Polyester film

Abstract

To provide a polyester film that does not show interference colors when mounted on a display device, such as a big-screen liquid crystal display, and also has high flexibility.SOLUTION: A polyester film has a flexural rigidity of 0.2 mN or more and 5.0 mN or less in both the longitudinal direction and the width direction, determined with a loop stiffness tester, and also has a chroma ΔC of 0.0 or more and 0.5 or less.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a polyester film.

Background Art

[0002] Thermoplastic resin films, particularly polyester films, have excellent properties such as mechanical properties, electrical properties, dimensional stability, transparency, and chemical resistance, and are thus widely used as base films in many applications such as magnetic recording materials and packaging materials. In particular, in recent years, in the fields of flat panel displays and touch panels, the demand for various optical films such as cover films for preventing damage to the surface of display devices of flexible displays has been increasing. The practical application of an image display device using a self-luminous body called an Organic Light Emitting Diode (hereinafter referred to as an "organic electroluminescence display device") has been progressing. This organic electroluminescence display device uses a self-luminous body, so it is not only excellent in visibility and response speed compared to conventional liquid crystal display devices, but also does not require an auxiliary lighting device such as a backlight, so it can be thinned and made flexible as a display device. For this reason, the development of flexible display devices that can be folded and rolled up has been accelerating, and fold resistance is required. In addition, for the use of polarizer protection films, replacement from conventional TAC (triacetyl cellulose) films to polyester films has been actively studied for the purpose of cost reduction. For example, as a method for improving scratch resistance for image display devices, a hard coat film in which hard coat layers are laminated on both surfaces of a base material containing a polyester film has been proposed (Patent Document 1). Also, for flexible displays, an antireflection film in which an antireflection layer is provided on at least one surface of a flexible transparent resin film containing a polyester film has been proposed (Patent Document 2).

[0003] In addition, when a biaxially oriented film is used as a polarizer protective film, due to the large birefringence in the plane direction and the thickness direction, rainbow unevenness can be visually confirmed when viewed obliquely. For this reason, in the biaxially oriented polyester films that have been conventionally studied, a technique is known in which the birefringence is kept low by suppressing the orientation in the plane direction to suppress rainbow unevenness (Patent Document 3). In addition, techniques have been devised in which the film is largely oriented in one axial direction and the birefringence is largely controlled so that rainbow unevenness is not visible to the naked eye (Patent Documents 4 and 5). Furthermore, for flexible display devices, an antireflection film has been proposed in which an antireflection layer is provided on at least one surface of a flexible transparent resin film including a polyester film (for example, Patent Document 6).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Summary of the Invention

Problems to be Solved by the Invention

[0005] When any of the above prior arts is used as a film for an image display device, it is characterized by a hard coat layer and an antireflection layer. The film serving as the base material does not consider bending resistance, and it has been difficult to apply it to flexible displays. In addition, in the method of suppressing the alignment in the plane direction, the suppression of rainbow unevenness when viewed obliquely is not sufficient, and in the method of greatly controlling birefringence, it has been difficult to develop it for foldable applications that are repeatedly bent due to the film thickness.

[0006] Therefore, an object of the present invention is to solve the above-mentioned drawbacks and provide a polyester film that has flexibility against bending and is difficult to visually recognize rainbow unevenness.

Means for Solving the Problems

[0007] In order to solve the above problems, the polyester film of the present invention has the following configuration. [1] A polyester film having a bending rigidity obtained from a loop stiffness tester of 0.2 mN or more and 5.0 mN or less in both the longitudinal direction and the width direction, and a color ΔC of 0.0 or more and 0.5 or less. [2] A polyester film having an aromatic dicarboxylic acid component and a diol component as main constituent components, wherein the aromatic dicarboxylic acid component has terephthalic acid or naphthalenedicarboxylic acid as the main acid component, and the diol component contains ethylene glycol as the main diol component. When the content of the secondary acid component A different from the main acid component selected from terephthalic acid, naphthalenedicarboxylic acid, isophthalic acid, and 1,4-cyclohexanedicarboxylic acid in the total acid components is a (mol%), and the content of the secondary diol component B selected from polyethylene glycol, cyclohexanedimethanol, 1,3-propanediol, isosorbide, and neopentyl glycol in the total diol components is b (mol%), the polyester film according to [1], which satisfies the following formulas (1) to (3). 0 ≦ a < 30 (1) 0 ≦ b ≦ 30 (2) 5 ≦ a + b ≦ 30 (3) [3] A polyester film having an aromatic dicarboxylic acid component and a diol component as main constituent components, wherein the content of naphthalenedicarboxylic acid in the total acid components as the aromatic dicarboxylic acid component is 70 mol% or more, the polyester film according to [1] or [2]. [4] The polyester film according to any one of [1] to [3], wherein the thermal shrinkage rate at 85 ° C is 1.0% or less in both the longitudinal direction and the width direction. [5] The polyester film according to any one of [1] to [4], wherein the front retardation Re of the film is less than 3000 nm. [6] The polyester film according to any one of [1] to [5], wherein the elongation absorption parameter is 0.7 or more and 1.0 or less. [7] The polyester film according to any one of [1] to [6], which is mounted on a polarizing plate. [8] The polyester film according to any one of [1] to [7], which is mounted on a display. [Effect of the Invention]

[0008] The polyester film of the present invention can display with high quality even when mounted on a display device such as a flexible display, and has flexibility, so it is particularly suitable for use in a display that involves repeated bending. [Brief Description of the Drawings]

[0009]

Figure 1

Figure 2

[0010] The polyester film of the present invention is a polyester film having a bending rigidity obtained from a loop stiffness tester of 0.2 mN or more and 5.0 mN or less in both the longitudinal direction (MD direction) and the width direction (TD direction), and a color ΔC of 0.0 or more and 0.5 or less. Hereinafter, this polyester film will be described in detail.

