Polyester film

Abstract

To provide a polyester film excellent in flexure resistance and flatness.SOLUTION: A polyester film whose young's modulus in a direction (a direction) in which the young's modulus is the largest is 2.8 GPa or larger, and elongation absorption parameters in the a direction and a direction (b direction) orthogonal to the a direction are 0.7 or larger and 1.5 or smaller.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] In recent years, image display devices using self-luminous materials called organic light-emitting diodes (hereinafter referred to as "organic electroluminescent display devices") have been increasingly put to practical use. Compared to conventional liquid crystal display devices, organic electroluminescent display devices are superior in terms of visibility and response speed because they use self-luminous materials. Furthermore, they do not require auxiliary lighting devices such as backlights, making it possible to make the display devices thinner and more flexible. For this reason, the development of flexible displays that can be foldable or rolled up is accelerating, and bending resistance is also required for cover films that prevent scratches on the display device surface.

[0003] For example, a hard coat film in which hard coat layers are laminated on both sides of a substrate containing a polyester film has been proposed as a method for improving scratch resistance for image display devices (Patent Document 1). Also, an antireflection film in which an antireflection layer is provided on at least one side of a flexible transparent resin film containing a polyester film has been proposed for flexible displays (Patent Document 2). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-61750 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-75869 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the films used for image display devices described in Patent Documents 1 and 2 are characterized by a hard coat layer and an antireflection layer, and the flex resistance of the substrate film is not taken into consideration, making it difficult to apply them to flexible displays. Furthermore, it has been found that when the film is made flexible in order to improve its flex resistance, a new problem arises in that the flatness deteriorates when a curable resin layer such as a hard coat layer is applied to the film as a scratch-resistant layer.

[0006] Therefore, an object of the present invention is to solve the above-mentioned problems and to provide a polyester film that can achieve both bending resistance and flatness. [Means for solving the problem]

[0007] In order to solve the above problems, the polyester film of the present invention employs the following means. (1) The Young's modulus in the direction in which the Young's modulus is greatest (direction a) is 2.8 GPa or more, and the extension absorption parameters in the direction a and the direction perpendicular to the direction a (direction b) are both 0.7 or more and 1.5 or less. (2) The heat shrinkage starting temperature obtained from the heat shrinkage curve is 65°C or higher and 120°C or lower in both the a direction and the b direction. (3) The bending hysteresis 2HB determined by the following measurement method shall be 0.04 gf·cm / cm or more and 0.10 gf·cm / cm or less in both directions a and b. [Method for measuring bending hysteresis 2HB] Using a multipurpose bending tester (KES-FB2) manufactured by Kato Tech, bending hysteresis 2HB is measured in the a direction and the b direction under the following conditions. The bending hysteresis 2HB is measured at a curvature of -2.5 to +2.5 cm. -1 is considered as one cycle, and the value obtained in the fifth cycle (repeated five times) is used. Number of repetitions: 5 (5th cycle of adopted data) SENS:2×5 Curvature:-2.5~+2.5cm-1 Deformation speed: 0.50cm -1 / sec Sample size: a direction x b direction = 20cm x 20cm (4) The bending hardness measured by the following measurement method is 0.06 gf cm in both a and b directions. 2 / cm or more 0.12gf·cm 2 / cm or less. [Method of measuring bending hardness] Using a multipurpose bending tester (KES-FB2) manufactured by Kato Tech, bending hardness is measured in the a direction and the b direction under the following conditions. The bending hardness is measured at a curvature of -2.5 to +2.5 cm. -1 is considered as one cycle, and the value obtained in the fifth cycle (repeated five times) is used. Number of repetitions: 5 (5th cycle of adopted data) SENS:2×5 Curvature:-2.5~+2.5cm -1 Deformation speed: 0.50cm -1 / sec Sample size: a direction x b direction = 20cm x 20cm (5) The weight average molecular weight Mw is 20,000 or more and 50,000 or less. (6) The number of bending cycles in the direction a before breaking is 200,000 or more and 1,000,000 or less. (7) The film thickness is 5 μm or more and 30 μm or less. (8) The static friction coefficient of at least one surface is 0.2 or more and 0.5 or less. (9) At least one surface has a surface resistivity of 1 × 10 12 Must be Ω / □ or less. (10) To be used in flexible displays. [Effects of the Invention]

[0008] According to the present invention, by controlling the Young's modulus and the elongation absorption parameter within specific ranges, a polyester film that can achieve both bending resistance and flatness can be provided. Such a polyester film can be particularly suitably used as a cover film for flexible displays such as organic electroluminescence display devices. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 10 is a schematic diagram showing an evaluation of winding resistance. DETAILED DESCRIPTION OF THE INVENTION

[0010] The polyester film according to the present invention will be described in detail below with reference to embodiments.

[0011] In a preferred embodiment of the polyester film of the present invention, the Young's modulus in the direction (a-direction) in which the Young's modulus is greatest is 2.8 GPa or more. Young's modulus indicates the rigidity of the film and is an index of maintaining flatness when a curable resin such as a hard coat is applied to the film. A Young's modulus of 2.8 GPa or more in the a-direction makes it possible to maintain flatness when a curable resin such as a hard coat is applied to the film. If the Young's modulus in the a-direction is less than 2.8 GPa, the flatness may be poor when hard coated. From the viewpoint of flex resistance, the Young's modulus in the a-direction is preferably less than 5.5 GPa. From the viewpoint of flatness, the Young's modulus in the b-direction, which is the direction perpendicular to the a-direction, is preferably 2.8 GPa or more. From the viewpoint of flatness, the Young's modulus in the b-direction is preferably 3.5 GPa or more, and most preferably 3.8 GPa or more. Although there is no particular upper limit, the Young's modulus in the b-direction is preferably 10.0 GPa or less. To achieve a Young's modulus of 2.8 GPa or more in the a-direction, it is preferable to use a resin primarily composed of polyethylene terephthalate, polyethylene naphthalate, or polycyclohexylene dimethylene terephthalate as the polyester. Here, "mainly" refers to a polyester structural unit of 75 mol% or more, and homopolyester is most preferable from the viewpoint of increasing the Young's modulus. The film-forming conditions can be controlled by using the above resin and stretching it to 3.5 times or more. Since the Young's modulus increases as the film is oriented in the stretching direction, the direction stretched most significantly becomes the a-direction. For example, by setting the stretching ratios in a certain direction (uniaxial direction) and the direction perpendicular thereto to 3.5 times or more, and the area ratio to 12.25 times or more, the Young's modulus can be increased to 2.8 GPa or more in both the a-direction and the b-direction. Since the Young's modulus improves as the stretching ratio increases, it can be adjusted appropriately depending on the required function. However, since an excessively high stretching ratio can cause breakage during film formation, the stretching ratio is preferably 4.0 times or less, and the area ratio is preferably 16.0 times or less.When the polyester film of the present invention is used in a roll-up flexible display, it is preferable to use it so that the a-direction is the roll-up direction, since this makes use of the excellent flex resistance during repeated roll-up, resulting in good long-term durability.

[0012] The polyester film of the present invention is preferably a biaxially oriented polyester film, which can be obtained by biaxially stretching the polyester film by the method described below.

