer (manufactured by Atago Co., Ltd., NAR-4T, measurement wavelength 589 nm), and the absolute value of the difference in refractive index between the two axes (|Nx-Ny|) was taken as the refractive index anisotropy (ΔNxy). The retardation (Re) was calculated from the product (ΔNxy × d) of the refractive index anisotropy (ΔNxy) and the film thickness d (nm).

[0092] (6) Thickness direction retardation (Rth) The thickness direction retardation is a parameter indicating the average retardation obtained by multiplying the two birefringences ΔNxz (=|Nx-Nz|) and ΔNyz (=|Ny-Nz|) when viewed from the cross section of the film in the thickness direction by the film thickness d. Nx, Ny, Nz and the film thickness d (nm) were determined in the same manner as in the retardation measurement, and the average value of (ΔNxz × d) and (ΔNyz × d) was calculated to determine the thickness direction retardation (Rth).

[0093] (Production Example 1 - Polyester A) The temperature of the esterification reactor was increased to 200°C, and 86.4 parts by mass of terephthalic acid and 64.6 parts by mass of ethylene glycol were charged. While stirring, 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 were charged as catalysts. The temperature was then increased under pressure, and a pressurized esterification reaction was carried out under conditions of a gauge pressure of 0.34 MPa and 240°C. The esterification reactor was then returned to normal pressure, and 0.014 parts by mass of phosphoric acid was added. The temperature was then increased to 260°C over 15 minutes, and 0.012 parts by mass of trimethyl phosphate was added. After 15 minutes, the mixture was dispersed using a high-pressure disperser. After 15 minutes, the resulting esterification reaction product was transferred to a polycondensation reactor, and a polycondensation reaction was carried out under reduced pressure at 280°C.

[0094] After the polycondensation reaction was completed, the resin was filtered through a Naslon filter with a 95% cutoff diameter of 5 μm, extruded from a nozzle in the form of a strand, cooled and solidified using cooling water that had been previously filtered (pore diameter: 1 μm or less), and cut into pellets. The intrinsic viscosity of the resulting polyethylene terephthalate resin (A) was 0.62 dl / g, and it contained substantially no inert particles or internally precipitated particles. (Hereinafter, this will be referred to as PET (A)).

[0095] (Production Example 2 - Polyester B) 10 parts by mass of dried ultraviolet absorber (2,2'-(1,4-phenylene)bis(4H-3,1-benzoxazinon-4-one) and 90 parts by mass of particle-free PET (A) (intrinsic viscosity of 0.62 dl / g) were mixed and the mixture was kneaded using an extruder to obtain ultraviolet absorber-containing polyethylene terephthalate resin (B) (hereinafter abbreviated as PET (B)).

[0096] (Production Example 3 - Preparation of Adhesion-Modifying Coating Liquid) By carrying out a transesterification reaction and a polycondensation reaction by a conventional method, the dicarboxylic acid component (relative to the total dicarboxylic acid component) was 46 mol% terephthalic acid, 46 mol% isophthalic acid, and 8 mol% sodium 5-sulfonatoisophthalate, and the glycol component (relative to the total glycol component) was 50 mol% ethylene glycol and 50 mol% neopentyl glycol. A water-dispersible sulfonate metal base-containing copolymerized polyester resin was prepared. Next, 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 then heated and stirred. When the temperature reached 77 ° C., 5 parts by mass of the water-dispersible sulfonate metal base-containing copolymerized polyester resin was added, and the mixture was stirred until no lumps of resin remained. The resin aqueous dispersion was then cooled to room temperature to obtain a uniform water-dispersible copolymerized polyester resin solution having a solids concentration of 5.0% by mass. Furthermore, 3 parts by mass of aggregated silica particles (Sylysia 310, manufactured by Fuji Silysia Co., Ltd.) were dispersed in 50 parts by mass of water, and then 0.54 parts by mass of an aqueous dispersion of Sylysia 310 was added to 99.46 parts by mass of the water-dispersible copolymer polyester resin liquid, and 20 parts by mass of water was added while stirring to obtain an adhesive property-modifying coating liquid.

[0097] (Example 1) <Production of Polyester Film 1 for Protecting Polarizer> 90 parts by mass of particle-free PET (A) resin pellets and 10 parts by mass of PET (B) resin pellets containing an ultraviolet absorber were used as raw materials for the intermediate layer of the base film. They were dried under reduced pressure (1 Torr) at 135 ° C for 6 hours, and then fed to extruder 2 (for intermediate layer II). PET (A) was dried by a conventional method and fed to extruder 1 (for outer layer I and outer layer III), respectively, and melted at 285 ° C. These two polymers were each filtered through a stainless steel sintered filter medium (nominal filtration accuracy 10 μm particle 95% cut) and stacked in a two-type three-layer confluence block. The resulting mixture was extruded into a sheet shape from a die, and then wrapped around a casting drum with a surface temperature of 30 ° C. using an electrostatic casting method, cooled, and solidified to produce an unstretched film. At this time, the discharge rate of each extruder was adjusted so that the thickness ratio of layer I, layer II, and layer III was 10:80:10.

