Hard coat film for foldable displays and its applications

Abstract

To provide a hard coat film for a folding type display which prevents a folding crease and a crack in a folding part, is affected by a fine crack of an easily adhesive resin layer, and is excellent in suppressing iris-like color (interference spot).SOLUTION: A hard coat film for a folding type display has an easily adhesive resin layer and a hard coat layer on at least one surface of a polyester film having a thickness of 10 to 80 μm in this order, wherein the easily adhesive resin layer is obtained by curing a composition containing at least one kind of compound selected from a titanium compound and a zirconium compound, and a polyester resin, and has the easily adhesive resin layer, and the polyester film before the hard coat layer is laminated satisfies a characteristic within a specific range.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a hard coat 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 to the hard coat film for the 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 device 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] While methods for increasing durability include partially varying the film thickness (see Patent Document 2), these methods suffer from the problem of poor mass production capabilities.

[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.

[0009] On the other hand, the aforementioned hard coat film also requires visibility and aesthetic appeal. Therefore, to suppress glare and iridescent coloration (interference spots) caused by reflected light when viewed from any angle, it is common practice to provide a multilayer anti-reflective layer on top of the hard coat layer, consisting of layers of high and low refractive index layers stacked on top of each other. However, recently, three-wavelength fluorescent lamps have become mainstream to reproduce daylight color, making interference spots caused by reflected light more visible. Furthermore, there is a growing demand for cost reduction through the simplification of the anti-reflective layer. Therefore, there is a need for a hard coat film that can suppress interference spots as much as possible even without an added anti-reflective layer.

[0010] As mentioned above, a method has been proposed to suppress interference spots by providing one or two optical adjustment layers with adjusted refractive indices on a polyester film. However, with polyester films, durability against repeated bending must be considered. For example, optical adjustment layers containing excessive metal nanoparticles have not yielded satisfactory results because interference spots are generated by microcracks originating from the nanoparticles. [Prior art documents] [Patent Documents]

[0011] [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 project] [Problems that the invention aims to solve]

[0012] The present invention aims to solve the problems of conventional display components as described above, and to provide a foldable display that is excellent in mass production 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 hard coat film for a foldable display that does not cause fold marks or cracks at the folded part, and furthermore, can effectively suppress iridescent coloration (interference spots) that are affected by fine cracks in the easily adhesive resin layer, etc. [Means for solving the problem]

[0013] In other words, the present invention consists of the following configuration. 1. A hard coat film for a foldable display, comprising a polyester film with a thickness of 10 to 80 μm, having an easy-adhesion resin layer and a hard coat layer sequentially on at least one side, wherein the easy-adhesion resin layer is formed by curing a composition containing at least one compound selected from titanium compounds and zirconium compounds, and a polyester resin, and the polyester film having the easy-adhesion resin layer before lamination of the hard coat layer satisfies the following conditions (1) to (4). (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 hard coat film for a foldable display according to the first description, wherein the refractive index of the easy-adhesion resin layer is lower than the refractive index in the bending direction and the refractive index in the folding direction of the polyester film having the easy-adhesion resin layer before lamination of the hard coat layer, and is higher than the refractive index of the hard coat layer. 3. The polyester film for a foldable display according to the first or second aspect, wherein the refractive index of the easily adherable resin layer satisfies the following conditions (5) and (6). (5) The refractive index of the easily adherable resin layer is lower than the refractive index in the bending direction of the polyester film before laminating the hard coat layer having the easily adherable resin layer, and the difference in refractive index is greater than 0 and not more than 0.07. (6) The refractive index of the easily adherable resin layer is lower than the refractive index in the folding direction of the polyester film before laminating the hard coat layer having the easily adherable resin layer, and the difference in refractive index is not less than 0.080 and not more than 0.150. 4. The polyester film for a foldable display according to any one of the first to third aspects, wherein the polyester resin contained in the easily adherable resin layer is a polyester resin containing a naphthalenedicarboxylic acid component as at least a part of the dicarboxylic acid component constituting the polyester resin. 5. The hard coat film for a foldable display according to any one of the first to fourth aspects, wherein the total light transmittance of the polyester film before laminating the hard coat layer having the easily adherable resin layer is 85% or more, the haze is 3% or less, and the maximum heat shrinkage rate is 6% or less. 6. The hard coat film for a foldable display according to any one of the first to fifth aspects, wherein the thickness of the hard coat layer is 1 to 50 μm. 7. The foldable display in which the hard coat film for a foldable display according to the sixth aspect is arranged as a surface protection film so that the hard coat layer is located on the surface, and a single continuous hard coat film is arranged through the folding part of the foldable display. 8. A portable terminal device having the foldable display according to the seventh aspect.

Advantages of the Invention

[0014] The foldable display using the hard coat film for a foldable display of the present invention can maintain mass productivity while the hard coat film does not crack at the folding portion and does not cause deformation after repeated folding. Further, in addition to cracks at the folding portion, peeling at the interface between the hard coat and the easy-adhesive resin layer, and peeling at the interface between the easy-adhesive resin layer and the polyester film, iridescent colors (interference fringes) caused by fine cracks and the like can be effectively suppressed, and image distortion at the folding portion of the display does not occur. A portable terminal device equipped with the foldable display using the hard coat film as described above provides beautiful images, is rich in functionality, and is excellent in convenience such as portability.

Brief Description of the Drawings

[0015] [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 hard coat film for a foldable display in the present invention.

Embodiments for Carrying Out the Invention

[0016] (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, LCDs, organic ELs, and inorganic ELs having a foldable structure are preferred. In particular, organic ELs and inorganic ELs that can reduce the layer structure are particularly preferred, and organic ELs with a wide color gamut are even more preferred.

[0017] (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 a 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 part. The surface protection film, 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.

[0018] The hard coat film for foldable displays of the present invention may be used in any part of a foldable display. Below, using an organic EL display as an example, a typical configuration of a foldable display and the parts in which the hard coat film of the present invention may be used will be described. In the following, the hard coat film for foldable displays of the present invention may simply be referred to as the hard coat film of the present invention.

[0019] (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.

[0020] (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 hard coat 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.

[0021] (Circular polarizer) A circular polarizer suppresses the degradation of image quality caused by the reflection of external light by internal display components. A circular polarizer comprises a linear polarizer and a phase difference plate. The linear polarizer has a protective film on at least the side of the polarizer that is visible. A protective film may also be present on the side of the polarizer opposite to the visible side, and the phase difference plate may be directly laminated onto the polarizer. The phase difference plate can be a resin film having a phase difference, such as polycarbonate or cyclic olefin, or a resin film with a phase difference layer made of a liquid crystal compound. The hard coat film of the present invention can be used as a polarizer protective film. In these cases, if the base film of the hard coat 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.

[0022] (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 hard coat 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 hard coat film of the present invention is used. The hard coat layer is provided on the surface of the foldable display with the viewing side facing outwards. Note that the hard coat layer may be provided on both sides.

[0023] (Protective film on the back) It is also preferable to provide a protective film on the back side of the display. The hard coat film of the present invention can be used as this protective film on the back side.

[0024] The hard coat 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 hard coat 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.

[0025] Furthermore, the hard coat film of the present invention is not necessarily required for all of the above-mentioned applications of foldable displays. In foldable displays, hard coat 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.

[0026] When the base film constituting the hard coat 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.

[0027] Polyester resins used in polyester films, which are the base films for hard coat 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.

[0028] When a polyester copolymer is used as the polyester film, which is the base film for a hard coat 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.

[0029] Furthermore, in the production of polyester film, which is the base film for hard coat 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 and makes 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.

[0030] The polyester film, which is the base film for the hard coat film, preferably has a thickness of 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 offers superior flexibility, processability, and handling.

[0031] The surface of the hard coat 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 of the polyester film before laminating the hard coat layer after laminating the easily adhesive resin layer 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.

[0032] As mentioned above, for the purpose of reducing haze, it is preferable that the surface irregularities of the film are not too large. However, in order to provide a certain degree of slipperiness from the standpoint of handling, 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 as an easy-adhesion resin layer during the film-forming process.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] For polyester films, the total light transmittance in a state having an easy-adhesion resin layer but no hard coat layer 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 film is preferably 85% or higher in order to increase the total light transmittance of the hard coat film described later. 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.

[0038] The maximum heat shrinkage rate after heat treatment at 150°C for 30 minutes in a polyester film having an easily adhesive resin layer but without a hard coat layer 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. While a lower maximum heat shrinkage rate is preferable, it is preferably -1% or more, and preferably 0% or more. A negative value here means expansion after heating, and a value of -1% or more is preferable as it indicates good flatness.

[0039] In order to provide sufficient pencil hardness to the hard coat film for the foldable display of the present invention, it is preferable that the polyester film has an easily adhering resin layer and, in a state without a hard coat layer, has the following characteristics. In conventional base polyester films, it is thought that the pencil hardness of the hard coat film decreased after lamination of the hard coat layer due to deformation in the thickness direction of the film. In the present invention, it is preferable that the indentation depth after removal of the test force in the thickness direction using the dynamic ultramicro hardness tester described later for the base polyester film in the above state be within a specific range. In the pencil hardness evaluation of the hard coat film using the above base polyester film, it is preferable that a high hardness can be achieved. The indentation depth after removal of the test force in the thickness direction of the base polyester film in the above state is preferably 1.5 μm or less, more preferably 1.4 μm or less, and even more preferably 1.3 μm or less. If the indentation depth after unloading the test force (the final amount of deformation under load) is 1.5 μm or less, the pencil hardness evaluation of the hard coat film after lamination of the hard coat layer can be performed by making the film less prone to deformation in the thickness direction, thus increasing the pencil hardness. Increasing the pencil hardness of the hard coat 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 generally better, 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.

