Polyester film and foldable display device including same

Abstract

To provide a polyester film capable of exhibiting enhanced surface hardness, along with excellent transparency, flexibility, folding characteristics, and the like, and a laminate and a foldable display device each comprising the same.SOLUTION: A polyester film is disclosed. 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%, and the thickness is 80 to 150 μm.SELECTED DRAWING: Figure 2

Description

[Technical field]

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

[0002] Recently, in the field of mobile devices where both large screens and portability are required, foldable display devices that can flexibly fold in response to external forces have been attracting attention. Such foldable display devices have the advantage that they can be folded to reduce size when not in use to improve portability, 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 on the inside of the folding direction, and an out-folding type (2) in which the screen is located on the outside of the folding direction (see FIG. 3). As the transparent substrate 200 applied to the cover window of these foldable display devices, initially, polyimide-based films were 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 superior properties compared to polyimide-based films, so ultra-thin glass (UTG) is mostly used in the in-folding type (1).

[0004] However, the ultra-thin glass (UTG) used as the transparent substrate 200 still has a high possibility of cracking due to external impact, so a protective film 100 is applied to the surface of the transparent substrate 200 to achieve impact mitigation, shatterproofing, scratchproofing, etc.

[0005] Polyester films are mainly used as the protective film 100. However, polyester films currently developed have limitations in that they cannot provide a satisfactory level of 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, if the polyester film becomes too thick, cracks will occur at the repeatedly folded portions (a and b in FIG. 3) and the folding characteristics will deteriorate, making it difficult to increase the thickness of the polyester film, and therefore there is a limit to ensuring the required surface hardness. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Korean Patent Publication No. 2021-0059629 Summary of the Invention [Problem to be solved by the invention]

[0007] The embodiment provides a polyester film that is excellent in transparency, flexibility, folding properties, etc., and can also exhibit improved surface hardness.

[0008] Also, the realization example provides a laminate and a foldable display device manufactured using the polyester film. [Means for solving the problem]

[0009] In order to solve the above problem, according to an embodiment, a polyester film is provided in which, when a first direction and a second direction perpendicular to each other are defined within 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%, and the thickness is 80 μm to 150 μm.

[0010] According to another embodiment, a laminate is provided which includes a transparent substrate and a polyester film disposed on the transparent substrate, the polyester film having a yield elongation in at least one of the first direction and the second direction, the yield elongation being 2.2% to 2.6%, and a thickness being 80 μm to 150 μm, when a first direction and a second direction perpendicular to each other are defined within the plane of the film.

[0011] According to another embodiment, a foldable display device is provided, which includes a foldable display panel, a transparent substrate disposed on the foldable display panel, and a polyester film disposed on the transparent substrate, wherein the polyester film has a yield elongation of 2.2% to 2.6% in at least one of the first and second directions perpendicular to each other within the plane of the film, and a thickness of 80 μm to 150 μm. Effect of the Invention

[0012] According to the above embodiment, by controlling the yield elongation of the polyester film within a specific range, the polyester film can have excellent flexibility and folding properties even if it is relatively thick, and thus the polyester film can exhibit improved surface hardness.

[0013] Therefore, the polyester film according to the embodiment can be applied as a protective film for a foldable display device, and can efficiently prevent the foldable display device from being scratched or absorbing impacts applied thereto while maintaining its original properties even after repeated folding multiple times. [Brief description of the drawings]

[0014] [Figure 1] FIG. 1 is a reference diagram for explaining the yield elongation of a polyester film according to an embodiment. [Diagram 2] FIG. 2 is a reference diagram (cross-sectional view) for explaining a foldable display device according to an embodiment. [Diagram 3]FIG. 3 is a schematic diagram showing an in-folding type and an out-folding type foldable display device. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] The present invention will be described below with reference to examples. Note that the examples are not limited to the contents disclosed below, and may be modified in various forms as long as the gist of the invention is not changed.

[0016] In this specification, a description that a certain component is formed above / below another component, or is connected or joined to each other includes all of the components being formed, connected or joined directly between these components or indirectly via other components. Furthermore, it should be understood that the reference to above / below each component may change depending on the direction in which the object is observed.

[0017] As used herein, the term "comprising" is intended to embody certain features, regions, steps, steps, elements, and / or components, and does not exclude the presence or addition of other features, regions, steps, steps, elements, and / or components, unless specifically stated to the contrary.

