Polyester film, and polarizing plate comprising the same

Abstract

Provided is a polyester film which, when applied to an image display device, has less generation of rainbow, and which is useful in improving the durability of a polarizing plate, and a polarizing plate including the polyester film. The polyester film of the present invention has an in-plane retardation Re(590) of 100 nm or less, and a linear expansion coefficient in a first direction of 0 / °C to 4.0 x 10 ‑5 -6 / °C. In one embodiment, the absolute value of the difference between the linear expansion coefficient in the first direction and the linear expansion coefficient in a second direction orthogonal to the first direction is 1.0 x 10 ‑5 -6 / °C or less.

Description

[0001] This application is a divisional application of an application with the application date of November 5, 2019, the application number of 201980090775.3, and the invention title of "Polyester Film, and Polarizing Plate Containing the Same". TECHNICAL FIELD

[0002] The present application relates to a polyester film and a polarizing plate containing the same. BACKGROUND

[0003] In an image display device (for example, a liquid crystal display device, an organic EL display device), a polarizing plate is disposed on at least one side of a display unit due to its image forming method. In recent years, image display devices have a tendency to further diversify in function and use, and are required to be able to withstand use in more severe environments. A polarizing plate generally has a configuration in which a polarizing member is sandwiched by two protective films, and as the protective film, cellulose triacetate, an acrylic resin, a cyclic olefin resin, or the like is widely used. On the other hand, from the viewpoint of durability as described above, it has been proposed to use a polyester film such as polyethylene terephthalate (PET) or polyethylene naphthalate (PEN) having excellent mechanical properties, chemical resistance, and moisture barrier properties as a protective film for a polarizing member (for example, Patent Literature 1). However, although the polyester film has excellent mechanical properties, it has birefringence, and sometimes becomes a cause of deterioration of visual recognition such as the generation of a rainbow. In particular, with the high brightness and high color purity of image display devices in recent years, this problem of rainbow is significant.

[0004] On the other hand, with a polarizing plate conventionally often used, which is configured using a protective film formed of cellulose triacetate, an acrylic resin, or a cyclic olefin resin, a crack is sometimes generated in the polarizing member due to a change in temperature. In recent years, with the thinning of image display devices, the thinning of the film of the polarizing member is required, and on the other hand, image display devices that are expected to be used at high temperatures are increasing, and thus, there is a strong demand for a polarizing plate that does not generate a crack in the polarizing member and has excellent durability.

[0005] PRIOR ART DOCUMENTS

[0006] PATENT LITERATURE

[0007] Patent Literature 1: Japanese Patent Application Laid-Open No. 8-271733 SUMMARY

[0008] PROBLEMS TO BE SOLVED BY THE INVENTION

[0009] The present application has been made to solve the above-described conventional problems, and its main object is to provide a polyester film in which the generation of rainbow is less when applied to an image display device, and which contributes to the improvement of the durability of a polarizing plate.

[0010] Solution to problem

[0011] The in-plane retardation Re(590) of the polyester film of the present application is 100 nm or less, and the linear expansion coefficient in the first direction is 0 / °C to 4.0 x 10 -5 / °C.

[0012] In one embodiment, the absolute value of the difference between the linear expansion coefficient in the first direction and the linear expansion coefficient in the second direction orthogonal to the first direction is 1.0 x 10 -5 / °C or less.

[0013] In one embodiment, the orientation function (f) of the polyester film is 0.01 to 0.24.

[0014] In one embodiment, the polyester film is formed of polyethylene terephthalate and / or modified polyethylene terephthalate.

[0015] In one embodiment, the modified polyethylene terephthalate contains a constitutional unit derived from diethylene glycol, 1,4-butanediol, 1,3-propanediol, or isophthalic acid.

[0016] In one embodiment, the crystallinity of the polyester film, as determined based on DSC, is 30% or more.

[0017] According to another aspect of the present application, there is provided a polarizing plate. The polarizing plate comprises: a polarizing member, and the polyester film described above disposed on one side of the polarizing member.

[0018] In one embodiment, the absolute value of the difference between the linear expansion coefficient of the polyester film in the first direction and the linear expansion coefficient of the polarizing member in a direction parallel to the first direction, and the absolute value of the difference between the linear expansion coefficient of the polyester film in the second direction orthogonal to the first direction and the linear expansion coefficient of the polarizing member in a direction parallel to the second direction are each 3.5 x 10 -5 / °C or less.

[0019] In one embodiment, the thickness of the polarizing member is 20 μm or less.

[0020] In one embodiment, the polarizing plate further comprises an easy-adhesion layer disposed on the polarizing member side of the polyester film.

[0021] In one embodiment, the easy-adhesion layer contains fine particles.

[0022] In one embodiment, the thickness of the easy-adhesion layer is 0.35 μm or less.

[0023] In one embodiment, the refractive index of the easy-adhesion layer is 1.55 or less.

[0024] Effects of the Invention

[0025] According to the present application, by setting the in-plane retardation to 100 nm or less and the linear expansion coefficient in a prescribed direction to 0 / °C to 4.0 x 10 -5 / °C, a polyester film that is less likely to generate rainbow when applied to an image display device and that contributes to improvement in durability of the polarizing plate can be provided. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 A cross-sectional view of a polarizing plate according to one embodiment of the present application.

[0027] Figure 2 A cross-sectional view of a polarizing plate according to another embodiment of the present application.

[0028] REFERENCE NUMERALS

[0029] 10 polarizing member

[0030] 20 polyester film

[0031] 30 adhesive layer

[0032] 40 easy-adhesion layer

[0033] 100, 200 polarizing plate DETAILED DESCRIPTION

[0034] Hereinafter, preferred embodiments of the present application will be described, but the present application is not limited to these embodiments.

[0035] A. Polyester Film

[0036] The in-plane retardation Re(590) of the polyester film described above is 100 nm or less. In the present application, by setting the in-plane retardation Re to such a range, a polyester film that is less likely to generate rainbow when applied to an image display device can be obtained. The in-plane retardation Re(590) of the polyester film is preferably 80 nm or less, more preferably 60 nm or less, and further preferably 55 nm or less. When it is in such a range, the above-described effects are more remarkable. Note that the smaller the in-plane retardation Re(590) is, the more preferable it is, and the lower limit thereof is ideally 0 nm, and it can be, for example, 1 nm. Note that the in-plane retardation Re(λ) is the in-plane retardation of a film measured with light having a wavelength of λ nm at 23°C. Therefore, Re(590) is the in-plane retardation of a film measured with light having a wavelength of 590 nm. For Re(λ), when the thickness of a film is set to d (nm), it is calculated by the formula: Re(λ) = (nx - ny) x d. Here, nx is the refractive index in the direction in which the in-plane refractive index is the largest (i.e., the slow axis direction), and ny is the refractive index in the direction in the plane that is orthogonal to the slow axis.

[0037] The linear expansion coefficient of the polyester film in the first direction is 0 / °C to 4.0 x 10 -5 / °C. When it is in this range, a polyester film that is effective in protecting a polarizing element when laminated to the polarizing element and that prevents the polarizing element from developing cracks can be obtained. The linear expansion coefficient of the polyester film in the first direction is preferably 0.2 x 10 -5 / °C to 4.0 x 10 -5 / °C, more preferably 1.0 x 10 -5 / °C to 4.0 x 10 -5 / °C, further preferably 1.5 x 10 -5 / °C to 4.0 x 10 -5 / °C, particularly preferably 2.0 x 10 -5 / °C to 4.0 x 10 -5 / °C, most preferably 2.5 x 10 -5 / °C to 3.8 x 10 -5 / °C. When it is in this range, the above effects are more pronounced. In one embodiment, the above first direction corresponds to the direction of conveyance (MD) when the polyester film is manufactured. The linear expansion coefficient can be determined by TMA measurement according to JIS K 7197.