[0040] 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 for polyester films that have an easy-adhesion resin layer but no hard coat layer. As a means of reducing the refractive index to 1.520 or less, as will be described later, examples of conditions that can be set 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 that allows control of these properties to a desirable range.

[0041] The non-hard-coated surface of the hard-coated film of the present invention can be treated to apply an adhesive or to improve adhesion with the hard-coated layer.

[0042] Surface treatment methods include, for example, sandblasting, solvent treatment to create uneven surfaces, and oxidation treatments such as corona discharge, electron beam irradiation, plasma treatment, ozone / ultraviolet irradiation, flame treatment, chromic acid treatment, and hot air treatment, and can be used without any particular limitations.

[0043] Furthermore, it is preferable to improve adhesion by using an adhesion-enhancing layer such as an easy-to-adhere resin layer. The easy-to-adhere resin layer can be any resin without particular limitations, such as acrylic resin, polyester resin, polyurethane resin, or polyether resin, and can be formed by a general coating method, preferably a so-called in-line coating formulation.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] (Regarding the refractive index in the bending direction) In the present invention, the refractive index of the polyester film having an easy-adhesion resin layer and before lamination of the hard coat layer is preferably 1.590 to 1.620 in at least one direction of the longitudinal direction (machine flow direction) and width direction, and more preferably 1.591 to 1.600. Furthermore, the refractive index of the polyester film having an easy-adhesion resin layer and before lamination of the hard coat layer 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 of the longitudinal direction and 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 described above. If the refractive index is 1.590 or higher, there is no risk of cracking in the direction of the fold after the bending test described later, and of course, no breakage will occur, 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. Note that the difference in refractive index between the polyester film with an easy-adhesion resin layer and before lamination of the hard coat layer and the polyester film alone without an easy-adhesion resin layer is negligibly small.

[0050] By having an easy-adhesion resin layer and controlling the refractive index in at least one direction, either the longitudinal direction (machine flow direction) or the width direction, of the polyester film before lamination of the hard coat layer 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.

[0051] 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.

[0052] 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.

[0053] (Regarding the refractive index in the direction of the folding part) The refractive index of the polyester film having the above-mentioned easy-adhesion resin layer, before lamination of the hard coat layer, is preferably 1.670 to 1.700 in the direction perpendicular to the direction in which the refractive index is 1.590 to 1.620. That is, it is preferable that the refractive index in the direction perpendicular to the bending direction (the direction of the folded part) is 1.670 to 1.700. Setting it to 1.670 to 1.700 can reduce deformation when folded in the bending direction. Setting it to 1.700 or less can suppress cracking or breakage in the direction of the folded part. Setting it to 1.670 or more can improve flexibility in the bending direction and improve 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).

[0054] (Regarding the refractive index in the direction of thickness) It is preferable that the refractive index in the thickness direction of the polyester film, which has an easy-adhesion resin layer and is not laminated with a hard coat layer, be 1.520 or less. By keeping the refractive index at 1.520 or less, it is possible to suppress the decrease in hardness of the film surface even if the refractive index in the bending direction is designed to be low, thereby achieving both flexibility and surface hardness. Keeping it at 1.520 or less reduces the indentation depth after unloading the test force in the thickness direction, and improves the hardness of the film surface, especially the pencil hardness of the hard coat film after the hard coat layer is laminated. More preferably, it is 1.515 or less, even more preferably 1.510 or less, particularly preferably 1.505 or less, and most preferably 1.500 or less. A low refractive index in the thickness direction is preferable, but for stable production, it is preferable to have a refractive index of 1.3 or more, and even more preferably 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 and folding directions. However, in order to control the refractive index in the thickness direction while controlling the refractive index in the bending and width directions to a desirable range, it is preferable to set the conditions while checking the balance of each process condition in the film formation process.

[0055] Methods for controlling the refractive index in the thickness direction to the above range include setting the stretching preheating temperature, stretching temperature, and stretching ratio in the bending direction, the stretching preheating temperature and stretching temperature in the folding direction, 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 the 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°C to 120°C. The stretching ratio in 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 for high-magnification stretching, multi-stage stretching may be used. In this 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.

[0056] (Regarding the density of polyester film) The polyester film, which has an easy-adhesion resin layer and is laminated with a hard coat layer, has a density of 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. Note that the density of the polyester film having the easy-adhesion resin layer before lamination of the hard coat layer is negligibly small compared to the density of the polyester film alone without the easy-adhesion resin layer.

[0057] It is preferable that the bending direction of the polyester film 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.

[0058] Furthermore, in the present invention, the polyester film has an easy-adhesion resin layer and is prepared before laminating the hard coat layer. (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.

[0059] 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, controlling the refractive index and density of the film within the above-mentioned preferred range is a particularly preferred embodiment. 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-coated film after lamination of the hard-coat layer.

[0060] 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 end of the film is gripped with a clip, guided into a hot air zone heated to 75 to 120°C, dried, and then stretched 4.0 to 6.0 times, more preferably 4.4 to 6.0 times, in the width direction. Subsequently, it is guided into a heat treatment zone at 180 to 240°C, and heat treatment can be performed for 1 to 60 seconds. During this heat treatment process, if necessary, relaxation treatment of 0 to 10% may be performed in the width direction or the longitudinal direction.

[0061] The intrinsic viscosity of the polyester film preferably ranges from 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 is unlikely to occur. 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.

[0062] (Easy-adhesive resin layer) In the present invention, in order to improve the adhesiveness between the polyester film and the hard coat layer, etc., it is preferable to laminate an easy-adhesive resin layer on the polyester film. The easy-adhesive resin layer can be obtained by so-called in-line coating, in which a coating solution for forming the easy-adhesive resin layer is applied to one or both sides of an unstretched or uniaxially stretched film in the longitudinal direction, heat-treated and dried as necessary, and further stretched 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-adhesive layer is preferably controlled to 0.005 to 0.20 g / m 2 It is preferable to manage it to. 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.

[0063] The resin to be included in the coating solution used for laminating the easy-adhesion layer can be any resin, such as polyester resin, polyether polyurethane resin, polyester polyurethane resin, polycarbonate polyurethane resin, or acrylic resin, without any particular limitations. However, it is preferable to include polyester resin in terms of high adhesion to the polyester film and refractive index. Furthermore, among the dicarbon and diol components that make up the polyester resin, it is preferable to have a polyester resin copolymerized with a naphthalenedicarboxylic acid component as at least a part of the dicarboxylic acid component, which can increase the refractive index of the easy-adhesion resin layer. In addition, a crosslinked structure may be formed in the binder resin contained in the easy-adhesion resin layer in order to improve the adhesion durability of these easy-adhesion resin layers. 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, and self-crosslinking polyurethane resins may also be included. Two or more crosslinking agents may also be used in mixture form. Due to the properties of the above-mentioned inline coat, it is preferable to coat with an aqueous coating solution, and the resins and crosslinking agents are preferably water-soluble or water-dispersible resins or compounds.

[0064] The polyester resin contained in the easily adhesive resin layer is preferably a linear polyester composed of a dicarboxylic acid component and a diol component (glycol component).

[0065] Examples of the dicarboxylic acid components mentioned above include terephthalic acid, isophthalic acid, phthalic acid, 2,6-naphthalenedicarboxylic acid, 4,4-diphenyldicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, adipic acid, sebacic acid, phenylindanedicarboxylic acid, and dimer acid. Two or more of these components can be used. Furthermore, small amounts of unsaturated polybasic acids such as maleic acid, fumaric acid, and itaconic acid, and hydroxycarboxylic acids such as p-hydroxybenzoic acid and p-(β-hydroxyethoxy)benzoic acid can be used together with these components. The proportion of unsaturated polybasic acid components and hydroxycarboxylic acid components is 10 mol% or less, preferably 5 mol% or less.

[0066] By incorporating a naphthalenedicarboxylic acid-derived component as the dicarboxylic acid component of the above-mentioned polyester resin, the refractive index increases, making it easier to control the iridescent coloration under fluorescent light. Furthermore, it becomes possible to improve the heat and humidity resistance. Of course, the polymerization and copolymerization processes for incorporating the naphthalenedicarboxylic acid component into the polyester can be either the direct polymerization method or the transesterification method, and the dicarboxylic acid component, such as naphthalenedicarboxylic acid, may be introduced in the form of its ester derivative.

[0067] As the naphthalenedicarboxylic acid described above, 2,6-naphthalenedicarboxylic acid is preferred. The proportion of the naphthalenedicarboxylic acid component in the total dicarboxylic acid components constituting the polyester resin is preferably 20 mol% or more, more preferably 30 mol% or more, even more preferably 50 mol% or more, and even more preferably 60 mol% or more. A proportion of 20 mol% or more is preferred because it has a significant effect in increasing the refractive index of the easily bondable resin layer. The proportion of the naphthalenedicarboxylic acid component in the total dicarboxylic acid components constituting the polyester resin may be 100 mol%, but it is more preferable to be 95 mol% or less for the flexibility of the easily bondable resin layer.

