Polyester film and foldable display device comprising the same

Abstract

To provide a polyester film capable of exhibiting enhanced surface hardness, along with excellent transparency, flexibility, and folding characteristics, and a laminate and a foldable display device comprising the same.SOLUTION: Provided is a biaxially stretched polyester film, wherein when a first direction and a second direction perpendicular to each other are defined in the plane of the film, a yield elongation in at least one of the first direction and the second direction is 2.2% to 2.6%, the first direction is a longitudinal direction (MD), the second direction is a transverse direction (TD), and the thickness is 80 μm to 150 μm.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] Embodiments relate to a polyester film that is excellent in transparency, flexibility, folding properties, etc. and can exhibit improved surface hardness, a laminate including the same, and a foldable display device.

Background Art

[0002] Recently, in the field of mobile devices where large screen and portability are simultaneously required, foldable display devices that can be flexibly folded in response to an external force have attracted attention. Such foldable display devices have the merit that they can be folded and made small for high portability when not in use, and can be unfolded to realize a large screen when in use.

[0003] The foldable display device has been developed into an in-folding type (1) in which the screen is located inside in the folding direction and an out-folding type (2) in which the screen is located outside in the folding direction (see FIG. 3). As the transparent base material 200 applied to the cover window of these foldable display devices, initially, a polyimide-based film was mainly used in the in-folding type (1), and ultra-thin glass (UTG) was mainly used in the out-folding type (2). However, recently, the physical properties of ultra-thin glass have been improved and it has shown superior properties compared to polyimide-based films. Therefore, most ultra-thin glass (UTG) is also used in the in-folding type (1).

[0004] However, the ultra-thin glass (UTG) used as the transparent base material 200 still has a high possibility of cracks and the like occurring due to external impacts. Accordingly, a protective film 100 is applied to the surface of the transparent base material 200 to achieve shock mitigation, splash prevention, scratch prevention, etc.

[0005] As the protective film 100, a polyester film is mainly used. However, currently developed polyester films have limitations in that they cannot exhibit a level sufficient to satisfy the surface hardness that a protective film should have. That is, in order to increase the surface hardness, it is necessary to increase the thickness of the polyester film. However, when the thickness of the polyester film increases, cracks occur at the portions (a and b in FIG. 3) that are repeatedly folded, and the folding characteristics deteriorate. Therefore, it is difficult to increase the thickness of the polyester film, and thus there is a limit in securing the required surface hardness.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] An embodiment aims to provide a polyester film that is excellent in transparency, flexibility, folding characteristics, etc., and can exhibit improved surface hardness.

[0008] Also, an embodiment aims to provide a laminate and a foldable display device manufactured using the polyester film.

Means for Solving the Problems

[0009] According to an embodiment for solving the above problems, when defining a first direction and a second direction perpendicular to each other in the plane of the film, a polyester film is provided in which the yield elongation rate of at least one of the first direction and the second direction is 2.2% to 2.6%, and the thickness is 80 μm to 150 μm.

[0010] According to another implementation example, a laminate is provided that includes a transparent substrate and a polyester film disposed on the transparent substrate. When the polyester film defines a first direction and a second direction perpendicular to each other in the plane of the film, at least one of the yield elongation rates in the first direction and the second direction is 2.2% to 2.6%, and the thickness is 80 μm to 150 μm.

[0011] According to still another implementation example, a foldable display device is provided that includes a foldable display panel, a transparent substrate disposed on the foldable display panel, and a polyester film disposed on the transparent substrate. When the polyester film defines a first direction and a second direction perpendicular to each other in the plane of the film, at least one of the yield elongation rates in the first direction and the second direction is 2.2% to 2.6%, and the thickness is 80 μm to 150 μm.

Advantages of the Invention

[0012] According to the implementation example, by controlling the yield elongation rate of the polyester film within a specific range, even if the thickness of the polyester film is relatively thick, it can be excellent in flexibility and folding characteristics. Thereby, the polyester film can exhibit improved surface hardness.

[0013] Therefore, the polyester film according to the implementation example is applied as a protective film for a foldable display device, and even when multiple repeated foldings are performed, it can efficiently prevent the impact applied to the foldable display device from being alleviated and scratches from occurring while maintaining the original characteristics.

Brief Description of the Drawings

[0014]

Figure 1

Figure 2

Figure 3

[0015] Hereinafter, the invention will be described with reference to implementation examples. Note that the implementation examples are not limited to the content disclosed below, and can be modified into various forms without changing the gist of the invention.