[0038] In the present application, by setting the in-plane retardation Re(590) and the linear expansion coefficient in the first direction to the above ranges, a polyester film that is less likely to generate rainbow when applied to an image display device and that contributes to improvement in durability of the polarizing plate (prevention of crack generation) can be provided. More specifically, the above polyester film can be a stretched film obtained by a stretching process, and by appropriately adjusting the manufacturing conditions in the stretching process, the in-plane retardation Re(590) and the linear expansion coefficient in the first direction can be well-balancedly controlled, and as a result, a polyester film having excellent characteristics for a polarizing member protective film from the viewpoints of rainbow and durability as described above can be obtained. One of the reasons for obtaining such an excellent polyester film is that, by optimization of the manufacturing conditions, not only the linear expansion coefficient of the polyester film is well-adjusted, but also the crystallinity (crystallinity, orientation) of the polyester-based resin is well-adjusted. As the above manufacturing conditions, stretching conditions (stretching temperature, stretching ratio, stretching speed, MD / TD stretching sequence), preheating temperature before stretching, heat treatment temperature after stretching, heat treatment time after stretching, relaxation ratio in the MD / TD directions after stretching, and the like can be exemplified. Focusing on the optimization of the various manufacturing conditions to well-balancedly control the in-plane retardation Re(590) and the linear expansion coefficient in the first direction is one of the great achievements of the present application. The stretching temperature, the stretching ratio, and the stretching speed can be appropriately adjusted for MD / TD, respectively. Note that TD means a direction orthogonal to the conveying direction (MD) at the time of manufacturing the polyester film.

[0039] The linear expansion coefficient of the above polyester film in a second direction orthogonal to the above first direction is preferably 0 / °C to 4.0 x 10 -5 / °C, more preferably 1.0 x 10 -5 / °C to 4.0 x 10 -5 / °C, further preferably 1.5 x 10 -5 / °C to 4.0 x 10 -5 / °C, particularly preferably 2.0 x 10 -5 / °C to 4.0 x 10 -5 / °C, most preferably 2.5 x 10 -5 / °C to 3.8 x 10 -5 / °C. In one embodiment, the above second direction corresponds to TD. In addition, the linear expansion coefficient in the second direction can also be controlled by the manufacturing conditions of the polyester film, particularly the stretching conditions and the heat treatment conditions.

[0040] The absolute value of the difference between the linear expansion coefficient in the first direction and the linear expansion coefficient in the second direction of the polyester film is preferably 1.0 x 10 -5 / °C or less, more preferably 0.7 x 10 -5or less. When such a range is obtained, the effect of preventing the polarizing member from developing cracks is remarkable.

[0041] The polyester film described above preferably has a crystallinity of 30% or more, more preferably 40% or more, and further preferably 50% or more, as measured by differential scanning calorimetry (DSC). The upper limit of the crystallinity is, for example, 70%. When such a range is obtained, a polyester film that is excellent in heat resistance and mechanical properties and is suitable as a polarizing member protective film can be obtained.

[0042] In the polyester film described above, it is preferable that the arrangement of the crystalline portion be uniform in the plane. For example, it is preferable that the polyester film be adjusted so that the amount of the crystalline portion in a first direction and in a second direction orthogonal to the first direction be the same degree. When such adjustment is made, a polyester film that is small in in-plane retardation and is less likely to develop rainbow can be obtained. The arrangement of the crystalline portion can be adjusted by the stretching conditions at the time of production of the polyester film. In addition, the arrangement of the crystalline portion can be evaluated by the degree of crystalline orientation as measured by X-ray diffraction (XRD).

[0043] The orientation function (f) of the polyester film described above is preferably 0.01 to 0.24, and more preferably 0.05 to 0.20. The orientation function indicates the orientation of the crystallization of the polyester film in a first direction (for example, the conveying direction (MD) at the time of production of the polyester film), f = 1 when the orientation is complete in the first direction, f = 0 when it is random, and f = -0.5 when the orientation is complete in a second direction orthogonal to the first direction. When the orientation function (f) is in the range described above, a polyester film that is particularly less likely to develop rainbow when applied to an image display device and that contributes to improvement in the durability of the polarizing plate (prevention of crack development) can be provided. For the orientation function (f), for example, a Fourier transform infrared spectrometer (FT-IR) is used, polarization is used as the measuring light, and the measurement is performed by attenuated total reflection spectrometry (ATR). Specifically, the measurement is performed by causing the measuring light to be incident on the sample in a state in which the vibration direction of the measuring light (polarization) is parallel to the surface of the polyester film sample and in a state in which the vibration direction of the measuring light (polarization) is orthogonal to the surface of the polyester film sample, and the intensity I of the peak at 1340 cm -1 (peak of crystallization) of the obtained spectra is calculated by the following equation. Here, the intensity I is the value at 1340 cm -1 / 1410 cm -1 with the peak at 1410 cm -1 as the reference peak.

[0044] f = (3<cos 2 θ> - 1) / 2

[0045] = [(R - 1)(R0+ 2)] / [(R+ 2)(R0- 1)]

[0046] = (1 - D) / [c (2D + 1)]

[0047] = -2 x (1 - D) / (2D + 1)

[0048] wherein,

[0049] c = (3 cos 2 β - 1) / 2

[0050] β = 90 deg

[0051] θ: angle of the molecular chain with respect to the stretching direction

[0052] β: transition dipole moment with respect to the molecular chain axis (β = 0)

[0053] R0= 2 cot 2 β

[0054] 1 / R = D = (I / / ) / (I⊥)

[0055] I⊥: absorption intensity when measured in a state in which the vibration direction of the measuring light (polarized light) is orthogonal to the surface of the polyester film sample

[0056] I / / : absorption intensity when measured in a state in which the vibration direction of the measuring light (polarized light) is parallel to the surface of the polyester film sample

[0057] The thickness of the polyester film described above can be appropriately set according to the purpose, the desired in-plane retardation, and the like. The thickness of the phase difference film is typically 10 μm to 100 μm, preferably 20 μm to 80 μm, and further preferably 20 μm to 50 μm.

[0058] The total light transmittance of the polyester film described above is preferably 80% or more, more preferably 85% or more, further preferably 90% or more, and particularly preferably 95% or more. The haze of the polyester film described above is preferably 1.0% or less, more preferably 0.7% or less, further preferably 0.5% or less, and particularly preferably 0.3% or less.

[0059] The moisture permeability of the polyester film is preferably 100 g / m 2 or less, more preferably 50 g / m 2 or less, and further preferably 15 g / m 2 or less. When the range is such, a polarizing plate having excellent durability and moisture resistance can be obtained.

[0060] The polyester film of the present application is formed of a polyester-based resin. The polyester-based resin can be obtained by condensation polymerization of a carboxylic acid component and a polyhydric alcohol component.