[0013] In a preferred embodiment of the polyester film of the present invention, the stretch absorption parameters in both the a-direction and the direction perpendicular to the a-direction (the b-direction) are 0.7 to 1.5. The stretch absorption parameter here refers to a value obtained by Evaluation Method (9) "Stretch Absorption Parameter" described in the Examples. It is an index of how easily a film shrinks in the direction perpendicular to the stretching direction when stretched. It was conceived and discovered based on the r-value obtained from the composition strain ratio test method for thin metal sheet materials described in JIS Standard Z254:2008. Flexible displays are expected to be subjected to repeated winding and folding loads, such as bending and winding, and how to absorb these loads is important. The inventors focused on the stretch absorption parameter as an index of the absorption of loads from repeated bending and winding, and found that controlling the stretch absorption parameter within the range of the present invention results in excellent flex resistance, such as repeated winding. The larger the stretch absorption parameter, the better, and it is preferably 0.8 or greater. It is also preferably 1.0 or greater, and more preferably 1.2 or greater. If the stretch absorption parameter is less than 0.7, the bending resistance may be poor, and it is difficult to achieve a stretch absorption parameter exceeding 1.5 when a polyester film is used.

[0014] In order to set the stretch absorption parameters in the a-direction and the direction perpendicular to the a-direction (b-direction) to 0.7 or more and 1.5 or less, it is considered important to form a dense film bulk structure and enhance the effect of absorbing external forces such as bending through molecular chain entanglement. Based on this principle, it can be controlled by the film production conditions ((1) stepwise stretching, (2) cooling to 50°C or less after biaxial stretching of the film), the plane orientation coefficient, the intrinsic viscosity of the film, and the minute endothermic temperature peak Tmeta. This is explained in detail below.

[0015] First, as a condition for biaxial stretching of a film, it is important to perform stepwise stretching (for example, stretching in three or more stages) in the stretching direction of each axis in order to form a dense molecular structure. For example, when performing primary stretching in three stages, if the stretching ratios in the three stages are MD1, MD2, and MD3, respectively, it is important to set the stretching ratios as follows to form a dense bulk structure. MD1: 1.03 times or more and 1.8 times or less. MD2: 1.05 times or more and 1.12 times or less. MD3: 3.0 times or more, and MD3 accounts for 80% or more of the total draw ratio in the first axis (MD1×MD2×MD3). The total stretching ratio in the first axis is 3.5 times or more.

[0016] Furthermore, when biaxial stretching is performed in three stages, and the stretching ratios in the three stages are designated TD1, TD2, and TD3, respectively, it is important to set the stretching ratios as follows to obtain a dense bulk structure. TD1: 1.5 times or more and 2.2 times or less. TD2: 1.3 times or more and 1.8 times or less. TD3: 1.2 times or more and 1.5 times or less. The total biaxial stretching ratio (TD1 x TD2 x TD3): 3.5 times or more.

[0017] Furthermore, the stretch absorption parameter tends to increase in the range where it shows a positive value as the planar orientation coefficient increases; therefore, a planar orientation coefficient of 0.164 or greater is preferred. Increasing the planar orientation coefficient is advantageous in terms of increasing the stretch absorption parameter, but may also deteriorate dimensional stability. Therefore, it should be adjusted appropriately according to the required thermal properties. However, if the minute endothermic temperature peak Tmeta is observed in the range above 200°C, it is difficult to achieve a stretch absorption parameter of 0.7 to 1.5, even if the planar orientation coefficient is controlled. Furthermore, the planar orientation coefficient can be controlled by the stretch ratio, and can be increased by increasing the stretch ratio. To achieve a stretch absorption parameter of 0.8 or greater in the a direction and the direction perpendicular to the a direction (b direction), it is important to maintain a planar orientation coefficient of at least 0.166. While there is no particular upper limit, a planar orientation coefficient greater than 0.17 can frequently cause tearing during stretching, resulting in poor productivity.

[0018] In the present invention, it is important to include a heat treatment step after the step of cooling the biaxially stretched film to 50°C or less. In conventional polyester film manufacturing methods, after sequential biaxial stretching or simultaneous biaxial stretching, heat treatment is performed directly in the heat treatment step without cooling to 50°C or less for productivity and thermal efficiency reasons. However, to form a dense film bulk structure and promote molecular chain entanglement, it is important to cool the film to 50°C or less once after biaxial stretching. While the detailed mechanism is unknown, it is presumed that stabilizing the crystalline structure formed by biaxial stretching at 50°C or less before heat treatment densifies the film bulk structure, making it easier to control the elongation absorption parameter to 0.7 to 1.5 or less. Therefore, if heat treatment is performed without cooling to 50°C or less after biaxial stretching, it may be difficult to control the elongation absorption parameter to 0.7 to 1.5 or less. It is more preferable to include a heat treatment step after the step of cooling to 35°C or less after biaxial stretching.

[0019] In the present invention, increasing the intrinsic viscosity of the film tends to increase the extensional absorption parameter. The extensional absorption parameter can be kept within a suitable range by controlling it together with other factors, but the intrinsic viscosity of the film is preferably 0.66 dL / g or more and 1.15 dL / g or less. Note that increasing the intrinsic viscosity tends to increase the filtration pressure during the extrusion process for film formation, which may reduce mass productivity. The intrinsic viscosity of the film can be adjusted by the intrinsic viscosity of the resin used. There is no particular upper limit, but if the filtration pressure increases during the extrusion process for film formation, it becomes necessary to reduce the discharge rate, which may reduce mass productivity. The intrinsic viscosity of the film can be adjusted by the intrinsic viscosity of the resin used.

[0020] As described above, in the present invention, it is important to have a minute endothermic temperature peak Tmeta of less than 200°C or to have no minute endothermic temperature peak Tmeta at all in order to achieve an elongational absorption parameter of 0.7 or more and 1.5 or less. With the intrinsic viscosity and film-forming stretching conditions within the preferred ranges of the present invention, it is important to have a Tmeta of less than 195°C in order to achieve an elongational absorption parameter of 0.8 or more; it is important to have a Tmeta of less than 185°C in order to achieve an elongational absorption parameter of 1.0 or more; and it is important to have a Tmeta of less than 175°C in order to achieve an elongational absorption parameter of 1.2 or more. A minute endothermic temperature peak Tmeta of less than 200°C can be controlled by setting the maximum heat history temperature (usually referring to the heat treatment temperature) in the film production process to 200°C or less.

[0021] From the viewpoint of dry heat resistance, the polyester film of the present invention preferably has a heat shrinkage initiation temperature of 65°C or higher and 120°C or lower in both the a-direction and the b-direction, as obtained from a heat shrinkage curve. To control the temperature within this range, the Young's modulus in the a-direction is adjusted to 2.8 GPa or higher, and then heat treatment is performed so that Tmeta is 180°C or higher. More preferably, the heat shrinkage initiation temperature is 70°C or higher and 100°C or lower in both the a-direction and the b-direction. To control the temperature within this range, the first axis stretching temperature ED1 during biaxial stretching is set to be 10°C or higher than the glass transition temperature Tg of the resin, and then heat treatment is performed so that the second axis stretching temperature ED2 satisfies the following formula and Tmeta is 180°C or higher. |ED1-ED2|≦5℃ In the case of stepwise stretching, ED1 and ED2 refer to the temperatures TD1 and TD2 in the respective stretching steps.