[0018] All numbers and expressions expressing quantities of components, reaction conditions, and so forth described herein, unless otherwise indicated, should be understood to be modified in all instances by the term "about."

[0019] In this specification, the terms "first", "second", etc. 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 the drawings may be exaggerated for illustrative purposes and may differ from the actual size.

[0021] The polyester film used as a protective film needs to be relatively thick to increase the surface hardness in order to improve the effects of impact mitigation, scratch prevention, etc., but as the polyester film becomes thicker, there is a problem that the folding properties deteriorate. Here, in order to increase the surface hardness of the polyester film, a hard coat layer is formed on the surface, but a satisfactory level of surface hardness is not obtained due to limitations on the components of the hard coat layer and the coating thickness.

[0022] As a result, the inventors conducted various studies to ensure that the folding properties are maintained even when the thickness of the polyester film is increased, and as a result, they confirmed that the folding properties can be ensured even when the polyester film is relatively thick by controlling the yield elongation of the polyester film within a specific range.

[0023] In the following examples, a polyester film that is relatively thick and therefore capable of exhibiting high surface hardness, as well as excellent folding properties, flexibility, transparency, etc., a laminate including the same, and a foldable display device are described.

[0024] [Polyester film] <Physical properties of polyester film> The polyester film according to the embodiment may have a yield elongation of 2.2% to 2.6%. Specifically, when a first direction and a second direction perpendicular to each other are defined in the plane of the film, the yield elongation 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 is within the above range, a polyester film having excellent folding characteristics, high surface hardness, and a required level of elongation may be provided. Specifically, when the yield elongation of the polyester film is less than 2.2%, the folding characteristics may be deteriorated, and when it exceeds 2.6%, the elongation is significantly reduced, and the polyester film may be easily broken in the process of pulling or bending.

[0025] The yield elongation may refer to the tensile deformation rate at the yield point, which means the point at which elasticity is lost and permanent deformation begins. In this case, since the yield point of a polyester film, which is a polymeric material, is not clearly defined, the tensile deformation rate at the 0.2% offset yield point was defined as the yield elongation rate of the polyester film in the examples. The 0.2% offset yield point may refer to the point where the linear portion where the tensile elongation (strain) increases linearly from 0% meets the point of 0.2% of the gauge length (focal length) of the test specimen before tension (see FIG. 1).

[0026] Meanwhile, the first direction may be any direction in the plane of the polyester film, and the second direction may be a direction perpendicular to the first direction in the plane of the film. Specifically, the first direction may be the machine direction (MD) or transverse direction (TD) of the polyester film, and the second direction may be the transverse direction (TD) or machine direction (MD) perpendicular thereto. More specifically, the first direction may be the machine direction (MD) of the polyester film, and the second direction may be the transverse direction (TD) of the polyester film.

[0027] According to an embodiment, the yield elongation may be a yield elongation (E1) in the machine direction (MD). That is, the polyester film according to the embodiment may be a polyester film having a yield elongation (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%), which satisfies the following formula (1):

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

[0029] Specifically, in the formula (1), the E1 / E2 ratio can 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] In addition, the polyester film according to the embodiment may be a polyester film having a yield elongation (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 satisfying the following formula (2).

[0031] 0.01≦D1×E1 / T≦0.1 ... (2) In the formula (2), D1 is the stretch ratio in the machine direction (MD), E1 is the yield elongation 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 embodiment satisfies the above formula (1) and / or formula (2), it has excellent folding characteristics and high surface hardness, and also exhibits a required level of elongation, thereby ensuring flexibility.

[0034] Meanwhile, the polyester film according to the embodiment may have a thickness deviation of 3 μm or less. Specifically, the polyester film may 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] In addition, the polyester film according to the embodiment may have an elongation percentage of 40% or more. Specifically, the polyester film may have an elongation percentage of 40% to 200%, 50% to 190%, 60% to 180%, 70% to 170%, 80% to 160%, or 100% to 160%. When the elongation percentage is within the above range, the polyester film may have excellent flexibility and folding properties.

[0036] In addition, the polyester film according to the embodiment may have a haze of 2.0% or less. Specifically, the polyester film may 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 may have excellent transparency and optical properties.