[0011] The polyester film of the present invention refers to a film mainly composed of polyester as the resin constituting the film. The polyester referred to in the present invention has a dicarboxylic acid constituent component and a diol constituent component. In the present invention, the constituent component refers to the smallest unit that can be obtained by hydrolyzing polyester.

[0012] Examples of the dicarboxylic acid constituent component constituting the polyester of the present invention include aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 1,8-naphthalenedicarboxylic acid, 4,4'-diphenyldicarboxylic acid, 4,4'-diphenyletherdicarboxylic acid, 5-sodium sulfoisophthalic acid, phenylendodicarboxylic acid, anthracenedicarboxylic acid, phenanthrenedicarboxylic acid, 9,9'-bis(4-carboxyphenyl)fluorene acid; aliphatic dicarboxylic acids such as malonic acid, succinic acid, glutaric acid, adipic acid, suberic acid, sebacic acid, dodecanedioic acid, dimer acid, eicosanedioic acid, pimelic acid, azelaic acid, methylmalonic acid, ethylmalonic acid; alicyclic dicarboxylic acids such as adamantanedicarboxylic acid, norbornenedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, decalindicarboxylic acid; and ester derivatives thereof. Among them, it is preferable to use an aromatic dicarboxylic acid component as the main constituent component. These structural units derived from dicarboxylic acids may be used alone or in combination of two or more, and further, oxyacids such as hydroxybenzoic acid may be partially copolymerized.

[0013] As the structural unit of the diol component constituting the polyester of the present invention, for example, ethylene glycol, 1,3-butanediol, 1,4-butanediol, tetramethylene glycol, 1,6-hexanediol, 1,2-cyclohexanedimethanol, 1,3-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, diethylene glycol, triethylene glycol, polyalkylene glycol, isosorbide, 1,2-propanediol, 1,3-propanediol, neopentyl glycol, 1,5-pentanediol, 1,7-heptanediol, 2,2-bis(4-hydroxyethoxyphenyl)propane, spiroglycol and the like can be mentioned. These structural units derived from diols may be used alone or in combination of two or more.

[0014] The bending rigidity obtained from the loop stiffness tester of the polyester film of the present invention is 0.2 mN or more and 5.0 mN or less in both the longitudinal direction and the width direction. By having a bending rigidity of 5.0 mN or less, it is excellent in bending resistance such as repeated bending. From the viewpoint of bending resistance, the smaller the value of the bending rigidity, the more preferable it is, and it is more preferable that the bending rigidity is 4.0 mN or less in both the longitudinal direction and the width direction. Also, from the viewpoint of handleability, the bending rigidity is preferably 0.2 mN or more in both the longitudinal direction and the width direction. In order to control the bending rigidity within the above range, it is most effective to set the film thickness to 10 μm or more and 50 μm or less, and it is more preferable that the film thickness is 10 μm or more and 40 μm or less. Furthermore, in order to control the bending rigidity to be low, it is a preferable method to reduce the Young's modulus, and it is preferable that the Young's modulus in both the longitudinal direction and the width direction is 0.5 GPa or more and 5.0 GPa or less, and it is more preferable that it is 0.5 GPa or more and 4.0 GPa or less. Incidentally, if the film thickness is too thick, it tends to be difficult to make the bending rigidity 5.0 mN or less regardless of the Young's modulus of the polyester film. As long as it does not inhibit the effects of the present invention, a plasticizer or an elastomer may be added to reduce the Young's modulus. Examples of the elastomer include Hytrel manufactured by DuPont. However, as will be described later, when reducing ΔC, it is an effective method to use 70 mol% or more of the total acid component of the polyester resin as naphthalenedicarboxylic acid, but with such a resin composition, the bending rigidity tends to be even larger. Therefore, when containing 70 mol% or more of naphthalenedicarboxylic acid with respect to the total acid component, from the viewpoint of making the bending rigidity small, it is preferable that the film thickness of the polyester film is 40 μm or less.

[0015] The polyester film of the present invention has a color difference ΔC of 0.0 or more and 0.5 or less. By controlling ΔC within this range, moiré unevenness can be suppressed when the film is used as a screen protection film or a polarizer protection film, and high visibility can be ensured without coloring not only when viewed from the front direction but also when viewed from a viewing angle inclined by 60° to 80° from the normal direction of the film surface. The lower the ΔC, the more effectively moiré unevenness can be suppressed, and it is more preferable that ΔC is 0.0 or more and 0.3 or less. The value of ΔC can be controlled by the front retardation of the film (the front retardation of the film is a value obtained as |refractive index in the slow axis direction - refractive index in the fast axis direction| × film thickness. Hereinafter, the front retardation of the film may be referred to as Re.) and the Nx coefficient obtained from the refractive index of the film (the Nx coefficient is a value obtained as |refractive index in the slow axis direction - refractive index in the fast axis direction| / |refractive index in the fast axis direction - refractive index in the thickness direction|. Hereinafter, the Nx coefficient may be referred to as Nx.). The larger the values of Re and Nx, the smaller the value of ΔC. However, even if either Re or Nx is sufficiently large, if the other value is small, it may not be possible to achieve ΔC ≦ 0.5. Specifically, ΔC is controlled to be 0.5 or less when the combination of Re and Nx satisfies the following formula (4), and ΔC is controlled to be 0.3 or less when the combination of Re and Nx further satisfies formula (5). (Re - 710) × (Nx - 0.18) - 2300 ≧ 0 (4) (Re - 1700) × (Nx - 0.44) - 1600 ≧ 0 (5) As a method for increasing Re, first, increasing the film thickness can be mentioned. However, as described above, when the film thickness is large, the bending rigidity becomes large, so it is preferable to control the film thickness within the aforementioned range. Hereinafter, a method for controlling Re and Nx within this film thickness range to make ΔC 0.5 or less will be described.

