Polyester film and its applications

Abstract

To provide a laminated film for a foldable display that prevents image distortion at the folded portion after repeated folding. [Solution] A laminated film for a foldable display, having a hard coat layer on one side of a polyester film with a thickness of 10 to 80 μm, and an adhesive layer on the side of the polyester film opposite to the side having the hard coat layer, wherein the polyester film satisfies the following conditions. (1) Refractive index in the bending direction is 1.590 to 1.620 (2) The refractive index in the direction of the folding part is 1.670 to 1.700 (3) The refractive index in the thickness direction is 1.520 or less. (4) Density is 1.380 g / cm³ 3 That's all. (Here, the bending direction refers to the direction perpendicular to the fold when folding the polyester film.)

Description

[Technical Field]

[0001] The present invention relates to a laminated film for a foldable display, a foldable display, and a mobile terminal device, and more particularly to a foldable display and a mobile terminal device that are less prone to image distortion due to film deformation even after repeated folding, and a laminated film that can provide the aforementioned foldable display. [Background technology]

[0002] With the advancement of thin-film and lightweight mobile devices, mobile devices such as smartphones have become widely popular. While mobile devices are required to have various functions, convenience is also a key requirement. Therefore, popular mobile devices need to be small, around 6 inches in size, as they are designed to be easily operated with one hand and to be stored in clothing pockets.

[0003] On the other hand, tablet devices with screen sizes ranging from 7 to 10 inches offer high functionality, as they are intended for use not only with video content and music, but also for business, drawing, and reading. However, they cannot be operated with one hand, and their portability is poor, posing challenges to convenience.

[0004] To achieve these goals, a method has been proposed to make the display more compact by connecting multiple displays together (see Patent Document 1). However, because the bezel remains, the image is cut off, resulting in reduced visibility, which has prevented its widespread adoption.

[0005] Therefore, in recent years, mobile devices incorporating flexible displays and foldable displays have been proposed. With this method, images are not interrupted, and the device can be conveniently carried as a mobile terminal equipped with a large screen display.

[0006] In conventional displays and mobile devices without a folding structure, the surface of the display could be protected with a non-flexible material such as glass. However, in folding displays, when the folding part forms a single display surface, it is necessary to use a flexible hard coat film or similar material that can protect the surface. However, in folding displays, certain areas that are folded are repeatedly bent, causing the film in those areas to deform over time, leading to problems such as distortion of the image displayed on the display. In addition to the surface protection film, folding displays use films in various parts, such as polarizing plates, phase difference plates, touch panel substrates, substrates for display cells such as organic EL, and protective materials on the back, and these films also need to be durable against repeated folding.

[0007] Therefore, methods for partially changing the film thickness have been proposed (see Patent Document 2), but these have the problem of being unsuitable for mass production.

[0008] Furthermore, methods for adjusting the refractive index of polyester film in the bending direction have also been proposed (see Patent Document 3), but there was a problem that as the refractive index in the bending direction was lowered, the pencil hardness when hard coat was applied decreased, resulting in a decrease in the surface protection function of the display. Also, while lowering the refractive index in one direction improved deformation when folded, it increased the uniaxial orientation in the folding direction, leading to problems such as cracks occurring or breakage at the folded part. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Japanese Patent Publication No. 2010-228391 [Patent Document 2] Japanese Patent Publication No. 2016-155124 [Patent Document 3] International Publication No. 2018 / 150940 [Overview of the Initiative] [Problems that the invention aims to solve]

[0010] The present invention aims to solve the problems of conventional display components as described above, and to provide a foldable display that is easy to mass-produce and does not cause distortion of the image displayed at the folded part after repeated folding, and a mobile terminal device equipped with such a foldable display, by providing a laminated film for a foldable display that does not cause creases or cracks at the folded part. [Means for solving the problem]

[0011] In other words, the present invention consists of the following configuration. 1. A laminated film for a foldable display, having a hard coat layer on one side of a polyester film with a thickness of 10 to 80 μm, and an adhesive layer on the side of the polyester film opposite to the side with the hard coat layer, wherein the polyester film satisfies the following conditions. (1) Refractive index in the bending direction is 1.590 to 1.620 (2) The refractive index in the direction of the folding part is 1.670 to 1.700 (3) The refractive index in the thickness direction is 1.520 or less. (4) Density is 1.380 g / cm³ 3 That's all. (Here, the bending direction refers to the direction perpendicular to the fold when folding the polyester film.) 2. The laminated film for a foldable display according to the first description, wherein the elastic modulus of the polyester film in the bending direction is 2.7 GPa or less, and the elastic modulus of the polyester film in the folding direction is 4.5 GPa or more. 3. The laminated film for a foldable display according to the first or second above, wherein the thickness of the adhesive layer is 1 to 50 μm. 4. A laminated film for a foldable display according to any one of the first to third descriptions above, wherein the thickness of the hard coat layer is 1 to 50 μm. 5. A foldable display comprising the laminated film for foldable displays according to the first to fourth aspects, wherein the laminated film is disposed as a surface protection film such that the hard coat layer is located on the surface, and a single continuous laminated film is disposed through the folding portion of the foldable display. 6. A portable terminal device having the foldable display according to the fifth aspect.

Advantages of the Invention

[0012] The foldable display using the laminated film for foldable displays of the present invention maintains mass productivity, and the laminated film does not crack at the folding portion, does not deform after repeated folding, and does not cause image distortion at the folding portion of the display. A portable terminal device equipped with the foldable display using the laminated film as described above provides beautiful images, is rich in functionality, and is excellent in convenience such as portability.

Brief Description of the Drawings

[0013] [Figure 1] It is a schematic diagram for showing the bending radius when the foldable display in the present invention is folded. [Figure 2] It is a schematic diagram for showing the bending direction of the polyester film constituting the laminated film for foldable displays in the present invention.

Modes for Carrying Out the Invention

[0014] (Display) The display referred to in the present invention generally refers to a display device. Examples of the types of displays include LCD, organic EL display, inorganic EL display, LED, FED, etc. Among them, LCD, organic EL, and inorganic EL having a foldable structure are preferable. In particular, organic EL and inorganic EL that can reduce the layer structure are particularly preferable, and organic EL with a wide color gamut is more preferable.

[0015] (Foldable display) A foldable display is a single continuous display that can be folded in half or in other ways for portability. Folding reduces the size by half, improving portability. The bending radius of the foldable display is preferably 5 mm or less, and more preferably 3 mm or less. A bending radius of 5 mm or less allows for a thinner design when folded. A smaller bending radius is generally better, but a smaller bending radius makes it more prone to creases. A bending radius of 0.1 mm or more is preferred, but it may also be 0.5 mm or more, or even 1 mm or more. Even with a bending radius of 1 mm, a sufficiently thin design can be achieved for practical portability. The bending radius when folded is measured at the location indicated by reference numeral 11 in the schematic diagram of Figure 1, and refers to the inner radius of the folded portion. The surface protection film, which is one application of the laminated film of the present invention described later, may be located on the outside or inside of the folded foldable display. Furthermore, the foldable display may be tri-fold, quad-fold, or even rollable, and all of these fall within the scope of the foldable display as defined in this invention.

[0016] The laminated film for folding displays of the present invention may be used in any part of a folding display. Below, using an organic EL display as an example, a typical configuration of a folding display and the parts in which the laminated film of the present invention may be used will be described. Hereinafter, the laminated film for folding displays of the present invention may simply be referred to as the laminated film of the present invention.

[0017] (Foldable OLED display) The essential component of a foldable organic EL display is the organic EL module, but additional components such as a circular polarizer, touch panel module, surface protective film, and back protective film may be provided as needed. (OLED module) A typical OLED module consists of electrodes, an electron transport layer, an emissive layer, a hole transport layer, and transparent electrodes.

[0018] (Touch panel module) It is preferable for mobile devices to have a touch panel. When an organic EL display is used, it is preferable that the touch panel module is located on top of the organic EL display or between the organic EL module and the circular polarizing plate. The touch panel module has a transparent substrate such as a film and transparent electrodes placed on it. The laminated film of the present invention can be used as this transparent substrate. When used as a transparent substrate for a touch panel, it is preferable to provide a refractive index adjustment layer.