[0037] In addition, the polyester film in the example has a surface hardness of 120N / mm 2 Specifically, the polyester film has a surface hardness of 120 N / mm 2 ~180N / mm 2 , 125N / mm 2~175N / mm 2 , 130N / mm 2 ~170N / mm 2 , 135N / mm 2 ~170N / mm 2 , or 140N / mm 2 ~160N / mm 2 When the surface hardness is within the above range, the polyester film can be used efficiently as a protective film.

[0038] In addition, the polyester film according to the embodiment may have an in-plane retardation (Ro) of 4000 or less, 3500 or less, or 3000 or less. Specifically, the polyester film may have an in-plane retardation (Ro) of 500 to 4000, 600 to 3500, or 700 to 3000. When the in-plane retardation (Ro) is within the above range, rainbow unevenness occurring in the polyester film may be minimized.

[0039] In addition, the polyester film according to the embodiment may have a thickness direction retardation (Rth) of 6000 or more, 7000 or more, or 8000 or more. Specifically, the thickness direction retardation (Rth) of the polyester film may be 6000 to 14000, 7000 to 13000, or 8000 to 12000. Since the thickness direction retardation (Rth) is within the range, rainbow unevenness occurring in the polyester film may be minimized.

[0040] The in-plane retardation (Ro) can be a parameter defined as the product (Δnxy×d) of the refractive index anisotropy (Δnxy=|nx−ny|) of two orthogonal axes on one surface of the film and the thickness (d) of the film.

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

[0042] <Characteristics of polyester film> The polyester film according to the embodiment may have a thickness of 80 μm or more. Specifically, the polyester film may 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. By having the thickness within the above range, even if the film is repeatedly folded many times (e.g., more than 200,000 times), the occurrence of cracks at the folded portion is minimized, and a polyester film exhibiting high surface hardness can be provided.

[0043] In addition, the polyester film according to the embodiment may be a biaxially stretched film. Specifically, the polyester film may be a film stretched in the machine direction (MD) with a stretch ratio (D1) of 2.5 to 3.0 and stretched in the transverse direction (TD) with a stretch ratio (D2) of 3.5 to 5.5. The stretch ratio (D1) in the machine direction (MD) may be specifically 2.5 to 2.9, 2.5 to 2.8, or 2.5 to 2.7. The stretch ratio (D2) in the transverse direction (TD) may be specifically 3.5 to 5.4, 3.7 to 4.9, or 4.3 to 4.8. By having the stretch ratios (D1, D2) in the machine direction (MD) and / or the transverse direction (TD) within the above ranges, even if the thickness of the polyester film is relatively large (e.g., 80 μm or more), the yield elongation is controlled to a desired level, and a polyester film having excellent folding properties can be provided.

[0044] In addition, the polyester film according to the embodiment may have a ratio (D2 / D1) of the stretch ratio (D2) in the transverse direction (TD) to the stretch ratio (D1) in the longitudinal direction (MD) of 1.3 to 1.8. Specifically, the ratio (D2 / D1) may 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. By having the ratio (D2 / D1) within the above range, a polyester film having excellent folding properties, appearance, and optical properties may be provided.

[0045] The polyester film according to the embodiment may exhibit excellent folding properties not only at room temperature but also at low temperatures. Specifically, the polyester film according to the embodiment may not crack when repeatedly folded at -20°C with a bending radius of 1.5 mm 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 includes a polyester resin. The polyester resin may be a homopolymer resin or a copolymer resin obtained by polymerization (polycondensation) of a dicarboxylic acid and a diol. The polyester resin may also be a mixture of the homopolymer resin and the copolymer resin.

[0047] The dicarboxylic acid may be specifically, but is not limited to, 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, dodecadicarboxylic acid, or a combination thereof.

[0048] The diol may be specifically, 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 may be an aromatic polyester resin having excellent crystallinity, and more specifically, a resin containing polyethylene terephthalate (PET) resin as a main component.

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

[0051] <Production method of polyester film> The polyester film according to the embodiment may be manufactured through a process including biaxial stretching at a specific stretch ratio condition and heat treatment at a specific temperature.