[0068] Within the limits of achieving the effects of the present invention, further glycol components in the polyester resin may include ethylene glycol, 1,3-propane glycol, propylene glycol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol, diethylene glycol, 1,4-cyclohexanedimethanol, xylene glycol, dimethylolpropionic acid, glycerin, trimethylolpropane, poly(ethyleneoxy)glycol, poly(tetramethyleneoxy)glycol, alkylene oxide adducts of bisphenol A, etc., and two or more of these can be used.

[0069] Furthermore, considering the weight of the crosslinking agent in the easily bondable resin layer, flexibility tends to be lost, and there is a risk that cracks may occur in the easily bondable resin layer after the bending test. In such cases, it is preferable to include a dicarboxylic acid component represented by the following formula (1) and / or a diol component represented by the following formula (2) in the polyester resin. (1) HOOC - (CH2)n - COOH (wherein n is an integer between 4 and 10) (2) HO-(CH2)n-OH (wherein n is an integer between 4 and 10)

[0070] Thus, by including dicarboxylic acid and / or diol components having carbon components of a specific length, flexibility can be imparted to the polyester resin, making it easier to maintain the coated film even after bending tests, and suppressing cracks originating from particle aggregates. Examples of the dicarboxylic acid component in formula (1) include adipic acid, sebacic acid, and azelaic acid. Examples of the diol component in formula (2) include 1,4-butanediol and 1,6-hexanediol.

[0071] Polyester resin can be used in the form of water, a water-soluble organic solvent (for example, an aqueous solution containing less than 50% by mass of alcohol, alkyl cell solver, ketone, or ether), or an organic solvent (for example, toluene, ethyl acetate, etc.) that has been dissolved or dispersed in it.

[0072] When polyester resin is used as an aqueous coating solution, water-soluble or water-dispersible polyester resin is used. To achieve such water solubility or water dispersion, it is preferable to copolymerize a compound containing a sulfonic acid base or a compound containing a carboxylic acid base.

[0073] The number-average molecular weight of the polyester resin is preferably 5,000 to 40,000, from the viewpoint of coating film strength and ease of water dispersion. More preferably it is 10,000 to 30,000, and particularly preferably 12,000 to 25,000.

[0074] The solid content of the resin polyester resin in the solid content of the easily adhesive resin layer is preferably 20% by mass or more and 90% by mass or less, from the viewpoint of adhesion and refractive index adjustment. More preferably, it is 30% by mass or more and 80% by mass or less. The polyester resin may be a single type or a blend of two or more types. In the case of a blend of two or more types, the total composition of the polyester resin components is preferably as described above.

[0075] (urethane resin) The urethane resin that can be used in the easy-adhesion resin layer contains at least a polyol component and a polyisocyanate component as constituent elements, and further contains a chain extender as needed. The above urethane resin is a polymer compound in which these constituent elements are copolymerized mainly by urethane bonds. Including polycarbonate polyol as a constituent element of the urethane resin is one preferred form because it can give flexibility to the coating film. These constituent elements of the urethane resin can be identified by nuclear magnetic resonance analysis or other methods.

[0076] (Polyurethane resin with a polycarbonate backbone) It is preferable that the diol component, which is a constituent of the polyurethane resin having a polycarbonate skeleton, contains an aliphatic polycarbonate polyol that has excellent heat resistance and hydrolysis resistance. In the optical applications of the present invention, it is preferable to use an aliphatic polycarbonate polyol from the viewpoint of preventing yellowing.

[0077] Examples of aliphatic polycarbonate polyols include aliphatic polycarbonate diols and aliphatic polycarbonate triols, but aliphatic polycarbonate diols are preferably used. Examples of aliphatic polycarbonate diols that are components of the urethane resin of the present invention include aliphatic polycarbonate diols obtained by reacting one or more diols such as ethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 3-methyl-1,5-pentanediol, 1,6-hexanediol, 1,9-nonanediol, 1,8-nonanediol, neopentyl glycol, diethylene glycol, dipropylene glycol, 1,4-cyclohexanediol, and 1,4-cyclohexanedimethanol with carbonates such as dimethyl carbonate, diphenyl carbonate, ethylene carbonate, and phosgene.

[0078] Examples of polyisocyanates that are components of the urethane resin of the present invention include aromatic aliphatic diisocyanates such as xylylene diisocyanate, alicyclic diisocyanates such as isophorone diisocyanate and 4,4-dicyclohexylmethane diisocyanate and 1,3-bis(isocyanatemethyl)cyclohexane, aliphatic diisocyanates such as hexamethylene diisocyanate and 2,2,4-trimethylhexamethylene diisocyanate, or polyisocyanates obtained by pre-adding these compounds, one or more of them, with trimethylolpropane or the like. The above-mentioned polyisocyanates are preferable for optical applications where high transparency is required and there is no problem of yellowing. Furthermore, such polyisocyanates are preferable because the coating film does not become too rigid, stress due to shrinkage and swelling of photocurable resins can be relieved, and adhesion is maintained.

[0079] To impart water solubility to urethane resin, sulfonic acid (salt) groups or carboxylic acid (salt) groups can be introduced (copolymerized) into the urethane molecular backbone. Since sulfonic acid (salt) groups are strongly acidic and their hygroscopic properties can make it difficult to maintain moisture resistance, it is preferable to introduce weakly acidic carboxylic acid (salt) groups. Nonionic groups such as polyoxyalkylene groups can also be introduced.

[0080] To introduce carboxylic acid (salt) groups into urethane resin, for example, a polyol compound having carboxylic acid groups, such as dimethylolpropionic acid or dimethylolbutanoic acid, is introduced as a copolymer component and neutralized with a salt-forming agent. Specific examples of salt-forming agents include ammonia, trialkylamines such as trimethylamine, triethylamine, triisopropylamine, tri-n-propylamine, and tri-n-butylamine, N-alkylmorpholines such as N-methylmorpholine and N-ethylmorpholine, and N-dialkylalkanolamines such as N-dimethylethanolamine and N-diethylethanolamine. These can be used individually or in combination of two or more.

[0081] When a polyol compound having a carboxylic acid (salt) group is used as a copolymer component to impart water solubility, the molar ratio of the polyol compound having a carboxylic acid (salt) group in the urethane resin is preferably 3 to 60 mol%, and more preferably 5 to 40 mol%, when the total polyol component of the urethane resin is considered to be 100 mol%. A molar ratio of 3 mol% or more is preferable because it provides good water dispersibility. Furthermore, a molar ratio of 60 mol% or less is preferable because it maintains water resistance and heat and humidity resistance.

[0082] In the present invention, the glass transition temperature of the urethane resin is preferably less than 0°C, and more preferably less than -5°C. A glass transition temperature of less than 0°C is preferable because it easily provides suitable flexibility from the viewpoint of stress relaxation of the coated layer.

[0083] To form a crosslinked structure within the easily adhering resin layer, the easily adhering resin layer may contain a crosslinking agent. Including a crosslinking agent further improves adhesion under high temperature and high humidity conditions. Specific examples of crosslinking agents include urea-based, epoxy-based, melamine-based, isocyanate-based, oxazoline-based, and carbodiimide-based agents. Among these, melamine-based, isocyanate-based, oxazoline-based, and carbodiimide-based crosslinking agents are preferred due to their long-term stability of the coating solution and their effect on improving adhesion under high temperature and high humidity treatment. Furthermore, catalysts and other appropriate measures may be used as needed to promote the crosslinking reaction.

[0084] When a crosslinking agent is included in the easy-to-adhere resin layer, the crosslinking agent content is preferably 5% by mass or more and 50% by mass or less of the total solid components of the coated layer. More preferably, it is 10% by mass or more and 40% by mass or less. If it is 10% by mass or more, the strength of the resin in the easy-to-adhere resin layer is maintained and adhesion under high temperature and high humidity conditions is good. If it is 40% by mass or less, the flexibility of the resin in the coated layer is maintained and adhesion after repeated folding tests at room temperature and under high temperature and high humidity conditions is preferred.

[0085] In the present invention, it is preferable that the easily adhering resin layer contains at least one compound selected from titanium compounds and zirconium compounds. The iridescent coloration (interference spots) of the hard coat film is said to occur because of the large difference between the refractive index of the polyester film substrate (e.g., 1.62 to 1.65) and the refractive index of the hard coat layer made of acrylic resin or the like (e.g., 1.52). In order to prevent the occurrence of interference spots by reducing the refractive index difference between the laminates, it is important to control the refractive index of the easily adhering resin layer so as to reduce the refractive index difference between the polyester film and the easily adhering resin layer, and between the easily adhering resin layer and the hard coat layer. When controlling the refractive index of the easily adhering resin layer, which consists of the main component binder resin or particles, the control becomes easier by including the above-mentioned compound with a high refractive index. Examples of titanium compounds include water-soluble titanium chelate compounds, water-soluble titanium acylate compounds, titanium oxide, and titanium chloride, among which titanium dioxide (titania) is preferably used. Examples of zirconium compounds include water-soluble zirconium chelate compounds, water-soluble zirconium acylate compounds, zirconium acetate, zirconium hydroxide, and zirconium oxide, among which zirconium dioxide (zirconia) is preferably used. The aforementioned compounds with a high refractive index are also preferably in particulate form.

[0086] The average particle size of metal oxide particles such as zirconium dioxide is preferably between 5 nm and 150 nm. More preferably between 10 nm and 100 nm, and even more preferably between 30 nm and 70 nm.