[0019] (Circular Polarizer) The circular polarizer suppresses the reflection of external light by internal display components, which can degrade image quality. The circular polarizer comprises a linear polarizer and a phase difference plate. The linear polarizer has a protective film on at least the viewing side of the polarizer. The polarizer may also have a protective film on the side opposite to the viewing side, and the phase difference plate may be directly laminated onto the polarizer. The phase difference plate can be made of a resin film having a phase difference, such as polycarbonate or a cyclic olefin, or a resin film with a phase difference layer made of a liquid crystal compound. The laminated film of the present invention can be used as a polarizer protective film. In these cases, if the base film of the laminated film of the present invention is a polyester film, it is preferable that the slow phase axis direction of the polyester film is parallel or perpendicular to the absorption axis direction of the polarizer. A deviation of up to 10 degrees, preferably 5 degrees, from this parallelism or perpendicularity is acceptable.

[0020] (Surface protective film) If the display is subjected to an impact from above, the circuits of the organic EL module or touch panel module may break, so a surface protective film is often provided. The laminated film of the present invention is used as this surface protective film. Surface protective films include cover windows that are incorporated into the outermost surface of the display, and aftermarket films that can be applied, peeled off, and replaced by the user, but in either case, the laminated film of the present invention is used. It is provided on the surface of a foldable display with the hard coat layer facing the viewing side. Note that the hard coat layer may be provided on both sides.

[0021] (Protective film on the back) It is also preferable to provide a protective film on the back side of the display. The laminated film of the present invention can be used as this protective film on the back side.

[0022] The laminated film of the present invention may be any other type of film used in the folding portion of a foldable display component. Among these, the laminated film of the present invention is preferably used as a cover window surface protection film, an after-surface protection film, a base film for a touch panel module, and a back surface protection film. Furthermore, it is preferably used as a cover window surface protection film and an after-surface protection film.

[0023] Furthermore, the laminated film of the present invention is not necessarily required for all of the above-mentioned applications of foldable displays. For foldable displays, laminated films based on polyester film, polyimide film, polyamide film, polyamide-imide film, polycarbonate film, acrylic film, triacetylcellulose film, cycloolefin polymer film, polyphenylene sulfide film, polymethylpentene film, etc., can be used as appropriate.

[0024] When the base film constituting the laminated film of the present invention is a polyester film, it may be a single-layer film made of one or more types of polyester resins, or when two or more types of polyester are used, it may be a multilayer film or a super-multilayer laminated film with a repeating structure.

[0025] Polyester resins used in polyester films, which are the base films of laminated films, include, for example, polyethylene terephthalate, polybutylene terephthalate, polyethylene-2,6-naphthalate, or polyester films made of copolymers mainly composed of these resin components. Among these, stretched polyethylene terephthalate film is particularly preferred in terms of mechanical properties, heat resistance, transparency, and cost.

[0026] When a polyester copolymer is used as the polyester film, which is the base film of a laminated film, examples of the dicarboxylic acid component of the polyester include aliphatic dicarboxylic acids such as adipic acid and sebacic acid; aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, and 2,6-naphthalenedicarboxylic acid; and polyfunctional carboxylic acids such as trimellitic acid and pyromellitic acid. Examples of the glycol component include fatty acid glycols such as ethylene glycol, diethylene glycol, 1,4-butanediol, propylene glycol, and neopentyl glycol; aromatic glycols such as p-xylene glycol; alicyclic glycols such as 1,4-cyclohexanedimethanol; and polyethylene glycol with an average molecular weight of 150 to 20,000. The preferred mass ratio of copolymer components in the copolymer is less than 20% by mass. When it is less than 20% by mass, film strength, transparency, and heat resistance are maintained, which is preferable.

[0027] Furthermore, in the production of the polyester film that serves as the base film for the laminated film, the intrinsic viscosity of at least one type of resin pellet is preferably in the range of 0.50 to 1.0 dl / g. An intrinsic viscosity of 0.50 dl / g or higher is preferable because it improves the impact resistance of the resulting film, making it less likely for the internal circuitry of the display to break due to external impact. On the other hand, an intrinsic viscosity of 1.00 dl / g or lower is preferable because it prevents the filtration pressure of the molten fluid from rising too much, making it easier to operate the film production stably.

[0028] The thickness of the polyester film, which is the base film of the laminated film, is preferably 10 to 80 μm, and more preferably 25 to 75 μm. A thickness of 10 μm or more provides an effect of improving pencil hardness and impact resistance, while a thickness of 80 μm or less is advantageous for weight reduction and also offers excellent flexibility, processability, and handling properties.

[0029] The surface of the laminated film of the present invention may be smooth or uneven, but since it is used as a surface cover for displays, a decrease in optical properties due to unevenness is undesirable. The haze is preferably 3% or less, more preferably 2% or less, and most preferably 1% or less. If the haze is 3% or less, the visibility of the image can be improved. The lower limit of the haze is better, but from the standpoint of stable production, it is preferably 0.1% or more, and may also be 0.3% or more.

[0030] As mentioned above, for the purpose of reducing haze, it is preferable to have less surface irregularity on the film. However, in order to provide a certain degree of slipperiness from a handling perspective, irregularities can be formed by compounding particles into the base polyester film or by coating the base polyester film with a particle-containing coating layer during the film-forming process.

[0031] Known methods can be used to incorporate particles into a base polyester film. For example, the particles can be added at any stage of polyester production, but preferably, they can be added as a slurry dispersed in ethylene glycol or the like at the esterification stage, or after the transesterification reaction is complete but before the polycondensation reaction begins, to allow the polycondensation reaction to proceed. Alternatively, this can be done by using a vented kneading extruder to blend a slurry of particles dispersed in ethylene glycol or water with the polyester raw material, or by using a kneading extruder to blend dried particles with the polyester raw material.

[0032] In particular, a method is preferred in which aggregated inorganic particles are homogeneously dispersed in a monomer liquid that will become part of the polyester raw material, and then the filtered solution is added to the remaining polyester raw material before, during, or after the esterification reaction. With this method, since the monomer liquid has low viscosity, homogeneous dispersion of particles and high-precision filtration of the slurry can be easily performed, and when added to the remaining raw material, the particles dispersibility is good and new aggregates are less likely to be formed. From this viewpoint, it is especially preferable to add it to the remaining raw material in a low-temperature state before the esterification reaction.

[0033] Furthermore, by obtaining a polyester containing particles beforehand and then kneading and extruding the resulting pellets with pellets that do not contain particles (masterbatch method), the number of protrusions on the film surface can be further reduced.

[0034] Furthermore, the polyester film used as the base material may contain various additives, within a range that maintains a desirable range of total light transmittance. Examples of additives include antistatic agents, UV absorbers, and stabilizers.

[0035] The total light transmittance of the polyester base film is preferably 85% or higher, and more preferably 87% or higher. A transmittance of 85% or higher is sufficient to ensure adequate visibility. The total light transmittance of the polyester base film is preferably 85% or higher in order to increase the total light transmittance of the laminated film. While a higher total light transmittance of the polyester film is desirable, from the standpoint of stable production, it is preferably 99% or lower, and may also be 97% or lower.

[0036] The maximum heat shrinkage rate of the polyester film used as the base material after heat treatment at 150°C for 30 minutes is preferably 6% or less, and more preferably 5% or less. A heat shrinkage rate of 6% or less can suppress flatness defects such as curling and waviness during HC processing. A lower heat shrinkage rate is generally better, but it is preferably -1% or more, and preferably 0% or more. A negative value here means that the material expanded after heating, and flatness defects may occur even if the shrinkage rate falls below -1%.