[0061] As the carboxylic acid component, aromatic dicarboxylic acids, aliphatic dicarboxylic acids, alicyclic dicarboxylic acids can be exemplified. As the aromatic dicarboxylic acid, for example, terephthalic acid, isophthalic acid, benzylmalonic acid, 1,4-naphthalene dicarboxylic acid, diphenyl dicarboxylic acid, 4,4'-oxybenzoic acid, 2,5-naphthalene dicarboxylic acid can be exemplified. As the aliphatic dicarboxylic acid, for example, malonic acid, dimethylmalonic acid, succinic acid, glutaric acid, adipic acid, trimethyl adipic acid, pimelic acid, 2,2-dimethyl glutaric acid, azelaic acid, sebacic acid, fumaric acid, maleic acid, itaconic acid, thiodipropionic acid, diglycolic acid can be exemplified. As the alicyclic dicarboxylic acid, for example, 1,3-cyclopentane dicarboxylic acid, 1,2-cyclohexane dicarboxylic acid, 1,3-cyclopentane dicarboxylic acid, 1,4-cyclohexane dicarboxylic acid, 2,5-norbornane dicarboxylic acid, adamantane dicarboxylic acid can be exemplified. The carboxylic acid component can be a derivative such as an ester, a chloride, an anhydride, including, for example, 1,4-cyclohexane dicarboxylic acid dimethyl ester, 2,6-naphthalene dicarboxylic acid dimethyl ester, isophthalic acid dimethyl ester, terephthalic acid dimethyl ester, and terephthalic acid diphenyl ester. The carboxylic acid component can be used alone or in combination of two or more.

[0062] As the polyol component, dihydric alcohols can be exemplified as representative. As the dihydric alcohol, aliphatic diols, alicyclic diols, aromatic diols can be exemplified. As the aliphatic diol, for example, ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, 1,3-propanediol, 2,4-dimethyl-2-ethylhexane-1,3-diol, 2,2-dimethyl-1,3-propanediol (neopentyl glycol), 2-ethyl-2-butyl-1,3-propanediol, 2-ethyl-2-isobutyl-1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 3-methyl-1,5-pentanediol, 2,2,4-trimethyl-1,6-hexanediol can be exemplified. As the alicyclic diol, for example, 1,2-cyclohexane dimethanol, 1,3-cyclohexane dimethanol, 1,4-cyclohexane dimethanol, spiro glycol, tricyclodecane dimethanol, adamantane diol, 2,2,4,4-tetramethyl-1,3-cyclobutane diol can be exemplified. As the aromatic diol, for example, 4,4'-thiodiphenol, 4,4'-methylene diphenol, 4,4'-(2-norbornylidene)diphenol, 4,4'-dihydroxydiphenyl, o-dihydroxybenzene, m-dihydroxybenzene, and p-dihydroxybenzene, 4,4'-isopropylidene phenol, 4,4'-isopropylidene bis(2,6-dichlorophenol), 2,5-naphthalene diol, and p-xylene glycol can be exemplified. The polyol component can be used alone or in combination of two or more.

[0063] As the polyester-based resin, polyethylene terephthalate and / or modified polyethylene terephthalate is preferably used, and polyethylene terephthalate is more preferably used. When these resins are used, a polyester film having excellent mechanical properties and less generation of rainbow can be obtained. Polyethylene terephthalate and modified polyethylene terephthalate can be used in mixture.

[0064] As the modified polyethylene terephthalate, for example, modified polyethylene terephthalate containing a constitutional unit derived from diethylene glycol, 1,4-butanediol, 1,3-propanediol, or isophthalic acid can be exemplified. The proportion of diethylene glycol in the polyol component is preferably more than 0 mol% and 10 mol% or less, and more preferably more than 0 mol% and 3 mol% or less. The proportion of 1,4-butanediol in the polyol component is preferably more than 0 mol% and 10 mol% or less, and more preferably more than 0 mol% and 3 mol% or less. The proportion of 1,3-propanediol in the polyol component is preferably more than 0 mol% and 10 mol% or less, and more preferably more than 0 mol% and 3 mol% or less. The proportion of isophthalic acid in the carboxylic acid component is preferably more than 0 mol% and 10 mol% or less, and more preferably more than 0 mol% and 8 mol% or less. When it is in such a range, a polyester film having good crystallinity can be obtained. Note that the mol% described above is mol% with respect to the total amount of all the repeating units of the polymer.

[0065] The weight average molecular weight of the polyester-based resin is preferably 10,000 to 100,000, and more preferably 20,000 to 75,000. When it is in such a weight average molecular weight, a film that is easy to handle at the time of molding and has excellent mechanical strength can be obtained. The weight average molecular weight can be measured by GPC (solvent: THF).

[0066] In one embodiment, a polyester film with an easy-adhesion layer is provided. The easy-adhesion layer contains, for example, a water-based polyurethane and an oxazoline-based crosslinking agent. Details of the easy-adhesion layer are described, for example, in Japanese Patent Application Publication No. 2010-55062. The entire description of this publication is incorporated herein by reference.

[0067] In one embodiment, the easy-adhesion layer described above contains any appropriate fine particles. By forming an easy-adhesion layer containing fine particles, it is possible to effectively suppress blocking that occurs at the time of winding. The fine particles can be inorganic fine particles or organic fine particles. As inorganic fine particles, for example, inorganic oxides such as silicon dioxide, titanium dioxide, aluminum oxide, and zirconium oxide; calcium carbonate; talc; clay; calcined kaolin; calcined calcium silicate; hydrated calcium silicate; aluminum silicate; magnesium silicate; and calcium phosphate can be exemplified. As organic fine particles, for example, silicone-based resins, fluorine-based resins, and (meth)acrylic-based resins can be exemplified. Among these, silicon dioxide is preferable.

[0068] The particle diameter (number average primary particle diameter) of the above-mentioned fine particles is preferably 10 to 200 nm, and further preferably 20 to 60 nm.

[0069] The thickness of the above-mentioned easy-adhesion layer is preferably 2 μm or less, more preferably 1 μm or less, and further preferably 0.35 μm or less. When the thickness is in such a range, a polyester film with an easy-adhesion layer which does not easily obstruct the optical properties of other members when applied to an image display device can be obtained.

[0070] In one embodiment, the refractive index of the above-mentioned easy-adhesion layer is preferably 1.45 to 1.60. When the refractive index is in such a range, a polyester film with an easy-adhesion layer which does not easily obstruct the optical properties of other members when applied to an image display device can be obtained. In one embodiment, the refractive index of the above-mentioned easy-adhesion layer is 1.54 or more.

[0071] In one embodiment, the above-mentioned polyester film can have an anti-blocking layer on at least one side thereof. The constitution of the anti-blocking layer can adopt the constitution of the above-mentioned easy-adhesion layer described in the above. The anti-blocking layer preferably contains the above-mentioned fine particles.

[0072] (Method for producing a polyester film)

[0073] The above-mentioned polyester film can be obtained by a forming step of forming a film-forming material (resin composition) containing the above-mentioned polyester-based resin into a film shape, and a stretching step of stretching the film formed in the forming step. Preferably, the stretching step includes a preheating treatment of the film before the film stretching, and a heat treatment after the film stretching. In one embodiment, the polyester film is provided in a long strip shape (or a shape cut from a long strip).

[0074] The film-forming material can contain an additive other than the above-mentioned polyester-based resin, and can contain a solvent. As the additive, any appropriate additive can be used according to the purpose. As specific examples of the additive, a reactive diluent, a plasticizer, a surfactant, a filler, an antioxidant, a preservative, an ultraviolet absorber, a leveling agent, a thixotropic agent, an antistatic agent, a conductive material, a flame retardant can be exemplified. The number, kind, combination, and amount of the additive can be appropriately set according to the purpose.