[0016] Re increases by increasing the refractive index difference between the slow axis direction and the fast axis direction (hereinafter, the refractive index difference between the slow axis direction and the fast axis direction may be referred to as the in-plane refractive index difference). To increase the in-plane refractive index difference, Re increases by adopting a stretching method with enhanced uniaxial anisotropy. However, since Nx takes a larger value as the refractive index in the thickness direction is larger, the more the film is not surface-oriented, the larger the value it takes. From the perspective of suppressing surface orientation, it is preferable that the total stretching ratio is smaller, and Nx takes a large value. From the above points, it is necessary to stretch under suitable stretching conditions to satisfy Equation (4) with both Re and Nx being large values. In the case of a film stretched only in one axial direction, the stretching ratio needs to be 2.5 to 5.0 times, and it is more preferable that the stretching ratio is 3.0 to 5.0 times. To satisfy Equation (4) by biaxial stretching, after setting the total stretching ratio to 8.0 times or less, the stretching ratio in the other direction with respect to the stretching ratio in one axial direction should be 2.0 times or more, and the stretching ratio in the direction with a high stretching ratio should be 3.0 times or more. It is more preferable that the stretching ratio in the direction with a low stretching ratio is 1.5 times or less. In addition, from the perspective of maintaining the balance between Re and Nx and satisfying Equation (4) while keeping the film thickness within the aforementioned range, it is preferable that Re is less than 3000 nm.

[0017] In the heat treatment process after stretching, from the perspective of suppressing the surface orientation of the film and increasing the Nx coefficient, it is preferable that the heat treatment temperature is equal to or lower than the melting point of the constituent resin - 50°C, and it is more preferable that it is equal to or lower than the melting point of the constituent resin - 60°C. On the other hand, from the perspective of preventing deformation during heating such as processing after mounting on a polarizing plate with thermal dimensional stability, the thermal shrinkage rate at 85°C is preferably 1.0% or less in both the longitudinal direction and the width direction. To make the thermal shrinkage rate 1.0% or less in both the longitudinal direction and the width direction, it is necessary to set the heat treatment temperature to 150°C or higher. From the above, the heat treatment temperature in the heat treatment process is preferably 150°C or higher and the melting point of the constituent resin - 50°C, and more preferably 150°C or higher and the melting point of the constituent resin - 60°C or lower.

[0018] The polyester film of the present invention is a polyester film mainly composed of an aromatic dicarboxylic acid component and a diol component, and preferably the aromatic dicarboxylic acid component contains terephthalic acid or naphthalenedicarboxylic acid as a main acid component, and the diol component contains ethylene glycol as a main diol component. In the present invention, the term "main acid component" refers to the component contained in the largest amount as an acid component of the polyester, and the term "secondary acid component" refers to the acid component of the polyester other than the main acid component. Furthermore, the term "main diol component" refers to the component contained in the largest amount as a diol component of the polyester, and the term "secondary diol component" refers to the diol component of the polyester other than the main diol component.

[0019] To increase Nx, it is preferable to incorporate a copolymerization component into the constituent resin to increase amorphousness and suppress planar orientation. The polyester film of the present invention preferably contains a secondary acid component or secondary diol component other than the aforementioned main acid component and main diol component (hereinafter, the secondary acid component and secondary diol component may be collectively referred to as copolymerization component). When the main acid component is terephthalic acid, isophthalic acid, 1,4-cyclohexanedicarboxylic acid, or naphthalenedicarboxylic acid is preferred. When the main acid component is naphthalenedicarboxylic acid, terephthalic acid, isophthalic acid, or 1,4-cyclohexanedicarboxylic acid is preferred. When the main acid component is naphthalenedicarboxylic acid, polyethylene glycol, cyclohexanedimethanol, 1,3-propanediol, isosorbate, or neopentyl glycol is preferred. Among these, 1,3-propanediol or neopentyl glycol is preferred as the secondary diol component, as it introduces a twisted structure into the molecular chain, thereby increasing the thickness-direction refractive index and reducing ΔC. On the other hand, if the amount of copolymerization component is too large and the amorphousness is too high, the thermal dimensional stability of the film will decrease.