[0037] In order to provide sufficient pencil hardness to the laminated film for the foldable display of the present invention, it is preferable that the polyester film used as the base film has the following characteristics. In conventional cases, when evaluating the pencil hardness of a hard coat film (laminated film) after laminating a hard coat layer with a conventional base polyester film, it is thought that the pencil hardness decreases because the film deforms in the thickness direction. In the present invention, it is preferable to set the indentation depth after removing the test force in the thickness direction using the dynamic ultramicrohardness tester described later for the base polyester film to a specific range. In the evaluation of the pencil hardness of a hard coat film using the above base polyester film, it is preferable to be able to achieve high hardness. The indentation depth after removing the test force in the thickness direction of the base polyester film is preferably 1.5 μm or less, more preferably 1.4 μm or less, and even more preferably 1.3 μm or less. When the indentation depth after removing the test force (the final amount of deformation under load) is 1.5 μm or less, the film is less likely to deform in the thickness direction when evaluating the pencil hardness of a hard coat film (laminated film) after laminating a hard coat layer, and the pencil hardness can be increased. Increasing the pencil hardness of the hard coat film (laminated film) makes it less likely for scratches and dents to occur on the display surface, improving the visibility of the display. While a lower indentation depth after unloading the test force is preferable, a depth of 0.3 μm or more is preferable, and even more preferably 0.5 μm or more, in terms of stable production and saturation of the effect.

[0038] To reduce the indentation depth after the test load is removed, it is effective to adjust the refractive index in the thickness direction to 1.520 or less. As a means of reducing the refractive index to 1.520 or less, as will be described later, examples of conditions include adjusting the stretching ratio in the bending and folding directions to a high level, setting the stretching temperature in the bending and folding directions to a low level, and setting the heat-fixing temperature to a high level, while keeping other physical properties, such as the refractive index in the bending and folding directions, within a range where they can be controlled to a desirable range.

[0039] The elastic modulus in the bending direction of the polyester film base material is preferably 2.7 GPa or less, more preferably 2.6 GPa or less, and even more preferably 2.5 GPa or less. Reducing the elastic modulus in the bending direction reduces the stress applied to the hard coat layer when the hard coat film (laminated film) is bent. A lower elastic modulus in the bending direction improves flexibility, but since this impairs the flatness of the display surface, 1.8 GPa or higher is preferred. The elastic modulus in the folding direction is preferably 4.5 GPa or higher, more preferably 4.6 GPa or higher, and even more preferably 4.7 GPa or higher. A higher elastic modulus in the folding direction helps maintain the flatness of the display surface during display fabrication. A higher elastic modulus in the folding direction is preferable, but from the viewpoint of film formation, 8.0 GPa or less is preferred.

[0040] The polyester film described above can be manufactured, for example, through a polymerization step in which inorganic particles are homogeneously dispersed in a monomer liquid that will be part of the polyester raw material, filtered, and then added to the remainder of the polyester raw material to polymerize the polyester; and a film forming step in which the polyester is melt-extruded into a sheet through a filter, cooled, and then stretched to form a base film.

[0041] Next, we will explain in detail the manufacturing method of the polyester film that serves as the base material, using polyethylene terephthalate (PET) pellets as the raw material for the base film, but this is not the only method. Furthermore, the number of layers, such as single-layer or multi-layer construction, is not limited.

[0042] PET pellets are mixed in a predetermined ratio, dried, and then supplied to a known molten lamination extruder. The pellets are extruded through a slit-shaped die into a sheet, and then cooled and solidified on a casting roll to form an unstretched film. For single-layer films, one extruder is sufficient. However, when manufacturing multi-layer films, two or more extruders, two or more manifolds or confluence blocks (for example, confluence blocks with a rectangular confluence section) are used to laminate multiple film layers constituting each outermost layer. Two or more sheets are then extruded from a die and cooled on a casting roll to form an unstretched film.

[0043] In this case, during melt extrusion, it is preferable to perform high-precision filtration to remove foreign matter contained in the resin at any location where the molten resin is maintained at approximately 280°C. The filter material used for high-precision filtration of the molten resin is not particularly limited, but a stainless steel sintered body filter material is preferred because it has excellent performance in removing aggregates mainly composed of Si, Ti, Sb, Ge, and Cu, as well as high-melting-point organic matter.

[0044] Furthermore, the filtration particle size of the filter media (initial filtration efficiency 95%) is preferably 20 μm or less, and particularly preferably 15 μm or less. If the filtration particle size of the filter media (initial filtration efficiency 95%) exceeds 20 μm, foreign matter larger than 20 μm cannot be sufficiently removed. Although high-precision filtration of molten resin using a filter media with a filtration particle size (initial filtration efficiency 95%) of 20 μm or less may reduce productivity, it is preferable for obtaining a film with fewer protrusions caused by coarse particles.

[0045] (Regarding the refractive index in the bending direction) In the present invention, the refractive index of the polyester film used as the base material is preferably 1.590 to 1.620 in at least one direction, either the longitudinal direction (machine flow direction) or the width direction, and more preferably 1.591 to 1.600. Furthermore, the refractive index of the polyester film in the bending direction is preferably 1.590 to 1.620, and more preferably 1.591 to 1.600. Here, the bending direction refers to the direction perpendicular to the folding portion (reference numeral 21) assumed in the application of a foldable display, as shown by reference numeral 22 on the polyester film (reference numeral 2) in Figure 2. A refractive index of 1.590 to 1.620 in at least one direction, either the longitudinal direction or the width direction, is preferable because it reduces deformation when repeatedly folded and does not risk degrading the image quality of the foldable display. A refractive index of 1.591 to 1.600 is more preferable. Of course, it is preferable that this direction is the bending direction as described above. A refractive index of 1.590 or higher prevents cracking in the folded direction after the bending test described later, and of course, prevents breakage, thus maintaining good visibility of the display. The refractive index of the polyester film can be effectively adjusted by adjusting the stretching ratio and stretching temperature. In addition, a relaxation process in the stretching direction and multi-stage stretching may be used to adjust the refractive index. When performing multi-stage stretching, it is preferable to make the stretching ratio of the second and subsequent stages higher than that of the first stage.

[0046] By controlling the refractive index of the polyester film in at least one of the longitudinal direction (machine flow direction) and the width direction within the above range, and more preferably by controlling the refractive index in the bending direction within the above range, fatigue due to compressive stress applied to the inside of the fold during folding can be reduced. Fatigue due to compressive stress is thought to occur mainly in the crystalline parts, and the fewer crystals there are in the bending direction, the less fatigued the film is. Therefore, it is thought that by lowering the refractive index, the amount of oriented crystals in the bending direction is reduced, thereby suppressing compressive fatigue.

[0047] Furthermore, creep caused by tensile stress on the outside of the fold during folding can be suppressed by reducing the refractive index. Fatigue due to tensile stress is thought to occur mainly in the amorphous region, where repeated stress causes alignment of molecular chains and deformation. It can be inferred that the fewer molecular chains aligned in the bending direction, the less deformation due to alignment. Also, since fatigue due to tensile stress can be suppressed by having fewer amorphous regions, a higher degree of crystallinity, i.e., a higher density, is preferable.

[0048] In the present invention, it is preferable to stretch the unstretched polyester sheet in at least one direction, either the longitudinal direction (machine flow direction) or the width direction, to 1.2 to 2.0 times, and more preferably 1.7 to 2.0 times. Furthermore, it is preferable that the stretching direction is the bending direction as described above. A stretching ratio of 1.2 times or more is preferable because there is no deformation during post-processing such as hard coat coating, and a stretching ratio of 2.0 times or less is preferable because there is no thickness unevenness in the film. The stretching temperature is preferably 75 to 120°C, and more preferably 75 to 105°C. Conventional known methods such as hot air heating, roll heating, and infrared heating can be used as heating methods during stretching. By setting the stretching temperature to 75 to 120°C, it is possible to prevent large thickness unevenness caused by stretching at the above stretching ratio. In addition, by stretching at the lowest possible temperature within the range where large thickness unevenness does not occur as described above, the refractive index in the thickness direction can be reduced.