[0075] As the method of forming a film from the film-forming material, any appropriate forming process can be used. As specific examples, compression molding, transfer molding, injection molding, extrusion molding, blow molding, powder molding, FRP molding, flow coating (for example, flow casting), calender molding, hot pressing, and the like can be exemplified. Extrusion molding or flow coating is preferable. This is because the smoothness of the obtained film can be improved, and good optical uniformity can be obtained.

[0076] The stretching method of the film is preferably biaxial stretching, more specifically, sequential biaxial stretching or simultaneous biaxial stretching. The simultaneous biaxial stretching is preferred. By stretching the film by the simultaneous biaxial stretching, the in-plane retardation Re(590) and the linear expansion coefficient can be well balanced, and a polyester film in which the generation of rainbow is particularly less can be obtained. The sequential biaxial stretching or the simultaneous biaxial stretching is preferably performed using a tenter stretching machine. Thus, the stretching direction of the film is preferably the length direction (MD) and the width direction (TD) of the film.

[0077] The stretching temperature is preferably Tg + 5°C to Tg + 50°C, more preferably Tg + 5°C to Tg + 30°C, further preferably Tg + 5°C to Tg + 10°C, with respect to the glass transition temperature (Tg) of the film. By stretching at such a temperature, a polyester film in which the in-plane retardation Re(590) and the linear expansion coefficient are well balanced can be obtained. In addition, a polyester film having excellent transparency can be obtained.

[0078] The stretching ratio in the MD is preferably 1.1 times to 5 times, more preferably 1.1 times to 4 times, further preferably 1.5 times to 3.5 times, particularly preferably more than 2 times and 3.2 times or less. When it is in such a range, a polyester film in which the in-plane retardation is controlled in a desired range and which has good crystallinity can be obtained.

[0079] The stretching ratio in the TD is preferably 1.1 times to 5 times, more preferably 1.1 times to 4 times, further preferably 1.5 times to 3.5 times, particularly preferably more than 2 times and 3.2 times or less. When it is in such a range, a polyester film in which the in-plane retardation is controlled in a desired range and which has good crystallinity can be obtained.

[0080] The ratio of the stretching ratio in the MD to the stretching ratio in the TD (stretching ratio in TD / stretching ratio in MD) is preferably 0.7 to 1.5, more preferably 0.8 to 1.2, further preferably 0.9 to 1. When it is in such a range, a polyester film in which the generation of rainbow is particularly less can be obtained.

[0081] The stretching speed in the MD is preferably 5% / sec to 400% / sec, more preferably 5% / sec to 150% / sec, further preferably 8% / sec to 150% / sec, further preferably 8% / sec to 100% / sec, particularly preferably 8% / sec to 80% / sec, most preferably 8% / sec to 60% / sec. When it is in such a range, a polyester film having excellent optical properties and good crystallinity can be obtained.

[0082] The stretching speed of the TD is preferably from 5% / sec to 150% / sec, more preferably from 5% / sec to 100% / sec, further preferably from 8% / sec to 100% / sec, particularly preferably from 8% / sec to 80% / sec, most preferably from 8% / sec to 60% / sec. When the stretching speed is within such a range, a polyester film having excellent optical properties and good crystallinity can be obtained.

[0083] The temperature of the preheating treatment is preferably from 80°C to 150°C, more preferably from 90°C to 130°C. In addition, the time of the preheating treatment is preferably from 5 seconds to 100 seconds, more preferably from 10 seconds to 100 seconds, further preferably from 15 seconds to 80 seconds. When the time of the preheating treatment is within such a range, a polyester film having excellent optical properties and good crystallinity can be obtained.

[0084] The temperature of the heat treatment is preferably from 100°C to 250°C, more preferably from 120°C to 200°C, further preferably from 130°C to 180°C. When the temperature of the heat treatment is within such a range, a polyester film having excellent transparency and good crystallinity can be obtained. The time of the heat treatment is preferably from 1 second to 50 seconds, more preferably from 2 seconds to 50 seconds, further preferably from 2 seconds to 40 seconds, particularly preferably from 5 seconds to 40 seconds, most preferably from 8 seconds to 30 seconds. When the time of the heat treatment is within such a range, a polyester film having excellent transparency and good crystallinity can be obtained.

[0085] B. Polarizing plate

[0086] Figure 1 A cross-sectional view of a polarizing plate according to one embodiment of the present application is shown. The polarizing plate 100 has a polarizing member 10 and a polyester film 20 disposed on one side of the polarizing member 10. As the polyester film 20, the polyester film of the present application described in item A above is used. An arbitrary appropriate other polarizing member protective film can be disposed on the other side of the polarizing member, or no polarizing member protective film can be disposed. In one embodiment, the polarizing member 10 and the polyester film 20 (or other polarizing member protective film) are laminated by means of an adhesive layer 30.

[0087] In one embodiment, the above-described polarizing plate can be applied to an image display device in such a manner that the side on which the above-described polyester film is disposed is the visually recognized side. In addition, in the case where the above-described polarizing plate is applied to a liquid crystal display device, the polarizing plate having the polyester film can be disposed on the visually recognized side of the liquid crystal cell, or on the back side.

[0088] As the polarizing member, an arbitrary appropriate polarizing member can be used. For example, the resin film forming the polarizing member can be a single-layer resin film, or a laminate of two or more layers.

[0089] As a specific example of the polarizing member composed of a single layer of a resin film, there can be mentioned one obtained by performing dyeing treatment based on a dichroic substance such as iodine or a dichroic dye and stretching treatment on a hydrophilic polymer film such as a polyvinyl alcohol (PVA)-based film, a partially formaldehyde-esterified PVA-based film, an ethylene-vinyl acetate copolymer-based partially saponified film, and the like; a dehydrated product of PVA, a dehydrochlorinated product of polyvinyl chloride, and the like. From the viewpoint of excellent optical properties, a polarizing member obtained by dyeing a PVA-based film with iodine and performing uniaxial stretching is preferably used.

[0090] The above-described dyeing based on iodine is performed, for example, by immersing a PVA-based film in an aqueous iodine solution. The stretching ratio of the above-described uniaxial stretching is preferably 3 to 7 times. The stretching can be performed after the dyeing treatment or can be performed while dyeing. Alternatively, the dyeing can be performed after the stretching. As needed, the PVA-based film is subjected to swelling treatment, crosslinking treatment, cleaning treatment, drying treatment, and the like. For example, by washing the PVA-based film with water before dyeing, not only dirt and an anti-blocking agent on the surface of the PVA-based film but also the PVA-based film can be swelled to prevent uneven dyeing.

[0091] As a specific example of the polarizing member obtained using the laminate, there can be mentioned one obtained using a laminate of a resin substrate and a PVA-based resin layer (PVA-based resin film) laminated to the resin substrate, or a laminate of a resin substrate and a PVA-based resin layer formed by coating on the resin substrate. The polarizing member obtained using the laminate of the resin substrate and the PVA-based resin layer formed by coating on the resin substrate can be produced, for example, by coating a PVA-based resin solution on a resin substrate and drying it to form a PVA-based resin layer on the resin substrate, thereby obtaining a laminate of the resin substrate and the PVA-based resin layer; and performing stretching and dyeing on the laminate to produce the PVA-based resin layer as a polarizing member. In the present embodiment, the stretching typically involves immersing the laminate in an aqueous boric acid solution and stretching it. Further, the stretching can further involve, as needed, aerial stretching of the laminate at a high temperature (e.g., 95°C or higher) before the stretching in the aqueous boric acid solution. The obtained laminate of the resin substrate / polarizing member can be used as it is (i.e., the resin substrate can be used as a protective layer of the polarizing member), or the resin substrate can be peeled from the laminate of the resin substrate / polarizing member, and a suitable protective layer as needed can be laminated to the peeled surface and used. Details of such a method for producing a polarizing member are described, for example, in Japanese Patent Application Publication No. 2012-73580. The entire description of this publication is incorporated herein by reference.