[0087] By designing the average particle size of the metal oxide fine particles to be within the above range, film haze can be reduced. Furthermore, it is desirable that the particulate metal compound be designed with a particle size lower than the film thickness of the easily adhesive resin layer in order to obtain a buffering effect when repeatedly folded, and a particle size that does not easily cause particle aggregation is preferable. Suppressing particle aggregation is desirable because it prevents the easily adhesive resin layer from becoming the starting point for cracks, etc., when folded.

[0088] The amount of particulate metal compound added is preferably designed to be between 0.1% by mass and less than 15% by mass relative to the easily adhesive resin layer, in order to provide a buffering effect when the easily adhesive resin layer is folded, and conversely, to prevent the initiation of cracks due to particle aggregation. Furthermore, a concentration of 0.5% by mass and 14% by mass or less is preferable, and more preferably 1% by mass and 13% by mass or less. Adding particles within this range makes it difficult for aggregated particles to form in the easily adhesive resin layer, and since cracks do not form in the easily adhesive resin layer when repeatedly folded by these aggregated particles, interference spots are less likely to occur after folding tests, which is preferable.

[0089] In the easily bondable resin layer, it is preferable that the proportion of particulate metal compounds in the folding direction is lower than in the bending direction. Since fine cracks that occur during repeated folding tend to occur in the folding direction where load is applied in the thickness direction, it is thought that crack formation can be suppressed by reducing the particle frequency in the folding direction. It is thought that reducing the particle frequency can reduce the particle aggregation rate and the frequency of crack propagation. The proportion of particles in each direction can be confirmed by transmission electron microscopy (TEM) observation of the cross-section.

[0090] As a method for changing the directional proportions of particulate metal compounds in the easily adhesive resin layer, formation by an in-line coating method is preferred. In the in-line coating method, there is a step of stretching the coating solution containing the easily adhesive resin material in at least one direction after coating, so it is thought that the proportions of particles in the easily adhesive resin layer can be changed by adjusting the stretching ratio.

[0091] A dispersant may be used in combination with particulate metal compounds to prevent aggregation. The dispersant used in this invention can be any polymer compound that can maintain the binder resin consisting of an emulsion, dissolve or disperse the crosslinking agent described later, and disperse metal oxide fine particles.

[0092] Specifically, known polymer dispersants such as polyvinyl, polyacrylic acid, polycarboxylic acid, and polyurethane can be used. More specifically, as polyvinyl polymers, polyvinyl alcohol, polyvinylpyrrolidone, polyvinyl acetate, polyvinyl esters, etc. and copolymers thereof can be used; as polyacrylic acid polymers, polyacrylic acid, sodium polyacrylate, ammonium polyacrylate, etc. and copolymers thereof can be used; as polycarboxylic acid polymers, polycarboxylic acid, sodium polycarboxylate, ammonium polycarboxylate, etc. and copolymers thereof can be used; as polyurethane polymers, polyurethane, etc. and copolymers thereof can be used, and copolymers with these copolymers or sulfonic acid polymers can also be used. Among these, acrylic resins such as polyacrylic acid polymers are preferred as dispersants for metal fine particles.

[0093] It is more preferable that the metal oxide particles have a dispersant on part or all of their surface. The inclusion of a dispersant has the effect of suppressing the aggregation of metal oxide particles. By including particles to which the easily adhesive resin layer is applied, stress relaxation during repeated folding tests can be expected while maintaining the transparency of the coating film. The inclusion of a dispersant is preferable because, depending on the coating film formation process, aggregation of metal oxide particles in the easily adhesive resin layer is less likely to occur, thus reducing the likelihood of cracking.

[0094] The method for surface-treating metal oxide particles with acrylic resin is not particularly limited, but specific examples include adding a pre-mixed mixture of metal oxide particles and acrylic resin to a solvent and then dispersing it, or sequentially adding metal oxide particles and acrylic resin to a solvent and then dispersing them.

[0095] Equipment that can be used for these dispersions includes dissolvers, high-speed mixers, homomixers, mixers, ball mills, roll mills, sand mills, paint shakers, SC mills, annular mills, pin mills, etc.

[0096] The amount of dispersant added to the metal oxide particles is preferably between 5% by mass and less than 40% by mass, based on the mass of the metal oxide particles. When the amount of dispersant added is 5% by mass or more, it is preferable because a good dispersion state of the metal oxide particles can be obtained in the easily adhesive resin layer. When the amount of dispersant added is less than 40% by mass, it is preferable because it is easier to adjust the refractive index of the easily adhesive resin layer while taking advantage of the characteristics of the metal oxide particles. Furthermore, an amount of dispersant added between 10% by mass and 30% by mass is even more preferable.

[0097] In this invention, the refractive index of the easily adhesive resin layer is designed to be within a certain range, and by satisfying the thin-film interference principle, iridescent coloration (interference spots) can be reduced. Furthermore, by filling the easily adhesive resin layer with a certain amount of the metal compound used for refractive index adjustment, it is believed that damage to the easily adhesive resin layer during repeated folding is mitigated.

[0098] The thickness of the easily adhesive resin layer that can suppress iridescent coloration (interference spots) should be adjusted so as to satisfy the equation 2nd = λb / 4. Here, n is the refractive index of the easily adhesive resin layer, d is the thickness of the easily adhesive resin layer, and λb is the bottom wavelength of the reflectance spectrum, which can be appropriately set within the range of 450 to 650 nm.

[0099] In the present invention, the polyester film having an easy-adhesion resin layer is designed so that the refractive index differs in the bending direction and the folding direction. Therefore, it is preferable to control the refractive index of the easy-adhesion resin layer considering each direction.

[0100] For a polyester film having an easy-adhesion resin layer and before lamination of a hard coat layer, it is desirable that the refractive index of the easy-adhesion resin layer be controlled to be low relative to the refractive index in the bending direction of the polyester film. The range of the refractive index difference between the easy-adhesion resin layer and the bending direction is preferably greater than 0 and 0.070 or less. More preferably between 0.005 and 0.065. Even more preferably between 0.010 and 0.060. It is preferable that the refractive index of the easy-adhesion resin layer is lower than that of the polyester film in the bending direction, as this reduces the refractive index difference with the laminated hard coat layer, effectively suppressing iridescent coloration (interference spots). It is also preferable that the refractive index difference with the polyester film is 0.070 or less, as this prevents the refractive index difference from becoming too large, effectively suppressing iridescent coloration (interference spots). In the bending direction, compressive stress is applied when the easy-adhesion resin layer is bent inward, and tensile stress is applied when it is bent outward. Therefore, it is preferable to design the resin component with a higher proportion of amorphous resin rather than a higher proportion of crystalline resin, and it is also preferable to add an appropriate amount of metal compound to compensate for the refractive index.

[0101] It is desirable that the refractive index of the easy-adhesion resin layer be controlled to be low even with respect to the refractive index in the direction of the folded portion of the polyester film before lamination of the hard coat layer, which has an easy-adhesion resin layer. The range of the difference between the refractive index of the easy-adhesion resin layer and the refractive index in the direction of the folded portion is preferably 0.080 or more and 0.150 or less. More preferably 0.085 or more and 0.14 or less. Even more preferably 0.090 or more and 0.13 or less. When the refractive index is lower than that in the direction of the folded portion of the polyester film, the refractive index difference with the laminated hard coat layer becomes small, and iridescent coloration (interference spots) is effectively suppressed, which is preferable. Also, when the refractive index difference is 0.150 or less, the refractive index difference with the polyester film does not become too large, and iridescent coloration (interference spots) is effectively suppressed, which is preferable.

[0102] It is preferable to add particles to the easy-adhesion layer to impart 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. To give the coated layer appropriate slipperiness, silica particles with an average particle size of 200 nm to 700 nm are particularly preferred.

[0103] Furthermore, the amount of particles added to impart slipperiness is preferably less than 1% by mass relative to the easily adhesive resin layer. When it is less than 1% by mass, there are few particles in the easily adhesive resin layer that are larger than the thickness of the layer, which is preferable because it makes it difficult for cracks to propagate when folded. A further preferable embodiment is 0.5% by weight or less.

[0104] As for the method of applying the coating solution, known methods similar to those described above for the coating layer can be used. 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.

[0105] (Hard coat layer) When the polyester film of the present invention is used as a surface protective film to protect a foldable display, it is preferable that at least one of its surfaces has a hard coat layer. The hard coat layer is preferably positioned on the display surface side of the polyester film when used in the display. The resin forming the hard coat layer is not particularly limited and can be siloxane-based, inorganic hybrid-based, acrylic-based, urethane acrylate-based, polyester acrylate-based, epoxy-based, etc. Furthermore, two or more materials can be mixed and used, or particles such as inorganic fillers or organic fillers can be added.

[0106] Examples of resins that form the hard coat layer include compounds having (meth)acrylate-based functional groups, such as polyester (meth)acrylate, urethane (meth)acrylate, epoxy (meth)acrylate, and silicone (meth)acrylate, as well as compounds having functional groups with unsaturated double bonds, such as allyl groups and vinyl groups. In addition, polyfunctional monomers may be used in combination to increase the hardness of the hard coat layer. Examples of polyfunctional monomers include trimethylolpropane tri(meth)acrylate, hexanediol (meth)acrylate, tripropylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritol hexa(meth)acrylate, 1,6-hexanediol di(meth)acrylate, and neopentyl glycol di(meth)acrylate. The above materials may be used individually or mixed together.