[0049] (Regarding the refractive index in the direction of the folding part) The refractive index of the polyester film used as the base material, in the direction perpendicular to the direction in which the refractive index is 1.590 to 1.620, is preferably 1.670 to 1.700. That is, the refractive index in the direction perpendicular to the bending direction (the direction of the folded part) is preferably 1.670 to 1.700. Setting it to 1.670 to 1.700 reduces deformation when folded in the bending direction. Setting it to 1.700 or less suppresses cracking or breakage in the direction of the folded part. Setting it to 1.670 or higher improves flexibility in the bending direction and improves surface hardness. 1.680 to 1.695 is more preferable. Methods for adjusting the refractive index in the direction perpendicular to the bending direction include the stretching ratio, stretching preheating temperature, stretching temperature, multi-stage stretching, and film relaxation. The stretching ratio is preferably 4.0 to 6.0 times, and more preferably 4.4 to 6.0 times. Furthermore, the preheating temperature for stretching in the direction perpendicular to the bending direction is preferably 70 to 110°C. When performing multi-stage stretching in the direction perpendicular to the bending direction, it is preferable to increase the stretching ratio of the second and subsequent stages compared to the first stage. Film relaxation may be performed by 1 to 10% in both the machine flow direction (longitudinal direction) and the perpendicular direction (width direction).

[0050] (Regarding the refractive index in the direction of thickness) The refractive index in the thickness direction of the polyester film used as the base material is preferably 1.520 or less. More preferably 1.515 or less, even more preferably 1.510 or less, particularly preferably 1.505 or less, and most preferably 1.500 or less. By setting it to 1.520 or less, even if the refractive index in the bending direction is designed to be low, a decrease in the hardness of the film surface can be suppressed, and both flexibility and surface hardness can be achieved. Setting it to 1.520 or less reduces the indentation depth after the test force is removed in the thickness direction, and the hardness of the film surface, especially the pencil hardness of the hard coat film after the hard coat layer is laminated, can be improved. A low refractive index in the thickness direction is preferable, but for stable production, 1.3 or more is preferable, and it may even be 1.4 or more. Particularly preferable is 1.410 or more. The above range can be achieved by increasing the stretching ratio in both the bending direction and the folding direction, but in order to control the refractive index in the thickness direction while controlling the refractive index in the bending direction and width direction to a preferred range, it is preferable to set the conditions while checking the balance of the conditions of each process in the film manufacturing process.

[0051] Methods for controlling the refractive index in the thickness direction of the polyester film base material to the above range include setting the stretching preheating temperature, stretching temperature, and stretching ratio in the bending direction, stretching preheating temperature, stretching temperature, multi-stage stretching, high-magnification stretching, or heat-fixing temperature. The stretching preheating temperature in the bending direction is preferably 70°C to 110°C. The stretching temperature in the bending direction is preferably 75°C to 120°C. The stretching ratio in the bending direction is preferably 1.2 to 2.0 times, and more preferably 1.7 to 2.0 times. By lowering the stretching temperature and stretching at a low stretching ratio, the refractive index in the thickness direction can be effectively reduced while maintaining flexibility in the bending direction. The stretching preheating temperature in the folding direction is also preferably 75°C to 110°C. The stretching temperature is preferably 75 to 120°C. The stretching ratio of the folding section is preferably 4.0 to 6.0 times, and more preferably 4.4 to 6.0 times. The refractive index in the thickness direction can be effectively reduced while maintaining or reducing the refractive index in the bending direction. As a method of high-magnification stretching, multi-stage stretching may be used. In that case, it is preferable to make the stretching magnification of the second stage higher than that of the first stage in order to effectively control the refractive index. Alternatively, a method of stretching again after the crystallization process may be used. Accelerated stretching, in which the stretching speed is increased from the beginning to the end of the stretching process, may also be used. The preferred heat-fixing temperature is 180-240°C. Heat-fixing promotes oriented crystallization in the stretching direction, which can lower the refractive index in the thickness direction. The reason why lowering the refractive index in the thickness direction improves the hardness of the film surface is not entirely clear, but it is thought that aromatic compounds such as benzene rings within the molecular chain are oriented in the planar direction, which suppresses deformation caused by stress in the thickness direction.

[0052] (Regarding the density of polyester film) The density of the polyester film used as the base material is 1.380 g / cm³. 3 Preferably, it is 1.383 g / cm³. 3 It is more preferable that the above is true. 1.380 g / cm³ 3By doing so, flexibility can be improved, and the surface hardness of the film, particularly the pencil hardness of the hard coat film after the hard coat layer has been laminated, can be improved. A higher density is preferable, although this is somewhat affected by the presence or absence of particles in the film, but 1.40 g / cm³ is preferable. 3 The following is preferable: By setting the heat-fixing temperature during film formation to 180-240°C, crystallization can be promoted and the density can be effectively increased.

[0053] It is preferable that the bending direction of the polyester film used as the base material corresponds to the longitudinal direction (machine flow direction). This makes it easier to lower the refractive index in the bending direction at the biaxial stretching stage and improve flexibility. In other words, it is preferable to stretch the unstretched polyester sheet in the longitudinal direction at a stretching ratio of 1.2 to 2.0 times, more preferably 1.7 to 2.0 times, to obtain a polyester film. Furthermore, it is preferable to stretch it in the width direction at a stretching ratio of 4.0 to 6.0 times, more preferably 4.4 to 6.0 times.

[0054] Furthermore, in the laminated film of the present invention, the polyester film that serves as the base material (1) Refractive index in the bending direction is 1.590 to 1.620 (2) The refractive index in the direction of the folding part is 1.670 to 1.700 (3) The refractive index in the thickness direction is 1.520 or less. (4) Density is 1.380 g / cm³ 3 That's all. It is particularly preferable to simultaneously possess the four characteristics described above. However, even within the range of preferred manufacturing conditions described above, if the combination of conditions is not optimal within each preferred manufacturing condition range, such as a draw ratio of 1.4 times or less in the bending direction, a draw ratio of less than 4.4 times in the folding direction, and a heat-fixing temperature of 220°C or less, it may not be possible to obtain a product that satisfies all four characteristics simultaneously. In such cases, the four characteristics can be simultaneously satisfied by fine-tuning any of the conditions or a combination thereof, such as increasing the draw ratio in the bending direction to 1.7 times or more, increasing the draw ratio in the folding direction to 4.4 times or more, increasing the heat-fixing temperature to around 230°C, or lowering the draw temperature in the bending direction and / or the folding direction.

[0055] To adjust film-forming properties, film strength, thermal dimensional stability, and appearance defects, any film-forming method such as stretching, relaxation, heat fixing, or surface treatment may be used. However, in this invention, it is particularly preferable to control the refractive index and density of the polyester film used as the base material within the above-mentioned preferred range. By controlling the refractive index and density within the preferred range, it is possible to provide a polyester film suitable for foldable displays that exhibits superior flexural resistance and surface hardness compared to conventional films, and in particular, high pencil hardness of the hard coat film after lamination of the hard coat layer.

[0056] Specifically, for example, after sufficiently vacuum-drying PET pellets, they are supplied to an extruder, melt-extruded into a sheet shape at about 280°C, cooled and solidified to form an unstretched PET sheet. The obtained unstretched sheet is stretched 1.2 to 2.0 times, more preferably 1.7 to 2.0 times, in the longitudinal direction with a roll heated to 75 to 120°C to obtain a uniaxially oriented PET film. Further, the ends of the film are gripped with clips and led into a hot air zone heated to 75 to 120°C. After drying, it is stretched 4.0 to 6.0 times, more preferably 4.4 to 6.0 times, in the width direction. Subsequently, it is led into a heat treatment zone at 180 to 240°C and can be heat-treated for 1 to 60 seconds. During this heat treatment process, a relaxation treatment of 0 to 10% may be performed in the width direction or the longitudinal direction as necessary.