[0016] In this specification, the description that a certain component is formed above / below another component, or is connected or coupled to each other, includes all cases where these components are directly or indirectly formed, connected, or coupled via other components. Further, it should be understood that the reference for above / below of each component can vary depending on the direction of observing the object.

[0017] In this specification, the description of "including" is for specifying a specific characteristic, region, stage, process, element, and / or component, and does not exclude the existence or addition of other characteristics, regions, stages, processes, elements, and / or components unless otherwise stated.

[0018] All numbers and expressions indicating the amounts of components, reaction conditions, etc. described in this specification should be understood to be modified by the term "about" in all cases unless otherwise stated.

[0019] In this specification, terms such as first and second are used to describe various components, and the components should not be limited by the terms. The terms are used for the purpose of distinguishing one component from another.

[0020] The size of each component in this drawing may be exaggerated for the purpose of explanation and may be different from the actual size applied.

[0021] The polyester film used as a protective film needs to have a relatively large thickness to increase its surface hardness in order to enhance effects such as shock mitigation and scratch prevention. However, when the thickness of the polyester film increases, there is a problem that the folding property deteriorates. Here, in order to increase the surface hardness of the polyester film, a hard coat layer is formed on its surface. However, due to limitations in the components and coating thickness of the hard coat layer, a satisfactory level of surface hardness has not been obtained.

[0022] As a result, the inventors conducted various studies to ensure the folding property even when the thickness of the polyester film was increased. As a result, it was confirmed that by controlling the yield elongation rate of the polyester film within a specific range, the folding property can be ensured even at a relatively large thickness.

[0023] In the following examples, a polyester film, a laminate including the same, and a foldable display device that can exhibit a high surface hardness due to its relatively large thickness and are excellent in folding property, flexibility, transparency, etc. will be described.

[0024] [Polyester Film] <Physical Properties of Polyester Film> The polyester film according to the implementation example may have a yield elongation rate of 2.2% to 2.6%. Specifically, when defining a first direction and a second direction perpendicular to each other in the plane of the film for the polyester film, the yield elongation rate of at least one of the first direction and the second direction may be 2.2% to 2.6%, 2.21% to 2.6%, 2.22% to 2.6%, 2.25% to 2.6%, 2.28% to 2.58%, 2.29% to 2.57%, or 2.3% to 2.55%. When the yield elongation rate is within the above range, a polyester film with excellent folding properties, high surface hardness, and showing a required level of elongation can be provided. Specifically, if the yield elongation rate of the polyester film is less than 2.2%, the folding properties may deteriorate, and if it exceeds 2.6%, the elongation rate may significantly decrease, and the polyester film may be easily cut during the process of being pulled or bent.

[0025] The yield elongation rate may mean the tensile deformation rate at the yield point, which means the point where the elastic force is lost and permanent deformation begins. At this time, since the yield point of the polyester film, which is a polymer material, is not clearly distinguishable, in the examples, the tensile deformation rate at the 0.2% offset yield point was defined as the yield elongation rate of the polyester film. The 0.2% offset yield point may mean the point where the linear part where the tensile strain increases linearly from 0% is moved to the point of 0.2% of the gauge length (focal length) of the test piece before tension and meets (see Figure 1).

[0026] On the other hand, the first direction may be any direction in the plane of the polyester film, and the second direction may mean a direction perpendicular to the first direction in the plane of the film. Specifically, the first direction may be the length direction (MD) or the width direction (TD) of the polyester film, and the second direction may be the width direction (TD) or the length direction (MD) perpendicular thereto. More specifically, the first direction may be the length direction (MD) of the polyester film, and the second direction may be the width direction (TD) of the polyester film.

[0027] According to the implementation example, the yield elongation rate may be the yield elongation rate (E1) in the machine direction (MD). That is, the polyester film according to the implementation example has a yield elongation rate (E1) in the machine direction (MD) of 2.2% to 2.6% (specifically 2.21% to 2.6%, 2.22% to 2.6%, 2.25% to 2.6%, 2.28% to 2.58%, 2.29% to 2.57%, or 2.3% to 2.55%), and may be a polyester film that satisfies the following formula (1).

[0028] 1.1 ≦ E1 / E2 ≦ 1.4... (1) In the formula (1), E1 is the yield elongation rate in the machine direction (MD), E2 is the yield elongation rate in the transverse direction (TD).

[0029] Specifically, in the formula (1), the E1 / E2 ratio may be 1.14 to 1.39, 1.15 to 1.37, 1.18 to 1.35, 1.18 to 1.33, 1.19 to 1.30, or 1.19 to 1.28.