[0052] Specifically, the method for producing a polyester film according to the embodiment includes the steps of forming an unstretched sheet from a polyester resin, biaxially stretching, and heat treating, thereby producing a polyester film having a thickness of 80 μm to 150 μm and 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 in the plane. More specifically, in the embodiment, in order to obtain a polyester film exhibiting specific physical properties, the extrusion and casting temperature of the polyester resin may be adjusted, and the preheating temperature during stretching, the stretch ratio for each direction, the stretching temperature, the stretching speed, etc. may be adjusted, and the heat treatment temperature and relaxation rate during heat treatment and relaxation after stretching may be adjusted, which will be described below.

[0053] First, a 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 unstretched sheet thus formed may be subjected to a biaxial stretching step after preheating. The unstretched sheet may be preheated within a range of Tg+5°C to Tg+50°C based on the glass transition temperature (Tg) of the polyester resin, specifically, 70°C to 100°C. When preheated within this range, the unstretched sheet has flexibility suitable for biaxial stretching, and breakage of the sheet during biaxial stretching can be efficiently prevented.

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

[0056] The stretching speed is not particularly limited, but may be specifically 6.5 m / min to 8.5 m / min. The stretching temperature is not particularly limited, but may be specifically 125°C or higher, 130°C or higher, or 135°C or higher (e.g., 125 to 160°C, 130 to 155°C, 135 to 150°C, or 135 to 145°C).

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

[0058] In addition, the ratio (D2 / D1) of the stretch ratio (D2) in the transverse direction (TD) to the stretch ratio (D1) in the longitudinal direction (MD) is 1.3 to 1.8, specifically, 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 having excellent transparency and flexibility and an optimal range of yield elongation can be obtained.

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

[0060] The film obtained through the heat treatment step may be subjected to a relaxation step in which the film is relaxed in the machine direction (MD) and / or the transverse direction (TD). The relaxation may 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. The relaxation may be performed at a relaxation rate of 1% to 10%, 2% to 7%, or 3% to 5%.

[0061] Thereafter, the film obtained through the relaxation step may be subjected to a cooling step, which may be performed at a temperature 50° C. to 150° C. lower than the temperature at which the heat treatment step is performed.

[0062] [Laminate] The laminate according to the embodiment includes a transparent substrate and a polyester film, and the specific description thereof is as follows.

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

[0064] Specifically, the polymer film may be a polyimide film. The polyimide film may include a polyimide polymer obtained by polymerizing one or more compounds selected from the group consisting of a diamine compound, a dianhydride compound, and a dicarbonyl compound. The polyimide polymer may 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 may be an aromatic diamine compound containing an aromatic structure. Specifically, the diamine compound may be, but is not limited to, 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.

[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, but is not limited to, 2,2'-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA), biphenyl-tetracarboxylic acid dianhydride (BPDA), or a combination thereof.

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

[0068] For example, the polyimide polymer may include a repeating unit represented by the following Formula 1, but is not limited thereto.

[0069] [Chemical formula 1] JPEG0007671804000001.jpg29128 In the above Chemical Formula 1, n is an integer of 1 to 400.

[0070] The polyimide polymer may contain one or more repeating units represented by the following chemical formulas 2 to 4, but is not limited thereto.

[0071] [Chemical formula 2] JPEG0007671804000002.jpg31128 In the above Chemical Formula 2, x is an integer of 1 to 400.

[0072] [Chemical formula 3] JPEG0007671804000003.jpg3281 In the above Chemical Formula 3, y is an integer of 1 to 400.

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

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

[0075] The transparent substrate may have a surface hardness of HB or higher and a light transmittance of 80% or higher at a wavelength of 550 nm. The transparent substrate may also have a yellowness index of 5 or less and a haze of 2% or less based on a thickness of 50 μm.

[0076] The polyester film included in the laminate according to the embodiment may be a protective film for protecting the transparent substrate. When a first direction and a second direction perpendicular to each other are defined in the plane of the film, the polyester film has a yield elongation of 2.2% to 2.6% in at least one of the first direction and the second direction, 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 embodiment described above.

[0077] According to an embodiment, the laminate including the transparent substrate and the polyester film may be folded repeatedly until peeling occurs 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. When the number of repeated folding times is within the above range, peeling does not occur even with frequent folding, and therefore the laminate may be advantageously applied to a foldable display device.

[0078] Specifically, the laminate according to the embodiment has excellent transparency, flexibility and folding properties, and includes the above-mentioned polyester film, and therefore has improved surface hardness, and can be usefully applied as a cover material for a foldable display device.