[0022] The polyester film of the present invention preferably has a bending hysteresis 2HB of 0.04 gf·cm / cm or more and 0.10 gf·cm / cm or less in both the a-direction and the b-direction. Having a bending hysteresis 2HB within the above range results in excellent dynamic flex resistance. Here, the bending hysteresis 2HB is a value determined by the measurement method described below, and is an index of flexural resilience. If the bending hysteresis 2HB exceeds 0.10 gf·cm / cm, dynamic flex resistance deteriorates, and if it is less than 0.04 gf / cm / cm, the flatness may be poor when a curable resin layer is provided.

[0023] During dynamic bending tests, the stress applied to a film is compressive on the inside of the film when bent and tensile on the outside. Therefore, to ensure bending resistance, it is important for the film to be resistant to stresses in both the in-plane and thickness directions. To control the bending hysteresis 2HB to 0.10 gf·cm / cm or less, for example, if a polyester film is used with a high in-plane orientation, resistance in the in-plane direction—i.e., resistance to tension on the outside of the bent portion during dynamic bending—is achieved. However, this results in a decrease in the refractive index in the thickness direction, which results in a lack of resistance to compression on the inside of the bent portion, making it impossible to achieve a bending hysteresis 2HB of 0.10 gf·cm / cm or less. Therefore, to achieve a bending hysteresis 2HB of 0.10 gf·cm / cm or less, a thickness-direction refractive index of 1.46 or greater is preferred. Furthermore, to ensure bending resistance in the in-plane direction, it is preferable to control the in-plane orientation coefficient, which indicates the degree of orientation in the in-plane direction (the average refractive index in the a and b directions minus the thickness-direction refractive index), to 0.166 or greater and 0.17 or less.

[0024] The in-plane and thickness refractive indices and plane orientation coefficient of a film can be adjusted by the film-forming conditions, such as the stretching temperature, areal stretch ratio, and post-stretching heat treatment temperature, as well as the crystallinity of the resin. For example, in a polyester film production method using sequential biaxial stretching, a thickness-direction refractive index of 1.46 or higher can be achieved by setting the longitudinal stretching temperature above the resin's glass transition temperature but not exceeding the glass transition temperature +20°C, setting the width-direction stretching temperature above the resin's glass transition temperature but not exceeding the glass transition temperature +20°C but not exceeding the glass transition temperature +60°C, and setting the areal stretch ratio to between 10x and 16x. Furthermore, the higher the crystallinity, the lower the thickness-direction refractive index after stretching tends to be. Resin crystallinity can be controlled, for example, by incorporating copolymerization components. For example, when polyethylene terephthalate is used as a polyester, its crystallinity can be reduced by copolymerizing it with isophthalic acid, naphthalenedicarboxylic acid, cyclohexanedimethanol, spiroglycol, or the like.

[0025] The polyester film of the present invention has a bending hardness of 0.06 gf cm in both the a-direction and the b-direction, as determined by the measurement method described below, from the viewpoint of maintaining flatness when a curable resin layer such as a hard coat layer is applied. 2 / cm or more 0.12gf·cm 2 / cm or less. More preferably, the bending hardness in both the a-direction and the b-direction is 0.08 gf cm 2 / cm or more 0.12gf·cm 2 / cm or less. The bending hardness tends to increase as the film thickness increases and as the film orientation increases. By adjusting the thickness and film orientation according to other required properties, the bending hardness can be controlled within the above range.

[0026] From the viewpoints of handleability, scratch resistance, flex resistance, and flatness when a curable resin is applied, the polyester film of the present invention preferably has a film thickness of 5 μm to 30 μm, more preferably 9 μm to 28 μm, and most preferably 12 μm to 26 μm. Furthermore, since a thickness exceeding 30 μm tends to result in a very high bending hysteresis 2HB regardless of the material, a thickness of 30 μm or less is preferred.

[0027] The polyester film of the present invention is primarily composed of polyester. Examples of glycols or derivatives thereof that provide the polyester include aliphatic dihydroxy compounds such as ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, and neopentyl glycol; polyoxyalkylene glycols such as diethylene glycol, polyethylene glycol, polypropylene glycol, and polytetramethylene glycol; alicyclic dihydroxy compounds such as 1,4-cyclohexanedimethanol and spiroglycol; aromatic dihydroxy compounds such as bisphenol A and bisphenol S; and derivatives thereof.

[0028] Examples of dicarboxylic acids or their derivatives that can be used to produce the polyesters of the present invention include aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, 2,6-naphthalenedicarboxylic acid, diphenyldicarboxylic acid, diphenylsulfonedicarboxylic acid, diphenoxyethanedicarboxylic acid, and 5-sodiumsulfonedicarboxylic acid, aliphatic dicarboxylic acids such as oxalic acid, succinic acid, adipic acid, sebacic acid, dimer acid, maleic acid, and fumaric acid, alicyclic dicarboxylic acids such as 1,4-cyclohexanedicarboxylic acid, and hydroxycarboxylic acids such as parahydroxybenzoic acid, as well as derivatives thereof. Examples of dicarboxylic acid derivatives include esters of dimethyl terephthalate, diethyl terephthalate, 2-hydroxyethyl methyl terephthalate, dimethyl 2,6-naphthalenedicarboxylate, dimethyl isophthalate, dimethyl adipate, diethyl maleate, and dimethyl dimerate.

[0029] In the polyester composition of the present invention, it is preferable that 80 mol% or more of the glycol units are structural units derived from ethylene glycol, more preferably 85 mol% or more, and most preferably 90 mol% or more. Furthermore, it is preferable that 80 mol% or more of the dicarboxylic acid units are structural units derived from terephthalic acid, more preferably 85 mol% or more, and most preferably 90 mol% or more. Furthermore, instead of using a single resin, other polyester resins may be mixed. For example, mixing polybutylene terephthalate can decrease the Young's modulus, while adding polyethylene naphthalate tends to increase the Young's modulus. Mixing can be performed appropriately depending on the required properties.

[0030] The polyester film of the present invention preferably has a weight-average molecular weight (Mw) of 20,000 or more and 50,000 or less, more preferably 20,000 or more and 27,500 or less. As described above, a weight-average molecular weight within this range increases molecular chain entanglement, making it easier to control the elongational absorption parameter within the range of the present invention compared to films obtained under the same film-forming conditions. There are no particular limitations on the method for achieving a weight-average molecular weight (Mw) of 20,000 or more and 50,000 or less. For example, the higher the intrinsic viscosity of the polyester used as a raw material, the higher the weight-average molecular weight (Mw). Increasing the melt extrusion temperature above the melting point of the resin tends to lower the weight-average molecular weight (Mw). Furthermore, increasing the residence time between feeding into the extruder and bleeding from the die tends to lower the weight-average molecular weight (Mw). Therefore, a preferred weight-average molecular weight (Mw) can be achieved by controlling these conditions. A suitable method is to use a polyester resin with an intrinsic viscosity of 0.70 or more, control the extrusion temperature to the melting point of the resin + 40°C or less, and set the residence time from when the polyester is fed into the extruder until it bleeds out of the die to 10 minutes or less. From the viewpoint of increasing the weight-average molecular weight Mw, it is more preferable to use a resin with an intrinsic viscosity of 0.80 or more.

[0031] The polyester film of the present invention preferably has a bending resistance in the a-direction of 200,000 to 1,000,000 times before breaking. Having a bending resistance in this range improves the resistance to repeated bending. Therefore, a roll-up flexible display using the a-direction as the winding direction is preferred because it takes advantage of the excellent bending resistance during repeated winding and has good long-term durability. This can be controlled by setting the film's plane orientation coefficient to 0.165 to 0.170.