[0030] Also, the polyester film according to the implementation example has a yield elongation rate (E1) in the machine direction (MD) of 2.2% to 2.6% (specifically 2.21% to 2.6%, 2.22% to 2.6%, 2.25% to 2.6%, 2.28% to 2.58%, 2.29% to 2.57%, or 2.3% to 2.55%), and may be a polyester film that satisfies the following formula (2).

[0031] 0.01 ≦ D1 × E1 / T ≦ 0.1... (2) In the formula (2), D1 is the draw ratio in the machine direction (MD), E1 is the yield elongation rate in the machine direction (MD), T is the thickness of the polyester film.

[0032] Specifically, in the formula (2), the D1×E1 / T ratio ((D1×E1) / T ratio) can be 0.02 to 0.098, 0.03 to 0.095, 0.04 to 0.094, 0.05 to 0.092, 0.06 to 0.091, or 0.07 to 0.09.

[0033] When the polyester film according to the realization example satisfies the formula (1) and / or the formula (2), it has excellent folding properties, high surface hardness, shows a required level of elongation rate, and can ensure flexibility.

[0034] On the other hand, the polyester film according to the realization example can have a thickness deviation of 3 μm or less. Specifically, the polyester film can have a thickness deviation of 2.5 μm or less, 2.3 μm or less, 2.2 μm or less, 2 μm or less, 1.8 μm or less, or 1.5 μm or less.

[0035] Also, the polyester film according to the realization example can have an elongation rate of 40% or more. Specifically, the polyester film can have an elongation rate of 40% to 200%, 50% to 190%, 60% to 180%, 70% to 170%, 80% to 160%, or 100% to 160%. When the elongation rate is within the above range, the polyester film can have excellent flexibility and folding properties.

[0036] Also, the polyester film according to the realization example can have a haze of 2.0% or less. Specifically, the polyester film can have a haze of 0.4% to 2.0%, 0.4% to 1.8%, 0.5% to 1.5%, 0.5% to 1.3%, or 0.6% to 1.0%. When the haze is within the above range, the polyester film can have excellent transparency and optical properties.

[0037] Also, the polyester film according to the realization example can have a surface hardness of 120 N / mm 2 or more. Specifically, the polyester film can have a surface hardness of 120 N / mm 2 to 180 N / mm 2 or 125 N / mm 2~175 N / mm 2 、130 N / mm 2 ~170 N / mm 2 、135 N / mm 2 ~170 N / mm 2 、 or 140 N / mm 2 ~160 N / mm 2 It can be. When the surface hardness is within the above range, the polyester film can be efficiently used as a protective film.

[0038] In addition, the in-plane retardation (Ro) of the polyester film according to the implementation example can be 4000 or less, 3500 or less, or 3000 m or less. Specifically, the in-plane retardation (Ro) of the polyester film can be 500 - 4000, 600 - 3500, or 700 - 3000. When the in-plane retardation (Ro) is within the above range, the rainbow unevenness generated in the polyester film can be minimized.

[0039] Also, the thickness direction retardation (Rth) of the polyester film according to the implementation example can be 6000 or more, 7000 or more, or 8000 or more. Specifically, the thickness direction retardation (Rth) of the polyester film can be 6000 - 14000, 7000 - 13000, or 8000 - 12000. When the thickness direction retardation (Rth) is within the range, the rainbow unevenness generated in the polyester film can be minimized.

[0040] Note that the in-plane retardation (Ro) can be a parameter defined by the product (△nxy × d) of the anisotropy of the refractive indices (△nxy = |nx - ny|) of two axes perpendicular to each other on one surface of the film and the thickness (d) of the film.

[0041] Also, the thickness direction retardation (Rth) can be a parameter defined as the average of the retardations obtained by multiplying the film thickness (d) by two birefringences △nxz (= |nx - nz|) and △nyz (= |ny - nz|) when viewed from the cross-section in the thickness direction of the film.

[0042] <Properties of the polyester film> The polyester film according to the implementation example can have a thickness of 80 μm or more. Specifically, the polyester film can have a thickness of 80 μm to 150 μm, 83 μm to 140 μm, 85 μm to 135 μm, 90 μm to 120 μm, 95 μm to 110 μm, or 95 μm to 105 μm. When the thickness is within the above range, even if repeated folding is performed a large number of times (for example, 200,000 times or more), the occurrence of cracks at the folding site can be minimized, and a polyester film with high surface hardness can be provided.