[0020] From the above, when the content of the secondary acid component A different from the main acid component selected from terephthalic acid, naphthalenedicarboxylic acid, isophthalic acid, and 1,4-cyclohexanedicarboxylic acid is a (mol%) with respect to the total acid components, and the content of the secondary diol component B selected from polyethylene glycol, cyclohexanedimethanol, 1,3-propanediol, isosorbide, and neopentyl glycol is b (mol%) with respect to the total diol components, it is preferable to satisfy the following formulas (1) to (3) from the viewpoint of achieving a balance between Nx and the thermal shrinkage rate, and more preferably to satisfy (1)’, (2)’, and (3)’. 0 ≦ a < 30 (1) 0 ≦ b ≦ 30 (2) 5 ≦ a + b ≦ 30 (3) 1 ≦ a ≦ 25 (1)’ 1 ≦ b ≦ 25 (2)’ 8 ≦ a + b ≦ 25 (3)’ Also, from the viewpoint of increasing Re, it is preferable that 70 mol% or more of the acid components are naphthalenedicarboxylic acid. By containing 70 mol% or more of naphthalenedicarboxylic acid, the in-plane refractive index difference tends to be large. As a result, even at a low draw ratio, a high Re can be achieved, making it easier to increase the Nx coefficient.

[0021] The polyester film of the present invention preferably has a stretch absorption parameter of 0.7 or more and 1.0 or less, more preferably 0.8 or more and 1.0 or less. The stretch absorption parameter, as used herein, refers to the value obtained by (11) stretch absorption parameter described in the section on property measurement methods and effect evaluation methods, and is an index indicating how easily a film shrinks in a direction perpendicular to the stretching direction when stretched. When a polyester film is used in a flexible device, it is expected that the polyester film will be subjected to repeated bending loads. Therefore, it is preferable that the polyester film has durability against such loads (flex resistance). By controlling the stretch absorption parameter within this range, this load can be reduced, resulting in excellent bending resistance. By setting the stretch absorption parameter at or above the aforementioned lower limit, the load during bending can be sufficiently absorbed. On the other hand, by setting the stretch absorption parameter at or below the aforementioned upper limit, excessive deformation during bending can be prevented. To set the stretch absorption parameter at or above the aforementioned lower limit, it is preferable to cool the film to 50°C or less after stretching and then perform a heat treatment, and more preferably to 35°C or less. In order to make the stretch absorption parameter equal to or less than the upper limit mentioned above, a method of stretching the film by 1.05 times or more in both the longitudinal direction and the width direction can be mentioned.

[0022] The polyester film of the present invention may be a single-layer polyester film or a laminated polyester film. For example, by providing a slippery layer containing particles on both surface layers of the polyester film for the purpose of imparting the film with handleability and scratch resistance, the amount of particles added that would otherwise reduce the total light transmittance can be reduced throughout the film, thereby imparting slipperiness and suppressing the decrease in total light transmittance. Furthermore, the polyester film of the present invention may be laminated with various functional layers to the extent that the effects of the present invention are not impaired. For example, a laminate sheet having a layer containing a curable resin on at least one side of the polyester film of the present invention can be used. By forming a laminate sheet having a layer containing a curable resin on at least one side of the polyester film of the present invention, it is possible to enhance the scratch prevention effect against impact from the curable resin layer side. Furthermore, an easy-adhesion layer may be provided on the surface. The provision of an easy-adhesion layer improves the adhesion of the curable resin, etc.

[0023] The polyester film of the present invention may also contain various additives, such as antioxidants, heat stabilizers, weather stabilizers, ultraviolet absorbers, organic lubricants, pigments, dyes, organic or inorganic fine particles, fillers, antistatic agents, and nucleating agents, to the extent that they do not deteriorate the properties of the film.

[0024] Next, a preferred method for producing the film of the present invention will be explained below, but the present invention should not be construed as being limited to such an example.

[0025] The resin constituting the polyester film is a copolymer of the aforementioned acid component and diol component, which is fed into a vented twin-screw extruder and melt-extruded. The extruder is preferably circulated under a nitrogen atmosphere, with an oxygen concentration of 0.7% by volume or less and a resin temperature of 240°C to 320°C. The extruder is then passed through a filter or gear pump to remove impurities and to equalize the extrusion rate, before being discharged into a sheet form onto a cooling drum through a T-die. The sheet polymer is then adhered to the casting drum using a static electricity application method, in which high-voltage electrodes are used to statically bond the resin to the cooling drum; a casting method, in which a water film is formed between the casting drum and the extruded polymer sheet; a casting drum temperature below the glass transition point of the polyester resin to bond the extruded polymer; or a combination of these methods. The sheet polymer is then cooled and solidified to obtain an unstretched film. Among these casting methods, the static electricity application method is preferred when using polyester, from the standpoints of productivity and flatness.

[0026] The polyester film of the present invention is preferably stretched in at least one direction from the viewpoints of heat resistance, dimensional stability, mechanical strength, flatness, and thickness unevenness. When biaxially oriented, the film can be obtained by stretching an unstretched film in the longitudinal direction and then in the width direction, or by a sequential biaxial stretching method in which an unstretched film is stretched in the width direction and then in the longitudinal direction, or by a simultaneous biaxial stretching method in which the film is stretched in the longitudinal and width directions almost simultaneously. Alternatively, the film may be further stretched in the longitudinal or width direction after biaxial orientation.

[0027] In such a stretching method, the stretching ratio in the first axis is preferably 1.05 to 2.0 times, or 2.5 to 4.0 times. If the stretching ratio in the first axis is more than 2.0 times but less than 2.5 times, thickness unevenness may increase and the phase difference may vary. The stretching speed is preferably 5,000% / min to 100,000% / min. The preheating temperature is preferably at least 30°C below the glass transition temperature of the resin and not more than 10°C above the glass transition temperature of the resin, and the stretching temperature is preferably at least 40°C above the glass transition temperature of the resin.