[0032] The polyester film of the present invention preferably has a static friction coefficient of 0.2 to 0.5 on at least one surface to prevent scratches when wound up, such as in a roll-up display. The static friction coefficient can be controlled by incorporating particles into the film. For example, at least one surface layer preferably contains inorganic particles with an average particle size of 0.005 μm to 10 μm and / or organic particles at 0.01% by mass or more. From the perspective of display transparency, the particle content is preferably 3.0% by mass or less. Examples of inorganic particles that can be used include wet and dry silica, colloidal silica, aluminum silicate, titanium oxide, calcium carbonate, calcium phosphate, barium sulfate, aluminum oxide, mica, kaolin, and clay. Examples of organic particles that can be used include particles containing components such as styrene, silicone, acrylic acids, methacrylic acids, polyesters, and divinyl compounds. Among these, inorganic particles such as wet and dry silica, alumina, and calcium carbonate, and particles containing styrene, silicone, acrylic acid, methacrylic acid, polyester, divinylbenzene, etc., are preferred. Furthermore, two or more of these inorganic and organic particles may be used in combination. If the static friction coefficient is less than 0.2, the film may become slippery and poorly handleable. On the other hand, if the static friction coefficient exceeds 0.5, the surface may be easily scratched when wound up, as is the case with rollable displays.

[0033] The polyester film of the present invention has a surface resistivity of 1×10 on at least one surface. 12 It is preferably Ω / □ or less, and particularly preferably 0.01×10 12 (Ω / □) or more 1×10 12 (Ω / □) or less. By having a surface resistivity in this range, it is possible to prevent electrostatic damage such as preventing dust adhesion, adhesion of products due to static electricity, and destruction of precision electronic circuit material elements due to static electricity discharge. A surface resistivity of 1×10 12If the resistance exceeds 0.01×10 (Ω / □), static electricity is likely to be generated in the film, and foreign matter is likely to adhere to the surface of the film of the present invention during the process of laminating other layers, which may result in defects. 12 If the resistivity is below Ω / □, the film may become conductive, which is undesirable. To control the surface resistivity within this range, the surface can be subjected to corona treatment or by laminating an easy-adhesion resin layer with a surface free energy of 38 mN / m or more on at least one side.

[0034] A preferred embodiment of the polyester film of the present invention is a laminate sheet having a layer containing a curable resin on at least one side of the polyester film, which can enhance the effect of suppressing scratches caused by impact from the curable resin layer side, making the polyester film suitable for use in organic electroluminescence display devices.

[0035] Here, the term "curable resin" refers to a resin that forms a crosslinked structure and hardens when exposed to heat or light. The curable resin is not particularly limited, but is preferably a thermosetting resin or an ultraviolet-curable resin. Specific examples include organic silicone-based, polyol-based, melamine-based, epoxy-based, multifunctional acrylate-based, urethane-based, isocyanate-based, organic-inorganic hybrid-based (a composite material of an organic material and an inorganic material), and silsesquioxane-based resins having a curable functional group. More preferred are epoxy-based, multifunctional acrylate-based, organic-inorganic hybrid-based, and silsesquioxane-based resins. Even more preferred are multifunctional acrylate-based, organic-inorganic hybrid-based, and silsesquioxane-based resins.

[0036] Preferred examples of the polyfunctional acrylate and silsesquioxane resins used as the curable resin include polyfunctional acrylate monomers, oligomers, urethane acrylate oligomers, alkoxysilanes, alkoxysilane hydrolysates, and alkoxysilane oligomers. Examples of polyfunctional acrylate monomers include polyfunctional acrylates having two or more (meth)acryloyloxy groups per molecule and modified polymers thereof. Specific examples include pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol tri(meth)acrylate, dipentaerythritol tetra(meth)acrylate, dipentaerythritol penta(meth)acrylate, dipentaerythritol hexa(meth)acrylate, trimethylolpropane tri(meth)acrylate, and pentaerythritol triacrylate hexanemethylene diisocyanate urethane polymers. These monomers can be used alone or in combination.

[0037] In the present invention, the layer containing the curable resin preferably contains one or more types of particles. Here, the particles may be either inorganic or organic particles, but the inclusion of inorganic particles is preferred for improving surface hardness. The inorganic particles are not particularly limited, but examples include metal or semimetal oxides, silicides, nitrides, borides, chlorides, and carbonates. Specifically, at least one type of particle selected from the group consisting of silica (SiO), aluminum oxide (AlO), zinc oxide (ZnO), zirconium oxide (ZrO), titanium oxide (TiO), antimony oxide (SbO), and indium tin oxide (InO+SnO) is preferred. When particles are introduced to improve surface hardness, the particle diameter is preferably 1 nm or more and 300 nm or less. To achieve a higher level of both surface hardness and bending resistance, the particle diameter is more preferably 50 nm or more and 200 nm or less, and even more preferably 100 nm or more and 150 nm or less. The particle size referred to here refers to the number average particle size, which means the particle size observed in the cross section of the film. If the shape is not a perfect circle, the particle size is the value converted to a perfect circle with the same area. Here, the number average particle size Dn can be calculated by the following steps (1) to (4). (1) First, a cross section of the film is cut in the thickness direction using a microtome without crushing it, and a magnified image is obtained using a scanning electron microscope. At this time, the cut is made parallel to the a direction of the film. (2) Next, for each particle observed in the cross section of the image, its cross-sectional area S is calculated, and the particle size d is calculated using the following formula. d=2×(S / π) 1 / 2 (3) Using the obtained particle diameter d and the number of resin particles n, Dn is calculated using the following formula. Dn=Σd / n where Σd is the sum of particle diameters within the observation surface, and n is the total number of particles within the observation surface. (4) The above (1) to (3) are carried out at five different locations, and the average value is the number-average particle size of the particles. 2 The above evaluation is carried out in the above areas.

[0038] Furthermore, the content ratio of the curable resin to the particles is preferably particle / resin=20 / 80 to 80 / 20 by mass. If the particle / resin ratio is less than 20 / 80, the surface hardness may be insufficient, and if it exceeds 80 / 20, the flex resistance may decrease. In order to achieve a higher level of both surface hardness and flex resistance, the particle / resin mass ratio is more preferably 30 / 70 to 70 / 30, and even more preferably 40 / 60 to 60 / 40.

[0039] Next, a specific example of a method for producing the polyester film of the present invention will be described. Here, polyethylene terephthalate is used as the resin constituting the film, but the present invention is not limited to this example.

[0040] First, polyethylene terephthalate resin with an intrinsic viscosity of 0.85 g / d is dried and pre-crystallized, then fed into a single-screw extruder and melt-extruded. The resin temperature is preferably controlled at 265-290°C. Next, the resin is passed through a filter or gear pump to remove impurities and equalize the extrusion rate, and then discharged into a sheet form from a T-die onto a cooling drum. The sheet polymer is then adhered to the casting drum using one of several methods: an electrostatic 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 method in which the casting drum temperature is adjusted to the glass transition point of the polyester resin (glass transition point -20°C) 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, when using polyester, the electrostatic application method is preferred from the standpoints of productivity and flatness.