[0043] Also, the polyester film according to the implementation example can be a biaxially stretched film. Specifically, the polyester film can be a film stretched in the machine direction (MD) at a draw ratio (D1) of 2.5 to 3.0 and stretched in the transverse direction (TD) at a draw ratio (D2) of 3.5 to 5.5. Specifically, the draw ratio (D1) in the machine direction (MD) can be 2.5 to 2.9, 2.5 to 2.8, or 2.5 to 2.7. Also, the draw ratio (D2) in the transverse direction (TD) can be 3.5 to 5.4, 3.7 to 4.9, or 4.3 to 4.8. When the draw ratios (D1, D2) in the machine direction (MD) and / or the transverse direction (TD) are within the above range, even if the thickness of the polyester film is relatively thick (for example, 80 μm or more), the yield elongation rate can be controlled to the required level, and a polyester film with excellent folding properties can be provided.

[0044] Also, for the polyester film according to the implementation example, the ratio (D2 / D1) of the draw ratio (D2) in the transverse direction (TD) to the draw ratio (D1) in the machine direction (MD) can be 1.3 to 1.8. Specifically, the ratio (D2 / D1) can be 1.3 to 1.75, 1.3 to 1.65, 1.33 to 1.6, 1.35 to 1.55, or 1.4 to 1.52. When the ratio (D2 / D1) is within the above range, a polyester film with excellent folding properties, appearance, and optical properties can be provided.

[0045] The polyester film according to such an embodiment can exhibit excellent folding properties not only at normal temperature but also at low temperature. Specifically, the polyester film according to the embodiment may not generate cracks during repeated folding at -20°C with a bending radius of 1.5 mm for 100 times or more, 1000 times or more, 10,000 times or more, 50,000 times or more, 100,000 times or more, 150,000 times or more, or 200,000 times or more.

[0046] <Polyester film composition> The polyester film according to the embodiment contains a polyester resin. The polyester resin may be a single polymer resin or a copolymer resin obtained by polymerizing (polycondensing) a dicarboxylic acid and a diol. Further, the polyester resin may be a resin in which the single polymer resin or the copolymer resin is mixed.

[0047] Specifically, the dicarboxylic acid may be terephthalic acid, isophthalic acid, orthophthalic acid, 2,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, diphenylcarboxylic acid, diphenoxyethanedicarboxylic acid, diphenylsulfonecarboxylic acid, anthracenedicarboxylic acid, 1,3-cyclopentanedicarboxylic acid, 1,3-cyclohexanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, hexahydroterephthalic acid, hexahydroisophthalic acid, malonic acid, dimethylmalonic acid, succinic acid, 3,3-diethylsuccinic acid, glutaric acid, 2,2-dimethylglutaric acid, adipic acid, 2-methyladipic acid, trimethyladipic acid, pimelic acid, azelaic acid, sebacic acid, suberic acid, dodecanedicarboxylic acid, or a combination thereof, but is not limited thereto.

[0048] Specifically, the diol may be, but is not limited to, ethylene glycol, propylene glycol, hexamethylene glycol, neopentyl glycol, 1,2-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, decamethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 2,2-bis(4-hydroxyphenyl)propane, bis(4-hydroxyphenyl)sulfone, or a combination thereof.

[0049] Specifically, the polyester resin is an aromatic polyester resin with excellent crystallinity, and more specifically, it may be a resin mainly composed of polyethylene terephthalate (PET) resin.

[0050] As an example, the polyester film according to the implementation example may contain about 85% by weight or more, 90% by weight or more, 95% by weight or more, or 99% by weight or more of the PET resin. Further, the polyester film may contain other polyester resins in addition to the PET resin. Specifically, the polyester film may contain polyethylene naphthalate (PEN) resin at 15% by weight or less. More specifically, the polyester film may contain the PEN resin at 0.1% by weight to 10% by weight, or 0.1% by weight to 5% by weight.

[0051] <Method for manufacturing polyester film> The polyester film according to the implementation example can be manufactured through a process including biaxial stretching under specific stretching ratio conditions and heat treatment at a specific temperature.

[0052] Specifically, the method for manufacturing a polyester film according to the implementation example includes the steps of forming an unstretched sheet from a polyester resin, biaxially stretching it, and heat-treating it. As a result, a polyester film with a thickness of 80 μm to 150 μm and a yield elongation rate of at least one of the first direction and the second direction perpendicular to each other in the plane of 2.2% to 2.6% can be manufactured. More specifically, in the implementation example, in order to obtain a polyester film exhibiting specific physical properties, the extrusion and casting temperatures of the polyester resin are adjusted, the preheating temperature during stretching, the stretching ratio for each direction, the stretching temperature, the stretching speed, etc. are adjusted, and the heat treatment temperature and relaxation rate during heat treatment and relaxation after stretching are adjusted. The process may go through the following steps, which will be described as follows.