[0107] When ultraviolet light is the active energy ray used to cure the hard coat layer, it is preferable to add a photopolymerization initiator. The photopolymerization initiator may be a radical polymerization system, a cationic polymerization system, or a mixture of cationic and radical polymerization systems, but a radical polymerization system is particularly preferred because it has a high reaction rate and excellent productivity. Examples of ultraviolet radical polymerization initiators include alkylphenones, benzoins, phosphine oxides, ketals, anthraquinones, thioxanthones, azo compounds, peroxides, 2,3-dialkyldione compounds, disulfide compounds, fluoroamine compounds, aromatic sulfonium compounds, titanocenes, and phenyl oxyacetates, which may be used individually or in combination of two or more. More specific examples include carbonyl compounds such as acetophenone, 2,2-diethoxyacetophenone, p-dimethylacetophenone, p-dimethylaminopropiophenone, benzophenone, 2-chlorobenzophenone, 4,4'-dichlorobenzophenone, 4,4'-bisdiethylaminobenzophenone, Michler ketone, benzyl, benzoin, benzoin methyl ether, benzoin ethyl ether, benzoin isopropyl ether, methylbenzoyl phosphate, p-isopropyl-α-hydroxyisobutylphenone, α-hydroxyisobutylphenone, 2,2-dimethoxy-2-phenylacetophenone, and 1-hydroxycyclohexylphenyl ketone; sulfur compounds such as tetramethylthiuram monosulfide, tetramethylthiuram disulfide, thioxanthone, 2-chlorothioxanthone, and 2-methylthioxanthone; and peroxide compounds such as benzoyl peroxide and di-t-butyl peroxide. The amount of photopolymerization initiator added can be in the range of 0.1 parts by mass or more, more preferably 1 part by mass or more, 30 parts by mass or less, and more preferably 20 parts by mass or less, per 100 parts by mass of the active energy ray curable resin. An amount of 0.1 parts by mass or more is preferable as it allows for high hardness of the hard coat layer. An amount of 30 parts by mass or less is also preferable as it prevents yellowing of the hard coat layer and ensures sufficient hardening of the hard coat layer.

[0108] Furthermore, various additives can be included within a range that does not impair the performance of the hard coat. Examples of such additives include polymerization inhibitors, crosslinking agents, antistatic agents, adhesion improvers, antioxidants, leveling agents, coupling agents, defoaming agents, fillers, solvents, anti-glare agents, anti-reflective agents, inorganic fillers, and organic fillers.

[0109] The refractive index of the hard coat layer is preferably smaller than that of the easy-adhesion resin layer, from the viewpoint of suppressing iridescent coloration (interference spots) of the hard coat film.

[0110] (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 allows for sufficient curing. To increase pencil hardness, a thickness of 5 μm or more is even more preferable. Furthermore, by keeping the thickness to 50 μm or less, curling due to hard coat curing shrinkage can be suppressed, improving the handling properties of the film.

[0111] (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.

[0112] (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.

[0113] (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.

[0114] (Characteristics of the hard coat layer) 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 transmittance of the hard coat film is preferably 85% or higher, more preferably 87% or higher, and even more preferably 88% or higher. If the transmittance is 85% or higher, sufficient visibility can be obtained. The total light transmittance of the hard coat film is generally preferable as it is higher, but from the standpoint of stable production, it is preferable to have 99% or less, and may also be 97% or less. In addition, the haze of the hard coat film is generally preferable to be low, preferably 3% or less. The haze of the hard coat film is 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 haze is generally preferable as it is lower, but from the standpoint of stable production, it is preferable to have 0.1% or more, and may also be 0.3% or more.

[0115] 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, and an anti-static layer is also preferably applied in the present invention.

[0116] 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 a refractive index 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 the refractive index adjustment layer, or a separate refractive index adjustment layer may be laminated. [Examples]

[0117] Next, the present invention will be described using examples and comparative examples. First, the method for evaluating characteristic values ​​performed with the polyester film of the present invention is shown below.

[0118] (1) Intrinsic viscosity After crushing and drying the film or polyester resin, it was 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.

[0119] (2) Density The density was measured according to the method compliant with JIS K7112:1999 (density gradient pipe method). (Unit: g / cm³) 3 ). (3) Average particle size of particles in polyester film or easy-to-adhere resin layer The method involves observing the particles in the cross-section of the film using a scanning electron microscope, observing 50 particles, and using the average value to determine the average particle diameter. For irregularly shaped particles that are not spherical, the particle diameter can be calculated as the equivalent diameter. The equivalent diameter is obtained by dividing the area of ​​the observed particle by pi (π), calculating the square root, and multiplying by two. (The unit depends on the average particle diameter, but nm is mainly used).

[0120] (4) Refractive index of polyester film having an easy-adhesion resin layer before lamination of the hard coat layer In accordance with JIS K7142:2008 "Method for measuring the refractive index of plastics (Method A)" Using an Abbe refractometer (NAR-4T, manufactured by Atago Corporation, measurement wavelength 589 nm), the refractive index in the longitudinal direction, the refractive index in the width direction, and the refractive index in the thickness direction of a polyester film having an easy-adhesion resin layer before lamination of a hard coat layer were determined.

[0121] (5) Refractive index of the easily bonded resin layer The refractive index of the easy-adhesion resin layer can be calculated by fitting the reflection spectrum measured using a spectrophotometer (product name "UV-3150", manufactured by Shimadzu Corporation) with the reflection spectrum calculated from an optical model of the thin film using Fresnel coefficients. To prevent back surface reflection, a black vinyl tape wider than the measurement spot area (for example, product name "Yamato Vinyl Tape NO200-38-21", manufactured by Yamato, 38 mm wide) should be attached to the side of the PET substrate opposite to the side on which the easy-adhesion resin layer is formed (the back surface; if easy-adhesion resin layers are formed on both sides, the surface of the easy-adhesion resin layer on the side on which the refractive index is not to be measured) before measurement.

[0122] (6) 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.

[0123] (7) Flexural resistance of polyester film samples (flexural radius 0.5 mm) Using the same method as the bending test described above, the bending radius was set to 0.5 mm and the display was bent 200,000 times at a speed of 1 time / second. Here, Figure 1 is a schematic diagram to show the bending radius when a foldable display is folded, and considering the case where a polyester film is placed on the inner surface of the folded state, the bending test is performed model by setting the location indicated by reference numeral 11 in Figure 1 to 0.5 mm. The outer film surface of the bent part was observed with a digital microscope (HIROX RH8800) at 700x magnification to check for the presence or absence of wrinkles (cracks). Separate from the above-mentioned visual test of bending resistance with a bending radius of 1.5 mm, this test, with a reduced bending radius of 0.5 mm, is intended to evaluate the foldable display in a state closer to the actual usage condition, where the hard coat layer and other components are laminated or attached. This test is intended to detect minute defects that are difficult to detect by visual inspection, such as defects that make the display prone to breakage or cracking, separate from the visual observation with a bending radius of 1.5 mm. ○: No defects on the outer surface of the film when bent. ×: Wrinkles (cracks) can be seen on the film surface on the outside of the break or bend.

[0124] (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 × 10 6 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

[0125] (9) Total light transmittance, haze A polyester film laminated with an easy-adhesion resin layer was used as a sample and measured using a haze meter (NDH5000, manufactured by Nippon Denshoku Industries Co., Ltd.).

[0126] (10) Maximum thermal contraction A sample film of polyester film laminated with an easy-adhesion resin layer was cut to 10 mm vertically x 250 mm horizontally. Marks were made at 200 mm intervals along the longer side, aligned with the direction of measurement, and the distance between the marks A 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 between the marks B 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 positions that divide the width of the sample film into three equal parts, and the average value 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 (%).

[0127] (11) Refractive index of the hard coat layer The refractive index of the hard coat layer can be calculated by fitting the reflection spectrum measured using a spectrophotometer (product name "UV-3150", manufactured by Shimadzu Corporation) with the reflection spectrum calculated from an optical model of a multilayer thin film using Fresnel coefficients. The reflectance of the hard coat layer is measured by applying the hard coat composition to a 50 μm thick polyethylene terephthalate (PET) substrate without an easy-adhesion resin layer, curing it to form a hard coat layer with a thickness of 1 to 10 μm, and then attaching a black vinyl tape with a width larger than the measurement spot area (for example, product name "Yamato Vinyl Tape NO200-38-21", manufactured by Yamato, 38 mm wide) to the back surface of the PET substrate opposite to the surface on which the hard coat layer is formed, in order to prevent back surface reflection.

[0128] (12) Improvement of interference spots in hard coat films (iridescent coloration) A hard-coat film was cut to an area of ​​50 mm (film width direction) x 110 mm (film length direction) to create a sample film. A black glossy tape (Nitto Denko, vinyl tape No. 21; black) was attached to the side of the obtained sample film opposite the hard-coat layer. The hard-coat side of this sample film was placed facing upwards, and a three-wavelength daylight white light (National Palook, FL 15EX-N 15W) was used as a light source. The film was observed from an oblique angle above at a position where the strongest reflection was visible (distance from the light source 40-60 cm, angle of 15-45° to the film surface).

[0129] The results of visual observations will be ranked according to the following criteria. The observations will be conducted by five people familiar with the evaluation, and the rank with the most votes will be adopted as the evaluation rank. In the event of a tie between two ranks, the middle rank among the three will be adopted. For example, if there are two votes each for ◎ and ○ and one vote for △, ○ will be adopted. If there is one vote for ◎ and two votes each for ○ and △, ○ will be adopted. If there are two votes each for ◎ and △ and one vote for ○, ○ will be adopted. ◎: No iridescent coloration is visible from any angle. ○: A slight iridescent sheen can be seen from certain angles. △: Slight iridescent coloration is observed. ×: Clear iridescent coloration is observed.