[0041] The unstretched film obtained in the casting process can be stretched in the longitudinal direction and then in the width direction, or in the width direction and then in the longitudinal direction, using a sequential biaxial stretching method, or in a simultaneous biaxial stretching method, in which the film is stretched in the longitudinal and width directions almost simultaneously. In such a biaxial stretching method for a film, for example, it is important to stretch the film in multiple stages in the stretching directions of each axis in order to form a dense molecular structure. For example, when the primary stretching is performed in three stages, and the stretching ratios in the three stages are designated MD1, MD2, and MD3, respectively, it is important to set the ratios as follows to obtain a dense bulk structure. MD1: 1.03 times or more and 1.8 times or less. MD2: 1.05 times or more and 1.12 times or less. MD3: 3.0 times or more, and MD3 accounts for 80% or more of the total draw ratio in the first axis (MD1×MD2×MD3). The total stretching ratio in the first axis is 3.5 times or more.

[0042] Furthermore, when biaxial stretching is performed in three stages, and the stretching ratios in the three stages are designated TD1, TD2, and TD3, respectively, it is important to set the ratios as follows to obtain a dense bulk structure. TD1: 1.5 times or more and 2.2 times or less. TD2: 1.3 times or more and 1.8 times or less. TD3: 1.2 times or more and 1.5 times or less. The total biaxial stretching ratio (TD1 x TD2 x TD3): 3.5 times or more.

[0043] In the present invention, from the viewpoint of achieving a Young's modulus of 2.8 GPa or more and preventing thickness unevenness, it is preferable to stretch the film 3.5 times or more in each direction, and when the planar orientation coefficient is set to 0.165 to 0.17, the areal stretching ratio is preferably set to 12.25 to 16 times. Furthermore, the stretching temperature is preferably set to a level that does not cause stretching unevenness, and from the viewpoint of achieving a thickness direction refractive index of 1.46 or more, for example, when a sequential biaxial stretching method is adopted in which stretching is performed in the longitudinal direction and then in the width direction, it is preferable that the preheating temperature in the longitudinal direction be not less than the glass transition temperature of the resin -20°C and not more than the glass transition temperature +0°C, and the stretching temperature be not less than the glass transition temperature of the resin +30°C, and it is preferable that the preheating temperature in the width direction be not less than the glass transition temperature of the resin -10°C and not more than the glass transition temperature +20°C, and the stretching temperature be not less than the glass transition temperature of the resin +60°C.

[0044] The polyester film of the present invention is preferably heat-treated after biaxial stretching. Heat treatment can be performed by any conventional method, such as in an oven or on a heated roll. However, as described above, in the present invention, in order to form a dense film bulk structure and enhance molecular chain entanglement, it is important to once cool the biaxially stretched film to 50°C or less before heat treatment. After cooling to 50°C or less after biaxial stretching, heat treatment may be performed, as described above, on the condition that the small endothermic temperature peak Tmeta is less than 200°C. Heat treatment can improve dimensional stability. To achieve a small endothermic temperature peak Tmeta of less than 200°C, the heat treatment temperature should be set to 200°C or less. Heat treatment can also be performed by dividing the film into multiple zones and gradually increasing and decreasing the temperature, or by slightly stretching the film to approximately 1.01 to 1.2 times the width direction during the heat treatment process. The heat treatment time can be set to any value within a range that does not deteriorate the properties, and is preferably 10 to 60 seconds, more preferably 15 to 30 seconds. Furthermore, the heat treatment may be performed by relaxing the film in the longitudinal and / or transverse directions.

[0045] Furthermore, when laminating a layer containing a curable resin to the polyester film of the present invention, it is preferable to perform a corona treatment on the surface or to laminate an easy-adhesion resin layer having a thickness of 10 nm to 500 nm and a surface free energy of 38 mN / m or more on at least one side in terms of adhesion to the layer. Methods for forming the easy-adhesion resin layer include coating the film surface with an easy-adhesion resin (composite melt extrusion, hot melt coating, in-line or offline coating from a solvent other than water, or a water-soluble and / or water-dispersible resin), and surface lamination of a similar composition or a blend thereof. Among these, an in-line coating method, in which a coating agent is applied to one side of the film before the orientation crystallization is complete, stretched in at least one direction, and heat-treated to complete the orientation crystallization, is preferred from an industrial standpoint for forming a uniform coating. Furthermore, when an easy-adhesion resin layer is provided by coating, the resin used to provide the easy-adhesion resin layer is not particularly limited. For example, acrylic resins, urethane resins, polyester resins, olefin resins, fluorine-based resins, vinyl resins, chlorine-based resins, styrene-based resins, various graft resins, epoxy resins, silicone resins, etc. can be used, and mixtures of these resins can also be used. From the viewpoint of adhesion, polyester resins, acrylic resins, or urethane resins are preferably used. When a polyester resin is used as an aqueous coating liquid, a water-soluble or water-dispersible polyester resin is used. To achieve such water solubility or water dispersion, it is preferable to copolymerize a compound containing a sulfonate group or a compound containing a carboxylate group. When an acrylic resin is used as an aqueous coating liquid, it must be dissolved or dispersed in water, and a surfactant (for example, but not limited to, polyether compounds) may be used as an emulsifier.

[0046] In addition, various crosslinking agents can be used in combination with the resin in the adhesive resin layer used in the present invention to further improve adhesion. Melamine-based, epoxy-based, and oxazoline-based resins are commonly used as crosslinking agent resins. Examples of particles contained in the resin layer of the present invention include inorganic particles and organic particles, but inorganic particles are more preferred because they improve lubricity and blocking resistance. Examples of inorganic particles that can be used include silica, alumina, kaolin, talc, mica, calcium carbonate, and titanium.

[0047] The polyester film of the present invention has excellent flex resistance and flatness after application of a curable resin layer, and therefore can be particularly suitably used as a cover film for an organic electroluminescent display device. By using the polyester film as a cover film for an organic electroluminescent display device, scratches on the display device surface can be prevented without impairing the flexibility of the display device. Furthermore, in addition to optical films, the polyester film can also be used as various cover films utilizing the properties of the present invention, as a construction film for packaging and the like, as a preferred embodiment. [Example]

[0048] The present invention will be described below with reference to examples, but the present invention is not limited to these examples. Various properties were measured by the following methods.

[0049] (1) Film thickness When measuring the overall thickness of the film, a dial gauge was used to measure the thickness at any five positions on a sample cut out of the film, and the average value was calculated.

[0050] (2) Composition of the resin that makes up the film The film was dissolved in hexafluoroisopropanol (HFIP), 1 H-NMR and 13 The content of each monomer residue and by-product diethylene glycol is quantified using C-NMR.

[0051] (3) Glass transition temperature and melting point Tm, Tmeta of the resin that constitutes the polyester film The glass transition temperature [°C], melting point Tm [°C], and minute endothermic peak temperature Tmeta [°C] were measured and analyzed in accordance with JIS K7121 (1987) using a differential scanning calorimeter robot DSC-RDC6220 manufactured by Seiko Instruments Inc. and thermal analysis rheology system software "Muse" manufactured by SII NanoTechnology Inc. Specifically, when a 5 mg sample was heated from 25°C to 300°C at 20°C / min, the temperature at the apex of the exothermic peak obtained from the DSC curve was taken as Tcc, the temperature at the apex of the endothermic peak obtained from the DSC curve was taken as the melting point Tm, and the small endothermic peak observed at the midpoint between Tm and Tcc was taken as Tmeta. The glass transition temperature was determined from the point where the curve of the stepwise change in the glass transition of the differential scanning calorimetry chart intersected with a line equidistant in the vertical direction from the line extended from each baseline.