[0079] [Foldable display device] The foldable display device according to the embodiment includes a foldable display panel, a transparent substrate, and a polyester film, which will be described in detail below.

[0080] The foldable display panel included in the foldable display device according to the implementation may be an organic light-emitting display (OLED) panel, a quantum dot light-emitting display (QLED) panel, etc. As an example, FIG. 2 is a schematic diagram of a foldable display device 10 including a foldable display panel 300, which may include a front polarizer 310 and an organic light-emitting display panel 320.

[0081] The front polarizer 310 may be disposed on a front surface of the organic light emitting display panel 320. Specifically, the front polarizer 310 may be attached to a surface of the organic light emitting display panel 320 on which an image is displayed.

[0082] The organic light emitting display panel 320 displays an image by self-emitting light in pixel units. The organic light emitting 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 each corresponding to a pixel. 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 may apply a current to each of the organic light emitting units to drive the organic light emitting substrate.

[0083] The transparent substrate 200 included in the foldable display device 10 according to the embodiment is disposed as a transparent cover on the foldable display panel 300. 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 embodiment is disposed as a protective film on the transparent substrate 200. The polyester film 100 has substantially the same configuration and characteristics as the polyester film according to the above embodiment.

[0085] The foldable display device 10 according to the implementation may be of an in-folding type or an out-folding type depending on the direction in which it is folded.

[0086] The foldable display device according to this embodiment has excellent folding characteristics because the polyester film is applied as a protective film, and can have excellent life and reliability even when external force is applied to the surface for a long time or many times (e.g., use of a touch pen).

[0087] (Example) The present embodiment will be described in more detail below with reference to examples, although the scope of the present embodiment is not limited to these examples.

[0088] <Production 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 using an extruder to produce an unstretched sheet. The unstretched sheet was preheated and biaxially stretched while adjusting the stretch ratio in each of the machine direction (MD) and transverse direction (TD) to obtain a stretched sheet. The stretched sheet was then heat-set, relaxed at a relaxation rate of 3% to 5% for 3 to 10 seconds, and then cooled to produce a polyester film having a thickness of 100 μm. In this case, the preheating temperature, stretch ratio in each direction, stretch ratio ratio, stretching temperature, and heat-setting temperature were adjusted as shown in Table 1 below.

[0089] (Comparative Examples 1 to 11) Each polyester film having a thickness of 100 μm was prepared in the same manner as in Example 1, except that the preheating temperature, stretch ratio in each direction, stretch ratio ratio, stretching temperature, and heat setting temperature were adjusted as shown in Table 1 below.

[0090] Comparative Example 12 A polyester film having a thickness of 50 μm was prepared by the same process as in Example 1, except that the preheating temperature, stretch ratio for each direction, stretch ratio ratio, stretching temperature, and 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 to a width of 15 mm and a length of 100 mm to prepare a test piece, and the yield elongation in the length direction (E1) and the yield elongation in the width direction (E2) were measured using an Instron universal testing machine, and the results are shown in Table 2. 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) The polyester film was sampled over a length of 10 cm in the machine direction (MD) and across the entire width in the transverse direction (TD), and the thickness deviation was measured using a thickness profiler (SOLVETECH, PR2000-B-220 device), with the results shown in Table 2. The test standard was ASTM D8136-7, and the thickness was recorded in the transverse direction (TD) based on the results, and the maximum and minimum thickness deviations were calculated.

[0094] (Test Example 3. Elongation Evaluation) A polyester film was cut into test pieces having a length of 100 mm in the direction to be measured (e.g., the machine direction (MD)) and a length of 15 mm in the direction perpendicular to the direction (e.g., the transverse direction (TD)). The elongation was measured under the following conditions using a universal testing machine (UTM, INSTRON 5566A device), and the results are shown in Table 2 below. Specimen focal (gauge) distance: 50mm Measurement temperature: normal temperature Pulling speed: 50mm / min The test piece was continuously tensioned until it broke, and the tensile percentage at the point where it broke was expressed as elongation (%).

[0095] (Test Example 4. Haze Evaluation) The polyester film was cut into a size of 100 mm x 100 mm to prepare a test piece, and the haze was measured using a haze meter (Gardner BYK), and the results are shown in Table 2. At this time, samples were taken from a total of 10 places, and the haze was calculated as the average of the remaining values ​​excluding the maximum and minimum values ​​from the measured values.