[0053] First, the polyester resin is extruded to form an unstretched sheet. The extrusion of the polyester resin can be carried out at 230°C to 300°C, 240°C to 290°C, or 250°C to 280°C.

[0054] The formed unstretched sheet may undergo a biaxial stretching process after preheating. The preheating of the unstretched sheet is carried out within the range of Tg + 5°C to Tg + 50°C based on the glass transition temperature (Tg) of the polyester resin. Specifically, it can be carried out at 70°C to 100°C. When preheating is carried out within this range, the unstretched sheet has suitable flexibility for biaxial stretching and can effectively prevent the sheet from breaking during biaxial stretching.

[0055] The biaxial stretching of the preheated unstretched sheet can be carried out by the simultaneous biaxial stretching method or the sequential biaxial stretching method. Specifically, the unstretched sheet can be biaxially stretched in the machine direction (MD) and the transverse direction (TD) by the sequential biaxial stretching method in which the unstretched sheet is first stretched in one direction and then stretched in the direction perpendicular to that direction.

[0056] The speed at which the stretching is performed is not particularly limited, but specifically, it can be 6.5 m / min to 8.5 m / min. Also, the temperature at which the stretching is performed is not particularly limited, but specifically, it can be 125°C or higher, 130°C or higher, or 135°C or higher (for example, 125 to 160°C, 130 to 155°C, 135 to 150°C, or 135 to 145°C).

[0057] The draw ratio in the longitudinal direction (MD) is 2.5 to 3.0, and specifically, it can be 2.5 to 2.9, 2.5 to 2.8, or 2.5 to 2.7. Also, the draw ratio in the width direction (TD) is 3.5 to 5.5, and specifically, it can be 3.5 to 5.4, 3.7 to 4.9, or 4.3 to 4.8. When the draw ratios in the longitudinal direction (MD) and the width direction (TD) are within the above ranges, a polyester film with uniform thickness and having a yield elongation rate within an optimal range can be obtained.

[0058] Also, the ratio (D2 / D1) of the draw ratio (D2) in the width direction (TD) to the draw ratio (D1) in the longitudinal direction (MD) is 1.3 to 1.8, and specifically, it can be 1.3 to 1.75, 1.3 to 1.65, 1.33 to 1.6, 1.35 to 1.55, or 1.4 to 1.52. When the ratio (D2 / D1) is within the above range, a polyester film with excellent transparency and flexibility and having a yield elongation rate within an optimal range can be obtained.

[0059] The film obtained through the stretching step may undergo a heat treatment step. The heat treatment can be performed at a temperature of 180°C or higher, 195°C or higher, or 195°C to 230°C for 0.2 minutes to 1 minute, or 0.4 minutes to 0.7 minutes.

[0060] The film obtained through the heat treatment step may undergo a relaxation step of being relaxed in the longitudinal direction (MD) and / or the width direction (TD). The relaxation can be performed at a temperature of 150°C to 250°C for 1 second to 1 minute, 2 seconds to 30 seconds, or 3 seconds to 10 seconds. Also, the relaxation can be performed at a relaxation rate of 1% to 10%, 2% to 7%, or 3% to 5%.

[0061] Subsequently, the film obtained through the relaxation process may undergo a cooling process. The cooling can be performed at a temperature 50°C to 150°C lower than the temperature at which the heat treatment process is performed.

[0062] [Laminate] The laminate according to the implementation example includes a transparent substrate and a polyester film. Specifically explaining this, it is as follows.

[0063] The transparent substrate included in the laminate according to the implementation example can be a substrate having transparency and flexibility. Specifically, the transparent substrate can be a polymer film or a glass substrate.

[0064] The polymer film can specifically be a polyimide-based film. The polyimide-based film can include a polyimide-based polymer obtained by polymerizing one or more selected from the group consisting of a diamine compound, a dianhydride compound, and a dicarbonyl compound. The polyimide-based polymer can include an imide repeating unit derived from the polymerization of a diamine compound and a dianhydride compound and / or an amide repeating unit derived from the polymerization of a diamine compound and a dicarbonyl compound.

[0065] The diamine compound can be an aromatic diamine compound containing an aromatic structure. Specifically, the diamine compound can be 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl (TFDB), oxydianiline (ODA), p-phenylenediamine (pPDA), m-phenylenediamine (mPDA), p-methylenedianiline (pMDA), bis(4-aminophenyl)fluorene (FDA), or a combination thereof, but is not limited thereto.