[0052] (4) Intrinsic viscosity 0.1 g of sample was weighed out to within 0.001 g accuracy and dissolved in 10 mL of o-chlorophenol by heating at 100°C for 30 minutes. The solution was cooled to room temperature, and 8 mL of the solution was placed in an Ostwald viscometer placed in a water bath at 25°C, and the number of seconds it took for the solution to pass the marked line was measured (A seconds). Similarly, 8 mL of o-chlorophenol alone was used in an Ostwald viscometer placed in a water bath at 25°C, and the number of seconds it took for the solution to pass the marked line was measured (B seconds). The intrinsic viscosity was calculated using the following formula:

[0053] IV=-1+[1+4×K×{(A / B)-1}] 0.5 / (2×K×C) Here, K is 0.343 and C is the concentration of the sample solution (g / 100 mL).

[0054] (5) Planar orientation coefficient fn of polyester film Using sodium D line (wavelength 589 nm) as a light source and methylene iodide as a mounting solution, the refractive indices in the a-direction, b-direction, and thickness direction of the film (nMD, nTD, nZD, respectively) were measured at 25°C using an Abbe refractometer 4T (manufactured by Atago Co., Ltd.) in accordance with JIS K7142 (2014) Method A. The refractive index of the test piece used was 1.74. The plane orientation coefficient (fn) of layer B was calculated from the obtained refractive index using the following formula (1): fn=(nMD+nTD) / 2-nZD (1) (6) Weight average molecular weight Mw To prepare the sample solution, 5 mL of hexafluoroisopropanol containing sodium trifluoroacetate was added to 3 mg of sample, and the mixture was gently stirred at 40°C for 3 hours. The mixture was then filtered using a 0.5 μm filter. The weight-average molecular weight was measured under the following conditions using gel permeation chromatography (GPC) (detector: differential refractive index detector RI-8020, manufactured by Tosoh Corporation) with hexafluoroisopropanol containing sodium trifluoroacetate as the solvent. Column: Shodex HFIP-LG (φ8.0 mm x 5 cm, Showa Denko) Two Shodex HFIP-806M (φ8.0mm x 30cm, Showa Denko) Flow rate: 0.5mL / min Column temperature: 40℃ Injection volume: 0.2mL Molecular weight calibration: Monodisperse polymethyl methacrylate (PMMA) (standard sample) manufactured by Showa Denko.

[0055] (7) Thickness of the curable resin layer and the easy-adhesion resin layer The thickness of the curable resin layer on the film was measured by observing the cross section using a transmission electron microscope (TEM). The thickness of the curable resin layer was read from an image taken with a TEM at a magnification of 100,000 times. The thicknesses of the curable resin layer and the easy-adhesion resin layer were measured at a total of 10 points, and the average value was used. Note that the observation magnification may be other than 100,000 times as long as the thickness can be measured.

[0056] (8) Young's modulus Strip samples, 150 mm long and 10 mm wide, were rotated clockwise in 10° increments, and the Young's modulus was measured for 18 samples rotated from 0° to 170°. The direction with the highest Young's modulus was designated the a-direction, and the direction perpendicular to this was designated the b-direction. Young's modulus was measured using an Instron-type tensile tester according to the method specified in JIS Z1702 (1994). Measurements were performed under the following conditions, with 10 samples being measured for each, and the average value was calculated. Measuring device: Orientec Co., Ltd. automatic film strength and elongation measuring device "Tensilon" (registered trademark) 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.

[0057] (9) Elongation absorption parameters A rectangular sample 150 mm long and 10 mm wide was cut out so that the film's a-direction and b-direction were the length directions, and the shrinkage in the width direction at 60% elongation in each direction was measured using a universal testing machine, and the elongation absorption parameter was calculated using the following formula. The tensile test was performed using an Instron type tester in accordance with the method specified in JIS Z1702 (1994). The measurement was performed under the following conditions, with 10 samples, and the test was stopped after each sample was stretched to 60% elongation, and the initial length L0 and the length at 60% elongation L 60 , the initial value in the width direction is W0, and the width length at 60% elongation is W 60 The elongation absorption parameter was calculated using the following formula. 60 Select the position where W0 and W 60 The value extracted from the position where the minimum value is obtained is used. Measurements can also be made using a universal projector if necessary. Universal testing machine: Orientec Co., Ltd.'s automatic film strength and elongation measuring device "Tensilon" (registered trademark) 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. Extension absorption parameter = -(ln(W 60 / W0) / (ln(L 60 / L0)) *ln: natural logarithm (10) Bending hysteresis 2HB Using a multipurpose bending tester (KES-FB2) manufactured by Kato Tech Co., Ltd., measurements were carried out in the a and b directions under the following conditions to obtain the bending hysteresis 2HB. The bending hysteresis 2HB was measured at a curvature of -2.5 to +2.5 cm. -1 The data obtained in the fifth cycle (five repetitions) was used. Number of repetitions: 5 (5th cycle of adopted data) SENS:2×5 Curvature:-2.5~+2.5cm -1 Deformation speed: 0.50cm -1 / sec Sample size: a direction x b direction = 20cm x 20cm (11) Bending hardness Using a multipurpose bending tester (KES-FB2) manufactured by Kato Tech Co., Ltd., measurements were carried out in the a and b directions under the following conditions to obtain the bending hardness. The bending hardness was measured at a curvature of -2.5 to +2.5 cm. -1 The data obtained in the fifth cycle (five repetitions) was used. Number of repetitions: 5 (5th cycle of adopted data) SENS:2×5 Curvature:-2.5~+2.5cm -1 Deformation speed: 0.50cm -1 / sec Sample size: a direction x b direction = 20cm x 20cm (12) Coefficient of static friction Using a slip tester manufactured by Toyo Seiki Co., Ltd., the initial resistance value when two films were rubbed together was measured in accordance with JIS-K7125 (1999), and this was taken as the static friction coefficient μs. The samples were rectangular, 80 mm wide and 200 mm long, and three sets (six pieces) were cut from the roll so that the length direction of the rectangle was in the a direction. Three measurements were taken, and the average value was calculated.

[0058] (13) Scratch resistance during winding The polyester film of the present invention was subjected to a rubbing test using a rubbing tester under the following conditions to determine its scratch resistance. The same polyester film was used as the sample and the rubbing material. Evaluation environment conditions: 25°C, 60% RH Rubbing material: polyester film of the present invention Wrap the band around the scraping tip (1cm x 1cm) of the tester that comes into contact with the sample and secure it in place. Travel distance (one way): 13cm, Scrub speed: 13cm / sec Load: 100g / cm 2 , Tip contact area: 1cm x 1cm, number of rubs: 5 times. After rubbing, the back of the sample was painted with oil-based black ink, and scratches on the rubbed area were visually observed using reflected light and evaluated according to the following criteria: The above test was repeated three times for evaluation. ○: No scratches are visible at all. △: Slight scratches are visible. ×: There are scratches that are obvious at a glance.

[0059] (14) Surface resistivity The film is left to stand at a temperature of 23°C and a relative humidity of 65% for 24 hours to condition the humidity, and then the surface resistivity is measured under the same conditions using a digital ultra-high resistance / microcurrent meter (Advantest R8340A) at an applied voltage of 100 V. Five measurements are taken, and the average value is used as the surface resistivity (Ω / □).