[0130] (13) Flexural testability of hard coat film A sample film was prepared by cutting a hard coat film to a size of 50 mm in the width direction and 110 mm in the flow direction. Using the obtained sample film, a bending test was performed 200,000 times at a speed of 1 time / second, with the bending radius set to 3.0 mm, by folding the film so that the easy-adhesion resin layer and the hard coat layer were on the inside, similar to the bending test method described above. After the test, the bent portion of the sample film 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. ×: Cracks or fold marks are observed in the sample.

[0131] (14) Observation of interference spots (iridescent coloration) after bending test of hard coat film The same observations as for interference spot improvement were performed on the bent portion of the sample film after the bending test of the hard-coated film described above. Specifically, black glossy tape (Nitto Denko, vinyl tape No. 21; black) was attached to the side of the obtained sample film opposite the hard-coated layer. The hard-coated side of this sample film was placed facing upwards, and a three-wavelength daylight white light (National Palook, FL 15EX-N 15W) was used as a light source. The film was observed from an oblique angle above at a position where the strongest reflection was observed (distance from the light source 40-60 cm, angle of 15-45° to the film surface). The results of the visual observation were ranked according to the following criteria. The observations were performed by five people familiar with the evaluation, and the rank with the most votes was adopted as the evaluation rank. In the case of a tie between two ranks, the middle of the three divided ranks was adopted. For example, if there are 2 people each with ◎ and ○ and 1 person with △, we will select ○. If there is 1 person with ◎ and 2 people each with ○ and △, we will select ○. If there are 2 people each with ◎ and △ and 1 person with ○, we will select ○. ◎: No iridescent coloration is visible from any angle. ○: A slight iridescent sheen can be seen from certain angles. △: Slight iridescent coloration is observed. ×: Clear iridescent coloration is observed.

[0132] This test is conducted with the aim of detecting interference spots caused by minute adhesion defects such as delamination and cracks at the interface between the hard coat layer, the easy-adhesion resin layer, and the polyester film layer, by evaluating the interference spots.

[0133] (15) Pencil hardness The pencil hardness of the hard coat film was measured using a sample, 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 hardness of 3H or higher was considered acceptable.

[0134] (Preparation of polyethylene terephthalate pellets (R1)) 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 from the second esterification reactor was continuously removed from the system and supplied to the third esterification reactor. Further, an EG solution containing TMPA in an amount that resulted in 20 ppm of P atoms 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 reactor 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 (R1) with an intrinsic viscosity of 0.62 dl / g.

[0135] (Preparation of polyethylene terephthalate pellets (R2)) The manufacturing process for polyethylene terephthalate pellets (R1) 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 (R2).

[0136] (Preparation of polyethylene terephthalate pellets (R3)) Polyethylene terephthalate pellets (R1) 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 (R3) with an intrinsic viscosity of 0.75 dl / g.

[0137] (Polymerization of copolymerized polyester resins) Polymerization of copolymer polyester resins (a1) to (a3) ​​for forming an easily adhesive resin layer was carried out as follows. In a stainless steel autoclave equipped with a stirrer, thermometer, and partial reflux condenser, 410.3 parts by mass of dimethyl 2,6-naphthalenedicarboxylate, 46.3 parts by mass of sebaciic acid, 42.5 parts by mass of dimethyl sodium 5-sulfoisophthalate, 175.6 parts by mass of ethylene glycol, 29.2 parts by mass of diethylene glycol, 204.2 parts by mass of 1,6-hexanediol, and 0.5 parts by mass of tetra-n-butyl titanate were charged, and a transesterification reaction was carried out from 160°C to 220°C for 4 hours. The temperature was then raised to 255°C, the reaction system was gradually depressurized, and the reaction was carried out under reduced pressure of 30 Pa for 1 hour and 30 minutes to obtain copolymer polyester resin (a1). The obtained copolymer polyester resin was pale yellow and transparent. The composition of copolymer polyester resin (a1) is shown in Table 1.

[0138] Furthermore, copolymer polyester resins (a2) and (a3) ​​with the compositions listed in Table 1 were obtained in the same manner by changing the raw materials. The composition and weight-average molecular weight results measured by 1H-NMR for these copolymer polyester resins are shown in Table 1.

[0139] [Table 1]

[0140] (Preparation of aqueous dispersion of copolymerized polyester resin) In a reactor equipped with a stirrer, thermometer, and reflux device, 25 parts by mass of copolymer polyester resin (a1) and 15 parts by mass of ethylene glycol t-butyl ether were added and heated at 110°C, and the resin was stirred to dissolve it. After the resin was completely dissolved, 60 parts by mass of water were gradually added to the polyester solution while stirring. After the addition, the liquid was cooled to room temperature while stirring to prepare an aqueous dispersion of the copolymer polyester (Aw-1) with a solid content of 25% by mass and a milky white color. Similarly, aqueous dispersions were prepared using copolymer polyester resins (a2) to (a3) ​​instead of copolymer polyester resin (a1), and these were designated as aqueous dispersions (Aw-2) to (Aw-3).

[0141] (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 9.03 parts by mass of triethylamine was added to obtain a polyurethane prepolymer solution. Next, 450 g of water was added to a reaction vessel equipped with a homodisperser capable of high-speed stirring, and the temperature was adjusted to 25°C for 2000 min. -1 While stirring and mixing, the isocyanate-terminated prepolymer was added and dispersed in water. Subsequently, under reduced pressure, a water-soluble polyurethane resin (B-1) with a solid content of 35% by mass was prepared by removing acetonitrile and a portion of the water.

[0142] (Preparation of aqueous solution of self-crosslinked polyurethane resin) 100 parts by mass of a polyester diol (OHV: 2000 eq / ton) consisting of adipic acid, 1,6-hexanediol, and neopentyl glycol (molar ratio: 4 / 2 / 3) was mixed with 41.4 parts by mass of xylylene diisocyanate. The mixture was reacted at 80-90°C for 1 hour under a nitrogen stream, then cooled to 60°C, and 70 parts by mass of tetrahydrofuran were added to dissolve it, yielding a urethane prepolymer solution (NCO / OH ratio: 2.2, free isocyanate groups: 3.30% by mass). Subsequently, the urethane prepolymer solution was brought to 40°C, and then 45.5 parts by mass of a 20% by mass aqueous solution of sodium bisulfite was added and the mixture was reacted at 40-50°C for 30 minutes with vigorous stirring. After confirming the disappearance of the free isocyanate group content (in terms of solid content), the solution was diluted with emulsified water to obtain an aqueous solution of self-crosslinked polyurethane resin (B-2) containing isocyanate groups blocked with 20% by mass of sodium bisulfite.

[0143] (Polymerization of blocked isocyanate compounds) In a flask equipped with a stirrer, thermometer, and reflux condenser, 100 parts by mass of a polyisocyanate compound having an isocyanurate structure derived from hexamethylene diisocyanate (Duranate TPA, manufactured by Asahi Kasei Chemicals), 55 parts by mass of propylene glycol monomethyl ether acetate, and 30 parts by mass of polyethylene glycol monomethyl ether (average molecular weight 750) were charged and held at 70°C for 4 hours under a nitrogen atmosphere. The reaction mixture temperature was then lowered to 50°C, and 47 parts by mass of methyl ethyl ketoxime were added dropwise. The infrared spectrum of the reaction mixture was measured, and the disappearance of the absorption of the isocyanate group was confirmed, yielding a block polyisocyanate solution with a solid content of 75% by mass.

[0144] (Preparation of water-dispersible blocked isocyanates) Water was added to the block polyisocyanate solution obtained above to obtain a block polyisocyanate aqueous dispersion (C-1) with a solid content of 40% by mass.

[0145] (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 (C-2) with a solid content of 40% by mass.

[0146] (Melamine-based crosslinking agent) As a melamine-based crosslinking agent, DIC Corporation's Bekkamin® M-3 (solid content concentration 60%) was used (melamine-based crosslinking agent (C-3)).

[0147] (Polymerization of oxazoline-based crosslinking agents) A flask equipped with a thermometer, nitrogen gas inlet tube, reflux condenser, dropping funnel, and stirrer was filled with a mixture of 58 parts by mass of deionized water and 58 parts by mass of isopropanol as an aqueous medium, and 4 parts by mass of polymerization initiator (2,2'-azobis(2-amidinopropane) dihydrochloride). Meanwhile, a mixture of 16 parts by mass of 2-isopropenyl-2-oxazoline as a polymerizable unsaturated monomer having an oxazoline group, 32 parts by mass of methoxypolyethylene glycol acrylate (average number of moles of ethylene glycol added: 9 moles, manufactured by Shin-Nakamura Chemical Co., Ltd.), and 32 parts by mass of methyl methacrylate was filled into the dropping funnel and added dropwise over 1 hour at 70°C under a nitrogen atmosphere. After the addition was complete, the reaction solution was stirred for 9 hours and cooled to obtain a water-soluble resin (C-4) having an oxazoline group with a solid content of 40% by mass.