[0066] The dianhydride compound may be an aromatic dianhydride compound containing an aromatic structure or an alicyclic dianhydride compound containing an alicyclic structure. Specifically, the dianhydride compound may be 2,2'-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA), biphenyl-tetracarboxylic acid dianhydride (BPDA), or a combination thereof, but is not limited thereto.

[0067] The dicarbonyl compound may be an aromatic dicarbonyl compound containing an aromatic structure. Specifically, the dicarbonyl compound may be terephthaloyl chloride (TPC), 1,1'-biphenyl-4,4'-dicarbonyl dichloride (BPDC), isophthaloyl chloride (IPC), or a combination thereof, but is not limited thereto.

[0068] As an example, the polyimide-based polymer may contain a repeating unit represented by the following Chemical Formula 1, but is not limited thereto.

[0069] [Chemical Formula 1] In Chemical Formula 1, n is an integer from 1 to 400.

[0070] In addition, the polyimide-based polymer may contain one or more of the repeating units represented by the following Chemical Formulas 2 to 4, but is not limited thereto.

[0071] [Chemical Formula 2] In Chemical Formula 2, x is an integer from 1 to 400.

[0072] [Chemical Formula 3] In Chemical Formula 3, y is an integer from 1 to 400.

[0073] [Chemical Formula 4] In the chemical formula 4, y is an integer from 1 to 400.

[0074] On the other hand, the glass substrate can specifically be ultra-thin glass (UTG). Such a transparent substrate can have a thickness of 20 μm to 500 μm, 30 μm to 300 μm, or 40 μm to 100 μm.

[0075] In addition, the transparent substrate can have a surface hardness of HB or higher and a light transmittance of 80% or higher at a wavelength of 550 nm. Also, based on a thickness of 50 μm, the transparent substrate can have a yellowness of 5 or less and a haze of 2% or less.

[0076] The polyester film included in the laminate according to the implementation example can be a protective film that protects the transparent substrate. When defining a first direction and a second direction perpendicular to each other in the plane of the film, such a polyester film has a yield elongation rate of at least one of the first direction and the second direction of 2.2% to 2.6% and a thickness of 80 μm or more (specifically 80 μm to 150 μm). Specifically, the polyester film has substantially the same configuration and characteristics as the polyester film according to the foregoing implementation example.

[0077] According to the implementation example, the laminate including the transparent substrate and the polyester film can have a repeated folding number of 100 or more, 1000 or more, 10,000 or more, 50,000 or more, 100,000 or more, 150,000 or more, or 200,000 or more until peeling occurs. When the repeated folding number is within the above range, peeling does not occur even with frequent folding, so it can be advantageously applied to a foldable display device.

[0078] Specifically, the laminate according to the implementation example is excellent in transparency, flexibility, and folding characteristics, includes the aforementioned polyester film, and has improved surface hardness, so it can be usefully applied as a cover material for a foldable display device.

[0079] [Foldable Display Device] The foldable display device according to the implementation example includes a foldable display panel, a transparent substrate, and a polyester film. Specifically explaining this, it is as follows.

[0080] The foldable display panel included in the foldable display device according to the implementation example can be an organic light-emitting diode (OLED) panel, a quantum dot light-emitting diode (QLED) panel, or the like. As an example, FIG. 2 schematically shows a foldable display device 10 including a foldable display panel 300. The foldable display device 10 may include a front polarizing plate 310 and an organic light-emitting diode display panel 320.

[0081] The front polarizing plate 310 can be disposed on the front surface of the organic light-emitting diode display panel 320. Specifically, the front polarizing plate 310 can be adhered to the surface of the organic light-emitting diode display panel 320 where an image is displayed.

[0082] The organic light-emitting diode display panel 320 displays an image by self-emission in pixel units. Such an organic light-emitting diode display panel 320 may include an organic light-emitting substrate and a driving substrate. The organic light-emitting substrate may include a plurality of organic light-emitting units respectively corresponding to pixels. Each of the organic light-emitting units includes a cathode, an electron transport layer, a light-emitting layer, a hole transport layer, and an anode. The driving substrate is drivingly coupled to the organic light-emitting substrate. That is, the driving substrate can apply a current to each of the organic light-emitting units so that the organic light-emitting substrate is driven.

[0083] The transparent substrate 200 included in the foldable display device 10 according to the implementation example is disposed on the foldable display panel 300 as a transparent cover. Such a transparent substrate 200 has substantially the same configuration and characteristics as the transparent substrate described in the laminate.