[0057] The intrinsic viscosity of the polyester film serving as the base material is preferably in the range of 0.50 to 1.0 dl / g. When the intrinsic viscosity is 0.50 dl / g or more, the impact resistance is improved, and it is preferable that disconnection of the internal circuit of the display due to external impact hardly occurs. On the other hand, when the intrinsic viscosity is 1.00 dl / g or less, the filtration pressure rise of the molten fluid does not become too large, and film production is stable, which is preferable.

[0058] (Easy adhesion layer) In the present invention, in order to improve the adhesiveness between the polyester film serving as the base material and the adhesive layer, hard coat layer, etc., it is also preferable to laminate an easy adhesion layer on the polyester film. The easy adhesion layer can be obtained by applying a coating solution for forming the easy adhesion layer on one or both sides of an unstretched or longitudinally uniaxially stretched film, heat-treating and drying as necessary, and further stretching in at least one direction that has not been stretched. Heat treatment can also be performed after biaxial stretching. The final coating amount of the easy adhesion layer is preferably controlled to 0.005 to 0.20 g / m 2 . When the coating amount is 0.005 g / m 2 or more, adhesiveness is obtained, which is preferable. On the other hand, when the coating amount is 0.20 g / m 2 or less, blocking resistance is obtained, which is preferable.

[0059] The resin to be included in the coating solution used for laminating the easy-adhesion layer can be any resin without particular limitation, such as polyester resins, polyether polyurethane resins, polyester polyurethane resins, polycarbonate polyurethane resins, or acrylic resins. Examples of crosslinking agents to be included in the coating solution for forming the easy-adhesion layer include melamine compounds, isocyanate compounds, oxazoline compounds, epoxy compounds, and carbodiimide compounds. Two or more of each can also be used in mixture form. Due to the nature of inline coatings, these are preferably applied with a water-based coating solution, and the resins and crosslinking agents are preferably water-soluble or water-dispersible resins or compounds.

[0060] It is preferable to add particles to the easy-adhesion layer to provide slipperiness. The average particle size of the fine particles is preferably 2 μm or less. If the average particle size exceeds 2 μm, the particles tend to fall off the easy-adhesion layer. Examples of particles to be included in the easy-adhesion layer include inorganic particles such as titanium dioxide, barium sulfate, calcium carbonate, calcium sulfate, silica, alumina, talc, kaolin, clay, calcium phosphate, mica, hectorite, zirconia, tungsten oxide, lithium fluoride, and calcium fluoride, as well as organic polymer particles such as styrene-based, acrylic-based, melamine-based, benzoguanamine-based, and silicone-based particles. These may be added to the easy-adhesion layer individually, or two or more may be added in combination.

[0061] Furthermore, known methods similar to those used for the coating layer can be used for applying the coating solution. Examples include the reverse roll coating method, gravure coating method, kiss coating method, roll brush method, spray coating method, air knife coating method, wire bar coating method, and pipe doctor method, and these methods can be used individually or in combination.

[0062] (Hard coat layer) When the laminated film of the present invention is used as a surface protective film to protect a foldable display, it is preferable that the polyester film, which is the base material, has a hard coat layer on at least one surface. It is preferable that the hard coat layer is positioned on the display surface side of the polyester film when used in the display. To suppress curling, the hard coat layer may be provided on both sides. The resin used to form the hard coat layer is not particularly limited and can be acrylic, siloxane, inorganic hybrid, urethane acrylate, polyester acrylate, epoxy, etc. Furthermore, two or more materials can be mixed and used, or particles such as inorganic fillers or organic fillers can be added.

[0063] (Film thickness of the hard coat layer) The thickness of the hard coat layer is preferably 1 to 50 μm. A thickness of 1 μm or more is preferable as it allows for sufficient curing and increases pencil hardness. Furthermore, by limiting the thickness to 50 μm or less, curling due to hard coat curing shrinkage can be suppressed, improving the handling properties of the film.

[0064] (Application method) The hard coat layer can be applied using various methods, including Meyer bar coating, gravure coating, die coating, and knife coating, without any particular limitations, and can be appropriately selected depending on viscosity and film thickness.

[0065] (Curing conditions) For curing the hard coat layer, methods such as curing with energy rays like ultraviolet light or electron beams, or curing with heat can be used. However, curing methods using ultraviolet light or electron beams are preferred in order to reduce damage to the film.

[0066] (Pencil hardness) The pencil hardness of the hard coat layer is preferably 3H or higher, and more preferably 4H or higher. A pencil hardness of 3H or higher ensures that it is not easily scratched and does not reduce visibility. Generally, a higher pencil hardness of the hard coat layer is preferable, but 9H or lower is acceptable, 8H or lower is acceptable, and even 6H or lower can be used without practical problems.

[0067] (Characteristics of laminated films) The hard coat layer in this invention can be used to protect the display by increasing the pencil hardness of the surface as described above, and a high transmittance is preferable. The total light transmittance of the laminated film is preferably 85% or more, and more preferably 88% or more. Sufficient visibility can be obtained with a transmittance of 85% or more. While a higher total light transmittance of the laminated film is generally preferable, from the standpoint of stable production, it is preferably 99% or less, and may also be 97% or less. Furthermore, the haze of the laminated film is generally preferable to be low, and preferably 3% or less. A haze of 2% or less is more preferable, and most preferably 1% or less. If the haze is 3% or less, the visibility of the image can be improved. While a lower haze is generally preferable, from the standpoint of stable production, it is preferably 0.1% or more, and may also be 0.3% or more.

[0068] The hard coat layer may also have other functions added to it. For example, a hard coat layer with added functionality such as an anti-glare layer having a certain pencil hardness, an anti-glare anti-reflective layer, an anti-reflective layer, a low-reflection layer, a scratch-resistant layer, and an anti-static layer is also preferably applied in the present invention.

[0069] Furthermore, a hard coat layer may be provided when used as a base film for a touch panel module. When an ITO layer, for example, is used as the transparent electrode layer for a touch panel module, it is preferable to provide an opacity adjustment layer between the base film and the transparent electrode layer in order to make the electrode pattern less visible. In this case, the hard coat layer itself may also serve as a refractive index adjustment layer, or a separate refractive index adjustment layer may be laminated.

[0070] (Adhesive layer) Preferably, the polyester film substrate of the laminated film has an adhesive layer on the side opposite to the side with the hard coat layer. The material constituting the adhesive layer is not particularly limited, and conventional adhesives such as rubber-based, acrylic-based, and polyolefin-based adhesives can be used. Examples of rubber-based adhesives include natural rubber, butadiene rubber, isoprene rubber, and styrene-based block copolymer elastomers such as SBS, SIS, SEBS, and SEPS. Examples of acrylic-based adhesives include crosslinked copolymers of methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, pentyl (meth)acrylate, hexyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate. Examples of crosslinking agents include isocyanate compounds, epoxy compounds, metal chelating agents, and compounds containing multiple double bonds. Examples of polyolefin-based adhesives include ethylene-propylene rubber such as EPM and EPDM, as well as those made by using these as soft segments and polyethylene or polypropylene as hard segments, and blends of ethylene-propylene rubber with polyethylene or polypropylene. The tackiness can be adjusted by adding known tackifiers, softeners, etc., to the adhesive layer. It can also be adjusted by adjusting the molecular weight of the base polymer used. The adhesive layer may also contain other components. Examples of other components include additives (i.e., auxiliary agents), such as dyes, pigments, antistatic agents, antioxidants, light stabilizers, UV absorbers, neutralizing agents, nucleating agents, epoxy stabilizers, lubricants, antibacterial agents, flame retardants, and plasticizers.

[0071] (Film thickness of the adhesive layer) The thickness of the adhesive layer is 1 μm to 50 μm, preferably 2 μm to 40 μm, and more preferably 4 μm to 30 μm.

[0072] (Coating method) The adhesive solution may be coated and dried, or in the case of acrylics, monomers or oligomers may be coated and then cured by radiation, or a substrate-less optical adhesive sheet (OCA) may be transferred.

[0073] The adhesive strength (peel strength) should preferably be adjusted depending on the material to which it is attached. In the case of folding displays, if the adhesive strength is weak, the bending part will lift, so the adhesive strength to the glass after 24 hours is preferably 10 to 40 N / 25 mm.