[0092] The thickness of the polarizing member is, for example, 1 μm to 80 μm. In one embodiment, the thickness of the polarizing member is preferably 20 μm or less, further preferably 3 μm to 15 μm. When the polyester film of the present application is used, the cracking of the polarizing member can be effectively prevented, and therefore a thin polarizing member can be used even in a severe environment such as high temperature or large temperature change.

[0093] The polarizing member and the polarizing member protective film (polyester film) can be laminated with the aid of an arbitrary appropriate adhesive layer. It is preferable that the adhesive layer be formed of an adhesive composition containing a polyvinyl alcohol-based resin.

[0094] In the above-described polarizing plate, the absolute value of the difference between the linear expansion coefficient of the polyester film in the first direction and the linear expansion coefficient of the polarizing member in the direction parallel to the first direction is preferably 3.5 x 10 -5 / °C or less, more preferably 3.3 x 10 -5 / °C or less, further preferably 3.0 x 10 -5 / °C or less. When the range is such, the cracking of the polarizing member can be prevented even in a severe environment such as high temperature or large temperature change. The lower limit of the absolute value of the difference between the linear expansion coefficient of the polyester film in the first direction and the linear expansion coefficient of the polarizing member in the direction parallel to the first direction is more preferably small, and can be, for example, 0.5 x 10 -5 / °C.

[0095] In the above-described polarizing plate, the absolute value of the difference between the linear expansion coefficient of the polyester film in the second direction (direction orthogonal to the first direction) and the linear expansion coefficient of the polarizing member in the direction parallel to the second direction is preferably 3.5 x 10 -5 / °C or less, more preferably 3.3 x 10 -5 / °C or less, further preferably 3.0 x 10 -5 / °C or less, particularly preferably 2.5 x 10 -5 / °C or less. When the range is such, the cracking of the polarizing member can be prevented even in a severe environment such as high temperature or large temperature change. The lower limit of the absolute value of the difference between the linear expansion coefficient of the polyester film in the second direction and the linear expansion coefficient of the polarizing member in the direction parallel to the second direction is more preferably small, and can be, for example, 0.5 x 10 -5 / °C.

[0096] In one embodiment, the absolute value of the difference between the linear expansion coefficient of the polyester film in the first direction and the linear expansion coefficient of the polarizing member in the direction parallel to the first direction, and the absolute value of the difference between the linear expansion coefficient of the polyester film in the second direction (direction orthogonal to the first direction) and the linear expansion coefficient of the polarizing member in the direction parallel to the second direction are each 3.5 x 10 -5 / °C or less (preferably 3.3 x 10 -5below). By such a range, even in a severe environment of high temperature, large temperature change, or the like, cracking of the polarizing member can be prevented.

[0097] Figure 2 A cross-sectional view of a polarizing plate according to another embodiment of the present application. The polarizing plate 200 further has an easy-adhesion layer 40 disposed on the side of the polarizing member 10 of the polyester film 20. In one embodiment, the polyester film A with the easy-adhesion layer is disposed on the polarizing member 10 in such a manner that the easy-adhesion layer 40 is on the side of the polarizing member 10. As the easy-adhesion layer, the easy-adhesion layer described in item A above can be used.

[0098] C. Image display device

[0099] The above-described polarizing plate can be applied to an image display device. As representative examples of the image display device, a liquid crystal display device, an organic electroluminescence (EL) display device can be cited. The image display device adopts a constitution known in the industry, and thus detailed explanation is omitted.

[0100] Example

[0101] Hereinafter, the present application will be specifically described by way of examples, but the present application is not limited by these examples. The measuring method of each property in the examples is as follows. Note that, unless otherwise specified, "parts" and "%" in the examples are on a weight basis.

[0102] (1) In-plane retardation Re (590)

[0103] The polyester film used in the examples and comparative examples was cut into a length of 5 cm and a width of 5 cm, and used as a measurement sample. The in-plane retardation was measured using "Axoscan" manufactured by Axometrics, Inc. for the measurement sample. The measurement wavelength was 590 nm, and the measurement temperature was 23°C.

[0104] (2) Coefficient of linear expansion

[0105] The coefficient of linear expansion of the polyester film and the polarizing member was measured based on JIS K 7197 using a thermal mechanical analysis device "TMA7100" manufactured by Hitachi High-Tech Science Corporation, at a rate of 10°C / minute from 30°C to 150°C, and the deformation amount of the test film at each temperature was measured. Then, the coefficient of linear expansion of the film was calculated from the deformation amount in the temperature range of 30°C to 70°C. Note that, the case where the film size becomes larger (expansion) as the temperature rises was recorded as positive (+), and the case where the film size becomes smaller (contraction) as the temperature rises was recorded as negative (-).

[0106] The linear expansion coefficient of the polyester film was measured in the MD (first direction) and TD (second direction). For the polarizing member, the linear expansion coefficient was measured in the direction parallel to the MD and in the direction parallel to the TD in the polarizing plate.

[0107] (3) Crystallinity

[0108] The crystallinity of the polyester film used in the examples and comparative examples was measured by differential scanning calorimetry (DSC). The heat generation and the heat of fusion observed in the temperature rise of the sample to 300°C at a rate of 10°C / min were determined, and the crystallinity was determined by the following equation. Note that the heat generation and the heat of fusion were measured using Q-2000 manufactured by TA instruments.

[0109] Crystallinity (%) = (heat of fusion obtained in the measurement - heat generation obtained in the measurement) / heat of fusion of 100% crystalline polyethylene terephthalate (119 mJ / mg) x 100

[0110] (4) Orientation function (f) of the polyester film

[0111] For the orientation function (f) of the polyester film, Fourier transform infrared spectrometer (Frontier FT IR manufactured by PerkinElmer, trade name "Frontier FT IR") was used, and the intensity I of the peak at 1340 cm

[0112] Specifically, the measurement was performed by making the measurement light incident on the sample in a state in which the vibration direction of the measurement light (polarization) was parallel to the surface of the polyester film sample and in a state in which the vibration direction of the measurement light (polarization) was orthogonal to the surface of the polyester film sample, and the intensity I of the peak at 1340 cm -1 (Crystalline peak) of the obtained spectrum was calculated by the following equation. Here, the intensity I is the value of 1340 cm -1 / 1410 cm -1 of the reference peak at 1410 cm -1 .