[0084] The polyester film 100 included in the foldable display device 10 according to the implementation example is disposed on the transparent substrate 200 as a protective film. Such a polyester film 100 has substantially the same configuration and characteristics as the polyester film according to the aforementioned implementation example.

[0085] The foldable display device 10 according to the implementation example can be an in-folding type or an out-folding type according to the folding direction.

[0086] Since such a polyester film is applied as a protective film in the foldable display device according to the implementation example, it exhibits excellent folding characteristics and can be excellent in its lifespan and reliability even when an external force is applied to the surface over a long time or a large number of times (for example, when using a touch pen).

[0087] (Example) Hereinafter, the present implementation example will be described in more detail with reference to examples. However, the scope of the present implementation example is not limited by these examples.

[0088] (Manufacture of Polyester Film) (Examples 1 to 11) Polyethylene terephthalate (PET) resin was extruded and cast at a temperature of about 250°C to 280°C by an extruder to produce an unstretched sheet. The unstretched sheet was preheated and biaxially stretched while adjusting the stretching ratio in each of the longitudinal direction (MD) and the width direction (TD) to obtain a stretched sheet. Next, the stretched sheet was heat-fixed, relaxed at a relaxation rate of 3% to 5% for 3 seconds to 10 seconds, and then cooled to manufacture polyester films each having a thickness of 100 μm. At this time, the preheating temperature, the stretching ratio in each direction, the ratio of the stretching ratios, the stretching temperature, and the heat-fixing temperature were adjusted as shown in Table 1 below.

[0089] (Comparative Examples 1 to 11) A polyester film with a thickness of 100 μm was produced through the same process as in Example 1, except that the preheating temperature, the draw ratio in each direction, the ratio of draw ratios, the drawing temperature, and the heat setting temperature were adjusted as shown in Table 1 below.

[0090] (Comparative Example 12) A polyester film with a thickness of 50 μm was produced through the same process as in Example 1, except that the preheating temperature, the draw ratio in each direction, the ratio of draw ratios, the drawing temperature, and the heat setting temperature were adjusted as shown in Table 1 below.

[0091] [Table 1]

[0092] (Test Example 1. Yield elongation evaluation) The polyester film was cut into test pieces with a width of 15 mm and a length of 100 mm. Using a universal material testing machine from Instron, the yield elongation rate (E1) in the length direction and the yield elongation rate (E2) in the width direction were measured respectively, and the results are shown in Table 2 below. At this time, the gauge length of the test piece (sample) was set to 50 mm, the tensile speed was set to 50 mm / min, and a 0.2% offset (yield point) was applied.

[0093] (Test Example 2. Thickness deviation evaluation) A 10-cm length was taken from the polyester film in the length direction (MD) and the full width in the width direction (TD). Using a thickness profiler (SOLVETECH, PR2000-B-220 device), the thickness deviation was measured, and the results are shown in Table 2 below. At this time, the test standard followed ASTM D8136-7. After recording the thickness in the width direction (TD) according to the results, the deviation between the maximum thickness and the minimum thickness was calculated.

[0094] (Test Example 3. Elongation rate evaluation) The polyester film was cut into test pieces with a length of 100 mm in the direction to be measured (e.g., the machine direction (MD)) and a width of 15 mm in the direction perpendicular thereto (e.g., the transverse direction (TD)). The elongation rate was measured using a universal material testing machine (UTM, INSTRON 5566A device) under the following conditions, and the results are shown in Table 2 below. Test piece focus (gage) length: 50 mm Measurement temperature: room temperature Tensile speed: 50 mm / min The tensile rate at the point where the test piece was continuously pulled until it broke was expressed as the elongation rate (%).

[0095] (Test Example 4. Haze evaluation) The polyester film was cut into test pieces with a size of 100 mm × 100 mm. The haze was measured using a haze meter (Gardner BYK). The results are shown in Table 2 below. At this time, samples were taken from a total of 10 locations, and the haze was calculated as the average of the values excluding the maximum and minimum values among the measured values.

[0096] (Test Example 5. Surface hardness evaluation) The polyester film was cut into test pieces with a size of 100 mm × 100 mm. The surface hardness was measured using a nano hardness measurement device (Micro Materials, Nano Test Vantage Platform device). The tip of the nano hardness measurement device was penetrated to a depth of 3 μm, and the hardness measured at this time was used as the surface hardness value.