[0148] (Zirconia particles) In a 3-liter glass container, 2283.6 g of pure water and 403.4 g of oxalic acid dihydrate were added and heated to 40°C to prepare a 10.72% by mass oxalic acid aqueous solution. While stirring this aqueous solution, 495.8 g of zirconium oxycarbonate powder (ZrOCO3, manufactured by AMR International Corp., containing 39.76% by mass in terms of ZrO2) was gradually added and mixed for 30 minutes, after which it was heated at 90°C for 30 minutes. Next, 1747.2 g of a 25.0% by mass tetramethylammonium hydroxide aqueous solution (manufactured by Tama Chemical Industry Co., Ltd.) was gradually added over 1 hour. At this point, the mixture was in a slurry state, and ZrO2 The slurry contained 4.0% by mass in terms of equivalent content. This slurry was transferred to a stainless steel autoclave container and subjected to hydrothermal treatment at 145°C for 5 hours. The product after this hydrothermal treatment was completely sol-formed with no undissolved gelatin. The obtained sol contained 4.0% by mass as ZrO2, had a pH of 6.8, and an average particle size of 19 nm. Furthermore, the transmittance measured when the sol was adjusted to a ZrO2 concentration of 2.0% by mass with pure water was 88%. When the particles were observed with a transmission electron microscope, most of them were aggregated particles of primary ZrO2 particles around 7 nm in size. 4000 g of the zirconia sol with a ZrO2 concentration of 4.0% by mass obtained by the above hydrothermal treatment was washed and concentrated using an ultrafiltration apparatus while gradually adding pure water to obtain 953 g of zirconia sol with a ZrO2 concentration of 13.1% by mass, a pH of 4.9, and a transmittance of 76% at a ZrO2 concentration of 13.1% by mass.

[0149] After washing and concentrating as described above, 300 g of zirconia sol with a ZrO2 concentration of 13.1% by mass was obtained. To this, 3.93 g of 20% by mass citric acid aqueous solution and 11.0 g of 25% by mass tetramethylammonium hydroxide aqueous solution were added, and the mixture was further concentrated using an ultrafiltration apparatus to obtain 129 g (D-1) of high-concentration zirconia sol with a ZrO2 concentration of 30.5% by mass. This obtained high-concentration zirconia sol had a pH of 9.3 and an average particle size of 19 nm. Furthermore, this zirconia sol was free of precipitates and remained stable for more than one month under conditions of 50°C.

[0150] (Zirconia aqueous dispersion) The zirconia sol obtained above was mixed with a polyacrylic acid dispersant (Toagosei Co., Ltd.: Aron A-30SL) to prepare a zirconia aqueous dispersion with a solid content of 13% by mass. Of the solid content, 3% by mass was the dispersant and 10% was zirconia, resulting in zirconia aqueous dispersion D-1.

[0151] (Titania particles) A white slurry with a pH of 9.5 was prepared by mixing 12.09 kg of an aqueous titanium tetrachloride solution containing 7.75% by mass of titanium tetrachloride (manufactured by Osaka Titanium Technologies Co., Ltd.) on a TiO2 basis with 4.69 kg of aqueous ammonia (manufactured by Ube Industries, Ltd.) containing 15% by mass of ammonia. Next, this slurry was filtered and washed with pure water to obtain 9.87 kg of hydrated titanate cake with a solid content of 10% by mass. To this cake, 11.28 kg of aqueous hydrogen peroxide (manufactured by Mitsubishi Gas Chemical Co., Ltd.) containing 35% by mass of hydrogen peroxide and 20.00 kg of pure water were added, and the mixture was heated at 80°C for 1 hour with stirring. Finally, 57.52 kg of pure water was added to obtain 98.67 kg of an aqueous titanium peroxide solution containing 1% by mass of titanate peroxide on a TiO2 basis. This aqueous titanium peroxide solution was transparent yellowish-brown and had a pH of 8.5.

[0152] Next, 4.70 kg of cation exchange resin (manufactured by Mitsubishi Chemical Corporation) was mixed with 98.67 kg of the titanic acid peroxide aqueous solution, and 12.33 kg of potassium stannate aqueous solution containing 1% by mass of potassium stannate (manufactured by Showa Chemical Co., Ltd.) on an SnO2 basis was gradually added under stirring. After separating the cation exchange resin that had incorporated potassium ions, etc., it was placed in an autoclave (manufactured by Pressure Glass Industry Co., Ltd., 120 L) and heated at a temperature of 165°C for 18 hours.

[0153] Next, the obtained mixed aqueous solution was cooled to room temperature and then concentrated using an ultrafiltration membrane apparatus (Asahi Kasei Corporation, ACV-3010) to obtain 9.90 kg (D-2) of an aqueous dispersion sol containing titanium-based fine particles with a solid content of 10% by mass. When the solid matter contained in the sol thus obtained was measured by the method described above, it was found to be titanium-based fine particles (primary particles) consisting of a composite oxide containing titanium and tin, having a rutile-type crystalline structure. Furthermore, when the content of metal components contained in these titanium-based fine particles was measured, the oxide equivalents of each metal component were 87.2% by mass for TiO2, 11.0% by mass for SnO2, and 1.8% by mass for K2O. The pH of the mixed aqueous solution was 10.0. Furthermore, the aqueous dispersion sol containing the titanium-based fine particles was a transparent milky white, the average particle size of the titanium-based fine particles contained in this aqueous dispersion sol was 35 nm, and the distribution frequency of coarse particles with a particle size of 100 nm or more was 0%. Furthermore, the refractive index of the obtained titanium-based nanoparticles could be estimated to be 2.42.

[0154] (Titania aqueous dispersion) The titanium-based fine particles obtained above were mixed with a polyacrylic acid dispersant (Toagosei Co., Ltd.: Aron A-30SL) to prepare a titania aqueous dispersion with a solid content of 13% by mass. Of the solid content, 3% by mass was used as the dispersant and 10% was titania to obtain titania aqueous dispersion D-2.

[0155] (Zirconia / titania mixed dispersion) The zirconia sol obtained above was mixed with titania-based fine particles and a polyacrylic acid dispersant (Toagosei Co., Ltd.: Aron A-30SL) to prepare a zirconia / titania mixed aqueous dispersion with a solid content of 13% by mass. Of the solid content, 3% by mass was used as the dispersant, and a mixed dispersion D-3 was obtained with a ratio of 7.5% zirconia and 2.5% titania.

[0156] (Silica particles) As silica particles, colloidal silica with a particle size of 40 nm and a solid content concentration of 30% by mass was used as D-4.

[0157] (Silica particles) To impart slipperiness, silica particles with a particle size of 450 nm and a solid content concentration of 40% by mass were used as D-5.

[0158] (Surfactants) To improve the leveling properties of the coating film when forming an easily adhering resin layer, a silicone-based surfactant with a solid content concentration of 100% by mass was used as E-1.

[0159] (Preparation of coating solution for easy adhesion layer formation) The following coating agents were mixed to prepare coating solution P-1. Water 47.52 parts by mass Isopropanol 25.00 parts by mass Polyester resin (Aw-1) 17.75 parts by mass Water-dispersible blocked isocyanate compound (C-1) 4.76 parts by mass Zirconia / titania mixed aqueous dispersion (D-3) 4.88 parts by mass Silica particles (D-5) 0.06 parts by mass (Silica sol with average particle size of 450 nm, solid content concentration of 40% by mass) Silicone-based surfactant (E-1) 0.03 parts by mass (Silicone-based, solid content concentration 100% by mass)

[0160] Similarly, coating solutions P-2 to P-14 were prepared using the mixing ratios of each coating agent as shown in Table 2.

[0161] [Table 2]

[0162] (Example 1) Polyethylene terephthalate pellets (R1) were supplied to an extruder and melted at 285°C. This polymer was filtered through a stainless steel sintered filter medium (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 in contact 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 resin layer forming coating liquid (P-1) 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 0.09 g / m². 2 The material was then adjusted to achieve the desired result. After that, it was guided into a tenter and preheated at 105°C, then stretched transversely to 4.0 times its original size at 95°C, the width was fixed, and heat-set at 230°C for 5 seconds, and then relaxed by 4% in the width direction at 180°C to obtain a polyethylene terephthalate film with a thickness of 50 μm.

[0163] A hard coat coating solution (H-1 below) is applied to one side of the polyethylene terephthalate film having the above-mentioned easy-adhesion resin layer using a Meyer bar so that the film thickness after drying is 10 μm. After drying at 80°C for 1 minute, ultraviolet light is irradiated (cumulative light intensity 200 mJ / cm²). 2 ), a hard coat film was obtained.

[0164] (Coating solution for hard coat layer formation: H-1) 95 parts by mass of a urethane acrylate hard coat agent (Arakawa Chemical Industries, Ltd., Beamset® 577, 100% solids content), 5 parts by mass of a photopolymerization initiator (BASF Japan, Irgacure® 184, 100% solids content), and 0.1 parts by mass of a leveling agent (Bic Chemie Japan, BYK307, 100% solids content) were mixed and diluted with a toluene / MEK = 1 / 1 solvent to prepare a hard coat layer forming solution (H-1) with a solids content of 40%.

[0165] (Examples 2-3) A polyester film and a hard coat film were obtained in the same manner as in Example 1, except that the longitudinal stretching ratio was changed as shown in Table 3.

[0166] (Example 4) A polyester film and a hard coat film were 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.

[0167] (Examples 5-14) A polyester film and a hard coat film were obtained in the same manner as in Example 1, except that the coating liquid for forming the easily adhesive resin layer was changed to P-2 to P-11, as shown in Table 3.

[0168] (Examples 15, 16) As shown in Table 4, polyester films and hard coat films were obtained in the same manner as in Example 1, except that hard coat coating solution H-1 was replaced with H-2 and H-3 below.