[0074] A release film may be provided on the surface of the adhesive layer (the opposite side of the polyester film).

[0075] (Release film) As the release film for the adhesive sheet, any commonly used release film can be used as appropriate. There are no particular limitations on the resin of the base film; any resin that forms a resin film, such as polyester, polycarbonate, polyamide, polyimide, polyamide-imide, polystyrene, triacetylcellulose, polypropylene, and cyclic polyolefins, can be used without restriction. Among these, polyester is preferred in terms of mechanical strength, heat resistance, and supply stability, and polyethylene terephthalate is even more preferred. The film may be an unstretched film or a stretched film. If it is a stretched film, it may be a uniaxially oriented film or a biaxially oriented film. Among these, biaxially oriented polyethylene terephthalate film is preferred.

[0076] If the base film itself has release properties, it can be used as is as a release film. Alternatively, surface treatments such as corona treatment, plasma treatment, or flame treatment may be performed to adjust the release properties.

[0077] (Release layer) Furthermore, a release layer may be provided on the base film. Examples of release layers include silicone-based, amino resin-based, alkyd resin-based, and long-chain acrylic resin-based release layers, and their composition and type can be appropriately selected according to the required release force.

[0078] When a release layer is provided, an easy-adhesion layer may also be provided on the base film. As the easy-adhesion layer, conventionally used materials such as polyester, acrylic, and polyurethane can be used with each base film, and can be selected according to the base film and release layer being used.

[0079] (Antistatic layer) A release film antistatic layer may be provided. Examples of release layer antistatic agents include quaternary ammonium salts, conductive polymers such as polyaniline and polythiophene, needle-shaped metal fillers, conductive high refractive index fine particles such as tin-doped indium oxide fine particles and antimond-doped tin oxide fine particles, and combinations thereof. Polyester, polyurethane, polyamide, acrylic, etc., can be used as the binder resin. The antistatic layer is provided on the release surface and may be located below the release layer, or it may be provided on the opposite side of the release surface. [Examples]

[0080] Next, the present invention will be described using examples and comparative examples. First, the method for evaluating characteristic values ​​implemented in the present invention is shown below.

[0081] (1) Intrinsic viscosity of polyester A polyester film or polyester resin was crushed and dried, then dissolved in a mixed solvent of phenol / tetrachloroethane = 60 / 40 (mass ratio). After removing inorganic particles from this solution by centrifugation, the flow time of the solution at a concentration of 0.4 (g / dl) and the flow time of the solvent alone were measured using an Ubbelohde viscometer at 30°C. The intrinsic viscosity was calculated from the ratio of these times using Huggins' equation, assuming that Huggins' constant is 0.38.

[0082] (2) Flexural resistance of polyester film samples (flexural radius 1.5 mm) A polyester film sample measuring 20 mm in width and 110 mm in flow direction was prepared. Using a no-load U-shaped stretch tester (Yuasa System Equipment Co., Ltd., DLDMLH-FS), the bending radius was set to 1.5 mm, and the sample was bent 200,000 times at a speed of 1 time / second. At that time, the sample was fixed at a position 10 mm from both ends of the long side, and the bending area was 20 mm x 90 mm. Here, Figure 1 is a schematic diagram to show the bending radius when a foldable display is folded, and considering the case where the polyester film is placed on the inner surface of the folded state, the bending test was performed model by setting the location indicated by reference numeral 11 in Figure 1 to 1.5 mm. After the bending process was completed, the sample was placed on a flat surface with the inside of the bend facing downwards and observed visually. ○: No cracks or deformation were observed in the sample. ×: The sample has cracks or creases, and when placed horizontally, it lifts up by more than 5mm.

[0083] (3) Flexural resistance of laminated film samples (flexural radius 3.0 mm) Using the same method as the bending test described above, the hard coat film was bent 200,000 times at a speed of 1 time / second with a bending radius of 3.0 mm. In cases where the hard coat layer was only on one side, tests were performed with the hard coat layer on the inside and with the hard coat layer on the outside. ○: No cracks in the hard coat layer, no deformation observed in the hard coat film. ×: Fracture, cracks visible in the hard coat layer, or deformation visible in the hard coat film.

[0084] (4) Refractive index of polyester film In accordance with JIS K 7142:2014 "Method for measuring the refractive index of plastics (Method A)", the refractive index in the longitudinal direction, the refractive index in the width direction, and the refractive index in the thickness direction were determined using an Abbe refractometer (ATAGO Corporation, NAR-4T, measurement wavelength 589 nm).

[0085] (5) Pencil hardness Laminated films were used as samples and measured in accordance with JIS K 5600-5-4:1999, with a load of 750g and a speed of 1.0mm / s. In this invention, a score of 3H or higher was considered acceptable.

[0086] (6) Total light transmittance, haze The measurement was performed using a haze meter (NDH5000, manufactured by Nippon Denshoku Industries Co., Ltd.).

[0087] (7) Density The density was measured according to the method compliant with JIS K 7112:1999 (density gradient pipe method). (Unit: g / cm³) 3 ).

[0088] (8) Depth of indentation after unloading of test force The sample was cut into approximately 2 cm squares and fixed to an 18 × 18 mm microcover glass (manufactured by Matsunami Glass Co., Ltd.) with the opposite side of the measurement surface using adhesive (Cemedine® High Super 30). After fixing, it was left at room temperature for more than 12 hours, and then the indentation depth (μm) after unloading the test force was measured using a dynamic ultramicrohardness tester "DUH-211" (manufactured by Shimadzu Corporation) under the following conditions. <Measurement Conditions> Test mode: Load-unload test Indenter used: 115 degree ridge angle, triangular pyramid indenter Indenter modulus: 1.140 × 10⁶ N / mm 2 Indenter Poisson's ratio: 0.07 Test force: 50mN Load speed: 4.44mN / sec Load holding time: 2sec Unloading holding time: 0sec

[0089] (9) Maximum thermal contraction A sample film was cut to 10 mm x 250 mm, and marks were made at 200 mm intervals along the longer side, aligned with the desired measurement direction. The distance A between the marks was measured under a constant tension of 5 g. Next, the sample film was left unloaded in an oven at 150°C for 30 minutes, then removed from the oven and cooled to room temperature. After that, the distance B between the marks was determined under a constant tension of 5 g, and the thermal shrinkage rate (%) was calculated using the following formula. Note that the thermal shrinkage rate was measured at three equal points along the width of the sample film, and the average of the three points was taken as the thermal shrinkage rate (%). Thermal shrinkage rate (%) = [(AB) × 100] / A The sample film was cut separately in both the bending and folding directions, with the length and width differing, and measurements were taken. The data in the direction with the larger measurement value was taken as the maximum thermal shrinkage rate (%).

[0090] (10) Tensile modulus (Young's modulus (unit: GPa)) The tensile modulus of polyester film in the bending and folding directions was measured at 23°C in accordance with JIS K7127:1999.

[0091] (Preparation of polyethylene terephthalate pellets (a)) As the esterification reactor, a continuous esterification reactor consisting of a three-stage complete mixing tank equipped with a stirrer, a partial condenser, a raw material inlet, and a product outlet was used. TPA was supplied at 2 tons / hr, EG at 2 moles per mole of TPA, and antimony trioxide was added in an amount that resulted in 160 ppm of Sb atoms relative to the generated PET. This slurry was continuously supplied to the first esterification reactor of the esterification reactor and reacted at atmospheric pressure at 255°C with an average residence time of 4 hours. Next, the reaction product in the first esterification reactor was continuously removed from the system and supplied to the second esterification reactor. EG distilled off from the first esterification reactor was supplied to the second esterification reactor at 8% by mass relative to the generated polymer (generated PET). Furthermore, an EG solution containing magnesium acetate in an amount that resulted in 65 ppm of Mg atoms relative to the generated PET, and an EG solution containing TMPA in an amount that resulted in 20 ppm of P atoms relative to the generated PET were added, and the reaction was carried out at atmospheric pressure at 260°C with an average residence time of 1.5 hours. Next, the reaction product in the second esterification reaction vessel was continuously removed from the system and supplied to the third esterification reaction vessel. Furthermore, an EG solution containing TMPA in an amount such that the P atom content was 20 ppm relative to the generated PET was added, and the reaction was carried out at atmospheric pressure at 260°C with an average residence time of 0.5 hours. The esterification reaction product generated in the third esterification reaction vessel was continuously supplied to a three-stage continuous polycondensation reactor for polycondensation, and then filtered through a stainless steel sintered filter material (nominal filtration accuracy, 90% cut of 5 μm particles) to obtain polyethylene terephthalate pellets (a) with an intrinsic viscosity of 0.62 dl / g.