[0113] f = (3<cos 2 θ>-1) / 2

[0114] = [(R-1)(R0+2)] / [(R+2)(R0-1)]

[0115] =(1-D) / [c(2D+1)]

[0116] =-2x(1-D) / (2D+1)

[0117] wherein,

[0118] c = (3 cos 2 β - 1) / 2

[0119] β = 90 deg

[0120] θ: angle of the molecular chain with respect to the stretching direction

[0121] β: transition dipole moment with respect to the molecular chain axis (β = 0)

[0122] R0 = 2 cot 2 β

[0123] 1 / R = D = (I / / ) / (I⊥)

[0124] I⊥: absorption intensity when measured in a state where the vibration direction of the measuring light (polarized light) is orthogonal to the surface of the polyester film sample

[0125] I / / : absorption intensity when measured in a state where the vibration direction of the measuring light (polarized light) is parallel to the surface of the polyester film sample

[0126] (5) Rainbow

[0127] A liquid crystal cell was taken out from a liquid crystal TV "45UH7500" manufactured by LGD Corporation, and the polarizing plate on the backlight side was peeled off. The polarizing plates obtained in the examples and comparative examples were attached to the surface of the liquid crystal TV from which the polarizing plate was peeled off, with the absorption axis of the polarizing element being on the short side of the liquid crystal TV, with the aid of an adhesive. The liquid crystal cell to which the polarizing plates obtained in the examples and comparative examples were attached was set again, and the TV was turned on to white display.

[0128] The liquid crystal TV that was turned on was visually confirmed in all directions at an angle of 60° from the polar angle, and the presence or absence of rainbow was observed. The evaluation was performed based on the following criteria.

[0129] O: no rainbow was observed

[0130] Δ: rainbow was slightly observed

[0131] X: rainbow was obviously observed

[0132] (6) Dimensional change

[0133] The polyester films used in the examples and comparative examples were cut into 100 mm x 100 mm. The dimensions of the cut films were accurately measured with a CNC image measuring machine QuickVision (QV606) manufactured by Mitutoyo Corporation. Thereafter, the films were put into a heating oven at 100°C for 24 hours, taken out, and the dimensions were accurately measured again, and the dimensional change was calculated.

[0134] (7) Crack test (thermal shock acceleration test)

[0135] A cold and hot shock tester (manufactured by ESPEC) was used to evaluate the polarizing plates obtained in the examples and comparative examples.

[0136] The polarizing plates obtained in the examples and comparative examples were cut into 50 mm in width by 150 mm in length. At this time, samples in which the absorption axis direction of the polarizing member was parallel to the width (short side) of the cut polarizing plate and samples in which the transmission axis direction of the polarizing member was parallel to the width (short side) of the cut polarizing plate were prepared. The surface of the polarizing plate on which the protective film (polyester film) was not laminated was adhered to 0.5 mm-thick non-alkali glass using an acrylic adhesive, and the samples were prepared.

[0137] The obtained samples were placed in the test area of the cold and hot shock tester, and the temperature in the test area was decreased from room temperature to -40°C over 30 minutes. Subsequently, the temperature in the test area was increased to 85°C over 30 minutes, and then decreased to -40°C over 30 minutes again. This process of increasing the temperature from -40°C to 85°C and decreasing the temperature to -40°C again was defined as one cycle, and after 100 cycles and 200 cycles were repeated, the laminates were taken out, and the presence or absence of cracks was confirmed visually, and evaluated according to the following criteria.

[0138] O: No cracks were observed even after 200 cycles were repeated.

[0139] Δ: No cracks were observed after 100 cycles were repeated, but cracks were generated after 200 cycles were repeated.

[0140] X: Cracks were generated after 100 cycles were repeated.

[0141] [Manufacturing Example 1] Manufacture of Polyester Film A

[0142] After a polyester resin (polyethylene terephthalate, manufactured by Bell Polyester Products, Inc., IV value: 0.75 dl / g (measured at a concentration of 0.4 g / dl in a mixed solvent solution of phenol: 1,1,2,2,-tetrachloroethane = 6:4)) was vacuum-dried at 100°C for 10 hours, a non-crystalline polyester resin film having a thickness of 200 μm was prepared using a film forming device equipped with a single-screw extruder (manufactured by Toyo Precision Machinery Co., Ltd., screw diameter 25 mm, barrel set temperature: 280°C), a T die (width 500 mm, set temperature: 280°C), a cooling roll (set temperature: 50°C), and a winding machine.

[0143] The obtained amorphous polyester-based resin film was subjected to simultaneous biaxial stretching using a stretcher KARO IV manufactured by Bruckner Co. to obtain a polyester film A (in-plane phase Re(590): 89 nm, thickness: 22 μm). The stretching ratio was set to 3 times in the length direction (MD) and 3 times in the width direction (TD). The stretching temperature was set to 90°C, and the stretching speed was set to 10% sec in both MD and TD. In addition, after the stretching treatment, heat treatment was performed at 140°C for 10 seconds while maintaining the size.

[0144] [Manufacturing Example 2] Manufacture of Polyester Film B

[0145] The stretching speed was set to 50% sec in both MD and TD, and otherwise, the same operation as in Manufacturing Example 1 was performed to obtain a polyester film B (in-plane phase Re(590): 41 nm, thickness: 22 μm).

[0146] [Manufacturing Example 3] Manufacture of Polyester Film C

[0147] After vacuum drying a polyester resin (polyethylene terephthalate, manufactured by Bell Polyester Products, Inc., modification amount of isophthalic acid: 2.5 mol% (with respect to the total number of moles of the entire repeating units of the polymer), modification amount of diethylene glycol: 1.0 mol% (with respect to the total number of moles of the entire repeating units of the polymer), IV value: 0.77 dl / g (measured in a mixed solvent solution of phenol: 1,1,2,2,-tetrachloroethane = 6:4, concentration 0.4 g / dl) at 100°C for 10 hours, a film forming apparatus having a single screw extruder (manufactured by Toyo Precision Machine Co., Ltd., screw diameter 25 mm, barrel set temperature: 280°C), a T die (width 500 mm, set temperature: 280°C), a cooling roll (set temperature: 50°C), and a winding machine was used to produce an amorphous polyester-based resin film having a thickness of 200 μm.

[0148] The obtained amorphous polyester-based resin film was subjected to simultaneous biaxial stretching using a stretcher KARO IV manufactured by Bruckner Co. to obtain a polyester film C (in-plane phase Re(590): 52 nm, thickness: 22 μm). The stretching ratio was set to 3 times in the length direction (MD) and 3 times in the width direction (TD). The stretching temperature was set to 85°C, and the stretching speed was set to 50% sec in both MD and TD. In addition, after the stretching treatment, heat treatment was performed at 180°C for 10 seconds while maintaining the size.

[0149] [Manufacturing Example 4] Manufacture of Polyester Film D

[0150] A polyester film D (in-plane phase Re(590): 68 nm, thickness: 35 μm) was obtained by the same procedure as in Production Example 1, except that the stretching ratio was set to 2.5 times in the length direction (MD) and 2.5 times in the width direction (TD), and the stretching speed was set to 30% sec in both MD and TD.

[0151] [Production Example 5] Production of Polyester Film E

[0152] A polyester film D (in-plane phase Re(590): 68 nm, thickness: 35 μm) was obtained by the same procedure as in Production Example 1, except that the stretching ratio was set to 2.5 times in the length direction (MD) and 2.5 times in the width direction (TD), and the stretching speed was set to 30% sec in both MD and TD.

[0153] [Production Example 6] Production of Polyester Film F

[0154] A polyester film F (in-plane phase Re(590): 114 nm, thickness: 25 μm) was obtained by the same procedure as in Production Example 1, except that the stretching speed was set to 2% sec in both MD and TD.