[0098] Next, a coating amount of 0.08 g / m2 was applied to both sides of the unstretched PET film by the reverse roll method. ２ After the adhesiveness modifying coating liquid was applied so that the thickness of the coated film became 1 / 2 mm, the coated film was dried at 80° C. for 20 seconds.

[0099] The unstretched film with this coating layer formed was introduced into a tenter stretching machine, and while the film's edges were held with clips, it was introduced into a hot air zone at 105°C and stretched 4.0 times in the TD. It was then heat-treated at 180°C for 15 seconds. The film was then relaxed and cooled to 100°C, after which it was slightly stretched in the width direction. The clips holding both ends of the film were subsequently released, the edge portions were cut off, and the film was taken up with a tension of 350 N / m to obtain a jumbo roll of uniaxially oriented PET film. The resulting jumbo roll was divided into two equal slit rolls (L (left side) and R (right side)). The slit roll positioned on the left side yielded a polyester film 1 for protecting a polarizer, measuring 80 μm in thickness, 1500 mm in width, and 1500 m in length. The Re was 8160 nm, the Rth was 9520 nm, and the NZ coefficient was 1.67. The conditions for the relaxation treatment, the small stretching ratio, etc. are as shown in Table 1.

[0100] <Preparation of Liquid Crystal Panel> A polyester film 1 for protecting a polarizer was attached to one side of a polarizer made of PVA, iodine, and boric acid so that the transmission axis of the polarizer was parallel to the TD of the polyester film 1 for protecting a polarizer. A TAC film (manufactured by Fuji Film Corporation, thickness: 60 μm) was attached to the other side of the polarizer to prepare a polarizing plate. The angle difference between the maximum heat shrinkage direction of the polyester film 1 for protecting a polarizer and the TD direction was within 25 degrees, and the a-direction was set to the TD direction.

[0101] <Preparation of Liquid Crystal Panel> A liquid crystal panel was removed from a 65-inch IPS liquid crystal television using a 0.4 mm thick glass substrate for the liquid crystal cell. The light source side and viewer side polarizing plates were peeled off from the liquid crystal panel, and instead, the polarizing plates prepared above were attached to the liquid crystal cell via a commercially available optical pressure-sensitive adhesive sheet so that the transmission axis of the polarizer coincided with the transmission axis direction of the polarizing plate before peeling, thereby preparing a liquid crystal panel.

[0102] The polarizing plate was attached to the liquid crystal cell so that the polyester film 1 for protecting a polarizer was located on the far side (opposite side) from the liquid crystal cell.

[0103] Examples 2 to 11, Comparative Examples 1 to 6 In the production of the polyester film for protecting a polarizer, polyester films 2 to 17 were produced in the same manner as in Example 1, except that the relaxation rate, slight stretching ratio, etc. were set as shown in Table 1, and polarizing plates and liquid crystal panels were produced using these films. Except for Film 10, which had Re of 6630 nm and 7735 nm and an NZ coefficient of 1.67, the Re, Rth, and NZ coefficient of each film were almost the same as those of Film 1.

[0104]

[0105] From the results shown in Table 1, it was found that the polarizing plate using the polyester film for protecting a polarizer of the present invention can suppress light leakage while suppressing warping of the panel, compared to the polarizing plate of the comparative example.

[0106] According to the present invention, it is possible to provide a polarizer protective film, a polarizing plate, and a liquid crystal display device that can suppress warping of a liquid crystal panel while simultaneously suppressing light leakage. In particular, it is possible 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 while simultaneously suppressing light leakage, even when the liquid crystal panel is placed in a high-temperature environment for a long period of time. Furthermore, it is possible to provide an economically advantageous polyester film for protecting a polarizer with improved productivity, since the film produced has good flatness and can be widely used as a polarizer protective film.

Claims

1. A polyester film for protecting polarizers, wherein the thermal shrinkage rate of the film before and after heat treatment at 80°C in a first direction (direction a) is T1 (%), and the thermal shrinkage rate of the film before and after heat treatment at 150°C in the same direction as direction a (direction a) is T2 (%), and the following conditions (1) and (2) are satisfied for a polyester film for protecting polarizers. T1 ≥ 0.2% ... (1) 1.0<T2 / T1≦5.0 (2)

2. When the thermal shrinkage rate of the film in direction b, which is perpendicular to direction a, before and after 80°C heat treatment is T3 (%), the following conditional equations (3) and (4) are satisfied for the polyester film for polarizer protection according to claim 1. T3 ≥ 0.06% ... (3) 1.0<T1 / T3≦6.0 (4)

3. The polyester film for protecting polarizers according to claim 1, wherein the retardation is 3000 to 30000 nm.

4. A polyester film for protecting polarizers according to claim 1, wherein the thickness is 25 to 200 μm.

5. The polarizer protective polyester film according to claim 1, wherein the polarizer protective polyester film has at least one layer selected from the group consisting of a hard coat layer, an anti-reflective layer, a low-reflective layer, an anti-glare layer, and an anti-reflective anti-glare layer on at least one surface.

6. A polarizing plate having a polarizer protective polyester film according to any one of claims 1 to 5 on at least one surface of the polarizer.

7. An image display device having a polarizing plate as described in claim 6.

8. A liquid crystal display device having a polarizing plate as described in claim 6.