[0028] The biaxial stretching ratio is preferably 2.8 to 4.0 times, and more preferably 3 to 3.8 times. The biaxial stretching speed is desirably 5,000% / min to 100,000% / min. The preheating temperature is preferably at least 20°C below the glass transition temperature of the resin - and not more than 20°C above the glass transition temperature of the resin, and the stretching temperature is preferably at least the glass transition temperature of the resin and not more than 60°C above the glass transition temperature of the resin.

[0029] In the present invention, the film may be heat-treated after each stretching step. Heat treatment can be performed by any conventional method, such as in an oven or on a heated roll. The heat treatment temperature is preferably 100°C or higher and less than the melting point (Tm) of the constituent resin. When heat treatment is performed after each step, the highest heat treatment temperature is preferably performed after the final stretching step, from the viewpoint of stretchability. Furthermore, considering dimensional stability, the highest heat treatment temperature is preferably 150°C or higher, and from the viewpoint of suppressing planar orientation, it is preferably controlled to less than the melting point (Tm) of the constituent resin minus 50°C. The heat treatment time can be any value within a range that does not deteriorate the properties, and is preferably 5 to 60 seconds, more preferably 10 to 40 seconds, and most preferably 15 to 30 seconds. In order to control the elongation absorption parameter, this heat treatment step is preferably performed after cooling the film temperature to 50°C or lower, and more preferably to 35°C or lower.

[0030] Furthermore, in order to improve the adhesion to the polarizer, it is also possible to perform corona treatment on at least one side or coat an easy-adhesion layer. As a method of providing the coating layer in-line during the film manufacturing process, a method of uniformly applying a dispersion of a coating layer composition in water on a film that has been uniaxially stretched at least once using a metering wire bar, a gravure roll, etc., and drying the coating agent while stretching is preferable. At that time, the thickness of the easy-adhesion layer is preferably 0.01 μm or more and 1 μm or less. Also, various additives such as antioxidants, heat stabilizers, ultraviolet absorbers, infrared absorbers, pigments, dyes, organic or inorganic particles, antistatic agents, nucleating agents, etc. may be added to the easy-adhesion layer. The resin preferably used for the easy-adhesion layer is preferably at least one resin selected from acrylic resins, polyester resins, and urethane resins from the viewpoints of adhesiveness and handleability. Furthermore, it is also preferably annealed at 90 to 200 °C.

[0031] Since the polyester film of the present invention has no visible rainbow unevenness under specific conditions and is flexible, when mounted on an image display device that has been subjected to repeated bending, such as a cover of a display or curved surface processing, the exhibition of interference colors is suppressed. Taking advantage of its characteristics, it can also be bonded to a PVA sheet (polarizer) created by containing and orienting iodine in PVA and used as a polarizing plate.

[0032] Since the polyester film of the present invention is excellent in bending resistance, it can be particularly preferably used as a flexible device used in a foldable display or the like. For example, by applying it as a cover film for an organic electroluminescence display device, it is possible to prevent damage to the surface of the display device without impairing the flexibility of the display device. Also, not only optical films, but also various cover films and industrial material films such as packaging applications that utilize the characteristics of the present invention are mentioned as preferable embodiments.

[0033] (Method for Measuring Characteristics and Method for Evaluating Effects) The method for measuring the properties and the method for evaluating the effects in the present invention are as follows.

[0034] (1) Composition of polyester The film was hydrolyzed with alkali, and each component was analyzed by gas chromatography or high-performance liquid chromatography, and the composition ratio was determined from the peak area of each component. An example is shown below. The dicarboxylic acid constituent components and other constituent components were measured by high-performance liquid chromatography. The measurement conditions can be analyzed by known methods, and an example of the measurement conditions is shown below. In addition, the measurement was carried out after filtering and separating the inorganic particles. Apparatus: Shimadzu LC-10A Column: YMC-Pack ODS-A 150×4.6mm S-5μm 120A Column temperature: 40°C Flow rate: 1.2 ml / min Detector: UV 240 nm The quantification of the diol constituent components and other constituent components can be analyzed by known methods using gas chromatography. An example of the measurement conditions is shown below. Apparatus: Shimadzu 9A (manufactured by Shimadzu Corporation) Column: SUPELCOWAX-10 capillary column 30 m Column temperature: 140°C to 250°C (heating rate 5°C / min) Flow rate: Nitrogen 25 ml / min Detector: FID.

[0035] (2) Intrinsic viscosity of polyester The intrinsic viscosity of the polyester resin was measured at 25°C using an Ostwald viscometer after dissolving the polyester in orthochlorophenol. In the case of a laminated film, the intrinsic viscosity of each layer alone can be evaluated by scraping off each layer of the film according to the laminated thickness.

[0036] (3) Film thickness The film thickness was measured at five arbitrary positions with 10 films stacked using a dial gauge in accordance with JIS K7130 (1992) A-2 method. The average value was divided by 10 to obtain the film thickness.

[0037] (4) Refractive index Using the sodium D line (wavelength 589 nm) as the light source, methylene iodide as the mounting liquid, and an Abbe refractometer 4T (manufactured by Atago Co., Ltd.) at 25°C, the refractive indices in the longitudinal, width, and thickness directions of the film were measured in accordance with JIS K7142 (2014) A method. A test piece with a refractive index of 1.74 was used.