[0060] (15) Number of bending failures Using an MIT folding endurance tester (Mize Co., Ltd. No. 702), samples cut to a length (measurement direction) of 110 mm and a width of 15 mm were subjected to a bending test in accordance with JIS P8115 (2001) at a load of 1000 g, a bending angle of 135° left and right (R: +135°, L: -135°), a bending speed of 175 times / min, and a chuck tip R: 0.38 mm. The number of times the film was bent until it broke was taken as the number of times to break. The test was performed three times, and the average value was used.

[0061] (16) Dynamic bending resistance For the polyester film of the present invention, a sample cut to a width of 108 mm and a length of 112 mm was used in a U-shaped stretch tester (DLDMLH-FS manufactured by Yuasa System Instruments). The sample was attached to the end of the tilt clamp with the tilt clamp in a horizontal position and the stroke direction was in the length direction of the sample. The sample was then bent 10,000 times at a test speed of 60 r / min, a test stroke of 60 mm, and a face-to-face distance of 3 mm. After the test, the sample was evaluated based on the reflected light from a fluorescent lamp and its appearance as follows: A: No change in appearance was observed, and no distortion of reflected light was observed. B: No change in appearance, but distortion of reflected light was observed. C: Bending lines were clearly observed on the exterior.

[0062] (17) Winding resistance by mandrel test A film cut into a size of 15 mm x 30 mm (a direction x b direction) was conditioned for 24 hours at 23°C and 60% RH, and then placed as shown in Figure 1 so that the center of the base of the mandrel testing machine (Ueshima Seisakusho) was aligned with the center of the film. A 1.0 mm diameter test rod was placed, and the film was rewound 180° and left to stand for 24 hours. The film was then removed, quickly placed on a flat floor, and the height of the film lifted from the ground was recorded. The same measurement was performed on the film cut into a size of 15 mm x 30 mm (b direction x a direction), and the height of the film lifted was also recorded. Five measurements were taken for the film cut into a size of 15 mm x 30 mm (a direction x b direction) and the film cut into a size of 15 mm x 30 mm (b direction x a direction). The rewinding resistance was determined based on the average lift height of a total of 10 measurements, as follows:

[0063] Less than 0.5mm: ◎ 0.5mm or more and less than 0.8mm: Yes 0.8mm or more and less than 1.0mm:□ 1.0mm or more and less than 1.2mm: △ 1.2mm or more: × (18) Flatness after applying curable resin The curable resin Q described below in (Preparation of Curable Resin Q) was applied using a slot die coater while controlling the flow rate so that the thickness after drying would be 5 μm, and the film was dried at 100°C for 1 minute to remove the solvent. Next, a high-pressure mercury lamp was used to irradiate the film with the curable resin at 300 mJ / cm. 2 The polyester film was irradiated with ultraviolet light to obtain a curable resin laminate sheet. As the flatness of the obtained laminate sheet after coating of the curable resin layer, (a) wrinkles and (b) curls were evaluated as follows. (a) Wrinkles No wrinkles, good flatness of the entire sheet: ◎ There is a slight distortion in the film, but it does not affect the film's usability: ○ There are some obvious distortions in the film, but it doesn't affect the film's usability. The entire film is wavy and unusable: × (b) Curl The film was cut into A4 size pieces and placed on a flat surface, and the height it rose above the ground was evaluated as follows: No film lifting: ◎ The film is less than 10mm above the ground: The film is lifted from the ground by 10mm or more but less than 15mm: △ If the film rises above the ground by more than 15mm: ×

[0064] (Mixing of hardening resin Q) Silica particles with a particle size of 100 nm (organosilica sol manufactured by Nissan Chemical Industries, Ltd.) and a multifunctional acrylate ("KAYARAD" (registered trademark) PET30 manufactured by Nippon Kayaku Co., Ltd.) were mixed in a mass ratio of 50:50, and the mixture was diluted with a toluene / MEK mixed solvent (mass ratio of 50:50) to prepare a curable resin.

[0065] (19) Antistatic effect The curable resin laminate sheet of each polyester film obtained by the method described in (18) was cut into a square of 20 cm (400 cm2), and then, a Toray X35S black film was laid as a base, and the cut-out sample was placed on top of it. Under a fluorescent light environment of 500 to 1,000 lux, the major diameter of defects was visually measured using a JIS P8208 / P8145 general dot cage, and the number of defects of 0.1 mm or more was counted and evaluated as follows. ○: 10 or more pieces / 400cm 2 It was. △: Less than 10 pieces / 400cm 2 It was.

[0066] (20) Heat shrinkage starting temperature obtained from the heat shrinkage curve Using a thermomechanical measuring device, TMA / SS6100 (Seiko Instruments Inc.), a load of 3 g was applied to a sample with a sample width of 4 mm and a sample length (distance between chucks) of 20 mm. The sample was heated from room temperature to 170°C at a heating rate of 5°C / min, and held at 170°C for 10 minutes. A thermal shrinkage curve was obtained when the sample was heated from room temperature to 170°C. The obtained thermal shrinkage curve was then differentiated with respect to the measurement time to obtain the DTMA. The temperature at which the DTMA became 0 was defined as the thermal shrinkage initiation temperature.

[0067] (21)Dry heat resistance Screen printing was performed on the film surface. The printing was performed using U-PET (517) ink and SX270T screen manufactured by Mino Group Co., Ltd. at a squeegee speed of 300 mm / sec and a squeegee angle of 45°. The film was then dried in a hot air oven at 80°C for 5 minutes to obtain a film for evaluation. The appearance of the obtained film for evaluation was evaluated according to the following criteria. A: No wrinkles were observed even after drying, and the appearance was good. B: Some wrinkles were observed after drying, but the appearance was good. C: Wrinkles were observed after drying, but were at a level that would not pose a practical problem for a printed film.

[0068] (Polyester manufacturing) The polyester resin used for film formation was prepared as follows.

[0069] (Polyester A) Polyethylene terephthalate resin (intrinsic viscosity 0.75) containing 100 mol % terephthalic acid as the dicarboxylic acid component and 100 mol % ethylene glycol as the glycol component.

[0070] (Polyester B) Polyethylene terephthalate resin (intrinsic viscosity 0.70) containing 100 mol % terephthalic acid as the dicarboxylic acid component and 100 mol % ethylene glycol as the glycol component.

[0071] (Polyester C) Polyethylene terephthalate resin (intrinsic viscosity 0.80) containing 100 mol % terephthalic acid as the dicarboxylic acid component and 100 mol % ethylene glycol as the glycol component.

[0072] (Polyester D) Polyethylene terephthalate resin (intrinsic viscosity 0.85) containing 100 mol % terephthalic acid as the dicarboxylic acid component and 100 mol % ethylene glycol as the glycol component.

[0073] (Polyester E) Polybutylene terephthalate resin (intrinsic viscosity 1.2) containing 100 mol % terephthalic acid as the dicarboxylic acid component and 100 mol % 1,4-butanediol as the glycol component.

[0074] (Particle Master A) Polyethylene terephthalate particle master (intrinsic viscosity 0.65) containing calcium carbonate particles with an average particle size of 1.2 μm at a particle concentration of 1 mass% in polyester A.