[0092] (Preparation of polyethylene terephthalate pellets (b)) The manufacturing process for polyethylene terephthalate pellets (a) was carried out in the same manner as above, except for adjusting the residence time of the third esterification reaction, to adjust the intrinsic viscosity to 0.580 dl / g and obtain polyethylene terephthalate pellets (b).

[0093] (Preparation of polyethylene terephthalate pellets (c)) Polyethylene terephthalate pellets (a) were subjected to solid-phase polymerization at 220°C under reduced pressure of 0.5 mmHg using a rotary vacuum polymerization apparatus to produce polyethylene terephthalate pellets (c) with an intrinsic viscosity of 0.75 dl / g.

[0094] (Polymerization of urethane resin) In a four-necked flask equipped with a stirrer, a Liebig condenser, a nitrogen inlet tube, a silica gel drying tube, and a thermometer, 72.96 parts by mass of 1,3-bis(isocyanatemethyl)cyclohexane, 12.60 parts by mass of dimethylolpropionic acid, 11.74 parts by mass of neopentyl glycol, 112.70 parts by mass of polycarbonate diol with a number average molecular weight of 2000, and 85.00 parts by mass of acetonitrile and 5.00 parts by mass of N-methylpyrrolidone as solvents were added. The mixture was stirred at 75°C for 3 hours under a nitrogen atmosphere, and it was confirmed that the reaction solution reached the predetermined amine equivalent. Next, the reaction solution was cooled to 40°C, and then 9.03 parts by mass of triethylamine was added to obtain polyurethane prepolymer D solution. Next, in a reaction vessel equipped with a homodisperser capable of high-speed stirring, 450 g of water was added, the temperature was adjusted to 25°C, and the isocyanate-terminated prepolymer was added and dispersed in water while stirring at 2000 min-1. Subsequently, a water-soluble polyurethane resin (A) with a solid content of 35% by mass was prepared by removing some of the acetonitrile and water under reduced pressure.

[0095] (Polymerization of water-soluble carbodiimide compounds) In a flask equipped with a thermometer, nitrogen gas inlet tube, reflux condenser, dropping funnel, and stirrer, 200 parts by mass of isophorone diisocyanate and 4 parts by mass of the carbodiimide catalyst 3-methyl-1-phenyl-2-phosphorene-1-oxide were added and stirred at 180°C for 10 hours under a nitrogen atmosphere to obtain isocyanate-terminated isophorone carbodiimide (degree of polymerization = 5). Next, 111.2 g of the obtained carbodiimide and 80 g of polyethylene glycol monomethyl ether (molecular weight 400) were reacted at 100°C for 24 hours. Water was gradually added at 50°C to obtain a yellow, transparent, water-soluble carbodiimide compound (B) with a solid content of 40% by mass.

[0096] (Preparation of coating solution for easy adhesion layer formation) The following coating agents were mixed to create the coating solution. Water 16.97 parts by mass Isopropanol 21.96 parts by mass Polyurethane resin (A) 3.27 parts by mass Water-soluble carbodiimide compound (B) 1.22 parts by mass Particles 0.51 parts by mass (Silica sol with average particle size of 40 nm, solid content concentration of 40% by mass) Surfactant 0.05 parts by mass (Silicone-based, solid content concentration 100% by mass)

[0097] (Preparation of hard coat coating solution a) 100 parts by mass of hard coat material (Opstar® Z7503, manufactured by JSR, concentration 75%) was mixed with 0.1 parts by mass of leveling agent (BYK307, manufactured by BIK Chemie Japan, concentration 100%), and diluted with methyl ethyl ketone to prepare hard coat coating solution a with a solid content of 40% by mass.

[0098] (Preparation of hard coat coating solution b) 95 parts by mass of pentaerythritol triacrylate (manufactured by Shin-Nakamura Chemical Industry Co., Ltd., A-TMM-3, 100% solids content), 5 parts by mass of photopolymerization initiator (manufactured by BASF Japan, Irgacure® 907, 100% solids content), and 0.1 parts by mass of leveling agent (manufactured by BIC Chemie Japan, BYK307, 100% solids content) were mixed and diluted with a toluene / MEK = 1 / 1 solvent to prepare a 40% by mass hard coat coating solution b.

[0099] (Example 1) Pellet(a) of polyethylene terephthalate was fed into an extruder and melted at 285°C. This polymer was filtered through a stainless steel sintered filter (nominal filtration accuracy, 95% cut of 10 μm particles), extruded in sheet form through a die, and then cooled and solidified using an electrostatic casting method by contacting it with a casting drum at a surface temperature of 30°C to produce an unstretched film. This unstretched film was uniformly heated to 75°C using a heating roll, and then heated to 85°C with a non-contact heater to perform 1.4 times roll stretching (longitudinal stretching). The above-mentioned easy-adhesion layer forming coating solution was applied to both sides of the obtained uniaxially oriented film using the roll-coating method, and then dried at 80°C for 20 seconds. The coating amount after final drying (after biaxial stretching) was 0.06 g / m². 2 The material was adjusted to achieve the following. Then, it was guided into a tenter and preheated at 105°C, followed by 4.0x transverse stretching at 95°C. The width was then fixed, and heat setting was performed at 230°C for 5 seconds. Finally, it was relaxed by 4% in the width direction at 180°C to obtain a 50 μm thick polyethylene terephthalate film. On one side of the prepared film, hard coat coating solution a was applied using a Meyer burr to achieve a dry film thickness of 5 μm. After drying at 80°C for 1 minute, ultraviolet light was irradiated (cumulative light intensity 200 mJ / cm²). 2 A hard coat film was obtained. Subsequently, a double-sided adhesive sheet (Lintec Corporation, product name "OPTERIA MO-3006C", thickness: 25 μm) was attached to the side of the polyethylene terephthalate film that did not have a hard coat layer to obtain a laminated film. The evaluation results are shown in Table 1.

[0100] (Examples 2-3) A polyester film was obtained in the same manner as in Example 1, except that the longitudinal stretching ratio was changed as shown in Table 1, and a laminated film was obtained.

[0101] (Example 4) A polyester film was obtained in the same manner as in Example 1, except that the stretching ratio in the width direction was changed to 4.4 times and the heat setting temperature was changed to 220°C, and a laminated film was obtained.

[0102] (Examples 5-6) A polyester film was obtained in the same manner as in Example 4, except that the stretching ratio in the longitudinal direction was changed as shown in Table 1, and a laminated film was obtained.

[0103] (Example 7) A polyester film was obtained in the same manner as in Example 1, except that the stretching ratio in the width direction was changed to 5.5 times and the heat setting temperature was changed to 190°C, and a laminated film was obtained.

[0104] (Examples 8-9) A polyester film was obtained in the same manner as in Example 7, except that the stretching ratio in the longitudinal direction was changed as shown in Table 1, and a laminated film was obtained.

[0105] (Example 10) In the manufacturing process of Example 5, a polyester film was obtained in the same manner as in Example 5, except that after stretching in the longitudinal direction, a 10% relaxation heat treatment was performed at 100°C. A laminated film was then obtained.