[0097] (Test Example 6. Folding property evaluation) After cutting a polyester film into a size of 50 mm in width and 200 mm in length to prepare test pieces, a folding property was evaluated by conducting a folding repetition test 200,000 times in the length direction (MD) (using a U-shaped expansion and contraction test apparatus manufactured by Yuasa Corporation), and the results are shown in Table 2 below. At this time, it was set to fold 60 times per minute with a bending radius of 1.5 mm, and folding 200,000 times was carried out at normal temperature and low temperature (-20°C), respectively. After the completion of 200,000-fold folding, the presence or absence of crack generation on the surface of the folding part was confirmed by a scanning electron microscope (SEM), and when cracks occurred, it was indicated as "○", and when cracks did not occur, it was indicated as "×".

[0098]

Table 2

[0099] Referring to Table 2 above, it was confirmed that in Examples 1 to 11 where the yield elongation rate of the polyester film was in the range of 2.2% to 2.6%, even at a relatively thick thickness of 100 μm, the flexibility, transparency, and folding properties were all excellent.

Explanation of Signs

[0100] 10: Foldable display device 100: Polyester film 200: Transparent substrate 300: Foldable display panel 310: Front polarizing plate 320: Organic light-emitting display panel

Claims

1. When a first direction and a second direction perpendicular to each other are defined in the plane of the film, the yield elongation in at least one of the first direction and the second direction is 2.2% to 2.6%, The first direction is a machine direction (MD) and the second direction is a cross-machine direction (TD), A biaxially oriented polyester film having a thickness of 80 μm to 150 μm.

2. 2. The biaxially oriented polyester film according to claim 1, wherein the stretch ratio in the machine direction (MD) is 2.5 to 3.

0.

3. 2. The biaxially oriented polyester film according to claim 1, wherein the stretch ratio in the transverse direction (TD) is 3.5 to 5.

5.

4. 2. The biaxially oriented polyester film according to claim 1, having a thickness of 90 μm to 120 μm.

5. The stretch ratio in the machine direction (MD) (D １ The stretch ratio in the transverse direction (TD) to the ２ ) ratio (D ２ / D １ 2. The biaxially oriented polyester film according to claim 1, wherein the tensile strength (T) is 1.3 to 1.

8.

6. The yield elongation in the machine direction (MD) (E １ ) is 2.2% to 2.6%, The biaxially oriented polyester film according to claim 1, which satisfies the following formula (1): 1.1≦E １ / E ２ ≦1.4 ... (1) In the formula (1), E １ is the yield elongation in the machine direction (MD), E ２ is the yield elongation in the transverse direction (TD).

7. The yield elongation in the machine direction (MD) (E １ ) is 2.2% to 2.6%, The biaxially oriented polyester film according to claim 1, which satisfies the following formula (2): 0.01≦D １ ×E １ / T≦0.1 ... (2) In the formula (2), D １ is the stretch ratio in the machine direction (MD), E １ is the yield elongation in the machine direction (MD), T is the thickness of the polyester film.

8. The thickness deviation is 3 μm or less, The elongation is 40% or more, The haze is 2.0% or less, Surface hardness: 120N / mm ２ The biaxially oriented polyester film according to claim 1 .

9. 2. The biaxially oriented polyester film according to claim 1, which does not crack when repeatedly folded at -20° C. and with a bending radius of 1.5 mm 200,000 times or more.

10. 10. The biaxially oriented polyester film of claim 1 comprising a polyethylene terephthalate (PET) resin.

11. A transparent substrate; and a biaxially oriented polyester film disposed on the transparent substrate, The biaxially oriented polyester film has a yield elongation of 2.2% to 2.6% in at least one of a first direction and a second direction perpendicular to each other within the plane of the film, the first direction being a machine direction (MD), the second direction being a transverse direction (TD), and the thickness being 80 μm to 150 μm.

12. The laminate of claim 11 , wherein the transparent substrate is a polyimide-based film or ultra-thin glass (UTG).

13. A foldable display panel; A transparent substrate disposed on the foldable display panel; and a biaxially oriented polyester film disposed on the transparent substrate, The biaxially stretched polyester film has a yield elongation of 2.2% to 2.6% in at least one of the first and second directions, the first direction being a machine direction (MD), the second direction being a transverse direction (TD), and the thickness being 80 μm to 150 μm, when a first direction and a second direction perpendicular to each other are defined in the plane of the film.

14. forming an unoriented sheet from a polyester resin; biaxial stretching; and heat treating the mixture, The film obtained by the heat treatment is then relaxed. A method for producing the biaxially oriented polyester film according to claim 1.

15. The stretching speed in the biaxial stretching step is 6.5 m / min to 8.5 m / min; The stretching temperature in the biaxial stretching step is 125 to 160° C. The method of claim 14.

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