[0169] (Coating solution for hard coat layer formation: H-2) 30 parts by mass of pentaerythritol tritetraacrylate (Toagosei Co., Ltd., Aronics® M-306, 100% solids), 65 parts by mass of polyester acrylate (Toagosei Co., Ltd., Aronics® M9050, 100% solids), 5 parts by mass of photopolymerization initiator (BASF Japan, Irgacure® 907, 100% solids), and 0.1 parts by mass of leveling agent (Bic Chemie Japan, BYK307, 100% solids) were mixed and diluted with a toluene / MEK = 1 / 1 solvent to prepare a 40% by mass hard coat coating solution (H-2).

[0170] (Coating solution for hard coat layer formation: H-3) 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 BI-Chemie Japan, concentration 100%), and diluted with methyl ethyl ketone to prepare a hard coat coating solution (H-3) with a solid content of 40% by mass.

[0171] (Comparative Example 1) A polyester film and a hard coat film were obtained in the same manner as in Example 1, except that the film was stretched only in the width direction and not in the longitudinal direction, resulting in uniaxial stretching.

[0172] (Comparative Example 2) A polyester film and a hard coat film were obtained in the same manner as in Example 4, except that the film was stretched only in the width direction and not in the longitudinal direction, resulting in uniaxial stretching.

[0173] (Comparative Examples 3-7) A polyester film and a hard coat film were obtained in the same manner as in Example 1, except that the heat setting temperature was changed to 220°C and the PET pellets and thickness were as shown in Table 1.

[0174] (Comparative Example 8) A polyester film and a hard coat film were obtained in the same manner as in Example 1, except that the stretching ratio in the longitudinal direction was changed to 3.4 times.

[0175] (Comparative Examples 9-11) A polyester film and a hard coat film were obtained in the same manner as in Example 1, except that the coating solution was changed to P-12 to P-14 as shown in Table 3.

[0176] The evaluation results are shown in Tables 3 and 4.

[0177] [Table 3]

[0178] [Table 4]

[0179] As shown in Examples 1 to 16, when the refractive index of the polyester film having an easy-adhesion layer is within a certain range, and a hard coat layer is provided on the easy-adhesion resin layer of the polyester film formed by curing a composition containing at least one compound selected from titanium compounds and zirconium compounds and a polyester resin, the pencil hardness of the hard coat is also satisfied, and the interference spot improvement after continuous bending tests is also good.

[0180] In each example, when the cross-section was observed with a transmission electron microscope, it was found that the proportion of particles contained within the easily adhesive resin layer was smaller in the width direction (folded portion) than in the longitudinal direction (bending direction).

[0181] In Comparative Examples 1 to 8, where the film-forming conditions for the polyester film were changed, the refractive index of the polyester film having the easily adhering resin layer was not within the optimal range, and therefore, the resulting hard coat film did not meet the requirements for a satisfactory pencil hardness.

[0182] In Comparative Example 9, where the easy-adhesion resin layer was modified, neither the titanium compound nor the zirconia compound was contained, and the refractive index of the easy-adhesion resin layer was insufficiently adjusted. As a result, no improvement in interference spots was observed even before the bending test of the hard coat film. Furthermore, the dispersibility of the silica used in place of the metal compound was poor, and deterioration of interference spots was observed after the bending test.

[0183] In Comparative Example 10, where the easy-adhesion resin layer was modified, improvement in interference spots before the bending test was observed. However, the dispersibility of the silica, which was replaced with a titanium compound instead of a zirconia compound, was poor, and after the bending test, the interference spots worsened due to the occurrence of cracks in the easy-adhesion resin layer.

[0184] In Comparative Example 11, where the easy-adhesion resin layer was modified, the absence of polyester resin necessitated the addition of a large amount of titanium / zirconia mixed oxide to adjust the refractive index. As a result, aggregation occurred in the metal oxides within the coating, leading to a deterioration of interference spots after the bending test.

[0185] The hard coat films from each example and comparative example were 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 hard coat film was positioned 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 displays using the hard coat films from each example 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 hard coat films from each comparative example appeared to develop image distortion at the folding portion of the display as usage increased, and interference patterns appeared, resulting in poor visibility, which was undesirable. In addition, dents and scratches were observed on the surface of some displays. [Industrial applicability]

[0186] A foldable display using the hard coat film for foldable displays of the present invention maintains mass producibility while preventing deformation of the hard coat 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 hard coat 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]

[0187] 1: Foldable display 11: Bending radius 2: Polyester film for surface protection of foldable displays 21: Folding section 22: Bending direction (direction perpendicular to the folding part)

Claims

1. A hard coat film for a foldable display, comprising a polyester film with a thickness of 10 to 80 μm, having an easy-adhesion resin layer and a hard coat layer sequentially on at least one side, The easy-to-adhere resin layer is formed by curing a composition containing at least one compound selected from titanium compounds and zirconium compounds, fine particles, polyester resin, and surfactant. At least one compound selected from the titanium compound and the zirconium compound is: The average particle diameter is between 5 nm and 150 nm. The total amount of the titanium compound and zirconium compound is, relative to the easy-to-adhere resin layer, The concentration is 0.1% by mass or more and less than 15% by mass. The fine particles have an average particle size of 2 μm or less (excluding the titanium compound and the zirconium compound), The aforementioned surfactant is a silicone-based surfactant. The polyester film is a stretched polyethylene terephthalate film. The intrinsic viscosity of the resin pellets forming the polyester film is greater than 0.58 dl / g and less than 0.75 dl / g. The polyethylene terephthalate film comprises a copolymer of aromatic dicarboxylic acid and glycol. The film having the polyester film and the easy-adhesion resin layer is formed by applying the composition that forms the easy-adhesion resin layer to the surface of the polyester film, and then heat-setting it at a temperature of 230°C to 240°C. The aforementioned polyester film is The refractive index in the bending direction is 1.569 or more and 1.609 or less. The refractive index in the direction of the folding part is 1.679 to 1.

684. The refractive index in the thickness direction is 1.509 to 1.

516. The density is 1.385 to 1.

387. (Here, the bending direction is the longitudinal direction of the polyester film, The direction of the folded portion is the width direction of the polyester film; In a film having the polyester film and an easily adhesive resin layer, The refractive index and density measured under the condition that the easy-adhesion resin layer is formed such that the amount of coating after drying is 0.005 to 0.20 g / m2 satisfy the following conditions: The refractive index in the bending direction is 1.590 to 1.

620. The refractive index in the direction of the folding part is 1.670 to 1.

700. The refractive index in the thickness direction is 1.520 or less. Density is 1.380 g / cm³ ３ That's all. (Here, the bending direction is the longitudinal direction of the film having the polyester film and the easy-adhesion resin layer, The direction of the folded portion is the width direction of the film having the polyester film and the easy-adhesion resin layer; The polyester resin contained in the aforementioned easy-to-adhere resin layer is a polyester resin in which at least a portion of the dicarboxylic acid component is a naphthalenedicarboxylic acid component, among the dicarboxylic acid component and diol component that constitute the polyester resin. Hard coat film for foldable displays.

2. The hard coat film for a foldable display according to claim 1, wherein the refractive index of the easy-adhesion resin layer is lower than the refractive index in the bending direction and the refractive index in the folding direction of the polyester film having the easy-adhesion resin layer before lamination of the hard coat layer, and higher than the refractive index of the hard coat layer.

3. The hard coat film for a foldable display according to claim 1, wherein the refractive index of the easy-adhesion resin layer satisfies the following conditions; The refractive index of the easy-adhesion resin layer is lower than the refractive index in the bending direction of the polyester film having the easy-adhesion resin layer and before lamination of the hard coat layer, and the difference in refractive index is greater than 0 and 0.07 or less. The refractive index of the easy-adhesion resin layer is lower than the refractive index in the direction of the folded portion of the polyester film having the easy-adhesion resin layer and before lamination of the hard coat layer, and the difference in refractive index is 0.080 or more and 0.150 or less.

4. The polyester film having the aforementioned easy-adhesion resin layer has a total light transmittance of 85% or more, a haze of 3% or less, and a maximum heat shrinkage rate of 6% or less before lamination of the hard coat layer. However, the maximum thermal shrinkage rate is the shrinkage rate measured after leaving the sample film unloaded in an oven at 150°C for 30 minutes and then cooling it to room temperature. A hard coat film for a foldable display according to claim 1.

5. The composition forming the easy-to-adhere resin layer further comprises a crosslinking agent, the content of which the crosslinking agent is 5% by mass or more and 50% by mass or less of the total solid components of the easy-to-adhere resin layer. The hard coat film for a foldable display according to claim 1, wherein the average particle size of the fine particles contained in the easy-adhesion resin layer is 200 nm or more and 700 nm or less.

6. The aromatic dicarboxylic acid contained in the polyethylene terephthalate film comprises at least one selected from terephthalic acid, isophthalic acid, phthalic acid, or 2,6-naphthalenedicarboxylic acid. The hard coat film for a foldable display according to claim 1, wherein the glycol contained in the polyethylene terephthalate film comprises at least one selected from ethylene glycol, diethylene glycol, 1,4-butanediol, propylene glycol, or neopentyl glycol.

7. A foldable display in which the hard coat film for a foldable display according to claim 1 is arranged as a surface protective film such that the hard coat layer is positioned on the surface, and a single continuous hard coat film is arranged through the folding portion of the foldable display.

8. A portable terminal device having a foldable display as described in claim 7.

Images