[0075] (Particle Master B) Polyethylene terephthalate particle master (intrinsic viscosity 1.1) containing calcium carbonate particles with an average particle size of 1.2 μm at a particle concentration of 1 mass% in polyester E.

[0076] (Mixing of easy-adhesion resin P) The easy-adhesion resin layer to be laminated on the surface of the film was prepared as follows.

[0077] Resin solution (a): A solution obtained by mixing 70 parts by mass of an aqueous coating liquid of a polyester resin consisting of the acid components and diol components of terephthalic acid (88 mol%), 5-sodium sulfoisophthalic acid (12 mol%), and ethylene glycol (100 mol%), which are acid components, with 30 parts by mass of an aqueous dispersion of a polyester resin consisting of the acid components of terephthalic acid (50 mol%), isophthalic acid (49 mol%), and 5-sodium sulfoisophthalic acid (1 mol%), and the diol components of ethylene glycol (55 mol%), neopentyl glycol (44 mol%), and polyethylene glycol (molecular weight: 4000) (1 mol%), which are acid components, and diol components. Crosslinker (b): Methylol group-type melamine crosslinker Crosslinking agent (c): oxazoline group-containing crosslinking agent Particles (d): Water dispersion of colloidal silica particles with a particle diameter of 150 nm Particle (e): Aqueous dispersion of colloidal silica particles with a particle diameter of 300 nm Fluorine-based surfactant (f): "Megafac" (registered trademark) F-444 manufactured by DIC Corporation These were mixed in a solid mass ratio of (a) / (b) / (c) / (d) / (e) / (f) = 47 parts by mass. The mixture was mixed in a ratio of 19 parts by weight, 20 parts by weight, 4.9 parts by weight, 0.7 parts by weight, and 0.1 parts by weight.

[0078] The refractive index of the adhesive resin P after drying was 1.57.

[0079] (Mixing of hardening resin Q) Silica particles with a particle size of 100 nm (organosilica sol manufactured by Nissan Chemical Industries, Ltd.) and a multifunctional acrylate ("KAYARAD" (registered trademark) PET30 manufactured by Nippon Kayaku Co., Ltd.) were mixed in a mass ratio of 50:50, and the mixture was diluted with a toluene / MEK mixed solvent (mass ratio of 50:50) to prepare a curable resin.

[0080] (Examples 1 to 8, 10 to 19, Comparative Examples 1 to 6) The resin species and particle master species were mixed in the amounts listed in Table 1 and fed into an extruder. The mixture was then melted at the extruder temperature listed in Table 1 and extruded into a sheet form from a T-die onto a cooling drum controlled at 25°C. A 0.1 mm diameter wire electrode was used to apply static electricity, and the sheet was attached to the cooling drum to obtain an unstretched sheet. The sheet was then quenched with a cooling roll controlled at 20°C, stretched in the MD direction with each roll at the stretching temperature and stretch ratio listed in Table 2, and then cooled once (intermediate cooling). Next, both sides of this uniaxially stretched film were subjected to a corona discharge treatment to adjust the film's wet tension to 55 mN / m. Both sides of the film were coated with the easy-adhesion resin P, stretched in the TD direction at the stretching temperature and stretch ratio listed in Table 2 in a first oven tenter, and cooled to the temperature listed in Table 2 via intermediate cooling. The film was then heat-treated at the heat-treatment temperature listed in Table 2 and relaxed in the width direction in the tenter of a second oven to obtain a film with the thickness listed in Table 1. The physical properties of the obtained films are as shown in Tables 3 and 4, with the MD direction being the a direction and the TD direction being the b direction. The examples were excellent in bending resistance and flatness after coating with a curable resin.

[0081] Example 9 A polyester film was obtained in the same manner as in Example 5, except that no adhesive resin P was applied. As in the other Examples, the film had excellent flex resistance and flatness after application of the curable resin.

[0082] [Table 1]

[0083] [Table 2]

[0084] [Table 3]

[0085] [Table 4] [Industrial Applicability]

[0086] The polyester film of the present invention has a Young's modulus and an elongation absorption parameter within specific ranges, and therefore can be particularly suitably used as, for example, a cover film for a flexible image display device.

Claims

1. A polyester film having a Young's modulus of 2.8 GPa or more and less than 5.5 GPa in the direction (a-direction) in which the Young's modulus is greatest, an elongational absorption parameter in both the a-direction and a direction (b-direction) perpendicular to the a-direction being 0.7 or more and 1.5 or less, an intrinsic viscosity of the film being 0.66 dl / g or more and 1.15 dl / g or less, and containing a resin primarily composed of any one of polyethylene terephthalate, polyethylene naphthalate, and polycyclohexylene dimethylene terephthalate.

2. 2. The polyester film according to claim 1, wherein the heat shrinkage initiation temperature obtained from the heat shrinkage curve is 65° C. or higher and 120° C. or lower in both the longitudinal and transverse directions.

3. 3. The polyester film according to claim 1, wherein the bending hysteresis 2HB measured by the following method is from 0.04 gf·cm / cm to 0.10 gf·cm / cm in both the a-direction and the b-direction. [Method for measuring bending hysteresis 2HB] Using a multipurpose bending tester (KES-FB2) manufactured by Kato Tech Co., Ltd., bending hysteresis 2HB is measured in the a direction and the b direction under the following conditions. The bending hysteresis 2HB is measured at a curvature of -2.5 to +2.5 cm. －１ is defined as one cycle, and the value obtained in the fifth cycle (repeated five times) is defined as the value obtained in the fifth cycle. Number of repetitions: 5 (5th cycle of adopted data) SENS: 2 x 5 Curvature: -2.5 to +2.5 cm －１ Deformation speed: 0.50 cm －１ / sec Sample size: a direction x b direction = 20 cm x 20 cm

4. The bending hardness measured by the following measurement method is 0.06 gf cm in both the a direction and the b direction. ２ / cm or more 0.12gf・cm ２ 4. The polyester film according to claim 1, wherein the thickness of the polyester film is 1 / cm or less. [Method for measuring bending hardness] Using a multipurpose bending tester (KES-FB2) manufactured by Kato Tech Co., Ltd., bending hardness is measured in the a direction and the b direction under the following conditions. The bending hardness is measured at a curvature of -2.5 to +2.5 cm. －１ is defined as one cycle, and the value obtained in the fifth cycle (repeated five times) is defined as the value obtained in the fifth cycle. Number of repetitions: 5 (5th cycle of adopted data) SENS: 2 x 5 Curvature: -2.5 to +2.5 cm －１ Deformation speed: 0.50 cm －１ / sec Sample size: a direction x b direction = 20 cm x 20 cm

5. 5. The polyester film according to claim 1, wherein the weight average molecular weight Mw is 20,000 or more and 50,000 or less.

6. 6. The polyester film according to claim 1, wherein the number of bending cycles in the a direction to break is 200,000 to 1,000,000.

7. 7. The polyester film according to claim 1, having a thickness of 5 μm or more and 30 μm or less.

8. 8. The polyester film according to claim 1, wherein the static friction coefficient of at least one surface of the polyester film is 0.2 or more and 0.5 or less.

9. The surface resistivity of at least one surface is 1×10 １２ 9. The polyester film according to claim 1, wherein the elastic modulus is Ω / □ or less.

10. The polyester film according to any one of claims 1 to 9, which is used for a flexible display.

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