[0038] (5) Longitudinal direction, width direction, slow axis direction, fast axis direction In the present invention, the flow direction during film production is defined as the longitudinal direction, and the direction perpendicular to the longitudinal direction is defined as the width direction. For the slow axis direction and the fast axis direction, with an arbitrary direction as the 0° reference, the refractive indices from 0 to 360° were measured at 15° intervals, and the direction with the highest refractive index was defined as the slow axis direction, and the direction perpendicular to it was defined as the fast axis direction. When the longitudinal direction and the width direction are unknown, it is assumed that the longitudinal direction coincides with the slow axis direction, and the width direction coincides with the fast axis direction. The refractive index was measured in accordance with JIS K7142 (2014) A method using the sodium D line (wavelength 589 nm) as the light source, methylene iodide as the mounting liquid, and an Abbe refractometer 4T (manufactured by Atago Co., Ltd.) at 25°C. A test piece with a refractive index of 1.74 was used. (6) Re, Nx coefficient The Re and Nx coefficients were calculated from the following formulas (6) and (7) using the film thickness obtained by the method described in (3), the refractive indices in the longitudinal, width, and thickness directions obtained by the method described in (4), and the refractive indices in the slow axis direction and the fast axis direction obtained by the method described in (5).

[0039] Re = (refractive index in the longitudinal direction - refractive index in the width direction) × film thickness (6) Nx coefficient = |refractive index in the slow axis direction - refractive index in the fast axis direction| / |refractive index in the fast axis direction - refractive index in the thickness direction| (7) (7) Glass transition temperature Tg, melting point Tm In accordance with JIS K7121 (1999), using the differential scanning calorimeter robot DSC-RDC6220 manufactured by Seiko Instruments Inc., and for data analysis, using the thermal analysis rheology system software "Muse" manufactured by SII NanoTechnology Inc., the glass transition temperature Tg [°C] and melting point Tm [°C] were measured and analyzed in accordance with JIS K-7122 (1987 edition). Using 5 mg of the polyester film of the present invention as a sample, the temperature at the peak of the endothermic peak obtained from the DSC curve when the temperature was raised from 25°C to 300°C at a rate of 20°C / min was taken as the melting point Tm [°C]. The glass transition temperature Tg [°C] was determined from the point where a straight line equidistant from the vertical axis direction from the extended straight line of each baseline intersects the curve of the step-like change portion of the glass transition in the step-like change portion of the differential scanning calorimetry chart. When the melting point was not observed, it was designated as "ND". Also, when there were multiple crystal melting peaks, the temperature with the largest absolute value of the heat quantity was adopted respectively.

[0040] (8) Measurement of spectral transmittance and calculation of ΔC The film was cut into a length of 4.0 cm and a width of 5.0 cm. The film was sandwiched between two polarizing plates (Kenis Corporation, thin polarizing film, size S, transmittance 0.43, polarization ratio 0.9999, product code 1-115-0820) so that the slow axis of the film was parallel to the absorption axis of the polarizing plate. Measurements were performed using a Hitachi High-Technologies Corporation (formerly Hitachi High-Technologies Corporation) U-4100 spectrophotometer equipped with the attached variable angle transmission accessory for the U-4100 spectrophotometer. A light source mask of 8 mm (H) x 5 mm (W) was used, and a normal-incidence detector with a mirror surface was used to measure the light transmittance from 400 to 800 nm. The measurement conditions were a scanning speed of 600 nm / min, a sampling pitch of 1 nm, and high-resolution measurement mode. During measurement, the angle of the angle-variable transmission accessory was set to 60°, and the sample was positioned so that the angle between the film's slow axis direction and the horizontal plane was α°, based on the state in which the polarizing plate's absorption axis was aligned with the horizontal plane (α = 10, 20, 30, 40, 50, 60). Baseline measurements were performed in the same manner as above, using two polarizing plates stacked together with their absorption axes aligned. From the spectral transmittance spectrum obtained, tristimulus values (X, Y, Z) were calculated according to JIS Z8701, using a 2° visual field and D65 as the standard light. The obtained tristimulus values were substituted into equations (8) to (10) to calculate ΔCα at a measurement angle of α. ΔCα was calculated when α = 10, 20, 30, 40, 50, and 60°, and the average of these values was calculated as ΔC.

[0041]

number

[0042] (9) 85℃ heat shrinkage rate The film was cut into rectangular samples measuring 150 mm in length and 10 mm in width in both the longitudinal and transverse directions. Marked lines were drawn on the samples at 100 mm intervals (50 mm from the center to both ends), and the samples were heat-treated by hanging a 3 g weight and placing them in a hot air oven heated to 85°C for 6 hours. The gauge length after heat treatment was measured, and the heat shrinkage was calculated from the change in the gauge length before and after heating using the following formula (8). Measurements were taken on 10 samples, and the average value was calculated. The gauge length was measured using a Nikon universal projector (PROFILE PROJECTOR V16A).

[0043] Heat shrinkage rate (%) = {(gauge line distance before heat treatment) - (gauge line distance after heat treatment)} / (gauge line distance before heat treatment) × 100 (8).