[0155] [Production Example 7] Production of Polyester Film G

[0156] A polyester film G (in-plane phase Re(590): 54 nm, thickness: 50 μm) was obtained by the same procedure as in Production Example 1, except that the stretching ratio was set to 2 times in both MD and TD, and the stretching speed was set to 2% sec in both MD and TD.

[0157] [Production Example 8] Production of Polyester Film H

[0158] After vacuum drying a polyester resin (polyethylene terephthalate, manufactured by Bell Polyester Products, Inc., modification amount of isophthalic acid: 6.8 mol% (with respect to the total number of moles of all repeating units of the polymer), modification amount of diethylene glycol: 1.0 mol% (with respect to the total number of moles of all repeating units of the polymer), IV value 0.85 dl / g (measured in a mixed solvent solution of phenol: 1, 1, 2, 2-tetrachloroethane = 6:4, concentration 0.4 g / dl) at 100°C for 10 hours, an amorphous polyester resin film having a thickness of 200 μm was produced using a film production apparatus equipped with a single-screw extruder (manufactured by Toyo Precision Machine Co., Ltd., screw diameter 25 mm, barrel set temperature: 280°C), a T die (width 500 mm, set temperature: 280°C), a cooling roll (set temperature: 50°C), and a winding machine.

[0159] The obtained amorphous polyester-based resin film was subjected to simultaneous biaxial stretching with a stretching machine KARO IV manufactured by Bruckner, to obtain a polyester film H (in-plane phase Re(590): 110 nm, thickness: 22 μm). The stretching ratio was set to 3 times in the length direction (MD) and 3 times in the width direction (TD). The stretching temperature was set to 90°C, and the stretching speed was set to 10% / sec in both MD and TD. In addition, after the stretching treatment, heat treatment was performed at 140°C for 10 seconds while maintaining the size.

[0160] [Production Example 9] Production of Polyester Film I

[0161] After vacuum drying a polyester resin (polyethylene terephthalate, manufactured by Bell Polyester Products, Inc., modification amount of isophthalic acid: 2.5 mol% (relative to the number of mol of the total repeating units of the polymer), modification amount of diethylene glycol: 1.0 mol% (relative to the number of mol of the total repeating units of the polymer), IV value: 0.77 dl / g (measured in a mixed solvent solution of phenol: 1,1,2,2,-tetrachloroethane = 6:4, concentration 0.4 g / dl) at 100°C for 10 hours, a polyester film I (in-plane phase Re(590): 17 nm, thickness: 50 μm) having a thickness of 50 μm was produced using a film production device provided with a single screw extruder (manufactured by Toyo Precision Machine Co., Ltd., screw diameter 25 mm, barrel set temperature: 280°C), a T die (width 500 mm, set temperature: 280°C), a cooling roll (set temperature: 50°C), and a winding machine.

[0162] [Production Example 10] Production of Polarizing Member A

[0163] A long roll of a polyvinyl alcohol (PVA)-based resin film (manufactured by KURARAY CO., LTD., product name "PE3000") having a thickness of 30 μm was uniaxially stretched in the length direction so that the length direction became 5.9 times using a roll stretching machine, and at the same time, swelling, dyeing, crosslinking, and cleaning treatments were performed, and finally, a drying treatment was performed, to produce a polarizing member A having a thickness of 12 μm.

[0164] Specifically, in the swelling treatment, the stretched to 2.2 times while being treated in pure water at 20°C. Next, in the dyeing treatment, the stretched to 1.4 times while being treated in an aqueous solution of iodine and potassium iodide having a weight ratio of 1:7 adjusted so that the single transmittance of the obtained polarizing member becomes 45.0% at 30°C. Further, the cross-linking treatment was a 2-stage cross-linking treatment, in the first stage of which the stretched to 1.2 times while being treated in an aqueous solution in which boric acid and potassium iodide were dissolved at 40°C. The boric acid content of the aqueous solution in the first stage of the cross-linking treatment was 5.0% by weight, and the potassium iodide content was 3.0% by weight. In the second stage of the cross-linking treatment, the stretched to 1.6 times while being treated in an aqueous solution in which boric acid and potassium iodide were dissolved at 65°C. The boric acid content of the aqueous solution in the second stage of the cross-linking treatment was 4.3% by weight, and the potassium iodide content was 5.0% by weight. In addition, in the washing treatment, an aqueous solution of potassium iodide at 20°C was used. The potassium iodide content of the aqueous solution in the washing treatment was set to 2.6% by weight. Finally, in the drying treatment, drying was performed at 70°C for 5 minutes to obtain the polarizing member A.

[0165] [Example 1]

[0166] As the base material, an amorphous isophthalic acid copolymerized polyethylene terephthalate (IPA copolymer PET) film (thickness: 100 μm) having a long strip shape and a water absorption rate of 0.75% and a Tg of 75°C was used. A single face of the base material was subjected to corona treatment, and an aqueous solution containing polyvinyl alcohol (degree of polymerization: 4200, saponification degree: 99.2 mol%) and acetyl acetyl-modified PVA (degree of polymerization: 1200, acetyl acetyl-modification degree: 4.6%, saponification degree: 99.0 mol% or more, manufactured by Japan Synthetic Chemical Industry Co., Ltd., trade name "GOHSEFIMER (registered trademark) Z200") at a ratio of 9:1 was applied to the corona-treated face at 25°C and dried to form a PVA-based resin layer having a thickness of 11 μm, and a laminate was produced.

[0167] The obtained laminate was subjected to free-end uniaxial stretching to 2.0 times in the longitudinal direction (lengthwise direction) at different circumferential speeds between rollers in an oven at 120°C (overhead assisted stretching).

[0168] Next, the laminate was immersed in a non-dissolving bath (an aqueous boric acid solution obtained by compounding 4 parts by weight of boric acid with respect to 100 parts by weight of water) at a liquid temperature of 30°C for 30 seconds (non-dissolving treatment).

[0169] Next, the laminate was immersed in a dyeing bath at a liquid temperature of 30°C while adjusting the iodine concentration and the immersion time so that the polarizing plate becomes a predetermined transmittance. In this example, the laminate was immersed in an aqueous iodine solution obtained by compounding 0.2 parts by weight of iodine and 1.5 parts by weight of potassium iodide with respect to 100 parts by weight of water for 60 seconds (dyeing treatment).

[0170] Next, the laminate was immersed in a cross-linking bath (an aqueous solution of boric acid obtained by mixing 3 parts by weight of boric acid and 3 parts by weight of potassium iodide with respect to 100 parts by weight of water) at a liquid temperature of 30°C for 30 seconds (cross-linking treatment).

[0171] Subsequently, while the laminate was immersed in an aqueous solution of boric acid (an aqueous solution obtained by mixing 4 parts by weight of boric acid and 5 parts by weight of potassium iodide with respect to 100 parts by weight of water) at a liquid temperature of 70°C, it was stretched in the longitudinal direction (lengthwise direction) at a total stretch ratio of 5.5 times between rollers having different circumferential speeds (uniaxial stretching in water).

[0172] Subsequently, the laminate was immersed in a washing bath (an aqueous solution obtained by mixing 4 parts by weight of potassium iodide with respect to 100 parts by weight of water) at a liquid temperature of 30°C (washing treatment).

[0173] In addition, the polyester film A produced in Production Example 1 was subjected to corona treatment, and a water solution in which 15.2 wt% of "SUPERFLEX 210R" manufactured by the First Industrial Pharmaceutical Co., Ltd. and 2.7 wt% of "WS-700" manufactured by Nippon Shokubai Co., Ltd. were dissolved was applied in a manner such that the film thickness after drying was 300 μm, and drying was performed at 80°C for 1 minute to obtain a polyester film A with an easy-adhesion layer.