[0096] (Test Example 5. Surface Hardness Evaluation) The polyester film was cut into a size of 100 mm x 100 mm to prepare a test piece, and the surface hardness was measured using a nano hardness measuring device (Micro Materials, Nano Test Vantage Platform device), and the results are shown in Table 2. The tip of the nano hardness measuring device was allowed to penetrate to a depth of 3 μm, and the hardness measured at this time was taken as the surface hardness value.

[0097] (Test Example 6. Evaluation of folding properties) The polyester film was cut into a width of 50 mm and a length of 200 mm to prepare a test piece, and then a folding test (using a U-shaped stretch tester manufactured by YUASA) was carried out in the length direction (MD) to evaluate the folding characteristics, and the results are shown in Table 2 below. At this time, the bending radius was set to 1.5 mm and the folding was carried out 60 times per minute, at room temperature and at a low temperature (-20°C). After the folding was completed 200,000 times, the surface of the folded part was checked for the presence or absence of cracks using a scanning electron microscope (SEM), and the occurrence of cracks was indicated by "○" and the absence of cracks was indicated by "×".

[0098] [Table 2] JPEG0007671804000007.jpg75165

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

[0100] 10: Foldable display device 100: Polyester film 200: Transparent base material 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), 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 ) is 1.3 to 1.8, The yield elongation in the machine direction (MD) (E １ ) is 2.2% to 2.6%, The following formula (1) and the following formula (2) are satisfied, The thickness is 80 μm to 150 μm, A polyester film that does not crack when repeatedly folded 200,000 times or more at a bending radius of 1.5 mm at -20°C: 1.1≦E 1 / E 2 ≦1.4 ... (1) 0.01≦D １ ×E １ / T≦0.1 ... (2) In the formula (1), E1 is the yield elongation in the machine direction (MD), E2 is the yield elongation in the transverse direction (TD), 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.

2. The first direction is a machine direction (MD) and the second direction is a cross-machine direction (TD), 2. The polyester film according to claim 1, wherein the stretch ratio in the machine direction (MD) is 2.5 to 3.

0.

3. The first direction is a machine direction (MD) and the second direction is a cross-machine direction (TD), 2. The polyester film according to claim 1, wherein the stretch ratio in the transverse direction (TD) is 3.5 to 5.

5.

4. 2. The polyester film according to claim 1, which has a thickness of 90 μm to 120 μm.

5. 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 polyester film according to claim 1 .

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

7. A transparent substrate; A polyester film disposed on the transparent substrate, The 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) and the second direction being a transverse direction (TD), and a stretch ratio (D １ The stretch ratio in the transverse direction (TD) to the ２ ) ratio (D ２ / D １ ) is 1.3 to 1.8, and the yield elongation in the machine direction (MD) (E １ ) is 2.2% to 2.6%, satisfies the following formula (1) and the following formula (2), has a thickness of 80 μm to 150 μm, and does not crack when repeatedly folded 200,000 times or more at −20° C. with a bending radius of 1.5 mm: 1.1≦E 1 / E 2 ≦1.4 ... (1) 0.01≦D １ ×E １ / T≦0.1 ... (2) In the formula (1), E1 is the yield elongation in the machine direction (MD), E2 is the yield elongation in the transverse direction (TD), 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 laminate according to claim 7 , wherein the transparent substrate is a polyimide-based film or ultra-thin glass (UTG).

9. A foldable display panel; A transparent substrate disposed on the foldable display panel; A polyester film disposed on the transparent substrate, The 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) and the second direction being a transverse direction (TD), and a stretch ratio (D １ The stretch ratio in the transverse direction (TD) to the ２ ) ratio (D ２ / D １ ) is 1.3 to 1.8, and the yield elongation in the machine direction (MD) (E １ ) is 2.2% to 2.6%, satisfies the following formula (1) and the following formula (2), has a thickness of 80 μm to 150 μm, and does not crack when repeatedly folded 200,000 times or more at −20° C. with a bending radius of 1.5 mm: 1.1≦E 1 / E 2 ≦1.4 ... (1) 0.01≦D １ ×E １ / T≦0.1 ... (2) In the formula (1), E1 is the yield elongation in the machine direction (MD), E2 is the yield elongation in the transverse direction (TD), 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.

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