[0044] (10) Young's modulus of polyester film Strip samples 150 mm long and 10 mm wide were cut out in the longitudinal and transverse directions of the film. Young's modulus was measured using an Instron-type tensile tester according to the method specified in JIS Z1702 (1994). Measurements were carried out under the following conditions, with 10 samples being measured and the average value being calculated. Measuring device: Orientec Co., Ltd.'s automatic film strength and elongation measuring device "Tensilon AMF / RTA-100" Sample size: width 10 mm x test length 50 mm Pulling speed: 300 mm / min Measurement environment: temperature 23℃, humidity 65%RH (11) Bending rigidity Using a loop stiffness tester (manufactured by Toyo Seiki), a rectangular sample with a width of 5 mm and a length of 100 mm in the width direction was made into a ring, and the load required to give a displacement of 10 mm at a displacement speed of about 3.5 mm / second was measured. The obtained value was taken as the flexural rigidity (mN) in the longitudinal direction. Similarly, the same measurement was performed on a rectangular sample with a length of 5 mm and a width of 100 mm in the longitudinal direction, and the obtained value was taken as the flexural rigidity (mN) in the width direction. The flexural rigidity in the longitudinal direction and the flexural rigidity in the width direction were each measured 5 times, and the average value of all these values was taken as the flexural rigidity (mN).

[0045] (12) Elongation absorption parameter A strip-shaped sample with a length of 150 mm and a width of 10 mm was cut out and used so that the film width direction and the longitudinal direction became the length direction. The shrinkage rate in the width direction at an elongation of 40% was measured in each direction with a universal testing machine, and the elongation absorption parameter was calculated by the following formula. The tensile test was measured using an Instron type according to the method specified in JIS Z1702 (1994). The measurement was carried out under the following conditions. With a sample number of 10, after each was pulled until the elongation reached 40%, the test was stopped. When the initial length was L0, the length at an elongation of 40% was L40, the initial value in the width direction was W0, and the width length at an elongation of 40% was W40, the elongation absorption parameter was obtained from the following calculation formula. From the measurement sample, the positions where L0 and L40 were the maximum values were selected, and the values where W0 and W40 were the minimum values were extracted. It is also possible to measure using a universal projector as needed. Also, the elongation absorption parameter is the larger value of the width direction and the longitudinal direction, and when it breaks during the test, the values are calculated with L40 / L0 = 1.4 and W40 = W0. Universal testing machine: Film strength and elongation automatic measuring device "Tensilon AMF / RTA-100" manufactured by Orientec Co., Ltd. Sample size: Width 10 mm × test length 50 mm Tensile speed: 300 mm / min Measurement environment: Temperature 23°C, humidity 65%RH. Elongation absorption parameter = -(ln(W40 / W0) / (ln(L40 / L0)) ※ln: Natural logarithm.

[0046] (13) Visibility evaluation Cut out samples with a size of 110 mm in two width directions (film width 210 mm) and 300 mm in the longitudinal direction from the central part in the width direction of the film. Then, sandwich and bond the film with two polarizing plates (manufactured by Kenis Co., Ltd., polarizing film thin S size, transmittance 0.43, polarization ratio 0.9999) so that the longitudinal direction of the film is parallel to the absorption axis of the polarizing plate to form a test piece. For the bonding, a laminator roll set at 85 °C was used. The visibility from an angle of 60° with respect to the normal direction of the test piece plane was confirmed when the created test piece was placed on an LED light source (A3-101 manufactured by Tri-Tech).

[0047] ◎: Almost no interference color is observed.

[0048] ○: Although a slight interference color is observed, there is no problem in practical use.

[0049] △: A slight interference color is observed, but there is no problem in practical use.

[0050] ×: The interference color is clearly observed and is not suitable for display applications.

[0051] (14) Flexural fatigue resistance evaluation Using a U-shaped expansion and contraction tester (DLDMLH-FS manufactured by Yuasa System Equipment Co., Ltd.), with the direction in which the bending hysteresis 2HB in the film plane is the smallest as the longitudinal direction, a sample cut out to a length of 112 mm and a width of 108 mm was attached to the end of the tilt clamp with the tilt clamp in a horizontal state and the stroke direction being the sample length direction. It was bent 10,000 times at a test speed of 60 r / min, a test stroke of 60 mm, and a surface distance of 3 mm. After the test, the sample was attached to a black mount board, and then the film state was observed by holding a three-wavelength fluorescent lamp of F10 above it, and the evaluation was carried out according to the following criteria. ◎: There was no change in appearance, and no distortion of the reflected light was observed. ○: When looking closely, a bending line can be confirmed, but there is no problem in practical use. △: A slight bending line can be confirmed in terms of appearance, but there is no problem in practical use. ×: A bending line was clearly observed in the appearance.

[0052] (15) Heat resistance evaluation of the film Samples obtained by laminating a polyester film and a PVA sheet with a laminating nip roll were heat-treated at 85 °C for 6 hours, and the judgment was made as follows based on the appearance. ○: No wrinkles or slack were observed in terms of appearance. △: Slight wrinkles were confirmed in terms of appearance, but there is no problem in practical use. ×: Wrinkles were clearly observed in the appearance.

[0053] (16) Handling property A film with a width of 300 mm and a length of 200 m (6 inches, 350 mm long core roll) was prepared, and it was wound back onto a 3-inch, 350 mm long core under the following conditions, and evaluated according to the following criteria while varying and increasing the conveyance speed and tension. ○: No breakage or chipping occurred even when wound back at a speed of 10 m / min and a conveyance tension of 70 N / m. ×: Breakage or chipping occurred when wound back at a speed of 10 m / min and a conveyance tension of 70 N / m.

Example

[0054] (Manufacture of polyester) The polyester resin used for film formation was prepared as follows.