[0174] A PVA-based resin water solution ("GOHSEFIMER (registered trademark) Z-200" manufactured by Nippon Shokubai Co., Ltd., resin concentration: 3 wt%) was applied to the surface of the PVA-based resin layer of the above laminate, and the above polyester film A with an easy-adhesion layer was attached. The obtained laminate was heated in an oven maintained at 60°C for 5 minutes. Subsequently, the substrate was peeled from the PVA-based resin layer to obtain a polarizing plate (polarizing member B (transmittance: 42.3%, thickness: 5 μm) / protective film (polyester film)). Note that the polyester film A and the polarizing member B were laminated in a manner such that the MD direction of the polyester film A was substantially parallel to the absorption axis direction of the polarizing member.

[0175] The obtained polarizing plate was subjected to the above evaluations (2) to (7). The results are shown in Table 1.

[0176] [Example 2]

[0177] A polarizing plate was obtained in the same manner as in Example 1, except that the polyester film B obtained in Production Example 2 was used instead of the polyester film A produced in Production Example 1.

[0178] The obtained polarizing plate was subjected to the above evaluations (2) to (7). The results are shown in Table 1.

[0179] [Example 3]

[0180] The single-side of the polarizing member A obtained in Production Example 10 was coated with a PVA-based resin aqueous solution (manufactured by Nippon Shokubai Co., Ltd., trade name "GOHSEFIMER (registered trademark) Z-200", resin concentration: 3% by weight) and the polyester film B obtained in Production Example 2 was attached thereto. The resulting laminate was heated in an oven maintained at 60°C for 5 minutes to obtain a polarizing plate.

[0181] The resulting polarizing plate was subjected to the above evaluations (2) to (7). The results are shown in Table 1.

[0182] [Example 4]

[0183] The polyester film C obtained in Production Example 3 was used instead of the polyester film A produced in Production Example 1, and otherwise, the same operation as in Example 1 was performed to obtain a polarizing plate.

[0184] The resulting polarizing plate was subjected to the above evaluations (2) to (7). The results are shown in Table 1.

[0185] [Example 5]

[0186] The polyester film D obtained in Production Example 4 was used instead of the polyester film A produced in Production Example 1, and otherwise, the same operation as in Example 1 was performed to obtain a polarizing plate. The resulting polarizing plate was subjected to the above evaluations (2) to (7). The results are shown in Table 1.

[0187] [Example 6]

[0188] The polyester film E obtained in Production Example 5 was used instead of the polyester film A produced in Production Example 1, and otherwise, the same operation as in Example 1 was performed to obtain a polarizing plate. The resulting polarizing plate was subjected to the above evaluations (2) to (7). The results are shown in Table 1.

[0189] [Comparative Example 1]

[0190] The polyester film F obtained in Production Example 6 was used instead of the polyester film A produced in Production Example 1, and otherwise, the same operation as in Example 1 was performed to obtain a polarizing plate.

[0191] The resulting polarizing plate was subjected to the above evaluations (2) to (7). The results are shown in Table 1.

[0192] [Comparative Example 2]

[0193] The polyester film F obtained in Production Example 6 was used instead of the polyester film A produced in Production Example 1, and otherwise, the same operation as in Example 3 was performed to obtain a polarizing plate.

[0194] The resulting polarizing plate was subjected to the above evaluations (2) to (7). The results are shown in Table 1.

[0195] [Comparative Example 3]

[0196] A polarizing plate was obtained in the same manner as in Example 1, except that the polyester film G obtained in Production Example 7 was used instead of the polyester film A produced in Production Example 1.

[0197] The obtained polarizing plate was subjected to the above evaluations (2) to (7). The results are shown in Table 1.

[0198] [Comparative Example 4]

[0199] A polarizing plate was obtained in the same manner as in Example 1, except that the polyester film H obtained in Production Example 8 was used instead of the polyester film A produced in Production Example 1.

[0200] The obtained polarizing plate was subjected to the above evaluations (2) to (7). The results are shown in Table 1.

[0201] [Comparative Example 5]

[0202] A polarizing plate was obtained in the same manner as in Example 1, except that the polyester film J (manufactured by Toyobo Co., Ltd., trade name "Cosmoshine A4100", in-plane phase Re(590): 7800 nm, thickness: 75 μm) was used instead of the polyester film A produced in Production Example 1.

[0203] The obtained polarizing plate was subjected to the above evaluations (2) to (7). The results are shown in Table 1.

[0204] [Comparative Example 6]

[0205] A polarizing plate was obtained in the same manner as in Example 1, except that the polyester film K (manufactured by Mitsubishi Chemical Corporation, trade name "T100-J25", in-plane phase Re(590): 525 nm, thickness: 25 μm) was used instead of the polyester film A produced in Production Example 1.

[0206] The obtained polarizing plate was subjected to the above evaluations (2) to (7). The results are shown in Table 1.

[0207] [Comparative Example 7]

[0208] A polarizing plate was obtained in the same manner as in Example 1, except that the polyester film I obtained in Production Example 9 was used instead of the polyester film A produced in Production Example 1.

[0209] The obtained polarizing plate was subjected to the above evaluations (2) to (7). The results are shown in Table 1.

[0210] [Table 1]

[0211]

Claims

1. A polyester film, which is a single-layer film, having an in-plane phase difference Re(590) of less than 100 nm and a coefficient of linear expansion in the first direction of 0 / ℃~4.0×10⁻⁶. -5 / ℃, The absolute value of the difference between the linear expansion coefficient in the first direction and the linear expansion coefficient in the second direction orthogonal to the first direction is 0.7 × 10⁻⁶. -5 / ℃ below, The orientation function (f) of the polyester film is 0.01 to 0.

24. The polyester film is formed from polyethylene terephthalate and / or modified polyethylene terephthalate, and the polyester film is a stretched film.

2. The polyester film according to claim 1, wherein, The modified polyethylene terephthalate comprises building blocks derived from diethylene glycol, 1,4-butanediol, 1,3-propanediol, or isophthalic acid.

3. The polyester film according to claim 1 or 2, wherein the crystallinity determined by DSC is 30% or more.

4. A polarizing plate comprising: a polarizing element and a polyester film as described in any one of claims 1 to 3 disposed on one side of the polarizing element.

5. The polarizing plate according to claim 4, wherein, The absolute value of the difference between the linear expansion coefficient of the polyester film in the first direction and the linear expansion coefficient of the polarizer in the direction parallel to the first direction, and the absolute value of the difference between the linear expansion coefficient of the polyester film in the second direction orthogonal to the first direction and the linear expansion coefficient of the polarizer in the direction parallel to the second direction, are both 3.5 × 10⁻⁶. -5 / ℃ below.

6. The polarizing plate according to claim 5, wherein, The thickness of the polarizing element is less than 20 μm.

7. The polarizing plate according to any one of claims 4 to 6, further comprising an easy-to-adhere layer disposed on the polarizing side of the polyester film.

8. The polarizing plate according to claim 7, wherein, The adhesive layer contains microparticles.

9. The polarizing plate according to claim 7, wherein, The thickness of the easy-to-adhere layer is less than 0.35 μm.

10. The polarizing plate according to claim 7, wherein, The refractive index of the easy-to-adhere layer is below 1.55.

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