[0106] (Example 11) In the manufacturing process of Example 5, a polyester film was obtained and a laminated film was obtained in the same manner as in Example 5, except that the clips were released at 200°C after heat setting and relaxation heat treatment was performed in the longitudinal and width directions. In the longitudinal direction, the tenter speed and winding roll speed were adjusted so that the relaxation rate was 3%. Relaxation in the width direction was left free.

[0107] (Example 12) A polyester film was obtained in the same manner as in Example 1, except that the temperature during longitudinal stretching was changed to 75°C and the heat-fixing temperature was changed to 220°C, and a laminated film was obtained.

[0108] (Example 13) A polyester film was obtained in the same manner as in Example 1, except that the temperature during longitudinal stretching was changed to 75°C, the stretching ratio was changed to 1.2 times, and then the stretching ratio in the width direction was changed to 5.0 times. A laminated film was then obtained.

[0109] (Example 14) A polyester film was obtained in the same manner as in Example 3, except that the longitudinal stretching was performed in two stages, with the first stage stretching ratio being 1.2 times and the second stage stretching ratio being 1.67 times. A laminated film was then obtained. The total longitudinal stretching ratio was approximately 2.0 times.

[0110] (Example 15) A polyester film was obtained in the same manner as in Example 5, except that the preheating temperature during widthwise stretching was changed to 95°C and the heat setting temperature was changed to 190°C, and a laminated film was obtained.

[0111] (Example 16) A polyester film was obtained in the same manner as in Example 2, except that the widthwise stretching was performed in two stages, with the first stretching ratio set to 1.5 times and the second stretching ratio set to 4.0 times, and the heat setting temperature was changed to 190°C. A laminated film was then obtained. The total widthwise stretching ratio was 6.0 times.

[0112] (Examples 17-18) A polyester film was obtained in the same manner as in Example 2, except that the thickness was changed as shown in Table 1, and a laminated film was obtained.

[0113] (Example 19) A polyester film was obtained in the same manner as in Example 1, except that the heat relaxation treatment in the width direction was not performed during the manufacturing process of Example 1, and a laminated film was obtained.

[0114] (Example 20) After preparing an unstretched film in the same manner as in Example 1, the unstretched film was preheated in a tenter at 75°C and then transversely stretched to 1.4 times its original size at 85°C. The above-mentioned easy-adhesion layer forming coating solution was applied to both sides of the resulting uniaxially oriented film by the roll-coating method, and then dried at 80°C for 20 seconds. The coating amount after final drying (after biaxial stretching) was 0.06 g / m². 2The material was adjusted to achieve the following: It was uniformly heated to 105°C using a heating roll, then heated to 95°C with a non-contact heater and roll-stretched to 0.4 times its original size (longitudinal stretching). The width was fixed and heat-set at 230°C for 5 seconds to obtain a polyethylene terephthalate film with a thickness of 50 μm. The subsequent steps were the same as in Example 1 to obtain a laminated film.

[0115] (Example 21) A polyethylene terephthalate film with a thickness of 50 μm was obtained in the same manner as in Example 1, and then a hard-coated film was obtained by applying hard-coat coating solution b. A laminated film was then obtained in the same manner as in Example 1.

[0116] (Comparative Example 1) A polyester film was obtained in the same manner as in Example 1, except that it was stretched only in the width direction and not in the longitudinal direction, resulting in uniaxial stretching. A laminated film was then obtained.

[0117] (Comparative Example 2) A polyester film was obtained in the same manner as in Example 7, except that it was stretched only in the width direction and not in the longitudinal direction, resulting in uniaxial transverse stretching, and a laminated film was obtained.

[0118] (Comparative Examples 3-7) A polyester film was obtained in the same manner as in Example 1, except that the heat-fixing temperature was changed to 220°C and the PET pellets and thickness were as described in Table 1, and a laminated film was obtained. As described above, Comparative Examples 3 to 7 had lower heat-fixing temperatures than Example 1, and the combinations of conditions for the elongation ratio in the longitudinal and width directions were not optimal within the preferred condition range. As shown in Table 1, the refractive index in the thickness direction increased, the indentation depth after unloading the test force was greater, and the pencil hardness after lamination of the hard coat layer was lower compared to each example.

[0119] (Comparative Example 8) A polyester film was obtained in the same manner as in Example 1, except that the longitudinal stretching ratio was changed to 2.7 times and the heat-fixing temperature was changed to 220°C, and a laminated film was obtained.

[0120] (Comparative Example 9) A polyester film was obtained in the same manner as in Example 1, except that the stretching ratio in the longitudinal direction was changed to 3.4 times, and a laminated film was obtained.

[0121] (Comparative Example 10) A polyester film was obtained in the same manner as in Example 4, except that the heat-fixing temperature was changed to 100°C, and a laminated film was obtained.

[0122] (Comparative Example 11) A polyester film was obtained in the same manner as in Example 13, except that the longitudinal stretching temperature was changed to 130°C, and a laminated film was obtained.

[0123] (Comparative Example 12) A polyester film was obtained in the same manner as in Example 1, except that the preheating temperature in the width direction was changed to 120°C, and a laminated film was obtained.

[0124] A laminated film was bonded to an organic EL module via a 25 μm thick adhesive layer to create a smartphone-type foldable display that could be folded in half at the center, corresponding to the bending radius in Figure 1, with a radius of 3 mm. The laminated film was arranged on the surface of a single continuous display through the folding portion, with the hard coat layer positioned on the surface of the display. The laminated films used in each embodiment satisfied the functionality and visibility requirements of a smartphone that could be folded in half at the center for portability. Furthermore, the surface did not dent under external force. On the other hand, the foldable displays using the laminated films in each comparative example appeared to develop image distortion at the folding portion of the display as usage frequency increased, which was undesirable. In addition, dents and scratches were observed on the surface of some of them.

[0125] [Table 1] [Industrial applicability]

[0126] A foldable display using the laminated film for foldable displays of the present invention maintains mass producibility while preventing deformation of the laminated film located on the surface of the foldable display after repeated folding, thus preventing image distortion at the folded portion of the display. In particular, a portable terminal device or image display device equipped with a foldable display using the laminated film of the present invention as a surface protective film provides beautiful images, is highly functional, and offers excellent portability and other conveniences. [Explanation of Symbols]

[0127] 1: Foldable display 11: Bending radius 2: Polyester base film constituting the laminated film for foldable displays 21: Folding section 22: Bending direction (direction perpendicular to the folding part)

Claims

1. A hard coat layer is provided on one side of the polyester film. A laminated film for a foldable organic EL display, having an adhesive layer on the surface of the polyester film opposite to the surface having a hard coat layer, wherein the polyester film satisfies the following conditions. (1) Refractive index in the bending direction is 1.590 to 1.620 (2) The refractive index in the direction of the folding part is 1.670 to 1.700 (3) The refractive index in the thickness direction is 1.520 or less. (4) Density of 1.380 g / cm³ ３ That's all. (5) Polyester film is polyethylene terephthalate film (Here, the bending direction refers to the direction perpendicular to the fold when folding the polyester film.)

2. The laminated film for a foldable organic EL display according to claim 1, wherein the elastic modulus of the polyester film in the bending direction is 2.7 GPa or less, and the elastic modulus of the folded portion in the folding direction is 4.5 GPa or more.

3. The laminated film for a foldable organic EL display according to claim 1, wherein the polyester film does not crack or deform when bent 200,000 times at a speed of 1 time / second with a no-load U-shaped stretch tester (Yuasa System Equipment Co., Ltd., DLDMLH-FS) set to a bending radius of 1.5 mm.

4. The laminated film for a foldable organic EL display according to claim 1, wherein the thickness of the polyester film is 10 to 80 μm, the thickness of the adhesive layer is 1 to 50 μm, and the thickness of the hard coat layer is 1 to 50 μm.

5. A foldable organic EL display in which a laminated film for a foldable organic EL display according to claims 1 to 4 is arranged as a surface protective film such that a hard coat layer is positioned on the surface, wherein a single continuous laminated film is arranged across the foldable portion of the foldable organic EL display.

6. A portable terminal device having a foldable organic EL display as described in claim 5.

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