[0055] (Polyester A) A copolyester resin (intrinsic viscosity 0.8 dl / g) in which the 2,6-naphthalenedicarboxylic acid component is 90 mol% and the isophthalic acid component is 10 mol% as the dicarboxylic acid component, and the ethylene glycol component is 90 mol% and the neopentyl glycol component is 10 mol% as the diol component.

[0056] (Polyester B) A copolyester resin (intrinsic viscosity 0.8 dl / g) in which the 2,6-naphthalenedicarboxylic acid component is 100 mol% as the dicarboxylic acid component, the ethylene glycol component is 94 mol% as the diol component, and the neopentylene glycol component is 6 mol%.

[0057] (Polyester C) A copolyester resin (intrinsic viscosity 0.8 dl / g) in which the 2,6-naphthalenedicarboxylic acid component is 84 mol% and the isophthalic acid component is 16 mol% as the dicarboxylic acid components, and the ethylene glycol component is 84 mol% and the neopentylene glycol component is 16 mol% as the diol components.

[0058] (Polyester D) A copolyester resin (intrinsic viscosity 0.8 dl / g) in which the terephthalic acid component is 90 mol% and the isophthalic acid component is 10 mol% as the dicarboxylic acid components, and the ethylene glycol component is 90 mol% and the 1,3-propanediol component is 10 mol% as the diol components (Polyester E) A copolyester resin (intrinsic viscosity 0.8 dl / g) in which the 2,6-naphthalenedicarboxylic acid component is 90 mol% and the isophthalic acid component is 10 mol% as the dicarboxylic acid components, and the ethylene glycol component is 90 mol% and the polyethylene glycol component is 10 mol% as the diol components.

[0059] (Polyester F) A polyester resin (intrinsic viscosity 0.65 dl / g) in which the terephthalic acid component is 100 mol% as the dicarboxylic acid component and the ethylene glycol component is 100 mol% as the diol component (Particle Master A) A polyethylene terephthalate particle master (intrinsic viscosity 0.65 dl / g) containing aggregated silica particles with a number average particle diameter of 2.2 μm at a particle concentration of 2 mass% in Polyester A.

[0060] (Examples 1 to 13, Comparative Examples 1 to 10) With the composition as shown in Table 1, the raw materials were supplied to a vent co-rotating twin-screw extruder with an oxygen concentration of 0.2% by volume, melted at the temperatures shown in Table 1 for the extruder cylinder temperature (extrusion temperature), short tube temperature, and die temperature, and discharged in sheet form onto a cooling drum controlled to the temperature shown in Table 2 through a T-die. At that time, electrostatic application was performed using a wire-shaped electrode with a diameter of <0.1> mm to obtain an unstretched sheet that adhered to the cooling drum. Subsequently, a polyester film was obtained under the film-forming conditions shown in Table 2. All of the films obtained in the examples were excellent in visibility, flex resistance, and handleability. On the other hand, the comparative examples were inferior in any one of visibility, flex resistance, and handleability.

[0061]

Table 1

[0062]

Table 2

[0063]

Table 3

[0064]

Table 4

Industrial Applicability

[0065] Since the polyester film of the present invention uses a polyester resin having a specific composition and has a spectral transmittance within a specific range under specific conditions, it can be used for optical films such as flexible displays that are excellent in flex resistance, difficult to visually recognize rainbow unevenness, and excellent in heat resistance.

Explanation of Signs

[0066] 1: Light source 2: Measurement sample 3: Direct incidence detector 4: Angle of the angle-variable transmission device 5: Angle between the film longitudinal direction and the horizontal plane 6: Longitudinal direction of film 7: Horizontal plane a: Orthogonal direction to the surface direction of measurement sample 2

Claims

1. A polyester film having a flexural rigidity obtained from a loop stiffness tester of 0.2 mN or more and 5.0 mN or less in both the longitudinal direction and the width direction, and a color difference ΔC of 0.0 or more and 0.5 or less, wherein the polyester film is a polyester film mainly composed of an aromatic dicarboxylic acid component and a diol component, the aromatic dicarboxylic acid component contains naphthalenedicarboxylic acid as the main acid component, and the diol component contains ethylene glycol as the main diol component, the content of the secondary acid component A different from the main acid component selected from terephthalic acid, naphthalenedicarboxylic acid, isophthalic acid, and 1,4-cyclohexanedicarboxylic acid is a (mol%), when the content of the secondary diol component B selected from polyethylene glycol, cyclohexanedimethanol, 1,3-propanediol, isosorbide, and neopentyl glycol is b (mol%) with respect to the total diol component, the following formulas (1) to (3) are satisfied, 0 ≦ a < 30 (1) 0 ≦ b ≦ 30 (2) 5 ≦ a + b ≦ 30 (3) A polyester film in which the content of naphthalenedicarboxylic acid with respect to the total acid component as the aromatic dicarboxylic acid component is 70 mol% or more.

2. The polyester film according to claim 1, having a heat shrinkage rate at 85 °C of 1.0% or less in both the longitudinal direction and the width direction.

3. The polyester film according to claim 1 or 2, having a front retardation Re of the film of less than 3000 nm.

4. The polyester film according to any one of claims 1 to 3, having an elongation absorption parameter of 0.7 or more and 1.0 or less.

5. The polyester film according to any one of claims 1 to 4, mounted on a polarizing plate.

6. The polyester film according to any one of claims 1 to 4, mounted on a display.

Images